SELECTED TOPICS IN APPLIED PHYSICS

Japanese Journal of Applied Physics 52 (2013) 110115

Nano Electronics and Devices

http://dx.doi.org/10.7567/JJAP.52.110115

How Contacting Electrodes Affect Single -Conjugated Molecular Electronic States:
Local Density of States of Phthalocyanine Nanomoleculeson MgO(001), Cu(111), Ag(001), Fe(001), and Mn(001)
Shuhei Nakashima, Yuhei Yamagishi, Kenji Oiso, and Toyo Kazu Yamada
Graduate School of Advanced Integration Science, Chiba University, Chiba 263-8522, Japan
E-mail: toyoyamada@faculty.chiba-u.jp
Received April 30, 2013; accepted July 24, 2013; published online October 18, 2013
Single molecules have attracted much interest as new materials for future spin electronic devices; however, many open questions still remain.
One of them is how the electronic local density of states (LDOS) of single molecules is affected when they are in contact with electrodes. We showa systematic study of the LDOS of -conjugated phthalocyanine (H2 Pc) nanomolecules adsorbed on various electrodes, namely, (1) MgO(001)thin films grown on Ag(001), (2) noble metals of Cu(111) and Ag(001), and (3) 3d magnetic metals of Fe(001) and Mn(001), adupting scanningtunneling spectroscopy techniques with an ultrahigh-vacuum scanning tunneling microscopy setup at room temperature. Since MgO thin films cut
the electronic coupling from the substrate Ag(001), we could observe H2 Pc molecular states at 1:5 and +1.0 eV. H2 Pc molecules on the noble
metal substrates form a pattern with a square unit cell of about 1:5  1:5 nm2 and have similar LDOS peaks near the Fermi energy. Strong
hybridizations between the substrate 3d spin-polarized states and the molecular  orbitals produce new molecular states of H2 Pc molecules
adsorbed on Fe(001) and Mn(001) near the energy positions of the Fe(001) minority spin state and the Mn(001) majority spin state, respectively.
# 2013 The Japan Society of Applied Physics

1. Introduction

The realization of a sustainable society is nowadays one of
the most important issues in saving our environment. Since ahuge increase in the human population in the next decades
is predicted, a reduction in energy consumption is required.
Energy consumption in small electronic devices will playa major role. The minimization of electronic devices is
expected to be a trump. This is the reason why nanoscienceand nanotechnology have been markedly advanced over thelast decades. Single molecular electronics and spintronics
are considered to be new breakthroughs in the developmentof future nanodevices with low energy consumption, lowcost, and high performance.1–9)
However, many open questions still remain in realizing
single molecular electronic devices. One of them is how theelectronic local density of states (LDOS) of single molecules
is aﬀected when the molecules are brought into contact with
substrate electrodes. The highest occupied molecular orbital(HOMO) and lowest unoccupied molecular orbital (LUMO)
of molecules in the gas phase are frequently calculated
by molecular orbital simulations. Previously, the electronic
states of molecules adsorbed on metallic electrodes wereassumed to be the same as those of molecules in the gas
phase. This assumption is acceptable for thick bulk-likemolecular ﬁlms. Surface and interface eﬀects have been
considered to be minor problems.
However, for future nanoelectronic devices, surface andinterface eﬀects determine the electronic properties. Studies
of single molecules and molecular monolayer ﬁlms havebeen carried out using noble metal substrates.3,5–11) Quantum
spin states of single molecular magnets have been widely
studied.3,4,6–9) Electronic couplings from the noble metal
substrates have been considered not to aﬀect strongly thequantum spin states of the molecules. However, we haverecently found that properties of Fe-based spin-crossover
molecules are aﬀected by the electronic coupling from a
Cu(001) substrate.5) We succeeded in detecting molecular
low-spin and high-spin states of the spin-crossover mole-cules by inserting a CuN layer between the substrate andthe molecules to cut the coupling from the substrate, whichmeans that even with noble metal electrodes, electronicor spin states of single molecules can be modiﬁed. Such a
decoupling layer is required to observe intrinsic molecularorbitals. MgO (2–3 ML) and NaCl (2 ML) thin ﬁlms havealso been used as decoupling layers.12–14)
On the other hand, strong electronic couplings between
substrates and molecules can also be useful for new
molecular spintronics. Strong hybridizations between 3d
spin-polarized LDOS and molecular -orbitals result in amagnetoresistance eﬀect through a single molecule,1,2) i.e.,
spin current through single molecules can be controlled byswitching the magnetization directions of magnetic electrodes.
In this work, we report a systematic study of the LDOS
of low-dimensional -conjugated phthalocyanine (H2 Pc)
molecules on various substrates by scanning tunnelingspectroscopy techniques with an ultrahigh-vacuum scanningtunneling microscopy (STM) setup at room temperature.
MgO(001) ultrathin ﬁlms, noble metal Cu(111) and Ag(001)surfaces, and 3d magnetic metal Fe(001) and Mn(001)surfaces were used as substrate electrodes.
2. Experimental Procedure

