- Email: [email protected]
Spatially resolved surface-related exciton polariton dynamics in a single ZnO tetrapod
Accepted Manuscript Spatially resolved surface-related exciton polariton dynamics in a single ZnO tetrapod Fangfang Sun, Liaoxin Sun, Bo Zhang, Hailong Wang PII:
S0038-1098(17)30397-6
DOI:
10.1016/j.ssc.2017.12.003
Reference:
SSC 13337
To appear in:
Solid State Communications
Received Date: 14 August 2017 Revised Date:
1 December 2017
Accepted Date: 4 December 2017
Please cite this article as: F. Sun, L. Sun, B. Zhang, H. Wang, Spatially resolved surface-related exciton polariton dynamics in a single ZnO tetrapod, Solid State Communications (2018), doi: 10.1016/ j.ssc.2017.12.003. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
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Spatially resolved surface-related exciton polariton dynamics in a single ZnO tetrapod
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Fangfang Suna,b , Liaoxin Sunb,∗, Bo Zhangb , Hailong Wanga,∗∗ a
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Provincial Key Laboratory of Laser Polarization and Information Technology,Department of Physics,Qufu Normal University,Qufu 273165,China b State Key Laboratory of Infrared Physics, Shanghai Institute of Technical Physics, Chinese Academy of Sciences, Shanghai 200083, China
Abstract
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The band-edge emission lifetime in a single ZnO tetrapod is studied by using the time-resolved confocal micro-photoluminescence (TR-µPL) spectroscopic technique at room temperature. By performing µPL and TR-µPL mapping along the tapered arm of tetrapod, we observe whispering gallery mode (WGM) polaritons and find that the predominant radiative lifetime of exciton polaritons decreases linearly with increasing the surface-to-volume ratio of the sample. This behavior is ascribed to the surface electric field induced enhancement of the radiative decay rate of the exciton-like polaritons coupling with LO phonons. Keywords: polariton, lifetime, LO phonon, whispering gallery mode 1. Introductions
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The strong coupling between light and excitons in semiconductor microcavities, which forms “exciton polariton”, has been widely studied in the past decade.[1, 2, 3] Recently, great efforts are devoted to the studies of room temperature polaritons, which are essential for the polariton-based devices applications[4, 5, 6]. ZnO, with large exciton binding energy (∼ 60 meV) and strong oscillator strength (which are much larger than those of GaAs, CdTe etc.), shows great potential in developing room temperature polariton devices.[7, 8] Just now, room temperature exciton-polaritons ∗ ∗∗
Corresponding author: [email protected] Corresponding author: [email protected]
Preprint submitted to Solid State Communications
December 1, 2017
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(polariton lasing) in a ZnO nanowire and a planar microcavity have been reported.[8, 9, 10, 11] Triggered by the development of ZnO nano-fabrication techniques, the optical properties of some ZnO nanostructures, e.g., tetrapod, nanowire, nanobelt, nanodisk, are re-visited by many groups.[12, 15, 14, 16, 13, 17] Especially, the PL decay mechanism is attracting great interests due to the increasing demands for efficient ultraviolet light emitting devices. The dependence of exciton radiative decay rate with the length of ZnO nanorod and surface-related lifetime of donor bound excitons have been reported, respectively.[18, 19] The very long decay time of exciton PL in ZnO tetrapods is also observed.[20, 21] However, in the previous work, for studying the influence of size or morphology on the PL decay rate, a series of samples synthesized under different conditions were used. This unavoidably brings the uncertainty in ensuring the same crystalline quality for the samples, while the crystalline quality plays a key role in the dynamical properties of carriers. Besides, one can usually get an space-averaged effect rather than local information on excitons (exciton polaritons) dynamics, mixed PL signals coming from amounts of size-varied nanorods or nanowires may hinder better understanding of the underlying physics of carriers dynamics. In this work, we carry out the TR-µPL measurements on a single ZnO tetrapod. These measurements rule out the concerns mentioned above and provide local information on the intrinsic optical properties of ZnO nanostructures. The WGMs polaritons are clearly observed and fitted very well by using plane wave function and coupled oscillator model. By performing time-resolved PL scanning along the c-axis of one tapered arm of ZnO tetrapod, we find that the predominant radiative lifetime of exciton polaritons decreases with increasing the surface-to-volume ratio of the tapered arm. This behavior, which can be described by a linear equation, is ascribed to the surface electric field induced enhancement of radiative recombination of exciton polaritons coupling with LO phonons. 2. Samples and Experimental methods
The high quality ZnO tetrapods (each nanorod axle is parallel to its c-axis) were synthesized by vapor-phase transport method.[8] For the TRµPL measurements on a single tetrapod, we first disperse the tetrapods in ethanol by sonicating and then transfer them onto a transparent quartz slide. The TR-µPL of single tetrapod is collected by using a confocal microscopic 2
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Figure 1: (a) Optical image of a single ZnO tetrapod investigated in our experiments. (b) the corresponding SEM image.
