Highly Enhanced Exciton Recombination Rate by Strong Electron

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Highly Enhanced Exciton Recombination Rate by Strong Electron-phonon Coupling in Single ZnTe Nanobelt Qing Zhang, Xinfeng Liu, MUHAMMAD IQBAL BAKTI Utama, Jun Zhang, María de la Mata, Jordi Arbiol, Yun Hao Lu, Tze Chien Sum, and Qihua Xiong Nano Lett., Just Accepted Manuscript • DOI: 10.1021/nl3037867 • Publication Date (Web): 21 Nov 2012 Downloaded from http://pubs.acs.org on November 27, 2012

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Highly Enhanced Exciton Recombination Rate by Strong Electron-phonon Coupling in Single ZnTe Nanobelt Qing Zhang1, Xinfeng Liu1, Muhammud Iqbal Bakti Utama1, Jun Zhang1, María de la Mata2, Jordi Arbiol2,3, Yunhao Lu4, Tze Chien Sum1, *, Qihua Xiong1,5,* 1

Division of Physics and Applied Physics, School of Physical and Mathematical Sciences, Nanyang

Technological University, Singapore 637371 2

Institut de Ciència de Materials de Barcelona, ICMAB-CSIC, Campus de la UAB, 08193 Bellaterra,

Catalonia, Spain 3

Institució Catalana de Recerca i Estudis Avançats (ICREA), 08010 Barcelona, Catalonia, Spain.

4

Department of Materials Science and Engineering, Zhejiang University, Hangzhou 310058, China.

5

Division of Microelectronics, School of Electrical and Electronic Engineering, Nanyang

Technological University, Singapore 639798 *To whom correspondence should be addressed. Email address: [email protected] and [email protected].

1

ACS Paragon Plus Environment

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Abstract Electron-phonon coupling plays a key role in a variety of elemental excitations and their interactions in semiconductor nanostructures. Here we demonstrate that the relaxation rate of free excitons in a single ZnTe nanobelt (NB) is considerably enhanced via a non-thermalized hot-exciton emission process as a result of an ultra-strong electron-phonon coupling. Using time-resolved photoluminescence (PL) spectroscopy and resonant Raman spectroscopy (RRS), we present a comprehensive study on the identification and the dynamics of free/bound exciton recombination and the electron-phonon interactions in crystalline ZnTe NBs. Up to tenth order longitudinal optical (LO) phonons are observed in Raman spectroscopy, indicating an ultra-strong electron-phonon coupling strength. Temperature-dependent PL and RRS spectra suggest that electron-phonon coupling is mainly contributed from LH free excitons. With the presence of hot-exciton emission, two time constants (~80 ps and ~18 ps) are found in photoluminescence decay curves, which are much faster than those in many typical semiconductor nanostructures. Finally we prove that under high excitation power, amplified spontaneous emission (ASE) originating from the electron-hole plasma occurs, thereby opening another radiative decay channel with an ultrashort lifetime of few ps. KEYWORDS: ZnTe Nanobelts, Exciton Dynamics, Electron-phonon Coupling, Photoluminescence, Resonant Raman Spectroscopy

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The understanding and control of the exciton relaxation properties in semiconductor nanostructures is of fundamental scientific interest because of their direct relevance to the practical applications in linear and nonlinear optoelectronic and photovoltaic devices.1-6 The exciton relaxation process and associated relaxation dynamics are highly dependent on the band structure, the types of phonons involved and their respective coupling strength to electrons or excitons.7-9 In polar semiconductors (such as ZnO, CdS, GaAs, etc), free excitons are strongly coupled to LO phonons via Fröhlich interaction, which can lead to a much faster carrier radiative rate than the nonpolar semiconductors.10-12 During a typical intravalley relaxation process, the photoexcited carriers lose their excess energy to crystal lattice by emission of either longitudinal optical (LO) or acoustic phonons, relaxing to the k~0 momentum states.7 Since the typical relaxation time for LO and acoustic phonons are on the scale of ~ 100 fs to ~100 ps, respectively, which is much shorter than the exciton recombination time scale (~1 ns), the emission of a non-equilibrium elementary excitation produces “hot-excitons” that have been observed in these polar semiconductors.13-16 The “hot-exciton” emission considerably enhances the radiative rate of the carriers. However, in most of as-reported II-VI semiconductors such as ZnO, CdS and etc, the exciton decay path is still dominated by thermal-equilibrium recombination processes, thus the decrease of exciton decay rate by electron-phonon coupling is limited and the exciton lifetime is on the scale of several hundreds of pico-seconds or even longer. Recently, through coating a layer of SiO2/Ag shell to CdS nanowire, Cho et al. succeeded in tuning the recombination process in CdS nanowires from one at thermal-equilibrium to a hot-exciton recombination process through plasmon enhanced exciton-phonon coupling effect, resulting in a decrease of the exciton lifetime by a factor of ~ 1000.3 Herein we demonstrate a very strong electron-phonon coupling in ZnTe NB with which the radiative

