Article pubs.acs.org/Langmuir
Uniform Plasmonic Response of Colloidal Ag Patchy Particles Prepared by Swinging Oblique Angle Deposition Layne Bradley* and Yiping Zhao Department of Physics and Astronomy, The University of Georgia, Athens, Georgia 30602, United States S Supporting Information *
ABSTRACT: The plasmonic property of Ag patchy particles fabricated using a colloid monolayer and oblique angle deposition shows significant variations due to the multidomain nature of the monolayer. A swinging oblique angle deposition method is proposed to create uniform patchy particles. Both numerical calculations and experiment show that when the swinging angle is larger than 90°, the resulting plasmonic patchy particles have similar morphology and demonstrate uniform optical response that does not depend on the monolayer domain orientation. These uniform patchy plasmonic particles have great potential for plasmonic-based applications.
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INTRODUCTION
Patchy particles are a variety of nanomaterial in which a portion of the surface of a nanoparticle is covered by another material. Such particles have been used in a wide range of applications such as nanoelectronics,1 drug delivery,2,3 or serving as building blocks for assembling crystals and other complex structures.4−7 In plasmonic-based applications such as optical biosensing8−10 and nanophotonics,11,12 patchy particles have proven useful due to the strong effect that the shape of noble metal patches can have on the plasmonic properties at the nanometer scale.13−17 Using colloidal nanoparticles as a host, it has been shown that these properties, such as localized surface plasmon resonance (LSPR), have a strong response to small changes in the shape of the noble metal patches deposited on the surface.12,18−20 The broad range of applications for patchy particles is reflective of the many fabrication methods or synthesis strategies used to make them. One popular method is to combine self-assembled colloidal monolayers with oblique angle deposition (OAD) as illustrated in Figure 1.10,21−23 OAD is a well-known physical vapor deposition method in which the incident angle between the deposition flux and the surface normal of the substrate is large. This large incident angle (θ) exaggerates geometrical shadowing effects on the surface. When performed on close-packed colloidal monolayers, OAD creates patches whose shape is strongly dependent on the incident angle and the domain orientation (φ) of the colloids.21,22 During the monolayer fabrication, the large-area monolayer will consist of many randomly oriented domains.9,11 As an example, according to our own experience, the monolayers of 500 nm diameter polystyrene beads formed by an air−liquid interface method frequently have an average domain size of approximately 60 μm2 (or about 300 beads). When performing OAD on a multidomain colloidal monolayer, © 2016 American Chemical Society
Figure 1. OAD deposition on close-packed colloidal substrates. Angle of incidence, θ, is the angle between vapor flux and the normal to the substrate surface. The colloid domain angle, φ, is the angle between the vapor flux and the apex of the hexagon. Because of the 6-fold rotational symmetry, the angular range of unique domains is 0° ≤ φ < 60°.
the shape of the patches on any individual bead is directly related to the domain orientation with respect to the vapor direction due to different shadowing effects from neighboring beads (Figure 1, right panel). According to known plasmonic theory, plasmonic particles of different shape and size will have significantly different optical response.24 Thus, for optical measurements, if the illuminating light source has a beam that is smaller than the domain size, the optical response of measurements from different locations will not be the same; or, if the beam is large and covers multiple monolayer domains, Received: March 12, 2016 Revised: April 25, 2016 Published: April 29, 2016 4969
DOI: 10.1021/acs.langmuir.6b00980 Langmuir 2016, 32, 4969−4974
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motion has on the optical response of the patchy particles. Because of the simple fabrication methods that are employed and the uniformity that is achieved, we believe these structures have a great potential for use in plasmonic-based applications.
