Ligand-Dependent Morphology and Optical Properties of Lead Sulfide

Oct 4, 2016 - ABSTRACT: Small angle X-ray scattering, atomic force microscopy, optical microscopy, and absorption and lumines- cence spectroscopy have...
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Ligand-Dependent Morphology and Optical Properties of Lead Sulfide Quantum Dot Superlattices Elena V. Ushakova,*,† Sergei A. Cherevkov,† Aleksandr P. Litvin,† Peter S. Parfenov,† Dominika-Olga A. Volgina,† Igor A. Kasatkin,‡ Anatoly V. Fedorov,† and Alexander V. Baranov† †

ITMO University, 49 Kronverkskiy pr., Saint-Petersburg, 197101 Russia Saint Petersburg State University, 7-9 Universitetskaya nab., Saint-Petersburg, 199034 Russia

‡

ABSTRACT: Small angle X-ray scattering, atomic force microscopy, optical microscopy, and absorption and luminescence spectroscopy have shown that the type and amount of surface organic ligands have a strong impact on the morphology and optical properties of 2D superlattices formed by lead sulfide quantum dots (QDs) of different sizes deposited on a glass substrate. This allows control of the process of QD self-assembly, the homogeneity of the superlattices, and the interparticle distance by optimization of the amount of surface ligands. It has been found that annealing and aging also influence the stability of the structural and optical properties of the superlattices.

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INTRODUCTION Nowadays semiconductor nanostructured materials are actively used in a number of applications in photonics, photovoltaics, biology, and medicine.1−4 Colloidal nanocrystals or quantum dots (QDs) are an important class of semiconductor nanostructures which offer the unique possibility of tuning the energy band gap from the near IR to the visible region by varying the QD size and chemical composition. Colloidal QDs can form ordered superlattices (SLs) on solid substrates by means of self-assembly. Such structures are commonly used as an active element in photodetecting or light harvesting devices and phototransistors. The quantum dot superlattices (QDSLs) formed by colloidal QDs may be further used to design multicomponent layered heterostructures for high-efficiency cascade solar cells.5−7 Currently, solar cells based on QDs have relatively low efficiency, up to 9.6%,8 as compared to Si analogues. This value is limited by the optimization of the device architecture, the use of hot charge carriers, and increasing efficiency of the charge recombination process.9 The recombination processes can be suppressed by efficient QD surface passivation that reduces the number of trap states,10−12 by increasing the mobility of the charge carriers13,14 or by improving the SL morphology and optical properties.15,16 Recent research has shown that QD surface passivation can be efficiently achieved by the proper choice of the type of organic and inorganic molecules covering the surface of the QDs in colloidal solutions. However, the role of the QD ligands and their impact on the SL morphology and optical properties is still an open question. Narrow band gap lead sulfide QDs are currently most widely used for the fabrication of NIR photodetectors and third generation solar cells.8,13,17,18 With an energy band gap of 0.41 © 2016 American Chemical Society

eV, a high extinction coefficient, small and equal values of electron and hole masses, and the realization of multiexciton generation regime, these materials offer great perspectives for both light harvesting and charge transport.19,20 Although the formation of PbS QD superlattices has been observed by several research groups,15,16,21−24 a comprehensive analysis of their formation and aging as well as of optical properties is still absent. In this work we report the effect of the type and amount of organic ligands on the morphology and optical properties of densely packed structures formed by PbS QDs of different sizes on a glass substrate and the stability of the morphology and optical properties under aging and annealing procedures.

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EXPERIMENTAL SECTION Materials. PbS QDs with sizes ranging from 3.2 to 8.4 nm were synthesized by hot-injection in an organic solution.25 Oleic acid (OA) molecules were used as organic ligands. The type and amount of ligand was varied by ligand exchange. This is usually performed by chemical treatment of the QD surface in colloidal solutions.15,16,26 This method was used to change the amount of OA molecules on the QD surface or to replace them by trioctylphosphine oxide (TOPO). Briefly, QDs were precipitated with methanol and then redispersed in tetrachloromethane (TCM) with the addition of the required amount of ligand molecules. The successful ligand exchange in the QD samples was confirmed by FTIR spectra. It was found that three procedures of treatment of the initial QD solution with Received: August 1, 2016 Revised: September 30, 2016 Published: October 4, 2016 25061

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Figure 1. QDM0T1 samples: (a) SAXS patterns of the samples with different QD diameters, DQD. The inset shows a comparison of the SAXS patterns of samples formed from the initial QD solution (dashed line) and QD3.4M0T1. (b) Transmission microphotograph of the QD3.2M0T1. Scale bar is 50 μm. (c) Sketch of the structure formed by TOPO-capped QDs. (d) Absorption (ABS) and normalized photoluminescence (PL) spectra of QD5.3M0T1 compared with those of the initial QD solution.

