Lattice-Strain-Induced Slow Electron Cooling Due to Quasi-Type-II

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Lattice Strain Induced Slow Electron Cooling due to Quasi Type-II Behavior in Type-I CdTe/ZnS Nanocrystals Sourav Maiti, Tushar Debnath, Partha Maity, and Hirendra N. Ghosh J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.5b02420 • Publication Date (Web): 16 Mar 2015 Downloaded from http://pubs.acs.org on March 18, 2015

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The Journal of Physical Chemistry

Lattice Strain Induced Slow Electron Cooling due to Quasi Type-II Behavior in Type-I CdTe/ZnS Nanocrystals Sourav Maiti, Tushar Debnath, Partha Maity and Hirendra N. Ghosh* Radiation and Photochemistry Division, Bhabha Atomic Research Centre, Mumbai 400085, India * E-mail: [email protected]. Tel: +91-22-25593873, Fax: (+) 91-22-25505331/25505151.

Abstract: Detailed analysis on charge separation energetics and dynamics for CdTe/ZnS

nanocrystals have been carried out with varying shell thickness to elucidate quasi type-II behavior in a standard type-I system. Red shift in the absorption-photoluminescence spectra and increase of the excited state lifetime in the core/shell nanocrystals with thick ZnS shell (2ML and 4ML of ZnS) indicate quasi type-II behavior due to charge separation. Separation of charge arises as the lattice strain at the core/shell interface alters the conduction band energy levels for both core and the shell in an opposite way extending the electronic wave-function towards the shell. To find out the energetics of the charge separation the steady state spectra were analyzed in the realm of Marcus theory to reveal charge separation occurring in the inverted region with -ΔG°ET> λ. Slow electron cooling as observed from ultrafast transient absorption measurements with increasing shell thickness also confirms electron being decoupled from the hole as the electronic wave-function spreads out to the shell. Consistent with the Marcus theory analysis the separation of charge is clearly exhibited in the nanocrystal with highest ZnS shell thickness as the excitonic bleach shows slower electron cooling rate and increased amplitude of slow recovery component in the red region of transient absorption spectrum.

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Keywords: CdTe/ZnS nanocrystals, Quasi type II core-shell, lattice strain, ultrafast transient absorption, Auger cooling.

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1. Introduction: Semiconductor nanocrystals have emerged as a versatile material for solar energy conversion, light emitting devices and bio-imaging due to their shape, size and surface dependent opto-electronic properties originating from quantum confinement. 1-8 This charge carrier confinement can be tuned by suitable surface bound surfactants, impurity doping and through introducing a shell material either in a type-I structure where both the electrons and holes are confined only in the core or through type-II structure where one of the charge carriers leaks to the shell leading to charge separation.9-21 The charge separated state in type-II nanostructures have been a subject of intense investigation due to its long-lived nature.13,15,20,22-25 Scholes et. al. have analyzed the steady state absorption and emission spectra to demonstrate the energetics of charge transfer state in CdSe/CdTe type-II nano-rods with the help of Marcus theory concluding charge separation occurs in the Marcus inverted region.26 The dynamics of the charge transfer state has also been analyzed in detail for different type-II nanocrystals utilizing femtosecond transient absorption spectroscopy. 14-19 Burda et.al. have studied both energetics and dynamics of charge transfer in CdTe/CdSe core/shell spherical nanocrystals demonstrating charge separation in the inverted region with ~400 fs build up time for the CT state. 16,19 Further, Zamkov et. al. have inferred the dynamics of charge transfer state in ZnSe/CdS/ZnSe nanobarbells along with the Marcus inverted behaviour of charge transfer from ZnSe to CdS. 17 Although, formation of type-I or type-II nanocrystals depends entirely on the band alignment of the core and shell materials, Nie et. al. have shown type-I to type-II conversion is feasible utilizing lattice strain at the core/shell interface when a compressive shell is put over soft CdTe core.27 The lattice strain at the core/shell interface tends to alter the conduction bandalignment of both core and the shell material leading to charge carrier leaking. However, the 3



