Chemical Tailoring of Band Offsets at the Interface ... - ACS Publications

Article pubs.acs.org/JPCC

Chemical Tailoring of Band Offsets at the Interface of ZnSe−CdS Heterostructures for Delocalized Photoexcited Charge Carriers Amit Dalui,† Arup Chakraborty,‡ Umamahesh Thupakula,† Ali Hossain Khan,† Sucheta Sengupta,† Biswarup Satpati,§ D. D. Sarma,†,∥,⊥ Indra Dasgupta,‡ and Somobrata Acharya*,† †

Centre for Advanced Materials and ‡Department of Solid State Physics, Indian Association for the Cultivation of Science, Jadavpur, Kolkata 700032, India § Surface Physics and Material Science Division, Saha Institute of Nuclear Physics, 1/AF Bidhannagar, Kolkata 700064, India ∥ Solid State and Structural Chemistry Unit, Indian Institute of Science, Bangalore 560012, India ⊥ Council of Scientific and Industrial Research-Network of Institutes for Solar Energy (CSIR-NISE), New Delhi 110001, India S Supporting Information *

ABSTRACT: Monocomponent quantum dots (QDs) possess limited electron−hole delocalization capacity upon photoexcitation that suppresses the efficiency of photoenergy harvesting devices. Type II heterostructures offer band offsets at conduction and valence bands depending upon the band gaps of the constituent QDs which largely depend on their sizes. Hence, by keeping the size of one constituent QD fixed while varying the size of the other QD selectively, the band offsets at the interface can be engineered selectively. We report on the tuning of band offsets by synthesizing component size modulated heterostructures composed of a fixed sized ZnSe QD and size tuned CdS QDs with variable band gaps. The resultant heterostructures show spontaneous charge carrier separation across the interface upon photoexcitation depending on the extent of band offsets. Formation mechanism, epitaxial relationship, and the intrinsic nature of interface of the heterostructures are investigated. Experimental results are corroborated with ab initio electronic structure calculations based on density functional theory. Spontaneous charge carrier delocalization across the interface depends on the magnitude of band offsets, which facilitates fabrication of QD sensitized solar cells (QDSSCs). Improved device performances of QDSSCs in comparison to the limited photon-to-current conversion efficiencies of monocomponent QDs demonstrates the significance of band offsets for natural charge carrier separation.

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INTRODUCTION Heterostructures composed of chemically distinct semiconductor components are unique class of materials suitable for a number of applications based on the principle of charge carrier migration mechanism at the interface.1−8 Unlike tuning the electronic properties by changing the size or shape of monocomponent nanocrystals,9−13 the electronic properties of heterostructures rely on the energy states at the interface.14−22 Heterostructures consisting of two different monocomponent semiconductor quantum dots (QDs) possesses band offsets at conduction band (CB) and valence band (VB) depending upon the relative alignments of the energy levels. The presence of band offsets results in a spatially indirect band gap for type II heterostructures which is smaller than the band gaps of both constituent QDs. The band offsets at the CB and VB are determined by the band gaps of the constituent QDs which largely depends on the sizes of the QDs.23−25 Hence, by varying the size of one of the constituent QDs while retaining the other size unchanged, the band offset at the interface can be tuned selectively. Such nanostructures allow spontaneous charge carrier (electron and hole) delocalization across the interface depending on the band offset retarding the charge © 2016 American Chemical Society

recombination, which is beneficial for photovoltaic applications.26−28 Additionally, superior band gap tunability, in comparison to the monocomponent QDs, can be achieved in heterostructures depending upon the extent of band offsets leading to improved photon conversion efficiencies in the solar spectrum. Both the band offset and band gap tunable features are beneficial for fabricating QD sensitized solar cells (QDSSCs) where electron injection rates into oxides or hole transport in the QD/redox electrolyte plays the key role for improving device efficiencies.29−34 However, creating such heterostructures with selective band offset at the interface is synthetically challenging. We report on the band offset control at the interface by synthesizing a fixed sized ZnSe QDs (∼4.5 nm) and three different sized CdS QDs (2.6, 4.2, and 5.6 nm). Synthesis of size-tuned CdS QDs allowed us varying the band gaps of CdS component selectively while the fixed size of ZnSe QDs prefixes the band gap of the other component within the heteroReceived: January 29, 2016 Revised: May 2, 2016 Published: May 2, 2016 10118

DOI: 10.1021/acs.jpcc.6b00986 J. Phys. Chem. C 2016, 120, 10118−10128

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The Journal of Physical Chemistry C

