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20. Supported by NSF grant DMR 9984102 and DOE

Nanobelts of Semiconducting
OxidesZheng Wei Pan,1 Zu Rong Dai,1 Zhong Lin Wang1,2*
Ultralong beltlike (or ribbonlike) nanostructures (so-called nanobelts) weresuccessfully synthesized for semiconducting oxides of zinc, tin, indium, cadmium, and gallium by simply evaporating the desired commercial metal oxide
powders at high temperatures. The as-synthesized oxide nanobelts are pure,
structurally uniform, and single crystalline, and most of them are free fromdefects and dislocations. They have a rectanglelike cross section with typical
widths of 30 to 300 nanometers, width-to-thickness ratios of 5 to 10, andlengths of up to a few millimeters. The beltlike morphology appears to be a
distinctive and common structural characteristic for the family of semiconducting oxides with cations of different valence states and materials of distinct
crystallographic structures. The nanobelts could be an ideal system for fully
understanding dimensionally confined transport phenomena in functional oxides and building functional devices along individual nanobelts.
Binary semiconducting oxides, such as ZnO,
SnO2, In2O3, and CdO, have distinctive properties and are now widely used as transparent
conducting oxide materials (1) and gas sensors(2). For example, fluorine-doped SnO2 film iswidely used in architectural glass applications
because of its low emissivity for thermal infrared heat (1). SnO2 nanoparticles are regarded as
one of the most important sensor materials for
detecting leakage of several inflammable gases owing to their high sensitivity to low gas
concentrations (2). Tin-doped indium oxide(In2O3:Sn, ITO) film is an ideal material for flat
panel displays because of its high electrical conductivity and high optical transparency (1), andZnO is regarded as an ideal alternative material
for ITO because of its lower cost and easieretchability (1). The current studies of semiconducting oxides have been focused on two-dimensional films and zero-dimensional nanoparticles, which can be readily synthesized withvarious well-established techniques such as
sputtering (for films) and sol-gel (for particles).
In contrast, investigations of wirelike semiconducting oxide nanostructures are cumbersome
because of the unavailability of nanowire
structures.
As stimulated by the novel properties of
School of Materials Science and Engineering, 2Schoolof Chemistry and Biochemistry, Georgia Institute ofTechnology, Atlanta, GA 30332– 0245, USA.
1*To whom correspondence should be addressed. Email: zhong.wang@mse.gatech.edu

carbon nanotubes, wirelike nanostructures haveattracted extensive interest over the past decade
because of their great potential for addressing
some basic issues about dimensionality andspace-confined transport phenomena as well as
applications (3). Besides nanotubules (4, 5),
many other wirelike nanomaterials, such as carbides [SiC (6–8) and TiC (6)], nitrides [GaN (9,10) and Si3N4 (11)], compound semiconductors(12, 13), element semiconductors [Si (14–16)
and Ge (14)], and oxide [Ga2O3 (17) and MgO(18)] nanowires, have been successfully fabricated. In geometrical structures, these nanostructures can be classified into two maingroups: hollow nanotubes and solid nanowires,
which have a common characteristic of cylindrical symmetric cross section. Here, we report
another group of distinctly different semiconducting oxide nanostructures that have a rectangular cross section, in correspondence to a beltlike (or ribbonlike) morphology. The oxides
with the nanobelt morphology cover cations
with different valence states and materials withdifferent crystallographic structures, and it
seems to be a common structural characteristic
for the family of semiconducting oxides.
Our synthesis is based on thermal evaporation of oxide powders under controlled conditions without the presence of catalyst (19). The
desired oxide powders were placed at the centerof an alumina tube that was inserted in a horizontal tube furnace, where the temperature,pressure, and evaporation time were controlled.
In our experiments, except for the evaporation

grant DEFG0286ER45237.016 (B.C.), by a FeodorLynen Fellowship from the Alexander von Humboldt Foundation (C.B.), and by a Monbusho Grantin-Aid for Scientific Research on Priority Area (Innovative Synthetic Reactions) and the Japan Society for the Promotion of Science–NSF Exchange
Program.
21 November 2000; accepted 8 February 2001

