Defects can induce drastic changes of the electronic properties of two-dimensional transition metal dichalcogenides and influence their applications. It is still a great challenge to characterize small defects and correlate their structures with properties. Here, we show that tip-enhanced Raman spectroscopy (TERS) can obtain distinctly different Raman features of edge defects in atomically thin MoS2, which allows us to probe their unique electronic properties and identify defect types (e.g., armchair and zigzag edges) in ambient. We observed an edge-induced Raman peak (396 cm−1) activated by the double resonance Raman scattering (DRRS) process and revealed electron–phonon interaction in edges. We further visualize the edge-induced band bending region by using this DRRS peak and electronic transition region using the electron density-sensitive Raman peak at 406 cm−1. The power of TERS demonstrated in MoS2 can also be extended to other 2D materials, which may guide the defect engineering for desired properties.
Electrochemical
tip-enhanced Raman spectroscopy (EC-TERS) is a
powerful technique for the in situ study of the physiochemical properties
of the electrochemical solid/liquid interface at the nanoscale and
molecular level. To further broaden the potential window of EC-TERS
while extending its application to opaque samples, here, we develop
a top-illumination atomic force microscopy (AFM) based EC-TERStechnique
by using a water-immersion objective of a high numerical aperture
to introduce the excitation laser and collect the signal. This technique
not only extends the application of EC-TERS but also has a high detection
sensitivity and experimental efficiency. We coat a SiO2 protection layer over the AFM-TERS tip to improve both the mechanical
and chemical stability of the tip in a liquid TERS experiment. We
investigate the influence of liquid on the tip–sample distance
to obtain the highest TERS enhancement. We further evaluate the reliability
of the as-developed EC-AFM-TERS technique by studying the electrochemical
redox reaction of polyaniline. The top-illumination EC-AFM-TERS is
promising for broadening the application of EC-TERS to more practical
systems, including energy storage and (photo)electrocatalysis.
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