Aliovalent Doping in Colloidal Quantum Dots and Its Manifestation on

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Aliovalent doping in colloidal quantum dots and its manifestation on their optical properties: surface attachment versus structural incorporation Alexandros Stavrinadis, J. S. Pelli Cresi, F. D'Acapito, Cesar Magen, F. Boscherini, and Gerasimos Konstantatos Chem. Mater., Just Accepted Manuscript • DOI: 10.1021/acs.chemmater.6b01445 • Publication Date (Web): 12 Jul 2016 Downloaded from http://pubs.acs.org on July 17, 2016

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Chemistry of Materials

Aliovalent doping in colloidal quantum dots and its manifestation on their optical properties: surface attachment versus structural incorporation Alexandros Stavrinadis1, J.S. Pelli Cresi2, F. d’Acapito3, César Magén4, F. Boscherini2,3, Gerasimos Konstantatos1,5,*

1

ICFO-Institut de Ciencies Fotoniques, The Barcelona Institute of Science and Technology,

08860 Castelldefels (Barcelona), Spain 2

Department of Physics and Astronomy, University of Bologna, Italy

3

Istituto Officina dei Materiali, Consiglio Nazionale delle Ricerche, c/o ESRF, Grenoble,

France 4

Laboratorio de Microscopías Avanzadas (LMA), Instituto de Nanociencia de Aragon (INA)

- ARAID and Departamento de Fisica de la Materia Condensada, Universidad de Zaragoza, 50018 Zaragoza, Spain 5

ICREA—Institució Catalana de Recerca i Estudis Avançats, Passeig Lluís Companys 23,

08010 Barcelona, Spain *[email protected]

Abstract Doping colloidal quantum dots (CQDs) with aliovalent cations is a promising yet underexplored approach to control the optoelectronic properties in CQDs. In CQD doping, kinetics determine whether a dopant element will incorporate into the host crystal structure, while thermodynamics dictate the mechanism of dopant incorporation. Here we show that those mechanisms can be readily monitored by simple optical measurements and XRD studies in CQD ensembles. Based on this, we outline the critical role of dopant solubility limit in CQD doping, bridging the gap between nanocrystalline and bulk semiconductors. Finally we present a combined simulation and XAFS data study to shed new insights on the origin of charge compensation upon doping in CQD materials that has thus far limited high doping efficacy even under efficient dopant incorporation schemes.

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1. INTRODUCTION Colloidal quantum dots (CQDs) is a class of nanomaterials whose unique optical properties has brought about a revolution in solution processed photovoltaics1,2, light emitting diodes3,4 and photodetectors5–7. These applications require photoactive semiconductor components with accurately controlled optical and electronic properties. The lack of control over the electronic properties of CQD solids, in particular their doping character, still acts as a roadblock towards more advanced device architectures and functionalities. Doping CQDs with aliovalent elements that can act as electron donors or acceptors is a very promising approach. Although aliovalent doping has been the cornerstone in doping single crystalline semiconductors to control carrier-type and density, very few prior works report electronic doping with aliovalent cations for CQD solids and a complete understanding of the mechanisms at play has yet to be developed. Several semiconducting CQDs (CdSe, InAs, PbS) have been doped with a range of elements either during the synthesis,8–12 postsynthetically in solution13–15, or via solid state surface treatments16–19. Yet a complete picture on the factors that determine the efficacy and mechanism of dopant incorporation in the host as well as the corresponding optoelectronic effects doping causes, still remain elusive. Dopant incorporation, the primary prerequisite step in doping, is governed by both kinetics and thermodynamics20–24. A relevant kinetic factor is whether the dopant precursor reacts fast enough during CQD growth, compared to the precursors providing the host CQDs´ elements20,21. Dopant incorporation is further determined by whether the dopant stays long enough on the surface of the as-grown CQD to react with the host lattice and be overcoated by the latter22. On the other hand, thermodynamics dictate the energy required for the formation of a specific dopant complex within the host lattice and thus its occurrence likelihood23,24. Following aliovalent dopant incorporation, a question that still remains is what determines the location of a dopant in the host material. The dopant site, e.g. whether the dopant is located on the surface or the core and whether it is substitutional or interstitial, will affect its optoelectronic impact on the CQD solid. An additional major question is how aliovalent dopants impact the CQD´s optical properties and structure. While the dopant site, along with the dopant´s valency and ionization energy, determine the doping character, other dopant-induced electronic defects may hinder active electronic doping11,15. The identification of macroscopically measured material properties, in


