Synthesis and Characterization of ZnO Nanoflowers

Solution Deposition∗

GAO Hai-Yong(高海永)∗∗,YAN Fa-Wang(闫发旺),ZHANG Yang(张扬),LI Jin-Min(李晋闽),

ZENG Yi-Ping(曾一平)

Semiconductor Lighting Technology Research and Development Centre,Institute of Semiconductors,Chinese Academy

of Sciences,Beijing100083

(Received11November2007)

ZnO nanoflowers are synthesized on AlNfilms by solution method.The synthesized nanoflowers are composed of nanorods,which are pyramidal and grow from a central point,thus forming structures that areflower-shaped as a whole.The nanoflowers have two typical morphologies:plate-like and bush-like.The XRD spectrum corresponds to the side planes of the ZnO nanorods made up of the nanoflowers.The micro-Raman spectrum of the ZnO nanoflowers exhibits the E2(high)mode and the second order multiple-phonon mode.The photoluminescence spectrum of the ZnO nanoflowers exhibits ultraviolet emission centred at375nm and a broad green emission centred at526nm.

PACS:61.46.Hk,81.05.Dz,81.10.Dn

ZnO is recognized as a versatile material with po-tential applications in photodetectors,optically gated switches,ultraviolet light-emitting diodes(LEDs), and chemical sensors due to its large direct bandgap (3.37eV),piezoelectricity,and high exciton bind-ing energy(60meV).[1]One-dimensional(1D)ZnO nanostructures such as nanorods,[2]nanotubes,[3] nanowires[4]and nanobelts,[5]has stimulated growing interest owing to their potential applications in laser emission,nanoscale actuators,and nanoscale hetero-junctions.A variety of methods have been employed for the synthesis of ZnO nanomaterials,such as high-temperature vapour-liquid-solid or thermal evapora-tion processes.[6,7]In those processes,either high vac-uum and temperature or some catalysts such as gold nanoparticles are required.Recently,aqueous solu-tion deposition approaches based on wet chemical and bottom-up processes have been reported to synthesize ZnO nanorods.[8]The solution deposition is simple, inexpensive,and can be carried out at low tempera-ture.We successfully synthesized ZnO nanorods on GaN-based LED wafer by solution deposition.[9]AlN has a very small or even negative electron affinity,so electrons can be easily extracted from its surface to a vacuum under an electricfield.[10]ZnO nanostructures also havefield emission performance.[11]It is expected that novel emitter or planar display devices is the pos-sible applications for ZnO nanostructures grown on AlN.In this work,we synthesize ZnO nanoflowers on AlNfilms in aqueous solution at low temperature.

The AlNfilms were grown on sapphire(0001)sub-strates using a metal organic chemical vapour depo-sition(MOCVD)system.The thickness of the crack-free AlNfilms was1µm.The details of the growth of AlNfilms have been presented elsewhere.[12]Zinc acetate(Zn(CH3COO)2·2H2O)wasfirst dissolved in deionized water with a concentration of0.023M in a container made of polytetrafluoroethylene.Ammo-nium hydroxide(NH4OH)was then added until the pH reaching about10.The AlNfilms were immersed in the solution and the container was put into a stain-less steel autoclave.The autoclave was sealed and put into a oven,and the temperature was set at100◦C. The growth time was two hours.The morphologies and structures of the samples were characterized by a coldfield emission scanning electron microscope(FE-SEM,Hitachi S-4800)equipped with an energy dis-persive x-ray(EDX)spectroscope and a x-ray diffrac-tometer(XRD,Rigaku D/max-RB).A JY-HR800Ra-man spectrometer was used to measure the micro-Raman spectra of the samples.Photoluminescence (PL)measurement was performed with a He-Cd laser (325nm)used as the source of excitation.

Figure1shows an SEM micrograph of the ZnO nanoflowers grown on the AlNfilm.Many beauti-ful nanoflowers can be seen.The nanoflowers forming the crystallinefilm are white colour observed by naked eye and not easy to be erased.There are two typical types of nanoflowers.One has a plate-like morphol-ogy as shown in rectangle a,where six petals grow symmetrically towards six directions laterally,and the seventh petal grows vertically to the substrate.The other has a bush-like morphology as shown in rectan-gle b,where large numbers of outstanding petals grow radially from the centre in many directions.The mor-phologies of the other nanoflowers are between these two types.

