J. Phys. Chem. C 2008, 112, 1473-1478
1473
Effect of Surface Chemistry on Mechanical Energy Dissipation: Silicon Oxidation Does Not Inherently Decrease the Quality Factor Amy M. Richter, Debodhonyaa Sengupta, and Melissa A. Hines* Department of Chemistry and Chemical Biology, Cornell UniVersity, Ithaca, New York 14853-1301 ReceiVed: May 22, 2007; In Final Form: October 17, 2007
The rate of energy dissipation in megahertz-range micromechanical silicon resonators is unaffected by the controlled oxidation of one-half monolayer of surface sites, thereby proving that silicon oxidation does not inherently decrease the quality factor (Q). Homogeneously mono-oxidized surfaces were prepared by the controlled reaction of dodecyl aldehyde with H-terminated silicon surfaces to form dodecoxy-terminated surfaces (C12H25O-Si). The existence of an approximately half monolayer of Si-O-R species (the maximum allowed by steric constraints) was confirmed by infrared spectroscopy. As a control, dodecyl-terminated surfaces (C12H25-Si) were prepared by the reaction of 1-dodecene with H-terminated surfaces. Infrared spectroscopy showed that the density and ordering of the alkyl chains on the dodecoxy- and dodecyl-terminated surfaces were nearly identical, and no subsurface oxidation was detected on either surface. Dodecoxy- and dodecylterminated resonators displayed similar quality factors both immediately after functionalization and after extended exposure to vacuum and H2O-saturated air. Although the dodecoxy-terminated resonators appeared to adsorb more mass (as evidenced by frequency shifts), the increased adsorption did not have a deleterious effect on resonator quality. The relatively high qualities of mono-oxidized resonators stand in stark contrast to the low qualities displayed by resonators terminated with native or chemical oxides. This result suggests that oxide defect species may play a role in mechanical energy dissipation at surfaces.
Q ) x1 + x2
1. Introduction By virtue of their small masses, micro- and nanoelectromechanical (MEMS and NEMS) resonators show great promise in chemical and biological sensing applications. For example, nanometer-scale resonators capable of detecting subattogram (1000 Ω-cm, float-zone Si(111) wafers using a combination of optical lithography, reactive ion etching, thermal oxidation, and anisotropic wet etching as described in ref 8. Each wafer was diced to produce more than 100 6 mm × 6 mm chips; each chip contained ∼20 resonators of each size. Each resonator consisted of a hexagonal mass suspended by two 4-µm-long, 440-nm-wide silicon wires over a micrometers-deep triangular chasm. Three hexagonal mass sizes were used: 6.6, 5.2, and 3.7 µm (measured across the flats). The entire resonator (mass and wires) had a uniform thickness in the 280-330 nm range. The exact thickness depended on the anisotropic etch duration and could be accurately determined from measurements of the resonant frequency.8 Since the fabrication procedure introduced small inhomogeneities in device dimensions across the wafer, samples cut from adjacent regions were used when direct comparisons of mechanical properties (i.e., f and Q) were necessary. 2.2. Surface Functionalization. After fabrication and wafer dicing, the resonators were supported by pillars of silicon, and the tops and sides of the resonators were coated with a thick, protective thermal oxide. Before functionalization, each chip was cleaned using a modified RCA clean as described in ref 18 and thoroughly rinsed in deionized water (Millipore MilliQ). The supporting silicon pillars were removed with a 5 min etch in 70 °C, 50% w/v KOH (Transene) and then rinsed. The protective oxide was removed with a 1.5 min immersion in room-temperature buffered oxide etchant (Mallinckrodt-Baker, a 5:1 mixture of aqueous NH4F:HF), and the chips were thoroughly rinsed. At this stage, the resonators were suspended, and all exposed surfaces were fully terminated by a monolayer of H atoms, as verified by infrared spectroscopy (vide infra). Dodecoxy (C12H25O-) monolayers were synthesized by the thermal reaction of neat 1-dodecanol19 (Fluka, 98.5%) or dodecyl aldehyde20 (Aldrich, 92%) with H-terminated silicon surfaces, whereas dodecyl (C12H25-) monolayers were synthesized by the thermal reaction of neat 1-dodecene (Aldrich, 96%) with H-terminated silicon surfaces.21 The reagents were first dried under vacuum over activated molecular sieves (1-dodecanol, 1-dodecene and dodecyl aldehyde) or sodium (1-dodecanol) for at least 12 h, distilled at reduced pressure, and then stored under inert atmosphere prior to use. Freshly cleaned and prepared H-terminated samples were placed in clean, dry glass vials; immersed in the appropriate reagent under inert atmosphere; sealed; and reacted at elevated temperature for 1 h (1-dodecanol, 165 °C; dodecyl aldehyde, 165 °C; 1-dodecene, 200 °C). After