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particular optical ones, that may be associated with such imperfect doping cases, remains even more elusive. In this work, these questions are addressed for the case of aliovalent doping of PbS CQDs with a first exciton absorption peak position in the range of 1.29-1.45 eV and size dispersity (as measured by TEM) of approximately 12-14% (8-10% as deduced from optical absorption spectra). This material is used in high performance CQD optoelectronics, yet little is known about doping it with aliovalent cations and in particular trivalent cations that should act as ndopants upon substitutional doping. Our original hypotheses, which are supported by the aforementioned literature, are that: 1) doping incorporation should be primarily determined by reaction kinetics, and 2) thermodynamics dictate the incorporation site of the dopant. We subsequently utilize these hypotheses to select trivalent doping elements that are expected to behave differently in terms of incorporation efficiency. Our final goal is to study the impact of “successful” -in terms of incorporation efficiency- doping on the CQDs´ macroscopically probed basic structural and optical properties. We chose trivalent atoms to n-type dope PbS CQDs, inspired by prior reports of efficient in terms of physical incorporation, yet not very efficacious in terms of free electron generation, doping of Bi in PbS CQDs11. Bismuth however is difficult to monitor with characterization techniques appropriate for doping studies like X-ray absorption spectroscopy (XAS)25. Having Bi fluorescence emission lines close to those of Pb, an analysis based on XAS on these systems would be extremely difficult as the Bi signal would be overwhelmed by that of Pb. To circumvent this problem, we have identified In and Sb as more suitable markers. In and Sb, having emission features far from those of Pb, are more easily detected. Furthermore, Sb belongs to the same group as Bi while In does not, thus different doping behavior is expected for the two cases despite the fact that both have preferred oxidation state of +3 and similar ionic radii (0.76 Å and 0.8 Å for Sb3+ and In3+ respectively with VI coordination26). Indeed, from bulk semiconductor alloy studies it is known that while Sb2S3 can form a solid solution with PbS, preserving the PbS structure for Sb/Pb atomic ratios up to approximately 3%27,28, the same does not apply for In2S3 for which such miscibility has not been observed29. There are two important factors affecting active dopant incorporation by substitution in a host semiconductor structure, based on thermodynamics and kinetics: The first one is the extra energy needed for the dopant ion to replace the native ion22,24,30. Considering the different standard enthalpies of formation at 298.5 K of Sb2S3 (-141.8 kJ/mol)31 and In2S3 (-427 kJ/mol)32, and taking into account theoretical studies on the “doping bottleneck” effect for 3 ACS Paragon Plus Environment


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bulk semiconductors33,34, the lower formation energy of In2S3 compared to Sb2S3 suggests that achieving solubility of In3+ in PbS via cation substitution would be more energetically demanding than Sb3+. The second factor related to dopant incorporation is the time required for the dopant reaction/incorporation as compared to the timescale in which nucleation and growth of the host crystal takes place21,22,30. For cation doping of PbS CQDs, this overall kinetic factor would depend on the relative reaction rates of Sb3+ and In3+ individually with S2- as compared to the reaction rate of the latter with Pb2+. Considering that S2- and Pb2+ are categorized as a mild Lewis base and acid respectively21, Sb3+ (like Bi3+) as a borderline acid and In3+ as a hard acid, a higher reaction rate of S2- with Sb3+ would favor higher Sb3+ incorporation as compared to In3+. In short, initial considerations of either thermodynamic or kinetic aspects of the doping process both indicate that doping PbS CQDs with In3+ would be more difficult than doping with Sb3+. To further elucidate the role of the synthetic method in determining dopant incorporation in CQDs, we investigated different synthetic and doping methods based on in-situ and postsynthetic dopant-incorporation schemes. While the comparison between In and Sb is performed on the basis of adding the respective dopant precursor (metal acetate) to the standard Pb-oleate-containing precursor commonly used for the PbS CQD synthesis35, for the case of Sb we further investigated two additional synthetic approaches: (i) adding a small amount of oleylamine (OLA) in the precursor, as a secondary CQD ligand which -as described in more detail in the Results section- should bind preferentially to Pb instead to Sb and thus act as a relatively (as opposed to oleic acid) dopant insensitive regulator of the CQD growth process and thus CQD size, to demonstrate engineering of doped-CQDs´ properties beyond dopant density concentration, (ii) a post synthetic doping approach in which premade PbS CQDs are doped by reflux in a Sb-oleate precursor, aiming for surface-localized doping via a mild cation exchange process (CX)13,15.

2. Experimental Methods Chemicals Lead (II) oxide (99.999%), hexamethyldisilathiane (TMS) (synthesis grade), 1-octadecene (technical grade 90%), oleylamine (technical grade 90%), Indium (III) Acetate (99.99% trace metals basis), tetrabutylammonium iodide (TBAI, reagent grade, 98%), 1,2-ethanedithiol 4 ACS Paragon Plus Environment

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(EDT, technical grade >90%) and toluene (anhydrous, 99.8%) were purchased from Sigma Aldrich. Antimony (III) acetate (99.999%, metals basis) was purchased from Alfa Aesar. Acetone, acetonitrile and methanol were purchased from Panreac. Synthesis of PbS and doped PbS CQDs Oleic acid-capped PbS QDs with a first excitonic peak at 900–980 nm were synthesized with a modified version of a standard method35, using a standard Schlenk line as follows: 0.45 g of PbO was dissolved in 1.5 ml oleic acid and 3 ml octadecene under vacuum (0.2 × 10−1 mbar) overnight (16 h) at 95°C. Afterwards, 15 ml of octadecene were further added. Then, under Ar atmosphere, the temperature was raised to 120 °C, 0.21 ml TMS in 10 ml octadecene were injected, and the final solution was left to cool down to 36 °C. Then CQDs were isolated/purified by precipitation upon the addition of excess acetone, centrifugation and redispersion in toluene in normal atmospheric conditions. This cleaning process was repeated twice either with acetone or with methanol. In:PbS and Sb:PbS CQD syntheses were performed in the same way, with the addition of the appropriate amount of animony acetate (or indium acetate) in the original lead-precursor solution. The synthetic method also containing oleylamine in the precursor (OLA method), refers to the aforementioned standard method with the addition of 100 µl of oleylamine to the original cation precursor. Post synthetic doping of PbS CQDs with antimony, also referred to as the CX (cation exchange) method, was performed as follows: PbS CQDs dispersed in toluene (126 g/l) were transferred into a N2 glovebox. Using a Schlenk line, antimony acetate, the amount of which was selected according to the weight/molar amount of the PbS to be used and the intended Sb/Pb atomic precursor ratio (e.g. 0.005 g antimony acetate for the 4% Sb/Pb sample), was dissolved in 2 ml oleic acid and 4 ml octadecene at 95oC under vacuum overnight. The next morning the atmosphere of the Sb-precursor was switched from vacuum to Ar, and the temperature was lowered to 90oC, at which point 0.8 ml of the PbS CQD/toluene solution was added by injection. Subsequently, heating was removed and the flask was left to cool down. The final CQD product was cleaned similarly as above.

