Exceptional electronic transport and quantum oscillations in thin bismuth crystals grown inside van der Waals materials.

Confining materials to two-dimensional forms changes the behaviour of the electrons and enables the creation of new devices. However, most materials are challenging to produce as uniform, thin crystals. Here we present a synthesis approach where thin crystals are grown in a nanoscale mould defined by atomically flat van der Waals (vdW) materials. By heating and compressing bismuth in a vdW mould made of hexagonal boron nitride, we grow ultraflat bismuth crystals less than 10 nm thick. Due to quantum confinement, the bismuth bulk states are gapped, isolating intrinsic Rashba surface states for transport studies. The vdW-moulded bismuth shows exceptional electronic transport, enabling the observation of Shubnikov-de Haas quantum oscillations originating from the (111) surface state Landau levels. By measuring the gate-dependent magnetoresistance, we observe multi-carrier quantum oscillations and Landau level splitting, with features originating from both the top and bottom surfaces. Our vdW mould growth technique establishes a platform for electronic studies and control of bismuth's Rashba surface states and topological boundary modes1-3. Beyond bismuth, the vdW-moulding approach provides a low-cost way to synthesize ultrathin crystals and directly integrate them into a vdW heterostructure.

Proximity-induced chiral quantum light generation in strain-engineered WSe2/NiPS3 heterostructures.

Quantum light emitters capable of generating single photons with circular polarization and non-classical statistics could enable non-reciprocal single-photon devices and deterministic spin-photon interfaces for quantum networks. To date, the emission of such chiral quantum light relies on the application of intense external magnetic fields, electrical/optical injection of spin-polarized carriers/excitons or coupling with complex photonic metastructures. Here we report the creation of free-space chiral quantum light emitters via the nanoindentation of monolayer WSe2/NiPS3 heterostructures at zero external magnetic field. These quantum light emitters emit with a high degree of circular polarization (0.89) and single-photon purity (95%), independent of pump laser polarization. Scanning diamond nitrogen-vacancy microscopy and temperature-dependent magneto-photoluminescence studies reveal that the chiral quantum light emission arises from magnetic proximity interactions between localized excitons in the WSe2 monolayer and the out-of-plane magnetization of defects in the antiferromagnetic order of NiPS3, both of which are co-localized by strain fields associated with the nanoscale indentations.

Enhanced Exciton-to-Trion Conversion by Proton Irradiation of Atomically Thin WS2.

Defect engineering of van der Waals semiconductors has been demonstrated as an effective approach to manipulate the structural and functional characteristics toward dynamic device controls, yet correlations between physical properties with defect evolution remain underexplored. Using proton irradiation, we observe an enhanced exciton-to-trion conversion of the atomically thin WS2. The altered excitonic states are closely correlated with nanopore induced atomic displacement, W nanoclusters, and zigzag edge terminations, verified by scanning transmission electron microscopy, photoluminescence, and Raman spectroscopy. Density functional theory calculation suggests that nanopores facilitate formation of in-gap states that act as sinks for free electrons to couple with excitons. The ion energy loss simulation predicts a dominating electron ionization effect upon proton irradiation, providing further evidence on band perturbations and nanopore formation without destroying the overall crystallinity. This study provides a route in tuning the excitonic properties of van der Waals semiconductors using an irradiation-based defect engineering approach.

Manipulating Interlayer Excitons for Near-Infrared Quantum Light Generation.

Interlayer excitons (IXs) formed at the interface of van der Waals materials possess various novel properties. In parallel development, strain engineering has emerged as an effective means for creating 2D quantum emitters. Exploring the intersection of these two exciting areas, we use MoS2/WSe2 heterostructure as a model system and demonstrate how strain, defects, and layering can be utilized to create defect-bound IXs capable of bright, robust, and tunable quantum light emission in the technologically important near-infrared spectral range. Our work presents defect-bound IXs as a promising platform for pushing the performance of 2D quantum emitters beyond their current limitations.

Enabling Oxidation Protection and Carrier-Type Switching for Bismuth Telluride Nanoribbons via in Situ Organic Molecule Coating.

