Bigelow_2002_Surface-Science

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Role of surface and interface science in chemical

vapor deposition diamond technology

L.K. Bigelow a, M.P. D’Evelyn b,*

a Bigelow Consulting Corporation, Boylston, MA 01505, USA, formerly with Norton Diamond Film of Saint-Gobain

Industrial Ceramics, USAb

General Electric Corporate Research and Development, P.O. Box 8, Schenectady, NY 12301, USA

Received 15 August 2000; accepted for publication 3 May 2001

Abstract

Diamond is well known as the hardest material in nature. It also has other unique bulk physical and mechanical

properties, such as very high thermal conductivity and broad optical transparency, which enable a number of new

applications now that large areas of diamond can be fabricated by the new diamond plasma chemical vapor deposition

(CVD) technologies. However, some of the most interesting properties of diamond, including the ability to be grown

over large areas by CVD processes, result not from its bulk properties but from its special and unique surface chemistry.

The surface chemistry derived properties are as remarkable as the bulk properties, and in the end may enable the

development of new applications, technologies, and industries which are at least as important as those based on thebulk properties. Some of these surface properties are extreme chemical inertness, low surface energy, low friction co-

efficients, negative electron affinity, biological inertness, and high over-voltage electrode behavior. The surface science

and some of the interesting ongoing research in these areas are explored and illustrated, and unresolved questions are

highlighted. Ó 2001 Elsevier Science B.V. All rights reserved.

Keywords: Models of surface kinetics; Chemical vapor deposition; Growth; Surface chemical reaction; Surface energy; Diamond;

Polycrystalline surfaces

1. Introduction

Diamond is best known for its qualities as a

gemstone and for its use as an industrial abrasive.

As a gem, diamond is highly valued because it has

a crystal structure that can be cleaved to bring out

the brilliance and fire created by its high refractive

index and optical transparency. As an industrial

abrasive, it is unique because of its unmatchedhardness, which makes it the abrasive of choice to

grind or cut any other hard material, and the only

one that can be used to mechanically polish dia-

mond itself.

The development of large area coatings of dia-

mond by plasma chemical vapor deposition (CVD)

processes has made it possible to utilize of some of

diamond’s unique bulk and surface properties in

new applications that were not possible with the

small stones found in nature, or with high-pressure

Surface Science 500 (2002) 986–1004

www.elsevier.com/locate/susc

* Corresponding author. Tel.: +1-518-3877133; fax: +1-518-

3877563.

E-mail addresses: [email protected] (L.K. Bigelow),

[email protected] (M.P. D’Evelyn).

0039-6028/01/$ - see front matter Ó 2001 Elsevier Science B.V. All rights reserved.

P II: S0 0 3 9 -6 0 2 8 (0 1 )0 1 5 4 5 -X



synthetic diamond. Many of these new potential

opportunities stem not from the bulk properties

important in gemstones and abrasive applications,

but from the surface chemistry and surface prop-erties of diamond. It is the objective of this re-

view article to examine a few of the most interesting

of these new opportunities, including specula-

tive ones, and to discuss the surface science issues

which must be explored to understand and develop

them into commercially successful new techno-

logies.

2. Diamond growth overview

Many of the most interesting features of dia-

mond, including the CVD growth process itself,

derive from the special surface chemistry of dia-

mond. These surface chemistry related properties

and processes in the end may enable the develop-

ment of new applications, technologies, and in-

dustries which are at least as important as those

based on diamond’s strength and hardness. Some

of these special surface properties are extreme

chemical inertness, low surface energy, low friction

coefficients, negative electron affinity, biological

inertness, and unique behavior when used as anelectrode in aqueous solutions.

Diamond is one of the most stable chemical

structures in nature, but graphite has an even more

favorable free energy of formation than diamond

under normal ambient pressures and temperatures.

The Gibbs free energy of formation at room

temperature and 1 atm is 2.9 kJ/mol more negative

for graphite than for diamond [1]. When carbon

condenses under other than extreme pressures, the

formation of graphite is thermodynamically fa-

vored instead of the growth of diamond. If onewere to try to transform graphite to diamond at

room temperature, a pressure of at least 16,000

atm would be required, although the kinetics of

transformation would of course be vanishingly

slow at this temperature. However, at higher

temperatures the pressure stability region of

graphite expands even further, and even higher

pressures are required to favor the growth of dia-

mond. At 1700 K in excess of 50 kbar are required

to reach the diamond stability region, which cre-

ates a serious technical barrier to the growth of

synthetic diamond by high pressure processes.

Despite these barriers, synthetic diamond is now

grown on a large scale in high-pressure pressesunder diamond-stable conditions by dissolution of

graphite in metal ‘‘solvent-catalysts’’ and precipi-

tation of diamond. This process was developed in

the mid 1950s [2] and commercialized by General

Electric, followed soon after by De Beers and

others. The worldwide market for synthetic su-

perabrasives, including diamond and cubic boron

nitride grit, polycrystalline diamond and cubic

boron nitride (manufactured under high pressures

and high temperatures), and diamond and cubic

boron nitride powder, is approximately one billion

US dollars per year and continues to expand sig-

nificantly.

Questions remain concerning how diamond

forms in nature. Conventional models have dia-

mond growing over geological eons, 200 or more

kilometers below the surface of the earth under

high temperatures and pressures, and subsequently

being transported to the surface by primary vol-

canic intrusions. In South Africa, the type of vol-

canic rock commonly associated with diamonds is

called kimberlite, after the area where it was first

discovered. In the 1970s, in Australia, diamondswere also discovered in a significantly different type

of volcanic intrusion known as lamproite [3]. More

recently, it has also been proposed that diamond

can form in the interior of large planets such as

Uranus and Neptune, whose atmospheres contain

CH4, water, and ammonia, through the conversion

of methane to diamond at pressures in the range of

10–50 GPa and 2000–3000 K [4]. The authors

propose that this process of conversion to diamond

even produces enough energy to contribute to the

observed energy budget of these planets!

