Investigation of the Exchange Kinetics and Surface Recovery of Cd x

Article pubs.acs.org/cm

Investigation of the Exchange Kinetics and Surface Recovery of CdxHg1−xTe Quantum Dots during Cation Exchange Using a Microfluidic Flow Reactor Stephen V. Kershaw,*,† Nema M. Abdelazim,† Yihua Zhao,‡,# Andrei S. Susha,† Olga Zhovtiuk,† Wey Yang Teoh,‡ and Andrey L. Rogach† †

Department of Physics and Materials Science & Centre for Functional Photonics (CFP), City University of Hong Kong, Hong Kong, Hong Kong SAR ‡ Clean Energy and Nanotechnology Laboratory, Joint Laboratory for Energy and Environmental Catalysis, School of Energy and Environment, City University of Hong Kong, Hong Kong, Hong Kong SAR S Supporting Information *

ABSTRACT: Detailed analyses of coupled photoluminescence, emission lifetime, and absorption measurements have been made on the products of cation exchange reactions between CdTe nanocrystals and Hg2+ salt/ligand solutions in a microfluidic flow reactor and capillary measurement cell to probe the reaction kinetics over the seconds to hours time scale and to establish the influence of the reaction conditions on the spatial distribution of the mixed cations within the resulting CdxHg1−xTe colloidal quantum dots. The establishment of the evolution of the radiative and nonradiative rates allowed the recovery of the emission quantum yield in CdxHg1−xTe quantum dots to be quantified to almost 50% and the necessary time scales to be determined for each set of reaction conditions. The reaction kinetics showed clear indication of a fast surface exchange process followed by a slower internal rearrangement of the cation distribution.

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work9 has shown that Cd2+/Hg2+indiffusion in the CdxHg1−xTe system is not quite so fast and there is a question as to the limit of cation exchange possible and the influence that the reaction conditions may have on the spatial distribution of cations, both immediately after the treatment with Hg2+ salt and in the period of days to weeks afterward, until an equilibrium distribution is reached. The mechanisms for the cation indiffusion process have been the subject of a great deal of both experimental and theoretical investigation and discussion. In general, there are several mechanisms that may assist indiffusion: if the cations concerned (both incoming and displaced) are small enough they may benefit from interstitial sites in the lattice; larger cations may require cation vacancies in order to progress through the structure. Chakraborty et al.15 have also shown how dissimilarly charged cations may benefit from the local charge stabilization of defects which can then assist cation diffusion and substantially accelerate the indiffusion rate. Such diffusion mechanisms are well-known in bulk semiconductors and have been the subject of wide ranging studies for many different host lattices and dopants.16−18 Which type of mechanism is at play is heavily dependent on the cation and lattice dimensions and geometry, but in semiconductor QDs, there is potentially more

INTRODUCTION There have been many reports of ion exchange reactions in colloidal quantum dots (QDs)1−5 and in II−VI based QD materials in particular.6−10 Some of the earliest observations of cation exchange stemmed from attempts to make QD quantum wells by forming multilayer core−shell−shell heterostructures,6 where it was seen that cation indiffusion between different layers was also occurring, leading to radial cation concentration gradients rather than discrete semiconductor layers with abrupt interfaces.7 In some materials, this indiffusion process may take place, albeit slowly, at room temperature,8 while in materials with higher cation/anion bond enthalpies thermal activation11 may be required to increase ionic mobilities. In the case of chalcogenide QDs, it was established that cation exchange may occur almost completely topotaxially,12 with the anion lattice providing the structural integrity to maintain the QD size and shape. Where the starting morphology and that of the product may differ, the transformation may be localized, with induced strain limiting complete transformation.13 In other cases, possibly depending on the nanocrystal size, even changes of morphology may not be a barrier to complete exchange.12 In the case of the CdxHg1−xTe system, the matching morphologies and very similar lattice constants of CdTe and HgTe (0.6482 and 0.6460 nm, respectively) mean that lattice strain due to exchange is virtually absent, further ensuring topotaxiality. In cases such as the CdS/Ag+ system, exchange of Cd2+ for 2Ag+ occurs very rapidly as observed by Chan et al.14 Our previous © 2017 American Chemical Society

Received: October 24, 2016 Revised: March 3, 2017 Published: March 3, 2017 2756

DOI: 10.1021/acs.chemmater.6b04544 Chem. Mater. 2017, 29, 2756−2768

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Chemistry of Materials

due to its superior control of fluid, mass, and energy transport.24−27 In order to study the kinetics during the early stage of the present cation exchange process, we have taken advantage of the rapid mixing and the controlled mass transport in a continuous flow microfluidic reactor. In this case, the CdTe QD starting solution and the Hg2+ salt/ligand solution were injected from two side channels, respectively, of a “T” shaped microfluidic junction, where the mixing and reaction happens immediately, and the cation exchange then proceeds along the continuous flow of the reaction fluid in the outlet arm. The reaction products are then transported under laminar flow along the outlet capillary into the spectrometer. Such an arrangement was used by Chan et al.14 to probe millisecond scale exchange dynamics on the CdS/Ag2S system, while monitoring the structure of the products by synchrotron X-ray scattering. In our experiments, the rate of exchange was somewhat slower and we probed PL properties and the evolution of absorption spectra on reaction time scales from 10 s to 80 min after mixing, simply by varying the flow rate of reactants through the system. This approach gives us insight into the reaction kinetics and shows the development of a very strong nonradiative recombination pathway, in some cases up to hundreds of times faster than in the initial CdTe QDs, which rapidly subsides later in the exchange process. The study sheds light on the best way to avoid strong disparities between radiative and nonradiative recombination and to promote recovery of the PL QY for the CdxHg1−xTe QDs given from the disruption during the initial phase of the cation exchange.

