Enhanced Photoluminescence of ZnO LangmuirâBlodgett Films on

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Enhanced Photoluminescence of ZnO Langmuir−Blodgett Films on Gold-Coated Substrates by Plasmonic Coupling Nóra Á brahám† and Imre Dékány*,†,‡ †

Supramolecular and Nanostructured Materials Research Group of the Hungarian Academy of Sciences, H-6720 Szeged, Aradi vértanúk tere 1, Hungary ‡ Department of Medical Chemistry, University of Szeged, H-6720 Szeged, Aradi vértanúk tere 1, Hungary ABSTRACT: The interaction between a nanostructured metal and a semiconductor can lead to advantageous enhancement of the emission properties of the semiconductor, which is important for light-emitting diode applications. Herein, we present a study of the interaction between ZnO nanoparticles (3 nm) and two types of gold nanostructures: 10-nm particles and 50-nm-thick films. Photoluminescence emission measurements were performed to characterize the effects of the gold on the emission properties of ZnO and to study the dependence of the interaction on the structure of Au. Stearic acid Langmuir−Blodgett layers were introduced onto the film to control the distance between the noble metal and the semiconductor. The largest (12-fold) enhancement in the emission intensity due to plasmonic coupling was observed for a continuous Au film with one spacer layer, that is, a 2.2-nm distance between the metal and the ZnO.

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INTRODUCTION The study of the properties of noble-metal nanostructures is a fast-developing area of nanoscience.1,2 Combination of metal nanostructures with other organic or inorganic substances can result in the appearance of new features or the enhancement of an advantageous property of the material under investigation. The unique plasmonic properties of noble-metal nanoparticles and films make them suitable for many modern applications such as light-emitting diodes,3 catalysis,4,5 and the rapidly expanding area of biomedical applications regarding diagnostics and therapeutics.6,7 Currently, ZnO is a popular material because it is inexpensive and easy to prepare and it has many advantageous properties (optical properties due to the quantum size effect).8−12 The photoluminescence (PL) properties of ZnO nanostructures of different morphologies have been widely studied. In most cases, ZnO quantum dots exhibit UV and visible emissions. The UV emission is associated with the direct recombination of the photoinduced charge carriers; the mechanism of the visible emission is not yet completely understood.13 The visible emission is related to surface defect states, but there is no agreement in the literature regarding the nature of the defects. Most authors refer to oxygen vacancies,14−16 but zinc vacancies,17 oxygen interstitials,18 zinc interstitials,19 and antisite oxygen20 have also been proposed. The picture becomes more complex when a nanosized noble metal is combined with a semiconducting material. Plasmonic features of the metal can influence the emission properties of the semiconductor. Nanostructures composed of Au and ZnO have been synthesized and their photoluminescence properties investigated by some authors.21−27 Because of the different plasmonic properties of noble-metal particles and films, the © XXXX American Chemical Society

properties of these nanostructures must be discussed separately. Localized surface plasmons (LSPs) of noble-metal nanoparticles can be excited with light, resulting in resonance of the plasmons, which is called localized surface plasmon resonance (LSPR). The resonance energy depends on the nanoparticle size and shape and the chemical environment (which includes the characteristics of the surrounding medium and also the presence of other molecules or particles that can induce plasmonic interactions). LSPs of particles can induce short-range interactions within the distance of a few nanometers.1,2 Surface plasmon polaritons (SPPs) are the excited surface plasmons in a continuous metal film. SPPs can propagate along the surface of the metal and induce an evanescent field. This evanescent field can have an effect over much longer distances, typically a few tens of nanometers.1 Subramanian and co-workers24 published a detailed study on the photoluminescence of ZnO particles in ethanol, including charging events in ZnO with and without metal. They found that electron accumulation quenches the visible emission of ZnO and showed that excess charge could be removed from the particles with dissolved oxygen. Metal- (Pt- and Au-) capped ZnO particles showed different properties: The ohmic-type interaction between Pt and ZnO facilitates discharge of electrons into the electrolyte, whereas in the case of Au-capped ZnO particles, the electrons are distributed between the metal and the semiconductor, resulting in Fermi-level equilibration and suppression of the UV emission. Received: February 19, 2012 Revised: May 13, 2012

