Influence of barrier height on carrier dynamics in strained InxGa1-xAs/GaAs quantum wells

Extraordinary approach to further boost plasmonic NIR-SERS by cryogenic temperature-suppressed non-radiative recombination

We report an effective strategy to promote the near-infrared surface-enhanced Raman scattering spectroscopy (NIR-SERS) activity by boosting the photon-induced charge transfer (PICT) efficiency at cryogenic temperature. Based on as-prepared Au/Ag nano-urchins (NUs) with abundant surface defects, the extremely low temperature (77 K) can significantly weaken the metallic lattice vibration and reduce the recombination of thermal phonons and photoexcited electrons, then accelerate the migration of energetic electrons. It enables the NIR-SERS detection limit of dye molecules to be achieved at 10^-17 M, which is nearly three orders of magnitude better than that at room temperature. The present work provides a new, to the best of our knowledge, approach for ultra-trace NIR-SERS bioanalysis.

Tightly-bound trion and bandgap engineering viaγ-ray irradiation in the monolayer transition metal dichalcogenide WSe2

Afterγ-ray irradiation treatment, a monolayer tungsten diselenide could be transitioned into an n-doped semiconductor due to the anion vacancies created by the radiation. Transmission electron microscope studies showed clear chemical modulation with atomically sharp interface. Change in the lattice vibrational modes induced by passivation of oxygen is captured by Raman spectroscopy. The frequency shifts in both in-plane and out-of-plane modes are dependent linearly on the oxidation content. We observe a negative trion, which is a neutral exciton bound with an electron, in the photoluminescence spectra. The binding energy of this trion is estimated to be ∼90 meV, making it a tightly bound exciton. The first-principles calculation suggests that an increase in the anion vacancy population is generally accompanied by a transition from a direct gap material to an indirect one. This opens up a new venue to engineer the electronic properties of transition metal dichalcogenides by using irradiation.

Low temperature-boosted high efficiency photo-induced charge transfer for remarkable SERS activity of ZnO nanosheets

Improving the photo-induced charge transfer (PICT) efficiency is the key factor for boosting the surface-enhanced Raman scattering (SERS) performance of semiconductor nanomaterials. Introducing plentiful surface defect states in porous ZnO nanosheets (d-ZnO NSs) effectively provides additional charge transfer routes for highly efficient PICT within the substrate-molecule system. Significantly, an interesting phenomenon of low temperature-boosted SERS activity of these d-ZnO NSs is consequently observed. The enhanced SERS activity can be attributed to the efficient PICT processes due to the significant reduction of non-radiative recombination of surface defects at a low temperature. This is carefully investigated through combining in situ low-temperature SERS measurements with temperature-dependent photoluminescence (PL) emission spectroscopy. Our results clearly demonstrate that the weakened lattice thermal vibration at a low temperature effectively suppresses the phonon-assisted relaxation and reduces carrier traps, resulting in the increase of PL intensity. The decreased traps of photo-induced electrons at surface defect states effectively facilitate the PICT efficiency within the substrate-molecule system. An ultrahigh enhancement factor of 7.7 × 105 and low limit of detection (1 × 10-7 M) for a 4-mercaptopyridine molecule at a temperature of 77 K are successfully obtained. More importantly, the low temperature-enhanced SERS effect is also obtainable in other metal oxide semiconductors, such as d-TiO2 and d-Cu2O nanoparticles. To the best of our knowledge, this is the first time the low temperature-boosted SERS activity of semiconductors has been observed. This study not only provides a deep insight into the chemical SERS mechanism, but also develops a novel strategy for improving semiconductor SERS sensitivity. The strong SERS activity at a low temperature reported here may open new avenues for developing non-metal SERS substrates with new functionalities, especially for the research on cryogenic sensing and hypothermal medicine.

Direct-bandgap emission from hexagonal Ge and SiGe alloys

Silicon crystallized in the usual cubic (diamond) lattice structure has dominated the electronics industry for more than half a century. However, cubic silicon (Si), germanium (Ge) and SiGe alloys are all indirect-bandgap semiconductors that cannot emit light efficiently. The goal¹ of achieving efficient light emission from group-IV materials in silicon technology has been elusive for decades^2-6. Here we demonstrate efficient light emission from direct-bandgap hexagonal Ge and SiGe alloys. We measure a sub-nanosecond, temperature-insensitive radiative recombination lifetime and observe an emission yield similar to that of direct-bandgap group-III-V semiconductors. Moreover, we demonstrate that, by controlling the composition of the hexagonal SiGe alloy, the emission wavelength can be continuously tuned over a broad range, while preserving the direct bandgap. Our experimental findings are in excellent quantitative agreement with ab initio theory. Hexagonal SiGe embodies an ideal material system in which to combine electronic and optoelectronic functionalities on a single chip, opening the way towards integrated device concepts and information-processing technologies.

Quick Optical Identification of the Defect Formation in Monolayer WSe2 for Growth Optimization

Bottom-up epitaxy has been widely applied for transition metal dichalcogenides (TMDCs) growth. However, this method usually leads to a high density of defects in the crystal, which limits its optoelectronic performance. Here, we show the effect of growth temperature on the defect formation, optical performance, and crystal stability in monolayer WSe₂ via a combination of Raman and photoluminescence (PL) spectroscopy study. We found that the defect formation and distribution in monolayer WSe₂ are closely related to the growth temperature. These defect density and distribution can be controlled by adjusting the growth temperature. Aging experiments directly demonstrate that these defects are an active center for the decomposition process. Instead, monolayer WSe₂ grown under optimal conditions shows a strong and uniform emission dominated by neutral exciton at room temperature. The results provide an effective approach to optimize TMDCs growth.