Article pubs.acs.org/cm
Room Temperature Synthesis of HgTe Quantum Dots in an Aprotic Solvent Realizing High Photoluminescence Quantum Yields in the Infrared Nema M. Abdelazim,† Qiang Zhu,‡ Yuan Xiong,† Ye Zhu,§ Mengyu Chen,‡ Ni Zhao,‡ Stephen V. Kershaw,*,† and Andrey L. Rogach† †
Department of Physics and Materials Science and Centre for Functional Photonics (CFP), City University of Hong Kong, Kowloon, Hong Kong S.A.R. ‡ Department of Electronic Engineering, The Chinese University of Hong Kong, Shatin, New Territories, Hong Kong S.A.R. § Department of Applied Physics, The Hong Kong Polytechnic University, Kowloon, Hong Kong S.A.R. S Supporting Information *
ABSTRACT: A computer controlled, automated synthesis method has been used to grow HgTe quantum dots (QDs) entirely at room temperature, using an aprotic solvent, dimethyl sulfoxide. The growth is carried out with small iterative additions of the Te precursor, which allows frequent sampling of the products to assess the growth trajectory in terms of the relationship between the QD concentration and QD diameters as the reaction proceeds. As such, this approach is a useful tool to develop a detailed understanding of the growth process and to work toward optimizing the reaction conditions in terms of the quality of the resulting QDs. HgTe QDs with emission spectra ranging up to 3000 nm and with photoluminescence quantum yields of up to 17% at 2070 nm have been produced by this method. Although coupling of the exciton to ligand vibrations is inevitable in this energy range, attention to the growth conditions and QD quality can influence the detailed coupling mechanisms, with fewer carrier traps reducing the extent of polaron mediated coupling. The influence of reaction conditions such as ligand-to-cation ratios and rate of Te precursor addition upon the onset of QD aggregation has been also examined. The method is readily up-scalable and has been employed to produce HgTe QD materials for infrared photodetectors.
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INTRODUCTION
synthesis may have to offer in terms of improved IR photodetector performance.15−17 In the past few decades, much effort was put into the development of synthetic methods to produce nanoparticles of different sizes and shapes;18−21 however, only a few studies22,23 deal with particle formation mechanisms in the IR and the range of synthetic approaches and the level of understanding has not reached the same level of sophistication as that for other QDs (e.g., refs 24 and 25). Perhaps one reason for the slower development of synthetic knowledge has been the absence of detailed information in particular on the particles’ size and concentration during the growth process to allow a picture of the growth trajectories to be derived. Until now, various approaches have been developed for the synthesis of HgTe QDs including colloidal synthesis in organic5,26 or aqueous media.5,27 Among the synthetic routes for the production of HgTe QDs, colloidal growth in organic solvents based on “hot injection” has been the most successful
The optical and electronic properties of infrared (IR) emitting colloidal quantum dots (QDs) such as mercury and lead chalcogenides can be tuned via particle growth to the optimum spectral region for a variety of applications.1 In the visible and UV spectral range, organic dye molecules can have near 100% photoluminescence quantum yield (PLQY); however, in the IR range, QDs offer significant advantages over dyes and are by comparison highly photostable.2,3 Inorganic QDs, composed of semiconductor materials, have a higher absorption coefficient than conventional dyes,4 band-gap tunability by size control,5 multiple exciton generation may be exhibited,6 and for photodetector applications, they can offer efficient charge separation and good stability.7,8 Among the various IR QDs, mercury telluride (HgTe) seems to be an ideal material because it is a semimetal in the bulk with zero band gap, and so quantum confinement in principle allows tuning through the full range of IR wavelengths.9,10 This attractive property has seen HgTe investigated in detail by several groups, most notably that of Guyot Sionnest,11−14 while we have recently focused on the benefits that our low temperature DMSO-based © 2017 American Chemical Society
Received: June 24, 2017 Revised: August 29, 2017 Published: August 30, 2017 7859
DOI: 10.1021/acs.chemmater.7b02637 Chem. Mater. 2017, 29, 7859−7867
Article
Chemistry of Materials and effective approach for producing larger HgTe QDs9,11,26 with band gaps extending out to several μm. However, the reaction requires a rapid homogeneous reaction which is hard to achieve in large volume reaction vessels while the longer wavelength product also suffers from low PLQY.28 This also brings an inherent complication and difficulties in consistent synthesis and the production of good quality materials.29,30 Aqueously grown HgTe QDs tend to aggregate as the particle size increases, and this hinders their further growth when the average diameter of QDs exceeds 4 nm.27 For such small QDs, Ostwald ripening driven by heating at around 70−80 °C has to be performed to extend the QD diameters and push the QD emission further to longer wavelengths.10 HgTe QDs have been successfully used in IR photodetectors,12,13,15 and work continues to improve on their performance, addressing both materials14,16 and