Unravelling the Role of Surface Traps on Carrier Relaxation and

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C: Physical Processes in Nanomaterials and Nanostructures

Unravelling the Role of Surface Traps on Carrier Relaxation and Transfer Dynamics in Ultrasmall Semiconductor Nanocrystals Supriya Ghosh, Dushyant Kushavah, and Suman Kalyan Pal J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b07222 • Publication Date (Web): 27 Aug 2018 Downloaded from http://pubs.acs.org on August 28, 2018

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The Journal of Physical Chemistry

Unravelling the Role of Surface Traps on Carrier Relaxation and Transfer Dynamics in Ultrasmall Semiconductor Nanocrystals Supriya Ghosh, Dushyant Kushavah and Suman Kalyan Pal* School of Basic Sciences and Advanced Material Research Center, Indian Institute of Technology Mandi, Kamand 175005, H. P., India. *

Corresponding Author: Tel.: +91 1905 267040; Fax: +91 1905 267009; E-mail: [email protected]

Abstract Charge carrier trapping by the surface defects of colloidal semiconductor nanocrystal (NC) is a ubiquitous process, which limits the performance of NC-based photovoltaic and photocatalytic devices. Although several empirical approaches led to the enhancement of device efficiency via passivation of trap states, a systematic and unified description of trapping mechanism remains obscure. In this contribution, we present a detailed experimental investigation of the carrier dynamics of CdSe NCs with varying concentration of surface traps by means of time-resolved photoluminescence (PL) and absorption. Our study reveals that the rate of carrier cooling becomes faster as trap density increases because of the increased hot carrier trapping. A comparative dynamical study is also presented to demonstrate how trap states influence electron injection process. This enhanced understanding of the role of trap sates on charge carrier dynamics can provide valuable insight toward the rational development of more efficient NC-based optoelectronic devices. Keywords: Ultrasmall nanocrystals, carrier relaxation, trap states, time-resolved emission, transient absorption 1 ACS Paragon Plus Environment

The Journal of Physical Chemistry 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

1. Introduction Semiconductor nanocrystals (NCs)1-2 have attracted a great deal of attention over the last two decades owing to their diverse applications that span bioimaging3-5 light emitting diodes6 and solar cells.7-12 Their appeal rests on high absorption coefficient,13 bandgap tunability,14 multiple exciton generation (MEG),15 large intrinsic dipole moment,16 and the facile, scalable and cost effective synthesis.17 The long lived charge separation remains one of the essential criteria for photovoltaic device applications. But the major drawback of NCs is fast charge recombination via nonradiative Auger processes, which dominate in smaller size particles due to higher Coulombic interaction between electron and hole.18 Various approaches such as creation of metal-semiconductor composite,19 semiconductorpolymer composite20 and semiconductor core-shell21 heterostructures had been adopted to minimize fast carrier recombination. However, photoinduced processes leading to efficient charge separation in semiconductor NCs remain somewhat elusive. A deeper understanding of such most basic processes in semiconductor NCs might be a valuable asset to researchers aiming for the enhancement of photovoltaic performance towards Shockley-Queisser limit. Surface trapping has long been recognized as the key factor in controlling the electronic and optical properties of NCs.12, 22 Traps states of semiconductor NCs play an important role in determining the fate of electronically excited states. Many of the photophysical properties like hot electron cooling, charge injection and recombination could be influenced by trap states.23 Hence, a detailed knowledge of surface states, the mechanism by which they trap charges and affect different processes in NCs could be beneficial for the improvement of device efficiencies. A number of theoretical calculations and experimental measurements have been carried out for understanding carrier trapping dynamics in semiconductor NCs. Mooney et al.24 studied PL from core and surface (trap) 2 ACS Paragon Plus Environment

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states of NCs and found that the variation of PL quantum yield (QY) with temperature depends upon the fractional change in the population of NCs rather than change of radiative and nonradiative decay rates. Scholes and co-workers25 investigated charge carrier trapping dynamics by temperature dependent PL measurements. According to them both charge trapping and detrapping are thermodynamically activated processes and the activation energy barrier for detrapping process depends upon size of the nanocrystals as well as nature of the trap states. On the other hand, Galland et al.26 observed two different types of PL blinking, which was rationalized by the charge fluctuations in the core and surface states of the NCs. Kambhampati and co-workers

