Παρουσίαση Μεταπτυχιακής Διπλωματικής Εργασίας του κ. Μίνωα Χαιρέτη

27 Οκτωβρίου 2023

ΠΑΝΕΠΙΣΤΗΜΙΟ ΚΡΗΤΗΣ

ΤΜΗΜΑ ΕΠΙΣΤΗΜΗΣ ΚΑΙ ΤΕΧΝΟΛΟΓΙΑΣ ΥΛΙΚΩΝ

 

ΠΑΡΟΥΣΙΑΣΗ ΜΕΤΑΠΤΥΧΙΑΚΗΣ ΔΙΠΛΩΜΑΤΙΚΗΣ ΕΡΓΑΣΙΑΣ

 

Τίτλος

«Fabrication and Characterization of 2D Halide Perovskite Crystals for Polaritonic Devices»  

του Μίνωα Χαιρέτη

μεταπτυχιακού φοιτητή του Τμήματος Επιστήμης και Τεχνολογίας Υλικών του Πανεπιστημίου Κρήτης

 

Επιβλέπων: Παύλος Σαββίδης

 

Τετάρτη 1 Νοεμβρίου 2023 Ώρα 10:00

H παρουσίαση θα πραγματοποιηθεί στην αίθουσα Τηλε-εκπαίδευσης (Ε130), στο κτήριο του Τμήματος Μαθηματικών και Εφαρμοσμένων Μαθηματικών, του Πανεπιστημίου Κρήτης

Abstract

« The main aim of this work was to fabricate high quality thin 2D Halide perovskite crystals and use them to demonstrate strong light-matter coupling in various device geometries. We explored two alternative methods to fabricate crystals, namely, liquid exfoliation and solution method. We characterized the material structure using XRD as well as microscope/SEM techniques and employ optical measurements such as PL, Reflectivity, Power and Temperature dependence, to characterize exciton emission properties in this material. Liquid exfoliation method produced relatively small sized perovskite crystals not compatible with their introduction inside wavelength sized cavity. The second method, provided us with the much better results both in crystal size and structure as well as excitonic emission. Τhe successfully optimized material was then incorporated inside optical cavity and on top of a Bragg reflector. Large angle white light reflectivity measurements at room temperature revealed the appearance of resonant mode residing at the surface of the Bragg mirror whose coupling with overlaying perovskite excitons was studied.  Finally, strong-light matter coupling at room temperature was obtained when incorporating such crystals inside DBR mirror-based cavities demonstrating the great promise these materials pose for real world polaritonic applications.».

Παρουσίαση Μεταπτυχιακής Διπλωματικής Εργασίας του κ. Κοσμά Γιάνναρη (αλλαγή ώρας και αίθουσας)

20 Οκτωβρίου 2023

Τίτλος

«Development of Nanohybrid Polyurethane Coatings with Self-healing Properties»  

του Κοσμά Γιάνναρη

μεταπτυχιακού φοιτητή του Τμήματος Επιστήμης και Τεχνολογίας Υλικών του Πανεπιστημίου Κρήτης

 

Επιβλέπουσα: Κυριακή Χρυσοπούλου

 

Παρασκευή 20 Οκτωβρίου 2023, Ώρα 18:00

H παρουσίαση θα πραγματοποιηθεί στην αίθουσα Β2, στο κτήριο του Τμήματος Χημείας, του Πανεπιστημίου Κρήτης

Abstract

In recent years, self-healing materials have been a subject of increased research interest due to their ability to self-repair damages either autonomically or by external stimuli, extending their lifetime and therefore assisting in the reduction of waste. Polyurethanes represent one of the most promising materials to use towards this direction. In particular, waterborne polyurethane dispersions (WPUDs) are widely used in coatings giving the additional advantage of the reduction of volatile organic compounds (VOCs) in the environment in comparison with the traditional coatings which are solvent-based. At the same time, inorganic and / or graphitic nano-additives are widely used to improve the mechanical, thermal, optical and electronic properties of a polymeric matrix resulting in polymer nanocomposites with an overall optimized behavior.