the optical signal will be a combination of the responses from the various illuminated domains, i.e., one would expect broadened LSPR peaks. When considering how this lack of uniformity may effect plasmonic-based sensing applications, variations in the response from different locations would lead to less consistent, and thus less accurate, measurements; and broadened peaks would result in decreased sensitivity since minor changes in commonly measured parameters, such as peak location or intensity, might be lost. To reduce the domain effects, many works have focused on producing a large single-domain colloid monolayer. One of the earliest and most common techniques is to use an external force, such as in a Langmuir trough, to cause the beads to align in a compact formation. There have also been attempts to modify the surface of the spheres prior to the formation of the monolayer in order to engineer a preferential alignment for a specific geometry.7 For the creation of patchy particles, altering the surface of the substrate to serve as a template for the colloid monolayer formation has also been attempted.25,26 The surface modification can either be a chemical or physical modification that serves to confine the colloids into predesigned geometries.27 These techniques have been shown to effectively form bead monolayers well-suited for the creation of uniform patchy particles, but they all have significant costs either in equipment, materials, or preparation time. Here we propose to combine a simple colloid monolayer fabrication method with a swinging OAD strategy to produce patchy particles with uniform size, shape, and plasmonic response. The addition of a swinging motion to OAD is a wellknown technique which has been explored for the creation of uniform nanorods.28,29 As depicted in Figure 2, swinging the
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EXPERIMENTAL SECTION
Materials. 500 nm diameter PS nanospheres (Bangs Laboratories, 07307) were used as the monolayer template. The monolayer was deposited onto cleaned glass slides (Gold Seal) and silicon pieces (University Wafer). Silver pellets (99.999%) were purchased from Kurt J. Lesker Company. Deionized (DI) water (18 MΩ) was used throughout the experiments. Fabrication Procedures. The formation of the colloidal monolayer has been reported in detail previously.29 In brief, 500 nm diameter polystyrene beads were prepared in a water−alcohol solution. A glass Petri dish was thoroughly cleaned and filled so that a thin layer of water covered the bottom of the dish. Drops of the PS bead suspension were made to fall into the dish at a regular rate. The beads from each drop spread out across the surface of the water and contributed to the floating monolayer. Cleaned substrates were placed in the water below the monolayer, and the water was removed. The monolayer of polystyrene beads adhered onto the substrates as they dried. The colloid monolayer substrates were placed into a custom-built ebeam deposition system and rotated to an incident angle of θ = 80°. Multiple depositions were performed varying the azimuthal rotation of the samples to provide total swinging angles of Δφ = 0°, 30°, 60°, 90°, 120°, 180°, 240°, 270°, 300°, 330°, and 360° with total deposited thicknesses of 45.8, 49.6, 55.0, 57.3, 61.1, 68.7, 76.3, 80.1, 84.0, 87.8, and 91.6 nm, respectively. The variation in deposited Ag thickness was calculated to approximately maintain the same thickness per deposited area on the beads. For each sample, Ag was deposited at a rate of 0.1 nm/s. The azimuthal rotation rate was varied based on the predetermined values for Δφ and the thickness so that the samples swung between −Δφ/2 and Δφ/2 a total of 25 times during the deposition. Characterization Procedures. The domain size of the monolayer and the domain effects on the shape of the deposited material were investigated with a scanning electron microscope (SEM, FEI Inspect F). The optical properties of the samples were measured using two different UV−vis systems. A home-built microscopic UV−vis spectrometer, described in another publication29 and having an optical spot size of about 40 μm2, was used to collect the local transmission spectra from 12 randomly selected areas on the sample. For comparison, a commercial UV−vis−NIR (Jasco V-570) spectrophotometer, with an illuminating spot size of about 6 mm2, was used to observe the multidomain optical properties. The differences between each pair of local transmission spectra were quantified by calculating a correlation coefficient using the built-in Matlab function CORR. The input to this function was a matrix in which the columns were the 12 transmission spectra collected with the microscopic UV−vis spectrometer. The output from the CORR function was a diagonally symmetric square 12 by 12 matrix, and each element of the matrix is a Pearson’s linear correlation coefficient, Cmn, between the spectra Tm(λ) and Tn(λ), which is calculated by
Figure 2. The addition of an azimuthal swinging motion during the deposition affects the shape of the deposited material. Δφ is the angle range through which the sample is rotated from an initial domain orientation (φ0).