27-30

established a link between

surface chemistry and optical properties of NCs. Their study reveals that one can control the surface PL by varying the temperature and surface ligand. The surface PL is a result of electron transfer process between core to surface state and can be rationalized by semiclassical Marcus-Jortner model. But, Califano et al.31 proposed that Auger mediate trapping mechanism is responsible for trapping in NCs. In a later work, Boehme et al.32 demonstrated that classical Marcus model or semiclassical Marcus-Jortner model can only be used to explain electron transfer for shallow trap, where reorganization energy is almost equal to the energy difference between excitonic and surface state. But, for the deep trap case, where energy difference between excitonic and surface state is much higher than reorganization energy, the trapping mechanism follow Auger assisted model. In addition, they found that both lifetime and QY of excitonic PL of NCs increase with decreasing the density of trap states. Moreover, the influence of trap passivation or reduction of density of trap states on charge recombination dynamics in colloidal NCs has been also investigated experimentally17,

33

However, none of the existing reports is general enough; either they

are purely qualitative34-36 or highly empirical25, 37-38 to account for the behavior of surface states in the presence of an electron acceptor. A systematic and quantitative study is very

3 ACS Paragon Plus Environment


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important in order to have a deeper insight into the contribution of trapping processes to charge carrier dynamics in NCs in the absence and presence of an electron acceptor. It is well known that decreasing particle size increases specific surface area and introduces trap states at the surface of NCs.31,

39

Ultrasmall sized (99%, HPLC grade), absolute ethanol (>99% AR, S D Fine Chem Limited), 3-mercapto propionic acid (MPA), trioctylphosphine oxide (TOPO), dodecylphosphonic acid (DDPA), cadmium oxide (CdO), selenium powder (Se), triocetylphosphine (TOP), hexanol (C6H14O) and methanol (CH4O) from Sigma-Aldrich. All reagents were used without further purification. 2.2 Synthesis of CdSe NCs. The method of CdSe NC preparation was adapted from that reported by Bowers et al.40 In this method, 3 g trioctylphosphine oxide (TOPO), 0.25 g dodecylphosphonic acid (DDPA) and 0.064 g (0.5 mmol) cadmium oxide (CdO) were mixed in a 100 mL three neck round-bottom flask on a stir-plate with a heating mantle. For probing the temperature, a temperature sensor was inserted in one of the side necks and other side of the flask was closed with a rubber septum. Argon gas was flown through the central neck of the flask in order to maintain inert atmosphere. The reaction flask was heated to 150 °C during purging. The purge needle was removed and heating was continued till 300 °C. When the solution became opaque (brown to clear and colourless), 2.5 ml of 0.2 M selenium triocetylphosphine solution (Se:TOP) was injected into the solution. Within few minutes after adding Se:TOP, the solution became yellow colour. The flask was then immediately cooled to prevent further growth. Then prepared NCs were precipitated with methanol in 50 mL centrifuge tubes and collected via centrifugation at 6000 rpm for three minutes. Then it was dried in the centrifuge tubes and redispersed in 6



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mL hexanol. The dispersion was centrifuged at 6000 rpm for 20 min. The supernatant containing the CdSe NCs was gradually pour into a clean tube and precipitated with methanol. Later, the precipitation was collected followed by centrifugation at 6000 rpm for 20 min. Finally, solid NCs were dried, dissolved in toluene, and stored in the dark. 2.3. Synthesis of ZnO NCs. Nanometer-sized ZnO particles were prepared following a method reported elsewhere.44-45 For a typical preparation, 0.995 g of Zn(OAC)2. 2H2O was dissolved in 100 mL of absolute ethanol and heated up for proper dissolution by forming a clear solution. Then 2.8 ml of 25% (w/w) TMAOH in methanol was added with constant injection rate into it under vigorous stirring condition in a water bath at room temperature for the nucleation of uniform small sized particles. Obtained ZnO NC dispersion was stirred for 30 minutes with MPA (0.012 mmol, 1.28 mg) and 2 mL of HPLC grade methanol. The mixture was centrifuged at 6000 rpm for 5 minutes, and then supernatant was discarded. The resulting precipitate was redissolved in HPLC grade methanol and bifunctional MPA-capped ZnO was obtained. 2.4. Spectroscopic and microscopic techniques. The absorption spectra of the samples at room temperature (300 K) were recorded in a quartz cuvette of 1 cm path length using a Shimadzu

UV−vis

2450

spectrophotometer.

