Faculty of Physics University of Warsaw > Events > Seminars > Solid State Physics Seminar

Solid State Physics Seminar

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room 0.06, Pasteura 5 at 10:15

mgr Mahwish Sarwar (Institute of Physics, Polish Academy of Sciences, Warsaw, Poland)

"Crystal Lattice Damage and Recovery of Rare-Earth implanted beta-Ga2O3"

-Ga2O3 as a wide bandgap semiconductor (4.8eV) with unique properties, is a potential candidate for various optoelectronic applications, such as high-power LED, high-end displays, car sensors, and UV photodetectors. High radiation resistance paved the way for it to be used as a material working in extremely radiative and harsh environments. Such conditions induce damage in the crystal lattice, which eventually affects the device’s efficiency and lifetime. In addition, optical tuning of -Ga2O3 for example, by doping with rare-earth (RE) ions is important for its prospective applications. This material has especially proven itself as a suitable host for RE ions. Ion implantation is an efficient technique for doping material due to its many advantages, such as controlled dopant concentration and depth location. However, it creates lattice damage and can cause optical inactivation of RE dopant; therefore, annealing is necessary. To address this problem, we implanted -Ga2O3 single crystals with different RE ions and annealed them in various conditions. Based on Channeling Rutherford Backscattering Spectrometry (RBS/c) accompanied by McChasy computer simulations we analyzed defects evolution after implantation and we were able to determine for the first time the defect accumulation curve for this material. Two different damage regions were revealed, indicating the complexity of defects after irradiation, and their different reaction to rapid thermal annealing (RTA). We observed that defect accumulation process in this material occurs in an unusual way and, for some fluences, it results in appearance of a new crystallographic phase, -Ga2O3, that is formed about 60 nm below the crystal surface as confirmed by TEM images. We also observed that the lattice structure recovery level after annealing decreases with increasing ion fluence, and above a particular fluence (< 0.5 dpa), complete RTA resistance was noticed. Further, for selected fluence, various annealing conditions were tested for comprehensive studies of recovery and the optical response of RE to structural changes. Acknowledgments The work was performed within the co-financed international project of the MNiSW (5177/HZDR/2021/0) and Helmholtz Zentrum Dresden Rossendorf project (20002208-ST), supported also by the NCN project UMO-022/45/B/ST5/02810.

room 0.06, Pasteura 5 at 10:15

prof. dr hab. Franciszek Krok (Wydział Fizyki, Astronomii i Informatyki Stosowanej, Uniwersytet Jagielloński)

Self-Organized Gold Nanoislands on MoS₂: growth, characterization, and nanomanipulation

The interaction between metal nanoparticles and transition metal dichalcogenides (TMDs) opens new possibilities in optoelectronics and nanoengineering. A promising approach for fabricating well-defined nanostructures is the self-organization of metal deposits on semiconductor surfaces, which enables precise control over their properties at the atomic level.In the first part of the presentation, we will explore the growth and structural characterization of gold nanoislands on MoS₂ surfaces, formed via deposition on both pristine and synthetic bilayer MoS₂/SiO₂ substrates. High-resolution scanning probe techniques, including scanning tunneling microscopy (STM), scanning electron microscopy (HR-SEM), and high-angle annular dark field scanning transmission electron microscopy (HAADF-STEM), reveal the crystallographic alignment of the Au nanoislands with the underlying MoS₂ lattice (Fig. 1). The resulting structures exhibit a characteristic triangular morphology, with lateral sizes ranging from 10–50 nm and thicknesses of 2–8 nm, tunable by deposition parameters.A key aspect of these systems is their weak interaction with the MoS₂ substrate, making them ideal candidates for nanoscale manipulation. In the second part, we will present STM- and AFM-tip-induced manipulation experiments, demonstrating controlled displacement of individual nanoislands along the zigzag directions of the MoS₂ lattice. Additionally, we will discuss the role of substrate defects, such as exfoliated grooves and vacancies, in influencing manipulation behavior. The ability to construct well-ordered nanopad systems through deterministic manipulation methods offers exciting perspectives for molecular nanoelectronics and nanoscale device integration.

room 0.06, Pasteura 5 at 10:15

prof Paulina Płochocka (Laboratoire National des Champs Magnétiques Intenses, UPR 3228, CNRS-UGA-UPS-INSA, Grenoble and Toulouse, 38042 France Department of Experimental Physics, Faculty of Fundamental Problems of Technology, Wroclaw University of Science and Technology, Wroclaw)

Excitonic landscape in layered semiconductors

The optical properties of low-dimensional semiconductor nanostructures are often governed by excitons – quasi-particles formed by a photo-generated electron and hole bound together by Coulomb attraction. Here I will explore the excitonic landscape in 2D semiconductors, and van der Waals heterostructures, where excitonic effect are strongly enhanced. One prominent example is the 2D Ruddlesden–Popper metal-halide perovskites (2DP), where the soft, polar, and low-symmetry lattice creates a unique environment for electron-hole interactions. I will also demonstrate that the van der Waals nature of 2DP allows for easy integration with other 2D materials, particularly transition metal dichalcogenides. Finally, I will discuss the excitonic properties in homo-bilayer transition metal dichalcogenides where the interaction between two dipolar excitons with opposite dipole moments can lead to the formation of a new type of interlayer exciton, namely a quadrupolar exciton.

