Effect of Nano-Ni Catalyst on the Growth and Characterization of Diamond Films by HFCVD

1 Department of Chemistry and Biochemistry, Northern Illinois University, Dekalb, IL 60115, USA 2 Chemical Vapor Deposition Division, Research and Development Department, Kinik Company, No. 64, Chung-San Road, Ying-Ko, Taipei 239, Taiwan 3 Department of Chemistry, Tamkung University, Tamsui, Taipei 239, Taiwan 4 Department o f Mechanical Engineering, Northern Illinois University, Dekalb, IL 60115, USA


Introduction
Recently, Ni nanoparticles were utilized for catalytic synthesis of carbon nanotube (CNT) by CVD because the graphitic layer covering Ni nanoparticles would transform into CNT under a controlled temperature [1,2].On the other hand, the catalytic effect of Ni also was illustrated to inhibit CVD diamond nucleation and growth, though diamond may nucleate and grow on the amorphous carbon or graphitic interlayer initially formed on Ni in a low-pressure methanehydrogen environment [3][4][5][6][7].Consequently, under a wellcontrolled growing condition, CNT and diamond can be simultaneously and selectively grown on Ni-coated and diamond powder-seeded Si substrates [8].
However, diamond powders and/or mixed with transition metal powders of size in micron scale (Ni of 3 ∼ 5 μm in size, Ti, Cu, Fe, etc.) have also been processed as catalysts to enhance the initial nucleation density for diamond film growth by ultrasonication seeding and HFCVD [9,10].The catalytic enhancement was explained to result from the dissociative chemisorption of CH 4 at the Ni catalytic sites.Since the catalytic effect of Ni nanoparticles is expected to improve substantially due to higher surface to volume ratio as compared with Ni particles in micron size, the Ni nanoparticles would be an ideal catalytic seeds for the growth of nanocrystalline diamond (NCD) film by HFCVD due to the enhanced nucleation density with the assistance of Ni nanoparticles.The as-grown NCD films are expected to have a better surface roughness (Ra) as compared to microcrystalline diamond films due to smaller crystallites.The NCD films with better Ra would have many potential applications such as cutting tools.

Diamond Powder Solution.
First, 0.37 wt% of PVP was added into a 46 g EG solution and mixed by a magnetic stirrer for at least 1.5 hours.Second, 1 wt% of fluorosurfactant (FS, Dupont FS-510) was added into the PVP/EG solution and mixed for 30 minutes.Third, 0.005 wt% of nanodiamond seed (ND, Carbo-Tec Dynaget M3D) or diamond powder (NDP, SP-DP Maple Canada Group) was added into the FS/PVP/EG solution and well dispersed by a sonication dismembrator (Fisher Scientific Model-100) with 15 W power for 30 seconds.Res.Inc.) using as-prepared nano-Ni/EG and diamond powder/EG solutions.The modified Si substrates were used for diamond films deposition by HFCVD system (Sp3 Inc., Model 500).The deposition pressure was controlled at 18 ∼ 20 Torr, containing a gas mixture of 2.3 vol% methane (99.995% purity) and hydrogen (99.95% purity) under a total flow rate of 3000 sccm.The distance between the tungsten (W) filament and Si substrates was adjusted at 10 mm.The W filament temperature was controlled at 2000 ∼ 2200 • C. The temperature of Si substrates was about 650 ∼ 700 • C in the seeding stage and about 800 ∼ 850 • C in the growing stage.In this work, an initial nucleation time of 1.5 hours and a total deposition time of 48 hours were employed.

Size Distribution of Nano-Ni Catalysts with PVP in EG
by DLS and AFM.DLS results indicated a mean diameter of around 50 nm (Figure 1) for both 1 mM (dash blue) and 2 mM (solid red) of nano-Ni catalysts with 0.37 wt% PVP in EG solutions.In the insert of Figure 1, the AFM morphology of the nano-Ni catalysts spun-coated on Si substrate also showed the particle size around 50 ∼ 70 nm.Those nano-Ni catalysts were used to assist diamond film growth on Si by HFCVD.
Journal of Nanomaterials

