There is increasing interest in tuning the optical and optoelectronic properties of semiconductor nanostructures using metal nanoparticles in their applications in light-emitting and detection devices. In this work we study the effect of a dielectric Al2O3 gap layer (i.e., spacer) on the interaction of ZnO nanowires with metal nanoparticles. The Al2O3 spacer thickness is varied in the range of 1–25 nm using atomic layer deposition (ALD) in order to tune the interaction. It is found that ~5 nm is an optimum spacer thickness common for most metals, although the enhancement ratio of the near-bandedge emission differs among the metals. Consistent results are obtained from both photoluminescence (PL) and cathodoluminescence (CL) spectroscopies, with the latter being applied to the optical properties of individual semiconductor/metal nanoheterostructures. The interaction is primarily proposed to be related to coupling of ZnO excitons with local surface plasmons of metals, although other mechanisms should not be ruled out.
Radiative coupling between surface plasmons (SPs) and excitons in a nanoscale optical emitter can lead to fundamental quantum optical phenomena, such as the Purcell effect  and vacuum Rabi or normal mode splitting [2–4]. SPs can increase the density of states and thus the spontaneous emission rate in semiconductors [5–8]. Modulations of the spontaneous emission in this SP coupling regime have been successfully realized in various quantum optical systems [9–13]. Semiconductor-dielectric-metal trilayers are archetypal systems for the study of exciton-plasmon-polaritons  or exciton-plasmon conversion , as well as enhancement of spontaneous light emissions in, e.g., InGaN [10, 16]. In such systems the sandwiched dielectric layer modulates the coupling strength between the two semiconductor excitons and metal SPs due to wave function overlap.
As a typical semiconductor emitter, ZnO has a direct band gap of 3.37 eV and a large exciton binding energy of 60 meV at room temperature. Well-aligned ZnO nanowires (NWs) are being widely studied for their application in light emitting diodes, UV optical switches, and nanoscale lasers . As-grown ZnO NWs usually have a high density of defects or impurities which will trap carriers and result in low luminescence efficiency [18, 19]. Recently, a number of groups have reported that, by coating ZnO with either a metal or dielectric layer, the near-bandgap emission (NBE) intensity can be either increased [20–26], or quenched [27–29]. In particular, direct metal capping appears to be an effective way of enhancing the NBE due to SP coupling [20–23, 26, 30–33]. At the same time, the direct contact of metal with ZnO complicates the interpretation of the interaction mechanism. A number of possible physical processes could occur at the metal-ZnO interface including, for example, charge transfer [24, 28, 29, 34], unintentional H doping , metal-induced gap states , and metal diffusion . It is known that in exciton-plasmon coupling for enhancement of fluorescence, a dielectric spacer layer is usually necessary to prevent emission quenching caused by coupling to lossy surface waves. An insulating dielectric layer inserted between ZnO and the metal layer, as adopted in the present study, will retard direct electron flow, and therefore be more appropriate for the study of exciton-plasmon coupling. So far there are limited studies dedicated to the localized SP-coupling-induced enhancement of ZnO luminescence. Lawrie and co-workers studied the effect of MgO spacers on the coupling of SP polaritons on Ag films with polycrystalline ZnO films, and established Purcell enhancement of NBE as well as visible band enhancement through dipole-dipole scattering [38, 39]. Very recently, Liu and co-workers reported the effects of Ag localized SPs on the UV electroluminescence from ZnO/GaN heterojunction light-emitting diodes by inserting MgO spacer layers, and found a similar enhancement trend . No systematic investigation on the spacer-thickness dependence of the luminescence of ZnO NWs has previously been reported.