In this work, nanohybrid polyurethane coatings were developed and their self-healing properties were evaluated. 2D materials like graphitic carbon nitride, g-C3N4, graphene oxide, GO, and Ti3C2Tx which belongs to the newly emerged class of Mxenes were dispersed in the polymeric matrix. The intrinsic self-healing mechanism of the WPUD can be improved by the presence of the inorganic compounds as they are anticipated to provide more hydrogen bonds due to the nitrogen and/or oxygen functional groups that exist on their surface or due to their ability to dissipate heat faster and thus increase the mobility of the polymer chains. Following the nano-additives synthesis, the materials were thoroughly characterized via X-ray Diffraction, XRD, Differential Scanning Calorimetry, DSC, as well as Raman and Infrared Spectroscopy, ATR-FTIR, measurements. WPUD nanocomposites were fabricated in different concentrations ranging between 0.05 - 1% wt. to investigate the effect of the additive content on the final properties. The PU/g-C3N4 nanocomposites were prepared via solid-liquid mixing while the PU/GO and PU/Ti3C2Tx via liquid-liquid mixing to avoid sedimentation problems. All nanocomposites were structurally and thermally characterized and no difference was observed in comparison to the properties of the pure PU due to the small amount of the nano-additives. Finally, coatings of all nanocomposites were fabricated by drop casting on silicon wafers. The coated surfaces were scratched creating deep and shallow cracks, using a razor blade and their self-healing efficiency was evaluated in two different temperatures (i.e., 75oC and 90oC). The healing procedure was monitored utilizing a Polarized Optical Microscope equipped with a Linkam heating stage. A quantification of the healing results was attempted by using mean grey value analysis. Similarities and differences were observed depending on the additive used; in certain cases, the self-healing ability of the pure polyurethane was found enhanced in the nanocomposites and the healing rate was found much higher, due to either better heat dissipation and / or the enhanced ability for hydrogen bond formation.

Development of the polymer nanocomposite coatings with enhanced self-healing efficiency is anticipated to enhance their usage as varnishes in wooden floors as well as paints in the car industry.

Παρουσίαση Μεταπτυχιακής Διπλωματικής Εργασίας της κ. Γιονίντας Μπούση

19 Οκτωβρίου 2023

μεταπτυχιακής φοιτήτριας του Τμήματος Επιστήμης και Τεχνολογίας Υλικών του Πανεπιστημίου Κρήτης

Τίτλος

Synthesis of Multifunctional Protein-Polymer Conjugates Using Oxygen Tolerant Approaches

Επιβλέπουσα Καθηγήτρια: Καλλιόπη Βελώνια

 

H παρουσίαση θα πραγματοποιηθεί την Τετάρτη 25 Οκτωβρίου 2023, Ώρα 14:00,  στην αίθουσα Β2 του Τμήματος Χημείας, του Πανεπιστημίου Κρήτης

Abstract

Protein-polymer conjugates are hybrid materials that have the potential to exhibit properties of both the biomolecule and the synthetic polymer. As such, they are expected to find application in a variety of sectors such as medicine, biotechnology, diagnostics among other.

Focus in this Thesis was the synthesis of multifunctional protein-polymer conjugates using different oxygen tolerant techniques. An oxygen-tolerant, aqueous copper-mediated polymerization approach, and an alternative, ligand-free methodology was used to synthesize protein-polymer conjugates and triblock bioconjugates in high yields. The effect of hydrophobic/hydrophilic ratio, chemical composition and temperature on the self-assembly of the biohybrid products were evaluated. Finally, a significant contribution into the development of a new, oxygen-tolerant methodology for the synthesis of protein-polymer conjugates via organocatalysis is presented in this Thesis. This greener and more sustainable approach was used to synthesize enzyme bioconjugates and evaluate their catalytic activity.

