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Nature and decay pathways of photoexcited states in CdSe and CdSe/CdS nanoplatelets Lucas Tijn Kunneman, Juleon M. Schins, Silvia Pedetti, Hadrien Heuclin, Ferdinand C. Grozema, Arjan J. Houtepen, Benoit Dubertret, and Laurens D. A. Siebbeles Nano Lett., Just Accepted Manuscript • DOI: 10.1021/nl503406a • Publication Date (Web): 04 Nov 2014 Downloaded from http://pubs.acs.org on November 5, 2014

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Nature and decay pathways of photoexcited states in CdSe and CdSe/CdS nanoplatelets

Lucas T. Kunneman †, Juleon M. Schins †, Silvia Pedetti ‡, Hadrien Heuclin §, Ferdinand C. Grozema †, Arjan J. Houtepen †, Benoit Dubertret ‡, Laurens D.A. Siebbeles *†

† Optoelectronic Materials Section, Department of Chemical Engineering, Delft University of Technology, Julianalaan 136, 2628 BL Delft, The Netherlands ‡ LPEM, ESPCI-ParisTech, PSL Research University, CNRS, Sorbonnes Université, UPMC Paris 06, 10 rue Vauquelin, 75005 Paris, France § Nexdot, 10 rue Vauquelin, 75005 Paris, France

ACS Paragon Plus Environment

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Abstract

The nature and decay dynamics of photoexcited states in CdSe core-only and CdSe/CdS core/shell nanoplatelets was studied. The photophysical species produced after ultrafast photoexcitation are studied using a combination of time-resolved photoluminescence (PL), transient absorption (TA) and terahertz (THz) conductivity measurements. The PL, TA and THz exhibit very different decay kinetics, which leads to the immediate conclusion that photoexcitation produces different photophysical species. It is inferred from the data that photoexcitation initially leads to formation of bound electron-hole pairs in the form of neutral excitons. The decay dynamics of these excitons can be understood by distinguishing nanoplatelets with and without exciton quenching site, which are present in the sample with close to equal amounts. In absence of a quenching site the excitons undergo PL decay to the ground state. In nanoplatelets with a quenching site, part of the initially produced excitons decays by hole trapping at a defect site. The electron that remains in the nanoplatelet moves in the Coulomb potential provided by the trapped hole.

Keywords: Nanoplatelet, excitons, trapping, photoluminescence, pump-probe spectroscopy, THz spectroscopy


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Two-dimensional materials attract a great deal of attention due to their intriguing physical and chemical properties, which offer promising prospects for optical and electronic applications.1, 2 Semiconductor nanosheets and nanoplatelets prepared by colloidal synthesis are of particular interest, due to versatility in chemical composition, as well as the possibility of cheap processing from solution.3 Atomically flat colloidal cadmium chalcogenide nanoplatelets (NPLs) have been prepared with a thickness of a few monolayers only.4 Since the thickness is less than the bulk exciton Bohr radius, effects of quantum confinement can be exploited to tune the optical absorption and photoluminescence (PL) spectra. Such NPLs are of interest for application in LEDs,5 lasers,6-8 polarized emitters9 and field effect transistors.10 In recent years different cadmium chalcogenide NPL architectures have been synthesized, including core/shell11-13 and core/crown14 structures. Electron and hole states in CdSe nanosheets have been described on basis of an k ⋅ p approach4, 15 and more recently with a tight-binding (TB) model.16 Figure 1 schematically shows electron and hole states near the band gap of CdSe nanosheets. Analogous to bulk CdSe one can distinguish the conduction band (CB), the heavy hole (HH) band and the light hole (LH) band. Effects of quantum confinement cause the calculated effective masses of electrons and holes in thin nanosheets to differ from bulk, see the caption of Figure 1.16 Both quantum confinement and the low dielectric constant of the environment of colloidal CdSe nanosheets lead to calculated exciton binding energies of a few hundred millielectronvolts, which is more than one order of magnitude larger than for bulk CdSe.15, 16 In agreement with this, photoexcitation of CdSe NPLs leads to formation of excitons rather than free charge carriers.17 Electron-hole exchange interactions lead to the occurrence of bright and dark exciton states in NPLs with an energetic