Material characterization The optical absorption spectra of 5 g /l or lower CQD/toluene solutions were measured with a Varian 5000 UV–vis-NIR spectrophotometer. For each of the presented optical absorption 5 ACS Paragon Plus Environment


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and photoluminescence comparative studies, the CQD/toluene solution concentration was the same for all samples within each individual dopant/method series studied. The centre and half width at half maximum (HWHM) of the first exciton peak for each sample, as seen in optical absorption measurements plotted against the optical energy E, were determined by fitting a Gaussian function setting as a low energy limit a measurement point located far from the peak (above 0.82eV) and as a short wavelength limit a measurement point located below the apparent position of the peak. Fittings were computed using the respective computational tool of Origin 8.1 software. Using the obtained peak parameters, the optically deduced dispersion σ of the average diameter dCQD of the CQDs was calculated using the following formula36: =

( ) √22 1ℎ ℎ

assuming that the average diameter dCQD of the CQDs is correlated to the optical energy Eo of the first exciton absorption peak according to the formula reported by Moreels et al.37 for PbS CQDs: = 0.41 +

0.0252

1 + 0.283

The photoluminescence (PL) spectra were obtained using a Horiba JobinYvon iHR550 fluorolog system equipped with a Hamamatsu R5509-73 photomultiplier tube detector and an integrated sphere (Quanta-phi) with liquid cell/cuvette holder. PL was acquired using excitation at wavelengths in the 650-750nm range and was always shorter compared to the position of the CQDs first exciton absorption peak. The excitation wavelength and intensity were the kept the same for all samples within each of the studied dopant/method series. All PL spectra were further normalized to the absorption of the samples at the excitation wavelength, as measured by the same set-up. Transmission electron microscopy (TEM) for samples made by drop-casting very thin CQD solutions on lacey carbon films was performed with a JEOL 2100 microscope operated at 200 kV and with an FEI Tecnai F30 microscope equipped with a field emission gun and operated at 300 kV. Scanning TEM images in high-angle annular dark-field mode (STEMHAADF) and Electron Energy Loss Spectroscopy (EELS) line profiles for chemical mapping of Sb in Sb:PbS CQDs were acquired in an FEI Titan 60–300 microscope operated at 300 kV 6 ACS Paragon Plus Environment

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and equipped with a probe aberration corrector to provide a spatial resolution below 1 Å. To minimize the negative influence of the CQDs´ organic ligand coating on the quality of images, before conducting STEM-HAADF imaging, as deposited CQD samples on the lacy carbon films were further immersed in a 10 g/l tetrabutylammonium iodide (TBAI)/ methanol solution for 60s and further washed with pure methanol. It is known that this process removes oleic acid from the surface of the dots and passivates the later with iodine anions16,17. Indeed, we found that this process resulted in a significant reduction of the carbon contamination observed by STEM-HAADF imaging, while the inter-dot spacing was reduced (dots appeared to be touching each other) and small scale super-lattice ordering of the dots (10-15 nm scale) was observed. XRD spectroscopy on several micron-thick QD films, which were additionally flattened by pressing against a glass surface, was performed using a Panalytical XPERT-PRO diffractometer equipped with Cu K-alpha (1.540598 Å) source and operated at a continuous mode with a 0.0394deg step size. Gaussian fittings of the (220) reflections were conducted using Origin 8.1 software. For each fitting, straight lines connecting the data points at 39o and 47o 2θ angles -which are located outside the peak- were considered as a linear background. The Gaussian fitting provides the centre of the peak. This is used to calculate the specific plane spacing dhkl according to Bragg’s law: λ=2*dhkl*sin(θ) where λ is the X-ray wavelength. We further calculate the lattice constant a of the crystal, from the relationship: a=(h2+k2+l2 ) 1/2*dhkl considering h=2, k=2, l=0 ICP-OES analysis of CQD powders dissolved in nitric acid was performed with a PerkinElmer Optima 3200 RL.

XAFS characterization, analysis and related DFT calculations



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Samples for XAFS were prepared by drop casting 150 µl of CQD/toluene solutions on clean glass microscope slides (12.5x25mm), with the concentration of the solutions varied in the 100-150 g/l range. In and Sb K edge XAFS measurements were performed at the “LISA” BM-08 beam line38,39 of the European Synchrotron Radiation Facility (ESRF) in Grenoble, France. The samples were measured in the fluorescence mode using a dynamically sagitally focusing Si (311) monochromator40, a 12-element hyper pure Ge detector and associated digital electronics with a 0.5 µs peaking time41. Higher order harmonics were eliminated using a pair of Pt – coated mirrors (Ecutoff = 32 keV). All measurements were performed at 80 K in order to reduce thermal damping of the signal. XAFS spectra in the extended energy range (EXAFS) were processed according to standard procedures using Athena 42. The pre-edge region was fitted with a linear function while the post – edge region was fitted with a quadratic polynomial to simulate the atomic cross section. As a first approximation, the energy origin for the energy – to – wavenumber conversion (!) was chosen as the maximum of the first derivative of the absorption

spectrum. The Fourier Transforms reported in Fig. A were performed in the range " = 3.0 −

11.5 Å&' with a k2 weight.

