electron correlation in new materials and nanosystems, 2007, p.429

Electron Correlation in New Materials and
Nanosystems
NATO Science Series
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Series II: Mathematics, Physics and Chemistry – Vol. 241
Electron Correlation in New Materials
and Nanosystems
edited by
Kurt Scharnberg
University of Hamburg, Germany
and
Sergei Kruchinin
Bogolyubov Institute for Theoretical Physics,
Kiev, Ukraine
Published in cooperation with NATO Public Diplomacy Division
Proceedings of the NATO Advanced Research Workshop on
Yalta, Ukraine
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Electron Correlation in New Materials and Nanosystems, held in
19 23 September 2005.
TABLE OF CONTENTS

PART I. Quantum nanodevices 1
Transport properties of fullerene nanodevices
Nanoscale studies on metal-organic interfaces
9
N. Chandrasekhar
Electron-electron interaction in carbon nanostructures
A. I. Romanenko, O. B. Anikeeva, T. I. Buryakov, E. N. Tkachev,
A. V. Okotrub, V. L. Kuznetsov A. N. Usoltseva, A. S. Kotosonov
Single-level molecular rectifier
Yu.G. Naidyuk, I.K. Yanson, D.L. Bashlakov, V.V. Fisun,
R.I. Shekhter
PART II. Superconductivity
II.1 Magnesium diboride and the two-band scenario
v
ix
3
Preface
N.L. Saini, L. Simonelli
23
37
59
71

73
93
A. Fujiwara, Y. Matsuoka, N. Inami, E. Shikoh
E.G. Petrov
O.P. Balkashin, L.Y. Triputen, A. Konovalenko, V. Korenivski,
S. Akutagawa, T. Muranaka, J. Akimitsu
Magnetic unipolar features in conductivity of point contacts between
A. Bianconi, M. Filippi, M. Fratini, E. Liarokapis, V. Palmisano,
Shape resonances in the interband pairing in nanoscale
normal and ferromagnetic D-metals (Co, Ni, Fe)
modulated materials
Superconductivity in magnesium diboride and its related materials
vi CONTENTS
Magnetic and microwave properties of the two gap
superconductor MgB
2
T. Dahm
magnesium diboride
T. Örd, N. Kristoffel, K. Rägo
Nanosize two-gap superconductivity
H. Nagao, H. Kawabe, S. P. Kruchinin
Exact solution of two-band superconductivity in ultrasmall grains
H. Kawabe, H. Nagao, S. P. Kruchinin
II.2 Cuprate and other unconventional superconductors
Experimental evidence for a transition to BCS superconductivity
in overdoped cuprates
G. Deutscher
C
low-T
C

Ariando, H. J. H. Smilde, C. J. M. Verwijs, G. Rijnders,
D. H. A. Blank, H. Rogalla, J. R. Kirtley, C. C. Tsuei,
H. Hilgenkamp
Anisotropic resonance peak in orthorhombic superconductors
D. Manske, I. Eremin
Dynamical spin susceptibility in the underdoped cuprate
superconductors: DDW state and influence of orthorhombicity
J P. Ismer, I. Eremin, D. K. Morr
Disorder effects in d-wave superconductors
C. T. Rieck, K. Scharnberg, S. Scheffler
103
107
117
129
141
149
175
187
199
Free energy functional and critical magnetic fields anisotropy in
Experiments using high-T versus Josephson contacts
CONTENTS vii
First principles calculations of effective exchange
integrals for copper oxides and isoelectronic species
K. Yamaguchi, Y. Kitagawa, S. Yamanaka, D. Yamaki,
Microscopic evidence of the FFLO state in the strongly-correlated
superconductor CeCoIn
5
probed by
115

