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Nanoscale Frictional Properties of Mixed Alkanethiol Self-Assembled Monolayers on Au(111) by Scanning Force Microscopy: Humidity Effect Lingyan Li, Shengfu Chen, and Shaoyi Jiang* Department of Chemical Engineering, University of Washington, Seattle, Washington 98195 Received September 19, 2002. In Final Form: November 26, 2002 We present a quantitative study of the nanoscale frictional properties of uniform mixed self-assembled monolayers (SAMs) of dodecanethiol and 11-mercapto-1-undecanol on Au(111) as a function of surface composition using scanning force microscopy (SFM) at a variety of relative humidities. Surface properties are varied from hydrophobic to hydrophilic by adjusting the surface composition of mixed SAMs. Results show that the frictional properties of mixed SAMs are affected by both surface properties and relative humidities. At lower relative humidity, there is a clear relationship between surface composition and friction coefficient. Thus, it may be possible to determine the local surface composition of a mixed SAM from its friction coefficient measured by SFM under a controlled environment. At higher relative humidity or in water, the friction coefficient is not sensitive to changes in surface composition. Due to the significant effect of the scanning environment, one must be very careful when measuring and analyzing frictional properties by SFM in air.
* To whom correspondence should be addressed. E-mail: sjiang@ u.washington.edu.
have lower friction coefficients than their short-chain counterparts. The difference in the size of the methyl and trifluoromethyl terminal groups causes the difference in friction.13 SFM has also been applied to the study of the frictional properties of silanes8,14-16 and alkanethiol/Au(111) with chemically modified tips.15-19 Recently, the humidity effect on frictional properties of SAMs has been studied by Zhang et al. for alkyl/Si20 and by Tian et al. for mica and methyl-terminated alkylsilane/SiO2.21 Most of the previous investigations focused on surfaces covered by single-component SAMs. However, technological applications may rely on mixed monolayers that comprise SAMs with different chain lengths (physical roughness) or terminal groups (chemical roughness). One of the advantages of mixed SAMs is that chemical and structural properties of a surface can be controlled by adjusting the abundance, type, and spatial (both normal and lateral) distribution of tail groups.22 We have systematically studied the phase behavior of mixed SAMs on Au(111) and were able to prepare molecular-scale mixed SAMs containing several terminal groups, such as acetic acid, hydroxyl, and amine,23 for various chain lengths and compositions using the kinetically trapped method.24
(1) Carpick, R. W.; Salmeron, M. Chem. Rev. 1997, 97, 1163. (2) (a) Bhushan, B.; Israelachvili, J. N.; Landman, U. Nature 1995, 374, 607. (b) Bliznyuk, V. N.; Everson, M. P.; Tsukruk, V. V. J. Tribol. 1998, 120, 489. (3) (a) Maboudian, R. Surf. Sci. Rep. 1998, 30, 207. (b) Maboudian, R.; Howe, R. T. J. Vac. Sci. Technol., B 1997, 15, 1. (4) Kiely, J. D.; Houston, J. E.; Mulder, J. A.; Hsung, R. P.; Zhu, X. Y. Tribol. Lett. 1999, 7, 103. (5) Binnig, G.; Quate, C. F.; Gerber, C. Phys. Rev. Lett. 1986, 56, 930. (6) Mate, C. M.; McClelland, G. M.; Erlandsson, R.; Chiang, S. Phys. Rev. Lett. 1987, 59, 1942. (7) Bhushan, B.; Kulkarni, A. V.; Koinkar, V. N.; Boehm, M.; Odoni, L.; Martelet, C.; Belin, M. Langmuir 1995, 11, 3189. (8) Meyer, E.; Overney, R.; Brodbeck, D.; Howald, L.; Luthi, R.; Frommer, J.; Guntherodt, H.-J. Phys. Rev. Lett. 1992, 69, 1777. (9) Xiao, X.; Hu, J.; Charych, D. H.; Salmeron, M. Langmuir 1996, 12, 235. (10) Lio, A.; Charych, D. H.; Salmeron, M. J. Phys. Chem. B 1997, 101, 3800. (11) McDermott, M. T.; Green, J.-B. D.; Porter, M. D. Langmuir 1997, 13, 2504. (12) Li, L.; Yu, Q.; Jiang, S. J. Phys. Chem. B 1999, 103, 8290. (13) Kim, H. I.; Koini, T.; Lee, T. R.; Perry, S. S. Langmuir 1997, 13, 7192.
