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Surface plasmon polaritons (SPPs) are electromagnetic excitations existing at the interface between a metal and a dielectric. SPPs provide a promising path in nanophotonic devices for light manipulation at the micro and nanoscale with applications in optoelectronics, biomedicine, and energy harvesting. Recently, SPPs are extended to unconventional materials like graphene, transparent oxides, superconductors, and topological systems characterized by linearly dispersive electronic bands. In this respect, 3D Dirac and Weyl semimetals offer a promising frontier for infrared (IR) and terahertz (THz) radiation tuning by topologically-protected SPPs. In this work, the THz-IR optical response of platinum ditelluride (PtTe2) type-II Dirac topological semimetal films grown on Si substrates is investigated. SPPs generated on microscale ribbon arrays of PtTe2 are detected in the far-field limit, finding an excellent agreement among measurements, theoretical models, and electromagnetic simulation data. The far-field measurements are further supported by near-field IR data which indicate a strong electric field enhancement due to the SPP excitation near the ribbon edges. The present findings indicate that the PtTe2 ribbon array appears an ideal active layout for geometrically tunable SPPs thus inspiring a new fashion of optically tunable materials in the technologically demanding THz and IR spectrum.

Silicene has recently been revisited as a 2D material with potential thermoelectric applications. Here, the thermal properties of supported silicene are determined by optothermal Raman spectroscopy. Single and multilayer silicene is grown either directly on silver (Ag) substrates or on an intermediate tin (Sn) monolayer, introduced to reduce the influence of the substrate on the physical properties of silicene. Experimental values of an effective cross-plane thermal conductivity of 0.5 W/mK are obtained for silicene on Ag and 0.3 W/mK for silicene on stanene-Ag. The values of the interfacial thermal conductance, on the other hand, are 0.3 and 0.5 GW/m2K, respectively. Heterostack engineering is confirmed as a versatile strategy for extracting relevant physical parameters in silicene and for modulating the thermal response in the 2D limit.

Dall'isolamento del grafene, la nuova classe dei materiali bidimensionali (2D) si è continuamente ampliata, aprendo la strada a molte opportunità nell'evoluzione delle nanotecnologie. In questo gruppo di materiali, gli Xeni sono una classe emergente di materiali simili al grafene, sintetizzati mediante epitassia da fasci molecolari (MBE), che mostrano proprietà eccezionali grazie alla loro natura 2D. Tra questi, il silicene gioca un ruolo cruciale, grazie alla potenziale facilità di integrazione nell'attuale tecnologia al silicio, mentre lo stanene presenta interessanti proprietà, soprattutto legate all'accoppiamento spin-orbita (SOC) e alla sua possibile integrazione come isolante topologico 2D. In questo quadro, lo scopo di questo lavoro di tesi è legato allo studio e all'ottimizzazione della reale manipolazione di Xeni, come il silicene e lo stanene, dal punto di vista del processo, al fine di discutere e risolvere le principali problematiche versola loro implementazione in strutture di dispositivi. In particolare, i campioni considerati sono di tre tipi: silicene cresciuto su Ag(111) su mica, eterostrutture di silicene-stanene cresciute su Ag(111) su mica e campioni di stanene cresciuti su substrato InSb. A differenza del grafene, l'ibridazione mista sp2-sp3 degli Xeni, e in particolare del silicene, comporta un'elevata reattività ambientale che porta alla degradazione del materiale. Lo sviluppo di schemi di incapsulamento affidabili è fondamentale per la scoperta di nuove possibili applicazioni; pertanto, un ruolo chiave in questo lavoro è legato alle eterostrutture di silicene-stanene, in cui il silicene è inserito tra uno strato Al2O3 di protezione sulla superficie superiore e lo stanene su Ag(111) sulla faccia inferiore. L'introduzione di uno strato di stanene ha permesso di sviluppare uno schema di incapsulamento completo per proteggere il silicene dalla degradazione in condizioni ambientali quando viene disassemblato dal substrato di crescita. Co, Since the isolation of graphene, the new class of two-dimensional (2D) materials has been continuously expanded, paving the way for many opportunities in the evolution of nanotechnology. In this materials group, Xenes are an emerging class of graphene-like materials, synthesized by molecular beam epitaxy (MBE), that show exceptional properties due to their 2D nature. Among these, silicene plays a crucial role, as a possible future integration in the current silicon technology, and stanene presents interesting properties, especially related to the spin-orbit coupling (SOC) and its possible integration as a 2D topological insulator. In this framework, the purpose of this work of thesis is related to the study and optimization of the actual manipulation of Xenes, such as silicene and stanene, from a processing point of view, in order to discuss and solve the main issues toward the implementation into device structures. In particular, the considered samples are of three types: silicene grown on Ag(111)-on-mica substrate, heterostructures of silicene-stanene grown on Ag(111)-on-mica and samples of few-layer stanene grown on InSb substrate. Unlike graphene, the mixed sp2-sp3 hybridization of Xenes, and in particular of silicene, results in a high environmental reactivity leading to material degradation. The development of reliable encapsulation schemes is fundamental for the discovery of new possible applications, hence, a key role in this work is related to the vertically stacked silicene-stanene heterostructures, where silicene is sandwiched in between an Al2O3 capping layer on the top surface and the stanene on Ag(111) crystal at the bottom face. The introduction of a stanene buffer layer enabled to develop an all-around encapsulation scheme to protect silicene against degradation under environmental conditions when disassembled from the native substrate. By combining different chemical and structural techniques, such as X-ray photoelectron spectroscopy (XPS) and Raman

