Polyvinyl pyrrolidone (PVP, MW ∼29000), silver nitrate (AgNO₃, ) and ascorbic acid (AA, ) were purchased from Sigma–Aldrich, USA. Gold chloride trihydrate ( , ) and hydrochloric acid (HCl, ) were purchased from Sinopharm Chemical Reagent Co., Ltd., China. The CdSe/ZnS quantum dots (QDs) solution was purchased from the Suzhou Xingshuo Nanotech Co., Ltd., China. Ultrapure water from Milli-Q (Millipore, USA, resistivity ) source was used throughout the experiments.

The structural features of the gold seeds, NS and CdSe/ZnS QDs were detected with scanning electron microscope (SEM, Zeiss Ultra Plus, Germany) and transmission electronic microscopy (TEM; Fei Tecnai T20, USA). Extinction spectra were measured with the fiber optic spectrometer (NOVA, Ideaoptics Technology Ltd., China). The femtosecond Ti:Sapphire laser system (Legend-F-1k, 800 nm, 1 kHz, 100 fs) was used in the excitation pulses experiment. The 400-nm pumping beam was realized through a β-barium borate (BBO) crystal. A fast optical multichannel analyzer (OMA, Spectra Pro-300i) was used to collect the emitted beam at the edge of the sample.

A 100-mL solution with concentration of 1-mmol/L solution was heated to boiling. Then 15-mL trisodium citrate solution with a mass fraction of 1% under vigorous stirring was added. The gold seed solution was obtained by heating the reaction solution for 15 minutes, then reserved after cooling.

A 100- HCl solution with 1-mol/L concentration and 1-mL seeds solution were added to 100-mL solution with 0.25-mmol/L concentration in turn under stirring of 700 rpm. Then, 1-mL AgNO₃ solution with 30-mmol/L concentration and 500- AA solution with 100-mmol/L concentration were added simultaneously. After 30-s reaction, the color of the solution changed rapidly, and finally the gold NS samples were obtained. Finally, the gold NS solution was centrifuged at 4500 rpm for 45 minutes, then the precipitate was re-dispersed into 10-mL water.

A rapid assistance-free self-assembly method was used for the fabrication of gold NS film. A pre-cleaned indium tin oxide (ITO) glass slab was put into the untreated gold NS solution. After 12 h, the ITO glass slab was removed from the solution and the surface was flushed with water. Finally, the gold NS thin film was fabricated for the thermal stability experiment.

The 0.7-mL CdSe/ZnS QDs ethanol solution with 0.3 mg/mL was fully mixed with 0.1-g PVP and stirred for 30 min at 600 rpm. Then 300- ultrasonic treated gold NS solution was added to the mixed solution. To prevent gold NS aggregation, the mixed solution was stirred for 1 h and then was left still for 12 h. Then the mixed solution was spinned onto the pre-cleaned ITO glass for 60 s at 3000 rpm. Finally, the prepared thin film sample was put into the drying box for 6 h at 60 °C. The PVP constituted a matrix for the CdSe/ZnS QDs gain medium and immobilized the embedded NPs in the dried thin film. The gold NSs were embedded and immobilized in a gain medium film composed of PVP and CdSe/ZnS QDs. Meanwhile, the film without gold NS was prepared under the same process conditions.

The dispersion characteristics of QDs are very important for the preparation of uniform random laser gain medium thin films. To evaluate the dispersion of CdSe/ZnS QDs, we measured their dispersion properties using TEM, as shown in Fig. 1(a). The size of the CdSe/ZnS QDs displayed in the TEM image is remarkably uniform. The corresponding absorption (black line) and emission spectra (red line) of the CdSe/ZnS QDs are illustrated in Fig. 1(b). The emission peak of CdSe/ZnS QDs is located at ∼628 nm. The position of emission peak can be tuned by varying the size of CdSe/ZnS QDs.^[33] To evaluate the gold seeds we synthesized, the morphologies of the gold seeds were measured by TEM, as shown in Fig. 1(c). The TEM image indicates the gold seeds that were monodispersed in the solution. To determine the size uniformity of the gold seeds, we made a statistical analysis of the particles size for the TEM image, as displayed in the inset of Fig. 1(c). The results indicated that the seed size is mainly between 20 nm and 30 nm. Then, by testing the absorption spectra of the gold seeds (Fig. 1(d)), we further studied their LSPR properties. As shown in Fig. 1(d), the LSPR peak of gold seeds is located at about 520 nm, which is consistent with the small nanospheres that we have observed.^[34]

Fig. 1.