Quantitative data analysis was based on simulated signals calculated using the FEFF code43 and non – linear fitting using ARTEMIS42 based on the results of the ab-initio structural modeling44. The atomic coordinates of the Sb vacancy complex were used as a starting point to fit the XAFS spectra of the Sb:PbS samples. The spectra were fitted 42 with a combination of an oxide component (calculated as described above) and single scattering contributions due to the 1st and 2nd shell of the Sb vacancy complex, calculated using FEFF43. Fitting parameters were the relative amplitude of the oxide phase, an energy origin shift, distance variations and Debye Waller factors for the oxide contribution and 1st and 2nd shell of the vacancy complex; the fitting range was " = 2.0 − 11.5 Å&' , ) = 1.0 − 4.5 Å with a k2 weight. Best fits were found for small distance variations (a few 0.01 Å) and reasonable Debye Waller factors (~ 0.003 Å for the 1st shell and ~ 0.007 Å for the 2nd one). The fit of the 0.5% Sb:PbS sample is reported in figure 1c.


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DFT simulations of dopant complexes. We performed ab – initio calculations of the equilibrium atomic structure of three candidate complexes involving the dopants: simple substitutional in a Pb site (,-./ , , = 01 or 4), two substitutional dopants plus a Pb vacancy

(2,-./ + 5-.66 ) and two substitutional dopants plus a S interstitial (2,-./ + 0766 ); the simple substitutional site is electrically charged while the other two are neutral. The structure and formation energy of these complexes were calculated within the framework

of the Density Functional Theory (DFT) using the VASP code45. The PbS structure46 was described with a rhombohedral cell and a supercell of dimensions 3 × 3 × 3 (54 atoms, side a =12.7 Å, 8 = 9 = : = 60°) was considered for the calculations. Structural relaxation consisted in minimizing the total energy of the supercell by varying its dimensions and internal atomic coordinates. Calculations were done with projector augmented wave (PAW) pseudopotentials and the exchange-correlation functional used was the generalized gradient approximation (GGA)47. Plane waves were considered with a cut-off energy of 650 eV. The Brillouin zone was sampled using a 4 × 4 × 4 k-point mesh chosen with the Monkhorst-Pack scheme48. At each ionic step, the electronic structure was optimized until attaining a convergence of the total energy within 10-6 eV, whereas the ionic positions were optimized until Hellman Feynman forces were below 5 × 10-4 eV/ Å. The convergence of the cell energy (checked by comparing calculations with coarser k meshes 2 × 2 × 2 and 3 × 3 × 3) was about 0.01 eV. The validity of the method was checked on pure PbS, for which we found a cubic lattice parameter of aDFT = 5.997 Å that compares well (+ 1%) with the experimental value of aexp=5.934 Å46. The local structural parameters (number of atoms, N, at a given distance) around the dopant atoms obtained are listed in Supporting Table S1. To calculate the formation energies of the complexes, the chemical potential of each element involved (Sb, In S, Pb) was derived by supposing a chemical equilibrium among the following compounds: (Sb2S3 --structure from49--, S8 --structure from50--, PbS) and (In2S3 – structure from

51

--, S8, PbS) . The formation energy of each compound was calculated by

DFT and from these values the chemical potentials µS, µPb, µIn, µSb were derived. This scenario corresponds to a growth in S-saturated conditions with the metals at their minimum chemical potential. Successively, the formation energies of the various complexes were calculated as differences between the cell energy obtained by DFT and the sum of the chemical potentials times the number of related atoms in the cell52. Supplementary table S2 summarizes the formation energies of the complexes.



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Determination of the oxide ratio: The first aspect of data analysis focused on the determination of the oxide ratio. As previously mentioned, all the In doped samples exhibited an XAFS spectrum which indicated a predominantly oxide environment. As a confirmation, the first shell contribution of these spectra was fitted with an In – O first shell signal simulated on the basis of the crystallographic structure of crystalline In2O3, with excellent agreement, as illustrated in supplementary figure S6a for the 2 % sample. This fit was performed in the range " = 3.0 − 10.0 Å&' , ) = 1.0 − 2.5 Å with a k2 weight. This confirms that in all In – doped samples the dopant is bound to O atoms, either in an oxide or an oleate phase; distinguishing between the two is impossible based on XAFS. In order to estimate the fraction of Sb atoms in the oxide phase in the Sb doped samples, the XAFS spectra were fitted with a combination of simulated signals relative to Sb –O and Sb – S first shell single scattering signals. As a first approximation, the Sb – O signal was obtained from a simulation based on the crystallographic structure of crystalline Sb2O3. The Sb – S first shell signal was obtained from the DFT simulations described below. Fits were performed in the range " = 2.0 − 10.5 Å&' , ) = 1.0 − 2.5 Å with a k2 weight using as fitting parameters the relative amount of Sb in the two phases, assuming coordination numbers equal to 3 for Sb – O and 4 for Sb – S signals, respectively; in view of these approximations the obtained relative fraction of Sb atoms in the two phases must be considered as an indication and can be affected by a systematic errors of the order of 20 %. Very good fits were obtained in all cases; as an example we report a fit for the 2% Sb:PbS OLA sample in supplementary figure S6b. Supplementary Table S3 reports the oxide fraction (as a percentage) in Sb doped samples.

