Open Access
Add to BibSonomy
Abstract
Here we report a novel hybrid structure composing of microsphere array (MA), Al nanoparticles (Al-NPs), ZnO thin film (luminescence layer), Au nanoparticles (Au-NPs), and substrate (sapphire) for ultraviolet (UV) luminescence enhancement of planar wide bandgap semiconductor film. The plasmonic sandwich structure of Al-NPs/ZnO/Au-NPs boosts the hot electron state density in the conduction band by electron trapping from deep-defect level of ZnO and localized surface plasmon resonances (LSPRs) coupling around dual metallic NPs, enhancing UV emission and suppressing visible emission efficiently. The dielectric microsphere array capping on the plasmonic sandwich structure further increases UV emission intensity via photonic nanojets, optical whispering-gallery modes (WGMs), and directional antenna effect, by which the interaction between photon and exciton is strengthened. The contribution of microsphere cavity coupling with LSPRs to UV luminescence enhancement is therefore revealed. The maximum enhancement ratio of up to two orders of magnitude (~250-fold) is achieved by the optimized 5.06-μm-diameter-MA/Al-NPs/ZnO/Au-NPs/sapphire structure and the UV emission is highly directional with a divergent angle of ~5°. The present work provides a simple and easily-prepared structure incorporating optical WGMs and LSPRs to manipulate UV luminescence of planar wide-bandgap semiconductors for potential optoelectronic applications.
© 2019 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
The considerable demands in information science and technology motivate the development of the 3rd wide bandgap semiconductors, e.g. GaN, ZnO, SiC, ZnS, etc., due to their superior optoelectronic properties used in ultraviolet (UV) detectors [1,2], LEDs [3,4], piezoelectric elements [5,6], solar cells [7,8], low-threshold UV lasers [9–11], etc. Compared with the 1st- and the 2nd- generation semiconductors, the 3rd-generation semiconductors possess a wider bandgap, higher exciton binding energy, and higher saturated electron drift velocity, etc. As the representative of the 3rd semiconductors, ZnO has attracted a great attention owing to the direct bandgap (~3.37 eV) with a large exciton binding energy (60 meV) at room temperature. It has been widely applied in UV optoelectronic devices [12–14]. The typical photoluminescence (PL) spectrum of ZnO consists of two bands, i.e. near-band-edge (NBE) excitonic emission in the UV band and deep-defect-related levels (DL) emission in the visible (VIS) band [15]. The improvement of UV emission and suppression of VIS emission is critical for ZnO-based low-threshold UV-LEDs/lasers.
In the past decades, surface plasmons (SPs) have been widely utilized to enhance photoluminescence of ZnO by combining with noble metal film or nanoparticles (NPs) [16–20]. A variety of metals, e.g. Au, Ag, Al, Pt, etc., has been employed and the corresponding enhancement mechanisms have been revealed in previous studies [21–27]. Li et al. found the dependence of localized surface plasmon resonances (LSPRs) coupled emission on light-extracting direction in the Ag/ZnO/Si structure, where the maximum enhancement ratio of intensity (ERI) was 15 for downward extraction [28,29]. Koleva et al. demonstrated the enhanced blue-shifted photoluminescence of ZnO nanolayers by plasmonic resonance around Ag NPs [30]. Xu et al. achieved 12-fold enhancement for UV emission in ZnO/Ag film by thermal annealing [31]. Xiao et al. realized great emission enhancement in an Ag-NPs/SiO2/ZnO film sandwiched structure, where the enhancement was attributed to the resonant coupling between LSPRs and spontaneous emission of ZnO [32,33]. Jiang et al. found that the interaction between incident light and LSPRs enhanced the UV emission and suppressed the VIS emission in the Au/ZnO/sapphire and ZnO/Au/sapphire structures [34]. Xu et al. decorated ZnO microrod with Au NPs and first observed the dramatic enhancement of spontaneous and stimulated emission [35,36]. Pan et al. also exhibited 84-fold enhancement of UV emission and 8.3-fold enhancement of internal quantum efficiency in the non-polar ZnO thin films coated with Al/AlOx [37]. Unfortunately, the coupling condition and inherent energy loss in SPs limited the ERI of UV emission in one order of magnitude for high-quality crystalline ZnO luminescence layers [23,33,38].
In addition to metals, dielectric micro/nano-structures regulate the light with a wide spectral band due to their low absorptance from UV to near-infrared. Fused silica microsphere array (MA) is the typical representative of dielectric metamaterials with several extraordinary properties for light manipulation in microscale, i.e. photonic nanojets [39–41], optical whispering gallery modes (WGMs) [42,43], and directional antenna effect [44,45]. The microsphere cavity has demonstrated the capability in the fields of single-nanoparticle detection [46,47], fluorescence enhancement [48–52], Raman scattering enhancement [53–58], super-resolution imaging [59–62], etc. In our previous work, we have demonstrated PL enhancement by a dielectric MA capping on the ZnO thin film grown on a non-metallic substrate [51]. The mechanism of PL enhancement by the dielectric MA was proposed, for the first time [52]. The PL enhancement for high-quality single-crystal ZnO bulk capping with a MA was also characterized, in which the effects of microsphere diameter, excitation power, tilting angle, and ambient temperature on enhancement ratio were investigated sophisticatedly and in comparison with LSPRs enhancement by Au-NPs [63]. However, the dielectric MA coupled with metallic NPs for the purpose of UV-PL enhancement has not been studied so far. The combination of LSPRs in metallic NPs with photonic nanojets, optical WGMs and directional antenna effect in dielectric microsphere cavity would achieve a giant enhancement in the UV band for the wide bandgap luminescence semiconductors.
In this work, a novel hybrid structure, composing of dielectric MA/Al-NPs/ZnO/Au-NPs/substrate, was proposed. It combined dielectric microsphere cavity with dual metallic NPs to form a sandwich structure for UV-PL enhancement and VIS-PL suppression. It paves a simple way to manipulate the interaction between photon and exciton as well as excitation and emission light fields for wide bandgap semiconductors as high-efficient light sources in future.
