Ultrafast Carrier Dynamics of Photo-Induced Cu-Doped CdSe

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C: Physical Processes in Nanomaterials and Nanostructures

Ultrafast Carrier Dynamics of Photo Induced Cu Doped CdSe Nanocrystals Avisek Dutta, Rajesh Bera, Arnab Ghosh, and Amitava Patra J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b05422 • Publication Date (Web): 27 Jun 2018 Downloaded from http://pubs.acs.org on July 6, 2018

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The Journal of Physical Chemistry

Ultrafast Carrier Dynamics of Photo Induced Cu Doped CdSe Nanocrystals

Avisek Dutta, Rajesh Bera, Arnab Ghosh, and Amitava Patra* Department of Materials Science, Indian Association for the Cultivation of Science, Kolkata 700 032, India

*To whom correspondence should be addressed. E-mail: [email protected] Phone: (91)-33-2473-4971, Fax: (91)-33-2473-2805

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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

ABSTRACT The understanding of ultrafast carrier relaxation process in doped semiconductor quantum dots is very important for their potential applications in light emitting diodes (LEDs), optoelectronics. Here, we have studied the change in electronic properties of Cu doped CdSe QDs upon light illumination. The light induced effect leads to the enhancement of the band edge decay time and reduces the decay time of the dopant emission due to photo-corrosion of Cu doped CdSe QDs. The bleaching recovery kinetics and the hot electron cooling dynamics have been studied by using femtosecond transient absorption spectroscopy. It is observed that the electron cooling process of doped CdSe QDs is dependent on the dopant concentration and the cooling kinetics of doped CdSe QDs are found to be slower than undoped QDs. After light irradiation, the cooling processes of hot electron and recovery process in doped systems are modified.


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INTRODUCTION Incorporation of transition metal ions as dopants into semiconductor nanocrystals (NCs) has attracted a great deal of interest for various applications in the areas of bio-labeling, lightemitting devices, photo-catalysis, and spintronics.1-5 Successful doping can be done by various methods such as pre-doped single source precursor, nucleation doping, growth doping, ion exchange and diffusion, and so forth.6-9 Copper doped quantum dots (QDs) serve as a model system for studying the electronic structure and photophysics of doped semiconductor NCs.10-15 However, the fundamental photophysical properties of Cu:CdSe NCs are still not clear because the emission property of copper doped CdSe is dependent on the oxidation state of copper present in QDs. There is a debate on the oxidation state of copper ion whether it is in Cu (+1) state or Cu (+2) state. Klimov et al. reported that Cu is present in the QDs as Cu (+2) prior to photo-excitation11, 16 and Gamelin et al. claimed that Cu (+1) is present in the copper doped CdSe NCs.17-18 It is noted that d level of Cu ion lies in between the bang gap of the host CdSe. The advantage of copper doped QDs over Mn doped CdSe is the tuning of emission.19-20 In the Mn doped QDs, the PL arises due to d-d transition i.e. from 4T1 to 6A1 transition of Mn at ~580 nm.21-23 However, for the Cu doped CdSe QDs, the modified band gap of the host can tune the emission of the QDs from 600-800 nm. Both excitonic and dopant emission appear in presence of divalent copper (3d9, Cu2+) doped QDs. After excitation, the electron moves from valance band (VB) to conduction band (CB) of QDs and the band to band emission of QDs generates when CB electron recombines with holes of VB. The dopant emission appears when the photoexcited electron of conduction band (CB) of QDs comes down to the atomic level of Cu2+ (d9). After excitation of Cu+1(d10) doped QDs system, the hole in VB goes to the atomic level of Cu1+ via non-radiative pathway. Then, the electron of CB comes to Cu

+1

state to give NIR PL

emission without excitonic emission. Interestingly, the Cu dopants play an important role on the exciton dynamics in ultrafast time scale for Cu-doped semiconductor NCs which is well studied.24 Zhang et al. have reported the charge carrier dynamics for Cu-doped ZnSe NCs and they investigated the influence of Cu doping on the luminescence decay of the band-edge emission.8 Banin at al. have used transient absorption spectroscopy to understand the electronic structure and dynamics of Cu doped InAs QDs where doped states were situated below the conduction band of the QDs.24-26 Ultrafast spectroscopic study is required for better understanding the exciton dynamics in such doped system. To best of our knowledge, there is no 3 ACS Paragon Plus Environment


