Physics

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Shiv Nadar University

Herein, the structural, optical, and electrical properties of Au-induced crystallization in amorphous germanium (a-Ge) thin films are presented for future solar energy material applications. The a-Ge/Au thin films on quartz substrate were prepared via the e-beam and thermal evaporation techniques, respectively. The as-prepared samples were irradiated with 100 MeV Ni7+ ions at different fluences of 1 × 1012 ions per cm2, 5 × 1012 ions per cm2, 1 × 1013 ions per cm2 and 5 × 1013 ions per cm2. The ion irradiation did not result in the crystallization of Ge; instead, an increase in ion fluence decreased the Au crystallite size, which ultimately deteriorated the Au crystallization. The ion-irradiated samples were post-annealed for 4 h at a fixed temperature of 300 °C to achieve the crystallization of Ge. The post-annealing of the ion-irradiated samples led to the crystallization of Ge, where the crystallization of Ge was better for the samples irradiated at high fluence, leading to a smaller grain size corresponding to a high grain boundary, and hence high defect sink density. The energy bandgap of crystalline Ge was determined using reflectance and transmittance spectra, which was found to be low (∼0.64 eV) at higher fluence. The field-emission scanning electron microscopy and atomic force microscopy analyses show that the surface microstructure changed for the post-annealed samples irradiated at different fluences. The role of the fractal-like surface microstructure in multiple scattering of light on the film surface is discussed. Thus, the lower values of resistivity and sheet resistance as well as good optical properties of Ge make it a promising material for future poly-Ge-based solar cell applications. © 2020 The Royal Society of Chemistry.

World University root node

(SoNS) School of Natural Sciences

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CrystEngComm

Crystallization of Ge in ion-irradiated amorphous-Ge/Au thin films

The surface dynamics of TiO2 thin films, evolving under the implantation of 50 keV Ti ions, have been investigated. The morphological evolution, as investigated with atomic force microscopy, delineates a surface smoothening by ion implantation.The nanoscale structures at surfaces also undergo a size reduction. Scaling formalism has been applied to understand this temporal and spatial dynamics by estimating the scaling exponents (α,β and γ) via Height-height correlation function (HHCF) and power spectral density (PSD) investigations. The roughness exponent α (0.5 &lt; α &lt; 1) indicates these ion irradiated surfaces to be of self affine nature. Also, they evolve into locally smoother surfaces with increasing fluence. Negative growth exponent (β) also delineates surface smoothening. Exponent, γ, is observed to increase from ~2 at lower ion fluences to 2.5 at the highest fluence. This behavior suggests that diffusion is predominantly controlling the dynamical evolution of the ion irradiated TiO2 surfaces at the highest fluence, similar to that in the bulk case. © 2020 Elsevier B.V.

Nuclear Instruments and Methods in Physics Research, Section B: Beam Interactions with Materials and Atoms

Dynamic Surface evolution and scaling studies on TiO2 thin films by Ti ion implantation

In this work, we have explored the sensing capabilities of a monolayer 1T-HfTe2 nanosheet for the environmental hazardous gases like CO, CO2, NO, NO2, NH3, and N2O, along with common environmental gases like O2, N2, H2, and H2O, using first-principles density functional theory and nonequilibrium Green's function (NEGF) based methods. Through a detailed study of structural, electronic, vibrational, and device properties, we analyzed the strength of interactions and charge transfers between the adsorbent surface and the adsorbate molecules and revealed monolayer 1T-HfTe2 to be selective to effectively sense the NO gas at very low bias. In the case of NO (NO2 and O2) adsorption, we identified a significant (moderate) amount of charge transfer with the 1T-HfTe2 nanosheet surface which leads to a considerable (moderate) change in the electronic structure of that surface, that in turn affects the other properties of the surface also. In particular, a semimetal to metal transition in the case of adsorption of NO, NO2, and O2 is noticed, with more prominance in the case of NO, which we believe to be a major reason behind the higher interaction of these gases with the 1T-HfTe2 nanosheet and a selective sensing for NO. Our device simulations to attain the I-V characteristics for the pristine 1T-HfTe2 surface and all the adsorbed ones give a direct way to measure the resistance change due to adsorption of these gases, which is a basic tool to detect gas in a "resistance sensor", and can also be measured experimentally. These calculations clearly indicate a significant (small) increase in the current when NO (NO2 and O2) was adsorbed on the nanosheet of 1T-HfTe2 and almost no increase for the rest of the examined gases. All together, by understanding the basic underlying principles, this work unlocks the potential selective NO gas sensing capability of the less explored transition metal dichalcogenide (TMD) 1T-HfTe2. Our work also reveals it to be an efficient NO sensor that works even at low voltage, leading to less power consumption, and can effectively find an application in environmental monitoring and various areas of the medical field. © 2020 American Chemical Society.

