Role of ZnS Segment on Charge Carrier Dynamics and

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Role of ZnS Segment on Charge Carrier Dynamics and Photoluminescence Property of CdSe@CdS/ZnS Quantum Rods Pushpendra Kumar, Rajeev Ray, Patrick Adel, Franziska Luebkemann, Dirk Dorfs, and Suman Kalyan Pal J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.7b12223 • Publication Date (Web): 24 Feb 2018 Downloaded from http://pubs.acs.org on February 24, 2018

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The Journal of Physical Chemistry

Role of ZnS Segment on Charge Carrier Dynamics and Photoluminescence Property of CdSe@CdS/ZnS Quantum Rods

Pushpendra Kumar,a Rajeev Ray,a Patrick Adel,b Franziska Luebkemann,b Dirk Dorfs,b* Suman Kalyan Pal,a* a

School of Basic Sciences and Advanced Material Research Center, Indian Institute of

Technology Mandi, Kamand 175005, H.P, India. b

Institut für Physikalische Chemie und Elektrochemie, Leibniz Universität Hannover, Callinstr.

3A, Raum 205, D-30167 Hannover, Germany Corresponding Authors: *[email protected], *[email protected]

ABSTRACT Growing a wide bandgap shell on bare core and/or core@shell materials is a fascinating idea for improving the photoluminescence (PL) efficiency and stability. An epitaxially grown shell adds another degree of complexity to the system and modulates the excited state relaxation dynamics, which remain poorly understood. Employing time-resolved PL and femtosecond transient absorption (TA) spectroscopy, we present a thorough study on charge carrier dynamics of CdSe@CdS and CdSe@CdS/ZnS quantum rods (QRs). Various excitation wavelengths were used to identify the contribution of individual segment towards the optical properties of the QRs. 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

Our femtosecond TA measurements provide a clear evidence of excitation migration from CdS as well as ZnS to CdSe core within few picoseconds of photoexcitation. The excitons recombine faster in the CdSe moiety of the CdSe@CdS/ZnS than that of the CdSe@CdS QRs via an extra decay path. The interband trap states that are created via the formation of extended defects due to lattice strain relaxation (or ion exchange during the formation of ZnS segment) in CdSe@CdS/ZnS QRs could provide the additional decay channel leading to low PL intensity and quantum yield (QY). We believe that our study will help to develop a strategy for enhancing PL efficiency through energy funneling across semiconductor heterojunctions and to understand charge carrier dynamics in nanoheterostructures.

1. INTRODUCTION Developing a thin shell of a wide band gap semiconductor on the outer surface of the emitting nanocrystals allows substantial improvement of their stability and tunability of the PL. Such types of core@shell nanocrystals exhibit efficient PL with stability superior to single phase nanoparticles and organic dyes and are of practical interest for light emitting devices and biological imaging.1, 2 Because of the visible PL from CdSe, a continuous attention was paid to CdSe-based colloidal heteronanostructures in the past decades to understand their optical and electrical properties. The opto-electronic properties of these materials are strongly dependent on their composition, shape, and dimensions.3, 4 A number of materials are available for designing shells around CdSe core, but a greater attention has been devoted to CdS because it provides a high flexibility to the shell growth due to the small lattice mismatch between CdSe and CdS (~ 4%).5 Tetrapod 6, plate

7

and rod

8

-shaped CdS shells surrounding a CdSe spherical core have

been reported in recent past. The core/shell CdSe/CdS structures enable electron−hole wave 2 ACS Paragon Plus Environment

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function overlap engineering due to the small conduction band offset between CdSe and CdS.9, 10 It is reported in the literature11, 12 that CdS shell on CdSe core helps to remove the surface states, and with increasing shell thickness, PL QY increases tremendously.13, 14 These structures were used as fluorescent reporters in bioanalysis and microscopy15,

