1. Introduction

Ultraviolet light sources, such as light-emitting and laser diodes, have attracted significant interest from researchers owing to their unique optoelectronic properties and widespread applications, including as ultraviolet sensors, in catalysis, and in communication and sterilization devices, among others [1–5]. To realize miniaturized, module-based, integrated, multifunction optoelectronic devices with high reliability, semiconductor nanostructures have been employed to fabricate high-performance ultraviolet light sources; examples of these low-power devices include highly integrated devices, high-density secondary storage, and highly sensitive optoelectronic sensors [6–10]. However, it is still a challenging task to fabricate ultraviolet light sources, especially stable, high-efficiency sources, owing to issues such as multiple surface defects, low luminescence efficiency, and poor quality factor [11–16].

Due to direct wide bandgap (3.37 eV) and a large exciton binding energy (60 meV), ZnO has drawn much attention for its potential applications in ultraviolet optoelectronic devices [7,15,17–19]. While possessing natural optical microcavities, progress in lasing or stimulated emission has been achieved from a variety of low dimensional ZnO micro/nanostructures. For instance, as-synthesized hexagonal ZnO microstructures have been widely applied to construct natural whispering gallery mode (WGM) microcavities because of multiple total internal reflection at the ZnO/air boundary. Thus, ZnO micro/nanostructures can provide promising candidates to achieve ultraviolet lasers with high $Q$-factor and low threshold [20–23]. Plasmonics is an area of research that merges the fields of optics and nanoelectronics; in particular, it involves research of light with relatively large free-space wavelengths that are confined to the nanometer scale, thereby enabling a family of low-dimensional functional optoelectronic devices [9,24–27]. Furthermore, nanostructured metals with outstanding plasmonic response have been introduced to improve luminescence efficiency, as well as the enhancement of the $Q$-factor of ZnO micro/nanostructures based optical microresonators [12,28–31]. Among the noble metal nanostructures, those of Ag, Al, and Ga are the most studied materials because of their unique ultraviolet plasmonic characteristics and wide availability. Nevertheless, these noble metal nanostructures are affected by oxidation and have poor stability in complex and harsh conditions, such as in aerobic and aqueous environments; thus, the search for noble metals that allow for plasmonic performance with high stability, controlled sizes, and plasmonic energy-tunability in the ultraviolet spectrum is ongoing [32–38]. Recently, Rh nanostructures for use as ultraviolet plasmonic materials have been successfully synthesized [39–41]. Incorporating Rh nanostructures in low-dimensional wide bandgap semiconductors might present a suitable approach for the fabrication of high-performance optoelectronic devices in the short wavelength region that still needs to be further exploited [9,35].

Accordingly, in this work, Rh nanocubes (RhNCs) of high purity and controlled sizes with a plasmonic response in the ultraviolet spectral region were successfully synthesized. To demonstrate the ultraviolet plasmonic behavior of the fabricated RhNCs for prospective applications, individual ZnO microwires were prepared with RhNCs deposition (RhNCs@ZnO MWs). In addition to significantly enhancing photoluminescence (PL) emission, the RhNCs plasmons also modulated the whispering gallery mode (WGM) lasing characteristics of the ZnO MWs, leading to a reduction in their lasing threshold, enhancement in lasing output, and higher $Q$-factor compared with bare ZnO MWs. Furthermore, to investigate the working principle, related time-resolved PL (TRPL) and numerical simulations were performed. The as-prepared RhNCs with a controlled size, energy-tunable plasmons, and high stability have the potential for use in practical applications that require unique ultraviolet plasmonic characteristics, including stable, high-efficiency optoelectronic devices operating in the short-wavelength spectral region.

