Temperature-Dependent Photoluminescence of Ag2Se Quantum Dots

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Temperature-Dependent Photoluminescence of Ag2Se Quantum Dots Changyin Ji,† Yu Zhang,*,†,‡ Tieqiang Zhang,‡ Wenyan Liu,† Xiaoyu Zhang,† Hongzhi Shen,† Yu Wang,† Wenzhu Gao,‡ Yiding Wang,† Jun Zhao,§ and William W. Yu*,†,§ †

State Key Laboratory on Integrated Optoelectronics and College of Electronic Science and Engineering, Jilin University, Changchun 130012, China ‡ State Key Laboratory of Superhard Materials and College of Physics, Jilin University, Changchun 130012, China § Department of Chemistry and Physics, Louisiana State University, Shreveport, Louisiana 71115, United States ABSTRACT: The time-resolved photoluminescence spectroscopy was employed to analyze the optical properties of Ag2Se quantum dots with different diameters at temperatures of 80− 360 K. The photoluminescence lifetime measurement disclosed that in the low-energy electronic structure there were two dominating emissive “in-gap” states associated with surface defect and intrinsic states, which were further confirmed by Gaussian fitting of the photoluminescence spectra. The temperature-dependent emission peak energy was fitted to phenomenological equations to extract the average phonon energy, the Huang−Rhys factor, and the excitonic acoustic phonon coupling coefficient. The relatively large phonon energy and small Huang−Rhys factor were demonstrated, which induced the small variation of emission peak energy in the low-temperature range. Meanwhile, the photoluminescence line width increased with temperature and was analyzed based on the standard equation describing the temperature dependence of the width of the ground state exciton. The variation of both the photoluminescence peak and line broadening was mostly due to the exciton to acoustic phonon coupling.

1. INTRODUCTION Increasing study has been recently applied on I−VI colloidal semiconductor quantum dots (QDs) that are promising material for the applications of near-infrared (NIR) sensors,1 medicine, biology,2−4 and solar cells5 due to their narrow band gaps, i.e., 0.9, 0.15, and 0.67 eV, for Ag2S, Ag2Se, and Ag2Te, respectively.6−8 Similar to the IV−VI and II−V QDs, the emission efficiencies of I−VI QDs are generally high, and their emission wavelength can be finely tuned by modulating the particle size.9−13 Though several synthesis methods of Ag2S, Ag2Se, and Ag2Te QDs have been proposed, their physical characterizations have not been well documented. The sizedependent molar extinction coefficients and transition energies of Ag2Se QDs were reported by Langevin et al.,14 which showed that the molar extinction coefficients followed a power law with QD size, providing a simple approach to analyze the QD concentration in a solution.11,12,15 On the other hand, the change in the photoluminescence (PL) intensity with temperature is always determined by the temperature-related nonradiative recombination or the evolution of the PL mechanism. Hence, the temperature dependence of PL characteristics is of great importance for the investigation of the recombination processing. Both extinction and PL properties have relations with the band energy of bulk semiconductors that vary with temperature © XXXX American Chemical Society

and reflect multiple recombination transition processes, and several temperature dependence studies have been done on the QDs of CdSe, CdSe/ZnS, ZnSe, PbS/CdS, PbSe, Ag2Te, InP/ ZnS, and ZnCuInS/ZnSe/ZnS.16−25 For the understanding of radiative and nonradiative processes associated with high average acoustic phonon energy, we report here the investigation on the temperature-dependent emission peaks of Ag2Se QDs. The emission peaks were measured with temperature change, and the data were fitted using the empirical expressions to achieve the Huang−Rhys factor as well as the average phonon energy. The PL broadening was fitted with temperature change to describe the temperaturerelated 1S exciton in the QDs.26 The average temperature coefficient was thus determined to be −2.48 × 10−4 eV/K (dE/ dT).

