Article pubs.acs.org/cm
Controllable Localized Surface Plasmonic Resonance Phenomena in Reduced Gold Oxide Films Yu-Lun Liu, Cheng-Yi Fang, Chen-Chieh Yu, Tai-Chi Yang, and Hsuen-Li Chen* Department of Materials Science and Engineering, National Taiwan University, No. 1, Sec. 4, Roosevelt Road, Taipei, 10617 Taiwan (R.O.C.) S Supporting Information *
ABSTRACT: In this study, we used reactive sputtering to prepare large-area, homogeneous gold oxide (AuOx) films, which we then subjected to reduction processes under ambient conditions at room temperature to obtain reduced AuOx films featuring embedded Au nanoparticles (NPs). We analyzed these films in terms of both their material characteristics and optical properties. For the first time, we obtained the optical constants of AuOx films directly deposited on glass and Si substrates. Moreover, we observed localized surface plasmon resonance (LSPR) phenomena from the visible to the near-infrared (NIR) during the formation of the Au NPs in the AuOx film. The plasmonic properties of the Au NPs embedded in the AuOx films were much different from those of chemically synthesized Au NPs. We found that the LSPR wavelength of the Au NPs embedded in the AuOx films could be tuned from 700 to 980 nm by varying the duration of the reduction process. Moreover, the reduced AuOx films could be applied as surface-enhanced Raman spectroscopy (SERS)-active substrates. The minimum detection was achieved down to a concentration of 10−10 M (R6G) when using the reduced AuOx film-based substrate−a level comparable with those of other kinds of SERS substrates prepared using more complicated methods. The rapid method proposed herein provides large-areas films with tunable resonance wavelengths for applications as SERS-active substrates in optical and chemical sensing.
■
shapes, densities, and surrounding media.13 Localized surface plasmon resonance (LSPR) on noble metal NPs featuring a high density of charge carriers has been exploited in optoelectronic devices, biosensing, photothermal therapy, and Raman scattering.14−16 Raman scattering is a practical label-free method for chemical and biological analysis. Although it can be a useful technology for sensing different specimens, the low inelastic scattering intensity can make detection very difficult. The LSPR of NPs plays an important role in surface-enhanced Raman scattering (SERS), where the Raman signals of molecules adsorbed to the NPs can be enhanced dramatically through strong near-field plasmon resonance.17,18 Moreover, tunability of the LSPR wavelength is attractive because different LSPR wavelengths are required for various laser sources in the visible and near-infrared (NIR) regimes.19 Generally, NIR laser sources are superior at suppressing the fluorescence of analyzed molecules when using near-infrared surface-enhanced Raman scattering (NIR-SERS), although the longer excitation wavelength decreases the efficiency of Raman scattering.20 To increase the Raman intensities of the NIR-SERS signals at longer excitation wavelengths, it is necessary to design LSPR structures in the NIR regime. Accordingly, a rapid, inexpensive strategy for preparing large-area SERS substrates that operate in the NIR regime would be very attractive. LSPR wavelengths
INTRODUCTION Although bulk gold (Au) is a noble metal that is barely reactive, nanosized Au has many applications that take advantage of its unique chemical and physical properties;1−3 for example, chemical stability and excellent electronic properties make nanosized Au useful in electronic devices, biosensors, electrodes, and catalysts.4−6 Recently, gold oxide (AuOx) has attracted attention for application as an interfacial layer material, particularly because its work function of approximately 5.5 eV is much higher than that of a pristine Au film (ca. 4.7 eV).7−9 A material with such a high work function can be used to modify the hole-injection barrier between the active layer and the transporting electrode in an organic device.8,9 In previous studies, AuOx films have been obtained through anodic oxidation and ultraviolet (UV) ozone and oxygen plasma treatments of the surfaces of pure Au substrates.7,10−12 Because unstable and very thin (ca. 0.2 nm) AuOx layers were formed on the surfaces of these pure Au substrates, it has been very difficult to characterize these AuOx materials and to observe their reduction behavior. Therefore, the development of reliable methods for the preparation of uniform and sufficiently thick AuOx films will be necessary if we are to further study and apply them in practical devices. Until now, only few investigations of the reduction behavior of AuOx films through both material and optical analyses have been demonstrated. The surface plasmon resonance (SPR) phenomena exhibited by metallic nanoparticles (NPs) are affected by their sizes, © 2014 American Chemical Society
Received: October 1, 2013 Revised: January 22, 2014 Published: February 5, 2014 1799
dx.doi.org/10.1021/cm403227w | Chem. Mater. 2014, 26, 1799−1806
Chemistry of Materials
Article
Figure 1. (a, b) Top-view and (c, d) cross-sectional SEM images of AuOx films prepared through reactive-sputtering deposition: (a, c) as-deposited AuOx film; (b, d) AuOx film after storing under ambient conditions for six months. The red circles in (d) highlight the Au NPs. Scale bars in (a−c), 100 nm; in (d), 50 nm. (e, f) EDX analysis of the chemical composition of (e) the as-deposited AuOx film and (f) the AuOx film after being stored under ambient conditions for six months.
