Fluorescence Dynamics and Stochastic Model for Electronic

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Fluorescence Dynamics and Stochastic Model for Electronic Interaction of Graphene Oxide with CdTe QD in Graphene Oxide-CdTe QD Composite Simanta Kundu, Suparna Sadhu, Rajesh Bera, Bipattaran Pramanik, and Amitava Patra J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/jp405712p • Publication Date (Web): 03 Sep 2013 Downloaded from http://pubs.acs.org on September 6, 2013

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The Journal of Physical Chemistry

Fluorescence Dynamics and Stochastic Model for Electronic Interaction of Graphene Oxide with CdTe QD in Graphene OxideCdTe QD Composite**

Simanta Kundu, Suparna Sadhu, Rajesh Bera, Bipattaran Paramanik 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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Abstract The development of graphene oxide (GO) /semiconductor quantum dots (QDs) hybrid composite remains a frontier area of research to design optoelectronic, photovoltaic and light harvesting devices based on electron transfer process. Therefore, the examination of electron transfer process from QDs to GO as a function of the number of sites of QD and the mean fractional surface coverage of QD by GO sheet with changing the size of QD and concentration of GO is an important issue to manipulate the performance of devices. Here, we have assembled graphene oxide-CdTe QD composite by the attachment of positively charged cysteamine capped CdTe QDs with negatively charged GO. The structural changes due to electronic interaction of graphene oxide with QDs have been evaluated using Raman spectroscopy. The shifting of G - band and increase of ID/IG intensity ratio reveal the electron transfer from excited QDs to GO. The fluorescence dynamics of QD has been investigated by time resolved fluorescence spectroscopy and the electron transfer rate (2.24 x 108 s-1to 1.18 x 108 s-1) is found to be decreased with increasing the size of QDs. We analyze the decays of fluorescence by assuming a binomial distribution of number of available sites of QD and the mean fractional surface coverage of QD by GO sheet which control the quenching process. Analysis suggests that the average number of available sites (152 to 396) increases, the mean fractional surface coverage and the total quenching rate (1.3 x 108 s-1 to 0.18 x 108 s-1) are decreased with increasing the size of QD. It is noteworthy that ~6 fold increase in the photocurrent is found in this composite device under light illumination. Such graphene oxide-QD functional materials open up new possibilities in solar energy conversion, photovoltaic and various potential applications.

Keywords: Graphene oxide. quantum dot. quenching. fluorescence dynamics. binomial distribution. photocurrent.


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Introduction: Graphene-semiconductor composite nanostructures have been recently emerged as a new class of functional materials because of their potential applications i.e. solar energy conversion, optoelectronic devices, catalysis and sensing.1,2,3,4,5

It is evident that graphene is a two

dimensional atomically thin sheet of sp2 hybridized carbon which has unique physical and electronic properties. Recently, oxygenated form graphene oxide (GO) has provided a new avenue towards the graphene-based composites due to its good dispersibility in both polar and nonpolar solvents.6 The recent emergence of the electronic interaction of graphene or GO with electron donor/acceptor molecules or QD has stimulated a lot of interest because that causes significant modification in the electronic structures and properties of graphene. The drastic fluorescence quenching of aromatic molecules by graphene due to electron transfer has been confirmed in recent studies.7,8 Recently, Majima and co workers9 have studied the interfacial electron transfer dynamics by single-molecule fluorescence spectroscopy in a GO-dye hybrid system. However, in case of QD-graphene composite, the PL quenching of QD is explained either by electron /energy transfer or charge transfer.1,

10,11,12

The energy transfer from QD to

graphene has been reported for graphene/QD composite system.13 Recently, the energy transfer from CdSe/ZnS QD to graphene by single molecular spectroscopy has been reported by Brus et al.14 The existence of charge-transfer as well as electronic and magnetic interactions between graphene and adsorbed semiconducting oxide or magnetic nanoparticles has been reported.15 Kamat et al.11 have demonstrated that both electron and energy transfer occur from photoexcited CdSe QD to graphene oxide. Ultrafast electron transfer process has been reported in grapheneCdS composite system.10 Again, the charge separation of graphene-CdTe composite materials drastically improves as compared to that of CdTe QD has been confirmed by using ultrafast spectroscopy.16 It is also reported that the large photocurrent with fast response time due to efficient electron transfer from QD to graphene has been reported Kwon et al.17 Theoretical foundation for understanding the interfacial electron-hole separation in QD/graphene system has been reported.18 Again, photovoltaic

device based on graphene-CdSe composite has been

demonstrated.19 Thus, significant progress has been made on the electronic interaction of graphene/QD composite but the mechanism of the fluorescence quenching in the composite systems has not been fully understood. So far there is no knowledge, how the number of available sites of QD and the mean fractional surface coverage of QD by GO sheet in GO/QD 3 ACS Paragon Plus Environment


