Synthesis and optical study of crystalline GaP nanoflowers

Article abstract of DOI:10.1063/1.1875732

GaP nanoflowers composed of numerous GaP nanowires are synthesized through heating InP and Ga2O3 powders. Crystalline GaP nanowires growing from Ga-rich particles have a cubic structure, uniform diameters of ~300 nm, and lengths from several to tens of micrometers. Typically, an individual GaP nanowire displays a hexagonal prism-like morphology with 〈111〉 as the preferential growth direction. Cathodoluminescence measurements show that GaP nanoflowers and GaP nanowires emit at ~600 and ~750 nm, respectively. Additional low-intensity emission peaks are observed for GaP nanoflowers at ~450 nm.

Full text of DOI:10.1063/1.1875732

Synthesis and optical study of crystalline GaP nanoflowers
B. D. Liu, Y. Bando, C. C. Tang, D. Golberg, R. G. Xie, and T. Sekiguchi
Citation: Applied Physics Letters 86, 083107 (2005); doi: 10.1063/1.1875732
View online: http://dx.doi.org/10.1063/1.1875732
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APPLIED PHYSICS LETTERS 86, 083107 ͑2005͒
Synthesis and optical study of crystalline GaP nanoflowers
B. D. Liua͒ and Y. Bando
Graduate School of Pure and Applied Sciences, University of Tsukuba, Tsukuba, Ibaraki 305-0005, (Japan)
and Advanced Materials Laboratory, National Institute for Materials Science, Namiki1-1, Tsukuba,
Ibaraki 305-0044, Japan
C. C. Tang and D. Golberg
Advanced Materials Laboratory, National Institute for Materials Science, Namiki1-1, Tsukuba,
Ibaraki 305-0047, Japan
R. G. Xie and T. Sekiguchi
Nanomaterials Laboratory, National Institute for Materials Science, Namiki1-1, Tsukuba,
Ibaraki 305-0047, Japan
͑Received 4 October 2004; accepted 21 January 2005; published online 18 February 2005͒
GaP nanoflowers composed of numerous GaP nanowires are synthesized through heating InP and
Ga2O3 powders. Crystalline GaP nanowires growing from Ga-rich particles have a cubic structure,
uniform diameters of ϳ300 nm, and lengths from several to tens of micrometers. Typically, an
individual GaP nanowire displays a hexagonal prism-like morphology with ͗111͘ as the preferential
growth direction. Cathodoluminescence measurements show that GaP nanoflowers and GaP
nanowires emit at ϳ600 and ϳ750 nm, respectively. Additional low-intensity emission peaks are
observed for GaP nanoflowers at ϳ450 nm. © 2005 American Institute of Physics.
͓DOI: 10.1063/1.1875732͔
Recently, nanostructural semiconducting materials with
various structures and morphologies have received much at-
tention due to their technological applications, intriguing
properties, and quantum size effects.1–3 It has been found
that the nanostructure properties may have strong depen-
dence on size, morphology, and shape.4,5 As an important
semiconductor material, gallium phosphide ͑GaP͒, a wide
band gap semiconductor ͑Eg=2.26 eV at room temperature͒,
is of special interest due to its excellent properties and po-
tential applications in electrical and optical devices.1,6,7 To
date, various GaP structures such as nanoparticles,
nanowires, nanobelts, and “nanotrees” have been synthesized
via different synthesis processes.8–11 Another structure, a
“nanoflower”, has been observed in SiC, MoS2, and NbS2,
etc.12–14 All these intriguing structures have stimulated us to
focus on the synthesis for GaP nanoflowers and analysis of
their morphologies. In this letter, the flower-like GaP nano-
material has been synthesized and its structure, morphology,
and optical properties are systemically investigated.
sive spectrometer ͑EDS͒. The cathodoluminescence ͑CL͒
spectra were recorded in a charge-coupled device ͑spectra
view 2D; Jobin Yvon, France͒.
The structure and crystallinity of as-synthesized products
were examined by x-ray diffraction. As shown in Fig. 1, the
product is well crystallized and may be indexed to a cubic
GaP phase with the lattice constant of a=0.545 nm, match-
ing that of a bulk material ͑see JCPDs Card No. 32-0397͒
within the experiment error.
