Random lasers have drawn considerable attention in recent years for the light multiscattering-based feedback mechanism, which is different from conventional cavity-based lasers , , , , , . Various random lasers have been demonstrated based on dielectric nanoparticle scattering , ,  and metal nanoparticle surface plasmonic scattering , , , . In particular, random lasing based on localized surface plasmon resonances (LSPR) have demonstrated the characteristics of low threshold and high performance , , , , , , , , , , , , . For example, Popov et al.  found that gold nanoparticles enhanced the gain in random laser system containing dyes and gold nanoparticles. To achieve random lasing with lower threshold and multicolor emissions, nanogap-based random lasing was proposed by choosing the gold-silver (Au-Ag) bimetallic porous nanowires with abundant nanogaps that provide strong feedback or gain channels for coherent lasing from dye molecules , . Based on the strong confinement characteristic of the nanogaps, Au-Ag bimetallic nanowire-based random lasers operate at a low threshold and have high Q-factors in a wide visible spectral range . However, the syntheses of Au-Ag nanowires need a relatively long growth period of about several hours and are generally limited by multistep protocols. The size of Au-Ag nanowires is also not easy to be controlled, which is not good for optimizing the performance of random lasers. Its bimetallic character is not convenient to research on the effect of the nanogap metal component on stimulated emission. Therefore, the size-controllable and easily processable pure metallic nanoparticles are vastly needed for high-performance and multicolor random lasers.
Considering the antibacterial activity and special optical performance of silver, silver nanoparticle synthesis and application have attracted a lot of attentions from researchers. Silver nanoparticles have been used as scatterers in random lasing , , . However, these nanoparticles have simple morphology and no nanogaps. To further improve the performance of nanogap-based random laser, it is necessary to fabricate silver nanoparticles with abundant nanogaps to play the role of scattering and greatly enhance the local electromagnetic field in a random system. As a typical nanoparticle with abundant nanogaps, silver nanoflowers (Ag NFs) have demonstrated superior antibacterial activity against Pseudomonas aeruginosa, Streptococcus faecalis, and Escherichia coli , . Besides, Ag NFs have shown a remarkable enhancement effect in surface-enhanced Raman scattering , , , , , , , . However, there are no reports on Ag NF-based random laser that are beneficial to broadening the scope of random lasing applications.
In this work, Ag NFs with nanosheet aggregations are fabricated in a short growing period of several minutes by a rapid one-step solution-phase synthesis method. Controllable nanogap density is achieved by changing the concentration of polyvinylpyrrolidone (PVP), leading to the fine tuning of surface morphology. Fabricated Ag NFs are used as broadband scattering centers to achieve hot-spot effect-based random systems. The abundant nanogaps randomly distributed within Ag NFs and the interparticle coupling effect of Ag NFs can provide high enhancement of electromagnetic field and demonstrate a broadband surface plasmonic spectrum over the whole visible range. These characteristics lead to Ag NF-based random laser operating with an extremely low threshold of 0.24 MW cm−2 and high-quality factor of more than 10,000, which are improved by a factor of 630% and 1667% with respect to the silver nanowire-based cases, respectively. The coherent colorful random lasing covering the visible range is realized using the dye molecules oxazine (Oz; red), Coumarin 440 (C440; blue), and Coumarin 153 (C153; green). Moreover, the growing period of Ag NFs is shortened to several minutes. All these features show that Ag NFs are highly efficient broadband scatterers for high-performance random lasers.
2 Results and discussion
Ag NFs are fabricated in a growing period of 5 min using the ice-water bath method as reported previously  under different PVP concentrations (Section 4). The nanostructures of the synthesized Ag NFs are characterized by scanning electron microscopy (SEM) as shown in Figure 1A–H, presenting Ag NFs with abundant nanogaps. A close inspection reveals that these Ag NFs are actually composed of Ag nanosheets (Figure 1E–H). The diameters of Ag NFs can be systematically controlled from 1200±10 nm (Figure 1A and E) to 1000±15 nm (Figure 1B and F) and from 500±18 nm (Figure 1C and G) to 360±20 nm (Figure 1D and H) by increasing the concentration of PVP in reaction solution from 23 to 46 and 115 to 137 mm, respectively. The thickness of the nanosheets increases simultaneously from 21±3 to 51±8 nm by increasing the PVP concentration from 23 to 137 mm. Smaller particles and larger nanosheets induce less nanogaps. As a result, the density and amount of nanogaps within Ag NFs are both decreased by increasing the PVP concentration. That is, the nanogap density can be controlled by varying the concentrate of PVP as well as the size of Ag NFs.
