Uniform and high-quality submicrometer tubes of GaS layered crystals

Article abstract of DOI:10.1063/1.2093924

GaS, group III-VI semiconductor compound, is known to possess a layered structure. In this letter, uniform and high-quality GaS submicrometer tubes have been synthesized via a simple high-temperature thermal reaction route. Each GaS tube is uniform in size, and has length up to tens of microns and outer diameter of ～200-900 nm; some of the tubes are partially filled with liquid metallic Ga rods. Photoluminescence spectrum reveals that the GaS tubes have two strong emission bands centered at ～585 and ～615 nm. Possible reaction processes and a rolling-up growth mechanism of as-grown GaS tubes were briefly discussed.

Full text of DOI:10.1063/1.2093924

Uniform and high-quality submicrometer tubes of GaS layered crystals
J. Q. Hu, Y. Bando, J. H. Zhan, Z. W. Liu, and D. Golberg
Citation: Appl. Phys. Lett. 87, 153112 (2005); doi: 10.1063/1.2093924
View online: http://dx.doi.org/10.1063/1.2093924
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APPLIED PHYSICS LETTERS 87, 153112 ͑2005͒
Uniform and high-quality submicrometer tubes of GaS layered crystals
J. Q. Hua͒
International Center for Young Scientist (ICYS), National Institute for Materials Science (NIMS),
Namiki 1-1, Tsukuba, Ibaraki 305-0044, Japan
Y. Bando, J. H. Zhan, Z. W. Liu, and D. Golberg
Advanced Materials Laboratory, National Institute for Materials Science (NIMS), Namiki 1-1, Tsukuba,
Ibaraki 305-0044, Japan
͑Received 11 May 2005; accepted 22 August 2005; published online 7 October 2005͒
GaS, group III–VI semiconductor compound, is known to possess a layered structure. In this letter,
uniform and high-quality GaS submicrometer tubes have been synthesized via a simple
high-temperature thermal reaction route. Each GaS tube is uniform in size, and has length up to tens
of microns and outer diameter of ϳ200–900 nm; some of the tubes are partially filled with liquid
metallic Ga “rods.” Photoluminescence spectrum reveals that the GaS tubes have two strong
emission bands centered at ϳ585 and ϳ615 nm. Possible reaction processes and a rolling-up
growth mechanism of as-grown GaS tubes were briefly discussed. © 2005 American Institute of
Physics. ͓DOI: 10.1063/1.2093924͔
Following the discovery of a carbon nanotube,1 an active
research has been launched with respect to the formation of
tubular structures in other layered or anisotropic crystals.
Tenne et al.2 succeeded in the preparation of WS2 and MoS2
nanotubes through a reaction of thin tungsten films or Mo2O3
with H2S at elevated temperatures. The results have first im-
plied that the materials possessing a layered structure, other
than graphite, are indeed able to form energetically stable
nanotubes under favorable growth conditions. During the last
decade, various approaches were developed to synthesize a
variety of layered tubes ͑composed of molecular layers͒, in-
cluding BxCyNz,3 BN,4 NiCl2,5 VOx,6 InS,7 MS2 ͑M=Ti, Zr,
Hf, Nb, and Ta͒,8 Bi,9 and ReS2.10 Nowadays, these inor-
ganic tubes with interesting properties and potential applica-
tions constitute an important domain of the nano- and micro-
structural family.
GaS, group III–VI semiconductor compound, is known
to possess a layered structure, which consists of double lay-
ers of metal atoms sandwiched between double layers of
nonmetal atoms, i.e., the layers contain four sheets of gal-
lium and sulfur atoms stacked along the c axis in the se-
quence S–Ga–Ga–S, and there are two layers in the unit cell.
The bonding between two adjacent layers is of the Van der
Waals type, whereas the bonding within a layer is predomi-
nantly covalent.11 Due to the layered nature of GaS, the ex-
istence of the stable tubular structure of this material has
been predicted theoretically.12 Very recently, Hu et al. re-
ported the synthesis of GaS nanotubes by annealing the GaS
lamellar precursor.13 Later Gautam et al. synthesized GaS
nanotubes through a laser and thermally induced exfoliation
method.14 Compared to the GaS bulk crystals, those synthe-
sized GaS nanotubes have a low crystal quality, which would
negatively affect their semiconducting performance in the
real devices. Herein we report on the growth and optical
properties of uniform and high-quality GaS submicrometer
tubes synthesized via a simple high-temperature thermal re-
action route.