ACS Applied Nano Materials

First-Principles Investigation of the 1T-HfTe2Nanosheet for Selective Gas Sensing

Flexible and low-power consuming integrated circuits are some of the basic requirements for smart wearable devices. High mobility solution-processed organic field-effect transistors (OFETs) have the potential to make a big impact in printed electronic circuits, but their overall performance is currently limited by unusually high threshold voltages (Vth). Herein, systematic optimization of donor–acceptor conjugated polymer, based on dithienothiophene (DTT) and thiophene-flanked diketopyrrolopyrrole (DPP), namely, PDPPT–DTT, OFETs by application of self-assembled monolayers (SAMs) at the semiconductor–dielectric, and semiconductor–metal interfaces is reported. The results clearly exhibit that simultaneous application of octyltrichlorosilane (OTS) as semiconductor–dielectric interface modifying layer and pentafluorobenzene thiol (PFBT) as semiconductor–metal interface modifying layer results in significantly lower Vth and subthreshold slope values from −14.07 V and 13.26 (V Dec−1) to +1.06 V and 7.11 (V Dec−1), respectively. This tailored approach is also beneficial in enhancing hole mobility values by an order of magnitude from 0.01 to 0.5 cm2 V−1 s−1 along with the possibility of switching from hole accumulation mode (Vth = −3.75 V) to depletion mode (Vth = +1.06 V) through device engineering. Simultaneous interface engineering reveals OFET electronic properties can be fine-tuned for robust circuits and low power electronic applications. © 2020 Wiley-VCH GmbH

Physica Status Solidi (A) Applications and Materials Science

Reduced Threshold Voltages and Enhanced Mobilities in Diketopyrrolopyrrole–Dithienothiophene Polymer-Based Organic Transistor by Interface Engineering

Graphene quantum dots (GQDs) possess excellent optical and electrical properties that can be used in a wide variety of application. Synthesis of hybrid nanoparticles with GQDs have been known to improve the properties further. Therefore, in this method, graphene quantum dots –gold (GQD-Au) hybrid nanoparticles were synthesized using GQDs which reduces HAuCl4.3H2O to Au nanoparticles on its surface at room temperature. The GQDs with self-passivated layers were synthesized by microwave assisted hydrothermal method using glucose as a single precursor. The synthesis process does not involve the use of harmful chemicals. The whole synthesis process of GQD and GQD-Au hybrid nanoparticles takes only five minutes. The synthesized GQDs have been extracted using citrate in order to increase the stability of the hybrid nanoparticles for up to four weeks. The size of the synthesized GQD-Au hybrid nanoparticles is in the range of 5–100 nm and were found to be luminescent under UV-A illumination. The merit of the following method over other synthesis techniques include its rapidity, ease of preparation, and no requirement of elaborate synthesis procedures and/or harmful chemicals. The GQD-Au hybrid nanoparticles can be used in several applications such as luminescent coatings for glass and windowpanes for automobiles, etc. The reducing property of GQDs can further be utilized for the reduction of various metal salts (AgNO3) and organic dyes (methylene blue and methyl orange). It presents a method/protocol-development of the luminescent GQD-Au hybrid particles of size ~ 5–100 nm. The GQD-Au hybrid particles find potential applications in luminescent coating applications. © 2020

MethodsX

Engineering of luminescent graphene quantum dot-gold (GQD-Au) hybrid nanoparticles for functional applications

New families of non-parity-time-symmetric complex potentials with all-real spectra are derived by the supersymmetry method and the pseudo-Hermiticity method. With the supersymmetry method, we find families of non-parity-time-symmetric complex partner potentials, which share the same spectrum as base potentials with known real spectra, such as the (complex) Wadati potentials. Different from previous supersymmetry derivations of potentials with real spectra, our derivation does not utilize discrete eigenmodes of base potentials. As a result, our partner potentials feature explicit analytical expressions, which contain free functions. With the pseudo-Hermiticity method, we derive a new class of non-parity-time-symmetric complex potentials with free functions and constants, whose eigenvalues appear as conjugate pairs. This eigenvalue symmetry forces the spectrum to be all-real for a wide range of choices of these functions and constants in the potential. Tuning these free functions and constants, phase transition can also be induced, where conjugate pairs of complex eigenvalues emerge in the spectrum. © 2020 Author(s).

