Article pubs.acs.org/JPCC
Thermal Stability of Gold Nanorods for High-Temperature Plasmonic Sensing Nicholas A. Joy,† Brian K. Janiszewski,† Steven Novak,† Timothy W. Johnson,‡ Sang-Hyun Oh,‡ Ananthan Raghunathan,† John Hartley,† and Michael A. Carpenter*,† †
College of Nanoscale Science and Engineering, University at Albany−State University of New York, 257 Fuller Road, Albany, New York 12203, United States ‡ Department of Electrical and Computer Engineering, University of Minnesota, Minneapolis, Minnesota 55455, United States S Supporting Information *
ABSTRACT: There are many potential sensing applications for Au nanorods due to a tunable localized surface plasmon resonance (LSPR) frequency that changes with aspect ratio. However, their application at high temperatures is limited due to a shape change that can take place well below the melting point of bulk Au, driven by a reduction in surface energy. A method of stabilizing Au nanorods is provided here by encapsulating them with a 15 nm capping layer of yttria stabilized zirconia (YSZ). After annealing rods with nominal dimensions of 100 × 44 nm to a temperature of 600 °C, small reductions in length were observed, but the rods remained stable for all subsequent sensing tests at 500 °C, which amounted to 80 h. It was shown with a separate sample that the rod geometry can be preserved even up to 800 °C over a 12 h annealing period, although a significant shortening of the rod length occurred, leaving a void space in the YSZ. The sensing response of both the transverse and the longitudinal LSPR peaks was monitored for H2, CO, and NO2 exposures in an air background at 500 °C. In all cases, the longitudinal LSPR peak shows a larger shift upon gas exposure than does the transverse peak.
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the AuNPs themselves18,19 because previous work has shown that the catalytic activity of AuNPs depends on the particle size,20,21 shape,22,23 and exposed facets.24 These factors provide interesting areas for research due to the ease of which they can be controlled. For example, a nanorod shows both transverse and longitudinal LSPR peaks, the latter of which for Au nanorods (AuNR) can be tuned from the visible to IR wavelengths simply by changing the size and aspect ratio of the rod. Besides providing different sensing regimes, there is an aspect-ratio dependence on the longitudinal peak shift to changes in the surrounding dielectric environment,25 which may provide an opportunity to distinguish between analytes that are detected on the basis of different mechanisms (e.g., charge transfer versus dielectric constant). In addition, the size and shape dependencies on the catalytic properties of Au may favor some reactions over others and therefore provide a means of improving selectivity of the response. This may be beneficial in sensor arrays where the different array elements could show differences in the response to the same exposure conditions. A challenge arises when putting this idea into practice for hightemperature applications as AuNRs will change shape to minimize their surface energy even at temperatures less than one-fourth the melting point of bulk Au.26 The approach described here is to encapsulate the AuNRs with YSZ prior to annealing at high temperatures, and in general encapsulation in
INTRODUCTION Plasmonic sensing is a well-known method of optical detection based on the resonant oscillation of conduction-band electrons that can respond to changes in the surrounding dielectric environment, temperature, adsorbates, and charge density of the metal.1−4 Of the many potential plasmonic sensing applications, one target area of interest for this work is for high-temperature emissions sensing of combustion environments as there are limited industrially available sensors. For this type of harsh environment application, gold is the metal of choice, not only for its plasmonic properties, but also because of its resistance to oxidation even at high temperatures. A common approach in this case is to use gold nanoparticles (AuNPs) because they show localized surface plasmon resonance (LSPR) in the visible range and there is no angle dependence of the incident light for spherical particles as there is with other techniques such as the Kretschmann configuration. Stability of the AuNPs at high temperature has been demonstrated by embedding them in a metal oxide matrix, even after hundreds of hours of gas exposures at 500 °C.5,6 Hightemperature plasmonic sensing has already been demonstrated for gases such as H2, CO, NO2, H2S, and CH4.7−13 The focus of much of the work in this area has been to improve the selective sensing response, either through the use of different oxide matrix materials such as CeO2,14 yttria stabilized zirconia (YSZ), TiO2,15 NiO−SiO2,16 TiO2−NiO,17 or BaO,11 or through data analysis using multivariate techniques like principal component analysis or linear discriminant analysis.14,15 An additional area of investigation is in the effect of © XXXX American Chemical Society
Received: January 18, 2013 Revised: May 6, 2013