2. EXPERIMENTAL SECTION 2.1. Chemicals. Silver acetate (AgAc, 99.99%), Se powder (100 mesh), tetradecylphosphonic acid (TDPA, 98%), tri-noctylphosphine (TOP, 90%), and tetrachloroethylene (99%) were purchased from Alfa Aesar. 1-Octanethiol (≥98.5%), oleic Received: January 31, 2015 Revised: June 1, 2015

A

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Figure 1. PL spectra (red) and UV−vis (blue) of Ag2Se QDs recorded at 300 K for three sizes. Insets: the TEM photographs of QDs.

178.1 meV for 3.2, 3.7, and 4.2 nm Ag2Se QDs, respectively. These values are very different from direct excitonic recombination that has a Stokes shift no more than 100 meV for excitonic recombinations.31 These large Stokes shifts reveal the existence of an unconventional in-gap state, corresponding to the surface defect or trap states as nonradiative recombination centers, which has been observed in other kinds of QDs.32 The time-resolved PL (TRPL) of 3.7 nm Ag2Se QDs was performed and is shown in Figure 2a. It can be clearly seen that

acid (OAc, 90%), 1-octadecene (ODE, 90%), and oleylamine (OAm, 90%) were purchased from Acros. Hexane, methanol, and acetone were purchased from Aldrich. 2.2. Synthesis of Ag2Se QDs. The Ag2Se QDs were prepared with an approach modified from previous reports.27,28 Typically, 0.1 mmol of AgAc, 1.64 mmol of 1-octanethiol, and 5 mL of ODE were added into a 50 mL three-neck flask. The system was degassed with N2 for 10 min and was then heated to 433 K under vigorous stirring. After that, a TOP−Se solution (0.1 mmol of Se dissolved in 1.5 mL of TOP under N2 atmosphere) was swiftly injected into the hot solution. The solution color changed from yellow to black meaning the formation of Ag2Se QDs. The temperature was kept at 403 K for the further growth of Ag2Se QDs. The reaction was stopped by adding 10 mL of toluene, and a raw product with particle size of 3.7 nm was obtained. The growth temperature could be varied at 381 or 413 K to achieve QDs with sizes of 3.2 or 4.2 nm, respectively. To monitor the growth of the nanoparticles, solution samples of small quantities were taken at different reaction times for absorption and PL measurements. The raw QD products were extracted with methanol at least three times and then redispersed in 1 mL of hexane and precipitated by 2 mL of methanol and 1 mL of n-butyl alcohol with centrifugation (10 000 rpm for 5 min).10,29,30 The purified Ag2Se QDs were finally dispersed in n-hexane or tetrachloroethylene for further characterizations. 2.3. Characterizations. A Shimadzu UV-3600 spectrophotometer was used to record the absorption spectra of the Ag2Se QDs. The PL properties of QDs in tetrachloroethylene were measured on an Omni-λ300 monochromotor/spectrograph. A TECNAI F30ST TEM was used to observe the morphology and size of the QDs. The TEM specimens were prepared by placing 4 μL of QD solution (hexane) on carbon-film-coated copper grids and left to dry at room temperature. Timecorrelated single-photon counting (TCSPC) measurements were performed on a mini-τ fluorescence spectrometer (Edinburgh Photonics) with an EPL405 laser diode. The highest repetition rate of the EPL405 laser diode was 10 MHz (100 ns separation). However, when measuring the decay curves, 1 μs separation was chosen to avoid the PL accumulation. Film thickness measurement was carried out on DEKTAK 150 Vecco.

Figure 2. (a) PL decay curve of 3.7 nm Ag2Se QDs (fitting parameters: A1 = 6673, A2 = 23088, τ1 = 245 ns, τ2 = 59 ns) and (b) two-Gaussian fitting curve of PL spectrum of 3.7 nm Ag2Se QDs at 80 K; PL1 and PL2 were the luminescence bands of the intrinsic and surface defect states, respectively.

the PL decays are not single-exponential, indicating two components with different decay time according to the following fitting formula ⎛ t ⎞ ⎛ t⎞ I(t ) = A1 exp⎜ − ⎟ + A 2 exp⎜ − ⎟ ⎝ τ2 ⎠ ⎝ τ1 ⎠