Raman Spectra of Diamond
Films.Molecular properties of both front and backsides of diamond films (CVDD-ND: blue; CVDD-Ni: red; CVDD-NDP: green) were examined by Raman scattering as shown in Figure 3, where the backside gave the chemical characteristics of the diamond film at the initial growth stage.An amorphous carbon phase starts to form at the initial stage as observed by a small broad band at 1520 cm −1 on the backside (the insert in Figure 3(b)).This is then transformed to give the diamond structure as displayed by a small peak around 1334 cm −1 on the front side (the insert in Figure 3(a)).In all cases, the crystalline diamond peak at 1334 cm −1 is only weakly observed, suggesting the % and size of diamond in film composition is low and small.From Figure 3(a), Raman spectra of front-sides of diamond films indicate a better crystalline diamond feature in CVDD-ND than the others as evidenced by XRD in Figure 2. Contrarily, the backside of CVDD-Ni diamond film shows the most amorphous feature at the early growth.
All spectra showed an intense band at 2270 cm −1 and a shoulder band at 2800 cm −1 , which have been attributed to several origins: (1) a photoluminescence peak or a color center due to silicon defects in diamond films [11,12], and (2) the combinational overtone Raman band of a short conjugation chain of transpolyacetylene (t-PA), which may grow around the grain boundaries or surface of diamond films [13][14][15][16][17].The origin (2) is preferred since these Raman bands have also been observed for the diamond films grown on nonsilicon substrates [18].Moreover, in Figure 3, the intensity of 2270 cm −1 peak is even stronger on the front than on the backsides of diamond films grown on Si by HFCVD.This indicates that the 2270 cm −1 peak is not due to the Si substrate defects, but the t-PA formed around the grain boundaries or surface of diamonds.It is worthwhile to mention that the backside of diamond film catalyzed by nano-Ni (CVDD-Ni, red) gives the lowest peak intensity of Raman band at 2270 cm −1 , indicating that the formation of t-PA is low at the initial growth of diamond phase due to nano-Ni catalysts.This may come from the catalytic hydrogenation that converts t-PA into methylene-like chains in the presence of nano-Ni catalysts.1800 cm −1 ) and carbon dioxide (2350 cm −1 ) is noticed in the spectra [19].The CH and CC vibrational features of diamond films are located in the spectral ranges of 2600 cm −1 to 3200 cm −1 and 650 cm −1 to 1400 cm −1 , respectively, as shown in Figure 5.

ATR/FT-IR Spectra of Diamond Films
In Figure 5(a), the sp 3 -CH 2 asymmetric and symmetric vibrations at 2918 cm −1 and 2850 cm −1 are distinctively observed in CVDD-Ni diamond films.Meanwhile, the additional sp 3 -CH vibration at 2894 cm −1 is found in CVDD-NDP diamond films [20].The incorporation of more hydrogen in CVDD-Ni diamond films may come from the catalytic dissociation of hydrogen which leads to hydrogenation of t-PA into methylene-like chains in the presence of nano-Ni, as evidenced by Raman spectra in Figure 3. CVDD-NDP-Ni diamond films indicate weak combinational features of CVDD-Ni and CVDD-NDP diamond films.However, it is almost featureless for CVDD-ND diamond film in the same CH vibrational region of 2600 cm −1 to 3200 cm −1 .These spectroscopic observations might imply a better crystallinity of diamond crystals with less hydrogen included in CVDD-ND diamond films as evidenced by XRD in Figure 2. In Figure 5(b), the similar C-C stretching features between 1000 ∼ 1400 cm −1 and C-H bending characteristics of disubstituted alkene-like structure around 870 cm −1 are observed in those diamond films [21].

X-Ray Photoelectron Spectroscopy of Diamond Films.
In Figure 6, the XPS C 1s spectra of CVDD-NDP, CVDD-ND, CVDD-Ni, and CVDD-NDP-Ni diamond films (from bottom to top) are deconvoluted by Gaussian multiple peaks fitting into two main components with the binding energies (BE) of 281.2 ∼ 281.3 eV (for sp 2 carbon bonding) and 281.6 ∼ 284.0 eV (for sp 3 carbon bonding).Interestingly, the measured C 1s binding energies of our diamond films are relatively lower than the accepted 284.4 eV for diamond [22], after a calibration by referring to Cu 2p 3/2 peak in XPS spectra.
The highest binding energy of sp 3 -C 1s peak and narrower distribution of CVDD-NDP diamond film suggest more sp 3 -C bonding present in the diamond film surface than the others.On the other hand, the binding energies of sp 2 -C 1s peaks are similar to all diamond films.However, the stronger sp

Effect of Nano-Ni on the Growth Mechanism of Diamond Films.
When CH 4 /H 2 gas mixtures were used for diamond film growth by HFCVD, the dominant mechanistic species above H-terminated diamond growing surfaces were observed to be methyl radicals and acetylene [7].The CH 4 generates methyl radical and H 2 provides the H source for hydrogen abstraction on diamond growing surfaces.The growth of diamond crystals proceeded through the successive addition of methyl radicals and followed by a C-C bridging of two CH 3 • bonded on adjacent carbon radical sites of diamond growing surface after hydrogen abstraction.Therefore, the concentration ratio of reactive species H/CH 3 • plays several key roles in the growth of CVD diamond films.For example, H can be responsible for (1) activation of H-terminated diamond growing surfaces by H-abstraction, (2) etching or suppression of graphite and disordered carbons phases, (3) saturating dangling bonds on the diamond growing surfaces, and (4) incorporation into diamond films leading to the formation of amorphous hydrogenated carbon a-C:H or t-PA phases in diamond films [13][14][15][16][17].
The above mechanism is in agreement with of our experimental results of diamond films assisted by nanocatalysts and HFCVD using CH 4 /H 2 gas mixtures.The compositions of diamond films show (1) crystalline diamond and amorphous (locally mixed sp 3 /sp 2 -C configuration) carbon phases and (2) sp 2 -bonded carbon clusters (short chain polyethylene such as t-PA in diamond films).Particularly, in our nano-Ni assisted growth of diamond films, the enhanced formation of  H atoms by a catalytic dissociation of H 2 on nano-Ni surface can subsequently activate the hydrogen abstraction reaction, saturate the dangling bonds on the diamond growing surfaces, and hydrogenate t-PA into methylene-like phases.This growth mechanism would lead to carbon deposition surrounding nano-Ni catalysts through the initial formation of glassy or amorphous carbons overlayers and followed by the nucleation of diamond crystals [3].Meanwhile, the formation of the amorphous carbon layer on nano-Ni catalyst surface may passivate the possible conversion of diamond to graphite in the presence of nano-Ni.From there on, new diamond nuclei grew on amorphous carbon layer, leading to the formation of spherical cauliflower-like microcrystalline diamond particles prior to coalescence into continuous diamond films.

