Infrared reflection spectroscopy of thin films on highly oriented pyrolytic graphite

Area of Science:

• Materials Science
• Spectroscopy
• Surface Science

Background:

• Highly oriented pyrolytic graphite (HOPG) is a unique material with anisotropic properties.
• External reflection infrared (IR) spectroscopy is a powerful technique for surface analysis.
• Understanding substrate properties is crucial for optimizing spectroscopic measurements.

Purpose of the Study:

• To investigate the suitability of HOPG as a substrate for mid-infrared external reflection spectroscopy.
• To determine the optical constants of HOPG and understand its semimetallic behavior.
• To evaluate HOPG's performance compared to traditional metallic and dielectric substrates.

Main Methods:

• Reflection measurements of clean HOPG surfaces using polarized light at varying incidence angles and frequencies.
• Determination of anisotropic optical constants (refractive index 'n' and absorption index 'k').
• Comparison of experimental spectra with spectral simulations for different surface coverages (clean, physisorbed, chemisorbed).

Main Results:

• Anisotropic optical constants 'n' and 'k' were determined for HOPG, revealing its semimetallic nature.
• HOPG exhibits intermediate reflectivity between metals and dielectrics.
• Adsorbate spectra on HOPG show asymmetric peak shapes, characteristic of a transition between metallic and dielectric substrates.

Conclusions:

• HOPG behaves similarly to a metal in terms of spectral sensitivity and surface selection rules for IR spectroscopy.
• HOPG is a suitable substrate for external reflection infrared measurements.
• The semimetallic properties of HOPG provide a unique platform for surface analysis.