25Self-assembledMonolayers for

ControllingHydrophobicity and/or

Friction and Wear

25.1 Introduction25.2 A Primer to Organic Chemistry

Electronegativity/Polarity • Classification and Structure of Organic Compounds • Polar and Nonpolar Groups

25.3 Self-assembled Monolayers: Substrates, Organic Molecules, and End Groups in the Organic Chains

25.4 Tribological Properties25.5 Conclusions

25.1 Introduction

Reliability of microdevices, also commonly referred to as microelectromechanical systems (MEMS), aswell as magnetic storage devices (which include magnetic rigid disk drives, flexible disk drives, and tapedrives), require the use of molecularly thick films for protection of sliding surfaces (Bhushan et al., 1995a;Bhushan, 1996, 1998a, 1999a). A solid or liquid film is generally necessary for acceptable friction andwear properties of sliding interfaces. A small quantity of liquid present between some surfaces cansubstantially increase the static friction as a result of formation of menisci or adhesive bridges (Bhushan,1999b). Smooth surfaces in contact, and in the presence of a small amount of liquid, generally tend toadhere or stick strongly and can exhibit high static friction (Bhushan, 1996). It becomes a major concernin devices operating at ultralow loads as the liquid-mediated adhesive force may be on the same orderas the external load and may lead to high friction and wear.

The source of the liquid film can be either a preexisting film of liquid and/or capillary condensates ofwater vapor from the environment. If the liquid wets the surface (0 ≤ θ < 90°, where θ is the contactangle* between the liquid-vapor interface and the liquid-solid interface at the solid-liquid-vapor three-phase contact line and surface (Figure 25.1(a)), the liquid surface is thereby constrained to lie parallelwith the surface (Ulman, 1995), and the complete liquid surface must therefore be concave in shape(Figure 25.1(b)). Surface tension results in a pressure difference across any meniscus surface, referred to

*The direct measurement of contact angle is most widely made from sessile drops. The angle is generally measuredby aligning a tangent with the drop profile at the point of contact with the solid surface using a telescope equippedwith a goniometer eyepiece.

Bharat BhushanThe Ohio State University

as capillary pressure or Laplace pressure, and is negative for a concave meniscus (Bhushan, 1999b). Thenegative Laplace pressure results in an intrinsic attractive (adhesive) force that depends on the interfaceroughness (local geometry of interacting asperities and number of asperities), surface tension, and contactangle. During sliding, frictional effects — due not only to external load but also to intrinsic adhesiveforce — need to be overcome. High measured static friction force contributed largely by liquid-mediatedadhesion (meniscus contribution) is generally referred to as stiction. One of the ways to minimize theeffect of liquid-mediated adhesion, in addition to an increase in surface roughness, is to select hydrophobic(water-fearing) rather than hydrophilic (water-loving) surfaces. If a liquid lubricant film needs to beapplied for control of friction, stiction, and wear, it should be thin (about half of the composite roughnessof the interface) to minimize the stiction contribution (Bhushan, 1998b, 1999b). Thus, for ultra-smoothsurfaces with RMS roughness on the order of a few nanometers, molecularly thick liquid films are requiredfor liquid lubrication.

If the interface requires significant sliding (e.g., in micromotors, microactuators, and magnetic diskdrives), the interface should exhibit low friction and wear properties. High static friction or high adhesionis also one of the major problems, because the devices are used over a range of humidities. Some devicesmay require little relative motion but the surfaces may come in close proximity, in which case stictioncan be a major issue. An example is an optical switch using oscillating mirrors to switch optical fibers;oscillating mirrors sometimes contact the substrate and stiction can be an issue. In these cases, hydro-phobicity of the surface is the major requirement.

Silicon used in the construction of microdevices can be treated to make it hydrophobic or hydrophilic.A hydrophilic surface is desirable if a hydrophobic film is to be anchored on it. Bulk silicon and polysiliconfilm are dipped in hydrofluoric acid (HF) or hydrogen peroxide (H2O2) to generate either a hydrophobicor hydrophilic surface, respectively (e. g., Maboudian, 1998; Scherge and Schaefer, 1998). In HF etchingof silicon, hydrogen passivates the silicon surface by saturating the dangling bonds and results in ahydrogen-terminated silicon surface that is responsible for less adsorption of water. However, afterexposure of the treated surface to the environment, it re-oxidizes, which can adsorb water and thus thesurface again becomes hydrophilic. H2O2 treatment generates a thin oxide layer (Si-Ox) that is receptiveto water.

FIGURE 25.1 (a) Schematic of a sessile drop on a solid surface and the definition of contact angle; and (b) formationof meniscus bridges as a result of liquid present at an interface.

The surfaces can be either treated or coated with a liquid of low surface tension or a certain solid filmto make them hydrophobic and/or to control friction, stiction, and wear. The classical approach tolubrication uses freely supported multimolecular layers of liquid lubricants (Bowden and Tabor, 1950;Bhushan, 1996, 1999a,b; Bhushan and Zhao, 1999). Boundary lubricant films are formed either byphysisorption, chemisorption, or chemical reaction. The physisorbed films can be either monomolecularor polymolecular thick. The chemisorbed films are monomolecular, but stoichiometric films formed bychemical reaction can be multilayered. In general, the stability and durability of surface films decreasein the following order: chemically reacted films, chemisorbed films, and physisorbed films. A goodboundary lubricant should have a high degree of interaction between its molecules and the sliding surface.As a general rule, liquids are good lubricants when they are polar and thus able to grip solid surfaces (orbe adsorbed). Polar lubricants contain reactive functional end groups. Boundary lubrication propertiesare also dependent on molecular conformation and lubricant spreading. It should be noted that liquidfilms with a thickness on the order of a few nanometers may be discontinuous and may deposit in anisland form with nonuniform thickness and lateral resolution on the nanometer scale.