3. Surface chemistry of chemical vapor deposition

diamond growth

Growth of diamond by CVD [5,6] is driven by

the chemistry and kinetics of gas-phase and sur-

face reactions, rather than thermodynamics. Dia-

mond is typically grown using a dilute mixture of

a hydrocarbon such as methane in hydrogen.

L.K. Bigelow, M.P. D’Evelyn / Surface Science 500 (2002) 986–1004 987



Additional species, such as oxygen, nitrogen, flu-

orine, and chlorine, are sometimes used, but con-

sideration of the mechanistic role of the latter

species is beyond the scope of this article. Atomichydrogen is generated by inputting power into the

gas phase, for example, by means of a white-hot

filament, a microwave discharge, or an arc dis-

charge, which in turns reacts with the hydrocarbon

to form reactive species such as CH3 and C2H2.

These species are then transported to the substrate

surface, where the reactive precursors adsorb and

are subsequently converted to diamond by a series

of reactions that are still not completely under-

stood. Surface chemistry plays a crucial role in

determining the growth rate and quality of dia-

mond films, and surface science methods have

contributed significantly to the current level of

understanding of the growth mechanism.

Virtually all commercial CVD diamond is

polycrystalline, but on the atomic scale growth

takes place on single-crystal grains. The symmetry

of individual crystallites can be best understood by

reference to the crystal structure of diamond,

shown in Fig. 1. Each carbon atom is bonded to

four neighbors, and these tetrahedra are arranged

in such a way that eight atoms fit in a cube whose

edge is the lattice constant (3.567AA). Facets on

crystals always occur perpendicular to the slowest-

growing crystallographic directions, which in CVD

diamond normally correspond to the so-called

[1 0 0] and [1 1 1] crystallographic directions [7].

Referring to Fig. 1, the ideal-structure (1 0 0) sur-

face (perpendicular to the [1 0 0] direction) com-

prises vertically oriented zig-zag chains of atoms;

as each new atomic layer is added the direction of

the chains rotates by 90°. On ideal-structure (1 1 1)

surfaces each surface atom is bonded to three

atoms below to form an array of puckered hexa-gons. Square (1 0 0) and hexagonal or triangular

(1 1 1) facets characteristic of individual diamond

crystals (Fig. 1(d)) are also present in polycrys-

talline CVD diamond films, as shown in Fig. 2.

The properties of the film, including strength, abra-

sion resistance, fracture toughness, transparency,

surface roughness, etc., are determined by the

microstructure, including grain size and orienta-

tion, stresses, dislocations, and incorporation of

impurities. These impurities can include elements

such as nitrogen, or metals, or non-diamond

phases such as carbon atoms in a graphitic bond-

ing configuration, known as sp2 carbon. The

microstructure is in turn determined by the de-tailed surface growth mechanism, including the

relative growth rates in the [10 0] and [11 1]

directions, v1 0 0 and v1 1 1, respectively, and the

relative rates of generation of new crystallographic

directions (twinning or re-nucleation) versus con-

tinued growth in a given direction (homoepitaxy)

[8]. Examples of CVD diamond surface structures

that are produced by different growth chemistries

and growth conditions are shown in Figs. 2 and 3.

The polycrystalline structure of the diamond in

Fig. 2(a) is typical for many commercial coatings

on cutting tool inserts or rotary tools. The square

(1 0 0) diamond crystal faces shown in a surface

view in Fig. 2(b) and in a fracture cross-section in

Fig. 3 were grown within a transitional parameter

space between nearly pure (1 0 0) facet growth

and a mode which generates a microcrystalline

diamond morphology. The microcrystalline mor-

phology can be seen between the (1 0 0) faces in

Fig. 2, and the (1 0 0) growth columns in Fig. 3.

Our current level of understanding of the

atomic-scale mechanism of diamond CVD results

from a close interplay of experiment and modeling,although there are a number of basic questions

that remain unanswered. Under typical diamond

growth conditions there is good evidence that most

of the carbon atoms originate from methyl radi-

cals, CH3 [6,9–11]. However, diamond growth

from acetylene, C2H2, occurs with a lower reaction

probability [12,13], and under more extreme con-

ditions atomic carbon, C2, and alkyl halides can

act as diamond precursors, although the latter are

beyond the scope of this article.

What is the molecular-level mechanism bywhich CH3 and/or C2H2 become incorporated into

(1 0 0) and (1 1 1) diamond surfaces? To provide

a starting point for kinetic growth models, sev-

eral groups have examined hydrogenated (1 0 0)

and (11 1) diamond surfaces or facets after

quenching from growth conditions by scanning

tunneling microscopy (STM). On the (1 0 0) sur-

face, a number of authors have observed a (2Â 1)

reconstruction [14–16], that is, a doubling of the

surface unit cell size in one direction, as shown in

988 L.K. Bigelow, M.P. D’Evelyn / Surface Science 500 (2002) 986–1004



Fig. 4(a). (1 1 1) surfaces are often highly defective

on the micron scale, but unreconstructed (1 Â 1)

surface domains, that is, with a surface unit cell

that is indistinguishable from the ideal structure

(Fig. 1(c)), have been observed by several groups

[15,17], as shown in Fig. 4(b). These surface

structures are well understood. The linear features

on the (1 0 0)-2Â 1:H surface comprise rows of

C–C dimers, in which each surface carbon atom is

bonded to one hydrogen atom and has two back-

bonds to the diamond lattice, as shown schemati-

cally in Fig. 5(a). The orientation of the dimer

rows in adjacent atomic layers is rotated by 90°

due to the symmetry of the lattice. The length of

the monohydride dimer bond has been measured

as 1.60 AA by dynamical low energy electron dif-

fraction (LEED) [18], in excellent agreement with

theory [19–21]. The vibrational modes of the C–H

Fig. 1. Lattice structure of diamond. (a) Cubic unit cell, showing fourfold bonding to each carbon atom. (b) (1 0 0) surface (ideal

structure), comprising vertically oriented zig-zag chains of first- and second-layer atoms. (c) (1 1 1) surface (ideal structure), com-

prising puckered hexagons of first- and second-layer atoms. (d) Isolated diamond crystal, with square (1 0 0) and hexagonal (1 1 1)

facets.