room 0.06, Pasteura 5 at 10:15

prof dr hab. Elżbieta Guziewicz (Institute of Physics, Polish Academy of Sciences, Warsaw)

Acceptor complexes in ZnO – effect of strain, surface proximity and carbon content

As recently established in experimental and theoretical works, defect complexes including native point defects, hydrogen and possibly a dopant determine electrical conductivity of ZnO in a wide range as they introduce shallow and deep donor and acceptor states in the bandgap. This knowledge seems discouraging because the control of conductivity, and in particular the achievement of acceptor conductivity, requires the simultaneous tuning of point defects and dopants, which is difficult to achieve. Our investigations indicate, however, that this goal seems to be achievable, but it requires very conscious approach to the growth procedure and appropriate post-processing. In the presentation we will show the experimental cathodoluminesence (CL) and scanning photoelectron microscopy (SPEM) measurements performed on ZnO:N layers that allow sampling of a single microcrystallite in the films cross-section [1-3]. The results account for grouping acceptor and donors in separate domains/crystallites. This phenomenon is found to be very common provided the ZnO films are grown under O-rich conditions and nitrogen is introduced in the form of the –NH chemical group. Density Functional Theory calculations point out that the complexes involving zinc vacancy (VZn), hydrogen and nitrogen provide complexes-related acceptor states [4]. Hydrogen stabilizes formation of the VZnNO complex, but the appearing VZnNOH complex is found to be a deep acceptor. DFT calculations show that compressive strain and/or surface proximity facilitate the formation of acceptor complexes, so they are easily created under appropriate strain/microstrain conditions or near the surface. This theoretical finding may explain the origin of acceptor and donor grouping in different crystallites. Additionally, a closer insight into the electronic structure of a single crystallite performed by scanning photoemission experiment reveals that carbon present in a form of –CH group facilitates the formation of acceptor states, as seen by the modification of the valence band shape. According to DFT calculations, CiHx group is stable in the ZnO crystal lattice and creates C-H-O bond states. The calculated migration properties show that complexes such as VZn(NH)O are easily formed in the presence of the interstitial CiH2 group as it leads to lowering of the migration energy of VZn by 0.8 eV and to 0 eV in the ZnO and ZnO:N, respectively [5]. Acknowledgements The study was supported by the Polish NCN Project DEC-2018/07/B/ST3/03576 [1] E. Guziewicz, O. Volnianska et al., Phys. Rev. Appl. 18 044021 (1-13) (2022)[2] E. Guziewicz, E. Przezdziecka et al., ACS Appl. Mat. Interfaces 9, 26143-26150 (2017)[3] S. Mishra, B.S. Witkowski et al., Phys. Stat. Sol. A 2022, 2200466 (1-11)[4] O. Vonianska, V.Yu. Ivanov, L. Wachnicki, E. Guziewicz, ACS Omega 2023, 8, 43099[5] E. Guziewicz, S. Mishra, M. Amati, L. Gregoratti and O. Volnianska, Nanomaterials 2025, 15, 30

room 0.06, Pasteura 5 at 10:15

dr hab. Maciej Rogala, prof. UL (Wydział Fizyki i Informatyki Stosowanej Uniwersytetu Łódzkiego.)

Two-dimensional crystalline metal oxides – synthesis, modification, and applications of a-MoO3 with a focus on electronics properties

MoO₃ is widely utilized in organic electronics for modifyingtransparent electrodes. Its high work function enables effective energylevel alignment between different layers in OLED and OPV systems. Whilemost studies focus on determining the minimum thickness required for thefunctional properties, this seminar will challenge that approach byemphasizing the crucial role of crystallinity rather than layerthickness in optimizing performance. The presentation will introduce real two-dimensional crystalline MoO₃layers grown on graphene-like substrates [1]. A comprehensive analysisof their morphology, chemical composition, and electronic structure willbe provided by STM, AFM, XPS, UPS, TEM, STS and KPFM techniques.Additionally, the potential for nanoscale modifications will be explored[2], with particular attention given to the challenges associated withmeasuring the work function of ultra-thin functional layers [3]. [1] D.A. Kowalczyk, M. Rogala et al. 2D Mater. 8 025005 (2021) [2] A. Nadolska, M. Rogala et al. Crystals, 13 905 (2023) [3] D.A. Kowalczyk, M. Rogala et al. ACS Appl. Mater. Interfaces, 1444506 (2022) *Work was supported by the National Science Centre, Poland2020/38/E/ST3/00293

room 0.06, Pasteura 5 at 10:15

dr Eunika Zielony (Department of Experimental Physics, Wroclaw University of Science and Technology)

