W
e live in a world full of useful organic materials and surfaces. In addition to those in biological systems, a growing number of polymeric materials, whose surfaces provide key mechanical and chemical properties, are being used in materials technology. Most liquids used as interfacial lubricants are mixtures of organic molecules, as are liquid crystal displays. Moreover, researchers in the rapidly expanding field of biotechnology are using organic surfaces for protein adsorption and molecular recognition. Despite the importance of organic surfaces, they have not yet been subjected to the same type of molecular-level scrutiny as have metal and oxide surfaces and interfaces. This Report reviews the surface science techniques that appear most suitable for characterizing organic surfaces. We discuss 13techniques, many of them developed in recent years, that provide information about composition and structure on the molecular level: secondary ion mass spectroscopy (SIMS),X-ray photoelectron spectroscopy (XPS), ultraviolet photoelectron spectroscopy (UPS), lowenergy electron diffraction (LEED), highresolution energy electron loss spectros-
Scott S. Perry Gabor A. Somorjai University of California,Berkeley
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0003 2700/94/0366 -403A/$04.50/0 0 1994 American Chemical Society
A variety of su flace
analysis techniques can be used to obtain composition and structural informa ti0n copy (HEELS), Raman spectroscopy (RS) ,Fourier transform infrared spectroscopy (FT-IR), sum frequency generation (SFG) , scanning tunneling microscopy (STM), atomic force microscopy (AFM), surface force measurements (SFM), contact angle measurements, and neutron reflectivity measurements. The first two techniques, SIMS and XPS, provide information about surface composition and the chemical environment and bonding of surface species. UPS ‘probes the density of electronic states in the valence band of materials and can provide a spectroscopic fingerprint of many organic compounds. LEED and HREELS are electron-scattering techniques that are uniquely suited to yield the structure of the surface monolayer. RS, FT-IR, and SFG are optical techniques used to study solid-gas interfaces at high pressures along with solid-solid
and solid-liquid “buried” interfaces. STM, AFM, and SFM provide atomic-scale resolution of surface structure, the forces between molecules at surfaces and, in some cases, measurements of hardness and friction properties of the surface region. Contact angle measurement is a classical technique that provides information about surface thermodynamic properties. Finally, neutron reflectivity measurements probe the density of a surface region and can be used to determine concentration profiles in polymer blends. We will discuss the operating principles of each technique and the associated instrumentation, and then give examples of the types of data that can be gathered about organic surfaces. In addition, we provide references to more detailed reviews of the individual techniques for the interested reader. In this broad discussion of techniques suitable for the study of organic surfaces, it should be noted that such surfaces can take many difEerent forms, ranging from monolayers of molecules adsorbed on inorganic or metallic substrates, to supported thin films of organic material, to the actual surface of bulk compounds. Not all techniques will be applicable to every type of surface. In addition, the necessity of vacuum conditions for several of the techniques will also restrict their application to some materials. Finally, the techniques discussed here have varying degrees of sur-
Analytical Chemistry, Vol. 66, No. 7, April 1, 1994 403 A
face sensitivity and can probe molecules at a range of depths into the surface region. With these considerations, it appears that combinations of techniques may be necessary to provide a wellrounded picture of the surface properties of organic systems. SIMS SIMS is a powerful probe of organic and polymer surface composition. Noble gas ions (typically argon) or atomic metal ions (e.g., Cs', Ga') impinge on the surface at well-defined energies in the range of 130 keV and eject secondary ions from the surface (Figure la). Of the particles sputtered from the surface, only a small fraction are ionized, either positively or negatively; these secondary ions are detected in a mass spectrometer. In turn, the fragments of a given mass are assigned to specific compositional fragments and, through careful calibration studies, are related to the molecular structure of the surface. A complete discussion of the op erating principles of SIMS can be found in the introductory text by Werner ( I ) , The positive-ion spectra of polypropy-
lene and polyacetylene shown in Figure l b are examples of SIMS studies of organic surfaces (2).Only the relative intensities of the common mass fragments are used to distinguish the two polymers containing only carbon and hydrogen. For organic materials containing additional elements (e.g., 0, C1, F), mass fragments unique to these elements would be observed in the spectrum, providing additional means of identification.The use of standards and calibration in SIMS studies is essential for proper identification and quantification. The high sensitivity of the mass spectrometric detection scheme allows surface species to be detected at concentrations of - lo9 molecules/cm2. Because the concentration of a surface monolayer is usually lOI5 molecules/cm2, SIMS detection limits approach 1ppm. In addition, SIMS provides extreme surface sensitivity because it detects charged ions; ionic fragments generated beneath the surface plane have a high probability of neutralization before escaping the surface and are therefore not detected. The sputtering nature of this tech-
Figure llSIYS. (a) Instrumentation. (b) Positive-ion secondary ion mass spectra of polypropylene and polyacetylene. (Adapted from Reference 3.)
