論文 / 著書情報
Article / Book Information
題目(和文)
Title(English)

Design and control of the metal-molecule interface for highly conductive
single molecular junctions

著者(和文)

金子哲

Author(English)

Satoshi Kaneko

出典(和文)

学位:博士（理学）,学位授与機関:東京工業大学,
報告番号:乙第4127号,
授与年月日:2016年4月30日,
学位の種別:論文博士,
審査員:木口 学,河内 宣之,腰原
智昭

伸也,大島

康裕,河合

Citation(English)

Degree:,Conferring organization: Tokyo Institute of Technology,
Report number:乙第4127号,
Conferred date:2016/4/30,
Degree Type:Thesis doctor,
Examiner:,,,,,

学位種別(和文)

博士論文

Type(English)

Doctoral Thesis

Powered by T2R2 (Tokyo Institute Research Repository)明雄,西野ThesisDesign and control of themetal-molecule interface for highly
conductive single molecular junctions

Submitted to the Tokyo Institute of Technology
for the degree of Doctor of Science

Satoshi Kaneko

Department of Chemistry
Graduate School of Science and Engineering
Tokyo Institute of Technology

January 2016

Contents

1

2

Introduction

1

1.1

Single molecular junctions

1

1.2

Electron transport depending on metal species and molecule species

2

1.3

Role of interface in electron transport of single molecular junctions

3

1.4

Anchoring groups and direct-binding technique4

1.5

Purpose of this thesis

5

References

7

Theoretical background

11

2.1

Electric states in one-dimensional conductor

11

2.2

Electron transport in nano-sized conductor

13

2.3

Landauer-Büttiker formula

14

2.4

Electron transport in molecular junction

17

2.5

Coupling between electron transport and vibration

21

2.5.1

Inelastic electron tunneling spectroscopy

21

2.5.2

Point-contract spectroscopy

22

2.5.3

3

4

2

2

d I/dV spectrum behavior

23

References

24

Experimental concepts and techniques

26

3.1

Fabrication of single molecular junction

26

3.2

Break-down and re-formation of the metal nano contact

27

3.3

Experimental setup30

3.3.1 MCBJ setup30

3.3.2 Two-terminal conductance measurements

32

3.3.3

32

dI/dV measurements

References

33

Design of the interaction on single benzene molecular junction

34

4.1

Introduction

35

4.2

Experiments

35

4.3

Results and discussion

35
i

4.4

4.3.1

Conductance value and configuration

35

4.3.2

High and well-defined conductance value

38

Conclusions

39

References
5

Design of interface structure of single molecular junction utilizing sphericalendohedral Ce@C82 metallofullerenes

41

5.1

Introduction

42

5.2

Experiments

43

5.3

Results and discussion

43

5.3.1

Conductance and I-V characteristics

43

5.3.2

Formation process

48

5.4

6

40

Calculations

50

5.5 Conclusions

54

References

54

Anchoring groups enclosed in π-conjugated system in N2 molecule

58

6.1

Introduction

59

6.2

Experiments

60

6.3

Results and discussion

60

6.3.1

Pt electrodes

60

6.3.2

Cu electrodes

66

6.4

Conclusions

71

References
7

72

Controlling the electrical property of highly conductive single pyrazine molecularjunction

74

7.1

Introduction

75

7.2

Experiments

76

7.3

Results and discussion

77

7.3.1 Conductance77

7.3.2 dI/dV measurements and NEXAFS measurements

79

7.4 Calculations

81

7.4.1 Methods

81

7.4.2 Structure and conductance82

7.4.3 Vibrations

87
ii

8

7.5 Control of the electric conductance88

7.6 Conclusions

90

References

91

General conclusions

95

Acknowledgements

98

Achievements

100

iii

1 Introduction
1.1 Single molecular junctions
With the increasing concern regarding the limited amount of fossil fuel, there have been many
attempts to realize a more effective utilization of existing supplies. Dennard proposed that thescaling down of semiconductors would lead to energy savings and improved performance of theelectronic devices (1). Dennard’s scale theory has had a significant impact on the electricindustry, and has contributed towards the miniaturization of electronic devices. Recently
developed micro-fabrication techniques have accelerated the miniaturization of electronic
devices. Up to now, the integration ratio has been increasing, as predicted by Moore (2).
According to Moore’s law, the size of electronic components has decreased to the order of a few
nanometers. In the near future, it is expected that the size of devices will further decrease to theatomic scale; however, it is impossible to fabricate sufficient insulator layers on the atomic scale.
It is believed that we have approached practical limits with respect to the top-down method for
fabricating electronic devices. Therefore, the use of single atoms and molecules has attracted
much attention as a bottom-up method for the fabrication of minimum size electronic devices.
Aviram and Ratner predicted a single-molecule rectifier in 1974 (3), which triggered a variety ofstudies to fabricate a “metal/single molecule/metal structure” called a single molecular junction,and to investigate electron transport on the single molecular junction.
In the 1990s, van Ruitenbeek successfully measured the electric conductance of a single gold
atomic junction utilizing the break-junction method with an elastic metal substrate (4). This
research accomplished the measurement of the quantum conductance of 1 G0 (2e2/h = 13 kΩ).
This is very different from the figure of the bulk metal that obeys Ohm’s law. Further
investigation has been done with respect to a single hydrogen molecule that is placed between
platinum electrodes. Reed also measured the conductance of a single 1,4-benzenedithiol via
gold-sulfur connection by using scanning tunneling micro spectroscopy (STM) (5). Theirmethod, which uses sulfur atoms, enables us to fabricate the metal-molecular junction: The
sulfur atom works as an anchoring group and enables us to form a rigid bond between themolecular backbone and metal electrodes. The concept of the anchoring group and synthesis
technique led to the development of a variety of single molecular junctions that have properties
such as negative differential conductance (6, 7), rectification (8, 9), and switching (10, 11).
Recently, not only the conductance value, but also a variety of physical quantities has been
investigated for many single molecular junctions. Current-voltage (I-V) characteristics have1

revealed molecular level position (12, 13). Thermopower measurements (14-16) were
performed considering the concept that a single molecular junction has potential for applicationin thermoelectric materials. Thermopower in a single molecular junction revealed the carriers
and the energy difference between the molecular orbital and the fermi level of 1,4-benzene
dithiol and 4-4’ bipyridine, and fullerene (15-17). The differential conductance measurement
addressed the effect of the electron-vibration coupling on the current in a single molecularjunction (18-20). Shot-noise measurements revealed the transmission probability of thehydrogen single molecular junction (21) and the magnetic state in the Pt atomic chain (22).
These efforts made single molecular junctions more attractive from a scientific perspective as asystem that provides fundamental physical phenomena in nanoscale systems in addition to thepotential that is realized.
This research flow indicates that the high conductive single molecular junction is an attractive
system in the science and engineering fields. It is clear that high conductivity is a practicalbenefit for electric compounds. Highly conductive single molecular junctions reduce energy
consumption in the on-states, and they produce strong signals to be extracted. In addition to its
engineering benefit, a highly conductive single molecular junction is attractive because of the
manifestation of its novel property as a meta-molecule system because molecular orbital andmetal atomic states are effectively hybridized in highly conductive molecular junctions.

1.2 Electron transport depending on metal species and molecule
species
In order to discuss the electron transport of the single molecular junction, the contribution fromeach part of the single molecular junctions is separated into several parts, namely, the molecular
backbone, interfaces, and metal electrodes. The molecular backbone determines the size of theenergy gap between the highest occupied molecular orbital (HOMO) and lowest unoccupied
molecular orbital (LUMO). Because the Fermi energy gap is located between the HOMO andLUMO in a usual single molecular junction, a smaller energy gap between the HOMO andLUMO results in a smaller energy difference between the molecule’s orbital energy and EF,
leading to higher conductivity. A simple way of reducing the HOMO-LUMO energy gap is touse π-conjugated molecules for the molecular backbone. The π-conjugated molecule has asmaller HOMO-LUMO gap compared with a single molecular junction with a saturatedhydrocarbon system. However, recent studies have revealed that increasing aromatic property
does not always result in an increased conductivity of the single molecular junction (23, 24).
2

The effective energy difference between the molecular orbital and EF is important for highconductivity. The type of metal used for electrodes also affects the conductivity. The Fermi
energy positions, the density of states at the Fermi energy, and the number of channels involvedin the transport are different from the metal types. For example, transition metals such as Pt
have large density of states at the Fermi energy, and these metals tend to have multiple channels,
which lead to high conductivity compared with gold, silver, and copper.

1.3 Role of interface in electron transport ofsingle molecular junctions
Another area that contributes to electron transport is the interface between molecules and metals.
The effect of the metal-molecule interface is also discussed in bulk-sized organic conductors
such as light emitting diodes, because the interface structure determines the charge injection of
the organic conductor. Briefly, the charge injection depends on the dipole on the metal surface,electric affinity, and ionic potential of the adsorbed molecule (25-27). In a single molecularjunction, configuration of a molecule between electrodes, as well as the adsorption site, plays adecisive role in the electron conductance. Even when we keep the same molecules between
metal electrodes, the conductance value drastically changes depending on the molecularconfiguration. For example, it is reported that 1,4’-benzenedithiol molecular junction, which isone of the most widely investigated molecular junctions, showed a conductance value in therange of 0.0001-0.1 G0 (5, 28-32). It was reported that the variability of the conductance values
originate from the variable configuration that molecules have in the single molecular junction.
Previous studies have revealed that changes in the molecular configuration change reduce theenergy level of the molecules that are bridged between electrodes (7, 33). The energy shift of themolecule induced the conductance change of the molecular junction. The molecularconfiguration changes the orbital coefficients at the connecting point. The large orbital
confidents induce the effective hybridization between the metal and molecule, leading to a large
electric coupling. The effect of configurational change on the anisotropic orbital distribution
was investigated utilizing an H2TPP molecule (7). The H2TPP molecule had two conductancestates depending on the orbital configuration of the HOMO-2 orbital (7). Heiss et al. precisely
investigated the electron-coupling effect on the conductance using STM. The tilt angle of themolecule affects the coupling between the phenyl ring and metal electrodes (34). Kitaguchi
similarly investigated the metal-molecule interaction between the phenoxy molecule and Cu
substrate (35). A strong interaction via oxygen atom forms a strong bond between phenoxy and3

Cu. On the other hand, there is a somewhat weak interaction between the phenyl ring and the tip
compared to that of the substrates. The interaction between the phenyl ring and tip is precisely
controlled by the tilt angle of the phenyl ring. The conductance value is also affected by theelectric states of the metal atom near the oxygen atom. Queck et al. observed a mechanicalswitch using bipyrizine molecule (11).
The single molecular junction shows a variety of conductance values depending on theconnecting points. A theoretical study showed that the benzene molecule connected with thepara position has a different conductance value from benzene with a meta position because of
the quantum-interface effects (36), which are based on the property of the electron as waves.
When electron waves propagate through the two terminal ring-shaped mesoscopic structures,the partial electron wave at each branch interferes with each other destructively or
constructively depending on the phase of the electron waves. A theoretical study gives a simple
example for the benzene molecule. The wave functions constructively interfere in the para
position; however, they interfere destructively in the meta position. In order to realize
constructive interference, it is important to have the same sign for C and C* in Eq. (2.42) (37).
The molecules that have a cross-conjugated system are expected to exhibit the destructive effectby their conductance values. Compounds that have cross-conjugated systems showed distractive
quantum-interference effects (38).
As mentioned above, single molecular junctions tend to have a variety of conductance values
for specific single molecular junction, and most studies have not determined the molecularconfiguration in the single molecular junction. Uncertainty about the molecular configuration,and not the fixed conductance value, is an obstacle to discussing the electron transport of thesingle molecular junction and fabrication of electronic components.

1.4 Anchoring groups and direct-binding technique
In order to fabricate single molecular junctions with fixed conductance values, many attempts
have been made to determine how to connect the molecular backbone to metal electrodes. In asingle molecular junction, the molecular backbone is connected to metal electrodes using a partcalled an anchoring group. The main role of anchoring groups is to connect the molecular
backbone to metal electrodes. The other roles of anchoring groups are to separate the molecular
backbone from metal electrodes, and to protect the property of an isolated molecule in a single
molecular junction. One of the most famous anchoring groups is thiol. The strength of thegold-thiol bond is about 1 eV, which enables it to form stable bonds, and we can fabricate photo4

switches, rectifiers and other electronic components by protecting the original molecule’s
property.
With respect to anchoring groups, systematic studies about the benzene derivatives revealed
that the amine, cyano, and carboxyl groups showed well-defined conductance values (39-41).
The hybridized states originating from the lone pair in amine helps the single molecular junction
to maintain fixed conductance value with different tilt angles (39, 41). Recent synthesizing
techniques have been developed to fabricate single molecular junctions having three anchoringgroups at each side, and an anchoring group containing fullerene molecules.
Actually, these anchoring groups enabled us to successfully fabricate a single molecularjunction with fixed conductance values and to reproduce the function of an isolated molecule ina single molecular junction; however, some issues still need to be overcome. The observed
phenomena are similar to those in isolated states with reduced performance. For example, thefabricated photo switching utilizing diarylethene derivatives showed a conductance value of0.001 G0, resulting in a low on-off ratio (10). Because anchoring groups prevent hybridization
between the molecular backbone and metal electrodes in order to protect the intrinsic propertyof the molecular backbone, they also function as obstacles to electron transport (42). As long as
we use the anchoring group in a single molecular junction, there remain limitations in terms of
our ability to improve the electron transport.
In this study, I eliminated the concept of the anchoring group, and aimed to hybridize themolecular orbital with metal electric states. Kiguchi et al. demonstrated that benzene molecules
showed high conductance values with Pt electrodes (43). The direct binding via the π orbital inaromatic molecules enables molecular orbitals to be effectively hybridized with the orbitals of
metals, leading to not only improvements in the conductance value, but also makes the single
molecular junction a novel material. However, it is difficult to fabricate the single molecularjunction with a well-defined conductance value without anchoring groups. In highly conductive
single molecular junctions, molecular orbitals are hybridized with metal electric states. Highly
conductive single molecular junctions have potential as novel materials whose properties aredifferent from those of isolated molecules and metal species.

1.5 Purpose of this thesis
The purpose of this thesis is to fabricate a highly conductive single molecular junction with awell-defined conductance value, and to control its electric property by designing the interface
structure between a metal and molecule. In order to clarify the molecular configuration and5electric states of a single molecular junction, I investigated the fabricated single molecularjunction by performing inelastic tunneling electron spectroscopy or using current-voltage
characteristics as well as conductance measurements.
In this thesis, I focused on the direct-binding technique utilizing a π-conjugated moleculewith the aim of fabricating a highly conductive single molecular junction. In chapter 4, I present
details regarding the fabrication of a benzene molecular junction with a well-defined
conductance value, focusing on the interaction between benzene and metal. By designing theinterface to have proper interaction between metal and benzene, the molecular configurationalchange can be limited. Proper interaction enables us to form a single molecular junction only inthe most stable configuration, which leads to a well-defined conductance value. On the other
hand, the metal-molecule interface is designed such that it maintains the conductance value evenwhen the molecular configuration changes, as discussed in chapter 5. Fullerene, which has aspherical shape, is expected to have a fixed conductance value with configurational changes. In
chapters 6 and 7, the anchoring part is enclosed in the π-conjugated system in order to control
the electric property of the single molecular junctions. The nitrogen atom in the π-conjugatedsystem is expected to maintain a high conductivity with a fixed molecular configuration. The
lone pair on the nitrogen atom is orthogonal to the π system, which is expected to have twostable configurations, a π-connected one and a lone-pair-connected one. By controlling theinterface structure, it is expected to control the electric property of the single molecular junction.
In chapter 6, the use of the nitrogen molecule is investigated as the simplest π-conjugatedsystem that includes nitrogen atoms. The formation and electric transport of nitrogen molecules
are investigated for each metal species. In chapter 7, the pyrazine molecular junction isinvestigated using Pt electrodes. By controlling the metal-molecule distance, the molecularconfiguration can be controlled, as well as the electrical properties. The two conductance states
are fabricated depending on the configuration of the pyrazine molecule. The configuration of thepyrazine molecule is investigated by the dI/dV spectrum to determine the configuration of thebenzene molecule. The high and low conductance states were assigned to the configuration with
its p orbital being both parallel and vertical to the binding direction. Finally, the electricalproperties of the single molecular junction are controlled by controlling the interface structure,and the two atomic configurations were successfully switched using an external force. The
ability to control the electrical property using a mechanical force is a unique property of thesingle molecular junction because it is difficult to change the electrical property of a conductor
using an external force. The tiny structural change in the interface induced the drastic change inthe conductance value of the single molecular junction. The observed behavior may enable us to6

apply the single molecular junction as a novel material whose molecular orbital strongly
correlates with metal orbitals.

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10

2 Theoretical background
2.1 Electric states in one-dimensional conductor
One-dimensional conductors have different conductance states when compared withthree-dimensional conductors (1, 2). Here, we consider a one-dimensional conductor that has ananoscale in the x and y directions, but is infinite in the z direction. The Schorödinger equation
can be expressed as in Eq. (2.1).

[−

ℏ2 2
∇ + 𝑉(𝑅)] 𝜑(𝑅) = 𝐸𝜑(𝑅) … (2.1)
2𝑚

Because the potential in the z direction is zero, the system can be described as a free electron
model. The three-dimensional potential V(R) can be described as V(x, y). The wave function in
the z direction can be described as φ = φ(x,y)exp(ikzz), and the Schorödinger equation isexpressed as in Eq. (2.2) and Eq. (2.3).

[−ℏ2 𝜕 2𝜕2
𝜕2( 2 + 2 + 2 ) + 𝑉(𝑥, 𝑦)] 𝜑(𝑥, 𝑦)exp(𝑖𝑘𝑧 𝑧) = 𝐸𝜑(𝑥, 𝑦)exp(𝑖𝑘𝑧 𝑧) … (2.2)
2𝑚 𝜕𝑥
𝜕𝑦
𝜕𝑧

ℏ2 𝑘𝑧2 ℏ2 𝑑 2
𝑑2[−( 2 + 2 ) + 𝑉(𝑥, 𝑦)] 𝜑(𝑥, 𝑦)exp(𝑖𝑘𝑧 𝑧) = 𝐸𝜑(𝑥, 𝑦)exp(𝑖𝑘𝑧 𝑧) … (2.3)
2𝑚
2𝑚 𝑑𝑥
𝑑𝑥
The wave function is satisfied by Eq. (2.4).
ℏ2 𝑘𝑧2[] exp(𝑖𝑘𝑧 𝑧) = 𝐸𝑧 𝑒xp(𝑖𝑘𝑧 𝑧) … (2.4)
2𝑚ℏ2 𝑘 2

Therefore, 𝐸𝑧 = 2𝑚𝑧 in the z direction. On the other hand, the wave function is satisfied by Eq.
(2.5) in the x and y directions.

