Ultrathin two-dimensional inorganic materials: new opportunities for solid state nanochemistry.

Graphene and graphene-like two-dimensional (2D) nanomaterials, such as black phosphorus (BP), transition metal carbides/carbonitrides (MXene) and transition metal dichalcogenides (TMD), have been extensively studied in recent years due to their unique physical and chemical properties. With atomic-scale thickness, these 2D materials and their derivatives can react with ROS and even scavenge ROS in the dark. With excellent biocompatibility and biosafety, they show great application potential in the antioxidant field and ROS detection for diagnosis. They can also generate ROS under light and be applied in antibacterial, photodynamic therapy (PDT), and other biomedical fields. Understanding the degradation mechanism of 2D nanomaterials by ROS generated under ambient conditions is crucial to developing air stable devices and expanding their application ranges. In this review, we summarize recent advances in 2D materials with a focus on the relationship between their intrinsic structure and the ROS scavenging or generating ability. We have also highlighted important guidelines for the design and synthesis of highly efficient ROS scavenging or generating 2D materials along with their biomedical applications.

Grain boundaries (GBs) modulate the macroscopic properties in polycrystalline materials because they have different atomic and electronic structures from the bulk. Despite the progress on the understanding of GB atomic structures, knowledge of the localized electronic band structures is still lacking. Here, we experimentally characterized the atomic structures and the band gaps of four typical GBs in α-Al2O3 by scanning transmission electron microscopy and valence electron energy-loss spectroscopy (EELS). It was found that the band gaps of the GBs are narrowed by 0.5-2.1 eV compared with that of 8.8 eV in the bulk. By combing core-loss EELS with first-principles calculations, we elucidated that the band gap reductions directly correlate with the decrease of the coordination numbers of Al and O ions at the GBs. These results provide in-depth understanding between the local atomic and electronic band structures for GBs and demonstrate a novel electronic-structure analysis for crystalline defects.

First-principles study of structure, electronic properties and stability of tungsten adsorption on TiC(111) surface with disordered vacancies

Obtaining electronic and atomic structure of material simultaneously is very important for developing the nano-technology. In this paper, we demonstrate that atomic and electronic structure of an interface can be extracted with combination of Gerchberg-Saxton indirect microscopy and electron spectroscopy imaging (ESI) technique. Basically, Gerchberg-Saxton algorithm includes two projections. Projection in the real space is a maximum entropy (ME) de-convolution process and in reciprocal space is an amplitude substitution process. It has been shown that Gerchberg-Saxton algorithm can extend the structural resolution to near 0.1nm. An application case of Gerchberg-Saxton algorithm to solve the atomic structure for 3C-polytypic SiC boundary is shown.ESI spectrum processed by FFT interpolation, maximum entropy de-convolution and wavelet transformation allow us to extract 2-dimensional map of the sp2/sp3 with a sub-nanometer resolution. Grain boundary and interface at SiC are good candidates for this study, since the bond distance of Si-C is slightly less than 0.1nm which is not routinely resolvable using a FEG TEM and Si-L (99eV) and C-K-edges (283 eV) locate in a reasonable energy range. The resultant electronic structure can be compared with that calculated using WIEN97. An example of quantitative analysis on 2-dimensional sp3/sp2 map deduced from the C K-edge of ESI spectra acquired from 6H-SiC is given.

The studies of the properties of graphene synthesized on the surface of epitaxial films of cubic single-crystal silicon carbide preliminarily grown on Si(001) wafers have been reviewed. These studies were supported by the Russian Foundation for Basic Research, project no. 17-02-01139. The results of these studies demonstrate that graphene layers synthesized on β-SiC/Si(001) substrates have the atomic structure and electronic properties of a quasi-freestanding graphene sheet. Continuous graphene layers with a preferential direction of nanodomain boundaries, which is determined by the orientation of steps on the initial surface, can be synthesized on vicinal SiC(001) substrates. The possibility of controlled growth of mono-, bi-, and trilayer graphene on β-SiC/Si(001) wafers has been demonstrated. The studies have shown the opening of a transport gap and a high positive magnetoresistance in a parallel magnetic field in an ordered system of graphene nanoribbons on the vicinal SiC(001) surface. It has been shown that the functionalization of graphene with organic compounds changes the electronic properties of graphene on SiC(001), modifying it to a semiconductor with given properties, which allows applications in modern micro- and nanoelectronics.