404 A Analytical Chemistry, Vol. 66,No. 7, April 1, 1994
nique creates advantages and disadvantages in studying organic surfaces. With high incident beam currents, material is rapidly removed from the surface and depth profiles of the surface region can be obtained. Although depth profiling is important in some studies, the high flux of ions can also lead to deleterious reaction and degradation of the organic or polymer surface itself. Sputtering effects can be minimized by reducing the incident beam current-a technique referred to as static SIMS. An additional advantage of sputtering is realized when the ion source is focused on a small spot and rastered across the surface. By selectively detecting mass fragments as a function of raster position, compositional maps of the surface can be generated. Tarlov and Newman have imaged a photopatterned, two-component, self-assembled monolayer at micrometer resolution, which demonstrates the m a p ping capabilities of SIMS (3). XPS
In XPS, X-rays of suitable energy (in the range 2 1000 eV) eject photoelectrons from the surface and near-surface regions. These photoelectrons include those emitted directly from core and valence levels of the organic material as well as electrons emitted indirectly through an Auger process. (XPS focuses on the emission of core-level electrons.) The emitted electrons can be detected as a function of energy and angle of emission. The energy of the photoemitted electrons is a function of the core-level energy state from which they were generated and therefore provides elemental specificity. Because core-level energy states often are perturbed by local bonding energy states, this process also provides a measure of the chemical environment of the elements. Furthermore, angle-resolved detection of the emitted electrons probes the depth distribution of species in the surface region, Electrons ejected within a dense material must escape to be detected. The farther the electrons must travel within the material, the lower the probability of their detection. In XPS, electrons detected along grazing angles to the surface are more likely to be ejected from the near-surface region or from the surface monolayer; therefore, they provide
ecific information. Conlarger concentration emitted from deeper e at exit angles closer to is angular dependence vely treated in detail by
asurements are
the principle of X-ray roscopy and a sche 1XPS instrument. XPS is
oks describe the princiof XPS (5,6).The applithe study of polymer
surface with these photons
layer, revealing the sp molecules at the surface and the bonding ate to the underlying s
s of Teflon surfaces
air with increasing laser defluorination, branchation (7). The X-ray at lower binding energies ncreasing laser irradican be deconvoluted
only used apparatus, and poly(pheny1ene) sulfide (8).Again, photoelectrons are emitted from different depths within the surface region depending on the energy of the emitted elec-
urface as a function of exposure
tical simulations of In the theoretical different surface
-
a
-
L, or 2r
n information ar orescent scree
lectrons requires this surface structure of benzene adsorbed on a rhodium(ll1) surface in a improved version of the apparatus lower electron beam current for the inci-
ermined by LEED surface
A schematic diffraction pattern that might arise from an adsorbed of an organic molecule is also Figure 4. The diffraction pattern is
ons are
igure 4. Schematic of an
metal atoms outward in a direc
interatomic distances of atoms in t
.
