Origin and Dynamics of Highly-Efficient Broadband

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Origin and Dynamics of Highly-Efficient Broadband Photoluminescence of Aqueous Glutathione-Capped Size-Selected Ag-In-S Quantum Dots Oleksandr Stroyuk, Alexandra Raevskaya, Felix Spranger, Oleksandr Selyshchev, Volodymyr Dzhagan, Steffen Schulze, Dietrich R.T. Zahn, and Alexander Eychmüller J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b00106 • Publication Date (Web): 21 Jan 2018 Downloaded from http://pubs.acs.org on January 21, 2018

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The Journal of Physical Chemistry

Origin and Dynamics of Highly-Efficient Broadband Photoluminescence of Aqueous Glutathione-Capped Size-Selected Ag-In-S Quantum Dots Oleksandr Stroyuka,b*,Alexandra Raevskayaa,b, Felix Sprangera, Oleksandr Selyshchevc, Volodymyr Dzhaganc,d, Steffen Schulzec, Dietrich R.T. Zahnc, Alexander Eychmüllera a

Physical Chemistry, TU Dresden, 01062 Dresden, Germany b

L.V. Pysarzhevsky Institute of Physical Chemistry,

National Academy of Sciences of Ukraine, Kyiv, 03028, Ukraine c

Semiconductor Physics, Chemnitz University of Technology, 09107 Chemnitz, Germany d

V. E. Lashkaryov Institute of Semiconductors Physics,

National Academy of Sciences of Ukraine, Kyiv, 03028, Ukraine

Corresponding author: *

Dr. Oleksandr Stroyuk, Physikalische Chemie, Technische Universität Dresden, Bergstraße 66b, 01062 Dresden,

Germany, Tel. +49(0)351 463 34351; Laboratory of Organic Photovoltaics and Electrochemistry, L.V. Pysarzhevsky Institute of Physical Chemistry, National Academy of Sciences of Ukraine, Kyiv, Prosp. Nauky 31, 03028, Ukraine, Tel. +38(0) 44 525 02 70, e-mail: [email protected]; [email protected]

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Abstract. 2–3-nm size-selected glutathione-capped Ag-In-S (AIS) and core/shell AIS/ZnS quantum dots (QDs) were produced by precipitation/redissolution from an aqueous colloidal ensemble. The QDs reveal broadband photoluminescence (PL) with a quantum yield of up to 60% for the most populated fraction of the core/shell AIS/ZnS QDs. The PL band shape can be described by a self-trapped exciton model implying the PL band being a sequence of phonon replica of a zero-phonon line resulting from strong electron-phonon interaction and a partial conversion of the electron excitation energy into lattice vibrations. It can be concluded that the position and shape of the PL bands of AIS QDs originate not from energy factors (depth and distribution of trap states), but rather from the dynamics of the electronphonon interaction and the vibrational relaxation in the QDs. The rate of vibrational relaxation of the electron excitation energy in AIS QDs is found to be size-dependent, increasing almost twice from the largest to the smallest QDs.

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Introduction

In recent years ternary In-based I-III-VI metal-chalcogenide quantum dots (QDs), including Cu-In-S (CIS), Zndoped CIS, Ag-In-S (AIS) and Zn-doped AIS (ZAIS) emerged as an attractive alternative for the binary II-VI and IV-VI QDs, in particular those containing acutely toxic metal ions, such as CdX and PbX (X = S, Se, Te)1–5. The ternary QDs combine highly efficient photoluminescence (PL) in the visible and near IR ranges with an unprecedentedly broad variability of the optical properties achievable by complex changes of the QD composition, size, doping, and variations in the surface ligand shell1–3,6–8. As compared to binary CdX and PbX QDs, the ternary compounds typically exhibit very broad PL bands with a spectral width of hundreds of milli-electronvolts. This property compromises in some ways the perspectives of such QDs in applications requiring a high purity of the emitted light1,6,9. At the same time, a large spectral width (or full width at half maximum – FWHM) of the PL bands results in spectacular visual changes of the emission color even at comparatively tiny variations of the PL band position, allowing to produce multi-colored and bright luminophors attractive for bio-medical labeling and multi-colored cell tracking6,7,9,10. The most brightly emitting ternary QDs are typically produced by the heating-up and hot-injection protocols elaborated earlier in great detail for binary metal-chalcogenide QDs. These syntheses utilize high-boiling-point organic solvents (such as octadecene or trioctylphosphine) and long-chain surface-capping ligands (1-dodecanethiol or oleylamine) to precisely control the size and surface perfection of the forming QDs2,3,7,10,11. As the bio-sensing applications require the QDs to be rendered water-soluble and ready for the conjugation to various bio-molecules, the as-prepared luminescent QDs need to be subjected to ligand exchange with water-soluble agents and phase transfer from the original organic oils to water2,9–12. The most efficient phase transfer and stabilization in aqueous solutions is typically achieved with multifunctional transfer agents such as mercaptoacids (thioglycolyc and mercaptoacetic acid), cysteine or glutathione (GSH) that can bind strongly to the QD surface by a mercapto-group and simultaneously prohibit the QD aggregation in water due to the electrostatic repulsion between the carboxyl anions11,12. As compared to other water-soluble multi-functional ligands (mercapto-acids, cysteine, etc.) GSH exhibits an excellent resistivity towards oxidation and hydrolysis13 and provides GSH-capped QDs with a high stability against aggregation. GSH was used to transfer PbS13or CIS14 QDs from organic media to water via ligand exchange. This approach is very popular for binary II-VI metal-chalcogenide QDs, however, in the case of ternary I-III-VI compounds it results in a considerable loss of the PL efficiency13,15. For example, the phase transfer of CIS QDs originally capped with oleylamine with a PL quantum yield (PL QY) of 70% to water mediated by GSH yields aqueous QDs with a PL QY of only 34%15. In this view, attempts of direct syntheses of luminescent GSH-stabilized QDs in aqueous media have constantly been conducted. In particular, successful syntheses were reported for GSH-capped ZnS16, CdS17,18, PbS19, doped ZnS20 and CdxZn1–xS21, HgS22, CdSe23, ZnSe24–27QDs, alloyed ZnSexTe1-x28, CdxZn1–xSe27 and CdxZn1–xTe29 QDs, as well as Ag2Se30 and CdTe31–36 QDs. Gluthatione can act not only as a surface-binding ligand but also as a precursor for the in situ formation of a ZnS shell25,32 as well as a reducing agent for Se precursors in the syntheses of metal selenide QDs30. Recently, GSH was shown to serve as an efficient stabilizer for direct and aqueous syntheses of composition- and size-selected ternary I-III-VI QDs, such as CIS37–40, AIS15,41, and ZAIS42. Luminescent AIS QDs were reported to form when treating bovine serum albumine-stabilized Ag2S QDs with an In3+ complex with GSH43. The treatment results in a simultaneous In3+ incorporation into the QD structure and a surface ligand exchange. The capability of GSH to form stable complexes and even coordination polymers with metal cations can be used to tune finely the size and size distribution of the final metal chalcogenide QDs, as reported for CdS QDs18. The capping with the functionality-rich GSH allows for the post-synthesis conjugation of QDs to bio-molecules while preserving the PL efficiency. Such QD-biomolecule conjugates were used as luminescent contrast agents for living 3



