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Materials Chemistry and Physics 101 (2007) 387–394

Effects of deposition temperature and chemical composition on the ZnOcrystal growth on the surface of Pd catalyst through

electroless chemical reaction

J.Y. Lee a, S. Horiuchi a, H.K. Choi b,∗a Nanotechnology Research Institute, National Institute of Advanced Industrial Science and Technology (AIST),

1-1-1, Higashi, Tsukuba, Ibaraki 305-8565, Japanb Department of Manufacturing Technology and Standards, Agency for Technology & Standards,

Jungang-dong, Gwacheon-city, Gyeonggi-do 427-716, Republic of Korea

Received 31 August 2005; received in revised form 23 March 2006; accepted 4 July 2006

bstract

Zinc oxide (ZnO) was site-selectively grown on the palladium (Pd) catalyst through the electroless deposition process under mild conditions,nd the effects of deposition temperature and chemical composition on the ZnO crystal growth were investigated. ZnO crystals were synthesizedn the UV-patterned Pd catalysts in the aqueous solutions of various dimethylamine borane (DMAB)/Zn(NO3)2 ratio at 30–70 ◦C. The site-elective deposition was confirmed by X-ray photoelectron spectroscopy (XPS) data and elemental maps of Pd, Zn and oxygen in energy-filteringransmission electron microscopy (EFTEM), and the crystal morphology was observed by scanning electron microscopy (SEM). A strong nearand emission at around 390 nm and a weak green emission at around 470 nm were observed in the photoluminescence (PL) spectrum. The ZnOrystals were grown in the following three steps: (1) ZnO fibrils were generated on the Pd catalysts and became sphere-like particles, (2) hexagonal

urtzite crystals initiated to grow from the sphere-like particles, and (3) the crystals grew in two directions—longitudinal and lateral growthsiving rod-type or needle-type hexagonal crystals. It was found that longitudinal growth rate increased with increasing deposition temperature orMAB/Zn(NO3)2 ratio.2006 Elsevier B.V. All rights reserved.

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eywords: Electroless deposition; Zinc oxide; Palladium catalyst; UV-patternin

. Introduction

Zinc oxide (ZnO) is an attractive and promising mate-ial with various interesting characteristics such as wide bandap of 3.37 eV, high exciton binding energy of 60 meV, lowower threshold for optical pumping, and high transmittance ofV–visible rays (about 80%) at room temperature, in addition toigh mechanical, thermal and chemical stability and absence ofoxicity. Therefore, many researchers have investigated in ordero apply these properties to the practical fields such as trans-arent conductive devices, solar cell windows, high-resolution

eld emission display, ultraviolet or blue laser diodes, piezoelec-

ric semiconductor, surface acoustic wave devices, etc., and theyave also developed many deposition techniques for ZnO [1–4].

∗ Corresponding author. Tel.: +82 25037974; fax: +82 25071923.E-mail address: hyeongki@ats.go.kr (H.K. Choi).

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254-0584/$ – see front matter © 2006 Elsevier B.V. All rights reserved.oi:10.1016/j.matchemphys.2006.07.003

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Recent scientific and technological researches on the ZnO-elated materials have focused on the synthesis and character-zation of raw materials and devices with increasingly smallimensions. There are many well-known methods such as sput-ering [5], metal organic chemical vapor deposition (MOCVD)6], molecular beam epitaxy (MBE) [7], and pulsed laser depo-ition (PLD) [8], etc. However, these techniques have seriousemerits that the deposition condition is too violent to pre-erve the substrates without damage and it is hard to obtainigh-resolution pattern edges in the etching step due to tear-ng of the ZnO film during the lift-off process [2,9,10]. So,

any techniques have been developed to overcome the demerits,nd site-selective electroless deposition technique processed inmild aqueous condition is concerned as one of them, for it has

any following merits [11–13]: (1) it is a simple and economical

ow-energy process, (2) it allows us to obtain uniformly thickoatings of metallic, dielectric, and semiconducting materialsn both conducting and non-conducting substrates, (3) it can be

388 J.Y. Lee et al. / Materials Chemistry and Physics 101 (2007) 387–394

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pplied to the complex three dimensional surface profiles andardly-accessible surface, and (4) the deposited thin films andhe substrates are safe from thermal stress because it is processed

nder 100 ◦C.

