Emergent Inhomogeneity and Nonlocality in a Graphene Field-Effect Transistor on a Near-Parallel Moiré Superlattice of Transition Metal Dichalcogenides.

At near-parallel orientation, twisted bilayers of transition metal dichalcogenides exhibit interlayer charge transfer-driven out-of-plane ferroelectricity. Here, we report detailed electrical transport in a dual-gated graphene field-effect transistor placed on a 2.1° twisted bilayer WSe2. We observe hysteretic transfer characteristics and an emergent charge inhomogeneity with multiple local Dirac points evolving with an increasing electric displacement field (D). Concomitantly, we also observe a strong nonlocal voltage signal at D ∼ 0 V/nm that decreases rapidly with increasing D. A linear scaling of the nonlocal signal with longitudinal resistance suggests edge mode transport, which we attribute to the breaking of valley symmetry of graphene due to the spatially fluctuating electric field from the underlying polarized moiré domains. A quantitative analysis suggests the emergence of finite-size domains in graphene that modulate the charge and the valley currents simultaneously. This work underlines the impact of interfacial ferroelectricity that can trigger a new generation of devices.

Bi4TaO8Cl as a New Class of Layered Perovskite Oxyhalide Materials for Piezopotential Driven Efficient Seawater Splitting.

Piezocatalytic water splitting is an emerging approach to generate "green hydrogen" that can address several drawbacks of photocatalytic and electrocatalytic approaches. However, existing piezocatalysts are few and with minimal structural flexibility for engineering properties. Moreover, the scope of utilizing unprocessed water is yet unknown and may widely differ from competing techniques due to the constantly varying nature of surface potential. Herein, we present Bi4TaO8Cl as a representative of a class of layered perovskite oxyhalide piezocatalysts with high hydrogen production efficiency and exciting tailorable features including the layer number, multiple cation-anion combination options, etc. In the absence of any cocatalyst and scavenger, an ultrahigh production rate is achievable (1.5 mmol g-1 h-1), along with simultaneous generation of value-added H2O2. The production rate using seawater is somewhat less yet appreciably superior to photocatalytic H2 production by most oxides as well as piezocatalysts and has been illustrated using a double-layer model for further development.

Metavalent Bonding in GeSe Leads to High Thermoelectric Performance.

Orthorhombic GeSe is a promising thermoelectric material. However, large band gap and strong covalent bonding result in a low thermoelectric figure of merit, zT≈0.2. Here, we demonstrate a maximum zT≈1.35 at 627 K in p-type polycrystalline rhombohedral (GeSe)0.9 (AgBiTe2 )0.1 , which is the highest value reported among GeSe based materials. The rhombohedral phase is stable in ambient conditions for x=0.8-0.29 in (GeSe)1-x (AgBiTe2 )x . The structural transformation accompanies change from covalent bonding in orthorhombic GeSe to metavalent bonding in rhombohedral (GeSe)1-x (AgBiTe2 )x . (GeSe)0.9 (AgBiTe2 )0.1 has closely lying primary and secondary valence bands (within 0.25-0.30 eV), which results in high power factor 12.8 µW cm-1 K-2 at 627 K. It also exhibits intrinsically low lattice thermal conductivity (0.38 Wm-1 K-1 at 578 K). Theoretical phonon dispersion calculations reveal vicinity of a ferroelectric instability, with large anomalous Born effective charges and high optical dielectric constant, which, in concurrence with high effective coordination number, low band gap and moderate electrical conductivity, corroborate metavalent bonding in (GeSe)0.9 (AgBiTe2 )0.1 . We confirmed the presence of low energy phonon modes and local ferroelectric domains using heat capacity measurement (3-30 K) and switching spectroscopy in piezoresponse force microscopy, respectively.

Ferroelectric Instability Induced Ultralow Thermal Conductivity and High Thermoelectric Performance in Rhombohedral p-Type GeSe Crystal.