Thermoelectric materials with high electrical conductivity and low thermal conductivity (e.g., Bi2Te3) can efficiently convert waste heat into electricity; however, in spite of favorable theoretical predictions, individual Bi2Te3 nanostructures tend to perform less efficiently than bulk Bi2Te3. We report a greater-than-order-of-magnitude enhancement in the thermoelectric properties of suspended Bi2Te3 nanoribbons, coated in situ to form a Bi2Te3/F4-TCNQ core-shell nanoribbon without oxidizing the core-shell interface. The shell serves as an oxidation barrier but also directly functions as a strong electron acceptor and p-type carrier donor, switching the majority carriers from a dominant n-type carrier concentration (∼1021 cm-3) to a dominant p-type carrier concentration (∼1020 cm-3). Compared to uncoated Bi2Te3 nanoribbons, our Bi2Te3/F4-TCNQ core-shell nanoribbon demonstrates an effective chemical potential dramatically shifted toward the valence band (by 300-640 meV), robustly increased Seebeck coefficient (∼6× at 250 K), and improved thermoelectric performance (10-20× at 250 K).

Visualizing Grain Statistics in MOCVD WSe2 through Four-Dimensional Scanning Transmission Electron Microscopy.

Using four-dimensional scanning transmission electron microscopy, we demonstrate a method to visualize grains and grain boundaries in WSe2 grown by metal organic chemical vapor deposition (MOCVD) directly onto silicon dioxide. Despite the chemical purity and uniform thickness and texture of the MOCVD-grown WSe2, we observe a high density of small grains that corresponds with the overall selenium deficiency we measure through ion beam analysis. Moreover, reconstruction of grain information permits the creation of orientation maps that demonstrate the nucleation mechanism for new layers-triangular domains with the same orientation as the layer underneath induces a tensile strain increasing the lattice parameter at these sites.

Tuning of the electronic and vibrational properties of epitaxial MoS2through He-ion beam modification.

Atomically thin transition metal dichalcogenides (TMDs), like MoS2with high carrier mobilities and tunable electron dispersions, are unique active material candidates for next generation opto-electronic devices. Previous studies on ion irradiation show great potential applications when applied to two-dimensional (2D) materials, yet have been limited to micron size exfoliated flakes or smaller. To demonstrate the scalability of this method for industrial applications, we report the application of relatively low power (50 keV)4He+ion irradiation towards tuning the optoelectronic properties of an epitaxially grown continuous film of MoS2at the wafer scale, and demonstrate that precise manipulation of atomistic defects can be achieved in TMD films using ion implanters. The effect of4He+ion fluence on the PL and Raman signatures of the irradiated film provides new insights into the type and concentration of defects formed in the MoS2lattice, which are quantified through ion beam analysis. PL and Raman spectroscopy indicate that point defects are generated without causing disruption to the underlying lattice structure of the 2D films and hence, this technique can prove to be an effective way to achieve defect-mediated control over the opto-electronic properties of MoS2and other 2D materials.

Thermoelectric properties of antimony selenide hexagonal nanotubes.

Antimony selenide (Sb2Se3) is a material widely used in photodetectors and relatively new as a possible material for thermoelectric applications. Taking advantage of the new properties after nanoscale fabrication, this material shows great potential for the development of efficient low temperature thermoelectric devices. Here we study the synthesis, the crystal properties and the thermal and thermoelectric transport response of Sb2Se3 hexagonal nanotubes (HNT) in the temperature range between 120 and 370 K. HNT have a moderate electrical conductivity ∼102 S m-1 while maintaining a reasonable Seebeck coefficient ∼430 µV K-1 at 370 K. The electrical conductivity in Sb2Se3 HNT is about 5 orders of magnitude larger and its thermal conductivity one half of what is found in bulk. Moreover, the calculated figure of merit (ZT) at room temperature is the largest value reported in antimony selenide 1D structures.

1D to 2D Transition in Tellurium Observed by 4D Electron Microscopy.

