Orbital Band Research Articles

Trends in structural, electronic and elastic properties of Ti2AC (A = Al, Zn, Cu, Ni) based on late transition metals substitution

Recently, the substitutional incorporation of late transition metals in the “A” sublattice of MAX phases further extends the physical properties and diversity of this class of materials. In this study, the structural, electronic and elastic properties of Ti2AC (A = Al, Zn, Cu, Ni) were investigated using the density functional theory. The calculated results reveal that a large number of d orbital bands of Zn, Cu, and Ni atoms accumulate around −6.5 eV, −2.5 eV, and −2.0 eV, respectively, leading to stronger ionic and metallic characteristics for Ti2ZnC, Ti2CuC, and Ti2NiC. Three Ti2AC (A = Zn, Cu, Ni) phases are ductile and consequently damage tolerant, whereas Ti2AlC is brittle in nature. When Al is replaced by Zn, Cu, and Ni, there is a remarkable decrease of hardness and shear modulus, indicating high machinability. Ti2ZnC, Ti2CuC, and Ti2NiC are expected to be promising thermal barrier coating materials.

Enhancing Soil Mapping with Hyperspectral Subsurface Images generated from soil lab Vis-SWIR spectra tested in southern Brazil

The Remote Sensing (RS) products involving the visible, near-infrared and short-wave infrared (Vis–NIR-SWIR, simply Vis-SWIR) wavelengths are applicable for spatial prediction of soil properties, usually in the process called Digital Soil Mapping (DSM). The multispectral optical orbital sensors lack spectral resolution, while the Proximal Soil Sensing (PSS) offers many application possibilities. The limited spectral resolution and soil cover are the main challenging aspects for DSM. The objective of the study was to combine the soil lab Vis-SWIR spectra within DSM technique to create new hyperspectral images to use as covariates in a novel method of soil mapping. The advantages of the method were demonstrated throughout the spatial prediction of the Total Carbon (TC) content on soils of the upper part of Itatiaia National Park (INP), (Rio de Janeiro and Minas Gerais States, southeastern Brazil). The covariates applied were multispectral orbital bands from Sentinel 2, relief with its terrain derivations, geology and geomorphology. Using uppermost soil horizon lab Vis-SWIR spectra information from 72 points in the upper part of INP, 130 hyperspectral images were generated, in other words, Hyperspectral Subsurface Image. The validation methods were 8-fold cross-validation (CV) and External Validation (EV) running alongside within Random Forest algorithm, where randomness was locked. The DSM reaches an average CV coefficient of determination (R2) of 0.39 and Root Mean Square Error (RMSE) of 4.6, while creating a new model with a set of hyperspectral images gave a CV R2 of 0.60 and RMSE of 4.06. The EV also presented an advantage for the second approach. The results support the method as the first strong integration between PSS and DSM, named by the authors as Hyperspectral Soil Mapping (HSM), which proved to be more efficient for mapping TC due to the pure soil lab Vis-SWIR spectra signal (free of atmospheric influence and vegetation cover). The method can be applied in dense or sparsely vegetated areas, for agricultural or conservation purposes, for various soil properties, with each and every wavelength.

Cytotoxicity and Antibacterial Potentials of Mixed Ligand Cu(II) and Zn(II) Complexes: A Combined Experimental and Computational Study.

