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A ﬂux induced crystal phase transition in thevapor–liquid–solid growth of indium-tin oxide
nanowires†
Gang Meng,a Takeshi Yanagida,*a Hideto Yoshida,a Kazuki Nagashima,a Masaki Kanai,aFuwei Zhuge,a Yong He,a Annop Klamchuen,a Sakon Rahong,a Xiaodong Fang,bSeiji Takedaa and Tomoji Kawaia
Single crystalline metal oxide nanowires formed via a vapor–liquid–solid (VLS) route provide a platform not
only for studying fundamental nanoscale properties but also for exploring novel device applications.
Although the crystal phase variation of metal oxides, which exhibits a variety of physical properties, is aninteresting feature compared with conventional semiconductors, it has been diﬃcult to control the
crystal phase of metal oxides during the VLS nanowire growth. Here we show that a material ﬂux
critically determines the crystal phase of indium-tin oxide nanowires grown via the VLS route, although
thermodynamical parameters, such as temperature and pressure, were previously believed to determine
the crystal phase. The crystal phases of indium-tin oxide nanowires varied from the rutile structures(SnO2), the metastable ﬂuorite structures (InxSnyO3.5) and the bixbyite structures (Sn-doped In2O3) when

Received 24th February 2014
Accepted 3rd April 2014

only the material ﬂux was varied within an order of magnitude. This trend can be interpreted in terms ofthe material ﬂux dependence of crystal phases (rutile SnO2 and bixbyite In2O3) on the critical nucleation
at the liquid–solid (LS) interface. Thus, precisely controlling the material ﬂux, which has been

DOI: 10.1039/c4nr01016g

underestimated for VLS nanowire growths, allows us to design the crystal phase and properties in the

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VLS nanowire growth of multicomponent metal oxides.

1

Introduction

Single crystalline nanowires composed of metal oxides oﬀer aninteresting research platform not only for investigating fundamental nanoscale properties of metal oxides but also for
exploring novel engineering applications by utilizing the uniqueand robust properties in air and/or water.1–4 Among various
nanowire growth methods, a vapor–liquid–solid (VLS) method
has proven great potential to synthesize well-dened single
crystalline nanowires. In the VLS nanowire growth, the size andspatial position of nanowires are readily controllable byadjusting the size and spatial position of the metal catalyst.5–7 In
addition, the heterostructures along axial or radial directions
can be sequentially designed by using the VLS process.8–11 These
unique and fascinating features of the VLS method are notattainable by other nanowire growth methods.12aInstitute of Scientic and Industrial Research, Osaka University, 8-1 Mihogaoka,
Osaka, Ibaraki, Japan. E-mail: yanagi32@sanken.osaka-u.ac.jp; Fax: +81-06-68794295; Tel: +81-06-6879-4294
bAnhui Institute of Optics and Fine Mechanics, Chinese Academy of Sciences, Hefei
230031, China† Electronic supplementary information (ESI) available: STEM mapping of In and
Sn in an ISO nanowire (Fig. S1) and homogeneity of a uorite ISO phase at the fulllength of a nanowire (Fig. S2). See DOI: 10.1039/c4nr01016g

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Metal oxides exhibit various fascinating physical properties
including ferroelectric, ferromagnetism, transparent conducting, superconducting and others, which are not attainable byconventional semiconductors.13–16 The crystal phase variation of
metal oxides plays an important role on a variety of suchfascinating physical properties. The crystal phase variation foroxide thin lms has been controlled by varying the surrounding
thermodynamical conditions including temperature and pressure.17,18 The crystal phase variation should also be appropriately controlled for the VLS nanowire growth of metal oxides toattain the desired physical properties. Unfortunately, the wide
range control of thermodynamical conditions, which has been
applied to oxide thin lm formations, is not applicable to the
VLS nanowire growth due to the narrow window of surrounding
conditions required for the VLS nanowire growth.19 In thisstudy, we propose a strategy to control the crystal phase of metaloxides during VLS nanowire growth. We demonstrate that amaterial ux critically determines the crystal phase stability
during the VLS nanowire growth of indium-tin oxides. Although
thermodynamical conditions were previously believed todetermine the crystal phase of VLS nanowires, we found that theslight ux diﬀerence induces the completely diﬀerent crystal
phases of indium-tin oxide nanowires.