It is needed to carry out the electroless deposition processhat the substrate’s surface should be given the sensitization andubsequent activation by catalyst prior to deposition. So, many

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ig. 2. (a–c) TEM micrographs showing the dispersion of Pd nanoparticles preparedatterned by UV dose (10 J cm−2) through a photomask. (a) A low magnification iespectively. (d–f) SEM micrographs showing the ZnO nanoparticles deposited on thatio = 0.1. (d) A low magnification image reproducing the photomask. (e and f) Maghowing the ZnO patterns and the magnified image after removal of missing depositf DMAB/Zn(NO3)2 ratio = 1.0.

rning on the Pd catalyzed PMMA film.

esearchers have investigated to advance the micro and nanopat-erning of catalysts by combining photo-chemical, chemical,atalytic, and self-assembled block copolymer tools in order

o get site-selective crystals grown on the patterned catalysts1,2,11,13–15]. For example, Saito et al. [2,13] fabricated anlectroless ZnO micropattern on the Pd catalyst selectively bymploying a SAM of photo-patterned phenyltrichlorosilane.

by a 5 min exposure to Pd(acac)2 vapor within a PMMA thin film (ca. 20 nm)mage. (b and c) Magnified images of the UV irradiated and masked regions,e Pd catalysts at 50 ◦C for 10 min in the aqueous solution of DMAB/Zn(NO3)2

nified images deposited on (b and c), respectively. (g and h) SEM micrographsed particles, which were deposited at 50 ◦C for 20 min in the aqueous solution

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V light through a photomask changed the phenyl groups ofhe SAM to OH groups, and the Pd catalysts were selectivelyttached on the surface of phenyl groups in SnCl2 and PdCl2queous solutions. Sugimura et al. [15] reported the Ni patternn the SAM of octadecyltrimethoxysilane with UV-patternedd catalysts which were produced in PdCl2/HCl/HF solution.oriuchi et al. [16–18] also developed a simple dry process for

he patterning metal nanoparticles in polymer films through theeduction of a metal complex used as a precursor. Palladium(II)is(acetylacetonato), denoted as Pd(acac)2, was vaporized initrogen atmosphere at 180 ◦C. Then, the vapor penetrated intoolymethylmethacrylate (PMMA) film treated with UV throughphotomask and it simultaneously reduced to the metallic Pd

anoparticles in the UV irradiated regions with rare formationn the UV masked regions.

In this study, we synthesized ZnO crystals on the Pd catalystsite-selectively by combining the UV-patterning of Pd nanopar-icles on PMMA thin film and electroless chemical reaction onhe Pd surface, and the effects of various conditions of reactantomposition, deposition temperature, and time were estimated.

. Experiments

Palladium(II) bis(acetylacetonato) denoted as Pd(acac)2 was purchased fromohnson Matthey Materials Technology and was recrystallized in acetone. Poly-ethylmethacrylate was obtained from Aldrich Chemical Company, Inc. whose

umber-average molecular weight, Mn, was 350,000. It was precipitated twicerom methylene chloride into methanol for purification. Zinc nitrate (Zn(NO3)2)nd dimethylamine borane (DMAB) were purchased from Wako Pure Chemicalndustries, Ltd.

To pattern Pd nanoparticles within the PMMA film through the reduction ind(acac)2, the following procedure was used [16–18], as shown in Fig. 1: Firstly,0 nm thick PMMA film was spin-coated on silicon wafer, dried at 60 ◦C for 2 h,nd patterned with UV irradiation at the dose of 10 J cm−2 through a photomaskith 5 �m × 5 �m square windows. Then, it was put into the glass reactor of

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ig. 3. (a) XPS spectra for the ZnO particles prepared on Pd catalysts at 50 ◦C forMAB/Zn(NO3)2 ratio = 1.0, where Pd catalysts were prepared on the UV irradiatedFTEM for the ZnO particles prepared at 50 ◦C for 5 min, and (c–e) were the Pd, Zn

nd Physics 101 (2007) 387–394 389

d(acac)2 and maintained at 180 ◦C for 5 min in N2 atmosphere. In this stage,ost Pd clusters were positioned within the UV-patterned area.

