Bhushan B. Nanotribology and Nanomechanics: An Introduction

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910 Bharat Bhushan

Table 17.3. Names and formulae of selected alcohols, ethers, phenols and thiols

Name Formula

Alcohols R

−

e.g., methanol CH

ethanol CH

Ethers R

−



e.g., dimethyl ether CH

−

diethyl ether CH

−O−CH

Phenols C

OH or

Thiols

−

e.g., methanethiol CH

The letters R− and R



− represent an alkyl group. The R− groups in

ethers can be the same or diﬀerent and can be alkyl or aromatic (Ar)

groups

Table 17.4. Names and formulae of selected aldehydes and ketones

Name Formula

Aldehydes RCHO or

RCH

ArCHO or

Ar C H

e.g., methanal or formaldehyde HCHO

ethanal or acetaldehyde CH

CHO

Ketones RCOR



RCR'

RCOAr or

RCAr

ArCOAr or

Ar C Ar

e.g., butanone or methyl CH

COCH

ethyl ketone

The letters R and R



represent an alkyl group and Ar represents an

aromatic group

17 Self-Assembled Monolayers (SAMs) for Controlling Adhesion 911

Table 17.5. Names and formulae of selected carboxylic acids and esters

Name Formula

Carboxylic acid

RCOOH or

RCOH

ArCOOH or

Ar C OH

e.g., methanoic acid HCOOH

(formic acid)

ethanoic acid CH

COOH

(acetic acid)

octadecanoic acid CH

(CH

)

COOH

(stearic acid)

Esters

RCOOR



RCO

acid

alcohol

e.g., methyl propanoate CH

COOCH

The letter R represents an alkyl group and Ar represents an aro-

matic group

The letter R represents a hydrogen, an alkyl group or an aromatic

group and R



represents an alkyl group or an aromatic group

Table 17.6. Names and formulae of selected organic nitrogen compounds (amides and

amines)

Name Formula

Amides RCONH

RCNH

e.g., methanamide HCONH

(formamide)

ethanamide (acetamide) CH

CONH

Amines RNH

e.g., methylamine CH

ethylamine CH

The letter R represents an alkyl group or aromatic group

912 Bharat Bhushan

(ArCOOH).Carboxylic acidswith evennumbersof carbon atoms,n (rangingfrom 4

to about 20), are called fatty acids (for example, n = 10, 12, 14, 16 and 18 are called

capric acid, lauric acid, myristic acid, palmitic acid, and stearic acid, respectively).

Esters are alcohol derivatives of carboxylic acids, where the hydrogen of the

hydroxyl group is replaced with an alkyl or aryl group. Therefore, their general

formula is RCOOR



, where R can be a hydrogen, alkyl group or aromatic group,

and R



may be an alkyl group or aromatic group but not a hydrogen. Esters are

found in fats and oils. Some examples are shown in Table 17.5.

Table 17.7. Some examples of polar (hydrophilic) and nonpolar (hydrophobic) groups

Name Formula

Polar

Alcohol (hydroxyl)

−

Carboxylic acid

−

COOH

Aldehyde

−

COH

Ketone

RCR

Ester

−

COO

−

Carbonyl

Ether R

−

Amine

−

Amide

C NH

Phenol

Thiol

−

Trichlorosilane SiCl

Nonpolar

Methyl

−

Triﬂuoromethyl

−

Aryl (benzene ring)

The letter R represents an alkyl group

17 Self-Assembled Monolayers (SAMs) for Controlling Adhesion 913

Table 17.8. Organic groups listed in increasing order of polarity

Alkanes

Alkenes

Aromatic hydrocarbons

Ethers

Trichlorosilanes

Aldehydes, ketones, esters, carbonyls

Thiols

Amines

Alcohols, phenols

Amides

Carboxylic acids

Amides and Amines

Amides and amines are organic compounds containing nitrogen. Amides are nitro-

gen derivatives of carboxylic acids where the carbon atom of the carbonyl group

is bonded directly to the nitrogen atom of an

−

NHR, or

−

group in-

stead of the oxygen of a hydroxyl group. The characteristic structure of an amide is

RCONH

An amine is a substitutedammoniamoleculewhere at least oneof the hydrogens

attached to nitrogen is replaced with an alkylor aryl group.It thereforehas a general

structure of RNH

NH or R

N, where R is an alkyl or aromatic group. Some

examples are shown in Table 17.6.

