Showing papers by "Abhishek K. Singh published in 2016"
TL;DR: Using density-functional theory, the microscopic understanding gained here unveils the challenges in exfoliation and controlling the functionalization of MXene, which is essential for its practical application.
Abstract: MXene, a two-dimensional layer of transition metal carbides/nitrides, showed great promise for energy storage, sensing, and electronic applications. MXene are chemically exfoliated from the bulk MAX phase; however, mechanistic understanding of exfoliation and subsequent functionalization of these technologically important materials is still lacking. Here, using density-functional theory we show that exfoliation of Ti3C2 MXene proceeds via HF insertion through edges of Ti3AlC2 MAX phase. Spontaneous dissociation of HF and subsequent termination of edge Ti atoms by H/F weakens Al–MXene bonds. Consequent opening of the interlayer gap allows further insertion of HF that leads to the formation of AlF3 and H2, which eventually come out of the MAX, leaving fluorinated MXene behind. Density of state and electron localization function shows robust binding between F/OH and Ti, which makes it very difficult to obtain controlled functionalized or pristine MXene. Analysis of the calculated Gibbs free energy (ΔG) shows...
186 citations
TL;DR: The charge transfer causes an upward shift in the Dirac point with respect to Fermi level at a rate of 15.7 meV GPa(-1) as a function of applied hydrostatic pressure, leading to heavy p-type doping in graphene.
Abstract: A unique way of achieving controllable, pressure-induced charge transfer doping in the graphene/MoS2 heterostructure is proposed. The charge transfer causes an upward shift in the Dirac point with respect to Fermi level at a rate of 15.7 meV GPa(-1) as a function of applied hydrostatic pressure, leading to heavy p-type doping in graphene. The doping was confirmed by I2D /IG measurements.
46 citations
TL;DR: An intensive study on the chemical transformation of MAX phase into MXene sheets by inserting HF, alkali atoms and LiF in Nb4AlC3 MAX phase is performed, showing that at 500-650 K, the Li-MXene bond indeed breaks leaving a pristine MXene sheet behind.
Abstract: Synthesis of pristine MXene sheets from MAX phase is one of the foremost challenges in getting a complete understanding of the properties of this new technologically important 2D-material. Efforts to exfoliate Nb4AlC3 MAX phase always lead to Nb4C3 MXene sheets, which are functionalized and have several Al atoms attached. Using the first-principles calculations, we perform an intensive study on the chemical transformation of MAX phase into MXene sheets by inserting HF, alkali atoms and LiF in Nb4AlC3 MAX phase. Calculated bond-dissociation energy (BDE) shows that the presence of HF in MAX phase always results in functionalized MXene, as the binding of H with MXene is quite strong while that with F is weak. Insertion of alkali atoms does not facilitate pristine MXene isolation due to the presence of chemical bonds of almost equal strength. In contrast, weak Li-MXene and strong Li-F bonding in Nb4AlC3 with LiF ensured strong anisotropy in BDE, which will result in the dissociation of the Li-MXene bond. Ab initio molecular dynamics calculations capture these features and show that at 500-650 K, the Li-MXene bond indeed breaks leaving a pristine MXene sheet behind. The approach and insights developed here for chemical exfoliation of layered materials bonded by chemical bonds instead of van der Waals can promote their experimental realization.
33 citations
TL;DR: In this article, the phonon Boltzmann transport formalism and density functional theory based calculations are used to show that stanene has a low thermal conductivity, indicating anisotropy in thermal transport.
Abstract: Using the phonon Boltzmann transport formalism and density functional theory based calculations, we show that stanene has a low thermal conductivity. For a sample size of 1 × 1 μm2 (L × W), the lattice thermal conductivities along the zigzag and armchair directions are 10.83 W m−1 K−1 and 9.2 W m−1 K−1 respectively, at room temperature, indicating anisotropy in thermal transport. The low values of thermal conductivity are due to large anharmonicity in the crystal resulting in high Gruneisen parameters, and low group velocities. The room temperature effective phonon mean free path is found to be around 17 nm indicating that the thermal transport in stanene is completely diffusive in nature. Furthermore, our study reveals the relative importance of the contributing phonon branches and that, at very low temperatures, the contribution to lattice thermal conductivity comes from the flexural acoustic (ZA) branch and at higher temperatures it is dominated by the longitudinal acoustic (LA) branch. We also show that the lattice thermal conductivity of stanene can further be reduced by tuning the sample size and creating rough surfaces at the edges. Such tunability of lattice thermal conductivity in stanene suggests its applications in thermoelectric devices.
