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Quantitatively Probing the Means of Controlling Nanoparticle Assembly on Surfaces Jonathan M. Patete,† Xiaohui Peng,† Joseph M. Serafin,‡ and Stanislaus S. Wong*,†,§ †
Department of Chemistry, State University of New York at Stony Brook, Stony Brook, New York 11794-3400, United States Department of Chemistry, St. John’s University, 8000 Utopia Parkway, Queens, New York 11439-0001, United States § Condensed Matter Physics and Materials Sciences Department, Brookhaven National Laboratory, Building 480, Upton, New York 11973, United States ‡
bS Supporting Information ABSTRACT: As a means of developing a simple, cost-effective, and reliable method for probing nanoparticle behavior, we have used atomic force microscopy to gain a quantitative 3D visual representation of the deposition patterns of citrate-capped Au nanoparticles on a substrate as a function of (a) sample preparation, (b) the choice of substrate, (c) the dispersion solvent, and (d) the number of loading steps. Specifically, we have found that all four parameters can be independently controlled and manipulated in order to alter the resulting pattern and quantity of as-deposited nanoparticles. From these data, the sample preparation technique appears to influence deposition patterns most broadly, and the dispersion solvent is the most convenient parameter to use in tuning the quantity of nanoparticles deposited onto the surface under spin-coating conditions. Indeed, we have quantitatively measured the effect of surface coverage for both mica and silicon substrates under preparation techniques associated with (i) evaporation under ambient air, (ii) heat treatment, and (iii) spin-coating preparation conditions. In addition, we have observed a decrease in nanoparticle adhesion to a substrate when the ethylene glycol content of the colloidal dispersion solvent is increased, which had the effect of decreasing interparticlesubstrate interactions. Finally, we have shown that substrates prepared by these diverse techniques have potential applicability in surface-enhanced Raman spectroscopy.
1. INTRODUCTION As the demand for smaller and more efficient sensors, highdensity data storage media, and similar devices continues to grow, the reproducible ability to assemble nanoscale components reliably into a definable aggregate with adequate precision has become an issue of pressing interest. That is, the bottom-up approach using nanosized constituents to construct a functional, logical architecture is certainly a viable route toward generating practical nanoscale devices.1 With this in mind, in this article, we seek to address fundamental questions concerning nanoparticle assembly, which is a function of the particle concentration, size, and distribution. Namely, what factors can be reliably used to manipulate nanoparticle deposition onto a substrate? Previous research has found that deposition patterns of nanoparticles on a substrate can be complex. Specifically, deposition patterns are dependent upon interactions among the particles, the substrate, the solvent, and the ambient environment (including fluid). In the particular case of the solvent, properties such as surface tension, vapor pressure, polarity, and viscosity can play critical roles in nanoparticle deposition by affecting the r 2011 American Chemical Society
colloidal stability and particle flow within the evaporating droplet. Additional relevant intermolecular and surface forces include but are not limited to van der Waals forces, hydrogen bonding, ionic and covalent bonding, electrostatic attraction and repulsion, wettability, capillary forces, and surface tension.2,3 A number of elegant experiments involving electron-beam-induced deposition,4 template-assisted self-assembly,5 the exploitation of chemical interactions between chiral ligands on nanoparticle surfaces,1 and physical manipulation using an atomic force microscope (AFM) tip6 have been reported as viable means of controlling nanoparticle assembly. Furthermore, electrostatic interactions can be tailored to probe and control the assembly of colloidal gold and silver nanoparticles.712 The crystallographic mismatch between the nanoparticles and the substrate has also been manipulated to direct the assembly of metal nanoparticles onto a substrate.13,14 Nevertheless, Received: December 23, 2010 Revised: March 20, 2011 Published: April 14, 2011 5792
