Journal of ELECTRONIC MATERIALS DOI: 10.1007/s11664-016-4444-2 Ó 2016 The Minerals, Metals & Materials Society Optical and Phonon Characterization of Ternary CdSexS1Àx Alloy Quantum Dots L.A THI ,1,4 N.D CONG,2 N.T DANG,1,5 N.X NGHIA,1,3 and V.X QUANG1 1.—Institute of Research and Development, Duy Tan University, Danang, Vietnam 2.—Department of Physics, Hanoi University of Science, Hanoi, Vietnam 3.—Institute of Materials Science, Vietnam Academy of Science and Technology, Hanoi, Vietnam 4.—e-mail: anhthigl25@gmail.com 5.—e-mail: dangtoan2107@gmail.com Ternary CdSexS1Àx alloy quantum dots (QDs) were synthesized using a wet chemical method Their morphology, particle size, structural, optical, and vibrational properties were investigated using transmission electron microscopy, x-ray diffraction, UV–Vis, fluorescence and Raman spectroscopy, respectively The optical and vibrational properties of the QDs can be controlled by adjusting the Se/S molar ratio The absorption and emission peaks shift to a longer wavelength range when increasing the Se content The presence of two CdSe-like and CdS-like longitudinal optical phonon modes was observed The dependencies of the optical and phonon modes on the Se content are discussed in detail Key words: Optical properties, CdSexS1Àx, ternary alloys, quantum dot INTRODUCTION Semiconductor nanocrystals (NCs) are of a primary interest to several research fields because of their unique complex optical properties Due to quantum confinement effects such as the sizedependent optical and electronic properties, NCs can find a wide-range practical applications in photovoltaics,1 light-emitting diodes,2 photocatalysis, bioassays, and electronics,3,4 solar cells, biological labeling, and sensors.5,6 Nevertheless, for some specific applications, multiple characteristics are required to appear in a single system, for example, very small NCs are desirable for in vivo imaging Additionally, for multiplexing experiments, a range of NC size is required to achieve a specific range of fluorescence colors The size of NCs also plays an important role, especially, when nanocrystals are incorporated into larger superstructures such as mesoporous materials in photovoltaics However, binary alloy NCs cannot satisfy these requirements; therefore, alloy nanocrystals are used as a solution.7 It was shown that the physical properties of alloy (Received October 9, 2015; accepted March 1, 2016) NCs can be adjusted by the chemical composition and the particle size In recent years, II–IV quantum dots (QDs) such as CdSxSe1Àx, CdxZn1ÀxS, CdxZn1ÀxSe, CdSexTe1Àx, HgSe xS1Àx, etc., have attracted much attention by researchers because of their complex physical phenomena and potential practical applications.8 These QDs exhibit high photoluminescence quantum yield (PLQY),9 and low blinking rates.10 It was found that the QDs synthesized with trioctylphosphine (TOP), tributylphosphine, or trioctylphosphine oxide are oxidation-sensitive and toxic In addition, the size of the QDs prepared by another alternative approach from precursors with low reactivity can be controlled by the reaction time.6 Gradient and homogeneous alloy NCs exhibit strong different optical properties.11–13 However, most previous studies have focused only on gradient and not on homogenous alloy nanocrystals The difficulty in alloy nanocrystal research lies in devising a synthetic scheme to produce the desired alloy structure to be homogeneous or gradient In pursuance of homogeneous alloys, the growth rates of the two constituent materials must be equal14 and the growth of one constituent must not impede the growth of the other In addition, the Thi, Cong, Dang, Nghia, and Quang structure and bonding of the two materials must be sufficiently similar to allow their easy mixing, otherwise the formation of segregated structures such as core/shell or two different binary nanocrystals may occur To demonstrate homogeneous alloying, x-ray powder diffraction (XRD) and electronmicroscopy techniques are often combined with elemental analyses.15–17 However, these methods are only reliable for larger QDs and can only address a small number of nanocrystals Alternatively, ensemble level methods, which were applied to small QDs, are also applicable Tangi Aubert and co-workers have introduced Raman spectroscopy to analyze homogeneous alloying in colloidal QDs.16 They tracked the frequency and the width of the longitudinal optical phonon resonance as a function of composition using Raman spectroscopy and showed that the combination of both precursors yields homogeneous CdS1ÀxSex QDs.16 In this work, we synthesized QDs only with cadmium, sulfur, and selenium precursors, which were dissolved in Octadecene The approach is used to fabricate homogeneous alloys, which are weakly dependent on these above ligands We also studied the composition-dependent optical properties of the ternary CdSexS1Àx QDs (x from to 1) The homogeneity of the alloys CdSexS1Àx QDs was verified by