Low-Temperature Photoluminescence Studies of CsPbBr3 Quantum

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Low Temperature Photoluminescence Studies of CsPbBr Quantum Dots Aparna Shinde, Richa Gahlaut, and Shailaja Mahamuni J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.7b02982 • Publication Date (Web): 13 Jun 2017 Downloaded from http://pubs.acs.org on June 15, 2017

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The Journal of Physical Chemistry

Low Temperature Photoluminescence Studies of CsPbBr3 Quantum Dots Aparna Shinde, Richa Gahlaut, and Shailaja Mahamuni* Department of Physics, S.P. Pune University, Pune 411 007, India *E-mail: [email protected]

ACS Paragon Plus Environment

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Abstract Cesium lead halide pervoskite semiconductors are being extensively studied due to unprecedentedly high luminescence efficiency and concomitant narrow emission line width. Here, we report photophysical properties of CsPbBr3 quantum dots having different sizes. Notably 5.5 nm sized CsPbBr3 quantum dots reveal95 % photoluminescence (PL) quantum yield at room temperature. Moreover, signature of the stimulated emission is observed at low temperature for excitation fluence as low as ∼4.16 µW (Xe lamp excitation). Even though, CsPbBr3 quantum dots reveal the red shift in band gap at low temperature, similar to the single crystal, the exciton-phonon interaction is profoundly affected by the quantum size effects. Temperature dependent optical studies reveal an anomalous decrease in exciton-LO phonon coupling in small sized quantum dots aside from expected higher exciton binding energy. Observed stimulated emission in low sized CsPbBr3 quantum dots has implications in realizing quantumdot based laser.


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Introduction Past

two

years

have

witnessed

outstanding

luminescence

properties

of

trihalideperovskitenanocrystals (NCs). Inorganic perovskites are stable than their organic analogues.1 Notably narrow emission line width with photoluminescence (PL) quantum efficiency reaching 90% or higher2 make them ideal candidates for light emitting and light harvesting applications.3-6In fact, the prototype light emitting diodes with narrow emission width which can be operated at low turn-on voltage are prepared5,7,8in various laboratories.Moreover, the forbidden gap of CsPbX3 is tunable from visible through ultraviolet by using different halide (X: I, Br, Cl) ions.9,10 Facile synthesis routes are being developed11,12to obtain desired value of forbidden gap by post-synthesis protocols while retaining the size of NCs. Furthermore, CsPbX3 is believed to have a clean forbidden gap that is free of notorious trap states.2 Semiconductor quantum dots (QDs), particularly well-studied CdSe based quantum structures are foreseeing competition with the perovskite (CsPbX3) QDs. Initial efforts revealed yet different photophysical behavior of CsPbX3. In contrast to common semiconductors, with decrease in temperature the forbidden gap narrows down due to contraction of lattice that is instrumental in stabilizing valence energy levels in a peculiar way. Strangely, CsPbBr3 single crystal shows13photoluminescence emission at higher energy than the absorption feature, which is, however, not the case for quantum dots. Moreover, the temperature dependent PL studies reveal two step thermal quenching in emission intensity of single crystal; the first stage activation energy is attributed to the phonon energy. It would be intriguing to examine the photoluminescence properties of CsPbBr3 quantum dots as a function of temperature. Indeed temperature dependent photophysical properties of CsPbBr3 NCs are addressed14,15albeit scarcely, despite the importance of electron-phonon interaction in deciding PL emission line width, hot electron decay dynamics, and charge carrier mobility.


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In addition, charge carrier cooling on non-resonant photon absorption is governed by electron-phonon interaction. A great discrepancy in the value of optical phonon energy even in the case of highly luminescent CsPbBr3 QDs is seen. For instance, 11 nm sized CsPbBr3 QDs are reported to have value of phonon energy to be 36 meV,16 30 meV,17 4.6 meV18which is far different than 16 meV reported for the single crystal CsPbBr3.13,19Further, relative contribution of different scattering mechanisms such as acoustic and optical phonons is yet to be explored. The exciton binding energy reaching 120 meV associated with the giant oscillator strength of CsPbBr3nanoplatelets is reported.14Notablymultiexciton interactions were examined by the transient absorption spectroscopy (TA) on the CsPbBr3 QDs. In 11 nm sized QDs, biexciton binding energy ~ 30 meV was observed.20 The pump-probe transient absorption spectroscopy on ~8.6 nm sized QDs show the higher order multiexciton complexes.21 Further, variation in the binding energy of biexcitons with QD size is also reported. The confinement (size variation 3.5 nm – 13.1 nm) dependent TA study reveals the highest biexciton binding energy (100 meV) for QDs having size ~ 7.1 nm which is nearly equal to Bohr diameter of CsPbBr3.22Such high biexciton binding energyand high absorption coefficient22with large photoluminescence quantum yield (PLQY) makes it inexpensive optical gain material. The observed stimulated emission is a manifestation of the biexciton recombination. In this process, exciton annihilation leads to the population inversion of a radiative biexciton state, which then undergoes stimulated emission to the one-exciton level upon interaction with other photon.23 At low incident fluence, stimulated emission appears as a weak feature. With theincrease in the intensity of incident radiation, the stimulated emission features rises in intensity.24The advantage of CsPbX3 is the lasing action occurs at low threshold (5 µJ/cm2)25 and tuning the laser wavelength in blue, green and red region just by