3. Results and discussion

Doping efficiency The dopant M/Pb ratio in the CQD samples was measured via inductively coupled plasma optical emission spectroscopy (ICP-OES) and compared to the respective ratio of the cation precursors used for the CQD synthesis, as shown in Figure 1a. For all series, the dopant amount in the CQDs increases almost linearly, with increasing dopant precursor concentration. However, indium, which reaches an atomic ratio In/Pb = 1% for a corresponding precursor ratio of 10 %, barely remains in the final materials. On the contrary, Sb incorporates in the material much more efficiently, resulting in more than 7% in the 10 ACS Paragon Plus Environment

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product for a 10% precursor ratio. This difference between the two dopants is the most striking indication of their different doping efficiencies. Sb dopes efficiently the final CQD material for all three synthetic methods used, yet the highest degree of Sb doping is observed for the post-synthetic doping approach (CX) for which Sb is expected to be located on the surface of the CQDs. The effect of dopant incorporation on the overall structure of the CQDs was evaluated via XRD. XRD patterns show absence of dopant related secondary phases (supplementary Figure S1), and no systematic relative change in the various XRD peaks intensities with increasing doping is observed. However, a detailed analysis of the 220 peak´s position (examples of analysed peaks are shown in supplementary Figure S2) indicates the appearance of a small yet systematic decrease of the PbS lattice constant with increasing dopant concentration, for all dopants and doping methods, as shown in Figure 1b. Three different causes of the decreased lattice constant should be considered: (i) a decrease in the size of the CQDs as has been previously observed for the case of PbS

53

, (ii) compressive surface stress induced by

“shelling” of the CQD surface with dopant species, (iii) incorporation of the dopant in the CQDs. Information about the change of either the crystal (CQD) size or of stress as a function of doping can be obtained from the change of FWHM of the XRD 220 peaks with increasing dopant ratio, as presented in Figure 1c. It must be noted that the changes observed in FWHM are not attributed to systematic experimental errors such as instrument broadening and slight configurational differences like beam focusing, sample penetration and goniometer optics. That is because all measurements were performed successively under the same configuration of the XRD instrument, all samples were of similar dimensions and morphology, and were positioned accurately using a dial gauge that allows for a ±10 µm maximum displacement. No significant peak broadening was observed for the respective position and maximum displacement and 2Θ ranges when calibrating the instrument and ensuring beam focusing using a silicon oxide powder reference sample. Thus, for our samples an increase of the FWHM as compared to the un-doped particles signifies a decrease of the CQD size and/or an increase of crystal strain. To further distinguish between the aforementioned factors we also have to consider the samples´ optical absorption exciton peak as a function of doping as shown in Figure 1d-f; a shift of the exciton peak position should primarily depend on the CQD size. The different series studied here help us evaluate the importance and synergistic action for each of the aforementioned effects. For the In dopant incorporation is minimum (as seen in Figure 1a), and a significant CQD size decrease is 11 ACS Paragon Plus Environment


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evident from figure 1d. The observed decrease of the lattice constant by 0.1-0.2 Å agrees well with previous reports for similar size-induced changes of the lattice constant of PbS CQDs53. Regarding the Sb CX series, in which as indicated by its absorption spectra (supplementary figure S3) the CQD size does not change appreciably, the small lattice constant change (Figure 2b) is caused by Sb adsorbed on the CQD surface, thus creating compressive stress as indicated by the small increase of the XRD FWHM and the small decrease of the lattice constant. We now turn our attention to the two series in which the CQDs are doped with Sb during the synthesis (with or without OLA). These series present the highest increase in the lattice constant with increased doping (Figure 1b). In those cases however, neither stress nor CQD size can explain the aforementioned change of the lattice constant. This is corroborated by the fact that for these series the FWHM of the 220 peak does not increase with doping, but quite the contrary: it decreases (figure 1c) rather significantly for Sb/Pb up to 4, then for Sb/Pb up to 10 it either increases again slightly (Sb series) or decreases more but at smaller rate (Sb, OLA series). The optical absorption spectra (Figure 1e,f) do not allow us to attribute the aforementioned XRD FWHM values to an increase of the CQDs size, since for both series the exciton peak is blue-shifted with increased doping. In fact, for the Sb OLA series for which the largest FWHM decrease is observed, we did additional size analysis using electron microscopy imaging (supplementary figure S4) and found that the CQD average size does not change appreciably ranging from 2.74 nm for the undoped sample to 3.02 nm for the most heavily doped (10%) sample, with a nearly constant size standard deviation of ±0.38 nm which would translate to a size dispersity ranging from 13.8 to 12.5% respectively. Therefore, for these series the decrease of the lattice constant should be attributed to a homogenous lattice distortion caused by incorporation of the Sb within the CQDs. We propose that Sb dissolves rather homogenously within the CQDs for Sb/Pb up to 4% and yields a material with a somewhat sharper XRD spectra; for higher concentrations (Sb/Pb=10%), Sb starts accumulating on the surface of the CQDs. When it does so, the CQDs are strained and this induces broadening of the FWHM of the XRD peaks. A remaining question is why the decrease of the FWHM in the Sb OLA series is more significant compared to the simple Sb series (no OLA). We propose that the answer is related to the different evolution of the CQD size with doping for the two series. The respective change is more dramatic when no OLA is included in the reaction. That manifests in a decrease of the CQD size. This would tend to increase the FWHM of the XRD peak, yet this effect is – 12 ACS Paragon Plus Environment

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seemingly- completely dissipated by the aforementioned structural impact from dilution of Sb within the PbS structure. Consequently, there is little change of the 220 peak´s FWHM across the Sb doping concentration study for the particular series.