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splitting of a few millielectronvolts.18 Although slower at cryogenic temperatures, the spin-flip transition between bright and dark exciton states at room temperature occurs on a timescale of picoseconds.18 Since this is much faster than exciton decay to the ground state, bright and dark states are in thermodynamic equilibrium for the timescales considered in the present work. The PL decay dynamics of CdSe NPLs and CdSe/CdZnS core/shell NPLs was found to be multi-exponential11, 19 and NPLs were found to exhibit blinking behavior analogous to quantum dots.20-22 This indicates that exciton decay pathways other than recombination to the ground state exist. The studies reported in the present paper aim to provide information about the decay pathways of excitons in CdSe core-only and CdSe/CdS core/shell NPLs. To this end species produced by photoexcitation were detected by time-resolved measurements of PL, changes in optical absorption (TA) and terahertz (THz) conductivity. PL is sensitive to neutral excitons, while THz conductivity measurements can be used to detect free mobile charge carriers. TA measurements are sensitive to both the HH-CB and the LH-CB transition,23, 24 which makes it possible to distinguish excess electrons from holes.


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Figure 1. In-plane band diagram of CdSe NPLs showing the energies E ( k ) of the conduction band (CB), heavy hole (HH) band and light hole (LH) band, as a function of the in-plane wavevector k. The band offsets are not to scale. For the 5 monolayer thick CdSe NPLs studied in the present work, the effective mass of the CB electron obtained from tight-binding calculations is m* = 0.22 m0 , with m0 the free electron mass.16 Interestingly, for these NPLs the calculated inplane effective mass of the HH amounts to m* = 0.41 m0 , which is slightly smaller than the value of m* = 0.50 m0 for the LH.

2. Results and discussion 2.1. Structural and optical characterization of nanoplatelets Studies were carried out on CdSe core-only NPLs dissolved in n-hexane and on CdSe/CdS core/shell NPLs dissolved in toluene. The NPLs were passivated with oleic acid and synthesized according to procedures described previously.4, 13 The CdSe NPLs have a thickness of 1.5 nm (5


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CdSe monolayers) and lateral dimensions of 38 ×10 nm 2 , as inferred from TEM.13 The CdSe/CdS NPLs consist of a 1.5 nm thick CdSe core, which is covered on both sides by a shell 5-6 monolayers of CdS, while the lateral dimensions are 40 ×12 nm 2 . A TEM image of each type on NPL is shown in Figure 2. The samples were kept in airtight Hellma QS cuvettes and stored in nitrogen atmosphere in a glove box when not used. The steady state optical absorption obtained with a Lambda 40 UV-VIS spectrometer from Perkin-Elmer exhibits two distinct peaks, see Figure 2. The low (high) energy peak is due to electronic excitation from the HH (LH) valence band state to the CB state. The energies of the HH-CB and LH-CB transitions are influenced by quantum confinement, since the 1.5 nm thickness of the CdSe core is significantly smaller than the 5.6 nm CdSe bulk exciton Bohr radius. The absorption spectrum of the CdSe/CdS core/shell NPLs is red-shifted as compared with the spectrum of CdSe NPLs. This red-shift reflects a reduced effect of quantum confinement due to spatial extent of the electron into the CdS shell.11 The 12 nm width of the photoluminescence (PL) spectrum of CdSe NPLs in Figure 2, obtained with a PTI Quantamaster after photoexcitation at 400 nm, is typical for the well-defined thickness of 1.5 nm. The 25 nm width of the CdSe/CdS NPLs PL is larger than that of CdSe NPLs, which has been attributed to larger electron-phonon coupling in CdS as compared to CdSe, and fluctuations in the position of mirror charges in the less well-defined surface of the CdSe/CdS NPLs.11 The transient absorption (TA) spectra in Figure 2 were obtained after photoexcitation with a 180 fs pulse at 400 nm Light Conversion Pharos pump laser and detection with a Ultrafast Systems Helios delivering 150 fs pulses at a repetition rate of 5 kHz. The pump fluence was taken sufficiently low, so that higher-order recombination of photogenerated species does not affect the decay kinetics of the relative change in absorption,