study on the influence of light irradiation on the decay kinetics of copper doped QDs. Here, we highlight the influence of light irradiation on the change of photophysical properties of Cu doped CdSe nanocrystal. We also investigated how dopant concentration influences on the electron cooling dynamics and bleaching kinetics by using femtosecond transient absorption spectroscopy (TAS). Moreover, the tuning of the emission color from red to bright yellow is highlighted after photo irradiation of the Cu doped CdSe nanocrystals. MATERIALS Cadmium acetate dihydrate (Cd(OAc)2).2H2O), stearic acid and copper (II) nitrate (Cu (NO3)2), oleic acid (OlAc) (90%), 1-octadecene (ODE, 90%), trioctylphosphine (TOP, 90%), and Se pellets were purchased from Sigma Aldrich. Tetramethyl ammonium hydroxide pentahydrate (TMAH, 98%), ethanol, toluene, hexane were obtained from Spectrochem. Copper stearate (CuSt2) was synthesized and purified by the following procedure similar to the literature reports published previously (Supporting information). EXPERIMENTAL SECTION Synthesis of Cu doped CdSe QDs Cu:CdSe NCs were synthesized using a hot injection method in organic solvent. Briefly, Cd(OAc)2.2H2O (0.126 g, 0.5 mmol), oleic acid (2 mL), and ODE (5 mL) were degassed under vacuum at 1400C under argon for 10-15 min, which produced a clear, colorless solution of cadmium oleate (CdOl2). The Se precursor solution was prepared by dissolving 789 mg of Se powder in 5 mL of TOP and 5 mL of ODE under Argon atmosphere. CdOl2 was allowed to cool at room temperature and added 1% and 5% Cu source (CuSt2 in mol % with respect to Cd2+) under continuous Argon flow to produce blue color solution. The temperature of the flask was raised to 1800C at a constant rate of 100C/minute and 1 mL of 1 mmol of TOP-Se solution was rapidly injected. The reaction was allowed to proceed for 5 minutes which produced reddish color solution of copper doped CdSe QDs. Then, the crude was allowed to cool at room temperature and added dry ethanol/acetone mixture to the crude mixture followed by centrifugation for purification. The supernatant was discarded and the pellets were dried under vacuum, and re-dispersed in dry toluene for further studies. The synthetic method of undoped CdSe QDs was given in supporting information.


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Light irradiation of Cu:CdSe QDs Light illumination of QDs was performed by following methods. A xenon lamp (300 watt) was used as the light source. The dissolved QDs in dry toluene, was put into a 10 ml beaker with continuous stirring and kept in front of the light source for 2 hrs. Aliquots were collected at constant time interval for various spectroscopic studies. CHARACTERIZATION Powder X-ray diffraction (XRD) was recorded by using Bruker D8 Advance powder diffractometer having Cu Kα radiation (1.5418 Å). Transmission electron microscopy (TEM) measurements were done by using a JEOL, JEM-2100F at an operating voltage of 200 kV. TEM samples were prepared by drop casting of QDs solution in toluene on carbon coated Au grid followed by the evaporation of the solvent. The XPS measurements were carried out by using an Omicron Nanotechnology instrument. Room-temperature optical UV- visible absorption spectra were recorded by UV−Vis spectrophotometer (Shimadzu). Steady state photoluminescence studies were carried out by using a Fluoro Max-P (Horiba Jobin Yvon) luminescence spectrophotometer. The samples were excited at 371 nm using a pulse diode Nano LED (IBH Nanoled-07) in an IBH Fluorocube apparatus during the time correlated single photon counting (TCSPC) measurements. The repetition rates were 500 KHz and 1 MHz depending upon the lifetime range of the samples. The fluorescence decays were analyzed using IBH DAS6 software. The following equation was used to analyze the experimental time resolved fluorescence decays, P (t ) : n

P (t ) = b + ∑ α i exp( − i

t

τi

)

(1)

Here, n is the number of discrete emissive species, b is a baseline correction (“dc” offset), and αi and τi are the pre-exponential factors and excited-state fluorescence lifetimes associated with the ith component, respectively. For multi-exponential decays the average lifetime,〈τ〉, was calculated from the following equation: n

< τ >= ∑ β iτ i

(2)

i =1

Where β i = α i / ∑ α i and ai is contribution of the decay component. Room temperature EPR spectra were recorded at a microwave frequency on a JEOL instrument. In our femtosecond 5 ACS Paragon Plus Environment


transient absorption spectrophotometer (TAS) setup,27 a mode-locked Ti:sapphire oscillator (Seed laser, Mai-Tai SP, Spectra Physics) generates pulses of ~ 80 fs duration with a wavelength of 800 nm, at a repetition rate of 80 MHz. The energy required for amplifying the seed pulse is supplied by a separate pump laser (Nd:YLF laser, 527 nm, ASCEND EX, Spectra-Physics). The Spitfire Ace amplifier system (consist of optical stretcher, regenerative amplifier and optical compressor) amplify low-energy laser pulses to mJ energy level. The output from the amplifier was (800 nm, 500 (74 ± 5) % >500 (25.2 ± 2) % >500 (29 ± 2) %

700 ± 0.75 (100%) 400 ± 0.69 (100%)