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dx.doi.org/10.1021/jp301641u | J. Phys. Chem. C XXXX, XXX, XXX−XXX

The Journal of Physical Chemistry C

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Au-coated ZnO films were prepared by Li et al.23 They observed a large enhancement in the UV emission and almost complete quenching of the visible emission, which they explained in terms of direct charge transfer between Au and ZnO. LSPs at the metal/semiconductor interface make the transfer of the electrons in ZnO from defect states to the conduction band possible because of the resonant interaction, facilitating UV emission. Lawrie, Haglund, and Mu21,22 published a profound work on the effect of Au and Ag films on the PL emission properties of a 70-nm-thick ZnO film (prepared by electron beam evaporation) with and without MgO spacer layers. They observed enhancement of both the UV and visible emissions and proposed different explanations for the coupling between ZnO and the metal depending on the structure of the metal (particles or films) and the type of emission [donor−acceptor pair (visible) and band-edge (UV) emissions]. The proposed mechanism for visible emission is LSP−dipole−dipole scattering for both particulate and quasicontinuous films, whereas the interactions responsible for the change in UV emission are LSPs and hot-electron tunelling for particulate films and SPPs from Purcell enhancement for quasicontinuous films. In this work, we studied the effects of different gold nanostructures on the photoluminescence properties of ZnO quantum dots in solid-supported samples. We focused not only on the change in intensity of the emission but also on the shift in the maximum. We prepared two series of samples: one with gold nanoparticles spray-coated on a quartz surface and one with a thick (50-nm), continuous gold film on a glass surface. In addition to the structure of gold (surface-coated versus continuous), the distance between the gold and the ZnO was controlled with varying numbers of stearic acid spacer layers to study the distance dependence of the interaction. The plasmonic interaction between gold (nanoparticles or continuous films) and ZnO was characterized with photoluminescence measurements.

where M is the atomic weight of Au (M = 197 g/mol), c is the concentration of Au3+ ions in the initial solution (in our case, c = 0.2 mM), ρAu is the density of Au assuming a uniform fcc structure (ρAu = 19.3 g/cm3), and d is the diameter of the particles (which are considered to be spheres). Gold-Coated Substrates. Two types of gold-coated substrates were used in this work, and accordingly, two series of samples were prepared. In the first case, the substrate was a 50-nm-thick gold-film-coated glass substrate purchased from Platypus Technologies (type AU.0500.ALSI). This type of substrate was hydrophilized by being immersed in 2 g/100 mL ethanolic solution of 2-mercaptoethanol (Fluka, >99%) for 4 h, then rinsed with ethanol, and finally dried in nitrogen. In the second series, quartz was used as the substrate (cleaned in piranha solution), and gold nanoparticles were spray-coated onto its surface. For the spray-coating process, the original Au nanoparticle sol was used; no further treatment was applied. The films were then annealed at 400 °C in air for 3 h. For technical reasons, atomic force microscopy (AFM) measurements were carried out on glass-supported samples that were prepared and treated in the same way as the quartz-supported samples. Preparation of Langmuir−Blodgett Films of Stearic Acid and ZnO Particles on Solids. The Langmuir−Blodgett (LB) technique was used to prepare multilayer coatings on gold-coated substrates. For spreading, a 0.3 mg/mL stearic acid (Reanal, a.r.) solution in chloroform (Sigma-Aldrich, Chromasolv, ≥99.9%) was made. Spreading sols of ZnO nanoparticles were obtained by diluting the ethanolic sol with chloroform at a volume ratio of 1:2. Surface pressure versus surface area isotherms were recorded before film preparation in a Kibron MicroTroughS Langmuir trough and were used to determine the surface pressure of the deposition, which is the key factor in Langmuir−Blodgett film transfer.34 A representative isotherm of ZnO particles was published in our previous work.29 The isotherms obtained for stearic acid were in good agreement with earlier results by other authors,35 so they are not published here. Transfer of the monolayers onto solid substrates was carried out at constant applied surface pressure of 2 mN/m in the case of ZnO nanoparticles and 30 mN/m in the case of stearic acid. Glass slides coated with a 50-nm-thick gold film and quartz slides coated with gold nanoparticles were used as substrates for LB film preparation. The deposition of stearic acid layers was carried out in Z-type deposition mode (head-totail orientation).35 The structures of the prepared films in the two series are shown in Figure 1. Sample notation is also indicated in this figure. Characterization of ZnO and Gold Particles. Particles were visualized by transmission electron microscopy (TEM; FEI Tecnai G2 20 X-TWIN) on carbon-coated copper grids. Samples of ZnO were obtained from compressed Langmuir

