Nanocrystalline Diamond Coatings

Mei WangDepartment of Physics and Materials Science, City University of Hong Kong,Hong Kong, and Department of Materials Science and Engineering, ShandongUniversity, Jinan, China

Paul K. ChuDepartment of Physics and Materials Science, City University of Hong Kong,Hong Kong, China

AbstractThis entry describes the common chemical vapor deposition techniques to produce (ultra) nanocrystallinediamond (UNCD/NCD) coatings and discusses the associated mechanisms including the roles of hydrogenand argon in the processes. UNCD/NCD coatings are commonly doped with boron and nitrogen, and thecorresponding electronic, electrochemical, and optical properties are discussed. The surface properties of thematerials can be tailored using different surface modification techniques, and the use of UNCD/NCDcoatings in micro-electromechanical systems and electron emission devices is described.

INTRODUCTION

Owing to the unique combination of physical, chemical,and mechanical properties such as high thermal conductiv-ity, high hardness, large band gap, optical transparency,stability against chemical reagents, high mechanical stabil-ity, and corrosion resistance, diamond films have attractedconsiderable scientific and technological interests. How-ever, conventional diamond coatings are too rough and thevariable surface morphology has hampered wider indus-trial, biomedical, and consumer product applications. Inthis respect, smoother films with decreased crystal size aretechnologically better.

Nanocrystalline diamond (NCD) coatings are typicallyproduced in a hydrogen (H2)-rich chemical vapor deposi-tion (CVD) environment. The materials have grain sizesranging from a few nanometers to a hundred nanometers,which tend to increase with film thickness, and very smallamounts (<5%) of sp2-bonded carbon in the form of defectsor grain boundaries. Ultra nanocrystalline diamond(UNCD) coatings were first fabricated by CVD under H2-poor and argon (Ar)-rich conditions by Gruen andcoworkers in 1994. UNCD is generally characterized bycrystalline grains 2–5 nm in size (independent of film thick-ness) surrounded by a layer of nondiamond carbon con-necting the grains and possesses a significant sp2-bondedcarbon content of around 10%. Gruen observed that whenthe crystalline size was reduced, the percentage of carbonatoms located in the grain boundaries increased drastically,thus affecting the mechanical and electrical properties ofthe NCD coatings. Therefore, in addition to possessing therobust chemical, mechanical, and thermal properties typical

of conventional CVD microcrystalline diamond coatings,NCD and UNCD coatings have other distinctive propertiesincluding the smaller grain size, smoother and more uni-form surface morphology, lower threshold for field emis-sion, and smaller macroscopic friction coefficients. UNCDcoatings are also electrically conductive due to the nondia-mond matrix. Both NCD and UNCD can be readily dopedwith nitrogen or boron to produce high conductivity andhave applications in nano/micro-electromechanical sys-tems (N/MEMS), tribology, biotechnology, electrical andelectrochemical devices, and optical devices. This entrypresents a concise review encompassing CVD deposi-tion, doping, modification, properties, and applications ofNCD coatings. The burgeoning materials and field haveattracted much research interests, and readers are referredto other reviews including the deposition techniques,growth mechanism, and mechanical properties for moreinformation.

COATING DEPOSITION

Techniques

Diamond coatings are typically deposited by CVD. Theplasma necessary for the deposition is activated by hotfilaments (HFs) heated to 2,200–2,800°C,[1–6] direct cur-rent (DC),[7,8] or microwave (MW).[9–13] MWCVD andHFCVD are the most commonly used to grow diamondthin films. While MWCVD offers more versatility in thegas choice and process control, HFCVD is more attrac-tive economically and easier to operate and maintain.

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In addition, HFCVD can be readily upscaled to substrateslarger than 400 mm but it is difficult for MWCVD systems.In HFCVD, the chemical stability of the substrate, deposi-tion parameters as well as precursor gases determines thegrowth rate and quality of the diamond layers.[14] InMWCVD, NCD deposition is typically conducted in amethane (CH4)/H2 mixture.[15–17] H2 can be replaced par-tially or completely by Ar[18] or N2

[19,20] by reducing thepressure,[21] MW power, or substrate temperature[22–24] orby applying a bias voltage during deposition.[25,26] Thevarious CVD techniques have been reviewed.[21,27–29] Inaddition, compact NCD coatings have been prepared by ahigh-pressure high-temperature (HPHT) process similar tothe one used in the commercial production of syntheticdiamond.[30–34] In this entry, diamond coatings producedby CVD techniques are described.