Surface Chemistry, Mechanical Energy Dissipation cooling, the samples were rinsed vigorously in CH2Cl2 (Mallinckrodt), then immediately loaded into the testing apparatus or infrared spectrometer for analysis. 2.3. Surface Infrared Analysis. The composition and packing density of the surface monolayers were characterized with infrared absorption spectroscopy in both the transmission and multiple internal reflection (MIR) geometries.22 Although the MIR geometry is much more sensitive to surface absorption bands, strong multiphonon absorption limits the transparency of the silicon substrate below 1500 cm-1. The resonator chips were too small for direct spectroscopic analysis, so larger samples (15 mm × 38 mm × 500 µm) were cut from the same batch of silicon wafers and coprocessed with the resonator chips. (The front and back surfaces of the resonators, which comprised 80-90% of the surface area, are well-modeled as a flat Si(111) surface, as has been shown by previous AFM experiments8 as well as atomic-scale studies of KOH/Si(111) etching.23) The short sides of the spectroscopic samples were beveled at 45° to enable MIR analysis. In the MIR geometry, incident light from a Fourier transform infrared spectrometer was focused onto one bevel. The light then underwent ∼75 internal reflections before exiting the opposite bevel and passing through a ZnSe polarizer. In the transmission geometry, the incident light was focused on the center of the sample at 74° from the surface normal. The light was subsequently focused onto a mercury-cadmiumtelluride (MCT; MIR geometry) or deuterated-triglycine-sulfate (DTGS; transmission geometry) detector. Spurious interference fringe in the spectra, which was caused by multiple reflections from the highly polished wafer faces, was removed computationally.24 The spectra presented here were all referenced to a freshly prepared H-terminated surface. 2.4. Mechanical Testing. After functionalization, the resonator chips were mounted on a piezoelectric ceramic actuator, loaded into an ion-pumped vacuum chamber and evacuated to 10-8 Torr. The mechanical properties of the resonators were quantified by driving the piezoelectric ceramic at a variable frequency while monitoring the displacement of the resonator with a laser optical interferometer. The frequency and quality factor of each resonator was extracted from a best-fit Lorentzian to the measured response function. The reported mechanical properties represent the averaged response of three to four resonators from a single functionalized chip. Replicate experiments produced similar results; however, these results were not included in the average due to small, process-induced variations in mechanical response from chip to chip. 3. Results 3.1. Monolayer Characterization. Spectra of H-terminated surfaces (not shown) displayed a single, sharp, p-polarized absorption band centered at 2083.6 cm-1 which was assigned to the Si-H stretch vibration on flat Si(111) terraces in accordance with previous studies.25 No oxidation or hydrocarbon contamination was observed. Spectra of dodecyl-terminated surfaces, formed by the reaction of 1-dodecene with H-terminated surfaces, were consistent with the formation of well-packed monolayers with no surface oxidation. Three characteristic absorption bands were observed in the C-H stretch region (2820-2980 cm-1), as shown by the dashed line in Figure 3. These bands correspond to the symmetric (∼2850 cm-1) and antisymmetric (∼2920 cm-1) methylene and antisymmetric methyl (∼2960 cm-1) stretch modes.26 In condensed hydrocarbons, the energy of the antisymmetric methylene stretch mode is sensitively dependent on the order and packing of the molecules.27 For example, the
J. Phys. Chem. C, Vol. 112, No. 5, 2008 1475
Figure 3. A comparison of the C-H stretch region of the infrared absorption spectrum of Si(111) surfaces terminated with a monolayer of dodecoxy (blue) and dodecyl (red) moieties. The comparable absolute intensities and line positions of these spectra are indicative of very similar density and packing of the alkyl chains. The spectra were taken at 1 cm-1 resolution in the MIR geometry.
