With organic electronics rapidly maturing, interest in

molecular electronics growing, and prospects of

single-molecule electronics improving, there is an

increasing need to understand the interactions and

electronic structure at interfaces between molecular

and nonmolecular phases1-3. The level of activity in

new materials for organic light-emitting diodes,

organic field-effect transistors, and organic solar

cells; optical or transport characterization of organic

thin films; and fabrication of other organic electronic

devices has risen dramatically over the past decade4-

6. This work has demonstrated that interfaces play a

critical role because they determine, to a large

extent, the properties and performance of materials

and devices. Interfaces become even more important

as device dimensions decrease in nanoelectronics and

single-molecule electronics.

While the properties of conventional metal,

semiconductor, or insulator interfaces are fairly well

understood, many fundamental aspects of molecular

interfaces are not. The influence of basic aspects on charge

carrier injection and transport, such as the molecule-

substrate bond, molecular orientation, geometric structure,

energy levels, and the very nature of the molecule, e.g.

conjugated/aromatic (π) bonds versus saturated (σ) bonds,

remain to be investigated systematically. Our lack of

understanding of the complete picture limits our ability to

model and control the properties of devices comprising single

molecules, molecular monolayers, or multilayered bulk films.

by David Cahen1,*, Antoine Kahn2, and Eberhard Umbach3

Energetics of molecular interfaces

1Department of Materials and Interfaces,

Weizmann Institute of Science,

Rehovoth, Israel 76100

*E-mail: david.cahen@weizmann.ac.il

2Department of Electrical Engineering,

Princeton University,

Engineering Quadrangle, Olden Street

Princeton, NJ 08544, USA

3Experimental Physics II,

University of Würzburg,

97074 Würzburg, Germany

July/August 200532 ISSN:1369 7021 © Elsevier Ltd 2005

Transport of charge carriers through interfaces is

crucial to all electronic and optoelectronic devices, in

particular devices based on organic molecular films

and, especially, monomolecular layers and single

molecules. The energetics of molecular interfaces are

exceedingly important, therefore, and must be

understood in detail so that we can model and

control their behavior. This knowledge, however, is

not always sufficient, as the very physics of charge

carrier transport through molecular interfaces

remains, at times, unclear. This article provides an

overview of the main issues being researched actively

in the field of interfaces involving organic molecules,

and points out areas where progress has been made

and where basic questions remain unanswered.

REVIEW FEATURE

Key issuesUnderstanding carrier transport between an electrode and a

molecule or molecular film implies an understanding of:

• The electronic energy level(s) of the molecule that are

relevant to injection into and transport through the

molecule or film, and their relative position(s) with respect

to the energy levels of the electrode;

• The chemical and electronic interaction between the

molecule and the surface, e.g. chemical bonding7, overlap

of substrate and molecular wavefunctions, and local

structural and potential modification of the surface; and

• The role of molecular structure, e.g. orientation and

conformation.

Other issues relevant to the behavior of interfaces are:

• The effects of neighboring molecules on the energy levels

and electronic structure of the molecule on which the

injected charge is localized, e.g. (de)polarization or

relaxation effects;

• The bonding and ordering of molecules at the interface,

which strongly determine the growth, morphology, and

order of the layers, and hence the film properties; and

• The nature of the second (top) contact, which depends

critically on the way it is deposited and can affect the

original molecules/substrate interface.

Several features are often directly correlated to the

properties of the interface and, as a result, determine the

optical and transport properties. These are:

• The molecular orientation, and local and long-range order;

• The size and orientation of domains; and

• The density of defects, including centers for trapping of

charges and nonradiative decay.

In the following sections, we review some of these issues,

assess our general understanding, and mention outstanding

problems that need to be addressed in order to make tangible

progress in the field of molecular electronics.

Investigation methodsMethods of choice for determining the electronic structure of

molecular interfaces are surface/interface techniques that

have a proven track record with ‘classical’ inorganic electronic

materials. Of course, the methods mentioned can only be

exemplary and reflect, to some extent, the authors’ expertise.