characterization. In this study, we deposited the AuOx films using a reactive RF magnetron sputtering process. After their deposition, we stored the AuOx films at room temperature, exposed to atmospheric conditions. We then performed analyses of the AuOx films after various storage durations. To compare the surface morphologies of the AuOx films before and after reduction, we recorded SEM images of the as-deposited AuOx films and after their storage for six months. Figure 1a,c displays top-view and cross-sectional images, respectively, of an as-deposited AuOx film having a thickness of 60 nm; its surface was very flat. After six months of storage, many nanostructured particles were present on the surface of the AuOx film [Figure 1b]; the cross-section view of AuOx film [Figure 1d] reveals particles having a size of approximately 10 nm (marked by red circles). The presence of these NPs suggested that they had formed from the Au atoms of the AuOx film during the reducing process. Therefore, we used EDX spectroscopy to characterize the AuOx films. Figure 1e and 1f display the measured EDX spectra of the as-deposited AuOx film and of the film after six months of storage. For the as-deposited AuOx film, in addition to the signals of the Au and Si atoms, we observe a strong signal for O atoms, confirming the oxidation of the Au atoms. In Figure 1f we observe that some of the O atoms had escaped from the AuOx film after six months of storage; in other words, some of the AuOx species had reduced to form Au NPs that had become embedded in the AuOx film.
depend mainly on the shapes of the metal NPs and on the nature of the surrounding dielectric media.13 Irregularly shaped metal NPs prepared through chemical reactions can induce LSPR in longer wavelength regimes, but the particle density can be difficult to control through solution processes.21−23 In this study, we prepared AuOx thin films using reactive sputtering, which could deposit large-area and homogeneous AuOx films. We then conducted detailed optical and materials analyses of the as-deposited and reduced AuOx films. We investigated the optical properties of the AuOx films to determine their optical constants (refractive indices, extinction coefficients) from the visible to the NIR regime. We also experimentally and computationally investigated the special LSPR phenomena that occur when an AuOx film is gradually reduced to form Au NPs that are closely embedded within or on the surface of the AuOx film. We found that the Au NPs could be formed on the reduced AuOx films with variable LSPR wavelengths that could yield very high SERS activities. The embedded Au NPs also generated many hot spots and resonated in the visible and NIR regimes, very useful properties for SERS applications.
■
RESULTS AND DISCUSSION
According to previous studies, Au cannot be oxidized readily and AuOx films are not stable at room temperature.24,25 For precise observation of the reduction of AuOx, we aim to prepare films having sufficient thicknesses for material 1800
dx.doi.org/10.1021/cm403227w | Chem. Mater. 2014, 26, 1799−1806
Chemistry of Materials
Article
According to the literature, 25−28 the gold oxide is thermodynamically unstable because of the positive heat of formation (+19.3 kJ/mol). Therefore, the gold oxide is unstable at room temperature. For natural reduction in ambient, the reduction mechanism is associated with the decomposition of the gold oxide. When the oxygen atoms are directly desorbed from gold oxide film, atomic oxygen will recombine with another oxygen atom to form the molecular oxygen (O2) and then escape from the surface of the gold oxide film. To characterize the material structures and reduction behavior of the AuOx films, we used TEM to observe the AuOx film after six months of storage. Figure 2a,b displays the
diffraction result is similar to the process of forming Ag nanostructures through reduction of AgOx films.29,30 Figure 2c provides a schematic representation of the reduction of a AuOx thin film to form a Au NP-embedded AuOx film. The Au NPs presumably resulted from phase separation of the reduced AuOx film. We found that the size and density of the Au NPs increased upon increasing the reduction time; meanwhile, the thickness of the AuOx film decreased. The resulting Au NPs were distributed uniformly over the whole area of the AuOx film. Figure 3 displays XPS spectra of a pure Au film, an asdeposited AuOx film, and a AuOx film after six months of
Figure 3. (a) Au 4f and (b) O 1s XPS spectra of a pure Au film, an asdeposited AuOx film, and a AuOx film that had been reduced under ambient conditions for six months.
