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Preparation of fluorescent CdTe@CdS core@shell quantum dots using chemical free gamma irradiation method
Journal of Luminescence 192 (2017) 17–24
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Preparation of fluorescent CdTe@CdS core@shell quantum dots using chemical free gamma irradiation method S.P. Rajua, K. Hareeshb, S. Chethan Paia, S.D. Dholec, Ganesh Sanjeeva, a b c
MARK
⁎
Microtron Centre, Department of Physics, Mangalore University, 574199, India School of Physics, University of Western Australia, Crawley, Perth, WA 6009, Australia Department of Physics, Savithribai Phule Pune University, 411007, India
A R T I C L E I N F O
A B S T R A C T
Keywords: Fluorescence Core@shell QDs Gamma rays Irradiation method TEM PL spectroscopy
This study investigates pH dependent gamma radiation effects on Mercaptopropionic Acid (MPA) capped CdTe quantum dots (QDs). Gamma radiation, as fast and chemical free catalyser, causes CdS Shell formation on MPA capped CdTe QDs at very high alkaline environment. UV–vis absorption spectroscopic results showed that, with increase in gamma radiation doses, the absorption peak of CdTe QDs decreased to lower wavelength (decrease in particle size) in both acidic & neutral medium (as synthesised), and it increased towards longer wavelength in basic medium indicating the possibility of growth of CdS shell over CdTe core QDs. X-ray diffractogram studies have revealed that the peaks of CdTe QDs shift towards higher angles after irradiation, supporting the notion of the core@shell particle structure of CdTe@CdS. TEM images confirmed the extra stretch around core particles from CdS material resulted in increase of average size of QDs. EDX study revealed the chemical composition of material system. X-ray photoelectron spectroscopic results revealed the shift in the binding energy of both Cd and S thereby indicating the change in the coordination condition after irradiation promoted the particles’ formation of core@shell in nature. The systematic photoluminescence spectroscopic investigation at all possible pH values against ascended gamma radiation doses revealed the formation of CdS shell over CdTe core at optimum pH value (i.e. only at basic pH 12). Contrarily, photo-oxidation of QDs’ was observed at neutral (and below) pH values towards gamma irradiation. The quantum yield is increased only after introducing external shell source material at highest alkaline media confirming the formation of CdS shell on CdTe core.
1. Introduction Quantum dots (QDs) have got attention due to their unique optoelectronic properties [1]. QDs are special case of nanoparticles can be grown in colloidal form with varied band gap in optical region using single material. Therefore, QDs play many materials’ role in specialised applications like solar cells [2,3], LEDs [4,5], Sensors [6,7], Bio-markers [8,9] etc. Such versatile materials are highly dispersible at the time of application for quality performance. Besides, they are high quantum yield fluorescent materials resistant to photo-bleaching with better spectroscopic properties. Therefore, they are the better materials than traditionally used dyes which lack in such quality parameters for the above mentioned applications [10]. Size (few nanometers) of this quantum systems are within their exciton Bohr radius to be called as Quantum dots. Among many QDs, CdTe QD has received attention because of its band gap tunability particularly for solar cell applications. Moreover, it is known for its fast injection of electrons into active layer of TiO2 compared to CdSe and ⁎
Corresponding author. E-mail address: ganeshsanjeev@rediffmail.com (G. Sanjeev).
http://dx.doi.org/10.1016/j.jlumin.2017.06.019 Received 22 January 2017; Received in revised form 6 June 2017; Accepted 8 June 2017 Available online 12 June 2017 0022-2313/ © 2017 Elsevier B.V. All rights reserved.
other materials [11]. Additionally, colloidal CdTe QDs capped with thiols (as hydrophilic capping agents) synthesised in aqueous media have found applications in bio-systems. This skips another burden of indispensible step i.e., dispersing QDs in to water when they are grown in organometallic solvents. Such steps are complicated and inefficient to utilize QDs in bio-applications. Moreover, capping agents are responsible for the reduction of non-radiative recombination of excitons’ (electron-hole pairs). This increases the radiative fluorescence decay from QDs as they passivate surface dangling bonds (or called as defects). Besides, these cappers help to synthesis particles along with maintaining their colloidal stability. Therefore, passivation with more in number of capping agents leads to increase in quantum yield and colloidal stability. However, there is practical inefficiency of intense capping as alignment of thiol molecules on non-uniform QD's surface may create hydrophobic chemical patches. These patches restrict intense passivation (called steric hindrance) at the time of synthesis, despite the careful selection of capping molecule [12]. In account of the above, just grown such core QDs hardly achieves intense passivation for
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literature. This study shows CdS shell can be grown even in the case of CdTe QDs capped with MPA at highest stable alkaline value (pH 12) only after introducing cadmium and sulphur source material with controlled gamma radiation doses. Moreover this work also concludes that QDs grown in a pH value need not be suitable for shell growth of other material on the same QD core. The gamma radiation assisted synthesised CdTe@CdS core@shell QDs capped with MPA are of high quality QDs which can be applied in many applications such as solar cells or sensors or biomarkers etc.
the above quality properties. Among all other addressed solutions for the above problems, inorganic shell growth on bare QD surface is a popular way out after synthesis of QDs. Core@shell QDs of type I and II enable bigger band gap materials (shell) growth on smaller band gap (core) QDs or viseversa irrespective of their materials’ groups [13,14]. This sort of choosy shell growth leads to manufacturing ease in creation of nanoscale semiconducting junctions with reduced photo-bleaching for multiple applications. This also helps in reduction of cytotoxicity (if bio-friendly shells like ZnS are grown) when hazardous elements are used in synthesis. Therefore, inorganic shell growth from multiple materials with controlled morphometric aspects is required for having good quality QDs. Shelling of core CdTe QDs with CdS material is suitable to fix the above problem. This combination of materials’ is more advantageous as CdS possesses lower lattice mismatch (< 10%) with the core CdTe compared to other materials. Therefore, such atomic passivation leads to flexible spread and epitaxial coverage of CdS shell over CdTe QDs surface. This would saturate unpassivated chemical sites there by reducing steric hindrance even though surface possesses nonuniform corners where capping molecules are not allowed to reach. CdS shells were grown over CdTe [15], CdSe [16], HgSe [17], HgTe [18], ZnSe [19], ZnTe [20] core QDs, using different methods like reflux method [15], Aerosol Flow method [21], Microwave method [22], etc. All these methods use either harsh chemicals with different expensive setup or elevated temperature with time consuming complicated procedures. Therefore, it is needed to establish a method which avoids such complications. Nowadays, the shell growth by radiolytic method [23,24] has been extensively used for its simplicity. Radiation is a clean catalyser with no chemical contamination or usage of elevated temperature. This potential method is also used as catalyser for synthesis [25–27] and other intervention steps in material processing [28,29]. Therefore, irradiation can be a facile method to get shell formation over pre-synthesised core QDs. There are few reports in literature on the utility of both ionising and non-ionising radiation in material engineering especially in the field of nanoparticles. Most of them have used UV radiation for the purpose which is time consuming [23] and leads to increase the sample temperature. This would create some problems such as continuous