Solid films are also commonly used for controlling hydrophobicity and/or friction and wear. Hydro-phobic films have nonpolar terminal groups (to be described later) that repel water. These films havelow surface energy (15 to 30 dyn/cm) and high contact angle (θ ≥ 90°), which minimize wetting (e.g.,Zisman, 1959; Schrader and Loeb, 1992; Neumann and Spelt, 1996). It should be noted that these filmsdo not totally eliminate wetting. Multimolecularly thick (few tenths of a nanometer) films of conventionalsolid lubricants have been studied. Hansma et al. (1992) reported the deposition of multimolecularlythick, highly oriented PTFE films from the melt or vapor phase, or from solution by a mechanicaldeposition technique, by dragging the polymer at controlled temperature, pressure, and speed against asmooth glass substrate. Scandella et al. (1994) reported that the coefficient of nanoscale friction of MoS2

platelets on mica, obtained by exfoliation of lithium intercalated MoS2 in water, was a factor of 1.4 lessthan that of mica itself. However, MoS2 is reactive to water, and its friction and wear properties degradewith an increase in humidity (Bhushan, 1999b). Amorphous diamond-like carbon (DLC) coatings canbe produced with extremely high hardness and are commercially used for wear applications (Bhushan,1999c). Their largest application is in magnetic storage devices (Bhushan, 1996). Doping of the DLCmatrix with elements such as silicon, fluorine, oxygen, and nitrogen (Dorfman, 1992; Grischke et al.,1995) influences the contact angle (or surface energy) and tribological properties. Oxygen and nitrogenreduce the contact angle (or increase the surface energy), due to the strong polarity formed when theseelements are bonded to carbon. On the other hand, silicon and fluorine increase the contact angle from70 to 100° (or reduce the surface energy from 20 to 40 dyn/cm), making them hydrophobic (Butter et al.,1997; Grischke et al., 1998). Nanocomposite coatings with a diamond-like carbon (a-C:H) network anda glass-like (a-Si:O) network are deposited using a PECVD (plasma-enhanced chemical vapor deposition)technique in which plasma is formed from a siloxane precursor using a hot filament. For a fluorinatedDLC, CF4 as the fluorocarbon source is added to an acetylene plasma. In addition, fluorination of DLCcan be achieved by the post-deposition treatment of DLC coatings in a CF4 plasma. Fluorine- and silicon-containing DLC coatings mainly reduce their polarity due to the loss of sp2-bonded carbon (polarizationpotential of the involved π electrons) and dangling bonds of the DLC network. Silicon and fluorine areunable to form double bonds, so they force carbon into an sp3 bonding state (Grischke et al., 1998).Friction and wear properties of both silicon-containing and fluorinated DLC coatings have been reportedto be superior to that of conventional DLC coatings (Donnet et al., 1997; Kester et al., 1999). These filmsmay not be well-organized or uniform, and are too thick for microdevices.

Organized and dense molecular-scale layers of, preferably, long-chain organic molecules are knownto be superior lubricants on both macro- and microscales as compared with freely supported multimo-lecular layers (Bhushan et al., 1995b; Bhushan, 1999a). Common methods to produce monolayers andthin films are by Langmuir–Blodgett (LB) deposition and by chemical grafting of organic molecules torealize self-assembled monolayers or SAMs (Ulman, 1991, 1996). In the LB technique, organic moleculesfrom suitable amphiphilic molecules are first organized at the air-water interface and then transferredto a solid surface to form mono- or multilayers (Zasadzinski et al., 1994). In the case of SAMs, the

functional groups of molecules chemisorb onto a solid surface, which results in the spontaneous forma-tion of robust, highly ordered and oriented, dense monolayers chemically attached to the surface (Ulman,1996). In both cases, the organic molecules used have well-distinguished amphiphilic properties (hydro-philic functional head and a long hydrophobic, aliphatic tail) so that adsorption of such molecules onan active inorganic substrate leads to their firm attachment to the surface. Direct organization of SAMson the solid surfaces allows coating in the tight areas, such as the bearing and journal surfaces in anassembled bearing. The weak adhesion of classical LB films to the substrate surface restricts their lifetimesduring sliding, but certain SAMs of films can be very durable (Bhushan et al., 1995b). As a result, SAMsare of great interest in tribological applications.

An overview of unbonded and chemically bonded multimolecular layers of liquid lubricants can befound in various references (Bhushan, 1996, 1999a,b; Bhushan and Zhao, 1999). This chapter focuseson SAMs for low friction and wear and/or low stiction (with high hydrophobicity). SAMs are producedby various organic precursors. First, a primer to organic chemistry is presented, followed by an overviewof suitable substrates, organic molecules, and end groups in the organic chains for SAMs, and then anoverview of tribological properties and some concluding remarks.

25.2 A Primer to Organic Chemistry

All organic compounds contain the carbon (C) atom. Carbon, in combination with hydrogen, oxygen,nitrogen, and sulfur results in a large number of organic compounds. The atomic number of carbon is6, and its electron structure is 1s2 2s2 2p2. Two stable isotopes of carbon, 12C and 13C, exist. With fourelectrons in its outer shell, carbon forms four covalent bonds, with each bond resulting from two atomssharing a pair of electrons. The number of electron pairs that two atoms share determines whether ornot the bond is single or multiple. In a single bond, only one pair of electrons is shared by the atoms.Carbon can also form multiple bonds by sharing two or three pairs of electrons between the atoms. Forexample, the double bond formed by sharing two electron pairs is stronger than a single bond and alsoshorter than a single bond. An organic compound is classified as saturated if it contains only the singlebond, and unsaturated if the molecules possess one or more multiple carbon-carbon bonds.

