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: . ., . ., . ., - . ., . ., ., . . - , CdSe // . . . 2, . . . 2016. 2. . 3–11.
F o r c i t a t i o n: Ronishenko B. V., Antanovich A. V., Prudnikau A. V., Fedo- syuk A. A., Bercu N. B., Molinari M., Artemyev M. V. Elec- trophoretic deposition of hydrophobic dSe quantum dots, nanorods and nanoplatelets from their colloidal solutions in nitrobenzene. Vestnik BGU. Ser. 2, Khimiya. Biol. Geogr. 2016. 2. P. 3–11 (in Engl.).
: – . – . – . – . – . – . – - .
A u t h o r s: Bohdan Ronishenko, student at the faculty of chemistry. ronishenko@rambler.ru Artsiom Antanovich, junior researcher. antanovi4@gmail.com Anatol Prudnikau, junior researcher. nanochem@mail.ru Aleksandra Fedosyuk, junior researcher. sasha-f18@mail.ru Nicolae Bogdan Bercu, postgraduate student. nicolae-bogdan.bercu@univ-reims.fr Michael Molinari, associate professor. michael.molinari@univ-reims.fr Mikhail Artemyev, head of the laboratory. m_artemyev@yahoo.com
546+544.7
ELECTROPHORETIC DEPOSITION OF HYDROPHOBIC1 CdSe QUANTUM DOTS, NANORODS AND NANOPLATELETS FROM THEIR COLLOIDAL SOLUTIONS IN NITROBENZENE
B. V. RONISHENKO a, A. V. ANTANOVICH a, A. V. PRUDNIKAU a, A. A. FEDOSYUK a, N. B. BERCU b, M. MOLINARI b, M. V. ARTEMYEV a
aResearch Institute for Physical Chemical Problems of the Belarusian State University, Leningradskaya street, 14, 220030, Minsk, Republic of Belarus
bLaboratory of Research in Nanosciences (LRN EA4682), University of Reims, Champagne-Ardenne, Reims, France
We studied optical properties and morphology of thin electrophoretically deposited films of CdSe nanocrystals of different dimensionality (quantum dots, nanorods, nanoplatelets) with the help of optical absorption spectroscopy and atomic force microscopy. Utilization of nitrobenzene, an organic solvent with high dielectric constant allows rapid and complete electrophoretic deposition of CdSe nanocrystals of different types (quantum dots, nanorods, nanoplatelets) from their colloidal solutions. The high speed of the
1The article published in author’s edition.
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electrophoretic deposition leads to the formation of thin porous films with irregular structure. The type of studied nanocrystals does not have a significant impact on the morphology of electrophoretically deposited films. Such porous quantum dot films can be perspective for photovoltaic structures due to potentially increased contact area between quantum dots film and a conducting polymer used as a counter-electrode.
Key words: electrophoresis; nanocrystals; CdSe; nitrobenzene; atomic force microscope.
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Electrophoretic deposition (EPD) of semiconductor, metal, oxide nanocrystals from their organic or aqueous colloidal solutions is a versatile method for preparation of thin crack-free films with well controlled thickness [1; 2, p. 131–155, 267–294; 3]. Thin, matrix-free films composed of semiconductor nanocrystals can be implemented in electroluminescent and photovoltaic panels where electrical contact between neighboring nanocrystals and the absence of such defects, as pinholes and cracks are important parameters [4 –7]. While CdSe nanocrystals can be effectively deposited from aqueous solutions, it usually requires an additional step of solubilization of the as-synthesized hydrophobic nanocrystals in water which can deteriorate the optical properties of nanocrystals [8–11]. Moreover, the direct aqueous synthesis of CdSe nanocrystals is limited by the nanocrystal size (small to medium), shape (spherical) and core structure.