room to maneuver for example due to possible lattice dilation due to strain (relaxation) effects, increased defect densities,19 and the dynamic effects on cation migration of confined low frequency modes in the phonon density of states20 including surface and breathing modes. While cation diffusion is clearly enhanced in small QDs relative to the bulk material, there is still much discussion about the precise diffusion mechanism(s), and indeed, these may vary according to the geometry of the host lattice, the cation sizes, and respective charges. Groeneveld et al.11 have investigated the indiffusion of Zn2+ and Cd2+ in heterostructures, starting from ZnSe core QDs in organic solvents treated with Cd2+. Different exchange temperatures and thermal sequences allowed them to form CdSe/ZnSe core/shell QDs, cation gradient QDs, and uniformly distributed CdxZn1−xSe alloyed QDs. The control was attributed to the thermal activation of Zn Frenkel defect pairs (a Zn interstitial, Zni, paired with a Zn vacancy, VZn). The zincblende structure has two interstitial sites per unit cell, one tetrahedral and one octahedral,21,19 but thermal activation is required to form the defect pair and to drive the migration of the defect through the lattice by site hopping. In bulk CdxHg1−xTe, the dominant vacancies are cation vacancies and Te antisite defects. Using Hg−Cd indiffusion coefficients in CdxHg1−xTe given by Shaw16 for a nominal composition of x = 0.9, the equivalent diffusion rate in the bulk form would be approximately 0.48 × 10−4 e−1.91eV/kT cm2s−1. For the same diffusion rate (10−47 cm2s−1) as in the CdxZn1−xSe case,11 this would equate to an absolute temperature of just 207 K suggesting a substantially lower onset temperature for indiffusion in CdxHg1−xTe QDs compared with the CdZnSe system. The potentially higher diffusivity alone is insufficient to form the basis for a room temperature Frenkel pair diffusion model in CdxHg1−xTe. Directly mapping the spatial distribution of Cd2+ and Hg2+cations in small (e.g., 3−5 nm) QDs is a difficult task. The difference in contrast between Cd and Hg atoms in HRTEM is very slight, and as already mentioned, the lattice constants of CdTe, HgTe, and alloy compositions in between are almost indistinguishable. Elemental mapping using EDX is poor due to the very low signals from such small structures. Recent work by Smith et al.22 has shown a way to indirectly infer the type of structure (at least by distinguishing between outright core−shell and alloy cases) from the relative oscillator strengths of the lowest lying transitions near the band edge. The latter are derived from fitted absorption spectra. However, information on band edge oscillator strengths can also be derived from radiative transition rates,23 and we have used both methods here to determine the likely cation structural dynamics during and after the exchange process. Furthermore, we have determined both the radiative and nonradiative recombination rates throughout the exchange process, along with the PL quantum yields (PL QY). We have investigated different strategies for ion exchange in aqueous solution where Hg2+ ion is introduced as the perchlorate salt, Hg(ClO4)2, solubilized in alkaline solutions by a range of different water-soluble thiols. The effect of Hg2+ salt/ligand concentration, choice of thiol and solution pH on exchange rate, degree of exchange, and resulting cation structures have been studied. Following reaction kinetics in the early stages of exchange on static samples is not possible until the pace of the reaction slows. Microfluidic technology transposes the transient kinetics to a steady state spatial distribution along the length of the flow and is a good platform to study the mechanism of nanoparticle nucleation and growth,

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EXPERIMENTAL SECTION

All chemicals (cadmium acetate hydrate (Cd(Ac)2·2H2O), Uni-Chem; sodium tellurite 499% (Na2TeO3), Aldrich; thioglycolic acid AR grade (TGA), Accuchem; sodium hydroxide AR grade (NaOH), Accuchem; sodium borohydride >98% (NaBH4), Aldrich; mercury perchlorate 98% (Hg(ClO4)22.5H2O), Aldrich; 1-thioglycerol >97% (1-TG), Sigma; mercaptopropionic acid >99% (MPA), Accuchem; mercaptoethylamine >98% (MEA), Sigma; 2,3,-dithiopropanol >97% (DTP), Accuchem) were used as received from the suppliers. Milli-Q deionized water was used in all experiments. Note that Cd and Hg salts and their solutions are potentially toxic by ingestion and should be handled with normal laboratory hygiene and safety precautions. Gloves, safety spectacles, and labcoats should be used at all times, and waste materials should be properly disposed of via a registered and responsible chemical waste processing company. CdTe QD Synthesis. The water based CdTe QD starting material was prepared according to the method reported by Wu et al.28 0.4 mmol of Cd(Ac)2·2H2O was dissolved in 50 mL of water in a twonecked flask, and 36 μL of TGA was injected into the solution. The pH was adjusted to 10.5 using 1 M NaOH solution, and 0.08 mmol of Na2TeO3 was dissolved in 50 mL of Milli-Q water and then added to the cadmium precursor solution followed by 160 mg of NaBH4; the mixture was allowed to react for 5 min. Afterward, the flask was connected to a condenser and refluxed at 100 °C under open-air conditions. The Cd2+/TGA/TeO32− molar ratio was 1:1:0.2. After several hours of refluxing, during which the QD size increased, the solution PL peak reached 623 nm and at this point the growth was terminated by cooling the solution. The QY of the final solution measured after storage for several months in a refrigerator was 55%. Prior to use in the alloying experiments, the as-stored CdTe QDs were 5 times diluted and heat treated with a small addition of sodium bicarbonate (5 g/500 mL) for 30 min at 80 °C so that QD clusters formed during the initial synthesis10 were broken apart and the PLQY improved. The core diameter of the CdTe QDs was determined from the solution absorption spectrum (peak at 603 nm) to be 4.0 nm using the sizing curve data of Kamal et al.29 2757


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Chemistry of Materials Ion Exchange. For ion exchange experiments, a number of mercury salt solutions were made with different ligands and ligand concentrations and, in the case of 1-TG stabilized solutions, different pH values. The ligands are required in order to ensure the mercury or displaced cadmium does not precipitate as an insoluble salt at high solution pH. The ligand thus serves to deliver the Hg2+ to the QD surface and assists in the removal of the displaced Cd2+ ions. Inevitably during the ion exchange process, some of the added ligand will reequilibrate with the TGA originally used to synthesize the CdTe QDs which may affect the long-term colloid stability. For this reason, the exchange ligand concentration was adjusted in each case to ensure the colloid did not precipitate on at least a 2−3 week time scale. During the reaction, equal volumes of the diluted CdTe QD solution and the Hg2+ salt/ligand solution were mixed together in a microflow reactor with the mixing experiments typically covering 0−80 min reaction time scales. For longer reaction times, equal volumes were simply mixed in a sample vial and allowed to stand for up to several weeks. The Hg2+ salt solutions were prepared as follows: typically, around 0.44 to 0.61 mmol of ligand was added to a stirred 50 mL volume of water, and the pH was raised to >10 by the addition of a few drops of 1 M NaOH solution. A separate solution of 50 mg of Hg(ClO4)2 2.5 H2O dissolved in 10 mL of pH neutral water was then added while maintaining the pH above 10 (above 9.2 for the lower pH solution). Once all of the latter was added, the pH was fine-tuned to the final required value (10.8 in most cases and 9.6 in one case). This stock was used neat or diluted as required. In one case for MPA, a 4.6 times more concentrated solution was used. Table 1 gives a list of the Hg2+/ ligand solutions at the strengths used in the ion exchange experiments, and the same sample name convention is used throughout the rest of this paper.

