Silicene: Compelling Experimental Evidence for Graphenelike Two-Dimensional Silicon

The influence of structure on the electronic properties of $\pi$-conjugated systems

A combination of scanning tunneling microscopy, angle-resolved photoelectron spectroscopy, ab initio random structure searching, and density functional theory electronic structure calculations was applied to elucidate the atomic arrangement and electron band structure of the $(\mathrm{Au},\mathrm{Al})/\mathrm{Si}(111)2\ifmmode\times\else\texttimes\fi{}2$ two-dimensional compound formed upon Al deposition onto the mixed $5\ifmmode\times\else\texttimes\fi{}2/\sqrt{3}\ifmmode\times\else\texttimes\fi{}\sqrt{3}$ Au/Si(111) surface. It was found that the most stable $2\ifmmode\times\else\texttimes\fi{}2$-(Au, Al) compound incorporates four Au atoms, three Al atoms, and two Si atoms per $2\ifmmode\times\else\texttimes\fi{}2$ unit cell. Its atomic arrangement can be visualized as an array of meandering Au atomic chains with two-thirds of the Al atoms incorporated into the chains and one-third of the Al atoms interconnecting the chains. The compound is metallic and its electronic properties can be controlled by appropriate Al dosing since energetic location of the bands varies by $\ensuremath{\sim}0.5\phantom{\rule{0.16em}{0ex}}\mathrm{eV}$ during increasing of Al contents. The $2\ifmmode\times\else\texttimes\fi{}2$-(Au, Al) structure appears to be lacking the ${C}_{3v}$ symmetry typical for the hexagonal lattices. The consequence of the peculiar atomic structure of the two-dimensional alloy is spin splitting of the metallic states, which should lead to anisotropy of the current-induced in-plane spin polarization.

In this work, the structural, mechanical, electronic, optical and magnetic properties of the RhNbSb half‐Heusler compound were examined and analyzed using density functional theory (DFT). According to the results, type I atomic arrangement is structurally the most stable for the compound. In all three of its atomic arrangement types, the compound is mechanically stable and ductile according to the analysis of its mechanical properties. Generalized gradient approximation (GGA) + U approach was applied in addition to GGA approach, where U is Hubbard parameter, to increase the accuracy of results in electronic band structure, density of states (DOS), and magnetic moments. Therefore, electronic band structure and DOS calculations demonstrate that the compound exhibits metallic properties in both its type I and type II atomic arrangements with GGA predictions. However, under GGA + U calculations, the compound becomes half metal when in type I but it still reflects metallic nature when in type II. The compound’s half‐metallic nature in type I under the GGA+U method suggests that it may be a good fit for spintronics applications in this type I of its atomic arrangement. The calculated total magnetic moment of the compound under GGA + U approach exactly fits with Slater–Pauling rule of half‐metallic nature in its type I atomic arrangement, a result that supports half‐metallic nature of the compound in type I atomic arrangement for electronic properties under GGA + U prediction. Furthermore, the compound might be taken into consideration for optoelectronic applications according to the results of computed optical characteristics.

In this work, the structural, electronic, and dynamical properties of novel half-Heusler (HH) alloy AlNiP are studied by applying first-principles density functional theory (DFT). The structural properties of AlNiP in the LiAlSi-type crystal structure with three different phases (α-phase, β-phase, and γ-phase) in different atomic arrangements are explored. The ‘γ-phase’ of AlNiP is found to be energetically stable among three. Moreover, the calculated ferromagnetic (FM), non-magnetic (NM) and anti-ferromagnetic (AFM) state for AlNiP favours the ‘NM’ state. The overlapping of the valence band and conduction band signifies the metallic characteristics of the alloy at 0 GPa and high pressure 38 GPa. The lattice dynamics of novel HH alloy AlNiP have also been investigated by applying DFT followed by density functional perturbation theory (DFPT). The phonon calculation suggests the face-centred cubic phase of HH alloy AlNiP is stable at ambient condition and also at high pressure of 38 GPa. For the confirmation of this stability, the phonon density of states has also been calculated at zero and higher value of pressure. The calculated results are further interpreted in the light of the similar type of alloys.