2. Experiment
According to our pilot experiments, it was found that the LSPRs-related PL enhancement was independent from the luminescence film thickness greater than 300 nm. In order to achieve the maximum ERI of PL, two different metallic NPs were selected, i.e. Al-NPs on top surface and Au-NPs on bottom surface of ZnO film forming Al-NPs/ZnO/Au-NPs structure. Such a sandwich layers with dual metallic NPs can stimulate LSPRs in a wide spectral band [23]. Then the MA covered onto the sandwich structure and the sapphire/silicon was used as the substrate for practical applications. Figure 1(a) shows the MA/Al-NPs/ZnO/Au-NPs/substrate structure and the typical morphologies of MA, Al-NPs, ZnO thin film and Au-NPs, respectively. The substrate was first cleaned ultrasonically in acetone, ethanol and deionized water for 15 mins each to remove surface contaminations. The Au-NPs were then sputtered onto the surface of substrate. Afterwards, ZnO thin film with the thickness of ~300 nm was grown by pulsed laser deposition (PLD) according to our previous work [52]. Then the top surface of ZnO thin film was coated with Al-NPs. The Au- and Al-NPs sputtering was performed at room temperature with the current of 5 mA and pressure of 8 Pa. The sputtering time was varied in 0 s, 10 s, 30 s, 50 s, 70 s, and 90 s to control the size and gap of Au- and Al-NPs, respectively. The ZnO thin film grown on substrate without metallic NPs was also prepared as the control. Finally, the fused silica (FS) microspheres with diameter of 1.49, 2.47, 3.98, 4.86, 5.06, 6.10, 6.46 and 7.27-μm were self-assembled onto the Al-NPs/ZnO/Au-NPs/substrate sandwich structure by drop-coating [51]. The hybrid MA/Al-NPs/ZnO/Au-NPs/substrate structure was therefore prepared.
Fig. 1 Experimental configuration for acquisition of PL spectra from MA/Al-NPs/ZnO/Au-NPs/substrate structure. (a) Diagram of hybrid dielectric MA/metallic NPs sandwich structure and surface morphology of MA, Al-NPs, ZnO thin film and Au-NPs. (b) Schematic of spectroscopic setup. (c) XRD pattern of ZnO thin film grown on the sapphire substrate with/without Au-NPs.
The PL spectra were acquired by a house-made micro-PL spectroscope coupled with a 325-nm CW laser (KIMMON IK3301R-G) and a high-resolution monochromator (HORIBA iHR550). Figure 1(b) shows the schematic of the spectroscopic setup. The excitation laser was focused by a 3 × objective with NA of 0.08, by which the power arriving onto the sample was 0.5 mW. The ion sputtering coater (KYKY, XBC-12) was used to deposit Au/Al-NPs. The surface and cross-sectional morphologies of ZnO thin film as well as Au/Al-NPs were characterized by scanning electron microscopy (SEM, HITACHI UHR FE-SEM SU8220). The crystalline structures of ZnO/sapphire with/without Au-NPs were analyzed by X-ray diffraction (XRD, PERSEE, PGENERAL XD-3). Figure 1(c) shows the XRD pattern of ZnO thin film grown on the sapphire substrate with/without Au-NPs, where the (002) and (004) crystalline planes indicated the highly c-axis oriented growth of ZnO thin film. The absorption spectrum of Au- and Al-NPs were measured by a UV/VIS/NIR spectrophotometer (Shimadzu UV-2401).
A finite element method (FEM) algorithm using the commercial software of COMSOL Multiphysics (licensed by COMSOL Co. Ltd.) was employed to study the electric fields of excitation and emission light from the hybrid luminescence structure. A 2D model was developed to simulate the LSPRs, photonic nanojets, optical WGMs, and optical directional antenna effect on PL enhancement. The refractive indexes of microsphere, ZnO thin film and sapphire substrate were set as 1.48, 1.69, and 1.76 at 325 nm as well as 1.47, 1.73 and 1.76 at 376 nm, respectively. The extinction coefficient of ZnO thin film was 0.15 at 325 nm and 0.03 at 376 nm [64]. The complex permittivities of Au- and Al-NPs were set as −0.19 + 4.74i and −14.27 + 2.21i at 325 nm as well as −0.80 + 4.76i and −19.38 + 3.56i at 376 nm, respectively [65]. The periodic boundary conditions were applied in the model to simulate the microsphere array. A 325 nm wavelength plane wave was used as the excitation source to obtain the focusing strength, whereas an electrical dipole with a central wavelength at 376 nm located at the microsphere bottom was employed as the PL emission.
3. Results and discussion
3.1 UV-PL enhancement in Al-NPs/ZnO/Au-NPs sandwich structure
The evolutions of PL spectra of ZnO thin film grown on sapphire and silicon substrates with different Au-NPs sputtering time were first studied. It is well known that the LSPRs are very sensitive to the size and gap of NPs. Figures 2(a) and 2(b) show the morphologies of Au-NPs on sapphire and Si substrates. The small Au-NPs were sparsely distributed onto the substrate surface in the sputtering time of 10 s. The size of NPs was then increased from 10 nm to 16 nm whereas the gap was reduced from 8 nm to 3 nm with the sputtering time increasing to 30 s. Further increasing the sputtering time up to 50 s resulted in the formation of Au islands with the size greater than 100 nm and gradual mergence as an Au film, suppressing the effect of LSPRs on UV emission enhancement. Therefore, the sputtering time of 30 s was optimal for the Au-NPs formation between substrate and ZnO thin film. In Figs. 2(c) and 2(d), it can be clearly seen that Au-NPs at the bottom side of ZnO thin film enhanced the UV emission and suppressed the VIS emission. It is attributed to the energy transfer from deep-defect level emission to LSPRs for hot-electrons flowing into the conduction band, which enhances NBE-related spontaneous emission [23,66]. The variations of ERI for UV and VIS emission (i.e. ERIUV and ERIVIS) as well as normalized ERI to VIS band (i.e. ERIUV/VIS) are shown in the insets of Figs. 2(c) and 2(d). The ERIUV was increased up to the maximum of 5.2 folds at the optimal sputtering time of 30 s for both substrates. Meanwhile the ERIVIS was suppressed down to ~0.5 folds due to the absorption band of Au-NPs near 550 nm. It should also be noted that the Au-NPs at the sputtering time of 30 s isolated the effect of substrate on PL emission and therefore, the underlying Au-NPs made the ZnO thin film suitable to any substrate even with significant lattice mismatch (e.g. silicon) for practical applications.