heating up of samples for very long time or any other solvent related degradation problems which may reduce quality of QDs for long term usage. This has made us to carry out the study using ionising radiation as there are no specific reports on the growth of CdS shell on CdTe core in few minutes. The present study utilizes ionising gamma radiation assisted method to grow CdS shell over CdTe QDs. A few minutes of expose to gamma radiation leads to fast and intense CdS shell formation as the energy of gamma is moderately high compared to UV light. Moreover, gamma radiation source provides continuous and uniform radiation from all directions so that all samples are being irradiated for having high uniformity shell formation. Additionally, CdTe QDs capped with MPA molecules were suffering from photo-oxidation rather than shell formation upon irradiation as reported in literature [13,30]. On the other hand, shell formation to this QDs is important as MPA capped to CdTe QDs leads to higher crystallinity, quantum yield and much lower initial size distribution, than many other thiol molecules (such as TGA, GSH, TGH) [31]. In view of the above, a detailed investigation of all parameters involved in shell growth which are not supporting shelling process, in this case, is a combined objective of the present study. This also helps to see the possibility of extended studies on growth of different types of core@ shell QDs such as CdTe@ZnS, CdTe@CdS@ZnS etc and thicknesses of inorganic shells using different capping molecules through irradiation method. This required investigation includes synthesis parameters like variation of pH of the solution environment or availability of shell source materials or successful catalization by irradiation or all the above. Unfortunately, the reports on the comprehensive investigation of pH dependent irradiation effects on CdTe QDs are very sparse in
2. Experimental section 2.1. Materials Analytical grade chemicals were used as it is without any further purification. 3-Mercaptopropionic acid, Sodium borohydrate, TriSodium citrate dehydrate were procured from MERCK. Sodium tellurite (99% pure) and Cadmium chloride (99%) were received from ALDRICH and FLUKA respectively. Rhodamine B was from Sigma Aldrich (Quantum Yield = 0.35 in water and this was used as reference dye for relative quantum yield measurements). Double distilled water was used throughout the experiments. 2.2. Synthesis of MPA capped CdTe QDs MPA Capped CdTe QDs were synthesised using hydrothermal synthesis method and the detailed procedure is given in supplementary information. In brief, molar ratio of Cd:Te:MPA::1:0.25:8 was maintained during synthesis of QDs. 100 mg Tri-Sodium citrate dehydrate and 50 mg Sodium Borohydrate were dissolved in 40 ml double distilled water as per the systematic synthesis protocol. As prepared solution was taken in air tighten Teflon autoclave, with stainless steel housing. This autoclave was maintained at 180 °C for 11 min to grow suitable size of QDs. The pH of as grown MPA capped CdTe colloidal QD solution was measured as 7 soon after the synthesis. 2.3. Irradiation of MPA capped CdTe QDs The pH of as prepared CdTe QDs was adjusted in the range from 4 to 12 by the addition of HCl and 3 M NaOH drop wise slowly to set acidic and basic conditions respectively. These solutions were taken in different polypropylene bottles and exposed to Co-60 gamma radiation source with a dose rate of 9.5 kGy/h for different doses in the range from 0.3 to 20 kGy. The details of Co-60 gamma radiation chamber are given in Supplementary information. 2.4. Characterization UV–Vis spectroscopic (UV–Vis) studies of all the samples were carried out using Shimadzu UV − 1800 – Spectrophotometer. XRD analysis was done from Rigaku Miniflex 300 X-ray Diffractometer with Cu-kα as a radiation (wavelength = 1.5406 Å). Photoluminescence spectroscopy (PLS) was carried out using Perkin Emler, LS 55. The surface morphology was recorded using TEM of model Tecnai G2 U-thin 200 kV, LaB6 filament. X-ray photoelectron spectroscopic (XPS) was carried out using Omicron EA 125 analyzer at room temperature in an ultra-high vacuum. Relative quantum yield of QDs was calculated using Rhodamine B as reference dye. 2.5. Sample preparation 2.5.1. TEM-Imaging Dots were cleaned using propanol and dispersed in ethanol with required dilution. So-dispersed dots were drop casted on copper grid and dried at room temperature for two days. These grids were directly mounted on sample holder of the TEM setup. 18
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Fig. 1. Capture of QDs in vials with varied pH values of solution.
2.5.2. PL-Spectroscopy Samples were characterized in colloidal (in water) form only. 3. Results and discussion Fig. 1 depicts the colloidal solution of CdTe QDs at different pH values from pH 4 to pH 12. When pH was brought down to appreciable acidity, protonation competes with capping agents’ bonding on QD's surface leading to oxidation. This reduces colloidal stability which resulted in agglomeration and precipitation of particles. It can be clearly seen from Fig. 1 as pH number 5 (and bellow pH values) was ended in precipitation of QDs followed by divorcing solvent. Colloidal stability is important in the case of shell formation around individual particles. Higher Colloidal stability frees individual QDs to open their surfaces for atomic shell coverage. This is very difficult or impossible when QDs are in congested or precipitated form. Higher pH values (≥ 6) lead to stable colloidal stability as capping molecules binding dominates against protonation. This can be clearly seen from Fig. 1. Therefore, the solution with pH≥6 is highly colloidal with greater photo-emission. On the other hand, variation of pH in pre or post synthesised nanoparticles may cause appreciable change in size (hydrodynamic), quantum yield as well as stability of nanoparticles [32]. Often, it may hinder nanoparticle growth itself if the pH environment is not suitable. Furthermore, pH causes variation in zeta potential, agglomeration, network formation, photo-oxidation and sensitisation of surface of colloidal QDs for different catalysers [33–35]. Hence, shell formation is hardly achieved if there is weak colloidal desperation caused by above reasons. Therefore, this study mainly focuses on possibility of CdS shell formation with different pH (from 6 to 12) against gamma radiation to find out the proper pH number for CdS shell formation and also to answer why photo-oxidation is taking place in the case of MPA capped CdTe QDs when irradiated. This kind of investigations need proper understanding of the process of radiation induced changes (primary or secondary) in the growth media. The water as the growth media undergoes radiolysis due to irradiation forming many free radical species such as H*, H2O2*, H2O*, H3O*, etc. The formation of these free radicals may lead to following two cases. (1) The formation of these free radicals will vary the pH of solution which may affect QD stability as these free radicals will directly interact with surface of CdTe QDs. This interaction may eventually lead to photo-oxidation and reduction in QD size along with decreased colloidal stability when exposed to gamma radiation. (2) Otherwise, these free radicals may create favourable environment for the shell growth by releasing shell source materials resulting an increase in the size of particles. Therefore, variation in colloidal environment needs to be monitored while irradiating QDs. The mechanism of the chemical reaction which may occur during gamma irradiation in acidic and basic environment is given in Scheme 1. As can be seen from Scheme 1(b), at basic environment, S2− ions get released from MPA molecule as a result of gamma irradiation. These sulphur ions combined with free Cd2+ ions in the solution to form CdS shell around the CdTe QDs. When the chemical environment turns to acidity, MPA molecules on the surface of QDs were trimmed off by repeated protonation to cause removal of capping boundary (as in Scheme 1(a)) assisted by gamma radiation. This finally leaves QDs without capping agents in solution. These bare QDs without capping
Scheme 1. The reaction mechanism during gamma irradiation in (a) acidic environment and (b) basic environment.