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rate of a single bare ZnTe NBs is significantly enhanced (~ 18 ps), involving the so-called “hot-exciton” emission. Zinc telluride (ZnTe), a direct gap semiconductor with a bandgap of 2.26 eV at room temperature, is one of the most promising materials for optoelectronic devices in the pure green region.17 It has many unique applications in the domains of terahertz generation and detection, radiation detector, acousto-optic devices and photorefractive substrates in optical data processing, as well as in other telluride compounds.17-20 A clear understanding of its excitonic properties (generation, decay, transport, etc) is particularly important for improving the performance those optoelectronic devices. However, unlike other II-VI semiconductor nanostructures such as ZnO, CdS, CdTe, whose exciton properties have been extensively studied, the excitonic dynamics of ZnTe nanostructures are rarely explored, especially in the time domain.5, 21-22 Till now many experimental and theoretical efforts have been devoted to the growth and steady-state PL characterizations of ZnTe epilayers and nanostructures.17-18,

23-27

In epilayer structures, the lattice mismatch and

difference in thermal expansion coefficient with the substrate induces a source of strain, structural defects and element doping states, giving rising to the difficult and controversial assignment of the emission17, 28-29 The crystal quality of ZnTe fabricated so far in nanowire and nanocrystal forms are also not as good as that of other II-VI semiconductors.18, 24, 27 ZnTe in nanobelt (NB) form may circumvent this problem effectively.19 Our research is motivated to obtain a clear understanding of mechanism and nature of the exciton recombination dynamics of ZnTe materials following photoexcitation. In particular, the mechanism of electron-phonon coupling and its reduced non-equilibrium hot-exciton emission are explored in energy and time domains. In this paper, we investigate the exciton dynamics in the ZnTe NB between the regions of 4


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spontaneous and amplified spontaneous emission (ASE) using steady state and time-resolved photoluminescence spectroscopy (TRPL) and resonant Raman spectroscopy (RRS). The origins of the excitons are elucidated and the emission peaks are assigned with the support of first-principle calculations. We find that LO phonons are strongly coupled to the LH free excitons, leading to the emission of the tenth order LO phonons (10th-LO). The decay rate of the excitons is highly enhanced with the occurrence of hot-exciton process.

Figure 1．．Characterization of the as-grown ZnTe nanobelt on a Si wafer. (a) SEM image. The inset is a side-view of single nanobelt with a thickness ~ 80 nm. (b) Low magnification TEM image. (c) High-resolution TEM image. (d) Power spectrum (FFT) obtained from the HRTEM image in (c). Figure 1a-b display typical scanning electron microscopy (SEM, JEOL 7001F) and low magnification transmission electron microscopy (TEM, JEOL JSM-7001F) images of the as-grown NBs (see methods in supporting information). The thickness of the nanobelts is from 40 nm to 300 nm and the width is on the scale of 100 nm ~10 µm. The high-resolution TEM (HRTEM) image displayed in Figure 1c shows that the ZnTe NB has a single zincblende crystalline structure. The clear and sharp diffraction spots in the power spectra obtained from HRTEM images (Figure 1d)

