Two-dimensional Honeycomb Research Articles

Geometry-dependent acoustic higher-order topological phases on a two-dimensional honeycomb lattice

Higher-order topological states, as emergent topological phases of matter, originating from condensed matter physics, have sparked a vibrant exploration of topological insulators. Their topologically protected multidimensional localized states are typically associated with nontrivial bulk band topology, and the significant impact of lattice geometry is unconsciously overlooked. Here, we construct coupled acoustic cavities on a two-dimensional honeycomb lattice to investigate the sensitivity of higher-order topological modes to the variations of edge contour. Fractional charge is utilized to accurately predict topological modes with distinct topological orders, in spite of the minimal bulk bandgaps inherent in the honeycomb lattice and bound states in the continuum. It is found that the presence and absence of the first-order and higher-order topological modes in the same topological phase are tightly linked to the sample boundaries, which can be demonstrated by both theoretical analysis and numerical calculation. Our study also discusses potential physical realization of geometry-dependent topological states across different platforms, providing inspiration for the prospective application of topological devices in acoustics.

On the plasmonic properties of a graphene nanoribbon and noble metal composite array

Graphene, composed of sp2 hybrid orbitals, is a hexagonal nanomaterial with a two-dimensional honeycomb lattice structure. Nowadays, thanks to its great electronic, optical, and thermal properties, there have been many promising applications based on graphene nanostructures. Under irradiations of external electromagnetic fields, the free electrons on graphene surfaces can be excited to oscillate collectively, resulting in highly localized surface plasmons. This phenomenon may provide foundations for designs of tunable plasmonic devices, therefore, graphene-based nanostructures have been widely studied so far. Besides, investigations on noble metals is another hot topic in the plasmonic community. With irradiations of incident lights, collective oscillations of free electrons on noble metals’ surfaces can also be induced, forming surface plasmons. The plasmonic characteristics can be adjusted by varying the parameters of the metals, including the geometries, thicknesses, as well as dimensions, etc. In this paper, a composite array consisting of graphene nanoribbons and noble metals is proposed, and its plasmonic properties are studied at mid-infrared wavelengths. In the structure, graphene nanoribbons are placed in between two SiO2 layers, and gold semi-cylinders and silver nanofilms are placed above the upper SiO2 layer, respectively. Using the finite difference time domain method, the light transmittance and electric field of the array are investigated, by varying the graphene’s Fermi energy and layer number, the noble metals’ sizes, as well as the period of the array, respectively. The results show that a plasmonic resonance feature is revealed in the 4–18 μm wavelength range, and the resonance characteristics can be tuned by adjusting the above parameters. The composite array structure proposed in this work may provide us with a new platform for the design of plasmonic devices in the mid-infrared regime.

Large-scale Production of Few-Layer Reduced Graphene Oxide by the Rapid Thermal Reduction of Graphene Oxide and Its Structural Characterization

Graphene, a carbon allotrope, is a two-dimensional honeycomb of carbon atoms. Although graphene is a thin material, it is the strongest material known on Earth thanks to the strong carbon bonds in its structure. It is stated that the strength of these carbon bonds in graphene is about 100 times stronger than steel. In this study, graphite was first converted into graphene oxide (GO) by the Improved Hummers method, which is one of the methods suitable for large-scale production. Reduced graphene oxide (RGO) was obtained from the synthesized GOs by thermal reduction. TGA, FTIR, XRD, XPS, Raman, BET, and SEM analyses were used to characterize GO produced using the improved Hummers method and RGO reduced by thermal methods. TGA measurements show that RGO produced using the thermal approach had a lower mass loss than graphite oxidized using the improved Hummers process. This shows that the GO sample prepared using the improved Hummers approach contains a considerable number of distinct oxygen-containing groups. The novelty of the modified Hummers' method lies in its enhanced efficiency in producing graphene oxide through reduced thermal reaction times and improved scalability compared to the original approach in the literature. The C:O ratio of the GO and RGO samples was determined by XPS to be 1.88 and 11.17, respectively. The ID/IG ratio obtained by Raman analysis was 0.973. In addition, RGO's BET surface area was discovered to be 543.6 m2 g-1. These findings demonstrated that graphite was successfully oxidized by an improved Hummers method, and the resulting GO was thermally converted to few-layer RGO.

Growing sp2 materials on transition metals: calculated atomic adsorption energies of hydrogen, boron, carbon, nitrogen, and oxygen atoms, C2 and BN dimers, C6 and (BN)3 hexamers, graphene and h-BN with and without atomic vacancies.