To realize a homogeneous coating of NWs with a dielectric spacer in this work, we applied atomic layer deposition (ALD) to achieve fine control over the Al2O3 spacer thickness for ZnO NW/plasmonic metal nanoparticle heterostructures. It is found, in complementary photo- and cathodoluminescence spectroscopic studies (of NW arrays and singular semiconductor/metal heterostructures, respectively) that the NBE can either be enhanced or quenched depending on the spacer thickness, and a maximum metal nanoparticle-induced enhancement occurs at around 5 nm ALD Al2O3. This observation (peak PL enhancement around spacer thickness of 5 nm) is proposed to be a result of competition of a nanoscale hybrid plasmonic mode with other emission mechanisms .
2.1 Sample preparation
Well-aligned ZnO NW arrays were grown from thoroughly mixed ZnO-graphite source powders on gold-coated GaN/a-plane sapphire substrates via chemical vapor deposition. Full details of the growth process can be found in Ref. . The as-grown samples were cut into small pieces for subsequent depositions of Al2O3 and metal. Coating with Al2O3 to the desired thickness was realized by ALD (Beneq TFS-200), using trimethylaluminum (TMA) and water as the aluminum and oxygen sources respectively, at 200°C with a film growth rate of ~0.13 nm/cycle. The thickness was determined based on the linear growth rate. The outer layers of metal nanoparticles were applied via DC sputtering (JFC-1600, JEOL). Some samples were directly sputtered with Au without the intermediate Al2O3 spacer for control experiments. The morphology of the NWs was observed using both scanning electron microscope (SEM) and transmission electron microscope (TEM).
2.2 Photoluminescence characterization
Room-temperature PL measurements were carried out using the 325 nm line of a continuous-wave He-Cd laser for excitation at an incident angle of 45° and spectra were recorded in the surface-normal direction. The laser power was 10 mW and the spot size around 1 mm2. At least three measurements were taken on each sample at different locations on the surface to verify uniformity.
2.3 Cathodoluminescence characterization
The excitation of NWs for CL measurements was performed using an SEM (CamScan CS3200 with LaB6 cathode). All CL measurements were performed by exciting the sample with a 30 kV electron beam at a beam current of ~1.2 nA. Acquisition times varied from 0.01 to 0.5 s depending on the signal intensity: times of 0.5, 0.1 and 0.01 s were used for pristine ZnO NWs, ZnO/Al2O3/Pt, and ZnO/Al2O3/Au NWs, respectively. The electron beam was focused onto the sample through a small hole in a short-working-distance parabolic mirror mounted directly above the sample, which collected emitted light over approximately half of the available hemispherical solid angle. The mirror directs the light out of the SEM chamber to a hyperspectral light collection system (spectrometer and liquid-nitrogen-cooled CCD detector). This arrangement allows one to build CL maps of the NWs with combined spatial and spectral information about the sample (each point in the map, which has a spatial resolution of ~20 nm, contains the full spectrum of the emitted light) [43, 44].
3 Results and discussion
Vertically aligned ZnO NWs, with a diameters in the range of 80–140 nm and a typical height of 1 μm, are fabricated via epitaxial growth on lattice-matching GaN layers. As-grown ZnO NWs have very smooth surfaces, which are maintained after application of Al2O3 spacer layers by virtue of the ALD process. After metal sputtering, the NW surfaces are decorated with a discontinuous layer of nanoparticles, as shown in Figure 1. In this example the gold nanoparticle diameter is about 5 nm (produced by sputtering at 10 mA for 20 s); the metal particle size increases with higher sputtering currents and/or longer sputtering times.
Figure 2 presents a comparison of the PL spectra from various stages of the NW production and coating process: for a pristine ZnO NW array; a ZnO NW array coated by ALD with Al2O3; an array coated with an Al2O3spacer layer and metal nanoparticles; and an array coated directly with metal nanoparticles (i.e., without an Al2O3 spacer). Results are presented here for gold nanoparticles, but the pattern of behavior is similar for other metals. The as-grown, pristine ZnO NWs exhibit extremely weak UV emission at 378 nm and strong broadband defect emission centered at 510 nm. This is likely due to a high density of surface defect states within the bandgap, which leads to strong band bending and a significant reduction in the rate of radiative exciton recombination [8, 18, 41]. After coating with Al2O3, the visible defect-related emission is somewhat suppressed, while the level of UV emission remains unaffected. In previous PL studies of thinly Al2O3- or polymer-coated ZnO NWs [25, 45], a surface passivation effect was proposed to explain a slight enhancement of UV emission with or without a decrease in visible emission. In the present study, the PL spectra show only a slight change in NBE after Al2O3 or direct metal coating, indicating that surface passivation is a marginal effect in relation to the optical properties.