Nucleation

Diamond nucleation on nondiamond surfaces without pre-treatment is usually very difficult and slow. Small nucle-ation densities typically lead to poor film propertiesespecially thermal conductivity and roughness.[22,35,36]

Nucleation in the early growth stages is vital to film growthand morphology.[37–43] One important pretreatment is sub-strate activation performed by scratching with diamondpowder.[44,45] The process embeds small residual diamondparticles into the substrate resulting in higher nucleationdensities and better uniformity[46–48] possibly due to thelarger surface-to-volume fraction of fine scratches. Alter-natively, denser nucleation can be achieved by immersingthe substrate into a suspension of diamond particles fol-lowed by ultrasonic treatment.[37,39,41] This yields moreuniform film growth and smoother films. The nucleationmechanism is similar to the abrasion process in which theultrasound causes similar damage to the substrate as theembedded diamond fragments. Contrary to abrasion tech-niques, the nucleation density actually increases with par-ticle size because the very small diamond particles haveinsufficient momentum to damage the substrate or embedthe diamond fragments.[37] Another approach to enhancethe diamond nucleation density is to coat the substratewith as many diamond particles as possible. They act asseeds for subsequent epitaxial growth and the basicmechanism is electrostatic attraction of fine particles tothe substrate. The last technique has advantages suchas no substrate damage, 3-D possibilities, and simpleup-scaling. It has been shown that the nucleation densityincreases with decreasing dimensions of the seedingmaterials.[40,49]

Carbon-derived interlayers have been shown to increasethe local nucleation density due to the higher surface stickingcoefficient, larger carbon content, or higher carbon growthspecies surface mobility;[50,51] that is, a mechanism whichenhances the surface carbon concentration or provides acarbon diffusion barrier should be beneficial.[38,51–55]

Furthermore, the more sp3 content the interlayer has, thebetter is nucleation.[56]

A special nucleation technique called bias-enhancednucleation (BEN) can be applied to conductive substrates.In this method, the substrate is negatively biased to100–250 V DC with respect to the chamber or a secondinternal electrode.[57–62] According to the report by Leeet al.,[57] the bias voltage shows more pronounced effectson improving the electron field emission capacity of dia-mond than doping with boron. It can be ascribed to theincreased grain boundaries which are highly conductiveand good emission sites. Sharda et al.[58] and Jianget al.[59] have extended the use of the bias voltage over thegrowth period to enhance secondary nucleation of dia-monds that leads to the formation of NCD.[57]

Larger nucleation densities can be accomplished bycombining the aforementioned techniques.[62–67] For exam-ple, Rotter et al. have developed multistage nucleationstarting with a short growth step on the mirror silicon (Si)surface to produce a thin carbon layer followed by treat-ment in an ultrasonic bath containing a suspension of nano-diamond particles.[62] This process embeds diamondfragments into the carbon layer, and subsequently, the car-bon layer becomes a high concentration source of carbonfor particle growth and is completely removed during theearly stages of growth. Sumant et al. have reported a similarapproach by first exposing the substrate to a hydrocarbonplasma for a short time to induce the formation of siliconcarbide at the diamond/Si interface along with a thin, uni-form layer of hydrogenated amorphous carbon on top. Thisamorphous carbon layer enables the formation of a uniformand dense layer of nanodiamond seed particles spread overthe substrate in the second step.[66] Lee et al. have shownthat a 50-nm thick Si overlying layer on the nucleatedsubstrate can increase the nucleation density by almost twoorders of magnitude.[67] The nucleation kinetics of dia-mond on the Mo-coated Si followed by an ultrasonic seed-ing step with nanosized detonation diamond powders isincreased 10 times compared to blank Si.[68] Naguib et al.have reported that thin layers of tungsten can enhance thenucleation density on account of the formation of carbide atthe interface impeding carbon diffusion into the sub-strate,[65] although this metal interlayer may be undesirablein optical coatings.[69]

Growth Mechanism

Most research activities have hitherto focused on the use ofAr/CH4 (with or without H2) to deposit NCD andUNCD.[70–74] Alternatively, NCD can be produced usinghigh concentrations of CH4 in H2

[75] or CH4/H2 containingN2

[76,77] and/or O2.[78] In all these cases, H2 is the carrier

gas, CH4 is the carbon source, and the other gases supplythe dopants.[77–79] The role played by O2,

[80] N2,[81] halo-

gen,[82] and other chemistry[83,84] may not be critical to thebasic understanding of diamond growth. It is generally

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regarded as perturbation of the CH4/H2 chemistry. In thissection, we will discuss the Ar/CH4/H2 chemistry and con-sider diamond growth on existing diamond surfaces.

Role of CH4

There are two main procedures and mechanisms pertain-ing to the preparation of NCD coatings.[29,64,85–89] The C2

insertion mechanism was proposed by Gruen and cow-orkers.[85] There have been follow-up studies about thismechanism[86,87] and the role of Ar in the growth processwill be discussed later. It has been proposed that NCDfilms can be synthesized by substrate BEN in conven-tional CH4/H2 plasma CVD.[58] When the substrate isbiased negatively, cations such as H and CHx play animportant role in the subplantation model,[88,89] and it willbe discussed later in this entry in the section discussingabout the role of H2 in diamond fabrication.