Figure 2. SLs QDM0A1: (a) SAXS patterns of SLs with different QD diameters, DQD. The inset shows a comparison of the SAXS patterns of QDSLs formed from the initial QD solution (dashed line) and SL QD3.4M0A1. (b) Transmission light microphotograph. Scale bar is 50 μm. (c) Sketch of the structure formed by OA-capped QDs. (d) Absorption (ABS) and normalized photoluminescence (PL) spectra of SL QD3.4M0A1 in comparison with initial QD solution.

Methods. The spectral analysis of the samples was performed using a Shimadzu UV-3600 spectrophotometer and an IR spectrofluorimeter.28,29 The type and amount of organic molecules on the QD surface were determined using a Bruker Tenzor II FTIR spectrophotometer. Small angle X-ray scattering (SAXS) and wide angle X-ray scattering (WAXS) curves were recorded with a Bruker D8 Discover X-ray diffractometer using a parallel beam of Cu Kα radiation in reflection θ−2θ mode. The QD core size was calculated from peak width by Sherrer’s equation.30 The distance between QD centers (SL lattice constant), L, was calculated from the angular position of the interference peak according to Bragg’s law. The QD ensemble morphology was studied using a Zeiss Merlin electron microscope operated at 15 kV with the working distance WD = 2.7 mm (STEM mode) and an NT-MDT atomic force microscope (AFM). Transmission light microphotographs of the samples were obtained with an Olympus BX-51 confocal microscope equipped with a 100× (NA = 0.95) objective. All spectral measurements and sample imaging were performed in multiple areas of each sample to obtain representative data.

methanol led to almost complete removal of OA molecules from the QD sample. SLs were formed by the self-organization of QDs on a glass substrate. The glass slides were previously washed with acetone, boiled in isopropanol for 10 min and finally allowed to dry at ambient conditions. To form SLs, 5 μL of the appropriate QD solution were dropped three times on the substrates preheated to 50 °C, resulting in a concentrated film with lateral size up to 5 mm. Each sample was then placed on a stove at 50 °C to evaporate the solvent. Different samples were designated as QDDDMPXNN, where DD is the QD diameter (in nm), MP indicates methanol treatment, where P is the number of times the sample has been treated, and XNN is the type of ligand (X: T, TOPO or A, OA) with NN being the molar ratio of ligand molecules to Pb atoms on the QD surface. For example, QD3.2M3A0.2 corresponds to a SL sample formed by 3.2 nm QDs obtained from QD solution treated three times with methanol; OA molecules were used as capping agent with the molar ratio OA/Pb = 1/5. Each set of samples MPXNN included four specimens formed by QDs of different diameters. It should be noted that QD solutions with a high amount of OA molecules (OA/Pb molar ratios >0.5) formed drops of a OA/TCM mixture which were nonevaporable. High-temperature annealing of the sample in vacuum at the boiling point of the organic ligands27 leads to a decrease in the interparticle distances in the SL, thus resulting in an increased conductivity. However, there is a lack of information on the morphology and optical properties of the thermally treated SLs. In this work, SLs were obtained from the initial OA-capped QDs in colloidal solutions as described above and then annealed at 380 °C for 1 and 30 min in vacuum.