STM and spectroscopy (STS) experiments were performed at 300 K in ultrahigh-vacuum (UHV; base pressure<8  109 Pa). STS measurements were performed by
opening the feedback loop of the tunneling current, i.e.,
ﬁxing the tip–sample separation (z). The tunneling current
was measured at each pixel by varying the sample voltage
within 1:5 V. IðV Þ curves averaging more than 1000 singleIðV Þ curves were used in this study. Diﬀerential conductivity (dI=dV ) curves were obtained by numerical diﬀerentiation of the IðV Þ curves. Although a lock-in ampliﬁer
was used to obtain the dI=dV signals indicated in ourprevious papers,1) here we do not use a lock-in ampliﬁer sothat we can distinguish between real and pseudogaps aroundthe Fermi energy. A dI=dV curve normalized by its ﬁtted

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3. Results and Discussion

Firstly, we investigated the LDOS of -conjugated H2 Pc
molecules adsorbed on MgO(001) ultrathin islands witha thickness of 2 ML grown on atomically ﬂat Ag(001)terraces. (MgO in bulk is known to be insulator.) The MgO
thin ﬁlm was used to suppress electronic coupling between
Ag(001) and the molecules. Using such decoupling layers,
we observed true electronic structures of H2 Pc molecules.
Figure 1(a) shows an STM topographic image of MgOislands on the Ag(001) substrate. A dI=dV map obtained
at +2 V in the same area as Fig. 1(a) shows a strong
black-white contrast [see Fig. 1(b)]. Dark areas correspondto MgO islands, which clearly indicate that the LDOSof MgO islands is much lower than that of Ag(001).
Figure 1(c) shows a dI=dV curve obtained on MgO islands.
dI=dV values at around the Fermi energy are approximately
0 nA/V, which indicates a very low LDOS. dI=dV increases
exponentially below 2 eV and above +1.5 eV. This experimental result is comparable to the dI=dV spectrum obtainedin Ref. 14. Schintke et al. reported that MgO (1–10 ML) thinﬁlms have an unoccupied state at around +2.5 eV, an
occupied state at around 4:0 eV, and a gap width of about
6.2 eV.14) (The gap width of bulk MgO is 7.8 eV.) Thin ﬁlms
have smaller gaps. The theoretically calculated LDOS ofMgO (2–3 ML) thin ﬁlms is very low near the Fermi energy,
but it starts to increase at around +1.5 and 1:5 eV, i.e.,the calculation showed a gap width of about 3 eV.14) The
calculated LDOS nicely ﬁts our experimentally obtained

(a)

0.3

(b)

(c)
Occupied
LDOS

Unoccupied
LDOS

gap

0.0
-3
(d)

