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Ivan Verzhbitskiy

Publications and source records attributed to Ivan Verzhbitskiy.

13 recordsLinked to original sources

Context-Aware Deep Learning for Defect Classification in Atomic-Resolution STEM

Artificial intelligence is rapidly advancing materials characterization, yet most applications in electron microscopy rely solely on image contrast, overlooking the chemical and experimental context that shapes image formation. This limitation makes defect classification inherently ambiguous, as similar contrasts can arise from different materials or imaging conditions. Here we develop a context-aware learning framework that integrates image-derived contrast with metadata describing composition, beam energy, and detector geometry. Using a systematically constructed dataset of ~55 million simulated patches spanning 576 cases across 96 doped monolayer transition-metal dichalcogenides, we show that conditioning on contextual variables transforms defect classification from an ill-posed image-only task into a well-posed, physically grounded problem. The framework achieves over 98% accuracy on simulations and near-human agreement on experimental data, with a 94% reduction in posterior entropy. By emphasizing contextual grounding over architectural complexity, this approach links experimental image contrast to the underlying chemical and imaging conditions, supporting physically grounded defect assignments and a general pathway toward multimodal AI models for autonomous materials characterization.

cond-mat.mtrl-sci

Ferroelectric brightening of spin forbidden dark excitons in a WSe2/hybrid perovskite heterostructure

Long-lived dark excitons in monolayer WSe2 present promising candidates for carrying spin and valley information, but their optical access and spin manipulation have conventionally required the use of strong external magnetic fields. Here, using a ferroelectric hybrid perovskite heterostructure, we leverage the ferroelectric proximity effect to break the WSe2's in-plane rotational symmetry and brighten the spin-forbidden dark excitons under zero magnetic field conditions. Furthermore, we show that the twist angle between the WSe2 and perovskite crystals controls the ferroelectric coupling strength and valley-contrasting polarization. Our proposed mechanism, supported by a four-band tight-binding model, suggests that the ferroelectric proximity effect induces an asymmetric intersublattice interaction, generating an effective in-plane spin-orbit coupling (SOC) field that rotates spin/valley polarization and brightens dark excitons. Our work establishes ferroelectric proximity coupling as an electrically reconfigurable, magnetic-field-free strategy for spin exciton control in two-dimensional semiconductors.

cond-mat.mtrl-sci

Electrical control of valley polarized charged biexcitons in monolayer WS$_2$

Excitons are key to the optoelectronic applications of van der Waals semiconductors with the potential for versatile on-demand tuning of properties. Yet, their electrical manipulation is complicated by their inherent charge neutrality and the additional loss channels induced by electrical doping. We demonstrate the dynamic control of valley polarization in charged biexciton (quinton) states of monolayer tungsten disulfide, achieving up to a sixfold increase in the degree of circular polarization under off-resonant excitation. In contrast to the weak direct tuning of excitons typically observed using electrical gating, the quinton photoluminescence remains stable, even with increased scattering from electron doping. By exciting at the exciton resonances, we observed the reproducible non-monotonic switching of the charged state population as the electron doping is varied under gate bias, indicating a coherent interplay between neutral and charged exciton states.

cond-mat.mes-hall

Symmetry breaking and spin-orbit coupling for individual vacancy-induced in-gap states in MoS2 monolayers

Spins confined to point defects in atomically-thin semiconductors constitute well-defined atomic-scale quantum systems that are being explored as single photon emitters and spin qubits. Here, we investigate the in-gap electronic structure of individual sulphur vacancies in molybdenum disulphide (MoS2) monolayers using resonant tunneling scanning probe spectroscopy in the Coulomb blockade regime. Spectroscopic mapping of defect wavefunctions reveals an interplay of local symmetry breaking by a charge-state dependent Jahn-Teller lattice distortion that, when combined with strong (~100 meV) spin-orbit coupling, leads to a locking of an unpaired spin-1/2 magnetic moment to the lattice at low temperature, susceptible to lattice strain. Our results provide new insights into spin and electronic structure of vacancy induced in-gap states towards their application as electrically and optically addressable quantum systems.