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Langmuir despite some tangible degree of control, self-assembled arrays have yet to achieve the same uniformity and regularity as lithographic patterning.13 Therefore, it is important to achieve an adequate understanding of the factors that govern the behavior of self-assembled nanocrystal deposition patterns, such as the formation of a 2D submonolayer array of organically passivated metal nanoparticles.15 From this knowledge, we may be able to choose which parameters are most appropriate for tailored assembly purposes. In our experiments, we have used an atomic force microscope (AFM) to gain a quantitative 3D visual representation regarding the discrete effects independently associated with (a) sample preparation, (b) substrate choice, (c) the dispersion solvent, and (d) the number of loading steps with respect to the assembly of citrate-capped Au nanoparticles. In effect, all four of these parameters were found to be important in determining the net result of nanoparticle deposition. Specifically, we have evaluated nanoparticle assembly in two dimensions by determining the percentage of the area covered by particles. To analyze the assembly on the vertical scale (z axis), we have not only measured the height of the particles and aggregates but also obtained surface roughness measurements from these height images. These methods of analysis have provided us with the ability to quantify the extent of nanoparticle deposition on a surface in all three dimensions without necessarily requiring either any regularity or order within these nanoparticle geometries. In fact, we can obtain information about the uniformity of the deposition patterns as well as the degree to which these particles form aggregates from our data. In this report, we chose to consider two commonly used substrates with different physicochemical properties, namely, silicon (111) and muscovite mica. Silicon is a semiconducting material most commonly used in electronic applications such as integrated circuits and photovoltaics. The exposed surface of a silicon wafer is terminated with an amorphous native oxide layer that is vulnerable to contamination by organics.16 Therefore, a cleaning procedure is often employed to provide a substrate with uniform surface chemistry. The silicon wafers used in this experiment were treated with a standard piranha etch protocol that imparts a fresh hydrophilic silicon oxide layer. As a result, however, the topography of the wafer becomes more irregular and an increase in surface roughness has been observed.16 By contrast with silicon, muscovite mica is a silicate mineral with the general chemical formula of KAl3Si3O10(OH)1.8F0.2 that readily cleaves along the crystallographic (001) plane as a result of the weak interaction between the potassium ions and the aluminosilicate layers.17 The result is a relatively flat, charged polar surface that subsequently reacts with airborne contaminants, such as water vapor and carbonaceous gases.18,19 This material has excellent insulating properties, having a thermal conductivity of 0.46 W m1 K1 and a relative permittivity of about 6 to 7.20,21 The systems that we have looked at may be used in electronic and optical applications, such as surface-enhanced Raman spectroscopy (SERS)-based sensing devices.12,2022 Because our aim has been focused toward gaining fundamental knowledge to be used for subsequent applications in nanoscale devices, our objective has been to generate reliable qualitative and quantitative predictions about nanocrystal assembly on a substrate using basic, standard processing conditions. We have successfully investigated and elucidated the influence of the sample preparation technique, substrate choice, dispersion solvent, and number of loading steps upon nanoparticle deposition patterns.
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2. EXPERIMENTAL SECTION 2.1. Chemicals and Materials. Hydrogen tetrachloroaurate(III) hydrate (99.999%) was purchased from Alfa Aesar with no additional purification steps. Muscovite mica and precut silicon (111) substrates were ordered from Ted Pella Inc. Mica surfaces were cleaved using double-sided tape immediately prior to deposition with colloidal gold. Silicon wafers were rinsed with water, cleaned in piranha solution (2:1 H2SO4/H2O2) for 30 min at 6080 °C, rinsed, and ultimately sonicated in ultrapure water having a resistivity of 18 MΩ cm. Prior to use, wafers were dried in argon. This cleaning protocol is known to remove all organic contaminants, thereby leaving a fresh silicon oxide layer on the surface.16 Both mica and silicon surfaces were characterized by contact angle measurements using a Rame-Hart model 120 contact angle goniometer. 