Two-dimensional Honeycomb Structure Research Articles

Z-shaped gate tunnel FET with graphene channel: An extensive investigation of its analog and linearity performance

This work analyzes a graphene channel Z-shaped gate tunnel FET's (ZTFET) analog, and linearity performance. This research aims to introduce graphene with a two-dimensional honeycomb structure that is anticipated to be a strong challenger for the upcoming generation of semiconductor devices. The ZTFET with graphene channel provides a 3-decade increase in ON current, indicating a notable improvement in gate capacitance and transconductance compared to the conventional silicon channel. This improvement further leads to better linearity and analog/RF performance. We delved into various linearity and Radio Frequency (RF) figure-of-merits, including gmn, VIP2, VIP3, IIP3, 1‐dB compression point, GBWP, TFP, unity gain cut‐off frequency, and maximum oscillation frequency. The results of the new GC-ZTFET are compared with those of the traditional ZTFET to establish its superiority. The GC-ZTFET outshines other device structures when speaking of linearity, RF performance, and current-carrying capability.

Half-metallic ferromagnetism with high critical temperatures in Substitutionally Doped Rare-Earth 2D Germanene

This work delves deep into the exploration of Rare-earth (RE) replacement doping germanene monolayers, where RE elements stand out for their fascinating electronic and magnetic nature derived from the elusive f-orbitals. Employing density functional theory (DFT) calculations, we unveil the electronic and magnetic characteristics of doped germanene systems. With its two-dimensional honeycomb structure, germanene holds immense promise for a wide range of applications within group IV materials. Through the discussion, we deeply uncover how the substitution of two specific concentrations of RE-atoms, namely La, Ce, Pr, Nd, Sm and Pm, transforms semi-metallic non magnetic germanene to half-metallic ferromagnets with remarkably high critical temperatures, when Hubbard correction is considered. These revelations not only deepen our comprehension of integrating f-orbitals into germanene but also open up exciting avenues for advancements in spintronic devices and beyond.

Cluster Odd-Parity Multipoles by Staggered Orbital Ordering in Locally Noncentrosymmetric Crystals

Odd-parity multipoles in crystals manifest themselves not only in their peculiar electronic orderings but also in unconventional parity-violating physical phenomena. We here report the emergence of odd-parity multipoles by considering staggered orbital orderings in a locally noncentrosymmetric crystal system with the global inversion center but without the inversion center at each lattice site. We show that various odd-parity multipoles, such as the electric toroidal monopole, electric dipole, and electric toroidal quadrupole, are realized depending on the type of orbital orderings in the one-dimensional zigzag chain. Such odd-parity multipoles give rise to an antisymmetric spin splitting in the electronic band structure with the aid of the relativistic spin–orbit coupling. We also show that similar states with odd-parity multipoles are realized in other locally noncentrosymmetric crystals, such as the two-dimensional honeycomb and three-dimensional diamond structures.

Anilato-Based Coordination Polymers with Slow Relaxation of the Magnetization: Role of the Synthetic Method and Anilato Ligand.

We have prepared and characterized three coordination polymers formulated as [Dy2(C6O4Cl2)3(fma)6] ⋅ 4.5fma (1) and [Dy2(C6O4X2)3(fma)6] ⋅ 4fma ⋅ 2H2O with X=Br (2) and Cl (3), where fma=formamide and C6O4X2 2-=3,6-disubstituted-2,5-dihydroxy-1,4-benzoquinone dianion with X=Cl (chloranilato) and Br (bromanilato). Compounds 1 and 3 are solvates obtained with slow and fast precipitation methods, respectively. Compounds 2 and 3 are isostructural and only differ in the X group of the anilato ligand. The three compounds present (6,3)-gon two-dimensional hexagonal honey-comb structures. Magnetic measurements indicate that the three compounds show slow relaxation of the magnetization at low temperatures when a continuous magnetic field is applied, although with different relaxation times and energy barriers depending on X and the crystallisation molecules. Compounds 1-3 represent the first examples of anilato-based lattices with formamide and field-induced slow relaxation of the magnetization.