CH4 in the precursor mixture has a profound effect onthe morphology and properties of the diamond coat-ings.[90] The nucleation density, nucleation rate, and adhe-sion strength of the diamond coatings can be enhanced byincreasing the CH4 concentration.[91] Fig. 1a and 1bshows the synchrotron near edge extended X-ray absorp-tion fine structure spectroscopy of NCD films preparedusing different CH4 concentrations from 1% to 100%. Itwas calculated that the sp2 carbon concentration of thesamples increases the steadily from 2% to 25% in thesurface (based on TEY) and from 1% to 23% in the bulk(based on FY) when the CH4 concentration increasesfrom 1% to 100%.[90]

Role of H2

H2 plays a vital role in the CVD processes.[92] In par-ticular, atomic hydrogen is an essential ingredient of the

vapor used to produce the films,[21] and a small amountof H2 improves aggregation of the NCD films.[64] Thetransition between microcrystalline diamond and UNCDcan be controlled by varying the amount of H2 in theplasma.[86,88] Fig. 2 illustrates the microstructure of thediamond films during the transition from the microcrys-talline structure to the nanocrystalline one as the H2

concentration increases from 0.5% to 1.5%. At H2 con-centrations of 1% and 1.5%, the film appeared asUNCD. However, there is a sharp transition between1% and 0.5% H2, where not only diamond but also anoncrystalline carbonaceous structure was grown.[92]

According to the conventional CVD diamondmodel,[27,92,93] H atoms created during the activationprocess react with the hydrocarbon precursor creatinga complex mixture of hydrocarbon species includingreactive carbon-containing radicals. The H atoms alsoabstract H2 from the surface of CH bonds thereby cre-ating surface radical sites. These radical sites occasion-ally react with gas-phase carbon-containing radicalscreating adsorbed carbon species. Finally, the atomichydrogen and, to a lesser extent, other gaseous speciesreact with sp or sp2 carbon sites on the surface, convert-ing them into sp3-bonded carbon. However, the sp2 car-bon atoms are etched back into the gas phase at a highrate by the H2-rich plasma during NCD growth, and thisexplains why the sp2 content in the film is rather small(<5%).[94] The grain size, surface morphology, and sur-face roughness of the NCD films prepared in H2-richplasmas depend on the film thickness. Furthermore,plasma with a small H2 concentration produces UNCDfilms with about 10% grain boundaries.[88] By reducingthe atomic H2 concentration, etching of sp2 carbonatoms is retarded giving rise to nonepitaxial crystallites,re-nucleation as well as smaller grain size. However, ifthe H2 concentration is reduced while the hydrocarbon

Fig. 1 NEXAFS of NCD filmssynthesized with different meth-ane concentrations: (a) TEYof CK-edge and (b) FY of C K-edge.Source: From Yang, Yang,et al.[90] ©2007 Elsevier. Rep-rinted with permission.

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content in the plasma is increased, nondiamond phasesare formed and it may eventually lead to completeabsence of the diamond phase.[95] Zhou et al. havereported diamond growth based on the C2 insertionmechanism.[86]

Role of Ar

In Ar-rich plasma systems, Ar atoms play an importantrole in the production of C2 which is generally regarded asthe growth species of NCD.[21,86] In C2H2/H2/Ar plasmas,the major production channel of C2 is the direct dissocia-tion of C2H2 from collisions with Ar as follows: C2H2 +Ar → C2 + H2 + Ar. In CH4/H2/Ar plasmas, the dissoci-ation of CH4 from collisions with metastable Ar results inthe formation of a significant amount of CHx (x = 0–3)radicals that eventually produce C2H2 by recombination.Subsequent dissociation due to collisions with Ar finallyproduces C2 dimers in the same manner.[96,97] The inten-sity of C2 relative to H in C2H2/H2/Ar plasmas was 2 to3 times higher than that in CH4/H2/Ar plasmas for theequivalent composition.[64]

The Ar concentration shows obvious effects on the prop-erties of NCD such as grain size,[86,98] morphology,[98]

surface smoothness[86] as well as deposition rate.[86,98] Forinstance, a smaller crystallite size down to the nanometerscale can be accomplished by adding a large amount of Arto CH4 or H2/CH4 in an MW plasma[21,86] without usingion bombardment.[99] Zhou et al. have demonstrated a tran-sition from microcrystalline diamond to NCD in the rangebetween 2 and 97 vol% Ar in the Ar/H2/CH4 plasmas.[86]

The microcrystalline grain size and columnar growth havebeen observed from films produced using small concentra-tions of Ar in the reactant gases, but when the Ar concen-tration exceeds a threshold (*90 vol%), themicrocrystalline diamond films are converted totally toNCD, as shown in Fig. 3a.[86] It is because of the differentmechanism from CH3 in the H2-rich plasma to C2 in an Arplasma with a small H2 content.

