Underpotential Deposition of Cadmium on ... - ACS Publications

Article Cite This: J. Phys. Chem. C 2019, 123, 931−939

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Underpotential Deposition of Cadmium on Colloidal CdSe Quantum Dots: Effect of Particle Size and Surface Ligands Yauhen Aniskevich,†,‡ Artsiom Antanovich,† Anatol Prudnikau,†,∥ Mikhail V. Artemyev,† Alexander V. Mazanik,§ Genady Ragoisha,† and Eugene A. Streltsov*,‡ †

Research Institute for Physical Chemical Problems of the Belarusian State University, Minsk 220006, Belarus Faculty of Chemistry and §Energy Physics Department, Faculty of Physics, Belarusian State University, Minsk 220030, Belarus

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S Supporting Information *

ABSTRACT: Electrochemistry of quantum dots (QDs) can provide useful information on their redox behavior and energy states. Here, we present a study of the surface-limited electrochemical deposition of cadmium adatoms (underpotential deposition or upd) on electrophoretically formed films of CdSe QDs of different sizes (2.4−6.3 nm) capped with different ligands (oleate and sulfide). Oleate-capped QD films were shown to have a weak electrochemical response in upd reaction, whereas sulfide-capped QDs demonstrated a significant increase in cathodic current that was attributed to enhanced surface accessibility and improved interparticle electron transfer. Cadmium upd onset potential in sulfide-treated QD films was found to be markedly size dependent. The increase of QD size results in the positive shift of upd onset potential which is in agreement with the change in lowest unoccupied molecular orbital position. The results imply that the proposed method utilizing electrochemical surface-limited reaction on QDs can be applied to probe energy states of chalcogenide semiconductor nanoparticles.

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INTRODUCTION

because the electrode potential and the electron energy are interrelated

Quantum dots (QDs) with their tunable electronic, optical, and chemical properties are attractive objects for designing functional materials and devices including solar cells,1−3 photodetectors,4,5 advanced catalysts,6−8 electroluminescent structures,9−11 and so forth. The applicability of QDs in such applications is determined by the energy of occupied and unoccupied orbitals which control the optical transitions, charge transfer, and nanoparticle redox chemistry. The valence and conduction band edge energies [highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) energy] on the absolute energy scale were measured by different techniques, including electrochemical or photoelectrochemical methods,12−17 ultraviolet photoelectron spectroscopy,18−20 and tunneling spectroscopy.21−23 Electrochemical methods are relatively simple; they provide information on QD redox behavior24,25 and allow determining so-called electrochemical band gap of QDs.12,14,24 The basic electrochemical technique involves cyclic polarization of QDs on conducting substrate or in dissolved state in electrolyte solution. This method was applied to characterize redox behavior of CdS,12 CdSe,13,14,24,26−29 Cu−Zn−S/Se30 QDs, and so forth. The electrochemical response of QDs typically presents as several peaks on cyclic voltammogram with the current onset potentials treated as HOMO and LUMO positions on the electrochemical scale of electrode potentials © 2018 American Chemical Society

Energy = −E ·e − Eref ·e

(1)

where Energy is electron energy versus vacuum, eV, E electrode potential, V, versus reference electrode, e elementary charge and Eref is absolute electrode potential of the reference electrode versus vacuum, V [Eref = 4.44 V for the standard hydrogen electrode (SHE)].31 The reduction process is considered as the injection of electron into LUMO and oxidationas the electron removal from HOMO.14 In fact, these processes are often accompanied by the electrochemical degradation of QDs.12 In the case of irreversible degradation of QDs, the peak position in cyclic voltammogram becomes dependent on several factors such as potential scan rate, side reaction, and so forth.32,33 To the best of our knowledge, reversible electrochemical behavior of chalcogenide QDs was observed earlier only for mercury chalcogenide nanoparticles.34 Chalcogenide nanoparticles characterization was also performed using other electrochemical and spectroelectrochemical methods.15−17 Volk et al.15 applied energy-resolved electrochemical impedance spectroscopy for studying elecReceived: October 23, 2018 Revised: November 30, 2018 Published: November 30, 2018 931