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2 Experimental section
Indium-tin oxide nanowires were fabricated by the vapor–liquid–solid (VLS) method using pulsed laser deposition(PLD).19–25 Prior to the nanowire growth, the chamber was
evacuated to a base pressure of 5  106 Pa. Then the oxygen
and Ar mixed gas (the molar ratio of O2 : Ar ¼ 1 : 106) was
introduced into the chamber. The total pressure of the chamber
was maintained to be 10 Pa. Aer heating the MgO(100)substrate (with 0.7 nm Au layer) up to the growth temperature of
750  C, the SnO2 and In2O3 mixed target (the molar ratio of
Sn : In ¼ 80 : 20) was ablated using an ArF excimer laser (l ¼
193 nm, repetition rate ¼ 10 Hz). The supplied metal uxes were
controlled by tuning the laser power from 25 mJ to 60 mJ. We
analyzed the supplied metal uxes by measuring the volume ofthin lms deposited under room-temperature, where the evaporation events on the substrate were negligible. The volume ofthin lms was measured using a surface proler (Alpha-Step
IQ), and then the metal uxes were estimated by the material
density.19 The compositions of supplied uxes were measured
using an electron probe micro-analyzer (EPMA, JEOL, JXA8800R). Further details of the nanowire growth process can beseen elsewhere.24,25 The morphologies of nanowires wereobserved using the eld emission scanning electron microscope(FESEM, JEOL, JSM-7001F). The crystallinity, phase and element
concentration of nanowires were examined using the highresolution transmission electron microscope (HRTEM, JEOLPaper

JEM-ARM200F), equipped with energy dispersive spectroscopy(EDS) and selected area electron diﬀraction (SAED). The transport properties of nanowires were measured with the semiconductor parameter analyzer (Keithley 4200SCS).

3 Results and discussion
Fig. 1 shows the eﬀect of material ux on the SEM, TEM, SAEDand EDS data of fabricated indium-tin oxide nanowires. The
material ux was varied from 2  1017 to 16  1017 (metal atomscm2 s1) within an order of magnitude, and the other conditions were set to be constant. These nanowires were grown by
using the tin-rich target of Sn : In in the molar ratio of 80 : 20.
When the total material ux was relatively low – 2 1017 cm2 s1, the fabricated nanowires only showed the rutile
crystal phase (space group: P42/mnm) of SnO2 without thesignicant indium component, as shown in EDS data of Fig. 1a.
When we increased the material ux to 6  1017 cm2 s1, thefabricated nanowires exhibited the metastable uorite crystal
m) of InxSnyO3.5 (so-called ISO phase26).
phase (space group: Fm3
The uorite phase was conrmed by electron diﬀraction analysis taken along [001] and [011] directions, as shown in theupper and lower SAED data in Fig. 1b. Note that these nanowires are entirely composed of the metastable ISO crystal phase,
which is in sharp contrast to previous reports that showed
the partial ISO crystal phase within nanowires.24,26 The
detailed microstructural analysis of ISO nanowires can be seenFig. 1 SEM, TEM, HRTEM, SAED and EDS data of indium-tin oxide nanowires when varying the supplied Sn/In ﬂuxes: (a) data for the ﬂux of 2 1017 cm2 s1. (b) Data for the ﬂux of 6  1017 cm2 s1, the upper ED data were taken along [001] and the lower data were taken along the [011]orientation. (c) Data for the ﬂux of 16  1017 cm2 s1, the upper ED data were taken along the [001] and the lower one was taken along the [011]orientation.

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in ESI – Fig. S1.† When the material ux was further increased
to 16  1017 cm2 s1, the fabricated nanowires were of abixbyite crystal phase (space group: Ia3) of the conventional Sndoped In2O3 (ITO), as shown in Fig. 1c. Thus, interestingly thematerial ux strongly inuences the crystal phase of fabricatednanowires even when using the constant of other experimental
congurations for VLS nanowire growths. Although the tin
concentration of the target source was constant for all VLS
nanowire growths, the tin concentration incorporated into
nanowires was signicantly diﬀerent when varying only thematerial uxes, as seen in the EDS data of Fig. 1. Fig. 2 showsthe eﬀect of material ux on the series of tin concentration data
incorporated into nanowires, which were measured by TEMEDS. We dene the tin concentration (%) as (Sn/(Sn + In)) 100. Other growth conditions were the same as those for thegrowth experiments of Fig. 1. Below 4  1017 cm2 s1 of thematerial ux, the tin concentration was almost 100% with therutile crystal phase of SnO2. In the middle material ux range(6  1017–14  1017 cm2 s1), the tin concentration was around
30% with the uorite crystal phase of metastable ISO. For thehigher material ux range – 16  1017 cm2 s1, the tin
concentration decreased to around 20% with the bixbyite crystal
phase of ITO. It is noted that these structural and composition
variations occur within an order of magnitude of the material
ux. These experimental data highlight that the material ux,
which was previously believed to aﬀect solely the growthrate,19,27 determines the crystal phase of nanowires formed via
the VLS route.
Here, we discuss the inherent mechanisms of how thematerial ux aﬀects the crystal phase of indium-tin oxide
nanowires during the VLS growth. Firstly, we examine thecomposition diﬀerence between the target source and thematerial species supplied onto the substrate when the material