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EXPERIMENTAL METHODS Preparation of ZnO Particles. ZnO colloidal nanocrystals were prepared according to the method of Meulenkamp,28 as described also in our previous work.29 Five millimoles of Zn(Ac)2.2H2O (Fluka, a.r.) was dissolved in 50 mL of boiling ethanol (Molar Chemicals, a.r.). After the solution had been cooled rapidly to 0 °C, 50 mL of ethanolic solution containing 7 mmol of LiOH·H2O (Sigma, a.r.) was added dropwise, and the mixture was stirred for an additional 2 h while temperature was kept below 4 °C. The resulting transparent sol was stored at ∼5 °C. Preparation of Gold Nanoparticles. Gold particles were prepared by the widely used Turkevich method:30,31 reduction of Au3+ ions (HAuCl4 was purchased from Sigma-Aldrich) with trisodium citrate (Sigma-Aldrich) in aqueous medium. The preparation was carried out with 0.2 mM HAuCl4 and a 1:5 molar ratio of Au3+ to citrate. It is important to calculate the average number of nanoparticles in the gold nanodispersion for film preparation with the spray-coating technique.32,33 The easiest way to calculate the average number of particles (N) in a given volume (V) is with the equation N=

6McV ρAu d3π

Figure 1. Schematic representation and sample notation of the members of the two series. [SA(n) represents varying numbers (n) of stearic acid (SA) layers.]

(1) B



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films at the air/water interface and were dried at 100 °C in air for 2 h. Samples of gold nanoparticles were made by dropping a small amount of the sol onto a copper grid and allowing it to dry in air. UV−vis absorbance spectra of Au sols were measured with an Ocean Optics USB2000 spectrophotometer in a quartz cuvette. Photoluminescence spectra of ZnO particles were recorded with 50-fold-diluted sols on a Horiba Jobin Yvon FluoroMax-4 fluorescence spectrometer using a 350-nm excitation source. PL emission enhancement was calculated by dividing the integrated emission intensity of the sample by the integrated emission intensity of the ZnO film without Au. Characterization of Hybrid Nanostructured Films. The structure of the 50-nm-thick gold film and the spray-coated gold nanoparticle film were examined by AFM on a Nanoscope III atomic force microscope from Digital Instruments with a piezo scanner capable of a deflection of 12.5 μm in the x and y directions and 3 μm in the z direction using a tapping-type silicon tip (Veeco Nanoprobe Tips, model RTESP, 125 μm, 300 kHz). X-ray diffraction (XRD) measurements were performed on a Philips PW 1830 diffractometer using Cu Kα radiation at a voltage of 40 kV and a current of 35 mA. The Scherrer equation was used to determine lattice parameters from the half-width of the diffraction peaks

kλ d= β cos Θ

Figure 2. Representative HRTEM image of ZnO nanoparticles.

