arXiv subjects

A. Vasilev

Publications and source records attributed to A. Vasilev.

7 recordsLinked to original sources

arXiv2026

Laser induced optical reconfiguration in Ge_Sb_Te Films with composition dependent response

Phase change Ge_Sb_Te (GST) materials exhibit pronounced optical contrast and tunability driven by structural transformations, enabling a diverse range of photonic and optoelectronic applications. GST materials undergo reversible amorphous crystalline phase transitions during which the refractive index and extinction coefficient increase significantly in the crystalline phase across a broad spectral range. However, a systematic correlation between local composition, crystalline microstructure, and broadband optical response within as deposited crystalline GST films has not been established, particularly for films spanning various compositions within a single growth process and in the absence of amorphous-crystalline transitions. Here, we report a systematic study of composition resolved optostructural property relationships in as-deposited crystalline GST films spanning Ge3Sb2Te6, Ge2Sb2Te5, and GeSb2Te4 within a single CVD process, avoiding intermediate phase transitions. This enables direct correlations between composition, microstructure, morphology, and broadband optical response across 400-1700 nm. Specifically, the compositional gradient results in a shift of the absorption minimum from 815.9 nm to 889.8 nm, accompanied by a 5.9 fold change in the intensity, reflecting the strong dependence of optical behavior on composition and microstructure. We further show that femtosecond laser irradiation enables spatially selective tuning of the optical response. These findings establish crystalline GST films as a platform for broadband-tunable, spatially programmable photonic elements, controllable through both compositional design and localized laser processing.

cond-mat.mtrl-sci↗

arXiv2026

Polyvinylpyrrolidone planarized liquid crystalline 1T-WS2/rGO hybrid nanocomposites-based humidity sensing platform

Two-dimensional hybrid nanocomposites combining graphene-like materials and transition metal dichalcogenides (TMDCs) are created using various synthesis methods and are vital for environmental sensing due to the synergistic effects of their components. These hybrid materials offer enhanced sensitivity, selectivity, surface-to-volume ratio, tunable electronic properties, strong interaction with analytes, and fast detection capabilities, which address the limitations of individual materials. This is due to their outstanding surface-to-volume ratio, tunable electronic properties, strong interactions with analytes, as well as their high sensitivity, selectivity, and fast detection capabilities, which are crucial for environmental sensing. Herein, a hydrothermally synthesized and polyvinylpyrrolidone (PVP)-stabilized 1T phase tungsten disulfide/reduced graphene oxide, 1T-WS2/PVP/rGO, hybrid nanocomposite is reported. The intrinsic liquid crystalline behavior of the synthesized composite enables the formation of highly uniform films for effective humidity sensing. The optostructural characterization and key performance parameters, including response and recovery times, of the 1T-WS2/PVP/rGO hybrid nanocomposite-based structure are comprehensively analyzed and studied. This is the first to report on the synthesis of a 1T-WS2/PVP/rGO hybrid nanocomposite, its liquid crystalline phase formation, and application in humidity sensing. The proposed sensing platform introduces a novel approach to humidity sensing using 1T-WS2/PVP/rGO liquid crystalline films, which combine the metallic advantages of 1T-WS2, the stabilizing role of PVP and the conductive framework of rGO into aligned LC structures for enhanced sensitivity, rapid response and environmental robustness.

physics.app-ph↗

arXiv2025

Revisiting THz absorption in GO and rGO liquid crystalline films

With a swift progress in modern high-throughput communication systems, security, sensing and medicine utilizing THz range technologies, the demand for easy-to-fabricate, lightweight and high-performance absorbing materials has increased drastically. Notably, traditional approaches of eliminating unwanted radiation based on metasurfaces often face fabrication challenges limiting their practicality. In this study, we propose a straightforward approach for fabricating graphene oxide (GO) and reduced graphene oxide (rGO) liquid crystalline (LC) films via the vacuum filtration method and investigate their THz absorption characteristics. Here, the presence of LC phase in our electrochemically exfoliated GO and rGO LC films was confirmed by ellipsometric characterization. THz time-domain spectroscopy (TDS) measurements reveal that these films possess a low reflectance and transmittance confirming their strong absorptive properties within 0.4 - 1.6 THz frequency range for 2 micrometer thick GO and rGO LC films. Particularly, the GOLC film shows 37 % average absorption at a thickness of 2.12 micrometer, which is 221 times smaller than the central wavelength. Similarly, the rGOLC film reaches 50 % absorption with a 1.68 micrometer thickness, 279 times smaller than the central wavelength. These findings provide valuable insights for development of GO- and rGO-based LC THz absorbers with highly tunable properties due to the ordering of GO flakes. Specifically, the LC phase of GO contributes to the formation of more uniform films with enhanced absorption due to the compact stacking and denser packing, compared to conventional GO films with randomly oriented GO flakes.

