Abstract
The chapter reports photoluminescence (PL) and an energy transfer dynamic in a hybrid heterostructure consisting of an Ag nanoparticle (NP) layer and Cd0.08Zn0.92O/ZnO quantum well (QW). The observed PL quenching was closely related to electronic states of excitons confined in the QW. The PL quenching of the QW emission was only observed at low temperatures which excited carriers were radiatively recombined due to excitonic localization derived from fluctuated energy potentials in the QW. In contrast, delocalization of excitons from the QW with increasing temperature resulted in disappearance of the PL quenching. Time-resolved PL measurements revealed a decay rate of PL from the QW emission through the presence of energy transfer from the QW to Ag NP layer. The temperature-dependent energy-transfer rate was similar to that of the radiative recombination rate. The Ag NP layer surface showed a visible light absorption caused by localized surface plasmons (LSPs), which was very close to the PL peak energy of the QW. These results indicated that the excitonic recombination energy in the QW was nonradiatively transferred to Ag NP layer owing to energy resonance between the LSP and the QW. These phenomena could be explained by a surface energy transfer mechanism.
Keywords
- silver
- plasmon
- energy transfer
- exciton
- quantum well
1. Introduction
Semiconductor-based quantum wells (QWs) with metallic nanostructures, such as nanodots, nanoparticles and nanogratings, have received much attention as promising hybrid structures towards fabrications of plasmon-coupled emitting devices. In particular, the energy interactions between surface plasmons (SPs) and QW emitters have been investigated in relation to exciton-plasmon coupling [1, 2, 3, 4, 5]. SPs can effectively capture dipole oscillator energy in QWs. As a result, spontaneous decay rates of light emissions from QWs are remarkably modified in close proximity to metallic nanostructures [6]. These phenomena usually lead to enhancing or quenching processes. Enhanced luminescence is promoted by localized electromagnetic fields (near-fields) induced by SPs on metallic nanostructured surfaces. On the other hand, quenching of luminescence is attributed to various energy dissipation mechanisms based on plasmon damping in metals, such as plasmonic absorptions, excitonic diffusions, and electron-hole pair excitations [7, 8, 9]. Dissipation processes related to these mechanisms are more significant than the nonradiative recombination in QWs, which results in remarkable decrease of quantum efficiency of QWs. An elucidation of luminescent quenching is a key issue for the fabrications of the plasmon-based emitting devices on QWs.
Detailed investigations concerning photoluminescent (PL) quenching have been demonstrated hybrid structures of metallic nanoparticles and semiconductor quantum dots (QDs) [10, 11, 12, 13, 14]. The quenching effect has been utilized to bio-photonic applications such as chips for the detections of DNA, protein molecules, and energy transfer assays [15]. In the cases of Au nanoparticles (NPs)–CdSe QDs, the efficient PL quenching has been closely related to the spectral overlap of plasmon absorptions of Au NPs and light emissions from the QDs, as well as the separation length between Au nanoparticles and the QDs. The recombination energy of confined excitons in the QDs has been nonradiatively transferred to the Au NPs, leading to shortening lifetimes of light emissions from the QDs, which could be explained by both mechanisms of Förster energy transfer (FRET) [16] and surface energy transfer (SET) processes [17]. A FRET process induced between Au NPs and QDs is based on a dipole-dipole interaction between Au NPs and QDs. A dipole-dipole interaction is known as short-range coupling with
Recently, energy coupling between QWs and SPs has been studied on InxGa1-xN-based and CdxZn1-xO-based QWs with Ag nanostructures. Some studies demonstrated PL enhancement [18], while others reported quenched PL [19, 20]. For InxGa1-xN and CdxZn1-xO alloys, it is known that electronic states in QWs are present either in the form of bound electron–hole pairs (localized excitons) or free electrons and holes (free excitons), as realized by carrier localization or delocalization, respectively. These electronic states give an influence to the quantum efficiency of InxGa1-xN and CdxZn1-xO QWs, which is further dependent on temperature [21, 22, 23]. High quantum efficiency is based on excitonic localization at defect sites such as interface disorders and atom fluctuations. In the concrete cases of InxGa1-xN QWs with Ag nanostructures, it was reported that the plasmon-coupled PL properties at room temperature were suppressed at low temperatures [24]. Furthermore, it was confirmed that the PL quenching became strong with increasing quantum efficiency of InxGa1-xN QWs [25]. These results resulted in the conjecture that the PL quenching is closely attributed to the electronic states of InxGa1-xN QWs in the presence of Ag nanostructures. However, the influence of plasmonic fields on PL quenching has, for the most part, remained undocumented at present. Accordingly, the origin of nonradiative energy transfer contributing to PL quenching is not fully understood.
