1. Introduction

On account of high durability and pony size, low dimensional ultraviolet light emitting devices have aroused highly interests among various scientific and commercial applications, including medical treatment, solid-state illumination, photolithography technique, biological sterilization and so on [1–5]. In particular, ultraviolet light emitting diodes (LEDs) via hyperpure narrow band are competitive candidates for the ever-increasing demands among various fields due to great luminescent monochromaticity, high stability, and independent of automatic color filter [6–11]. So far, hyperpure ultraviolet LEDs have been extensively reported in many kinds of materials and devices, including organic materials, perovskite, wide band gap semiconductors, etc.; together with various micro/nanostructures, including quantum dot, micro/nanowires and planar or vertical structures [12–17]. However, the development of organic materials and perovskite structures inevitably suffer from non-environmentally friendly (toxicity), poor stability and short working lifetime, which greatly limit the application of the above structures based ultraviolet LEDs. Among those, wide band gap semiconductors, such as II-VI oxides (ZnO, Ga$_2$O$_3$, and MgO), and III-V nitrides (GaN and AlN), are considered as the most competitive candidates for realizing ultraviolet LEDs via hyperpure narrow band due to high stability, low cost, low-power consumption, and environmentally friendly properties [18–23].

Over the past decades, much concerns are directed toward purifying light-emission of broad bandgap semiconductor ultraviolet LEDs, and tremendous efforts being made to narrow the spectral line width are summarized as follows: (i) Fabricating core-shell micro-/nanostructures: for example, in 2015, Liu et al., reported a high performance n-ZnO/p-GaN heterostructure ultraviolet LED using ZnO-SiO$_2$ core-shell nanorods array [24]. However, it is still a challenge to control the shell morphology, crystal phase, interface structure in the device fabricating procedure [25,26]. (ii) Adding insulating layer (electron-blocking interlayer): various dielectric layers containing MgO [27], AlN [28], Al$_2$O$_3$ [29], hBN [30], SiO$_2$ [31], have been utilized as electron-blocking interlayer to improve the lighting performance and emission purity of the n-ZnO/p-GaN heterojunction LEDs. In this regard, it is hard to precisely modulate the thickness of the interlayer between n-ZnO and p-GaN, which will affect the carrier injection and decrease the luminescence efficiency [23,32]. (iii) Introducing metallic nanostructures: recently, Ag nanowires and Rh nanostructures with ultraviolet plasmonic properties have been successfully synthesized and applied in constructing ultraviolet LEDs [33–36]. Though, the synthesis technology of those metal nanostructures are commonly wet chemical methods. The remanent organic solutions could decrease the stability of heterojunction devices and limit the application of devices [37–39]. Besides, plasmons induced the generation of hot electrons has been employed to construct n-ZnO:Ga/p-GaN based ultraviolet LED via hyperpure narrow band [40–42]. Nevertheless, the difficulties, including the excitation of localized plasmons, the generation of hot electrons, and the efficiency of hot electrons transferred into the nearby semiconductors, would inevitably influence the device characteristics [43,44]. Therefore, it is urgent to explore a controllable, repeatable and high efficient method to develop ultraviolet LED with hyperpure and narrow spectra.

In this study, individual ZnO microwires via Ga-doping (ZnO:Ga MWs) were utilized to fabricate near-ultraviolet LEDs, with p-type GaN film acting as hole transporting layer. The fabricated LEDs can exhibit bright electroluminescence (EL) peaking at 400.0 nm, with a line width of approximately 35 nm. In particular, by incorporating Pt nanoparticles (PtNPs) on ZnO:Ga MWs, hyperpure ultraviolet LEDs with narrower linewidth were realized. Compared with a bare wire based luminescence device, plasmon-enhanced EL lighting performances can be obtained in the presence of PtNPs, such as an obvious blueshift (from 400.0 to 377.0 nm), an enhanced light output (about 4-fold), and a remarkable reduction of line width (narrowing from 35 to 10 nm). Profit from PtNPs deposition, typical near-band-edge (NBE) emission of ZnO:Ga MW governed the EL characters of the fabricated LEDs. The working mechanism can be attributed to the excitation of PtNPs ultraviolet plasmons, and coupled with the neighboring ZnO:Ga. Therefore, with the aid of incorporating PtNPs via desired ultraviolet plasmonic properties, the realization of current-driven hyperpure ultraviolet LEDs could offer a simple, direct and effective scheme to construct light emitting devices via Ultra-narrowed line width in the ultraviolet wavelengths, together with the improvement of luminescence performance.