Figure 4. A comparison of the Si-O stretch region of the infrared absorption spectrum of Si(111) surfaces terminated with (a) a dodecyl monolayer, (b) a dodecoxy monolayer (aldehyde synthesis), and (c) a native oxide. The absorption at 1097 cm-1 is attributed to a Si-O stretch vibration. The dodecoxy- and dodecyl-functionalized surfaces show no sign of subsurface oxidation. The spectra were taken at 2 cm-1 resolution in transmission.
increasing disorder that results from the solid-to-liquid-phase transition leads to a 6-8 cm-1 increase in the energy of this band. The energy of the antisymmetric methylene stretch mode is therefore often used as an indicator of monolayer order. Dodecyl-terminated surfaces typically displayed an antisymmetric methylene stretch at 2921.8 cm-1, which is in good agreement with the 2921.2 cm-1 transition reported by Linford et al. 21 for well-ordered monolayers. As shown by Figure 4a, these monolayers did not absorb in the Si-O stretch region (900-1200 cm-1), which is consistent with the known oxidation resistance of these monolayers. For comparison, Figure 4c shows the Si-O stretch region for a native-oxide-terminated surface. The spectra of dodecoxy-terminated surfaces prepared by the thermal reaction of dodecyl aldehyde with H-terminated surfaces were consistent with the formation of dense, well-ordered
1476 J. Phys. Chem. C, Vol. 112, No. 5, 2008
Figure 5. A comparison of the C-H stretch region of the infrared absorption spectrum of dodecoxy-terminated Si(111) surfaces functionalized using dodecyl aldehyde (blue) and 1-dodecanol (red). The decreased intensity of the entire spectrum and blue shift of the antisymmetric methylene stretch band are indicative of the relatively low density of monolayers prepared with the alcohol precursor. The spectra were taken at 1 cm-1 resolution in the MIR geometry.
monolayers. As shown by the solid line in Figure 3, the peak intensities and positions of the C-H stretch bands were nearly identical to those observed on dodecyl-terminated surfaces. The slightly lower energy of the antisymmetric methylene stretch (2921.2 cm-1) and higher overall intensity of the C-H stretch bands show that the dodecoxy monolayers are at least as dense and well-ordered as the dodecyl monolayers. As shown by Figure 4b, these monolayers also displayed a sharp absorption at 1097 cm-1, which is tentatively assigned to the O-C stretch vibration in analogy with recent studies of methoxy-terminated Si(111) surfaces.28 In the CH3O-Si study, first-principles calculations were used to assign a sharp band at ∼1080 cm-1 to the coupled motion of the C-O stretch and CH3 rock modes. Because of its near vertical orientation, the corresponding Si-O stretch vibration was not observed at 74° incidence. At nearnormal incidence, the Si-O stretch vibration was observed at ∼1050 cm-1. In contrast, dodecoxy-terminated surfaces prepared by the thermal reaction of 1-dodecanol with H-terminated surfaces were of considerably lower quality, as shown by the peak intensities and positions of the C-H stretch bands in Figure 5. Dodecoxy monolayers formed from the reaction of alcohol had consistently lower absorbance than those produced from aldehydes, suggesting the formation of sparser monolayers. This conclusion is consistent with the ∼3.5 cm-1 blue shift of the methylene stretch vibration, which is indicative of less dense monolayers. Although numerous attempts were made to further purify and dry the alcohol and to vary the reaction conditions, we were unable to improve the quality (i.e., order and density) of the dodecoxy monolayers synthesized from 1-dodecanol. This negative result is consistent with the findings of previous researchers who have shown that alkoxy monolayers formed from the thermal20 and photochemical29 reaction of alcohols are less chemically stable than those produced by the reaction of aldehydes and are less dense and well-packed.29 3.2. Mechanical Properties of Functionalized Resonators. 3.2.1. Dodecoxy Monolayers Synthesized from Dodecyl Aldehyde. When the aldehyde-based synthesis was used, dodecoxy-
Richter et al.