27 citations
TL;DR: In this paper, the changes in electronic and transport properties of graphene bilayer caused by sliding one of the layers are investigated using density functional theory (DFT), which leads to a drastic transition in electronic structure from linear to parabolic dispersion.
25 citations
TL;DR: In this paper, the authors reported a large band gap opening in the van der Waals heterostructure of graphene and graphane (hydrogenated graphene) under normal compressive (NC) strain.
Abstract: Using first-principles calculations, we report a large band-gap opening in the van der Waals heterostructure of graphene and graphane (hydrogenated graphene) under normal compressive (NC) strain. In the presence of graphane, interlayer charge transfer from graphene to graphane triggers the intralayer charge redistribution in graphene, breaking the equivalence of the two sublattices. This chiral symmetry breaking, however, is not strong enough to split the Dirac cone. The application of the NC strain enhances the inter- and intralayer charge transfer leading to a splitting of the Dirac cone, reflected as a redshift of the $G$ peak in Raman spectra. With strain, the band gap increases monotonically and attains a maximum of 0.74 eV at 20% strain, which is the largest ever reported splitting of a Dirac cone in graphene. Tight-binding analysis demonstrates that the applied strain changes the on-site interactions of carbon atoms belonging to a particular sublattice of graphene, thereby breaking the chiral symmetry leading to the opening of a band gap. A sufficiently large band gap with linear dispersion of Dirac bands in the graphene/graphane heterostructure constitutes promising features for room-temperature electronic and optical devices.
21 citations
TL;DR: The magnetic ordering in the material was found to be stable up to higher pressures, making pressure a tool to tune the electronic property without perturbing the magnetic property.
Abstract: While pressure-induced changes in the electronic, magnetic, and optical properties of Cu–Cl hybrid perovskites have been studied intensively, the correlation between these properties and pressure-induced structural changes is still vaguely understood. Here, by first-principles calculations on a model system (EDBE)[CuCl4] (EDBE = 2,2′-(ethylenedioxy)bis(ethylammonium)) (a Cu–Cl hybrid perovskite), we correlate the evolution of a series of exciting physical properties with pressure while resolving some of the long-standing debates on the fundamental electronic nature of this important class of material. The material shows two structural phase transitions and an anisotropy in compressibility with increasing pressure. After a critical pressure of 17 GPa, the structure becomes highly symmetric, thereby suppressing the Jahn–Teller distortions. At zero pressure, mapping the optical transitions with the Laporte selection rules, lower and higher energy excitations are found to be of Mott–Hubbard (MH) and charge tr...
21 citations
TL;DR: In this paper, the authors reported a simultaneous increase in both electrical conductivity and thermopower under hydrostatic pressure, which can enable the development of efficient thermoelectric devices in the moderate to high temperature range.
Abstract: The inverse coupled dependence of electrical conductivity and thermopower on carrier concentration presents a big challenge in achieving a high figure of merit. However, the simultaneous enhancement of electrical conductivity and thermopower can be realized in practice by carefully engineering the electronic band structure. Here by taking the example of Bi2S3, we report a simultaneous increase in both electrical conductivity and thermopower under hydrostatic pressure. Application of hydrostatic pressure enables tuning of electronic structure in such a way that the conductivity effective mass decreases and the density of states effective mass increases. This dependence of effective masses leads to simultaneous enhancement in electrical conductivity and thermopower under n-type doping leading to a huge improvement in the power factor. Also lattice thermal conductivity exhibits very weak pressure dependence in the low pressure range. The large power factor together with low lattice thermal conductivity results in a high ZT value of 1.1 under n-type doping, which is nearly two times higher than the previously reported value. Hence, this pressure-tuned behaviour can enable the development of efficient thermoelectric devices in the moderate to high temperature range. We further demonstrate that similar enhancement can be observed by generating chemical pressure by doping Bi2S3 with smaller iso-electronic elements such as Sb at Bi sites, which can be achieved experimentally.