2. Expriemtntal and simulation section

$\mathbf {Fabrication\ of\ a\ ZnO:Ga\ MW\ covered\ by\ PtNPs}$: Individual ZnO:Ga MWs were synthesized by a chemical vapor deposition (CVD) method [45–47]. In this study, high-purity powders of ZnO, Ga$_{2}$O$_{3}$ and C with specific ratio of 10 : 1 : 11, were used as the precursor mixtures. The precursor mixtures were stored in a ceramic boat, with a Si-wafer placed on the top of the boat, serving the substrate to collect the MW. During the preparing procedure, high purity $Ar$ (99.99%, 125 sccm) was injected into the furnace chamber, and treated as the protecting gas, leading to the oxygen-deficient environment. As the reaction is accomplished, individual MWs can be obtained around the Si substrate. A MW was picked out, and placed on a cleaned glass-slide. In particles, serving as electrodes, were fixed the end-sides of the MW. Secondly, Pt nanofilms were sputtered onto the ZnO:Ga MW via radio-frequency (RF) magnetron sputtering technology (the sputtering time $\sim$ 300 s). By injecting current into the MW, bright fluorescent emission can be seen, with the luminescence regions located at the center of the wire. Due to Joule heating effect, the Pt nanofilms located at the center, can be annealed into physically isolated nanoparticles. Therefore, a ZnO:Ga MW covered by PtNPs (PtNPs@ZnO:Ga) was successfully prepared.

$\mathbf {Device\ fabrication}$: The procedure of fabricating the heterostructured device is concluded as follows [34,40,48]: (i) MgO nanofilms with a thickness of about 15 $\mu$m were prepared on the two sides of a GaN substrate by using an electron-beam evaporation system; (ii) Ni/Au (30/30 nm) were deposited on the p-GaN substrate using an electron-beam evaporation system, then followed by an annealing treatment in air; (iii) a MW was placed at the middle area of the GaN substrate, with bilateral MgO nanofilms serving as insulating layers; (iv) an ITO glass was placed at the top of the MW, leading to the fabrication of the heterojunction luminescence device. In the device architecture, the ITO layer and Ni/Au were utilized as electrodes for current injection.

$\mathbf {Finite\ difference\ time\ domain\ (FDTD)\ simulation}$: During the simulation, a perfectly matched layer (PML) was utilized to absorb the outgoing waves. Additionally, the near-field intensity $|\mathbf {E}|^2$ distribution for a single PtNP on a ZnO:Ga MW substrate during ultraviolet lasing excitation was also investigated by numerical simulation. In the model, a ZnO:Ga MW with hexagonal cross-section on a quartz substrate was constructed. The width of the MW $D$ = 15 $\mu$m, the size of the PtNP $d$ = 75 nm, and the calculated wavelength $\lambda$ = 370 nm; the corresponding refractive indices of ZnO:Ga n$_{ZnO:Ga}$ = 2.35, quartz substrate n$_{quartz}$ = 1.5, and air n$_{air}$ = 1.0. The complex relative permittivity for Pt at the excitation wavelength is denoted by $\varepsilon _{Pt}$ = -3.1 + 1.83i. In addition, the models were illuminated at normal incidence.

3. Results and discussions

The as-synthesized ZnO:Ga MWs were tested by using a scanning electron microscopy (SEM). Figure 1(a) shows a typical SEM photograph of an individual ZnO:Ga MW, exhibiting hexagonal cross-section, straight side walls and smooth surfaces (the diameter $D$ is 15 $\mu$m). As displayed in the inset of Fig. 1(a), the single wire exhibits near-perfect hexagon-shaped cross section. As our previous work demonstrated, individual MWs can be employed to fabricate incandescent-type EL devices [49]. As the injection current reached a certain value, we can see bright and green EL emission at the center of the wire. By incorporating Pt nanofilms via radio-frequency magnetron sputtering technique (the sputtering time: 300s), the single wire based incandescent-type emission device started to emit red lighting. The single ZnO:Ga MW decorated by Pt nanofilms was characterized. As exhibited in Fig. 1(b), the evaporated Pt nanofilms located in the emission region can be annealed into physically isolated nanoparticles [46,49]. An enlarged SEM image of the prepared PtNPs, is displayed in Fig. 1(c). The average size of these NPs is evaluated to be 75 nm. Besides, these obtained PtNPs also illustrate hemispherical structure. Thus, an individual ZnO:Ga MW decorated by physically isolated PtNPs via uniform distribution, can be prepared.