25.2.1 Electronegativity/PolarityWhen two different kinds of atoms share a pair of electrons, a bond is formed in which electrons areshared unequally: one atom assumes a partial positive charge and the other a negative charge with respectto each other. This difference in charge occurs because the two atoms exert unequal attraction for thepair of shared electrons. The attractive force that an atom of an element has for shared electrons in amolecule or polyatomic ion is known as its electronegativity. Elements differ in their electronegativities.A scale of relative electronegatives, in which the most electronegative element, fluorine, is assigned avalue of 4.0, was developed by L. Pauling. Relative electronegativities of the elements in the PeriodicTable can be found in most undergraduate chemistry textbooks (e.g., Hein et al., 1997). The relativeelectronegativity of the nonmetals is high as compared to that of metals. The relative electronegativitiesof selected elements of interest here with high values are presented in Table 25.1.

The polarity of a bond is determined by the difference inelectronegativity values of the atoms forming the bond. If theelectronegativities are the same, the bond is nonpolar and theelectrons are shared equally. In this type of bond, there is noseparation of positive and negative charge between atoms. Ifthe atoms have greatly different electronegativities, the bond isvery polar. A dipole is a molecule that is electrically asymmet-rical, causing it to be oppositely charged at two points. As anexample, both hydrogen and chlorine need one electron to formstable electron configurations. They share a pair of electrons inhydrogen chloride, HCl. Chlorine is more electronegative and

TABLE 25.1 Relative Electronegativity of Selected Elements

Element Relative Electronegativity

F 4.0O 3.5N 3.0Cl 3.0C 2.5S 2.5P 2.1H 2.1

therefore has a greater attraction for the shared electrons than does hydrogen. As a result, the pair ofelectrons is displaced toward the chlorine atom, giving it a partial negative charge and leaving thehydrogen atom with a partial positive charge (Figure 25.2). However, the entire HCl molecule is electri-cally neutral. The hydrogen atom with a partial positive charge (exposed proton on one end) can beeasily attracted to the negative charge of other molecules, and this is responsible for the polarity of themolecule. A partial charge is usually indicated by δ, and the electronic structure of HCl is given as:

Similar to the HCl molecule, HF is polar and both behave as a small dipole. On the other hand, methane(CH4), carbon tetrachloride (CCl4), and carbon dioxide (CO2) are nonpolar. In CH4 and CCl4, the fourC-H and C-Cl polar bonds are identical; and because these bonds emanate from the center to the cornersof a tetrahedron in the molecule, the effects of their polarities cancel each other. CO2 (O�C�O) isnonpolar because carbon-oxygen dipoles cancel each other by acting in opposite directions. Symmetricmolecules are generally nonpolar. Water (H-O-H) is a polar molecule. If the atoms in water were linear,as in CO2, two O-H dipoles would cancel each other, and the molecule would be nonpolar. However,water has a bent structure with an angle of 105° between the two bonds; this is responsible for waterbeing a polar molecule.

25.2.2 Classification and Structure of Organic Compounds

Table 25.2 presents selected organic compounds grouped into classes.

25.2.2.1 Hydrocarbons

Hydrocarbons are compounds that are composed entirely of carbon and hydrogen atoms bonded to eachother by covalent bonds. Saturated hydrocarbons (alkanes) contain single bonds. Unsaturated hydrocar-bons that contain carbon-carbon double bonds are called alkenes, and those with triple bonds are calledalkynes. Unsaturated hydrocarbons that contain an aromatic ring (e.g., the benzene ring) are calledaromatic hydrocarbons.

25.2.2.1.1 Saturated Hydrocarbons: AlkanesAlkanes, also known as paraffins, are saturated, straight- or branched-chain hydrocarbons, with onlysingle covalent bonds between the carbon atoms. The general molecular formula for alkanes is CnH2n+2,

FIGURE 25.2 Schematic representation of the formation of a polar HCl molecule.

H Cδ δ+ −

: :. .

. .

l

where n is the number of carbon atoms in the molecule. Each carbon atom is connected to four otheratoms by four single covalent bonds. These bonds are separated by angles of 109.5° (the angle by linesdrawn from the center of a regular tetrahedron to its corners). Alkane molecules contain only carbon-carbon and carbon-hydrogen bonds, which are symmetrically directed toward the corners of a tetrahe-dron. Therefore, alkane molecules are essentially nonpolar.

Common alkyl groups have the general formula Cn H2n+1 (one hydrogen atom less than the corre-sponding alkane). The missing H atom can be detached from any carbon in the alkane. The name of thegroup is formed from the name of the corresponding alkane by replacing -ane with the -yl ending. Someexamples are shown in Table 25.2(a).

25.2.2.1.2 Unsaturated HydrocarbonsUnsaturated hydrocarbons consist of three families of compounds that contain fewer hydrogen atomsthan the alkane with the corresponding number of carbon atoms, and contain multiple bonds betweencarbon atoms. These include alkenes (with carbon-carbon double bonds), alkynes (with carbon-carbontriple bonds), and aromatic compounds (with benzene rings that are arranged in a six-membered ringwith one hydrogen atom bonded to each carbon atom and three carbon-carbon double bonds). Someexamples are shown in Table 25.2(a).

25.2.2.2 Alcohols, Ethers, Phenols, and Thiols

Organic molecules with certain functional groups are synthesized for their desirable properties. Alcohols,ethers, and phenols are derived from the structure of water by replacing the hydrogen atoms of waterwith alkyl groups (R) or aromatic (Ar) rings. For example, phenol is a class of compounds having ahydroxyl group attached to an aromatic ring (benzene ring). Organic compounds that contain the -SHgroup are analogs of alcohols, and known as thiols. Some examples are shown in Table 25.2(b).