Models of superconductivity in Sr
RuO
4
High-Tc superconductivity of cuprates and ruthenates
J. D. Dow, D. R. Harshman, A. T. Fiory
Doping dependence of cuprate coherence length, supercarrier
effective mass, and penetration depth in a two-component scenario
N. Kristoffel, T. Örd, P. Rubin
Order parameter collective modes in unconventional superconductors
Vortex matter and temperature dependence of the Ginzburg-Landau
phenomenological lengths in lead nanowires
G. Stenuit, J. Govaerts, S. Michotte, L. Piraux
Angular dimensional crossover in superconductor  normal
metal multilayers
S. L. Prischepa, C. Attanasio, C. Cirillo
superconductor-ferromagnet proximity systems
J. F. Annett, B. L. Gyorffy, M. Krawiec
PART III. Spintronics
Kondo effect in mesoscopic systems
A. N. Rubtsov, M. I. Katsnelson, E. N. Gorelov, A. I. Lichtenstein
223
In-NMR 235
251
263
275
283
293
303
317
325

327
T. Kawakami, M. Okumura, H. Nagao, S. P. Kruchinin
2
K. Kumagai, K. Kakuyanagi, M. Saitoh, S. Takashima,
M. Nohara, H. Takagi, Y. Matsuda
T. Dahm, H. Won, K. Maki
Andreev states and spontaneous spin currents in
Peter Brusov, Pavel Brusov
viii CONTENTS
1/f noise and two-level systems in Josephson qubits
A. Shnirman, G. Schön, I. Martin, Y. Makhlin
Single-electron pump: device characterization and
Zero-bias conductance through side-coupled double quantum dots
J. Bonþa, R. Žitko
A.N. Lavrov, A.A. Taskin, Y. Ando
L. Alff
C
H. Hori, Y. Yamamoto, S. Sonoda
Large magnetoresistance effects in novel layered Rare Earth Halides
R.K. Kremer, M. Ryazanov, A. Simon
intrinsic magnetic-field-effect transistor
Spin-orbital ordering and giant magnetoresistance in cobalt oxides:
343
linear-response measurements 357
371
381
401
417
431
R. Schäfer, B. Limbach, P. vom Stein, C. Wallisser

Ferrimagnetic double perovskites as spintronic
materials 393
ferromagnetism in gallium manganase
nitrides based on resonation properties of impurities in semiconductors
Author index
Subject index
A possible model for high-T
433
PREFACE
These proceedings reflect much of the work presented and extensively
discussed in a stimulating and congenial atmosphere at the NATO
Advanced Research Workshop “Electron correlation new materials and
nanosystems”, held at the “Yalta” Hotel, Yalta, Ukraine from 19-23
September 2005. The lively discussion sessions in the evenings,
unfortunately, could not be included in the proceedings but in some sense
they were continued during a rigorous refereeing process which lead to
substantial modifications of many contributions. This refereeing process,
together with the request by the publisher “to have those articles which
have been written by non-native English speakers carefully proofread and,
if necessary, corrected by a native English speaker working closely with the
editor”, caused considerably delay in the submission of these proceedings
to the publisher. On the other hand, given this extra time, several
participants who in Yalta had declined to submit manuscripts, could be
persuaded to present their latest research in these proceedings after all.
Other authors used this opportunity to update their manuscripts. So, on
average these proceedings represent state of the art research as of the
summer of 2006 rather than September 2005.
Since neither of us work in an English-speaking environment, we tried
to enlist the help of referees to meet the publisher’s request for carefully
proofreading manuscripts. We would like to use this opportunity to express

our sincere thanks to the referees for the help we got.
Thanks are also due to Johny Sebastian from Springer’s texsupport,
who modified the original style file in accord with the editor’s wishes.
These changes helped in particular to squeeze many manuscripts on an
even number of pages and thus reduce the physical size of this tome
substantially.
The topics discussed included a wide range of novel materials with
emphasis on superconductors, mesoscopic and nanostructured systems like
quantum wires, quantum dots, nanotubes and various hybrid structures
involving ferromagnets and superconductors or organic substances and
metals. Studies of these systems were presented which addressed the
problems of understanding the fundamental physical processes as well as
their applications to quantum computing and spintronics. The workshop
closed with a session on various types of sensors. Most contributions were
presented orally, but in order not to overload the program and to leave
ix
x PREFACE
enough time for discussion, there was also a poster session. Some of the
papers presented as posters have been included in these proceedings.
This workshop addressed a range of topics rather wider than is usual.
While the relaxed atmosphere provided by this Black Sea resort with its
natural beauty and architectural gems, filled with captivating history,
encouraged and facilitated the communication with colleagues having
rather different backgrounds, the editors decided to focus the proceedings
more sharply and thus not to include the sessions on sensors, for which
only two manuscripts had been submitted.
We are grateful to members of the International Advisory Committee
A. Balatsky and I.Yanson for their consistent help and suggestions.
We would like to thank the NATO Public Diplomacy Division,
Collaborative Programmes Section, for the essential financial support