Size Exclusion Chromatography SEC), Native Polyacrylamide Gel Electrophoresis (PAGE), Nuclear Magnetic Resonance Spectroscopy (NMR), Fourier Transform Infrared Spectroscopy (FT-IR), Thermogravimetric analysis (TGA) and Field Emission Scanning Electron Microscopy (FE-SEM) were used characterize the products.

 

Παρουσίαση Διδακτορικής Διατριβής κ. Μαρίας Ψαρρού

17 Οκτωβρίου 2023

Επιβλέπουσα Καθηγήτρια: Μαρία Βαμβακάκη

(Σύμφωνα με το άρθρο 95, παρ. 3 του Ν. 4957/2022, ΦΕΚ 141 τ. Α΄/21.7.2022)

 

Την Τρίτη 24 Οκτωβρίου 2023 και ώρα 14:00 στην αίθουσα Τηλεκπαίδευσης Ε130 του Τμήματος Μαθηματικών και Εφαρμοσμένων Μαθηματικών του Πανεπιστημίου Κρήτης, θα γίνει η δημόσια παρουσίαση και υποστήριξη της Διδακτορικής Διατριβής της υποψήφιας διδάκτορος του Τμήματος Επιστήμης και Τεχνολογίας Υλικών κ. Μαρίας Ψαρρού, με θέμα:

 

«Responsive Polymer Nanostructures and Hydrogels with Photo/Acid Sensitive Linkages for Biomedical Applications»

 

Περίληψη

“Stimuli–sensitive or “smart” polymers have attracted increasing interest for applications in rapidly burgeoning research fields, including biotechnology and nanomedicine, electronic devices, and others. The present PhD thesis focuses on the synthesis of novel, stimuli–degradable polymers, polymer networks and polymer–drug conjugates for potential use in drug delivery or polymer recycling.

First, we investigated thioketal and thioacetal bonds as a new family of photolabile linkages. Linear main–chain poly(thioketals) were synthesized via polycondensation and their photodegradation mechanism upon exposure to UV light was examined. The chemical and macromolecular characteristics, of the synthesized polymers were characterized by SEC and ¹H NMR spectroscopy. Next, a facile chemical platform for the synthesis of photodegradable and thermo–reversible, model thioacetal hydrogels comprising poly(ethylene glycol) (PEG) elastic chains is presented. The viscoelastic properties of the hydrogels, their photodegradation under UV exposure, and reversible formation upon heating were investigated by dynamic shear rheology. Mechanistic insights into the photodegradation mechanism of the hydrogels were gained by ¹H NMR spectroscopy kinetic studies on a model, small molecule compound.

Next, the synthesis, characterization and photochemical properties of different main–chain photodegradable poly(acylhydrazones) with photo–sensitivity ranging from the UV to the visible light range was investigated. First, a novel light– and acid–cleavable, main–chain poly(acylhydrazone) copolymer was synthesized via a step–growth reaction of a dihydrazide monomer, with a dibenzaldehyde modified poly(ethylene glycol) affording a hydrophilic alternating copolymer. The water–soluble poly(acylhydrazone) was conjugated with a hydrophobic anticancer drug, doxorubicin (DOX), affording an amphiphilic polymeric prodrug which formed spherical nanostructures in water. The synergistic effect of light–mediated degradation and acid–induced hydrolysis of the acylhydrazone bonds along the polymer chains and the release kinetics of the drug were investigated. In addition, poly(acylhydrazones) using a PEG diacylhydrazide macromonomer and judiciously selected aromatic dialdehydes or diketones as the comonomers were synthesized and the effect of the aromatic comonomers on the polymerization kinetics, self-assembly and photophysical properties was examined. Finally, the synthesis, self-assembly properties and photodegradation of alternating poly(acylhydrazone) multiblock copolymers comprising hydrophilic PEG blocks and hydrophobic PDMS blocks is presented.

