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
Trang 1writing and hydrothermal synthesis
X.D Guoa,b,⁎ , H.Y Pia, Q.Z Zhaoa, R.X Lia
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
a b s t r a c t
a r t i c l e i n f o
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 offlowerlike ZnO nanostructures are synthesized Our results indicate that annealing the samples could reduce nonradiative related defects and greatly increase luminescence ef
ficien-cy The formation mechanisms offlowerlike 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 Extensive work has been devoted to
synthesize one- and two-dimensional (1D and 2D) nanostructures
such as nano-particles, -wires, -belts, -tubes, -rings, -springs, -bows,
-combs, -disks, etc.[1–9] Compared to 1D and 2D nanostructures,
com-plex 3D architectures may offer opportunities to explore novel
proper-ties of nanocrystals and be employed as novel building blocks to
fabricate more complicated and advanced 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 developed
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 reactants may be the simplest and
most effective way to prepare highly crystalline products at low
temper-atures[13,14] This method allows considerable influence of reaction
spe-cies on thefinal size and morphology of the as-synthesized samples on a
large scale
To fabricate nanostructures spatially located, researchers have
dem-onstrated a few techniques based on an assembly method under the
con-trol of external forces Xu et al and Kim et al have reported a technique
for growing vertically aligned ZnO nanowire (NW) arrays on a silicon substrate coated with ZnO seeds by electron beam lithography [15,16] Aizenberg investigated the combination of self-assembled monolayers (SAMs) and micro contract printing to controlled micropat-terns of calcite crystals on surfaces with controlled location[17] Zhou
et al have reported a selective growth of ZnO nanorod arrays by using proton beam writing[18] Kim et al have presented an approach for the preparation of ZnO nanowire arrays by combining laser-interference lithography for templating and a chemical-vapor-transport process for nanowire growth[19]
In this paper, we report the growth of 3Dflowerlike ZnO nanostruc-tures on GaN/LiAlO2substrates by combining laser direct writing and hy-drothermal method In contrast with someflowerlike structures which formed by self-assembly technologies using nanoparticles[17], nanorods
show uniform 3D structuredflowers 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 of flow-erlike ZnO nanostructures, were investigated The mechanism for the formation offlowerlike nanostructures was discussed The 3D flowerlike ZnO nanostructures reported here could be important for applications in the transistor, optoelectronics,field emission, and gas sensing[22]
2 Experimental
nanostructures on GaN/LiAlO2substrate The process involved three steps
⁎ 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.
Trang 22.1 Substrate preparation
The GaN/LiAlO2substrate was prepared by depositing about 2.0μm
thick GaNfilm on LiAlO2with 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
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 system A commercial 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 experiment The sample was mounted on a computer-controlled
xyz translation stage The surface of the sample was positioned
perpen-dicular to the propagation direction of the incident laser beam in the
focal plane of a 100× objective lens (NA= 0.8) The number of pulses
de-livered to the sample was controlled via an electromechanical shutter,
and the laser pulse energy was measured by a pyroelectric detector
2.3 Hydrothermal growth
The nutrient solution was prepared from an aqueous solution
contain-ing zinc nitrate [Zn(NO3)2·6H2O] and hexamethyltetramine [(CH2)6N4,
HMT] at a molar ratio of 1:1 and zinc concentration of 0.025 mol/L
Subse-quently, the substrates were immersed downward into the reaction
solu-tion and heated at a constant temperature of 90 °C in a water bath for 1 h
with continuous stirring After deposition, the substrate was thoroughly
washed with deionized water and dried in air at room temperature
The morphology and composition of the as-prepared products were
characterized by afield emission scanning electron microscopy
(FE-SEM) (JEOL JSM-6700F) which was equipped with an energy-dispersive
spectroscopy (EDS) facility TEM images, SEAD pattern, and HRTEM
im-ages were taken on a JEOL-2010 transmission electron microscope The
μ-PL spectra were recorded at room temperature using the 325 nm
exci-tation line (Renishaw inVia) from a He–Cd laser
3 Results and discussions
ob-served that the diameter of the obtainedflower was approximately
10μm and the flower consisted of 2D nanosheets Here, the substrate
was irradiated by 20 fs laser pulses with an energy of 1μJ There were
laser induced nanoripples which appear around theflower, a
phe-nomenon which was also found and discussed in our previous work
irradiated laser pulses as 250, 20 and 8, the ablated area can be
adjusted and subsequent growth of ZnOflowers changed correspond-ingly We successfully obtained ZnOflowers with diameters of 10 μm,
5μm, 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 offlowers, 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 theflower and the ripples, as labeled“A” and “B” inFig 2(a), respectively It can be seen that theflower 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 inFig 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 nonradiaradia-tive 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 ZnOflowerlike 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 ZnOflowerlike 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 inFig 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 theflowerlike 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 ofFig 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 inFig 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 theflowerlike ZnO nanos-tructures, systematic time-dependent experiments illustrating the evo-lution of the structure 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
Trang 3for 5 min, some sprouts grew out from the irradiated dot (Fig 4(a)).
When the reaction time extends to 10 min, the sprouts grew larger
and formed petals (Fig 4(b)) After 30 min, a smallflower was formed
When the reaction time is increased to 2 h, the 3Dflowerlike
nanostruc-tures appear, and almost no impurities can be observed To determine
the appropriate reaction time, the effect of longer reaction times, up to
12 h, has been investigated Results show that the reaction time exceed-ing 2 h will not brexceed-ing about evident structural and morphological modi-fications With reaction time increasing, the concentration of ZnO nuclei decreases conversely, and the growth velocity of ZnO nanopetals de-creases along with the reduced concentration As a result, the morphol-ogy changes very little after a certain period Additional growth steps
350 400 450 500 550 600 650 700
Wavelength (nm)
as-grown sample annealed sample
Energy
B
A
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
Trang 4produce a secondary structure, as shown inFig 4(f) Wefirst obtained
flower similar to that ofFig 2(a) After hydrothermal growth proceeded
for 1 h, we took the substrate out of the solution, carefully washed it with
deionized water at room temperature, and then put the substrate back
into the solution for 10 min The structure obtained in this way is
shown inFig 4(f) It can be seen that numerous ZnO nanodots arefilled
in the space between the petals of theflower In general, the formation of
ZnO nanocrystals can be divided into two processes: nucleation and
growth When the substrate was taken out of the solution and washed
with deionized water, the temperature of the substrate decreased
rapid-ly, thus many ZnO nuclei attached on the petals of theflower When the
substrate was put back into the solutions, these nuclei would evolve into
polyhedral seeds and further grow into hexagonal nanodots In this case,
the seeds tend to take a single-crystal in an attempt to minimize the total
surface energy of the system
The growth of ZnOflowerlike 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 irradiaabsorp-tion 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−] 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] In our
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 Zn2+and OH−, the Zn(OH)2and/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 interspacesfilling process to
give afinal 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 3Dflowerlike ZnO nanos-tructures by combining laser direct writing and hydrothermal growth method The control of the as-fabricated ZnOflowerlike 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 thefield of fabrication of nano-electronic devices
Acknowledgments This work wasfinancially 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
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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