ZnSe and size-tuned CdS QDs. As-synthesized ZnSe and CdS QDs are purified and redispersed in high boiling point solvent ODE with an equimolar ratio in a three-necked round-bottom flask. A fixed amount of OA (∼0.3 mmol) was added to the mixture, and the whole mixture was degassed for 15 min at room temperature. Then the reaction mixture was annealed at 180 °C for 1 h. The reaction mixture shows yellowish color at room temperature which gradually turns from orange to dark red during annealing at 180 °C. Then it was cooled down to room temperature, and the product was precipitated out by adding acetone. Purification was done by acetone and finally with a acetone−chloroform mixture. Heterostructures can be dispersed in a variety of organic solvents like chloroform, toluene, hexane, etc. An identical reaction condition was maintained for the synthesis of three different sized heterostructures using ZnSe QDs and three different sized CdS QDs. Three different heterostructures are prepared using fixed sized ZnSe QDs and using one of the size-tuned CdS QDs. Characterization. A UV−vis absorption spectrum was obtained out using a Shimadzu 2550 UV−vis spectrophotometer. Fluorescence spectroscopy was performed with a Horiba Jobin Yvon nanolog spectrofluorometer. All optical measurements were performed at room temperature under ambient conditions. TEM images and EDX spectra were measured on a JEOL-JEM 2010 electron microscope using a 200 kV electron source. STEM (HAADF) were taken on a UHR-FEG-TEM, JEOL; JEM 2100 F model using a 200 kV electron source. For chemical mapping of NCs, a FEI, TECNAI G2F30, S-TWIN microscope operating at 300 kV equipped with a GATAN Orius B CCD camera was used. Energy filtered TEM images for chemical mapping were acquired using a Gatan imaging filter. Chemical maps from Cd−N (67 eV) and Se−M (57 eV) edges were obtained using the jump ratio method by acquiring two images (one post-edge and one preedge), respectively, with a energy slit width of 8 eV. Specimens were prepared by drop-casting the NCs solution in chloroform on a carbon-coated copper grid, and the grid was dried under air. XRD of the samples was taken by a Bruker D8 Advance powder diffractometer, using Cu Kα (λ = 1.54 Å) as the incident radiation. The cationic percentage was determined by ICP-AES using a PerkinElmer Optima 2100 DV machine. The purified QDs were redispersed in chloroform. The chloroform was dried off. Then the dried QDs were digested in concentrated nitric acid. The nitric acid solution of the sample was diluted with Milli-Q water to carry out the measurement. The fluorescence lifetime measurements were carried out using time-correlated single-photon counting (TCSPC) from Horiba Jobin Yvon with nano-LED-340 (pulse duration of 1 ns) excitation source. The J−V curves of devices were measured using a Keithley 2420 source meter under the 1 sun illumination with power 1000 W/m2. The device area was 0.64 cm2. Computational Details. Electronic structure calculations are carried out using density functional theory (DFT) as implemented in Vienna ab initio simulation package (VASP).37,38 We have used plane wave basis set with energy cutoff of 500 eV along with projector augmented wave (PAW) method.39 The exchange-correlation term in DFT was treated within the generalized gradient approximation (GGA) with Perdew−Burke−Ernzerhof (PBE) functional. QDs and heterostructures are simulated in a large cell with ∼12 Å vacuum in all there directions to get rid of the interaction between periodic

structures. Control over the band offsets have been achieved by attaching synthetically one of the size-tuned CdS QDs with the fixed sized ZnSe QDs. Selective tuning of band offsets allows natural delocalization of photoexcited charge carriers across the interface. By utilizing the natural charge carrier separation capabilities, we have fabricated QDSSCs, which shows the maximum power conversion efficiency for the heterostructures with the largest band offset. All of the heterostructures show improved device efficiencies in comparison to the monocomponent constituent QDs signifying the role of band offsets for photoenergy conversion.

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EXPERIMENTAL SECTION Chemicals. Cadmium oxide (CdO), 1-octadecene (ODE), S-powder, zinc acetate dihydrate (Zn(OAc)2·2H2O, 99%), selenium powder (100 mesh, 99.5%), oleic acid (OA, technical grade, 90%), sulfur powder (Sigma), 3-mercaptopropanoic acid (MPA), titanium tetrachloride (TiCl4, Sigma), titanium tetraisoproxide (TTIP), anatase TiO2 nanoparticles (size 20 nm, Sigma), acetic acid, α-terpeneol, and ethyl cellulose were used. All of the reagents were used as received. Synthesis of ZnSe QDs. ZnSe QDs were synthesized by a similar method reported by Zhang et al. with trivial modification.35 Typically, a mixture (12 mL in total) of Zn(Ac)2·2H2O (0.044 g, 0.2 mmol), Se powder (0.008 g, 0.1 mmol), oleic acid (0.14 mL, 0.4 mmol), and 12 mL of ODE was loaded in a 50 mL three-necked round-bottom flask fitted with an air-cooled condenser. Consequently, the reaction flask was heated to 120 °C and kept at this temperature for 1 h under stirring and degassing. The temperature was then raised to 300 °C at a rate of 4.5 °C/min. The growth time was counted once the temperature reached 300 °C and annealed for 3 h at this temperature. The reaction was then cooled down to room temperature. QDs were extracted by washing three times with a chloroform−methanol mixture to remove the unreacted precursors and metal oleate. Finally, the product was precipitated out by adding acetone. The product was further purified by acetone and finally with a chloroform−acetone mixture. The purified product was redispersed in ODE for further use. Synthesis of CdS QDs. Size-tuned CdS QDs were synthesized following the method of Peng et al. with a slight modification.36 In a typical synthesis, CdO (0.1 mmol), 0.5 mmol of OA (Cd:OA = 1:5), and 5 mL of ODE were degassed for 10 min at room temperature. The mixture was heated to 300 °C to obtain a clear solution. A solution of sulfur (0.0016 g, 0.05 mmol) in 1 mL of ODE was quickly injected into this hot solution, and the reaction mixture was allowed to cool to 250 °C, at which the growth of CdS QDs was carried out. Annealing for 2 min at 250 °C resulted in CdS QDs of ∼2.6 nm diameter. CdS QDs of other diameter were prepared following the similar protocol but varying Cd:OA molar ratio and annealing time. CdS QDs of ∼4.2 nm diameter was prepared using Cd:OA molar ratio 1:20 and annealing for 4 min at 250 °C. CdS QDs ∼5.6 nm in diameter were prepared by using a Cd:OA molar ratio of 1:50 and 4 min annealing time. All CdS QDs were extracted by washing with a chloroform−methanol mixture for three times to remove the unreacted precursor and the metal oleate. The reaction mixture was precipitated out with acetone and centrifuged at 10 000 rpm for 5 min. The purified products were redispersed in ODE for further use. Synthesis of ZnSe−CdS Heterostructures. Heterostructures have been synthesized using the presynthesized purified 10119


