Controllable Fabrication of ZnO Microspheres for Whispering Gallery

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Controllable Fabrication of ZnO Microsphere for Whispering Gallery Mode Microcavity Yanjun Liu, Chunxiang Xu, Zhu Zhu, Daotong You, Ru Wang, Feifei Qin, Xiaoxuan Wang, Qiannan Cui, and Zengliang Shi Cryst. Growth Des., Just Accepted Manuscript • DOI: 10.1021/acs.cgd.8b00716 • Publication Date (Web): 16 Jul 2018 Downloaded from http://pubs.acs.org on July 18, 2018

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Crystal Growth & Design

Controllable Fabrication of ZnO Microsphere for Whispering Gallery Mode Microcavity Yanjun Liu, Chunxiang Xu*, Zhu Zhu, Daotong You, Ru Wang, Feifei Qin, Xiaoxuan Wang, Qiannan Cui and Zengliang Shi State Key Laboratory of Bioelectronics, National Demonstration Center for Experimental Biomedical Engineering Education, School of Biological Science and Medical Engineering, Southeast University, Nanjing 210096, P. R. China

Abstract: Whispering gallery mode (WGM) microcavity made of ZnO microsphere is an ideal platform for photonic integration and optical biological detection. But the fabrication and the formation mechanism of ZnO microsphere is still challenging. In this paper, we present a controllable fabrication strategy to obtain zinc oxide (ZnO) microspheres with smooth surface, good crystallinity and excellent optical properties for improved WGM lasing performance by taking advantage of the cooperative process between reactants, annealing and metal nanoparticles decoration. The synergic effect between the hexamethylenetetramine (C6H12N4) and citrate was experimentally observed on the formation of ZnO microspheres. Their crystallinity and surface flatness can be improved by gradually decreasing the heating rate. Due to the electron transfer process in metal/semiconductor hybrid structure assisted by the localized surface plasmon resonance, Au NPs can match well with the photon energy of the defect emission of ZnO microsphere and induced one order of magnitude increasement in lasing intensity compared to the bare one. Our results hold a promise for the application of ZnO microspheres in the construction of UV laser and WGM sensing technology.

Key words: ZnO Microsphere, Controllable Fabrication, Formation Mechanism, Whispering-Gallery Mode

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1. Instruction Whispering gallery mode (WGM) microcavity can lead to strongly optical resonance through the inner wall total reflection of light, which significantly enhances the light-matter interactions. At present, many new type lasers, such as GaN,1 GaAs2 and perovskite,3,

4

have excellent luminescence properties. ZnO has been

demonstrated to be an ideal material of WGM microcavity since ZnO is a direct band gap semiconductor with a wide band gap (3.37 eV), large excitonic binding energy (60 meV) and high-symmetric crystal structure. 5-7 The appealing lasing performance from ZnO WGM microcavity makes it has a promising development in various ultraviolet (UV) light-emitting devices and laser diodes. What’s more, ZnO has been employed as a propitious material in biosensors because of good biocompatibility, excellent electrochemical activity and electron transport capacity, especially its high isoelectric point (IEP, 9.5) performance.8, 9 Interestingly, the geometry of synthesized ZnO micro/nanostructures can be controlled and be very flexible, the shapes of combs,10 tubes,11 hexagonal microrods,12 microbelts,13 plates,14 microtowers,15 microneedles16 and spheres17 have been widely reported. Generally speaking, to reduce the optical loss for more efficient optical confinements and lower lasing threshold, an isotropic and smooth microstructure like microspheres with high Q factor is highly desired. Many spherical microstructures, such as CsPbBr3 microsphere,3 polystyrene microsphere,18 polyphenyl ether droplet and bovine serum albumin sphere19 have been reported with high Q lasing. Some of them can be used in biosensing, because WGM microcavity will be ultra-sensitive to the external disturbance, including the deformation of microcavity caused by tiny stress and the refractive index change caused by the adsorption of biological molecular.19 But the most materials are either poor in stability or require external gain. ZnO microstructure as natural WGM microcavity doesn’t require external gain media and complex processing. And resent reports have shown ZnO WGM microcavity can realize ultraviolet electroluminescence20 as well as interfacial refractive index


