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A deep understanding of hot carrier (HC) dynamics is important to improve the performance of optoelectronic devices by reducing the thermalization losses. Here, we investigate the hot hole cooling ...

The simultaneous occurrence of gigantic piezoelectricity and Rashba effect in two-dimensional materials are unusually scarce. Inversion symmetry occurring in MX3 (M= Ti, Zr, Hf; X= S, Se) monolayer...

Using first-principles calculations, we demonstrate a combination of two emergent fields, type II van der Waals heterostructures and Janus structures, for the purpose of optimizing the harvesting o...

Interfaces of heterostructures are routinely studied for different applications. Interestingly, monolayers of the same material when interfaced in an unconventional manner can bring about novel properties. For instance, CdS monolayers, stacked in a particular order, are found to show unprecedented potential in the conversion of nanomechanical energy, solar energy, and waste heat into electricity, which has been systematically investigated in this work, using DFT-based approaches. Moreover, stable ultrathin structures showing strong capabilities for all kinds of energy conversion are scarce. The emergence of a very high out-of-plane piezoelectricity,

A density functional framework has been used to explore the geometrical stability and electronic properties of phosphorene sheet. The non-equilibrium Green’s function approach has also been utilize...

Organic quinones conjugated with a conductive support like graphene via a covalent and/or non-covalent approach are emerging as low-cost and sustainable alternatives to conventional pseudocapacitive materials because of their fast and reversible redox kinetics. Herein, for the first time, reduced graphene oxide (rGO) networks functionalized with the eco-friendly dopamine (DA) moiety in a combined covalent and non-covalent manner have been explored by a facile hydrothermal synthesis method as high-performance pseudocapacitive materials. Further, a unique in situ electrochemical polymerization approach has been undertaken in an attempt to boost the overall storage capacity as well as cycling stability. Electrochemical tuning of the active material can result in an enhancement in the specific capacitance via dual improvement in faradaic and capacitive current as compared to the initial xerogel material, highlighting the significance of the in situ process. The electrode material shows a highest specific capacitance (CSP) of 348 F g−1 at a current density of 0.5 A g−1 in 1 M H2SO4 while retaining 60% of its initial capacitance even at a very high current density of 200 A g−1. A metal-free all-solid-state symmetric supercapacitor was developed with the in situ electropolymerized active material and the as-fabricated device exhibits an excellent CSP of 218 F g−1 at 0.325 A g−1 in 1 M PVA/H2SO4 gel electrolyte with a maximum energy density of 30.3 W h kg−1 and power density up to 13 kW kg−1. Most importantly, the material exhibited an extraordinary stability of 53 000 charge–discharge cycles while retaining 94% of its initial capacitance, while the device also shows an excellent capacitance retention of 92% even after 10 000 continuous cycles. Simultaneously, the DFT study was amended to understand the covalent and non-covalent interaction of the redox species, its charge storage mechanism and charge density distribution as well as to calculate the density of states.

The response of the electronic properties of the HfN2 monolayer to external perturbation, such as strain and electric fields, has been extensively investigated using density functional theory calculations for its device-based applications and photocatalysis. The HfN2 monolayer is found to be a semiconductor showing a direct band gap of 1.44 eV, which is widely tunable by 0.9 eV via application of biaxial strain. Furthermore, the tunability in the band edges of the HfN2 monolayer straddling the water redox potential under a biaxial strain of ±10% makes it suitable for solar energy harvesting via photocatalytic applications over a wide range (0–7) of pH. The band gap can be decreased by 29.8% under a biaxial tensile strain of 10%. Upon incorporation of spin orbit coupling (SOC) a large spin splitting at the conduction band (Δc ∼ 314 meV) and a small splitting at the valence band (Δv ∼ 32 meV) are noted, which is attributable to the orbital composition of the band edges. The spin splitting in the band edges is found to be adjustable via biaxial compressive strain. The strain dependent mechanical properties and stability reveal the ability of the HfN2 monolayer to withstand a large magnitude of strain of up to ±10%, thereby bringing about a giant tunability in its Young modulus (Y) from 66 N m−1 to 283 N m−1, which is gainfully exploitable in flexible electronics. The tunability in Y over such a wide range has not been observed in other 2D materials. Moreover, the HfN2 monolayer undergoes a transition from a semiconducting to a metallic state under the application of a normal electric field or gate voltage of 0.48 V A−1, which may potentially serve as the OFF (semiconducting) and ON (metallic) state in devices. Interestingly, an electric field of such intensity has been realized experimentally using pulsed ac field technology. Such a small gate voltage will greatly lower its power consumption. The electronic origin of this transition from the OFF to the ON state is found to arise from unoccupied NFEG (Nearly Free Electron Gas) states. A HfN2 monolayer based tunnel field effect transistor (t-FET) is proposed herewith as a model device for low-power digital data storage, thereby paving new avenues in flexible electronics and memory devices.

A stable ultrathin 2D van der Waals (vdW) heterobilayer, based on the recently synthesized boron monophosphide (BP) and the widely studied molybdenum disulfide (MoS2), has been systematically explored for the conversion of waste heat, solar energy, and nanomechanical energy into electricity. It shows a gigantic figure of merit (ZT) > 12 (4) for p (n)-type doping at 800 K, which is the highest ever reported till date. At room temperature (300 K), ZT reaches 1.1 (0.3) for p (n)-type doping, which is comparable to experimentally measured ZT = 1.1 on the PbTe-PbSnS2 nanocomposite at 300 K, while it outweighs the Cu2Se-CuInSe2 nanocomposite (ZT = 2.6 at 850 K) and the theoretically calculated ZT = 7 at 600 K on silver halides. Lattice thermal conductivity (κl ≈ 49 W m-1 K-1) calculated at room temperature is lesser than those of black phosphorene (78 W m-1 K-1) and arsenene (61 W m-1 K-1). The nearly matched lattice constants in the commensurate lattices of the constituent monolayers help to preserve the direct band gap at the K point in the type II vdW heterobilayer of MoS2/BP, where BP and MoS2 serve as donor and acceptor materials, respectively. An ultrahigh carrier mobility of ∼20 × 103 cm2 V-1 s-1 is found, which exceeds those of previously reported transition metal dichalcogenide-based vdW heterostructures. The exciton binding energy (0.5 eV) is close to those of MoS2 (0.54 eV) and C3N4 (0.33 eV) single layers. The calculated power conversion efficiency (PCE) in the monolayer MoS2/BP heterobilayer exceeds 20%. It surpasses the efficiency in MoS2/p-Si heterojunction solar cells (5.23%) and competes with the theoretically calculated ones, as listed in the manuscript. Furthermore, a high optical absorbance (∼105 cm-1) of visible light and a small conduction band offset (0.13 eV) make MoS2/BP very promising in 2D excitonic solar cells. The out-of-plane piezoelectric strain coefficient, d33 ≈ 3.16 pm/V, is found to be enhanced 4-fold (∼14.3 pm/V) upon applying 7% vertical compressive strain on the heterobilayer, which corresponds to ∼1 kbar of hydrostatic pressure. Such a high out-of-plane piezoelectric coefficient, which can tune top-gating effects in ultrathin 2D nanopiezotronics, is a relatively new finding. As BP has been synthesized recently, experimental realization of the multifunctional, versatile MoS2/BP heterostructure would be highly feasible.