Besides CdSe quantum dots, other types of quantum-confined nanocrystals like nanorods and nanoplatelets can be electrophoretically deposited from their colloidal solutions in organic solvents [12, 13]. Since hydropho- bic CdSe nanocrystals dissolve well only in non-polar organic solvents like chloroform, toluene, hexane (as well as tetrahydrofurane and pyridine) or their mixtures and are not soluble in water and alcohols, the electrophoretic deposition is usually performed in those solvents [9, 14 –19]. Solvent and surface properties of CdSe nanocrystals play an important role in the EPD efficiency. The main factor governing the EPD is the surface charge of nanocrystals produced by polar groups of surface stabilizers like oleic acid, trioctylphosphine oxide (TOPO), pyridine etc. [12, 13, 6]. The relative surface charge of nanocrystals depends on the protonation/ deprotonation of charged surface groups which is enhanced in polar solvents with high dielectric constants like water, acetonitrile and alcohols. In non-polar solvents, like toluene and hexane, the surface charge is intrinsically low and in turn requires application of high electric field strength of the order of hundreds of volts per centimeter of the interelectrode distance. The duration of EPD in non-polar solvents may also take much time, of the order of tens of minutes, that impedes practical implementation of this process. The addition of solvents with high dielectric constants like acetonitrile or acetone partially helps to increase the EPD speed and decrease the applied voltage [7, 13]. However, both acetone and acetonitrile are poor solvents for CdSe nanocrystals stabilized with hydrophobic molecules like TOPO or oleic acid and provoke the aggregation and precipitation of nanocrystals when added in large proportions.

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Here, we demonstrate that the use of nitrobenzene, an organic solvent with high dielectric constant, allows rapid and complete EPD of CdSe nanocrystals of different types (quantum dots, nanorods, nanoplatelets) from their colloidal solutions. Nitrobenzene has the dielectric constant about 35 and forms stable colloidal solutions of CdSe core quantum dots, nanorods and nanoplatelets stabilized with oleic acid.
Experimental section Hydrophobic core CdSe quantum dots (QDs, d = 3,3 nm), nanorods (NRs, d × L = 3,2 × 16,0 nm2) and
nanoplatelets (NPLs, 5 ML CdSe, L × W = 10 × 20 nm2) were prepared by standard high temperature methods described elsewhere [20, 21] and stored in chloroform. All types of nanocrystals were capped with oleic acid prior to use in the experiments with EPD. To do that the crude nanocrystals were precipitated out three times with methanol and re-dispersed in a fresh portion of 15 % v/v solution of oleic acid in chloroform. The obtained colloidal solutions were stirred for 48 h and the nanocrystals were precipitated with isopropanol in order to remove the excess of oleic acid. After centrifugation at 5000 rpm for 5 min the nanocrystals were dispersed in nitrobenzene, which was purified by distillation in vacuum prior to use. The obtained colloidal solution was sonicated for 5 min and centrifuged at 5000 rpm for 5 min to remove large aggregates. The average concentration of nanocrystals was about 5 mg/ml. The resulting colloidal solution of CdSe nanocrystals in nitrobenzene was transferred into 2 mm quartz cuvette.
The EPD process was conducted on two flexible Indium tin oxide (ITO) electrodes on 150 µm polyethylene terephthalate (PET) foil (Aldrich) with the surface resistivity of 60 /sq. The ITO electrodes were separated by a Teflon spacer to create approximately 1,5 mm of inter-electrode distance. The high DC voltage power supply for gel-electrophoresis was used to apply the necessary voltage to both electrodes. In order to achieve a pseudo-pulse EPD regime we connected a 200 µF capacitor parallel to the power supply output. After charging the capacitor at 450 V, the electrical circuit was closed and the EPD occurred quickly and completely. After 1 min the system was switched off, the electrodes were removed from cuvette, consecutively rinsed with nitrobenzene and iso-propanol and then dried.
The optical absorption spectra of deposited films were registered by Ocean Optics HR 2000 spectrometer. The examination of the morphology of deposited nanocrystalline films was performed with a Dimension 3100 AFM (Bruker). All measurements were carried out at room temperature.
Results and discussion Figure 1 shows three representative images of ITO/
PET electrodes with electrophoretically deposited thin films of CdSe QDs, NRs and NPLs. Figure 1 demonstrates that all types of nanocrystals form homogeneous films on the surface of ITO electrodes. It is important to note that the EPD takes place only on anodes, while cathodes remain practically uncovered with nanocrystals. Earlier, similar asymmetrical EPD was observed for oleic acid-capped CdSe NPLs dispersed in hexane/acetone mixture [13]. At the same time CdSe QDs capped with TOPO molecules deposit electrophoretically at both electrodes in a similar amount [7, 16, 19]. The nitro-group of nitrobenzene acts as proton acceptor that interacts with carboxyl groups of oleic acid on the surface of CdSe nanocrystals creating weak negative surface charge. The presence of negative surface charge drastically increases the efficiency of EPD in nitrobenzene which together with high dielectric constant allows for rapid and complete deposition of CdSe nanocrystals within few seconds.