0
E- EF [eV]

0.2

3

(e)

dI/dV[nA/V]

tunneling probability function (T ) corresponds to the sample
LDOS.15–18)
Nonmagnetic tungsten (W) tips were electrochemicallyetched from W wires ( ¼ 0:3 mm, purity 99.9%) in air
using KOH aq., while the etching process was monitored by
optical microscopy. Subsequently, the tip was rinsed with
hot water, then transferred into the load lock chamber of our
STM setup. The apex of the W tip was sputtered with Neþ in
UHV and annealed at temperatures of up to 2000 K to obtaina clean apex. Fcc-Cu(111) and fcc-Ag(001) single crystals(purity 99.999%) were used as conductive noble metal
substrates. A clean and atomically ﬂat surface was obtained
after several sputtering and annealing cycles in UHV. A
MgO(001) ultrathin ﬁlm was grown on Ag(001). Mg was
deposited on Ag(001) at 500 K at an oxygen (purity
99.995%) pressure of 1  104 Pa. A clean and atomicallyﬂat bcc-Fe(001) surface was obtained on Fe-whisker single
crystals by Arþ sputtering and annealing cycles up to 870 K.
Mn(001) ultrathin ﬁlms were grown on an Fe(001)-whisker
single crystal substrate at 370 K.
H2 Pc molecules were used as a representative of
-conjugated molecules. Commercial H2 Pc powder (Alfa
Aesar, purity 95%) was puriﬁed by sublimation at 653 K
and recrystallization at 473 K at a pressure of 1  103 Pa(yield 30%). Nuclear magnetic resonance (NMR) andinfrared spectra conﬁrmed the absence of impurities after
the puriﬁcation. The clean H2 Pc powder was placed in a
crucible that was set in a molecule chamber and heatedto 550 K. The H2 Pc powder was deposited on single crystal
surfaces at 300 K by opening the shutter valve between the
preparation and molecule chambers. The deposition rate was
0.03 ML/min.

dI/dV[nA/V]

Jpn. J. Appl. Phys. 52 (2013) 110115

HOMO

LUMO
gap

0.0
-3

0
E- EF [eV]3

Fig. 1. (a–c) Obtained on the surfaces of MgO(001) bilayer islands grownon Ag(001). (a) STM topographic image (55  28 nm2 , Vs ¼ þ2:8 V,
I ¼ 50 pA). (b) dI=dV map at +2.0 V. MgO islands appear dark. (c) dI=dV
curve obtained on MgO islands at Vs ¼ þ2:8 V and I ¼ 500 pA. (d) and(e) Obtained on the surfaces of 1 ML H2 Pc molecules on MgO(001) bilayer
islands. (d) STM topographic image (15  15 nm2 , Vs ¼ þ2:8 V,
I ¼ 50 pA). The inset image is a model of a H2 Pc single molecule.
(e) dI=dV curve obtained on 1 ML H2 Pc on MgO islands at Vs ¼ þ2:8 V
and I ¼ 200 pA.

dI=dV curve in Fig. 1(c). To obtain spectroscopy curves
with a better signal-to-noise ratio, we used low-noise current
ampliﬁers and cables with low capacitance (about 50 pF/m),
which reduced the total current noise of our homemade STM
setup down to about 200 fA. The current noise amplitude in
dI=dV was below 0.001 nA/V.18) Although, in Fig. 1(c),dI=dV values at around the Fermi energy are very low,
we can detect the tunneling electrons from the substrate
Ag(001), i.e., when we set the sample bias voltage, forexample at +1 V, we can observe the Ag(001) surface
through the MgO thin ﬁlm while paying attention to avoid
crashing the tip into the MgO thin ﬁlm. (We needed to set
the current at 1–5 pA.) Therefore, the dI=dV conductance in
Fig. 1(c) near the Fermi energy is not zero, but the LDOS
between 2 and +1.5 eV is very low [gap in Fig. 1(c)] since
the LDOS near the Fermi energy of Ag(001) decays rapidly
into the MgO ﬁlm. The 2 ML MgO ﬁlm likely cuts theelectronic coupling from Ag(001). If we observe new LDOS
peaks within the gap when we deposit H2 Pc molecules on
the MgO thin ﬁlms, these new peaks can be identiﬁed as
molecular HOMO and LUMO states.
We deposited 1 ML H2 Pc molecules on MgO islands. An
observed STM image is shown in Fig. 1(d), in which single
H2 Pc molecules connect with each other and form molecular
chains. dI=dV curves were measured on the molecules.
Within the MgO gap, we found two peaks. H2 Pc molecules
have a LUMO peak at +1.0 eV and a HOMO peak at1:5 eV, as shown in Fig. 1(e). Furthermore, since dI=dV
values between 0:8 and +0.6 eV are very low (close to
0 nA/V), we consider that the molecules on the MgO ﬁlms