Interaction between GaN nanowires and oxide shells for enhanced light emission

Gallium nitride (GaN) nanowires (NWs) grown by molecular beam epitaxy on Si substrates hold significant potential for optoelectronic applications due to their exceptional structural quality, high luminescence efficiency, reduced lattice mismatch strain to the substrate, and a high thermal stability. However, their advantages are often compromised by external conditions. To mitigate this issue, shells are frequently employed to protect the NWs from air exposure, and thus ensuring high system efficiency. In this study, core-shell GaN-AlOx/HfOx NWs were investigated, with oxide coatings applied through atomic layer deposition. Scanning electron microscopy was used to examine the morphology and structure of the NWs. To analyze the changes in the GaN core’s crystal lattice, strain was calculated based on outcomes of X-ray diffraction, photoluminescence (PL), and Raman spectroscopy. Consistent results were obtained by all these techniques, supported by statistical analysis, which revealed statistically significant differences in the calculated strain. The potential of NWs for optoelectronic applications was highlighted through PL and cathodoluminescence (CL) spectra and maps at room- and lower temperatures. Both experiments showed that the shells effectively protect NWs from photodegradation and enhance luminesce efficiency by passivating surface states, enabling core field screening through carrier accumulation, preventing carrier tunneling to surface states, minimizing strain, and inducing a flat-band effect. Interestingly, the highest emission intensity was observed for NWs with the thinnest shells (1-5 nm), while thicker shells (over 5 nm up to 20 nm) resulted in lower PL and CL signals. The reduction in signal intensity may be due to an increasing strain gradient, generated defects, and light scattering. Our results clearly show that the discussion on the optimal shell thickness is essential for proper design of the NW structures. R. Szymon, E. Zielony, et al., Small 2024, 2401139, 1-10.

room 0.06, Pasteura 5 at 10:15

dr Arka Karmakar (Faculty of Physics, University of Warsaw)

Effect of Resonant Overlaps Between the Optical Bandgaps and Large Twist Angle in the Interlayer Energy Transfer Process

In the post silicon era, type-II transition metal dichalcogenides (TMDCs) heterostructures (HSs) are predicted to be the building blocks for the next generation (opto)electronic device applications. Competing interlayer charge (CT) and energy transfer (ET) processes control the photocarrier relaxation pathways in the semiconducting TMDC HSs. In the type-II TMDC HSs, CT process dominates due to its fast timescale (< 100 fs) as compared to the ET process (≤ 1 ps). In this talk, I will present our recent results from an ongoing project to create an understanding on different complex aspects of the ET processes in the TMDC HSs. In the first part of my talk, I will begin with our ET exploration with the HS made by the monolayers (1Ls) of MoSe2 and ReS2 [1]. In this work, we showed that despite forming the type-II HSs an ET process from the ReS2 to MoSe2 layer dominates over the traditional fast interlayer CT process, due to the resonant overlap between the optical bandgaps between the two materials. This ET process resulted ~ 400% MoSe2 photoluminescence (PL) enhancement from the HS area without any charge-blocking interlayer. Upon completely blocking the CT process by placing a charge-blocking interlayer we observed more than one order magnitude higher MoSe2 PL emission from the HS area. This motivated us to explore the effect of resonant overlap between the high-lying absorption states between different materials. Traditionally ET is observed from a higher-to-lower energy state (thermodynamically favorable). In a type-II HS formed between 1Ls of MoS2 and WSe2, high-lying absorption states (band-nesting regions) have resonant overlaps at cryogenic temperature [2]. This enables to observed a unique ET process from the lower-to-higher bandgap (WSe2-to-MoS2) material with a thin charge-blocking interlayer. In the second part, I will talk about the effect of large twist angle in the ET process [3]. Interlayer CT process happens due to the excitonic wavefunctions overlaps. Whereas, the long-distance ET happens mainly via dipole-dipole interaction. By creating large twist angles (~ 30°-60°) in MoSe2 homobilayers fabricated with stacking exfoliated layers on top of the flakes grown by the chemical vapor deposition (CVD), we effectively reduce the CT process. The ~ 3% strain induced lattice mismatch also changes CVD bandgap from direct-to-indirect, making the CT even weaker and allowing the ET process to take over the carrier relaxation channels. References: [1] A. Karmakar et al., Dominating Interlayer Resonant Energy Transfer in Type-II 2D Heterostructure, ACS Nano 16, 3861 (2022). [2] A. Karmakar et al., Excitation-Dependent High-Lying Excitonic Exchange via Interlayer Energy Transfer from Lower-to-Higher Bandgap 2D Material, Nano Lett. 23, 5617 (2023). [3] A. Karmakar et al., Twisted MoSe2 Homobilayer Behaving as a Heterobilayer, Nano Lett. 24, 9459 (2024).

room 0.06, Pasteura 5 at 10:15

prof. Robert Bogdanowicz (Gdansk University of Technology Faculty of Electronics, Telecommunication and Informatics)

Is CVD Diamond Ready to be an Electronic Material ?