 

Fig. 1. (a) SEM image of a ZnO:Ga MW. Inset, the MW illustrates hexagonal cross section. (b) SEM image of ZnO:Ga MW covered by physically isolated PtNPs. And the enlarged SEM image of the PtNPs is shown in (c). (d) Normalized extinction spectrum of as-synthesized PtNPs. Inset, SEM image of PtNPs prepared on the quartz substrate. (e) $I$-$V$ curves of a ZnO:Ga MW before and after decorated by PtNPs. (f) PL spectra of a ZnO:Ga MW before and after decorated by PtNPs.

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Optical property of the as-synthesized PtNPs was studied by an ultraviolet-visible extinction spectroscopy via an UV-6300 spectrophotometer. The corresponding extinction spectrum is displayed in Fig. 1(d). From the figure, the major extinction peak positions at around 365 nm in ultraviolet wavelengths. The SEM photograph of PtNPs is also displayed in the inset of Fig. 1(d). It can be inferred that PtNPs possessing localized surface plasmonic energies in the ultraviolet region is successfully synthesized [49,50]. The electrical transport properties of a ZnO:Ga MW before and after decorated by PtNPs were tested via Keysight semiconductor device analyzer (B1500A) at room temperature. As depicted in Fig. 1(e), $I$-$V$ curves present linear behaviors, illustrating good ohmic contact formed between ZnO:Ga MWs and In electrodes. Besides, an observably enhanced electronic transport property was also achieved. While, the optical properties of single ZnO:Ga MW not covered and covered by PtNPs were also performed by using He-Cd laser (325 nm) as excitation source via a LabRAM-UV Jobin-Yvon spectrometer. The major PL wavelength of single ZnO:Ga MW peaks at around 378 nm in ultraviolet spectral region, reflecting a typical NBE emission of ZnO:Ga MW (Fig. 1(f), the blue solid line). Meanwhile, an ignorable visible emission was also observed, which may result from the surface defects of the as-synthesized ZnO:Ga MW (for example: oxygen vacancies) [40,49]. By introducing PtNPs, a huge enhancement of the NBE-type emission is acquired. The enhanced mechanism has been studied, and can be put down to good overlapping between the NBE emission of the ZnO:Ga MW and the localized surface plasmonic energy of the PtNPs, yielding the coupling interaction between ZnO:Ga excitons and ultraviolet plasmons of PtNPs [49,51].

Due to the modulation of PtNPs on optical and electrical properties, a ZnO:Ga MW before and after covered by PtNPs, were used to construct heterojunction emission devices, with p-type GaN film acting as hole transporting layer. In the LED architecture, ITO glass and Ni/Au served as the electrodes for the carrier injection [34,40,51]. Electrical characterization of the fabricated single MW heterostructure devices were performed. As exhibits in Fig. 2(a) (the black solid line), the $I$-$V$ curve of the n-ZnO:Ga MW/p-GaN heterojunction exhibits good rectifying diode-like characteristic. The illustration in inset is a schematic of the device. As we previously reported, ITO can make a well-defined ohmic contacting with ZnO:Ga MW; while, Ni/Au (30/30 nm) being evaporated on p-type GaN film, can also form ohmic contact. Thus, the LED-like rectifying properties can be assigned to the heterojunction, which being created between n-ZnO:Ga MW and p-type GaN film [34,48]. From the plotted $I$-$V$ curve, the turn-on voltage is evaluated to be 3.38 V [28,48,52]. Upon injecting current, the single MW LED gives out near-ultraviolet light. The signals of emitted photons were recorded via a PIXIS 1024BR CCD detection system. Figure 2(b) displays the EL spectra of the fabricated LED, with the input current varying in range of 0.90-13.95 mA. As shown in the figure, the major EL wavelengths peak at around 400.0 nm in the near-ultraviolet spectrum, and the spectral line width is exacted to be about 35 nm.

 

Fig. 2. (a) $I$-$V$ curves of fabricated light-emitting devices made of a ZnO:Ga MW before and after covered by PtNPs, together with p-GaN film acting as the hole injecting layer. Inset: schematic architecture of the heterojunction device containing a PtNPs@ZnO:Ga MW and p-GaN film. In the device configuration, Ni/Au and ITO served as the electrodes for the carrier injection. (b) EL spectra of n-ZnO:Ga MW/p-GaN film based LED, with the input current varying from 0.90 to 13.95 mA. (c) EL spectra of n-PtNPs@ZnO:Ga MW/p-GaN film based LED, with the input current varying from 0.35 to 12.80 mA. (d) The main EL peak wavelengths of the fabricated LEDs versus the input current. (e) The spectral linewidth of the fabricated LEDs versus the input current. (f) Integrated EL intensity of the fabricated devices versus the input current.

