Controllable growth of flowerlike ZnO nanostructures by combining laser direct writing and hydrothermal synthesis X.D. Guo a,b, ⁎ , H.Y. Pi a , Q.Z. Zhao a , R.X. Li a a Shanghai Institute of Optics and Fine Mechanics, Chinese Academy of Sciences, Shanghai 201800, China b Department of Physics and Technology, University of Bergen, Bergen 5007, Norway abstractarticle info Article history: Received 19 July 2011 Accepted 1 September 2011 Available online 7 September 2011 Keywords: Crystal growth Microstructure Scanning/transmission electron microscopy (STEM) Hydrothermal method A method combing laser direct writing (LDW) and hydrothermal growth was developed for the synthesis of flowerlike three-dimensional (3D) ZnO nanostructures. By controlling the parameters of hydrothermal syn- thesis (reaction time and reaction temperature) and laser irradiation (pulse energy, irradiated time, and focus conditions), different sizes of flowerlike ZnO nanostructures are synthesized. Our results indicate that annealing the samples could reduce nonradiative related defects and greatly increase luminescence efficien- cy. The formation mechanisms of flowerlike ZnO nanostructures are also discussed. Such a mild synthesis route can be extended to fabricate complex 3D architectures of other materials. © 2011 Elsevier B.V. All rights reserved. 1. Introduction In the past decades, ordered nanostructures with controlled surface area and crystal morphologies have attracted great interest because the morphologies of most nanostructures can effectively tune their intrinsic chemical and physical properties. Exte nsive work has been devot ed to synthesize one- and two-dimensional (1D and 2D) nanostructures such as nano-parti cles, -wires, -belts , - tubes, -rings, -springs, -bows, -combs, -disks, etc. [1–9]. Compared to 1D and 2D nanostructures, com- plex 3D architectures may offer opportunit ies to explore novel proper- ties of nanocrystals and be employed as novel building blocks to fabricate more complicated and a dvanced materials. Up to now, various vapor methods such as thermal evaporation [10], chemical vapor depo- sition [11],vapor–liquid–solid (VLS) assisted [12], have been develop ed to prepare oriented nanostructures, but these methods typically require high temperatures and vacuum conditions, which limit the choice of sub- strate and the economic viability of high-volume production. In compar- ison with traditional vapor deposition approaches, the mild hydrothermal process using thermal treatment of the r eactants may be t he simplest and most effective way to prepare hig hly crystalline products at low temper- atures [13, 14]. This method a llows considerable influe nce of reaction spe- cies on the final size and morphology of the as-synthesized samples on a large scale . To fabricate nanostructures spatially loca ted, resea rche rs have dem- onstrated a few technique s based on an assem bly method under the con- trol of external forc es. Xu et al. and Kim et al. have reported a technique for growing verticall y aligned ZnO nanowire (NW) arrays on a silicon substrate coated with ZnO seeds by electron beam lithography [15,16]. Aizenberg in vestigated the combina tion of self-assembled monolayers (S AMs) and micro contract printing to controlled micro pat- terns of calcite crystals on surfaces with controlled location [17].Zhou et al. have reported a sele ctive growth of Z nO nanorod arr ays by using proton beam writing [18].Kimetal.havepresented an approach for the preparation of ZnO nan owire arrays by combining laser -interference lithography f or templating and a chemical-v apor-transport process for nanowire growth [19] . In this paper, we report the growth of 3D flowerlike ZnO nanostruc- tures on GaN/LiAlO 2 substrates by combining laser direct writing and hy- drothermal method. In contrast with some flowerlike structures which formed by self-assembly technologies using nanoparticles [17],nanorods [20], and nanobelts [21] as building blocks, t he structures