Study of conduction mechanisms of InSeSb nano-chalcogenide alloys.

The electrical conduction mechanisms for bulk samples of In0.1Se0.9-xSbx(x= 0, 0.04, 0.08 and 0.12) nano-chalcogenide system, synthesized by the melt-quenching technique are investigated through current-voltage (I-V) characteristics. For the detailed study of conduction mechanism pellets of bulk samples are prepared. A thorough examination of electrical conductivity is done in the temperature range of 295-318 K and 0-50 V voltage range. FromI-Vmeasurements it is observed that samples are showing ohmic nature at lower field and non-ohmic nature at relatively higher field values. The temperature dependence of DC conductivity is analyzed using the Arrhenius relationship which is found to increase with Sb content. The value of activation energy and pre-exponential factor are calculated, which revealed that the conduction is due to the hopping of charge carriers among the localized states. Different parameters of Mott's variable range hopping such as degree of disorderT0, density of localized statesN(EF), hopping distance (Rhop), and hopping energy (W) are calculated. For the high field conduction process Poole-Frenkel, and Schottky processes are studied.

Ultralow lattice thermal conductivity in type-I Dirac MBene TiB2.

MBenes, the emergent novel two-dimensional family of transition metal borides have recently attracted remarkable attention. Transport studies of such two-dimensional structures are very rare and are of sparking interest. In this paper Using Boltzmann transport theory with ab-initio inputs from density functional theory, we examined the transport in TiB2MBene system, which is highly dependent on number of layers. We have shown that the addition of an extra layer (as in bilayer BL) destroys the formation of type-I Dirac state by introducing the positional change and tilt to the Dirac cones, thereby imparting the type-II Weyl metallic character in contrast to Dirac-semimetallic character in monolayer ML. Such non-trivial electronic ordering significantly impacts the transport behaviour. We further show that the anisotropic room temperature lattice thermal conductivity κLfor ML (BL) is observed to be 0.41 (0.52) and 2.00 (2.04) Wm-1K-1for x and y directions, respectively, while the high temperature κL(ML 0.13 Wm-1K-1and BL 0.21 Wm-1K-1at 900 K in x direction) achieves ultralow values. Our analysis reveals that such values are attributed to enhanced anharmonic phonon scattering, enhanced weighted phase space and co-existence of electronic and phononic Dirac states. We have further calculated the electronic transport coefficients for TiB2MBene, where the layer dependent competing behaviour is observed at lower temperatures. Our results further unravels the layer dependent thermoelectric performance, where ML is shown to have promising room-temperature thermoelectric figure of merit (ZT) as 1.71 compared to 0.38 for BL.

Hydrogenation driven ultra-low lattice thermal conductivity inß12borophene.

Borophene gathered large interest owing to its polymorphism and intriguing properties such as Dirac point, inherent metallicity, etc but oxidation limits its capabilities. Hydrogenated borophene was recently synthesised experimentally to harness its applications. Motivated by experimental work, in this paper, using first-principles calculations and Boltzmann transport theory, we study the freestandingß12borophene nanosheet doped and functionalised with hydrogen (H), lithium (Li), beryllium (Be), and carbon (C) atoms at differentß12lattice sites. Among all possible configurations, we screen two stable candidates, pristine and hydrogenatedß12borophene nanosheets. Both nanosheets possess dynamic and mechanical stability while the hydrogenated sheet has different anisotropic metallicity compared to pristine sheet leading to enhancement in brittle behaviour. Electronic structure calculations reveal that both nanosheets host Dirac cones (DCs), while hydrogenation leads to shift and enhancement in tilt of the DCs. Further hydrogenation leads to the appearance of additional Fermi pockets in the Fermi surface. Transport calculations reveals that the lattice thermal conductivity changes from 12.51 to 0.22 W m-1 K-1(along armchair direction) and from 4.42 to 0.07 W m-1 K-1(along zigzag direction) upon hydrogenation at room temperature (300 K), demonstrating a large reduction by two orders of magnitude. Such reduction is mainly attributed to decreased phonon mean free path and relaxation time along with the enhanced phonon scattering rates stemming from high frequency phonon flat modes in hydrogenated nanosheet. Comparatively larger weighted phase space leads to increased anharmonic scattering in hydrogenated nanosheet contributing to ultra-low lattice thermal conductivity. Consequently, hydrogenatedß12nanosheet exhibits a comparatively higher thermoelectric figure of merit (∼0.75) at room temperature along armchair direction. Our study demonstrates the effects of functionalisation on transport properties of freestandingß12borophene nanosheets which can be utilised to enhance the thermoelectric performance in two-dimensional (2D) systems and expand the applications of boron-based 2D materials.