Direct and inverse photoemission spectroscopies

Ultraviolet photoemission spectroscopy (UPS) has been used

very successfully for mapping filled ‘frontier’ orbital states8,9,

in particular the highest occupied molecular orbital (HOMO)

of the molecule at the interface (Fig. 1). Inverse

photoemission spectroscopy (IPES), while not as widely used,

gives the most straightforward measurement of empty

‘frontier’ orbital states, in particular the lowest unoccupied

molecular orbital (LUMO) of the molecule at the

interface10-12. The combination of UPS measurement of the

HOMO and IPES measurement of the LUMO of an organic

monolayer or film is used to determine the single particle

gap, or transport gap, Et of the molecular materials. By

determining the position of the Fermi level of the metal

contact on the same energy scale (with respect to a common

reference level), the energy barrier for electron and hole

injection across the interface, i.e. the band offset, can be

derived directly (Fig. 2). Using this information, one can

achieve one of the most important goals of these interface

studies, namely the correlation of the interface barrier with

experimental electrical characteristics obtained from current-

voltage, capacitance-voltage, or internal photoemission

measurements13,14. For contacts to ‘thick’ (tens of

nanometers) molecular films, relatively good agreement is

obtained between results from photoemission experiments

and barrier heights derived from electrical characteristics15. In

these ‘thick’ systems, the current corresponds to carrier

injection into ‘relaxed’ electron or hole polaron states, and

charge transport is well described by carrier hopping. Yet,

precise determination of the energy of the transport states

62 64 66 68 70 72 74 76 78 80 82

4Å α-NPD on Au

clean Au

Kinetic energy (ev)

10Å α-NPD on Au

20Å α-NPD on Au

α-NPD on Au

hν= 82 eV

EF

HOMO

13579111315 -1 -3 -5

Binding Energy w.r.t. EF (eV)

Inte

nsit

y (a

.u.)

62 64 66 68 70 72 74 76 78 80 82

4Å α-NPD on Au

clean Au

Kinetic energy (ev)

10Å α-NPD on Au

20Å α-NPD on Au

α-NPD on Au

hν= 82 eV

EF

HOMO

13579111315 -1 -3 -5

Binding energy with respect to EF (eV)

Inte

nsit

y (a

.u.)SubstrateSubstrate

hνe-

θ

(a)

(b)

(c)

Fig. 1 (a) Deposition of a molecular film on a metallic substrate. (b) Schematic of a

photoemission spectroscopy experiment. (c) UPS as a function of film thickness for a film

of N,N’-bis-(1-naphthyl)-N,N’-diphenyl1-1,1-biphenyl1-4,4’-diamine (α-NPD)

deposited on Au. The bottom spectrum corresponds to the clean Au substrate and

provides the position of the Fermi level (EF). The two vertical lines show the position of EF

and the onset of the HOMO. Their energy separation gives the barrier for hole injection

from the metal into the molecular film φh (shown schematically in Fig. 2). The photon

energy is 82 eV (synchrotron radiation source).

July/August 2005 33

generally remains difficult because of multiple contributions

to the width of the spectroscopic peaks of the frontier orbital

states, i.e. the HOMO and LUMO, from static and dynamic

disorder, incomplete screening on the time scale of the

photoemission process, the evaluation techniques, and

experimental resolution. Thus, in spite of the considerable

progress achieved so far, there is still a need to improve

measurement methodology and data analyses. The

comparison of experimental spectra with theoretical

calculations of filled and empty states16 remains a valuable

method to achieve a better understanding and a more

accurate determination of state energies.

In contrast to the case of ‘thick’ film systems, there is still

considerable uncertainty in correlating transport across

junctions involving single molecules or molecular monolayers

with the electronic structure of the molecule and interfaces.

While it is often assumed that tunneling is the dominant

mechanism, this is not obvious in all cases17. The type of

tunneling that should be used to describe the transport,

e.g. ‘through-bond’ or ‘through-space’, also remains to be

determined for many systems3. Evidence is accumulating that

these crucial details depend not only on the specific system,

but also on the way the contacts are prepared18-21,

cleanliness of the substrate surface, purity of the molecular

materials, and, in the case of monolayers, on the quality of

the layer. It is becoming clear that the interaction between

the molecule(s) and the substrate (be it metal or

semiconductor) can be a dominant factor and control

transport across the interface22-24. Fig. 3 illustrates this for a

metal/molecules/semiconductor system. It shows

schematically how breaking a chemical bond between

molecule and electrode changes the potential profile across

the molecule. This acute sensitivity to the nature of the bond

has been used to prepare molecule-controlled, room-

temperature negative differential resistance devices25,26.

Because widespread awareness of the importance of

molecule-electrode bonds is relatively recent, comparisons

between early transport data and those transport data with

photoemission data is often problematic.

As mentioned above, the combination of UPS and IPES

provides a measure of Et for a material. Et is the minimum

energy of formation of a separated, uncorrelated free

electron and hole, and is associated with the transport of

single particles in the solid (Fig. 4). The optical gap Eopt, on

the other hand, corresponds to the onset of optical

absorption and formation of a bound electron-hole pair, or

exciton (Fig. 4). Because of the strong localization of carriers

in molecular solids (see below), the exciton is generally an

on-molecule excitation (Frenkel exciton) or an excitation

shared by neighboring molecules (charge-transfer exciton).