DOI: 10.1021/acs.jpcc.8b10318 J. Phys. Chem. C 2019, 123, 931−939

Article

The Journal of Physical Chemistry C tronic structure of PbS nanocrystals. Spittel et al.16 proposed potential-modulated absorption spectroscopy for evaluation of the absolute energy level positions in QD from absorption changes upon polarization. Malashchonak et al. employed photocurrent measurements to evaluate LUMO positions of CdSe QDs.17 In this study, to avoid interference of QD redox degradation during electrochemical determination of HOMO−LUMO band structure, we used specific surface-limited electrochemical reaction, namely, metal underpotential deposition (upd). The upd is the electrochemical deposition of metal atoms (adatoms) on metal or semiconductor electrode above the reversible potential of the corresponding bulk metal deposition.35−39 The driving force for upd is determined by the specific affinity of metal adatoms to the substrate electrode.37 The upd is a well-studied phenomenon for bulk metallic electrodes35−37,40,41 that was demonstrated to be very sensitive to properties of the substrate35,36,38,42 and in the case of metal nanoparticles was shown to be size-dependent.43−46 Recently, we have demonstrated the possibility of Cd underpotential deposition on nanostructured CdS and CdSe films fabricated via chemical bath deposition (CBD).47 The CBD films consisted of nanocrystals with a broad size distribution and mean diameter of 3, 6, and 12 nm determined by deposition parameters. Individual nanocrystals in such films are close-packed which provides efficient electrical contact between neighboring crystals and also makes them suitable to use as electrodes for electrochemical study. However, the broad size distribution of CdS and CdSe nanocrystallites in CBD films hinders precise examination of the size effects. On the other hand, monodisperse colloidal QDs with tunable composition, size, and shape, such as ones obtained by high-temperature organometallic route,48−52 require additional procedures to make them suitable for electrochemical measurements. Several methods of QD film preparation such as QD attachment using bifunctional linkers, drop-casting, layer-by-layer growth, electrophoretic deposition (EPD), and so forth24,53−59 were presented in literature. Here, we utilized EPD of CdSe QDs of different diameters from their colloidal solution in nitrobenzene which was successfully used earlier60 to produce homogeneous films with good adhesion to the substrate. The immediate use of such films as electrodes for investigation of the QD size effect in cadmium upd was hindered by weak interparticle electron transfer, so we exchanged bulky organic capping ligands for much smaller sulfide ions in the QD film which enabled cadmium upd as a QD size-sensitive reaction and provided additional information on the effect of ligands.

the anode and EPD was completed in 5 min, as indicated by the complete discoloration of nitrobenzene solution. A new portion of QD solution was used for preparation of each electrode. After the deposition, samples were rinsed for 20 s in methanol to remove nitrobenzene residuals and dried in air in the dark. Ligand Exchange Procedure. The exchange of oleic acid by sulfide was performed by immersing CdSe QD films in 0.2 mg mL−1 solution of Na2S in methanol for 1 h. The electrodes were afterward removed from solution, rinsed twice for 20 s in methanol, and dried in air. Electrochemical Measurements. Electrochemical experiments were performed in a conventional one-compartment 3electrode electrochemical cell with Pt-sheet counter electrode (Metrohm 3.109.0790) and an Ag|AgCl|KCl (sat) as a reference electrode (EVL-1M3.1 Gomel Plant of Measuring Devices) using Gamry Reference 600 potentiostat. The reference electrode was connected with the cell through a salt bridge with K2SO4. CdSe QD films on FTO were used as working electrodes. FTO glasses were kept in warm solution of H2O2 and NH3, rinsed with distilled water, and dried before the EPD. Blank electrolyte for cyclic voltammetry (CV) was 0.5 M K2SO4 aqueous solution. The electrolyte for Cd underpotential deposition study contained additionally 10 mM of CdSO4. Physical Characterization Methods. Raman spectra were obtained using a Nanofinder HE (LOTIS TII, BelarusJapan) confocal spectrometer. DPSS CW laser emitting at 532 nm was used for excitation. Optical power was attenuated down to 50 μW to avoid sample damage. Spectral resolution and accuracy in determination of Raman shifts were better than 2.5 cm−1. Optical absorption spectra were recorded using an Ocean Optics HR-2000+ spectrometer equipped with an Ocean Optics DH-2000 white light source. Chemicals. Nitrobenzene was supplied by Sigma-Aldrich (99%) and was distilled from P2O5 prior to use. Methanol was Fisher Chemical HPLC grade and was used without further purification. CdSO4·8/3H2O (≥99.0%, Sigma-Aldrich) and K2SO4 (≥99.0%, Sigma-Aldrich) were used for preparing electrolytes. Na2S·9H2O (99.99%, Sigma-Aldrich) was used for ligand exchange solution preparation.