Energy-dispersive spectroscopy (EDS) shown in Figure 1I demonstrates that the nanoparticles (Figure 1A) are composed of pure silver as the silver element possesses 100% at 3 keV, except for the background element. To reveal the crystalline structure of the nanoporous Ag NFs, powder X-ray diffraction (XRD) is employed as shown in Figure 1J. The four characteristic diffraction peaks in the XRD pattern can be indexed to (111), (200), (220), and (311) planes of the face-centered cubic with lattice constant of 4.086 Å, which is in good agreement with the traditional value of Ag (PDF 040783). The peak at the diffraction angle of 38.116° is most intense, which can be attributed to the largest exposed area of the corresponding facets. Interestingly, the intensity ratios of (111):(200) diffraction peaks are among 3.18 and 3.73 for the as-prepared Ag NFs (Figure S1, Supporting Information), which are bigger than the intensity ratio 2.5 of the standard Ag diffraction database. The result indicates that the produced Ag NFs exhibit a preferred orientation along the (111) plane in the process of growing randomly along the radial directions.
To better understand the formation mechanism of Ag NFs, we monitor the morphology evolution of the nanoparticles taken out at different reaction times as shown in Figure 2A–D. As the reaction time is 1 s (Figure 2A), the obtained Ag NFs with a diameter of 600 nm are assembled by the nanosheets with a thickness of 50 nm. Increasing the reaction time to 2 s (Figure 2B), the particles grow bigger, whereas the thickness of the assembled nanosheets decreases to about 20 nm. When the reaction time is further increased to 3 s (Figure 2C) and 5 s (Figure 2D), the particles become bigger, whereas the nanosheets become thinner and narrower and the margins thus become smaller. Based on the above phenomenon, a growth mechanism of Ag NFs is hypothesized as four distinguishable stages presented in Figure 2E. First, the Ag+ is reduced to Ago by ascorbic acid and the reduced Ago is partially blocked in the aqueous solution by PVP and citric acid (CA). Second, the Ago agglomerates into particles with random pilot on its surface and the nanoparticles maintain as quasi-spheres as the shear force supplied by constant magnetic stirring . Third, the newly reduced surplus Ago agglomerates selectivity on the nanosheets by the assistance of the Ostwald ripening process and thus forms new nanosheets along the orientation, which fills the margins among the existing nanosheets. The loose flower-like particles on the random pilot are obtained with the assistance of CA. Last, the whole loose flower-like particles act as subseeds and thus accelerate the formation of compact 3D Ag NFs via a further growth process . Thus, the final product is assembled into quasi-spheres composed of abundant nanosheets.
To investigate the property of the local electromagnetic field for the produced Ag NFs, two simplified models are constructed for 500 nm NF and 800 nm NF as shown in Figure 3A and B. The electric field distributions of Ag nanoparticles at YZ panel are systematically calculated at different incidental wavelengths of 435, 540, 575, and 640 nm, which are in the luminescence band of C440, C153, rhodamine 6G (R6G), and Oz, as shown in Figure S2 (Supporting Information; 500 nm) and Figure S3 (Supporting Information; 800 nm). The field enhancement factor (FEF) in the nanogaps changes with the incident frequency, particle morphology, and size. The maximal FEF of 500 nm Ag NF reaches 31.5 at 435 nm, which is seven times bigger than that of the simple Ag nanosphere under the corresponding wavelength. The maximal FEF of 800 nm NF is enhanced by 9.94 times regarding that of the simple Ag nanosphere at 575 nm. That is, the local electromagnetic field can be greatly enhanced by the hot-spot effect induced by the nanogaps. Considering the molecules of dyes randomly distributed around the nanoparticles, the normalized average electric intensity is defined as(1)
where E is the electric intensity near the surface of Ag NF and ∫ds is the integral along the surface of Ag NF. The normalized average electric intensity is calculated and shown in Figure 3C and D. Ag NFs demonstrate obvious superiority in electromagnetic field enhancement to nanoparticles with the same diameter over the whole visible range. In addition, nanogaps formed between/among different Ag NFs can still give additional significant enhancement of the local electromagnetic field in a relatively large region . These two factors are the fundamental of the nanoflower-based random lasers operating at an extremely low threshold.
