NANO LETTERS
Tapping Mode Atomic Force Microscopy for Nanoparticle Sizing: Tip−Sample Interaction Effects
2002 Vol. 2, No. 9 945-950
Yuval Ebenstein, Eyal Nahum, and Uri Banin* Institute of Chemistry, the Farkas Center for Light Induced Processes and the Center for Nanoscience and Nanotechnology, The Hebrew UniVersity of Jerusalem, Jerusalem 91904, Israel Received June 27, 2002; Revised Manuscript Received July 24, 2002
ABSTRACT The use of tapping mode atomic force microscopy as a sizing tool for nanoparticles is investigated. Height of CdSe nanocrystals measured on different substrates is found to be significantly lower than the particle diameters as determined by transmission electron microscopy. Only minor height variations were found upon particle surface ligand exchange, but significant height variations were found for particles imaged on surfaces with different chemical natures. The distorted height data is attributed to shifts of the tip resonance frequency during imaging, induced by the attractive capillary forces near the substrate. Particle height variations observed when using different drive frequencies of the cantilever corroborate this assignment.
Nanostructures manifest size dependent properties with both basic and applied significance.1 An important example is provided by semiconductor nanocrystals that exhibit significant changes with size of the optical and electronic properties, due to the quantum confinement effect.2 Accurate sizing of such structures is crucial for the understanding and exploitation of these properties. Here we examine the use of tapping mode atomic force microscopy (TMAFM)3 for sizing of nanoparticles and investigate effects related to tip-sample interaction processes on the measured particle height. Sizing of nanoparticles with diameters below 10 nm is commonly done using transmission electron microscopy (TEM), with either medium or high resolution, providing direct information on the average size and the size distribution. These techniques can cause sample degradation due to the interaction with the electron beam. These techniques are also difficult to implement for nanostructures of materials with low atomic number, due to the low contrast presented in the TEM images. Using HRTEM, it is possible to identify the crystalline structure of the particles, but amorphous regions are not easily resolved.4 Additional techniques such as small-angle X-ray scattering5 and dynamic light scattering6 provide size information that is only indirect. The atomic force microscope (AFM)7 is a powerful tool for the characterization of nanostructures,8,9 and its ease of operation and widespread accessibility make it an interesting candidate for nanoparticle sizing purposes. AFM methods are expanding rapidly allowing for the measurement of electrical10 and * Corresponding author:
[email protected] 10.1021/nl025673p CCC: $22.00 Published on Web 08/06/2002
© 2002 American Chemical Society
optical11 properties of single particles, correlated with the topographic information, and in these cases sizing information is essential. This, along with the ability of AFM to resolve nanostructures of a wide selection of materials with little intrusion, provides potential advantages for using AFM as a sizing tool for nanoparticles. The basic mode of AFM operation is contact mode in which height information is deduced from the deflection of the cantilever as the sharp tip scans the sample while maintaining contact with its surface. Nanometric particles that are not tightly bound to the substrate are often dragged along the surface during scanning, due to the lateral force applied by the tip.12 This problem is overcome by the use of tapping mode AFM. In this mode of operation, the cantilever is oscillated close to its resonance frequency and the tip taps the surface only periodically, reducing significantly the lateral force. The reduction of the applied force also enables imaging of soft samples such as polymers and biological specimens without inducing irreversible damage. Anomalous height data have been reported for various samples imaged in TMAFM. Height reduction was observed for biomolecules such as G-wire DNA,13 and the effect was attributed to compression of the soft matter due to the pressure applied by the tip. For inorganic materials such as Cu clusters on SiOx,14 Au clusters on Al2O3,15 and magnetite nanoparticles on mica,16 reduction of height was also reported, but for such hard materials physical compression by the tip is unlikely. To further examine the utility of TMAFM for nanocrystal sizing we present a study of the effects of various factors on the measured height of CdSe