cell tracking and labeling18,30,31,35,38,39–43. By capping QDs selectively with L or D optical isomers of GSH, chirally active QDs can be produced for focused cell toxicity and sensing studies33. The nature of the broadband PL of ternary CIS and AIS still remains a field of discussion, because these QDs seem to reveal different electron-hole recombination mechanisms as compared to both the corresponding bulk ternary semiconductors and the broadly studied II-VI binary QDs1,8,44,45. In particular, a generally accepted model of trap-statemediated radiative recombination typically evoked to account for the large FWHM and Stokes shifts of the PL bands is currently questioned and revised in terms of its applicability to the ternary QDs1,44. A deeper insight into the mechanisms of PL emission requires to probe separately the QD composition, size, doping, and surface capping, which is a challenge still to be met in the case of ternary and quaternary In-based QDs. Of great help in this direction can be synthetic approaches allowing one of the above-mentioned factors to be varied while leaving others untouched, such as for example a sizeselective precipitation/redispersion introduced by us recently for thioglycolate (TGA)-capped aqueous AIS QDs46. Themethod was found to yield a series of QDs with the same synthesis history (produced from a single “parental” ensemble), the same composition, the same ligand shell, only but varying in size and, consequently, the optical properties. This allows information on the size-dependent absorption/emission behavior of such QDs to be extracted. In the present paper we demonstrate the feasibility of the size-selection for the separation of an original colloidal ensemble of aqueous GSH-capped AIS QDs into a broad series of fractions containing QDs with different average sizes and considerably focused size distributions as compared to the parental ensemble. The QDs were found to have similar compositions allowing discriminating the influence of size on the spectral QD characteristics. We focus on the discussion of the nature of the broadband PL revealed by such selected QDs with the FWHM being almost independent on the QD size and size distribution. We argue that this inherent property of ternary In-based QDs should be interpreted in terms of a self-trapped exciton and strong electron-phonon interaction, rather than in the frame of the conventional model of a trapmediated donor-acceptor recombination.

Material and methods Aqueous colloidal AIS and AIS/ZnS QDs stabilized by GSH are produced similarly to our previous report on TGA-capped QDs46 in a reaction between GSH complexes of metal cations with sodium sulfide in the presence of ammonium hydroxide at 96–98 °C. In a typical procedure, 2.4 mL 0.5 M aqueous GSH solution is added to 2.5 mL deionized water followed by 0.8 mL 1.0 M aqueous InCl3 solution (containing 0.25 M HNO3 to assure the complete solubility of the indium salt). In 5 min of stirring 1.0 mL 5.0 M NH4OH is introduced. All sequential additions are performed at intense stirring. The addition of ammonia induces the formation of a white precipitate dissolving after 2–3min of refluxing of the mixture. Then, 2.0 mL 0.1 M aqueous AgNO3 solution is introduced followed by rapid addition of 1.0 mL 1.0 M aqueous Na2S solution and 0.25 mL 2.0 M aqueous solution of ammonium citrate. The synthesis is completed by heating at 96–98 °C for 60 min in a temperature-regulated bath. The total volume of the “parental” AIS QD solution is 10 mL. The next step aims at the formation of a ZnS shell. For this, 3.0 mL 0.5 M GSH solution is mixed with 0.4 mL 5.0 M NH4OH and 1.6 mL of aqueous 1.0 M Zn(CH3COO)2 solution (containing 0.01 M HNO3) with a constant magnetic stirring. The resulting solution (5.0 mL) is mixed with 10 mL of the parental AIS colloid and heated at 96–98 °C for 10 min. Both AIS and AIS/ZnS colloids can then be subjected to purification by adding a batch volume (15 mL) of 2propanol. The addition results in the precipitation of the QDs, the precipitate can be separated by centrifugation (2 min at 10000 rpm) and redissolved in the required volume of deionized water. 4


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Size-selective precipitation is performed by the step-wise addition of 2-propanol to the parental AIS or AIS/ZnS QD solution till the formation of a precipitate. This is always done in steps of 1.0 mL 2-propanol. The precipitate is separated by centrifugation and redissolved in 1.0 mL of deionized water. The procedure is repeated 8–10 times to produce separate fractions of colloidal AIS (AIS/ZnS) QDs. The QDs are characterized by the absorption, PL and X-ray photoelectron (XPS) and inductively-coupled plasma optical emission (ICP-OES) spectroscopies, transmission electron microscopy (TEM), and X-ray diffraction (XRD). The details are provided in the Supporting Information (SI) file.

