ABSTRACT
Investigations of single and bilayers of bismuth are one of the most thrusting areas of research in contemporary condensed matter physics and material sciences. This is because such ultrathin layers of bismuth host interesting exotic electronic properties, which are important from both fundamental science and future application perspectives. In the past, many inorganic processes for the synthesis of single and bi-layers of bismuth were reported using physical and chemical vapor deposition techniques. The ultrathin films deposited are found to interact electronically with the substrates due to their proximity to the substrate surface. We introduce a new and easy organic channel for the synthesis of the bismuth multi-bilayers in ambient conditions. Bismuth stearate multi-bilayer thin films are deposited on the hydrophobic silicon and hydrophilic glass substrates using the Langmuir-Blodgett technique. Optical absorption spectroscopy measurements in the infrared region provided information on various bond structures present in those bismuth stearate thin films. Specular x-ray reflectivity (XRR) experiments and their analysis of such thin films unambiguously show the highly periodic stacking of bismuth bilayers along the surface-normal directions within the multilayer film structure. Model-based microstructural analysis of the XRR data further shows that each bilayer of bismuth is well separated (3.5 nm) from other bismuth bilayers by hydrocarbon chains. At these separations,the electronic states of the bismuth bilayers are expected to be non-interacting with each other. The morphology of the surface obtained from field emission scanning electron microscopy supports the XRR analysis. A bandgap of 3.2 eV was obtained for such bismuth stearate thin films from the optical spectroscopy measurements in the UV-visible range. The large separations between the bismuth-bilayers and between the substrate and the bismuth bilayers are expected to minimize the electronic interactions between them.
ABSTRACT
Angular resolved photoemission spectroscopy in combination with ab initio calculations show that trace amounts of carbon doping of the Bi_{2}Se_{3} surface allows the controlled shift of the Dirac point within the bulk band gap. In contrast to expectation, no Rashba-split two-dimensional electron gas states appear. This unique electronic modification is related to surface structural modification characterized by an expansion of the top Se-Bi spacing of ≈11% as evidenced by surface x-ray diffraction. Our results provide new ways to tune the surface band structure of topological insulators.
ABSTRACT
In this work nanoclusters formed in a Pt/Ni/C multi-trilayer by the ion-irradiated method of synthesis are characterized. In particular, an attempt to understand the role of interfaces in the synthesis is made. With this objective, ion-irradiation-induced structural changes in a Pt/Ni/C multi-trilayer using X-ray absorption spectroscopy (at the Ni K-edge) in conjunction with the X-ray standing-wave technique are investigated. The XANES analysis identifies chemical binding at pristine Ni/C and Ni/Pt interfaces, in contrast with physical adsorption at the Pt/C interface. The chemical nature of the interfaces determines their relative stability with respect to irradiation and controls the extent of metallic diffusion. The most interesting structural change, upon irradiation, is the disruption of the Pt/C interface and subsequent migration of Pt atoms towards pre-diffused Ni atoms within the C layer, leading to the formation of Ni-centered Ni-Pt bimetallic nanoclusters (with Ni:Pt = 60:40). These clusters are highly disordered beyond their nearest neighbor and find wide-scale applications as, for example, magnetic devices etc. The implications of these findings on the design goals are discussed.
ABSTRACT
Microstructural characterization of a synthetic periodic and graded Pt/Ni/C multilayer system by X-ray reflectivity and ion scattering techniques is presented. The experimental reflectivity data are fitted with a theoretical multi-trilayer model with graded periodicity which increases from substrate to film surface along the surface normal direction. The increase in periodicity is found to be due to a linear increase in C-layer thickness from the bottom to the top, with a change of slope nearly at the middle of the multilayer stack. The thicknesses of Pt and Ni layers, the variation of C-layer thickness with depth, interface roughness of Pt/Ni, Ni/C, C/Pt interfaces are determined from the analysis of the reflectivity data. Rutherford backscattering spectrometry measurements were also made on the same sample. Simulated Rutherford back scattering spectrometry data using the parameters obtained from the analysis of the X-ray reflectivity data agree well with the measured Rutherford backscattering spectrum.
