DSpace at VNU: Luminescent ZnS:Mn thioglycerol and ZnS:Mn ZnS core shell nanocrystals: Synthesis and characterization

Optical Materials 35 (2012) 136–140 Contents lists available at SciVerse ScienceDirect Optical Materials journal homepage: www.elsevier.com/locate/optmat Luminescent ZnS:Mn/thioglycerol and ZnS:Mn/ZnS core/shell nanocrystals: Synthesis and characterization Tran Thi Quynh Hoa a, Le Thi Thanh Binh a, Le Van Vu a, Nguyen Ngoc Long a,⇑, Vu Thi Hong Hanh b, Vu Duc Chinh b, Pham Thu Nga b a b Faculty of Physics, Hanoi University of Science, 334 Nguyen Trai Road, Thanh Xuan District, Hanoi, Viet Nam Institute of Materials Science, Vietnamese Academy of Science and Technology, 18 Hoang Quoc Viet Road, Cau Giay District, Hanoi, Viet Nam a r t i c l e i n f o Article history: Received 15 February 2012 Received in revised form July 2012 Accepted 14 July 2012 Available online 11 September 2012 Keywords: ZnS:Mn/thioglycerol ZnS:Mn/ZnS Core/shell nanocrystals Absorption Photoluminescence Photoluminescence excition a b s t r a c t The synthesis and photoluminescent properties of Mn2+-doped ZnS nanocrystals coated with an organic shell of thioglycerol and an inorganic shell of ZnS are reported in this paper The photoluminescence spectra of bare ZnS:Mn nanocrystals exhibited a dominant ultraviolet–violet emission peaked at the wavelength range of 395–450 nm and an weak orange emission peaked at the wavelength range of 580–600 nm The ultraviolet–violet emission was attributed to the surface defect states The orange emission was assigned to the 4T1–6A1 transition of Mn2+ ions These two channels of radiative recombination compete with each other The coating ZnS:Mn nanocrystals with the thioglycerol shells or the ZnS shells reduced the surface defects and led to the enhancement of the emission of Mn2+ ions On the other hand, the overcoating ZnS:Mn nanocrystals by thioglycerol shell restricted the growth of the nanocrystals, while the overcoating ZnS:Mn nanocrystals by ZnS shells made the band edge of the ZnS:Mn/ZnS core/shell nanocrystals shift to the lower energy side (the red shift) compared with the bare ZnS:Mn nanocrystals as observed in both the absorption and the photoluminescence excitation spectra Ó 2012 Elsevier B.V All rights reserved Introduction It is well known that there are two techniques for fabrication of strong photoluminescent nanocrystalline materials First technique is the introducing some impurities into nanocrystals (NCs) In the existing literature much effort has been devoted to the doping nanocrystals, for example, CdS:Mn [1], CdS:Eu [2], ZnO:Co,Ni [3], ZnSe:Mn [4], ZnS:Cu [5], ZnS:Pb,Cu [6], and ZnS:Mn [7–13] Second technique is the coating semiconductor NCs with organic or inorganic materials It has been shown that the coating NCs improves the photoluminescence quantum yields by passivating surface nonradiative recombination sites Since the 1990s up to now many articles have been devoted to core/shell and core/multishell NCs, for example, CdS/hexadecylamine (HDA) [14], ZnS/thioglycerol (TG) [15], ZnSe:Mn/TG [16], CdSe/ZnS [17], CdSe/ZnSe [18], CdSe/CdS [19], CdSe/CdS/ZnS, CdSe/ZnSe/ZnS [20], CdSe/CdZnS/ ZnS [21], and InP/ZnS [22] Currently, few reports on synthesis of ZnS:Mn/ZnS core/shell NCs appeared An enhanced luminescence was observed in Mn:ZnS/ZnS quantum dots synthesized by a reverse micelle route [23], a high-boiling solvent process [24], and a nucleation-doping strategy [25] However, the effects of the ZnS shell on the absorption (ABS), photoluminescence (PL), and ⇑ Corresponding author E-mail address: longnn@vnu.edu.vn (N.N Long) 0925-3467/$ - see front matter Ó 2012 Elsevier B.V All rights reserved http://dx.doi.org/10.1016/j.optmat.2012.07.018 photoluminescence excitation (PLE) spectra of core/shell nanocrystals