Electrical, Thermal and Spectroscopic Characterization of Bulk Bi2Se3 Topological Insulator

We report electrical (angular magneto-resistance, and Hall), thermal (heat capacity) and spectroscopic (Raman, x-ray photo electron, angle resolved photo electron) characterization of bulk Bi2Se3 topological insulator, which is being is grown by self flux method through solid state reaction from high temperature (950C) melt and slow cooling (2C/hour) of constituent elements. Bi2Se3 exhibited metallic behaviour down to 5K. Magneto transport measurements revealed linear up to 400% and 30% MR at 5K under 14 Tesla field in perpendicular and parallel field direction respectively. We noticed that the magneto-resistance (MR) of Bi2Se3 is very sensitive to the angle of applied field. MR is maximum when the field is normal to the sample surface, while it is minimum when the field is parallel. Hall coefficient (RH) is seen nearly invariant with negative carrier sign down to 5K albeit having near periodic oscillations above 100K. Heat capacity (Cp) versus temperature plot is seen without any phase transitions down to 5K and is well fitted (Cp = gammaT + betaT3) at low temperature with calculated Debye temperature (ThetaD) value of 105.5K. Clear Raman peaks are seen at 72, 131 and 177 cm-1 corresponding to A1g1, Eg2 and A1g2 respectively. Though, two distinct asymmetric characteristic peak shapes are seen for Bi 4f7/2 and Bi 4f5/2, the Se 3d region is found to be broad displaying the overlapping of spin - orbit components of the same. Angle-resolved photoemission spectroscopy (ARPES) data of Bi2Se3 revealed distinctly the bulk conduction bands (BCB), surface state (SS), Dirac point (DP) and bulk valence bands (BVB) and 3D bulk conduction signatures are clearly seen. Summarily, host of physical properties for as grown Bi2Se3 crystal are reported here.Comment: 6 Pages Text + Figs; Comments Suggestions welcom

Influence of active layer thickness on electrical properties of P3HT/n-Si based hybrid heterostructure

In the present study, we analyze the effect of active (organic) layer thickness on the optical and electrical properties of poly 3-hexylthiophene/n-silicon hybrid hetero-structure. The organic/inorganic sandwiched heterojunction have been prepared via spin-coating of poly 3-hexylthiophene film onto an oxide passivated Si substrate at room temperature. The device structure has been fabricated via depositing silver and aluminum contacts on Poly 3-hexylthiophene and n-silicon layers, respectively. The optical and electrical properties of the fabricated heterostructures have been examined by varying the active layer thickness from 50 to 120 nm. Photoluminescence measurements displayed a sharp intense peak at 578 nm corresponding to characteristic poly 3-hexylthiophene band-to-band transition. Enhancement in forward current and reduction in leakage current was observed with increased active layer thickness. It has been observed that employing an active layer thickness of 100 nm, the device produces enhanced forward currents with low leakage currents which leads to the formation of high quality heterojunction and demonstrates better performance of the device

Electrical, thermal and spectroscopic characterization of bulk Bi2Se3 topological insulator

We report the electrical (angular magnetoresistance and Hall), thermal (heat capacity) and spectroscopic (Raman, X-ray photoelectron, angle-resolved photoelectron) characterization of a bulk Bi2Se3 topological insulator, which was grown by self-flux method through solid-state reaction from high-temperature (950 ◦C) melt and slow cooling (2 ◦C/h) of constituent elements. Bi2Se3 exhibited metallic behaviour down to 5 K. Magnetotransport measurements revealed linear up to 400 and 30% magneto-resistance (MR) at 5 K under a 14-T field in perpendicular and parallel field directions, respectively. We noticed that the MR of Bi2Se3 is very sensitive to the angle of the applied field. The MR is maximum when the field is normal to the sample surface, while it is minimum when the field is parallel. The Hall coefficient (RH) is seen nearly invariant with a negative carrier sign down to 5 K albeit having near-periodic oscillations above 100 K. The heat capacity (Cp) versus temperature plot is seen without any phase transitions down to 5 K and is well fitted (Cp = γT + βT 3) at low temperature with a calculated Debye temperature (θD) value of 105.5 K. Clear Raman peaks are seen at 72, 131 and 177 cm−1 corresponding to A1 1g, E2g and A2 1g, respectively. Though two distinct asymmetric characteristic peak shapes are seen for Bi 4f7/2 and Bi 4f5/2, the Se 3d region is found to be broad, displaying the overlapping of spin-orbit components of the same. Angle-resolved photoemission spectroscopy (ARPES) data of Bi2Se3 revealed distinctly the bulk conduction bands (BCB), surface state (SS), Dirac point (DP) and bulk valence bands (BVB), and 3D bulk conduction signatures are clearly seen. Summarily, a host of physical properties for the as-grownBi2Se3 crystal are reported here