The growth of graphene and hexagonal boron nitride on hot transition metal surfaces involves the adsorption of precursor molecules, and their dissociation and assembly into two-dimensional honeycomb lattices. In a recent account it was found that h-BN may be distilled on a rhodium metal surface, which yields higher quality h-BN [Cun et al., ACS Nano, 2020, 15, 1351]. In this context, we calculated in a systematic approach the adsorption energies and sites of hydrogen, boron, carbon, nitrogen, and oxygen atoms and from the site dependence the activation energy for diffusion. Existing computed values of the solvation energy into the bulk were compared to the present ones with our calculation scheme and found to be in good agreement. For the distinction of different systems we introduce the concepts of epiphilicity and epiphobicity. The adsorption energies and stabilities of the C2 and BN dimers, the C6 and (BN)3 ring-hexamers and the graphene and h-BN monolayers allow the prediction of the performance of different substrates in chemical vapor deposition (CVD) processes for the growth of graphene and h-BN. Finally, vacancy creation energies were calculated as a criterion for the stability of graphene and h-BN on metallic substrates.

Two-dimensional honeycomb lattice structure for underwater acoustic cloaking using pentamode materials

This research article presents an innovative and novel approach to achieve underwater acoustic cloaking using a two-dimensional honeycomb lattice structure with pentamode materials in the kHz frequency range. Underwater acoustic cloaking holds substantial importance in various applications, such as marine engineering, imaging, and military operations, making the development of an efficient underwater acoustic shell imperative. The proposed cloak consists of a pentamode titanium material honeycomb lattice embedded in an air background, submerged in water. To attain effective camouflage and regulate the phase and energy flow, impedance matching was applied to the overall geometry of the structure. By fine-tuning the structural parameters of the cloaking shell, derived from the effective mass velocity and density for recovering reflected waves, impedance matching with water was ensured. Through simulation calculations and optimization design, the average total scattering cross-section of the acoustic cloak is determined to be 0.1. The results demonstrate that the pentamode material-based cloaking approach is not only suitable and efficient in achieving the cloaking phenomenon but also enhances operator flexibility. The operating frequency bandwidth of the acoustic cloaking system is approximately 8 kHz for lattice constant a = 5 mm. These findings pave the way for further advancements in underwater acoustic cloaking technologies.

Γ-Graphyne: A Promising Electron Acceptor for Organic Photovoltaics

The most significant discovery of recent years is graphene, which consists of a single layer of sp2-hybrid carbon atoms arranged in a two-dimensional (2D) honeycomb lattice nanostructure.Graphynes – a two-dimensional materials similar to graphene, but composed of sp-hybridized carbons periodically integrated into a sp2-hybridized carbon network. Structure of graphynes were first theoretically proposed in 1987 by Baughman et al.[1] More than 20 years later, in 2010, Li et al. developed the first successful methodology for creating γ-graphdiyne films [2]. Despite the significant attention from the scientific community, only recently Hu et al. reported the synthesis of γ -graphyne with unified long-range crystalline structure [3].We demonstrate that γ -graphyne is an efficient electron acceptor due to its low LUMO and its ability to delocalize an excess charge. Moreover, in contrast to other 2D sheets like graphene its electronic properties are not sensitive to vacancy defects. Investigation of photoinduced electron transfer processes in complexes of γ -graphyne with typical electron-accepting and electron-donating partners shows that the lowest excited states with donors are charge separated states formed by the electron transfer from partner to γ -graphyne. This electron transfer is thermodynamically favorable and occurs on nano-to-picosecond time scale. In contrast, electron transfer from γ -graphyne to strong electron acceptors such as fullerenes or arylenediimides is unlikely [4].

Benzene-1,4-Di(dithiocarboxylate) Linker-Based Coordination Polymers of Mn2+, Zn2+, and Mixed-Valence Fe2+/3.