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changing the chemical composition / halide substitution26 is feasible. Recently, two photon lasing is also demonstrated27-29 in CsPbBr3 NCs at a low threshold. In the present work, photoluminescence line width is examined for three different sized CsPbBr3 QDs in a temperature between 10 K through 300 K. This temperature range clearly discerns the electron-phonon interaction regimes into two sections; thermal energy less than LO phonon energy at lower temperature while the thermal energy close to the LO phonon energy near room temperature. Acoustic phonon energy is also estimated from the experimental data. Further, electron-phonon contribution is evaluated for varying sized CsPbBr3 QDs and is discussed. Interestingly, the present experiments demonstrate stimulated emission from 5.5 nm sized CsPbBr3 QDs at low temperature for unprecedentedly low excitation intensity. Results and Discussion Synthesis of CsPbBr3 QDs is carried out by slightly modifying the hot injection protocol.2Three batches of QDs having sizes 5.5 ± 0.98 nm (PQD-5.5), 7.3 ± 1.1 nm (PQD7.3) and 10.1 ± 1.8 nm (PQD-10.1) are synthesized at different injection temperatures (X-ray diffraction patterns and transmission electron micrographs are given in the supporting information, Figure S1 and Figure S2). FTIR spectrum (Figure S3) confirms presence of oleic acid and oleylamine on the surface of QDs, the molecules which stabilize it in non polar solvents. The optical absorption features of PQD-5.5 appear at 2.53 eV and 2.59 eV, while PQD-7.3 and PQD-10.1 reveal single feature at 2.45 eV and 2.41 eV respectively (Figure 1). Nominal value of the Bohr exciton diameter in CsPbBr3 being 7.0 nm,30 charge carriers in PQD-5.5 are strongly confined. In fact, it is a quantum structure having only about 5 unit cells. Strongly confined structure of PQD-5.5 opens up heavy hole (HH)-electron and light hole (LH)-electron transitions31corresponding to the optical absorption feature at 2.53 eV and 2.59 eVrespectively.


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Figure1 illustrates the optical absorption and photoluminescence emission spectra (with excitation wavelength 400 nm; 3.1 eV) of different sized CsPbBr3 QDs. Very narrow emission lines were observed for CsPbBr3 QDs. With decreasing size of QDs, PL broadening is observed. The room temperature emission lines show value of FWHM to be 132 meV, 99 meV and 95 meV for PQD-5.5, PQD-7.3 and PQD-10.1 respectively. The point to be noted is, even if size distribution of PQD-5.5 is narrower, emission line width is larger; which will be discussed later. The optical absorption spectrum of PQD-5.5 reveals features at 2.59 eV (strong) and 2.53 eV. The deconvoluated PL spectrum recorded at 300 K (inset of Figure 1a) shows features at A: 2.59 eV, B: 2.53 eV, and C: 2.47 eV (the highest intensity). Feature A is attributed to electron-light holetransition, and feature B to electron-heavy hole transition. These two distinct transitions are observed in optical absorption as well as in PL due to removal of degeneracy in strongly confined quantum dots. Similar splitting in HH and LH levels in nanoplatelets is recently reported.31Intense feature C at 2.47 eV is due to bound exciton transition. It may be noted that in contrast to single crystal studies, optical absorption is located at higher energy than PL emission feature. PL spectra of PQD-5.5, PQD-7.3 and PQD-10.1 CsPbBr3 NCs recorded at 10 K comprise of four, two and one emission features respectively (Figure 2).In spite of having large size distribution in PQD-10.1, PL spectra at 10 K show only one peak; this fact suggests that the number of peaks observed is not due to the size distribution but appear due to the quantum confinement. As mentioned earlier, the Bohr exciton diameter of CsPbBr3 being 7 nm, PQD5.5 are in the strong confinement regime and can bedeconvoluted in to four PL features (Figure 2a). At low temperature, PL emission spectra reveal another feature D at 2.38 eV. Literature survey indicates, at high incident intensity, the stimulated emission appears through biexcitonic recombination in CsPbBr3 QDs24,25,28at room temperature. For example, CsPbBr3 QDs sized 9.0 nm, show hump like feature at low energy side of spontaneous