ZnO was synthesized on the Pd-patterned PMMA/Si wafer inn(NO3)2/DMAB solution. The chemical ratio of DMAB to Zn(NO3)2 washanged in the range of 0.1–1 and reaction temperature was maintained at each0–70 ◦C, and reaction time was changed from 5 to 30 min at each temperature.inally, the miss-deposited ZnO was removed from the Si surface together withMMA film in acetone.

To confirm the site-selective ZnO deposition on the Pd catalysts, energy-ltering transmission electron microscopy (EFTEM, LEO922 operated by anccelerating voltage of 200 kV) was used. It could also serve conventionalEM images and elemental maps. Chemical composition of the depositednO was analyzed by X-ray photoelectron spectroscopy (XPS, PHI’s Quan-

um 2000 Scanning ESCA Microprobe), the crystal morphology was observedy scanning electron microscopy (SEM, Philips XL30 FE-SEM). The num-er of Pd nanoparticles and the diameter and length of ZnO nanorods weretatistically estimated by image processing software (analySIS, Soft Imagingystem Co. Ltd.). The photoluminescence (PL) spectrum was taken with anxcitation wavelength of 325 nm from a spectrofluorometer (JASCO, Model:P-6500).

. Results and discussion

To observe the top-view of Pd pattern on the UV-treatedMMA thin film, 20 nm thickness of PMMA film was coatedn a cleaved NaCl single crystal followed by UV irradiation andd patterning, following the same procedure of Section 2. And

hen, it was lifted-off from the NaCl by dipping it into waternd was mounted on a 600 mesh TEM copper grid, which wasreliminary coated with carbon film by steeping in the carbon-oated NaCl into water. TEM image in Fig. 2a reproduced thehotomask pattern that the bright region was due to the lower

olymer density induced by the scission and gasification ofMMA chains under UV stimulation [17,18]. The distributionensity of Pd nanoparticles in the UV irradiated region was farigher than that in the UV masked region, as shown in Fig. 2b

(i) 0 min, (ii) 10 min, (iii) 20 min, and (iv) 30 min in the aqueous solution ofregion by an exposure to Pd(acac)2 vapor for 5 min. (b) A zero-loss image in

, and oxygen elemental maps for (b), respectively.

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nd c, respectively, where black dots were Pd catalysts in theluster form. It was due to that C C double bond generatedy the UV irradiation may have enhanced the reduction rate ofd(acac)2 to Pd element [17,19].

SEM image in Fig. 2d revealed by the ZnO micropatterneproducing the Pd pattern of Fig. 2a deposited at 50 ◦C for0 min in the aqueous solution of DMAB/Zn(NO3)2 ratio = 0.1.ost ZnO particles were deposited with an excellent site-

electivity on the white regions that was precisely fitted with theV-patterned regions. However, some particles were the miss-

eposited ZnO on the UV masked regions. Fig. 2e and f werehe magnified SEM images of ZnO nanoparticles generated inhe UV irradiated and masked regions, respectively. The aver-ge particle size and deposition percent for UV irradiated region

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ig. 4. SEM images showing ZnO morphology synthesized at (a) 30 ◦C, (b) 40 ◦C,egions. All were synthesized on the patterned Pd/PMMA thin film by 5 min exposuor 10 min.

nd Physics 101 (2007) 387–394

ere 49.2 nm and 80.7%, and those for UV masked region were4.9 nm and 24.5%, respectively. Therefore, the miss-depositednO should be removed in acetone to get good patterns as shown

n Fig. 2g and h deposited at 50 ◦C for 20 min in the aqueousolution of DMAB/Zn(NO3)2 ratio = 1.0.

