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Growth behavior of β-Ga2O3 nanomaterials synthesized by catalyst-free thermal evaporation

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2010 Phys. Scr. 2010 014079

(http://iopscience.iop.org/1402-4896/2010/T139/014079)

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Phys. Scr. T139 (2010) 014079 (6pp) doi:10.1088/0031-8949/2010/T139/014079

Growth behavior of β-Ga2O3nanomaterials synthesized bycatalyst-free thermal evaporationKwon Koo Cho1, Gyu Bong Cho1, Ki Won Kim1 and Kwang Sun Ryu2

1 School of Materials Science and Engineering, ERI and i-cube center, Gyeongsang National University,900 Gazwadong, Jinju, Gyeongnam 660-701, Korea2 Department of Chemistry, University of Ulsan, Ulsan 680-749, Korea

E-mail: kkcho66@gnu.ac.kr

Received 25 December 2009Accepted for publication 19 January 2010Published 28 May 2010Online at stacks.iop.org/PhysScr/T139/014079

AbstractVarious kinds of β-Ga2O3 nanomaterials such as nanowires, nanorods, nanobelts, nanosheetsand nanocolumns have been successfully synthesized by simple evaporation of galliumpowder with no assisted catalyst in a flow of argon gas. The as-synthesized materials werepure, structurally uniform, single crystalline with monoclinic β-Ga2O3 structure (space group:C2 m−1) and free from defects. The synthesized nanomaterials were deposited with a growthorder of nanocolumn/nanorod, nanowire/nanobelt and nanosheet with synthesis time. Thenucleation site was looked over in detail. We present evidence that the surface, edge and tip ofpreviously grown β-Ga2O3 nanomaterials again provide a nucleation site of new β-Ga2O3

nanomaterials. Because no metal catalysts were introduced into our growth, a vapor–liquid–solid (VLS) growth is not the likely process in this work, indicating that the observednanomaterials were grown via a vapor–solid (VS) mechanism.

PACS numbers: 81.07.−b, 81.16.−c, 81.20.−n

(Some figures in this article are in colour only in the electronic version.)

1. Introduction

The synthesis of single-crystalline semiconducting nano-materials having one- or two-dimensional morphologieshas attracted increasing interest in recent years on the basisof nanoscience and nanotechnology, due to their size ormorphology-related properties and potential applicationsin nanodevices [1, 2]. Much effort has been made tosynthesize semiconductor nanomaterials. Recently, severalkinds of binary oxide materials such as ZnO [3], SnO2 [4],MgO [5], SiO2 [6], In2O3 [7], NiO [8] and Ga2O3 [9–11]were synthesized as nanowires, nanorods, nanobelts andnanosheets. Among them, monoclinic gallium oxide(β-Ga2O3) with a wide band gap (Eg = 4.8 eV) [12] hasbeen known to exhibit both conduction and luminescenceproperties [13, 14]. It has a variety of applications, includingas a transparent conducting oxide, an optical emitter forUV and a high-temperature stable gas sensor [14, 15]. Sofar, the syntheses of one- and two-dimensional β-Ga2O3

nanowires [16, 17], nanorods [18], nanobelts [19] andnanosheets [20] have been successfully achieved throughmethods including physical evaporation, arc discharge, laserablation, vapor chemical reaction, catalytic-assisted processesand carbon nanotube-assisted routes [21–25].

The growth of metal oxide nanomaterials has beenexplained by the following three steps: vaporization ofraw material, condensation of vapor onto a catalyst or asubstrate, and growth of nanomaterials at the condensedsite. In the growth mechanism of many metal oxidenanomaterials, there are two well-accepted mechanisms:the vapor–liquid–solid (VLS) mechanism [26] and thevapor–solid (VS) mechanism [27]. The most remarkablesign of the VLS growth mechanism as a catalyst-assistedprocess is that the solidified droplet was observed at theend of synthesized products. On the other hand, there isno catalyst at the tip of nanomaterials synthesized by theVS growth mechanism. Namely, the growth mechanism hasbeen classified by the presence (VLS mechanism) or not

0031-8949/10/014079+06$30.00 1 © 2010 The Royal Swedish Academy of Sciences Printed in the UK

Phys. Scr. T139 (2010) 014079 K K Cho et al

Figure 1. Schematic depiction of the experimental setup.