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	Fig. 1. (color online) (a) TEM image and (b) normalized absorption and emission spectra of the CdSe/ZnS QDs, and (c) TEM image and (d) normalized absorption spectrum of the gold seeds.

To obtain high-quality random laser, we prepared gold NS nanostructures with many sharp apexes by the seed-growth method. Figure 2(a) shows the TEM image of gold NS. As shown in Fig. 2(a), the gold NS was well dispersed and each particles had several apexes nanostructures distributed on the surface. To observe its microscopic morphology more clearly, the high magnification of single gold NS TEM image is shown in the inset of Fig. 2(a). The central nucleusʼs size of the gold NS is about 50 nm, while the sharp apexes are of varying lengths. This multi-tip nanostructure has a great scattering characteristic. The apexes position will produce a large number of hot spots. All of them can enhance the light emission of fluorophore. To estimate the LSPR properties of this kind of gold NS, we have further measured the absorption spectrum of the gold NS solution. As shown in Fig. 2(b), the LSPR peak of the gold NS is about 810 nm. Compared with the gold seeds, the LSPR peak of the gold NS has a remarkable red shift, which is due to the increase of particle size.

Fig. 2.

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	Fig. 2. (color online) (a) TEM image of the gold NS. The inset of panel (a) corresponds to higher magnification TEM image. (b) The normalized absorption spectra of gold nanostars (NS). (c) Illustration of the pump and detection of emission from the edge of the gain medium film.

In our experiment, we used a typical vertical pumping, lateral detection experimental setup, as shown in Fig. 2(c). In the setup, the femtosecond Ti:sapphire laser system (Legend-F-1k, 800 nm, 1 kHz, 100 fs, Coherent, USA) was used as the pump light source. The 400-nm pumping beam was realized through a BBO crystal. In the signal detection, we use a fast optical multichannel analyzer (OMA, Spectra Pro-300i, Princeton instruments, USA) to collect the emitted beam at the edge of the sample. Meanwhile, a 532-nm high pass filter is used to suppress the scattering light from the pump light source.

Anisotropic MNP, especially those with lots of sharp-apex nanostructures, have strong near field enhancement properties due to their lightning rod effects.^[35] However, the near field enhancement is also accompanied by the generation of heat, which will have an impact on metal nanostructures, especially on the sharp apexes.^[36] Once the morphology of metal nanostructures degenerates due to the surrounding high-temperature environment, it will directly affect the stability of the system and even cause serious performance degradation. Therefore, it is necessary to check the relationship between the morphology of MNP and temperature, which can be used to guide some experiments related to high heat. We compared the SEM images of the gold NS at different annealing temperatures. It can be seen from Fig. 3(a) that when the gold NS was annealed at 150 °C, the morphology of the gold NS can still be maintained well compared with the TEM image in Fig. 2(a). With the increase of annealing temperature, the apexes morphology of the gold NS surface gradually degenerates. When annealed at a temperature of 250 °C, the apexes morphology of the gold NS surface has become less apparent, as shown in Fig. 3(c). Moreover, when the annealing temperature increased to 300 °C, the apexes nanostructures of the gold NS disappeared completely and transformed into nanospheres (Fig. 3(d)). The results indicate that the apexes nanostructures of the synthesized gold NS can keep its shape when annealing at 200 °C. Because Ag has a lower melting point than gold, Ag nanoparticles are more susceptible to temperature. The results in the literature showed that the morphology of Ag nanoparticles began to change when heating at 90 °C,^[24] while that of gold NS changed at about 200 °C. Therefore, we believe that the gold material is more suitable than silver for some experiments in high temperature environment.

Fig. 3.

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	Fig. 3. (color online) SEM images of the gold NS films after annealing in air atmospheres. The samples were annealed at: (a) 150 °C, (b) 200 °C, (c) 250 °C, and (d) 300 °C for 5 minutes, respectively.

