Japanese Journal of Applied Physics
Vol. 45, No. 3B, 2006, pp. 1956–1961#2006 The Japan Society of Applied Physics

Electronic Structures of Individual Poly(3-hexylthiophene) Nanowires
on Hydrogen-Terminated Si(100) Surfaces
Yasuhiko T ERADA, Hidemi SHIGEKAWA, Yuji S UWA1 , Seiji H EIKE1 ,Masaaki F UJIMORI1 and Tomihiro HASHIZUME1
Institute of Applied Physics, CREST, 21st COE, University of Tsukuba, Tsukuba, Ibaraki 305-8573, Japan
1
Advanced Research Laboratory, Hitachi, Ltd., Hatoyama, Saitama 350-0395, Japan(Received July 5, 2005; accepted September 4, 2005; published online March 27, 2006)

Electronic structures of individual conducting polymers, poly(3-hexylthiophene)s (P3HTs), ﬁxed on a hydrogen-terminatedSi(100) surface have been examined by scanning tunneling microscopy/spectroscopy and ﬁrst-principles calculations within
the density-functional approach. The calculations reveal that the electronic structure of the polymer is only weakly inﬂuenced
by the substrate, which ensures that the ﬁxed polymers maintain a conduction property similar to that of isolated polymers.
The current–voltage curves for the substrate-molecule-tip junction show rectiﬁcation characteristics, indicating the carrier
doping in the ﬁxed polymers. [DOI: 10.1143/JJAP.45.1956]
KEYWORDS: scanning tunneling microscopy/spectroscopy (STM/STS), first-principles calculation, electronic structures,conducting polymer, P3HT, Si(100)1.

Introduction

Electronics using organic molecules may have a great
advantage in mass-producing homogenous and ﬂexible
devices at low cost by chemical synthesis, over traditional
semiconductor electronics.1) Although much progress has
been made in synthesizing individual molecular components, there still remain immense and unsolved diﬃculties in
economical fabrication of complete electronic circuits at themolecular level. A possible solution is hybrid molecularelectronic devices,1) comprising molecular components andwires that connect to metal electrodes fabricated on asemiconductor substrate. In such hybrid devices, organic
molecules are desirable for intermediate wires connecting
molecular components and metal electrodes because theywould exhibit similar electronic structures and hence can beutilized to tune their interfacial energy barriers.2)
Contact between molecular wires (or components) andsubstrates, however, could be a serious problem that canlimit the performance and stability of the devices. The
presence of the substrate surface may have an inﬂuence onelectronic structures of the embedded molecules as a resultof bond formation and may alter the designed performanceof molecular wires. Therefore, understanding of the contact
eﬀect on the electronic structures is a key issue to designinghybrid devices.
Poly(3-hexylthiophene)s (P3HTs) ﬁxed on a hydrogenterminated Si(100) surface [H-Si(100)] could be a model
system to investigate energy structures of molecular wires
embedded on substrate surfaces because it is likely that thehydrogen passivation of the surface enables the preservationof the energy structures of the P3HT wires. Moreover, it has
been suggested that the embedded P3HTs can be utilized formolecular wires, together with fabrication techniques thatenable the patterning of microscopic electrodes in the planar
conﬁguration on H-Si(100) surfaces.3–5) We have investigated the electronic structures of individual P3HTs ﬁxed on
H-Si(100) using scanning tunneling microscopy and spectroscopy (STM/STS).6) The energy-band diagram and

Author to whom correspondence should be addressed.
E-mail address: terada@bk.tsukuba.ac.jp

conduction model have been proposed for the H-Si(100)/
P3HT/STM-tip system. The assumptions in the previous
paper6) are (i) no band bending of bulk silicon, (ii) a band
picture for the electronic structure of P3HT, (iii) a weakinteraction between P3HT and H-Si(100), and (iv) biasdependent molecular orbital (MO) levels. Although theprevious model has given an explanation consistent with theexperimental results, some of the assumptions were diﬃcult
to be proved by the experiments alone. To give acomplementary explanation, we calculate here the electronic
structures of the P3HT/H-Si(100) system within the densityfunctional approach. We reexamine the previous model inaccordance with the calculations and conﬁrm the validity ofthe model. We then explain the experimental results
consistently.
2.

