Magnetism in AV_{3}Sb_{5} (A=Cs, Rb, and K): Origin and Consequences for the Strongly Correlated Phases.

The V-based kagome systems AV_{3}Sb_{5} (A=Cs, Rb, and K) are unique by virtue of the intricate interplay of nontrivial electronic structure, topology, and intriguing fermiology, rendering them to be a playground of many mutually dependent exotic phases like charge-order and superconductivity. Despite numerous recent studies, the interconnection of magnetism and other complex collective phenomena in these systems has yet not arrived at any conclusion. Using first-principles tools, we demonstrate that their electronic structures, complex fermiologies and phonon dispersions are strongly influenced by the interplay of dynamic electron correlations, nontrivial spin-polarization and spin-orbit coupling. An investigation of the first-principles-derived intersite magnetic exchanges with the complementary analysis of q dependence of the electronic response functions and the electron-phonon coupling indicate that the system conforms as a frustrated spin cluster, where the occurrence of the charge-order phase is intimately related to the mechanism of electron-phonon coupling, rather than the Fermi-surface nesting.

Correction to Stable Sulfuric Vapor Transport and Liquid-Sulfur Growth on Transition Metal Dichalcogenides.

[This corrects the article DOI: 10.1021/acs.cgd.2c01318.].

Stable Sulfuric Vapor Transport and Liquid Sulfur Growth on Transition Metal Dichalcogenides.

Transition metal dichalcogenides (TMDs) are an emergent class of low-dimensional materials with growing applications in the field of nanoelectronics. However, efficient methods for synthesizing large monocrystals of these systems are still lacking. Here, we describe an efficient synthetic route for a large number of TMDs that were obtained in quartz glass ampoules by sulfuric vapor transport and liquid sulfur. Unlike the sublimation technique, the metal enters the gas phase in the form of molecules, hence containing a greater amount of sulfur than the growing crystal. We have investigated the physical properties for a selection of these crystals and compared them to state-of-the-art findings reported in the literature. The acquired electronic properties features demonstrate the overall high quality of single crystals grown in this work as exemplified by CoS2, ReS2, NbS2, and TaS2. This new approach to synthesize high-quality TMD single crystals can alleviate many material quality concerns and is suitable for emerging electronic devices.

1D/2D Hybrid Te/Graphene and Te/MoS2: Multifaceted Broadband Photonics and Green-Energy Applications.

We highlight the enhanced electronic and optical functionalization in the hybrid heterojunction of one-dimensional (1D) tellurene with a two-dimensional (2D) monolayer of graphene and MoS2 in both lateral and vertical geometries. The structural configurations of these assemblies are optimized with a comparative analysis of the energetics for different positional placements of the 1D system with respect to the hexagonal 2D substrate. The 1D/2D coupling of the electronic structure in this unique assembly enables the realization of the three different types of heterojunctions, viz. type I, type II, and Z-scheme. The interaction with 1D tellurene enables the opening of a band gap of the order of hundreds of meV in 2D graphene for both lateral and vertical geometries. With both static and time-dependent first-principles analysis, we indicate their potential applications in broadband photodetection and absorption, covering a wide range of visible to infrared (near-IR to mid-IR) spectrum from 380 to 10 000 nm. We indicate that this 1D/2D assembly also has bright prospects in green-energy harvesting.

All-Electrical High-Sensitivity, Low-Power Dual-Mode Gas Sensing and Recovery with a WSe2/MoS2 pn Heterodiode.

