Polarized tip-enhanced Raman spectroscopy at liquid He temperature in ultrahigh vacuum using an off-axis parabolic mirror

TL;DR

This paper presents a novel cryogenic ultrahigh vacuum tip-enhanced Raman spectroscopy system with high numerical aperture optics, enabling detailed vibrational studies of nanomaterials and potentially bulk solids at liquid helium temperatures.

Contribution

The work introduces a home-built LHe-UHV-TERS system with an off-axis parabolic mirror, high polarization control, and cryogenic operation, advancing the study of bulk and surface materials.

Findings

Achieved polarization-dependent Raman spectra of carbon nanotubes at 15 K.

Observed enhancement factors up to 10^7.

Demonstrated system's capability for studying bulk and surface materials.

Abstract

Tip-enhanced Raman spectroscopy (TERS) combines inelastic light scattering well below the diffraction limit down to the nanometer range and scanning probe microscopy and, possibly, spectroscopy. In this way, topographic and spectroscopic as well as single- and two-particle information may simultaneously be collected. While single molecules can now be studied successfully, bulk solids are still not meaningfully accessible. It is the purpose of the work presented here to outline approaches toward this objective. We describe a home-built, liquid helium cooled, ultrahigh vacuum tip-enhanced Raman spectroscopy system (LHe-UHV-TERS). The setup is based on a scanning tunneling microscope and, as an innovation, an off-axis parabolic mirror having a high numerical aperture of approximately $0.85$ and a large working distance. The system is equipped with a fast load-lock chamber, a chamber for the \textit{in situ} preparation of tips, substrates, and samples, and a TERS chamber. Base pressure and temperature in the TERS chamber were approximately $3\times 10^{-11}$~mbar and 15~K, respectively. Polarization dependent tip-enhanced Raman spectra of the vibration modes of carbon nanotubes were successfully acquired at cryogenic temperature. Enhancement factors in the range of $10^7$ were observed. The new features described here including very low pressure and temperature and the external access to the light polarizations, thus the selection rules, may pave the way towards the investigation of bulk and surface materials.