Experimental

Silicon samples were cut from a commercial n-typeSi(100) wafer (1– 6 mcm) doped with arsenic. Methods
for preparing the H-terminated surface and W tips aredescribed elsewhere.4) P3HTs were dissolved in chloroform
solvent and the solution was pulse injected onto H-Si(100)using a pulse valve (General Valve) as previously reported.7,8) After removing the residual solvent by annealing thesilicon substrate, we obtained STM images and conﬁrmedthat isolated chains of P3HT were successfully mounted on
the clean H-Si(100) surface.
All the STM/STS measurements were carried out at room
temperature under a base pressure of the STM chamber of1  108 Pa. During STS measurements, the STM tip
position was held constant at the height established by theconstant current feedback conditions of Vs and I, and thenthe I–V curve was acquired within the disabled feedback
loop. At each position, the corresponding I–V curve wasaveraged over 32 scans to increase the signal-to-noise ratio.
A simultaneously acquired topographic image ensured an
accurate assessment of the spatial position of each spectrum.
The typical numbers of topographic and spectroscopic data
points were 256  256 and 64  64, respectively. The
conductance spectra, dI=dVs, were numerically diﬀerentiated from the measured I–Vs.
The calculations were based on the density functional

1956

Jpn. J. Appl. Phys., Vol. 45, No. 3B (2006)

Y. T ERADA et al.

theory. We have employed the generalized gradient approximation (GGA)9) and used the plane-wave-based ultrasoft
pseudopotentials.10,11) The energy cutoﬀ was taken as
20.25 Ry. The convergence criterion of the geometry
optimization was that all of the forces acting on each atom
were within 1  103 hartree/a.u. The polymers wereassumed to exhibit an inﬁnite periodic structure with a unit
cell including two pentagons. The H-Si(100) surface wasrepresented by a slab model containing ﬁve layers of Si
atoms. The remaining dangling bonds of an unreconstructed
Si surface on the back of the slab were terminated with H
atoms.
3.

Results

3.1 STM image and STS spectra
Figure 1(a) shows a typical topographic image of a
P3HT/H-Si(100) surface. Isolated chains of P3HT are
observed in this ﬁgure, and the width, length, and heightare in good agreement with the typical values in our previous
STM observations.7,8) The current maps at diﬀerent substratevoltages are shown in Figs. 1(b)–1(d). There is a distinct
diﬀerence in the maps between the positive and negative
bias voltages. The maps at negative bias [Figs. 1(c) and
1(d)] exhibit little contrast, whereas that at positive bias
[Fig. 1(b)] exhibits a distinct contrast, reﬂecting the diﬀerence in the I–V curves between P3HT and H-Si(100). The
dark and bright areas in Fig. 1(b) are assigned to the P3HT
and H-Si(100) surface sites, respectively.
To quantitatively discuss the I–V diﬀerence, the I–V data
were averaged over the dark (P3HT) and bright [H-Si(100)]
areas in Fig. 1(b). Figures 1(e) and 1(f) show the resulting I–
V curves and dI=dV spectra for P3HT (open circles) and HSi(100) (solid circles). The I–V and dI=dV curves for HSi(100) form symmetric shapes about a symmetry axis of
Vs ¼ 0:40 V and exhibit a conductance gap at 0:94 <Vs < 0:13 V.
In contrast to those for the H-Si(100) surface, the I–V
curve and dI=dV spectrum for P3HT [Figs. 1(e) and 1(f)] arehighly asymmetric and have two features: (i) a conductance
gap at 0:94 < Vs < 0:73 V that stretches out toward thepositive bias axis, in contrast to that of H-Si(100), and (ii)current suppression at positive bias, revealing a rectiﬁcation
characteristic.
3.2 Calculations
The calculations for the P3HT/H-Si(100) system are time
consuming because of a large unit cell. We ﬁrst examine thepossibility that P3HT can be substituted for polythiophene(PT) that has a less-complicated chemical structure. It is
likely that the similarity in chemical structure between PT
and P3HT results in electronic structures similar to eachother. As expected, the calculated bands and density of states(DOS) of PT [Figs. 2(a) and 2(b)] and P3HT [Figs. 3(a) and3(b)] are quite similar, especially near the Fermi levels. For
both PT and P3HT, the amplitudes of wave functions near
the Fermi levels [Figs. 2(d)–2(g) and 3(d)–3(g)] are locatedat the pentagon sites, indicating their conducting (or semiconducting) properties along the backbones. Their bandfeatures only diﬀer at the levels much lower than the Fermi
level [Figs. 2(h) and 3(h)]. The low-energy bands for P3HT
[Fig. 3(h)] are attributed to a side chain of the hexyl group.