Dopant complex In order to further determine the incorporation site of the dopants in the PbS CQD, we used XAFS

25,54–56

at the dopant atoms’ K – edge. We use the extended part of the spectrum

(EXAFS) and not XANES for the analysis because the XANES features at the K edge of Sb (and In) are very broad due to the limited core hole lifetime (9.2 eV for Sb)57, making this approach of limited applicability in this specific case (see also Supplementary Figure S5b). Details on the experiment, data treatment and analysis are reported in experimental methods section. All the In doped samples exhibited EXAFS spectra relative exclusively to an oxide phase (supplementary figure S6), thus confirming that In does not incorporate in the PbS CQDs, rather it aggregates/attaches on their surface and there is prone to oxidation. In Figure 2a we report the Magnitude of the Fourier Transforms (FTs) of Sb K – edge EXAFS for 1% doping for the Sb:PbS, Sb:PbS OLA and Sb:PbS CX samples. The peak at ) ≅ 1.5 Å is attributed to Sb – O bonds due either to Sb oxide or Sb oleate phases while the one at ) ≅ 2 Å is due to Sb – S bonds, as confirmed by detailed fitting. Figure 2a shows that in the CX sample most of the Sb is found in an oxide phase, thus confirming that the CX method leads to shelling of the CQDs with Sb containing species which are also prone to oxidation. The same figure though also acts as proof that in the other two Sb series, a significant (albeit not complete) fraction of the Sb enters the CQDs forming bonds with S and is protected from oxidation.

Figure 2b shows FTs for a set of Sb:PbS samples for Sb concentrations ranging from 0.5 % to 10 %. For ) ≤ 3 Å spectra are rather similar; however, the region between 3.5 Å and 4.5 Å becomes gradually more structured as the dopant concentration decreases. This interatomic distance range is that expected for the Sb – Pb second coordination shell for substitutional Sb in PbS. The appearance of a more structured signal in this R range with decreasing dopant concentration is suggestive of the formation of a well-defined dopant complex. At higher dopant concentration this is most probably obscured by the formation of a variety of more complex structures and / or precipitation of Sb – S phases.



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In order to gain insight in the nature of the incorporation site of Sb in PbS CQD we have performed an advanced analysis of the 0.5% Sb:PbS spectrum, basing our approach on ab – initio Density Functional Theory (DFT) simulations. We have considered three candidate complexes: a simple substitutional dopant in a Pb site (,-./ , , = 01 or 4), two substitutional

dopants plus a Pb vacancy (2,-./ + 5-.66 , the “vacancy complex”) and two substitutional

dopants plus a S interstitial (2,-./ + 0766 , the “interstitial complex”); the simple substitutional site is electrically charged while the other two are charge compensated (neutral). The atomic geometry of these complexes is schematically reported in Figures 2d-f along with the

calculated formation energies (detailed geometrical parameters and calculated energies in supplementary Tables S1 and S2, and additional observations in supplementary discussion S1). Here, additional complexes (e.g. based on interstitial dopants) are not considered since most probably these would have high formation energies, as already demonstrated in the case of 2,-./ + 0766 . Regarding the complexes we have considered, our calculations show that: (i) The Sb vacancy complex has the lowest formation energy compared to all others, (ii) the In vacancy complex has a significantly higher formation energy compared to the Sb one, (iii) both interstitial complexes involving either Sb or In have a higher formation energy compared to all others, and (iv) for simple substitution of Pb with Sb or In, the formation energies are very close. This last point indicates that the inefficiency of In incorporation is not thermodynamically hindered, but that it is likely inhibited by its reduced reactivity rate with sulfur as compared to lead and antimony since indium is a harder Lewis acid. On the other hand, for Sb, that does incorporate in the CQDs, the formation energy related to every possible complex should determine which complex will indeed be formed. In order to confirm the formation of the Sb vacancy complex the atomic coordinates obtained from the DFT calculations were used as an input for the analysis of the Sb EXAFS data for the 0.5% Sb:PbS sample, using non-linear fitting. From our analysis, a poor comparison with the experiment was found for the simple substitutional complex (see Fig. S7). Instead, a good agreement is found for the neutrally charged vacancy complex 201-./ + 5-.66 . The 0.5% Sb:PbS spectrum is best fitted (methodology described in experimental methods) with a combination of an oxide phase and the 201-./ + 5-.66 complex as reported in Figure 2c (supplementary table S3). A number of features confirm our fitting model: it perfectly reproduces the first shell peak at 2 Å and the second shell region is also well fitted (the 2.53.5 Å region which however contains little physical meaning).


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Overall our data and analysis demonstrate that in Sb:PbS samples, significant amount of Sb is incorporated in a well-defined site in the PbS matrix. The fact that a charge neutral complex is thermodynamically favored over a pure substitutional complex also suggests that while PbS CQDs are heavily loaded with dopants, only a small fraction of them form (pure substitutional) complexes that would donate electrons in the host. The relative number of dopant atoms in the oxide phase was further determined by quantitative fitting (between 1.0 and 2.5 Å)., and that number ranged from a minimum of ∼ 10 % for low doped Sb:PbS and Sb:PbS OLA samples to ∼ 90 % for Sb:PbS CX samples. Examples of fittings performed for oxide analysis are shown in supplementary Figure S6, and determined oxide ratios for all dopants and series are given in supplementary Table S3. Sb at high doping concentrations (Sb/Pb=10%) is mostly oxidized due to exceeding the solubility limit of Sb in the PbS cores, similar to bulk studies27. As a result, the excess Sb stays on the surface of the CQDs and is prone to oxidation. A similar effect has been recently presented for InAs CQDs doped with Ag 55. Before closing the discussion on Sb incorporation in PbS, it must be mentioned that this was also confirmed at a single dot level using electron energy loss spectroscopy (EELS) on heavily doped 10% Sb:PbS CQDs, the surface of which was passivated with iodide. Iodide was purposely applied as a surface ligand replacing oleic acid in order to reduce the samples´ organic content, which is detrimental for this measurement. In spite of the small amount of Sb doping in the CQDs, Figure 2g shows a cumulative spectrum showing the presence of the Sb M4,5 edge. Furthermore, integrated intensities of Sb signal along the line profile marked in Figure 2h demonstrates that Sb signal appears in the CQDs positions and it is virtually absent in the spaces between them.