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emission peak under low excitation intensity of pulsed laser. With increasing excitation intensity the hump like feature evolves into very sharp peak (FWHM ~5 nm) which corresponds to the stimulated emission. In the present case, a hump like structure is seen in PQD-5.5 at 2.38 eV (peak D) at the lowest energy at 10 K. The emission is due to the biexcitonic recombination. To confirm the origin of biexcitonic recombination in PQD-5.5, we have calculated the biexcitonic binding energies (Eb) based on the energy separation between biexcitonic peak position (2.38 eV) and other emission peak positions observed at higher energies viz. at 2.55 eV, 2.48 eV and 2.42 eV. The calculated biexcitonic binding energies are 170 meV, 100 meV and 40 meV for 2.55 eV, 2.48 eV and 2.42 eV levels respectively. Among these three values of biexcitonic binding energies, 40 meV is in good agreement with the reported value24which also suggests that feature D (2.38 eV) is a radiative recombination from the biexcitonic level to the excitonic level. Further, on excitation of PQD-5.5 with pulsed laser (1.8 mW) at 10 K, the hump type feature evolves into a sharp and intense peak (Figure S4) which confirms that the peak D corresponds to the stimulated emission through recombination of biexcitons. PL experiments were carried out under low excitation intensity. As mentioned in the literature Auger recombination process takes place at high excitation intensity24,32therefore Auger recombination process is not considered in our experiment and will be a topic of next report.The fact that the biexcitonic binding energy is larger than the thermal energy at room temperature is encouraging and needs to be explored further to stabilize biexcitons at room temperature which would be advantageous in optoelectronic devices. The schematic diagram (Figure 3) of the recombination process is constructed on the basis of observed PL spectrum of PQD-5.5 at 10 K. Feature located at 2.55 eV (peak A) and 2.48 eV (peak B) corresponds to the electron-light hole and electron-heavy hole transitions. The excitonic energy level (X) is present below the LUMO level by an amount ~ 50 meV which


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is attributed to peak C. (2X) is the biexciton energy level having energy double to that of level X. Biexcitonic recombination takes place from level XX to level X and is attributed to stimulated emission (peak D). The energy difference between levels 2X and XX is the binding energy of biexciton. At 10 K, PL emission spectra of PQD-7.3, reveal two emission features (Figure 2b). Higher energy component A is due to the free excitonicrecombination and the low energy component B originates from the bound exciton recombination which is a dominant recombination at room temperature. In PQD-10.1, PL emission is mainly due to bound excitons, while free exciton transition is not seen. Biexciton binding energy in smaller QDs is higher than largesized QDs; therefore the probability of re-absorption of stimulated photon is less in smaller QDs. Consequently, stimulated emission is observed at 10 K, only in case of PQD-5.5 NCs under excitation power as low as ~4.16 µW. Unlike to common tetrahedral semiconductors such as CdSe33 and GaAs,34 PL peak position red shifts with decreasing temperature for perovskite NCs. A representative example of the variation in peak energy with temperature from 10 K through 300 K for PQD-5.5 is shown in Figure 4a. It is known that thermal expansion and electron-phonon interaction contribute to the change in the forbidden gap of semiconductor with temperature. Typically, semiconductor shows blue shift in the band gap as temperature decreases due to the prevailing electron-phonon interaction. In CsPbBr3, thermal expansion is a dominating factor. Consequently, with increasing temperature, thermal expansion of the lattice tends to decrease the interaction between two valence orbitals (s and p) which leads to narrowing of the valance band width and hence the forbidden gap increases.17,35 It may be worthwhile to mention here that the orthorhombic structure of CsPbBr3 is slightly distorted structure from ideal cubic perovskite structure. In CsPbBr3perovskite structure, Cs is present at the corner of cube with Pb at the center and Br at the face-center position of


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cube. Br forms octahedron around Pb. It is reported that36the valence band involves hybridized Br 4p and Pb 6s orbitals and conduction band comprises of Pb 6p orbitals. The optical properties of CsPbBr3 are therefore governed by PbBr6 octahedra.37,38 The temperature dependent PL study gives information about physical parameters of QDs such as exciton binding energy (EB), longitudinal optical phonon energy (Eph), inhomogeneous broadening (Γo) and exciton-phonon coupling strength (Γop). As expected, PL intensity for all peaks decreases with increasing temperature. The PL intensity as a function of temperature is plotted and fitted using an Arrhenius equation,

