Designing Coupled Quantum Dot with ZnS-CdSe Hybrid Structure for

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C: Physical Processes in Nanomaterials and Nanostructures

Designing Coupled Quantum Dot with ZnS-CdSe Hybrid Structure for Enhancing Exciton Lifetime Debadrita Bhattacharya, Sulay Saha, Vishnu Prasad Shrivastava, Raj Ganesh S Pala, and Sri Sivakumar J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b01210 • Publication Date (Web): 30 Mar 2018 Downloaded from http://pubs.acs.org on March 30, 2018

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The Journal of Physical Chemistry

Designing Coupled Quantum Dot with ZnS-CdSe Hybrid Structure for Enhancing Exciton Lifetime # # ҂ #, †,* Debadrita Bhattacharya, Sulay Saha, Vishnu Prasad Shrivastava, Raj Ganesh S. Pala, and Sri #, †, ┴, ‡, * Sivakumar # Department of Chemical Engineering, Indian Institute of Technology, Kanpur, 208016, India ҂

Department of Physics, Indian Institute of Technology, Kanpur, 208016, India

†

Materials Science Programme, Indian Institute of Technology, Kanpur, 208016, India ┴ Centre for Environmental Science and Engineering, Indian Institute of Technology, Kanpur, 208016, India ‡ Centre for Nanoscience and Soft Nanotechnology, Indian Institute of Technology, Kanpur, 208016, India * Corresponding authors: [email protected], [email protected]

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Abstract Fabrication of coupled quantum dot (CQD) may provide an excellent platform for the realization of high end opto-electronic applications as well as quantum information processing. CQD can be synthesized by well-known cation exchange method and less explored ‘nanoparticle fusion’ method, in which the latter involves the coupling between constituting facets of two different semiconductors. Herein, we elucidate the mechanistic formation pathway of different heterostructures with ZnS and CdSe quantum dots (QD) with an emphasis on the formation of CQD comprised of bicompartmental Janus structure (i.e. Janus structure consisting of two compartments with the two QDs coupled) via ‘nanoparticle fusion’. With increase in the ratio of Cd/Zn from 0.9:1 1.3:1 2.5:1 4:1 12:1, we observe the evolution of structure from CdZnSeS alloy Acorn Janus Bicompartmental JanusA with ZnS Zinc Blende (ZB) – CdSe Wurtzite (Wz) Bicompartmental JanusB (ZnS/CdSe-both ZB) and eventually to CdZnSeS alloy core - CdSe thick shell. Interestingly, the CQD possess two distinct emission bands (570/630 nm) in which 570 nm emission arises from the formation of new electronic states due the strong coupling between the two QDs whereas 630 nm is the characteristic CdSe emission. Further, the coupling enhances the exciton lifetimes of 570/630 nm emission (Bicompartmental JanusA - 36/31 ns and Bicompartmental JanusB - 41/94.8 ns) which can be exploited in various applications. Further, the DFT simulations provide the heuristics behind the formation of certain heterostructure with strain and interfacial energies of particular facets dictating the morphology during coupling of QDs.


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1. Introduction Development of coupled quantum dot (CQD) has attracted considerable interests due to their unique electronic/optical properties useful in modern electronic (e.g. quantum information processing1-3 and field effect transistors), energy (e.g. solar cells and solar hydrogen generation), 4,5

biological (e.g. biolabelling and biosensors),

6-7

and optical (lasers, sensors etc.)

8-10

applications. CQD (sometimes referred as artificial molecules)11 consists of two different semiconductor quantum dots which are interlinked face to face through a facet specific attachment. The reduction in the interdot spacing enhances the interaction between the electronic wavefunctions of the constituent QDs which produces cooperative physical phenomena such as formation of new intermediate electronic states,12 charge transport, and long-range optical tunability13-16 with enhanced carrier lifetimes.13 The extent of coupling is mainly determined by the size, morphology, composition of constituent QDs, and facet attachment between the quantum dots. Further, the mechanistic formation pathway of CQD is also not well understood. In this regard, ZnS-CdSe CQD quantum dot can be an attractive candidate for the above said applications in which ZnS is a wide band gap semiconductor (3.7 eV at 300 K) whereas CdSe QDs possess lower band gap (1.75 eV) and coupling of such particles promote separation of electron and holes in certain particle geometries. Both the semiconductors have been well exploited as individual QDs17-18 as well as core/shell structure (CdSe-ZnS) to achieve enhanced optical properties. Further, ZnS-CdSe CQD can acquire improved optical properties (e.g. enhanced lifetime, tunable emission spectra) compared to the above said morphologies which has not been explored till to date.



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Herein, we report the fabrication of ZnS-CdSe CQD with enhanced optical properties (high lifetime and tunable emission spectra). The CQD can be fabricated by different methods such as, well known cation exchange method (cation exchange between two semiconductors consisting of same anion and two different captions),15 less explored nanoparticle fusion method,13, 19-21 and self-assembly. The extent of coupling is determined by the fabrication method, in which nanoparticle fusion method can have better control over the above mentioned processes because coupling is facilitated via surface specific attachment through chemical bonding. The coupling between particular specific crystallographic facets of the constituent quantum dots can be tuned by varying the reaction parameters such as co-solvent ratio, choice of surfactant, temperature, and ratio of precursors. In the current report, we have prepared ZnS-CdSe CQD through a nanoparticle fusion method consisting of two steps in which ZnS seed is formed in the first step and subsequently CdSe-ZnS CQD dot is formed by adding the prerequisite amount of Cd and Se precursors in the solution in the second step. Interestingly, different morphologies have been obtained with varying Cd:Zn ratios such as alloy (0.9:1), Acorn Janus (1.13:1), bicompartmental JanusA (2.5:1), bicompartmental JanusB (4:1), and alloy core - CdSe thick shell (12:1). The bicompartmental JanusA and JanusB are considered as CQD in which coupling between CdSe and ZnS leads to formation of distinct interface that enhance exciton lifetime. In all the cases, the crystal phase of ZnS is Zinc Blende (ZB) and CdSe is Wurtzite (WZ) phase except JanusB consisting of both the constituting QDs in ZB phase. We note that the current methodology provides better control over the size and morphology of QDs compared to the existing synthesis procedures to obtain Cu2S-ZnS (one-pot synthesis), ZnSe-CdSe (blending at high temperature) and PbSe-CdSe (cation exchange) CQD.13,

15, 21-22

Among the different

morphologies, CQD (i.e. bicompartmental JanusA and JanusB) possess better optical properties,