2. Experimental

2.1 Synthesis of RhNCs

An improved slow-injection polyvinylpyrrolidone (PVP)-directed polyol process (one of wet-chemical methods) was used to synthesize Rh nanostructures. First, 2 mL of KBr (0.25 M solution in EG) was added into a 50-mL boiling flask, which was then sealed and heated at 160$^\circ$C for 40 min. Then, 15 ml of RhCl$_3$ (0.025 M solution in EG) and 15 ml of PVP (0.1125 M in EG) were injected simultaneously into the KBr solution using a two-channel syringe pump at a rate of 1 mL/h; during this addition that lasted 15 h for the growth of RhNCs, the reaction mixture was kept at 160$^\circ$C with slow magnetic stirring. After this reaction, the obtained suspension was naturally cooled to room temperature, after which the final product was washed with acetone and ethyl alcohol several times and then centrifuged. And the sample was dispersed in ethyl alcohol for further use. In addition, the size of RhNCs was tuned by adjusting the amount of precursor during synthesis (RhCl3 (0.025 M) and PVP (0.1125 M)) [39,40]. Moreover, to ensure that the as-prepared RhNCs have high stability, we verified that the as-synthesized RhNCs can maintain their shape and optical property when the colloid was placed in a drying box at 100$^\circ$C for about 3 months.

2.2 Fabrication of single RhNCs@ZnO MWs

Individual ZnO MWs with hexagonal cross-section were synthesized via a chemical vapor deposition (CVD) method [17,42]. Then, a single ZnO MW was selected and transferred onto a quartz substrate, after which RhNCs were deposited onto it using a spin-coating technique, followed by an annealing process at 150$^\circ$C for about 30 min. The residual organics were removed, while leaving RhNCs selectively deposited on the surfaces of the ZnO MW. Thus, single ZnO MW with RhNC deposition (RhNCs@ZnO) was fabricated [7,18,35].

2.3 Characterization and analysis instruments

RhNCs were characterized using a transmission electron microscope (TEM); meanwhile, their ultraviolet-visible and near-infrared spectra were recorded using a UV-6300 spectrophotometer. The individual ZnO MWs prepared with and without RhNC deposition were also characterized using a scanning electron microscope (SEM). The PL emissions of the MWs were measured via a LABRAM-UV Jobin Yvon spectrometer with the 325 nm line of a He-Cd laser as the excitation source. Furthermore, optically pumped lasing measurements were also performed. The excitation laser (excitation wavelength: 355 nm; repetition rate: 1 kHz; pulse length: 100 fs) was generated using an optical parametric amplifier (OPERA SOLO) with a Ti:sapphire laser (coherent), which was then focused onto the body of the wire through a confocal micro-PL system (Olympus BX53). A spectrometer (SpectraPro-2500i, Acton Research Corporation) was placed to record and analyze the resulting optical spectra. To measure the spectral-temporal response of individual ZnO MW covered by RhNCs, time-resolved photoluminescence (TRPL) measurements were performed using an optically triggered streak camera system (C10910, Hamamatsu) at 295 nm resulting from frequency doubling of the fundamental fs pulsed laser at 590 nm with a repetition rate of 1 kHz (OperA Solo, Coherent). All the measurements were performed at room temperature.

3. Results and discussion

Figure 1(a) shows a typical TEM image of the RhNCs that clearly highlights their highly uniform shape and size (the edge length $\sim$ 35 nm). The optical absorption property of the RhNCs characterized using a UV-6300 spectrophotometer is depicted in Fig. 1(b). In particular, a dominant peak was observed at 370 nm in the ultraviolet spectral band, which yielded ultraviolet localized surface plasmon resonance [39,40]. Figure 1(c) shows an SEM image of the as-prepared single ZnO MWs (the diameter of the MW $D$ = 10 $\mu$m) with its hexagonal cross-section shown in the inset. Furthermore, a single ZnO MW with deposited RhNCs is shown in Fig. 1(d). Figure 1(e) shows an enlarged SEM image of the deposited RhNCs; from the figure, it can be observed that the RhNCs were uniformly deposited on the surfaces of the MW. To explore the effect of RhNCs on the optical properties of single ZnO MW, PL emission measurements of single ZnO MW prepared with and without RhNCs deposition were performed using the 325 nm line of a He-Cd laser as an excitation source; these measurements are shown in Fig. 1(f). It can be observed that the near-band-edge (NBE) emissions of ZnO dominated the PL spectra; however, it was accompanied by a weaker broadband defect-related emission at about 510 nm. With the incorporation of RhNCs, the NBE emission of ZnO was considerably improved with a two-fold increase in the enhancement ratio. Moreover, the defect-related emission was slightly suppressed. Thus, the noticeable enhancement in PL can be attributed to the deposited RhNCs on the ZnO MWs [12,28,30].