16

and as optoelectronic

components in photovoltaics, light emitting diodes (LEDs), and liquid crystal displays (LCDs).17-20 An interesting class of nanocrystals is dot-in-a-rod (QD@RD or QD@QR), in which a spherical seed of a semiconductor nanocrystal is embedded within another rod-shaped semiconductor material. A spherical CdSe core embedded closer to one end in CdS rods across the rod length shows improved PL QY. 21 Furthermore, the QD@QRs emission is anisotropic or polarized, whereas the PL from quantum dots (QDs) is isotropic.22, 23 Therefore, in the last few years, QD@QRs have increasingly replaced QDs in many of the commercially available II/VI semiconductor nanocrystals based products.24 CdSe@ZnS and CdSe@CdS heterostructures show type-I band alignment which effectively confines the charge carriers to the CdSe part of the structure.25 In order to acheive better stability and enhencement in PL yield of core/shell structures, the length and diameter are commonly covered by a coordinatively bound shell of wide band gap nanomaterials. ZnS is a nontoxic, chemically stable wide bandgap (3.8 eV for the bulk material) semiconductor. Because of its higher bandgap than CdSe, it provides better confinement of charge carriers in the CdSe core when used as a shell material. In addition, a good electronic insulation of the fluorescent part of the QDs or QRs can be achieved by ZnS. Typically, a thin shell (6 ± 3 Å) made of ZnS around the CdSe cores helps to passivate the surface and enables dramatic enhancement of PL QY from less than 10% to more than 50%.26 Talapin et al.27 suggested that ZnS provides an excellent confinement of the charge carriers and a

3 ACS Paragon Plus Environment


tolerable lattice mismatches at each interface in CdSe/CdS/ZnS and CdSe/ZnSe/ZnS. ZnS shell was repoted to increase the PL QY of CdSe/CdS core/shell nanorods by ten folds.28 Femto second TA spectroscopic study suggested that the reason for such large increase of QY in the CdSe/CdS/ZnS is the elimination of electron trapping states via surface state passivation by ZnS shell.28 Baranov et al.29 suggeated that a defect-free core/shell interface is more important for producing strong luminescence than an increase of the shell thickness of the ZnS. Lomascolo and coworkers30 investigated ultrafast processes to understand the effect of the shell thickness on stimulated emission and photoinduced absorption transitions in CdSe/CdS/ZnS core/shell nanorods. According to them, the density of high energy defects in CdSe/CdS nanorods is reduced with ZnS coating. The reduction is more for thicker shells, however, thick shells introduce new midgap defect states due to strain relaxation at the core-shell interface. High PL efficiency was also reported for double shell nanorods of CdSe. In such structures, the outer ZnS shell that is grown epitaxially acts as potential barrier to confine the charge carriers inside the CdSe/CdS or the CdSe/ZnSe regions leading to a high PL yield of the final core@shell@shell nanorods.31 Moreover, Deka et al.32 have shown that the presence of trap states at the CdS surface can be reduced through growth of ZnS, which resulted in enhancement of PL QY as the probability of charge carrier decay at CdSe emitting states increases in CdSe/CdS/ZnS double shell nanorods. In contrary, we noticed reduction in PL property of CdSe in segmented CdSe@CdS/ZnS (dot@rod/shell) nanoheterostructures.33

Here, we undertake a thorough investigation of the charge carrier dynamics in segmented CdSe@CdS/ZnS (eNR) dot-in-a-rod using femtosecond pump-probe technique. We compare these ultrafast processes with that of CdSe@CdS QRs (NR) to develop a deeper insight into the


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effect of ZnS on the carrier dynamics and hence PL property. The role of CdS and ZnS moieties were also investigated by exciting these sites with different excitation wavelengths. Ultrafast excitation migration from both CdS and ZnS to CdSe site is quite evident from TA kinetic measurements. Excitons live longer in CdS site in the presence of ZnS segment suggesting reduction of surface trapping. However, in the CdSe moety excitons decay faster in CdSe@CdS/ZnS QRs than in CdSe@CdS leading to low PL efficiency. We attribute the rapid depopulation of excitons in CdSe@CdS/ZnS QRs to their non-radiative recombination via the midgap states created due to strain relaxation at the interface between the segments.