SEM images reveal that the as-synthesized products are
of flower-like morphology, as shown in Fig. 2͑a͒. As an ex-
ample, Fig. 2͑b͒ presents a complete GaP nanoflower. Nu-
merous individual GaP nanowires grew densely from its in-
ner core. Generally, an individual GaP nanowire is straight
and has a uniform diameter of ϳ300 nm and length ranging
from several to tens of micrometers. High-magnification
SEM image, shown in Fig. 2͑c͒, reveals that each GaP nano-
wire possesses a hexagonal prism morphology with each
hexagonal prism layer stacking along the axis direction. The
GaP nanoflowers were synthesized in a conventional re-
sistance furnace through heating Ga2O3 and InP ͑1:1͒ starting
reactants in a flow of NH3. The starting materials were mixed
completely and put into the center of a quartz tube, then the
reactants were heated to 1000 °C under an Ar flow ͓200 stan-
dard cubic centimeters per minute ͑sccm͔͒, and Ar was sub-
sequently substituted by NH3 ͑300 sccm͒ as the temperature
increased to 1150 °C. After the whole system was maintained
at 1150 °C for 1 h, dark-brown fibers deposited within the
quartz tube ͑downstream͒. The as-synthesized products were
examined by means of an x-ray diffractometer ͑RINT-2200V,
40 V/40 mA with Cu K␣ radiation͒, a scanning electron mi-
croscope ͓͑SEM͒ JEOL, JSM-6700F͔, and a high-resolution
field emission transmission electron microscope ͓͑TEM͒
JEOL, JEM-3000F͔ equipped with an x-ray energy disper-
a͒
Electronic mail: Liu.baodan@nims.go.jp
FIG. 1. XRD pattern of as-synthesized GaP nanoflowers.
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0003-6951/2005/86͑8͒/083107/3/$22.50 86, 083107-1 © 2005 American Institute of Physics
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083107-2
Liu et al.
Appl. Phys. Lett. 86, 083107 ͑2005͒
FIG. 3. Representative TEM images of ͑a͒ a GaP nanoflower and ͑b͒ an
individual GaP nanowire and ͑c,d͒ corresponding EDS spectra. ͑e͒ HRTEM
image of a GaP nanowire.
FIG. 2. ͑a,b͒ Low-magnification and ͑c͒ high-magnification SEM images of
͑as indicated by DTA experiments, not shown here͒ and a
large number of liquid Ga droplets are carried to a low tem-
perature zone by the flowing gas and congregate into Ga-rich
particles in an In-vapor atmosphere. The P atoms are ab-
sorbed onto the surface of Ga particles and bond with Ga
atoms. This results in GaP nanowires densely protruding
from the surface of Ga-rich particles. The absence of In in
the Ga-rich particles and GaP nanowires is possibly due to
the lattice mismatch between In, Ga, and GaP structures.
TEM image of a representative GaP nanowire protruding
from a GaP nanoflower is presented in Fig. 3͑b͒. Typically,
the GaP nanowires have a rough surface, a diameter of
ϳ300 nm, and a length reaching several micrometers. EDS
analysis, Fig. 3͑d͒, confirms that the wires consist of P and
Ga atoms in an atomic ratio ϳ1:1. They have a marginal
content of O as a result of insurmountable surface oxidation
during the growth process.
GaP nanoflowers.
inset of Fig. 2͑c͒ shows that some GaP hexagonal prisms
may intersect.
The structure and chemical composition of GaP
nanoflowers were further examined by TEM and EDS. The
typical TEM image of a GaP nanoflower is depicted in Fig.
3͑a͒. The GaP nanoflower consists of a massive core of
ϳ4 ␮m and numerous nanowires as petals with lengths
ranging from several to dozens of micrometers. Chemical
composition analysis performed on the core verifies that the
core is enriched in Ga and depleted in P, as displayed by
EDS spectrum in Fig. 3͑c͒. Surprisingly, no traces of In are
detected in the core within the EDS resolution limit. How-
ever, it should be noted that the source InP material is
thought to play a key role for the formation of GaP nanof-
lowers. Although the exact growth mechanism for the GaP
nanoflowers has not yet been fully understood, a vapor–
liquid–solid process is assumed to cause the growth. When
the temperature is increased above the melting point of InP
͑1060 °C͒, it starts to decompose into In and P atoms. Mean-
Figure 3͑e͒ displays a representative high-resolution
TEM ͑HRTEM͒ image of a GaP nanowire with a hexagonal
prism-like morphology. HRTEM and electron diffraction
͑ED͒ analyses performed on an individual nanowire imply a
face-centered-cubic structure. The intrafringe separations in
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while, Ga2O3 transforms to a liquid Ga droplet at 1100 °C
the directions parallel and perpendicular to the wire axis are
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083107-3
Liu et al.