Figure 6. The initial quality factors of three different sizes of dodecoxy- (circles) and dodecyl-terminated (squares) resonators show no systematic differences. For comparison, the average quality of all resonators is indicated by the dashed line.
Figure 7. The quality factors of dodecoxy- (circles) and dodecylterminated (squares), 5.2-µm-wide resonators as a function of time. For the first 134 h, the resonators were kept in vacuum (10-8 Torr). The resonators were then stored in a closed container of air at 1 atm pressure and 100% humidity for 96 h and retested in vacuum.
functionalized resonators displayed mechanical properties very similar to those of dodecyl-functionalized resonators. For example, Figure 6 compares the initial quality factors of three different resonator sizes. No systematic difference in initial quality factor between dodecoxy-functionalized and dodecylfunctionalized resonators was observed. The long-term stability of the dodecoxy-functionalized resonators was also very good. As shown by the data for 5.2-µmwide resonators in Figure 7, neither the dodecoxy- or the dodecyl-functionalized resonators displayed significant degradation in their quality factors over 134 h in vacuum. After this time, the resonators were removed from vacuum, placed in a sealed bell jar of room air at 100% humidity for an additional 96 h, and retested. On average, the air exposure resulted in a 15-20% drop in the quality factor, which is roughly consistent with previous experiments on long-chain-alkyl-functionalized resonators.9,11 Although the dodecoxy-functionalized resonators were marginally more resistant to air-induced Q degradation, this difference may not be statistically significant. Interestingly, the long-term stability of the functionalized resonators cannot be (completely) explained by the expected adsorption-resistant properties of the monolayers. As explained
Surface Chemistry, Mechanical Energy Dissipation
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Figure 8. The fractional frequency shift of dodecoxy- (circles) and dodecyl-terminated (squares), 5.2-µm-wide resonators as a function of time. (See caption of Figure 7 for testing details.) Negative frequency shifts are presumably due to adsorption-induced changes in resonator mass. For illustrative purposes, the right-hand axis converts the fractional frequency shift into the effective monolayers of O atoms required to generate this shift (see text.)
in the introduction, the resonance frequencies of these devices are very sensitive to changes in adsorbed mass. Figure 8 displays the fractional frequency shift,
∆ f f(t) - f(0) ) f f(0)
(3)
for 5.2-µm-wide functionalized resonators as a function of time, t. When the resonators are stored in vacuum, the resonant frequencies are very stable, indicating little or no adsorption from vacuum. This behavior should be contrasted with that of H-terminated resonators, which display a dramatic negative frequency shift under similar conditions.9 In moist air, the negative frequency shifts in Figure 8 suggest significant adsorption from the gas phase. The dodecyl-terminated resonators display a moderate frequency shift, whereas the dodecoxyterminated devices display a much larger shift. The frequency shift is, of course, insensitive to the identity of the adsorbate(s); however, the magnitude of the shift can be used to estimate the total adsorbed mass.8 Since these masses are quite small, they are most conveniently expressed in terms of the effective coverage of a specific adsorbate. For example, if O atoms were adsorbing uniformly over the entire exposed surface of the resonator, the monolayers of adsorbed O, θO, would be given by
θO ) -
( )(
)(
∆f mSi 2 DT f mO 0.314 nm 4T + D
)
(4)