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on a Vistec VB300 Gaussian EBL tool operating at 100 kV. The beam current was set to 5 nA, which yields a theoretical spot size of approximately 7 nm. The exposure dose was set to 1100 μC/cm2 with a beam step size (pixel size) of 5 nm. After the exposure, the Cr layer was chemically etched away, and the PMMA was developed using methyl isobutyl ketone and isopropyl alcohol (1:3). Next, 6 nm of Ti followed by 20 nm of Au was deposited by electron-beam evaporation without breaking vacuum between the two depositions. Lift-off of the remaining PMMA was done in boiling acetone leaving the Ti/ Au rods on the YSZ surface. A final 15 nm capping layer of YSZ was deposited by PVD using the same deposition conditions as described for the YSZ underlayer. Modeling of the transverse and longitudinal absorbance peaks has been performed by the finite-difference time-domain (FDTD) method using full 3-D FDTD simulations to verify the origin of the resonances. The simulations were done with a commercial software package (Fullwave, RSoft Design Group) and contained two nanorods with periodic boundary conditions to simulate an infinite array. A pulsed wave was incident from above the nanorods, and transmitted power was collected below the rods and then Fourier transformed to obtain the transmission spectra. Simulations were done with both TE- and TM-polarized light, and the results were averaged to simulate unpolarized light. Incremental changes in both the length and the width of the rods were made and correlated to shifts in peaks of the spectra. Grid sizes of 1 nm were used in the inplane dimensions, while a grid size of 10 nm graded down to 1 nm at the nanorod was used in the dimension perpendicular to the substrate. The optical constants used for Au were originally from Palik and then fit to a Lorentz/Drude model by Rakic.38 Sensing tests were performed by exposing the sample to H2, CO, and NO2 at a temperature of 500 °C in a background of dry air. Exposures took place in a quartz flowtube furnace for 1 h intervals with a constant total flow rate of 2000 sccm. A quartz tungsten halogen light illuminated the sample with unpolarized white light, and a lens was used to focus an image of the sample onto the 10 μm entrance slit of an Oriel Instruments MS257 imaging spectrograph. The spectrum was collected with a Peltier cooled CCD detector yielding an overall spectral resolution of ∼0.7 nm. The LSPR peak positions served as the sensing signal and were found by fitting a Lorentzian function to each of the two peaks.
other metal oxides of small grain sizes should work equally as well. This method is shown to stabilize the rod geometry up to 600 °C at atmospheric pressures, and even up to 800 °C with a change in length but still maintaining rod geometry. There have been theoretical studies on the thermal stability of AuNRs,27−29 and a number of papers discussing the stability of AuNRs exposed to femto- to nanosecond laser pulses,26,30−33 but relatively few papers on the prolonged thermal stability of encapsulated AuNRs. Of these, the most relevant to this work examined the use of a titanate shell to stabilize chemically synthesized single-crystalline AuNRs for detection of H2 at 300 °C.34 From annealing tests, it was reported that UV-cured titanate increased the thermal stability of the rod geometry to 400 °C, which was suggested to be due to the densification of the titanate shell after UV treatment. Another sensing-based study concerning titania coated AuNRs reported thermal stability throughout a 1 h anneal at 300 °C. These samples were exposed to various liquid solvents at ambient temperatures and showed a shift in the plasmon peaks that were characteristic of a change in the surrounding dielectric function, but reversible sensing tests were not performed under hightemperature conditions.35 A separate work compared the stability of Ag and AuNRs capped with a thin carbon shell and heated under vacuum. In the vacuum environment, carboncoated AuNRs were stable to at least 670 °C, while uncoated AuNRs completely changed shape within hours at 500 °C.36 AuNRs have also been stabilized with surface ligands while encapsulated in polymers such as poly (methyl methacrylate) PMMA;37 however, shape change occurred as the temperature was increased above the glass transition temperature of the PMMA. In all related work that could be found in the literature, wet chemistry synthesis methods were used to produce the AuNRs.26,30−37 In the present study, they were made by an electron-beam lithography (EBL) process, allowing for precise control and uniformity of the rod dimensions, shapes, and interparticle spacings. One difference with this process is that a thin adhesion layer is commonly used (in this case Ti) to promote Au adhesion during the lift-off step. Also, the asdeposited rods are not single-crystalline like chemically synthesized rods. However, EBL allows for a uniform rod distribution on the surface, which avoids issues due to clustering such as plasmon interactions between closely spaced particles. Furthermore, removal of surface stabilizing chemicals and those surface species used to control the interparticle spacings of the deposited particles, prior to encapsulation in metal oxides, is not necessary.
