Article Cite This: J. Phys. Chem. C 2017, 121, 24345-24351
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In Situ Monitoring of Cation-Exchange Reaction Shell Growth on Nanocrystals Annina Moser, Maksym Yarema, Weyde M. M. Lin, Olesya Yarema, Nuri Yazdani, and Vanessa Wood* Laboratory for Nanoelectronics, Department of Information Technology and Electrical Engineering, ETH Zurich, Gloriastrasse 35, CH-8092 Zurich, Switzerland S Supporting Information *
ABSTRACT: We demonstrate how in situ monitoring of the photoluminescence during shell growth around colloidal nanocrystals (NCs) can be used to develop a detailed and quantitative model for this process. We apply it here to study cation-exchange based growth of ZnS on a Cu−In−Se NC to form Cu−In−Se/ZnSe1−xSx alloyed NCs. We determine that this process begins with the Zn precursor binding to the outer layer of the NC followed by diffusion of Zn cations into successive atomic monolayers of the NC. At temperatures below 100 °C, Zn cations can only diffuse into the outermost atomic monolayer of the Cu−In−Se NCs. At growth temperatures above 100 °C, the second monolayer also becomes thermally accessible and can be filled with Zn cations. Our results provide an understanding of cation-exchange shell growth at the atomic level via optical analysis. The approach and mathematical model described here can be applied to other core/shell nanostructures and allows selection of optimal synthesis conditions to achieve desired core/shell design for a specific application.
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While diffusion is well studied for bulk materials18 and thin films,19 cation-exchange reactions for colloidal NCs remain largely underexplored.12 Several groups have previously performed studies of cationexchange processes.13,20 For example, Cossairt et al. analyzed the multistage Cd2+ diffusion mechanism in ZnTe nanorods by measuring the elemental composition of aliquots during the cation-exchange reaction.21 Here, we show how the multistage nature of a cationexchange shell formation can be tracked using in situ PL measurements. The time evolution of the emission spectra during the synthesis is analyzed as a function of growth time, temperature, and injection rate. As a model system, we select indium-rich group I−III−VI NCs. With the growth of a ZnS or ZnSe shell through the cation-exchange method, PL quantum yields of Cu−In−S, Cu−In−Se, Ag−In−S, and Ag−In−Se NCs16,22−25 can be increased from 20 to 30% to exceed 80%,26,27 making them of interest for use in optical and optoelectronic devices28−30 and in bioimaging applications.31,32 For the Cu−In−Se/ZnSe1−xSx NCs we focus on here, zinc and sulfur (or selenium) precursors can be added just after synthesis of Cu−In−Se (CISe) core-only NCs33 or in a second step after purification and redissolving of the CISe NCs.26,27,31,34 The average size of NCs remains nearly constant during the cation-exchange shell growth, which means the CISe cores are partially replaced by the shell, explaining the blue-shift of PL spectrum (i.e., shift toward higher energies).15,16,34,35 Due
INTRODUCTION For over 20 years, it has been known that shell growth on colloidal NCs significantly improves photoluminescence (PL) efficiencies.1 Shells not only passivate electronic surface defects but also alter the electronic confinement, which can displace the electronic density away from the surface and allow versatile tuning of optical and electronic properties. Since then, a considerable amount of research has been devoted to developing new shell synthesis routes and expanding the range of semiconductor material combinations.2,3 Multiple,4 gradient alloyed,5 and “giant”6,7 shells have led to record PL efficiencies due to the suppression of nonradiative processes, such as Auger recombination.8 Using NCs in solid-state devices introduces additional challenges, such as high electric fields, charge injection, and transport.9,10 Tailoring of the band structure via shell growth can be used to overcome these challenges.9 Cation-exchange reactions are commonly used to grow a shell on NCs, providing a way to improve their optical and electronic properties while keeping their size and shape unchanged.11−14 Depending on the reaction parameters and the miscibility of phases, heterostructures or complex homogeneous alloys can be prepared via cation-exchange, which are not otherwise accessible in a single synthesis step.15 It is understood that cation-exchange reactions in NCs involve diffusion of cations and are enabled by favorable thermodynamic properties.11 At higher reaction temperatures, more incoming cations can diffuse into the NC core,13,16 and smoother interfaces (i.e., increased alloying between core and shell materials) are observed.17 These effects point to thermally activated processes in the cation-exchange growth mechanism. © 2017 American Chemical Society
Received: August 28, 2017 Revised: October 6, 2017 Published: October 11, 2017 24345
DOI: 10.1021/acs.jpcc.7b08571 J. Phys. Chem. C 2017, 121, 24345−24351
Article
The Journal of Physical Chemistry C to the multicationic nature of the Cu−In−Se/ZnSe1−xSx NCs, it is challenging to quantify the type of shell (i.e., extent of alloying and shell thickness) that has been produced or understand how shell growth conditions influence these parameters. Our in situ PL approach allows us to identify distinct steps in the multistage cation-exchange growth of the ZnSe1−xSx shell and calculate the activation energies associated with each step. We determine that the cation-exchange growth mechanism consists of a surface reaction of the Zn precursor, followed by sequential solid-state diffusion of Zn atoms into successive atomic monolayers. These insights enable the selection of growth condition to achieve a specific atomic composition and core/shell alloying.
