Peculiarities Of Electronic Structure Research Articles

Stability of 2D Crystals from Tessellation

AbstractReticular chemistry has been a cornerstone in the design of novel 2D materials. Despite numerous possibilities for topological arrangements, only a few with high symmetry can form stable networks. Here, starting from 2D carbons, four types of highly stable tessellations are discovered, which consist of chains of non‐hexagonal rings separated by hexagonal ribbons. A modified Read–Shockley model is established to perfectly describe the stability of these highly stable frameworks, which is based on the interaction between non‐hexagonal rings. Moreover, these four types of tessellations and the modified Read–Shockley model are found to be of general validity in designing highly stable 2D materials, which is verified by the calculations on polymorphs of boron nitride and molybdenum disulfide. Besides, among the studied 2D carbon allotropes, two semi‐metallic structures with highly anisotropic Dirac cones and one semimetal with a Dirac nodal line at the Fermi level are discovered, as protected by their D2h symmetry. Spin‐orbital coupling is further found to open small bandgaps for these three Dirac structures, making them nontrivial topological insulators. The in‐depth understanding of the stability of 2D crystals in this study provides a new way for rational design of 2D crystals that may show peculiar electronic structures.

Endohedral metallofullerene molecular nanomagnets.

Magnetic lanthanide (Ln) metal complexes exhibiting magnetic bistability can behave as molecular nanomagnets, also known as single-molecule magnets (SMMs), suitable for storing magnetic information at the molecular level, thus attracting extensive interest in the quest for high-density information storage and quantum information technologies. Upon encapsulating Ln ion(s) into fullerene cages, endohedral metallofullerenes (EMFs) have been proven as a promising and versatile platform to realize chemically robust SMMs, in which the magnetic properties are able to be readily tailored by altering the configurations of the encapsulated species and the host cages. In this review, we present critical discussions on the molecular structures and magnetic characterizations of EMF-SMMs, with the focus on their peculiar molecular and electronic structures and on the intriguing molecular magnetism arising from such structural uniqueness. In this context, different families of magnetic EMFs are summarized, including mononuclear EMF-SMMs wherein single-ion anisotropy is decisive, dinuclear clusterfullerenes whose magnetism is governed by intramolecular magnetic interaction, and radical-bridged dimetallic EMFs with high-spin ground states that arise from the strong ferromagnetic coupling. We then discuss how molecular assemblies of SMMs can be constructed, in a way that the original SMM behavior is either retained or altered in a controlled manner, thanks to the chemical robustness of EMFs. Finally, on the basis of understanding the structure-magnetic property correlation, we propose design strategies for high-performance EMF-SMMs by engineering ligand fields, electronic structures, magnetic interactions, and molecular vibrations that can couple to the spin states.

Tin/Tin Oxide Nanostructures: Formation, Application, and Atomic and Electronic Structure Peculiarities.

For a very long period, tin was considered one of the most important metals for humans due to its easy access in nature and abundance of sources. In the past, tin was mainly used to make various utensils and weapons. Today, nanostructured tin and especially its oxide materials have been found to possess many characteristic physical and chemical properties that allow their use as functional materials in various fields such as energy storage, photocatalytic process, gas sensors, and solar cells. This review discusses current methods for the synthesis of Sn/SnO2 composite materials in form of powder or thin film, as well as the application of the most advanced characterization tools based on large-scale synchrotron radiation facilities to study their chemical composition and electronic features. In addition, the applications of Sn/SnO2 composites in various fields are presented in detail.

Ultrafast charge transfer enhanced nonlinear optical properties of CH3NH3PbBr3 perovskite quantum dots grown from graphene

Abstract Halide perovskite quantum dots (PQDs) have exhibited significantly superior nonlinear optical properties compared to traditional semiconductor materials thanks to their peculiar physical and electronic structures. By further improving the nonlinear optical properties of PQDs, it is expected to adapt to ultrafast photonics applications. This work reported the nonlinear optical properties of methylammonium lead bromide-graphene (CH3NH3PbBr3-G) composites synthesized by growing CH3NH3PbBr3 quantum dots directly from a graphene oxide lattice. Our experiments indicate that the combined advantages of the ultrafast charge transport properties from graphene and the strong charge generation efficiency of perovskite can be integrated together. The CH3NH3PbBr3-G composite exhibited enhanced saturable absorption properties with large modulation depth and very low saturation intensity. The transient absorption spectra and carrier dynamics analysis revealed that the enhancement of the saturated absorption properties of the composites mainly arose from the ultrafast charge transfer between G and CH3NH3PbBr3 which promoted the coupling between different states. The results pave the way for the design of optical switches or mode lockers based on saturable absorbers with good performance.