(14) Liu, Y.; Wu, T.; Evans, D. F. Langmuir 1994, 10, 2241. (15) Clear, S. C.; Nealey, P. F. J. Colloid Interface Sci. 1999, 213, 238. (16) Moser, A.; Eckhardt, C. J. Thin Solid Films 2001, 382, 202. (17) (a) Frisbie, C. D.; Tozsnyai, L. F.; Noy, A.; Wrighton, M. S.; Lieber, C. M. Science 1994, 265, 2071. (b) Noy, A.; Frisbie, C. D.; Rozsnyai, L. F.; Wrighton, M. S.; Lieber, C. M. J. Am. Chem. Soc. 1995, 117, 7943. (18) Green, J.-B. D.; McDermott, M. T.; Porter, M. D.; Siperko, L. M. J. Phys. Chem. 1995, 99, 10960. (19) Fujihira, M.; Tani, Y.; Furugoti, M.; Akiba, U.; Okabe, Y. Ultramicroscopy 2001, 86, 63. (20) Zhang, L. Z.; Li, L. Y.; Chen, S. F.; Jiang, S. Y. Langmuir 2002, 18, 5448. (21) Tian, F.; Xiao, X.; Loy, M. M. T.; Wang, C.; Bai, C. Langmuir 1999, 15, 244. (22) (a) Bain, C. D.; Evall, J.; Whitesides, G. M. J. Am. Chem. Soc. 1989, 111, 7155. (b) Bain, C. D.; Whitesides, G. M. J. Am. Chem. Soc. 1989, 111, 7164. (23) Li, L. Y.; Chen, S. F.; Jiang, S. Molecular-Scale Mixed Alkanethiol Monolayers of Different Terminal Groups on Au(111) by Low-Current Scanning Tunneling Microscopy. Langmuir, accepted. (24) Chen, S. F.; Li, L. Y.; Boozer, C. L.; Jiang, S. Langmuir 2000, 16, 9287.
1. Introduction Understanding molecular-scale adhesion, friction, lubrication, and wear is crucial to modern technologies, such as micro- or nanoelectromechanical systems (MEMS/ NEMS) and hard disk drives.1,2 Self-assembled monolayers (SAMs) are one of the strategies used for minimizing stiction and reducing adhesion and friction in MEMS/ NEMS.3,4 The scanning force microscope5,6 is an ideal instrument to study nanoscale forces. The advantage of scanning force microscopy (SFM) is that it allows accurate measurements of forces applied in both horizontal and normal directions to a surface. In the past decade, SFM has been applied to study frictional properties of various materials ranging from Langmuir-Blodgett films to SAMs.7-13 These studies have established the application of SFM to nanotribology. The most commonly studied system of SAMs is an alkanethiol on a gold substrate. Several studies have shown the correlation of the frictional properties of self-assembled alkanethiols with their chain lengths10-12 and terminal groups.13 Long-chain monolayers
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Recently, Beake et al. studied the friction and adhesion of mixed SAMs of dodecanethiol (C12)/mercaptoundecanoic acid (C10COOH) and C12/octadecanethiol (C18) to model chemical and mechanical heterogeneities, respectively.25 Measurements of nanoscale friction and adhesion based on calculated force constants are unlikely to yield reproducible results with different tips since these calculations require the dimensions and relevant elastic moduli of the cantilever, which are not easy to measure. Frictional forces measured by different groups may differ by as much as 1 order of magnitude for the same system under the same load.10,11 Thus, comparative studies are often carried out using the same tip. However, a fundamental understanding of nanoscale friction requires a quantitative analysis. Recently, we combined the two-slope and the added-mass methods for lateral and normal force calibrations.12 The combined method was applied to measure frictional force versus load curves for several alkanethiols on Au(111). Results show that the friction coefficients for the same alkanethiol system, but with different tips, differ by less than 15%, indicating the reliability of the combined method. In this work, mixed SAMs of dodecanethiol (C12) and 11-mercapto-1-undecanol (C11OH) on Au(111) are chosen so as to vary the surface properties from hydrophobic to hydrophilic by adjusting the surface composition of mixed SAMs.26,27 We applied the combined lateral and normal force calibration method to investigate the effects of surface properties and scanning environments (in air with controlled humidity or in water) on frictional properties. SFM measurements were performed on an atomically flat terrace to ensure that atomic-scale friction was measured. In several previous reports,17,25 friction loops (traceretrace cycles) were obtained in micrometer scale, within which there existed several atomic gold steps. 