Impact of capping agent on the electron transfer dynamics of CdTe QDs with methyl viologen

Journal of Luminescence 178 (2016) 356–361

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Impact of capping agent on the electron transfer dynamics of CdTe QDs with methyl viologen Mariadoss Asha Jhonsi a,n, Sekar Thulasi a, Arunkumar Kathiravan b,n a b

Department of Chemistry, B.S. Abdur Rahman University, Chennai 600048, Tamil Nadu, India National Centre for Ultrafast Processes, University of Madras, Taramani Campus, Chennai 600113, Tamil Nadu, India

art ic l e i nf o

a b s t r a c t

Article history: Received 13 November 2015 Received in revised form 7 June 2016 Accepted 8 June 2016 Available online 14 June 2016

The primary process in quantum dot solar cells is electron transfer between excited state QDs and semiconductor. There are several key factors controlling this electron transfer process including capping agents. Therefore fundamental understanding of capping agent dependent electron transfer dynamics is much needed. Here, we have employed two capping agents namely thioglycolic acid (TGA) and mercapto propionic acid (MPA) in which the later has one additional methylene group in its structure which plays a significant role on electron transfer process. Hence, we have studied the electron transfer dynamics of TGA and MPA capped CdTe QDs with methyl viologen (MV) in aqueous medium by using steady state and time resolved absorption and fluorescence techniques. The results show that MV quenches the fluorescence of CdTe QDs which is capping agent dependent. The obtained quenching rate constant for TGA and MPA capped CdTe QDs are 8.04
1010 M  1s  1 and 1.66
1010 M  1s  1 respectively. From the quenching rate constant values, it is clear that there is a significant role of capping agent on electron transfer process. This is due to MPA passivate the surface of QDs more than TGA, which slow down the electron transfer process from QDs to MV. Moreover, the observed radical cation (MV þ ) from transient absorption measurements confirms that the fluorescence quenching of CdTe QDs by MV is attributed to electron transfer. These experimental results will help to understanding the behavior of QDs with various acceptors towards the applications of quantum dot solar cells. & 2016 Elsevier B.V. All rights reserved.

Keywords: Capping agent CdTe QDs Fluorescence quenching Electron transfer Viologen

1. Introduction Quantum dots (QDs) are defined as a class of quasi-zerodimensional nanoparticles in which carrier motion is restricted in all three spatial dimensions [1]. Semiconductor QDs have been widely explored in fundamental research due to their optical properties, such as narrow and tunable emission spectra, high quantum yields, and photochemical stabilities [2,3]. Particularly, cadmium based QDs such as CdS, CdSe and CdTe were used in many applications, such as in light emitting devices, photovoltaic cells, photonics, transistors and biological labeling [4–8]. In particular, exploiting QDs as solar harvesters constitutes a promising approach toward low-cost third-generation solar cells owing to their band gap tunability, high absorption coefficient, solution processability, and multiple exciton generation possibilities [7–12]. Among them, Cadmium telluride (CdTe) QDs are the subject of intense studies due to they absorb throughout the visible and n

Corresponding authors. E-mail addresses: [email protected] (M.A. Jhonsi), [email protected] (A. Kathiravan). http://dx.doi.org/10.1016/j.jlumin.2016.06.022 0022-2313/& 2016 Elsevier B.V. All rights reserved.

near-infrared region, and display narrow and widely tunable photoluminescence together with a high stability against photobleaching when compared with other QDs and traditional organic dyes [7–9]. Moreover, CdTe is one of the leading thin film materials for photovoltaic (PV) applications. The advantage of using CdTe in thin-film solar cells lies on the fact that it is a direct band gap material with high absorption compared to silicon and the ease of device processing and high stability [10]. Generally, bare QDs are not stable in water or organic solvents and thus need surface capping reagents to help them stabilize. However, surface reagents such as organic molecules with long alkyl chains, polymers, or inorganic shells decelerate the rate of electron transfer (ET) between the excited QDs and electron acceptors because of the increased distance from the QD core to the adsorbates [13,14]. There are reports available for the effect of capping agent and size of QDs on the excited state electron transfer studies [15–17] which clearly indicates that the properties of molecular linkers can significantly influence the deactivation pathways of QDs and the efficiency of interfacial electron-transfer reactions between QDs and other molecules. For instance, Weiss reported the photoinduced electron transfer (PET) rate for a Cds QDs-Viologen complex. They demonstrate the use of transient