The purpose of this review is to focus from an experimental point-of-view on the new physical properties of some of the thinnest superconducting films that can be fabricated and studied in situ nowadays with state-of-the-art methods. An important characteristic of the films we address is that the underlying electronic system forms a two-dimensional electron gas (2DEG). Up to now there are only few of these systems. Such true 2D superconductors can be divided into two classes: surface-confined or interface-confined films. Because the second types of films are burried below the surface, they are not accessible to purely surface-sensitive techniques like angular-resolved photoemission spectroscopy (ARPES) or scanning tunneling spectroscopy (STS). As a consequence the bandstructure characteristics of the 2DEG cannot be probed nor the local superconducting properties. On the other hand, in situ prepared surface-confined films are nowadays accessible not only to ARPES and STS but also to electrical transport measurements. As a consequence surface-confined systems represent at present the best archetypes on which can be summarized the new properties emerging in ultimately thin superconducting films hosting a 2DEG, probed by both macroscopic and microscopic measurement techniques. The model system we will widely refer to consists of a single atomic plane of a conventional superconductor, like for example lead (Pb), grown on top of a semiconducting substrate, like Si(111). In the introductory part we first introduce the topic and give historical insights into this field. Then in the section , we introduce useful concepts worked out in studies of so-called ‘granular’ and ‘homogeneous’ superconducting thin films that will be necessary to understand the role of non-magnetic disorder on 2DEG superconductors. In this section, we also briefly review the superconducting properties of crystalline Pb/Si(111) ultrathin films grown under ultrahigh vacuum (UHV) conditions in order to illustrate their specific properties related to quantum-size effects. In the next section we review the growth methods and structural properties of the presented 2DEG surface-confined superconductors. In section , we review the electronic structure and Fermi surface properties as measured by macroscopic ARPES and confront them to ab initio DFT calculations based on the characterized atomic structures of the monolayers. The following section reviews the macroscopic properties inferred from in situ electrical transport measurements methods, including attempts to study the Berezinsky–Kosterlitz–Thouless 2D regime. In the last section , we summarize the emerging local spectroscopic properties measured by STS. These latter demonstrate variations of the local superconducting properties at a scale much shorter than the superconducting coherence length due to a combined effect of non-magnetic disorder and two-dimensionality. Further peculiar local spectroscopic effects are presented giving evidence for the presence of a mixed singlet-triplet superconducting order parameter induced by the presence of a strong Rashba spin–orbit coupling term at the surface. These local signatures will be discussed along with ARPES and transport measurements in parallel high magnetic field on closely related systems. Finally, we present in anisotropic Pb and In monolayers the peculiar role played by atomic steps on vortex properties, leading to the observation by STS of mixed Abrikosov–Josephson vortices in agreement with in situ macroscopic transport measurements. From the overview of all recent experimental and theoretical results it appears that these surface 2D superconductors, such as one monolayer of Pb on Si(111), are ideal templates to engineer and realize topological superconductivity.

Structural, electronic, dynamic, optic and elastic properties of MgScGa via density functional theory

TiO2/Cu2O coupled oxide films in Cl− ion containing solution: Volta potential and electronic properties characterization by scanning probe microscopy

The radical molecules have attracted significant attention from researchers because their electronic and spin properties can be controlled via the structure and heteroatoms. With the advancements in on-surface synthesis, it has become possible to conduct spin engineering at the single-molecule level. Here, we investigate the controllable polarized and electronic states of 4,8,12-trioxotriangulene adsorbed on AuSix/Au(111) surfaces with a combination of scanning tunneling microscopy (STM) operated at 4.3 K and density functional theory calculations. A rich variety of STM topographic contrasts of the molecule reveals various interactions between the molecule and the substrate. We found that the magnetic and electronic properties can be modulated through tip-induced formation and dissociation of a Si-O bond. This finding may pave the way for advancements in molecular spintronics.

Edges and point defects in layered dichalcogenides are important for tuning their electronic and magnetic properties. By combining scanning tunneling microscopy (STM) with density functional theory (DFT), the electronic structure of edges and point defects in 2D‐PtSe2 are investigated where the 1.8 eV bandgap of monolayer PtSe2 facilitates the detailed characterization of defect‐induced gap states by STM. The stoichiometric zigzag edge terminations are found to be energetically favored. STM and DFT show that these edges exhibit metallic 1D states with spin polarized bands. Various native point defects in PtSe2 are also characterized by STM. A comparison of the experiment with simulated images enables identification of Se‐vacancies, Pt‐vacancies, and Se‐antisites as the dominant defects in PtSe2. In contrast to Se‐ or Pt‐vacancies, the Se‐antisites are almost devoid of gap states. Pt‐vacancies exhibit defect induced states that are spin polarized, emphasizing their importance for inducing magnetism in PtSe2. The atomic‐scale insights into defect‐induced electronic states in monolayer PtSe2 provide the fundamental underpinning for defect engineering of PtSe2‐monolayers and the newly identified spin‐polarized edge states offer prospects for engineering magnetic properties in PtSe2 nanoribbons.

CrSi2 crystallites on Si(110)

Increasing the stability of perovskites is essential for their integration in commercial photovoltaic devices. Halide mixing is suggested as a powerful strategy toward stable perovskite materials. However, the stabilizing effect of the halides critically depends on their distribution in the mixed compound, a topic that is currently under intense debate. Here we successfully determine the exact location of the I and Cl anions in the CH3NH3PbBr3–yIy and CH3NH3PbBr3–zClz mixed halide perovskite lattices and correlate it with the enhanced stability we find for the latter. By combining scanning tunneling microscopy and density functional theory, we predict that, for low ratios, iodine and chlorine incorporation have different effects on the electronic properties and stability of the CH3NH3PbBr3 perovskite material. In addition, we determine the optimal Cl incorporation ratio for stability increase without detrimental band gap modification, providing an important direction for the fabrication of stable perovskite devices. The increased material stability induced by chlorine incorporation is verified by performing photoelectron spectroscopy on a half-cell device architecture. Our findings provide an answer to the current debate on halide incorporation and demonstrate their direct influence on device stability.