TABLE 25.2(a) Names and Formulae of Selected Hydrocarbons

Name Formula

Saturated hydrocarbonsStraight-chain alkanes: CnH2n+2

e.g., Methane CH4

Ethane C2H6 or CH3CH3

Alkyl groups: CnH 2n+1

e.g., Methyl –CH3

Ethyl –CH2CH3

Unsaturated hydrocarbonsAlkenes: (CH2)n

e.g., Ethene C2H4 or CH2�CH2

Propene C3H6 or CH3CH�CH2

Alkynes:e.g., Acetylene HC�CH

Aromatic hydrocarbonse.g., Benzene

25.2.2.3 Aldehydes and Ketones

Both aldehydes and ketones contain the carbonyl group, C�O, a carbon-oxygen double bond. Aldehydeshave at least one hydrogen atom bonded to the carbonyl group, whereas ketones have only alkyl oraromatic groups bonded to the carbonyl group. The general formula for the saturated homologous seriesof aldehydes and ketones is CnH2nO. Some examples are shown in Table 25.2(c).

25.2.2.4 Carboxyl Acids and Esters

The functional group of carboxylic acids is known as a carboxyl group and represented as -COOH.Carboxylic acids can be either aliphatic (RCOOH) or aromatic (ArCOOH). The carboxylic acids witheven numbers of carbon atoms, n, ranging from 4 to about 20 are called fatty acids (e.g., n = 10, 12, 14,16, and 18 are called capric acid, lauric acid, myristic acid, palmitic acid, and stearic acid, respectively).

TABLE 25.2(b) Names and Formulas of Selected Alcohols, Ethers, Phenols, and Thiols

Name Formula

Alcohols: R–OHe.g., Methanol CH3OHEthanol CH3CH2OH

Ethers: R–O–R′e.g., Dimethyl ether CH3–O–CH3

Diethyl ether CH3CH2–O–CH2CH3

Phenols C6H5OH or OH

Thiols: –SHe.g., Methanethiol CH3SH

Note: The letters R- and R′- represent an alkyl group.The R- groups in ethers can be the same or different andcan be alkyl or aromatic (Ar) groups.

TABLE 25.2(c) Names and Formulas of Selected Aldehydes and Ketones

Name Formula

Aldehydes:

O�

RCHO or R–C–HO�

ArCHO or Ar–C–He.g., Methanal or formaldehyde HCHOEthanal or acetaldehyde CH3CHO

Ketones: O�

RCOR′ or R–C–R′O�

RCOAr or R–C–ArO�

ArCOAr or Ar–C–Are.g., Butanone or methyl ethyl ketone CH3COCH2CH3

Note: The letters R and R′ represent an alkyl group, and Arrepresents an aromatic group.

Esters are alcohol derivates of carboxylic acids. Their general formula is RCOOR′, where R can behydrogen, an alkyl group, or an aromatic group, and R′ can be an alkyl or aromatic group, but not ahydrogen. Esters are found in fats and oils. Some examples are shown in Table 25.2(d).

25.2.2.5 Amides and Amines

Amides and amines are organic compounds containing nitrogen. Amides are nitrogen derivates ofcarboxylic acids. The carbon atom of a carbonyl group is bonded directly to the nitrogen atom of an-NH2, -NHR, or -NR2 group. The characteristic structure of amide is RCONH2.

An amine is a substituted ammonia molecule that has the general formula RNH2, R2NH, or R3N,where R is an alkyl or aromatic group. Some examples are shown in Table 25.2(e).

25.2.3 Polar and Nonpolar Groups

Table 25.3(a) summarizes polar and nonpolar groups commonly used in the construction of hydrophobicand hydrophilic molecules. Table 25.3(b) lists the relative polarity of selected polar groups (Mohrig et al.,1998). Thiol, silane, carboxylic acid, and alcohol (hydroxyl) groups are the most commonly used polar

TABLE 25.2(d) Names and Formulae of Selected Carboxylic Acids and Esters

Name Formula

Carboxylic acida:

O�

RCOOH or R–C–OHO�

ArCOOH or Ar–C–OHe.g., Methanoic acid (formic acid) HCOOHEthanoic acid (acetic acid)Octadecanoic acid (stearic acid)

CH3COOHCH3(CH2)16COOH

Estersb: O�

RCOOR′ or R–C–O–R′

acid alcohol

e.g., Methyl propanoate CH3CH2COOCH3

a The letter R represents an alkyl group and Ar represents anaromatic group

b The letter R represents a hydrogen, alkyl group, or aromaticgroup; and R′ represents an alkyl or aromatic group.

TABLE 25.2(e) Names and Formulae of Selected Organic Nitrogen Compounds (Amides and Amines)

Name Formula

Amides:

O�

RCONH2 or R–C–NH2

e.g., Methanamide (formamide) HCONH2

Ethanamide (acetamide) CH3CONH2

Amines:H

RNH2 or R–NH

R2NHR3N

e.g., Methylamine CH3NH2

Ethylamine CH3CH2NH2

Note: The letter R represents an alkyl or aromatic group.

{ {

anchor groups for attachment to surfaces. Methyl and trifluoromethyl are commonly used end groupsfor hydrophobic film surfaces.

