Structure, stability and electronic properties of bimetallic atomic chains of Au–Ag and Au–Pt

The surface atomic structure of Si atomic chains grown on the Si/Cu(110) surface alloy has been investigated by a combination of different experimental techniques. For Si coverages below 0.5 monolayers the low-energy electron diffraction (LEED) pattern shows a $c(2\ifmmode\times\else\texttimes\fi{}2)$ reconstruction, corresponding to the formation of a surface alloy. Upon further Si deposition, the LEED pattern evolves toward a (2\ifmmode\times\else\texttimes\fi{}2)-like structure with streaks along the [001] direction. Scanning tunneling microscopy (STM) images show the presence of linear atomic Si chains on top of the surface alloy layer. We present an atomic model for the surface termination based on the STM images and on the main atomic directions of the (2\ifmmode\times\else\texttimes\fi{}2)-like phase found by a simple analysis of the $\mathrm{Si}2p$ full hemispherical x-ray photoelectron diffraction patterns. This model consists of linear atomic Si chains running along the [110] surface direction formed on top of the surface alloy. The chains present small (2\ifmmode\times\else\texttimes\fi{}2) domains, which are not in phase with respect to each other. After heating the (2\ifmmode\times\else\texttimes\fi{}2)-like phase up to 250 \ifmmode^\circ\else\textdegree\fi{}C, a quasi-(3\ifmmode\times\else\texttimes\fi{}4) structure is developed. This structure consists of similar chains exhibiting a different periodicity. Furthermore, we have used synchrotron radiation photoemission (x-ray and ultraviolet photoemission spectroscopy) to gather information about the electronic structure of the atomic chains.

We report structure and electronic properties of Au-Pd, Au-Pt and Au-Ag bimetallic atomic chains absorbed on NiAl(110) and Cu(110) substrate. It is found that the presence of substrate significantly influences the electronic structure of the chains. Atoms of single chains of Au-Pd, Au-Pt and Au-Ag bind more strongly with Ni atoms of NiAl substrate, as compared with Cu atoms in Cu(110). The interaction between chain atoms is found stronger than the chain-substrate atoms, when chains are placed on Cu substrate, while it is other way round in case of chains on NiAl substrate. Effect of change in positions of atoms in bimetallic chains in presence of substrate is studied by placing double chains of Au-Pd, Au-Pt and Au-Ag on Cu (110) substrate in three different configurations. It is found that Au-Pd and Au-Pt bimetallic chains stabilize in double zigzag topology, when placed on Cu (110) substrate. While Au-Ag chains exhibit ladder topology on Cu(110) substrate. Ferromagnetism that is observed in ground state of free standing chains of Au-Pd and Au-Pt is not found when chains are absorbed on NiAl(110) and Cu(110) substrate. It is likely that the interaction between chain and substrate atoms results to zero magnetic moment.

The electronic and optical properties of bimetallic single atomic linear chains of Au-Pd have been investigated within Density functional theory frame work. Dielectric matrix has been computed within random phase approximation for atomic chains with different compositions. The results of bimetallic atomic chains of Au-Pd are compared with the pristine Au and Pd atomic chains. It has been found that doping the pristine chains affects the real and imaginary part of dielectric function which is greatly dependent on the composition of bimetallic chains.

We have theoretically studied the possible formation of evenly mixed Pt–X, Au–X, Pd–X bimetallic atomic chains (ACs) with X = Co, Fe, and Ni. The results show that Pt–Fe ACs are the most energetically favorable. First principles calculations revealed that the energy of alloy formation is dependent on the d-band filling of the magnetic component (Fe, Co, and Ni). Thus, we found that formation of stable evenly mixed ACs with Ni atoms as the magnetic component is not possible. Moreover, we found that the alloying energy is dependent on the geometry of the AC. We found that the energy of alloy formation remains unchanged in linear ACs and drastically decreases by ∼1 eV under chain contraction, accompanied by a transition of the AC from a linear to a zigzag configuration. Our electronic structure calculations revealed the emergence of a ferromagnetic transition under stretching of the AC in all the bimetallic chains with Fe atoms as the magnetic component (Pt–Fe, Pd–Fe, and Au–Fe) from the ferromagnetic to the antiferromagnetic state.

We study the stability and structure of self-assembled atomic chains on Si(111) induced by monovalent, divalent, and trivalent adsorbates, using first-principles total-energy calculations and scanning tunneling microscopy. We find that only structures containing exclusively silicon honeycomb or silicon Seiwatz chains are thermodynamically stable, while mixed configurations, with both honeycomb and Seiwatz chains, may be kinetically stable.

