1. Introduction

Incandescent light bulb is an electric light with a filament, when voltage is applied onto the bulb, the filament will be heated to such a high temperature that it glows with visible light. Light bulbs have been widely used as in household and commercial lighting, for portable lighting such as table lamps, car headlamps, and flashlights, and for decorative and advertising lighting the major lighting source of human for over 150 years, and contributed greatly to the human society [1–4]. The fluorescent mechanism of the light bulbs is black-body radiation, in which the filament is heated to a relatively high temperature, thus visible radiation will be generated. In particular, by means of nanometer technology and modern processing technique, a large fraction of the thermal radiation emitted from hot objects can be tailored. The fabrication of a plain incandescent tungsten filament, which surrounded by a cold-side nanophotonic interference system, can provide a platform to reflect the emitted infrared lighting and transmit visible lighting. The luminous efficiencies of this kind of photonic-crystal emitters at moderate temperatures can be achieved as high as $\sim$ 40$\%$, which can be nearing a limit for lighting applications [5–10].

In addition, extensive research over the past decades has concentrated on the development of on-chip light sources. The emitter at nanometer level, can create light in small structures on the surface of a chip. This monolithic nanoscale light sources have enabled plenty of applications, such as high-performance communication, low cost lighting and smart displays [11–16]. It is crucial to develop fully integrated photonic circuits that do with light what is now done with electric currents in semiconductor integrated circuits. Researchers have developed many approaches, such as graphene-based light sources. This kind of lighting emitters endow a new sense of the oldest and simplest artificial light source, the incandescent light bulb can be integrated onto a chip. However, from the above fluorescence mechanism, one can learn that the filament should be contained in vacuum to prevent the oxidization; and it is hard to adjust the fluorescence color of the bulb since it is heavily dependent on the temperature of the filament [17–22]. In addition, individual Ga-doped ZnO microwires (ZnO:Ga MWs) with hexagonal cross section were prepared, which being employed to construct typical incandescent light sources. By comparison, single micro/nanowires with quadrilateral cross section can provide a platform to enhance the multifunctionality, practicability and power of modulating the transport features of electronic, excitonic and photonic, as well as optical properties [23–28]. Therefore, multi-functional products have attracted much attention as the rapid development of science and technology. The development of synthesized approach to prepare semiconductor micro/nanowires with well-defined crystal structures is a challenging task in a predictable manner, together with the imperious requirement on the fabrication of future multifunctional devices [29–31].

In this work, Ga-doped ZnO microwires (ZnO:Ga MWs) with quadrilateral cross section were successfully prepared. In addition to be served as ultraviolet materials, single ZnO:Ga MW can be utilized to construct typical incandescent light source. By adjusting Ga-incorporation, the dominating incandescent lighting peaks can tuned in the visible spectral band. Interestingly, the incorporation of metal nanostructures decoration, the light-emission features can be further modulated. In particular, dual-color emitters, aligned array-type emitters can also be fabricated. Compared with tungsten-based incandescent lamp filament, the incandescent lighting of single ZnO:Ga MW can pave the way towards the realization of commercially viable, compatibility with silicon-based materials, and demonstrated the potential benefits of large-scale electrification. Therefore, individual ZnO:Ga MWs based incandescent emitters can undertake a new function of the oldest, affordable and easily prepared light sources. In addition, individual ZnO:Ga MWs being treated as efficient light sources can open up intriguing scientific questions, and possible applications of one dimensional linear, transparent, flexible displays and optical interconnects with electronic circuits.