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RESULTS AND DISCUSSION QD Films Preparation. CdSe core QDs of different average sizes were synthesized according to the standard procedure.61 For the electrochemical measurements, we prepared CdSe QD films via EPD on transparent conducting FTO electrodes from colloidal solutions of oleate-capped QDs in nitrobenzene. Oleic acid capping on CdSe QDs impeded electron transfer in the films which resulted in low and noisy current in the upd of cadmium. To increase film conductivity, we removed oleic acid from the QD surface through a ligand exchange procedure with inorganic ligands. First, we treated EPD films by immersing them in methanol and bubbling H2Se gas. While ligand exchange process was evidenced by absorption spectra, we also noticed the formation of small amount of elemental selenium. As selenium could be further involved in cadmium upd,62 we performed a similar procedure with more stable sulfides. Sulfide ions are well-known robust inorganic ligands for QDs and may be applied both to colloidal solutions63,64 and solid QD films.65−68 In experiments, the asdeposited CdSe QD films were immersed in 3 mM solution of

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METHODS CdSe Quantum Dots Synthesis and Preparation of Films. CdSe core QDs of different average sizes were synthesized according to the standard procedure reported in ref 61 QD films were prepared via EPD of CdSe QDs from their colloidal solution in nitrobenzene onto fluorine-doped tin oxide (FTO)/glass electrodes. Colloidal solutions of QDs in nitrobenzene were prepared by adding 1−20 μL of a crude CdSe QD solution in CHCl3 to 1 mL of freshly distilled nitrobenzene and sonicating for 10 min. Both FTO/glass cathode and the anode were immersed in 1 mL of colloidal solution of QDs in purified nitrobenzene, and 500 V dc potential was applied. A film of QDs was deposited only onto 932


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The Journal of Physical Chemistry C

inherent to bulk CdS (302 cm −1 ), 71 and its shape demonstrates essential asymmetrical low-frequency broadening, which is a result of the strong phonon confinement effect.72 Note that similar LO peak position and shape were observed for CdSe−CdS core−shell nanoparticles.73 Raman peaks at 205 and 411 cm−1 correspond to scattering on one and two LO phonons in CdSe, respectively.69 The LO phonon frequency of 205 cm−1 is slightly lower than that for bulk CdSe (210 cm−1) because of spatial confinement of phonons in QDs.69,74,75 Electrochemistry. Electrochemical experiments with electrophoretically deposited QD films were conducted in a conventional one-compartment three-electrode electrochemical cell with Pt foil as a counter electrode and Ag|AgCl|KCl (sat.) as a reference electrode (+0.202 V vs SHE at 20 °C). Blank cyclic voltammograms were recorded in aqueous 0.5 M K2SO4 electrolyte solution, whereas the electrolyte for the underpotential deposition additionally contained 10 mM CdSO4. All solutions where purged with nitrogen for 10 min prior to measurements. The cyclic potential scans started from open circuit potential down to −0.75 V and reversed to the positive direction to avoid bulk cadmium phase deposition which started below −0.75 V. Figure 2 shows typical cyclic voltammograms for oleate- and sulfide-capped 4.5 nm CdSe QDs in blank and Cd2+-containing solutions. In both cases, the current in the blank electrolyte was small and showed the cathodic peak upon addition of CdSO4 to the electrolyte similarly to Cd upd on CdSe CBD films.47 Sulfide-treated QDs showed nearly fivefold increase in the peak current as compared to oleate-capped QDs. The enhanced electrochemical response can be explained by improved electric contacts between neighboring QDs after the exchange of bulky oleic ligand for sulfide. We observed no oxidation peak in the studied potential range which points to the irreversible character of upd on CdSe QDs. Similar irreversible behavior has been observed earlier for chemically deposited CdSe and CdS films.47 Furthermore, chemical irreversibility of the upd in the first potential cycle was confirmed by the absence of the cathodic peak in the second and subsequent cycles. This was due to occupation by cadmium of almost all sites on QD surface available for Cd2+ reduction in the first upd run (see, Figure S2 in the Supporting Information).

