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Probing Exciton Move and Localization in Solution-Grown Colloidal CdSexS1x Alloyed Nanowires by Temperature- and Time- Resolved Spectroscopy Gaoling Yang, Zongwei Ma, Haizheng Zhong, Shuangyang Zou, Cheng Chen, Junbo Han, and Bingsuo Zou J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.5b07198 • Publication Date (Web): 14 Sep 2015 Downloaded from http://pubs.acs.org on September 18, 2015

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The Journal of Physical Chemistry C is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

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The Journal of Physical Chemistry

Probing Exciton Move and Localization in Solution-Grown Colloidal CdSexS1-x Alloyed Nanowires by Temperature- and TimeResolved Spectroscopy Gaoling Yang,† Zongwei Ma,‡ Haizheng Zhong,†* Shuangyang Zou,† Cheng Chen,‡ Junbo Han,‡* and Bingsuo Zou§* †

Beijing Key Laboratory of Nanophotonics and Ultrafine Optoelectronic Systems,

School of Materials Science & Engineering, Beijing Institute of Technology, 5 Zhongguancun South Street, Haidian District, Beijing, 100081, China ‡

China

Wuhan

National

High

Magnetic

Field

Center

and

School

of

Physics, Huazhong University of Science and Technology, Wuhan, 430074, China §

Micro Nano Technology Centre, School of Physics, Beijing Institute of Technology,

5 Zhongguancun South Street, Haidian District, Beijing, 100081, China

1

ACS Paragon Plus Environment


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ABSTRACT

Colloidal semiconductor nanowires are interesting materials with polarized optical feature for optoelectronics devices. Previously, we observed an interesting photoluminescence enhancement in colloidal alloyed CdSexS1-x nanowires. In the present work, low temperature steady-state and time-resolved photoluminescence spectra were applied to understand the photoluminescence enhancement in these CdSexS1-x alloyed nanowires. The band-edge emission and surface-defect emission of alloyed CdSexS1-x nanowires, observed in low temperature photoluminescence spectra, show different changing trend with the variation of their composition. Moreover, the radiative lifetime for band-edge emission and surface-defect emission reveals an opposite changing trend with the variation of temperature. These findings suggest that the variation of photoluminescence quantum yields with composition is determined by the competition between exciton move and localization. If the carriers are localized in the interior of nanowires, the migration of photo-induced excitons to their surface will be prohibited and more probability for radiative recombination at band edge occurred.

KEYWORDS: photoluminescence, CdSexS1-x, alloyed, exciton recombination

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INTRODUCTION

Semiconductor alloyed nanostructures elucidates tremendous electrical and optical properties due to continuous tunability of their bandgaps,1-3 which offer rich platforms for fundamental spectroscopic studies,4,5 as well as the exploration in lasers,6-8 light emitting diodes,9,10 photodetectors,11,12 solar cells13,14 and field-effect transistors.15 As the kinetics study of the photogenerated electron-hole pair (the exciton) in semiconductor alloyed nanostructures is benefit to the device optimization and the development of such devices, a better understanding of the exciton relaxation pathway is important. However, unlike the rapid development of the application exploration, spectroscopic studies of semiconductor alloyed nanostructures are limited.16 CdSexS1-x is particularly attractive alloyed system and has been widely studied in nanoscience due to broad bandgap tunability from 2.4 eV (CdS) to 1.7 eV (CdSe).17-20 However, there is very limited study on their photoluminescence (PL) dynamics to understand composition dependent behaviors of radiative and nonradiative recombination of photoexcited excitons in CdSexS1-x nanostructures.21-24 In case of nanocrystals, Rosenthal et al. conducted the studies of CdSexS1-x nanocrystals with different size and compositions using ultrafast fluorescence upconversion spectroscopy, and the results indicate that the surface states were strongly correlated to the relaxation process of photoexcited carriers.21,

22

As for

nanostructures prepared by vapor phase methods, Zhang et al. revealed an enhanced band-edge emission in CdSexS1-x nanobelts due to the reduction of surface effect.23 Recently, we have demonstrated ultralong homogeneous colloidal CdSexS1-x alloyed 3



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nanowires (NWs) with color tunable and polarized emissions.25 An optimized PL quantum yield (QY) up to 8.8% was reported, which is much higher than the QYs of binary pure CdS or CdSe NWs (mostly less than 1%). In principle, a clear understanding of the nature of exciton dynamics in these alloyed NWs should be important to further improve their PL properties.

