Cds Quantum Dots: Aqueous Synthesis, Spectroscopic and Microscopic Investigation

Tremendous interest has been growing for Semiconductor quantum dots (QDs) in recent years due to their promising applications in optical devices, solar cells, biosensors, and biological imaging 1−8. Luminescent QDs possesses many advantages compared to common organic fluorophores such as high photoluminescence (PL), high quantum yield (QY), tunable emission wavelength, multiplexing capabilities, and high photostability against photobleaching. CdS, CdSe, and CdTe that are Group II−VI semiconductor QDs have extensively studied because of the simplicity with which their emission in the visible range can easily tune by changing their diameter and advances in their fabrication methods. CdS is one of the important direct-band semiconductors with a bandgap (Eg) of 2.42 eV. CdS QDs employed for photoelectric conversion in solar cells, in light-emitting diodes (LEDs) for flat-panel displays 9−12 and for biological applications 13, 15. Unique properties of QDs depend on the diameter, hence controlling size and size distribution as well as crystallinity and surface defects are crucial for tailoring its properties. The properties of such smaller nano-system extremely depend on the confinement of the quasi particles (electrons, holes, and phonons, etc.) down to radii less than 10 nm. The nonlinear optical properties and the quick response time make the quantum confined CdS system tempting for the production of various optoelectronic devices such as solar cells, laser diodes, LEDs, photomultiplier, photoconductors, and wavelength converters. The electron–phonon coupling in CdS is of prime importance because its strength increases with decreasing crystallite size. Compared to the electronic states, there is not much information available on the vibrational modes of nanocrystal CdS quantum dots (QDs) 16. There are several methods developed for the production of CdS QDs. The organometallic approach and its alternatives, which is based on the high temperature decomposition of organometallic precursors quickly injected in to the well-stirred and hot organic solvents, generally provide the best quality QDs [17-22]. However, for most of the biological applications, the QDs must be soluble in water. Therefore, ligand exchange is a way to achieve water solubility. Small thiol-based molecules generally used to substitute hydrophobic capping agents like trioctylphosphine oxide (TOPO) or long-chain amines used to control the growth process at high temperature. Moreover, some chemicals used in this route are highly toxic, pyrophoric and/or very expensive. Finally, the organometallic approach is extremely inappropriate in terms of scaling up for large-scale industrial

the success of organic synthetic routes, aqueous routes have also developed to tackle problems associated with phase transfer. The depletion in optical properties (diminishing fluorescence QY and photo stability) could be avoided by direct synthesis in an aqueous medium. In the aqueous synthesis of CdS QDs, small hydrophilic thiols such as thioglycolic acid (TGA), 3-mercaptopropionic acid (MPA), and thioglycerol (TG) are normally used as a surface stabilizing agent, which produce nanomaterial with good emission efficiencies 17, 23−30. In both the synthetic route, nucleation takes place just after the injection of precursor, and continues until the temperature and the precursor concentration drop below a critical threshold. New synthetic methods that do not require the injection of precursors have also been developed. Some of the reports have been recently published on one-pot non-injection routes to semiconductor nanocrystals either via the organometallic method 31−35, in aqueous solution 36, 37, or under microwave irradiation 38. Surfactants are amphiphilic molecule which possess a polar head group and nonpolar tail, which forms micelles in the solution which can act as support or template for the growth of smaller moieties i.e. Quantum dots, When micellar solutions are used, the aqueous solution forms a nanoreactor and CdS is present in the core of the micelle 39, 40. Spherical micelles formed in the aqueous medium. When CTAB used as a capping agent, it results in spherical shapes because the nucleation and growth of the particle occurs in the cavity of the micelle obtained39, 40. These micelles have a diameter range in nanometers, so there is possibility that the growing nano-objects can entangle within these vesicles and form a stable dispersion. Despite of high utility of the surfactants in controlling the growth i.e. size and optical properties of metallic nanoparticles, there are very few reports regarding the surfactant mediated assembly of CdS QDs. Therefore, an attempt has been made for studying the effect of surfactant on the stability and growth of CdS quantum dots stabilized by thioglycolic acid in cationic and anionic micellar media.

Cadmium chloride pentahydrate (CdCl2.5H2O), thioglycoloic acid (SHCH2CH2COOH), thiourea (NH2CSNH2), cetyltrimethyl ammonium bromide (CTAB), sodium dodecyl sulphate (SDS) and sodium chloride (NaCl) were procured from Sigma Aldrich and used without further purification. All the experiments were performed using Millipore water obtained from milliQ system.