Type II van der Waals (vdW) heterobilayers of six group III chalcogenide monolayers (MX; M = Ga, In; X = S, Se, Te) have been systematically investigated and compared with the pristine monolayers v...

A stable 2D van der Waals (vdW) heterobilayer, constituted by boron monophosphide (BP) and Gallium Nitride (GaN) monolayers, has been explored for different kinds of energy conversion and nanoelectronics. The nearly matched lattice constants of GaN and BP are commensurate with each other in their lattice structures. The out-of-plane inversion asymmetry coupled with the large difference in atomic charges between the GaN and BP monolayers induces in the heterobilayer a giant out-of-plane piezoelectric coefficient (|d33|max ≈ 40 pm V-1), which is the highest ever reported in 2D materials of a finite thickness. It is much higher than the out-of-plane piezoelectric coefficient reported earlier in multilayered Janus transition metal dichalcogenide MXY (M = Mo, W; X, Y = S, Se, Te) (|d33|max = 10.57 pm V-1). Such a high out-of-plane piezoelectricity found in a BP/GaN heterobilayer can bring about gigantic strain-tunable top gating effects in nanopiezotronic devices based on the same. Moreover, electron mobility (∼104 cm2 V-1 s-1) is much higher than that of transition metal dichalcogenides and conventional semiconductors. The origin of low lattice thermal conductivity (κL ∼ 25.25 W m-1 K-1) in BP/GaN at room temperature, which is lower than that of black phosphorene (78 W m-1 K-1), buckled arsenene (61 W m-1 K-1), BCN (90 W m-1 K-1), MoS2 (34.5 W m-1 K-1) and WS2 (32 W m-1 K-1) monolayers, has been systematically investigated via phonon dispersion, lattice thermal conductivity, phonon lifetime and mode Grüneisen parameters. The valence band maximum (VBM) and conduction band minimum (CBM) arising from GaN and BP monolayers respectively result in a type II vdW heterobilayer, which is found to be thermodynamically favorable for photocatalytic water splitting in both acidic and neutral media. The exciton binding energies are comparable to those of MoS2 and C3N4 single layers, while the absorbance reaches as high as ∼105 cm-1 in the visible wavelength region. The emergence of high piezoelectricity, high carrier mobility, low lattice thermal conductivity and photocatalytic water splitting abilities in the proposed vdW heterobilayer signifies enormous potential for its versatile applications in nanoscale energy harvesting, e.g., nano-sensors in medical devices, future nanopiezotronics, 2D thermoelectrics and solar energy conversion.

The coupling of piezoelectric properties with Rashba spin-orbit coupling (SOC) has proven to be the limit breaker that paves the way for a self-powered spintronic device (ACS Nano, 2018, 12, 1811-1820). For further advancement in next-generation devices, a new class of buckled, hexagonal magnesium-based chalcogenide monolayers (MgX; X = S, Se, Te) have been predicted which are direct band gap semiconductors satisfying all the stability criteria. The MgTe monolayer shows a strong SOC with a Rashba constant of 0.63 eV A that is tunable to the extent of ±0.2 eV A via biaxial strain. Also, owing to its broken inversion symmetry and buckling geometry, MgTe has a very large in-plane as well as out-of-plane piezoelectric coefficient. These results indicate its prospects for serving as a channel semiconducting material in self-powered piezo-spintronic devices. Furthermore, a prototype for a digital logic device can be envisioned using the ac pulsed technology via a perpendicular electric field. Heat transport is significantly suppressed in these monolayers as observed from their intrinsic low lattice thermal conductivity at room temperature: MgS (9.32 W m-1 K-1), MgSe (4.93 W m-1 K-1) and MgTe (2.02 W m-1 K-1). Further studies indicate that these monolayers can be used as photocatalytic materials for the simultaneous production of hydrogen and oxygen on account of having suitable band edge alignment and high charge carrier mobility. This work provides significant theoretical insights into both the fundamental and applied properties of these new buckled MgX monolayers, which are highly suitable for futuristic applications at the nanoscale in low-power, self-powered multifunctional electronic and spintronic devices and solar energy harvesting.

Semiconducting indium selenide (InSe) monolayers have drawn a great deal of attention among all the chalcogenide two-dimensional materials on account of their high electron mobility; however, they suffer from low hole mobility. This inherent limitation of an InSe monolayer can be overcome by stacking it on top of a boron phosphide (BP) monolayer, where the complementary properties of BP can bring additional benefits. The electronic, optical, and external perturbation-dependent electronic properties of InSe/BP hetero-bilayers have been systematically investigated within density functional theory in anticipation of its cutting-edge applications. The InSe/BP heterostructure has been found to be an indirect semiconductor with an intrinsic type-II band alignment where the conduction band minimum (CBM) and valence band maximum (VBM) are contributed by the InSe and BP monolayers, respectively. Thus, the charge carrier mobility in the heterostructure, which is mainly derived from the BP monolayer, reaches as high as 12 × 103 cm2 V−1 s−1, which is very much desired in superfast nanoelectronics. The suitable bandgap accompanied by a very low conduction band offset between the donor and acceptor along with robust charge carrier mobility, and the mechanical and dynamical stability of the heterostructure attests its high potential for applications in solar energy harvesting and nanoelectronics. The solar to electrical power conversion efficiency (20.6%) predicted in this work surpasses the efficiencies reported for InSe based heterostructures, thereby demonstrating its superiority in solar energy harvesting. Moreover, the heterostructure transits from the semiconducting state (the OFF state) to the metallic state (the ON state) by the application of a small electric field (∼0.15 V Å−1) which is brought about by the actual movement of the bands rather than via the nearly empty free electron gas (NFEG) feature. This thereby testifies to its potential for applications in digital data storage. Moreover, the heterostructure shows strong absorbance over a wide spectrum ranging from UV to the visible light of solar radiation, which will be of great utility in UV—visible light photodetectors.