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-

suggest the good quality of the ZnTe NB. The growth direction of the ZnTe NB is [111] .7 The observed lattice fringes on HRTEM images correspond to 0.352 nm and 0.305 nm for (111) and (002) interplanar distances, respectively, matching with a calculated cell parameter of 0.61 nm (a=b=c).19 Raman scattering spectroscopy is a powerful technique to study the phonons in the semiconductor nanostructures. The peak position and lineshape parameters carry important information concerning the crystalline quality, structural defects, novel phonon states associated with size or anisotropic shape.4, 30-31 In particular, resonant Raman scattering (RRS) is effective to probe the electron-phonon interactions and electronic structures.12-13, 32-34 The overtones/fundamental Raman peak intensity ratio in RRS spectra can be used to evaluate the electron-phonon coupling strength.12,

32-34

In ZnTe

nanorods, we have recently observed a strong electron-phonon coupling with a Huang-Rhys factor ~ 3 using RRS.32 Nevertheless, a few more key questions remain to be addressed. For instance, how does the exciton contribute to the LO phonon scattering cross section? What is the outcome of the exciton dynamics due to such strong electron-phonon coupling strength?

Figure 2．．Steady-state PL and Raman spectra of a single ZnTe nanobelt. (a) Room temperature PL spectrum, excited by a 473 nm laser. (b) Room temperature resonant Raman spectrum, excited by a 6


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514 nm laser. (c) Resonant Raman spectrum at room temperature, excited by a 532 nm laser. (d) Schematic diagram of the hot-exciton emission (red curves) and normal exciton (blue curves) emission processes. When the NB is excited by a 473 nm laser, only one emission band is observed with a peak at 548.4 nm (ca. 2.26 eV), as shown in Figure 2a,.35 As the NBs is excited by a 514 nm laser, in addition to the PL emissions, several narrow peaks are observed corresponding to phonon emissions of ZnTe. Three peaks located at 110, 120 and 145 cm-1 are assigned to ELO, A1 and ELO/ETO modes of the crystalline Te phase as reported in ZnTe NRs.32 Seven other peaks located at 205, 410, 615, 820, 1025, 1230 and 1435 cm-1, nearly the multiple of ωLO = 205 cm-1, are the corresponding n-th order (n = 1, 2, 3, 4, 5, 6, 7) LO phonon emission peaks respectively. Compared to the appearance of high-order LO peaks, Raman peaks of TO phonons are not resolved, suggesting the dominant Fröhlich interaction in ZnTe NBs.32 By introducing the idea of the “cascade relaxation model”, in which hot-excitons serve as intermediate states, Gross et al. and Klein et al. explained the multiphonon Raman spectroscopy in II-VI semiconductor compounds.36-37 This process can be qualitatively described as follows (indicated by red curves in Figure 2d): a primary photon with energy of ћω0 creates a hot-exciton together with the emission of one LO-phonon, then the exciton is scattered multiple times by LO-phonons into states separated by ћωLO. Finally there is an indirect annihilation of the exciton with emission of one more LO-phonon and a secondary photon with energy ћω0-nћωLO37. The RRS process expressed above is very similar to that of real-state hot-exciton emission. It is argued that the two processes originate from different physical phenomena: in hot-exciton emission process the participation of real exciton states is required but not in multiphonon resonant scattering.13,

15

However, so far it is still hard to distinguish them in the optical spectrum.13 The hot-exciton emission 7


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also consists of a series of narrow lines shifted by integer numbers of LO phonons from the excitation energy.13 Leite et al. ascribed the LO emissions of CdS crystal to RRS, however Permogorov et al. suggested that only part of the emissions were associated with hot-exciton emissions.10, 13 Martin et al. suggested that the inelastic scattering of light in CdS crystals occurs as a cascade process containing aspects of hot-exciton luminescence and Raman scattering.36 In some cases such as exciton-polariton states involved emissions, it is meaningless to distinguish the two processes.13 In ZnTe NBs, it is more likely that RRS and hot-exciton emission process occur simultaneously. Firstly, the opening up of fast hot-exciton emission channel will accelerate the decay of exciton in excited states extensively.3,