proportional to the shift of the transition energy from the bulk band gap Eg for that particular alloy composition: Γi = 2δr(ℏωi − Eg )

where δr is a broadening factor related to the QD size distribution Δr = Rδr. Thus, only one adjustable independent peak width parameter, δr, is needed to specify all the component peak widths. This requires that the composition of the alloy QDs is first determined so that the bulk bandgap Eg of the matching alloy in eq 1 can be determined from the bowing curve for CdxHg1−xTe. The bulk bandgap of CdxHg1−xTe alloys can be calculated using the expression due to Nemirovsky and Finkman:31 Eg = −0.337 + 1.948x + 6.006 × 10−4T (1 − 1.89x) (2)

and the variation, showing the transition from semiconductor to semimetal, is given in Figure S2. The constrained peak fitting equation can then be written as, n

Abs(E) =

sample name

vol./ weight

Hg(ClO4)2 (g)

dilution (or X concentration) factor

1TG-pH10.8 1TG/5-pH10.8 1TG/10-pH10.8 1TG/50-pH10.8 1TG-pH9.6 MPA-pH10.8 MPAx4.6-pH10.8 MEA/5-pH10.8 MEA/10-pH10.8 DTP/5-pH10.8

0.57 0.57 0.57 0.57 0.57 0.61 2.81 0.44 0.44 0.50

50 μL 50 μL 50 μL 50 μL 50 μL 50 μL 230 μL 50 mg 50 mg 50 μL

0.050 0.050 0.050 0.050 0.050 0.050 0.230 0.050 0.050 0.050

1 5 10 50 1 1 X 4.6 5 10 5

⎛

(E − Ei)2 ⎞ ⎟⎟ ⎝ w(Ei − Eg ) ⎠

∑ aiexp⎜⎜− i=1

(3)

where Ei is the position of fitted peak i, ai is the corresponding peak amplitude, Eg is the bulk bandgap of the same composition alloy, and w is a single common width parameter for all peaks. Constraints were added for the peak positions, Ei, amplitudes, ai, and the width, w. The average cation stoichiometric ratio of the alloyed CdxHg1−xTe QDs can be determined from changes in the absorption at short wavelength, i.e., at much higher energies than the bandgap where confinement effects are small, excitonic effects are weak, and the Maxwell-Garnett effective medium theory may be applied.32,29,33,34 The absorption then has a linear dependence on the ratio of the Hg/Cd content, varying between the two values for pure HgTe and pure CdTe. Kamal et al.29 give the extinction coefficient for CdTe at 410 nm along with the spectral dependence in the short wavelength range. Lhuillier et al.35 give the corresponding absorption cross section for HgTe at 415 nm. Assuming no change in the cation to anion ratio and given that the lattice parameter for CdTe and HgTe is almost identical (to within 0.3%), a simple expression for the composition x (in CdxHg1−xTe) can be derived from the 415 nm absorptions of the pure CdTe QDs and the alloy QDs:

Table 1. Hg2+/Ligand Solution Strengths Used in Ion Exchange Experiments mmol ligand

(1)

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RESULTS A typical set of absorption spectra of CdxHg1−xTe QDs following the ion exchange process is shown in Figure 1a. As can be seen in Figure 1a, the red-shifting absorbance trend with increasing reaction residence time reflects the ion exchange between incoming Hg2+ and outgoing Cd2+ resulting in the narrow bandgap CdxHg1−xTe. The absorption curves were fitted to a set of Gaussian peaks (on an energy scale; Figure 1b) following a constrained peak fitting procedure given by Smith et al.22 The approximate peak positions of the component Gaussian peaks were determined by taking the fourth derivative of the energy spectra, and the fitting process allowed one to search within +50 meV of these estimated positions. The number of free parameters for the set of peaks was reduced by making use of the peak width relationship due to Klimov.30 The inhomogeneous width Γi of an excitonic peak due to size broadening is assumed to be

⎛ A QD − A CdTeQD ⎞ ⎟⎟ 1 − x = ⎜⎜ ⎝ 4.934A CdTeQD ⎠

(4)

where AQD is the measured absorption of the QD sample and ACdTe QD is the absorption of the pure CdTe QDs. This is similar to the expression given by Smith et al.22 but with a different denominator taking into account the slightly different comparison wavelengths in each case (here, for convenience, we use 415 nm rather than 410 nm). Figure 2a shows a typical set of composition trajectories for a number of different Hg2+ concentrations, using 1-TG as the exchange ligand in the microreactor capillary flow experiments. Similar curves for other experiments (different added ligands and Hg 2+ concentrations) are shown in Figure S3. Note however that at this stage there is no indication as to how the exchanged 2758


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Figure 1. (a) Typical absorption spectrum data sets for CdxHg1−xTe QDs from the microreactor mixing/capillary flow system, given for the various reaction residence times and compared to CdTe QDs. (b) Example of an absorption spectrum fitted to multiple Gaussian peaks following the procedure of Smith et al.22

Figure 2. Typical changes in composition of CdxHg1−xTe QDs during microreactor/flow ion exchange reactions. (a) Composition changes over time for various Hg2+/ligand solution concentrations (a stock solution of 0.57 mmol of ligand/50 mg of Hg(ClO4)2 in 50 mL of water was diluted 1× (1TG-pH10.8), 5× (1TG/5-pH10.8), 10× (1TG/10-pH10.8), and 50× (1TG/50-pH10.8) and mixed with the CdTe QD solution flow in equal volumes). (b) For comparison, the equivalent number of monolayers of Hg2+(for the same amounts of material as in (a)) that could be formed by exchange with Cd2+ working inward from the QD surface is shown.