Two-dimensional MoS2 gas sensors have conventionally relied on a change in field-effect-transistor (FET) channel resistance or in the Schottky contact/pn homojunction barrier. We demonstrate an enhancement in sensitivity (6×) and dynamic response along with a reduction in detection limit (8×) and power (104×) in a gate-tunable type-II WSe2(p)/MoS2(n) heterodiode gas sensor over an MoS2 FET on the same flake. Measurements for varying NO2 concentration, gate bias, and MoS2 flake thickness, reinforced with first-principles calculations, indicate dual-mode operation due to (i) a series resistance-based exponential change in the high-bias thermionic current (high sensitivity), and (ii) a heterointerface carrier concentration-based linear change in near-zero-bias interlayer recombination current (low power) resulting in sub-100 µW/cm2 power consumption. Fast and gate-bias tunable recovery enables an all-electrical, room-temperature dynamic operation. Coupled with the sensing of trinitrotoluene (TNT) molecules down to 80 ppb, this study highlights the potential of the WSe2/MoS2 pn heterojunction as a simple, low-overhead, and versatile chemical-sensing platform.

Anisotropic nonlinear optical response in a graphene oxide-gold nanohybrid.

In this Letter, we report for the first time, to the best of our knowledge, the anisotropic optical response in a graphene oxide (GO)-gold (Au) nanohybrid. Polarization-sensitive nonlinear optical absorption measurements revealed that nanohybrids are highly anisotropic, (ß⊥-ß‖)≈28cm/GW, which is more than one order of magnitude higher than that of control GO (2 cm/GW). The first-principle analysis of absorbance at nanohybrid interfaces with varying functional ligand concentrations corroborates with the experimentally observed intrinsic linear anisotropy. Thus, this Letter enables new routes to realize smart and high-performing nonlinear optical systems selectively and directionally such as tunable optical limiters and optical data processing devices.

Combinatorial Large-Area MoS2/Anatase-TiO2 Interface: A Pathway to Emergent Optical and Optoelectronic Functionalities.

The interface of transition-metal dichalcogenides (TMDCs) and high-k dielectric transition-metal oxides (TMOs) had triggered umpteen discourses because of the indubitable impact of TMOs in reducing the contact resistances and restraining the Fermi-level pinning for the metal-TMDC contacts. In the present work, we focus on the unresolved tumults of large-area TMDC/TMO interfaces, grown by adopting different techniques. Here, on a pulsed laser-deposited MoS2 thin film, a layer of TiO2 is grown by atomic layer deposition (ALD) and pulsed laser deposition (PLD). These two different techniques emanate the layer of TiO2 with different crystallinities, thicknesses, and interfacial morphologies, subsequently influencing the electronic and optical properties of the interfaces. Contrasting the earlier reports of n-type doping at the exfoliated MoS2/TiO2 interfaces, the large-area MoS2/anatase-TiO2 films had realized a p-type doping of the underneath MoS2, manifesting a boost in the extent of p-type doping with increasing thickness of TiO2, as emerged from the X-ray photoelectron spectra. Density functional analysis of the MoS2/anatase-TiO2 interfaces, with pristine and interfacial defect configurations, could correlate the interdependence of doping and the terminating atomic surface of TiO2 on MoS2. The optical properties of the interface, encompassing photoluminescence, transient absorption and z-scan two-photon absorption, indicate the presence of defect-induced localized midgap levels in MoS2/TiO2 (PLD) and a relatively defect-free interface in MoS2/TiO2 (ALD), corroborating nicely with the corresponding theoretical analysis. From the investigation of optical properties, we indicate that the MoS2/TiO2 (PLD) interface may act as a promising saturable absorber, having a significant nonlinear response for the sub-band-gap excitations. Moreover, the MoS2/TiO2 (PLD) interface had exemplified better phototransport properties. A potential application of MoS2/TiO2 (PLD) is demonstrated by the fabrication of a p-type phototransistor with the ionic-gel top gate. This endeavor to analyze and perceive the MoS2/TiO2 interface establishes the prospectives of large-area interfaces in the field of optics and optoelectronics.

Broken symmetries and the related interface-induced effects at Weyl-system TaAs in proximity of noble metals.