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Figure 1. Schematic of band offset tuning using fixed sized ZnSe QD and size-tuned CdS QDs for three different heterostructures (ZnSe−CdS1, ZnSe−CdS2, and ZnSe−CdS3). The valence bands (VBs) and conduction bands (CBs) are showm for the fixed size ZnSe QD and size tuned CdS QDs. Corresponding conduction band (ΔECB1,2,3) and valence band (ΔEVB1,2,3) offsets are denoted by double headed arrows, respectively. Electrons from VB of ZnSe and CdS QDs will move into the respective CBs (indicated by gray color up arrows) upon photoexcitation with photon energy hν. Because of presence of offsets at the interface of ZnSe−CdS heterostructure, the photoexcited CB electrons (− sign) of ZnSe will move into the CB of CdS (marked by blue arrow). Simultaneously, the holes (+ sign) from the CdS VB will move to the ZnSe VB (marked by blue arrow) due to the presence of valence band offset. The transition gaps and indirect recombination pathways of the heterostructures are indicated by ΔEs (ΔE1, ΔE2, ΔE3) and green, yellow, and red dotted arrows. Because of the presence of different CB offsets associated with heterostructure of size modulated CdS QDs, the indirect trasnsition energy gaps are different (ΔE1 > ΔE2 > ΔE3).

images. Only a Γ-centered single k-point is used for the calculations. We have relaxed the atomic positions to minimize the Hellman−Feynman forces on each atom with the tolerance value of 0.01 eV/Å. Preparation of MPA-Capped Water-Soluble QDs. Ligand exchange was performed following the method reported in the previous literature.40 Typically, MPA (0.212 g, 0.2 mmol) was dissolved in 0.3 mL of Milli-Q water and 1.0 mL of methanol. The pH of the solution was adjusted to 12 by adding 40% NaOH solution. The MPA−methanol solution was then added dropwise into the 6.0 mL of QDs chloroform solution (containing 0.5 mmol of QDs) and stirred for 30 min to precipitate the QDs. Then 10 mL of water was added into the mixture and stirred for another 30 min. The solution was finally separated into two phases where QDs were transferred into the water. The underlying organic phase was discarded, and the aqueous phase containing the QDs was collected. The free MPA ligand in the QDs aqueous solution was then isolated by precipitating the QDs by the addition of acetone. The supernatant was discarded, and the pellet was then redissolved in water for further use. All pure QDs and heterostructures were phase transferred in the same way for application of QDSSCs. TiO2 Electrode Preparation. Fluorine-doped tin oxide (FTO) glass (Sigma-Aldrich, 2 mm thick, 8 Ω/cm) was cleaned by sonication in detergent solution for 30 min followed by sonication in Milli-Q water, acetone, and isopropanol sequentially for 20 min each. Then it was dried in hot-air oven. Three separate layers of TiO2 films were deposited on the FTO glass. First, a TiO2 layer was deposited on FTO glass by treating the FTO glass in 40 mM TiCl4 solution at 70 °C for 30 min and washing with water and ethanol. Then a compact layer of TTIP in slightly acidic isopropanol was spin-casted (1000 rpm) onto the above TiCl4-treated FTO and dried at 120 °C for 1 h. A TiO2 paste was then deposited as active layer by the doctor blade technique. The TiO2 paste was prepared