storage. Figure 3a displays the Au 4f spectra obtained from the three different samples. The as-deposited AuOx sample provided two clear peaks at binding energies of 85.8 and 89.5 eV, which are very different from the Au 4f7/2 signal at 84 eV and the 4f5/2 signal at 87.6 eV of the pure Au film. These two clear peaks of the as-deposited AuOx sample indicate the characteristic behavior of the Au3+ state; previous studies have found similar peaks in Au surfaces oxidized through UV/ozone treatment.31,32 In contrast with the previous reports, however, the as-deposited AuOx films in this study exhibited much stronger shifts in the peaks relative to the original Au 4f peaks. Our measured results indicate that a large amount of AuOx formed in the as-deposited sample; thus, the oxygen plasma interacted strongly with the Au atoms during the reactive sputtering process. After storing for six months, we found that the positions of these two signals shifted to lower values and the original Au peaks appeared. Figure 3b displays the O 1s spectra obtained from the three different samples. For the asdeposited AuOx sample, we found that the Au atoms were obviously oxidized, with a strong peak at 530.4 eV representing the O 1s signal of AuOx.32 After six months of storage, the intensity of the O 1s peak decreased. This result is consistent
Figure 2. (a) TEM image of a AuOx film that had been reduced under ambient conditions for six months; inset: enlarged image. (b) HRTEM image and SAED pattern of the reduced AuOx film; the analysis region is indicated by the white circle in the inset to (a). (c) Schematic representation of the formation of Au NPs on the surface of a AuOx film during the reduction process.
TEM and HR-TEM images of a AuOx film after six months of storage. From Figures 1c,d and 2a, we find that the thickness of the AuOx film decreased from 60 to 33 nm after performing the reduction process for six months. The high-magnitude TEM image in the inset to Figure 2a reveals NPs embedded within the AuOx film. The regions with a relatively high image contrast of the NPs indicate where they might contain a large amount of Au atoms. Moreover, we also characterized the Au NPs using HR-TEM [Figure 2b]; the lattice spacing from the electron diffraction fringes (ca. 0.286 nm) matches the (111) interspacing of face-centered cubic of Au. The selected-area electron diffraction (SAED) pattern of the Au NPs [inset to Figure 2b] confirmed their polycrystalline structure. This 1801
dx.doi.org/10.1021/cm403227w | Chem. Mater. 2014, 26, 1799−1806
Chemistry of Materials
Article
Figure 4. (a) Transmittance and reflectance spectra of as-deposited 60-nm-thick AuOx and pure Au films deposited on glass substrates. (b) Optical constants [refractive index (n); extinction coefficient (k)] of the AuOx and pure Au films. (c) Transmittance spectra of the as-deposited AuOx film after it had been stored for various durations. The purple and yellow lines display the transmittance spectra of 32- and 60-nm-thick pure Au films, respectively. (d) Reflectance spectra of an as-deposited 60-nm-thick AuOx film on a Si substrate and after different periods of storage. The purple and yellow lines are the reflectance spectra of 32- and 60-nm-thick pure Au films, respectively.
We also employed the transmittance and reflectance spectra of the AuOx films to study the reduction process and the behavior of the Au NPs forming in the AuOx films. In addition to the optical spectrum of the as-deposited AuOx film, we also measured the time-dependent optical behavior while the AuOx thin films were gradually reduced. We prepared as-deposited AuOx films on glass substrates and then measured the transmittance and reflectance spectra after different durations of the reduction process. Figure 4c displays transmittance spectra of the as-deposited AuOx and pure Au films of identical thickness (60 nm). The transmittance of the as-deposited AuOx film was much higher than that of the pure Au film, and the transmission decreased gradually upon increasing the storage time. We also found that the transmission near 700 nm dipped after two months of storage; this dip shifted to longer wavelength (ca. 750 nm) after six months of storage. Moreover, after six months of storage, the transmittance spectrum of the AuOx film was very different from that of pure Au film, implying that the AuOx film did not completely transform to a pure Au film. Figure 4d displays reflection spectra of pure Au and AuOx films coated on Si substrates after different reduction times. The measured reflectance of the pure Au film was much higher than that of the AuOx films. After two months of storage, a reflection peak appeared near 700 nm; this signal shifted to longer wavelength (ca. 780 nm) after six months of storage, presumably because of the existence of Au NPs from reduction of the AuOx film. We attribute both the reflection peaks and transmission dips to the LSPR phenomena of the Au NPs,