analyzing Raman spectra Their optical properties and structural evolution during the mechanical alloying process are discussed in detail EXPERIMENTAL DETAILS Materials We made use of cadmium oxide (CdO, 99.99%, Sigma-Aldrich), sulfur powder (Se, 99.99%, SigmaAldrich), selenium powder (Se, 99.99%, SigmaAldrich), octadecene (ODE, 90%, Sigma-Aldrich), and oleic acid (OA, 9999%, Sigma-Aldrich) All chemicals were used as received without any further purification Synthesis of CdSexS12x Quantum Dots In a typical synthesis, 30 mg (2.4 mmol) of CdO, 90 mL of ODE and 2.3 mL of OA were placed in a 100 mL three-neck flask and heated to 250°C until the CdO was dissolved The sulfur stock solution was prepared by mixing 0.0058 g of sulfur in mL of ODE and heating to 100°C for 15 The selenium stock solution was prepared by dissolving 0.048 g of Se in mL of ODE and then heating to 180°C for several hours All of these reactions were conducted in nitrogen gas After forming the Se2À and S2À ion precursor solutions, they were cooled down to room temperature and mixed together The mixture was then injected into the cadmium solution, which was being stirred at 260°C Five CdSexS1Àx samples with different S/Se molar ratios x (x = 0, 0.25, 0.5, 0.75 and 1) were prepared Then the solution in isopropanol was centrifuged at a speed of 5000 rpm and then re-dispersed in toluene Characterization Transmission electron microscopy (TEM) images were collected using a JoelJEM1010 operating at 80 kV Absorption spectra were recorded using a Varian-Cary-5000 Ultraviolet–Visible (UV–Vis) spectrophotometer Photoluminescence (PL) spectra were measured by a Jobin–Yvon Flurolog FL3-22 fluorescence spectrophotometer with the wavelength of the exciting radiation of 400 nm Raman spectra (Ra) were recorded by an XploRA spectrometer (Horriba Jobin–Yvon) with laser wavelength of 532 nm (2.3 eV) X-ray powder diffraction (XRD) was conducted on a Siemen D5005 x-ray diffractometer using Cu-Ka1 radiation with wavelength of 0.15406 nm For Ra and XRD measurements, the samples were deposited on glass slides All of these measurements were performed at room temperature RESULTS AND DISCUSSION Structure Analysis The TEM images of CdSe0.25S0.75 and CdSe0.75S0.25 with growth time of h are shown in Fig 1a and b, respectively It can be seen that the nanoparticles have spherical shape with good monodispersity and are uniformly distributed The calculated average particle size is nm for x = 0.25 and 7.5 nm for x = 0.75, implying the increase of the mean particle size with the increase of the Se content Figure 2a shows the room-temperature x-ray powder diffraction (XRD) patterns of CdSexS1Àx The data analysis shows that the peak positions of the first member of the CdSexS1Àx system CdS correspond to the ‘‘zincblend’’ crystal structure, whereas those of the end member CdSe correspond to the ‘‘wurtzite’’ crystal structure.18 The presence of the diffraction peaks corresponding to both ‘‘wurtzite’’ and ‘‘zincblend’’ structures are observed in the XRD patterns of the intermediate alloys with x = 0.25, 0.5, and 0.75, which indicates the coexistence of these phases in these compositions The diffraction peaks shift toward higher 2h angles upon increasing the Se content, which corresponds to a decrease of the lattice parameter of the CdSexS1Àx system The analysis shows that the lattice parameter of CdSexS1Àx depends on the Se content following from Vegard’s linear relation for ternary alloys, i.e., aCdSex S1Àx ẳ xaCdSe ỵ xịaCdS as shown in Fig 2b, where aCdSex S1Àx is the lattice constant for a particular composition of the alloy defined by x; aCdSe and aCdS are the lattice constants of CdSe and CdS, respectively A linear dependence on cation composition x of the lattice spacing indicates that Vegard’s law is followed and again demonstrates the formation of homogeneous alloys.16 Optical and Phonon Characterization of Ternary CdSexS1Àx Alloy Quantum Dots Fig Typical TEM images of CdSe0.25S0.25 (a) and CdSe0.75S0.25 (b) alloy QDs Fig (a) XRD patterns and (b) dependence of lattice constant of the ternary CdSexS1Àx alloy QDs on Se content x Fig (a) UV–Vis and PL spectra of CdSexS1Àx alloys QDs, (b) Dependence of PL peak position on Se content Thi, Cong, Dang, Nghia, and Quang Fig (a) The second derivative of the optical absorption spectra of CdSe, (b) Dependence of calculated band gap of CdSexS1Àx alloys QDs on Se content Fig Raman spectra of ternary CdSexS1Àx QDs with different Se content Optical Properties Figure 3a represents the absorption and photoluminescence (PL) spectra of CdSexS1Àx alloy QDs It can be seen that both of the first exciton absorption peaks in the UV–Vis and PL spectra of CdSexS1Àx shift to the red with the increase of the Se content reflecting the concomitant change of the band gap As a result, by changing the composition of S and Se, one can easily cover a wide range of optical properties The optical absorption spectra shift from 2.56 eV to 1.79 eV to match up to the