without which this meeting could not have taken place. Thanks are also due
to the National Academy of Science of Ukraine and the Ministry of Ukraine
for Education and Science for support.
Kurt Scharnberg and Sergei Kruchinin
August 2006.
PART I

Quantum Nanodevices
TRANSPORT PROPERTIES OF FULLERENE NANODEVICES
Toward the New Research Field of Organic Electronic Devices
Akihiko Fujiwara
*
, Yukitaka Matsuoka, Nobuhito Inami, Eiji Shikoh
School of Materials Science, Japan Advanced Institute of Science and
Technology, 1-1 Asahidai, Tatsunokuchi, Ishikawa 923-1292, Japan, and
CREST, Japan Science and Technology Agency, 4-1-8 Honchou,
Kawaguchi, Saitama 332-0012, Japan
Abstract. We report transport properties of C
60
thin film field-effect transistors (FETs) with
a channel length of several-ten nanometers. Nonlinear drain current I
D
versus source-drain
voltage V
DS
characteristics were observed at room temperature. We discuss this
phenomenon in terms of the crossover from a diffusive conductance in bulk regime to a
coherent one in the nanometer scale.
Key words: Fullerene; Nanodevice; Organic electronics; Transport properties; Field-effect

transistor; Crossover
1. Introduction
The miniaturization of transistors enables us to put a billion transistors on a
chip operating with the clock periods of a billionth of a second. However,
as transistors get smaller in size, there are many undesirable effects, such as
short-channel effects and the increase of off-currents. Moreover, quantum
effects will become significant. To overcome these effects, a device based
on a new principle of operation, such as a single-electron transistor (SET)
1
,
is required. In recent years, C
60
has attracted considerable attention as the
material for an island of the SET because it can be regarded as an ideal
quantum dot by itself. C
60
is a closed cage, nearly spherical molecule
consisting of 60 carbon atoms with a diameter of about one nanometer. Its
high symmetry results in a unique electronic structure, such as the three-
fold degenerate lowest-unoccupied-molecular orbital (LUMO) and the five-
fold degenerate highest-occupied-molecular orbital (HOMO)
2
. In addition,
______
*
Corresponding author: Akihiko Fujiwara; e-mail:
K. Scharnberg and S. Kruchinin (eds.),
Electron Correlation in New Materials and Nanosystems,
c
 2007 Springer.

3
3–8.
4
the electronic structure of crystalline C
60
is hardly modified from that of a
free C
60
molecule, namely, a molecular orbital, because crystalline C
60
is a
nearly ideal molecular crystal with van der Waals interaction. The
quantized electronic levels are conserved even when C
60
is in a cluster or a
crystalline state.
A FET is a macroscopic system with dominant classical effects,
whereas an SET is a nano-scale system with dominant quantum mechanical
effects. The transport properties of C
60
thin film FETs with a channel of
several-decades of micrometers
3,4
and of the C
60
SET with an island of
several nanometers
5
have been reported. The device structures of a FET and
a SET are qualitatively different in inorganic devices. However, in organic