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sensing21 and ultrasensitive label-freely dopamine detection by utilizing its high-quality resonance.22 However, ZnO microsphere is usually difficult to fabricate because of its wurtzite crystal structure. Based on previous reports,17, 23 synthesis of ZnO microsphere can rarely be controllable and the formation mechanism is largely unclear, which needs further investigation. We notice that hydrothermal technique is a simple and effective method to synthesize different micro/nanostructures. The products synthesized by hydrothermal method have the advantages of high purity and controllable size, and their dispersibility is excellent. The possible reason is that the construction of atoms or molecules can be realized at micro/nanometer level by choosing the suitable hydrothermal conditions, such as the reactants, surfactants and their concentration.24 And annealing can further depress the defects inside to improve the crystallinity of ZnO microsphere for WGM microcavity. The performance of a device is crucial for its applications. In the scenario of ZnO microsphere as a lasing microcavity, the near-band edge (NBE) UV emission of ZnO microsphere is usually accompanied by a distinct visible emission due to the defects, which limits its application in the construction of UV laser and devices. As a result, WGM lasing performances achieved in an experiment are usually worse than expected. To solve this problem, we take advantage of the noble-metal nanoparticles (NPs) decoration, which has been proved to be an efficient method to reduce the threshold and enhance the NBE emissions by utilizing the localized surface plasmon resonance (LSPR) effect.25 The extinction spectrum of Au NPs matches well with the defect emission of ZnO, which can improve the NBE of ZnO by LSP-assisted electron transfer. In this paper, we first design a controllable synthesis strategy with hydrothermal technique, where different reactants and their proportion as well as heating rate were employed to control the morphology and improve the crystallinity of ZnO microspheres. Then, we further discuss the growth mechanism in hydrothermal technique aiming for a smooth high-quality ZnO microsphere. At last, to reduce the lasing threshold and enhance the NBE emissions of ZnO, optimized sputtered Au NPs ACS Paragon Plus Environment


were chosen in view of their extinction spectra, which match better with the defect emission of ZnO. Our results hold a promise for the application of ZnO microspheres in the construction of UV laser and WGM sensing technology.

2. Synthesis and Characterization 2.1 Fabrication of ZnO and ZnO/Au microspheres Here, ZnO microspheres were synthesized by hydrothermal method. The 0.05 M of ZnCl2 and 0.05 M of hexamethylenetetramine (C6H12N4) solution 40 mL were mixed and stirred until the chemicals completely dissolved in deionized water. After that, the 0.02 M of Na3C6H5O7·2H2O (Na3-citrate) in 20 mL deionized water was added to the above solution. After that, the mixed solution was put in to a tightly closed glass bottle in an oven at 90°C for 1.5 h. After that, the collected product was washed three times with ethanol and deionized water respectively and then dried in the oven at 45°C for 30 min. To estimate the role of reactants in the formation process of ZnO microsphere and understand the mechanism deeply, different reactants were replaced in the control experiments. Repeated the above experiment by changing C6H12N4 to 2 ml of NH3·H2O. Then replaced ZnCl2 with Zn (CH3COO) 2·2H2O to carry out the experiment as first part. Another sample was obtained without the addition of Na3-citrate. And ZnO samples were also collected at 5, 20, 60 and 90 min of the hydrothermal reaction. Based on the understanding of the formation mechanism and to control the surface of products, the content of Na3-citrate was changed. The amount of Na3-citrate added to the solution was set as 0.01~ 0.05 M and keep the content of ZnCl2 and C6H12N4 unchanged. To depress the defects inside, improve the crystallinity and control the surface structure of ZnO microsphere, the dried ZnO samples were further annealed in an ambient atmosphere with the heating rate of 0.6~3.6 °C/min from room temperature to 450 °C. In order to reduce the threshold and enhance the NBE emissions of ZnO, Au NPs were optimized to allow their extinction spectra to match better with the defect


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emission of ZnO.11 The Au NPs were decorated on ZnO microspheres by using ion sputtering method for comparison with the bare ZnO microspheres. The sputtering current and pressure were 14 mA and 40 Pa, and the sputtering time was 15 s, 30 s, 45 s and 60 s, respectively. 2.2 Characterization The field emission scanning electron microscopy (FESEM, Carl Zeiss Ultra Plus), X-ray diffraction (XRD, Siemens D5005) and energy dispersive X-ray spectroscopy (EDX, EX-250) were used to characterize the morphology, crystal structure and elemental distribution of the samples. The spontaneous and stimulated spectra of sample were measured using a micro-PL system. The femtosecond pulse laser (pulse duration 150 fs, repetition rate 1000 Hz, operates at 325 nm) was focused on the sample through an optical microscope (OLYMPUS BX53F). The spectra were collected through an optical multichannel analyzer (Acton SP2500i). Au NPs were sputtered on the surface of ZnO microspheres by the sputtering system (Ion Sputter Hitachi E-1010) and their absorption spectra were revealed by the UV-visible spectrophotometer (Shimadzu UV-2600). Theoretical calculation and simulations were carried out using the finite element method. Time-resolved photoluminescence (TRPL) experiments were performed by an optically triggered streak camera system (C10910, Hamamatsu) at 325 nm with a repetition rate of 1 kHz (Opera Solo, Coherent).