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in particular lifetime of exciton is higher compared to other morphologies. We believe that formation of distinct alloyed interface between constituting QDs in the bicompartmental JanusA and JanusB leads to the formation of intermediate metastable electronic state which traps the charge-carrier and simultaneously detraps them to return to the original state. As a result, holeelectron pair interaction is delayed which leads to delayed luminescence/enhancement of excited state lifetime for e- in the interface to a significantly high extent in both the CQDs (lifetime ~ 40 ns), compared to the other heterostructures synthesized by us. Moreover, the lifetime of excited state e- of CdSe has significantly increased in case of bicompartmental Janus heterostructures, especially in JanusB (lifetime ~ 94 ns), as compared to the maximum obtained lifetime value for CdSe QD (lifetime ~ 14 ns) core23 or, CdSe/ZnS core/shell heterostructure (~ 9 ns).24 In JanusB, CdSe is in ZB phase and this might result in higher lifetime of CdSe compared to the lifetime WZ-CdSe in JanusA, supported by other reports too.25-26 The reaso n behind the enhanced lifetime value in case of Janus heterostructures compared to the other heterostructures needs to be further investigated and this can lead to an interesting field of research in near future for structure dependent PL properties. To understand the mechanistic aspect of heterostructure formation considering ZnS-CdSe system, several characterization techniques have been used such as XRD (X-ray diffraction), TEM (Transmission electron microscopy) & HRTEM (High-resolution transmission electron microscopy), photoluminescence (PL) studies, and XPS (X-ray photoelectron spectroscopy). Further, the experimental results have been analysed with the help of the DFT (Density functional theory) to rationalize heterostructure morphology. The simulations suggest that the interface formation of a high surface energy facet of seed (or core) and a low surface energy facet of shell leads to formation of CQD with bicompartmental Janus structure via ‘nanoparticle



fusion’ mechanism, whereas the interface comprising of a low surface energy facets of seed and a low surface energy facet of shell or even a high energy facet of shell would result in core/shell structure. We believe that the insights borne out of these studies will improve our understanding behind the intriguing mechanism behind the formation of CQD which is of prime interest to enhance the number of total sites involved in coupling and results in suitable interface formation between the constituent components of the couple QDs.27-28 2. Methods 2.1 Materials

Cadmium oxide (CdO, ≥99.99%), zinc acetate (Zn(CH3COO)2, 99.999%), sulfur powder (S, 99.98%), selenium powder (Se, 99.99%), 1-octa-decene (ODE, 90%), oleic acid (OA, 90%), trioctylphosphine (TOP, 97%), anhydrous chloroform (99.8%) were used as purchased from Sigma Aldrich. Hexane (95.0%) and acetone (99.5%) were purchased from Fisher chemicals. 2.2 Synthesis of ZnS core

In the present work, ZnS seed particles are synthesized through hot injection method by the reaction of Zn-acetate (12 mmole) and TOP (surfactant) - Sulfur complex in the presence of OA (surfactant) and ODE solvent (≥ 310 °C). Zn-acetate is dissolved in 21.6 ml OA and 60 ml ODE in a 500 ml flask. The mixture was heated at 150 °C, degassed with purging N2 gas and further heated to 310 °C to form a clear solution. After obtaining the solution, 12 mmole S powder (0.38g) in 9 ml TOP and 10 ml ODE was injected and 310°C was maintained for 1 hour. The obtained ZnS quantum dots are purified by adding 120ml chloroform: acetone (1:2) followed by centrifugation at 5000 rpm for 5 minutes (5 times) to separate out the ZnS nanoparticles and disperse in n-hexane. The obtained ZnS powder was dried and kept as stock (stock-1) for using in the second step. 6 ACS Paragon Plus Environment

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2.3 Synthesis of ZnS-CdSe heterostructures

Heterogeneous quantum dot with CdSe/ZnS was prepared by varying precursor solution of CdSe with respect to the amount of ZnS seed and the reaction annealing temperature was kept constant (240°C). The different composition ratios such as Cd:Zn 0.9:1, 1.3:1, 2.5:1, 4:1 and 12:1 result in different morphologies. Total amount of different precursors used in the synthesis are given in Supporting Information Table S1. It can be noted the concentration of Cd/Se precursor was varied with fixed ZnS seed and to do so the concentration of ligand, solvent and surfactant are also varied. As in each case, OA and ODE used for solvation of ZnS QD was kept constant, the rest of the OA and ODE was added to CdO for making the solution of Cd-precursor. The required amount of Cd-precursor and Se-precursor solutions were added in the ZnS seed solution (stock-1) in stages. In the second step followed by the synthesis of ZnS seed, two clear stock solutions are prepared in which CdO is dissolved in OA/ODE mixture (260° C) and Se powder in TOP (100° C). Then, as a typical synthesis procedure, dried ZnS powder (1 mmole) is dissolved in 5 ml ODE (solvent) and 5 ml OA (ligand) in a 250 ml two-necked flask and the resulting solution was heated at 240°C. At this temperature, Cd-stock solution was quickly injected into the reaction flask and the temperature was maintained for 15 minutes. After that, Se-stock solution was added and allowed the nanoparticles to grow for 15 minutes (240°C). QDs are purified by adding required amount of chloroform: acetone mixture (Supporting Information Table S1) followed by centrifugation at 5000 rpm for 5 minutes. 3. Results and discussion The synthesis of CQD (CdSe-ZnS) heterostructure is achieved through a seed-mediated two-pot synthesis (details of the synthesis conditions are provided in the experimental section). Pure ZnS seed was prepared first and in the second step CQD (designated as Bicompartmental JanusA and



JanusB) was synthesized by adding required amount of precursor solution of CdSe with respect to the total amount of ZnS to maintain the certain ratio of Cd:Zn (i.e. 2.5:1 for JanusA, 4:1 for JanusB). Apart from the CQD, different heterostructures (alloy, acorn and alloy core - thick CdSe shell) can be formed with variable precursor ratios (Supporting information, Table S1). The obtained heterostructures are further analyzed for its structural and photoluminescence features using a variety of characterization techniques such as TEM, HRTEM, XPS, XRD and PL which have been discussed in detail in following sections: TEM and HRTEM analysis

Figure 1 shows a representative set of TEM images of the synthesized nanoparticles of all compositions while the insets show the corresponding HRTEM images. TEM image of pure ZnS nanoparticles (size ~ 8 nm) with high monodispersity is shown in Figure 1a with the inset HRTEM representing the lattice fringes. The calculated lattice spacing (0.31 nm) of ZnS, shown in Supporting Information Figure S1, confirms the formation of highly crystalline ZB phase. However, if Cd and Se precursors are added during the synthesis, a new feature begins to appear on the particles that is more evident in the HRTEM image (inset) with a slightly higher ratio of Cd:Zn precursor ratio (2.5:1 and 4:1). With the Cd:Zn precursor ratio 0.9:1, a near uniform size distribution of the particle are obtained as evident from the TEM image (Figure 1b). However, it is difficult to discern a clear interface between ZnS and CdSe as evident from the inset of Figure 1b, suggesting probable formation of an alloy structure. In addition to that, a detailed statistical analysis of lattice spacing’s confirms the heterogeneity in the structure with no distinct peak of any of the components (Figure 2). Further increase in the Cd: Zn precursor ratio (1.3:1) leads to the formation of the Acorn Janus structure (i.e. Janus structure consisting of two compartments with the smaller compartment partly covered by the bigger one) with monodisperse size