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EXPERIMENTAL SECTION Materials. CuCl (anhydrous, ≥99.99%), InCl3 (anhydrous, 99.999%), diethylzinc (97%), selenium (99.99%), and tri-noctylphosphine (TOP, 97%) were purchased from STREM chemicals; sulfur (99.5%), oleic acid (techn., 90%), toluene (≥99.7%), ethanol (≥99.8%), and methanol (≥99.9%) from Sigma-Aldrich; lithium bis(trimethylsilyl)amide (LiN[Si(CH3)3]2, 95%) and squalane (99%) from Acros. General Remarks. All syntheses are carried out in an airfree environment using standard Schlenk line technique. Injection mixtures and stock solutions are prepared in a N2filled glovebox. A stock solution of 1 M sulfur in TOP (i.e., TOP:S) is prepared by dissolving respective amounts of sulfur in TOP at room temperature. Caution: diethylzinc and its solutions should be handled with care due to high toxicity and f lammability of the substance. Scaled-Up Synthesis of Cu−In−Se Core-Only Nanocrystals. To enable comparison of the different shell syntheses, we prepare a single batch of CISe NC cores. Six grams of Cu− In−Se NCs with a stoichiometry of Cu3In5Se9 and an average size of 3.3 ± 0.4 nm are fabricated using the upscaling technique described previously. 36 In short, a solution containing 20 mmol of CuCl and 20 mmol of InCl3 in 120 mL of TOP is heated to 305 °C in a 1 L three-neck flask. An injection mixture, containing 80 mmol of Se and 120 mmol of LiN[Si(CH3)3]2 in 120 mL of TOP, is swiftly added from the connected funnel using underpressure-governed hot-injection technique.36 The temperature drops to 210−220 °C, and the reaction is terminated after 2 min of growth time by cooling the reactor to room temperature with pressurized air and later (at temperatures 100 NCs. X-ray diffraction measurements are made on a Rigaku SmartLab 9 kW System with rotating Cu anode and 2D solidstate detector HyPix-3000SL.
time. The 2D map (Figure 3B) plots the shift of PL peak energy during the cation-exchange process relative to that of the core-only CISe NCs. For temperatures below 150 °C, an initial red-shift of the PL peak is visible, followed by a shift in peak positions to higher energies. At higher temperatures only a blue-shift of the PL peak is observed. Surface Reaction and Subsequent Diffusion. We propose that this initial red-shift of the PL spectrum is due to an increase of the NC size, coming from the attachment of Zn precursor molecules to the NC surface (inset of Figure 3C). Indeed, small red-shifts (tens of millielectronvolts) are typically observed when shell material grows epitaxially around the core NCs.7 For all growth temperatures, the red-shift of PL peak position is 41 meV. This can be explained by the fact that our starting core NCs are the same for all shell growth syntheses and that the process is limited by the number of atomic sites that can be populated by Zn. The rate of the PL red-shift with time, which we fit with an exponential decay, exhibits a notable temperature dependence (Figure 3C). An Arrhenius-type fit of the PL peak red-shift rates vs 1/T (Figure 3D) indicates an activation energy of 0.43 eV for the surface reaction of Zn precursor during the cationexchange process (full description of fitting methods in the Supporting Information). Furthermore, from the slow- and fastinjection syntheses at 120 °C, we observe that the red-shift of the PL peak energy occurs at similar rates for both normal- and fast-injection speeds (Figure S4). We conclude that, for the chosen reaction parameters of the temperature series, the surface reaction of Zn precursor is limited by surface reaction kinetics and not the availability of the precursor in the solution. For temperatures above 150 °C, the red-shift associated with the surface reaction is already from the beginning of the
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RESULTS AND DISCUSSION To begin our investigation of the cation-exchange mechanism, we consider the long injection time temperature series (Figure S3). Figure 3A shows temperature-normalized PL peak positions for each growth temperature as a function of reaction 24347
DOI: 10.1021/acs.jpcc.7b08571 J. Phys. Chem. C 2017, 121, 24345−24351
Article
The Journal of Physical Chemistry C
Figure 4. (A) Relative blue-shift of PL peak position for the long injection time temperature series after subtraction of the red-shift contribution and single/double-exponential growth fits. (B,C) PL shift rates and pre-exponential factors as derived from fits in (A). (D) Cationic composition of Cu− In−Se/ZnSe1−xSx core/shell nanocrystals for the short injection time temperature series. (E) Calculated number of zinc and sulfur atoms in a single nanocrystal. (F) Illustration of cation-exchange process, including surface attachment of Zn atoms and temperature-dependent solid-state Zn diffusion through subsequent atomic monolayers.