Fig. 1. (a) STM topographic image of P3HT/H-Si(100) (20  20 nm2 ,Vs ¼ 2 V, I ¼ 0:05 nA). (b)–(d) Spatial maps of I obtained by recording
I–V curves simultaneously with topographic image shown in (a). The
maps were taken at (b) Vs ¼ þ1 V, (c) Vs ¼ 1 V, and (d) Vs ¼ 2:5 V.
(e) Spatially averaged I–V curves for P3HT (open circle) and H-Si(100)(solid circle). (f) dI=dV curves numerically diﬀerentiated from the I–V
curves shown in (e) for P3HT (open circle) and H-Si(100) (solid circle).

Although the side chain aﬀects the structural properties suchas the twisting of the molecular backbone [see inset of
Fig. 3(c)], the conduction property is mainly related to thebands near the Fermi energy. Thus, we hereafter substitutepolythiophene (PT) for P3HT in the calculations.
We then calculate the band structures and DOS for HSi(100) [Fig. 4(c)] and the PT/H-Si(100) system [Fig. 4(d)]along with the geometry optimization [Figs. 4(a) and 4(b)].
A 4  4 unit cell of silicon coincidentally has a similar
length with the size of four pentagons of PT in the optimized
structure, and hence it is found that only a vertical extensionof a unit cell is suﬃcient for calculations. Figure 4(d)reveals that the valence band (VB) of PT (drawn by a dashedline), located in the band gap of H-Si(100), forms a shapesimilar to that of the isolated PT-VB and is not aﬀected by
the presence of H-Si(100). This indicates the absence ofstrong bonds between PT and H-Si(100). This is also
conﬁrmed by the calculations of the wave functions (Fig. 5).
The amplitudes of the highest occupied molecular orbital

1957

Jpn. J. Appl. Phys., Vol. 45, No. 3B (2006)

Y. T ERADA et al.

2

2

0

0

-2 d
EF
e
-4

EF

d
(a)

(b)

Γ

X

-4 f
g
h
-6

X

-8

Density of states

Γ

(c)

(f)

(c)

Density of states
(f)

(d)

(g)

(d)

(g)

(e)

(h)

(e)

(h)

Fig. 2. Calculations of energy structures of PT. (a) Band structures.
(b) DOS. (c) Optimized geometry. (d)–(h) Amplitudes of wave functionsat -point in (d) LUMO, (e) HOMO, (f) HOMO-1, (g) HOMO-2, and(h) HOMO-3 states. The corresponding band structures are shown in (a).

(HOMO), HOMO-1, and HOMO-2 states [Figs. 5(b)–5(d)]are located at either the PT or H-Si(100) site, except for the
LUMO state exhibiting a slightly overlapping amplitude[Fig. 5(a)].
4.

(b)

e

f
-6 g
h
-8

(a)

Discussion

4.1 Model
On the basis of the calculations, we examine the modelthat was previously proposed.6) We have assumed (i) the
absence of the band bending in the bulk silicon, (ii) the band
picture for the electronic structure of P3HT, (iii) a weakinteraction between P3HT and H-Si(100), and (iv) biasdependent molecular energy-band levels. Here, we ignore
the band-bending eﬀect as previously discussed because thesilicon we used is so highly doped (2  1019 cm3 ) that the
depletion layer is suﬃciently thin (1 nm) to enable electrons
to tunnel through it.12) The band picture for P3HT energy
levels is a consequence of the calculations under theperiodical boundary condition, as revealed by the continuous
PT-DOS on H-Si(100) [Fig. 4(d)]. The weak interaction
between P3HT and H-Si(100) is conﬁrmed by the calcu-