[−

ℏ2 𝑑2
𝑑2
( 2 + 2 ) + 𝑉(𝑥, 𝑦)] 𝜑(𝑥, 𝑦) = 𝐸𝑥,𝑦 𝜑(𝑥, 𝑦) … (2.5)
2𝑚 𝑑𝑥
𝑑𝑥

When we assume a rectangular conductor, 𝜑(𝑥, 𝑦), 𝐸𝑥,𝑦 , are the same as the eigen function and11eigen energy, respectively, in quantum wells. Hence, the eigen energy of a one-dimensional
conductor can be expressed as follows:
𝐸 = 𝐸𝑖,𝑗 +ℏ2 𝑘𝑧2… (2.6)2𝑚

φ = exp(ikzz) … (2.7)
Figure 2.1(a) is the band distribution based on Eq. (2.6) (1). The parabola for each index
notation i, j is called an electric sub band. The density of states of the sub band Dij is expressed
as follows, and the density of states can be described as in Figure 2.1(b) (1).

𝐷𝑖𝑗 =
𝐷𝑖𝑗 =

4𝐿
ℎ𝜐𝑖,𝑗

𝑑𝑁𝑖,𝑗 𝑑𝑘… (2.8)𝑑𝑘 𝑑𝐸(𝐸 > 𝐸𝑖,𝑗 ) … (2.9)

𝐷𝑖,𝑗 = 0 (𝐸 < 𝐸𝑖,𝑗 ) … (2.10)
where vi,j is the velocity of the electron in the i, j sub band. When we have different sub bands,
the sub bands can be called “modes” and “channels”.

Figure 2.1: Schematic image showing (a) electric states and (b) density of states of
one-dimensional conductor.

122.2 Electron transport in nano-sized conductor

Figure 2.2: Schematic image of (a) diffusive and (b) ballistic electron transport.
The electron transport of an atomic-sized conductor is different from that of a bulk-sized
conductor. In a bulk-sized conductor, the electron transport is diffusive, as shown in Figure
2.2(a). The electrons scatter at scattering point such as impurities and defects. The conductanceof the bulk-sized conductor obeys Ohm’s law, and it is proportional to the area S and electric
conductivity σ, as shown in Eq. (2.11).
G = σS / L…(2.11)

When the size of the conductor reaches an atomic scale, electron transport is influenced by
the conductor size compared with the mean free path (l) and phase-coherence length (lφ.). The
mean free path, which represents the distance travelled by an electron between collisions, isabout 100 nm. On the other hand, lφ represents the distance within which the phase information
is preserved. For gold, lφ is of the scale of 1 μm. Therefore, the single atomic and molecularjunctions discussed in this thesis are smaller than l, lφ. In this region, electrons travel ballisticallythough the conductor, as shown in Figure 2.2 (b).

13

2.3 Landauer-Büttiker formulaHere, we consider the electron transport through an atomic-sized conductor utilizing thescattering approach. In this approach, the sample works as a scattering point, which connects tothe left (L) and right (R) electrodes over which the electron scattering is negligible, as shown in
Figure 2.3 (3).

Figure 2.3: Schematic view of scattering approach for two terminal conductors.
The electrodes work as a reservoir that is the source of carriers, which are determined by the
Fermi-Dirac distribution function. The Fermi-Dirac distribution function is described asfollows:𝑓α =1
𝐸 − 𝜇α
exp [
]+1
𝑘B 𝑇α

… (2.12)

where the parameter μα and Tα, represents the chemical potential and temperature of the left andright electrodes, respectively. kB is the Boltzmann constants. As discussed in the previous section,
the wave function can be expressed separately in the longitudinal and transverse directions, andits energy is expressed as in Eq.(2.6). The electron transport can be separated into incoming andoutgoing states depending on the sample for each electrode. In order to express incoming andoutgoing states, we introduced creation and annihilation operators. For the outgoing states, we
expressed the creation and annihilation operators as ̂aαn (E), â†αn (E). For incoming states, we
expressed the operators as b̂αn (E), b̂†αn (E). The index, n represents the quantum number todescribe the quantum channels. The two operators were connected via the scattering matrix S,

14

âL1 (E)b̂L1 (E)( … ) = 𝑺 ( … ) … (2.13)
âRNR (E)b̂RNR (E)

The scattering matrix S is described using the transmitting (t) and reflecting states (r) as follows:
𝑆 = (𝑟𝑡𝑡 ′ ) … (2.14)𝑟′

The dimensions of each element are characterized by the quantum number of each electrode,which is related to the number of channels. The dimensions of each element are NL × NL, NR ×
NR, NR × NL, and NL × NR, for r, r’, t, and t’, respectively. In total, the dimensions of thescattering matrix are (NL + NR) × (NL + NR). The flux conservation and time-reverse symmetry
leads to the unique character of unitarily and symmetricity of the scattering matrix. The current
operator in the left electrodes can be described using the field operator ΨL.

𝐼̂L (z. t) =

ℏ𝑒
∂
∂ †
̂ L† (𝒓, t) Ψ
̂ L (𝒓, t) − ( Ψ̂ (𝒓, t)) Ψ̂ L (𝒓, t)] … (2.15)∫ d𝒓𝐓 [Ψ
2i𝑚
∂z
∂z L

̂ L (𝒓, t) = ∫ d𝐸e−i𝐸t/ℏ ∑
Ψ
i𝐸t

NL

χLn (𝒓𝐓 )n=1 √2πℏυLn (𝐸)̂ L† (𝒓, t) = ∫ d𝐸e ℏ ∑ΨNL[âLn ei𝑘Ln z + b̂Ln e−i𝑘Ln z ] … (2.16)

χ ∗Ln (𝒓𝐓 )n=1 √2πℏυLn (𝐸)[â†Ln e−i𝑘Ln z + b̂†Ln ei𝑘Ln z ] … (2.17)

rT is the transverse coordination and z is the coordination along the leads. χLn (𝒓𝐓 ) are thetransverse wave functions. Because the observable values do not change drastically, especiallynear the Fermi energy, we can neglect the energy dependence of the quantities. For the condition
where the physical quantity does not depend on the energy, the current operator is expressed asfollows:

𝐼̂L (t) =𝑒′∑ ∫ d𝐸d𝐸′ei(𝐸−𝐸 )t/ℏ [ â†Ln (𝐸 ′ )âLn (𝐸) − b̂†Ln (𝐸 ′ )b̂Ln (𝐸)]2πℏ n… (2.18)

where n̂+Ln (E) is the operator of the occupation number of the incident electrons in the left
leads, n̂−
Ln is the operator of the occupation number of the out-going electrons in lead L inchannel n, and n̂+̂−
Ln is expressed as Eq. (2.19) and (2.20).
Ln (E), n15n̂+̂ Ln (𝐸′) â†Ln (𝐸) … (2.19)
Ln (𝐸) = â̂†n̂−Ln (𝐸) = bLn (𝐸′) bLn (𝐸) … (2.20)
Now, E’ = ħΩ + E, and the integral over Ω gives𝐼̂L (t) =𝑒∑ ∫ 𝑑𝐸[n̂+̂−
Ln (𝐸, t)] … (2.21)
Ln (𝐸, t) − n
2πℏ 𝑛

n̂±
Ln (𝐸, t) are the time-dependent occupation numbers. Eq. (2.21) shows that the current can be
expressed as the difference in the occupation number in the electrodes. Eq. (2.21) is thenre-expressed using the scattering matrix s,′

𝐼̂L (t) =i(𝐸−𝐸 )t𝑒′
∑ ∑ ∫ 𝑑𝐸𝑑𝐸′ e ℏ â†𝛼𝑚 (𝐸 )𝐴𝑚𝑛̂𝛽𝑛 (𝐸 ′ ) … (2.22)
𝛼𝛽 (𝐿; 𝐸, 𝐸 )a
2πℏ
αβ mn

†
′
′
𝐴𝑚𝑛
𝛼𝛽 (𝐿; 𝐸, 𝐸 ) = 𝛿𝑚𝑛 𝛿𝛼𝐿 𝛿𝛽𝐿 − ∑ 𝑆𝐿𝛼;𝑚𝑘 (𝐸)𝑆𝐿𝛽;𝑘𝑛 (𝐸 ) … (2.23)𝑘Note, that α and β represent the electrodes R and L,respectively. SLα;mk(E) is the element of thescattering matrix that relates b̂Lm (E) to âαk (E). At the equilibrium, the mean square of theannihilation operator is〈â†𝛼𝑚 (𝐸 )â𝛽𝑛 (𝐸 ′ )〉 = 𝛿𝛼𝛽 𝛿𝑚𝑛 𝛿(𝐸 − 𝐸 ′ )𝑓𝛼 (𝐸)… (2.24)

The mean current in the junction is derived from Eq. (2.22).

〈𝐼𝐿 〉 =𝑒∫ 𝑑𝐸𝑇𝑟[𝑡 † (𝐸)𝑡(𝐸)][𝑓𝐿 (𝐸) − 𝑓𝑅 (𝐸)]2πℏ

… (2.25)

The matrix t is the off-diagonal block of the scattering matrix, and tmn = SRL;mn. At the limit ofzero temperature and small voltage, the electric conductance G is𝐺=

𝑒2
𝑇𝑟[𝑡 † (𝐸𝐹 )𝑡(𝐸𝐹 )]ℎ… (2.26)

The matrix [𝑡 † (𝐸)𝑡(𝐸)] can be diagonalized. The eigenvalues of 𝑡 † (𝐸)𝑡(𝐸) give thetransmission probability, Tn, and the mean current and conductance are expressed as follows:16

〈𝐼𝐿 〉 =𝑒∫ 𝑇𝑛 𝑑𝐸[𝑓𝐿 (𝐸) − 𝑓𝑅 (𝐸)] … (2.27)
2πℏ
𝐺=

𝑒2
∑ 𝑇𝑛 … (2.28)ℎ

When we consider the spin degeneracy, the Landauer-Büttiker expression is derived as𝐺=2𝑒 2∑ 𝑇n … (2.29)ℎ

The conductance is expressed using only the transmission probability. Note that the conductanceof the bulk metal depends on the temperature. 2e2/h is referred to as the quantum unit, and isexpressed as G0, which has a conductance of about 77 μS or 13 kΩ. If the transmission
probability of all transmission channels is 1, the conductance is an integer multiple value of G0.

2.4 Electron transport in molecular junction
The model for molecular conductors was proposed by Paulsson. In this model, a moleculebehaves like a quantum dot (4). We assume that the electron tunneling with double barriers (5)is as shown in the upper panel of Figure 2.4.

Figure 2.4: Schematic image of resonant-tunneling model with double barrier and schematic
image of single molecular junction.

17

The scattering matrix for barriers 1 and 2 are expressed as follows:𝑟
𝑆1 = ( 1
𝑡1

𝑡′1
),
𝑟′1

𝑟
𝑆2 = ( 2𝑡2𝑡′2) … (2.30)
𝑟′2Both S1 and S2 are unitary, and the amplitude A of the wave function is given by Eq. (2.31).
𝑡1 𝑡2 𝑒 𝑖𝑘𝑊𝐴=… (2.31)
1 − 𝑟2 𝑟′1 𝑒 2𝑖𝑘𝑊
W is the distance between two barriers, and k is the momentum of the electron. The
transmission probability T0 is given by the square of the amplitude.

𝑇0 =

𝑇1 𝑇2
1 − 2√𝑅1 𝑅2 cos(2𝑘𝑊 + 𝛿) + 𝑅1 𝑅2… (2.32)

Tγ and Rγ are the transmission and reflection at barriers 1 and 2, respectively. δ is the declinationof r2 and r’. T0 is a maximum at

2kW + δ = 2πl (l is an integer)…(2.33)

Eqs. (2.32) and (2.33) imply that the energy in the quantum dot is discrete. When the energy of
the incident electron is equal to the discrete energy level, T0 has a maximum at the resonanceposition. Now, T1 and T2 are small, and the resonant position shift from φ = φ0 to φ = φ0 + δ. The
momentum k is expressed by k = [2m(E+εφ)/ħ2]1/2, and k is shifted to k + (ε/vFħ). The
transmission probability T0 is expressed by

𝑇0 ≈

𝑇1 𝑇2
2𝑊𝜀 2
(1 − √𝑅1 𝑅2 ) + √𝑅1 𝑅2 ( ℏ𝑣 )
𝐹
2

𝑇0 ≈

𝛤1 𝛤2
(𝛤 +𝛤 )2
𝜀2 + 1 4 2
𝛤𝑖 =… (2.35)ℏ𝑣𝐹 𝑇𝑖… (2.36)
2𝑅

18… (2.34)

Here, Гi is the transition probability for the transition when an electron travels from the discretelevel to the electron reservoir. The resonance is expressed by the Lorenz curve. The width Г isexpressed as the summation of the transition probabilities, Г1 and Г2. Considering the single
transmission channel, the conductance value is given by

𝐺=

2𝑒 2
𝛤1 𝛤2
ℎ 𝜀 2 + (𝛤1 +𝛤2 )2 /4… (2.37)

In single molecular junction systems, the double barriers are comparable to tunnel barriers
between the molecule and electrodes. The discrete energy level is comparable to the energy
level of the molecule. In the molecular junction, Г1 is described by the relaxation time (4) as

Гi = eħ/τi …(2.38)
Γ represents the line width of the energy level after interacting with metals, as shown in Figure2.5. When the electric coupling increases, the relaxation time decreases, leading to a larger Г,which means that Г represents the electric coupling between the metal and molecule (4).

Figure 2.5: Schematic image metal electric states and molecular level.
The Fermi energy is located between HOMO and LUMO. Because the energy level near the
Fermi level plays an important role in electron transport, the discrete level that we refer to is
HOMO or LUMO. When the energy difference between HOMO and EF is smaller than thatbetween LUMO and EF, the carrier is a hole. Otherwise, the carrier is electron. Eq. (2.37)
indicates that the smaller energy difference between the molecule’s orbital and EF, as well as thelarger coupling Γ result in a large conductance value in the molecular junction.
Although the scattering approach explains physical quantities such as the conductance,
thermopower, and shot noise, it is difficult to express the properties of an individual single
molecular junction. In the scattering approach, it is difficult to contain the intrinsic physicalproperty of a molecule and metal electrodes. In most cases, the transmission probability of the19single molecular junction is expressed using advanced and retarded non-equilibrium greenfunction GR/A.



T (E)  Tr  L E G R E  R E G A E 

…(2.39) …(2.40)

G R / A E   E     LR / A E    RR / A E 

1where I and H are the unit matrix and Hamiltonian matrix of the molecule, respectively. Γ isexpressed by the self-energy ΣL(R)R/A.

L R   i  RL  R    LA R 

 …(2.41)

The real part of the self-energy gives the energy shift of the resonance position in Figure 2.5,
and the imaginary part gives the broadening of the energy level. ΓL(R) represents the broadening
function. For a weak coupling system, Green’s function can be obtained from the zeroth Green’s
functions that are calculated separately for the electrodes and the molecule.
0  R / A
E   k
GRL

C R ,k C L* ,kE   k  i…(2.42)where CR,k, εk, and η represent the k-th molecule’s orbital coefficient at site R, the k-th
molecule’s orbital energy, and the infinitesimal value, which is determined by the relationshipbetween Green’s function and the DOS. The term CC* represents the orbital distribution of amolecule that is bridged between metal electrodes. The NEGF method explained the
conductance of graphene, the single molecular junction, and many nano-sized conductors (6).
Eqs. (2.39) – (2.42) express the physical effects expressed by scattering models. The
transmission probability derived by the Green function explained the effects of the orbitalalignments and DOS of the metal electrodes.

202.5 Coupling between electron transport and vibration
2.5.1 Inelastic electron tunneling spectroscopy
In single molecular junctions, it is important to evaluate the interface structure. Althoughscanning electron microscopy (SEM) images are successfully observed for gold atomic contacts,
their observation is limited to fullerene molecules in studies regarding single molecularjunctions (7). To identify a single molecule, Stipe demonstrated the differential conductancemeasurement of acetyl molecule on the Cu surface. The injected electron excites the vibration of
the C-H stretching mode of the acetylene molecule (8), as shown in Figure 2.6. The first
derivative of the differential conductance dependence is called IETS (inelastic electron
tunneling spectroscopy). In this section, the differential conductance behavior in an
atomic-sized conductor is discussed.

Figure 2.6: IETS of acetylene molecule on Cu surface (8). Copyright (1998) by the American
Association for the Advancement of Science.
The electron-vibration coupling in the tunneling regime is described, as shown in Figure 2.7.
When V < Vk, the electron tunnels are from the left electrode (L) to the right electrode (R).
When the electron obtains sufficient energy to excite the vibration (eVk > hω), the inelastic
tunneling channel opens. The opening of the additional channel slightly increases the
conductance of the junction. Therefore, the peak structure is observed in the d2I/dV2 spectrum inthe tunneling regime.
In order to excite the vibration by tunneling electrons, the vibration should be coupled withthe molecular orbital, which mainly contributes to the transport of electrons. The tunnelingcurrent that is enhanced by electron-vibration coupling is expressed as Eq. (2.43).
21

Itunnel = <Ψin|M|Ψfm>… (2.43)
Ψin and Ψfm are the initial and final states of inelastic scattering. M represents theelectron-vibration coupling matrix.

Figure 2.7: (a) Schematic image of electron transport in tunneling regime. (b) Typical dI/dV
spectrum in the tunneling regime.

2.5.2 Point-contact spectroscopy
In the case of nano-sized conductors with ballistic electron transport, the dI/dV spectrum iscalled point-contact spectroscopy (PCS). In the ballistic transport regime, energy dispersion is
described using a free electron model, as shown in Figure 2.8(a). In the ballistic regime, anelectron occupies the states having momentum from -kz to +kz without a bias voltage. Note, thatkz is the momentum in the direction that is parallel to the atomic junction. At the bias voltage ofVk, the electron obtains sufficient energy to excite the vibration mode. Then, the electron transits
from the occupied states having a momentum of kz to the unoccupied states having a momentumof –kz, which means that electrons are back-scattered, leading to a reduction in the conductancevalue. Therefore, a dip structure is observed at the vibration energy in the d2I/dV2 spectrum inthe ballistic regime, as shown in Figure 2.8(b).

22Figure 2.8: (a) Energy dispersion in the ballistic regime. (b) Typical dI/dV spectrum in theballistic regime.

2.5.3 d2I/dV 2 spectrum behavior
In a realistic single molecular junction, the differential conductance spectrum behavior isgoverned by the transmission probability T, where we assume a single channel. The scattering
approach also explains the limited understanding of the dI/dV spectrum behavior. In the ballistic
regime, the ongoing states are fully occupied because almost all electrons travel from the left toright electrodes. At Vk , the only states to which the electrons can transit are the unoccupied
incoming states, leading to a reduction of the conductance. On the other hand, in the tunnelingregime, almost all electrons are reflected, and the incoming states are occupied. At Vk,, the only
states to which electrons can transit are unoccupied outgoing states, leading to an increase in the
conductance value. Paulsson et al. explained the d2I/dV2 spectrum behavior depending on thesymmetricity (α) of the junction and transmission (9, 10), as shown in Figure 2.9. When we
have the symmetric junction (α = 1), the dI/dV response behavior changes from the peak to dip
at T = 0.5. This theory was experimentally verified with an H2O molecule and1,4-benzenedithiol molecule, which have single channels (11, 12).