(VS mechanism) of the nanoparticles at the tip of grownnanomaterials. Theoretically, one kind of aligned catalystonly produces a single type of nanomaterial, but someresearch has reported that two or three kinds of nanomaterialswere synthesized at the same time [16, 28, 29]. Theseresults more frequently occur while synthesizing binary oxidenanomaterials, and there are many reports related to thisresult. Xiang et al [28] reported that Ga2O3 nanowires andnanobelts were synthesized at the same time in the presenceof Au catalyst, and Yang et al [29] have reported that threekinds of Ga2O3 nanomaterials (nanowires, nanobelts andnanosheets) were synthesized at the same time by usingthe catalytic assistance of Au. Besides the referred cases ofresearch, for the most part research related to the synthesis ofbinary oxide nanomaterials, especially the growth mechanismof Ga2O3 nanomaterials, is only being judged by whether thesolidified globule catalysts exist at the tip of nanomaterialsor not. In the case of nanomaterials synthesized by the VLSmechanism (that is suspending a catalyst at the end), thenucleation site is a ‘catalyst’, and there have been manyrelated reports [30, 31]. However, reports on the nucleationsite of nanomaterials without a catalyst at the tip or bottomhave been very limited.

In this work, we surveyed the nucleation site ofnon-catalyst-assisted nanomaterials. To induce all synthesizedmaterials that can be grown by the ‘VS’ mechanism, wedid not employ a catalyst. We chose Ga2O3 among thementioned binary oxide nanomaterials, because there havebeen many reports that Ga2O3 can be synthesized with variousmorphologies (wires, rods, belts, sheets, ribbons, etc). Wesuccessfully obtained various kinds of Ga2O3 nanomaterialsby the non-catalyst-assisted method and tried to discoverpainstakingly the nucleation site of these morphologies.

2. Experiment

The growth process is based on simple thermal evaporationof pure gallium metal powder (99.999%, Sigma-Aldrich)without a catalyst. A bare p-type silicon wafer with a crystaldirection of (001) was used as the substrate and an aluminaplate (50 × 30 × 3 mm) was used as the template. The cutsilicon substrate and alumina template were ultrasonicallycleaned for 30 min each with acetone, pure ethanol (99.9%,Duksan) and finally distilled water. After that, to prevent

Figure 2. XRD pattern of the products obtained from the substratesynthesized at 900 ◦C for 30 min.

oxidation and contamination, cleaned substrates were driedin a vacuum oven at 60 ◦C for 24 h. In a typical synthesisprocess, an appropriate amount of gallium powder was loadedonto an alumina template, and bared silicon substrate wasplaced at a distance of 10 mm from gallium powder. Thispreparation (hereafter ‘a total system’) was used as a totalsetting of the synthesis. In this work, a conventional horizontalelectronic resistance furnace mounted by a quartz tube with aninner diameter of 47 mm was used. The synthesis procedureand apparatus of Ga2O3 nanomaterials were slightly differentfrom those of previous work. In former work, as mentionedthe total system was located in the center of the horizontaltube furnace in advance of applying heat. This method hasa shortcoming in adjusting the exact reaction time betweena precursor and a substrate as well as in controlling themid-term reaction in the track of rising temperature in afurnace. However, in this work we designed a substrateconveying system to adjust the exact reaction time so that wecould observe the total synthesis process of various kinds ofnanomaterials. The synthesis apparatus is depicted in figure 1.After heating the furnace to the intended temperature, a totalsystem was located at position B by the substrate conveyingbar and synthesized exactly for 10, 20 and 30 min; afterthat it dragged out to initial position A and then cooleddown to room temperature. The synthesis temperature inthis work was set to 900 ◦C, and 500 sccm of pure Ar gas(99.99%) was used as a carrier gas. The resulting sample

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Phys. Scr. T139 (2010) 014079 K K Cho et al

Figure 3. FE-SEM images of the resulting materials synthesized at 900 ◦C for (a) 10 min, (b) 20 min and (c) 30 min.

was characterized by x-ray diffraction (XRD), field-emissionscanning electron microscopy (FE-SEM) and high-resolutiontransmission electron microscopy (HRTEM).

3. Results and discussion

β-Ga2O3 materials were grown on silicon substrate withouta catalyst. After the reaction, white-colored materials weredeposited on the silicon substrate. The overall structure of theas-synthesized product was characterized by XRD. Figure 2exhibits the typical XRD pattern of the products synthesizedfor 30 min at 900 ◦C. The peak positions of diffractionpatterns turn out to be in good agreement with those ofmonoclinic β-Ga2O3 structure with lattice constants a =

1.223, b = 0.304, c = 0.580 nm and β = 103.7◦ reported forthe bulk β-Ga2O3 crystal (JCPDS card no. 11-370). It shouldbe noted that no diffraction peaks derived from impuritieshave been found within the detection limit. The XRD resultsindicated that the as-synthesized products are highly pureand single-phase β-Ga2O3. Miller indices of β-Ga2O3 weremarked in this pattern.