Nano-enhanced Drug Delivery of Dacarbazine using Heteroatoms (B, N, P, S) doped Ag-Functionalized Silicene Nanomaterials: Insight from Density Functional Theory

Cancer remains a major global health concern, necessitating the development of novel and more effective treatment strategies. This research focused on exploring the potential of silicene as a nano-drug delivery platform. Silicene, a two-dimensional honeycomb structure, has garnered attention as an alternative to graphene, germanenes, and stanenes due to its comparative advantages in interfacing with micro or nano electronic devices. In this study, we investigated the co-doping of Ag-doped silicene with B, N, P, and S to evaluate their potential as adsorbents for delivering dacarbazine (DCB). Density functional theory (DFT) calculations at the ωB97XD/def2SVP level of theory were utilized to analyze their sensitivity, conductivity, stability, and reactivity. The geometry optimization results revealed that the introduction of B, N, P, and S as co-dopants significantly reduced the Ag52—Si30 bond in the Ag-functionalized silicene nano surface from 2.589 Å to a range of 2.241–2.074 Å. Likewise, a similar post-co-doping magnitude reduction effect was observed in the energy gaps, with the interactions ranging from 3.1186—3.7325 eV. Regarding adsorption characteristics, the Ead values indicated physisorption in the B, N, and P-co-doped interactions and chemisorption in the S-co-doped system, with values of 28.399, 147.445, 235.100, and -141.345 kcal/mol, respectively. After incorporating the basis set superposition error (BSSE) correction to the calculated adsorption energies, the adjusted values were obtained as follows: dcb_B@AgSi, dcb_P@AgSi, and dcb_N@AgSi exhibited 28.400, 135.103, and 147.446 kcal/mol, respectively. Meanwhile, dcb_S@AgSi displayed an adsorption energy of -142.344 kcal/mol. Furthermore, analyzing the results using QTAIM and NCI revealed the presence of non-covalent interactions, as well as partial and covalent interactions. This study sheds light on the promising therapeutic potential of B, N, P, and S co-doped Ag-functionalized silicene nano systems as efficient nano-drug delivery agents for dacarbazine (DCB). The insights gained from this research could pave the way for the development of advanced drug delivery systems with enhanced sensitivity and stability.

Programmable Polariton Topological Insulators All-Optically Controlled by the Stark Effect.

Efficiently and flexibly manipulating unidirectional edge states is key to developing topological insulators as functional devices. In this work, we propose an all-optical method that utilizes the valley-selective optical Stark effect to realize programmable topological insulators. We pattern a two-dimensional honeycomb structure in an exciton-polariton platform resulting from a strong light-matter coupling in a monolayer transition metal dichalcogenide. The optical Stark effect is induced to generate a pseudo magnetic field, combined with spin-orbit coupling to form the topological one-way edge states of the polariton. On account of the ultrafast switching speed and precisely spatial controllability of the optical Stark effect, two applications, i.e., ports ratio tunable polariton splitter and programmable polariton router, were demonstrated, showing designable and rewritable functionality of all-optically controllable polariton topological insulators. This study paves the way to robustly and intelligently control/form polaritonic and spintronic devices for future classical and quantum information processing and application.

Characterization of two-dimensional cellular elastic topological insulators based on regular-hexagon carriers

An elastic topological insulator with pseudo-spin characteristics is designed based on honeycomb lattice phononic crystals with positive hexagonal carriers, which can realize path defect immunity and backscattering suppression transmission characteristics. By introducing a positive hexagon carrier with a certain size at the narrow diameter junction of the two-dimensional honeycomb structure to achieve symmetry breaking, a four-fold accidental degeneracy point can be obtained by adjusting the cell parameters. The main variable of the primitive cell is the hexagonal carrier side length [Formula: see text]. It is found that the four-fold Dirac point can be opened and a band gap can be formed by contracting the positive hexagonal carrier. Inversion of the energy band occurred in the separated two-fold degenerate state, for which the transformation of mediocre state and nonmediocre state had been realized so that the structure with acoustic pseudo-spin and topological edge state could be obtained. Based on the principle of body-edge state correspondence, the topologically protected edge acoustic transmission is simulated by the construction of the edge states combined with the connection of different structural systems. Further, different elastic phonon crystal structures are constructed, and the characteristics of path defect immunity and back-scattering suppression of elastic wave propagation by topological edge states are verified. The designed elastic topological insulators have great application prospects in the regulation of elastic waves.