Na2S in methanol for 1 h, rinsed with fresh methanol, and dried in air. Figure 1 shows the influence of Na2S treatment on the optical absorption and Raman spectra of the CdSe QD films on FTO electrodes.

Figure 1. (a) Schematic representation of the ligand exchange of oleic acid by sulfide ions on the surface of CdSe QDs. (b) Optical absorption spectra of CdSe QD films on FTO electrodes before (black) and after (red) ligand exchange. (c) Raman spectra of CdSe QD films on FTO electrodes (2.8 nm in diameter CdSe QDs) before (black) and after (red) ligand exchange.

Figure 1b shows that the ligand exchange with sulfide ions results in the red shift and spectral broadening of the first excitonic transition similar to that reported in refs64,65,67,68 where such spectral shifts were attributed to changes in potential barrier and dielectric confinement due to the formation of surface Cd−S bonds. Indeed, after sulfide treatment, Raman spectra in Figure 1c demonstrates the appearance of a new peak at 280 cm−1 corresponding to longitudinal optical (LO) phonon of the Cd−S layer.69,70 The spectral position of this peak significantly differs from the value

Figure 2. Cyclic voltammograms of 4.5 nm CdSe QD film electrodes: (a) before, (b,c) after sulfide treatment. Black curves correspond to blank scans in 0.5 M K2SO4 solution, the red curvesto cadmium upd in 0.5 M K2SO4 + 10 mM of CdSO4 solution. dE/dt = 5 mV s−1. Geometric surface area was 0.13 cm2 for (a,b), and 0.25 cm2 for (c). 933


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3d), which points to the surface-limited character of the reaction. Another important conclusion which follows from Figure 3 is that the scan rate should be fixed when upd potentials are compared on different electrodes. Here, we used 5 mV s−1 scan rate which was sufficient for investigation of samples in reasonable time, whereas considerably higher scan rates were avoided because of the CV distortion caused by Ohmic resistance, capacitive current, and so forth. Influence of the Film Thickness. The thickness of films of 2.8 nm CdSe QD was varied by changing the concentration of colloidal nitrobenzene QD solutions used for EPD. The deposition was performed for the fixed time (5 min) that provided complete deposition of QDs onto the electrodes. Figure 4a presents optical absorption spectra that were used to estimate the thickness of thus obtained QD layers (details

Figure 2c shows the effect of slightly more negative potential of the scan reversal (compared to Figure 2b) which resulted in starting of bulk cadmium cathodic deposition followed by the anodic oxidation (anodic peak at −0.68 V) of bulk cadmium in the reverse scan. The potential range in further examination was limited by the region of underpotential deposition to avoid effects of bulk cadmium deposition. The exchange of oleate by sulfide also resulted in improved reproducibility of the Cd upd from sample to sample (see, Figures S3 and S4 in the Supporting Information). Good reproducibility as well as the increased electrochemical response in the case of sulfide-capped QDs was crucial for accurate electrochemical characterization of size effects in the upd. Therefore, further experiments have been carried out with the sulfide-capped QD films. Scan Rate Influence. Peaks in cyclic voltammograms can result either from surface-limited character of the electrochemical reaction or from nonstationary diffusion of the dissolved electroactive species in electrolyte solution. The origin of the current peak can be established on the basis of the relationship between the peak current and the scan rate. The peak current that results from diffusion is proportional to square root of the scan rate, whereas linear dependence of the peak current on the scan rate results from the current confinement by number of surface sites available for the electrochemical reaction.32,33 Figure 3a presents cyclic

Figure 4. (a) Absorption spectra of 2.8 nm CdSe QD films of different thickness prior to the sulfide treatment and (b) corresponding cyclic voltammograms of cadmium upd from 0.5 M K2SO4 + 10 mM of CdSO4 electrolyte after the sulfide treatment. dE/ dt = 5 mV s−1.