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RESULTS AND DISCUSSION A cross section depicting the final structure is shown in Figure 1. The nominal rod dimensions used in this study were 100 × 44 nm, chosen so the longitudinal LSPR peaks would be well within the limits of the spectrometer. The structure shown in Figure 1 was annealed in an Ar environment while monitoring the absorbance spectrum in situ during which time the grains within the polycrystalline YSZ capping layer and the AuNRs grow. Because the refractive index of the YSZ likely changes during annealing, shifts in the LSPR absorbance peaks include effects of both dielectric and geometry changes. For this reason, ex situ analysis was performed using environmental scanning electron microscopy (ESEM), and the annealing was performed in 100 °C steps, from 300 to 600 °C, with 6 h at each temperature, cooling to room temperature after each step. Figure 2a shows the room temperature series of absorbance spectra following each annealing step.
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EXPERIMENTAL METHODS The fabrication procedure is outlined as follows. A quartz substrate was cleaned by sonicating in acetone for several minutes and blown dry. A 65 nm base layer of YSZ was then deposited by RF magnetron physical vapor deposition (PVD) using a 99.9% pure YSZ (5 wt % Y2O3) target in an ∼2 mTorr Ar process pressure. The sample was then annealed at 970 °C for 3 h in Ar. PMMA 495 A2 was used as the positive electronbeam resist and spin coated on the YSZ base layer at 2000 rpm for 60 s. The sample was then baked on a hot plate at 180 °C for 60 s resulting in a resist thickness of approximately 70 nm. This was followed by deposition of a 20 nm Cr layer using electron-beam evaporation. Because quartz is nonconducting, the purpose of the Cr layer was to eliminate charging during the EBL write process. Patterning of the rod features was done B
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spacing of the AuNRs because it appears at the same spot on multiple spectra. The absorbance spectrum from FDTD modeling is compared to the sample spectrum in Figure 3. To match
Figure 1. Cross-sectional depiction through the short axis of a Ti/Au nanorod encapsulated in YSZ.
Both the transverse and the longitudinal peak positions redshift as a result of the first anneal at 300 °C, while a blue-shift takes place for each subsequent anneal. The reason for this is not the focus of this Article, but it is likely due to opposing factors, for example, densification of the YSZ, which would increase the refractive index and lead to red-shift, decreasing aspect ratio of the AuNRs, which would lead to a blue-shift in the longitudinal peak, and a decrease in the effective dielectric constant around the AuNRs due to separation of the Au−YSZ interface (as will be discussed later), which would result in a blue-shift. The peak widths, which are related to dampening of the surface plasmon, consistently decrease with each annealing step. This may be related to the number of defects and dangling bonds in the YSZ at the Au interface, as well as the small Au grain sizes within the unannealed polycrystalline AuNR.39 Also, as is discussed later, it is thought that the Ti adhesion layer, which is present in the unannealed AuNRs, diffuses away during the annealing steps, and so damping caused by Ti should decrease, resulting in a reduced peak width. To test the effect of the adhesion layer on the line width, FDTD simulations were performed with and without the 6 nm Ti adhesion layer. The results are shown in Figure 2b. It can be seen that the addition of the 6 nm Ti layer increases the line width and lowers the absorbance. This is similar to what is seen experimentally. Finally, the small kink in the spectra at ∼950 nm is thought to be due to diffraction of light off the regular
Figure 3. Experimental and simulated absorbance spectra from the array of AuNRs. The experimental spectrum was taken at room temperature after the final 600 °C anneal, but before the start of gas exposures.