In semiconductor NWs, the surface has multiple effects because of the considerable large surface area and long axis of the NWs. Excitons can move freely along the longitudinal axis, significantly increasing their chances of meeting surface trap sites.26-29 It has been learned that the nonradiative recombination of the charge carriers to surface traps in semiconductor nanostructures could lead to the reduction of PLQYs.30-34 On the other hand, in alloyed nanostructures, because of the statistically distributed fluctuations of an average potential induced by compositional disorder, localized excitons received more attentions as most of excitons are trapped in potential valleys.35 Therefore, it becomes complicated but more interesting if these excitons move and localization processes compete with each other as a function of composition or temperature in NWs, which is almost related with our observed PL enhancement in these alloyed NWs.

In the present study, we have conducted detailed and systematic investigations on the optical properties and carrier dynamics of CdSexS1-x alloyed NWs by means of temperature-dependent and time-resolved PL spectroscopy. At low temperature, surface-defect-related emissions appeared and showed different changing trends with

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composition varied. At the same time, it is obvious that the exciton radiative lifetime increases

by increasing

the temperature. As for the

decay process of

surface-defect-related emission, a decrease of the PL lifetime was observed with temperature increasing. These results suggest that the variation of QYs for CdSexS1-x alloyed NWs is likely correlated with the competition between exciton move and localization.

EXPERIMENTAL SECTION

Synthesis and materials characterizaitons. CdSexS1-x NWs with varied compositions

were

prepared

following

our

previous

publications.25

The

as-synthesized NWs were purified and dispersed in toluene for further characterizations and spectroscopic measurements. For routine characterizations, UV−vis absorption and steady state PL spectra were measured at room temperature using

Hitachi

U-3010

spectrophotometer

and

FP-6600

spectrophotometer,

respectively. The structural morphology was carried out by JEM-2100F transmission electron microscope. Scanning transmission electron microscopy-energy dispersive spectroscopy (STEM-EDS) maps were recorded with a Tecnai G2 F20 TEM (FEI) equipped with a DX-4 analyzer (EDAX) operating at an acceleration voltage of 150 kV. X-ray diffraction (XRD) investigation was carried out at room temperature with a Bruker D8 diffractometer using Cu radiation (wavelength = 1.5406 Å). Optical measurements. To study the optical properties, the NWs solutions were dropcasted on a glass substrate to form a thin film. PL spectra were measured by 5



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using a frequency-doubled Ti:sapphire mode locked laser (Mira 900, Coherent) with an excitation light of 405 nm. In order to analyze the time-integrated photoluminescence, an EM-CCD (Andor DU970P) through a monochromator (Andor SR500) was employed. The time-tagged time-resolved mode of the PicoHarp 300 was used to record the excitation pulses and the arrival times of all photons with a time stamp.

RESULTS AND DISCUSSION

Composition varied CdSexS1-x alloyed NWs were synthesized as described previously.25 Figure 1a shows both the absorption band edge and PL emission peak of alloyed NWs continuous red shifted along with the increase of Se content, reflecting the concomitant change of the band gap. The presence of chemical composition was determined by EDX analysis. Figure 1b shows the PL QYs of CdSexS1-x alloyed NWs as a function of Se content. It is noticed that after a strong increase from nearly about 1% to 4 % for the composition x=0.2, the percentage of PL QY making up to 8.5 % at x=0.33. This percentage gradually declines to 1 % at x=0.8. TEM images of the as-fabricated colloidal CdSexS1-x NWs are shown in figure 1c, 1d and figure S1. All the samples have diameters of about 16 nm, (CdSe0.25S0.75, 16.5 ± 3 nm (±18.1%, 90NWs); CdSe0.33S0.67, 16 ± 3 nm (±18.7%, 80NWs); CdSe0.42S0.58, 16 ± 2 nm (±12.5%, 60NWs); CdSe0.53S0.47, 16 ± 2 nm (±12.5%, 110NWs)), the diameter deviation is about 12-18%, which indicates the high quality of our samples.36-40 The typical samples have lengths of several tens of micrometers. No obvious change of the