2.2. Synthesis of Gold Nanoparticles. Gold nanoparticles were synthesized using the Turkevich citrate reduction method.23 Specifically, 50 mL of aqueous 3.314 104 M hydrogen tetrachloroaurate(III) hydrate was heated to boiling at 96.5 °C. The gold cations therein were reduced by the dropwise addition of 1.842 mL of 2.497 102 M aqueous sodium citrate so as to yield a deep-red solution of citratecapped gold nanoparticles after an additional 30 min of heating and stirring. The reaction solution was then diluted to 100 mL in a volumetric flask and subsequently divided into four equal parts. The colloids were isolated upon centrifugation at 7000 rpm for 55 min and were subsequently redispersed in 7 mL of ethanol, water, or ethylene glycol. The concentration of gold nanoparticles was calculated to be ∼2 109 particles/μL for each colloidal suspension generated. The asprepared gold nanoparticles were subsequently characterized using a number of techniques such as scanning electron microscopy (SEM), transmission electron microscopy (TEM), UVvisible spectroscopy, and energy-dispersive X-ray spectroscopy (EDS) to confirm their composition, size distribution, and morphology. 2.3. Sample Preparation. For air-evaporated samples, 10 μL of colloidal gold was placed onto the substrate and allowed to dry for at least an hour at room temperature. For heat-treated samples, 10 μL of colloidal gold was placed on a substrate and then warmed in an oven for 5 min at 80 °C. The sample was then immediately cooled in a freezer at 18 °C for 10 min in order to quench its thermal activity and halt particle movement. For spin-coated samples, 10 μL of colloidal gold was spin coated onto the substrate at 3000 rpm for 30 s. All other experimental parameters, such as particle concentration, temperature, and pressure, were kept constant. Unless otherwise stated, ethanol was used as the dispersion solvent for all experiments. 2.4. AFM Imaging Parameters. Images were taken in Tapping Mode with a Digital Instruments Multimode scanning probe microscope. Force modulation-etched silicon tips (FESP) with a spring constant of 36.8 N/m and a resonance frequency of about 6875 kHz were used. The magnitude of amplitude oscillation was kept at a minimal level for each sample though not necessarily constant because each sample may require a different amount of force for optimal and accurate imaging. All AFM images were taken at the center of the sample substrate and measure 3 3 μm2 in area with a height-scale threshold of 20 nm in order to maintain self-consistency within our data analysis. A minimum of 15 images were analyzed for each data set, and error bars in the graphs represent the standard deviation around the mean value. Control samples (Figure S5) were prepared with clean solvents, and the average roughness of untreated surfaces was subtracted out from the raw data such that the final values represent only the contribution to the roughness associated with as-deposited nanoparticles. A description of the data analysis is described in the Supporting Information section. 2.5. SERS Measurements. To determine the viability of using these as-prepared substrates as SERS devices, the Raman spectra of rhodamine 6G (R6G) were recorded on a WiTec Alpha300 Raman 5793
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Figure 1. Representative TEM image of Au nanoparticles in citrate solution (A) with a histogram representing particle diameter measurements (222) of as-prepared nanoparticles (B) accompanied by (C) the UVvisible spectra of as-prepared Au nanoparticles dispersed in the reaction solution, ethylene glycol, water, and ethanol, respectively. microscope equipped with an excitation wavelength of 532 nm and a 600 g/mm grating under a 20 objective optical lens. The spectra shown represent an average of five spectra taken from different locations on the substrate. In addition, each individual spectrum was recorded as an average of 10 signal accumulations with a 10 s integration time. The laser power was set to the minimum value required to observe the Raman signal in order to prevent the degradation of the probe molecule. Specifically, it was kept constant at this same value (in the mW range) for all samples. Active substrates were prepared by depositing 20 μL of a 1 104 M R6G aqueous solution onto the nanoparticle thin films and allowing these aliquots to dry overnight.