with that shown in Figure 3a, suggesting that some portion of the AuOx film underwent reduction to form Au NPs after six months of storage. To study the optical characteristics of the AuOx films, we used an optical spectrometer to measure the optical spectra of pure Au and AuOx films of identical thickness (60 nm) on glass substrates. Figure 4a displays the measured transmittance and reflectance spectra of the pure Au and as-deposited AuOx films. The AuOx film exhibited much lower reflectance and higher transmission than those of the pure Au film. The high transmittance is an important feature of AuOx films for application in the hole-transport layers of light emitting diodes. The refractive index (n) and extinction coefficient (k) of AuOx films can be obtained from the ellipsometric and optical thin film models. As displayed in Figure 4b, the optical constants (n, k) of the AuOx film were very different from those of the pure Au film: the AuOx film possessed a much higher refractive index and a lower extinction coefficient than those of the pure Au film from the visible to the NIR regime. Previous studies of the optical constants of ultrathin AuOx films (ca. 2 nm) have generally been performed on the surfaces of Au substrates,33,34 possibly leading to difficulties because of the instability of the ultrathin AuOx films; the reduction process should affect the optical constants and thickness of the AuOx films. In this experiment, the optical constants of AuOx films deposited directly on both glass and Si substrates having a thickness larger than 50 nm were determined in both the visible and the NIR regimes. 1802
dx.doi.org/10.1021/cm403227w | Chem. Mater. 2014, 26, 1799−1806
Chemistry of Materials
Article
which gradually formed through reduction of the AuOx film and were embedded within it. Measuring transmittance and reflectance is a convenient real-time means of characterizing nanostructured metals in different surroundings. To understand the optical behavior of the Au NPs embedded in AuOx films we applied the 3D-FDTD method to simulate in the system. Figure 5a provides a schematic representation of Au NPs partially embedded in a 60-nm-thick AuOx film deposited
on a Si substrate. In this model, we assumed the embedded depth of the particle in the AuOx film to be half of the particle diameter and that the bottom hemisphere of the Au NP came into close contact with the AuOx film without any air gaps. First, we simulated the reflectance spectra from the visible to the NIR regime, with the optical constants for AuOx used in the simulation being based on the measured results in Figure 4b. To determine the effect of the particle size of the Au NPs, we simulated reflectance spectra of Au NPs of various particle sizes embedded within a AuOx film. As displayed in Figure 5b, a reflection peak appeared in the spectrum; this peak became more intense and underwent a red-shift upon increasing the particle size. The inset to Figure 5b displays the particle distributions in the simulation setup. The reflection peak shifted to a wavelength of 790 nm when the particle size increased to 10 nm. To further clarify the cause of the strong peaks in the reflectance spectra, Figure 5c displays the electric field intensity distributions of Au NPs embedded in AuOx films under different incident wavelengths; here, a brighter color relates to stronger electric field amplitude. Au NPs having diameters of 8 and 10 nm exhibited LSPR phenomena at wavelengths of 720 and 790 nm, respectively, leading to strong electric field enhancements around the NPs. The particles were, however, off-resonant at a longer wavelength of 1000 nm; thus, no obvious electric field enhancements appeared at this wavelength. Therefore, we attribute the strong peaks in the reflectance spectra to the LSPR phenomena of the Au NPs embedded in the AuOx films. Notably, the experimental data in Figure 4d are in good agreement with the simulations, implying that the interface between the Au NPs and the AuOx film featured close contact without air gaps. We further simulated the optical properties of Au NPs placed on a glass substrate and surrounded with air. Supporting Information Figure S1a provides a schematic representation of the simulation model. Supporting Information Figure S1b displays the simulated size-dependent reflectance spectra of Au NPs on glass in the visible and NIR regimes. We found that the LSPR peaks of the Au NPs did not alter obviously with the particle sizes (6, 8, or 10 nm), and the LSPR peaks were all located around 520 nm. On the other hand, the LSPR wavelengths of the Au NPs embedded in AuOx films were much more sensitive to the their particle sizes (Figure 5b); therefore, we could tune the LSPR wavelength by changing the particle size of the Au NPs based on the different reduction durations. Furthermore, the high refractive index (ca. 2.7) of the AuOx films meant that the reduced AuOx films featuring embedded Au NPs possessed the longer LSPR wavelengths than that of chemically synthesized Au NPs in air or water.31,32 Supporting Information Figure S1c displays the electric field intensity distribution of Au NPs on a glass substrate illuminated under light with different wavelengths. A brighter color relates to stronger electric field amplitude. Because the LSPR wavelengths of 8-nm and 10-nm Au NPs on glass were both near 520 nm, they demonstrated the large electric field intensity at the wavelength of 520 nm.35,36 And the particles were offresonant at a longer wavelength of 750 and 1000 nm; thus, no obvious electric field enhancements appeared at these wavelengths [Supporting Information Figure S1c]. SERS is a technique for increasing the intensity of Raman signals to provide greater molecular information about analytes. Because the LSPR wavelengths of our reduced AuOx films could be tuned through variations of the reduction duration, we suspected that they would be useful for SERS measurements