Electrochemical and magnetic properties of nanostructured CoMn2O4 and Co2MnO4

In this study, we have focused on the synthesis of cobalt manganite nanostructures using a simplistic hydrothermal route. We have explored these spinels as alternative low-cost bifunctional electrocatalysts for oxygen reduction/evolution reactions (ORR/OER). Herein, we have developed energy-saving, facile and rapid synthetic methodologies for highly active spinel electrocatalysts. Two spinel phases, cubic Co2MnO4 and tetragonal CoMn2O4 have been successfully obtained by tuning the stoichiometric ratio of Co and Mn salts respectively. These CoMn2O4 and Co2MnO4 nanocubes have been used as bifunctional catalysts towards OER and ORR. Electrocatalytic experiments show that cubic Co2MnO4 nanocubes show five times higher activity towards ORR than tetragonal CoMn2O4 nanocubes while the tetragonal phase is a better electrocatalyst towards OER than the cubic Co2MnO4 phase. XPS studies revealed two types of oxygen (lattice O and surface adsorbed O species like OH-) and the efficiency of the catalyst could be related to the binding affinity of oxygen. This explains the better catalytic activity of cubic Co2MnO4 which has a large percentage of adsorbed oxygen species. The stability of the catalyst was confirmed by carrying out TEM studies on a sample after carrying out 25 cycles. Magnetization experiments reveal that both the tetragonal CoMn2O4 as well as cubic Co2MnO4 show hysteresis at 10 K and 100 K without reaching saturation, which confirms an existing ferrimagnetic order in the samples. Both the tetragonal and cubic phases show T-c similar to 110 K and 150 K respectively

Structural, vibrational and electronic properties of CuO nanoparticles synthesized via exploding wire technique

The study of mixed phase Cu/Cu2O/CuO nanoparticles synthesized by Exploding Wire Technique has been recently reported by us. Aiming to achieve single phase CuO nanoparticles, the mixed phase Cu/Cu2O/CuO nanoparticles were subjected to annealing at different temperature and time durations in oxygen environment. In this article, we discussed two samples; two phase Cu2O/CuO and single phase pure CuO nanoparticles obtained by annealing at 500 degrees C and 900 degrees C for 10 h. Rietveld refinement and Williamson-Hall analyses revealed formation of pure phase of CuO at 900 degrees C with an average crsytallite size of 27.6 nm. Irregular shape of nanoparticles with average size of 8 nm was observed by Transmission Electron Microscopy. Selected Area Electron Diffraction pattern matches with standard interplanar distance of CuO. Fourier Transform Infrared and Micro-Raman (mu R) spectra exhibit broadening of vibrational modes; indicative of pure phase CuO at 900 degrees C. Extensive X-ray Photoelectron Spectroscopy analysis revealed that the percentage contributions of Cu1+ and oxygen vacancy (V-O) decreases whereas; Cu2+ and interstitial oxygen (O-1) enhances on increasing the annealing temperature from 500 degrees C to 900 degrees C and thus, resulting the pure phase formation of CuO nanoparticles. Notably, through our analyses we propose an electronic band structure diagram on the basis of valance band maximum, as obtained by XPS and the band gap energy as estimated via UV-visible spectroscopy for mixed phase of Cu2O/CuO (1.6 +/- 0.02 eV) and pure phase of CuO (1.3 +/- 0.02 eV) nanoparticles