The morphologies were characterized by FE-SEM.Various nanostructures such as nanowires, nanorods,nanobelts, nanosheets and nano-sized column-like structureswere observed. Figure 3 shows the morphology of thesample synthesized at 900 ◦C for 10, 20 and 30 min,respectively. It can be seen from the FE-SEM image infigure 3(a) synthesized for 10 min that the product consistsof column-like (hereafter ‘nanocolumns’) and rod-likestructures. The main morphological type is the polyangularnanocolumn, which posseses diameter and length regularlywithin 300–500 nm and around 3 µm, respectively, andit was densely deposited on the substrate. The inset offigure 3(a) shows a typical polyangular structure of theas-synthesized nanocolumn. There are limited reports on theformation of such column-like structures using the chemicalor thermal vapor deposition method. Recently, Yang et al [29]synthesized column- and sheet-like β-Ga2O3 structures ona gold-coated Si substrate by thermal evaporation of the Gadroplet. They suggested that the different morphologies of theproducts are strongly dependent on the concentration gradientof the vapor sources with the distance between sourcematerial and synthesis position. Also, Jung et al [32] reportedthat besides β-Ga2O3 nanowires, nanobelts and nanosheets,nano- and microcolumns were obtained by the thermalannealing of compacted gallium nitride (GaN) powder in

flowing nitrogen. They suggested that the morphology ofthe nanostructures may be determined by the degree ofsupersaturation in the voids derived by compacting GaNpowder. In this work, we did not use any catalysts orassistance materials that can work as a seed of nanomaterialgrowth. The growth of nanocolumns on the surface of thesilicon substrate might be explained by the following process.Vapor is condensed and then nucleated on the silicon surface.As the nucleated nanomaterials grow further, they begin tooverlap and impinge on other neighboring crystals, givingrise to nano- or microcolumns. Synthesized substrate alsoconsists of comparatively thin nanomaterials (see the arrowmarked in figure 3(a)) with spherical cross section. Thediameter and length of the products are in the range of70–150 nm and 1–3 µm, respectively. We suggest that theproduct can be classified as a ‘nanorod’. We suppose thatthe nanorods could be formed from isolated nucleation fromother neighboring nucleation. Various kinds of morphologieswere found at the substrate synthesized for 20 min, as shownin figure 3(b). The nanorods and nanocolumns shown infigure 3(a) were also found at the substrate synthesizedfor 20 min and their morphology and size are similar tothose of figure 3(a). Besides nanorods and nanocolumns,nanowires with a diameter of 30–70 nm and nanobelts witha width of 200–400 nm were found locally. Figure 3(c)shows the resulting materials synthesized for 30 min. Thesubstratum morphology of the sample is mostly the same asthe 20 min synthesized result, but partially there was a denselyaggregated region that is composed of various kinds (mostlynanobelts) of nanomaterials. The nanobelts and nanowires inthis region are very long compared to the 20 min synthesizednanomaterials, and some materials are even several hundredmicrometers. Moreover, many nanosheets with a width ofone to several micrometers were found. But except for thisregion, it seemed that there is no lateral growth after 20 minby examining the morphology of the substratum.

To verify the nucleation site of the above-mentionednanomaterials, a more detailed investigation of FE-SEMwas carried out. Figure 4 displays various growth casesof the nanomaterials. The nucleation points of nanowires,nanorods and nanobelts are taken at the sample synthesizedfor 20 min, and nanosheets are obtained from the 30 minsynthesized substrate. Three nucleation cases of nanowires,which are derived from the tip of the nanobelt, the sideedge of the nanobelt and the side edge of the nanocolumn,were detected, as shown in figures 3(a)–(c). The diameters ofnanowires ranged from 30 to 70 nm and the lengths ranged

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Phys. Scr. T139 (2010) 014079 K K Cho et al

Figure 4. FE-SEM images of the nucleation site of nanomaterials synthesized at 900 ◦C for (a–i) 20 min and (j–l) 30 min: (a) a nanobeltderived from the tip of the nanobelt, (b) a nanowire synthesized from the side edge of the nanobelt, (c) a nanowire and nanorod synthesizedfrom the side edge of the nanocolumn, (d) a nanorod derived from the tip edge of the nanocolumn, (e) nanorods grown from the surfaceof substrate directly, (f) one piece of nanowire, nanobelt and nanorod synthesized from the side plane of one nanocolumn, (g) a nanobeltsynthesized from the tip edge of the nanocolumn, (h) a nanobelt and sheet synthesized from the side edge and tip edge of the nanobelt,(i) a nanobelt grown from the side plane of the nanocolumn, (j) a squared nanosheet synthesized by connecting two pieces of nanocolumn,(k) a rectangular nanosheet synthesized from the side plane of the nanobelt and (l) triangular nanosheets derived from the side plane of thenanobelt.