Fig. 2 Evolution of PL intensity with sputtering time of Au-NPs on various substrates. SEM images of (a) Au-NPs on sapphire and (b) Au-NPs on silicon structures with different sputtering time. PL spectra of ZnO thin films with/without underlying Au-NPs at different sputtering time (0 s, 10 s, 30 s, 50 s, 70 s and 90 s) on (c) sapphire and (d) silicon substrate. The insets are ERI of PL in UV and VIS bands as well as normalized ERI to VIS emission, respectively.
The Al-NPs were then deposited onto the ZnO/Au-NPs/substrate to form the Al-NPs/ZnO/Au-NPs/substrate sandwich structure. The Al-NPs/ZnO/substrate without underlying Au-NPs was also prepared as the control. The morphologies of Al-NPs decoration at different sputtering time is shown in Fig. 3(a). Figure 3(b) exhibits that the Al-NPs on the top surface of ZnO thin film can further enhance the UV-PL emission from the ZnO/Au-NPs/substrate and ZnO/substrate structure. The inset of Fig. 3(b) is the evolution of ERIUV, ERIVIS and ERIUV/VIS with Al-NPs sputtering time increasing. The ERIUV achieved the maximum of 6.89-fold and the DL emission intensity was completely suppressed down to the background noise level at the sputtering time of 32 s. The longer sputtering time resulted in the high density of NPs on top surface blocking the excitation light arriving onto the ZnO, causing the reduction of ERI in both UV and VIS bands. The ERIUV/VIS revealed that the optimal structure of Al-NPs/ZnO/Au-NPs/sapphire with Al-NPs and Au-NPs sputtering around 30 s, by which the maximum normalized enhancement ratio was 36.53 folds. The Al-NPs/ZnO/Au-NPs sandwich structure achieved the effective UV-PL enhancement and VIS-PL suppression for the ZnO thin film.
Fig. 3 Evolution of PL intensity with sputtering time of Al-NPs on ZnO/Au-NPs/sapphire and ZnO/sapphire samples. (a) SEM images of Al-NPs with different sputtering time (24 s, 28 s, 32 s, 36 s, 40 s). (b) PL spectra of ZnO thin film with/without Al-/Au-NPs. The insets are ERIUV, ERIVIS and ERIUV/VIS, respectively.
In order to understand the mechanism of PL enhancement in the Al-NPs/ZnO/Au-NPs sandwich structure, the corresponding diagram of energy level was proposed as shown in Fig. 4(a). The conduction band, valence band and deep-defect level of ZnO are located at −4.19 eV, −7.39 eV, −5.09 eV with respect to the absolute vacuum scale (AVS), respectively [35]. The Fermi energy level of Au and Al are −5.1 eV and −4.3 eV to AVS, respectively [35,67]. Considering the Fermi level of Au close to the deep-defect level of ZnO, the electrons trapped by the deep defects flows into the Au fermi level and then are excited by LSPRs due to resonant energy matching with DL emission, as shown in Fig. 4(b). The electron states at the interface of Au-NPs and ZnO was therefore increased and the hot electrons transiting into the conduction band of ZnO improved the recombination rate of NBE-related spontaneous emission [23]. On the top surface, the LSPRs were stimulated due to the absorption of excitation and emission light in UV band by Al-NPs, as shown in Fig. 4(b). The excitation light harvesting and Purcell’s effect in the plasmonic nanocavity (i.e. nanogaps between Al-NPs [27,68]) resulted in the number of free-excitons increased and therefore boosted UV-PL emission intensity, as shown in Fig. 3(b).
Fig. 4 Mechanism of PL enhancement in Al-NPs/ZnO/Au-NPs sandwich structure. (a) Schematic of energy coupling between LSPRs of Au/Al-NPs with UV and VIS emissions of ZnO. (b) Absorption spectra of Au- and Al-NPs from UV to VIS band.
3.2 Effect of dielectric microsphere array on PL enhancement by capping on Al-NPs/ZnO/Au-NPs sandwich structure
The mechanism of ZnO UV-PL enhancement by dielectric microsphere array (MA) has been revealed in our previous work [51,52,63]. The microsphere cavity inherently possesses several extraordinary properties, i.e. photonic nanojets, optical WGMs and directional antenna effect, which can further boost the ZnO UV-PL by coupling with LSPRs. The microsphere array was self-assembled onto the Al-NPs/ZnO/Au-NPs/sapphire. The effect of microsphere diameter on PL enhancement from the hybrid structure is shown in Fig. 5, in which the diameters were 1.49 μm, 2.47 μm, 3.98 μm, 4.86 μm, 5.06 μm, 6.10 μm, 6.46 μm and 7.27 μm, respectively. It can be seen that the UV and VIS emission intensities were further enhanced up to 36.04 folds and 8.28 folds, respectively, by capping 5.06-μm-diameter MA onto the plasmonic sandwich structure. The maximum ERIUV/VIS was hence calculated to be 36.04/8.28~4.35. Although the optimal diameter was same as our previous work for bare ZnO thin film. The ERIUV from the hybrid structure was ~3-fold higher than that of bare ZnO thin film without LSPRs coupling [51,52]. It confirmed the strengthened regulation of light field in the hybrid microsphere cavity/dual metallic NPs structure.
