A mild reduction-phosphidation approach to nanocrystalline GaP

Article abstract of DOI:10.1016/j.ssc.2004.09.023

Nanocrystalline gallium phosphide (GaP) has been prepared through a reduction-phosphidation by using Ga, PCl₃ as gallium and phosphorus sources and metallic sodium as reductant at 350°C. The XRD pattern can be indexed as cublic GaP with the lattice constant of a = 5.446 A?. The TEM image shows particle-like polycrystals and flake-like single crystals. The PL spectrum exhibits one peak at 330 nm for the as-prepared nanocrystalline GaP.

Full text of DOI:10.1016/j.ssc.2004.09.023

Solid State Communications 132 (2004) 667–671
www.elsevier.com/locate/ssc
A mild reduction–phosphidation approach to nanocrystalline GaP
*
Luyang Chen, Tao Luo, Mingxing Huang, Yunle Gu, Liang Shi, Yitai Qian
Department of Chemistry and Structure Research Laboratory, University of Science and Technology of China, Hefei,
Anhui 230026, People’s Republic of China
Received 7 July 2004; received in revised form 30 August 2004; accepted 11 September 2004 by A. Pinczuk
Available online 28 September 2004
Abstract
Nanocrystalline gallium phosphide (GaP) has been prepared through a reduction–phosphidation by using Ga, PCl3 as gallium
and phosphorus sources and metallic sodium as reductant at 350 8C. The XRD pattern can be indexed as cublic GaP with the
˚
lattice constant of aZ5.446 A. The TEM image shows particle-like polycrystals and flake-like single crystals. The PL spectrum
exhibits one peak at 330 nm for the as-prepared nanocrystalline GaP.
q 2004 Published by Elsevier Ltd.
PACS: 61.14.Qp
Keywords: A. Gallium phosphide; B. Chemical synthesis; C. Transmission electron microscopy; C. X-ray diffraction
1. Introduction
important way to prepare GaP is the metal-organic route.
Dougall and co-workers [8] have reported that GaP
semiconductor clusters can be synthesized in zeolite by
using the reaction (Me3)Ga and PH3. At refluxing
temperature, GaP nanocrystallines were obtained from
GaCl3 and P(SiMe3)3 or (Na/K)3P and GaX3 in toluene or
other solvent, respectively [9,10]. Another important
method for preparing GaP is the solid-state metathesis, in
which GaX3 (XZF, Cl, I) and Na3P are used as the raw
materials [11,12].
Semiconductor nanoparticles are interesting materials
because they have chemical and physical properties
different from those of the corresponding bulk and isolated
atoms or molecules [1–4]. Residing in the mesoscopic
regime between the molecular and bulk regimes, semicon-
ductor nanocrystals have unique electronic, magnetic and
optical properties due to their small size and large surface-
to-volume ratio [5,6]. Recently, quantum size effects on the
optical properties have been predicated to be more evident
in the III–V group semiconductor nanomaterials than that in
II–VI semiconductor nanomaterials [7]. Therefore, it is
important to further study the synthesis and properties of
III–V group semiconductor nanomaterials under mild
conditions.
Herein, we provide a route to prepare nanocrystalline
GaP at 350 8C by the reduction–phosphidation method, in
which phosphorus trichloride (PCl3) and gallium (Ga) are
used as source materials and metallic sodium (Na) as the
reductant. The reaction can be expressed as follows:
Ga CPCl3 C3Na 3ÿ5ÿ0ÿ/ÿ8C GaP C3NaCl
GaP is an indirect gap semiconductor and has many
important applications in microelectronic devices. The most
2. Experimental procedure
* Corresponding author. Tel.: C86 551 3601589; fax: C86 551
3607402.
All the manipulations were carried out in a dry glove box
with Ar flowing. In the typical process, 0.01 mol Ga,
0.01 mol PCl3 and 0.03 mol Na were placed into a stainless
E-mail addresses: chendavy@mail.ustc.edu.cn (L. Chen),
ytqian@ustc.edu.cn (Y. Qian).
0038-1098/$ - see front matter q 2004 Published by Elsevier Ltd.
doi:10.1016/j.ssc.2004.09.023

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L. Chen et al. / Solid State Communications 132 (2004) 667–671
ethanol for several times to remove the impurities. The final
product was vacuum-dried at 50 8C for 3 h. Dray-black
powders were obtained.
X-ray powder diffraction (XRD) pattern was carried out
on a Rigaku Dmax-gA X-ray diffractometer with Cu Ka
˚
radiation (lZ1.54178 A). The morphology of GaP was
examined from transmission electron microscopy (TEM)
images taken with a Hitachi H-800 transmission electron
microscope. X-ray photoelectron spectra (XPS) were
recorded on a VGESCALAB MKII X-ray photoelectron
spectrometer, using non-monochromatized Mg Ka X-rays
as the excitation source. The ultraviolet and visible light
(UV–vis) spectra and photoluminescence (PL) spectra were
recorded on JGNA Specord 200 (Japan) PC UV–vis
spectrophotometer and Hitachi 850 fluorescence spectro-
photometer, respectively.
Fig. 1. XRD pattern of the GaP sample.
steel autoclave with a quartz liner. And then, the autoclave
was sealed and heated at 350 8C for 6 h, followed by
naturally cooling to room temperature. The product in the
quartz liner was washed with distilled water and absolute
3. Results and discussion
Fig. 1 shows the XRD pattern of the product. All of the
Fig. 2. XPS spectra of the GaP sample.