dimer species are coupled, producing symmetric

and asymmetric stretch modes at 2919 and 2899

cmÀ1, respectively, as observed by surface infrared

spectroscopy [22], again in excellent agreementwith theory. The (1 1 1)-1Â 1:H surface comprises

a simple termination of the bulk lattice, with one

hydrogen atom bonded to each surface carbon

atom, as shown schematically in Fig. 5(b). The

C–C bond lengths are essentially unperturbed, as

established by dynamical LEED [23] and X-ray

diffraction [24], and the C–H bond produces a

sharp vibrational mode at 2832 cmÀ1, as observed

by infrared–visible sum frequency generation [25]

and infrared spectroscopy [26].

Diamond growth models beginning from the

(10 0)-2Â 1:H and (11 1)-1Â 1:H surface struc-

tures have been proposed by a number of authors,

employing methyl radicals [27–29], acetylene [30],or both [31–33] as assumed growth precursors.

The rate constants for most surface reactions

were estimated from analogous gas-phase reac-

tions, and relatively good agreement with experi-

mental growth rates has been achieved. A number

of global features of the surface growth mecha-

nism are well established. Incident hydrogen atoms

abstract (i.e., an Eley–Rideal mechanism) hydro-

gen from surface C–H bonds, creating dangling-

bond sites at which hydrocarbons can adsorb.

Further surface H-abstraction and rearrangement

reactions form new C–C bonds and remove the

remaining H atoms, incorporating the adsorbed

species into the diamond lattice. However, beyond

this global understanding a number of basic ques-

tions remain under active discussion. What are the

roles of different growth precursors, especially

CH3 and C2H2? Are etching (removal of adsorbed

CH x species) and/or surface migration important

during growth and in producing smooth surfaces?

We consider these questions briefly in the re-

mainder of this section but defer others, such as:

why are $104 hydrogen-atom surface reactionsrequired for each atom of diamond deposited [6]?

The most definitive test of a diamond growth

model is to test its predictions on all experimen-

tally observed surfaces, including the growth rate

as a function of temperature and gas composition

and the surface morphology. This program rep-

resents a challenge to theory based on kinetics of

elementary surface reactions because of the large

range of time scales involved and the large number

of atoms necessary to adequately address ques-

tions of surface morphology. To date this test hasbeen performed only by Battaile et al. [32,33], who

considered growth from both CH3 and C2H2 and

employed a sophisticated computer simulation

method capable of sampling a wide range of time

scales. These authors achieved reasonable agree-

ment with experimental homoepitaxial growth ki-

netics data in the [1 0 0] and [1 1 1] directions [7,34]

with two key findings. First, reasonable growth

rates in the [1 1 1] direction could only be achieved

if a significant level of C2H2 was present in the gas

Fig. 2. (a) Typical polycrystalline diamond coating on a cutting

tool. Growth steps, twinning, and apparent re-nucleation at

crystal plane intersections are visible. (b) Surface morphology

of CVD diamond grown in a transitional parameter space be-

tween dominant (1 0 0) (square facets) and microcrystalline

structures.




Fig. 3. Fracture cross-section of the film in Fig. 2(b), showing the columnar growth of the (1 0 0) crystals, and the interspersed mi-

crocrystalline diamond. The nucleating substrate surface is at the bottom of the micrograph.

Fig. 4. STM images of (a) (1 0 0)-2Â 1 domains and (b) (1 1 1)-1Â 1 domains on CVD-grown diamond surfaces (from Ref. [15], with

permission). In (a) the surface unit cell is doubled in size in the direction perpendicular to the rows — hence the (2Â 1) designa-

tion — whereas in (b) the surface unit cell is identical to that of an ideal surface where the atoms are frozen in place at their bulk

positions (cf. Fig. 1(c)).




phase in order to nucleate new layers, even though

most of the growth resulted from CH3. Second,

[1 0 0] growth was significantly faster than experi-

ment unless etching of CH x species, by a combi-

nation of H-induced reactions and simple thermal

desorption, was assumed to be relatively facile.

The latter conclusion was supported by high-level

ab initio calculations of the energetics of several

key surface intermediates.

Experimental tests of these conclusions is madedifficult by the presence of both CH3 and C2H2 in

virtually all diamond CVD reactors. However,

Martin and co-workers [11,12,35] have shown that

it is possible to grow high-quality diamond in

a flow tube, where the gas velocity is sufficiently

high that reactions interconverting CH3 and C2H2

are negligible in the region of diamond growth.

Recently, D’Evelyn et al. [35] have shown that

diamond growth from H and CH3 can produce

well-faceted cubo-octahedra, as shown in Fig. 6(a).

The observation of comparable growth rates inthe [10 0] and [1 1 1] directions –– implied by the

shapes of the crystals –– despite a very low partial

pressure of C2H2, appears to contradict the first

conclusion of Battaile et al. Growth from H and

C2H2 produced octahedra with a high concentra-

tion of contact twins, as shown in Fig. 6(b). The

latter results demonstrate that CH3 and C2H2 are

not interchangeable in the growth mechanism and,

in particular, that defect-free incorporation into

the lattice is less likely with C2H2, at least on the

(1 1 1) surface. The apparent absence of a nucle-

ation requirement by 2-carbon species during [1 1 1]

growth suggests that the steady-state growth sur-

face has a significant coverage of adsorbed species,

i.e., does not closely resemble the (1 1 1)-(1Â 1)

surface, which in turn suggests that etching reac-

tions may not as facile as modeled by Battaile et al.