When the NWs are directly coated with Au nanoparticles, the UV emission of the ZnO/Au structures is enhanced slightly while the visible emission is drastically quenched. This is consistent with previous reports on direct metal capping of ZnO nanostructures [26, 30, 34]. Finally, the ZnO/Al2O3/Au samples present a UV peak that is significantly enhanced at an optimal dielectric thickness of 5 nm. This gigantic increase in the NBE intensity is also observed for other metals (Al, Ag and Pt).
The defect-related surface states of the as-grown ZnO NWs cause strong band bending and charge depletion, therefore reducing the chances for exciton recombination near the surface. With Al2O3/metal coating, the defect emissions are reabsorbed by the metals, and simultaneously the plasmon/exciton coupling increases the recombination probability between electrons outside the depletion region and holes within the depletion region.
It is found that the broad visible emission of as-grown ZnO NWs is almost completely eliminated when the metal nanoparticle layer is present, with or without the ALD spacer. The quenching is independent of the thickness of the Al2O3 spacer from 0 to at least 25 nm (see more below), indicating that the mechanism responsible for the quenching of visible emissions is not related to SP coupling, but more likely to metal plasmonic absorption. As the metal nanoparticles have a broad plasmonic absorption peak covering the visible range, the ~500 nm emissions from the ZnO NW core can be absorbed by the metal particles.
In order to systematically study the thickness effect of the spacer layer on UV emission, ZnO NWs were coated with different ALD Al2O3 spacer thicknesses (5, 10 and 25 nm) and sputter-coated with one of four different metals. To ensure comparability across the samples, each batch of samples were cut from the same pristine ZnO NW sample. Data for gold and silver are presented in Figure 3 (semi-log scale), as they are the two metals of most common interest. The spectra for Pt and Al show very similar trends and thus not presented herein. It is found that the enhancement of UV emissions is most significant at an Al2O3 thickness of ~5 nm for all metals; further increase to 10 and 25 nm results in sharp reduction of emission intensity. Table 1 lists the enhancement ratios (defined as the ratio of UV emission intensity of the Al2O3/metal-coated NWs to that of the pristine ZnO NWs) for the four types of metal at the spacer thickness of g=5 nm. Substantial PL enhancement in ZnO and other wide bandgap semiconductors by Ag and Al plasmons has been observed previously [10, 40]. Maximum enhancement is expected for resonant coupling between metal SPs and ZnO excitons when the plasmon frequency lies near 3.3 eV (380 nm). For Ag and Al which have localized plasma frequencies in the near UV range, the enhancement ratios are expected to be greater than for Au and Pt, which is not exactly the case in Table 1. The most likely reason for this discrepancy is the different particle size. Although the metals were sputtered under the same condition, the nanoparticle sizes and associated UV extinction coefficients vary significantly from one to the next (the particle sizes roughly estimated from TEM images are ~5 nm for Au, 15 nm for Ag, and 3 nm for Pt. As for Al, the size is indistinguishable probably because of oxidation). The observed PL enhancement of Al is nearly 10 times that of other metals. This could be related to the natural oxidation of the Al nanoparticles, which added an extra Al2O3 layer surrounding the Al core and gave an extra enhancement.
To further examine how sensitively the enhancement depends on Al2O3 spacer thickness, and to check if there exists a competing passivation effect at small (g<5 nm) Al2O3 thickness, a set of control samples with the same gold nanoparticles but different Al2O3 thicknesses were prepared and characterized by PL spectroscopy. Figure 4 shows the UV peak intensity increases monotonically with increasing spacer thickness in the range of g=1–5 nm, with the visible emission being heavily suppressed in all cases.