presented here show uniform 3D structured flowers with nanosheets-constructed net- work morphology. Through laser direct writing, we can achieve posi- tion-controlled growth of ZnO structures only on patterned areas where GaN layer was exposed. Several parameters including both hydro- thermal growth and laser irradiation, which affect on the growth o f flow- erlike ZnO nanostructures, were investigated. The mechanism for the formation of flowerlike nanostructures was discussed. The 3D flowerlike ZnO nanostructures r eported here could be important for applications in the transistor, optoelect ronics, field emission, and gas sensing [22]. 2. Experimental Fig. 1 shows the schematic diagram of the selective growth of ZnO nanostructures on GaN/LiAlO 2 substrate. The process involved three steps. Materials Letters 66 (2012) 377–381 ⁎ Corresponding author at: Department of Physics and Technology, University of Bergen, Bergen 5007, Norway. Tel.: +47 94803087; fax: + 47 55589440. E-mail address: xiaodong.guo@ift.uib.no (X.D. Guo). 0167-577X/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.matlet.2011.09.008 Contents lists available at SciVerse ScienceDirect Materials Letters journal homepage: www.elsevier.com/locate/matlet 2.1. Substrate preparation The GaN/LiAlO 2 substrate was prepared by depositing about 2.0 μm thick GaN film on LiAlO 2 with low temperature GaN buffer layers by metal–organic chemical vapor deposition (MOCVD). The as-prepared substrate was cleaned by a standard cleaning, and then a 1-μm-thick layer of PMMA (polymethyl methacrylate) was spin coated on the sub- strate at a rotation speed of 4000 rounds per minute. After that, the sub- strate was baked on a hot plate at 100 °C for 10 min. 2.2. Laser direct writing Micropatterning of the as-prepared substrate was conducted by a femtosecond laser sy stem . A com merci al regenerative amplified Ti: Sapphire laser (RegA 9000, Coherent) that emits linearly polarized light with pulse duration of 150 fs and a repetition rate of 1 kHz was used in this experime nt. The sample was mounted on a computer-controlle d xyztranslationstage.Thesurfaceofthesamplewaspositionedperpen- dicular to the propagation direction of the incident laser beam in the focal plane of a 100× objective lens (NA= 0.8). The numbe r of pulses de- livered to the sample was controlled via an electromechanical shutter, and the laser pulse energy was measured by a pyroelec tric detector. 2.3. Hydrothermal growth The nutrient solution w as prepared from an aqueous solution contain- ing zinc nitrate [Z n(NO 3 ) 2 ·6H 2 O] and hexame thyltetramine [(CH 2 ) 6 N 4 , HMT] at a molar ratio of 1:1 and zinc concentr ation of 0.025 mol/L. Subse- quently, the substrates wer e immersed downward into the react ion solu- tion and heated at a constant temperature of 90 °C in a water bath for 1 h with continuous stirrin g. After deposition, the substrate was thorough ly washed with deionized water and dried in air at room temperature. The morphology and composition of the as-prepared products were characterized b y a fie ld em ission scannin g electro n microsc opy (FE- SEM) (JEOL JSM-6700F) which was equipped with an energy-dispersive spectroscopy (EDS) facility. TEM images, SEAD pattern, and HRTEM im- agesweretakenonaJEOL-2010transmissionelectronmicroscope.The μ-PL spectr a were recorded at room temperature using th e 325 nm exci- tation line (Renishaw inVia) from a He–Cd laser. 