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Register a new userOn the analysis of island shape evolution from diffuse x-ray scattering of organic thin films and the implications for growth
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Understanding the growth of organic semi-conducting molecules with shape anisotropy is of high relevance to the processing of optoelectronic devices. This work provides insight into the growth of thin films of the prototypical rodlike organic semiconductor diindenoperylene on a microscopic level, by analyzing in detail the film morphology. We model our data, which were obtained by high-resolution grazing incidence small angle x-ray scattering (GISAXS), using a theoretical description from small angle scattering theory derived for simple liquids. Based on form factor calculations for different object types we determine how the island shapes change in the respective layers. Atomic force microscopy measurements approve our findings.
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X-ray diffraction studies of the bond-orientational order in the hexatic-B phase of 75OBC and 3(10)OBC compounds are presented. The temperature evolution of an angular profile of a single diffraction peak is analyzed. Close to the hexatic-B-smectic-A transition these profiles can be approximated by the Gaussian function. At lower temperatures in the hexatic-B phase the profiles are better fitted by the Voigt function. Theoretical analysis of the width of diffraction peaks in three-dimentional (3D) hexatics is performed on the basis of the effective Hamiltonian introduced by Aharony and Kardar. Theoretical estimations are in good agreement with the results of x-ray experiments.
Zinc oxide (ZnO) epitaxial thin films grown on c-sapphire substrates by pulsed laser deposition were investigated using angle and polarization-resolved photoluminescence spectroscopy. Side-emission spectra differed significantly from surface-emission spectra in exhibiting dominant, narrow, polarization-resolved peaks. These spectral features were attributed to leaky substrate modes in the layers. Observations were first verified using transmission calculations with non-adjustable parameters, which took into account the dispersion, the anisotropy of the ZnO refractive index and the dependence on film thickness. Results were consistent with Fabry-Perot-like interference being the origin of the distinctive ZnO luminescence observed at grazing incidence angles. A second analysis, based on the source terms method, was used in order to retrieve the bulk emission properties, including the wavelength-dependent quantum yield and the emission anisotropy. While ZnO thin films were considered here, this analysis method can be extended to any luminescent thin film of similar geometry, demonstrating the potential of leaky mode analysis for probing passive and active material properties.
The profile of suspended silicon nitride thin films patterned with one-dimensional subwavelength grating structures is investigated using Atomic Force Microscopy. We first show that the results of the profilometry can be used as input to Rigorous Coupled Wave Analysis simulations to predict the transmission spectrum of the gratings under illumination by monochromatic light at normal incidence and compare the results of the simulations with experiments. Secondly, we observe sharp vertical deflections of the films at the boundaries of the patterned area due to local modifications of the tensile stress during the patterning process. These deflections are experimentally observed for various grating structures and investigated on the basis of a simple analytical model as well as finite element method simulations.
We use real-time reflection high energy electron diffraction intensity oscillation to establish the Te-rich growth dynamics of topological insulator thin films of Bi2Te3 on Si(111) substrate by molecular beam epitaxy. In situ angle resolved photoemission spectroscopy (ARPES), scanning tunneling microscopy and ex situ transport measurements reveal that the as-grown Bi2Te3 films without any doping are an intrinsic topological insulator with its Fermi level intersecting only the metallic surface states. Experimentally, we find that the single-Dirac-cone surface state develops at a thickness of two quintuple layers (2 QL). Theoretically, we show that the interaction between the surface states from both sides of the film, which is determined by the penetration depth of the topological surface state wavefunctions, sets this lower thickness limit.
To ensure the practical application of atomically thin transition metal dichalcogenides, it is essential to characterize their structural stability under external stimuli such as electric fields and currents. Using vacancy monolayer islands on TiSe2 surfaces as a model system, for the first time we have observed a shape evolution and growth from triangular to hexagonal driven by scanning tunneling microscopy (STM) electrical stressing. The size of islands shows linear growth with a rate of (3.00 +- 0.05) x 10-3 nm/s, when the STM scanning parameters are held fixed at Vs = 1.0 V and I = 1.8 nA. We further quantified how the growth rate is related to the tunneling current magnitude. Our simulations of monolayer island evolution using phase-field modeling are in good agreement with our experimental observations, and point towards preferential edge atom dissociation under STM scanning driving the observed growth. The results could be potentially important for device applications of ultrathin transition metal dichalcogenides and related 2D materials subject to electrical stressing under device operating conditions.
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