17.2.3 Polar and Nonpolar Groups

Table 17.7 summarizes the polar and nonpolar groups commonly used to construct

hydrophobic and hydrophilic molecules. Table 17.8 lists the relative polarities of

selected polar groups [64]. Thiol, silane, carboxylic acid, and alcohol (hydroxyl)

groupsare the polaranchor groupsmost commonlyused to attach to surfaces.Silane

anchor groups are commonly used for Si or SiO

surfaces because

−

bonds

are strong. Methyl and triﬂuoromethyl are commonly used as end groups for hy-

drophobic ﬁlm surfaces.

17.3 Self-Assembled Monolayers: Substrates, Spacer Chains;

and End Groups in the Molecular Chains

SAMs are formed as a result of the spontaneous self-organization of functional-

ized organic molecules into stable, well deﬁned structures on substrate surfaces,

Fig. 17.3. The ﬁnal structure is close to or at thermodynamic equilibrium and so it

tends to form spontaneously and it rejects defects. SAMs consist of three building

914 Bharat Bhushan

Surface terminal group

Spacer chain

Chemical bond

at the surface

Surface-active

head group

Interchain

van der Waals

and electrostatic

interactions

Self-assembly molecules

Substrate

A single chain

Substrate

Fig. 17.3. Schematic of a self-

assembled monolayer on

a surface and associated

forces

groups: a head group that binds strongly to a substrate, a surface terminal (tail or

end) group that constitutes the outer surface of the ﬁlm, and a spacer chain (back-

bone chain) that connects the head and surface terminal groups. The SAMs are

named according to the surface terminal group (as opposed to the spacer chain and

the head group), or the type of compound formed at the surface. In order to con-

trol hydrophobicity, adhesion, friction, and wear, it should strongly adhere to the

substrate and the surface terminal group of the organic molecular chain should be

nonpolar. To obtain strong attachment of the organic molecules to the substrate, the

head group of the molecular chain should contain a polar terminal group that as-

sociates with the surface in an exothermic process (energies on the order of tens

of kcal/mol); in other words it results in an apparent pinning of the head group to

a speciﬁc site on the surface through a chemical bond. Furthermore, their molecular

structures and the presence of crosslinking both have a signiﬁcant eﬀect on their

friction and wear performance. The substrate surface should have a high surface

energy (hydrophilic), so that there is a strong tendency for molecules to adsorb on

the surface.The surfaceshould be highly functional, with polar groups and dangling

bonds (generally unpairedelectrons), so that it can react with organicmolecules and

providea strong bond.Because of the exothermichead group–substrateinteractions,

molecules try to occupy every available binding site on the surface, and during this

process they generally push together molecules that have already been adsorbed.

This process results into the formation of ordered molecular assemblies. The inter-

actions between molecular chains are van der Waals or electrostatic in nature, with

energies on the order of a few (< 10) kcal/mol (exothermic). The molecular chains

in SAMs are not perpendicular to the surface; the tilt angle depends on the anchor

group as well as on the substrateand the spacer group.For example,the tilt angle for

alkanethiolate on Au is typically about30−35

◦

with respect to the substrate normal.