A new microwave-enhanced synthesis method for the production of tellurium nanostructures is reported-with control over products from the 1D regime (sub-5 nm diameter nanowires), to nanoribbons, to the 2D tellurene regime-along with a new methodology for local statistical quantification of the crystallographic parameters of these materials at the nanometer scale. Using a direct electron detector and image-corrected microscope, large and robust 4D scanning transmission electron microscopy datasets for accurate structural analysis are obtained. These datasets allow the adaptation of quantitative techniques originally developed for X-ray diffraction (XRD) refinement analyses to transmission electron microscopy, enabling the first demonstration of sub-picometer accuracy lattice parameter extraction while also obtaining both the size of the coherent crystallite domains and the nanostrain, which is observed to decrease as nanowires transition to tellurene. This new local analysis is commensurate with global powder XRD results, indicating the robustness of both the new synthesis approach and new structural analysis methodology for future scalable production of 2D tellurene and characterization of nanomaterials.

Isotope Effect in Bilayer WSe2.

Isotopes of an element have the same electron number but differ in neutron number and atomic mass. However, due to the thickness-dependent properties in MX2 (M = Mo, W; X = S, Se, Te) transition metal dichalcogenides (TMDs), the isotopic effect in atomically thin TMDs still remains unclear especially for phonon-assisted indirect excitonic transitions. Here, we report the first observation of the isotope effect on the electronic and vibrational properties of a TMD material, using naturally abundant NAWNASe2 and isotopically pure 186W80Se2 bilayer single crystals over a temperature range of 4.4-300 K. We demonstrate a higher optical band gap energy in 186W80Se2 than in NAWNASe2 (3.9 ± 0.7 meV from 4.41 to 300 K), which is surprising as isotopes are neutral impurities. Phonon energies decrease in the isotopically pure crystal due to the atomic mass dependence of harmonic oscillations, with correspondingly longer E2g and A21g phonon lifetimes than in the naturally abundant sample. The change in electronic band gap renormalization energy is postulated as being the dominant mechanism responsible for the change in optical emission spectra.

Giant Mechano-Optoelectronic Effect in an Atomically Thin Semiconductor.

Transition metal dichalcogenides (TMDs) are particularly sensitive to mechanical strain because they are capable of experiencing high atomic displacements without nucleating defects to release excess energy. Being promising for photonic applications, it has been shown that as certain phases of layered TMDs MX2 (M = Mo or W; X = S, Se, or Te) are scaled to a thickness of one monolayer, the photoluminescence response is dramatically enhanced due to the emergence of a direct electronic band gap compared with their multilayer or bulk counterparts, which typically exhibit indirect band gaps. Recently, mechanical strain has also been predicted to enable direct excitonic recombination in these materials, in which large changes in the photoluminescence response will occur during an indirect-to-direct band gap transition brought on by elastic tensile strain. Here, we demonstrate an enhancement of 2 orders of magnitude in the photoluminescence emission intensity in uniaxially strained single crystalline WSe2 bilayers. Through a theoretical model that includes experimentally relevant system conditions, we determine this amplification to arise from a significant increase in direct excitonic recombination. Adding confidence to the high levels of elastic strain achieved in this report, we observe strain-independent, mode-dependent Grüneisen parameters over the entire range of tensile strain (1-3.59%), which were obtained as 1.149 ± 0.027, 0.307 ± 0.061, and 0.357 ± 0.103 for the E2g, A1g, and A21g optical phonon modes, respectively. These results can inform the predictive strain-engineered design of other atomically thin indirect semiconductors, in which a decrease in out-of-plane bonding strength may lead to an increase in the strength of strain-coupled optoelectronic effects.

Polyelectrolyte-Assisted Oxygen Vacancies: A New Route to Defect Engineering in Molybdenum Oxide.