M.A. Jhonsi et al. / Journal of Luminescence 178 (2016) 356–361

2. Experimental 2.1. Materials and methods Thioglycolic acid (TGA, 98%), Mercapto propionic acid (MPA, 98%), CdCl2.2.5H2O (99.99%), tellurium powder (99.997%), sodium borohydride (95%) and Methyl viologen dichloride hydrate (98%) were purchased from Sigma–Aldrich and used as such without further purification. Other chemicals and solvents were of analytical grade and purchased from LOBA Chemicals (India). Milli-Q water was used to prepare the samples for spectral measurements. The synthesis of thiol-capped colloidal CdTe QDs was performed by using CdCl2 and NaHTe as precursors followed by the previously reported methods with some modifications [24–26]. The optimized reaction condition, characterization (XRD and TEM) and Tauc plot for the prepared QDs were given in the supporting information (Figs. S1, S2 and S3). The particle size of the prepared QDs is provided in Table 1. X-ray powder diffraction patterns were recorded on a Rigaku MiniFleXll-C system using CuKα radiation (λ ¼ 1.5418 Å) radiation and a graphite monochromator in the diffracted beam. TGA and MPA capped CdTe QDs samples were recorded in the form of powder. A scan rate of 1° min  1 was applied to record a pattern in the 2θ ¼25° in the range of 2θ ¼ 20–80°. Morphology of the synthesized TGA and MPA-capped CdTe QDs were investigated by high resolution transmission electron microscopy (HRTEM, JEOL

Table 1 Absorption (λmax), emission (λemi) wavelength, particle size from powder XRD, TEM, absorption measurements and band gap energy (Eg) of prepared TGA and MPA capped QDs. Parameters

TGA–CdTe

MPA–CdTe

λmax (nm) λemi (nm) a Band gap (eV) b Particle size c Particle size d Particle size e Radii

522 576 2.15 2.7 3.1 2.84 1.90

536 583 2.12 2.9 3.3 3.07 1.98

a

From Tauc relation. From Scherer equation. From TEM measurement. d From absorption measurement. e From Brus equation. b c

100

Transmittance (%)

absorption to simultaneously investigate the PET process of a single QD–ligand couple and quantify the affinity of the ligand for the QD surface under the conditions of the PET experiment [18]. Later, they describes a study of the rates of photoinduced electron transfer (PET) from CdSe. QDs to poly(viologen) within thin films, as a function of the length of the ligands passivating the QDs. They have shown, for the first time, a dramatic decrease of the PET rate upon increasing the length of the ligands on the QDs [19]. In 2013, the same author reported the evidence for a through-space pathway for electron transfer from QDs to carboxylate-functionalized viologens [20]. The rate constant for PET from colloidal CdS QDs to alkylcarboxylatefunctionalized viologens is independent of the number of methylene groups in the alkyl chain (n). The insensitivity of the electron transfer rate constant to the length of the functional groups on the viologen suggests that a “through-space” pathway. Kamat et al. reported that the electron transfer between methyl viologen radicals and graphene oxide [21]. The methyl viologen radicals are capable of transferring electrons to graphene oxide and partially restore the sp2 network. Lian et al. reported the interfacial charge separation and recombination in Core/Shell QDsmolecular acceptor [22]. They concluded that, III–V and II–VI semiconductors provides a promising approach for optimizing their light harvesting and charge separation for solar energy conversion applications. Yang et al. reported the supramolecular self-assembly and photophysical properties of pillar [5] arenestabilized CdTe QDs mediated by viologens [23]. CP[5]As could efficiently trap bridged bis(MV) inside the cavities through host– guest binding interactions, and thus efficiently prevent the electron transfer from CdTe QDs to bridged bis(MV). Based on the context, we would like to investigate the effect of thiol capping agents on CdTe QDs surface and their excited state properties. For electron transfer studies, we have chosen a well known electron acceptor namely methyl viologen (MV), since it has similar reduction potential compared with TiO2 conduction band. In addition, steady state and time resolved techniques are used to probe the role of capping agent on the electron transfer between CdTe QDs and MV.

