Environ. Sci. Technol. 2003, 37, 2813-2819
Investigation of Surface Changes of Nanoparticles Using TM-AFM Phase Imaging RONG DONG AND LIYA E. YU* Department of Chemical and Environmental Engineering, National University of Singapore, 4 Engineering Drive 4, Singapore 117576, Singapore
Tapping-mode AFM (TM-AFM) phase imaging was utilized to characterize the surface changes of nanosize particles, in regard to the effects of different amounts of condensed water and organic coatings on particle surfaces. Model nanoparticles were continuously examined under various relative humidity (RH) levels by concurrently obtaining both topographic and phase images. The condensed water appeared to soften particle surfaces and to increase tipsample attractive interaction over relatively stiff surfaces, which were shown with dark phase contrasts and negative phase shift values in phase images. Under high RH, a massive amount of water gave the particles a droplet-like surface, which reversed the original negative phase shifts to positive values with bright contrasts. Glutaricacid coatings provided a compliant surface with high viscosity resulting in a dark phase contrast, whereas water droplets containing relatively low viscosity gave a bright phase contrast and positive phase shift. Overall, our results show that it is essential to describe the physical properties of a sample surface as solid, soft, or droplet-like material in order to derive a meaningful understanding of the surface changes of nanosize particles based on TM-AFM phase images. In contrast to other phase imaging studies, this work clearly correlates continuous surface changes with phase images, demonstrating a promising approach to characterize environmental nanoparticles.
Introduction Nanoparticles play an important role in the atmosphere because they can serve as seed aerosols leading to subsequent physical and chemical transformations, which may have adverse effects on the atmospheric environment and on human health. Tapping mode atomic force microscope (TMAFM) has been adopted for investigating the surface properties of atmospheric aerosols. While various studies have discussed particle morphology and surface reactions mainly based on TM-AFM height images (1-4), little information on phase images has been provided. TM-AFM phase images are generated based on the phase lag between the oscillation frequency of cantilever and driver. Since the phase lag reflects the interactions between the scanning tip and the sample surface, phase images contain the advantage of demonstrating the surface properties of particle samples. However, more phase images of nanoparticles are needed to serve as a reference for direct observations and for further understanding of the complicated correlations between tip-sample interactions and resultant phase images. * Corresponding author phone: +65 6874 6474; fax: +65 6779 1936; e-mail:
[email protected]. 10.1021/es034071k CCC: $25.00 Published on Web 05/17/2003
2003 American Chemical Society
The interactions between a TM-AFM scanning tip and sample surface mainly consist of attractive and repulsive forces, which result in different phase shifts or phase lags. To investigate the force effects on the phase shift, various research groups have described the oscillation behaviors of the AFM scanning tip by measuring dynamic force curves and developing mathematical models (5-11). Repulsive forces occurring between the scanning tip and relatively stiff surfaces tended to enhance the phase shift toward the positive direction, while attractive forces resulted in a negative phase shift (5, 12, 13). Bar et al. (14) further pointed out that the force experienced by the AFM cantilever (or scanning tip) at the lower turning point of oscillation (LPTO) played the most influential role in the resultant phase images. While chemical characteristics such as the hydrophilicity/hydrophobicity feature of a sample surface also appeared to affect the phase shift (15, 16), most studies focused on the effects of the physical properties of sample surfaces on the phase shift. Young’s modulus was employed primarily to indicate the physical property or stiffness of a sample surface. It was consistently found that samples with a higher elastic modulus (stiffer surface) would result in larger (brighter) positive phase shift. In addition, Magonov et al. (7) found that phase images not only distinguished multiple components but also showed the density distribution of a single component on sample surfaces. Although the phase-image-associated mathematical models were developed to