Weyl semimetal TaAs, congenially accommodating the massless Weyl fermions, furnishes a platform to observe a spontaneous breaking of either the time-reversal or the inversion symmetry and the concurrent genesis of pairs of Weyl nodes with significant topological durability. Former experimental analysis, which reveals that the near-zero spin-polarization of bulk TaAs, experiences a boost in proximity of point-contacts of non-magnetic metals along with the associated tip-induced superconductivity, provides the impetus to study the large-area stacked interfaces of TaAs with noble metals like Au and Ag. The primary outcomes of the present work can be listed as follows: (1) First-principles calculations on the interfacial systems have manifested an increment of the interface-induced spin-polarization and contact-induced transport spin-polarization of TaAs in proximity of noble metals; (2) In contrast to the single interface, for vertically stacked cases, the broken inversion symmetry of the system introduces a z-directional band-dispersion, resulting in an energetically separated series of non-degenerate band crossings. The simultaneous presence of such band-crossings and spin-polarization indicated the coexistence of both broken time reversal and inversion symmetries for metal-semimetal stacked interfaces; (3) quantum transport calculations on different device geometries reveal the importance of contact geometry for spin-transport in TaAs devices. Lateral contacts are found to be more effective in obtaining a uniform spin transport and larger transport spin polarization; (4) the phonon dispersion behaviour of TaAs displays a closure of band-gap with the associated increase of phonon-density of states for the acoustic modes in proximity of lateral contacts of noble metals.

Intriguing electronic and optical prospects of FCC bimetallic two-dimensional heterostructures: epsilon near-zero behavior in UV-Vis range.

A higher superconducting critical temperature and large-area epsilon-near-zero systems are two long-standing goals of the scientific community, having an explicit relationship with the correlated electrons in localized orbitals. Motivated by the recent experimental findings of the strongly correlated phenomena in nanostructures of simple Drude metallic systems, we have theoretically investigated some potential bimetallic FCC combinations having close resemblance with the experimental systems. The explored systems include the large-area interface to the embedded and doped two-dimensional (2D) combinatorial nanostructures. Using different effective single-particle first-principles approaches encompassing density functional theory (DFT), time-dependent DFT (TDDFT), phonon and DFT-coupled quantum transport, we propose some interesting correlated prospects of potential bimetallic nanostructures like Au/Ag and Pt/Pd. For the 2D doped and embedded nanostructures of these systems, the DFT-calculated non-trivial band-structures indicate the interfacial morphology-induced band localization. The calculated Fermi-surface topology of the nanostructures and the corresponding nesting behavior may be emblematic to the presence of instabilities, such as charge density waves. The optical attributes extracted from the TDDFT calculations result in near-zero behavior of both real and imaginary parts of the dynamical dielectric response in the ultra-violet to visible (UV-Vis) optical range. In addition, low-energy intra-band plasmonic oscillations, as present for individual metallic surfaces, are completely suppressed for the embedded and doped nanostructures. The TDDFT-derived electron-energy loss spectra manifest the survival of only inter-band transitions. The presence of soft phonons and dynamic instabilities is observed from the phonon-dispersion of the nanostructured systems. Quantum transport calculations on the simplest possible device made out of these bimetallic systems reveal the generation of highly transmitting pockets over the cross-sectional area for some selected device geometry. We envisage that, if scrutinized experimentally, such systems may unveil many fascinating interdisciplinary aspects of orbital chemistry, physics and optics, promoting their relevant applications in many diverse fields.

A combined spectroscopic and ab initio study of the transmetalation of a polyphenol as a potential purification strategy for food additives.

Recently, metal exchange (transmetalation) techniques have become popular for the post-synthesis modification of metal organic complexes (MOCs). Here, we have explored the possibility of toxic metal ion (mercury (Hg)) exchange from a model polyphenol, curcumin, which is a very important food additive, using a much less toxic counterpart (copper). While the attachment of different metals on the polyphenol was confirmed using a picosecond resolved fluorescence technique, the surface plasmon resonance (SPR) band of the Ag nanoparticle (NP) was employed as a tool to detect uncoupled Hg ions in aqueous media. Furthermore, a microscopic understanding of the experimental observations was achieved through density functional theory (DFT) based theoretical studies. The presence of Cu ions in the vicinity of Hg-curcumin, upon ground state optimization, was observed to extrude most of the Hg from the curcumin complex and replace its position in the complex. The study may find relevance in the development of a purification strategy for food additives heavily contaminated with toxic metals.