seen. It should be noted that the QD concentration for the first (pure CdTe QDs) measurement is halved once the subsequent mixing process is started due to the 50/50 dilution in the Hg2+ salt/ligand stream. In the case shown, the recovery of the PL intensity is just starting to become apparent at longer reaction times. Higher concentrations of Hg2+/ligand solution lead to a much stronger and rapid drop in intensity, but the PL recovery may commence sooner. The PL spectra can be fitted to two or three Gaussian peaks (energy scaled), as shown in the examples given in Figure S5, which is convenient for calculations of (total) peak positions, spectral widths, and integrated intensities, particularly for weaker, noisier spectra obtained when using higher Hg2+ concentrations to achieve a more rapid shift in bandgap. Note: We do not make any interpretation of

Hg2+ is distributed within the QDs, i.e., whether it remains located at the surface, diffuses with a radial profile into the QDs, or is truly randomly distributed throughout the QD volume. However, with the knowledge of the QD diameter and the composition, it is possible to calculate the number of equivalent monolayers as if the Hg2+ ions had remained at or as close as possible to the surface as shown in Figure 2b. This will be discussed later when considering how the spatial distribution evolves during the exchange process. Figure S4 shows the rate at which the steady state PL changes during the flow reaction when pure CdTe QD solution is mixed with equal volumes of 1TG-pH10.8 Hg2+/1-TG solution. The initial PL is rapidly quenched, and at the same time, a continual and smooth shift to longer wavelengths is 2759


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Chemistry of Materials the fitted Gaussian components in terms of separate emission processes, rather the fitted forms are a convenient means to smooth and parametrize the curves for analysis of the overall (dominant) peak position and integrated areas. For the steady state PL measurements, a 405 nm laser was used to excite the flowing solution and, therefore by determining the relative integrated PL intensities of the starting CdTe QD material divided by the linear absorption at 405 nm from the absorption flow measurements, the relative PLQY of the latter can be determined. This has been compared with the corresponding ratio for the ion exchange mixing product in order to calculate the change in PLQY relative to the CdTe QD starting material against reaction times as shown in Figure 3 for a range of different Hg2+/1-TG concentrations.

spectral width is related to both QD size and compositional polydispersities and by the variation in PLQY across these combined distributions. The QDs in the wings of these distributions may not necessarily have the same PL QY as those near the center leading to a possible apparent narrowing of the ensemble integrated PL spectrum.36,37 As the cation exchange proceeds (shifts to lower energies), the effect of composition polydisperisty may be expected to become more significant, possibly accounting for the rise going to lower energies seen for the 1TG-pH10.8 data set. PL decay profiles were measured immediately following each steady state PL spectrum for each consecutive flow rate. The CdTe QD starting material shown in Figure 5 typically showed a decay mostly dominated by a 25−30 ns exponential term with a small contribution from a slower component. After the start of the ion exchange process, the overall decay rate was faster and more multiexponential in nature. CdTe PL transients could be fitted to biexponential decay functions while three decay terms were generally necessary for Hg2+ containing QDs. These multiexponential decay parameters were then used to derive average PL decay times.

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DISCUSSION While the main and most obvious appeal of ion exchanged QDs is their bandgap tunability combined with the ability to start with a defined shape, size, and size distribution template2 (in this case quasi-spherical CdTe QDs) and preserve those features after the exchange process, there are some interesting and practical details that remain to be explored. To investigate some of these points, we ran ion exchange measurements with a range of different water-soluble thiols (1-TG, MPA, MEA, and DTG) and at a range of pH and Hg 2+ salt/ligand concentrations, subject to the constraint that the QD colloid remains stable at least over the duration of the experiments (up to 80 min) and preferably longer term (several weeks thereafter). A very illuminating visualization of the different aspects of the ion exchange process can be made by calculating the radiative and (approximate or apparent) nonradiative recombination rates at different reaction times using the corresponding capillary flow absorption, PL, PL lifetime data, and the absolute PLQY of the CdTe starting material. The radiative recombination rate, kr, is given as,

Figure 3. Relative PLQY trajectories (normalized to the CdTe starting material) for a number of different Hg2+/1-TG solution concentrations during flow ion exchange. At the highest concentration, the initial drop in QY is the greatest but the recovery has already started during the 80 min time scale. In the other cases, the recovery occurs but on longer time scales than those shown here (as followed in the static mixing experiments).

Useful correlations can be observed in the steady state PL variation during the ion exchange process. Figure 4a shows the almost linear relationship between the PL peak energy of CdxHg1−xTe QDs and the peak position of the lowest energy fitted Gaussian peak derived from the absorption measurements. This would suggest a simple linear relationship between the (room temperature) Stokes shift, ΔES, and the QD bandgap in this regime. For the case of the 1-TG samples treated at pH 10.8, in terms of the PL peak energy, Epl, ⎛ 0.155Epl − 0.384 ⎞ ΔES = ⎜ ⎟ 0.845 ⎝ ⎠

kr =

QY τavg

(6)

and the apparent nonradiative rate, knr, as, k nr =

(5)

1 − kr τavg

(7)

Here, we neglect the fact that some of the distribution of QDs may in fact be dark or nonemissive on the time scale over which the TCSPC measurements are made (the inverse of the pulse repetition frequency) and, so, may effectively never be sampled in the PL measurements.38 This should not affect the calculation of the radiative rate, but the nonradiative rate will be overestimated.38 However, the variation in the apparent nonradiative rates that we report below spans several decades, and this is far greater than the error due to neglecting the dark fraction correction. It would not be feasible to determine the dark fraction experimentally39,40 in a parallel flow experiment, so here, we simply use the uncorrected apparent nonradiative

Figure 4b shows the PL fwhm/PL peak wavelength variation during the ion exchange process for a set of samples with the same ligand and solution pH but a range of concentrations. In percentage terms, the PL spectra are similar in normalized width to the starting material for the largest shift in PL position (bandgap), but interestingly, in the case shown, there is a slight narrowing of the relative spectral widths, irrespective of the pace with which the exchange process is carried out; i.e., the widths seem to sit on a universal curve, allowing for some experimental variation due to noisy PL spectra in the early stages for the highest Hg2+ concentration reaction. The PL 2760


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Figure 4. (a) Correlation between PL peak energy and the first (band edge) Gaussian peak position of CdxHg1−xTe QDs from fitted absorption spectra. Data from several different ion exchange experimental runs are combined and follow the same trend indicating a simple linear relationship between room temperature Stokes shift and QD bandgap energy (Epl = 0.384 + 0.845Eabs,G1, where Epl is the PL peak energy and Eabs,G1 is the energy of the first fitted Gaussian at the band edge of the absorption spectrum). (b) Influence of the ion exchange process on PL spectral widths. The plot shows the PL fwhm normalized by the PL peak wavelength. For limited Hg2+ uptake, a slight dip in the relative width is seen followed by a slow rising trend as the bandgap is further lowered, possibly indicating greater influence from compositional polydispersity as progressively more Hg2+ is absorbed.