Taking into account the results discussed thus far we can conclude that: 1) The different incorporation efficiency of In and Sb is most likely dictated by the kinetics of the synthetic reaction growth. 2) For Sb which does incorporate in PbS effectively, thermodynamics dictate its introduction as substitutional impurity accompanied by additional cation vacancies that yield a charge neutral doping mechanism. 3) A dopant-host solubility limit, similar to the case of bulk crystals, is confirmed for Sb.



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4) Post synthetic treatment of the CQDs with Sb does not yield effective incorporation of the dopant in the CQD core, thus dopant-induced optical effects should also be related to dopants (and electronic states) located on the surface of the CQDs.

Optical signatures of doping efficiency As already discussed, In- and Sb- doping schemes were found to impact differently the optical absorbance (Abs.) of the QDs as shown in Figures 1d-f, as well as their PL as shown in Figures 3a,b . Comparison of Figure 1d with Figures 1e,f shows that a progressive increase of Sb leads to a shift of the exciton Abs. peak to higher energies with simultaneous broadening and an overall smearing of the exciton peak. A progressive increase of In also shifts the exciton peak, yet it does not cause any broadening or smearing. At the same time, while the PL with In doping remains unaffected or even slightly increases as shown Fig. 3a, the PL of Sb, as shown in Fig. 3b, is quenched with increasing dopant concentration. The aforementioned effects confirm that Sb enters the CQD structure while In does not. The progressive shift of the exciton peak to higher energies with increasing dopant concentration can be ascribed to the reduction of the free oleic acid concentration during the synthesis of the CQDs, since part of the oleic acid is consumed in forming M-oleate complexes36, in agreement with prior reports58. For our experiments, considering that In and Sb, both having a valency of +3, should consume the same amount of oleic acid, equimolecular amounts of the two dopants should reduce the size and thus blue-shift the excitonic peak of the produced CQDs in the same manner. To further examine the interpretation of the absorption spectra, Figure 3c presents the peak position with varying M/Pb precursor atomic ratios (peak position identified by fitting the peak with a Gaussian function). For both In and Sb, the peaks shift linearly with the M/Pb ratio following nearly the same rate dependence, further corroborating our hypothesis that those shifts are due to oleic acid consumption by the dopant precursors. On the other hand, when oleylamine is introduced in the synthetic reaction, it provides additional -and independent to the presence of dopant- control over the size of the nanocrystals, suppressing the shift of the exciton peak position in the Sb:PbS OLA series. This happens because oleylamine (OLA) is considered a softer (borderline) Lewis base compared to oleic acid59. We thus expect it to have greater affinity for the softer Lewis acid among Pb and Sb; that would be Pb. This is why oleylamine acts as relatively dopant insensitive regulator of the CQD growth process and thus CQD size. 16 ACS Paragon Plus Environment

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The shape of the exciton Abs. peak of the doped CQDs, which is different for the Sb and In series, may depend on a variety of factors: i) the effect of the doping process on the kinetics of the CQD growth and thus the CQDs´ size dispersion, ii) the effect of the dopant incorporation on the electronic structure of the CQDs, iii) the variation of the dot-to-dot dopant concentration according to a Poisson distribution

60

.

These factors should be

irrelevant for the case of the In:PbS as long as indium does not participate in the CQD structural formation. Evidence for this is provided by the optically deduced size dispersion σ, calculated using the centre of the exciton absorption peak and FWHM values (details in experimental methods) of the In:PbS CQDs, which does not increase with increasing In/Pb precursor ratio (Figure 3d). Indium is thus considered to affect the growth of PbS only by consuming some of the oleic acid, and its presence in the CQDs is limited to surface attachment. The latter may be responsible for the increase of the PL for the In-doped CQDs as seen in Figure 3a. On the other hand, the optically deduced dispersion σ for Sb:PbS does increase with increasing Sb/Pb ratio (Figure 3d) for both the OLA and the neat (no OLA) series, and we consider this as an optical signature of Sb incorporation in the PbS CQDs. Comparison of the exciton peak position and dispersion evolution for the Sb:PbS with and without oleylamine (OLA) - illustrated in Figures 3c,d - shows that OLA addition affects the evolution of the peak position by making it less pronounced, but does not affect the peak dispersion with increasing Sb:Pb ratio. OLA thus serves as a way to fine-tune the size of the CQDs without affecting further Sb incorporation. In our discussion thus far, we have avoided explicitly assigning the increase of the optically deduced σ of the Sb-doped samples to an actual increase of the CQD size dispersion because: i) the PL peak of all Sb:PbS samples (Figures 1e and suppl. Figure S1b) is not blue shifted despite the fact their absorption peak is blue-shifted and that both absorption and PL of In:PbS are also blue shifted. ii) The size histograms of supplementary Figure S4 don’t exhibit a spread in the size distribution. iii) Absorption peak smearing is also observed for the CX series (supplementary Figure S3), for which post synthetic doping conditions are unlikely to lead to an increase of size dispersion of CQDs, and iv) as discussed above, the FWHM of the 220 XRD peak is decreased with increasing Sb amount (Figure 1c) . Therefore, to explain the exciton optical absorption peak for the Sb-doped CQDs, we propose that the specific feature is associated with effective broadening of the band-edges of the dots upon doping, due to the addition of dopant (impurity) states. These states should be mostly localized and therefore should not be affected by quantum confinement as much as the delocalized states of the host 17 ACS Paragon Plus Environment