2. Experimental section

2.1 The synthesis of individual ZnO:Ga MWs with quadrilateral cross section

Individual ZnO:Ga MWs with perfect quadrilateral cross section were fabricated using chemical vapor deposition (CVD) method, with a mass of the product can be collected around Si-wafer inside a alundum boat. To prepare ZnO:Ga MWs, high-purity precursor mixture of ZnO (99.99$\%$), Ga$_2$O$_3$ (99.999$\%$) and C (99.99$\%$) powders served as the source materials. Taken the weight ratio of ZnO : Ga$_2$O$_3$ : C = 10 : 1 : 11 for instance, Ga$_2$O$_3$ high-purity powder was utilized as the Ga-incorporation source [26,32]. Firstly, the carrier gas composed of argon ($Ar$: 99.99$\%$, 180 sccm) was introduced to clean furnace chamber, leading to the formation of oxygen-deficient environment. Secondly, the precursor mixture was placed at the reaction temperature zone, which being served as the highest temperature reaction zone of the horizontal alundum tube reactor of the engineered reaction temperatures gradient zones. The furnace chamber should be raised up to 1150 $^\circ$C, with the temperature ramp rate 25$^\circ$C/min. Owing to the different carbothermal threshold temperature of ZnO (900 $^\circ$C) and Ga$_2$O$_3$ (1100 $^\circ$C), when the temperature zone reached 900 $^\circ$C, Zn-vapor can be produced. Because of the anoxic environment, the Zn-vapor could be accumulated around Si-substrate, and then being adsorbed by the substrate. The Zn-vapor could be oxidized to form ZnO quasiparticle nanofilms and then being served as the seed layer. Continuously to increase the temperature beyond 1100 $^\circ$C, Ga-vapor can also be produced by means of carbothermal reduction. Protected by carrier gas, a relatively large amount of Zn-vapor and slight Ga-vapor could be accumulated around the Si-substrate, which is helpful to mix Zn-vapor and Ga-vapor sufficiently. When the growing temperature keeping at 1150 $^\circ$C, excess vapor mixture of Zn-vapor and Ga-vapor were accumulated around the Si-substrate. Then incorporated with residual O$_2$ in the furnace chamber, individual Ga-doped ZnO MWs could be gradually formed around the Si-substrate, with the pre-deposited ZnO quasiparticle nanofilms being served as the seed layer.

By adjusting the growing conditions, such as the weight of the precursor mixture, the growing temperature and the growing time, the size of the MWs could be modulated. Additionally, increasing Ga$_2$O$_3$ weight ratio in the precursor mixture can be employed to modulate the Ga-doping concentration in the as-synthesized ZnO:Ga MWs. Afterwards, high-purity oxygen ($O_2$: 20 sccm) should be introduced into the furnace chamber, which can be used to improve the single-crystalline quality of the as-prepared MWs [25,28,33–35]. In the context, individual ZnO MW with controlled Ga-doping concentration can be prepared, with five different weight ratios of the precursor mixtures of ZnO:Ga$_2$O$_3$:C, such as 11:1:12 (sample-1), 10:1:11 (sample-2), 9:1:10 (sample-3), 8:1:9 (sample-4), and 7:1:8 (sample-5), respectively.

In particular, to guarantee that the as-prepared MWs possessing perfect quadrilateral cross section, the necessary and sufficient conditions should be satisfied. (1) High purity Ar should be introduced to clean the furnace chamber, leading to the anoxic environment. (2) The temperature ramp rate of heating should be as fast as possible (The temperature ramp rate of the tube furnace employed in our laboratory $\leq$ 25 $^\circ$C/min). Thus, Zn-vapor could be mixed with Ga-vapor sufficiently. (3) Additionally, in our experiments, the growth temperature maintained at 1150 $^\circ$C, which was much higher than the one in the typical carbothermal reaction. In the synthesized procedure, the formation of high concentrations of Zn-vapor would be predominated due to oxygen deficit growing condition, resulting in the cross section shape changing [23,24,28,29]. Therefore, comparatively higher growing temperature and hypoxia vapor-solid growth environment can be supposed to be the determinant factor, leading to the subsequent preparation of individual ZnO:Ga MWs with quadrilateral cross section.

2.2 The fabrication of single ZnO:Ga MW based incandescent-type light source

Single MW was selected from the corundum boat, and then transferred onto quartz substrate. Indium (In) particles served as the electrodes was fixed onto both ends of the wire, leading to the fabrication of single MW based metal-semiconductor-metal (MSM) structure. Thereby, a typical incandescent light source on account of single MW was constructed. Afterwards, Au (99.99$\%$) quasiparticle nanofilms was deposited on the MWs using a radio-frequency magnetron sputtering technique [25,26,29,34]. In addition, with the aid of metal mask, Au quasiparticle nanofilms could be deposited on the MW periodically.