A 40 mg of cadmium chloride and 22.8 mg of thiourea (TU) dissolved in 30ml of water to prepare Cd2+/thiourea precursor solution. 20ml of thioglycolic acid (TGA) solution containing (30 µl of TGA) then added to the precursor solution and the pH of the reaction mixture adjusted to 10.5 with 1.0 M NaOH (Scheme-1). The typical molar ratio of Cd2+: TU: TGA was 1:2:3 in our experiments. The solution was then nitrogen bubbled for 30 minutes and then transferred to three necked round bottom flask, which is connected into the heating mantle with condenser. The completely nitrogen bubbled solution was then set up on the reflux for the nucleation and growth of CdS QDs. The growth of CdS QDs was monitored via UV-visible spectra with the reaction time. After being refluxed at the 100 0C for 12 h the yellow, green QDs was removed from the mantle and cooled down to room temperature, then transferred to a conical flask and precipitated with acetone. This precipitate washed 3-4 times with acetone and dried to get crystalline powder. The purified crystalline powder used for the further studies. The CdS QDs solutions with 1µM concentration were prepared by redispersing the CdS QDs powder in millipore water, and then different concentration of CTAB and SDS introduced in to the QDs solution to study the effect of surfactant on QDs stability and further growth.

UV-Visible spectra were monitored using Thermoscintific evolution-300 spectrophotometer operated at a resolution of 2 nm. Fluorescence spectra were recorded using an Agilent fluorescence spectrophotometer (G9800AA). The FTIR spectra of thioglycolic acid and thioglycolic stabilized CdS quantum dots (TGA@CdS QDs) measured with Shimadju IR Affinity 8400S FTIR Spectrometer. TEM measurements performed on a JEOL, JEM-2100F, operated at accelerating voltage 200kV. SEM analysis has been carried out by ZEISS EVO (EVO 18) Series Scanning Electron Microscope. X-ray

RESULTS AND DISCUSSION

Figure 1 A shows the representative set of refluxing time dependent (1-7 h) UV−vis spectra of TGA stabilized CdS QDs prepared using a Cd2+: TU: TGA molar ratio of 1:2:3 and Cd2+ concentration of 5 mM. The absorption band of bulk CdS positioned at 515 nm 41. Desired growth of CdS QDs achieved after 7 h refluxing. All samples showed well-defined first excitonic peaks between 330 to 392 nm with different refluxing time (Table 1) and attributed to 1sh-1se excitonic transitions 42. These values signify that the shift in the absorption peak corresponding to the increase in particle diameter of the CdS QDs with the duration of the hydrothermal process.

Ostwald ripening which causes a slow defocusing of size distribution 20. Later on the bands become sharper as reaction time elapsed. The sharp absorption features are indicative of monodispersed particles. The variation of the absorption bands indicates that the particles grow rapidly as the reaction time increased. The corresponding absorption edges (obtained by the intersection of the sharply decreasing region of the spectrum with the baseline) 43 are located between 315 to 423 nm for reactions performed during 1 to7 h refluxing, which corresponds to band gaps of 3.93 to 2.92 eV, respectively (Figure 2). A comparison with the value of bulk CdS that have an absorption edge located at 515 nm (2.42 eV) indicates quantum confinement in all the sprepared nanocrystals. Using these band gap values, the particle diameters were calculated following the formula of Brus (Eq.1) based on the effective mass approximation 44−47,

Eg is the band gap of the bulk material (2.42 eV), h is the reduced Plank’s constant (6.58 × 10−16 eV), r is the radius of spherical nanocrystals, e is the charge of the electron (1.6 × 10−19 C), ε is the semiconductor dielectric constant (5.7), mec and mhv are effective masses of the electrons and holes, respectively and mo is the free electron mass (mo = 9.11 × 10−28 g). With the effective masses of electrons (mec = 0.19mo) and holes (mhv = 0.8mo), diameters of 1.5 to 4.1 ± 0.9 nm were found for CdS@TGA QDs obtained after 1 to 7 h reaction, respectively (Table 1). The deviation of 0.9 nm corresponds to ±10 nm for the determination of the absorption edges. The sizes of the CdS QDs were also estimated from the UV−vis absorption spectra by Peng’s empirical equations 48, 49 and were found to be in good accordance with results obtained from the Brus formula (1.3 to 3.9 ± 0.9 nm for CdS@TGA QDs obtained after 1 to 7 h reaction, respectively). CdS QDs prepared by the organometallic TOP and TOPO based approach and its alternatives usually exhibit fluorescence emission dominated by the band-edge emission 48-55. Generally, for CdS QDs with diameters ranging from 2.0 -5.3 nm, the PL emission is located between 375 and 460 nm 56. Figure 3 shows fluorescence spectra of CdS@TGA quantum dots corresponding to the absorption spectra (Figure 1A). It is evident that the PL emission peak position varies with refluxing time from 511 to 598 nm (Table 1) in a similar manner as the absorption peak position and CdS QDs diameter varies. The considerable variation between the emission peak position and the absorption edge indicates that the present CdS QDs exhibit trap-state