The Coulomb attraction between the paired electron and hole

is large, and the binding energy of the exciton EB is

significantly larger than in inorganic semiconductors such as

Si, GaAs, or ZnO. The consensus from experimental results

obtained over the past few years with various techniques, as

well as from theory10,11,27,28, is that EB = 0.5-1.5 eV in

molecular solids and 0.2-0.5 eV in polymers29,30. It is

typically ~10 meV in inorganic solids. As EB is the energy

REVIEW FEATURE

July/August 200534

Molecular film

Metal

IE

Evac

HOMO

LUMO

∆

EF

EA

φe

φh

Evac

Et

φM

Interfacialdipole

Fig. 2 Energy diagram of an interface between a molecular film and a metal surface. The

ionization energy IE, electron affinity EA, and transport gap Et of the organic film are

defined. The electron and hole injection barriers φe and φh correspond to the energy

difference between EF and LUMO and HOMO, respectively. The effect of an interfacial

dipole layer with energy offset ∆ is shown as well.

Ev

Ec

HOMO

LUMO

Evac

EF

metal

semi-conductor

metalEv

Ec

HOMO

LUMO

Evac

EF

semi-conductor

-1 -0.5 0 0.5 1-1 -0.5 0 0.5 1

Cu

rren

t (a

.u.)

Cu

rren

t (a

.u.)

Voltage[V]

Chemicallybound

Physical contactonly

EAφM

φMEA

Fig. 3 Current-voltage characteristics (left) showing the impact of a chemical bond at the

contact between molecule and electrode. The band diagrams on the right correspond to

(top) a molecule chemically bound to the metal electrode, and (bottom) a molecule with

physical contact only. The current-voltage curves refer to systems described in25,26.

REVIEW FEATURE

necessary to break the pair to create an uncorrelated electron

and hole on separate molecules, Et is larger than Eopt by a

significant amount. The difference between transport and

optical gaps could be similar for molecular monolayers and

single molecules, although the proximity of the substrate and

the absence of neighboring molecules in the latter case are

expected to affect polarization and the transport gap31.

Especially in the case of chemical bonding, the monolayer

molecules may have considerably different electronic

properties than those in the ‘thick’ molecular film. Moreover,

an optical gap may be difficult to determine in the first or

second molecular layer as a result of very effective optical

quenching32. At this point, however, there are insufficient

reliable data to draw definite conclusions on this issue.

Throughout this overview and in all contemporary

descriptions of molecule/contact interfaces, it is assumed

that electronic equilibrium is achieved – an assumption that

is questionable for thicker molecular films. The issue of

electronic equilibrium between molecules and electrodes is

especially relevant when doing electron spectroscopy, e.g.

UPS, IPES, or related techniques mentioned below, on

molecular monolayers and films. Electron spectroscopies

involve charge flow that may lead, under some conditions, to

significant charging on the time scale of the experiment33-35,

particularly on fairly insulating materials like nonconjugated

organic molecules and in thick films. These conditions must

be recognized to prevent significant errors in the evaluation

of the interface energetics. Recent work on dipolar layers

shows that equilibrium is generally achieved when doing

electron spectroscopy on single layers of conjugated

(π-bonded) molecules36, but the situation may well be

different for layers of saturated (σ-bonded) molecules.

Other spectroscopic techniques

Further techniques include two-photon photoemission

(2PPE)37,38 and near-edge X-ray absorption fine structure

(NEXAFS or XANES), which have proven to be powerful tools

for investigating energetics of organic molecular interfaces.

2PPE provides information on Et or Eopt of the molecular

material, depending on whether the first of the two

excitations is inter- or intramolecular, respectively39,40. In the

former case, the hole state is measured by a direct two-

photon excitation of an electron from the HOMO to the

vacuum, while the electron state is measured by first

populating the LUMO with an external electron, e.g. from the

metal substrate, followed by photoemission from the LUMO.

Very good agreement with Et measured by UPS/IPES has been

demonstrated41. In the latter case, the first photon excites an

electron from the HOMO to the LUMO, forming an exciton

on the neutral molecule and populating the empty state

across Eopt, and the second photon breaks the electron-hole

pair. 2PPE is therefore a very powerful and versatile tool for

investigating the energetics of metal/molecule systems. If

performed in the time-resolved mode, 2PPE becomes a

powerful tool for probing the dynamics of charge exchange

and excitation in organic systems40.