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Specifically, by incorporating PtNPs, the fabricated single MW heterojunction device were tested. The corresponding $I$-$V$ curve is illustrated in Fig. 2(a) (the red solid line). From the curve, significantly increased electrical property is obtained. Except diode-like rectifying behavior, the turn-on voltage of the PtNPs@ZnO:Ga MW LED is estimated to be 2.77 V, which a little smaller than that of bare ZnO:Ga MW based heterojunction device. Regrettably, we can see that much more leakage current generated in the reverse voltage. It is mainly attributed to the enhancement of the electrical properties of the heterojunction device covered by PtNPs, leading to the increase of conductivity from the fabricated n-PtNPs@ZnO:Ga/p-GaN heterojunction LED [34,53]. Similarly, EL characterization of single PtNPs@ZnO:Ga MW LED was implemented. As shown in the EL spectra (see Fig. 2(c)), a strong EL emitting at 377.0 nm can be obtained. Especially, the line width of the EL spectra exhibits a significant narrowing to be about 10 nm. Hence, upon introducing PtNPs, the acquired EL characters of the fabricated LED is dominated by the typical NBE luminescence of ZnO:Ga.

Further to investigate the effect of PtNPs on the EL features of the fabricated heterojunction LEDs, the main EL wavelengths versus the driven current are plotted in Fig. 2(d). Compared with the n-ZnO:Ga MW/p-GaN heterojunction LED, the main EL peaks exhibit observable blueshift from 400 to 377.0 nm in the ultraviolet wavelengths. Additionally, a little redshift of the major EL peaks for the fabricated n-PtNPs@ZnO:Ga MW/p-GaN heterojunction LED, may be assigned to the bandgap shrinkage induced by heating effect via increasing the input current. The line width of heterojunction LEDs versus the injection current are plotted in Fig. 2(e). Clearly, the line width also illustrates a narrowing from 35 to 10 nm [34,40]. Figure 2(f) exhibits the integrated EL intensity versus the input current of the heterojunction LEDs, which before and after covered by PtNPs. By contrast, the EL intensities of the fabricated PtNPs@ZnO:Ga MW LED were dramatically enhanced under various input currents [28,52]. Hence, the decoration of PtNPs can be used to mediate the EL performances of the single MW LEDs, including significant enhancement of light output, observable blueshift of the major EL peaks, and the remarkable reduction of the line width.

Moreover, EL characterization of the fabricated single MW LED was recorded by using a CCD digital camera. Figure 3(a) illustrates that, increasing the input current in the range of 4–13 mA, observable bright and blue-violet light emission from the n-ZnO:Ga MW/p-GaN heterojunction LED, can be captured. The luminescence region is observed across the total length of the wire, and increases in their brightness. By decorating PtNPs on the ZnO:Ga MW, significantly enhanced light-brightness of the single wire based LED can be acquired. Figure 3(b) shows the corresponding optical microscope EL video screenshots in the range of 4-10 mA. Clearly, dazzling and pure violet radiance can be observed from the single PtNPs@ZnO:Ga MW LED device. The modulation of PtNPs on the EL characteristics are in consistent with the blue shift of the major EL peaks of the LED devices [28,53]. Thus, by introducing PtNPs with appropriate sizes, we can achieve a EL performance-enhanced n-ZnO:Ga MW/p-GaN film LED device.

 

Fig. 3. (a) Optical photographs of single ZnO:Ga MW based LED, with the injection current ranging from 4.0–13.0 mA. (b) Optical photographs of single PtNPs@ZnO:Ga MW based LED, with the input current ranging from 4.0–10.0 mA.