nanocrystals. We examined height variations upon changes in the substrate hydrophobicity, nature of the particle surface ligands, and the cantilever drive frequency. From this we infer a mechanism for distortion of nanocrystal height data extracted from TMAFM. We focused our investigation on the prototypical system of CdSe nanocrystals prepared using high-temperature pyrolysis of organometallic precursors in coordinating solvents.17,18 The advantage of CdSe nanocrystals for this study is the ability to achieve narrow size distributions over a broad range of diameters. The average diameter and the size dispersion can be easily monitored from the position and width of the absorption peaks, respectively. Samples for AFM imaging were prepared by spin casting a drop of nanocrystal solution on the desired substrate. TMAFM images were acquired using a commercial AFM (DI bioscope with NanoScope 3a controller) and standard silicon tips (MikroMasch, NSC1119). The z piezo was calibrated using 2 nm steps formed in etched muscovite mica.20 Images were first flattened using the DI software algorithm, (excluding the particles from the flattened area) and then analyzed automatically using commercial image processing software (Image Metrology, SPIP) to yield height histograms of the imaged particles. The height extracted from this method was in good agreement with the height determined manually from cross sections. Our TMAFM measurements showed that the height data is highly sensitive to scanning parameters such as the tapping amplitude and force. Variations of a few nanometers in height and even complete contrast reversal (particles appearing as holes) were measured for different scanning parameters and tips. The dependence of height data on these experimental parameters is rather complex and is under investigation. To eliminate these effects, each part of the experiments reported below was performed using the same tip and scanning parameters. The cantilever was driven at the resonance frequency peak as measured in free air unless stated otherwise. The measurements were performed on two nanocrystal samples designated (a) and (b), with the first absorption peak at 595 and 620 nm, respectively. A characteristic AFM image and the corresponding height histogram for ∼250 particles of sample (a) are presented in Figure 1. Clearly, the lateral size of the particles imaged by the AFM is large, due to the convolution effect of the tip, while the height should reflect the particle size more accurately. Figure 2 (top frame) compares Gaussian fits to the height distribution histograms of sample (a), imaged by TMAFM on two different substrates, and by TEM. Trace I represents particle heights imaged by TMAFM on glass cover slips (Thompson) that were RCA treated21 to become highly hydrophilic (contact angle for a 5 µL droplet of water was 12°), and the average height is 1.6 nm. Trace II represents particles imaged by TMAFM on glass cover slips that were made hydrophobic using hexamethyldisilazane (HMDS)22 (contact angle of 63°), and the average height is 2.8 nm. Trace III represents the histogram of diameters from ∼600 particles imaged by TEM, and the average diameter is 5.0 nm. A striking feature is that the TMAFM measurements 946
Figure 1. 3D representation of a typical 1 µm2 TMAFM image of sample (a) on mica. The height data deduced from several such images (∼250 particles) is plotted as a histogram from which mean height and size distribution was obtained (top).
on both substrates yield significantly smaller height data compared to the TEM data. Similar height reduction was also observed for commercially available gold colloids (diameter: 5.2 nm, Pelco) imaged on freshly cleaved mica.23 Moreover, particles imaged on the RCA treated glass appear lower than the same particles imaged on HMDS treated glass. This effect was so significant that when imaging smaller particles on the RCA treated substrate it was often impossible to clearly resolve the particles from the background. To characterize the tip-sample interactions, force curves were measured as shown in Figure 2 (bottom frame) presenting the deflection of the tapping cantilever as the tip was approached to the surface and then retracted. As the z piezo withdraws from the surface, the cantilever is deflected until its pulling force is sufficient to detach the tip from the surface force field. The larger displacement needed to detach the tip from the RCA treated glass indicates that the attractive forces on the tip are significantly larger compared with the HMDS treated glass. To check the effect of the particle surface chemistry on the net tip-sample interaction, we modified the surface of the particles utilizing known ligand exchange procedures.24-26 Trioctylphosphine-oxide (TOPO), dodecylamine, pyridine, and mercaptoacetic acid were used as capping ligands. The first two ligands manifest hydrophobic nature to the nanocrystal surface, while pyridine effectively removes the TOPO ligands leaving the particle surface nearly bare. Mercaptoacetic acid treatment renders the particle soluble in water through manifesting a hydrophilic surface. In the case of pyridine and mercaptoacetic acid, the efficiency of the ligand exchange was verified by the solubility of the modified particles in pyridine and water respectively, indicating a sufficient surface exchange to modify the chemical nature of the particle surface. Dodecylamine capping was confirmed by a significant increase in the fluorescence intensity of the particles. Particles with the four different surface ligands were Nano Lett., Vol. 2, No. 9, 2002
Figure 3. Gaussian fits to the height distribution histograms of sample (a) with modified surface ligands. Trace I: particles capped with mercaptoacetic acid. Trace II: dodecylamine capping. Trace III: trioctylphosphine-oxide capping. Trace IV: pyridine.