The orthorhombic phase of GeSe, a structural analogue of layered SnSe (space group: Pnma), has recently attracted attention after a theoretical prediction of high thermoelectric figure of merit, zT > 2. The experimental realization of such high performance in orthorhombic GeSe, however, is still elusive (zT ≈ 0.2). The rhombohedral phase of GeSe, a structural analogue of GeTe (space group: R3m), previously stabilized at high pressure (2 GPa) and high temperature (1600 K), is promising due to its theoretically predicted ferroelectric instability and the higher earth abundance of Se compared to Te. Here, we demonstrate high thermoelectric performance in the rhombohedral crystals of GeSe, which is stabilized at ambient conditions by alloying with 10 mol % AgBiSe2. We show ultralow lattice thermal conductivity (κL) of 0.74-0.47 W/mK in the 300-723 K range and high zT ≈ 1.25 at 723 K in the p-type rhombohedral (GeSe)0.9(AgBiSe2)0.1 crystals grown using Bridgman method. First-principles density functional theoretical analysis reveals its vicinity to a ferroelectric instability which generates large anomalous Born effective charges and strong coupling of low energy polar optical phonons with acoustic phonons. The presence of soft optical phonons and incipient ferroelectric instability in (GeSe)0.9(AgBiSe2)0.1 are directly evident in the low temperature heat capacity (Cp) and switching spectroscopy piezoresponse force microscopy (SS-PFM) experiments, respectively. Effective scattering of heat carrying acoustic phonons by ferroelectric instability induced soft transverse optical phonons significantly reduces the κL and enhances the thermoelectric performance in rhombohedral (GeSe)0.9(AgBiSe2)0.1 crystals.

Ultrathin Free-Standing Nanosheets of Bi2O2Se: Room Temperature Ferroelectricity in Self-Assembled Charged Layered Heterostructure.

Ultrathin ferroelectric semiconductors with high charge carrier mobility are much coveted systems for the advancement of various electronic and optoelectronic devices. However, in traditional oxide ferroelectric insulators, the ferroelectric transition temperature decreases drastically with decreasing material thickness and ceases to exist below certain critical thickness owing to depolarizing fields. Herein, we show the emergence of an ordered ferroelectric ground state in ultrathin (∼2 nm) single crystalline nanosheets of Bi2O2Se at room temperature. Free-standing ferroelectric nanosheets, in which oppositely charged alternating layers are self-assembled together by electrostatic interactions, are synthesized by a simple, rapid, and scalable wet chemical procedure at room temperature. The existence of ferroelectricity in Bi2O2Se nanosheets is confirmed by dielectric measurements and piezoresponse force spectroscopy. The spontaneous orthorhombic distortion in the ultrathin nanosheets breaks the local inversion symmetry, thereby resulting in ferroelectricity. The local structural distortion and the formation of spontaneous dipole moment were directly probed by atomic resolution scanning transmission electron microscopy and density functional theory calculations.

Modulating capacitive response of MoS2 flake by controlled nanostructuring through focused laser irradiation.

Unlike graphene nanostructures, various physical properties of nanostructured MoS2 have remained unexplored due to the lack of established fabrication routes. Herein, we have reported unique electrostatic properties of MoS2 nanostructures, fabricated in a controlled manner of different geometries on 2D flake by using focused laser irradiation technique. Electrostatic force microscopy has been carried out on MoS2 nanostructures by varying tip bias voltage and lift height. The analysis depicts no contrast flip in phase image of the patterned nanostructure due to the absence of free surface charges. However, prominent change in phase shift at the patterned area is observed. Such contrast changes signify the capacitive interaction between tip and nanostructures at varying tip bias voltage and lift height, irrespective of their shape and size. Such unperturbed capacitive behavior of the MoS2 nanostructures offer modulation of capacitance in periodic array on 2D MoS2 flake for potential application in capacitive devices.

Unconventional superconductivity at mesoscopic point contacts on the 3D Dirac semimetal Cd3As2.

Three-dimensional (3D) Dirac semimetals exist close to topological phase boundaries which, in principle, should make it possible to drive them into exotic new phases, such as topological superconductivity, by breaking certain symmetries. A practical realization of this idea has, however, hitherto been lacking. Here we show that the mesoscopic point contacts between pure silver (Ag) and the 3D Dirac semimetal Cd3As2 (ref. ) exhibit unconventional superconductivity with a critical temperature (onset) greater than 6 K whereas neither Cd3As2 nor Ag are superconductors. A gap amplitude of 6.5 meV is measured spectroscopically in this phase that varies weakly with temperature and survives up to a remarkably high temperature of 13 K, indicating the presence of a robust normal-state pseudogap. The observations indicate the emergence of a new unconventional superconducting phase that exists in a quantum mechanically confined region under a point contact between a Dirac semimetal and a normal metal.

Transport spectroscopy on trapped superconducting nano-islands of Pb: signature of unconventional pairing.