2. Experimental Section Materials. Dodecanethiol and 11-mercapto-1-undecanol were purchased from Aldrich Chemical Co. (Milwaukee, WI) and used as received. Gold shots (99.99%) were purchased from D. F. Goldsmith. Glass beads (10-30 µm) from Polyscience were used for the normal force calibration. The faceted SrTiO3(305) surface annealed for 20 h at 1100 °C in flowing oxygen as discussed in the literature28 was used for the lateral force calibration. Glassware for the preparation of SAMs was cleaned with chromic acid cleaning solution (Fisher Scientific). Monolayer Preparation. A freshly cleaved mica sample (Asheville-Schoonmaher Mica Co.) was placed into the chamber of the BOC Edwards AUTO306 thermal evaporator and preheated at 325 °C for 2 h before gold deposition. The typical evaporation rate was 0.1-0.3 nm/s, and the thickness of gold films ranged from 150 to 200 nm. The gold-coated substrates were then annealed in the chamber at 325 °C for half an hour. Before immersion into alkanethiol-containing ethanol solutions, the gold-coated substrates were further annealed in a H2 flame. The Au(111) surface prepared following the above method has low contamination and flat Au(111) terraces as large as 300 nm × 300 nm according to our atomic force microscopy (AFM) measurements. SAMs were formed by immersing the gold-coated substrate in the preheated ethanol solution of alkanethiol (1 mM) overnight.23,24 Prior to imaging, all of the SAM samples were rinsed extensively with ethanol and dried under a stream of N2. (25) Beake, B. D.; Leggett, G. J. Langmuir 2000, 16, 735. (26) (a) Bain, C. D.; Biebuyck, H. A.; Whitesides, G. M. Langmuir 1989, 5, 723. (b) Bain, C. D.; Whitesides, G. M. Angew. Chem., Int. Ed. Engl. 1989, 28, 506. (27) Laibinis, P. E.; Whitesides, G. M. J. Am. Chem. Soc. 1992, 114, 1990. (28) Sheiko, S. S.; Mo¨ller, M.; Reuvekamp, E. M. C. M.; Zandbergen, H. W. Phys. Rev. B 1993, 48, 5675.
Langmuir, Vol. 19, No. 3, 2003 667 SFM Measurements. Experiments were performed with a MultiMode NanoScope III AFM having a commercial Si3N4 cantilever of a nominal spring constant of 0.58 N/m (Digital Instruments, Santa Barbara, CA). The instrument was allowed to equilibrate thermally in the home-built chamber for ∼1 h after mounting a sample. The microscope was operated in a constant force mode, where the tip was scanned back and forth at 90° along the same horizontal line (slow scan direction disabled) while a feedback loop holds the externally applied load constant. Hence, the normal force signal and the lateral force signal can be measured simultaneously. The normal force between the tip and the sample, FN, was estimated from the force versus z-displacement curves, with FN ) 0 designated as the point at which the tip breaks contact with the surface. Thus, FN is the summation of adhesion and external load. Therefore, when FN is decreased to zero, frictional force should reach zero. The frictional force, FL, was measured from the plots of friction signal versus lateral displacement (i.e., friction loops) while the scanning speed was fixed at 1.2 µm/s. A friction versus load curve was obtained by varying FN. The combined added-mass and two-slope method was described previously.12 In short, the normal spring constant of a microfabricated cantilever was determined by measuring the resonant frequencies when different small spheres (e.g., glass beads) were added at the end of the cantilever.29 The tip was used as a micromanipulator to pick up a sphere. Several different sized spheres were used. The results showed very good linear relationship between the added mass and (2πν)-2 as expected. The lateral force was calibrated using the SrTiO3(305) sample proposed by Ogletree et al.30 The calibration was made by sliding the tip across the surface of the known slopes (i.e., facets (101) and (103) of the SrTiO3(305) crystal) and measuring the lateral force signals as a function of the normal force signal. With this in situ calibration method, the lateral force signal in voltage from the photodiode can be converted to lateral force directly without knowing the lateral optical deflection sensitivity. The piezo scanner sensitivity for the x-y directions was calibrated from the lattice images of mica and Au(111), while the z piezo calibration was done using the faceted SrTiO3(305) surface30 and a monatomic step height