Despite extensive studies on coupled spin and valley physics, occurrence of these properties is currently limited to transition-metal dichalcogenides (TMDCs) and graphene. Therefore, exploration of materials beyond TMDCs and graphene is necessary for a further advancement in valleytronics. In this work, the $\mathrm{Hf}{\mathrm{N}}_{2}$ monolayer, a theoretically reported semiconductor having a direct band gap has been investigated in-depth for its applications in valleytronics and spintronics. It exhibits a large valley spin splitting (VSS) \ensuremath{\sim}314 meV at the conduction band (CB) due to strong spin-orbit coupling (SOC). Such a large VSS at the CB is unique and comparable to that of tungsten-based dichalcogenide monolayers at its valence band (VB). The spin splitting is nearly insensitive to small in-plane strain, whereas the optical transition frequencies between the spin states $({\ensuremath{\omega}}_{\mathrm{up}},\phantom{\rule{4pt}{0ex}}{\ensuremath{\omega}}_{\mathrm{down}})$ in the CB and VB is effectively strain tunable. Inspired by the experimentally observed higher exciton lifetime 1.8 ns in $\mathrm{MoS}{\mathrm{e}}_{2}/\mathrm{WS}{\mathrm{e}}_{2}$ type-II van der Waals heterostructure (vdWH) than in the monolayers \ensuremath{\sim}2.1 (2.5) $ps$ in monolayer $\mathrm{Mo}{\mathrm{S}}_{2}$ $(\mathrm{W}{\mathrm{S}}_{2})$, a type-II (vdWH) prototype has been demonstrated by stacking $\mathrm{Hf}{\mathrm{N}}_{2}$ monolayer over $1\mathrm{H}\ensuremath{-}\mathrm{WS}{\mathrm{e}}_{2}$ monolayer for an extended lifetime of valley polarized excitons and large magnitude of spin splitting at both CB and VB. The elastic properties highlight the robustness of $\mathrm{Hf}{\mathrm{N}}_{2}$ monolayer and vdWH, as the bending modulus and critical buckling strain are found to be superior to that in graphene. Further, carrier mobility calculated using the deformation potential theory in $\mathrm{Hf}{\mathrm{N}}_{2}/\mathrm{WS}{\mathrm{e}}_{2}$ is as high as ${\ensuremath{\mu}}_{h}=16\phantom{\rule{4pt}{0ex}}\ifmmode\times\else\texttimes\fi{}{10}^{3}\phantom{\rule{0.16em}{0ex}}\mathrm{c}{\mathrm{m}}^{2}{\mathrm{V}}^{1}{\mathrm{s}}^{\ensuremath{-}1}$. Moreover, this work introduces the $\mathrm{Hf}{\mathrm{N}}_{2}$ monolayer as an exceptionally promising valleytronic material having valley properties complementary to that of Group VI TMDC. This is a report on valley contrasting physics and VSS at the CB, which would strongly motivate experimentalists to synthesize and explore this predicted two-dimensional material.

Phonons in crystalline solids are of utmost importance in governing its lattice thermal conductivity (k L). In this work, k L in hafnium (Hf) dichalcogenide monolayers has been investigated based on ab initio DFT coupled to linearized Boltzmann transport equation together with single-mode relaxation-time approximation. Ultra-low k L found in HfS2 (2.19 W m-1 K-1), HfSe2 (1.23 W m-1 K-1) and HfSSe (1.78 W m-1 K-1) monolayers at 300 K, is comparable to that of the state-of-art bulk thermoelectric materials, such as, Bi2Te3 (1.6 W m-1 K-1), PbTe (2.2 W m-1 K-1) and SnSe (2.6 W m-1 K-1). Gigantic longitudinal-transverse optical (LO-TO) splitting of up to 147.7 cm-1 is noticed at the Brillouin zone-centre (Γ-point), which is much higher than that in MoS2 single layer (∼2 cm-1). It is driven by the colossal phonon-electric field coupling arising from the domination of ionic character in the interatomic bonds and Born effective or dynamical charges as high as 7.4e on the Hf ions, which is seven times that on Mo in MoS2 single layer. Enhancement in k L occurs in HfS2 (2.19 to 4.1 W m-1 K-1), HfSe2 (1.23 to 1.7 W m-1 K-1) and HfSSe (1.78 to 2.2 W m-1 K-1) upon the incorporation of the non-analytic correction term. Furthermore, the mode Gruneisen parameter is calculated to be as high as ∼2.0, at room temperature, indicating a strong anharmonicity. Moreover, the contribution of optical phonons to k L is found to be ∼12%, which is significantly high than that in single-layer MoS2. Large atomic mass of Hf (178.5 u), small phonon group velocities (4-5 km s-1), low Debye temperature (∼166 K), low bond and elastic stiffness (Young's modulus ∼75 N m-1), small phonon lifetimes (∼6 ps), low specific heat capacity (∼17 J K-1 mol-1) and strong anharmonicity are collectively found to be the factors responsible for such a low k L. These findings would be immensely helpful in designing thermoelectric interconnects at the nanoscale and 2D material-based energy harvesters.