Figure 5. (a) Typical drop in the measured PL lifetimes and increasingly multiexponential decay character of CdxHg1−xTe QDs during the ion exchange process (shown for solution 1TG/10-pH10.8). (b) The corresponding average PL lifetime behavior during a number of ion exchange runs at different Hg2+/1-TG concentrations.

the final measurement in the capillary was at 80 min reaction time, but an additional measurement was made on a separately made up stock of identical materials after 1 week (which was further red-shifted). Figure 6 displays a number of interesting features: the apparent nonradiative rate initially shows a marked Hg2+ concentration dependent increase in some cases well over 2 orders of magnitude, resulting in a dramatic drop in PLQY. In fact, the ratio of the maximum knr excursion to the initial knr value for the CdTe starting material shows a strong linear dependence on the concentration of the Hg2+/ligand solution

rate as a comparative measure of the reduction in the QY due to the combined effects of nonradiative recombination channels and nonparticipating QDs as a guide to how disruptive the exchange process is and how fast recovery proceeds. The evolution of the two recombination rates is shown in Figure 6 for a number of ion exchange runs using Hg(ClO4)2/ 1-TG as the Hg2+source at a range of concentrations (1:1 volumes of CdTe QDs mixed with Hg2+ stock solution diluted by 1×, 5×, 10×, and 50×). Rather than reaction time, the results are shown as a function of the PL peak position, with the latter shifting to the red as the reaction proceeded. In each case, 2761


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Recovery of the radiative rate toward the alloy-trend line may then indicate a softening of the Hg2+ concentration profile and an internal redistribution of cations. The mechanisms for such a possible cation indiffusion process will be discussed later. We note that the divergence of the kr values from the alloy trend line is (mostly) Hg2+ concentration dependent, and in the case of the lowest Hg2+ concentration, the final value lies quite close to the calculated alloy value. The data (Figure 6) for the 1-TG solutions (at pH 10.8) appear to show two choices: the exchange process can be driven fast by using high Hg2+ concentrations and obtaining a large bandgap shift but possibly in the process forming a core− shell or radial Hg2+ composition gradient, which may gradually flatten out over time, or may persist to some extent, or the exchange may be carried out more gradually using lower Hg2+ concentrations with the cation distribution staying closer to that of an alloy. Finally, for Figure 6, we note one off-trend anomaly. For the final measurement (at 1 week) of the 1-TG/ 10-pH10.8 Hg2+solution, the PL peak energy value appears too high (red shift slower than expected in comparison with the other measurements). The final nonradiative rate is higher and the corresponding radiative rate lower than would be expected by comparison with the other three sets of data in this group. While the capillary measurements for the rest of this Hg2+ concentration data set do not appear to have been adversely affected, it appears that the separate solution stored in the sample vial may be affected during storage. The reason for the apparent retardation of the exchange process for this final point measurement can be traced to the apparent onset of QD aggregation as highlighted under the light scattering measurements discussed later. pH Dependence. The same type of analysis was applied for ion exchange with 1-TG at a slightly lower pH (9.6) to see if the degree of dissociation of the thiol group to thiolate ion and therefore the binding strength of the ligand to the free Hg2+ had any significant effect on the exchange. The results are compared with the previous measurement at the same Hg2+/ ligand concentration at pH 10.8 in Figure 7a. The initial impact on the nonradiative rate is lowered slightly while the radiative rate drops rapidly and to a greater extent. However, differences between the two sets of data are less at longer exchange times, and at the longest times, the radiative rate for the lower pH case is slightly better than for the higher pH case. Once more however, the radiative rate is still lower than would be expected for a simple uniformly distributed alloy. Figure 7a suggests that reducing the ligand/Hg2+ binding strength in this way may accelerate the exchange process a little, and though the initial disturbance to the radiative rate is greater, it may start to recover sooner. The limitation with this approach is that dropping the pH of the exchange solution may reduce the colloid stability in the longer term, and dropping it below pH 9 will certainly lead to instability even in the short term. Choice of Ligands. We recall that, when the ligand used to solvate the Hg2+ salt at high pH differs from the one used to stabilize the starting CdTe QDs, both cation and ligand exchange will occur. If a high concentration of a more weakly binding ligand is introduced, this may reduce the stability of the colloid depending on the extent and rate of the ligand exchange. Conversely, displacement in favor of a strongly binding ligand at the QD surface may retard the exchange rate. Figure 7b compares the exchange processes using Hg2+/MPA solutions at two different salt/ligand concentrations. MPA is a

Figure 6. Radiative (filled symbols and solid lines) and nonradiative (open symbols and dashed lines) recombination rates of CdxHg1−xTe QDs for a range of Hg2+ salt/1-TG ion exchange runs derived from microreactor/capillary absorption and PL measurements. The gray shaded box shows the range of the final nonradiative recombination rates, while the horizontal dotted line shows the typical starting value (for pure CdTe QDs). The dark red line is the extrapolated radiative rate calculated according to Fermi’s Golden Rule (eq 8) as if there were no changes in the overlap of electron and hole wave functions during the exchange, i.e., just assuming that the local field factor and QD refractive index affected the radiative rate.

