Femtosecond-Laser Nanostructuring of Black Diamond Films under Different Gas Environments

At a Glance

Metadata	Details
Publication Date	2020-12-17
Journal	Materials
Authors	M. Girolami, A. Bellucci, Matteo Mastellone, S. Orlando, Valerio Serpente
Institutions	Institute of Nanostructured Materials, Sapienza University of Rome
Citations	11
Analysis	Full AI Review Included

Executive Summary

This research validates a cost-effective, ambient-pressure manufacturing workflow for high-performance black diamond films, traditionally requiring complex Ultra-High Vacuum (UHV) conditions.

Core Achievement: Demonstrated that femtosecond (fs) laser nanostructuring of polycrystalline diamond under a constant Helium (He) flow achieves optical properties nearly identical to those fabricated in UHV.
Value Proposition: Eliminates the need for expensive and complex UHV systems, paving the way for large-scale, cost-effective production of black diamond solar absorbers.
Performance Metric: The He-treated film (TM3-He) achieved a solar absorptance (α_s) of 86.2%, closely matching the optimal UHV reference value of 88.4%.
Mechanism of Success: He flow ensures a uniform distribution of Laser Induced Periodic Surface Structures (LIPSS) across the polycrystalline surface, crucial for efficient light trapping.
Failure Analysis (Air/N₂): Treatments under compressed air or N₂ flow resulted in irregular, terraced surfaces with deep cracks along grain boundaries, leading to poor LIPSS uniformity and significantly lower α_s (~79%).
Structural Integrity: The He-treated sample exhibited compressive stress (Raman peak upshift), consistent with optimal UHV-fabricated black diamond films, indicating high structural quality.

Technical Specifications

Parameter	Value	Unit	Context
Target Material	Polycrystalline CVD Diamond	N/A	Thermal Management (TM) Grade
Laser Wavelength (λ_fs)	800	nm	Ti:sapphire system
Pulse Duration	~100	fs	Ultrafast regime
Repetition Rate (f)	1	kHz	Fixed parameter
Laser Spot Diameter (2w)	150	µm	1/e² width
Single Pulse Fluence (Φ_p)	4.44	J/cm²	Above ablation threshold (~3 J/cm²)
Total Accumulated Fluence (Φ_A)	5.0	kJ/cm²	Optimal fluence for 1D LIPSS
Horizontal Scan Speed (v_x)	1	mm/s	Raster pattern speed
LIPSS Periodicity (Λ)	170 ± 10	nm	Measured structure size (Theoretical: 166 nm)
Solar Absorptance (α_s) - He	86.2	%	Best ambient condition result (TM3-He)
Solar Absorptance (α_s) - UHV Ref.	88.4	%	Previous optimal UHV result
Solar Absorptance (α_s) - Air	79.1	%	Result for TM1-air
Diamond Raman Peak (TM3-He)	1335.1	cm^-1	Compressive stress (Upshifted from 1332.0 cm^-1)
Diamond Raman Peak (TM1-Air)	1326.3	cm^-1	Tensile stress (Downshifted)
LIPSS Depth	480 ± 20	nm	Inferred from tilted SEM images

Key Methodologies

The fabrication process utilized a linearly polarized fs-pulsed laser system in a raster scanning pattern on polycrystalline CVD diamond samples at ambient pressure.

