Gallium arsenide waveguides as a platform for direct mid-infrared vibrational spectroscopy
At a Glance
Section titled âAt a Glanceâ| Metadata | Details |
|---|---|
| Publication Date | 2020-03-31 |
| Journal | Analytical and Bioanalytical Chemistry |
| Authors | Julian Haas, Robert Stach, Claudia Kolm, Rudolf Krska, Boris Mizaikoff |
| Institutions | UniversitÀt Ulm, BOKU University |
| Citations | 11 |
| Analysis | Full AI Review Included |
Technical Documentation & Analysis: Gallium Arsenide Waveguides for Mid-Infrared Spectroscopy
Section titled âTechnical Documentation & Analysis: Gallium Arsenide Waveguides for Mid-Infrared SpectroscopyâExecutive Summary
Section titled âExecutive SummaryâThis research paper evaluates Gallium Arsenide (GaAs) as a versatile alternative to traditional materials (Silicon, Diamond) for mid-infrared (MIR) Attenuated Total Reflection (ATR) spectroscopy, demonstrating its utility in trace-level chemical sensing.
- Core Finding: GaAs ATR waveguides exhibit high efficiency, showing a normalized efficiency approximately 40% higher than the SCD diamond benchmark for sodium acetate detection.
- Application Success: The study successfully demonstrated Surface-Enhanced Infrared Absorption (SEIRA) using immobilized Gold Nanostars (AuNSts) on the GaAs surface for the trace detection of the mycotoxin Aflatoxin B1 (AFB1).
- Signal Amplification: A SEIRA enhancement factor of approximately 4x was achieved, leading to a 14% improvement in the Limit of Detection (LOD) for AFB1 (from 4000 ppb to 3500 ppb).
- Material Comparison: While GaAs offers high efficiency and a wide transparency window (10,000-667 cm-1), diamond remains the superior choice for applications requiring extreme chemical resilience and a wide pH range (1-14).
- 6CCVD Value Proposition: 6CCVD specializes in high-purity Single Crystal Diamond (SCD) with ultra-smooth polishing (Ra < 1 nm) and custom metalization, providing the ideal platform for replicating and significantly extending this high-sensitivity SEIRA research in chemically demanding environments.
Technical Specifications
Section titled âTechnical SpecificationsâThe following hard data points were extracted from the comparative analysis of ATR waveguide materials and the SEIRA detection results.
| Parameter | Value | Unit | Context |
|---|---|---|---|
| SCD Diamond Thickness (Used) | 250 | ”m | ATR Waveguide Disk |
| GaAs Thickness (Used) | 300 | ”m | ATR Waveguide Disk |
| Diamond Refractive Index (n1) | 2.4 | N/A | At 1000 cm-1 |
| GaAs Refractive Index (n1) | 3.3 | N/A | At 1000 cm-1 |
| Diamond pH Range Suitability | 1 - 14 | N/A | Superior chemical resilience |
| GaAs pH Range Suitability | 3 - 12 | N/A | Limited by chemical stability |
| Diamond Penetration Depth (dp) | 0.70 | ”m | At 6 ”m wavelength |
| GaAs Penetration Depth (dp) | 0.73 | ”m | At 6 ”m wavelength |
| Normalized Efficiency (GaAs) | 221 | N/A | 40% higher than Diamond (155) |
| SEIRA Enhancement Factor (AFB1) | Approx. 4 | N/A | AuNSt-modified GaAs vs. bare GaAs |
| LOD (AFB1, SEIRA) | 3500 | ppb | AuNSt-modified GaAs surface |
| AuNSt Average Feret Diameter | 55 ± 30 | nm | Nanostar size for SEIRA |
Key Methodologies
Section titled âKey MethodologiesâThe experiment focused on surface preparation and functionalization of the GaAs ATR crystal to enable SEIRA detection.
- Waveguide Preparation: SCD Diamond (250 ”m), Si (380 ”m), and GaAs (300 ”m) disks were laser-cut. GaAs disks were cleaned via sonication, followed by immersion in concentrated HCl (32%) to remove native oxide.
- Surface Activation: The GaAs surface was activated by depositing 40 ”L of concentrated HCl (32%) for 60 s, followed by rinsing with dry, degassed ethanol to suppress oxide regrowth.
- Linker Immobilization (SAM): A self-assembled monolayer (SAM) was formed by immersing the activated GaAs chips in a 2 mM ethanolic solution of thiol-terminated polyethylene glycol (HS-PEG-SH) for 12 hours. Thiol groups strongly bind to GaAs.
- Plasmonic Nanostar Synthesis: Gold Nanostars (AuNSts) were synthesized using a modified seed-mediated growth protocol (citrate reduction).
- AuNSt Immobilization: The SAM-modified GaAs chips were immersed in the AuNSt suspension for 12 hours, followed by rinsing with water and drying under N2 stream.