13-14, 37

Experimentally, we have observed significant

shortening of carrier radiative rate which is demonstrated not due to surface states (to be discussed later). Additionally, only two LO emission lines are observed below the center of PL emission peaks in Raman spectra taken at both 532 nm and 514 nm excitation cases, similar to that was observed in CdS crystals.10 Lastly, the intensity differences of the high-order LO overtones are not as large as those obtained from the multiphonon Raman scattering process.13 When the NB is excited by 532 nm excitation, the 3th-LO intensity is almost the same with 2th-LO which is in good agreement with hot-exciton emissions (Figure 2c). The exciton/carrier decay dynamics is highly dependent on the type, population and symmetry of excitons.9 To obtain a clear understanding of the nature and origin of excitons in ZnTe NBs, we had carried out low-temperature steady-state PL spectroscopy. Figure 3a-b shows the PL spectra of a single ZnTe NB under the excitation of a 473 nm laser. As the temperature decreases, the PL emission peak blue shifts and the peak width becomes narrower and narrower. When the temperature is around 90 K, the PL emission band splits into several peaks, with an additional new band around 8


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2.30 eV，which we assign to DAP emission – to be discussed in detail later. As the temperature decreases to 30 K or even lower, more spectral details and finer emission features are resolved. The PL emission at 13 K (Figure 3a) is dominated by near-band edge emission around 525 nm (ca. 2.37 eV). In some literatures, strong Y emissions around 600 – 700 nm was observed due to strains or elemental doping.38-39 In the as-grown NBs, no such similar emission was observed in this region (data not shown here), justifying the absence of strain or extrinsic doping. To accurately determine the values of position and full width at half-maximum (FWHM) of each peak, we fit the experimental data (open blue circles) with least-squares fit of multiple Lorentzian line shape function (solid red curve). The green solid curves depict each deconvolved Lorentzian PL bands. The degeneracy of the conduction bands and valence bands are two-fold (Γ6c) and four-fold (Γ8v), respectively, in zincblende semiconductors.40 Due to a large spin-orbit splitting, only excitons with the hole in the topmost (Γ8) valence band are observed. The ground state of this exciton splits to three types of exciton states-Γ3, Γ4 and Γ5, while only the Γ5 exciton state is dipole allowed.40 Due to the strains in the crystal, the fourfold of valence band is reduced, leading to the splitting of Γ5 into heavy hole (HH) and light hole (LH) free excitons respectively.40 Previous reflectance and PL spectroscopy have shown that the ground-state (1S) heavy hole free exciton (XHH1S) and light hole free exciton (XLH1S) of ZnTe epilayer are located 2.379 and 2.375 eV at 2 K respectively.28, 39 In our PL spectrum at 13 K, the two most noticeable peaks are located at 2.377 and 2.372 eV, therefore we assign them to be free exciton XHH1S and XLH1S, respectively. The assignment can be justified based upon two reasons. Firstly, no luminescence lines are found above the two peaks as the temperature increases from 13 K to 300 K. Previous studies have reported bound emission peaks around 2.375 eV due to neutral acceptors caused by extrinsic elemental doping (Li, Ag, Cu).22, 36-37 However, in those 9


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studies, strong free exciton emissions were observed in the high energy part of the bound exciton emission. With increasing of temperature, the bound exciton emission decreases rapidly because of the

ionization

effect

and

the

free

exciton

emission

thus

becomes

dominant.

The

temperature-dependent emission position E(T) of the two peaks (Figure 3c) can be well fitted with Varshni function E(T) = E0+T2α/(β+T), where α = 0.6 meV/K, β = 160 K, in good agreement with previous studies, underpinning the validity of the assignment.41 The emission peak near 2.368 eV is weaker than XHH1S and XLH1S. This emission feature has been reported by many other groups and assigned to I1, referring to excitons bound to a neutral shallow acceptor. The source of this peak is still controversial. Extrinsic element diffusion from substrate, including P, As, Li Ag and Cu, has been reported to introduce shallow acceptors giving rise to the emission peak ~ 2.368 eV.17, 28, 42 Unlike its epilayer counterpart, which has a lot of element doping defects, stoichiometric defects dominate in nanowires and nanobelts grown by vapor method.4-5 During synthesis, stoichiometric defects consisting of zinc (VZn) and telluride (VTe) vacancies, interstitials such as ZnI and TeI, as well as anti-sites denoted as ZnTe and TeZn will be unintentionally formed. We conduct a calculation of the electronic energy level of intrinsic defects in zincblende ZnTe (see Figure S1, supporting information). The Zn vacancy VZn related states have the lowest energy above the top of valence band thus can serve as a shallow acceptor. According to Erbarut’s calculation on band structures of ZnTe, Zn neutral vacancy can induce a bound electronic state with an energy ~10 meV above the valence band edge, which is consistent with our experimental result (~ 7 meV).43 Hence, we tentatively attribute this peak to originate from intrinsic shallow acceptor levels Zn vacancy. All the other stoichiometric defects are related to deep levels,