357

80 60

TGA MPA TGA-CdTe MPA-CdTe

40 20 0 3000

2500

2000

1500

1000

-1

Wavenumber(cm ) Fig. 1. FT-IR spectra of TGA, MPA and capped CdTe QDs.

JEM-1230 with accelerating voltage of 120 kV). FT-IR spectra were obtained by using JASCO FT-IR ATR 6300 spectrometer at room temperature in the range of 4000–400 cm  1. The samples were placed in a liquid cell between two windows (CaF2). Mirror velocity is 0.3 cms  1 and numbers of co-added scans are 4 then total collection times is less than 2 min. Absorption spectra were recorded using Perkin-Elmer Lamda 25 UV–visible spectrophotometer. The fluorescence quenching measurements were carried out with Perkin-Elmer LS 45 spectrofluorometer. The excitation and emission slit width (each 5 nm) and scan rate (200 nm min  1) were maintained constant for all the measurements. For fluorescence studies, more diluted solutions were used to avoid spectral distortions due to the inner-filter effect and emission reabsorption. Transient absorption experiments were carried out using nanosecond laser flash photolysis (Applied Photophysics, UK). The third harmonic (355 nm) of a Q-switched Nd:YAG laser (Quanta-Ray, LAB 150, Spectra Physics, USA) with 8 ns pulse width and 150 mJ pulse energy was used to excite the samples. The transients were probed using a 150 W pulsed xenon lamp, a Czerny-Turner monochromator, and Hamamatsu R-928 photomultiplier tube as detector. The transient signals were captured with an Agilent infiniium digital storage oscilloscope, and the data were transferred to the computer for further analysis. For laser flash photolysis studies, samples were purged with argon gas for about 45 min prior to the laser irradiation. Time-resolved fluorescence decays were obtained by the time correlated singlephoton counting (TCSPC) technique exciting the sample at 400 nm. Data analysis was carried out by the software provided by IBH

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TGA CdTe_Abs MPA CdTe_Abs TGA CdTe_Emi MPA CdTe_Emi

1.0 0.8

10

Log Counts

Normalized Intensity

1.2

0.6 0.4

10

IRF TGA-CdTe MPA-CdTe

10

0.2 10

0.0 450

500

550

600

650

700

0

750

Wavelength (nm) Fig. 2. Normalized absorption and emission (λexi ¼ 400 nm) spectra of TGA and MPA capped CdTe QDs.

(DAS-6), which is based on deconvolution techniques using nonlinear least-squares method and the quality of the fit is ascertained with the value of χ2 o1.2.

3. Results and discussion

ð1Þ

30

40

50

Fig. 3. Nanosecond fluorescence decay of TGA–CdTe and MPA–CdTe QDs are monitored at 575 nm and 583 nm respectively. Table 2 Time resolved fluorescence data. Samples

τ1 (ns)

A1 (%)

τ2 (ns)

A2 (%)

τ3 (ns)

A3 (%)

o τ 4 (ns)