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composite influence on the PL quenching of QD. The photoluminescence quenching process of QD on the surface of GO should depend on number of available sites of QD on the surface of GO and the mean fractional surface coverage of QD by GO sheet and these two parameters again depend on the size of QD and concentration of GO. To our knowledge, these issues are not well addressed so far. Weiss et al.20,21 have done extensive work on exciton dissociation with QDorganic molecules system and have developed model for determining the rate constant of charge separation using binomial distribution of the number of charge acceptors adsorbed per QD within an ensemble of QDs. Recently, Pansu et al.22 have described the quenching dynamics of CdSe QD by using a binomial distribution of quenchers. We have recently developed model for determining energy transfer rate and carrier relaxation rates of QD using Poisson distribution of the number of quenchers per QD within an ensemble of QDs.23,24 Herein, CdTe QDs are electrostatically linked to the GO sheet through a ligand therefore a small portion of QD is available for the interaction on GO sheet. Thus, the number of available sites on QD surface for the interaction with GO sheet is small which varies with changing the size of QD. In case of QDorganic molecule system, heterogeneous distribution of quenchers on the surface of QD is seen because the zero dimensionality of quencher molecule. However, in case of QD-GO composites, GO sheet acts as 2D quencher. Thus, we believe the number of sites on QD interacts with GO follow the binomial distribution instead of Poisson distribution. Herein, we have attached positively charged cysteamine capped CdTe QD with negatively charged GO sheet and the structural changes are investigated by Raman spectroscopy. The quenching process has been investigated by steady state and time resolved fluorescence spectroscopy. We have analyzed the decays of the fluorescence of QD in GO-CdTe QD composite by using a proposed stochastic model to estimate the number of sites available and the fractional surface coverage of QD by GO sheet. The photoconductivity properties of GO-QD composites with visible illumination have been studied to understand transport properties of GOQD composite. Such GO-QD composite may have great potential for optoelectronics, photovoltaic, light harvesting and sensing applications. Experimental Section: Preparation of graphene oxide: Graphene oxide (GO) was obtained using the Hummer’s method.25 Briefly, 2 gm of graphite powder (lobachemie, 60 mesh) were mixed with 1 gm of NaNO3 (Merck) and 50 ml of 4 ACS Paragon Plus Environment

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H2SO4 (Merck) and the mixture was cooled down to 0oC. 6 gm of KMnO4 was added slowly maintaining the temperature below 5oC. The cooling bath was removed then and stirred at room temperature for half an hour. Then, 100 ml of water was added and the temperature was increased to 90oC and kept it for one hour. The mixture was diluted with another 300 ml of water and then treated with 30% H2O2 (Merck) solution until the effervescence was ceased. The solution was left undisturbed for overnight and then the supernatant was descanted. The brown precipitate was centrifuged and washed with 10% HCl solution for several times and finally with water. The Product was dispersed in water by sonication for 45 min and then large particles were centrifuged out at 3000 rpm. The remaining particles were centrifuged at 12000 rpm and collected and dried at vacuum. 5 mg of this powder was dispersed in 10 ml of de-ionized water by sonication, giving rise to 0.5 mg/ml brown dispersion of GO which was stable for months. The XRD pattern and UV absorption spectrum of GO are given in the supporting information. Synthesis of water soluble cysteamine capped CdTe QD: Colloid of CdTe QD was synthesized using the reaction between CdCl2 and NaHTe solution with some modification.26,27 At first, NaHTe was prepared by adding 0.53 mM of NaBH4 (Merck) and 0.18 mM of Te powder (Aldrich) in a round bottom flask, containing 2 mL of MilliQ water in inert atmosphere. On the other hand 0.50 mM of CdCl2.2.5H2O (Aldrich) and 1mM of cysteamine (Aldrich) were dissolved in 70 mL of MilliQ water. The pH of the cysteamine solution was maintained to 5.5 by adding required amount of HCl. The mixture was bubbled by N2 for 45 minutes to make O2 free. Then 0.06 mM of NaHTe was immediately injected into the mixture under vigorous stirring for 30 mins followed by refluxing at 110° C in aerial atmosphere. A solution of different size was taken out from the reaction mixture with the growth time of 15, 30 and 50 min. These nanocrystals are stable for more than three months when stored at 10°C. CdTe QD - GO composite preparation The desired amounts of GO stock dispersion were added to the diluted QD solution with typical GO concentrations between 5 – 25 µgm/ml. The final concentrations of cysteamine capped CdTe QD in the stock solutions was 1.2 µM for all the solutions. Three different sets were prepared for the three QD solutions. The positively charged cysteamine capped CdTe QDs are attached with negatively charged GO surface. All the solutions were kept for 5 hours at room temperature for stabilization before the characterization. 5 ACS Paragon Plus Environment