Fig. 5 PL enhancement of MA-capped Al-NPs/ZnO/Au-NPs/sapphire and corresponding ERI with various microsphere diameters. (a) PL spectra and (b) ERIs of PL in UV and VIS band using MAs with different microsphere diameters.
In order to understand the mechanism of PL enhancement in the hybrid microsphere cavity/dual metallic NPs structure, the electric fields in the UV band for excitation and luminescence emission were numerically simulated. The light focusing distribution and far-field emission pattern were analyzed. Figure 6(a) shows the electric field of excitation light focused at the shadow side of a single microsphere. Considering the Mie scattering of microsphere and the exciton behavior of NBE and DL emissions, the ERI by microsphere focusing, ERIf, can be calculated as [52]
where |E(r)|2 is the intensity of electric filed on the surface of plasmonic sandwich structure, r is the radius of microsphere, and γ is the exponent determined by IPL∝Pγ (where IPL is the PL intensity and P is the excitation power [69]). Figure 6(b) shows that the fitted γ of NBE and DL emission were ~1.077 and ~0.758, confirming the typical free-exciton and bound-exciton behaviors, respectively. According to Eq. (1), the theoretical contribution of microsphere focusing effect to ERI of UV-PL emission was ~1.39.Fig. 6 PL enhancement mechanisms by MA capped plasmonic sandwich structure. (a) Focused excitation intensity distribution at the shadow side of 5.06-μm-diameter MA. (b) Experimental PL spectra of Al-NPs/ZnO/Au-NPs/sapphire plasmonic sandwich structure using the excitation power of the 325 nm laser in the range of 2-500 μw, where the inset shows the integrated PL peak intensities at 376 nm and ~550 nm as a function of excitation power in a double logarithmic plot. (c) Optical WGMs of UV-PL emission in the MA/Al-NPs/ZnO/Au-NPs/sapphire structure. (d) Electric field intensity and far-field polar distribution of UV-PL emission from the plasmonic sandwich structure to free space with/without MA capping.
The fused silica microsphere inherently possessed good transmittance in UV and VIS band. The scattered excitation light at shadow side of microsphere could therefore be trapped in the microsphere due to the solid immersion lens effect and formed WGMs increasing the excitation cross-sectional area in ZnO thin film [52]. As a result, the PL intensity was enhanced due to the multi-interaction of the excitation light at the contact point of MA with the sandwich structure. The ERI by excitation light trapping in the microsphere, ERIex, is thereby expressed as
where EAl-NPs/ZnO/Au-NPs(θ) and EMA/Al-NPs/ZnO/Au-NPs(θ) are angular vectors of electric fields without/with MA in the far-field, respectively. According to our previous work [52], the inherent absorption of ZnO film was 82.30% at 325 nm. According to Eq. (2), The MA trapped excitation light increasing the absorption from 82.30% to 96.57%, 98.56%, 98.47%, 99.49%, 99.15%, 99.72%, 99.51% and 99.78% by the microsphere diameter of 1.49-μm, 2.47-μm, 3.98-μm, 4.86-μm, 5.06-μm, 6.10-μm, 6.46-μm and 7.27-μm, respectively. For the microsphere with the diameter of 5.06 μm, the reflectivity was reduced by 21 folds for the ERIex up to ~1.20.The microsphere is also a typical optical cavity supporting (WGMs) with a high Q-factor by trapping PL emission at 376 nm via total internal reflection. Figure 6(c) shows the WGMs in a 5.06-μm-diameter microsphere capping on the sandwich structure. The WGMs resulted in Purcell’s effect boosting the NBE-related emission [51]. It is well known that ERI by Purcell’s effect is related to Q/V, where Q and V are the quality factor and mode volume of the microsphere cavity, respectively [70]. According to numerical simulation, the Q factors of MA on Al-NPs/ZnO/Au-NPs/sapphire and on ZnO/sapphire are 1232 and 465, respectively. The light coupling between microsphere cavity and Al-NPs enhanced the Q factor of ~2.65 folds, as shown in Fig. 6(c), by which the interaction between photon and exciton was boosted. Further considering the MA/ZnO/sapphire in the previous work where the ERIWGM was 1.51 [52], the ERIWGM of MA/Al-NPs/ZnO/Au-NPs/sapphire was estimated to be 1.51 × 2.65≈4.00.
