Origin of surface-induced visible light absorption of nanostructured diamond

At a Glance

Metadata	Details
Publication Date	2025-10-29
Journal	MRS Bulletin
Authors	A. Bellucci, Alexandre Chemin, Tristan Petit, Eleonora Bolli, Veronica Valentini
Institutions	Stanford University, Institute of Structure of Matter
Analysis	Full AI Review Included

Origin of Surface-Induced Visible Light Absorption of Nanostructured Diamond (Black Diamond)

Technical Analysis and Material Solutions by 6CCVD

This document analyzes the research concerning the origin of enhanced visible light absorption in nanostructured Single-Crystal Diamond (SCD), often termed “Black Diamond” (BD). The findings confirm that surface-localized sp² defects, induced by femtosecond laser processing, are the primary drivers for subbandgap photon absorption, validating diamond’s potential for high-efficiency solar energy conversion devices, such as Photon-Enhanced Thermionic Emission (PETE) cathodes.

Executive Summary

Core Achievement: Demonstrated a ~10⁴-fold enhancement in visible light absorption in nanostructured SCD (“Black Diamond”) compared to pristine samples, crucial for solar energy applications.
Mechanism Identified: Enhanced absorption is primarily due to the formation of sp²-like carbon defects (carbon reconstruction and C-O bonds) confined to the uppermost atomic layers of the diamond surface.
Key Defect States: XAS and PDS correlated two major subbandgap transitions originating from the Valence Band Maximum (VBM) at 1.25-1.3 eV (sp² C=C) and 2.5-2.7 eV (C-O bonds).
Fabrication Method: The material was created using femtosecond laser-induced periodic surface structures (1D-LIPSS) on (100) SCD, followed by strong chemical oxidation.
Material Performance: The calculated absorption coefficient (α) for the BD layer alone reaches extremely high values, ranging from 5 x 10⁵ to 1 x 10⁶ cm^-1 in the visible spectrum.
Device Relevance: The study confirms the viability of defect-engineered diamond for opto-electronic and solar-energy-conversion applications, provided long-term thermal and structural stability of these engineered states can be ensured.

Technical Specifications

Parameter	Value	Unit	Context
Diamond Bandgap (E_g)	5.47	eV	Intrinsic property of SCD
LIPSS Periodicity	170 ± 10	nm	1D-LIPSS structure on (100) surface
Laser Pulse Duration	100	fs	Ti:Sapphire femtosecond laser source
Laser Wavelength	800	nm	Used for nanostructuring
Impinging Radiation Dose	2.1	kJ cm^-2	Fixed dose for LIPSS fabrication
Boron Implantation Dose	1 x 10¹⁵	cm^-2	For buried p-type layer formation
Peak Boron Concentration	~3 x 10²⁰	cm^-3	Located ~90 nm below the surface
BD Layer Absorption Coefficient (α)	5 x 10⁵ to 1 x 10⁶	cm^-1	Calculated for the BD layer in the visible spectrum
Absorption Enhancement	~10⁴-fold	N/A	Increase compared to pristine SCD
Key Defect Transition (B)	1.25 - 1.3	eV	sp² carbon reconstruction states (from VBM)
Key Defect Transition (C)	2.5 - 2.7	eV	Carbon-oxygen bonds (from VBM)

Key Methodologies

The Black Diamond (BD) samples were fabricated using a multi-step process combining CVD material preparation, ion implantation, laser nanostructuring, and chemical treatment:

Substrate Selection: Standard-grade Single-Crystal Diamond (SCD) plates (4.5 x 4.5 x 0.5 mm³, (100) orientation) were used as the starting material.
Buried p-type Layer: Boron ion implantation was performed at 40 keV with a dose of 1 x 10¹⁵ cm^-2 to create a p-type layer peaking ~90 nm below the surface.
Surface Nanostructuring (LIPSS): A Ti:Sapphire femtosecond laser (100 fs, 800 nm, 1 kHz) was used in high vacuum (<10^-7 mbar) to create 1D-LIPSS structures with a periodicity of 170 nm.
Chemical Etching: Samples were treated in a boiling, strongly oxidizing solution (H₂SO₄/HClO₄/HNO₃) to remove graphitic debris and oxidize the surface, which is critical for stabilizing the sp² defects.
Contact Fabrication: Femtosecond laser-induced graphitization was used to create an array of 10,000 columns (100 x 100, 40 µm pitch) for distributed contacts, reducing device series resistance.
Surface Analysis: Advanced surface-sensitive techniques were employed, including Tip-Enhanced Raman Spectroscopy (TERS), X-ray Absorption Spectroscopy (XAS), and depth-resolved X-ray Photoelectron Spectroscopy (XPS).