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spectroscopy (JY-Horiba LabRam HR800) equipped with the time-correlated single-photon-counting system (TCSPC). The spatial resolution and the time resolution are about 1 µm and sub-100 ps, respectively. The excitation source is a picosecond (ps) laser with a wavelength of 371 nm and a repetition rate of 1 MHz. 3. Experimental results and Discussion
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Figure 1 shows the optical image of a single ZnO tetrapod investigated in our experiments and the corresponding scanning electron microscopy (SEM) image. The length scale of one arm of tetrapod is depicted in both optical image and SEM. The typical ZnO tetrapod usually has four tapered arms and the cross section of arm is inerratic hexagon. In this hexagonal structure, the light can be well confined due to the total reflection effect and forming WGMs.[8] To give a quantitative analyses on the experimental results, we first extract the relation of the radius (R) and the scanning positions (x) along the ZnO tapered arm shown in Fig. 2(a). The experimental result can be well fitted by a linear equation, which is written as follows: R(x) = 3.6 × 102 − 4.5x 3
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Figure 2: (a)The relation between the radius and the scanning position x, the blue line are fitting result by linear equation. (b) The PL spectra observed by scanning excitation measurement along the c-axis of one arm of tetrapod from the position 10 µm to 30 µm. (c) The radius dependent WGMs energy shift are extracted from PL spectra, the dash-dot curves are simulation results
Figure 2(b) shows the spectra measured at the positions from 10 µm to 30 µm. The band-edge emission modulated by the WG microcavity is clearly observed. The band-edge emission is usually attributed to the free
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excitons transitions and their LO phonon replicas. However, considering the strong interaction between excitons and light in ZnO, strictly speaking, the PL signal is originated from the free excitons and exciton-like polaritons transitions accompanied by emitting LO phonons,[22] which has been verified and studied in Ref[8]. Following our previous work, the WGM polaritons can be well described by plane wave function for pure WGMs combined with coupled oscillator model for exciton-light strong coupling. Plane wave function for WGMs in hexagonal microcavity can be written: (2)
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√ hc 6 R= √ [N + arctan(β 3n2 − 4)] π 3 3nE
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Here, R is the radius of circumscribing circle of the WG microcavity. h, c, n are the Planck constant, light speed in vacuum and the refractive index, respectively. The integer N is the WGM order. The factor β reflects polarization, β=n for TE (the electrical component of light E⊥c-axis) and β=1/n for TM (E//c-axis) mode. And the coupled oscillator model for polaritons in the strong coupling regime can be expressed as: ∑
2 2 ωj,L − ωj,T ε(ω) = ε∞ (1 + Ωj 2 ) ωj,T − ω 2 − iωγj j=A,B,C
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where, ε∞ is the background dielectric constant, A, B, C are three different polarized excitons around ZnO bandedges, which strongly coupled with TE or TM polarized WGMs.The detailed descriptions of prefactor Ωj for TE and TM polarizations can be found elsewhere.[23] ω j,T and ω j,L are the transverse and longitudinal resonance frequencies, and γ j is the damping constants. By using three equations (1)-(3) above, the radius-dependent WGM polaritons energy shift can be fitted and the results are shown in Figure 2(c). The simulated results (the dash-dot curves) match with experiments very well, which demonstrates a series of resonant modes observed in PL coming from the WGM polaritons caused by strong coupling of pure WGMs and A, B and C excitons. To further understand the polaritons’ dynamic properties, TCSPC combined with µPL technique, a powerful tool to investigate the dynamics of carriers in nano-sized semiconductor structures, is used. Thanks to this useful time-resolved technique, we obtain the typical TR-µPL spectrum observed at the wavelength of 382 nm and the position of 10 µm in Fig. 3(a). The result shows that the TR-µPL consists of two decay processes which can be fitted 5
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by biexponential function, I(t) = I0 (A exp(−t/τf ) + B exp(−t/τs )), where A, B are weighing coefficients and A + B = 1. The fast decay (τf ∼ 0.38 ns) is commonly ascribed to the nonradiative process, while the slow decay time (τs ∼ 3.6 ns) is attributed to the radiative process. This result is consistent with the previously reported decay time recorded by a streak camera system, in which the radiative recombination induced slow decay rate is well verified by examining the temperature dependence of PL linewidth and lifetimes of the excitons.[20] This long PL lifetime demonstrates the superior optical quality of our ZnO tetrapod. Moreover, we can also get the weighing coefficients of radiative decay (A = 86%) and nonradiative decay (B = 14%), respectively, by fitting the experimental result. These values indicate that the PL decay is mainly governed by the radiative recombination process. To further explore the dynamical behavior of exciton polartions in the single ZnO tetrapod, we perform the TR-µPL mapping measurements centered at wavelength of 382 nm along the c-axis of one arm with a step length of 1-2 µm. The detailed experimental results are shown in Fig. 3(b). It can be seen that the PL decay rate is gradually enhanced as the scanning position moved from 10 µm to 30 µm. That implies that the lifetime of exciton polaritons is strongly dependent on the radius of tapered arm. For a better understanding of the PL dynamical properties, we fit all these TR-µPLs by biexponential equations. Based on the above measured reults and equation (1) of R versus the scanning positions, we plot both the radiative lifetime τs and the nonradiative lifetime τf as function of the surface-to-volume ratio (1/R) in Fig. 3(c). The radiative lifetime decreases gradually from 3.6 ns at root part to less than 1 ns at tip-top part, which can be described by a linear equation
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1.6 × 103 (4) R This unambiguously demonstrates that the radiative lifetime is linearly proportional to the ratio of surface-to-volume of one arm of tetrapod. This behavior is attributed to the surface electric field induced enhancement of radiative recombination of exciton-like polaritons accompanied by emitting of LO phonons.[24] In nanostructured semiconductors, a charged surface layer is formed by the transfer of electrons from interior donor defects to the surface states. If an exciton polariton locates very close to the charged surface layer, the electric field would lead to the displacement of electron and hole in the exciton component of polariton. A nice consequence of this situation is the τs = 8.7 −
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Figure 3: a) The typical TR-µPL observed at scanning position 10 µm, the fitting result is shown as blue curve. The instrument response function (IRF) of TCSPC is shown. (b) The TR-µPL spectra measured along the c-axis of one arm from scanning position 10 µm to 30 µm. (c) The radiative decay lifetime (τs ) and nonradiative decay lifetime (τf ) are labeled by red balls and blue circles, respectively. The relation of τs versus 1/R can be described by a linear equation, which is shown as red line. (d) Wavelength dependence of τs and τf .