Results and Discussion

Optimization of the synthesis of parental AIS colloids. The composition of AIS QDs can be expressed as the molar ratio xAg:xIn:xS of the corresponding components. We varied systematically the xAg, xIn, and xS parameters, the concentration of the capping agent, the temperature and the duration of the thermal treatment, as well as the mode and the conditions of the ZnS shell formation to achieve the highest possible PL QY. Similar to the previously studied TGA-capped AIS QDs46, an increase in the Ag/In ratio results in an increase of the integral absorbance of the colloidal AIS-GSH solutions as well as in a steady shift of the absorption band edge to lower energies (SI, Fig. S1a). In contrast to AIS-TGA colloids, the present GSH-based systems demonstrate quite distinct absorption band thresholds allowing for a reliable estimation of the bandgap Eg by re-plotting the spectra in Tauc coordinates for direct transitions and deducing the intercepts with the abscissa. The bandgap was found to decrease monotonically from around 2.8 eV to ~2.1 eV as the Ag/In ratio was increased from 0.05 to 0.5 (Fig. 1a, curve 1). The Ag 4d orbitals have a strong contribution to the valence band (VB) “top” near the edge1,44 and, therefore, an increase in the silver content expectedly results in an increase in the absorption and a bandgap narrowing. The AIS-GSH QDs emit PL in broad bands with the position and intensity strongly being dependent on the silver content (SI, Fig. S1b). As the Ag/In ratio is increased, the PL band peak shifts to lower energies from ~2.1 eV to 1.8 eV (Fig. 1a, curve 2), while the PL intensity increases, reaching a maximum at an Ag/In ratio around 0.25 and then decreases for Ag-richer QDs (Fig. 1b, curve 3). Similarly, the spectral parameters of the PL band depend considerably on the S/Ag ratio, while the In content is kept constant (SI, Fig. S2a). The PL intensity increases with the growth of the S/Ag ratio, displays a maximum at S/Ag ~ 5 and then decreases for lower silver contents (SI, Fig. S2b). When plotted as a topographic surface, both dependences of the PL intensity on the sulfur and silver contents produce a peak corresponding to Ag/S ratios from 1:4 to 1:5 (Fig. 1b). In this view, we selected QDs with a composition corresponding to xAg:xIn:xS = 1:4:5 for the following studies. The AIS QDs form in a reaction between GSH complexes with Ag+ and In3+ and hydrosulfide ions at 96–98 °C. The duration of such a thermal treatment is found to affect the optical properties of the QDs in quite a decisive manner. In particular, the absorption band edge of AIS QDs (Ag:In:S = 1:4:5) shows a steady shift to lower energies as the treatment duration is increased (SI, Fig. S3a), the bandgap shrinking from 2.85 eV before heating to ~2.3 eV after a 80 min treatment (Fig. 1c, curve 1), most probably related to a growth of the QDs. The PL band maximum EPL follows this trend showing a shift from ~2.1 eV to 1.85 eV (Fig. 1c, curve 2). The modest PL shift contrasts sharply with the drastic increase in PL intensity with heating time, almost doubling after the 80 min treatment (Fig. 1c, curve 3). This observation shows that, besides increasing the mean QD size, the heating results in an increase of the lattice ordering of the AIS-GSH QDs and the

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elimination of defects responsible for non-radiative electron-hole recombination. As the spectral changes become negligible for times longer than 60 min this treatment duration was adopted for the further studies of the AIS-GSH QDs.

Figure 1. (a, c) Bandgap (curve 1), PL band maximum (curve 2), and integral PL intensity (curve 3) of AIS QDs as a function of molar Ag/In ratio (a) and the duration of the thermal treatment (c); (b) Variation of PL intensity of AIS QDs with xAg and xS at a constant xIn = 4; (d) Absorption spectra (curves 1,2) and PL spectra (curves 1/,2/) of AIS QDs (curves 1) and AIS/ZnS QDs (curves 2) produced at the same Ag:In:S:Zn ratio of 1:4:5:0 and 1:4:5:8, respectively.

Covering of AIS QDs with a ZnS shell was found to be a rather efficient way of enhancing the PL emission of AIS-TGA QDs46. This approach works also for the present AIS-GSH system, the enhancement factor reaching ~2 (Fig. 1d). The formation of a ZnS shell results in a small shift of EPL to higher energies (Fig. 1d) as a result of the well-reported diffusion of Zn2+ ions from the shell into the AIS lattice1,2. The latter trend becomes more pronounced with an increased Zn/In ratio (SI, Fig. S4), however, a saturation of the PL enhancement is achieved already at Zn:In = 3:1. The colloidal AIS and AIS/ZnS QDs produced in this way revealed a very high stability toward aggregation and oxidation. In addition, the PL characteristics remained unaltered for several months of shelf storage. Keeping the component ratios constant, the molar concentration of the colloids can be raised by an order of magnitude with the stability and the absorption/PL characteristics of the QDs remaining the same (when normalized to the concentration, SI, Fig. S5).

Size-selection of AIS-GSH QDs. The addition of an ample excess (volumetric) of 2-propanol results in the instant coagulation of colloidal GSH-capped AIS and AIS/ZnS QDs, similar to their TGA-stabilized analogues46. This procedure can be used for batch separation of all AIS QDs from the parental solution for purification purposes as the coagulate can be readily redissolved in deionized water yielding again stable colloidal solutions. If the 2-propanol addition is performed in portions, a selective precipitation of the largest QDs in the colloidal ensemble can be achieved. By separating the coagulate and repeating this procedure, numerous fractions of QDs differing in the optical properties, and, therefore, in the average size, can be produced from the parental colloid. Figure 2a exemplifies the absorption spectra of eight fractions produced from a “standard” parental AIS QD colloid (Ag:In:S = 1:4:5, 60 min heating). As the fraction number increases from 1 to 8 the absorption threshold of the AIS 6


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QDs shifts to lower wavelengths (higher energies) clearly indicating that the size of selectively precipitated QDs decreases resulting in an enhancement of the spatial exciton confinement and a widening of the bandgap. The bandgap of sizeselected AIS QDs is found to increase in a seemingly linear manner with the fraction number from 2.3 eV (fraction No. 1) to almost 3.0 eV for the smallest QDs in the fraction No. 8 (Fig. 2a, inset). The same trend can be observed for AIS/ZnS QDs with the bandgap increasing from 2.3 eV to 3.1 eV for the last fraction No. 8. A larger Eg for AIS/ZnS QDs as compared to the “core” AIS QDs in the respective fraction numbers (starting from 4–5 and higher) indicates that the incorporation of Zn2+ is obviously more efficient for smaller QDs than for larger ones, in accordance with the XPS data discussed below. Performing the size-selection with the same alcohol-to-water ratios we found that for a certain composition of the colloid (precursor concentration, GSH content, Ag:In:S ratio, Zn:In ratio) we can obtain colloidal solutions with highly reproducible bandgaps (and, therefore average sizes) and PL characteristics. This reproducibility allows us to refer to different fractions directly by their bandgap, not by the euphemeric fraction number or the non-descriptive 2propanol/water volumetric ratio. At the same time, by adjusting the composition and concentration of the original parental colloid we can vary the number of the separated fractions increasing to 10 (such QDs are used in some of the experiments in the present paper) and even further to a record 18 fractions (see a photograph in SI, Fig. S6a).