Silicene or the two-dimensional (2D) graphene-like silicon allotrope has recently emerged as a promising candidate for various applications in nanotechnology. However, concerns on the silicene stability still persist to date and need to be addressed aiming at the fabrication of competing and durable silicene-based devices. Here, we present an all-around encapsulation methodology beyond the current state-of-the-art silicene configuration, namely silicene sandwiched in between a capping layer (e.g., Al2O3) and the supporting substrate (e.g., Ag). In this framework, the insertion of one or two sacrificial 2D Sn layers enables the realization of different atomically thin encapsulation schemes, preserving the pristine properties of silicene while decoupling it from the growth template. On one hand, the epitaxy of a 2D Sn layer before silicene allows for the removal of the Ag substrate with no effect on silicene which in turn can be easily gated, for example, with an oxide layer on its top face. On the other hand, a full 2D encapsulation scheme, where top and bottom faces of silicene are protected by 2D Sn layers, gives rise to an atomically thin and cm2-scaled membrane preventing degradation of silicene for months. Both schemes thus constitute an advancement for the silicene stability and encapsulation in ambient conditions, paving the way to further exploitation in flexible electronics and photonics.

Due to their superior mechanical properties, 2D materials have gained interest as active layers in flexible devices co-integrating electronic, photonic, and straintronic functions altogether. To this end, 2D bendable membranes compatible with the technological process standards and endowed with large-scale uniformity are highly desired. Here, it is reported on the realization of bendable membranes based on silicene layers (the 2D form of silicon) by means of a process in which the layers are fully detached from the native substrate and transferred onto arbitrary flexible substrates. The application of macroscopic mechanical deformations induces a strain-responsive behavior in the Raman spectrum of silicene. It is also shown that the membranes under elastic tension relaxation are prone to form microscale wrinkles displaying a local generation of strain in the silicene layer consistent with that observed under macroscopic mechanical deformation. Optothermal Raman spectroscopy measurements reveal a curvature-dependent heat dispersion in silicene wrinkles. Finally, as compelling evidence of the technological potential of the silicene membranes, it is demonstrated that they can be readily introduced into a lithographic process flow resulting in the definition of flexible device-ready architectures, a piezoresistor, and thus paving the way to a viable advance in a fully silicon-compatible technology framework.