NEXAFS involves the excitation of an electron from a core

level into unoccupied orbitals. Hence, it not only provides

information about the chemical state of the molecule and the

participation of previously unoccupied orbitals in the

intermolecular or interface bonding42, but also enables the

exact determination of the LUMO energy. Moreover, it allows

determination of the molecular orientation43,44. It is also a

local probe that yields information about the spatial

distribution of unoccupied orbitals within the molecule. The

availability of third-generation synchrotron sources has

improved the energy resolution to 20-50 meV for the edges

of light elements. This now allows the study of electron-

vibronic coupling45, intermolecular interaction, and potential

anharmonicities46. Because NEXAFS has much higher

resolution (one order of magnitude) and much higher

sensitivity (two to four orders of magnitude; also causing less

July/August 2005 35

Gas phase

I

A

P-

P+

−pE

+pE

vacuumE

Condensed phase

Molecular ion

LUMO

HOMO

Et = I - A – (P+ + P-) Eopt

Neutralmolecule

Optical absorption

+ + -

-+

+

-

-

-

-

--

--

+

+

+

+

+

+

-+ Optical absorption

Single electron

Single hole

(a)

(b) (c)

Fig. 4 (a) Schematic of a molecular film with a single hole (upper left) and a single

electron (lower right) surrounded by the polarization they induce in neighboring

molecules. Also shown is an optically excited on-molecule exciton (upper right).

(b) Energy levels of the single electron and single hole in the isolated gas-phase molecule

and the condensed-phase molecule. The energy levels of the latter are shifted with

respect to those of the former by the polarization (P-, P+) induced by the charge. The

electron and hole levels define the transport gap Et. (c) Optical gap Eopt of the organic

material, defined as the energy necessary to create an on-molecule exciton.

radiation damage) than IPES, NEXAFS seems preferable for

the determination of unoccupied orbitals. Yet, the unclear

influence of the core hole makes it difficult to reach

unambiguous conclusions on the energetics.

Contact potential difference measurements

This method, which uses a Kelvin probe, provides a totally

noninvasive means of investigating energetics of surfaces and

incipient interfaces47. The technique gives a precise measure

of the work function of the surface under investigation,

relative to the work function of the probe. For a given

surface, changes in work function and, in the case of

semiconductors, of electron affinity, ionization potential, and

band bending, can be followed as a function of surface

treatment, e.g. molecule adsorption, in various ambient

atmospheres or ultrahigh vacuum (UHV). The latter provides

the necessary link with photoemission data, which, by the

nature of the method, reflects the situation in vacuum.

Comparison between measurements in ambient and UHV

leads to an assessment of the effects of ambient exposure on

the contacts. This is of considerable importance for two

critical issues of molecular devices, i.e. their fabrication

environment and stability.

Spectroscopies based on scanning probe techniques

Methods like scanning tunneling spectroscopy (STS)31,48,49

add considerable insight by measuring filled and empty

transport levels with molecular spatial resolution. The

techniques can be seen as the energy spectroscopy

complement to contact-mode atomic force or scanning

tunneling microscopy for measuring current transport

through single (or small numbers of) molecules50,51. Because

other methods provide an average over large ensembles of

molecules, comparing the results with STS measurements

provides crucial information on inhomogeneities at interfaces

(see below). An alternative approach to gauge the

homogeneity of the interface relies only on scanning probe

measurements by collecting and comparing many single-

molecule or small-domain experimental results22.

A difficulty inherent to scanning tunneling techniques,

especially STS, is in understanding the degree to which the

high electric field developed between the tip and the surface

(or molecule) drops mostly in the physical gap or modifies

the energy levels of the molecule or molecular film. This issue

is not restricted to molecular solids, and band dragging is a

well known phenomenon when performing STS to measure

inorganic semiconductor band gaps48. STS has been

performed on single molecular layers and has produced

results in good agreement with UPS/IPES techniques31. For

thicker films, STS with penetration of the tip into the film has

been performed successfully for molecular materials and

polymers29,49, although the invasive tip-molecule contact

raises some questions about the chemical interaction

between tip and molecules and the specific energy levels

probed by the technique.

Finally, a frequent difficulty with these and most other

types of measurements, especially for thicker films, is that

the interface relevant for current transport is buried below a

layer that is thicker than the probing depth of the technique.