absorption spectra of CdS and CdSe QDs as shown in Fig 3b The overlaid spectra clearly indicate that the Stokes shifts for all the alloy QDs are dependent on the composition but not on the particle size Figure 3b shows that the PL emission peak position for the Se containing QDs shifts from 2.43 eV (510 nm) to 1.74 eV (712 nm), according to the Se/S composition Since, based on the TEM result, the sizes of the CdSexS1Àx QDs with different Se/S ratios prepared at the same growth time are quite similar, the red shift of the PL emission peak could be due to the change of chemical composition.19 In order to determine the band gaps Eg of the alloys, the second derivatives of the optical absorption spectra of the CdSexS1Àx QDs were used to calculate Eg as shown in Fig 4a Figure 4b shows the calculated band gaps, which correspond to the first exciton absorption peaks of these alloys with that at x = 0.25, 0.5, and 0.75 appearing at 2.38 eV, 2.33 eV, and 2.1 eV, respectively The band gaps of the alloy QDs decrease with the increase of the Se content, which is consistent with the previous results.20 The room-temperature Raman spectra of CdSexS1Àx are shown in Fig To evaluate the longitudinal optical (LO) and the surface optical (SO) phonons, the elemental Raman bands are fitted by a sum of several Gaussian–Lorentzian peaks The fitting curves and the peak parameters are shown in Fig and Table I, respectively The peaks located near 300 cmÀ1 and 200 cmÀ1 correspond to the LO and the SO phonons of the CdSe-like and CdS-like modes in alloy QDs.21 With the increase of the Se content, the LO1 CdSe-like phonon mode shifts from 188 cmÀ1 to 199 cmÀ1, whereas the LO2 CdS-like shifts from 295 cmÀ1 to 287 cmÀ1 (Fig 6d) Additionally, the presence of the higher order phonon modes 2LO1 (from 405 cmÀ1 to 389 cmÀ1), 2LO2 (from 581 cmÀ1 to 595 cmÀ1) and LO1 + LO2 (near 490 cmÀ1)22 are also observed as listed in Table I The observation of the phonon vibrational modes corresponding to both CdS and Optical and Phonon Characterization of Ternary CdSexS1Àx Alloy Quantum Dots Fig GaussianÀLorentzian fitting of the Raman spectra (a) CdSe, (b) CdSe0.5S0.5, (c) CdS, (d) LO, SO frequency shifts of ternary CdSexS1Àx QDs with different Se contents (0 £ x £ 1) Table I The Raman scattering phonon frequencies (cm21) of ternary CdSexS12x alloys (0 £ x £ 1) derived by fitting Raman spectra with a Gaussian–Lorentzian function Sample CdS CdSe0.25S0.75 CdSe0.5S0.5 CdSe0.75S0.25 CdSe LO1 188.07 188.94 190.3 199.45 LO2 296.17 291.24 288.31 288.23 SO1 167.83 172.06 172.9 192.27 SO2 290.68 283.87 264.86 260.96 2LO1 380.25 383.16 389.21 405.65 2LO2 595.46 586.69 581.61 584.18 FWHM LO1 19.74 19.90 24.01 14.32 FWHM LO2 8.96 15.01 17.87 18.75 First-order CdSe-like and CdS-like longitudinal and surface optical phonons are labeled LO1, SO1 and LO2, SO2, respectively The second order CdSe-like and CdS-like longitudinal optical phonons are labeled 2LO1 and 2LO2, respectively FWHM is the full width at half maximum of a peak CdSe is consistent with the other published results for binary CdS/CdSe nanocrystals.23 The frequency of the SO1 CdSe-like and SO2 CdS-like phonon increases from 167 cmÀ1 to 192 cmÀ1 and from 260 cmÀ1 to 290 cmÀ1 when increasing the Se content, respectively Here we derived the SO modes due to the asymmetrical broadening of their Raman peaks with the increase of the Se content Thus, the variation of the S/Se ratio leads to the shifting of Raman bands When the content of Se varies, the linear shifts of the LO1 and SO1 frequencies to higher energy and the LO2 and SO2 frequencies to lower energy are observed (Fig 6d) Since the formation of minority domains results in Thi, Cong, Dang, Nghia, and Quang independent peaks at the CdSe or CdS frequency, this strongly supports a homogeneous distribution of Se and S atoms over the QDs Simultaneously this result can be explained if we consider phonon confinement effects in the alloys and strain effects at the QDs interfaces, which may influence the vibrational frequencies Homogeneous alloying is further confirmed by the overall increase of the full width at half maximum (FWHM) of the peak with respect to the pure compounds This indicates the homogenous distribution of S and Se in this composition Other S/Se ratios result in an asymmetric and subsequent increase of the FWHM, which is in agreement with the expectations for homogeneous alloys.6 CONCLUSIONS In this work, we have presented a method to synthesize homogenous ternary CdSexS1Àx alloys QDs The optical and structural properties of the samples were studied in detail The coexistence of two ‘‘wurtzite’’ and ‘‘zinc-blend’’-type structural phases in the samples was observed The absorption band edges show a red shift as the Se content increases The band gap of CdSexS1Àx QDs shows an approximately linear dependence on the Se concentration, which is similar to the case of the band edge of PL spectra The homogeneity of 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