Materials

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19 pages, 6056 KiB

Open AccessArticle

Evidence of sp²-like Hybridization of Silicon Valence Orbitals in Thin and Thick Si Grown on α-Phase Si(111)√3 × √3R30°-Bi

by David Garagnani, Paola De Padova, Carlo Ottaviani, Claudio Quaresima, Amanda Generosi, Barbara Paci, Bruno Olivieri, Mieczysław Jałochowski and Mariusz Krawiec

Materials 2022, 15(5), 1730; https://doi.org/10.3390/ma15051730 - 25 Feb 2022

Cited by 4 | Viewed by 1753

Abstract

One-monolayer (ML) (thin) and 5-ML (thick) Si films were grown on the α-phase Si(111)√3 × √3R30°-Bi at a low substrate temperature of 200 °C. Si films have been studied in situ by reflection electron energy loss spectroscopy (REELS) and Auger electron spectroscopy, as [...] Read more.

One-monolayer (ML) (thin) and 5-ML (thick) Si films were grown on the α-phase Si(111)√3 × √3R30°-Bi at a low substrate temperature of 200 °C. Si films have been studied in situ by reflection electron energy loss spectroscopy (REELS) and Auger electron spectroscopy, as a function of the electron beam incidence angle α and low-energy electron diffraction (LEED), as well as ex situ by grazing incidence X-ray diffraction (GIXRD). Scanning tunneling microscopy (STM), and scanning tunneling spectroscopy (STS) were also reported. The REELS spectra, taken at the Si K absorption edge (~1.840 KeV), reveal the presence of two distinct loss structures attributed to transitions 1s→π* and 1s→σ* according to their intensity dependence on α, attesting to the sp²-like hybridization of the silicon valence orbitals in both thin and thick Si films. The synthesis of a silicon allotrope on the α-phase of Si(111)√3 × √3R30°-Bi substrate was demonstrated by LEED patterns and GIXRD that discloses the presence of a Si stack of 3.099 (3) Å and a √3 × √3 unit cell of 6.474 Å, typically seen for multilayer silicene. STM and STS measurements corroborated the findings. These measurements provided a platform for the new √3 × √3R30° Si allotrope on a Si(111)√3 × √3 R30°-Bi template, paving the way for realizing topological insulator heterostructures from different two-dimensional materials, Bi and Si. Full article

(This article belongs to the Special Issue Multilayer and Hybrid Two-Dimensional Materials)

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13 pages, 4586 KiB

Open AccessArticle

Temperature-Dependent Growth and Evolution of Silicene on Au Ultrathin Films—LEEM and LEED Studies

by Tomasz Jaroch and Ryszard Zdyb

Materials 2022, 15(4), 1610; https://doi.org/10.3390/ma15041610 - 21 Feb 2022

Cited by 6 | Viewed by 1661

Abstract

The formation and evolution of silicene on ultrathin Au films have been investigated with low energy electron microscopy and diffraction. Careful control of the annealing rate and temperature of Au films epitaxially grown on the Si(111) surface allows for the preparation of a [...] Read more.

The formation and evolution of silicene on ultrathin Au films have been investigated with low energy electron microscopy and diffraction. Careful control of the annealing rate and temperature of Au films epitaxially grown on the Si(111) surface allows for the preparation of a large scale, of the order of cm², silicene sheets. Depending on the final temperature, three stages of silicene evolution can be distinguished: (i) the growth of the low buckled phase, (ii) the formation of a layered heterostructure of the low buckled and planar phases of silicene and (iii) the gradual destruction of the silicene. Each stage is characterized by its unique surface morphology and characteristic diffraction patterns. The present study gives an overview of structures formed on the surface of ultrathin Au films and morphology changes between room temperature and the temperature at which the formation of Au droplets on the Si(111) surface occurs. Full article

(This article belongs to the Special Issue Multilayer and Hybrid Two-Dimensional Materials)

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14 pages, 3338 KiB

Open AccessArticle

Tuning Ferromagnetism in a Single Layer of Fe above Room Temperature

by Ryszard Zdyb, Krisztián Palotás, Eszter Simon, Tomasz Jaroch and Zbigniew Korczak

Materials 2022, 15(3), 1019; https://doi.org/10.3390/ma15031019 - 28 Jan 2022

Cited by 1 | Viewed by 1838

Abstract

The crystallographic and magnetic properties of an Fe monolayer (ML) grown on 2 ML Au/W(110) substrate are studied with spin-polarized low-energy electron microscopy, density functional theory, and relativistic screened Korringa–Kohn–Rostoker calculations. The single layer of iron atoms possesses hexagonal symmetry and reveals a [...] Read more.