3. Results and discussion

3.1 The synthesis of individual ZnO:Ga MWs with quadrilateral cross section

As previously reported that, micro/nanostructured ZnO crystal generally has a wurtzite structure and usually presents a natural hexagonal cross section, which naturally serves as a whispering gallery mode (WGM) lasing microcavity owing to its high reflective index [35–37]. The emitted photons can travel circularly in the hexagonal cross section owing to the total internal reflection at the ZnO/air boundary. Inspired by its unique geometry, many efforts have been developed to synthesize the hexagonal ZnO micro/nanostructures. By comparison, typical Fabry-Perot (F-P) mode laser was also observed on account of single nanowire, with both end facets served as two reflecting mirrors [30]. Meanwhile, F-P mode microlasers can be realized on account of ZnO-based structures, which possessing quadrilateral cross section, with bilateral sides served as the reflecting mirrors. Such unique quadrilateral ZnO nano/microstructures can modulate the photons and/or excitons to produce multiple emission modes, which may provide a potential platform to expand the application in producing low-threshold lasing, color-by-design light emission diodes, exciton-polariton emission, et al [23,28,29,33].

As described in the experiment section, ZnO:Ga MWs with quadrilateral cross section were successfully synthesized. The synthesized procedure is schematically illustrated in Fig. 1(a). The as-prepared procedure is proposed as following: Firstly, to guarantee that the growth procedure operated under hypoxia environment, high-purity $Ar$ (180 sccm) should be introduced into the horizontal alundum tube reactor; Secondly, after the precursor mixtures of ZnO, Ga$_2$O$_3$ and C high-purity powders placed at the reaction temperature region, the horizontal alundum tube reactor should be raised up to 1150 $^\circ$C as soon as possible (In our experimental, the temperature ramp rate denoted as 25 $^\circ$C/min); Thirdly, after maintained about 30 min, a mass of individual ZnO:Ga MWs can be collected around the Si substrate. Figure 1(b) displayed the optical photograph of the as-synthesized ZnO:Ga MWs. It indicated that a mass of MWs can be collected around the Si substrate, as well as the wall of the alundum boat, with the length of the wires can be up to 2 cm. Figure 1(c) illustrated SEM image of the as-synthesized ZnO:Ga MWs, which possessed highly smooth surfaces. A single ZnO:Ga MW was also characterized, as displayed in Fig. 1(d). Additionally, amplified SEM image of the MW, which possessed perfect quadrilateral cross section was also demonstrated in Fig. 1(e). In addition, by adjusting the growing conditions, such as the weight of the precursor mixture, the growing temperature and the growing time, the size of the MWs could be modulated. Therefore, individual ZnO:Ga MWs with controlled sizes, quadrilateral cross section and well-crystallized were successfully prepared [26,28,29,35,38].

 

Fig. 1. The synthesis of individual ZnO:Ga MWs: (a) Schematic diagram of the synthesized procedure for the ZnO:Ga MWs via a CVD method. (b) Optical photograph of the as-synthesized ZnO:Ga MWs, the product located around Si-substrate, as well as the alundum boat wall. (c) SEM image of ZnO:Ga MWs array. (d) SEM image of single ZnO:Ga MW. (e) SEM image of single ZnO:Ga MW possessing quadrilateral cross section.

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Taken a single ZnO:Ga MW selected from Sample-1 for example, elemental mapping using energy-dispersive X-ray spectroscopy (EDX) was carried out, as shown in Fig. 2(a). It indicated that EDX elemental mappings of Zn, Ga and O species respectively, were uniform throughout the MW. Thus, with the aid of Ga$_2$O$_3$ served as the doping source, Ga-dopant could be incorporated into ZnO. To characterize the as-synthesized ZnO:Ga MWs, the high resolution transmission electron microscopy (HRTEM) images of MWs selected from sample-1 illustrated that the as-prepared ZnO:Ga MW has an atomically resolved wurtzite structure and a growth direction along (002) direction, as demonstrated in Fig. 2(b) [32]. The interspacing between Zn, O and Ga planes was also determined to about 0.275 nm, which is slightly larger than that in intrinsic undoped ZnO MW (0.261 nm), indicating the lattice expansion due to the substitution of Ga for Zn, as revealed in Fig. 2(c) [26,35,39,40]. The X-ray diffraction (XRD) pattern of the as-prepared ZnO:Ga MWs was investigated, with the diffraction angle ranging from 20$^\circ$ to 80$^\circ$, as indicated in Fig. 2(d). All the peaks are in 100$\%$ phase matching with the ZnO hexagonal phase of JCPDF No. 36-1451 [38–41]. The little shift of the diffraction peaks may be attributed to the impurity atom concentration of Ga being estimated to be lower than $\sim$ 1$\%$, while the overlap between Ga K and Zn L in the EDX spectra [26,32].

