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Diamond nanorods from nanocrystalline diamond films

Sobia Allah Rakha, Yu Guojun �, Zhou Xingtai, Ishaq Ahmed, Dezhang Zhu, Jinlong Gong

Shanghai Institute of Applied Physics, Laboratory of Nuclear Analysis Techniques (LNAT), Chinese Academy of Sciences, Shanghai 201800, People’s Republic of China

a r t i c l e i n f o

Article history:

Received 7 November 2008

Accepted 25 March 2009

Communicated by M. SkowronskiAvailable online 1 April 2009

PACS:

68.65.–k

81.15.Gh

68.37.–d

81.07

68.55

Keywords:

A1. Electron Microscopy

A2. Hydrogen plasma etching

A3. Hot filament CVD

B1. Nanocrystalline diamond

B1. Diamond nanorods

a b s t r a c t

Diamond nanorods (DNRs) have been prepared by hydrogen plasma post-treatment of nanocrystalline

diamond films in radio-frequency (RF) plasma-assisted hot-filament chemical vapor deposition. Single-

crystal diamond nanorods with diameters of 3–5 nm and with lengths up to 200 nm grow under

hydrogen plasma irradiation of nanocrystalline diamond thin film on the Si substrate at high

temperatures. The DNRs growth occurs from graphite clusters. The graphite clusters arises from the

etching of diamond carbon atoms and from the non-diamond phase present in the parent film. The

graphite clusters recrystallized to form nanocrystalline diamonds which further grow for diamond

nanorods. The negative applied bias and surface stresses are suggested to support one-dimensional

growth. The growth direction of diamond nanorods is perpendicular to the (111) crystallographic

planes of diamond. The studies address the structure and growth mechanism of diamond nanorods.

& 2009 Elsevier B.V. All rights reserved.

1. Introduction

Recent strong scientific and technological interest in nanos-tructured carbon materials, such as nanotubes and nanowires, hasbeen motivated by the diverse range of physical properties thesesystems exhibit and also their unusual quantum properties andpotential applications as nanoconnectors and nanoscale devices.The production of CNT and other sp2-bonded nanocarbons hasbeen successfully realized by a variety of techniques, the synthesisof diamond nanowires (nanorods) of nanometer diameters(the one-dimensional sp3-configured analogue of CNT) turnedout to be much more difficult. Using ab initio calculations, thefundamental stability of diamond nanowires (DNW) was exploreddepending on size and crystallographic direction, [1,2] and thisobject was predicted to be stable (energetically favored) atdiameters ranging from 2.7 to 9 nm. The DNWs are expected topossess unique mechanical and electronic properties, in particu-lar: the predicted brittle fracture force and zero strain stiffness forDNW with radii greater than about 1–3 nm exceed those for CNT[3]. The electronic structure of DNW has been theoreticallyconsidered by Barnard et al. [4]. They find that the band gap of

diamond nanowires is narrower than that of bulk diamond;moreover it varies with the surface morphology, diameter and theorientation of the principle axis. It has been predicted that theiruse in structural applications [5] make them a potentially usefulcommodity.

Experimentally, Aligned diamond nanowhiskers have beensuccessfully formed using air plasma etching of polycrystallinediamond films [6]. Dry etching of the diamond films withmolybdenum deposits created well-aligned uniformly dispersednanowhiskers up to 60 nm in diameter with a density of 50/Am2

[7]. Diamond nanocylinders with a diameter of �300 nm havebeen synthesized on anodic alumina template by microwaveplasma-assisted chemical vapor deposition and by nucleatingwith 50 nm diamond particles at the bottom of membrane holes[8] although no crystalline orientation were observed. Nanorodsof single-crystalline diamond with a diameter of 50–200 nm werefabricated by a microfabrication method combining a microwaveplasma treatment with a reactive ion etching method [9]. Furtherprogress in this field were made recently, DNWs were observed byGruen et al. [10] in ultrananocrystalline diamond (UNCD) filmsgrown in Ar-rich microwave plasma with nitrogen added in theprocess gas. Vlasov et al. [11] reported on the synthesis of hybriddiamond–graphite nanowires produced by microwave plasmachemical vapor deposition (MWCVD). Carbon nanotube andnanowire growth from spherical carbon nanoparticles, without

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Journal of Crystal Growth

0022-0248/$ - see front matter & 2009 Elsevier B.V. All rights reserved.

doi:10.1016/j.jcrysgro.2009.03.035

�Corresponding author. Tel./fax: +86 2159552539.

E-mail address: yuguojun@sinap.ac.cn (G.J. Yu).

