# Dehydrogenation vs Apparent Hydrogenation: Unraveling the Mechanisms of He and O2 Plasma Etching on Colloidal Nanocrystal Films

## TL;DR

This paper introduces a reliable method to measure organic ligand removal from nanoparticles using Raman spectroscopy, revealing different chemical effects of He and O2 plasmas.

## Contribution

A novel calibration method for Raman spectroscopy using IBA data to quantify ligand removal in plasma-treated nanocrystal films.

## Key findings

- He plasma causes rapid dehydrogenation and cross-linking with faster C–H signal loss than actual carbon removal.
- O2 plasma unexpectedly leads to 'apparent hydrogenation' due to oxidative fragmentation and stabilization of methyl-terminated fragments.

## Abstract

Removing organic
ligands from colloidal nanoparticles is critical
for fabricating solid-state devices, yet accurately quantifying this
removal remains a significant analytical challenge. Here, we establish
a robust and accessible method for this quantification by calibrating
Raman spectroscopy against precise ion beam analysis (IBA) for nanoparticle
assemblies (CNAs) processed by helium (He) and oxygen (O2) plasmas. We demonstrate that the calibration curves are remarkably
independent of plasma power and pressure, depending critically only
on the choice of feed gas. He plasma induces rapid dehydrogenation
and cross-linking, evidenced by a much faster decrease in the C–H
Raman signal relative to the actual carbon loss. Conversely, O2 plasma leads to a surprising “apparent hydrogenation”,
where the carbon backbone is removed significantly faster than the
C–H signal diminishes. This counterintuitive effect is explained
by a serial mechanism of oxidative fragmentation; β-scission
cleaves the alkyl chains, and subsequent stabilization steps enrich
the remaining film with hydrogen-rich methyl-terminated fragments,
while carbon is efficiently removed as volatile CO. This work provides
calibrated functions that enable the rapid determination of absolute
carbon content in processed CNAs using simple Raman spectroscopy with
uncertainties of ∼8% for O2 and ∼12% for
He plasma, offering a vital tool for both process diagnostics and
fundamental studies of plasma–matter interactions in colloidal
nanocrystal films.