 

Fig. 2. (a) EDX elemental mappings of Zn, O and Ga species, respectively. (b) HRTEM image of the as-synthesized ZnO:Ga MW. (c) Amplified SEM image of the as-synthesized ZnO:Ga MW. (d) XRD patterns of as-prepared ZnO:Ga MWs. The peaks correspond to wurtzite ZnO structure with lattice constants $a$ = 3.25 $\dot {A}$ and $c$ = 5.21 $\dot {A}$, consistent with JCPD Card No. 36-1451. (e) Temperature-dependent PL emission spectra of the as-prepared ZnO:Ga MW. (f) Temperature-dependent $I$-$V$ characteristic curves of single ZnO:Ga MW.

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To characterize the optical properties of single ZnO:Ga MW, temperature-dependent photoluminescence (T-PL) measurement was carried out using a He-Cd laser with an excitation wavelength of 325 nm. The resulting PL spectra demonstrated one dominating emission peaks centered around 3.348 eV at 110 K, accompanied with negligible visible emissions, as shown in Fig. 2(e). It also illustrated that the ultraviolet emission dominated the T-PL spectra with increasing the temperature ranging from 110 to 300 K. Additionally, with reducing the temperature, several fine features of the lighting could be evolved in the near band edge spectral region. Taken the PL spectrum at 110 K for example, the dominant emission peak centered at 3.348 eV can be attributed to typical donor-bound excitons ($D^o X$). On the lower energy side of this emission peak, the emission peaks located at 3.305 and 3.233 eV could be ascribed to the free electron to acceptor ($FA$) and donor-acceptor-pair ($DAP$) transitions, respectively [25,29,33]. With increasing the temperature, more and more free excitons appeared, and then occupy the ground states. At room temperature, the emission peaks centered around 3.296 eV can be ascribed to the recombination of free excitons [26,32,36,39].

The electronic transport measurement on account of as-synthesized single ZnO:Ga MW was carried out at room temperature. Taken a single ZnO:Ga MW with the size of length $\sim$ 5 mm and the diameter $d$ $\sim$ 10 $\mu$m for example, single MW based MSM structure was fabricated, with In served as the electrodes. Linear $I$-$V$ curves indicated ohmic contacts forming between the as-prepared MW and In electrodes. The resistivity of the MW was calculated to be about 9.0 $\times$ 10$^{-5}\Omega \cdot m$. Significantly, when measured at lower temperature, the resistivity reduced to be about 3.0 $\times$ 10$^{-5}\Omega \cdot m$. Compared with undoped ZnO MWs, Ga-doping has significant influence on the electrical properties of the as-prepared ZnO:Ga MWs. A further increase in the doping concentration may result in degenerately Ga-doped ZnO MWs that illustrated quasi-metallic behavior [25,32,35]. Additionally, to distinguish the contributions of Ga-doping and intrinsic defects (such as oxygen vacancies) on the electronic transport behavior of the as-synthesized MW, an annealing at 1150 $^\circ$C for 30 min in the oxygen-enriched environment were also performed, but little changes could be obtained. The enhanced n-type conduction of the as-synthesized ZnO:Ga MWs were indeed dominated by the incorporation of Ga-dopant [26,29,32]. Therefore, individual ZnO:Ga MWs possessed well-crystallized, and excellent electronic transport characteristics were successfully prepared via a simple CVD method.

3.2 Wavelength-tuning emissions from individual ZnO:Ga MWs based incandescent sources

Interestingly, when the applied bias exceeded a certain value, bright and visible light-emitting can be captured, with the emission regions located towards the center of the wire. The lighting features could be analogous to tungsten filament lamp [25,26,29,34]. Thus, a typical incandescent light source was proposed and demonstrated employing single ZnO:Ga MW as the fluorescence filament. A schematic structural illustration of the light-emitting device is depicted in Fig. 3(a). The device is involving a single ZnO:Ga MW and two contact electrodes. When a bias applied onto the MW beyond certain value, bright fluorescence can be observed, with the emission region located at the center of the wire. Thus, single MW can function like the filaments in incandescent light bulb, with the advantage that they operate in an ambient air environment [17,26]. To explore the influence of Ga-incorporation on the electronic transport properties, individual ZnO:Ga MWs with identical sizes were selected from the samples ranging from Sample-1 to Sample-5, respectively. The $I$-$V$ characteristics curves illustrated that increasing Ga$_2$O$_3$ in the source mixtures can be utilized to tune n-type conduction, as shown in Fig. 3(b). Taken a single ZnO:Ga MW selected from Sample-1 for example, bright and green lighting can be observed when the injection current beyond certain value, with the emission regions located towards the center of the wire. The emitted photons were collected, with the dominating lighting peaks centered around 500 nm (see Figs. 3(c) and 3(d)).