Figure 2. Absorption (a,b) and PL spectra (c,d) of size-selected AIS QDs (a,c) and AIS/ZnS QDs (b,d) in fractions No. 1 through 8. Inserts in (a,b): bandgap Eg of QDs as a function of the fraction number. Light-gray curves in (c,d) correspond to the PL spectra of the parental non-fractionated colloids. (e,f) Photographs of size-selected AIS (e) and AIS/ZnS (f) colloids taken under UV illumination (365 nm). Ag:In:S:Zn = 1:4:5:8.

To evaluate the population of QDs in different fractions we took the optical density of the size-selected QDs far from the absorption band edge (at 300 nm) as a quantitative measure of the molar AIS concentration. This evaluation is regarded as a first approximation, because the strong quantum confinement in small AIS QDs can strongly affect not only 7



the band-edge absorption but also the selection rules and the molecular unit-weighted oscillator strengths of all electron transitions in the QDs. It is found that the first and second fractions of AIS QDs are the most populated ones in terms of the AIS concentration, while the amount of QDs in the following fractions declines rapidly being ~20 times lower for fraction No. 8 as compared with the most populated fraction No. 2 (SI, Fig. S6b). In the case of AIS/ZnS, again, the fraction No. 2 is the most populated, while the concentration of the QDs decreases in a much steeper way than for AIS QDs (SI, Fig. S6c) and the last fractions No. 7 and 8 contain two-orders of magnitude less AIS than the most populated fraction No. 2. The size-selected AIS QDs reveal X-Ray diffraction patterns typical for chalcopyrite Ag-In-S with the reflex intensity gradually decreasing with an increase in the fraction number (SI, Fig. S7). The latter fact indicates a gradual disordering of the QD lattice with decreasing size, most probably due to the lattice relaxation to accommodate the growing surface-to-volume ratio. The estimations based on the Scherrer equation show the AIS QDs in the fractions No. 7–8 being smaller than 2 nm, while larger QDs (2.5–3.0 nm) are found in the first two fractions. Therefore, the size variation in the produced series of size-selected AIS-GSH QDs is as narrow as that for the previously reported TGA-capped AIS QDs46. The GSH-stabilized QDs are found to be challenging objects for the TEM imaging due to sample charging impeding a precise focusing of the electron beam. As a result, only vague TEM pictures were obtained (SI, S8a, b) showing, nevertheless, that the QD size in fraction No. 1 (around 3 nm) is larger than the QD size in fraction No. 7 (2 nm and smaller). It should be noted that the problem of obtaining good TEM images of GSH-capped metal chalcogenide QDs synthesized directly in water seems to be a more general character as this difficulty has been experienced before for different semiconductors including binary and ternary chalcogenides16,18,19,22–26,28,34. A HRTEM image of the parental AIS QDs shows the presence of crystalline domains ranging in size from ~1.5 nm to 4 nm (SI, Fig. S8c). The colloidal solutions are also found to form films of high uniformity and optical quality when spin-coated onto silicon substrates. An atomic force microscopic image of the surface of such a film produced from the fraction No. 7 (SI, Fig. S8d) reveals an average roughness of around 2 nm resembling the size derived from the TEM and XRD measurements and attesting the very uniform character of the produced films. A basic issue for the non-stoichiometric AIS QDs is to discriminate the evolution of the optical properties caused by the QD size variation from variations of the QD composition in different fractions. We subjected a series of ten sizeselected fractions of AIS/ZnS QDs to a XPS study aiming at the evaluation of possible compositional deviations. The highresolution photoelectron spectra in the ranges of Zn2p, Ag3d, and In3d core level emissions (SI, Fig. S9a-c) show corresponding doublets at the positions typical for Zn2+ (1020/1043 eV), Ag+ (367/373 eV), and In3+ (444/451 eV)47,48. The S2p range (SI, Fig. S9d) reveals two doublets at 161.4/162.6 eV and 163.4/164.6 eV characteristic for the lattice sulfide and the mercapto-group bound to the QD surface, respectively. The relative contribution of the latter component increases somewhat (by around 30%) from fraction No. 1 to fraction No. 10 in line with the increase of the total QD surface/volume ratio, but no steady trend throughout the entire series could be obtained due to the large scatter in data. Figure 3a shows a set of atomic In/Ag (blue bars), Zn/In (green bars), and S/In (red bars) ratios calculated from the corresponding areas in the high-resolution XPS spectrafor ten fractions of AIS/ZnS-GSH QDs. The indium-to-silver ratio which most strongly affects the optical properties of the size-selected QDs is found to vary between 2.4 and 3.0 and, with regard to the experimental error, can be assumed to be almost constant throughout the series studied. Inspecting the data we can safely assume that the observed changes in the absorption spectra (as well as the changes in the PL spectra discussed below) originate from the QD size variation between the fractions and not from variations of the silver-to-indium ratio. As this ratio is smaller than the one set at the synthesis of the parental colloid (Ag:In = 1:4) we can also assume that In3+ is partially bound in a complex with GSH, which remains unprecipitated in the parental solution. 8


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Figure 3. (a) Atomic ratios of components of size-selected AIS/ZnS QDs as a function of the bandgap (corresponding fraction numbers are given in the figure) as determined by XPS (Ag:In:S:Zn = 1:5:10:10, 10 fractions); (b) PL QY of colloidal AIS QDs (squares 1) and AIS/ZnS QDs (squares 2) as a function of the QD bandgap. Ag:In:S:Zn = 1:4:5:8 (8 fractions).

The ratio of zinc to indium is found to increase steadily from around 1–1.5 for the first fractions to 2.1–2.2 for the last fractions, most probably due to the presence of Zn2+-GSH complexes on the QD surface and the incorporation of Zn2+ ions into the QD lattice. Both factors are expected to increase with decreasing QD size resulting in the observed trend. Similar In-to-Ag and Zn-to-In ratios were derived from the ICP-OES measurements of the size-selected AIS/ZnS QDs (SI, Fig. S10), the values being reasonably similar for all fractions, varying around an average In/Ag = 3.0 and Zn/In = 2.3. The S-to-In ratio is found to increase considerably, from around 4 for the first fractions to ~7 for the smallest QDs, which can also be accounted for by the increase in the total QD surface area.