To demonstrate the emission properties of the CdSe/ZnS QDs doped random laser sample with gold NS as scatters, we have carried out experimental measurement on them. The random lasing spectrum collected at different pump fluences is shown in Fig. 4(a). In Fig. 4(a), we only observe the emission of the fluorescence signal at low pump fluences (below the threshold). When the pump fluences are higher than the laser threshold (0.8 mJ/cm²), the single-mode random lasing signals were detected. The random lasing comes from the incoherent feedback in the device. With the increase of pump fluences, the random lasing was obtained when the gain is greater than the loss. Meanwhile, the full width at half maximum (FWHM) of random lasing becomes narrower with the increase of pump fluences. The apex nanostructure of a gold nanostar (NS) has a large number of hot spots; many investigations^[37,38] have proved that the electromagnetic enhancement factor of spots can reach more than 10⁹. Therefore, the hot spot effect produced by the gold NS can greatly enhance the emission efficiency of QDs, thus enhancing the random laser signal. In our study, the surface plasmon resonance spectrum of the gold NS is not overlapping completely well with the emission wavelength band of QDs. The reason is due to the complex morphology and anisotropic apex distribution. As a result, the resonant wavelengths of different hot spots on the gold NS are completely different. This will lead to the resonance peaks spreading in a wide wavelength range corresponding to different dipole moments. Although their resonance wavelengths depend on the macroscopical distribution of average size, which is not overlapping completely with the emission wavelength of QDs, it still enables the generation of many hot spots contributing to the electromagnetic enhancement at the emission wavelength.

Fig. 4.

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	Fig. 4. (color online) (a) Lasing emission spectra of the gain medium film with gold NS under different pump fluences. (b) Lasing emission intensity as a function of the increasing pump fluence. The inset shows that the FWHM of lasing emission peak is 6 nm.

To evaluate the effect of pump fluence on the intensity of emission, we further studied the relationship between them, as shown in Fig. 4(b). As can be seen from Fig. 4(b), when the pump fluence exceeds the threshold value 0.8 mJ/cm², the emission intensity shows a steep linear increase. However, with the increase of the pump fluence, the emission intensity reaches a saturation state, and the increasing trend becomes slower. We believe that the reasons for this saturation phenomenon are as follows. During the process of random laser pumping, both Auger recombination and optical gain amplification exist. When the pump fluence is low, the Auger recombination effect is very small, and the optical gain amplification is dominant for the device. However, with the increase of pump fluence, more electrons will reverse to high energy levels. When the number of electrons in the high energy level reaches a certain number, there will be a significant Auger recombination effect, and the recombination rate will increase rapidly with the increase of the number of electrons. When the pump fluence further increases, the Auger recombination and optical gain amplification effect will reach a dynamic equilibrium, and the random laser intensity will be saturated with the increase of the pump fluence.^[39,40] As illustrated in the inset of Fig. 4(b), an FWHM of only 6 nm was obtained. We have calculated that the quality factor (Q) of the random is 105, according to , where λ and are the wavelength and line width of the random lasing peak, respectively. The results indicate that the closed loop has a high Q.

To determine the role of gold NS in random laser devices, we have further studied a kind of reference sample, that is, disordered gain medium thin film without gold NS. The emission spectrum of the two samples at 2.5-mJ/cm² pumping fluence is displayed in Fig. 5. From Fig. 5, we can see clearly that for the sample without doping gold NS, the emission signal is the only fluorescence spectra. But when the gold NS were doped, we obtained a single-mode random lasing signal. Therefore, the gold NS act not only as scatterers, but also further enhance the emission of CdSe/ZnS QDs because of the strong electric field on the surface. In addition, during the process of synthesizing QDs, a layer of CdS was added before the shell fabrication for ensuring the lattice matching between the CdSe core and the ZnS shell. In fact, in the random laser experiment, we adopted a quasi-type-II CdSe/ZnS core–shell QDs structure. For this quasi-type-II structure, there are spatially separated electrons and holes between the core and the shell structures. Because of the high local charge densities, the excitons are repulsive to each other, moving upward the Coulomb interaction energy of the exciton–exciton. Therefore, the emission of random laser is blue-shifted when it is pumped.^[41,42]

Fig. 5.

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	Fig. 5. (color online) Emission characteristics of gain medium thin films. For two films, spectra were recorded at the same pump fluence of 2.5 mJ/cm2. The reference film without gold NS shows ASE (red line). The film containing gold NS exhibits high intensity lasing (black line).