 

Fig. 3. Typical incandescent light source based on single ZnO:Ga MW: (a) Schematic illustration of electrically biased single ZnO:Ga MW based incandescent-type light source, with the light emission located at the center of the wire. (b) $I$-$V$ characteristics curves of individual ZnO:Ga MWs with controlled Ga-incorporation. (c) Normalized EL emission spectra of individual ZnO:Ga MWs based incandescent-type light sources. (d) Optical micrographic images of bright visible light emission from individual ZnO:Ga MWs based incandescent-type light sources.

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Furthermore, with increasing the Ga-doping concentration, the dominating emission wavelengths can be tuned from 500 nm to 625 nm in the visible spectral band, as indicated in Fig. 3(c). By comparison, the spectral linewidth of the emission spectra became progressive spectral broadening with increasing the Ga-doping concentration, which may be caused by the narrow-bandgap effect and the donor impurity energy introduced in ZnO:Ga MWs [32,35,40]. The fabrication of single ZnO:Ga MW based fluorescent filament emitter was achieved, with the optical microscopic images demonstrated in Fig. 3(d). The optical microscopic photographs of EL emission were also captured by a digital camera (see Fig. 3(d)). By increasing the Ga$_2$O$_3$ weight ratios in the reaction mixtures, the emitted lighting from single MW based incandescent sources can be consequently tuned from the green-lighting, yellow-lighting, to red-lighting [26,29]. It is also significantly important to develop a new preparation of electroluminescence (EL) lighting sources with tunable wavelength and size-controlled. By adjusting the length of MWs, lighting emitters can be tuned from elongated lighting sources to spot lighting sources. Meanwhile, owing to the absence of rectification characteristics, relevant electrical measurement results illustrated that the alternating current-driven light-emitting functions excellently on the ZnO:Ga MWs [17,19,21,26]. The fluorescent lighting of single ZnO:Ga MW based incandescent-type source can be clearly observed by naked eyes in normal indoor lighting conditions. Therefore, it is clearly established that Ga-doping is a powerful means to tune the emission color by controlling the dopant concentration in ZnO:Ga MWs [8,25,26,40].

To expoit the influence of Ga-incorporation on incandescent-type lighting, single undoped ZnO MW was also prepared. Nevertheless, no lighting signal could be captured, even if the applied bias as high as 300 V [42]. Thus, the incandescent-type lighting can be attributed to Ga-related impurity levels [25,26,29]. Additionally, compared with ZnO-based ultraviolet light sources, individual ZnO:Ga MWs based light sources functions much like an incandescent light bulb [5,29,33,43]. This new type of broadband light emitter can be integrated into chips and will pave the way towards the realization of flexible, and transparent displays, and ZnO-based on-chip optical communications. Consequently, the as-prepared ZnO:Ga MWs with controlled Ga-doping concentration can be utilized to construct wavelength-tuning exceptionally incandescent light bulbs. To exploit the possible lighting principle, it can be found out that the lighting principle of single ZnO:Ga MW based incandescent source is a Joule-heating facilitated non-equilibrium carrier recombination process due to the outstanding n-type conduction and well-crystallized. While, EL emission characteristics also illustrated that the dominating emission peaks of light-emitting can be derived from Ga-dopant induced impurity band. Electron injection is the key factor of understanding the EL lighting process. It leads to a "hotspot" at the center, heating induced non-equilibrium hot electrons can be generated, and then staying confined to the small hotspot towards the center of MWs. Once energetic electrons exceed bandgap of ZnO, lighting could be produced [17,25,26,29].