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To study the modulation of PtNPs on the carrier recombination of the devices, the collected EL spectra were fitted by using Gaussian functions. Taken a representative emission spectrum of n-ZnO:Ga MW/p-GaN heterojunction LED (input current: 6.0 mA) into account, the spectrum is decomposed into three distinct subbands, which position at around 377.6 nm, 406.3 nm and 449.2 nm, respectively. And each emission band can be corresponded to a particular radiative recombination procedure. The ultraviolet emission emitting at around 377.6 nm, could be ascribed to the typical NBE emission of the ZnO:Ga MW; while the blue-violet emission band centered at around 406.3 nm, could be attributed to interfacial recombination at the ZnO:Ga/GaN interface, in which the electrons come from ZnO:Ga MW, and holes originate from the p-type GaN film; finally, the emission band positions at around 449.2 nm could be originated from the transitions from the conduction band or indefinitely shallow donors to Mg-related acceptor levels in the p-type GaN film [28,54]. As the input current increasing, the corresponding emission intensity of the decomposed subpeaks illustrate near linear increase, as shown in Fig. 4(b). Furthermore, a schematic of the energy band alignment diagram of the n-ZnO:Ga MW/p-GaN heterojunction LED is described in Fig. 4(c). The value of conduction band offset (CBO) and valence band offset (VBO) were calculated to be 0.29 eV and 0.27 eV, respectively. In consequence, the radiative recombination of the n-ZnO:Ga MW/p-GaN heterojunction LED is mainly distributed at the interface of n-ZnO:Ga MW and p-type GaN film.

 

Fig. 4. (a) Gaussian deconvolution analyses of a representative EL spectrum from single ZnO:Ga MW based LED at the input current of 6.0 mA. (b) The emission intensity of three sub-bands versus the input current from single ZnO:Ga MW based LED. (c) The energy band alignment diagram of the n-ZnO:Ga MW/p-GaN heterojunction LED. (d) Gaussian deconvolution analyses of a representative EL spectrum from single PtNPs@ZnO:Ga MW based LED at the input current of 6.0 mA. (e) The emission intensity of three sub-bands versus the input current from single PtNPs@ZnO:Ga MW based LED. (f) The energy band alignment diagram of the n-PtNPs@ZnO:Ga MW/p-GaN heterojunction LED.

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By introducing PtNPs, the EL spectrum of the fabricated LED at the driven current of 6.0 mA, was also fitted by using Gaussian functions. As shown in Fig. 4(d), the EL spectrum is also decomposed into three distinct subbands, which position at around 377.0 nm, 387.3 nm and 448.3 nm, respectively. By comparison, the ultraviolet emission peaking at 377.0 nm, dominates the EL spectrum, accompanying with ignored emissions the other emission subbands of 387.3 and 448.3 nm. Figure 4(e) displays the intensity of the three sub-bands as a function of injection current. It can be clearly seen that light intensity of the ultraviolet emission increased sharply. Moreover, the energy band alignment of n-PtNPs@ZnO:Ga/p-GaN heterojunction LED is plotted in Fig. 4(f). The value of corresponding CBO and VBO were calculated to be 0.10 eV and 0.08 eV, respectively. According to the explanation, the radiative recombination of the n-PtNPs@ZnO:Ga MW/p-GaN heterojunction LED is mainly confined at the side of n-ZnO:Ga MW [34,40]. Therefore, the incorporation of PtNPs via appropriate sizes can be utilized to engineer the band alignment of the fabricated n-ZnO:Ga MW/p-GaN heterojunction, resulting in the modulation of recombination EL achieved in the single ZnO:Ga MW gain media.

To further study the modulation of PtNPs on lighting characters of the fabricated n-ZnO:Ga MW/p-GaN film heterojunction LED, the EL spectra of the devices not decorated and decorated by the metal NPs, are compared under the input current of 6.0 mA. As depicted in Fig. 5(a), the relative EL intensity is enhanced by a ratio of near four times via the decoration of PtNPs on the wire. Apart from the PtNPs-enhanced EL, an obvious blue shift of the main emission peak from the LED covered by PtNPs, was also observed (See Fig. 5(b)). As illustrated in the figure, that the spectral profile of PL (PtNPs@ZnO:Ga MW) is similar to the EL of the constructed n-PtNPs@ZnO:Ga MW/p-GaN heterojunction LED. Therefore, typical NBE luminescence of ZnO:Ga MW dominates the EL of the n-PtNPs@ZnO:Ga MW/p-GaN heterojunction LED. In the device configuration, single PtNPs@ZnO:Ga MW primarily serves as the functional zone for the radiative recombination of carriers (electrons from n-PtNPs@ZnO:Ga and holes from p-GaN) during the LED working procedure [34,35]. As we mentioned above, PtNPs with appropriate sizes can function as plasmonic materials in the ultraviolet regions. To probe into the ultraviolet plasmonic response, the photoconductive behaviors of the constructed heterojunction device not decorated, and decorated by PtNPs, were performed in both the presence and absence of ultraviolet illumination (the incident wavelength is $\lambda$ = 370.0 nm, and the light intensity is 5.0 mW/cm$^{2}$). Figure 5(c) exhibit $I$-$V$ characteristics under dark and ultraviolet illumination. For the device covered by PtNPs, it is obviously noticed that higher photocurrent can be achieved than that of the bare MW based device [55,56]. To investigate the influence of PtNPs on the optoelectronics of the n-ZnO:Ga MW/p-GaN LED device, the enhanced characters can be assigned to the excitation of localized surface plasmon resonances of the deposited PtNPs on the wires. On account of the good overlapping between major extinction peak of the PtNPs and NBE-type emission of ZnO:Ga MW, the coupling interaction between ZnO:Ga excitons and PtNPs-plasmons can occur. Therefore, the incorporation of PtNPs can be utilized to increasing the interband transition of ZnO:Ga MWs, leading to engineering the energy band alignment and improving the bandgap recombination of the heterojunction.