2. EXPERIMENTAL METHODS 2.1. Systhesis of NR and eNR. NR and eNR were prepared following established chemical methods reported elsewhere.33 As a representative for exchanged nanorods (eNR), we chose a sample in which 25% of the CdS was exchanged to ZnS. Transmission electron microscopy (TEM) analysis including energy dispersive x-ray spectroscopy (EDX) elemental mapping in high-angle annular dark field scanning transmission electron microscopy (HAADF-STEM) mode of the investigated eNR sample are shown in Figure S1(Supporting Information). Because of the preference of the rod tip in the first step of the cation exchange, the exchange occurs mainly at one end of the rod till around 40% of ion exchange. The successful ion exchange from CdS to ZnS of the nanorods can be well seen in the Zn/Cd elemental map shown in figure S1C (Supporting Information). Therefore, ZnS cell is expected to form at one end of the eNR sample (inset of Figure 1). Figure S2 (Supporting Information) shows the picture of NR and eNR in toluene under room light and UV (365 nm) illumination. The length and diameter estimated from TEM images (SI Figure S3 and S4) were 35.0 ± 7.1 nm and 5.8 ± 1.2 nm for NR and 34.5 ± 4.5 5 ACS Paragon Plus Environment


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and 5.7 ± 0.9 nm for eNR, respectively. The TEM images infer that both the length and diameter of the nanoheterostructure are slightly reduced in eNR. 2.2. Microscopic Technique. STEM-HAADF and EDX measurements were conducted using a JEOL JEM-2100F, equipped with a field emission gun operated at 200 kV. Elemental analysis was recorded by EDX analysis in STEM mode. TEM and HR-TEM images were performed by using a FEI Tecnai G2 F20, equipped with a field emission gun operated at 200 kV. Samples were prepared by dropcasting 10 µL cleaned sample on a carbon coated copper grid (300 mesh) purchased from QUANTIFOIL.

2.3. Steady-state and Time-resolved Spectroscopic Techniques. Steady-state UV-vis absorption and emission measurements were carried out using a Shimadzu UV-2450 spectrophotometer and Cary Eclipse fluorescence spectrophotometer (Agilent Technologies), respectively. TA

measurements were accomplished by femtosecond pump-probe TA

spectrometer equipped with a femtosecond laser from Spectra Physics, USA.34,

35

The laser

consists of a mode-locked Ti:sapphire oscillator (Mai Tai, Spectra Physics), which produces femtosecond pulses of wavelength centered at 800 nm. The oscillator output was amplified in a Ti:sapphire regenerative amplifier (Spitfire Ace, Spectra Physics), which was pumped by 527 nm light from a frequency-doubled Nd:YLF laser (Empower, Spectra Physics). The regenerative amplifier delivers 800 nm laser pulses (∼35 fs) of energy ∼4 mJ at a repetition rate of 1 kHz. Ampifier output was then splitted into two components. While one beam was sent to an optical parametric amplifier (TOPAS Prime, Spectra Physics) that generates pump pulses, the second one was focused onto a moving CaF2 crystal through a delay unit for white light continuum 6 ACS Paragon Plus Environment

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(350−800 nm) generation. The pump and probe beams of near collinear geometry were used to achieve a greater overlap between the two to produce a TA spectrum of high signal-to-noise ratio. The intensity of the incident excitation beam was controlled by using an adjustable neutral density filter before the sample cell (path length of 2 mm). The probe pulses were detected with pump blocked and unblocked conditions using a 500 Hz mechanical chopper. TA spectra were recorded by CCD arrays after dispersion by a grating spectrograph (Acton Spectra Pro SP 2358). The group velocity dispersion (GVD) of WLC spectra was compensated by a chirp correction program. TA kinetic traces were measured using two well-aligned photodiodes of variable gain by controlling the relative delay between the pump and probe pulses with the help of a stepper motor driven optical delay line. Data analysis were carried out in Origin Lab, Matlab and in a software package provided by Pascher Instrument.

3. RESULTS AND DISCUSSION 3.1. Optical Properties of NR and eNR. Both NR and eNR samples were first examined by steady-state absorption and PL spectroscopy. Figure 1a and b depict the static absorption and emission spectra of the NR and eNR samples, respectively. The absorption spectrum of the NR shows a maximum at around 610 nm, which can be assigned to the characteristic absorption band of the CdSe dot. The shoulder at 475 nm belongs to the excitonic band of the CdS body of the rods. On the other hand, eNR exhibits a slightly blue-shifted absorption spectrum where the signature peak of CdSe core appears at 600 nm and CdS rod at 470 nm. PL spectra of both NR and eNR were recorded for two different excitations (310 and 410 nm). Fig. 1b shows

