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

Enhanced Local and Nonlocal Photoluminescence of Organic Rubrene Microrods using Surface Plasmon of Gold Nanoparticles: Applications to Ultrasensitive and Remote Biosensing

Nonlocal photoluminescence (PL) signal transfer through semiconducting nanostructures has been intensively studied for its potential applicability in photonic circuits, optical communications, and optical sensing. In this study, organic semiconducting rubrene microrods (MRs) were synthesized and hybridized with functionalized gold nanoparticles (Au-NPs) to optimize both their optical and biosensing properties. The steady-state local PL intensity of the rubrene MR was considerably enhanced by the Au-NPs’ hybridization due to the energy-transfer effect from the surface plasmon (SP) coupling. It was clearly observed that the nonlocal PL signal-transfer efficiency of rubrene/Au-NPs hybrid MRs drastically increased along crystalline axes with the aid of the SP effect. The coupling of exciton polaritons in the luminescent rubrene MR with the SP as well as the scattering effect contribute to the variation of the exciton decay rate, resulting in a change in the PL signal-transfer efficiency for the hybrid MRs. The enhancement of the local and nonlocal PL emission of the rubrene/Au-NPs hybrid MRs was applied to ultrasensitive and remote biosensing. We observed PL signal transfer of fluorescent-dye attached DNA along the MR and successfully detected target-DNA with a concentration of 100 picomole using rubrene/Au-NPs/probe–DNA hybrid MR

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

Observation of Charge Transfer in Heterostructures Composed of MoSe₂ Quantum Dots and a Monolayer of MoS₂ or WSe₂

Monolayer transition metal dichalcogenides (TMDs) are atomically thin semiconductor films that are ideal platforms for the study and engineering of quantum heterostructures for optoelectronic applications. We present a simple method for the fabrication of TMD heterostructures containing MoSe₂ quantum dots (QDs) and a MoS₂ or WSe₂ monolayer. The strong modification of photoluminescence and Raman spectra that includes the quenching of MoSe₂ QDs and the varied spectral weights of trions for the MoS₂ and WSe₂ monolayers were observed, suggesting the charge transfer occurring in these TMD heterostructures. Such optically active heterostructures, which can be conveniently fabricated by dispersing TMD QDs onto TMD monolayers, are likely to have various nanophotonic applications because of their versatile and controllable properties

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

Simple Chemical Treatment to n‑Dope Transition-Metal Dichalcogenides and Enhance the Optical and Electrical Characteristics

The optical and electrical properties of monolayer transition-metal dichalcogenides (1L-TMDs) are critically influenced by two dimensionally confined exciton complexes. Although extensive studies on controlling the optical properties of 1L-TMDs through external doping or defect engineering have been carried out, the effects of excess charges, defects, and the populations of exciton complexes on the light emission of 1L-TMDs are not yet fully understood. Here, we present a simple chemical treatment method for n-dope 1L-TMDs, which also enhances their optical and electrical properties. We show that dipping 1Ls of MoS₂, WS₂, and WSe₂, whether exfoliated or grown by chemical vapor deposition, into methanol for several hours can increase the electron density and also can reduce the defects, resulting in the enhancement of their photoluminescence, light absorption, and the carrier mobility. This methanol treatment was effective for both n- and p-type 1L-TMDs, suggesting that the surface restructuring around structural defects by methanol is responsible for the enhancement of optical and electrical characteristics. Our results have revealed a simple process for external doping that can enhance both the optical and electrical properties of 1L-TMDs and help us understand how the exciton emission in 1L-TMDs can be modulated by chemical treatments

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

Augmented Quantum Yield of a 2D Monolayer Photodetector by Surface Plasmon Coupling

Monolayer (1L) transition metal dichalcogenides (TMDCs) are promising materials for nanoscale optoelectronic devices because of their direct band gap and wide absorption range (ultraviolet to infrared). However, 1L-TMDCs cannot be easily utilized for practical optoelectronic device applications (e.g., photodetectors, solar cells, and light-emitting diodes) because of their extremely low optical quantum yields (QYs). In this investigation, a high-gain 1L-MoS₂ photodetector was successfully realized, based on the surface plasmon (SP) of the Ag nanowire (NW) network. Through systematic optical characterization of the hybrid structure consisting of a 1L-MoS₂ and the Ag NW network, it was determined that a strong SP and strain relaxation effect influenced a greatly enhanced optical QY. The photoluminescence (PL) emission was drastically increased by a factor of 560, and the main peak was shifted to the neutral exciton of 1L-MoS₂. Consequently, the overall photocurrent of the hybrid 1L-MoS₂ photodetector was observed to be 250 times better than that of the pristine 1L-MoS₂ photodetector. In addition, the photoresponsivity and photodetectivity of the hybrid photodetector were effectively improved by a factor of ∼1000. This study provides a new approach for realizing highly efficient optoelectronic devices based on TMDCs