[Cu(C15H9O4)(C12H8N2)O2C2H3]·3H2O (1) and [Zn(C15H9O4)(C12H8N2)]O2C2H3 (2) have been synthesized and characterized by ultraviolet-visible (UV-vis) spectroscopy, Fourier transform infrared (FTIR) spectroscopy, mass spectrometry, thermogravimetric analysis/differential thermal analysis (TGA/DTA), X-ray diffraction (XRD), scanning electron microscopy-energy-dispersive X-ray spectroscopy (SEM-EDX), and molar conductance, and supported by density functional theory (DFT) and time-dependent DFT (TD-DFT) calculations. Square pyramidal and tetrahedral geometries are proposed for Cu(II) and Zn(II) complexes, respectively, and the XRD patterns showed the polycrystalline nature of the complexes. Furthermore, in vitro cytotoxic activity of the complexes was evaluated against the human breast cancer cell line (MCF-7). A Cu(II) centered complex with an IC50 value of 4.09 μM was more effective than the Zn(II) centered complex and positive control, cisplatin, which displayed IC50 values of 75.78 and 18.62 μM, respectively. In addition, the newly synthesized complexes experienced the innate antioxidant nature of the metal centers for scavenging the DPPH free radical (up to 81% at 400 ppm). The biological significance of the metal complexes was inferred from the highest occupied molecular orbital-lowest unoccupied molecular orbital (HOMO-LUMO) energy band gap, which was found to be 2.784 and 3.333 eV, respectively for 1 and 2, compared to the ligands, 1,10-phenathroline (4.755 eV) and chrysin (4.403 eV). Moreover, the molecular docking simulations against estrogen receptor alpha (ERα; PDB: 5GS4) were strongly associated with the in vitro biological activity results (E B and K i are -8.35 kcal/mol and 0.76 μM for 1, -7.52 kcal/mol and 3.07 μM for 2, and -6.32 kcal/mol and 23.42 μM for cisplatin). However, more research on in vivo cytotoxicity is suggested to confirm the promising cytotoxicity results.

Spinel lithium titanate anode/polyether sulfone nanocomposite synthesized by pulsed laser ablation method for optoelectronic applications

Nowadays, to increase the usage of energy storage applications like electric cars or stationary storage, the cost of these manufacturers must be reduced. Our present study focuses on an alternate electrode production approach to suit the needs of today's lithium ion battery’s cost efficiency by using an eco-friendly method, the pulsed laser ablation method in liquid media technique, which was used for the first time to synthesize spinel lithium titanate anode, Li4Ti5O12 nanoparticles (LTO NPs), and incorporate them with polyether sulfone (PES) in just one step to form a PES/LTO nanocomposite. The evidence from XRD showed that the nanocomposite film is formed as a crystalline phase from a cubic spinel structure corresponding to LTO, with crystalline sizes around 9.4 nm. Furthermore, SEM revealed a semi-spherical distribution of LTO NPs throughout the PES matrix. Also, the elemental analysis provides the elemental peaks for C, S, Ti, and O, and no other elemental peaks do, confirming their purity. Moreover, the FT-IR investigation affirmed the interaction between PES and LTO NPs via the sulfone group with the breakage of the sulfur and oxygen double bond and the formation of a new link between SOLa and SOTi that may be responsible for the emergence of this band. Also, the absorption study confirmed the formation of localized states between occupied and unoccupied molecular orbital bands is made feasible by the chemical linkages between PES chains and LTO NPs. As a result of the dielectric investigation, LTO NPs are a good choice for usage as dopants to enhance the electrical characteristics of PES polymer. Overall, the PES/LTO nanocomposite films' improved dielectric and optical properties make them appropriate for energy storage applications.

Identification of Borophosphene/graphene heterostructure as anode for Li-ion Batteries and its origin

The development of two-dimensional (2D) material or heterostructure as an anode material is necessary to enhance the electrochemical performance of Li-ion batteries (LIBs) but finding the correct combination is a challenge. In the present work, using First principles study, we have proposed borophosphene (BP-ML) and graphene-based multilayer heterostructure as a possible anode material. We have found that BP-ML and graphene-based heterostructure are conductive in nature. Here, we have also investigated the role of Pz (π) and Py (σ) atomic orbital bands of BP-ML and graphene. On Li intercalation, charge transfer is mainly site and interface definitely which helps to improve the specific capacity. The specific capacity of the proposed heterostructure varies from 546 to 427 mA h/g. For maximum Li confirmation, the volume expansion of these heterostructures is about 14–16%. The presence of graphene helps to maintain the open-circuit voltage (OCV) of heterostructure on an average 0.7 V. Also, helps to support the diffusion barrier energy in the range of 0.27–0.71 eV. This proposed 2D heterostructure could be the future material for the LIB's anode material.

