Charge Transport Calculation along Two-Dimensional Metal/Semiconductor/Metal Systems

Two-dimensional transistors are promising candidates for the next generation of nanoscale devices. Like the other alternatives, they also encounter problems such as instability under standard condition (STP), low channel mobility, small band gaps, and difficulty to integrate metal contacts. The latter poses a great challenge since metal/semiconductor interface significantly affects the transistor‘s performance. Some of these obstacles can be solved by using two-dimensional transition metal di-chalcogenides (TMDC) materials. In this study, we performed charge transport calculation based on density functional theory (DFT) followed by wave dynamics to evaluate the performance of six two-dimensional TMDC metal/semiconductor/metal systems. Each semiconductor monolayer was laterally connected, at both ends to metal contacts consisting of VS₂ or FeS₂ monolayers. We found that charge transport was more efficient in systems containing a CrS₂ semiconductor monolayer compared to systems with MoS₂ or WS₂ as the semiconductor monolayer. The electronic characterization of the monolayer TMDC materials by DFT estimates well the trend in charge transport efficiency calculated using wave packet dynamics.     

Nanowire encapsulation with polymer for electrical isolation and enhanced optical properties

Light management and electrical isolation are essential for the majority of optoelectronic nanowire (NW) devices.Here,we present a cost-effective technique,based on vapor-phase deposition of parylene-C and subsequent annealing,that provides conformal encapsulation,anti-reflective coating,improved optical properties,and electrical insulation for GaAs nanowires.The process presented allows facile encapsulation and insulation that is suitable for any nanowire structure.In particular,the parylene-C encapsulation functions as an efficient antireflection coating for the nanowires,with reflectivity down to ＜1％ in the visible spectrum.Furthermore,the parylene-C coating increases photoluminescence intensity,suggesting improved light guiding to the NWs.Finally,based on this process,a NW LED was fabricated,which showed good diode performance and a clear electroluminescence signal.We believe the process can expand the fabrication possibilities and improve the performance of optoelectronic nanowire devices.     

Compton profile and charge transfer study in intermetallic Ti–Al system

The electronic structure of three intermetallic alloys namely Ti₃Al, TiAl and TiAl₃ in terms of Compton profiles is reported in this work. Directional Compton profiles are calculated for all the three alloys employing CRYSTAL code within the framework of density functional theory. The spherically averaged theoretical values are compared with the measurements made using 59.54 keV gamma-rays from Am²⁴¹ source. The calculations are in overall agreement with measurements in all cases. The measurements are also compared with the superposition of LCAO profiles of elemental solids. For Ti₃Al and TiAl₃ the LCAO values show better agreement whereas for TiAl the synthesized LCAO values are closer to the experiment. Effect of titanium 3d electrons is clearly visible in intermediate range of momentum in the Ti rich alloy. Charge transfer in the three alloys has also been estimated following the superposition of experimental profiles of Ti and Al metals. Comparison of Compton spectra of Ti₃Al, TiAl and TiAl₃ with the superposition of the Compton spectra of elemental constituents suggests a charge transfer of 2.8, 0.9 and 0.6 electron per Al atom, respectively. Such large values seem unreasonable and, therefore, this approach cannot be used for any reliable determination of the charge transfer in this system.     

Electronic Properties of ZnO: Band Structure and Directional Compton Profiles

The electronic band structure and directional Compton profiles (DCPs) of ZnO are studied in this work. Calculations are performed considering a set of three schemes based on density functional theory (DFT), the Hartree–Fock (HF) method, and a hybrid scheme. All band structures predict direct bandgaps. The best agreement with experiment is, however, shown by the hybrid scheme. The three schemes are also applied to compute DCPs along [100], [110], and [001] directions. These are compared with measurements made on single crystals of ZnO employing a 59.54 keV gamma-ray Compton spectrometer. Calculations overestimate the momentum density in the low-momentum region while underestimate the anisotropies. Positions of extremes in anisotropies deduced from calculations are well reproduced by the measured anisotropies in some cases. Within the experimental limits, the DCPs from the HF method are in better agreement with the measurements compared with DFT.     

Photovoltaic properties of GaAsP core-shell nanowires on Si(001) substrate

We report on the growth and electro-optical studies of photovoltaic properties of GaAsP nanowires. Low density GaAsP nanowires were grown by Au assisted MOVPE on Si(001) substrates using a two step procedure to form a radial p-n junction. The STEM analyses show that the nanowires have cubic structure with the alloy composition GaAs?.??P?.?? in the nanowire core and GaAs?.??P?.?? in the shell. The nanowire ensembles were processed in the form of sub-millimeter size mesas. The photovoltaic properties were characterized by optical beam induced current (OBIC) and electronic beam induced current (EBIC) maps. Both OBIC and EBIC maps show that the photovoltage is generated by the nanowires; however, a strong signal variation from wire to wire is observed. Only one out of six connected nanowires produce a measurable signal. These strong fluctuations can be tentatively explained by the variation of the resistance of the nanowire-to-substrate connection, which is highly sensitive to the quality of the Si-GaAsP interface. This study demonstrates the importance of the spatially resolved charge collection microscopy techniques for the diagnosis of failures in nanowire photovoltaic devices.     

Optimized contrast enhancement for tumor detection

Magnetic resonance imaging (MRI) is a real assistant for doctors. It provides rich information about anatomy of human body for precise diagnosis of a diseases or disorder. But it is quite challenging to extract relevant information from low contrast and poor quality MRI images. Poor visual interpretation is a hindrance in correct diagnosis of a disease. This creates a strong need for contrast enhancement of MRI images. Study of existing literature shows that conventional techniques focus on intensity histogram equalization. These techniques face the problems of over enhancement, noise and unwanted artifacts. Moreover, these are incapable to yield the maximum entropy and brightness preservation. Thus ineffective in diagnosis of a defect/disease such as tumor. This motivates the authors to propose the contrast enhancement model namely optimized double threshold weighted constrained histogram equalization. The model is a pipelined approach that incorporates Otsu's double threshold method, particle swarm optimized weighted constrained model, histogram equalization, adaptive gamma correction, and Wiener filtering. This algorithm preserves all essential information recorded in an image by automatically selecting an appropriate value of threshold for image segmentation. The proposed model is effective in detecting tumor from enhanced MRI images.     

Retraction Note: Offline scripting-free author identification based on speeded-up robust features

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