L. Chen et al. / Solid State Communications 132 (2004) 667–671
669
Fig. 3. TEM images and SAED patterns of the GaP sample.
seven peaks at d-spacings of 3.131, 2.714, 1.921, 1.640,
˚
1.570, 1.361, 1.250 A can be indexed as cubic GaP ((111),
reference valued for GaP [13,14]. The P2p core (Fig. 2(c))
shows two peaks, one at 129.6 eV corresponding to P from
GaP and the other at 133.7 eV corresponding to the oxidized
P species [13]. Because of the higher surface areas are much
more easily oxidized when they are exposed to air. Thus, the
phosphorus atoms on the surface of the product may turn to
oxide phosphorus gradually, resulting in the oxidized
phosphorus species in the XPS spectrum. Alivisatos et al.
have observed the similar phenomenon in InP nanocrystals
[15].
(200), (220), (311), (222), (400), (331)). The lattice
˚
constants are aZ5.446 A, in good agreement with aZ
˚
5.448 A (JCPDS card# 12-0191). No evidence of Ga2O3,
NaCl and other impurities are observed.
Further evidence for the formation of GaP and analysis
of as-prepared powder’s surface nature, the sample was also
characterized by XPS spectrum. Higher resolution spectra
taken of the Ga2p, Ga3d and P2p regions were shown in Fig.
2. The Ga 2p3/2 peak (Fig. 2(a)) at 1117.2 eV and Ga 3d
peak (Fig. 2(b)) at 19.40 eV are also consistent with the
The morphology of as-prepared GaP was observed by
the TEM. The sample was prepared by placing a drop of

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L. Chen et al. / Solid State Communications 132 (2004) 667–671
as-prepared nanoscale GaP shows a shoulder at 360 nm and
a shallow tail that extends out to about 700 nm. Bulk GaP is
an indirect semiconductor with an indirect band gap of
2.22 eV (559 nm) and a direct band gap of 2.78 (446 nm).
We attribute the steep absorption and shoulder at 360 nm to
a direct transition in the GaP quantum dots (QDs); the
shallow tail region above 600 nm is attributed to the indirect
transition. Micic et al. have researched the similar situation
in GaP nanocrystals [16]. Fig. 4(b) shows the room
temperature PL spectrum from as-prepared GaP. The
excitation wavelength is 230 nm and the filter wavelength
is 310 nm. It is clear that one emitting peak enters at 330 nm
and this originates from generally intrinsic band gap PL of
GaP. Compared with the luminescent spectral behavior of
the bulk GaP, the emission feature in PL spectrum is
attributed to the presence of defects of either interface or
interior in nanocrystalline GaP.
The formation process of GaP may be illustrated as
follows:
PCl3 C3Na/P C3NaCl
Ga CP/GaP
In this reaction, nascent phosphorus is generated in the
reaction of PCl3 (boiling pointZ76 8C) by metallic sodium
(melting pointZ98 8C). Then nascent phosphorus and initial
gallium combine to form GaP.
The influences of reaction temperature and reaction time
on the formation of the GaP were investigated. If the
temperature was lower than 250 8C, the yield of GaP was
low. If the temperature is higher than 400 8C, the grain size
grew easily up. The reaction time of longer than 6 h at
400 8C did not obviously affect the crystallinity and the
yield. However, when reaction time was less than 4 h, the
reaction was incomplete.
Fig. 4. (a) UV–vis and (b) PL spectra of the GaP sample.
dilute alcohol suspension of GaP that was dispersed by
ultrasonication onto a carbon-coated copper grid for
observation in an electron microscope. In Fig. 3(a), the
material on copper grid mainly exhibits uniform particle
with about 20 nm in size. In fact, the XRD pattern shows the
material is not whole nanosized by Debeye Scherrer
calculation. GaP on copper grid coming from dilute alcohol
suspension is only a part of smaller material, so the TEM
image shows nanoscale particles. In Fig. 3(b), the diffraction
4. Conclusions
In summary, we have succeeded in synthesizing cubic
GaP through a reduction–phosphidation by using Ga, PCl3
and Na at 350 8C for 6 h. XRD pattern reveals the
synthesized GaP crystallined in cubic crystal structure.
TEM image shows particle-like polycrystal and flake-like
single crystalline. PL spectrum exhibits one peak at 330 nm
of the as-prepared nanocrystalline GaP.
˚
rings from inner to outer, at d-spacings of 3.13, 1.92, 1.64 A,
match hexagonal GaP (111), (220), (311) planes, in good
agreement with the XRD results. From Fig. 3(c) it can be
seen that the sample also consists of flake-like morphology.
The corresponding SAED pattern (Fig. 3(d)) of GaP flakes is
consistent with the single crystalline nature.
Acknowledgements
Fig. 4(a) shows the absorption spectrum of a sample
dispersed in ethanol by sonication. The absorption spectrum
This work is supported by the Chinese National Natural
Science Foundation.

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