ZnS

APPLIED PHYSICS LETTERS 97, 193104 共2010兲

Luminescence properties of In„Zn…P alloy core/ZnS shell quantum dots Ung Thi Dieu Thuy,1,2 Peter Reiss,2 and Nguyen Quang Liem1,a兲 1

Institute of Materials Science (IMS), Vietnam Academy of Science and Technology (VAST), 18 Hoang Quoc Viet, Cau Giay, Hanoi, Vietnam 1000 2 DSM/INAC/SPrAM (UMR 5819 CEA-CNRS-Université Joseph Fourier)/LEMOH, CEA Grenoble, 17 Rue des Martyrs, 38054 Grenoble Cedex 9, France

共Received 15 September 2010; accepted 21 October 2010; published online 10 November 2010兲 Chemically synthesized InP/ZnS core/shell quantum dots 共QDs兲 are studied using time-resolved photoluminescence spectroscopy and x-ray diffraction. Zinc stearate, which is added during the synthesis of the InP core, significantly improves the optical characteristics of the QDs. The luminescence quantum yield 共QY兲 reaches 60%–70% and the emission is tunable from 485 to 586 nm by varying the Zn2+ : In3+ molar ratio and growth temperature. The observed increased Stokes shift, luminescence decay time, and QY in the presence of Zn are rationalized by the formation of an In共Zn兲P alloy structure that causes band-edge fluctuation to enhance the confinement of the excited carriers. © 2010 American Institute of Physics. 关doi:10.1063/1.3515417兴 Because of the high surface-to-volume ratio and the quantum confinement effect, semiconductor quantum dots 共QDs兲 or nanocrystals 共NCs兲 have become interesting objects for both fundamental research and practical applications.1–5 Among III-V semiconductors, InP has attracted much interest because it is a direct gap material with a band gap of 1.27 eV, which is suitable for getting visible emission in the quantum confinement regime. For the synthesis of III-V materials, Wells et al.6 originally developed a dehalosylilation reaction applied to InAs and GaAs, which has been transposed to InP.7 Many other synthetic routes have been reported in the meanwhile.8–13 Nowadays, mainly fatty acids are used as stabilizers of indium salts 共e.g., indium acetate or indium chloride兲 that react with tris共trimethylsilyl兲phosphine, P共TMS兲3 in the noncoordinating solvent 1-octadecene.14–16 As-prepared InP NCs generally show weak luminescence because of the existence of nonradiative relaxation channels originating from surface states. After overcoating with a ZnS shell,15–17 InP NCs become highly luminescent. In a previous paper, we reported the synthesis of InP/ZnS NCs by a heating up single-step method yielding samples with a fluorescence quantum yield 共QY兲 exceeding 60%.16 In this method, all 共In, P, Zn, and S兲 precursors are mixed at room temperature and then quickly heated to 250– 300 ° C. The presence of Zn-stearate and dodecanethiol in the reaction mixture results in the formation of ZnS at elevated temperature 共⬎230 ° C兲. Recent progress in the elucidation of the internal structure of the obtained QDs by means of synchrotron X-ray photoelectron spectroscopy revealed that they consist of an alloyed InPZnS core covered by a thin ZnS shell.18 This structure also explains the efficient energy transfer by dipoledipole interactions observed for close-packed QDs produced by the one-pot method.19 In this letter, we discuss on the contribution of zinc to the improvement of the optical properties of In共Zn兲P/ZnS QDs synthesized using a two-step method. In contrast to the above cited method,16 not only dodecanethiol but also zinc stearate is added during the InP core NC synthesis, while the a兲

Author to whom correspondence should be addressed. Electronic mail: [email protected].