19 citations
18 Feb 2016
TL;DR: In this article, the authors show that while the electronic structure and indirect band gap of SnS2 do not change significantly with the number of layers, they can be reversibly tuned by applying biaxial tensile (BT), BC, and normal compressive (NC) strains.
Abstract: Controlled variation of the electronic properties of. two-dimensional (2D) materials by applying strain has emerged as a promising way to design materials for customized applications. Using density functional theory (DFT) calculations, we show that while the electronic structure and indirect band gap of SnS2 do not change significantly with the number of layers, they can be reversibly tuned by applying biaxial tensile (BT), biaxial compressive (BC), and normal compressive (NC) strains. Mono to multilayered SnS2 exhibit a reversible semiconductor to metal (S-M) transition with applied strain. For bilayer (2L) SnS2, the S-Mtransition occurs at the strain values of 17%,-26%, and -24% under BT, BC, and NC strains, respectively. Due to weaker interlayer coupling, the critical strain value required to achieve the S-Mtransition in SnS2 under NC strain is much higher than for MoS2. From a stability viewpoint, SnS2 becomes unstable at very low strain values on applying BC (-6.5%) and BT strains (4.9%), while it is stable even up to the transition point (-24%) in the case of NC strain. In addition to the reversible tuning of the electronic properties of SnS2, we also show tunability in the phononic band gap of SnS2, which increases with applied NC strain. This gap increases three times faster than for MoS2. This simultaneous tunability of SnS2 at the electronic and phononic levels with strain, makes it a potential candidate in field effect transistors (FETs) and sensors as well as frequency filter applications.
17 citations
TL;DR: In this article, the spin-orbit coupling (SOC) is ignored, and the authors theoretically propose topologically non-trivial states in a recently experimentally discovered superconducting material CaSn$_3.
Abstract: Based on the first-principles calculations, we theoretically propose topologically non-trivial states in a recently experimentally discovered superconducting material CaSn$_3$. When the spin-orbit coupling (SOC) is ignored, the material is a host to three-dimensional topological nodal-line semimetal states. Drumhead like surface states protected by the coexistence of time-reversal and mirror symmetry emerge within the two-dimensional regions of the surface Brillouin zone connecting the nodal lines. When SOC is included, unexpectedly, each nodal line evolves into two Weyl nodes (W1, W2) in this centrosymmetric material. Berry curvature calculations show that these nodes occur in a pair and act as either a source or sink of Berry flux. The material also has unique surface states in the form of Fermi arcs, which unlike other known Weyl semimetal, form closed loops of surface states on the Fermi surface. Our theoretical realization of topologically non-trivial states in a superconducting material paves the way towards unraveling the interconnection between topological physics and superconductivity.
12 citations
TL;DR: In this article, it was shown that the intrinsic size effect in ferroelectrics can as well manifest in terms of switching the ground state from one ferroelectric phase (tetragonal with polarization along [001]) to another ferro electric phase (rhombohedral with polarisation along [111]).
Abstract: In general, crystallite size reduction is known to suppress the ferroic order (polarization or magnetization) in ferroelectric and magnetic systems. Using free particles of a giant tetragonality $(c/a--1=0.18)$ ferroelectric alloy $\mathrm{BiFe}{\mathrm{O}}_{3}\text{\ensuremath{-}}\mathrm{PbTi}{\mathrm{O}}_{3}$ as the model system, here we show that the intrinsic size effect in ferroelectrics can as well manifest in terms of switching the ground state from one ferroelectric phase (tetragonal with polarization along [001]) to another ferroelectric phase (rhombohedral with polarization along [111]). In this particular case, because of the strong coupling of the magnetic and structural degrees of freedom, a magnetic order also sets in below the critical size, making it a size induced magnetoferroelectric transformation. The driving force for this unusual transformation is argued to be the large depolarizing and domain wall energies in the tetragonal phase.