The recent downturn in gem-quality synthetic diamond production presents a unique opportunity to redirect sophisticated CVD systems and skilled process engineers toward electronic and quantum applications. While CVD diamond has long promised revolutionary capabilities for electronics and quantum computing, widespread adoption remains constrained by the limited availability of large-area, high-quality substrates at commercial scale.Significant advances in materials engineering have emerged through novel approaches to diamond synthesis and modification. The incorporation of deuterium during growth has revealed fascinating insights into isotope effects on diamond's electronic properties, while innovative diamond foil/membrane technology has enabled unprecedented control over strain engineering and doping profiles. These developments have been complemented by breakthroughs in surface termination and interface engineering, crucial for electronic device integration.The field now stands at a critical turn point where fundamental research into growth mechanisms, defect formation, and carrier transport can benefit from sophisticated synthesis capabilities. Key research priorities include understanding the role of plasma chemistry in controlling impurity incorporation, elucidating mechanisms of extended defect formation during heteroepitaxial growth, and developing novel approaches to selective doping and defect engineering. These fundamental investigations are essential for realizing diamond's potential in quantum and electronic applications, where precise control over material properties at the atomic scale is paramount.

room 0.06, Pasteura 5 at 10:15

dr hab. Grzegorz Muzioł (Institute of High Pressure Physics Unipress PAS)

Advancements in growth of InGaN by MBE for novel optoelectronic devices

In this talk we will explore recent advancements in the growth of InGaN by molecular beam epitaxy (MBE). A modification in the MBE system allowed us to obtain molecular beam fluxes of unprecedentedly high magnitude, previously unused in MBE of any other material system. This breakthrough has allowed us to investigate a new growth regime – high temperature growth. We will demonstrate that growing at higher temperatures reduces the incorporation of point defects and enhances the quantum efficiency of InGaN layers. Additionally, we will highlight the unique opportunities offered by the MBE technique in growing III-nitride heterostructures. Specifically, the ability to create low-resistivity tunnel junctions opens the door to new device concepts, such as light-emitting devices that operate in both forward and reverse bias [1], distributed feedback laser diodes with high spectral purity [2], and laser diode stacks for high-power operation [3], to name just a few. Lastly, we will address the challenges and future directions for MBE. The foundation for creating highly efficient structures via MBE has been established, and we anticipate that continued improvements in growth conditions will pave the way for the commercialization of novel devices that can only be achieved through the MBE technique. [1] M. Żak, G. Muziol, et al., Nature Communications 14, 7562 (2023). [2] G. Muziol, M. Hajdel, et al., Optics Express 28, 35321 (2020).[3] M. Siekacz, G. Muziol, et al., Optics Express 27, 5784-5791 (2019).

room 0.06, Pasteura 5 at 10:15

mgr Aleksander Rodek (IFD UW)

Two-dimensional excitons interacting with Fermi sea of carriers

Understanding the fundamental properties of exciton physics in the presence of 2D carrier gas in Transition Metal Dichalcogenides (TMDs) has been the subject of interest for many works in recent years. The development of electrostatically-gated heterostructures of these materials facilitated precise control of free carrier density, which opened the way for the studies of strong exciton-carrier interactions present in TMDs, while also leading to such milestone achievements in the field of correlated electron systems like the observation of the Wigner crystal phase or the significant enhancement of the magnetic susceptibility. Despite, however, all of this progress, there are still limited investigations into the particular influence of the Fermi sea of carriers on the optical response and dynamics of the exciton states, especially in the ultrafast limit. In this work, I will present the results of resonant pump-probe spectroscopy measurements of MoSe2 monolayer heterostructures with tunable electron density. This experimental approach will allow me to showcase various optical effects stemming from exciton-carrier interactions in two dimensions. Special focus will be put on the influence of free electrons on different relaxation mechanisms, and how their presence leads to increased decay rates of excitons and intervalley scattering of carriers. Furthermore, by presenting comparative measurements of heterodyne Four-Wave-Mixing spectral interferometry, I will also discuss the dependence of coherent exciton properties on the increasing density of the Fermi sea. Analysis of the observed changes in the homogeneous and inhomogeneous broadenings of exciton transitions shows that free carriers lead to increased exciton decoherence rates and screening of the spatial dielectric disorder. This valuable finding may also explain the observed strengthening of the coherent coupling between charged and neutral exciton states.

room 0.06, Pasteura 5 at 10:15

prof. Agata Kamińska (Institute of Physics, Polish Academy of Sciences, PAN , Faculty of Mathematics and Natural Sciences, Cardinal Stefan Wyszynski University,Institute of High Pressure Physics ‘Unipress’, PAN)

“Ultraviolet color centers in various polytypes of boron nitride - high-pressure study”