Adsorption of single metallic atoms on self-assembled molecular domain of terephthalic acid

Cobalt and gold atoms have been deposited on self-assembled molecular domains of terephthalic acid and are studied using low temperature scanning tunneling microscopy and DFT calculations. These two types of atoms present different adsorption properties as well as peculiar electronic structures. Co atoms are firmly adsorbed on the molecules while Au is highly dependent of the presence of diffusing hydrogen atoms. These latter can also favour the possible desorption of the adatoms or change their adsorption site depending on the presence of defective molecules. The results open the way to new investigations concerning the use of single atoms or molecules for chemistry at the subnanometer scale.

Nonlinear Photonics Using Low‐Dimensional Metal‐Halide Perovskites: Recent Advances and Future Challenges

Low-dimensional metal-halide perovskites have exhibited significantly superior nonlinear optical properties compared to traditional semiconductor counterparts, thanks to their peculiar physical and electronic structures. Their exceptional nonlinear optical characteristics make them excellent candidates for revolutionizing widespread applications. However, the research of nonlinear photonics based on low-dimensional metal-halide perovskites is in its infancy. There is a lack of comprehensive and in-depth summary of this research realm. Here, the state-of-the-art research progress related to third-and higher-order nonlinear optical properties of low-dimensional metal-halide perovskites with diverse crystal structures from 3D down to 0D, together with their practical applications, is summarized comprehensively. Critical discussions are offered on thefundamental mechanisms beneath their exceptional nonlinear optical performancefrom the physics viewpoint, attempting to disclose the role of intrinsic attributes (e.g., composition, bandgap, size, shape, and structure) and external modulation strategies (e.g., developing core-shell structures, transition metal ion doping, and hybridization with dielectric microspheres) in tuning the response. Additionally, their potential applications in nonlinear photonics, nonlinear optoelectronics, and biophotonics are systematically and thoroughly summed up and categorized. Lastly, insights into the current technical challenges and future research opportunities of nonlinear photonics based on low-dimensional metal-halide perovskites are provided.

Computational discovery of stable phases of graphene and h-BN van der Waals heterostructures composed of group III–V binary compounds

We propose stable structures of graphene and hexagonal BN (h-BN) van der Waals (vdW) heterostructures in conjunction with conventional group III–V binary compounds on the basis of density functional calculations. The calculations demonstrate that the binary compounds in graphene and h-BN superlattices are stabilized by forming the double-layer honeycomb (DLHC) structure, different from its stable structure in the bulk phase. Due to peculiar electronic structures of group III–V binary compounds in the DLHC structure, various types of electronic properties are obtained depending on the combination of two-dimensional materials (graphene and h-BN) and group III–V binary compounds. The calculated results suggest that diverse combinations and exotic electronic properties could be tailored in vdW heterostructures including group III–V compounds.

Surface Coordination of Multiple Ligands Endows N-Heterocyclic Carbene-Stabilized Gold Nanoclusters with High Robustness and Surface Reactivity.

Deciphering the molecular pictures of the multi-component and non-periodic organic-inorganic interlayer is a grand technical challenge. Here we show that the atomic arrangement of hybrid surface ligands on metal nanoparticles can be precisely quantified through comprehensive characterization of a novel gold cluster, Au44 (i Pr2 -bimy)9 (PA)6 Br8 (1), which features three types of ligands, namely, carbene (1,3-diisopropylbenzimidazolin-2-ylidene, i Pr2 -bimy), alkynyl (phenylacetylide, PA), and halide (Br), respectively. The delicately balanced stereochemical effects and bonding capabilities of the three ligands give rise to peculiar geometrical and electronic structures. Remarkably, despite its complex and highly distorted surface structure, cluster 1 exhibits unusual catalytic properties and yet it is highly stable, both chemically and thermally. Moreover, rich reactive sites on the cluster surface raise the prospect of bio-compatibility (as it can be functionalized to yield water-soluble derivatives) and bio-applications.