Three new coordination polymers (CPs) constructed from the linker 1,4-di(dithiocarboxylate) (BDDTC2-)─the sulfur-analog of 1,4-benzenedicarboxylate (BDC2-)─together with Mn-, Zn-, and Fe-based inorganic SBUs are reported with description of their structural and electronic properties. Single-crystal X-ray diffraction revealed structural diversity ranging from one-dimensional chains in [Mn(BDDTC)(DMF)2] (1) to two-dimensional (2D) honeycomb sheets observed for [Zn2(BDDTC)3][Zn(DMF)5(H2O)] (2). Gas adsorption experiments confirmed a 3D porous structure for the mixed-valent material [Fe2(BDDTC)2(OH)] (3). 3 contains a 1:1 ratio of Fe2+/3+ ions, as evidenced by 57Fe Mössbauer, X-band EPR, and X-ray absorption spectroscopy. Its empirical formula was established by elemental analysis, thermal gravimetric analysis, infrared vibrational spectroscopy, and X-ray absorption spectroscopy in lieu of elusive single-crystal X-ray diffraction data. In contrast to the Mn- and Zn-based compounds 1 and 2, the Fe2+/3+ CP 3 showed remarkably high electrical conductivity of 5 × 10-3 S cm-1 (according to van der Pauw measurements), which is within the range of semiconducting materials. Overall, our study confirms that sulfur derivatives of typical carboxylate linkers (e.g., BDC) are suitable for the construction of electrically conducting CPs, due to acceptedly higher covalency in metal-ligand bonding compared to the electrically insulating carboxylate CPs or metal-organic frameworks. At the same time, the direct comparison between insulating CPs 1 and 2 with CP 3 emphasizes that the electronic structure of the metal is likewise a crucial aspect to construct electrically conductive materials.

Half-valley semimetal and largest inverse topological caloric effect at topological quantum criticality in the chiral-symmetric AIII class

Half-valley semimetal (HVSM) and single-valley states are the hallmark of valleytronics in two-dimensional honeycomb lattice materials, but their quasi-one-dimensional analog that takes advantage of quantum manipulation has not yet been realized. We propose a double-helical ladder model described by a coupled double Su–Schrieffer–Heeger chain, wherein the interchain coupling controlled by magnetic flux induces time-reversal and particle-hole symmetry breaking and preserves only the chiral symmetry, which is classified into the AIII symmetry class. It realizes valley polarization, single-valley topological insulator, and HVSM as the topological quantum criticality (TQC), signaling well valley filter or valve effects. Furthermore, the TQC produces the largest inverse topological caloric effect accompanied by a T-linear relation of isothermal entropy change at ultra-low temperatures. Our findings not only open alternative perspectives for multifunctional quantum devices in valleytronics but also shed light on the thermodynamic characterization of TQC and promote the rapid development of topological quantum refrigeration technology.

Adsorption and inactivation of pollutant molecules on the hexagonal Borophene/Al(111) superstructure

The Borophene deposited on Al(111) substrates presents a hexagonal lattice configuration – known as honeycomb –. Due to the outstanding surface reactivity of two-dimensional honeycomb materials, they have been considered promising candidates for gas sensor devices. In this work, we assess the feasibility of using borophene/Al(111) as a sensing device for pollutant molecules like nitrogen dioxide (NO2), nitric oxide (NO), carbon dioxide (CO2), carbon monoxide (CO), sulfur dioxide (SO2), and dimethyl sulfoxide (DMSO). Our results show that borophene/Al(111) is a good gas-trapping agent. It can also dissociate the toxic molecules into their atomic components, which are incorporated into the borophene structure. Almost all pollutant molecules were successfully inactivated, except CO2, which remained physically adsorbed on the sheet. Electrostatic potential maps help examine the surface evolution along the adsorption and inactivation process. Our results indicate that the borophene/Al(111) superstructure is suitable for environmental remediation.

Simulation of Interaction Processes of C20 Fullerene with Graphene

Graphene, a carbon sheet one atom thick, with carbon atoms arranged in a two-dimensional honeycomb configuration, has a number of intriguing properties. Fullerenes are a promising material for creating electro-active elements in solar cells and active layers in thin-film organic transistors. A computer model of the C20 fullerene molecule was constructed using the energy minimization method with the second-generation Brenner potential (REBO). A computer model of "infinite" defect-free graphene was built, designed to consider the process of adsorption of a C20 fullerene molecule on its surface. To study adsorption process computer models of fullerene and "infinite" graphene were approached to the required distance with a different set of geometric arrangement of fullerene with respect to the graphene surface. It has been established that the adsorption of fullerene C20 on the surface of graphene can be carried out in three different ways, differing in the number of interacting fullerene and graphene atoms. The binding energies and adsorption lengths for C20 fullerene molecules adsorbed on the graphene surface in different ways are calculated. The way of adsorption corresponding to the highest binding energy and the shortest adsorption length was revealed.