A

Cary

Eclipse

fluorescence

spectrophotometer (Agilent Technologies) was used for the measurements of the photoluminescence. The emission was detected at right angle of excitation light to avoid stray light. The time-resolved emission measurements were carried out by using time correlated

single

photon

counting

(TCSPC)

spectrometer,

Fluorolog

Modular

Spectrofluorometer (HORIBA Scientific). The samples were excited with 407 nm light from a nanoLED. A photomultiplier tube (PMT) based detector was used for detection of the emitted photons through a monochromator. The instrument response of the TCSPC set-


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up was measured by collecting the scattered light from a ludox suspension and found to be ~1 ns. Reduced ߯ ଶ , Durbin–Watson (DW) parameter and residuals were used to judge the goodness of the fit. Transmission electron microscopy (TEM) images of the NC samples were recorded by an instrument (Tecnai G2 F20, FEI Co., USA) operated at an accelerating voltage of 200 kV. The energy dispersive spectroscopy (EDS) was performed in the same TEM instrument with an X-ray detector (Bruker, Co., Germany). Interplaner distance from FFT image was measured with Digital Micrograph (TM) software. 2.5. Femtosecond transient absorption (TA) spectroscopy. The femtosecond TA spectrometer used in this work has been previously described elsewhere.46-47 In brief, a Ti:sapphire regenerative amplifier (Spitfire ace, Spectra Physics) seeded by an oscillator (Mai Tai SP, Spectra Physics) was used as light source. The laser output from the amplifier having a central wavelength 800 nm, pulse width < 35 fs and energy 4 mJ per pulse was used to generate pump and probe pulses. A white light continuum (WLC) probe in the visible wavelength range was generated by sending a small fraction of 800 nm focused beam through a sapphire crystal. In order to obtain stable WLC probe, iris and neutral density filters were used for adjusting the intensity of 800 nm light. Spatial overlapping of pump and probe pulses were made at the sample position. To eliminate low frequency laser noises, probe was splitted into two beams and detected as sample and reference separately. The detection of probe pulses was performed under pump blocked and unblocked conditions with a mechanical chopper of rotational frequency 500 Hz. TA spectra were recorded by CCD arrays after dispersion using a grating spectrograph (Acton spectra Pro SP 2358). Chirp correction programme (Pascher Instrument) was used to compensate the group velocity dispersion (GVD) of WLC spectra. TA kinetic traces were recorded by using two well-aligned photodiodes of variable gain controlling the relative delay between 7 ACS Paragon Plus Environment


pump and probe pulses with the help of a stepper motor driven optical delay line. The TA kinetic data were analyzed by using a software package provided by Pascher Instrument. 3. Results and discussion 3.1. Characterization of NCs. Before exploring the carrier dynamics it is very important to monitor the ground state optical properties of NCs. For our study, we choose three different CdSe NCs of varying trap density: CdSe-1 (high trap density), CdSe-2 (intermediate trap density) and CdSe-3 (low trap density). Trap density was varied by changing the surface to volume ratio of the NCs. The surface to volume ratio decreases with increasing the diameter of NCs resulting reduced trap density. Figure 1a shows the steady state absorption spectra of CdSe-1 (orange), CdSe-2 (blue) and CdSe-3 (olive) NCs. The 1S excitonic peak is apparent at 445, 470 and 518 nm for three NCs, respectively. Sharp band-edge absorption peaks suggest narrow size distribution of CdSe NCs. Size of CdSe NCs can be found out from the following equation 13 D (CdSe) = (1.6122×10-9)λ4-(2.6575×10-6)λ3+(1.6242×10-3)λ2-(0.4277)λ+(41.57) where D is the diameter of NCs and λ is the wavelength of the first excitonic peak of the corresponding sample. Estimated diameters of CdSe-1, CdSe-2 and CdSe-3 NCs from 1S excitonic peak positions are 1.97, 2.06 and 2.52 nm, respectively. These values are very close to those obtained from Brus equation.48-49 The size of the NCs falls within the quantum confinement regime, which is apparent from the confinement induced shift in the absorption spectra (figure 1a). TEM image and EDS measurement further confirm the formation of tiny CdSe particles (Figure 2 and Figure S1). Figure 2a shows that particles


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Intensity (Norm.)

