Vivek Kalihari
IIT Bombay - Bombay, India; 2005
B.S. Materials Science
Research: Study of Epitaxial Relations in Polycrystalline Organic Semiconductor Films by Transverse Shear Microscopy
Polycrystalline organic semiconductor films play a central role in organic electronics because their inherent order, relative to amorphous films, facilitates more efficient charge transport. In analogy with conventional semiconductors (e.g., poly-Si), the electrical performance of polycrystalline organic semiconducting layers is sensitive to grain morphology and alignment, as well as to defects. Indeed, recognition of the importance of microstructure has led to extensive structural characterization of organic semiconductor films by X-ray diffraction, and optical, electron, and scanning probe microscopy. Yet there are still many aspects of organic semiconductor microstructure that are not well understood and detailed correlations with transport are rare. One surprising bottleneck to understanding microstructure-property relationships has been the difficulty of producing clear images of grains in extremely thin, coalesced layers of organic semiconductors on technologically relevant substrates, such as gate dielectrics, which are critical components of organic field effect transistors (OFETs).
In my thesis work, I have been focusing on the development of a novel microscopy technique, which we termed Transverse Shear Microscopy (TSM), to produce striking, high contrast images of grain size, shape, and orientation in films of polycrystalline organic materials. The ability of image grain orientation is a key feature of TSM and the resulting Grain Orientation Maps substantially enhance the possibilities for quantitative analysis of microstructure. Using TSM, we have established for the first time the presence of both epitaxial and non-epitaxial domains in ultrathin polycrystalline layers of pentacene grown on SiO2. The microstructure of pentacene films is particularly important, as pentacene is a benchmark organic semiconductor for OFETs. Epitaxial domains in the second pentacene molecular layer exhibit unusual type-II coincidence with respect to the first pentacene monolayer, while the third and subsequent layers show commensurism with their respective underlayers. In addition, lateral force microscopy and Kelvin probe force microscopy reveal that the epitaxial domains have significantly less friction and more positive surface potential. Collectively, these findings open up exciting new possibilities for understanding the growth of crystalline organic semiconductor films and for correlating microstructure with electrical performance in devices.
Publications
"Observation of Unusual Homo-Epitaxy in Ultrathin Pentacene Films and Correlation with Surface Electrostatic Potential," V. Kalihari, D. J. Ellison, G. Haugstad, C. D. Frisbie, Adv. Mater. 2009 accepted
"Grain Orientation Mapping of Polycrystalline Organic Semiconductor Films by Transverse Shear Microscopy," V. Kalihari, E. B. Tadmor, G. Haugstad, C. D. Frisbie, Adv. Mater. 2008, 20, 4033-4039
"Tetracene Air-Gap Single Crystal Field-Effect Transistors," Y. Xia, V. Kalihari, C. D. Frisbie, N. K. Oh, J. A. Rogers, Appl. Phys. Lett. 2007, 90, 162106
"Magnetic Properties and the Investigation of Possible Interstitial Occupancy by Small Atoms in some R3Ni6X6 Compounds (R = rare earth; X = Si, Al)," V. Kalihari, S. K. Dhar, Journal of Magnetism and Magnetic Materials 2005, 294, 40-48