The presence of oxygen vacancy sites fundamentally affects physical and chemical properties of materials. In this study, a dipole-containing interaction between poly(diallyldimethylammonium chloride) PDDA and α-MoO3 is found to enable high-concentrations of surface oxygen vacancies. Thermal annealing under Ar resulted in negligible reduction of MoO3 to MoO3- x with x = 0.03 at 600 °C. In contrast, we show that the thermochemical reaction with PDDA polyelectrolyte under Ar can significantly reduce MoO3 to MoO3- x with x = 0.36 (MoO2.64) at 600 °C. Thermal annealing under H2 gas enhanced the substoichiometry of MoO3- x from x = 0.62 to 0.98 by using PDDA at the same conditions. Density functional theory calculations, supported by experimental analysis, suggest that the vacancy sites are created through absorption of terminal site oxygen (Ot) upon decomposition of the N-C bond in the pentagonal ring of PDDA during the thermal treatment. Ot atoms are absorbed as ionic O- and neutral O2-, creating Mo5+-vO· and Mo4+-vO·· vacancy bipolarons and polarons, respectively. X-ray photoemission spectroscopy peak analysis indicates the ratio of charged to neutral molybdenum ions in the PDDA-processed samples increased from Mo4+/Mo6+ = 1.0 and Mo5+/Mo6+ = 3.3 when reduced at 400 °C to Mo4+/Mo6+ = 3.7 and Mo5+/Mo6+ = 2.6 when reduced at 600 °C. This is consistent with our ab initio calculation where the Mo4+-vO·· formation energy is 0.22 eV higher than that for Mo5+-vO· in the bulk of the material and 0.02 eV higher on the surface. This study reveals a new paradigm for effective enhancement of surface oxygen vacancy concentrations essential for a variety of technologies including advanced energy conversion applications such as electrochemical energy storage, catalysis, and low-temperature thermochemical water splitting.

Nanoscale self-assembly of thermoelectric materials: a review of chemistry-based approaches.

This review is concerned with the leading methods of bottom-up material preparation for thermal-to-electrical energy interconversion. The advantages, capabilities, and challenges from a material synthesis perspective are surveyed and the methods are discussed with respect to their potential for improvement (or possibly deterioration) of application-relevant transport properties. Solution chemistry-based synthesis approaches are re-assessed from the perspective of thermoelectric applications based on reported procedures for nanowire, quantum dot, mesoporous, hydro/solvothermal, and microwave-assisted syntheses as these techniques can effectively be exploited for industrial mass production. In terms of energy conversion efficiency, the benefit of self-assembly can occur from three paths: suppressing thermal conductivity, increasing thermopower, and boosting electrical conductivity. An ideal thermoelectric material gains from all three improvements simultaneously. Most bottom-up materials have been shown to exhibit very low values of thermal conductivity compared to their top-down (solid-state) counterparts, although the main challenge lies in improving their poor electrical properties. Recent developments in the field discussed in this review reveal that the traditional view of bottom-up thermoelectrics as inferior materials suffering from poor performance is not appropriate. Thermopower enhancement due to size and energy filtering effects, electrical conductivity enhancement, and thermal conductivity reduction mechanisms inherent in bottom-up nanoscale self-assembly syntheses are indicative of the impact that these techniques will play in future thermoelectric applications.

Thermoelectric properties and thermal tolerance of indium tin oxide nanowires.

Highly crystalline indium tin oxide (ITO) nanowires were grown via a vapor-liquid-solid method, with thermal tolerance up to ∼1300 °C. We report the electric and thermoelectric properties of the ITO nanowires before and after heat treatments and draw conclusions about their applicability as thermoelectric building blocks in nanodevices that can operate in high temperature conditions. The Seebeck coefficient and the thermal and electrical conductivities were measured in each individual nanowire by means of specialized micro-bridge thermometry devices. Measured data was analyzed and explained in terms of changes in charge carrier density, impurities and vacancies due to the thermal treatments.

Effect of cobalt alloying on the electrochemical performance of manganese oxide nanoparticles nucleated on multiwalled carbon nanotubes.