the particles, the result for the number of particles is 3.73 × 1012 nanoparticles. Characterization of Stearic Acid Langmuir−Blodgett Layers. Langmuir−Blodgett layers of stearic acid (SA) were used as spacers between Au and ZnO in the films. The main advantages of these layers are precise determination of the layer thickness and good control over the preparation process, so that the distance between Au and ZnO was well-defined and could be tuned very easily. Multilayers of SA were characterized by XRD. The diffraction pattern recorded for six layers of SA is presented in Figure 4. (We measured XRD patterns of SA films with fewer layers, but the intensity of the reflected beam was very low.) The diffractogram shows the characteristic peaks of a well-ordered, layered structured sample. In addition to the first-order Bragg peak (d001), the second- (d002), third- (d003), fourth- (d004), and fifth- (d005) order reflections can be seen in the diffractogram. The registered peaks are located at 2.02°, 4.08°, 6.18°, 8.24°, and 10.3° 2θ angles, indicating interplanar distances of 43.7, 21.64, 14.3, 10.72, and 8.58 Å, respectively. The Scherrer equation was used to calculate the layer thickness of the stearic acid multilayer considering all five peaks, and the average of the thicknesses obtained for the different peaks was 4.4 nm. The length of a stearic acid molecule can be calculated approximately by means of the bond lengths between the atoms: L = 0.127 × (nc − 1) + 0.1 + 0.24, where nc is the number of C atoms in the molecule (nc = 18), 1.27 nm is the length of a C−C bond in zigzag conformation, 0.1 nm is the length of a C−H bond, and 0.24 nm is the length of the C−O− H group.36 The length value for one molecule was thus calculated as L = 2.5 nm, which is in good agreement with previous results.36 Considering the length of one SA molecule to be L = 2.5 nm, a result for the interplanar distance can be obtained only when the value is to be applied to a bilayer. In the case of Z-type deposition, the molecules are in a head-to-tail orientation which is thermodinamically less stable than the head-to-head/tail-to-tail orientation, which is realized during the Y-type deposition. Some authors have already shown35,37,38 that it is possible that the molecules overturn in the deposited

(2)

where d is the lattice plane distance of the powder, λ is the wavelength of Cu Kα radiation, β is the full width at halfmaximum, Θ is the Bragg diffraction angle, and k is a constant. UV−vis absorbance spectra (Ocean Optics CHEM 2000 UV− vis spectrometer) were measured for the spray-coated goldparticle-based samples using air as the reference. Photoluminescence spectra of the ZnO-containing films were measured on a Horiba Jobin Yvon FluoroMax-4 fluorescence spectrometer with a special sample holder using a 350-nm excitation source.

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RESULTS AND DISCUSSION Characterization of ZnO Nanoparticles. We published a detailed characterization of ZnO nanoparticles in terms of size, crystallinity, aggregation behavior, and photoluminescence properties in our previous work.29 Herein, we present a highresolution transmission electron microscopy (HRTEM) image for visualizing the particles. The HRTEM image shown in Figure 2 indicates spherical particles with a diameter of about 3 nm. Characterization of Au Nanoparticles. The gold nanoparticles used in this work were visualized by TEM (Figure 3a), and the mean size and size distribution of the particles were calculated (Figure 3b). The particles were spherical in shape and about 10 nm in size. The size distribution curve was very narrow, indicating that a dispersion with monodisperse particles was obtained. The UV−vis absorbance spectrum of the sol is presented in Figure 3c. One strong plasmon absorption band was observed at 518 nm, which means that the particles were spherical in shape and individually dispersed in the sol. No aggregation was observed. The average number of particles was calculated for our Au nanodispersion by means of eq 1. If we consider a 1 mL volume of the dispersion and d = 10.2 nm as the average diameter of C



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Figure 4. (a) XRD pattern of a six-layer Langmuir−Blodgett film of stearic acid, (b) tilted position of stearic acid molecules and proposed structure of the six-layer stearic acid Langmuir−Blodgett film.