Hexagonal boron nitride (hBN) is a wide bandgap semiconductor which was synthesized already in the 19th century, but only recently a high quality, single crystals with macroscopic millimetric size were produced [1], leading to the realization of a light-emitting device operating in the deep UV [2]. This achievement paved the way for applications of hBN to advanced optoelectronics, making it to be considered a challenger of aluminum nitride [3]. Furthermore, Bourrelier et al. reported in 2016 single photon emission of color centers emitting at 4.1 eV [4]. However, the question of the nature of the defect giving rise to this behavior is still under debate. In order to contribute to the elucidation of the origin of such emission, we performed high hydrostatic pressure studies of the low-temperature photoluminescence of bulk h-BN crystals and other BN stacking sequences, i.e. the Bernal bBN) and rhomboedral (rBN) forms (polytypes) in the ~ 3 – 4 eV spectral region using the diamond anvil cell technique. The results showed that the emission energy decreased with pressure less sensitively than the bandgap [5,6]. This behavior, distinct from the shift of the bandgap is typical of deep traps.Theoretical calculations of pressure dependencies of various defect levels in hBN and other BN polytypes demonstrated that some of the observed UV lines are associated with carbon-related defects, and their pressure behavior depends strongly on BN polytype. Our results show that tuning the stacking sequence in different polytypes of a given crystal provides unique “fingerprints” contributing to the identification of defects in 2D materials.[1] K. Watanabe et al., Nat. Mater., 3, 404 (2004). [2] K. Watanabe et al., Int. J. Appl. Ceram. Technol., 8, 977 (2011). [3] H.X. Jiang et al., ECS J. Solid State Sci. Technol., 6, Q3012 (2017).[4] R. Bourrellier et al., Nano Lett,. 16 4317 (2016). [5] K. Koronski et al., Superlattices and Microstructures, 131, 1 (2019).[6] J. Plo et al, https://arxiv.org/pdf/2405.20837, submitted to Phys. Rev. X (2024).

room 0.03a, Pasteura 5 at 10:15

prof. dr hab. Marek Godlewski (Instytut Fizyki Polskiej Akademii Nauk)

High-k Oxides – Properties and Applications

Ultra-thin layers of wide-gap oxides (so-called "high-k oxides") deposited by the Atomic Layer Deposition (ALD method) are currently used in electronics, as gate oxides and elements of the new generation of semiconductor memories, in photovoltaics, as barrier layers or as anti-reflective layers, and in opto-electronics, in thin films electroluminescence devices or as anti-reflective coatings.During the seminar I will discuss the advantages of the ALD deposition method, and then the fascinating properties of so-called high-k oxides. Thin layers of high-k oxides are deposited using the ALD at low-temperature and the process is optimized to ensure their best stoichiometry. Unfortunately, in the following annealing processes of electronic structures, defects resulting from deviations from the ideal stoichiometry can be generated - such as oxygen vacancies or broken bonds. In some extreme cases the concentration of these defects can be high, which can generate significant ionic conductivity associated with the diffusion of oxygen ions. In the case of hetero-structures, the properties of oxides determine the accumulation of charges in the near-surface layer of the semiconductor. The generation of defects at the dielectric/semiconductor interface (and within the insulating layer) limits the efficiency and reliability of electronic components, by increasing the leakage current and reducing the mobility of carriers in the semiconductor. Thus, it is necessary to ensure a low concentration of defects at the interface. I will discuss selection of oxides for a given application and still many open questions regarding their properties.

room 0.06, Pasteura 5 at 10:15

dr hab Piotr Wojnar, prof IFPAN (Institute of Physics Polish Academy of Sciences)

"Molecular beam epitaxy of indium selenide and antimony triselenide: 2D and 1D van der Waals semiconductors"

Since the discovery of graphene two-dimensional (2D) atomic crystals characterized by strong in-plane covalent bonds and weak interlayer van der Waals forces became one of the leading topics in the condensed matter physics. Indium selenide is a semiconductor which occurs in different crystalline structures, the most of which belong to the family of 2D semiconductors. They attract the interest due to outstanding electronic and optical properties, which are prospective in view of applications in next generation electronic and photonic devices. In the first part of the seminar it will be demonstrated how to fabricate optically active indium selenide thin layers by molecular beam epitaxy and to control its crystalline phase by changing the growth conditions. An attempt to obtain optically active crystal phase heterostructures involving γ-InSe, γ-In2Se3 and β-In2Se3 crystal phases in a well-controlled manner will be described. The second part of the seminar will be devoted to the molecular beam epitaxy of antimony triselenide (Sb2Se3). This semiconductor belongs to the family of one dimensional (1D) van der Waals semiconductors which attract the research interest due to the possibility of downscaling semiconducting channels in transistors even down to the one atomic chain limit. It crystalizes in the stibnite (orthorhombic) crystalline structure consisting of 1D ribbons held together by weak Se-Se van der Waals interactions. It will be demonstrated that self-assembled, highly anisotropic nanostructures spontaneously form in the molecular beam epitaxy process of antimony triselenide on GaAs substrates. With increasing growth time all three dimensions: the length, the width and the height of these nanostructures increase simultaneously, with length being usually one order of magnitude larger than the two other parameters. These 1D nanostripes have all the orientation parallel to the substrate surface and preserve the epitaxial relation with the substrate. The fabrication of well-ordered arrays of horizontal nanostripes aligned in directions defined by the orientation of the substrate is perspective in view of the development of electronic circuits and networks composed of interconnected nanostructures.