MnO is an electrically insulating material which limits its usefulness in lithium ion batteries. We demonstrate that the electrochemical performance of MnO can be greatly improved by using oxygen-functional groups created on the outer walls of multiwalled carbon nanotubes (MWCNTs) as nucleation sites for metal oxide nanoparticles. Based on the mass of the active material used in the preparation of electrodes, the composite conversion-reaction anode material Mn1-x Co x O/MWCNT with x = 0.2 exhibited the highest reversible specific capacity, 790 and 553 mAhg-1 at current densities of 40 and 1600 mAg-1, respectively. This is 3.1 times higher than that of MnO/MWCNT at a charge rate of 1600 mAg-1. Phase segregation in the [Formula: see text] nanoparticles was not observed for x ≤ 0.15. Capacity retention in x = 0, 0.2, and 1 electrodes showed that the corresponding specific capacities were stabilized at 478, 709 and 602 mAhg-1 respectively, after 55 cycles at a current density of 400 mAg-1. As both MnO and CoO exhibit similar theoretical capacities and MnO/MWCNT and CoO/MWCNT anodes both exhibit lower performance than Mn0.8Co0.2O/MWCNT, the improved performance of the [Formula: see text] alloy likely arises from beneficial synergistic interactions in the bimetallic system.

Block Copolymer-Assisted Solvothermal Synthesis of Hollow Bi2MoO6 Spheres Substituted with Samarium.

Hollow spherical structures of ternary bismuth molybdenum oxide doped with samarium (Bi2-xSmxMoO6) were successfully synthesized via development of a Pluronic P123 (PEO20-PPO70-PEO20)-assisted solvothermal technique. Density functional theory calculations have been performed to improve our understanding of the effects of Sm doping on the electronic band structure, density of states, and band gap of the material. The calculations for 0 ≤ x ≤ 0.3 revealed a considerably flattened conduction band minimum near the Γ point, suggesting that the material can be considered to possess a quasi-direct band gap. In contrast, for x = 0.5, the conduction band minimum is deflected toward the U point, making it a distinctly indirect band gap material. The effects of a hollow structure as well as Sm substitution on the absorbance and fluorescence properties of the materials produced increased emission intensities at low Sm concentrations (x = 0.1 and 0.3), with x = 0.1 displaying a peak photoluminescence intensity 13.2 times higher than for the undoped bulk sample. Subsequent increases in the Sm concentration resulted in quenching of the emission intensity, indicative of the onset of a quasi-direct-to-indirect electronic band transition. These results indicate that both mesoscale structuring and Sm doping will be promising routes for tuning optoelectronic properties for future applications such as catalysis and photocatalysis.

Effects of surface band bending and scattering on thermoelectric transport in suspended bismuth telluride nanoplates.

A microdevice was used to measure the in-plane thermoelectric properties of suspended bismuth telluride nanoplates from 9 to 25 nm thick. The results reveal a suppressed Seebeck coefficient together with a general trend of decreasing electrical conductivity and thermal conductivity with decreasing thickness. While the electrical conductivity of the nanoplates is still within the range reported for bulk Bi2Te3, the total thermal conductivity for nanoplates less than 20 nm thick is well below the reported bulk range. These results are explained by the presence of surface band bending and diffuse surface scattering of electrons and phonons in the nanoplates, where pronounced n-type surface band bending can yield suppressed and even negative Seebeck coefficient in unintentionally p-type doped nanoplates.

Thermal conductivity and phonon transport in suspended few-layer hexagonal boron nitride.

The thermal conductivity of suspended few-layer hexagonal boron nitride (h-BN) was measured using a microbridge device with built-in resistance thermometers. Based on the measured thermal resistance values of 11-12 atomic layer h-BN samples with suspended lengths ranging between 3 and 7.5 µm, the room-temperature thermal conductivity of a 11-layer sample was found to be about 360 W m(-1) K(-1), approaching the basal plane value reported for bulk h-BN. The presence of a polymer residue layer on the sample surface was found to decrease the thermal conductivity of a 5-layer h-BN sample to be about 250 W m(-1) K(-1) at 300 K. Thermal conductivities for both the 5-layer and the 11-layer samples are suppressed at low temperatures, suggesting increasing scattering of low frequency phonons in thin h-BN samples by polymer residue.