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energetics and dynamics of this strain induced charge separation is still obscure. To shed light on what was unexplained we have provided an in depth analysis of strain induced charge separation energetics and dynamics in type-I CdTe/ZnS nanocrystals employing both steady state and time resolved measurements. To demonstrate quasi type-II behavior through strain induced charge carrier leaking we have synthesized two different CdTe/ZnS nanocrystals with varying shell thickness from CdTe core through successive ionic layer adsorption and reaction (SILAR) method using literature reports with some modifications.28,29 The CdTe core is termed (a) and core/shell nanocrystals are termed as (b) and (c) for 2 monolayer (ML) and 4 ML of ZnS thickness, respectively with 1ML being ~0.32 nm. According to bulk band-alignment CdTe/ZnS nanocrystals form type-I structure where both electron and hole remain confined in the CdTe core. However, the ~19% lattice mismatch at the core/shell hetero-interface (lattice parameters for CdTe and ZnS are 6.48Å and 5.41Å, respectively) creates lattice strain on both CdTe core and ZnS shell in an opposite way which alters the band arrangement in the nanocrystals. 2. Experimental Section: a. Materials: Cadmium oxide (CdO, 99.5%), tellurium tellur (Te, 99.99%), zinc acetate (ZnAc2, 99.9%), sulfur powder (S, 99.99 %), oleic acid (90%), tri-octyl phosphine (TOP, 90%) and octadecene (ODE) (90%) was purchased from Sigma-Aldrich and used without further purification. AR grade chloroform and AR methanol were used for cleaning the nanocrystals. Spectroscopic grade chloroform was used for transient absorption measurements.

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b. Synthesis and Characterization of CdTe core and CdTe/ZnS core/shell nanocrystals: CdTe Nanocrystals of diameter ~4.6 nm were synthesized according to synthetic methodology reported by Peng et.al.28 In brief, a mixture of 2 mmol of CdO, 8 mmol of oleic acid and 8 ml of 1-octadecene (ODE) was heated to 260°C under nitrogen to form colorless cadmium oleate. 1 mmol of Te dissolved in TOP and ODE was then injected swiftly to the reaction mixture. The nanocrystals were grown at 240°C. When desired size was reached, the reaction mixture was cooled down and quenched with chloroform. The nanocrystals were precipitated with methanol from chloroform solution for three times. Finally, the CdTe nanocrystals were re-dispersed in chloroform for further experimentation. ZnS shell was synthesized on top of CdTe core using successive ionic layer adsorption and reaction (SILAR) method following previous literature with few modifications. At first, Znoleate solution of desired amount was prepared by dissolving Zinc acetate tetrahydrate in oleic acid and ODE at 180°C. Similarly, sulfur was dissolved in TOP and ODE to prepare S-TOP. The CdTe nanocrystals dispersed in ODE were heated to the 220°C for shell growth. Required amount of the Zn-oleate solution and S-TOP solution for 1 ML shell growth were alternatively injected to the nanocrystal solution. After each injection the solution was allowed to grow for 10 minutes. When desired shell thickness is reached, the reaction mixture was cooled down and the nanocrystals were cleaned using the same procedure for CdTe core. c. Transient absorption measurements: Details of the experimental setup for transient absorption measurements can be found elsewhere.30 5



3. Results and Discussion: a. Spectral evolution of CdTe core and CdTe/ZnS core/shell nanocrystals: Figure 1A compares the UV-Vis absorption and emission spectra of all the nanocrystals. The CdTe core has the 1st exitonic absorption peak around 665 nm with emission centered around 685 nm. The size of the CdTe was found to be ~4.6 nm which correlates well with the sizing curve provided by Peng et.al.31 Formation of ZnS shell on top of CdTe core gradually red shifts both the absorption and emission spectra accompanied with increase in the excited state

A c b a

Intensity (a.u.) Counts

photoluminescence lifetime as shown in Figure 1B. The spherical morphology, monodispersed

Absorbance

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B c'

3

10

b'

a' 2

10

L

1

500

600

700

800

10

0

20

Wavelength (nm)

40

60

80

Time (ns)