Journal of Crystal Growth 311 (2009) 3332–3336

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any metal catalyst was reported by Botti et al. [12]. Their resultsshowed that instead of metal catalysts, amorphous, largelyhydrogenated carbon nanoparticles were the special and effectivecatalyst for nanowire or nanotube growth. Our group reported[13] on the synthesis of single-crystalline high-aspect-ratioDNWs (those with a high ratio of length to diameter) with adiameter of 4–8 nm and lengths of several hundred nanometersby a prolonged (20 h) treatment of CNTs in a microwave hydrogenplasma. In that case, the DNWs turned out to be covered byamorphous carbon.

One-dimensional diamond–graphite nanostructures are ofinterest for the study of their physical and chemical properties,which can differ from those of bulk diamond and graphite. In thiswork, we report on the synthesis of diamond nanorods (DNRs) byhydrogen plasma post-treatment of nanocrystalline diamondfilms. These nanostructures consist of single-crystalline diamondwith diameters of 3–5 nm and with lengths up to 200 nm.The growth mechanism along with microscopic studies is carriedout here.

2. Experimental details

The nanocrystalline diamond thin (500 nm) films weredeposited on Si substrates in a hot-filament chemical vapordeposition (HFCVD) reactor in CH4/H2 mixtures with 1.5% CH4 at750 1C and at a pressure of 25 Torr. The Si wafer was scratchedwith diamond paste to facilitate the deposition of diamondparticles. The as-prepared sample of NCD film is then placed intothe radio-frequency plasma-enhanced chemical vapor deposition(PECVD). Hydrogen with a flow rate of 50 sccm was introducedinto the reaction chamber to give a dynamic hydrogen pressure of3 Torr (400 Pa). In order to activate and dissociate hydrogen gas,the filament temperature was set at 2200 1C. Plasma with a powerof 250 W was initiated at a substrate temperature of 800 1C(1073 K). The activated atomic hydrogen reacted with substrate togive volatile reaction products that in turn reacted on the sample.After reaction for 11 h, the specimens were cooled down invacuum. Scanning electron microscope (SEM, LEO-1530VP),high-resolution transmission electron microscope (HRTEM, JEOLJEM-2011 operated at 200 kV) were used to examine the samples.The specimens for transmission electron microscope (TEM)observations were prepared by scratching the deposited samplewith a tweezer, and putting them on a copper grid.

3. Results and discussion

Scanning electron microscope (SEM) image (Fig. 1(a)) showsthe morphology of thin nanocrystalline diamond film. MicroRaman spectrum of this film in Fig. 1(b) shows peaks at 1136, 1331,1477 and 1554 cm�1. The peak at 1332 cm�1 is indicativeof CVD diamond. A peak at 1554 cm�1 is referred as G peak,which is a characteristic of sp2 carbon [14]. The peaks at1136 cm�1 and 1477 cm�1 are observed in nanodiamond thinfilms and can be a convenient probe for nanodiamond thinfilms [15]. A SEM image in Fig. 2(a) shows the sample afterhydrogen post-plasma treatment for 11 h. A high-resolutionSEM image (Fig. 2(b)) shows that the nanorods with thediameters less than 10 nm and lengths greater than 200 nmwere formed on �100-nm-diameter-sized rods. The sample wasobtained after hydrogen plasma treatment of nanocrystallinediamond film at 800 1C. Observations of the HRSEM imageindicate small particles on the thick rod and rods are emergingfrom these particles. These are diamond nanoparticles asconfirmed by HRTEM.

Fig. 3 shows transmission electron microscopy image of therod formed. The nanorod has a diameter of 3–5 nm and withlength up to �200 nm. To unravel the structure of the nanowiresHRTEM has been performed. HRTEM demonstrates a cleardifference between the two types of structures as identified bySEM (Fig. 2). Fig. 4 shows the presence of two different crystallinestructures: diamond nanorod structure (characterized byinterplanar distance d111=0.205 nm) and graphite structurecharacterized by d0001=0.334 nm. It can be seen from the figurethat the rod is emerging from diamond cluster. The inset showsthe (111) planes with a spacing of 0.205 nm and Fourier transform(FFT) with the spots representing the {111} spacing of diamond.The diameter of diamond nanorod is in good agreement with thetheoretical prediction that diamond nanorods occupy a stabilityrange from approximately 2.7–9 nm in diameter [2]. Fig. 5 showsanother nanorod, with (111) planes (spacing, 0.205 nm) areoriented perpendicular to the growth direction (white arrow) inHRTEM image. The inset shows the (111) planes with a spacing of0.205 nm and Fourier transform with the spots representing the{111} spacing of diamond. The Si structure seen in the imagearises from scratching the sample for TEM grid. It is observed forSi nanowires in literature for wire growth axis parallel [16] as well

Fig. 1. (a) SEM image of nanocrystalline diamond film used for post-treatment by

hydrogen plasma and (b) Raman spectrum of nanocrystalline diamond film used

for post-treatment by hydrogen plasma.

