Plasmonic Carbon-Dot-Decorated Nanostructured Semiconductors for

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Plasmonic Carbon Dots Decorated Nanostructured Semiconductors for Efficient and Tunable Random Laser Action Wei-Cheng Liao, Yu-Ming Liao, Chuan-Tsung Su, Packiyaraj Perumal, Shih-Yao Lin, Wei-Ju Lin, Cheng-Han Chang, Hung-I Lin, Golam Haider, Chiao-yun Chang, ShuWei Chang, Cheng-Yen Tsai, Tien-Chang Lu, Tai-Yuan Lin, and Yang-Fang Chen ACS Appl. Nano Mater., Just Accepted Manuscript • DOI: 10.1021/acsanm.7b00061 • Publication Date (Web): 29 Dec 2017 Downloaded from http://pubs.acs.org on December 29, 2017

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ACS Applied Nano Materials

Plasmonic Carbon Dots Decorated Nanostructured Semiconductors for Efficient and Tunable Random Laser Action Wei-Cheng Liao,‡ Yu-Ming Liao,‡ Chuan-Tsung Su,† Packiyaraj Perumal, Shih-Yao Lin, Wei-Ju Lin, Cheng-Han Chang, Hung-I Lin, Golam Haider, Chiao-Yun Chang,§ Shu-Wei Chang, Cheng-Yen Tsai, Tien-Chang Lu,§ Tai-Yuan Lin,*† and Yang-Fang Chen*

Department of Physics, National Taiwan University, Taipei 10617, Taiwan

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ABSTRACT: Carbon dots emerge as popular materials in various research fields, including biological and photovoltaic areas, while there lacks significant reports related to their applications in laser devices, which play a significant role in our daily life. In this work, we demonstrate the first controllable random laser assisted by the surface plasmon effect of carbon dots. Briefly, carbon dots derived from candle soot are randomly deposited on the surface of gallium nitride (GaN) nanorods to enhance the ultraviolet fluorescence of GaN and generate plasmonically enhanced random laser action with coherent feedback. Furthermore, potentially useful functionalities of tunable lasing threshold and controllable optical modes are achieved by adjusting the numbers of carbon dots, enabling for optical communication and identification technologies. In addition to providing an efficient alternative for plasmonically enhanced random laser devices with simple fabrication and low cost, our work also paves a useful route for the application of environmentally friendly carbon dots in optoelectronic devices.

KEYWORDS: random lasers, carbon nanodots, gallium nitride nanorods, surface plasmon resonance, controllability, nanostructure


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Random lasers are a kind of mirror-less laser formed in disordered gain medium, which have drawn wide attention due to their unique physical mechanism and possible applications in many kinds of aspects. 1-6 The classification of random lasers can be distinguished by their spectral and temporal coherence.7 Different from conventional lasers, random lasers using multiple scattering for their optical feedback usually feature angle-unrestricted, easy-fabrication, and low-cost properties.8 Despite numerous superiorities, random lasers are restricted by their poor controllability due to the randomness of multiple scattering. Therefore, manipulation of random lasers, especially on their threshold, optical modes, and wavelength, has intrigued wide research interest.9-14 Besides, to improve the performance of random lasers, noble metallic nanostructures are traditionally utilized as scattering centers for plasmonically enhanced random lasing.15-19 However, introduction of noble metal suffers from relatively high cost, and most of them are not environmentally friendly. Carbon dots (C-dots) known as “fluorescence carbon” were first obtained by Xu et al. in 2004.20 These discrete and quasi-spherical carbon nanoparticles with sizes below 10 nm greatly impress people with their strong fluorescence which shows obvious dependence on sizes and excitation wavelength. 21-22 In addition, C-dots are regarded superior to conventional semiconductor quantum dots owing 3 ACS Paragon Plus Environment