A recent study52 of a metal/polymer interface via ballistic

electron emission microscopy (BEEM) is a first attempt to

probe the buried interface with spatial resolution. So far,

however, methodologies involving measurements as a

function of increasing film thickness have proven to be the

most practical way to circumvent the problem. While the

buried interface is less of a problem for monolayers, making

measurements as a function of partial monolayer coverage

can yield important information on the evolution from the

free contact surface to the interface of interest53.

Theory and experimentThe comparison between experimental and theoretical results

on interface electronic structures plays a crucial role in the

process of understanding contact/molecule interfaces. The

fundamental difficulty here is that electrons in molecules are

described using localized electron models, while electrons in

the metal or semiconductor contact are described using a

delocalized picture with periodic boundary conditions.

Combining these two approaches poses a challenge for

achieving a consistent, unified energy level model for

interfaces. Current methods use one of the two descriptions:

the localized54 or delocalized approach55. The latter is

problematic for single molecule work because of the periodic

boundary condition. The former requires the use of large

clusters to obtain an electron density at the Fermi level and

describe the metal contact correctly. In both cases, the

problem is one that can be overcome with enough

computational power or the development of fast algorithms.

Relevant energy levelsOne of the most important, and so far unresolved, issues in

single-molecule transport is to identify the specific energy

REVIEW FEATURE

July/August 200536

REVIEW FEATURE

levels that are involved in the process. All the research efforts

described above are aimed at providing the ‘right’ energy

levels/bands for defining barriers to electronic charge carrier

injection and transport. This task is easiest if the molecules

are inert toward the contacting material, i.e. the only

interactions are of the van der Waals type. The problem is

more complex when stronger chemical interactions take

place, even if electronic equilibrium (see above) is assumed.

The choice of relevant energy levels for injection and

transport in a macroscopic inorganic metal/semiconductor

system is clear. These levels include the metal Fermi level and

the top of the valence band (for holes) or bottom of the

conduction band (for electrons) of the semiconductor. For a

thick organic semiconductor layer, injection and transport

involve carriers (polarons) that thermalize to the lowest-

energy transport states. The valence and conduction band

edges of the inorganic semiconductor are replaced by the

transport states, i.e. the HOMO and LUMO levels of the

molecule, measured as described above. Good correlation

between interface energetics and injection and transport data

is generally obtained in these cases15. In metal/single

molecule or molecular monolayer/metal systems, however,

questions remain concerning the transport mechanism(s), the

electron-molecule interaction, and the time scale involved in

the process. The nature and energy of the molecular orbitals

involved in injection and transport are not known a priori. In

the Au-S-alkyl chain-S-Au system, the highest occupied alkyl

orbital is more than 3 eV below the Au Fermi level. If the

transport experiments are interpreted in terms of carrier

tunneling through a square symmetric barrier, the height of

the barrier is, at most, 1.5-2 eV (assuming an effective

electron mass of 1)56. The only occupied level that might

fit this picture is the one localized on the S at the very

interface with Au57,58. However, if the tunneling barrier is

determined by this molecular level, the mode of transport of

the electron through the rest of the molecules remains

unclear.

This very important example shows the fundamental

problem faced in predicting the barrier for charge carrier

transport between molecules and contacts. Resolving these

basic issues also requires a consistent understanding of both

the occupied and unoccupied energy levels at the interface,

as well as variations in electrical potential across the

interface. Without this, distinguishing between nonresonant

(through-space) and resonant (through-bond) tunneling

becomes very difficult. Finally, thermionic emission can also

become important with semiconducting contacts13,59.

Interacting versus noninteractingcontacts The contacts that are easiest to describe, and whose

properties are most reliably predicted, are those between a

substrate and a noninteracting molecule. In metal/organic

film interfaces, where the metal-molecule interaction is not

dominated by a strong chemical reaction, progress toward

predicting interface energetics and injection barriers has used

models related to interface induced density of states60. The

physical proximity of the metal and organic semiconductor

broadens the molecular energy levels and results in a quasi-

continuum of states in the band gap of the organic film in

the first molecular layer or two, i.e. the induced density of

interface states. The relative position of the contact Fermi

level and charge neutrality level of this induced density of

states defines charge transfer across the interface and the

interface energetics. These models, in accord with

experimental results61,62, clearly point out that metal/organic

interfaces generally do not follow the Mott-Schottky model63

and that, as is found for their inorganic counterparts, their

electronic structure is dominated by charge exchange

between the contact material and interface states. Exceptions

to the departure from the Mott-Schottky model have been

seen with polymer/metal interfaces. However, these systems

are generally fabricated ex situ from a liquid phase, where the

metal surface is fundamentally changed with respect to the

intrinsic surfaces considered above.