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as perpendicular to crystallographic planes of wire. It can also beseen from Fig. 5 that the rod is emerging from diamond clusterslike the diamond nanorod shown in Fig. 4(a). TEM images showthat these nanorods are crystalline, have uniform diameters andare originated from diamond nanoclusters. No amorphous carbon[13] or metal oxide [17] shell was found around the nanowire/rodsas observed by HRTEM. The amorphous shell was suggested toplay crucial role in one-dimensional growth of nanowires/rods.Alfredo et al. [18] also observed Si nanowires without amorphousshell. The rods have finite length because there is no incominghydrocarbon gas source. Fig. 6 shows TEM image of the thick rod(shown in SEM image) indicating clusters forming the rod. Theseclusters have lattice spacing of 0.34 nm. This separation agreeswell with the lattice spacing of (0 0 0 1) planes in graphite. Theinset shows FFT image with the spots representing the (0 0 0 1)spacing of graphite.

The results on the etching of thin diamond film in hydrogenplasma allow us to consider the following explanation for thephenomena observed. The role of hydrogen plasma is consideredimportant for the formation of hydrocarbon reactive species fromcarbon presented in the substrate. Under hydrogen plasmatreatment at high temperature, carbon atoms sputtered out oretched by hydrogen plasma from non-diamond phase as well asfrom the diamond phase in the film will be reacted with hydrogento form reactive hydrocarbons and deposited again to form

amorphous carbon clusters. The crystallization subsequentlyoccurred, which is mediated by the insertion of hydrogen atomsinto the loosely bound amorphous carbon matrix or strained C–Cbonds as the hydrogen atoms diffuse through the amorphouscarbon similar to the crystallization in amorphous silicon [19].The gaseous hydrocarbon radicals (CHx) are produced. Thesehydrocarbon radicals transform to graphite-like structure under

Fig. 2. (a) SEM image of hydrogen plasma treatment of nanocrystalline diamond

film at temperature of 800 1C for 11 h to form DNRs and (b) HRSEM image

indicating diamond clusters and DNRs on graphite rod.

Fig. 3. TEM image of DNR grown after hydrogen plasma treatment at 800 1C

temperature for 11 h.

Fig. 4. HRTEM image indicating the DNR emerging from diamond nanocluster. The

wire principle axis is perpendicular to crystallographic planes. The image shows

the diamond (111) lattice planes with a d-spacing of 0.205 nm. Inset shows the

(111) planes with a spacing of 0.205 nm and FFT for {111} nanorod diamond

planes. Graphite planes with a d-spacing of 0.334 nm are also indicating.

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hydrogen plasma treatment as is discussed by Singh [20]. Thecontinues etching with hydrogen plasma at 800 1C in the presenceof applied bias of �150 V and the impinging flux of positivehydrogen ions abstract the hydrogen and resulted in thick rod-likestructures composed of graphite clusters (Fig. 6). The diamondnanocrystallites that can be seen in SEM and TEM images on thegraphite rods are the result of recrystallization under H plasmabombardment by rearrangement of carbon atoms from graphiteclusters towards the (111) crystallographic plane of the diamondcrystal, as it is with the lowest surface-free energy in the crystal

[21]. Single-crystal DNRs began to grow from diamond nanocrys-tallites through thermally activated processes or through surfacegrowth mechanism [22].

On the graphite rods, the nuclei’s of diamond that stoodseparately with their growth direction normal to the substratesurface underwent fast growth [21]. The recrystallized regionswill act as nuclei for further growth of diamond in one dimension.The negative bias is helpful for one-dimensional growth. The biasprobably provides directional diffusion of the hydrocarbonradicals along the growing diamond protrusions [23,24]. Anotherimportant factor is specific surface stresses [25] that may exist inthe substrate may influence preferential one-dimensional growthof the diamond nanoparticles in the direction perpendicular to theSi rod axis.