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to numerous advantages such as water solubility, simple fabrication, low cost, chemical inertness, high photo-stability, low toxicity, and biocompatibility.23 Nowadays, C-dots have found important and wide applications in various fields including bioimaging, biolabeling, drug delivery, photo-catalyst, photovoltaic cells, and photodetectors, etc.24-31 Simultaneously, surface plasmons of C-dots were first reported in 2015.32 Since C-dots are generally compositions of amorphous and graphitic carbons, surface plasmons can be observed among sp2 hybridized carbon atoms resembling graphene plasmons. 33 Discovery of C-dots plasmons thus creates another powerful route for advanced optoelectronic applications and serves as an alternative substitute for metal particles. In this work, we discover that C-dots provide suitable platform for the coupling between the excitation of surface plasmon resonance and the optical transition in gallium nitride (GaN). This effect lends a novel method to control random lasing action from GaN nanorods (GNRs) based on C-dots surface plasmons. More specifically, the as-prepared vertically aligned GNRs, which exhibit amplified spontaneous emission (ASE) without resonant feedback under optical excitation, are randomly deposited with sp2 hybridized C-dots on their surface. These C-dots are derived from candle soot,34 synthesized through a typical combustion oxidation method with oxidative acid treatment as shown in Scheme 4 ACS Paragon Plus Environment

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1 (see more details in the Methods). Due to the cooperative effect of light scattering and field enhancement in the vicinity of C-dots, the random walk of light in disordered GNRs active medium are turned into random closed-loop cavities for coherent feedback. As a result, the lasing behavior can be controlled between ASE and coherent random lasing, and the lasing threshold can be manipulated within a wide range by depositing different amount of C-dots. These novel functionalities are potentially useful in various fields such as optical communications, colorful imaging, identification technologies or biological counting.8-9,35-36 Besides, this simple strategy provides a comparatively accessible way for plasmonically enhanced light-emitting devices. Compared with previous works, in which manipulation of random lasers optical modes and threshold are achieved in dye solution or dye-doped polymer films,10-11,15,37 the integration of C-dots and GNRs exhibits more potential for electrically-driven laser devices and solid-state lightning. Since GaN material was previously demonstrated biocompatible and low-toxicity,38 more possible applications of controllable random laser devices for biological use are therefore foreseeable. Results and Discussion The top and side-view scanning electron microscope (SEM) images of the pristine GNRs we use are provided in Figures 1a and 1b, respectively. The 5 ACS Paragon Plus Environment


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nanorods are randomly grown on the sapphire substrate. The average length, diameter, and distance between nanorods are approximately 750 nm, 300 nm, and 350 nm, respectively. The scales are comparable with the emission wavelength of GaN (~370 nm) used in our experiments. These GNRs simultaneously serve as active medium and scattering centers, which lay the foundation for ASE to occur. Compared with pristine GaN film, light that is multiply scattered by GNRs undergoes longer time in the gain materials, hence is more beneficial for laser action.

It is important to notice that ASE occurs when light that originates from spontaneous emission is amplified through the process of stimulated emission in gain medium.8,39 Namely, ASE does consist of stimulated emission. ASE is often classified as lasing without mirrors, which can occur in the absence of well-defined optical cavities.8 That is, ASE could take place in completely transparent active materials where light rays can propagate freely, or in randomly scattering environment where light can be amplified via multiple scattering process. Particularly, those photons that are multiply scattered in the gain medium can have a strong emission output. The mean frequency is mainly determined by the gain curve of the active materials. This kind of process is incoherent and only provides intensity or energy amplification. From this perspective, ASE includes those non-resonant laser-like behaviors arising from 6 ACS Paragon Plus Environment

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multiple scattering of light in gain materials. Based on the above arguments and previous published reports, 7-8,39-43 it is reasonable to regard the non-resonant feedback random lasers as part of ASE.