devices they are the same: the difference is only the size. This comes from
two factors. One is the electronic structure. It originates from the molecular
orbital even in the crystal and hardly depends on the size as discussed
above. Another is the existence of the barrier at the contact between the
channel area and the electrodes for the electron conduction. It acts as the
tunnel barrier for an SET and as the Schottky barrier for an FET. The latter
is not favorable but cannot be excluded so far. In organic devices, therefore,
the marginal electronic states between a macroscopic system and a nano-
scale one are expected (Fig. 1). In this work, to clarify the C
60
device
properties in this marginal area, we have investigated the transport
properties of C
60
thin film FETs with a channel length of ca. 20 nanometers.
Figure 1. Schematic overview of organic electronics.
2. Experimental details
Figure 2 shows the schematic cross section of the fabricated C
60
thin film
FET with a diagram of the measurement setup. The Au source and drain
electrodes with thickness of 100 nm were fabricated on the 400 nm SiO
2
layer that was made on the surface of a heavily doped n-type silicon wafer,
A. FUJIWARA ET AL.
FULLERENE NANODEVICES
5
using an electron-beam lithography method. The doped silicon layer of the
wafer was used for a gate electrode. The distance between source and drain
electrodes, i.e. the channel length of the fabricated C

60
thin film FETs was
approximately 20 nm.
Figure 2. Schematic cross section of the fabricated C60 thin film FET (700 nm
channel length) with a diagram of the measurement setup.
A typical scanning electron microscope (SEM) image of fabricated
60
(99.98 %) was used for the formation of the thin films channel layer. A C
60
thin film of 150 nm thickness was formed on the SiO
2
layer using vacuum
(< 10
-4
Pa) vapor deposition at the deposition rate of 0.01 nm/s.
It is well known that the n-type organic semiconductor is very sensitive
to chemically and physically adsorbed O
2
and/or H
2
O molecules, which can
generate traps of electrons and suppress carrier transport
6-8
. Therefore,
before measurements, the samples were annealed at 120 ºC under 10
-3
Pa
for a few days. The drain and gate electrodes were biased with dc voltage
sources and the source electrode was grounded. The transport properties of
C

60
FETs were measured at room temperature under 10
-4
Pa without
exposure to air after annealing.
source and drain electrode is shown in Fig. 3. Commercially available C
Gate
Drain
A
Source
V
DS
I
D
V
G
SiO
2
C
60
thin film
Channel length
6
Figure 3. Typical scanning electron microscope (SEM) image of fabricated source
and drain electrode.
3. Results and discussion
Figure 4 shows the source-drain voltage V
DS
dependence of the drain
current I

D
. I
D
increases nonlinearly with increasing V
DS
and is enhanced by
V
G
: the I
D
versus V
DS
curves are almost symmetrical. The symmetrical
characteristics can be related to those observed in the SET operation rather
than the FET operation in which a pronounced asymmetric I
D
versus V
DS
response is observed. As for the V
G
dependence, an enhancement of I
D
is
similar to the FET operation.
It is worth noting that the device structures of the C
60
FET
3,4,9,10
and the
C

60
SET
5
are the same in principle. Weak contact between the inorganic
metal electrodes and organic semiconductor, acting as tunnel barrier in the
SET, exists even in the FET as Schottky barrier, although no such an
obstacle exists in the inorganic FETs. Therefore, a reduction (an expansion)
in size of the FET (SET) will lead to the appearance of the SET (FET)
mode of operation in organic devices. The device size shown in Fig. 3 is
about 20 nm and is of the same order of characteristic size in which the
quantum effect is observed. In addition, the device operation is, in part,
similar to that observed in both the SET and FET. On the other hand, the
devices with the channel length of about 50 - 700 nm operate as a FET
10,11
.
Considering the characteristics of devices and their size-dependence it is
A. FUJIWARA ET AL.
FULLERENE NANODEVICES
7
plausible that the crossover from the FET character to that of the SET takes
place around a channel length of 20 nm. More detailed and systematic
experiments on the crossover from the macroscopic behavior (the FET
operation) to the microscopic quantum behavior (the SET operation) will
clarify the mechanism of electron transport in organic materials.
Figure 4. I
D
versus V
DS
plots for V
G