Figure 1: (A) Steady state absorption and emission spectra of (a) CdTe core, (b) CdTe/ZnS-2ML and (c) CdTe/ZnS-4ML. (B) Emission decay trace of (a') CdTe core, (b') CdTe/ZnS-2ML and (c') CdTe/ZnS-4ML after 540 nm excitation. L denotes the lamp profile. size and high crystallinity with uniform shell growth for core/shell nanocrystals were revealed by HRTEM (Figure S1, SI). The high QY combined with HRTEM analysis, increase in QY and PL lifetime with shell thickness suggest that shell growth is homogeneous and uniform around CdTe core. Further analysis of the TEM images and support for the structure claimed are provided in the supporting information. Each ML deposits ~0.32 nm of shell so that the size of the nanocrystals after 4-ML of shell growth is 7.2 (±0.5) nm as measured from TEM (Figure S1, 6


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SI).Occurrence of single emission band for all core/shell nanocrystals excludes the possibility of interfacial alloying during high temperature synthesis (Figure S2, SI). The emission spectra is symmetrical in shape with FWHM~ 45 nm for thickest shell ruling out a very broad size distribution or inhomogeneous nucleation and growth for core/shell nanocrystals. Almost fourfold increase in the quantum yield after shell coating indicates better surface passivation with ZnS shell removing the surface traps on the CdTe core. Table 1 lists all the relevant parameters for CdTe core (a) and CdTe/ZnS core/shell nanocrystals (b and c, respectively). Broadening of absorption spectra with structureless non-distinct band-edge features and increase in the excited state lifetime are indicative of charge carrier separation leading to a quasi type-II behavior. As the electronic wavefunction extends towards the shell while the hole resides in the core, the electron-hole wavefunction overlap decreases which causes the aforementioned effects. In the following section this will be discussed in detail. The strain effect at the core/shell interface can be modeled using continuum elasticity theory for CdTe/ZnS-4ML nanocrystal.27, 32 The strain profile using this model (Figure S3, SI) shows the CdTe core is under isotropic compressive strain in all directions. However, for the shell the strain is tensile in the tangential direction and compressive in radial direction. Therefore, for a softer core like CdTe which is easily compressible, now on formation of ZnS shell compresses the core, thereby increasing the conduction band energy. 27 Moreover, as the shell experiences tangential expansion, its conduction band-edge moves lower in energy. Due to these two concerted and opposite effects the core and shell conduction band-edges move in opposite direction converting a type-I alignment to a type-II (or quasi type-II) one.27 Therefore, the electronic wave function extends out to the shell leading to charge separation in a type-I structure. This strain induced mechanism is different from that in the ZnSe/CdSe core/shell 7



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nanocrystals where type-I to type-II conversion is achieved through changing the electron-hole confinement energies by varying the shell thickness. 33 Theoretical modeling along with cyclic voltammetric measurements have been used to determine bandedge positions for CdTe, CdTe/CdSe and CdSe/CdTe nanocrystals.34, 35 According to Nanavati et. al. our CdTe core has conduction and valence bandedge at -3.54 and -5.40eV (vs. vacuum), respectively.34 Theoretical modeling for strained CdTe/CdS NCs with~10% lattice mismatch have revealed that conduction band offset is much more affected than valence band offset under strain.36 Although analysis of bandedge positions either through theoretical modeling or electrochemistry analysis is beyond the scope of this work, we can safely mention that in case of CdTe/ZnS also the conduction band offset changes more than the valence band offset. Table 1: Excitonic absorption, luminescence, quantum yield and excited state lifetimes for CdTe core (a) and CdTe/ZnS core/shell nanocrystals with 2 and 4 ML of shell thickness (b and c, respectively). Material

λabsExciton

emExciton

QY (%)

τPL

CdTe (a) CdTe/ZnS-2ML (b) CdTe/ZnS-4ML (c)

665 nm ~695 nm (broad) ~700 nm (broad)

685 nm 710 nm 720 nm

8 19 31

22.8 ns 30.2 ns 32.6 ns

b. Marcus theory analysis: To concretize our idea of charge separation in a type-I system we analyzed the steady state spectra in terms of Marcus theory to find out the energetics of charge separation. In Marcus theory 37, depending upon the free energy difference between the reactants and products (∆G°ET) and the reorganization energy (λ) three regions for electron transfer are identified: normal region (-∆G°ET < λ) where rate increases with increase in ∆G°ET, barrierless region (-∆G°ET = λ) in which rate is maximum, and inverted region (-∆G°ET> λ) where rate decreases with increase in 8