23

Figure 2.9: d2I/dV2 spectrum behavior depending on the symmetry and transmission (10).
Copyright (2008) by the American Physical Society.
The dI/dV curve also shows a characteristic structure when the tunneling electron induceselementary process of reaction triggered by the vibration excitation (13-16). When the vibration
coordination is the same as the reaction coordination, the reaction is induced by vibration with alarger energy than the reaction barrier. The hydrogen dissociation reaction of the trans-2-butene
molecule on Pd (110) was observed (14). When the vibration coordination is not equal to thereaction coordination, the reaction is induced by the coupling between tunneling electrons andvibration modes along the reaction coordination. In this mechanism, the CO molecule diffuseson the Cu substrate by the injection of the tunneling electrodes (15). In a single molecularjunction, the transition between two meta-stable structures is observed by the injection of thetunneling electrons. The peak structure was observed in the dI/dV spectrum.
It has been reported that when a magnetic impurity exists in the nano-contacts, a peak or dip
is observed at V = 0, which is due to the Kondo effect. The Kondo effects is explained in termsof the many-body scattering problem. In a metallic atomic contact, this zero bias anomaly isalso observed for Pt atomic junctions.

References
1.

C. Kittel, Introduction to Solid State Physics. (Wiley, US, ed. 8, 2004).

2.

J. H. Davies, The Physics of Low-dimensional Semiconductors An Introduction.
24

(Camgridge university press, Cambridge, UK, 1997).
3.

Y. M. Blanter, M. Buttiker, Shot noise in mesoscopic conductors. Phys. Rep. 336, 1-166(2000).

4.

M. Pausson, F. Zahid, S. Datta, Handbook of Nanoscience, Engineering, andTechnology. (CRC Press US, 2003).

5.

H. Fukuyama, シリーズ物性物理の新展開「メゾスコピック系の物理」. (maruzen,
1996).

6.

T. Tada, K. Yoshizawa, Quantum transport effects in nanosized graphite sheets.
Chemphyschem 3, 1035-1037 (2002).

7.

M. Yoshida, Y. Kurui, Y. Oshima, K. Takayanagi, In-situ observation of the fabrication
process of a single shell carbon fullerene nano-contact using transmission electron
microscope-scanning tunneling microscope. Jpn. J. Appl. Phys. 46, L67-L69 (2007).

8.

B. C. Stipe, M. A. Rezaei, W. Ho, Single-molecule vibrational spectroscopy andmicroscopy. Science 280, 1732-1735 (1998).

9.

T. Shimazaki, Y. Asai, Theoretical study of the lineshape of inelastic electron tunnelingspectroscopy. Phys. Rev. B 77, 5428 (2008).

10.

M. Paulsson, T. Frederiksen, H. Ueba, N. Lorente, M. Brandbyge, Unified descriptionof inelastic propensity rules for electron transport through nanoscale junctions. Phys.
Rev. Lett. 100, 226604 (2008).

11.

O. Tal, M. Krieger, B. Leerink, J. M. van Ruitenbeek, Electron-vibration interaction insingle-molecule junctions: from contact to tunneling regimes. Phys. Rev. Lett. 100,
196804 (2008).

12.

Y. Kim, T. Pietsch, A. Erbe, W. Belzig, E. Scheer, Benzenedithiol: A broad-range
single-channel molecular conductor. Nano Lett. 11, 3734-3738 (2011).

13.

K. Motobayashi, Y. Kim, H. Ueba, M. Kawai, STM を用いた表面吸着分子のアクショ
ンスペクトル. 表面科学

14.

9, 597-602 (2011).

Y. Kim, T. Komeda, M. Kawai, Single-molecule reaction and characterization by
vibrational excitation. Phys. Rev. Lett. 89, 126104 (2002).

15.

T. Komeda, Y. Kim, M. Kawai, B. N. J. Persson, H. Ueba, Lateral hopping of molecules
induced by excitation of internal vibration mode. Science 295, 2055-2058 (2002).

16.

Y. Sainoo, Y. Kim, T. Okawa, T. Komeda, H. Shigekawa et al., Excitation of molecularvibrational modes with inelastic scanning tunneling microscopy processes: Examination
through action spectra of cis-2-butene on Pd(110). Phys. Rev. Lett. 95, 246102 (2005).

25

3 Experimental concepts and techniques
3.1 Fabrication of single molecular junction
In order to investigate the electron transport of a single molecular junction, a single moleculeshould be trapped in the metal nanogap. Metal nanogaps are commonly fabricated using the
break-junction technique. The metal electrodes are broken down and the nanogap can be formed
just after the break-down of the metal electrodes. The molecule around the nanogap can move tothe nanogap, and then a single molecule is placed between the metal atoms. The metal nanocontacts are broken by the tip movement of the STM, bending of the elastic substrate, electricbias, and electro-static gate. The applied bias at the nano contact induces the movement of metal
atoms in the electron-migration method, and the external force was applied by electro static
gating. With the STM-break-junction method, the lateral movement of the STM-tip fabricatesthe nanogap. In this thesis, mechanically controllable break-junction (MCBJ) technique isapplied (1). In this MCBJ technique, the nano contact is broken by mechanical bending, asshown in Figure 3.1. The advantage of the MCBJ technique is the stability of the molecularjunction because it is free from thermal drift.

Figure 3.1: Schematic image of the MCBJ technique.
26

3.2 Break-down and re-formation of the metal nano contact
Yanson et al. demonstrated the MCBJ technique using Au electrodes (2), and showed theelectric conductance behavior during the breaking process of Au nano contacts. Figure 3.2(a)shows the time course of the electric conductance of the Au nano contact during the breaking
process, which is called the conductance trace. The electric conductance is gradually reduced ina step-wise fashion. The plateaus are observed at a conductance value that is the integer multipleof the quantum unit of G0 (2e2/h). Figure 3.2(b) shows the conductance histogram, which wasconstructed from seveal thousand traces. The peaks in the conductance histogram represent thepreferred conductance states that are formed during the breaking process. Because the Au
atomic contact has only one channel and perfect transmission, the conductance values of 3, 2,
and 1 G0 correspond to the atomic junctions made of 3, 2, and 1 atoms, respectively.

Figure 3.2: (a) Conductance trace, (b) Conductance histogram of the Au nano contact with thebias voltage of 100 mV.
Note that the Au atomic junction forms the linear atomic chain, while the other metals such as
Ag and Cu do not form the linear atomic chain (3). Yanson et al. also showed the formation of
the Au linear atomic chain by constructing a histogram of the plateau length. As shown in
Figure 3.3(a), the peak represents the atomic contact with a length of 1, 2, and 3 atoms (2).
Ohnishi also verified the formation of a linear atomic chain with a length of a few atoms (4), as
shown in Figure 3.3(b). The Au atomic junction is composed of two parts; the linear atomic
27

chain and the atomic electrodes. The inter-atomic distance is about 2.5 Å. This property of thetendency to form a linear atomic chain is limited to 5d metals such as Au, Pt, and Ir. On theother hand, 4d metals do not form linear atomic chains (3, 5-8). Figure 3.3(c) is a plateau length
histogram measured by Ag electrodes (5). In 5d metals, the relative effects increase the effective
coupling of the s states and d states, leading to the stabilization of the low-dimensional states (3,8).

Figure 3.3: (a) Plateau length histogram obtained from over 10000 traces for the Au atomic
junction. (b) TEM image of the Au atomic chain (4). Copyright (1998) by the Nature PublishingGroup. (c) Plateau length histogram for Ag atomic junction (5). Copyright (2006) by theAmerican Physical Society.
Figure 3.4(a) is a trace of the breaking process and re-formation process. After the break
down of the atomic chain, the atomic electrodes also shrink. The gap size is estimated to be a
few Angstroms. When the bias supply to the piezo is stopped, the elastic substrate return to theinitial states, leading to the re-formation of the nano contacts. The separation, which is required
to reform the nano contacts, equals the size of the nanogap, which is the summation of thelength of the linear atomic chain and that of the shrinkage of the atomic electrodes. The relation
between the return length and the plateau length is shown in Figure 3.4(b). The shrinkage of theelectrodes is estimated to be 5 Å. Even though no linear atomic chain is not formed, a nanogap
with a size of 5 Å is formed.

28

Figure 3.4: (a) Conductance trace observed during breaking and formation processes. (b)Correlation of the plateau length and return length (2). Copyright (1998) by the Nature
Publishing Group.
In the presence of a molecule, the plateau is observed the after the break-down of the metal
atomic contact, as shown in Figure 3.5(a) (9). The plateau originates from the rearrangements ofthe gold electrodes at the atomic electrodes, as well as the configurational change of themolecule. The change in the conductance of the single molecular junction was also studied by
Bruot et al. with 1,4-benzenedithiol (10). Figure 3.5(b) shows the variation in conductance ofthe benzene molecular junction. In the region of 2-7, the conductance is decreases with thedecrease of the electric coupling between the molecule and gold electrodes. At points 1 and 8,
the conductance value increases as the HOMO energy of benzenedithiol approaches the Fermi
energy of the Au electrodes (10).

Figure 3.5: Conductance trace observed in the presence of molecule. (a) Benzene diamine (9)Copyright (2008) by the American Chemical Society. (b) Benzenedithiol (10). Copyright (2012)by the American Chemical Society.

293.3 Experimental setup
3.3.1 MCBJ setup
MCBJ sample was fabricated by typical procedure. The notched metal wire (φ = 0.1) is placedon the elastic substrate (phosphor blond, t = 1 mm). The sample was mounted on sample stage
at a three-point bending configuration. The substrate is roughly bent by pushing using thedriving shaft. The mechanical vibration is decoupled at the fork blade part by disconnecting thedriving shaft, and the mechanical bending is precisely controlled by a piezo element. The
vertical movement of the piezo is converted to the horizontal movement of the metal wire at thecenter part. The conversion ratio rd is expressed as follows (1, 11):𝑟d =𝛿𝑑 3𝑢ℎ= 2 … (3.1)
𝛿𝑧ℎwhere each parameter is described as in Figure 3.6 (f). The conversion factor rd is estimated tobe ~10-3, which is sufficient to control the separation on a sub-nanometer scale.
For low-temperature measurements, I used the measurement system that is described in
Figure 3.2(a)-(f). The custom-made chamber was evacuated and dipped in the liquid helium
chamber. The temperature of the MCBJ substrate reached 10 K. The target molecule wasintroduced via a heated capillary.
For room-temperature measurements, I used the measurement system described in Figure
3.6(e). The chamber was evacuated with a turbo molecular pump (TMP: 250l/s) or sputter-ion
pump (IP: 270l/s). Because the mechanical vibration from the TMP affected the conductancemeasurements of the single molecular junctions, the chamber was evacuated by only IP during
the conductance measurements. The base pressure reached 2  10-7 Pa.

30

Figure 3.6: (a) Image of MCBJ system for low-temperature measurement. (b) Custom-madechamber for low-temperature measurement. (c) Image of the chamber around the sample. (d)Schematic image of the MCBJ system. (e) Custom-made chamber for room-temperature
measurement. (f) Schematic image for MCBJ substrate and two-terminal conductancemeasurement.

31

3.3.2 Two-terminal conductance measurements
The metal wire was connected to the copper wire using silver paste, and the copper wire wasconnected to the measurement devices via coaxial cable. Electric conductance was measured
with the given DC bias voltage. The bias voltage was sourced by a PCI-MIO-16XE-10 AD
converter (National Instrument) and the current was measured using a Keithley 428
picoammeter. In this setup, the electric noise level was below 2  103 G0 under an applied bias
voltage. Typically, the metal contact was repeatedly formed and broken at 6 Hz. In a single
conductance trace, 2500 data points were recorded in 80 ms. At least 1000 traces were recorded
for each sample. Artificial selection of a specific trace was not performed during theconstruction of the histogram.

3.3.3 dI/dV measurements
dI/dV measurements were performed using the lock-in techniques, as shown in Figure 3.7. The
differential conductance was measured using the lock-in technique with an AC modulation witha 1-mV amplitude and a 7.777-kHz frequency. The conductance was monitored for a fixedcontact configuration during a sweep of the DC bias.

Figure 3.7: Schematic image for dI/dV measurement.

32

References
1.

N. Agrait, A. L. Yeyati, J. M. van Ruitenbeek, Quantum properties of atomic-sized
conductors. Phys. Rep. 377, 81-279 (2003).

2.

A. I. Yanson, G. R. Bollinger, H. E. van den Brom, N. Agrait, J. M. van Ruitenbeek,
Formation and manipulation of a metallic wire of single gold atoms. Nature 395,783-785 (1998).

3.

R. H. M. Smit, C. Untiedt, A. I. Yanson, J. M. van Ruitenbeek, Common origin forsurface reconstruction and the formation of chains of metal atoms. Phys. Rev. Lett. 87,
266102 (2001).

4.

H. Ohnishi, Y. Kondo, K. Takayanagi, Quantized conductance through individual rowsof suspended gold atoms. Nature 395, 780-783 (1998).

5.

W. H. A. Thijssen, D. Marjenburgh, R. H. Bremmer, J. M. van Ruitenbeek,
Oxygen-enhanced atomic chain formation. Phys. Rev. Lett. 96, 026806 (2006).

6.

R. H. M. Smit, C. Untiedt, G. Rubio-Bollinger, R. C. Segers, J. M. van Ruitenbeek,
Observation of a parity oscillation in the conductance of atomic wires. Phys. Rev. Lett.
91, 076805 (2003).

7.

S. R. Bahn, K. W. Jacobsen, Chain formation of metal atoms. Phys. Rev. Lett. 87,
266101 (2001).

8.

B. Hammer, J. K. Norskov, Why gold is the noblest of all the metals. Nature 376,
238-240 (1995).

9.

M. Kiguchi, S. Miura, T. Takahashi, K. Hara, M. Sawamura et al., Conductance of
single 1,4-benzenediamine molecule bridging between Au and Pt electrodes. J. Phys.
Chem. C 112, 13349-13352 (2008).

10.

C. Bruot, J. Hihath, N. J. Tao, Mechanically controlled molecular orbital alignment insingle molecule junctions. Nat. Nanotechnol. 7, 35-40 (2012).

11.

J. C. Cuevas, E. Scheer, Molecular Electronics: An Introduction to Theory andExperiment World Scientific Series in Nanotechnology and Nanoscience (World
Scientific Publishing Co. Pte. Ltd., Singapore, 2010).

33

4

Design of the interaction on single benzene
molecular junction

In this chapter, I focused on the interaction between the molecule and metal, which enabled
the fabrication of a single molecular junction with a well-defined conductance value utilizing aπ-conjugated molecule. To do this, it is important to limit the molecule’s configuration. Here, I
fabricated a benzene molecular junction with a well-defined electric conductance value bychoosing an appropriate metal-molecule interaction. Ag and Au are expected to interact morewith the benzene molecule compared to Pt. By choosing an appropriate interaction, I fabricateda single benzene molecular junction with a high and well-defined conductance value using Agelectrodes.

The contents in this chapter have been published in: Satoshi Kaneko, Tomoka Nakazumi, Manabu
Kiguchi, “Fabrication of the well-defined single benzene molecule junction using Ag electrodes”
The Journal of Physical Chemistry Letters 1, 3520–3523 (2010).

34

4.1 Introduction
As mentioned in chapter 1, highly conductive single molecular junctions have attracted
widespread attention because of the potential in the development of novel materials, as well as
their application to electronic devices. Recent studies revealed that a highly conductive single
benzene molecular junction can be prepared by direct binding to Pt electrodes without using ananchoring group (1, 2). The conductance of the Pt/benzene/Pt junction was 0.1-1 G0, which wascomparable to that of the metal atomic contact. The direct binding of a π-conjugated molecule
to metal electrodes is a promising technique for obtaining a conducting molecule wire inmolecular electronic devices. Although the single Pt/benzene/Pt junction is highly conductive, itdoes not show a fixed conductance value. The variance of the conductance of the Pt/benzene/Pt
junction was a factor of 10 (1). In order to develop the single molecular electronics, it isstrongly desired to fabricate well-defined single molecular junctions showing high and fixed
conductance values. In the case of the Pt/benzene/Pt junction, the junction can take various
atomic configurations during stretching because of the strong metal-molecule interaction. In thischapter, I focus on the less reactive metals Au and Ag. I investigated the conductance of the Au
and Ag contacts in benzene atmosphere at 10 K. A single benzene molecular junction wasformed for the Ag electrode, but there was negligible formation of a benzene molecular junction
for the Au electrodes. The single Ag/benzene/Ag junction was characterized by inelastic
electron tunneling spectroscopy (IETS) (1, 3), which provides information on the vibration
mode between the benzene molecule and the Ag electrode.

4.2 Experiments
A single benzene molecular junction was fabricated utilizing the MCBJ technique at 10 K, as
mentioned in chapter 3. The benzene molecule was introduced via a heated capillary. The
electric conductance of the Au and Ag nano contacts were measured in the presence of thebenzene molecule at 10 K.

4.3 Results and discussion
4.3.1 Conductance value and configuration
Figure 4.1 shows a conductance trace and histogram measured during the breaking process ofthe silver electrode in the presence of the benzene molecule. The conductance trace had a35

plateau at 0.25 G0 (Figure 4.1(a)). The prominent peak at 0.25 G0 appeared in the corresponding
conductance histogram, as shown in Figure 4.1(b). From three different samples, theconductance value was determined to be 0.24 ± 0.08 G0, which was in agreement with thecalculated conductance value (4). In order to confirm the presence of the benzene molecule andconfiguration of the benzene molecule, I measured the dI/dV spectra at a conductance value of
0.24 G0. The observed conductance value agreed with the results of the theoretical calculation
study. Figure 4.2 shows the dI/dV and d2I/dV2 spectrum at the conductance value of 0.2 G0. I
observe an abrupt increase in the conductance at ±42 mV in the dI/dV spectrum, and aprominent peak was observed in the d2I/dV2 spectrum. The abrupt increase in the conductanceoriginated from the opening of the new conductance channel while exciting the vibration modes
in molecular junctions. For clean Ag contacts, the vibration energy was observed around 10
meV and 20 meV in point-contact spectroscopy (5). The energy of the internal vibration modeof benzene is higher than 50 meV (6). In a previous study, inelastic tunneling spectroscopy was
performed for a benzene molecule attached on the surface of Ag (110) (7). The vibration mode
between benzene and the silver surface was observed at 41 meV when the benzene wasabsorbed on the kink site. The observed energy agreed with the vibration mode between thesilver and benzene molecule. The dI/dV spectrum clearly shows that the benzene molecule wasplaced between silver electrodes.

Figure 4.1: (a) Conductance trace measured with the bias voltage of 0.1 V. (b) Conductancehistogram constructed from 1000 traces.

36Figure 4.2: dI/dV spectrum and d2I/dV2 spectrum observed at 0.2 G0 in the presence of thebenzene molecule.

Figure 4.3: Conductance trace and histogram measured for Au electrodes in the presence ofbenzene. The bias voltage was 100 mV.

37

On the other hand, neither plateaus nor peaks were observed in the histogram for gold
electrodes below 1 G0 for Au electrodes, which means that a single benzene molecule was notformed using Au electrodes (Figure 4.3). The conductance behavior and dI/dV spectra showedthat an Ag/benzene/Ag single molecular junction formed with a well-defined conductance valueof 0.24 G0. On the other hand, a single benzene molecular junction was not formed between Au
gold electrodes.