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Characterization: Morphological Studies was performed in an AFM, VEECO, dicp-II and the detailed structural features were investigated by a high-resolution transmission electron microscopy (HRTEM; JEOL 2100). Zeta potential was measured in Malveron Zetasizer instrument. Raman spectra were recorded in solution via 900 scattering by exciting the sample with an Ar+ ion laser source of 48 mW power at the sample with T64000 model made of Horiba Jobin Yvon. Raman spectra of the samples were recorded at room temperature in solution phase. The light irradiation dependent study was done after different irradiation times with a white light source compiled with a < 420 nm cut off filter. Room temperature optical absorption spectra were taken by a UV-vis spectrophotometer (SHIMADZU). Room temperature photoluminescence studies were carried out using a Fluoro Max – P (Horiba Jobin Yvon) luminescence spectrometer. For time correlated single photon counting (TCSPC) measurement, the samples were excited at 375 nm by picoseconds NANOLED IBH-375. The fluorescence decays were collected on Hamamatsu MCP photomultiplier. The following expression was used to analyze the experimental time resolved fluorescence decays, P(t) n

P (t ) = b + ∑ α i exp ( − t / τ i )

(1)

i

Here, n is the number 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 (n), the average lifetime,〈τ〉, was calculated from equation 2. 〈τ 〉 =

n

∑

i =1

n

α i τ i2 / ∑ α iτ i

(2)

i =1

The device for the photocurrent measurement was prepared on ITO coated glass substrate. First a wetting layer (~ 50 nm) of PEDOT: PSS from 1:1 ethanol mixture was spin coated at 2000 rpm and heated in a vacuum at 100 0C. Then the concentrated GO and QD mixture was spin coated at 1500 rpm from 1:1 water and ethanol mixture followed by heating at 100 0C for 1 hr in vacuum. And the procedure was repeated for 5 times to ~ 300 nm thickness of the film. Then an electrode of Al of ~ 100 nm thickness was deposited using a vacuum depositor. The effective area for current accumulation is ~6 mm2. The device was put into a chamber, and an electrometer 6 ACS Paragon Plus Environment

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(Keithley 6517B) was connected between the two electrodes. The I-V measurement was performed with the electrometer at room temperature. Photocurrent measurement was performed with a 150 W Newport-Stratfort Solar Simulator model 76500 using white light under 1 sun (AM 1.5 G) illumination at 100 mW cm-2.

Results and Discussion: Structural Study: The GO surfaces are fully decorated with quantum dots (QDs) because CdTe QDs are positively charged and GO surface is negatively charged. The negative charge of GO surface is due to presence of carboxylic acid groups which is confirmed by zeta potential value. The zeta potential value of GO is -20.2 mV, indicating high negative charged surface of GO. On the other hand, the zeta potential value of cysteamine capped CdTe QDs is +28.6 mV, indicating the positive charge of QDs. Therefore, the positively charged QDs are preferentially attached with negatively charged GO surface through electrostatic interaction. It is clearly seen from TEM image (Fig. 1a) that the dimension of GO flakes is less than 1µm and the high resolution TEM image (figure 1b) confirms the GO flakes are very thin. The line analysis of the AFM image (fig. 1c) of GO confirms that the thickness of as prepared GO flakes is around 1.15 nm, indicating the formation of mono layer of GO which is in good agreement with the previous reported values for the monolayer GO.28 The measured sizes of the synthesized QDs are found to be 2.4 (±0.2) nm, 3.5 (±0.2) nm and 4.8 (±0.3) nm, respectively from TEM measurement (supporting information fig. S1). The attachment of 2.4 nm QD on GO surface has been confirmed by TEM and AFM images (Figure 2). The TEM images of the other two hybrid solution with 3.5 nm QD and 4.8 nm QD are given in the supporting information (fig. S2). The structural change of GO due to attachment of QDs on GO surface has been investigated by Raman spectroscopy. Figure 3 shows the Raman spectra of as prepared GO and GO-QDs composite solutions excited with a 514.5 nm Ar+ ion laser. A weak band at 1352 cm-1 (D -band) and a strong band at 1628 cm-1 (G-band) are observed for as prepared GO sample. The ID/IG intensity ratio of as prepared GO solution is 0.28 which is a hallmark of structural change of GO network. The D-band is due to longitudinal plane phonon vibration or k-point phonons of A1g symmetry and the G-band is due to zone centre phonons of E2g symmetry.29 It is noted that the G band of as prepared GO is shifted to higher energy as compare with the G band of the 7 ACS Paragon Plus Environment


