Lattice Strain Controls the Carrier Relaxation Dynamics in CdxZn1âxS

Article pubs.acs.org/JPCC

Lattice Strain Controls the Carrier Relaxation Dynamics in CdxZn1−xS Alloy Quantum Dots Suparna Sadhu and Amitava Patra* Department of Materials Science, Indian Association for the Cultivation of Science, Kolkata-700 032, India S Supporting Information *

ABSTRACT: Here, we demonstrate the impact of lattice strain on the carrier relaxation of CdxZn1−xS alloy nanocrystals. In alloy nanocrystals, the difference in the lattice constants of their constituents can induce a lattice strain that varies with the composition of alloy. We have analyzed the decay curves using a multiexponential decay function, and it is found that the average lifetime decreases with increasing the concentration of Zn in CdxZn1−xS alloy nanocrystals. A stochastic model of carrier relaxation dynamics of CdxZn1−xS alloy nanocrystals has been proposed to estimate the values of the radiative relaxation rate, nonradiative relaxation rate, and number of trap states. Results show that the radiative and nonradiative relaxation rates and the number of trap states increase with increasing the lattice strain of the alloy nanocrystals. Such alloy nanocrystals should have great potentials for nonlinear optical properties, photovoltaic devices, and solar cell.

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INTRODUCTION There has been a growing interest on utilizing semiconductor quantum dots (QDs) for technological applications such as efficient solar energy conversion,1−4 light-emitting diodes,5−7 photovoltaic cells,8,9 laser diodes,10 and so on. Lattice strain has a large influence on the optical properties of semiconductor materials at nanoscale.11−15 In nanocrystalline solids, straininduced changes in lattice parameters alter the intrinsic interatomic distances and modify the energy levels of bonding electrons that leads the changes in their electronic and optical properties.16 Semiconductor core/shell nanocrystals are a class of strained materials where lattice mismatch between the core and shell alters its various properties.12,17−19 The epitaxial deposition of a compressive shell material (ZnS, ZnSe, ZnTe, CdS, or CdSe) on a small and soft nanocrystalline core (CdTe) can dramatically change the conduction and valence band energies of both the core and the shell. Lattice-mismatchinduced strain leads to spatial separation of electrons and holes and prolongs the excited-state lifetimes.12 It is known that the interface strain has a great contribution on the modulation of band gap in coherent wurtzite CdSe/CdTe core/shell nanowire (NW) heterostructures.18 Again, it is reported that the blinking behavior of colloidal core/shell QDs has been influenced by lattice strain.19 Although the effect of strain on semiconductor devices has been widely studied,20,21 no attention has been paid to understanding the influence of lattice strain on the carrier relaxation dynamics of colloidal semiconductor nanocrystals. The detailed understanding of the carrier relaxation dynamics is essential because it dictates the overall efficiency in various optoelectronics, photovoltaic, light-harvesting, and sensing applications.22 Recently, semiconductor alloy QDs (ABxC1−x) have attracted much attention in these technological applications because of their unique composition-dependent optical and electronic properties without changing the particle size.23−25 Lattice mismatch is expected in alloy nanocrystals due © 2012 American Chemical Society

to the difference in the lattice constants of their constituents, and this lattice mismatch can induce a lattice strain. Recently, a few studies26−28 have been reported on the excited state lifetime of colloidal ternary alloy QDs, but the impact of strain on carrier dynamics of ternary alloy QDs is still unrevealed. Here, we have tuned the lattice strain by varying the composition of CdxZn1−xS alloy nanocrystals, and the impact of lattice strain on carrier relaxation dynamics has been discussed. We have studied the decay dynamics by using time-resolved spectroscopy and analyzed the decay curves using multiexponential decay function. Finally, we propose a stochastic model to estimate the radiative and nonradiative relaxation rate and the number of trap states and compare these decay parameters with experimental data. Finally, we try to correlate the radiative and nonradiative relaxation rates and the number of trap states with lattice strain developed in CdxZn1−xS alloy QDs.