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RESULTS AND DISCUSSION

QD Ensembles from TOPO-Capped QDs. SAXS patterns of all of the QDM0T1 samples are shown in Figure 1a. Very wide interference peaks (fwhm of the peak in 2θ is larger than 0.4 deg) are observed for the QDs of diameter smaller than 5.3 nm, and no interference peak is found for the 8.4 nm QDs. The mean interparticle distances in the samples QD3.2M0T1 and QD5.3M0T1 were 12.6 and 17.0 nm, respectively. The interparticle distance in the QD ensembles formed by TOPO-capped QDs is significantly larger than that in the 25062

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The Journal of Physical Chemistry C samples obtained from the initial QD solution (inset in Figure 1a). The large widths of the SAXS peaks suggest no long-range ordering in these samples. This is confirmed by the microphotographs in Figure 1b. The QDs are assembled in dendrites which are typical of the structures formed with TOPO molecules31 (see Figure 1c). The absorption band of these samples was found to be blueshifted as compared to the initial QD solution. This is likely caused by the oxidation of the surface during the formation of ensembles on the substrate at ambient conditions, resulting in shrinkage of the QD core. The PL band is slightly broadened, and its peak position is almost the same as compared to the spectrum of the solution. An increased PL Stokes shift is expected for the QDs with smaller core size.32 These absorption and PL band modifications are illustrated in Figure 1d for the sample QD5.3M0T1. QDSLs from OA-Capped QDs. The SAXS patterns of SLs QDM0A1 of different diameters are shown in Figure 2a. In contrast to the TOPO-capped QDs, a narrow peak corresponding to the densely packed ordered QD ensemble is observed in each pattern. Its position and width depend on the lattice period and homogeneity of SL, respectively.15 The peak shifts from 1.65 to 1.02 deg with increasing QD size, and the interdot distance decreases from 2.1 to 1.7 nm. The narrowing of the diffraction peak of SLs QDM0A1 as compared to SLs formed by the initial QDs shown in the inset in Figure 2a indicates improved SL homogeneity in the QDM0A1 samples. These samples contain SLs with lateral sizes up to tens of microns as shown in microphotograph in Figure 2b and sketched in Figure 2c. SAXS patterns show that the interdot distance in these samples is larger than 1 nm. This distance is too large for an efficient charge transfer along the SL,32 and therefore these SLs are not well suited for photovoltaic applications. Absorption and PL spectra of the SLs from OA-capped QDs were compared with those from the initial QD solution (Figure 2d). In all samples the absorption peak is blue-shifted by more than 100 nm, which is much larger than the corresponding shift in the SLs formed by TOPO-capped QDs. This is due to the formation of those SLs in “oily” drops with an excess of OA molecules that led to a stronger oxidation of QD surface and, therefore, to a more pronounced reduction in the QD core diameter. However, the PL peak undergoes the same shift. The reason for this is not yet clear. The most probable explanation may be related to the fact that the dielectric constant of the “oily” drop differs substantially from that of the initial QD solution. Although OA surface ligands appeared most suitable for the formation of homogeneous SLs, the effect of the amount of capping molecules on the morphology of the SLs remains unclear. Different scientific groups studied the effect for the QDs with sizes less than 3.5 nm21,22 or larger than 10 nm.24 In this work we tried to fill this gap. To optimize the morphology of the SLs, we used consecutive treatments with small amounts of methanol to decrease the number of OA molecules present on the surface and thus decrease the interparticle distance in SLs. The reduction of the quantity of OA molecules was supported by FTIR spectra of the treated samples (see inset in Figure 3a) showing a decrease in the intensity of the C−H lines at 2800−3000 cm−1 associated with OA molecules. The distance between the particles in the SLs formed by OA-

Figure 3. Interparticle distance in SLs as a function of the number of chemical treatments of the QD surface. The inset in (a) FTIR spectra of SL QD4.3 with different concentrations of OA. STEM images of SLs formed by 4.3 nm OA-capped QDs: (b) initial, (c) M3A0. The inset in c is a STEM image of a close-packed QD structure; scale bar is 50 nm.