Figure2(a)shows a detailed image of some nano-

∗Supported by National High-tech R&D Programme of China under Grant No2006AA03A102.∗∗Email:hygao@semi.ac.cn

c 2008Chinese Physical Society an

d IOP Publishing LtdNo.2GAO Hai-Yong et al.1

Fig.1.SEM micrograph of the synthesized ZnO nanoflowers grown on AlNfilms.

Fig.2.SEM and EDX of some nanoflowers:(a)high magnification SEM image,(b)corresponding EDX spec-trum.

flowers.Both plate-like and bush-like nanoflowers can be seen clearly.Each nanoflower is several mi-crometres in size.The nanorods that make up the nanoflowers are hexagonal and pyramidal.The aver-age length of the nanorods is about2.5µm,and the side length of the hexagonal cross-section at the centre of the nanorod is about300nm.Thus it can be esti-mated that the ratio between the vertical and lateral growth rate is about9.62.Figure2(b)shows the corre-sponding EDX spectrum.Peaks at0.52keV,1.02keV, 1.49keV,8.62keV,and9.58keV are attributed to the O-Kα,Zn-Lα,Al-Kα,Zn-Kα,and Zn-Kβenergy peaks,respectively.Only peaks associated with Zn and O atoms are seen in the EDX spectrum,except for the Al-related peaks that come from the AlNfilm. This reveals that the nanoflowers,in this case,are in-deed ZnO.

Figure3shows a detailed SEM image of one nanorod.The growth of the nanorod is step-layered, and become gradually thinner up to the tip,where a hexagonal mesa is formed.The side length of the hexagonal mesa is about45nm.Every layer of ZnO nanorods coincides with the structure of the AlN and ZnO(0001)plane,which are both hexagonal.

The nanorod appears to be slightly bent,which is due to the accumulation of the secondary electrons of the SEM system during the measurement.

Fig.3.Detailed SEM image of one of the nanorods that make up nanoflowers.

Figure4(a)is a schematic illustration of a hexagonal plate-like nanoflower corresponding to the nanoflower marked by a in Fig.1.The angle between two neighbour lateral nanorods is60◦.The lattice con-stants of AlN are a=0.3111nm,and c=0.4980nm. The lattice constants of ZnO are a=0.3249nm,and c=0.5206nm.Thus the lattice mismatch between AlN and ZnO is about4.4%in the a direction and 4.54%in the c direction.ZnO nanoflowers can grow on AlN anyway,even though ZnO cannot always grow vertically.The so-called self-nucleation process may contribute to the formation of ZnO nanoflowers on AlNfilms by solution deposition.[13]The mechanism of the formation of ZnO crystals is as follows:[14,15] Zn(CH

3

COO)

2

·2H2O↔Zn2++2CH3COO−

+2H2O,(1) NH4OH↔NH+4+OH−↔NH3+H2O,(2) Zn2++4NH3↔Zn(NH3)2+4,(3) Zn(NH3)2+4+2OH−↔ZnO+4NH3+H2O,

(4)

Zn2++2OH−↔Zn(OH)

2

.(5) In the progress of the formation of ZnO,the2GAO Hai-Yong et al.Vol.25 complex Zn(NH3)2+4is formedfirst by mixing

Zn(CH3COO)2·2H2O and ammonia hydroxide.With

an increase of temperature,the complex Zn(NH3)2+4

will be dehydrated.Then,the ZnO nucleuses were

formed,and crystals begin to grow into the nanorods.

Changing Zn2+and pH will provide different amounts

of ZnO nucleuses.Low Zn2+or PH(<9)will decrease

the concentration of Zn(NH3)2+4.At high Zn2+or

pH(>12),Zn(OH)2will be formed(Eq.(5)).Zinc

acetate concentration0.023M,temperature100◦C,

pH=10are almost the optimized conditions in our

experiments.After a ZnO nucleus is synthesized on AlN,it serves as a nucleation centre from which ZnO nanorods grow.When there is only one nucleus,the nanorods grow out from it symmetrically in six direc-tions parallel to the substrate and one direction per-pendicular to the substrate.In this way the plate-like nanoflowers are formed.When there are many nu-cleuses assembled together,there

will be many nucle-ation centres,and many nanorods grow radially from different centres.In this way the bush-like nanoflowers are formed.Of course,most nanoflowers are between the plate-like and bush-like types.Figure4(b)gives a schematic illustration of the part near the tip of a nanorod.The cone angle of the nanorod is about20◦. The ratio between the vertical growth rate and lateral growth rate is about9.62as presented above.The growth rate along the 0001 direction is usually the fastest.[15]The{0001}facets with a fast growth rate disappear easily,resulting in the appearance of slower growing facets.However,{0001}facets do not disap-pear completely in this situation.There is always a hexagonal mesa left at the end of the nanorods,which may suggest that the energy of the nanorod is the lowest when it grows to the length mentioned above.