device structure17 related topics. For photodetection, both radiative and nonradiative recombination are unwanted competing channels which alike remove photogenerated carriers. Ideally both processes should be slower than carrier dissociation and extraction. However, with this caveat, materials with higher PLQY in solution are still desirable, as this at least ensures that the nonradiative recombination rate is not excessively high compared with the radiative rate which is to a large extent determined by QD size for a given material (considering coreonly QDs and leaving aside the use of heterostructures). Thus, high PLQY in solution, coupled with a slow net recombination rate, is best for photodetection. For emissive applications (i.e., QD LEDs) of course, the competition between radiative and nonradiative recombination is of paramount importance, and ideally, the former should be as fast as possible. Fermi’s Golden Rule, however, dictates that radiative recombination runs more slowly with increasing emission wavelength, and this is true for all materials, whether bulk or nanocrystalline. In addition, although bulk semiconductor LEDs emitting in the >2 μm range are available, their wavelength range is typically limited to a few hundred nm by the choice of material and composition (alloys). The band gap of HgTe QDs has already been extended out to several μm (though the PLQY of these coreonly QDs remains very low at long wavelengths still). There is still plenty of room for QD fluorophores to make an impact in the IR range above 2 μm where all materials struggle to emit more efficiently or brightly than thermal sources for example. Due to the limitations of the organic solvent, hot-injection approach and the aqueous method, a room temperature, aprotic solvent, gas-injection synthesis method has been developed here which in some ways combines the benefits of both forementioned methods. We systematically investigated the nucleation and growth of HgTe QDs synthesized in an aprotic solvent dimethyl sulfoxide (DMSO), with sizes of 3 nm up to 5 nm using a stepwise approach. While the aprotic solvent, DMSO, can support the solvation of ionic and many organic materials, it is less able to support as widespread hydrogen bonding as water. The monofunctional ligand selected, 2furanmethanethiol (FMT), likewise is less able to support strong interactions other than via the thiol group. These choices reduce the tendency of the QDs to aggregate at later stages in the growth. FMT was chosen because it is a short ligand and can be readily dissolved and is stable in DMSO on reaction time scales. For prolonged growth, repeated iterations of electrochemically generated H2Te gas injection (bubbled into the flask in a stream of Ar gas; see the Supporting Information, SI 1), each followed by typically 20−30 min
waiting time, were applied. With this gas multiple step-injection method, the PL peak can be extended into the 2−3 μm range or even longer while maintaining relatively high PLQYs (e.g., up to 17% at 2070 nm in this work), which are comparable to or even better than those of well-developed Hg chalcogenide nanocrystals28 in this size range. Besides the high PLQY, another advantage of this synthesis method is that it can generate a large quantity of solution with high QD concentrations. We typically use a 300−500 mL batch size, and larger volumes can easily be produced with the same method. As the synthesis is relatively slow, it allows the QD size and concentration to be monitored during the reaction, along with identification of any QD aggregates that may tend to form in the later stages of the growth. We adopted an iterative synthesis method, adding H2Te precursor in small stages, followed by a waiting period to allow consumption of the reactant before measurements. In this manner, we gained insight into the particle formation mechanism in order to understand the influence of the reaction parameters on the final QD products and their optical properties. The process in its present form is slow enough to allow for meaningful feedback from the optical characterization; i.e., H2Te and Hg2+ and ligand concentrations could be adjusted to “steer” the synthesis to some degree, though, in the work presented here, we have not taken advantage of this possibility. The growth could be conducted at a faster rate (by reducing or eliminating the waiting periods), but here, we aimed to characterize aliquots of the reaction mixture as the synthesis developed. We have chosen to conduct the growth at room temperature to demonstrate that high temperatures may not necessarily be a prerequisite to growing large sized good quality HgTe QDs. Since nanocrystal reaction kinetics are sensitive to the chemical behavior of ligands coordinated to the surfaces and to precursor concentrations,24 we first examined the effect of FMT:Hg2+ ratios on QD concentration, QD core size, QD hydrodynamic diameters (with regard to clustering onset), PL spectra, and PLQY. Each of these properties was determined for aliquots removed from the reaction flask after each iteration of the multistep growth process. The results allow a selection of the optimum ligand:cation ratio for a given outcome, i.e., best PLQY, fastest particle growth, rapid increase in QD concentration, and degree of QD aggregation. In a second set of syntheses, we examined the effect of the H2Te dose rate upon the growth trajectory.
