Albert V. Davydov

Two-dimensional Cu nanostructures containing noncanonical face-centered cubic/hexagonal close-packed (fcc/hcp) heterophase boundaries are attractive for CO2 electroreduction but remain difficult to prepare by wet-chemical methods. Here, we report a high-temperature solution reduction route to Cu nanodisks with a thickness of 26.5 ± 2.3 nm and a diameter of 85.6 ± 3.7 nm. Transmission electron microscopy reveals abundant fcc/2H and fcc/4H interfaces on the nanodisk side surfaces, where the 2H and 4H polytypes exhibit AB and ABCB stacking, respectively. In CO2-saturated 0.1 M KHCO3, the nanodisks achieve a methane Faradaic efficiency of 60.4% at −1.25 V vs the reversible hydrogen electrode and outperform fcc Cu nanoparticle and microsheet controls tested at the same catalyst loading. Density functional theory calculations indicate that the free-energy penalty for forming the *CHO intermediate is smaller on the fcc/2H and fcc/4H models than on fcc Cu. These results support a thermodynamic contribution of heterophase interfaces to the observed methane selectivity; the calculations do not establish a reduction in kinetic activation barriers. This solution-phase route therefore provides access to phase-engineered Cu nanodisks and enables structure−property relationships relevant to CO2-to-CH4 electrocatalysis.

Spatially varying strain provides a route to control exciton transport and localization in two-dimensional semiconductors, but resolving these processes requires correlated spatial, spectral, and temporal measurements. Here, we investigate multilayer GaSe suspended over nanopillar arrays using photoluminescence spectroscopy, Raman mapping, and wavelength-resolved fluorescence lifetime imaging microscopy (FLIM). Raman mapping confirms localized tensile strain around the nanopillars, while FLIM reveals complementary emission patterns: free-exciton emission at 615–625 nm forms annular distributions around the pillars, whereas red-shifted strain-localized emission at 630–670 nm is concentrated toward the highly strained pillar regions. For GaSe on Al2O3-coated nanopillars, the strain-localized emission exhibits biexponential decay with characteristic lifetimes near 1.20 and 1.46 ns, together with an increase in the slow lifetime toward the pillar apex and a slow-to-fast amplitude ratio of 7.68. In contrast, the corresponding red-shifted emission on metallic Cu nanopillars is predominantly single exponential, highlighting the influence of the local interface on exciton recombination. The combined spatial redistribution, strain-correlated spectral redshift, and systematic lifetime evolution support strain-guided exciton funneling in multilayer GaSe. These results establish wavelength-resolved FLIM as an effective approach for mapping strain-dependent exciton dynamics and demonstrate nanopillar-induced strain as a means to engineer exciton localization in layered semiconductors.

Heteroepitaxy is the predominant mode of thin-film growth for gallium nitride (GaN) in LEDs, RF electronics and power electronic applications. A major factor hindering GaN from wider adoption in the higher-voltage regime is the pervasiveness of threading dislocations generated during heteroepitaxial growth on lattice and thermally mismatched substrates. In this work, the dislocation populations of commercially-obtained GaN HEMT structures grown on sapphire, silicon and SiC substrates are characterized. Spatial maps of threading dislocations are obtained using the SEM-based technique of electron channeling contrast imaging (ECCI) which allows for the comparison of defect densities across the HEMT wafers. It is demonstrated that the sapphire and silicon-grown HEMT structures have similar dislocation densities of approximately 3 × 109 cm-2 whereas the SiC-based structure has a density around 71% lower at 8.7 × 108 cm-2. Through further variation of ECCI operating conditions, determination of relative concentration of edge dislocations to dislocations with a screw component is obtained in the SiC HEMT structure.

In this work, we investigate exciton dynamics and localized light emission in two-dimensional (2D) multilayer GaSe through strain and defect engineering. We used three methods to engineer defect-localized excitons: nanopillar-induced localized tensile strain, hydrostatic pressure quenching, and UV laser irradiation. All three methods produce low-energy localized emission with photoluminescence saturation behavior versus the excitation power, indicating highly localized point-defect states. These results demonstrate that exciton emission in multilayer GaSe can be effectively controlled through complementary strain and defect engineering, highlighting its potential for tunable optoelectronic and quantum photonic applications.

Engineered defects in multilayer 2D materials can be embedded beneath the sample surface, providing enhanced stability of their optical response. In multilayer transition-metal dichalcogenides (TMDs) (MoS2 and MoSe2), we study defect-assisted emission involving chalcogen-vacancy-related states, where strain and anisotropic thermal or mechanical perturbations are used to modify carrier lifetimes and level alignment in indirect-gap systems. Polarization-dependent emission associated with these defect-related transitions is observed. In layered group-III chalcogenides, particularly GaSe and InSe, defects are introduced through rapid thermal quenching and UV laser irradiation. We report photoluminescence (PL) tunability, emission enhancement, polarization effects, and spatially resolved PL lifetime distributions associated with free excitons, strain-bound excitons, and defect-localized excitons. In mechanically strained GaSe, bound excitons exhibit longer PL lifetimes than free excitons, correlating with enhanced emission intensity, while such polarization effects are absent under thermally induced strain. Spatial variations in PL lifetime consistent with strain-induced exciton energy gradients are observed and discussed in the context of exciton redistribution in strained regions. At low temperatures, thermal activation energies for exciton escape from defect-localized states in GaSe and InSe are extracted, and spectrally sharp emission features in GaSe are characterized. Finally, UV laser writing is used to create grating patterns with spatially modulated defect densities in GaSe, enabling controlled modification of local optical emission.