on the Au(111) substrate. The atomic force microscope on a vibration base was enclosed in the home-built environment-controlled chamber and sealed with a silicon rubber base. Relative humidity was controlled by injecting either dry nitrogen or water-saturated nitrogen into the chamber and varied from 80% to 10%. The temperature inside the chamber was about 28 °C. The gas inlet was sealed during measurements. For friction measurements in solution, a fluid cell for contact-mode AFM was used. Deionized water was transferred into the fluid cell by syringe. After each experiment, the fluid cell was rinsed thoroughly with ethanol and copious amounts of deionized water and blown dry with nitrogen. X-ray Photoelectron Spectroscopy (XPS). XPS spectra were obtained on a Surface Science Instruments (SSI) S-Probe ESCA using a 1000 µm spot size. An aluminum KR 1,2 monochromatized X-ray source (1486.6 eV) was used to stimulate photoemission. The energy of the emitted electrons was measured with a hemispherical energy analyzer at the pass energy of 150 eV. Spectra were collected with the analyzer at 55° with respect to the surface normal of the sample. Typical pressures in the analysis chamber during spectral acquisition were 10-9 Torr. SSI data analysis software was used to calculate elemental compositions from the peak areas.
3. Results and Discussion Characterization of C12/C11OH. Two types of pure SAMs (C12 and C11OH) and their 1:3, 1:1, and 3:1 mixtures on Au(111) were studied in this work. C12 (hydrophobic) and C11OH (hydrophilic) are similar in chain length but different in terminal group. Topographic images and simultaneously captured frictional images (29) Cleveland, J. P.; Manne, S. Bocek, D.; Hansma, P. K. Rev. Sci. Instrum. 1993, 64, 403. (30) Ogletree, D. F.; Carpick, R. W.; Salmeron, M. Rev. Sci. Instrum. 1996, 67, 3298.
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Figure 1. SFM frictional image (50 nm × 50 nm) of pure C12 SAMs on Au(111) and its corresponding FFT spectra.
were acquired carefully from 20 × 20 nm2 to 5 × 5 µm2 scales. All of these results clearly show the uniform structure of the two-component monolayers. Figure 1 shows the SFM frictional image of C12 SAMs on Au(111) and its corresponding fast Fourier transform (FFT) spectra. Panels a and b of Figure 2 show the SFM frictional images of mixed C12/C11OH (1:1) and C12/C11OH (3:1) SAMs coadsorbed onto Au(111) from solution, respectively. The FFT spectra of these images clearly show a periodicity of 0.5 nm, which is the same as that of pure alkanethiols. This indicates that all the systems studied here have compact packing structures. Previously, we systematically studied the phase behavior of mixed SAMs of alkanethiols on Au(111).24,31 We carried out a configurational-bias Monte Carlo simulation study of the preferential adsorption and phase segregation of alkanethiol mixed SAMs on Au(111) and found that phase segregation occurs when two components in mixed SAMs have a difference in chain length of more than three carbon atoms,31 which is consistent with our experiment results.24 Recently, we reported two new methods to prepare molecular-scale uniform mixed SAMs on Au(111). One method is the coadsorption of thiols on Au(111) at relatively high solution temperatures.24 The uniform mixed SAMs formed at higher temperature are kinetically trapped. The other method is the coadsorption of mixed asymmetric and symmetric disulfides.32 For coadsorption involving asymmetric disulfides, when the preferential adsorption of long-chain pairs occurs, each long-chain moiety will bring in a short chain due to the structure of the asymmetric disulfide. The short chains around a longchain block further aggregation of long chains, thus reducing the possibility of phase segregation. Both methods are convenient and useful to prepare surfaces with controlled chemical and structural properties at the molecular level. We applied the kinetically trapped method to the preparation of several mixed SAMs containing various terminal groups, such as acetic acid, hydroxyl, and amine.23 Results show that uniform mixed SAMs can (31) Shevade, A. V.; Zhou, J.; Zin, M. T.; Jiang, S. Langmuir 2001, 17, 7566. (32) Chen, S. F.; Li, L. Y.; Boozer, C. L.; Jiang, S. J. Phys. Chem. B 2001, 105, 2975.
