Diamond films are used in several fields such as biosensors, electrochemical sensors, and ion-sensitive field effect transistors. The diamond’s surface termination impacts its properties. For instance, oxygen-terminated and hydrogen-terminated diamond surfaces act as electrical insulators and p-type conductors, respectively. This switch between oxygen termination and hydrogen termination at the surface is both reversible and manageable.
Oxygen-terminated diamond surfaces are manufactured using methods including, but not restricted to, contact with boiling oxidizing acid or oxygen plasma, UV/ozone treatment, or cathodic treatment.
Hydrogen-terminated surfaces have traditionally been manufactured using monatomic hydrogen, with one of the following:
- Hot filament methods can cause contamination of the diamond film via deposition of material from the filament.
- Plasma methods can lead to unnecessary etching of the diamond film.
- Electrochemical methods can only be applied to realize hydrogenation of electrically conductive diamond films.
Diamond film hydrogenation
Using molecular hydrogen, diamond film hydrogenation has also been obtained by exposing diamond surfaces to H2 at high temperatures (>500ºC) under vacuum conditions. Conversely, the operational constraints of high vacuum restrict the degree to which hydrogenated diamond surfaces can be manufactured on an industrial scale through this method.
In association with Hasselt University, Delft University of Technology have attempted the hydrogenation of chemical vapor deposited (CVD) diamond films at atmospheric pressures using H2 to enhance the viability of manufacturing hydrogenated-diamond films on an industrial scale.
X-ray photoelectron spectroscopy (XPS), carried out using the Thermo Scientific™ K-Alpha+™ XPS system, was applied to compare the surface chemical composition of the diamond film hydrogenated using molecular hydrogen (H2-treated) with the diamond film hydrogenated utilizing a more traditional monatomic hydrogen plasma method (H plasma-treated). For reference purposes, spectra from a diamond film oxidized using UV/ozone were also obtained.
In this analysis, oxidation of the diamond films was performed using UV/ozone treatment, under ambient conditions for about 4 hours. Diamond film hydrogenation by H-plasma treatment happened in a plasma reactor at approximately 700ºC and 3500 W for a period of 5 minutes, hydrogenation by exposure to a H2 flow was conducted in a non-plasma quartz tube, reactor heated at about 850°C for a period of 20 minutes at ambient pressure and subsequently cooled to room temperature, as the flow of H2 was maintained.
Undoped nanocrystalline diamond films are grown on quartz using the CVD technique and subjected to ambient conditions for several days. C 1s core level and valence band spectra of these diamond films were obtained by utilizing Al Ka X-rays with a spot size of 400 µm, together with the patented K-Alpha+ charge compensation that was used during all spectral acquisition. C 1s core level spectra were obtained at a pass energy (PE) of 50 eV, with a step size of 0.1 eV and using 10 repeat scans. Furthermore, valence band spectra were obtained from undoped nanocrystalline diamond films after oxidation by UV/ozone treatment and then hydrogenation by exposing to H2.
Thermo Scientific™ Avantage™ software was used to process and analyze the acquired XPS spectra. Background correction of the C 1s core level spectra employed the “Smart” base line subtraction function and the rectified profiles fitted to 70/30 Gaussian/Lorentzian convolution peak(s).
On qualitative comparison of the fitted C 1s spectra, a clear resemblance was seen between the hydrogenated diamond films grown by H-plasma treatment (Figure 2) and H2-treatment (Figure 3), while the UV/ozone treated film spectrum (Figure 1) had a different appearance with fitted parts due to oxidized carbon at ~286.5 and ~288 eV, probably C-O and C=O, respectively. At 284 and 285 eV, fitted peaks indicating C-C bonding and called as “Peak A” and “Peak B,” were seen in all three samples.
Figure 1. UV/ozone-treatment C 1s scan. 10 scans, 1m 35.5s, 400 µm, CAE 50, 0.10 eV.
Figure 2. H-plasma treatment C 1s scan. 10 scans, 1m 35.5s, 400 µm, CAE 50, 0.10 eV.
Figure 3. H2-plasma treatment C 1s scan. 10 scans, 1m 35.5s, 400 µm, CAE 50, 0.10 eV.
Varying ratios of Peak A to Peak B between the hydrogenated diamond films (H plasma-treated and H2-treated, Figure 2 and Figure 3) and the oxidized diamond film (UV/ozone-treated, Figure 1) were linked to an increase in the energy barrier for electron emission in the oxidized sample, This was done by researchers who developed the H2-treated films.
In the C 1s spectra of the hydrogenated films, a small oxidized part continues to be present and this could be due to incomplete oxidation of the diamond on exposure of the films to surrounding conditions. Moreover, the occurrence of a small Peak A part in the oxidized film spectrum shows that complete oxidation does not happen, probably due to roughened areas that are not completely exposed to ozone during treatment, while the opposite is true in the case of hydrogenated films.
Valence band spectra of hydrogenated and oxidized films, undoped nanocrystalline diamond films (Figure 4 and Figure 5) were also obtained and qualitatively compared.
Figure 4. Valence brand spectra of UV-treatment (red) H-plasma treated (green) 175 nm thick diamond films. 50 scans, 70 m 20.0s, 400 µm, CAE 100, 0.20 eV.
Figure 5. Valence brand spectra of UV-treatment (red) H-plasma treated (green) 115 nm thick diamond films. 50 scans, 70 m 20.0s, 400 µm, CAE 100, 0.20 eV.
It was noticed that spectra from the hydrogenated films migrated to a lower binding energy by about 1.0-1.5 eV when compared to the oxidized films. A reduction in the intensity of the peak at about 25 eV - assigned as photoemission signal from the Oxygen 2s orbital by the researchers who conducted this study - is found in the hydrogenated films.
With the aid of XPS technique, the surface chemical composition of diamond films hydrogenated using a new, ambient pressure treatment comprising H2 gas, was effectively studied. Comparisons with a hydrogenated film developed through a traditional H-plasma treatment were then made. Further, comparison with UV/ozone treated diamond films shows that oxidized carbon species are either eliminated and/or decreased during H2-treatment. There were no inconsistencies in the C 1s core level spectra of these films and this signifies the validity of H2-treatment as a method for diamond surface hydrogenation that can be conducted at ambient pressure. The band bending effect was also noticed within the valence band spectra.
This information has been sourced, reviewed and adapted from materials provided by Thermo Scientific – X-Ray Photoelectron Spectroscopy.
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