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∆G°ET. Marcus analysis have been carried out to demonstrate the existence of a charge separated state arising as the electron leaks to the shell due to staggered band alignment for different typeII nanocrystals such as CdSe/CdTe nanorods, CdTe/CdSe nanocrystals, ZnSe/CdS/ZnSe nanobarbells.17,19,26 Here, we have put a step forward in utilizing Marcus theory to show the existence of a charge separated (CS) state in a type -I system. For the highest ZnS shell thickness

2.5 Absorption

Emission

CS CdTe

A

B

2.0 0

ACS

G (eV)

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


ECS

ACdTe

1.5

G

ET

E

0

G

1.0

0

G

E

CS

max CS

-1

750

Wavelength(nm)

GS

CS

0.0

700

CdTe

CdTe

0.5

ECdTe

650

max

0

-0.5 0.0 0.5 1.0

1

Bath Polarization

Figure 2: (A) Deconvolution of absorption spectra (solid lines) and emission spectra (dotted lines) for CdTe/ZnS-4ML nanocrystal. The black line is the original data whereas the red line represents the fitted data. (B) Marcus charge transfer analysis showing potential energy diagrams for ground state, photo-excited CdTe dominated state and charge separated state (CS). These are non-adiabatic free energy curves. (inset) Adiabatic free energy curves with showing mixing of CdTe and CS states. (CdTe/ZnS-4ML) the absorption and emission spectra, converted to corresponding lineshape plots were deconvoluted (Figure 2A). Fitting of the absorption spectra clearly depicts two distinct absorption peaks, one at 665 nm and the other at 705 nm. The first one can be assigned to absorption dominated by CdTe itself (ACdTe: CdTe state) as this is very close to CdTe core absorption. The other peak at 705 nm is due to a charge separated (ACS: CS state) state which is 9



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much red shifted as compared to CdTe core absorption. The emission spectra also gives rise to two emission bands after deconvolution, the short wavelength peak at 717 nm (E CdTe) comes mainly from the CdTe state whereas longer 728 nm peak arises due to emission from the CS state (ECS). The reorganization energy (λ) for each state were calculated which were further used to plot potential energy diagram for each state as plotted in Figure 2B. Details of the analysis and deconvolution can be found in the supporting information (Figure S4, SI). Reorganization energy for CS state (λCS) and free energy change for electron transfer (-∆G°ET) were calculated to be 30 and 70 meV, respectively. As -∆G°ET >λCS, the charge separation occurs in the Marcus inverted region which is depicted in Figure 2B. This tallies with the previous analysis of charge separation in type-II nanocrystals where Marcus inverted behavior was found to be common due to small reorganization energy resulting from weak exciton-phonon coupling.26 One noteworthy feature here is, in the emission spectra the CdTe state emission is slightly broader compared to sharp CS state emission. In the absorption spectra also the oscillator strength of CS state absorption is marginally greater than CdTe state absorption. Similar features was previously observed in a CdSe/CdTe nanorod and was explained in terms of strong mixing between these two states.26 Therefore, we performed adiabatic analysis for the charge separation which incorporates state mixing between CdTe state and CS state according to methods developed previously.19,26 Figure 2B inset shows the adiabatic free energy curves. As the CdTe and CS states mix the absorption spectra gets split into two bands each of which has a distinct emission band. The mixing of the two states is in well accord with the quasi-type-II behavior we want to demonstrate in a type-I system. As the type-II behavior is intrinsic to the system in CdTe/CdSe nanocrystals due to staggered band alignment, 1ML (~0.4 nm) shell is enough to