[33]. Further support for a more complex steady-

state growth surface comes from recent modeling

work [36] and the dependence of STM surfaceimages on post-growth annealing/etching in atomic

hydrogen [37].

Most diamond growth models assume that hy-

drocarbon precursors either become incorporated

into the lattice where they stick initially or else

desorb, i.e., surface migration is negligible. Yet the

facets of as-grown CVD diamond surfaces are

normally relatively smooth on the nanometer-to-

micron scale, and surface diffusion is known to be

important in the production of smooth surfaces of

many materials. Random growth on arbitrarysurface sites should produce atomically rough

surfaces, in contradiction to experiment. Frenk-

lach and co-workers [29,38] have proposed that

surface migration is significant during [1 0 0] dia-

mond growth, at least on the 5–20 AA length scale,

and facilitates the growth of dimer rows and

smooth surfaces. These predictions are supported

by semi-empirical and high-level quantum chemi-

cal calculations of the structures and energetics

of various surface intermediates and reactions

Fig. 5. Atomic structure models of (a) (1 0 0)-2Â 1 and (b) (11 1)-1Â 1 diamond surfaces. Light circles: H atoms. Larger, smaller dark

circles: top-layer, lower-layer C atoms. The 90° rotation in the direction of the dimer rows in (a) as the surface height is raised by one

atomic layer is a simple consequence of the symmetry of the diamond lattice (Fig. 1(b)).




[29,38]. Direct evidence for surface migration

during growth is so far lacking, but it is not clear

that the degree of smoothness observed on CVD-

grown facets can be accounted for in the absence

of migration.

Separate experiments in hydrogen plasmas

(etching, rather than growth conditions) suggest

that etching and surface migration can occur under

growth conditions, but this remains to be firmly

established. Hydrogen plasma treatments can pro-

duce smooth (1 0 0) and (1 1 1) surfaces under someconditions [39–43], and can roughen [42–44] or

produce faceted pits [45,46] under other condi-

tions. Most authors have attributed the smoothing

phenomenon to etching. However, Rawles et al.

[42,43] found that the size of diamond particles

was unchanged during H-atom-induced facet for-

mation and was independent of particle packing

density and gas flow rates, and argued that this

observation supported surface migration but was

inconsistent with etching and etching/regrowth

mechanisms.

Definitive answers to the questions posed above

require further experimental and theoretical surface

science investigation. An improved fundamental

understanding of the CVD growth mechanism

should enable improvements in the quality and

decreases in the cost of CVD diamond. Particularly

valuable would be in situ measurement of the cov-

erage and type of surface hydrogen during growth;

quantification of the evolution of nanometer-scale

surface roughness; and the kinetics of etching and

surface migration induced by hydrogen atoms.

4. Properties

The chemical composition of the diamond sur-

face is important in determining certain physical

properties and for commercial applications. These

properties include friction coefficient, surface elec-

trical conductivity, surface energy for wetting and

bonding, negative electron affinity, and plasma

erosion resistance. As discussed above, the surface

bond structure and tendency to reconstruct de-pends sensitively on the crystal orientation, growth

mode and surface treatment process.

4.1. Surface bonding

In this section we discuss the bonding and en-

ergetics of several diamond surface structures that

are or might be important for applications. More

comprehensive reviews can be found elsewhere

[19,47–49].

Fig. 6. Diamond crystals grown in a flow-tube at 800 °C from

(a) HþCH3; (b) HþC2H2. The crystal in (a) is well faceted,

with (1 0 0) and (1 1 1) facets of similar area, which implies that

methyl radicals alone are capable of growing diamond in [10 0]

and [1 1 1] directions at similar rates. The crystal in (b) is ter-

minated by highly defective (1 1 1) surfaces only, which indi-

cates that [1 1 1] growth from acetylene leads to many defectsand is considerably slower than growth in the [1 0 0] direction.




Hydrogenated diamond surfaces are important

for applications as well as in CVD growth. Several

types of studies have demonstrated that the surface

C–H bond is essentially identical to that in alk-anes, with a bond energy of about 96 kcal/mol [50].

As noted above, the vibrational frequencies of

adsorbed hydrogen agree closely with theory. We

also saw in the previous section that abstraction of

surface hydrogen by gas-phase atomic hydrogen

plays a crucial role in the growth mechanism.

CVD modeling studies have assumed an activation

energy of about 7 kcal/mol [30,51], similar to that

in analogous gas phase abstraction reactions.

Krasnoperov et al. [52] measured an identical ac-

tivation energy for the surface abstraction reaction

on polycrystalline CVD diamond at pressures that

were high enough to reach thermal equilibrium

between the surface and the impinging gas-phase

species. A much smaller apparent activation en-

ergy has been observed under ultrahigh vacuum

conditions [53,54], which indicates that transla-

tional energy in the incoming H atom is more ef-

fective in surmounting the activation barrier than

is thermal motion of the surface atoms. The

sticking coefficient for atomic hydrogen on bare

surface sites appears to be approximately one.

The rates of desorption of H2 from the (1 0 0)-2Â 1:H and (1 1 1)-1Â 1:H monohydride surfaces

are approximately proportional to the surface

concentration of hydrogen atoms. This behavior is

unusual on surfaces –– hydrogen desorption rates

are more typically proportional to the square of

the surface concentration. The corresponding ac-

tivation energies for hydrogen desorption are 80–

88 kcal/mol [55,56] and 85–92 kcal/mol [25,57],

respectively, consistent with a C–H bond energy

near 96 kcal/mol. Following desorption the sur-

faces adopt a 2Â 1 reconstruction, comprisingC@C dimers linked by a r bond and a weak p

bond on (1 0 0) or p-bonded –C–C– chains on

(1 1 1) [19,47–49]. Naively, the simplest desorption

mechanism on the (1 0 0) surface would involve H

atoms on a dimer moving together and forming

an H–H bond while breaking two C–H bonds

and forming a surface p bond. However, such a

mechanism is symmetry forbidden [58] and, by

analogy to hydrocarbons, a much higher activa-

tion energy (120 kcal/mol for H3C–CH3 !