In this study, I focus on CdxZn1-xO QWs with visible PL for studies of plasmon-exciton coupling. To date, we have investigated band-edge luminescence of CdxZn1-xO QWs in order to understand optical properties of excitonic recombination in the QWs [26, 27, 28]. CdxZn1-xO QWs along the polar axis produce internal fields due to piezoelectric and spontaneous polarizations between well and barrier layers. In the case, we must consider two kinds of radiative processes. One process involves excitonic localization because of spatial cadmium fluctuations. The cadmium-rich regions have functions of radiative recombination centers of localized excitons at potential minima formed in CdxZn1-xO wells. The other process is related to the quantum-confinement Stark effect (QCSE) generated by an internal field induced in wells [29]. For a well whose width is greater than the excitonic Bohr diameter of the free exciton (
This chapter reports the temperature-dependent optical dynamics of PL quenching in a CdxZn1-xO QW placed in the vicinity of Ag NP layer. The samples in this work do not exhibit PL enhancement at any temperature, which is convenient for optical studies of quenching behaviors. In our case, pulsed optical excitations are used to evaluate the dynamics of energy transfer and other competing processes concerning carrier recombination in the QW. To an effort to understand the exciton-plasmon coupling, two kinds of relaxation steps in the QW were considered, namely, radiative or nonradiative recombination of excited carriers (electrons and holes). These physical states can be alternatively realized by changing the temperature. In addition, a ZnO spacer of variable thickness spatially separated the QW from the Ag nanostructures on top of the QW structure, which was capable of revealing subsequent energy transfer processes.
2. Fabrications and evaluations of CdxZn1-xO QWs
Single Cd0.08Zn0.92O/ZnO quantum wells (QWs) were deposited on O-polar ZnO (000-1) substrates using a pulsed laser deposition method [30]. ArF excimer laser pulses (193 nm, 3 Hz and 1 J/cm
The cross-sectional images of scanning transmittance electron microscopy (STEM) revealed the local structure of a QW with a well width (
3. Optical and structural properties of assembled Ag nanoparticles
Figure 2(a) shows a surface AFM image for the Ag structure fabricated on the QW. Ag layer consisted of small nanoparticles. The homogeneity of assembled Ag NPs was identified from a fast Fourier transform pattern (inset of Figure 2(a)). The extinction spectrum of the Ag NP layer showed a peak top at 2.60 eV, relating to a LSP resonance (Figure 2(b)) [31]. The photon energy of the LSP overlapped with that of the QW emission. This can produce an efficient energy interaction between the LSP and the QW. In addition, the reflection spectra in Figure 2(c) revealed that the Ag NP layer exhibited weak reflection intensity at 2.60 eV, i.e., the extinction of the Ag NP layer was almost dominated not by light scattering but by absorption. These optical properties were derived from a LSP field generated on the Ag NP layer surface [32]. Therefore, an increase of light absorption in the QW due to light extraction by reflection from the metal “mirror” is very small.
Figure 3(a) shows the cross-section TEM image of local structure of the Cd0.08Zn0.92O QW and the Ag NP layer at atomic scale. No threading dislocation was seen at the interface between the buffer layer and the substrate. A detailed structure of the Ag NP layer was clearly obtained. Figure 3(b) shows a cross-section TEM image focused on a heterointerface between Ag NP and ZnO layers. The Ag NP layer was composed of homogeneous alignment structure of Ag NPs with lateral and vertical sizes of 20 nm. Each Ag NP was directly located on the ZnO capping layer with a very flat surface. No thin-layered Ag nanostructure was found at the interface. In an effort to measure the local structure of the QW, a STEM was used to observe a Z-contrast image. Spatial separation between the QW and the Ag NP layer was clearly identified. The interface of the well and capping layers was found to be smooth. This also indicated that excited carriers generated in the QW were spatially separated from the Ag NP layer at nanoscale.
