Eur Phys J Appl Phys (2013) 64: 10402 DOI: 10.1051/epjap/2013130011 THE EUROPEAN PHYSICAL JOURNAL 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 Center for Materials Science, Faculty of Physics, VNU University of Science, 334 Nguyen Trai, Thanh Xuan, Hanoi, Vietnam Faculty of Physics, Ho Chi Minh City University of Science, 227 Nguyen Van Cu, HCM City, Vietnam Received: January 2013 / Accepted: March 2013 Published online: October 2013 – c EDP Sciences 2013 Abstract Indium phosphide semiconductor materials (InP) have various applications in the field of semiconductor 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 properties 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 photoluminscence (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 Introduction Nanostructured materials are an important research object of science and technology Compared with the bulk materials, the nanomaterials exhibit some new unique properties due to the quantum confinement effect, which can bring various promising applications in science and technology 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 phosphide (GaP), have emission wavelengths ranging from the blue region to the near infrared one, they are promising alternatives to the CdSe-based nanoparticles for applications 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) Hence, it can be expected to easily prepare InP NCs exhibiting 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 trioctylphosphine oxide (TOPO) and trioctylphosphine (TOP) at elevated temperatures [3], reactions of indium acetate (In(Ac)3 ) with in situ generated gaseous PH3 from Ca3 P2 [4], reactions of InCl3 , yellow phosphorus (P4 ) and reducing agent (NaBH4 ) at low temperatures [5], and reactions of precursor chemicals (InCl3 , P4 , KBH4 ) with assistance of ultrasound [6] It has been well established that the ultrasonic irradiation introduces a variety of physical and chemical effects deriving from acoustic cavitation [7] Such cavitation behavior, 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 irradiation Structural and optical properties of the synthesized InP NCs were examined Experimental International Workshop on Advanced Materials and Nanotechnology 2012 (IWAMN 2012) a e-mail: hmtphysics@gmail.com All the chemicals used in our experiment, including indium chloride, yellow phosphorus and sodium borohydride, 10402-p1 The European Physical Journal Applied Physics by a Shimadzu UV 2450 PC spectrometer PL measurements were performed on a spectrometer Fluorolog FL 3-22 Jobin-Yvon-Spex, USA used 450 W xenon lamp as an excitation source Results and discussion Fig Flowchart for preparing InP NCs by sonochemical method are of analytic grade without further purification The typical procedure of InP nanocrystal preparation used in our experiment was as follows (Fig 1): 0.74 g (2.5 mmol) of InCl3 ·4H2 O was dissolved in 25 mL of ethanol and 0.62 g (5 mmol) of P4 was dissolved in 25 mL of toluene In order 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 NaBH4 was 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 irradiation under ambient air for 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 cm in depth directly 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, dilute hydrochloric acid (HCl) solution and distilled water, and finally dried at 60 ◦ C in argon atmosphere for 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%) solution, n-butanol, TOPO and H2 O 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 analyzed by X-ray diffraction (XRD) using an X-ray diffractometer Siemens D5005, Bruker, Germany, with Cu-Ka1 (λ = 0.154056 nm) radiation The surface morphology of the samples was observed by using a JEOL JEM 1010 transmission electron microscope The composition of the samples was determined by an energydispersive X-ray spectrometer (EDS) Oxford ISIS 300 attached to the JEOL-JSM5410 LV scanning electron microscope Raman measurements were carried out by using LabRAM HR 800, Horiba spectrometer with 632.8 nm excitation The UV-vis absorption spectra were obtained In our experiment the InP was formed from the mixtures of InCl3 ·4H2 O, P4 and NaBH4 in the mixed solvents of ethanol and toluene under the high intensity ultrasonic irradiation In this reaction solution, first NaBH4 reduces In3+ in the dissolved InCl3 ·4H2 O 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 + 6B2 H6 + 6H2 The XRD patterns of as-prepared NCs synthesized at different temperatures are shown in Figure 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 1) and (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 increasing 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), (2 