, with I on ( I off )


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the transmitted probe fluence with the pump laser on (off). During the first few picoseconds after the pump pulse, the amplitude and shape of the ∆A spectra change, due to cooling of hot electrons and holes to states near the bottom of the CB and the top of the HH valence band, respectively. This effect is absent when exciting at the band edge. In what follows, the focus is on thermalized excitons and charges at band edge states. The transient spectra at pump-probe delay times exceeding 5 ps remain constant in shape, and are shown in Figure 2. The ∆A spectra are negative (enhanced transmission or bleach) near the HH-CB and the LH-CB transitions. Bleach results from the presence of a free hole in the valence band or a free electron in the conduction band, as well as from a bound electron-hole pairs in the form of an exciton. Bright excitons also contribute to bleach by stimulated emission. Hence, TA probes all charges and excitons, in contrast to PL which is only sensitive to bright excitons.


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Figure 2. Steady state optical absorption and photoluminescence spectra, together with transient change, ∆A, in optical absorption after photoexcitation of CdSe NPLs (left panel) and CdSe/CdS core/shell NPLs (right panel). The ∆A spectra were obtained by averaging over probe delay times of 10-50 ps after photoexcitation. The layered composition of the NPLs is schematically shown at the top of the Figure, while the bottom row shows the corresponding TEM images.


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2.2. Decay kinetics of photoluminescence and transient optical absorption Insight into the nature and decay mechanism of photogenerated excitons and charges was obtained from time-resolved PL and transient absorption measurements, see Figure 3. The PL transients in Figure 3 were obtained with a Lifespec (Edinburgh Instruments), by photoexcitation with 90 ps pulses at 405 nm with a repetition rate of 1 MHz and detection at the wavelength for which the PL spectra in Figure 2 are maximum. The PL decay kinetics are bi-exponential with a half-life time of a few hundred picoseconds for the CdSe NPLs and more than one nanosecond for the CdSe/CdS NPLs, in agreement with previous measurements.11, 19 The observation that the PL decay kinetics is bi-exponential can be due to the fact that not all NPLs behave identically on the timescale of PL. Indeed, it has been found previously that single NPLs exhibit blinking of the PL intensity, with a switching time between blinking ON and OFF states of the order of a millisecond.19 Hence, during the 5 nanoseconds time interval of the measurements in Figure 3 a NPL is either in the ON state or in the OFF state. This can be taken into account by distinguishing two types of NPLs, with decay pathways depicted in Figure 4. One fraction of NPLs contains an exciton quenching site, while the other fraction is free from an exciton quenching site. Note, that on longer timescales individual NPLs switch between the ON and the OFF state,11, 19 while the fraction of the ensemble of NPLs in each state is constant. The presence of NPLs with and without an exciton quenching site also explains that the change of the optical absorption, ∆A, measured at the maximum of the bleach of the HH-CB transition, decays much slower than the PL, see Figure 3. Exciton quenching can result in the presence of an excess electron or hole in the NPL, which contributes to the optical bleach and not to PL. An excess charge carrier can result from exciton quenching by charge transfer to a surface defect, or from Auger recombination of an exciton by promoting an already trapped surface charge to the


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CB or HH valence band. On basis of the results of the theoretical analysis described below, decay via Auger recombination is considered unlikely, see Section 2.4. As discussed in the Introduction, the interconversion between bright and dark exciton states occurs on a picosecond timescale. Therefore, thermodynamic equilibrium between bright and dark has been achieved on the longer timescale of the PL and ∆A decay in Figure 3. Consequently, the presence of dark excitons cannot explain that the decay kinetics of ∆A differ from that of PL. This allows for the approximation of describing the bandedge exciton with a single state called X. Note, that thermal exciton dissociation will not give rise to a significant amount of free charges, since the exciton binding energy in CdSe NPLs is of the order of hundreds of millielectronvolt.15, 16 To get further insight into the decay pathways of excitons, the PL and ∆A decay kinetics were analyzed theoretically, as described in Section 2.3.