Many Body Aspects on the Lattice Vibrational Properties of Germanene and Silicene along Highly Symmetry Directions

The investigation of lattice vibrational properties in monolayer two-dimensional honeycomb lattices comprising 2D group-IV semiconductor materials such as Germanene and Silicene (Low Buckled structure) holds immense significance in the realm of research, owing to their potential integration into the forthcoming electronic sector for information technology. The ductile nature of 2D group-IV semiconductor materials enables seamless integration with existing semiconductor technology on substrates, setting them apart from Graphene. Our primary objective revolves around comprehending the lattice vibrational properties of Germanene and Silicene (LB) through the examination of their honeycomb and buckled lattice structures. We use the Adiabatic Bond Charge Model with a Python program to derive vibrational frequencies at the Г points along highly symmetrical directions. Moreover, we extensively analyze the acoustical and optical contributions to the lattice vibrational frequencies. We anticipate that the phonon frequencies along the Г‒M direction for 2D atomically thin Germanene and Silicene will yield reasonably similar outcomes to those other research groups achieve.

Lattice thermal conductivity and thermoelectric properties of two-dimensional honeycomb monolayer of CdO

The lattice thermal conductivity of the CdO monolayer was investigated by density functional theory in combination with Boltzmann transport theory and it was estimated about 3.346 Wm−1K−1 at T = 300 K. Comparing this result with the one reported for the bulk CdO, it was revealed that the lattice thermal conductivity of the monolayer is lower than the bulk CdO. The lower lattice thermal conductivity of the CdO monolayer was attributed to the lower phonon lifetime and phonon group velocity in comparison to the bulk counterpart. Considering the lower lattice thermal conductivity of the monolayer, higher thermoelectric efficiency was estimated in comparison with the reported efficiency for the bulk CdO. At T = 900 K, the maximum zT = 0.76 can be found in p-type for the hole concentrations of 7.2 × 1012 cm−2.

Structural Properties of Graphene

Graphene, a flat monolayer of carbon atoms tightly packed into a two-dimensional (2D) honeycomb lattice with six carbon atoms, are of two types due to their bonding. Due to its structural uniqueness, properties of the grapheme is found to be very different and important in application point of view. Hence, the authors have studied the structural properties as co-ordination number, coordinates, distances of nearest atoms from 1st to 5th, analytically using two dimensional coordinate system taking one atom of a unit cell at the origin. Furthermore, packing fraction is also calculated, as the property is very important to study absorption/adsorption of different atoms into the sheet of it.

Anomalous Nernst effect in honeycomb and kagome magnet LaCo5 at room temperature

Magnetic order-induced time reversal symmetry breaking in the kagome lattice is predicted to generate Weyl semimetal and Chern insulating phases. The two-dimensional (2D) honeycomb lattices with spin orbit coupling (SOC) and ferromagnetism are also known to host the Chern insulating phase. Here, we study the transport properties of the ferromagnetic topological material LaCo5 crystals with TC = 840 K, which is the highest among the reported magnetic topological materials up to now. It is composed of 2D Co2 kagome layers that are separated by La–Co1 layers, in which the Co1 atoms exhibit a honeycomb structure. Both the 2D kagome and honeycomb layers exhibit out-of-plane magnetization. The large intrinsic anomalous Hall effect and anomalous Nernst effect of about 4.6 μVK−1 are observed in the ab plane with the B//c configuration at room temperature, which can be attributed to non-zero Berry curvature according to the first-principles calculations. Our results enrich the magnetic topological materials with honeycomb and kagome lattices, providing a good platform for the further study of the QAHE. The excellent thermoelectric performance, high TC, nontoxicity, and low cost also make LaCo5 a promising candidate for the applications of thermoelectric generators and heat flow sensors at high temperatures.

Recent progress on valley polarization and valley-polarized topological states in two-dimensional materials

Valleytronics, using valley degree of freedom to encode, process, and store information, may find practical applications in low-power-consumption devices. Recent theoretical and experimental studies have demonstrated that two-dimensional (2D) honeycomb lattice systems with inversion symmetry breaking, such as transition-metal dichalcogenides (TMDs), are ideal candidates for realizing valley polarization. In addition to the optical field, lifting the valley degeneracy of TMDs by introducing magnetism is an efficient way to manipulate the valley degree of freedom. In this paper, we first review the recent progress on valley polarization in various TMD-based systems, including magnetically doped TMDs, intrinsic TMDs with both inversion and time-reversal symmetry broken, and magnetic TMD heterostructures. When topologically nontrivial bands are empowered into valley-polarized systems, valley-polarized topological states, namely valley-polarized quantum anomalous Hall effect can be realized. Therefore, we have also reviewed the theoretical proposals for realizing valley-polarized topological states in 2D honeycomb lattices. Our paper can help readers quickly grasp the latest research developments in this field.