TL;DR: In this article, the d-band center of a Sc-metallacarborane (MCB) was tuned electronically to enhance gas adsorption selectivity, which can be efficiently ulitized in various catalytic, sensing and gas storage systems.
TL;DR: In this paper, the dynamics of surface adsorption of various chemicals on ultra-thin single crystalline gold nanowires (AuNW) through sensitive resistance fluctuation spectroscopy measurements coupled with ab initio simulations are studied.
Abstract: In this letter we present systematic studies of the dynamics of surface adsorption of various chemicals on ultra-thin single crystalline gold nanowires (AuNW) through sensitive resistance fluctuation spectroscopy measurements coupled with ab initio simulations. We show that, contrary to expectations, the adsorption of common chemicals like methanol and acetone has a profound impact on the electrical transport properties of the AuNW. Our measurements and subsequent calculations establish conclusively that in AuNW, semiconductor-like sensitivity to the ambient arises because of changes induced in its local density of states by the surface adsorbed molecules. The extreme sensitivity of the resistance fluctuations of the AuNW to ambient suggests their possible use as solid-state sensors. Published by AIP Publishing.
TL;DR: Soil and seed treatments with Trichoderma resulted in lowering the wilt disease and considerably increased grain yield of chick pea over farmers' practice and outlines the need for its popularization in larger areas.
Abstract: Chickpea is one of the important pulse crops of the Faizabad district among pulses which is affected by the Fusarium wilt ( Fusarium udum ) disease causing severe yield loss. An eco-friendly integrated disease management technology, particularly use of Trichoderma as seed and soil treatment with rotted FYM have been evaluated on large area of farmers' field during Rabi 2012-13 and 2013-14 in the Madhupur, Anjana and Magalsi villages of Faizabad district (Uttar Pradesh) through on farm trial. Grain yield of chick pea and incidence of wilt were significantly less under Trichoderma treatment and was much superior over the control. Soil and seed treatments with Trichoderma resulted in lowering the wilt disease and considerably increased grain yield of chick pea over farmers' practice. The use of Trichoderma for the management of wilt was very much appreciated by the farmers. The success of Trichoderma in the target villages outlines the need for its popularization in larger areas.
Journal Article•
TL;DR: Close suction drainage prevents post- operative surgical site (Incision) infection, reduces hospital stay and the wound healing time is also less when closed suction drain is used in patients undergone elective laparotomies in comparison to simple abdominal closure.
Abstract: INTRODUCTION: Wound infection continues to represent a major problem, both in terms of how they affect the outcome of surgical procedure, and for their impact on length of hospital stay and cost of treatment. Moreover, post-operative wound infection or surgical site infection (SSI) delays return to gainful employment and prolong hospital stay. The objective of closed suction drainage of an established surgical site wound collection is to facilitate treatment of such collection with the aim of increasing patient’s comfort, decreasing patient morbidity and decreasing the length of hospitalization. AIMS/OBJECTIVES: 1. To evaluate the role of closed suction drainage in reduction of surgical site infection in elective laparotomy wounds as compared to laparotomy wounds closed without suction drainage. 2. To study the effect of closed suction drainage in reducing the hospital stay. METHODS: Study group consists of patients with suction drain in wound closure, while controls were patients with simple abdominal closure. Sample size- Group A – Patients with suction drain in wound closure – 50 cases Group B – Patients with simple abdominal closure – 50 cases A Prospective study was conducted at Dr. D.Y.Patil Medical College & Hospital and Research Centre Pimpri, Pune, for a period of 3 years from 2013 to 2016. RESULT: Closed suction drainage prevents post- operative surgical site (Incision) infection ,reduces hospital stay and the wound healing time is also less when closed suction drain is used in patients undergone elective laparotomies in comparison to simple abdominal closure. CONCLUSION: 1. Closed suction drainage prevents post- operative surgical site (Incision) infection significantly. 2. Closed suction drainage reduces hospital stay in a patient undergone elective laparotomy, which is significantly more where closed suction drain is not used. 3. Obese patients with subcutaneous fat are more likely to develop Surgical site infection.