The influence of coupling between chains on the conductivity of atomic carbon chains

We studied plasmonlike resonances in one-dimensional (1D) atomic chain systems by using time-dependent density-functional theory (TDDFT) and local density functional theory. Recent TDDFT studies have shown the coexistence of longitudinal and transverses collective plasmonlike resonances in the atomic chains of simple and noble metals. Such atomic chains contain only a few atoms. The induced polarization occurs along the atomic chain in longitudinal mode and perpendicular to the atomic chain in transverse mode. To understand the emergence of plasmonlike resonance in 1D atomic chains better, we studied carbon chains in which plasmonic resonances are not expected to occur. We used TDDFT to study the emergence of collective resonances in various forms of carbon chains, cumulenes ${\mathrm{C}}_{n}{\mathrm{H}}_{4}$, polyynes ${\mathrm{C}}_{n}{\mathrm{H}}_{2}$, and alkenes ${\mathrm{C}}_{n}{\mathrm{H}}_{n+2}$. The excitation energy and dipole oscillation strengths of these systems were determined through TDDFT by using the turbomole package. We determined how collective plasmonlike resonances arise from single-electron excitations when the number of electrons increases as the carbon chain lengthens. The collective excitation behavior is then compared with that of metallic atomic chains. Our TDDFT results showed longitudinal collective modes for cumulenes and polyynes, as well as for finite-length chains. These collective excitations exhibit the same behavior as that of longitudinal ``plasmon'' previously identified in sodium and silver chains, although polyynes are gapped in the long chain limit. Such longitudinal excitations are absent in alkenes. However, unlike metal atomic chains, carbon chains exhibited no transverse collective mode. The band structure of periodic atomic chains was calculated by using the standard local density functional method. These structures were used to interpret the results and to relate the single-electron excitation to the collective plasmonlike response. Within the one-particle quantum-well picture, the longitudinal mode in the linear atomic chain arises from intraband transition with $\ensuremath{\Delta}q=1$, where $q$ is the quantum number of quantum wells. $\ensuremath{\Delta}q=1$ intraband transitions can be found in metallic (e.g., Na) chains and in carbon chains (cumulenes and polyynes), such longitudinal collective mode is rather ``generic''. Meanwhile, the transverse modes of the sodium chains are attributed to interband transitions with an even $\ensuremath{\Delta}q$ (dominated by $\ensuremath{\Delta}q=0$), and such transverse collective excitations only form if the allowed $\ensuremath{\Delta}q=0$ transitions occur between bands that are parallel to each other. Such bands can be found in simple metals, but not in carbon chains.

Aluminum Nitrides (AlN) thin films have been deposited ‎on glass substrates using DC-magnetron sputtering technique for different back pressure. The piezoelectric and related properties of highly c-axis oriented AlN films fabricated by dc planar magnetron sputtering have been calculated. Experimental results show that highly c-axis oriented AlN films can be fabricated by dc planar magnetron sputtering. X-ray powder diffraction (XRD) technique shows that AlN thin films exhibit a hexagonal structure. Vienna Ab initio Simulation Package (VASP) within the framework of density functional theory (DFT) and generalized gradient approximation (GGA) was used to investigate the structural and electronic properties of hexagonal AlN structures. The experimental lattice parameters of the as prepared thin films are found to be in good agreement with ab-initio calculated parameters. UV–Vis spectrophotometer measurements are performed to investigate the optical properties of AlN thin films. We found that the refractive index of AlN thin films exhibits values ranging between 2.1 and 2.2. Furthermore, the elastic, piezoelectric and dielectric tensors of AlN crystal are calculated using VASP. The dynamical Born effective charge tensor is reported for all atoms in the unit cell of AlN. The value of the principle component of electronic contribution to the static dielectric tensor of AlN is found to be ≈ 4.68 that is in good agreement with the experimental static dielectric constant. In addition, clamped-ion piezoelectric tensor is calculated. The diagonal components of the piezoelectric tensor are found to be e_33=1.784 C/m^2 and e_31=-0.8 C/m^2. The large values of the piezoelectric coefficients show that polar AlN crystal exhibits a strong microwave piezoelectric effect. Furthermore, the components of the elastic moduli tensor are calculated. The extraordinary electronic, optical, piezoelectric and elastic properties make AlN thin films potential candidates for several opto-electronic, elastic, dielectric and piezoelectric applications.

To meet the demands for functional layers in inverted flexible perovskite solar cells, high-performance formamidinium-based perovskite solar cells, and high-performance photodetectors in future applications, it is crucial to appropriately reduce the bandgap of third-generation wide-bandgap semiconductor materials. In this study, we first optimized doping sites through Ag-Cl and Ag-S configurations to establish stable substitution patterns, followed by density functional theory (DFT) calculations using the Generalized Gradient Approximation with the Perdew-Burke-Ernzerhof (GGA-PBE) functional, implemented in the Vienna Ab initio Simulation Package (VASP). A plane-wave basis set with a cutoff energy of 450 eV and a 3 × 4 × 3 Γ-centered k-mesh were adopted to investigate the effects of Mg-Cl, Mg-S, Zn-Cl, and Zn-S co-doping on the structural stability, electronic properties, and optical characteristics of β-Ga2O3. Based on structural symmetry, six doping sites were considered, with Ag-S/Cl systems revealing preferential occupation at octahedral Ga(1) sites through site formation energy analysis. The results demonstrate that Mg-Cl, Mg-S, Zn-Cl, and Zn-S co-doped systems exhibit thermodynamic stability. The bandgap of pristine β-Ga2O3 was calculated to be 2.08 eV. Notably, Zn-Cl co-doping achieves the lowest bandgap reduction to 1.81 eV. Importantly, all co-doping configurations, including Mg-Cl, Mg-S, Zn-Cl, and Zn-S, effectively reduce the bandgap of β-Ga2O3. Furthermore, the co-doped systems show enhanced visible light absorption (30% increase at 500 nm) and improved optical storage performance compared to the pristine material.

