Peng Miao

Ultrafast Surface-Plasmon-Induced Photodimerization of p-Aminothiophenol on Ag/TiO2 Nanoarrays

Surface‐plasmon‐induced photocatalysis has been of increasing interest as a thriving new paradigm for catalytic reactions, while the efficiency of such kind of reactions remains very limited on metal nanostructures. Herein, an ultrafast photodimerization of p‐aminothiophenol (PATP) into p,p′‐dimercaptobenzene (DMAB) was witnessed on the hybrid system of Ag/TiO2 by visible‐light laser excitation, even under inert gas atmosphere. Hot electrons generated from plasmonic decay would transfer to the conduction band of TiO2, which enables the holes (generated in the metal nanoparticles) to trigger the immediate PATP–DMAB conversion. We believe such ultrafast reaction process, with highly enhanced efficiency, will greatly promote the research and applications of the surface‐plasmon‐induced or assisted catalytic reactions.

Fabrication of arrayed triangular micro-cavities for SERS substrates using the force modulated indention process

Based on the tip-based continuous indentation process, a novel method for the fabrication of periodic arrayed triangular micro-cavities on copper (Cu) surfaces is presented. The indentation force is modulated and the indentation speed and moving velocity of the precision stage used to drive the workpiece are optimized to improve the machining efficiency. The deformation property of the pile-ups at the side of the pyramidal cavity is studied. Due to the overlap of the pile-ups of adjacent micro pyramidal cavities, two and three dimensional arrayed nanostructures are successfully achieved. Then, the structured Cu (110) surface is used as a surface-enhanced Raman scattering (SERS) substrate with the rhodamine 6G (R6G) probe molecule as the detecting target in the present study. Experimental results show that the Raman intensity is enhanced with a decrease in the moving velocity of the precision stage. SERS enhancement factors within the range of 105 to 3 × 106 are achieved on the structured Cu (110) surface, which demonstrates that this method is reliable, replicable, homogeneous and inexpensive for the fabrication of SERS substrates.

Galvanic replacement-mediated synthesis of hollow Cu2O–Au nanocomposites and Au nanocages for catalytic and SERS applications

The galvanic replacement reaction (GRR) involves a corrosion process that is driven by the difference in the electrochemical potentials of two species. Here we demonstrate the synthesis of hollow Cu2O–Au nanocomposites via a GRR process between Cu2O and HAuCl4, and subsequent conversion of the hollow Cu2O–Au nanocomposites into Au nanocages that are actually assembled of ∼10 nm Au nanoparticles. It is interesting to find that Cu2O nanocubes produced from reductive solution chemistry are actually transformed from Cu(OH)2 nanowire precursors, and the Cu2O particle size is inversely proportional to the reaction temperature. A time-dependent TEM study of the GRR process between Cu2O and HAuCl4 indicates that this reaction involves evolution of an internal hollow core and surface precipitation of Au nanoparticles, which allows the formation of hollow Cu2O–Au nanocomposites. Comparing the properties of hollow Cu2O–Au nanocomposites and Au nanocages, it is determined that the hollow Cu2O–Au nanocomposites are more catalytically active in the reduction of 4-nitrophenol into 4-aminophenol in the presence of NaBH4, and Au nanocages are two orders of magnitude more sensitive in SERS detection of the target molecule, methylene blue. We believe the findings in this work may render a better understanding of the preparation and GRR process of Cu2O nanomaterials.