H2C@CH2 þH2 [59]) might be expected. The dy-

namical nature of the H2 desorption mechanism is

as yet unknown. The first-order kinetics imply that

hydrogen tends to cluster together on the surfacerather than being randomly distributed, but the

roles of p-bonding on the clean surface, bond

strain, surface diffusion, and perhaps other factors

remain to be elucidated.

The existence of a p bond (albeit highly strained)

on the clean (1 0 0)-2Â 1 surface suggests that so-

called Diels–Alder chemistry –– reaction between

one molecule with a C@C rþ p bond and a sec-

ond molecule with two C@C bonds separated by a

C–C r bond to form a cyclic structure –– might be

possible, creating new possibilities for functional-

ization of diamond surfaces. Recent studies have

shown this to be the case. Adsorption of 1,3-buta-

diene (H2C@CH–CH@CH2) occurs readily and

produces the expected –CH2 –CH@CH–CH2 –

product, as shown by infrared spectroscopy [60]

and high-resolution electron energy loss spectro-

scopy [61]. There is a twist, however, which ap-

parently results from the highly strained nature of

the surface p bond. Adsorption of a simple alkene

(cyclopentene), which is nominally symmetry for-

bidden (the analogous molecular reaction does not

take place) also occurs on the diamond surface[62], albeit with a low sticking coefficient.

Hydrogenated diamond surfaces are stable in

oxygen or air up to a temperature of about 300 °C,

above which the surface hydrogen is progressively

replaced by surface oxygen species [63].

The chemistry of oxygen-containing species on

diamond surfaces is important and, for the most

part, analogous to that in organic molecules. Un-

fortunately, perhaps, the details are fairly complex.

Oxygenated diamond surfaces typically comprise a

combination of bridge-bonded –O– and linear>C@O species and, if some surface hydrogen is

also present, –OH and –COOH groups [63–66].

Upon heating, adsorbed oxygen desorbs only as

carbon containing gas products, including CO and

CO2, starting at desorption temperatures near 600

°C [67]. Desorption occurs over a wider range of

temperatures for oxygen than for hydrogen, re-

flecting a range of binding energies as would be

expected from the range of structures. Frenklach

et al. have determined the activation energy for




desorption to be 45 kcal/mol by b-scission of a C–

C@O backbond [68].

Halogen termination of diamond might be ex-

pected to be very stable and to have useful prop-erties, by analogy to alkyl halides and TeflonTM.

However, in contrast to the alkane-like behavior

described above, the chemistry of halogens on dia-

mond surfaces shows large departures from the

behavior of analogous alkyl halides, which have

C–X bond energies of about 116 or 82 kcal/mol for

fluorine or chlorine, respectively. Experimental

evidence indicates that desorption of F and Cl is

atomic rather than recombinative [69]. Based on

bond energies calculated with a cluster analogue of

diamond and assuming a normal pre-exponential

factor, one would predict that less than 10% of a

monolayer would desorb upon heating to tem-

peratures of 1200 or 700 °C for F or Cl, respec-

tively [70]. In stark contrast, F and Cl begin

‘‘oozing’’ (desorbing) from the surface at or below

room temperature. Fluorine begins desorbing near

room temperature but residual F has been found

by some authors to remain at temperatures as high

as 1200 °C [69,71,72]. For chlorine on diamond

(1 0 0) and (1 1 1) desorption was observed over the

range À50 to 300 °C [69], while on diamond

powder a significantly higher desorption temper-ature range was observed, %300–1100 °C [73]. The

extremely broad range of desorption temperatures

imply a strongly coverage-dependent bond energy,

as low as 20 kcal/mol at full coverage. The bond

energies for F and Cl have been calculated as 103

and 48 kcal/mol at monolayer coverage [74],

somewhat reduced from the low-coverage results

[70]. However, it is clear that further contributions

to the destabilization of C–X bonds at high cov-

erage remain to be understood, whether from

steric repulsion, dipole repulsion, a decreasingability of the surface to donate charge to the more

electronegative halogen atoms, or other effects.

4.2. Surface energy

Surface termination of the diamond has im-

portant implications for many industrially impor-

tant processes such as metallization and bonding

to diamond. If the diamond is annealed at 1000 °C

in ultrahigh vacuum, the adsorbed hydrocarbons,

hydrogen and oxide species are removed, and the

surface reconstructs. A clean surface of C–C sat-

urated bonds is actually quite stable and unreac-

tive, at least by comparison to other materials suchas silicon. Fabis has reported on the effect of

plasma treatment of cleaned diamond surfaces

with H, O, and CF4 plasmas on the surface energy

of the diamond, and the impact that has on

polymer adhesion for electronics packaging [75].

The surface energy and the availability of elec-

trons for further bonding to materials on the

surface is strongly affected by these species termi-

nating the surface, as illustrated by the coverage

dependence of C–X bond energies. Fabis measured

the surface energy of diamond after various sur-

face treatments, using a series of liquids of differ-

ent surface tensions, following the work of Kaifu

and Komai [76]. As shown in Table 1, the O ter-

minated surface is the most wettable, and is also

hydrophilic.

Processing treatments which create polar oxy-

gen functionalities such as carbonyl (C@O) and

ether (C–O) are a key requirement for strong

bonding of polymers to diamond. In contrast,

clean high temperature annealed (1000 °C in 10À10

Torr) surfaces with C@C saturated bonds, hy-

drogen terminated surfaces, and even fluorineterminated surfaces are hydrophobic and are re-

sistant to wetting or bonding. XPS analyses re-

ported by Fabis indicate C–O and C@O surface

functional groups, and C–F (with some possible

C–O residuals) are present on the surfaces treated

with CF4 plasmas.

