Sub-nanometer-thick native sp2 carbon on oxidized diamond surfaces

TL;DR

This study uses angle-resolved XPS to analyze the ultra-thin sp2 carbon layer on oxidized diamond surfaces, revealing a 0.4 nm thick layer predominantly bonded to oxygen, which is difficult to detect with conventional methods.

Contribution

It demonstrates a novel application of angle-resolved XPS to precisely measure the sub-nanometer sp2 carbon layer on diamond surfaces, advancing surface characterization techniques.

Findings

Identified a 0.4 nm thick sp2 carbon layer on oxidized diamond surface.

Showed that oxygen mainly bonds to the sp2 carbon layer, not directly to diamond.

Used peak-fitting and Auger spectra analysis for detailed surface composition.

Abstract

Oxygen-terminated diamond has a wide breadth of applications, which include stabilizing near-surface color centers, semiconductor devices, and biological sensors. Despite the vast literature on characterizing functionalization groups on diamond, the chemical composition on the shallowest portion of the surface (< 1 nm) is challenging to probe with conventional techniques like XPS and FTIR. In this work, we demonstrate the use of angle-resolved XPS to probe the first ten nanometers of (100) single-crystalline diamond, showing the changes of the oxygen functional groups and the allotropes of carbon with respect to depth. With the use of consistent peak-fitting methods, the peak identities and relative peak binding energies were identified for sp2 carbon, ether, hydroxyl, carbonyl, and C-H groups. For the oxygen-terminated sample, we also quantified the thickness of the sp2 carbon layer situated on top of the bulk sp3 diamond bonded carbon to be 0.4 $\pm$ 0.1 nm, based on the analysis of the Auger electron spectra and D-parameter calculations. These results indicate that the majority of the oxygen is bonded to the sp2 carbon layer on the diamond, and not directly on the sp3 diamond bonded carbon.