4. PL modulation and excitonic localization
Figure 4(a) shows the steady-state PL spectra of uncoated and Ag-coated Cd0.08Zn0.92O QW as a function of temperature. A cw InGaN laser (403 nm) was selected as excitation source to perform steady-state PL of light emission from the QW. The luminescence is dispersed by a single monochromator glazed at 500 nm. The PL intensity was dependent on the presence of Ag NP layer. At a temperature of 10 K with the Ag-coated QW, the intensity of the PL emission from the QW (
where
Figure 5(a) shows the time-resolved PL (TRPL) signals at 10 K taken at the PL peak energy of the uncoated and Ag-coated QWs. The TRPL signals obeyed bi-exponential decays represented by
5. Nonradiative energy transfer
For the uncoated QW, excited carriers undergo by radiative (
The
The calculated transfer efficiency achieved was as high as 34% at 10 K and then decreased quickly at high temperatures above 75 K (Figure 6(b)). Thus, the long PL lifetime of the QW emission at the low temperatures can make QW-LSP coupling highly probable. However, a short PL lifetime could not provide a sufficient energy transfer rate, resulting in no change in PL intensity of the QW emission.
6. Spatial length of quenching efficiency
The relationship between a separation length (
where
where
Previous studies on quenching related to metallic-induced luminescence have suggested two different models. One is the contact model proposed by Choosing
As another aspect, participation of the LSP involving the PL quenching was explored using a flat Ag layer. The RD-sputtered Ag layer showed a very flat surface with a roughness of 1 nm and had no LSP absorption (inset of Figure 7(b)). In this case, the value of
7. Discussion
The PL enhancement of InxGa1-xN/GaN QWs with Ag nanostructures has been observed near room temperature (RT), relating to the electronic states in the QWs. Lu
In contrast, PL quenching of the CdxZn1-xO QW with Ag NP layer in this work showed the opposite tendency. The transfer efficiency enhanced with decreasing temperature. The temperature dependence of the energy transfer rate was similar to that of the radiative decay rate (Figure 6(a) and 6(b)). Therefore, the recombination energy of localized excitons in the QW is partially consumed by nonradiative energy transfer to the Ag NP layer, resulting in quenching PL. This originates from an overlap of photon energy between the QW emission and the LSP absorption. This situation has been also observed on Au NP–CdSe QDs [40]. The difference in energy transfer processes between PL enhancement and quenching is attributed to electronic states in a QW placed in vicinity of metallic nanostructures, that is, localization or delocalization of excited carriers. It is suggested that radiative QW emission due to excitonic localization contributes to PL quenching, which supported the past reports that PL suppression became strong with decreasing temperature as well as with increasing quantum efficiency on InxGa1-xN QWs. The prevention of localized excitons in the QW would be desired to reduce PL quenching phenomenon if possible toward efficient plasmon-coupled emitting devices.
8. Conclusion
The remarkable PL quenching in the hybrid structure of CdxZn1-xO QW with Ag NP layer was observed at low temperatures, which was strongly dependent on the electronic states in the QW. The quenching effect was found in the temperature region in which excited carriers radiatively recombined owing to excitonic localization. On the other hand, delocalization of excitons with increasing temperature led to a decreased quenching efficiency. Energy transfer for the PL quenching was generated when the photon energy of the QW emission overlapped with that of LSP absorption. In addition, the spatial separation of the QW from the Ag NP layer revealed that the PL quenching showed a long-range length obeyed almost, which could be explained by a SET process. The comparative study used the Ag NP layers with different character clarified that the origin of PL quenching was attributed to the local LSP field induced on Ag NP layer. As a consequence, it was indicated that QW-LSP coupling played an important role in quenching PL. This coupling required existence of a pair of trapped carriers “localized excitons” as an electronic state in the QW. On the other hand, the quenching effect was not observed at the high temperatures at which excited carriers exist as free electrons and holes in the QW. Then, QW emission was dominated by a decay rate that was exhibited in the PL as a nonradiative contribution. It was identified that the electronic states of a QW played an important role in quenching PL.
Acknowledgments
This research was supported by a grant-in-Aid for Exploratory Research (No. 15 K13331) and Scientific Research (B) (No. 25289084).
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