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which could possibly account for the origins of the red band emission in the PL spectra observed in some other literature (no deep level emission is observed here).23, 28, 39 The broader peak at 2.352 eV, which locates just below I1, is identified as the LO phonon replicas of XHH1S, with the emission of one LO (i.e., ELH1S - hωLO ) with approximately 26 meV.32 The broad band at 2.32 eV is approximately two LO phonons (i.e., ELH1S -2 hωLO ) away from XHH1S. However, it splits into two peaks (2.328 eV and 2.319 eV). A series of dual-peaks around the bands, which is followed by 2.303 eV and 2.278 eV, clearly demonstrate the characteristic multiple phonon replicas with an energy spacing of approximately one LO phonon. Similar features have been observed previously and assigned to electrons bound to the acceptor (i.e., e-A) and holes that are bound to the donors (i.e. D-A), respectively.17, 35, 42, 44-45

Energy (eV) 2.40

2.38

2.36

(a)

X LH

Energy (eV)

2.34 2.30

1S

λex =473 nm @ 13K, 57 µW

eA DA

2.32

2.25

2.20

2.30 2.40

2.38

2.36

2.34

2.32

(b)

2.15

2.30 240K

λex =473 nm @ 13K, 20 µW

180K

Intensity (a. u.)

150K 1S

X HH

120K

I1

eA-LO DA-LO DA-2LO DA-3LO DA-4LO DA-5LO

540

560

90K 80K 70K

580

Wavelength (nm)

60K

10LO 9LO

1S

X HH-LO

50K

Zn-Related e-A D-A

30K 13K

520

2.40

1S

XHH

525

530

Wavelength (nm)

535

540

-4 2

EX =2.378-6x10 T /(160+T) 1S

520

525

530

535

Wavelength (nm)

540

(d)

70K

HH

Energy (eV)

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2.36 X1S LH

60K

(c)

50K

2.32

2.28

40K 30K

-4 2

EX =2.373-6x10 T /(170+T) 1S

2.24

LH

0

80

160 240 Temperature (K)

320

522

524 526 Wavelength (nm)

528

Figure 3 Low temperature PL spectroscopy and analysis. (a) PL spectrum taken at 13 K with an excitation wavelength of 473 nm and a power of 57 µW. The inset is the zoom-in PL spectrum expanded between 540 and 580 nm with a weaker excitation power (20 µW). (b) Temperature-dependent PL spectra. The red and blue arrows guide the red-shift of the free exciton XHH1S and XLH1S as the increasing of temperature. (c) Temperature dependence of the XHH1S and XLH1S peak energy. The solid curves are fitting results based on the Varshni equation. (d) Zoom-in PL 11


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spectra in range from 520 nm to 530 nm. The black and red arrows indicate the 10th-LO and XLH1S positions, respectively. Two narrow peaks (FWHM~1 meV) can be resolved at 2.362 eV and 2.387 eV, with Stokes shift energies of 10hωLO and 9hωLO compared with incident laser energy. The peak positions show no dependence on temperature, therefore we attribute them to the secondary emission of 10th-LO and 9th-LO. To our best of knowledge, this is the highest order LO overtones reported in this material. More interestingly, the intensity of XLH1S is enhanced dramatically as the XLH1S energy is in resonance with 9th-LO phonons at 40 K, as shown in Figure 3d. Similar feature was found in CdS nanowire with Ag serving as shell by Cho et al. and was ascribed to the dominant of hot-exciton emission generation.3 Next we discuss the temperature-dependent resonant Raman spectroscopy. We would like to address which exciton is involved in the strong electron-phonon coupling in ZnTe nanostructures. By varying the temperature, the exciton emission position is tuned in resonance to the incidence and scattering radiation energy such that the intensity of each Raman peak can be accurately probed. According to the Albrecht A term, the Raman scattering cross section for an n-th Stokes LO phonon process is given by ∞

nm mi R (ω ) = µ ∑ m = 0 Eex + nhω LO − hω0 + iΓ n

2

2

4

where µ is the electronic transition dipole moment, i

and m

(1)

denotes the initial states and the

intermediate vibration level in the excited state, respectively.11 Eex is electronic transition energy, and