Recombination dynamics of indirect excitons in hexagonal BN epilayers containing polytypic segments grown by chemical vapor deposition using carbon-free precursors

Hexagonal (h) BN is a semiconductor that crystallizes in layers of a two-dimensional honeycomb structure. Since hBN exhibits high quantum efficiency (QE) near-band edge emission at around 5.8 eV in spite of the indirect bandgap, hBN has a potential for the use in deep-ultraviolet light emitters. For elucidating the emission dynamics of indirect excitons (iXs) in hBN, spatially and temporally resolved luminescence measurements were carried out on hBN epilayers grown using carbon-free precursors. In addition to major μm-side flat-topped (0001) hBN columnar grains, sub-μm-scale polytypic segments were identified, which were likely formed by certain growth instabilities. The hBN domains exhibited predominant emissions of phonon-assisted fundamental iXs at 5.7–5.9 eV and a less-pronounced 4.0-eV emission band. The photoluminescence lifetime (τPL) for the iX emissions was 54 ps, which most likely represents the midgap recombination lifetime (τMGR) for an iX reservoir. Because τPL did not change while the cathodoluminescence (CL) intensity increased with temperature above 100 K, both the immobile character of iXs and strong exciton–phonon interaction seem significant for procreating the high QE. The CL intensity and τPL of the 5.5 eV band monotonically decreased with temperature, indicating that τPL represents τMGR, most probably a nonradiative lifetime, around the real states. Equally significant emissions at 6.035 eV at 12 K and 6.0–6.1 eV at 300 K were observed from the polytypic segments, most probably graphitic bernal BN, which also exhibited negligible thermal quenching property.

Topological corner states in acoustic honeycomb structure

In recent years, a new type of topological insulator, termed higher-order topological insulator, has attracted tremendous research interest. Such exotic lower-dimensional topological boundary states have been extended and reproduced in classical systems, such as optics and acoustics. In this paper, a two-dimensional acoustic honeycomb structure with a triangle resonant cavity is numerically studied. Topological phase transition is induced by gradually adjusting the intracell and intercell coupling, and then the topological phase is used to construct a second-order topological insulator. The topological properties of second-order topological insulators can be characterized by using the quantized quadrupole moments. When quantized quadrupole <inline-formula><tex-math id="M4">\begin{document}$ {Q_{ij}} = 0 $\end{document}</tex-math><alternatives><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="5-20211848_M4.jpg"/><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="5-20211848_M4.png"/></alternatives></inline-formula>, the system is trivial, while <inline-formula><tex-math id="M5">\begin{document}$ {Q_{ij}} = 1/2 $\end{document}</tex-math><alternatives><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="5-20211848_M5.jpg"/><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="5-20211848_M5.png"/></alternatives></inline-formula>, the system is topologically nontrivial. We investigate the acoustical higher-order states of triangular and hexagonal structures, respectively. The gapped zero-dimensional corner states are observed in both structures, but the robustness properties of the corner states emerge only in the hexagonal structures but not in the triangular-shaped ones. The topological corner modes will offer a new way to robustly confine the sound in a compact acoustic system.

Effect of electric field on two-dimensional honeycomb structures from group (III–V)