simulate the tip-sample interactions on hard surfaces, they remained insufficient to describe those on compliant sample surfaces. Unlike those of stiff samples, the characteristics of compliant samples seemed to be reflected differently in phase images. Tamayo and Garcı´a (15, 17) simulated the behavior of an AFM cantilever and demonstrated that phase contrast appeared to be independent of sample elasticity without considering viscous interaction during tip-sample contacts. Similar findings were also reported by Bar et al. (18), who studied compliant sample surfaces. Most TM-AFM studies discussed the phase images of compliant surfaces by examining polymer samples, while polymers containing very different surface characteristics led to diverse findings. For example, the Young’s modulus of various polymers covered a wide range of elasticity, spanning from 2 MPa to 9 GPa. In addition, Spatz et al. (19) found that a sample surface made of the same polymer material exhibited different softness and phase images. Furthermore, the computational results of Dubourg and Aime´ (20) showed that the adhesion force between the scanning tip and simulated soft materials significantly affected the resultant phase shifts. These are concrete indications of the complications of measuring compliant samples using AFM. The surface properties of droplet-like samples differ from those of soft polymers; two studies (15, 21) showed that the phase shift over liquidlike samples were affected primarily by viscoelastic and wetting behavior of samples as well as the tip-sample adhesion forces. It is necessary to note that most of the observations were based on model simulation, while only a few groups conducted phase imaging on droplets of CaCl2 and P2O5 on mica (22) and of glycerin on pyrolitic graphite (15, 23). Hence, obtaining meaningful data on phase images of droplet-like samples appears particularly challenging, mainly for the following reasons: (1) limited knowledge of tip-sample interactions has been provided; (2) few phase images of nanosize droplets are available; and (3) little technology has been developed to characterize the surface properties of nanosize particles. VOL. 37, NO. 12, 2003 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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In this study, TM-AFM phase images of nanosize aerosols are presented to analyze the effects of condensed water and organic coatings on particle surface. To allow a direct comparison of phase images obtained under different environments, all the samples were scanned based on similar TM-AFM operation parameters. The condensed water on particles appeared to soften sample surfaces, increase tipsample attractive force, and make particles droplet-like. An additional attempt was made to show that TM-AFM phase images reflected particles coated with organic materials, comprising different polarity and hygroscopicity.
TABLE 1. Size Range of Individual Nanoparticle Samples core particles
azelaic-acid-coated particles
glutaric-acid-coated particles
30-50 nm
30-55 nm
80-125 nm
Experimental Section Particle Sample Preparation. The model aerosols were produced from an aerosol generation system similar to the Sinclair-Lamer generator (24). The generation system consists of two major units, a nuclei-generation and an organicvaporization unit. Solid (NH4)2SO4 (0.5 g, Merck, Germany) was held in a 500-mL flask and vaporized by a heating mantle with a gas-phase temperature of about 150-160 °C. A nitrogen stream at a flow rate of 4 L per minute was introduced into the nuclei-generation unit to carry the condensed (NH4)2SO4 nuclei to another 500-mL flask in the organic-vaporization unit. A silicon oil bath was used to increase the temperature of the second flask to generate the desired organic vapor. The organic compounds held in the second flask were approximately 60 mg of glutaric acid (C5 R,ωdicarboxylic acid, Sigma-Aldrich, U.S.A.) or 50 mg of azelaic acid (C9 R,ω-dicarboxylic acid, Sigma-Aldrich, U.S.A.). The mixtures containing the (NH4)2SO4 nuclei and gaseous organic species entered a long double-walled air condenser, so that organic vapor can spontaneously condense on inorganic nuclei forming desired organic-coated aerosols. The model aerosols produced were collected for 10 min on a freshly cleaved mica substrate attached at the exit point of the air condenser. The chemical composition of our inorganic nuclei was verified to be ammonium sulfate ((NH4)2SO4), based on the results of bulk analysis via ion chromatography (IC, Metrohm, Herisau, Switzerland) and the