Raman Study of the Diamond to Graphite Transition Induced by the Single Femtosecond Laser Pulse on the (111) Face

At a Glance

Metadata	Details
Publication Date	2022-12-29
Journal	Nanomaterials
Authors	А.А. Khomich, V. V. Kononenko, Oleg S. Kudryavtsev, E.V. Zavedeev, А. В. Хомич
Institutions	Institute of Radio-Engineering and Electronics, Prokhorov General Physics Institute
Citations	20
Analysis	Full AI Review Included

Executive Summary

This research successfully demonstrated the controlled synthesis of highly ordered graphite (HOG) layers on the diamond (111) face using single femtosecond UV laser pulses, offering a pathway to high-quality conductive patterning.

Optimal Processing Window: A specific laser fluence range of 4-6 J/cm² was identified as optimal, yielding graphitic layers with maximum structural perfection (lowest defect concentration, narrowest G-band FWHM).
Graphitization Regimes: Three distinct graphitization regimes were characterized based on laser fluence: (1) nonablative (optical penetration limited), (2) customary ablative (graphitization wave velocity limited), and (3) bulk graphitization (simultaneous transformation across the excited volume).
Structural Identification: Raman spectroscopy revealed that the optimal sp² phase is best described as folded multilayer graphene (crumpled graphenic sheets), rather than highly disordered nanocrystalline graphite, which is typical for (100) irradiation.
Defect Control: The study established a clear correlation between laser fluence and structural defects, allowing for precise control over the resulting carbon phase, ranging from “under-graphitized” diamond to “perfect” HOG and “damaged” graphite.
(111) Face Advantage: Utilizing the (111) face minimizes internal stresses during the diamond-to-graphite transformation, which is crucial for achieving the observed high degree of crystalline perfection in the conductive layer.

Technical Specifications

Parameter	Value	Unit	Context
Substrate Material	Natural Type IIa Diamond	N/A	(111) cleaved face
Nitrogen Concentration	< 10¹⁷	cm^-3	Sample purity
Laser Type	Ti:Al₂O₃ (3^rd Harmonic)	N/A	Single pulse irradiation
Pulse Duration	120	fs	At half maximum
Wavelength	266	nm	UV radiation
Photon Energy	4.65	eV	Two-photon absorption regime
Gaussian Beam Radius (w_g)	2.21	µm	1/e level
Graphitization Threshold (F_th)	1.81	J/cm²	Surface graphitization initiation
Optimal HOG Fluence Range	4 - 6	J/cm²	Maximum structural perfection (low I_D/I_G ratio)
Bulk Graphitization Threshold	8	J/cm²	Initiation of transformation in the crystal bulk
Max Graphitized Thickness	~200	nm	Measured via Raman attenuation method
HOG G-band FWHM (Optimal)	27 - 30	cm^-1	Typical for Highly Oriented Graphite (HOG)
Thermal Diffusivity (Diamond)	~10	cm²/s	High value prevents immediate thermal equilibrium
Thermal Diffusivity (Graphite)	~0.1	cm²/s	Low value causes heat to be “locked” near the surface

Key Methodologies

Substrate Preparation: A natural Type IIa diamond monocrystal was used. The (111) face was prepared by mechanical splitting, ensuring low nitrogen concentration (< 10¹⁷ cm^-3).
Femtosecond Laser Irradiation: A single pulse (120 fs, 266 nm) from a Ti:Al₂O₃ laser system was focused onto the (111) surface using an aspherical lens (NA = 0.1). Fluence was varied widely (1-45 J/cm²).
Crater Depth Measurement: To determine the depth of the graphitization front, samples were annealed in air at 600 °C to oxidize and remove the sp² phase. Crater depth was then measured using white light interferometry and Atomic Force Microscopy (AFM).
Structural Characterization (Raman Spectroscopy): Confocal Raman spectroscopy (473 nm excitation) was used to analyze the sp² phase structure. The spectral range was expanded (200-2300 cm^-1) to detect low- and high-frequency defect bands (455, 810, and 1890 cm^-1).
Raman Data Decomposition: Spectra were fitted using a four-band decomposition procedure (G, D, D’, and A bands). The diamond peak was subtracted using a Breit-Wigner-Fano profile to account for mechanical stresses at the interface.
Thickness Estimation: Graphitized layer thickness was estimated non-destructively by measuring the attenuation of the diamond Raman line intensity due to linear absorption by the overlying graphitic layer.

Commercial Applications

Diamond Electronics and Interconnects: Creation of highly conductive, low-resistance electrodes and interconnects directly patterned onto diamond substrates, crucial for high-power, high-frequency, and extreme environment electronics.
Quantum Sensing and Computing: Precise fabrication of conductive structures (e.g., gates or contacts) on (111) diamond surfaces, optimizing alignment and emission intensity for Nitrogen-Vacancy (NV) centers.
Carbon Composite Materials: Development of ordered carbon composite matrices consisting of conductive (HOG) and dielectric (diamond) components for applications in photonics and terahertz optics.
Shallow Junction Devices: Enabling the formation of efficient p-n junctions and other active elements in diamond electronics by leveraging the superior dopant incorporation efficiency of the (111) face.
Micro- and Nanofabrication: Utilizing the highly localized and non-thermal nature of femtosecond processing for creating micron-scale conductive patterns and microfluidic circuits in transparent materials.

View Original Abstract

The use of the ultrafast pulse is the current trend in laser processing many materials, including diamonds. Recently, the orientation of the irradiated crystal face was shown to play a crucial role in the diamond to graphite transition process. Here, we develop this approach and explore the nanostructure of the sp2 phase, and the structural perfection of the graphite produced. The single pulse of the third harmonic of a Ti:sapphire laser (100 fs, 266 nm) was used to study the process of producing highly oriented graphite (HOG) layers on the (111) surface of a diamond monocrystal. The laser fluence dependence on ablated crater depth was analyzed, and three different regimes of laser-induced diamond graphitization are discussed, namely: nonablative graphitization, customary ablative graphitization, and bulk graphitization. The structure of the graphitized material was investigated by confocal Raman spectroscopy. A clear correlation was found between laser ablation regimes and sp2 phase structure. The main types of structural defects that disrupt the HOG formation both at low and high laser fluencies were determined by Raman spectroscopy. The patterns revealed give optimal laser fluence for the production of perfect graphite spots on the diamond surface.

Tech Support

Original Source

DOI: https://doi.org/10.3390/nano13010162

References

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