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diameters and lengths was observed for all the samples with the variation of Se/S precursor ratios, therefore, the PL tuning was attributed to the variation of composition. Figure 1e shows the elemental mappings of Cd, Se and S in the alloyed CdSexS1-x NWs with x of 0.25 from STEM-EDS measurements, which confirms the formation of alloyed structures.

Figure 1. (a) UV-vis and PL spectra of CdSexS1-x alloyed NWs in toluene with different components. (b) Plot of the composition with PL QYs. (c, d) TEM images of CdSexS1-x alloyed NWs with x= 0.25 and (e) their corresponding STEM-EDS mappings. The temperature dependent PL spectra of CdSexS1-x (0 ≤ x ≤ 1) alloyed NWs from 5 K to 290 K are shown in figure 2. Some similar trends are observed for CdSexS1-x alloyed NWs with different compositions: a red-shift as well as a broadening of the emission peak with the increase of temperature. More strikingly, a distinct peak appears at the longer wavelength region with the temperature decreasing, which is particularly evident for the sample with Se composition (x) of 0.25, 0.53 and 1, negligible on CdSe0.42S0.58, and entirely vanished for CdSe0.33S0.67 (see Figure 2a-2f).

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The near-band-edge emission varied from 550 nm to 650 nm with the composition varying, while the longer wavelength peak is attributed to surface-defect-related emission.23 More specifically, for CdSe0.25S0.75, only band-edge peak is observed at room temperature. As temperature decreases, the surface emission appears at about 150 K and is comparable with the band-edge peak when temperature decreases to 70 K. Similar behaviors have been observed for CdSe0.53S0.47 and pure CdSe NWs, except for lower intensity of surface-defect-related emission than that of band-edge emission band in the entire temperature range. For CdSe0.42S0.58 and CdSe0.33S0.67, the surface-defect-related emission is not obvious or even disappeared.

Figure 2. PL spectra of colloidal CdSexS1-x alloyed NWs with different components: x=0.25 (a), x=0.33 (b), x=0.42 (c), x=0.53 (d) and x=1 (e), at different temperatures from 5 K to 290 K. (f) PL spectra of compositions varied alloyed NWs at 5K.

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The change of the PL spectra as a function of temperature T was further investigated. Figure 3 shows fitting results of the PL spectra recorded at different temperatures using a single Gaussian function. We also plotted the extracted excitonic emission peak energies of CdSexS1-x alloyed NWs with different compositions versus temperatures, as shown in figure 3a. As expected, all the PL peak energies shift toward to lower energy as temperature increases due to the enhanced exciton-phonon coupling and lattice expansion.41 The experimental data could be fitted by the empirical Varshni law for the band gap:

= − ,42, 43 where is

the energy gap at 0 K, and and are constants related to the material. It is necessary to know that this equation was firstly derived for the infinite crystal and can be adapted for polycrystalline semiconductors and colloidal nanocrystals.44 Our results can be well fitted by the Varshni equation and the fittings parameters , , and are tabulated in table 1. Table 1. Eg is the bandgap at room temperature, Eg0 is the energy gap at 0 K, is the temperature coefficient, is a fitting parameter that is close to the Debye temperature, is the LO-Phonon and exciton-surface energy, Represents the coupling strength of exciton-LO-Phonon and exciton-surface, is the coupling strength of exciton-acoustic-phonon. Sample

Eg Eg0 (eV) (eV)

CdSe0.25S0.75 2.2

α -4

(10 eV/K)

2.289 4.32±0.26

β (K)

ELO+S (meV)

ΓLO+S (meV)