cond-mat.mes-hall

Laser-induced topological spin switching in a 2D van der Waals magnet

Two-dimensional (2D) van der Waals (vdW) magnets represent one of the most promising horizons for energy-efficient spintronic applications because their broad range of electronic, magnetic and topological properties. Of particular interest is the control of the magnetic properties of 2D materials by femtosecond laser pulses which can provide a real path for low-power consumption device platforms in data storage industries. However, little is known about the interplay between light and spin properties in vdW layers. Here, combining large-scale spin dynamics simulations including biquadratic exchange interactions and wide-field Kerr microscopy (WFKM), we show that ultrafast laser excitation can not only generate different type of spin textures in CrGeTe$_3$ vdW magnets but also induce a reversible transformation between them in a toggle-switch mechanism. Our calculations show that skyrmions, anti-skyrmions, skyrmioniums and stripe domains can be generated via high-intense laser pulses within the picosecond regime. The effect is tunable with the laser energy where different spin behaviours can be selected, such as fast demagnetisation process ($\sim$250 fs) important for information technologies. The phase transformation between the different topological spin textures is obtained as additional laser pulses are applied to the system where the polarisation and final state of the spins can be controlled by external magnetic fields. We experimentally confirmed the creation, manipulation and toggle switching phenomena in CrGeTe$_3$ due to the unique aspect of laser-induced heating of electrons. Our results indicate laser-driven spin textures on 2D magnets as a pathway towards ultrafast reconfigurable architecture at the atomistic level.

cond-mat.mes-hall

Liquid Metal Printed Ultrathin Oxides for Monolayer WS2 Top-Gate Transistors

Two-dimensional (2D) semiconductors are promising channel materials for continued downscaling of complementary metal-oxide-semiconductor (CMOS) logic circuits. However, their full potential continues to be limited by a lack of scalable high-k dielectrics that can achieve atomically smooth interfaces, small equivalent oxide thicknesses (EOT), excellent gate control, and low leakage currents. Here, we report liquid metal printed ultrathin and scalable Ga2O3 dielectric for 2D electronics and electro-optical devices. We directly visualize the atomically smooth Ga2O3/WS2 interfaces enabled by the conformal nature of liquid metal printing. We demonstrate atomic layer deposition compatibility with high-k Ga2O3/HfO2 top-gate dielectric stacks on chemical vapour deposition grown monolayer WS2, achieving EOTs of ~1 nm and subthreshold swings down to 84.9 mV/dec. Gate leakage currents are well within requirements for ultra-scaled low-power logic circuits. Our results show that liquid metal printed oxides can bridge a crucial gap in scalable dielectric integration of 2D materials for next-generation nano-electronics.

cond-mat.mtrl-sci

Data-driven discovery of high performance layered van der Waals piezoelectric NbOI2

Using high-throughput first-principles calculations to search for layered van der Waals materials with the largest piezoelectric stress coefficients, we discover NbOI2 to be the one among 2940 monolayers screened. The piezoelectric performance of NbOI2 is independent of thickness, and its electromechanical coupling factor of near unity is a hallmark of optimal interconversion between electrical and mechanical energy. Laser scanning vibrometer studies on bulk and few-layer NbOI2 crystals verify their huge piezoelectric responses, which exceed internal references such as In2Se3 and CuInP2S6. Furthermore, we provide insights into the atomic origins of anti-correlated piezoelectric and ferroelectric responses in NbOX2 (X = Cl, Br, I), based on bond covalency and structural distortions in these materials. Our discovery that NbOI2 has the largest piezoelectric stress coefficients among 2D materials calls for the development of NbOI2-based flexible nanoscale piezoelectric devices.

cond-mat.mtrl-sci

Mode-center Placement of Monolayer WS$_{2}$ in a Photonic Polymer Waveguide

Effective integration of 2D materials such as monolayer transition metal dichalcogenides (TMDs) into photonic waveguides and integrated circuits is being intensely pursued due to the materials' strong exciton-based optical response. Here, we present a platform where a WS$_{2}$-hBN 2D heterostructure is directly integrated into the photonic mode-center of a novel polymer ridge waveguide. FDTD simulations and collection of photoluminescence from the guided mode indicate that this system exhibits significantly improved waveguide-emitter coupling over a previous elastomer platform. This is facilitated by the platform's enhanced refractive-index contrast and a new method for mode-center integration of the coupled TMD. The integration is based on a simple dry-transfer process that is applicable to other 2D materials, and the platform's elastomeric nature is a natural fit to explore strain-tunable hybrid-photonic devices. The demonstrated ability of coupling photoluminescence to a polymer waveguide opens up new possibilities for hybrid-photonic systems in a variety of contexts.

physics.optics

Measuring valley polarization in two-dimensional materials with second-harmonic spectroscopy