PL

spectra with 310 nm excitation, while spectra corresponding to 410 nm excitation is presented in 7 ACS Paragon Plus Environment


Figure S5. The PL peak of NR appears at 618 nm, but it is blue shifted by ~7 nm for eNR. Very small Stoke’s shift of the PL spectra suggest that PL (from both NR and eNR) mainly originates from the band-edge of CdSe dot. The blue shift of the PL band of eNR is consistent with the blue shift in the absorption spectra. Such blue shift in absorption and emission spectra is possibly caused by a slight change in dimension of the eNR and/or strong lateral quantum confinement induced by the growth of ZnS segment. Interestingly, PL peaks do not depend on excitation wavelength. In case of NR, we excited CdS rod at both the wavelengths, but in eNR, at 410 nm we excited CdS rod, while ZnS segment was mainly excited during 310 nm excitation. The observed excitation independent PL suggest that excitations migrate from both ZnS segment and CdS rod to CdSe core and emit PL due to radiative recombination. A significant quenchng in PL intensity of eNR with respect to NR is observed for both the excitations (Figure 1b and S5). We measured PL QY (ΦPL) of both NR and eNR samples using the reference rhodamine 6G (Figure S6). PL quantum yield of eNR is found to be almost half of NR (Figure S6). In order to ensure PL quenching in eNR, time-resolved PL measurements for

(a) (b) Figure 1. (a) Absorption and (b) PL spectra of NR and eNR following excitation at 310 nm. Inset shows the structures of NR and eNR. 8 ACS Paragon Plus Environment

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both NR and eNR were carried out using various excitations (at 255, 368 and 454 nm). PL lifetimes of NR and eNR are independent of excitation wavelength (Figure S7). The average lifetimes (߬average) of NR and eNR are found to be ~ 16 and ~ 12 ns, respectively. We have determined radiative recombination times (߬radiative) from ߬average and ΦPL by using the following relation, 36 ߬radiative = ߬average / ΦPL

(1)

߬radiative is estimated to be 30.3 and 43.6 ns for NR and eNR, respectively. It should be noted that the value of ߬radiative for NR is comparable to that of CdSe/CdS dot-in-rod core/shell nanostructure.36 Low PL QY in spite of high radiative lifetime indicates dominancy of nonradiative decay channel over radiative decay in eNR. These observations suggest the presence of extra nonradiative decay pathways leading to low PL QY in eNR as compared to NR.

3.2. Femtosecond TA Studies. Energy funneling from ZnS segment or CdS rod to CdSe dot in dot-rod-shell structure is clearly evident from steady state PL measurements. The lower PL QY of eNR than NR could be due to difference in the charge carriers dynamics. In order to gain insight about the carriers relaxation mechanism femtosecond TA measurments were performed following excitations at 310 and 410 nm. The chirp-free TA spectra of NR show two negative absorption bands with peak maxima at 475 and 630 nm as shown in Figure 2. Comparison of TA spectra with steady state absorption spectrum suggests that the absorption features at 475 and 630 nm could be attributed to ground state bleach (GSB) associated with CdS rod and CdSe core, respectively. These are basically the excitonic bleach due to 1S(e)−1S3/2(h) (1Se) electronic



transitions.37, 38 Fig. 3 dipicts the TA spectra of eNR ( at various time delays) with two main bleach features around 466 and 620 nm. Because of the presence of two similar bands in the steady state absorption spectrum, the bleach bands in eNR could be assigned to the excitonic bleach due to 1S(e)−1S3/2(h) (1Se) electronic transition in CdS rods and CdSe core, respectively. The filling of quantized electronic states (following Pauli’s exclusion principle) in nanocrystals affects transitions associated with the linear absorption spectrum and leads to the bleaching of the corresponding optical transitions.39. Prevoius TA studies showed that the exciton bleach signals in cadmium chalcogenide QDs, NRs and nanosheets aries mainly due to the statefilling.40-43 Therefore, the origin of the blue and red bleach bands (for NR and eNR) could be attributed to the state-filling transitions of both electron and hole in CdS rod and CdSe dot, respectively.44, 45 The appearance of CdSe bleach band (for 310 nm excitation), while exciting mainly CdS (for NR) and ZnS (for eNR), indicates the migration of excitons to CdSe core. It is evident from the TA spectra of both NR and eNR indicates that the negative signal in the high energy side (CdS bleach) grows in 2-3 ps, whereas the low energy bleach band (CdSe bleach) forms in about 6 ps after excitation. The slow build up of the bleach signal of the low energy band further suggests the exciton migration to CdSe core from CdS rod and/or ZnS segment.30, 46 A photoinduced absorption (PA) band is appeared around 360 nm at very early time (~ 500 fs) for NR (Figure 2a). According to previous reports,28, 46 CdSe/CdS core/shell nanorods possesses high energy surface trap states. The PA or excited state absorption in such nanorods could be assigned to transitions of trapped charge carriers to higher-energy continuum states.28, 46 Here, the growth of the PA signal is faster than the bleach at 475 nm, but their decays are comparable (Figure S8). These observations suggest that the positive band at 360 nm can be attributed to PA from trap states present close to the CB of CdS rod. However for eNR, PA band