both the transverse and the longitudinal peak positions, the rod width was set to 50 nm (estimated from postanneal ESEM), and the length was adjusted to 85 nm. It was also necessary to decrease the refractive index of the YSZ to n = 1.53, which serves as an approximation for the effective dielectric constant immediately surrounding the nanorods. ESEM images in Figure 4a and b show the AuNRs after liftoff prior to capping with YSZ, and after YSZ capping and the final 600 °C anneal, respectively. Some edge roughness around the perimeter of the rods can be seen after the lift-off step in Figure 4a. This is due to tearing of a continuous metal film around the perimeter of the rods during the lift-off process. In Figure 4b, even though the rods are capped with YSZ, secondary electron contrast differences can still be seen from Au underneath the capping layer when using a high enough electron accelerating voltage. After annealing to 600 °C, some of the rods started to shorten, leaving a void under the YSZ cap as indicated by the arrows in Figure 4b. Shortening of the rods
Figure 2. Absorbance spectra of the AuNRs, (a) experimentally measured at room temperature after annealing at each of the listed temperatures and (b) simulated with and without a 6 nm adhesion layer below each AuNR. C
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Figure 4. Plan view ESEM images of the AuNRs, (a) prior to capping with YSZ or annealing, and (b) after capping with YSZ and annealing to 600 °C. Arrows point to spots where the Au has separated from the YSZ capping layer as a result of annealing.
Figure 5. 300 × 30 nm rods after annealing to 800 °C. (a) Top view where a shortening of the rods is seen from the contrast of Au under the YSZ capping layer. (b) Cross section at 52° off the surface normal showing the Au and void space surrounded by YSZ. Pt was deposited to make a clean cut with a Ga focused ion beam.
would cause a blue-shift in the longitudinal peak position, but another important factor is the creation of a new Au interface with a much lower refractive index than that of YSZ. This reduction in the effective dielectric environment around the AuNRs would cause a blue-shift in both peaks, with the longitudinal peak shifted more than the transverse. To confirm that a shortening of the rods leads to the formation of voids, a separate study was done using 300 × 30 nm rods with the same cross-section as shown in Figure 1 except having 30 nm of YSZ for both the base and the capping layer on a Si substrate. The sample was annealed in a manner similar to that already described, but to a maximum temperature of 800 °C with 12 h at each step. Figure 5 shows a representative top view and cross-section through the long axis of a rod that clearly shows a significant void space where the gold was contained prior to annealing. In our studies, nanorods without a metal oxide overcoat layer are not thermally stable at 500 °C, while a thicker overcoat layer appears to increase the maximum temperature achieved prior to large changes in the nanorod. Optimal thicknesses for specific sensing or other thermally elevated applications will be the focus of future studies.
The question that Figures 4 and 5 raise is: What happened to the gold that filled the void space prior to annealing? Because there is no obvious increase in the short axes of the rods as seen in Figure 5, it seems there is a reduction of volume in the AuNR as a result of annealing. One possible explanation for this is a loss of the Ti layer due to diffusion. From secondary ion mass spectroscopy (SIMS) depth profiling, which is available in the Supporting Information, it was found that Ti readily diffuses through the Au, even at temperatures as low as 150 °C, which is supported by previous studies.40 X-ray photoelectron spectroscopy (data not shown) has also indicated that the Ti is in the form of an oxide. Thus, in the limiting scenario that all of the Ti diffuses to the boundaries of the nanorod forming a mixed oxide with YSZ, the loss of the Ti layer would represent a loss of 23% ± 3% of the original volume of the nanorod. Measurement of the reduced lengths of AuNRs similar to those in Figure 5a represents a 39% ± 4% volume loss assuming only the length changes, which is more than can be accounted for by the Ti alone. However, as the drive to reduce surface energy causes the rod to shorten, it may also push out against the YSZ causing the shape to bulge. Supposing the volume of the Au in the rod is conserved while the volume of Ti in the rod is lost, an D
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Figure 6. Exposure plots showing the change in the transverse and longitudinal peak positions relative to the start of the exposures for (a) H2, (b) CO, and (c) NO2. 8-point adjacent averaging was used to smooth the NO2 transverse peak trace due to the low signal-to-noise ratio.