The last few decades have seen an immense development of interest in two-dimensional materials, which can be integrated in either mono-, bi-, tri- or tetra-layer form. The tuning of physical properties of 2D materials is a prevalent challenge since they disclose enormous opportunities for electronic and opto-electronic devices. The functionality of two dimensional (2D) materials can be enhanced through several external factors such as strain, defects, doping, electric field, stacking, and mechanical stress, etc. Recently, the two-dimensional III-V (III = B, Al, Ga, In, Tl and V = N, P, As, Sb, and Bi) monolayers have been theoretically discovered. The present research has been designed to study the variations in their properties under a small range of external electric field. Previous studies have been highlighted which suggested that electric field can be an effective approach to improve the physical properties of 2D materials. In this paper, the influence of an external electric field, in the range of −0.3 VÅ−1 to 0.3 VÅ−1, on the structural, electronic and optical properties of III-V monolayers has been investigated using the first-principles calculations in the context of density functional theory (DFT). The results suggest that monolayers retain their geometrical properties. However, the electric field enhances the strength of covalent bonds. The results also disclose that the bandgap of these monolayers can be widely tuned with the help of electric field. As the electric field is raised, few of the monolayers exhibit transition from semiconducting to conducting behavior. Moreover, the optical absorption is also observed to modify with electric field. The amount of absorption and the energy at which electronic transitions occur vary uniformly with electric field. The transparency of few monolayers in visible region alongwith maximum absorption in UV region ensures their potential for Visible-blind UV detectors, flat panel displays and window layers in solar cells. The bandgap-type transition promise their application in low-power electronic devices and widely tunable bandgap shows their potential for various opto-electronic devices. The findings of this paper focus on the contribution of III-V monolayers to the development of more efficient electronic and opto-electronic devices.

Adatom-induced dislocation annihilation in epitaxial silicene

The transformation of the stripe domain structure of spontaneously-formed epitaxial silicene on ZrB2 thin films into a single-domain driven by the adsorption of a fraction of a monolayer of silicon was used to investigate how dislocations react and eventually annihilate in a two-dimensional honeycomb structure. The in-situ real time scanning tunneling microscopy monitoring of the evolution of the domain structure after Si deposition revealed the mechanisms leading to the nucleation of a single-domain island into a domain structure through a stepwise reaction of partial dislocations. After its nucleation, the single-domain island extends by the propagation of edge dislocations at its frontiers. The identification of this particular nucleation-propagation formation of dislocation-free silicene sheet provides insights into how crystallographic defects can be healed in two-dimensional materials.

Nitric Oxide Scattering off Graphene Using Surface-Velocity Map Imaging

We investigated the scattering of nitric oxide, NO, off graphene supported on gold. This is of fundamental importance to chemistry as collisions are the necessary first step to chemical reactions on graphene, and nitric oxide molecules are inherently radicals, with the potential to bond to graphene. We directed a molecular beam of NO in helium onto graphene and detected the directly scattered molecules using surface-velocity map imaging. In contrast to previous scattering studies off graphite, which detected only a modest reduction of the translational energy of the NO, we observe a loss of ∼80% of the molecules’ kinetic energy. Our classical molecular dynamics simulations still predict a loss of ∼60% of the translational energy in the scattering process. This energy appears to partly go into the NO rotations but mostly into collective motion of the carbon atoms in the graphene sheet. At 0° incidence angle, we also observe a very narrow angular scattering distribution. Both findings may be unique to pristine graphene on gold as (1) the two-dimensional (2D) honeycomb structure is perfectly flat and (2) the graphene is only loosely held to the gold at a distance of 3.4 Å, thus it can absorb much of the projectiles’ kinetic energy.

Phonon transport properties of two dimensional group-III nitrides (BN, AlN, and GaN)

In this work, the lattice thermal conductivity of two-dimensional honeycomb structures of BN, AlN, and GaN studied using first-principles density functional theory and solving the linearized Boltzmann transport equation by relaxation time approximation. Investigation of the influence of the phonon modes on the thermal conductivity revealed that ZA, and LA + TA + ZO modes have the most and TO + LO have the least contribution to the thermal conductivity of BN whereas in AlN, the contribution of ZA decreases while the effect of TO + LO increases. In GaN, the TO + LO branches Play the major role on the thermal conductivity. The observed differences in the lattice thermal conductivity of these monolayers were explained by considering the variations of their frequency-dependent phonon lifetimes, group velocities, and heat capacities. In BN monolayer, the group velocity and phonon lifetime of the acoustic phonons found to be higher whereas the heat capacity of it's LO and TO branches was lower than the monolayers of AlN and GaN and therefore, the contribution of the acoustical phonon branches to thermal conductivity has become more important. However in GaN, the LO and TO phonons revealed the highest phonon lifetime and specific heat resulting the dominance of the contribution of these phonon modes to the total thermal conductivity.