Nanoscaleux was varied. This is because the material composition
supplied from the target might be altered when the laser powerof ablation is varied to change the material ux. We measured
the material composition supplied onto the substrate by performing the composition measurements of lms depositedunder room temperature, where the evaporation events are
negligible.19,28 As shown in the inset of Fig. 2, the measured tin
concentration was nearly consistent with the nominal target
composition (80%) independent of the material ux variation.
Thus, we can exclude the scenario based on the composition
variation of the supplied species on the crystal phase variationin the VLS nanowires of indium-tin oxides.
Next, we consider the nucleation probability of indium andtin at the liquid–solid (LS) interface to explain the crystal phasevariations of nanowires. This is because, in principle, the crystal
nucleation at the LS interface determines the crystal phase of
VLS grown nanowires.29 When both indium and tin elements
exist within the metal catalysts,30 the crystals formed at the LSinterface can be various crystal phases comprised of indium, tin
and oxygen. If the diﬀerent crystal phases have diﬀerent criticalmaterial uxes for nucleation,31 the critical ux dependence of
diﬀerent crystal phases might explain the present experimental
results of Fig. 1 and 2.
Although the complete understanding requires the knowledge as to the thermodynamics and kinetics of nucleation of allcomponents of Sn–In–O–Au (including the phase diagram),such information of multicomponent systems is unfortunately
not available. As an alternative experimental approach to obtainan insight into the nucleation events of tin and indium
elements at the LS interface, we measured the critical material
ux for the nucleation at the LS interface of the parent
compounds – In2O3 and/or SnO2. We measured the growth rateof nanowires when varying the material uxes, as shown in

Fig. 2 The incorporated tin concentration data of nanowires, measured by EDS, when increasing the supplied Sn/In ﬂux. The detailed crystal
structural information of obtained single crystalline nanowires was shown on the right side. The inset ﬁgure shows the tin concentration data ofthin ﬁlms deposited at room temperature which were measured by EPMA.

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NanoscaleFig. 3 Critical ﬂuxes for nucleation of the parent compounds (bixbyite
In2O3 and rutile SnO2 nanowires) when varying the material ﬂuxes.

Fig. 3. The x-axis of Fig. 3 is the metal ux of each purecomponent, whereas the x-axis of Fig. 2 is the total metal ux ofthe mixed supply. The same experimental congurations

Paper(temperature, pressure and others) in Fig. 1 and 2 were
employed for these nanowire growth experiments. As can beseen, the critical ux for the nucleation of In2O3 nanowires islower than that for SnO2 nanowires. This diﬀerence is due totheir diﬀerent energy barrier heights for the nucleation at the LSinterface, as discussed in previous studies.24,31 This experimental trend of Fig. 3 can be utilized to semi-quantitatively
explain the crystal phase variation of Fig. 1 and 2 in the VLS
growth in terms of the critical material ux values, as given
below.
For the total mixed ux range below 4  1017 cm2 s1 (i.e., 0.8
 1017 cm2 s1 for indium and 3.2  1017 cm2 s1 for tin), only
the SnO2 crystal phase can nucleate at the LS interface. This isbecause the material ux for tin only exceeds the critical ux for
SnO2, whereas the material ux for indium is still below thecritical ux for the In2O3 crystal phase. In this range, the supplied
indium species must evaporate to the vapor phase during the VLS
growth. In addition, indium species within Au catalysts cannot be
incorporated into rutile SnO2 nanowires due to low solubility.24
For the total mixed ux range around 4–6  1017 cm2 s1, thematerial ux for indium can exceed the critical ux for In2O3
when considering the use of the present indium 20% target. i.e.,
the indium ux is 0.8–1.2  1017 cm2 s1, which is interestingly
in good agreement with the critical ux for In2O3 nanowires inFig. 4 Transport properties of single crystalline ISO nanowires. (a) A schematic image of the transport measurement system for ISO nanowires.
(b) A typical I–V curve of ISO nanowires using the four-probe measurement system. The inset ﬁgure shows the resistivity distribution data, andthe inset image shows the SEM data of the fabricated measurement system. (c) Typical temperature dependence data of resistivity (r–T) of ISO
nanowires and (d) Field eﬀect measurement data. The inset shows the conﬁguration of a ﬁeld eﬀect transistor (FET).