Absorbance (Norm.)

1.0

(a)

1.2 0.9 0.6 0.3 0.0 350

1.25

400

450 500 550 Wavelength (nm)

600

0.8 0.6 0.4 0.2

450

250 (c)


750

(d)

200

1.00

Intensity

0.75 0.50

150 100 50

0.25 0.00 350

(b)

0.0

650

Absorbance

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


400


0

600

450


750

Figure 1. (a) Normalized electronic absorption spectra of CdSe-1 (orange), CdSe-2 (blue) and CdSe-3 (olive) NCs. (b) Normalized emission spectra of CdSe-1 (orange), CdSe-2 (blue) and CdSe-3 (olive) NCs. (c) In phase photoinduced absorption spectra of CdSe-2 NCs in the absence (blue) and presence of ZnO NCs (black). (d) Steady state emission spectra of CdSe-2 NCs in the absence (blue) and presence (black) of ZnO NCs. are spherical, well defined and uniform in size. Lattice fringes are clearly seen in high resolution images of some NCs (figure 2b). Inset of Figure 2b shows Fast Fourier Transform (FFT) of one of the atomically resolved NCs. We estimated the interplanar spacing (d) from the FFT pattern and found to be 0.351 and 0.334 nm, which corresponds to (002) and (101) planes of hexagonal CdSe lattice, respectively.



(a)

Figure 2. TEM images of CdSe-3 NCs showing tiny particles (a) and corresponding FFT pattern (b). Inset shows interplaner distances of 0.351 and 0.334 nm, which corresponds to (002) and (101) planes of CdSe wurtzite structure (JCPDS card number 00-002-0330) within experimental error. Scale bars are 10 and 5 nm in image (a) and (b), respectively. Steady state photoluminescence (PL) spectra of CdSe NCs are depicted in figure 1b. PL spectrum of each NC contains two bands; one corresponds to near-monochromatic band edge (excitonic) emission and another one is associated with broad red shifted trap emission.6 Different quantum yield of trap emission is a consequence of varying trap densities in these NCs. As diameter of the NCs reduces, trap emission becomes more prevalent and eventually dominates the overall NC emission. Next, we study the interaction of CdSe NCs with ZnO NCs of diameter 3.2 nm.44 The steady state absorption and PL spectra of ZnO NCs are shown in figure S2. While first


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excitonic absorption peak is observed around 328 nm, PL maximum appears at 370 nm. Small Stokes shift of the PL band infers that unlike CdSe NCs, PL in ZnO NCs originates from band edge. This observation also indicates that density of trap state is negligible in ZnO NCs.

Figure 3. Energy level alignment of CdSe-2 and ZnO NCs showing the possibility of electron transfer. In order to investigate the interaction between CdSe and ZnO NCs, we measured absorption and PL spectra of CdSe NCs in the absence and presence of ZnO NCs (concentration ratio ~1:1). Absorption spectra shows a small change, while PL (both band edge and trap) is almost quenched indicating the possibility of energy and/or charge transfer from CdSe NCs to ZnO NCs (Figure1c and 1d). Similar PL quenching is observed for other CdSe NCs (Figure S3). We looked into the energy band positions for both CdSe and ZnO NCs (Figure 3) to find the reason of PL quenching. According to previous report,50-51 the typical value of the valence band maximum (VBM) of CdSe NCs is -6.09



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eV, which is independent of the particle size. The energy of the conduction band minimum (CBM) of CdSe NCs was obtained by adding the band gap value (calculated from absorption spectrum) to VBM energy. Energy values of VBM and CBM of ZnO NCs were taken from the previous reports for the particles of same size.49, 52 It is clear from Figure 3 that the CBM of CdSe NCs is at energetically higher position than the CBM of ZnO NCs. Therefore, photoinduced electron transfer from CB of CdSe to ZnO NC is thermodynamically feasible and could lead to the PL (both excitonic and trap) quenching of CdSe NCs. However, injection of trapped electron to ZnO NC could also lead to the quenching of CdSe trap PL. It is worth noting that the persistent red shifted PL even in the presence of ZnO NCs indicates that surface traps are located rather deep in the excitonic band gap. This could be the reason unlike previously reported NCs,25-26, 53 we observe a stable PL from the trap states of ultrasmall CdSe NCs. 3.2. Time-resolved PL measurements. Figure 4a shows decay kinetics of excitonic PL from CdSe-1 (orange), CdSe-2 (blue) and CdSe-3 (olive) NCs following 389 nm excitation. It is clear from the figure that PL decay gets faster with the decrease in particle size. We fitted the decay traces multiexponentially and have presented the results in Table 1. The PL decay kinetics obtained for this type of NCs have been previously reported.54 The decay transients are nonmonoexponential in nature, and a biexponential or a triexponential

function

typically

required

to

describe

the

decay

well.