(1)

where A1 and A2 are fractional contributions of the PL decay lifetime of τ1 and τ2, respectively. The short lifetime (τ1) of tens of nanoseconds can be attributed to the recombination of the “intrinsic” state, while the longer one (τ2) can be ascribed to the surface-related recombination. The normalized products ( f i) of Ai (normalized amplitude) and τi (time constant) of two components can be concluded using the formula of f i = τiAi/ ∑τiAi, which were 54% and 46% for τ1 and τ2, respectively. This result was further confirmed by a two-Gaussian fitting of PL spectra as shown in Figure 2b. Because Ag2Se is a band edge emission material, the PL1 peaked at 0.975 eV, corresponding to the intrinsic recombination, while surface defect-related PL2 (GS) with slightly smaller energy was located at 0.950 eV. Their contribution ratio was similar to the result from TRPL measurements. The two components contributed different parts to the PL line intensity: the intrinsic state is a rapidly thermally quenched part, but the surface state is relatively stable

3. RESULTS AND DISCUSSION Figure 1 shows the absorption and PL spectra of three Ag2Se QDs at 300 K. As the particle size increased, both the PL and excitonic absorption peaks shifted to red. The Stokes shifts (the energy difference between the peak positions of the first excitonic absorption and the PL band) decreased with the increasing size of Ag2Se QDs, which were 362.0, 247.8, and B

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Figure 3. PL spectra of Ag2Se QDs with different particle sizes at temperatures of 80−360 K.

to temperature. Figure 3 shows the proportion verified in total PL lines as the function of temperature. According to the temperature-dependent PL spectra of Ag2Se QDs at 80−360 K (Figure 3), the shift of emission peak energy, the decline of PL intensity, as well as the broadening of the PL spectra varied with the temperature increase. Measurements on QD films with thickness of 1.2, 1.4, 1.7, and 2.8 μm showed that the PL spectrum was independent of thickness. As Figure 3 shows, the peak position was stable at low temperatures but shifted to red with an average coefficient of −2.48 × 10−4 eV/K when the temperature was higher than 200 K. This phenomenon is quite different from that of other QDs.23,25 Therefore, there is a different mechanism, and it is necessary to analyze the phonon energy and the coupling of exciton and phonon. By comparison of experimental data with fitting results, we can generate internal interactions such as coupling and transition mechanisms. The emission peaks of Ag2Se QDs with three particle sizes changed with temperature (Figure 4a), which can be nicely

Table 1. Fitting Parameters of the PL Peak Energy As a Function of Temperature (Figure 4a and Figure 4b) Based on Equation 2 and Equation 3 size (nm)

Eg(R, 0) (eV)

α(R) (meV)

β(R) (K)

S (Huang−Rhys factor)

⟨ℏω⟩ (meV)

4.2 3.7 3.2

0.895 1.038 1.240

0.230 0.245 0.265

190 178 161

0.45 0.47 0.56

16.1 16.2 16.3

with temperature is similar to that of the temperaturedependent band gap shrinkage of bulk material, due to the quantum confinement effect. Another equation38 was also used to fit the data shown in Figure 4. This equation improves the Varshni equation theoretically39 since the parameters used in this equation are related to an intrinsic interaction within semiconductors, i.e., the electron−phonon coupling ⎡ ⎛ ⟨ℏω⟩ ⎞ ⎤−1 Eg (R , T ) = Eg (R , 0) − 2S⟨ℏω⟩⎢exp⎜ ⎟ − 1⎥ ⎢⎣ ⎝ kBT ⎠ ⎥⎦ (3)