As is well-known that, light-emitting from electrically biased single semiconductor component (such as single micro/nanowire, nanotube, naonobelt and so on) can be explained by several different scenarios: a bipolar EL mechanism where the electrons and holes are injected simultaneously into the structures, such as EL emissions from p-n, p-i-n junction [29,35,37,43,44]; a unipolar impact excitation process by hot carriers (e.g., metal-semiconductor based Schottky junction) [45]; and thermal light emission due to Joule heating effect [6,17]. By comparison, a bipolar EL mechanism and Schottky junction based EL emission mechanism can be ruled out due the lack of rectification characteristic. Until now, a straightforward identification of the temperature, which induced by thermal resistance effect at nanometer-scale volume, still remains elusive at this stage. This viewpoint has been affirmed by previously papers reported, such as single graphene microbelt based filament-type light source, novel incandescent tungsten filament surrounded by a cold-side nanophotonic interference system, and so on [5,6,17,19,42]. According to the blackbody radiation theory, the visible radiation characters revealed that high-temperature $\sim$ 3000 K could be calculated. Especially, the shorted the dominating emission wavelength, the higher the temperature is needed. Due to single ZnO nano/microwires based light source, the fatal factor is that the as-synthesized ZnO nano/microwires can endure the temperature as high as 1000 K [25,26,32,42]. Thus, Joule heating effect induced blackbody radiation can be ruled out for the light-emitting mechanism from electrically biased single ZnO:Ga MW based incandescent-type light source. In addition, the experimental observations, for example, increasing the injected current, a little redshift of the EL emissions can be observed; Increasing Ga-doping concentration in ZnO MWs can also bring redshift of the EL emissions; Together with the enhancement EL emissions with reducing the environment temperature. Once again, thermal light-emission mechanism can be ruled out for the lighting working principle on account of single ZnO:Ga MW based light source [25,26,29,34]. Therefore, the incandescent-type lighting principle from single ZnO:Ga MW can be attributed to electrical-injection induced the non-equilibrium electron-hole recombination, with electrons located in the Ga-related impurity levels.

3.3 Color-switchable EL emissions from single ZnO:Ga MW, with Au quasiparticle nanofilms deposition periodically

Furthermore, the incorporation of metal nanostructures deposited on semiconductors can be employed to modulate optical and electrical properties of low dimensional materials and devices, such as the passivation of surface defect states, the enhancement the emission efficiency of the luminescent materials, the modulation of electronic transport properties and so on [46–48]. For example, due to the electric-field confinement and enhancement of localized surface plasmon resonances, the lighting efficiency of emission materials and devices can be enhanced by incorporating metal nanostructures. Especially, tunable plasmonic resonances with controlled size, shape and dielectric environment of the nanostructures, can afford potential platform to modulate the performance of luminescent materials [41,49]. As previously reported that, the introduction of metal quasiparticle decoration can be utilized to improve ultraviolet emissions of single ZnO:Ga MW; Meanwhile, while increasing the sputtering time of metal nanostructures decoration can also be employed to enhance n-type conduction [25,29,50,51].

Taken a single ZnO:Ga MW selected from Sample-1 for instance, bright and green light-emitting can be seen from electrically biased single MW based incandescent-type light source. After introducing Au quasiparticle nanofilms deposition (Supporting time: 30 s), bright and red-lighting can be observed, with the red-emission regions located towards the center of the wire. Further to increasing the injection current beyond certain value, green-lighting signals can also be captured. Especially, increasing the sputtering time of Au quasiparticle nanofilms deposition to be 60 s, the optical characterization of single Au@ZnO:Ga MW based incandescent-type light source illustrated that bright and red-lighting can be recorded, without no green-lighting signal being detected. Therefore, the sputtering time of Au quasiparticle nanofilms deposition denoted as 60 s was employed to modulate the emission features of single ZnO:Ga MW based incandescent-type light source [25,29]. Afterwards, another single ZnO:Ga MW (Sample-1) was selected to construct typical incandescent light source; And then introducing Au quasiparticle nanofilm deposited on the one segment of the wire, thus, a new typical incandescent light source composed of red and green lighting strung together was fabricated.