Pre-Cleaning: Samples were subjected to a standard cleaning procedure using a strongly oxidizing mixture (HNO₃:H₂SO₄:HClO₄, 1:1:1 volume ratio) at boiling point for 15 minutes, followed by ultrasonic cleaning in acetone and 2-propanol.
Laser Setup: A Ti:sapphire laser system (800 nm, 100 fs, 1 kHz) was used, focused to a 150 µm spot diameter.
Scanning Parameters: The laser scanned an 8 x 8 mm² area at a constant speed (1 mm/s) with a vertical shift (Δy = 20 µm) to ensure a precise total accumulated fluence (Φ_A) of 5.0 kJ/cm².
Gas Environment Control: A constant flow of gas (Compressed Air, N₂, or He) was directed towards the sample surface. The flow angle was maintained close to 90° to the surface normal to prevent spatially dependent boundary layer thickness.
Post-Treatment: Samples were cleaned again using the oxidizing mixture to remove debris generated by the ablation process.
Morphological Analysis (SEM): FE-SEM was used to evaluate LIPSS formation and uniformity. A thin (~2 nm) Au coating was applied prior to SEM to prevent sample charging.
Structural Analysis (Raman): Raman spectroscopy (514.5 nm Ar⁺ laser) was performed to assess structural modifications, crystalline quality (FWHM), and internal stress (peak shift).
Optical Analysis (Spectrophotometry): Absorptance (α) spectra were derived from measured transmittance (τ) and hemispherical reflectance (ρ) in the 0.25-2.5 µm range, used to calculate the solar absorptance (α_s) under AM 1.5 global-tilt irradiance.

Commercial Applications

This technology is highly relevant for applications requiring robust, high-efficiency solar absorption and thermal stability, particularly where manufacturing cost reduction is critical.

Concentrated Solar Power (CSP) Receivers: Black diamond films offer outstanding solar absorptance (α_s > 85%) across the solar spectrum, making them ideal for high-efficiency solar thermal collectors.
Photon-Enhanced Thermionic Emission (PETE) Devices: Black diamond is suitable for use as a cathode material in PETE solar cells, which operate at very high temperatures (Ref. [4-6]).
Solar Thermionic-Thermoelectric Generators (ST2G): Used as the primary solar absorber component in hybrid energy conversion systems that require materials stable at extreme heat (Ref. [37]).
High-Temperature Electronics/Coatings: Diamond’s inherent thermal management properties, combined with enhanced optical absorption, make it valuable for protective or functional coatings in harsh thermal environments.
Industrial Laser Processing: The successful transition from UHV to ambient He flow enables the integration of black diamond fabrication into standard, high-throughput industrial laser processing lines, reducing capital expenditure and operational complexity.

View Original Abstract

Irradiation of diamond with femtosecond (fs) laser pulses in ultra-high vacuum (UHV) conditions results in the formation of surface periodic nanostructures able to strongly interact with visible and infrared light. As a result, native transparent diamond turns into a completely different material, namely “black” diamond, with outstanding absorptance properties in the solar radiation wavelength range, which can be efficiently exploited in innovative solar energy converters. Of course, even if extremely effective, the use of UHV strongly complicates the fabrication process. In this work, in order to pave the way to an easier and more cost-effective manufacturing workflow of black diamond, we demonstrate that it is possible to ensure the same optical properties as those of UHV-fabricated films by performing an fs-laser nanostructuring at ambient conditions (i.e., room temperature and atmospheric pressure) under a constant He flow, as inferred from the combined use of scanning electron microscopy, Raman spectroscopy, and spectrophotometry analysis. Conversely, if the laser treatment is performed under a compressed air flow, or a N2 flow, the optical properties of black diamond films are not comparable to those of their UHV-fabricated counterparts.

Tech Support

Original Source

DOI: https://doi.org/10.3390/ma13245761

References

2015 - Absorptance enhancement in fs-laser-treated CVD diamond [Crossref]
2016 - Infrared absorption of fs-laser textured CVD diamond [Crossref]
2016 - Optimization of black diamond films for solar energy conversion [Crossref]
2017 - Graphite distributed electrodes for diamond-based photon-enhanced thermionic emission solar cells [Crossref]
2016 - Black diamond for solar energy conversion [Crossref]
2012 - Femtosecond laser-induced periodic surface structures [Crossref]
2017 - Laser-induced periodic surface structures on bismuth thin films with ns laser pulses below ablation threshold [Crossref]
2017 - Femtosecond laser induced robust periodic nanoripple structured mesh for highly efficient oil-water separation [Crossref]
2020 - Femtosecond laser fabrication of LIPSS-based waveplates on metallic surfaces [Crossref]