[21,29,85,86] The depositionrate of NCD declines significantly to 0.2 μm h−1 at 80–99vol% Ar (1% CH4/Ar), as shown in Fig. 3b.[86] It can beexplained by that diamond growth based on the C2 insertionmechanism involving H2 has lower activation energy thanthat without H2.

DOPANTS IN NCD AND THEIR PROPERTIES

Boron is one of the common dopants used to produce con-ducting NCD electrodes. Boron atoms having a covalentradius close to that of carbon are substitutional in the dia-mond lattice. The Bohr’s radius (3.1 × 10−10 m) of boron iscomparable with the lattice constant of diamond (3.56 ×10−10 m) and so it behaves like a typical shallow accep-tor.[100] The electrical properties of doped NCD are influ-enced in a complex manner by the boron concentration,lattice H2, nondiamond sp2 impurities, and defect den-sity.[101] Doping with boron leads to conventional valenceband conduction, and excellent electrical conduction can beattained at high boron levels. Bulk boron doping of NCDcan in fact yield superconductivity, if metallic doping isachieved.[102,103] Boron-doped NCD coatings exhibit supe-rior electrochemical properties over other conventionalelectrode materials including low-capacitive backgroundcurrents, wide potential window in aqueous media, andgood corrosion resistance in harsh environments.[104–106]

As shown in Fig. 4[107] boron-doped NCD films exhibitexcellent electrochemical properties boasting a wide work-ing potential window, low-voltammetric background cur-rent, and good response to Fe(CN)6

3−/4−, Ru(NH3)62+/3+,

IrCl62−/3−, and methyl viologen without pretreatment. The

voltammetry curves obtained from all the couples arequasi-reversible indicating that boron-doped NCD has asufficient density of electronic states over the wide poten-tial range to support rapid electron transfer. Other stud-ies[103] have shown that boron as well as sp2 carbonphases in the grain boundaries governs the optical absorp-tion process. Hence, while the electronic properties depend

(A)

0.5 μm

0.5 μm

(D)

(B)

(C)

(E)

(F)

0.5 μm

Fig. 2 Scanning electron microscopic (SEM) images of the dia-mond film prepared in Ar/CH4 with (a) 0.5% H2, (b) 1% H2, and(c) 1.5% H2 in the plasma and low-resolution transmission elec-tron microscopic (TEM) images: (d), (e), and (f) corresponding to(a), (b), and (c), respectively.Source: From Birrell, Gerbi, et al.[92] ©2006 IOP Publishing.Reproduced with permission. All rights reserved.

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on boron, the optical properties are mainly determined bygrain boundaries.

In comparison, UNCD films exhibit different conduc-tivity behavior due to the significant sp2 concentration.When doped with nitrogen, the conductivity increasesdramatically due to an increase in the density of states

associated with π-bonding.[108] The π-states have been the-oretically predicted to be symmetrical around the Fermienergy but with significantly lower energy than the σ-statesdue to sp3 bonding.[108,109] In UNCD coatings, nitrogendoes not act as a donor (nitrogen is a deep n-type dopantat 1.7 eV from the conduction band minimum) and thus

Fig. 4 Cyclic voltammetric i–Ecurves for (a) Fe(CN)6

3−/4−, (b)Ru(NH3)6

2+/3+, (c) IrCl62−/3−,

(d) methyl viologen (MV+2/+) inpotassium chloride, (e) 4-tert-butylcatechol, and (f) Fe2+/3+ inperchloric acid for a NCD elec-trode (scanning rate = 100 mV/s;electrode geometric area =0.2 cm2).Source: From Show, Witek,et al.[108] ©2003 American Che-mical Society. Reprinted withpermission.

Fig. 3 (a) Raman spectra of the as grown films and (b) growth rates of diamond deposited using Ar/H2/CH4 plasmas containing differentAr concentrations.[86]

Source: From Zhou, Gruen, et al.[86] ©1998 AIP Publishing LLC. Reprinted with permission.

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does not significantly influence the conductivity of theUNCD films. Instead, a large concentration of nitrogen inUNCD enhances the transport path related to a highly con-ductive subsystem of π-states located in the grain bound-aries.[109] Therefore, nitrogen affects the conductivityindirectly and it is more related to structural changes ratherthan doping. Nitrogen-doped NCD coatings possess semi-metallic electronic properties over a potential range from atleast −1.5 to 1.0 V vs. saturated calomel electrode(SCE).[110] The electrodes, like boron-doped NCD, exhibita wide working potential window, low background current,and high degree of electrochemical activity in redox sys-tems such as Fe(CN)6

3−/4−, Ru(NH3)62+/3+, IrCl6

2−/3−, and

methyl viologen. It has also been found that doping withnitrogen can improve the emission properties[111] and has aprofound influence on the optical transparency of the films,as shown in Fig. 5.[112] Nitrogen-doped UNCD shows thelargest absorption coefficient because the addition of nitro-gen increases the gain boundary volume fraction, and thegrain boundaries have a large influence on the optical prop-erties of the NCD films.