capped QDs after different numbers of methanol treatments is shown in Figure 3a. SAXS patterns show that the methanol treatment influences the SL morphology. As shown in Figure 3a, the step-by-step ligand removal with minimal additions of methanol results in the desired decrease in the interparticle distance in SLs of all QD sizes. In SLs formed by the smallest dots (D = 3.3 nm) the interdot distance is around 1.8 nm that is close to the OA chain length. The methanol treatment of the QD surface does not lead to notable changes in the SL morphology. In the SLs formed by medium sized QDs (D = 4.3 and 5.3 nm) the methanol treatment leads to improved homogeneity of the SLs and shorter interdot distances, down to 0.8 and 0.5 nm, respectively. Smaller interdot distances allow fast charge carrier transport by a tunneling/hopping mechanism along the SLs and thus provide an opportunity to utilize these SLs as active components of photovoltaic devices.33,34 In SLs formed by the 8.4 nm QDs, the interdot distance gradually decreases down to about 1 nm upon QD surface treatment. The structural modification of the SL is confirmed by the STEM images shown in Figure 3b and 3c for the OA-capped SLs QD4.3. The SLs formed by the initial QD show domains with a hexagonal 2D arrangement of the particles (Figure 3b). The size of the domains varies from 15 to 30 nm. The QD4.3M3A0 sample contains two types of areas: submicron sized close-packed 3D structures (Figure 3c) and 2D structures surrounding those large structures (inset in Figure 3c). It can be seen that the QDs connect to each other and form chains which appear as black curves. The step-by-step chemical treatment results in the modification of the PL spectra of the samples, while their absorption spectra remain virtually unchanged. This is shown in Figure 4 for the superlattice QD4.3MPA0. It can be seen that the position of the absorption band remains nearly unchanged and close to that of the QD initial solution with an increasing number of surface treatments. This shows that the proposed protocol of mild chemical treatment does not destroy the QD surface with reduction of its size. In contrast, compared to that of the QD initial solution, the PL band of the SLs undergoes an increasing red shift with an increasing number of methanol treatments. This suggests that the shift is caused by energy transfer between closely packed QDs within the SL.35 The shorter the interparticle distance in the SL, the larger the PL shift. At the same time, the PL decay time is shortened with increasing number of methanol treatments (inset in Figure 4). This is possibly related to the increasing amount of surface traps with the removal of OA 25063

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Figure 5c and Figure 5d. When compared to the images of the initial and methanol-treated SLQDs, this suggests that the methanol treatments destabilize the QD solution that leads to the assembly of the QD into large close-packed structures, not the layered structures. Our findings show that the morphology and optical properties of SLs formed by the 4.3 and 5.3 nm QDs can be substantially controlled by the number of OA molecules on the particle surface. The effect of methanol treatment and OA addition on the absorption and PL band shift in SLQD4.3 and SLQD8.4 is illustrated in Figure 5e. It can be seen that the increase in the number of OA molecules on the QD surface results in a blue shift of the absorption and PL bands in comparison with those of the initial QD solution. The shift is more pronounced for the QDs of medium size. The presence of OA molecules on the QD surface leads to a decrease in the QD core size which may indicate strong oxidation and formation of a PbO layer on the QD surface. At the same time, the ligand amount on the surface of larger QDs does not strongly influence their absorption and PL band positions. This may be caused by smaller surface-tovolume ratio in larger QDs, and, hence, lower impact of the ligand layer on their optical properties. SLs formed by the smallest QDs have very broad absorption and PL bands with large shifts. This is probably related to the high surface-tovolume ratio in smaller dots. However, the values of the bandwidth and shifts are strongly scattered that makes the control of optical properties of the SL formed by the smallest QDs questionable. Impact of Aging on the QDSL Optical Properties. For successful utilization, the nanostructured materials based on QDs must be stable at ambient conditions. It was shown that the most harmful process influencing the long-time stability of a QD ensemble is the oxidation of the QD surface.36−38 However, the influence of OA on the long-time stability of the optical properties of SL was not studied. We have found that the SL absorption and PL band positions were unstable

Figure 4. Absorption (ABS) (solid curves) and PL (dashed curves) spectra of SL QD4.3MPA0: P = 1 (black), P = 2 (green), P = 3 (magenta). Absorption and PL spectra of the initial QD solution (blue curves) are shown for comparison. Inset: PL decay of QDSLs.