ate-induced restructur-
the surface plane are more strongly coupled to the incident electrons and give to high intensities in the vibrational sp
trum of benzene and deuterated benzene ned by HREELS. From assignment ibrational peaks, the orientation of
le1 to the surface ar
ith the surface can be deteren compared with optical specs, the high resolution of this
-15 eV impinges on the sample and ckscattered from the surface (Figure . In the scattering process, millielectronIt energy losses arise from vibrational
changes in the intensi bands are observed with changes in incident and scattering angle of the ele terinFprocess and energy of the vibrational peaks, one can obtain information about the structure and orientation of organic films or adsorbed molecules. A full
e in
pectrum of adsorbed Reference 13. Figure 6 shows the vibrational spec-
Chemical bonds oriented normal to
tion can sometimes li tional band separations and shifts can be 0 cm-'. An additional limitation e charging of insulating overlayers
1365
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Deuterated benzene
1 11% I 4 6
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DCc
Benzene
HC'
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FT-IR are typically consi bulk characterization. H cal
1. 66,No. 7,April 1,
proper experimental conditions these techniques can be used as powerful probes of surface composition and vibrational structure. Figure 8 shows the experimental geometry used in surface RS. As in the cases of LEED and HREELS, RS is most useful for deriving surface-specific information from thin films (on the order of monolayers) of organic adsorbates or polymers supported on metallic or semiconducting substrates. Typically, a noble gas ion laser provides the visible excitation source, which is focused to a tight spot on the sample. The light scattered from the surface is
collected and dispersed by a monochromator before being directed to a sensitive optical detector. By either scanning the monochromator or using a multichannel detector, the vibrational spectrum of the organic surface can be obtained by monitoring the intensity of the Raman scattered light versus its wavelength or photon energy distribution. Because Raman scattering typically is a weak process, close attention must be paid to the experimental configuration. A full discussion of these details can be found in the review by Campion (17). The Raman spectrum of a mixed film
43u
(- 20 A thick) of polyimide monomers, oxydianiline (ODA) and pyromelliticdianhydride (PMDA) , adsorbed on Ag (111) (18)is shown in the lower half of Figure 9. Individual vibrational bands characteristic of the two components are clearly resolved along with two bands (1410 and 1504 cm-') that arise from a reaction of PMDA with the silver substrate. When the film of monomers is heated, polyimide is formed through a condensation reaction and the characteristic imide vibrational bands are observed (upper portion of Figure 9). These studies demonstrate the usefulness of RS in following surface reactions of organic adsorbates and pathways of polymerization. RS also has demonstrated unique qualifications for the study of various forms of carbon solids. The unique vibrational signatures of diamond, graphite, and amorphous carbon as well as other forms of carbon arise from lattice vibrations (phonons) of the carbon structure. In recent years the technology that uses microwave plasmas or other energetically excited sources of carbon to produce diamond thin films has become important in the coatings industry. RS is perhaps the best technique to distinguish among various forms of carbon found in these processes; it is used extensively throughout
3'
Figure 7. HREELS spectra of o-xylene on Pt(ll1) at different annealing temperatures. (Adapted with permission from Reference 16.)
408 A Analytical Chemistry, Vol. 66, No. 7, April 1, 1994
Figure 8. Instrumental setup for Raman spectroscopy.
the field. Although this application of RS probes bulk structure of the carbon, it provides enough sensitivity to study the properties of very thin films used as coatings. In many applications of RS to organic materials, fluorescence induced by the incident radiation is orders of magnitude more intense than the Raman scattered light, which precludes obtaining the vibrational information. A modification of RS called waveguide ET-RS minimizes the fluorescence of many organics by using infrared excitation (19). By working at longer wavelengths (lower energy), the
fluorescence is not excited; however, the cross sections for Raman scattering are drastically reduced. For this reason, special excitation and collection schemes must be used. Fist, the excitation is coupled into a thin film of a supported polymer overlayer so that the light is propagated through the organic. This produces multiple reflections at the substrate and air interfaces, which makes possible the sampling of large amounts of material. To maximize collection efficiency, an elongated fiber-optic bundle is placed very near the surface from which Raman scattering is generated (Fig-
1790
Figure 9. Raman spectroscopy of polymers.
FT-IR spectroscopy
To derive surface-specific information from organic materials, FT-IR spectroscopy is most frequently carried out in one of two modes: reflection absorption IR spectroscopy (RAIRS) or attenuated total internal reflection (ATR) IR spectroscopy (Figure 11).Back (ZU) and Chabal(Z1) have reviewed the application of FT-IR to the study of thin films through these a p proaches. In a RAIRS configuration, the
''
-
The Raman spectrum of the two monomers of polyimide coadsorbed on Ag(l11) at 120 "C shows the characteristic bands of the monomers as well as two bands arising from PMDA bonded to the silver substrate. After the sample is annealed to 200 "Cto produce the condensation polymerization,the spectrum displays the characteristic imide bands at 1391 and 1788 cm-'. (Adapted with permission from Reference 18.)