cond-mat.mtrl-sci↗

arXiv2025

L-Cysteine Polymorph Coatings for THz Sensing Metasurfaces

The electromagnetic response of metasurfaces can be intentionally engineered by carefully designing their unit cells. Narrowband metasurfaces, characterized by high quality factors, can serve as an efficient platform for biosensing in optical to THz regimes, to explore new structural forms of biomolecules, further elevating the capabilities of THz sensing technologies. In this study, an all-metallic metasurface featuring structural asymmetry is proposed to analyze L-Cysteine with orthorhombic and monoclinic crystallographic structures. The influence of the concentration of L-Cysteine in the monoclinic phase on the THz response of the metasurface was performed using a drop-casting method. Our findings reveal a noticeable frequency shift as analyte concentration raises, underscoring the potential for high sensitivity in detecting biomolecules. Thus, this research demonstrates that asymmetric THz metasurfaces offer rather promising detection capabilities at lower THz frequencies between two crystallographic structures of L Cysteine, with a Q-factor of 37, and a sensitivity of 1.430 THz ml/g. We believe that our results can be of great value to the development of biosensors for improved material characterization at low frequencies.

physics.optics↗

arXiv2025

Dual Effect of L-Cysteine on the Reorientation and Relaxation of Fe3O4-Decorated Graphene Oxide Liquid Crystals

Here, we study the dynamics of Fe3O4-decorated L-Cysteine-functionalized GOLC director under an external magnetic field and analyze L-Cysteine's influence on the reorientation and relaxation time of the director. In particular, Fe3O4 nanoparticles were synthesized by solution-combustion method and added for altering orientational properties of GO which we synthesized electrochemically and functionalized by L-Cysteine. In addition, a comprehensive comparison of the director behaviour of GOLC and Fe3O4-decorated L-Cysteine-GOLC was undertaken to verify the tunability of the aforementioned systems. Furthermore, we demonstrate dual-effect of L-Cysteine on the magnetic field-induced alignment and relaxation time of GOLC systems, namely decrease in reorientation time and at the same time increase in relaxation time. Besides, micropattern creation and controlling in the drying drops of GOLC (net- and knit-like, flower-like, radial- and parallel-strip etc.) using a magnetic field were shown. The results of our studies could facilitate the fabrication of ordered and patterned tunable GOLC assemblies for a range of advanced applications. GOLC was undertaken to verify the tunability of the aforementioned systems. Furthermore, we demonstrate dual-effect of L-Cysteine on the magnetic field-induced alignment and relaxation time of GOLC systems, namely decrease in reorientation time and at the same time increase in relaxation time. Besides, micropattern creation and controlling in the drying drops of GOLC (net- and knit-like, flower-like, radial- and parallel-strip etc.) using a magnetic field were shown. The results of our studies could facilitate the fabrication of ordered and patterned tunable GOLC assemblies for a range of advanced applications.

cond-mat.soft↗

arXiv2024

Improving the Electro-Optical Properties of MoS$_2$/rGO Hybrid Nanocomposites Using Liquid Crystals

Hybrid systems of two-dimensional (2D) materials (such as graphene-family materials and 2D transition metal dichalcogenides) are attracting much attention due to their distinctive optoelectronic, thermal, mechanical, and chemical properties. The application perspectives of these materials in various fields further expand when enriching those with liquid crystals (LCs) primarily due to their enhanced tunability and functionality. In this study, we report on the hydrothermal synthesis of hybrid nanocomposites composed of MoS$_2$ and rGO and discuss tuning possibilities of their electro-optical properties by incorporating thermotropic LCs. In particular, we demonstrate that the incorporation of 5CB LC increases the sensitivity and charge storage efficiency of the hybrid nanocomposites. In addition, we also present the responsivity, detectivity, and response time properties of the hybrid nanocomposites of MoS$_2$/rGO, both with and without the inclusion of nematic LCs. Furthermore, we demonstrate that the system exhibits a 5CB-induced photocurrent switching effect. We believe the findings will open new doors for applications of these materials in optoelectronics and photonics.

cond-mat.mtrl-sci↗

arXiv2024

E-Beam Induced Micropattern Generation and Amorphization of L-Cysteine-Functionalized Graphene Oxide Nano-composites

The evolution of dynamic processes in graphene-family materials are of great interest for both scientific purposes and technical applications. Scanning electron microscopy and transmission electron microscopy outstand among the techniques that allow both observing and controlling such dynamic processes in real time. On the other hand, functionalized graphene oxide emerges as a favorable candidate from graphene-family materials for such an investigation due to its distinctive properties, that encompass a large surface area, robust thermal stability, and noteworthy electrical and mechanical properties after its reduction. Here, we report on studies of surface structure and adsorption dynamics of L-Cysteine on electrochemically exfoliated graphene oxides basal plane. We show that electron beam irradiation prompts an amorphization of functionalized graphene oxide along with the formation of micropatterns of controlled geometry composed of L-Cysteine-Graphene oxide nanostructures. The controlled growth and predetermined arrangement of micropatterns as well as controlled structure disorder induced by e beam amorphization, in its turn potentially offering tailored properties and functionalities paving the way for potential applications in nanotechnology, sensor development, and surface engineering. Our findings demonstrate that graphene oxide can cover L-Cysteine in such a way to provide a control on the positioning of emerging microstructures about 10-20 um in diameter. Besides, Raman and SAED measurement analyses yield above 50% amorphization in a material. The results of our studies demonstrate that such a technique enables the direct creation of micropatterns of L-Cysteine-Graphene oxide eliminating the need for complicated mask patterning procedures.

cond-mat.mtrl-sci↗