molecules from the QD surface that provides additional nonradiative channels for energy dissipation. For quantitative estimations, we have also investigated the effect of addition of a known amount of OA molecules to the QD surface after three procedures of methanol treatment. The increase in the amount of OA molecules was estimated from FTIR spectra. In the SLs formed by the smallest and largest dots studied, the addition of OA molecules did not lead to noticeable changes in the SL morphology, as demonstrated by the SAXS pattern presented in Figure 5a for SLs formed by 8.4 nm QDs. In the SLs formed by medium sized QDs (4.3 and 5.3 nm) the addition of OA molecules results in a slightly larger interdot distance. This is illustrated in Figure 5b for SLs formed by 4.3 nm QDs. With a small increase in the OA amount, the QDs form 2D lattices which are arranged like layers on the substrate. This is illustrated by the STEM and AFM images of the sample SL QD4.3M2A0.2 with the interdot distance of 1.1 nm, shown in

Figure 5. SAXS patterns of SLs as a function of quantity of OA molecules on QD surface formed by (a) 8.4 nm and (b) 4.3 nm QDs. Dashed lines (0 OA, 1 OA, and 2 OA) show the expected angles calculated by Bragg’s law for SLs with interparticle distances equal to 0, 1, and 2 times the length of an OA molecule, respectively. (c) STEM and (d) AFM images of SL QD4.3M2A0.2, the scale bar in the STEM image is 50 nm. (e) Absorption (ABS) (squares) and PL (circles) shift dependence on the number of chemical treatments of the QD surface for the SLs formed by 4.3 nm (green) and 8.4 nm (brown) QDs. 25064

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The Journal of Physical Chemistry C over a period of 3 and 6 months after the SL preparation. Typical changes in the band positions with time are shown in Figure 6 for the SLQD4.3 and SLQD8.4 samples.

Figure 6. Shifts of the absorption (ABS) (black squares) and PL (red circles) bands of SLs with different chemical treatment (MPANN) after 3 months of aging: (a) SL QD4.3, (b) SL QD8.4.

Figure 7. WAXS patterns of SL QD4.3 with different numbers of chemical treatments of the QD surface after 3 months aging. The asprepared SL formed by the initial QD solution (black curve) is shown for comparison.

For the SLs formed by the medium sized dots, the absorption peak is blue-shifted after three months and remains unchanged on further aging. The shift is proportional to the amount of the OA molecules in the QD solution that is schematically shown for SL QD4.3 in Figure 6a. The maximal blue shifts of 152 and 281 meV for the dots of diameters 4.3 and 5.3 nm, respectively, are observed for QDM1A0.5. At the same time, the total removal of OA molecules by methanol treatment (M3A0 sample, bottom line in Figure 6a) results also in the increase in absorption shift due to “opening” of the dot surface to oxidation by chemical treatment. Analogous changes are observed in the PL bands. These facts are illustrated in Figure 6a for the SL QD4.3 samples. The samples QD8.4MPA0 were stable in time, and the number of methanol treatments did not affect the absorption peak position considerably. Blue shift of the absorption peak increased with increasing amount of OA molecules on the QD surface. The shift was nonlinear with time, and the peak position stabilized after 3 months of aging. The largest shifts of 225 meV were observed for the sample QD8.4M1A0.5 with the maximal OA amount. This is illustrated in Figure 6b for the SL QD8.4 samples. Thus, we can conclude that the mild methanol treatment of the QD solution does not lead to a strong QD surface deterioration followed by oxidation. In contrast, passivation of the QD surface with additional OA molecules leads to a strong surface oxidation and even quenching of the PL. The change of Stokes shift can be estimated from the slopes of the dashed lines in Figure 6. For the medium sized QDs, the increase in Stokes shift along with the absorption and PL shifts is observed (Figure 6a, dashed lines). This can be explained by the size-dependent energy structure PbS QDs and a strong sizedependent Stokes shift for the QD size smaller than 5 nm.32 In the case of larger QDs the Stokes shift is slightly increased (see Figure 6b). This also corresponds to the observed sizedependence of the Stokes shift. A reduction of the QD core size resulting from the aging process is supported by WAXS data. An increase in the fwhm of the peaks is observed for all samples treated three times with methanol after 3 months of aging that reflects the small decrease in the QD core size. This is illustrated in Figure 7 for the SL QD4.3 samples with different amounts of OA molecules on the QD surface. The oxidation of the QD surface during aging is more pronounced for the samples with increased amount of OA molecules. The WAXS pattern of the SL QD4.3M2A0.5 in Figure