-
ure loa). The collected light is focused onto the slits of a conventional FT-IR spectrometer for detection. The benefits of this arrangement are clearly demonstrated in Figure lob, where the Raman spectra, collected in waveguide and normal geometries, are shown for a thin polystyrene film. Although this approach avoids fluorescence excitation,it inherently requires a relatively thick (- 1pm) sample to act as the waveguide, thereby probing the bulk structure of the film.
Figure 10. Waveguide FT=Raman spectroscopym (a) Instrumental setup. (b) Comparison of the spectra of polystyrene obtained in waveguide and normal geometries. (Adapted from Reference 19.)
Analytical Chemistry, Vol. 66, No. 7, April I, 1994 409 A
incident IR radiation is focused onto an external metal surface at glancing angles of incidence (- 3").These conditions are required for appropriate electric fields to exist at the surface and lead to strict vibrational surface selection rules. Adsorbates on the surface absorb IR radiation at frequencies equal to that of their molecular vibrations. The reflected light is dispersed and detected by conventional FTmethods to yield an absorption spectrum. In the ATR mode, the incident light is coupled into a single crystal (typically silicon, germanium, or zinc selenide), which is optically transparent at these wavelengths. Incident and exit angles are designed to propagate the light through the crystal by making multiple reflections from the faces of the crystal. In this way, an evanescent field extends from both faces of the crystal, and molecules within the field absorb radiation at frequencies equal to that of their molecular vibrations. Like RAIRS, an ATR configuration makes use of conventional ET detection schemes. Spectra of organic materials have been collected in an ATR mode by deposition of thin films onto the crystal faces or by placement of materials in intimate contact with the crystal surface. Growing films directly onto the ATR crystals holds the advantage of having control over the thickness of the film sampled. The RAIR spectra of two long-chain alkanoic acids bound to an aluminum oxide surface are shown in Figure 12. The two adsorbate systems of this study (22) are distinguished by the termination of the chains with either a methyl or a vinyl group. Methyl termination of the chains produces features at 2966 and 2880 cm-' (Figure 12a), whereas the vinyl end group leads to characteristic absorptions at - 3000 cm-l (Figure 12b). Other regions of the spectrum provide information of backbone conformation and head group chemistry. In addition to molecular identification, polarization studies of the asymmetric and symmetric stretches of the methylene backbone and methyl or vinyl end groups of these chains allow orientations with respect to the surface plane to be determined. SFG
Sum frequency generation is a nonlinear optical technique that provides extreme 410 A
surface sensitivity in the vibrational study of organic surfaces. In this technique, two laser beams, one of IR and the other of visible wavelength, are focused onto a surface (Figure 13).Physical properties of an absorbing medium, namely the nonlinear susceptibility, can lead to the generation of light intensity at the sum of the
Figure 11- FT=IRspectroscopy. (a) Reflection absorption IR spectroscopy (RAIRS) and (b) attenuated total internal reflection (ATR) excitation geometries.
3600
3100
Y
I 2700
31 00
Wave number
~~~~~~~-~~
Figure 12. RAIRS spectra of Langmuir-Blodgett films. (a) Methyl and (b) vinyl terminated LangmuirBlodgett films adsorbed on an aluminum oxide surface. (Adapted from Reference 22.)