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Figure 3. A) Time-resolved PL measured at the maximum of the PL spectrum (553 nm, red line), and transient optical absorption, ∆A, at the maximum of the bleach of the HH-CB transition (549-551 nm, green line) of CdSe NPLs, normalized at 5 ps. The dashed black lines are the result of fitting the kinetic scheme depicted in Figure 4, with equation (4) describing the PL and equation (5) the HH-CB bleach kinetics. B) Fitted time-evolution of excitons in NPLs without a quenching site, f X0 (blue), excitons in NPLs with a quenching site, f X1 (red), and quenched excitons fQ1 (green). C) Time-resolved PL (664 nm, red line) and ∆A (657-665 nm, green line) for CdSe/CdS NPLs, similar to Figure 3 A. D) Fitted time-evolution of (quenched) excitons, similar to Figure 3B.

2.3. Analysis of exciton decay pathways The kinetics of exciton decay was described according to the schemes in Figure 4. Photoexcitation of NPLs leads initially to formation of excitons, with a fraction f X0 in a NPL without exciton quenching site, and a fraction f X1 in a NPL with a quenching site. Excitons in NPLs without a quenching site only undergo (non-)radiative decay to the ground state with rate

k X −GS , so that

f X0 ( t ) = f X0 ( 0 ) exp ( −k X−GS t ) . (1)

where f X0 ( 0 ) is the initial fraction of excited NPLs without an exciton quenching site. Excitons in a NPL with a quenching site can also decay non-radiatively with a rate k X−Q to a state with an excess charge carrier at a band edge state of the NPL, yielding


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((

))

f X1 ( t ) = f X1 ( 0 ) exp − k X−GS + k X−Q t .

(2)

The charge carrier decays to the ground state with rate kQ−GS . These processes yield for the fraction of initially excited NPLs with excess charges in a band edge state of the NPL

fQ1 ( t ) = f X1 ( 0 )

((

))

k X−Q  exp −kQ−GS t − exp − k X−GS + k X−Q t  ,  k X−GS + k X−Q − kQ−GS 

(

)

(3)

where it was assumed that photogeneration does not directly yield charge carriers, which implies that f X0 ( 0 ) + f X1 ( 0 ) = 1 , and fQ1 ( 0 ) = 0 .

Figure 4. Left: Excitons in a NPL without a quenching site undergo first-order (non-)radiative decay to the ground state with rate k X −GS . Right: Excitons in a NPL with quenching site decay to the ground state in competition with exciton quenching with rate k X−Q , yielding an excess charge


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carrier at a band edge state. The excess charge carrier decays to the ground state with rate kQ−GS . The color coding of the states is consistent with the species in Figure 3 B and D.

The PL amplitude is proportional to the population of excitons in NPLs without and with quenching sites, yielding bi-exponential decay kinetics according to

SPL ( t ) ∝ f X0 ( t ) + f X1 ( t ) .

(4)

The magnitude of the bleach of the HH-CB transition is determined by the exciton population and the population of quenched excitons. The bleach due to excitons results from reduced absorption and stimulated emission, while the bleach due to a quenched exciton in a NPL stems from reduced absorption only. The magnitude of the bleach is then given by

HH −CB S∆A ( t ) ∝ fX0 ( t ) + f X1 ( t ) +

σQ 1 f (t ) , σX Q

(5)

with σ Q σ X the ratio of the bleach cross section of an excess charge and an exciton. An optimal fit of equations (4) and (5) to the experimental data in Figure 3 was obtained by adjusting the values of the rate constants in Figure 4, the fraction of NPLs without a quenching site f X0 ( t = 0 ) and the ratio σ Q σ X . The optimal values found for these fit parameters are presented in Table 1. The agreement between the experimental data and the fits in Figure 3