3. Results and discussions Fig. 2(a) shows a typical SEM image of a single ZnO flower. It is ob- served that the diameter of the obtained flower was approximately 10 μm and the flower consisted of 2D nanosheets. Here, the substrate was irradiated by 20 fs laser pulses with an energy of 1 μJ. There were laser induced nanoripples which appear around the flower, a phe- nomenon which was also found and discussed in our previous work [23,24]. Fig. 2(b) clearly demonstrates that by tuning the number of irradiated laser pulses as 250, 20 and 8, the ablated area can be adjusted and subsequent growth of ZnO flowers changed correspond- ingly. We successfully obtained ZnO flowers with diameters of 10 μm, 5 μm, and 3 μm, respectively. Controlling flower density is another important aspect in spatial organization; this can be achieved by using a computer-controlled xyz translation stage. Fig. 2(c) shows a matrix of flowers, the distance between the flowers can be mediated through the pre-determined sites. Surprisingly, it was found that while the substrate was irradiated in line-scan mode, the flowers formed corresponding long range architecture along the irradiated lines (Fig. 2(d)). Fig. 2(e) shows the EDS analysis of the flower and the ripples, as labeled “A” and “B” in Fig. 2(a), respectively. It can be seen that the flower is mainly composed of Zn and O, and the ripples contain Ga and N. C in the EDS spectrum originates from a thin C layer sputtered on the sample for obtaining clear SEM images. The micro- PL spectra measured at room temperature from the as-grown ZnO nanostructures before and after annealing are shown in Fig. 2(f). A weak ultraviolet (UV) light emission peak and strong visible light emission was observed from the as-grown ZnO. It is well known that the excited light emission intensity is determined by both radia- tive and nonradiative recombinations. To reduce and restructure non- radiative related defects, we chose an annealing temperature of 500 °C in air. It can be seen that annealing of the ZnO flowerlike struc- tures significantly improved the UV light emission at 380 nm wave- length, and successfully decreased the yellow and green band emission. The as-grown ZnO flowerlike structures were prepared by the hydrothermal method which could introduce excess zinc or oxy- gen vacancies [18]. Our results indicate that annealing the samples could reduce nonradiative related defects and greatly increase lumi- nescence efficiency. The high magnification SEM image shown in Fig. 3(a) demonstrates the detailed structural information of the sample. We found that the ob- served structure is constructed by many nanosheets with an average thickness of 100 nm. Further insight into the morphology and micro- structure of the flowerlike ZnO nanostructures were gained by using TEM and high-resolution TEM. Fig. 3(b) presents a TEM image for a typ- ical isolated ZnO nanosheet obtained by ultrasonic dispersion of the as- prepared sample in ethanol. Enlarged view of the rectangular area in panel shows that each petal has a dense structure, where the smooth surface consists of inter connected nanoparticles (Fig. 3(c)). The elec- tron diffraction pattern (inset of Fig. 3(c)) recorded from the edge of the nanopetals displays several concentric diffraction rings and some regular diffraction spots, indicating the polycrystalline nature of the petals. The high-resolution TEM image exhibits well-resolved two- dimensional lattice fringes, as shown in Fig. 3(d). It can be concluded that the nanoparticles themselves are single-crystalline, whereas the whole hierarchical structures are polycrystalline due to the anisotropic assembly of the building blocks. To understand the growth mechanism of the flowerlike ZnO nanos- tructures, systematic time-dependent experiments illustrating the evo- lution of the str ucture were carried out. When the reaction proceeds Fig. 1. Fabrication process of 3D flowerlike ZnO nanostructures: deposition of a PMMA layer on a GaN/LiALO 2 substrate, laser processing and hydrothermal growth of ZnO nano- structure. The right side shows a CCD image of the substrate after laser processing. 