The GaS submicrometer tubes were prepared in a verti-
cal high-frequency induction furnace, which was described
in detail elsewhere.15 A typical procedure was performed as
follows: a graphite crucible containing a mixture of Ga2O3,
ZnS, and activated carbon powders was heated at high speed
to 1400–1500 °C and kept at this temperature for 1.5 h, and
then rapidly cooled to room temperature. The whole process
was conducted in an Ar atmosphere under ambient pressure
and a flow rate of 0.5–1.0 L/min.
After the synthesis, a C fiber thermo-insulating layer was
densely covered with a greenish-yellow product. X-ray pow-
der diffraction ͑XRD͒ spectrum, Fig. 1, conforms that as-
synthesized product contains only one crystalline phase,
that is, hexagonal GaS ͓JCPDS ͑30-0576͒; a=3.587 Å and
c=15.492 Å; space group: P63/mmc͔ with a layered struc-
ture. Scanning electron microscopy examination showed that
the GaS products were straight tubes with lengths of up to
ϳ tens of micrometers.
Most of the tubes have uniform diameters and wall
thicknesses throughout whole lengths, as shown in Fig. 2͑a͒,
in which this tube has a length of ϳ90 ␮m. Some tubes have
outer diameters of ϳ600–900 nm and wall thicknesses of
200–300 nm ͓Fig. 2͑b͔͒; the others have diameters of
ϳ200–300 nm and wall thicknesses of ϳ60–100 nm ͓Fig.
2͑c͔͒. In some cases, Fig. 2͑d͒, the GaS tubes are partially
filled with liquid metallic Ga “rods” ͓Ga may remain liquid
͑bulk Ga: mp of 29.8 °C͒ at lower T being confined with a
nanotube͔;16 the Ga-filling occupies more than ϳ40–60% of
the entire tube cavity. A small portion of the tubes has a very
thick wall ͑ϳ300 nm in thickness͒ and a very narrow central
channel ͑ϳ30 nm in diameter͒, as shown in Fig. 2͑e͒. Most
of the tubes appear to display open tip ends, as depicted in
Fig. 2͑f͒. Energy-dispersive x-ray spectroscopy revealed that
the tubes consisted of gallium and sulfur in an atomic ratio
1:1; both the elemental map and spatially resolved elemental
profile directly highlight a tubular geometry of the present
GaS material ͑not shown͒.
Figure 3 shows high-resolution transmission electron mi-
croscopy ͑TEM͒ images and electronic diffraction ͑ED͒ pat-
terns of a GaS tube with the outer and inner diameters of
ϳ250 and ϳ80 nm, respectively. Figure 3͑a͒ is the lattice
a͒
Electronic mail: HU.Junqing@nims.go.jp
0003-6951/2005/87͑15͒/153112/3/$22.50
87, 153112-1
© 2005 American Institute of Physics
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153112-2
Hu et al.
Appl. Phys. Lett. 87, 153112 ͑2005͒
FIG. 1. XRD spectrum of as-grown GaS layered crystals.
image of a GaS tube wall. As seen from the image, the wall
is composed of regularly ordered molecular layers. Figure
3͑b͒ displays an ED pattern taken from the wall. It resembles
a double diffraction pattern, i.e., the electrons scattered by
mutually parallel and very thin layers with the basal planes
perpendicular to the electron beam-͓001͔ zone partially over-
lap those scattered by the tube’s longitudinal edge with the
basal planes parallel to the beam-͓010͔ zone. The strong
͕10l͖ diffraction spots of the ͓010͔ zone located at l= ͑2n
+1͒ do correspond to a hexagonal crystal. Figure 3͑c͒ shows
an ED pattern taken from a tube cavity, that can be indexed
as the ͓001͔ zone axis of a GaS crystal. Figure 3͑d͒ depicts a
high-resolution TEM ͑HRTEM͒ image of the GaS layers
whose ͑004͒ lattice fringes with a d spacing of 0.387 nm
within the wall are parallel to the tube axis. Figure 3͑e͒ is a
HRTEM image taken from the cavity region. The image and
the d spacing ͑ϳ0.31 nm͒ well match those of a hexagonal
GaS crystal viewed along the ͓001͔ zone axis; the Moiré
fringes are due to the overlapping of the top and bottom
FIG. 3. HRTEM images and ED patterns of a GaS tube. ͑a͒ Overall lattice
image of the tube wall. ͑b͒, ͑c͒ ED patterns taken from the tube wall and
cavity, respectively. ͑d͒, ͑e͒ HRTEM images taken from the tube wall and
cavity, respectively.