according to the method reported in the literature.41 The film was kept at room temperature for 5 min and then dried at 80 °C for 20 min. The TiO2 electrodes were annealed at 450 °C for 30 min. The scattering layer of TiO2 was deposited on top of the active layer by treating the TiO2 film with 40 mM TiCl4 solution at 70 °C for 30 min. Then it was rinsed with water and ethanol followed by annealing at 450 °C for 30 min. Quantum Dots Sensitization and Fabrication of Solar Cells. QDSSC is fabricated following a method previously reported in the literature.42 QDs were immobilized on TiO2 film by drop-casting the MPA-capped QDs aqueous dispersion onto the TiO2 film and staying for drying the solvent. Such ex situ postsynthesis ligand exchange approach have been found to be effective for high loading QDs on mesoporous TiO2 film.42 Then it was rinsed with water and ethanol sequentially. After deposition, the QDs sensitized TiO2 films were coated with ZnS for two cycles by alternate dipping into the methanolic solution of 0.2 M Zn(OAc)2·2H2O and 0.2 M Na2S solution for 1 min per dip. The role of ZnS coating is to reduce the recombination of electrons from TiO2 into the electrolyte. ZnS treatment does not only suppress recombination at the mesoporous TiO2/electrolyte or QD/electrolyte interface but also affects the FTO/electrolyte interface.43,44 The cell were constructed by assembling the brass-based Cu2S counter electrode and QD-sensitized TiO2 electrode using a 50 μm thick Scotch spacer with a binder clip. The Cu2S counter electrodes were prepared by immersing brass foil in 1.0 M HCl solution at 75 °C for 30 min. Polysulfide aqueous solution is used as electrolyte, consisting of 2.0 M Na2S, 2.0 M S, and 0.2 M KCl. The brass foil was vulcanized immediately.

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RESULTS AND DISCUSSION

The tuning of band offsets of heterostructures by utilizing the band gap variation of one constituent QD while retaining the other fixed is shown schematically in Figure 1. A fixed sized ZnSe QD is coupled with size-tuned CdS QDs (CdS1, CdS2, 10120


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Figure 2. TEM images of (a) ZnSe QDs (b−d) CdS1, CdS2, and CdS3 QDs. The HRTEM images are shown in the insets of (a)−(d) showing the lattice spacing of cubic ZnSe and CdS QDs. The interplanar distances of 0.32 ± 0.01 nm (a, inset) is observed corresponding to the (111) planes of cubic bulk ZnSe (JCPDS #80-0021). HRTEM images (b−d, inset) of size-tuned CdS QDs show well-resolved lattice planes with interplanar distances of 0.34 ± 0.02 nm corresponding to (111) cubic bulk CdS (JCPDS #80-0019). UV−vis absorption and photoluminescence (PL) spectra of (e) ZnSe QDs and (f) three different sized CdS QDs.

and CdS3) consisting of different band gaps. The fixed size of ZnSe QD prefixes the band gap of one of the constituent component, while variable sizes of CdS QDs tune the band gap of the other constituent block within the heterostructures in a selective manner. Chemical attachment of ZnSe QD with CdS QDs creates differences in the potential (the band offsets) at the CB and VB depending on the relative band alignment (Figure 1). The size-tuned CdS QDs allow tuning the band offsets selectively at the interface of two constituent QDs. We designed ZnSe−CdS heterostructures in two distinct synthesis steps. First, ZnSe QDs and size-tuned CdS QDs were synthesized as starting materials (Experimental Section for synthesis details).35,36 Next, purified ZnSe and one of the sizetuned CdS QDs were redispersed in the high boiling point solvent 1-octadecene (ODE) in equimolar ratio to obtain heterostructures of three different band offsets. Transmission electron microscopy (TEM) image reveals a diameter of ∼4.5 ± 0.3 nm for ZnSe QDs and (Figure 2a and Figure S1a, Supporting Information, for size distribution histogram). TEM images of size-tuned CdS QDs show diameter of ∼2.6 ± 0.3 nm (CdS1), ∼4.2 ± 0.3 nm (CdS2), and ∼5.6 ± 0.4 nm (CdS3) (Figure 2b−d and Figure S1b−d for size distribution histogram). High-resolution TEM (HRTEM) shows wellresolved lattice planes of QDs corresponding to bulk cubic ZnSe and CdS (Figure 2a−d, insets). The UV−vis absorption and photoluminescence (PL) spectra of ZnSe and size-tuned CdS QDs show band-edge transitions with sharp full width at half-maxima indicating narrow size distribution of the precursor QDs (Figure 2e,f). We have calculated the band gaps of the precursor QDs from the absorption onsets. The ZnSe QDs show band gap of 2.86 eV while the sizetuned CdS QDs exhibit band gaps of 3.04 eV (CdS1), 2.82 eV (CdS2), and 2.60 eV (CdS3). A significant overlap between the first absorbing and emitting states is found for the monocomponent QDs. The narrow size distributions prefix the band-edges of the corresponding monocomponent QDs. Three distinct sets of heterostructures are prepared by the