Table 1

Surface energy measurements of CVD diamond as a function of

surface treatment of polished samples (Ra¼ 10 nm), from

Fabis [75]

Surface treatment XPS func-

tional group

Surface energy

(dynes/cm)

1000 °C: 1200 s, at 10À10

Torr

C–C 22

H-plasma: 1800 s, 1 KW, C–H 28

2.45 GHz

CF4-plasma: 1800 s, 1 KW, C–F (C–O) 38

13.56 MHz

O-plasma: 1800 s, 1 KW, C–O, C@O 50

13.56 MHz




The surface activity of the polymer or metal

to be bonded to diamond is of course the other

half of the equation which determines the actual

strength of the bond to a treated diamond surface.The adherence of an epoxy polymer used in elec-

tronics plastics packaging under a series of boil/

peel cycles was found by Fabis to directly correlate

with the surface energy measurement in Table 1.

Surface treatment of the diamond after deposition

to increase its surface energy is also critical to the

adherence of blanket or patterned metallizations

for the fabrication of thermal management sub-

strates for electronic devices.

5. Applications

5.1. Surface factors in diamond friction

CVD diamond is known to have a low friction

coefficient under appropriate conditions. This is a

function of its surface termination state, and is af-

fected by prior processing and by the environment

and conditions of frictional contact, as well as the

surface roughness and crystal structure of the dia-

mond faces. In vacuum, diamond-on-diamond

sliding contact eventually leads to an increase infriction from initial values of 0.1 to 1.4, with sig-

nificant surface damage occurring in the process

[77]. This can be attributed first to shearing and

fracture of local high spots, and then to the removal

under high local forces of the critical surface termi-

nating species, such as O or H. Those species, when

present reduce the surface energy, and prevent local

atomic attraction or actual bonding across the in-

terface of carbon atoms at contact asperities.

Those surface chemistries and structures of

the diamond surface that promote low frictionalproperties are of major importance in bearing and

seal applications. These are presently under de-

velopment. They take advantage of the strong

bonding of diamond to its surface terminating

species, which reduces local bonding with the

contacting frictional face and reduces frictional

forces. In many applications, the chemical inert-

ness and low surface energy of diamond can also

prevent the build-up of deposits that would nor-

mally reduce seal life.

5.2. Diamond electrodes

The use of doped diamond electrodes in elect-

rochemical analysis and in electrochemical syn-thesis and destruction is an area of increasing

research interest and potential industrial impor-

tance. CVD diamond has three advantages over

conventional graphite electrodes used in electro-

chemistry, according to Angus et al. [78]. These are

chemical stability and robustness, large potential

window for water stability, and low background

current densities. Boron doping at low concen-

trations produces p-type semi-conductivity at an

acceptor level 0.37 eV above the valence band [79].

At high B concentrations, in the range of 1020 –1021

cmÀ3, diamond becomes a semi-metal with con-

ductivities as high as 10À3 X cm. These conduc-

tivities can be reached with B/C ratios in the gas

greater than about 500 ppm [78].

Conductive CVD diamond can be grown on

substrates such as Si, W, and Mo by doping with

boron in the gas phase. Angus et al. [78] has sur-

veyed the literature relating the ratio B/C in the

gas phase to B/C in the grown diamond. He found

a wide range of rates of incorporation for different

deposition processes, but the amount of B incor-

porated in the diamond was usually proportionalto the ratio in the gas for a given deposition ex-

periment. In general, boron incorporation was

greater in hot filament depositions than in micro-

wave growth, and was greater on (1 1 1) surfaces

than (1 0 0) surfaces. It is speculated that there may

be more activated species on the surface of the

diamond in microwave deposition, which reduces

the concentration of adsorbed boron on the sur-

face. The presence of oxygen also greatly reduces

the incorporation ratio of boron, apparently by

oxidation of boron to B2O3 [80].Highly conductive diamond is desirable for

electrosynthesis and electrodestructive applica-

tions, while electrochemistry can utilize diamond

of lower conductivity. Further, higher quality di-

amond with less incorporation of non-diamond

carbon produces a broader window of water sta-

bility for electrochemistry (Fig. 7).

This figure shows the difference in range of

water stability in electrolysis experiments in 0.5 M

H2SO4 for high quality diamond, low quality




diamond, platinum, and highly oriented pyrolytic

graphite (HOPG). In this type of experiment, one

varies the electrical potential and measures the

current that flows as substances dissolved in an

aqueous solution break down or dissociate or as

the water itself is dissociated into hydrogen and

oxygen. Electrochemical reactions of dissolved

species can only be observed usefully if the water it-

self is not decomposing! For high quality diamond,

an aqueous solution evolves hydrogen at a poten-

tial of À1.25 V and oxygen at a potential of þ2.3

V, a much wider range than the corresponding

Fig. 7. Effect of type of electrode (diamond/platinum/HOPG), and of diamond quality on electrode potential range for water

decomposition. (from Angus et al. [78], reproduced by permission of the publisher of New Diamond and Frontier Carbon Tech-

nology, MYU K.K.). The horizontal axes show the electrochemical potential applied to the diamond electrodes relative to astandard hydrogen reference electrode (SHE). The current flow (vertical axes) increases sharply at the potential at which breakdown

of the aqueous electrolyte into hydrogen (negative potential) or oxygen (positive potential) takes place. Note that the high quality

diamond electrode (a), has the widest range of negative and positive potential before breakdown of the aqueous solution (À1.25 to