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distribution which has been confirmed by the presence of contrast difference in respective TEM and HRTEM images as observed in Figure 1c. The inset shows ‘Acorn’ type of Janus structure with one region of ZnS (Size ~ 8 nm, same as that of the core ZnS) and the “half covered shell” corresponds to CdSe (Size ~ 4 nm) with a distinct interface between the two constituting QDs. Further, Fast Fourier transformation (FFT) generated using ImageJ software of the heterostructure reveals the diffraction spots of (111) facet of ZnS (ZB) and (002) facet of CdSe (WZ) which clearly confirms the formation of Acorn type structure (Figure 3). With Cd: Zn precursor ratio (2.5:1 and 4:1), the “half covered shell” of CdSe in Acorn grows to a bigger size (size ~8 nm) and forms bicompartmental Janus structure (JanusA and JanusB respectively) with the two compartments, ZnS and CdSe QDs, coupled in between (insets in Figure 1d and Figure 1e respectively). It is clearly evident that the formation of an interface between CdSe and ZnS is more prominent in JanusB than JanusA. Moreover, the lattice spacing’s calculated from the corresponding HRTEM image (Figure 1e) and FFT pattern confirms the formation of ZB- ZnS and ZB-CdSe (Figure 4). Further increase in Cd:Zn precursor ratio (Cd:Zn::12:1) leads to the formation of alloy core (CdZnSeS) - thick shell (CdSe) structure which is confirmed from the TEM image (Figure 1f). HRTEM images suggest that the size of pure ZnS decreases in this case compared to the starting ZnS seed. In addition to that, it is concluded that alloy core (evident from indistinguishable lattice spacing’s in the interface of ZnS and CdSe) and a thick CdSe shell is formed. This indicates the possibility of ion exchange between core ZnS and CdSe shell leading to the formation of an alloyed core. FFT image confirms the presence of ZnS (ZB) and CdSe (WZ) phases (Figure 5). In summary, the CQD structure appears to evolve as the cationic feeding ratio emerges to a certain critical concentration ratio (Cd:Zn = 2.5:1 and 4:1) and then



disappears as more Cd is added. The nanoparticles seem to have highly uniform size and size distribution in all the samples. XRD Analysis

The crystal structures of QDs are determined from the XRD analysis and the corresponding deconvoluted peaks are given in Figure 6a and 6b respectively. XRD of the ZnS starting precursor seed shows the formation of ZB phase. As Cd is introduced, a subtle broadening of the peaks in the range of 25-30ᶱ and 46-50ᶱ take place as compared to the seed of ZnS which is more prominent in the case of Acorn (Cd:Zn=1.3:1), JanusA (Cd:Zn=2.5:1) and JanusB (Cd:Zn=4:1) structure. The diffraction patterns of Acorn and JanusA show the presence of peaks corresponding to WZ-CdSe planes of (100), (002), and (101). Further, the peak at 48° corresponds to the plane of (220) which confirms the presence of ZB-ZnS phase and the peak at 28° corresponding to ZB-ZnS (111) overlaps with WZ-CdSe (101) peak as evident from the deconvolution plot. In contrast, XRD of JanusB structure does not possess peaks corresponding to (100) and (101) and the peaks are sharper compared to the earlier cases. These observations suggest that the CdSe formed as a ZB phase along with ZB-ZnS structure which has been confirmed by HRTEM images (Figure 1e). However, XRD of core - shell structure shows the presence of the characteristics peaks of WZ-CdSe. However, peaks corresponding to (111) and (220) facets of ZnS are not clearly visible. This may be due to the formation of WZ-CdSe as the thick shell over ZnS seed. Furthermore, the presence of ZB-ZnS core is confirmed from HRTEM image in Figure 1f. The structural features are further characterised by XPS and PL. In contrast to the above samples, the characteristic peaks of CdSe in WZ or ZB phase are absent in alloy (Cd:Zn=0.9:1). However, the peaks are broadened with a small shift towards the smaller angle. This can be attributed to the formation of alloy structure with a little increase in the lattice parameter than pure ZnS due to partial substitution of Zn and S surface atoms with larger Cd and 10 ACS Paragon Plus Environment

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Se atoms respectively29. This further supports the corresponding HRTEM image (Figure 1b). ZnS retains the ZB phase even after the heterostructure formation. It is to be mentioned that to identify the phases very clearly from XRD data is challenging due to strong overlap of the peaks corresponding to WZ and ZB phases of CdSe. It is necessary to combine XPS and HRTEM analyses with XRD, so that the ultimate structure of such complicated nanomaterial are unambiguously determined. XPS analysis

To unravel the structural heterogeneity of these QDs, investigation with X-Ray photoelectron spectroscopy (XPS) has been performed. High resolution scan between 50-60, 155-170, 400418, and 1015-1025 eV have been performed to identify signature peaks of Se 3d, S 2p/Se 3p, Cd 3d and Zn 2p. Moreover, the elemental composition is deduced from the XPS spectra and shown in Table 2. The Se 3d electrons feature the characteristic peaks around ~55 eV as shown in Figure 7 (red trace for Se 3d5/2 and blue trace for Se 3d3/2). The deconvoluted core level spectra for Acorn-Janus (Figure 7a) and bicompartmental-JanusB (Figure 7b) QDs clearly indicate the presence of Se 3d electrons. A small peak shift towards higher binding energy is exhibited in the case of JanusB structure (Cd:Zn=4:1). This observation can be rationalized with the help of Table 2 in which the elemental composition ratio based on percentage atomic concentrations is tabulated. In this table the overall composition ratio, within the XPS penetration depth, is presented by normalizing the compositions with respect to Cd. Our analysis yields that the amount of Se is higher in JanusB than it is in Acorn structure. Since, the amount of surrounding electropositive Zn-3p electrons (the amount of S is nearly invariant in both the cases) are lesser in the vicinity of Se-3d electrons in JanusB, the overall binding energy shifts in higher energy in case of JanusB structure compared to the Acorn one. Similarly, core - shell QD 11 ACS Paragon Plus Environment