The electronic and optical properties of the Fe,Co,Ni and Cu doped ZnO monolayer photocatalyst

The structural, electronic, and optical properties of optimized Fe, Co, Ni and Cu doped ZnO monolayer in a two-dimensional honeycomb structure are analyzed based on the density functional theory. The stability is testified by analysis of phonon dispersion curves. The variations of electronic and optical properties of Fe, Co, Ni and Cu doped ZnO monolayer under in-plane and out-of-plane strain effect are also investigated. The results show that tunable band structures and optical properties of Fe, Co, Ni and Cu doped ZnO monolayer, such as indirect–direct band gap transition, shrink and enlargement of band gaps can be achieved by applying strain, which indicates a possible new route for tailoring the electronic properties of ultrathin nanofilms through strain engineering.

In-plane strain-free stanene on a Pd2Sn(111) surface alloy

Here we report the growth of laterally strain-free stanene, that is a two-dimensional (2D) honeycomb structure of tin atoms, on a Pd(111) crystal terminated by a ${\mathrm{Pd}}_{2}\mathrm{Sn}$ surface alloy. The atomic geometry and the electronic structure have been thoroughly investigated by scanning tunneling microscopy (STM), low-energy electron diffraction (LEED), Auger electron spectroscopy, high-resolution synchrotron radiation photoemission spectroscopy, and advanced first principles calculations. The STM images clearly reveal the epitaxial growth of honeycomb stanene on the preformed ${\mathrm{Pd}}_{2}\mathrm{Sn}$ surface alloy. LEED patterns clearly show commensurate (\ensuremath{\surd}3\ifmmode\times\else\texttimes\fi{}\ensuremath{\surd}3)R30\ifmmode^\circ\else\textdegree\fi{} spots, corresponding to a lattice constant of 0.47 nm, in perfect accord with the cell size of free-standing stanene. The measured very low buckling, within 20 pm, is derived from section profiles of atomic-scale STM images. This contrasts the significantly larger buckling of free-standing stanene, possibly due to a STM tip effect as well as a non-negligible interaction with the underlying surface alloy. The electronic structure exhibits a characteristic 2D band with parabolic dispersion, which is in good accordance with electronic structure calculations in density functional theory.

Tunable auxeticity and isotropic negative thermal expansion in three-dimensional lattice structures of cubic symmetry

A class of three-dimensional lattice structures of cubic symmetry is designed by spatially assembling and merging a number of two-dimensional re-entrant honeycomb structures. We demonstrate that the effective thermoelastic properties of these structures are widely tunable by tailoring the microstructural geometry and/or constituent materials. Particularly, they possess negative effective Poisson’s ratios. If the inclined and non-inclined walls are of dissimilar thermal expansion coefficients, they can further attain isotropic negative or positive effective thermal expansion coefficients with a magnitude as large as several to dozens of times that of the constituent materials. Owing to these novel properties, they are well suitable as structural or functional components in engineering systems for preventing or harnessing thermal and/or mechanical deformation.

Surface-enhanced resonance Raman scattering of MoS2 quantum dots by coating Ag@MQDs on silver electrode with nanoscale roughness

Molybdenum disulfide quantum dots (MQDs) possess unique optical and electronic properties owing to their typical microstructures, such as two-dimensional (2D) honeycomb structure, quantum confinement, and surface passivation. However, the microstructures that have been experimentally investigated are far from being sufficient evidence for clearly understanding the electronic and optical mechanism due to the lack of effective detection methods. Here, pure MQDs were fabricated through the laser ablation of bulk MoS2 along the orientation parallel to the MoS2 layers, which is essential for surface-enhanced Raman scattering (SERS) measurement. Surface-enhanced resonance Raman scattering (SERRS) is introduced to dramatically enhance the Raman signals of MQDs by coating Ag@MQDs on a silver electrode with nanoscale roughness and highlights much more abundant Raman peaks corresponding to the MQD microstructures of the hexagonal crystalline domains, passivation electron-donating groups, defects, and disorders than those highlighted by normal Raman scattering (NRS) and resonant Raman scattering (RRS). Furthermore, owing to the adsorption of MQDs via oxygen-containing passivation groups on the silver surface, SERRS highlights more vibrations associated with passivation structures than Fourier transform infrared (FTIR) spectra that are usually applied to provide crucial evidence for the existence of passivation groups. Abundant Raman peaks derived from the defects and disorders of MQDs are also observed. Additionally, several second-order Raman peaks that are related to the hexagonal structure of MQDs can also be observed and are induced by defects, disorders, and adsorption on the silver surface. This work indicates that SERRS has advantages over high-resolution transmission electron microscopy (HRTEM), FTIR, NRS and RRS in characterizing the microstructures of MQDs.

