In this report we present a simple method to synthesize InP nanocrystals from inorganic precursors such as indium chloride InCl3, yellow phosphorus P4, reduction agent NaBH4 at low tempe
Trang 1P HYSICAL J OURNAL
APPLIED PHYSICS
Regular Article
Synthesis of indium phosphide nanocrystals by sonochemical method and survey of optical properties
Ho Minh Trung1,2,a, Nguyen Duy Thien1, Le Van Vu1, Nguyen Ngoc Long1, and Truong Kim Hieu2
1 Center for Materials Science, Faculty of Physics, VNU University of Science, 334 Nguyen Trai, Thanh Xuan, Hanoi, Vietnam
2 Faculty of Physics, Ho Chi Minh City University of Science, 227 Nguyen Van Cu, HCM City, Vietnam
Received: 3 January 2013 / Accepted: 8 March 2013
Published online: 4 October 2013 – c EDP Sciences 2013
Abstract Indium phosphide semiconductor materials (InP) have various applications in the field of
semi-conductor optoelectronics because of its advantages But the making of this material is difficult due to the
very weak chemical activity of In element In this report we present a simple method to synthesize InP
nanocrystals from inorganic precursors such as indium chloride (InCl3), yellow phosphorus (P4), reduction
agent NaBH4 at low temperature with the aid of ultrasound Structural, morphological and optical
prop-erties of the formed InP nanocrystals were examined by transmission electron microscopy (TEM), X-ray
diffraction (XRD), energy dispersed X-ray analysis (EDS), Raman scattering, absorption and
photolumin-scence (PL) spectroscopy After the surface treatment of InP nanocrystals with liquid hydrofluoric (HF)
acid, the luminescence spectra have an enhanced intensity and consist of the peaks in the region from
500 nm to 700 nm The intensity of these peaks strongly depends on the concentration and etching time
of HF
1 Introduction
Nanostructured materials are an important research
ob-ject of science and technology Compared with the bulk
materials, the nanomaterials exhibit some new unique
pro-perties due to the quantum confinement effect, which can
bring various promising applications in science and
tech-nology and in the life as well While most studies in the
area of nanomaterials focus on II–VI semiconductor
nanocrystals (NCs) [1], the studies and applications of
III–V compound NCs are rather sparse as compared to
II–VI NCs This is because the synthesis of colloidal
III–V nanoparticles is more difficult than for II–VI NCs
The reason for this is that III–V semiconductor
compounds are more covalent, and high temperatures are
required for their synthesis However, because III–V NCs,
for example indium phosphide (InP) and gallium
phos-phide (GaP), have emission wavelengths ranging from the
blue region to the near infrared one, they are promising
alternatives to the CdSe-based nanoparticles for
applica-tions such as light-emitting diodes (LEDs), photovoltaic
cells, bio-labeling, etc On the other hand, among the
III–V compounds, bulk InP has a narrow band
gap (1.35 eV) and, in particular, an exciton Bohr radius
of 11.3 nm [2], which is larger than that of CdSe (3.5 nm)
International Workshop on Advanced Materials and
Nanotechnology 2012 (IWAMN 2012)
a e-mail:hmtphysics@gmail.com
Hence, it can be expected to easily prepare InP NCs ex-hibiting a strong quantum confinement effect
Until now, many routes have been employed for synthesis of InP NCs, including thermolysis reactions of indium chloride (InCl3) and P(Si(Me3 3) in trioctylphos-phine oxide (TOPO) and trioctylphostrioctylphos-phine (TOP) at elevated temperatures [3], reactions of indium acetate (In(Ac)3) with in situ generated gaseous PH3 from
Ca3P2 [4], reactions of InCl3, yellow phosphorus (P4) and reducing agent (NaBH4) at low temperatures [5], and re-actions of precursor chemicals (InCl3, P4, KBH4) with assistance of ultrasound [6]
It has been well established that the ultrasonic irradi-ation introduces a variety of physical and chemical effects deriving from acoustic cavitation [7] Such cavitation be-havior, i.e., the formation, growth and implosive collapse
of bubbles, has been used extensively to generate novel materials with unusual properties
In the present paper we prepared InP NCs by using the reaction of indium chloride and yellow phosphorus as the In and P precursors, respectively, and a reducing agent (sodium borohydride) with the aid of ultrasound irradia-tion Structural and optical properties of the synthesized InP NCs were examined