temperature, which was determined on the basisof the melting point of the oxides used, we kept
the following parameters constant: evaporation
time, 2 hours; chamber pressure, 300 torr; andAr flowing rate, 50 standard cubic centimeters
per minute. During evaporation, the products
were deposited onto an alumina plate placed atthe downstream end of the alumina tube. The
as-deposited products were characterized andanalyzed by x-ray diffraction (XRD) (PhilipsPW 1800 with Cu K␣ radiation), scanning electron microscopy (SEM) (Hitachi S800 FEG),
transmission electron microscopy (TEM) [Hitachi HF-2000 FEG at 200 kV and JEOL 4000EX
high-resolution TEM (HRTEM) at 400 kV], andenergy-dispersive x-ray spectroscopy (EDS).
Thermal evaporation of ZnO powders (purity: 99.99%; melting point: 1975°C) at 1400°C
for 2 hours resulted in white woollike productsthat formed in high yield on the surface of thealumina plate. SEM observations reveal that theproducts consist of a large quantity of wirelikenanostructures with typical lengths in the rangeof several tens to several hundreds of microm-Fig. 1. Ultralong nanobelt structure of ZnO(with wurtzite crystal structure). (A) SEM imageof the as-synthesized ZnO nanobelts obtained
from thermal evaporation of ZnO powders at1400°C. (B) XRD pattern recorded from theZnO nanobelts.

www.sciencemag.org SCIENCE VOL 291 9 MARCH 20011947REPORTSeters; some of them even have lengths on
the order of millimeters (Fig. 1A). EDS
microanalysis and powder XRD measure-ment (Fig. 1B) show that the sample iswurtzite (hexagonal) structured ZnO withlattice constants of a ⫽ 3.249 Å and c ⫽Fig. 2. TEM and HRTEM
images of ZnO nanobelts
showing their geometrical shape. (A to C) TEM
images of several straightand twisted ZnO nanobelts, displaying theshape characteristics ofthe belts. (D) Cross-sectional TEM image of aZnO nanobelt, showinga rectanglelike cross section with width-to-thickness ratio of ⬃9. The
cross-sectional TEM specimen was prepared by slicing nanobelts embedded
in epoxy with an ultramicrotone. (E) TEM image of a nanobelt growing along [011៮ 0], showing only one stacking
fault present in the nanobelt; this type of nanobelt is the only one thathas a defect. The inset
shows the electron diffraction pattern. (F) HRTEM image from box 1 in(E), showing a clean andstructurally perfect surface (the amorphouscontrast seen above the surface is from the amorphous carbon film used to support the nanobelt for
TEM imaging). (G) HRTEM image from box 2 in (E), showing the stacking fault.
Fig. 3. Superlong nanobelt structure of SnO2
(with rutile crystal structure). (A) SEM image ofthe as-synthesized SnO2
nanobelts, showing ahigh volume percentage(⬎ 95%) of SnO2 nanobelts and a small amountof Sn nanoparticles as indicated by arrows. (B toD) TEM images of SnO2
nanobelts with straightand twisted shapes. An
enlargement of a broken
nanobelt is inserted in (B)to display the rectanglelike cross section of thebelt. The beltlike shape isfurther verified by an enlargement of the boxedregion in (C) as redisplayed in (D); the widthto-thickness ratio is ⬃5.
(E) HRTEM image of a
SnO2 nanobelt showingthat the nanobelt is single crystalline and freefrom dislocation and defects. (Inset) The corresponding electron diffraction pattern recorded with electron beam perpendicular to the long axis of abelt, showing the growth direction to be [101] (note the rutile structure of the sample). The SnO2nanobelts shown in (A), (B), and (E) were obtained from thermal evaporation of SnO2 powders at1350°C; the SnO2 nanobelts shown in (C) and (D) were obtained from thermal evaporation of SnOpowders at 1000°C, but they preserve the same crystal structure and same growth morphology.