agents attract each other to cause agglomeration. This leads to drastic quenching of PL emission followed by increase of surface defects and precipitation of QDs. However, photochemistry is complex to predict exact mechanism of the reaction when inorganic nanoparticles are on discussion. However, results obtained from spectroscopic analysis help to analyse the phenomenon to support the proposed reaction mechanism and core@shell hypothesis. UV–vis spectra of gamma irradiated CdTe QDs at different pH values are shown in Fig. 2. The absorption peak of CdTe QDs shifts towards lower wavelength at pH 6 (blue shifting) indicating decrease in the particle size followed by narrowed quantum confinement. Similar kind of variation was observed when CdTe QDs were irradiated at pH 7. However, when CdTe QDs were irradiated at pH 10 and 12, the absorption peak slightly shifts towards higher wavelength side (red shifting) indicating the possibility of CdS shell formation. This is due to alteration of conduction band alignment in core and shell materials together by inducing lattice strain. This is reflected in redshift (within limit) of absorption spectra (discussed below in detail). This is mainly due to the increase in pH values. In addition to this, the absorption spectra were observed with clear and wider redshift at pH 12, compared to that at pH 10. Thus, it clearly indicates that the higher alkalinity (pH 12) results in thicker shell formation. This contrast behaviour of irradiated QD solutions’ for acidic and alkaline pH numbers can be seen in Fig. S1 in the Supporting information. The irradiated QDs in vials at pH 7 & 6 (after a few days) are colloidally unstable with reduced fluorescence emission. On the other hand, QDs at pH 12 condition were found with high colloidal stability with greater photo-emission (appear in red colour to naked eyes). Typical X-ray diffractogram (Fig. 3) shows its pattern for pristine and gamma irradiated QDs’ at a dose of 1.2 kGy (this particular dose shown greater PL emission property for QD with colloidal stability towards gamma irradiation which will be discussed later) maintained at pH 12. Pristine CdTe showed peaks at 25°, 42° and 49° respectively corresponding to (111), (220) and (311) planes which is in agreement with the standard diffraction pattern for cubic CdTe material (JCPDS file no. 03-065-1047). XRD pattern (Zinc blende structure) of the core and shell materials is almost same with shift in their peak position for their bulk form. These three peaks were proportionately shifted to higher angles after irradiation. The FWHM value of peaks’ for irradiated (CdTe@CdS) samples was increased compared to pristine CdTe QD's. This is due to induce of lattice strain from core@shell structure in order to grow epitaxial junction. This crystal structure modification was further confirmed with SEAD ring's pattern. SEAD pattern is complimentary for XRD studies. This study was conducted while taking TEM images. Three 19
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Fig. 2. Absorption spectra of MPA capped CdTe QDs at different pH values (6, 7, 10 & 12) for varied gamma radiation doses.
particle size is increased to 4.5 nm after irradiation confirming the formation of CdS shell over CdTe core. Inset of Fig. 4(a1 & b1) shows of single particles of CdTe @CdS and CdTe QDs respectively. Samples were imaged with TEM for dried samples. Optical spectra were taken for samples in liquid form. This phase (dried and liquid) difference would also result in change of particle size. Particles in water possessed larger size due the increase of hydrodynamic radius around individual particle which is created from extra ionic boundary. This boundary will be removed when they are dried. However, such increase or decrease of particle size due to the above process is proportional in liquid and solid phases for the actual variation in the particle size. Fig. S3 of TEM image shows core@shell particles with 5 nm resolution in the Supporting information. XPS is surface sensitive tool to analyse nature of chemical bonding over the surface of materials. Sulphur and Cadmium are the shell materials for CdS layer formation on CdTe QDs surface. Cadmium is common element in both core and shell materials of CdTe@CdS QDs. Furthermore, Tellurium and Sulphur are purely core and surface related elements respectively attached to Cadmium. Therefore, shift in binding energy of Cadmium and Sulphur before and after irradiation at pH 12 gives the information of CdS shell formation. Fig. 6(a) shows high resolution XPS spectra of S2P of CdTe before and after irradiation. The Sulphur peak of pristine CdTe is deconvoluted into two peaks viz., S 2P3/2 and S 2P1/2 at 163.86 eV and 161.63 eV respectively. These doublet peaks are separated by 2.23 eV with intensity of 1:2 ratios. For gamma irradiated CdTe QDs, these sulphur related doublet peaks were shifted towards lower energy levels compared to pristine CdTe i.e., at 163.09 eV (S2P3/2) and 160.25 eV (S2P1/2) respectively. Such shift in the binding energy of sulphure confirmed the process of surface modification of CdTe QDs with CdS shell by irradiation and which is in agreement with the reported literature [23]. Additionally, Fig. 5(b) shows high resolution XPS spectra of Cadmium for pristine and gamma irradiated CdTe QDs. Peaks of Cadmium also evolved with doublet peaks. As can be seen from Fig. 6(b), Cd showed two peaks at
Fig. 3. XRD pattern of QDs before (CdTe) and after irradiation (CdTe@CdS).
major and broad rings which are situated with common centre can be clearly seen in the inset of Fig. 4(a & b). These three rings correspond to the same crystal planes 111, 220 and 311 respectively (from shorter radius to larger) attributed in XRD peaks at mentioned angles. These rings features were also altered after the formation of the CdS shell corroborating XRD pattern to confirm the modification of material structure. Fig. S2 of EDX spectra for core and core@shell materials in the Supporting information reveal the chemical composition of samples before and after irradiation respectively. Similar kind of variation in the diffraction pattern was observed during the growth of core@shell QDs in literature [36]. This provides further support for the formation of the CdS shell on the CdTe Core. TEM images of CdTe QDs (at pH 12) before and after irradiation at a dose of 1.2kGy are shown in Fig. 4. The average particle size of CdTe QDs before irradiation is found to be around 4 nm. However, this 20
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Fig. 4. TEM images and size distribution of QDs before (b) and after (a) irradiation respectively at pH 12. SAED pattern of QDs for the same can be found in the inset of (a) and (b). The top inset (a1) shows the shelled structure.
Fig. 5. XPS spectra for shelled and unshelled QDs (before and after irradiation). Sulphure (a) and Cadmium (b) Doublet spectra.
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Fig. 6. Variation of PL spectral intensity (a) & wavelength (b) of MPA Capped CdTe QDs with varied gamma radiation doses.
when CdS shell is grown on CdTe core epitaxially by reducing the bandgap of the whole core@shell system. This lattice strain is increased with increase in thickness (epitaxially) of the shell material. This small lattice strain is may not be seen through TEM images as the lattice mismatch is only around 10%. However, this induced lattice strain is enough to modify PL emission peak position due to the above reason (conduction band edge alteration). Therefore slight redshift is the indication of shell growth (can be used as measurement of shell thickness also with increase in redshift to higher wavelength) [37]. Therefore, the shifting of emission signal position towards higher wavelength indicates growth of bigger particles and increase in intensity of emission signal is a measure of intense passivation of QDs. Therefore, increase in both PL peak intensity and PL peak wavelength indicates growth of bigger particles with intense passivation with a shell against gamma radiation doses. It can be seen from Fig. 6(a & b) that such increase in peak intensity and redshift of wavelength is observed more at pH 12 (with shell source material) where the absorption spectral shift was also clear at this pH number 12 towards higher wavelength ( see Fig. 2(d)). At pH 6 environment, abrupt reduction of intensity with short lived curves in PL spectra at very lower gamma doses itself is due to higher instability of particles to gamma radiation at this situation (curve pH 6 in Fig. 6(a & b)). This kind of instability is due to photo-oxidation of QDs against irradiation resulted in separation of solvent and particles at this pH environment corroborating the UV–vis spectroscopic results (Fig. 2 for pH 6). The particles might have got agglomerated and precipitated due to removal of capping agents assisted with gamma radiation at this pH value. This result, also, supports the proposed radiolysis reaction mechanism for lower pH values against gamma radiation doses where CdTe QD was left un-passivated. The un-passivated QDs were joined together, finally, turned to bulky material. Therefore, pH6 is not favourable for the shell growth. Primarily, stability of QDs in solution is required the most to grow a shell on their surfaces using irradiation method. However, irradiation of QDs at pH 7 was also not showed much improvement in PL properties against gamma doses as shown in the same figure (Note: Tall head PL intensity of QDs was observed at this pH 7 situation among all other pH values. The PL graph of wavelength & intensity at different pH values is given in the Supporting information (Fig. S5). Therefore, PL Spectral shift of pH 6 and 7 has not followed a trend and ended their curves as well at important doses for the shell formation (Fig. 6(a & b)). This irregularity of spectral shift with sudden reduction of intensity in both the cases suggests that these lower and
404.89 eV and 411.77 eV corresponding to Cd 3d1/2 and Cd 3d5/2 respectively with peak to peak separation as 6.88 eV which is also in agreement with the reported values [25]. It can also be seen from Fig. 5(b) that Cd peaks shifted towards higher values of binding energies, i.e. to 405.33 eV and 412.28 eV after irradiation compared to pristine CdTe QDs. This shift in the binding energy confirms the formation of CdS shell on CdTe core as the binding energy of CdS shell is different from CdTe core due to change in the co-ordination condition [25]. In order to give a better explanation, PL studies were carried out for gamma irradiated CdTe QDs at different pH values such as 6, 7, 10 and 12 (with and without shell source material). The corresponding PL spectra for all values of pH are shown in Fig. S4 (Supporting information) and the variation of PL peak intensity & PL peak wavelength with gamma radiation doses are shown in Fig. 6(a) and (b) respectively. Furthermore, graph of FWHM (inset 7(a)) and relative Quantum Yield (QY) (inset 7(b)) against gamma doses with PL curves of optimised pH 12 condition for shell formation (with shell source materials) are shown in Fig. 7. Slight redshift in PL emission spectra indicates further growth of CdTe QDs’ with different material (CdS as shell). Such redshift in CdTe QDs’ is due to alteration of conduction band edge alignment from induced lattice strain in both materials. Such lattice strain is developed
Fig. 7. PL spectra of gamma irradiated QDs after introduction of Cd+MPA complex at pH 12. Inset: Variation of FWMH (a) and normalized Quantum Yield (b) with radiation dose.