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enhancement of exciton-LO phonon coupling and thus the radiative decay of exciton-like polaritons. In our experiments, a ps laser, whose energy is higher than that of band-gap of ZnO, is used as an excitation source. In this one photon excitation, the penetration depth of laser photon is about 50 nm, implying that most of excitons are generated in a thin layer region beneath the surface.[25] And subsequently they relax to exciton-polariton states.[26] The dynamics of exciton polaritons would be easily influenced by the surface electric field. With decreasing the radius of tapered arm, the ratio of the charged surface to the uncharged interior region increases, which results in the stronger influences of surface electric field on the radiative recombination of exciton-like polartions. Besides, the nonradiative decay as 7
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function of 1/R is also examined. It shows that the nonradiative decay rate changes very little as the 1/R decreases. This indicates that the nonradiative recombination centers should not be the surface-related defects, but may be native defects or defect complexes in the body.[27] Here, we also present the dynamics of exciton polaritons as function of the emission wavelength (Fig. 3(d)). Both radiative and nonradiative decay rates vary very little in the band-edge emission region. This behavior can be well explained by a dynamical mechanism of exciton polaritons. The excitonlike polaritons decay through LO phonons to become photon-like polaritons, whose lifetime is sub-ps determined by the lifetime of photon component in the material. This extremely short lifetime is beyond the time-resolution of our TCSPC. Therefore, the lifetime of the broad band-edge emission can only reflect the decay rate of exciton-like polartions, which should be independent on the wavelength as our experimental observation. 4. Conclusions
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In conclusion, we present the µPL spatial scanning and TR-µPL measurements on a single tapered ZnO tetrapod at room temperature. The WGM polaritons are clear observed and described by using plane wave function combined with coupled oscillator model. The TR-µPL can be well fitted by biexponential decay curve, which is composed of the predominant radiative decay process and a negligible nonradiative decay process. By careful scanning excitation with time-resolved detection along the tapered arm of tetrapod, we find that the radiative lifetime of exciton polaritons decreases linearly with the surface-to-volume ratio increasing. This dynamical behavior is attributed to the surface electric field influence on the radiative recombination of exciton polaritons. Our results quantify the surface-related dynamics of carriers in a single nano-sized ZnO structure which may be of great importance for design of high efficient nanostructured ZnO-based optoelectronic devices. Acknowledgement This work was supported by the National Natural Science Foundation of China (11474297, 11674343, 61674096). L. Sun acknowledges Youth Innovation Promotion Association CAS (2016221) and Shanghai Committee of Science and Technology (17ZR1444000). 8
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References
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[1] J. Kasprzak, M. Richard, S. Kundermann, A. Baas, P. Jeambrun, J. M. J. Keeling, F. M. Marchetti, M. H. Szyma´ nska, R. Andr´e, J. L. Staehli, V. Savona, P. B. Littlewood, B. Deveaud, and Le Si Dang, Nature(London) 443, 409 (2006). [2] A. Amo, D. Sanvitto, F. R. Laussy, D. Ballarini, E. del Valle, M. D. Martin, A. Lemˆıtre, J. Bloch, D. N. Krizhanovskii, M. S. Skolnick, C. Tejedor, and L. Vi˜ na, Nature 457, 291 (2009).
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[3] S. Christopoulos, G. Baldassarri H¨oger von H¨ogersthal, A. J. D. Grundy, P. G. Lagoudakis, A. V. Kavokin, and J. J. Baumberg, Phys. Rev. Lett. 98, 126405 (2007).