shell has a ZnSe1−xSx gradient composition. Combining EDX results with known densities of materials and size of NCs (3.15 nm), we estimate the number of Zn and S atoms per NC using the mathematical description given in the Supporting Information (Figures 4E and S9B). This information enables us to rule out possible explanations for the two diffusion processes. Previous literature on I−III−VI thin films suggests several mechanisms for solid-state zinc diffusion.19 The two diffusion processes observed (Figures 4A−C) could be associated with, e.g., different Zn diffusion mechanisms (interstitial vs substitutional diffusion), differences between Zn-to-Cu and Zn-to-In substitution (i.e., ZnCu and ZnIn, respectively), or by competing cation- and anion-exchange processes. We rule out all of these explanations, considering that the high-energy process is insignificant below 100 °C (Figure 4C). Since a significant amount of Zn cations are present in the structure at low temperatures (>25% of cations, Figure 4D), the diffusion cannot be attributed to being solely interstitial. This indicates that substitutional diffusion is prevalent in all cases, so assigning the low-energy process to interstitial site diffusion and highenergy process to substitution is therefore not physical. Due to the larger ionic size of In3+, we assume that the ZnIn substitution process is more energy intense than ZnCu.39 However, even for low temperatures, a considerable amount of indium is replaced with zinc so linking the two processes to the zinc substitution mechanisms is not coherent with our observations. Finally, negligible amounts of sulfur below 150 °C (Figure 4E) rule out anion-exchange as the cause of the blue-shift. Furthermore, comparing ex situ PL emission energies with elemental composition shows a clear dependence on zinc content, while no trend is visible for sulfur content (Figure S10). We therefore need a different explanation for the two activation energies associated with Zn incorporation. In this study, CISe core NCs have a radius of less than three unit cells,
reaction outweighed by the strong shift to higher energies and therefore not visible in the PL spectra. This blue-shift of the PL emission spectrum during shell growth indicates an increased electronic confinement of the core NCs, which we associate with solid-state diffusion of Zn into the NC, replacing Cu and In ions. To analyze the PL peak position blue-shifts, we subtract the red-shift contribution attributed to the Zn surface reaction (Figure 4A, calculation details in the Supporting Information). The blue-shift is larger for higher growth temperatures, indicating more efficient diffusion processes with increasing temperatures. For high temperatures, curves of the blue-shift as a function of reaction time are best fit with a double-exponential, whereas low temperature curves show single-exponential behavior (Figure 4A). This suggests two different diffusion processes. Arrhenius plots of the two sets of exponents reveal the activation energies of a low- and a high-energy process as 0.18 and 0.51 eV, respectively (Figure 4B). Their corresponding pre-exponential factors describe the contribution of each process at the different shell growth temperatures (Figure 4C). Elucidating the Diffusion Mechanism. To elucidate the origins of the two distinct activation energies associated with Zn incorporation into the NCs, we determine the size and composition for all NCs from the two temperature series. Transmission electron microscopy (TEM) and X-ray diffraction data indicate that the size of Cu−In−Se/ZnSe1−xSx core/shell NCs is approximately equivalent to that of core-only CISe NCs (Figures S5 and S6) and that the crystal structure remains a zinc blende type34 before and after the growth of Zn chalcogenide shell (Figures S7 and S8). Figures 4D and S9A show the cationic compositions, calculated by energy-dispersive X-ray spectroscopy (EDX), as a function of reaction temperature for short and long injection times, respectively. While the content of Zn increases with higher temperatures, the indium and copper content decreases for both series. Sulfur content remains consistently smaller than that of Zn, indicating that the 24348
DOI: 10.1021/acs.jpcc.7b08571 J. Phys. Chem. C 2017, 121, 24345−24351
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growth temperatures promote a relaxation of the structure via Zn diffusion, decreasing the lattice strain. Following this hypothesis, structural strain reaches a maximum when the first monolayer is completed with Zn atoms, while inner layers remain unchanged (which is the case at approximately 100 °C, Figure 4C). When the filling of the second layer is kinetically accessible (above 100 °C), the lattice strain can partially relax (Figure 5B).