coupling of ZnSe and CdS1 (ZC1), ZnSe and CdS2 (ZC2), and ZnSe and CdS3 (ZC3). The constituent QDs of ZC1 and ZC3 can be easily identified from the TEM images due to the size differences of ZnSe, CdS1, and CdS3 QDs, while this feature is not prominent for ZC2 owing to the nearly equal sizes of ZnSe and CdS2 QDs (Figure 3a−c and Figure S2a−f). The HRTEM images of ZC1, ZC2, and ZC3 reveal distinct interplanar spacing of ∼0.32 ± 0.02 and ∼0.34 ± 0.02 nm corresponding to (111) planes of cubic ZnSe and CdS, respectively (Figure 3d−f). The constituent QDs within the heterostructures are further verified by the statistical analysis of lattice planes using DM3 software from JEOL (Figure S3). Additional structural confirmations are obtained from the fast Fourier transformation images (Figure 3g−i) and selected area electron diffraction patterns (Figure S4). We have probed the elements within the heterostructure using EFTEM (Figure 3j−m) techniques.45 The elemental analysis by using energy dispersive X-ray spectroscopy (EDX) confirms the presence of all the four elements (Zn, 24.4%; Se, 27.8%; Cd, 25.2%; S, 22.6%) in a stoichiometric ratio (Figure S5 and Table S1). The cationic composition is further evaluated by inductively coupled plasma atomic emission spectroscopy (ICP-AES) (Table S2). The atomic ratio of the cations is found to be consistent with EDX measurements (Tables S1 and S2). A careful observation of HRTEM images of ZC1, ZC2, and ZC3 reveals an angle of ∼58.5° at the interface, suggesting that the (3−11) planes of CdS couple with (111) planes of ZnSe to form heterostructures (Figure 3n). Notably, the coupling between the above sets of planes implies a lattice mismatch of ∼41.2%, which is energetically unfavorable to form a stable heterojunction with sharp interface. However, the partial alloying at the interface may be expected to impose stability to the heterostructures.46−48 Indeed, partial alloying at the interface is evidenced from the powder X-ray diffraction (XRD) patterns (Figure 4a). The XRD patterns of ZnSe and size-tuned CdS QDs match with 10121


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Figure 3. (a−c) TEM images of three different heterostructures with fixed sized ZnSe QDs and size-tuned CdS QDs: (a) ZC1, (b) ZC2, and (c) ZC3. (d−f) HRTEM images of ZC1, ZC2, and ZC3, respectively. (g−i) FFT patterns of ZC1, ZC2, and ZC3, respectively. (j−m) Chemical mapping of the constituent elements Cd and Se using energy filtered TEM (EFTEM) where (j) is the unfiltered TEM image; (k) and (l) are the EFTEM images collected using the jump ratio method using Cd−N (67 eV) and Se−M (57 eV) energies with a energy slit of 8 eV width; (m) composite image of Cd and Se. (n) A reconstructed model of heterostructure showing (111) planes of ZnSe makes an angle of 58.5° with the (3− 11) planes of CdS.

bulk cubic structures with peaks corresponding to (111), (220), and (311) reflections. The XRD patterns of the heterostructures also reveal the cubic crystal phases; however, higher order reflections appear in between ZnSe and CdS reflections owing to the contributions from the corresponding planes or due to possible strain in the lattices.2 Additionally, (311) peaks of the heterostructures show a gradual shift toward higher angles with increasing sizes of CdS QDs. Interestingly, a new peak at an angle of ∼39° appears for ZC1, ZC2, and ZC3 which does not correspond to the cubic reflections of precursor QDs. A comparison of JCPDS files reveals that the peak position and

intensity of this newly developed peak correspond to the ZnCdS phase (Table S3), suggesting partial alloy formation at the interface. ZnSe QDs shows blue emission whereas size-tuned CdS QDs show blue to violet color depending upon QD sizes (Figure S6). ZC1, ZC2, and ZC3 display longer range fluorescence in comparison to pure QD components showing green, yellow, and red color, respectively (Figure S6). UV−vis absorption and PL spectra of three different sets of heterostructures (Figure 4b) show development of new peaks in the range of 2.54−2.17 eV which do not correspond to either 10122


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extent forming an alloyed interface while the larger portion of these QDs retain their original electronic states. The overlapping of absorption and PLE bands for confirms that the lower energy PL transitions are originated from the low energy states at the interfaces of the heterostructures (Figure S8). Carrier recombination dynamics are probed by measuring time-correlated single photon counting (TCSPC) technique. All fluorescence lifetime decay curves follow multiexponential decay profile (Table S4). Comparison of lifetime decay profiles of pure ZnSe, CdS QDs, and corresponding heterostructures shows slower decay rates for ZC1, ZC2, and ZC3 in comparison to the pure QDs (Figure 4e). Excited state lifetimes are found to be 44 ns for ZnSe QDs and 49, 51, and 33 ns for CdS1, CdS2, and CdS3 QDs, respectively. In comparison, longer lifetimes of 160, 195, and 413 ns are observed for ZC1, ZC2, and ZC3, respectively (Table S4). Indeed, a slow radiative recombination is expected from the heterostructures in comparison to monocomponent QDs due to delocalization of charge carriers driven by the band offsets.2−4 In accordance, ZC3 consisting of the biggest size CdS3 with the largest band offsets shows the longest lifetime among all heterostructures. In addition, we have observed other transition pathways for the heterostructures. For example, the 195 ns lifetime of ZC2 (by monitoring PL energy at 2.3 eV) along with another component of ∼80 ns was found by monitoring a different PL energy of 2.85 eV (Figure S10). Interestingly, this shorter lifetime component of ZC2 is comparable to pure ZnSe or CdS QDs, suggesting the existence of other radiative channels through the ZnSe or CdS sections within the heterostructures which supports the PLE observations. However, the longer components of lifetime of heterostructures indicate that radiative recombination mechanism across the ZnSe−CdS interface is governed by the band offset formed by adjoining ZnSe and CdS QDs. One might anticipate that pure ZnSe or CdS QDs may partly dissolve in hot ODE releasing monomers, which in turn induces cation exchange reaction resulting in the observed lower energy transitions.49 The other possibility is the “selfrecognition” of ZnSe or CdS QDs to grow into larger sized QDs leading to the lower energy PLs.50 Control experiments using either pure ZnSe or CdS QDs do not show shift of peak positions in optical spectroscopies, suggesting stability of individual QDs under the same reaction conditions of heterostructure formation (Figure S11). The physical mixture of an equimolar ratio (1:1 mM) of ZnSe to CdS QDs in ODE shows characteristic optical features of pure QDs, indicating that the observed lower energy transitions do not originate from the energy transfer but rather are the characteristic features of heterostructures (Figure S12a,b). We have carried out XRD measurements of physical mixture of ZnSe and CdS3 QDs before and after annealing at 180 °C for 1 h (Figure S12c). Comparison of the XRD reveals the appearance of a peak at ∼39°, the same peak observed for the heterostructures earlier (Figure 4a), suggesting an attachment of ZnSe and CdS3 QDs upon annealing. These interfacial alloyed states are not predominant quantity-wise; however, they may dominantly contribute to the observed PL/PLE/PL lifetime processes. These observations confirm that the annealing leads to the coupling of ZnSe and CdS QDs, which results in new electronic transitions in comparison to pure QDs. To identify the effect of annealing temperature on the interface formation, we have heated up the physical mixture of ZnSe and CdS2 up to 250 °C while peaking up aliquots at different temperatures during the