Fig. 3. Calculations of energy structures of P3HT. (a) Band structures.
(b) DOS. (c) Optimized geometry. (d)–(h) Amplitudes of wave functionsat -point in (d) LUMO, (e) HOMO, (f) HOMO-1, (g) HOMO-2, and(h) HOMO-3 states. The corresponding band structures are shown in (a).
The chemical structure of P3HT is shown in the inset of (c).

lations as discussed before. The absence of the strong bonds
between the molecule and the substrate surface results in thebias-dependent molecular bands,12–14) because the electricﬁeld inside the molecule is poorly screened and hence shifts
the molecular electrostatic potential.
Under these assumptions, we have modeled the energyband diagram for the H-Si(100)/P3HT/W-tip junction(Fig. 6) as previously proposed.6) Although silicon band
bending is shown in this ﬁgure, we assume Si ¼ 0 as in caseof the H-Si(100)/W-tip system. Taking the molecular band
shift into account, the shift of the electrostatic potential on
P3HT is roughly given by ðVS  Si Þ  VS under a bias
VS , where  denotes the average molecular potential underunit applied bias.12) The parameter  is determined by thegeometry of the junction; it is approximately 0.5 when an
STM tip touches the molecule and decreases with increasing
tip–molecule distance.9) The energy gap of P3HT is assumed
to be a typical experimental value in the literature, 2.1 eV.15)
We adjust the location of the molecular bands under zero
bias in such a way that P3HT-VBM is located in the silicon

1958

Jpn. J. Appl. Phys., Vol. 45, No. 3B (2006)

Y. T ERADA et al.

(a) V S = 0

(a)
(b)

∆φ

H

ECBM

Si

EC
EF[Si]
EV

EF[W]

EVBM

4
3

(c)

Si

2
EF

P3HT

W tip

(b) V S < 0

0
-1

Γ

Jx

K

Jy

Γ

DOS

3

(d)

2

ECBM

EC
EF[Si]

EF A
1 B

-VS

EVBM

EV

EF[W]

C D

0
-1

Γ

Jx

K

Jy

Γ

|Vs|

DOS

φSi
η(|Vs|−φSi)
Si

Fig. 4. Calculations of energy structures of H-Si(100) and PT on HSi(100). [(a) and (b)] Optimized geometry of PT on H-Si(100) in (a) top
view and (b) side view. (c) Band structure and DOS of H-Si(100).
(d) Band structure and DOS of PT on H-Si(100). The dashed line
corresponds to states attributed to PT.

P3HT(1−η)(|Vs|−φSi)W tip(c) V S > 0
ECBM

(a)

(b)

EC
EF[Si]
EV

Si

(d)

+VS

charging
effect

φSi

(c)

EF[W]

EVBM

η(Vs−φSi)P3HT(1−η)(Vs−φSi)

Vs

W tip

Fig. 6. Energy band diagrams in H-Si(100)/P3HT/STM tip junction. EC ,
EV , ECBM , and EVBM are energy levels of silicon-CBM, silicon-VBM,
P3HT-CBM, and P3HT-VBM, and EF [Si] and EF [W] are Fermi energy
levels of silicon and the W tip, respectively. Si and  are silicon band
bending and the work-function diﬀerence between silicon and tungsten,respectively. Electrostatic potential proﬁles are also shown at the bottomin (b) and (c).

Fig. 5. Amplitudes of wave functions calculated for PT on H-Si(100) at
-point in (a) LUMO, (b) HOMO, (c) HOMO-1, and (d) HOMO-2 states,
respectively.

band gap as revealed by the calculations [Fig. 4(d)]. The
location agrees well with the expectations in our previous
paper.6)

As a result of the molecular band shift, the conductance
gap in the molecular I–V is determined by the shift of either
CBM or VBM of P3HT, and the closer band edge to either
CBM or VBM of silicon determines the conduction behavior. In our case, therefore, P3HT-VBM is expected todominate the conduction. This is also in good agreementwith the previous conclusion that the conductance gap isdetermined by the shift of P3HT-VBM.6)

1959

Jpn. J. Appl. Phys., Vol. 45, No. 3B (2006)