Static PL of the size-selected GSH-stabilized AIS and AIS/ZnS QDs. The size-selective precipitation/redispersion produces a series of colloidal solutions differing considerably by their PL characteristics. The size-selected AIS and AIS/ZnS QDs emit in broad bands (Fig. 2c,d), similar to those of the original parental solutions, but the PL band position and intensity (normalized to the QD concentration in each fraction) depend on the fraction number. In the case of AIS QDs the PL band maximum shifts from ~660 nm for the first fraction to ~560 nm for the last fraction in the series, while the PL intensity is reduced by about an order of magnitude (Fig. 2c). A similar trend is observed for AIS/ZnS QDs exhibiting PL maxima at ~620 nm for fraction No. 1 and ~520 nm for fraction No. 8 and a similar ~6-fold reduction in the PL intensity (Fig. 2d). The PL bands are characterized by a similar spectral width (FWHM) of around 0.35‒0.40 eV for all fractions. Visually the PL emission color changes from orange-red to green in the series of size-selected AIS QDs (Fig. 2e) and from orange to bluish-green for AIS/ZnS QDs (Fig. 2f). We measured the absolute PL quantum yields (QYs) for all fractions using two different setups equipped with integrating spheres to assure a high reproducibility. It is found that the highest PL QY of 32% and 60% are observed for the most populated fractions No. 2 of AIS and AIS/ZnS QDs, respectively (Fig. 3b). The twice as large PL QY of the core/shell AIS/ZnS QDs as compared to the bare AIS clearly indicates the passivating effect of the zinc sulfide shell, most probably arising from both the elimination of surface defects and possible lattice imperfections due to the Zn2+ diffusion into the AIS core. As the AIS QD size decreases (and the bandgap increases) the PL QY decreases down to only a few percent for the least populated fractions No. 7, 8 (Fig. 3b). The core/shell AIS/ZnS QDs show a similar trend, but in this case the fractions with the smallest QDs reveal distinctly higher PL QYs of around 10%, showing that the effect of passivation with ZnS is stronger for smaller QDs, in accordance with the conclusions derived from the absorption spectra and the XPS results.

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The nature of the broadband PL of ternary CIS and AIS QDs, similar to copper- and silver-doped binary cadmium chalcogenide QDs, still remains a subject of discussion1,44, and the proposed interpretations are quite controversial. Most frequently, the broadband PL of CIS/AIS QDs is interpreted in a “conventional” way by applying a model of donoracceptor (D‒A) recombination1,44,49‒53. This model implies that the PL emission originates from the radiative recombination between trapped electrons and holes and the spectral width of the PL band is a function of at least three distributions: (i) a distribution of sizes; (ii) D and A trap level distributions by “depth” (i.e. an energy gap between the corresponding band edge and the local trap levels in the bandgap), and (iii) a distribution of distances separating the trapped electrons and the holes within each single QD, the latter distance being limited by the maximum QD size in the ensemble, i.e. (iii) can partially correlate with (i). In terms of the D‒A model the energy of PL quanta EPL can be expressed as

EPL = Eex − ( D− + D+ ) +

e2 εR

(1)

where Eex is the exciton energy (or bandgap), D+ and D‒ are the “depths” of the hole and electron traps, while the e2/εR describes the Coulombic electron-hole interaction depending on the distance R between the charge carriers (ε is the dielectric constant of the QD material). A size distribution of the QDs in a colloidal ensemble results in a distribution of Eex, while an inhomogeneity of the QD lattice and many possible origins of the electron and hole traps contribute to the distribution of the D+ and D‒ energies. Finally, the trapped carriers situated closer to each other are characterized by a higher Coulombic interaction when recombining, thus generating a PL with a higher energy than those pairs situated further apart. All three factors are believed to contribute to the formation of broad PL bands1,44,49‒53. As the closer carrier pairs recombine faster the D‒Aemitting QDs typically exhibit a dependence of the PL lifetime on the PL observation wavelength (energy) – the lower the PL energy the longer the PL lifetime. Such dependences are usually taken as an unambiguous proof of the D‒A type PL both for binary and ternary metal chalcogenide QDs1,8. However, when applied to ternary AIS and CIS QDs, especially those subjected to size-selection, the D‒A model manifests certain inconsistencies that, combined together, show the need for a more realistic approach for the description of PL phenomena. The recent progress in the synthesis of CIS/AIS QDs with strictly tailored morphology, phase, and composition allows producing colloidal QDs with low dispersities2,3,7,10,12,54 showing, nevertheless, broad PL bands like their polydisperse analogs. The size selection applied in the present work is also expected to considerably narrow the size distribution separating the original parental ensemble into numerous fractions with different average sizes. However, the size-selected AIS and AIS/ZnS QDs reveal the same or even somewhat larger FWHMs of the PL bands as the parental colloids, showing no indications of band narrowing (Fig. 2c, d). Finally, recent studies on single CIS and AIS QDs showed them to emit in equally broad bands with the band maximum position55,56 changing randomly but within the PL range of the whole QD ensemble. All these observations allow the size-dependence of Eex (Eg) to be excluded as a major contribution to the spectral width of ternary QDs. The idea of a broad distribution of trap states by depth also seems to be impaired by various static PL measurements. A broad depth distribution implies a large number of available defects, but the defects inherently can participate both in radiative and non-radiative recombinations. The highest PL QY of 60% in the present work is very close to the best results typically reported for AIS/ZnS QDs produced by heating up/hot injection syntheses51,52,57. Even higher PL QYs were reported for larger (5‒6 nm) alloyed ZAIS QDs reaching up to 79%58 indicating that we still have room for improvement of our aqueous synthesis. In the frame of the D–A model the broad PL is assumed to originate from a broad distribution of energies and distances of the trapped carriers. Hence, we should expect the spectral parameters of the PL bands of the AIS QDs to be 10