In the last part of this thesis, the development of hybrid mRNA delivery systems, comprising polymer coated superparamagnetic iron oxide nanoparticles (SPIONs), is presented. The SPIONs were coated with modified natural polymers, namely oxidized dextran and quaternized chitosan. The biocompatibility of the magnetic carriers in the presence and absence of a magnetic field was tested. Owning to the presence of the cationic (quaternized chitosan) or aldehyde (oxidized dextran) groups on the particle surface, mRNA was bound via electrostatic interactions or covalent bonds, respectively, and its transfection efficiency was examined.”

Παρουσίαση Διδακτορικής Διατριβής κ. Αντωνίου Κόρδα

17 Οκτωβρίου 2023

Επιβλέπουσα: Ανθή Ρανέλλα

(Σύμφωνα με το άρθρο 95, παρ. 3 του Ν. 4957/2022, ΦΕΚ 141 τ. Α΄/21.7.2022)

 

Την Τρίτη 24 Οκτωβρίου 2023 και ώρα 10:30 στην αίθουσα Β2 του Τμήματος Χημείας του Πανεπιστημίου Κρήτης, θα γίνει η δημόσια παρουσίαση και υποστήριξη της Διδακτορικής Διατριβής του υποψήφιου διδάκτορος του Τμήματος Επιστήμης και Τεχνολογίας Υλικών  κ. Αντωνίου Κόρδα, με θέμα:

 

               «3D Scaffolds for Neural Regeneration»

 

Περίληψη

“Disease and trauma are primary causes of damage and malfunction of the Nervous System (NS). Tissue Engineering (TE) is a field that aims to alleviate such issues and develop strategies to repair damaged tissues. TE utilizes scaffolds that mimic the native tissue and provide substrates for cell cultures that would later be introduced to the damaged site and restore function.

To restore NS, the basic concept of TE is often combined with other strategies such as co-culture of multiple cell types and electrical stimulation (ES). The combination of scaffolds fabricated via Two-Photon Polymerization and the co-culturing of glial Schwann (SW10) and neuronal N2a cells revealed the interactions between the two cell lines in the in vitro experiments in order to develop an in vitro experimental model for PNS studies. The co-culture environment was favorable for the growth of longer neurites for extended experimental periods over the respective N2a cultures and scaffold topography affected axon directionality, highlighting the benefits of scaffolds and co-culture environments towards N2a differentiation. Furthermore, ES was applied to NE-4C stem cells cultured on flat glasses and simple groove arrays to study the different cell fates towards neurons or glial cells in CNS recovery. NE-4C cells formed neurospheres which developed tracts that were influenced by groove topography while astrocytes (glia) were also present. Additionally, the use of specific markers for both neurons and glia revealed the effect of ES on the expression of said markers, towards the understanding of NE-4C behavior and development of functional neuronal networks.”

Παρουσίαση Διδακτορικής Διατριβής κ. Χρυσάνθης-Πηνελόπης Αποστολίδου

17 Οκτωβρίου 2023

Επιβλέπουσα Καθηγήτρια: Άννα Μητράκη

(Σύμφωνα με το άρθρο 95, παρ. 3 του Ν. 4957/2022, ΦΕΚ 141 τ. Α΄/21.7.2022)

 

Την Δευτέρα 23 Οκτωβρίου 2023 και ώρα 12:00 στην αίθουσα Τηλεεκπαίδευσης Ε130 του Τμήματος Μαθηματικών και Εφαρμοσμένων Μαθηματικών του Πανεπιστημίου Κρήτης, θα γίνει η δημόσια παρουσίαση και υποστήριξη της Διδακτορικής Διατριβής της υποψήφιας διδάκτορος του Τμήματος Επιστήμης και Τεχνολογίας Υλικών κ. Χρυσάνθης-Πηνελόπης Αποστολίδου, με θέμα:

 

«Responsive Self-Assembled Peptide Biomaterials and Applications»

«Αποκρίσιμα Αυτοοργανωμένα Πεπτιδικά Βιοϋλικά και Εφαρμογές»