4.3.2 High and well-defined conductance value
The observed conductance value of 0.24 G0 is improved by ten times when compared with an
ordinal single molecular junction utilizing anchoring groups. The previous theoretical research
revealed the effect of removing the anchoring groups (8). In the case of the 1,4-benzenedithiol(BDT) molecule, the BDT molecule is placed between the electrodes with a benzene ringparallel to the electrodes at the most stable configuration. The HOMO mainly contributes to theelectron transport in the single molecular junction because the HOMO is located near the Fermi
energy (9). The HOMO of the BDT molecule consists of a p orbital on the sulfur atom, which isperpendicular to the direction of the electron transport. The orbitals that can couple to the porbital on sulfur atoms is limited to the s or p states of carbon in the molecular back-bone, andthese carbon states are not hybridized with sulfur estates, leading to small electron transmission
from the benzene ring to the anchoring groups. On the other hand, the benzene molecule isplaced between electrodes with a p orbital parallel to the bonding direction. In the case of thebenzene molecule, LUMO mainly contributes to the electron transport. The geometry at themost stable configuration, as shown in the inset of Figure 4.2, illustrates that the π-conjugatedorbitals hybridize directly with the conduction channels at the electrodes. The LUMO of thebenzene molecule is effectively hybridized with the orbital on the metal electrodes in a wide
energy range. An effective hybridization between the molecule and metal electrodes leads to ahigh conductance value.
The observed conductance value for the benzene molecule utilizing Ag electrodes was
reduced compared with that of benzene between Pt electrodes: The benzene molecule had aconductance value of 1 G0~0.1 G0 (1). The Pt/benzene/Pt molecular junction was considered,
where the benzene molecule showed a conductance value of 1 G0 at the most stableconfiguration, and the benzene molecule can take many configurations between Pt electrodes.
The interaction between benzene and Pt is considered much stronger than that between benzene
and Ag. The bonding energy at the most stable configuration of the benzene molecule to thePt(111) surface is 1.2 eV (10); however, the bonding energy at the most stable configuration of38

the benzene molecule to the Ag(111) surface is 0.05 eV(11). The larger interaction between themetal and benzene molecule enables benzene to have many configurations between metalelectrodes. On the other hand, proper interaction between Ag and benzene formed a single
molecular junction only with the most stable configuration. The reason for the difference in the
conductance value of Ag/benzene/Ag junction compared with that of the Pt/benzene/Pt junction
is explained by the density of the states on the metal atom at the Fermi energy, and the strengthof the interaction between the metal and benzene. The density of the states of the Pt at the Fermi
energy is larger than that of Ag. The larger density of states and interaction result in a larger Γ in
Eq. (2.37), leading to a larger conductance value.
The difference in the formation of the single-molecular junction may have originated fromthe Au linear atomic chain formation. As mentioned in the previous section, Au tends to form alinear atomic chain, but Ag does not. The benzene molecule was placed between the metalelectrodes after the break-down of the Ag atomic contact. On the other hand, when the Au linear
atomic chain was formed, a large nanogap was formed for Au electrodes. The benzene molecule
was not able to bridge the nanogap after the break-down of the linear atomic chain. As
mentioned in the previous paragraph, the benzene should have been attached to the Au
electrodes with a benzene ring facing the Au electrodes, as shown in Figure 4.4 (a-b).

Figure 4.4: Schematic image of the breaking process of the Ag (a) and Au (b) nano contacts.

4.4 Conclusions
I investigated the conductance behavior of the Au and Ag contacts in benzene atmosphere at 10
K. While a single Ag/benzene/Ag junction was formed, there was negligible formation of asingle Au/benzene/Au junction. The Ag/benzene/Ag junction showed a fixed conductance value39

of 0.24 G0, indicating the formation of a well-defined single molecular junction. The junction
was characterized by IETS, which showed the vibration mode between the benzene moleculeand the Ag electrodes. IETS confirmed the formation of a single Ag/benzene/Ag junction. The
present study revealed that the moderate metal-molecule interaction is favorable to realizing awell-defined single molecular junction.

References
1.

M. Kiguchi, O. Tal, S. Wohlthat, F. Pauly, M. Krieger et al., Highly conductive
molecular junctions based on direct binding of benzene to platinum electrodes. Phys.
Rev. Lett. 101, 046801 (2008).

2.

M. Kiguchi, Electrical conductance of single C60 and benzene molecules bridging
between Pt electrode. Appl. Phys. Lett. 95, 073301 (2009).

3.

M. Kiguchi, R. Stadler, I. S. Kristensen, D. Djukic, J. M. van Ruitenbeek, Evidence fora single hydrogen molecule connected by an atomic chain. Phys. Rev. Lett. 98, 146802(2007).

4.

Y. Li, P. Wei, M. L. Bai, Z. Y. Shen, S. Sanvito et al., Contact geometry and electronictransport properties of Ag-benzene-Ag molecular junctions. Chem. Phys. 397, 82-86(2012).

5.

Y. G. Naidyuk, I. K. Yanson, Point-contact spectroscopy. Springer Series in Solid-State
Sciences (Springer-Verlag New York, New York, 2005), vol. 145.

6.

R. A. Wolkow, M. Moskovits, A comparative study of the electron energy loss
spectrum and the surface‐enhance Raman spectrum of benzene adsorbed on silver. J.
Chem. Phys. 96, 3966 (1992).

7.

J. I. Pascual, J. J. Jackiw, Z. Song, P. S. Weiss, H. Conrad et al., Adsorbate-substrate
vibrational modes of benzene on Ag(110) resolved with scanning tunnelingspectroscopy. Phys. Rev. Lett. 86, 1050 (2001).

8.

J. Ferrer, V. M. Garcia-Suarez, Tuning the conductance of molecular junctions:
Transparent versus tunneling regimes. Phys. Rev. B 80, 085426 (2009).

9.

H. Song, Y. Kim, Y. H. Jang, H. Jeong, M. A. Reed et al., Observation of molecularorbital gating. Nature 462, 1039-1043 (2009).

10.

M. Saeys, M. F. Reyniers, G. B. Marin, M. Neurock, Density functional study ofbenzene adsorption on Pt (111). J. Phys. Chem. B 106, 7489-7498 (2002).

11.

A. Bilic, J. R. Reimers, N. S. Hush, R. C. Hoft, M. J. Ford, Adsorption of benzene on
copper, silver, and gold surfaces. J Chem Theory Comput 2, 1093-1105 (2006).

40

5

Design of interface structure of single
molecular junction utilizing sphericalendohedral Ce@C82 metallofullerenes

In this chapter, I discuss the fabrication of a highly conductive single molecular junction by
designing an interface structure. It is expected that the spherical shape would enable us toobtain a well-defined conductance value because the configurational change does not affect theelectron transport. Here, I investigate the electron transport of a single endohedral Ce@C82
metallofullerene. The endohedral Ce atom modified the electronic states of fullerene andshowed the potential for application to electronic devices.

The contents of this chapter have been published in: Satoshi Kaneko, Lu Wang, Guangful Luo, Jing
Lu, Shigeru Nagase, Satoru Sato, Michio Yamada, Zdenek Slanina, Takechi Akasaka, Manabu
Kiguchi, “Electron transport through single endohedral Ce@C82 metallofullerenes”Physical Review
B 86, 155406 (2012).

41

5.1 Introduction
Highly conductive molecular junctions have been fabricated by the direct binding of
π-conjugated organic molecules to metallic electrodes (Pt, Au, Ag) without the use of anchoringgroups (1-5).The conductive π orbital directly hybridizes with the orbital of the metal electrodes,
resulting in high conductivity. Moreover, the single fullerene molecular junction preferentiallyshows a fixed conductance value because of its spherical molecular shape in the break-junction
process (3, 5).
Of the fullerene-related materials, endohedral metallofullerenes (i.e., fullerenes containing
metals inside the carbon cage) have received significant attention owing to their remarkableelectrical and magnetic properties, and their possible applications in future molecular electronics(6, 7). The interaction between the metal and the carbon cage plays an important role indetermining their unique properties, which distinguishes them from their counterparts withempty carbon cages. Based on these interests, as well as the unique characteristics of single
fullerene molecular junctions, electron transport through a single metallofullerene molecule has
been investigated by several groups (8-12). Senapati et al. investigated the electronic structureand electron transport through a single Gd@C82 molecule using theoretical calculations(8). The
conductance of the single Gd@C82 molecular junction was found to be smaller than that of C82
because of the reduction of the transmission of the conducting channels caused by the charge
localization. Yasutake et al. investigated the electron transport through a single Tb@C82
molecule on an octanethiol/Au(111) surface via scanning tunneling microscopy (STM) (9).
Here, the switching of the single Gd@C82 molecular orientation caused by the interaction
between the electric dipole moment of the Gd@C82 molecule and an external electric field wasobserved. However, to the best of our knowledge, electron transport through the single
metallofullerene bridge between metal electrodes has never been directly measured.
In this chapter, I discuss a single metallofullerene molecular junction, and I investigate its
electron transport properties using the MCBJ technique in an ultra-high vacuum (UHV). The
single Ag/Ce@C82/Ag molecular junction showed a high and fixed conductance value of 0.28
(0.05) G0, which was much larger than that of single molecular junctions having anchoringgroups (< 0.01G0), while only half that of the single C60 molecular junction of 0.5 G0. The
reduction in the conductance of the single Ce@C82 molecular junction compared with that of thesingle C60 molecular junction was explained in terms of the localization of electrons in the C82
cage using the ab initio quantum transport calculations. In the case of the Au electrodes, thesingle Ce@C82 molecular junction was not formed by the break-junction technique because the42

Ce@C82 molecule could not be trapped in the large Au nanogap that was formed just after
breaking the Au contacts.

5.2 Experiments
Soot containing Ce@C82 was prepared according to the reported procedure (7) employing the
DC arc-vaporization method using a composite anode containing graphite and cerium oxidewith the atomic ratio of Ce/C equal to 2.0%. The composite rod was subjected to an arc
discharge as an anode under a pressure of 150 Torr He. The raw soot was collected andextracted with 1,2,4-trichlorovenzene solvent for 15 h. The soluble fraction was injected into the
HPLC; a PYE column (20 mm  250 mm i.d.; Cosmosil, Nacalai Tesque Inc.) was used in thefirst step and a Buckyprep column (20 mm  250 mm i.d.; Cosmosil, Nacalai Tesque Inc.) in thesecond step to give pure Ce@C82.
A single Ce@C82 molecular junction was fabricated by utilizing the MCBJ technique at room
temperature, as mentioned in chapter 3. A notched Au or Ag wire was used as electrodes. The
MCBJ substrate was placed in the UHV chamber. 1 ML (mono-layer) Ce@C82 was depositedon the Au and Ag contacts with a Knudsen cell before stretching the contacts. The distance
between the Knudsen cell and the sample was 200 mm. The temperature of the sample was 300
K, and the pressure was kept below 5  10-6 Pa during the deposition process. The amount ofdeposited Ce@C82 was monitored with a thickness monitor.

5.3 Results and discussion

5.3.1 Conductance and I-V characteristics
Figure 5.1 shows the typical conductance traces and conductance histograms during the
breaking of the Au or Ag contacts before and after introduction of Ce@C82. The stretch length isthe displacement between the stem parts of the metal electrodes that were fixed with epoxy
adhesive. Before the introduction of Ce@C82, immediately preceding the separation of the Au
and Ag electrodes, the conductance was 1 G0, which corresponded to Au and Ag atomic
contacts (13, 14). The corresponding conductance histograms showed clear 1 G0 peaks. Then,after the introduction of Ce@C82 to the Ag contacts, the 1 G0 plateau was elongated in the43

conductance traces, and the 1 G0 peak was enhanced in the conductance histograms. A new
sequence of steps appeared in the conductance traces at the lower-conductance region (Figure5.1(a)). The conductance value of the steps was an integer multiple of 0.2~0.3 G0. The
corresponding conductance histogram showed a peak around 0.3 G0 (Figure 5.1(b)). There were
only a few steps in the conductance traces below 0.1 G0, and no features were observed in the
conductance histograms below 0.1 G0. These experimental results suggest that the Ce@C82
molecule was trapped between the Ag nanogap, and the steps in the traces and peak in thehistogram showing values of 1  and 2  0.2~0.3 G0 could be ascribed to the bridging of one
and two Ce@C82 molecules between Ag electrodes. From repeated measurements, theconductance of the single Ag/Ce@C82/Ag molecular junctions was determined to be 0.28
(0.05) G0, which was much larger than that of single molecular junctions having anchoringgroups (< 0.01 G0) (15-19). The appearance of the sharp peak in the conductance histogram also
showed the formation of the single molecular junction with a fixed conductance value. The
conductance measurements represented a single Ce@C82 molecular junction exhibiting a highand fixed conductance value, which was fabricated using direct-binding techniques.

44Figure 5.1: Typical conductance traces (a,c) and histograms (b,d) of Ag and Au contacts after
the introduction of Ce@C82. The black traces and histograms indicate the values obtained for theclean Ag or Au contacts. The metal electrodes were Ag (a,b) and Au (c,d). The intensity of the
conductance histograms was normalized with the number of the conductance traces. The bin
size was 0.004 G0.

While the single Ce@C82 molecular junctions were formed using Ag electrodes, they were
not formed using Au electrodes by the direct-binding technique. No plateaus or features wereobserved in the conductance traces (Figure 5.1(c)) or histograms (Figure 5.1(d)) below 1 G0 for
Au contacts after the introduction of Ce@C82. The elongation of the 1 G0 plateau in the
conductance traces and the enhancement of the 1 G0 peak in the conductance histogram wereobserved, as is the case with the Ag contacts after the introduction of Ce@C82.
The current-voltage (I-V) curves were measured for the single Ag/Ce@C82/Ag molecularjunctions by a direct current (DC) method, where each curve was recorded at fixed electrodes
for a separation at 300 K. The DC voltage was scanned from -0.2 V to +0.2 V (positive scan), or
45

from +0.2 V to -0.2 V (negative scan) with a step voltage of 0.004 V. In the present study, theI-V curves showing similar plots with positive and negative scans were adopted. Figure 5.2
shows the I-V curve and the first derivative for the single Ag/Ce@C82/Ag molecular junction,
where the I-V curves were obtained by averaging the six I-V curves. The symmetric upward stepin the conductance was observed at 0.12 V. This conductance enhancement around 0.12 V
was observed for three distinct samples. The conductance enhancement is explained that the
Fermi level of the Ag electrode approaches the molecular level of Ce@C82 by increasing thebias voltage. Although a simple comparison between the threshold energy and molecular levelis not appropriate for a single molecular junction (20, 21), there is some relationship between
the threshold energy and the energy difference between the molecular orbital and the Fermi
level. In the case of a single Au/benzenedithiol/Au molecular junction, a conductanceenhancement was observed at a bias voltage of 1 V (22). The energy difference between the
Fermi level and HOMO (conduction orbital) was 1.2 eV for the Au/benzenedithiol/Au
molecular junction, which agreed with the threshold voltage of 1 eV (23). Because the energy
difference between the Fermi level of the Ag electrode and the conduction orbital of Ce@C82 ismuch smaller than the Au/benzenedithiol/Au molecular junction because of the small energy
gap, and the threshold voltage decreased to a low value, which may lead to the appearance ofthe conductance enhancement at 0.12 V in I-V curves.

46

Figure 5.2: (a) I-V curve and (b) the first derivative for the single Ag/Ce@C82/Ag molecularjunction. The I-V curve was measured using a direct current (DC) method at 300 K.

Using a similar direct-binding technique, the conductance of a single C60 molecule trapped in
the Au and Ag nanogaps has been measured under UHV at 300 K in a previous study (5). The
conductance values of the single Au/C60/Au junction and Ag/C60/Ag junctions were 0.3(0.1)
and 0.5(0.1)G0, respectively. The conductance of the single fullerene molecule was reduced by
the introduction of Ce metal, which appeared to be contradictory to the corresponding reductionin the HOMO-LUMO gap (6). In the simple-tunneling model (13, 14), the conductance of thesingle molecular junction increases with the decrease in the energy difference between the
Fermi level and the conduction orbital related to the HOMO-LUMO gap.

47

Table 5.1: Summary of the conductance for the single C60 and Ce@C82 molecular junctions.
SystemConductance

Au/Ce@C82/Au-

Ag/Ce@C82/Ag

0.28 (0.05)G0

Au/C60/Au

0.3(0.1) G0

Ag/C60/Ag

0.5(0.1)G0

5.3.2 Formation process
The Au/Ce@C82/Au junction was not formed, whereas Au/C60/Au and Ag/Ce@C82/Ag
junctions were formed. These experimental results can be explained by the difference in theelectrode gap distance of the nanogap formed by breaking the metal electrodes. Figure 5.3
shows the length histogram of the 1 G0 plateau, which corresponds to the Au or Ag atomic
contact (13, 14). Because the probability of the appearance of the 1 G0 plateau was very low fora clean Ag contact, the length histograms were not evaluated. For the clean Au, Au/C60/Au, andAg/Ce@C82/Ag junctions, the Au and Ag atomic contacts (1 G0) broke within 0.3 nm. In
contrast, the Au atomic contact could be stretched up to 1.4 nm after the introduction of
Ce@C82. Most of the junction stretched more than 0.5 nm. Because the Au-Au distance was
0.255 nm, the 1.4-nm stretching breadth indicates the formation of an atomic chain. The
theoretical calculation results showed that the interaction between the molecule and the Au
electrode was larger for Ce@C82 than for C60 (see the discussion about the binding energy in thetheoretical part). Because of the large interaction between Ce@C82 and Au, the Au atomic chain
was highly stabilized in the presence of Ce@C82 molecules (24), while no long atomic chainswere formed for the clean Au and Au in the presence of C60 at 300 K. After breaking the Au
atomic chain, the nanogap was formed between the Au electrodes. The gap size was the sum of
the length of the Au atomic chain (< 0.5 nm) and the contraction of the electrode (~0.5 nm) (25).
The single Ce@C82 molecule could not be trapped in the Au nanogap because of its large gap
size (~0.9 nm), as shown in Figure 5.3.

48

Figure 5.3: (a) Length histogram of the 1 G0 plateau for the Au and Ag contacts at the bias
voltage of 100 mV. The blue, red, green, and black curves are the results of the Au/Ce@C82/Au,
Ag/Ce@C82/Ag, Au/C60/Au junction, and clean Au contacts, respectively. The inset shows the
typical conductance trace of the Au contacts before the introduction of Ce@C82 at a bias voltage
of 0.1 V. The intensity of the length histograms was normalized to the number of the
conductance traces. The length histograms were constructed without data selection from 1000
conductance traces of breaking metal contacts. (b) Schematic view of the breaking process of
the Au and Ag contacts. While a single molecular junction was formed for the Ag contact, nosingle molecular junction was formed for the Au contact because of the large nanogap.