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Jpn. J. Appl. Phys. 52 (2013) 110115

0.04

(b)

dI/dV [nA/V](a)

Occupied
LDOS

0.00
-1.5
5

0.0
E- EF [eV]

1.5

0.0
E- EF [eV]

1.5

0.0
E- EF [eV]

1.5

0.0
E- EF [eV]

1.5

(d)

(dI/dV)/T [arb. unit](c)

Unoccupied
LDOS

0
-1.5
4

(f)

dI/dV [nA/V]

(e)

0
-1.5
(g)

1.8

(h)

(dI/dV)/T [arb. unit]

have a gap width of about 1.4 eV, i.e., the molecules on the
MgO ﬁlms can have semiconductor-like properties.
Secondly, we deposited H2 Pc molecules on noble metal
substrates of fcc-Ag(001) and fcc-Cu(111). Electronic
coupling from the noble metals to the molecules will aﬀect
molecular electronic states.
Figure 2(a) shows an STM topographic image obtained ona clean and atomically ﬂat Cu(111). Atomic terraces larger
than 20 nm are observed. Figure 2(b) shows an experimentally obtained dI=dV curve on a clean terrace. A surface
state peak is observed at around 200 meV [marked by anarrow in Fig. 2(b)]. This peak is known to be an sp surface
state and electrons on Cu(111) show free-electron-likebehavior.19,20) Unlike the MgO(001) surface, Cu(111) surface has a higher LDOS near the Fermi energy.
H2 Pc molecules were deposited on this Cu(111) surface atroom temperature. Single molecules diﬀused on the Cu(111)
surface and formed two-dimensional islands or ﬁlms, whichindicates that the coupling between Cu(111) and themolecules is not strong. Figure 2(c) shows a molecularpattern obtained on the terrace. Single dots correspond tosingle molecules. The molecules have a unit cell size of
about 1:5  1:5 nm2 , which is very similar to the patternobserved on FePc ﬁlms (1 ML) on Cu(111).11) Similar
patterns were even observed on CuPc ﬁlms on Au(111).10)
These suggest that phthalocyanine molecules adsorbed on a
sixfold-symmetry fcc(111) noble metal substrate form anordered pattern with a square unit cell. The fact that themolecular patterns are independent of the noble metal
substrate also indicates a weak coupling between themolecules and the noble metals.
Tunneling spectroscopy measurements were performedon this surface. Figure 2(d) shows the obtained LDOS ofthe molecular ﬁlm. New peaks were found near the Fermi
energy at 300, +300, and +600 meV. In the case of
decoupling, MgO layers cut the electronic coupling from thesubstrate, and HOMO and LUMO are separated by about
2.5 eV; however, owing to the coupling, HOMO and LUMO
peaks appear near the Fermi energy, which increases theLDOS at the Fermi energy, and H2 Pc molecules on Cu(111)
have metal-like properties.3)
Figures 2(e)–2(h) show STM/STS results obtained on
Ag(001) substrates. Figure 2(e) shows an STM topographic
image obtained on a clean and atomically ﬂat Ag(001)surface. Atomic terraces are observed.
Figure 2(f ) shows a dI=dV curve obtained on Ag(001). In
contrast to the case of Cu(111), there is no clear LDOS peaksince Ag(001) does not have a surface state near the  point
and bulk bands are dominant near the Fermi energy.21,22)[Outermost electron shells of Ag(001) are 4d and 5s electron
orbitals.]
We deposited H2 Pc molecules on the Ag(001) surface
at room temperature. Similarly to the case of the Cu(111)
surface, H2 Pc molecules diﬀused on the Ag(001) surface andformed a two-dimensional ﬁlm. Figure 2(g) shows an STM
topographic image obtained on the 1 ML H2 Pc ﬁlm grownon Ag(001). A molecular pattern with a square unit cell withdimensions of about 1:5  1:5 nm2 can be observed in theenlarged image in Fig. 2(g); the dimensions are the same as
those of the 1 ML H2 Pc molecules on Cu(111). This result issurprising. Since fcc(001) has a diﬀerent symmetry from