As is well known from ‘classical’ metal-semiconductor

contacts, reactive contacts deviate from behavior predicted

on the basis of standard models and energy levels/bands of

the isolated components. Electronic states resulting from the

interface chemistry generally dominate the energetics64. The

same is true for contacts to molecules65. In the latter case,

the energetics even depend on the chemical bonding, the

molecular orientation, and co-adsorbates. The resulting

interface dipole, which essentially determines the band

offsets, may vary between -1 eV and +1 eV or more, even for

the same molecule and substrate in different adsorbate

bonds66. In molecular electronics involving nonconjugated

molecules, the issue is complicated by the finding that

considerably more efficient charge transport is found if the

interface is reactive, i.e. if the molecule and substrate form a

July/August 2005 37

chemical bond, than if it is chemically nonreactive (see

Fig. 3)3. This means that the most attractive molecular

electronics systems might be those whose behavior cannot

be predicted from knowledge of the isolated components,

emphasizing the crucial importance of interface studies.

If the polar character of the substrate-molecule bond can

be estimated, then a model of interface dipoles67 that

reproduces the behavior of a wide range of ‘classical’ metal-

semiconductor contacts is likely to be another useful step.

These dipoles are important for the energy level alignment at

the interface (see Fig. 2). Another factor that needs to be

considered in constructing an energy level diagram is that

bonding at the interface leads to the formation of new

molecule/substrate states. This has been well known in

surface science for decades, and can be described in the

simplest way by the Andersen-Newns (molecular orbital

interactions) model, yielding energy levels different from

those of the components. The occurrence of such states is

quite common for π-bonded aromatic molecules with

functional groups, even though bonding may preferentially

involve frontier orbitals located on the aromatic part68. The

new interface orbitals may insert gap states that facilitate

charge carrier tunneling by introducing a chemisorbed

molecular monolayer between the substrate and the

transport layer21. Thus, one can argue that only knowledge of

the exact chemistry and structure (molecular orientation and,

for monolayers, lateral order) of the interface will allow a

true description of its electronic structure.

Single molecule versus monolayerIn addition to the interface mechanisms mentioned above,

the energetics of the contact between a given substrate and a

molecule will be affected in two major ways by the presence

of neighboring molecules surrounding the central molecule.

The first effect is the stabilization (lowering) of the energy

level of the injected charge because of electronic polarization

of the metallic contact (i.e. by the image charge) and of

adjacent molecules, especially if they are π-bonded69. The

electron density in the metal and the molecular layer

responds very rapidly to the presence of the excess charge

and ‘screens’ it to some extent. In the bulk of the molecular

film, this screening, or polarization, accounts for the >1 eV

difference between the ionization energy (or electron

affinity) of a single gas-phase molecule and the molecule in

the condensed phase. The screening by the image charge is

even larger70. On the metal surface, the difference in

polarization between a single molecule and a molecule in a

saturated, packed monolayer can be as large as a few

hundred millielectronvolts31,71-73, depending on the molecule

polarizability and packing structure. In the case of

chemisorption, there may be an effective charge exchange

between metal and molecule after creation of a charge in the

chemisorbed molecule. This exchange leads to charge

delocalization and, as a result, the difference between an

ionized molecule in the gas and chemisorbed phases may

amount to a few electronvolts. The second effect, which is

crucial in layers of intrinsically dipolar molecules or

chemisorbates involving considerable charge transfer, is

depolarization. When dipolar molecules pack closely on a

surface, dipole-dipole repulsion causes a change in charge

distribution within the molecules or between the molecules

and the substrate. The former is well known and has been

described by Topping74,75. Molecular depolarization, brought

about by molecular distortion, has been observed for dipolar

silanes on SiOx/Si76,77. Depolarization via charge transfer

between molecules and a metal substrate was invoked to

explain unusual giant magnetism in normally diamagnetic

molecules of a normally diamagnetic substrate78.

Another effect altogether has been suggested to explain

the much higher current densities obtained with single

molecules than with monolayers of identical molecules. The

lower energies of vibrational modes of single molecules

versus a molecular ensemble, presumably because of steric

hindrance, make these modes more readily accessible for

charge transport via hopping79. It is likely that the strength of

bonding to the substrate plays an important role in both the

ability to have hopping transport and molecular distortion, an

issue that remains to be explored.

Interface polarization and equilibriumThe importance of molecular and interface dipoles has

already been noted. Here, we briefly dwell on the issue of

interface polarization and how it affects interface energetics.

The concept of interface dipoles can explain how molecules

equilibrate electronically with the contacting material.

