Jingbiao Cui

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.

We investigate the effects of electron beam energy on defect formation in monolayer graphene using Raman spectroscopy and the local activation model. Monolayer graphene samples were irradiated at beam energies ranging from 1 to 30 keV, and the evolution of Raman D, G, and D’ peaks was monitored as a function of electron fluence. For all energies, the ID/IG and ID′/IG ratios exhibited a two-stage behavior, initially increasing with fluence before decreasing at higher defect densities, consistent with the local activation model predictions. Notably, the maximum ID/IG ratio decreased and shifted to a higher fluence with increasing beam energy. At lower beam energies (1–2 keV), an additional broad Raman peak was observed near the G peak, attributed to carbonaceous film deposition from secondary electrons (SEs). Defect formation was modeled using the local activation model with adjustments to account for SE-induced dissociation of surface adsorbates, showing good agreement with experimental data. Parameters A and B, which represent the incident electron fluence required to generate a defect-active region and the fraction of dissociable adsorbates, respectively, showed systematic trends with beam energy. Postirradiation annealing studies were conducted to determine activation energies for defect healing via Arrhenius analysis. The extracted activation energies (0.31–0.48 eV) are consistent with sp3-type defects such as hydrogen and hydroxyl groups with attached water molecules. These findings highlight the critical role of beam energy and SE yield in the defect engineering of graphene and demonstrate the utility of the local activation model in quantitatively describing beam-induced disorder.

In this work, we report on defect generation in multilayer GaSe through hydrostatic pressure quenching and UV laser irradiation. The Raman line width from the UV 266 nm irradiated sample is much wider than that in pressure-quenched GaSe, corresponding to a wider defect energy distribution range in the former sample than the latter. After quenching from 11.2 GPa, three photoluminescence (PL) peaks from defect states are observed at 657, 681, and 695 nm at a low temperature of 93 K. Defect-related peaks at 649, 694, 750, and 774 nm also appear in low-temperature PL spectra after UV laser irradiation, with a nonmonotonous intensity dependence on irradiation duration. There are common features in defects produced by these two methods: the PL peaks with the lowest energy are sharp, and their PL intensities increase linearly with the excitation laser power and saturate above a certain excitation laser power. These two features are similar to those in defects for single-photon emission (SPE) in other 2D materials at even lower temperatures. Fluorescence lifetime imaging shows distinguished short (2.3 ns) and long (75.6 nm) lifetimes of the 695 nm PL line in pressure-quenched GaSe. The density functional theory predicts defect energy levels related to Se vacancy.

We study the effects of electron irradiation on suspended graphene monolayers and graphene supported on SiO2 substrates in the range 5.0 × 1015–4.3 × 1017 electrons/cm2. The suspended graphene monolayers are exfoliated over SiO2 substrates containing micrometer-sized holes, with graphene completely covering the hole, and are referred to as graphene drums. The irradiation was performed using a scanning electron microscope at 20–25 keV electron energy. We observe a two-stage behavior for the ID/IG, ID′/IG, and ID/ID′ ratios as a function of the average distance between defects, LD, where ID, IG, and ID′ are the intensities of the Raman D, G, and D′ peaks, respectively. Good fits to the dependence of the ratios on LD are obtained using the local activation model equation. The fits are used to characterize the defects at high defect densities. We also carried out annealing studies of samples irradiated to the first stage and used an Arrhenius plot to measure activation energies for defect healing, Ea. We measured Ea = 0.90 eV for the graphene drums, consistent with the hydroxyl groups; for supported graphene, we measured Ea = 0.36 eV, consistent with hydrogen adsorbates. We also studied the surface of the drums using atomic force microscopy and found no observable holes after irradiation and annealing. Our results show that the local activation model is useful in characterizing the defects in graphene drums.

Encapsulating Cs₄PbBr₆ quantum dots in silicon nano-sheets not only stabilizes the halide perovskite, but also takes advantage of the nano-sheet for a compatible integration with the traditional silicon semiconductor. Here, we report the preparation of un-passivated Cs₄PbBr₆ ellipsoidal nanocrystals and pseudo-spherical quantum dots in silicon nano-sheets and their enhanced photoluminescence (PL). For a sample with low concentrations of quantum dots in silicon nano-sheets, the emission from Cs₄PbBr₆ pseudo-spherical quantum dots is quenched and is dominated with Pb²⁺ ion/silicene emission, which is very stable during the whole measurement period. For a high concentration of Cs₄PbBr₆ ellipsoidal nanocrystals in silicon nano-sheets, we have observed Förster resonance energy transfer with up to 87% efficiency through the oscillation of two PL peaks when UV excitation switches between on and off, using recorded video and PL lifetime measurements. In an area of a non-uniform sample containing both ellipsoidal nanocrystals and pseudo-spherical quantum dots, where Pb²⁺ ion/silicene emissions, broadband emissions from quantum dots, and bandgap edge emissions (515 nm) appear, the 515 nm peak intensity increases five times over 30 min of UV excitation, probably due to a photon recycling effect. This irradiated sample has been stable for one year of ambient storage. Cs₄PbBr₆ quantum dots encapsulated in silicon nano-sheets can lead to applications of halide perovskite light emitting diodes (PeLEDs) and integration with traditional semiconductor materials.