All measurements were performed at room temperature.

3. Results and Discussion 3.1 Nanostructure formation of ZnO microsphere Fig. 1 shows the morphologies and the corresponding XRD patterns of ZnO structures under different hydrothermal conditions, such as different reactants and their concentrations. Various structures were synthesized with different reactants as shown in Fig. 1(A)-(D). Fig. 1(A) depicts a ZnO microsphere with ZnCl2, C6H12N4 and Na3-citrate as reactants. In Fig. 1(B), nanosheets of ZnO were obtained as



C6H12N4 was replaced by NH3·H2O. Fig. 1(C) suggests that transforming the Zn2+ source from ZnCl2 to Zn (CH3COO) 2·2H2O doesn’t change the spherical morphology of ZnO. However, the ZnO is found to have a rod-shaped structure (Fig. 1(D)) in the case without the addition of Na3-citrate. For above ZnO samples, the XRD patterns illustrate the diffraction peaks at 2θ=31.029°, 33.697°, 35.506°, 46.812°, 55.811°, 62.278°, 67.298°, and 68.609°, corresponding to crystallographic plane (100), (002), (101), (102), (110), (103), (112), and (201) respectively. This demonstrates all ZnO products belong to a hexagonal phase (JCPDS NO. 89-1397). The results indicate that C6H12N4 and Na3-citrate play important roles to form ZnO microspheres when using ZnCl2 or Zn (CH3COO) 2·2H2O as zinc source.

Fig. 1 SEM images of ZnO products and correspond XRD patterns. The hydrothermal reaction solutions are (A) ZnCl2, C6H12N4 and Na3-citrate (B) ZnCl2, NH3·H2O and Na3-citrate (C) Zn(CH3COO) 2·2H2O, C6H12N4 and Na3-citrate (D) ZnCl2 and C6H12N4


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The Na3-citrate acts a pivotal role as a blocking, capping, and orienting material in the growth of ZnO.

26, 27

The hydroxyl and carboxyl groups in the citrate ion attract

Zn2+ ions and control the growth direction of ZnO structures. In order to further study the formation process of ZnO microspheres, ZnO samples (with ZnCl2 and 0.038 M Na3-citrate) were collected at 5, 20, 60 and 90 min of reaction and their morphologies are shown in Fig. 2(A), (B), (C) and (D), respectively. It is found that several microspheres scattered on particle-shaped layers after the reaction for 5 min as shown in Fig. 2(A). More spheres appear and particles decrease significantly when the reaction time is prolonged to 20 min (Fig. 2(B)). After the reaction for 60 min, discrete solid microspheres are formed (Fig. 2(C)). Fig. 2(D) exhibits the further development of ZnO microspheres under the hydrothermal conditions for 90 min.

Fig. 2 SEM images of ZnO with different reaction time for (A) 5 min (B) 20 min (C) 60 min (D) 90 min, (E) the growth mechanism diagram of ZnO microsphere

The growth mechanism of the ZnO microsphere is schematically depicted in Fig. 2 (E) based on the morphological observations of the ZnO structures. In the mixture of ZnCl2, C6H12N4 and Na3-citrate, the hydroxyl and carboxyl groups in Na3-citrate attract Zn2+ ions to form Na3-citrate coupled with Zn2+ (Zn:citrate). During the thermal treatment at 90 °C, the formaldehyde (HCHO) gas is generated from



C6H12N4.28 The Zn:citrate precursors cover the HCHO gas bubbles to serve as a seed layer for ZnO growth on the bubble surface. That was why the ZnO products did not present as microspheres but sheets (Fig. 1(B)) once the C6H12N4 was replaced with NH3·H2O. At the beginning of reaction, the most bubbles are not strong enough to hold of obtained ZnO seed, which results in the formation of the ZnO particles (Fig. 2(A)). Eventually, the discrete solid ZnO microspheres are fabricated.24