peak shift in both cases. Although there is no striking difference in the response between the reducing gases H2 and CO in this case, it is thought that selectivity and/or sensitivity may be enhanced through further investigation of the particle geometry, crystallographic orientation of the Au, and arrangement of the plasmonic structures. The rods were quite stable throughout 80 h of testing at 500 °C. Figure 8 shows a comparison of the absorbance spectra at 500 °C before and after each of the gas exposure tests. The longitudinal peak position shifted only a few nanometers over the course of the experiments, and the shift was toward longer wavelengths, opposite the trend during annealing. While the longitudinal peak red-shifted, the transverse LSPR peak blueshifted by about 6 nm. This is an interesting result, although the cause is not yet known.
expansion of about 3.8 nm in both the width and the height of the original dimensions could explain the reduction of the length as seen. Although a change of this magnitude is too small to confirm with SEM, it is thought that both loss of Ti and bulging of the Au contribute to the shortening of the AuNRs in the long axis. The AuNRs in Figure 4b were used for preliminary sensing tests by monitoring changes in both the transverse and the longitudinal LSPR peak positions during gas exposures in a background of dry air at 500 °C. Figure 6a−c shows the peak response as a function of time during gas exposures to H2, CO, and NO2, respectively, for the gas concentrations listed in the figures. Two replications were performed for each of the five concentrations to show repeatability. Baseline correction was performed on all traces in Figure 6 to eliminate drift in the response and allow an overlaid comparison between the transverse and longitudinal modes. In each case, the longitudinal LSPR peak shows a greater change than the transverse peak for each analyte exposure. Comparisons of peak shifts upon gas exposure for the individual concentrations are best seen in the calibration curves shown in Figure 7. Exposure to NO2 itself was unique because the transverse peak showed a very minimal response while the longitudinal peak had a clear signal change. The reason for this is still a matter of investigation as previous Au−YSZ nanocomposite films have shown response to NO2 down to the low ppm levels,10 which was also expected for the transverse peak response. For H2 and CO exposures, a clear response was seen from both peaks, while the longitudinal peak again showed a larger
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CONCLUSIONS
Encapsulation of EBL-patterned gold nanorods with YSZ has been shown to stabilize the rod geometry at high temperatures. At 600 °C, most of the 100 × 44 nm rods maintain their original geometry, but a significant percentage begin to show small reductions in length. Annealing higher aspect ratio, 300 × 30 nm rods, up to 800 °C showed a significant reduction in length, but the rod geometry was still maintained. It is thought that the reduction in length is due to (1) a loss of a portion of the Ti adhesion layer due to diffusion followed by reshaping of the Au to fill the enclosed volume with reduced surface area, and (2) a small amount of bulging in the shorter axes of the rod. Preliminary sensing tests at 500 °C show a response of E
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Figure 7. Calibration curves showing the shift in LSPR peak position for both transverse and longitudinal LSPR peaks for different concentrations of (a) H2, (b) CO, and (c) NO2.
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ASSOCIATED CONTENT
S Supporting Information *
SIMS depth profiling text and plots. This material is available free of charge via the Internet at http://pubs.acs.org.
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS We gratefully acknowledge the help of Dr. Tom Murray with FIB cross-sectioning. This work was supported by the United States Department of Energy National Energy Technology Laboratory under contract number DE-FE0007190 as well as the National Science Foundation [PN: 1006399]. Any opinions, findings, and conclusions or recommendations expressed in this publication are those of the authors and do not necessarily reflect the views of the United States Department of Energy National Energy Technology Laboratory.
Figure 8. Absorbance spectra taken at 500 °C in air before and after each gas exposure test. The order of exposure tests was CO, H2, and then NO2.
both LSPR peaks to H2 and CO, while the response to NO2 is seen mainly in the longitudinal peak with only a very weak signal change in the transverse peak. After the initial annealing steps, the rods were stable throughout 80 h of gas exposures at a temperature of 500 °C in an air background at atmospheric pressure. This method of stabilizing nanopatterned features may be extended to other geometries that could provide new opportunities for plasmonic sensing utilizing structures that create enhanced electric field zones, which may benefit sensitivity and reactivity of the analytes.
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