have not been studied or incompletely studied In this paper, we coated the ZnS:Mn NCs with organic (TG) and inorganic (ZnS) shells The study was focused on the effect of the shell upon ABS, PL and PLE spectra The results showed that the surface states in our ZnS:Mn/TG and ZnS:Mn/ZnS core/shell NCs acted as radiative recombination centers competing with the Mn2+ optical centers Our experiment results indicated that the TG or ZnS shells could reduce the surface defects and led to the enhancement of luminescence of Mn2+ ions In addition, the TG shell acted as a stabilizer, decreasing the growth speed of the nanocrystals, while the ZnS shells caused the red shift of the band edge of the ZnS:Mn/ZnS NCs due to the increase of NC size Experimental 2.1 Synthesis of ZnS:Mn/TG core/shell NCs Typically, in order to prepare ZnS nanocrystals doped with at.% Mn, 2.634 g (0.012 mol) of Zn(CH3COO)2Á2H2O, 29.785 g (0.12 mol) of Na2S2O3Á5H2O, and 0.029 g (0.12 mmol) of Mn(CH3COO)2Á4H2O were mixed in 160 mL deionized water The mixture solution was magnetically stirred for 30 at room temperature to produce an optically clear solution The mixture solution temperature was then raised to 96 °C and was kept T.T.Q Hoa et al / Optical Materials 35 (2012) 136–140 constant at that temperature for 55 At that time 20 mL of this mixture solution was taken to obtain bare ZnS:Mn nanocrystalline sample (denoted by 55 no TG sample) In order to synthesize thioglycerol (TG)-coated ZnS:Mn NCs, mL of 11 M TG (C3H8O2S) solution was quickly injected into the above mixture solution In this case TG could be used as good ligands and a stabilizer This last solution was under steady stirring at constant temperature of 96 °C After 5, 10, 15, 30, 60, 180, and 360 since the time of injecting TG, every 20 mL of solution was taken to obtain ZnS:Mn/TG core/shell nanocrystalline samples (denoted by 55 + TG 05 sample, etc.) The produced solutions were centrifuged and washed many times with double distilled water The final powder products were dried in air at 60 °C for 12 h 2.2 Synthesis of ZnS:Mn/ZnS core/shell NCs The TOPZn (0.4 M) and TOPMn (0.4 M) stock solutions were prepared by adding Zn(CH3COO)2Á2H2O and MnCl2Á4H2O into trioctylphosphine (TOP), respectively For preparation of a basic mixture, 3.33 g of hexadecylamine (HDA) and 6.65 g of trioctylphosphine oxide (TOPO) were put into a three neck reaction flask closed and filled with N2 gas The mixture solution was magnetically stirred at 220 °C for 15 Then the appropriate amounts of TOPMn and TOPZn stock solutions and 0.21 mL of hexamethyldisilathiane, also known as bis(trimethylsilyl)sulfide ((TMS)2S), one after another 10 were quickly injected into the basic mixture solution During this process, the mixture solution was being stirred at 220 °C After that, the temperature was reduced to 120 °C under stirring for 15 The desired amount of received solution was taken and dispersed into the same quantity of toluene to obtain ZnS:Mn NCs The doping concentration of Mn2+ was 0.5, 1.0, 5.0, and 10.0 at.% of Zn2+ in ZnS In order to prepare ZnS:Mn/ZnS core/shell structure, it is necessary to calculate the required amount of shell precursors to obtain the desired shell thickness Our calculation is as follows [27]: The ZnS NCs have a face-centered-cubic structure with a lattice constant a There are Zn atoms and S atoms in a unit cell Suppose that the radius of the ZnS:Mn core NCs is r, the number of Zn and/ or S atoms in one ZnS NC will be: pr 3 nat ZnSịcore ẳ a3ZnS 1ị If the amount of Zn and/or S precursors used for preparation of the core NCs is m (mol), the mole amount of ZnS core NCs synthesized will be (assume all reactants have been consumed): nmol ZnSịcore ẳ m nat ZnSịcore 2ị When the thickness of one monolayer (ML) is d, the number of Zn