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EXPERIMENTAL SECTION
Chemicals. Mercury acetate (Accuchem, >98%), furanmethanethiol (FMT, Sigma-Aldrich, 98%), toluene (Sigma-Aldrich, >99.7%), methanol (Sigma-Aldrich >99.8%), tetrachloroethylene (TCE, Fisher, >99.9%), dodecanethiol (DDT, Accuchem, >98%), formamide (BDH, >99.5%), and dimethyl sulfoxide (DMSO, Fluka, >99%) were all used as received from the suppliers. The mercury salt is toxic by ingestion, and the materials should be handled using normal lab precautions (gloves, lab coat, safety glasses). While very little H2Te is generated at any time during this procedure and should not be vented outside the reaction flask, it is nonetheless advisible to carry out these types of syntheses in an extracted fume hood. Characterization. The sequence of measurements made on each aliquot during each synthesis run is shown schematically in SI 2 (Figure S2). Absorption spectra were recorded on a Shimadzu UV 3600 spectrometer in the range 380−3000 nm. The short wavelength absorption cross section at 415 nm31 was used to determine the progress of the reaction in terms of the synthetic yield of QDs for
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DOI: 10.1021/acs.chemmater.7b02637 Chem. Mater. 2017, 29, 7859−7867
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Chemistry of Materials aliquots taken at various time intervals. At such short wavelengths, the reaction solvent, DMSO, is sufficiently transparent for measurements. For the longer wavelength range up to 3 μm (i.e., for spectra up to and including the band-gap region), TCE was used as the solvent and as the absorption reference material. TCE is the most preferable solvent for optical measurements in that range but necessitates a solvent and ligand transfer step to move the QDs from DMSO/FMT into TCE/ DDT (see SI 2, Figure S3). PL and PLQY measurements were made on an Edinburgh Instruments FLS920P spectrometer equipped with an integrating sphere. HgTe QDs emitting in the near-infrared (up to 1.6 μm) were measured with an InGaAs photon counting photomultiplier tube on one port cooled by liquid nitrogen vapor (−80 °C). For longer wavelengths (>1.2 μm), a liquid nitrogen cooled InSb photodiode was used. The excitation source was an 880 nm steady state laser which was mechanically chopped for the InSb photodiode based measurements. The absolute PLQY was determined for samples emitting at short wavelengths, from 1.2 to 1.6 μm, using the integrating sphere with subthreshold excitation from the 880 nm laser. For longer wavelengths, the relative integrated PL intensity normalized by the absorbance at the same excitation wavelength of 880 nm was determined in order to estimate the PLQY. By overlapping the relative PLQY measurements with the absolute PLQY measurements on the shorter wavelength samples, the scaling of the relative PLQY measurements on the longer wavelength samples could be performed. The optical density of all of the samples was adjusted to be as similar as possible, with the actual absorption values used to fine-tune the normalizations. The hydrodynamic size of the QDs and any onset of particle clustering in the colloids were measured by dynamic light scattering (DLS) using a Malvern Instruments ZetaNanosizer machine. Before DLS measurements, the solutions were passed through a 20 nm pore size filter. Samples were loaded into a quartz cuvette, and five series of measurements were performed, and the mean result taken. The average size, the size distribution, and the shape of HgTe QDs were also evaluated by high-resolution transmission electron microscopy. The images were measured on a Jeol JEM-2100F TEM/STEM electron microscope operated at 200 kV. The samples were prepared by diluting QD/TCE solutions and filtering with 0.2 μm and then 20 nm pore size hydrophobic PTFE filters, placing roughly 50 μL on a carbon-coated Cu grid and then standing to dry over several minutes. Synthesis. The aprotic solvent gas-injection synthesis of HgTe QDs is a modified version of the aqueous synthetic method previously reported.5,32,33 By choosing aprotic DMSO as the reaction solvent to replace water, we can minimize the tendency of QDs to aggregate and still obtain larger sized particles that remain freely dispersed. An electrochemical cell was used to provide H2Te gas buffered in a stream or Ar as a precursor. The electrolysis current was supplied by a power supply unit (PSU) operating in current control mode. The Hg2+ source, Hg2+(CH3COO−)2, was dissolved in the DMSO solvent, along with the FMT ligand prior to starting the reaction, and the solution was deaerated for 1 h before adding the first portion of H2Te. For longer duration syntheses, the addition of further Hg2+ salt solution in DMSO was provided for by the use of a syringe pump and catheter to introduce it into the reaction flask. The PSU and syringe pump were controlled by a LabView program. The reaction was carried out under argon in a 500 mL three-neck reaction