In this work, the surface potential (VS) of exfoliated MoS2 monolayers on Au nanostripe arrays with period of 500 nm was investigated using Kelvin probe force microscopy. The surface morphology showed that the suspended MoS2 region between neighboring Au stripes underwent tensile-strain. In the dark, the VS of the MoS2 region on the Au stripe (VS-Au) was larger than that of the suspended MoS2 region (VS-S). However, under green light illumination, VS-Au became smaller than VS-S. To explain the VS modification, band diagrams have been constructed taking into consideration not only the local strain but also the electronic interaction at the MoS2/Au interface. The results of this work provide a basis for understanding the electrical properties of MoS2-metal contacts and improving the performance of MoS2-based optoelectronic devices.

A growth technique to directly prepare two-dimensional (2D) materials onto conventional semiconductor substrates, enabling low-temperature, high-throughput, and large-area capability, is needed to realize competitive 2D transition-metal dichalcogenide (TMD)/three-dimensional (3D) semiconductor heterojunction devices. Therefore, we herein successfully developed an atmospheric-pressure plasma-enhanced chemical vapor deposition (AP-PECVD) technique, which could grow MoS2 and WS2 multilayers directly onto PET flexible substrate as well as 4-in. Si substrates at temperatures of <200 °C. The as-fabricated MoS2/Si and WS2/Si heterojunctions exhibited large and fast photocurrent responses under illumination of a green light. The measured photocurrent was linearly proportional to the laser power, indicating that trapping and detrapping of the photogenerated carriers at defect states could not significantly suppress the collection of photocarriers. All the results demonstrated that our AP-PECVD method could produce high-quality TMD/Si 2D–3D heterojunctions for optoelectronic applications.

We prepared MoS 2 monolayers on Au nanodot (ND) and nanohole (NH) arrays. Both these sample arrays exhibited enhanced photoluminescence intensity compared with that of a bare SiO 2 /Si substrate. The reflectance spectra of MoS 2 /ND and MoS 2 /NH had clear features originating from excitation of localized surface plasmon and propagating surface plasmon polaritons. Notably, the surface photovoltages (SPV) of these hybrid plasmonic nanostructures had opposite polarities, indicating negative and positive charging at MoS 2 /ND and MoS 2 /NH, respectively. Surface potential maps, obtained by Kelvin probe force microscopy, suggested that the potential gradient led to a distinct spatial distribution of photo-generated charges in these two samples under illumination. Furthermore, the local density of photo-generated excitons, as predicted from optical simulations, explained the SPV spectra of MoS 2 /ND and MoS 2 /NH. We show that the geometric configuration of the plasmonic nanostructures modified the polarity of photo-generated excess charges in MoS 2 . These findings point to a useful means of optimizing optoelectronic characteristics and improving the performance of MoS 2 -based plasmonic devices.

Exciton–polaritons (EPs) can be formed in transition metal dichalcogenide (TMD) multilayers sustaining optical resonance modes without any external cavity. The self-hybridized EP modes are expected to depend on the TMD thickness, which directly determines the resonance wavelength. Exfoliated WS2 flakes were prepared on SiO2/Si substrates and template-stripped ultraflat Au layers, and the thickness dependence of their EP modes was compared. For WS2 flakes on SiO2/Si, the minimum flake thickness to exhibit exciton–photon anticrossing was larger than 40 nm. However, for WS2 flakes on Au, EP mode splitting appeared in flakes thinner than 10 nm. Analytical and numerical calculations were performed to explain the distinct thickness-dependence. The phase shifts of light at the WS2/Au interface, originating from the complex Fresnel coefficients, were as large as π/2 at visible wavelengths. Such exceptionally large phase shifts allowed the optical resonance and resulting EP modes in the sub-10-nm-thick WS2 flakes. This work helps us to propose novel optoelectronic devices based on the intriguing exciton physics of TMDs.

Metal and transition-metal dichalcogenide (TMD) hybrid systems have been attracting growing research attention because exciton–plasmon coupling is a desirable means of tuning the physical properties of TMD materials. Competing effects of metal nanostructures, such as the local electromagnetic field enhancement and luminescence quenching, affect the photoluminescence (PL) characteristics of metal/TMD nanostructures. In this study, we prepared TMD MoS2 monolayers on hexagonal arrays of Au nanodots and investigated their physical properties by micro-PL and surface photovoltage (SPV) measurements. MoS2 monolayers on bare Au nanodots exhibited higher PL intensities than those of MoS2 monolayers on 5-nm-thick Al2O3-coated Au nanodots. The Al2O3 spacer layer blocked charge transfer at the Au/MoS2 interface but allowed the transfer of mechanical strain to the MoS2 monolayers on the nanodots. The SPV mapping results revealed not only the electron-transfer behavior at the Au/MoS2 contacts but also the lateral drift of charge carriers at the MoS2 surface under light illumination, which corresponds to nonradiative relaxation processes of the photogenerated excitons.

Solution‐processed Cu(In,Ga)(S,Se)2 (CIGS) has a great potential for the production of large‐area photovoltaic devices at low cost. However, CIGS solar cells processed from solution exhibit relatively lower performance compared to vacuum‐processed devices because of a lack of proper composition distribution, which is mainly instigated by the limited Se uptake during chalcogenization. In this work, a unique potassium treatment method is utilized to improve the selenium uptake judiciously, enhancing grain sizes and forming a wider bandgap minimum region. Careful engineering of the bandgap grading structure also results in an enlarged space charge region, which is favorable for electron–hole separation and efficient charge carrier collection. Besides, this device processing approach has led to a linearly increasing electron diffusion length and carrier lifetime with increasing the grain size of the CIGS film, which is a critical achievement for enhancing photocurrent yield. Overall, 15% of power conversion efficiency is achieved in solar cells processed from environmentally benign solutions. This approach offers critical insights for precise device design and processing rules for solution‐processed CIGS solar cells.

Fabrication of Cu(In,Ga)(S,Se) 2 (CIGSSe) absorber films from environmentally friendly solutions under ambient air conditions for use in solar cells has shown promise for the low-cost mass production of CIGSSe solar cells. However, the limited power conversion efficiency (PCE) of these solar cells compared with their vacuum-processed counterparts has been a critical setback to their practical applications. This study aims to fabricate solution-processed CIGSSe solar cells with high PCEs by incorporation of Ag into the precursor layer of the CIGSSe absorber films. The results showed that Ag doping promoted grain growth by accelerating Se uptake, irrespective of the location within the CIGSSe film. Nevertheless, uniform Ag doping formed crevices that lowered the PCE of the cells, while centrally localizing the doped Ag prevented the formation of crevices, resulting in high PCEs up to 15.3%. Our results demonstrate that carefully doping Ag into a selected area of the precursor layer of the CIGSSe films can realize solution-processed chalcopyrite solar cells with high PCE.

Understanding the excitonic processes at the interfaces of fluorescent π-conjugated molecules and metal electrodes is important for both fundamental studies and emerging applications. Adsorption configurations of molecules on metal surfaces significantly affect the physical characteristics of junctions as well as molecules. Here, the electronic structures and optical properties of molecular assemblies/Au interfaces were investigated using scanning probe and photoluminescence microscopy techniques. Scanning tunneling microscopy images and tunneling conductance spectra suggested that the self-assembled molecules were physisorbed on the Au surface. Visible-range photoluminescence studies showed that Au thin films modified the emission spectra and reduced the lifetime of excitons. Surface potential maps, obtained by Kelvin probe force microscopy, could visualize electron transfer from the molecules to Au under illumination, which could explain the decreased lifetime of excitons at the molecule/Au interface.