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very slight variation in FWMH with increased emission intensity till dose 1.2 kGy reflects the quality of shelled QDs. The variation in FWMH is shown in inset of Fig. 7(a). Further, relative quantum yield was calculated for CdTe QDs exposed to gamma radiation for both the cases (with and without shell source materials) at pH 12. The inset of Fig. 7(b) shows the plot of normalized Quantum Yield for the variation of gamma radiation dose. As can be seen from Fig. 7(b), the Quantum Yield (QY) has increased with increase in dose up to 1.2 kGy and then it has decreased for further doses in case of CdTe QDs with shell source materials. However, it has shown almost constant QY with increased doses, in case of irradiated CdTe QDs without shell source materials. It can also be observed that the QY increased almost linearly with dose up to 1.2 kGy indicating the epitaxial growth of CdS shell on CdTe QDs. After, 1.2 kGy, the QY decreased drastically and is different from QY of unshelled CdTe QDs. This indicates the complete quenching of QY for higher doses as CdS shell growth continues for increased gamma doses towards non-epitaxial manner. The constant value of QY for CdTe without shell source materials depicts simply the stabilisation of particles against radiation without shell formation.
neutral pH values are not suitable for the CdS shell formation even though particles were grown at this range of pH values in this case of MPA capped CdTe QDs. This is due to photo-oxidation of CdTe QDs rather than shell formation at less than or equal to neutral pH number of media against irradiation in the case of MPA capped CdTe QDs. The photo-oxidation is mainly because of production of reactive species from irradiation which caused protonation of MPA capping molecule as explained above. Such protonation lead to detachment of capping molecules on the QD surface and finally, resulted in agglomeration. In contrast to lower pH values, there was comparative improvement in pH 10 with continuous red shift and reduced rate of decrease in PL signal with extended curve's length for irradiated QDs in Fig. 6(a) says alkalinity helps particles to withstand during gamma irradiation. However, much intensified redshift with decreased PL intensity does not claim shell formation but agglomeration. Reduction of pH value of water when it was irradiated with gamma radiation was observed as shown in Supporting information (Fig. S6). This pH variation upon irradiation caused by production of reactive free radical species as mentioned earlier towards gamma irradiation of water. Therefore, shell growth may not be possible if the alkalinity is not strong enough to quench so-produced radical species in the water make a favourable environment for the shell growth. Therefore, pH 10 is also not sufficient for inorganic passivation even though there was comparative improvement in PL data. This suggests adopting a higher alkaline environment as there was appreciable sign of improvement in the pH values of alkalinity. As can be seen from Fig. 6, very slight redshift and saturation in PL peak intensity with decreased reduction rate were observed with increase in gamma radiation doses at pH 12. This suggests CdTe QDs were stable at this pH value against gamma radiation for the shell formation. Nevertheless, shell formation leads to increase in PL intensity with suitable redshift [36] which was absent even at this pH number. This clearly says pH 12 stabilizes QDs against irradiation but shortage of source of shell material is hindering the shell formation. Therefore, another set of CdTe QDs were irradiated by introducing new cadmium (CdCl2) and sulphur (MPA) sources to compensate shortage of source materials for shell formation as suspected. Complex of Cd+MPA as source materials with precursor amount 1 ml of CdCl2 form stock solution (365 mg/50 ml) and 20 µl MPA were added together to 40 ml of MPA capped CdTe QDs for irradiation at pH 12. This step completes all the requirements for CdS shell formation process. Systematic redshift in wavelength with doubled PL intensity is observed for a dose of 1.2 kGy and it can be seen in the Fig. 6(a & b) at pH 12 with shell source material. During gamma irradiation, the released S2- ions from MPA molecules (extra added) would combine with free Cd2+ ions (extra added) available in the solution and then they diffused through capping fence to form CdS shell around CdTe QDs (Note: these samples were utilised, as mentioned above, for TEM, XPS & XRD studies to correlate the shell formation). At the same experimental conditions, no increase of intensity was observed when the CdTe QDs were irradiated without Cd and S source materials at pH 12 environment (Fig. 6(a)). This clearly confirms that, such increase in PL intensity with redshift is due to the formation of CdS shell on CdTe QDs. This validates our proposed mechanism of radio-chemistry of QDs shown in scheme B for alkaline situation supported by other supporting studies mentioned above. The PL emission intensity was increased with radiation doses till 1.2 kGy and then decreased for later doses. Furthermore, moderate redshift in PL wavelength was observed till 1.2 kGy suggesting the upper dose limit to control the shell thickness. After 1.2 kGy, the PL intensity decreased along with very larger redshift indicating the non-epitaxial growth of CdS shell which finally resulted in bigger size QDs with increase of surface defects lead to amplified non-radiative emission pathways. Fig. 7 shows the PL intensity at different wavelengths for CdTe QDs irradiated with shell source materials at pH 12. Symmetric PL curves of
4. Conclusions The CdS shell formation from gamma assisted irradiation method depends on pH values of the colloidal media of MPA capped CdTe QDs. The higher pH values resulted in redshift (enabled CdS shell formation) whereas the lower pH values in blueshift (lead to photo-oxidation) of absorption spectra. Shifts in the XRD peaks towards higher angles and variation of features of rings in SAED pattern before and after irradiation supports CdS shell formation notion. TEM images revealed the extension of particle size with CdS shell on bare CdTe QDs after irradiation at pH 12 as there was an increase in average particle size after irradiation. Modification of chemical environment of Cadmium and Sulphur bonding on surface of core QDs is reflected in XPS spectra after irradiation at pH 12 confirmed the growth of CdS layer around CdTe QD. The PL Spectral shift which was patterned according to UV–vis spectral shift for all domains of pH values supports the radiolysis mechanism. Blue shift for acidic & neutral (as synthesised) and redshift for higher alkaline conditions were recorded confirming the increase of size (redshift) after irradiation at alkaline pH values. Increase of quantum yield until 1.2 kGy only after introducing shell source materials confirms the epitaxial CdS shell formation. pH 12 was concluded as optimum pH value suitable for CdS shell formation even though QDs were grown in neutral pH condition in the case of MPA capped CdTe QDs. Therefore, this also concludes that the pH value in which QDs were grown need not to be only the suitable value for shell growth by different materials. Acknowledgements Authors acknowledge CARRT, Mangalore University for extending gamma irradiation facility. The authors thank BRNS for supporting the work under a research project (No. 34/14/73/2014-BRNS/10493). Appendix A. Supplementary material Supplementary data associated with this article can be found in the online version at http://dx.doi.org/10.1016/j.jlumin.2017.06.019. References [1] [2] [3] [4]
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Journal of Luminescence journal homepage: www.elsevier.com/locate/jlumin
Preparation of fluorescent CdTe@CdS core@shell quantum dots using chemical free gamma irradiation method S.P. Rajua, K. Hareeshb, S. Chethan Paia, S.D. Dholec, Ganesh Sanjeeva, a b c
MARK
⁎
Microtron Centre, Department of Physics, Mangalore University, 574199, India School of Physics, University of Western Australia, Crawley, Perth, WA 6009, Australia Department of Physics, Savithribai Phule Pune University, 411007, India
A R T I C L E I N F O
A B S T R A C T
Keywords: Fluorescence Core@shell QDs Gamma rays Irradiation method TEM PL spectroscopy
This study investigates pH dependent gamma radiation effects on Mercaptopropionic Acid (MPA) capped CdTe quantum dots (QDs). Gamma radiation, as fast and chemical free catalyser, causes CdS Shell formation on MPA capped CdTe QDs at very high alkaline environment. UV–vis absorption spectroscopic results showed that, with increase in gamma radiation doses, the absorption peak of CdTe QDs decreased to lower wavelength (decrease in particle size) in both acidic & neutral medium (as synthesised), and it increased towards longer wavelength in basic medium indicating the possibility of growth of CdS shell over CdTe core QDs. X-ray diffractogram studies have revealed that the peaks of CdTe QDs shift towards higher angles after irradiation, supporting the notion of the core@shell particle structure of CdTe@CdS. TEM images confirmed the extra stretch around core particles from CdS material resulted in increase of average size of QDs. EDX study revealed the chemical composition of material system. X-ray photoelectron spectroscopic results revealed the shift in the binding energy of both Cd and S thereby indicating the change in the coordination condition after irradiation promoted the particles’ formation of core@shell in nature. The systematic photoluminescence spectroscopic investigation at all possible pH values against ascended gamma radiation doses revealed the formation of CdS shell over CdTe core at optimum pH value (i.e. only at basic pH 12). Contrarily, photo-oxidation of QDs’ was observed at neutral (and below) pH values towards gamma irradiation. The quantum yield is increased only after introducing external shell source material at highest alkaline media confirming the formation of CdS shell on CdTe core.