also (Cd: Zn::12:1) shows the presence of Se 3d5/2 and Se 3d3/2 peaks which confirms the formation of Se-X (X=Zn/Cd) bond formation. In contrast, Se 3d3/2 peak is absent in the case of alloy QD (Cd:Zn=0.9:1). In contrast, Se 3d3/2 peak is absent in the case of alloy QD (Cd:Zn=0.9:1). The notable point is that the peak shape is symmetric in case of alloy and asymmetric in JanusB. We have deconvoluted the peak of Se 3d with the help of the Casa XPS software keeping the relative sensitivity factor (RSF) constant i.e. 3:2 for the two peaks 3d5/2 and 3d3/2, although FWHM changes during the fitting process. As the peak hump was missing in the symmetric peak of Se 3d3/2 in case of the alloy structure, the fitting did not yield any peak for this. This raises a possibility of the presence of elemental Se or Se weakly bound to cation, rather than any particular Se-X bond which matches with the existing report that peak splitting is nonexistent in pure Se.30 However, the peak shifted ~0.4 eV towards the lower binding energy with respect to the binding energy of pure Se, owing to interaction with other elements in the vicinity of Se. Further, the XPS peaks between 155-170 eV have been deconvoluted to investigate the presence of S and Se elements (Figure 8). XPS spectra of Acorn structure shows the presence of spin-orbit doublet features of S 2p3/2 (162.2 eV) and S 2p1/2 (163.5 eV) with a small shift towards higher binding energy compared to pure ZnS seed due to increase in presence of Se 3p electrons. In addition to that, it also shows the presence of characteristic Se 3p3/2 (160.6 eV) and Se 3p1/2 (166.4 eV) peaks. Further, Figure 9 shows the XPS analysis of Cd 3d spectra in which the peaks at 405 eV and 411 eV show the presence of Cd 3d5/2 and 3d3/2 peaks. The peaks in Figure 9a are deconvoluted to indicate Cd atoms bound to Se/S atoms (Cda) and surface Cd atoms (Cdb) attached to adsorbents (-OH) and ligands (oleic acid). These are indicated by sharp peak and broad regions respectively. Similar peaks were observed in the case of JanusB structure (Figure


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9b) however, the ratio of Cda/Cdb is higher and the peaks are observed in the higher binding energy compared to Acorn type. Using the appropriate relative sensitivity factor, we have determined the ratio of Cda:Cdb and is compared with the exiting reports.26 From the above spectra, it is evident that the bicompartmental JanusB structure having ZB-CdSe retains more Cd2+ surface-other element bonding, compared to Acorn (Janus) type with less Cd2+ surfaceother element bonding due to the presence of WZ-CdSe structure. Therefore, the ratio of Cda:Cdb is lesser in ZB –CdSe in comparison to WZ-CdSe, which is also supported by the literature.26 Thus, it suggests that XPS also can unveil the phase formation in the CdSe nanocrystals. Further, the core - shell structure also shows the similar spectra however with Cda : Cdb ratio lying between Acorn and Janus type (Figure 9d). In alloy QD, Cdb is absent which indicates that Cd does not take part in strong bonding (Figure 9c). In addition, the core level spectra of Zn 2p3/2 around 1022 eV for all the samples (Figure 10) were analysed. Acorn (Figure 10b) and JanusB (Figure 10c) structures show two convoluted peaks in which the broad region is attributed to Zne (surface Zn atoms attached to adsorbents and ligand oleic acid).and the sharp peak is attributed to ZnS (Zn atom bound to Se/S atoms) indicated by blue and red traces, respectively. We note that the Zne is very minimal or absent in the case of pure ZnS seed (Figure 10a) and alloy structure (Figure 10d). In contrast, Zn peaks are absent in core - shell structure (Figure 10e) due to low depth penetration of XPS which supports again the formation of pure CdSe shell (Table 2). In summary, we deduce the following conclusions from the XPS spectra. (1) Presence of Zn 2p and S 2p peaks confirm the formation of pure ZnS crystal further supported by XRD and HRTEM images. (2) In case of alloyed QD, all the elements are present except S in the outer surface. Furthermore, absence of any strong Se-X (X=Cd/Zn) bond also indicates the possibility of formation of an alloy. Likewise, core level spectra of core/shell QD also reveals the absence



of S in outer shell and in addition to that Zn is also absent. Hence, it is concluded that shell is comprised of pure CdSe and the core is CdZnSeS alloy. Absorbance spectra and Photoluminescence analysis

The absorption spectrum for ZnS (Supporting Information Figure S2) shows the characteristic excitation peak (λex = 325 nm). But for all the heterostructures, coupling of ZnS with CdSe results in generation of new high-energy peak (λex = 270 nm) originating from new state developed at the interface which dominates over the characteristic CdSe peak (λex = 460 nm). Further, photoluminescence studies were performed to investigate the luminescence characteristics. Figure S3 and Figure 11 show the emission spectra of ZnS and all the heterostructured samples respectively. The emission peak for seed ZnS QDs fabricated is consistent with the characteristic strong band edge photoluminescence (PL) band at 452 nm i.e. λem = 452 nm for λex = 325 nm. On the other hand, all ZnS-CdSe heterostructures are excited with both the wavelengths 270 nm as well as 460 nm and their corresponding emission spectra patterns do not show much difference. A distinct optical feature is observed in the bicompartmental Janus QD’s (JanusA - Cd: Zn::2.5:1 and JanusB - Cd:Zn::4:1). They show two distinct emission bands around 570 and 630 nm in which the 630 nm band is attributed to CdSe, whereas 570 nm can be attributed to emission band arising due to coupling to a large extent between ZnS/CdSe resulting in new electronic state at the interface. It is to be noted that this band is absent in all other morphologies of heterostructure. A similar observation has been noted in the case of ZnSe/CdS CQD reported by Sengupta et.al.13 From this we can deduce that increase in Cd precursor concentration leads to formation of CQD with enhanced interdot diffusion. In contrast, Acorn Janus structure shows very weak interface emission along with strong CdSe emission which further suggests weak coupling and an incomplete ZnS/CdSe 14 ACS Paragon Plus Environment

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interface formation. Further, alloyed core – thick CdSe shell structure also shows negligible emission band at 570 nm and CdSe emission (at 670 nm) with red shift due to lesser quantum confinement effect arising from bigger size of the heterostructure. However, alloy structure (Cd:Zn::0.9:1) shows a single emission band (at 630 nm) with a negligible interface emission band which can be attributed to emission arising due to the formation of alloy as supported by XRD. Further, the excited state lifetimes of 630 nm and 570 nm have been estimated from their emission decay profile (Supporting Information Figure S4) and tabulated (Table 1) by triexponential decay fit.31 It shows that enhanced average exciton lifetime for the CdSe peak is observed for both the couple dots (31 ns for JanusA and 94 ns for JanusB). The enhancement in lifetime in JanusB compared to JanusA may be attributed to the ZB structure of CdSe in JanusB, while CdSe is in WZ in the other structure. We note that previous reports also clearly suggest that ZB-CdSe shows better PL properties compared to WZ structure.25-26 Moreover, both the Janus structures possess enhanced exciton lifetime (~40 ns) corresponding to 570 nm emission (arising from the interface). The increase in lifetime is consistent with the delayed electron-hole pair overlap due to formation of CQD between ZnS-CdSe QDs. Furthermore, Acorn QD (Cd:Zn::1.3:1) the lifetime is almost same for λem = 570 nm as well as for λem = 630 nm (~6ns). The low lifetime at interface peak (λem = 570 nm) indicates incomplete interface formation as predicted by HRTEM and XPS analysis. Similarly, in case of alloyed QD (Zn:Cd::1:0.9) the lifetime is low for λem = 630 nm (CdSe peak) (~9ns). Furthermore, for λem = 570 nm absence of exciton lifetime indicates non existent ZnS-CdSe interface supported by HRTEM as well as XPS analysis. Likewise, absence of interface as well as insufficient coupling in alloyed core - thick shell QD (Cd:Zn::12:1) can be verified. Further, the excited state lifetime (~9ns) corresponding to 670 nm emission (characteristic CdSe peak) is also lesser in core - shell heterostructure



compared to bicompartmental Janus structures. Therefore, the overall results further support the XRD and HRTEM analyses that in CQD a distinct alloyed interface formation is feasible which leads to a new emission wavelength (λem = 570 nm) due to enhanced interdot diffusion. Density Function Theory (DFT) based analysis on growth of different heterostructures:

To rationalize certain aspects regarding the formation of different morphologies, density functional theory (DFT) based simulations have been performed. The two-step synthetic procedure is always started with ZnS as the seed and we observe that CdSe grows on top of ZnS particles, which suggests that heterogeneous nucleation of CdSe is favorable compared to homogeneous nucleation. The morphology of ZnS seeded particles is expected to play an important role and is determined by its surface energies via the Wulff construction but the ratio of facets on the exposed surface of ZnS is expected to vary depending upon the particle size and reaction conditions. The surface energy analysis of individual low-index surfaces of ZB-ZnS indicates that the surface of ZnS QDs would be constituted with mostly low energy (110) facets though reduced amount of (001) and (111) facets can also be present (Supporting Information Table S2). The above results are in agreement with other computation and experimental results that have been performed on ZnS surfaces.32-33. In a solution reaction containing Cd- and Se-precursors, ZnS particles are susceptible to ion-exchange reaction on the surface resulting in alloy formation in surface. The reactivity of different ZnS surfaces will vary depending upon its surface structure and we have investigated all the low index surfaces of ZnS for ion-exchange reaction with Cd and Se atoms. The chemical state of Cd and Se atom in reaction solution is of much debate and we have used following reactions to explore the viability of ion-exchange reaction on ZnS surfaces in Equation (1) and (2).


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ZnS (hkl) + Cdb Cds-ZnS(hkl) + Znb

(1)

ZnS (hkl) + Seb Ses-ZnS (hkl) + Sb

(2)

In the above given expressions, h, k and l correspond to miller indices of planes and have values of 0, 1,… Cds and Ses refers to the surface substituted atoms in ZB-ZnS nanoparticles. The subscript ‘s’ refers to surface atoms of ZnS nanoparticles. Cdb, Znb, Sb and Seb are the elemental bulk structures of Cd, Zn, S and Se respectively. The formation energies of ion-exchange reaction on different facets of ZnS are summarized in Table 3. The calculation suggests that limited ion-exchange reaction will occur on stable (110) surfaces and (001) and (111) surfaces are more susceptible to favorable exothermic ion-exchange reaction. Therefore, this suggests that alloyed interface formation will not take place along the (110) surface of ZnS. On the other hand a distinct alloyed interface formation is feasible on CdSe-ZnS heterostructures as Cd and Se would have the propensity to diffuse through (001) and (111) surfaces of ZB-ZnS nanoparticles and the ion-exchange reaction will increase extensively with increase in the concentration of Cd- and Se-precursors in the reaction solution. This further supports the experimental observation that with moderately high Cd/Se concentration, ion exchange reaction takes place resulting in bicompartmental Janus structure (both JanusA and JanusB) with the alloyed interface between the two compartment of ZnS - CdSe and with very high Cd/Se concentration (Cd:Zn::12:1) alloyed core is formed. The experimental results also suggest that (110) facet of ZnS does not couple with any of CdSe facets. Intermixing of cations and anions will reduced strain along the interface and allow for the formation of interfaces with lesser defects. High-strain on the interface is more conducive for island-growth rather than epitaxial film formation. Since CdSe can have both WZ and ZB-phases, we have calculated the



interfacial strain for low-index facets of both CdSe phases with low-index facets of ZB-ZnS and summarized in Table 4. It is found that ZB-CdSe facets produce large interfacial strain during film formation over ZBZnS surfaces despite having similar crystal structure. This is because a large mismatch of lattice parameters of ZB-ZnS and ZB-CdSe. However, low-index facets of WZ-CdSe form less-strained interfaces over ZB-ZnS than considered in the previous case (Table 4). The most stable (110) facet of ZB-ZnS does not form any low-strain interface with WZ-CdSe, though less stable (111) facet of ZB-ZnS forms with (001), (100) and (110) surfaces of WZ-CdSe. This would enable island-growth of WZ-CdSe over ZB-ZnS seed nanoparticles and result in bicompartmental JanusA type of heterostructures on low Cd- and Se-concentration in the reaction solution (Cd:Zn::2.5:1). The computational results points to the lower possibility of synthesizing core/shell structure with thin shell of CdSe over ZnS core unless ligand-assisted growth allows for the synthesis of ZnS with higher surface energy (111) facet. The exposure of ZnS (111) surface can even allow for the stabilization of rocksalt-CdSe (111) thin films through pseudomorphic growth as predicted by theoretical studies.34 With the increase in Cd and Se molar feed, it is expected that greater number of monolayers (ML) would be grown over seed ZB-ZnS nanoparticles. The growth kinetics, morphology and constituting phases of heterostructures would be less influenced by the seed nanoparticle as the thickness of shell increases and the morphology of the heterostructures would be more governed by the surface energetics of overgrowing CdSe nanoparticles (Supporting Information Table S3). Among lower index facets of WZ-CdSe, (110) facet is the most stable and would constitute the majority of the surfaces which is also supported by other reports. This indicates that conformalisland-growth over the seed would be in perpendicular to (110) facet of CdSe. However, the