Using a density-functional scheme, we investigate the electronic, structural, and vibrational properties of amorphous silicon nitride. Through a Car-Parrinello molecular-dynamics simulation, we generate a model structure formed mainly by a network of ${\text{SiN}}_{4}$ tetrahedra a large fraction of which are edge sharing. Only a small fraction of atoms are overcoordinated and undercoordinated. First, the structural properties such as angular distributions, atomic arrangements in first-neighbor shells, the neutron total structure factor, the radial distribution function, and pair-correlation functions are examined. Next, the electronic properties are analyzed by considering the quasiparticle density of states which is calculated through the $GW$ method. Good agreement is found with experimental data when available. Successively, we focus on a range of vibrational spectra. First, the vibrational density of states is analyzed in terms of its decomposition into N and Si contributions. Then, we investigate the Born effective charge tensors, the high-frequency, and static dielectric constants and calculate the real and imaginary parts of the dielectric function in the infrared. Therefrom we obtain the infrared-absorption spectrum and the refractive index that are found to be in accord with experimental measurements. Moreover, we address the Raman spectrum which is compared with available experimental data. Electronic structure and vibrational properties of the point defects present in our model are also discussed. Density-functional and $GW$ schemes appear to be appropriate for modeling materials based on silicon nitride. In particular, our modeling of silicon nitride achieved a successful level of comparison with experiments. This allows us to infer that $a{\text{-Si}}_{3}{\text{N}}_{4}$ features a high content of edge-sharing tetrahedra, which are absent in the crystalline phases of silicon nitride at ambient conditions.

Self-assembled strategy has been proven to be a promising vista in constructing organized low-dimensional nanostructures with molecular precision and versatile functionalities on solid surfaces. Herein, we investigate by a combination of scanning tunneling microscopy (STM) and dispersion-corrected density functional theory (DFT), the adsorption of tetracene molecules on the silver substrate and the mechanism mediating the self-assembly on Ag(110). As expected, ordered domain is formed on Ag(110) after adsorption with adjacent molecules being imaged with alternating bright or dim pattern regularly. While such behavior has been assigned previously to the difference of molecular adsorption height, herein, it is possible to investigate essentially the mechanism leading to the periodic alternation of brightness and dimness for tetracene adsorbed on Ag(110) thanks to the consideration of Van der Waals (vdW) dispersion force. It is demonstrated that the adsorption height in fact is same for both bright and dim molecules, while the adsorption site and the corresponding interfacial charge transfer play an important role in the formation of such pattern. Our report reveals that vdW dispersion interaction is crucial to appropriately describe the adsorption of tetracene on the silver substrate, and the formation of delicate molecular architectures on metal surfaces might also offers a promising approach towards molecular electronics.

Classical carbenes are highly reactive species that have traditionally been regarded as transient intermediates, making direct structural characterization a challenge. Among them, N‐heterocyclic carbenes (NHCs) stand out for their stability and broad applicability, yet atomic‐scale insights into their structure and electronic properties remain limited. Here, we report the on‐surface single‐molecule characterization of a free NHC deposited on ultrathin insulating NaCl layers on Au(111), enabling direct investigation of a reactive carbene in its free form. Using a combination of scanning tunneling microscopy (STM), atomic force microscopy (AFM), and density functional theory (DFT), we resolve its molecular structure and frontier orbital with sub‐molecular resolution. We further demonstrate the reactivity of the free NHC through its on‐surface complexation with a gold atom. These results demonstrate that real‐space techniques enable direct insights into the structure and reactivity of free NHCs, helping to connect molecular properties with their broader functional applications.

Classical carbenes are highly reactive species that have traditionally been regarded as transient intermediates, making direct structural characterization a challenge. Among them, N-heterocyclic carbenes (NHCs) stand out for their stability and broad applicability, yet atomic-scale insights into their structure and electronic properties remain limited. Here, we report the on-surface single-molecule characterization of a free NHC deposited on ultrathin insulating NaCl layers on Au(111), enabling direct investigation of a reactive carbene in its free form. Using a combination of scanning tunneling microscopy (STM), atomic force microscopy (AFM), and density functional theory (DFT), we resolve its molecular structure and frontier orbital with sub-molecular resolution. We further demonstrate the reactivity of the free NHC through its on-surface complexation with a gold atom. These results demonstrate that real-space techniques enable direct insights into the structure and reactivity of free NHCs, helping to connect molecular properties with their broader functional applications.