where mSi and mO are the atomic masses of silicon and oxygen, respectively, 0.314 nm is the Si(111) bilayer spacing, and D and T are the width and thickness of the paddle, respectively. This quantity is shown on the right-hand axis of Figure 8. Irrespective of the exact identity of the adsorbates, Figure 8 shows that the dodecoxy-terminated resonators are able to tolerate a significant amount of adsorption with little degradation in quality factor. Since both the dodecoxy and dodecyl monolayers present the same chemical functionality to the gas phase and since both sets of resonators were exposed to exactly the same environment, the dramatic difference in adsorption between the two surface terminations is puzzling. We suggest two possible causes for this difference. First, the dodecoxyterminated surfaces may be more susceptible to chemical attack
than dodecyl-terminated surfaces. Since the infrared absorption data suggest that the alkyl regions of the monolayers have very similar configuration and density, any differences in reactivity would presumably be due to differences in the interfacial bonding (i.e., Si-C vs Si-O-C) or interfacial electrostatics. The second possibility is that the differences in adsorption are due to different monolayer packing or density on the sides or edges (or both) of the resonator, regions of the device that may not be well-modeled by the flat Si(111) surface used in the spectroscopic investigations. We note, however, that these regions account for only 10-20% of the surface area of the resonator (depending on resonator size). Importantly, these regions are (nominally) Si{110} planes, which are expected to have reactivity that is similar to the majority Si{111} surfaces. 3.2.2. Dodecoxy Monolayers Synthesized from 1-Dodecanol. Dodecoxy-terminated resonators functionalized with the 1-alcohol precursor had dramatically different performance from those functionalized with the aldehyde. These resonators displayed uniformly low quality factors (∼10 000 typ), which is perhaps not surprising, given the low density of these monolayers. We speculate that trace H2O in the alcohol leads to partial oxidation of the interface, as suggested by the AFM images of Boukherroub et al.,20 and that this oxidation leads to lowered quality factors. To test this hypothesis, repeated attempts were made to dry the alcohol (e.g., using molecular sieve, sodium or trichlorosilane), but no improvement in mechanical performance was observed. Because of their poor initial quality factors, a detailed lifetime analysis was not performed on these resonators. 4. Discussion These results conclusively show that silicon oxidation does not inherently lead to increased mechanical energy dissipation. When silicon surfaces are oxidized in a controlled, strain-free manner by the introduction of one-half monolayer of Si-O-R surface groups, increased dissipation is not observed. This result is important, because it opens the door to a much broader range of functionalization chemistries for high-performance sensor applications. If oxidized silicon is not inherently dissipative, why do resonators terminated with a thin chemical oxide have such poor mechanical performance (i.e., low Q)? What chemical species is leading to the increased mechanical energy dissipation? These surface oxides are not homogeneous SiO2; they contain a mixture of strained and relaxed SiO2, silicon suboxides, and surface silanol (Si-OH).30 The oxides also have a very high density of electronic defects (∼1013 cm-2).31 (For comparison, the well-annealed Si-SiO2 interfaces found in MOS transistors have defect densities of ∼1010 cm-2.30) The increased mechanical energy dissipation in oxidized resonators cannot be attributed to the presumed majority species (unstrained SiO2) for two reasons. First, macroscopic