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is blue shifted to 350 nm. The rise time of this band is slower than 475 nm bleach, but similar to 630 nm signal (Figure S9). These results indicate that unlike NR, PA signal in eNR originates mainly from defect sates that are closer to the CB of CdSe core. Creti et al.28, 30 observed defect states, which are resonant to the midgap states of CdSe core in CdSe/CdS/ZnS core/shell nanorods and ascribed them to lattice strain relaxation. In eNR, similar midgap defect states may form and contribute to PA. Furthermore, it is apparent from TA spectra (Figure S10) that the recovery of CdSe bleach band in eNR is faster than NR. The reason for this faster recovery could be faster recombination of excitons via midgap states. The role of such midgap states and their origin in eNR are discussed in section 3.5 (vide infra). Additionnaly, close inspection of the DA spectra (Fig. 2 and 3) suggest that although the bleach signal vanishes at 655 nm at early times (before 1 ps), it extends up to 680 nm at longer delay times. The delayed negative signal at the red end of the TA spectra could be attributed to the stimulated emission (SE) from the CdSe core.

(a)

(b)

Figure 2. TA spectra of NR in the visible spectral region (excitation wavelength ~ 310 nm): (a) early time (500 fs to 3 ps) and (b) later time (3 ps to 500 ps).



(a)

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(b)

Figure 3. TA spectra of eNR in the visible spectral region (excitation wavelength ~ 310 nm): (a) early time (500 fs to 3 ps) and (b) later time (3 ps to 500 ps).

In order to elucidate detail exciton dynamics in NR and eNR, TA kinetics were measured at their respective bleach bands using 100µJ/cm2 pump energies and two different (310 and 410 Table 1. Parametres obtained from the fitting of TA kinetics System

1

2

λpump

λprobe (nm)

߬growth (%)

߬growth (%)

߬1 (%)

߬2 (%)

߬3 (%)

310

475

360 fs (100)

-

4.78 ps (42)

128 ps (32)

1.74 ns (26)

630

550 fs (78)

4.8 ps (22)

316 ps (42)

1.37 ns (58)

475

140 fs (100)

-

3.6 ps (47)

122 ps (30)

1.73 ns (23)

630

505 fs (82)

3.7 ps (18)

330 ps (38)

1.66 ns (62)

-

475

595 fs (100)

-

3.8 ps (48)

160 ps (24)

1.73 ns (28)

630

530 fs (95)

3.85 ps (5)

57 ps (32)

234 ps (28)

1.14 ns (40)

475

156 fs (100)

-

3.6 ps (44)

165 ps (26)

1.70 ns(30)

630

610 fs (93)

3.6 ps (7)

56 ps (18)

277 ps (37)

NR 410

310 eNR 410


1.46ns (49)

