Near-Edge X-Ray Absorption Fine Structure of Ultrananocrystalline Diamond / Hydrogenated Amorphous Carbon Films Prepared by Pulsed Laser Deposition

The atomic bonding configuration of ultrananocrystalline diamond (UNCD)/hydrogenated amorphous carbon (a-C:H) films prepared by pulsed laser ablation of graphite in a hydrogen atmosphere was examined by near-edge X-ray absorption fine structure spectroscopy. The measured spectra were decomposed with simple component spectra, and they were analyzed in detail. As compared to the a-C:H films deposited at room substrate-temperature, the UNCD/a-C:H and nonhydrogenated amorphous carbon (a-C) films deposited at a substrate-temperature of 550◦C exhibited enhanced π∗ and σ∗C≡C peaks. At the elevated substrate-temperature, the π∗ and σ∗C≡C bonds formation is enhanced while the σ∗C–H and σ∗C–C bonds formation is suppressed. The UNCD/a-C:H film showed a larger σ∗C–C peak than the a-C film deposited at the same elevated substratetemperature in vacuum. We believe that the intense σ∗C–C peak is evidently responsible for UNCD crystallites existence in the film.


Introduction
Ultrananocrystalline diamond (UNCD) films comprising diamond crystallites with diameters less than 10 nm and an amorphous carbon matrix [1,2] have attracted considerable attention from the physical and technological viewpoints because of the following features: (i) some properties resembling those of diamond and sp 3 -rich amorphous carbon, so-called diamond-like carbon (DLC); (ii) smooth surface [3], which is contrastive to that of polycrystalline diamond; (iii) higher temperature stability as compared to that of DLC; (iv) unique optical and electrical properties owing to a large number of grain boundaries in the film [4].Here, the grain boundaries exactly mean the interfaces between UNCDs and between an amorphous carbon matrix and UNCDs.It has been theoretically predicted that a large number of grain boundaries generate additional energy states between the bottom of the conduction band and the top of the valence band of diamond [5].Experimentally, a large absorption coefficient caused by the direct optical bandgap with a value of 2.2 eV that might be due to grain boundaries has been reported [6].As for the electrical properties, the n-type conduction of nitrogen-doped UNCD films is realized by the grain boundary conductions [7].The effects of the grain boundaries on the film properties are hot topics specific to the UNCD films.The local atomic structure in the UNCD films significantly influences their physical properties.However, thus far, there have been a few researches on the local atomic structure [8,9].
Raman spectroscopy has ever been widely utilized for the characterization of carbon-based materials [10,11].However, this technique has the following shortcomings for the structural measurement of mixed sp 2 -and sp 3 -bonded carbon materials.(i) There is the large difference in the Raman cross-section between sp 2 and sp 3 bonds.The Raman cross-section for graphitic features can be up to 50 times compared to that of diamond [12].This results in the extremely large sensitivity to the sp 2 bonds as compared to the sp 3 bonds [13].(ii) Long range ordering factors in the materials indirectly contribute to the spectrum in Raman spectroscopy.Since the wavelength of an incident probe beam is on the order of micrometers, the sensitivity is strongly dependent on the wavelength of the incident beam and the crystallite size in materials.The detectable crystallite size has a critical value that depends on the wavelength of the incident beam.An ultraviolet incident beam is required for the detection of nanocrystalline diamond.From these shortcomings, for amorphous or nanocrystalline carbon wherein sp 2 and sp 3 bonds coexist, the qualification of the sp 2 and sp 3 bond fractions in their mixed materials is difficult to be reliably performed.
On the other hand, Near-edge X-ray absorption fine structure (NEXAFS) spectroscopy can overcome these problems.This spectroscopy probes the final-state wave function near the excited atom wherein transitions from the C 1s core level to the unoccupied states area are caused by X-ray photon absorption.The dominant interaction terms could be approximated to those between the localized bonds.Thus the NEXAFS spectrum can roughly be decomposed with the simple component spectra [14,15].
NEXAFS spectroscopy has been successfully utilized for investigating the local bonding configurations within amorphous and nanocrystalline carbon.For amorphous carbon films, the spectrum has been decomposed into the components spectra originating from π * and σ * bonds, and the bonding configuration structure has been studied in detail [16,17].The sp 2 fraction estimation method that has been established in electron energy loss spectroscopy (EELS) is applicable to the NEXAFS measurement [18,19].For hydrogenated amorphous carbon [15,20] and nitrogendoped amorphous carbon films [21,22], the bonding configurations of hydrogen and nitrogen atoms with carbon atoms have been investigated.For UNCD films, on the other hand, there have been a few researches thus far [9,23].
As above-mentioned, the UNCD films have the complicated structure including the grain boundaries, which must have a significant role on the unique physical properties of the UNCD films.In this paper, we discuss on the atomic bonding configuration of UNCD/hydrogenated amorphous carbon (a-C:H) composite films by analyzing the NEXAFS spectra.Here we call our films "UNCD/a-C:H" films since UNCD crystallites are exactly surrounded by an a-C:H matrix.The peak decomposition of the measured spectrum revealed the specific bonding configuration to the UNCD/a-C:H films.