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ZnS shell growth is carried out in a second distinct step at lower temperature. By varying the Zn2+ : In3+ molar ratio 共from 0:1 to 2:1兲 and the reaction temperature 共250 or 300 ° C兲 during core InP NC growth, QDs emitting in the range of 485–586 nm were obtained, which exhibit after ZnS shell growth QYs of 60%–70%. The addition of zinc carboxylates—in particular of zinc undecylenate—during the synthesis of InP QDs has been previously investigated by Nann and co-workers.17 They concluded that zinc carboxylates played the role of surface ligands for InP NCs, while the possibility of lattice doping by zinc was ruled out. Although quantitative data concerning the internal structure of ⬍2 nm InP NCs is not available to date, we present an ensemble of spectroscopic studies, namely, UV-visible absorption, steady-state photoluminescence 共SSPL兲, and timeresolved photoluminescence 共TRPL兲, which clearly point at the formation of an In共Zn兲P alloy structure. Figure 1 shows the absorption and SSPL spectra of two

FIG. 1. 共Color online兲 Absorption and SSPL spectra of InP/ZnS core/shell QDs 共i.e., the Zn2+ : In3+ ratio equals to 0:1兲 and of typical alloy In共Zn兲P/ZnS QDs 共Zn2+ : In3+ ratio of 1:1兲. The inset shows the PL peak shift as a function of the Zn2+ : In3+ ratio at two different reaction/shelling temperatures.

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In共Zn兲P/ZnS QDs samples synthesized in the same conditions except the change of the Zn2+ : In3+ molar ratio being 0:1 关Figs. 1共a兲 and 1共b兲兴 and 1:1 关Figs. 1共c兲 and 1共d兲兴, respectively. The In共Zn兲P core was synthesized at 300 ° C followed by shelling with ZnS at 285 ° C for 1 h. For the InP/ ZnS 共i.e., Zn2+ : In3+ = 0 : 1兲 QDs, the excitonic absorption appears as a shoulder and the PL spectrum is composed of two components: one peak at 586 nm is assigned to originate from InP QDs while the one peaking near 470 nm is believed to originate from the ZnS shell.20 The QY of this kind of QDs is rather low, around 1%. Also, the full width at half maximum 共FWHM兲 of the PL spectrum is comparably large 共90 nm兲. The Stokes shift in this case is of 280 meV. For the samples with the presence of Zn in the core QD reaction solution, a pronounced excitonic absorption and strong, rather narrow luminescence was observed at 443 and 510 nm, respectively, i.e., at much shorter wavelengths in comparison with the first case. The sharp excitonic absorption and narrow excitonic emission 共FWHM⬃ 50 nm兲 indicate the high crystalline quality of the synthesized In共Zn兲P/ZnS QDs. Also in this case, the Stokes shift is much larger, namely, 370 meV, than in the case without using Zn for the core NC synthesis. For various Zn contents in In共Zn兲P/ZnS QDs, we observed a systematic redshift of the SSPL peak with decreasing Zn2+ : In3+ molar ratio and with increasing reaction/growth temperature, as shown in the inset of Fig. 1. The experimental results indicate that zinc stearate not only affects the optical properties but also affects the growth kinetics of the InP NCs. When using the same zinc concentration, a higher growth temperature resulted in bigger particles emitting at longer wavelengths. However, at high reaction temperatures, Ostwald ripening leads to an increased size dispersion and consequently to the broadening of the PL peak line width. By changing the Zn2+ : In3+ molar ratio in the precursor solution and reaction/growth temperature, the excitonic and the emission peak could be conveniently tuned over a wide spectral range of 420–515 and 485–586 nm, respectively, corresponding to a mean size of InP core in the range of 1.5–2.5 nm.21 The optical characteristics obtained from the absorption and SSPL measurements imply that alloy In共Zn兲P core NCs were formed rather than pure InP NCs. The larger Stokes shift of the In共Zn兲P/ZnS as compared to that of InP/ZnS is an evidence for band fluctuation due to alloying the lattice structure by replacing Zn into the In site, as similarly observed in ZnSSe thin film and other alloy QDs.22,23 Summarizing, the experimental results obtained so far clearly indicate the formation of In共Zn兲P alloy NCs in the presence of zinc, which possess a larger band gap energy than InP. The alloy structure facilitates subsequent ZnS shell growth due to a “smoother” change of the lattice parameters at the interface between the In共Zn兲P core and the ZnS shell than in the case of a pure InP core. The suppression of InP defect states including P dangling bonds results in a significant enhancement of the QY of the alloy In共Zn兲/ZnS QDs.24 To study further the role of Zn in the optical properties of In共Zn兲P/ZnS, we performed TRPL measurements on a series of samples with different Zn concentrations. From these measurements, it is possible to trace the PL intensity at a certain delay from the excitation moment to exactly determine the instant PL intensity of each spectral component in a complex emission. Figure 2 illustrates the TRPL spectra of the InP/ZnS real core/shell QDs 关Fig. 2共a兲兴 and the alloy

Appl. Phys. Lett. 97, 193104 共2010兲

FIG. 2. 共Color online兲 TRPL spectra taken from 共a兲 InP/ZnS core/shell QDs 共without Zn兲 and 共b兲 alloy In共Zn兲P/ZnS QDs with a Zn2+ : In3+ ratio of 1:1 synthesized under the same conditions.