Analytical Chemistry, Vol. 66,No. 7, April 1, 1994
incident frequencies in the absence of inversion symmetry (see Reference 23 for a complete discussion). Because inversion symmetry is broken at an interface of two media, excitation of molecules adsorbed on surfaces will lead to sum frequency intensities. Sum frequency vibrational spectra are generated by scanning one of the frequencies (IR) through the energy range of vibrational excitations. When the IR excitation is near the energy of a molecular vibration, the nonlinear susceptibility will exhibit a resonance and lead to intensity at the sum of this frequency and the visible frequency. Because SFG is generated only from the adsorbates immediately at the surface and does not require the subtraction of signals from the bulk phase, it provides a sensitive probe of the organic surface properties and of the molecules at the buried interface (solid-solid and solid-liquid interfaces). An example of sum frequency generation studies of an organic thin film is shown in Figure 14. The top vibrational spectrum is obtained from pentadecanoic acid adsorbed on a water surface at low concentrations (24).At these low concentrations, there are not enough intermolecular interactions to cause the chains of the pentadecanoic acid to pack. Therefore, evidence for kinks in the molecular chains adsorbed in a random fashion is observed through increased intensity of the CH, stretching modes at 2850 cm-l, and the diminished intensity of the terminating methyl stretches at 2875 and 2940 cm-l. The lower spectrum shows a similar long-chain organic [CH, (CH,) 17 (Me) N+(CH,), Si(OMe),Cl-] (DMOAP) adsorbed on glass in the absence (black) and presence (red) of the liquid crystal molecule 4"-octyl-4-cyanobiphenyl (8CB). The spectrum of DMOAP adsorbed on glass looks very much like that of the pentadecanoic acid at low coverages on water, indicating that DMOAP is not adsorbed in a close-packed, wellordered monolayer. However, when 8CB is coadsorbed onto the same surface, the intensity of the CH, stretching mode at 2850 cm-l is greatly diminished and points to the ordering effect of the liquid crystal molecule. These studies clearly indicate the potential of SFG in probing organic surface composition and orientation.
,
A Sample
Figure 13. Schematic of the SFG technique.
by bringing a metallic tip under piezoelectric control to within angstroms of a surface. At these small distances electrons can tunnel from the tip to the surface or from the surface to the tip on application of a bias voltage. A constant tip-sample separation is maintained by electronic feedback on this tunneling current. By scanning the tip (or sample) parallel to the surface, a map of tunneling current variations across the surface is produced. Alternatively, the tip can be scanned while holding a fixed tunneling current to produce a “topographic” image of the surface from the control voltages of the feedback loop. The images generated from the variFigure 14. SFG spectroscopy of ations of the tunneling current are reprelong-chain organic molecules. sentative of differences in the local density (a) Spectrum of pentadecanoic acid at low concentrations on a water surface. (b) Spectra of states across the surface. of DMOAP adsorbed on glass with (red) and Currently, specific assignment of without (black) the liquid crystal molecule “bumps” and “holes” in the images to dif8CB. The coadsorption of 8CB leads to packing of the DMOAP chains. (Adapted with ferent atoms relies on theoretical calculapermission from Reference 24.) tions of the local density of states and assumptions about the true nature of the tip. Although the theory of STM is in its inSTM fancy, it is clear that the observation of The scanning tunneling microscope (Figthese surface images is providing a direct ure 15) is a powerful’toolfor directly view into the behavior of many surfacestudying the real-space structure and beadsorbate systems. For example, with havior of surfaces and molecular adsorknowledge of the tip geometry and the bates. Ogletree and Salmeron have rechemical identity of the adsorbed moleviewed the study of solid surfaces with cule, bond distances in the adsorbates can STM (25).Frommer has reviewed the be obtained under some conditions. Comuse of STM and AFM in organic chemisbined theoretical and experimental studtry (26) and Hansma et al. have discussed ies are under way in many laboratories to the specific applications of STM to biologi- make the STM technique a quantitative cal issues (27).Atomically resolved immeasure of bond distances as well as ages of the surface region are generated bond geometry.
Because STM relies on the conduction of electrons through the sample, many organic surfaces are not suitable for study due to their insulating properties. This technique, however, has been successfully applied to thin layers of organic adsorbates. An example of the capabilities of STM is shown in Figure 16, where the image of benzene is coadsorbed with carbon monoxide on a rhodium(ll1) surface (28) as compared with a surface structure obtained through LEED studies (11).The benzene molecules are imaged with molecular threefold symmetry; the CO molecules appear as small maxima between the larger benzene molecules. By observing defects in this ordered structure at step edges, local adsorption sites could be identified in the scanning tunneling microscope image. STM differs from other surface characterization tools in that it gives high spatial resolution and imaging of individual molecules on the surface. It holds great promise for studies of surface monolayers under a variety of conditions, including high pressures, high temperatures, and liquid environments that are not accessible with LEED and HEELS. Moreover, because we obtain a real image with STM, long-range order is not necessary. Individual molecules can be detected; therefore defect sites and molecules at defect sites are clearly visible with STM. Because chemical processes often occur at these defect sites, STM is a promising technique for applications in surface chemistry, especially organic surface chemistry. AFM
The atomic force microscope, an adaptation of the STM approach, is based on the
Figure 15. STM.