The exfoliation of covalent organic frameworks into covalent organic nanosheets (CONs) not only helps to reduce fluorescence turn-off phenomena but also provides well-exposed active sites for fast response and recovery for various applications. The present work is an example of rational designing of a structure constructed by condensing triaminoguanidinium chloride (TG

Piezoelectric nanomaterials have been emerging as flagship materials for harvesting nanoelectromechanical energy. Pristine, semiconducting 1T-MX2 (M = Zr and Hf; X = S, Se, and Te) monolayers are intrinsically centrosymmetric, and hence non-piezoelectric. This inversion symmetry is broken in their Janus monolayer (non-centrosymmetric) structures, leading to the emergence of a high degree of piezoelectricity in them. This brings along a new dimension in nanoscale piezoelectricity, as the origin of this piezoelectricity is predominantly ionic in nature, in contrast to the 1H-MoS2 monolayer, where it is of electronic character. DFT calculations reveal the piezoelectric coefficient (d22 = 4.68–14.58 pm V−1) in these Janus monolayers to be much higher than that in single layer 1H-MoS2 (d11 = 2.99 pm V−1). 9% uniaxial tensile strain applied along the arm-chair direction is found to raise d22 in HfSSe Janus monolayers to 123.04 pm V−1, which reaches the level of piezoelectric coefficients in the state-of-the-art perovskites. The major contribution of the ionic component to the piezoelectric coefficient is attributable to the predominance of ionic character in the interatomic bonds in these monolayers, which arises from the decoupled band edges, i.e., no hybridization between the band edge states (chalcogen-p and metal-d). Contrarily, 1H-MX2 (M = Mo and W; X = S, Se, and Te) monolayers with coupled band edges are held together mainly by covalent bonds, resulting in the dominance of electronic contribution to piezoelectricity. The nature of band edges causes a lower deformation potential for electrons in 1T Hf and Zr based dichalcogenide monolayers and their Janus structures with respect to 1H-MX2 (M = Mo and W; X = S, Se, and Te) monolayers. This induces a much higher electron mobility in the former than in 1H-MX2 (M = Mo and W; X = S, Se, and Te) monolayers. The carrier mobility calculated using Lang et al.'s formalism [Phys. Rev. B, 2016, 94, 235306] agrees well with the experimentally measured electron mobility. Our predictive findings underscore the imminent need to synthesize these 1T-MX2 semiconducting Janus structures to induce a high level of piezoelectricity together with robust electron mobility.

2D semiconducting transition-metal-dichalcogenides (TMDCs) have drawn a surge of research interests in ultra-thin nanoelectronics and optoelectronics, owing to their moderate charge carrier mobility. Due to a large variation in reported data on charge carrier mobilities of these free-standing monolayers, a more accurate determination of carrier mobilities is a must. This article highlights room temperature carrier mobilities calculated in group-VI B TMDC (i.e., MX2 (M = Mo, W; X = S, Se, Te)) monolayers via a systematic comparison of reliable models used within the framework of density functional theory. Additionally, piezoelectric effects on carrier mobility have been incorporated. The robust formulation by Lang et al. [Phys. Rev. B, 2016, 94, 235306] gives the most accurate estimation, where the carrier (i.e. electron or hole) mobility and the ratio of hole to electron mobility reach closest to the experimentally measured value. The higher mobility of holes with respect to electrons is attributed to the smaller deformation potential of the holes, while smaller carrier effective masses and larger elastic moduli cause larger carrier mobility in WX2 relative to MoX2. Carrier mobility is found to drop from S to Te and also from W to Mo in MX2 on account of the decrease in the elastic moduli, which is consistent with the increase in the macroscopic static dielectric constant. Erroneously high electron mobility (>1000 cm2 V−1 s−1), which is two orders of magnitude higher than the experimental ones, has been disputably reported earlier for MoTe2 and WS2 monolayers. It owes its origin to the ultrasmall deformation potential found therein. High (∼1.4–2.3) hole to electron mobility ratio in MX2 monolayers is found to occur in our work as MoSe2 > WS2 > MoS2 > WSe2 > WTe2 > MoTe2. This large carrier mobility ratio will facilitate an efficient separation of electron–hole pairs, which is particularly useful in optoelectronic applications, e.g., photovoltaics, photocatalysis and artificial photosynthesis. In all terms including optical absorbance, a WS2 monolayer is found to be most suitable for photophysical processes. Using the Nernst formulation, water splitting and CO2 photoreduction on WS2 are found to be favorable over a wide range of pH.

High photovoltaic efficiency with a staggered type II band alignment has been found in the heterobilayers of relatively less explored dialkali metal oxides and chalcogenides, using DFT-D3 and HSE06 functional. Out of all the type II van der Waals (vdW) heterostructures which are possible through a combination of any two individual monolayers stacked on top of one another, K2O/Cs2O, K2S/Cs2S, and K2Se/Cs2S heterobilayers are found to exhibit a high-power conversion efficiency (PCE) of 21.56%, 19.31%, and 16.43% respectively along with high carrier mobility (>104 cm2/V.s) and high visible light absorption coefficient (α ~ 105 cm−1). Therefore, these heterobilayers will be highly useful in photovoltaic applications. The PCE in the tandem solar cells comprising of the wider band gap, Na2Te/K2Se, in the top cell, and the narrower bandgap, K2O/Cs2O, in the bottom cell, is observed to reach close to 30%. The binding energies in K2O/Cs2O, K2S/Cs2S, and K2Se/Cs2S heterobilayers are found to be −36.38, −28.99, and −29.52 meV/A2 at the interlayer layer distance of 3.30 A, 2.75 A, and 4.0 A respectively, at equilibrium. The large magnitude of negative binding energy indicates the exothermicity and experimental feasibility in the process of formation of these vdW heterostructures.

Besides the intrinsic semiconducting direct band gap in monolayer MoS2 (ML-MoS2), piezoelectricity arises in it due to the broken inversion symmetry. This underscores the need to unveil the simultaneous response of piezoelectric and semiconducting properties to different modes of strain. The present study explores a synergic coupling between these two properties in adaptive nanopiezotronic devices, using density functional theory. Out of the different strain types studied, shear strain and uniaxial tensile strain applied along the zigzag direction are found to be most effectual in fortifying the piezoelectric properties in ML-MoS2. Shear strain is found to raise both the piezoelectric stress (e11) and strain (d11) coefficients by 3 orders of magnitude, while uniaxial tensile strain increases the same by 2 orders of magnitude for an applied mechanical strain of 5%. The effect is found to be even stronger upon reaching the elastic limit, which is found to lie within 5–10% strain for different strain modes s...