Sensitized ZnO nanorod assemblies to detect heavy metal contaminated phytomedicines: spectroscopic and simulation studies.

The immense pharmacological relevance of the herbal medicine curcumin including anti-cancer and anti-Alzheimer effects, suggests it to be a superior alternative to synthesised drugs. The diverse functionalities with minimal side effects intensify the use of curcumin not only as a dietary supplement but also as a therapeutic agent. Besides all this effectiveness, some recent literature reported the presence of deleterious heavy metal contaminants from various sources in curcumin leading to potential health hazards. In this regard, we attempt to fabricate ZnO based nanoprobes to detect metal conjugated curcumin. We have synthesized and structurally characterized the ZnO nanorods (NR). Three samples namely curcumin (pure), Zn-curcumin (non-toxic metal attached to curcumin) and Hg-curcumin (toxic heavy metal attached to curcumin) were prepared for consideration. The samples were electrochemically deposited onto ZnO surfaces and the attachment was confirmed by cyclic voltammetry experiments. Moreover, to confirm a molecular level interaction picosecond-resolved PL-quenching of ZnO NR due to Förster Resonance Energy Transfer (FRET) from donor ZnO NR to the acceptor curcumin moieties was employed. The attachment proximity of ZnO NR and curcumin moieties depends on the size of metals. First principles analysis suggests a variance of attachment sites and heavy metal Hg conjugated curcumin binds through a peripheral hydroxy group to NR. We fabricated a facile photovoltaic device consisting of ZnO NR as the working electrode with Pt counter electrode and iodide-triiodide as the electrolyte. The trend in photocurrent under visible light illumination suggests an enhancement in the case of heavy metal ions due to long range interaction and greater accumulation of charge at the active electrode. Our results provide a detailed physical insight into interfacial processes that are crucial for detecting heavy-metal attached phytomedicines and are thus expected to find vast application as sensors for the detection of selective metal contaminants.

Few-Layer MoS2 p-Type Devices Enabled by Selective Doping Using Low Energy Phosphorus Implantation.

P-type doping of MoS2 has proved to be a significant bottleneck in the realization of fundamental devices such as p-n junction diodes and p-type transistors due to its intrinsic n-type behavior. We report a CMOS compatible, controllable and area selective phosphorus plasma immersion ion implantation (PIII) process for p-type doping of MoS2. Physical characterization using SIMS, AFM, XRD and Raman techniques was used to identify process conditions with reduced lattice defects as well as low surface damage and etching, 4X lower than previous plasma based doping reports for MoS2. A wide range of nondegenerate to degenerate p-type doping is demonstrated in MoS2 field effect transistors exhibiting dominant hole transport. Nearly ideal and air stable, lateral homogeneous p-n junction diodes with a gate-tunable rectification ratio as high as 2 × 10(4) are demonstrated using area selective doping. Comparison of XPS data from unimplanted and implanted MoS2 layers shows a shift of 0.67 eV toward lower binding energies for Mo and S peaks indicating p-type doping. First-principles calculations using density functional theory techniques confirm p-type doping due to charge transfer originating from substitutional as well as physisorbed phosphorus in top few layers of MoS2. Pre-existing sulfur vacancies are shown to enhance the doping level significantly.

Interfacial n-Doping Using an Ultrathin TiO2 Layer for Contact Resistance Reduction in MoS2.