from 10 to several tens of micrometers. Three nucleationcases of nanorods, which are grown from the tip edge of thenanocolumn, the surface of the substrate directly and the sideplane of the nanocolumn, are shown in figures 4(d)–(f). Theirdiameters are approximately similar to those of nanowires,but we distinguish nanorods from nanowires by measuringthe length. This is because it seemed that there is no lateralgrowth on the nanorod after 20 min. These nanorods have verystraight shape and are about a few micrometers long. In thecase of nanobelts, there are three nucleation sites, the tip edgeof the nanocolumn, the side plane or tip edge of the previouslyformed nanobelt and the side edge of the nanocolumn, asshown in figures 4(g)–(i). Nanosheets were mostly createdfrom the side edge of the nanocolumn and nanobelt, as shownin figures 4(j) and (l). Some nanosheets were generated fromthe tip edge of the nanocolumn and grown into a squaredsheet shape by connecting the other adjacent grown sheet,as shown in figure 4(j). The widths of these nanosheetsin this work varied from 500 nm to several micrometers.The thicknesses of the nanosheets varied in the range of3–30 nm; hence they are mostly opaque or translucent, butsome of them are so thin that they appear transparent underan electron beam. Each white arrow in figure 4 indicates the

nucleation points of various nanomaterials. We suppose thatnew nucleation sites are formed on the surface, edge andtip of previously grown nanomaterials by condensation ofcontinuously supplied gallium sources, and it provides againa nucleation site of new nanomaterials. But the relationshipbetween a new nucleation site and the morphology of grownnanomaterials has not yet been properly investigated in thiswork, and is in progress. Figure 5 depicts the TEM images ofa nanosheet. A low magnified image of β-Ga2O3 nanomaterialreveals that its geometrical shape is plate-like morphologywith a width of 500 nm, as shown in figure 5(a). Moredetailed crystal structure was investigated by HR mode andSAED. The HRTEM image and the SAED pattern of theGa2O3 nanobelt displayed in figures 5(b) and (c) indicatethat the nanobelt is single crystalline with no defects. Thecorresponding SAED pattern in figure 5(c) with a [111] zoneaxis also reveals that the nanobelt is monoclinic β-Ga2O3 andits growth direction is 〈110〉.

Since we did not use a catalyst, we assume that all typesof nanomaterials found in this work have been grown by theVS mechanism, and we suggest that there is a typical growthorder of the nanomaterials by evaluating figures 4 and 5.

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Phys. Scr. T139 (2010) 014079 K K Cho et al

Figure 5. (a) TEM image of the nanosheet synthesized from the side edge of the nanorod, (b) HRTEM image of (a) and (c) SAED patternof (b).

So to speak, various kinds of nanomaterials were synthesizedin order of

1. Nanocolumns and nanorods directly grown from thesurface of the substrate.

2. Nanobelts/rods/wires synthesized from the tip edge of thenanocolumn.

3. Nanobelts/sheets synthesized from the side edge of thenanocolumn.

4. Nanowires/rods/belts/sheets derived from previouslyformed nanomaterials.

In previous studies, more attention was paid to thesynthesis and microstructure analysis of Ga2O3 nanomaterialswith various morphologies, while a few were related tothe nucleation site grown by the VS mechanism. Thepresent work reveals the nucleation sites of various β-Ga2O3

nanostructures, and indicates useful academic information forunderstanding the growth behavior and synthesis of one- ortwo-dimensional nanostructures with various morphologies.

4. Conclusions

In summary, various kinds of one- or two-dimensional Ga2O3

nanomaterials have been prepared by simple evaporationof Ga powder without any catalyst in argon ambient.We could define that the various nanomaterials weresynthesized in order of (i) nanocolumns and nanorodsgrown from the surface of the substrate directly, (ii)nanobelts/nanorods/nanowires synthesized from the tipedge of the nanocolumn, (iii) nanobelts/sheets synthesizedfrom the side edge of the nanocolumn and finally (iv)nanowires/rods/belts/sheets derived from previously formednanomaterials, in sequence. The diameters of nanocolumnsranged regularly within 300–500 nm and lengths were around3 µm, the diameters of nanowires ranged from 30 to 70 nmand lengths were in the range from 10 to several tensof micrometers, and the diameters of nanorods were inthe range 70–150 nm and lengths were ∼ 3 µm, the widthsof nanobelts and nanosheets were 200–400 nm and one toseveral micrometers, respectively. We found that previouslyformed nanomaterials provide a new nucleation site. Wealso noticed that the preformed nanocolumn can be a goodnucleation site of various kinds of nanomaterials. The growth

mechanism of referred nanomaterials in this work wascontrolled by the VS mechanism. We suppose that the resultsof the nucleation site on various nanomaterials can be usefulacademic information for understanding the growth behaviorand synthesis of one- or two-dimensional nanostructures withvarious morphologies.

Acknowledgment

This work was supported by the National ResearchFoundation (KRF) grant funded by the Korea government(MEST) (no. 2009-0083818) and second stage of BK21.

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