Such

multiexponential decay is a consequence of significant trap mediated, nonradiative recombination. The mean excitonic lifetime (τaverage) of CdSe-1 (3.84 ns) is much less compared to CdSe-2 (6.93 ns) and CdSe-3 (14.9 ns) NCs. The reason for faster PL decay in small sized particles could be carrier trapping by large number of defect states present in small NCs.


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10000

10000 8000

(b)

(c)

6000 Counts

100

IRF

50

1000

Counts

100

10000

(a)

1000

Counts

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


100 Time (ns)

150

200

4000

2000

50

100 150 Time (ns)

200

15

30

45 60 Time (ns)

75

Figure 4. (a) PL decay profile of CdSe-1 (orange), CdSe-2 (blue) and CdSe-3 (olive). Excitation wavelength is 389 nm and emission wavelengths are 474, 485 and 528 nm for CdSe-1, CdSe-2 and CdSe-3 NCs, respectively. (b) PL decay kinetics (at 485 nm) of CdSe2 NCs in absence (blue) and presence (black) of ZnO NCs. (c) PL decay kinetics (at 600 nm) of CdSe-2 NCs in the absence (blue) and presence (black) of ZnO NCs. Solid red lines are fitted curves using triexponential function. Table 1. Fitting Parameters of (Excitonic) PL Kinetics of CdSe NCs without and with ZnO NCs.

Sample

τ1 (ns)

τ2 (ns)

τ3 (ns)

τaverage (ns)

CdSe-1

0.92

6.42

48.8

3.84

(70%)

(27%)

(3%)

CdSe-

0.30

2.24

7.89

1/ZnO

(80%)

(16%)

(4%)

CdSe-2

0.66

8.35

49.5

(73%)

(18%)

(10%)

CdSe-

0.30

2.24

7.89

2/ZnO

(80%)

(16%)

(4%)


0.91

ket (s-1)a

8.4 × 108

6.93

0.91

9.5 × 108

90


CdSe-3

1.47

16.3

74.8

(57%)

(31%)

(12%)

CdSe-

0.30

2.85

3/ZnO

(98%)

(2%)

a k et

-

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14.9 0.35

2.8 × 109

= [τ average (CdSe / ZnO)]−1 − [τ average (CdSe)]−1

We measured PL (excitonic) dynamics of CdSe NCs in the presence of ZnO NCs to know about their effect. PL kinetics of CdSe NCs becomes faster in the presence of ZnO NCs (Figure 4b and S4) leading to decreased PL lifetime (Table 1). The mean quenched lifetime (τaverage ) of CdSe NCs in presence of ZnO NCs is 0.91, 0.91 and 0.35 ns for CdSe1, CdSe-2 and CdSe-3, respectively. The quenching of average lifetime of excitonic PL could be attributed to the electron transfer to ZnO NCs. As the exciton binding energy in CdSe NCs is relatively high,55-56 it is expected that electron has to overcome a Coulombic barrier while injecting into ZnO NCs. Thus, it is reasonable to consider that cold electron injection is a relatively slow process and does not interfere the carrier relaxation in CdSe NCs.56 The rate (ket) of cold electron injection can be obtained by comparing the average PL (excitonic) lifetimes of CdSe NCs in the absence and presence of ZnO NCs. The estimated values of ket are given in Table 1. Cold electron injection rate is found to be increased by about four times with increasing the size (decreasing the trap density) of the CdSe NC. These observations suggest that the density of trap state present in the donor NC highly influence the rate of cold electron transfer. Figure 4c shows kinetics of trap emission (at 600 nm) of CdSe-2 NCs without and with ZnO NCs. Average lifetime of trap PL of CdSe NCs is much higher than that of lifetime of band edge PL. Long lifetime of trap PL of CdSe NCs is attributed to the slow recombination of trapped carriers. Moreover, the average lifetime of trap PL of CdSe NCs 14 ACS Paragon Plus Environment