TL;DR: This was the first kind of study conducted on these remote islands to know the prevalence of different ocular diseases and the important point to be seen in this study was unawareness of glaucoma and cataract load.
Abstract: Background Andaman and Nicobar Island is a group of 572 islands with a population of 3,80,500. The only service hospital as well as govt hospital is situated at its capital, Portblair. The islands are interconnected only by sea passage and to reach the capital by this route it sometimes takes two days. Methods The objective of this study is to assess the spectrum of different ocular diseases in remote islands of Andaman and Nicobar. Author had conducted various camps on these remote islands as a community outreach programme over two years. Results Top eye diseases found were refractive error (35%), cataract (22%), conjunctival infammatory disease (15%), glaucoma cases (07%), pterygium (07%), NLDO (02%) and uveitis (02%). Out of total 132 cataract patients screened, 32 patients (approximately 24%) were operated in INHS Dhanvantari at free of cost with help of NPCB (National Programme for Control of Blindness). Conclusion This was the first kind of study conducted on these remote islands to know the prevalence of different ocular diseases. The important point to be seen in this study was unawareness of glaucoma and cataract load. No optometrist was posted in the PHCs to provide primary eye care like prescribing glasses, which is also a big concern.
01 Sep 2016
TL;DR: In this article, inelastic finite element analysis was used to investigate the shakedown and ratcheting behavior of a cylindrical vessel with multiple radial crossholes and the effect of opening radius, thickness ratio and availability of ligament between two openings.
Abstract: Shakedown and ratcheting behaviour of thick cylinder with multiple radial crossholes is investigated in this paper by inelastic finite element analysis. Effect of opening radius (ri/Ri), thickness of the vessel i.e. thickness ratios (R0/Ri) and availability of ligament between two openings (L/1) is studied. In order to normalise the applied pressure, limit load for thick cylindrical vessel without any opening was considered and it was found to be highly conservative. Later, interaction diagram normalised with respect to the limit load for thick cylinder taking into account the effects of opening was plotted. In order to distinguish between plastic shakedown and ratcheting, thickness advantage is taken into consideration. A more practical assessment technique is also discussed where the maximum range of thermal stress is computed through finite element analysis and the results were found to be in accordance with the Bree diagram. However the elastic-plastic shakedown boundary was found to be affected significantly with the incorporation of discontinuities and with changes in the parameters.
01 Jan 2016
TL;DR: TENS shows a very promising effect in controlling the postoperative pain, there was a significant reduction in pain in Hernioplasty and Appendicectomy surgeries but minimal in Cholecystectomy, and side effects of analgesics drugs are reduced to the minimal level.
Abstract: Introduction: The management of post-operative pain continues to be a significant aspect of a post-operative patient care in surgical practice. TENS is a simple, non-invasive analgesia technique which is used in health care settings, due to its ease of administration, rapid onset, it is cheaper compared to long term use of drugs, analgesics & also few side effects/ drug interactions. The method being non-toxic and non-invasive, the physiological & psychological dependence does not develop with its use. The advantage of TENS is that the problems of respiratory depression, sedation, hypotension seen with narcotics are avoided & reduced. This study is being undertaken to study the efficacy of TENS in management of post operative pain. Aims & Objectives: 1. To assess the effectiveness of TENS in the management of post-operative pain. 2. To determine the role of TENS in reducing the incidence of postoperative pain associated complications. 3. To compare the effectiveness of TENS with postoperative analgesic group and patients receiving both TENS and analgesia. Materials & Methods: The study included 120 patients who underwent surgeries like Appendicectomy, hernioplasty and cholecystectomy and were divided in three groups. The detailed case history of one hundred twenty cases was recorded, clinical examination and investigations carried out. Post-operative pain was assessed with visual analogue score and functional activity score from POD 1 to POD 7 and associated postoperative complications. All these factors were used to monitor the progress and assess the recovery of the patient. Depending on use of TENS or analgesics or both and divided in groups A, B and C. Conclusions: TENS shows a very promising effect in controlling the postoperative pain, there was a significant reduction in pain in Hernioplasty and Appendicectomy surgeries but minimal in Cholecystectomy. TENS also shows that side effects of analgesics drugs are reduced to the minimal level.