In this talk we’ll present two stories related to the tip enhanced Raman scattering (TERS) imaging of 2D semiconductors. First, we’ll demonstrate recent results of TERS imaging of 1-4-layer ReS2 exfoliated to template-stripped gold. Conventional far-field Raman spectra of ReS2 feature up to 18 peaks within the 0-500 cm-1 range, while the intensity of individual peaks strongly depends on the mutual orientation of the polarization vector in the laser focus and the crystalline axes within the ReS2 plane. We demonstrate that unlike the far-field response, the gap-mode TERS spectra of ReS2 on gold do not change as a result of the sample rotation in the XY plane. We also show that in addition to the regular vibrational modes we can also observe the layer breathing modes in multilayer ReS2 crystals. In the second part we'll discuss the possible reasons for an unprecedented preferential enhancement of the 995 cm-1 mode in TERS spectra of α-MoO3 and how the fact that this mode falls within the Type I reststrahlen band may affect the TERS response and open the possibilities for the IR-Raman crosstalk.

Wide bandgap semiconductors, including gallium nitride (GaN), are crucial for next-generation power electronics due to their superior properties compared to silicon. However, challenges in their fabrication, particularly with epitaxial regrowth for vertical diode device structures, can introduce defects that impact performance. In this study, we report on vertical GaN pn diodes that exhibit an exceptionally large hysteresis loop with on/off ratios exceeding nine orders of magnitude, a behavior absent in devices grown in situ without changing growth reactors. We demonstrate that these devices exhibit robust endurance, maintaining a stable memory window for over 60 000 switching cycles at room temperature (20 °C), and remain partially viable up to at least 275 °C. Through analytical characterization, including secondary ion mass spectrometry and electroluminescence, we provide evidence that the hysteresis behavior is linked to a defect-rich interface region containing silicon and carbon impurities. To further investigate the underlying mechanism, we developed a technology computer aided design model that successfully replicates the observed hysteresis and identifies the likely mechanisms involved, key attributes of which include interfacial barriers, fixed negative charge, and positive charge trapping. These findings highlight a critical challenge for traditional GaN power devices while also demonstrating their potential for novel high-temperature memory applications.

Multiferroics host simultaneous and coupled ferroic orders, allowing disparate external stimuli to induce abrupt transformations in their structures and properties. Creating multiferroicity in two-dimensional (2D) van der Waals (vdW) platforms would add the elements of strong quantum confinement, enhanced quasiparticle excitations, and wide tunability to the capabilities of these systems. In this work, we constructed vdW heterostructures of ferromagnetic triiron gallium ditelluride (Fe ₃ GaTe ₂ ) and ferroelectric copper indium thiophosphate (CuInP ₂ S ₆ ) to integrate their respective orders. We observed strong interferroic coupling at room temperature by demonstrating ferroelectrically reconfigurable magnetic anisotropy of 2D Fe ₃ GaTe ₂ . The interferroic magnetoelectricity diminished with increasing Fe ₃ GaTe ₂ thickness, revealing the interfacial nature of heterostructure multiferroicity. Our discovery of all-vdW heterostructure multiferroicity opens the door to the artificial assembly of vdW layers for designer 2D multiferroics.

We measure the temperature-dependent in-plane thermal conductivity, κ∥(T), of high-purity γ-InSe bulk single crystals and exfoliated thin flakes (30–50 nm) from 50 to 300 K. Our bulk results agree with prior bulk reports and provide a reproducible reference for phonon transport. In the literature, cross-plane thermal conductivity of supported InSe flakes shows relatively modest variation, whereas reported room-temperature in-plane values for supported flakes span a wide range, including outliers that exceed bulk despite much smaller thickness. In our measurements, the flake κ∥ at room temperature is lower than bulk, as expected, but exhibits substantial sample-to-sample variability; despite being thinner, intrinsic flakes show higher κ∥ than doped flakes, consistent with reduced impurity scattering and improved flake uniformity. We analyze our data using a Callaway-type phonon-scattering model in which substrate interactions (including plausible strain-related renormalization of acoustic parameters) contribute to the observed spread near room temperature. However, below ∼150 K, the measured trends cannot be captured without invoking changes in sound velocity far larger than realistic strain levels, indicating that additional mechanisms beyond simple strain renormalization are required at low temperature. These measurements provide low-temperature κ∥ benchmarks for γ-InSe flakes and constrain how much of the reported room-temperature spread can be explained by thickness, nonuniformity, and substrate effects alone.