Electronic and elastic correlations in AlB2-type two-dimensional hexagonal MBenes.

With the advent of MXenes as two-dimensional (2D) materials beyond graphene, non carbonic 2D materials analogically referred as MBenes have significantly attracted researchers' attention. Such 2D MBenes remains largely unexplored. Here, we systematically investigate electronic and elastic properties of 2D transition metal (TM) based AlB2-type hexagonal MBenes consisting of a honeycomb networked graphene like boron layer embedded with diverse TM atoms at center. First we determine the thermodynamic, dynamic, thermal, and mechanical stability of MBenes, considering a wide range of 3d, 4d, and 5d TM elements. Electronic and elastic calculations are performed for stable MBenes in order to parameterize and investigate the interdependence of properties. Elastic calculations predicts the brittle-ductile nature and bond character of MBenes while unraveling the in-plane auxetic behavior. Our electronic calculations predict the metallic band nature for 2D VB2, NbB2, TaB2, and WB2along with previously reported dirac points in 2D TiB2, FeB2, ZrB2, and HfB2. The elastic and electronic calculations clearly indicates the non-directional metallic bonds and intrinsically ductile nature of 2D-FeB2distinct from other MBenes. Subsequently we performed a covariance analysis to assess the correlation amongst the observables of interest and further establish the interdependence of the properties. Our calculations for elastic correlations also suggests that mechanical brittle-ductile nature and auxetic behavior of MBenes can be tuned by strain engineering of the elastic constants. Our results further suggests that strong correlations between Poisson ratio and d state electrons can be utilized to tune the auxetic behavior by careful doping of the materials. Our work demonstrates the weak elastic-electronic correlations, suggesting that the strain engineering can be utilized for the tailored behavior of MBenes for practical applications. Thus, our systematic analysis of the mechano-elastic and electronic properties of 2D hexagonal MBenes and their correlations advance our understandings of emergent 2D family.

Electronic structure and defect-induced luminescence study of phase-stabilized t-ZrO2 nanocrystals.

Luminescent tetragonal-ZrO2 (t-ZrO2 ) nanocrystals were synthesized using an optimized combustion method without post-synthesis annealing and characterized using X-ray diffraction, electron microscopy, Raman spectroscopy, X-ray photoelectron spectroscopy, UV-Vis. spectroscopy, photoluminescence spectroscopy, thermoluminescence (TL), and vibrating sample magnetometry. The as-synthesized t-ZrO2 nanocrystals have a bandgap of 4.65 eV and exhibit defect-assisted blue emission (Commission Internationale de I'Elcairage coordinates 0.2294, 0.1984) when excited at 270 nm. The defect states were qualitatively and quantitatively analyzed using TL after irradiating nanocrystals with γ- and UV radiations at various doses. The TL glow curves show intense emission in the high-temperature region from 523 to 673 K for both UV- and γ-irradiated samples; however, another less-intense TL peak was also observed in the low-temperature region from 333 to 453 K with γ irradiation at higher doses, indicating the formation of shallow trapping states. The activation energies, frequency factor, and order of kinetics were estimated using the computerized glow curve deconvolution method for the shallow and deep traps for γ- and UV-irradiated samples. The present study shows that phase-stabilized t-ZrO2 nanocrystals are potential candidates for luminescence-based applications.