attributed to the recombination of charge carriers trapped in the surface states and is related to the size of CdS QDs 56−59. The smaller particles leads larger energy band gap, which in turn results more blue shift in fluorescence band. Fluorescence QYs of the samples have been determined using the fluorescein as fluorescence standard (94% QY in 0.1 M NaOH), reached up to 32% for the sample prepared during 7 h of refluxing and were independent of the excitation wavelength between 300 and 400 nm. PL QYs values increase continuously with increasing duration of refluxing time (12.5 - 32.5% (Table 1) for 1 to 7 h of refluxing, respectively). The increase in fluorescence QYs with nanocrystal growth of CdS QDs prepared via organometallic approach is recently reported 60. In addition to this, it can also be observed that the full-width-at-half-maximum (FWHM) of fluorescence spectra is 147 to 119 nm, which is 4-5 times wider than the emission peaks of CdS QDs prepared in organic medium at elevated temperature 48-55. This is the typical behavior of CdS QDs prepared via aqueous synthetic route. Furthermore, the fluorescence spectra (figure 3) is very fine, which indicates the particles with very less surface defects. Table 1. Showing the variation of absorption and emission maxima, FWHM, quantum yield, band gap and size of the quantum dot at different reflux time. λa = absorption maxima, λe = emission maxima, a* = Size calculated through brush equation for corresponding λa

1 330 511 147 12.5 3.93 1.5 0.9 2 378 533 154 13.2 3.20 2.9 0.9 3 384 556 149 14.1 3.05 3.4 0.9 4 388 572 151 15.2 3.04 3.6 0.9 5 388 577 130 20.0 2.99 3.8 0.9 6 392 590 124 25.3 2.97 3.9 0.9 7 392 598 119 32..5 2.92 4.1

Figure 1B shows the evolution of the first excitonic peak in absorption bands with growth for different molar ratio of Cd2+: TGA from 1:2, 1:3, and 1:4. In these reactions, the Cd2+: TU molar ratio kept constant at 1:2 and the variation in molar ratio of capping agent on growth kinetics has been studied. As can be seen, the variation of band position with refluxing time indicates the growth of the particle. The exponential growth of quantum dot has been observed with refluxing time, which seems that QDs growth follows first order kinetics. The first order rate constant for the growth of TGA@CdS QDs has been calculated using equation 2. The kobs values for different molar ratio (Cd2+: TGA) 1:2, 1:3, and 1:4 are 8.19x10-5 s-1, 5.78x10-4 s-1, and 2.21x10-4 s-1, respectively. The kobs value suggested that the particle growth rate is maximum at 1:3 of Cd2+ to TGA molar ratio.

(2)

The increase in the ratio of TGA to Cd2+: TU results the red shift of absorption and fluorescence spectra (Figure S1) which indicates the formation of larger CdS nanocrystallites with more surface defects than those prepared with smaller Cd2+:TGA ratios (1:2 or 1:3). In the meantime, the fluorescence intensity showed a trend to increase first and decrease later, with maximum intensity at Cd2+/TGA = 1:3. The fluorescence QYs of CdS@TGA QDs prepared with Cd2+: TGA ratios of 1:2, 1:3, and 1:4 found to be 16.2, 32.0, and 5.04%, respectively. This behavior can be explained by the fact that increasing the amount of

Figure 4. (A) Absorption spectra of CdS@TGA quantum dots in presence of 1mM CTAB (Inset Absorption spectra of CdS@TGA after a week) (B) plot of Absorbance Vs Time showing effect of CTAB on growth (C) fluorescence spectra of CdS@TGA quantum dots in presence of CTAB (1) just after the adding of CTAB (2) after a week. Micellar arrangement can enhance the stability of QDs by preserving the nano-objects inside the micelle and promotes the monodispersity. Figure 4B shows the sequential set of UV-visible spectra of CTAB induced CdS@TGA QDs. These spectra suggested a growth pattern in the cationic micellar medium after introduction of CTAB in to the micromolar solution of CdS@TGA QDs. The absorption spectrum rises dramatically with a change in spectral pattern (Inset Figure 4) which indicates the micellar incorporation over CdS@TGA QDs. The growth pattern of CdS@TGA QDs with time for 2 different concentrations of CTAB over its CMC (1mM and 2mM) indicates that the [CTAB] is directly proportional to the growth rate (Figure 4B) (1.8x 10-3 s-1 in case of 2mM bands (Inset Figure 4A), one at shorter wavelength 350 nm and other at longer wavelength at 415 nm, indicates the formation of 2 different sized CdS QD which are well supported by the fluorescence spectra (Figure 4C).

