Nanostructured InGaN Quantum Wells as a Surface-Enhanced Raman Scattering Substrate with Expanded Hot Spots

Abstract

Surface-enhanced Raman scattering (SERS) is a technique that can deliver label-free, real-time, and multiplex detection of target molecules. However, the development of this potential tool has been impeded by an obstacle: reliability. Because SERS detection relies on the very localized (< 10 nm) hot spot, severe intensity fluctuation occurs as the molecule thermally diffuses in and out of the tiny spot, making it difficult to quantify the information of analytes. Here, we address the problem by greatly expanding the effective area of a hot spot. The breakthrough is realized by a few layers of conformally nanostructured InGaN, which introduce wafer-scale charge coupling at the molecule/metal/semiconductor interface. These additional coupling channels interconnect plasmonic nanojunctions, rendering the SERS-active surface spreading over 1200 μm2 in Raman mapping. The result allows us to capture trace molecules with increased chances and stabilized signals, paving a way for SERS to enter real-life applications.

Controlling the Electron Concentration for Surface-Enhanced Raman Spectroscopy

“…When more InGaN layers are embedded below the surface, the increased piezoelectric polarization induces stronger internal electric fields within the QWs, resulting in a steeper band tilting, and thus more electrons are drawn toward the surface. As the QWs are excited by the laser for SERS measurement, a certain amount of the much increased electrons (due to laser pumping) should be transferred to Au by tunneling or overcoming the Schottky barrier . The agreement between CV measurement and band simulation demonstrates that carrier concentration below the substrate surface can be managed by the QW number.…”

“…For the 0QW sample, the least and deepest electrons are due to the depletion region formed by the Schottky barrier at the Au/n-GaN interface . With the QWs, electrons are confined at shallow depths from the surface and can be transferred to the Au surface upon laser excitation . Although the three QW samples are capped by the same 1.6 nm GaN layer, Figure a shows that increasing the QWs further pushes electrons toward the surface.…”

“…The attractive performances come from several merits of GaN in SERS sensing, including uniform Raman signals, chemical inertness, and high surface carrier density due to strong crystal polarizations . It is recently found that growing InGaN/GaN quantum wells (QWs) by metal–organic chemical vapor deposition (MOCVD) is an effective tactic to control the surface carrier density for SERS . The nitride QWs not only enhance the Raman signals of analytes but also significantly expand the hot spots.…”

“…The nitride QWs not only enhance the Raman signals of analytes but also significantly expand the hot spots. These findings are ascribed to a wafer-scale charge coupling at the SERS-active regions . Because the structure and position of QWs can be precisely tuned by MOCVD, one can vary the surface electron concentration by adjusting the growth parameters.…”

“…The facilities (MOCVD, e-beam evaporator, and furnace) used to produce the SERS substrates all yield wafer-scale surface morphology and NP coverage (Figure S6). Spectra in Figure b were obtained by subtracting the fluorescent background of R6G from the raw spectra . Raman peaks at 612, 770, 1368, and 1650 cm –1 belong to different vibration modes of R6G .…”