 

Fig. 1. (a) TEM image of the as-prepared RhNCs. (b) Absorption spectrum of the as-prepared RhNCs. (c) SEM image of a single ZnO MW with perfect hexagonal cross-section (inset). (d) SEM image of a single RhNCs@ZnO MW; the as-synthesized RhNCs were randomly deposited with a uniform distribution on the surface of the MW. (e) Enlarged SEM image of RhNCs deposited on the surface of the ZnO MW. (f) PL spectra of single ZnO MW prepared without, and with RhNCs deposition.

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It is well-known that semiconductor nano/microstructures with a hexagonal cross-section can provide a promising platform to realize an optical WGM microresonantor wherein light waves propagate circularly on its inner walls because of multiple total internal reflections at the semiconductor/air boundary [21,29,43,44]. The excitation power-dependent optical measurements of single MW prepared with and without RhNCs deposition were performed using a confocal micro-PL ($\mu$-PL) spectroscopy system, see the experimental section. The PL spectra of a single bare ZnO MW exhibited broadband spontaneous radiation as the pumping power density $I_{exc}$ was increased from 100 to 153 kW/cm$^2$, as shown in Fig. 2(a). The wavelength of the dominant emission peak is located at 387.0 nm with a spectral linewidth of about 12 nm. When $I_{exc}$ was increased to 170 kW/cm$^2$, several sharp emission peaks appeared in the spontaneous emission spectrum of the ZnO MW, indicating that an optical microresonator with specific modes can be formed using single ZnO MW with a hexagonal cross-section [20,21,42]. Moreover, as $I_{exc}$ was increased to more than 185 kW/cm$^2$, the PL spectra were dominated by a series of sharp emission lines, which increased in number of resonator mode and the emission intensity as $I_{exc}$ was increased.

 

Fig. 2. (a) PL spectra of a single ZnO MW as a function of pumping fluence $I_{exc}$ varying from 100 to 220 kW/cm$^2$. (b) Integrated PL emission intensity and spectral FWHM as functions of pumping fluence in a single, bare ZnO MW. The lasing threshold was estimated to be about 162 kW/cm$^2$. (c) PL spectra of a single RhNCs@ZnO MW for pumping fluence $I_{exc}$ ranging from 47 to 190 kW/cm$^2$. (d) Integrated PL emission intensity and spectral FWHM for the same pumping fluence range in the same ZnO MW covered by RhNCs. The lasing threshold $P_{th}$ was estimated to be about 100 kW/cm$^2$. (e) The comparison of lasing spectra of the single ZnO MW covered without and with RhNCs deposition, with the pumping fluence of 170 kW/cm$^2$. (f) RhNCs-plasmon enhanced ratio of integrated PL intensity versus the pumping fluence of the single ZnO MW covered without, and with RhNCs deposition.

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Integrated PL emission intensity and spectral full width at half maximum (FWHM) as functions of pumping fluence $I_{exc}$ were depicted in Fig. 2(b). The nonlinear features of the PL emission above and below the threshold revealed that optically pumped lasing action can occur in the ZnO MWs, which, in turn, indicated that multi-mode lasing action was produced and confined in the hexagonal cross-section of the MWs [20,21,30]. Moreover, the spectral FWHM dramatically reduced from 12 to 0.11 nm above a lasing threshold of 162 kW/cm$^2$ (the spectral FWHM are the average values of the each lasing spectra); while, the mode spacing between two adjacent subpeaks is almost the same, indicating the same waveguiding origin of each emission modes. Additionally, the typical two-stage-shape pumping fluence-dependent emission relationship also suggested a distinct transition from spontaneous emission to stimulated radiation. Therefore, the lasing threshold of $P_{th}$ $\sim$ 162 kW/cm$^2$ is in good agreement with the appearance of the narrow subpeaks around the pumping fluence. In terms of the lasing mechanisms, it was observed that the lasing oscillations belonged to the WGM-type, wherein the sidewalls of the MWs served as the reflective surfaces of the optical concentrator that caused the total internal reflection.