495.4 Calculations
The following are the first-principle calculations for the single C60 and Ce@C82 molecularjunctions in order to shed light on the experimental observations of single molecular junctions.
The structure for free Ce@C82 was optimized by employing the density functional theory (DFT)
using the B3LYP functional (26-28) and the SDD basis (with the SDD effective core potential)
for the Ce atom and 3-21G, 6-31G*, 6-31+G*, or 6-311G* basis for the C atom, as
implemented in the Gaussian program package (29). There was little variation in the structurewith the basis set on the C atom, and the optimized free structure was used as a starting
geometry in the following conductance calculations. The ground-state structure of Ce@C82 has
a C2v symmetry, and the Ce atom is located at an off-cage-center position adjacent to ahexagonal ring along the C2 axis of the cage with an average distance of 2.534 Å from the ringcarbons at the B3LYP/6-311G*~SDD computational level. The B3LYP functional is the mostcommonly used DFT functional for fullerene calculations; if it is checked, e.g., at the
M06-2X/6-311G*~SDD level (26, 27), the shortest Ce-C contact has a similar value of 2.524 Å.
In the optimized free Ce@C82 structure, more than two electrons are moved from the metal tothe cage (measured by the Mulliken atomic charges, although the charge transfer depends on thecomputational treatment and charge definition (29). Transport property calculations were
performed using the non-equilibrium Green’s function (NEGF) formalism and DFT as
implemented in the ATK 2008.10 code (30, 31). A single- basis set plus polarization with amesh cutoff energy of 300 Ry, norm-conserving pseudopotentials, and a local density
approximation (LDA) was employed.
In this study, a two-probe model (Figure 5.4) was adopted, where either a C60 or Ce@C82
molecule was sandwiched between semi-infinite gold or silver electrodes with a (111) surface
cross-section. Hexagonal 5 × 5 × 3 unit cells of 14.40 × 14.40 × 7.06 Å3 and 14.45 × 14.45 ×
7.08 Å3 containing 75 atoms were used for the Au and Ag electrodes, respectively. The k-pointsof the electrodes were set to Monkhorst-Pack 1 × 1 × 50. The structures were optimized until
the maximum atomic force was less than 0.03 eV/Å. The conductance of the single molecularjunction depends on the atomic configuration of the metal electrodes (13-15). In the present
study, the (111) surface of Au and Ag electrodes was investigated. Although the absolute
conductance value depends on the geometry of the surface, the relative conductance value is notsensitive to the geometry of the surface when compared to that of the same geometry of thesurface (32). Hence, the results obtained from the (111) surface of Au and Ag electrodes
should be robust with respect to the geometry of the surface. Two configurations of Ce@C82
were considered between electrodes. In configuration-α (Figure 5.4(b)), the C2 axis of Ce@C82
50

is parallel to the surfaces of the electrodes, whereas in configuration-β (Figure 5.4(c)), theCe@C82 molecule is rotated 90 degrees so that the C2 axis is perpendicular to the electrodesurfaces. The distance between the molecule and the nearest metallic atoms was determined by
optimizing the geometry of the molecule on top of the corresponding metallic surface. The
relative position of the Ce atom in the carbon cage was also optimized.
The following are the calculations for the binding energy, which is defined as Eb = (Es + Em) ES+M, where ES+M, ES, and EM are the total energies for the electrode-slabs plus molecule, only
the electrode-slabs, and the isolated molecule, respectively. The distance between the moleculeand the nearest metallic atoms was determined by optimizing the molecule on top of thecorresponding metallic surface. The calculated binding energies of the Au/C60/Au,
Au/Ce@C82/Au (α), Au/Ce@C82/Au (β), Ag/C60/Ag, Ag/Ce@C82/Ag (α), and Ag/Ce@C82/Ag(β) are 1.20 eV, 1.28 eV, 1.31 eV, 1.14 eV, 1.26 eV, and 1.23 eV, respectively. These values
were much larger than the physisorption energy, and weak chemical bonds would be formed
between Ce@C82 and the Au or Ag electrodes.
The electronic properties of electrodes were calculated with periodic conditions using the
DFT scheme, and Green’s function, together with the self-energy of the central-scattering region,
was obtained using the standard NEGF method. An energy criterion of 1 × 10-5 eV was used in
all of the self-consistent calculations. Finally, the electric current I under a bias voltage Vbias wascalculated using the Landauer-Büttiker formula, as shown in Eq. (2.25) (33): The
molecular-projected self-consistent Hamiltonian (MPSH) energy spectra and the corresponding
MPSH orbitals were also calculated to illustrate the mechanism of the electron transport. The
calculated conductance values for the Au/C60/Au and Ag/C60/Ag junctions are 0.506 and 0.790
G0, respectively, compared with the corresponding experimental values of 0.3(0.1) and0.5(0.1) G0. The calculated conductance of the Au/Ce@C82/Au (α), Au/Ce@C82/Au (β),
Ag/Ce@C82/Ag (α), and Ag/Ce@C82/Ag (β) are 0.159, 0.048, 0.212, and 0.063 G0, respectively.
The Mulliken population analysis revealed that 1.10, 1.12, 1.11, and 1.12 electrons weretransferred from the Ce atom to the carbon cage in the Au/Ce@C82/Au(α), Au/Ce@C82/Au(β),
Ag/Ce@C82/Ag(α), and Ag/Ce@C82/Ag(β) configurations, respectively. The conductancehistogram of the Ag/Ce@C82/Ag junction showed a single peak at 0.28 G0, indicating theformation of one stable atomic configuration. The comparison between the calculation andexperimental results suggests that the Ag/Ce@C82/Ag (α) junction would be preferentiallyformed under current experimental conditions. When the Ag/Ce@C82/Ag (β) junction is
stretched, the junction would break because of its weak interaction between the benzene

51molecule and Ag electrodes at 300 K. Actually, the calculated conductance value (0.212 G0) of
the Ag/Ce@C82/Ag (α) is indeed much closer to the experimental value of 0.28 G0 than thevalue of 0.063 G0 for Ag/Ce@C82/Ag (β). Both the experiments and calculations indicate thatthe conductance of the Ag/Ce@C82/Ag junction was smaller than that of the Ag/C60/Ag
junction; the measured and calculated conductance of the former was smaller than that of thelater by 44% and 73%, respectively.

Figure 5.4: Structure models, transmission spectra (black), and MPSH energy spectra (red), andisosurfaces of the MPSH orbitals at zero bias voltage for the systems with (a) Ag/C60/Ag, (b)Ag/Ce@C82/Ag(α), and (c) Ag/Ce@C82/Ag(β). Red ball: Ce; blue ball: Au; grey ball: C.

52

Figure 5.5: Structure models, transmission spectra (black), and MPSH energy spectra (red), andisosurfaces of the MPSH orbitals at zero bias voltage for the systems with (a) Au/C60/Au, (b)Au/Ce@C82/Au(α) , and (c) Au/Ce@C82/Au(β). Red ball: Ce; yellow ball: Au; grey ball: C.

The transmission spectra and MPSH energy spectra of Ag/C60/Ag and Ag/Ce@C82/Ag were
investigated, as shown in Figure 5.4. The corresponding results of Au/C60/Au and
Au/Ce@C82/Au are shown in Figure 5.5. The electron transmissions are due mainly tocontributed by MPSH states above the Fermi level (EF), which are LUMO – LUMO+2 for C60
systems and LUMO – LUMO+5 for Ce@C82 systems. The small energy difference between
LUMO – LUMO+5 for Ce@C82 and the Fermi level may result from the conductanceenhancement around 0.12 V, which was observed for Ag/Ce@C82/Ag (see Figure 5.2). The
transmission coefficients of the major peaks in C60 systems are about 2.5 times larger than thosein the Ce@C82 systems. Meanwhile, the transmission peaks in C60 systems are also much wider
than those of the Ce@C82 systems. The transmission coefficients above EF when C60 was used,
as opposed to Ce@C82, are attributed to the fact that the electrons are significantly moredelocalized in C60 than those in Ce@C82. As show, in Figures 5.4 and 5.5, the LUMO andLUMO+1 MPSH of Ce@C82 are more localized than those of C60, and this is most likely
53because of the interactions between the Ce atoms, which are located at the off-center position of
the cage, and the carbon atoms around it. For this reason, the conductance of C60 systems wasseveral times larger than that of systems involving Ce@C82. In addition, after performing therotation of the Ce@C82 from configuration-α to configuration-β, it was revealed that the MPSH
states contribute less to the connection between the electrodes, and consequently, induce smaller
conductance value in configuration-β.

5.5 Conclusions
In this study, I investigated a single Ce@C82 molecule bridging between metal electrodes. I
fabricated single Ce@C82 molecular junctions using the MCBJ technique at 300 K under UHV.
The electric conductance of the single Ce@C82 molecule bridging between Ag electrodes was
0.28(0.05) G0. I successfully fabricated a single metallofullerene molecular junction showing ahigh and fixed conductance value using the direct-binding technique. The conductance of thesingle Ce@C82 molecule was smaller than that of the single C60 molecule bridging between Ag
electrodes, which was supported by theoretical calculations. The localization of electrons in the
Ce@C82 molecule explained the reduction of the conductance of the single Ce@C82 molecularjunction. While the single Ce@C82 molecular junction was formed for the Ag electrodes, it was
not formed for the Au electrodes. The Ce@C82 molecule could not be trapped in the large
nanogap, which was formed after breaking the Au electrodes.

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A. J. Perez-Jimenez, Molecular electronics with endohedral metallofullerenes: The test
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N. Agrait, A. L. Yeyati, J. M. van Ruitenbeek, Quantum properties of atomic-sized
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A. Mishchenko, L. A. Zotti, D. Vonlanthen, M. Burkle, F. Pauly et al., Single-moleculejunctions based on nitrile-terminated biphenyls: A promising new anchoring group. J.
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57

6 Anchoring groups enclosed in π-conjugatedsystem in N2 molecule

In the previous chapter, the fabrication of a single molecular junction with a high andwell-defined conductance value was discussed by designing the interaction between themolecule and metal and the spherical structure. In order to control the electric property of thesingle molecular junction, it is important to control the interface structure of single molecularjunctions. The anchoring groups enclosed in the π-conjugated system fixed the connecting pointwith high conductivity. Here, I focus on the π-conjugated system with a connecting part.
Nitrogen is one of the promising anchoring groups having a fixed conductance value, as
mentioned in the Introduction section. In this chapter, I focus on the nitrogen molecule, which
has a π electron and a lone pair. First, I investigate the formation of the molecular junction
depending on the interaction between the molecule and metals.

The contents of this chapter have been published in: Satoshi Kaneko, Jinjiang Zhang, Jianwei Zhao,
Manabu Kiguchi, “Electronic conductance of platinum atomic contact in a nitrogen atmosphere” The
Journal of Physical Chemistry C 117, 9903-9907 (2013), and Satoshi Kaneko, Yuuga Nakamura,
Jingiang Zhang, Xiongbo Yang, Jianwei Zhao, Manabu Kiguchi, “Formation of single Cu atomic
chain in nitrogen atmosphere” The Journal of Physical Chemistry C 119, 862–866 (2015).

58

6.1 Introduction
In the previous section, the highly conductive single molecular junction showed a fixed value
utilizing a π-conjugated molecule by properly designing the interface. The next step is to control
the electric property of a single molecule by controlling the interface structure. In order tocontrol the interface structure, it is important to fix the anchoring point, which keeps its electric
property unless a proper perturbation is applied. It has been reported that nitrogen is one of themost promising anchoring groups that show a fixed conductance value. In this chapter, I focus
on the N2 molecule, which is the simplest molecule that has an anchoring group in the
π-conjugated system.
The N2 molecule is also useful as an impurity in metal linear atomic chains (LACs), and theincorporated or adsorbed molecule modifies the electric property of the metal atomic chain. An
interesting interaction and adsorption properties could appear for the metal nanowires, because
the atomic and electronic structures of the metal nanowires are different from those of bulk
metals. This is because of the decrease in the number of neighboring atoms. A previous study
revealed the formation process of the Au atomic wire in a UHV utilizing transmission electron
microscopy (TEM). Interestingly, the interatomic distance between Au atoms was extremely
long (0.35~0.4 nm) compared with that of bulk (0.25 nm) (1). The long interatomic distance has
been explained by the formation of a zigzag structure, as well as the presence of impurity atoms,
such as C, H, or O in the nanowire. The theoretical calculation showed that the metal nanowire
could be stabilized by the adsorption of atoms or molecules on the metal nanowire, or theincorporation of the atoms or molecules into the nanowire. The experimental formation of Ag
and Co atomic wires in the presence of hydrogen at 4 K has been reported, although clean silver
and cobalt do not form atomic wires (2-4). In the case of Pt nanowires, the formation of thesingle hydrogen molecular junction was revealed by PCS and IETS, shot-noise measurements,
and theoretical calculations (5, 6). When N2 molecules interact with transition metals such as Pt,
the reactivity of N2 molecules is enhanced, (e.g., ammonium synthetic reaction (7)). The
reactivity of N2 could increase further for metal nanowires because of the decrease in thenumber of neighboring atoms. It is expected that the electrical property of nitrogen, which isinert in isolated states, will significantly vary. On the other hand, the interaction between Cu
nanowire and nitrogen has been investigated using an ab initio total energy calculation (8, 9).
The calculation results predicted a tendency of N and N2 to make strong bonds in Cu nanowires,leading to the formation of longer Cu atomic wires.
In this chapter, I consider Pt and Cu nanowires in the presence of N2 molecules. Pt was more59

reactive than Cu because of the higher density of states at the Fermi level (10-14). The geometry
and electronic structures were investigated using an MCBJ technique and theoretical calculation.
The conductance and vibration spectroscopy measurement combined with the theoretical study
revealed the formation of the single N2 molecular junction. The conductance of the single N2
molecular junction was 1 G0, which was close to that of the metal atomic contact.

6.2 Experiments
Experiments were performed using the MCBJ technique at 10 K, as mentioned in chapter 3. A
notched Pt and Cu wire (0.1 mm in diameter, 10 mm in length) were used for the electrode, as
mentioned previously. N2 gas (>99.9995 vol.%, several μmol) was introduced via a heatedcapillary.

6.3 Results and discussion
6.3.1 Pt electrodes
Figure 6.1(a) shows typical conductance traces representing variations in the conductance of Pt
contacts during the breaking process of the metal wire. A stepwise fashion represented theatomic structural transition of the contact. The plateau at 1.5 G0 corresponded to the
conductance of a Pt atomic contact, which agreed with the previous result (15). As mentioned inchapter 2, the electrical conductance through the metal atomic contact was expressed byG  2e 2 / h Ti

where Ti is the transmission-coefficient probability of the i-th conductancechannel. The Pt atomic contact has a conductance of 1.5-2 G0 (16), depending on the atomic
configuration of the contact. In the presence of nitrogen, a plateau was observed at 1 G0 in
addition to the plateau at 1.5 G0. The conductance histogram constructed from one thousand
conductance traces showed peaks at both 1 G0 and 1.5 G0 (Figure 6.1(b)). The appearance of the
1 G0 peak indicated that the atomic configuration with conductance of 1 G0 is more likely to beformed during the breaking process.

60

Figure 6.1: (a) Conductance trace before (black) and after (red) the introduction of N2 gas,measured at the bias voltage of 100 mV. (b) Conductance histogram constructed from 1000
conductance traces before (black) and after (red) introduction of N2 gas. The bin size was 0.004
G0. The count was normalized by the number of total breaking processes.

In order to investigate the atomic configuration of the stable atomic contact with 1 G0, thedI/dV and d2I/dV2 spectra were measured. Figure 6.2(a) represents a typical dI/dV spectrum
measured at a conductance value around 1 G0. A symmetric conductance enhancement wasobserved around ± 40 meV, which originates from electron-vibration interaction in the contact(5, 10, 11, 17).
In order to accurately determine the vibration energy, 41 dI/dV spectra were collected for thecontact in the conductance regime ranging from 0.05 G0~1.2 G0, resulting in the energy
histogram presented in Figure 6.2(b). The distribution could be fitted by a Gaussian function,
whose peak center was 43 meV and full-width at half maximum was 8 meV. As the energy of 43
meV was larger than the phonon energy of Pt (18), and much smaller than the energy of N2
internal vibration modes (19), the vibration mode of 43 meV would correspond to the vibration
mode between the Pt electrode and nitrogen. The existence of nitrogen in the Pt contact was
revealed by the vibration spectroscopy. Further discussion about the atomic structure andvibration mode are given in the next section related to theoretical calculation part.
61

Figure 6.2: (a) Typical dI/dV spectrum of the Pt-N2 system showing a conductance of close to 1
G0. (b) Histogram of the vibration energy of the Pt-N2 system constructed from 41 dI/dV
spectra.

It is worth discussing the shape of the dI/dV and d2I/dV2 spectra. While the symmetricconductance enhancement was observed in the dI/dV spectrum, as in Figure 6.2(a), theconductance suppression was also observed in the dI/dV spectrum. Figure 6.3 shows the
distribution of the conductance enhancement and suppression in the dI/dV spectrum for the
Pt-N2 system as a function of the conductance and energy in the conductance range from 0.05
G0 to 1.2 G0. Conductance enhancement was observed more frequently than conductancesuppression. There was no correlation between the conductance and shape of the dI/dV spectrum.
The previous theoretical study predicted that the conductance enhancement is observed if thetransmission probability is below 0.5, and conductance suppression is observed otherwise (20).
This phenomenon was observed in H2O (21) and benzendithiole (22). The discrepancy may be
explained by the difference in the number of channels. In systems such as H2O molecules,
electron transport occurs through a single channel and the molecule-metal coupling issymmetric (20-22). On the other hand, in a Pt-N2 system, more than one channel contributes to62the electron transport, as shown in Figure 6.6 above. The existence of multi channels may affectthe shape of the dI/dV spectrum. Based on theoretical calculations conductance enhancement
can be induced by nontransmitting d channels, which are coupled to the transmitting s channel
via transverse modes in the Pt-H2 system (23).

Figure 6.3: Distribution of the conductance enhancement and suppression in the dI/dV spectrum
for the Pt-N2 system as a function of the conductance and energy for conductance values
ranging from 0.05 G0 to 1.2 G0.

In order to investigate the structure of the molecular junction, theoretical calculations were
performed for four possible structures (inset in Figure 6.4). The inset of Figure 6.4 shows the
structural models investigated in this study. The left and right Pt electrodes were constructedfrom metal bulk oriented in the (111) direction and an atomically sharp tip. The tips weremodeled as 4-atom tetrahedrons. In the model named “atomic” (Pt-N), the atomic nitrogen wasplaced between Pt electrodes. The other models (Pt-N2) represent the N2 molecule placed
between the Pt electrodes with its molecular axis vertical (“bridge,” “vertical”) or parallel(“parallel”) to the junction axis. While the N2 molecule was placed on the line of two top Pt
atoms in “vertical,” the N2 molecule was placed above the Pt-Pt bond in “bridge.” The geometryof each model was optimized using first-principle computer code Atomistix Toolkit (ATK)software (quantum-wise A/S) (24, 25), during which the bulk of electrodes was fixed while theN atom (or N2 molecule) and the two tips were relaxed. The total energy of the system wascalculated as a function of the distance between electrodes, where the distance was the length
between two Pt atoms at the top of the tetrahedrons. The electron-transport behavior of the63models at stable distances was also investigated. During these calculations, double-ξ with apolarization basis set and single-ξ with a polarization basis set were chosen for the N atoms and
Pt atoms, respectively.
Models that consist only of an N atom (or N2 molecule) and two 4-atom Pt tetrahedrons wereused for the calculation of the vibration energy, before which geometric optimization was
carried out at a b3lyp level using lanl2dz basis set with Gaussian software (26). During thegeometric optimization, each 4-atom Pt tetrahedron was kept as a group, while the distance
between two tetrahedrons and the configuration of the N atom (or N2 molecule) was relaxed.
Figure 6.4 represents the total energy of the junctions as a function of the distances. The
stable states appeared at 3.65 Ǻ, 2.50 Ǻ, 4.29 Ǻ, and 5.03 Ǻ for “atomic,” “bridge,” “vertical,”and “parallel” configurations, respectively. The “bridge” and “parallel” configurations weremuch more stable than that of “vertical” configuration. The conductance was calculated to be
1.34, 2.91, 1.12, and 0.98 G0 for “atomic,” “bridge,” “vertical,” and “parallel” configurations,
respectively. The vibration energy for each junction was investigated. Table 1 shows a summaryof the vibration modes around 40 meV, which was observed in the present study. For the“atomic” and “vertical” configurations, there were no vibration modes around 40 meV. For the“parallel” configuration, there were three vibration modes (42.2, 44.1, and 45.0 meV) around 40
meV. These vibration modes are depicted in Figure 6.5. Based on the experimental andtheoretical calculation, the Pt-N2 system showing a conductance of close to 1 G0 was assigned tobe the “parallel” configuration. Theoretical calculation also revealed that the N-N bond wasstretched for the Pt-N2 junction. While the N-N bond is 1.09 Å for an isolated N2 molecule, theN-N bond was 1.16 Å in the molecular junction because of the interaction between the moleculeand metal electrodes.