measurement of an X-ray photoelectron spectroscopy (XPS). Generated core particles were collected on a Teflon-coated glass fiber filter (Gelman Sciences, Michigan, U.S.A.) for 16 h, followed by ultrasonication in 10 mL of ultrapure DI water before the IC analysis. The anions were separated by a Metrosep Anion Dual 2 column (4.6 × 75 mm) using an eluent consisting of 3.2 mM of Na2CO3 and 1.0 mM of NaHCO3 at a flow rate of 0.8 mL/ min. The cations were carried through a Metrosep Cation 1-2 column (4.6 × 75 mm) with an eluent comprising 4 mM of tartaric acid and 1 mM of dipicolinic acid at a flow rate of 0.8 mL/min. To confirm the chemical composition shown by the bulk analysis, the generated inorganic nuclei were collected on mica for 15 h, followed by an XPS scan using an AXIS HSi spectrometer (Kratos Analytical Ltd., UK) coupled with a monochromatized Al KR X-ray source (1486.6 eV photons) at a constant dwell time of 100 ms and a pass energy of 40 eV. Pure solid ammonium sulfate and pure ammonium bisulfate standards were scanned under the same conditions as described above to serve as references. Consistent with the results obtained from the bulk IC analysis, the XPS measurements demonstrated that the N/S atomic concentration ratio of our generated core particles was around 2, ascertaining that our inorganic nuclei were (NH4)2SO4. A JSM-5600 scanning electronic microscope (SEM, JEOL Ltd., Japan) coupled with an energy-dispersed X-ray (EDX) detector was used to verify the presence of glutaric-acid coatings based on the elemental composition over core particles. This demonstrated that our generation system was reliable when producing organic-coated particles. 2814
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FIGURE 1. RH history of (NH4)2SO4 core particles, azelaic-acidcoated particles, and glutaric-acid-coated particles. The designated numbers correspond to the figures discussed in this paper. TM-AFM Measurements. A NanoScope IIIa Multimode AFM (Digital Instruments, Santa Barbara, CA) in a tapping mode was employed to image the nanoparticle samples. A silicon probe and a 125-µm long cantilever comprising a lever force constant of 40 N/m and resonant frequency of about 300 kHz were utilized. All the AFM images were obtained with a scan rate of 0.5 or 1 Hz over a selected area in the dimension of 2 µm × 2 µm or 1 µm × 1 µm. Table 1 shows the size ranges of individual particle samples, which were measured based on AFM topographic images. To expose individual samples to an environment with elevated relative humidity (RH), a nitrogen stream at a flow rate of 2-4 L per minute was introduced through an impinger containing deionized water to bring moisture to an acrylic study chamber, which enclosed the AFM system and studied samples. To provide a dry environment, a nitrogen stream at a higher flow rate of 4-6 L per minute was directly sent to the AFM chamber. The RH level in the detection chamber was measured by an electronic hygrometer (Extech Instruments, China) with an accuracy of ( 1% RH. Figure 1 shows the RH trend of our study chamber as a function of time for imaging individual samples.
Results and Discussion Phase Images of (NH4)2SO4 Aerosol. The phase images in this study provide information on changes in particle surfaces, complementary to the topographic images for the examination of water condensation and evaporation processes of (NH4)2SO4 aerosols. Figure 2 shows the three-dimensional height images and phase images of (NH4)2SO4 particles under various RH levels. The profiles adjacent to individual phase images show the phase shift corresponding to the line across the phase images. The phase images shown in Figure 2 demonstrated how different amounts of water condensed on the (NH4)2SO4 aerosols changed the surface property. At 59% RH, the phase image in Figure 2a presents that the (NH4)2SO4 particles comprised dark frame, corresponding to the dips in the phase-shift profile. The phase image in Figure 2b demonstrates expanded dark frames along with the enlarged negative phase shifts when RH increased to 93%, indicative of increased water condensation. The corresponding 3-D topographic image in Figure 2b also shows that water condensation at 93% RH mainly concentrated around, rather than fully covered the particles. This confirms that the expanded dark frames around the particles given in the phase