Appl. Phys. Lett. 86, 083107 ͑2005͒
vealed by EDS analysis. The low intensity peak in the visible
range ͑ϳ450 nm͒ was observed for GaP nanoflowers; this
peak was not seen for the GaP nanowires though. A similar
supra-band-gap emission has also been reported by van Driel
et al.17 in porous GaP and Cullis et al.18 in porous Si. This
emission was usually attributed to an oxide-originated defect
rather than to a size confinement effect. The GaP nanoflow-
ers, may be easily oxidized due to the high content of Ga in
their inner cores. These oxide-originated defects could be
eliminated by subsequent hydrogen passivation treatment.18
In summary, GaP nanoflowers have been synthesized by
means of a simple evaporation of InP and Ga2O3 powders.
The GaP nanoflowers are crystalline and possess a cubic
structure. The growth is assumed to start from the surface of
Ga-rich particles and controlled by a vapor–liquid–solid
mechanism. An individual GaP petal has a hexagonal prism
morphology with a diameter of ϳ300 nm and a length of
several micrometers or more. TEM analysis implies that the
GaP nanowires grow along the ͗111͘ direction. Optical mea-
surements reveal a redshift at ϳ600 nm for both GaP nano-
flowers and GaP nanowires due to impurity atoms absorbed
on the wire surface. Additional emission peak at ϳ750 nm
for GaP nanoflowers and nanowire has also been observed,
possibly due to radiative recombination by way of a donor–
acceptor pair in the band gap. The authors envisage that the
flower-like GaP structure may be valuable for future nanode-
vice design.
FIG. 4. CL spectra of GaP nanoflowers and a nanowire measured at room
temperature.
0.314 and 0.274 nm, respectively, being in a good agreement
with the ED data. They correspond to the ͑111͒ and ͑220͒
planes of a GaP cubic structure. Further, based on the HR-
TEM image and ED pattern, it could be concluded that the
hexagonal prism structure observed in the SEM image ͓Fig.
2͑c͔͒ grows through stacking the ͕111͖ planes along the pref-
erential ͗111͘ orientation and the six planes in the GaP prism
correspond to the ͕220͖ planes.
Optical properties of the GaP nanoflowers were finally
studied. Figure 4 shows room-temperature CL spectra taken
from the GaP nanoflowers and an individual GaP nanowire.
The characteristic emission peak at around 600 nm is de-
tected for both GaP nanoflowers and the individual nanowire.
This shows an obvious redshift compared to the bulk and
porous GaP materials ͑ϳ550 nm͒.15 It should be noted that
the redshift may possibly result from some impurity effects,
e.g., atomic vacancies on the wire surface, rather than from
the size effects ͑nanowire diameters are rather big
ϳ300 nm͒. N-doped GaP nanobelts with a low N content
have been reported to show a redshift.10 The present GaP
nanoflowers were synthesized with the assistance of NH3.
There may be some N atoms absorbed on the surface, though
no N traces were detected during the EDS analysis within its
resolution limit. In addition, the similar emission peaks for
nanoflowers and the nanowire were observed at ϳ750 nm.
The peak intensity for the nanoflowers was higher compared
with such characteristic peaks in an individual GaP nano-
wire. This broad peak was also observed during cathodolu-
minescence and photoluminescence studies at room tempera-
ture for standard GaP, which was attributed to radiative
recombination by way of a donor–acceptor pair in the band
gap.15,16 This interpretation also looks reasonable for the
present nanoscale GaP materials. The difference in light
emission intensity for GaP nanoflowers and nanowires
͑ϳ750 nm͒ may result from the different Ga content, as re-
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