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graphite,30, 31, 32 indicating the extensive oxidation of the graphitic network. In the present study, the G band shifts to lower frequency after attachment with QDs and the maximum red shifting (13 cm-1) of the G band is observed for GO- QD (3.5 nm) hybrid system. This red shifting of the G band indicates the softening of the phonons which occurs due to electron enrichment in the GO network. The ID/IG intensity ratio increases after attachment with the QD. The ID/IG intensity ratio of the prepared GO solution is 0.28, whereas the ratio increases to 0.43, 0.50 and 0.45 after attaching of 2.4 nm QD, 3.5 nm QD and 4.8 nm QD with GO sheet, respectively. The increase in ratio (ID/IG) after attaching the QD indicates the modification of GO network due to defect centers. It reveals that the structural change of GO sheet occurs due to attachment of QDs on the GO surface. Steady State and Time - Resolved Fluorescence Study The absorption and photoluminescence spectra of three different QDs aqueous solution are shown in fig. 4. The absorption peaks are at 505 nm, 514 nm and 526 nm for 2.4 nm QD, 3.5 nm QD and 4.8 nm QD, respectively and the corresponding emission peaks are at 526 nm, 536 nm and 556 nm, respectively at the excitation of 375 nm. The inset of fig. 4b shows the color of the solutions under normal light and under UV- light. It is interesting to note that PL quenching of QDs is observed in GO-CdTe QD composites. A systematic study on quenching phenomenon of QDs with changing the concentration of GO has been investigated. Figure 5 represents the PL spectra of the pure 3.5 nm QD solution and with gradual increase of GO concentration at 375 nm excitation wavelength. The fluorescence quenching of the QD along with blue shifting of the PL band is observed with increasing of GO concentration. The blue shifting of the emission maxima is due to the heterogeneity in the composite solution which is confirmed by a study of the excitation spectra as a function of the emission wavelength. The photoluminescence quenching of 3.5 nm QD varies from 23% to 75% with changing the concentration of GO from 5 to 25 µgm/ml. The photoluminescence quenching of 2.4 nm QD varies from 30% to 72% with changing the GO concentration from 5 to 25 µgm/ml (supporting information fig. S3A). For 4.8 nm QD, it varies from 19% to 65% with changing the concentration of GO from 5 to 25 µgm/ml (supporting information fig. S3B). It is evident that PL quenching occurs either by energy transfer or electron transfer process, i.e. due to nonradiative relaxation of exciton. To confirm the energy/electron transfer process from QD to GO, time resolved fluorescence study is performed because decay time measurements are more sensitive than PL 8 ACS Paragon Plus Environment

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quenching efficiencies where errors come from the fluctuations in the lamp intensity. The interaction of QDs with GO sheet can be monitored by measuring the exciton lifetime, which is measured by time-correlated single-photon counting (TCSPC). We measured pulsed excitation (375 nm) to measure the decay times of these QDs at their maximum PL peak. Figure 6 shows the time-resolved fluorescence decay curves of 3.5 nm QD without and with changing the concentration of GO. The decay profiles are well fitted with tri-exponentials. The shortening of average decay time of QD without GO is observed in presence of GO. The values are 4.03 ns, 2.85 ns, 2.26 ns and 1.23 ns for without GO, and after addition of 5, 15 and 25 µgm/ml GO, respectively. It is seen that the decay time of the 3.5 nm QD decreases gradually with the increasing concentration of GO which is in agreement with the PL quenching of QD. It is to be noted that the faster component (τ1) decreases from 0.70 ns to 0.18 ns for the gradual increase of GO concentration from 5-25 µgm/ml and other two components are varied from 5.50 ns to 3.92 ns and 25.20 ns to 21.16 ns. It is seen that the faster component of the decay time decreases rapidly with increasing GO concentration, indicating the electron transfer process. The decay time parameters are given in Table 1. All decay curves and the decay time values for 2.4 nm QD and 4.8 nm QD are given in the supporting information (fig. S4, Table S1 & S2). The decay time of 2.4 nm QD decreases from 2.72 ns to 1.17 ns and the decay time decreases from 7.37 ns to 2.14 ns for 4.8 nm QD with increasing the concentration of GO. Therefore, it is clear from this study that fluorescence decay dynamics of QDs are modified by GO sheet due to their electronic interaction. Thus, the size of QDs and the concentration of GO are played an important role. The PL quenching and the shortening of decay time definitely indicate the nonradiative quenching process of QD due to electronic interaction with GO. This nonradiative relaxation process mainly occurs either via electron transfer or energy transfer processes. The possibility of the energy transfer is less, because there is no spectral overlap between emission spectrum of the QD and absorption spectrum of the GO. Hence, the electron transfer mechanism plays a role in the nonradiative relaxation of the QDs in GO-QD composite. The change of nonradiative rate of excited state deactivation can be calculated by the following equation: KNRD = 1/(τ (QD-GO)) – 1/(τQD)

(3)

Where τQD is the decay time of the pure QD and τGO-QD is the decay time of the QDs in presence of 15 µgm/ml concentration of GO. The change of nonradiative relaxation rate is found to be 2.24 x 108 s-1, 1.94 x 108 s-1 and 1.18 x 108 s-1 for 2.4 nm QD, 3.5 nm QD and 4.8 nm QD, 9 ACS Paragon Plus Environment