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decreases after adding ZnO NCs. As we have already mentioned in the previous section, the quenching of the lifetime of trap PL could be due to injection of CB and/or trapped electrons of CdSe NC to ZnO NC. 3.3. Pump-probe measurements. Owing to comparatively low time resolution of the TCSPC set up, several fast processes like hot electron cooling, hot electron trapping etc. cannot be captured in PL measurements. Femtosecond transient absorption (TA) measurements were carried out for ultrasmall to small sized CdSe NCs to demonstrate the effect of surface trap states on charge carrier dynamics. Figure 5a depicts the TA spectra of CdSe-2 NCs. TA spectra for CdSe-1 and CdSe-3 are shown in figure S5. The acquired TA spectra of CdSe NCs (Figure 5 and S5) reflect the shape of steady state absorption spectra. The dominant feature is the bleach corresponding to the lowest absorption band due to the electron population in the 1S state after excitation. For the CdSe-2 NCs, we observe a ground state bleach band at 470 nm. Similar bleach band is observed at 450 and 515 nm for CdSe-1 and



Figure 5. (a) TA spectra of CdSe-2 NCs without (blue) and with ZnO NCs (black). (b) Normalized TA kinetics of CdSe-1 (orange), CdSe-2 (blue) and CdSe-3 (olive) NCs. Solid red lines are the fitted curves using multiexponential function. Excitation wavelength is 410 nm for both CdSe-1 and CdSe-2 NCs, while 480 nm for CdSe-3 NCs. All kinetics were probed at their respective band edge transitions. Excitation fluence per pulse was 8.6

× 1013 photons cm-2. (c) Illustration of carrier dynamics in CdSe NCs in the presence of ZnO NCs. Purple, orange, black and dark red arrows denote the pump transition, probe transition, depopulation of electron from higher excited state (HES) of the conduction band to trap states, and the depopulation of electron from CBM to trap states and CBM of ZnO NCs, respectively. (d) TA kinetics of CdSe-2 (blue) and CdSe-2/ZnO (black) NC assembly monitored at 470 nm.


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CdSe-3 NCs, respectively (Figure S5). The persistent bleach signal allows us to monitor the dynamical events by focusing on the TA kinetics at maximum bleach positions. It is worth noting that a minor shift is observed in the bleach band position when CdSe NCs are attached to ZnO NCs (Figure 5a). This can be ascribed to both the change of the NC environment when attached to ZnO and a possible coupling effect between two NCs.57-58 Further, we measured TA kinetics at 450, 470 and 515 nm (1S exciton position) for CdSe-1, CdSe-2 and CdSe-3 NCs, respectively (Figure 5b). All kinetic traces were fitted multiexponentially and fitting parameters are given in Table 2. A faster growth of bleach signal is observed for the smallest size particle (CdSe-1) and the signal growth is getting slower with increasing the particle size (figure S5). The rise time of the bleach signal is 150 fs for CdSe-1, while for CdSe-3 it is 300 fs (Table 2). These observations indicate that depletion of the population of hot electron in ultrasmall NCs is much higher in compare to small size NCs. In ultrasmall NCs, every photon may not result into an electron in CBM as some of the hot electrons created immediately after absorption can be cooled directly into trap states. Kambhampati39 thoroughly discussed hot carrier relaxation in semiconductor NCs including hot carrier trapping. In fact the surface trapping of hot carriers is regarded as one of the reasons for intermittent PL (blinking) from NC.26, 53 Such additional cooling pathway of hot carriers leads to the fast rise of the ground state bleach. With increasing particle size, density of trap states and correspondingly amplitude of hot trapping decreases resulting slow rise of the bleach signal. For efficient harvesting of hot electrons, the rate of electron transfer from the donor to the acceptor material must compete with intraband cooling in the donor. Fast cooling of hot electrons in small sized CdSe NCs infers that the possibility of hot electron harvesting from ultrasmall CdSe NCs in a photovoltaic environment is less.