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[4] Pallab Bhattacharya, Thomas Frost, Saniya Deshpande, Md Zunaid Baten, Arnab Hazari, and Ayan Das, Phys. Rev. Lett. 112, 236802 (2014). [5] Johannes D. Plumhof, Thilo St¨oferle, Lijian Mai, Ullrich Scherf and Rainer F. Mahrt, Nat. Mater. 13, 247 (2014).
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[6] Xiaoze Liu, Tal Galfsky, Zheng Sun, Fengnian Xia, Erh-chen Lin, YiHsien Lee, St´ephane K´ena-Cohen and Vinod M. Menon, Nat. Photon. 9, 30 (2015). [7] Marian Zamfirescu, Alexey Kavokin, Bernard Gil, Guillaume Malpuech, and Mikhail Kaliteevski, Phys. Rev. B. 65, 161205R (2002).
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[8] Liaoxin Sun, Zhanghai Chen, Qijun Ren, Ke Yu, Lihui Bai, Weihang Zhou, Hui Xiong, Z. Q. Zhu, and Xuechu Shen, Phys. Rev. Lett. 100, 156403 (2008).
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[9] W. Xie, H. Dong, S. Zhang, L. Sun, W. Zhou, Y. Ling, J. Ju, X. Shen and Z. Chen, Phys. Rev. Lett. 108, 166401 (2012).
[10] Jun-Rong Chen, Tien-Chang Lu, Yung-Chi Wu, Shiang-Chi Lin, WeiRein Liu, Wen-Feng Hsieh, Chien-Cheng Kuo, and Cheng-Chung Lee, Appl. Phys. Lett. 94, 061103 (2009).
9
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[11] F. Li, L. Orosz, O. Kamoun, S. Bouchoule, C. Brimont, P. Disseix, T. Guillet, X. Lafosse, M. Leroux, J. Leymarie, M. Mexis, M. Mihailovic, G. Patriarche, F. R´everet, D. Solnyshkov, J. Zuniga-Perez, and G. Malpuech, Phys. Rev. Lett. 110, 1956406 (2013). [12] A. B. Djuriˇsi´c, Y. H. Leung, Wallace C. H. Choy, and Kok Wai Cheah, Appl. Phys. Lett. 84, 2635 (2004). [13] W. M. Kwok, A. B. Djuriˇsi´c, Y. H. Leung, W. K. Chan, and D. L. Phillips, Appl. Phys. Lett. 87, 223111 (2005).
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[14] G. H. Du, F. Xu, Z. Y. Yuan, and G. Van Tendeloo, Appl. Phys. Lett. 88, 243101 (2006).
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[15] X. L. Xu, F. S. F. Brossard, D. A. Williams, D. P. Collins, M. J. Holmes, R. A. Taylor, and X. Zhang, Appl. Phys. Lett. 94, 231103 (2009). [16] Min Gao, Wenliang Li, Yang Liu, Quan Li Qing Chen, and Lian-Mao Peng, Appl. Phys. Lett. 92, 113112 (2008). [17] Chinkyo Kim, Yong-Jin Kim, Eue-Soon Jang, Gyu-Chul Yi, and Hyun Ha Kim, Appl. Phys. Lett. 88, 093104 (2006).
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[18] Sangsu Hong, Taiha Joo, Won II Park, Yong Ho Jun, and Gyu-Chul Yi, Appl. Phys. Lett. 83, 4157 (2003). uell, M. R. Wagner, A. Hoffmann, A. Cornet, and J. [19] J. S. Reparaz, F. G¨ R. Morante, Appl. Phys. Lett. 96, 053105 (2010).
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[20] S.-K. Lee, S. L. Chen, D. Hongxing, L. Sun, Z. Chen, W. M. Chen, and I. A. Buyanova, Appl. Phys. Lett. 96, 083104 (2010).
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[21] Yongchun Zhong, Aleksandra B. Djuriˇsi´c, Yuk Fan Hsu, Kam Sing Wong, Gerhard Brauer, Chi Chung Ling, and Wai Kin Chan, J. Phys. Chem. C. 112, 16286 (2008).
[22] Douglas Magde, and Herbert Mahr, Phys. Rev. Lett. 24, 890 (1970). [23] J. Lagois, Phys. Rev. B. 23, 5511 (1981). [24] Takeshi Hiral, Nobuhito Ohno, Yoshiyuki Harada, Taku Horii, Yuji Sawada, and Tadashi Itoh, Appl. Phys. Lett. 93, 041113 (2008). 10
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[25] G. E. Jellison, Jr. and L. A. Boatner, Phys. Rev. B. 58, 3586 (1998). [26] T. Onuma, K. Hazu, A. Uedono, T. Sota, and S. F. Chichibu, Appl. Phys. Lett. 96, 061906 (2010).
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[27] D. C. Look, G. C. Farlow, Pakpoom Reunchan, Sukit Limpijumnong, S. B. Zhang, and K. Nordlund, Phys. Rev. Lett. 95, 225502 (2005).
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ACCEPTED MANUSCRIPT 1. The time-resolved PL scanning measurements on a single tapered ZnO tetrapod at room temperature were proposed and conducted for the first time. 2. The WGM polaritons are clearly observed and described by using plane wave function combined with coupled oscillator model.