Figure 2a shows the transmission electron microscope (TEM) image of a single C-dot with a round shape. The size is about 8 nm. Besides, optical characteristics of the C-dots are measured and demonstrated in Figures 2b and 2c. Absorbance of the as-prepared C-dots is notably high in the UV region with a long tail extended to visible and infrared region as shown in Figure 2b. This extremely broad absorption spectrum covering from UV to infrared region implies an excellent capability of exploiting light with various wavelength.21 Similar to the absorption spectrum, the photoluminescence contains a broad-band emission with a peak centered at 470 nm. All the above results are in good agreement with previous reports.23

Lasing spectra of pristine GNRs and C-dots/GNRs composites under 266 nm pulsed laser excitation with different pumping intensities have been performed. Typical amplified spontaneous emission is characterized by a threshold in the power conversion and spectral narrowing. 8,44 These behaviors are shown clearly in Figure 3 and Figure 4. In Figure 3a, the peak of ASE is centered at 371 nm with approximately 2 nm full width of half maximum 7 ACS Paragon Plus Environment


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(FWHM). In contrast to the reference pristine GNRs arrays which exhibit only pronounced ASE even at high pumping energy density, the C-dots decorated GNRs arrays exhibit the quite different emission spectra (Figure 3b) with several distinct features. First, the luminescence intensity is remarkably amplified. Second, several discrete lasing peaks with linewidth less than 1 nm emerge over emission band at pumping energy density higher than 9.04 mJcm-2. The positions, intensities, and numbers of sharp lasing peaks fluctuate randomly. The observed phenomenon is in accordance with the characteristics of coherent random lasing action presented in previous works.8,45-46 which could be resulted from the coherent feedback provided by the multiple scattering events forming the closedloop paths and leading to the exceeding of amplification over the loss along such a loop path.44-45 Accordingly, the C-dots decorated GNRs arrays exhibit the typical behaviors of coherent-feedback random lasers. Figure 3c and 3d show the top-view SEM images of a single GNR without and with C-dots corresponding to Figure 3a and 3b, respectively. C-dots, owing to their high aqueous solubility, can be easily removed from GNRs using deionized water and deposited repeatedly. Therefore, we can use this simple and straightforward method to control the modes of random lasing between coherent and incoherent feedback,


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which may be applicable to several interdisciplinary fields, including pattern identification, information processing, biostatistics, and wearable optics. Figure 4a provides further evidence for lasing behavior from GNRs with and without decorated C-dots. The correlation between emission intensity and optical excitation energy density shows an obvious superlinear transition from low pumping intensity to high pumping intensity, indicating the typical threshold behavior of laser action. However, with the introduction of C-dots, higher emission intensity is observed from C-dots decorated GNRs arrays, and the lasing threshold for C-dots decorated sample is substantially lower than that for the bare one, which are 9.04 mJcm-2 and 16.81 mJcm-2, respectively. Thus, the deposition of C-dots into the GNRs arrays play a vital role in the transition from ASE to coherent random laser action. Interestingly, Figure 4b depicts our experiment results of the lasing threshold (Ith) dependence on the amount of Cdots (N) deposited on the surface of GNRs, which shows that the lasing threshold decreases due to deposition of more C-dots. The thresholds corresponding to 10 μl, 15 μl, 20 μl, 30μl, 40 μl, and 50 μl of deposited C-dots solution are 9.04 mJcm2

, 8.54 mJcm-2, 6.28 mJcm-2, 5.9 mJcm-2, 4.22 mJcm-2, and 3.89 mJcm-2,

respectively. Using C-dots as scattering centers provides us a new perspective for the optimization of lasing threshold, which is important for developing novel 9 ACS Paragon Plus Environment


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light sources in the fields of image display, security and optoelectronics. However, the threshold would not keep decreasing without limit with the amount of C-dots being increased. An incensement of threshold could occur at certain increasing scatterers density similar to the previous reports.15 It is because across a critical C-dots density, the excessive scattering events will prevent the formation of coherent closed loops for laser action to occur. In addition, the imperceptible influence of absorption due to denser C-dots as shown in Figure 2b can obstruct the amplification from coherent closed-loops, such that the threshold will start to increase as provided in supplementary information (Figure S1). The formation of closed-loop cavities usually arises from localized modes of light induced by strong multiple scattering in disordered gain materials.44 However, to satisfy the Ioffe-Regel criterion necessary for strong scattering (i.e. kls

Plasmonic Carbon-Dot-Decorated Nanostructured Semiconductors for

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