be prepared at higher temperatures for various terminal groups, chain lengths, and compositions. Moreover, SAMs formed at higher temperatures contain larger domains and fewer defects than those formed at lower temperatures.33,34 Homogeneous mixed C12/C11OH SAMs on Au(111) were prepared at higher temperatures in this work. These SAMs have large domains and compact structures as other uniform mixed SAMs studied previously.23,24 The surface composition of C11OH was determined from XPS by comparing the intensity of the O(1s) signal in mixed SAMs to that of SAMs derived from pure C11OH. The mole fraction of C11OH on the surface versus in ethanol solution is shown in Figure 3. Adsorption of C12 is favored over that of C11OH, particularly at low mole fraction. This result is similar to that reported by Bain et al.26 Frictional Properties of C12/C11OH. We studied nanoscale frictional properties of mixed SAMs of C12/ C11OH. We zoomed into a 20 × 20 nm2 atomically flat area from a 500 × 500 nm2 larger scan every time in order to measure frictional properties. This ensures that what we measure is truly nanoscale frictional properties of homogeneous mixed C12/C11OH SAMs on an atomically flat surface. Frictional force versus load curves were measured using SFM with calibrated tips under a controlled environment. Figure 4 compares the frictional properties of C12, C11OH, and their mixtures at a relative humidity (RH) of 10% (a), 20% (b), 31% (c), and 50% (d) and in deionized water (e). Frictional forces were measured for total loads up to 50 nN. Higher loads than 50 nN will cause damage on the monolayers as observed previously.35 From Figure 4, one can see that the friction coefficient of C11OH SAMs is much higher than that of C12 SAMs at a RH smaller than 31% while friction coefficients are similar for both C11OH and C12 SAMs at a RH greater than 50% and in water. Figure 5 presents the friction coefficients for C11OH, C12/ C11OH (1:1), and C12 under different relative humidities. (33) Yamada, R.; Wano, H.; Uosaki, K. Langmuir 2000, 16, 5523. (34) Li, L. Y.; Chen, S. F.; Jiang, S. Protein Adsorption on Alkanethiolate Self-Assembled Monolayers: Nanoscale Surface Structural and Chemical Effects. Langmuir, submitted. (35) Liu, G.-Y.; Salmeron, M. B. Langmuir 1994, 10, 367.
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Figure 3. Mole fraction of C11OH on the surface versus in ethanol solution for mixed C12/C11OH SAMs. Surface compositions were determined from XPS by scaling the O(1s) signal of mixed SAMs to that of pure C11OH SAMs. The error bars correspond to the standard deviations. The diagonal straight line represents the ideal case where no preferential adsorption occurs.
Figure 2. SFM frictional image (20 nm × 20 nm) of mixed C12/C11OH SAMs on Au(111) when coadsorbed from (a) 1:1 and (b) 3:1 solutions.
For the hydrophilic monolayers, the friction coefficient decreases quickly with the increase of relative humidity and gradually approaches a constant. A similar trend for the effect of humidity on the friction coefficient was observed in our previous SFM measurements20 and molecular dynamics (MD) simulations36 for the system of alkyl/Si(111) and in SFM measurements of other systems, such as (hydrophilic) mica.21,37 Our previous simulation results show that with the increase of water molecules between two hydrophilic surfaces, water molecules start to show a layering behavior.36 The layering behavior of water molecules contributes to the decrease of the friction coefficient. On hydrophilic surfaces, when humidity increases, more and more water molecules will be con(36) Zhang, L. Z.; Jiang, S. Y. J. Chem. Phys. 2002, 117, 1804. (37) Hu, J.; Xiao, X. D.; Ogletree, D. F.; Salmeron, M. Surf. Sci. 1995, 327, 358.