Fig. 3 SEM images of ZnO with different concentration of Na3-citrate (A) 0.01 M (B) 0.02 M (C) 0.03 M (D) 0.038M (E) 0.05 M and different heating rate of (F) 3.6 °C/min (G) 1.2 °C/min (H) 0.9 °C/min (I) 0.75 °C/min (J) 0.6 °C/min

To obtain a high-quality resonance, ZnO microspheres need extremely smooth surfaces and good crystallinity. As we have demonstrated the importance of the Na3-citrate to form ZnO microspheres, different concentrations of Na3-citrate were employed here to smooth the surface of the microspheres. Fig. 3(A)-(E) shows ZnO structures grown with different concentration of Na3-citrate. The ZnO microsphere composed of nanosheets are grown with 0.01 M of Na3-citrate (Fig. 3(A)). Introducing of Na3-citrate 0.02 M, the structure is found to be microsphere architecture with rough surface (Fig. 3(B)). As the concentration of Na3-citrate is ACS Paragon Plus Environment

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increased to 0.03 M, the surface of ZnO microsphere becomes smoother (Fig. 3(C)). With 0.038 M of Na3-citrate (Fig. 3(D)), the as-grown ZnO exhibits smooth surface of spherical shape. When the added concentration of Na3-citrate is greater than 0.05 M, the surface of ZnO microsphere becomes rough again (Fig. 3(E)). There is an optimized concentration of 0.038 M here for the smooth ZnO microspheres. The ZnO microspheres mentioned later are all fabricated with 0.038 M of Na3-citrate. However, a large number of defects exits in the ZnO microsphere fabricated by hydrothermal method and annealing provides an effective technique to improve the crystallinity of material. The crystallization is dependent on the heating rate, which further affects the surface structure of ZnO. A smooth surface is required for ZnO microsphere to realize and improve the WGM lasing. For this purpose, a series of heating rates were adopted to control the crystallization and optimize the surface. The obtained ZnO samples are annealed from room temperature to 450 °C in an ambient atmosphere with heating rate of 0.6~3.6 °C/min, and the SEM images are shown in Fig. 3(F)-(J). With the heating rate of 3.6 °C/min, the microsphere exhibit porous surface as shown in Fig. 3(F). When slow down the heating rate (1.2, 0.9 and 0.75 °C/min), the surface of ZnO microsphere gradually becomes smooth, as shown in Fig. 3(G)-(I). Finally, a fairly smooth ZnO microsphere with high crystallinity is formed when the heating rate down to 0.6 °C/min as shown in Fig. 3(J). The PL performances of ZnO microspheres before and after annealing are further characterized in the next section, and demonstrate indirectly the improved crystal quality of ZnO microspheres.

3.2 Optical properties of ZnO microsphere cavity Under the same excitation power of 0.305 mW, the PL spectra of ZnO microspheres before and after annealing are depicted in Fig. 4(A). ZnO exhibit a wide defect emission before annealing. To understand the origins of the light emission, a typical PL spectrum of ZnO before annealing is decomposed into three Gaussian parts as shown in the upper spectrum where, the emission peak at 390 nm, 437 nm and 507 nm account for the NBE emission of ZnO, defects emissions from interstitial Zn (Zni) and oxygen vacancy (VO), respectively.29, 30 It suggests that there is a number of Zni ACS Paragon Plus Environment


and VO formed in the fast hydrothermal process.31, 32 However, as shown in Fig. 4 (A), the emission peak at 437 nm from Zni decreases distinctly due to the elimination of Zni by the crystallization from annealing process. The annealed ZnO microsphere shows an obvious NBE emission centered at around 388 nm and a defect emission centered at 549 nm.

Fig. 4 (A) PL spectra of ZnO microspheres before and after annealing (0.6 °C/min) and Gaussian fits of the PL spectra of corresponding ZnO measured at 0.305 mW (B) Lasing emission spectra under different excitation power density for an individual ZnO microsphere, insert is the dependence of the emission intensity on the pumping power density (C) The optical microscopy image of the individual ZnO microsphere (D) The electrical field distribution when resonance wavelength is set as 389.5 nm