þ2.3 V). The low surface energy of the hydrogen-terminated surface of diamond may contribute to this behavior. The potential

range observed with lower quality diamond (b) is less than with high quality diamond, probably because the defects on the surface

are graphitic in behavior, and are more reactive. The platinum (c) and HOPG (d) electrodes result in the breakdown of the

electrolyte at lower potential limits than either of the diamond types. The importance of this behavior of diamond electrodes is that

electrochemical methods can be used to react and destroy many chemicals, such as contaminants in waste water, before actually

decomposing the water itself. Electrochemistry with CVD diamond electrodes therefore has potential as a method of waste water

treatment. CVD diamond electrodes can also precipitate heavy metal from waste water, and can power a number of chemical

reactions in water, other solutions, and molten salts, while often exhibiting much greater resistance to attack and corrosion than

other materials. Diamond electrodes can be used to detect and measure a much larger range of dissolved substances than electrodes

of other materials by measuring the breakdown characteristics of the compounds dissolved in the aqueous electrolyte. Examples of

substances that have been detected via diamond electrodes include polyalkylene glycol, dopamine, azide, sulfa drugs, and hista-

mines.




values of 0.0 and þ1.5 V observed with a platinum

electrode. Non-diamond carbon, which is present

in larger amounts in lower quality diamond re-

duces the range of accessible potentials to nearthat observed with HOPG. The high reaction

stability of the diamond surface has been attrib-

uted to hydrogen termination [81] which, as dis-

cussed in the previous section, leads to a low

surface energy and hydrophobic behavior. How-

ever, even diamond surfaces that have been oxi-

dized by local generation of oxygen at high

positive potentials exhibit high sensitivity and

stability, which makes diamond suitable as an

electrode for electrochemical determinations under

oxidizing conditions, for example, dopamine in

ascorbic acid [81]. The importance of diamond

quality in resisting attack during electrochem-

ical processes is pointed out by DeClements and

others, following the results of their research on an-

odic polarization in concentrated KOH [82]. Low

quality diamond films are attacked at the grain

boundaries, but separate high quality film samples

showed no signs of corrosion or morphological

damage under the same experimental conditions.

Diamond electrodes can be used for analyzing

dissolved compounds that cannot be detected with

other electrodes. This is possible because the wateris dissociated at a lower potential with these other

electrodes than are the dissolved compounds. For

example, the presence of polyalkylene glycol can

be analyzed using diamond electrodes, whereas

platinum electrodes cause electrolysis of the water

before oxidation of the polyalkylene glycol, and

the reaction potential for the glycol is not reached

[78]. With diamond, the peak heights are linear

with the glycol concentration. The wide potential

window and low baseline current of diamond

make this possible.Other electroanalytical applications have been

demonstrated on diamond that are unreliable or

impossible on glassy carbon (GC). Some of these

are detection or measurement of azide, polyam-

ines, sulfa drugs, histamines, and NADH, a co-

enzyme used in several dehydrogenase-based

biosensors [81].

A great deal of interest and research activity is

focussed on commercially developing the electro-

chemical processing capability of diamond elec-

trodes, both for, (1) the destruction of pollutants

and for, (2) the generation of desireable chemical

compounds and elements. For example, efficient

fluorination of 1,4-diflurobenzene using borondoped diamond has been demonstrated by Okino

et al. [83]. The diamond electrodes show much

higher stability in this process than Pt or HOPG

[83]. Compton has recently showed diamond is

an excellent material for the sonoelectrochemical

production of hydrogen peroxide from oxygen

with its combination of mechanical properties and

electrode characteristics [84]. The effect of hydro-

gen plasma pre-treatment of the diamond surface

in sonoelectrochemical processing of dioxygen has

also been studied [85].

Industrial applications of the diamond elec-

trode technology are under active development for

pollution control, for example in treatment of

waste water effluent from industrial plants. The

oxidation of organic compounds to CO2 at the

surface of a diamond electrode has been demon-

strated with a current efficiency greater than

85% [86]. Experiments on the oxidation of phenol

(an example of an aromatic compound), and acetic

acid (an example of an aliphatic compound)

were successful in reducing these contaminants to

low levels (less than 3 ppm in the case of phenol).The electrodes were not damaged or poisoned

after weeks of operation. Cyanide can also be

eliminated, especially in the presence of chlo-

ride ions, which help to catalyse the oxidation re-

action.

Large area (50Â 60 cm2) boron doped dia-

mond electrodes on substrate materials such as

titanium, zirconium, niobium, tantalum, tungsten,

and graphite have been developed [87]. Examples

of boron doped diamond electrodes produced on

these materials by the Fraunhofer Institute areshown in Fig. 8. Further research to understand

the details of the oxidation and reduction reactions

at the diamond surface will lead to electrodes op-

timized for industrial applications. The reduction

of cadmium and copper from aqueous solutions

has also been demonstrated [88]. These toxic

metals precipitate out as fine particulates which do

not adhere to the diamond electrode, thus per-

mitting easy disposal of the precipitated metals,

and re-use of the electrodes. The clean surface that




remains is a direct result of the low surface energy

and chemical stability of diamond.

6. Future research directions

Some opportunities for improved fundamental

understanding of diamond surface and growth

chemistry have already been given. In this section

we enumerate some novel applications.

6.1. Negative electron affinity

Diamond has the property that with certain

types of surface termination, such as clean or hy-drogen terminated surfaces, the conduction band

minimum lies above the vacuum level at the sur-

face, and the diamond exhibits negative electron

affinity. This means there is no activation barrier

to ejection of an electron from the surface, in

contrast to most other materials, which require an

applied potential at the surface or a high temper-

ature to initiate electron emission into space. This

behavior has created considerable research interest

[89–91] because it could potentially lead to highly

efficient cold cathode emission devices, and prod-

ucts such as large area flat panel displays. The

market for field emission displays is potentially

very large market because the devices would bebright (‘‘sun visible’’) and highly energy efficient.

However, the performance of diamond in field

emission display development to date has been

disappointing because the density of emission sites

has been too low, and the emission current has

also been too low. A factor in this is the lack so far

of an effective way to n-type dope the diamond to

support significant emission from NEA surfaces or

from impurity based conduction sites. Nanotubes

appear at present to be the most promising ap-

proach to field emission sources for flat paneldisplays.