resonators fabricated from pure silica have exceptionally low dissipation.32 Second, Wang has shown that silicon resonators terminated with a 100-nm-thick, well-annealed thermal oxide have significantly lower dissipation than similar resonators terminated with a 1.3-nm-thick chemical oxide.33 The relatively high quality factors of dodecoxy-terminated resonators suggest (but do not conclusively prove) that the increased dissipation is not due to surface silanol, as this species is chemically similar to the alkoxides studied here. Unfortunately, silanol-terminated resonators cannot be probed directly, as efforts to produce silanol-functionalized silicon surfaces with no bare silicon atoms (i.e., dangling bonds) or subsurface oxidation have been unsuccessful.34
1478 J. Phys. Chem. C, Vol. 112, No. 5, 2008 The two most likely candidates for the highly dissipative species therefore appear to be defect species, either strained SiO-Si species or silicon “dangling bonds.” These defect species may (or may not) be associated with electronic defects in the oxide. Defect-induced dynamic mechanical energy dissipation is well-known and well-studied in bulk materials.17 In brief, the resonant motion of the oscillator leads to periodic strain fields and thus periodic deformation of the bonds surrounding the defect. As a result, the deformation typically changes the defect’s electronic and vibrational energy levels. If this deformation puts the defect in a nonequilibrium state, the defect will relax to its equilibrium state with a characteristic relaxation time. If this relaxation time is comparable to the period of the resonator (neither much longer nor much shorter), the relaxation will occur with a phase lag to the applied stress. This phase lag inherently leads to mechanical energy dissipation and so-called “internal friction.” In a few cases, the specific structure of the dissipative defect is known. For example, boron-induced electronic defects are a significant source of mechanical energy dissipation in silicon at cryogenic temperatures.35 Recent ab initio models have also been used to study divacancy-induced dissipation in bulk silicon.36 Can the known density of interfacial electronic defects at the chemical oxide/silicon interface account for the dissipation observed in oxidized resonators? This is a difficult question to answer, as there is little information on the specific cross sections of individual defects in the literature. Nevertheless, it is important to realize that the small sizes of the resonators lead to high effective concentrations of electronic defects. For example, our resonators have a typical surface-area-to-volume ratio of 5 × 104 cm-1. A surface density of 1013 electronic defects/cm2 therefore corresponds to an effective bulk density of 5 × 1017 cm-3, which is almost two orders of magnitude larger than the 1.1 × 1016 cm-3 density at which boron defects made a significant contribution to dissipation in macroscopic silicon resonators.35 Electronic defects at the silicon/silicon oxide interface are therefore a plausible candidate for the dissipative species. 5. Conclusions The quality factors and long-term stabilities of megahertzrange micromechanical silicon resonators terminated by dense, close-packed dodecyl and dodecoxy monolayers have no systematic differences. Since these two surface functionalities differ by a single oxygen atom, this result shows that the rate of mechanical energy dissipation in megahertz-range micromechanical silicon resonators is not measurably affected by the controlled mono-oxidation of approximately one-half of the silicon surface atoms. Although previous measurements have shown that the growth of chemical or native oxides leads to increased mechanical energy dissipation, the low dissipation of Si-OR-functionalized surfaces suggests that this disipation is not due to unstrained mono-oxidized species (e.g., Si-O-H). Acknowledgment. This work was supported by the Cornell Center for Materials Research, a Materials Research Science