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show the charge separated state.16,19 Whereas in CdTe/ZnS nanocrystals the strain induced charge separation requires 4ML (~1.3 nm) of shell to demonstrate the charge separated behavior. c. Femtosecond transient absorption measurements: Ultrafast transient absorption measurements were carried out to justify the aforementioned arguments of charge separation. The nanocrystals dispersed in chloroform were excited with 400 nm (3.1 eV) laser beam and the pump induced changes in the absorption (ΔOD) was recorded over a broad spectrum range (530 to 775 nm) along with the kinetic traces at the corresponding wavelengths. The pump power was kept low enough (400 ps (1%) components. The first two ~ 5ps and ~50 ps components of bleach recovery are mainly due to carrier trapping, whereas the slow >400 ps component arises due to electron trapping in competition with the radiative recombination.40-42 Increase in the amplitude of long decay component signifies electron is spatially separated from hole leading to slower recombination rate and/or removal of electron traps.18,40 For core/shell nanocrystals the electron cooling time slows down to 1.7 ps for 4-ML of ZnS shell. The slowing down of this Auger assisted electron cooling is indicative of decoupling of electron from hole and will be discussed in subsequent sections. The amplitude of the slow recovery component (>400 ps) increases to 17% upon shell growth which is presumably due to charge separation and removal of surface traps as a result of better surface passivation by ZnS. All the kinetic fitting parameters are tabulated in Table 2. Previously, Burda and co-workers have reported shell thickness dependent transient absorption spectra of CdTe/CdSe type-II nanocrystals.16 They have shown increase in electron cooling time from 200fs to 400fs upon shell growth which is attributed to separation of electron 13



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Table 2: Exponential fitting parameters of bi-exponential growth (τg) and tri-exponential recovery (τr) of core and core/shell nanocrystals at 665 nm. For CdTe/ZnS-4ML the fitting parameters at 700 nm are also given. The percentages at the parenthesis represent amplitude of the corresponding exponential functions. Sample

Wavelength (nm)

τ1g

τ2g

τ1r

τ2r

τ3r

CdTe

665

400ps

(65%)

(35%)

(78%)

(21%)

(1%)

400ps

(40%)

(60%)

(40%)

(44%)

(16%)

400ps

(35%)

(65%)

(30%)

(53%)

(17%)

400ps

(35%)

(65%)

(25%)

(55%)

(20%)

CdTe/ZnS

665

(2ML) 665 CdTe/ZnS (4ML)

700

and hole wave-function as the electronic wave-function delocalizes to shell due to type-II alignment. The conduction band electron cools down from higher excited state to 1S e state by transferring its energy to the valence band hole through an Auger assisted mechanism. 43,44 Energy transfer from electron to hole has been directly confirmed with the help of time resolved luminescence and terahertz spectroscopy. 45 The excited hole then relaxes by releasing its energy to phonons through dense valence band levels. The Auger assisted cooling requires electron-hole wave-function overlap [].46 Thus decoupling the electron from hole leads to slower electron cooling. Slow electron cooling in type-II CdTe/CdS and CdTe/ZnTe was reported 14


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earlier due to spatial separation of electron from hole. 18,47,48 When electron is decoupled from the hole, the hot electron can still cool down either through ligand vibrations (if a suitable vibrational mode is available) or intermediate trap states or both. Also, using hole trapping surfactants the electron cooling time was slowed down as trapping the hole minimizes electron-hole wavefunction overlap.44,49 Electron cooling can be further slowed down to ~ 1 ns if hole trapping surfactants are put on a type-II structure in which electron traps are carefully passivated. 50 In our case, due to strain induced band gap tuning the electron extends towards the shell decreasing the electron-hole wavefunction overlap. We have not changed the surfactant in any of the NCs. All the NCs synthesized are oleic acid capped. Therefore, the effect of surfactant is not an issue in our case. The ZnS shell passivates the surface defects of the CdTe core and introduces considerable strain to extend the electron towards the shell. Therefore, the slow electron cooling observed here demonstrates that the electronic wave-function actually spreads out to the shell in a type-I structure due to lattice strain leading to a quasi-type-II structure as predicted by Nie et. al.27 In fact if there is charge separation in a type-I system, it should get manifested in the wavelength dependent bleach dynamics. As discussed previously, we deconvoluted the absorption spectra of CdTe/ZnS-4ML nanocrystal for Marcus analysis to CdTe dominated state (~665 nm) and charge separated state (~700 nm). Therefore, to further confirm our arguments we monitored the probe wavelength dependent bleach dynamics of CdTe/ZnS-4ML nanocrystal at 665 nm (CdTe dominated state) and 700 nm (charge separated state) which are shown in Figure 4, inset. The kinetic traces are fitted as described earlier-with a bi-exponential growth and triexponential recovery functions. The kinetic parameters are summarized in Table 2. Two observations are consistent with the idea of charge separation. The hot electron cooling time 15