As indicted by the pump-fluence dependent PL spectra shown in Fig. 2(c), the same single ZnO MW incorporated with RhNCs were also optically pumped. At a low pumping fluence of 102 kW/cm$^2$, several sharp subpeaks appeared abruptly at the low energy edge of the broad spontaneous emission band with a spectral FWHM of about 0.09 nm (Still, the spectral FWHM are the average values of the each lasing spectrum). The sharp narrowing of the spectral linewidth of each emission subpeak was comparable with that of the bare ZnO MW, thereby suggesting the occurrence of lasing action. Each subpeak can be considered as an optical WGM-type microresonator; thus, single RhNCs@ZnO MW with a hexagonal cross-section can be utilized to fabricate WGM-type microcavities. Moreover, the integrated PL intensity and spectral FWHM for different pumping fluence $I_{exc}$ values were also plotted, as shown in Fig. 2(d). The nonlinear characteristics of the observed PL intensity were well-fitted with two linear functions for spontaneous emission and lasing regimes with an estimated lasing threshold $P_{th}$ of about 100 kW/cm$^2$; this estimation was also in good agreement with the appearance of the narrow subpeaks around the pumping fluence threshold [28,29]. It is clear that the lasing threshold of the single RhNCs@ZnO MW (100 kW/cm$^2$) reduced obviously compared with that of the bare ZnO MW (162 kW/cm$^2$), which suggests that lasing action can be relatively more easily achieved using the MW covered by RhNCs [12,28,30].

Furthermore, to investigate the modulation effect of RhNCs on the lasing characteristics of the single ZnO MWs, the lasing spectra of the same ZnO MW with and without RhNCs deposition under the same pumping fluence of 170 kW/cm$^2$ were plotted and compared (see Fig. 2(e)). By comparison, the enhancement in the lasing intensity of the ZnO MW was more than fifteen-fold after RhNC deposition on it. Moreover, a slight blue shift in the lasing band was also observed, which also could be attributed to the deposited RhNCs. The enhancement ratio of the integrated PL intensity of the samples with increasing excitation power density is depicted in Fig. 2(f). As can be observed from the figure, the enhancement ratio increases rapidly, and then levels off at about two times the original magnitude as the pumping fluence $I_{exc}$ approaches 190 kW/cm$^2$. The phenomenon could be understood in terms of a screening effect caused by excess carriers generated at a high excitation power density. Therefore, the deposition of RhNCs on single ZnO MWs presents a potential approach to achieve a lower-threshold laser [12,28].

Figure 3(a) depicts the relationship between the lasing emission spectrum, lasing mode number and refractive index for a single ZnO MW (the pumping fluence $I_{exc}$: 170 kW/cm$^2$). The lasing emission peak positions were in good agreement with the theoretical modes using which the excited mode number; and refractive index of the MW could be deduced using the plane wave model for the optical WGM microcavity [20,21,44]. The average FWHM of the lasing peak was about 0.11 nm; thus, the corresponding $Q$-factor was estimated to be about 3650 according to the formula $Q$ = $\lambda /\Delta \lambda$, where $\lambda$ is the lasing wavelength. Moreover, the lasing peak wavelength-dependent refractive index and lasing mode numbers were also plotted for the ZnO MW with RhNC deposition under the same excitation conditions (see Fig. 3(b)). In addition to significant enhancement in the lasing intensity of the ZnO MW on RhNC deposition, more lasing modes with mode numbers $\mathbf {N}$ in the range of 169–154 were observed, which were in accordance with the lasing band ranging from 388.0 to 399.0 nm. Furthermore, the $Q$-factor was also enhanced to about 4000.