17 Self-Assembled Monolayers (SAMs) for Controlling Adhesion 915

Table 17.9. Selected substrates and precursors commonly used to create SAMs

Substrate Precursor Binding with Substrate

Au R SH (thiol) RS

−

Au Ar SH (thiol) ArS

−

Au RSSR



(disulﬁde) RS

−

Au RSR



(sulﬁde)

Si/SiO

, glass RSiCl

(trichlorosilane) Si

−

Si (siloxane)

Si/Si

−

H RCOOH (carboxyl) R

−

Metal oxides RCOOH (carboxyl) RCOO

−

...MO

(e.g., Al

,SnO

,TiO

)

R represents alkane (C

2n+2

) and Ar represents aromatic hydrocarbon. These consist of

various surface-active headgroups, usually with a methyl terminal group

Table 17.9 lists selected systems that have been used to form SAMs [51]. The

spacer chain of the SAM is often an alkyl chain ((

−

)

) or a derivatized alkyl

group. By attaching diﬀerent terminal groups to the surface, the ﬁlm surface can

be made to attract or repel water. Commonly used surface terminal groups in hy-

drophobic ﬁlms with low surface energy, in the case of a single alkyl chain, include

nonpolar methyl (

−

) or triﬂuoromethyl (

−

) groups. For hydrophilic ﬁlms,

commonly used surface terminal groups include alcohol (

−

OH) or carboxylic acid

(

−

COOH) groups. Commonly used surface-activehead groups include thiol (

−

SH),

silane (such as trichlorosilane or

−

SiCl

) and carboxyl (

−

COOH) groups. The sub-

strates most commonly used are gold, silver, platinum, copper, hydroxylated (acti-

vated) surfaces of SiO

on Si, Al

on Al, and glass.

The substrate surface should be clean before deposition. For silicon substrates,

a concentrated HF solution (typically 49% HF) is commonly used to remove the

oxide layer, and then the surface is rinsed with deionized water [45,46]. Hydrogen

passivates the surface by saturating the dangling bonds resulting in a hydrogen-

terminated silicon surface with hydrophobic properties. In the deposition of mul-

timolecularly thick polymer ﬁlms with nonpolar ends, hydrophobic substrates can

lead to a coated surface with a high contact angle, and are therefore preferred. The

substrate should be hydrophilic for SAM deposition in order to ensure strong in-

terfacial bonds with head groups. Hydroxylation of oxide surfaces is carried out to

make them hydrophilic. Silicon and other metals get oxidized and hydroxylated to

some degree when exposed to the environment. Bulk silicon, polysilicon ﬁlm or

SiO

ﬁlm surfaces are commonly treated by immersing them in Piranha solution

(a mixture of typically 3 : 1 v/v98%H

:30%H

) at temperatures of about

◦

C for about 30 min, followed by rinsing in deionized water, to produce a hy-

droxylated silica surface [23,45,46]. Piranha solution also removes any organic and

metallic contaminants, whereas HF would not necessarily remove organics. Oxy-

gen plasma is another technique used to hydroxylate SiO

as well as polymer sur-

faces [43,44]. For complex silicon geometries, oxygen plasma may be preferable.

Figure 17.4 shows schematics of surfaces after various surface treatments. Surfaces

916 Bharat Bhushan

Fig. 17.4. Schematic show-

ing HF-treated silica and the

hydroxylation process that oc-

curs on silica and elastomeric

surfaces treated with Piranha

solution and oxygen plasma,

respectively

after piranha or oxygen plasma treatment remain hydrophilic for a few hours to

about a day and become hydrophobic when they come into contact with carbon.

They can retain hydrophilicity longer in dry nitrogen. To retain their hydrophobic-

ity, the polymers are generally stored in DI water. Surfaces treated with HF remain

hydrophobic for about 2–3h and retain their hydrophobicity longer in dry nitrogen.

Epitaxial Au ﬁlm on glass, mica or single-crystal silicon (produced by e-beam

evaporation) substrates are commonly used because they can be deposited on

smooth surfaces as an atomically ﬂat and defect-free ﬁlm. To get the organic

molecules to pack together and provide a better ordering, the substrate should be

chosen such that the cross-sectional diameter of the spacer chain is equal to or

smaller than the distance between the anchor groups attached to the substrate. In

the case of alkanethiol ﬁlm, the advantage of Au substrate over SiO

substrate is

that it results in better ordering because the cross-sectional diameter of the alkane

molecule is slightly smaller than the distance between the sulfur atoms attached to

the Au substrate (≈0.53nm). The thickness of the ﬁlm can be controlledby varying

the length of the hydrocarbon chain, and the surface properties of the ﬁlm can be

modiﬁed by the terminal group.