TGA–CdTe MPA–CdTe a MOA–CdTe

1.4 2.3 1.7

15 13 44

14.8 13.5 9.4

85 87 25

– – 0.2

– – 31

12.8 12.1 3.18

a

3.1.2. Absorption spectra of QDs UV–visible absorption spectra of TGA and MPA capped CdTe QDs shows a band at 522 nm and 536 nm respectively [Fig. 2], which are assigned to the first excitonic transition (1se–1sh). A well resolved absorption maximum is observed which indicates a narrow size distribution of capped CdTe QDs. MPA capped QDs are bathochromically shifted while comparing with the TGA capped QDs, this is due to extra methylene group present in the MPA which can passivate the CdTe surface more than that of TGA. These results suggest that the diameter of prepared QDs depends on the capping agents. Therefore, the particle size (D) of prepared QDs is estimated by using Eq. (1). Further, using D value one can determine the concentration of prepared CdTe QDs from Eq. (2) [29]:     3 2 D ¼ 9:8127
10–7 λ  1:7147
10–3 λ þ ð1:0064Þλ  ð194:84Þ

20

Time (ns)

3.1. Characterization of QDs

Data obtained by fitting of fluorescence decay [Fig. S7].

0.30 0.25

Absorbance

3.1.1. FTIR spectra of QDs There are reports found for the binding of thiol and carboxyl groups on the surface of QDs [16,17,27,28]. So in order to check the presence of functional groups and binding nature of QDs with capping agents, we have done the FT-IR analysis (Fig. 1). The spectra of TGA and MPA have a characteristic absorption band ranging from 2550 cm  1–2700 cm  1, which belongs to –SH stretching vibration. However, this band is completely disappeared while capping on QDs surface, which clearly indicates that the formation of complex between cadmium and –SH group, which contributes to the passivation of CdTe surface. Moreover, in all the cases, the carbonyl (C¼O) stretching vibration is observed in the region of 1600–1700 cm  1, which clearly explains the carboxyl group of capping agent is free. This is an additional support when both carboxyl and thiol groups were present in the capping ligand then the bonding will occur preferably between the Cd and thiol group of the capping agent and not with the carboxyl group. So the FT-IR results clearly enlighten that CdTe QDs surface is covered by capping agents.

10

TGA_CdTe TGA_CdTe + 0.5 mM MV TGA_CdTe + 2.5 mM MV

0.20 0.15 0.10 0.05 0.00 400

450

500

550

600

650

700

Wavelength (nm) Fig. 4. Absorption study of a) TGA–CdTe QDs and b) MPA–CdTe QDs in presence of 0.5 and 2.5 mM MV.

A ¼ ϵbc; ϵ ¼ 10043ðDÞ2:12

ð2Þ

where D (nm) is the particle size of QDs and λ (nm) is the wavelength of first excitonic absorption peak of corresponding QDs (522 nm and 536 nm for TGA and MPA capped CdTe respectively). Based on Eq. (2), diameter of the prepared QDs has been calculated and it is found to be 2.84 nm and 3.07 nm for TGA and MPA capped QDs respectively. These results suggest that there is a significant effect of capping agent on the particle size. In Eq. (2), A is the absorbance at the peak position of the first excitonic absorption peak, c (M) is the molar concentration of capped CdTe QDs, b (cm) is the path length of the radiation beam used (1 cm, fixed) for recording the absorption spectrum, ε (M  1cm  1) is the extinction coefficient per mole of CdTe QDs at the first excitonic absorption band. By using particle size (D) obtained from Eq. (2), the extinction coefficient (ε) values are calculated and it

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25

Table 3 Fluorescence quenching data.

20

Intensity

359

0.0 mM MV 0.5 mM MV 1.0 mM MV 1.5 mM MV 2.0 mM MV 2.5 mM MV

15

10

Samples

a

TGA–CdTe MPA–CdTe MOA–CdTe

1.03
103 M  1 2.01
102 M  1 2.75
101 M  1

KSV

b

c

8.04
1010 M  1 s  1 1.66
1010 M  1 s  1 d 8.64
109 M  1 s  1

 0.88  0.68  0.96e

kq

ΔGet (eV)

a

Stern–Volmer constant. Quenching rate constant. c Free energy change. d Calculated using the lifetime of MOA–CdTe (3.18 ns). e Calculated using potential of MOA–CdTe (2.62 eV). b

5 whereas, the longer components are quenched fluorescence by a non-radiative processes due to deep traps, respectively.