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3. The radiative lifetime of exciton polaritons decreases linearly with the surface-to-volume ratio increasing, this behavior can be ascribed to the surface
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electric field enhanced the radiative recombination of exciton polaritons.
S0038-1098(17)30397-6
DOI:
10.1016/j.ssc.2017.12.003
Reference:
SSC 13337
To appear in:
Solid State Communications
Received Date: 14 August 2017 Revised Date:
1 December 2017
Accepted Date: 4 December 2017
Please cite this article as: F. Sun, L. Sun, B. Zhang, H. Wang, Spatially resolved surface-related exciton polariton dynamics in a single ZnO tetrapod, Solid State Communications (2018), doi: 10.1016/ j.ssc.2017.12.003. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
ACCEPTED MANUSCRIPT
Spatially resolved surface-related exciton polariton dynamics in a single ZnO tetrapod
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Fangfang Suna,b , Liaoxin Sunb,∗, Bo Zhangb , Hailong Wanga,∗∗ a
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Provincial Key Laboratory of Laser Polarization and Information Technology,Department of Physics,Qufu Normal University,Qufu 273165,China b State Key Laboratory of Infrared Physics, Shanghai Institute of Technical Physics, Chinese Academy of Sciences, Shanghai 200083, China
Abstract
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The band-edge emission lifetime in a single ZnO tetrapod is studied by using the time-resolved confocal micro-photoluminescence (TR-µPL) spectroscopic technique at room temperature. By performing µPL and TR-µPL mapping along the tapered arm of tetrapod, we observe whispering gallery mode (WGM) polaritons and find that the predominant radiative lifetime of exciton polaritons decreases linearly with increasing the surface-to-volume ratio of the sample. This behavior is ascribed to the surface electric field induced enhancement of the radiative decay rate of the exciton-like polaritons coupling with LO phonons. Keywords: polariton, lifetime, LO phonon, whispering gallery mode 1. Introductions
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The strong coupling between light and excitons in semiconductor microcavities, which forms “exciton polariton”, has been widely studied in the past decade.[1, 2, 3] Recently, great efforts are devoted to the studies of room temperature polaritons, which are essential for the polariton-based devices applications[4, 5, 6]. ZnO, with large exciton binding energy (∼ 60 meV) and strong oscillator strength (which are much larger than those of GaAs, CdTe etc.), shows great potential in developing room temperature polariton devices.[7, 8] Just now, room temperature exciton-polaritons ∗ ∗∗
Corresponding author: [email protected] Corresponding author: [email protected]
Preprint submitted to Solid State Communications
December 1, 2017
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(polariton lasing) in a ZnO nanowire and a planar microcavity have been reported.[8, 9, 10, 11] Triggered by the development of ZnO nano-fabrication techniques, the optical properties of some ZnO nanostructures, e.g., tetrapod, nanowire, nanobelt, nanodisk, are re-visited by many groups.[12, 15, 14, 16, 13, 17] Especially, the PL decay mechanism is attracting great interests due to the increasing demands for efficient ultraviolet light emitting devices. The dependence of exciton radiative decay rate with the length of ZnO nanorod and surface-related lifetime of donor bound excitons have been reported, respectively.[18, 19] The very long decay time of exciton PL in ZnO tetrapods is also observed.[20, 21] However, in the previous work, for studying the influence of size or morphology on the PL decay rate, a series of samples synthesized under different conditions were used. This unavoidably brings the uncertainty in ensuring the same crystalline quality for the samples, while the crystalline quality plays a key role in the dynamical properties of carriers. Besides, one can usually get an space-averaged effect rather than local information on excitons (exciton polaritons) dynamics, mixed PL signals coming from amounts of size-varied nanorods or nanowires may hinder better understanding of the underlying physics of carriers dynamics. In this work, we carry out the TR-µPL measurements on a single ZnO tetrapod. These measurements rule out the concerns mentioned above and provide local information on the intrinsic optical properties of ZnO nanostructures. The WGMs polaritons are clearly observed and fitted very well by using plane wave function and coupled oscillator model. By performing time-resolved PL scanning along the c-axis of one tapered arm of ZnO tetrapod, we find that the predominant radiative lifetime of exciton polaritons decreases with increasing the surface-to-volume ratio of the tapered arm. This behavior, which can be described by a linear equation, is ascribed to the surface electric field induced enhancement of radiative recombination of exciton polaritons coupling with LO phonons. 2. Samples and Experimental methods
The high quality ZnO tetrapods (each nanorod axle is parallel to its c-axis) were synthesized by vapor-phase transport method.[8] For the TRµPL measurements on a single tetrapod, we first disperse the tetrapods in ethanol by sonicating and then transfer them onto a transparent quartz slide. The TR-µPL of single tetrapod is collected by using a confocal microscopic 2
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spectroscopy (JY-Horiba LabRam HR800) equipped with the time-correlated single-photon-counting system (TCSPC). The spatial resolution and the time resolution are about 1 µm and sub-100 ps, respectively. The excitation source is a picosecond (ps) laser with a wavelength of 371 nm and a repetition rate of 1 MHz. 3. Experimental results and Discussion
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Figure 1 shows the optical image of a single ZnO tetrapod investigated in our experiments and the corresponding scanning electron microscopy (SEM) image. The length scale of one arm of tetrapod is depicted in both optical image and SEM. The typical ZnO tetrapod usually has four tapered arms and the cross section of arm is inerratic hexagon. In this hexagonal structure, the light can be well confined due to the total reflection effect and forming WGMs.[8] To give a quantitative analyses on the experimental results, we first extract the relation of the radius (R) and the scanning positions (x) along the ZnO tapered arm shown in Fig. 2(a). The experimental result can be well fitted by a linear equation, which is written as follows: R(x) = 3.6 × 102 − 4.5x 3