 

Fig. 3. Relationship between the lasing emission spectrum and refractive index of a single ZnO MW (the diameter of the MW $D$ = 10.0 $\mu$m) (a) before and (b) after RhNCs deposition at the same pumping fluence $I_{exc}$ of 170 kW/cm$^2$. (c) Lasing wavelength-dependent mode number of a ZnO MW prepared with and without RhNCs deposition. (d) Lasing wavelength-dependent $Q$-factor of ZnO MWs prepared with and without RhNCs deposition.

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Figure 3(c) illustrates the relationship between the each lasing subpeaks and corresponding mode number of the single ZnO MW. Based on these results, it is clearly revealed that the introduction of RhNCs on the MW can improve optical field confinement and PL enhancement. Moreover, the $Q$-factor trends of the single ZnO-MW-based optical microresonators with and without RhNC deposition were also plotted, as shown in Fig. 3(d). From the figure, it can be inferred that RhNC deposition on the ZnO MW can be utilized to improve the $Q$-factor of the latter. Therefore, the incorporation of RhNCs presents a potential approach to modulate the lasing characteristics of individual ZnO MWs, including the enhancement of their optical output intensity, reduction in their threshold for stimulated radiation, and increase in their $Q$-factor [12,20,28,30].

The exploitation on the working principle of RhNCs and their ultraviolet plasmonic effect on the WGM lasing characteristics of single ZnO MWs were performed [12,17,18]. The normalized optical absorption spectrum of RhNCs and PL spectrum of a single ZnO MW are depicted in Fig. 4(a). From this figure, it can be observed that the main peak originated from the localized surface plasmon resonance of RhNCs matches well with the measured wavelength of PL emission peaks of the single ZnO MW, leading to the enhancement of the spontaneous and stimulated emissions from the ZnO MWs [12,28,29]. Figure 4(b) shows the room-temperature TRPL of the ZnO MWs prepared with and without RhNCs deposition (See the experimental section). The fitting of TRPL decay curves involved a deconvolution between the instrument response function and a monoexponential radiative decay. The decay lifetime $\tau$ was defined by the formula, $I(t)$ = $I_0$ $\exp (-t/\tau )$, where $I_0$ was the normalization constant. It was observed that the lifetime of the ZnO MW covered by RhNCs ($\sim$ 169 ps) was clearly shorter than that of the bare ZnO MW ($\sim$ 206 ps). This suggests that the incorporation of RhNCs deposition can be employed to facilitate the exciton recombination rate of ZnO MW, which is ascribed to the influence of the deposited RhNCs. Localized surface plasmons of the RhNCs excited under the illumination of the 355 nm fs-pulsed laser, can immensely enhance the performance of the optical microcavity by the plasmons coupling between the decorating RhNCs and ZnO MWs. Therefore, the efficient excitation of ultraviolet plasmons for the deposited RhNCs and corresponding ultraviolet plasmonic effects are further applied to a hexagonal ZnO MW to improve its WGM lasing performance [20,28,29,31].

 

Fig. 4. (a) Normalized intensity of the absorption spectrum of RhNCs along with the PL spectrum of a single ZnO MW. (b) Room-temperature TRPL decays of bare ZnO and RhNCs@ZnO MWs. The simulated optical standing wave field distributions in the WGM microcavity of (c) a single, bare ZnO MW and (d) that covered by RhNCs (d). (e) Electrical-field distribution of single RhNC at the $x$-$y$-plane. (f) Electrical-field distribution at the $x$-$z$ plane of the cross section along the RhNC/ZnO interface.