Fig. 3a shows XPS spectra for the ZnO particles, preparedor Pd catalysts at 50 ◦C for (i) 0 min, (ii) 10 min, (iii) 20 min,nd (iv) 30 min in the aqueous solution of DMAB/Zn(NO3)2atio = 1.0, where Pd catalysts were prepared on UV-irradiatedMMA film through the exposure to Pd(acac)2 vapor for 5 min.

ig. 3a(i) displays the characteristic peaks for the core levels ofd3d at 341.1 and 335.5 eV, and that for O1s at 532.9 eV. When

he deposition time was 10 min in Fig. 3a(ii), two Pd3d peaksisappeared and new peaks for Zn2p were found at 1045.5 eV for

(c) 50 ◦C, (d) 60 ◦C, and (e) 70 ◦C on the UV irradiated and masked (insets)re to Pd(acac)2 vapor in the aqueous solution of DMAB/Zn(NO3)2 ratio = 0.4

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n2p1/2 and 1022.3 eV for Zn2p3/2 with a separation of 23.2 eVetween the two peaks, which was ascribed to the Zn2p state20]. Zn2p3/2 was well-known as the characteristic peak for then2+ in ZnO. As deposition time increased, so do the peaks forn. However, O1s peak was almost constant regardless of thenO deposition or the increment of the time. It was for the fact

hat the oxygen in the carbonyl group of PMMA was covered byhe oxygens of ZnO. Other peaks were attributed to the C andi elements in the PMMA and Si wafer.

In order to observe the site-selective deposition of ZnO crys-als on the Pd catalysts, EFTEM and elemental mapping analysesere carried out for the ZnO sample synthesized at 50 ◦C for

0 min in the ratio of DMAB/Zn(NO3)2 = 1 solution that it wasrepared via the similar procedure of Section 2: 20 nm thick-ess of PMMA thin film was coated on the NaCl single crystallate by spin coater and dried sufficiently. Then, it was treated by

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ig. 5. SEM images showing ZnO nanostructures deposited on the UV irradiated reg0 min, (c′) 20 min, (d′) 60 min at 50 ◦C, and for (a′′) 5 min, (b′′) 10 min, (c′′) 20 minhere Pd pattern was prepared by 5 min exposure to Pd(acac)2 vapor.

nd Physics 101 (2007) 387–394 391

0 J cm−2 of UV light and exposed to Pd(acac)2 vapor for 5 min.he Pd patterned PMMA was moved onto the 600 mesh carbon-oated TEM grid and ZnO was synthesized on the TEM grid at0 ◦C for 5 min in the aqueous solution of DMAB/Zn(NO3)2atio = 1.0.

Fig. 3b was the zero-loss image for the UV irradiated region inFTEM, where the zero-loss image in EFTEM was the same in

he conventional TEM. The sphere-like ZnO nanoparticles con-acted with one another. The site-selective deposition of ZnOrystals on the Pd catalysts was confirmed by the Pd map forcattered white dots and Zn. Oxygen maps aggregated whiteanoparticles were shown in Fig. 3c–e, respectively. Those maps

ere created by the “three-window method” using the two back-round images at 295 ± 10 and 320 ± 10 eV and a core lossmage at 415 ± 10 eV for the Pd map, the two backgroundmages at 950 ± 20 and 980 ± 20 eV and a core loss image at

ion for (a) 5 min, (b) 10 min, (c) 20 min, (d) 60 min at 40 ◦C, for (a′) 5 min, (b′), (d′′) 60 min at 70 ◦C in the aqueous solution of DMAB/Zn(NO3)2 ratio = 0.4,

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n(06ttregion in each DMAB/Zn(NO3)2 ratio, suggesting that mostZnO crystals were deposited with an excellent site-selectivityon the Pd patterns. It was attributed to the inclined distributionof Pd catalysts within the UV irradiated regions, as shown in

92 J.Y. Lee et al. / Materials Chemi

150 ± 20 eV for the Zn map, and the two background images at90 ± 10 and 515 ± 10 eV and a core loss image at 535 ± 10 eVor the oxygen map [21]. Zn and oxygen maps precisely over-apped the zero-loss image and they almost well-agreed to the Pd

ap. These elemental distributions confirmed that the depositedanoparticles were ZnO crystals, which site-selectively grew onhe Pd catalysts.