 

Fig. 5. (a) Comparison of EL intensity of the fabricated LEDs with and without PtNPs decoration, under the same injection current of 6.0 mA. (b) Normalized intensity of PL from a ZnO:Ga MW, ELs of the fabricated LEDs before and after covered by PtNPs. (c)The comparison of photocurrent of the fabricated LEDs before and after covered by PtNPs, under ultraviolet light illumination (upper, violet and blue) and dark (lower, green and orange). (d) Simulated three-dimensional near-field intensity distribution of a PtNP, which placed on the ZnO:Ga MW. (e) Simulated plasmon mapping and electric-field distribution of a PtNP prepared on ZnO:Ga MW (top view). (f) Electric-field distribution of the cross section at the PtNP/ZnO:Ga interface.

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The plasmonic influence of PtNPs on the devices were also performed by using theoretical simulation via finite-different time-domain (FDTD) method. A three-dimensional model composed of a PtNP placed on a ZnO:Ga MW was built up. Illustration in Fig. 5(d), simulated near field intensity distribution $|\mathbf {E}|^2$ for resonant illumination is achieved. Large enhancements of incident radiation intensity are mainly localized at the bilateral sides of the Pt-nanohemisphere at the PtNP/ZnO:Ga interface. The calculated spatial distributions of charge and electric-field $|\mathbf {E}|^2$-intensity at $x$-$y$-plane are depicted in Fig. 5(e). As shown in the figure, we can observe that the spatial distribution of surface charges can oscillate across the two opposite sides of the nanoparticle as the incident light wavelength illuminated near the resonant peak of PtNPs plasmons. Accordingly, the corresponding $|\mathbf {E}|^2$-intensity illustrates that the maximal electric-field intensities are primarily focused at both ends of the PtNP symmetrically.

Figure 5(f) displays the optical intense field concentration of the PtNP/ZnO:Ga composite structure at $x$-$z$-plane. It is observed that the simulated evanescent field distribution of the excited particle can infiltrate into ZnO:Ga. The calculated result demonstrates that the electric field intensity damped rapidly with the increasing distance between the metal particle and the neighboring ZnO:Ga MW [46,49]. Hence, PtNP-plasmon induced near-field at the PtNPs/ZnO:Ga interface, which is formed by the polarization and $\mathbf {K}$-vector, can intensively re-distribute into the contacting area between the PtNPs and ZnO:Ga MW. Upon ultraviolet excitation, the as-synthesized PtNPs on the wire can lead to the coupling interaction between PtNPs-plasmons and ZnO:Ga excitons, resulting in the improvement of the ultraviolet emission of ZnO:Ga. Therefore, the deposited PtNPs on ZnO:Ga MW can accelerate the radiation recombination rate of electron-hole pairs in LED devices [35,40].

4. Conclusion

In conclusion, working as ultraviolet plasmonic source, the incorporation of PtNPs can be utilized to modulate the interband transition of ZnO:Ga MWs, leading to engineering the energy band alignment and improving the bandgap recombination in the constructed n-ZnO:Ga MW/p-GaN heterojunction devices. The increased EL performances, containing significantly enhanced light output, an obvious blueshift of main EL peak wavelengths, and significant reduction of the line width, are achieved. When operated upon electrical-excitation, the recombination of electron-hole pairs can occur in the ZnO:Ga MW gain medium, leading to fabricating a relative pure ultraviolet light sources. Compared with the previously reported experimental schemes, introducing PtNPs with desired ultraviolet plasmonic response can offer a simple, direct and workable scheme to construct ultraviolet light emitting devices with hyperpure and narrow line width, together with the improvement of luminescence characteristics.