The crystallographic and magnetic properties of an Fe monolayer (ML) grown on 2 ML Au/W(110) substrate are studied with spin-polarized low-energy electron microscopy, density functional theory, and relativistic screened Korringa–Kohn–Rostoker calculations. The single layer of iron atoms possesses hexagonal symmetry and reveals a ferromagnetic order at room temperature. We experimentally demonstrate the possibility of tuning the Curie temperature and the magnitude of magnetization of the Fe monolayer by capping with Au. Taking into account several structural models, the calculation results mostly show ferromagnetic states with enhanced magnetic moments of Fe atoms compared to their bulk value and a further increase in their value after covering with Au. The theoretically calculated Curie temperatures are in fair agreement with those obtained in the experiments. The calculations, furthermore, found evidence for the presence of frustrated isotropic Fe–Fe exchange interactions, and a discussion of the structural effects on the magnetic properties is provided herein. Full article

(This article belongs to the Special Issue Multilayer and Hybrid Two-Dimensional Materials)

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11 pages, 6364 KiB

Open AccessArticle

Surface Transport Properties of Pb-Intercalated Graphene

by Markus Gruschwitz, Chitran Ghosal, Ting-Hsuan Shen, Susanne Wolff, Thomas Seyller and Christoph Tegenkamp

Materials 2021, 14(24), 7706; https://doi.org/10.3390/ma14247706 - 13 Dec 2021

Cited by 11 | Viewed by 2888

Abstract

Intercalation experiments on epitaxial graphene are attracting a lot of attention at present as a tool to further boost the electronic properties of 2D graphene. In this work, we studied the intercalation of Pb using buffer layers on 6H-SiC(0001) by means of electron [...] Read more.

Intercalation experiments on epitaxial graphene are attracting a lot of attention at present as a tool to further boost the electronic properties of 2D graphene. In this work, we studied the intercalation of Pb using buffer layers on 6H-SiC(0001) by means of electron diffraction, scanning tunneling microscopy, photoelectron spectroscopy and in situ surface transport. Large-area intercalation of a few Pb monolayers succeeded via surface defects. The intercalated Pb forms a characteristic striped phase and leads to formation of almost charge neutral graphene in proximity to a Pb layer. The Pb intercalated layer consists of 2 ML and shows a strong structural corrugation. The epitaxial heterostructure provides an extremely high conductivity of

σ = 100

mS/□. However, at low temperatures (70 K), we found a metal-insulator transition that we assign to the formation of minigaps in epitaxial graphene, possibly induced by a static distortion of graphene following the corrugation of the interface layer. Full article

(This article belongs to the Special Issue Multilayer and Hybrid Two-Dimensional Materials)

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18 pages, 2316 KiB

Open AccessArticle

Spectroscopic Analysis of Rare-Earth Silicide Structures on the Si(111) Surface

by Simone Sanna, Julian Plaickner, Kris Holtgrewe, Vincent M. Wettig, Eugen Speiser, Sandhya Chandola and Norbert Esser

Materials 2021, 14(15), 4104; https://doi.org/10.3390/ma14154104 - 23 Jul 2021

Cited by 4 | Viewed by 2076

Abstract

Two-dimensional rare-earth silicide layers deposited on silicon substrates have been intensively investigated in the last decade, as they can be exploited both as Ohmic contacts or as photodetectors, depending on the substrate doping. In this study, we characterize rare-earth silicide layers on the [...] Read more.

Two-dimensional rare-earth silicide layers deposited on silicon substrates have been intensively investigated in the last decade, as they can be exploited both as Ohmic contacts or as photodetectors, depending on the substrate doping. In this study, we characterize rare-earth silicide layers on the Si(111) surface by a spectroscopic analysis. In detail, we combine Raman and reflectance anisotropy spectroscopy (RAS) with first-principles calculations in the framework of the density functional theory. RAS suggests a weakly isotropic surface, and Raman spectroscopy reveals the presence of surface localized phonons. Atomistic calculations allow to assign the detected Raman peaks to phonon modes localized at the silicide layer. The good agreement between the calculations and the measurements provides a strong argument for the employed structural model. Full article

(This article belongs to the Special Issue Multilayer and Hybrid Two-Dimensional Materials)

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15 pages, 9635 KiB

Open AccessArticle

Topological Atomic Chains on 2D Hybrid Structure

by Tomasz Kwapiński and Marcin Kurzyna

Materials 2021, 14(12), 3289; https://doi.org/10.3390/ma14123289 - 14 Jun 2021

Viewed by 2039

Abstract

Mid-gap 1D topological states and their electronic properties on different 2D hybrid structures are investigated using the tight binding Hamiltonian and the Green’s function technique. There are considered straight armchair-edge and zig-zag Su–Schrieffer–Heeger (SSH) chains coupled with real 2D electrodes which density of [...] Read more.