275.7±5.3 27.0±0.3 17.5±0.5 12.9±0.6

CdSe0.33S0.67 2.15 2.225 3.50±0.18

218.7±8.1 26.7±0.4 7.3±0.3

CdSe0.42S0.58 2.12 2.191 3.40±0.23

208.9±3.7 9.2±0.1

CdSe

175.6±6.4 32.4±0.5 36±0.9

1.98 2.04

3.75±0.17

σ (ηeV/K)

21.1±1.1

11.8±0.4 43.6±1.5 17.5±0.8 9



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Previous works on temperature dependent PL studies of bulk materials or semiconductor nanocrystals have shown that the broadening of emission band could be divided into three parts, one inhomogeneous broadening part, and two other homogeneous broadening parts coming from the optical and acoustic phonon-exciton interaction.45 In NWs, the mechanisms for bandwidth broadening include acoustic-phonon scattering, longitudinal optical (LO) phonon scattering and exciton-surface scattering. Based on above analysis, the total bandwidth broadening are composed of a temperature independent intrinsic part , and temperature dependent exciton-surface scattering and exciton-phonon, expressed as: =

+ +

/"# $% 46

, where represents the inhomogeneous peak

width at zero temperature, is the exciton-acoustic-phonon coupling coefficient,

represents the LO-phonon coupling and exciton-surface scattering coefficient, is the LO-phonon and exciton-surface energy. The temperature dependence of PL FWHM for alloyed CdSexS1-x NWs can be well fitted, as shown in figure 3b, and the fitting results are summarized in table 1. In figure 3b, the dashed lines come from the inhomogeneous broadening and acoustic-phonon scattering ( = +

).46 It is indicated that for CdSe0.25S0.75 and CdSe, the acoustic-phonon contributes up to 130K very significantly, but from 130 K onward, the predominant LO phonon and exciton-surface account for the further increase of FWHM. For CdSe0.33S0.67 and CdSe0.42S0.58, the dominant contribution to the line broadening comes from the coupling of exciton to acoustic-phonon, exciton-surface has little effect on these two samples, which is consistent which the discussed PL results above. 10


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Figure 3. Temperature dependent PL peak and FWHMs for CdSexS1-x alloyed NWs with different components. The solid lines are fits using the Varshni law and exciton-phonon model for the temperature variation of the energy and FWHM, respectively. The dashed lines represent the inhomogeneous broadening and the acoustic-phonon scattering fitted by using = + .

The recombination dynamics of photo-generated electron-hole pairs in II-VI CdSe and CdTe colloidal semiconductor NWs have been studied by applying single NW spectrascopy,47-49 revealing the role of size and excitation intensity in exciton

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recombination dynamics. Generally speaking, there are several possible mechanisms that may contribute to electron-hole pair recombination dynamics in NWs including radiative recombination, surface-interrelated nonradiative recombination, as well as Auger-assisted ionization and Auger recombination (high excitation intensity). 47,50,51 The resulting alloyed CdSexS1-x NWs have an average radius of ~8 nm, which is much larger than the Bohr radius of CdSe (aB = 5.6 nm) and CdS (aB = 2.9 nm). Surprising blue shift was observed in these CdSexS1-x NWs, especially for CdSe NWs. In comparison to the bandgap of 1.74 eV for bulk materials, the resulting CdSe NWs with diameter of 16 nm have a bandgap of 1.98 eV. This phenomenon is inconsistent with the previous reports and may be explained to the formation of one-dimentional excitons due to surface depletion induced quantum confinement.52,53 The observed blue shift implies that exciton recombination are dominant in these CdSexS1-x NWs. As discussed in the introduction, the compositional disorder could lead to statistically distributed fluctuations of an average potential,35 localized excitons are likely generated due to the presence of potential valleys, which have been demonstrated in CdSxSe1-x platelets,54 GaAs1-xPx alloyeds,55 and ZnSe1-xTex bulk alloyeds.56 These potential valleys can localize photo-induced excitons and prevent them from moving to the surface, which may account for the reduction of surface-defect-related emission. We therefore proposed that exciton localization and exciton move are competition in the exciton relaxation process. To better understand the exciton relaxation dynamics, figure 4 illustrates a schematic diagram to summarize the level diagram along with its associated rate process. Basically, upon excitation, the photo-induced excitons 12