Negative Poisson\u2019s ratio in two-dimensional honeycomb structures

Negative Poisson’s ratio (NPR) in auxetic materials is of great interest due to the typically enhanced mechanical properties, which enables plenty of novel applications. In this paper, by employing first-principles calculations, we report the emergence of NPR in a class of two-dimensional honeycomb structures (graphene, silicene, h-BN, h-GaN, h-SiC, and h-BAs), which are distinct from all other known auxetic materials. They share the same mechanism for the emerged NPR despite the different chemical composition, which lies in the increased bond angle (θ). However, the increase of θ is quite intriguing and anomalous, which cannot be explained in the traditional point of view of the geometry structure and mechanical response, for example, in the framework of classical molecular dynamics simulations based on empirical potential. We attribute the counterintuitive increase of θ and the emerged NPR fundamentally to the strain-modulated electronic orbital coupling and hybridization. It is proposed that the NPR phenomenon can also emerge in other nanostructures or nanomaterials with similar honeycomb structure. The physical origin as revealed in our study deepens the understanding on the NPR and would shed light on future design of modern nanoscale electromechanical devices with special functions based on auxetic nanomaterials and nanostructures.

Metal frame structures with controlled anisotropic thermal conductivity

The summation law (kxx+kyy+kzz=(1−ε)ks) for those structures made of the same metal frames has been generalized in terms of the thermal conductivity tensor kij=∑n=1Nsnks(g→n·e→i)(g→n·e→j) in order to identify the conditions for orthotropic thermal conductivity and the level of the anisotropy of thermal conductivity in metal frame structures. A novel design strategy was proposed for controlling the anisotropy of the effective thermal conductivity tensors in various metal frame structures. Both three-dimensional lattice frame structurers and two-dimensional prismatic cellular honeycomb structures were considered to achieve their desired levels of the anisotropy in the thermal conductivity tensors. The relationships between the level of the anisotropy and frame design parameters are found for rectangular frame structures, deformed octet-truss structures, deformed tetrakaidecahedron structures and various kinds of honeycomb structures. The relationships analytically obtained for these structures agree well with the full 3D numerical results, and thus can serve as a theoretical basis to design the anisotropy of the effective thermal conductivity in metal frame structures according to their particular thermal applications.

High graphene permeability for room temperature silicon deposition: The role of defects

Graphene (Gr) is known to be an excellent barrier preventing atoms and molecules to diffuse through it. This is due to the carbon atom arrangement in a two-dimensional (2D) honeycomb structure with a very small lattice parameter forming an electron cloud that prevents atoms and molecules crossing. Nonetheless at high annealing temperatures, intercalation of atoms through graphene occurs, opening the path for formation of vertical heterojunctions constituted of two-dimensional layers. In this paper, we report on the ability of silicon atoms to penetrate the graphene network, fully epitaxially grown on a Ni(111) surface, even at room temperature. Our scanning tunneling microscopy (STM) experiments show that the presence of defects like vacancies and dislocations in the graphene lattice favor the Si atoms intercalation, forming two-dimensional, flat and disordered islands below the Gr layer. Ab-initio molecular dynamics calculations confirm that Gr defects are necessary for Si intercalation at room temperature and show that: i) a hypothetical intercalated silicene layer cannot be stable for more than 8 ps and ii) the corresponding Si atoms completely lose their in-plane order, resulting in a random planar distribution, and form strong covalent bonds with Ni atoms.