Figure 4A shows the experimental extinction spectra of different Ag NF suspensions. All the extinction spectra exhibit broadband characteristic over the whole visible spectral range without any obvious resonance peak , , which is in accord with the simulated results (Figure S4, Supporting Information). Using the broadband enhancement effect induced by the abundant randomly distributed nanogaps with different random sizes in Ag NFs as the scatterers to supply strong gain for optical feedback , , the dyes C440, C153, R6G, and Oz are chosen as the gain media to build colorful random lasing systems, labeled as RL-C440, RL-C153, RL-R6G, and RL-Oz, separately. The photoluminescence spectra of these dyes are centered at approximately 435, 540, 575, and 640 nm, whereas the absorption spectra are centered at approximately 380, 430, 520, and 610 nm, respectively, as shown in Figure 4B. The optical processes of photoluminescence and absorption are in the visible range and thus enhanced by Ag NFs.
Figure 5A shows the emission spectra of the random laser with gain material R6G under different pump power densities, whereas the pump pulses run at 532 nm as shown in Figure S5 (Supporting Information). When the pump power density is 0.18 MW cm−2, there is only broad spontaneous emission (Figure 5A, black curve). When the pump power density is higher than the threshold Eth=0.24 MW cm−2, there appear several sharp spikes with linewidth of subnanometer (Figure 5A, red curve). The peak intensities increase rapidly with increasing pump power density. There are more spikes that emerged in the spectra as the pump power densities further increase, as shown in Figure 5A (blue and olive curves). The appearance of sharp spikes indicates building the coherent feedback in random system, resulting from the strong feedback and large enhancement of the local electromagnetic field induced by the nanogaps within Ag NFs , , . The probability of coherent random lasing , , defined as the ratio of the number of spectra with coherent modes to the total number of spectra captured, is used to determine the threshold of coherent random lasing as shown in Figure 5B. When the pump power density is below the threshold of 0.24 MW cm−2, the probability of random lasing is zero. Increasing the power density, the probability quickly rises from 0 to 1 and remains close to 1. The working threshold of Ag NF-based random lasers is improved by 630% relative to that of the traditional Ag nanowire-based samples , as induced by the unique nanoporous morphology of Ag NFs. Figure 5C demonstrates the linewidth of the random lasing mode at 568.83 nm, which is 0.048 nm as described as full-width at half-maximum (FWHM; Δλ). The corresponding quality factor (defined as λpeak/Δλ) of 11,851 is improved by 1667% (625%) with respect to that of the Ag nanowires  (nanoparticles )-based random lasers and is comparable to other lasing systems , . The stability of the random laser is concluded in Figure S6 (Supporting Information) by demonstrating the threshold variations of the 1-year-stored Ag NF-based random lasers at different times, displaying a stable threshold behavior with little fluctuations around 0.24 MW cm−2, which is the same as that of the random laser shown in Figure 5B. The good stability shows that Ag NFs are the excellent scatterers in random lasers.
To further show the effects of the nanogaps on the performance of random lasing, the dependency of the thresholds of random lasers on the diameter of Ag NFs is carefully studied as shown in Figure 5D. The mass concentration remains unchanged to ensure that Ag NF volume fraction and thus the gain volume are fixed. The threshold decreases from 0.35 MW cm−2 to a minimum of 0.24 MW cm−2 as Ag NFs diameter increases from 360 to 1000 nm. This is because that the density and amount of nanogaps within Ag NFs with the diameter of 1000 nm reach maximum as demonstrated by SEM and thus provide the strongest feedback and gain channels for random lasing. This phenomenon is in accordance with the results of previous studies , , . The increment of threshold with further increasing particle size (>1000 nm) may result from the fact that the particles agglomerate and precipitate during the measurement. The corresponding emission spectra with multiple sharp peaks and the threshold behaviors of the random lasers are shown in Figure S7 (Supporting Information). All random lasers exhibit the property of coherent feedback and low threshold.