SURFACE MODIFICATION OF NCD COATINGS

Because the surface properties of materials depend on sur-face termination, NCD can be modified to tailor the surfaceproperties. The common surface modification techniquesare described in this section.

Plasma Treatment

Generally, the as-deposited NCD possesses an H2-termi-nated surface. This is because CVD processes normallyend with an H2 plasma treatment or are carried out in anH2-containing atmosphere.[113] H2 induces a hydrophobicbehavior[114] and negative electron affinity χ of −0.7 to−1.3 eV for a NCD sample.[115] H2 also produces subsur-face carriers which render the diamond surface semicon-ducting.[116] Fig. 6a[117] shows the high-resolution X-rayphotoelectron spectroscopy C 1s spectra of NCD after H2

plasma treatment, and π-bonded carbon and C–H bondcan be observed. H2 termination is normally very stablein air for months, although gradual replacement of H2 byoxygen (O) from air has been reported.[118]

Fig. 5 Optical absorption spectra calculated from photothermaldeflection spectroscopy for NCD and UNCD coatings.Source: From Williams & Nesládek.[101] ©2006 John Wiley andSons.

Fig. 6 XPS C 1s high-resolutionspectra showing various compo-nents on the NCD surface afterdifferent functionalization treat-ments: (A) hydrogenation; (B)UV/ozone oxidation; (C) photo-chemical amination; and (D)C3F8 plasma fluorination.Source: From Wang, Ishii,et al.[118] ©2012 American Che-mical Society. Reprinted withpermission.

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O plasma treatment produces O-containing functionalgroups such as –OH, –O–C–O–, –C=O, and –COOH onthe NCD surface.[117,119–121] These O-containing functionalgroups are hydrophilic enabling biologically active speciesto link relatively easily. In particular, carboxyl andhydroxyl termination can provide a flexible platform forsurface functionalization via linker molecules leading todiverse functional termination in advanced surface engi-neering.[119] Fig. 7a[117] depicts the scheme for immobili-zation of aptamer probes based on carboxyl termination inbiosensor applications. However, an oxidized surface haspositive electron affinity and it is nonconductive due tohigh work function, thus limiting electronic applica-tions.[117,122,123] After longer O plasma treatment, plasmaetching of the NCD surface can be achieved. This has beenutilized to produce a patterned diamond surface for use as aporous mask.[123]

Fluorination of NCD has been performed in the pres-ence of cold plasma containing fluorine-containing gases

such as octafluoropropane (C3F8), tetrafluoromethane(CF4), or sulfur hexafluoride (SF6).

[124,125] As shownin Fig. 6d, a fluorine-terminated NCD surface withextraordinary hydrophobic properties can be produced.The termination can react with Grignard reagents andamine-forming alkylated and aminated NCD, respec-tively.[126,127] According to the report by Mangeneyet al., fluorinated NCD can be used to obtain alkyl,amino, and amino acid NCD derivatives by nucleophilicdisplacement of fluorine. Moreover, fluorinated diamondelectrodes are promising in reactions requiring verynegative potentials, for example, deposition of somemetals and nitrate reduction.[128] Amine-terminatedsurfaces can be obtained by NH3 plasma treatment ofH2-terminated diamond.[129–131] Plasma treatment canalso be employed to graft specific functionalities suchas halogen-containing molecules[132] and organic filmscontaining specific functional groups,[133] as shown inFig. 7b.

Fig. 7 Schematic showing the various steps: (i) initialization of NCD-biosensing surface with probe aptamers by functionalization ofCOOH and NH2 on (A) carboxylated and (B) aminated surface, respectively; (ii) immobilization of PDGF-binding aptamers onto the NCDsurface; (ii) specific conjugation between introduced PDGF proteins and immobilized aptamer probes; and (iii) attachment of Cy5-taggedsignaling aptamer onto PDGF proteins.Source: From Wang, Ishii, et al.[118] ©2012 American Chemical Society. Reprinted with permission.