64Figure 6.4: Total energy of (a) Pt-N and (b) Pt-N2 as a function of the distance between
electrodes.

Figure 6.5: Three possible vibration modes having a “parallel” configuration.

Table 6.1: Summary of calculations
Model

AtomicBridgeVertical

Parallel

Most Stable Distance (Å)

3.65

2.50

4.29

5.03

1.34

2.91

1.12

0.98

-

37.4, 44.1 meV-Conductance at 0V(G0)
Vibration Modes near 40
meV

42.2, 44.1, 45.0
meV

In order to obtain a better understanding of the conductance at zero bias, the transmission
coefficient at the Fermi level was investigated. It was determined that two dominant channels
transfer electrons through the junction, and their transmission probabilities are 0.458 and 0.454,
respectively. From the transmission spectrum (Figure 6.6a), It was roughly predicted that thesetwo dominant channels originate from the tail of the broad peak at -0.1 eV. Figure 5b shows the65main channels of -0.1eV; it is determined that these π-type channels are from the coupling
between orbitals (dxz and dyz) of the Pt atom and orbitals (px and py) of the N atom (27), which
supports the dI/dV spectrum behavior.

Figure 6.6: (a) Transmission spectrum at zero bias, (b) Two dominant channels of the peak at-0.1 eV.

6.3.2 Cu electrodes
Figure 6.7(a) represents conductance traces of the Cu contacts measured during the breaking
process. Before the introduction of nitrogen, a plateau was observed at 1.0 G0, representing theformation of the Cu atomic contact. The 1.0 G0 peak was observed in the conductancehistogram, as shown in Figure 6.7(b). After the introduction of N2, a plateau was observed
around 0.9 G0 in the conductance traces, and a prominent peak was observed at 0.9 G0 in thecorresponding histogram. The conductance of the center of the peak was estimated to be
0.86±0.06 G0 by averaging the conductance values for five distinct samples. The intensity of the
0.86 G0 peak for the N2/Cu contacts increased twofold compared with the 1.0 G0 peak for the Cu
contacts. The peak shift and enhancement of the peak intensity in the conductance histogramindicate that the conductance of the Cu atomic junction is reduced and stabilized after the
introduction of N2. Because the plateau length around 1.0 G0 observed after the introduction of
N2 is elongated in Figure 6.7(a), it is considered that the elongation of the plateau length
contributes to the enhancement of the peak intensity in the conductance histogram. In order toestimate the plateau length quantitatively, a plateau length histogram was constructed. The
plateau length was defined as the length between the first and last points showing a conductance66

of 0.67–1.0 G0 in the conductance traces. Figure 6.8 shows plateau length histograms obtained
before and after the introduction of N2. Before the introduction of N2, a single prominent peak isobserved, and the plateau length is a maximum of 0.5 nm. This behavior represents Cu form
only atomic contact but hardy form atomic chain without molecule, which agrees with previous
study (2, 28). On the other hand, the plateau was stretched to a maximum of 1.5 nm, andmultiple peaks were observed after the introduction of N2. The plateau length histogram wasfitted with four Gaussian functions. The average distance between the peaks is 0.25 ± 0.06 nm,
which is close to the interatomic distance of bulk 0.25 nm Cu crystal. The elongation of theplateau length and multiple peaks in the plateau length histogram indicates that Cu forms asingle atomic chain with a length of 2–3 atoms.

Figure 6.7: Conductance traces (a) and histograms (b) of Cu contacts before (black) and after(red) the introduction of N2. Bias voltage: 50 mV.

67

Figure 6.8: Plateau length histogram before (black) and after (blue) the introduction of N2.
Green and Red line: fitting by Gaussian function.

Next the formation of the linear Cu atomic chain was investigated. Figure 6.9 shows aschematic of the simulation, in which we used several groups of models to describe the possible
structures appearing at different stages during the elongation of a Cu LAC in N2 atmosphere. In
the models, the left and right Cu electrodes were constructed from metal bulk oriented in the[111] direction, which was also used in conductance calculations. Tips were modeled as two
opposite tetrahedrals, which represent the broken section of the nanowire. The N2 molecule
adhered to the broken section in two ways, i.e., parallel or vertical to the stretch direction. The
models were stretched quasi-statically with the first-principle computer code Atomistix Toolkit(ATK) software.

68

Figure 6.9: Schematic diagram for the simulation of a stretch of the Cu single atomic chain in
N2 atmosphere. The arrows show the possible routes.
The evolution of the energy and conductance of the models belonging to different stages areplotted in Figure 6.10. The stretch length is defined as the distance between electrodes. The totalenergy profile is presented by setting the lowest energy point as zero. As models A1 and A2
have one fewer Cu atoms than the others, neither of them is plotted in the figure. The V-shaped
energy curves reflect the fact that there is an energy barrier between stages. If there is a weakconnection in the LAC, it cannot be transformed to a new stage, and will rupture. By connecting
three different stages, a typical conductance trace similar to those in Figure 6.7(a) was produced.
The calculated trace has a plateau at 0.9 G0, which agrees with the conductance measurement.
The plateau in the cyan region contains a small portion of stage A and a large portion of stage B,
which provides possible atomic structures corresponding to the conductance steps around 0.86
G0, which was observed experimentally. In other words, the conductance peak at 0.86 G0 in
Figure 6.7(b) corresponds to the structure in which the Cu atomic junction is attached to the N2
molecule, while clean LACs (atomic contacts) showed conductance values around 1.0 G0. The
conductance steps observed around 0.2–0.5 G0 (the 7th and 8th traces in Figure 6.7) may be
explained by structures such as C1 or C2, in which N2 is inserted between Cu atoms;
nevertheless, such a structure did not contribute to the peak in the conductance histogram of
Figure 6.7.

69

Figure 6.10: Evolution of total energy (upper panel) and differential conductance at zero bias
(lower panel) during the quasi-static stretch.

Based on both the experimental and theoretical results, it was considered that the adsorptionof the N2 molecules contribute to strengthening the Cu–Cu bond and the formation of Cu LACs.
A previous theoretical study showed that the adsorption of molecules stabilizes the high-energy
state of a metal atomic junction (29). The presence of adsorbates on the LAC reduces the energyof the surface in low coordination states, which may enable us to form metal LACs. Anotherrole of the N2 molecule in the formation of LACs is the direct support of the Cu–Cu bond. On
the other hand, there is another possible scenario for the formation of a Cu atomic chain. Figure
6.11 shows the strength of the interaction between N-Cu and Cu-Cu during the stretch. The
strength of the Cu–Cu interaction (the binding energy of B0) decreases rapidly, while that of the
N–Cu interaction (the binding energy of B2) remains small and constant until 2.61 nm, where
the Cu–Cu distance is so great that the N2 starts to be inserted between the atoms. The rapid
decrease in the interaction between the Cu atoms at a certain separation indicates that Cu atomic
junction is brittle without N2 molecule. On the other hand, the presence of N2 makes theconnection more flexible and increases the junction’s durability; this makes it easy to extract
70

another atom from the tip. In an experimental scenario, i.e., a dynamic process, the atoms do nothave enough time to reach the most energetically stable configuration, such as C2. The Cu
nanowire tends to rupture at some weak Cu–Cu connections where no N2 molecules areattached.

Figure 6.11: Comparison between N–Cu interaction (B2) and Cu–Cu interaction (B0).

6.4 Conclusions
The electronic conductance of the platinum-copper contact in nitrogen atmosphere has been
investigated using both an experimental technique and theoretical calculations. A single N2
molecule was placed between Pt electrodes in parallel. The conductance of the single N2
molecular junction was 1 G0, which was close to that of the metal atomic contact. Two dominantchannels originating from the coupling between the N2 molecule and electrodes contribute to theelectron transport. On the other hand, the N2 molecule was not placed between copper
electrodes. With the support of the N2 molecule, the linear Cu atomic chain was formed,
although Cu does not form a linear atomic chain without molecules. The lone pair in thenitrogen atom formed Pt-N2 bonds, and successfully formed a Pt/N2/Pt single molecularjunction.

71

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E. P. M. Amorim, E. Z. da Silva, Effect of light impurities on the electronic structure ofcopper nanowires. Phys. Rev. B 85, 155407 (2012).

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M. Kiguchi, O. Tal, S. Wohlthat, F. Pauly, M. Krieger et al., Highly conductive
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S. Kaneko, T. Nakazumi, M. Kiguchi, Fabrication of a well-defined single benzene
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M. Kiguchi, S. Miura, K. Hara, M. Sawamura, K. Murakoshi, Conductance of single
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M. Kiguchi, H. Nakamura, Y. Takahashi, T. Takahashi, T. Ohto, Effect of anchoringgroup position on formation and conductance of a single disubstituted benzene
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M. Kiguchi, T. Nakazumi, K. Hashimoto, K. Murakoshi, Atomic motion in H-2 and D-2
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R. H. M. Smit, C. Untiedt, A. I. Yanson, J. M. van Ruitenbeek, Common origin forsurface reconstruction and the formation of chains of metal atoms. Phys. Rev. Lett. 87,
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N. Agrait, A. L. Yeyati, J. M. van Ruitenbeek, Quantum properties of atomic-sized
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B. C. Stipe, M. A. Rezaei, W. Ho, Single-molecule vibrational spectroscopy andmicroscopy. Science 280, 1732-1735 (1998).

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M. Paulsson, T. Frederiksen, H. Ueba, N. Lorente, M. Brandbyge, Unified descriptionof inelastic propensity rules for electron transport through nanoscale junctions. Phys.
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O. Tal, M. Krieger, B. Leerink, J. M. van Ruitenbeek, Electron-vibration interaction insingle-molecule junctions: from contact to tunneling regimes. Phys. Rev. Lett. 100,
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Y. Kim, T. Pietsch, A. Erbe, W. Belzig, E. Scheer, Benzenedithiol: A broad-range
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73

７ Controlling the electrical property of highly
conductive single pyrazine molecular junction

A N2 molecule was successfully placed between Pt electrodes. The lone pair of N2 formed asingle molecular junction with a high and well-defined conductance value. In this chapter, I
investigate the application of a pyrazine molecule between the Pt electrode. Pyrazine molecules
are expected to have two conductance states. One is the configuration where a π orbital isparallel to the bonding direction. The other is the configuration where a p orbital is vertical tothe bonding direction. Here, bi-stable states were fabricated and their configurations were
investigated in terms of their conductance and differential conductance, near edge X-ray finestructure, and theoretical calculations. Defined configurations were switched by an external
force. The electric property of the single molecular junction was controlled by changing theinterface structure.

The contents of this chapter have been published in: Satoshi Kaneko, Manabu Kiguchi,“Investigation on the pyrazine molecular junction studied by conductance measurement and nearedge X-ray absorption fine structure” Fullerenes, Nanotubes and Carbon Nanostructures 22,
166-172 (2014), and Satoshi Kaneko, Carlo Motta, Gian Paolo Brivio, Manabu Kiguchi,“Mechanically controllable bi-stable states in highly conductive single pyrazine molecular junction”
Nanotechnology 24, 315201 (2013).

74

7.1 Introduction
The utilization of bi-stable states in single molecular junctions as memory, switches, and othercomponents is a key issue in the realization of molecular electronics. Molecular-based switches
are advantageous because of their small energy consumption as slight structural changes are
sufficient to control the conductance of the molecular junction, and such changes can be
induced by tiny mechanical forces, electric fields, and photo irradiation, etc. (1-5). For example,
the mechanophore molecule was proposed as a distortion sensor whose color is changed by the
deformation of the molecule (1, 2). In addition, a high on/off ratio has been measured for logicalcircuits using rotaxane molecules (3) and suspended graphene by the application of voltage
pulses (4). Recently, several single molecular switches showing bi-stable states have beenreported using photochromic molecules, 1,6-hexanedithiol, 4,4’-bipyridine, and other molecules,
thanks to the recent development of experimental techniques (e.g., scanning tunnelingmicroscope (STM) break junction and mechanically controllable break junction (MCBJ)) (6-8).
In particular, single bipyridine molecular switches have been extensively studied because Au-N
bonds provide fixed conductance values (8-14), which is very important to obtaining reliable
single molecular switches. However, there remain unresolved problems in this field. First, theconductance value of the on-state of the single molecule switch is too low, namely below
10-1~10-2 G0 (G0 = 2e2/h) (7, 8). Second, until the present, all of the evidence for the switching
behavior has been obtained indirectly. The conductance measurements can only provide
information on the conductance switching, and the mechanism at the base of the transition
between bi-stable states is often not clear.
Because it was revealed that nitrogen in a π-conjugated system works well to fabricate single
molecular junction with high and well-defined conductance values, the next target is to fabricate
the bi-stable electric states and to control the bi-stable states utilizing the pyrazine molecule. In
this chapter, I discuss the fabrication of highly conductive single molecular junctions showing
bi-stable states, and I investigate the switching mechanism using a combination of experimental
and theoretical techniques. In order to improve the conductance of the single molecularjunctions, I first address the metal-molecule interface. Typically, in conventional single
molecular junctions, the molecules are attached to the metal electrodes via anchoring groups(e.g., thiol, amine) (15-20). Although a stable single molecular junction can be prepared in thisway, it has been reported that the anchoring group acts as a resistive spacer, leading to low
conductivity, as mentioned in chapter 1. Recently, the direct binding of π-conjugated molecules
to metal electrodes has been found to be a promising technique to improve the conductance of
single molecular junctions (21-29). Indeed, an efficient overlap between π orbitals of themolecule and the metal states leads to high conductivity. In this chapter, the use of a pyrazine
75

single molecular junction is discussed. In this system, the π orbitals of pyrazine, including the porbital of nitrogen, can directly bind to the metal electrodes. In addition, the presence ofnitrogen atoms in the pyrazine molecular backbone can fix the binding site with metalelectrodes, forming a non-directional σ bond between the lone pair of the nitrogen atom and themetal. This realizes the efficient coupling between the molecule and the tip, allowing for stableconductance values (17). Earlier theoretical calculations on jellium electrodes predicted the
conductance of the pyrazine molecular junction as high as 1 G0 (10). Next, I focus on the metalelectrodes. In the simple tunneling model, the conductance of single molecular junction depends
on the local density of states (LDOS) of the contact metal atoms at the Fermi level (30).
Differently from Au, Pt displays a narrow 5d band located at EF, and consequently, a larger
density of states (DOS) occurs. Thus, molecular orbitals can effectively be hybridized with the dorbital of Pt. For this reason, single molecular junctions using Pt as metal electrodes are
expected to have high conductivity. Actually, such increases in the conductance of single
molecular junction have been reported for various molecules including benzenedithiol (19),benzene (21, 22), and H2 (31, 32).
This chapter shows highly conductive pyrazine/Pt junctions using the MCBJ technique at 10
K. The conductance measurements display two distinct conductance states of 1.0 G0 and 0.3 G0,
which are two orders of magnitude larger when compared to single molecular junctions withanchoring groups, including pyrazine-related molecular junctions. By combining theexperimental conductance results, IETS measurements and theoretical calculation revealed thatthese two states are due to different geometrical configurations of pyrazine, which exhibit large
and small coupling with the electrodes. I also demonstrated that the controlled torsion of themolecule is capable of switching between the two conducting regimes.

7.2 Experiments
Pyrazine was placed between Pt electrodes using the MCBJ technique at 10 K, following themethod mentioned in chapter 3. Pyrazine (Wako 97%) was degassed by performing freezing andpumping cycles, as well as the thawing technique. Both before and after the introduction of thepyrazine, the conductance was measured with a bias voltage of 300 mV during the stretchingprocess. The differential conductance was measured by a lock-in technique with an AC
modulation with amplitude of 1 mV and a frequency of 7.777 kHz. The conductance wasmonitored for a fixed contact configuration during the sweep of the DC bias from -80 mV to+80 mV.
The NEXAFS (near edge X-ray absorption fine structure) spectra were measured at the soft76

X-ray beam line BL-7A in the Photon Factory at the Institute of Materials Structure Science,
Tsukuba, Japan. The photon energy was calibrated with respect to the C 1s to π* peak positionof highly oriented pyrolytic graphite (HOPG) at 285.5 eV. After preparing the clean Pt(111)
surface by repeated cycles of Ar sputtering and annealing, the multilayer of the pyrazine film
was deposited on the Pt surface by the introduction of pyrazine gas at a substrate temperature
below 240 K. The monolayer film was prepared by heating the sample up to 300 K.

7.3 Results and discussion
7.3.1 Conductance
Figure 7.1(a) displays typical conductance traces as a function of the stretch length, which is thedisplacement between the stem parts of the Pt electrodes that are fixed with epoxy adhesive.
Before the introduction of pyrazine, the conductance decreases in a stepwise fashion with thestretching of the junction, showing the last plateaus around 1.5 G0. The corresponding
conductance histogram (Figure 7.1(b)) presents a peak around 1.5 G0, which corresponds toclean Pt atomic contacts. After the introduction of pyrazine, steps appear around 1.0 G0 and 0.3
G0. Some steps have positive or negative slopes and display fluctuations in the conductance,
which are caused by the small change in the atomic configuration of the junction. Note that the
0.3 G0 step appears after the 1 G0 step, as shown more clearly in the last two traces in Figure
7.1(a). The 1.5 G0 peak, which is characteristic of the Pt atomic contacts (21), is suppressed in
the conductance histogram (Figure 7.1(b)), and a larger weight is distributed in the entire range
below that value. The new features, which are due to the introduction of pyrazine, signal theexistence of two distinct conductance states. This indicates that the clean Pt atomic contacts are
hardly formed, and that pyrazine molecules would bridge between Pt electrodes. Theirconductance was determined to be 1.0 G0 (high-conductance state, HC) and 0.27 G0 (lowconductance state, LC) by numerically averaging the conductance values in the histograms thatwere measured for four distinct samples. Some features observed above 1.5 G0 may haveoriginated from a Pt atomic contact covered with pyrazine molecules.

77

Figure 7.1: (a) Conductance traces. (b) Conductance histograms of the Pt contacts before (black
dotted curve) and after (red curve) the introduction of pyrazine. The bias voltage is set to 300
mV. The conductance histogram is constructed without data selection from 1000 conductancetraces of breaking metal contacts. The intensity of the conductance histograms is normalized by
the number of conductance traces. The bin size was 0.004 G0.