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strongly dependent on temperature. For an ensemble of traps there exist states separated in energy close to kT (k is the Boltzmann constant) and therefore a re-population of traps at room temperature is expected with the charge carriers coming from deeper traps to shallower traps under the influence of the thermal lattice energy45,60. As the temperature is lowered, the de-trapping becomes slower, resulting in a red shift and a narrowing of the trap-state-mediated PL bands. However, the reported studies of temperature dependences of ternary CIS and AIS QDs50,57 showed no effects of this sort to occur with the spectral width of the PL band remaining essentially the same in the whole temperature range studied. Our preliminary study of the temperature dependence of the PL of the size-selected AIS/ZnS QDs (to be published elsewhere) also showed the PL band width being essentially the same in the broad T range from 280 to 10 K (SI, Fig. S11) where “freezing” of the traps can be expected. A possible contribution of the distribution of the trapped carriers by distance within single QDs can also be questioned in the light of the reports on the dependence of the PL emission of AIS QDs on the excitation power50,52,61 as well as by the studies on size-selected AIS QDs reported by us46. Indeed, as the size of AIS QDs becomes smaller we should expect a narrowing of the spectrum of possible distances between the trapped electrons and holes which is not the case for the size-selected QDs showing the same FWHM of the PL band in the entire range of sizes. Moreover, the smallest QDs (d< 2 nm) have only a few lattice planes and a broad distribution of D–A distances is not realistic. Additionally, if a distribution of distances between the trapped electrons and holes is assumed, an effect of the PL excitation intensity on the population of the electronic states (corresponding to different e–h distances) is expected, i.e. a narrowing of the PL band would be expected as the excitation intensity is reduced. This was found for AIS QDs with the PL excitation power varied by several orders of magnitude50,52,61 indicating that the concept of D‒A recombination with differently separated e–h pairs need to be revised at least for the present case of the ternary In-based chalcogenide QDs1,44,49. Daniel Gamelin et al. proposed that the PL emission of copper-doped cadmium chalcogenide QDs as well as ternary CIS QDs (that can be viewed as the ultimate case of copper doping when each lattice metal ion is Cu+) can be described by a self-trapped exciton model44, originally introduced for silver halide QDs and then suggested by Louis Brus et al. to account for a specific PL behavior of ultra-small CdS QDs62. The model is based on the assumption of a strong electron-phonon interaction and active participation of vibrational modes of the QD lattice in the fate of the photogenerated charge carriers. One of the carriers, typically the valence band hole, is assumed to be bound to a local state (Cu+ dopant in copper-doped QDs or any M+ ion in silver halides, CIS, or AIS QDs44) resulting in a considerable local distortion of the lattice stabilizing the trapped carrier. Due to strong coupling to the lattice vibrations a portion of the excitation energy is dissipated as phonons, typically as longitudinal optical (LO) phonons because a trapping site is assumed to consist of a central metal ion in the tetrahedral “shell” of four anions and the trapped charge carrier is coupled to metal – anion stretching vibrations44,50,52. The radiative recombination of a free electron with a trapped hole results, therefore, in a series of phonon replicas of the zero-phonon line EZPL generally observed as a broad PL band for silver halide QDs, Cu-doped CdS (CdSe), CIS, and AIS QDs44. The energy En and relative intensity In of each phonon replica forming the PL band can be estimated in the frame of a configuration coordinate model50,52 as En = EZPL – nħω,

In =

S ne−S n!

11


(2)

(3)


where n is the number of emitted phonon, ħω is the phonon energy, S is the Huang-Rhys factor equal to the average number of phonons emitted before the PL event. The case of n = S corresponds to the peak energy EPL of the PL band, that is EPL = EZPL – Sħω. Hamanaka et al. were first to apply this model to simulate the PL spectrum of 2.6 nm AIS QDs50. Their fitting yielded a realistic value of the AIS LO phonon, a large Huang-Rhys factor of 23, and a EZPL value corresponding to a lowintensity excitonic feature in the absorption spectrum. These observations indicate that the experimental PL spectra can be modeled using empirically determined values – EZPL equal to the bandgap and the LO phonon energy derived from Raman spectra, while the Huang-Rhys factor S can be estimated from the Stokes shift between the Eg and EPL normalized to the phonon energy. The present size-selected AIS-GSH QDs (a series of ten fractions) were found to exhibit different Stokes shifts varying from 330 meV for fraction No. 1 to 630 meV for fraction No. 10 showing that the average number of phonons emitted prior to the PL event is twice as large for the smallest QDs as for the largest QDs (Fig. 4a, b). In terms of the selftrapped exciton model these observations imply that the vibrational relaxation of the excitation energy occurs 2 times faster in the smallest QDs. As smaller QDs have a larger surface-to-volume ratio, the above observation may indicate that the hole can be trapped by the surface shell of the ligands. Indeed, earlier we reported on the ultra-small (1.8–2.0 nm) CdS QDs stabilized by surface Cd2+ complexes with polyethyleneimine (PEI)63 or mixed complexes with TGA and ammonia64, exhibiting similar broadband and intense PL. We found that such QDs are crystalline and extremely stable indicating a high lattice perfection. A moderate heating of such CdS colloids resulted in a strong PL quenching, but the PL intensity was completely restored after cooling. We interpreted this effect as a result of the thermally activated dissociation of the surface Cd2+ complex species participating in the radiative recombination and the formation of aqua-complexes prone to non-radiative energy dissipation. A similar situation can take place in the present case of GSH-stabilized AIS QDs because this ligand provides the same combination of mercapto- and amino-groups as in the case of CdS-TGA-NH3 QDs. Our preliminary experiments showed that the PL intensity of AIS-GSH QDs indeed drops by an order of magnitude after heating of colloidal solution from RT to 60–70 °C, and this effect is completely reversible upon cooling (a detailed account on the temperature effects is beyond the scope of the present paper and will be reported elsewhere). Therefore, we can assume that the layer of surface-capping GSH complexes can act as a “collective” trap for the photogenerated hole and the vibrational relaxation becomes expectedly faster for the smaller QDs with a larger surface-to-volume ratio. A strong contribution of both direct exciton-ligand coupling and indirect surface charge trapping events are currently discussed as possible mechanisms of a vibrational loss of the excitation energy65. We tried to reconstruct the PL spectra of two fractions of size-selected AIS QDs (Nos. 1 and 10) using the Eg values derived from the corresponding absorption threshold (Eg1 = 2.40 eV and Eg10 = 2.92 eV, Fig. 4a, b), ħω = 38 meV (the energy corresponding to the strongest phonon peak in the Raman spectra46), and a Huang-Rhys factor of 9 (No. 1) and 17 (No. 10) calculated from the corresponding Stokes shifts. The modeled spectra (bars in Fig. 4a, b) appeared to be much narrower than the experimental PL spectra, with the difference between the modelling and experiment being much larger for fraction No. 1. The differences between the model and the observed PL indicate the existence of an additional factor contributing to the PL band broadening. To investigate the possible nature of this factor we modelled the PL spectrum of previouslystudied CdS-PEI QDs characterized by a very narrow and sharply resolved excitonic absorption feature63 indicating a very narrow QD size distribution (SI, Fig. S12a). The modelling using the CdS LO phonon energy, EZPL = 3.50 eV and S = 27 (derived from the Stokes shift) showed a perfect match with the experimental PL spectrum indicating a good applicability of the model and the chosen parameters for the case of QD ensembles with low polydispersity. Therefore, the mismatch 12