Y. T ERADA et al.

4.2 Conduction gaps in I–V curves
In accordance with the model, we explain the experimental results as follows. We ﬁrst consider the I–V curve at the
H-Si(100) sites [Figs. 1(e) and 1(f)]. Ignoring the bandbending eﬀect, the measured conductance gap (1.07 V) for
H-Si(100) is attributed to the band gap of bulk silicon. The
gap is shifted toward the negative Vs axis because the Fermi
energy EF [Si] of bulk silicon is close to the energy level ofbulk-silicon-CBM.
We then consider the I–V curve at the P3HT sites. The
measured I–V curve shows the wide conductance gap andthe rectiﬁcation. The conductance gap can be explained by
the P3HT-VB-based conduction model. Under a slight
positive or negative bias, the band gap of P3HT prevents
electrons from tunneling through the junction. Under a large
negative bias, the tip Fermi energy EF [W] becomes lower
than the silicon-VBM level, EV [Fig. 6(b)]. Since the P3HTVB levels exist in between them, electrons in the P3HT-VB
tunnel into the tip empty states while the P3HT-VB levels
are ﬁlled by the silicon-molecule coupling, which is stronger
than the molecule-tip coupling. Under a large positive bias,
on the other hand, the P3HT-VBM level, EVBM , becomes
higher than the silicon-CBM level, EC [Fig. 6(c)]. In thiscase, the P3HT-VB levels are emptied out and holes areformed in P3HT-VB, leading to the electron tunneling fromthe tip ﬁlled states. According to our previous investigation,6) the threshold for tunneling is given by the conditions
EV > EF [W] for Vs < 0 and EC < EVBM for Vs > 0.
4.3 Origins of rectiﬁcation
We then explain the suppression of the current at positivebias. In our previous paper,6) we discussed three possible
reasons for the current suppression: (i) the diﬀerent couplingof the P3HT-MOs to the silicon orbitals, (ii) the silicon band
bending, and (iii) the charging eﬀect of P3HT. Among them,
we have concluded that the former two are unlikely and thecharging eﬀect is likely. In addition to the charging eﬀect,
there is one other important reason that has not been
discussed previously. The forth and prominent reason for the
suppression is the diﬀerence in the number of states of P3HT
that contribute to current ﬂow between negative and positive
biases. The electrons tunnel via the P3HT-VB levels thathave higher energies than EF [W] at negative bias, while via
the P3HT-VB levels that have higher energies than EC at
positive bias. Therefore, the number of states that contributeto tunneling is larger at negative bias than at positive bias. In
the Tersoﬀ–Hamann approximation,16) tunneling conductance can be generally expressed by dI=dV / S ðeV  EF Þ,
where S and EF are the DOS and Fermi energy of a surface.
Using this approximation, the molecular conductance isroughly given by dI=dV / ð1  ÞP3HT ðð1  ÞeVS Þ forVS ð Va Þ < 0 and dI=dV / P3HT ðeVS Þ for VS ð Vb Þ >0, where P3HT is the DOS of P3HT, and Va (< 0) and Vb(> 0) are the threshold voltages for tunneling. Since weassume 0 <   0:5, we can derive the relationship that the
conductance dI=dV is larger for VS < 0 than for VS > 0,provided that P3HT is constant. Considering the simplicity
of our theoretical approach, this accounts well for the current
suppression at positive bias.
The charging eﬀect is weak compared with the DOS
eﬀect but not negligible. The eﬀect of diﬀerential charging

on the I–V asymmetry has previously been discussed indetail17) for a system where two contacts are asymmetric.
When this is the case, one contact is much stronger than theother. At negative bias, the stronger silicon–P3HT contact
tends to ﬁll the P3HT-VB levels, leading to no energy shiftof P3HT-MO levels due to the charging. At positive bias,
however, the P3HT-VB levels tend to be emptied out by thestronger silicon–P3HT contact, leading to the positive
charging of P3HT, which shifts all the energy levels down.
This shift, not present for negative bias, would postpone theonset of conduction and eﬀectively stretch out the voltage
axis toward the positive bias region.
The above argument indicates that the P3HT molecule ispositively doped at positive substrate bias. We therefore
conjecture that the STM tip serves as a gate electrode toenable the p-type P3HT doping. Applying the negative bias
to the tip would cause the modulation of the conduction
behavior through the P3HT backbone.
5.