SAMs are usually produced by immersing a substrate in the solution containing

a precursor (ligand) that reacts with the substrate surface or by exposing the sub-

strate to the vapor of the reactive chemical precursor [24]. A schematic of a vapor

deposition system is shown in Fig. 17.5. Samples are placed in a quartz reaction

tube. A silane bubbler is used to introduce gaseous silane into the quartz reaction

tube, which is placed in an oven at a controlled temperature. An inert gas (N

)

is used as the carrier gas. A by-product condenser is used to trap the by-products

and/or unreacted silane.

Some SAMs have been widely reported. SAMs of long-chain fatty acids

2n+1

COOH or (CH

)(CH

)

COOH (n = 10, 12, 14 or 16) on glass or alumina

substrates have been widely studied since the 1950s [12,20,21,24,25].Probably the

17 Self-Assembled Monolayers (SAMs) for Controlling Adhesion 917

flow

Silane

bubbler

Oven Quartz reaction tube

Exhaust

By-products

condenser

Liquid N

Substrate

Temperature-

controlled

oil bath

Fig. 17.5. Schematic showing vapor phase deposition system for silane SAMs [43]

most studied SAMs to date are n-alkanethiolate (n-alkyl and n-alkane are used in-

terchangeably)monolayersCH

(CH

)

−

preparedby the adsorptionof alkanethiol

−

(CH

)

SH solutiononto Au ﬁlm [39–41,51], and n-alkylsiloxanemonolayers pro-

duced by the adsorption of n-alkyltrichlorosilane (

−

)

SiCl

onto hydroxylated

Si/SiO

substrate with siloxane (Si

−

Si) binding; see Fig. 17.6 [43,44,65]. (Note

that “siloxane” (Si

−

Si) refers to the bond, while “silane” (Si

2n+2

), which de-

scribes covalently bonded compounds containing the elements Si and other atoms

or groups such as H and Cl to form SiH

and SiCl

, refers to the head group of

the precursor. These terms are used interchangeably.) Jung et al. [66] have pro-

duced organosulfur monolayers – decanethiol (CH

)(CH

)

SH and didecyl sul-

ﬁde CH

(CH

)

−

(CH

)

–onAuﬁlms.Geyer et al. [67], Bhushan and

Liu [39], Liu et al. [40], and Liu and Bhushan [41] have produced monolayers of

1, 1



-biphenyl-4-thiol(BPT) on Au surfaces, where the spacer chain of the ﬁlm con-

sists of two phenyl rings with hydrogen end groups. Bhushan and Liu [39] and

End group,

e.g., CH

–(CH

)–

Silica

Silicon substrate (100)

Fig. 17.6. Schematics of

a methyl-terminated, n-

alkylsiloxane monolayer on

Si/SiO

918 Bharat Bhushan

Liu and Bhushan [41] have also reported monolayers of 4, 4



-dihydroxybiphenyl

on Si surfaces. Bhushan et al. [43, 46], Kasai et al. [44], Lee et al. [45] and Ta o

and Bhushan [48] have produced perﬂuoroalkylsilaneon Si surfaces and Tambe and

Bhushan [47] have produced alkylphosphonate on Al surfaces.

17.4 Tribological Properties of SAMs

The basis for the molecular design and tailoring of SAMs should be complete

knowledge of the interrelationships between the molecular structures and tribolog-

ical properties of SAMs, as well as a deep understanding of the adhesion, friction

and wear mechanisms of SAMs at the molecular level. Friction and wear proper-

ties of SAMs have been studied on the macro- and nanoscale. Macroscale tests are

conducted using a so-called pin-on-disk tribotester apparatus, in which a ball speci-

men slides against a lubricated ﬂat specimen [9,10]. Nanoscale tests are conducted

usinganatomicforce/friction force microscope (AFM/FFM) [4, 5, 9, 10]. In the

AFM/FFM experiments, a sharp tip of radius 5–50nm slides against a SAM speci-

men. A Si

tip is commonly used for friction studies and a Si or natural diamond

tip is commonly used for scratch, wear and indentation studies.