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EXPERIMENTAL SECTION Materials and Methods. The detailed synthesis method for the preparation of Cd0.73Zn0.27S, Cd0.62Zn0.38S, Cd0.52Zn0.48S, Cd0.44Zn0.56S, and Cd0.31Zn0.69S alloy QDs has been reported in our previous publication.29 For the preparation of Cd0.52Zn0.48S sample, 0.066 g of cadmium acetate and 0.0548 g of zinc acetate were added to 5 mL of oleylamine in a two-necked roundbottom flask, and the mixture was heated to 150 °C under Ar flow for 20 min to form a clear solution. At this temperature, an excess amount of S powder (dissolved in 2.5 mL of oleylamine) was swiftly injected into the hot reaction mixture under gentle stirring. The reaction mixture was kept at the desired growth temperature (150 °C). After 2 h, the reaction was quenched by Received: May 21, 2012 Revised: June 23, 2012 Published: June 28, 2012 15167

dx.doi.org/10.1021/jp304901w | J. Phys. Chem. C 2012, 116, 15167−15173

The Journal of Physical Chemistry C

Article

the addition of a large volume of anhydrous toluene into the reaction mixture. The resulting alloy nanocrystals in toluene solution were precipitated out by using ethanol and isolated by centrifugation and decantation. A series of CdxZn1−xS QD alloy samples (x = 0.73, 0.62, 0.44, 0.31) were also prepared under the same experimental conditions by varying the initial molar ratios of Cd and Zn precursors. Characterization. The compositions of the alloy QD were determined by means of a Shimadzu atomic absorption spectrophotometer AA-6300, equipped with a lamp for Cd and Zn at wavelengths 228.8 and 213.9 nm, respectively. The crystalline phases of the nanoparticles were identified by X-ray diffraction (XRD) using a Siemens model D 500, powder X-ray diffractometer using a Cu Kα source (1.5418 Å radiation). Room temperature optical absorption spectra and the emission spectra of all samples were recorded on a Shimadzu UV−vis spectrophotometer and a Fluoro Max-P (HORIBA JOBIN YVON) luminescence spectrometer, respectively. For the time correlated single photon counting (TCSPC) measurements, the samples were excited at 375 nm by a picoseconds diode laser (IBH Nanoled-07) with 1 MHz repetition rate in an IBH Fluorocube apparatus. The pulse duration is , |ex> and |tr> represent ground 15170



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are represented by red solid lines in Figure 4b−f, and the values of parameters are given in Table 2. The radiative recombination rate (kr) varies from 0.012 to 0.12 ns−1 as the Cd mole fraction changes from 0.73 to 0.31. The nonradiative rate due to the trapping of charge carriers in each trap state (knr) and the number of trap states (mt) are increased upon decreasing the Cd mole fraction (Table 2). It is already reported39 that nonradiative recombination rate is increased with increasing the lattice strain, which is associated with defects at the heterostructure interfaces resulting from the lattice constant mismatch. Here, the lattice mismatch is ∼7% between zinc blende CdS and ZnS in CdxZn1−xS alloy QDs, which induces a lattice strain that is associated with the nonradiative rate. The compressive lattice strain within CdxZn1−xS alloy QDs increases with decreasing the Cd mole fraction and creates more trap states (mt) in low Cd content CdxZn1−xS QDs. It reveals that this large numbers of trap states are responsible for the higher values of the nonradiative recombination rate (knr) in low Cd content CdxZn1−xS QD. The values of the rate (knr′) due to the luminescence quenching process by NC local environment and the values of the average number of quenchers (mt′) surrounding the NC are very close for different CdxZn1−xS alloy QDs, indicating that the local environment (e.g., surface capping ligand, inter-NC energy transfer) around the NC has no influence on the carrier relaxation dynamics of different CdxZn1−xS alloy QDs (Table 2). A slight deviation of these values is seen from the values calculated from stochastic model. The quantum yield measurements on nonexponential decay systems are not so easy. Driel et al.38 observed the deviation of the emission decay curve when both radiative and nonradiative decays are independently distributed. As a result, the emission quantum efficiency is distributed, not expressed by a single number. The distribution of the total decay rates weighted with the radiative rates. Thus, we have used the above method to measure the quantum yields and measured the values of radiative and nonradiative rates from quantum yield. In order to find out the relation between the lattice strain and decay parameters, we have plotted the radiative recombination rates (kr), the average number of trap states (mt), and the nonradiative relaxation rates due to the trap states (knr) against the lattice strain in Figure 6. It is interesting to note that the values of decay parameters (kr, mt, knr), i.e., radiative recombination rate, nonradiative relaxation rates, and the average number trap states are increased with increasing the compressive lattice strain. We have estimated some parameters independently and check the validity of this kinetic model (Supporting Information, Figure S4). Analysis reveals that the lattice strain plays an important role on the carrier relaxation dynamics of alloy QDs, which is new observation in this study.