In order to examine the influence of surface stabilizing groups on EPD of CdSe nanocrystals we chose one type of CdSe nanocrystals (the nanorods) and conducted the ligand exchange on their surface. Two aliquots of CdSe NRs capped with oleic acid in chloroform were purified by triple deposition/redispersion procedure in order to remove the excess of oleic acid. Dodecanethiol and oleylamine (Aldrich) were added afterwards to purified colloidal solutions of CdSe NRs in chloroform to achieve 1 % volume concentration. Both solutions were stirred overnight and the nanocrystals were precipitated with methanol. The deposition/redispersion procedures were repeated two times in order to remove an excess of thiol or amine. Then, CdSe NRs were re- dissolved in nitrobenzene and used for EPD as described above.
Fig. 1. Optical images of ITO/PET electrodes (anodes) with thin films of CdSe QDs (a), NRs (b)
and NPLs (c), electrophoretically deposited from their colloidal solutions in nitrobenzene (V = 450 V, t = 1 min)
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Figure 2 shows representative images of ITO/PET electrodes with electrophoretically deposited thin films of CdSe NRs capped with oleic acid, dodecanthiol and oleylamine. Figure 2 demonstrates again that the EPD occurs only on anodes leaving cathodes practically uncapped. Surprisingly, dodecanethiol-capped CdSe NRs show almost the same EPD efficiency as oleic acid-capped ones. This result points to the importance of the ability of surface ligands to deprotonate and form negative surface charge. Contrarily, CdSe NRs stabilized with oleylamine do not demonstrate any EPD effect, while proton-accepting nitrogroups of nitrobenzene do not interact with amino-groups of oleylamine leaving CdSe NRs practically un-charged.
Figure 3 presents optical absorption spectra of CdSe nanocrystals in colloidal solutions and EPD thin films and demonstrates that EPD process does not affect the character of the optical transitions of the nanocrystals. The spectral positions and shapes of main absorption peaks of colloidal solutions and EPD films are almost identical thus indicating that the size, shape and chemical composition of CdSe nanocrystals remain unchanged.
The morphology of the as-obtained films was investigated using atomic force microscopy in contact and tapping modes. At first, we studied the roughness of initial ITO film on PET substrate in order to understand how it can affect the roughness of the final EPD CdSe films. Figure 4 shows three AFM images of a bare ITO film before EPD. From fig. 4 one may infer that the average surface roughness of ITO film is below 3 nm and may be neglected into the following.
Figure 5 shows four AFM images of CdSe QDs electrophoretically deposited onto the same ITO/PET films. One can see that EPD CdSe QDs film is highly porous and consists of submicron-size granules formed by aggregates of CdSe QDs. The surface roughness of EPD QDs film is around 80–90 nm which is much higher that the surface roughness of ITO film. Therefore the topography of the ITO substrate does not affect the morphology of EPD films.
Fig. 2. Optical images of CdSe NRs electrophoretically deposited onto ITO/PET anodes (right strips) and cathodes (left strips) from their colloidal solutions in nitrobenzene. CdSe nanorods were capped with oleic acid (a),
dodecanthiol (b) and oleylamine (c) (V = 450 V, t = 1 min)
Fig. 3. Optical absorption spectra of CdSe QDs (a), NRs (b) and NPLs (c) in colloidal solutions in nitrobenzene (1) and EPD films (2) (V = 450 V, t = 1 min)

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Fig. 4. AFM images in tapping mode of the surface of a ITO/PET film before EPD of CdSe nanocrystals: 5 × 5 µm2 area (a) (3,7 nm RMS roughness); 2 × 2 µm2 area (b) (2,2 nm RMS roughness);
1 × 1 µm2 AFM 3D image of the uncoated area of the sample (c)
Fig. 5. AFM images of EPD CdSe QDs films: 10 × 10 µm2 area (a) (94 nm RMS roughness), 5 × 5 µm2 area (b) (86 nm RMS roughness), 2D (c) and 3D (d) (52 nm RMS roughness) images of 1 × 1 µm2 area (V = 450 V, t = 1 min)
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Fig. 6. The depth profile (b) over the large hole in AFM image of EPD CdSe QDs film (a)
Fig. 7. AFM images in tapping mode of EPD CdSe NRs on ITO/PET substrate: 10 × 10 µm2 area (a) (249 nm RMS roughness), 5 × 5 µm2 area (b) (136 nm RMS roughness), 2D (c) and 3D (d) (67 nm RMS roughness) images of 2 × 2 µm2 area (V = 450 V, t = 1 min)