Ag NFs with abundant nanogaps are used to achieve colorful coherent random systems of RL-C440 (blue), RL-C153 (green), and RL-Oz (red), as shown in Figure Figure 6. The three random lasers present obvious sharp spikes in the emission spectra induced by the coherent feedback. The insets are the photographs of the corresponding random laser, exhibiting the property of random lasing with blue, green, and red, respectively. The linewidth of the random lasing modes from each system are 0.046 nm (at 436.23 nm), 0.050 nm (at 544.03 nm), and 0.062 nm (at 642.63 nm) as shown in 6D–F, respectively. The corresponding Q factors are calculated as 10,906, 10,881, and 11,475 based on the linewidth. The thresholds of the three lasers are measured and shown in Figure 6G–I. It can be seen that the thresholds of the three random lasers are 0.18 MW cm−2 (RL-C440), 0.78 MW cm−2 (RL-C153), and 0.14 MW cm−2 (RL-Oz), respectively, which are all considerably low for random lasers.
To intuitively demonstrate the significant improvement of the produced Ag NFs for colorful random lasers, a property table of the surface plasmonic random lasers is carefully illustrated in Table 1 by comparing the synthetic method of the scatterers, the pump source, the threshold, the linewidth, and the emission band of the random lasing. Compared to the traditional surface plasmonic resonance random lasers  and the reported “hot-spot” effect-based random lasers , , Ag NF-based random lasers have the minimal working threshold of 0.24 MW cm−2 and the smallest linewidth of 0.048 nm when pumped by the nanosecond pulses. Good performance is induced by the interparticle coupling effect and the unique morphology of Ag NFs with the relative small size and abundant nanogaps, which can efficiently enhance the local electromagnetic field. Further, the randomness of the nanogaps in distribution and size makes the produced Ag NFs as broadband plasmonic scatterers in random lasers covering the whole visible range (350–750 nm). Moreover, Ag NFs are obtained in a growing period of several minutes by a one-step solution-phase synthesis method, which is the most simple and rapid among these scatterers of Ag nanowires, gold nanostars , Au-Ag bimetallic nanowires , and Ag NFs. In particular, the growing time can be improved by ~70 times with respect to the gold  and/or Au-Ag bimetallic nanowires . Therefore, Ag NFs provide predominant abilities for achieving high-performance random lasers covering the whole visible range.
In summary, Ag NFs with abundant nanogaps are first proposed as broadband plasmonic scatterers to achieve high-performance random lasers covering the whole visible range. The synthesized mechanism of Ag NFs by the ice-water bath method is proposed by analyzing the morphology of nanoparticles obtained at different reaction times in our experiments. The concentration of nanogaps and the size of Ag NFs could be well controlled by changing the concentration of PVP in the fabrication process. These nanogaps and spiky tips near the surface of Ag NFs and the interparticle coupling effect greatly enhance the local electromagnetic field over the whole visible range. Random lasers with extremely low threshold (0.24 MW cm−2) and high Q-factors (~10,000) are achieved using Ag NF dispersion with the dye R6G. The capability of Ag NFs in the enhancement of colorful lasing resonance covering the visible range is further demonstrated using various laser dyes as gain media, such as C440, C153, and Oz. The rapid, flexible, and one-step solution-phase synthesis of these nanostructures enables the easy realization of the nanogap-based random lasers and provides a way for efficient multicolor laser system operation and photon energy harvesting.
4.1 Preparation of Ag NFs
In our experiments, AgNO3 (99.9999%; Aldrich), PVP (Mw=1,300,000; Aladdin), ascorbic acid (Aldrich), and CA (99.9%; Aladdin) are used as received. AgNO3 aqueous solution (1 ml, 0.5 m), PVP solution (1 ml, 0.3 m), and CA solution (0.1 ml, 0.25 m) are separately added into deionized water (10 ml) in a 25 ml beaker at every 10 min with a magnetic stirring in an ice-water bath. The concentration of PVP is calculated in terms of the repeating units. After 15 min, ascorbic acid aqueous solution (C6H8O6, 1 ml, 0.5 m) is then quickly injected into the vigorously stirred mixture. The added ascorbic acid is kept equimolar to that of silver ions. The solution became gray or khaki immediately in accordance with the color of the added acid and a large quantity of Ag nanoparticles are produced in a few seconds. After 5 min, the mixed solution is added in 100 ml ice water and then centrifuged to terminate the reaction. The nanoparticles are obtained through the multicycles of wash-centrifuge-redisperse purification to remove excess PVP and extreme small Ag nanoparticles. The final product is stored in ethanol to form the Ag nanoparticle suspension. For comparison, the preparation of Ag particles with different concentrations of PVP (23, 46, 115, and 137 mm) is also carried out.