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respectively for 15 µgm/ml concentration of GO which is consistent with the estimated rate of electron transfer (6.7 x 108 s-1) in CdSe-GO composites, upon illumination.11 Non-radiative relaxation rate is found to decrease with increasing the size of QD which is consistent with previous results.19,33 It is well known that GO will accept electrons readily from excited state QDs because of oxidation state of carbon in the GO network. Kamat et al. have reported the reduction of GO to reduced graphene oxide (rGO) due to electron transfer from CdSe QDs to GO under photoexcitation.11 Here, Raman study has been used to understand the electron transfer process. Figure 7 shows the Raman spectra of GO-3.5 nm QD composite solution under different times of light irradiation. It is interesting to note that the red shifting of G band and increase in ID/IG intensity ratio are observed with increase of light irradiation time. The G band shifts from 1624 cm-1 to 1602 cm-1 and D band appears at 1350 cm-1 after 60 minutes of light irradiation The ID/IG intensity ratio changes from 0.50 to 0.87. The change of the ID/IG ratio and the shifting of the G band are found to be fast in first 25 minutes of light irradiation and then it becomes slow thereafter. The shifting of the G-band and increase in ID/IG intensity ratio suggest the reduction of GO due to electron transfer from photoexcited QDs to graphene oxide, which is consistent with previous results.34 The D mode in the carbon system arises due to the K-point phonons of A1g symmetry and the strength of the D-mode is proportional to the probability of finding a sixfold ring in the lattice. Thus, the development in the D peak indicates the ordering in the sp2 network. It is already discussed that the ID/IG intensity ratio increases when QDs are attached with the GO due to electronic interactions with QDs. During the light irradiation, an excited state electron transfer occurs from QDs to GO and GO reduces to reduced GO (rGO). Due to this reduction, sp2 cluster domains become more populated and ordered which increase the D-band intensity. The shifting of the G-band to lower frequencies is due to the presence of scattered olefin bonds in the network which have higher vibrational frequency.29 It is also reported that another defect D' band is located at 1620 cm-1 which is partially merged with G band.31 During the electron transfer from QDs to GO, a reduction of the GO occurs followed by formation of aromatic ring structure and elimination of defects, resulting the shifting of G-band to the lower frequencies. Thus, an electron transfer from excited QDs to GO is further confirmed by the Raman study. The plausible pathways for the reduction of GO are given below: CdTe + hν → CdTe (e + h)

(4) 10 ACS Paragon Plus Environment

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CdTe (e) + GO → CdTe + rGO

(5)

It is noted that the ID/IG intensity ratio and the shifting of G band of 3.5 nm QD is higher than 2.4 nm QD and 4.8 nm QD with same light irradiation time (supporting information fig. S5). This difference is due to different kinds of nonradiative relaxation processes for different sized QDs for their different trap states.16 Another reason is the band alignment of QDs with the Fermi level of GO. Kinetic Model: To understand the interaction between CdTe QDs and GO sheet in GO-CdTe composite system, we analyzed the decay curves by using a stochastic kinetic model. Recently, kinetic model has been proposed to understand the carrier relaxation dynamics of different shapes35 and alloy nanocrystals36 where the ensemble averaged decay curve of the excited semiconductor quantum dots is given by equation: I t = I 0 exp{− k0t − mt [1 − exp(− knr t )] − mt' [1 − exp(− knr/ t )]}

(6)

Here, k0 is the radiative recombination rate, knr is the nonradiative rate constant for each surface trap and knr_ is the rate due to the luminescence quenching process by QDs local environment (e.g. solvent or any other impurities). mt and mt´ are the average number of surface trap states participating in nonradiative relaxation process and the average number of quenchers surrounding the QDs, respectively. Herein, CdTe QDs are electrostatically linked to the GO sheet through a ligand therefore a small portion of QD is available for the interaction on GO sheet. Thus, the number of available sites on QD surface for the interaction on GO sheet is small which depends on the size of QD. Here, it is assumed that the number of sites on QD interact with GO follow the binomial distribution instead of Poisson distribution. Binomial distribution (eq. 7) describes the fraction of surface of QD participating in quenching process. N m Am =  (θ ) (1 − θ ) N −m m 

(7)

where Am is the probability of finding a QD attached to GO sheet through m sites, N is the number of available sites on the QD surface through which GO sheet is attached, θ is the mean fractional surface coverage of QD by GO sheet. Therefore, the ensemble averaged decay curve of the fluorescence quenching of excited QDs attached on GO sheet is given by eq. 8.



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N

I t = I 0 exp{− k0t − mt [1 − exp(− knr t )] − m [1 − exp(− k t )]}∑ Am exp(− mkq )t ' t

/ nr

(8)

m=0

where kq is the rate of quenching for each attached site to GO. Substituting Am in eq. 8 yields  N N  I t = I 0 exp{−k0t − mt [1 − exp(−knr t )] − mt' [1 − exp(−knr/ t )]}  ∑   (θ ) m (1 − θ ) N − m exp(−mkq t )  (9)  m=0  m  

After simplification (detailed derivation given in the supporting information), I t = I 0 exp{− k 0 t − mt [1 − exp( − k nr t )] − mt' [1 − exp( − k nr/ t )]} 1 + (exp( − k q t ) − 1)θ 

N

(10)