The measured conductance values were compared with those of other similar single
molecular junctions, where π-conjugated molecules are directly bound to metal electrodes. The
single Pt/benzene/Pt molecular junction showed conductance values of 1.0-0.2 G0 depending on
the atomic configuration of the benzene single molecular junctions (21). The single
Ag/benzene/Ag molecular junction showed a fixed conductance value of 0.2 G0, reflecting aweak benzene-Ag interaction compared with a strong benzene-Pt interaction (22). The
conductance values of the single Au/C60/Au, Ag/C60/Ag, and Pt/C60/Pt molecular junctions were0.3 G0, 0.5 G0, and 0.7 G0, respectively (23, 24). The direct-binding technique has also been
applied to endohedral Ce@C82 metallofullerenes, ethylene and acetylene, and other molecules(25, 26). These single molecular junctions showed high conductance values around 1.0-0.2 G0,
while the pyrazine single molecular junction shows conductance values of 1.0 G0 and 0.3 G0.
The latter are close to those of other single molecular junctions fabricated using thedirect-binding technique.

787.3.2 dI/dV measurements and NEXAFS measurements
While conductance traces provide evidence of a bi-stable molecular junction, further insight into
the conduction properties can be obtained by examining the IETS. Such measurements as afunction of the voltage across the single molecular junction were performed for both HC and LC
regimes, keeping the electrode separation fixed. Figure 7.2 displays the differential conductanceand its derivative for HC and LC regimes. The arrows denote shallow symmetric upward
features in the differential conductance around 30 mV for the LC state and 60 mV for that of theHC (Figure 7.2(a)). Note that there are clear peaks in the derivatives (Figure 7.2(b) and 7.2(c)).
In addition, an asymmetric background in the differential conductance (Figure 7.2(a)) was
observed. The asymmetric feature may be attributed to the interference of electrons scattered on
defects in the leads (33-35), asymmetric coupling between the molecule and metal leads, andother effects. The conductance enhancement is explained by the opening of an additionaltunneling channel for electrons that give energy to a vibration mode (36, 37), as previously
mentioned. Because the phonon modes of Pt are below 20 meV (T mode: 6, 10 meV, L mode:
20 meV) (38), the two observed features correspond to vibration modes of the Pt-molecule bondor internal molecular ones, which confirms that pyrazine was placed between the Pt leads. The
different bias voltages of HC and LC vibrations reflect two distinct configurations. Here, I
discuss the shape of the IETS. In the present study, a peak appeared in d2I/dV2 curves even in
the HC regime. Based on the theory of single molecular junctions, there should be a peak in the
d2I/dV2 curves if the transmission probability is below 0.5, and a dip in the d2I/dV2 curves if thetransmission probability is above 0.5 (39). In such a model, electron transport occurs through asingle channel and the molecule-metal coupling is symmetric (39). The shape of the IETS is
discussed in the latter theoretical part in terms of the number of channels.
Figure 7.3 shows the C-K and N-K edge NEXAFS spectra of the monolayer and multilayer
pyrazine film on a Pt surface taken at X-ray incidence angles of 55 for the C-K edge, and 30
for the N-K edge to the Pt surface. The NEXAFS spectra were normalized with respect to thevalue of the edge jump, where the intensity was proportional to the number of carbon or
nitrogen atoms. The C-K edge NEXAFS spectra of the multilayer pyrazine film showed peaks
at 284 eV, 287 eV, and 304 eV, which correspond to the C 1s to π*, the C 1s to π*, and the C 1s
to σ* transitions, respectively (40, 41) . The N-K edge NEXAFS spectra of the multilayer
pyrazine film showed peaks at 397 eV and 407eV, which correspond to the N 1s to π* and the N
1s to σ* transitions, respectively (40, 41). While the C-K edge NEXAFS spectra of themonolayer film was similar to that of the multilayer film, the N-K edge NEXAFS spectra of themonolayer film changed from that of the multilayer film. The broadening of the π* peak was
clear for the N-K edge NEXAFS spectra of the monolayer film. Because the π* peak of the N-K
79

edge NEXAFS spectra reflected the electronic structure of the p orbital of the N atoms of thepyrazine molecule, the broadening of the π* peak of the N-K edge NEXAFS spectra indicatedthat the pyrazine molecule was adsorbed on the Pt surface via the N atoms of pyrazine
molecules.

Figure 7.2: (a) Differential conductance spectra of single pyrazine molecular junctions and theirderivatives taken at (b) high- and (c) low-conductance regimes.

80Figure 7.3: (a) C-K edge and (b) N-K edge NEXAFS spectra for pyrazine films grown on
Pt(111) taken at X-ray incidence angles of 55 for the C-K edge, and 30 for the N-K edge to
the Pt surface. The black and red curves are the result of the thick layer and monolayer,
respectively.

7.4 Calculations
7.4.1 Methods
The electronic structure calculations were carried out using the first-principle self consistentmethod implemented in the SIESTA code (42, 43), which uses a basis set of numerical atomic
orbitals. Conductance calculations were done within the formalism of the non-equilibrium
Green's function combined with the DFT, as implemented in the TranSIESTA package (44). The
exchange-correlation energy is described by the Perdew-Burke-Ernzerhof (PBE) generalized
gradient approximation (GGA) (45). The potential used was Troullier-Martins, norm-conserving
pseudopotentials in the fully separable Kleimann and Bylander form with channels having coreradii of 2.60, 1.25, 1.48, and 1.25 Bohr for Pt, C, N, and H, respectively. A single-ζ polarizedbasis set of numerical atomic orbitals was used, with cut-off radii that have been determined by
imposing confinement energy of 0.01 Ry. The energy cutoff for a real-space mesh was set to 200
Ry. These values have been tested both for bulk Pt and isolated pyrazine.
817.4.2 Structure and conductance
Ab-initio calculations were performed to analyze the switching mechanism of pyrazine/Pt single
molecular junctions. In order to determine the most suitable contact configuration to be adopted,
the adsorption energies for different structures were calculated. Several adsorption sites wereconsidered, namely, the adsorption on top of a pyramidal Pt4 cluster, on the hollow and bridge
sites of a Pt3 cluster, and on the hollow and top sites of the clean Pt(111) surface. In Figure 7.4,
the relaxed structures as well as the adsorption energies for all five configurations are reported.
The most stable contact configuration is the pyramidal Pt4 electrode, which is placed on the topof Pt(111). The surface top adsorption is unfavorable compared to the Pt4 top one because of thesteric repulsion between the molecule and the surface Pt atoms. The calculations also suggestthat pyrazine adsorbed on a hollow site is unstable, and that N selectively forms a bond on the
top of a Pt atom. The NEXAFS study for the pyrazine molecules on the Pt surface supports theformation of N-Pt bonds, as mentioned in the previous section.

Figure 7.4: Adsorption energies of pyrazine on different electrode geometries: (a) on top of Pt4
cluster, on (b) hollow site and (c) bridge site of a Pt3 cluster, on the (d) top site, and (e) hollow
site of the clean Pt(111) surface.

According to these results, the tips were modeled as composed by two Pt4 pyramids placed on
top of a 3 × 3 Pt(111) surface. In order to describe the pyrazine junction, a periodic supercell
was considered, as shown in Figure 7.5(e) and (f). The density matrices were calculated using a
3 × 3 k-mesh in the plane orthogonal to the junction axis (i.e., the longest cell dimension in
Figure 7.5(f)), while the transmission spectra were instead computed using a 5 × 5 k-mesh.
Preliminary results indicate that the transmission functions converged with respect to the lateralcell size. The slab contains 10 atomic layers. First, the single electrode containing the tip isrelaxed, and then I used the obtained geometry to build the junction. All the subsequentrelaxations were performed by keeping all of the electrodes’ atomic positions fixed, andallowing the Pt pyramidal atoms as well as the pyrazine ones to relax until the residual forces82are smaller than 0.04 eV/Å.
The configuration dependency of the properties was measured, as shown in Figure 7.5. In
Figure 7.5, the distance D is defined as the distance between the two outermost Pt surfaces.
Here, dTips is the distance between the apical Pt atoms, and α1,2 are defined as the angles between
the straight line of the N-N molecular backbone and that defined by the N-Pt bonds (as shown in
the inset). Eform is the junction formation energy, which is calculated as Eform =EPt-molecule-Pt-2EPt-Emolecule, and T(EF) is the transmission function at the Fermi level. By observing
Figure 7.5(a), two regimes can be distinguished. An HC regime is observed for distances D
lower than 14.5 Å, where the conductance is within the range of 1.3 G0 and 1.5 G0. In thisregime, the tilt angles α1,2 are lower than 120°, meaning that the molecule sits in a tilted
configuration with respect to the junction axis. The electronic coupling between the molecule
orbitals and the d orbitals of the Pt tip is favored due to the orientation of the molecule, andcorrespondingly the transmission function increases inversely with α1,2. The transmission does
not change within the range between D = 13.0 Å and 14.5 Å, and it shows an HC plateau. In therange between 14.3 Å and 14.7 Å, a rapid rotation of the molecule is assisted, which overcomesthe energy barrier (Figure 7.5(b)) to reach the local energy minimum corresponding to the LC
state.

83

Figure 7.5: Left (a-d): Pyrazine/Pt junction properties plotted against the surface distance D, (a)transmission function T(EF), (b): junction formation energy Eform, (c): bending angles α1,2, (d):distance dTips between the outermost Pt atoms and its derivative d'. Right (e-f): geometry of the
Pyrazine/Pt junction for the calculation. Top (e) and front (f) view of supercell used fortheoretical calculation. (g): geometry of the central part of the junction during the stretchingprocess. The α1,2 is highlighted by blue and red. The following atomic color scheme was used :grey = Pt, yellow = C, cyan = H, light blue = N.

84

In this regime, the pyrazine is aligned with the junction axis, i.e., α1,2 are equal to 180°; this isreflected in a lower coupling between the molecule orbitals and those of Pt. As a consequence, it
was revealed that the transmission T(EF) assumes fractional values, and its intensity is fairly
constant and equal to 0.7 G0 for a range of about 2 Å. This is in agreement with the appearanceof the LC feature in the conductance histogram (Figure 7.1(b)). When D is larger than 16.5 Å,
the Pt-N bond is broken, causing an abrupt decay from T(EF) to zero, while the binding energy
also tends to zero. During the junction elongation, dTips increases almost linearly (Figure 7.5(d)),
while the peaks in its derivative signal transitions between different regimes. The two
experimental conductance states of the HC and LC states were assigned to the two regimes, as
highlighted by first-principles calculations. In agreement with the conductance traces, the LC
plateau appears after the HC one and extends for a longer range. Note that the distance D
between the two plateaus is comparable to the experimental ones in Figure 7.1(a). The
differences between theory and experiments regarding the conductance intensities are due to theintrinsic limitations of the ground-state DFT applied to electronic transport, as theunderestimation of the HOMO-LUMO gap and the self-interaction error (46). In fact, even withthe prototypical case of the Au/benzenedithiol/Au junction, the DFT-NEGF approachoverestimates any experimental conductance by at least one order of magnitude (47). In thiscase, no significant discrepancies were found because the system shows a strong
metal-molecule coupling. It was found that the molecule stretching along the minimum-energy
path obtained a bi-stable junction where conductance results are in fairly good agreement withthe experimental ones. For this reason, an average over several molecular configurations was
not performed.
In order to obtain a more comprehensive view of the electronic properties, in Figure 7.6,
calculations were performed to show the projected density of the states’ (PDOS) onto the basis
orbitals associated with the N and C species and the 5d orbitals of Pt for a representative state ineach conductance regime (D = 13.5 Å for the solid line and D = 15.3 Å for the dashed line).
Calculations reveal that in both cases, the pyrazine HOMO orbital is highly hybridized with the
Pt states, and it provides the main contribution for the conductance at EF. At the same time, theD = 15.3 Å junction shows more pronounced peaks, which indicates a lower hybridization of
the molecular orbitals with respect to the D = 13.5 Å case. As mentioned previously, this isbecause of the different bending angles that are formed by the pyrazine with the junction.
Accordingly, in the upper panel, one can see that the overall conductance for D = 13.5 Å islarger not only near EF, but also over a broader energy range. In fact, the transmission
coefficient depends on the metal-molecule coupling. For the bent structure, the coupling islarger because of a higher hybridization of the pyrazine orbitals, which are polarized85

orthogonally to the molecular plane, and the metal d orbitals, which stem from theunder-coordination of the surface Pt atoms. The result of the bending is a higher overlap
between the respective wave functions, which leads to larger hybridization and conductance.

Figure 7.6: Upper panel: transmission function T(E) of Pt/pyrazine junctions for high- andlow-conductance structures calculated for two representative distances D, namely 13.5 Å (blacksolid line: HC) and 15.3 Å (blue dashed line: LC). Lower panel: PDOS of the same structureson the C and N orbitals and on the Pt 5d orbital.
To obtain information about the individual channel contributions to the conduction, theeigenchannels were calculated (48) for these two reference configurations. From the results, inthe HC regime, two channels are open and equally contribute to the charge transport, while only
one is present in the LC regime. In Figure 7.7, such transport channels are depicted to provide
an intuitive picture of transport. For D = 15.3 Å, the symmetry and the spatial distribution of thechannel has the same character of the π-type HOMO pyrazine, while for D = 13.5 Å, the twochannels display a stronger interaction between the states of the Pt atoms and those of themolecule.
86

Figure 7.7: Isosurface plots of most of the transmitting eigenchannels of the high- andlow-conductance structures at two representative distances D. (a) and (b) first and second
eigenchannel for D = 13.5 Å (HC). (c) single eigenchannel for D = 15.3 Å (LC). The real part ofthe scattering state is shown.

7.4.3 Vibrations
In this section, the shape of the IETS is discussed. In the present study, a peak appears in the
d2I/dV2 curves even for the HC regime, as similarly discussed in chapter 6. It was verified that
more than one channel contributes to the electron transport through the single pyrazine molecule
bridging Pt electrodes at the HC regime (see above), and this may explain the appearance of thepeak in the d2I/dV2 curves, even for the HC regime (1 G0). The number of conductance channels
can be experimentally determined by multi-Andreev reflection or shot-noise measurement (49,50).
Theoretical calculations can help to identify the vibrational modes, which are activated in the
IETS. The DFT calculations were performed in order to calculate the vibrational modes of thepyrazine/Pt junctions for the geometries obtained at different D. The finite Pt clusters wereconsidered to represent the electrodes and all the Pt atoms were fixed, which was justified by
the large difference between the mass of Pt and that of the other species involved. Figure 7.8
shows the stretching dependence of the relevant vibration modes. The lowest three modes
involve rigid vibrations of the entire molecule with respect to the Pt junction. As a consequence,
their energy is strongly affected by stretching (D). At distances D > 15 Å, two possible
87

candidates for the LC mode were observed with IETS. Namely, these modes involve a shufflingof the molecule in the axial direction, and in the frustrated rotation of the molecule with respect
to the axis of its plane. Unlike the third rigid mode (light blue on line) shown in Figure 7.8, theirenergy dependence on D is the same. Their vibrational energy increases while the molecule isprogressively rotated, and slowly decreases once it is coaxial with the junction. On the other
hand, the possible candidates for the HC mode are two internal vibrations of the pyrazine.

Figure 7.8: Stretching dependence of the relevant vibrational modes of pyrazine/Pt junctions
involved in experiments. The insets represent the characteristics of the three lower energy
modes, which involve a collective vibration of the molecule with respect to the Pt junction. The
two dotted lines correspond to the experimental vibrational energies for HC and LC
configurations.

7.5 Control of the electric conductance
It should be noted that pyrazine can be anchored to the Pt tips in two ways, namely by its N
atom and by its two C atoms. The N-anchoring approach found in this study is always
energetically favored. However, at a low D, the bending of the molecule also enables the88realization of the C-anchoring with an energy cost of less than 0.1 eV. On the other hand, forlarger tip separations (D > 14.5 Å), the molecule sits coaxially with the junction and the Clinking is unfavorable, as the steric repulsion between the H and Pt would cost more than 1 eV.
From the calculations, in the case of C-anchoring for D = 13.2 Å, the transmission T(EF) is
approximately 1.5 G0. This further confirms the presence of two regimes associated with thegeometrical features of the pyrazine.
After showing the application of pyrazine single molecular junction on Pt leads, the existenceof bi-stable structures, and characterizing their atomic configurations, a mechanically controlled
conductance switching was demonstrated. Mechanical elongation/compression was repeated byapplying an AC voltage (triangle wave) to the piezo element. The resulting displacement
amounts to 1.0 nm at a modulation frequency of 0.4~0.8 Hz. Figure 7.9 displays two examples
of conductance switching of the pyrazine single molecular junction. The curve (a) shows thetransition between the HC and LC states and breaking the junction, while curve (b) shows thetransition only between the HC and LC states. The conductance changes from high to low in a
discrete manner in response to the smooth triangle-wave perturbation. This confirms that
conductance switching indeed occurs via transitions between two specific metal-moleculecontact configurations following mechanical controllable modulation.

Figure 7.9: Sample pyrazine switching conductance traces, collected while applying the linear ramps(dashed black line) measured at a 100 mV applied bias. The displacement is 1.0 nm, and themodulation frequency is (a) 0.8 and (b) 0.4 Hz.

89

7. 6 Conclusions
In this chapter, I investigated the electron transport of a pyrazine molecule using Pt electrodes.
Break-junction experiments suggest the existence of two distinct stable states that exhibit high(1.0 G0) and low (0.3 G0) conductance values, the latter appearing at larger stretching. The
experimental results of the conductance versus stretch length show two plateaus, which aresignatures of the formation of two different molecule-Pt states. In addition, the conducting
traces highlight the existence of bi-stable states that display two high conductivities, the lower
one at about 0.3 G0 and the higher one at about 1.0 G0. Mechanical manipulation of the junction
by the application of an AC voltage to the piezo element shows that the switch can be reversibly
moved from one state to the other, and that lower conductance is displayed at larger distances.
The effect of the vibration mode on the conductance of such junctions has been measured by
IETS. In the spectra, two peaks were found at 30 meV and 60 meV, and their vibrational origin
was proven by the measured relatively linear temperature dependence of the width of the IETS
derivative of the peak at 30 meV. It is interesting to note that there are two eigenchannels of thetransmission matrix with the largest eigenvalues, whose magnitude is just below 0.5. This may
explain why peaks were observed in the IETS derivatives, and not dips. In support of theexperiments, I also performed DFT first-principle calculations of both the junction geometry
and of the conductance to simulate the junction elongation. To do this, I followed the minimum
energy path. By optimizing the configuration of the system, I found that pyrazine was coupled
to a Pt4 cluster. Then, by controlled torsion of the molecule from a bent shape in which the N-Pt
axis forms and angle of about 90 degrees with the molecular one to one in which such angle is
180 degrees, the two configurations corresponding to the two bi-stable states were obtained. The
calculated conductance value was 1.3 G0 at the shorter distance and 0.7 G0 at the longer distance.
The DFT calculations revealed that the two stable states can be ascribed to different geometricalconfigurations in which the pyrazine axis is bent and coaxial with the junction, whichcorrespond with high and low couplings of the molecular orbitals with the electrodes,
respectively. The lowest energy configuration successfully provided a good qualitative estimateof the variation of the system geometry due to stretching of the molecule-electrode coordinate,
and of the formation of the bi-stable states. In conclusion, the highly conductive bi-stablemolecular junction was constructed. The Pt/pyrazine/Pt molecular junction exhibitedwell-defined structures, where the Pt electrodes are covalently bonded directly to the C
backbone without an intervening link group.