Experimental spectroscopic and quantum computational investigation and molecular docking analysis of N-benzyloxycarbonyl-l-serine – an anticancer agent

This work presents the nature of N-benzyloxycarbonyl-l-serine through quantum chemical calculations and spectral analysis. The optimized geometry and vibrational frequencies were computed by density functional theory. The geometric parameters were theoretically obtained and those values were compared with experimental data. The calculated vibrational wavenumbers of the heading compound display exact agreement with the recorded spectrum. The frontier molecular orbital band gap energy confirmed that the heading compound has a good reactivity and stability of the molecule. The theoretical UV-Vis spectrum turned into analysis in gas and with different solvents. The nuclear magnetic resonance spectrum was calculated. Each evaluation was in comparison with the experimental ones. The molecular electrostatic potential energy map is an effective model to identify electrophilic and nucleophilic sites. Further, natural bond orbital and Mulliken charge analysis had been additionally calculated. Drug-likeness studies were carried out. This result shows that the heading compound has a good biological activity. Anticancer activity was tested based on the molecular docking evaluation and it was diagnosed that the heading compound can act as a potential inhibitor of liver cancer.

An estimate of expected economic losses from satellite collisions with orbital debris

We estimate the expected annual cost of satellite collisions with orbital debris and find that the expected annual loss for all orbits was $86-103 million in 2020. Nearly all expected losses, $79-102 million, were in LEO, and these losses represent approximately 0.16% of the value of operational satellites in that orbit. Over 70% of losses occurred in the 600-900 km orbital band. Commercial satellites’ share of expected losses is approximately one-third of expected total losses. These findings may have implications for policy, in particular relating to the market for active debris removal.

Structural and Spectral Properties of 4-(5-methyl-[1, 2, 4] triazolo [1, 5-a] pyrimidine-7-yloxy) phthalonitrile: Analysis by TD-DFT Method, ADME Analysis, and Molecular Docking Simulations

In this study, 4-(5-methyl-[1,2,4]triazolo[1,5-a]pyrimidine-7-yloxy) phthalonitrile (coded as MTPPN) was chosen as the phthalonitrile compound and the quantum chemical and in-silico studies have been done. First, the basis set of the time dependent density functional theory (TD-DFT) method was used and the boundary orbital energies and band gap calculations of the molecule were performed. Analysis of atoms in molecules (AIM) theoretical calculations is presented to learn about electron density and bond critical point. In addition, absorption, distribution, metabolism, and excretion analyzes (ADME) were performed for the drug potential of the compound. On some enzymes effect of MTPPN compound was examined. The docking score was obtained for AChE, BChE, α-GLY proteins -7.864, -6.848, and -5.511 kcal/mol, respectively. MTPPN gave a good inhibitory performance in an in-silico study as a drug candidate.

A new impact assessment model to integrate space debris within the life cycle assessment-based environmental footprint of space systems