Origin of the Ultrafast Response of the Lateral Photovoltaic Effect in Amorphous MoS2/Si Junctions

The lateral photovoltaic (LPV) effect has attracted much attention for a long time because of its application in position-sensitive detectors (PSD). Here, we report the ultrafast response of the LPV in amorphous MoS2/Si (a-MoS2/Si) junctions prepared by the pulsed laser deposition (PLD) technique. Different orientations of the built-in field and the breakover voltages are observed for a-MoS2 films deposited on p- and n-type Si wafers, resulting in the induction of positive and negative voltages in the a-MoS2/n-Si and a-MoS2/p-Si junctions upon laser illumination, respectively. The dependence of the LPV on the position of the illumination shows very high sensitivity (183 mV mm-1) and good linearity. The optical relaxation time of LPV with a positive voltage was about 5.8 μs in a-MoS2/n-Si junction, whereas the optical relaxation time of LPV with a negative voltage was about 2.1 μs in a-MoS2/p-Si junction. Our results clearly suggested that the inversion layer at the a-MoS2/Si interface made a good contribution to the ultrafast response of the LPV in a-MoS2/Si junctions. The large positional sensitivity and ultrafast relaxation of LPV may promise the a-MoS2/Si junctions applications in fast position-sensitive detectors.

Fabrication of gold-coated PDMS surfaces with arrayed triangular micro/nanopyramids for use as SERS substrates

Using the tip-based continuous indentation process, arrays of three-dimensional pyramidal cavities have been successfully machined on a copper template and the structures were successfully transferred to a polydimethylsiloxane (PDMS) surface using a reverse nanoimprinting approach. The structured PDMS surface is coated with a thin Au film, and the final substrate is demonstrated as a surface-enhanced Raman spectroscopy (SERS) substrate. Rhodamine 6G (R6G) was used as a probe molecule in the present study to confirm the SERS measurements. Arrays of micro/nanostructures of different dimensions were formed by the overlap of pyramidal cavities with different adjacent distances using the tip-based continuous indentation process. The effects of the reverse nanoimprinting process and coating process on the final topography of the structures are studied. The experimental results show that the Raman intensity of the Au-film-coated PDMS substrate is influenced by the topography of the micro/nanostructures and by the thickness of the Au film. The Raman intensity of 1362 cm−1 R6G peak on the structured Au-film-coated PDMS substrate is about 8 times higher than the SERS tests on a commercial substrate (Q-SERS). A SERS enhancement factor ranging from 7.5 × 105 to 6 × 106 was achieved using the structured Au-film-coated PDMS surface, and it was demonstrated that the method proposed in this paper is reliable, replicable, homogeneous and low-cost for the fabrication of SERS substrates.

SERS-Based Plasmon-Driven Reaction and Molecule Detection on a Single Ag@MoS2 Microsphere: Effect of Thickness and Crystallinity of MoS2

Plasmon‐driven catalysis on a single particle has attracted great attention in recent years, while the relationship between reaction efficiency and substrate remains to be deeply studied. Here, we demonstrate the fabrication of a novel Ag@MoS2 core‐shell single particle SERS substrate by coating MoS2 film on Ag microspheres through the pulsed laser deposition (PLD) technique, where the thickness and crystallinity of MoS2 can be effectively controlled by the PLD time and temperature. It is revealed that both the thickness and crystallinity of MoS2 can greatly influence the hot electron transfer process, and thus the plasmon‐driven reaction (4‐NTP into DMAB) efficiency and Raman enhancement of dye molecules on Ag@MoS2 substrates. Generally, high crystallinity and thin thickness of MoS2 can lead to accelerated plasmon‐driven reaction efficiency and greatly enhanced Raman signals of target molecules. This study opens up a new avenue for broadening the research area of the plasmon‐driven catalysis and Raman enhancement on the hybrid system of two‐dimensional materials and metal nanostructures.

