Layered materials are quasi-2D crystals with highly anisotropic bonding, consisting of atomic layers with strong in-plane covalent bonding and held together by weak out-of-plane van der Waals interaction. The weaker forces between neighboring layers make them easy to cleave into atomically thin materials and stable in such thin forms. The cleavage in ultra-high vacuum conditions, in combination with different electronic spectroscopies, allows to investigate the vibrational and electronic properties minimizing contaminations, such as oxidation. These systems are attracting much interest in the scientific community due to the rich spectrum of fascinating properties associated to the low-dimensional structure, making them interesting for potential applications in a variety of fields, including nanoelectronics, spintronics, optoelectronics, photonics and energy storage. In this PhD project, the electronic properties and the collective excitations (plasmons, phonons, excitons, …) of different layered materials have been investigated by means of a few electronic spectroscopies, as in particular high-resolution electron energy loss and angle-resolved ultraviolet photoemission spectroscopies (HREELS and ARPES). Different types of layered systems have been considered: nanoporous graphene (NPG), black phosphorus (BP), 1T-TiSe2 and 4Hb-TaSe2 - as examples of transition-metal dichalcogenides (TMDs). NPG is a free-standing 3D graphene arrangement formed by few weakly interacting layers. The evolution of the structural and electronic properties in the conversion from pristine NPG to its deuterated phase has been investigated by combining HREELS experiments, and state-of-the-art ab initio simulations. The frequency of the C-D stretching modes depends on the specific structural configuration of D adsorption; comparison with DFT calculation allows to prove that the deuterated structure is 2-side. Furthermore, HREELS reveals the onset of the electronic transition upon deuteration, demonstrating the opening of a semiconducting gap at 3.25 eV, which is in substantial agreement with GW calculations, and indicates the relevant role of excitons. Moreover, the Dirac plasmon properties in the alkali metal-doped NPG sample have been investigated. Upon K-doping, HREEL spectra show a clearly defined and asymmetric loss feature, related to the plasmon excitation, which shows a blue-shift and a marked broadening upon increasing potassium doping. Alkali doping has been also exploited to investigate the Stark effect in BP, which consists of stacked layers of phosphorene held together by weak van der Waals forces. The giant Stark effect affects both the energy and the spatial localization of the valence and conduction band, which extends for several layers in the subsurface region. HREELS has been employed to study the plasmon of the valence band holes in the pristine sample and that of the conduction band electrons in the Cs-doped BP, determining the plasma frequency and the thickness of the n-doped layer and allowing to estimate the charge distribution and the electric field inside the material. Finally, TMD is a class of quasi-2D layered crystals with MX2 structure (where M is a transition metal atom of groups 4-10 and X a chalcogen atom), which offers a novel opportunity to investigate the onset of correlated electronic phases associated to the charge density wave (CDW) lattice reconstructions. Momentum-resolved HREELS has been used to follow plasmon and phonon dispersions of 1T-TiSe2, aiming to unravel the possible excitonic-insulating nature of its (2×2) CDW phase transition. Furthermore, (√13×√13) CDW features are observed in ARPES for the first time on the 4Hb-TaSe2.

I materiali cristallini quasi-2D sono caratterizzati da forti legami covalenti nel piano, mentre i diversi piani sono tenuti assieme da interazioni deboli di tipo Van der Waals. Le forze deboli tra strati adiacenti rendono questi cristalli facili da sfaldare: le strutture ottenute, costituite anche da pochi strati atomici, risultano stabili. La sfaldatura in condizioni di ultra alto vuoto permette di minimizzare l’ossidazione, e quindi di studiare, mediante diverse spettroscopie elettroniche, le loro proprietà elettroniche e vibrazionali. Questi sistemi stanno suscitando un grande interesse nella comunità scientifica poiché presentano un ampio spettro di proprietà inusuali e affascinanti, associate alla bassa dimensionalità, con applicazioni potenziali in vari campi di ricerca. In questo progetto di tesi si sono sfruttate alcune spettroscopie elettroniche, come in particolare la spettroscopia di perdita di energia di elettroni (HREELS) e di fotoemissione ultravioletta risolta in angolo (ARPES), per analizzare le proprietà elettroniche e le eccitazioni collettive (plasmoni, fononi, eccitoni, …) di alcuni materiali quasi-2D. Sono stati studiati in particolare: grafene nanoporoso (NPG), fosforo nero (BP), 1T-TiSe2 e 4Hb-TaSe2 - esempi di dicalcogenuri di metalli di transizione (TMDs). L’NPG è una tipologia di grafene free-standing, caratterizzato da una morfologia 3D costituita da pochi strati debolmente interagenti. La modifica delle proprietà strutturali ed elettroniche fra la fase pura dell’NPG e quella deuterata è stata studiata combinando esperimenti HREELS e simulazioni ab-initio. La frequenza dello stretching del legame C-D è sensibile alla configurazione strutturale specifica dell'adsorbimento del deuterio: il confronto con calcoli DFT ha consentito di dimostrare che il deuterio si lega in entrambi i lati del grafene. Inoltre, a seguito della deuterazione, compare nello spettro HREEL una struttura elettronica che dimostra l'apertura di un gap a 3.25 eV, in sostanziale accordo con i conti GW, indicando un ruolo rilevante degli eccitoni. Sono state inoltre studiate le proprietà del plasmone di Dirac in campioni di NPG drogati con metalli alcalini. A seguito del drogaggio con potassio, gli spettri HREEL mostrano un picco di perdita definito ed asimmetrico, associato all'eccitazione del plasmone, che si sposta a energie maggiori e si allarga all'aumentare del drogaggio. Il drogaggio mediante alcalini è stato inoltre sfruttato per studiare l'effetto Stark nel BP, materiale costituito da strati sovrapposti di fosforene. L'effetto Stark gigante influenza sia l'energia sia la localizzazione spaziale della banda di valenza (BV) e di conduzione (BC), che si estende per diversi strati sotto la superficie. L’HREELS è stato impiegato per studiare il plasmone delle lacune della BV nel campione pulito e quello degli elettroni della BC nel BP drogato con cesio, determinando la frequenza di plasma e lo spessore dello strato drogato n e permettendo di stimare la distribuzione di carica e il campo elettrico all'interno del materiale. Infine, i TMD sono una classe di cristalli quasi-2D con una struttura del tipo MX2 (dove M indica un atomo di un metallo di transizione appartenente ai gruppi 4-10 e X quello di un calcogeno), caratterizzati dalla comparsa di fasi elettroniche collettive associate ad una modulazione della densità di carica (CDW), legata alla ricostruzione del reticolo cristallino. Al fine di svelare la possibile natura di isolante eccitonico della fase CDW (2×2) del 1T-TiSe2, le dispersioni dei fononi e del plasmone sono state misurate mediante la spettroscopia HREEL risolta in momento. Sono state inoltre misurate per la prima volta mediante ARPES la mappa di Fermi e le bande caratteristiche della fase CDW (√13×√13) sul 4Hb-TaSe2.