3. RESULTS AND DISCUSSION 3.1. Structural Characterization of Colloidal Gold. Analysis by TEM imaging, shown in Figure 1A, revealed that as-prepared nanoparticles, dispersed in a citrate reaction solution, were spherical in nature and possessed an average diameter of 17.6 ( 3.0 nm with a range of 11.325.7 nm. As-prepared nanoparticles were relatively monodisperse, as illustrated by the histogram shown in Figure 1B. To ensure that solvent effects were not greatly affecting size distributions and morphology, particles were independently dispersed in ethanol, water, and ethylene glycol and were found to possess diameter ranges of 17.8 ( 2.7, 16.5 ( 2.6, and 17.5 ( 4.0 nm, respectively. Representative TEM
images and histograms for these colloids can be found in the Supporting Information section (Figures S1S3). The UVvisible spectra of our colloidal solutions shown in Figure 1C reveal the characteristic surface plasmon resonance (SPR) peak for monodisperse colloidal gold in the visible region, located at 530 nm.24,25 The breadth of the peak can be ascribed to the presence of polydispersity (e.g., multiple size domains that arise from imperfect stabilization by the citrate capping agent25). The preservation of size, morphology, and stability is evidenced by a lack of change either in the peak position/intensity in the UVvisible data (Figure 1C) or in the TEM results across the entire sample range. A representative SEM image of as-prepared citrate-capped Au nanoparticles and the corresponding EDS spectrum are given in the Supporting Information section (Figure S4). From these data, one can observe that as-synthesized Au nanoparticles were very small and could be readily dispersed onto the silicon sample substrate. The EDS spectrum highlights the presence of a silicon peak that can be ascribed to the underlying substrate and a small carbon peak that can be attributed to impurities from within the instrumental setup as well as the citrate capping agent. The remaining peaks are derived from our gold nanoparticle sample, suggesting a very high purity. 3.2. Surface Energy Determination by Contact Angle Measurements. Advancing contact angles for the piranha-etched silicon 5794
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substrates were measured using water and glycerol. For mica, ethylene glycol and propylene glycol yielded the most reliable contact angle measurements within the precision and accuracy of the instrument. The OwensWendt method26,27 was employed to calculate the surface energy of these two substrates. This method relates the surface energy of a solid to the contact angle (θ) by the following equations qffiffiffiffiffiffiffiffiffiffi qffiffiffiffiffiffiffiffiffiffi! γL ð1 þ cos θÞ ¼ 2
γdL γdS þ
p p
γL γS
ð1Þ
p
γS ¼ γdS þ γS
ð2Þ
where γL is the surface tension of the liquid and γdL and γpL are its dispersion and polar components, respectively. The surface energy of our substrates (γs) is calculated by the sum of its polar (γps ) and dispersion (γds ) components that can be determined from eq 1. Values for the surface tension, dispersion component, and polar component of water, glycerol, ethylene glycol, and propylene glycol are shown in Table 1 as per literature data.2,27 Moreover, as shown in Table 1, surface energies of mica and piranha-etched silicon were calculated to be 135 and 64.4 mJ/m2, respectively. Table 1. Surface Energy Calculations for Mica and Silicon γps surface
liquid
silicon water glycerol mica
ethylene glycol
γds
γs
2
contact angle (mJ/m ) (mJ/m2) (mJ/m2) 29.1 ( 1.9°
39.8
24.6
64.4
117.3
17.7
135
12.1 ( 2.3° 29.3 ( 1.8°
propylene glycol 41.9 ( 1.8°
3.3. Assembly of Gold Nanoparticles Dispersed in Ethanol as a Function of Sample Preparation Conditions. We have
systematically tested the level of control over the deposition of as-prepared gold nanoparticles dispersed in ethanol under distinctive sample preparation techniques. By varying the deposition technique, we drastically affect the mechanism by which the solvent evaporates and, ultimately, the deposition pattern. For example, it has been shown that ring patterns tend to appear as a result of a faster evaporation time, which can occur under conditions such as elevated temperature.28 One group has suggested that each annular ring arises from the pinning of the rim of an opening hole by a sufficient number of particles in a process governed by evaporation, hole nucleation, and fluid flow.29 3.3.1. Effect of Sample Preparation Conditions on a Silicon Substrate. The representative AFM image in Figure 2A suggests that a majority of nanoparticles, deposited onto a silicon wafer by air evaporation, form many large aggregates. However, the size and number of these aggregates can be drastically reduced under both heating (Figure 2B) and spin-coating (Figure 2C) preparative conditions. Silicon wafers that have been treated with piranha solution but do not contain gold nanoparticles have served as a background and are themselves shown as a control sample in the Supporting Information section (Figure S5b). The average heights of particle aggregates in the center of the silicon wafer were noted to be 25.5 ( 14.7, 22.6 ( 14.1, and 19.1 ( 10.4 nm for air-evaporated, thermally heated, and spin-coated samples, respectively. Surface coverage determined by pixel analysis suggested that 24.6, 10.7, and 0.603% of the silicon substrate was covered by gold nanoparticles for air-evaporated, thermally heated, and spin-coated samples, respectively. The corresponding trend in our roughness analysis was similar, with
Figure 2. AFM height images (3 3 μm2) of Au nanoparticles dispersed in ethanol and deposited onto piranha-etched silicon by (A) air-evaporation, (B) heat-evaporation, and (C) spin-coating techniques. (D) Corresponding particle surface coverage measurements. 5795
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Figure 3. Representative SEM images of Au nanoparticles in ethanol deposited on a silicon wafer by (A, B) air-evaporation, (C, D) heat-evaporation, and (E, F) spin-coating techniques. The centers of the substrate are shown in A, C, and E, and the outer edges are shown in B, D, and F.
