Studies of Substrate Mediated Modification of Chemical Vapour Deposition Graphene

  • Khadisha Zahra

Student thesis: Phd

Abstract

In its pure form pristine graphene has an intrinsic zero band gap energy and low reactivity. Research has shown that nitrogen is a prime candidate for tailoring graphene’s innate properties. Due to nitrogen’s similar atomic size to carbon, it is a promising candidate for heteroatom substitution and has already been shown to demonstrate great potential as an electrocatalyst. Molecular nitrogen (N2) has also been shown to passivate the underlying substrate, thus forming neutral quasi-freestanding graphene on its growth substrate. This thesis demonstrates the simplicity in substrate mediated modification of graphene grown by chemical vapour deposition (CVD) on copper. Both commercial (BGT Materials Ltd.) and lab grown samples were characterised using near-ambient pressure X-ray photoelectron spectroscopy (NAP-XPS), Raman and atomic force microscopy (AFM). NAP-XPS was used to probe the development of nitrogen dopants. Pristine CVD graphene was annealed in ammonia (NH3), leading to the discovery of N2 Further experiments indicated that the reaction was oxide mediated: the reduction of NH3 over heated Cu2O formed N2 and water. This occurred at low pressure (0.6 mbar NH3) with an onset temperature of ~520 K and a N2 de-intercalation temperature of >520 K to 595 K. AFM measurements revealed smooth protrusion in the height and dark regions (i.e. less stiff) in the corresponding stiffness images. Thus, pressurised graphene nanobubbles (GNBs) had been formed. Statistical data of AFM measurements where used to assess the shape and aspect ratio of the GNBs. They were shown to be in agreement with the Universal Shape previously described in the literature, though the aspect ratio was higher than previous reports and included a step-like transition. This is believed to be due to increased interaction with the rough copper substrate and surface pinning of the graphene film. Variations in nitrogen doped graphene species and atomic concentration (at.%) post ammonia treatment were shown to depend on the duration of substrate oxidation. The process could be tailored to maximise GNB formation (10 min) or dopant species (60 min). Both in-situ and ex-situ doped samples, could also be further modified through thermal treatments, as pyridinic dopants appear less stable than graphitic + pyrrolic dopants. We have successfully intercalated nitrogen-containing molecules (NH3) beneath the graphene surface, simultaneously forming nitrogen doped graphene and N2 filled GNBs via a post-synthesis technique. This provides a simple approach to controlling nitrogen incorporation and studying dopant species, enabling greater functionality for customised devices in the future.
Date of Award31 Dec 2023
Original languageEnglish
Awarding Institution
  • The University of Manchester
SupervisorRobert Dryfe (Supervisor) & Alex Walton (Supervisor)

Keywords

  • AFM
  • Atomic Force Microscopy
  • Raman
  • NH3
  • Universal Shape
  • Oxidation
  • Thermal Stability
  • Ammonia
  • Doping
  • Nitrogen
  • Copper
  • Intercalation
  • Graphene
  • X-ray Photoelectron Spectroscopy
  • XPS
  • Near-Ambient Pressure X-ray Photoelectron Spectroscopy
  • NAP-XPS
  • Graphene Nanobubbles
  • GNBs
  • Chemical Vapour Deposition
  • CVD

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