Direct monitoring of plasmon resonances in a tip-surface gap of varying width

We have studied the dependence of tip-enhanced Raman (TER) spectra composed of discrete Raman lines and an intense Lorentzian-shaped background on varying interparticle distance. For increasing tip-sample separation, we observe a remarkable blueshift of the background maximum that is ascribed to an energy shift of the localized surface plasmon resonance. In addition, we report on a steep background and Raman band intensity decrease within 5--100. 3em{0ex}nm tip retraction that highlights the underlying near-field enhancement mechanism of TER spectroscopy.