Conclusions

In summary, we investigated the electronic structures ofthe H-Si(100)/P3HT/STM-tip system experimentally andtheoretically. The measured I–V curves reveal (i) the shift ofthe onset voltage toward the positive substrate bias axis, incontrast to the curves for the H-Si(100)/tip system, and (ii)the suppression of the current at positive substrate bias. The
ﬁrst-principle calculations reveal that the molecular energy
bands are little aﬀected by the presence of H-Si(100) and themolecular VB is located in the silicon band gap. In
accordance with the calculations, we examined the modelfor the electrostatic potential proﬁle across the H-Si(100)/
P3HT/W-tip junction that was previously proposed, andconﬁrmed the validity of the model. The measured I–V
asymmetry is explained by the shift of the P3HT-VB levels
driven by the STM tip potential. The suppression of thecurrent arises from the diﬀerence in the number of states of
P3HT that contribute to current ﬂow between negative andpositive biases. The tunneling at positive bias causes the
charging of P3HT, indicating the possibility of electrical
doping of the polymer wire by applying a negative voltage to
the STM tip.
Acknowledgement
Part of the study was performed through Special Coordination Funds for Promoting Science and Technology of the
Ministry of Education, Culture, Sports, Science and Technology of Japan.

1960

1) C. Joachim, J. K. Gimzewski and A. Aviram: Nature (London) 408(2000) 541.
2) I. H. Campbell, S. Rubin, T. A. Zawodzinski, J. D. Kress, R. L. Martin,
D. L. Smith, N. N. Barashkov and J. P. Ferraris: Phys. Rev. B 54
(1996) R14321.
3) J. W. Lyding, T.-C. Shen, J. S. Hubacek, J. R. Tucker and G. C. Abeln:
Appl. Phys. Lett. 64 (1994) 2010.
4) T. Hashizume, S. Heike, M. I. Lutwyche, S. Watanabe, K. Nakajima,
T. Nishi and Y. Wada: Jpn. J. Appl. Phys. 35 (1996) L1085.
5) M. Fujimori, S. Heike, Y. Terada and T. Hashizume: Nanotechnology
15 (2004) S333.
6) Y. Terada, K. Miki, M. Fujimori, S. Heike, Y. Suwa and T.
Hashizume: J. Appl. Phys. 97 (2005) 124302.
7) Y. Terada, B.-K. Choi, S. Heike, M. Fujimori and T. Hashizume: NanoJpn. J. Appl. Phys., Vol. 45, No. 3B (2006)

Y. T ERADA et al.

Lett. 3 (2003) 527.
8) Y. Terada, B.-K. Choi, S. Heike, M. Fujimori and T. Hashizume:
J. Appl. Phys. 93 (2003) 10014.
9) J. P. Perdew, K. Burke and Y. Wang: Phys. Rev. B 54 (1996) 16533.
10) D. Vanderbilt: Phys. Rev. B 41 (1990) 7892.
11) K. Laasonen, A. Pasquarello, R. Car, C. Lee and D. Vanderbilt: Phys.
Rev. B 47 (1993) 10142.
12) T. Rakshit, G.-C. Liang, A. W. Ghosh and S. Datta: Nano Lett. 4(2004) 1803.

13) S. Datta, W. Tian, S. Hong, R. Reifenberger, J. I. Henderson and C. P.
Kubiak: Phys. Rev. Lett. 79 (1997) 2530.
14) N. P. Guisinger, M. E. Greene, R. Basu, A. S. Baluch and M. C.
Hersam: Nano Lett. 4 (2004) 55.
15) T.-C. Chung, J. H. Kaufman, A. J. Heeger and F. Wudl: Phys. Rev. B
30 (1984) 702.
16) J. Tersoﬀ and D. R. Hamann: Phys. Rev. Lett. 50 (1983) 1998.
17) F. Zahid, A. W. Ghosh, M. Paulsson, E. Polizzi and S. Datta: Phys.
Rev. B 70 (2004) 245317.

1961