1948

5.206 Å, consistent with the standard values
for bulk ZnO (20).
TEM images reveal that the geometricalshape of the ZnO nanostructures is a belt (Fig. 2,A to C) that is distinct in cross section from thepreviously reported nanotubes (4, 5) and nanowires (6–18). Each nanobelt has a uniform widthalong its entire length, and the typical widths ofthe nanobelts are in the range of 50 to 300 nm.
No particle was observed at the ends of thenanobelts. A ripplelike contrast observed in theTEM image is due to strain resulting from thebending of the belt. To further verify the morphological characteristic of the ZnO nanobelts, across-sectional TEM image taken from a nanobelt is given in Fig. 2D, which exhibits a rectanglelike cross section. The typical thickness
and width-to-thickness ratios of the ZnO nanobelts are in the range of 10 to 30 nm and ⬃5 to10, respectively, as determined from several
tens of nanobelts. HRTEM and electron diffraction show that the ZnO nanobelts are structurally uniform and single crystalline but with two
different growth directions. The nanobelt, growing along [0001] and enclosed by ⫾(21៮ 1៮ 0) and⫾(011៮ 0) facets, shows no defect and no dislocation; the one growing along [011៮ 0] and enclosed by ⫾(0001) and ⫾(21៮ 1៮ 0) facets is also
dislocation free but with only a single stackingfault that is parallel to the axis and runs throughout the entire length of the nanobelt (Fig. 2, Eand G). The surfaces of the nanobelts are clean,
atomically sharp, and without any sheathedamorphous phase (Fig. 2F).
The beltlike geometrical morphology is acommon structural characteristic for functionaloxides with different crystallographic structures,at least for the ones we studied. For example,
single crystalline SnO2 nanobelts of rutile structure can consistently be synthesized by thermal
evaporation of either SnO2 powders (purity:99.9%; melting point: 1630°C) at 1350°C orSnO powders (purity: 99.9%; melting point:1080°C) at 1000°C. After evaporation, similar
white fuzzlike products were deposited on thealumina plate, whether the source material was
SnO2 or SnO. SEM imaging (Fig. 3A) and EDS
analysis show that the products are composed oflarge quantities of ultralong SnO2 nanobelts(with lengths up to on the order of millimeters)
and a small fraction of Sn nanoparticles (asindicated by arrows in Fig. 3A) (21). XRD
patterns from the as-synthesized nanobelt samples show the rutile type structure with lattice
constants of a ⫽ 4.722 Å and c ⫽ 3.184 Å,
which are consistent with those of bulk SnO2(20). TEM images (Fig. 3, B to D) display thecharacteristic shape of the SnO2 nanobelts (notethe rectanglelike cross section of a broken nanobelt in Fig. 3B and a side view of a nanobelt
displayed in Fig. 3D). The ripplelike strain contrast can also be seen in SnO2 nanobelts. Eachnanobelt is uniform in width and thickness, andthe typical widths of the SnO2 nanobelts are in
the range of 50 to 200 nm. Cross-sectional TEM

9 MARCH 2001 VOL 291 SCIENCE www.sciencemag.org

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observations show that the cross sections of theSnO2 nanobelts are rectanglelike, with typical
width-to-thickness ratios of ⬃5 to 10. An HRTEM image (Fig. 3E) reveals that the nanobelts
are single crystalline and dislocation free. The
electron diffraction pattern (inset in Fig. 3E)
indicates that the SnO2 nanobelt grows along[101] and it is enclosed by ⫾(010) and ⫾(101៮ )crystallographic facets.
Nanobelts of In2O3 with C–rare earth crystal
structure (20) were also synthesized by ourmethod (Fig. 4A). The evaporation of In2O3
powders (purity: 99.99%; melting point:⬃1920°C) at 1400°C yields In2O3 nanobelts.
TEM observations show that most of the In2O3
nanobelts have uniform width and thickness
along their lengths (Fig. 4B). However, somenanobelts exhibit a sharp shrinkage in width
while the thickness is preserved and form abridgelike structure (Fig. 4C), which may be a
candidate for transport measurement from anoxide nanobridge. Typically, the In2O3 nanobelts have widths in the range of 50 to 150 nm
and lengths of several tens to several hundredsof micrometers. Electron diffraction (inset, Fig.
4B) analysis shows that the In2O3 nanobelts are
single crystalline and grow along 具100典, thesurfaces being enclosed by {100}.
Nanobelts of CdO with NaCl cubic structure(20) were also synthesized by evaporating CdO
powders (purity: 99.998%; melting point:1430°C) at 1000°C. Besides CdO nanobelts,
many single crystalline CdO sheets with sizes
Fig. 4. Ultralong, high-purity nanobelts of (A to C)
In2O3 (with C–rare earthcrystal structure) and (D toG) CdO (with NaCl cubic
crystal structure). (A) SEM
image of the as-synthesized
In2O3 nanobelts. (B) TEM
image of the In2O3 nanobelts and an electron diffraction pattern (inset),showing the single crystalline, defect-free structureof the nanobelts. The nanobelts grow along [100]. (C) Ananobelt with an abruptly
reduced width. This structure is frequently observedfor In2O3. (D) SEM image ofthe as-synthesized CdO
nanobelts and sheets. (Eand F) TEM images and acorresponding electron diffraction pattern of the single crystalline CdO nanobelts growing along [100].
(G) A broken CdO nanobelt
as a result of focused electron beam illumination.