Predicting interface dipoles requires understanding the cause

of their formation and origin. The simplest approach is

intuitive and based on chemistry and electronegativity

differences. Both atomic and group electronegativities can be

used. We can again consider the much-studied Au-thiol

REVIEW FEATURE

July/August 200538

REVIEW FEATURE

systems to see the main problem with this approach. Several

electronegativity scales are available and it is not obvious

a priori which one to use. On each side of the Au-S bond are

a molecular system (ending with the S atom) and a metallic

one (Au). If we use the Phillips-van Vechten scale80, derived

for nonmolecular systems, the bond is found to be quite

polar with S negative compared to Au. The same holds if we

use the Allred-Rochow scale81. However, if we use the

Pauling scale80,81, which was derived for isolated molecules,

the bond is nonpolar – more in line with the interpretation of

experimental data82,83. Alternatively, we can turn to

quantum mechanics, foregoing the convenience of intuition.

We can hope that, as theoretical descriptions of the

molecular (localized)/nonmolecular (delocalized electron)

interface develop, theory will provide easy-to-use predictive

tools beyond those based on electronegativity.

The critical role of interface polarization is driven home by

our recent results that show total inversion of free molecule

dipole moments as a result of molecule-metal contact

interaction (see Fig. 5)18,20. This phenomenon is most

remarkable, as it does not occur upon chemical binding of the

molecule to the substrate, but upon metal contact deposition

on the molecule, i.e. at a supposedly noninteracting interface.

Interface homogeneity There are generally two ways to approach this issue. One is

to start with ‘ideal conditions’, i.e. a highly ordered, single-

crystalline substrate surface; a highly ordered, well-

characterized interface layer; one or more epitaxially grown

homogeneous molecular layers; and a carefully prepared

second (top) contact, and then measure transport, optical, or

(opto)electronic properties. This approach is well-established

in inorganic semiconductor physics and surface science, but

very difficult to achieve in organic hybrid systems because of

the difficulties of preparing homogeneous epitaxial layers and

top contacts. Some of these steps have been achieved with

organic systems, i.e. the preparation of highly ordered

molecular interface (mono)layers on highly ordered substrate

surfaces84-87. Even the homogeneous and epitaxial growth of

an organic film on a metal substrate can be achieved in

specific cases88,89. The final step, i.e. deposition of

homogeneous top contacts onto the molecule(s) and

molecular films, without damaging the molecules for the

measurements of transport properties is still problematic. As

discussed elsewhere in this issue, several approaches are

being pursued19,90,91, but deposition on epitaxial films has, by

and large, not yet been realized. Nevertheless, studies of the

first steps have already shown how complicated, for instance,

interface preparation can be. For example, depending on

temperature, coverage, deposition rate, and co-adsorbate,

deposition of a molecule on a substrate can result in several

different geometric structures with significantly different

chemical bonding92 or molecular orientation93, resulting in

very different interface dipoles66.

The second approach starts from a ‘real’ system and

attempts to correlate its homogeneity with other properties.

Because most techniques for interface characterization

average over macroscopic dimensions, extracting information

on structural heterogeneity is problematic. Scanning probe

methods can alleviate the problem if the measurement does

not interfere with the molecular structure (e.g. if used in

noncontact mode). For inorganic metal/semiconductor

interfaces, the effect of inhomogeneity on electronic

transport across the interface has been studied thoroughly

and fairly accurate models have been developed94,95. For

molecular contacts, the problem is exacerbated by the

limited purity of many of the compounds normally used in

research96. The degree of homogeneity is likely influenced by

the strength of binding of the molecules to the surface. Thus,

Bias [V]0.2 0.3 0.4 0.5 0.6 0.7

0

100

200

300

400

500

Cu

rren

t [

Me H

CN

bare

CN > H > Me

Bias [V]0.2 0.3 0.4 0.5 0.6 0.70.2 0.3 0.4 0.5 0.6 0.7

0

100

200

300

400

500

0

100

200

300

400

500

Cu

rren

t [C

urr

ent [

Me H

CN

bare

CN > H > Me

Cu

rren

t (µ

A)

CO2HHO2C

OOCC OO

XX

n-GaAs

Au

OO OO

X X

0 0.1 0.2 0.3 0.4 0.5

0

2

4

6

8

10

CN

H

Me

bare

CN < H < Me

Cu

rren

t (µ

A)

Bias [V]0 0.1 0.2 0.3 0.4 0.50 0.1 0.2 0.3 0.4 0.5

0

2

4

6

8

10

0

2

4

6

8

10

CN

H

Me

bare

CN < H < Me

Cu

rren

t (µ

A)

Bias [V]

∆EA = EA - ∆φmol.dipole

∆EA = EA + ∆φmol.dipole

∆φb = M −∆EAsemiconductor

B

A

φ

Fig. 5 Effect of the nature of the molecule-metal electrode contact on current-voltage

characteristic of Au/molecules/n-GaAs diodes (molecule shown at top left; device

schematic at bottom left). The molecules, which are chemically bound to the GaAs, are

changed systematically. ∆φb is the change in Schottky diode barrier height. ∆φmol.dipole is

the change in electrostatic potential as a result of the interfacial molecular dipole layer.

