Muhammad Arslan Shehzad

Nematic Liquid Crystal on a Two Dimensional Hexagonal Lattice and its Application

We have studied the alignment of liquid crystal adsorbed onto graphene and hexagonal boron nitride by using a polarized optical microscope. From the experimental data, it was found that there were 6 different alignment orientations of the liquid crystal molecules on a single crystal substrate. This result has never been reported and is quite different from other previous results. As the hexagonal lattice has a threefold rotational symmetry, three different alignment orientations were expected, but our result seems counter-intuitive. We explain this result considering the bending of the tail of the liquid crystal molecules. Using this anchoring effect with six accurate discrete angles, a novel non-volatile display can be developed with micron-scale pixel size, due to the molecular level accuracy of the alignment.

A progressive route for tailoring electrical transport in MoS2

Typically, molybdenum disulfide (MoS2) synthesized by chemical vapor deposition (CVD) is polycrystalline; as a result, the scattering of charge carriers at grain boundaries can lead to performances lower than those observed in exfoliated single-crystal MoS2. Until now, the electrical properties of grain boundaries have been indirectly studied without accurate knowledge of their location. Here, we present a technique to measure the electrical behavior of individual grain boundaries in CVD-grown MoS2, imaged with the help of aligned liquid crystals. Unexpectedly, the electrical conductance decreased by three orders of magnitude for the grain boundaries with the lowest on/off ratio. Our study provides a useful technique to fabricate devices on a single-crystal area, using optimized growth conditions and device geometry. The photoresponse, studied within a MoS2 single grain, showed that the device responsivity was comparable with that of the exfoliated MoS2-based photodetectors.

Study of Grains and Boundaries of Molybdenum Diselenide and Tungsten Diselenide Using Liquid Crystal

Direct observation of grains and boundaries is a vital factor in altering the electrical and optoelectronic properties of transition metal dichalcogenides (TMDs), that is, MoSe2 and WSe2. Here, we report visualization of grains and boundaries of chemical vapor deposition grown MoSe2 and WSe2 on silicon, using optical birefringence of two-dimensional layer covered with nematic liquid crystal (LC). An in-depth study was performed to determine the alignment orientation of LC molecules and their correlation with other grains. Interestingly, we found that alignment of liquid crystal has discrete preferential orientations. From computational simulations, higher adsorption energy for the armchair direction was found to force LC molecules to align on it, compared to that of the zigzag direction. We believe that these TMDs with three-fold symmetric alignment could be utilized for display applications.

Layer-modulated, wafer scale and continuous ultra-thin WS2 films grown by RF sputtering via post-deposition annealing

Tungsten disulfide (WS2) is a layered semiconducting material with a tunable bandgap that is promising in next generation nanoelectronics as well as energy harvesting devices. In this study, we presented a continuous and wafer-scale uniform WS2 layer preparation technique through sulfurization of a RF-sputtered WO3 film. Various characterization techniques were employed in order to investigate the structural and physical properties of the WS2 films. It was observed that the thickness of WS2 films could be controlled by tuning the sputtering time. The fabricated WS2 transistors exhibited high mobility values of ∼17 and 37–38 cm2 V−1 s−1 and on/off ratios in the range of ∼104 and 104–105 for 80–100 s-sputter time and 120–140 sputter time, respectively, which is in the maximum range for CVD-grown WS2 FETs with an SiO2 gate oxide. Photoresponse was also studied for a few layers of WS2 on a transparent quartz substrate and it was observed that the photosensitivity was linearly dependent on bias voltage. The proposed growth technique is attractive for next-generation transparent and nanoelectronic devices, as well as for other potential applications.