Γ is the homogeneous linewidth of electronic state.11 The resonant Raman scattering occurs when the denominator becomes zero. Thus, the LO overtone intensity will be sensitive to the exciton

12


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energy Eex and its linewidth Γ . Figure 4a shows the RRS spectra of single ZnTe NB taken at temperature from 13 K to 240 K. Since the LO overtones are much stronger than the other Raman peaks such as SO and TO, the Raman intensity has been logarithmically scaled such that other Raman peak and fine features can be resolved clearly. The observation of SO phonons in Raman scattering is due to the breaking of translational symmetry in nanostructures, which has been discussed in detail in a wide range of nanowires and nanobelts.30, 32, 46 With increasing temperature, more and more LO overtones are resolved. The intensity of those Raman peaks located near or at XLH1S is stronger than that of the other peaks, while the red arrows in Figure 4a indicate the position of XLH1S. The trends can be seen clearly from the integrated Raman intensity as a function of temperature (Figure 4b). In the temperature region of 13 K-90 K, the exciton linewidth Γ is almost the same (also seen in Figure S2, supporting information), thus the Raman intensity is mainly determined by the exciton energy Eex. The intensity profile of each LO, TO+LO and 2LO peak show a single maximum, where ELH1S = hω0 -hω ph (ph = LO, LO+TO and 2LO), suggesting that LO are mainly coupled to XLH1S.34 Since hωLO < hωLO +TO < hω2 LO , energy of the exciton in strong resonance with LO+TO (Eex = hω0 -hωLO+TO ) is higher than that of 2LO (Eex = hω0 -hω2LO ). The exciton energy decreases with the increasing of temperature, thus the highest Raman intensity of LO+TO is obtained at a lower temperature (30 K) than that of 2LO (60 K). λexc= 514 nm

1LO

2LO

(b)

4LO

3LO

5LO

2LO+TO 205 K LO+TO 121 K 60 K 30 K LO+SO

200

400

LO LO+TO 2LO

Raman Intensity (a.u.)

(a) Log10Intensity (a.u.)

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12 K

600

800

1000

Raman Shift (cm-1)

0

50

100

150

Temperature (K)

13


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Figure 4．Multi-phonon scattering analysis. (a) Resonant Raman spectra of single ZnTe nanobelt taken at different temperatures. The red arrows indicate the XLH1S position at each temperature. (b) Integrated intensity ratio of LO, LO+TO and 2LO modes as a function of temperature. A hallmark of hot-exciton process is its fast decay rate.3 The typical recombination time constant of general non-thermalized exciton recombination is around ~1 ns.5 In hot-exciton emission process, the excess energy of the exciton is dissipated mainly by the emission of LO phonons, leading to a recombination time scale of tens of picoseconds.3 Transient PL spectroscopy, with varied pump power density, has been conducted to probe the exciton recombination dynamics of ZnTe NBs. As Figure 5a-b shows, the PL decay curve can be well fitted by a double exponential decay equation as follows,

I (t ) = I1 exp(−t / τ1 )+I 2 exp(−t / τ 2 ) ,

(2)

where τ1 and τ2 are time constants for two decay paths, I1, I2 are the corresponding amplitude of PL intensity. When the pumping fluence is below 5 µJ/cm2, τ1 and τ2 are 80 and 18 ps, the respective amplitudes I1 and I2 are 0.18 and 0.82. The two time constants, especially τ2, are much smaller than PL emission lifetime reported in many other semiconductors.5, 7, 13 In nanostructures, the PL lifetime could be reduced by surface states related non-radiative process, with a reported decay time still on the order of ~ 100 ps.1 In order to rule out the influence of surface state, a 200 nm thick PMMA layer was spin-coated onto the ZnTe NBs sample substrate. The transient PL spectrum (Figure 5b, red curve) is almost the same as that of bare ZnTe NB, suggesting that the two short time constants are intrinsic properties. The free exciton lifetime of ZnTe was considered to be < 100 ps, however the exact value was not obtained in that experiment due to their instrument limit.47 Here τ1 is measured to be ~ 80 ps, which is in good agreement with above study, thus we attribute it to the free exciton lifetime. The faster decay path, with a time constant τ2 ~ 18 ps, has much higher amplitude (0.82) 14