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between the modeling results and the PL spectra of AIS-GSH QDs can arise from a larger size distribution in the corresponding fractions compared to CdS-PEI. This assumption also agrees with a lower mismatch for the smaller QDs in fraction No. 10 that is obviously characterized by a narrower size distribution as compared to the first fraction of QDs. To account for a possible size distribution of the QDs in size-selected fractions we adopted a method of the determination of an average Eg from the first derivative of the absorption spectrum near the band edge proposed by Pesika et al. for ZnO QDs66. This method allows a more realistic average bandgap to be obtained for an ensemble of QDs in the regime of strong spatial exciton confinement as compared with the tangent method, the latter providing a lower limit of Eg. The average bandgap values for fractions Nos. 1 and 10 determined by the derivative method (see example in SI, Fig. S12b) are found to be higher (Eg1 = 2.65 eV, Eg10 = 3.29 eV) providing larger Stokes shifts (Fig. 4c, d) and the Huang-Rhys factors of S1 = 15 and S10 = 27. The reconstruction of the PL spectra using these corrected values results in a much better match between the modeled and the experimental spectra showing a very good applicability of the self-trapped exciton model to the size-selected AIS QDs.

Figure 4. Normalized absorption and PL spectra (solid curves) of size-selected colloidal AIS QDs from fractions No. 1 (a,c) and No. 10 (b,d) combined with the results of PL spectra modeling (bars) without (a,b) and with (c,d) the correction on the QD size distribution. Ag:In:S:Zn = 1:5:10:10.

These results also attest that the position and shape of the PL bands of AIS QDs do not originate from trap depth distributions but rather from the dynamics of the electron-phonon interaction and the vibrational relaxation in the ultrasmall QDs coupled to ligand molecules. In this view, the discussed QDs differ strongly from larger “regular” nanoparticles and bulk semiconductors where the broadband or multi-band emission is associated with different defects and recombination mechanisms. The possibilities of a drastic difference in the origin of the PL between bulk CuInS2 and CIS QDs and a size dependence of the stability of self-trapped excitons were discussed in a recent review by Gamelin et al. for Cu-doped QDs44 It was suggested that this model becomes even more appropriate as the size of ternary QDs is decreased.

Time-resolved photoluminescence of the size-selected GSH-stabilized AIS and AIS/ZnS QDs. The PL decays of size-selected GSH-stabilized AIS (AIS/ZnS) QDs reveal a distinct multi-exponential character (Fig. 5a) typical for ternary metal-chalcogenides1,15,41,51–53,57. The PL decay curves of CIS and AIS QDs are typically fitted with linear combinations of two–three single-exponential functions to derive a weighted average PL lifetime15,41,51,52,57. No real physical model of the radiative events in the QDs is assumed in this approach and the lifetimes of separate fitting components cannot be 13



Page 14 of 21

associated with any specific recombination channel67. As the PL decay curves of the present size-selected AIS and AIS/ZnS QDs have essentially the same shape (Fig. 5a) and show no break points or bends, that might indicate two (or more) different emitting species, we opted to avoid the fitting and adopted a conventional radiative lifetime (corresponding to the 1/e reduction of the PL intensity) as a measure of the PL decay rate. The PL lifetime τ of AIS and AIS/ZnS QDs is found to depend on the QD bandgap (Fig. 5b) decreasing from around 330 ns for the largest QDs in fraction No. 1 (Eg1 = 2.32 eV) to ~90 ns for the smallest QDs in fraction No. 10 (Eg10 = 2.95 eV). Covering of AIS with a ZnS shell did not affect the PL lifetime in fractions 1–6, while increasing τ somewhat for the last two fractions of the smallest QDs. The latter effect can be accounted for by a more massive penetration of Zn2+ ions into the lattice of the smallest QDs, in accordance with the results of absorption spectroscopy and XPS.

Figure 5. (a) PL decay curves of size-selected colloidal AIS/ZnS QDs (fraction numbers are given on the figure) registered in the maxima of corresponding PL bands; (b–d) radiative lifetime (b), radiative recombination rate constant (c) and non-radiative recombination rate constant (d) of colloidal AIS QDs (squares 1) and AIS/ZnS QDs (squares 2) as a function of the QD bandgap. Ag:In:S:Zn = 1:4:5:8.

By combining stationary measurements of the PL QY with the kinetic data on τ we can extract values of the rate constants of the radiative recombination kr and the non-radiative recombination knr as a function of the respective Eg. The radiative QD lifetime is inversely proportional to the sum of the rate constants of all recombination routes (eq. (4)), while the PL QY represents the fraction of the radiative channels in the entire set of recombination events (eq. (5)):

τ =

1 k r + k nr

(4) ,

PLQY =

kr k r + k nr

(5)

After a combination of eq. (4) and (5) we can evaluate the radiative recombination rate constant

kr =

PLQY

(6),

τ

and then calculate the rate constant of the non-radiative recombination processes. 14