Moreover, the ERI resulted from directional antenna of MA was attributed to the solid immersion lens effect, by which the PL emission was collected and redirected. The ERIantenna was determined by the microsphere diameter and NA of objective [42,52,55,63]. The ERIantenna in the hybrid structure is therefore calculated by [52]
Figure 6(d) shows the typical electrical field and far-field emission polar patterns from Al-NPs/ZnO/Au-NPs/sapphire capping with/without MA. It can be clearly seen that the MA coupled majority of PL energy from the plasmonic sandwich structure into the free space with a narrow divergent angle. The calculated ERIantenna for 1.49-μm, 2.47-μm, 3.98-μm, 4.86-μm, 5.06-μm, 6.10-μm, 6.46-μm and 7.27-μm-diameter MAs were 3.95, 4.13, 4.48, 5.42, 4.90, 3.59, 3.59 and 3.97 folds, respectively.The total ERI of PL by MA capped on the plasmonic sandwich structure is thereby given by
According to above-mentioned UV-PL enhancement contribution via light focusing, antireflection, WGMs, and directional antenna effects, the theoretical ERI was theoretically calculated to be 32.69 folds in the 5.06-μm-diameter-MA/Al-NPs/ZnO/Au-NPs/sapphire structure, in good agreement with experimental result, i.e. ~36.04 folds. Further considering the enhancement via plasmonic sandwich structure, the maximum ERIUV in the optimal hybrid MA/Al-NPs/ZnO/Au-NPs/sapphire structure was 6.89*36.04~250 folds and the ERIVIS was about 1.56 folds. In order to simplify the description and highlight the quantum efficiency enhancement, the ERIUV/VIS was used in the following discussion.3.3 Normalized PL spectra and unidirectional emission strength in the hybrid PL enhancement structure
Figure 7(a) shows the PL spectra in the hybrid MA/Al-NPs/ZnO/Au-NPs/sapphire, plasmonic Al-NPs/ZnO/Au-NPs/sapphire and ZnO/sapphire structures, of which the intensity of VIS emission is normalized. The maximum total ERIUV/VIS using 5.06-μm-diameter MA was ~160 folds, in which the ERIUV/VIS by plasmonic Au-/Al-NPs and by MA were 36.53 and 4.35, respectively, as discussed above. The superior property of the hybrid structure was of unidirectional emission, as shown in Fig. 6(d). In order to demonstrate the light emission regulation, the effect of tilting angle on ERIUV/VIS was investigated, as shown in Fig. 7(b). For the plasmonic sandwich structure without MA, the ERIUV/VIS kept constant with the tilting angle of 0-80° due to the homogeneous scattering by the disordered metallic NPs. However, a threshold of tilting angle at ~5° existed when the MA was capped onto the sandwich structure, where the ERIUV/VIS was constant within the threshold and then rapidly decreased with tilting angle further increasing. It should be noted that the threshold angle was related to the objective as following [54]
where n0 is the refractive index of ambient media and NA is the numerical aperture of objective. As theoretical estimation demonstrated in Fig. 6(d), the MA/Al-NPs/ZnO/Au-NPs/sapphire structure demonstrated angle-dependent emission owing to the optical antenna effect of microsphere. The two main lobes of PL emission in the far field was confined in the critical angle of ~4.59° calculated by Eq. (5). It validated the experimental result as shown in Fig. 7(b). The hybrid structure would therefore be beneficial to planar luminescence devices with highly directional emission in practical applications.Fig. 7 Normalized PL spectra and unidirectional emission from hybrid MA/Al-NPs/ZnO/Au-NPs/sapphire structure. (a) Normalized PL spectra and (b) effect of tilting angle on ERIUV/VIS in various ZnO-based luminescence setups.
4. Conclusions
The present work demonstrated a hybrid structure, composing of MA, dual metallic NPs, luminescence layer and substrate, for giant luminescence enhancement. The maximum ERI was ~250 folds for UV-PL emission of planar ZnO film in the hybrid 5.06-μm-diameter-MA/Al-NPs/ZnO/Au-NPs/sapphire structure. The PL spectra in the Al-NPs/ZnO/Au-NPs plasmonic sandwich structure and the absorption spectra of Au-/Al-NPs confirmed the luminescence enhancement mechanism by LSPRs. The plasmonic sandwich structure contributed the hot electron transferring to conduction band of ZnO via electron trapping from deep-defect levels and resonant energy coupling, by which the free-exciton recombination rate was boosted and the deep-defect-bound exciton transition was suppressed. The UV-PL was further enhanced by capping MA onto the plasmonic sandwich structure via photonic nanojets, optical WGMs of excitation light and UV-PL emission, as well as directional antenna effect. The excitation light focused and trapped in the MA increased the excitation cross-sectional area. The part of PL energy was enrolled for Purcell’s effect for UV emission enhancement and the rest was redirected into the free space with a narrow emission divergence. This work paves a new way to improve UV emission from the planar wide-bandgap semiconductors by simply capping MA on plasmonic sandwich structure. It would be beneficial for design of energy-saving on-chip luminescence devices with highly-directional emission for optoelectronic applications in future.