Figure 5. (a) Schematic representation of Au NPs embedded on a AuOx film. (b) Simulated reflectance spectra of Au NPs (particle sizes: 4, 6, 8, and 10 nm) embedded on a AuOx surface. (c) 3D-FDTD simulations of electric field intensities around Au NPs (sizes: 8 and 10 nm) under light of different wavelengths. 1803
dx.doi.org/10.1021/cm403227w | Chem. Mater. 2014, 26, 1799−1806
Chemistry of Materials
Article
Figure 6. (a) Reflection spectra of reduced AuOx films after being subjected to various reduction processes. (b) Raman spectra of the probe material (R6G, 10−6 M) on different substrates. (c) Raman spectra of R6G (10−4, 10−6, 10−8, 10−10 M) adsorbed on the surfaces of reduced AuOx films; inset: expanded view of the Raman spectrum of 10−10 M R6G. (d) Raman spectra of 10−7 R6G on chemically synthesized Au NPs and a Au NP-embedded AuOx film. (e) Extinction spectra of a Au NP-embedded AuOx film and chemically synthesized Au NPs.
spectroscopy. For instance, many analytes, especially those of organic or biological materials, are fluorescent species. Exciting these samples with a green laser (532 nm) may promote fluorescence, which might overlap and hide the Raman signals. To avoid such interference, Raman spectroscopy techniques such as surface-enhanced infrared Raman scattering (SEIRS) and surface-enhanced resonance Raman scattering (SERRS) often apply a red-light laser (e.g., 632.8 nm) or an NIR laser (e.g., 785 nm) to generate Raman signals.40 These laser sources possess photon energies lower than those of green-light or other shorter-wavelength laser sources; therefore, a red-light or NIR laser might not promote the electronic transitions resulting in fluorescence. Nonetheless, the intensity of Raman scattering also decreases dramatically when using an excitation source of longer wavelength, since the probability of inducing Raman scattering is proportional to (1/wavelength).4 Accordingly, SERS substrates having red-light or NIR resonance wavelengths are excellent candidates for Raman enhancement. In this study, we could tune the LSPR wavelength of the Au NPs embedded in the AuOx film from 700 to 980 nm to match the excitation light source, thereby allowing maximum SERS signals to be obtained through optimization of the LSPR wavelength.
performed using excited lasers with different wavelengths. The AuOx films we prepared in this study reduced slowly under ambient conditions. Therefore, we used chemical reduction to shorten the reduction time, immersing as-deposited AuOx films into an ethanol solution to chemically reduce the AuOx films rapidly.37 When the ethanol solution is applied, the atomic oxygen will react with ethanol and generate the acetaldehyde (product of the oxidation of ethanol) and thus increase the reduction rate of gold oxide.37−39 Therefore, the reacting rate of the chemical reduction method (immersing gold oxide into ethanol solution) is much faster than that of the natural reduction method (placing the gold oxide in ambient). By varying the immersion time from 30 to 150 min, we obtained AuOx films with different degrees of reduction. Figure 6a presents reflectance spectra of the naturally and chemically reduced films. The LSPR wavelengths of the reduced AuOx films were readily and widely tunable from approximately 700 to 980 nm, a very convenient feature for many LSPR-based applications requiring wavelengths from the visible to the NIR regime. Raman signals are generated when molecules interact with incoming photons. Typically, the incident light source is a narrow-bandwidth laser. The selection of the excitation wavelength of the laser is an important issue for Raman 1804
dx.doi.org/10.1021/cm403227w | Chem. Mater. 2014, 26, 1799−1806
Chemistry of Materials
Article
material characterization techniques. We obtained the optical constants, from the visible to the NIR regime, for AuOx films on different substrates. From optical measurements, it is possible to perceive the formation of Au NPs on the surface of the AuOx film; the reflectance spectra of the reduced AuOx films were in good agreement with those obtained through simulation. The LSPR wavelengths of the Au NP-embedded AuOx films were much longer than those of chemically synthesized Au NPs. We developed SERS substrates based on these reduced AuOx films for highly sensitive detection. We could tune the LSPR wavelengths of the Au NP-embedded AuOx films from 700 to 980 nm merely by varying the duration of the reduction process, allowing maximum SERS signals to be achieved through optimization of the LSPR wavelength. Our substrate based on a reduced AuOx film exhibited minimum detection at 10−10 M (R6G), a concentration comparable with those provided by other SERS substrates prepared using more complicated processing. Overall, our rapid method provides large-area AuOx films having tunable resonance wavelengths for use as SERS-active substrates; we suspect that these films will also be useful in various optical and chemical sensing applications.