= 0 V (circle) and 4 V (triangle).
4. Conclusion
We have investigated the transport properties in short-channel C
60
thin film
FETs. The I
D
versus V
DS
plots showed symmetric nonlinear characteristics.
This phenomenon can be interpreted as the crossover between a diffusive
conductance in bulk regime and a coherent one in the nanometer scale. The
marginal area is estimated to fall around 20 nm.
Acknowledgements
The authors are grateful to Dr. M. Akabori, Professor S. Yamada, and the
technical staffs of the Center for Nano-materials and Technology at the
Japan Advanced Institute of Science and Technology for use of the
electron-beam lithography system and other facilities in the clean rooms, as
well as for technical support. This work is supported in part by the JAIST
International Joint Research Grant, the Grant-in-Aid for Scientific Research
(Grant Nos. 16206001, 1731005917, 17540322) from the Ministry of
Education, Culture, Sports, Science and Technology (MEXT) of Japan, and
the NEDO Grant (Grant No. 04IT5) form the New Energy and Industrial
Technology Development Organization (NEDO), and the Kurata Memorial
-60
-40
-20
0
20
40

60
-3 -2 -1 0 1 2 3
0 V
4 V
V
DS
(V)
I
D
(pA)
8
Hitachi Science and Technology Foundation, the Support for International
Technical Exchange from TEPCO Research Foundation.
References
1. H. Grabert, M. H. Devoret, Single Charge Tunneling, NATO ASI Series vol. 294,
Plenum Press, New York, 1992.
2. M. S. Dresselhaus, G. Dresselhaus, P. C. Eklund, Science of Fullerenes and Carbon
Nanotubes (Academic Press, New York, 1996).
3. R. C. Haddon, A. S. Perel, R. C. Morris, T. T. M. Palstra, A. F. Hebard, R. M. Fleming,
C
60
thin film transistors, Appl. Phys. Lett. 67, 121-123 (1995).
4. K. Horiuchi, K. Nakada, S. Uchino, S. Hashii, A. Hashimoto, N. Aoki, Y. Ochiai, M.
Shimizu, Passivation effects of alumina insulating layer on C
60
thin-film field-effect
transistors, Appl. Phys. Lett. 81, 1911-1912 (2002).
5. H. Park, J. Park, A. K. L. Lim, E. H. Anderson, A. P. Alivisatos, P. L. McEuen,
Nanomechanical oscillations in a single-C
60

transistor, Nature 407, 57-60 (2000).
6. A. Hamed, Y. Y. Sun, Y. K. Tao, R. L. Meng, P. H. Hor, Effects of oxygen and
illumination on the in situ conductivity of C
60
thin films, Phys. Rev. B 47, 10873-10880
(1993).
7. B. Pevzner, A. F. Hebard, M. S. Dresselhaus, Role of molecular oxygen and other
impurities in the electrical transport and dielectric properties of C
60
films, Phys. Rev. B
55, 16439-16449 (1997).
8. R. Könenkamp, G. Priebe, B. Pietzak, Carrier mobilities and influence of oxygen in C
60
films, Phys. Rev. B 60, 11804-11808 (1999).
9. S. Kobayashi, T. Takenobu, S. Mori, A. Fujiwara, Y. Iwasa, Fabrication and
characterization of C
60
thin-film transistors with high field-effect mobility, Appl. Phys.
Lett. 82, 4581-4583 (2003).
10. Y. Matsuoka, N. Inami, E. Shikoh, A. Fujiwara, Transport properties of C
60
thin film
FETs with a channel of several-hundred nanometers, Sci. Technol. Adv. Mater. 6, 427-
430 (2005).
11. Y. Matsuoka, N. Inami, E. Shikoh, A. Fujiwara, unpublished.
A. FUJIWARA ET AL.
NANOSCALE STUDIES ON METAL-ORGANIC INTERFACES
N. Chandrasekhar ()
Institute of Materials Research and Engineering, 3 Research Link,
Singapore 117602