- Spectroscopy: Comparative ATR measurements were performed using a Bruker Vertex 70 FT-IR spectrometer equipped with a BioATR II cell and an LN2-cooled MCT detector. Spectra were recorded at 2 cm-1 resolution, averaging 200 scans.
- Analyte Detection: Aflatoxin B1 (AFB1) was dissolved in a 2:1 methanol/chloroform mixture. Solutions were added to the sample tray and allowed to evaporate (10 min) before measurement.
6CCVD Solutions & Capabilities
Section titled â6CCVD Solutions & CapabilitiesâThe research successfully validates the use of semiconductor materials for integrated ATR sensing platforms, particularly highlighting the need for high-quality substrates and precise surface modification for SEIRA enhancement. 6CCVDâs MPCVD diamond materials offer superior performance characteristics essential for advancing this research into robust, real-world applications.
Applicable Materials
Section titled âApplicable MaterialsâTo replicate or extend this research, 6CCVD recommends the following materials:
- Optical Grade Single Crystal Diamond (SCD): Required for high-performance ATR sensing where chemical inertness and physical resilience are paramount (e.g., pH 1-14 environments). SCD provides a wider operational window than GaAs (pH 3-12) and avoids the spectral limitations of Si (vibrational modes around 1200 cm-1).
- Polycrystalline Diamond (PCD): Suitable for large-area, cost-effective ATR platforms, especially where the application requires wafers up to 125 mm in diameter.
- Boron-Doped Diamond (BDD): For electrochemical sensing applications integrated alongside ATR, BDD offers unmatched stability and conductivity.
Customization Potential
Section titled âCustomization PotentialâThe paper utilized specific disk dimensions (250 ”m to 380 ”m thickness) and relied on external synthesis and immobilization of AuNSts for SEIRA. 6CCVD directly addresses these requirements with advanced fabrication services:
| Research Requirement | 6CCVD Customization Capability | Technical Advantage |
|---|---|---|
| Specific Waveguide Geometry | Custom Dimensions and Thickness | We provide SCD wafers from 0.1 ”m up to 500 ”m thickness, and PCD wafers up to 125 mm, allowing precise tuning of the angle of incidence and number of internal reflections (N) to maximize sensitivity. |
| Ultra-Smooth Surface Finish | Precision Polishing (Ra < 1 nm) | Our SCD polishing achieves surface roughness (Ra) < 1 nm, significantly better than standard commercial crystals, which is critical for reproducible SAM formation and uniform plasmonic nanostructure immobilization required for high-fidelity SEIRA. |
| Integrated Plasmonic Structures | In-House Metalization Services | We offer internal deposition of thin films (Au, Pt, Ti, Pd, Cu) directly onto the diamond surface. This capability enables researchers to move beyond solution-based AuNSt immobilization to direct fabrication of highly controlled plasmonic structures (e.g., nanostars or seed layers) for superior and reproducible SEIRA enhancement factors (potentially > 1000x). |
| Global Supply Chain | Global Shipping (DDU/DDP) | We ensure reliable, fast delivery of custom diamond components worldwide, simplifying logistics for international research teams. |
Engineering Support
Section titled âEngineering SupportâThe successful implementation of SEIRA relies heavily on the synergy between the waveguide material, surface chemistry (thiol binding), and plasmonic structure integration.
6CCVDâs in-house PhD team can assist researchers in optimizing material selection for similar Mid-Infrared Chem/Biosensor projects. We provide consultation on:
- Selecting the optimal diamond grade (SCD vs. PCD) based on required transparency and cost.
- Designing custom ATR geometries (thickness and angle) to maximize the evanescent field penetration depth (dp).
- Developing robust metalization schemes for integrated plasmonic enhancement layers.
For custom specifications or material consultation, visit 6ccvd.com or contact our engineering team directly.
View Original Abstract
Abstract During recent years, mid-infrared (MIR) spectroscopy has matured into a versatile and powerful sensing tool for a wide variety of analytical sensing tasks. Attenuated total reflection (ATR) techniques have gained increased interest due to their potential to perform non-destructive sensing tasks close to real time. In ATR, the essential component is the sampling interface, i.e., the ATR waveguide and its material properties interfacing the sample with the evanescent field ensuring efficient photon-molecule interaction. Gallium arsenide (GaAs) is a versatile alternative material vs. commonly used ATR waveguide materials including but not limited to silicon, zinc selenide, and diamond. GaAs-based internal reflection elements (IREs) are a new generation of semiconductor-based waveguides and are herein used for the first time in direct spectroscopic applications combined with conventional Fourier transform infrared (FT-IR) spectroscopy. Next to the characterization of the ATR waveguide, exemplary surface reactions were monitored, and trace-level analyte detection via signal amplification taking advantage of surface-enhanced infrared absorption (SEIRA) effects was demonstrated. As an example of real-world relevance, the mycotoxin aflatoxin B1 (AFB1) was used as a model analyte in food and feed safety analysis.