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Individual ZnO microsphere was required to further discuss its optical properties, the separation process was as follows: Firstly, the 20 mg ZnO microspheres were dispersed in 30 ml alcohol, and then 20 µl of diluted solution were sprayed on silicon substrate with pipette. Finally, the silicon substrates with ZnO microspheres were placed in 60 ℃ oven for 15 min. Fig. 4(B) shows the lasing emission spectra under different excitation power densities for an individual ZnO microsphere with diameter of 1.2 µm. At a low pumping power density (71.72 to 91.66 W/cm2), the spectrum displays a weak spontaneous emission band centered at 389.5 nm. Sharp peaks emerge out once the pumping power density reaches to 104.45 W/cm2. When the pumping power density increases to 129.46 W/cm2, two strong lasing modes emerge. The full width at half-maximum (FWHM) is about 1.185 nm for the strongest resonant peak located at 389.5 nm in the emission spectrum and the Q factor is calculated to be 329 (the definition Q = λ/∆λ, where λ and ∆λ are the peak wavelength and the fwhm, respectively). The inset of Fig. 4(B) plots the dependence of the lasing emission intensity on the excitation power density on the ZnO microsphere and the threshold power density is estimated 98.23 W/cm2. The Fig. 4(C) is the optical microscopy image of the individual ZnO microsphere, which emits out bright blue-violet light in the dark field image under the excitation of 325 nm UV laser. To further verify its WGM lasing and study the lasing behaviors for ZnO microsphere, the finite element method was employed to simulate the optical field distributions of optical mode in the microcavity, which provides an effective feedback approach with whispering gallery mode. The refractive index of ZnO was set as 2.48 and the lasing wavelength is set as 389.5 nm, respectively. Electrical field distribution of the equator plane is demonstrated in the Fig. 4(D), a clear horizontal mode can be observed.

3.3 Plasmon-enhanced WGM lasing of ZnO microsphere cavity However, the NBE UV emission of ZnO is suppressed because of a large number of defects. Our research group has been engaged in the systematic research in the field of metal nanoparticles coupling assisted ZnO WGM lasing for many years, and we have shown the utilizing LSPR of Au NPs is an effective way to improve the UV emission as well as inhibit the defect emission of ZnO.

11, 14, 33-35


The photon energy


of the defect emission from ZnO can match with the LSPR energy of Au NPs very well to generate a resonant energy coupling process.36 As shown in Fig. 5 (A), based on the resonant coupling between Au LSP and ZnO defect emission, the electrons of Au NPs can be excited to a higher energy state, and then transfer back to the conduction band of ZnO to enhance the NBE of ZnO.11, 37 An important factor for the enhancement of NBE intensity depends on the overlap between the defect emission spectrum of ZnO and the extinction spectrum of Au NPs, which affects the coupling efficiency and strength. The size and distribution of Au NPs have a strong influence on the localized surface plasmon coupling.

Fig. 5 (A) Energy band diagram of Au/ZnO system showing the LSPR coupling between ZnO microsphere and Au NPs (B) the SEM images of Au NPs with different sputtering time (C) UV-visible absorption spectra of Au NPs with different sputtering time (D) PL spectrum of ZnO microspheres with different sputtering time of Au NPs

Next, the optical properties of Au NPs are investigated by adjusting the Au sputtering time. The SEM images of Au NPs on silicon substrates with different sputtering time are shown in Fig. 5(B). The size of Au NPs is 5 nm, 10 nm and 20 nm as the sputtering time increases from 15 s to 30 s and 45 s, respectively. Some Au NPs merge together with the size of 50 nm while the sputtering time prolongs to 60 s. Fig. 5(C) exhibits the absorption spectra of Au NPs with different sputtering time on


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quartz substrates. The absorption peaks are located at 528 nm, 551 nm, 558 nm and 588 nm corresponding to the sputtering time of 15 s, 35 s, 45 s and 60 s, respectively. There is a red-shift of the absorbance of Au NPs when the sputtering time is from 15 s to 60 s. Au NPs with absorption peak at 551 nm is the closest to the photon energy of the defect emission of ZnO microsphere (defect peak at 549 nm). This result indicates that the LSPR energy of Au NPs with sputtering time 30 s will match better with the defect emission energy of ZnO.11 Under the same excitation power, the PL spectra of bare ZnO and ZnO/Au microspheres with different sputtering time of Au NPs on silicon substrates are shown in the Fig. 5(D). Corresponding to Fig. 5(C), these two samples were sputtered at the same time. When the sputtering time is 30 s, the defect peak intensity reduces and the NBE emission reaches a maximum value.