0) and (3 1) diffraction planes, respectively, in cubic sphalerite InP crystal The lattice constant determined from Fig XRD patterns of InP nanocrystalline powders prepared at different temperatures 10402-p2 H.M Trung et al.: Synthesis and optical properties of InP nanocrystals Fig EDS patterns of InP nanocrystalline powders Fig Typical Raman scattering spectrum of the InP nanocrystals Fig 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 powders are shown in Figure 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 Figure indicates that the InP NCs agglomerated into the bigger spherical nanoparticles which have a broad size distribution Figure 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 observation of the TO and LO lattice vibration 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 Figure 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 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)2 versus hν the absorption becomes stronger (lines (b)) No excitonic structure is observed in Figure For InP NCs the absorption spectra without excitonic structure were observed in the NCs with diameter larger than 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 semiconductor 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(Eg − hν)1/2 , where A is a constant and Eg is the band gap of the material The plot of (αhν)2 versus hν for the InP NCs dispersed in ethanol is represented in the inset of Figure 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 10402-p3 The European Physical Journal Applied Physics Fig 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 days (The line at 540 nm is an emission of HF containing solution.) Fig 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 shows the PL spectra of the InP NCs that had undergone a treatment with various amounts of HF at etching time of days Figure 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 increase 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, dangling bounds, etc.) acting as non-radiative recombination centers Conclusion Fig PL spectra of the InP NCs that had undergone a surface treatment at etching time of 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 exhibited a very weak PL After the surface treatment with liquid HF, the InP NCs show an enhanced PL, which consists 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 InP NCs have been synthesized by sonochemical method using the precursors such as indium chloride, yellow phosphorus and reduction agent XRD analysis indicated that the InP NCs possess face-centered-cubic crystal structure with a lattice constant a = 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) References Nanocrystal Quantum Dots, V.I Klimov (Ed.), 2nd edn (CRC Press, Taylor & Francis Group, Boca Raton, London, New York, 2010) 10402-p4 H.M Trung et al.: Synthesis and optical properties of InP nanocrystals L Langof, L Fradkin, E Ehrenfreund, E Lifshitz, O.I Micic, A.J Nozik, Chem Phys 297, 93 (2004) A.A Guzelian, J.E.B Katari, A.V Kadavanich, U Banin, K Hamad, E Juban, A.P Alivisatos, R.H Wolters, C.C Arnold, J.R Heath, J Phys Chem 100, 7212 (1996) L Li, M Protiere, P Reiss, Chem Mater 20, 2621 (2008) U.T.D Thuy, T.T.T Huyen, N.Q Liem, P Reiss, Mater Chem Phys 112, 1120 (2008) B Li, Y Xie, J Huang, Y Liu, Y Qian, Ultrason Sonochem 8, 331 (2001) Y Mastai, A Gedanken, in The Chemistry of Nanomaterials: Synthesis, Properties and Applications, Vol 1, C.N.R Rao, A Muller, A.K Cheetham (Eds.), (Wiley-VCH Verlag GmbH & Co KGaA, Weinheim, 2004), p 113 H Yang, W Yin, H Yang, Y Song, J Phys Chem Solids 69, 1017 (2008) O.I Micic, J.R Sprague, C.J Curtis, K.M Jones, J.L Machol, A.J Nozik, H Giessen, B Fluegel, G Mohs, N Peyghambarian, J Phys Chem 99, 7754 (1995) 10 E.J Johnson, in Semiconductors and Semimetals, R.K Willardson, Albert C Beer (Eds.), Vol 3, Optical Properties of III-V Compounds (Academic Press, New York, London, 1967), p 153 11 S Xu, J Ziegler, T Nann, J Mater Chem 18, 2653 (2008) 12 O.I Micic, J Sprague, Z Lu, A.J Nozik, Appl Phys Lett 68, 3150 (1996) 10402-p5 ... InP transverse -optical (TO) mode and longitudinal -optical (LO) lattice vibration modes, respectively The observation of the TO and LO lattice vibration modes once again indicates that the InP NCs... The lattice constant determined from Fig XRD patterns of InP nanocrystalline powders prepared at different temperatures 10402-p2 H.M Trung et al.: Synthesis and optical properties of InP nanocrystals. .. exciton emission and the long wavelength band was associated with a defect state-to-band recombination InP NCs have been synthesized by sonochemical method using the precursors such as indium chloride,