1. Introduction Quantum dots (QDs) have got attention due to their unique optoelectronic properties [1]. QDs are special case of nanoparticles can be grown in colloidal form with varied band gap in optical region using single material. Therefore, QDs play many materials’ role in specialised applications like solar cells [2,3], LEDs [4,5], Sensors [6,7], Bio-markers [8,9] etc. Such versatile materials are highly dispersible at the time of application for quality performance. Besides, they are high quantum yield fluorescent materials resistant to photo-bleaching with better spectroscopic properties. Therefore, they are the better materials than traditionally used dyes which lack in such quality parameters for the above mentioned applications [10]. Size (few nanometers) of this quantum systems are within their exciton Bohr radius to be called as Quantum dots. Among many QDs, CdTe QD has received attention because of its band gap tunability particularly for solar cell applications. Moreover, it is known for its fast injection of electrons into active layer of TiO2 compared to CdSe and ⁎
Corresponding author. E-mail address: ganeshsanjeev@rediffmail.com (G. Sanjeev).
http://dx.doi.org/10.1016/j.jlumin.2017.06.019 Received 22 January 2017; Received in revised form 6 June 2017; Accepted 8 June 2017 Available online 12 June 2017 0022-2313/ © 2017 Elsevier B.V. All rights reserved.
other materials [11]. Additionally, colloidal CdTe QDs capped with thiols (as hydrophilic capping agents) synthesised in aqueous media have found applications in bio-systems. This skips another burden of indispensible step i.e., dispersing QDs in to water when they are grown in organometallic solvents. Such steps are complicated and inefficient to utilize QDs in bio-applications. Moreover, capping agents are responsible for the reduction of non-radiative recombination of excitons’ (electron-hole pairs). This increases the radiative fluorescence decay from QDs as they passivate surface dangling bonds (or called as defects). Besides, these cappers help to synthesis particles along with maintaining their colloidal stability. Therefore, passivation with more in number of capping agents leads to increase in quantum yield and colloidal stability. However, there is practical inefficiency of intense capping as alignment of thiol molecules on non-uniform QD's surface may create hydrophobic chemical patches. These patches restrict intense passivation (called steric hindrance) at the time of synthesis, despite the careful selection of capping molecule [12]. In account of the above, just grown such core QDs hardly achieves intense passivation for
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literature. This study shows CdS shell can be grown even in the case of CdTe QDs capped with MPA at highest stable alkaline value (pH 12) only after introducing cadmium and sulphur source material with controlled gamma radiation doses. Moreover this work also concludes that QDs grown in a pH value need not be suitable for shell growth of other material on the same QD core. The gamma radiation assisted synthesised CdTe@CdS core@shell QDs capped with MPA are of high quality QDs which can be applied in many applications such as solar cells or sensors or biomarkers etc.
the above quality properties. Among all other addressed solutions for the above problems, inorganic shell growth on bare QD surface is a popular way out after synthesis of QDs. Core@shell QDs of type I and II enable bigger band gap materials (shell) growth on smaller band gap (core) QDs or viseversa irrespective of their materials’ groups [13,14]. This sort of choosy shell growth leads to manufacturing ease in creation of nanoscale semiconducting junctions with reduced photo-bleaching for multiple applications. This also helps in reduction of cytotoxicity (if bio-friendly shells like ZnS are grown) when hazardous elements are used in synthesis. Therefore, inorganic shell growth from multiple materials with controlled morphometric aspects is required for having good quality QDs. Shelling of core CdTe QDs with CdS material is suitable to fix the above problem. This combination of materials’ is more advantageous as CdS possesses lower lattice mismatch (< 10%) with the core CdTe compared to other materials. Therefore, such atomic passivation leads to flexible spread and epitaxial coverage of CdS shell over CdTe QDs surface. This would saturate unpassivated chemical sites there by reducing steric hindrance even though surface possesses nonuniform corners where capping molecules are not allowed to reach. CdS shells were grown over CdTe [15], CdSe [16], HgSe [17], HgTe [18], ZnSe [19], ZnTe [20] core QDs, using different methods like reflux method [15], Aerosol Flow method [21], Microwave method [22], etc. All these methods use either harsh chemicals with different expensive setup or elevated temperature with time consuming complicated procedures. Therefore, it is needed to establish a method which avoids such complications. Nowadays, the shell growth by radiolytic method [23,24] has been extensively used for its simplicity. Radiation is a clean catalyser with no chemical contamination or usage of elevated temperature. This potential method is also used as catalyser for synthesis [25–27] and other intervention steps in material processing [28,29]. Therefore, irradiation can be a facile method to get shell formation over pre-synthesised core QDs. There are few reports in literature on the utility of both ionising and non-ionising radiation in material engineering especially in the field of nanoparticles. Most of them have used UV radiation for the purpose which is time consuming [23] and leads to increase the sample temperature. This would create some problems such as continuous heating up of samples for very long time or any other solvent related degradation problems which may reduce quality of QDs for long term usage. This has made us to carry out the study using ionising radiation as there are no specific reports on the growth of CdS shell on CdTe core in few minutes. The present study utilizes ionising gamma radiation assisted method to grow CdS shell over CdTe QDs. A few minutes of expose to gamma radiation leads to fast and intense CdS shell formation as the energy of gamma is moderately high compared to UV light. Moreover, gamma radiation source provides continuous and uniform radiation from all directions so that all samples are being irradiated for having high uniformity shell formation. Additionally, CdTe QDs capped with MPA molecules were suffering from photo-oxidation rather than shell formation upon irradiation as reported in literature [13,30]. On the other hand, shell formation to this QDs is important as MPA capped to CdTe QDs leads to higher crystallinity, quantum yield and much lower initial size distribution, than many other thiol molecules (such as TGA, GSH, TGH) [31]. In view of the above, a detailed investigation of all parameters involved in shell growth which are not supporting shelling process, in this case, is a combined objective of the present study. This also helps to see the possibility of extended studies on growth of different types of core@ shell QDs such as CdTe@ZnS, CdTe@CdS@ZnS etc and thicknesses of inorganic shells using different capping molecules through irradiation method. This required investigation includes synthesis parameters like variation of pH of the solution environment or availability of shell source materials or successful catalization by irradiation or all the above. Unfortunately, the reports on the comprehensive investigation of pH dependent irradiation effects on CdTe QDs are very sparse in