where S stands for the Huang−Rhys factor; kB is the Boltzmann constant; and ⟨ℏω⟩ represents the average phonon energy. The fitting results are shown in Figure 4b and Table 1. The values of S indicated that the electron−phonon coupling increased with the reduced particle size and were in good agreement with the experimental data shown in Figure 1. The Eg(R, 0) that extracted from the fitting data of eq 2 and eq 3 was the same. Meanwhile, the values of ⟨ℏω⟩ were ascertained to be ∼16 meV and agreed with the longitudinal acoustic phonon energy of the bulk Ag2Se (∼17 meV), which can be achieved according to the reported Raman spectra.40−42 Therefore, the reduction of the energy gap with elevated temperature (Figure 4) is due to the coupling of the exciton and acoustic phonon. Besides, this average of phonon energy is very large compared to other QD materials; therefore, when the temperature is low, the acoustic phonon can hardly be excited. As a result, the coupling effect in Figure 4 is unconspicuous. According to the calculation of thermal motion energy, the acoustic phonon can be excited effectively, and the temperature dependence of the PL peak energy cannot be seen unless the temperature is above 197 K, which is compatible with the experimental results. Figure 5 extracts the full width at half-maximum (fwhm) of the PL spectra of Ag2Se QDs with three diameters for the temperature from 80 to 360 K. It can be observed that the fwhm shows almost no change at low temperature but increases quickly at high temperature, which is similar to most QDs. The PL broadening was partially inhomogeneous and partially homogeneous, originated from the exciton−phonon scattering,

Figure 4. Temperature-dependent PL peak energy for Ag2Se QDs with different size.

fitted by the widely used Varshni equation for the temperaturedependent emission peak energy33 Eg (R , T ) = Eg (R , 0) −

α (R )T 2 T + β (R )

(2)

where Eg(R, 0) stands for the band gap of Ag2Se QDs at 0 K; α(R) is a constant for the QDs about the temperature coefficient; and β(R) is the Debye temperature (θD) of the semiconductor at 0 K.34 This equation considers both variations of lattice parameter and temperature dependence of the electron−lattice interaction. Equation 2 was first proposed for an infinite crystal; still it can be introduced for the analysis of bulk semiconductors as well as QDs.21,34 Table 1 lists the fitting values of Eg(R, 0), α(R), and β(R) for Ag2Se QD samples. The values of α(R) and β(R) agreed with the reported temperature coefficient35 and Debye temperature36,37 for bulk Ag2Se, while the value of Eg(R, 0) was higher than that of the bulk Ag2Se material; the shift of PL band of the Ag2Se QDs C

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Figure 6. Integrated PL intensities of Ag2Se QDs with different sizes as a function of 1/kBT. Solid lines are the fitting results according to eq 5.

Figure 5. Temperature-dependent fwhm of the PL peaks for three Ag2Se QDs.

where IPL(T) is the integrated PL intensity at temperature T; I0 is the integrated PL intensity at 0 K; m stands for the amount of LO phonons involved in thermal escape of carriers; ELO is their energy; Ea is the thermal activation energy; and A and B are the ratios of the radiative lifetime in Ag2Se QDs and the capture time from emitting centers to nonradiative recombination centers. The best fitting results for the Ag2Se QDs are summarized in Table 3.

so it is important to examine the exciton−phonon scattering mechanism leading to the line broadening. The experimental data were fitted as eq 4. This equation is widely used to describe the temperature-dependent broadening of the excitonic peak in bulk semiconductors and QDs.21,43 The total line width is the sum of three factors: an inhomogeneous broadening factor and two homogeneous broadening factors coming from acoustic and optical phonon−exciton interactions, respectively.26 ⎡ ⎛E ⎞ ⎤ Γ(T ) = Γinh + σT + ΓLO⎢exp⎜ LO ⎟ − 1⎥ ⎢⎣ ⎝ kBT ⎠ ⎥⎦

Table 3. Fitting Parameters of PL Intensity As a Function of Temperature (Figure 6) Based on Equation 5

−1

(4)

where Γinh is the inhomogeneous line width of the QDs, which is temperature-independent and is caused by the fluctuations in composition, morphology, size, etc.; σ stands for the exciton− acoustic phonon coupling coefficient; ELO is the LO-phonon energy; and ΓLO is the exciton−LO−phonon coupling strength. The fitting data are given in Table 2, showing that an

ELO (meV)

ΓLO (meV)

σ (μeV/K)