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nm) excitations. All the kinetics were fitted multexponentially and results are presented in Table 1. 3.3. Ultafast Exciton Dynamics in NR. Firstly, we investigated the exciton dynamics of NR to learn about the electronic structures by measuring TA kinetics at bleach positions following excitations at 310 and 410 nm. Figure 4a displays normalised TA kinetics of NR measured at 475 and 630 nm following 310 nm excitation. The growth of CdS bleach signal (~ 475 nm) is faster than that of the CdSe bleach (~ 630 nm). Interestingly, TA kinetics at 630 nm grows till 6 ps, but by that time kinetics at 475 nm undergoes a decay followed by the initial rise (Figure 4b). We fitted TA kinetics at 475 nm multiexponentially with one growth component, 360 fs (100 %) and three decay components, ߬1= 4.78 ps (42 %), ߬2= 128 ps (32 %) and ߬3= 1.74 ns (26 %) (table1). The fast rise time (360 fs) is associated with the cooling of hot electron from higher energy level (1Pe or 1De) to lower energy level (1Se) of CdS rod (Scheme 1). The biexciton or

(a)

(b)

Figure 4. (a) TA kinetics of NR monitored at 475 and 630 nm with excitation at wavelength 310 nm. (b) Compression of TA kinetics of NR at early time monitored at 475 and 630 nm. Auger recombination time in CdS quantum dot is about 100 ps.47 Therefore, the observed lifetime of 128 ps could be assigned to the biexcitons that are formed in CdS. Excitons live 13 ACS Paragon Plus Environment


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couple of nanoseconds in CdS nanostructures48 suggesting that the slowest decay component appears most likely due to recombination of excitons. As trap states are present in the system, their contribution to the decay kinetics at 475 nm cannot be ruled out. On the other hand, TA kinetics at 630 nm can be fitted using biexponential growth with time constants ~ 550 fs (89 %) and ~ 4.8 (11 %) and two decay time constants, ߬ 1~ 316 ps (42 %) and ߬2 = 1.37 ns (58 %) (table 1). Interestigly, the second growth time constant (~ 4.8 ps) nicely matches with the fast decay time constant (~ 4.78 ps) of the CdS bleach signal. This observation infers that the excitons migrate from CdS rod to CdSe core with a characteristic time of 4.8 ps. It is worth noting that similar phenomenon was observed by Creti et al.28, 30 in the TA kinetics of CdSe/CdS dot/rod nanostructure, which they assigned as relaxation of excitons from higher to lower excited state. In addition, SE decay dynamics (at 660 nm) is comparable to bleaching dynamics at 630 nm (data not shown). This matching suggest that decay time constants, ߬1~ 316 ps (42 %) and ߬2 = 1.37 ns (58 %) are associated with the relaxation from the emitting excitonic state in CdSe core.

(a)

(b)

Figure 5. Compression of TA kinetics of NR monitored at (a) 475 and (b) 630 nm for two different excitations (310 and 410 nm).


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Next, we excited NR at the main excitonic band of CdS rod using 410 nm excitation and TA kinetics were measured at 475 and 630 nm. All the TA kinetics for 410 nm excitation including fitting curves are presented in Figure S11. TA kinetics were fitted with multiexponential functions and results are presented in table 1. The rise of TA signal (~140 fs) at 475 nm is much faster in case of 410 nm excitation (Figure 5a), while the rise at 630 nm is almost the same for both 310 and 410 nm excitations (Figure 5b). During excitation by 410 nm light, electrons get lifted very close to 1Se level of CdS and hence relaxes quickly to the band edge causing faster rise of the TA signal at 475 nm. Here, similar to 310 nm excitation, the fast decay time constant (~ 3.6 ps) of GSB of CdS matches well with the growth component (~ 3.7 ps) for GSB of CdSe. This observation further supports the possibility of photogenerated exciton migration from CdS to CdSe moiety in NR. The bleach signal at 475 nm recovers through both biexcitonic recombination (time constant 122 ps) and single excitonic recombination (time constant 1.73 ns). The fitting result (Table 1) of TA kinetics at 630 nm (excitation~ 410 nm) infers that relaxation dynamics in CdSe core is similar to that of the 310 nm excitation. Dynamics of excitons in NR for both the excitations (310 and 410 nm) are illustrated in Scheme 1. 3.4. Ultafast Exciton Dynamics in eNR. In order to study the influence of the ZnS segment on the carrier dynamics in eNR, we excite ZnS moiety using 310 nm laser light and record TA kinetics. Normalised TA kinetics measured at 475 and 630 nm (following 310 nm excitation) beloging to GSB of CdS rod and CdSe core, respectively along with fitted curves are presented in Figure 6a. Fitting of 475 nm TA kinetics yielded a single growth time constant, 595 fs and multiple decay time constants, ߬1= 3.8 ps (48 %), ߬2 = 160 ps (24 %) and ߬3 =1.73 ns (28 %) as shown in Table 1. The growth time (595 fs) is relatively larger than the growth time (360 fs) of