FIGURE 2. TM-AFM 3D-height images, phase images, and phase shifts of (NH4)2SO4 particles under (a) 59% RH, (b) 93% RH, and (c) 94% RH with a scan size of 1 µm × 1 µm and a scan rate of 1 Hz. image of Figure 2b were attributed to increased water condensation. The bright and dark contrast in the phase image of Figure 2a,b are consistent with the study conducted by Haugstad and co-workers (25, 26), who operated a TM-AFM at below resonance frequency to scan ultrathin films made of poly(vinyl alcohol) (PVA) on a mica substrate. When the tip penetrated a less viscous film and touched the core, they found that the AFM tip worked in the repulsive regime with a bright phase contrast, whereas a dark contrast indicated that the tip worked in the attractive regime. Hence, the bright phase contrast shown in Figure 2a,b suggests that the TMAFM tip penetrated through a relatively thin water film if there was any and tapped on the solid (NH4)2SO4 particles. The dark phase contrast surrounding the particle edges indicate the higher viscous/attractive force over the sample surfaces due to aggregated condensed water. Similarly, Schmitz et al. (1) also observed that particles deposited on a polyester foil contained distinguishable dark frames under ambient RH, and the dark frames were enhanced under a wet environment with 90% RH. The observations of dark frames around particles agree with the understanding of AFM scanning over a relatively stiff sample surface. In other words, the stronger attractive interactions imposed on a scanning tip caused a negative phase shift and reflected as a dark phase contrast, whereas the repulsive interactions between a tip and relatively stiff sample surface would cause a positive phase shift (bright phase contrast). Hence, the dark region surrounding the individual particles shown in the phase images of Figure 2a,b demonstrated the increased attractive force experienced by the scanning tip due to condensed water. While the condensed water increased the attractive force on samples surfaces, Colchero et al. (27) and Luna et al. (21) cautioned that a liquid neck formed between a scanning tip and condensed water on sample surfaces could also cause the dark frame in AFM phase images. Colchero et al. (27), who scanned highly oriented pyrolytic graphites (HOPG), considered that water vapor at room temperature could
spontaneously form a liquid neck bridging the tip and sample surface. Luna et al. (21) reported that the presence of a viscous liquid neck enforced an additional attractive force to pull the tip to the sample surface, causing a negative phase shift and dark phase contrast. The impact of the liquid neck became more apparent with increasing RH levels, which could even surpass the effects of the elastic or viscoelastic properties of sample surface on phase images (21). However, such liquid-neck formation should not significantly affect the phase images shown in Figure 2, because our AFM measurements were operated with a cantilever comprising a higher force constant, which should minimize the formation of the liquid neck (21). Therefore, the dark (negative) phase contrast (shift) demonstrated in Figure 2b was mainly due to the increased amount of water surrounding the particles. Upon exposure to high RH for a prolonged time, the dark frame of the (NH4)2SO4 particles with negative shifts in Figure 2b became slightly positive along with an increased phase shift over the particles (Figure 2c). The substantial increase in the phase shift was apparently due to more water that condensed on the particle surfaces during prolonged exposure under high RH. The topographic image in Figure 2c affirms that the increased water condensation, which appeared to fully cover the particles, reversed the negative phase shift of the dark frames to the positive value with corresponding bright phase contrast. Figure 3 shows the trend of the averaged phase shift of the dips displayed in the phaseshift profiles in Figure 2 to represent the changes in the dark frame around the particles. The enhanced errors recorded around 35 and 90 min mainly reflected the effects resulting from substantial changes of RH levels in the study chamber, which often produced AFM images with more noise and less resolution. The initial drop in the phase-shift trend echoes the expanding dark frame shown in Figure 2b. On the other hand, the increasing phase shift starting around 80 min eventually appeared with positive values as shown in Figure 2c. The increasing trend in the phase shift corresponded to the conversion of dark to bright phase contrast due to the VOL. 37, NO. 12, 2003 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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contact area. This resulted in the whitish phase contrast observed in Figure 2c.