Electronic states and atomic structure at the Pd2Si–Si interface have been investigated using AES and UPS in conjunction with transmission electron microscopy (TEM). By following the core and valence spectra from submonolayer to several atomic layers of Pd coverages, the changes in the partial state density of the Pd and Si atoms during interface formation have been observed. Stoichiometry calibration based on relative variation of Auger intensities reveals extra Si states existing within a few Å of the interface. Analysis of the spectral shapes and peak shifts shows that these Si states are derived from a Si-rich environment at the interface and are located mostly near the Si bandgap region. TEM lattice images reveal a structurally sharp Pd2Si–Si (111) interface with misfit dislocations and atomic steps present within several Å of the interface. Combining the spectroscopy and TEM results, we infer that the interface states originate from chemical bonds associated with the atomic defect structures produced by silicide formation.

Germanium−perovskite oxide heterostructures have a strong potential for next-generation low-voltage and low-leakage metal-oxide semiconductor field-effect transistors. We investigated the atomic structure and electronic properties of Ge on perfect and defective (001) SrTiO3 by first-principle calculations. The specific adsorption sites at the initial growth stage and the atomic structure of Ge on the SrTiO3 (001) substrate have been systematically investigated. The surface grand potential was calculated and compared as a function of the relative chemical potential. The complete surface phase diagram was presented. The energetically favorable interfaces were pointed out among the atomic arrangements of the Ge/SrTiO3 (001) interfaces. The atomic structure and electronic properties of the intrinsic point defects were calculated and analyzed for the Ge/SrTiO3 (001) interfaces.

Selective synthesis of Ni12P5 and Ni2P nanoparticles: Electronic structures, magnetic and optical properties

Two-dimensional (2D) materials hold great promise for future nanoelectronics as conventional semiconductor technologies face serious limitations in performance and power dissipation for future technology nodes. The atomic thinness of 2D materials enables highly scaled field-effect transistors (FETs) with reduced short-channel effects while maintaining high carrier mobility, essential for high-performance, low-voltage device operations. The richness of their electronic band structure opens up the possibility of using these materials in novel electronic and optoelectronic devices. These applications are strongly dependent on the electrical properties of 2D materials-based FETs. Thus, accurate characterization of important properties such as conductivity, carrier density, mobility, contact resistance, interface trap density, etc is vital for progress in the field. However, electrical characterization methods for 2D devices, particularly FET-related measurement techniques, must be revisited since conventional characterization methods for bulk semiconductor materials often fail in the limit of ultrathin 2D materials. In this paper, we review the common electrical characterization techniques for 2D FETs and the related issues arising from adapting the techniques for use on 2D materials.

Monodisperse faceted nanocrystals, with controllable shapes and sizes, have been becoming increasingly important for applications in catalysis, gas sensing, and energy conversion. Such highly shape sensitive and selective physical and chemical properties inherently stem from the atomic and electronic structures on the faceted surfaces. For elemental nanocrystals, the atomic structure on the surfaces is determined by the geometric shape itself. However, for compound materials such as alloys and complex oxides, the compositional segregation and different terminating lattice planes on the surfaces have to be taken into account. In order to understand the unique property and growth mechanism of these nanocrystals, atomic details on the faceted surfaces need to be studied on the atomic level. Strontium titanate (SrTiO 3 ), strontium zirconate (SrZrO 3 ) and their solid solutions (SrTi 1−x Zr x O 3 ) are important members in the class of perovskite structures with a general formula ABO 3 (Figure 1a). These materials are of great technological and fundamental importance not only because of their interesting properties, but also because of their ability to combine and to adjust these properties by chemical substitution with a wide variety of cations. However, despite the success of the synthesis of the {1 0 0}‐faceted BaTiO 3 , SrTiO 3 , and Ba 1−x Sr x TiO 3 nanocubes, whether the {1 0 0} facets of the nanocubes are terminated with AO (SrO) or BO 2 (TiO 2 ) is a question which still remains open for speculation and investigation. A comprehensive understanding of the growth mechanisms of these faceted nanocubes has not been achieved. Direct experimental evidence for the atomic structure on these nanocube surfaces has become one of the key steps in exploring the growth mechanisms. In this work, we report on detailed studies of monodisperse {1 0 0}‐faceted nanocubes of SrTi 1−x Zr x O 3 (x = 0.25 to 0.5) which were synthesized using the oil‐water two‐phase solvothermal method. The surface atomic structure of the monodisperse faceted nanocrystals is determined by means of aberration‐corrected high‐angle annular dark‐field scanning transmission electron microscopy (HAADF‐STEM). On the basis of the structural features on the faceted surfaces, a deeper insight into the growth mechanisms could be obtained.