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Fig. 3. Thus, once the nucleation for In2O3 at the LS interface
emerges, indium-rich uorite ISO nanowires were formed via the
VLS route. In addition, our experimental results demonstrate thatthe emerged nucleation of In2O3 fully suppressed the growth of
the SnO2 rutile phase at the LS interface. The preferential
nucleation of In2O3 at the LS interface can be interpreted in termsof the thermodynamical stability of each compound (e.g. themelting points of In2O3 – 1910  C and SnO2 – 1630  C).24
Considering the thermodynamical stability, tin species within
catalysts can be easily incorporated into the In2O3 – based
compounds rather than forming SnO2 crystal structures at the LSinterface, although the direct experimental evidence as to thephysical events at the LS interface should be examined by in situ
measurement techniques.32,33 Further increasing the mixedmaterial ux above 15  1017 cm2 s1 resulted in the change
from ISO to the ITO crystal phase, which can be qualitatively
explained in terms of the increased indium composition within
nanowires via the increased growth rate of In2O3 compounds at
the LS interface as seen in Fig. 3. Although the exact nucleation
mechanisms of multicomponent systems at the LS interface arestill open questions, the present simple model based on thepresence of diﬀerent critical material uxes for various crystal
phases gives a rigorous qualitative explanation for the materialux dependence on the crystal phase of indium-tin oxide VLS
nanowires. This approach should be applicable to various VLS
growths of multicomponent metal oxides to experimentallydesign the crystal phase.
Finally, we examine the transport properties of newly fabricated metastable ISO single crystalline nanowires. This isbecause the ISO nanowire fabricated in this study is the rst
demonstration that the metastable crystal phase of indium-tinoxides can be extracted as an entire single crystalline entity,34,35
as seen in the ESI – Fig. S2.† In previous works, ISO phases havebeen found as the secondary phase.24,26 Therefore, the physicalproperties of ISO single phases have never been measured todate. We measured the transport properties of fabricated ISO
single crystalline nanowires by utilizing the measurement
system, which has been reported elsewhere.24 As shown in
Fig. 4b, we found that the ISO nanowires are conductive(resistivity range of 104 U cm) with metallic behavior, provedby the resistivity–temperature (r–T) measurement shown in
Fig. 4c. The FET measurements (the back gate of 100 nm thick
SiO2) demonstrate the n-type behavior, as shown in Fig. 4d. In
fact, these trends are not consistent with the predictions of ISO
crystal phase properties by a rst principle calculation, where
they predicted the emergence of a semiconductor with thenarrow band gap for the ISO phase.36 One possible reason forthis is the composition diﬀerence between the present ISO andthat assumed in the calculation. Thus, the present material ux
control method oﬀers a strategy to explore new crystal phases
and their properties of multicomponent metal oxide nanowires
via the VLS route.

4 Conclusion
In summary, we demonstrated that the material ux criticallydetermines the crystal phase of VLS nanowires composed ofThis journal is © The Royal Society of Chemistry 2014

Nanoscaleindium-tin oxides although surrounding thermodynamicalparameters, including temperature and pressure, were previously believed to determine the crystal phase. The crystal phaseof indium-tin oxide nanowires varied from the rutile structure(SnO2), the uorite structure (ISO) and the bixbyite structure(ITO) when only the material ux was varied within an order ofmagnitude. The ux induced crystal phase variation can beinterpreted in terms of the material ux dependence of crystal
phases (rutile SnO2 and bixbyite In2O3) on the crystal nucleation
at the LS interface. The present experimental results highlightthat precisely controlling the material ux, which has been
underestimated, is a key issue to experimentally design the
crystal phase of multicomponent metal oxide nanowires via the
VLS route.

Acknowledgements
This study was supported by NEXT. T. K. was supported by the
FIRST program. A part of this work was supported by the “LowCarbon Research Network (Handai satellite)” of Ministry of
Education, Culture, Sports, Science and Technology (MEXT),
Japan.

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