The physical basis of the domain engineering in ferroelectrics and its application to lithium niobate crystals were reviewed. The unified kinetic approach to the domain structure evolution in electric field was formulated and its validity for understanding the variety of observed domain evolution scenarios was demonstrated. The kinetics and statics of the domain structure in the crystals of lithium niobate family including congruent, stoichiometric, and MgO doped ones have been discussed. The main stages of the periodical poling process and related problems have been pointed out. The basic poling techniques applied for creation of the periodical domain structures in bulk crystals and waveguides were compared. The recent applications of the periodically poled lithium niobate for light frequency conversion using second harmonic generation and optical parametric oscillation, excitation of the surface acoustic waves, and generation of terahertz radiation have been discussed. The special attention has been paid for achievements in fabrication of high-power optical parametric oscillation and integrated optical devices with periodically poled lithium niobate. The future trends in periodical poling and development of the nanodomain engineering which will allow to create the nanoscale domain patterns necessary for utilization of the new nonlinear interactions were reviewed.

The chemical vapor deposition (CVD) of molybdenum disulfide (MoS2) single-layer films onto periodically poled lithium niobate is possible while maintaining the substrate polarization pattern. The MoS2 growth exhibits a preference for the ferroelectric domains polarized "up" with respect to the surface so that the MoS2 film may be templated by the substrate ferroelectric polarization pattern without the need for further lithography. MoS2 monolayers preserve the surface polarization of the "up" domains, while slightly quenching the surface polarization on the "down" domains as revealed by piezoresponse force microscopy. Electrical transport measurements suggest changes in the dominant carrier for CVD MoS2 under application of an external voltage, depending on the domain orientation of the ferroelectric substrate. Such sensitivity to ferroelectric substrate polarization opens the possibility for ferroelectric nonvolatile gating of transition metal dichalcogenides in scalable devices fabricated free of exfoliation and transfer.

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Precise control of the domain structure in ferroelectric single crystals is one of the most important and challenging tasks in physics of ferroelectrics. So far, main part of investigations in this area was aimed at realization of high efficiency nonlinear optical interactions, such as second harmonic generation (SHG) and optical parametric oscillation. These applications require precise spatial variation of the spontaneous polarization, which distribution within the crystal is determined by the positions and orientations of the domain walls (DWs). Recently, the scientific attention has turned from the domains to the DWs themselves: these movable unit-cell-thick interfaces were proposed as the building blocks for reconfigurable nanoelectronics devices, because their properties can differ drastically from the bulk parent material [1, 2]. In particular, it has been shown that conductivity of the charged domain walls (CDWs) in ferroelectrics is many orders of magnitude higher, than that in the single domain state [3]. The utilization of the CDW as nanoelectronics devices requires three technological aspects to be developed: (1) controllable creation, (2) geometry tuning, and (3) removal. We present the experimental study of the CDW formation in congruent lithium niobate single crystals during polarization reversal using liquid and solid-state electrodes, as well as their combination [4]. It was shown that material of the electrode applied to the Z-polar surface is crucial. It was shown that CDW can be formed by two alternative procedures: (1) by forward switching for liquid electrode at Z+ polar surface and solid electrodeat Zone , (2) by backward switching for solid electrode at Z+ polar surface and liquid electrode at Zone. The obtained domain structure was investigated in the bulk of the crystal using scanning Cherenkov-type SHG microscopy [5]. For both used procedures the CDWs were formed when domains grew from Z+ polar surface covered by solid or liquid electrode towards Z-surface covered by solid electrode. Thus, existence of the solid electrode on Z-surface is the necessary condition for CDW formation. The created CDW can be transformed to almost neutral domain wall by application of the field pulse of the reverse polarity for both procedures. As a result, the tilt of the formed CDW can be tuned reversibly in the range from 0.2 to 1.2 degrees resulting in change between almost isolated and highly conductive states. When the tilt exceeded 1.2 degrees the CDW became jugged due to formation of additional spikes. For the second procedure the unusual partial backward switching during forward polarization reversal was revealed. Moreover, we have demonstrated that the created CDW can be used as a nanoelectronics channel for local electrolysis opening the additional possibilities for the ferroelectric lithography. The equipment of the Ural Center for Shared Use "Modern nanotechnology" Ural Federal University was used. The research was made possible in part by RFBR (grant 18-32-00641_mol_a).

The influence of the relative humidity on the domain growth during local switching by a conductive tip of a scanning probe microscope has been studied in Rb:KTP single crystals. The abnormal formation of the elongated hexagonal domains during the local switching in an axial-symmetric field has been attributed to the anisotropy of the domain wall velocity. The increase and subsequent decrease in a hexagonal domain aspect ratio as a function of the pulse duration for high humidity have been attributed to the influence of the high-resistivity water layer. We assume that the domain elongation by the fast walls is limited by the water layer representing the series resistance, which depends on the wall position, while the domain widening by the slow walls is limited by their mobility. The computer simulation of the spatial distribution of the electric field has demonstrated that the appearance of a water meniscus decreases the field close to the tip and increases it far from the tip. This result has allowed explaining the published alternative effects of humidity on the domain size: (1) decrease in size for small domains and (2) increase in size for large ones. The obtained results highlight the necessity of humidity control for domain engineering.

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