Opposite Polarity Surface Photovoltage of MoS₂ Monolayers on Au Nanodot versus Nanohole Arrays

Abstract

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.

Anomalous Light-Induced Charging in MoS2 Monolayers with Cracks

“…Interdigitated electrodes consisting of 50 nm Au and 10 nm Ti thin films were fabricated using photolithography and lift-off processes (Figure S1b and Supporting Information). On top of the NP-patterned substrates with the electrodes, CVD-grown MoS 2 monolayer flakes were transferred using a poly­(methyl methacrylate)-assisted wet transfer method as shown in Figure c,d . The Raman spectra of the as-grown MoS 2 flakes confirmed the preparation of high-quality monolayer samples (Figure S2a,b and Supporting Information).…”

“…The CPD measurements were carried out in a glove box filled with N 2 gas. , After loading, the samples were exposed to ultraviolet (UV) light with λ = 365 nm and a power density of 40 mW/cm 2 for 10 min. During this UV exposure, physisorbed environmental molecules (mostly O 2 and H 2 O) on the surface can be desorbed from the MoS 2 surface .…”

“…Transition metal dichalcogenides (TMDs), as atomically thin semiconductors with near-infrared and visible range band gap energies, have been increasingly studied for optoelectronic device applications. − Integration of two-dimensional (2D) TMDs on three-dimensional (3D) nanostructures leads to interesting physical phenomena including enhanced optical absorption, plasmon-exciton coupling, strain-mediated exciton funneling, and interfacial charge transfer . Proof-of-concept experiments can be attempted using exfoliated TMD flakes, which have random shapes and sizes.…”

“…Cracking occurs when applied stresses break the chemical bonds of TMD layers. Possible origins of mechanical stress are compositional modulation, mismatch of thermal expansion coefficients, and nonflat patterned substrates. − The strain states of TMD layers directly alter their band gap energies, followed by notable changes in the optical and electrical properties. − In addition to cracks, grain boundaries (GBs) are frequently observed as one-dimensional (1D) defects in TMD layers, and their influences on physical properties have been intensively investigated. − GBs cause scattering and trapping of charge carriers, hindering electrical conduction in extremely thin TMD layers. , The reduced electrical conductance can result in Joule heating-induced preferential oxidation at the GB region during current flow . Defects in TMD layers often limit device performance, but they can play beneficial roles in specific applications.…”

“…Kelvin probe force microscopy (KPFM) allows us to simultaneously measure the electrostatic force and surface morphology of a sample. ,, KPFM can detect the contact potential difference (CPD)the potential difference between the tip and samplewith nanoscopic spatial resolution. Light-induced surface charging can be represented by the surface photovoltage (SPV): SPV ≡ CPD L – CPD D , where CPD L and CPD D indicate the measured CPD data under illumination and in the dark, respectively. ,, The SPV signals reveal the polarity and magnitude of net charges generated by incident light. , Therefore, KPFM-based CPD measurements with and without light enable investigation of the spatial distribution of photogenerated charge carriers in nanostructures.…”