The CMC of SDS is found to be 6.39 mM under the reaction condition (Supporting information FigureS2). Therefore, a similar experiment was performed in the presence of 7 mM and 8.3 mM (CMC in aqueous medium) [61] SDS (Figure 5A and S2). We performed the above experiment at 7 mM as its CMC found to be decreased in presence of CdS@TGA QDs (Figure S2). No significant changes were observed as in case of CTAB. The reason might be the surface charge that QDs possess are negative (-SCH2CH2COO-) which can be well counter controlled by cationic surfactants. Stability of CdS@TGA QDs against salt has been studied by exposing the CdS@TGA QDs to different concentration of NaCl as well with saline (Figure 5B). Negligible change observed in spectral behavior, even after one week and no sign of precipitation observed, which indicates the stability of CdS@TGA QDs in salt environment and is in good agreement with the PL emission spectrum (Inset of Figure 5B). The FTIR spectra of thioglycolic acid and TGA-capped CdS QDs are shown in figure 6. The IR spectra of TGA-capped CdS QDs (red line) show significant changes as compared to thioglycolic acid (blue curve). The absence of stretching vibration of the thiol group ( = 2505cm-1) in QD indicates thiolated interaction of TGA onto the QD surface. Furthermore, a significant shift in the asymmetric stretching vibration of the carboxyl group (

Thioglycolic acid stabilized CdS (TGA@CdS) QDs were further characterized by X-ray powder diffraction (XRD) and transmission electron microscopy (TEM). Figure 7 shows the XRD pattern of CdS@TGA QDs of 2 different samples with different Cd2+ : TGA ratio. The broad peak indicates nano dimensions of the sample. With the increase in molar ratio, the peaks become more prominent and sharp due to the increase in nanocrystalline size. The less crystallinity observed in the sample prepared in 1:2 ratio, while higher crystallinity observed in 1:3 sample, which is in good agreement with the kinetic data. This significantly proves that higher growth rate produces higher crystalline material. The peaks are located at 2θ = 26.7, 43.8, and 51.3°, oriented along the (111), (220), and (311) directions are in good agreement with the JCPDS file 10−454 suggesting that the quantum dots are in cubic zinc blend form [63]. Figure 8 represents TEM images of the CdS QDs prepared under optimized conditions, with Cd2+: TGA 1:3. Insets of figure 8 show the corresponding selected area electron diffraction (SAED) pattern and frequency percentage of the CdS@TGA QDs. The images show that CdS@TGA QDs are spherical, well defined, and uniform in size and shape. The size distribution histogram for prepared nanocrystals suggested that the particles having size range 3-4 nm are present in large extent. The average sizes obtained from UV−visible spectra and TEM images resembles each other reasonably well. HR-TEM image of CdS@TGA QDs prepared after 7 h of refluxing illustrated, well-resolved lattice planes (Figure 8). From the lattice fringes of the micrograph, the interplanar spacing (d) was estimated to be 0.356 nm, which correlates to the miller indices (111) plane, i.e., 0.36 nm of the cubic phase. The cubic zinc blend structure of CdS nanoparticles was further evidenced by the SAED pattern. Scanning electron microscopic (SEM) image of CdS quantum dots shown in figure 8B reveals that CdS quantum dots are spherical in shape except few particles which are clumped together.

CdS quantum dots were prepared through aqueous route with a cubic zinc blend structure using cadmium chloride and thiourea as cadmium and sulfur source and thioglycolic acid as a stabilizing agent. Effect of different molar ratio of Cd2+ to TGA were found to affect the growth kinetics of quantum dots, which was found to be 5.78x10-4 s-1 for the molar ratio 1:3. In optimal condition, the fluorescence spectra of CdS quantum dots ranges between 490 to 594 nm during 7 h of refluxing and the best fluorescence quantum yield reached up to 32%. The band gap of quantum dots found to be dependent on refluxing time and

emission peaks (470 and 540 nm) indicates bidispersity in the presence of CTAB. However, in case of SDS fluorescence peaks completely vanishes, without significant change in absorption spectra. Different concentration of salt showed an insignificant effect on QDs aggregation.

Financial support from DST (New Delhi) Fast Track Project (SR/FT/CS-39/2011) New Delhi acknowledged with appreciation. We are thankful to sophisticated analytical instrument research facility (AIRF) Jawahar Lal Nehru University (JNU), New Delhi for TEM analysis. We are also grateful to head, School of Studies in Physics and Astrophysics, Pt. Ravishankar Shukla University, Raipur for FTIR analysis. Authors are thankful to the head, School of Studies in Chemistry, Pt. Ravishankar Shukla University, Raipur for providing laboratory facilities.

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