Conclusions
We have demonstrated the growth of diamond films assisted by ND, nano-Ni, NDP, and nano-Ni/NDP mixture by HFCVD using CH 4 /H 2 gas mixtures.The growth of diamond films assisted by nanocatalysts has followed the CH 3 • addition mechanism which leads to the observed spectral characteristics of diamond, amorphous carbon, and t-PA phases.The initial growth of diamond films seems to start with the 220 (or 110 ) facets which leads to the formation of diamond films of 111 pyramidal faceted diamond crystals at the end as shown by XRD.The TEM and SEM results indicated that diamond crystals were grown directly on both large NDP and small ND seeds, but the latter displayed a better crystallinity of diamond films.The Raman spectral band at 2270 cm −1 has been verified to originate from the short chain t-PA formed in diamond films.
It was illustrated that nano-Ni nanocatalysts can promote the formation of amorphous carbon but suppress t-PA phases at the initial growth of diamond films.The subsequent nucleation and growth of diamond crystals on the amorphous carbon layer leads to generation of the spherical diamond particles and clusters prior to coalescence into continuous diamond films.Moreover, a 36% reduction

2. 1 .
Preparation of Nanocatalysts 2.1.1.Nano-Ni Catalysts.Nano-Ni catalysts were prepared by a chemical reduction method and stabilized in ethylene glycol (EG) solutions containing poly(vinylpyrrolidone) (PVP) polymer.First, a 10 mL of 10 mM NaH 2 PO 2 • H 2 O (J. T. Baker 3740) in an EG solution and a 10 mL of 4 mM NiSO 4 • 6H 2 O (Fisher N73-100) in an EG solution were separately and freshly prepared and then well mixed by a magnetic stirrer for 30 minutes.Second, 0.37 wt% of PVP (SIGMA-ALDRICH 437190-500G) powder was added into the NaH 2 PO 2 • H 2 O/NiSO 4 • 6H 2 O/EG solution and mixed for at least 1.5 hour.Third, the final solution was heated under microwave (2.45 GHz, 900 W) for 3 minutes until the color of the solution changed from light green to black brown, indicating the formation of nano-Ni particles in solutions.

Figure 1 :
Figure 1: Particle size distribution by DLS and AFM of nano-Ni catalysts.

Figure 2
shows XRD patterns of the front-sides of diamond films (catalyzed by ND: blue; nano-Ni: redl; NDP: green; and a mixture of nano-Ni/NDP-brown), which are labeled as CVDD-ND, CVDD-Ni, CVDD-NDP, and CVDD-NDP-Ni, respectively.The XRD spectra (Figures 2(a), 2(b), and 2(c)) display three characteristic peaks around 44.3• , 75.6 • , and 91.8 • in 2θ, which are assigned to 111 , 220 , and 311 lattice planes of diamond crystals.The (220) reflection being considerably more intense than (111) reflection suggests the formation of pyramidal faceted diamond crystals as observed by SEM in Figure8.The XRD, for the backsides of diamond films are not presented here.Both 111 and 311 planes are not observed.However, a small bump around 75.5 • ( 220 plane) in 2θ of diamond crystal seems to come out at the initial growth stage.This observation may suggest the phase transition from the nondiamond to diamond structures during the growth of diamond films.As shown in Figure2, the crystallinity of CVDD-ND diamond films is better than the others.However, the crystallinity of CVDD-NDP-Ni diamond film is extremely weak.This may come from Ni catalytic graphitization of diamond powders, which facilitates the formation of amorphous phase in the early growth stage of diamond film, as shown to give an almost featureless XRD in Figure2.All XRD peaks observed in Figure2are quite broad (FWHM 0.3 • ∼ 0.5 • in 2θ), suggesting the formation of small diamond crystals.The observed XRD peaks displayed peak shifts of 2θ about 0.3 • ∼ 0.4 • relative to the corresponding theoretical positions that may imply the inhomogeneous stresses existed in diamond films.In Figure2(b), the peak shoulders around 75.8 • in 2θ can possibly result from the stacking faults.As in Figure 2(a), a shoulder around 44.3 • in 2θ for 111 peak for nano-Ni-assisted diamond films (CVDD-Ni) may be due to the lattice mismatching between diamond crystals and nano-Ni catalysts.

Figure 3 :
Figure 3: Raman spectra of front surfaces (a) and backsides (b) of diamond films.