Analytical Chemistry, Vol. 66, No. 7, April I , 1994 41 1 A
A schematic of the attractive and repulsive interactions encountered in AFM is shown in Figure 17. The forces that can lead to attractions of the tip to the surface include van der Waals attractions, capillary action, or electrostatic fields. When the tip and the sample are in contact, repulsive forces lead to deflection of the cantilever in an opposite direction. Signals optically detected from either type of deflection (attractive or repulsive) can be used as the feedback signal. The mechanical nature of the forces between tip and sample allows the study of conducting or insulating (e.g., many polymers and organic thin films) substrates. Based on mechanical interactions, an atomic force microscope can be operated in several modes to produce correlated information about the surface contact. The deflection of the cantilever normal to the surface as the tip is scanned over the sample produces a map of the surface toFigure 16. Comparison of STM imaging and LEED structure of pography much as in STM. Lateral deflecbenzene coadsorbed with CO on a tions of the cantilever can be monitored Rh(111) surface. simultaneously,revealing atomic-scale (a) STM image. (b) Structure of the friction between the tip and the sample. coadsorbed system calculated from LEED In addition, the force exerted on the tip I- V measurements. (attractive or repulsive) can be detected as a function of tipsample separation. The magnitude of attractive forces between the tip and sample and the degree of elastic and plastic deformation of the surface region can be determined from such force-distance curves. The interactions between an atomic force microscope tip and an organic substrate depend on the magnitude of forces within the contact. When the tip is brought into contact at low loads with an Figure 17. Schematic ordered organic overlayer (e.g., a longrepresentation of the attractive chain alkane in a Langmuir-Blodgett film and repulsive forces felt by an or self-assembledmonolayer), it probes atomic force microscope tip in only the topmost part of the chain. Deflecclose proximity to a surface. tions of the cantilever normal to the surThe force constant of the cantilever is represented by k. face reflect the structure of the outermost parts of the chain. The terminating methyl groups of selfassembled alkylthiols have been imaged physical principle of force interactions. in this fashion (30).As the load on the Instead of measuring tunnel currents between a sharp tip and a surface, AFM sample is increased, the microscope tip pushes through the organic layer and can monitors the mechanical interactions of the tip and the sample by optically detect- interact so strongly with the film that it ing the deflection of a flexible cantilever causes irreversible changes in the adsupporting an atomically sharp tip. A com- sorbed monolayers. Lateral interactions of plete review of the technique can be found the tip with the sample give a measure of the friction coefficientsand thus the luin References 26,27, and 29. 412 A Analytical Chemistry, Vol. 66, No. 7, April 1, 1994
bricity of organic films on the atomic scale. Data obtained from the operation of the AFM in the force-distance mode are shown in Figure 18.These studies depict the attractive and repulsive forces encountered by the tip on approaching various surfaces: alumina, a long-chain acid, a fluorinated long-chain acid, and Teflon (31). As the tip approaches an oxide surface, a strong and long-range attraction is observed, followed by the repulsive contact with the sample. On retraction of the tip, the attractive forces persist to even greater tipsample separations because of the “bonding,”or adhesion of the tip to the sample on contact. In the presence of a long-chainacid on alumina, the attractive forces on approach are much less dominant; however, on retraction they persist to even greater tipsample separations as the tip overcomes the capillary action of the organic. By terminating the acid chain with trifluoromethy1 end groups, the attractive and adhesive interactions are weakened. A similar but more extensive effect is observed as the tip approaches a Teflon surface where attractive forces are virtually absent. The slope of the repulsive section of the curve for Teflon reveals a greater elasticity as compared with the alumina surface, demonstrating the capability of AFM in measuring the mechanical properties of organic surfaces such as hardness and elastic and plastic deformations. SFA
The surface force apparatus (SFA) has a principle of operation very similar to that of the AFM; it monitors the physical interaction of two surfaces. In this case, however, the area of the two surfaces approaching one another at atomic distances is much larger. Well-defined, large areas of contact can be achieved only for surfaces with atomic flatness over the area of contact (Figure 19).Mica surfaces that display superior atomic smoothness because of their layered structure are bent in a cylindrical shape and approached at right angles. Atomically flat contact areas on the order of 10 pm in diameter can be achieved in this way. Analogous to the AFM approach curves, information about the surface con-
onitoring the forces sive) between the surlubricants under surfaces can be denerating an interfern them and monitoring
the two mica plates.
