Tip-enhanced Raman spectroscopy (TERS) on double perovskite La2CoMnO6 thin films: field enhancement and depolarization effects

Polarization-dependent tip-enhanced Raman spectroscopy (TERS) was carried out on double perovskite La2CoMnO6 (LCMO) thin films to determine the contributions of the enhancement by the excitation of surface plasmon polariton and tip-induced polarization effects. Due to B-site ordering and the resulting monoclinic P12(1)/n1 structure, the spectra exhibit a strong A(g) mode around 644cm(-1) originated from (Co/Mn)O-6 stretching vibrations. The TERS contrast C, defined as the ratio of the total-field (the sum of the near field and the far field) to the far-field intensity, up to C similar to 60-110 for a cross-polarized configuration (incident light polarization perpendicular to the detected one), reveals a possible breakdown of polarization-dependent selection rules for the Raman scattering due to a tip-induced near-field depolarization component. We disentangled a significant enhancement for all observed modes in a parallel configuration with a TERS contrast of C similar to 2-5 due to general near-field amplification, strongly affected also by the film thickness. With B-site ordering and highly dielectric behavior, LCMO therefore provides an excellent test system to study TERS phenomena on oxide surfaces. Copyright (c) 2016 John Wiley & Sons, Ltd.