I(T) = I ⁄1 + Ae

(1)

where I0 is intensity at 0 K, kB is Boltzmann constant. Figure 4b shows the fitted curve using equation (1) for PQD-5.5 with coefficient of determination (R2) ̴ 1 which indicates the goodness of fitted data. The calculated value of EB for PQD-5.5 is 50 meV. Emission line width plays a crucial role in optoelectronic devices and has two different contributions; the temperature independent inhomogeneous broadening due to the dispersion in size, shape, elemental composition etc. and the temperature dependent homogeneous broadening due to the interaction of excitons with acoustic phonons, optical phonons and impurities. The temperature dependence of emission line width is described by the Boson model,39 Γ T = Γ + σT + Γ / e

− 1 + Γ e

(2)

In equation (2) the first term Γ0 gives the inhomogeneous broadening and other three terms contribute to the homogeneous broadening. The second term is the linear contribution to the line width due to the exciton-acoustic phonon coupling. The third term defines the interaction of excitons with longitudinal optical (LO) phonons and last term denotes the contribution of impurities on the line width. Due to minimal trap states in CsPbBr3, the last term is neglected


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in further calculations. The experimental data for PQD-5.5 is fitted using equation (2) with R2~1 as shown in Figure 4c. The fitted values of inhomogeneous broadening (Γ0), LO phonon energy (Eph), coupling strength of exciton-LOphonon (Γop), coupling coefficient of excitonacoustic phonon (σ) are summarized in Table 1. On the similar lines, the experimental temperature dependent PL data of PQD-7.3 and PQD-10.1 is fitted (Figure S5, S6, S7) to evaluate the parameters. The exciton binding energies in the QDs are larger than thermal energy (26 meV) at room temperature, leading to bright luminescence. The exciton binding energies decrease with increasing size of quantum dot from 50 meV to 35 meV as a manifestation of quantum confinement. The observed large value of inhomogeneous broadening in PQD-5.5 is due to the multiple emission components which are distinctly resolved at low temperature. In homogeneous line width broadening, exciton-LO phonon coupling is dominant as compared to exciton-acoustic phonon coupling. The profound nature of exciton-LO phonon coupling is a consequence of polar nature and hence strong Frohlich interaction in CsPbBr3. Notably, at low temperature, acoustic phonon scattering is discernible. The exciton-LO phonon coupling constant is proportional to the squared absolute value of the difference in the Fourier transform of electron and holewave function.40 The increased exciton phonon coupling in strong confinement regime is due to the enhanced coupling of excitons with higher frequency phonons.40Further, with increase in nanocrystal radius, exciton- phonon coupling decreases up to certain radius and again shows enhancement in coupling above the critical value of the radius. This is because with increasing size, Coulomb interaction becomes more important and confinement effect decreases which enhance the difference in the envelope function for an electron and hole. Compared to PQD-5.5 in PQD10.1, Coulomb interaction predominates over confinement effect, consequentlyexciton-LO phonon coupling strength increases in PQD-10.1.


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The PLQY recorded at room temperature for PQD-5.5, PQD-7.3 and PQD-10.1 are 95%, 90% and 86% respectively. Rhodamine 6G dissolved in ethanol is taken as a reference dye for PLQY measurements. Further insight is gained from time resolved photoluminescence (TRPL) spectra. The decay curves of CsPbBr3 QDs are found to be best fitted using biexponential function which yields short (τ1) and long (τ2) lifetime component. Figure 5 indicates the reduction in the decay times with decreasing size of QDs. It is known that effective mass of electron and hole are nearly equal in CsPbBr3,2 therefore with increasing confinement (i.e. with decreasing size), overlap of electron hole wave function increases.41According to the Fermi’s golden rule, the transition probability and electron hole wave function are related to each other by the relation, λif =

ħ

|Mif|2ρf

Where, λif the transition probability from initial to final state and is inversely proportional to the lifetime, ħ is the reduced Plank’s constant, Mif = & '(∗ *'+ ,- is the matrix element of interaction of electron and hole and ρf is the density of final state. The transition probability will be larger if the coupling between initial and final state is stronger.Here, with decreasing size of PQD optical transition probability increases thereby decreasing the average fluorescence lifetime (τavg) for PQD-5.5. Decay time as estimated from fitting of the TRPL data, along with radiative (τr= τavg/ PLQY) and non-radiative (τnr= τavg/1-PLQY) recombination lifetimes are summarized in Table 2. The radiative andnon-radiative lifetimes were calculated by using PLQY and average PL lifetimeto check their contribution in total lifetime. In general, PLQY is the ratio of non-radiative recombination lifetime to the total recombination lifetime [PLQY = τnr/(τnr+τr)].42Therefore, the 19 to 6 times faster radiativerecombination lifetime than the corresponding non-radiative recombination lifetime result in the high PLQYs.