For the determination of the SERS properties, we adsorbed the well-established dye molecule R6G onto different substrates to probe the local electric field enhancement. Figure 6b reveals the enhanced Raman intensity of the R6G dye molecules that resulted from the near-field enhancement of the LSPR on the reduced AuOx films. To analyze the samples, we used a Raman microscope equipped with a laser having a wavelength of 785 nm. We chose this laser source to avoid fluorescence induced by the intense electronic transitions of R6G, the absorption band of which is located at 530 nm.41 We evaluated the SERS performances of four types of substrates: a Si wafer, a pure Au film, an as-deposited AuOx film and a reduced AuOx film prepared through chemical reduction for 60 min. Figure S2 (Supporting Information) displays an SEM image of the chemically reduced AuOx film; we observe many Au nanostructures closely distributed on the AuOx film that could generate many hot spots of electric field on the surface of the samples. In Figure 6b, no distinguishable SERS signals for R6G appeared on either the flat Au or Si substrates; the blue and green lines represent the Raman spectra of R6G on the AuOx films before and after reduction, respectively. Only the reduced AuOx sample provided an obvious SERS signal for the R6G molecules. All of the peaks in Figure 6b correspond to the Raman signals of R6G.42 We attribute the large signal enhancements induced by the reduced AuOx substrate to the strong intensities of the local electric fields around the Au NPs; in addition, the large number of Au NPs on the surface after reduction of the AuOx film resulted in many electromagnetic hot spots that enhanced the Raman signals of the analytes. Figure 6c displays SERS spectra of R6G adsorbed on the reduced AuOx film at concentrations ranging from 10−4 to 10−10 M. The reduced AuOx films exhibited good SERS performance, with a minimum detection ability (ca. 10−10 M) comparable with those of SERS substrates prepared using other complicated preparation techniques.43,44 Furthermore, we compared the influence of the LSPR wavelength on the SERS performance, using a 785-nm laser as the excitation light source. Figure 6d displays representative SERS spectra of Au NPs having a similar particle density, but in different surroundings, including chemically synthesized Au NPs (diameter: 13 nm) on a glass substrate and Au NPs (diameter: 10 nm) embedded in a reduced AuOx film. The SERS spectra reveal much better performance for the Au NPs embedded in the reduced AuOx film. To investigate the cause of the different performances of these two SERS substrates, we compared the extinction spectra of the two samples [Figure 6e]. The Au NPs embedded in the reduced AuOx surface exhibited a strong LSPR peak near 800 nm, whereas the chemically synthesized Au NPs induced LSPR at 520 nm. As reported previously, Raman signals will be enhanced dramatically when the LSPR wavelength lies between the excitation wavelength (here, 785 nm) and the wavelength of the Stokesshifted Raman photon.45 We believe that this phenomenon explains why the reduced AuOx films provided the much higher SERS intensity. In this study, tuning the LSPR wavelength, so that it matched the excitation wavelength of the laser, could be achieved simply by varying the duration of the reduction of the AuOx film.
■
EXPERIMENTAL SECTION
■
ASSOCIATED CONTENT
Materials. AuOx thin films were prepared from pure Au as a target and deposited with oxygen plasma through a radio frequency (RF) magnetron reactive sputtering process. The ratio of the gas flow rates is defined as f r = f(O2)/[f(O2) + f(Ar)]; f r was fixed at 0.9, where f(O2) and f(Ar) are the gas flow rates of O2 and Ar gas, respectively. The RF power and chamber pressure during the deposition process were fixed at 100 W and 0.3 Pa, respectively. Reduction Process. Two approaches were used to reduce the asdeposited AuOx films to form Au NP-embedded AuOx films: (i) a natural reduction process of storing the as-deposited AuOx films under a common environment at room temperature and (ii) immersing the AuOx films in an alcohol solution (99.5%) at room temperature and then drying under N2 gas. The latter process can dramatically increase the rate of reduction of AuOx films. Material Characterization and Simulation. The surface morphologies of the Au NP-embedded AuOx films were measured using scanning electron microscopy (SEM); the compositions of the AuOx films were characterized through energy-dispersive X-ray (EDX) analysis. Structural information (e.g., lattice spacing of the NPs) was obtained using high-resolution transmission electron microscopy (HRTEM). Because EDX data might not be accurate when measuring the concentrations of light elements, X-ray photoelectron spectroscopy (XPS) was also used to determine the compositions of the AuOx films. The reflectance and transmittance of the AuOx films at wavelengths in the region 400−1000 nm were measured using an optical spectrometer. The optical constants (refractive indices, extinction coefficients) of the AuOx films were obtained using the optical thin film model. The three-dimensional finite-difference time-domain (3DFDTD) method was used to simulate the near-field optical responses of the Au NP-embedded AuOx films. Raman spectroscopic measurements were performed using a conventional Raman microscope equipped with a diode laser. The samples for the SERS measurement were rhodamine 6G (R6G) dye molecules deposited on various substrates.