We have analyzed the decay curve of pure CdTe in absence of GO and determined the values of the parameters k0, mt, knr, mt´, knr_ by fitting the decays with eq. 6. Then, we have fitted the decay curves of the QDs in the presence of GO using eq. 10 by fixing k0, knr and knr_. The resulting fits are represented by red solid lines in fig. 8 and the decay fitting parameters are given in Table 2. The obtained value of the average rate of quenching by GO through each attached site (kq) is 0.136, 0.104 and 0.043 ns-1 and the average number of available sites (N) is 152, 240 and 396 for 2.4 nm QD, 3.5 nm QD and 4.8 nm QD, respectively. Thus, the number of available sites increases with increasing the size of QD due to change in surface area with size. Thus, the number of available sites of QD must have strong role on controlling the decay dynamics of QDs in GO-QDs composite. Another important parameter is the mean fractional surface coverage (θ) which is found to be increased from 1.1 x 10-3 to 3.3 x 10-3 with increasing the amount of GO from 5 µgm/ml to 25 µgm/ml for a fixed concentration of 3.5 nm QD. With increasing the concentration of GO, more numbers of QDs are attached with the GO surface and that increases of fractional area of interaction. It is seen that at a particular concentration of the GO (15 µgm/ml) and QD, the mean fractional surface coverage (θ) decreases with increasing the size of QDs, i.e. 6.4 x 10-3 for 2.4 nm QD, 2.4 x 10-3 for 3.5 nm QD and 1.0 x 10-3 for 4.8 nm QD. As the quenching time (1/kq) is not small compared to the measurement window, the last term of eq. 9 becomes (1- N.θ.kq ). This is approximated by exp (- N.θ.kq). Therefore, the product N.θ.kq is the meaningful quantity and which is the total quenching rate and depends on the number of available sites and the mean fractional surface coverage. Interesting to note that the total quenching rate (N.θ.kq) increases from 0.26 x 108 s-1 to 0.80 x 108 s-1 with increasing the amount of GO for 3.5 nm QD (Table 2). Thus, analysis reveals that the fractional surface coverage (θ) by


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GO sheet on the QD controls the quenching process of QD. All the data are given in Table 2. It is to be mentioned that that the total number of sites available for the electronic interaction in the surface of QD (N) increases and the fractional surface coverage (θ) decreases with increasing the size of QD. Therefore, the total quenching rate (N.θ.kq) decreases with increasing the size of QD, which is in consistent with the electron transfer rate. Thus, this model helps us to understand the photoluminescence quenching process of QD on the surface of GO as a function of the number of available sites of QD on the surface of GO and the mean fractional surface coverage of QD by GO sheet.

Photoconductivity Study Transport properties of GO-QD device under illumination have been investigated. Here, we measure photoconductivity properties of GO-QD hybrid device with visible illumination. Figure 9 shows the dark and photo current of the device prepared by GO- 3.5 nm QD composite system. Intrinsic graphene does not show any photocurrent under visible light. Thus, the change in photocurrent in GO-QD composites under visible illumination is due to electron transport from QD to GO. It is noteworthy that 5.9 fold increase of the photocurrent is found in the presence of light irradiation in the composite device. The increase of photocurrent is 3.9 fold and 5.1 fold for the GO-2.4 nm QD and GO- 4.8 nm QD composites (supporting information fig. S6). During light irradiation, an electron injection occurs from the excited QDs to the GO that will enhance the photocurrent. This can be explained as follows: a very thin layer of PEDOT: PSS is given on the ITO coated glass which blocks the electron flow towards ITO electrode. Then, a layer of the mixture of GO-QDs is formed with an upper Al electrode. Thus, the generated electron will be accepted rapidly by GO with the light irradiation and it will transfer to the Al electrode. On the other hand, the band alignment is such that the hole will be migrated to the ITO via PEDOT: PSS layer which generates enhanced photocurrent. This is given in the schematic diagram in the inset of fig. 9. Zhang et al. reported recently higher photon-to-electron conversion efficiency for 3.3 nm size of CdSe QD is higher than 2.7 nm and 4.1 nm CdSe QDs in the graphene-CdSe hybrid systems.19 In the present study, we observed larger photocurrent in 3.5 nm QD-GO system than 2.4 nm QD-GO and 4.8 nm QD-GO systems. This may be due to mismatch of band gap of QD’s with the Fermi gap of GO. We could not measure the band position of these water soluble CdTe QDs. Further investigation is required for detailed



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Page 14 of 31

understanding this hybrid system for improving performance and practical applicability of this device for future applications.

Conclusion: We have synthesized GO-QD composite by attaching positively charged QD with negatively changed GO and study their electronic interaction by Raman and time resolved fluorescence spectroscopy. The PL quenching of QD is mainly due to electron transfer from photoexcited QD to GO sheet and the electron transfer rate decreases with increasing the size of QD. A stochastic model has been proposed to quantify the average numbers of available site on the QD and the mean fractional surface coverage of QD by GO sheet because the total quenching rate is dependent on them. The total quenching rate is found to be decreased with increasing the size of QD. The enhancement of photocurrent in this hybrid device is found to suitable for potential applications in optoelectronic, light harvesting and solar cell systems.