2. Experimental section 2.1. Materials Analytical grade chemicals were used as it is without any further purification. 3-Mercaptopropionic acid, Sodium borohydrate, TriSodium citrate dehydrate were procured from MERCK. Sodium tellurite (99% pure) and Cadmium chloride (99%) were received from ALDRICH and FLUKA respectively. Rhodamine B was from Sigma Aldrich (Quantum Yield = 0.35 in water and this was used as reference dye for relative quantum yield measurements). Double distilled water was used throughout the experiments. 2.2. Synthesis of MPA capped CdTe QDs MPA Capped CdTe QDs were synthesised using hydrothermal synthesis method and the detailed procedure is given in supplementary information. In brief, molar ratio of Cd:Te:MPA::1:0.25:8 was maintained during synthesis of QDs. 100 mg Tri-Sodium citrate dehydrate and 50 mg Sodium Borohydrate were dissolved in 40 ml double distilled water as per the systematic synthesis protocol. As prepared solution was taken in air tighten Teflon autoclave, with stainless steel housing. This autoclave was maintained at 180 °C for 11 min to grow suitable size of QDs. The pH of as grown MPA capped CdTe colloidal QD solution was measured as 7 soon after the synthesis. 2.3. Irradiation of MPA capped CdTe QDs The pH of as prepared CdTe QDs was adjusted in the range from 4 to 12 by the addition of HCl and 3 M NaOH drop wise slowly to set acidic and basic conditions respectively. These solutions were taken in different polypropylene bottles and exposed to Co-60 gamma radiation source with a dose rate of 9.5 kGy/h for different doses in the range from 0.3 to 20 kGy. The details of Co-60 gamma radiation chamber are given in Supplementary information. 2.4. Characterization UV–Vis spectroscopic (UV–Vis) studies of all the samples were carried out using Shimadzu UV − 1800 – Spectrophotometer. XRD analysis was done from Rigaku Miniflex 300 X-ray Diffractometer with Cu-kα as a radiation (wavelength = 1.5406 Å). Photoluminescence spectroscopy (PLS) was carried out using Perkin Emler, LS 55. The surface morphology was recorded using TEM of model Tecnai G2 U-thin 200 kV, LaB6 filament. X-ray photoelectron spectroscopic (XPS) was carried out using Omicron EA 125 analyzer at room temperature in an ultra-high vacuum. Relative quantum yield of QDs was calculated using Rhodamine B as reference dye. 2.5. Sample preparation 2.5.1. TEM-Imaging Dots were cleaned using propanol and dispersed in ethanol with required dilution. So-dispersed dots were drop casted on copper grid and dried at room temperature for two days. These grids were directly mounted on sample holder of the TEM setup. 18
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Fig. 1. Capture of QDs in vials with varied pH values of solution.
2.5.2. PL-Spectroscopy Samples were characterized in colloidal (in water) form only. 3. Results and discussion Fig. 1 depicts the colloidal solution of CdTe QDs at different pH values from pH 4 to pH 12. When pH was brought down to appreciable acidity, protonation competes with capping agents’ bonding on QD's surface leading to oxidation. This reduces colloidal stability which resulted in agglomeration and precipitation of particles. It can be clearly seen from Fig. 1 as pH number 5 (and bellow pH values) was ended in precipitation of QDs followed by divorcing solvent. Colloidal stability is important in the case of shell formation around individual particles. Higher Colloidal stability frees individual QDs to open their surfaces for atomic shell coverage. This is very difficult or impossible when QDs are in congested or precipitated form. Higher pH values (≥ 6) lead to stable colloidal stability as capping molecules binding dominates against protonation. This can be clearly seen from Fig. 1. Therefore, the solution with pH≥6 is highly colloidal with greater photo-emission. On the other hand, variation of pH in pre or post synthesised nanoparticles may cause appreciable change in size (hydrodynamic), quantum yield as well as stability of nanoparticles [32]. Often, it may hinder nanoparticle growth itself if the pH environment is not suitable. Furthermore, pH causes variation in zeta potential, agglomeration, network formation, photo-oxidation and sensitisation of surface of colloidal QDs for different catalysers [33–35]. Hence, shell formation is hardly achieved if there is weak colloidal desperation caused by above reasons. Therefore, this study mainly focuses on possibility of CdS shell formation with different pH (from 6 to 12) against gamma radiation to find out the proper pH number for CdS shell formation and also to answer why photo-oxidation is taking place in the case of MPA capped CdTe QDs when irradiated. This kind of investigations need proper understanding of the process of radiation induced changes (primary or secondary) in the growth media. The water as the growth media undergoes radiolysis due to irradiation forming many free radical species such as H*, H2O2*, H2O*, H3O*, etc. The formation of these free radicals may lead to following two cases. (1) The formation of these free radicals will vary the pH of solution which may affect QD stability as these free radicals will directly interact with surface of CdTe QDs. This interaction may eventually lead to photo-oxidation and reduction in QD size along with decreased colloidal stability when exposed to gamma radiation. (2) Otherwise, these free radicals may create favourable environment for the shell growth by releasing shell source materials resulting an increase in the size of particles. Therefore, variation in colloidal environment needs to be monitored while irradiating QDs. The mechanism of the chemical reaction which may occur during gamma irradiation in acidic and basic environment is given in Scheme 1. As can be seen from Scheme 1(b), at basic environment, S2− ions get released from MPA molecule as a result of gamma irradiation. These sulphur ions combined with free Cd2+ ions in the solution to form CdS shell around the CdTe QDs. When the chemical environment turns to acidity, MPA molecules on the surface of QDs were trimmed off by repeated protonation to cause removal of capping boundary (as in Scheme 1(a)) assisted by gamma radiation. This finally leaves QDs without capping agents in solution. These bare QDs without capping
Scheme 1. The reaction mechanism during gamma irradiation in (a) acidic environment and (b) basic environment.