Surface Coordination of Multiple Ligands Endows N‐Heterocyclic Carbene‐Stabilized Gold Nanoclusters with High Robustness and Surface Reactivity

AbstractDeciphering the molecular pictures of the multi‐component and non‐periodic organic‐inorganic interlayer is a grand technical challenge. Here we show that the atomic arrangement of hybrid surface ligands on metal nanoparticles can be precisely quantified through comprehensive characterization of a novel gold cluster, Au44(iPr2‐bimy)9(PA)6Br8 (1), which features three types of ligands, namely, carbene (1,3‐diisopropylbenzimidazolin‐2‐ylidene, iPr2‐bimy), alkynyl (phenylacetylide, PA), and halide (Br), respectively. The delicately balanced stereochemical effects and bonding capabilities of the three ligands give rise to peculiar geometrical and electronic structures. Remarkably, despite its complex and highly distorted surface structure, cluster 1 exhibits unusual catalytic properties and yet it is highly stable, both chemically and thermally. Moreover, rich reactive sites on the cluster surface raise the prospect of bio‐compatibility (as it can be functionalized to yield water‐soluble derivatives) and bio‐applications.

Peculiarities of electronic structure and chemical bonding in iron and cobalt metal complexes of porphyrazine and tetra(1,2,5-thiadiazole)porphyrazine

The geometrical and electronic structures of iron and cobalt metal complexes of porphyrazine and tetra(1,2,5-thiadiazole)porphyrazine in ground and low-lying excited electronic states were determined by DFT calculations and the complete active space (CASSCF) method with following accounting dynamic correlation by multiconfigurational quasidegenerate second-order perturbation theory (MCQDPT2). A geometrical structure of D[Formula: see text] symmetry has been obtained for all four complexes. According to data obtained by the MCQDPT2 method, the complexes of cobalt and iron possess the ground states 2A[Formula: see text] and 3A[Formula: see text], respectively, and wave functions of the ground states have the form of a single determinant. It is shown that the crystal field theory (CFT) can be used to describe the sequence of electronic states of the investigated complexes. The nature of the bonds between metal atoms and nitrogen atoms has been described using the analysis of the electron density distribution in the frame of Bader’s quantum theory of atoms in molecule (QTAIM).

Peculiarities of electronic structure and composition in ultrasound milled silicon nanowires

The combined X-ray absorption and emission spectroscopy approach was applied for the detailed electronic structure and composition studies of silicon nanoparticles produced by the ultrasound milling of heavily and lowly doped Si nanowires formed by metal-assisted wet chemical etching. The ultrasoft X-ray emission spectroscopy and synchrotron based X-ray absorption near edges structure spectroscopy techniques were utilize to study the valence and conduction bands electronic structure together with developed surface phase composition qualitative analysis. Our achieved results based on the implemented surface sensitive techniques strongly suggest that nanoparticles under studies show a significant presence of the silicon suboxides depending on the pre-nature of initial Si wafers. The controlled variation of the Si nanoparticles surface composition and electronic structure, including band gap engineering, can open a new prospective for a wide range Si-based nanostructures application including the integration of such structures with organic or biological systems.

Polarization-resolved broadband time-resolved optical spectroscopy for complex materials: application to the case of MoTe2 polytypes.

Time-resolved optical spectroscopy (TR-OS) has emerged as a fundamental spectroscopic tool for probing complex materials, to both investigate ground-state-related properties and trigger phase transitions among different states with peculiar electronic and lattice structures. We describe a versatile approach to perform polarization-resolved TR-OS measurements, by combining broadband detection with the capability to simultaneously probe two orthogonal polarization states. This method allows us to probe, with femtoseconds resolution, the frequency-resolved reflectivity or transmittivity variations along two mutually orthogonal directions, matching the principal axis of the crystal structure of the material under scrutiny. We report on the results obtained by acquiring the polarization-dependent transient reflectivity of two polytypes of the MoTe2 compound, with 2H and 1T' crystal structures. We reveal marked anisotropies in the time-resolved reflectivity signal of 1T'-MoTe2, which are connected to the crystal structure of the compound. Polarization- and time- resolved spectroscopic measurements can thus provide information about the nature and dynamics of both the electronic and crystal lattice subsystems, advancing the comprehension of their inter-dependence, in particular in the case of photoinduced phase transitions; in addition, they provide a broadband measurement of transient polarization rotations.