Funding
National Natural Science Foundation of China (11504012, 11674018); Beijing Nova Program (Z171100001117101); Young Talent Program of Beijing Municipal Commission of Education.
Acknowledgments
The authors acknowledge the support by National Natural Science Foundation of China, Beijing Nova Program, and Young Talent Program of Beijing Municipal Commission of Education.
References
1. C. Soci, A. Zhang, B. Xiang, S. A. Dayeh, D. P. R. Aplin, J. Park, X. Y. Bao, Y. H. Lo, and D. Wang, “ZnO nanowire UV photodetectors with high internal gain,” Nano Lett. 7(4), 1003–1009 (2007). [CrossRef] [PubMed]
2. X. Fu, Z. Liao, Y. Zhou, H. Wu, Y. Bie, J. Xu, and D. Yu, “Graphene/ZnO nanowire/graphene vertical structure based fast-response ultraviolet photodetector,” Appl. Phys. Lett. 100(22), 223114 (2012). [CrossRef]
3. T. H. Yang and J. M. Wu, “Thermal stability of sol–gel p-type Al–N codoped ZnO films and electric properties of nanostructured ZnO homojunctions fabricated by spin-coating them on ZnO nanorods,” Acta Mater. 60(8), 3310–3320 (2012). [CrossRef]
4. P. Tao, Q. Feng, J. Jiang, H. Zhao, R. Xu, S. Liu, M. Li, J. Sun, and Z. Song, “Electroluminescence from ZnO nanowires homojunction LED grown on Si substrate by simple chemical vapor deposition,” Chem. Phys. Lett. 522, 92–95 (2012). [CrossRef]
5. Z. L. Wang and J. Song, “Piezoelectric nanogenerators based on zinc oxide nanowire arrays,” Science 312(5771), 242–246 (2006). [CrossRef] [PubMed]
6. S. Stassi, V. Cauda, C. Ottone, A. Chiodoni, C. F. Pirri, and G. Canavese, “Flexible piezoelectric energy nanogenerator based on ZnO nanotubes hosted in a polycarbonate membrane,” Nano Energy 13, 474–481 (2015). [CrossRef]
7. Q. Zhang, C. S. Dandeneau, X. Zhou, and G. Cao, “ZnO nanostructures for dye-sensitized solar cells,” Adv. Mater. 21(41), 4087–4108 (2009). [CrossRef]
8. L. Li, T. Zhai, Y. Bando, and D. Golberg, “Recent progress of one-dimensional ZnO nanostructured solar cells,” Nano Energy 1(1), 91–106 (2012). [CrossRef]
9. S. Chu, G. Wang, W. Zhou, Y. Lin, L. Chernyak, J. Zhao, J. Kong, L. Li, J. Ren, and J. Liu, “Electrically pumped waveguide lasing from ZnO nanowires,” Nat. Nanotechnol. 6(8), 506–510 (2011). [CrossRef] [PubMed]
10. J. Dai, C. X. Xu, and X. W. Sun, “ZnO-Microrod/p-GaN heterostructured whispering-gallery-mode microlaser diodes,” Adv. Mater. 23(35), 4115–4119 (2011). [CrossRef] [PubMed]
11. Q. Zhang, G. Li, X. Liu, F. Qian, Y. Li, T. C. Sum, C. M. Lieber, and Q. Xiong, “A room temperature low-threshold ultraviolet plasmonic nanolaser,” Nat. Commun. 5(1), 4953 (2014). [CrossRef] [PubMed]
12. Ü. Özgür, Y. I. Alivov, C. Liu, A. Teke, M. A. Reshchikov, S. Doğan, V. Avrutin, S. J. Cho, and H. Morkoç, “A comprehensive review of ZnO materials and devices,” J. Appl. Phys. 98(4), 41301 (2005). [CrossRef]
13. A. Janotti and C. G. Van de Walle, “Fundamentals of zinc oxide as a semiconductor,” Rep. Prog. Phys. 72(12), 126501 (2009). [CrossRef]
14. A. B. Djurisić and Y. H. Leung, “Optical properties of ZnO nanostructures,” Small 2(8-9), 944–961 (2006). [CrossRef] [PubMed]
15. Y. G. Wang, S. P. Lau, X. H. Zhang, H. H. Hng, H. W. Lee, S. F. Yu, and B. K. Tay, “Enhancement of near-band-edge photoluminescence from ZnO films by face-to-face annealing,” J. Cryst. Growth 259(4), 335–342 (2003). [CrossRef]
16. H. Raether, “Surface plasmons on smooth and rough surfaces and on gratings,” Springer-Verlag Berlin An, (2013).
17. C. Clavero, “Plasmon-induced hot-electron generation at nanoparticle/metal-oxide interfaces for photovoltaic and photocatalytic devices,” Nat. Photonics 8(2), 95–103 (2014). [CrossRef]
18. V. Amendola, R. Pilot, M. Frasconi, O. M. Maragò, and M. A. Iatì, “Surface plasmon resonance in gold nanoparticles: a review,” Journal of physics. J. Phys-Condens. Mat. 29(20), 203002 (2017).
19. W. L. Barnes, A. Dereux, and T. W. Ebbesen, “Surface plasmon subwavelength optics,” Nature 424(6950), 824–830 (2003). [CrossRef] [PubMed]
20. K. L. Kelly, E. Coronado, L. L. Zhao, and G. C. Schatz, “The optical properties of metal nanoparticles: the influence of size, shape, and dielectric environment,” J. Phys. Chem. B 107(3), 668–677 (2003). [CrossRef]
21. B. J. Lawrie, R. F. Haglund Jr., and R. Mu, “Enhancement of ZnO photoluminescence by localized and propagating surface plasmons,” Opt. Express 17(4), 2565–2572 (2009). [CrossRef] [PubMed]
22. W. Chamorro, J. Ghanbaja, Y. Battie, A. E. Naciri, F. Soldera, F. Mücklich, and D. Horwat, “Local structure-driven localized surface plasmon absorption and enhanced photoluminescence in ZnO-Au thin films,” J. Phys. Chem. C 120(51), 29405–29413 (2016). [CrossRef]
23. C. Xu, F. Qin, Q. Zhu, J. Lu, Y. Wang, J. Li, Y. Lin, Q. Cui, Z. Shi, and A. G. Manohari, “Plasmon-enhanced ZnO whispering-gallery mode lasing,” Nano Res. 11(6), 3050–3064 (2018). [CrossRef]
24. K. W. Liu, Y. D. Tang, C. X. Cong, T. C. Sum, A. C. H. Huan, Z. X. Shen, L. Wang, F. Y. Jiang, X. W. Sun, and H. D. Sun, “Giant enhancement of top emission from ZnO thin film by nanopatterned Pt,” Appl. Phys. Lett. 94(15), 151102 (2009). [CrossRef]