25.3 Self-assembled Monolayers: Substrates, Organic Molecules, and End Groups in the Organic Chains

Much of the research in preparation of SAMs has been carried out for the so-called soft lithographictechnique (Tien et al., 1998; Xia and Whitesides, 1998). This is a nonphotolithographic technique.Photolithography is based on a projection-printing system used for projection of an image from a maskto a thin-film photoresist, and its resolution is limited by optical diffraction limits. In soft lithography,an elastomeric stamp or mold is used to generate micropatterns of SAMs by contact printing (knownas microcontact printing or µCP (Kumar and Whitesides, 1993), by embossing (imprinting) (Chouet al., 1996), or by replica molding (Xia et al., 1996), all of which circumvent the diffraction limits of

TABLE 25.3(a) Some Examples of Polar (hydrophilic) and Nonpolar (hydrophobic) Groups

Name Formula

Polar:Alcohol (hydroxyl) –OHCarboxyl –COOHAldehyde –COHKetone O

�

R–C–REsterCarbonyl

–COO–C�O

Ether R–O–RAmine –NH2

Amide O�

–C–NH2

Phenol OH

ThiolTrichlorosilane

–SHSiCl3

Nonpolar:Methyl –CH3

Trifluoromethyl –CF3

Aryl (benzene ring)

Note: The letter R represents an alkyl group.

TABLE 25.3(b) Organic Groups Listed in Increasing Order of Polarity

AlkanesAlkenesAromatic hydrocarbonsEthersTrichlorosilanesAldehydes, ketones, esters, carbonylsThiolsAminesAlcohols, phenolsAmidesCarboxylic acids

photolithography. The stamps are generally cast from photolithographically generated patterned masters,and the stamp material is generally polydimethylsiloxane (PDMS). In µCP, the ink is a SAM precursorto produce nanometer-thick resists with lines thinner than 100 nm. Although soft lithography requireslittle capital investment, it is doubtful that it would ever replace well-established photolithography.However, µCP and embossing techniques can be used to produce microdevices that are substantiallycheaper and more flexible in choice of material for construction than conventional photolithography(e.g., SAMs and non-SAM entities for µCP and elastomers for embossing).

Other applications for SAMs are in the areas of bio/chemical and optical sensors, for use as drug-delivery vehicles, and in the construction of electronic components (Ulman, 1991; Service, 1994).Bio/chemical sensors require the use of highly sensitive organic layers with tailored biological propertiesthat can be incorporated into electronic, optical, or electrochemical devices. Self-assembled microscopicvesicles are being developed to ferry potentially lifesaving drugs to cancer patients. By getting organic,metal, and phosphonate molecules (complexes of phosphorous and oxygen atoms) to assemble themselvesinto conductive materials, these can be produced as self-made sandwiches for use as electronic compo-nents. The application of interest here is an application requiring hydrophobicity and/or low frictionand wear.

SAMs are formed as a result of spontaneous self-organization of functionalized, long-chain organicmolecules onto the surfaces of appropriate substrates into stable, well-defined structures (Figure 25.3).The final structure is close to or at thermodynamic equilibrium; as a result, it tends to form spontaneouslyand rejects defects. SAMs are named based on the terminal group, followed by the backbone chain andthe head group (or type of compound formed at the surface). For a SAM to control friction, wear, andhydrophobicity, it should be strongly adherent to the substrate, and the terminal group (tail group orthe group at the free end) of the organic molecular chain should be nonpolar. For strong attachment ofthe organic molecules to the substrate, the other end of the molecular chain (head group) should containa polar end group. The head group provides the more exothermic process (energies on the order of tensof kcal/mol); that is, it results in an apparent pinning of the head group to a specific site on the surfacethrough a chemical bond. Thus, the organic molecule of a SAM should contain a nonpolar end groupon one end and polar end group on the other. Furthermore, molecular structure and any crosslinkingwill have a significant effect on friction and wear performance. The substrate surface should have a highsurface energy, so that there will be a strong tendency for molecules to adsorb to the surface. The surfaceshould be highly functional, with polar groups and dangling bonds (generally unpaired electrons), sothat they can react with organic molecules and provide a strong bond. Because of the exothermic head

FIGURE 25.3 Schematic of a self-assembled monolayer on a surface and the associated forces.

group-substrate interactions, molecules try to occupy every available binding site on the surface; andduring this process, they generally push together molecules that have already adsorbed. The processresults in the formation of crystalline molecular assemblies. The interactions between molecular chainsare van der Waals or electrostatic in nature, with energies on the order of a few (<10) kcal/mol, andexothermic. The molecular chains in SAMs are not perpendicular to the surface; the tilt angle dependson the anchor group as well as on the substrate and the spacer group. For example, the tilt angle foralkanethiolate on Au is about 30 to 35° with respect to substrate normal. The requirement is that thespace be filled optimally.

SAMs are usually produced by immersing a substrate in the solution containing precursor (ligand)that is reactive to the substrate surface, or by exposing the substrate to the vapor of the reactive chemicalspecies (Ulman, 1991). Table 25.4 lists selected systems that have been used for the formation of SAMs(Xia and Whitesides, 1998). The backbone of a SAM is usually an alkyl chain (CnH2n+1) or a derivatizedalkyl group. By attaching different terminal groups at the surface (or surface group), the film surface canbe made to attract or repel water. The commonly used terminal group of a hydrophobic film with lowsurface energy, in the case of a single alkyl chain, is the nonpolar methyl (CH3) group; the other istrifluoromethyl (CF3). For a hydrophilic film, the commonly used terminal groups are alcohol (OH) orcarboxyl acid (COOH) groups. Surface-active head groups most commonly used are thiol (SH), silane(e. g., trichlorosilane or SiCl3), and carboxyl (COOH) groups. The substrates most commonly used aregold, silver, platinum, copper, hydroxylated (activated) surfaces of SiO2 on Si, Al2O3 on Al and glass, andhydrogen-terminated single-crystal silicon (H-Si). Hydroxylation of oxide surfaces is important to makethem hydrophilic. For example, thermally grown silica can be activated by sulfochromic treatment. Thesample is dipped in a solution consisting of 100 mL of concentrated H2SO4, 5 mL water, and 2 g potassiumbichromate for 3 to 15 min (Bhushan et al., 1995b). The sample is then rinsed with flowing pure water.This results in a surface with silanol groups (-Si-OH), resulting in a surface that is hydrophilic(Figure 25.4). Bulk silicon, polysilicon film, or SiO2 film surfaces can also be treated to produce anactivated silica surface by immersion in about three parts H2SO4 and one part H2O2 at temperaturesranging from ambient to 80°C. For organic molecules to pack together and provide a better ordering, a