0 500

550

600

650

700

750

3.2. Interaction of capped CdTe QDs with methyl viologen

Wavelength (nm) Fig. 5. Fluorescence quenching of TGA–CdTe QDs in presence of 0-2.5 mM of methyl viologen.

4

TGA-CdTe MPA-CdTe

I0 /I

3

2

1

0 0.0

0.5

1.0

1.5

2.0

2.5

3.0

-3

[MV] x 10 M Fig. 6. Comparison of Stern–Volmer plot for the fluorescence quenching of capped QDs by MV.

is found to be 9.5
104 M  1 cm  1 and 1.4
104 M  1 cm  1 for TGA and MPA capped QDs respectively. 3.1.3. Fluorescence spectra of QDs Fluorescence spectra of QDs are obtained at the excitation of 400 nm. Fluorescence of QDs is arises from the combination of band edge emission and electron–hole recombination due to the presence of surface defects and trap states on QDs [30]. As like absorption spectra, similar bathochromic shift is observed for MPA capped CdTe QDs [Fig. 2]. In addition, emission peaks are very sharp and narrow, which indicated that they had nearly monodisperse and uniformity in particle size [31], as already evidenced by TEM measurements [Fig. S2]. Further, time correlated single photon counting technique has been employed in order to understand the role of capping agents on the excited state lifetime. QDs are excited at 400 nm and decays are monitored at their emission maxima. The obtained fluorescence decays are presented in Fig. 3. Both decays are fitted with bi-exponential function [I(t)¼ a1exp(  t/τ1) þa2exp(  t/τ2)] and the fitted values are shown in Table 2. Both QDs are showing short and long lived components. According to the literature [32], the shorter component assigned to radiative (e–h) recombination processes due to surface defects

The reason for choosing methyl viologen is due to its electron accepting nature and it is having similar reduction potential value ( 0.44 V) of TiO2 conduction band. So, if the electron transfer from CdTe quantum dots (QDs) to MV is favorable, which can be used to relate such electron transfer process to the actual behavior of CdTe with TiO2, a promising material for solar cell devices. 3.2.1. UV–visible absorption studies Before studying the excited state interactions between QDs and methyl viologen (MV), it is necessary to know the type of interaction in the ground state. For instance, if there is an interaction in the ground state (i.e.) ground state complex, one could expect static quenching in the fluorescence measurements. If there is no interaction in the ground state, which may leads dynamic quenching. In that way, we have carried out optical absorption measurements of TGA capped CdTe QDs with low and high concentration of MV in water medium, which is shown in Fig. 4. The absorption spectra of QDs at both concentrations of MV exhibit the similar primary spectral features i.e. there is no increase or decrease in optical density of the absorption maxima, which clearly illustrate that there is no ground state association between the capped QDs and MV. MPA capped CdTe QDs also gave similar features which is shown in Fig. S4. Based on these results, one may expect dynamic quenching from the fluorescence measurements. 3.2.2. Fluorescence studies Fig. 5 shows the steady state fluorescence measurement of TGA capped CdTe QDs with increasing concentration of MV in water. From the figure it is clearly evident that the decrease in emission intensity of capped QDs in the presence of MV at the excitation wavelength of 400 nm. Similar type of quenching is observed for MPA capped CdTe QDs, which is shown in Fig. S5. The quenching of capped CdTe QDs fluorescence by MV can be analyzed by Stern– Volmer Eq. (3). I 0 =I ¼ 1 þ K SV ½Q 

