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Kravchenko, Ivan I

Publications and source records attributed to Kravchenko, Ivan I.

At least 19 records
All sourcesNASA NTRSDOE OSTI
DOE OSTI2016

Exploring Polarization Rotation Phase Instabilities in Super-Tetragonal BiFeO3 Epitaxial Thin Films and Their Technological Implications

Many functional properties of ferroelectrics are underlain by structural instabilities, which render these materials very susceptible to small alternating applied fields (electric, mechanical, etc.) through certain constitutive coupling relations, e.g., elastic compliance and piezoelectric response, and often such instabilities can be shifted by static applied fields thus meaning tunable dynamic properties. Structural instabilities are naturally accommodated on the brink of morphotropic phase boundaries (MPB s) where multiple phases of small energy difference coexist in different crystallographic forms. Canonical MPB is realized through compositional mixture, as is typically exemplified by Pb(Zr1-xTix)O3 solid solutions and relaxor ferroelectrics of (1-x)PbMg1/3Nb2/3O3-xPbTiO3. More recently, a strain-driven MPB has been discovered in BiFeO3 (BFO) thin films epitaxially grown on LaAlO3 (LAO) crystal substrates (which imposes about -4.5% in-plane strains). Such an MPB is in between a rhombohedral (R) phase that bulk BFO exhibits and a so-called super-tetragonal (T) phase, which name hints at its giant lattice axial ratio (c/a ~ 1.23) and accordingly high electric polarization (~1.5 C m-2). The discovery of an MPB in BFO has revealed another facet of this multiferroic system, further adding opportunities to its many exotic functionalities such as domain wall conduction, magnetoelectric and photovoltaic effects As with other MPB s, large electric-field induced strains as well as more underlying lattice softening effects are observed near this MPB promising piezoelectric-based applications. In addition, T-phase BFO itself shows distinct properties, e.g., electronic band gap and optical absorption, from the R-phase and the resultant switching effects between them may also be exploitable. However, unlike conventional ferroelectric oxides where the phases across an MPB usually have subtle difference caused by rotations of an ion off-centering polarization, the BFO system bears multiple structural degrees of freedom, in particular antiferrodistortive modes of oxygen octahedral tilt, and a multitude of structural transition paths are thereby facilitated. Moreover, since the MPB of BFO is driven by epitaxial strain, it is sensitive to the strain relaxation related to film thickness and growth conditions, and above some critical thickness the films appear in the form of a hierarchical mixed-phase microstructure involving several coexistent polymorphs with distinct lattice distortion (monoclinic phases) and tilts. Elastic and electrical heterogeneities are necessary consequences of such complex microstructure, which couple to the intrinsic order parameters and expectedly have profound influence on the structural dynamics and material properties. All these make it a demanding task to obtain a deep understanding of this MPB system on par with its application prospect. The thin-film material form also brings in experimental restrictions to the MPB phase transition studies of BFO since many pertinent techniques fail to operate at the nanoscale or suffer from formidably weak signals. Therefore, static structural characterizations using e.g. X-ray diffraction, electron microscopy and Raman scattering have prevailed thus far. Recently, we adapted band-excitation piezoresponse spectroscopy (BEPS) to probe the bias-induced R/T phase transition dynamics of BFO/SrTiO3 (STO) thin films, and revealed the soft-mode elastic behavior of the transition. The efficacy of our method, along with standard piezoresponse force microscopy (PFM), can be significantly leveraged by integrating a variety of local and/or global excitations, e.g., tip pressure (10 s GPa attainable), heating, photo-irradiation, available to modern scanning probe platforms. This can thus afford unique opportunities to survey the structural dynamics of ferroelectric materials coupled to those field variables, enabling rapid discovery or validation of their functional properties. In this study, we focus on the T-phase BFO/LAO system; we have examined its phase transition behavior not only due to local coaction of tip bias and loading force, but also under device-level global electric fields in a coplanar capacitor. The intrinsic elastic softening phenomena near the structural transitions are comprehensively revealed by BEPS and corroborated by phase-field modeling. Our findings may open a new pathway for technological utilization of the MPB phase instabilities of BFO.

BiFeO3↗
DOE OSTI2014

Direct Probing of Charge Injection and Polarization-Controlled Ionic Mobility on Ferroelectric LiNbO3 Surfaces

Mapping surface potential with time-resolved Kelvin Probe Force Microscopy (tr-KPFM) in LiNbO3 periodically-poled single crystal revealed activation of the surface ionic subsystem. Electric fields higher than certain threshold value but lower than the switching field induce injection of charge from the biased electrode, formation of an active region in its vicinity and uneven distribution of screening charge on the opposite ferroelectric domains. Tr-KPFM technique allows investigating these phenomena in details.

Strelcov, Evgheni↗
DOE OSTI2014

Controlled Vapor Phase Growth of Single Crystalline, Two-Dimensional GaSe Crystals with High Photoresponse

Abstract Compared with their bulk counterparts, atomically thin two-dimensional (2D) crystals exhibit new physical properties, and have the potential to enable next-generation electronic and optoelectronic devices. However, controlled synthesis of large uniform monolayer and multi-layer 2D crystals is still challenging. Here, we report the controlled synthesis of 2D GaSe crystals on SiO2/Si substrates using a vapor phase deposition method. For the first time, uniform, large (up to ~60 m in lateral size), single-crystalline, triangular monolayer GaSe crystals were obtained and their atomic resolution structure were characterized. The size, density, shape, thickness, and uniformity of the 2D GaSe crystals were shown to be controllable by growth duration, growth region, growth temperature, and argon carrier gas flow rate. The theoretical modeling of the electronic structure and Raman spectroscopy demonstrate a direct-to-indirect bandgap transition and progressive confinement-induced bandgap shifts for 2D GaSe crystals. The 2D GaSe crystals show p-type semiconductor characteristics and high photoresponsivity (~1.7 A/W under white light illumination) comparable to exfoliated GaSe nanosheets. These 2D GaSe crystals are potentially useful for next-generation electronic and optoelectronic devices such as photodetectors and field-effect transistors.

Li, Xufan↗
DOE OSTI2014

In situ formation of micron-scale Li-metal anodes with high cyclability

Scanning probe microscopy methods have been used to fabricate and cycle micron-scale Li anodes deposited electrochemically under nanofabricated Au current collectors. An average Li volume of 5 10^8 nm3 was deposited and cycled with 100 % coulombic efficiency for ~ 160 cycles. Integrated charge/discharge values agree with before/after topography, as well as in situ dilatometry, suggesting this is a reliable method to study solid-state electrochemical processes. In this work we illustrate the possibility to deposit highly cyclable nanometer thick Li electrodes by mature SPM and nanofab techniques which can pave the way for inexpensive nanoscale battery arrays.

Arruda, Thomas M↗