room 0.06, Pasteura 5 at 10:15

prof. Michał Nikodem (Faculty of Fundamental Problems of Technology, Wrocław University of Science and Technology)

"Antiresonant hollow-core fibers – a useful tool... also in gas sensing"

Antiresonant hollow-core fibers (AR-HCFs) have proven highly valuable across various applications. They are utilized in fields ranging from high-energy pulse delivery to ultra-low-loss and low-latency light transmission. In my presentation, I will explore how AR-HCFs can be effectively applied in laser spectroscopy, with a special emphasis on practical aspects of their use. I will show our experimental approach to laser-based trace gas sensing with AR-HCFs and highlight unique sensing configurations that are not achievable with conventional laser-based sensing methods.

room 0.06, Pasteura 5 at 10:15

dr inż. Marek Maciaszek (Wydział Fizyki Politechniki Warszawskiej Faculty of Physics, Warsaw University of Technology)

Ab initio study of carbon-related defects in hexagonal boron nitride: searching for single-photon emitters

Hexagonal boron nitride (h-BN) has long been used in nanotechnology as an insulating layer in heterostructures with graphene, owing to its wide bandgap (~6 eV). Recently, however, h-BN has gained significant attention due to the discovery of various color centers in its samples that exhibit single-photon emission. These single-photon emitters in h-BN are particularly attractive due to their excellent brightness and photostability, even at room temperature. Additionally, the wide bandgap of h-BN enables the hosting of single-photon emitters across a broad spectral range, from the infrared to the ultraviolet.While experiments suggest a correlation between the intensity of emission from many of these centers and the carbon content in h-BN samples, the chemical nature of most of the observed emitters remains unclear. To further advance the field, it is crucial to identify the point defects responsible for the observed emissions. First-principles calculations could play a pivotal role in achieving this goal.In my presentation I will discuss results of first-principles calculations focused on identifying three groups of emitters in h-BN, most likely associated with carbon defects: (i) ultraviolet emitters (ZPL 4.1 eV), (ii) so called “visible emitters” (ZPL between 1.6 and 2.1 eV), and (iii) blue emitters (ZPL 2.85 eV).

room 0.06, Pasteura 5 at 10:15

prof. dr hab. Włodzimierz Jaskólski (Institute of Physics, Faculty of Physics, Astronomy and Informatics, Nicolaus Copernicus University, Toruń, Poland)

On the robustness of gapless states in gated multilayer graphene

Gated multilayer graphene with rhombohedral stacking of layers exhibits a tunable energy gap. The stacking order can be changed when some layers are stretched, or delaminated and corrugated. In such cases, topological gapless states, connecting the valence and conduction band continua, appear at each valley. These states are localized at stacking domain walls (interfaces) that separate two different stacking orders. When the stacking change occurs along the zigzag direction, the Dirac cones at different valleys in the k-space are well separated. The gapless states are therefore valley-protected and provide one-dimensional and non-destructive valley currents that can flow along the stacking domain wall in opposite directions. Valley protection can be destroyed in the presence of atomic-scale defects. Here, we demonstrate the robustness of the gapless states to different defect-like perturbations of the multilayers. It is shown that some gapless states survive very strong distortions of the stacking domain walls. They persist when some layers are broken or partially removed, or even when vacancies or magnetic impurities are present at the stacking interface.

room 0.06, Pasteura 5 at 10:15

prof dr hab. Izabella Grzegory (Institute of High Pressure Physics Unipress PAS)