FIGURE 3. Averaged phase-shift of particle frames. substantial water condensation over the particle surfaces. This is the first report of such continuous observation of phase contrast reversal in proportion to the amount of water condensation. Herminghaus et al. (22), scanning water droplets, obtained a similar bright phase contrast. Magonov et al. (7) also reported the phase-contrast reversal over a soft sample surface of polyethylene (PE) on a silicon substrate and further concluded that a larger contact area between the tip and a softer surface was converted to a higher phase shift with brighter phase contrast. Thus it is not surprising that the liquidlike sample surfaces containing a substantial amount of condensed water provided a softer surface and larger
In addition to the effect of the tip-sample contact area, the phase images in Figure 2 further suggest that it is essential to differentiate whether a sample contains a stiff, viscoelastic, or droplet-like surface for proper AFM characterization. AFM measurements studying stiff sample surfaces have similarly demonstrated that the increased repulsive interaction between the tip and a sample surface containing a higher elastic modulus would give a larger phase shift with brighter phase contrast (7, 28). In contrast to the correlation between the stiff sample surfaces and phase shift, we learned from the results of Tamayo and Garcı´a (15) that for a fluidlike sample surface such as glycerin, the lower the viscosity, the more positive (brighter) the phase shift. In addition, the viscosity of viscoelastic materials seemed to compromise the influence of the elastic modulus on the overall phase shift. The viscosity of droplet-like samples appeared to dictate even the reflected phase shift (15). Since the substantial amount of condensed water altered the sample surfaces from being relatively stiff to droplet-like (Figure 2c), the fluidlike sample surface resulted in low viscosity because of a massive amount of water. The substantially enhanced phase shift and bright contrast due to a larger amount of condensed water can be noted as one of the characteristics of samples containing a very soft (or fluidlike) surface with low viscosity. Figures 2 and 3 also illustrate that the physical properties of particles changing from solid- to liquid-like surface triggered the phase reversal. This demonstrates that TM-AFM phase images can
FIGURE 4. TM-AFM height images, phase images, and phase shifts of azelaic-acid-coated (NH4)2SO4 particles under (a) 55% RH and (b) 92% RH with a scan size of 1 µm × 1 µm and a scan rate of 0.5 Hz. 2816
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FIGURE 5. TM-AFM height images, phase images, and phase shifts of glutaric-acid-coated (NH4)2SO4 particles under (a) 51% RH and (b) 93% RH with a scan size of 2 µm × 2 µm and a scan rate of 1.0 Hz. uniquely reflect the transition of the physical conditions over nanoparticle surfaces. It is worthwhile to note that well-characterized standard samples are needed to account for the effects of individual AFM scanning systems on generated phase images since the sensitivity of AFM operation systems would determine the extent of surface feature reflected in phase images (13). By manipulating operation parameters, phase reversal could be observed on the same AFM scanning position. Three research groups (29-31) scanned a variety of polymer samples consisting of soft surfaces on a silicon substrate and demonstrated phase contrast reversal by changing the system setpoint or free oscillation amplitude. James et al. (32) also utilized different parameters such as a setpoint to produce a wide range of valid phase images. Since similar operation parameters of the AFM system were utilized throughout our experiments, the variations in the phase images given in Figure 2 were solely attributed to the changes in surface properties. Hence, the phase images illustrated in Figure 2 can serve as the reference for future investigations on surface materials of nanoparticles using a TM-AFM operation condition similar to this study. Phase Images of Azelaic-Acid-Coated and Glutaric-AcidCoated (NH4)2SO4 Particles. To investigate how TM-AFM phase images of organic-coated nanoparticles varied, TMAFM height images, phase images, and corresponding phaseshift profiles of the azelaic-acid-coated and glutaric-acidcoated (NH4)2SO4 particles were assessed (Figures 4 and 5). Figure 4 shows a dark frame around the azelaic-acid-coated (NH4)2SO4 particles at 55% RH, similar to the pure (NH4)2SO4
aerosols given in Figure 2a. The azelaic-acid-coated (NH4)2SO4 aerosols exhibited smaller phase shift than the pure (NH4)2SO4 particles (Figure 2a), suggesting that the condensed organic material could soften the particle surfaces and/or increase tip-sample attractive interaction. At 92% RH, the azelaic-acid-coated (NH4)2SO4 particles in Figure 4b significantly grew in size as a consequence of substantial water uptake by the azelaic-acid-coated aerosols. Interestingly, the cross-sectional profile over the particle surface in Figure 4b contained consistent phase-shift values, suggesting that the particle surface was homogeneously covered by condensed water. The significant amount of condensed water at 92% RH could make the surface of the azelaic-acid-coated particles (Figure 4b) similar to that of the liquid droplets, exhibiting a surge in phase shift. Such changes in the surface properties under high RH agree with those of the pure (NH4)2SO4 aerosols shown in Figure 2c. This affirms that water droplets were reflected with bright contrast in phase images under our AFM operating system. Unlike that of the azelaic-acid-coated particles, the surface of the glutaric-acid-coated (NH4)2SO4 particles (Figure 5a) gave negative phase shifts in room condition at 51% RH. The dark phase contrast of the glutaric-acid-coated particles indicates that AFM tip experienced a strong attractive force on the particle surfaces could be due to the hydrophilic property and/or liquid-like condition of the glutaric-acid coatings. Compared to azelaic acid, glutaric acid is more polar and hygroscopic. Thus, the glutaric-acid coatings appeared to soften the particle surface and contribute to an attractive-force-dominant surface, which was consequently VOL. 37, NO. 12, 2003 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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TABLE 2. Sample Surface Properties and TM-AFM Phase Images little amount of condensed water sample