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separate into electrons and holes and the recombination may follow several pathways. The PL emission was determined by the competences between these processes. Once generated, photo-induced excitons can localize in the potential valleys (process і) and subsequently recombine to generate radiative emissions (process ііi). In the other case, photo-induced excitons move along the wires (process іі) to meet the surface defects (process іv) where trapped electrons recombine with trapped holes to give defect emissions (process v). These two processes can be strongly influenced by the composition. To analyze the compositional effects on the exciton recombination, we determined the PL decay curves under very low excitation intensity (15 nJ/cm2), as a consequence, Auger decay plays a limited role in the recombination process.

Figure 4. Schematic of the kinetic model of exciton recombination in NWs, the localization of photo-induced excitons (і), excitons move along NWs (іі), the radiative recombination of localized excitons (ііі), the trapping of electrons and holes (іv), and the radiative recombination of trapped electrons with trapped holes (v). We therefore performed time-resolved PL measurements as a function of temperature. Figure 5 shows the normalized near-band-edge excitons emission decay 13



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from 290 K to 10 K with 405 nm excitation. The temperature dependence of the average PL decay time is shown in figure 6, we can see that, the decay time is almost constant as temperature increases at a low temperature range from 5 K to 130 K. This phenomenon is an obvious characteristic of the exciton localization effect due to the constant radiative recombination at low temperature.57 However, at higher temperature, the decay time almost increase with increasing temperature. This can be explained to the thermally activation of the localized excitons into free excitons. In one-dimensional colloidal semiconductor NWs, under the influence of the momentum conservation requirement, radiative recombination only occurs in excitons with small momentum (k ≈ 0) at low temperature, at high temperature, much more excitons will populate states with large k, therefore they may not recombine radiatively, leading to a rapidly increase of excitonic radiative decay time.58,59

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Figure 5. Time-resolved near-band-edge excitons PL measurements vs temperature of (a) CdSe0.25S0.75 NWs, (b) CdSe0.33S0.67 NWs, (c) CdSe0.53S0.47 NWs and (d) CdSe NWs. The solid lines indicate the result of fitting.

Figure 6. Temperature dependence of the PL lifetime for (a) band-edge excitons emission of CdSe0.25S0.75 NWs, CdSe0.33S0.67 NWs, CdSe0.53S0.47 NWs and CdSe NWs and surface-defect-related emission for (b) CdSe0.25S0.75 NWs and CdSe NWs. To evaluate the PL recombination, the data are fitted by applying biexponential $,

$,

decay function as follows: &' = &% exp + / + &0 exp , and the decay constants -.

-

and amplitudes obtained from the fitting with varied detection compositions are summarized in figure S2. The decay constants for the first exponential decay 1% of 1-5 ns may originate from the immediate recombination of the free electrons and holes after excitation in the internal states. The second exponential decay process in alloyed NWs is found to have a decay constant of 20-100 ns, which is due to the localization process of exciton. To further investigate the decay process of the 15



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surface-defect-related

emission,

temperature-dependent

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time-resolved

PL

measurements were performed for surface-defect-related emission (Figure 7). In both of the samples studied, a decrease in the exciton lifetime with temperature is observed (Figure 6b), which can be attributed to the activation of trapping process at higher temperature. For CdSe0.25S0.75, the longest lifetime is obtained at about 50 K, which is also a sign of temperature dependent surface-defect related PL. Additionally, as shown in figure 2, the positions of surface-defect-related PL emission peaks do not change with temperature, indicating that the surface-defect-related emission primary comes from recombination of the travelling electrons and holes trapped by surface defects, rather than recombination of an electron from the surface relaxing state to the valence band.

