AmericanChemicalSociety.com

Organic/inorganic semiconductor interfaces have an important role in emerging electronic materials for application in organic light emitters and in organic photovoltaic solar energy harvesting.  Despite much work on these devices, there are many challenges that must be surmounted, most importantly the need to understand and control the electronic properties.  The creation of novel interfaces, including those based on new molecular-scale structures or new materials, leads to the opportunity to bring new functions to organic devices.  We have used scanning tunneling microscopy techniques to probe how organic/inorganic semiconductor interfaces are formed and to characterize their structure and electronic properties.  We have made significant progress in understanding the structure and electronic properties of a prototypical organic semiconductor/inorganic semiconductor interface consisting of a junction between pentacene and the nitrobenezene-terminated surface of silicon.  This interface is an example of the precise structures that can now be created in organic/inorganic systems, chosen because it is an excellent demonstration of the role of an interface molecule in determining the electronic properties of the junction.  Section 1 below describes a test of theoretical predictions of the structure of this interface a study of its electronic properties. Section 2 describes the use of similar organic thin film growth techniques to produce thin films of an organic ferroelectric material, which will allow us to expand the range of properties that can be used to control the flow of charge at these interfaces.

1. Structure and electronic properties of the pentacene-nitrobenzene-silicon interface

The properties of organic/inorganic interfaces depend sensitively on their composition and structure.  In order to create a prototypical interface from the ground up, we have developed an interface between the organic semiconductor pentacene and silicon in which a monolayer of the molecule nitrobenzene is located at the interface.  By doing this we have created a system in which the effects of the electronic properties of the intervening molecular layer can be precisely studied.  The process of creating this interface begins with the clean (001)-oriented silicon surface, which has an extremely well defined composition and structure, as shown in the ultrahigh vacuum scanning tunneling microscopy (STM) image in Fig. 1(a).  The surface is then exposed to nitrobenzene under ultrahigh vacuum conditions which forms a single-layer of reacted molecules.  A simultaneous theoretical study of the structure of nitrobenzene molecules attached to Si was conducted in collaboration with the research group of Prof. Manos Mavrikakis (UW-Madison).  By comparing theory and experiment, we have found that the disordered structure evident in the nitrobenzene/Si surface in Fig. 1(b) results from an incomplete conversion of the system to the lowest-energy configuration from the state reached by the initial attachment of the molecules.

Fig. 1 Formation of the Si (001)/nitrobenzene interface.  (a) STM image of the reconstructed Si (001) 2 x 1 surface starting point.  (b) STM image of the surface after attachment of the nitrobenzene molecular layer.

A molecular pentacene/nitrobenzene/Si junction is then created by depositing pentacene onto nitrobenzene-terminated Si surface.  The chemical attachment of nitrobenzene to the surface leads to an unreactive interface on which pentacene forms large smooth crystals, as shown in Fig. 2(a).

Fig. 2 (a) STM image of a the surface of a pentacene thin film grown on the nitrobenzene/Si surface.  (b) Scanning tunneling spectroscopy measurement of the normalized differential conductance dI/dV/(I/V) of the pentacene/nitrobenzene/Si junction, measured using the current flowing through the junction (I) and voltage (V) applied across it..

The electronic properties of the pentacene/nitrobenzene/silicon system can be probed using current voltage characteristics measured using scanning tunneling spectroscopy (STS).  The results of an STS measurement of the derivative of the current with respect to the current are shown in Fig. 2(b).  The derivative is normalized with respect to the total current in order to allow features at higher voltages to be seen.  The strong peak in the derivative of the current at -1.6 V in Fig. 2(b) results from negative differential resistance in this structure.  From this effect, we know that the nitrobenzene layer between the pentacene and silicon impedes the flow of charge and leads to resonant tunneling between the tip and the interface.  The existence of a strong barrier between pentacene and nitrobenzene provides insight into the design of monolayers that will allow charge to flow easily between organic and inorganic phases.

2. Organic ferroelectric thin film growth

The wide range of properties available in organic molecular materials presents a wide range of opportunities to tailor the electronic properties of interfaces.  We have expanded the range of materials available as organic thin film materials by developing a process that allows us to deposit thin films of a molecular organic ferroelectric material.  Ferroelectrics have a large permanent electric dipole moment, termed the remnant polarization.  By including this dipole moment at an interface it will be possible to modify the alignment of electronic states across the interface or to induce a permanent charge density in a subsequently grown semiconducting layer.  We have grown thin films of a prototypical organic ferroelectric material consisting of a cocrystal of the molecules phenazine and chloranilic acid.  The layers we produce have x-ray diffraction patterns indicative of the formation of the cocrystal on both Ni and Au, and which are clearly distinct from the diffraction patterns of the components (Fig. 3).  The ability to grow these layers provides the basis for using permanent ferroelectric dipole moments as an additional degree of freedom in manipulating the electronic properties of organic/inorganic semiconductor interfaces.

Fig. 3 X-ray diffraction q-2q scans with Cu Ka radiation for thin films of (a) chloranilic acid (H₂ca), (b) phenazine (Phz), and a Phz-H₂ca cocrystal on (c) Au and (d) Ni bottom electrodes.  Angles associated with reflections of the reported structure of each material are represented as vertical lines with heights proportional to the intensity reported in powder diffraction studies.  Reflections for both polymorphs of phenazine are shown in (b).