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Ion Implantation

Ion implantation is a good modification technique forNCD. Implanting nitrogen into NCD at room temperatureat 100 kV (ion implant fluence of 1016 cm−2) producessuper hydrophobic properties and ultra-low friction coeffi-cients contrary to the as-deposited coatings.[134,135] At asmall load of 10 mN, a large friction coefficient is observeddue to ploughing, fracture of the ball counter body, andabsence of carbonaceous transfer layer.[134] High-fluencenitrogen ion implantation into NCD reduces the diamondcrystallinity leading to swelling of the crystal lattice. Thecubic diamond grains in the film are converted into roundgrains due to sputtering by the nitrogen ions.[135] The NCDmorphology changes from “cauliflower-like” to a smootherone after nitrogen plasma ion immersion. The implantedNCD films exhibit improved electrical conductivity whilethe electron transfer kinetics decreases after nitrogenimplantation for 15 and 30 minutes.[136,137] After nitrogenion implantation, the field emission characteristics areimproved due to breaking of sp3 bonds, creation of amor-phous regions, and formation of defects.[136,138] The elec-trical conductivity increases based on the electrochemicalresponse observed from the treated NCD electrodes and thereduction of the work potential window for H2 evolution inthe acidic solution. Modification of NCD by B ion implan-tation yields good electronic properties (refer to the sectionon boron doping). Hu et al.[139] have performed oxygen ionimplantation into NCD at room temperature at 90 kV(implant fluence of 1 × 1014 cm−2) and found that theoxygen-implanted NCD exhibits n-type conductivitywhen the annealing temperature is above 800°C. In addi-tion, 30 kV-focused Ga+ ions are implanted into NCD toinvestigate the fluence dependence[140] Vanadium hasbeen suggested as a dopant in the n-type diamond semicon-ductor based on theoretical calculation. Experimentalresults indicate that implanting an element with a largeratomic volume into the NCD may be a feasible means tosynthesize otherwise difficult-to-dope semiconductors.[141]

Photochemical Functionalization

Photochemical functionalization is another to surfacemodification technique for diamond. The photochemicalreaction of alkenes and H-NCD is a common method fordiamond functionalization.[142] An H2-terminated dia-mond surface is obtained as grown or employing theaforementioned plasma chemistry. Photochemical modifi-cation is achieved with a long alkene chain illuminatedwith ultraviolet light at a wavelength of 254 nm under drynitrogen.[143] UV exposure (254 nm) of the H-NCD sur-face in the presence of terminal olefin molecules such asundecylenic acid[144] produces alkanes that are bound tothe diamond surface by C–C bonds.[145–147] The reactioncan be accelerated in the presence of electron-acceptinggroups such as CF3 in the molecule.[148] Photolysis of

perfluoroazooctane in the presence of NCD modifies thesurface by adding perfluorooctyl functional groups.[149–151]

Amine-terminated diamond surfaces have been produced inthis manner. Yang et al.[122] and Hartl et al.[123] have mod-ified H-NCD by a photochemical process to generate a sur-face layer with amino groups. The amine-NCD canimmobilize biomolecules such as proteins and DNA mole-cules in biotechnological applications.[122,123,147,151] Wanget al.[117] described a photochemical technique to graft dif-ferent functionalities including carboxyl, hydroxyl, ammine,and fluorine, as shown in Fig. 6b and 6c. Biosensors pro-duced on directly carboxylated and aminated NCD deliverhigh performance in terms of sensitivity, selectivity, long-term stability, and regeneration ability, demonstrating thatdirectly functionalized NCD films have large potential insensing platforms.

Chemical/Electrochemical Reactions

Chemical and electrochemical surface modification ofNCD using organic substances has also been performedby taking advantage of the carbon chemistry in the designof chemically modified surfaces.[152] A variety of func-tional groups has been introduced using primary alcoholgroups on hydroxylated nanodiamond using a chemicalapproach.[153] The reactivity of the terminal alcohol groupsis then exploited to create different functionalities on thediamond surface (halides, amines, cyanide, azide, andthiols). Silane-grafted nanodiamond particles are synthe-sized by functionalization of the hydroxylated diamondsamples and successfully applied as a novel solid phasein peptide synthesis.[154] Alternatively, long-chain carbox-ylic acid chlorides are reacted with hydroxylated diamondto introduce long alkyl groups in place of silane functionalgroups.[155] To graft oligomers with various functionalitiesincluding C(=O)OCH3, COOH, NH2, or aliphatic moieties,Chang et al. have used a graphitized surface[156] on H-UNCD. It is also possible to chemically graft (without elec-trochemical activation) diazonium salts by dipping thefreshly hydrogenated diamond surface in a saturated solu-tion of the diazonium for 72 hours.[157,158] Diazoniumchemistry is used to introduce brominated aryl groups ontothe diamond nanoparticles. The reaction leads to a radicalpolymerization reaction of tert-butyl methacrylate andsubsequent hydrolysis of the polymer brushes into poly(methacrylic acid) allows bovine serum albumin covalentimmobilization.[159] Electrochemical reduction of diazo-nium derivatives is an important route for surface functio-nalization of doped diamond and other substrates. Covalentfunctionalization of diamond surfaces through C–C bondscan be achieved by using aryldiazonium salts under rigor-ous ultrasonication.[128] Modification of NCD by alkenesfollowed by electrochemical reduction of diazonium saltshas been utilized to functionalize boron-doped UNCD dia-mond with amine groups.[160] Electroless grafting ontonanodiamond particles is an interesting alternative to

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electrochemical reduction of diazonium salts[161] becausenanoparticles cannot be grafted by electrochemical treat-ment when they are suspended in a liquid medium.Using a F2/H2 mixture at temperature varying from 150°Cto 470°C, Liu et al. have fluorinated nanodiamond.[149]