flask with a magnetic stirrer at room temperature. The H2Te electrolysis cell was calibrated gravimetrically before use to allow estimation of the molar yield of gas for a given electrolysis current and Ar flow rate (SI 1, Figure S1). The gas outlet from the main flask was vented into a measuring cylinder containing 1 M NaOH solution in order to absorb any H2Te not fully reacted in the main flask. Under the conditions described herein, there should be very little sign of this occurring, but should it happen, the NaOH solution would change from clear to pink/purple color initially. Since some of the reaction times were quite lengthy; we observed that, after a while, the concentration of the slightly volatile FMT ligand would begin to fall, with the rate depending on the Ar flow rate and the duration of the synthesis. We determined the exponential decay of the ligand concentration and factored in compensation for the ligand
loss by adding a small amount of ligand/DMSO solution periodically via a second syringe pump catheter arrangement. (see SI 4 for further details). The overall synthesis setup along with the typical reaction sequence is shown schematically in Figure S5. Typically, 300 mL of DMSO was added to a 500 mL flask, and deaerated in an argon gas stream for 1 h. A solution of mercury acetate (e.g., 0.4 mmol) dissolved in 10 mL of DMSO was added to the flask, and FMT (e.g., 0.8 mmol) was also added. In practice, we varied the exact amounts of Hg2+ salt and ligand over the course of a series of syntheses in order to exert control over the degrees of nucleation and particle growth, mainly to ascertain the best conditions for larger particle growth (longer wavelength emission). To start the reactions off, a burst of H2Te was generated at a high current (typically 100 mA) for a few minutes duration. This is termed a nucleation stage; although nucleation is not restricted totally to this step, it is definitely the dominant process. The initial stage is then followed by many further injections of H2Te, produced with lower electrolysis currents and for longer durations (e.g., 20−30 min per step). After each injection of gas, the stirred reaction mixture is allowed to stand for a further 20−30 min while the H2Te precursor is fully consumed. Prior to each injection of H2Te, the control program allowed the option to add further Hg2+ solution if required, and also to top up the FMT ligand to compensate for any loss during the preceding reaction round (see SI 3). Table S1 (in SI 5) gives a list of the reaction conditions for seven representative samples that will be discussed here. Samples 1−4 have different ligand ratios with the same nucleation and growth stage conditions. Samples 5−7 have the same ligand ratios with different synthetic conditions (growth stage currents and higher initial Hg2+ concentrations) which were relevant to the growth of longer wavelength emitting QDs. A further feature of the reaction control program was the use of a peristaltic pump and a third catheter line to allow reaction aliquots to be removed at programmed intervals and dispensed into sample bottles mounted on a carousel. Thus, after the wait period, following each injection of H2Te, the program could determine if a sample was required and collect one automatically. Typically, aliquots were collected after the nucleation stage, and each of the first few growth stages, but then after intervals of several growth stages as the reaction continued (e.g., nucleation, then stages 1, 2, 3, 4, 8, 12, 16, 20 28, 36, etc.). As with all syntheses involving heavy metal salts and solvents, standard handling precautions should be observed. In particular, solutions should be handled using gloves, lab coats, and safety spectacles to avoid skin or eye exposure, etc., and the synthetic procedure should be carried out under a fume hood. Waste materials should be disposed of via a properly regulated chemical waste disposal company.
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RESULTS AND DISCUSSION A number of syntheses were carried out, using different FMT:Hg2+ ratios and also at different H2Te dosage rates (by adjusting the electrolysis current). Figure 1 shows a typical series of PL spectra obtained by extracting aliquots from the reaction mixture periodically and transferring the QDs into TCE solvent and DDT ligand for measurement. Under the range of conditions used, the PL spectra were very broad and showed strong dips due to coupling to IR absorptions (mainly arising from the DDT ligand, but also any residual organic impurities after washing). The PL peak position can be used to estimate the average QD diameter using sizing curves from the literature. Kovalenko et al.10 cited a relationship between the band-gap energy Eg (in eV) and the QD diameter, d (in nm), as Eg = 0.3 + 0.2d −1 + 5.7d −2 7861
(1) DOI: 10.1021/acs.chemmater.7b02637 Chem. Mater. 2017, 29, 7859−7867
Article
Chemistry of Materials
normalization such that the maximum value of each gi term is