The transformation of diamond to graphite under hydrogenplasma is reported in literature [26]. Man et al. [26] suggested thatunder hydrogen plasma diamond grains converted to soft carbonspecies like graphite under the relatively high temperatureconditions. They further elaborated that the hydrogen plasmaetching enhanced carbon diffusion process, the carbon phase ofthe sample surface was changed from sp3 bonding to sp2 bonding.

DNRs from graphite are similar to diamond film growth fromgraphite nucleation first. The reports [27–29] show the formationof graphite before diamond nucleation. Lambrecht et al [30]suggested that diamond film can nucleate by the initial con-densation of graphite and subsequent hydrogenation along theedges of the graphite particles. The diamond growth then occursby the hydrogen abstraction and carbon incorporation reactions[27,31].

The growth of DNRs did not occur directly on Si substrate likein the case of carbon nanotube network [13]. It might be becauseof two reasons: (1) graphite rods proved suitable substrate, withonly carbon atoms present. It was suggested for Si nanowiresgrowth by Lee et al. [17] that the growth of nanorods occurredat the area near the cold finger, which suggested that thetemperature gradient provided the external driving force fornanowire formation and growth. Considering it valid for diamondnanostructures, helps to understand that because of temperaturegradient all regions in the sample are not suitable for wire/rodgrowth. (2) As was discussed by Sun et al. [13], in the early stagesonly diamond crystallization was performed. Then diamondnanocystallites growth and faceting might take place throughthermally activated process or surface growth mechanism [22]and further expose themselves for one-dimensional growth. Itimplied faceted nanoparticles helped for one-dimensional struc-ture. The proof of faceting is one-dimensional growth in particulardirection of these crystallites. These faceted nanoparticles areassumed to form after recrystallization of graphite. The transfor-mation of nanodiamond to carbon onions [32,33] or bucky-diamonds [34] by high-temperature annealing was observedboth experimentally and theoretically. These results indicate theexistence of a competition between transformation from graphitestructure to diamond, and in our case we observed both thetransformations i.e. from diamond to graphite and to diamondagain which further grow after crystallization and faceting to one-dimensional structure.

Fig. 7 is a schematic view for the nucleation of Si clusters,growth of Si rods along with the presence of diamond crystalliteson these rods and the growth of single-crystal DNWs underhydrogen plasma treatment of these diamond crystallites.

Literature observations showed that metal catalyst [16] or theamorphous largely hydrogenated carbon nanoparticles [12] werethe special and effective catalysts for nanotubes and nanowires/nanorods growth. Our results indicated that no metal catalyst oramorphous shell is needed for one-dimensional growth. Holmeset al. [16] also not observed any amorphous shell surrounding the

Fig. 5. High-resolution TEM image of another DNR. Inset shows the (111) planes

with a spacing of 0.205 nm and FFT with the spots representing the {111} spacing

of diamond.

Fig. 6. TEM image of the thick rod formed with graphite clusters. Inset shows FFT

with the spots representing the {0 0 0 1} spacing of graphite.

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wire obtained after using Au as a catalyst. Further experimentswill be needed to make definite conclusion for the competitionbetween the formation of diamond and the formation of graphitein the process of vapor-deposited diamond growth and also totailor these structures to the precise needs of nanotechnologists,and the present structures are not yet suitable for use in mostnanodevices.

4. Conclusions

In conclusion, single-crystalline diamond nanorods withdiameters of 3–5 nm and with lengths up to 200 nm have beensynthesized by hydrogen plasma post-treatment of nanocrystal-line diamond thin films. The graphite clusters were observedwhich recrystallized under hydrogen plasma to faceted nanocrys-talline diamonds. DNRs formed by the one-dimensional growth ofthese recrystallized nanodiamond clusters continues by surfacegrowth mechanism. The negative applied bias and surface stressesis suggested to support one-dimensional growth.

Acknowledgements

This work is financially supported by the Key Project of ChineseAcademy of Sciences Knowledge Innovation Program (Grant no.KJCX3. SYW. N10) and the National Natural Science Foundation ofChina (Grant no. 10375085).

References

[1] A.S. Barnard, S.P. Russo, I.K. Snook, Nano Lett. 3 (2003) 323.[2] A.S. Barnard, I.K. Snook, J. Chem. Phys. 120 (2004) 3817.