Elemental bulk lead (Pb) is a conventional type I, spin-singlet (s-wave) superconductor with a critical temperature T c = 7.2 K and a critical magnetic field H c = 800 Oe. However, it is known that at mesoscopic length scales, like in point-contact geometries, Pb shows significantly higher critical field, sometimes up to several Tesla. We have used this property to trap a small superconducting nano-droplet of Pb by forming a metallic point contact on Pb and then applying a magnetic field larger than 800 Oe that drives the bulk of the material non-superconducting. From systematic magnetic field dependent behaviour of the point-contact spectra measured across such a trapped island of Pb we show that the superconducting order parameter of mesoscopic Pb mixes non-trivially with magnetic field possibly due to the emergence of a local spin-triplet component at such length scales. From comparative studies with Nb-based point contacts we surmise that the strong spin-orbit coupling in Pb leads to the emergence of the unconventional component in the order parameter of mesoscopic Pb.

Self-oriented ß-crystalline phase in the polyvinylidene fluoride ferroelectric and piezo-sensitive ultrathin Langmuir-Schaefer film.

We report on the direct observation of ferroelectric switching and piezoelectric behaviour in ultrathin polyvinylidene fluoride (PVDF) films prepared by horizontal Langmuir-Schaefer (LS) technique. We have prepared pure ß-phase by just increasing the number of LS layers without using additional non-ferroelectric assisting agents. Edge-on oriented CH2-CF2 units of PVDF at the air-water interface enable self-orientation of ferroelectric dipoles by means of the hydrogen bonding network. Such restricted conformation of PVDF at the air-water interface results in an increased net dipole moment with the number of LS layers. The film's ferroelectric switching and piezoelectric sensitivity are demonstrated by hysteretic polarization switching loops and butterfly-loops, respectively. Successful circular domain writing on ultrathin LS film, down to 5 monolayers of PVDF, is demonstrated. The achievement of pure ß-phase of PVDF at room temperature without using any assisting agents may be promising for non-volatile memory and piezoelectric-based, ultrathin smart sensor devices.

Near Room Temperature Solvothermal Growth of Ferroelectric CsPbBr3 Nanoplatelets with Ultralow Dark Current.

CsPbBr3 exhibits outstanding optoelectronic properties and thermal stability, making it a coveted material for detectors, light-emitting diodes, and solar cells. Despite observations of ferroelectricity in CsPbBr3 quantum dots, synthesizing bulk ferroelectric CsPbBr3 crystals has remained elusive, hindering its potential in next-generation optoelectronic devices like optical switches and ferroelectric photovoltaics. Here, a breakthrough is reported: a novel solvothermal technique enabling the growth of ferroelectric CsPbBr3 nanoplatelets with lateral dimensions in the tens of micrometers. This represents a significant step toward achieving large-area ferroelectric CsPbBr3 crystals. Unlike traditional methods, this approach allows for growth and crystallization of CsPbBr3 in alcohol solutions by enhancing precursor solubility. This study confirms the ferroelectric nature of these nanoplatelets using second harmonic generation, electrical characterizations, and piezoresponse force microscopy. This work paves the way for utilizing ferroelectric CsPbBr3 in novel optoelectronic devices, significantly expanding the potential of this material and opening doors for further exploration in this exciting field.

Mild chemistry synthesis of ultrathin Bi2O2S nanosheets exhibiting 2D-ferroelectricity at room temperature.

Modern technology demands miniaturization of electronic components to build small, light, and portable devices. Hence, discovery and synthesis of new non-toxic, low cost, ultra-thin ferroelectric materials having potential applications in various electronic and optoelectronic devices are of paramount importance. However, achieving room-temperature ferroelectricity in two dimensional (2D) ultra-thin systems remains a major challenge as conventional three-dimensional ferroelectric materials lose their ferroelectricity when the thickness is brought down below a critical value owing to the depolarization field. Herein, we report room-temperature ferroelectricity in ultra-thin single-crystalline 2D nanosheets of Bi2O2S synthesized by a simple, rapid, and scalable solution-based soft chemistry method. The ferroelectric ground state of Bi2O2S nanosheets is confirmed by temperature-dependent dielectric measurements as well as piezoelectric force microscopy and spectroscopy. High resolution transmission electron microscopy analysis and density functional theory-based calculations suggest that the ferroelectricity in Bi2O2S nanosheets arises due to the local distortion of Bi2O2 layers, which destroys the local inversion symmetry of Bi2O2S.

Switchable Bulk Photovoltaic Effect in Intrinsically Ferroelectric 3D All-Inorganic CsPbBr3 Perovskite Nanocrystals.