Figure 4. (a) XRD patterns of ZnSe QDs, size-tuned CdS QDs and corresponding heterostructures. XRD patterns are compared with the standard JCPDS data files (black vertical lines for CdS, JCPDS #800019 and brown vertical lines for ZnSe, JCPDS #80-0021). The alloy peaks of heterostructures are marked with red asterisks. (b) Comparison of UV−vis absorption and PL spectra of ZC1, ZC2, and ZC3. (c) Band gap tunability by reaction annealing time at 180 °C for ZC1 (black squares), ZC2 (red dots), and ZC3 (blue up triangles) with error bars. (d) Comparison of PLE spectra of ZnSe QDs (black curves), size-tuned CdS QDs (red curves), and ZC1, ZC2, and ZC3 (blue curves). (e) Comparison of fluorescence lifetime decay curves of ZnSe QDs (black curve) and size-tuned CdS QDs (red curves) and heterostructures ZC1, ZC2, and ZC3 (green curves) monitoring PL maxima of monocomponent QDs and heterostructures.

ZnSe or CdS QDs (Figure 2e,f). These new PL features appear at 2.54 eV for ZC1, 2.32 eV for ZC2, and 2.17 eV for ZC3, suggesting formation of new band gaps for the heterostructures. In addition, the original high energy PL bands of the ZnSe and CdS QDs are significantly quenched, indicating a charge carrier population transfer to the newly developed states of heterostructures (Figure S7). The PL from ZC1, ZC2, and ZC3 is tunable with annealing time, suggesting increase in the extent of band offsets (Figure 4c). Eventually, PL can be tunable from ∼2 to ∼2.6 eV by changing the reaction annealing time. We have carried out photoluminescence excitation (PLE) spectra to elucidate the energy transition states that are involved in the PL processes. The PLE spectrum shows distinct overlap with the absorption spectrum (Figure S8). A comparison of PLE spectra of pure ZnSe and size-tuned CdS QDs with the heterostructures shows traces of ZnSe and CdS states along with the newly developed lower energy states of ZC1, ZC2, and ZC3 (Figure 4d). Additionally, the PLE measured using the lowest emission energy (∼1.9 eV) shows prominent low energy bands for the heterostructures which gradually retrace ZnSe or CdS QDs states upon increasing the excitation energies (Figure S9). These spectroscopic observations suggest that the ZnSe and CdS QDs adjoin to a certain 10123


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Figure 5. Total DOS of CdS QDs (black curve), ZnSe QDs (blue curve), heterostructures with sharp interface (red curve) and alloyed interface (green curve). Insets showing the corresponding model cluster used for the calculations where Cd, Zn, Se, S, and H atoms are represented in gray, magenta, green, yellow, and cyan spheres, respectively.