Mid-gap 1D topological states and their electronic properties on different 2D hybrid structures are investigated using the tight binding Hamiltonian and the Green’s function technique. There are considered straight armchair-edge and zig-zag Su–Schrieffer–Heeger (SSH) chains coupled with real 2D electrodes which density of states (DOS) are characterized by the van Hove singularities. In this work, it is shown that such 2D substrates substantially influence topological states end evoke strong asymmetry in their on-site energetic structures, as well as essential modifications of the spectral density function (local DOS) along the chain. In the presence of the surface singularities the SSH topological state is split, or it is strongly localized and becomes dispersionless (tends to the atomic limit). Additionally, in the vicinity of the surface DOS edges this state is asymmetrical and consists of a wide bulk part together with a sharp localized peak in its local DOS structure. Different zig-zag and armachair-edge configurations of the chain show the spatial asymmetry in the chain local DOS; thus, topological edge states at both chain ends can appear for different energies. These new effects cannot be observed for ideal wide band limit electrodes but they concern 1D topological states coupled with real 2D hybrid structures. Full article

(This article belongs to the Special Issue Multilayer and Hybrid Two-Dimensional Materials)

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20 pages, 1757 KiB

Open AccessArticle

Wiedemann–Franz Law for Massless Dirac Fermions with Implications for Graphene

by Adam Rycerz

Materials 2021, 14(11), 2704; https://doi.org/10.3390/ma14112704 - 21 May 2021

Cited by 14 | Viewed by 2553

Abstract

In the 2016 experiment by Crossno et al. the electronic contribution to the thermal conductivity of graphene was found to violate the well-known Wiedemann–Franz (WF) law for metals. At liquid nitrogen temperatures, the thermal to electrical conductivity ratio of charge-neutral samples was more [...] Read more.

In the 2016 experiment by Crossno et al. the electronic contribution to the thermal conductivity of graphene was found to violate the well-known Wiedemann–Franz (WF) law for metals. At liquid nitrogen temperatures, the thermal to electrical conductivity ratio of charge-neutral samples was more than 10 times higher than predicted by the WF law, which was attributed to interactions between particles leading to collective behavior described by hydrodynamics. Here, we show, by adapting the handbook derivation of the WF law to the case of massless Dirac fermions, that significantly enhanced thermal conductivity should appear also in few- or even sub-kelvin temperatures, where the role of interactions can be neglected. The comparison with numerical results obtained within the Landauer–Büttiker formalism for rectangular and disk-shaped (Corbino) devices in ballistic graphene is also provided. Full article

(This article belongs to the Special Issue Multilayer and Hybrid Two-Dimensional Materials)

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16 pages, 10438 KiB

Open AccessArticle

Evolution of Topological Surface States Following Sb Layer Adsorption on Bi₂Se₃

by Kris Holtgrewe, Conor Hogan and Simone Sanna

Materials 2021, 14(7), 1763; https://doi.org/10.3390/ma14071763 - 02 Apr 2021

Cited by 7 | Viewed by 2316

Abstract

Thin antimony layers adsorbed on bismuth selenide (

{Bi}_{2} {Se}_{3}

) present an exciting topological insulator system. Much recent effort has been made to understand the synthesis and electronic properties of the heterostructure, particularly the migration of the topological surface states [...] Read more.

Thin antimony layers adsorbed on bismuth selenide (

{Bi}_{2} {Se}_{3}

) present an exciting topological insulator system. Much recent effort has been made to understand the synthesis and electronic properties of the heterostructure, particularly the migration of the topological surface states under adsorption. However, the intertwinement of the topological surface states of the pristine

{Bi}_{2} {Se}_{3}

substrate with the Sb adlayer remains unclear. In this theoretical work, we apply density functional theory (DFT) to model heterostructures of single and double atomic layers of Sb on a bismuth selenide substrate. We thereby discuss established and alternative structural models, as well as the hybridization of topological surface states with the Sb states. Concerning the geometry, we reveal the possibility of structures with inverted Sb layers which are energetically close to the established ones. The formation energy differences are below 10 meV/atom. Concerning the hybridization, we trace the band structure evolution as a function of the adlayer-substrate distance. By following changes in the connection between the Kramers pairs, we extract a series of topological phase transitions. This allows us to explain the origin of the complex band structure, and ultimately complete our knowledge about this peculiar system. Full article

(This article belongs to the Special Issue Multilayer and Hybrid Two-Dimensional Materials)

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