colloid substrate azimuthally causes the domain orientation to rotate away from the initial orientation (φ0) through a prescribed angle (Δφ) during the deposition. Thus, during a deposition we can take advantage of the symmetry inherent in the close-packed colloidal monolayer and produce patchy particles with the desired uniformity without the complications of a prefabricated template or other special colloid layer preparation steps. In this paper we will discuss the results of swinging the monolayer substrate during the deposition. We explore the effect it has on the shape of the patches on the monolayer using both a predictive calculation and experimental verification. We also examine the influence the added swing
Cmn =
∑i (Tm(λi) − Tm)(Tn(λi) − Tn) ∑i (Tm(λi) − Tm)2 ∑i (Tn(λi) − Tn)2
(1)
where Tm and Tn are the mean values from the respective spectra. The value of Cmn ranges from 0 to ±1, and it provides a measure of the relationship between each pair of spectra: if two spectra are very similar, Cmn approaches 1; when the two spectra are very different, the value of Cmn is near zero. The average, C̅ , and standard deviation, σc, of the 66 unique Cmn values in the upper triangular part of the output matrix were then calculated for each sample. Numerical Calculations of Patchy Deposition. In-house Matlab code, as detailed in a previous publication,9 was used to predict the amount and shape of Ag deposited on a single-domain 4970
DOI: 10.1021/acs.langmuir.6b00980 Langmuir 2016, 32, 4969−4974
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Langmuir colloid monolayer while accounting for the shadowing effects of neighboring spheres. In the current work, 500 nm diameter closepacked spheres and an incident deposition angle of θ = 80° were used in all of the calculations. Calculations were performed varying the azimuthal swing angle from Δφ = 0° to Δφ = 360° at intervals of every 10° with a set of 12 distinct domains (φ0 = 0° to φ0 = 55° at an interval of 5°) for each Δφ. Because of the 6-fold rotational symmetry of the close-packed spheres, this covers the range of possible unique domains. To simulate the swing effect, the angle of the domain with respect to the deposition direction, φ, was rotated every 1° from φ = φ0 − Δφ/2 to φ = φ0 + Δφ/2 while aggregating the predicted Ag deposition on the surface of the nanospheres. To quantitatively compare the morphology of the predicted patches, the data from a single sphere from each patch calculation was used for two separate analyses. The first analysis involved the projected coverage of the patch over the upper half of the sphere. An intensity threshold was used to identify the patch boundary from a top-view image of the sphere (see Supporting Information). The coverage was calculated as the percentage of pixels from the sphere that were within the patch area. For each Δφ value, a standard deviation of the coverage values for the full set of associated φ0 was then calculated to provide a value for the variation based on swing angle. For the second analysis Matlab’s CORR2 function was used to calculate the correlation coefficient, C, between every pair of spheres from different φ0 but with the same Δφ. CORR2 calculates a two-dimensional Pearson’s correlation coefficient. For two m × n matrices, A(m,n) and B(m,n), the coefficient is calculated by
CAB =
30°. When there is no azimuthal swinging motion applied (i.e., Δφ = 0°), the shapes of the deposited material are very different for the separate domains. For φ0 = 0° the patch has a rectangle-like shape, whereas for φ0 = 15° it has more of a triangular projection, and for φ0 = 30° the patch is trapezoidal. The morphology of the patches changes significantly when a swinging motion is added. While the area of the sphere which is covered by a patch is nearly the same for all φ0 when Δφ = 60°, there is still significant difference in the shape. The patch for φ0 = 0° has an indentation in the middle and the sides are distended, while φ0 = 15° has the indentation shifted to one side, and φ0 = 30° has the sides trimmed. When the swing angle was increased to Δφ = 120°, the shapes from different domains become more similar, with the differences consisting just of some indentations and protuberances on the outer edge of the deposited material. With a swing angle of Δφ = 240°, the visible hemisphere of the beads are nearly completely covered, but the incomplete deposition on the side opposite from the flux does cause the shapes to differ slightly from one domain to another. When the swing angle is a complete rotation (Δφ = 360°, not pictured), the hexagonal shape of material on the spheres is identical for all domains, though with an orientation still reflecting the domain orientation. This trend of increasing similarity in shape with increased swing angle is quantitatively described by the average correlation coefficient, C̅ , of the patch morphology versus Δφ as shown in Figure 4 (black squares). This coefficient is a
∑m ∑n (A(m , n) − A̅ )(B(m , n) − B̅ ) ∑m ∑n (A(m , n) − A̅ )2 ∑m ∑n (B(m , n) − B̅ )2
(2) where A̅ and B̅ are the mean values of all the elements in the respective matrix. The in-house program saved the amount of predicted Ag on the surface of each sphere in a 360 × 180 element matrix (resolution of 1° in both latitude and longitude). The average correlation, C̅ , and standard deviation, σc, from all the unique C values for a given Δφ were then calculated.
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RESULTS AND DISCUSSION Morphology of Patches. Representative top-view images of the predicted Ag patches on the nanospheres for different initial monolayer domain angles (φ0) and swing angles (Δφ) are shown in Figure 3. The rotational symmetry of the closepacked spheres results in mirror images in the shape of the patches around φ0 = 30°; thus, we only discuss cases with φ0 ≤
Figure 4. Plots of the correlation coefficient versus swing angle based on the morphology of the calculated deposited shape (black) and based on measured transmission spectra (red). The similarity of the two confirms both the increase in uniformity of the samples with increased swing angle and the strong relationship of the plasmonic response to the material’s shape.