which is significantly smaller than typical diffusion reaction zones. Indeed, diffusion is known to operate differently for nanosized systems compared to bulk materials.40 For example, atomically sharp interfaces have been predicted for 2D thin films.41 In surface diffusion processes, each atomic monolayer may represent its own energetic barrier.42 We therefore propose that the low-energy process corresponds to the filling of the first atomic monolayer, while the high-energy process corresponds to filling of the second monolayer. Assuming a subsequent filling of the atomic monolayers, the first monolayer is filled with Zn atoms for all but the lowest temperatures (Figures 4E and S9B). This would explain the modest blue-shift for all temperatures below 100 °C. Filling of the second monolayer (i.e., the high-energy process) is only activated above 100 °C, and the extent it is filled increases linearly with temperature, according to the behavior of the preexponential factors. As soon as the second layer becomes accessible, the filling level of the first slightly decreases, which can be seen as a decrease of the first pre-exponential factor between 100 and 150 °C (Figure 4C). We attribute this observation to finite solid-state diffusion kinetics (i.e., a move of Zn ions between first and second atomic monolayer). When approaching the maximum filling of the second monolayer, also the first is filled again to a higher degree. The dotted lines in Figure 4E mark the number of cations or anions present in the first outermost or two outermost atomic monolayers of NC (calculation details in the Supporting Information). Intersections with these lines and the number of zinc correspond to the two temperatures 100 and 150 °C mentioned above, thus corroborating our proposed explanation of the stepwise diffusion. The evolution of PL peak width (full-width-at-half-maximum, fwhm) for the different reactions gives further evidence of a stepwise diffusion mechanism (Figure 5(A)). For temperatures
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CONCLUSION
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ASSOCIATED CONTENT
In summary, we used in situ PL measurements to study the cation-exchange process during the growth of Cu−In−Se/ ZnSe1−xSx core/shell NCs. At the beginning of the ZnSe1−xSx shell synthesis, a red-shift of emission spectra is observed, implying a size increase due to the surface attachment of zinc precursor molecules. The blue-shift of PL peak position can be associated with solid-state Zn diffusion through subsequent atomic monolayers of CISe NCs. While the surface monolayer of NCs can be exchanged with Zn ions at all shell growth temperatures between 70 and 174 °C, the second outermost atomic CISe monolayer only becomes thermally accessible at elevated temperatures above 100 °C. This multistage process is illustrated schematically in Figure 4F. All stages of the cation-exchange shell formation are thermally activated processes. The activation energy of the surface Zn reaction is 0.43 eV, while the activation energies of Zn diffusion into the first and second outermost atomic monolayers are 0.18 and 0.51 eV, respectively. The initial surface process exhibits a higher energetic barrier compared to cation-exchange in the surface atomic monolayer. This suggests that the surface reaction of Zn precursor is the limiting stage of the cation-exchange process and, once attached, Zn atoms easily diffuse into the outermost monolayer of the CISe NCs. Accordingly, the thickness of the ZnSe1−xSx shell is limited by temperature more than by reaction time or added zinc (Figure S10). For luminescent nanocrystals, in situ PL measurements during cation-exchange shell growth enable a fast and detailed analysis of nanoscale diffusion without disturbing the system during shell growth synthesis. The influence of single reaction parameters on distinct diffusion processes can be accessed and activation energies calculated. A better understanding of cationexchange process kinetics at the atomic level can enable better engineering of core/shell nanocrystals for luminescence applications, including biolabeling, optical downconversion, and electroluminescent devices.
Figure 5. (A) Full-width-at-half-maximum (fwhm) of the emission peak, mapped versus growth temperature and time for the long injection time temperature series. (B) Schematic explanation of PL width broadening through variable lattice strain due to the Zn diffusion through the atomic monolayers (MLs).
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.7b08571. Details of fitting methods and calculations; supporting Figures S1−S10 (PDF)
below 100 °C, the PL emission peak continues to broaden until the end of the reaction, whereas, for temperatures higher than 100 °C, the PL peak exhibits a maximal broadening followed by gradual decrease of the PL peak width. The higher the growth temperatures, the shorter is the time at which the largest fwhm occurs. We attribute the PL peak broadening to the strain of the core/shell structure, originating from the lattice mismatch between core and shell materials.34 Inserting more Zn atoms increases strain due to an abrupt concentration gradient. Higher
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. ORCID
Maksym Yarema: 0000-0002-2006-2466 Vanessa Wood: 0000-0002-1349-6557 24349
DOI: 10.1021/acs.jpcc.7b08571 J. Phys. Chem. C 2017, 121, 24345−24351
Article
The Journal of Physical Chemistry C Author Contributions
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The manuscript was written through contributions of all authors. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS The authors acknowledge support from the Swiss National Science Foundation through the Quantum Sciences and Technology NCCR and an ETH Research Grant. The authors thank David Norris for the SEM microscopy access and Mario Mü cklich for technical lab assistance. TEM and EDX measurements are performed at the Scientific Center for Optical and Electron Microscopy (ScopeM) of the Swiss Federal Institute of Technology, Zurich.
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