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lowest interfacial strain is found for (001)-WZ-CdSe facets and formation of these films would expose other less stable facets i.e. (100) along its perpendicular direction. This would make epitaxial film growth energetically unviable after certain critical thickness. The above analyses allow for certain rules that underlie the synthesized morphology of heterostructure. When the most stable surface of seed can form low-strain coherent interface with shell, then core - shell nanostructure would be formed. On the formation of low-strain interfaces with less stable surfaces of seed, bicompartmental JanusA structure would be formed via ‘nanoparticle fusion’ when the film thickness of shell is less. However, core - shell heterostructure may form on increase of shell thickness. In a nutshell we have experimentally observed that in a two step synthesis procedure, keeping ZnS seed fixed and Cd varied, several heterostructures can be obtained such as alloy, Acorn Janus, bicompartmental JanusA, bicompartmental JanusB and alloy core – thick CdSe shell (with an increasing Cd:Zn ratio in the same order). XRD analysis suggests that pure ZnS is in ZB phase which does not change its phase even after heterostructure formation. The deconvoluted XRD analysis suggests that with a low Cd:Zn ratio (1:0.9) alloy formation takes place. As the ratio increases, Acorn Janus structure formation (Cd:Zn::1.3:1) takes place with ZB-ZnS and WZ-CdSe. Further with a moderately high Cd:Zn ratio (2.5:1), formation of bicompartmental JanusA takes place with the same corresponding phases. On the contrary, with high Cd:Zn ratio (4:1) bicompartmental JanusB forms with both ZnS and CdSe in ZB phase instead of CdSe (WZ), as observed in previous cases. With further increase in the ratio (12:1) core - shell heterostructure formation takes place with ZnS (ZB) as core and CdSe (WZ) as shell. HRTEM analysis also supports the above mentioned heterotructure formation with more detailed structural information on the core/shell structure which basically consists of negligibly small ZnS region/alloy core -



thick CdSe shell. XPS analysis also supports the information obtained about the heterostructure formation. In addition to that, photoluminescence analysis of the heterostructures show that CQD with bicompartmental Janus structure show origin of a new emission band (not matching with either ZnS or, CdSe) at 570 nm. The maximum interfacial strain is possible in the bicompartmental Janus structure and hence interfacial strain could be the possible origin behind new emission band. In alloy or core-shell heterostructures, substitution reactions are observed which indicates that relaxation of interfacial strain leads to absence of the emission band. Further, there is less interfacial strain in acorn heterostructure with small interfacial area and here too emission band at 570 nm is not observed. Hence in our case, interfacial strain found to play big role in generation of emission bands. However, Dasgupta et al. has pointed out that strain plays insignificant role in the electronic structure of coupled quantum dots due to the small interfacial area in a CQD as we observe in acorn heterostructure.5 Due to the absence of conclusive proof on origin of new emission bands from our study, we have desisted from making further categorical comments.The perfect interface formation in case of CQD dot leads to reduction in the rate of radiative recombination of electron-hole pair and this results in enhanced radiative lifetime (~40 n) in both JanuA and JanusB in comparison to the other heterostructures, as observed for ZnSe-CdS CQD Sengupta et.al.13 The coupling of the CdSe (ZB/WZ) energy gap (energy gap of WZ/ZB almost invariant) with ZnS (ZB) offers band offset and the resulting transition gap is in between the energy gap of CdSe and ZnS, resulting in red-shifted PL compared to ZnS emission wavelength. The interesting point to be noted is that the lifetime for excited state of CdSe (emission wavelength 630 nm) also increases significantly in bicompartmental JanusB (94 ns) which is much higher than the already obtained value from the existing literature (CdSe QD with lifetime ~ 14 ns23 or, CdSe/ZnS core/shell heterostructure with


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lifetime ~ 9 ns).24 This could encourage the researches for further investigation of the intriguing pathway of the recombination of hole-electron pair in different heterostructure. Further DFT analysis assists in rationalize the mechanistic pathway for formation of these experimentally observed heterostructures- bicompartmental JanusA and core - shell. 4. Conclusions: Component ratio modulated ZnS-CdSe heteronanocrystals with different morphologies were successfully synthesized. In this case, the size of core ZnS dot was kept fixed with varying CdSe ratios. At a particular ratio of Cd:Zn, coupled quantum dots are formed. The coupled dots (JanusA and JanusB) show significantly high exciton lifetime compared to the other heterostructures obtained by us. Both JanusA and JanusB are formed by the coupling of two constituting quantum dots (ZnS and CdSe) through the facet specific attachment. In case of JanusA the constituting crystal structure for ZnS and CdSe are Zinc Blende (ZB) and Wurtzite (WZ) respectively while JanusB has ZB crystal structure for both the constituting QDs (ZnS and CdSe). Furthermore, it is deduced from DFT based simulations that apart from strain energy arising from lattice mismatch, the surface construction of core would dictate the heterostructure formation pathway. With the help of DFT analysis we came to conclusion that compositionally sharp interface modulates strain to generate coupled quantum dots. (001) and (110) facets of WZ-CdSe can grow over ZB-CdSe (111) with less strain and form a more coherent interface. Hence, unless ZnS (111) is the only exposed facet, which is possible only through “designer reaction conditions” (e.g. via specialized ligands), Janus/coupled heterostructure formation is preferred over core/shell heterostructure. This coupling might reduce the overall surface energy by elimination of some high energy facets and intermixing via cation exchange of two adjacent facets. Furthermore, partial removal of the surface-passivating ligands in the interface through coalescence leads to coupled structure.35 On the contrary, ZnS can grow over both CdSe (110) 21 ACS Paragon Plus Environment


and CdSe (001) which rationalizes the formation of core-shell heterostructure of WZ-CdSe@ZBZnS. The ‘nanoparticle fusion’ mechanism is a powerful technique for the formation of heterostructured nanomaterials and if the prerequisite conditions are understood better, then an extensive nanoparticles reaction library can be set up for accessing different multifunctional heterojunction nanomaterials. Characterization The formation of different morphologies were visualized by Transmission Electron Microscope (TEM) images using FEI Technai Twin microscope. High resolution TEM (HRTEM) images were taken on a UHR FEG-TEM, JEOL JEM-2100F electron microscope (electron source 200kV). FFT analyses and measurement of lattice spacing’s were done with the help of ImageJ software. XRD analyses were performed using a Siemens D5000 Bragg-Brentano θ–2θ diffractometer equipped with a diffracted-beam graphite monochromator crystal. The samples were taken in powder form. The data were collected over the 2θ range 15–85° with Cu-Kα (40 kV, 40 mA) radiation on slow scan rate with step size 0.04° 2θ with a counting time of 1.5 s step−1. XPS studies were done using PHI VersaProbe II Scanning XPS Microprobe (calibrated by setting C1s peak at 284.6 eV). I did the background correction by ‘Shirley Background Correction’ method for XPS analysis. Gaussian- Lorentzian function was used to deconvolute the obtained peaks. Photoluminescence measurements were recorded with an Edinburgh Instruments FLS 920 instrument using a 450-W Xe arc lamp was used and the emission spectra were corrected to eliminate the detector response. A red-sensitive Peltier element were used as the excitation source and cooled Hamamatsu R928-P PMT as detector respectively. The decay curves were obtained using time-correlated single-photon counting (TCSPC) and steady state fluorescence set up for all the heterostructures (Laser source is 460 nm). The UV-


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spectrophotometer, The Cary 5000 from Agilent technologies, was used to obtain the absorption spectra. Acknowledgement We acknowledge Supriyo Chakroborty for helping in HRTEM analysis in IACS, Kolkata and Prof. Prateek Sen (Department of Chemistry, IITK) for helping in lifetime measurement in IIT Kanpur. We acknowledge the support from the Department of Science and Technology, Government of India and nanoscience division, IITK. The authors declare no competing financial interests.