Structural, electronic and optical properties of concentration dependent indium (In) doped ZnO monolayer are studied in the framework of density functional theory. We have considered the various concentration of In (6.25, 12.50 and 18.75 atomic (at.) %) in ZnO monolayer. We have used Vienna Ab initio Simulation Package (VASP) with Projected Augmented Wave (PAW) pseudopotentials and generalized gradient approximation - Perdew, Burke and Ernzerhof (GGA-PBE) functional. Substitution of Zn with In atom is energetically favorable. The large sized In atom as compared to that of Zn induces the stress in ZnO monolayer. To reduce the stress, In atom protrudes out from the plane of ZnO monolayer. The computational model under DFT calculations with ultrasoft pseudopotential and PBE exchange correlation functional, showed that In atom remains in the plane of ZnO monolayer. Similar behavior is observed with the use of full-potential linearized augmented plane wave (FLAPW)-GGA approach. The effects of pseudopotential and exchange correlation functional are observed in terms of structural deformation in In-doped ZnO monolayer. It effectively modulates the band structure and optical properties. The computational codes, potentials, and exchange correlation functionals are playing major role in investigating the structural and electronic properties of various nanostructures.

Phonon ballistic transport in the atomic chains with different interface connections to the heat reservoir

ABSTRACTWe report the electronic structure and thermodynamic properties of platinum–lead oxides PbPt2O4 and Pb2PtO4. The band structure is calculated by the full-potential, full-relativistic local orbital minimum basis (FPLO) method in the local density approximation, including the generalized gradient approximation (GGA) and GGA+U, respectively. The values of gaps in Pb2PtO4 were estimated as 0.47, 1.02, and 0.52 eV in GGA, GGA+U, and the full-relativistic GGA, respectively. Pb2PtO4 is a semiconductor, and PbPt2O4 is a metallic conductor with the small values of the density of states at the Fermi level. The effect of chemical disorder in Pb2−xPt1+xO4 alloys is studied by the relaxation of the volume, shape, and the positions of atoms in the unit cell, using the Vienna Ab initio Simulation Package (VASP) method. The thermodynamic properties are computed in the quasi-harmonic Debye–Grüneisen model. The dependence of the thermodynamic parameters on the pressure and temperature is also presented.

Self-assembled arrays of atomic chains on Si(111) represent a fascinating family of nanostructures with quasi-one-dimensional electronic properties. These surface reconstructions are stabilized by a variety of adsorbates ranging from alkali and alkaline earth metals to noble and rare earth metals. Combining the complementary strength of dynamical low-energy electron diffraction, scanning tunneling microscopy and angle-resolved photoemission spectroscopy, we recently showed that besides monovalent and divalent adsorbates, trivalent adsorbates are also able to stabilize silicon honeycomb chains. Consequently silicon honeycomb chains emerge as a most stable, universal building block shared by many atomic chain structures. We here present the systematics behind the self-assembly mechanism of these chain systems and relate the valence state of the adsorbate to the accessible symmetries of the chains.

One-dimensional (1D) magnetic material systems have attracted widespread interest from researchers because of their peculiar physical properties and potential applications in spintronics devices. However, the synthesis of 1D magnetic atomic chains has seldom been investigated. Here, we developed an iodine-assisted vacuum chemical vapor-phase transport (I-VCVT) method, utilizing single-walled carbon nanotubes (SWCNTs) with 1D cavities as templates, and high-quality and high-efficiency fabrication of 1D atomic chains of CrCl3 was achieved. Furthermore, the structure of CrCl3 atomic chains in the confined space of SWCNTs was analyzed in detail, and the charge transfer between the 1D atomic chains and SWCNTs was investigated through spectroscopic characterization. A comprehensive study of the dynamic magnetic properties revealed the existence of spin glass states and freezing of the 1D CrCl3 atomic chains at around 3 K, which has never been seen in bulk CrCl3. Our work established an effective strategy for the control synthesis of 1D magnetic atomic chains with promising potential applications in further magnetic-based spintronics devices.

We report a first-principles investigation of spin polarized transport properties of Pt atomic chains in contact with two semi-infinite Pt slabs along the (111) direction. Our approach is based on the nonequilibrium Green's function coupled with density functional theory so that the Coulomb interaction is included in the calculation of current and shot noise on the Hartree level. For Pt atomic chains with different numbers of Pt atoms, we calculate the spin polarized I-V curve and shot noise. Our results show that the current increases almost linearly with bias for all Pt atomic structures. The calculated Fano factors are comparable to the recent experimental data and show sub-Poissonian behavior.