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slows down further as we move towards red end of the spectrum, from 1.7 ps at 665 nm to 2 ps at 700 nm. This implies electron is getting more and more decoupled from hole near the charge separated state. Also, in the recovery of bleach the 70 ps component at 665 nm gets replaced by longer 120 ps component and the amplitude of long decay (>400 ps) component increases towards 700 nm. This suggests slower rate of electron-hole recombination in the red region of the spectrum in accord with the idea of charge separation. Due to strain induced conduction band-edge modulation the electronic wave-function extends towards the shell whereas the hole still resides in the core resulting spatial separation of electron from hole. 27

Scheme 1: Schematic representation of electronic wave-function extending towards shell which results in the slow electron cooling observed for CdTe/ZnS nanocrystals. Scheme 1 summarizes the key findings of the paper. Photo-excitation generates electronhole pair (exciton) in the CdTe core. The photo-excited hot electron (e-) relaxes to 1Se by transferring its energy to hole (h+) which depends on electron-hole wave-function overlap. The 16


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hole relaxes through manifolds of phonon. Decoupling the electron from hole thus results in slow electron cooling. Lattice strain at the CdTe/ZnS interface alters the band alignment such that CdTe CB shifts to higher energy accompanied by ZnS CB moving downwards in energy (the shifting is shown by small arrows in the scheme). This results in the leakage of electron wavefunction to the shell leading to a quasi-type-II structure. The electron gets spatially separated from the hole resulting slow electron cooling and slow rate of recombination with hole as inferred from the transient absorption measurements. 4. Conclusion: In conclusion, CdTe/ZnS core/shell nanocrystals with varying shell thickness have been synthesized and characterized with steady state and time resolved techniques. With thick ZnS shell, gradual red shift in the absorption and photo-luminescence spectra are indicative of a quasi type-II alignment. Moreover, lack of band-edge like structure in the absorption spectra accompanied with increase in excited state lifetime of core/shell nanocrystals corroborate that charge carriers are spatially separated. In bulk, CdTe/ZnS core/shell hetero-structure exhibits type-I band alignment. However, the lattice strain arising due to prominent ~19% lattice mismatch at the core/shell interface alters the CB alignment of both core and shell to make a quasi-type-II structure where the electronic wave-function extends towards the shell leading to charge separation. Marcus analysis of the steady state data for thickest ZnS shell shows this strain mediated charge separation occurs in the inverted region due to small reorganization energy. There is a strong mixing between the CdTe dominated state and charge separated state which reconfirms the idea of charge separation. Further evidence for electron leaking comes from ultrafast transient absorption measurements which reveal slow electron cooling with thick ZnS shell implying electron being decoupled from hole. Extending electronic wave-function to 17



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the shell decreases electron-hole wave-function overlap, this slows down the Auger assisted hot electron cooling. In combination of Marcus theory and ultrafast transient absorption to probe charge separation energetics and dynamics, respectively we have clearly confirmed strain induced quasi type-II behavior in CdTe/ZnS type-I nanocrystals.

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] (Fax: (+) 91-22-25505331/25505151. Acknowledgement: This work was supported by “DAE-SRC Outstanding Research Investigator Award” (Project/Scheme No.: DAE-SRC/2012/21/13-BRNS) granted to Dr. H. N. Ghosh. S.M. and T. D. acknowledge CSIR and P.M. acknowledges DAE for research fellowship. We sincerely acknowledge Dr. D. K. Palit and Dr. B. N. Jagatap for their encouragement.

Supporting Information Available: Synthetic methodology for CdTe and CdTe/ZnS nanocrystals, HRTEM images and analysis, PL spectra after 250 nm excitation, details of Marcus theory analysis, deconvolution of absorption and PL spectra. This material is available free of charge via the Internet at http://pubs.acs.org.

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Lattice-Strain-Induced Slow Electron Cooling Due to Quasi-Type-II

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