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Figure 2: (a)The relation between the radius and the scanning position x, the blue line are fitting result by linear equation. (b) The PL spectra observed by scanning excitation measurement along the c-axis of one arm of tetrapod from the position 10 µm to 30 µm. (c) The radius dependent WGMs energy shift are extracted from PL spectra, the dash-dot curves are simulation results
Figure 2(b) shows the spectra measured at the positions from 10 µm to 30 µm. The band-edge emission modulated by the WG microcavity is clearly observed. The band-edge emission is usually attributed to the free
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excitons transitions and their LO phonon replicas. However, considering the strong interaction between excitons and light in ZnO, strictly speaking, the PL signal is originated from the free excitons and exciton-like polaritons transitions accompanied by emitting LO phonons,[22] which has been verified and studied in Ref[8]. Following our previous work, the WGM polaritons can be well described by plane wave function for pure WGMs combined with coupled oscillator model for exciton-light strong coupling. Plane wave function for WGMs in hexagonal microcavity can be written: (2)
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√ hc 6 R= √ [N + arctan(β 3n2 − 4)] π 3 3nE
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Here, R is the radius of circumscribing circle of the WG microcavity. h, c, n are the Planck constant, light speed in vacuum and the refractive index, respectively. The integer N is the WGM order. The factor β reflects polarization, β=n for TE (the electrical component of light E⊥c-axis) and β=1/n for TM (E//c-axis) mode. And the coupled oscillator model for polaritons in the strong coupling regime can be expressed as: ∑
2 2 ωj,L − ωj,T ε(ω) = ε∞ (1 + Ωj 2 ) ωj,T − ω 2 − iωγj j=A,B,C
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where, ε∞ is the background dielectric constant, A, B, C are three different polarized excitons around ZnO bandedges, which strongly coupled with TE or TM polarized WGMs.The detailed descriptions of prefactor Ωj for TE and TM polarizations can be found elsewhere.[23] ω j,T and ω j,L are the transverse and longitudinal resonance frequencies, and γ j is the damping constants. By using three equations (1)-(3) above, the radius-dependent WGM polaritons energy shift can be fitted and the results are shown in Figure 2(c). The simulated results (the dash-dot curves) match with experiments very well, which demonstrates a series of resonant modes observed in PL coming from the WGM polaritons caused by strong coupling of pure WGMs and A, B and C excitons. To further understand the polaritons’ dynamic properties, TCSPC combined with µPL technique, a powerful tool to investigate the dynamics of carriers in nano-sized semiconductor structures, is used. Thanks to this useful time-resolved technique, we obtain the typical TR-µPL spectrum observed at the wavelength of 382 nm and the position of 10 µm in Fig. 3(a). The result shows that the TR-µPL consists of two decay processes which can be fitted 5
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by biexponential function, I(t) = I0 (A exp(−t/τf ) + B exp(−t/τs )), where A, B are weighing coefficients and A + B = 1. The fast decay (τf ∼ 0.38 ns) is commonly ascribed to the nonradiative process, while the slow decay time (τs ∼ 3.6 ns) is attributed to the radiative process. This result is consistent with the previously reported decay time recorded by a streak camera system, in which the radiative recombination induced slow decay rate is well verified by examining the temperature dependence of PL linewidth and lifetimes of the excitons.[20] This long PL lifetime demonstrates the superior optical quality of our ZnO tetrapod. Moreover, we can also get the weighing coefficients of radiative decay (A = 86%) and nonradiative decay (B = 14%), respectively, by fitting the experimental result. These values indicate that the PL decay is mainly governed by the radiative recombination process. To further explore the dynamical behavior of exciton polartions in the single ZnO tetrapod, we perform the TR-µPL mapping measurements centered at wavelength of 382 nm along the c-axis of one arm with a step length of 1-2 µm. The detailed experimental results are shown in Fig. 3(b). It can be seen that the PL decay rate is gradually enhanced as the scanning position moved from 10 µm to 30 µm. That implies that the lifetime of exciton polaritons is strongly dependent on the radius of tapered arm. For a better understanding of the PL dynamical properties, we fit all these TR-µPLs by biexponential equations. Based on the above measured reults and equation (1) of R versus the scanning positions, we plot both the radiative lifetime τs and the nonradiative lifetime τf as function of the surface-to-volume ratio (1/R) in Fig. 3(c). The radiative lifetime decreases gradually from 3.6 ns at root part to less than 1 ns at tip-top part, which can be described by a linear equation
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1.6 × 103 (4) R This unambiguously demonstrates that the radiative lifetime is linearly proportional to the ratio of surface-to-volume of one arm of tetrapod. This behavior is attributed to the surface electric field induced enhancement of radiative recombination of exciton-like polaritons accompanied by emitting of LO phonons.[24] In nanostructured semiconductors, a charged surface layer is formed by the transfer of electrons from interior donor defects to the surface states. If an exciton polariton locates very close to the charged surface layer, the electric field would lead to the displacement of electron and hole in the exciton component of polariton. A nice consequence of this situation is the τs = 8.7 −
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Figure 3: a) The typical TR-µPL observed at scanning position 10 µm, the fitting result is shown as blue curve. The instrument response function (IRF) of TCSPC is shown. (b) The TR-µPL spectra measured along the c-axis of one arm from scanning position 10 µm to 30 µm. (c) The radiative decay lifetime (τs ) and nonradiative decay lifetime (τf ) are labeled by red balls and blue circles, respectively. The relation of τs versus 1/R can be described by a linear equation, which is shown as red line. (d) Wavelength dependence of τs and τf .