In共Zn兲P/ZnS QDs 关Fig. 2共b兲兴 prepared with the Zn2+ : In3+ molar ratio of 1:1 while keeping all other experimental conditions the same. At a delay time of 12 ns, the InP/ZnS core/shell structure exhibited InP excitonic emission at 586 nm along with a peak at around 470 originating from imperfections in the ZnS shell 关Fig. 2共a兲兴.20 It has to be noted that this defect emission peak shows a higher intensity than the peak arising from InP, in contrast to the SSPL measurements 共Fig. 1兲. The alloy In共Zn兲P/ZnS QDs only exhibits a single PL band at 510 nm. Also the evolution with delay time from the excitation moment of the two kinds of QDs is not the same due to differences in the decay time corresponding to each spectral component. For the InP/ZnS core/shell QDs, the evolution of both the 470 nm and the 586 nm bands with delay time depends on the recombination/transition rates in the ZnS shell and in the InP core, respectively. In addition, it is related to the transfer rate of charge carriers from the ZnS shell to the InP core.15 Figure 3共a兲 reveals that the decay time of the blue emission in ZnS is much shorter 共around 6 ns兲 than that of the InP NCs 共68 ns兲. Therefore, with a long delay time, we mainly observed the InP core luminescence. For the alloy In共Zn兲P/ZnS QDs 关Fig. 3共b兲, Zn: In= 1 : 1兴, the PL decays with two time constants: the shorter one 共38 ns兲 is related to recombination at surface states and the longer one 共85 ns兲 is responsible for the excitonic transition. By fitting to biexponential functions corresponding to the emissions from the two kinds of QD structures mentioned above, we have extracted the decay times for different zinc contents in the initial reaction mixture 共inset of Fig. 3兲. It is clearly seen that with increasing Zn concentration the decay times corresponding to both emission mechanisms increase. This is again in good agreement with the enhanced confinement of the excited charge carriers due to lattice fluctuation caused by the replacement of Zn for In. In other words, the excited charge carriers are confined longer and longer by lattice fluctuation with increasing zinc concentration in alloy In共Zn兲P/ ZnS QDs, corresponding to the longer observed decay times. In conclusion, alloy In共Zn兲P/ZnS core/shell QDs, synthesized in the presence of zinc during InP core NC synthe-


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efits from the Evariste Galois fellowship of the French Ministry of Foreign Affairs and from a funding of CEA-DRI. We further acknowledge financial support from the French Research Agency 共PNANO-07-NANO-044兲 and from CEA 共Technologies pour la Santé “TIMOMA2”兲. 1

FIG. 3. 共Color online兲 PL decay curves for 共a兲 InP/ZnS core/shell QDs and 共b兲 alloy In共Zn兲P/ZnS QDs measured at the emission peaks of InP and In共Zn兲P QDs. The deconvolution of each PL decay curve shows two components, tS and tL, both of which are longer with higher Zn concentration as presented in the inset.

sis, were studied by using XRD, absorption, and SSPL/TRPL techniques. The emission range of the alloy In共Zn兲P/ZnS QDs could be tuned from 485 to 586 nm 共FWHM 50 nm兲 by varying the Zn2+ : In3+ molar ratio and reaction temperature. Zinc stearate has a double role in the nucleation and growth process of InP QDs: 共i兲 as a surfactant influencing reaction kinetics; and 共ii兲 as a zinc precursor resulting in an In共Zn兲P alloy structure. The high fluorescence QY of 60%–70% is rationalized by the alloy structure of the Zn-containing InP/ ZnS QDs that causes the band-edge fluctuation to enhance the confinement of the excited carriers and consequently enhances the radiative transition probability. This work was partially supported by the National Foundation for Science and Technology Development 共NAFOSTED Vietnam, code 103.03.35.09兲. U.T.D.T. ben-

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