25. F. Qin, N. Chang, C. Xu, Q. Zhu, M. Wei, Z. Zhu, F. Chen, and J. Lu, “Underlying mechanism of blue emission enhancement in Au decorated p-GaN film,” RSC Advances 7(25), 15071–15076 (2017). [CrossRef]
26. H. Zhou, R. Deng, Y. Li, B. Yao, J. Qin, J. Song, Y. Li, Z. Ding, and L. Liu, “Giant enhancement of ultraviolet near-band-edge emission from a wide-bandgap oxide with dipole-forbidden bandgap transition,” J. Alloys Compd. 705, 492–496 (2017). [CrossRef]
27. S. Dellis, N. Kalfagiannis, S. Kassavetis, C. Bazioti, G. P. Dimitrakopulos, D. C. Koutsogeorgis, and P. Patsalas, “Photoluminescence enhancement of ZnO via coupling with surface plasmons on Al thin films,” J. Appl. Phys. 121(10), 103104 (2017). [CrossRef]
28. P. Cheng, D. Li, Z. Yuan, P. Chen, and D. Yang, “Enhancement of ZnO light emission via coupling with localized surface plasmon of Ag island film,” Appl. Phys. Lett. 92(4), 041119 (2008). [CrossRef]
29. P. Cheng, D. Li, X. Li, T. Liu, and D. Yang, “Localized surface plasmon enhanced photoluminescence from ZnO films: Extraction direction and emitting layer thickness,” J. Appl. Phys. 106(6), 63120 (2009). [CrossRef]
30. M. E. Koleva, A. O. Dikovska, N. N. Nedyalkov, P. A. Atanasov, and I. A. Bliznakova, “Enhancement of ZnO photoluminescence by laser nanostructuring of Ag underlayer,” Appl. Surf. Sci. 258(23), 9181–9185 (2012). [CrossRef]
31. Y. S. Liu, H. W. Lu, X. L. Xu, M. G. Gong, L. Liu, and Z. Yang, “Localized Surface Plasmons Enhanced Ultraviolet Emission of ZnO Films,” Chin. Phys. Lett. 28(5), 57803 (2011). [CrossRef]
32. X. H. Xiao, F. Ren, X. D. Zhou, T. C. Peng, W. Wu, X. N. Peng, X. F. Yu, and C. Z. Jiang, “Surface plasmon-enhanced light emission using silver nanoparticles embedded in ZnO,” Appl. Phys. Lett. 97(7), 071909 (2010). [CrossRef]
33. X. D. Zhou, X. H. Xiao, J. X. Xu, G. X. Cai, F. Ren, and C. Z. Jiang, “Mechanism of the enhancement and quenching of ZnO photoluminescence by ZnO-Ag coupling,” EPL-. Europhys. Lett. 93(5), 57009 (2011). [CrossRef]
34. Y. Zeng, Y. Zhao, and Y. Jiang, “Investigation of the photoluminescence properties of Au/ZnO/sapphire and ZnO/Au/sapphire films by experimental study and electromagnetic simulation,” J. Alloys Compd. 625, 175–181 (2015). [CrossRef]
35. Y. Wang, C. Xu, J. Li, J. Dai, Y. Lin, G. Zhu, and J. Lu, “Improved whispering-gallery mode lasing of ZnO microtubes assisted by the localized surface plasmon resonance of Au nanoparticles,” Sci. Adv. Mater. 7(6), 1156–1162 (2015). [CrossRef]
36. F. F. Qin, C. X. Xu, Q. X. Zhu, J. F. Lu, D. T. You, W. Xu, Z. Zhu, A. G. Manohari, and F. Chen, “Extra green light induced ZnO ultraviolet lasing enhancement assisted by Au surface plasmons,” Nanoscale 10(2), 623–627 (2018). [CrossRef] [PubMed]
37. Y. Zhou, S. Chen, X. Pan, and Z. Ye, “Photoluminescence enhancement in non-polar ZnO films through metallodielectric mediated Al surface plasmons,” Opt. Lett. 43(10), 2288–2291 (2018). [CrossRef] [PubMed]
38. W. H. Ni, J. An, C. W. Lai, H. C. Ong, and J. B. Xu, “Emission enhancement from metallodielectric-capped ZnO films,” J. Appl. Phys. 100(2), 26103 (2006). [CrossRef]
39. B. S. Luk Yanchuk, R. Paniagua-Domínguez, I. Minin, O. Minin, and Z. Wang, “Refractive index less than two: photonic nanojets yesterday, today and tomorrow,” Opt. Mater. Express 7(6), 1820 (2017). [CrossRef]
40. Z. Chen, A. Taflove, and V. Backman, “Photonic nanojet enhancement of backscattering of light by nanoparticles: a potential novel visible-light ultramicroscopy technique,” Opt. Express 12(7), 1214–1220 (2004). [CrossRef] [PubMed]
41. H. S. Patel, P. K. Kushwaha, and M. K. Swami, “Generation of highly confined photonic nanojet using crescent-shape refractive index profile in microsphere,” Opt. Commun. 415, 140–145 (2018). [CrossRef]
42. K. J. Vahala, “Optical microcavities,” Nature 424(6950), 839–846 (2003). [CrossRef] [PubMed]
43. K. W. B. W. Garrett, C G B, “Stimulated.\ emission into optical whispering modes of spheres,” Phys. Rev. Lett. 124(6), 1807 (1961).
44. A. Devilez, B. Stout, and N. Bonod, “Compact metallo-dielectric optical antenna for ultra directional and enhanced radiative emission,” ACS Nano 4(6), 3390–3396 (2010). [CrossRef] [PubMed]
45. A. E. Krasnok, A. E. Miroshnichenko, P. A. Belov, and Y. S. Kivshar, “All-dielectric optical nanoantennas,” Opt. Express 20(18), 20599–20604 (2012). [CrossRef] [PubMed]
46. X. Li, Z. Chen, A. Taflove, and V. Backman, “Optical analysis of nanoparticles via enhanced backscattering facilitated by 3-D photonic nanojets,” Opt. Express 13(2), 526–533 (2005). [CrossRef] [PubMed]
47. S. Yang, A. Taflove, and V. Backman, “Experimental confirmation at visible light wavelengths of the backscattering enhancement phenomenon of the photonic nanojet,” Opt. Express 19(8), 7084–7093 (2011). [CrossRef] [PubMed]
48. S. C. Hill, V. Boutou, J. Yu, S. Ramstein, J. P. Wolf, Y. Pan, S. Holler, and R. K. Chang, “Enhanced backward-directed multiphoton-excited fluorescence from dielectric microcavities,” Phys. Rev. Lett. 85(1), 54–57 (2000). [CrossRef] [PubMed]