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enhancement of exciton-LO phonon coupling and thus the radiative decay of exciton-like polaritons. In our experiments, a ps laser, whose energy is higher than that of band-gap of ZnO, is used as an excitation source. In this one photon excitation, the penetration depth of laser photon is about 50 nm, implying that most of excitons are generated in a thin layer region beneath the surface.[25] And subsequently they relax to exciton-polariton states.[26] The dynamics of exciton polaritons would be easily influenced by the surface electric field. With decreasing the radius of tapered arm, the ratio of the charged surface to the uncharged interior region increases, which results in the stronger influences of surface electric field on the radiative recombination of exciton-like polartions. Besides, the nonradiative decay as 7
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function of 1/R is also examined. It shows that the nonradiative decay rate changes very little as the 1/R decreases. This indicates that the nonradiative recombination centers should not be the surface-related defects, but may be native defects or defect complexes in the body.[27] Here, we also present the dynamics of exciton polaritons as function of the emission wavelength (Fig. 3(d)). Both radiative and nonradiative decay rates vary very little in the band-edge emission region. This behavior can be well explained by a dynamical mechanism of exciton polaritons. The excitonlike polaritons decay through LO phonons to become photon-like polaritons, whose lifetime is sub-ps determined by the lifetime of photon component in the material. This extremely short lifetime is beyond the time-resolution of our TCSPC. Therefore, the lifetime of the broad band-edge emission can only reflect the decay rate of exciton-like polartions, which should be independent on the wavelength as our experimental observation. 4. Conclusions
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In conclusion, we present the µPL spatial scanning and TR-µPL measurements on a single tapered ZnO tetrapod at room temperature. The WGM polaritons are clear observed and described by using plane wave function combined with coupled oscillator model. The TR-µPL can be well fitted by biexponential decay curve, which is composed of the predominant radiative decay process and a negligible nonradiative decay process. By careful scanning excitation with time-resolved detection along the tapered arm of tetrapod, we find that the radiative lifetime of exciton polaritons decreases linearly with the surface-to-volume ratio increasing. This dynamical behavior is attributed to the surface electric field influence on the radiative recombination of exciton polaritons. Our results quantify the surface-related dynamics of carriers in a single nano-sized ZnO structure which may be of great importance for design of high efficient nanostructured ZnO-based optoelectronic devices. Acknowledgement This work was supported by the National Natural Science Foundation of China (11474297, 11674343, 61674096). L. Sun acknowledges Youth Innovation Promotion Association CAS (2016221) and Shanghai Committee of Science and Technology (17ZR1444000). 8
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References
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[1] J. Kasprzak, M. Richard, S. Kundermann, A. Baas, P. Jeambrun, J. M. J. Keeling, F. M. Marchetti, M. H. Szyma´ nska, R. Andr´e, J. L. Staehli, V. Savona, P. B. Littlewood, B. Deveaud, and Le Si Dang, Nature(London) 443, 409 (2006). [2] A. Amo, D. Sanvitto, F. R. Laussy, D. Ballarini, E. del Valle, M. D. Martin, A. Lemˆıtre, J. Bloch, D. N. Krizhanovskii, M. S. Skolnick, C. Tejedor, and L. Vi˜ na, Nature 457, 291 (2009).
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[3] S. Christopoulos, G. Baldassarri H¨oger von H¨ogersthal, A. J. D. Grundy, P. G. Lagoudakis, A. V. Kavokin, and J. J. Baumberg, Phys. Rev. Lett. 98, 126405 (2007).