Figure 5. A schematic of various carrier relaxation processes in semiconductor QDs.

state, excited state and trap state, respectively. In semiconductor nanocrystals, the photoluminescence originates from the radiative recombination of charge carriers. In addition to the radiative recombination, there are several nonradiative relaxation processes including carrier trapping at QD defect states/surface states, quenching due to the NC local environment (e.g., interparticle energy transfer, charge transfer into ligand-based orbitals, ligand exchange at the QD surface), relaxation from trap states, and so on.22,42,43 A combination of all these processes gives rise to multiexponential emission dynamics that occurs over a nanosecond time scale. In our model, kr is the rate due to the radiative recombination, knr is the nonradiative rate due to the trapping of charge carriers in each trap state, and knr′ is the rate due to the luminescence quenching process by NC local environment. We also consider that nt is the number of trap states participating in the charge carrier trapping process and nt′ is the number of quenchers surrounding the NC. It is assumed that the distribution of the number of trap states and the distribution of number of quenchers surrounding the NC follows a Poisson distribution,41,44 namely, Φ(n) = (mn /n! )exp( −m)

(5)

Therefore, the ensemble-averaged decay curve of the excited semiconductor nanocrystals is given by α

I t = I0

α

∑ ∑

Φ(nt) Φ(nt′) exp[−(k r + ntk nr + nt′k nr′)

n t = 0 n t ′= 0

t]

(6)

After simplification, the final equation will be It = I0 exp{−k rt − mt [1 − exp(−k nrt )] − mt ′ [1 − exp( −k nr′t )]}

(7)

where mt and mt′ are the average number of trap states participating in the charge carrier trapping process and the average number of quenchers surrounding the NC, respectively. We have fitted the decay curves using eq 7, and the values of the parameters kr, knr, and knr′ are determined. The fitted curves

Table 2. Overview of the Values of Carrier Relaxation Parameters using the Stochastic Model system

lattice strain (magnitude)

reduced χ2

kr (ns−1)

mt

knr (ns−1)

mt′

knr′ (ns−1)

CdS Cd0.73Zn0.27S Cd0.62Zn0.38S Cd0.52Zn0.48S Cd0.44Zn0.56S Cd0.31Zn0.69S ZnS

0.178 0.148 0.195 0.213 0.281 0.3 0.151

0.86 0.92 0.9 0.87 0.89 0.9 0.91

0.016 0.012 0.027 0.043 0.076 0.12 0.31

1.4 0.89 2.2 3.1 3.9 5.3 4.1

0.81 0.76 0.97 1.23 1.43 1.87 0.53

1.6 1.9 1.8 1.5 2.1 2.3 0.3

0.071 0.066 0.07 0.074 0.068 0.072 0.1

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Phone: (91)-33-2473-4971. Fax: (91)-33-2473-2805. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS A.P. thanks to CSIR for generous funding.

Figure 6. Plot of the radiative recombination rates (kr), the number of trap states (mt), and the nonradiative relaxation rates due to the trap states (knr) against the lattice strain.

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CONCLUSIONS In conclusion, we have demonstrated the compositiondependent carrier relaxation dynamics of CdxZn1−xS alloy nanocrystals by using time-resolved spectroscopy. A stochastic model has been proposed to quantify the radiative and nonradiative relaxation rates. Finally, we have correlated the radiative and nonradiative rates and number of trap states with the lattice strain induced in CdxZn1−xS alloy nanocrystals. This new class of strain-tunable QDs opens up new possibilities in solar energy conversion, biological tagging, multicolor biomedical imaging, and various potential applications.

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ASSOCIATED CONTENT

S Supporting Information *

The HRTEM image of CdxZn1−xS alloy QDs, decay curves of pure CdS and ZnS (Figures S1−S3), and estimation of decay parameters by alternate method (Table S1 and Figure S4). This material is available free of charge via the Internet at http:// pubs.acs.org. 15172



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Lattice Strain Controls the Carrier Relaxation Dynamics in CdxZn1âxS

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Lattice Strain Controls the Carrier Relaxation Dynamics in CdxZn1âxS