Supporting Information: Supporting Information is available free of charge via the Internet at http://pubs.acs.org. Detailed DFT analysis, detailed precusor concentration used in experiments, several TEM and HRTEM images of ZnS quantum dot, absorption spectra of ZnS and ZnS-CdSe heterostructures, PL spectra of ZnS, decay analysis spectra, are provided in the Supporting Information. Author Information Corresponding Author

Sri Sivakumar, E-mail: [email protected] Raj Ganesh S. Pala, E-mail: [email protected] Orcid

Debadrita Bhattacharya: 0000-0002-8079-9638 23 ACS Paragon Plus Environment


Sulay Saha: 0000-0002-5201-5349 Vishnu Prasad Shrivastava: 0000-0002-5036-6196 Sri Sivakumar: 0000-0002-6472-2702 Raj Ganesh S. Pala: 0000-0001-5243-487X

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(7) Dubertret, B.; Skourides, P.; Norris, D. J.; Noireaux, V.; Brivanlou, A. H.; Libchaber, A. In Vivo Imaging of Quantum Dots Encapsulated in Phospholipid Micelles. Science 2002, 298, 1759-1762. (8) Chan, Y.; Steckel, J. S.; Snee, P. T.; Caruge, J.; Hodgkiss, J. M.; Nocera, D. G.; Bawendi, M. G. Blue Semiconductor Nanocrystal Laser. Appl. Phys. Lett. 2005, 86, 073102 (1-3). . (9) Doose, S. Optical Amplification from Single Excitons in Colloidal Quantum Dots. Small 2007, 3, 1856-1858. (10) Talapin, D. V.; Lee, J.-S.; Kovalenko, M. V.; Shevchenko, E. V. Prospects of Colloidal Nanocrystals for Electronic and Optoelectronic Applications. Chem. Rev. 2010, 110, 389-458. (11) Kouwenhoven, L. Coupled Quantum Dots as Artificial Molecules. Science 1995, 268, 14401441. (12) Kagan, C. R.; Murray, C. B. Charge Transport in Strongly Coupled Quantum Dot Solids. Nat. Nanotech. 2015, 10, 1013-26. (13) Sengupta, S.; Ganguli, N.; Dasgupta, I.; Sarma, D. D.; Acharya, S. Long-Range Visible Fluorescence Tunability Using Component-Modulated Coupled Quantum Dots. Adv. Mater. 2011, 23, 1998-2003. (14) Xu, X.; Hu, L.; Gao, N.; Liu, S.; Wageh, S.; Al‐Ghamdi, A. A.; Alshahrie, A.; Fang, X. Controlled Growth from ZnS Nanoparticles to ZnS–CdS Nanoparticle Hybrids with Enhanced Photoactivity. Adv. Func. Mater. 2015, 25, 445-454. (15) Zhang, J.; Chernomordik, B. D.; Crisp, R. W.; Kroupa, D. M.; Luther, J. M.; Miller, E. M.; Gao, J.; Beard, M. C. Preparation of Cd/Pb Chalcogenide Heterostructured Janus Particles Via Controllable Cation Exchange. ACS Nano 2015, 9, 7151-7163. (16) Oh, N.; Nam, S.; Zhai, Y.; Deshpande, K.; Trefonas, P.; Shim, M. Double-Heterojunction Nanorods. Nat. Commun. 2014, 5:3642. (17) Fang, X.; Zhai, T.; Gautam, U. K.; Li, L.; Wu, L.; Bando, Y.; Golberg, D. ZnS Nanostructures: From Synthesis to Applications. Prog. Mater. Sci. 2011, 56, 175-287. (18) Nirmal, M.; Dabbousi, B. O.; Bawendi, M. G.; Macklin, J. J.; Trautman, J. K.; Harris, T. D.; Brus, L. E. Fluorescence Intermittency in Single Cadmium Selenide Nanocrystals. Nature 1996, 383, 802-804.



(19) Nag, A.; Kundu, J.; Hazarika, A. Seeded-Growth, Nanocrystal-Fusion, Ion-Exchange and Inorganic-Ligand Mediated Formation of Semiconductor-Based Colloidal Heterostructured Nanocrystals. CrystEngComm 2014, 16, 9391-9407. (20) Ni, B.; Wang, X. Nanostructure Formation Via Post Growth of Particles. CrystEngComm 2015, 17, 6796-6808. (21) Dalui, A.; Chakraborty, A.; Thupakula, U.; Khan, A. H.; Sengupta, S.; Satpati, B.; Sarma, D. D.; Dasgupta, I.; Acharya, S. Chemical Tailoring of Band Offsets at the Interface of ZnSe– CdS Heterostructures for Delocalized Photoexcited Charge Carriers. J. Phys. Chem. C 2016, 120, 10118-10128. (22) Shimose, H.; Singh, M.; Ahuja, D.; Zhao, W.; Shan, S.; Nishino, S.; Miyata, M.; Higashimine, K.; Mott, D.; Koyano, M.; Luo, J.; Zhong, C. J.; Maenosono, S. Copper Sulfide– Zinc Sulfide Janus Nanoparticles and Their Seebeck Characteristics for Sustainable Thermoelectric Materials. J. Phys. Chem. C 2016, 120, 5869-5875. (23) Puntambekar, A.; Wang, Q.; Miller, L.; Smieszek, N.; Chakrapani, V. Electrochemical Charging of CdSe Quantum Dots: Effects of Adsorption Versus Intercalation. ACS Nano 2016, 10, 10988-10999. (24) Sukkabot, W. Variation in the Structural and Optical Properties of CdSe/ZnS Core/Shell Nanocrystals with Ratios between Core and Shell Radius. Physica B Condens Matter. 2014, 454, 23-30. (25) Xia, X.; Liu, Z.; Du, G.; Li, Y.; Ma, M. Wurtzite and Zinc-Blende CdSe Based Core/Shell Semiconductor Nanocrystals: Structure, Morphology and Photoluminescence. J. Lumin. 2010, 130, 1285-1291. (26) Subila, K. B.; Kishore Kumar, G.; Shivaprasad, S. M.; George Thomas, K. Luminescence Properties of CdSe Quantum Dots: Role of Crystal Structure and Surface Composition. J. Phys. Chem. Lett. 2013, 4, 2774-2779. (27) Shao, G.; Chen, G.; Yang, W.; Ding, T.; Zuo, J.; Yang, Q. Organometallic-Route Synthesis, Controllable Growth, Mechanism Investigation, and Surface Feature of PbSe Nanostructures with Tunable Shapes. Langmuir 2014, 30, 2863-2872. (28) Manna, L.; Scher, E. C.; Alivisatos, A. P. Synthesis of Soluble and Processable Rod-, Arrow-, Teardrop-, and Tetrapod-Shaped CdSe Nanocrystals. J. Am. Chem. Soc. 2000, 122, 12700-12706. 26 ACS Paragon Plus Environment