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than free exciton decay path (0.18). Thus it is difficult to assign the fast decay path to surface related non-radiative decay as that was commonly done in other nanostructures.1 Additionally, the amplitude of surface states related non-radiative decay path would decrease significantly under high power density since the surface states are full-occupied. However, here the amplitude of the fast decay path shows no noticeable decrease as the increasing of pumping fluence. The lifetime of the fast decay path (18 ps) is close to that was obtained in CdS/SiO2/Ag system (7 ps).3 It is therefore reasonable to attribute the fast decay path to hot-exciton emission.

Figure 5．Power-dependent time-resolved PL spectroscopy of single ZnTe nanobelt, pumped by a. 488 nm laser pulse with a duration of 150 fs and a repetition rate of 1 KHz. (a)–(b): Streak camera image (a) and time-resolved PL spectra (b) of an individual ZnTe nanobelt with a pumping fluence of 5 µJ/cm2. (c)–(d): Power-dependent streak camera images (c) and the corresponding time-resolved PL spectra (d) of ZnTe nanobelt. (e) The PL lifetimes extracted as function of power fluence. The green dash line indicates the threshold of amplified spontaneous emission. (f) Power dependent PL spectra of ZnTe nanobelt. (g) Intensity and peak position of PL emission as a function of pumping fluence extracted from (f) showing the amplified spontaneous emission. As the pumping fluence approaches 30 µJ/cm2, the overall emission decay becomes much faster (Figure 5c-e) and an additional shorter lifetime (τ3 ~ 5 ps) appears, due to the opening up of a multi-exciton decay channel by amplified spontaneous emission (ASE). The occurrence of ASE can 15


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be clearly seen in the corresponding steady-state PL spectra in Figure 5f. The emission intensity increases linearly as the pump fluence is increased from 5 µJ/cm2 to 30 µJ/cm2. When the pumping fluence is 35 µJ/cm2, an intensely sharp peak located at 552 nm appears with a FWHM of ~3 nm. The intensity of PL emission versus the pump fluence, plotted in the inset of Figure 5g, suggests an ASE threshold of ~32 µJ/cm2. The mechanism of ASE for II-VI semiconductor such as ZnO and CdS is exciton-exciton scattering in low temperature (T < ~ 70 K).48-49 At higher temperature, ASE is contributed by exciton-phonon scattering.48-49 As the concentration of exciton or carrier is larger than the critical Mott transition density, the Coulomb field between the electrons and holes gets screened, excitons will condense and form an electron-hole plasma with a smaller energy than that of excitons.50 In II-VI semiconductors, Mott density can be expressed as nm= (kbT/16πEbab3), where kb is the Boltzmann constant, Eb is the binding energy of exciton and ab is the Bohr radius.51 Substituting Eb = 13 meV and a0 = 5.2 nm, the Mott density of ZnTe at room temperature (300 K) is 3.4×1017 cm-3. Assuming that each absorbed photon can generate one electron-hole pair, the exciton density ne can be expressed as nex = F

( hω0 d )

.52 Since the pulsed width (150 fs) is much shorter than the exciton

decay time constants, the diffusion of excitons is not taken into consideration. The absorption coefficient of ZnTe is ~ 6×104/cm (wavelength: 480 nm), which means that ~74% of incident photons are absorbed by a 200 nm-thick ZnTe NB with a thickness d of ~200 nm. nex is calculated to be 4.6×1018 cm-3, larger than Mott transition critical density of ZnTe.53 Therefore, we attribute the ASE of ZnTe NB in room temperature to arise from the electron-hole plasma. As the pump fluence increases above threshold, band-gap renormalization takes effect, and the band gap is lowered as a function of the carrier density.54 As a result, the gain is shifted to the lower energy region and the 16


