Several artifacts might occur in TERS, the following section describes the different sources of parasitic TER spectra33. Carbon contamination can be produced by sample heating, ambient air adsorption on the tip itself or direct contamination from the sample34-35. For long acquisition times and when such contamination occurs, the collected Raman signal often average to two broad bands (D and G band) centered around 1350 cm-1 and 1580 cm-1, respectively, which can be easily recognized. Nevertheless, these broad background signals do not necessarily obscure the TERS spectrum of the material of interest, as this often can be identified easily as sharp Raman bands superimposed on the broad background signals. However, if this contamination leads to strong and sharp peaks that resemble the peaks from the spectral fingerprint of the analyte, it is much harder to recognize these peaks from the contamination signals. Possibilities to examine whether the observed spectrum belongs to a parasitic substance or not are, for example, to thoroughly compare the spectral pattern to conventional Raman spectroscopy or SERS data or to perform a time-dependent measurement, as contamination signals often fluctuate.
Furthermore, TER spectra often show a broadband background which is not related to carbon contaminations and whose origin is not yet fully understood. The most common explanation is that a plasmon-dependent photoluminescence from the rough metal surface (or from the tip–sample gap itself) accounts for the background36. A far-field fluorescence contribution from the sample itself can also occur and it may even be so strong that it can saturate the Raman spectrometer detector.
Large enhancement is a prerequisite for few- or singlemolecule measurements, but often the adsorbed molecules will not withstand such extreme field strengths! Extreme local intensities upon enhancement may become counterproductive; to overcome this problem, it is recommended to use a comparatively weak incident power of a few μW cm-2 (or a sufficiently large focus or short acquisition time) to avoid photobleaching, photochemical processes, sample heating, or desorption of molecules. Under continuous illumination, such effect can lead to the decomposition of the probed molecules. During this process, molecular bonds are broken and new ones are formed, thus accounting for the ‘‘blinking” of the Raman lines. These fluctuations in the Raman spectra, in number, intensity and wave-number position of the bands, can be interpreted in terms of ongoing chemical reactions leading to the modification in the carbon skeletal structure and in the nature of the chemical bonds35.