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shows that the processes in Figure 4 provide an adequate description of the dynamics of excitons and excess charges.

f X0 ( t = 0 )

σQ σ X

k X−GS (ns-1) k X−Q (ns-1)

kQ−GS (ns-1)

CdSe

0.22

3.11

≤0.02

0.33

0.71

CdSe/CdS

0.23

1.22

0.07

0.55

0.64

Table 1. Optimal values of parameters found from fits of the kinetic processes in Figure 4 to the PL and bleach of the HH-CB transition in Figure 3. Performing a fit with one of the values varied by 10% gives a result that agrees with the experimental data to within the noise and increases the sum of squared differences between fitted and experimental data by 5%. It is found from the fit that f X0 ( t = 0 ) = 0.33 , which implies that one third of the CdSe NPLs does not contain a quenching site. This value is almost twice as large for the CdSe/CdS NPLs, which indicates that the CdS shell reduces the number of quenching sites. The rate of charge transfer from an exciton to a quenching site, kX-Q, is about one order of magnitude larger than the rate of exciton decay to the ground state, kX-GS

( ~ 0.23 ns ) , for both CdSe and CdSe/CdS −1

NPLs. As a result, in the first few hundreds of picoseconds after photoexcitation a large fraction of excitons is quenched, which results in fast components in the decay kinetics of PL and bleach of the HH-CB transition. It can be seen in Figure 3B and Figure 3D that decay of the excited NPL with both an exciton and a quenching site, f X1 , corresponds with an increase of the fraction of excited NPLs with excess charges, fQ1 , on a timescale within a nanosecond. The PL at times exceeding one nanosecond is mainly caused by decay of excitons in NPLs without quenching site, given by f X0 in Figure 3B and D. The fraction of excess charges, fQ1 , does not decay


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significantly during the 5 ns time range considered, due to the small value of the charge recombination rate kQ-GS, which is only 0.07 ns-1 or less.

2.4. Exciton quenching leaves an electron in the conduction band Exciton quenching can involve charge transfer of either the electron or hole to a surface defect, or it can involve Auger recombination of an exciton by promoting an already trapped surface charge to the CB or HH valence band. The TA spectrum and decay kinetics provide evidence that exciton quenching leads to an electron in the conduction band subsequent to exciton quenching. Note that the LH-CB bleach is determined by a superposition of the exciton populations and the number of electrons in the CB, while it is not affected by the presence of holes in the HH valence band. Hence, in case exciton quenching removes the electron from the conduction band, the LH-CB bleach is given solely by the exciton populations:

LH −CB S∆A ( t ) ∝ fX0 ( t ) + fX1 ( t ) .

(6)

In contrast, if exciton quenching leaves an electron in conduction band, the LH-CB bleach is given by:

LH −CB S∆A ( t ) ∝ fX0 ( t ) + fX1 ( t ) + fQ1 ( t ) .

(7)

Figure 5 compares the measured LH-CB bleach with the kinetics calculated from equations (6) and (7), using the populations of excitons and charge-separated states from Figure 3B and Figure 3D. It is clearly seen that equation (7) (blue curve), reproduces the measured LH-CB bleach


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kinetics, while equation (6) (green curve) does not. Hence, exciton quenching leads to disappearance of the hole from the valence band, and presence of an electron in the CB.

Figure 5. A) Time-resolved bleach of the LH-CB transition for CdSe NPLs (519-522 nm, red curve), together with the bleach signal obtained from equation (6) for electron trapping (green curve) and from equation (7) for hole trapping (blue curve). B) Similar data for the LH-CB transition in CdSe/CdS NPLs (608-610 nm). The experimental data are reproduced for the case of hole trapping.

