ICMCTF2010 Session TS1: Experimental and Computational Studies of Molecular Materials and Thin Films

Driven by the recent success of using organic semiconductors as active materials for organic electronic applications detailed microscopic studies on structural and electronic properties of such soft materials have become a focus of scientific interest. Of particular interest are oligoacenes because of their ability to form crystalline phases which reveal remarkable high carrier mobilities and hence constitute well defined model systems. In view of the interrelation between intermolecular packing and electronic properties of such materials there is a fundamental interest in a precise control of the molecular packing and orientation in highly ordered thin films which is also of vital interest for an optimization of thin film devices such as organic field effect transistors (OFETs) where high charge carrier mobility is required.

In this talk I will discuss growth phenomena and structural properties of various organic semiconductor films prepared by molecular beam deposition under vacuum conditions onto different substrates surfaces. By combining various spectroscopic and diffraction techniques with microscopy the evolution of their resulting structure can be traced as a function of film thickness. It is found that the resulting molecular orientation and film morphology depend critically on the roughness and chemical termination of the substrate whereas growth rate and substrate temperature mainly affect the grain size. Films that were grown on differently pre-structured substrate surfaces indicate that the film morphology can be controlled to some extend (template effect) whereas the crystalline structure is usually not affected. Moreover, for many metal substrates a pronounced island growth occurs after completion of the first wetting layer or upon post deposition dewetting and result in non-contiguous films. Such dewetting phenomena can be effectively suppressed by first coating the substrate with self-assembled monolayers (SAMs). Possible driving forces for the appearance of the various film structures and strategies for a rational control of the microstructure of such organic films are discussed. Finally I will demonstrate that the concept of specific surface modifications can also be applied to the electrodes in bottom contact OFETs and causes largely improved device characteristics.

In organic transistors the interface between the semiconductor and dielectric is extremely important since it is where the vast majority of current flows. To optimize organic electronic devices it is critical to understand how to engineer the ideal dielectric surface. We show record charge carrier mobilities for more than 20 organic semiconductors on a crystalline octadecylsilane (OTS) monolayer surface. Mobilities as high as 5.0 and 3.0 cm2/Vs were achieved for C60 and pentacene transistors, respectively. [1,2] The semiconductor growth mode is the preferred 2D layer-by-layer growth on a crystalline dielectric modification self-assembled monolayer (SAM) whereas on an amorphous SAM the undesirable 3D growth is observed. 2D growth is preferred to 3D growth since less detrimental grain boundaries, which diminish current, are formed. SAM order and organic semiconductor growth mode and nucleation were critically investigated using atomic force microscopy (AFM), grazing incidence X-ray diffraction (GIXD) and Monte Carlo simulations. The interface-semiconductor interaction energy necessary to drive 2D growth was also calculated and important considerations about organic semiconductor nucleation density, stability, and thin film growth on OTS monolayer surfaces are discussed. [1-4]

[1] The Role of OTS Density on Pentacene and C60 Nucleation, Thin Film Growth, and Transistor Performance, Ajay Virkar, Stefan Mannsfeld, Joon Hak Oh, Michael F. Toney, Yih Horng Tan, Gang-yu Liu, J. Campbell Scott, Robert Miller, Zhenan Bao. Adv. Funct. Mater. 2009, 19, 1–9.

[2] Crystalline Ultra Smooth Self-Assembled Monolayers of Alkylsilanes for Organic Field-Effect Transistors, Yutaka Ito, Ajay Virkar, Stefan Mannsfeld, Joon Hak Oh, Michael Toney, and Zhenan Bao. J. Am. Chem. Soc., 2009, 131 (26), pp 9396–9404

[3] Organic Ajay A. Virkar, Stefan Mannsfeld, Zhenan Bao Semiconductor Growth and Morphology Considerations for Organic Thin Film Transistors. Ajay A. Virkar, Stefan Mannsfeld, Zhenan Bao and Natalie Stingelin. Submitted to Advanced Materials

[4] Energetics and Stability of Pentacene Thin Films on Amorphous and Crystalline Octadecylsilane Modified Surfaces. Ajay A. Virkar, Stefan Mannsfeld, and Zhenan Bao. Submitted to Journal of Materials Chemsitry – special issue on Interfaces in Organic and Molecular Electronics