and/or S atoms needed to coat one ZnS core NC with a shell of p ML is: nat ZnSịshell ẳ pẵr ỵ pdị3 r3 a3ZnS Â4 ð3Þ Therefore, the mole amount of Zn and/or S precursors needed for coating the as-prepared ZnS core NCs is: mol at nmol ZnSịshell ẳ nZnSịcore nZnSịshell 4ị Putting formulae (1)(3) into (4), we obtain: nmol ZnSịshell ẳ m ẵr ỵ pdị3 r3 r3 5ị In our experiment m = mmol, r = 2.47 nm, d is the spacing between the adjacent (1 1) lattice planes of cubic phase ZnS equal to 0.31 nm [12], p is the ML number of 2, 4, 6, and 137 For fabrication of ZnS:Mn/ZnS core/shell structure, first, ZnS:Mn core NCs were synthesized by the mentioned above method The temperature of ZnS:Mn core containing solution was kept at 200 °C In typical procedure for preparation of a ZnS shell with a thickness of ML, 2.45 mL of TOPZn stock solution and 0.2 mL of (TMS)2S were added drop wise into the colloidal solution containing ZnS:Mn core NCs After 15 of stirring, the suitable amount of the solution was taken and dispersed into the same quantity of toluene to obtain ZnS:Mn/ZnS core/shell NCs To receive ZnS:Mn/ ZnS NCs with more ZnS shell thickness, the similar processes were further repeated with different amounts of the stock solutions for shell growth The result of PL measurements indicated that the intensity of Mn2+ emission reached to maximum for the samples doped with at.% Mn At Mn2+ concentration of and 10 at.%, the Mn2+ emission disappeared because of the concentration quenching Therefore, in the current work, we performed coating only for the ZnS:Mn NCs with Mn concentration of 1.0 at.% and the shell thickness varied from to ML 2.3 Characterization of the samples Crystal structure of the NCs was analysed by using an X-ray diffractometer (XRD, SIEMENS D5005, Bruker, Germany) with Cu Ka1 (k = 1.54056 Å) irradiation The ultraviolet–visible (UV–vis) absorption spectra were obtained by a Shimadzu UV 2450 PC spectrometer The PL and the PLE spectra were measured at room temperature on a spectrofluorometer (Fluorolog FL 3-22 Jobin Yvon Spex, USA) with a 450 W xenon lamp as an excitation source Results and discussion 3.1 Structural characterization Fig shows typical powder XRD patterns for the ZnS:Mn/TG core/shell NCs with different TG-reaction times of 5, 10, 15, 30, 60, 180, and 360 (Fig 1a), and the ZnS:Mn/ZnS core/shell NCs with the ZnS shell thickness of 0, 2, 4, 6, and ML (Fig 1b) The diffraction peaks at 2h values of 28.7, 47.7 and 56.6 Å correspond to the (1 1), (2 0) and (3 1) diffraction planes, respectively It is obvious that all the peaks in the XRD patterns can be indexed to the cubic zinc blend phase of ZnS No other diffraction peaks are detected except for the ZnS related peaks The lattice constant calculated from XRD patterns is a = 0.540 ± 0.003 nm, in good agreement with the value in the standard card (JCPDS No 05-0566, a = 0.5406 nm) From Fig it can be seen that the diffraction lines are rather wide, indicating that the size of the NCs is small The average sizes of the ZnS nanocrystals were estimated by Scherrer’s formula [26]: L¼ 0:9k b cos h ð6Þ where b is the full width at half maximum (FWHM) in radians of the diffraction peaks, h is the Bragg’s diffraction angle and k is the wavelength for the Ka1 component of the employed copper radiation (1.54056 Å) The average sizes of the nanocrystals were estimated using (1 1) reflection peak to be 1.54, 2.09, 1.86, 2.03, 2.13, 2.16, 2.54 nm for the ZnS:Mn/TG with the TG-reaction times of 5, 10, 15, 30, 60, 180, and 360 min, respectively and 2.07, 2.16, 2.49, 2.30, 2.62 nm for the ZnS:Mn/ZnS NCs with the ZnS shell thickness of 0, 2, 4, 6, and ML, respectively Fig 1a also shows the XRD pattern of the ZnS:Mn NCs without TG coating prepared at 96 °C for 415 the same as the condition for preparing the (55 + TG 360 