By analogy to conventional environmental impacts, the potential release of debris or generation of fragments can be considered as the emission of an environmental stressor damaging the orbital ‘natural’ resource which supports space activities. Hence, it appears relevant to integrate systematically the impact of the emission of debris on the orbital resource within the life cycle impact assessment (LCIA) step to broaden the scope of life cycle assessment (LCA) for space systems. The main objective of this article is to propose a set of characterization factors to compute the impact caused by the generation of debris within the orbital environment. To do so, the proposed approach follows the methodology of emission-related characterization models in LCIA. the characterization model enables to link the emission of debris and final economic damages to space activities through a complete impact pathway including the fate of debris in downstream orbital compartments, the exposure of targeted space objects to this debris, and the economic damage in case of collision between the debris and the space object. The model is computed for different compartments of the low earth orbit (LEO) region thanks to a discretization of the orbital environment. Results show that the potential damages are the highest for orbital compartments located in the orbital bands of altitude/inclination: 550–2000 km/52–54°, 1,200–2000 km/86–88°, 400–2000 km/96–100°, because of the downstream location of Starlink constellation, OneWeb constellation, and earth observation satellites, respectively. The proposed set of CFs can be used in the LCA of different space systems in order to include impacts and damages related to space debris, along with other environmental impacts. This original development fully in line with the standardized LCIA framework would have potential for further integration into harmonised sector-specific rules for the European space sector such as product environmental footprint category rules (PEFCR).

Structural order-disorder in CaFe6Ge6 and Ca1-xCo6Ge6

Ternary alkaline earth transition metal germanides CaFe6Ge6 and Ca1-xCo6Ge6 have been synthesized and investigated. Depending on synthesis paths, different modifications have been found, all ordering variants of stuffed CoSn-type: CaFe6Ge6 with Y0.5Co3Ge3-type (P6/mmm, a = 5.1077(1) Å, c ​= ​4.0636(2) Å), MgCo6Ge6-type (P6/mmm, a = 5.1017(1) Å, c ​= ​8.1080(5) Å) or TbFe6Sn6-type (Cmcm, a = 8.150(2) Å, b ​= ​17.698(4) Å, c ​= ​5.116(1) Å) and Ca1-xCo6Ge6 with Y0.5Co3Ge3-type (P6/mmm, a = 5.1016(2) Å, c ​= ​3.9286(3) Å) or ScFe6Ga6-type (Immm, a = 8.830(2) Å, b ​= ​5.100(1) Å, c ​= ​7.855(2) Å). The main structural motif in Ca(Fe/Co)6Ge6 is Kagomé layers of Fe/Co atoms alternating with honeycomb layers of Ge atoms, where the hexagonal channels are filled with Ca atoms and Ge–Ge dumbbels. Up to date, calcium is the largest guest atom in this 166 structural family. The crystal chemistry and chemical bonding are discussed in terms of linear muffin-tin orbital band structure calculations and a topological analysis using the electron localization function.

Design Principles to Maximize Non‐Bonding States for Highly Tribopositive Behavior

AbstractThe development of highly tribopositive materials is crucial for realizing high‐performance triboelectric nanogenerators. In this study, a novel protocol to maximize the number of non‐bonding electrons with local dipoles for designing highly tribopositive materials is introduced, and nitrogen‐based dimethylol urea, diazolidinyl urea, and imidazolidinyl urea as promising tribopositive materials are suggested. The mechanism by which nitrogen‐based materials provide highly tribopositive properties is investigated using calculations based on density functional theory. Highly electronegative atoms, such as nitrogen and oxygen, attract electrons from neighboring atoms, resulting in the formation of negative local dipoles in the highest occupied molecular orbital band composed of non‐bonding electrons, thereby creating an electron‐donating environment. The proposed design protocol is confirmed by quantitatively investigating the triboelectric properties of nitrogen‐based materials, and analyzing the charge transfer characteristics of dimethylol urea based on dipole interactions.

Hydrogen storage in SiC, GeC, and SnC nanocones functionalized with nickel, Density Functional Theory—Study