Figure 2. (top frame) Gaussian fits to the size distribution histograms of sample (a). Traces I and II represent particle heights imaged by TMAFM on glass cover slips that were either RCA treated to become highly hydrophilic (I) or HMDS treated to become hydrophobic (II). Trace III represents the histogram of diameters of ∼600 particles imaged by TEM. (bottom frame) Force plots acquired on the hydrophobic glass substrate (dashed green line) and on the hydrophilic glass substrate (solid red line).
imaged on a freshly cleaved mica substrate with the same tip and similar scanning parameters. As is seen in Figure 3, all imaged samples present similar height within reasonable error, with the exception of the pyridine capped particles, which appear approximately 1 nm lower. This lower height may be attributed to the near removal of surface ligands in this case. Note that the average heights are larger than the ones observed in Figure 2 on the glass substrates, although still lower than the TEM size. This set of measurements was done using a different tip and scanning parameters, and a direct comparison with the data in Figure 2 cannot be made. Apparently, the influence of the chemical nature of the particle surface in this size regime is minor in comparison with the effect of the substrate surface chemistry. This is due to the small interaction area the particle has with the tip, which is governed by the particle geometry, compared to the interaction area of the surface with the tip, which is governed by the tip geometry. To a first approximation, the Nano Lett., Vol. 2, No. 9, 2002
AFM tip may be regarded as a sphere with diameter of a few tens of nanometers. This is supported by the lateral diameter of the imaged particles, which represents a convolution between the particle and the tip, and is on the order of 20 nm, while the actual particle diameters are only a few nanometers. Therefore, the net tip-particle interaction is expected to be much smaller than the tip-substrate interaction. To understand the effect of the substrate chemistry on the height data, it is first necessary to overview the principles of TMAFM operation. The feedback mechanism of TMAFM is controlled by the set-point amplitude ratio rsp ) Asp/A0, where A0 is the amplitude of the free oscillation at the drive frequency and Asp is the set-point amplitude. During scanning the amplitude of oscillation is maintained at Asp by adjusting the vertical tip-sample distance. When the tip scans over a higher topographical feature, the oscillation amplitude is reduced causing the feedback loop to raise the tip until the set-point amplitude (Asp) is reached, and in this way the surface topography is recorded. Additionally, the nature of tip-sample interactions may also affect the oscillation amplitude. In the case of a flat surface composed of two materials that apply a different attractive force on the tip, the regions exhibiting stronger attraction damp the amplitude of the oscillating cantilever and will appear higher in the TMAFM topography image as was observed for patterned self-assembled monolayers.27 As the tip is approached to the sample surface, the characteristics of the cantilever vibration, such as the amplitude, resonance frequency, and phase angle of vibration, change due to the tip-sample interaction. An elastic interaction acts as a spring, changing the spring constant of the cantilever to an effective value keff ) k + f ′, where f ′ is the sum of the force gradients of all attractive and repulsive forces acting on the cantilever.28 As a result of the tipsample interaction, the resonance frequency of the cantilever is altered to a new frequency ωeff, which is shifted by a factor 947
Figure 4. Schematic representation of the amplitude versus frequency curves and the corresponding tip oscillation amplitude at ω0 as the tip scans over the surface, the particle, and in free air (left). Recorded height is the sum of the topography and the relative amplitude damping between surface and particle (right).
(1 - f ′/k)1/2 with respect to ω0, the tip resonance frequency in free air. On the other hand, a dissipative force changes the damping constant of the cantilever, γ, and therefore the effective quality factor of the cantilever, defined by Q ) mω0/γ. The change of Q is reflected in the change of the amplitude at resonance.28 Both elastic and dissipative interactions are reflected in the amplitude change of the cantilever. To a first approximation, the amplitude of the interacting cantilever, Ainteracting(ω), is obtained by shifting the peak of Afree(ω) to a lower frequency (ωeff < ω0) when f ′ < 0, i.e., attractive forces cause the amplitude vs frequency curve to shift toward lower frequencies. The feedback loop samples the amplitude at a pre-set frequency; therefore a shift of the resonance curve is detected as a change of amplitude. Thus, when measured at ω0, the amplitude damping, ∆A(ω0) ) A0 - Ainteracting(ω0) for the case when f′ , k is written as27 ∆A ≈ A0
[(Qf′k ) - (2kf′ )r ] 2
2
sp
(1)
It is clear from eq 1, that the amplitude damping ∆A(ω0) increases with the increase of the attractive (negative) forces f ′ acting on the cantilever. The feedback mechanism of tapping mode works in such a way that a surface region of larger amplitude damping is recorded as higher in topography, and the apparent height is a sum of the true height of the imaged feature and the local amplitude damping. In the case of nanostructure sizing, the height is always measured relative to the substrate, and the relative amplitude damping between the substrate and the imaged nanostructure is added to the topographical height. A decrease in ∆A(ω0) (i.e., higher oscilation amplitude) when the tip is over the nanostructure relative to ∆A(ω0) when the tip is over the substrate will result in lower height than the true height, as illustrated in Figure 4. To further validate the importance of this effect for nanocrystal sizing, we imaged sample (b) using different driving frequencies, with the AFM feedback electronics set to detect the amplitude on different sides of the resonance curve. The results of this measurement are displayed in Figure 5 along with the resonance curve of the cantilever in free air where the drive frequencies are indicated (see top frame). Substantial changes in the height histograms are seen for the three frequencies. The particles imaged while on the 948
Figure 5. (top frame) Free air cantilever resonance curve. (bottom frame) Height distribution histograms of sample (b) using three drive frequencies: to the left (I, green curve), right (III, black curve), and at the center (II, red curve) of the free-air resonance curve as indicated by the arrows in the top frame. Measured height depends systematically on the drive frequency.