min) ZnS:Mn/TG NCs It can be seen that the diffraction lines of the ZnS:Mn NCs without TG 138 T.T.Q Hoa et al / Optical Materials 35 (2012) 136–140 Fig Typical powder XRD patterns for (a) the ZnS:Mn/TG NCs with different TGreaction time and (b) the ZnS:Mn/ZnS NCs with the ZnS shell thickness of 0, 2, 4, 6, and ML coating are much narrower than that of the ZnS:Mn/TG NCs The average size of the ZnS:Mn no TG NCs is 11.9 nm larger than that of the ZnS:Mn/TG NCs This fact indicated that the TG coating prevents the growth of ZnS:Mn NCs 3.2 Optical characterization 3.2.1 Absorption Temporal evolution of UV–vis absorption spectra at room temperature of the ZnS:Mn/TG NCs during the growth of NCs at 96 °C and the evolution of UV–vis spectra of the ZnS:Mn/ZnS NCs with different number of monolayers were investigated in detail It was found that all the spectra exhibit a sharp absorption edge and the absorption onset of the ZnS:Mn/TG and ZnS:Mn/ZnS NCs is shifted to the long wavelength side (the red shift) with increasing the TG-reaction time or the ZnS shell thickness, as usually observed in the core/shell NCs [27] It is well known that cubic ZnS is a direct-gap semiconductor [28] The relation between the absorption coefficients (a) and the incident photon energy (hm) for the case of allowed direct transition is written as follows [29]: ahm ¼ Aðhm À Eg Þ1=2 ð7Þ where A is a constant and Eg is the bandgap of the material The plots of (ahm)2 versus hm for the two kinds of core/shell NCs are represented in Fig By extrapolating the straight portion of the graph on hm axis at a = we found the bandgap Eg of the ZnS:Mn/TG NCs with the TG-reaction times of 0, 5, 10, 15, 30, 60, Fig The plots of (ahm)2 versus hm for (a) the ZnS:Mn/TG NCs with different TGreaction times and (b) the ZnS:Mn/ZnS NCs with different shell thicknesses The concentration of Mn in all the core ZnS:Mn NCs is at.% 180, and 360 to be 4.06, 4.06, 4.03, 4.01, 3.99, 3.95, 3.94, and 3.75 eV, respectively The Eg of the ZnS:Mn/ZnS NCs with the shell thicknesses of 0, 2, 4, 6, and ML was found to be 3.94, 3.93, 3.89, 3.85, and 3.83 eV, respectively It noticed that the values Eg of all the ZnS:Mn/ZnS NCs are higher than the bandgap value of 3.61 eV at room temperature for the cubic bulk ZnS [27] This fact is a clear evidence of the quantum confinement effect: In the case of ZnS:Mn/TG NCs, the band edge shift toward the lower energy side originated from increasing of the NCs size during their growth [14] In the case of ZnS:Mn/ZnS NCs, the red shift of band edge, in general, was due to a partial leakage of the exciton into the shell material [27] But in our case, the shell material was ZnS as same as the ZnS core, therefore, the observed red shift of band edge was assigned to the increase of NC size From the blue shift of the band edge (DEg), the nanocrystalline radius r could be determined using the Brus relation given as follows [30]: DEg ẳ EgNCị Egbulkị ẳ    p2 h 1:8e2 ỵ me mh 2r er ð8Þ where Eg(NC) and Eg(bulk) are the bandgaps of NC and bulk material, respectively; h  is the reduced Plank’s constant, e is the electron charge, mÃe and mÃh are the effective masses of electron and hole, respectively, e is the dielectric constant of the material In the SI units and the energy is calculated in eV, this formula is written as follows: DEg ẳ EgNCị Egbulkị ẳ    p2 h 1 1:8e ỵ 4pe0 er 2er me mh 9ị T.T.Q Hoa et al / Optical Materials 35 (2012) 136–140 where e0 ¼ 8:854 Â 10À12 FmÀ1 is the permitivity of free space For the ZnS mÃe ¼ 0:34m0 , mÃh ¼ 0:23m0 , and e ¼ 8:76, m0 is the free electron mass [31] Using the mentioned Eq (9), the nanoparticle radius was determined to be 2.16, 2.16, 2.23, 2.28, 2.33, 2.44, 2.47, and 3.50 nm for the ZnS:Mn/TG with the TG-reaction times of 0, 5, 10, 15, 30, 60, 180, and 360 