Combining the two sets of g-dependent data above generates the plot shown in inset Figure 4. A transition of the enhancement ratios occurs near g=5 nm, which is a very similar trend to that observed by Sorger et al. in the spacer thickness dependence of PL enhancement due to coupling with the waveguide plasmonic mode . In the Sorger work, dye molecules were placed inside a nanowire-film spacer with high optical localization, and the PL from the molecules formed a hybrid plasmonic polariton (HPP) waveguide mode. The PL enhancement peaked around g=10 nm because of a dominating nanoscale HPP mode over other emission channels, and quenching at smaller g . In our case, although the exact mechanism for the PL enhancement may differ from Sorger et al. the observed shape in Figure 4 (inset) can also be qualitatively explained by two competing effects: i) enhancement of the spontaneous emission rate with decreasing spacer thickness, and ii) an monotonically increasing coupling probability with increasing spacer thickness. The modal Purcell factor is the product of both and hence explains the maximum PL enhancement around 5 nm. The enhancement ratios approach zero at large (g>10 nm) spacer thickness, which, as expected, can be understood by the evanescent nature of the electric field of the SPs across the dielectric layer.
To complement PL studies of NW arrays, CL spectroscopy was employed to investigate the spatially-resolved luminescence properties of individual NWs (Figure 5). The versatility and high spatial-resolution of CL spectroscopy has established it as a powerful tool in the field of nanophotonics [46, 47], plasmonics [44, 48, 49], and more recently metamaterials . The NWs were transferred from the growth substrates (GaN/sapphire) into an ethanol solution and then dispersed on Si substrates. To ensure consistency of electron beam excitation conditions for CL analysis, both as-grown ZnO and metal-capped ZnO NWs with Al2O3 spacers were dispersed on the same Si substrate (see Figure 5A). CL spectra and corresponding intensity maps for bare ZnO NWs, ZnO NWs coated with 5-nm Al2O3/Pt, and ZnO NWs coated with 5-nm Al2O3/Au are presented in Figures 5d–f. The spectra were calibrated by assuming the intensity is proportional to the spectrum acquisition time. The CL results for individual NWs correspond well with PL data for NW arrays, revealing again the enhancement of UV emission is quenching of visible emission for metal-coated samples. The mapping result shows that defect emission is uniformly excited for electron injection points along the entire length of pristine ZnO NWs, but that UV emission is relatively less uniformly excited for all samples. This may indicate that the visible emissions originate mainly from the intrinsic defects on the NW surface while the UV emission intensity depends on the coating homogeneity of the metal particles.
Through a systematic study of the photo- and cathodoluminescence properties of metal-coated ZnO nanowires with an intermediate dielectric (Al2O3) spacer, we provide a strategy for tailoring the optical properties of ZnO NWs. Microscopic, spatially resolved cathodoluminescence measurements on individual NWs are entirely consistent with macroscopic photoluminescence data for NW arrays. The enhancement of band edge luminescence is found to be highly sensitive to the presence and thickness of the Al2O3 spacer: The metal-induced enhancement of UV emission with a dielectric spacer is dramatically larger than the case without spacer, and significant enhancements are obtained at an optimum Al2O3 thickness of ~5 nm for the four metals studied (Au, Ag, Al and Pt). This drastic enhancement of NBEs and elimination of defect emissions by metal-dielectric coating ZnO nanowires may have positive effect on their applications in UV detector, light emitting diodes, biosensors, and photovoltaics.
This work is supported by the following research grants: Ministry of Education (MOE) Tier 1 grants (M4011011.110, M4010802.110); an NTU start-up grant M4080514.110; a MOE Tier 3 grant, and the Engineering and Physical Sciences Research Council (grant EP/G060363/1).
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