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Table 1: Average lifetime data for different heterostructures

Sample Names

Average Lifetime (ns)

(precursor ratio)

λem (630 nm) @ λex (460 nm)

λem (570 nm) @ λex (460 nm)

Alloy (Cd:Zn::0.9:1)

8.6

0

Acorn (Cd:Zn::1.3:1)

6.2

6

Bicompartmental JanusA (Cd:Zn::2.5:1)

31.0

36

Bicompartmental JanusB (Cd:Zn::4:1)

94.8

41

Alloy core - thick shell (Cd:Zn::12:1) 6.8

0


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Table 2: Calculated elemental Composition from XPS data

Sample names (precursor ratio)

Composition ratio ( Cd:Zn:Se:S)

Alloy (Cd:Zn::0.9:1)

1: 0.000021: 0.27: 0

Acorn (Cd:Zn::1.3:1)

1: 0.27: 1.77: 1.45

Bicompartmental JanusB (Cd:Zn::4:1)

1: 0.11: 2.68: 1.47

Alloy core - thick shell (Cd:Zn::12:1)

1: 0 : 1.22:0



Page 30 of 43

Table 3: Heat of reaction for exchange reaction of Cd and Se atoms with low index facets of ZnS

Surface facet of Heat of reaction on exchange of Zn Heat of reaction on exchange of S ZnS (hkl)

with Cd (eV)

with Se (eV)

ZnS (hkl)+CdsCds-ZnS(hkl) + Znb

ZnS (hkl) + Ses Ses-ZnS (hkl) + Seb

001

-0.27

-0.86

110

0.68

1.55

111

-0.08

-3.45


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Table 4: Interfacial strain of low-index facets of CdSe (ZB and WZ) with low index facet of ZBZnS

ZB-ZnS (001)

ZB-ZnS (110)

ZB-ZnS (111)

ZB-CdSe (001)

8.2

12

9.4

ZB-CdSe (110)

6.4

8.2

5.7

ZB-CdSe (111)

4.5

19

8.2

WZ-CdSe (100)

5.2

5.8

3.9

WZ-CdSe (001)

4.5

6.2

0.9

WZ-CdSe (110)

17.0

8.4

2.7

WZ-CdSe (101)

7.3

11.0

6.0

WZ-CdSe (111)

6.0

11.0

4.1

௔

ܵ‫݊݅ܽݎݐ‬ଵଶ = (௔భ − 1ቁ where a1 is the lattice vector of material-1 and a2 is the lattice vector of material-2. మ



Figure 1: TEM images of nanoparticles synthesized with Cd:Zn molar feed ratios of (a) 0:1 (pure ZnS seed), (b) 0.9:1 (Alloy), (c) 1.3:1 (Acorn Janus), (d) 2.5:1 (bicompartmental JanusA), (e) 4:1 (bicompartmental JanusB) and (f) 12:1 (core - shell). Insets show the HRTEM images of the corresponding samples. The lighter region indicates ZnS phase and darker region shows CdSe phase in inset of (c) - (e). The region outlined by white line in inset of (f) indicates alloyed region.


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Figure 2: (a) HRTEM image of alloy structure with composition Cd:Zn::0.9:1 in which (A) and (B) show two alloy particles, (b) magnified image of the particle A and (c) lattice spacing’s calculated for selected area A 33 ACS Paragon Plus Environment


Figure 3: (a) HRTEM pattern (selected lighter region is ZnS and darker region corresponds to CdSe), (b) magnified HRTEM image and (c) corresponding FFT pattern for Acorn structure (Cd:Zn::1.3:1). 0.35 nm corresponds to (002) CdSe-Wz plane and 0.31 nm corresponds to (111) ZnS-ZB plane


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Figure 4: (a) HRTEM analysis for JanusB structure (Cd:Zn::4:1) and (b), (c) corresponding lattice spacing’s of ZnS and CdSe. Lattice spacing’s 0.31 nm corresponds to ZnS-ZB (111) plane and 0.35 nm corresponds to CdSe-ZB (111) plane. 35 ACS Paragon Plus Environment


Figure 5: FFT pattern of Alloy Core – thick CdSe shell heterostructure (Cd:Zn::12:1). The corresponding planes indicate the presence of CdSe wurtzite phase.


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Figure 6: (a) XRD analysis of nanoparticles synthesized with Cd:Zn molar feed ratios of 0:1 (pure ZnS), 0.9:1 (Alloy), 1.3:1 (Acorn), 2.5:1 (JanusA), 4:1 (JanusB), and 12:1 (core - shell). (b) Deconvoluted spectral analysis of Acorn, JanusA and JanusB. (‘ ’ indicates the ZB-ZnS, ‘ ’ indicates CdSe-Wz phase and ‘ ’ indicates ZB-CdSe phase.) 37 ACS Paragon Plus Environment


Figure 7: XPS counts from Se 3d spectra of nanoparticles synthesized with Cd:Zn molar feed ratios of (a) 1.3:1 (Acorn Janus), (b) 4:1 (bicompartmental JanusB), (c) 0.9:1 (Alloy) and (d) 12:1 (core-shell).


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Figure 8: XPS counts from S 2P and Se 3P spectra of nanoparticles synthesized with Cd:Zn molar feed ratios of (a) 0:1 (pure ZnS), (b) 1.3:1 (Acorn Janus), (c) 4:1 (bicompartmental JanusB), (d) 0.9:1 (Alloy) and (e) 12:1 (core-shell) (‘*’ –indicates the ‘S 2p3/2’ and ‘*’ –indicates the ‘S 2p1/2’) 39 ACS Paragon Plus Environment


Figure 9: XPS counts from Cd 3d spectra of nanoparticles synthesized with Cd:Zn molar feed ratios of (a) 1.3:1 (Acorn), (b) 4:1 (bicompartmental JanusB), and (c) 0.9:1 (alloy) and (d) 12:1 (core-shell) 40 ACS Paragon Plus Environment

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Figure 10: XPS counts from Zn 2p spectra of nanoparticles synthesized with Cd:Zn molar feed ratios of (a) 0:1 (pure ZnS), (b) 1.3:1 (Acorn Janus), (c) 4:1 (bicompartmental JanusB), (d) 0.9:1 (Alloy) and (e) 12:1 (core-shell) 41 ACS Paragon Plus Environment


Figure 11: Emission spectra of nanoparticles with different Cd:Zn ratios- Alloy (0.9:1), Acorn Janus (1.3:1), bicompartmental JanusA (2.5:1), bicompartmental JanusB (4:1), core-shell (12:1) excited with wavelength (a) 460 nm and (b) 270 nm; ‘ ’ denotes the CdSe characteristic peak and ‘ ’ denotes the interface peak.


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TOC Graphic:


Designing Coupled Quantum Dot with ZnS-CdSe Hybrid Structure for

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