employing ZC3 are compared with ZnSe and CdS3 QDSSCs (Figure S14b). The open circuit voltage (VOC) for ZC3 sensitized electrode is ∼0.54 V, which is greater than ZnSe (0.36 V) and comparable to CdS3 (0.56 V) sensitized electrodes (Figure S14b). However, the short circuit current (JSC) and power conversion efficiency of ZC1, ZC2, and ZC3 sensitized cells are greater than the corresponding ZnSe or sizetuned CdS QDSSc (Figure S14c and Table S5). In addition, we have compared the solar cell performance of our heterostructures with the reported ZnSe-CdS core−shell QDSSCs and TiO2/ZnSe/CdS thin film system.57−59 Our band offset controlled ZnSe−CdS asymmetric heterostructure shows better performance in all aspects of photovoltaic parameters. However, the efficiency is found to be lower in comparison to CdSe/CdTe based sensitized solar cells.60,61 Among the all heterostructures, ZC3 shows the highest JSC (17.4 mA/cm2) compared to ZC2 (14.5 mA/cm2) and ZC1 (13.6 mA/cm2) (Figure S14c and Table S5). Additionally, ZC3 shows the highest power conversion efficiency in comparison to ZC1 and ZC2 (Figure S15 and Table S5). Controlled band offsets depending on the sizes of CdS QDs in heterostructures drag the electrons and holes in opposite direction which enhances the charge carrier extraction efficiency considerably.57 Upon illumination, electrons from the VB of ZnSe and CdS QDs are promoted to the respective CBs of both the QDs. However, the CB electrons of ZnSe relax to the CB of CdS QDs due to the presence of band offset increasing the electron population density in the CB of CdS section. The populated electrons in the CB of CdS are transferred to the TiO2 matrix to enhance the device performances with the heterostructures. Accordingly, ZC3 with the largest band offset shows superior device performance. Another major issue is the broad range absorption by the heterostructures in the solar spectrum region in comparison to the monocomponent QDs which contributes to enhance incident photon-to-current conversion efficiencies. Devices fabricated with the heterostructures show fair photocurrent stability up to ∼12 min with efficient dark to photocurrent ratio (Figure S14c, inset). The photocurrent response to incident

reaction (Figure S13). New low energy absorption and PL features appear from 120 °C along with the characteristic optical features of precursor QDs, suggesting generation of new band gap in the heterostructures. Gradual shifts of the low energy transitions imply the change in the resultant band gap with the reaction annealing temperature. It is evident that the electronic structure at the interface plays a crucial role in tailoring the band offsets and the resultant transition gaps.51 Electronic levels of the ZnSe and CdS QDs after coupling are aligned in a way that the lowest energy states involve charge carrier population transfer across the interface. This originates new spatially indirect states at energies lower than the energy levels of pure ZnSe and CdS QDs. Photoexcitation populates electrons within CdS QDs due to band offset in the CB which ultimately relax to hole states confined in ZnSe QDs in the VB lowering down the resultant transition gap below the band gaps of individual QDs. The fixed size of ZnSe QDs fixes the band gap of one constituent component, while the size-tuned CdS QDs control the band gaps of the other constituent component within the heterostructure in a selective manner. Since the photoexcited electrons populated in CdS relax to hole states confined in ZnSe QDs, the size-tuned CdS QDs change the band offsets and delocalization of charge carriers selectively. Inspired by the natural charge carrier separation capacity of the heterostructures, we have fabricated quantum dot sensitized solar cells (QDSSCs) (Figure S14).29−34 Sandwich-type cells were constructed by FTO/QD-sensitized TiO2 film/ZnS/ polysulfide electrolyte/52−54 and Cu2S counter electrode (brass) using binder clips.55,56 In such configuration of QDSSCs, photoexcited electrons are injected into TiO2, while holes are collected by the redox couple (Figure S14a).55,56 Cross-sectional scanning electron microscope (SEM) image of ZC3 sensitized photoanode shows distinct layers of FTO, compact TiO2 layer, and active TiO2 layer adsorbed with ZC3 (Figure S14b, inset). The QDSSCs using pure ZnSe QDs and size-tuned CdS QDs were fabricated following the same procedure. Current density (J) versus voltage (V) characteristics of the QDSSCs 10124


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Figure 6. (a) Heterostructures for 1:(2/3) system where Cd, Zn, Se, S, and H atoms are represented in gray, magenta, green, yellow, and cyan spheres, respectively. The heterostructure mimics ZC1. (b) Energy resolved charge density plot for 1:(2/3) system. (c) Heterostructures for 1:1 system where Cd, Zn, Se, S, and H atoms are represented in gray, magenta, green, yellow, and cyan spheres, respectively. The heterostructure mimics ZC2. (d) Energy resolved charge density plot for 1:1 system. (e) Heterostructures with 1:2 system, where Cd, Zn, Se, S, and H atoms are represented in gray, magenta, green, yellow, and cyan spheres, respectively. The heterostructure mimics ZC3. (f) Energy resolved charge density plot for 1:2 heterostructures.

heterostructures from energy resolved charge density plots (Figure 6).24 Band offsets at the VB and CB are calculated to be 0.37 and 0.57 eV, respectively, for heterostructure (1:1 system) containing the same size of ZnSe and CdS clusters with pure interface (Figure 6c,d). The energy resolved charge density plot clearly reveals that the alloyed interface (Figure S18) is not sharp due to diffusion of states across the interface. This feature is the most prominent for the states belonging to the VB. The calculations further reveal that the nature of interface remains type II while band offset values change due to the alloying. The band offsets at the VB and CB are calculated to be 0.4 and 0.46 eV, respectively, for the ZnSe−CdS heterostructure with alloyed interface (Figure S18). Although there is a change in the band offsets due to interfacial alloying, importantly, our calculations reveal that alloying does not affect the transition band gaps. In order to study the effect of the variation of component size on the VB and CB offsets, calculations were carried out with a fixed size ZnSe and different sizes of the CdS QDs mimicking the synthetically obtained heterostructures (Figure 6). In one heterostructure, the number of atoms in the CdS QDs is twice in comparison to the number of atoms in the ZnSe QDs (1:2 system) and in another heterostructure, where the number of atoms in CdS QDs is two-thirds times the number atoms in the ZnSe QDs (1:(2/3) system). In the 1:(2/3) system, the CdS QDs consists of 40 Cd atoms and 39 S atoms