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longer wavelength modes are chosen and amplified, which can be seen in power dependence ASE peak position plotted in Figure 5g.50 When the temperature T is decreased from 300 K to 77 K, the free exciton lifetime increases slightly from 76 ps to 89 ps, however the hot-exciton emission related lifetime increase from 17 ps to 20 ps, as shown in Figure 6. The increase in the free exciton recombination lifetime with decreasing temperature could be ascribed to the suppression of the non-radiative decay channels (e.g. via a phonon-mediated process to the surface states) in the nanostructures. The non-radiative decay rate due to surface states is inversely proportional to exp(−Ea / kbT ) , where Ea is the activation energy for a non-radiative process.5 With decreasing temperature, the non-radiative decay rate reduces,

Figure 6．Time-resolved PL spectroscopy of single ZnTe nanobelt at different temperatures. (a)-(e):Temperature-dependent streak camera images (left frame) and the corresponding time-resolved PL spectra (right frame) from 80 K to 300 K. (f) The PL lifetimes extracted as a function of temperature. leading to a longer lifetime of free exciton at low temperature. Another possible reason is that the exciton-LO phonon coupling strength is enhanced as the increasing of temperature due to the lattice expansion, leading to a shorter lifetime at high temperature.55 As shown in Figure 6f, the hot-exciton emission rate, highly dependent on the exciton-LO coupling strength, becomes slower with decreasing temperature, thereby fulfilling the important role of exciton-LO coupling in the exciton 17


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recombination process in ZnTe NBs.

Conclusion We have investigated the exciton properties and dynamics of high quality single-crystalline ZnTe NB using time/temperature- resolved PL spectroscopy and resonant Raman spectroscopy. Free exciton and intrinsic defects, particularly Zn vacancy acceptors, determine the optical properties of ZnTe NBs. At high pumping fluence, electron-hole plasma become dominant and is responsible for ASE at room temperature. XLH1S is strongly coupled with LO phonons and a large number of excited electron-hole pairs release their excess energy by hot-exciton emission process. Two time constants corresponding to hot-exciton emission and thermalized exciton recombination are identified to be 18 ps and 80 ps through TRPL measurement. The surface dissipation process further proves that the short lifetime is an intrinsic property of ZnTe NBs. Our results advance the understanding of excitonic radiative properties with a strong electron-phonon coupling and non-equilibrium

hot-exciton emission in semiconductor nanostructures.

Acknowledgement The author Q.X. thanks the strong support from Singapore National Research Foundation through a Fellowship grant (NRF-RF2009-06) and a Competitive Research Program (NRF-CRP-6-2010-2). He also acknowledges very strong support from Ministry of Education via a Tier2 grant (MOE2011-T2-2-051), Nanyang Technological University via start-up grant (M58110061) and New Initiative Fund (M58110100). T. C. S. acknowledges the funding support from his NTU start-up grant (M58110068) and Academic Research Fund (AcRF) Tier 1 – RG 49/08 (M52110082). X. L.

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and T. C. S. also acknowledge the funding support from the Singapore National Research Foundation (NRF)

through

the

Competitive

Research

Program

(NRF-CRP5-2009-04)

and

the

Singapore-Berkeley Initiative for Sustainable Energy (SINBERISE) CREATE Programme. J.A. acknowledges the funding from the Spanish MICINN projects MAT2010-15138 (COPEON) and CSD2009 00013 IMAGINE and Generalitat de Catalunya (2009 SGR 770 and XaRMAE). Y.L. acknowledges the funding from National Nature Science Foundation of China (Grant No. 11004171).

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TOC Figure: Normal exciton

CB

λ ex=514 nm @ 294K

1LO

ω0 LO

Hot-exciton

Te

ω0 -ωLO

2LO

PL

3LO 4LO VB

ω0 -4ωLO

Te 1LO 2LO

-400

2LO 3LO 1LO

12 µ J/cm

2

30 µ J/cm

2

62 µ J/cm

2

75 µ J/cm

2

τ 3~ 5 ps

4LO

ω0 -2ωLO ω0 -3ωLO

τ1 ~ 80 ps τ 2 ~ 18 ps 6LO

5LO

7LO

400 800 1200 1600200 250 300 350 400 450 Decay Time (ps) Raman shift (cm-1)

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Highly Enhanced Exciton Recombination Rate by Strong Electron

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