function and obtained an angle of 62° with respect to the plane of the surface (see Figure 4b). Surface Topography of the Spray-Coated Au NP Film and the 50-nm-Thick Au Film. Au NPs spray-coated on a glass surface were visualized by AFM. Figure 5 shows a representative area of the sample. The surface of the glass was coated with particles, so the film had discontinuities. Crosssectional analysis showed single particles sitting on the surface of the substrate in close proximity to each other, with only a few particle associates. A root-mean-square (rms) surface roughness of 4.89 nm was measured for the area marked with a red square. Four large black holes appeared in the AFM image of the spray-coated substrate (see Figure 5). These are defects of the glass substrate that resulted from calcination at 400 °C. (Technical conditions did not allow for the preparation of spray-coated films on a quartz substrate for AFM measurements.) The surface structure of the 50-nm-thick gold film was also characterized by means of AFM measurements. Figure 6 presents a representative surface topography image and a crosssectional analysis of the sample. There is a continuous layer on the glass surface, and the layer is rather smooth, as an rms surface roughness of 1.78 nm was obtained for the area marked with the red square. AFM images were recorded for the stearic acid-coated substrates, as well. Figure 7 presents surface topographic images

Figure 3. (a) Representative TEM image, (b) calculated size distribution curve, and (c) UV−vis absorbance spectrum of the Au NP sol.

film, either in the aqueous subphase (during immersion for the next layer) or at ambient conditions. Overturning of the molecules results in a head-to-head orientation with a higher interplanar spacing. The thickness of a single Langmuir− Blodgett layer of stearic acid is one-half of the resulting value, namely, δ = 2.2 nm. The thickness of multiple layers of stearic acid can be calculated as multiples of the single-layer thickness. Based on this deduction, the distance between Au and ZnO can be varied at discrete multiple values of 2.2 nm. The thickness of a stearic acid LB layer (δ = 2.2 nm) is smaller than the length of the molecule (L = 2.5 nm), which means that molecules are not parallel to the surface normal. We calculated the tilt angle (γ) from the definition of the sine D



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Figure 5. AFM top-view image and cross-sectional analysis of spray-coated Au nanoparticles on a glass surface.

Figure 6. AFM top-view image and cross-sectional analysis of a continuous, 50-nm-thick gold film on a glass surface.

Figure 7. AFM surface topographic images of gold films coated with a double layer of stearic acid.

of the two types of gold films both coated with two Langmuir− Blodgett layers of stearic acid. The AFM images show very similar surface structures compared to the bare gold films, as expected. Gold nanoparticles deposited by the spray method are clearly visible on the surface, similarly to the uncoated sample. The surface topography of the 50-nm gold film did not change at all: there was almost no difference between the uncoated and stearic acid-coated films. Optical Properties of the Hybrid Nanostructured Films of Au and ZnO. Spectroscopic measurements were carried out to characterize the films of the spray-coated Au NPbased series. Figure 8 presents the UV−vis absorbance spectra. The spectrum of the spray-coated gold film shows the plasmon absorption band of Au NPs at 548 nm and the absorption of

ZnO around 350 nm. The plasmon absorption band of Au NPs is shifted to longer wavelengths compared to the spectrum measured for the aqueous dispersion (Figure 3c). The shift is most probably the result of multiple factors, but the separation of the contributions of all paramerers to the overall change is not at all unambigous. The change in interparticle distance and, therefore, in the appearance of interparticle interactions is the key factor in our opinion. In the aqueous sol, particles can move freely, and the interactions among them are minimal because of the large interparticle distance. In the solidsupported films, the particles are closer to each other, so interparticle interactions modify the position of the plasmon absorption band. A change in the refractive index of the surrounding environment could also modify the resonance E



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when one spacer layer of stearic acid was present in the film. An approximately 2-fold enhancement in the intensity was reached for no spacer layer between the Au NPs and the ZnO. The influence of a continuous, thick gold film on the PL emission properties of ZnO was also studied by PL measurements. Figure 10 shows the recorded emission intensities for

Figure 8. UV−vis absorbance spectra of a spray-coated Au film and a spray-coated Au film with a single-layer Langmuir−Blodgett film of ZnO particles on a quartz substrate.

energy. However, it is not easy to determine whether air with a lower refractive index or quartz with a higher refractive index than water is the dominant factor in terms of refractive index. The effect of Au nanoparticles on the emission properties of ZnO particles as a function of their distance was studied by photoluminescence measurements. The measured emission spectra in the visible and UV regions are shown in Figure 9 for