Influence of removing PMMA residues on surface of CVD graphene using a contact-mode atomic force microscope

For device fabrication based on 2D materials such as graphene, hexagonal boron nitride (hBN) and transition metal dichalcogenides (TMDs), polymethyl methacrylate (PMMA) is conventionally used in the wet transfer and lithography processes. All these processes are sources of polymer residue, which degrade the intrinsic electrical and optical properties of devices. In this work, we report the effect of mechanical cleaning via contact mode atomic force microscopy (AFM) on the surface morphology and electrical behavior of chemical-vapor-deposition grown graphene. An AFM tip with large contact force was used to scan, and multiple scanning was performed to remove the residues of PMMA. Raman mapping was incorporated to confirm the cleaning effect using AFM. Transconductance properties associated with a field-effect-transistor device based on the cleaned graphene were analyzed. It was observed that charge-neutrality point was shifted towards zero gate voltage and the charge carrier mobility was increased. We claim that our technique provides a facile route to fabricate devices with less polymer residue and higher efficiency.

Assessment of Genetic Diversity, Population Structure, and Evolutionary Relationship of Uncharacterized Genes in a Novel Germplasm Collection of Diploid and Allotetraploid Gossypium Accessions Using EST and Genomic SSR Markers

This study evaluated the genetic diversity and population structures in a novel cotton germplasm collection comprising 132 diploids, including Glossypium klotzschianum and allotetraploid cotton accessions, including Glossypium barbadense, Glossypium darwinii, Glossypium tomentosum, Glossypium ekmanianum, and Glossypium stephensii, from Santa Cruz, Isabella, San Cristobal, Hawaiian, Dominican Republic, and Wake Atoll islands. A total of 111 expressed sequence tag (EST) and genomic simple sequence repeat (gSSR) markers produced 382 polymorphic loci with an average of 3.44 polymorphic alleles per SSR marker. Polymorphism information content values counted 0.08 to 0.82 with an average of 0.56. Analysis of a genetic distance matrix revealed values of 0.003 to 0.53 with an average of 0.33 in the wild cotton collection. Phylogenetic analysis supported the subgroups identified by STRUCTURE and corresponds well with the results of principal coordinate analysis with a cumulative variation of 45.65%. A total of 123 unique alleles were observed among all accessions and 31 identified only in G. ekmanianum. Analysis of molecular variance revealed highly significant variation between the six groups identified by structure analysis with 49% of the total variation and 51% of the variation was due to diversity within the groups. The highest genetic differentiation among tetraploid populations was observed between accessions from the Hawaiian and Santa Cruz regions with a pairwise FST of 0.752 (p < 0.001). DUF819 containing an uncharacterized gene named yjcL linked to genomic markers has been found to be highly related to tryptophan-aspartic acid (W-D) repeats in a superfamily of genes. The RNA sequence expression data of the yjcL-linked gene Gh_A09G2500 was found to be upregulated under drought and salt stress conditions. The existence of genetic diversity, characterization of genes and variation in novel germplasm collection will be a landmark addition to the genetic study of cotton germplasm.

Visualizing Degradation of Black Phosphorus Using Liquid Crystals

Black Phosphorus (BP) is an excellent material from the post graphene era due to its layer dependent band gap, high mobility and high Ion/Ioff. However, its poor stability in ambient poses a great challenge for its practical and long-term usage. The optical visualization of the oxidized BP is the key and the foremost step for its successful passivation from the ambience. Here, we have conducted a systematic study of the oxidation of the BP and developed a technique to optically identify the oxidation of the BP using Liquid Crystal (LC). It is interesting to note that we found that the rapid oxidation of the thin layers of the BP makes them disappear and can be envisaged by using the alignment of the LC. The molecular dynamics simulations also proved the preferential alignment of the LC on the oxidized BP. We believe that this simple technique will be effective in passivation efforts of the BP, and will enable it for exploitation of its properties in the field of electronics.Black Phosphorus (BP) is an excellent material for post graphene era due to its layer dependent band gap, high mobility and high Ion/Ioff. However, its poor stability in ambient poses a great challenge in its practical and long-term usage. Optical visualization of oxidized BP is the key and foremost step for its successful passivation from the ambience. Here, we have done a systematic study of the oxidation of BP and developed a technique to optically identify the oxidation of BP using Liquid Crystal (LC). Interestingly we found that rapid oxidation of thin layers of BP makes them disappear and can be envisaged by using the alignment of LC. The molecular dynamics simulations also proved the preferential alignment of LC on oxidized BP. We believe that this simple technique will be effective in passivation efforts of BP and will enable it for exploitation of its properties in the field of electronics.