Περίληψη

“Smart biomaterials" are designed to respond to external stimuli like light, pH, and metals, offering diverse applications from bioimaging to anticancer and antimicrobial applications. Peptides are highlighted as promising components for these materials due to their self-assembly properties, biocompatibility, and bio-functionality. This dissertation investigates peptides in three contexts: first, as a light-responsive delivery system encapsulating porphyrins to create antimicrobial hydrogels, analyzing structure, mechanics, and antimicrobial properties. Second, peptides are used to chelate fluorescent molecules for cancer cell bioimaging, with successful coordination and cell penetration. Finally, biocompatible peptide-metal ion nanoparticles are explored for antibacterial and anticancer purposes, revealing their potential in tumoral environments. The study overall explores smart biomaterial development, focusing on peptides and their responsiveness to external stimuli.”

PhD student position at IESL

11 Οκτωβρίου 2023

In view of the forthcoming start of the HORIZON-EIC-2023-PATHFINDEROPEN-01-01 Project Glas-A-Fuels, entitled “Single-Atom Photocatalysts Enhanced by a Self-Powered Photonic Glass Reactor to Produce Advanced Biofuels”, we are looking to recruit a PhD student for the duration of four years. The experimental work will be implemented in the ULMNP laboratory of IESL-FORTH, while the provisional starting date would be February or March 2024. The main research activities will involve the synthesis, patterning, and characterization of functional composite inorganic oxide glasses.

The applicants are kindly asked to provide their CV to Dr. I. Konidakis (ikonid@iesl.forth.gr) and Dr. E. Stratakis (stratak@iesl.forth.gr).

Requirements:

B.Sc. and M.Sc. in physical sciences (Chemistry, Physics, Materials Science).

Useful links

FORTH: https://www.forth.gr/

IESL-FORTH: http://www.iesl.forth.gr/

ULMNP: http://stratakislab.iesl.forth.gr/ and https://www.iesl.forth.gr/en/research/ULNMP-Group

 

Παρουσίαση Διδακτορικής Διατριβής κ. Γεωργίου Βαϊλάκη

11 Οκτωβρίου 2023

Πρόσκληση σε Δημόσια Παρουσίαση της Διδακτορικής Διατριβής του

κ. Γεωργίου Βαϊλάκη

Επιβλέπων Καθηγητής: Γεώργιος Κοπιδάκης

(Σύμφωνα με το άρθρο 95, παρ. 3 του Ν. 4957/2022, ΦΕΚ 141 τ. Α΄/21.7.2022)

 

Την Πέμπτη 19 Οκτωβρίου 2023 και ώρα 10:00 στην αίθουσα Τηλεεκπαίδευσης Ε130 του Τμήματος Μαθηματικών και Εφαρμοσμένων Μαθηματικών του Πανεπιστημίου Κρήτης, θα γίνει η δημόσια παρουσίαση και υποστήριξη της Διδακτορικής Διατριβής του υποψήφιου διδάκτορος του Τμήματος Επιστήμης και Τεχνολογίας Υλικών κ. Γεωργίου Βαϊλάκη , με θέμα:

 

«Theoretical Study of Two-Dimensional Nanostructures.»

 

Περίληψη

“Intensive research efforts on two-dimensional (2D) materials of atomic thickness uncover interesting phenomena, exciting physics, and new possibilities for technological innovation. 2D materials show great promise in electronics, optoelectronics, sensing, catalysis, clean energy and environment applications. Following semimetal graphene (Gr), focus is on other stable 2D materials with varying electronic properties such as insulating hexagonal boron nitride (hBN), semiconducting transition metal dichalcogenides (TMDs), and superconducting iron selenide (Fe2Se2). The electronic properties of these materials are strongly affected by strain, nanostructuring, structural and chemical defects, and disorder. Layer by layer stacking of 2D materials gives rise to van der Waals heterostructures (VDWHs) of nanometer thickness and clean interfaces. Superconductivity of twisted bilayer Gr at the magic angle, interlayer excitons in TMD heterostructures and optoelectronic properties of Gr/TMD heterostructures, are examples, among others, where VDWHs significantly differ from their monolayer (ML) constituents. 2D nanostructures often exhibit extraordinary properties and present novel challenges for theory. Theoretical models can answer emerging fundamental questions and identify candidate materials with properties tailored for specific applications from all the range of unique 2D nanostructures. First-principles calculations, which provide solution to the quantum problem and are the basis for atomic-scale understanding of materials, become challenging when deviations from periodicity are strong.

