Gap-mode enhancement on MoS2 probed by functionalized tip-enhanced Raman spectroscopy

Abstract

Surface enhancement of molecular spectroscopic signals has been widely used for sensing and nanoscale imaging. Because of the weak electromagnetic enhancement of Raman signals on semiconductors, it is motivating but challenging to study the electromagnetic effect separately from the chemical effects. We report tip-enhanced Raman scattering measurements on Au and bulk MoS2 substrates using a metallic tip functionalized with copper phthalocyanine molecules and demonstrate similar gap-mode enhancement on both substrates. We compare the experimental results with theoretical calculations to confirm the gap-mode enhancement on MoS2 using a well-established electrostatic model. The functionalized tip approach allows for suppressing the background and is ideal for separating electromagnetic and chemical enhancement mechanisms on various substrates. Our results may find a wide range of applications in MoS2-based devices, sensors, and metal-free nanoscale bio-imaging.

Strong Near-Field Light–Matter Interaction in Plasmon-Resonant Tip-Enhanced Raman Scattering in Indium Nitride

“…The simplest plasmon-resonant TERS configuration is the one composed by a gold substrate and a gold tip, which is known in TERS literature as "gap mode" configuration and it is usually adopted for studying 2D materials and single molecules positioned inside the tip-substrate gap. 40,41,42 Fig. 3 (left) Raman shift values for phonon modes in the high-frequency region of the MEPA sample as a function of the spatial position along the line scan.…”

Resonant tip-enhanced Raman scattering by CdSe nanocrystals on plasmonic substrates

“…A dramatic increase of the TERS enhancement and a better lateral resolution, both resulting in higher contrast TERS imaging, can be achieved in the so-called gap-mode TERS. 24,25,27 In this mode, the organic or inorganic nanostructures are placed on a at or nanostructured metallic substrate in the gap between the sharp AFM tip and the metal substrate. 12 The gap mode is especially efficient when a nanostructured metal substrate is used, providing an extremely high local eld enhancement and its spatial connement.…”

Excitation-Tunable Tip-Enhanced Raman Spectroscopy

“…The finite-difference time-domain simulations were conducted with the commercial software package, Lumerical FDTD Solutions. The simulation set up is similar to that in our previous work 26 A main challenge for e-TERS was to find a tip that allows exciting a near-field between the tip and the substrate for a broad spectral excitation range in the visible 10,18,30,49 . Realizing e-TERS is impossible if the tip has to be replaced after a change of the excitation wavelength.…”

“…Prominent examples for nanoimaging and chemical analysis by TERS include the visualization of single carbon nanotubes (CNT's), defects in graphene, and single strands of RNA making TERS an invaluable tool in chemistry, biology and physics among others 10, 18-23 . TERS is based on the illumination of a sharp metallic atomic force microscopy (AFM) or scanning tunnel microscope (STM) tip with light and recording Raman spectra [24][25][26] . The incoming light excites localized surface plasmons (LSPs) in the tip and often also in the substrate [27][28][29][30] . The LSPs generate a strongly localized electromagnetic (EM) field (near-field) 10,18,21,31 , which has two functions in TERS.…”