Acknowledgement: DAE-SRC Investigator Award is gratefully acknowledged for financial support. SK, SS, RB and BP also thank CSIR for awarding fellowship. Prof. A. J. Pal and Mr. Sudip Kumar Saha of IACS are acknowledged for providing the photoelectrical measurement.

Supporting Information: Figure S1 TEM images of prepared CdTe QDs (a) 2.4 nm, (b) 3.5 nm and (c) 4.8nm, Figure S2 TEM images of the GO-3.5 nm QD (a) and GO-4.8 nm QD (b) composite solution, Figure S3 Emission spectra of the pure (A) 2.4 nm and (B) 4.8 nm QD pure (a) and after addition of (b) 5, (c) 10, (d) 15, (e) 20 and (f) 25 µgm/ml concentration of GO, Figure S4 Decay curves of (A) 2.4 nm and (B) 4.8 nm pure QD fluorescence (a) and with gradual increase of GO concentration (b) 5, (c) 15 and (d) 25 µgm/ml concentration of GO and the solid lines are the fitted decay curves, Table S1 Decay time calculation of 2.4 nm QD with increasing concentration of GO, Table S2 Decay time calculation of 4.8 nm QD with increasing concentration of GO, Detailed derivation of equation 10, Figure S5 Raman spectra of GO-2.4 nm QD (a), GO-3.5 nm QD (b) and GO-4.8 nm QD (c) composite solutions after 25 minute of visible light irradiation, Figure S6 (a) Dark and (b) photocurrent of GO – 2.4 nm QD (A) and GO – 4.8 nm QD composite systems. This material is available free of charge via the Internet at http://pubs.acs.org.


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** This paper is dedicated to Prof. C. N. R. Rao on his 80th birthday.


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Figure Captions: Figure 1. (a) TEM image of the GO sheets, (b) High resolution image of a single GO sheet and (c) AFM image of a single GO sheet and line analysis of the GO sheet showing the thickness of the sheet. Figure 2. (a) TEM image of the GO-2.4 nm QD composite, (b) High resolution image of the composite and (c) AFM image of the GO-2.4 nm QD composite system. Figure 3. Raman spectra of GO (a) and 2.4 nm QD (b), 4.8 nm QD (c) and 3.5 nm QD (d) GO (15 µgm/ml) composite in solutions.

Figure 4. (A) Absorption spectra and (B) emission spectra of the cysteamine capped quantum dot solutions and the inset of B shows the photographs of the solution under normal light and under uv-torch.

Figure 5. Emission spectra of the pure 3.5 nm QD (a) and after addition of (b) 5, (c) 10, (d) 15, (e) 20 and (f) 25 µgm/ml concentration of GO. Figure 6. Decay curves of 3.5 nm QD fluorescence (a) and with gradual increase of GO concentration (b) 5, (c) 15 and (d) 25 µgm/ml concentration of GO and the solid lines are the fitted decay curves by a tri-exponential decay. Figure 7. Raman spectra of 3.5 nm QD and GO (15 µgm/ml) composite with the increase of visible light irradiation time (a) 0 min, (b) 10 min, (c) 25 min, (d) 45 min and (e) 60 min. Figure 8. Decay curves of (A) 2.4 nm QD, (B) 3.5 nm QD and (C) 4.8 nm QD for pure (a) and there GO (15 µgm/ml) composites (b) fitted by the equation obtained from the model. Figure 9. (a) Dark and (b) photocurrent of GO - 3.5 nm QD composite system.



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Page 20 of 31

Table 1. Decay time parameters of 3.5 nm QD with increasing concentration of GO.

Concentration

χ2

of GO 0 µgm/ml

5 µgm/ml

15 µgm/ml

25 µgm/ml

a

1.32

1.30

1.38

1.21

τ1 a (ns)

τ2 a (ns)

τ3 a (ns)

τ

( α1 )

( α2 )

( α3 )

(ns)

0.70

5.50

23.18

4.03

(0.60)

(0.32)

(0.08)

0.45

4.82

23.10

(0.70)

(0.24)

(0.06)

0.38

4.18

21.16

(0.73)

(0.22)

(0.05)

0.18

3.92

25.20

(0.89)

(0.08)

(0.03)

±5% (error).