There is plenty of room for new structures at the bottom

Calculations of the atomic and electronic structure of Ca-Mg glasses and of the crystalline intermetallic compound ${\mathrm{CaMg}}_{2}$ are presented. For the glasses, the calculations are based on realistic models of the atomic structure constructed by a molecular-dynamics computer simulation linked to a steepest-descent potential-energy mapping. Interatomic forces are described in terms of pseudopotential-derived effective pair potentials. Our results for the atomic structure are in very good agreement with x-ray-diffraction data. They show that the topological short-range order in the glass is similar to the atomic arrangement in the crystalline Laves phase and best described as a highly defective tetrahedral close packing. The analogy between the glassy and the crystalline phases extends to the electronic structure. In both the crystalline and the amorphous alloys we find an essentially free-electron-like density of states (DOS), but close to the Fermi level the DOS is enhanced due to the onset of Ca 3d states. We find no evidence for the existence of a structure-induced minimum in the DOS near ${E}_{F}$. Our results are in good agreement with photoemission measurements and electronic specific-heat data.

Young's modulus plays a crucial role in determining the suitability of materials for various applications, including two-dimensional (2D) materials like graphene and transition metal dichalcogenides. Traditional indentation methods struggle with ultrathin 2D materials due to substrate effects. To overcome this, we propose using polydimethylsiloxane (PDMS) as a compliant substrate for atomic force microscopy force curves. This method, building on a 1950s analytical model, allows for accurate Young's modulus estimates by measuring flake thickness, applied force, and deformation. The results from our approach aligns well with existing literature for various 2D materials. PDMS, commonly used for mechanical exfoliation and transfer, offers an easily measurable Young's modulus, facilitating more efficient determinations. As the range of ultrathin materials grows, this platform enhances accessibility and efficiency in measuring Young's modulus, significantly contributing to the advancement of applications for these innovative materials.

In this thesis, the growth, the atomic structure as well as the electronic properties of self-assembled magic clusters and of thin rare earth silicide films on Si surfaces are investigated using scanning tunneling microscopy and spectroscopy. Self-assembled magic clusters on surfaces are promising structures to utilize the fascinating properties of clusters as zero-dimensional nanoobjects in future applications, e.g. in high density memory devices or in catalysis.
\nFor Sb on the Si(111)7×7 surface, the growth of pure Si clusters as well as of so-called Sb ringlike clusters is observed. The latter are found to exhibit different stoichiometries within the same basic structure by replacing Si atoms by Sb atoms, influencing strongly their electronic properties.
\nThe widely studied magic In and Ga clusters on Si(111) are used as a model system to study the detailed charge redistribution that takes place on the surface during the cluster formation as well as the effects of cluster accumulation on the electronic properties. In addition, the Si(557) surface, which is vicinal to Si(111), is used as a template to enable the growth of one-dimensional chains of magic clusters.
\nNanostructures induced by the rare earth metals Tb and Dy on Si surfaces are the material system most comprehensively studied in this thesis.
\nFor Dy and Tb on Si(111), the same two types of magic clusters are observed, which is related to the chemical similarity of the trivalent rare earth metals. These are the so-called off-center rare earth silicide cluster and the centered rare earth silicide clusters, for which the atomic structure could be identified and a semiconducting behavior is found by scanning tunneling spectroscopy. Moreover, the centered rare earth silicide clusters show a switching between two stable configurations.
\nOn the Si(001)2×1 surface, a completely different growth behavior is observed. Here, also magic Dy silicide clusters form, but the clusters show a one-dimensional ordering and assemble into chains consisting of different numbers of magic clusters.
\nThe thin film growth of Tb on Si(111) shows many similarities with the growth behavior observed for other trivalent rare earth metals in this parameter regime. For submonolayer Tb coverages, a 2√3×2√3 R30° superstructure, a 5×2 superstructure as well as nanorods showing either a 2×1 or a 4×1 superstructure on top are observed. For the 5×2 superstructure an existing structure model is refined and a detailed investigation is derived on how registry shifts lead to the different configurations of this structure. In the monolayer-to-multilayer regime,
\nthe growth of the TbSi2 monolayer with 1×1 periodicity and of the Tb3Si5 multilayer with √3×√3 R30° periodicity is found.

The atomic structure and optical properties of ferroelectric La-doped hafnium oxide (La:HfO2) thin films grown by the plasma-assisted atomic layer deposition were investigated. Using high resolution transmission electron microscopy, it was shown that the studied La:HfO2 film has a orthorhombic polar structure with the Pmn21 space group. It was found that the film exhibits ferroelectric properties. By means of x-ray photoelectron spectroscopy and specroellipsometry it was established that La:HfO2 consists of the HfO2 and La2O3 phases mixture. The specroellipsometry analysis with the Bruggeman effective medium approximation showed that the investigated La:HfO2 consists of 88% HfO2 and 12% La2O3. It is shown that etching La:HfO2 with argon ions leads to the oxygen vacancies generation in the near-surface region. These vacancies are generated mainly due to the knocking out of oxygen atoms to the interstitial positions, and the following annealing at 700 °C in vacuum for 1 h leads to the annihilation of that Frenkel defects.