Visualization 1: Enhancement of light-matter interaction and photocatalytic efficiency of Au/TiO₂ hybrid nanowires

Real-time view of Au growth along hybrid nanowires driven by local UV illumination. Originally published in Optics Express on 11 July 2016 (oe-24-14-15171

Spectroscopic Visualization of Grain Boundaries of Monolayer Molybdenum Disulfide by Stacking Bilayers

Polycrystalline growth of molybdenum disulfide (MoS₂) using chemical vapor deposition (CVD) methods is subject to the formation of grain boundaries (GBs), which have a large effect on the electrical and optical properties of MoS₂-based optoelectronic devices. The identification of grains and GBs of CVD-grown monolayer MoS₂ has traditionally required atomic resolution microscopy or nonlinear optical imaging techniques. Here, we present a simple spectroscopic method for visualizing GBs of polycrystalline monolayer MoS₂ using stacked bilayers and mapping their indirect photoluminescence (PL) peak positions and Raman peak intensities. We were able to distinguish a GB between two MoS₂ grains with tilt angles as small as 6° in their grain orientations and, based on the inspection of several GBs, found a simple empirical rule to predict the location of the GBs. In addition, the large number of twist angle domains traced through our facile spectroscopic mapping technique allowed us to identify a continuous evolution of the coupled structural and optical properties of bilayer MoS₂ in the vicinity of the 0° and 60° commensuration angles which were explained by elastic deformation model of the MoS₂ membranes

Composition-Tunable Synthesis of Large-Scale Mo_1–<i>x</i>W_<i>x</i>S₂ Alloys with Enhanced Photoluminescence

Alloying two-dimensional transition metal dichalcogenides (2D TMDs) is a promising avenue for band gap engineering. In addition, developing a scalable synthesis process is essential for the practical application of these alloys with tunable band gaps in optoelectronic devices. Here, we report the synthesis of optically uniform and scalable single-layer Mo_1–<i>x</i>W_<i>x</i>S₂ alloys by a two-step chemical vapor deposition (CVD) method followed by a laser thinning process. The amount of W content (<i>x</i>) in the Mo_1–<i>x</i>W_<i>x</i>S₂ alloy is systemically controlled by the co-sputtering technique. The post-laser process allows layer-by-layer thinning of the Mo_1–<i>x</i>W_<i>x</i>S₂ alloys down to a single-layer; such a layer exhibits tunable properties with the optical band gap ranging from 1.871 to 1.971 eV with variation in the W content, <i>x</i> = 0 to 1. Moreover, the predominant exciton complexes, trions, are transitioned to neutral excitons with increasing W concentration; this is attributed to the decrease in excessive charge carriers with an increase in the W content of the alloy. Photoluminescence (PL) and Raman mapping analyses suggest that the laser-thinning of the Mo_1–<i>x</i>W_<i>x</i>S₂ alloys is a self-limiting process caused by heat dissipation to the substrate, resulting in spatially uniform single-layer Mo_1–<i>x</i>W_<i>x</i>S₂ alloy films. Our findings present a promising path for the fabrication of large-scale single-layer 2D TMD alloys and the design of versatile optoelectronic devices

Simultaneous Hosting of Positive and Negative Trions and the Enhanced Direct Band Emission in MoSe₂/MoS₂ Heterostacked Multilayers

Heterostacking of layered transition-metal dichalcogenide (LTMD) monolayers (1Ls) offers a convenient way of designing two-dimensional exciton systems. Here we demonstrate the simultaneous hosting of positive trions and negative trions in heterobilayers made by vertically stacking 1L MoSe₂ and 1L MoS₂. The charge transfer occurring between the 1Ls of MoSe₂ and MoS₂ converted the polarity of trions in 1L MoSe₂ from negative to positive, resulting in the presence of positive trions in the 1L MoSe₂ and negative trions in the 1L MoS₂ of the same heterostacked bilayer. Significantly enhanced MoSe₂ photoluminescence (PL) in the heterostacked bilayers compared to the PL of 1L MoSe₂ alone suggests that, unlike other previously reported heterostacked bilayers, direct band transition of 1L MoSe₂ in heterobilayer was enhanced after the vertical heterostacking. Moreover, by inserting hexagonal BN monolayers between 1L MoSe₂ and 1L MoS₂, we were able to adjust the charge transfer to maximize the MoSe₂ PL of the heteromultilayers and have achieved a 9-fold increase of the PL emission. The enhanced optical properties of our heterostacked LTMDs suggest the exciting possibility of designing LTMD structures that exploit the superior optical properties of 1L LTMDs