Density functional theory(DFT) coupled to non-equilibrium green’s function has been performed to explore avenues to open band gap in graphene nanoribbons in order to reach a high current on/off ratio for their potential applications in transistors and futuristic nanodevices. Introduction of defects such as C-atom vacancies and doping with elements lying on either side of carbon in the periodic table have been invoked to engineer the band gap in narrow zig-zag nanoribbons. By varying the concentration of B and N dopants in zig-zag nanoribbon (ZGNR), their electronic structure is transformed to that of p-type and n-type semiconductor. A maximum band gap of 0.98 eV, 0.88 eV and 0.89 eV is achieved upon incorporating carbon-atom vacancies, boron doping and nitrogen doping respectively. Transport properties have been analyzed through the calculation of transmission spectrum and I–V characteristics. Doped and defective ZGNRs reported herewith show non-linearity in the current-voltage characteristics. The highest current value achieved by doping and defect is 16.5,39 μA for boron dopant, 18, 27 μA for nitrogen and 25.5, 30.2 μA for defects.

In the present work, density functional theory (DFT) combined with non-equilibrium Green’s function (NEGF) formalism is performed. The electronic properties (band structure and density of states) and transport properties (transmission spectrum and I–V characteristics) of armchair silicene nanoribbons (ASiNRs) doped with various elements, such as Al, Ga, In, Tl, P, As, Sb and Bi, are investigated. The negative differential resistance is observed for each doped ASiNR. The most geometrically stable structure and the maximum peak current to valley current (Ip/Iv) ratio is observed in indium (In) doped ASiNRs. Finally, In doped ASiNRs are proposed for field effect transistor (ASiNR-FET) formation using the high dielectric constant value of lanthanum oxide (La2O3 = 29) at different applied gate voltages (−0.1 to 0.4 V). The In doped ASiNR device shows a negative differential resistance phenomenon, which can be controlled by an applied gate voltage. It is found that doping with In in the electrodes and scattering region provides a higher drain current, and higher Ion/Ioff and Ip/Iv ratios. Our results have great application in digital devices and memory devices, and high frequency applications for future nanoelectronics.

Truxene is employed as a building block to successfully synthesize novel covalent organic frameworks (COFs). The condensation reaction between truxene (10,15-dihydro-5H-diindeno[1,2-a:1′,2′-c]fluorene, TX) and 1,4-phenylenediboronic acid (DBA) results in a crystalline COF with boron ester linkages (COF-TXDBA) and a surface area of 1526 m2 g−1, as confirmed by powder X-ray diffraction (PXRD) and Brunauer–Emmett–Teller (BET) surface area measurements. This is the first study where nanochannels generated by periodic COF planar layers are shown to ease the interactions of the boron ester linkages with the water molecules for efficient humidity sensing. The COF-TXDBA based % RH sensor exhibits a change of 3 orders of magnitude in impedance in the 11–98% RH range, with response and recovery times of 37 s and 42 s, respectively. The response transients measured by switching COF-TXDBA sensor back and forth in 4 loops of % RH range displays excellent reversibility of the sensor with a deviation of 2.3% in the switching process.

A nanohybrid material containing carbon-supported molybdenum carbide and nitride nanoparticles of size ranging from 8 to 12 nm exhibits excellent HER catalytic activity. This molybdenum based catalyst (MoCat) is designed as a highly efficient, low-cost (precious-metal-free), highly stable electrocatalyst for water electrolysis in acidic media, and synthesized using a simple methodology. These nanoparticles (β-Mo2C and γ-Mo2N) were produced in situ using a metal precursor and C/N source via a controlled solid state reaction. An overpotential of 96 mV for driving a current density of 10 mA cm−2 was measured for the MoCat catalyst, which is very close to that of commercially available Pt/C catalysts (61 mV).

Elastic-mechanical deformations are found to dramatically alter the electronic properties of monolayer (ML) $\mathrm{Mo}{\mathrm{S}}_{2}$; particularly, the low-energy Bloch bands are responsive to a directional strain. In this study, in-plane uniaxial deformation is found to drift the low-energy electron/hole valleys of strained $\mathrm{ML}\text{\ensuremath{-}}\mathrm{Mo}{\mathrm{S}}_{2}$ far away from $K/K'$ points in the Brillouin zone (BZ). The amount of drift differs notably from hole to electron bands, where the conduction band minimum (CBM) drifts nearly 2 times more than the valence band maximum (VBM) in response to a progressively increasing strain field (0--10%). The resulting strain-induced valley asymmetry/decoherence can lift the momentum degeneracy of valley carriers at the $K$ point, thereby affecting the low-energy valley excitations ($K$-valley polarization) in a strained $\mathrm{ML}\text{\ensuremath{-}}\mathrm{Mo}{\mathrm{S}}_{2}$ lattice. The quantum origin of this decoherent valley arises from the differences in the Bloch orbital wave functions of electron and hole states at the exciton band edges and their deformation under strain. A higher drift (g1.5 times) is noticed when strain is along the zigzag (ZZ) axis relative to the armchair (AC) axis, which is attributed to a faster decline in Young's modulus and Poisson's ratio (PR) along the ZZ direction. A similar valley drift only in the VBM of uniaxially strained $\mathrm{ML}\text{\ensuremath{-}}\mathrm{Mo}{\mathrm{S}}_{2}$ was reported in an earlier local density approximation (LDA) based density functional theory (DFT) study [Q. Zhang et al., Phys. Rev. B 88, 245447 (2013)], where a massive valley drift occurring at the CBM was fully overlooked. Moreover, the giant VBM drift reported therein is 6 times the drift observed in our DFT studies based on spin-orbit coupling (SOC) and Perdew-Burke-Ernzerhof generalized gradient approximation (PBE-GGA) functionals. The physical origin of valley drift has been ascertained in our thorough investigations. The robustness of our approach is substantiated as follows. With progressive increase in strain magnitude (0--10%), the band gap remains direct up to 2% uniaxial tensile strain, under SOC, which accurately reproduces the experimental strain-induced direct-to-indirect band gap transitions occurring at \ensuremath{\sim}2% strain. Based on LDA-DFT [Q. Zhang et al., Phys. Rev. B 88, 245447 (2013)], this crossover in band gap has been incorrectly reported to occur at a higher value of uniaxial strain of 4%. Moreover, the direct SOC band gap shows a linear redshift at a rate of 51--53 meV/(% of strain), under uniaxial tensile strain, which is in excellent quantitative agreement with experimentally observed rates in the redshift of direct excitonic transitions measured in several optical absorption and photoluminescence (PL) spectroscopy experiments. In addition, the Berry curvature \textohm{}($k$) of electron/hole bands gets significantly modulated in strained $\mathrm{ML}\text{\ensuremath{-}}\mathrm{Mo}{\mathrm{S}}_{2}$, where the intensity of the flux profile increases as a function of the magnitude of strain with an opposite drift around $K/K'$, when strained along the ZZ/AC direction. A strong strain-valley coupling leads to an enhancement in the strength of spin-orbit induced spin splitting of bands at VBM/CBM, which is sizably enhanced (\ensuremath{\sim}7 meV) simply by the strain-controlled orbital motions. Our findings are of prime importance in the valley physics of $\mathrm{Mo}{\mathrm{S}}_{2}$. Besides, the important theoretical insights emerging from this work will trigger further experimental investigations on $\mathrm{ML}\text{\ensuremath{-}}\mathrm{Mo}{\mathrm{S}}_{2}$ to realize its novel technological potential in nanoelectronics, spintronics, and valleytronics.