Fig.4.Schematic illustration of the synthesized nanoflowers:(a)plate-like nanoflower,(b)part of one nanorod near the tip.

Figure5shows the XRD spectrum of AlNfilm and ZnO nanoflowers.The peaks at2θ=36.00◦and41.◦in Fig.5(a)correspond to the diffraction of the AlN(0002)and Al2O3(0006)planes,respec-tively.The spectrum reveals that the as-deposited AlNfilm has a single crystal structure.The peaks at 2θ=31.79◦and36.10◦in Fig.5(b)correspond to the diffraction of the(10¯10)and(10¯11)planes of ZnO, respectively.The nanorods are grown in the c-axis orientation,but most of the nanorods are not perpen-dicular to the AlNfilm,as can be seen in Fig.1.They are parallel to the substrate or in other orientations. Then the peak corresponding to the(0002)plane of ZnO is very weak.In fact,the(10¯10)are the side planes of the hexagonal ZnO.Therefore the diffraction should correspond to the side planes of ZnO nanorods.

Fig.5.X-ray diffraction spectrum of the samples:(a) AlNfilms,(b)ZnO nanoflowers.

Figure6shows the micro-Raman scattering spec-tra of AlNfilms and ZnO nanoflowers recorded in backscattering geometry with no polarization detec-tion.Both AlN and ZnO belong to the C46v space group.According to group theory analysis at theΓpoint,the A1,E1,and2E2modes are Raman ac-tive.Raman shifts of245.9cm−1,654.7cm−1and 887.1cm−1in Fig.6(a)are assigned to E2(low fre-quency),E2(high frequency)and A1(LO)modes of AlN,respectively.Two weak peaks located at 416cm−1and750cm−1are assigned to the two A1g modes of the sapphire substrate.[16]Raman shifts of379cm−1,409cm−1,438.1cm−1and577cm−1in Fig.6(b)are assigned to A1(TO),E1(TO),E2(high) and A1(LO)modes of ZnO,respectively.The peak lo-cated at331cm−1is known to be the vibration modes due to the second order multiple-phonon scattering processes,and it should be assigned to E2(high)–E2(low)mode(multi phonon).The multiple-phonon scattering indicates the quantum confinement effect in the ZnO nanoflowers.[17]The peak at533cm−1 can be attributed to2B1(low)and LA overtones, and also corresponds to a second order process.[18] Among these peaks,the E2(high)mode correspond-ing to the band characteristic of a wurtzite phase has the strongest intensity and narrowest linewidth,which clearly demonstrates that the ZnO nanoflowers have wurtzite structure.

Figure7shows the room temperature PL spec-No.2GAO Hai-Yong et al.3

trum of the synthesized nanoflowers.The UV emis-sion centred at about375nm(3.31eV)corresponds to the near-band-edge emission(NBE),which originated from the recombination of free excitons.The broad green light emission centred at about526nm(2.36eV) is known as deep level emission(DLE),which is commonly attributed to the emission results from the radiative recombination of photo-generated holes with electrons occupying the oxygen vacancies.[19]The nanoflowers in our experiment were synthesized in a relatively low oxygen deficient ambient,so the green emission must be related to oxygen vacancies.The green emission suggests that ZnO nanoflowers can be Fig.6.Micro-Raman scattering spectra of the samples:

(a)AlNfilms,(b)ZnO nanoflowers.

Fig.7.Room temperature PL spectrum of ZnO nanoflow-ers grown on AlNfilms.potentially used as phosphors in solid state lighting (SSL)or in display technology when combined with thefield emission performance of AlNfilms.

In summary,ZnO nanoflowers have been success-fully synthesized on AlNfilms.Plate-like and bush-like are the two typical kinds of morphologies.Other nanoflowers are between these two types.The self-nucleation of ZnO nanoflowers has been discussed. The XRD spectrum corresponds to the side plane of the ZnO nanorods.The micro-Raman spectrum of the ZnO nanoflowers exhibits the E2(high)mode and the second order multiple-phonon mode.Ultraviolet emis-sion at375nm and broad green emission at526nm are observed in the PL spectrum of the ZnO nanoflowers. References

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