Refluxing of the diamond in sulfuric acid and nitric acid isa common method to introduce polar functional groups suchas hydroxyl, carboxylate, ester, and ether moieties onto thediamond surfaces.[151,162] The surface functionalities of acidrefluxed diamond can be further converted into other func-tional groups by converting the carboxylic acid moieties intoacid halides, followed by atom transfer radical polymeriza-tion (ATRP) in the presence of monomers and a radicalinitiator under deoxygenated conditions for a long reactiontime (>20 h). Polymethacrylate brushes have been attachedto nanodiamond by the so-called “grafting from” method ofATRP. Although several steps are necessary to attach theATRP initiators to the surface, the diamond polymer hybridsshow controlled dispersing properties.[149]

APPLICATIONS

Owing to its unique chemical, physical, andmechanical prop-erties, NCD can be used in a myriad of structural, biomedi-cal, and microelectronic applications.[117,122,123,126,136,144]

Micro-Electromechanical Systems

The superior physical, chemical, and tribomechanical prop-erties of diamond make it in principle an ideal material forMEMS, particularly in a harsh environment. However, thehigh intrinsic stress, rough surface, and inappropriate grainmorphology associated with conventional diamond filmsare drawbacks.[163] NCD and UNCD have distinct nanos-tructures but complementary properties.[134] Nowadays,diverse arrays of micro- and nanostructures have beendeveloped.[164–166] Wang et al. have produced NCD-based phononic structures (coupled mechanical resonatorarrays)[167] and UNCD-based resonators. Simple cantileverbeams fixed at both the ends have also been fabricated.[168]

These mechanical resonators have very high frequenciesdue to high stiffness (Young’s modulus) of NCD, and theyare excellent for mass and chemical sensors,[169] especiallyif the chemical termination of the surface can be con-trolled.[122,123,170] The grain boundaries doped with H2 ren-der NCD and UNCD as excellent dielectric materials, andtheir use as insulating dielectrics in radio-frequency MEMScapacitive switches has been demonstrated.[171]

Optical resonator structures and photonic crystals[172]

are also possible using NCD NEMS fabrication and even-tually may be integrated into mechanical resonator struc-tures for optical transduction of the mechanical motion. Thetechnology for fabricating monolithic UNCD AFM canti-levers with integrated tips has been developed,[144] andUNCD AFM probes, the first MEMS-type structures in the

market based on UNCD, are commercially available. ThisUNCDAFM tips exhibit practically no wear after extensivescanning on different surfaces as opposed to the high weargenerally observed from conventional Si- and Si3N4-basedAFM tips.[173]

Low-voltage MEMS/NEMS actuation can be achievedusing piezoelectric technology. Several piezoelectricmaterials such as langasite,[174] gallium phosphate,[175]

PZT,[176] ZnO[177] as well as AlN[178] have been combinedwith NCD to produce drivers for QCM, SAW, and canti-lever diamond structures.[179]

Electron Emission

NCD can be used as electron field-emitting materials thathave promising applications[136,180–182] due to low thresh-old field and strong emission current density.[181] Remark-able electron emission with turn-on fields of*1 V/µm andcurrent density of 4 × 10−4 amps/cm2 at 4 V/µm has beenachieved using NCD.[29] Zhu et al.[183] have reported ultra-low field emission from undoped nanostructured diamondand the emission behavior stems from the inherently defec-tive structure in the nanostructured diamond and low elec-tron affinity of the diamond surface.

Grain boundaries contain sp2-bonded carbon and pro-vide the conduction path for electrons, thus facilitatingelectron field emission.[29] Si field emitter arrays coatedwith UNCD emit electrons at as low as 2.6 V/mm.[184]

After exposure to H2, the field emission properties of theUNCD-coated Si field emitter arrays are enhanced. Thesignificant reduction in the turn-on voltage and increasein the emission current arise from the modification of theeffective work function at the localized emission sites.[185]

Nitrogen-doped NCD offers excellent field enhancementfactors on account of the small grain size, increased andcontrolled sp2 carbon content as well as n-type electricalconductivity and is a suitable electron-emitting material.Okano et al.[186] have obtained low-field electron emissionfrom nitrogen-doped diamond films prepared by HF CVD.The field emission properties of UNCD films are greatlyimproved after N ion implantation and annealing because Nions residing in grain boundaries can render the UNCDgrains semiconducting by charge transfer.[187]

Biotechnology

Because of the unique chemical, physical, electrical, andbiological properties, NCD has attracted much attention asa robust and stable bio-interface on implantable biosensorseven under harsh operating conditions. Surface terminationand dopants have been shown to be crucial to many appli-cations. NCD electrodes modified with DNA oligonucleo-tides have been used in field-effect biological sensors,[122]

and H-terminated NCD films offer an ideal platform forenzyme-based amperometric biosensors.[123] Biosensorsbased on carboxylated and aminated NCD films can deliver