are provided in the Supporting Information). While this method does not allow precise measurement of the film thickness due to undefined film morphology (porous vs closepacked), the overall film thickness is proportional to its optical density. Figure 4b shows cyclic voltammograms of cadmium upd on sulfide-treated QDs at different thickness of QD film. Cathodic peak current increases monotonously with the film thickness because of increase in total surface area of CdSe QDs and, correspondingly, number of sites available for Cd upd which provides evidence to the porous character of the electrophoretically deposited QD films. No significant change in the cathodic peak potential or the upd onset potential was observed with the variation of the film thickness. Size Effect in Cd Underpotential Deposition. After the optimization of experimental conditions, we studied the influence of CdSe QD diameter (2.4−6.3 nm) on the upd parameters. Cyclic voltammograms were recorded several times for every QD size using freshly prepared samples for reliability of the data. Figure 5a shows typical cyclic voltammograms of Cd upd on CdSe QD films formed by QDs of different diameters (for the convenience of comparison, each curve was normalized by its cathodic peak current). As the QD diameter increases from 2.4 to 6.3 nm, the potential of the cathodic peak attributed to cadmium upd shifts from −0.70 to −0.53 V with the corresponding upd onset potential increase from −0.6 to −0.4 V.

Figure 3. (a) Cyclic voltammograms of sulfide-treated films of 2.8 nm CdSe QDs at variable rates of the electrode potential scan in 0.5 M K2SO4 + 10 mM CdSO4 electrolyte, (b) the same data normalized by cathodic peak current; (c) peak potential and (d) peak current dependences on the scan rate. Geometric surface area of each film electrode was 0.30 ± 0.02 cm2.

voltammograms obtained on 2.8 nm CdSe QD electrode at different scan rates. Because upd is a chemically irreversible process, each cyclic voltammogram was recorded using a new electrode fabricated under the same conditions. Figure 3a shows that the cathodic peak current of cadmium upd on CdSe QDs increases with scan rate and the potential of the peak becomes more negative (Figure 3b,c). Our data is fitted well with the linear dependence on the scan rate (Figure 934


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for Cd2+ reduction. Thus, the smaller the particles are, the lower (more negative) electrode potential is required to induce the electron transfer from the FTO to the QDs. To validate the upd technique for determination of LUMO levels in colloidal QDs, we compared our results with the data presented in the literature on CdSe QDs characterization by electrochemical methods (Figure 7).13,16,17,24,27−29 The energy

Figure 5. (a) Cyclic voltammograms of CdSe films at different QD size: Cd upd from 0.5 M K2SO4 + 10 mM of CdSO4. dE/dt = 5 mV s−1. (b) upd onset potential vs diameter of CdSe QD. Error bars show intervals of 95% confidence.

Figure 5b presents the upd onset potential versus CdSe QD size. The onset potential of the upd was measured as shown in Figure S5 in the Supporting Information (4−5 samples of each size were used). The upd onset potential is size dependent and increases with the QD diameter. This electrochemical effect corresponds to the known decrease in the band gap energy of semiconductor QD with size which results from quantum confinement effect and the corresponding variation in the absolute position of the LUMO level.49,52 To determine the position of the LUMO level from the results of a CV experiment one can use two parameters:24 the redox peak values13,14,28,76 or redox onset potentials.77−82 According to Liu,24 the use of onset potentials rather than cathodic peak values is a more appropriate approach because the injection of electrons into the unoccupied levels starts when the potential of the electrode reaches the band edge. Note that the relation of the onset potential to the absolute position of the LUMO level is somewhat voluntary because it depends on the scan rate. Nevertheless, because the scan rate for each sample in our experiments has been fixed at 5 mV s−1, the onset potential can be considered as a useful parameter to study the size-dependent electrochemical properties of QD films. Figure 6 demonstrates the relationship between upd onset potential and LUMO position with respect to both electron energy and electrode potential scales. The quantum confinement effect in CdSe QDs results in the size dependence of the LUMO energy which controls the electron transfer required

Figure 7. Upd onset potentials and LUMO levels vs CdSe particle size determined by electrochemical methods. The black dashed line corresponds to a bulk CdSe CB minimum in contact with aqueous solution.83,85 The purple dashed line is a guide for the data obtained from Cd upd.

