A versatile scanning photocurrent mapping system to characterize optoelectronic devices based on 2D materials

The investigation of optoelectronic devices based on two-dimensional materials and their heterostructures is a very active area of investigation with both fundamental and applied aspects involved. We present a description of a home-built scanning photocurrent microscope that we have designed and developed to perform electronic transport and optical measurements of two-dimensional materials based devices. The complete system is rather inexpensive (<10000 EUR) and it can be easily replicated in any laboratory. To illustrate the setup we measure current-voltage characteristics, in dark and under global illumination, of an ultra-thin PN junction formed by the stacking of an n-doped few-layer MoS2 flake onto a p-type MoS2 flake. We then acquire scanning photocurrent maps and by mapping the short circuit current generated in the device under local illumination we find that at zero bias the photocurrent is generated mostly in the region of overlap between the n-type and p-type flakes.Comment: 9 pages, 3 figures, 1 table, supporting informatio

Identifying light impurities in transition metal dichalcogenides: the local vibrational modes of S and O in ReSe₂ and MoSe₂

Optical spectroscopy: vibrational fingerprints of impurities in ReSe2 and MoSe2 The Raman spectra of ReSe2-xSx alloys allows investigation of the distribution of substitutional sulfur atoms within the lattice of ReSe2. A team led by Lewis S. Hart at the University of Bath performed a combination of Raman spectroscopy at visible excitation wavelengths and density functional perturbation theory on ternary ReSe2-xSx alloys obtained by chemical vapor transport. The combination of a traditional normal incidence configuration and an “edge-on” geometry allowed identification of in-plane and out-of-plane Raman features, optically active in the respective experiments owing to their specific selection rules. The frequencies of the observed local vibrational modes arising from sulfur and oxygen atoms were observed to change as a function of the stoichiometric composition of the alloy, thus providing a means of monitoring such impurities. A similar effect was predicted for MoSe2-xSx alloys

Isolation and Characterization of Polymorphic Microsatellite Loci from Metapenaeopsis barbata Using PCR-Based Isolation of Microsatellite Arrays (PIMA)

The red-spot prawn, Metapenaeopsis barbata, is a commercially important, widely distributed demersal species in the Indo-West Pacific Ocean. Overfishing has made its populations decline in the past decade. To study conservation genetics, eight polymorphic microsatellite loci were isolated. Genetic characteristics of the SSR (simple sequence repeat) fingerprints were estimated in 61 individuals from adjacent seas of Taiwan and China. The number of alleles, ranging from 2 to 4, as well as observed and expected heterozygosities in populations, ranging from 0.048 to 0.538, and 0.048 and 0.654, respectively, were detected. No deviation from Hardy–Weinberg expectations was detected at either locus. No significant linkage disequilibrium was detected in locus pairs. The polymorphic microsatellite loci will be useful for investigations of the genetic variation, population structure, and conservation genetics of this species

Bound states of L-shaped or T-shaped quantum wires in inhomogeneous magnetic fields

The bound state energies of L-shaped or T-shaped quantum wires in inhomogeous magnetic fields are found to depend strongly on the asymmetric parameter $\alpha =W_{2}/W_{1}$, i.e. the ratio of the arm widths. Two effects of magnetic field on bound state energies of the electron are obtained. One is the depletion effect which purges the electron out of the OQD system. The other is to create an effective potential due to quantized Landau levels of the magnetic field. The bound state energies of the electron in L-shaped or T-shaped quantum wires are found to depend quadratically (linearly) on the magnetic field in the weak (strong) field region and are independent of the direction of the magnetic field. A simple model is proposed to explain the behavior of the magnetic dependence of the bound state energy both in weak and strong magnetic field regions.Comment: 4 pages, 4 figure

High open-circuit voltage in transition metal dichalcogenide solar cells

The conversion efficiency of ultra-thin solar cells based on layered materials has been limited by their open-circuit voltage, which is typically pinned to a value under 0.6 V. Here we report an open-circuit voltage of 1.02 V in a 120 nm-thick vertically stacked homojunction fabricated with substitutionally doped MoS2. This high open-circuit voltage is consistent with the band alignment in the MoS2 homojunction, which is more favourable than in widely-used TMDC heterostructures. It is also attributed to the high performance of the substitutionally doped MoS2, in particular the p-type material doped with Nb, which is demonstrated by the observation of electroluminescence from tunnelling graphene/BN/MoS2 structures in spite of the indirect nature of bulk MoS2. We find that illuminating the TMDC/metal contacts decreases the measured open-circuit voltage in MoS2 van der Waals homojunctions because they are photoactive, which points to the need of developing low-resistance, ohmic contacts to doped MoS2 in order to achieve high efficiency in practical devices. The high open-circuit voltage demonstrated here confirms the potential of layered transition-metal dichalcogenides for the development of highly efficient, ultra-thin solar cells