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NR. The absorption band at 320 nm of eNR belongs to the ZnS segment of the rod.33 Excitation at 310 nm not only creates hot carriers in the CdS moiety but also forms excitons in the ZnS segment. Unlike NR, both hot carrier relaxation and exciton migration from ZnS segment are expected to populate the CdS band edge in eNR. This is why the bleach signal at 475 nm for eNR takes longer time to grow than for NR at the higher energy excitation. Decay times ߬2 and ߬3 are very similar to that of NR and hence assigned to biexcitonic and excitonic recombinations, respectively in CdS rod.

(a)

(b)

(c)

(d)

Figure 6. (a) TA kinetics of eNR monitored at 475 and 630 nm for excitation wavelength ~ 310 nm. Comprison of TA kinetics of NR and eNR probed at 475 nm (b) and 630 nm (c) with 310 16 ACS Paragon Plus Environment

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nm excitation. (d) Early time TA kinetics of eNR measured at 475 nm for two different excitations. TA kinetics of GSB of CdSe (630 nm) possesses biexponential growth having time constants ~530 fs (95 %) and 3.85 ps (5 %) and triexponential decays with time constants ߬1= 57 ps (32 %), ߬2 = 234 ps (28 %) and ߬3 = 1.14 ns (40 %). Interestingly, the second growth time constant (~ 3.85 ps) is very close to the fast decay time constants (~ 3.8 ps) of CdS bleach signal at 475 nm (Table 1). These results suggest migration of exciton from CdS rod to CdSe core in eNR similar to NR. Frthermore, the bleach kinetics at 475 nm undergoes slow recovery, while 630 nm kinetics recovers faster in eNR than NR (Figure 6b, c). Slow decay of 475 nm kinetics in eNR could be attributrd to the reduction of surface traps due to ZnS segment (Scheme 1). In addition to the usual decay times (߬2 and ߬3), a relatively fast decay component (57 ps) is observed at 630 nm. This decay component was not present in the TA kinetics at 630 nm for NR when pumped by 310 nm light. The presence of this extra decay time indicates the existence of an additional recombination pathway in eNR. Moreover, different decay kinetics of SE (at 660 nm) than 630 nm bleach signal rules out a relaxation process of the emitting states (Figure S12). Slower recovery of bleach signal suggests that the extra decay channel in eNR is nonradiative in nature. Next, we use 410 nm laser excitation (to excite mainly CdS rod) and measure TA kinetics at 475 and 630 nm (Figure S11). Like 310 excitation, the CdS bleach signal at 475 nm of eNR possesses one growth and three decay time constants (Table 1). The growth componet (156 fs) is faster than that of 310 nm excitation (Figure 6d), but is similar to NR bleach at 475 nm for 410 excitation. Like NR, the fast growth of the CdS bleach signal in eNR could safely be assigned to hot carrier cooling in CdS rod. This means that pumping of eNR with 410 nm laser leads to 17 ACS Paragon Plus Environment


mainly the generation of high energy excitons in the CdS rod. Futhermore, same values of ߬1at 2

475 nm and ߬growth of 630 nm (Table 1) suggest exciton migration from CdS rod to CdSe dot in eNR too. Similar to 310 nm excitation, TA kinetics of eNR at 475 nm (for 410 nm excitation) undergoes slow recovery than NR suggesting reduced surface traps in eNR (Table 1, Figure S13 ). Recovery of TA signal of eNR at 630 nm (excitation 410 nm) is faster than that of NR and an additional fast decay time constant ߬1 = 57 ps was obtained by fitting the trace at 630 nm (Table 1). Similar fast decay component (Table 1) was observed in the TA kinetics at 630 nm for 310 nm excitation. We have already mentioned during the discussion of 310 nm excitation that the appearance of a short decay time in the bleach kintics at 630 nm in eNR is the indication of an additional recombination channel of the excitons in the CdSe core. All the dynamical processes that are taking place in eNR after photoexcitation are depicted in scheme 1.