Graphene-based van der Waals heterostructure (vdWH) comprising of HfN2 monolayer stacked over graphene has been designed and studied based on density functional theory. The vdWH forms a n-type Schottky contact with a Schottky barrier height (SBH) of 0.67 eV, while it exhibits p-type SBH of 0.93 eV. The response of SBH and electrical contact properties to external perturbation, such as, vertical strain and electric field has been investigated thoroughly. Under the application of strain and normal electric field within range of ±0.3 V/A, the type of electrical contacts, i.e., n/p type Schottky or Ohmic, is found to be interconvertible, while electron/hole doping in graphene is tunable by a doping carrier concentration of up to ~1013 cm−2, which lies between experimentally observed molecular doping (~1012 cm−2) and electrolytic gating (~1014 cm−2). Such an extremely high tunability in electrical contacts, doping carrier concentration along with its excellent optical response in the visible light region shows unrivalled potential of this vdWH in high performance graphene-based futuristic Schottky transistors with high on/off ratio, ultrathin phototransistor with high gain, low-power multivalued optical non volatile memory devices, and nanoelectronics.

It has been reported that N and B doping induce a quasi-bound state that suppresses the conduction in graphene nanoribbon (GNR)-based junctions, while an H defect or a pyridine-like N-atom (PN) substitution at the edge of the GNR does not affect the transmission close to the Fermi energy. However, these results may vary when the size of the functional unit of the GNR junction decreases to a molecular level. In this study, a defect is introduced to a test-bed architecture consisting of a polyacene bridging two zigzag GNR electrodes, which changes the molecular state alignment and coupling to the electrode states, and varies the equivalence between two eigen-channels at the Fermi level. It is revealed that B and N atom substitution, and H defects play a dual role in the molecular conductance, whereas the PN substitution acts as an ineffective dopant. The results obtained from density functional theory combined with the non-equilibrium Green’s function method aid in determining the optimal design for the GNR-based ultra-small molecular devices via defect engineering.

We have investigated the modeling of boron-substituted molecular single-electron transistor (SET), under the influence of a weak coupling regime of Coulomb blockade between source and drain metal electrodes. The SET consists of a single organic molecule (pyridine/pentane/1,2-azaborine/butylborane) placed over the dielectric, with boron (B) as a substituent. The impact of B-substitution on pyridine and pentane molecules in isolated, as well as SET, environments has been analyzed by using density functional theory-based ab initio packages Atomistix toolkit-Virtual NanoLab and Gaussian03. The performance of proposed SETs was analyzed through charging energies, total energy as a function of gate potential and charge stability diagrams. The analysis confirms that the B-substituted pentane (butylborane) and the boron-substituted pyridine (1,2-azaborine) show remarkably improved conductance in SET environment in comparison to simple pyridine and pentane molecules.

Recent synthesis of thin films of boron monophosphide (BP) motivates us to report herewith the lattice thermal conductivity calculated in nanoscale boron pnictides. The lattice thermal conductivity in BX (X = P, As, Sb) monolayers are calculated to be 51.3, 20.7 and 5.4 Wm−1K−1 respectively at 300 K. The ultra-low lattice thermal conductivity in BSb, which is comparable to that in SnSe, is attributed to low elastic and bond stiffness, low Debye temperature, small group velocity and large mode Gruneisen parameter near zone center. Phonon group velocities in BSb are comparable to that in SnSe and Bi2Te3. Up to 50% reduction in lattice thermal conductivity is noticed at 300 K upon shortening the phonon mean free path, which is realizable, e.g., in 1D nanoribbons, or via defects, vacancies, nanoengineering, etc. A comparative study based on different models to compute the lattice thermal conductivity will provide useful insights into the experimentally measured ones. The widest ever phononic gap (436 & 438 cm−1) is found in BAs and BSb monolayers, where the transverse and longitudinal optical phonons meet the criterion for the prevention of Klemens decay and hence, they can be gainfully exploited in hot-carrier solar cells. BX (X = P, As, Sb) monolayers show a direct bandgap and piezoelectricity. Nanoscale boron pnictides are promising materials in futuristic thermoelectrics, bolometers, third-generation solar cells and nanopiezotronics.

Electrical contacts arising at the van der Waals interface between boron pnictide (h-BP, h-BAs) and graphene monolayers have been systematically investigated using density functional theory. The electronic band structure of the individual monolayers is well preserved in the heterostructures constituted from them, indicating a weak van der Waals (vdW) interaction between them. BP monolayer is found to form n-type Schottky contact with a Schottky barrier height (SBH) of 0.4 eV in BP/graphene van der Waals heterostructure (vdWH), whereas a small p-type SBH of 0.16 eV occurs at BAs/graphene vdWH. The SBHs obtained for BX/graphene are lower than that of other transition metal dichalcogenide based graphene vdWH and electrical properties are found to be significantly tunable/transformable via small applied electric field of ±0.15 V/A. These insightful results will motivate experimentalists and technologists to design high performance graphene-based hybrid field-effect transistors (FET). Also, the vdWHs are found to show significant robustness, structural integrity and flexibility. The bending modulus of BP (As)/graphene is found to be lower than that in graphene/MoS2. The graphene/h-BN vdWH has been experimentally studied, while the thin films of h-BP have been recently synthesized; thereby substantiating the experimental feasibility in building up the heterostructures proposed in this work.