0.8
-1.5

Fig. 2. STM topographic images (a, c, e, g) and spectroscopy curves(b, d, f, h) corresponding to LDOS values obtained on (a, b) Cu(111),(c, d) 1 ML H2 Pc on Cu(111), (e, f ) Ag(001), and (g, h) 1 ML H2 Pc on
Ag(001). (a) 125  125 nm2 , Vs ¼ 2:0 V, and I ¼ 300 pA. (c) 12  12
nm2 , Vs ¼ þ870 meV, and I ¼ 1:1 nA. (e) 51  51 nm2 , Vs ¼ 2:0 V andI ¼ 50 pA. (g) 70  70 nm2 , Vs ¼ 900 mV, and I ¼ 500 pA. The inset(14  14 nm2 ) is an enlarged image showing a 2D molecular array with aperiodicity of about 1.5 nm.

fcc(111), we expected diﬀerent molecular patterns. These
results indicate weak coupling between Ag(001) and H2 Pc,
and it seems that Pc molecules physisorb on noble metal
surfaces.
The LDOS obtained on the 1 ML H2 Pc on Ag(001) isshown in Fig. 2(h). H2 Pc molecules have peaks at 300,+300, and +600 meV. These peak positions are almost
the same as the LDOS peak positions obtained on H2 Pc/
Cu(111). This proves that H2 Pc molecules do not chemisorb
on Cu(111) and Ag(001). Since HOMO and LUMO peaks
appear near the Fermi energy, 1 ML H2 Pc molecules on
Ag(001) could have metal-like properties.

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Jpn. J. Appl. Phys. 52 (2013) 110115

2(dI/dV)/T [arb. unit]

(a)

(b)

Occupied
LDOS

0
-1.5
5

0
-1.5
1.8

(dI/dV)/T [arb. unit]

(e)

1.5

0.0
E- EF [eV]

1.5

0.0
E- EF [eV]

1.5

0.0
E- EF [eV]

1.5

(f)

0.4
-1.5
(g)

0.0
E- EF [eV](d)(dI/dV)/T [arb. unit](c)

Unoccupied
LDOS

6

(h)

(dI/dV)/T [arb. unit]