light was evaluated by monochromatic incident photon-tocurrent efficiency (IPCE) measurements. Photocurrent response matches well the absorption spectrum (Figure S14d). The photocurrent also shows onsets at energy 2.7−2.9 eV corresponding to ZnSe and CdS QDs components and a lower energy transition threshold at 2.3 eV due to the formation of type II interface of the heterostructures. We have measured IPCE of ∼65% in the range of 2.3−3.1 eV using heterostructures as sensitizers (Figure S14d). In order to elucidate further the intrinsic nature of the interface of the heterostructures, we have carried out first principles electronic structure calculation using DFT as implemented in VASP.37−39 Model ZnSe−CdS heterostructure used for calculation consisting ZnSe and CdS clusters resembles the experimentally designed ZC2 heterostructure of same size of ZnSe and CdS QDs (Figure S16). Density of states (DOS) of the monocomponent QDs and heterostructures reveal that the band gap of heterostructures is smaller than either of the constituent QDs, indicating type II alignment of ZnSe−CdS interface (Figure 5 and Figure S16). Such reduced band gap for the heterostructures was inferred from the lower energy transitions observed in the optical spectra. From the charge density plots (Figure S17), VB is found to be contributed by the ZnSe section and CB arises from CdS section forming a type II interface. We have calculated the band offsets of CdS component size modulated ZnSe−CdS 10125


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lead to novel routes for other types of asymmetric heterostructures with different material combinations.

excluding the H passivators (Figure 6a). The band gap of 1:(2/ 3) heterostructure is calculated to be 2.28 eV, which is larger than the 1:1 system (Figure 6b). In the 1:2 system, CdS QDs consist of 104 Cd atoms and 104 S atoms, which is attached to ZnSe QDs consisting of 61 Zn atoms and 61 Se atoms excluding H passivators (Figure 6e). This model structure resembles the experimentally designed ZC3 heterostructure. Calculations reveal that the band gap between VB and CB states decreases by 0.14 eV upon increasing the size. The decrease in the band gap is due to quantum confinement as a result of the reduction of band gap for the larger CdS QD. The estimated VB and CB band offsets from energy resolved charge density plot are 0.20 and 0.89 eV, respectively (Figure 6f). It is to be noted that the band alignment at the interface of the heterostructures for different sizes of the CdS QDs remain type II in nature. A detail comparison of the band gaps and band offsets with different sizes of the CdS QDs is presented in the Table 1.

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* Supporting Information

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.6b00986. Data of materials synthesis, characterization, controlled experiments, photovoltaic and theoretical calculations (PDF)

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band gap (eV)

HOMO offset (eV)

LUMO offset (eV)

1:(2/3) 1:1 1:2

2.28 2.10 1.96

0.39 0.37 0.20

0.51 0.57 0.89

AUTHOR INFORMATION

Corresponding Author

*E-mail [email protected]; phone +91-33-24734971 (ext 1104). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS SERB (grant # EMR/2014/000664), DST India, is gratefully acknowledged for the financial support.

Table 1. Comparison of the Band Gap, HOMO Offset, and LUMO Offset of Heterostructures with Different Sizes of CdS Clusters with a Fixed Size ZnSe Cluster systems (ratio of the number of atoms in ZnSe and CdS clusters)

ASSOCIATED CONTENT

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These results suggest that no intermediate states are formed within the CB of CdS QDs and the VB of ZnSe QDs due to alloying, and the resultant transition band gap is defined by the CB of CdS QDs and the VB of ZnSe QDs. Our calculations reveal that the band gap and band offsets can be tuned by changing the size of the components and are in agreement with the PL spectra for heterostructures with different size of CdS QDs. These changes in the band offsets are attributed to the combined effect of quantum confinement and the nature of chemical bonding at the interface. Moreover, the band offsets of the heterostructures can be varied just by changing the size of one QD while retaining the other QDs size fixed.

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CONCLUSION Band offset modulation using various epitaxial techniques is well established. However, creating an efficient interface of two monocomoponent QDs to control band offsets through colloidal synthesis route is intriguing. We have shown that the band offset can be tailored selectively by varying the size of one constituent QD. The selective band offsets at the CB and VB offer natural delocalization of charge carriers across the interface upon photoexcitation. Additionally, the band gaps of the heterostructures can be tuned simultaneously over a broad range in comparison to the monocomponent QDs. Both these characteristics of charge carrier separation capability and band gap tunability contributed to the observed high photon to current conversion efficiency of the QDSSCs with heterostructures compared to the monocomoponent QDs. Although the efficiencies of the devices are low compared to the state-ofart of present solar cells, our results demonstrate the selective role of band offsets for photoenergy conversion. Attaching two individual QDs through selective facets with band offsets may 10126


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