4.2 3.7 3.2

50.3 51.4 52.5

161.1 153.5 142.1

35.8 36.8 38.2

overwhelming contribution to the line broadening comes from the exciton−acoustic phonon coupling. Consequently, for Ag2Se QDs at temperature of 80−360 K, both the line broadening and the line shift were predominantly caused by the exciton to acoustic phonon coupling. Temperature-dependent integrated PL intensity involves very complicated processes, such as thermal activation, thermal escape, and nonradiative relaxation. Figure 6 shows the PL intensities of Ag2Se QDs with the function of 1/kBT for the three sizes. We observed that the PL intensity decreased quickly as the temperature increased. Thinking about the radiative relaxation, the thermally activated nonradiative, and the thermal escape process, the solid curves were the fitting results using eq 544 I0 IPL(T ) = ⎡ ⎤−m E E 1 + A exp − k Ta + B⎢exp k LO − 1 ⎣ ⎦⎥ B BT

( )

Ea (meV)

ELO (meV)

Eescape (meV)

A

B

m

4.2 3.7 3.2

101.3 102.5 103.4

50.5 51.3 52.2

242.4 266.6 261.1

600.3 550.1 565.4

150.2 200.5 247.6

4.8 5.1 5.3

The Ea for all the samples was comparable with ΔEg(R, 0) values extracted from Varshni fitting data, which means the lowtemperature PL quenching originated from the thermally activated transition between the intrinsic and the defect states that influence the PL peak’s temperature-dependent properties. In our case, smaller QDs have higher density of defect states, and these defect states act as nonradiative carrier recombination centers. A thermally induced detrapping from a surface defect state to a weak radiatively coupled intrinsic state or to a rare thermally activated trapping may exist in this process. The nonradiative relaxation process with an activation energy (∼100 meV) led to a decline in the PL intensity, which agreed with the reported data in the literature.45 At high temperature, the thermal escape process occurred at the expense of PL intensity decay, where the absorbed LO phonons clearly increased with the decrease of particle diameter. This phenomenon is in good agreement with the increasing energy difference between adjacent states because of the strong confinement of small particle size, which results in an increase of the energy that a carrier has to take in to jump from one state to a higher one. This process can also be quantitatively compared by Eescape = m(ELO), and the Eescape value agrees with the two first absorption peaks (ΔE1,2) as shown in Figure 1 (m = 5, ∼246 mV in this work).

Table 2. Fitting Parameters of Temperature-Related fwhm of the PL Peaks (Figure 5) Based on Equation 4 size (nm)

size (nm)

4. CONCLUSIONS In summary, the size- and temperature-dependent spectra of Ag2Se QDs with different sizes were investigated. The emission peaks of QDs with three sizes were studied as a function of temperature from 80 to 360 K, and the data were fitted with

( )

(5) D

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two equations, from which the optimal fitting Huang−Rhys factor and average phonon energy were attained. The gained average phonon energy of Ag2Se was found to be relatively high compared to that of other QDs, which induced weak exciton− phonon coupling and the stable emission peak energy at low temperature. This conclusion also coincided with the PL spectrum, which could act as an explanation of the stable peak energy in the PL line at ultralow temperature. The fwhm of the PL peak was also analyzed with temperature from 80 to 360 K. The results indicated that the variations of both the band energy and the spectrum broadening for Ag2Se QDs were induced by the coupling of the exciton to acoustic phonon. Moreover, the thermal escape was estimated by the quantitative comparison of Eescape and ΔE1,2 with ELO, which provided a direct way to investigate the internal mechanism of emission.

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (Y. Zhang). *E-mail: [email protected] (W. W. Yu). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the following agencies and funds: the National Natural Science Foundation of China (61106039, 51272084, 61306078, 61225018, 61475062), the National Postdoctoral Foundation (2011049015), the Jilin Province Key Fund (20140204079GX), the Hong Kong Scholar Program (XJ2012022), the State Key Laboratory on Integrated Optoelectronics (IOSKL2012ZZ12), 3M, and NSF (1338346).

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DOI: 10.1021/acs.jpcc.5b01030 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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DOI: 10.1021/acs.jpcc.5b01030 J. Phys. Chem. C XXXX, XXX, XXX−XXX