2.85

2.26

1.23

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Table 2. Decay fitting parameters of QDs and QD-GO composites. Sample

χ2

mt

knr (ns-1)

mt'

knr' (ns-1)

0.99 0.013

0.89

0.51

3.84

0.045

QD + 5µgm/ml GO 0.99 0.013

0.82

0.51

3.58

QD + 15µgm/ml GO 0.98 0.013

0.72

0.51

QD + 25µgm/ml GO 0.98 0.013

1.13

0.51

2.4 nm QD

K0 (ns-1)

N

θ

kq (ns-1)

N.θ.kq (ns-1)

0.045

151

2.3x10-3

0.131

0.0455

2.96

0.045

147

6.4x10-3

0.139

0.1307

2.55

0.045

158

7.8x10-3

0.137

0.169

Average (error) 3.5 nm QD

152

0.136

(±0.003) (±0.074) (±0.079) (±0.064) (±0.007) (± 7) (±4.4x10-4) (±0.025) 0.96 0.014

0.64

0.50

2.94

0.043

QD + 5µgm/ml GO 0.98 0.014

0.79

0.50

2.46

0.043

231

1.1x10-3

0.102

0.0259

QD + 15µgm/ml GO 0.99 0.014

0.76

0.50

2.18

0.043

242

2.4x10-3

0.109

0.0633

QD + 25µgm/ml GO 0.98 0.014

1.05

0.50

1.27

0.043

246

3.3x10-3

0.1

0.0803

Average (error) 4.8 nm QD

240

0.104

(±0.002) (±0.058) (±0.061) (±0.11) (±0.012) (±10) (±2.0x10-4) (±0.025) 0.99 0.009

0.49

0.47

2.6

0.034

QD + 5µgm/ml GO 0.97 0.009

0.58

0.47

3.2

0.034

388 0.21x10-3

0.04

0.0032

QD + 15µgm/ml GO 0.96 0.009

0.93

0.47

2.7

0.034

403

1x10-3

0.046

0.0185

QD +25µgm/ml GO 0.99 0.009

1.29

0.47

1.63

0.034

398

2.7x10-3

0.043

0.046

Average (error)

396

0.043

(±0.001) (±0.05) (±0.06) (±0.20) (±0.008) (±14) (±0.2x10-4) (±0.008)



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Figure 1. (a) TEM image of the GO sheets, (b) High resolution image of a single GO sheet and (c) AFM image of a single GO sheet and line analysis of the GO sheet showing the thickness of the sheet.


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Figure 2. (a) TEM image of the GO-2.4 nm QD composite, (b) High resolution image of the composite and (c) AFM image of the GO-2.4 nm QD composite system.



G

Intensity (a.u.)

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

Page 24 of 31

ID/IG

D

0.50

d

0.45

c

0.43

b

0.28

a

1150

1400

1650

1900

Wavenumber cm-1

Figure 3. Raman spectra of GO (a) and 2.4 nm QD (b), 4.8 nm QD (c) and 3.5 nm QD (d) GO (15 µgm/ml) composite in solutions.


Page 25 of 31

a b

c

3.5 nm

4.8nm

Intensity (a.u.)

2.4 nm

Absobance

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


c b

a

350

400

450

500

550

600

650

475

525

Wavelength (nm)

575

625

Wavelength (nm)

Figure 4. (A) Absorption spectra and (B) emission spectra of the cysteamine capped quantum dot solutions (a) 2.4 nm QD, (b) 3.5 nm QD and (c) 4.8 nm QD and the inset of B shows the photographs of the solution under normal light and under uv-torch.



a

b

Intensity (a.u.)

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

Page 26 of 31

c

d e f

475

525

575

625

675

Wavelength (nm) Figure 5. Emission spectra of the pure 3.5 nm QD (a) and after addition of (b) 5, (c) 10, (d) 15, (e) 20 and (f) 25 µgm/ml concentration of GO.


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


Counts (in log)

Page 27 of 31

1000

a-d

100 0

10

20

30

40

Time (ns)

Figure 6. Decay curves of 3.5 nm QD fluorescence (a) and with gradual increase of GO concentration (b) 5, (c) 15 and (d) 25 µgm/ml concentration of GO and the solid lines are the fitted decay curves by a tri-exponential decay.



G D

ID/IG Intensity (a.u.)

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

Page 28 of 31

0.87

e

0.85

d

0.80

c 0.62

b

0.50

a

1150

1400

1650

1900

-1

Wavenumber (cm )

Figure 7. Raman spectra of 3.5 nm QD and GO (15 µgm/ml) composite with the increase of visible light irradiation time (a) 0 min, (b) 10 min, (c) 25 min, (d) 45 min and (e) 60 min.


Page 29 of 31

ln(counts)

(A)

9

7

a 5

b 0

10

20

30

40

30

40

30

40

9

ln(counts)

Time (ns) (B)

7

a b

5

0

10

20

Time (ns)

(C)

9

ln(counts)

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


7

a b

5

0

10

20

Time (ns)

Figure 8. Decay curves of (A) 2.4 nm QD, (B) 3.5 nm QD and (C) 4.8 nm QD for pure (a) and there GO (15 µgm/ml) composites (b) fitted by the equation obtained from the model.



b

2

1

Current (µ A)

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

Page 30 of 31

a 0

-1

-2

-1.0

-0.5

0.0

0.5

1.0

Voltage (V)

Figure 9. (a) Dark and (b) photocurrent of GO - 3.5 nm QD composite system.


Page 31 of 31

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TOC


Fluorescence Dynamics and Stochastic Model for Electronic

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