Fig. 6 (A) EDX spectrum and the elemental mapping image for the individual ZnO/Au microsphere SEM images of Au NPs with different sputtering time. Lasing emission spectra under same excitation power for ZnO microspheres before and after different Au sputtering decoration (B) 15 s (C) 30 s (D) 45 s and (E) 60 s, inserts are the SEM images of corresponding ZnO microspheres



The EDX spectrum and the elemental mapping in Fig. 6(A) depict the elemental distributions of Zn, O and Au within the structure of ZnO/Au clearly and the Au peaks are observed clearly in the EDX spectrum of ZnO/Au. It indicates that Au NPs are successfully decorated on the surface of the ZnO microsphere. In order to verify the inferred optimized sputtering time of Au for improving the WGM lasing performance of ZnO microsphere and understand the enhanced mechanism intuitively, four ZnO microspheres with similar diameter of 1.1 µm were employed to carry out the comparative experiments. Fig. 6(B)-(E) show the lasing emission spectra under the same excitation power for the ZnO microspheres with similar size before and after different Au sputtering decoration. It is obvious that Au NPs with sputtering time of 30 s have the most effective enhancement for the ZnO WGM lasing, which agrees well with the previous assumption. In other words, Au NPs with sputtering time of 30 s in this experiment is the best to match with the photon energy of the defect emission of ZnO microsphere.

Fig. 7 (A) Lasing emission spectra under different excitation power density for an individual ZnO/Au microsphere, insert is the dependence of the emission intensity on the pumping power density (B) schematic diagram of Au/ZnO system (C) the optical field distribution of ZnO/Au simulated by the finite element method (D) the normalized TRPL spectra and exponential decay fitting curve at the wavelength of 390 nm of ZnO microspheres before and after Au decoration


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Then, the Au NPs with sputtering time of 30 s were chosen to further analyze the lasing improvement for ZnO microsphere. Here Fig. 7 (A) shows the emission spectra of ZnO/Au microsphere (the same one analyzed in Fig. 4(B)) at different excitation power densities. The insert in Fig. 7 (A) plots the dependence of the lasing emission intensities on the excitation power density for ZnO/Au microsphere. The WGM lasing intensity for Au decoration is nearly ten-fold compared to the bare ZnO microsphere at the same excitation condition. The threshold power density of Au-decorated ZnO microcavity is estimated 78.9 W/cm2, which is lower than that of bare ZnO. A schematic diagram and optical field distribution of ZnO/Au are displayed in Fig. 7(B) and (C), the excited LSP can increase the electronic field at the interface between ZnO and Au NPs, and the hot spots can be seen clearly. A time-resolved photoluminescence (TRPL) measurement for ZnO and ZnO/Au was also employed to get more insights of this enhanced process as shown in Fig. 7(D). The normalized TRPL spectra decays can be fitted well by using the monoexponential function. The fitted decay time of bare ZnO is ~ 74.5 ps, which is shorter than ~ 95.9 ps of ZnO/Au. The longer decay time provides more intuitive evidence for the electron transfer.38 In brief, this plasmon-assisted electron transfer process results in the suppression of visible light and the NBE enhancement of ZnO.

4. Conclusions In summary, we have successfully synthesized the ZnO microsphere with smooth surface, improved crystallinity and excellent optical properties by designing a controllable fabrication strategy. Theoretically and experimentally, C6H12N4 and citrate have been proved to play important roles in the formation of ZnO microspheres in hydrothermal process. And their crystallinity can be improved without destroying the smooth surface by gradually decreasing the heating rate. Au NPs with sputtering time of 30 s matched best with the photon energy of the defect emission of ZnO microsphere in this experiment. Due to the LSPR effect of Au NPs and the electron transfer process, the significantly enhanced WGM lasing and reduced threshold of ZnO microspheres were observed after Au NPs decoration. Our results hold a promise



for application of ZnO in the construction of optoelectronic devices and WGM sensing technology.

Author Information Corresponding Author * E-mail:[email protected]

Notes The authors declare no competing financial interest.

Acknowledgements This work was supported by NSFC (61475035, 11734005 and 61704024), Science & Technology Project of Jiangsu Province (BE2016177, BK20170696), the Fundamental Research Funds for the Central Universities, and Collaborative Innovation Center of Suzhou Nano Science and Technology (SX21400213). Also, we thank the help of the National Demonstration Center for Experimental Biomedical Engineering Education (Southeast University).

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For Table of Contents Use Only: Controllable Fabrication of ZnO Microsphere for Whispering Gallery Mode Microcavity Yanjun Liu, Chunxiang Xu*, Zhu Zhu, Daotong You, Ru Wang, Feifei Qin, Xiaoxuan Wang, Qiannan Cui and Zengliang Shi

A strategy including controllable fabrication and Au decoration was designed to obtain smooth ZnO microsphere for Whispering Gallery Mode Microcavity.


Controllable Fabrication of ZnO Microspheres for Whispering Gallery

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