Figure 10. PL emission spectra of Au continuous-film-based samples in the (a) visible and (b) UV ranges.

varying numbers of layers of stearic acid spacers and, for comparison, the emission spectrum of ZnO. As can be seen, both the UV and the visible emissions became stronger upon the introduction of Au. The shift of the emission maximum and the enhancement of the emission intensity were evaluated as functions of the Au−ZnO distance (Figure 11). The wavelength of the emission has a maximum-type dependence on the spacer layer thickness, as a maximum shift of 7 nm was observed for two layers of stearic acid, which means a 4.4-nm distance between the metal and the ZnO. A large, 12-fold enhancement was measured for one spacer layer, corresponding to a 2.2-nm distance. Upon a further increase in the number of spacer layers, the intensity of the emission decreased nearly exponentially. The experimental observations show significant enhancement in the emission intensity of ZnO nanoparticles in the vicinity of Au. The results showed a marked difference for Au particles and continuous films: ZnO particles interacting with Au continuous films showed a much higher increase in intensity and a larger shift in emission. Multiple reasons might be responsible for this, including the difference in the amount of gold (the spray-coated gold film contained a much smaller mass of Au) or the different surface plasmon resonance properties of particles and films. However, it is not obvious that the increase in the mass of Au interacting with ZnO resulted in the increase of the emission intensity. Im and co-workers39 found that

Figure 9. PL emission spectra of Au NP-based samples in the (a) visible and (b) UV ranges.

the spray-coated Au NP-based samples. As can be seen, both the visible and the UV emissions became more intense because of the presence of gold, and the emission intensities were dependent on the distance between Au and ZnO particles. The strongest emission and, therefore, the highest enhancement were observed when no spacer layer was used. Figure 11 presents the wavelengths and the enhancements of the visible emission of the different samples as functions of the distance between Au and ZnO. There was a slight shift in the maximum F



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the visible emission intensity and the appearance of a weak UV emission were observed, but the effect was quite weak. This mechanism of coupling for this case is supported by the argument that photoinduced charge transfer leads to Fermilevel equilibration, which results in band-gap widening and, therefore, a shift of the emission maximum to shorter wavelengths.24,39 This phenomenon, namely, a 2-nm blue shift, was observed in this work (sample AuF/ZnO). The second argument for the presence of charge transfer is the low intensity of the emission: Because of the strong electric field, ZnO emission enhancement should have been observed. It is important to note that the largest shift and the greatest enhancement were measured for different samples for the two series: The maximum shift (two spacer layers for the AuF series and one spacer layer for AuNP series) was recorded with one more spacer layer than the largest enhancemet (one spacer layer for the AuF series and no spacer layer for the AuF series) for both series. The shift of the emission has been explained by the result of band-gap widening due to Fermi-level equilibration by some authors,24,39 but this mechanism can be responsible only for a blue shift of the maximum. If band-gap widening is excluded as the mechanism for the red shift, then two possibilities are left: band-gap narrowing or an unchanged band gap. Considering the causes leading to band-gap widening (Fermi-level equilibration), the Fermi level of ZnO must be higher than that of Au to achieve band-gap narrowing, which is quite plausible. The band gap of ZnO presumably does not change because of the enhanced electric field of the metal. If this is the case, then the energy level of the defect, playing a key role in the visible emission, must change. Whether this change in energy level is an increase or a decrease can be determined only after understanding the mechanism of ZnO visible emission. It is widely accepted that visible emission is induced by surface defects, and if one accepts the most cited hypothesis of the defects being oxygen vacancies, one must determine the transition responsible for visible emission: (1) van Dijken et al.15 proposed the transition from the conduction band to the deep acceptor level, whereas (2) Zhang et al.14 referred to a transition from a deep donor level to the valence band. We suggest that, if transition 1 is the key to the visible emission, then the defect energy level must increase, but if transiton 2 is the key, then the defect energy level must decrease.