Three-dimensional covalent networks of sp2 and sp3 C atoms: energetics and electronic properties of polymerized diphenylmethane and tetraphenylmethane

Based on the density functional theory with the generalized gradient approximation, we investigate geometric and electronic structures of three-dimensional covalent organic frameworks of polymerized diphenylmethane and tetraphenylmethane in which phenyl and biphenyl are arranged in a tetrahedral manner connected via methane vertexes. These three-dimensional covalent networks are energetically stable with the total energy of 90 and 65 meV atom−1 for diphenylmethane and tetraphenylmethane polymers, respectively, with respect to that of diamond. Polymerization reactions of diphenylmethane and tetraphenylmethane are endothermic with the reaction energy of 1.62 and 0.68 eV, respectively. These polymers have the peculiar electronic band structures in their valence and conduction states, which consist of the combination of the doubly degenerate flat band and two dispersive states forming a Dirac cone at the W point. The wavefunction analyzes and the simple tight-binding calculations reveal that the peculiar electron states are ascribed to the delicate balance between inter- and intra-phenyl/biphenyl π electron transfers, indicating that these polymers could be regarded as the three-dimensional kagome or pyrocroa networks with the internal degree of freedom.

(Invited) Synthesis of the Elusive Dimetallofullerenes

The metal-metal (M-M) bonding in dimetallofullerenes presents the peculiar electronic structures as well as the chemical and physical properties.[1-2] For Y and medium-size lanthanides, computations by Shinohara et al. showed that M2@C80 has one unpaired electron occupying the M–M bonding molecular orbitals, and the other electron is delocalized over the carbon cage.[3] Such molecules are elusive radicals. Recently, we have succeeded in the capture of the elusive M2@C80 (M=Y, Gd, Tb, Dy, Ho, and Er) as benzyl derivatives.[4] In this contribution, the synthesis and structural characterizations of these M2@C80–CH2Ph will be discussed. [1] A. A. Popov, S. Yang, L. Dunsch, Chem. Rev. 2013, 113, 5989-6113. [2] A. A. Popov, S. M. Avdoshenko, A. M. Pendas, L. Dunsch, Chem. Commun. 2012, 48, 8031-8050. [3] Z. Wang, R. Kitaura, H. Shinohara, J. Phys. Chem. C 2014, 118, 13953-13958. [4] F. Liu, D. S. Krylov, L. Spree, S. M. Avdoshenko, N. A. Samoylova, M. Rosenkranz, A. Kostanyan, T. Greber, A. U. B. Wolter, B. Büchner, A. A. Popov, Nat. Commun. 2017, 8, 16098.

Наноразмерные флуктуации потенциала в SiO-=SUB=-x-=/SUB=-, синтезированном плазмохимическим осаждением

The atomic structure and electron spectrum of a-SiOx:H films deposited on silicon and glass substrates by the plasma enhanced chemical vapor deposition method are considered in this paper. The film stoichiometric parameter «x» varied in the range from 0.57 to 2 depending on the oxygen supply to the reactor conditions. The film structures and the electronic structure peculiarities characterization, depending on the parameter «x» value, were carried out using a set of structural and optical techniques, as well as the ab initio quantum-chemical simulation for the model SiOx structure. It was established that the studied SiOx:H films mainly consist of silicon suboxide SiOy with SiO2 and amorphous Si clusters. Based on the spatial chemical composition fluctuations, the electron and hole potential fluctuations model for SiOx is proposed. The obtained results will allow a more accurate charge transport modeling in a-SiOx:H films, which is important for creating nonvolatile memory and memristor elements on the base of SiOx.

Phase Formation and Electronic Structure Peculiarities in the Al1 – xSi x Film Composites under the Conditions of Magnetron and Ion-Beam Sputtering

The peculiarities of the phase composition and electronic structure of aluminum–silicon composite films near the Al0.75Si0.25 composition obtained by the magnetron and ion-beam sputtering methods on a Si(100) silicon substrate are studied using the X-ray diffraction techniques and ultrasoft X-ray emission spectroscopy. In addition to silicon nanocrystals of about 25 nm in size, an ordered solid solution corresponding to the previously unknown Al3Si phase is formed in magnetron sputtering on a polycrystalline Al matrix. Films obtained by ion-beam sputtering of the composite target are found to be monophasic and contained only one phase of an ordered solid solution of aluminum silicide Al3Si of the Pm3m cubic system with the primitive cell parameter a = 4.085 A. However, subsequent pulsed photon annealing of the composite with different radiation doses from 145 to 216 J/cm2 gives rise to the partial decomposition of the Al3Si phase with the formation of free metallic aluminum and silicon nanocrystals with sizes in the range from 50 to 100 nm, depending on the pulsed photon radiation dose.