AbstractHydrogen is regarded as one of the most potential sustainable energy sources in the future. Applications include transportation. Still, the event of materials for its storage is difficult notably as a fuel in vehicular transport. Nanocones are a promising hydrogen storage material. Silicon, germanium, and tin carbide nanocones have recently been proposed as promising hydrogen storage materials. In the present study, we have investigated the hydrogen storage capacity of and nanocones functionalized with Ni. The functionalized Ni atom are found to be adsorbed on and with an adsorption energy of −5.56, −6.70, and −4.25 eV. The functionalized , and bind up to seven, six and four molecules of hydrogen with the adsorption energy of (−0.34, −0.35, and −0.26 eV) and an average desorption temperature of around 434, 447, and 332 K (ideal for fuel cell applications). The SiC, GeC, and SnC nanocones systems exhibit a maximum gravimetric storage capacity of 12.51, 7.78, and 4.08 wt%. We suggested that NiSiCNC and NiGeCNC systems can act as potential H2 storage device materials because of their higher H2 uptake capacity as well as their stronger interaction with adsorbed hydrogen molecules than NiSnCNC systems. The hydrogen storage reactions are characterized in terms of the charge transfer, the partial density of states, the frontier orbital band gaps, and isosurface plots. And electrophilicity are calculated for the functionalized and hydrogenated and nanocones.

Ferroelectricity modulates polaronic coupling at multiferroic interfaces

Physics of the multiferroic interfaces is currently understood mostly within a phenomenological framework based on screening of the polarization field and depolarizing charges. Additional effects still unexplored are the band dependence of the interfacial charge modulation and the associated changes of the electron-phonon interaction, coupling the charge and lattice degrees of freedom. Here, multiferroic heterostructures of the colossal-magnetoresistance manganite La1-xSrxMnO3 buried under ferroelectric BaTiO3 and PbZrxTi1-xO3 are investigated using soft-X-ray angle-resolved photoemission. The experimental band dispersions from the buried La1-xSrxMnO3 identify coexisting two-dimensional hole and three-dimensional electron charge carriers. The ferroelectric polarization modulates their charge density, affecting the coupling of the 2D holes and 3D electrons with the lattice which forms large Fröhlich polarons inherently reducing mobility of the charge carriers. Our k-resolved results on the orbital occupancy, band filling and electron-lattice interaction in multiferroic oxide heterostructures modulated by the ferroelectric polarization disclose most fundamental physics of these systems needed for further progress of beyond-CMOS ferro-functional electronics.

Orbital dependent band degeneracy and edge states in single layer and AA bilayer honeycomb lattice systems with p orbital degeneracy

We have investigated band structure and edge states of single and AA bilayer honeycomb lattice systems with p orbital degeneracy which would be relevant for relatively heavy metals such as Pb and Bi. The p electrons, which are triply degenerate in the atomic limit, form quadruple degeneracy at K point in a single layer honeycomb lattice. The spin-orbit interaction splits the quartet at K point into two singlets and one doublet which are mixtures of p x , p y and p z orbitals. In the AA bilayer honeycomb lattice, the Dirac node at K point is deformed into circular line node around it. With the intermediate interlayer coupling, there is a degenerate point along Γ-K with p z character. A p x /p y type degenerate point exists under the strong interlayer coupling. This degenerate point is robust against the spin-orbit interaction and the antiferromagnetic spin arrangement only along the intralayer bonds. Without SOI, localized edge states with p x /p y or p z character are created at the zigzag edges both in the single and AA bilayer systems. In the AA bilayer system with SOI, only the p x /p y edge states exhibit the Dirac-like dispersion.

(Digital Presentation) Angle-Resolved Photoluminescence Measurements of Rubrene Single Crystals to Study the Exciton Energy Dispersion