7 (blue) shows broadened peaks corresponding to the decrease in QD core size from 4.0 to 3.2 nm. The observed change in the particle sizes agrees with the change in the optical properties of the aged SLs. It should be noted that the samples of SL QD3.3MPA0.5 after 6 months of aging were transparent oily droplets. Thus, it can be concluded that increasing number of OA molecules causes chemical transformation of the quantum dot surface followed by almost complete disappearance of the PbS core. Our findings agree with the results of previous studies36−38 of the aging process in disordered ensembles of PbS QDs, where oxidation of QD surface was supposed to be the main driving force of QD aging. Thus, utilization of the QD layers as elements of photovoltaic devices requires appropriate protection to avoid oxidation of the QD core. Impact of Annealing on QDSL Morphology and Optical Properties. The effect of annealing on the morphology and optical properties of QDSLs was studied for the 4.3 nm QDs. In Figure 8 the SAXS patterns of the annealed SLs are shown together with those of the SLs formed from the initial QD solution. Figure 8a illustrates the evolution of the SAXS patterns of SLs formed by 4.3 nm QDs with the time of sample annealing. The interference peak corresponding to the ordered QD ensemble vanishes, and the optical properties of the SLs undergo changes in parallel. In Figure 8b the absorption and PL spectra of SL QD4.3 are compared with those of the initial 4.3 nm QD solution. After 1 min annealing, the absorption and PL bands become broader, and the PL intensity decreases. The same has been observed for other samples. The absorption and PL bands of all studied samples almost vanished after 30 min of annealing, which indicates practically total destruction of the PbS nanocrystals in the layer.

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CONCLUSIONS The morphology and optical properties of PbS QDSLs formed by QDs of different diameters depend on the chemical environment. The type of organic ligand on the surface strongly affects the process of QD self-assembly on the substrate. TOPO-capped QDs can only form dendrite-like structures with short-range ordering, while the optical properties of these samples are almost the same compared to those of the initial QD solution. OA-capped PbS QDs can assemble into highly ordered superlattices with different parameters depend25065

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Figure 8. (a) SAXS pattern of SLs formed by 4.3 nm QDs: initial (black), annealed for 1 min (red), annealed for 30 min (blue). (b) Absorption (ABS) (solid curves) and PL (dashed curves) spectra of SLQD4.3: initial (black), annealed for 1 min (red), annealed for 30 min (blue).

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ing on the amount of OA molecules in the initial colloidal solution. A maximal amount of OA molecules leads to the formation of a homogeneous lattice with relatively large interparticle distance (more than 1 nm) which prevents effective charge carrier transfer through the lattice. Chemical treatment of the QD surface improves homogeneity and shortens the interparticle distance in QDSLs. Ligand engineering is most effective for controlling morphology and optical properties of QDSLs with medium sized QDs. Mild treatment of QD colloidal solutions with methanol results in the formation of SLs with smaller interparticle distance, down to 0.4 nm, which is favorable for the charge carrier transfer. Investigation of the stability of the optical properties of SLs shows that the samples with a high content of OA molecules are more sensitive to the aging process, which results in oxidation of the QD surface and a decrease in the QD core size. At the same time, the almost complete removal of OA molecules also results in the oxidation of QD surface, an effect which is most prominent for SLs formed by medium-sized dots. Thus, we can conclude that there is an optimal amount of OA molecules which passivate the surface states while promoting the formation of a highly ordered lattice with minimal interparticle distance. Annealing leads to disintegration of superlattices. The QDrelated absorption and PL bands disappear, which indicates decomposition of QDs and formation of an inhomogeneous film on the substrate.

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REFERENCES

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Tel: +78124571780. Author Contributions

The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors thank the Ministry of Education of the Russian Federation (state task no. 3.109.2014/K) for financial support. A.P.L. thanks the Ministry of Education of the Russian Federation for financial support (scholarship of the President of the Russian Federation for young scientists and graduate students, CΠ-1841.2015.1). X-ray scattering experiments have been performed at the Centre for X-ray Diffraction Methods, Research Park, St. Petersburg State University. 25066

DOI: 10.1021/acs.jpcc.6b07734 J. Phys. Chem. C 2016, 120, 25061−25067

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DOI: 10.1021/acs.jpcc.6b07734 J. Phys. Chem. C 2016, 120, 25061−25067