rms roughness values of 6.62, 5.02, and 0.911 nm for airevaporated, thermally heated, and spin-coated samples, respectively. These values are illustrated for comparison in Figure 2D. The presence of elevated average height data, as compared with TEM measurements coupled with their relatively large standard deviations, for air-evaporated and heated samples indicates that significant vertical aggregation (e.g., islanding) is occurring, orthogonal to the plane of the surface. Under airevaporated conditions, as-prepared nanoparticles formed mostly large aggregates with the highest amount of surface coverage observed under our experimental conditions. The heating of air-evaporated samples produced a moderate surface coverage with a significant decrease in the level of aggregation as compared with the air-evaporated sample. By contrast, the height measurements for the spin-coated sample are consistent with the presence of a single layer of particles yielding the lowest degree of observed aggregation among the various sample preparation techniques employed. To explain the large variation in surface coverage as a result of the sample preparation technique, extensive characterization of the various substrates was performed using SEM. Whereas AFM is a relatively localized technique whose spatial scope is inherently limited by probe movement, SEM can more rapidly and effectively sample over much larger areas and thereby yield critical morphological insight into the deposition of nanoparticles in relatively inaccessible regions, such as the edges of substrates. For example, the SEM images presented in Figure 3A,B revealed that the
deposition pattern for air-evaporated samples is uneven, with the majority of the deposited nanoparticles concentrated at the center of the silicon substrate. By contrast, SEM images of the heated samples in Figure 3C,D showed the presence of “coffeering” patterns at the edges of the substrate. The deposition pattern for spin-coated samples was nonetheless noted to be relatively much more homogeneous, as illustrated in Figure 3E,F. 3.3.1.1. Air Evaporation. Though it is not a very controlled process,30 free air evaporation of a colloidal solution onto the substrate implies that all of the particles adhere to the surface in a process governed by evaporation kinetics, particle flux, and particle interactions with the liquidair interface. In essence, during drying, the concentration of the nanocrystals increases and the nanocrystals simultaneously experience stronger attractive forces due to reduced screening of the van der Waals forces between each other, which ultimately allows for the formation of aggregate structures.31 Thus, the deposition pattern has been found to depend on factors such as the initial concentration of the particle solute, the size of the suspended particles, the initial geometry of the drop (e.g., drop radius), and the time elapsed from the beginning of the drying process.32,33 Most importantly, we observe that our deposition patterns for air-evaporated and heat-evaporated samples have been influenced by fluid flow and particle migration within the droplet during the evaporation period. Deegan et al. have suggested that in the case of aqueous suspensions at room temperature, the contact line is pinned during the course of evaporation and solvent molecules at the 5796
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Figure 4. AFM height images (3 3 μm2) of Au nanoparticles dispersed in ethanol and deposited onto freshly cleaved mica by (A) air-evaporation, (B) heat-evaporation, and (C) spin-coating techniques. (D) Corresponding particle surface coverage measurements.