Although CdX (X = S, Se) has been mostly studied in the field of photocatalysis, photovoltaics, their intrinsic properties, such as, mechanical, piezoelectric, electron and phonon transport properties have been completely overlooked in buckled CdX monolayers. Ultra-low lattice thermal conductivity [1.08 W m-1 K-1 (0.75 W m-1 K-1)] and high p-type Seebeck coefficient [1300 μV K-1 (850 μV K-1)] in CdS (CdSe) monolayers have been found in this work based on first-principles DFT coupled to semi-classical Boltzmann transport equations, combining both the electronic and phononic transport. The dimensionless thermoelectric figure of merit is calculated to be 0.78 (0.5) in CdS (CdSe) monolayers at room temperature, which is comparable to that of two-dimensional (2D) tellurene (0.8), arsenene and antimonene (0.8), indicating its great potential for applications in 2D thermoelectrics. Such a low lattice thermal conductivity arise from the participation of both acoustic [91.98% (89.22%)] and optical modes [8.02% (10.78%)] together with low Debye temperature [254 K (187 K)], low group velocity [4 km s-1 (3 km s-1)] in CdS (CdSe) monolayers, high anharmonicity and short phonon lifetime. Substantial cohesive energy (∼4-5 eV), dynamical and mechanical stability of the monolayers substantiate the feasibility in synthesizing the single layers in experiments. The inversion symmetry broken along the z direction causes out-of-plane piezoelectricity. |d 33| ∼ 21.6 pm V-1, calculated in CdS monolayer is found to be the highest amongst structures having atomic-layer thickness. Superlow Young's modulus ∼41 N m-1 (31 N m-1) in CdS (CdSe) monolayers, which is comparable to that of planar CdS (29 N m-1) and TcTe2 (34 N m-1), is an indicator of its superhigh flexibility. Direct semiconducting band gap, high carrier mobility (∼500 cm2 V-1 s-1) and superhigh flexibility in CdX monolayers signify its gigantic potential for applications in ultrathin, stretchable and flexible nanoelectronics. The all-round properties can be synergistically combined together in futuristic applications in nano-piezotronics as well.

Selectivity of the electric field direction plays a vital role in modulating the phonon characteristics as well as electrical properties in low-dimensional materials. A comprehensive study on the effects of the direction-dependent electric field on MoS2 sample is reported herewith. The field-induced changes in the phonon characteristics and electronic band structure have been systematically investigated based on field responsive Raman and photoluminescence measurements. The atomistic insights obtained from density functional theory calculations have been correlated with the experimental observations to elucidate the underlying mechanism. The applied transverse electric field is found to be significantly more efficacious than the electric field applied vertically in altering the phonon signatures and bandgap in MoS2, where the electrostrictive response is found to arise from the field-induced alteration in metal–chalcogen interatomic bonds.

Control synthesis of graphene is the need of the hour for versatile applications in the energy sector. A novel yet facile mechanical exfoliation technique has been developed here for the synthesis of high-quality few-layer graphene (FLG)-like nanosheets from a biowaste material, peanut shells. Biowastes, on the other hand, is one of the most abundant natural resources for carbon, which needs to be recycled. Therefore, careful selection of biomass as well as proper synthesis engineering can provide a sustainable route to attain mass-scale graphene-like nanosheet production. Liquid-phase mechanical exfoliation in various solvent systems such as an organic solvent, isopropyl alcohol (IPA), water and hydrogen peroxide (as the best medium for cavitation), as well as an inorganic acidic solution of phosphoric acid which caused the exfoliation by intercalation, has been explored. Depending on the nature of solvents, reduction in dimensions of the as-derived exfoliated peanut-shell derived graphene-like nanosheets (EPSGs) occur, which ensures the number of layers of the sheets. In order to elucidate the role of exfoliating solvents, DFT-based computational studies have also been performed in detail. The as-synthesized EPSGs were tested for their subsequent applications in supercapacitor. The electrochemical results are consistent with the theoretical finding that organic solvents, like IPA, has a better exfoliating effect in the liquid-phase exfoliation and has shown excellent specific capacitance (323 F g−1) as well as energy density (101 W h kg−1).

Excellent photovoltaic abilities in a 2D excitonic solar cell based on staggered type-II van der Waals (vdW) hetero-bilayers comprising of semiconducting ZrS3 monolayer and monolayers of MS2 & MXY (M Mo, W; X, Y S, Se, Te; X ≠ Y) are reported herewith, using DFT-D2 and HSE06 functional. Studies on vdW hetero-bilayers of MX3/MX2 and MX3/M'Y2 have so far been conveniently avoided on account of their large lattice mismatch. The present work is the first attempt to address such hetero-bilayers constituted by monolayers of transition metal dichalcogenides and trichalcogenides. The nature of the band edges in ZrS3 and MS2 monolayers induces high electron and hole mobility in these individual monolayers, respectively, which has been combined synergistically in the hetero-bilayers consisting of them. The Power Conversion Efficiency (PCE) in ZrS3/MoS2, ZrS3/WS2, ZrS3/MoSeTe, ZrS3/WSTe, and ZrS3/WSeTe hetero-bilayers, calculated within the Anderson-limit, are found to reach as high as ~12%, 8%, 16%, 14%, and 14% respectively. The PCE of the hetero-bilayers reported herewith are much higher than the efficiency in MoS2/p-Si heterojunction solar cells (5.23%) and comparable to that of the theoretically proposed PCBM fullerene/BCN system (10–20%) and g-SiC2-based systems (12–20%) and the recently predicted TiNF/TiNBr (18%), TiNCl/TiNBr (19%), TiNF/TiNCl (22%) bilayer solar cell systems.

Oxygen vacancies play an important role in controlling the physical properties of a perovskite oxide. We report alterations in the electronic properties of a cubic perovskite oxide, namely, ${\mathrm{KTaO}}_{3}$, as a function of oxygen vacancies. The conducting surface of the ${\mathrm{KTaO}}_{3}$ single-crystal substrate has been realized via ${\mathrm{Ar}}^{+}$ irradiation. The band gap changes as a function of conductivity which is controlled by irradiation time, indicating the formation of defect states. Kelvin probe force microscopy suggests a sharp increase in the work function upon ${\mathrm{Ar}}^{+}$ irradiation for a short period of time followed by a monotonic decrease, as we increase the irradiation time. Our experimental findings along with theoretical simulations suggest a significant surface dipole contribution and an unusual change in the electronic band line-up of ${\mathrm{KTaO}}_{3}$ due to oxygen vacancies.