378 X.D. Guo et al. / Materials Letters 66 (2012) 377–381 for 5 min, some sprouts grew out from the irradiated dot (Fig. 4(a)). When the reaction time e xt ends to 10 min, th e sprouts grew larger and formed petals (Fig. 4(b)). After 30 min, a small flower was for med. When the reaction time is increased to 2 h, the 3D flowerlike nanostruc- tures appear, and almost no impurities can be observed. To determine theappropriatereactiontime,theeffectoflongerreactiontimes,upto 12 h, has been investigated. Results show that the reaction time exceed- ing 2 h will not bring about evi dent structural and morphological modi- fications. W ith r eaction ti me incre asing , the con centrati on of Z nO nucle i decreases conver sely, and the grow th veloci ty of ZnO nanopet als de- creases along with the reduced concentration. As a result, the morphol- ogy changes very little after a certain period. Additional growth steps A B (a) (b) 350 400 450 500 550 600 650 700 Normalized PL Intensity (A. U.) Wavelength (nm) as-grown sample annealed sample Energy B A (c) (d) (e) (f) Fig. 2. (a) SEM image of single flowerlike ZnO nanostructures, (b) SEM image of flowers with different sizes, (c) a matrix of flowers, (d) line-scan mode flowerlike nanostructure, (e) EDS spectrum of the structures shown in (a), A is flower, B is ripples, respectively. (f) Room temperature PL spectrum of the as-grown sample as well as after annealing. (d) (b) (a) (c) Fig. 3. (a) SEM image of an individual ZnO flower. (b)TEM image of a typical nanosheet. (c) Enlarged view that corresponds to the small frame area marked in the nanosheet, the inset is the SAED pattern of the nanosheets. (d) Corresponding high-resolution TEM image. 379X.D. Guo et al. / Materials Letters 66 (2012) 377–381 produce a secondary structure, as shown in Fig. 4(f). We first obtained flower similar to that of Fig. 2(a). After hydrothe rmal gr owth p roceed ed for 1 h, we took the substrate out of the solution, ca refully washed it with deionized water at room temperature, and then put the substrate back into the solution f or 10 min. The str ucture obtained in this way is shown in Fig. 4(f).ItcanbeseenthatnumerousZnOnanodotsarefilled in the space be twee n the petals of t he flower. In general, the formation of ZnO nanocrystals can be divided into two processes: nucl eation and growth.Whenthesubstratewastakenoutofthesolutionandwashed with de ionized water, the te mperature of the sub strate decreased rapid- ly, thus many ZnO nuclei attached on the petals of the flower. When the substrate was put back into the solutions, these n uclei would evolve into polyhedral seeds and further grow int o hexagonal nanodots. In this cas e, the seeds tend to take a single-crystal in an attempt to minimize the tot al surface energy of the system. The growth of ZnO flowerlike structures is controlled by nucleation and growth process in aqueous solution [25]. By spin coating, PMMA films can be engineered with typically a surface roughness under 1 nm. ZnO can hardly grow on the PMMA resist regions due to a lack of nucleation sites [16]. The absorption peak of PMMA is around 400 nm while the laser wavelength is 800 nm, thus two-photon absorp- tion took place during the irradiation of laser pulses. The PMMA at the laser illuminated regions was ablated, thus GaN was exposed to the so- lution during the hydrothermal growth, so the deposition starts only in the pre-illuminated sites. In the hydrothermal process, the negative na- ture of the growth unit [Zn(OH) 4 2− ] will lead to different growth rates of planes. When there is no organic additive in the solution, spherical ZnO particles easily developed because of the Ostwald ripening [26].Inour experiments, HMT is expected to serve as the organic template during the heating process to 90 °C, thus dynamically modifying the nucleation process. The substrate/crystal surface has a boundary layer of charged ions, the thickness of which is diffusion controlled. With increasing con- centrations of Zn 2+ and OH − ,theZn(OH) 2 and/or ZnO nuclei devel- oped under low precursor concentration and the action of HMT. In some studies, the formation of ZnO sheets and plates has been attribut- ed to a 1D branching and subsequent 2D interspaces filling process to give a final 3D structure [27]. In our case, the nanosheet might be the re- sult of the self-assembly of a number of active sites that trigger the nucleation at the interface, promoting the formation of petal crystals extending from the interface. 