4.2 Material characterization
The microscopic structures of Ag NFs and EDS measurement are characterized by SEM (Hitachi S4800 microscope). XRD measurements are carried out on a Shimadzu XRD-6100 diffractometer using a Cu Kα source (λ=0.154 nm).
4.3 Preparation of dye random lasing systems
In our experiment, C440 (0.5 mg ml−1), C153 (1 mg ml−1), R6G (0.15 mg ml−1), and Oz (0.16 mg ml−1) are separately dissolved in Ag NF suspensions with a mass concentration (0.018 mg ml−1) to make four disordered systems, which can be treated as random lasers of RL-C440, RL-C153, RL-R6G, and RL-Oz, separately. In the following experiments, the diameter of Ag NFs is chosen as 1000 nm unless otherwise noted.
All calculations are carried out using the commercial software COMSOL. The optical parameters of silver are chosen from Johnson and Christy’s experimental data . The excitation beam is a plane wave polarized along the y-axis and transmitted along the z-axis. Two different simplified models are built for Ag NF with different diameters. For the 500 nm NF, it possesses 100 nm hemispheres on the surface of a nanosphere with a core diameter of 400 nm. For the 800 nm Ag NF, it comprises silver nanocuboid distributed radially from the center. The nanocuboid is 400 nm in length and 80 nm in both width and height, with the intersection angle of 30° in the plane of X0Z0 and X0Y0. The normalized average electric field intensity and the extinction spectra are both implemented for different nanoparticles. It is worth noting that we have only simulated the maximal Ag NF with the diameter of 800 nm limited by the computer memory. However, we can obtain the conclusion that the more nanogaps and spiky tips Ag NFs possess, the bigger enhancement factor will be provided by Ag NFs.
4.5 Optical measurements
The experimental set-up is shown in Figure S5 (Supporting Information). The mixed suspension is collected in a cuvette of length 20 mm, width 10 mm, and height 45 mm. The dye system is then pumped vertically using a Q-switched frequency-doubled Nd:YAG-pulsed laser (Continuum PowerLite Precision 8000), a pulse duration of 8 ns, a repetition rate of 10 Hz, and an out-beam diameter of 8 mm. The wavelength is chosen as 532 nm for RL-R6G and 355 nm for RL-C440 and RL-C153. The 570 nm nanosecond pulsed laser is generated by Optical Parametric Oscillators pumped by a Q-switched Nd:YAG-pulsed laser to excite the RL-Oz. The emission spectra of the random lasing systems are collected horizontally by a Princeton Instruments Acton SP 2750 spectrometer with a high resolution of 0.01 nm. To determine the thresholds of the random lasing, the emission spectra are separately recorded for hundreds of times at each pump power density. The probability of coherent spectra is used to plot the threshold curves, which is calculated by measuring the ratio of the number of coherent spectra to the total spectral number of 300. In our experiment, the total spectra are collected at equal time intervals under each pump power density.
The following files are available free of charge.
The authors would like to acknowledge the National Natural Science Foundation of China (grant nos. 11574033, 11674032, 11074024, and 61275130), Beijing Higher Education Young Elite Teacher Project, and the Fundamental Research Funds for the Central Universities for their financial support. The authors would also like to thank Dr. Jun Zheng for the help with the simulations.
Author contributions: Q.C. designed the experiments and performed the experiments. Q.C. and X.L. performed the simulations. Q.C., X.S., and Z.W. wrote the paper. Z.W. supervised the project and conceived the study. All authors have given approval to the final version of the manuscript.
Competing interest of statement: The authors declare no competing financial interest.
Materials and methods, Figure S1–6: XRD patterns of Ag NFs fabricated under different concentration of PVP; Enhancement effect of electromagnetic field of Ag NFs; Extinction property of Ag NF; Experiment set-up for random laser systems; Stability of Ag NFs and the threshold of Ag NF-based random lasers; Spectra and threshold characteristics of random systems with different size Ag NFs; Spectra and threshold characteristics of random systems with different size Ag NFs; Stability of Ag NFs and the threshold of Ag NF-based random lasers.
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