Experimental
UNCD/a-C:H films were deposited on Si substrates at a substrate-temperature (T s ) of 550 • C and an ambient hydrogen pressure (P H2 ) of 53.3 Pa by pulsed laser deposition (PLD) using a graphite target.The detailed conditions have been indicated in our previous paper [6].For comparison, Photon energy (eV) Intensity (a.u.) NEXAFS was performed on the UNCD/a-C:H, a-C:H, and a-C films with the same thickness of 100 nm.NEXAFS spectra were taken in a total electron yield mode by measuring the current from the sample to ground using synchrotron radiation at the beam line 12 of SAGA Light Source.Highly  (a)  [26] 286 (b)  [15] 287.5 (a) [26] 289.3 (c) [24] 292.7 (d) [22] position 284 (b) [27] 287 (b) [27] 284.6 (b)  [15] (a) UNCD/a-C:H, (b) a-C, (c) nanocrystalline diamond (NCD), and (d) amorphous nitrogenated carbon.oriented pyrolytic graphite (HOPG) was used as a sample reference for the energy calibration.

Results and Discussion
The NEXAFS spectra of the UNCD/a-C:H film deposited at T s = 550 • C and P H2 = 53.3Pa, a-C film deposited at T s = 550 • C in vacuum, and a-C:H film deposited at T s = RT and P H2 = 53.3Pa are shown in Figures 1(a), 1(b), and 1(c), respectively.These spectra were normalized in intensity at 330 eV.The spectra consist of two major peaks.One peak centered at 285 eV is due to the π * bonding structure.The other peak positioned between 290 and 295 eV originates from the σ * bonding structure.In the photon energy range between 285 and 290 eV, there is the obvious difference in the spectrum between the films.This implies that several peaks are superpositioned in this range.
The sp 2 fraction was estimated using the following equation: where I π * sam and I π * ref are the area of the π * peak of the samples and reference HOPG, respectively, and I sam (ΔE) and I ref (ΔE) are the areas calculated under the remainder of each spectrum from 288.6 eV to 315 eV [24].The sp 2 fractions were estimated to be 58%, 67%, and 40% for the UNCD/a-C:H, a-C, and a-C:H films, respectively.The a-C:H film deposited at T s = RT shows the less sp 2 fraction as compared to the others deposited at T s = 550 • C. The sp 2 fraction of the UNCD/a-C:H film is smaller than that of a-C film, which might be mainly because of the existence of UNCD crystallites in the UNCD/a-C:H film.
In order to analyze the atomic bonding configuration in detail, the NEXAFS spectra were decomposed into component peaks.The decomposition was carried out based on the following assumptions.(a) The peak profile is predominantly responsible for the wave function; it cannot be dramatically changed between the similar kind of materials.(b) The peak profiles of the fitting component spectra were supposed to be approximately uniform between the samples.(c) The peak position was also almostly fixed because the bond length does not dramatically differ from those of similar materials.
