59 Search Results

Abstract The fine-tuning of topologically protected states in quantum materials holds great promise for novel electronic devices. However, there are limited methods that allow for the controlled and efficient modulation of the crystal lattice while simultaneously monitoring the changes in the electronic structure within a single sample. Here, we apply significant and controllable strain to high-quality HfTe ₅ samples and perform electrical transport measurements to reveal the topological phase transition from a weak topological insulator phase to a strong topological insulator phase. After applying high strain to HfTe ₅ and converting it into a strong topological insulator, we found thatmore » the resistivity of the sample increased by 190,500% and that the electronic transport was dominated by the topological surface states at cryogenic temperatures. Our results demonstrate the suitability of HfTe ₅ as a material for engineering topological properties, with the potential to generalize this approach to study topological phase transitions in van der Waals materials and heterostructures.« less

Lanthanum titanate glasses (17 La₂O₃-83 TiO₂) are fabricated via aerodynamic levitation melting. Here, nonlinear refraction and absorption were measured with the Z scan technique using ps optical pulses at 532 nm wavelength with peak intensities in the range of 0.25–2.5 GW/cm². The two-photon absorption coefficient (β) and the effective nonlinear refraction coefficient (⁠$$n^{eff}_{2}$$) are found to be, respectively, 2.19 cm/GW and 152 × 10^-16 cm²/W (85 × 10^-13 esu). The Raman gain is measured to be 106 × 10^-11 cm/W. The nonlinearity strength is found to be nearly 60 times larger in lanthanum titanate glasses relative to silica.

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. Here, 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.

The study reveals that a two-dimensional (2D) material as substrate for heterogeneous integration acts as a compliant substrate.

Industry is producing amazing new products every day from cell phones to aircraft and it’s all thanks to the creation of complicated materials. Now in an ideal world these materials would be perfect such as in this picture where all the atoms align. Perfect materials are great because they are easy to understand but in reality we are not living ideal world and most materials are far from perfect. We refer to the common imperfections in materials as defects. These defects are at the atomic level and can be anything from missing single atoms to entire neighborhoods of atoms thatmore » are arranged slightly differently such as shown here.« less

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-typemore » 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).« less

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.more » In this study, 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.« less

Focused ion beam (FIB) techniques have been frequently used to section metal-halide perovskites for microstructural investigations. However, the ion beams directly irradiating the sample surface may alter its properties far different from those of pristine, potentially leading to modified deterioration mechanisms under aging stressors. Here, we combine complementary approaches to measure the subsurface characteristics of polished perovskites and identify the chemical species responsible for the measured properties. Analysis of the experimental results in conjunction with Monte Carlo simulations indicates that atomic displacements and local heating occur in the first ≈15 nm of the subsurface of methylammonium lead iodide (MAPbI₃) bymore » glancing-angle Ar⁺ beam irradiation (4 kV at 3°). The lead-rich, iodine-deficient surface promotes rapid phase segregation under thermal aging conditions. On the other hand, despite the subsurface modification, our experiments confirm that the rest of the MAPbI₃ bulk retains the material integrity. Our observation supports that polished perovskites could serve in studying the properties of bulk or buried junctions far away from the altered subsurface with care.« less

Abstract Atomically thin transition metal dichalcogenides (TMDs), like MoS ₂ with 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) ⁴ He ⁺ ion irradiation towards tuning the optoelectronic properties of an epitaxially grown continuous film of MoS ₂ at the wafer scale, and demonstrate thatmore » precise manipulation of atomistic defects can be achieved in TMD films using ion implanters. The effect of ⁴ He ⁺ 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 MoS ₂ lattice, 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 MoS ₂ and other 2D materials.« less

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. Here, 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 withmore » 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.« less