min, respectively The nanocrystal radius was estimated to be 2.47, 2.50, 2.65, 2.82 and 2.92 nm for the ZnS:Mn/ZnS NCs with the shell thicknesses of 0, 2, 4, and ML, respectively It can be seen that the size values obtained from the band gap change are not in good agreement with those obtained from the XRD data This is because both Scherrer formula and Brus relation used for the size calculation are of approximation However, we chose the bigger value of 2.47 nm for Eq (5) in anticipation of the fact that the reactants for the shell growth have not been consumed It is worth noting from the above results that the size increase observed from the band gap change was smaller than that expected from calculation The reason for this most likely is that the reactants for the shell growth, in fact, were not consumed 3.2.2 Photoluminescence Fig shows the room temperature PL spectra excited with a wavelength of 336 nm for the ZnS:Mn/TG NCs with different TGreaction times and the ZnS:Mn/ZnS NCs with various numbers of shell monolayers It is found that in the PL spectra excited by the wavelength of 336 nm, for all the kinds of NCs, two emission peaks are observed One is a UV–violet emission peak (located at 432 or 401 nm) and another is an orange emission peak (located at 594 or Fig The room temperature PL spectra excited by the wavelengths of 336 nm for (a) the ZnS:Mn/TG NCs with different TG- reaction times and (b) the ZnS:Mn/ZnS NCs with the shell thickness of 0, 2, 4, 6, and ML 139 580 nm) These two radiative recombination channels compete with each other The UV–violet emission can be ascribed to the radiative recombination via surface defect states of NCs [24] The orange emission peak was attributed to the 4T1(4G) ? 6A1(6S) transitions within the 3d shell of Mn2+ ion [12] It is interestingly noticed that for ZnS:Mn NCs without coating with TG or ZnS shell, the UV–violet emission peak is dominant (lines a in Fig 3) As the TG-reaction time increases, the intensity of 432 nm emission peak significantly reduces, while the 594 nm emission peak becomes dominant and its intensity remarkably increases with increasing the TG-reaction time (lines b–h, Fig 3a) The change of the relative intensity of the two mentioned above emission peaks is clearly observed in Fig 3b as well For the bare ZnS:Mn NCs the 401 nm peak is dominant so that intensity ratio of the orange and the UV–violet peaks IO/IUV is 0.23 When an additional ZnS shell with the thickness of 2, and ML is grown on the core ZnS:Mn NCs, the intensity of the 401 nm emission peak decreases, whereas the intensity of the 580 nm emission peak increases so that the ratio IO/IUV raises up to 1.69, 2.74, and 3.81, respectively Thus, optimal thickness of ZnS coating to obtain a good PL is ML The further increasing the shell thickness up to ML causes a little decrease of the ratio IO/IUV ($3.53) This is because when the shell is too thick, inside the thick ZnS shell again appeared the structural defects (dislocations, gain boundaries), which play the role of nonradiative recombination sites reducing the PL intensity [14,20] Thus, by formation of an additional TG or ZnS shell with an appropriate thickness on the core ZnS:Mn NCs, the orange emission intensity of the ZnS:Mn/TG or the ZnS:Mn/ZnS core/shell NCs will be enhanced because of the passivating the surface defects due to a good ligand or a good match between core and shell lattice constants, which is in good agreement with previous reports [23,25] 3.2.3 Photoluminescence excitation The room temperature PLE spectra monitored at the 592 nm emission peak for the ZnS:Mn/TG NCs are illustrated in Fig 4a The PLE spectra exhibit both the excitation band due to the nearband-edge absorption (bands lower than 375 nm) and three excitation bands peaked at 430 nm (2.88 eV), 