A population imbalance at different valleys of an electronic system lowers its effective rotational symmetry. We introduce a technique to measure such imbalance - a valley polarization - that exploits the unique fingerprints of this symmetry reduction in the polarization-dependent second-harmonic generation (SHG). We present the principle and detection scheme in the context of hexagonal two-dimensional crystals, which include graphene-based systems and the family of transition metal dichalcogenides, and provide a direct experimental demonstration using a 2H-MoSe$_{2}$ monolayer at room temperature. We deliberately use the simplest possible setup, where a single pulsed laser beam simultaneously controls the valley imbalance and tracks the SHG process. We further developed a model of the transient population dynamics which analytically describes the valley-induced SHG rotation in very good agreement with the experiment. In addition to providing the first experimental demonstration of the effect, this work establishes a conceptually simple, com-pact and transferable way of measuring instantaneous valley polarization, with direct applicability in the nascent field of valleytronics.

physics.optics

Elastomeric waveguide on-chip coupling of an encapsulated MoS$_2$ monolayer

We propose a robust photonic platform for encapsulation and addressing of optically active 2D- and nano-materials. Our implementation utilises a monolayer of MoS$_2$ transition metal dichalcogenide embedded in an elastomeric waveguide chip. The structure is manufactured from PDMS using soft-lithography and capable of sustaining a single mode of guided light. We prove that this setup facilitates addressing of the 2D material flake by pumping it with polarised laser light and gathering polarisation-resolved photoluminescence spectra with the extinction ratio of 31, which highlights the potential for selection-rule dependent measurements. Our results demonstrate improved handling of the material and experimental simplification compared to other techniques. Furthermore, inherent elasticity of the host provides an avenue for direct mechanical coupling to embedded materials.

physics.app-ph

Characterization of the second- and third-harmonic optical susceptibilities of atomically thin tungsten diselenide

We report the first detailed characterization of the sheet third-harmonic optical susceptibility, $χ_{s}^{(3)}$, of tungsten diselenide (WSe$_{2}$). With a home-built confocal microscope setup developed to study harmonics generation, we map the second- and third-harmonic intensities as a function of position in the sample, pump power and polarization angle, for single and few layers flakes of WSe$_{2}$. We register a value of $|χ_{s}^{(3)}| \approx$ 0.91 $\times$ 10$^{-28}$ m$^{3}$ V$^{-2}$ at a fundamental excitation frequency of $\hbarω$ = 0.8 eV, which is comparable in magnitude to the third-harmonic susceptibility of other group-VI transition metal dichalcogenides. The simultaneously recorded second-harmonic susceptibility is found to be $|χ_{s}^{(2)}| \approx$ 0.70 $\times$ 10$^{-19}$ m$^{2}$ V$^{-1}$ in very good agreement on the order of magnitude with recent reports for WSe$_{2}$, which asserts the robustness of our values for $|χ_{s}^{(3)}|$.

physics.optics

Evidence for Fast Interlayer Energy Transfer in MoSe2/WS2 Heterostructures

Strongly bound excitons confined in two-dimensional (2D) semiconductors are dipoles with a perfect in-plane orientation. In a vertical stack of semiconducting 2D crystals, such in-plane excitonic dipoles are expected to efficiently couple across van der Waals gap due to strong interlayer Coulomb interaction and exchange their energy. However, previous studies on heterobilayers of group 6 transition metal dichalcogenides (TMDs) found that the exciton decay dynamics is dominated by interlayer charge transfer (CT) processes. Here, we report an experimental observation of fast interlayer energy transfer (ET) in MoSe2/WS2 heterostructures using photoluminescence excitation (PLE) spectroscopy. The temperature dependence of the transfer rates suggests that the ET is Förster-type involving excitons in the WS2 layer resonantly exciting higher-order excitons in the MoSe2 layer. The estimated ET time of the order of 1 ps is among the fastest compared to those reported for other nanostructure hybrid systems such as carbon nanotube bundles. Efficient ET in these systems offers prospects for optical amplification and energy harvesting through intelligent layer engineering.

cond-mat.mes-hall

Engineering bandgaps of monolayer MoS2 and WS2 on fluoropolymer substrates by electrostatically tuned many-body effects

Intrinsic electrical and excitonic properties of monolayer transition metal dichalcogenides are studied on CYTOP fluoropolymer substrates with greatly suppressed unintentional doping and dielectric screening. Ambipolar transport behavior is observed in monolayer WS2 by applying solid states backdates. The excitonic properties of monolayer MoS2 and WS2 are determined by intricate interplays between the bandage renormalization, Pauli blocking and carrier screening against carrier doping.

cond-mat.mtrl-sci