Visualizing symmetry-breaking electronic orders in epitaxial Kagome magnet FeSn films

Kagome lattice hosts a plethora of quantum states arising from the interplay of topology, spin-orbit coupling, and electron correlations. Here, we report symmetry-breaking electronic orders tunable by an applied magnetic field in a model Kagome magnet FeSn consisting of alternating stacks of two-dimensional Fe3Sn Kagome and Sn2 honeycomb layers. On the Fe3Sn layer terminated FeSn thin films epitaxially grown on SrTiO3(111) substrates, we observe trimerization of the Kagome lattice using scanning tunneling microscopy/spectroscopy, breaking its six-fold rotational symmetry while preserving the translational symmetry. Such a trimerized Kagome lattice shows an energy-dependent contrast reversal in dI/dV maps, which is significantly enhanced by bound states induced by Sn vacancy defects. This trimerized Kagome lattice also exhibits stripe modulations that are energy-dependent and tunable by an applied in-plane magnetic field, indicating symmetry-breaking nematicity from the entangled magnetic and charge degrees of freedom in antiferromagnet FeSn.

Two-Dimensional Silver-Chalcogenolate-Based Cluster-Assembled Material: A p-type Semiconductor.

We have synthesized and characterized a new two-dimensional honeycomb architecture resembling a single-layer of atomically precise silver cluster-assembled material (CAM), [Ag12(StBu)6(CF3COO)6(4,4'-azopyridine)3] (Ag12-azo-bpy). The interlayer noncovalent van der Waals interactions within the single-crystals were successfully disrupted, leading to the creation of this unique structure. The optimized Ag12-azo-bpy CAM demonstrates a valence band that is localized on the Ag12 cluster node situated near the Fermi energy level. This localization induces electron injection from the linker to the cluster node, facilitating efficient charge transportation along the plane. Exploiting this single-layer structure as a distinctive platform for p-type channel material, it was employed in a field-effect transistor configuration. Remarkably, the transistor exhibits a high hole mobility of 1.215 cm2 V-1 s-1 and an impressive ON/OFF current ratio of ∼4500 at room-temperature.

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.

Effects of intrinsic decoherence on quantum coherence and correlations between spins within a two-dimensional honeycomb lattice graphene layer system

This research delves into the influence of intrinsic decoherence on the behavior of quantum correlations and coherence between two interacting qubits in a graphene-based system. To evaluate the amount of nonclassical correlations in the system, we employ local quantum uncertainty (LQU), and to assess quantum coherence we use the relative entropy of coherence [Formula: see text] and [Formula: see text]-norm [Formula: see text]. We assume that the system is initially prepared in an extended-Werner-like (EWL) state, and we investigate how these quantifiers evolve over time and examine their sensitivity to various graphene layer system parameters, mixture parameter of the initial state and the intrinsic decoherence rate. Our results indicate that by adjusting the wave number operators, decreasing the intrinsic decoherence rate, and increasing the initial state mixing parameter, it is possible to enhance both quantum correlations and coherence within the two-dimensional honeycomb lattice system. In addition, we found that quantum coherence is more resilient to intrinsic decoherence than LQU, moreover, the [Formula: see text]-norm is more robust than the relative entropy of coherence.

The Ground State of Epitaxial Germanene on Ag(111).

Two-dimensional (2D) honeycomb lattices beyond graphene, such as germanene, appear very promising due to their outstanding electronic properties, such as the quantum spin Hall effects. While there have been many claims of germanene monolayers up to now, no experimental evidence of a honeycomb structure has been provided up to now for these grown monolayers. Using scanning tunneling microscopy (STM), surface X-ray diffraction (SXRD), and density functional theory, we have elucidated the Ge-induced reconstruction on Ag(111). We demonstrate that a powerful algorithm combining SXRD with STM allows us to solve a giant surface reconstruction with more than a hundred atoms per unit cell. Its extensive unit cell indeed consists of 98 2-fold or 3-fold coordinated Ge atoms, forming a periodic arrangement of pentagons, hexagons, and heptagons, with the inclusion of six dispersed Ag atoms. By analogy, we show that the reconstruction obtained by segregation of Ge through an epitaxial Ag/Ge(111) film possesses a similar structure, i.e., Ge pentagons/hexagons/heptagons with a few Ag atoms. Such an organization is more stable than that of pure Ge monolayers and can be assigned to the ground state of epitaxial germanene.