and electrode potential scales were related according to eq 1. To account for the influence of the experimental environment, we also calculated size-dependent LUMO energy of CdSe QDs in water using the data for bulk CdSe monocrystal measured in slightly acidic solution83 and adding the theoretical LUMO energy for 1Se level of CdSe QD calculated by Efros and Rosen.84 The resulting function is plotted on Figure 7 as a solid black curve. Figure 7 shows that the upd onset potentials are considerably higher (positioned lower on potential scale) than corresponding LUMO levels determined in the previous works.13,16,24,27−29 Also, one can see that reference electrochemical data are considerably scattered over the energy/ potential scales but clearly exhibit general pattern, that is, the LUMO level shifts to higher values with the decrease of the particle size. The dispersion of absolute energy levels can be explained by different preparation of electrodes, capping ligand, environment (type of electrolyte, etc.), or absorption/ desorption of different species.86−93 Therefore, strict comparison of our upd data with results obtained by other electrochemical methods is not straightforward because most electrochemical experiments were performed for nanoparticles capped with different organic ligands (TOPO, thiols, etc.) in different solvents. At the same time, Figure 7 shows that the change in upd potential with QD size reported in this paper follows a trend similar to theoretical data of Efros and Rosen. Our values are approximately 300 mV more positive than simulated LUMO position for the aqueous medium. The difference can be explained by Cd2+ adsorption because Cd2+ ions were shown to alter band edge positions of CdS bulk electrode92 and enrichment of CdSe QD surface with Cd2+ results in the increase of their redox potential for 400 mV.94 Another source of discrepancy might be the sulfide treatment of CdSe QDs

Figure 6. Schematic representation of electron transfer in Cd upd on CdSe QD films supported by FTO. The left part of the figure shows energy levels of the FTO substrate and CdSe QDs. The right part represents cyclic voltammogram of cadmium upd on 2.4 nm QD film with the electrode potential scale related to electron energy scale. The Fermi level EF of the FTO shifts upward in the cathodic scan. When the position of EF reaches the LUMO level of CdSe QD, the electron is transferred from the electrode to Cd2+ via the LUMO level of QD, which results in cadmium adatom deposition onto the CdSe QD. 935


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that induces red shift of absorption bands and alters LUMO energy. While such treatment complicates direct data juxtaposition, it was proved to be a crucial step for increasing film conductivity and obtaining reproducible results. Because beneficial effect of sulfide treatment is attributed to the removal of oleic acid from QD surface, future work will be focused on studying the influence of ligand exchange procedures, such as film treatment with other inorganic95,96 or short-chain organic ligands,97,98 hydrazine99 or pyridine,100 and so forth.

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CONCLUSION The electrochemical study of CdSe QD films formed via EPD from QD colloidal solution was carried out by introducing Cd2+ into the electrolyte solution as a reactant for surfacelimited reaction (upd) on QDs. Oleate ion exchange for sulfide ion was shown to strongly enhance the current of cadmium upd. The upd onset potential was shown to significantly depend on QD size and agree with LUMO energies for CdSe QDs in contact with aqueous solution. The effect is explained by control of the upd onset potential by LUMO. The discovered size effect in underpotential deposition of cadmium on CdSe QDs can be helpful for their absolute energy levels estimation and studying the redox behavior of the QDs.

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.8b10318. EPD setup, successive polarization of CdSe QD in Cd2+containing electrolyte, cyclic voltammograms of different CdSe QD samples, upd onset potential determination, and CdSe QD film thickness estimation (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Phone +375172095387. ORCID

Artsiom Antanovich: 0000-0001-8533-7347 Anatol Prudnikau: 0000-0002-4088-4942 Mikhail V. Artemyev: 0000-0002-6608-0002 Alexander V. Mazanik: 0000-0002-4725-0969 Eugene A. Streltsov: 0000-0003-2939-8502 Present Address ∥

Physical Chemistry, TU Dresden, Bergstraße 66b, 01062 Dresden, Germany. 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 M.A. acknowledges CHEMREAGENTS program. A.A. acknowledges BRFFI grant X17KIG-004. Y.A. acknowledges BRFFI grant X17M-003.

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REFERENCES

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