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

Efficient Energy Transfer (EnT) in Pyrene- and Porphyrin-Based Mixed-Ligand Metal–Organic Frameworks

Designing and synthesizing the ordered light-harvesting systems, possessing complementary absorption and energy-transfer process between the chromophores, are essential steps to accomplish successful mimicking of the natural photosynthetic systems. Metal–organic frameworks (MOFs) can be considered as an ideal system to achieve this due to their highly ordered structure, superior synthetic versatility, and tailorable functionality. Herein, we have synthesized the new light-harvesting mixed-ligand MOFs (MLMs, MLM-1–3) via solvothermal reactions between a Zr₆ cluster and a mixture of appropriate ratio of 1,3,6,8-tetrakis­(<i>p</i>-benzoic acid)­pyrene and [5,10,15,20-tetrakis­(4-carboxy-phenyl)­porphyrinato]-Zn­(II) ligands. The identical symmetry and connectivity of the two ligands of the MLMs was the key parameter of successful synthesis as a single MOF form, and the ample overlap between the emission spectrum of pyrene and the absorption spectrum of porphyrin provided the ideal platform to design an efficient-energy transfer (EnT) process within the MLMs. We obtained the nanoscale maps of the fluorescence intensities and lifetimes of microsize MLM grains for unambiguous visualization of EnT phenomena occurring between two ligands in MLMs. Moreover, due to complementary absorption and energy transfer between the two ligands in the MLMs, our MLMs performed as superior photoinduced singlet-oxygen generators, verifying the enhanced light-harvesting properties of the pyrene- and porphyrin-based MLMs

Local Enhancement of Exciton Emission of Monolayer MoS₂ by Copper Phthalocyanine Nanoparticles

Monolayer transition-metal dichalcogenides (1L-TMDs) provide ideal platforms to study light emission using two-dimensionally confined excitons. Recent studies have shown that the exciton emissions of 1L-TMDs can be conveniently modulated by developing heterostructures with zero-dimensional nanoparticles (NPs) or quantum dots. In this study, we synthesized organic semiconducting copper phthalocyanine (CuPc) NPs with sizes in the range of 30–70 nm by a re-precipitation method and decorated the chemical vapor deposition-grown 1L-MoS₂ with these NPs. This hybrid system exhibited a 6 times larger local photoluminescence (PL) at the positions of the CuPc NPs compared with the pristine 1L-MoS₂ sample. The PL enhancement and spectral modification of the 1L-MoS₂ decorated with CuPc NPs were attributed to the p-doping effect of the CuPc NPs, confirmed by spectral analysis and field-effect transistor measurements

Surface-Plasmonic-Coupled Photodetector Based on MAPbI₃ Perovskites with Au Nanoparticles: Significantly Enhanced Photoluminescence and Photodetectivity

Organic–inorganic metal halide perovskites (OMHPs) are promising active materials suitable for highly efficient solar cells, photodetectors, light-emitting diodes, and sensors. In this study, methylammonium lead iodide (MAPbI3) thin sheets (TSs) were synthesized as OMHPs using both hybrid vapor-solution method for optical study and anti-solvent solution method for the photodetector. π-Conjugated polyelectrolyte (π-CPE), such as poly(9,9-bis(4′-sulfonatobutyl)fluorene-alt-1,4-phenylene) potassium (FPS-K), was spin-coated on the MAPbI3 TS, and functionalized gold nanoparticles (Au-NPs) were hybridized. The laser confocal microscope photoluminescence (PL) intensity of the MAPbI3 TS was significantly enhanced after hybridization with Au-NPs/FPS-K, owing to the passivating effect of the FPS-K and the generation of extra-photoexcited charges by local surface plasmon resonance (LSPR) coupling with Au-NPs. These results were supported by the variation in the exciton lifetime measured from the time-resolved PL decay curves. The photocurrent of the MAPbI3 photodetector increased up to 1.1 × 104 times, and the photoresponsivity (R) and photodetectivity (D*) increased by 70 and 13 times, respectively, with the hybridization of Au-NPs/FPS-K. The highest D* of the Au-NPs/FPS-K/MAPbI3 photodetector was measured to be 7.5 × 1010 Jones at 735 nm excitation. The power and wavelength dependencies of R and D* for the MAPbI3 photodetector were also significantly improved by the Au-NPs/FPS-K hybrid. These results support the development of high-performance perovskite photodetectors utilizing LSPR coupling with the π-CPE layer

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

Effects of TiO₂ Interfacial Atomic Layers on Device Performances and Exciton Dynamics in ZnO Nanorod Polymer Solar Cells

The performances of organic electronic and/or photonic devices rely heavily on the nature of the inorganic/organic interface. Control over such hybrid interface properties has been an important issue for optimizing the performances of polymer solar cells bearing metal-oxide conducting channels. In this work, we studied the effects of an interfacial atomic layer in an inverted polymer solar cell based on a ZnO nanorod array on the device performance as well as the dynamics of the photoexcited carriers. We adopted highly conformal TiO₂ interfacial layer using plasma enhanced atomic layer deposition (PEALD) to improve the compatibility between the solution-prepared active layer and the ZnO nanorod array. The TiO₂ interfacial layer facilitated exciton separation and subsequent charge transfer into the nanorod channel, and it suppressed recombination of photogenerated carriers at the interface. The presence of even 1 PEALD cycle of TiO₂ coating substantially improved the short-circuit current density (<i>J</i>_sc), open circuit voltage (<i>V</i>_oc), and fill factor (FF), leading to more than 2-fold enhancement in the power conversion efficiency (PCE). The dynamics of the photoexcited carriers in our devices were studied using transient absorption (TA) spectroscopy. The TA results clearly revealed that the TiO₂ coating played a key role as an efficient quencher of photogenerated excitons, thereby reducing the exciton lifetime. The electrochemical impedance spectra (EIS) provided further evidence that the TiO₂ atomic interfacial layer promoted the charge transfer at the interface by suppressing recombination loss

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

Impeding Exciton–Exciton Annihilation in Monolayer WS₂ by Laser Irradiation

Monolayer (1L) transition metal dichalcogenides (TMDs) are two-dimensional direct-bandgap semiconductors with promising applications of quantum light emitters. Recent studies have shown that intrinsically low quantum yields (QYs) of 1L-TMDs can be greatly improved by chemical treatments. However, nonradiative exciton–exciton annihilation (EEA) appears to significantly limit light emission of 1L-TMDs at a nominal density of photoexcited excitons due to strong Coulomb interaction. Here we show that the EEA rate constant (γ) can be reduced by laser irradiation treatment in mechanically exfoliated monolayer tungsten disulfide (1L-WS₂), causing significantly improved light emission at the saturating optical pumping level. Time-resolved photoluminescence (PL) measurements showed that γ reduced from 0.66 ± 0.15 cm²/s to 0.20 ± 0.05 cm²/s simply using our laser irradiation. The laser-irradiated region exhibited lower PL response at low excitation levels, however at the high excitation level displayed 3× higher PL intensity and QY than the region without laser treatment. The shorter PL lifetime and lower PL response at low excitation levels suggested that laser irradiation increased the density of sulfur vacancies of 1L-WS₂, but we attribute these induced defects, adsorbed by oxygen in air, to the origin for reduced EEA by hindering exciton diffusion. Our laser irradiation was likewise effective for reducing EEA and increasing PL of chemically treated 1L-WS₂ with a high QY, exhibiting the general applicability of our method. Our results suggest that exciton–exciton interaction in 1L-TMDs may be conveniently controlled by the laser treatment, which may lead to unsaturated exciton emission at high excitation levels