6CCVD Solutions & Capabilities

6CCVD is uniquely positioned to supply the high-quality MPCVD diamond materials and customization services required to replicate, scale, and optimize this cutting-edge research into functional PETE and solar energy devices.

Applicable Materials

To successfully replicate and advance the “Black Diamond” research, 6CCVD recommends the following materials:

Material Specification	6CCVD Offering	Relevance to Research
Single Crystal Diamond (SCD)	Electronic Grade SCD (0.1µm - 500µm)	Provides ultra-low intrinsic defect density, ensuring that the intentionally induced sp² surface defects are the dominant absorption mechanism. Available in (100) orientation.
Boron Doped Diamond (BDD)	Heavy Boron Doped PCD/SCD	Essential for replicating the buried p-type layer (p/i/n structure). We offer precise control over doping concentration (up to 10²¹ cm^-3) and thickness profiles.
Polycrystalline Diamond (PCD)	Optical Grade PCD (up to 125mm)	For scaling up to larger area devices (e.g., 100 mm or 125 mm wafers) where the LIPSS technique can be applied across industrial-sized substrates.

Customization Potential

6CCVD’s in-house capabilities directly address the complex material requirements of this study:

Custom Dimensions and Thickness: The paper used small 4.5 x 4.5 mm plates. 6CCVD can supply SCD plates up to 10 x 10 mm and PCD wafers up to 125 mm in diameter, facilitating the necessary scale-up for practical solar applications. We offer custom substrate thicknesses up to 10 mm.
Ultra-Low Roughness Polishing: The starting material had Ra < 30 nm. 6CCVD provides superior polishing (Ra < 1 nm for SCD), ensuring an atomically smooth surface ideal for precise, uniform femtosecond laser nanostructuring (LIPSS).
Metalization Services: While the paper used fs-laser graphitization for contacts, 6CCVD offers internal metalization capabilities (Au, Pt, Pd, Ti, W, Cu) for creating robust, low-resistance electrical contacts post-nanostructuring and chemical treatment. We can deposit multi-layer stacks (e.g., Ti/Pt/Au) to customer specifications.

Engineering Support

The successful development of defect-engineered diamond relies on precise control over material purity and surface preparation. 6CCVD’s in-house PhD team specializes in MPCVD growth and post-processing optimization.

Material Selection for PETE: Our experts can assist researchers in selecting the optimal SCD or PCD grade, thickness, and doping profile necessary to maximize the photoelectronic gain and long-term stability required for high-temperature, high-flux PETE systems.
Surface Engineering Consultation: We provide technical consultation on how surface preparation (polishing, cleaning, and oxidation) impacts the subsequent formation and stability of beneficial sp² defects induced by laser nanostructuring.

Call to Action: For custom specifications or material consultation regarding defect-engineered diamond for solar or opto-electronic projects, visit 6ccvd.com or contact our engineering team directly. We offer global shipping (DDU default, DDP available) to ensure timely delivery of your critical materials.

View Original Abstract

Abstract Surface nanotextured diamond, named “black diamond,” absorbs efficiently visible light. This feature allows this material to be used in solar devices, such as the p/i/n photocathode under study in this work. Using optical and x-ray surface-sensitive spectroscopy techniques, this study establishes a correlation between the material’s optical properties and its chemical structure to elucidate the nature of light absorption and photocarriers’ generation. The analysis reveals that the surface states in the very first atomic layer of the diamond, such as carbon sp 2 reconstruction states and carbon oxygen bounds at 1.25 and 2.5 eV with respect to the maximum of the valence band, respectively, are the key factors for the enhancement of the visible light absorption with respect to pristine samples. These defects, together with a collective effect induced by the diffraction light trapping, could be useful for using black diamond in solar applications by exploiting the increase in the visible light absorption. Impact statement Diamond, an ultrawide bandgap semiconductor with an energy bandgap of 5. 47 eV, possesses exceptional thermal, mechanical, and electronic properties, making it a promising material for a wide range of applications. However, its intrinsic nature renders diamond natively visible blind, limiting its potential in solar-energy applications. Surface texturing using ultrashort laser pulses offers a way to enable diamond’s utilization in sunlight conversion. Nonetheless, it is crucial to understand the effects induced by laser treatments on its surface to tailor and optimize it as an active solar material. This study highlights the critical role of sp 2 defects formed in the uppermost layers of monodimensional laser-induced periodic surface structures (1D LIPSSs). These defects play a primary role in absorbing subbandgap photons. The recognition of the direct involvement of surface defects in the absorption mechanism paves the way for the development of highly efficient and fine- tuned nanotextured surfaces, designed to maximize the presence of these beneficial defects. Graphical abstract

Tech Support

Original Source

DOI: https://doi.org/10.1557/s43577-025-00971-2