Finally, we performed STM/STS measurements on single
H2 Pc molecules adsorbed on 3d magnetic metal substrates.
Here, we used bcc-Fe(001) and bct-Mn(001) substrates,
which are well-known model systems for ferromagnetic
and layerwise antiferromagnetic order substrates for STM
studies.17,23)
Figure 3(a) shows an STM topographic image obtained on
an Fe(001)-whisker single crystal substrate. Atomic terraces
larger than 100 nm are observed. The LDOS obtained bytunneling spectroscopy is shown in Fig. 3(b). A peak isobserved at +300 meV above the Fermi energy, which is
known to be a highly spin-polarized 3d minority spinstate.24)
Single H2 Pc molecules were deposited on this Fe(001)surface at room temperature. Figure 3(c) shows an STM
topographic image obtained on the surfaces of H2 Pc single
molecules on Fe(001). In contrast to the noble metal surfaces, single molecules adsorbed on Fe(001) do not diﬀuse
even at room temperature, which indicates a relatively
strong coupling between the Fe(001) substrate and the molecules. If the H2 Pc molecules chemisorb on Fe(001), molecular electronic structures are expected to change markedly.
The LDOS of H2 Pc/Fe(001) is shown in Fig. 3(d). Only
one clear peak can be observed at +300 meV; this energy
position is the same as that of the spin-polarized 3d surface
state of the Fe(001) substrate, i.e., the new molecular state
appears near the 3d surface state. A similar phenomenon
can be observed in the case of CoPc molecules on Co(111),
where Co(111) has a 3d spin-polarized surface state peakat 300 meV and the adsorbed CoPc has a spin-polarized
surface state peak at 150 meV; this energy position is close
to that of the Co(111) surface state. This is considered to be
due to the strong hybridization between the 3d state and the orbitals.20) If a similar hybridization occurs in the case ofFe(001) and H2 Pc, the +300 meV peak of H2 Pc/Fe(001)is likely spin-polarized. Since the new peak appears near
the Fermi energy, H2 Pc/Fe(001) could have metal-like
properties.
If the molecular  orbitals tend to hybridize strongly withthe 3d states, we will ﬁnd a new molecular state near
the energy position of the 3d state when we use diﬀerent
magnetic substrates. Here, we used Mn(001) thin ﬁlms.
One unique reason for using this Mn(001) thin ﬁlm is thatMn(001) has majority and minority spin states below andabove the Fermi energy, respectively. In the case of Fe(001),
molecular  orbitals likely couple with the minority spinstate at +300 meV. In the case of Mn(001), it is unclear
which spin state (minority or majority) is more favorable tocouple with H2 Pc molecular orbitals. Figures 3(e)–3(h)show STM/STS results obtained on the Mn(001) substrate.
The Mn(001) ﬁlm with a thickness of about 5 ML was grownon the Fe(001)-whisker single crystal. An STM topographic
image shows several atomic terraces on the surface due
to a layer-plus-island growth mode [Fig. 3(e)].16,17) LDOS
obtained on Mn(001) shows two peaks at 150 and+500 meV, which are attributed to spin-polarized 3dz2 -likemajority LDOS and minority LDOS, respectively [seeFig. 3(f )].17)
H2 Pc molecules were deposited on the Mn(001) ﬁlm atroom temperature. Similarly to the case of H2 Pc on Fe(001),
single molecules adsorbed on Mn(001) do not diﬀuse even at0-1.5

Fig. 3. (Color online) STM topographic images (a, c, e, g) andðdI=dVÞ=T curves (b, d, f, h) corresponding to LDOS values obtained on(a, b) Fe(001), (c, d) single H2 Pc molecules on Fe(001), (e, f ) Mn(001), and(g, h) single H2 Pc molecules on Mn(001). (a) 100  100 nm2 , Vs ¼ 1500
mV, and I ¼ 500 pA. (c) 20  20 nm2 , Vs ¼ 1000 mV, and I ¼ 200 pA.
(e) 100  100 nm2 , Vs ¼ 1500 mV, and I ¼ 80 pA. (g) 16  16 nm2 ,Vs ¼ 1500 mV, and I ¼ 200 pA.

room temperature [see Fig. 3(g)]. As we expected, 3d metals
seem to couple strongly with the molecules. The obtained
LDOS on H2 Pc/Mn(001) is shown in Fig. 3(h), in whicha new peak is observed at 300 meV close to the majority
peak position of the Mn(001) substrate. The calculated spinresolved LDOS of H2 Pc/Mn(001) was reported (supplementary information in Ref. 2). The peak at 300 meVcorresponds to the peak energy position of the molecularspin-polarized LUMO state, which is attributable to thestrong hybridization between the Mn(001) majority state
and the molecular LUMO state.2) These experimental and110115-4# 2013 The Japan Society of Applied Physics

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Jpn. J. Appl. Phys. 52 (2013) 110115

(b)

(c)

(d)

(e)

+1

+1

+1

+1

EF

EF

EF

EF

EF

-1

-1

-1

-1

Cu(111) H2Pc

Ag(001) H2Pc

Gap

+1

Gap

Energy [eV]

(a)