densed and thick water layers will be formed. The multiple layers of water molecules on hydrophilic surfaces function as a lubricant, thus reducing friction.38,39 As shown in Figure 5, for the hydrophobic monolayers, the friction coefficient remains the same as relative humidity increases. Tian et al.21 observed that friction varied with humidity for an octadecyltriethoxysilane (OTE) monolayer. They argued that the alkylsilane monolayer was not fully compact due to long-range disordering, and condensed water could penetrate through the film to the OTE/mica interface, leading to the variation of the friction coefficient with humidity on hydrophobic surfaces. Frictional properties depend on such factors as chain length and the size and the chemical nature of a terminal group. Since C12 and C11OH have similar chain lengths and sizes of the terminal group, the chemical nature of C12/C11OH SAMs is mainly responsible for the change in friction coefficient under different environments. It is known that hydrophobic surfaces have low surface energy while hydrophilic surfaces have high surface energy in dry air or in a vacuum. However, the surface energy of hydrophilic surfaces will be lowered due to adsorbed water layers as humidity increases. At higher relative humidity (e.g., RH ) 50%), low friction is observed for OHterminated SAMs due to adsorbed water films on the surface and for CH3-terminated SAMs due to the hydrophobic nature of the surface, leading to the similar friction coefficients for all mixed SAMs. Friction coefficients are plotted as a function of surface composition of C11OH at different RHs in Figure 6. Such plots can serve as a reference for probing local surface composition at a nanoscale domain from its friction coefficient measured by SFM. A number of techniques can establish the average composition of mixed SAMs on relatively larger areas. For example, XPS can ascertain surface composition over an area of the size of the X-ray spot. Spatial resolution is approximately in the range of (38) Freund, J.; Halbritter, J.; Horber, J. K. H. Microsc. Res. Technol. 1999, 44, 327. (39) Scherge, M.; Li, X.; Schaefer, J. A. Tribol. Lett. 1999, 6, 216.
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Figure 4. Frictional force vs load curves for C11OH (9), 1:3 C12/C11OH (0), 1:1 C12/C11OH (2), 3:1 C12/C11OH (4), and C12 (b) on Au(111) at a RH of (a) 10%, (b) 20%, (c) 31%, and (d) 50% and (e) in deionized water.
8-150 µm, depending upon the instrument.40 While SFM can be used for the direct visualization of phase-separated regions of nanoscale sizes in an organic monolayer, none of analytical techniques are available to determine surface composition in a nanoscale domain. Since frictional (40) Ratner, B. D.; Castner, D. G. In Surface Analysis - The Principal Techniques; Vickerman, J. C., Ed.; John Wiley & Sons Ltd.: Chichester, U.K., 1997.
properties depend on compositions, friction coefficients as a function of surface compositions can be predetermined over the entire composition range. Such a plot will serve as a reference in determining surface composition in a nanoscale domain by measuring its friction coefficient. To prepare such a reference plot, homogeneous mixed SAMs and quantitative friction measurements are essential. Thus, SFM is good not only for qualitatively
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Figure 5. Friction coefficients for C11OH (9), 1:1 C12/C11OH (2), and C12 (b) on Au(111) under different relative humidities by SFM. The error bars correspond to the standard deviations from six different measurements.
Figure 6. Friction coefficient vs surface composition of C11OH in air at a RH of 10% (9), 20% (0), 31% (2), and 50% (4) and in water (b). The error bars correspond to the standard deviations from six different measurements.
visualizing different phase regions but also for quantitatively determining local surface composition in a nanoscale domain.
of mixed SAMs are all similar, while at lower RH (e.g., 20% and 31%), the friction coefficient increases as the surface composition of C11OH in the mixed SAMs increases. Since there is a clear relationship between friction coefficient and surface composition at low RH, surface composition in a nanoscale domain could be determined by measuring its friction coefficient with SFM under a controlled environment. Quantitative measurements of frictional properties make this possible.
4. Conclusions We performed a quantitative study of nanoscale frictional properties of uniform mixed SAMs terminated with hydrophobic CH3 and hydrophilic OH groups using SFM. Quantitative measurements of nanoscale frictional properties were achieved by the combined normal and lateral force calibration method proposed in our previous work. For mixed C12/C11OH SAMs with both components having similar chain lengths and sizes of the terminal group, frictional properties are mainly determined by the chemical nature of the terminal group. The effects of the surface composition of mixed SAMs and the scanning environment on frictional properties were investigated. At higher RH (e.g., 50%) or in water, friction coefficients
Acknowledgment. This work has been supported by the National Science Foundation (CTS-9815436, CTS9983895, and CTS-0092699). XPS experiments were performed at the National ESCA and Surface Analysis Center for Biomedical Problems (NESAC/Bio), funded by National Institutes of Health Grant RR-01296 from the National Center for Research Resources. LA026575M