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To confirm the RhNC-plasmon-induced optical field confinement and PL enhancement at the RhNC-ZnO interface, numerical optical mode simulations were performed using the finite difference time domain (FDTD) method. For this simulation modeling, a single ZnO MW with a perfect hexagonal cross-section placed on the quartz substrate was constructed. In the modeling, the diameter of the ZnO MW, $D$ = 10.0 $\mu$m with the calculated wavelength $\lambda$ = 393 nm; the corresponding refractive indices of ZnO $n_{ZnO}$, quartz substrate $n_{quartz}$, and air $n_{air}$ are 2.35, 1.5, and 1.0, respectively. The complex relative permittivity for Rh at the wavelength of 370 nm is denoted as $\varepsilon _{Rh} = -15 + 8\textrm{i}$ [39–41]. Figure 4(c) shows $|E|^2$ field-distribution intensities along the hexagonal cross-section of a single, bare ZnO MW, wherein WGM-type patterns can be observed [30,42]. As the size of RhNCs being much smaller than the diameter of the ZnO MW (the edge length of RhNC of 35 nm $\ll$ the diameter of ZnO MW of 10 $\mu$m), together with hyperfine mesh of the RhNCs@ZnO MW with hexagonal cross-section, an ensemble simulations was constructed to account for the configurational averaging of the homogeneously random distribution of the nanocubes on the surfaces of the MW. The calculated $|E|^2$ field-distribution intensities along the hexagonal cross-section of a single RhNCs@ZnO MW were also recorded for the simulated optical path along the $x$-$y$ plane, as demonstrated in Fig. 4(d). From the figure, it can be observed that the light wave propagating circularly in the inner walls of the wire was greatly enhanced due to multiple total internal reflections at the RhNCs@ZnO/air boundary, suggesting that the performance of a single-ZnO-MW-based WGM optical microresonator is improved by the deposited RhNCs [12,28,29].

Moreover, to explore the influence of RhNCs on the enhancement of the WGM-type $|E|^2$ field-distribution, the electric-field distributions of a single RhNC (the edge length of 35 nm, the plasmon resonance wavelength of 370 nm) at the interface of RhNC-ZnO excited by 355 nm light illumination were extracted. Figure 4(e) shows the spatial distribution of the electric-field intensity of a single RhNC deposited on the MW. It is explicitly demonstrated that when the wavelength of incident light is near its plasmonic resonance peaks, notablly enhancement of electromagnetic field primarily concentrated at the endpoints of the deposited RhNCs [39,40,45]. According to the calculated result, the enhanced light absorption and high near-field enhancement in ZnO MW covered by RhNCs can be expected. Besides, the near field distribution of the RhNCs/ZnO hybrid structure from the cross section at $x$-$z$ plane were also extracted, as exhibited in Fig. 4(f). The calculated result reveals a rapidly decaying electric field enhancement with increasing distance from the RhNC to the nearby ZnO MW. Thus, RhNCs-plasmon induced near-field at the RhNCs/ZnO interface, which formed by the polarization and $\mathbf {K}$-vector, can intensively re-distribute into the contact area between the RhNCs and ZnO MW. It can lead to the coupling interaction between ZnO excitons and RhNCs plasmons, which is responsible for the enhancement of the NBE-type emission and WGM-mode lasing of single ZnO MW. More qualitatively, the excited RhNCs ultraviolet plasmons can be efficiently coupled with ZnO excitons, which together can then be coherently recirculated back into the plasmonic mode of the same RhNC [26,46,47]. Therefore, the ultraviolet plasmonic coupling of RhNCs with ZnO MW can suitably facilitate the radiation recombination rate of electron-hole pairs.

4. Conclusions

In summary, RhNCs with a controlled size and resonant energy-tunable plasmons in the ultraviolet spectral region were successfully prepared. Through RhNCs incorporation, where RhNCs act as ultraviolet plasmon sources, WGM lasing characteristics with high performance were achieved from individual ZnO MWs. Significant enhancement in the output lasing intensity with a lower lasing threshold and higher $Q$-factor were observed in ZnO MWs with RhNCs deposition compared with the bare ZnO MWs. The remarkable improvement in lasing performance can be attributed to the deposited RhNCs. Therefore, precise modulation of the size of noble RhNCs and associated plasmon-energy can be a prospective approach for understanding ultraviolet plasmonic behavior. Furthermore, Rh nanostructures have the potential for use in transformative and promising applications, such as in novel optoelectronic devices with high efficiency and high performance in the short wavelength region.