calculation of how well the shapes from different domains match to one another. As shown in Figure 4, C̅ increases nearly monotonically and approaching 1 with Δφ. As the swing angles increase from Δφ = 0° to Δφ = 60°, the correlation increases significantly from C̅ = 0.774 to C̅ = 0.951 due to the symmetry of the hexagonal close-packed structure of the colloids. After Δφ = 90° the correlation coefficient value only increases from C̅ = 0.984 through C̅ = 0.990 at Δφ = 120°, until it reaches the maximum with a value of C̅ = 0.999 at Δφ = 360°. There is a small dip in the correlation around the swing angle Δφ = 300° (C̅ = 0.995), where it drops from the local maximum of C̅ = 0.996 at Δφ = 270°. Still, for all Δφ ≥ 90° the correlation coefficient indicates that the shape of the deposited patchy particles are very uniform across different domains.
Figure 3. Images of the predicted morphology for depositions under different initial domain orientation (φ0) and azimuthal swing angle (Δφ) conditions. It can be seen that the shapes from different domains become more uniform with increased Δφ. 4971
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Figure 5. Graphs of the coverage area of the patch over the top hemisphere of the spheres calculated from the predicted morphology. (a) is the coverage percentage as a function of the swing angle (Δφ) for the initial domains φ0 = 0°, 5°, 10°, 15°, 20°, 25°, and 30°. (b) shows the variation between the coverage from these domains as a function of Δφ.
Initially, due to the 6-fold rotational symmetry of the closepacked spheres, it was hypothesized that the deposited material would have the same shape for every domain when a swing angle of Δφ = 60° was used. While this was not observed, as noted in the preceding discussions, in calculating the projected coverage area of the predicted patches on the top hemisphere of the colloid, there was an obvious effect related to this symmetry. This can be seen in the graphs of Figure 5. Figure 5a is a plot of the coverage versus Δφ for φ0 in the range of 0°− 30°, and Figure 5b plots the variation in the coverage between the different φ0. When the swing angle is an integer multiple of 60° (i.e., Δφ = n × 60°, n = 1, 2, 3, 4, 5, 6), the percent coverage for patches from different domains is nearly the same, as seen by the intersection of the lines in Figure 5a and the minima values in Figure 5b. While for Δφ ≠ n × 60° the variation of the coverage between different domains is larger, reaching maximal variation when Δφ = (2n + 1) × 30° (n = 0, 1, 2, 3, 4, 5). There is one exception in the data where the variation reaches a minimum at Δφ = 250° instead of 240°. This point coincides with the slope of the average coverage quickly beginning to plateau and thus is likely due to the coverage beginning to saturate beyond this swing angle which is why above Δφ = 240° the variation stays lower than it had been at any previous point. An explanation for the two preceding phenomena, the increasing agreement in shape and oscillating agreement in coverage, is depicted in Figure 6. The first four hexagonal rings of spheres in a close-packed arrangement are shown. We will consider the patch on the center sphere in the following
discussion, but any sphere within a domain can be viewed as being this center sphere. With a high incident angle, the shadowing of the spheres closest to the center has the greatest effect on the shape of the deposited patches. In the figure, spheres of the same color are at the same distance radially from the center sphere and thus have the same shadowing effect on it. The nonshaded areas represent the regions which will have a shadowing effect on the center point when swung by an angle of ±Δφ/2. Figure 6a depicts three domains (φ0 = 0°, φ0 = 30°, and φ0 = 45°) when Δφ = 60° (n = 1), while Figure 6b depicts two domains (φ0 = 0° and φ0 = 30°) for Δφ = 120° (n = 2). In either case, if one were to count the number of spheres of any color in one nonshaded region (adding together partial spheres) and compare it with the number of spheres of the same color in another domain with the same swing angle, they would be equal. For example, looking at the outer ring in Figure 6a, there is 1 purple sphere with light shading, 2 medium, and 1 dark in each domain; though, when φ0 = 0°, the lightest sphere is subdivided at the boundaries, and when φ0 = 30°, the darkest is. This consistency in the number of spheres is what results in the coverage being the same for domains when Δφ = n × 60°. It is obvious that if Δφ were slightly larger or smaller in either (i.e., Δφ ≠ n × 60°), the number of spheres of each color would not be the same between domains, and thus the coverage would differ. While the number is consistent between different domains in the figure, when viewing them from one boundary to the other, the order in which a sphere of a particular color is encountered changes. Looking again at the outer ring in Figure 6a and rotating counterclockwise, for example, when φ0 = 0°, the order is 1/2 light, medium, dark, medium, and then 1/2 light; but when φ0 = 30°, the order is 1/2 dark, medium, light, medium, and 1/2 dark. This phase difference in the order is what causes the variation in the shape of patches from different domains. Yet due to the rotational symmetry in the closepacked pattern, as Δφ increases, the phase difference is overcome by repetitions in the order. For example, rotating counterclockwise in Figure 6b, the sequence in spheres of medium shade, dark, medium, light, and medium appears in both domains. As Δφ continues to increase, the overlapping sequences between domains increase, resulting in the observed nearly monotonic increase in the correlation coefficient. All analysis that we have discussed up to this point was performed using the predicted results of the patches. In order to confirm how well these results correspond with actual depositions, representative top-view SEM images of different domains (φ0) and swing angles (Δφ) were taken. Figure 7 presents results from similar domains and swing angles as those