3 ± 0.05 (39.4 ± 3.5) % 3 ± 0.04 (27.8 ± 3.5) %

125 ± 0.55 (39 ± 2) % 125 ± 0.53 (39.2 ± 2) %

>500 (21.6 ±0.8)% >500 (33 ± 0.8) %

(a τg represents the growth time constants, and τr represents the bleaching recovery time constants) The slow component essentially represents electron-hole recombination. The amplitude of contribution corresponding to slower component which decreases with increasing doping concentration, which is consistent with TCSPC data, as band edge lifetime decreases with increasing percentage of doping. Decay kinetics before and after light irradiation (Figure 8) clearly show that decay is slower after light irradiation with increasing percentage of Cu doping. Interestingly, in presence of light, the contribution of slower component increases with increasing percentage of doping. Thus, the band edge lifetime is increased after light irradiation in TCSPC. Figure 9 displays schematically illustration of all the processes before and after light irradiation. For the un-doped CdSe QDs, trap states are less compared to doped CdSe QDs. No significant photo-corrosion is observed for the un-doped case. Consequently the band gap remains almost same after light irradiation. Figure 9B shows the relaxation processes of doped samples.


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Figure 9: Schematically illustration of all the processes of (A) undoped CdSe QDs and (B) Cu doped CdSe QDs before and after light irradiation for 120 minutes. In both the cases, after excitation electron-holes pair is generated and electron can go to upper excited states as the pump energy was higher than the band gap for 1S transition (e). Then the hot electron can relax its excess energy to the 1Se via cooling process (f) which represents growth of the 1S bleach. The bleaching recovery is observed where electron can depopulate from 1S state through radiative recombination (g) or to trap states (h). In (B), in presence of Cu+1 (d10), hole transfer to Cu i.e. decoupling of CB electron (i) is noticed. Then the electron of conduction band can move to Cu level due to Cu2+ (j) which results the depopulation of 1S state manifested faster recovery of 1S bleach.

Here, the band gap of the QDs increases upon light illumination and it influences carrier relaxation dynamics. In Cu doped CdSe QDs, copper is present near the VB of CdSe QDs and the cooling mechanism is different which is discussed above. In case of un-doped CdSe QDs, the electron excited to 1S state (path e) first, then the electron depopulates from the 1S state through recombination with hole present at VB or trap state (path g and h, Figure 9A). In



Cu doped CdSe QDs, Cu center can capture the hole from VB (path i, Figure 9B) which insists the recombination of CB electron with Cu (path j, Figure 9B). CONCLUSIONS In conclusion, we have demonstrated the effect of light irradiation on the ultrafast carrier dynamics of the Cu doped and un-doped CdSe QDs. In case of un-doped CdSe QDs, the PL intensity increases 3.4 times without any change in average decay time after 2 hrs light irradiation. In case of doped samples, the band edge emission of the copper doped CdSe QDs is increased during photo-irradiation. There is no significant change is decay time in case of undoped CdSe QDs. Interestingly, the decay time of dopant emission reduces from 71 ns to 28 ns for 1% Cu doped CdSe and from 87 ns to 37 ns for 5% Cu doped CdSe QDs after light irradiation.

From the femtosecond transient absorption spectroscopy, we have found the

hypsochromic shift with decreasing of the size of the doped QDs during photo illumination and the doping influences the electron cooling time. This photo induced effect in doped QDs can tune the color from red to bright yellow which might have potential applications in future.

ASSOCIATED CONTENT Supporting Information Preparation of copper stearate, synthesis of undoped CdSe QDs, XRD data of 5% Cu doped CdSe, Normalized UV-Vis absorption and steady-state PL spectra of 5% Cu doped CdSe QDs. Decay curves for band edge emission and dopant emission for 5% Cu doped CdSe QDs sample upon excitation of 370 nm. Normalized UV-Vis absorption and steady-state PL spectra of undoped CdSe QDs and PL decay emission for undoped CdSe QDs with time of illumination of light. Change in steady state absorption spectra and fluorescence spectra of undoped CdSe NCs during light irradiation. Steady state PL emission spectra of 5% Cu:CdSe NCs during light irradiation for normalized at dopant emission and normalized at band edge emission. UV-Vis spectra and steady state photoluminescence spectra of 1% Cu doped CdSe QDs in dark. PL decay emission at excitation of 370 nm for undoped CdSe QDs with time of illumination of light. Multiexponential TCSPC fitting parameters for CdSe, Cu- CdSe(1%), Cu-CdSe(5%) at the band edge emission and dopant emission. TA spectra of undoped CdSe QDs before and after light irradiation and kinetics of the ground state bleach dynamics for undoped CdSe QDs before and after light. 20 ACS Paragon Plus Environment

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This material is available free of cost from http//:pubs.acs.org. ACKNOWLEDGMENTS DST-TRC is gratefully acknowledged for the financial support. AD thanks DST-Inspire and RB thanks to IACS and AG thanks to CSIR for awarding the fellowship.



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Ultrafast Carrier Dynamics of Photo-Induced Cu-Doped CdSe

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