The electronic transport properties of the single ZnO:Ga MW, and then prepared with Au quasiparticle nanofilm deposition on one segment of the wire (The sputtering time: 60 s) were carried out. The $I$-$V$ characteristics curves illustrated linear and symmetric features, leading to the Ohmic contacts forming between the In electrodes and the MW. With the incorporation of Au quasiparticle nanofims decoration, n-type conductance improvement can be achieved, as indicated in Fig. 4(a). Additionally, increasing the applied bias onto single bare ZnO:Ga MW, green lighting can be captured, with the emission regions located towards the center of the wire. The emitted photons were recorded, with the dominating emission peaks centered around 500 nm, as indicated in Fig. 4(b). By incorporating Au quasiparticle nanofilms deposited on one segment of the wire, red-emission can be observed firstly when the injection current exceeded a certain value, with the lighting regions located at the segment of the wire covered by Au quasiparticles nanofilm. Continuous to increase the injection current, red-lighting became brighter and brighter. Additionally, green-lighting can also be captured, with the green-lighting regions located towards the bare segment of the wire. Therefore, green and red lighting side-by-side along the single ZnO:Ga MW can be achieved. Optical micrographic images of bright visible light emission from electrically biased single ZnO:Ga MW, with a segment of the wire covered by Au quasiparticle nanofilm decoration were collected, as indicated in Fig. 4(e).

 

Fig. 4. Light-emitting features from electrically biased single ZnO:Ga MW, with a segment of the MW covered by Au quasiparticle nanofilm decoration (the sputtering time: 60 s): (a) $I$-$V$ characteristics curves of single ZnO:Ga MW, with a segment of the MW covered by Au quasiparticle nanofilm decoration. (b) EL emission spectra from single ZnO:Ga MW based incandescent-type source. (c) EL emission spectra from incandescent-type light source composed of single ZnO:Ga MW, with one segment covered by Au quasiparticle nanofilm decoration. (d) Optical micrographic images of bright and green light emission from single ZnO:Ga MW based incandescent-type light source. (e) Optical micrographic images of bright visible light emission from electrically biased single ZnO:Ga MW, with a segment of the wire covered by Au quasiparticle nanofilm decoration.

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To characterize the emission characteristics, the emitted photons were collected. At lower injection current, the dominating emission wavelengths was firstly centered around 615 nm, which being completely consistent with the red lighting located towards the segment of the MW covered by Au quasiparticle nanofilms deposition [25,29,46]. When the injection current increased to 5.50 mA, another weaker emission centered around 505 nm can also be recorded. Further increasing the injection current ranging from 5.50 to 5.85 mA, in addition to the increasing brighter of the red-lighting, the green-lighting became gradually enhanced. Thus, the switchable of red lighting in series with green-lighting were achieved. The emitted photons were collected, with EL emission spectra illustrated that the dominant emission peaks centered at 505.0 nm and 603.5 nm respectively, as shown in Fig. 4(c). By contrast, the green-lighting peaks centered around 505 nm increased rapidly in intensity on increasing the injection current ranging from 5.50 to 5.85 mA.

Additionally, optical characterization of visible light emission was performed. Optical microscope images of single bare ZnO:Ga MW based incandescent-type emitter demonstrated that bright and green lighting can be captured, with the emission regions located towards the center of the wire, as demonstrated in Fig. 4(d). After introducing quasiparticle nanofilms deposition on one segment of the MW partially, red-lighting can be seen firstly when the injection current reached a certain value, with the emission regions located towards the area where Au quasiparticle nanofilms covering. With increasing the injection current, green-lighting can also be seen, with the emission regions located at the bare segment of the wire. Meanwhile, continue to increase the injection current, red lighting in series with green-lighting became brighter and brighter (see Fig. 4(e)). The emitted lighting is so intense enough to be observed even to the naked eye. Incorporated with the EL emission spectra, the emission features illustrated that the dominating emission peaks centered around 605 nm can be attributed to the modulation of the deposited Au quasiparticle nanofilms [25,29]. Therefore, the microsized red-lighting can be chained together with the other green-lighting with the aid of the incorporation of metal nanostructures deposited locally.

Further to modulate the influence of the deposited Au quasiparticle nanofilms on the emission features of single ZnO:Ga MW based incandescent-type emitter, fine-metal-mask (FMM) with the size at micrometer scale was adopted. By incorporating Au quasiparticle nanofilm decoration, red- and green-lighting alternately distributed along the MW can be captured when the applied bias exceeded a certain value. Additionally, by adjusting the sizes of the FMM, the dual color lighting regions can also be modulated. Optical micrographic images of green and red light emission from electrically biased single ZnO:Ga MW covered by Au quasiparticle nanofilm decoration periodically were collected with increasing the injection current. To probe into the influence of Au quasiparticle nanofilms deposition periodically on the electronic transport properties, the plotted $I$-$V$ curve clearly illustrated that the linear and symmetrical behavior revealed Ohmic contact characteristics can be formed between the MW and In electrodes. By comparing with the bare MW, the enhancement of the electronic transport properties can also be achieved, as depicted in Fig. 5(a).