Angle-resolved spectroscopic measurements provide information on the electronic structure of single crystals in reciprocal space. So far the single crystal of rubrene (RubSC), a representative of p-type organic semiconductor, has been investigated by using photoelectron spectroscopies and the highest occupied molecular orbital band energy dispersion of ~ 0.4 eV was characterized [1]. The degree of the dispersion was depended on the crystal axes [2]. The formation of energy band dispersion means that the hole carrier can be delocalized in certain crystal axes. An anisotropic hole carrier mobility is anticipated for RubSC and is indeed demonstrated in organic filed-effect transistors [3]. Considering the anisotropy of electronic structure and hole delocalization, it is questioned whether excitons can be also delocalized in RubSC. Delocalization of molecular excitons is relevant for organic devices, such as organic light-emitting diodes and organic photovoltaic cells, and is of fundamental interest. To investigate the possibility of such an exciton delocalization, we investigated angle-resolved photoluminescence (AR-PL) measurement RubSC using a newly built AR-PL apparatus.RubSC samples were prepared by using a physical vapor transport technique in a purified nitrogen stream, where “sublimed grade” (99.99% purity) source materials purchased from Sigma-Aldrich were used as received. AR-PL spectra were measured by a home-made apparatus consisting of a picosecond pulsed laser light source and some optical elements bought by Thorlabs, Inc. The excitation wavelength was 532 nm (the second harmonics of Nd:YAG laser; Ekspla, PL-2211) and the laser power was 4.9 mW with a (Gaussian) beam diameter of ~ 2 mm and a repetition rate of 1 kHz. The p-polarized light was prepared by using a half-wave plate and a beam splitter. The p-polarized light was focused on the rubrene single crystals using a convex lens (f=600 mm) at an incident angle of 34 degree (°) relative to the surface [(100) plane] normal of the RubSC. The emitted PL spectra were recorded by using a photonic multichannel spectrometer (PMA-12, Hamamatsu) connecting an optical bundled fiber (light-receiving area Φ 1mm), which was placed at a PL emission angle of 70° with a distance of 5.9 cm between RubSC and the fiber edge. To measure AR-PL spectra, the fiber mount was rotated step-by-step at a rotation center, which is a surface of rubrene single crystal. The resolution of the detected angle was estimated to be ±1degree. The alignment of the apparatus, namely, setting the in-plane and out-of-plane rotation center of the system, was done by using a line laser maker and digital cameras, which were used to check the light irradiation position on the crystals. All the measurements were done at 295 K in air.Figure 1(a) shows the results of AR-PL spectra of RubSC measured at an azimuthal angle (α, see in Fig.1(a)) of 200 °and 290°. For two spectra, the strongest peak was found at around 655 nm and the second strongest one is at around 705 nm. These peaks are consistent with those reported ones of RubSc; the largest one is the main PL band, and the second one is its vibronic progression. Note that photo-oxidization of the RubSC can possibly occur at the surface of the SC and, if in that case, its PL band would be seen at around 643 nm. However, our spectra do not show such a band and there was almost no irradiation time dependence, indicating the detected PL originates mostly from the bulk region of the RubSC. At the different α conditions, the main band shifted apparently, while the second one stayed unchanged. To more clarify the angle dependency, the second derivatives of all the data set of the AR-PL spectra were calculated and the results are shown in Fig. 1(b) as an intensity color map. It is clear that the first band shifts zigzag by every 90±10 °, while the second largest one is unchanged. The crystal structure of our RubSC grown by our way is orthorhombic and the angle between a and b axes of the RubSC is 90°. The periodicity of the turning point of the first band, i.e. 90°, is coincidently matched with the ab angle of the RubSC. This implies that the zigzag shift of the first PL band is related to the crystal structure of RubSC, whispering the detection of an energy dispersion of excitons of RubSC.Ref.[1] S. Machida, et al., Phys. Rev. Lett. 104, 156401 (2010). [2] Y. Nakayama, et al., Appl. Phys. Express 5,111601 (2012). [3] C. Reese and Z. Bao, Adv. Mater. 19, 4535 (2007). Figure 1

Influence of End-Capped Modifications in the Nonlinear Optical Amplitude of Nonfullerene-Based Chromophores with a D-π-A Architecture: A DFT/TDDFT Study.