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In addition to contrasting rise response of bleach signals (discussed above), the decay behaviour is also significantly different for different sized NCs. Decay kinetics of all three samples feature two decay components: a short (~3-4 ps) and a long (>200 ps). According to previous literature,31,

59

the short component is attributed to cold electron

trapping, while the long component is associated with the recombination of 1S excitons.17 As anticipated, CdSe-1 features very fast cold electron trapping (Table 2), which is getting slower with increasing the diameter of the NCs. These observations are consistent with substantial reduction of trap density with increasing the diameter of NCs. It is worth noting that we captured only a small fraction of the excitonic decay dynamics (till 600 ps) in TA measurements. Therefore, determination of excitonic lifetimes of CdSe NCs from TA kinetics would not be realistic. Table 2. Fitting Results of Bleach Kinetics of Different Sized CdSe NCs Mutiexponential Sample

CdSe-1

CdSe-2

CdSe-3

Rise

Kinetic Model

Decay

τr (ps)

τ1 (ps)

τ2 (ps)

0.15

2.41

>200

(100 %)

(76 %)

(24 %)

0.20

3.85

>200

(100 %)

(70 %)

(30 %)

0.30

4.5

>200

(100 %)

(54 %)

(46 %)

k10 kh -1 (ps ) (ps-1)

k10+kh (ps-1)

0.45

6.50

6.95

0.14

0.45

5.01

5.46

0.18

0.45

3.04

3.49

0.28

a

a

τr (ps)

τr=1/( k10+kh)

It is clear from the above discussion that carrier relaxation in NCs becomes complicated in the presence of trap states. At early times of the carrier dynamics, three 18 ACS Paragon Plus Environment

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processes such as cooling, hot and cold carrier trapping are involved (Figure 5c). These processes are usually investigated by comparing the TA kinetics measured at two pump wavelengths,39,

59

but here we measured S1 bleach kinetics for a single excitation

wavelength. To establish a quantitative relationship between rates of these processes, we used a simple rate equation model. According to this model, pump beam excite electrons from VBM to a higher excited state of the conduction band with population n1. From this higher excited state, hot electrons can cool at a rate characterized by k10 to CBM having population n0, which is detected by the probe beam. Alternatively, the electrons in higher excited state of the conduction band can undergo ‘hot trapping’ directly to trap states with a rate kh. Electron can also undergo ‘cold trapping’ from CBM to trap state (with rate kc). On the other hand, as the traps are deep inside the band gap, the chance of the release of carriers is very slim and hence the detrapping of carriers is neglected. Now, the evolution of the population of CBM can be expressed by the following coupled rate equations,

dn1 (t ) = −(k h + k10 )n1 (t ) dt

[1]

dn0 (t ) = k10 n1 (t ) − kc n0 (t ) dt

[2]

Equation (1) and (2) can be solved to yield

n0 (t ) =

k10 n1 (0 ) e −kct − e −(k10 +kh )t k10 + k h − k c

[

]

[3]

From equation 3, it is clear that the hot trapping rate constant kh appears both in the second exponential term, which determines the bleach rise time and in the denominator of the prefactor, which measures the bleach amplitude. Thus, the increased value of kh simultaneously decreases the bleach rise time and amplitude. The values of kc (τ1-1) have 19 ACS Paragon Plus Environment


already been estimated by fitting the TA kinetics using exponential functions (Table 2). These values of kc were used to fit bleach kinetics of different sized NCs using equation 3 to find the values of kh and k10. Fitting results are presented in Figure S7 and Table 2. For the three NCs studied, the values of k10 are very close with an average of 0.45 ps-1. On the other hand, fitting yields different values for kh, i.e., 6.50, 5.01 and 3.04 ps-1 for CdSe-1, CdSe-2 and CdSe-3, respectively. It should be noted that the rise time (τr) obtained from the kinetic model matches well with that estimated from multiexponential fitting of TA kinetics (Table 2). Similar picosecond-scale hot carrier trapping has been observed previously for a number of semiconductor NCs.17, 59 Moreover, it is clear from the fitting results that hot electrons exhibit subpicosecond dynamics, whereas a fraction of the cold electrons undergo fast trapping within few ps of laser excitation. It is worth noting that often the subnanosecond decays in colloidal NCs samples is attributed to multiple exciton generation (MEG), and the magnitude of the decay feature is used to calculate MEG yield.60 Our results infer that carrier trapping can be misinterpreted as MEG provided additional measurements are done. To separate out MEG from carrier trapping one has to measure bleach decays at sufficiently low excitation intensities below and above the MEG threshold. There should not be any subnanosecond bleach decay at below the MEG threshold. In addition, the bleach decay observed at the same low excitation intensities but above the MEG threshold should be monoexponential and characterized by the same decay rate found when there is more than one photon is generated per NC per pump pulse. To learn more about the electron injection process, we measured bleach kinetics of CdSe NCs attached to ZnO NCs. Figure 5d shows TA kinetics of CdSe-2 in the presence and absence of ZnO NCs. Decay kinetics of CdSe-1/ZnO and CdSe-3/ZnO NCs are shown in figure S8a and S8b, respectively. As we discussed before, rise of the bleach signal is associated with the cooling process of hot electrons. From Figure S6 and Table-2, 20 ACS Paragon Plus Environment