High Pressure in Physics and Engineering of Hexagonal Boron Nitride

The discoveries of graphene, topological insulators and other novel materials have opened a whole new area, namely two-dimensional electronics. This new frontier has spurred the rediscovery of boron nitride (BN), a member of the family of III-N compounds that has much promise for modern electronics. In this talk the hexagonal BN (hBN) will be introduced. In particular the use of high pressure for its synthesis and crystal growth as well for identification of selected defect centers will be highlighted.Like carbon, BN forms crystalline structures with different atomic configurations at different pressure-temperature conditions. At relatively low pressures, BN is thermodynamically stable in its hexagonal graphite-like structure, hBN. This form features monoatomic layers strongly bound in a honeycomb pattern, like graphene, and inter-bonded with weak van der Waals forces. The hBN, is well established, having been widely deployed for many years. It has found use as a crucible material, in the form of sintered ceramics that benefit from its high thermal and chemical stability, and as a solid lubricant, thanks to its layered structure. Now new opportunities beckon, with hBN under investigation as a potential platform for 2D electronics and quantum technology.While graphite and hBN have similarities in their crystalline form, they differ in electrical conductivity, with hBN behaving as a strongly insulating crystal. This strongly insulating characteristic is to be expected, given that hBN has a wide bandgap of 6 eV. In monolayer hBN this gap is direct, but it shifts to indirect as the number of layers increases. Surprisingly, regardless of whether this material has a direct or indirect band gap, the UV optical emission produced by hBN is extremely efficient. Employed in its two-dimensional form as very thin structures, hBN can be deployed as an excellent insulator or tunnelling dielectric barrier in devices based on graphene and other 2D heterostructures.Important opportunities arise in hBN due to defects within the material. These imperfections enable very interesting physical systems that provide single-photon emitters, or centres hosting quantum spin states with a long coherence time. The results of using high pressure for identification of important ultraviolet color centers in hBN will be presented.For both fundamental studies and the pursuit of new applications, it is critical to produce high-quality crystalline hBN, as this holds the key to uncovering its fascinating properties, as well as evaluating the predictability of theoretical models. Unfortunately, it is far from easy to grow crystalline hBN. Due to a melting temperature that exceeds 3000 °C, hBN, unlike silicon and GaAs, cannot be grown from its stoichiometric melt. Due to this limitation, two leading methods have emerged for the crystallization of hBN. One of them, involves very high pressures of 5 GPa or more, and metallic solutions, containing the likes of Ba, Mg and Ni. In contrast to GaN and InN, hBN does not require a high nitrogen pressure to suppress its decomposition at high temperatures. So the alternative –is based on growth on the surfaces of molten transition metal alloys containing boron, and involves a flow of nitrogen gas at atmospheric pressure. For both approaches, crystals are limited to the order of 1 mm in size. Another concern is the very small thickness of the hBN crystals grown on metal surfaces under atmospheric N2 pressure. To avoid this, we have focused recently, on the high N2 pressure approach to the crystal growth of hBN. Our expectation is that through the controlled increase of nitrogen solubility in molten metals, we will enhance the growth of hBN in the c-direction in the case of surface crystallization, and enable the production of thicker crystals. Our ultimate goal is to establish a new approach, where in contrast to crystallization of hBN on metal surfaces, crystals will be grown in the solution volume. That’s mirroring the technique that we honed for the production of high-quality single crystals of GaN with dimensions of more than 1cm. New emerging high pressure approaches: zone melting and ammonothermal will be also introduced.

room 0.06, Pasteura 5 at 10:15

dr inż. Konrad Wilczyński (Wydział Fizyki Politechniki Warszawskiej)

Theoretical studies of phonon properties in two-dimensional materials and their heterostructures including the lattice temperature

The purpose of this work is to study the vibrational properties of crystal lattices (phonons) in a choice of two-dimensional materials and their heterostructures on the grounds of quantum-mechanical simulations based on the density functional theory (DFT), with particular emphasis on the impact of the lattice temperature. The following two-dimensional structures (based on the transition metal dichalcogenides) will be considered: semiconducting single-layered 1H-MoS2 and 1H-WS2, multi-layered 2H-WS2 [1], 1H-MoS2/1H-WS2 heterostructures with different relative stacking between the layers, 1H-MoS2/graphene heterostructure, and highly anharmonic semi-metallic 1T-TiS2 material [2]. All the listed-above systems are significant from the point of view of their potential applications, enabling us to supplement physical properties of the famous graphene – from the point of view of its electrical, thermal, optical, mechanical, and chemical properties.The undertaken studies, based on rigorous theoretical grounds described in the literature of the 1960s, include the calculation of key effects induced by the anharmonicity of interatomic interactions, such as the thermal expansion of the structure and anharmonic phonon-phonon interactions (in particular three- and four-phonon processes) – affecting the temperature dependence of the effective phonon frequencies and their lifetimes. Each of the above-listed anharmonic effects has been studied separately, enabling us to better understand their nature in each studied two-dimensional material and their dependence on the structure’s geometry. The obtained theoretical temperature-dependent propagation parameters of main phonon modes agree very well with available spectroscopic measurement data, indicating the DFT calculations’ ability to well reproduce this physical property. Therefore, the proposed methodology might be successfully extended to more advanced problems such as complex multi-layered structures and other phonon-limited physical properties (e.g., thermal conductivity).Acknowledgments: Research was funded by the Warsaw University of Technology within the Excellence Initiative: Research University (IDUB) programme. We also acknowledge the usage of computer cluster DWARF at Warsaw University of Technology supported by the Polish National Science Center (NCN) under Contracts No. UMO-2017/26/E/ST3/00428 and UMO-2017/27/B/ST2/02792.References[1] K. Wilczyński, A.P. Gertych, K. Czerniak-Łosiewicz, J. Sitek, M. Zdrojek, “Phonon anharmonicity in multi-layered WS2 explored by first-principles and Raman studies”, Acta Materialia 240, 118299, (2022).[2] K. Wilczyński, A.P. Gertych, M. Zdrojek, “Explaining Mysterious “Shoulder” Raman Band in TiS2 by Temperature-dependent Anharmonicity and Defects”. J. Phys. Chem. C 127, 20870–20880 (2023).