interface are replenished by solvent molecules at the center of the droplet to maintain an equilibrium droplet shape within a fixed boundary.30,3436 This gives rise to an outward capillary flow within the droplet that carries dispersed solids to the edge of the contact line37 in a manner that has been found to depend upon the particle concentration, particle size, and distribution.38 In fact, the outward flow of fluids is thought to act dramatically to delay evaporation via reducing the flux at the main droplet edge. This ultimately leads to instability that breaks up longer and thinner droplets on a timescale that is shorter than that associated with the edge retraction alone created by evaporation.39 Moreover, this evaporation phenomenon arises from solvent substrate and solventair interactions including surface tension, solvent volatility, and effective heat transfer between the solvent and the surface that can be tuned to control the direction of the flow within the droplet.40,41 Our case of thermal heating, as compared with ambient air-evaporated samples, is reminiscent of a study in which the flow field and deposition patterns of particles could be altered by controlling temperature profiles through the patterned heating of the substrate either using resistive microheaters or through radiative heat transfer to the droplet surface.41 Evaporation provides the driving force for instability and irregular drying fronts and can introduce simultaneously particle concentration and temperature gradients.4244 The final observed pattern relies upon a sensitive balance of colloidal and capillary interactions.45 For example, nonaqueous colloidal suspensions under ambient conditions, such as our air-evaporated sample of ethanolbased colloidal gold, have been shown to demonstrate thermal Marangoni stresses that cause a reverse flow toward the center of the drop with a majority of the particles deposited at the center.41,4547 Under such conditions, the evaporative process may be associated with a thermal component, where evaporative
cooling and thermal conductivity lead to a temperature gradient. It was proposed that the apex of the evaporating droplet would be cooler than the contact line because of its distance from the substrate.30 This uneven temperature distribution induces a corresponding surface tension gradient throughout the drop so that the solvent, along with the nanoparticles, flows from areas of low surface tension to areas of higher surface tension.30,35,36,41,46 Our results for the air-evaporated silicon sample are consistent with these previous observations. 3.3.1.2. Thermal Heating. However, the increased evaporation rate, induced by heat treatment, generates an outward flow of fluid and particles from the center of the droplet, which has been observed to leave behind a coffee-ring deposition pattern.30,35,36,46 Our observation of a reversed fluid flow under elevated evaporation rates is consistent with Deegan’s explanation that at low particle concentrations (∼1001000 ppm), the direction of capillary fluid flow within the droplet is dominated by the higher evaporation rate at the pinned contact line, ultimately directing the nanoparticles to aggregate in a wedge region, thereby producing the coffee-ring pattern.35,48 Specifically, through SEM, we have observed a high degree of aggregation at defect sites on the outer perimeter of heat-treated silicon wafers. We surmise that the increased number of particles directed toward the outer perimeter of the droplet likely has allowed for more particles to be trapped in either cracks or other defect sites created during the piranha-etch cleaning step. 3.3.1.3. Spin Coating. By contrast, the spin-coating technique has been used in a wide variety of applications and is a very popular method for depositing homogeneous thin films.49,50 In essence, spin coating consists of four distinctive steps.50 First, the colloidal fluid is deposited as a droplet onto a fixed substrate. Second, the substrate is accelerated to a prescribed rotational 5797
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Figure 5. Representative SEM images of Au nanoparticles in ethanol deposited on mica by (A, B) air evaporation, (C, D) heat evaporation, and (E, F) spin coating. The centers of the substrate are shown in A, C, and E, and the outer edges are shown in B, D, and F.
speed and the liquid droplet spreads out to form a film that rotates at nearly the same rate as the substrate. Third, the film spreads outward and thins in a process controlled primarily by both the centrifugal force and the viscous shear force. Fourth, the film becomes sufficiently thin (e.g., on the order of the particle size) such that the evaporation process dominates, thereby further “diluting” the film. Theoretically, it is predicted that particles should preferentially collect within the film ridge near the moving contact line and that it may be possible to control the particle deposition profile and the aggregate size by changing the nature of the adhesive interactions of the particle and substrate materials as well as by modifying the rotation rate.50 Furthermore, nanoparticle deposition using the spin-coating apparatus is largely affected by solvent properties, including viscosity, volatility, and vapor pressure, as well as the particulate concentration and the spinner’s rotational speed, which we have set to be 3000 rpm. For example, high solvent viscosity can modify deposition by preventing the drop from attaining its equilibrium shape.35 As compared with air-evaporated samples, spin-coated samples will also undergo a more rapid rate of solvent evaporation with the additional complication that some of the solution, along with the nanoparticles, may be violently cast off the substrate in the process. Some nanoparticles thereby become irrevocably lost from the substrate itself, thereby resulting in fewer nanoparticles adhering to the surface. In fact, the spin-coating process is well known for generating relatively evenly distributed deposition
patterns of material, similar to the ones that we have observed herein.31,5153 Overall, we suggest that these factors are primarily responsible for the observation of reduced nanoparticle coverage associated with spin coating at the center of the silicon substrate. 3.3.2. Effect of Sample Preparation Conditions on a Mica Substrate. Because muscovite mica is a polar surface with a surface energy that is 2 times greater than that of silicon, as shown in Table 1, the colloidal droplet completely wets the surface, thereby resulting in a thin liquid layer with a contact angle