Abstract. We report ballistic electron emission microscopy (BEEM) studies on two metal
organic interfaces, Ag-polyparaphenylene (PPP) and Ag- Poly-1-methoxy-4-(2-
ethylhexyloxy)-p-phenylenevinylene (MEHPPV), and a metal-molecule interface Ag-
terthiophene-Au, which are evaporated, spin-coated, and self assembled on an Au film
respectively. All systems show spatially non-uniform carrier injection. Physical origins of
the non-uniform carrier injection and its implications are discussed. The observed injection
barriers are smaller than expected. We explain these using a model of metal induced gap
states. For the metal-molecule system, a WKB calculation is carried out and compared with
the experimental data. The results indicate that molecular levels are being accessed in the
BEEM experiment, since the measured currents are larger than a purely tunneling
contribution. Our results are consistent with previously published results on a similar
molecule. Implications for device applications are briefly discussed.
1. Introduction
Metal-organic (MO) interfaces have traditionally been investigated by
current-voltage (I-V), capacitance-voltage (C-V) and ultraviolet (UV)
spectroscopy, all of which average over macroscopic areas [1]. In contrast,
prototype devices incorporating molecules as active components are sub-
micron [2-4]. Recent work [4] has shown that nanoscale conductance
inhomogeneities can exist at MO interfaces. The physical origin of these
conducting filaments remains obscure. Filament growth and dissolution has
been identified as being responsible for the switching behavior in other
systems as well [5,6]. Lau et al. [4] report the observation of a single
switching center, and suggest a runaway process of filament growth driven
by increasing current density and/or electric field. Memory effects observed
in inorganic semiconductors [7] have been invoked to explain the behavior
of some organic devices [8,9]. Organic device configurations that have been
investigated to date are either Langmuir Blodgett (LB) films [4] or self-
assembled monolayers (SAM) [2,3]. At the present time, it is unclear
whether the inhomogeneities originate from microstructural perturbations
such as asperities at the interfaces with the contacting electrodes, or

and LB films are not rigid, and despite the implementation of precautionary
Key words: BEEM, Interfaces, Electronic transport, Manoscience
K. Scharnberg and S. Kruchinin (eds.),
Electron Correlation in New Materials and Nanosystems,
c
 2007 Springer.
9
whether they are an inherent electronic property of MO interfaces. SAM
9–21.
N. CHANDRASEKHAR
10
measures, it is uncertain whether the integrity of the organic is maintained
after deposition of the metal film [10]. For instance, in metal-inorganic
semiconductor (MIS) interfaces, unless the semiconductor surface is
prepared with care and the metal is chosen so that it is lattice matched, the
metal film is polycrystalline, causing significant variations in the electronic
transparency of the interface [11,12].
In this paper, we use ballistic electron emission microscopy and
spectroscopy to study charge transport across Ag-polyparaphenylene
oligomer (PPP), Ag-Poly-1-methoxy-4-(2-ethylhexyloxy)-p-phenylene-
vinylene (MEHPPV) and Ag-terthiophene (T3C4SH)-on-Au interfaces.
This technique allows us to determine the distribution of Schottky barrier
(SB) values with nanometer scale spatial resolution, unlike conventional
spectroscopy and current-voltage measurements that average over
millimeter areas. For the molecule, experimental results are compared with
a Wentzel Kramers Brillouin (WKB) calculation to discuss the tunneling
contribution in the measurement.
2. Ballistic electron emission microscopy (BEEM)
2.1. PRINCIPLE
A device configuration and schematic for BEEM, is shown in Fig. 1. An