used in the exchange reaction, as shown in Figure S6. This is further aggravated by a drop in the radiative recombination rate which after a time levels out and for the lower Hg2+concentration runs starts to show a recovery. For the lowest Hg2+ concentration, the radiative rate eventually exceeds the nonradiative rate and so the PLQY climbs back to just over 50%. In the other three cases, although the gap between the two narrows over time, the nonradiative rate always exceeds the radiative rate. The range of final nonradiative rates is marked by the gray box in Figure 6 while the starting value (in pure CdTe QDs) is shown by a horizontal dotted gray line, showing the extent of the shift in the nonradiative rates. The trend in the radiative rate, kr, is compared with what might be expected from Fermi’s Golden Rule (eq 8) if only the QD refractive index changed and the local field factor and transition frequency were similarly affected due to the change in cation ratio. The calculation also assumes a homogeneous alloy is formed (i.e., no segregation of electron and hole wave functions in a core− shell structure). The dark red line follows that trend starting from the measured radiative rate value for pure CdTe QDs. This assumes that the transition moment, |⟨f |p|i⟩|2, remained constant as might (approximately) be the case for the formation of a homogeneously distributed alloy. Changes in the QD permittivity and effective mass with composition may slightly change the electron−hole wave function overlap, but this would not be expected to make a substantial change. The fact that the radiative rates are always lower than this predicted level suggests that the magnitude of the transition moment and therefore the oscillator strength do in fact drop at least initially. This may indicate some degree of segregation of the two carriers in a (quasi-) type II or similar (e.g., gradient) heterostructure, caused by a Hg2+ rich surface region. ⎛ 3n 2 ⎞2 k r ∝ ⎜ 2 s 2 ⎟ ωns |⟨f |p|i⟩|2 ⎝ n1 + 2ns ⎠

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Figure 7. Radiative (filled symbols and solid lines) and nonradiative (open symbols and dashed lines) recombination rate trends for: (a) 1-TG at pH 10.8 (sample 1TG-pH10.8) and pH 9.6 (sample 1TG-pH9.6); (b) MEA at pH 10.8 (MEA/5-pH10.8 and MEA/10-pH10.8); (c) MPA at pH 10.8 (MPA-pH10.8 and MPAx4.6-pH10.8); (d) cross-comparison for different ligands (1TG/5-pH10.8, DTP/5-pH10.8, MEA/5-pH10.8, MPA-pH10.8).

concentration) is shown in Figure 7d. MEA, 1-TG, and MPA nonradiative rate behaviors are very similar, while the radiative rates show more of a spread, with 1-TG narrowing the gap between radiative and nonradiative processes the most. In contrast, the nonradiative rate curve for DTP, although similar to the others initially, remains at a high level thereafter and, therefore, leads to the poorest performance in terms of PLQY recovery. Oscillator Strengths and Cation Distribution. Further insight into the evolution of the cation distribution can be extracted from the absorption spectra measured in the microreactor/capillary arrangement. Again, we would like to establish if a core−shell, radial gradient, or uniformly distributed alloy is formed initially and if this changes after the initial uptake of Hg2+. As we mentioned earlier, and as shown in Figure 2b, the maximum amount of Hg2+ taken up in the present experiments equates to just under one monolayer equivalent if it were all to reside on the outermost lattice positions of the QDs.

thio-acid closely related to the TGA used to stabilize the starting CdTe QDs, so in this case, the aim was to see if using more of this similar ligand on the QDs and to solubilize the salt had any significant effect. Here again, the exchange process could be accelerated, in this case at higher concentrations, but the increased Hg2+ concentration pushed the nonradiative rate higher until well into the recovery phase. After 2 weeks, the high Hg2+ concentration solution recovers to a point where the radiative and nonradiative rates are almost equal (i.e., the PLQY is around 50%). In addition, at that point, the radiative rate is only a little below that expected for an alloy composition. Figure 7c shows the effect of using MEA as the Hg2+ solvating ligand. Adding the salt/ligand at the stock solution concentration destabilized the QDs within a few minutes, so the highest concentration that could be used was 5-fold diluted (MEA/5-pH10.8). A cross-comparison across all the ligands used (1-TG, MPA, MEA, and DTP) at pH 10.8 and Hg2+/ ligand stock concentration diluted 5-fold (except for the MPA exchange run which was made at the full (1×) stock 2763


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Chemistry of Materials On the basis of EMA calculations, Smith et al.22 show that for alloy QDs the relative oscillator strengths f1 rel and f 2 rel where,

firel =

Energy Levels and Structure. The possible transitions from the initial CdTe QD energy level structure to that for fully exchanged HgTe QDs are shown in Figure 9. In reality, it is

fiqd fCdTe

(9)

should be very similar, close to unity with f1 rel slightly higher and rising slowly with increasing Hg2+ content. On the other hand, for a core/shell structure (Hg2+ rich at the surface), f 2 rel should be larger than f1 rel and the two values diverge much more rapidly with Hg2+ content than for the core−shell case. Comparing these trends with the experimental data derived from the capillary absorption measurements (Figure 8), we see

Figure 9. Core/shell (top) and alloy (bottom) band structures during CdTe-to-HgTe alloying, assuming no change in particle diameter, i.e., topotactic exchange. Changing the composition may also alter the valence band offset relative to the HOMO level of the ligands, potentially changing the carrier trapping rate. Light gray boxes show the extent of the bulk conduction bands (for HgTe, CdTe, and alloys), and dark gray boxes show the corresponding bulk valence band positions. Red lines in the light gray boxes show the QD lowest conduction bands, and the corresponding red lines in the dark gray boxes are the highest QD valence bands. The bottom of the bulk conduction band for HgTe lies 0.13 eV within the top of the bulk valence band so the bottom of the bulk valence band is shown as a dashed line. The valence band offset between CdTe and HgTe is 0.4 eV.

Figure 8. Relative first (filled symbols) and second (open symbols) oscillator strengths (vs corresponding quantities for pure CdTe QDs according to eq 8) for 1-TG/pH 10.8 ion exchange run at several Hg2+/ligand concentrations.

two cases where f1 rel > f 2 rel and where they both remain fairly close together (1TG/5-pH10.8, 1TG/50-pH10.8), which is more consistent with alloy formation. For the highest Hg2+ salt/ligand case (1TG-pH10.8), f1 rel < f 2 rel and the difference becomes much greater as the Hg2+ content increases, consistent with the establishment of a core/shell like structure on the time scale covered. For sample (1TG/10-pH10.8), the relative oscillator strengths are again similar but their order, f 2 rel > f1 rel is not fully consistent with the alloy model. Overall, the oscillator strength trends support the radiative recombination rate trends described earlier for the 1-TG data set. Low Hg2+ concentrations, although shifting the bandgap more slowly, can maintain an alloy composition throughout the process, whereas the use of higher Hg2+ concentrations may shift Eg more rapidly, but may initially lead to a Hg2+ rich surface or even a core/shell structure. It is of interest to compare the relative oscillator strength predictions from EMA theory with absolute values ( fosc) derived from other sources. Kamal et al.29 give values for fosc for a range of different CdTe QD diameters, from which we can determine that for the 4.0 nm QDs in this study a fosc = 10.5 is predicted. For the same diameter HgTe QDs, the 2 band k · p model of Lhuillier et al.35 leads to a value of fosc = 4.2. On this basis, the relative value of the fosc for pure 4.0 nm diameter HgTe QDs should be 0.4 which may be somewhat lower than that expected from the EMA model for this composition.