Figure 20 shows the behavior of linear and branched alkanes under increasing forces as the two mica plates are squeezed together (32).The linear alkanes slip out
ration of the surfaces creased by the presence o The behavior of lubricated s tacts has also been studied as a function of relative motion of the surfaces. This is
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Analytical Chemistry, Vo/. 66, No. 7, A
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accomplished by translating the upper surface under piezoelectric control while monitoring the lateral forces between the two surfaces. Such measurements probe the dynamic properties, such as shear behavior and liquid-solid transitions, of molecularly thin organic films. In the near future, spectroscopic studies of the SFA
j
i
The measureme between a liquid and a solid technique for studying the surface or surface free energy of different als (Figure 21). This technique is t topic of several chapters in Adamso Physical Chemistry of Surfaces (33). an organic molecule is deposited on an surface it will wet
g indicates a smooth
liquid and the solid,
solid-gas (ysg), liquid-gas
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April 1, 1994
measuring the contact angles of a homologous series of liquids with a given surface and by plotting the contact angles as a function of the liquid surface tensions. Extrapolating xn to zero contact angle gives the critical surface tension of the solid and approximates its surface free energy. The critical tension of these surfaces changes as a function of the atomic substitution of the surface carbon or hydrogen atoms by heteroatoms. The wetting ability of the surface is important in applications that entail the interaction of a surface with an aqueous medium. Especially in bioorganic systems, the surface thermodynamic properties play a predominant role in controlling the adhesion and separation of various organic film structures at the molecular level. Neutron reflectivity measurements
The application of neutron reflectivity to the study of organic surfaces ranges from the study of liquid-vapor interactions to the determination of surface concentration gradients. Figure 22a shows a sche matic representation of an experimental apparatus used for the study of organic surfaces by neutron reflection. A wellcollimated beam of neutrons is scattered at grazing angles from an organic surface. At these angles, the neutron reflectivity of the surface region is a function of local mass density. The reflectivity of the surface region is measured as a function of neutron momentum and compared with a theoretically simulated reflectivity. An important input parameter of these calculations is the density profile of the organic layer. By studying carefully designed systems (containing selectively deuterated components), researchers can obtain the concentration profile and diffusion properties in polymer blends near an interface (34 1. In the top half of Figure 22b, the neutron reflectivity of a homogeneous mixture of polystyrene and deuterated poly(methyl methacrylate) (PMMA) is shown along with the concentration profile used to fit the data (upper right). When this film is annealed at 170 "Cfor 3 h, PMMA segregates to the surface as indicated by the increased density at the polymer-air interface (lower half of Figure 22b). This technique has somewhat limited applica-
bility because of the difficulty in obtaining neutron sources in many locations and laboratories. However, these studies demonstrate their unique power, which can be applied to the analysis of organic surfaces. We gratefully acknowledge all those who have allowed us to reproduce their work in this article. This work was supported in part by the Director, Office of Energy Research, Office of Basic Energy Sciences, Materials Sciences Division of the US. Department of Energy under contract no. DE-AC03-76SF00098.
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Scott S. P e w (left) is a postdoctoral associate in the Department of Chemistry at the University of California, Berkeley (Berkeley, CA 947201, where he is studying the tribological and adhesion properties of chemically vapordeposited diamond film. He received his B.S. degree in chemistry fiom Furman University and his Ph.D. in chemical physics fiom the University of Texas at Austin. Gabor A. Somodai is professor of chemistry at UC,Berkeley, and a faculty senior scientist at the Materials Science Division and group leader of the Catalysis Program of the Centerfor Advanced Materials at the Lawrence Berkeley Laboratory. After receiving his Ph.D. in chemistryfiom UC,Berkeley, in 1960, he joined the research stafof the IBM research center in Yorktown Heights, hY He joined the faculty of UC, Berkeley, in 1964.
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