2 Experimental
All the chemicals used in our experiment, including ind-ium chloride, yellow phosphorus and sodind-ium borohydride,
Trang 2Fig 1 Flowchart for preparing InP NCs by sonochemical
method
are of analytic grade without further purification The
typ-ical procedure of InP nanocrystal preparation used in our
experiment was as follows (Fig 1): 0.74 g (2.5 mmol) of
InCl3·4H2O was dissolved in 25 mL of ethanol and 0.62 g
(5 mmol) of P4 was dissolved in 25 mL of toluene In
or-der to completely dissolve the precursors, the above two
solutions were ultrasonically stirred for 15–60 min, using
a commercial ultrasonic cleaner Then 0.57 g (15 mmol) of
NaBH4was totally dissolved in 75 mL of ethanol and was
gradually dropped (1 mL/min) into the flask containing
the mixture of the above In and P precursor solutions At
the same time, the mixture was exposed to ultrasound
ir-radiation under ambient air for 4 h at the temperatures of
37, 47 and 57◦C Ultrasound irradiation was accomplished
with a high intensity ultrasound probe (Sonics VCX 750;
1.3 cm in diameter; Ti horn, 20 kHz, ultrasound power
density was 100 W/cm2) immersed 1 cm in depth
di-rectly in the reaction solution The mixture changed its
color gradually from yellow to black-brown The resulting
precipitates were separated by centrifugation (6000 rpm,
10–20 min), washed repeatedly with toluene, ethanol,
di-lute hydrochloric acid (HCl) solution and distilled water,
and finally dried at 60 ◦C in argon atmosphere for 2 h
The as-prepared InP NCs did not emit light or exhibited a
very weak PL Therefore, the InP NCs underwent surface
treatment with HF The HF-etching solution was prepared
by mixing appropriate amounts of aqueous HF (48%)
so-lution, n-butanol, TOPO and H2O The InP NCs were
added into the HF stock solution with different HF:InP
molar ratios under room light
Crystal structure of the synthesized products was
an-alyzed by X-ray diffraction (XRD) using an X-ray
diffrac-tometer Siemens D5005, Bruker, Germany, with
Cu-Ka1 (λ = 0.154056 nm) radiation The surface
mor-phology of the samples was observed by using a JEOL
JEM 1010 transmission electron microscope The
com-position of the samples was determined by an
energy-dispersive X-ray spectrometer (EDS) Oxford ISIS 300
attached to the JEOL-JSM5410 LV scanning electron
mi-croscope Raman measurements were carried out by using
LabRAM HR 800, Horiba spectrometer with 632.8 nm
excitation The UV-vis absorption spectra were obtained
by a Shimadzu UV 2450 PC spectrometer PL measure-ments were performed on a spectrometer Fluorolog FL 3-22 Jobin-Yvon-Spex, USA used 450 W xenon lamp as
an excitation source
3 Results and discussion
In our experiment the InP was formed from the mix-tures of InCl3·4H2O, P4and NaBH4in the mixed solvents
of ethanol and toluene under the high intensity ultra-sonic irradiation In this reaction solution, first NaBH4 re-duces In3+in the dissolved InCl3·4H2O to indium element, then the indium reacts with yellow phosphorus to form InP This process can be described by the following equation [5,6]:
4InCl3+12NaBH4+P4→ 4InP + 12NaCl + 6B2H6+ 6H2 The XRD patterns of as-prepared NCs synthesized at dif-ferent temperatures are shown in Figure2 It can be seen that the synthesis temperature is an important fact to affect the crystallization process of the nanocrystal At temperature of 37 ◦C no InP NCs could be formed, in addition, in the pattern one can clearly observe a strong diffraction peak at 32.9◦and a weak peak at 39.2◦, which correspond to the (1 0 1) and (1 1 0) diffraction planes
of the indium metal with the tetragonal phase structure, respectively Thus, at the temperature as low as 37 ◦C, only indium metal particles were created With increas-ing the temperature up to 47 ◦C, the reflective peaks related to In metal became weaker and the InP-related peaks began to appear When the synthesis temperature rose up to 57◦C, InP NCs could be favorably formed The peaks in the XRD patterns located at 2θ values of 26.2 ◦,
43.6◦ and 52.0◦ correspond to the (1 1 1), (2 2 0) and (3 1 1) diffraction planes, respectively, in cubic sphalerite InP crystal The lattice constant determined from
Fig 2 XRD patterns of InP nanocrystalline powders prepared
at different temperatures
Trang 3Fig 3 EDS patterns of InP nanocrystalline powders.