Ferroelectric all-inorganic halide perovskite nanocrystals with both spontaneous polarization and visible light absorption are promising candidates for designing ferroelectric photovoltaic applications. It remains a challenge to realize ferroelectric photovoltaic devices with all-inorganic halide perovskites that can be operated in the absence of an external electric field. Here we report that a popular all-inorganic halide perovskite nanocrystal, CsPbBr3, exhibits a ferroelectricity-driven photovoltaic effect under visible light in the absence of an external electric field. Pristine CsPbBr3 nanocrystals exhibit intrinsic ferroelectric key properties with a notable saturated polarization of ∼0.15 µC/cm2 and a high Curie temperature of 462 K, driven by the stereochemical activity of the Pb(II) lone pair. Furthermore, application of an external electric field allows the photovoltaic effect to be enhanced and the spontaneous polarization to be switched with the direction of the electric field. CsPbBr3 nanocrystals exhibit a robust fatigue performance and a prolonged photoresponse under continuous illumination in the absence of an external electric field. These findings establish all-inorganic halide perovskite nanocrystals as potential candidates for designing photoferroelectric devices by coupling optical functionalities and ferroelectric responses.

Universal Piezo-Photocatalytic Wastewater Treatment on Realistic Pollutant Feedstocks by Bi4TaO8Cl: Origin of High Efficiency and Adjustable Synergy.

Clean water is a fundamental human right but millions struggle for it daily. Herein, we demonstrate a new piezo-photocatalyst with immense structural diversity for universal wastewater decontamination. Single-crystalline Bi4TaO8Cl nanoplates with exposed piezoelectric facets exhibit visible-light response, piezoelectric behavior with coercive voltages of ±5 V yielding 0.35% crystal deformation, and pressure-induced band-bending of >2.5 eV. Using five common contaminants of textile and pharmaceutical industries, we show that the nanoplates can mineralize them in all piezocatalytic, photocatalytic, and piezo-photocatalytic approaches with efficiencies higher than most catalysts developed for just one contaminant. Their efficiencies for feedstocks differing over 2 orders of magnitude in concentrations, the highest to date, are also demonstrated to simulate real-life situations. These extensive studies established that combining piezocatalytic and photocatalytic approaches can lead to a tremendous synergy exceeding >45%. The origin of synergy has been illustrated for the first time using band-bending models and improved charge transfer from valence and conduction band electronic surfaces. We further quantified synergy across reactants, concentrations, and ultrasonic frequency and power to demonstrate their versatility and unpredictability. Finally, seven parameters that contribute to synergy but create unpredictability have been identified for the rational design of piezo-photocatalysts for wastewater treatment.

Tip-induced superconductivity.

It is widely believed that topological superconductivity, a hitherto elusive phase of quantum matter, can be achieved by inducing superconductivity in topological materials. In search of such topological superconductors, certain topological insulators (like, Bi2Se3) were successfully turned into superconductors by metal-ion (Cu, Pd, Sr, Nb etc) intercalation. Superconductivity could be induced in topological materials through applying pressure as well. For example, a pressure-induced superconducting phase was found in the topological insulator Bi2Se3. However, in all such cases, no conclusive signature of topological superconductivity was found. In this review, we will discuss about another novel way of inducing superconductivity in a non-superconducting topological material-by creating a mesoscopic interface on the material with a non-superconducting, normal metallic tip where the mesoscopic interface becomes superconducting. Such a phase is now known as a tip-induced superconducting (TISC) phase. This was first realized on Cd3As2in India. Following that, a large number of other topological materials were shown to display TISC. Since the TISC phase emerges only at a confined region under a mesoscopic point contact, traditional bulk tools for characterizing superconductivity cannot be employed to detect/confirm such a phase. On the other hand, such a point contact geometry is ideal for probing the possible existence of a temperature and magnetic field dependent superconducting energy gap and a temperature and magnetic field dependent critical current. We will review the details of the experimental signatures that can be used to prove the existence of superconductivity even when the 'text-book' tests for detecting superconductivity cannot be performed. Then, we will review various systems where a TISC phase could be realized.

2D weak anti-localization in thin films of the topological semimetal Pd[Formula: see text]Bi[Formula: see text]S[Formula: see text].

Pd[Formula: see text]Bi[Formula: see text]S[Formula: see text] (PBS) is a recently proposed topological semimetal candidate. However, evidence for topological surface states have not yet been revealed in transport measurements due to the large mobility of bulk carriers. We report the growth and magneto-transport studies of PBS thin films where the mobility of the bulk carriers is reduced by two orders of magnitude, revealing for the first time, contributions from the 2-dimensional (2D) topological surface states in the observation of the 2D weak anti-localization (WAL) effect in magnetic field and angle dependent conductivity measurements. The magnetotransport data is analysed within the 2D Hikami-Larkin-Nagaoka (HLN) theory. The analysis suggests that multiple conduction channels contribute to the transport. It is also found that the temperature dependence of the dephasing length can't be explained only by electron-electron scattering and that electron-phonon scattering also contributes to the phase relaxation mechanism in PBS films.