49. D. Gérard, J. Wenger, A. Devilez, D. Gachet, B. Stout, N. Bonod, E. Popov, and H. Rigneault, “Strong electromagnetic confinement near dielectric microspheres to enhance single-molecule fluorescence,” Opt. Express 16(19), 15297–15303 (2008). [CrossRef] [PubMed]
50. D. Gérard, A. Devilez, H. Aouani, B. Stout, N. Bonod, J. M. Wenger, E. Popov, and H. Rigneault, “Efficient excitation and collection of single-molecule fluorescence close to a dielectric microsphere,” J. Opt. Soc. Am. B 26(7), 1473–1478 (2009). [CrossRef]
51. Y. Yan, Y. Zeng, Y. Wu, Y. Zhao, L. Ji, Y. Jiang, and L. Li, “Ten-fold enhancement of ZnO thin film ultraviolet-luminescence by dielectric microsphere arrays,” Opt. Express 22(19), 23552–23564 (2014). [CrossRef] [PubMed]
52. L. Yang, Y. Yan, Q. Wang, Y. Zeng, F. Liu, L. Li, Y. Zhao, and Y. Jiang, “Sandwich-structure-modulated photoluminescence enhancement of wide bandgap semiconductors capping with dielectric microsphere arrays,” Opt. Express 25(6), 6000–6014 (2017). [CrossRef] [PubMed]
53. K. J. Yi, H. Wang, Y. F. Lu, and Z. Y. Yang, “Enhanced Raman scattering by self-assembled silica spherical microparticles,” J. Appl. Phys. 101(6), 063528 (2007). [CrossRef]
54. Y. Yan, C. Xing, Y. Jia, Y. Zeng, Y. Zhao, and Y. Jiang, “Self-assembled dielectric microsphere array enhanced Raman scattering for large-area and ultra-long working distance confocal detection,” Opt. Express 23(20), 25854–25865 (2015). [CrossRef] [PubMed]
55. C. Xing, Y. Yan, C. Feng, J. Xu, P. Dong, W. Guan, Y. Zeng, Y. Zhao, and Y. Jiang, “Flexible microsphere-embedded film for microsphere-enhanced raman spectroscopy,” ACS Appl. Mater. Interfaces 9(38), 32896–32906 (2017). [CrossRef] [PubMed]
56. T. W. Chang, X. Wang, A. Mahigir, G. Veronis, G. L. Liu, and M. R. Gartia, “Marangoni convection assisted single molecule detection with nanojet surface enhanced Raman spectroscopy,” ACS Sens. 2(8), 1133–1138 (2017). [CrossRef] [PubMed]
57. G. M. Das, A. B. Ringne, V. R. Dantham, R. K. Easwaran, and R. Laha, “Numerical investigations on photonic nanojet mediated surface enhanced Raman scattering and fluorescence techniques,” Opt. Express 25(17), 19822–19831 (2017). [CrossRef] [PubMed]
58. A. Arya, R. Laha, G. M. Das, and V. R. Dantham, “Enhancement of Raman scattering signal using photonic nanojet of portable and reusable single microstructures,” J. Raman Spectrosc. 49(5), 897–902 (2018). [CrossRef]
59. Z. Wang, W. Guo, L. Li, B. Luk’yanchuk, A. Khan, Z. Liu, Z. Chen, and M. Hong, “Optical virtual imaging at 50 nm lateral resolution with a white-light nanoscope,” Nat. Commun. 2(1), 218 (2011). [CrossRef] [PubMed]
60. Y. Yan, L. Li, C. Feng, W. Guo, S. Lee, and M. Hong, “Microsphere-coupled scanning laser confocal nanoscope for sub-diffraction-limited imaging at 25 nm lateral resolution in the visible spectrum,” ACS Nano 8(2), 1809–1816 (2014). [CrossRef] [PubMed]
61. L. Li, W. Guo, Y. Yan, S. Lee, and T. Wang, “Label-free super-resolution imaging of adenoviruses by submerged microsphere optical nanoscopy,” Light-Sci. 2(9), e104 (2013). [CrossRef]
62. P. K. Upputuri and M. Pramanik, “Microsphere-aided optical microscopy and its applications for super-resolution imaging,” Opt. Commun. 404, 32–41 (2017). [CrossRef]
63. Y. Yan, J. Liu, C. Xing, Q. Wang, Y. Zeng, Y. Zhao, and Y. Jiang, “Parametric study on photoluminescence enhancement of high-quality zinc oxide single-crystal capping with dielectric microsphere array,” Appl. Opt. 57(27), 7740–7749 (2018). [CrossRef] [PubMed]
64. C. Stelling, C. R. Singh, M. Karg, T. A. F. König, M. Thelakkat, and M. Retsch, “Plasmonic nanomeshes: their ambivalent role as transparent electrodes in organic solar cells,” Sci. Rep. 7(1), 42530 (2017). [CrossRef] [PubMed]
65. A. D. Rakić, A. B. Djurišić, J. M. Elazar, and M. L. Majewski, “Optical properties of metallic films for vertical-cavity optoelectronic devices,” Appl. Opt. 37(22), 5271–5283 (1998). [CrossRef] [PubMed]
66. K. Saravanan, R. Krishnan, S. H. Hsieh, H. T. Wang, Y. F. Wang, W. F. Pong, K. Asokan, D. K. Avasthi, and D. Kanjilal, “Effect of defects and film thickness on the optical properties of ZnO-Au hybrid films,” RSC Advances 5(51), 4813–4819 (2015). [CrossRef]
67. J. Lu, J. Li, C. Xu, Y. Li, J. Dai, Y. Wang, Y. Lin, and S. Wang, “Direct resonant coupling of Al surface plasmon for ultraviolet photoluminescence enhancement of ZnO microrods,” ACS Appl. Mater. Interfaces 6(20), 18301–18305 (2014). [CrossRef] [PubMed]
68. Y. Wu, Y. Dai, S. Jiang, C. Ma, Y. Lin, D. Du, Y. Wu, H. Ding, Q. Zhang, N. Pan, and X. Wang, “Interfacially Al-doped ZnO nanowires: greatly enhanced near band edge emission through suppressed electron-phonon coupling and confined optical field,” Phys. Chem. Chem. Phys. 19(14), 9537–9544 (2017). [CrossRef] [PubMed]
69. T. Schmidt, K. Lischka, and W. Zulehner, “Excitation-power dependence of the near-band-edge photoluminescence of semiconductors,” Phys. Rev. B Condens. Matter 45(16), 8989–8994 (1992). [CrossRef] [PubMed]
70. M. V. Artemyev, U. Woggon, R. Wannemacher, H. Jaschinski, and W. Langbein, “Light trapped in a photonic dot: microspheres act as a cavity for quantum dot emission,” Nano Lett. 1(6), 309–314 (2001). [CrossRef]