2.5. Exciton quenching by hole trapping and electron localization The presence of an excess electron in the CB subsequent to exciton quenching can be due to hole trapping or Auger recombination with an electron previously trapped on the surface (which may form a trion with the exciton). These two mechanisms can be distinguished, since they lead to different electron mobility. In case of hole trapping, the electron would move in the Coulomb potential of the localized hole. Conversely, Auger recombination involves promotion of an electron from a surface trap to the CB would lead to a free electron. An electron localized near a hole is less mobile than a free electron. Therefore an electron localized near a hole can be


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discerned from a free electron by optical pump-THz probe (OPTP) measurements. In OPTP, an electric field waveform of several picoseconds in duration is used to probe the complex conductivity of photoexcitations in the THz frequency domain.25-27 Figure 6 shows the kinetics of the OPTP signal ∆E / E0 as a function of pump-probe delay time. The fitted time-evolution of the (quenched) excitons discussed in section 2.3 is used to extract the THz mobilities of the exciton and the excess electron, as detailed in the Supporting Information. The measured THz mobility of a free electron is affected by scattering at the boundaries of the NPL, if the latter occurs on a timescale within the THz oscillation period. Scattering at domain boundaries enhances the phase lag between the electron motion and the oscillating THz field, which results in a lower real component and a larger imaginary component of the THz mobility. The THz mobility of free electrons that scatter at the boundaries of a NPL can be estimated using the theoretical model of Prins et al.28 According to their work, the measured AC mobility of a charge moving on a one-dimensional domain with length L is given by 29 ∞

µ AC (ω ) = 8 µINTRA ∑ k=0

ck−2 k Tµ 1 + ck2 B INTRA i eω L2 ,

(7)

ck = 2π ( k + 1 / 2 ) , i = −1 where ω is the radian frequency of the electric field, µ INTRA is the intrinsic mobility of the electron (i.e. the value for bulk), e the elementary charge, T the temperature and kB the Boltzmann constant. Using the electron mobility for bulk CdSe; i.e. µ INTRA = 600 cm2V-1s-1,30 gives a mobility averaged over the two lateral dimensions of the CdSe NPLs (10 nm x 38 nm) equal to µ AC = ( 20 + 72i ) cm 2 V-1s-1 for a frequency ω / 2π =0.5 THz. The imaginary part of the mobility obtained in this way is an order of magnitude larger than the experimental value. The


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large discrepancy leads to the conclusion that quenching of an exciton does not produce a free electron with a mobility comparable to the value for bulk CdSe., The lower electron mobility is likely results from localization of the electron near the hole due to Coulomb attraction. It is also possible that effects of quantum confinement on the electron effective mass, or scattering on (surface) defects cause the electron mobility in a NPL to be lower than in bulk. It cannot be excluded that excitons form a trion with an already trapped charge. The energy released by Auger recombination could then lead to release of the trapped charge.

Figure 6. ∆E / E0 at a probe frequency of ω p / ( 2π ) = 0.5 THz , as a function of pump-probe delay t. Both types of NPLs show a mixed real and imaginary response. The CdSe NPLs have an ingrowth followed by an almost constant signal, while the CdSe/CdS NPLs only show decay subsequent to excitation. The dashed black lines are the result of fitting the contributions from the excitons and quenched excitons, as described in the supporting information.

3. Conclusions A significant fraction of the CdSe and CdSe/CdS core/shell NPLs studied contains a defect at which excitons can become quenched by hole transfer to a defect site. This exciton quenching


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process leads to a decay of the PL on a timescale of hundreds of picoseconds for CdSe NPLs and somewhat more than one nanosecond for the CdSe/CdS NPLs. The electron in the CB is not moving freely throughout the NPL, as the Coulomb attraction to the trapped hole causes the electron to be localized.

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected]

ACKNOWLEDGEMENTS This research was supported by the Dutch Foundation for Fundamental research on Matter (FOM), in the program “Control over Functional Nanoparticle Solids”.

ASSOCIATED CONTENT Supporting Information  Determination of optical absorption cross section and effect of laser pump fluence on decay kinetics of transient absorption. Description and analysis of THz conductivity measurements. This material is available free of charge via the Internet at http://pubs.acs.org.


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