Local ferroelectric polarization switching driven by nanoscale distortions in thermoelectric [Formula: see text].

A remarkable decrease in the lattice thermal conductivity and enhancement of thermoelectric figure of merit were recently observed in rock-salt cubic SnTe, when doped with germanium (Ge). Primarily, based on theoretical analysis, the decrease in lattice thermal conductivity was attributed to local ferroelectric fluctuations induced softening of the optical phonons which may strongly scatter the heat carrying acoustic phonons. Although the previous structural analysis indicated that the local ferroelectric transition temperature would be near room temperature in [Formula: see text], a direct evidence of local ferroelectricity remained elusive. Here we report a direct evidence of local nanoscale ferroelectric domains and their switching in [Formula: see text] using piezoeresponse force microscopy(PFM) and switching spectroscopy over a range of temperatures near the room temperature. From temperature dependent (250-300 K) synchrotron X-ray pair distribution function (PDF) analysis, we show the presence of local off-centering distortion of Ge along the rhombohedral direction in global cubic [Formula: see text]. The length scale of the [Formula: see text] off-centering is 0.25-0.10 Å near the room temperatures (250-300 K). This local emphatic behaviour of cation is the cause for the observed local ferroelectric instability, thereby low lattice thermal conductivity in [Formula: see text].

Field induced hysteretic structural phase switching and possible CDW in Re-doped MoTe2.

Novel electronic systems displaying exotic physical properties can be derived from complex topological materials through chemical doping. MoTe2, the candidate type-II Weyl semimetal shows dramatically enhanced superconductivity up to 4.1 K upon Re doping in Mo sites. Based on bulk transport and local scanning tunnelling microscopy here we show that Re doping also leads to the emergence of a possible charge density wave phase in Re0.2Mo0.8Te2. In addition, the tunnellingI-Vcharacteristics display non-linearity and hysteresis which is commensurate with a hysteresis observed in the change in tip-height (z) as a function of applied voltageV. The observations indicate an electric field induced hysteretic switching consistent with piezoelectricity and possible ferroelectricity.

Spectroscopic evidence of mixed angular momentum symmetry in non-centrosymmetric Ru[Formula: see text]B[Formula: see text].

Superconducting crystals with a lack of inversion symmetry can potentially host unconventional pairing. However, till today, no direct conclusive experimental evidence of such unconventional order parameters in non-centrosymmetric superconductors has been reported. In this paper, through direct measurement of the superconducting energy gap by scanning tunnelling spectroscopy, we report the existence of both s-wave (singlet) and p-wave (triplet) pairing symmetries in non-centrosymmetric Ru[Formula: see text]B[Formula: see text]. Our temperature and magnetic field-dependent studies also indicate that the relative amplitudes of the singlet and triplet components change differently with temperature.

The pressure-enhanced superconducting phase of Sr[Formula: see text]-Bi[Formula: see text]Se[Formula: see text] probed by hard point contact spectroscopy.

The superconducting systems emerging from topological insulators upon metal ion intercalation or application of high pressure are ideal for investigation of possible topological superconductivity. In this context, Sr-intercalated Bi[Formula: see text]Se[Formula: see text] is specially interesting because it displays pressure induced re-entrant superconductivity where the high pressure phase shows almost two times higher [Formula: see text] than the ambient superconducting phase ( [Formula: see text] K). Interestingly, unlike the ambient phase, the pressure-induced superconducting phase shows strong indication of unconventional superconductivity. However, since the pressure-induced phase remains inaccessible to spectroscopic techniques, the detailed study of the phase remained an unattained goal. Here we show that the high-pressure phase can be realized under a mesoscopic point contact, where transport spectroscopy can be used to probe the spectroscopic properties of the pressure-induced phase. We find that the point contact junctions on the high-pressure phase show unusual response to magnetic field supporting the possibility of unconventional superconductivity.

Phase-incoherent superconducting pairs in the normal state of Ba(Fe(1-x)Co(x))2As2.

The normal state properties of the recently discovered ferropnictide superconductors might hold the key to understanding their exotic superconductivity. Using point-contact spectroscopy we show that Andreev reflection between an epitaxial thin film of Ba(Fe(0.92)Co(0.08))2As2 and a silver tip can be seen in the normal state of the film up to temperature T∼1.3T(c), where T(c) is the critical temperature of the superconductor. Andreev reflection far above T(c) can be understood only when superconducting pairs arising from strong fluctuation of the phase of the complex superconducting order parameter exist in the normal state. Our results provide spectroscopic evidence of phase-incoherent superconducting pairs in the normal state of the ferropnictide superconductors.