organic semiconductor is overlaid with a thin metal film (typically < 10 nm,
termed the base), with an ohmic contact on the opposite side (termed the
collector). The top metal film is grounded, and carriers are injected into it
using a scanning tunneling microscope (STM) tip. These carriers are
injected at energies sufficiently high above the metal’s Fermi energy, so
that they propagate ballistically before impinging on the interface. There is
spreading of carriers in the metal film due to mutual Coulomb repulsion as
buried Ag-PPP interface is studied.
Figure 1. Schematic for a ballistic emission emission microscopy experiment. The
NANOSCALE STUDIES OF METAL-ORGANIC INTERFACES
11
well as some scattering by imperfections. When the energy of the carriers
exceeds the Schottky/injection barrier, they propagate into the
semiconductor and can be collected at the contact on the bottom. Typically
the tunneling current is attenuated by a factor of 1000, so that collector
currents are in the picoampere range for tunneling currents of few nA.
Spectroscopy and imaging can be done on this structure, by monitoring the
collector current as a function of STM tip bias voltage at a fixed location, or
as a function of tip position at a fixed STM tip bias. One of the fundamental
advantages of BEEM is the ability to investigate transport properties of hot
electrons with high lateral resolution, typically at the nanometer scale.
2.2. EXPERIMENTAL
Choice of the base depends on the injection barrier that is to be measured.
We have chosen Ag, since most organics are hole transport materials and
the Fermi energy of Ag is favorably aligned with the highest occupied
molecular orbital (HOMO). The Ag film is nominally 10 nm thick. The
experiments were done at 77K in a home-assembled STM system. Sample
preparation and characterization have been discussed in one of our earlier
papers [13]. The current noise of the setup is typically 1 pA. Ag has been
shown to yield “injection limited” contacts for hole injection into the

polyparaphenylene/vinylene (PPV) family of organics [14]. It is important
to ensure that the Ag film is reasonably flat, since the BEEM actually
grounds the area of the metal investigated by the tip. Unless this
requirement is met, attempts to tunnel into patches of the metal film, which
are poorly connected, can lead to tip crashes.
3. Results
Figure 2a shows a raw current-voltage (I-V) spectroscopy over a 0 to 2 V
range for the Ag-PPP interface. Metal organic interfaces likely have a high
density of trap sites, and noise can be caused by trapping and release of
charge from these sites. Repeated acquisition of spectra at the same point
were found to damage the sample, as evidenced by instability of the
spectrum. Qualitatively, this curve is similar to BEEM spectra seen for MIS
interfaces. The Schottky or injection barrier is usually taken to be the point
where the collector current begins to deviate from zero.
Extraction of the SB from BEEM data, such as that shown in Fig. 3
requires modeling of the spectral shape. Bell and Kaiser [15] used a planar
tunneling formalism for determining the shape. The solid line is a Kaiser-
Bell fit to the raw data (dots), and has the form:
I
b
= A (V-V
o
)
n
(1)
N. CHANDRASEKHAR
12
where I
b
is the collector current, and V

o
is the injection barrier. We find that
the value of V
o
ranges from 0.3 to 0.5. This should be contrasted with the
injection barrier determined by the Schottky-Mott (SM) rule, which yields a
value of 0.9 V assuming alignment of vacuum levels for the metal and
organic. The exponent n varies from 2.76 to 3.13. This is substantially
higher than 2, which is commonly used to fit BEEM data for MIS
interfaces. However, this is not surprising, since the exponent n is
influenced by scattering at the interface. There will be more scattering at
the MO interface due to lattice mismatch, and non-conservation of
momentum vector (for MIS interfaces, conservation of k is usually
implicit). The V
o
values, extracted in this manner, are shown as a histogram
in Fig. 3(c). Substantial deviation of the barrier from the SM rule, and its
distribution are noteworthy and will be discussed later.
0.0 0.5 1.0 1.5 2.0
0
5
10
15
20
Current pA
Bias Volts
0.0 0.5 1.0 1.5 2.0
0
2
4