 

Fig. 5. Light-emitting features from electrically biased single ZnO:Ga MW, with Au quasiparticle nanofilm deposited on the MW periodically (the sputtering time: 60 s): (a) $I$-$V$ characteristics curves of single ZnO:Ga MW, and then covered by Au quasiparticle nanofilms periodically. (b) EL emission spectra from single ZnO:Ga MW based incandescent-type light source. (c) EL emission spectra from incandescent-type light source composed of single ZnO:Ga MW covered by Au quasiparticle nanofilm periodically. (d) Optical micrographic images of bright and green light emission from single ZnO:Ga MW based incandescent-type light source. (e) Optical micrographic images of green and red light emission periodically distributed along the wire from electrically biased single ZnO:Ga MW covered by Au quasiparticle nanofilm periodically.

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In a contrast, bright and green lighting can be observed from electrically biased single ZnO:Ga MW based incandescent source. Figure 5(b) illustrated that the dominating emission peaks centered around 518.5 nm. And then by incorporating Au quasiparticle nanofilms decoration periodically, the lighting color was apparently current-dependent. When the injection current lower than 6.35 mA, the predominant emission peaks centered around 640 nm, while red-lighting can be observed, with the emission regions located at the areas where Au quasiparticle nanofilms deposited. Thus, the lighting features were dominated by the segments of the wire covered by Au quasiparticle nanofilms. When the injection current exceeded 6.35 mA, green-lighting can also be captured, with the emisson regions located at the bare segments of the wire. In addition to the red emission peak centered around 635 nm, the other emission peaks centered around 512.5 nm became stronger and stronger with the injection current ranging from 6.5 to 7.0 mA. It can be observed that there are two main emissions, with the lighting peaks centered around 512.5 and 635 nm in the EL spectra, appeared successively with increasing the injection current. Thus, a hand of red-lighting, alternately in series with green-lighting can be achieved on account of single MW.

The optical characterization of visible light emission from electrically biased single ZnO:Ga MW was carried out, with the injection current denoted as 7.20 mA, as revealed in Fig. 5(d). Furthermore, the lighting from electrically biased single ZnO:Ga MW prepared with Au quasiparticle nanofilms deposition periodically were recorded, as shown in Fig. 5(e). When the injection current denoted as 6.35 mA, red-lighting dominated EL emission, with the lighting regions located at the zones where Au quasiparticle nanofilms deposition. Continue to increase the injection current, such as 6.50 mA and 6.60 mA, green-lighting can also be captured, with the emission regions located at the bare segments of the wire. Meanwhile, the red-lighting became brighter. Therefore, with the incorporation of Au quasiparticle nanofilms deposited periodically on the MW, the emission regions with alternate green and red lighting distributed periodically, can be achieved.

4. Conclusion

To summarize, individual ZnO:Ga MWs with quadrilateral cross section, controlled Ga-dopant concentration and well-crystallized were successfully synthesized. Apart from ultraviolet optical-electrical characteristics, the experimental finding provided a kind of novel light-emitting device similar in structure with traditional incandescent light bulb, but since the filament used are ZnO MWs instead of tungsten wires. The oxidization of the filament by oxygen can be avoided, thus the emission devices can operate in air ambient without vacuum. By adjusting Ga-incorporation, color-tunable lighting can also be achieved, with the dominating emission peaks tuned in the visible spectral band. The as-prepared single ZnO:Ga MW can endow the oldest and the most innovative incandescent-type filament lamp. Additionally, due to the lack of rectification characteristics, the lighting principle can be attributed to electrical-injection induced the non-equilibrium electron-hole recombination, with electrons located in the Ga-related impurity levels. Interestingly, by introducing Au quasiparticle nanofilms deposition, the lighting features of single ZnO:Ga MW based incandescent-type emitter can be modulated. When the incorporation of Au quasiparticle nanofilms deposition periodically, alternating green and red light-emission arrangement can be captured. Further efforts on the performance of the devices can be improved, they may find a variety of applications in labeling, signaling, displaying, or even lighting in the near future. Therefore, this kind of incandescent-type emitters can open new routes to on-Si-chip, small footprint, and high-speed light sources for highly integrated optoelectronics and photonics.