sidewalls of the tube. Further studies indicated that the axis
direction, i.e., the growth direction of the tube, is parallel to
the ͓120͔ crystallographic orientation of a GaS crystal, thus
being perpendicular to the ͕100͖ planes. Both the earlier
TEM imaging and electron diffraction suggest that the tube
grows through the successive rolling-up of molecular layers
with the ͑001͒ basal plane, as observed in other layered
structure tubes.17
From a chemical point of view, the formation of GaS
tubes was accomplished through the decomposition of
the source materials ͑Ga2O3+2C→Ga2O+2CO,2Ga2O
+4CO → 4Ga + C + 3CO2,18 2ZnS → 2Zn+S2, 2ZnS + C
19
→2Zn+CS2 ͒ followed by thermal reactions ͑2Ga+S2
→2GaS, and Ga2O+CS2→2GaS+CO͒ at a high tempera-
ture and in a carrying flow of pure Ar. Thus a very complex
chemical process is suggested to take place. Though the de-
tails of a rolling-up mechanism accounted for the tube
growth are not yet clear, we have observed the initial growth
stage at thin GaS layers ͑or nanosheets͒ during TEM exami-
FIG. 2. TEM images of the GaS tubes. ͑a͒ Overall view of a tube ͑low
magnification͒. ͑b͒ Larger and smaller ͑c͒ diameter uniform tubes. ͑d͒ A
Ga-filled tube. ͑e͒ A tube with thick wall and narrow central channel. ͑f͒ An
open-ended tube.
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153112-3
Hu et al.
Appl. Phys. Lett. 87, 153112 ͑2005͒
from minimum of conduction band by holes from deep ac-
ceptor centers.20,21
In conclusion, uniform GaS submicrometer tubes with
layered structures have been synthesized via a simple high-
temperature thermal reaction route. The cavities of these uni-
form GaS tubes may be further filled with different semicon-
ducting materials of various band gaps leading to interesting
electrical and optical nanodevices. In addition, theoretical
predictions have indicated that GaS nanotubes exhibit a
stable semiconducting gap,12 independent of their chirality
and converges rapidly with increasing diameter to that of the
two-dimensional layer. The present results suggest that the
designed simple method might be useful for the synthesis of
other tubular structures made of group III–VI semiconduc-
tors in order to meet the growing demands of nano- and
microscale science and technology.
FIG. 4. TEM images showing a rolling-up mechanism for as-grown GaS
tubes: ͑a͒ and ͑b͒ A rolling-up process along a GaS sheet’s edge. ͑c͒ and ͑d͒
A microfold of a GaS sheet at the tube end, as marked with arrows.
This work was performed through the Special Coordina-
tion Funds for Promoting Science and Technology from
MEXT, Japan.
nation. Figures 4͑a͒ and 4͑b͒ demonstrate thin GaS
nanosheets; a bending and rolling-up process ͑indicated by
arrows͒ along the edges results in the initial growth of a tube
͓therefore, TEM images and ED patterns are in agreement
with those taken from a tube cavity ͑not shown͔͒. Figures
4͑c͒ and 4͑d͒ ͓magnified framed area of Fig. 4͑c͔͒ show a
GaS tube whose tip-end domain can be considered to be a
microfold ͑indicated by arrows͒ of a thin nanosheet. The
observations are in accord with the reported growth process
of MoS2 nanotubes.2 For a GaS nanotube ͑both zigzag and
armchair types͒, based on density-functional tight-binding
theory, it is proposed that GaS nanotube grows towards the
value of two-dimensional hexagonal GaS sheet and is in con-
trast to carbon nanotube largely independent of the
chirality.12 However, the detailed chemical process and
growth mechanism of GaS submicrometer tubes require fur-
ther systematic investigations.
Figure 5 shows a room temperature photoluminescence
͑PL͒ spectrum recorded from the synthesized GaS tubes. It is
clear that two strong emission bands centered at ϳ585 nm
͑2.12 eV͒ and 615 nm ͑2.02 eV͒ are detected. The two bands
are in agreement with the PL emission band ͑of
ϳ656–539 nm, centered at ϳ582 nm͒ observed from the
GaS bulk single crystals;20 they probably are of impurity
nature and are due to the radiative recombination of electrons
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FIG. 5. Room temperature PL spectrum of as-synthesized GaS tubes.
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