We demonstrate a low and constant effective Schottky barrier height (ΦB ∼ 40 meV) irrespective of the metal work function by introducing an ultrathin TiO2 ALD interfacial layer between various metals (Ti, Ni, Au, and Pd) and MoS2. Transmission line method devices with and without the contact TiO2 interfacial layer on the same MoS2 flake demonstrate reduced (24×) contact resistance (RC) in the presence of TiO2. The insertion of TiO2 at the source-drain contact interface results in significant improvement in the on-current and field effect mobility (up to 10×). The reduction in RC and ΦB has been explained through interfacial doping of MoS2 and validated by first-principles calculations, which indicate metallic behavior of the TiO2-MoS2 interface. Consistent with DFT results of interfacial doping, X-ray photoelectron spectroscopy (XPS) data also exhibit a 0.5 eV shift toward higher binding energies for Mo 3d and S 2p peaks in the presence of TiO2, indicating Fermi level movement toward the conduction band (n-type doping). Ultraviolet photoelectron spectroscopy (UPS) further corroborates the interfacial doping model, as MoS2 flakes capped with ultrathin TiO2 exhibit a reduction of 0.3 eV in the effective work function. Finally, a systematic comparison of the impact of selective doping with the TiO2 layer under the source-drain metal relative to that on top of the MoS2 channel shows a larger benefit for transistor performance from the reduction in source-drain contact resistance.

Manifestation of long-range ordered state in layered VX2 (X = Cl, Br, I) systems.

We report the first-principles based density functional investigation of correlated magnetic behavior for the layered VX2 (X = Cl, Br, I) system both in bulk and monolayer forms. The bulk system stabilizes in the Néel 120° spin structure with the onset of long-range order at a very low temperature. However, a monolayer of the same system results in a two-dimensional long-range ordered spin-gel configuration in the a-b plane. The occurrence of this two-dimensional non-collinear long-range order for such hexagonal systems with honeycomb topology may provide an additional feature for controlling the use of such materials in magnetic memory devices.

High pressure structural and vibrational properties of the spin-gap system Cu2PO4(OH).

The structural and vibrational properties of the spin-gapped system Cu(2)PO(4)(OH) have been investigated at room temperature under high pressure up to ~20 GPa by Raman scattering and synchrotron-based x-ray diffraction and infrared (IR) spectroscopic measurements. The orthorhombic phase (space group Pnnm, z = 4) remains stable up to at least 7 GPa where it undergoes a weakly first order structural transition (with negligible volume drop) to a monoclinic phase (space group P2(1)/n, z = 4) with an abrupt monoclinic distortion. Refinement of atomic positions has been performed for the low pressure phase. The conspicuous changes in the vibrational spectra (Raman as well as far-IR) confirm this phase transition. At further higher pressures the monoclinic angle increases rapidly and the system transforms irreversibly into a disordered phase. Detailed vibrational analyses have been performed in the orthorhombic phase and pressure-induced structural evolution has been correlated with the vibrational modes corresponding to the Cu-O bonds. A strong negative pressure dependence of hydroxyl mode frequencies (as observed from the mid-IR absorption spectra) supports the pressure-induced structural disordering at higher pressures.

Spin interactions in mineral libethenite series: evolution of low-dimensional magnetism.

Interesting magnetic properties and spin-exchange interactions along various possible pathways in the half-integral spin quantum magnetic tetramer system: A(2)PO(4)OH (A=Co, Cu) are investigated. Interplay of structural distortion and the magnetic properties with the evolution of localized band structure explain the gradual transition from a three-dimensional antiferromagnet to a low-dimensional frustrated magnetic system along the series. A detailed study of the exchange mechanism in this system explores various possibilities of complex magnetic interaction. The electronic structure of this series, studied with the help of different appropriate density functional approaches such as Nth order muffin-tin orbital (NMTO) and plane-wave pseudopotential calculations incorporating onsite Coulomb repulsion (U), identifies the underlying magnetic exchange mechanism of this series. Thereafter a generalized minimal model spin-Hamiltonian is constructed for the low-dimensional system. Solution of this model Hamiltonian within first-order perturbation theory results in the evaluation of spin-gap in the spin-tetramer system. In addition, the effects of size confinement and volume reduction on the relevant exchange integrals and spin-gap of the low-dimensional system are also discussed.