most likely that the core−shell track is followed in the early stages of exchange and later a gradual shift to the alloy track occurs but with the gradation of this switch being dictated by factors such as the Hg2+ concentration, pH, and Hg2+/ligand binding strength, in other words, the rate at which Hg2+ is driven into the QD structure. The core/shell structure, with the inverted band gap of HgTe in the shell, will lead to localization of the electrons in that region.22 In contrast to many other core/shell structures, strain is not a very significant factor in CdTe/HgTe heterostructures. Both CdTe and HgTe are relatively “soft” with bulk moduli of 445 and 423 kBar, respectively.41 Both valence band and conduction band deformation potentials are relatively low, and since the lattice constants of the zincblende forms of each material are almost identical (0.6482 and 0.6460 nm), almost no strain arises. This allows the bulk bandgaps to be used without modification as a reasonable approximation for energy level calculations. As already mentioned, localization of one of the carriers (here electrons) in the shell will reduce the overlap of the lowest lying electron and hole wave functions, therefore reducing the transition moment and oscillator strength. This reduces the radiative recombination rate, which once other 2764


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there is relatively little data in the literature on ligand/QD band offsets. Kim et al.44 give a band offset for 1-TG to 3 nm HgTe QDs (−0.33 eV in their case), but for other QD alloy compositions, relatively little is known. In the case of core/shell structures, the confinement of holes to the CdTe core may be expected to reduce the hole trapping rate at thiol sites. This may improve the knr, but conversely, a core/shell structure would reduce kr, due to a reduced electron-hole wave function overlap; so, the net benefit from this point of view may not be so worthwhile. A further type of hole trapping mechanism in CdTe QDs has been discussed by Califano.45 Atomistic modeling studies have identified Auger mediated hole transfer to surface Te defects as an alternative mechanism by which holes may become trapped on the QD surface. Here, photogenerated electrons and holes may interact via fast carrier−carrier scattering processes resulting in re-excitation of the hole above its initial energy to a level that may then allow it to transfer to higher lying surface hole trap states, possibly ones not accessible by a simple direct mechanism from its previous lower energy state. The nonradiative recombination rate may therefore contain some contributions from trap states not actually located in the gap but lying just outside the QD VB. However, compositional and structural shifts in the QD VB during the exchange process will still have a bearing on transition rates into such trap states. Ion Exchange Mechanism. On the basis of experimental data in Figure 2a, we derived a value for an effective rate constant and the reaction order with respect to the Hg2+ concentration based on a simple reaction mechanism, as outlined in the Supporting Information. The former is in fact not a constant, and the value falls exponentially as the reaction proceeds. The latter has significant scatter during the reaction with an average value of 0.74. The variation in the effective rate constant suggests that a more complex multistep reaction mechanism is more appropriate and warrants more detailed investigation. The fact that the effective rate constant falls by over 100-fold during the first 80 min of reaction suggests that much slower processes, such as cation indiffusion within the QDs, dominate the exchange process at longer reaction times. In the present work, we have shown that, although the maximum amount of Hg2+exchanged for Cd2+ at most only slightly exceeds that required to form a single surface monolayer, there is evidence that, when driven in rapidly (with high Hg2+ salt/ligand concentrations), the Hg2+ initially accumulates at the surface and then gradually starts to disperse within the body of the QD. More gradual addition of Hg2+ shows characteristics of an alloy distribution of cations from the outset. Previous studies on CdHgTe materials7 (in that case HgTe cores with Cd2+ introduced) showed radial Hg2+ concentration gradients rather than abrupt core/shell interfaces. These factors suggest that there may be distinct phases to the ion exchange process with different kinetics, i.e., a fast surface exchange process governed by solution and surface chemistry, followed by slow cation indiffusion controlled by the energetics and mobilities of internal defect sites. In Figure 7a−d, we observed a rapid increase in the apparent nonradiative recombination rate which peaked and then more gradually declined, continuing to do so over several weeks after the start of the exchange reaction. The extent (maximum excursion) of this peak in knr scales almost linearly with the concentration of the Hg2+ solution, as shown in Figure S3. In terms of the ratio of incoming Hg2+ ions to potential surface cation sites, for the 1TG-pH10.8 series of exchange reactions,

contributions such as the changes in transition frequency and local field factor have been taken into consideration, can be taken as an indication of localization of one of the carriers. This was clearly observed for many of our samples, with the extent of the carrier segregation dependent on the rate at which Hg2+ was driven into the QDs. In most cases, the extent of the segregation softened with exchange reaction time, pointing to a slow carrier indiffusion process after the initial Hg2+ uptake at the surface. Bowing curves (Eg vs composition, x) are very useful when designing alloy materials. These are well-known for many bulk II−VI compounds but not necessarily so well established for the corresponding QD materials. Figure S8 shows the relationship between the bandgap energy (from the fitted absorption spectra) of CdxHg1−xTe QDs vs the composition x determined from the short wavelength absorption. Protière and Reiss42 have investigated the CdxZn1−xSe QD alloy system in detail and found that the optical bowing in their QDs matched that of the bulk material with a uniform but size dependent energy shift. They also make the comment that bowing is generally less acute in common anion systems than in common cation alloys. On this basis, one might expect a linear bowing curve for CdxHg1−xTe QDs, shifted relative to the bulk curve. The latter is also seen in Figure S8 and, within the limited range of compositions, appears to be in fairly good agreement with the Protière and Reiss hypothesis. The dashed line in Figure S8 joins the bandgap energies of CdTe QDs measured experimentally and that for pure HgTe QDs calculated from Lhuillier et al.’s35 sizing relationship, using Kamal et al.’s29 data to determine the CdTe QD diameter. (Apparent) Nonradiative Recombination Rate after Exchange. In Figures 6 and 7a−d, the apparent nonradiative rate undergoes a large excursion before returning to levels typically a little below 0.01 ns−1. This is a little higher than for the CdTe QD starting material (typically knr is around 0.01 to 0.03 ns−1). Here again, we emphasize that we can only derive an apparent nonradiative recombination rate as simple PLQY and lifetime measurements do not allow a full correction for any fraction of dark QDs that never emit light.38 It is worth considering what the eventual nonradiative rate might be, i.e., after all the surface disruption during the exchange process, since the value relative to the radiative rate will dictate the PLQY of the reaction products. Of course, the first point to establish is: has the surface recovered as much as possible? In this work, we have simply made measurements 1 or 3 weeks after the start of the exchange, which was taken as a compromise between allowing the surface to fully recover and not allowing potential colloid instability to further complicate the interpretation. There have been many studies of the surface trap mechanisms in II−VI QDs and CdTe in particular. Wuister et al.43 identified the location of the thiol ligand valence band (VB) relative to the VB of the CdTe QDs as a controlling factor for hole trapping. Several of the commonly used thiols are known to have HOMO levels close to the VB of CdTe QDs, some lying slightly above and some just below. Thus, a direct hole transfer to a surface thiol could be turned on or off simply by a compositional shift of the QD VB. The trapping rate would likewise be dependent on the offset between the thiol HOMO level and the CdTe VB. In a similar manner, exchange of the surface thiol during the exchange reaction may also shift the ligand/CdTe HOMO/VB offset, again modifying the hole trapping rate. While this is clearly an important factor, 2765