Journal of Mathematical Physics

New families of non-parity-time-symmetric complex potentials with all-real spectra

We successfully demonstrated a detailed and systematic enhancement of organic field effect transistors (OFETs) performance using dithienothiophene (DTT) and furan-flanked diketopyrrolopyrrole based donor–acceptor conjugated polymer semiconductor namely PDPPF-DTT as an active semiconductor. The self-assembled monolayers (SAMs) treatments at interface junctions of the semiconductor–dielectric and at the semiconductor–metal electrodes has been implemented using bottom gate bottom contact device geometry. Due to SAM treatment at the interface using tailored approach, the significant reduction of threshold voltage (Vth) from − 15.42 to + 5.74 V has been observed. In addition to tuning effect of Vth, simultaneously charge carrier mobility (µFET) has been also enhanced the from 9.94 × 10−4 cm2/Vs to 0.18 cm2/Vs. In order to calculate the trap density in each OFET device, the hysteresis in transfer characteristics has been studied in detail for bare and SAM treated devices. Higher trap density in Penta-fluoro-benzene-thiol (PFBT) treated OFET devices enhances the gate field, which in turn controls the charge carrier density in the channel, and hence gives lower Vth = + 5.74 V. Also, PFBT treatment enhances the trapped interface electrons, which helps to enhance the mobility in this OFET architecture. The overall effect has led to possibility of reduction in the Vth with simultaneous enhancements of µFET in OFETs, following systematic device engineering methodology. © 2020, The Author(s).

Scientific Reports

Electrode and dielectric layer interface device engineering study using furan flanked diketopyrrolopyrrole–dithienothiophene polymer based organic transistors

In this work, we consider the weighted difference of two independent complex Wishart matrices and derive the joint probability density function of the corresponding eigenvalues in a finite-dimension scenario using two distinct approaches. The first derivation involves the use of unitary group integral, while the second one relies on applying the derivative principle. The latter relates the joint probability density of eigenvalues of a matrix drawn from a unitarily invariant ensemble to the joint probability density of its diagonal elements. Exact closed form expressions for an arbitrary order correlation function are also obtained and spectral densities are contrasted with Monte Carlo simulation results. Analytical results for moments as well as probabilities quantifying positivity aspects of the spectrum are also derived. Additionally, we provide a large-dimension asymptotic result for the spectral density using the Stieltjes transform approach for algebraic random matrices. Finally, we point out the relationship of these results with the corresponding results for difference of two random density matrices and obtain some explicit and closed form expressions for the spectral density and absolute mean. © 2020 IOP Publishing Ltd.

Journal of Physics A: Mathematical and Theoretical

Spectral statistics for the difference of two Wishart matrices

Dissipative particle dynamics (DPD) simulations are performed to study the phase transition of sodium dodecyl sulfate (SDS) in aqueous solution, which is an anionic surfactant commonly known as sodium dodecyl sulfate. In this work, the aim is to find a coarse-grained minimal model suitable to produce the full phase diagram of SDS. We examine the coarse-grained models of SDS, which have been used in earlier computational studies to produce the phases as well as for finding the critical micelle concentration (CMC) of SDS. We contrast the results based on these models with the experimental observations to assess their accuracy. Our research also takes into account the importance of sodium ions, which come from the partial dissociation of SDS, when dissolved in water. The effect of sodium ion has not been considered explicitly in the computational work done so far using dissipative particle dynamics. In light of the above explorations, we propose new models for SDS and demonstrate that they successfully produce a compendious SDS phase diagram, which can precisely overlay the experimental results. Copyright © 2020 American Chemical Society.

ACS Omega

Phase diagram study of sodium dodecyl sulfate using dissipative particle dynamics

We consider an ensemble of random density matrices distributed according to the Bures measure. The corresponding joint probability density of eigenvalues is described by the fixed trace Bures-Hall ensemble of random matrices which, in turn, is related to its unrestricted trace counterpart via a Laplace transform. We investigate the spectral statistics of both these ensembles and, in particular, focus on the level density, for which we obtain exact closed-form results involving Pfaffians. In the fixed trace case, the level density expression is used to obtain an exact result for the average Havrda-Charvát-Tsallis (HCT) entropy as a finite sum. Averages of von Neumann entropy, linear entropy and purity follow by considering appropriate limits in the average HCT expression. Based on exact evaluations of the average von Neumann entropy and the average purity, we also conjecture very simple formulae for these, which are similar to those in the Hilbert-Schmidt ensemble. © 2019 IOP Publishing Ltd.