ð3Þ

where I0 and I are the fluorescence intensities of QDs in the absence and presence of MV, respectively. KSV is the Stern–Volmer constant and [Q] is the concentration of MV. According to Eq. (3), the Stern–Volmer constant (KSV) is obtained from the slope of the linear plot (Fig. 6). From the KSV, one can calculate the quenching rate constant (kq ¼KSV/τ) by using the lifetime (τ) of QDs. The values of KSV and kq were listed in Table 3. The obtained kq values are in the order of 1010 which is close to the diffusion-controlled rate constant limit, which clearly suggest that, dynamic quenching plays a major role in the fluorescence quenching of CdTe QDs by MV. Here, we assigned the quenching is due to electron transfer, since energy transfer is not possible due to mismatching energy

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0.04

3

0.03

TGA-CdTe/MV TGA-CdTe

IRF TGA-CdTe TGA-CdTe + 2.5 mM MV

2

10

ΔA

Log Counts

10

0.02

0.01

1

10

0.00

0

10

0

10

20

30

40

50

500

550

600

650

700

750

800

Wavelength (nm)

Time (ns)

Fig. 8. Transient absorption spectra of TGA–CdTe and with MV in water after excitation at 355 nm.

levels between CdTe and MV. It should be noted that, MV quenches the TGA capped CdTe efficiently than the MPA capped CdTe. This is due to additional methylene group present in the MPA blocks the quenching process and hence electron transfer from MPA capped CdTe to MV is deliberately slow.

3

Log Counts

10

IRF MPA-CdTe MPA-CdTe + 2.5 mM MV

2

10

1

10

0

10

0

10

20

30

40

50

Time (ns)

MPA-CdTe TGA-CdTe

Standard Deviation

MPA-CdTe+5mM MV TGA-CdTe+5mM MV

0

10

20

30

40

50

Time (ns) Fig. 7. Fluorescence decay profile for the quenching of a) TGA–CdTe QDs and b) MPA–CdTe QDs in presence 2.5 mM methyl viologen (c) residuals.

Table 4 Time resolved fluorescence quenching data. Samples TGA–CdTe MPA–CdTe

τ1 (ns) 0.5 1.1

A1 (%) 31 22

τ2 (ns) 3.9 7.2

3.2.3. Time resolved fluorescence studies The steady state fluorescence quenching measurement alone is not enough to confirm the observed dynamic type of quenching. In principle, fluorescence lifetime measurement is the most definitive method to distinguish the static and dynamic quenching process [33]. In this context, we have done fluorescence lifetime quenching measurements using TCSPC technique. Fig. 7 shows the fluorescence decay of TGA–CdTe QDs in the presence of highest concentration of MV (2.5 mM) in water. The fluorescence decay of QDs exhibited biexponential decay not only in dilute solutions and also in the presence of MV but the lifetime of QDs is much decreased in the presence of MV. The fitted values are given in Table 4. Similar behavior is observed for MPA–CdTe QDs with MV, which confirms that the dynamic type of quenching is operated in this system. In addition, the decrease in the fluorescence lifetime of capped QDs is directly related to the electron transfer [34]. Hence, the rate of electron transfer is calculated from the average lifetime of the quenched CdTe (Table 4). The obtained rate of electron transfer for TGA capped CdTe is much faster than the MPA capped CdTe. This result enlightens that there is a significant role of capping agent on the electron transfer between QDs and MV. 3.2.4. Calculation of free energy changes (ΔGet) for the electron transfer reactions The thermodynamics driving force (ΔGet) of electron transfer reactions can also be verified according to the well known Rehm–Weller expression [35]. The energy balance of a photoinduced electron transfer reaction is given by the Rehm–Weller equation which combines the oxidation potential (Eoxi) of the electron donor, the reduction potential (Ered) of the electron acceptor, an electrostatic correction term C and the excited state energy of the sensitizer. Therefore, Rehm– Weller Eq. (4) remains valid for measurements of fluorescence quenching through electron transfer.