TABLE 25.4 Selected Substrates and Precursors Commonly Used for SAMs Formation

Substrate Precursor Binding with Substrate

Au R SH (thiol) RS-AuAu Ar SH (thiol) ArS-AuAu RSSR′ (disulfide) RS-AuAu RSR′ (sulfide)Si/SiO2, glass R SiCl3 (trichlorosilane) Si-O-Si (siloxane)Si/Si-H RCOOH (carboxyl) R-SiMetal oxides (e.g., Al2O3, SnO2, TiO2) RCOOH (carboxyl) RCOO-….MOn

Note: R represents alkane (CnH2n+2) and Ar represents aromatic hydrocarbon. It consists ofvarious surface active headgroups, mostly with methyl terminal group.

FIGURE 25.4 Schematic showing the hydroxylation process occurring on a silica surface through a sulfochromictreatment.

substrate for given molecules should be selected such that the cross-sectional diameter of the backboneof the molecule is equal to or smaller than the distance between the anchor groups attached to thesubstrate. Epitaxial Au film on glass, mica, or single-crystal silicon produced by e-beam evaporation iscommonly used because it can be deposited on smooth surfaces as a film that is atomically flat anddefect-free. For the case of alkanethiolate film, the advantage of Au substrate over SiO2 substrate is thatit results in better ordering because the cross-sectional diameter of the alkane molecule is slightly smallerthan the distance between sulfur atoms attached to the Au substrate (~0.53 nm). The thickness of thefilm can be controlled by varying the length of the hydrocarbon chain, and the properties of the filmsurface can be modified by the terminal group.

Some of the SAMs have been widely reported. SAMs of long-chain fatty acids CnH2n+1COOH or (CH3)(CH2)nCOOH (n = 10, 12, 14, or 16) on glass or alumina are one class of films studied since the 1950s(Bowden and Tabor, 1950; Zisman, 1959). Probably the most studied SAMs to date are n-alkanethiolate*monolayers CH3(CH2)nS-prepared from adsorption of alkanethiol -(CH2)nSH solution onto an Au film(Xia and Whitesides, 1998) and n-alkylsiloxane** monolayers produced by adsorption of n-alkyltrichlo-rosilane -(CH2)nSiCl3 solution onto hydroxylated Si/SiO2 substrate (Wasserman et al., 1989) with siloxane(Si-O-Si) binding (Figure 25.5). SAMs of n-alkyltrichlorosilanes such as methyl-terminated, octadecyl-trichlorosilane (OTS) CH3(CH2)17SiCl3 compound on hydroxylated SiO2 on silicon wafer are also acommonly studied film. Jung et al. (1998) have produced organosulfur monolayers-decanethiol(CH3)(CH2)9SH, didecyl disulfide CH3(CH2)9S-S(CH2)9CH3, and didecyl sulfide CH3(CH2)9-S-(CH2)9CH3 on Au films. Geyer et al. (1999) have produced monolayers of 1,1′-biphenyl-4-thiol (BPT)on Au film surface, in which the backbone of the film consists of two phenyl rings with a hydrogen endgroup (Figure 25.6). Rings are expected to be stiff; therefore, their tribological properties are expectedto be superior to that of linear molecular structures. Crosslinking by low-energy electrons may furtherimprove mechanical and tribological properties.

FIGURE 25.5 Schematic of a methyl-terminated, n-alkylsiloxane monolayer on Si/SiO2.

*n-Alkyl and n-alkane are used interchangeably.**Siloxane (Si-O-Si) refers to the bond and silane (SinX2n+2 which includes a covalently bonded compounds

containing the elements Si and other atoms or groups such as H and Cl to form SiH4 and SiCl4, respectively) refersto the head group of the precursor. These terms are used interchangeably.

25.4 Tribological Properties

Friction and wear properties have been studied on the macro- and microscales. Macroscale tests areconducted using a so-called pin-on-disk test apparatus in which a ceramic (typically alumina or sapphire)ball is slid against a lubricated specimen (Bhushan, 1999b). Microscale tests are conducted using anatomic force/friction force microscope or AFM/FFM (Bhushan, 1999a). In AFM/FFM experiments, asharp tip of radius ranging from about 5 to 50 nm is slid against a lubricated specimen. An Si3N4 tip iscommonly used for friction studies, and a natural diamond tip is commonly used for wear and inden-tation studies.

In early studies, the effect of chain length of the carbon atoms of fatty acid monolayers on the coefficientof friction and wear using macroscale tests was studied by Bowden and Tabor (1950) and Zisman (1959).Zisman (1959) reported that for the monolayers deposited on a glass surface sliding against a stainlesssteel surface, there was a steady decrease in friction with increasing chain length. At a significantly longchain length, the coefficient of friction reaches a lower limit (Figure 25.7). He further reported thatmonolayers having a chain length below 12 carbon atoms behaved as liquids (poor durability); thosewith a chain length of 12 to 15 carbon atoms behaved like a plastic solid (medium durability); and thosewith chain lengths above 15 carbon atoms behaved like a crystalline solid (high durability).

FIGURE 25.6 Schematic of biphenylthiol (BPT) monolayer on Au and after electron-induced crosslinking. (FromGeyer, W. Stadler, V., Eck, W., Zharnikov, M., Golzhauser, A. and Grunze, M. (1999), Electron-induced crosslinkingof aromatic self-assembled monolayers: negative resists for nanolithography, Appl. Phys. Lett., 75, 2401-2403. Withpermission.)