Ab-initio density functional theory-based calculations have been performed on monolayer (ML) $$\text {MoS}_2$$ nanosheet to study the variation of its electronic properties with the application of uniaxial tensile and compressive strain along its two non-equivalent lattice directions, namely, the zig-zag and the arm-chair directions. Among all the strain types considered in this study, uniaxial tensile strain applied along the zig-zag direction is found to be the most efficacious, inducing a greater tunability in the band gap over a large energy range (from 1.689 to 0.772 eV corresponding to 0–9% of applied strain), followed by uniaxial tensile strain along arm-chair direction. In contrast, the ML– $$\text {MoS}_2$$ nanosheet is found to be less sensitive to the compressive strain applied uniaxially along both the arm-chair as well as zig-zag directions. Moreover, the charges on Mo and S atoms are not found to undergo considerable changes under the application of uniaxial strain, as the atomic motion along the other direction is free from any constraint.

In spite of iron being an abundant element present in soil, often crops show iron deficiency, because it is not present in phyto-available forms. External application is also not successful, because of soil carbonate/bicarbonate ions assisted conversion into unavailable oxy hydroxyl forms. Here the chitosan coating on iron powder loaded mesoporous silica have been found to control the above undesired soil reaction. The coating acts as gate, which close the pores in the presence of carbonate ions to keeps the iron in phyto-available forms, whereas open the pores in the presence of the uptake proton signal from the roots. This phenomenon of excellent control in the nutrient supply to plant is a complementary effect of the pH responsive chitosan and the mesoporous channel. Because in the absence of either of the component, the performance is reduced. In the tomato plant experiment, the ferric chelate reductase activity, an iron deficiency parameter, is reduced by ~20% in plants treated with FeSO4@MS@CH compared to the one treated with FeSO4@MS.

A systematic investigation of the selective catalytic conversion of poisonous HCN gas through hydracyanation of C2H2 activated on Au clusters, presented here for the first time, is of paramount importance from both scientific and technological perspectives. Hydracyanation of activated acetylene on an Au-cluster based catalyst leads to vinyl isocyanide (H2C[double bond, length as m-dash]CHNC) formation, a versatile chemical intermediate. Using density functional theory, bond activation of acetylene and selective catalytic hydracyanation of activated acetylene on small gold clusters Aun (n = 3-10) have been studied through a detailed analysis of the geometric and electronic structures. Different possible complexes of Aun-CHCH have been studied and two possible modes of adsorption of acetylene over the gold clusters, namely, the π- and di-σ modes, have been observed. The hydracyanation of the acetylene molecule is found to occur via the cleavage of one of acetylene triple bonds at the cost of formation of two Au-C bonds followed by the binding of HCN to the activated C[double bond, length as m-dash]C bond via nitrogen's lone pair. Preferential binding sites for HCN and C2H2 are analyzed through Fukui function calculations, frontier molecular orbital analysis and natural population charge distribution analysis. Based on adsorption energies, odd-sized Aun clusters are found to be significantly more favorable for C2H2 adsorption with the C-C bond stretching up to 1.31 A with respect to the C-C triple bond length of 1.21 A in the gas phase. The stretching frequency of adsorbed complexes, C2H2/Aun, (3460 cm(-1)), decreases notably relative to the frequency of the free acetylene molecule (7948 cm(-1)), which is a signature of the bond activation of the acetylene molecule over the Au clusters. The high adsorption energy of HCN on the Au9-C2H2 complex implies the considerable binding strength and activation of C2H2 and HCN on the Au9 clusters. Due to the importance of polymerization/cyclotrimerization of C2H2 in synthetic fiber industries, the capability of Au9 clusters to adsorb up to four acetylene molecules, (C2H2)n (n = 1-4), on adjacent sites without affecting the planarity/structure of Au9 is demonstrated here for the first time. Our findings provide valuable guidance for choosing a suitable substrate/support for realizing these Aun-catalyzed reactions in practical applications.

Strain and temperature induced tunability in the thermoelectric properties in monolayer MoS2 (ML-MoS2) has been demonstrated using density functional theory coupled to semi-classical Boltzmann transport theory. Compressive strain, in general and uniaxial compressive strain (along the zig-zag direction), in particular, is found to be most effective in enhancing the thermoelectric power factor, owing to the higher electronic mobility and its sensitivity to lattice compression along this direction. Variation in the Seebeck coefficient and electronic band gap with strain is found to follow the Goldsmid-Sharp relation. n-type doping is found to raise the relaxation time-scaled thermoelectric power factor higher than p-type doping and this divide widens with increasing temperature. The relaxation time-scaled thermoelectric power factor in optimally n-doped ML-MoS2 is found to undergo maximal enhancement under the application of 3% uniaxial compressive strain along the zig-zag direction, when both the (direct) electronic band gap and the Seebeck coefficient reach their maximum, while the electron mobility drops down drastically from 73.08 to 44.15 cm2 V-1 s-1. Such strain sensitive thermoelectric responses in ML-MoS2 could open doorways for a variety of applications in emerging areas in 2D-thermoelectrics, such as on-chip thermoelectric power generation and waste thermal energy harvesting.

MoS2 nanostructures, i.e., nanoribbons, nano-mesh, etc., may open different prospect of applications in nano-electronic and opto-electronic devices and sensors. However, the fabrication of these complicated nanostructures can be executed by using standard nano-patterning techniques such as lithography, printing, etc. Nevertheless, these standard techniques involve affluent multistep processes to optimize scalability, form factors and accuracy in the feature size. Herein, we demonstrate the fabrication of unique nano-structures on MoS2, such as nano-ribbons and nano-mesh, by a simple one-step process of direct laser writing using 532 nm low power focused laser. The minimum power required to etch a MoS2 layer for a 532 nm laser is found to be ∼6.95 mW and the minimum void size observed is ∼300 nm, which is very close to the diffraction limit of the laser used. Both the experimental and computational results have shown that the voids induced by laser etching always take a hexagonal or triangular shape, which...