In this work, we perform density functional theory (DFT) calculations for the atomic and electronic structure of defected 2D nanostructures, heterostructures consisting of combinations of TMD MLs, Gr, and other materials. Due to the large size of the simulation cells required, DFT calculations are very demanding and the results need careful interpretation using non-trivial computational tools. We present in detail the methods we develop for the construction of optimized simulation cells and for unfolding the electronic band structures from their Brillouin Zone (BZ). The effective band structure (EBS) produced allows for a clear and direct comparison between electronic properties of 2D defected nanostructures and heterostructures with their pristine or constituent MLs.

Applying our methodology to Gr/TMD and TMD/TMD heterostructures, several experimental observations are explained and predictions are made. Interlayer interactions in Gr/TMD have negligible effects. The Dirac cone of Gr remains unaffected, variations in the TMD electronic band gap are due to the minimal strain remaining in the simulation cells, and Fermi levels move closer to the conduction band minimum. In WX2/MoX2 VDWHs, where X = S, Se, interactions between TMD MLs result in hybridization of electronic states and energy eigenvalues split around the center of the BZ (Γ point). The magnitude of energy splitting depends on the interlayer distance and determines the valence band maximum at the center or the edge (K point) of the BZ. We find that interlayer transitions are more probable in WSe2/MoSe2 than in WS2/MoS2. In all VDWHs we examined, a small but universal redshift of the band gap for the TMDs is observed as opposed to a small blueshift for the Au/MoS2 heterostructure, in agreement with experiments. In hBN/TMD VDWHs, electronic properties of constituent MLs remain unaffected.

Our methodology also proves very useful in investigating defects and adsorption on 2D MLs. In conjunction with experiments, our DFT calculations show how n-doped WSe2 ML becomes a p-doped semiconductor via photochlorination. Energetics and EBSs show that chlorine fills chalcogen vacancies, neutralizing defect states close to the conduction band minimum and creating defect states close to valence band maximum. In another synergy with experiment, our DFT calculations show that Fe-vacancies in Fe2Se2 MLs give rise to a stretched lattice (which remains superconducting) at a relatively low energy cost. The absolute magnetic moment of the Fe atoms near the vacancies increases. Our EBS calculations demonstrate the effects of Fe vacancies in agreement with experiments. Sinking of the hole pocket and creation of new states above the hole pocket and below the electron pocket are produced by Fe vacancies. Finally, we present our theoretical results for hydrogen adsorption on Ni2P/CuCo2S4 heterostructure within another collaboration with experimentalists. Electron transfer towards the CuCo2S4 is related to the superior performance of the heterostructure as a catalyst for hydrogen evolution reaction.

Our DFT-based studies with the methodology we developed for building simulations and interpreting electronic band structures, combined with data from experiments, besides explaining observed phenomena, provide a general framework for making predictions which should be useful in future experiments and applications.”

Πρόσκληση σε Δημόσια Παρουσίαση της Διδακτορικής Διατριβής του κ. Εμμανουήλ Μανιδάκη

05 Οκτωβρίου 2023

Επιβλέπων Καθηγητής: Νικόλαος Πελεκάνος

(Σύμφωνα με το άρθρο 95, παρ. 3 του Ν. 4957/2022, ΦΕΚ 141 τ. Α΄/21.7.2022)

  Την Παρασκευή 13 Οκτωβρίου 2023 και ώρα 12:00 στην αίθουσα Τηλεεκπαίδευσης Ε130 του Τμήματος Μαθηματικών και Εφαρμοσμένων Μαθηματικών του Πανεπιστημίου Κρήτης, θα γίνει η δημόσια παρουσίαση και υποστήριξη της Διδακτορικής Διατριβής του υποψήφιου διδάκτορος του Τμήματος Επιστήμης και Τεχνολογίας Υλικών κ. Εμμανουήλ Μανιδάκη, με θέμα:  

«Development of Perovskite-Gallium Arsenide Double-Junction Photovoltaic Devices.»