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Fig. 8. AFM images in tapping mode of EPD CdSe NPLs on ITO/PET substrate: 10 × 10 µm2 area (a) (29 nm RMS roughness), 5 × 5 µm2 area (b) (28 nm RMS roughness), 2D (c) and 3D (d) (20 nm RMS roughness) images of 2 × 2 µm2 area (V = 450 V, t = 1 min)
Fig. 9. AFM images in tapping mode of EPD CdSe NRs on ITO/PET substrate: 10 × 10 µm2 area (a) (67 nm RMS roughness), 5 × 5 µm2 area (b) (40 nm RMS roughness) (V = 50 V, t = 10 min)
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Figure 6 shows the depth profile of large hole at the bottom right side of CdSe QDs film. The depth profile in fig. 6 can be used to estimate the average thickness of EPD CdSe QDs film. If one supposes that such hole can penetrate down to the ITO substrate, the average CdSe QDs film thickness is around 250 nm. However, the surface roughness of about 50 nm measured from fig. 6 makes such estimation rather crude.
Figure 7 presents four AFM images of electrophoretically deposited CdSe NRs film on ITO/PET substrate. Again, the EPD film of CdSe NRs shows porous irregular structure with the average surface roughness larger than 100 nm. The depth profile of a large hole at the upper left part of 2 × 2 µm2 area picture (not shown) indicates an average thickness of CdSe NRs films of about 250 nm. Therefore, we may conclude that in the case of QDs and NRs the structure of EPD films does not sufficiently depend on the type of studied nanocrystals.
Figure 8 shows four AFM images of electrophoretically deposited CdSe NPLs film on ITO/PET substrate. The morphology of NPLs film still remains irregular and porous with much larger number of holes as compared to QDs and NRs films. The average thickness of NPLs film measured from the depth profile of holes (not shown) is about 80 nm which is approximately three times less than the thickness of QDs and NRs films. The smaller thickness can be explained by less concentrated solution of CdSe NPLs in nitrobenzene, since the colloidal stability of NPLs is weaker that other types of nanocrystals due to the large lateral size. The smaller average roughness of CdSe NPLs films also can be due to smaller thickness of the film as compared to CdSe QDs and NRs. The larger amount of holes in CdSe NPLs films can be explained also by small thickness of EPD film or the specific interaction between CdSe NPLs during the EPD, when NPLs prefer to stick to deposited NPLs rather then cover bare ITO surface.
From the analysis of the AFM data on fig. 5, 7 and 8 we may conclude that all types of CdSe nanocrystals produce porous EPD films with irregular structure. Such porous structure can be formed due to very rapid pseudo-pulse electrophoretic deposition at high applied voltage. For example, the same CdSe NRs being de- posited at much lower voltage (50 V) and prolonged time formed more compact and less porous films of the same thickness (fig. 9). Therefore, the pseudo-pulse EPD regime can be used for the preparation of porous nanocrystalline films instead of compact and smooth ones, reported previously [7].
The use of nitrobenzene, an organic solvent with high dielectric constant, allows rapid and complete elec- trophoretic deposition of CdSe nanocrystals of different types (quantum dots, nanorods, nanoplatelets) from their colloidal solutions. The high speed of the electrophoretic deposition results in the formation of thin porous films with irregular structure. The type of studied nanocrystals does not have a significant impact on the morphology of electrophoretically deposited films. Highly porous films made from semiconductor nano- crystals, although undesirable in electroluminescence applications due to non-uniform charge transport, can be useful in various photovoltaic structures. The porous structure of quantum dot films can increase the efficiency of photoinduced charge extraction from individual quantum dots due to potentially increased contact area be- tween quantum dots film and a conducting polymer used as a counter-electrode.
B. Ronishenko, A. Antanovich, and A. Prudnikau acknowledge partial support from CHEMREAGENTS program. M. Artemyev acknowledges a support from the Region Champagne Ardenne as invited professor. N. B. Beru and M. Mo- linari acknowledge the DRRT Champagne Ardenne and the FEDER program for their support to the NanoMat platform.
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Received by editorial board 01.03.2016.