Fig. 4a–e shows the effects of deposition temperature on theorphologies of ZnO nanostructures on the UV irradiated andasked (insets) regions which were also prepared in the ratio

f DMAB/Zn(NO3)2 = 0.4 solution for 10 min. In this stage, itas better to compare the morphology in the UV masked area,ecause ZnO crystals of the UV masked area could grow withouthe hindrance of one another but there was no difference to thosef the UV irradiated sample except the deposition density. Theorphology of the ZnO synthesized at 30 ◦C was fibrillar shape

Fig. 4a) and that of 40 ◦C was similar fibrillar shape co-mixedith sphere-like shape (Fig. 4b). However, short faceted rod was

nitiated to grow at 50 ◦C (Fig. 4c) and some longer and facetedexagonal rod-like structure seemed to predominate at 60 ◦C ashown in Fig. 4d. At 70 ◦C, long and smooth hexagonal needlehapes were grown as shown in Fig. 4e.

In order to understand how the ZnO crystals grew, ZnO nanos-ructures synthesized at various conditions were compared, ashown in Fig. 5. All were created on the UV irradiated regionn the aqueous solution of DMAB/Zn(NO3)2 ratio = 0.4 for var-ous time at each temperature: (a) 5 min, (b) 10 min, (c) 20 min,d) 60 min at 40 ◦C, for (a′) 5 min, (b′) 10 min, (c′) 20 min, (d′)0 min at 50 ◦C, and for (a′′) 5 min, (b′′) 10 min, (c′′) 20 min,d′′) 60 min at 70 ◦C, where Pd pattern was prepared by 5 minxposure to Pd(acac)2 vapor. In the early stage of ZnO growtht 40 ◦C, fibrillar shape was initiated (Fig. 5a) and these fibrilsecame fat and sphere-like particles with increasing depositionime (Fig. 5b–d). The fibrillar ZnO at 30 ◦C shown in Fig. 3b isery little changed in the morphology and there was no sphere-ike particle even at 60 min, which is not shown in Fig. 5. At0 ◦C, sphere-like ZnO particles started to grow in very shortime of 5 min (Fig. 5a′). After 10 min of the growth (Fig. 5b′),

ost of them were changed to faceted rods, and they con-isted of short rounded hexagonal rods after 20 min (Fig. 5c′).

hen 60 min of the deposition time passed, smooth hexagonalanorods were produced (Fig. 5d′). In the case of 70 ◦C, thinnernd distorted hexagonal rods were generated (Fig. 5a′′ and b′′)nd grew in the length direction giving well defined hexagonaltructure, as the deposition time increased (Fig. 5c′′ and d′′).

From these results, the following three steps were suggestedor the growth of ZnO crystals: (1) ZnO fibrils were generated onhe Pd catalysts and became sphere-like particles, (2) hexagonalurtzite crystals initiated to grow from the sphere-like particles,

nd (3) the crystals grew in two directions—longitudinal and lat-ral growths, giving rod-type or needle-type hexagonal crystals22]. It was found that longitudinal growth rate increased withncreasing deposition temperature.

Fig. 6 shows room temperature PL spectra of ZnO nanos-ructures recorded at room temperature using an excitationavelength of 325 nm. All ZnO were deposited in the aqueous

olution of DMAB/Zn(NO3)2 ratio = 0.4 after 5 min exposure

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nd Physics 101 (2007) 387–394

o Pd(acac)2 vapor in the following conditions: (a) 20 min anda′) 60 min at 50 ◦C; (b) 20 min and (b′) 60 min at 60 ◦C; (c)0 min and (c′) 60 min at 70 ◦C. A strong near band emission wasound at around 390 nm in each spectrum except the shoulderf 50 ◦C related to the free-exciton recombination [6]. As depo-ition temperature and time increased, the intensity increased.y comparing with the SEM images in Fig. 5, it was found thatexagonal nanostructure was attributed to a strong near bandmission, but the peak was some broad even in the case of 60 mint 70 ◦C. A weak green emission was also observed at around70 nm in each spectrum, which was due to the radial recombi-ation of a photo-generated hole with an electron that belongedo a singly ionized oxygen vacancy in ZnO [6]. Its intensityas very little increased with increasing deposition temperature

