[Paper Review] Evidence for a pressure-induced phase transition of few-layer graphene to 2D diamond

Luiz G. P. Martins, Diego L. Silva+17 more|arXiv (Cornell University)|Oct 3, 2019

Graphene research and applications29 references4 citations

TL;DR

This study provides experimental and theoretical evidence for a pressure-induced phase transition in few-layer graphene to 2D diamond (diamondene) under hydrostatic conditions using water as a pressure-transmitting medium. High-pressure Raman and XRD data reveal a gradual top-bottom diamondization between 4–7 GPa, with the formation of hexagonal diamond structure consistent with diamondene, demonstrating a route to stable 2D ferromagnetic semiconductor with potential for spintronic applications.

ABSTRACT

We unveil the diamondization mechanism of few-layer graphene compressed in the presence of water, providing robust evidence for the pressure-induced formation of 2D diamond. High-pressure Raman spectroscopy provides evidence of a phase transition occurring in the range of 4-7 GPa for 5-layer graphene and graphite. The pressure-induced phase is partially transparent and indents the silicon substrate. Our combined theoretical and experimental results indicate a gradual top-bottom diamondization mechanism, consistent with the formation of diamondene, a 2D ferromagnetic semiconductor. High-pressure x-ray diffraction on graphene indicates the formation of hexagonal diamond, consistent with the bulk limit of eclipsed-conformed diamondene.

Motivation & Objective

• To investigate the feasibility of forming stable 2D diamond (diamondene) in few-layer graphene under high pressure.
• To determine the role of water as a pressure-transmitting medium in facilitating the phase transition.
• To characterize the structural evolution and symmetry breaking during the diamondization process.
• To establish a mechanistic understanding of the top-down diamondization pathway in few-layer graphene.
• To validate the formation of hexagonal diamond structure via high-pressure XRD and theoretical modeling.

Proposed method

• High-pressure Raman spectroscopy was performed on mechanically exfoliated few-layer graphene and graphite transferred onto 25-μm Si substrates using a diamond anvil cell (DAC) with water as the pressure-transmitting medium.
• High-pressure X-ray diffraction (XRD) was conducted at HPCAT, Advanced Photon Source, to probe the structural changes and confirm the formation of hexagonal diamond.
• Molecular dynamics (MD) simulations and density functional theory (DFT) calculations were used to model the phase transition mechanism and predict electronic properties.
• Raman spectra were analyzed for shifts in G and 2D bands, intensity ratios (I2D/IG), and linewidth changes to detect structural transitions.
• Optical imaging and powder diffraction were used to assess transparency and orientation effects in polycrystalline graphene samples.
• Comparative analysis of XRD patterns from graphene, Ice VII, and bulk hexagonal diamond was used to identify the new phase.

Experimental results

Research questions

• RQ1 Does high pressure induce a phase transition from few-layer graphene to 2D diamond in the presence of water?
• RQ2 What is the pressure range and mechanism of the diamondization process in few-layer graphene?
• RQ3 How does the use of water as a pressure-transmitting medium influence the phase transition kinetics and symmetry?
• RQ4 Is the resulting 2D phase consistent with the predicted diamondene structure, including its ferromagnetic and semiconducting properties?
• RQ5 Can high-pressure XRD and Raman spectroscopy jointly confirm the formation of hexagonal diamond in few-layer graphene?

Key findings

• A phase transition in 5-layer graphene and graphite was observed between 4–7 GPa via high-pressure Raman spectroscopy, marked by distinct changes in G and 2D band frequencies and linewidths.
• The pressure-induced phase became partially transparent and indented the silicon substrate, indicating extreme hardness consistent with diamond-like properties.
• High-pressure XRD confirmed the formation of hexagonal diamond (lonsdorite-like structure), matching the bulk limit of eclipsed-conformed diamondene.
• The transition proceeds via a gradual top-bottom diamondization mechanism, with surface layers transforming first, consistent with theoretical predictions of diamondene formation.
• The use of water as a pressure-transmitting medium enabled a more homogeneous and gradual phase transition compared to non-hydrostatic conditions.
• Powdered graphene samples showed extended phase transition ranges (up to ~24 GPa) due to inefficient vertical propagation from random stacking, unlike single-crystal-like flakes.