Junpeng Lu

Atomic Healing of Defects in Transition Metal Dichalcogenides

As-grown transition metal dichalcogenides are usually chalcogen deficient and therefore contain a high density of chalcogen vacancies, deep electron traps which can act as charged scattering centers, reducing the electron mobility. However, we show that chalcogen vacancies can be effectively passivated by oxygen, healing the electronic structure of the material. We proposed that this can be achieved by means of surface laser modification and demonstrate the efficiency of this processing technique, which can enhance the conductivity of monolayer WSe2 by ∼400 times and its photoconductivity by ∼150 times.

Improved Photoelectrical Properties of MoS2 Films after Laser Micromachining

Direct patterning of ultrathin MoS2 films with well-defined structures and controllable thickness is appealing since the properties of MoS2 sheets are sensitive to the number of layer and surface properties. In this work, we employed a facile, effective, and well-controlled technique to achieve micropatterning of MoS2 films with a focused laser beam. We demonstrated that a direct focused laser beam irradiation was able to achieve localized modification and thinning of as-synthesized MoS2 films. With a scanning laser beam, microdomains with well-defined structures and controllable thickness were created on the same film. We found that laser modification altered the photoelectrical property of the MoS2 films, and subsequently, photodetectors with improved performance have been fabricated and demonstrated using laser modified films.

Microlandscaping of Au Nanoparticles on Few-Layer MoS2 Films for Chemical Sensing

Surface modification or decoration of ultrathin MoS2 films with chemical moieties is appealing since nanointerfacing can functionalize MoS2 films with bonus potentials. In this work, a facile and effective method for microlandscaping of Au nanoparticles (NPs) on few-layer MoS2 films is developed. This approach first employs a focused laser beam to premodify the MoS2 films to achieve active surface domains with unbound sulfur. When the activated surface is subsequently immersed in AuCl3 solution, Au NPs are found to preferentially decorate onto the modified regions. As a result, Au NPs can be selectively and locally anchored onto designated regions on the MoS2 surface. With a scanning laser beam, microlandscapes comprising of Au NPs decorated on laser-defined micropatterns are constructed. By varying the laser power, reaction time and thickness of the MoS2 films, the size and density of the NPs can be controlled. The resulting hybrid materials are demonstrated as efficient Raman active surfaces for the detection of aromatic molecules with high sensitivity.

Optical and electrical applications of ZnSxSe1−x nanowires-network with uniform and controllable stoichiometry

Single crystalline ternary ZnS(x)Se(1-x) nanowires with uniform chemical stoichiometry and accurately controllable compositions (0≤x≤ 1) were synthesized through a simple and yet effective one-step approach with a specially designed modification. Energy-gap-tuning via compositional change was achieved for a direct band gap from 2.6 to 3.6 eV. Raman spectroscopy studies revealed typical two-mode behavior indicative of high miscibility in the alloyed compound. Moreover, the enhanced electrical-conductivity and gating effect behavior after the formation of ternary alloy enable their application in nano/micro-field effect transistor devices. In addition, the slow recombination rate in the photo-response process indicates their potential for photoelectric applications.

Engineering Bandgaps of Monolayer MoS2 and WS2 on Fluoropolymer Substrates by Electrostatically Tuned Many-Body Effects.

Intrinsic electrical and excitonic properties of monolayer transition-metal dichalcogenides can be revealed on CYTOP fluoropolymer substrates with greatly suppressed unintentional doping and dielectric screening. Ambipolar transport behavior is observed in monolayer WS2 by applying solid-state back gates. The excitonic properties of monolayer MoS2 and WS2 are determined by intricate interplays between the bandgap renormalization, Pauli blocking, and carrier screening against carrier doping.

Fluorescence Concentric Triangles: A Case of Chemical Heterogeneity in WS2 Atomic Monolayer

We report a novel optical property in WS2 monolayer. The monolayer naturally exhibits beautiful in-plane periodical and lateral homojunctions by way of alternate dark and bright band in the fluorescence images of these monolayers. The interface between different fluorescence species within the sample is distinct and sharp. This gives rise to intriguing concentric triangular fluorescence patterns in the monolayer. The novel optical property of this special WS2 monolayer is facilitated by chemical heterogeneity. The photoluminescence of the bright band is dominated by emissions from trion and biexciton while the emission from defect-bound exciton dominates the photoluminescence at the dark band. The discovery of such concentric fluorescence patterns represents a potentially new form of optoelectronic or photonic functionality.

Enhanced Photoresponse from Phosphorene–Phosphorene-Suboxide Junction Fashioned by Focused Laser Micromachining

Enhanced photoresponse is obtained from phosphorene-phosphorene-suboxide. A scanning focused laser beam is employed as a straightforward approach to convert part of a phosphorene film into phosphorene suboxide, creating a functional junction in situ on an optoelectronic device based on phosphorene. As a result, the photoelectrical properties of the optoelectronic device are significantly improved.

Hybrid Bilayer WSe2–CH3NH3PbI3 Organolead Halide Perovskite as a High‐Performance Photodetector

A high-performance 2D photodetector based on a bilayer structure comprising a WSe2 monolayer and CH3 NH3 PbI3 organolead halide perovskite is reported. High performance is realized by modification of the WSe2 monolayer with laser healing and perovskite functionalization. After modification, the output of the device was three orders of magnitude better than the pristine device; the performance is superior to that of most of the 2D photodetectors based on transition-metal-dichalcogenides (TMDs). This result indicates that combinatory TMDs-halide perovskite hybrids can be promising building blocks in optoelectronics.

Composition-dependent ultra-high photoconductivity in ternary CdSxSe1−x nanobelts as measured by optical pump-terahertz probe spectroscopy

AbstractWe employ optical pump-terahertz probe spectroscopy to investigate the composition-dependent photoconductivity in ternary CdSxSe1−x nanobelts. The observed carrier dynamics of CdS nanobelts display much shorter lifetime than those of ternary CdSxSe1−x nanobelts. This indicates the implementation of CdS nanobelts as ultrafast switching devices with a switching speed potentially up to 46.7 GHz. Surprisingly, ternary CdSxSe1−x nanobelts are found to exhibit much higher photoconductivity than binary CdS and CdSe. This is attributed to the higher photocarrier densities in ternary compounds. In addition, the presence of Se in samples resulted in prominent CdSe-like transverse optical (TO) phonon modes due to electron-phonon interactions. The strength of this mode shows a large drop upon photoexcitation but recovers gradually with time. These results demonstrated that growth of ternary nanostructures can be utilized to alleviate the high surface defect density in nanostructures and improve their photoconductivity.

Temperature and composition dependence of photoluminescence dynamics in CdSxSe1−x (0 ≤ x ≤ 1) nanobelts

CdSxSe1−x nanobelts with uniform and controllable composition are successfully fabricated by a one-step vapor-liquid-solid process. Temperature-dependent steady and time-resolved transient photoluminescence (PL) are studied for the obtained CdSxSe1−x nanobelts as well as pure CdS and CdSe nanobelts prepared in same method. For all samples, both band edge and surface state emission are found to contribute to the observed PL spectra. However, the emission originated from surface state is quite weak for CdSxSe1−x nanobelts as compared to CdS and CdSe. The reduced surface state emission in CdSxSe1−x nanobelts is ascribed to exciton localization due to random potential fluctuations. This attribution is further confirmed by the time-decay PL profiles where a very fast process of exciton transfer from interior to surface of nanobelts, observed in CdS and CdSe nanobelts, is absent for CdSxSe1−x nanobelts. These observations reveal that the surface effect of CdSxSe1−x nanobelts is less significant than that in CdS...