nd time, and it appeared even in the most excellent spectrumFig. 6c′). Therefore, in order to modify the optical property,he near band emission should be much sharper and the greenmission be disappeared by thermal annealing under reductivenvironment.

Fig. 7 shows the effect of DMAB/Zn(NO3)2 ratio on the ZnOanostructures deposited on the UV irradiated and the maskedinsets) regions: DMAB/Zn(NO3)2 ratio were (a) 0.1, (b) 0.2, (c).4, (d) 0.6, (e) 0.8, and (f) 1.0, respectively. All were deposited at0 ◦C for 10 min and Pd pattern was prepared by 5 min exposureo Pd(acac)2 vapor. The distribution density of ZnO particles onhe UV irradiated region was far higher than that of the masked

ig. 6. Photoluminescence spectra of ZnO nanostructures recorded at room tem-erature using an excitation wavelength of 325 nm. All ZnO were deposited inhe aqueous solution of DMAB/Zn(NO3)2 ratio = 0.4 after 5 min exposure tod(acac)2 vapor in the following conditions: (a) 20 min and (a′) 60 min at 50 ◦C;b) 20 min and (b′) 60 min at 60 ◦C; (c) 20 min and (c′) 60 min at 70 ◦C.

J.Y. Lee et al. / Materials Chemistry and Physics 101 (2007) 387–394 393

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ig. 7. SEM images showing ZnO nanostructures deposited on the UV irradiateatio: (a) 0.1, (b) 0.2, (c) 0.4, (d) 0.6, (e) 0.8, and (f) 1.0, respectively. All insets

ig. 2. In the DMAB/Zn(NO3)2 ratio = 0.1 and 0.2, the particlesostly consisted of rounded rectangular and very short hexag-

nal rods, and the longitudinal growth rate increased, givingonger hexagonal rods, as the ratio increased. The aspect ratioslength/diameter) of nanorods for Fig. 7a–f were 1.6, 1.7, 2.3,.8, 5.2, and 5.8, respectively, which were measured from thensets because the crystals generated on the UV masked regionf low Pd density could grow without overlapping one another.t meant that the longitudinal growth rate became faster than theateral growth rate, as DMAB/Zn(NO3)2 ratio increased.

. Conclusions

Zinc oxide (ZnO) site-selectively grew on the palladium (Pd)atalyst through the electroless deposition process under mild

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onditions, and the ZnO crystal growth was affected by depo-ition temperature and chemical composition. Patterned Pd cat-lysts were mainly produced on the UV irradiated polymethyl-ethacrylate region by the reduction of vaporized palladium(II)

is(acetylacetonato) denoted as Pd(acac)2. The site-selectiverowth of ZnO on Pd catalyst was confirmed by X-ray pho-oelectron spectroscopy data and elemental maps of Pd, Zn, andxygen in energy-filtering transmission electron microscopy.n XPS, Pd3d peaks at 341.1 and 335.5 eV disappeared andew Zn2p peaks at 1045.5 and 1022.3 eV appeared by the ZnOeposition, and the intensity of Zn2p peaks increased, as depo-

ition time increased. In EFTEM, the elemental maps of Pd,n, and oxygen were accurately overlapped to the TEM image.he ZnO crystals grew in the following three steps: (1) ZnObrils were generated on the Pd catalysts and became sphere-

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ike particles, (2) hexagonal wurtzite crystals initiated to growrom the sphere-like particles, and (3) the crystals grew in twoirections—longitudinal and lateral growths giving rod-type oreedle-type hexagonal crystals. It was found that longitudinalrowth rate increased with increasing deposition temperaturend DMAB/Zn(NO3)2 ratio.

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