Περίληψη "The perovskite solar cells represent today the most rapidly developed photovoltaic (PV) technology, as they combine low fabrication costs, high conversion efficiencies and the possibility to deposit on flexible substrates. On the other hand, the GaAs-based solar cells are still regarded as the reference technology in the PV industry, exhibiting the highest PV efficiencies of the field. In this thesis, we combine the benefits of the two material systems to provide a high-efficiency perovskite/GaAs tandem solar cell with enhanced characteristics. Accordingly, we have produced GaAs-based solar cell devices with PV efficiencies reaching ~15% values, comparing well with reported values for GaAs solar cells of similar design. We have developed optimized recipes for the deposition of every single layer of a full perovskite PV device, including the perovskite active layer, the electron and hole transporting layers, and the metal contacts. Specifically, we have successfully synthesized “red” perovskites with a gap around 650 nm needed in tandem perovskite/GaAs structures and have fabricated “red” perovskite solar cells with PV efficiency up to ~6.5%. This relatively modest value is most likely due to the “out-of-the-glovebox” deposition conditions in our laboratory. Nevertheless, combining a 1.77 eV perovskite solar cell provided by a fellow team along with our own GaAs solar cells, we managed to demonstrate a tandem 4-terminal device with a PV efficiency close to 23%, highlighting the benefit of the tandem configuration. Finally, we have shown that the deposition of MAPbI₃ on native GaAs substrates is able to generate a giant passivation effect on GaAs, an effect that appears to be fully reversible, in the sense that the perovskite layer can be easily washed away and the PL intensity and spectral features of the GaAs substrate are fully restored to their pristine condition."

Παρουσίαση διδακτορικού Γεωργίου Κουρμουλάκη

02 Οκτωβρίου 2023

Πρόσκληση σε Δημόσια Παρουσίαση της Διδακτορικής Διατριβής
του κ. Γεωργίου Κουρμουλάκη

Περίληψη

Two-Dimensional Transition Metal Dichalcogenides (2D-TMDs) exhibit remarkable optoelectronic properties, making them pivotal tools in the realization of advanced devices. Their direct bandgap nature, combined with the formation of stable excitons with binding energies on the order of hundreds of meVs, enables robust Photoluminescence (PL) emission even at room temperature. Owing to their atomic thickness and crystal symmetry, these materials are subject to specific optical selection rules in which valley and spin indices are intrinsically coupled creating a binary-like system which can be used for storing and processing information. Despite their resilience to mechanical deformation, 2D-TMDs are extremely sensitive to their dielectric environment.

In this thesis, we investigate how the optical characteristics of a monolayer of WS₂ are influenced by the substrate. We achieve this by transferring the two-dimensional materials onto a pre-patterned Si/SiO₂ substrate with cylindrical wells resulting in the creation of both suspended and locally strained regions within the same monolayer. We examine this sample configuration with means of optical spectroscopy and non-linear imaging. The distinct PL emission stemming from the neutral exciton (X⁰) in suspended regions highlights the impact of substrate. Furthermore, we show that Polarization-Resolved Second Harmonic Generation can be an all-optical tool to provide image patterns of the armchair orientation with strain signatures. In the last part of the thesis, we study the impact of biaxial tensile strain on the exciton energy and spin-valley polarization of monolayer WS₂ on top of Graphite. Our experimental results allowed us to extract the net effect of biaxial tensile strain to the depolarization, which we have attributed to the decrease of spin relaxation time due to an increase of the effective pseudospin precession frequency combined with the suppressed Κ-Λ scattering channel in the conduction band. Our results demonstrate that substrate modulation and mechanical deformation can be significant tools in tuning the properties of 2D-TMDs in the pursuit of developing novel optoelectronic devices.