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8 General conclusions
In this thesis, I addressed the fabrication of single molecular junctions showing high andwell-defined conductance values, and dealt with problems regarding the uncertainty anduncontrollable property of the interface between the molecule and metal in single molecularjunctions. Electric conductance measurement systems that are based on the mechanicallycontrollable break junction (MCBJ) technique enabled us to investigate the electron transport
property of single molecular junctions. The fabricated single molecular junctions werecharacterized not only in terms of their conductance, but also in terms of their I-V response or
dI/dV spectrum. These measurements enabled us to discuss the energy alignment or the interface
structure of the single molecular junctions.
In this thesis, I focused on a single molecular junction with a π-conjugated molecule withoutanchoring groups such as thiol or amine. In chapter 4, a benzene single molecular junction was
fabricated using Ag electrodes considering the strength of the interaction between the metal andmolecule. The proper interaction between benzene and Ag enables us to fabricate the benzene
molecular junction with a well-defined conductance value of 0.24 G0. The conductance value
improved by a factor of ten compared with that of a typical single molecular junction withanchoring groups. The interface structure of the single molecular junction was determined by
the dI/dV spectrum, which detected the vibration mode between benzene and Ag. The
conductance measurements, together with the dI/dV spectrum, revealed that an Ag/benzene/Ag
single molecular junction was formed only with the most stable configuration. In thisconfiguration, the benzene molecule binds to Ag electrodes with its p orbital parallel to thebonding direction. The proper interaction between the molecule and electrodes enabled us toform a single molecular junction with a well-defined conductance value. Then, in chapter5, I
focused on the fullerene molecule in order to fabricate a single molecular junction that shows ahigh and well-defined conductance value. The spherical shape of the Ce@C82 molecule
successfully formed a single molecular junction with a well-defined conductance value using
Ag electrodes. The I-V characteristics revealed the presence of a narrow energy gap between the
Fermi level of Ag and the fullerene molecular orbital, which results in a high conductivity of thefullerene molecule. The enclosed Ce atom reduced the conductance value by localizing theorbital of the fullerene cage compared with C60. It should be noted that Au did not form single
molecular junctions for both benzene and Ce@C82. This was explained by the formation of themetal linear atomic chain of the Au. The linear atomic chain formed a larger nanogap, which
prevented the formation of the single molecular junction. These studies with different electrodes95have revealed not only the strength of the interaction, but also that the stability of the
low-coordination states of metal plays an important role in the formation of single molecularjunctions.
After fabricating a single molecular junction with a high and well-defined conductance value
by properly designing the interface structure, I focused on controlling the electrical conductivityof the single molecular junction. In order to control the interface structure without reducing the
conductivity, the use of a nitrogen atom in the π-conjugated system was considered. First, in
chapter 6, the nitrogen molecule was investigated as the smallest π-conjugated system enclosed
the nitrogen atom using Cu and Pt electrodes. The single N2 molecular junction was formed
with Pt electrodes having a conductance value of 1 G0, which is comparable to the conductanceof metal atomic contacts. The high and well-defined conductance value of the N2 molecularjunction with Pt electrodes showed the possibility that the nitrogen atom works as an anchoring
point in highly conductive π-conjugated molecules with Pt electrodes. On the other hand, thesingle molecular junction was not formed with Cu electrodes. It should be noted that a linear Cu
atomic chain was formed with a few atomic lengths, although Cu does not form a linear atomic
chain without molecules. From these results, I observe that N2, which is inert in isolated states,
showed high conductivity and helped the formation of the linear atomic chain.
In chapter 7, I demonstrated the fabrication and control of bi-stable states for a Pt/pyrazine/Pt
molecular junction. First, the conductance was investigated during the breaking process.
Statistical analysis showed that the Pt/pyrazine/Pt single molecular junction had two
conductance states: 1.0 G0 and 0.3 G0. Then, the dI/dV spectrum detected vibration energies of
30 meV and 60 meV for each conductance state. The dI/dV spectrum combined with thetheoretical calculation revealed the configurations of the two conductance states. Finally, thetwo conductance states were switched by changing the distance between the two electrodes. The
two conductance states were successfully switched in the high-conductance region by an
external force. This result is unique for nano-sized systems because in bulk states, it is difficult
to change the electrical property with a mechanical force.
The findings in this thesis revealed that the electron transport property of a single molecularjunction can be controlled by designing a proper metal molecule interface. This finding
pertaining to the controllable electric transport in a highly conductive single molecular junction
results in the single molecular junction being the next stage in the development of nano-sized
electronic components. Moreover, because the molecular orbital in highly conductive single
molecular junction is significantly hybridized with a metal orbital, it is considered that such a96

single molecular junction is a unique nano-sized system. We may activate the inert molecule bysandwiching the molecule between a metal nanogap such as the Pt/N2/Pt junction, and induce anovel reaction by applying proper stimuli such as light, electric pulses, and mechanical force fora single molecular junction. In particular, because electromagnetic waves can be significantly
magnified at the nanogap, it is expected that a novel optical reaction will be observed in a single
molecular junction. In conclusion, the highly conductive single molecular junctions studied inthis thesis are expected to open the door to a new science field related to novel nano-sized
metal- molecule systems.

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Acknowledgements
This thesis would not have been possible without the involvement of people who I met during
the PhD course. The many interactions provide me with valuable knowledge, and motivated me
towards carrying out further investigations. I would therefore like to express my sincere
gratitude to everyone here.
I am grateful for the opportunity to have pursued the PhD course under the supervision ofProf. Manabu Kiguchi (Tokyo Institute of Technology), who introduced me to the science world
with great passion. His valuable advice and encouragement were a tremendous support. I would
therefore like to express my deep appreciation to Prof. Kiguchi.
I would also like to express my gratitude to all of the collaborators in this investigation. Theyare Prof. Takeshi Akasaka (University of Tsukuba), Dr. Michio Yamada (Tokyo Gakugei
University), and Dr. Zdenek Slanina (University of Tsukuba), who provided me with thewonderful molecule Ce@C82. Prof. Shigeru Nagase (Institute for Molecular Science), Dr.
Guangfu Luo (University of Wisconsin-Madison), Mr. Lu Wang, and Dr. Jing Lu (Peking
University) helped me with the theoretical calculations regarding the electron transport of
Ce@C82. Prof. Jianwei Zhao, Mr. Jinjiang Zhang, and Mr. Xiongbo Yang gave me thetheoretical support related to the formation and electron transport of the nitrogen single
molecular junction. Prof. Gian Paolo Brivio (University of Milano-Bicocca) and Dr. Carlo
Motta (Trinity College Dublin) provided me with theoretical insight about the electron transportof the pyrazine molecule.
Next, I appreciate the input by Prof. Toshiaki Enoki (Tokyo Institute of Technology), whoshared several suggestions and discussions about fundamental physics. I also thank Dr. Shintaro
Fujii (Tokyo Institute of Technology) for the discussions about single molecular junctions. Dr.
Kazuhito Tsukagoshi (National Institute for Materials Science, Japan) encouraged me andshared a lot of knowledge about nanotechnology techniques.
Special thanks also to Prof. Jan van Ruitenbeek (Leiden University), Dr. Manohar Kumer(Aalto University), Dr. Tadashi Shiota, and members of the AMC group at Leiden University.
My experience at Leiden University and the continuous fruitful discussion with them gave me a
lot of valuable information about atomic-sized conductors, and they were a great
encouragement.
The NEXAFS measurements presented in chapter 7 were performed at the Photon Factory in
the Institute of Materials Structure Science High Energy Accelerator Research Organization. I
appreciate the assistance by Prof. Amemiya and the faculty members in the Photon factory for
their kind help.

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Heartfelt thanks to the members and alumni of the Kiguchi-Nishino Laboratory at the Tokyo
Institute of Technology, especially Dr. Tomoaki Nishino, Dr. Santiago Marques-Gonzalez, Mr.
Rjuuji Matsushita, Mr. Ryoji Takahashi, Ms. Yu Li, Mr. Sho Suzuki, Ms. Akira Aiba, Dr.
Tomoka Nakazumi, Mr. Daigo Murai, Mr. Yuuga Nakamura, Chenyang Liu, and Ms. Tomoko
Kuroiwa for sharing their experiences with different scientific activities.
Finally, I would like to express my deep appreciation to my family for their unceasing
support.

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Achievements
List of Publications
1. Satoshi Kaneko, Daigo Murai, Santiago Marques-Gonzalez, Hisao Nakamura, Yuki
Komoto, Shintaro Fujii, Tomoaki Nishino, Katsuyoshi Ikeda, Kazuhito Tsukagoshi,
Managu Kiguchi, “Site selection in single-molecule junction for highly reproducible
molecular electronics” Journal of the American Chemical Society in press.
2. Yu Li, Firuz Demir, Satoshi Kaneko, Shintaro Fujii, Tomoaki Nishino, Alireza Saffarzadeh,George Kirczenow, Manabu Kiguchi, “Electrical conductance and structure of copper
atomic junctions in the presence of water molecules” Physical Chemistry Chemical Physics
17, 32436-32442 (2015).
3. Satoshi Kaneko, Yuuga Nakamura, Ryuuji Matsushita, Santiago Marques-Gonzalez,
Manabu Kiguchi, “Simultaneous measurement of electrical conductance and thermopowerof single benzeneditiol molecular junctions” Applied Physics Express 8, 065201 (2015).
4. Satoshi Kaneko, Yuuga Nakamura, Jingiang Zhang, Xiongbo Yang, Jianwei Zhao,
Manabu Kiguchi, “Formation of single Cu atomic chain in nitrogen atmosphere” The
Journal of Physical Chemistry C 119, 862–866 (2015).
5. Yu Li, Satoshi Kaneko, Shintaro Fujii, Manabu Kiguchi, "Symmetry of single hydrogenmolecular junction with Au, Ag and Cu Electrodes" The Journal of Physical Chemistry C
119, 19452-19457 (2015).
6. Ryoji Takahashi, Satoshi Kaneko, Shintaro Fujii, Manabu Kiguchi, “Photochromic
reaction of the diarylethene derivative on Au nanoparticles” Advances in Natural Sciences:
Nanoscience and Nanotechnology 6, 015006 (2015).
7. Ryuuji Matsushita, Satoshi Kaneko, Shintaro Fujii, Hisao Nakamura, Manabu Kiguchi,“Temperature dependence of the thermopower and its variation of the Au Atomic contact”
Nanotechnology 26, 045709 (2015).
8. Ryoji Takahashi, Satoshi Kaneko, Shintaro Fujii, Manabu Kiguchi, “Extension ofphotopolymerization region from the nanoscale to the macroscopic scale using a chemically
amplified photoresist” Bulletin of the Chemical Society of Japan 88, 277–282 (2015).
9. Chenyang Liu, Satoshi Kaneko, Yuki Komoto, Shintaro Fujii, Manabu Kiguchi, “Highly
conductive single naphthalene and anthracene molecular junction with well-defined
conductance” Applied Physics Letters 106, 103103 (2015).

100

10. Shintaro Fujii, Satoshi Kaneko, Liu Chenyang, Manabu Kiguchi, “Single naphthalene andanthracene molecular junctions using Ag and Cu electrodes in ultra high vacuum” Applied
Surface Science 354, 362-366 (2015).
11. Satoshi Kaneko, Manabu Kiguchi, “Investigation on the pyrazine molecular junction
studied by conductance measurement and near edge X-ray absorption fine structure”
Fullerenes, Nanotubes and Carbon Nanostructures 22, 166-172 (2014).
12. Chenyang Liu, Satoshi Kaneko, Yuki Komoto, Shintaro Fujii, Manabu Kiguchi,“Fabrication of single linear aromatic molecule junction with high formation probability”
Applied Physics Express 7, 105201 (2014).
13. Tomoka Nakazumi, Satoshi Kaneko, Manabu Kiguchi, “Electron transport properties of Au,
Ag, and Cu atomic contacts in a hydrogen environment” The Journal of Physical Chemistry
C 118, 7489-7493 (2014).
14. Si-Jia Hao, Vazhappilly Lona Joseph Joly, Satoshi Kaneko, Jun-ichi Takashiro, Kazuyuki
Takai, Hitoshi Hayashi, Toshiaki Enoki, Manabu Kiguchi, “Magnetic edge-states innanographene,

HNO3-doped

nanographene

and

its

residuecompoundsofnanographene-based nanoporous carbon” Physical Chemistry Chemical Physics 16,6273-6282 (2014).
15. Jun-ichi Takashiro, Yuki Kudo, Satoshi Kaneko, Kazuyuki Takai, Takafumi Ishii, Takashi
Kyotani, Toshiaki Enoki, Manabu Kiguchi, “Heat treatment effect on the electronic andmagnetic structures of nanographene sheets investigated through electron spectroscopy andconductance measurements” Physical Chemistry Chemical Physics 16, 7280-7289 (2014).
16. Satoshi Kaneko, Carlo Motta, Gian Paolo Brivio, Manabu Kiguchi, “Mechanicallycontrollable bi-stable states in highly conductive single pyrazine molecular junction”
Nanotechnology 24, 315201 (2013).
17. Satoshi Kaneko, Jinjiang Zhang, Jianwei Zhao, Manabu Kiguchi, “Electronic conductanceof platinum atomic contact in a nitrogen atmosphere” The Journal of Physical Chemistry C
117, 9903-9907 (2013).
18. Manabu Kiguchi, Satoshi Kaneko, “Single molecule bridging between metal electrodes”
Physical Chemistry Chemical Physics 15, 2253-2267 (2013).
19. 木口学, 松下龍二, 金子哲, 中住友香, “単一分子接合の作製およびその物性評価”

日本画像学会誌 52, 26-33 (2013).
20. Satoshi Kaneko, Lu Wang, G.uangful Luo, Jing Lu, Shigeru Nagase, Satoru Sato, Michio
Yamada, Zdenek Slanina, Takechi Akasaka, Manabu Kiguchi, “Electron transport through101

single endohedral Ce@C82 metallofullerenes” Physical Review B 86, 155406 (2012).
21. Tomoka Nakazumi, Satoshi Kaneko, Ryuuji Matsushita, Manabu Kiguchi, “Electric
conductance of single ethylene and acetylene molecules bridging between Pt electrodes”
The Journal of Physical Chemistry C 116, 18250-0855 (2012).
22. Manabu Kiguchi, Satoshi Kaneko, “Electron transport through single π-conjugatedmolecules bridging between metal electrodes” ChemPhysChem 13, 1116 -1126 (2012).
23. Ryuuji Matsushita, Satoshi Kaneko, Tomoka Nakazumi, Manabu Kiguchi, “Effect ofmetal-molecule contact on electron-vibration interaction in single hydrogen moleculejunction” Physical Review B 84, 245412 (2011).
24. 木口学，金子哲，中住友香，Oren Tal, Jan van Ruitenbeek, Juan Carlos Cuevas,“金属

電極に架橋したベンゼン，C60 単分子接合の構造および伝導特性の解明” 表面科学32, 331-336 (2011).
25. Satoshi Kaneko, Tomoka Nakazumi, Manabu Kiguchi, “Fabrication of the well-defined
single benzene molecule junction using Ag electrodes” The Journal of Physical Chemistry
Letters 1, 3520–3523 (2010).

International conference

1. Satoshi Kaneko, Manabu Kiguchi, Yuki Komoto, Daigo Murai, Kazuhito Tsukagoshi“Electron transport and surface enhanced Raman scattering at structurally well-defined
single 1,4-benzenedithiol” 31st European Conference on Surface Science, Barcelona, oral,
August, 2015.
2. Satoshi Kaneko, Manabu Kiguchi, Satoru Sato, Michio. Yamada, N. Mizorogi, Takeshi
Akasaka, Wang Lu, Shigeru Nagase, “Electrical conductance of single Ce@C82 molecule
bridging between metal electrodes” International School and Symposium on MolecularMaterials, Tokyo, poster, November, 2013, poster award.
3. Satoshi Kaneko, Manabu Kiguchi, Carlo Motta “Highly conductive pyrazine molecularjunction showing bi-stable states” International School and Symposium on MolecularMaterials and Devices, Durham, poster, September, 2012. poster award.
4. Satoshi Kaneko, Manabu Kiguchi, Tomoka Nakazumi, “Fabrication of the well defined
single benzene molecule junction using Ag electrodes” European Conference on Surface
Science, Wrocłow, August, 2011.
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5. Satoshi Kaneko, Manabu Kiguchi, Satoru Sato, Michio Yamada, Naomi Mizorogi,
Takeshi Akasaka, Wang Lu, Shigeru Nagase “Electrical conductance of single Ce@C82
molecule bridging between metal electrodes ” International Conference on Core Researchand Engineering Science of Advanced Materials, Osaka, June, 2010.

Domestic Conference

1. 金子哲, 村井大午, 小本祐貴, 塚越一仁, 木口学 “表面増強ラマンスペクトル・電流電圧特性同時計測による単分子接合の原子・電子構造解析” 2015 年真空・表面科学
合同講演会, つくば, 2015 年 12 月.
2. 金子哲, 鈴木翔, 木口学, “金ナノ構造体を用いたベンゼン誘導体の表面増強ラマン
散乱の界面構造依存性” 分子科学会, 東京, 2015 年 9 月.
3. 金子哲, 村井大午, 小本祐貴, 塚越一仁, 木口学 “表面増強ラマン分光法を活用した
1,4-ベンゼンジチオール単分子接合の電子輸送特性の解明” 応用物理学会, 神奈川,2015 年 3 月.
4. 金子哲, 中村優雅, Jinjiang Zhang, Jianwei Zhao, 木口学 “窒素分子を利用した銅単原子ワイヤーの安定形成” 徳島, 日本物理学会, 2013 年 9 月.
5. 金子哲，木口学，Lu Wang，Guangfu Luo，永瀬茂，佐藤悟，山田道夫，Zdenek，Slanina，Jing Lu，赤阪健 “金属内包フラーレン Ce@C82 単一分子の電子輸送特性” ナノスケ
ール分子デバイスセミナー, 東京, 2013 年 3 月.
6. 金子哲, 木口学, Carlo Motta “ピラジンを用いた高機能単分子スイッチの開発”

CSJ

化学フェスタ, 東京, 2012 年 10 月.
7. 金子哲, Carlo Motta, 木口学 “Pt 電極間に架橋されたピラジン単一分子接合の研究”
日本物理学会, 横浜, 2012 年 9 月.
8. 金子哲, 木口 学, “Pt 電極間に架橋された窒素単分子接合の研究” 日本物理学会,
2012 年, 3 月.
9. 金子哲 “窒素を含んだ単一分子の電子輸送特性の解明” ナノスケール分子デバイス
セミナー, 東京, 2012 年 3 月.
103

10. 金子哲，中住友香，木口学“規定された電気伝導度を示す π 共役単分子接合の探索”
日本物理学会, 秋季大会，富山，2011 年 9 月.
11. 金子哲，木口学，佐藤悟，山田道夫，溝呂木直美，赤阪健，永瀬茂， “金属電間に架橋した Ce@C82 単分子の電気伝導度特性”，日本物理学会第 65 回年次大会，岡山，
2010 年 3 月.

104