All other symbols are as described in Fig. 2. In panel (A), the change in EA is determined by

the free dipole moment, as explained elsewhere33,103. In panel B, the effect is inverted

because of the interaction between the molecules and the metal spill-over electron

density18,90, which is the result of intimate contact between metal and molecules.

July/August 2005 39

very strong bonding (e.g. phosphates to oxides) can leave

many molecules in nonequilibrium binding configurations,

while some weakly binding molecules (sulfides to most noble

metals) may not bind at all or only intermittently97. In both

cases, a variety of substrate-molecule orientations may

result, rather than the preferred one. The strength of

intermolecular interaction is important as it can force even

weakly binding molecules to order on a surface53,98,99.

Most molecular films include structural inhomogeneities

that have an impact on their electronic structure. Interface

inhomogenities can be a result of evolution of orientation in

the first two or three molecular layers closest to a metal

surface. The change in molecular orientation, from flat on the

metal surface to more vertical in the bulk, can give rise to

shifts of the frontier electronic levels by 100-200 meV, as

demonstrated by UPS100. These shifts can affect the

interpretation of spectroscopic measurements used to

determine interface energetics, as well as the charge injection

characteristics of the interface, in ways that have not yet

been fully evaluated. Another example concerns structural

defects, which result from growth constraints and can be

found in highly ordered molecular films101. These defects

affect charge carrier transport through ordered molecular

films. They also give rise to new electronic states, which can

extend into the band gap of the pristine material102.

Conclusions and summary While our understanding of metal/bulk organic interfaces

with either strong or weak chemical interaction allows clear

correlation of charge injection and transport with measured

interface energetics, the same cannot be said for single-

molecule or monomolecular film junctions. For these

interfaces, the presence or absence of charge transfer and

chemical bond formation matters considerably when looking

at interface transport. At the same time, single-molecule and

monomolecular layer interfaces, in particular those involving

strong chemical bonds, also appear to be the most interesting

for molecular electronics. Understanding their properties and

behavior will require work in both theory and experiment,

preferably in a coordinated manner.

The most exciting directions for theoretical work are in the

extension of existing concepts from inorganic systems to the

molecule/conductor interfaces, and toward bridging the

localized/delocalized electronic structure calculation gap. The

stronger the interactions at these molecule/substrate

interfaces, the more likely it is that knowledge of the

chemistry and structure of the molecular interface will be

necessary for accurate description of the electronic structure.

Unfortunately, though, this will come at the expense of

generalizing results to wider classes of interfaces.

On the experimental side, more of the reproducible, high-

quality data that are starting to be produced are needed. The

data are necessary to allow controlled comparisons between

systems prepared and measured in UHV and in ambient, as

well as to compare spatially averaged with spatially resolved

results. The latter will become progressively more important

as dimensions shrink and the heterogeneous nature of these

interfaces are more strongly expressed in the results.

In all cases, the critical issues for molecular electronic

devices remain the understanding and prediction of the

energy barriers for charge carrier injection, the homogeneity

of the barriers across the systems, and the development of

accurate theoretical tools to investigate these barriers. MT

AcknowledgmentsThis article is based to a large degree on what we learned from organizing, participating in,

and summarizing the 306th W. E. Heraeus Seminar on ‘Energetics of Interfaces between

Organic Molecules and Conductors’, held in March 2003 in Bad Honnef, Germany. We

thank the W. E. Heraeus Foundation for making that seminar possible and all the

participants for their contributions. DC thanks the G. M. J. Schmidt Minerva Centre for

Supramolecular Chemistry and the Philip M. Klutznick Research Fund at the Weizmann

Institute, and the Israel Science Foundation (Jerusalem) for support. AK thanks the US

National Science Foundation (NSF), grant DMR-0408589, the New Jersey Center for

Organic Optoelectronics, and the Princeton MRSEC of the NSF.

REVIEW FEATURE

July/August 200540

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