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Figure 7. Time-resolved surface-defect-related PL measurements vs temperature of (a) CdSe0.25S0.75 alloyed NWs and (b) CdSe NWs. The solid lines indicate the result of fitting. The influence of composition on the recombination dynamics of photogenerated exciton recombination for alloyed CdSe0.33S0.67 NWs was also investigated. Figure 8 shows decay lifetimes 1% and 10 as a function of selenium composition for alloyed CdSexS1-x NWs, the decay lifetimes and their respective weighting factors are listed in table S1. As shown in figure 8, both the fast (1% ) and slow (10 ) decay lifetimes increase at lower Se concentration and reached the maximum at x=0.33. Then the lifetimes begin to decrease for higher Se concentrations. This behavior indicates that 17



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carrier dynamics are not only temperature dependent, but also composition dependent. The varied composition introduce alloyed potential valleys to localize excitons in CdSe0.33S0.67. The exciton localization of NWs limits the opportunity of exciton migration to the surface as well as the surface defects induced nonradiative recombinations. This is consistent with the temperature dependent PL spectra results, as no surface defect emission for CdSe0.33S0.67 was observed due to exciton localization. This attribution is further confirmed by time-decay PL profiles where the decay lifetime for CdSe0.33S0.67 is significantly longer than other samples.

Figure 8. Exponential decay constants 1% and 10 as a function of selenium composition for CdSexS1-x alloyed NWs. Generally, the total PL emission is governed by the combination of radiative and non-radiative processes.60 The importance of alloying has been proved in reducing and suppressing surface trap sites of quantum dots though there are still no theoretical explanation.20,61 In colloidal alloyed semiconductor NWs, PL enhancement was also observed in comparison with pure CdS or CdSe NWs (less than 1%).31,38 The enhancement is achieved by exciton localization, under the action of potential valleys 18


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in alloyed NWs, the opportunity for exciton move to the surface greatly reduced, so most of carriers were localized in the interior of NWs and cannot migrated to the surface, therefore reduce the surface related nonradiative decays and give rise to more probability for rediative recombination at band edge. The PL emission depends on the ratio of the recombination rate of NWs and the localized rate in the traps. In alloyed NWs, photo-induced excitons recombine immediately or move to the surface, so more potential valleys will prevent exciton move to the surface and increase the proportion of effective recombination.

CONCLUSION

In conclusion, we report the temperature dependent PL properties of colloidal CdSexS1-x alloyed NWs and the corresponding analysis of their radiative recombination process. At low temperature, band-edge and surface-defect-related emissions were observed and varied with the change of composition. For CdSe0.42S0.58 and CdSe0.33S0.67, the surface-defect emission was not obvious or even disappeared. The changing of emission peak and the broadening of band-edge emisison were analyzed by fitting using empirical fomulars. The absence of surface-defect-related emissions in these CdSe0.42S0.58 and CdSe0.33S0.67 NWs can be explained by the competition between exciton move and localization, which dominates the PL recombinations in these alloyed NWs. This is also supported by the PL lifetime measurements. Based on experimental results, PL enhancement in these CdSexS1-x alloyed NWs can be explained as follow. When more carriers are localized in the 19



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interior of NWs and cannot migrate to the surface, radiative recombination at band edge is more likely to occur. This understanding implies that alloying is an effective strategy to improve the PL emission toward efficient nanostructure based optoelectronic devices.

ASSOCIATED CONTENT Supporting Information. High resolution TEM images and corresponding diameter distribution, and the correponding analysis of time-resolved PL decays. The supporting information is available free of charge on the ACS Publications website at http://pubs.acs.org. AUTHOR INFORMATION Corresponding Author *E-mail: [email protected]; [email protected]; [email protected] Notes The authors declare no competing financial interest. ACKNOWLEDGMENT The author H. Z. Zhong would like to thank Prof. Greg Scholes, Prof. Lin-Wang Wang and Dr. Kaifeng Wu for helpful discussions, and the author G. L. Yang would like to thank Dr. Chang Shuai for helpful manuscript comments. This work was financially supported by National Natural Science Foundation of China Grant (No. 21573018 and 21343005), Beijing Higher Education Young Elite Teacher Project (No. YETP1231), BIT International Graduate Exchange Program and the Open Research

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Fund of State Key Laboratory of Supramolecular Structure and Materials (No. sklssm2015029).

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TOC Figure

31


Probing Exciton Move and Localization in Solution ... - ACS Publications

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