466 nm (2.66 eV), and 496 nm (2.50 eV) The three excitation bands at 430, 466, and 496 nm are attributed to the absorption transitions from basic state 6A1(6S) to excited states 4T2(4D); 4E(4G), 4A1(4G); and T2(4G) within Mn2+ ion, respectively [12] The PLE spectra monitored at the 580 nm emission peak for the ZnS:Mn/ZnS NCs are depicted in Fig 4b For the bare ZnS:Mn NCs, the PLE spectrum exhibits an weak excitation band peaked at 381 nm (line a, Fig 4b) When the core ZnS:Mn NCs are overcoated with ZnS shells, the 381 nm excitation band becomes weaker (lines b–e, Fig 4b) From this fact, it is possible to infer that the 381 nm excitation band relates to the band-to-defect state absorption transitions It is worth noted that when the 381 nm excitation band becomes weaker, four weak excitation peaks located in the wavelength region of 380–500 nm can be clearly observed (inset of Fig 5) These four excitation peaks at 389 nm (3.10 eV), 425 nm (2.92 eV), 463 nm (2.68 eV), and 493 nm (2.51 eV) are assigned to the absorption transitions from basic state 6A1(6S) to excited states 4E(4D); 4T2(4D); 4E(4G), 4A1(4G); and 4T2(4G) within Mn2+ ion, respectively [12] The other excitation peaks appeared at the wavelengths lower than 360 nm are assigned to the band-to-band absorption transitions in the ZnS:Mn/ZnS NCs It should be noted that in addition to the weakening of the 381 nm excitation peak, the red shift of the absorption onset with increasing the shell thickness is observed as well, which is in good agreement with our results 140 T.T.Q Hoa et al / Optical Materials 35 (2012) 136–140 NCs possess a cubic zinc blend structure with lattice constant a = 0.540 ± 0.003 nm The PL spectra of the two core/shell NCs exhibited a UV–violet emission located at 395–450 nm, and an orange emission peaked at 580–600 nm These two radiative recombination channels compete with each other The UV–violet emission was attributed to the surface defect states The orange emission was assigned to the 4T1–6A1 transition of Mn2+ ions The coating ZnS:Mn NCs with a shell of organic (TG) or inorganic (ZnS) material enhanced the emission of Mn2+ ions due to passivating the surface defects In addition, the absorption and the PLE measurements indicated that the overcoating ZnS:Mn NCs by TG shell limited the growth of the nanocrystals, while the overcoating ZnS:Mn nanocrystals by ZnS shells made the band edge of the ZnS:Mn/ZnS core/shell nanocrystals shift to the lower energy side (the red shift) in comparison with the bare ZnS:Mn nanocrystals; the reason for this is the increase of NC size Acknowledgement This work is financially supported by Vietnam National University, Hanoi (TRIG A project No QGTD 10.24) References Fig (a) Normalized room temperature PLE spectra monitored at the 592 nm emission peaks of the ZnS:Mn/TG NCs with various TG-reaction times and (b) PLE spectra monitored at the 580 nm emission peaks of the ZnS:Mn/ZnS NCs with various numbers of shell ML Fig Normalized room 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Springer-Verlag, Berlin, New York, 1987 [32] B Mahler, P Spinicelli, S Buil, X Quelin, J.P Hermier, B Dubertret, Nat Mater (2008) 659  ... indicated that the overcoating ZnS: Mn NCs by TG shell limited the growth of the nanocrystals, while the overcoating ZnS: Mn nanocrystals by ZnS shells made the band edge of the ZnS: Mn/ ZnS core/ shell. .. measurements and the results of previous report for CdSe/CdS core/ shell NCs [32] Conclusions The ZnS: Mn/ TG and ZnS: Mn/ ZnS core/ shell NCs have been synthesized Both the synthesized ZnS: Mn/ TG and ZnS: Mn/ ZnS. .. TG or ZnS shell with an appropriate thickness on the core ZnS: Mn NCs, the orange emission intensity of the ZnS: Mn/ TG or the ZnS: Mn/ ZnS core/ shell NCs will be enhanced because of the passivating