Enhanced Light Emission from Monolayer Semiconductors by Forming Heterostructures with ZnO Thin Films

Monolayer transition-metal dichalcogenides (1L-TMDs) are atomically thin direct band gap semiconductors, from which the emission of light is determined by optical transitions of exciton complexes such as neutral excitons and trions. While the quantum yields of 1L-TMDs are quite low, the ability to control the populations of exciton complexes in 1L-TMDs through various doping processes is an interesting advantage, and provides ample possibilities for engineering the optical properties of these semiconductor monolayers. Here we demonstrate a simple method of controlling the populations of excitons and trions to enhance the light emission of 1L-TMDs by having them form heterostructures with ZnO thin films (TFs). 1Ls of MoS₂ or MoSe₂ showed up to 17-fold increases in photoluminescence (PL) when they were placed on ∼50 nm thick ZnO TFs. This enhancement of the PL was due to charge exchanges occurring through the 1L-TMD/ZnO interface. The PL enhancements and changes in the PL spectra of the 1L-TMDs were greater when the 1L-TMD/ZnO heterostructures were subjected to 355 nm wavelength laser excitation than when they were excited with a 514 nm wavelength laser, which we attributed to the onset of energy transfer by photoexcited excitons and/or the additional p-doping by photoexcited holes in ZnO. The p-doping phenomenon and the enhanced light emission of 1L-TMD/ZnO heterostructures were unambiguously visualized in spatially resolved PL and Raman spectral maps. Our approach using the 1L-TMD/ZnO TF heterostructure suggests that a rich variety of options for engineering the optical properties of 1L-TMDs may be made available by carrying out simple and intuitive manipulations of exciton complexes, and these endeavors may yield practical applications for 1L-TMDs in nanophotonic devices

Induced Transition of CdSe Nanoparticle Superstructures by Controlling the Internal Flow of Colloidal Solution

The self-assembly behaviors of flow-enhanced CdSe nanoparticle (NP) colloidal systems were investigated, which were systemically prepared by adding ethylene glycol (EG) or acetic acid (AA) to NP suspensions with deionized water (DI water) base. The additive solvents, which had higher boiling points and lower surface tensions than those of the DI water, modified the internal flow of the NP colloidal system, consequently affecting the morphologies of the generated NP superstructures after the full evaporation of their droplets. In flow-enhanced systems, NPs were formed into highly elongated dendrites that stretched from the center region to the edges along the direction of convective flow inside the droplet, while NPs in random drift system were easily aggregated to form cluster-shaped thick dendritic structures. When the volume fraction of EG was increased, the dominant superstructures were changed from dendrites to clusters, which can be mainly attributed to the changes in the dielectric properties of the NP droplets as evaporation proceeded because of the large discrepancy in the vapor pressures of EG and DI water. The balance between the interparticle potentials of electrostatic repulsion and van der Waals attraction was continuously altered, resulting in the formation of clusters with increasing EG ratio. Contrastively, the transition of superstructures could not be observed in the case of colloidal system prepared by mixing DI water and AA, which can be ascribed to the similar vapor pressures of the two solvents; the dielectric properties of the solution mixture was barely changed throughout the steady evaporation process, which resulted in the formation of uniformly distributed highly elongated dendrites. Polarization-dependent imaging experiments and photoluminescence measurements revealed that the stretched dendrites formed under the flow-enhanced conditions showed higher crystallinity than that of the clusters