SUPERLATTICES BASED ON LEAD SULFIDE QUANTUM DOTS

Oskolkov E.O., Ushakova E.V., Golubkov V.V., Litvin A.P., Parfenov P.S., Baranov A.V. 1) ITMO University, 49 Kronverkskiy pr., Saint-Petersburg, Russia.

2)Institute of Silicate Chemistry of Russian Academy of Sciences, 2 Adm. Makarova emb., Saint-Petersburg, Russia

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Introduction Experimental

Period of QDSLs Period of QDSLs after treatment

onclusions

References

For the past few years a lot of studies have been held on semiconductor nanocrystals absorbing and emitting in near-infrared region of spectrum, such as lead-chalcogenide quantum dots (QDs). These QDs can self-assemble into superstructures, namely, superlattices (QDSLs) [1]. The properties of the materials based on QDSLs depend strongly on distances between QDs. Therefore the establishment of the parameters of self-assembly process, which affects the morphology of superstructures based on QDs, is of great importance. This is the key to the ability of fabricating materials with definite and controllable parameteres and properties, for improving the photovoltaic devices in near-IR region of spectrum. Colloidal lead sulfate (PbS) quantum dots stand out due to their specific properties as large extinction coefficients, a large overlap of absorption spectrum with solar radiation spectrum, small effective masses of charge carriers, possibility of multiple exciton generation and hot-carrier extraction [3]. Despite large number of studies in this field, our knowledge of QD self-assembly is still far from being complete. The most important parameters, affecting this process are QD concentration, QD size, type and quantity of ligands and type of solvents. Studiyng these parameters effect on self-assembly leads to need for carrying out complex experimental data analysis and restrains the possibility of using of self-organized structures in advanced technology. Thus more detailed studies must be held to clarify some unknown aspects of QD self-assembly process and to get precise control of it. In this work we experimentally investigate the influence of the type and amount of ligands, together with temperature dependence, on QDSL period.

QDSLs formation PbS QDs were synthesized by hot-injection method. QD Diameters: 2.8 to 8.9 nm. Solvent: tetrachlormethane. The superlattices were obtained by the QD colloidal solution evaporation on the substrate. Substrate: a glass coverslide. To study the ligands influence on interparticle distances QDs with diameters of 6.3 nm were used. The stock QD solutions were treated the following way: 1) 100 l (acetone/isopropanol/methanol) + 50 l (QD stock solution), 2) stirring in the ultrasound bath. 3) centrifuging (to precipitate the nanocrystals) 4) redispersing in 50 l of TCM 5) stirring in the ultrasonic bath for 5-10 minutes 6) 15 l of obtained QD solution dripped on the glass substrate and dried in an ambient atmosphere at room

temperature

1. Nie Z., Petukhova A., Kumacheva E., Nature nanotechnology , 5(1), 15-25 (2009). 2. Quan Z. et al. Solvent-mediated self-assembly of nanocube superlattices //Journal of the American Chemical Society. 2014. . 136. . 4. . 1352-1359. 3. Rogach A. L. et al. InfraredEmitting Colloidal Nanocrystals: Synthesis, Assembly, Spectroscopy, and Applications //Small. 2007. . 3. . 4. . 536-557. 4. Ushakova E. V. et al. Self-organization of lead sulfide quantum dots of different sizes //SPIE Photonics Europe. International Society for Optics and Photonics, 2014. . 912625-912625-7.

As it was investigated earlier[4], the distances between QDs in superlattices depend linearly on QD diameter. The geometrical assembly of the structures does not depend on the QDs size. The period of ordering is of the same order as the QD diameter. The dependence slope shown in the Figure 4 by a dashed line is 1.28 1.1, though for the close-packed structures the slope should be strictly 1. The difference is likely caused by layer of organic molecules on the QD surface. This layer thickens as the QDs diameter increases. Thus the direct contact between QDs does not occur. The red line (x+1.8) shows the dependence of the layer thickness of ligands. The value of 1.8 nm corresponds to the oleic acid molecule size. The distances between QDs go up to ~3.4 nm for the larger-sized dots. This indicates an excessive amount of oleic acid molecules on QDs surface.

To determine the diameters of QDs and to study the superlattices spatial geometry the Small-Angle X-ray Scattering (SAXS) method was used. SAXS range: from 6 to 250 arcmin Setup: slit collimation geometry defining a beam of effectively infinite length Radiation: CuK radiation at wavelength of 1.54 Filter: Ni Angle resolution: ~1 arcmin The general scheme of this method is shown on Fig.1

Figure 1. Scheme of setup for SAXS measurements

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Figure 4. Correlation between QDs diameters and period of QDSLs

Figure 5. SAXS of treated PbS QDs

Figure 5 presents a comparison of the SAXS patterns. The obtained results indicate that the removal of the ligands leads to the shift of peaks positions to larger angles. At the same time, peaks become narrower as a higher ordering of QDSL takes place. Optical microscopy images (Figure 3) of QDSLs formed from QDs treated by acetone or isopropanol show that the obtained superstructures look more homogeneous than untreated. However, the attempt to remove ligands from the QD surface did not give us the ability to form close-packed QDSLs. The distance between the surfaces of the QDs was from 0.4 to 0.8 nm. This shows that the ligands were removed only partially.

Figure 3. Images of fabricated QDSLs from confocal microscope (Renishaw)

Figure 6. SAXS of treated PbS QDSLs

The annealing treatment of QDSLs was being held in vacuum at temperature of 380 C for periods of 1, 10 and 30 minutes. The QDSLs were fabricated by the method described in previous section. The given temperature and time intervals allow to vaporize the ligands from the QDs surfaces. The analysis of the the SAXS data for treated SLs showed that an increasing of the annealing time leads to more and more disordering in the initial QDSLs. And for the 30 minute annealing the QDSLs were destroyed completely and the sample represented the ensemble of isolated QDs on a substrate. This indicates that thermal treatment does not improve the QDSLs parameters such as ordering and homogenity. It is also can be assumed that complete ligand removal from the QD surface leads to disordering of the QD assemblies. Hence, the surface ligands affect much the QDSL formation process.

The morphology of QDSLs depends type and amount of ligands on QDs surfaces. Our study has shown that treatment with isopropanol provides formation of the most homogenous SLs with the minimal lattice period compared to other solvents used. The method of annealing of the QDSLs proved to be inefficient for QDSL improving, as the structures become less ordered along with the increasing of annealing time.

Acknowledgement This work was funded by grant 14.25.31.0002 and Government Assignment No. 3.109.2014/K of the Ministry of Education and Science of the Russian Federation. A.P.L. and E.V.U. thank the Ministry of Education and Science of the Russian Federation for support via the Scholarships of the President of the Russian Federation for Young Scientists and Graduate Students (20132015).

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Figure 2. Formation of close-packed structure