room 0.06, Pasteura 5 at 10:15

dr hab. Maciej Molas, prof. UW (Institute of Experimental Physics, Faculty of Physics, University of Warsaw, Warsaw, Poland)

„Excitons and phonons in van der Waals materials"

Semiconducting transition metal dichalcogenides (S-TMDs) based on molybdenum andtungsten, i.e. MoS2, MoSe2, MoTe2, WS2, and WSe2, are the most well-known representativesof van der Waals materials. Their most distinguished hallmark is the transition from indirect- todirect-band gap, when thinned down from a bulk to a monolayer (ML). The optical response ofS-TMDs is caused mainly by excitonic effects, even at room temperature, due to the largeexcitonic binding energy at the level of hundreds of meV. Moreover, the exciton-phononcoupling (EPC) is significant in thin layers of S-TMDs, which leads to the rich Raman scattering(RS) spectra under their resonant excitation conditions.In my talk I will give an overview on excitons and phonons apparent in high qualityS-TMD MLs and bilayers (BLs) encapsulated in hexagonal BN. The first part of the lecturewill cover experimental and theoretical investigations of excitonic properties in ML and the BLof MoSe2 [1]. The measured magnetic field evolutions of the reflectance contrast spectra of theMoSe2 ML and BL permit one to determine g-factors of intralayer A and B excitons, as well asthe g-factor of the interlayer exciton. The dependence of g-factors on the number of layers andexcitation state will be explained using first-principles calculations. It will be demonstrated thatthe experimentally measured ladder of excitonic s states in the ML can be reproduced using thek·p approach with the Rytova–Keldysh potential that describes the electron–hole interaction.On the contrary, the analogous calculation for the BL case requires taking into account the outof-plane dielectric response of the MoSe2 BL.The second part of my talk will be devoted to the study of phonons in the four S-TMDMLs, i.e. MoS2, MoSe2, WS2, and WSe2, using Raman scattering excitation (RSE), which is anexperimental technique in which the spectrum is made up by sweeping the excitation energywhen the detection energy is fixed [2]. It will be shown that the outgoing resonant conditionsof Raman scattering reveal an extraordinary intensity enhancement of the phonon modes, whichresults in extremely rich RSE spectra. The obtained spectra are composed not only of Ramanactive peaks, i.e. in-plane E’ and out-of-plane A’1, but the appearance of 1st, 2nd, and higherorder phonon modes is recognized. Moreover, the intensity profiles of the A’1 modes in theinvestigated MLs resemble the emissions due to neutral excitons measured in the correspondingPL spectra for the outgoing type of resonant Raman scattering conditions. This results indicatesthat the strength of EPC in S-TMD MLs strongly depends on the type of their ground excitonicstate, i.e. bright or dark, resulting in different shapes of the RSE spectra.[1] Ł. Kipczak, et al., 2D Materials 10, 025014 (2023).[2] M. Zinkiewicz, et al., npj 2D Materials and Applications 8, 2 (2024)

room 0.06, Pasteura 5 at 10:15

prof. dr hab. Grzegorz Karczewski (Institute of Physics, Polish Academy of Sciences)

Properties and applications of PbTe/CdTe nanocomposite

The wide energy gap II-VI semiconductor, CdTe, and the narrow energy gap IV-VI semiconductor, PbTe, have been studied for years, thus their properties are well known. Both crystallize in cubic lattice structures with very similar lattice constants, making epitaxy of one material on another simple. However, their crystal structures are different - for PbTe it is a rock salt structure, and for CdTe it is a zinc blende structure. The crystal structure mismatch makes PbTe and CdTe immiscible. The interfaces between areas of pure PbTe and CdTe are very sharp, however, decorated with a large number of dangling bonds that can act as traps for mobile carriers from PbTe. The goal of the seminar is to show that a mixture of nanometer-sized areas of pure PbTe and CdTe, i.e. a PbTe/CdTe nanocomposite, exhibits new or at least strongly modified features that lead to new functionalities and applications of the material. In particular, it will be recalled how perfectly shaped PbTe quantum dots are produced in a CdTe matrix and what their luminescent properties are. It will also be recalled that CdTe anti-dots introduced into PbTe improve its thermoelectric properties. The main part of the seminar will be devoted to the latest research results of high-temperature infrared detectors made of PbTe/CdTe nanocomposite. Two types of detectors will be presented: photoresistors and photodiodes. In the latter case, the p-n diodes are made of wide bandgap II-VI semiconductors with narrow bandgap PbTe nano-inclusions introduced into the depletion region during MBE growth. Such diodes can be used not only for infrared sensing, but also for two-color infrared photovoltaics.