Fig 4 TEM images of InP nanocrystalline powders.
the XRD patterns is a = 0.588 ± 0.002 nm, which is
in agreement with the standard values (a = 0.5869 nm,
JCPDS card No 32-0452)
Typical EDS spectra of the InP nanocrystalline
pow-ders are shown in Figure3 The EDS spectra of all the InP
samples exhibit the peaks related to elements In and P It
is noted that the oxygen (O) observed in the EDS spectra
is the residual not totally removed during washing It is
found that for the InP NCs prepared with the In:P molar
ratio of 1:2, the In:P atomic ratio was 1.42
The TEM image of InP powders depicted in Figure4
indicates that the InP NCs agglomerated into the bigger
spherical nanoparticles which have a broad size
distribu-tion
Figure5 shows typical Raman scattering spectrum of
the InP NCs The sharp scattering peaks around 306 and
339 cm−1, which are close to that of the bulk InP (TO:
304 cm−1, LO: 345 cm−1 [8]), are assigned to the InP
transverse-optical (TO) mode and longitudinal-optical
(LO) lattice vibration modes, respectively The
observa-tion of the TO and LO lattice vibraobserva-tion modes once again
indicates that the InP NCs were really formed
The room temperature UV-vis absorption spectrum of
the InP NCs dispersed in ethanol is presented in
Fig-ure 6 It can be seen that the ethanol (line (a)) almost
does not absorb the electromagnetic waves in the range of
250–900 nm When InP NCs are dispersed in ethanol,
Fig 5 Typical Raman scattering spectrum of the InP
nanocrystals
Fig 6 Typical UV-vis spectrum of the InP NCs dispersed
in ethanol The ethanol absorption spectrum is depicted for comparison The inset shows the plot of (αhν)2versushν.
the absorption becomes stronger (lines (b)) No excitonic structure is observed in Figure6 For InP NCs the absorp-tion spectra without excitonic structure were observed in the NCs with diameter larger than 4 nm [9] The reason for this mainly is a sufficiently wide size distribution which could easily mask excitonic peaks in quantum dots [9]
It is well known that cubic InP is a direct-gap semicon-ductor The relation between the absorption coefficients (α) and the incident photon energy (hν) for the case of
allowed direct transition is written as follows [9,10]:
αhν = A(E g − hν) 1/2 ,
whereA is a constant and E gis the band gap of the mater-ial The plot of (αhν)2versushν for the InP NCs dispersed
in ethanol is represented in the inset of Figure6 By using this plot we found the band gap of the InP NCs dispersed
in ethanol to be 2.74 eV Compared with the bulk InP
Trang 4Fig 7 PL spectra of the InP NCs prepared at 47 and 57◦C,
and that had undergone a surface treatment with HF (HF:InP
molar ratio = 22:1) for 9 days (The line at 540 nm is an
emission of HF containing solution.)
Fig 8 PL spectra of the InP NCs that had undergone a
sur-face treatment at etching time of 3 days with various HF:InP
molar ratios
band gap of 1.35 eV, the blue shift of 1.39 eV exhibits the
quantum confinement effect
The as-prepared InP NCs did not emit light or
exhib-ited a very weak PL After the surface treatment with
liquid HF, the InP NCs show an enhanced PL, which
con-sists of the peaks in the wavelength region from 500 nm to
700 nm (Fig.7) As seen from the figure, each PL spectrum
is the overlap of two emission bands: the short wavelength
one and the long wavelength one The similar PL spectra
were observed by other authors [2,11], in which the short
wavelength band was attributed to the exciton emission
and the long wavelength band was associated with a defect
state-to-band recombination
Fig 9 PL spectra of InP NCs at different etching times with
HF:InP molar ratio = 44:1
We have found that the PL intensity increases with HF concentration and etching time Figure 8 shows the PL spectra of the InP NCs that had undergone a treatment with various amounts of HF at etching time of 3 days Figure 9 depicts the PL spectra of the InP NCs that had undergone a treatment with HF (HF:InP = 44:1) at different etching times The reason for the observed in-crease in PL intensity, according to Micic et al [12], is due
to the HF-etching treatment of InP NCs, which removes
or passivates surface states (phosphorus vacancies, dan-gling bounds, etc.) acting as non-radiative recombination centers
4 Conclusion
InP NCs have been synthesized by sonochemical method using the precursors such as indium chloride, yellow phos-phorus and reduction agent XRD analysis indicated that the InP NCs possess face-centered-cubic crystal structure with a lattice constanta = 0.588 ± 0.002 nm The InP NCs
have spherical form with a broad size distribution The Raman scattering spectra exhibiting the InP TO (306 cm−1) and LO (339 cm−1) lattice vibration modes have indicated the formation of InP NCs as well The band gap of the InP NCs estimated from UV-vis absorption is about 2.74 eV The InP NCs after HF-etching treatment exhibit an enhanced PL with the intensity depending on the HF concentration and etching time
This work is financially supported by Ministry of Science and Technology of Vietnam (Project No 103.02.51.09 from NAFOSTED)
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