left side of the resonance curve (trace I, ω ω0). This is related to the shift in the resonance curve while scanning over the particle. For example, when the detection frequency is set to an initial value significantly lower than ω0, scanning will be performed Nano Lett., Vol. 2, No. 9, 2002
while it is still on the left side of the resonance curve even after the frequency shift due to the attractive force. On this side, the slope of the amplitude vs frequency curve is positive. When the tip scans over the particle, the curve is shifted back to the right, toward the free air curve, and the amplitude is further damped leading to the recording of higher topography. This shows that the frequency shift and the resulting amplitude damping can be partially compensated for by presetting the detection frequency of the feedback loop to a frequency lower than the free air resonance. These observations fully corroborate the mechanism proposed for the inaccurate nanocrystal heights recorded in TMAFM images. Under ambient conditions, the contamination layer composed mainly of water is present on all surfaces and leads to an attractive capillary force.9 The TMAFM measurements shown in Figure 2 were carried out with small A0 and large rsp (25 nm and 0.7, respectively, i.e., light tapping). Under these tapping conditions, capillary forces dominate the tip sample interactions. As the tip approaches the surface and enters the contamination layer, the cantilever resonance curve is shifted toward lower frequency due to the attractive capillary force. This shift causes the amplitude at the detected frequency ω0 to decrease. As the tip scans over a nanometric structure, there is a perturbation, causing a decrease of the capillary force, and a shift of the resonance back toward ω0, leading to a larger oscillation amplitude relative to the amplitude observed over the surface. Hence, the imaged structure appears lower than its actual height. A schematic illustration of the effect of attractive forces on the cantilever motion is presented in Figure 4. This mechanism explains the low height measured in TMAFM compared to TEM. The height difference between particles measured on the two substrates may be explained by the different hydrophobicity of the substrates. The RCA treated glass surface is rich with hydroxyl (OH) groups, which render it highly hydrophilic. HMDS treated glass on the other hand, is covered by methyl-terminated (CH3) chains, making it highly hydrophobic, as confirmed by the contact angle measurements. The tip of the Si cantilever is covered with SiO2 and is assumed to be significantly more hydrophilic than the CH3terminated surface. Thus the capillary force between the tip and the CH3 region should be much smaller than that between the tip and the OH region as is confirmed by the force measurement in Figure 2. It is reasonable that the contrast in tapping amplitude between tip-surface and tip-particle states will be smaller on the hydrophobic substrate compared with the hydrophilic one due to the smaller overall interaction leading to smaller shifts of the resonance curve, and therefore smaller reduction in the measured particle height. In conclusion, sizing nanoparticles with TMAFM in air is extremely sensitive to tip sample interactions, and specifically, to the attractive capillary forces induced on the tip by the contamination layer existing on surfaces measured in ambient conditions. These attractive forces cause the reduction of the acquired height in the nanometer scale. Particles in this size regime appear considerably lower than their true height. The force gradient near the imaged surface changes Nano Lett., Vol. 2, No. 9, 2002