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little remains known about the effect of nanocrystal size upon the indiffusion process. Indeed, further studies such as these, where different types of starting cation profile may be established and then followed, may be useful to explore such size related modifications. Bringing all of these features together, bandgap tuning, heterostructure control, high PLQY, colloid stability, and size control, and establishing how to manipulate them are key aspects of being able to make an impact in further solar cell, photodetector, and bioimaging applications of CdxHg1−xTe and similar QDs.

the excess was: 1TG-pH10.8, 4:1:1; TG/5-pH10.8, 1:1:1; TG/ 10-pH10.8, 0.4:1:1; 1TG/50-pH10.8, 0.1:1:1 (see Table S1). The initial peaking of knr rate may then be a measure of the surface disruption and an indication of the progress of the uptake of Hg2+ at the surface mediated mostly by solution kinetics and relative lattice stabilities for the surface layer of the QD (ionic diffusion to the surface, relative ligand binding kinetics, and lattice bond enthalpies). This is evidently the dominant process on the time scale of up to 1−2 h. A similar fast initial surface exchange dominated phase followed by a much slower internal indiffusion limited re-equilibration of the cation distribution has been reported by Bothe et al.46 in their studies of Cd2+ doping of PbSe and ZnSe QDs using similar or slightly higher dopant to incumbent cation ratios.

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ASSOCIATED CONTENT

S Supporting Information *

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The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.chemmater.6b04544. Microflow reactor and measurement schematic; bulk bandgap versus alloy composition curve; average composition vs reaction time curves; typical PL variation with reaction time; examples of fitted PL spectra; variation of the maximum knr normalized to the initial CdTe knr value versus relative Hg2+ solution strength for 1TG ligand solutions; hydrodynamic size distributions determined by dynamic light scattering for solutions after exchange reactions; bandgap versus composition for QDs treated with 1TG based solutions; table of Hg2+ solution cation to QD surface Cd2+ site ratios for 1TG based solutions; discussion of simple kinetic models (PDF)

CONCLUSIONS Our studies have shown that the kinetics of room temperature cation exchange in colloidal CdxHg(1−x)Te QDs may span the subsecond to days, weeks, and even longer time scales. Factors that influence the exchange rate when carried out in an aqueous medium include the relative hardness of the cations and ligands involved, along with the pH and the concentrations of the dots and the incoming cations. Although the exchange process is largely topotaxial, it may give rise to defects, which may be transient in nature and eventually heal or which may become fixed, leading to a far poorer photoluminescence quantum yield (PLQY) than the starting material or even quantum dot alloys grown directly by mixed cation synthesis. The impact on the PLQY was clearly profiled as initial disruption gave way to a recovery in performance that could clearly be optimized through a judicious choice of exchange rate. It was observed that slow (low Hg2+ concentration) reactions are consistent with a CdxHg1−xTe alloy structure from the outset, while more rapid reactions may initially yield Hg2+ rich surface layers which may subsequently indiffuse to form a more gradual transition from the Hg2+ rich surface layer to the CdTe core. Fitting of absorption spectra and analysis of the radiative recombination rate give independent information on the oscillator strengths of the band edge transitions which are strongly sensitive to the distribution of the cations and which allow discrimination of core−shell transitions from alloy or concentration gradient transition. PL measurements also allow the nonradiative rate to be determined, and the latter shows large variations during the exchange process in many cases, highlighting the surface disruption due to changes in the ligand binding and cation fluxes across the QD surface boundary. The effective equilibrium values of the radiative and nonradiative rates after one or more weeks establishes the eventual PLQY of the treated and red-shifted CdxHg1−xTe QDs. The investigation of the evolution of these rates and the dependence on the reaction conditions (reagent concentrations, pH, choice of ligand) allows for an optimum strategy for exchange (degree of red shift, maximization of PLQY, and selection of final cation distribution type) to be established. The establishment of the evolution of the radiative and nonradiative rates as presented in Figure 7 allowed the recovery of the PLQY in CdxHg1−xTe QDs to be quantified and the necessary time scales to be determined for each set of reaction conditions. The reaction kinetics shows clear indication of a fast surface exchange process followed by a slower internal rearrangement of the cation distribution. While the classification and energetics of defects responsible for cation indiffusion in bulk CdxHg1−xTe have been extensively explored,

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Stephen V. Kershaw: 0000-0003-0408-4902 Andrey L. Rogach: 0000-0002-8263-8141 Present Address #

Y.Z.: Key Laboratory of Optoelectronic Devices and Systems of Ministry of Education and Guangdong Province, College of Optoelectronic Engineering, Shenzhen University, Shenzhen, 518060, China. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The work is supported by the Research Grants Council of Hong Kong S.A.R. through the General Research Funds (Projects 104812, CityU 11302114, and CityU 11302714) and by the Project 9610350 of City University of Hong Kong.

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REFERENCES

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