on the order of several to several tens of micrometers were also formed (Fig. 4D). TheseCdO sheets usually have such shapes as rectangle, triangle, and parallelogram. The lengths ofthe CdO nanobelts are usually less than 100 ␮m,
and their widths are typically 100 to 500 nm(Fig. 4, E and F), substantially wider and shorter
than those of ZnO, SnO2, and In2O3 nanobelts.
As a result, the width-to-thickness ratios of CdO
nanobelts are usually greater than 10. The electron diffraction pattern (inset in Fig. 4F) showsthat the nanobelts grow along [100] and theirsurfaces are enclosed by ⫾(001) and ⫾(010)
facets. The symmetry patterns observed in thebright-field TEM images (Fig. 4F) are the bending contour in electron diffraction due to thebending of the nanobelt (22). Some nanobelts
were broken into two halves (Fig. 4G) during
TEM observation because of electron beam illumination, which is likely to be caused by theeasy cracking characteristic of the NaCl-typeionic structure of the nanobelt. Thus, it may bepossible to cut these nanobelts with a focusedelectron or ion beam, so that nanobelts withspecific lengths for nanodevice applications
could be fabricated.
Our studies also demonstrated that, besides
the above-mentioned four oxides that were usually regarded as the transparent conducting oxides, the beltlike nanostructures were also obtained from other oxide materials, such as
Ga2O3 (23) and PbO2.
Our TEM analysis shows that the growth of

the nanobelts may not be dominated by thevapor-liquid-solid (VLS) process (24) proposed
for the nanowires grown by a catalytic-assisted
technique (12–14, 25), in which a metal particle
is located at the growth front of the wire and acts
as the catalytic active site. Because the only
source material used in our synthesis is pureoxide powders and the composition of the oxide
nanobelts is similar or identical to the startingoxide, it is likely that the growth is governed bya vapor-solid process (18, 26), in which theoxide vapor, evaporated from the starting oxide
at a higher temperature zone, directly deposits
on a substrate at a lower temperature region andgrows into beltlike nanostructures.
The beltlike morphology is distinct fromthose of semiconductor nanowires. With a welldefined geometry and perfect crystallinity, thesemiconducting oxide nanobelts are likely to bea model materials family for a systematic experimental and theoretical understanding in the fundamental electrical, thermal, optical, and ionictransport processes in wirelike nanostructureswith the absence of dislocations and defects.
The semiconducting oxide nanobelts could be
doped with different elements and used for fabricating nanosize sensors based on the characteristics of individual nanobelts.
References and Notes

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19. A provisional patent has been filed by Georgia Tech
Research Cooperation.
20. F. S. Galasso, Structure and Properties of Inorganic
Solids (Pergamon, New York, 1970).
21. The Sn nanoparticles are adhered on the surfaces ofthe belts rather than at the ends, in contrast to thenanowires prepared by VLS reaction (12–14) thatalways terminate at one end with a catalytic nanoparticle. The formation of the Sn nanoparticles isbelieved to be related to the decomposition of theSnO2 or SnO vapor.
22. P. B. Hirsch, A. Howie, R. B. Nicholson, D. W. Pashley,M. L. Whelan, Electron Microscopy of Thin Crystals(Krieger, New York, 1977).
23. The Ga2O3 nanobelts can be prepared by either
heating mixed Ga⫺Ga2O3 powder at 1000°C or
heating GaN powders at 950°C. The products prepared from the later reaction consists of very longsingle crystalline Ga2O3 nanobelts and microscale
single crystalline Ga2O3 sheets.
24. R. S. Wagner, W. C. Ellis, Appl. Phys. Lett. 4, 89 (1964).
25. T. J. Trentler et al., Science 270, 1791 (1995).
26. G. W. Sears, Acta Metall. 3, 268 (1956).
8 December 2000; accepted 1 February 2001

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