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In highly luminescent (~86%) CdSe QDs, decay data with bi-exponential fit were observed where short lifetime (theoretically ~ 3 ns) is attributed to the initially populated core state and longer lifetime to the surface related emission.43,44On the contrary, in the present case, PLQY increases when the contribution of longer lifetime increases and shorter lifetime decreases. The peculiar trend indicates the contribution of surface states in higher PLQY (Table 2) of CsPbBr3 QDs. Nearly same value of the radiative lifetime and long lifetime (τ2) implies that excitonic emission observed in CsPbBr3 QDs involves shallow states, perhaps the surface states as well. Note that bound excitons are associated with these levels. The present findings indicate existence of trap states in CsPbBr3 QDs which is assumed to be minimal in earlier reports. Conclusions In summary, 5.5 nm-sized CsPbBr3 QDs reveal distinct optical properties. Optical absorption and PL spectra show splitting due to LH and HH levels. Low temperature PL spectra are rich in structure due to free excitons as well as bound excitons aside from the most interesting stimulated emission at low optical threshold. Temperature dependent PL studies are used to evaluate contribution of LO-phonon scattering and acoustic phonon scattering in different sized CsPbBr3 NCs. Unexpected decrease in the LO phonon-electron coupling that is observed in small sized CsPbBr3 QDs is understood on the basis of quantum confinement effects. Supporting Information Experimental method, X-ray diffraction and FTIR spectra of PQDs, transmission electron microscopy images of PQDs, PL spectrum with pulsed laser excitation for PQD-5.5 at 10K,fitting of temperature dependent PL experimental data of PQD-7.3 and PQD-10.1. Corresponding Author


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E-mail: [email protected] Acknowledgments This work is supported by the department of science and technology, New Delhi, India. AS and RG thank S.P. Pune University and University Grants Commission, New Delhi, respectively for the financial support.

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Table 1. Physical parameters obtained from fitting experimental temperature dependent PL data.

Samples

EB (meV)

Γo(meV)

Eph (meV)

Γop (meV)

σ (µeV/K)

PQD-5.5

50

43.5

16.2

49.4

23.9

PQD-7.3

48

26.1

18.5

81.6

15.5

PQD-10.1

34

23.8

20.4

98.1

8.9

EB– exciton binding energy,Γo– inhomogeneous contribution to linewidth, Eph – LO phonon energy, Γop– coupling strength of exciton-LO phonon, σ – coupling coefficient of excitonacoustic phonon


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Table 2. Decay parameters of PQDs

QY

τ1 (ns)

τ2 (ns)

(%)

(a1%)

(a2%)

2.1

4.4

(28%)

(72%)

2.8

8.54

(32%)

(68%)

5.71

20.44

(38%)

(62%)

Samples

PQD-5.5

PQD-7.3

PQD-10.1

95

90

86

τavg (ns)

τr (ns)

τnr (ns)

τr /τnr

4

4.2

81.1

0.051

7.7

8.6

77.4

0.111

18.3

21.3

130.9

0.162

(QY) – quantum yield, (τ1)-short and (τ2) long lived lifetime, a1 and a2 are the contributions for τ1andτ2respectively, (τavg) average decay lifetimes, (τr) radiative recombination lifetimes, (τnr)non-radiative recombination lifetimes


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Figure 1.Optical absorption (dotted line) and emission (continuous line) spectra of CsPbBr3QDs (a) PQD-5.5 (b) PQD-7.3 and (c) PQD-10.1 recorded at room temperature.


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Figure 2.PL spectra of (a) PQD-5.5 (b) PQD-7.3 and (c) PQD-10.1 at 10 K.


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Figure 3. Schematic of recombination processes in PQD-5.5 at 10 K


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Figure 4.Temperature dependent (a) emission energy of four PL emission peaks (b) PL intensity of peak C and (c) FWHM of peak C for PQD-5.5.


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Figure 5.Decay profile of (a) PQD-5.5, (b) PQD-7.3 and (c) PQD-10.1, here solid line represents fitted data.


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TOC GRAPHICS


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Low-Temperature Photoluminescence Studies of CsPbBr3 Quantum

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