-1

MgO H2Pc

Fe(001) H2Pc

Mn(001) H2Pc

PositionFig. 4. LDOS peak positions of H2 Pc molecules adsorbed on (a) MgO(001) thin ﬁlm (2 ML), (b) Cu(111), (c) Ag(001), (d) Fe(001)-whisker single crystal,and (e) Mn(001) thin-ﬁlm (about 5 ML) substrates. Solid lines denote surface state peak positions. Shaded areas denote the occupied density of state. Dottedlines denote onsets of increasing LDOS.

theoretical results prove that 3d spin-polarized surface states
couple strongly with the molecular  orbitals and that new
molecular spin states near the energy positions of the 3dstates are formed. In contrast to H2 Pc on noble metals, H2 Pc
on 3d magnetic metals have molecular states at diﬀerent
energy positions: +300 meV for H2 Pc/Fe(001), 300 meV
for H2 Pc/Mn(001), and 150 meV for H2 Pc/Co(111).20)
We could control the energy position of the H2 Pc molecularstate by choosing diﬀerent magnetic electrodes. Also, thepresence of the spin-polarized molecular state on H2 Pc/3d
metals means that nonmagnetic H2 Pc molecules exhibit
magnetic properties upon contact with 3d magnetic metals.
Since the new peak appears near the Fermi energy, themolecules adsorbed on Mn(001) could have metal-like
properties.
Figure 4 shows a summary of energy positions of thesurface state peaks obtained on the H2 Pc molecules adsorbed
on various electrodes. The MgO(001) thin ﬁlm grown on
Ag(001) has a very low LDOS (= gap) near the Fermi
energy. The electronic coupling from Ag(001) is decoupledby the 2 ML MgO thin ﬁlm. The dI=dV curve in Fig. 1(c)and the calculated LDOS in Ref. 14 showed that the 2 ML
MgO thin ﬁlm has a gap between 2 and +1.5 eV. The
H2 Pc molecules adsorbed on the MgO thin ﬁlm have
LDOS peaks at 1:5 and +1.0 eV, and the LDOS near the
Fermi energy is very low [Fig. 4(a)]. Since this electronic
structure is similar to the calculated electronic states ofH2 Pc molecules in the gas phase,25) we believe that the
MgO thin ﬁlm cuts the electronic coupling from the Ag
substrate.
H2 Pc single molecules adsorbed on the noble metal
substrates diﬀused and formed two-dimensional islands
or ﬁlms. In spite of the fact that diﬀerent types of noble
metal substrates were used, namely Ag(001), Cu(111), andAu(111),10,11) Pc molecules always formed a similar patternwith a square unit cell with dimensions of about 1:5 
1:5 nm2 , indicating that Pc molecules do not couple strongly
with noble metal substrates. Pc molecules likely physisorb
on noble metal surfaces. The LDOS values of H2 Pc adsorbed
on Ag(001) and Cu(111) show peaks at similar energy
positions of 300, +300, and +600 meV [see Figs. 4(b) and4(c)]. Since the molecular electronic states are not changedby the diﬀerent noble metal substrates, we could conclude

that the H2 Pc molecules physically adsorb on the noble
metal surfaces.
Figures 4(d) and 4(e) show the LDOS values of H2 Pc on
Fe(001) and Mn(001). H2 Pc molecules have new peaks near
the energy positions of the substrate 3d states. In contrast
to Figs. 4(b) and 4(c), the energy positions of molecularstates in Figs. 4(d) and 4(e) are changed by the diﬀerent
3d substrates. Another interesting point observed on the 3d
metal substrates is that, even at room temperature, H2 Pc
molecules do not diﬀuse on the substrate surface, which
was not observed on the noble metal and MgO ﬁlm surfaces.
The origin of these unique properties is considered to be
the stronger coupling between the molecular  orbitals andthe 3d magnetic metal substrates. Calculations also suggest
the strong hybridizations between the 3d states and themolecular  orbitals.2) H2 Pc molecules adsorbed on Fe(001)show an LDOS peak at the same energy position as the
3d minority spin state of the Fe(001) substrate, i.e., minority
spins can pass through the molecules. H2 Pc molecules
adsorbed on Mn(001) show a LUMO peak at an energy
position near the Mn(001) 3d majority spin state, i.e.,majority spins can pass through the molecules. Thesesuggest that -conjugated molecules adsorbed on magnetic
metals with 3d spin-polarized states likely have a spinpolarized molecular state at an energy position near thesubstrate 3d state. Such new properties of -conjugatedsingle molecules could be useful for near-future single
molecular spintronics.
4. Conclusions