agents attract each other to cause agglomeration. This leads to drastic quenching of PL emission followed by increase of surface defects and precipitation of QDs. However, photochemistry is complex to predict exact mechanism of the reaction when inorganic nanoparticles are on discussion. However, results obtained from spectroscopic analysis help to analyse the phenomenon to support the proposed reaction mechanism and core@shell hypothesis. UV–vis spectra of gamma irradiated CdTe QDs at different pH values are shown in Fig. 2. The absorption peak of CdTe QDs shifts towards lower wavelength at pH 6 (blue shifting) indicating decrease in the particle size followed by narrowed quantum confinement. Similar kind of variation was observed when CdTe QDs were irradiated at pH 7. However, when CdTe QDs were irradiated at pH 10 and 12, the absorption peak slightly shifts towards higher wavelength side (red shifting) indicating the possibility of CdS shell formation. This is due to alteration of conduction band alignment in core and shell materials together by inducing lattice strain. This is reflected in redshift (within limit) of absorption spectra (discussed below in detail). This is mainly due to the increase in pH values. In addition to this, the absorption spectra were observed with clear and wider redshift at pH 12, compared to that at pH 10. Thus, it clearly indicates that the higher alkalinity (pH 12) results in thicker shell formation. This contrast behaviour of irradiated QD solutions’ for acidic and alkaline pH numbers can be seen in Fig. S1 in the Supporting information. The irradiated QDs in vials at pH 7 & 6 (after a few days) are colloidally unstable with reduced fluorescence emission. On the other hand, QDs at pH 12 condition were found with high colloidal stability with greater photo-emission (appear in red colour to naked eyes). Typical X-ray diffractogram (Fig. 3) shows its pattern for pristine and gamma irradiated QDs’ at a dose of 1.2 kGy (this particular dose shown greater PL emission property for QD with colloidal stability towards gamma irradiation which will be discussed later) maintained at pH 12. Pristine CdTe showed peaks at 25°, 42° and 49° respectively corresponding to (111), (220) and (311) planes which is in agreement with the standard diffraction pattern for cubic CdTe material (JCPDS file no. 03-065-1047). XRD pattern (Zinc blende structure) of the core and shell materials is almost same with shift in their peak position for their bulk form. These three peaks were proportionately shifted to higher angles after irradiation. The FWHM value of peaks’ for irradiated (CdTe@CdS) samples was increased compared to pristine CdTe QD's. This is due to induce of lattice strain from core@shell structure in order to grow epitaxial junction. This crystal structure modification was further confirmed with SEAD ring's pattern. SEAD pattern is complimentary for XRD studies. This study was conducted while taking TEM images. Three 19
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Fig. 2. Absorption spectra of MPA capped CdTe QDs at different pH values (6, 7, 10 & 12) for varied gamma radiation doses.
particle size is increased to 4.5 nm after irradiation confirming the formation of CdS shell over CdTe core. Inset of Fig. 4(a1 & b1) shows of single particles of CdTe @CdS and CdTe QDs respectively. Samples were imaged with TEM for dried samples. Optical spectra were taken for samples in liquid form. This phase (dried and liquid) difference would also result in change of particle size. Particles in water possessed larger size due the increase of hydrodynamic radius around individual particle which is created from extra ionic boundary. This boundary will be removed when they are dried. However, such increase or decrease of particle size due to the above process is proportional in liquid and solid phases for the actual variation in the particle size. Fig. S3 of TEM image shows core@shell particles with 5 nm resolution in the Supporting information. XPS is surface sensitive tool to analyse nature of chemical bonding over the surface of materials. Sulphur and Cadmium are the shell materials for CdS layer formation on CdTe QDs surface. Cadmium is common element in both core and shell materials of CdTe@CdS QDs. Furthermore, Tellurium and Sulphur are purely core and surface related elements respectively attached to Cadmium. Therefore, shift in binding energy of Cadmium and Sulphur before and after irradiation at pH 12 gives the information of CdS shell formation. Fig. 6(a) shows high resolution XPS spectra of S2P of CdTe before and after irradiation. The Sulphur peak of pristine CdTe is deconvoluted into two peaks viz., S 2P3/2 and S 2P1/2 at 163.86 eV and 161.63 eV respectively. These doublet peaks are separated by 2.23 eV with intensity of 1:2 ratios. For gamma irradiated CdTe QDs, these sulphur related doublet peaks were shifted towards lower energy levels compared to pristine CdTe i.e., at 163.09 eV (S2P3/2) and 160.25 eV (S2P1/2) respectively. Such shift in the binding energy of sulphure confirmed the process of surface modification of CdTe QDs with CdS shell by irradiation and which is in agreement with the reported literature [23]. Additionally, Fig. 5(b) shows high resolution XPS spectra of Cadmium for pristine and gamma irradiated CdTe QDs. Peaks of Cadmium also evolved with doublet peaks. As can be seen from Fig. 6(b), Cd showed two peaks at
Fig. 3. XRD pattern of QDs before (CdTe) and after irradiation (CdTe@CdS).
major and broad rings which are situated with common centre can be clearly seen in the inset of Fig. 4(a & b). These three rings correspond to the same crystal planes 111, 220 and 311 respectively (from shorter radius to larger) attributed in XRD peaks at mentioned angles. These rings features were also altered after the formation of the CdS shell corroborating XRD pattern to confirm the modification of material structure. Fig. S2 of EDX spectra for core and core@shell materials in the Supporting information reveal the chemical composition of samples before and after irradiation respectively. Similar kind of variation in the diffraction pattern was observed during the growth of core@shell QDs in literature [36]. This provides further support for the formation of the CdS shell on the CdTe Core. TEM images of CdTe QDs (at pH 12) before and after irradiation at a dose of 1.2kGy are shown in Fig. 4. The average particle size of CdTe QDs before irradiation is found to be around 4 nm. However, this 20
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Fig. 4. TEM images and size distribution of QDs before (b) and after (a) irradiation respectively at pH 12. SAED pattern of QDs for the same can be found in the inset of (a) and (b). The top inset (a1) shows the shelled structure.
Fig. 5. XPS spectra for shelled and unshelled QDs (before and after irradiation). Sulphure (a) and Cadmium (b) Doublet spectra.
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Fig. 6. Variation of PL spectral intensity (a) & wavelength (b) of MPA Capped CdTe QDs with varied gamma radiation doses.
when CdS shell is grown on CdTe core epitaxially by reducing the bandgap of the whole core@shell system. This lattice strain is increased with increase in thickness (epitaxially) of the shell material. This small lattice strain is may not be seen through TEM images as the lattice mismatch is only around 10%. However, this induced lattice strain is enough to modify PL emission peak position due to the above reason (conduction band edge alteration). Therefore slight redshift is the indication of shell growth (can be used as measurement of shell thickness also with increase in redshift to higher wavelength) [37]. Therefore, the shifting of emission signal position towards higher wavelength indicates growth of bigger particles and increase in intensity of emission signal is a measure of intense passivation of QDs. Therefore, increase in both PL peak intensity and PL peak wavelength indicates growth of bigger particles with intense passivation with a shell against gamma radiation doses. It can be seen from Fig. 6(a & b) that such increase in peak intensity and redshift of wavelength is observed more at pH 12 (with shell source material) where the absorption spectral shift was also clear at this pH number 12 towards higher wavelength ( see Fig. 2(d)). At pH 6 environment, abrupt reduction of intensity with short lived curves in PL spectra at very lower gamma doses itself is due to higher instability of particles to gamma radiation at this situation (curve pH 6 in Fig. 6(a & b)). This kind of instability is due to photo-oxidation of QDs against irradiation resulted in separation of solvent and particles at this pH environment corroborating the UV–vis spectroscopic results (Fig. 2 for pH 6). The particles might have got agglomerated and precipitated due to removal of capping agents assisted with gamma radiation at this pH value. This result, also, supports the proposed radiolysis reaction mechanism for lower pH values against gamma radiation doses where CdTe QD was left un-passivated. The un-passivated QDs were joined together, finally, turned to bulky material. Therefore, pH6 is not favourable for the shell growth. Primarily, stability of QDs in solution is required the most to grow a shell on their surfaces using irradiation method. However, irradiation of QDs at pH 7 was also not showed much improvement in PL properties against gamma doses as shown in the same figure (Note: Tall head PL intensity of QDs was observed at this pH 7 situation among all other pH values. The PL graph of wavelength & intensity at different pH values is given in the Supporting information (Fig. S5). Therefore, PL Spectral shift of pH 6 and 7 has not followed a trend and ended their curves as well at important doses for the shell formation (Fig. 6(a & b)). This irregularity of spectral shift with sudden reduction of intensity in both the cases suggests that these lower and
404.89 eV and 411.77 eV corresponding to Cd 3d1/2 and Cd 3d5/2 respectively with peak to peak separation as 6.88 eV which is also in agreement with the reported values [25]. It can also be seen from Fig. 5(b) that Cd peaks shifted towards higher values of binding energies, i.e. to 405.33 eV and 412.28 eV after irradiation compared to pristine CdTe QDs. This shift in the binding energy confirms the formation of CdS shell on CdTe core as the binding energy of CdS shell is different from CdTe core due to change in the co-ordination condition [25]. In order to give a better explanation, PL studies were carried out for gamma irradiated CdTe QDs at different pH values such as 6, 7, 10 and 12 (with and without shell source material). The corresponding PL spectra for all values of pH are shown in Fig. S4 (Supporting information) and the variation of PL peak intensity & PL peak wavelength with gamma radiation doses are shown in Fig. 6(a) and (b) respectively. Furthermore, graph of FWHM (inset 7(a)) and relative Quantum Yield (QY) (inset 7(b)) against gamma doses with PL curves of optimised pH 12 condition for shell formation (with shell source materials) are shown in Fig. 7. Slight redshift in PL emission spectra indicates further growth of CdTe QDs’ with different material (CdS as shell). Such redshift in CdTe QDs’ is due to alteration of conduction band edge alignment from induced lattice strain in both materials. Such lattice strain is developed
Fig. 7. PL spectra of gamma irradiated QDs after introduction of Cd+MPA complex at pH 12. Inset: Variation of FWMH (a) and normalized Quantum Yield (b) with radiation dose.