Studio di proprietà elettroniche ed eccitazioni collettive di materiali quasi-2D mediante spettroscopie elettroniche risolte in angolo / Andrea Tonelli , 2024 Apr 15. 36. ciclo, Anno Accademico 2022/2023.

Studio di proprietà elettroniche ed eccitazioni collettive di materiali quasi-2D mediante spettroscopie elettroniche risolte in angolo

TONELLI, ANDREA
2024

Abstract

Layered materials are quasi-2D crystals with highly anisotropic bonding, consisting of atomic layers with strong in-plane covalent bonding and held together by weak out-of-plane van der Waals interaction. The weaker forces between neighboring layers make them easy to cleave into atomically thin materials and stable in such thin forms. The cleavage in ultra-high vacuum conditions, in combination with different electronic spectroscopies, allows to investigate the vibrational and electronic properties minimizing contaminations, such as oxidation. These systems are attracting much interest in the scientific community due to the rich spectrum of fascinating properties associated to the low-dimensional structure, making them interesting for potential applications in a variety of fields, including nanoelectronics, spintronics, optoelectronics, photonics and energy storage. In this PhD project, the electronic properties and the collective excitations (plasmons, phonons, excitons, …) of different layered materials have been investigated by means of a few electronic spectroscopies, as in particular high-resolution electron energy loss and angle-resolved ultraviolet photoemission spectroscopies (HREELS and ARPES). Different types of layered systems have been considered: nanoporous graphene (NPG), black phosphorus (BP), 1T-TiSe2 and 4Hb-TaSe2 - as examples of transition-metal dichalcogenides (TMDs). NPG is a free-standing 3D graphene arrangement formed by few weakly interacting layers. The evolution of the structural and electronic properties in the conversion from pristine NPG to its deuterated phase has been investigated by combining HREELS experiments, and state-of-the-art ab initio simulations. The frequency of the C-D stretching modes depends on the specific structural configuration of D adsorption; comparison with DFT calculation allows to prove that the deuterated structure is 2-side. Furthermore, HREELS reveals the onset of the electronic transition upon deuteration, demonstrating the opening of a semiconducting gap at 3.25 eV, which is in substantial agreement with GW calculations, and indicates the relevant role of excitons. Moreover, the Dirac plasmon properties in the alkali metal-doped NPG sample have been investigated. Upon K-doping, HREEL spectra show a clearly defined and asymmetric loss feature, related to the plasmon excitation, which shows a blue-shift and a marked broadening upon increasing potassium doping. Alkali doping has been also exploited to investigate the Stark effect in BP, which consists of stacked layers of phosphorene held together by weak van der Waals forces. The giant Stark effect affects both the energy and the spatial localization of the valence and conduction band, which extends for several layers in the subsurface region. HREELS has been employed to study the plasmon of the valence band holes in the pristine sample and that of the conduction band electrons in the Cs-doped BP, determining the plasma frequency and the thickness of the n-doped layer and allowing to estimate the charge distribution and the electric field inside the material. Finally, TMD is a class of quasi-2D layered crystals with MX2 structure (where M is a transition metal atom of groups 4-10 and X a chalcogen atom), which offers a novel opportunity to investigate the onset of correlated electronic phases associated to the charge density wave (CDW) lattice reconstructions. Momentum-resolved HREELS has been used to follow plasmon and phonon dispersions of 1T-TiSe2, aiming to unravel the possible excitonic-insulating nature of its (2×2) CDW phase transition. Furthermore, (√13×√13) CDW features are observed in ARPES for the first time on the 4Hb-TaSe2.
Spectroscopic investigation of electronic properties and collective excitations in layered materials
15-apr-2024
DE RENZI, Valentina
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