3.5. Effect of ZnS Segment. It is clear from figure 6b and S13 that the average decay time of the excitons in the CdS moiety of eNR is higher than that of NR. We have mentioned in section 3.2 that in NR there is high energy defect states (close to the CB of CdS) which trap charge carriers making carrier recombination faster in CdS rod. Slow (CdS) bleach recovery in eNR suggests that the inclusion of ZnS segment avoids carrier tarpping from the CB of CdS by reducing the surface states. Additionally, the excitonic lifetime is shorter in the CdSe moiety of the eNR than the NR suggesting the existence of an additional decay pathway in eNR. This extra recombination path is most presumable the origin of the less PL in eNR compared to NR. Moreover, we have seen that neither the PL decay nor the decay of CdSe bleach kinetics in eNR depends on the excitation wavelength. In other words, we can say that the exciton dynamics in the CdSe part of the rod 18 ACS Paragon Plus Environment

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does not really depend on the origin of excitons (whether they are created in CdS or ZnS moiety). These observations suggest that the extra decay path in eNR is most likely through midgap states (appear in the vicinity of the CB of CdSe) leading to nonradiative recombination of excitons present in the CdSe core. The difference in CdSe bleach recovery and SE decay kinetics is further support the presence of nonradiative decay path in eNR. The origin of the nonradiative decay channel in eNR could be associated to the structural changes occurred during the inclusion of ZnS segment. It is worth noting that CdS shell partially passivates the high energy surface traps of the CdSe core.

28, 30

The ZnS segment in eNR removes the surface traps from one end of the rod

where ZnS segment grows (Scheme 1) causing slow recombination of excitons in CdS rod. The lattice mismatch of CdSe with respect to CdS and Zns are 4.2% and 10.7%, respectively. These lattice parameters mismatch can lead to formation of misfit dislocations relaxing the nanocrystal structure and creation of extended defects at the interface.28, 29, 49 Therefore, the introduction of



Scheme 1. Schematic illustration of ultrafast processes occurring in NR and eNR following photoexcitation at 310 and 410 nm.

ZnS segment in CdSe@CdS dot@rod system can introduce in the midway of the bandgap of CdSe core additional states that acts as trapping or nonradiative recombination site. On the other hand, growth of ZnS segment via ion exchange procedure may also introduce midgap trap states. The fast decay time constant (~ 57 ps) of CdSe bleach in eNR is instead attributed to the carrier trapping rate by such interband states. Hence, band structure alteration due to addition of ZnS segment to the CdSe@CdS core shell system results in the reduction of PL.


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4. CONCLUSIONS In conclusion, we have studied exciton dynamics in both CdSe@CdS and CdSe@CdS/ZnS dot@rod structures. Although a fraction of excitons that are generated in CdS moiety recombine there nonradiatively, a substantial portion is migrated to CdSe dot leading to high PL from the core. Excitons generated in the ZnS segment are also transported to the CdSe core and contributed to the CdSe PL. Our femtosecond TA studies reveal that ZnS segment can not only reduce surface defects in CdSe@CdS/ZnS, but also introduce new defects which are resonant to the midgap states of CdSe. The interband trap states that are formed in CdSe@CdS/ZnS due to lattice strain relaxation and/or ion exchange cause nonradiative recombination of excitons leading to low PL QY.

Supporting Information EDX mapping of eNR, Photograph of NR and eNR dispersions in toluene under normal light and UV illumination, TEM images of NR and eNR, absorption and PL spectra of NR and eNR following 410 nm excitation, PL QY in toluene, PL decay kinetics and lifetimes at various excitations, comparison of TA kinetics of NR and eNR at various wavelengths following 310 and 410 nm excitations and enlarged TA spectra of NR and eNR corresponding to CdSe bleach.

Acknowledgements Financial support from the Council of Scientific and Industrial Research (CSIR), Government of India under Grant No. 03(1325)/14/EMR-II is gratefully acknowledged. Authors are thankful to Advanced Materials Research Centre (AMRC), IIT Mandi for the experimental facilities. Thanks to Juergen Caro and Armin Feldhoff for providing access to HR-TEM facilities. 21 ACS Paragon Plus Environment


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TOC Graphic:


Role of ZnS Segment on Charge Carrier Dynamics and

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