sample surface
core particles
stiff
azelaic-acid-coated particles glutaric-acid-coated particles
stiff; little organic coating over core particles compliant/droplet-like; organic coating with high viscosity
sample surface
phase image
dark frame (Figure 2a); expanded dark frame (Figure 2b) dark frame (Figure 4a)
droplet-like surface
bright contrast (Figure 2c) bright contrast (Figure 4b) bright contrast (Figure 5b)
dark surface (Figure 5a)
revealed as dark phase contrast shown in Figure 5a. This is consistent with the increased attractive force attributed to condensed water around the relatively stiff surfaces of the pure (NH4)2SO4 particles in Figure 2b. On the other hand, since atmospheric materials typically condense on particles (such as soot) as liquid-like film, the glutaric-acid coatings might cover the (NH4)2SO4 particles as fluid with high viscosity. In this case, the dark phase contrast of the glutaricacid coatings supports the idea that a liquid-like surface containing high viscosity (Figure 5a) would produce a more forceful negative phase shift; in contrast, the droplet-like particles covered with condensed water, which provided a less viscous liquid surface, were reflected with bright phase contrast (Figure 2c). The bright dots at the center of the particles shown in Figure 5a could indicate little presence of the glutaric-acid coating. The thin coverage of glutaric acid at the particle center might allow the scanning tip to easily penetrate the organic layer and to touch the particle core. In addition, the phase shift corresponding to the bright center of the glutaricacid-coated particles appeared to be similar to that of the core material shown in Figure 2b. This implies that the bright contrast could be due to the contact between AFM tip and stiff core particles. Similar to our observation of Figure 5a, bright dots were also shown in phase images by Haugstad et al. (25, 26), who scanned a single-component surface made of poly(vinyl alcohol) (PVA). They learned that changes in driving frequency eliminated the dots with bright phase contrast, which suggested an uneven distribution of PVA over the surface. Although we could obtain phase images showing homogeneous coatings of glutaric acid over core particles, the adjusted operating parameters may not be optimal for concurrently producing reliable topographic images. Besides, to obtain comparable phase images and to examine the effects of RH on particles containing different surface materials without additional interference from varied AFM measurement conditions, employing similar AFM operation parameters for examining different samples was preferred. After a prolonged exposure under 93% RH, the dark contrast of the glutaric-acid-coated particles was reversed to bright contrast due to the substantial water uptake over the particles (Figure 5b). Again, this aligns with the phase contrast of significant water covering the pure (NH4)2SO4 particles (Figure 2c) and azelaic-acid-coated particles (Figure 4b). Therefore, we conclude that substantial water condensation on particles can provide a droplet-like surface, leading to a positive phase shift and bright phase image contrast. Based on the results presented in this study, Table 2 summarizes the application of TM-AFM phase images for particles comprising different surface nature. Because chemical compounds comprise different hygroscopic properties, instead of using specific RH levels, we classify the phasecontrast response of particles based on little- vs substantialamount of water on particle surface. With less amount of water, phase images tend to show the original nature of particle surface. For particles containing relatively stiff surfaces, such as the initial core particles, the stronger elastic 2818
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droplet-like surface droplet-like surface with relatively low viscosity
properties with larger Young’s modulus tend to result in positive phase shift and bright contrast. Whereas, for particles containing compliant and droplet-like surfaces, such as the glutaric-acid-coated particles, the phase shift mainly depends on viscosity, because the higher viscosity and/or larger attractive force of a sample surface contributes to more negative phase shift with dark contrast in phase images. On the other hand, particles covered with a substantial amount of water would be scanned by a TM-AFM tip as droplet-like samples and reflected with bright phase contrast in phase images. Taken together, we demonstrate that TM-AFM phase images can be utilized to evaluate the amount of condensed water and to characterize the surface changes over organiccoated particles.
Acknowledgments Financial support provided by the National University of Singapore (Grant No.: R-279-000-091-112) is gratefully acknowledged.
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Received for review January 27, 2003. Revised manuscript received April 15, 2003. Accepted April 22, 2003. ES034071K
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