Abstract
In this work, we evaluate the dependence of tip-enhanced Raman (TER) spectra of a monolayer of thiophenol at a Au(111) electrode on the scanning tunneling microscope's tunneling current set-point and bias voltage parameters. We find an increase of the TER intensity upon set-point increase or bias decrease as expected from a gap-distance reduction. The relations obtained follow a theoretical model considering a simple gap-distance change when tuning the mentioned parameters. We find that the value of the bias voltage affects the TER intensity to a larger extent than the current set-point. Therefore it is advisable to work in a low-bias regime when aiming for ultrasensitive TER measurements.
Highlights
The most prominent bands of PhS are visible with a high signal-to-noise ratio (S/N) in all the spectra and are in agreement with previous tip-enhanced Raman spectroscopy (TERS) reports for PhS/Au(111).[9,13,27]
We have systematically studied the behaviour of scanning tunneling microscopy (STM)-tip-enhanced Raman (TER) spectra as a function of the bias voltage and tunneling current set-point in experiments in Ar and in water
For the PhS/Au(111) system, ITERS follows a dependence on the STM parameters expected from a simple gap-distance reduction upon set-point increase or bias decrease
Summary
In tip-enhanced Raman spectroscopy (TERS), the use of a scanning probe microscope tip as plasmonic antenna object yields the vibrational ngerprint of the system under study with sub-monolayer sensitivity and nanometric spatial resolution.[1,2] In the last decade, the capabilities of the technique in reaching single-molecule sensitivity with extremely high spatial resolution (in some cases in the sub-nm range) in air or in UHV conditions have been demonstrated.[3,4,5,6,7,8] Recently, TERS in liquid and electrochemical conditions has been achieved by a few groups, overcoming the experimental challenges TER studies at solid/liquid interfaces entail, such as the expected enhancement decrease due to beam aberrations and focus point distortion at the solid/liquid interface.[9,10,11,12,13,14,15] In-liquid experiments or/and single-molecule detection require high Raman enhancement factors and are usually performed with scanning tunneling microscopy (STM)-based TERS setups that typically provide enhancement factors that are a couple of orders of magnitude higher than atomic-force microscopy (AFM)-based TERS setups.[16 ]. One of the methods to maximize the enhancement in a TERS experiment is to minimize the gap distance between the tip and sample.[16,17,18,19,20] The TERS intensity, ITERS, is inversely proportional to the 8th to 10th power of the tip–sample separation:[18,20]
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