(Invited) Synthesis and Stabilization of the Unstable Dimetallofullerenes

The metal-metal (M-M) bonding in dimetallofullerenes presents the peculiar electronic structures as well as the chemical and physical properties.[1-2] For Y and medium-size lanthanides (Gd, Tb, Dy), computations by Shinohara et al. showed that M2@C80 has one unpaired electron occupying the M–M bonding molecular orbitals, and the other electron is delocalized over the carbon cage (Figure 1a).[3] Such molecules are unstable radicals. Recently, we have succeeded in the capture of the elusive M2@C80 (M=Y, Dy) as benzyl derivatives (Figure 1b). The structure was unambiguously determined (Figure 1c, d).[4] In this contribution, the synthesis, separation, and structural characterizations of the elusive M2@C80 will be presented and discussed. [1] A. A. Popov, S. Yang, L. Dunsch, Chem. Rev. 2013, 113, 5989-6113. [2] A. A. Popov, S. M. Avdoshenko, A. M. Pendas, L. Dunsch, Chem. Commun. 2012, 48, 8031-8050. [3] Z. Wang, R. Kitaura, H. Shinohara, J. Phys. Chem. C 2014, 118, 13953-13958. [4] F. Liu, D. S. Krylov, L. Spree, S. M. Avdoshenko, N. A. Samoylova, M. Rosenkranz, A. Kostanyan, T. Greber, A. U. B. Wolter, B. Büchner, A. A. Popov, Nat. Commun. 2017, 8, 16098. Figure 1

An Exceptionally Narrow Band-Gap (∼4 eV) Silicate Predicted in the Cubic Perovskite Structure: BaSiO3.

The electronic structures of 35 A2+B4+O3 ternary cubic perovskite oxides, including their hypothetical chemical compositions, were calculated by a hybrid functional method with the expectation that peculiar electronic structures and unique carrier transport properties suitable for semiconductor applications would be hidden in high-symmetry cubic perovskite oxides. We found unique electronic structures of Si-based oxides (A = Mg, Ca, Sr, and Ba, and B = Si). In particular, the unreported cubic BaSiO3 has a very narrow band gap (4.1 eV) compared with conventional nontransition-metal silicates (e.g., ∼9 eV for SiO2 and the calculated value of 7.3 eV for orthorhombic BaSiO3) and a small electron effective mass (0.3m0, where m0 is the free electron rest mass). The narrow band gap is ascribed to the nonbonding state of Si 3s and the weakened Madelung potential. The existence of the predicted cubic perovskite structure of BaSiO3 was experimentally verified by applying a high pressure of 141 GPa. The present finding indicates that it could be possible to develop a new transparent oxide semiconductor of earth abundant silicates if the symmetry of its crystal structure is appropriately chosen. Cubic BaSiO3 is a candidate for high-performance oxide semiconductors if this phase can be stabilized at room temperature and ambient pressure.

Xenon in Rigid Oxide Frameworks: Structure, Bonding and Explosive Properties of Layered Perovskite K4Xe3O12.

The tendency of high-valence xenon to form consolidated oxide structures is herein supported by the study of K4Xe3O12, the first example of a layered xenon perovskite. Xenon seems to be the only nontransition element that can adopt single-cation oxide perovskite frameworks. At the same time, peculiarities of electronic structure of xenon impose specific features on the bonding within a perovskite structure. Weak supramolecular interactions known as aerogen bonds are the linkers that maintain structural integrity of perovskite slabs in K4Xe3O12. The occurrence of aerogen bonding can provide an insight into the explosive properties of K4Xe3O12: the weakness of supramolecular interactions allows consideration of them as possible trigger bonds responsible for the detonation sensitivity of layered xenon perovskite.

Edge phonons in black phosphorus.

Black phosphorus has recently emerged as a new layered crystal that, due to its peculiar and anisotropic crystalline and electronic band structures, may have important applications in electronics, optoelectronics and photonics. Despite the fact that the edges of layered crystals host a range of singular properties whose characterization and exploitation are of utmost importance for device development, the edges of black phosphorus remain poorly characterized. In this work, the atomic structure and behaviour of phonons near different black phosphorus edges are experimentally and theoretically studied using Raman spectroscopy and density functional theory calculations. Polarized Raman results show the appearance of new modes at the edges of the sample, and their spectra depend on the atomic structure of the edges (zigzag or armchair). Theoretical simulations confirm that the new modes are due to edge phonon states that are forbidden in the bulk, and originated from the lattice termination rearrangements.