4. Conclusions We have demonstrated the fabrication of 3D flowerlike ZnO nanos- tructures by combining laser direct writing and hydrothermal growth method. The control of the as-fabricated ZnO flowerlike structures can be achieved by altering hydrothermal growth conditions as well as laser irradiation parameters. Possible mechanisms for the formation of different nanostructures have been proposed. We expect that the meth- odology for controlling the shape of ZnO nanocrystals demonstrated in this work could provide a great opportunity to fully explore their appli- cation in the field of fabrication of nano-electronic devices. Acknowledgments This work was financially supported by NSF of China (Grant Nos. 11174304, 61078080, and 61178024), Quanzhong Zhao acknowledges the sponsor from Shanghai Pujiang Program (Grant No. 10PJ1410600). The authors thank Prof. Helseth for valuable discussions. References [1] Hu ZS, Ramirez JE, Cervera BEH, Oskam G, Searson PC. J Phys Chem B 2005;109: 11209–14. [2] Huang MH, Wu Y, Feick H, Tran N, Weber E, Yang P. Adv Mater 2001;13:113–6. [3] Pan ZW, Dai ZR, Wang ZL. Science 2001;291:1947–9. [4] Yu HD, Zhang ZP, Han MY, Hao XT, Zhu FR. J Am Chem Soc 2005;127:2378–9. [5] Kong XY, Ding Y, Yang RS, Wang ZL. Science 2004;303:1348–51. [6] Gao PX, Ding Y, Mai W, Hughes WL, Lao CS, Wang ZL. Science 2005;309:1700–4. [7] Hughes W, Wang ZL. J Am Chem Soc 2004;126:6703–9. [8] Wang ZL, Kong XY, Zuo JM. Phys Rev Lett 2003;91:185502. [9] Xu CX, Sun XW, Dong ZL, Yu MB. Appl Phys Lett 2004;85:3878–80. [10] Shen G, Cho JH, Lee CJ. Chem Phys Lett 2005;401:414–9. [11] Muthukumar S, Gorla CR, Emanetoglu NW, Liang S, Lu YJ. Cryst Growth Des 2001;225:197–201. [12] Barrelet CJ, Wu Y, Bell DC, Lieber CM. J Am Chem Soc 2003;125:11498–9. [13] Breedon M, Kehagias Th, Shafiei M, Kalantar-zadeh K, Wlodarski W. Thin Solid Films 2009;518:1053–6. [14] Breedon M, Rahmani MB, Keshmiri SH, Wlodarski W, Kalantar-zadeh K. Mater Lett 2010;64:291–4. Fig. 4. SEM images of the samples at different times after the temperature reached 90 °C: (a) 5, (b) 10, (c) 30, (d) 120 min, respectively. (e) High-magnification SEM image of the flower. (f) ZnO flowerlike nanostructures after additional growth steps. 380 X.D. Guo et al. / Materials Letters 66 (2012) 377–381 [15] Xu S, Ding Y, Wei YG, Fang H, Shen Y, Sood AK, et al. J Am Chem Soc 2008;130:14958–9. [16] Kim YJ, Lee CH, Hong YJ, Yi GC. Appl Phys Lett 2006;89:163128–30. [17] Aizenberg J. Adv Mater 2004;16:1295–302. [18] Zhou HL, Shao PG, Chua SJ, Kan JA, Bettiol AA, Osipowicz T, et al. Cryst Growth Des 2008;8:4445–8. [19] Kim DS, Ji R, Fan HJ, Bertram F, Scholz R, Dadgar A, et al. Small 2007;3:76–80. [20] Sounart TL, Liu J, Voigt JA, Hsu JW P, Spoerke ED, Tian Z, et al. Adv Funct Mater 2006;16:335–44. [21] Lin KL, Chang J, Zhu YJ, Wu W, Cheng GF, Zeng Y, et al. Cryst Growth Des 2009;9: 177–81. [22] Compbell JL, Breedon M, Latham K, Kalantar-zadeh K. Langmuir 2008;24:5091– 8. [23] Guo XD, Li RX, Hang Y, Xu ZZ, Yu BK, Dai Y, et al. Appl Phys A 2009;94:423–6. [24] Guo XD, Li RX, Hang Y, Xu ZZ, Yu BK, Ma HL, et al. Mater Lett 2008;62:1769–71. [25] Xu F, Lu YN, Xie Y, Liu YF. J Phys Chem C 2009;113:1052–9. [26] Marqusee JA, Ross J. J Chem Phys 1983;79:373–8. [27] Park JH, Choi HJ, Choi YJ, Sohn SH, Park JG. J Mater Chem 2004;14:35–6. 381X.D. Guo et al. / Materials Letters 66 (2012) 377–381 . flowerlike ZnO nanos- tructures by combining laser direct writing and hydrothermal growth method. The control of the as-fabricated ZnO flowerlike structures can be achieved by altering hydrothermal growth. Controllable growth of flowerlike ZnO nanostructures by combining laser direct writing and hydrothermal synthesis X.D. Guo a,b, ⁎ ,. apor-transport process for nanowire growth [19] . In this paper, we report the growth of 3D flowerlike ZnO nanostruc- tures on GaN/LiAlO 2 substrates by combining laser direct writing and hy- drothermal