Figure 11. Evaluation of the visible emission characteristics of the two series of samples: (a) shift of the emission and (b) enhancement of the visible emission. Triangles represent samples of continuous Au film; tilted squares represent Au NP-based samples. The lines are only guides for the eyes; no fitting was applied.

increasing the surface coverage of ZnO nanorods with Au NPs led to a decrease of both the visible and UV emissions. Localized surface plasmons of Au nanoparticles can be excited through near-field interactions. ZnO nanoparticles in the excited state behave as an oscillating dipole that can induce a plasmon resonance in Au particles. The enhanced local electric field around the noble-metal nanoparticle can alter the emission properties of ZnO.22,40 We propose that the mechanism responsible for the enhancement of both the UV and visible emissions for the Au nanoparticle-based samples is the above-described dipole interaction, the process known as metal-enhanced fluorescence. Gold nanoparticles were located separately on the surface of the quartz substrate, so SPPs were not supported in these samples. LSPRs with much shorter interaction distances can induce enhanced emission only when there is direct contact between the metal and the ZnO, so a much weaker increase was reached compared to the continuous film. If a continuous film of noble metal is excited, surface plasmon polaritons are generated that can propagate along the metal/dielectric interface. These propagating waves result in an evanescent field that can induce enhanced light emission from ZnO through Purcell enhancement or dipole interactions.21,22 This mechanism is applicable for samples with spacer layers [AuF/SA(n)/ZnO]. The exponential dependence of the enhancement on the distance is in good agreement with the exponential decay of the evanescent field, further supporting the proposed mechanism. It was shown earlier that, if there is direct contact between Au and ZnO, then charge transfer can occur, which results in the enhancement of the band-edge emission and quenching of the visible emission.23,25,26 For the sample AuF/ZnO, when no spacer was present, charge transfer probably occurred from ZnO to the metal: A slight decrease in

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CONCLUSIONS Hybrid films of Au and ZnO were prepared by means of Langmuir−Blodgett and spray-coating techniques to characterize the plasmonic interaction between metal and semiconductor. The distance between Au and ZnO was controlled with varying numbers of stearic acid Langmuir−Blodgett layers. Photoluminescence experiments showed that the introduction of gold greatly influences the intensity and wavelength of the emission of ZnO due to plasmon coupling. The mechanism responsible for the emission enhancement was discussed: We propose that, depending on the structure of gold, a direct charge-transfer process or the effect of the enhanced electric field around the metal leads to both an increase in the emission intensity and a shift in the emission.

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AUTHOR INFORMATION

Corresponding Author

*Tel.: +36 62 544210. Fax: +36 62 544042. E-mail: i.dekany@ chem.u-szeged.hu. G



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Notes

(32) Khlebtsov, N. G. Anal. Chem. 2008, 80, 6620−6625. (33) Liu, X.; Atwater, M.; Wang, J.; Huo, Q. Colloids Surf. B 2007, 58, 3−7. (34) Kim, T.; Park, J.; Choi, J.; Kang, D. Thin Solid Films 1996, 284− 285, 500−504. (35) Petty, M. C. Langmuir−Blodgett Films. An Introduction; Cambridge University Press: Cambridge, U.K., 1996. (36) Lewis, G. N. J. Am. Chem. Soc. 1916, 38, 762−785. (37) Ehlert, R. C. J. Colloid Sci. 1965, 20, 387−390. (38) Angelova, A.; Ionov, R.; Gutberlet, T. Thin Solid Films 1997, 305, 309−315. (39) Im, J.; Singh, J.; Soares, J. W.; Steeves, D. M.; Whitten, J. E. J. Phys. Chem. C 2011, 115, 10518−10523. (40) Lakowicz, J. Anal. Biochem. 2005, 337, 171−194.

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors are very thankfull for the financial support of the Hungarian Scientific Fund (OTKA) no. K 73307. Project TÁ MOP-4.2.1/B-09/1/KONV-2010-0005, “Creating the Center of Excellence at the University of Szeged”, is supported by the European Union and cofinanced by the European Regional Fund.

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