Theory-assisted determination of nano-rippling and impurities in atomic resolution images of angle-mismatched bilayer graphene

Ripples and impurity atoms are universally present in 2D materials, limiting carrier mobility, creating pseudo–magnetic fields, or affecting the electronic and magnetic properties. Scanning transmission electron microscopy (STEM) generally provides picometer-level precision in the determination of the location of atoms or atomic 'columns' in the in-image plane (xy plane). However, precise atomic positions in the z-direction as well as the presence of certain impurities are difficult to detect. Furthermore, images containing moiré patterns such as those in angle-mismatched bilayer graphene compound the problem by limiting the determination of atomic positions in the xy plane. Here, we introduce a reconstructive approach for the analysis of STEM images of twisted bilayers that combines the accessible xy coordinates of atomic positions in a STEM image with density-functional-theory calculations. The approach allows us to determine all three coordinates of all atomic positions in the bilayer and establishes the presence and identity of impurities. The deduced strain-induced rippling in a twisted bilayer graphene sample is consistent with the continuum model of elasticity. We also find that the moiré pattern induces undulations in the z direction that are approximately an order of magnitude smaller than the strain-induced rippling. A single substitutional impurity, identified as nitrogen, is detected. The present reconstructive approach can, therefore, distinguish between moiré and strain-induced effects and allows for the full reconstruction of 3D positions and atomic identities

Synthesis and luminescent properties of PbS/SiO2 core-shell quantum dots

The research focuses on the development of techniques for creating core-shell structures, based on colloidal PbS quantum dots (PbS QDs) and establishing the influence of the dielectric SiO2 shell on the luminescent properties of PbS QDs. The objects of the study were PbS QDs with an average size of 3.0±0.5 nm, passivated with thioglycolic acid (TGA) and PbS/SiO2 QDs, based on them with an average size of 6.0±0.5 nm. When we passivated the PbS QD interfaces with thioglycolic acid molecules, there were two luminescence peaks at 1100 and at 1260 nm. It was found that increasing the temperature of the colloidal mixture to 60 °C provides an increase in the intensity of the long-wave peak. An analysis of the luminescence excitation spectra of both bands and the Stokes shift showed that the band at 1100 nm is associated with the radiative annihilation of an exciton, while the band at 1260 nm is due to recombination at trap levels. The formation of PbS/SiO2 QDs suppresses trap state luminescence, indicating the localization of luminescence centers predominantly at QD interfaces. The exciton luminescence at 1100 nm becomes more intensiv

Counter-Pandemic Vector of Remote Learning for University Students: Risks and Benefits of Educational Process Large-Scale Digitalization

The relevance of this study is due to the challenges of the global pandemic, which has led to the large-scale development of education digitalization. In this regard, this article is devoted to revealing the features of remote learning in the context of a global pandemic, identifying the risks and benefits of digital learning. The article reveals the discursive content of the counter-pandemic vector of distance learning for University students; defines the classification of risks and advantages of large-scale digitalization of students’ remote learning. Based on the results of the study, the authors justified the consolidated model of students distance learning, the counter-pandemic vector of which is a large-scale digitalization of the University educational process. The effectiveness of the model is proved by the results of its use in the process of large-scale digitalization of students’ remote learning in the context of a global pandemic. The materials of the article have practical application and can be useful in the development and implementation of digital educational complexes. They are recommended to teachers and students of the University, methodologists, curators, Tutors

Atomic-resolution visualization and doping effects of complex structures in intercalated bilayer graphene

Molecules intercalating two-dimensional materials form complex structures that have been characterized primarily by spatially averaged techniques. Here we use aberration-corrected scanning transmission electron microscopy and density-functional-theory (DFT) calculations to study the atomic structure of bilayer graphene (BLG) and few-layer graphene (FLG) intercalated with FeCl3. In BLG, we discover two distinct intercalated structures that we identify as monolayer FeCl3 and monolayer FeCl2. The two structures are separated by atomically sharp boundaries and induce large free-carrier densities on the order of 1013cm−2 in the graphene layers. In FLG, we observe multiple FeCl3 layers stacked in a variety of possible configurations with respect to one another. Finally, we find that the microscope's electron beam can convert the FeCl3 monolayer into FeOCl monolayers in a rectangular lattice. These results reveal the need for a combination of atomically resolved microscopy, spectroscopy, and DFT calculations to identify intercalated structures and study their properties

Trap state and exciton luminescence of colloidal PbS quantum dots coated with thioglycolic acid molecules

This work presents the results of studying the IR luminescence of colloid PbS quantum dots coated with molecules of thioglycolic acid. Luminescence of the sample was recorded using the InGaAs image sensor PDF 10C/M (ThorlabsInc., USA) and a diffraction monochromator with 600 mm-1 grating. To study the temperature dependence of luminescence, the sample was cooled in a nitrogen cryostat down to 80 К. A redistribution of the luminescence intensity between two peaks (1100 and 1280 nm) was identified upon a decrease in temperature. It was shown that an exciton absorption peak was present in the excitation spectrum for the short-wave luminescence peak, and the Stokes shift was ΔEstokes ~ 0.1 eV. On the contrary, the exciton peak was absent in the luminescence excitation spectrum of the long-wave band, and its red boundary was shifted towards the short-wave region, that provided the Stokes shift of more than 0.3 eV. It was concluded that the short-wave luminescence band appeared as a result of the radiative annihilation of an exciton, while the long-wave band appeared due to the recombination of charge carriers at trap states. Trap state luminescence was effectively excited upon direct absorption of the radiation by the luminescence centre. A three-level diagram was suggested that determined the IR luminescence of colloid PbS quantum dots coated with thioglycolic acid molecules

Spectral properties of hybrid associates of colloidal quantum dots Zn0.5Cd0.5S, europium tenoyltrifluoroacetonate and methylene blue

Hybrid associates formed from colloidal Zn0.5Cd0.5S quantum dots, passivated with thioglycolic acid, europium tenoyltrioteracetonate and methylene blue molecules, absorption, luminescence, IR and time-resolved spectroscopy technique are studied. The shift of the IR absorption bands of COO- and C=O groups of thioglycolic acid and europium thenoyltrifluoroacetonate has been detected. An increase in the efficiency of excitation of europium luminescence and a simultaneous increase lifetime of its luminescence upon adsorption on Zn0.5Cd0.5S quantum dots were found. Addition of methylene blue (thionine) molecules leads to quenching of the trap state luminescence of Zn0.5Cd0.5S and intracentric luminescence of Eu3+. A conclusion about the adsorption of Eu3+ on the surface of Zn0.5Cd0.5S quantum dots and the nonradiative energy transfer to methylene blue molecules was made

Spectral properties of hybrid associates of colloidal quantum dots Zn

Hybrid associates formed from colloidal Zn0.5Cd0.5S quantum dots, passivated with thioglycolic acid, europium tenoyltrioteracetonate and methylene blue molecules, absorption, luminescence, IR and time-resolved spectroscopy technique are studied. The shift of the IR absorption bands of COO- and C=O groups of thioglycolic acid and europium thenoyltrifluoroacetonate has been detected. An increase in the efficiency of excitation of europium luminescence and a simultaneous increase lifetime of its luminescence upon adsorption on Zn0.5Cd0.5S quantum dots were found. Addition of methylene blue (thionine) molecules leads to quenching of the trap state luminescence of Zn0.5Cd0.5S and intracentric luminescence of Eu3+. A conclusion about the adsorption of Eu3+ on the surface of Zn0.5Cd0.5S quantum dots and the nonradiative energy transfer to methylene blue molecules was made