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The radiative recombination rate is found to follow the PL QY, being maximal in fractions No. 2 of the AIS and the AIS/ZnS QDs (Fig. 5c), and then decreasing for smaller QDs. As with the PL QY, the incorporation of Zn2+ retards the kr decrease for the fractions No. 7,8 of the smallest AIS/ZnS QDs. One can see that the PL emission rate is generally by 60– 70% higher for the core/shell AIS/ZnS QDs as compared to bare AIS QDs in the corresponding fractions. In absolute values, the radiative recombination rate constant derived here for the size-selected AIS and AIS/ZnS QDs fall into the typical range of (0.1–1) µs–1 reported for such ternary QDs51,52,57. Surprisingly, the rate constant of the non-radiative recombination is found to be only slightly lower for the core/shell AIS/ZnS QDs than for their non-passivated AIS analogs in the first fractions with the highest PL QY (Fig. 5d). As the fraction number and Eg increase the knr difference between core AIS and core/shell AIS/ZnS QDs becomes more evident and for the smallest QDs the non-radiative recombination occurs almost two times slower after the QD passivation with the ZnS shell. These observations indicate that the PL enhancement effect of the ZnS shell may be interpreted as a result of both, an electron-hole interaction enhancement in the QD cores by the higher-bandgap outer ZnS layer as well as the formation of additional sites for the radiative recombination57, the latter effect becoming more pronounced for the smallest and Zn-enriched AIS/ZnS QDs. The PL lifetime is found to depend considerably on the PL registration wavelength, that is, on the PL energy. The emission rate increases by a factor of around two as the PL energy grows from around 1.95 eV to 2.35 eV and from 2.15 eV to 2.65 eV for the first (No. 1) and the last (No. 10) fractions studied, respectively (Fig. 6a). A complete set of the “lifetime – PL energy” dependences can be found in the SI (Fig. S13). Conventionally, such dependences are interpreted in terms of a higher probability of the radiative recombination between closer electron-hole (D–A) pairs1,44,50,52. The distancedependent Coulomb interaction between trapped charge carriers is supposed to contribute to the energy of the emitted PL quanta being higher for closer pairs. Based on the above discussion about the nature of the PL emission in AIS QDs we suggest an alternative interpretation of the “τ–hv” dependences which is consistent with the self-trapped exciton model and assumes the continuous dissipation of the excitation energy in the form of vibrational quanta till the event of radiative e–h recombination occurs. The longer a QD remains in the excited state, the larger portions of energy can be dissipated and the smaller the finally emitted PL energy is. In this interpretation, the PL energy loss divided by the difference in two radiative lifetimes between which the loss is observed, that is, the slope of a “τ–hv” plot, can be taken as a quantitative measure of the rate of phonon emission. Also, by normalizing this rate to the phonon energy we can evaluate an average time of a single phonon emission event.

Figure 6. (a) Radiative lifetime of size-selected colloidal AIS QDs from fractions no. 1, 6, and 10 as a function of the observation wavelength (PL energy); (b) average time of single phonon emission tsp (blue bars 1) and rate of the energy loss ∆hv/∆τ (red bars 2) determined for the size-selected AIS QDs as functions of the QD bandgap.

15



We find the rate of vibrational relaxation to be size-dependent increasing by a factor of almost two as the bandgap of AIS QDs increases from 2.4 eV to 2.92 eV (Fig. 6b, bars 2). The tsp time is also found to decrease from 18 ns for the largest AIS QDs to 9 ns for the smallest QDs (Fig. 6b, bars 1). These observations indicate that the phonon emission in the smallest AIS QDs occurs about two times faster than in the largest QDs in the studied series resulting in a larger relative PL energy loss during the time when the QDs remain in the excited state. In this view, the spectral parameters of the PL bands of AIS QDs are functions of two size-dependent parameters – the bandgap as a zero-phonon line of the PL spectrum and the rate of vibrational relaxation of a trapped charge carrier determining the maximum position and FWHM of the PL band. Hence, by using two different approaches, namely the analysis of the absorption/PL spectra in terms of the selftrapped exciton model and from the kinetic PL measurements we came to the same conclusion that the rate of vibrational relaxation in AIS QDs is size-dependent with its rate increasing approximately by a factor of two in the studied size range. Most probably, this size dependence also contributes to the above-discussed effect of the decrease of the radiative lifetime in series of size-selected AIS and AIS/ZnS QDs from the largest to the smallest QDs (see Fig. 5b).

Conclusions

We found that size-selective precipitation/redissolution can be successfully applied to GSH-based aqueous AIS and AIS/ZnS QDs producing broad (8–16 fractions) series of size-selected QDs with different emission colors and a maximum PL QY of 60% for the most populated fraction of the core/shell AIS/ZnS QDs which is among the highest reported for the direct aqueous synthesis of colloidal AIS QDs. We showed that the structure of the broad PL bands of AIS and AIS/ZnS QDs can be described by the model of self-trapped exciton implying that the PL band consists of a sequence of phonon replica of a zero-phonon line resulting from a strong electron-phonon interaction and a partial conversion of the excitation energy into lattice vibrations. These results also attest that the position and shape of the PL bands of AIS QDs originate not from energy factors (depth and distribution of trap states, etc.), but rather from the dynamics of the electron-phonon interaction and the vibrational relaxation in the QDs. The compliance with the self-trapped exciton model shows that the broadband PL emission in ternary AIS (and CIS44) QDs is an inherent property not necessarily associated with the defectiveness of the lattice and the QD surface and, therefore, we can expect the PL QY to be as high as reported for “ideal” cadmium chalcogenide QDs provided that the protocols of the synthesis and post-synthesis treatments are properly optimized. The rate of the conversion of the electronic excitation into vibrational modes in AIS QDs was found to be sizedependent, increasing almost twice from the largest to the smallest QDs in the studied size-selected series. This fact may indicate that the exciton trapping and coupling to the lattice vibrations results in a much higher distortion of the lattice of smaller QDs or, alternatively, that the trapping occurs on the QD surface periphery with the participation of the ligand shell which is expected to be of a higher efficiency for smaller QDs.

The Supporting Information (SI) file includes: details of characterization methods, absorption and PL spectral data on AIS and AIS/ZnS QDs produced at different Ag:In:S:Zn ratios and precursor concentrations, the mass distributions of QDs in the size-selected fractions, XRD patterns, TEM and AFM images of the original parental and the final size-selected AIS QDs, high-resolution XPS spectra of the AIS/ZnS QDs, In/Ag and Zn/In atomic ratios in the size-selected AIS/ZnS QDs obtained by ICP-OES, the results of modeling of the PL spectrum of CdS-PEI QDs, and “τ–hv” dependences for ten fractions of size-selected AIS QDs.

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Acknowledgements

This work was funded by the European Union’s Horizon 2020 research and innovation program under the Marie Skłodowska-Curie grant agreement No 701254 and by The Volkswagen Foundation (project “New functionalities of semiconductor nanocrystals by controllable coupling to molecules”).

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Origin and Dynamics of Highly-Efficient Broadband

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