2D MoS$_2$/Au interfaces for enhanced opto-electronic response with sub-bandgap photons

TL;DR

Atomically thin $MoS_2$ has a strong direct bandgap and large nonlinear susceptibility but weak device signals due to low absorption. The authors couple monolayer $MoS_2$ to core-shell Au nanoparticles to exploit localized surface plasmon resonances, generating a broadband supercontinuum that $MoS_2$ absorbs via resonant energy transfer. They observe a ~15-fold boost in photocurrent under sub-bandgap (NIR) excitation, and show that long-range energy transfer (FRET) from Au to $MoS_2$—not near-field field enhancement or tunneling—dominates the interfacial energy exchange, with $k_{ET} \propto 1/d^4$ and $\eta = 1/[1+(d/R_{2D})^4]$. This establishes a plasmonic 0D–2D energy-transfer platform, enabling extended MoS_2 photodetection into the NIR/telecom regime and guiding design of nanoparticle geometry and dielectric spacing to maximize photocurrent.

Abstract

Monolayer MoS$_2$ is a direct band gap semiconductor with potential applications in optoelectronics and photonics. MoS$_2$ also has a large optical nonlinearity. However, the atomic thickness of the monolayer limits the strength of the measured functional signals, such as the photocurrent or photoluminescence, in optoelectronic devices. Here, we show that photocurrent in monolayer MoS$_2$ can be induced by sub-band gap photons by depositing Au nanoparticles on it. In this system, the nonlinear light-matter interaction in Au nanoparticles enhanced by the localized surface plasmons results in the generation of supercontinuum, which is reabsorbed by MoS$_2$ due to efficient resonant energy transfer. Au nanoparticle assisted photocurrent is more than an order of magnitude larger than two-photon photocurrent in monolayer MoS$_2$. Optimization of the shape, size and composition of the nanoparticle has the potential to enhance the photocurrent significantly with the prospect of applications in the detection of NIR photons, and related technologies including optical telecommunication.

Figures (10)

• Figure 1: (A) Schematic of a device used in the experiments showing a monolayer MoS2 in contact with Au electrodes. The inset shows a photo of a device excited by a pulsed laser at 1030 nm. SHG from a flake of MoS$_2$ is indicated. (B) Image of flakes of MoS$_2$ on a sapphire substrate onto which the electrodes are deposited. (C) Theoretically calculated band structure of single-layer MoS$_2$ (adapted from Ref.GERBI2025). (D) Absorption (blue) and PL (red) spectra of the sample, showing a red shift in the PL with respect to the absorption. (E) Raman spectrum of the sample confirming that sample is composed of monolayer MoS$_2$.
• Figure 2: (A) Spectrum of the emission from monolayer MoS$_2$ induced by femtosecond pulses at 1030 nm (photon energy of 1.2 eV). A micrograph of the green emission visible to the naked eye is shown in the inset. The emission is dominated by SHG. A weaker emission, present in the dotted region, is shown in detail in (B). Three peaks are evident in the emission spectrum.
• Figure 3: (A) Fast Fourier Transforms (FFT) of different measured signals induced by excitation modulated at 2 kHz. (B) Comparison of signal and noise in the FFT of PC. (C) Intensity dependence of single photon PC and PL, and (D) intensity dependence of different nonlinear signals.
• Figure 4: (A) Top: Supercontinuum from Au nanoparticles on sapphire substrate irradiated by femtosecond pulses at 1030 nm. Bottom: Supercontinuum from Au nanoparticles on MoS$_2$. The spectra vary at different locations on the samples. (B) Intensity dependence of PC from MoS$_2$ and MoS$_2$ with Au nanoparticles. (C) Illustration of Au nanoparticles on MoS$_2$ showing the polarization of the light, regions around the particle where electric fields are computed using FDTD. (D) Electric field around the circumference of the nanoparticle on the plane parallel to MoS$_2$/substrate and passing through the center of the particle. (E) Electric field on the surface of the nanoparticle as a function of angle (0 to $\pi/2$) as the point is moved from the center of the particle to the base where it is in contact with MoS$_2$.
• Figure 5: (A) Band alignment of Au/SiO$_2$/MoS$_2$ heterostructure and (B) tunneling probability of electrons from Au to MoS$_2$ as a function of the energy of hot electrons and thickness of SiO$_2$.