FIGURE 25.7 Effect of chain length (or molecular weight) on coefficient of microscale friction of stainless steelsliding on glass lubricated with a monolayer of fatty acid and contact angle of methyl iodide on condensed monolayersof fatty acid on glass. (From Zisman, W.A. (1959), Friction, durability and wettability properties of monomolecularfilms on solids, in Friction and Wear, Davies, R. (Ed.), Elsevier, Amsterdam, 110-148. With permission.)

McDermott et al. (1997) studied the effect of alkyl chain length on the frictional properties of methyl-terminated n-alkylthiolate CH3(CH2)nS- films chemisorbed on Au(111) using an AFM. Through anexamination of the influence of the alkyl chain length with monolayers slid against Si3N4 tips, theyreported that the longer chain monolayers exhibited a markedly lower friction (Figure 25.8) and a reducedpropensity to wear than shorter chain monolayers. They conducted infrared reflection spectroscopy tomeasure the bandwidth of the methylene stretching mode [νa(CH2)], which exhibits a qualitative corre-lation with the packing density of the chains. It was found that the chain structures of monolayersprepared with longer chain lengths are more ordered and more densely packed in comparison to thoseof monolayers prepared with shorter chain lengths (Figure 25.8). They further reported that the abilityof the longer chain monolayers to retain molecular scale order during shear leads to a lower observedfriction. Monolayers having a chain length more than 12 carbon atoms, preferably 18 or more, aredesirable for tribological applications. (Monolayers with 18 carbon atoms, octadecanethiol or ODT, filmsare commonly studied.) Joyce et al. (1992) reported that these films have negligible adhesive film-(tungsten) tip interaction and complete passivation (hydrophobicity).

Xiao et al. (1996) and Lio et al. (1997) also studied the effect of the length of the alkyl chains on thefrictional properties of n-alkanethiolate films on gold and n-alkylsilane films on mica. Friction was foundto be particularly high with short chains of less than eight carbon atoms. Thiols and silanes exhibit similarfriction force for the same n when n > 11; while for n < 11, silanes exhibit higher friction, larger thanthat for thiols by a factor of about 3 for n = 6. The increase in friction was attributed to the large numberof dissipative modes in the less-ordered chains that occurs when going from a thiol to a silane anchoror when increasing n. Longer chains (n > 11), stabilized by van der Waals attraction, form more compactand rigid layers and act as better lubricants. Srinivasan et al. (1998) also reported that octadecyltrichlo-rosilane (C18) films deposited on oxidized (activated) polysilicon films reduced static friction by a factorof 20 with respect to the oxide films.

Fluorinated carbon (fluorocarbon) molecules are commonly used for lubrication (Bhushan, 1996b).Kim et al. (1997, 1999) studied frictional properties of fluorinated n-alkanethiolate films adsorbed onsingle-crystal gold film. They reported a factor of 3 increase in the friction in going from a hydrogenated(methyl-terminated) SAM (contact angle of 82° with methylene iodide) to the fluorinated (trifluorom-ethyl) SAM (contact angle of 78° with methylene iodide). The latter type of SAM surfaces can be producedas “Teflon-like” to have low surface energies. They suggested that fluorinated monolayers exhibit higherfrictional properties due to tighter packing at the interface, which arises from the larger van der Waals

FIGURE 25.8 Effect of chain length of methyl-terminated, n-alkanethiolate over Au film AuS(CH2)nCH3 on thecoefficient of microscale friction and peak bandwidth at half maximum (∆ν1/2) for the bandwidth of the methylenestretching mode [νa(CH2)]. (From McDermott, M.T., Green, J.B.D., and Porter, M.D. (1997), Scanning force micro-scopic exploration of the lubrication capabilities of n-alkanethiolate monolayers chemisorbed at gold: structural basisof microscopic friction and wear, Langmuir, 13, 2504-2510. With permission.)

radii of the fluorine atoms. Subsequent steric and rotational factors between adjacent terminal groupsgive rise to long-range multimolecular interactions in the plane of the CF3 groups. When these energeticbarriers are overcome in the film structure, more energy is imparted to the film during sliding and resultsin higher friction for the fluorinated films.

Liu et al. (1996) studied the effect of humidity on SAMs with hydrophobic surfaces (dihexadecyldim-ethylammonium, including two C16 substituents) and hydrophilic surfaces [(16, 16′-dihydroxydihexade-cyl)dimethylammonium, including two HOC16]. They reported that the friction force of the hydrophobicfilm (contact angle = 62°) is an order of magnitude lower than that of a hydrophilic film (contact angle =7°). NH2- and SO3H-terminated silane-based SAMs were deposited on activated single-crystal siliconand polysilicon films by Tsukruk et al. (1998). They used various polymer molecules for the backbone,and reported that these films exhibited high adhesion and friction in AFM experiments. Cappings ofNH2-modified surfaces (3-aminopropyltriethoxysilane) with rigid and soft polymer layers resulted in asignificant reduction in adhesion to a level lower than that of untreated surface.

Liu et al. (1996) also studied the effect of velocity on the coefficient of friction of these films. Theyfound that a peak in the friction vs. velocity measurements exists. Shearing on a surface monolayer ofcertain organic films may induce local phase transition in the film, which is related to its structure andbulk chain melting.