Photoresponse Enhancement in Monolayer ReS2 Phototransistor Decorated with CdSe–CdS–ZnS Quantum Dots

ReS2 films are considered as a promising candidate for optoelectronic applications due to their direct band gap character and optical/electrical anisotropy. However, the direct band gap in a narrow spectrum and the low absorption of atomically thin flakes weaken the prospect for light-harvesting applications. Here, we developed an efficient approach to enhance the performance of a ReS2-based phototransistor by coupling CdSe-CdS-ZnS core-shell quantum dots. Under 589 nm laser irradiation, the responsivity of the ReS2 phototransistor decorated with quantum dots could be enhanced by more than 25 times (up to ∼654 A/W) and the rising and recovery time can be also reduced to 3.2 and 2.8 s, respectively. The excellent optoelectronic performance is originated from the coupling effect of quantum dots light absorber and cross-linker ligands 1,2-ethanedithiol. Photoexcited electron-hole pairs in quantum dots can separate and transfer efficiently due to the type-II band alignment and charge exchange process at the interface. Our work shows that the simple hybrid zero- and two-dimensional hybrid system can be employed for photodetection applications.

Strong room-temperature emission from defect states in CVD-grown WSe 2 nanosheets

Monolayer transition metal dichalcogenides (TMDCs), as direct bandgap semiconductors, show promise for applications in ultra-thin flexible optoelectronic devices. However, the optical properties and device performance are greatly affected by defects, such as vacancies, present in these materials. Vacancies exist unavoidably in mechanically exfoliated or grown by chemical vapor deposition (CVD) monolayer TMDCs; therefore, their influence on the electric and optical properties of host materials has been widely studied. Here, we report a new defect state located at 1.54 eV, which is 70 meV lower than the neutral exciton energy in as-prepared MoS2 monolayers grown by CVD. This defect state is clearly observed in photoluminescence (PL) and Raman spectra at ambient conditions. PL mapping, Raman mapping, and atomic force microscopy analysis indicate a solid-vapor reaction growth mechanism of the defect state formation. During a certain growth stage, nuclei with the composition of WOxSey do not fully react with the Se vapor, leading to the defect formation. This type of defects permits radiative recombination of bound neutral excitons, which can make the PL intensity as strong as the intrinsic excitation. Our findings reveal a new way to tailor the optical properties of two-dimensional TMDCs without any additional processes performed after growth.

Phase transition induced Raman enhancement on vanadium dioxide (VO2) nanosheets

Semiconductor materials such as transition metal oxides and dichalcogenides have been increasingly explored as promising surface enhanced Raman scattering (SERS) platforms. This work demonstrates, for the first time, that VO2 nanosheet materials can be another SERS-active platform for molecule detection based on the chemical enhancement mechanism. A systematic study of the phase transition induced Raman enhancement on VO2 nanosheets reveals that increase in the crystal symmetry from VO2(B) to VO2(M) and to VO2(R) results in a dramatic decline in the enhancement ability of the probe molecules. On defect-rich VO2(B) nanosheets, a detection limit as low as 10−7 M can be reached for various dye molecules, confirming VO2 nanosheets as another kind of effective SERS-active semiconductor metal oxide. This study strongly indicates that a synergetic strategy of low-dimension and low-symmetry may be a new avenue for the design of highly efficient metal oxide-based semiconductor SERS substrates.

Photothermally Enhanced Plasmon‐Driven Catalysis on Fe5C2@Au Core–Shell Nanostructures

Plasmon‐driven catalysis has attracted great attention in recent years, but the reaction efficiency remains to be improved. Photothermal Fe5C2@Au core–shell nanostructures are fabricated through a self‐assembly process of Fe5C2 and Au nanoparticles (NPs) with the assistance of hexanethiol, which can be highly efficient surface enhanced Raman spectroscopy (SERS) platforms for the study of plasmon‐driven dimerization of 4‐aminothiophenol (4‐ATP) and 4‐nitrothiophenol (4‐NTP). As compared to bare Au NPs, much accelerated reaction kinetics can be achieved on the Fe5C2@Au core–shell nanostructures by quantitatively determining the Raman intensity of the ν(N=N) band in the generated 4,4′‐dimercaptobenzene (DMAB). The photothermal effect from the Fe5C2 NPs may lower the energy barrier and generate more hot electrons for the plasmon‐driven catalysis. This photothermal route may open up new avenues for enhancing the reaction rate and broadening the research area of the plasmon‐driven catalysis.