Influence of metallic surface states on electron affinity of epitaxial AlN films

The present article investigates surface metallic states induced alteration in the electron affinity of epitaxial AlN films. AlN films grown by plasma-assisted molecular beam epitaxy system with (30% and 16%) and without metallic aluminium on the surface were probed via photoemission spectroscopic measurements. An in-depth analysis exploring the influence of metallic aluminium and native oxide on the electronic structure of the films is performed. It was observed that the metallic states pinned the Fermi Level (FL) near valence band edge and lead to the reduction of electron affinity (EA). These metallic states initiated charge transfer and induced changes in surface and interface dipoles strength. Therefore, the EA of the films varied between 0.6-1.0 eV due to the variation in contribution of metallic states and native oxide. However, the surface barrier height (SBH) increased (4.2-3.5 eV) adversely due to the availability of donor-like surface states in metallic aluminium rich films

Nanostructured GaN and AlGaN/GaN heterostructure for catalyst-free low-temperature CO sensing

The use of expensive catalysts (e.g. platinum) and high operation temperature ( > 300 degrees C) has plagued the cost-effectiveness and thereby the commercialization of III-Nitride semiconductors based gas sensing technology. Inadequate research on the development of catalyst-free room-temperature CO sensing using GaN based structures is the critical reason behind the subjugation of this area. Therefore, in the present article, we aimed the development of GaN & AlGaN/GaN heterostructure based gas sensors for catalyst-free low-temperature CO sensing (at 100 ppm). To explore the underlying science behind such mechanism, the morphological, electronic and electrical properties of the devices were thoroughly investigated. The analysis revealed that CO sensing on GaN (and AlGaN/GaN heterostructure) is governed via the chemical nature of ambient-oxidation induced amorphous oxide layer (O-2(-), O2- or OH(- )species), which acts as donor/acceptor state at the surface. Besides, the critical device parameters like Schottky barrier height and electron accumulation associated with series resistance and leakage current (forward/reverse) displayed significant variation with temperature (27-250 degrees C) and perturbed the effective carrier transport/collection and ultimately the device efficiency. The study demonstrates that nanostructured surfaces can open avenues for the development of catalyst-free room temperature operating III-Nitride semiconductor based CO sensors

Effect of aging on transmittance, and effect of annealing temperature on CO2 sensing of ZnO thin film deposited by spin coating

Investigations have been made on the structural, morphological, optical, sensing, and aging characteristics of ZnO thin films created on glass substrate using the sol-gel spin coating process. Zinc acetate (ZnAc), ethanol, and stearic acid were employed as the starting materials, solvent, and stabilizer, respectively, to deposit the ZnO thin films on the blue star glass substrate. An UV–visible spectrophotometer is used to study the optical characteristics. With the aid of XRD and FESEM characterization techniques, the structural and morphological properties are studied. The results showed that the characteristics of ZnO thin films were strongly dependent on the annealing temperature. The XRD measurement indicated that the deposited film shows the hexagonal structure and preferential orientation along the (002) crystal plane with a grain size of 78.99 nm. The transmittance value of the ZnO films was relatively high up to 90 %. The result showed that the transmittance value is strongly dependent on the aging time of the sample. The SEM images show that morphology and texture are dependent on the annealing temperature. The SEM image also contains a foreign object or xenolith. The V-I characteristics show nonlinear behavior, and the resistance of the thin film is between 2800 and 5000 MΩ. The CO2 sensing at 450 °C is far better than that of at 750 °C. The response time at 450 °C is 20–40 s, the recovery time is better at 750 °C

Determination of band alignment at two-dimensional MoS2/Si van der Waals heterojunction

To understand the different mechanism occurring at the MoS2-silicon interface, we have fabricated a MoS2/Si heterojunction by exfoliating MoS2 on top of the silicon substrate. Raman spectroscopy and atomic force microscopy (AFM) measurement expose the signature of few-layers in the deposited MoS2 flake. Herein, the temperature dependence of the energy barrier and carrier density at the MoS2/Si heterojunction has been extensively investigated. Furthermore, to study band alignment at the MoS2/Si interface, we have calculated a valence band offset of 0.6660.17 eV and a conduction band offset of 0.4260.17 eV using X-ray and Ultraviolet photoelectron spectroscopy. We determined a type-II band alignment at the interface which is very conducive for the transport of photoexcited carriers. As a proof-of-concept application, we extend our analysis of the photovoltaic behavior of the MoS2/Si heterojunction. This work provides not only a comparative study between MoS2/p-Si and MoS2/n-Si heterojunctions but also paves the way to engineer the properties of the interface for the future integration of MoS2 with silicon