In order to make low cost, large area and flexible organic electronics, solution deposition of organic semiconductors (OSC) is contending to become an important method of producing organic field effect transistors (OFETs). A recently developed solution shearing process has demonstrated better OFET performance compared to simple drop casting or spin casting methods.¹ In this method, OSC solution is sandwiched between two substrates, a non wetting ‘shearing substrate’ and a wetting ‘device substrate’ that is heated to a controlled temperature. When the OSC solution is sheared by translating the shearing substrate, semiconductor crystallization occurs on the bottom device substrate. However, this method produced variable films in terms of thickness and electrical performance of OFETs, ranging from non-performing devices to devices with charge carrier mobilities as high as 0.5 cm²V^-1s^-1.² We have fabricated a new machine that can control the gap between the two substrates to an accuracy of tens of microns, control the temperature of the solution from -25°C to 220°C, and control the tilt angle between substrates. We first used the small molecule trimethyl-[2,2’;5’,2”;5”,2”] quarter-thiophen-5-yl-silane (4T-TMS) to investigate the thin film formation. Our work explores the changes in thin film morphology due to controlled change in experimental conditions. Changing experimental parameters such as casting temperature, shearing speed, gap width and substrate tilt angles gives rise to different thin film morphologies, which causes differences in carrier charge transport properties. We have shown that there is a local maximum for charge carrier mobility as a function of both gap width and shearing rate. By maintaining control over many variables present during solution shearing and by possessing a theoretical understanding of important parameters, it is now possible to create large, uniform and highly oriented crystalline thin films (~5 cm²) that show better charge carrier transport properties compared to other solution deposition methods. OSCs with different packing motifs can be used to determine how thin film morphology changes due to crystalline packing. We are looking at molecules that have edge to face, slip stack and brick wall packing motifs. Finally, anisotropy in charge carrier transport as a direct effect of thin film morphology is investigated.

1. H. A. Becerril, M. E. Roberts, Z. Liu, J. Locklin, and Z. Bao, Adv. Mater., vol. 20, no. 13,pp. 2588–2594, Jul. 2008.
2. Z. Liu, H. A. Becerril, M. E. Roberts, Y. Nishi, and Z. Bao, IEEE, vol. 56, no. 2, pp. 176-185, Feb. 2009.

The atomistic modeling of this heterojunction is a computational challenge: A combination of Molecular Dynamics simulation technique and carefully vetted intermolecular models for C60 and Pn can be used to study surface diffusional studies of small C60 clusters on a Pn surface, but not the deposition of many C60 particles. Mesocale Kinetic Monte Carlo (KMC) would allow the study of a monolayer or so, but to conduct such simulations requires an extensive library of energy barriers obtained from static simulations that cover every deposition and diffusion event that can be envisioned. It also requires a new KMC algorithm to capture the multiple lattices involved. If electronic properties are desired, ab initio calculations are needed, but computational resource constraints mean that this can only be done for a couple of molecules from a nanoscale part of this structure.

Using a combination of Molecular Dynamics/MM3- intermolecular potential models, molecular statics, KMC and ab initio calculations, we will illustrate the effect of two different morphologies for C60-Pn (thin film and bulk) on diffusional characteristics, potential energy surfaces, thin film growth and the final structure of the interface. We will show that the heterostructure is strained and defective compared to the two bulk materials and illustrate the impact of structure on the electronic properties.

We use Density Functional Theory (DFT) calculations in the generalized gradient approximation (GGA) to predict the properties of a number of novel Ti-M-N and V-M-N thin films in the B1 (NaCl) structure. The new compounds are obtained by alloying TiN and VN, with Ta, Nb, V, Mo and W, respectively Nb and W, in concentrations of 50 %. We evaluate the elastic moduli and constants for all these ternaries, perform a detailed analysis of their electronic structure, and compare these results with the corresponding properties of TiN and Ti_0.5Al_0.5N. Our calculations show that, in terms of hardness, these ternaries compare with TiN and Ti Al N, as we obtain comparable, respectively increased values, for the Young and bulk moduli, in most cases. Significantly, however, these novel compounds exhibit substantially lower values of the C₄₄ elastic constant and positive Cauchy pressures, i.e. they are considerably more ductile than TiN and Ti_0.5Al_0.5N. This unique combination of increased hardness and ductility, which is in contrast to the hardness/brittleness relationship typically found in hard coatings, is certainly relevant for applications in which high strength thin films/coatings are desired. In terms of electronic structure, our results reveal a layered charge density for all these ternaries, consisting in alternating high and low electron density regions, similar to that reported for MAX phase materials and other nanolaminates. This combination of metallic and ceramic properties is also evident in the density of states analysis we report. In order to fully understand the mechanism responsible for this interleaved arrangement of electrons, we carry out an improved crystal orbital overlap population (COOP) calculation and succeed in resolving energetically the bonding and antibonding contributions, of the first and second neighbors, to the chemical bonds in these compounds. Herein, we present the results of our COOP analysis, and based on this, we explain the observed trend in hardness and ductility as a result of the interaction between the e_g and t_2g sets of d orbitals characteristic to these ternaries.