We showed a systematic study on how the LDOS of lowdimensional -conjugated H2 Pc molecules is aﬀected by
diﬀerent substrate electrodes. LDOS values were measured
by using STM/STS techniques in UHV at room temperature.
The decoupling of a 2 ML MgO(001) thin ﬁlm from a
Ag(001) substrate and its eﬀects were examined. H2 Pc
molecules adsorbed on the MgO thin ﬁlm show H2 Pc molecular states at 1:5 and +1.0 eV. H2 Pc molecules adsorbed
on Cu(111) and Ag(001) have similar patterns with a square
unit cell (about 1:5  1:5 nm2 ) and similar LDOS peaks near
the Fermi energy. These results indicate a weak coupling
between the noble metal substrates and the H2 Pc molecules.
The molecules likely physisorbed on the noble metals.

110115-5# 2013 The Japan Society of Applied Physics

SELECTED TOPICS IN APPLIED PHYSICS

Jpn. J. Appl. Phys. 52 (2013) 110115

H2 Pc molecules was found to couple strongly with the 3dmagnetic metal substrates. H2 Pc molecules likely chemisorbed on Fe(001) and Mn(001). H2 Pc molecules on Fe(001)and Mn(001) could have new spin-polarized molecular statesnear the energy positions of the Fe(001) minority spin state
and the Mn(001) majority spin state, respectively.

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Acknowledgments

We thank Dr. H. Ishii, Dr. F. Shibahara, and Dr. Y. Noguchi
for assistance in the puriﬁcation of our molecules. The
low current noise was realized with the support of Dr. W.
Wulfhekel as well as with the technical support of Dr. T.
Schuh. Vacuum conditions in our STM setups have been
improved with the support of Dr. T. Uchihashi, Dr. I.
Arakawa, and Mr. Y. Shimokawa. We thank Dr. L. Gerhard
and Mr. N. Bachellier for carefully reading our manuscript.
This work was supported by JSPS KAKENHI Grant
Numbers 22810005, 23681018, Japan Science and Technology Agency (JST) Improvement of Research Environmentfor Young Researchers: Chiba University Young ResearchOriented Faculty Member Development Program in
Bioscience Areas, Chiba Univ. Global COE Program:
Advanced School for Organic Electronics, Casio
Science Promotion Foundation, Asahi Glass Foundation,
Hatakeyama Culture Foundation, The Association for the
Progress of New Chemistry, Iketani Science and Technology
Foundation, The Noguchi Institute, Ozawa Yoshikawa
Memorial Electronics Foundation, Research Foundation for
the Electrotechnology of Chubu, The Nakajima Foundation,
The Futaba Electronics Memorial Foundation, andShimadzu Foundation.

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Shuhei Nakashima is a student at the Graduate
School of Advanced Integration Science, Chiba
University.

Yuhei Yamagishi is a student at the Graduate
School of Advanced Integration Science, Chiba
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Toyo Kazu Yamada received the ﬁrst Ph. D.
degree from Gakushuin University in 2004 and thesecond Ph. D. degree from Radboud University
Nijmegen in 2005. In 2004, he was a Research
Fellow of the Japan Society for the Promotion of
Science (JSPS Research fellow PD), and from 2004
to 2008, he worked at Gakushuin University as
Assistant Professor. From 2008 to 2010, he worked
at Karlsruhe Institute of Technology as Alexandervon Humboldt Research Fellow. From 2010, he got
an Associate Professor position in Chiba University. In last 13 years, he hasstudied nanomagnets, single atoms, and single molecules by means of spinpolarized scanning tunneling microscopy and spectroscopy.

110115-6# 2013 The Japan Society of Applied Physics