Bhushan et al. (1995a,b) conducted detailed micro- and macroscale friction and wear studies on SAMsand LB films and their underlayers. The SAMs used were methyl-terminated octadecyldimethylchlorosi-lane (C18) films onto an Si(100) wafer covered with a thermally grown hydroxylated SiO2 layer(Figure 25.9). In these films, two of the chlorine atoms of trichlorosilane were replaced by methyl groups.This kind of silane still becomes bound to the silica surface, but crosslinking of the chains by siloxane(Si-O-Si) bond does not take place. This increases the space between the chains for more ordered layers.The structure of the LB film consists of an octadecylthiol (ODT)-coated gold sample (gold films thermallyevaporated onto single-crystal silicon) on top of which a single, upper inverted bilayer of zinc arachidate(C20, ZnA) was deposited by the LB technique (Figure 25.10). Note that ZnA is weakly bonded by vander Waals forces to the ODT underlayer. Macro- and microscale friction and microwear data are sum-marized in Table 25.5 and Figure 25.11. Note that the C18 film exhibits the lowest coefficient of microscalefriction of 0.018 as compared with other samples measured in this study (the coefficient of friction ofZnA film is ~0.03). The coefficient of microscale friction for ZnA film is comparable to that of the ODTlayer and lower than that of the Au film. The microwear resistance of C18 film is much better than ZnA

FIGURE 25.9 Schematic of a methyl-terminated octadecyldimethylchlorosilane monolayer on Si/SiO2.

and Au films, and is comparable with that of SiO2 film. The C18 films can withstand much higher thannormal load of 40 µN as compared with 200 nN for the case of ZnA films (Figure 25.12). Nanomechanicalstudies on SAMs have been conducted using an AFM (Salmeron et al., 1993; Bhushan et al., 1995b; Liuet al., 1998). Nanoindentation studies conducted by Bhushan et al. (1995b) showed that C18 films aremore rigid than ZnA films, which may be responsible for the high wear resistance of C18 films.

For comparison, macroscale friction and wear data on C18 and ZnA films are next presented (Bhushanet al., 1995b). The coefficient of friction values after they reach a steady-state value are presented inTable 25.5. Representative profiles of the coefficient of friction as a function of sliding distance (time)are presented in Figure 25.13. A significant increase in the coefficient of friction during sliding suggeststhat the interface has degraded and the inflection point in the friction curves is taken as the end of life.Note that C18 film exhibits the longest durability, as compared to ZnA and ODT films. Trends in mac-roscale friction and wear data are comparable to that in microscale data.

25.5 Conclusions

Exposure of microdevices to a humid environment results in condensates of water vapor from theenvironment. Condensed water or a preexisting film of liquid forms concave meniscus bridges betweenthe mating surfaces. The negative Laplace pressure present in the meniscus results in an intrinsic attractive

FIGURE 25.10 Schematic of the zinc arachidate LB film on ODT with a gold underlayer.

TABLE 25.5 Typical RMS Roughness and Coefficients of Microscale and Macroscale Friction Values of Various Samples

SampleRMS Roughness

(nm)a

Coefficient of Microscale Frictionb

Coefficient of Macroscale Frictionc

Si(100) 0.12 0.03 0.33SiO2/Si 0.21 0.03 0.19C18/SiO2/Si 0.16 0.018 0.07Au/Si 1.16 0.04 0.13ODT/Au/Si 0.92 0.03 0.14ZnA/ODT/Au/Si 0.55 0.03 0.16

a Measured on 1 µm × 1 µm scan area using AFM.b Si3N4 tip with a radius of about 30 to 50 nm at normal load in the range of 10 to

200 nN and scanning speed of 4 µm/s on a 1 µm × 1 µm scan area.c Alumina ball with 3-mm radius at normal loads of 0.1 N and average sliding speed

of 0.8 mm/s.

(adhesive) force that depends on the interface roughness, surface tension, and contact angle. The adhesiveforce can be significant in an interface with ultrasmooth surfaces and it can be on the same order as theexternal load if the latter is small, such as in magnetic storage devices and microdevices. Surfaces withhigh hydrophobicity can be produced either by surface treatment or by the application of hydrophobicfilms. In many applications, these films are expected to provide low friction and wear. To minimize highstatic friction (stiction), and/or because of small clearances, these films should be molecularly thick.Liquid films of low surface tension or certain hydrophobic solid films can be used. Ordered molecularassemblies with high hydrophobicity can be engineered using chemical grafting of various polymermolecules with suitable functional head groups and nonpolar terminal end groups. A class of self-assembled monolayers (or SAMs) with a polar group chemically attached to an activated substrate havebeen produced. Many of these films exhibit high hydrophobicity and low friction and wear. One of themost extensively studied SAM films is made of alkanethiol molecules, long hydrocarbon chains with asulfur atom at one terminus and a methyl group at another terminus for a hydrophobic film. A substrateof a gold (or silver) film is dipped into a solution of alkanethiol; the sulfur atoms attach to the gold. Thedistance between the sulfur atoms adsorbed on the surface is about the same as the cross-sectionaldiameter of the alkane molecule, resulting in a well-packed film. The film thickness can be controlled bythe length of the hydrocarbon chain and the properties of the film can be modified by the terminal group.Flexibility in choosing the backbone chain length, functional head groups, and nonpolar terminal groupenables the adaptability of the grafting process for lubrication purposes.

Based on limited tribological studies of SAM films, these films exhibit attractive hydrophobic andtribological properties. SAM films should find many tribological applications, including use in microdevices.

FIGURE 25.11 Microwear depth (a) as a function of normal load after one scan cycle, and (b) as a function ofnumber of scan cycles at the normal load indicated for various films using a square-pyramidal natural diamond tipwith a radius of about 100 nm in an AFM. (From Bhushan, B., Kulkarni, A.V., Koinkar, V.N., Boehm, M., Odoni,L., Martelet, C., and Belin, M. (1995b), Microtribological characterization of self-assembled and Langmuir-Blodgettmonolayers by atomic and friction force microscopy, Langmuir, 11, 3189-3198. With permission.)

FIGURE ne scan cycle (in Figure 25.11) for C18 and ΖnA films. Normalload an

25.12 Surface profiles showing the worn regions (center, 1 µm × 1 µm) after od wear depths are indicated in the figure.

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