Nonlinear optical (NLO) materials have several uses in many fields such as solid physics, biology, medicine, nuclear physics, and material research. Therefore, a series of nonfullerene-based derivatives (CC10D1–CC10D8) with a D−π–A configuration was planned for the NLO investigation using CC10R as the reference molecule with structural alternations at acceptor moieties. Natural bonding orbital (NBO), UV–vis spectra, frontier molecular orbitals (FMOs), global reactivity parameters (GRPs), transition density matrix (TDM), and density of states (DOS) were analyzed using the M06/6-311G(d,p) functional in chloroform solvent to understand the NLO responses of CC10R and CC10D1–CC10D8. The highest occupied molecular orbital (HOMO)–lowest unoccupied molecular orbital (LUMO) band gaps of CC10D1–CC10D6 were illustrated to be lower than that of CC10R, with the larger bathochromic shift (726.408–782.674 nm) resulting in a significant NLO response. Along with the band gap, the FMO method also identified an efficient interfacial charge transfer from D to A moieties via a π-bridge, which was further supported by the DOS and TDM map. Moreover, NBO calculations demonstrated that extended hyperconjugation and strong internal molecular interactions were important in their stabilization. The dipole moment (μ), linear polarizability ⟨α⟩, hyperpolarizability (βtotal), and second-order hyperpolarizability (γtotal.) were studied for CC10R and CC10D1–CC10D8. Among all of the derivatives, CC10D2 was proven to be the most appropriate candidate because of its suitable NLO behavior such as being well-supported by a reduced band gap (2.093 eV) and having a suitable maximum absorption wavelength (782.674 nm). Therefore, CC10D2 was reported to have a greater value of first hyperpolarizability (208 659.330 a.u.) compared with other derivatives and CC10R. For the second hyperpolarizability, a greater value was obtained for CC10R (5.855 × 107 a.u.), and its derivatives showed results comparable to that of the parent chromophore for γtotal. This theoretical framework reveals that structural customization with different acceptor units plays a significant role in obtaining attractive NLO materials for optoelectronic applications.

Hole-injection barrier across the intermolecular interaction mediated interfacial DNTT layer

The electronic structures and morphology of dinaphthothienothiophene (DNTT) thin films on highly oriented pyrolytic graphite (HOPG) and polycrystalline Au (pc-Au) substrates have been investigated using photoelectron spectroscopy and atomic force microscopy techniques to better correlate the charge-injection barrier with the interfacial structure. DNTT molecules follow Stranski–Krastanov-type growth mode, i.e., highly-dewetted islands/fibers above a relatively-wetted interfacial layer, on both the substrates, but distinctively different electronic structures. On the HOPG surface, the as-grown and thermal-annealed films show a broad and a sharp highest occupied molecular orbital (HOMO) bands, respectively. On the pc-Au surface, a significantly split HOMO band and a reduced hole-injection barrier are observed in a monolayer thick film, for the first time, which are absent in other thick films. Such splitting indicates a presence of a strong intermolecular interaction in the interfacial layer, presumably due to the frontier π-orbital overlapping arising from partially-covered densely-packed herringbone-like arranged molecules. The dewetted-fibers seem to form with up-right oriented molecules having negligible interaction, which desorb easily by thermal annealing to retain the highly-stable interfacial layer. The reduction of the hole-injection barrier at the DNTT/pc-Au interface, due to the formation of such interfacial layer having strong intermolecular interaction, has an immense importance in increasing the charge-injection efficiency from a practical electrode.

Heteroanionic Melilite Oxysulfide: A Promising Infrared Nonlinear Optical Candidate with a Strong Second-Harmonic Generation Response, Sufficient Birefringence, and Wide Bandgap.