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we observe that electron cooling time is getting slower (150 to 300 fs) with increasing the size of the NCs. As hot electron transfer (HET) and cooling are highly competitive processes, the former is expected to dominate over intraband cooling process for bigger sized CdSe NCs. The early time decay dynamics of the bleach signals (Figures 5d, S8a and S8b) are not affected by ZnO NC indicating that the fast surface trapping related decay components (~3-4 ps) remain unchanged in CdSe-ZnO NCs system. This observation infers that attachment of ZnO NCs does not diminish the surface traps of CdSe NCs. However, the long decay components (> 200 ps) become faster after attachment with ZnO NCs. This means that an extra electron depopulation pathway is introduced by ZnO NCs at longer time. The origin of such extra decay path is most presumably the cold electron injection from CdSe to ZnO NCs. A fraction of carriers that remains for longer time in CBM does not undergo subnanosecond trapping process, rather inject to ZnO NCs. We have already mentioned that determination of the exciton lifetime of CdSe NCs from our TA measurements would not be accurate because we monitored only partial dynamics of the cold exciton. For the same reason, cold electron injection rates were not estimated from TA measurements, rather determined from time-resolved PL (Table 1). We observe that electron injection rate (ket) decreases as one move from CdSe-3 to CdSe-1 NCs (Table-1). This observation can be rationalized by the reduction of trap density with increasing the diameter of NCs. In case of bigger particle a large fraction of carriers remains in the CBM for longer time because of less trapping, which in turn enhances the possibility of electron transfer to ZnO NCs leading to higher cold injection rate. Our results suggest that efficient cold electron injection from ultrasmall NCs is not possible, but small NCs, carrying almost no traps could be good candidates for cold electron harvesting. 4. Conclusions



In summary, we have systematically investigated the influence of surface trapping on ultrafast charge carrier dynamics in ultrasmall to small sized CdSe NCs in the absence as well as presence of ZnO NCs. By measuring subnanosecond carrier dynamics in NC samples of varying trap densities, we have been able to quantify carrier cooling and trapping rates. Our results reveal that both hot and cold electron trapping rates increase, whereas the rate of hot carrier cooling does not change with the increase in trap density (decrease in particle size). Steady state and time resolved emission studies confirm photoinduced (cold) electron transfer from CBM of CdSe to ZnO NCs. However, the possibility of the injection of trapped electrons from ultrasmall CdSe NCs cannot be ruled out. Cold electron transfer to ZnO NCs is found to be more efficient for CdSe-3 NCs having lowest trap density. A deeper understanding of how trap states impact the carrier relaxation in NCs has important implications for the interpretation of MEG and can provide valuable insight into how to rationally tune surface defects through materials engineering to optimize the performance of NC-based optoelectronic devices. Electronic supplementary information (ESI) TEM image and EDS of CdSe NCs, absorption and emission spectra of ZnO NCs, emission spectra, time-resolved emission kinetics and transient absorption spectra of CdSe and CdSe/ZnO NCs, comparison of the rise kinetics of bleach signals, and fitted transients of CdSe NCs of varying sizes. Acknowledgements The financial support from the DST (Grant No. DST/INT/SWD/VR/P-06/2014) under Indo-Swedish (DST-VR) joint scheme is highly acknowledged. The authors are highly thankful to IIT Mandi and AMRC (Advanced Material Research Centre) for research facilities. 22 ACS Paragon Plus Environment

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Unravelling the Role of Surface Traps on Carrier Relaxation and

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