A quantitative method, using temperature-controlled friction force microscopy (FFM), has been developed to determine the frictional (dissipative) character of thin polymer films. With this method variations in friction are sampled over micrometer-scale regions and are reduced to “friction histograms,” yielding the distribution of frictional forces on the surface. The temperature dependence of the mean value of the frictional distribution is correlated to the known glass-to-rubber transition (T g) and/or secondary relaxation mechanisms in films of poly(methyl methacrylate) (PMMA), poly(ethylene terephthalate) (PET), and polystyrene (PS). The dominant contribution to friction, on polymer films, was attributed to viscoelastic mechanical loss. Using equivalent time scales, measured T g's were lower than bulk polymer values. The frictional response of PMMA displayed time−temperature equivalence upon variation of scan-velocity and temperature. The rate dependence of the hindered rotation of the −COOCH3 group (β relaxation) in PMMA was consistent with Arrhenius type behavior, allowing calculation of an activation energy. The activation energy of the thin film was found to be lower than measured bulk energies.

The properties of organic‐semiconductor/insulator (O/I) interfaces are critically important to the operation of organic thin‐film transistors (OTFTs) currently being developed for printed flexible electronics. Here we report striking observations of structural defects and correlated electrostatic‐potential variations at the interface between the benchmark organic semiconductor pentacene and a common insulator, silicon dioxide. Using an unconventional mode of lateral force microscopy, we generate high‐contrast images of the grain‐boundary (GB) network in the first pentacene monolayer. Concurrent imaging by Kelvin probe force microscopy reveals localized surface‐potential wells at the GBs, indicating that GBs will serve as charge‐carrier (hole) traps. Scanning probe microscopy and chemical etching also demonstrate that slightly thicker pentacene films have domains with high line‐dislocation densities. These domains produce significant changes in surface potential across the film. The correlation of structural and electrostatic complexity at O/I interfaces has important implications for understanding electrical transport in OTFTs and for defining strategies to improve device performance.

Thin films of colloidal PbSe quantum dots can exhibit very high carrier mobilities when the surface ligands are removed or replaced by small molecules, such as hydrazine. Charge transport in such films is governed by the electronic exchange coupling energy (beta) between quantum dots. Here we show that two-dimensional quantum dot arrays assembled on a surface provide a powerful system for studying this electronic coupling. We combine optical spectroscopy with atomic force microscopy to examine the chemical, structural, and electronic changes that occur when a submonolayer of PbSe QDs is exposed to hydrazine. We find that this treatment leads to strong and tunable electronic coupling, with the beta value as large as 13 meV, which is 1 order of magnitude greater than that previously achieved in 3D QD solids with the same chemical treatment. We attribute this much enhanced electronic coupling to reduced geometric frustration in 2D films. The strongly coupled quantum dot assemblies serve as both charge and energy sinks. The existence of such coupling has serious implications for electronic devices, such as photovoltaic cells, that utilize quantum dots.

Strontium titanate (SrTiO3) is a foundational material in the emerging field of complex oxide electronics. Although its bulk electronic and optical properties are rich and have been studied for decades, SrTiO3 has recently become a renewed focus of materials research catalysed in part by the discovery of superconductivity and magnetism at interfaces between SrTiO3 and other non-magnetic oxides. Here we illustrate a new aspect to the phenomenology of magnetism in SrTiO3 by reporting the observation of an optically induced and persistent magnetization in slightly oxygen-deficient bulk SrTiO3-δ crystals using magnetic circular dichroism (MCD) spectroscopy and SQUID magnetometry. This zero-field magnetization appears below ~18 K, persists for hours below 10 K, and is tunable by means of the polarization and wavelength of sub-bandgap (400-500 nm) light. These effects occur only in crystals containing oxygen vacancies, revealing a detailed interplay between magnetism, lattice defects, and light in an archetypal complex oxide material.

Polycrystalline organic semiconductor films play a central role in organic electronics because their inherent order, relative to amorphous films, facilitates more efficient charge transport. Carrier mobilities in crystalline organic semiconductors are generally at least a factor of one hundred greater than in their amorphous counterparts, which is attractive for certain device applications, such as organic field effect transistors (OFETs), where higher charge mobilities result in better performance. [1][2][3][4][5] In analogy with conventional semiconductors (e.g., poly-Si), the electrical performance of polycrystalline organic semiconductor layers is sensitive to grain morphology and alignment, as well as to defects. [6][7][8][9][10][11] Indeed, recognition of the importance of microstructure has lead to extensive structural characterization of organic semiconductor films by X-ray diffraction, [12,13] and optical, [14] electron, [15][16][17][18] and scanning probe microscopy. [19,20] 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 microstructureproperty 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 OFETs.Here, we demonstrate that a novel scanning probe microscopy method, which we term Transverse Shear Microscopy (TSM), produces striking, high contrast images of grain size, shape, and orientation in films of polycrystalline organic materials. The ability to image grain orientation is a key feature of TSM and the resulting Grain Orientation Maps substantially enhance the possibilities for quantitative analysis of microstructure. For the ultrathin (1-2 nm) organic films we describe here, the grain orientation and shape recorded in the TSM images are difficult to visualize by any other microscopy method. Furthermore, by combining shear deformation experiments with theoretical analysis, we show that the mechanism of TSM orientation contrast originates from the intrinsic elastic anisotropy within individual grains. Thus, TSM has intriguing potential as a broadly applicable method for quantitative microstructure analysis, not only for organic semiconductors, but also for any suitably soft, crystalline material with a tensor modulus in the image plane. Our results substantially expand on an earlier report of TSM imaging, [19] in which we demonstrated orientation dependent contrast but did not analyze the film microstructure nor identify the imaging mechanism. In TSM, depicted in Figure 1A, the scanning direction of a force microscope probe tip is parallel to the cantilever axis, and the lateral deflection or twist of the cantilever is recorded. This mode of operation differs from the better-known lateral force microscopy (LFM) technique in one respect only, namely that in LFM the scanning ...