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[4] Pallab Bhattacharya, Thomas Frost, Saniya Deshpande, Md Zunaid Baten, Arnab Hazari, and Ayan Das, Phys. Rev. Lett. 112, 236802 (2014). [5] Johannes D. Plumhof, Thilo St¨oferle, Lijian Mai, Ullrich Scherf and Rainer F. Mahrt, Nat. Mater. 13, 247 (2014).
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[6] Xiaoze Liu, Tal Galfsky, Zheng Sun, Fengnian Xia, Erh-chen Lin, YiHsien Lee, St´ephane K´ena-Cohen and Vinod M. Menon, Nat. Photon. 9, 30 (2015). [7] Marian Zamfirescu, Alexey Kavokin, Bernard Gil, Guillaume Malpuech, and Mikhail Kaliteevski, Phys. Rev. B. 65, 161205R (2002).
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[8] Liaoxin Sun, Zhanghai Chen, Qijun Ren, Ke Yu, Lihui Bai, Weihang Zhou, Hui Xiong, Z. Q. Zhu, and Xuechu Shen, Phys. Rev. Lett. 100, 156403 (2008).
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[9] W. Xie, H. Dong, S. Zhang, L. Sun, W. Zhou, Y. Ling, J. Ju, X. Shen and Z. Chen, Phys. Rev. Lett. 108, 166401 (2012).
[10] Jun-Rong Chen, Tien-Chang Lu, Yung-Chi Wu, Shiang-Chi Lin, WeiRein Liu, Wen-Feng Hsieh, Chien-Cheng Kuo, and Cheng-Chung Lee, Appl. Phys. Lett. 94, 061103 (2009).
9
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[11] F. Li, L. Orosz, O. Kamoun, S. Bouchoule, C. Brimont, P. Disseix, T. Guillet, X. Lafosse, M. Leroux, J. Leymarie, M. Mexis, M. Mihailovic, G. Patriarche, F. R´everet, D. Solnyshkov, J. Zuniga-Perez, and G. Malpuech, Phys. Rev. Lett. 110, 1956406 (2013). [12] A. B. Djuriˇsi´c, Y. H. Leung, Wallace C. H. Choy, and Kok Wai Cheah, Appl. Phys. Lett. 84, 2635 (2004). [13] W. M. Kwok, A. B. Djuriˇsi´c, Y. H. Leung, W. K. Chan, and D. L. Phillips, Appl. Phys. Lett. 87, 223111 (2005).
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[14] G. H. Du, F. Xu, Z. Y. Yuan, and G. Van Tendeloo, Appl. Phys. Lett. 88, 243101 (2006).
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[15] X. L. Xu, F. S. F. Brossard, D. A. Williams, D. P. Collins, M. J. Holmes, R. A. Taylor, and X. Zhang, Appl. Phys. Lett. 94, 231103 (2009). [16] Min Gao, Wenliang Li, Yang Liu, Quan Li Qing Chen, and Lian-Mao Peng, Appl. Phys. Lett. 92, 113112 (2008). [17] Chinkyo Kim, Yong-Jin Kim, Eue-Soon Jang, Gyu-Chul Yi, and Hyun Ha Kim, Appl. Phys. Lett. 88, 093104 (2006).
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[18] Sangsu Hong, Taiha Joo, Won II Park, Yong Ho Jun, and Gyu-Chul Yi, Appl. Phys. Lett. 83, 4157 (2003). uell, M. R. Wagner, A. Hoffmann, A. Cornet, and J. [19] J. S. Reparaz, F. G¨ R. Morante, Appl. Phys. Lett. 96, 053105 (2010).
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[20] S.-K. Lee, S. L. Chen, D. Hongxing, L. Sun, Z. Chen, W. M. Chen, and I. A. Buyanova, Appl. Phys. Lett. 96, 083104 (2010).
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[21] Yongchun Zhong, Aleksandra B. Djuriˇsi´c, Yuk Fan Hsu, Kam Sing Wong, Gerhard Brauer, Chi Chung Ling, and Wai Kin Chan, J. Phys. Chem. C. 112, 16286 (2008).
[22] Douglas Magde, and Herbert Mahr, Phys. Rev. Lett. 24, 890 (1970). [23] J. Lagois, Phys. Rev. B. 23, 5511 (1981). [24] Takeshi Hiral, Nobuhito Ohno, Yoshiyuki Harada, Taku Horii, Yuji Sawada, and Tadashi Itoh, Appl. Phys. Lett. 93, 041113 (2008). 10
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[25] G. E. Jellison, Jr. and L. A. Boatner, Phys. Rev. B. 58, 3586 (1998). [26] T. Onuma, K. Hazu, A. Uedono, T. Sota, and S. F. Chichibu, Appl. Phys. Lett. 96, 061906 (2010).
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[27] D. C. Look, G. C. Farlow, Pakpoom Reunchan, Sukit Limpijumnong, S. B. Zhang, and K. Nordlund, Phys. Rev. Lett. 95, 225502 (2005).
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3. The radiative lifetime of exciton polaritons decreases linearly with the surface-to-volume ratio increasing, this behavior can be ascribed to the surface
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electric field enhanced the radiative recombination of exciton polaritons.