S Supporting Information *
■
Schematic representation of Au NPs on a glass substrate, simulated reflectance spectra of Au NPs (particle sizes: 4, 6, 8, and 10 nm) on a glass substrate, 3D-FDTD simulations, and SEM image. This material is available free of charge via the Internet at http://pubs.acs.org.
CONCLUSIONS We have developed a practical approach using reactive sputtering to prepare AuOx at room temperature. We studied the reduction of these AuOx films using both optical and 1805
dx.doi.org/10.1021/cm403227w | Chem. Mater. 2014, 26, 1799−1806
Chemistry of Materials
■
Article
(30) Tseng, M. L.; Huang, Y. W.; Hsiao, M. K.; Huang, H. W.; Chen, H. M.; Chen, Y. L.; Chu, C. H.; Chu, N. N.; He, Y. J.; Chang, C. M.; Lin, W. C.; Huang, D. W.; Chiang, H. P.; Liu, R. S.; Sun, G.; Tsai, D. P. ACS Nano 2012, 6, 5190−5197. (31) Jiang, P.; Porsgaard, S.; Borondics, F.; Köber, M.; Caballero, A.; Bluhm, H.; Besenbacher, F.; Salmeron, M. J. Am. Chem. Soc. 2010, 132, 2858−2859. (32) King, D. E. J. Vac. Sci. Technol. A 1995, 13, 1247−1253. (33) Vela, M. E.; Zerbino, J. O.; Arvia, A. J. Thin Solid Films 1993, 233, 82−85. (34) Cook, K. M.; Ferguson, G. S. J. Phys. Chem. C 2011, 115, 22976−22980. (35) Haes, A. J.; Zou, S. L.; Zhao, J.; Schatz, G. C.; Duyne, R. P. V. J. Am. Chem. Soc. 2006, 128, 10905−10914. (36) Haes, A. J.; Chang, L.; Klein, W. L.; Duyne, R. P. V. J. Am. Chem. Soc. 2005, 127, 2264−2271. (37) Ron, H.; Matlis, S.; Rubinstein, I. Langmuir 1998, 14, 1116− 1121. (38) Owens, C. M.; Roscoe, J. M. Can. J. Chem. 1976, 54, 984−989. (39) Ron, H.; Rubinstein, I. Langmuir 1994, 10, 4566−4573. (40) Kneipp, K.; Dasari, R. R.; Wang, Y. Appl. Spectrosc. 1994, 48, 951−955. (41) Zhao, J.; Jensen, L.; Sung, J. H.; Zou, S. L.; Schatz, G. C.; Duyne, R. P. V. J. Am. Chem. Soc. 2007, 129, 7647−7656. (42) Liu, G. L.; Lee, L. P. Appl. Phys. Lett. 2005, 87, 074101. (43) Li, X.; Zhang, Y.; Shen, Z. X.; Fan, H. J. Small 2012, 16, 2548− 2554. (44) Wi, J. S.; Tominaka, S.; Uosaki, K.; Nagao, T. Phys. Chem. Chem. Phys. 2012, 14, 9131−9136. (45) McFarland, A. D.; Young, M. A.; Dieringer, J. A.; Duyne, R. P. V. J. Phys. Chem. B 2005, 109, 11279−11285.
AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Fax: +886 2 23634562. Tel.: +886 2 33663240 (H. L. Chen). Notes
The authors declare no competing financial interest.
■
ACKNOWLEDGMENTS We thank the National Science Council, Taiwan, for supporting this study under Contracts NSC-100-2628-E-002-023-MY2 and NSC-100-2628-E-002-031-MY3.