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matrix24. Therefore, while these dopant states may not change (significantly) the impact of quantum confinement on the centre of the absorption peak, their presence will affect PL peak and intensity, because these states may mediate radiative recombination of photo-excited electron-hole pairs. This has been confirmed for PbS CQDs doped with bismuth61 and we expect it to also occur for Sb doping. In addition, charging of these states with electrons may affect the electronic structure of the host lattice and the oscillatory strength of band gap optical transitions. It is important to emphasize that such proposed dopant-induced states may not exclusively originate from inclusion of the dopant inside the CQD core, since they may as well be related to dopants located on the surface of the CQDs, as was observed for the CX series and in previous studies61. Based on the analysis of the optical properties, the following conclusions are outlined: 1) The efficient incorporation of trivalent dopants (Sb vs. In) in PbS CQDs is optically manifested by a combination of PL quenching and exciton absorption distortion. 2) When dopant incorporation is efficient (Sb vs. In) these optical effects (PL quenching and exciton absorption peak distortion) are present regardless of the incorporation sites of the dopants (surface vs. core for Sb doping). 3) The shape of the exciton absorption peak is more important in revealing dopant incorporation as compared to the position of the peak centre.

4. CONCLUSIONS

In this article we have shown that the efficiency and impact of doping CQDs with trivalent cations are very sensitive to the specific cation used. Significant structural features can be associated with whether the dopant is dissolved within the CQDs or simply attached to their surface. Furthermore, even when the dopant is incorporated within the CQDs, it may form charge neutral complexes. This may further explain why heavy charging of PbS CQDs via heterovalent cation doping has thus far remained elusive. While advanced experimental techniques are needed to confirm such rules in every individual host-dopant case, we have provided evidence that even in the case of having charge neutral dopant complexes, elemental doping efficiency may be monitored by two simple optical features: the shape of the optical absorption and the strength of the PL peaks for the band-edge transitions.


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Contributions of Authors. A.S. and G.K. designed and coordinated the research and edited the manuscript after receiving contributions, input and approval for publication from all co-authors. A.S. synthesized the CQDs, prepared samples for all characterization techniques, fabricated devices, and performed optical measurements and optoelectronic characterization of devices. F.B., J.P.C, F.d`A performed XAFS measurements, DFT calculations and respective analysis. C.M. performed STEM-HAADF and EELS measurements and respective analysis. G.K. supervised the study.

Acknowledgements We thank Giacomo Rossi for help during the experiment.

Funding Sources The research leading to these results has received funding from the Fundació Privada Cellex, and the European Community's Seventh Framework program (FP7-ENERGY.2012.10.2.1) under grant agreement 308997. We also acknowledge Financial support from the Spanish Ministry of Economy and Competitiveness (MINECO) and the “Fondo Europeo de Desarrollo Regional” (FEDER) through grant MAT2014-56210-R as well as the Severo Ochoa Programme for Centres of Excellence in R&D (SEV-2015-0522). This work was also supported by AGAUR under the SGR grant (2014SGR1548). Measurements were performed at ESRF within the public user program. 19 ACS Paragon Plus Environment


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Supporting Information

Supplementary figures with XRD spectra and related peak fitting examples, additional optical absorption and PL spectra, TEM and HAADF micrographs and CQD size analysis, FFT of XAFS spectra, XANES spectra, fits of the first shell XAFS signal, supplementary discussion regarding interatomic distances, supplementary tables regarding local structure around atoms, calculated energies of formation of complexes, percentage of Sb dopants in oxide phase.

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Figure 1 Efficiency and impact of doping process of structure and optical properties. (a) Comparison of dopant M/Pb atomic ratio for different dopants and synthetic methods. The measured from ICP-OES values of the CQD products are plotted against the calculated values of the precursors used for synthesis. (b) Comparison of the relative change of lattice constant of doped CQDs with increasing dopant concentration with respect to un-doped PbS CQDs, as calculated from the centre of the 220 XRD peak, and (c) respective change of the FWHM of the XRD 220 peak (lines are guides to the eye). (d) Opt. absorption of In:PbS showing blue shift or exciton peak with doping. (e) Opt. absorption of Sb:PbS showing blue shift and distortion with doping. (f) Opt. absorption of Sb:PbS synthesized with oleylamine (OLA) in the reaction showing distortion but little blue shift with doping. (Within each series of samples shown from (d) to (f), absorption spectra were taken for equal CQD/toluene solutions.)



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Figure 2 Incorporation of dopants in the CQDs. (a-c) Magnitude of the Fourier Transforms of Sb K-edge XAFS. (a) for 1% doped PbS CQD samples; (b) for Sb:PbS samples, various compositions; (c) data (black continuous line) and fit (red line with markers) for 0.5% Sb:PbS sample. (d-f) Graphical representations and calculated formation energies for In and Sb dopants of the local atomic structure of the three dopant complexes considered: (d) substitutional; (e) double substitutional plus Pb vacancy; (f) double substitutional plus S interstitial (Yellow: matrix S; dark grey: matrix Pb; purple: dopant (Sb or In); dark blue: S interstitial; light blue: Pb vacancy). (g) EELS spectrum around the Sb M4,5 edge position obtained averaging an EELS line profile acquired along the red arrow in (b) of the CQDs with 10% Sb:PbS. (h) Integrated intensity of the Sb M4,5 edge obtained along the line profile marked with a red arrow over the reference HAADF image. Scale bar is 5 nm.


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Figure 3 Optical signatures of the dopants´ bulk incorporation. (a) Normalized absorption photoluminescence (PL) of In:PbS showing lack of quenching with doping (PL spectra are normalized to absorption at excitation wavelength). (b) Normalized PL of Sb:PbS showing strong PL quenching with doping. (c) Change of Opt. absorption peak centre with varying doping concentration for different dopants/methods. (d) Change of optically deduced size dispersion with increasing doping for different dopants/methods.



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TOC


Aliovalent Doping in Colloidal Quantum Dots and Its Manifestation on

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