6
8
Current pA
Bias V
Figure 2. BEEM spectrum of (a) Ag-PPP interface, with Kaiser-Bell fit shown as
the solid line and (b) for Ag-MEHPPV interface. Each symbol represents one raw
spectrum. Only few points of the spectra for both organics are shown for clarity.
The solid line for MEHPPV is an average of over 20 individual spectra. See text
An STM image of the top Ag film, at 0.5 V and 1 nA is shown in Fig. 3(a).
The I-V and dI/dV enable choice of imaging conditions suitable to the
interface. For instance, based on the spectroscopy data, it is possible to
determine that bias voltage of 1 V should yield a measurable collector
current. Plots of the collector current as a function of the STM tip position
are images of electronic transparency of the interface. Such an image is
shown in Fig. 3(b). The image clearly indicates non-uniform transparency
of the interface over the region scanned by the STM. The bright spots
indicate transparent regions. The size of such regions appears to be 10
nanometers. BEEM studies of MIS interfaces often show a correlation
NANOSCALE STUDIES OF METAL-ORGANIC INTERFACES
13
Figure 2(b) shows raw and averaged BEEM spectra obtained from an
Ag-MEHPPV interface. When compared to spectra for the Ag-PPP
interface, two noteworthy differences are readily apparent. First, the noise
in the raw spectra is higher; and second, the current at 2V is much smaller.
The current is expected to be smaller, since the SB is 0.1 eV higher for Ag-
MEHPPV as compared to Ag-PPP. The higher noise is not surprising, since
a spin coated organic interface will be more disordered than an evaporated
organic interface. Increased disorder would imply a larger density of
trapping sites, and more noise. The quality of the spectral data on Ag-
MEHPPV precludes an analysis of the kind done above for Ag-PPP. The

phenyl rings in the spin coated MEHPPV are expected to lie in the plane of
the substrate, unlike those of PPP, where they are expected to lie
perpendicular to the plane of the substrate. This variation in geometry has
implications for charge transfer. The latter geometry is more conducive to
charge transfer, as shown by first principles theoretical calculations.
Figures 4(a) and (b) show STM and BEEM images of an approximately
150 nm square area for the Ag-MEHPPV system. The BEEM image of Ag-
MEHPPV also indicates nonuniform transparency of the interface. The
bright spots are the more transparent regions. The size of such regions is
typically a few nanometers. Due to coulomb repulsion, the injected charges
spread out in the metal base to as much as 5 nm. Further spreading results
when the charges cross over into the organic. The lateral resolution of
BEEM is determined by these factors. For MO interfaces, due to the lack of
k-conservation, the precise resolution of BEEM is difficult to determine.
Keeping this in mind, it is intriguing that isolated bright spots of lateral
extent less than 2 nm are seen in the BEEM current images. These likely
arise from interfacial defects which provide excess electronic states for the
charges. To summarize, spatial nonuni-formity of injection is observed for
both Ag-PPP and Ag-MEHPPV interfaces.
interface, assuming appropriate physical parameter values for PPP, using
standard equations from semiconductor physics [16]. For a dielectric
constant of 3, and a carrier concentration of 10
13
/cm
3
we obtain a field of
10
5
V/m. We note that these fields are at least two orders of magnitude
lower than the fields typically applied to organic devices during I-V

spectroscopy or operation.
between the STM, STM derivative and BEEM images. This is due to lateral
variation of the surface density of states [11,12]. In this work, the electronic
transparency of the interface and the surface morphology of the Ag film
have little correlation. It is possible to estimate the electric field at the
N. CHANDRASEKHAR
14
0.40 0.45 0.50
0
20
40
60
80
100
Number
Injection barrier, eV
Figure 3. (a) STM topography of 10 nm Ag film on PPP. The height variation is
1.2 nm. (b) corresponding BEEM current image, with full scale of 3.5 pA, at 0.8 V.
Scan size is 50 nm for both images. (c) Observed distribution of Schottky barrier
values.
Figure 4. (a) STM topography of 10 nm Ag film on MEHPPV. The height
variation is 2 nm. (b) corresponding BEEM current image, with full scale of 5 pA,
at 1.5 V. Scan size is 150 nm for both images.
We now discuss results on the terthiophene molecule. The terthio-phene
with the alkanethiol segment was deposited from solution (1 mM in
ethanol) and was immobilized onto a template-stripped gold surface,
prepared by the procedure of Wagner et al [17]. The Ag was evaporated
through a mechanical mask (1x2 mm). The film was deposited at a rate of
0.1 Å/s and has a thickness of 8 nm. The STM in Fig. 4(a) image shows the
granular structure of Ag deposited onto the molecule at 77 K. The BEEM

current image in Fig. 4(b) again shows non-uniform transparency of the
interface with bright spots that are a few nanometres in size. BEEM spectra
from the bright and dark regions show significant differences, and we this
below.