Richter et al. and Engineering Center of the National Science Foundation (DMR-0520404), and by the NSF under Award No. CHE0515436 and performed in part at the Cornell NanoScale Science and Technology Facility, a member of the National Nanotechnology Infrastructure Network, which is supported by the National Science Foundation (Grant ECS 03-35765). References and Notes (1) Ilic, B.; Craighead, H. G.; Krylov, S.; Senaratne, W.; Ober, C.; Neuzil, P. J. Appl. Phys. 2004, 95, 3694. (2) Yang, Y. T.; Callegari, C.; Feng, X. L.; Ekinci, K. L.; Roukes, M. L. Nano Lett. 2006, 6, 583. (3) Ekinci, K. L.; Yang, Y. T.; Roukes, M. L. J. Appl. Phys. 2004, 95, 2682. (4) Mihailovich, R. E.; MacDonald, N. C. Sens. Actuators, A 1995, 50, 199. (5) Carr, D. W.; Evoy, S.; Sekaric, L.; Craighead, H. G.; Parpia, J. M. Appl. Phys. Lett. 1999, 75, 920. (6) Yang, J.; Ono, T.; Esashi, M. Appl. Phys. Lett. 2000, 77, 3860. (7) Yang, J.; Ono, T.; Esashi, M. J. Vac. Sci. Technol. B 2001, 19, 551. (8) Wang, Y.; Henry, J. A.; Zehnder, A. T.; Hines, M. A. J. Phys. Chem. B 2003, 107, 14270. (9) Henry, J. A.; Wang, Y.; Hines, M. A. Appl. Phys. Lett. 2004, 84, 1765. (10) Wang, Y.; Henry, J. A.; Sengupta, D.; Hines, M. A. Appl. Phys. Lett. 2004, 85, 5736. (11) Henry, J. A.; Wang, Y.; Sengupta, D.; Hines, M. A. J. Phys. Chem. B 2007, 111, 88. (12) Neuwald, U.; Hessel, H. E.; Feltz, A.; Memmert, U.; Behm, R. J. Appl. Phys. Lett. 1992, 60, 1307. (13) Royea, W. J.; Juang, A.; Lewis, N. S. Appl. Phys. Lett. 2000, 77, 1988. (14) Webb, L. J.; Lewis, N. S. J. Phys. Chem. B 2003, 107, 5404. (15) Sander, D.; Ibach, H. Phys. ReV. B: Condens. Matter Mater. Phys. 1991, 43, 4263. (16) Bhiladvala, R. B.; Wang, Z. J. Phys. ReV. E: Stat. Phys., Plasmas, Fluids, Relat. Interdiscip. Top. 2004, 69, 036307-1-5. (17) Braginsky, V. B.; Mitrofanov, V. P.; Panov, V. I. Systems with small dissipation; Ph.D. Thesis, University of Chicago, Chicago, IL, 1985. (18) Flidr, J.; Huang, Y.-C.; Hines, M. A. J. Chem. Phys. 1999, 111, 6970. (19) Cleland, G.; Horrocks, B. R.; Houlton, A. J. Chem. Soc. Faraday Trans. 1995, 91, 4001. (20) Boukherroub, R.; Morin, S.; Sharpe, P.; Wayner, D. D. M.; Allongue, P. Langmuir 2000, 16, 7429. (21) Linford, M. R.; Fenter, P.; Eisenberger, P. M.; Chidsey, C. E. D. J. Am. Chem. Soc. 1995, 117, 3145. (22) Chabal, Y. J. Surf. Sci. Rep. 1988, 8, 211. (23) Garcia, S. P.; Bao, H.; Hines, M. A. Phys. ReV. Lett. 2004, 93, 166102-1661-4. (24) Faggin, M. F.; Hines, M. A. ReV. Sci. Instrum. 2004, 75, 4547. (25) Jakob, P.; Chabal, Y. J. J. Chem. Phys. 1991, 95, 2897. (26) Parikh, A. N.; Allara, D. L.; Azouz, I. B.; Rondelez, F. J. Phys. Chem. B 1994, 98, 7577. (27) Snyder, R. G.; Strauss, H. L.; Elliger, C. A. J. Phys. Chem. 1982, 86, 5145. (28) Michalak, D. J.; Rivillon, S.; Chabal, Y. J.; Este´ve, A.; Lewis, N. S. J. Phys. Chem. B 2006, 110, 20426. (29) Hacker, C. A.; Anderson, K. A.; Richter, L. J.; Richter, C. A. Langmuir 2005, 21, 882. (30) Higashi, G. S.; Chabal, Y. J. In Handbook of Semiconductor Wafer Cleaning Technology; Kern, W., Ed.; Noyes Publications: Park Ridge, NJ, 1993, pp 433-496. (31) Angermann, H. Anal. Bioanal. Chem. 2002, 374, 676. (32) Gretarsson, A. M.; Harry, G. M. ReV. Sci. Instrum. 1999, 70, 4081. (33) Wang, Y. Ph.D. Thesis, Cornell University, Ithaca, NY, 2004. (34) Rivillon, S.; Brewer, R. T.; Chabal, Y. J. Appl. Phys. Lett. 2005, 87, 173118. (35) Mihailovich, R. E.; Parpia, J. M. Phys. ReV. Lett. 1992, 68, 3052. (36) U ¨ stu¨nel, H.; Roundy, D.; Arias, T. A. Phys. ReV. Lett. 2005, 94, 025503.
