Origin of surface-induced visible light absorption of nanostructured diamond
At a Glance
Section titled âAt a Glanceâ| Metadata | Details |
|---|---|
| Publication Date | 2025-10-29 |
| Journal | MRS Bulletin |
| Authors | A. Bellucci, Alexandre Chemin, Tristan Petit, Eleonora Bolli, Veronica Valentini |
| Institutions | Helmholtz-Zentrum Berlin fĂŒr Materialien und Energie, Stanford University |
| Analysis | Full AI Review Included |
Executive Summary
Section titled âExecutive SummaryâThis study investigates the origin of enhanced visible light absorption in nanostructured diamond (Black Diamond, BD), validating its potential for high-efficiency solar energy applications, particularly Photon-Enhanced Thermionic Emission (PETE) photocathodes.
- Core Mechanism: The significant increase in visible light absorption is attributed to the formation of sp2-like defects (e.g., C=C bonds and trans-polyacetylene chains) confined to the uppermost atomic layers of the laser-induced periodic surface structures (1D-LIPSS).
- Defect Energy Levels: Spectroscopy correlates strong subbandgap absorption features to surface states located at approximately 1.25 eV and 2.5-2.7 eV above the Valence Band Maximum (VBM).
- Performance Metric: 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, representing a remarkable ~104-fold increase over pristine diamond.
- Defect Localization: Depth-resolved XPS confirms that the beneficial sp2 defects are highly localized, existing only within the first few nanometers of the surface, while the bulk remains highly crystalline sp3 diamond.
- Dual Contribution: The overall optical behavior is a combination of defect-state absorption and a collective light-trapping effect induced by the surface nanostructuring (diffraction gratings/mothâs eye mechanism).
- Application Validation: These findings confirm that black diamond is a viable alternative to conventional semiconductors for high-efficiency solar conversion systems, provided long-term thermal and structural stability of the engineered surface states can be demonstrated.
Technical Specifications
Section titled âTechnical Specificationsâ| Parameter | Value | Unit | Context |
|---|---|---|---|
| Intrinsic Diamond Bandgap (Eg) | 5.47 | eV | Pristine single-crystal diamond (SCD) |
| LIPSS Periodicity | 170 ± 10 | nm | 1D-LIPSS structure formed by fs-laser |
| Absorption Coefficient (BD Layer) | 5 x 105 to 1 x 106 | cm-1 | Calculated maximum in the visible spectrum |
| Absorption Enhancement | ~104 | -fold | Increase relative to pristine SCD |
| Primary Defect Absorption (B) | 1.25 - 1.3 | eV | Above VBM; attributed to sp2 C=C bonds |
| Secondary Defect Absorption (C) | 2.5 - 2.7 | eV | Above VBM; attributed to Ï*C=O transitions |
| C(1s) â Ï*C=C Transition (XAS) | 285.2 | eV | Unoccupied surface state energy |
| C(1s) â Ï*C=O Transition (XAS) | 286.5 | eV | Unoccupied surface state energy |
| Laser Pulse Duration | 800 | fs | Used for surface texturing |
| Laser Repetition Rate | 1 | kHz | Used for surface texturing |
| BD Layer Thickness (dBD) | 0.25 | ”m | Estimated thickness for absorption calculation |
| Substrate Thickness (dsub) | 475 | ”m | Estimated thickness for absorption calculation |
Key Methodologies
Section titled âKey MethodologiesâThe black diamond (BD) samples were fabricated using femtosecond (fs) laser texturing and characterized using a suite of surface-sensitive spectroscopy techniques to correlate structure and optical properties.
- Sample Preparation (Substrate): Single-crystal diamond (SCD) plates (4.5 x 4.5 x 0.5 mm3, optical grade) were used as the base material.
- Surface Texturing (1D-LIPSS): The SCD surface was irradiated using a Ti:Sapphire femtosecond laser (800 fs, 200 kHz repetition rate) to create 1D-LIPSS structures with a periodicity of 170 nm.
- Chemical Cleaning: Post-processing involved a wet chemical treatment using a 3:1 mixture of sulfuric acid and nitric acid (1.5 h at elevated temperatures) to remove graphitic debris and chemically oxidize the surface.
- Morphological Characterization (AFM): Atomic Force Microscopy confirmed the formation and periodicity of the 1D-LIPSS structures.
- Surface Chemistry Analysis (TERS): Tip-Enhanced Raman Spectroscopy provided nanoscale, surface-sensitive chemical mapping, confirming the presence of high-density sp2-related defects (D band, t-PA, C=C modes) localized on the nanostructured surface.
- Unoccupied State Analysis (XAS): X-ray Absorption Spectroscopy (C K-edge NEXAFS) was performed in Electron Yield (EY) mode to identify unoccupied intra-bandgap states, correlating them to Ï* transitions from sp2 C=C and C=O bonds.
- Subbandgap Absorption (PDS): Photothermal Deflection Spectroscopy measured the absorption coefficient (α) in the visible range (0.5 to 3.5 eV), successfully disentangling defect absorption from light scattering effects.
- Depth Profiling (XPS): Depth-resolved X-ray Photoelectron Spectroscopy was conducted by varying the incident photon energy (335 to 700 eV) to tune the probing depth, confirming the confinement of sp2 defects to the outermost atomic layers.
Commercial Applications
Section titled âCommercial ApplicationsâThe development of nanostructured black diamond with tailored surface defects opens pathways for high-performance devices in energy and optoelectronics.
- Solar Energy Conversion (High Concentration):
- Photon-Enhanced Thermionic Emission (PETE): Black diamond is optimized as a p/i/n photocathode material for PETE converters, which theoretically exceed Shockley-Queisser limits (up to 70.4% efficiency) by combining photovoltaic and thermal effects.
- Optoelectronics and Sensing:
- Visible Light Photodetectors: Utilizing the surface-induced subbandgap absorption to create diamond-based photodetectors sensitive to visible light, a capability traditionally absent in intrinsic diamond.
- High-Efficiency Antireflection Coatings: The 1D-LIPSS structures act as diffraction gratings, contributing to light trapping (mothâs eye mechanism), useful for high-performance optical windows and lenses in harsh environments.
- Advanced Thermal Management:
- Diamondâs exceptional thermal properties, combined with its new optical absorption capabilities, could lead to novel thermal energy harvesting or conversion systems.
View Original Abstract
Abstract Surface nanotextured diamond, named âblack diamond,â 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 materialâs optical properties and its chemical structure to elucidate the nature of light absorption and photocarriersâ 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 diamondâs 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