The decomposed spectra of the UNCD/a-C:H, a-C, and a-C:H films are shown in Figures 1(a), 1(b), and 1(c), respectively.The integral spectrum resulting from the decomposed component spectra is shown in red solid line.The jump of the C K-edge at the ionization potential was fitted with an error-function step, and the subtracted spectra are decomposed into Gaussian peaks [14,25].The spectra were decomposed with the component spectra due to π * C=C, σ * C-H, π * C≡C * , σ * C-C, σ * C=C, and σ * C≡C.Here, the a-C film showed the obviously wide π * C=C peak as compared to the other films.The π * C=C peak of the a-C film was decomposed into two.This implies that the bonds with different bond lengths coexist in the a-C film.Table 1 shows the peak position and full width at half maximum (FWHM) of the decomposed component spectrum.
There are two obvious differences between the UNCD/a-C:H and a-C films.One is that the σ * C≡C weakens and the σ * C-H peak appears for the UNCD/a-C:H film.The σ * C≡C bonds are partially replaced with the σ * C-H bonds by hydrogenation.The other is that the UNCD/a-C:H shows the greater σ * C-C peak than that of the a-C film.In the spectrum of the UNCD/a-C:H film, the σ * C-C peak is relatively intense as compared to the σ * C-H and π * C≡C peaks.This is different from the other spectra.And the UNCD crystallites were confirmed by TEM only in the UNCD/a-C:H film.We believe that the relatively intense σ * C-C peak is responsible for the existence of UNCDs.
In comparison with the a-C:H film, there are the following differences.(i) The UNCD/a-C:H film including the a-C film deposited at T s = 550 • C showed stronger π * and σ * C≡C peaks than the a-C:H film deposited at T s = RT.The C≡C bonds formation is enhanced at the elevated T s .
(ii) The a-C:H film showed intense σ * C-H and σ * C-C peaks instead of the π * and σ * C≡C peaks being weakened.Since the a-C:H film was deposited at T s = RT, the intense σ * C-H peak might result from a large amount of hydrogen in the film as compared to the UNCD/a-C:H film.At T s = RT, the σ * C-C bonds, instead of the C≡C bonds, were preferentially formed in the amorphous film.

Conclusion
The chemical bonding structure of the ultrananocrystalline diamond/hydrogenated amorphous carbon composite films prepared by pulsed laser deposition was investigated by nearedge X-ray absorption fine structure spectroscopy.The ultrananocrystalline diamond/hydrogenated amorphous carbon and amorphous carbon films deposited at the substratetemperature of 550 • C in 53.3 Pa hydrogen atmosphere and vacuum showed larger π * and σ * C≡C peaks than that of hydrogenated amorphous carbon film deposited at room substrate-temperature in 53.3 Pa hydrogen atmosphere.The ultrananocrystalline diamond/hydrogenated amorphous carbon composite films showed a greater σ * C−C peak as compared to the amorphous carbon film.This is evidently responsible for the existance of ultrananocrystalline diamond crystallites in the film.

Figure 1 :
Figure 1: NEXAFS spectra, which are decomposed with an errorfunction step and Gaussians, of (a) UNCD/a-C:H film deposited at T s = 550 • C and P H2 = 53.3Pa, (b) a-C film deposited at T s = 550 • C in vacuum, and (c) a-C:H film deposited at T s = RT and P H2 = 53.3Pa.

Table 1 :
Comparison of the peak positions and FWHMs of the decomposed component spectra of the UNCD/a-C:H films deposited at T s = 550 • C and P H2 = 53.3Pa, a-C films deposited at T s = 550 • C in vacuum, a-C:H films deposited at T s = RT and P H2 = 53.3Pa in this study, and carbon films in previous reports.