In early studies, the eﬀect of the chain length of the fatty acid monolayers on the

coeﬃcient of friction and wear on the macroscale was studied by Bowden and Ta-

bor [20]and Zisman [21].Zisman [21]reported that thereis a steadydecrease infric-

tion with increasing chain length for monolayersdeposited on a glass surface sliding

against a stainless steel surface. At a signiﬁcantly long chain length, the coeﬃcient

of friction reaches a lower limit, Fig. 17.7a. He also reported that monolayers with

a chain length of below 12 carbon atoms behave as liquids (poor durability), those

with chain lengths of 12–15 carbon atoms behave like plastic solids (medium dura-

bility), whereas those with chain lengths of above 15 carbon atoms behavelike crys-

talline solids (high durability). Investigations by Ruhe et al. [68] indicated that the

lifetime of the alkylsilane monolayer coating on a silicon surface increases greatly

with increasing chain length of the alkyl substituent. DePalma and Tillman [69]

showed that a monolayer of n-octadecyltrichlorosilane (n-C

SiCl

,OTS)isan

eﬀective lubricant on silicon.

With the development of AFM techniques, researchers have successfully char-

acterized the nanotribological properties of self-assembled monolayers [1,4,5,38–

44, 47]. Studies by Bhushan et al. [38] showed that C

alkylsiloxane ﬁlms ex-

hibit the lowest coeﬃcient of friction and can withstand much higher normal loads

during sliding than LB ﬁlms, soft Au ﬁlms and hard SiO

coatings. McDermott

et al. [70] studied the eﬀect of alkyl chain length on the frictional properties of

methyl-terminatedn-alkylthiolate CH

(CH

)

−

ﬁlms chemisorbedon Au(111)us-

ing an AFM. Theyreportedthat the longerchain monolayersexhibitmarkedlylower

friction and reduced propensity to wear than shorter chain monolayers, Fig. 17.7b.

These results are in good agreement with the macroscale results published by Zis-

man [21]. They also conducted infrared reﬂection spectroscopy in order to measure

17 Self-Assembled Monolayers (SAMs) for Controlling Adhesion 919

Molecular weight of lubricant

Coeffi-

cient of

friction

Contact

angle of

methylene

iodide on

monolayer

Total number of carbon atoms in molecular chain

0.4

0.3

0.2

0.1

81216202428

100 200 300

Contact angle

Friction

Coefficient of friction

2.0

1.5

1.0

0.5

0.0

Δν

1/2

(cm

–

) of ν

(CH

)

510152025

Fig. 17.7. (a)Eﬀect of chain length (or molecular weight) on coeﬃcient of macroscale fric-

tion of stainless steel sliding on glass lubricated with a monolayer of fatty acid, and contact

angle of methyl iodide on condensed monolayers of fatty acid on glass [21]. (b)Eﬀect of

chain length of methyl-terminated, n-alkanethiolate over Au ﬁlm AuS(CH

)

on the co-

eﬃcient of microscale friction and peak bandwidth at half-maximum (Δν

1/2

) for the band-

width of the methylene stretching mode [ν

(CH

)] [70]

the bandwidth of the methylene stretching mode [ν

(CH

)], which exhibits a quali-

tative correlation with the packing density of the chains. It was found that the chain

structures of monolayers prepared with longer chain lengths are more ordered and

more densely packed than those of monolayers prepared with shorter chain lengths.

Theyfurther reported thatthe ability of the longer chain monolayersto retain molec-

ular scale order during shear leads to a decreasein friction. Monolayerswith a chain

length of more than 12 carbon atoms – preferably 18 or more – are desirable for

tribological applications. (Incidentally, monolayers with 18-carbon- atom octade-

canethiol ﬁlms are often studied.)