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high sensing performance in terms of sensitivity, selectiv-ity, long-term stability, and regeneration ability, indicatingthat directly functionalized NCD films have big poten-tial.[117] Enzymatic[188] and nonenzymatic[189] glucose sen-sors based on the boron-doped NCD electrode boast highsensitivity and reproducibility, and NCD single photonsources and biomarkers have been fabricated.[190] An opti-cal sensor based on NCD loaded with horseradish peroxi-dase has been demonstrated to be highly sensitive towardhydrogen peroxide.[191]

NCD combines the virtues of surface smoothness, highcorrosion resistance, and biological tolerance. These favor-able features bode well for applications to medicine such assurgical tools and medical implants. NCD films induceimproved proliferation and differentiation of human osteo-blasts and neural stem cells compared to conventional poly-styrene petri dishes.[125] NCD has better resistance tobacterial colonization than medical steel and tita-nium.[122,192,193] Owing to the high wear resistance and lowcoefficient of friction of NCD, the amount of wear debrisgenerated during the joint functioning is reduced, therebyincreasing the life span of the prosthesis.[194] NCD can beused as a template for immobilization of active moleculesin biological and sensing applications.[117,122,123] The NCDsurface can also be modified by linking with antibody,human immunoglobulin G, which provide biomolecularrecognition capability and specificity characteristics. It canthus be used in biologically sensitive field effect transistor(Bio-FET).[195]

Electrode Applications

NCD is an outstanding material for electrochemical elec-trode, as it has a wide electrochemical window and lownoise,[107,110] and the utilization of NCD or UNCD in thisarea is an extension of implemented technology.[112,118]

Boron-doped NCD can be deposited on transparent sub-strates to produce a transparent electrochemical electrodesuitable for spectro-electrochemistry.[196,197] The reportedhigh conductivity of nitrogen-doped UNCD has spurredapplications to high-power electronic devices.[4] Boron-doped NCD electrode arrays prepared by microfabricationcan be used as electrochemical sensors, and they exhibitlow background currents, almost theoretically steady-statelimiting currents, and fast electron transfer rates close to0.01 cm s−1.[198] They have potential applications in elec-trophysiology, and boron-doped NCD electrodes havemany electroanalytical applications, e.g., determination ofsulfur-containing compounds,[199] post column detection ofnitrogen-containing molecules, and heavy metal analy-sis.[200–202] The electrode can be made to be highly con-ducting and active in electroanalytical processes in aqueousmedia, thus suitable for a wide range of processes includingoxidation of ferrocyanide, ascorbic acid, and nicotinamideadenine dinucleotide.[203] It has been shown that metal(gold and copper) deposition on NCD electrodes is fast and

efficient. The stripping efficiency (ratio of cathodic chargefor deposition to anodic charge for stripping) in these pro-cesses is close to unity under a wide range of conditions.

The pH- and ion-sensitive properties of an electrolyte-gateFET with diamond coatings have been investigated.[204,205]

For instance, a pH-sensitive electrolyte–diamond–insulator–semiconductor (EDIS) sensor with NCD coatings has beenreported by Christiaens et al.[206] The feasibility of using theEDIS structures with O-terminated NCD coatings in transdu-cers for multiparameter sensing of different (bio-) chemicalquantities such as pH, penicillin concentration as well ascharged macromolecules has been demonstrated.[207]

CONCLUSION

NCD is typically prepared using a CH4/H2 mixture or byreplacing partially or completely H2 with Ar, N2, and othergases. Mechanical pretreatment, seeding with particles, andchemical pretreatment can effectively enhance the diamondnucleation density on nondiamond substrates. The roles ofCH4, H2, and Ar in the growth of NCD are discussed. Byincreasing the CH4 concentration (from 1% to 100%), thegrowth rate, surface smoothness, and sp2 carbon content inthe film increase and the grain size decreases. H2 plays animportant role in stabilizing the sp3 carbon phase and etchingof the sp2 carbon phase. The Ar concentration shows obvi-ous effects on the grain size and deposition rate of NCD.Boron and nitrogen are common dopants in NCD coatings,and the doped materials possess good electronic properties,optical properties as well as electrochemical properties. Thequantitative methods for functionalization of NCD surfacesare also described. Incorporation of biological species intothe functionalized diamond surfaces in a controllable mannerenables the fabrication of biosensing devices which can beused in the fundamental study of living cell morphology incontact with the surface structures. NCD is an importantemerging material with immense potentials in different typesof applications especially pertaining to MEMS, electronemission, biotechnology, and electrodes.

ACKNOWLEDGMENTS

The authors thank the financial support from the HongKong PhD Scholars Program, Hong Kong Research GrantCouncil (RGC) General Research Funds (GRF) No. CityU112510 and the National Nature Science Foundation ofChina (No. 51002090).

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Nanocrystalline Diamond Coatings 873

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