ΔGet ¼ E1=2 oxi E1=2 red Eð0;0Þ þ C A2 (%) 69 78

oτ 4 (ns) 2.8 5.8

ket (s

1

) 8

3.5
10 1.7
108

E1/2oxi

ð4Þ

where, is oxidation potential of donor (0.83 V, 1.0 V for TGA and MPA capped CdTe QDs respectively, obtained from cyclic voltammetry measurements) E1/2red is reduction potential [36] of acceptor ( 0.44 eV vs NHE), E(0,0) is the excited state energy of CdTe (E0,0 value is estimated

M.A. Jhonsi et al. / Journal of Luminescence 178 (2016) 356–361

from the intersection of the absorption and emission spectra) and C is the Coulombic term. Since the solvent used is polar, the coulombic term in the above expression is neglected [37]. The calculated ΔGet values (Table 3) are highly exothermic [38] and hence, the electron transfer reaction of CdTe is thermodynamically favorable. A high exothermic of ΔGet are the incontestable proof for the electron-transfer mechanism.

Acknowledgments

3.2.5. Transient absorption measurements As like TCSPC technique, time-resolved absorption (TA) spectroscopy is also very useful technique to investigate the electron transfer process between donor and acceptor systems. If electron transfer from the excited state of donor to the acceptor occurs, subsequently the radical cation of donor or radical anion of the acceptor can be detected using this technique. Based on this strategy, here we have measured nanosecond -transient absorption spectra of CdTe QDs with 2.5 mM of MV after excitation at 355 nm under argon saturated condition which is shown in Fig. 8. In the presence of highest concentration of MV, TA spectrum exhibits a broad absorption in the range of 550–650 nm. It should be note here, no transient signal observed for CdTe QDs alone. Hence, the observed transient absorption peak (420–500 nm) is assigned to the radical cation (MV þ ) formed from MV2 þ [39–41]. The observed radical cation confirms that the fluorescence quenching of CdTe QDs by MV is attributed to electron transfer from QDs to MV.

Appendix A. Supplementary material

3.3. Effect of spacer length in capping agent From the above results, it is clear that there is a significant role on the electron transfer between CdTe and MV by capping agents (TGA and MPA). Further, we have the curiosity that what happened on the quenching order if we increase more methylene groups in the capping agents. In order to check the role of capping agent on the electron transfer, we have employed the mercaptooctanoic acid (MOA) capped CdTe quantum dots and the steady state absorption and fluorescence quenching measurements are meticulously performed. The detailed synthetic procedure of MOA–CdTe and characterization will be explored in near future. Now in the present work absorption spectral studies illustrate that there is no significant change in the absorption of MOA capped CdTe in presence of MV. So as like TGA and MPA capped CdTe, the MOA capped CdTe also not involve in the complexation with MV (Fig. S6a). Moreover, MPA capped CdTe shows a weak quenching from the fluorescence measurement (Fig. S6b). The calculated Stern–Volmer constant from the linear Stern–Volmer plot (Fig. S6c) is 2.75
101 M  1 which is two order of magnitude lesser than the other capping agents (Table 3). This result clearly point out the more numbers of methylene groups present in the capping agent blocks the quenching process and hence inferior electron transfer is observed between MOA capped CdTe and MV.

4. Conclusions In this work, we have demonstrated the fluorescence quenching of CdTe QDs by MV. Quenching of fluorescence is due to electron transfer and which is capping agent dependent. The obtained negative ΔGet value suggests that the electron transfer reaction of CdTe QDs with MV is thermodynamically favorable. Using transient absorption spectral studies, direct evidence for the electron transfer quenching was obtained. From this work, we gave an idea that longer the chain length of capping agent will slow down the electron transfer property of CdTe QDs. This fundamental results, definitely will give potential contribution towards the understanding of the excited state behavior of quantum dots with various electron donors as well as acceptors.

361

M.A.J. and S.T. thanks to DST-SERB, India for the project and fellowship respectively [Ref. No. CS-125/2013, Dt. 30/06/2014]. A.K. thanks to Department of Science and Technology, India for DSTINSPIRE Faculty Award.

Supplementary data associated with this article can be found in the online version at http://dx.doi.org/10.1016/j.jlumin.2016.06.022.

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