the effective spring constant of the oscillating cantilever, and this change in effective spring constant causes a shift of the amplitude vs frequency curve of the cantilever and thus a change in the acquired amplitude. As the tip is scanned over the surface the induced force is dynamically changing due to perturbations from topographical features. The amplitude versus frequency curve shifts back and forth causing a force induced “height” contribution to the topographic height of imaged nanostructures. This force induced height contribution is strongly dependent on the feedback loop acquisition frequency, and can be modified by variation of the drive frequency. Acknowledgment. We thank Taleb Mokari for assistance in the TEM measurements. This research was supported by the Israel Science Foundation (grant #99/00-12.5). Supporting Information Available: TMAFM image and corresponding size distribution histogram of 5.2 nm commercially available gold colloids. This material is available free of charge via the Internet at http://pubs.acs.org. References (1) Alivisatos, A. P. Science 1996, 271, 933. (2) Banin, U.; Cao, Y. W.; Katz, D.; Millo, O. Nature 1999, 400, 542. (3) Zhong, Q.; Innis, D.; Kjoller, R. W.; Elings, V. B. Surf. Sci. Lett. 1993, 290, L668. (4) Gerion, D.; Pinaud, F.; Williams, C. S.; Parak, J. W.; Zanchet, D.; Weiss, S.; Alivisatos, A. P. J. Phys. Chem. B 2001, 105, 8861. (5) Mattoussi, H.; Cumming, A. W.; Murray, C. B.; Bawendi, M. G.; Ober, R. J. Chem. Phys. 1996, 105, 9890. (6) Eichho¨fer, A.; von Ha¨nisch, C.; Jacobsohn, M.; Banin, U. Mater. Res. Soc. Symp. 2001, 636, D9.53.1-D9.53.9. (7) Binning, G.; Quate, C. F.; Gerber, C. Phys. ReV. Lett. 1986, 56, 930. (8) Magonov, S. N.; Whangbo, M.-H. Surface Analysis with STM and AFM; VCH: Weinheim, 1996. (9) Sarid, D. In Scanning Force Microscopy; Lapp, M., Stark, H., Eds.; Oxford University Press: New York, 1991. (10) Krauss, T. D.; O’Brien, S.; Brus, E. L. J. Phys. Chem. B 2001, 105, 1725. (11) Ebenstein, Y.; Mokari, T.; Banin, U. Appl. Phys. Lett. 2002, 80, 4033. (12) Junno, T.; Anad, S.; Deppert, K.; Montelius, L.; Samuelson, L. Appl. Phys. Lett. 1995, 66, 3295. (13) Marsh, T. C.; Vesenka, J.; Henderson, E. Nucl. Acids Res. 1995, 23, 697. (14) Kuhle, A.; Sorensen, A. H.; Bohr, J. J. Appl. Phys. 1997, 81, 6562. (15) Mahoney, W.; Schaefer, D. M.; Patil, A.; Andres, R. P.; Reifenberger, R. Surf. Sci. 1994, 316, 383. (16) Rasa, M.; Kuipers, B. W. M.; Philipse, A. P. J. Colloid Interface Sci. 2002, 250, 303. (17) Murray, C. B.; Norris, D. J.; Bawendi, M. G. J. Am. Chem. Soc. 1993, 115, 8706. (18) Peng, X.; Wickham, J.; Alivisatos, A. P. J. Am. Chem. Soc. 1998, 120, 5343. (19) Silicon cantilevers with typical resonance frequencies of 200-400 kHz and typical spring constants of 20-90 N/m. (20) Nagahara, L. A.; Hashimoto, K.; Fujishima, A.; Snowdenlfft, D.; Price, P. B. J. Vac. Sci. Technol. B 1994, 12, 1694. Etch pits with specific geometry are formed by wet etching the mica substrate in concentrated hydrofluoric acid. Steps of approximately 2 nm, corresponding to the molecular planes of mica, are formed along the long axis of these pits. (21) Glass coverslips were cleaned by sonication in detergent, washed, and then immersed for 15 min. in RCA solution (1: 1: 4, 30% H2O2: 30% NH4OH: high purity water) at 85 °C, washed with high purity water and dried with N2 flow. (22) Glass coverslips were cleaned by sonication in detergent, washed, dried with N2 flow and placed in a loosely sealed glass flask. Few drops of hexamethyldisil azane (HMDS) (Aldrich Chemical 37, 9212) were added to the flask and left to react overnight at 70 °C. 949
(23) Data may be viewed in the Supporting Information section. (24) Talapin, D. V.; Rogach, L. A.; Kornowski, A.; Haase, M.; Weller, H. Nano Lett. 2001, 1, 207. (25) Kuno, M.; Lee, J. K.; Dabbousi, B. O.; Mikulec, F. V.; Bawendi, M. G. J. Chem. Phys. 1997, 106, 9869. (26) Brandsch, R.; Bar, G.; Whangbo, M.-H. Langmuir 1997, 13, 6349.
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(27) Bar, G.; Thomann, Y.; Brandsch, R.; Cantow, H. J.; Whangbo, M.H. Langmuir 1997, 13, 3807. (28) Humphris, A. D. L.; Tamayo, J.; Miles, M. J. Langmuir. 2000, 16, 7891.
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