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very slight variation in FWMH with increased emission intensity till dose 1.2 kGy reflects the quality of shelled QDs. The variation in FWMH is shown in inset of Fig. 7(a). Further, relative quantum yield was calculated for CdTe QDs exposed to gamma radiation for both the cases (with and without shell source materials) at pH 12. The inset of Fig. 7(b) shows the plot of normalized Quantum Yield for the variation of gamma radiation dose. As can be seen from Fig. 7(b), the Quantum Yield (QY) has increased with increase in dose up to 1.2 kGy and then it has decreased for further doses in case of CdTe QDs with shell source materials. However, it has shown almost constant QY with increased doses, in case of irradiated CdTe QDs without shell source materials. It can also be observed that the QY increased almost linearly with dose up to 1.2 kGy indicating the epitaxial growth of CdS shell on CdTe QDs. After, 1.2 kGy, the QY decreased drastically and is different from QY of unshelled CdTe QDs. This indicates the complete quenching of QY for higher doses as CdS shell growth continues for increased gamma doses towards non-epitaxial manner. The constant value of QY for CdTe without shell source materials depicts simply the stabilisation of particles against radiation without shell formation.
neutral pH values are not suitable for the CdS shell formation even though particles were grown at this range of pH values in this case of MPA capped CdTe QDs. This is due to photo-oxidation of CdTe QDs rather than shell formation at less than or equal to neutral pH number of media against irradiation in the case of MPA capped CdTe QDs. The photo-oxidation is mainly because of production of reactive species from irradiation which caused protonation of MPA capping molecule as explained above. Such protonation lead to detachment of capping molecules on the QD surface and finally, resulted in agglomeration. In contrast to lower pH values, there was comparative improvement in pH 10 with continuous red shift and reduced rate of decrease in PL signal with extended curve's length for irradiated QDs in Fig. 6(a) says alkalinity helps particles to withstand during gamma irradiation. However, much intensified redshift with decreased PL intensity does not claim shell formation but agglomeration. Reduction of pH value of water when it was irradiated with gamma radiation was observed as shown in Supporting information (Fig. S6). This pH variation upon irradiation caused by production of reactive free radical species as mentioned earlier towards gamma irradiation of water. Therefore, shell growth may not be possible if the alkalinity is not strong enough to quench so-produced radical species in the water make a favourable environment for the shell growth. Therefore, pH 10 is also not sufficient for inorganic passivation even though there was comparative improvement in PL data. This suggests adopting a higher alkaline environment as there was appreciable sign of improvement in the pH values of alkalinity. As can be seen from Fig. 6, very slight redshift and saturation in PL peak intensity with decreased reduction rate were observed with increase in gamma radiation doses at pH 12. This suggests CdTe QDs were stable at this pH value against gamma radiation for the shell formation. Nevertheless, shell formation leads to increase in PL intensity with suitable redshift [36] which was absent even at this pH number. This clearly says pH 12 stabilizes QDs against irradiation but shortage of source of shell material is hindering the shell formation. Therefore, another set of CdTe QDs were irradiated by introducing new cadmium (CdCl2) and sulphur (MPA) sources to compensate shortage of source materials for shell formation as suspected. Complex of Cd+MPA as source materials with precursor amount 1 ml of CdCl2 form stock solution (365 mg/50 ml) and 20 µl MPA were added together to 40 ml of MPA capped CdTe QDs for irradiation at pH 12. This step completes all the requirements for CdS shell formation process. Systematic redshift in wavelength with doubled PL intensity is observed for a dose of 1.2 kGy and it can be seen in the Fig. 6(a & b) at pH 12 with shell source material. During gamma irradiation, the released S2- ions from MPA molecules (extra added) would combine with free Cd2+ ions (extra added) available in the solution and then they diffused through capping fence to form CdS shell around CdTe QDs (Note: these samples were utilised, as mentioned above, for TEM, XPS & XRD studies to correlate the shell formation). At the same experimental conditions, no increase of intensity was observed when the CdTe QDs were irradiated without Cd and S source materials at pH 12 environment (Fig. 6(a)). This clearly confirms that, such increase in PL intensity with redshift is due to the formation of CdS shell on CdTe QDs. This validates our proposed mechanism of radio-chemistry of QDs shown in scheme B for alkaline situation supported by other supporting studies mentioned above. The PL emission intensity was increased with radiation doses till 1.2 kGy and then decreased for later doses. Furthermore, moderate redshift in PL wavelength was observed till 1.2 kGy suggesting the upper dose limit to control the shell thickness. After 1.2 kGy, the PL intensity decreased along with very larger redshift indicating the non-epitaxial growth of CdS shell which finally resulted in bigger size QDs with increase of surface defects lead to amplified non-radiative emission pathways. Fig. 7 shows the PL intensity at different wavelengths for CdTe QDs irradiated with shell source materials at pH 12. Symmetric PL curves of
4. Conclusions The CdS shell formation from gamma assisted irradiation method depends on pH values of the colloidal media of MPA capped CdTe QDs. The higher pH values resulted in redshift (enabled CdS shell formation) whereas the lower pH values in blueshift (lead to photo-oxidation) of absorption spectra. Shifts in the XRD peaks towards higher angles and variation of features of rings in SAED pattern before and after irradiation supports CdS shell formation notion. TEM images revealed the extension of particle size with CdS shell on bare CdTe QDs after irradiation at pH 12 as there was an increase in average particle size after irradiation. Modification of chemical environment of Cadmium and Sulphur bonding on surface of core QDs is reflected in XPS spectra after irradiation at pH 12 confirmed the growth of CdS layer around CdTe QD. The PL Spectral shift which was patterned according to UV–vis spectral shift for all domains of pH values supports the radiolysis mechanism. Blue shift for acidic & neutral (as synthesised) and redshift for higher alkaline conditions were recorded confirming the increase of size (redshift) after irradiation at alkaline pH values. Increase of quantum yield until 1.2 kGy only after introducing shell source materials confirms the epitaxial CdS shell formation. pH 12 was concluded as optimum pH value suitable for CdS shell formation even though QDs were grown in neutral pH condition in the case of MPA capped CdTe QDs. Therefore, this also concludes that the pH value in which QDs were grown need not to be only the suitable value for shell growth by different materials. Acknowledgements Authors acknowledge CARRT, Mangalore University for extending gamma irradiation facility. The authors thank BRNS for supporting the work under a research project (No. 34/14/73/2014-BRNS/10493). Appendix A. Supplementary material Supplementary data associated with this article can be found in the online version at http://dx.doi.org/10.1016/j.jlumin.2017.06.019. References [1] [2] [3] [4]
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