[3] O. Shenderova, D. Brenner, R.S. Ruoff, Nano Lett. 3 (2003) 805.[4] A.S. Barnard, S.P. Russo, I.K. Snook, Phys. Rev. B 68 (2003) 235407.[5] K.E. Drexler, Nanosystems: Molecular, Machinery, Manufacturing and Com-

putation, Wiley, New York, 1992.[6] E.-S. Baik, Y.-J. Baik, D. Jeon, J. Mater. Res. 15 (2000) 923.[7] E.-S. Baik, Y.-J. Baik, S.W. Lee, D. Jeon, Thin Solid Films 295 (2000) 377.[8] H. Masuda, T. Yanagishita, K. Yasui, K. Nishio, I. Yagi, T.N. Rao, A. Fujishima,

Adv. Mater. 13 (2001) 247.[9] Y. Ando, Y. Nishibayashi, A. Sawabe, Diamond Relat. Mater. 13 (2004) 633.

[10] R. Arenal, P. Bruno, D.J. Miller, M. Bleuel, J. Lal, D.M. Gruen, Phys. Rev. B 75(2007) 195431.

[11] I.I. Vlasov, O.I. Lebedev, V.G. Ralchenko, Adv. Mater. 19 (2007) 4058.[12] S. Botti, R. Ciardi, M.L. Terranova, S. Piccirillo, V. Sessa, M. Rossi, Chem. Phys.

Lett. 355 (2002) 395.[13] L.T. Sun, J.L. Gong, Z.Y. Zhu, D.Z. Zhu, Suixia He, Adv. Mater. 16 (2004) 1849.[14] Y.M. LeGrice, R.J. Nemanich, J.T. Glass, Y.H. Lee, R.A. Rudder, R.J. Markunas,

Mater. Res. Symp. Proc. 62 (1990) 219.[15] M. Chhowalla, A.C. Ferrari, J. Robertson, G.A.J. Amaratunga, Appl. Phys. Lett. 76

(2000) 1419.[16] J.D. Holmes, K.P. Johnston, R.C. doty, B.A. Korgel, Science 287 (2000) 1471.[17] S.T. Lee, N. Wang, C.S. Lee, Mater. Sci. Eng. A 286 (2000) 16.[18] Alfredo M. Morales, Charles M. Lieber, Science 279 (1998) 208.[19] S. Sriraman, S. Agarwal, E.S. Aydil, D. Maroudas, Nature 418 (2002) 62.[20] J. Singh, J. Mater. Sci. 29 (1994) 2761.[21] J.S. Kim, M.H. Kim, S.S. Park, J.Y. Lee, J. Appl. Phys. 67 (1990) 33,354.[22] J.C. Angus, C.C. Hayman, Science 241 (1998) 643.[23] Q. Yang, C. Xiao, W. Chen, A. Hirose, Carbon 43 (2005) 748.[24] E.G. Gamaly, T.W. Ebbesen, Phys. Rev. B 52 (1995) 2083.[25] O.A. Shenderovaa, C.W. Padgett, Z. Hu, D.W. Brenner, J. Vac. Sci. Technol. B 23

(2005) 2457.[26] W.D. Man, et al., Diamond Relat. Mater. 16 (2007) 1455–1458.[27] J.C. Angus, et al., Philos. Trans. R. Soc. London A 342 (1993) 195–208.[28] P. Badzaig, W.S. Verwoerd, W.P. Ellis, Nature 343 (1990) 244–245.[29] M.M. Waite, S.I. Shah, Appl. Phys. Lett. 60 (1992) 2344–2346.[30] W.R.L. Lambrecht, C.H. Lee, B. Segall, J.C. Angus, Z. Li, M. Sunkara, Nature 364

(1993) 607.[31] J.E. Butler, R.L. Woodin, Philos. Trans. R. Soc. London A 342 (1993) 209–224.[32] S. Tomita, M. Fujii, S. Hayashi, K. Yaamoto, Chem. Phys. Lett. 305 (1999)

225.[33] G.D. Lee, C.Z. Wang, J. Yu, E. Yoon, K.M. Ho, Phys. Rev. Lett. 91 (2003) 265701.[34] J.Y. Raty, G. Galli, C. Bostedt, T.W. van Buuren, L.J. Terminello, Phys. Rev. Lett.

90 (2003) 037401.

Fig. 7. The schematic diagram for the formation of Si rods with graphite clusters embedded in these, and the growth of diamond nanorods under hydrogen plasma

irradiation of nanocrystalline diamond thin film on the Si substrate at high temperatures. (a) Plasma etching of the sample, exposed regions of the silicon surface and Si

clusters are formed, (c) followed by the formation of silicon carbide rods along with the graphite clusters, (d) graphite clusters transform to diamond nanoclusters under

hydrogen plasma and (e) recrystallized diamond nanocrystallites begin to grow for nanorods.

S.A. Rakha et al. / Journal of Crystal Growth 311 (2009) 3332–33363336