6.2. Bio-sensors and detectors

The surface and electronic properties of dia-

mond also make it a candidate for the develop-

ment of new bio-sensors and detectors, and

advanced diagnostic equipment such as in vivo

blood chemistry monitors. First of all, the extreme

inertness, especially of the hydrogen terminated

Fig. 8. Boron doped diamond electrodes on titanium, tantalum, niobium and graphite base material produced by the Fraunhofer

Institute, Braunschweig. (Reproduced by permission of New Diamond and Frontier Carbon Technology — from vol. 9, pp. 229–240.)




diamond surface, means that diamond is the most

inert and least likely to be rejected of all materials

for both structural and sensor applications in the

body. This bio-inertness and low tendency to in-duce blood clots has been reported many times.

Significant in vivo testing of products has not yet

begun, but there is growing activity to develop

diamond based body replacement parts, such as

permanent hip joint replacements and heart valve

components. This is an area for important future

research and commercial competition.

Troupe et al. have reported on diamond based

glucose sensors. The diamond electrodes have to

be boron doped to provide adequate conductivity

for use in this application [92]. The diamond was

doped by implantation with boron and annealed at

850 °C to provide adequate surface conductivity

(with a calculated 1Â 1020 atoms/cmÀ3 at the

surface). Stability of the diamond and the boron

in the lattice was measured by immersion in de-

ionized water for four weeks at 37 °C. The con-

centration of boron in the water reached only

4 ppb, indicating virtually no leaching of boron

from the diamond surface. Several designs were

tried. A device using an oxidized, activated dia-

mond surface, with specially selected adsorbed

bio-molecules to assist in the surface reactionsprovided the best sensitivity for glucose detection.

A range of new bio-agent and chemical detec-

tors have been proposed, based on the wide band

gap of diamond, in combination with its inert

and stable surface. A recently demonstrated tech-

nology with the potential to identify a wide

range of absorbed species on the diamond sur-

face and thereby detect dangerous bio-agents or

process-critical chemical constituents is called

charged-coupled deep level transient spectroscopy

(Q-DLTS). The principle is that adsorption of anycompound on the surface of the diamond will re-

sult in a change in the electronic states at the di-

amond surface, which can be characterized by its

effect on the transient response to imposed voltage

pulses. The voltage pulses may be applied by an

electrode grid on the surface of the diamond. The

surface potential energy and the density of trap-

ping centers will be characteristically different with

the adsorption of different species on the surface

of the diamond, and thus can be detected and

quantified by analyzing the response to the voltage

pulses. This surface analysis technique has been

demonstrated in a detector using DLC on quartz.

Adsorbed species (isopropyl alcohol and water)

can be differentiated by the transient response to

pulses from the array of inter-digital electrodes on

the surface [93]. A schematic of one design concept

using CVD diamond is shown in Fig. 9.

It is expected in this method that the effect of

different adsorbed species on the surface will beindependent of each other, so that many agents

can be detected by a single detector. Much work

remains to characterize and catalog the effect of

each adsorbate on diamond made with different

surface textures, surface treatments, and surface

terminations. However, the Q-DLTS method is

sensitive enough to detect a charge of 2000 elec-

trons on the surface, and may be able to detect

airborne contaminants in the parts per trillion

range. Bio-agents are typically transmitted by

Fig. 9. Schematic of detector grid and design for proposed

Q-DLTS chemical and bio-agent sensor using CVD diamond.

This detector is based on the effect that adsorbed species on the

surface of a diamond film have on the transient response to

electrical pulses applied to the surface. Cyclic bias pulses are

applied to the surface via a grid, changing the charge state of

the surface and of subsurface electronic trapping centers. The

surface charge is measured by integrating the current that flows

as the surface charge decays when the bias voltage is turned off.

The charging and decaying behavior is affected by adsorbed

molecules on the surface, which can be differentiated and

identified by their characteristic effects and decay signatures.

The frequency of the pulses is varied to produce curves of

charge versus pulse duration that are characteristic of the sys-

tem and the adsorbed species.




aerosols. The Q-DLTS sensor is sensitive to par-

ticles. It is hoped that bio-agents such as viral

agents, toxins and spores may be detectable di-

rectly or indirectly by developing a database of characteristic and differentiable effects on the

transient response of the diamond. Such systems,

designed with compact diamond detector sub-

strates and with an analytical database on-board

for each species of concern, should be easily por-

table. This would make the devices ideal for indi-

vidual use for working near hazardous chemicals,

or for field use to monitor and protect military and

civilian personnel against chemical or biological

agents.

The potential uses for disease control, hospital

and industrial air quality monitoring, and chemi-

cal process monitoring and control are staggering,

if the technology proves to be as sensitive as pro-

jected and also manufacturable and cost effective.

Surface science will play a key role in identifying

the important surface variables and treatments to

optimize sensitivity for different types of chemicals

and agents of interest. An added feature, which is a

benefit of the bulk properties of diamond is the

possibility to heat the detector, using a resistive

grid beneath the substrate or by another method.

This would desorb the adsorbed species from thesurface so the device could continuously monitor

and update the environment. It would become re-

useable over long periods in service.

7. Conclusion

A detailed understanding and application of

surface science is essential to the present industrial

use of diamond and to the development of the

exciting new products diamond will make possible.An example of an important practical problem

overcome by surface studies was reviewed earlier,

the problem of metallization of diamond. Until the

factors that affect the surface energy of diamond

were understood and the solutions applied, a great

deal of difficulty and inconsistency was experi-

enced by those trying to bond patterned or blanket

metallization to diamond. However, with appro-

priate pre-treatment, excellent adherence and reli-

able product performance has been achieved. It is

certain that the same principle will apply to the

development of many other new diamond prod-

ucts, including the selected ones we have reviewed

here.

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