■
REFERENCES
(1) Daniel, M. C.; Astruc, D. Chem. Rev. 2004, 104, 293−346. (2) Haiss, W.; Thanh, N. T. K.; Aveyard, J.; Fernig, D. G. Anal. Chem. 2007, 79, 4215−4221. (3) Murphy, C. J.; Sau, T. K.; Gole, A. M.; Orendorff, C. J.; Gao, J.; Gou, L.; Hunyadi, S. E.; Li, T. J. Phys. Chem. B 2005, 109, 13857− 13870. (4) Kamat, P. V. J. Phys. Chem. C 2007, 111, 2834−2860. (5) Luo, X.; Morrin, A.; Killard, A. J.; Smyth, M. R. Electroanalysis 2006, 18, 319−326. (6) Herzing, A. A.; Kiely, C. J.; Carley, A. F.; Landon, P.; Hutchings, G. J. Science 2008, 321, 1331−1335. (7) Rentenberger, S.; Vollmer, A.; Zojer, E.; Schennach, R.; Koch, N. J. Appl. Phys. 2006, 100, 053701. (8) Garcia, A.; Welch, G. C.; Ratcliff, E. L.; Ginley, D. S.; Bazan, G. C.; Olson, D. C. Adv. Mater. 2012, 24, 5368−5373. (9) Helander, M. G.; Wang, Z. B.; Greiner, M. T.; Liu, Z. W.; Qiu, J.; Lu, Z. H. Adv. Mater. 2010, 22, 2037−2040. (10) Horkans, J.; Cahan, B. D.; Yeager, E. Surf. Sci. 1974, 46, 1−23. (11) Gottfried, J. M.; Elghobashi, N.; Schroeder, S. L. M.; Christmann, K. Surf. Sci. 2003, 523, 89−102. (12) Wang, Z. B.; Helander, M. G.; Qiu, J.; Puzzo, D. P.; Greiner, M. T.; Hudson, Z. M.; Wang, S.; Liu, Z. W.; Lu, Z. H. Nat. Photonics 2011, 5, 753−757. (13) Kelly, K. L.; Coronado, E.; Zhao, L. L.; Schatz, G. C. J. Phys. Chem. B 2003, 107, 668−677. (14) Heo, M.; Cho, H.; Jung, J. W.; Jeong, J. R.; Park, S.; Kim, J. Y. Adv. Mater. 2011, 23, 5689−5693. (15) Fu, J. X.; Collins, A.; Zhao, Y. P. J. Phys. Chem. C 2008, 112, 16784−16791. (16) Dieringer, J. A.; Lettan, R. B.; Scheidt, K. A.; Duyne, R. P. V. J. Am. Chem. Soc. 2007, 129, 16249−16256. (17) Rycenga, M.; Camargo, P. H. C.; Li, W.; Moran, C. H.; Xia, Y. J. Phys. Chem. Lett. 2010, 1, 696−703. (18) Dasary, S. S. R.; Singh, A. K.; Senapati, D.; Yu, H.; Ray, P. C. J. Am. Chem. Soc. 2009, 131, 13806−13812. (19) Arnold, M. D.; Blaber, M. G. Opt. Exp. 2009, 17, 3835−3847. (20) Greeneltch, N. G.; Davis, A. S.; Valley, N. A.; Casadio, F.; Schatz, G. C.; Duyne, R. P. V.; Shah, N. C. J. Phys. Chem. Lett. 2012, 116, 11863−11869. (21) Moores, A.; Goettmann, F. New J. Chem. 2006, 30, 1121−1132. (22) Liang, Z. J.; Susha, A.; Caruso, F. Chem. Mater. 2003, 15, 3176− 3183. (23) Hao, F.; Sonnefraud, Y.; Dorpe, P. V.; Maier, S. A.; Halas, N. J.; Nordlander, P. Nano Lett. 2008, 8, 3983−3988. (24) Tsai, H. C.; Hu, E.; Perng, K.; Chen, M. K.; Wu, J. C.; Chang, Y. S. Surf. Sci. Lett. 2003, 537, L447−L450. (25) Ono, L. K.; Cuenya, B. R. J. Phys. Chem. C 2008, 112, 4676− 4686. (26) Bond, G. C. Catal. Today 2002, 72, 5−9. (27) Gerke, R. H.; Rourke, M. D. J. Am. Chem. Soc. 1927, 49, 1855− 1858. (28) Kim, J.; Samano, E.; Koel, B. E. Surf. Sci. 2006, 600, 4622−4632. (29) Wei, W.; Mao, X.; Ortiz, L. A.; Sadoway, D. R. J. Mater. Chem. 2011, 21, 432−438. 1806
dx.doi.org/10.1021/cm403227w | Chem. Mater. 2014, 26, 1799−1806