Figure 6. Considering a deposition direction that lines up with one of the depicted φ0, the nonshaded region reveals the area of neighboring beads that could have a shadowing effect on the center when rotating over the range of φ0 − Δφ/2 to φ0 + Δφ/2. (a) illustrates the case when Δφ = 60° and (b) illustrates Δφ = 120°. 4972
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Figure 7. SEM images of Ag deposited on 500 nm PS bead monolayers for different sweep angles (Δφ) and initial domain orientations (φ0). All scale bars represent 500 nm. Insets show single bead from calculations for comparison. Figure 8. Transmission spectra from samples with different swing angles. (a) Δφ = 0°, (b) Δφ = 60°, (c) Δφ = 120°, and (d) Δφ = 240°. The black solid lines represent local spectra from 12 locations on each sample. The orange solid line is the average of these 12 local spectra, and the red dotted line is the spectrum measured over a larger area which covers multiple domains.
presented for the predicted patches in Figure 3. When the samples were removed from the deposition chamber, the deposition direction was marked so that each domain orientation can be properly determined in successive measurements. The SEM images reveal patches that are very similar to those predicted in the calculations for each corresponding φ0 and Δφ. Under visual inspection between Figures 3 and 7 one observes that the overall coverage, shape, and variation between different domains of the actual depositions are captured in the predictions. Some slight deviations include the edges of the deposited patches not being as clearly defined, and there being small extra Ag nanoparticles in regions that are completely blank in the predicted samples. Yet, due to the similarity between the actual and predicted patches, additional calculations of the correlation coefficient were not performed from the SEM images. Instead, the optical properties of the deposited patches were used to investigate the uniformity of the samples. Optical Properties. Because of the sensitive dependence that LSPR exhibits on the shape and size of nanoparticles, it is expected that when the patchy particles vary significantly (due to the domain effects) that the optical properties of the material will similarly exhibit large variations. The small area measured by the microscopic UV−vis setup enables the observation and analysis of these variations. In Figure 8, the black curves are the transmission spectra measured from 12 random spots on samples that were prepared using four different swing angles, and the orange curve is the average of the 12 spectra. All of the samples show a valley around λ = 580 nm and peaks at around both λ = 500 and 650 nm. The spectra from the Δφ = 0° and Δφ = 60° samples show multipeaks at these wavelengths as well as an additional peak around λ = 750 nm that turns into more of a shoulder for Δφ = 120° and Δφ = 240°. The variation between spectra from the same sample is quite significant for swing angles of Δφ = 0° and Δφ = 60°, particularly around the peaks and in the near-infrared wavelength range. However, as the swing angle increases to Δφ = 120°, the variation decreases significantly and is nearly eliminated when Δφ = 240°. The calculation of the average correlation coefficient, C̅ , between the spectra from a single sample quantifies this variation well. When plotted together, the correlation coefficients from the transmission spectra (red circles) match well with those obtained from the morphological calculations as can be seen in Figure 4. The correlation increases quite rapidly from C̅ = 0.765 at Δφ = 0° until C̅ ≥
0.987 at Δφ ≥ 90°. This provides more evidence that with a swing angle Δφ ≥ 90° the Ag patches are very uniform, even from different domains. We can further investigate the plasmonic effects of the various domains by comparing the local transmission spectra collected with the microscopic UV−vis system with the spectrum taken by the Jasco system, which has a larger beam area and is shown as the red dashed curves in Figure 8. Because of this larger area, a single spectrum covers many different domains on the samples as well as the grain boundaries that separate them. As expected, this spectrum is a combination of the local spectra, being similar to their average in each case: the spectrum from the larger beam has a similar shape, with the same significant dips and peaks and the overall transmission percentage matching well for each sample. Some of the differences in the spectra are likely due to the grain boundaries of the monolayer. The beads affected by these boundaries are statistically few (