Automatic Morphological Subtyping Reveals New Roles of Caspases in Mitochondrial Dynamics

Morphological dynamics of mitochondria is associated with key cellular processes related to aging and neuronal degenerative diseases, but the lack of standard quantification of mitochondrial morphology impedes systematic investigation. This paper presents an automated system for the quantification and classification of mitochondrial morphology. We discovered six morphological subtypes of mitochondria for objective quantification of mitochondrial morphology. These six subtypes are small globules, swollen globules, straight tubules, twisted tubules, branched tubules and loops. The subtyping was derived by applying consensus clustering to a huge collection of more than 200 thousand mitochondrial images extracted from 1422 micrographs of Chinese hamster ovary (CHO) cells treated with different drugs, and was validated by evidence of functional similarity reported in the literature. Quantitative statistics of subtype compositions in cells is useful for correlating drug response and mitochondrial dynamics. Combining the quantitative results with our biochemical studies about the effects of squamocin on CHO cells reveals new roles of Caspases in the regulatory mechanisms of mitochondrial dynamics. This system is not only of value to the mitochondrial field, but also applicable to the investigation of other subcellular organelle morphology

The Study of Die Shape on Residence Time Distribution and Physical and Chemical Properties of Extrudates

In this research, four different die shapes (W,S,I and C) were designed to investigate the effects on residence time distributions. Also, the effects of process variables ( die shape, feed rate and screw speed) on the extrudate''s properties such as expansion ratios, bulk density, water solubility index, water absorption index, break force and puncture force were studied.The results of this research were:1. Extrusion die shape was most influential in residence time distribution. The S-shape die resulted the highest mean residence time and spread than other dies. The I-shape die had the greatest vessel dispresion number.2. The widthwise expansion ratio was highest in center and lowest in boundary, and this phenomena was caused by the flow pattern inside the dies.3. Mean residence time showed significant positive correlation with extrudate''s sectional expansion ratio, but significant negative correlation with extrudate''s longitudinal expansion, bulk density and puncture force. Spread of residence time distribution showed most significant positive correlation with extrudate''s sectional expansion ratio, however, significant negative correlation with extrudate''s bulk density and puncture force.4. Increasing feed rate resulted in an increase of extrudate''s sectional expansion ratio, but decrease extrudate''s longitudinal expansion, bulk density and puncture force. The increase of screw speed resulted in increasing extrudate''s sectional expansion ratio, however, decreasing extrudate''s puncture force.本研究設計四個形狀不同的縫口形模孔，分別為W、S、I、C形，以探討模 孔形狀對擠壓滯留時間分佈的影響，及模孔形狀、進料速率、螺軸轉速對 產品特性（寬度膨發率、橫向膨發率、截面積膨發率、縱向膨發、假密度 、水溶性指標、吸水性指標、最大破碎力、穿刺力）之影響。實驗結果如 下：1. 模孔形狀對滯留時間分佈有極顯著的影響，S形模孔的平均滯留時 間最長，滯留時間分佈變域最大，而I形模孔的擴散數最好。2. 四種形狀 產品的寬度膨發率都有中間較高兩側較低，且左右呈對稱的趨勢，此為模 孔中間物料的流速較快所導致。3. 平均滯留時間對產品截面積膨發率有 顯著的正相關，對縱向膨發有極顯著的負相關，對假密度與產品穿刺力有 顯著的負相關。滯留時間分佈變域對截面積膨發率有極顯著的正相關，對 假密度有極顯著的負相關，對穿刺力有顯著的負相關。4. 進料速率增加 會導致產品膨發率增加、縱向膨發、假密度及穿刺力減少。螺軸轉速增加 會導致產品膨發率增加、穿刺力減少

Optical and Transport Properties of Ni-MoS2

In this paper, MoS2 and Ni-MoS2 crystal layers were fabricated by the chemical vapor transport method with iodine as the transport agent. Two direct band edge transitions of excitons at 1.9 and 2.1 eV were observed successfully for both MoS2 and Ni-MoS2 samples using temperature-dependent optical reflectance (R) measurement. Hall effect measurements were carried out to analyze the transport behavior of carriers in MoS2 and Ni-MoS2, which indicate that the Ni-MoS2 sample is n-type and has a higher resistance and lower mobility than the MoS2 sample has. A photoconductivity spectrum was performed which shows an additional Ni doping level existing at 1.2 eV and a higher photocurrent generating only for Ni-MoS2. The differences between MoS2 and Ni-MoS2 could be attributed to the effect of Ni atoms causing small lattice imperfections to form trap states around 1.2 eV. The temperature-dependent conductivity shows the presence of two shallow levels with activation energies (84 and 6.7 meV in MoS2; 57 and 6.5 meV in Ni-MoS2). Therefore, the Ni doping level leads to high resistance, low mobility and small activation energies. A series of experimental results could provide useful guidance for the fabrication of optoelectronic devices using MoS2 structures