The achievement of balanced performance with a strong second-harmonic generation (SHG) response, proper birefringence, and wide band gap concurrently is a crucial but challenging task in infrared nonlinear optical (IR-NLO) crystals. Here, we theoretically confirmed that the heteroanionic oxysulfide tetrahedron would produce improved polarizability anisotropy and quadratic hyperpolarizability compared with the monoanionic oxide or sulfide tetrahedra. When this anion-mixing strategy was applied, melilite oxysulfide with the four representative members A2GeGa2OS6 (A = Ca, Sr) and Sr2MGe2OS6 (M = Zn, Cd) was successfully discovered as a promising IR-NLO material system. Remarkably, compared with the monoanionic melilite oxides, these compounds exhibited unbiased performances of proper birefringences (0.106-0.143), strong SHG responses (>10× melilite oxide Ba2CdGe2O7, 1.3-2.1× AgGaS2 (AGS) @1570 nm) with phase-matchable ability, wide band gaps (2.95-3.15 eV), and large laser-induced damage thresholds (LIDTs, 5.6-13.4× AGS). The high structure tolerance of melilite offers a great possibility to improve the SHG response via tuning the orbital composition and hybridization near band edges. This work provides an effective approach for the design of high-performance IR-NLO materials.

First-Principles Prediction of Superconductivity in High-Buckled Two-Dimensional Tin

Two-dimensional (2D) superconductors exhibit a variety of fascinating phenomena such as the Kosterlitz–Thouless–Berezinskii transition, electron quantum confinement effect, charge density wave, and topologically non-trivial band. Until now, the superconductivity of a classical elemental 2D system of tin has not yet been confirmed. Herein, using the microscopic theory of Bardeen, Cooper, and Schrieffer (BCS) and density functional theory calculations, we meticulously study the electron–phonon coupling, electronic structures, and phase transition of low-buckled (LB) stanene and high-buckled (HB) 2D tin. We demonstrate that the HB 2D tin exhibits remarkable multi-gap electron–phonon coupling superconductivity with critical temperatures approximating those of the conventional bulk tin and few-layer LB stanene. Our results clearly show the strain-induced transitions of the orbital occupation, band structure, and superconductivity from LB to HB structures. Our discovery of HB 2D tin as an excellent 2D superconductor should stimulate the experimental synthesis and characterization of its superconducting properties.

Optical Fingerprints of Nematicity in Iron-Based Superconductors

Nematicity, which refers to a phase of broken rotational but preserved translational symmetry, is underlined by the appearance of anisotropic properties and leaves remarkable fingerprints in all measurable physical quantities upon crossing the structural tetragonal-orthorhombic transition at Ts in several iron-based materials. Here, we review part of our own broadband optical investigations, addressing the impact of nematicity on the charge dynamics, as a function of temperature and of tunable applied stress, the latter acting as an external symmetry breaking field. We shall first focus our attention on FeSe, which undergoes a nematic (structural) transition without any subsequent onset of magnetic ordering below Ts. FeSe thus provides an opportunity to study nematicity without the limitations due to the reconstruction of the Fermi surface because of the spin-density-wave collective state in the orthorhombic phase, typical for several other iron-based superconductors. Our data reveal an astonishing anisotropy of the optical response in the mid-infrared-to-visible spectral range, which bears testimony of an important polarization of the underlying electronic structure in agreement with angle-resolved-photoemission-spectroscopy results. Our findings at high energy scales support models for the nematic phase resting on an orbital-ordering mechanism, supplemented by orbital selective band renormalization. The optical results at energies close to the Fermi level furthermore emphasize scenarios relying on scattering by anisotropic spin-fluctuations and shed new light on the origin of nematicity in FeSe. Moreover, the composition at which the associated Weiss temperature of the nematic susceptibility extrapolates to zero is found to be close to optimal doping (i.e., in coincidence with the largest superconducting transition temperature), boosting the debate to what extent nematic fluctuations contribute to the pairing-mechanism and generally affect the electronic structure of iron-based superconductors. The present review then offers a discussion of our optical data on the optimally hole-doped Ba0.6K0.4Fe2As2. We show that the stress-induced optical anisotropy in the infrared spectral range is reversible upon sweeping the applied stress and occurs only below the superconducting transition temperature. These findings demonstrate that there is a large electronic nematicity at optimal doping which extends right under the superconducting dome.