Two-dimensional Crystal Structure Research Articles

Isogeometric topology optimization for maximizing band gap of two-dimensional phononic crystal structures

A smooth and efficient isogeometric topology optimization (ITO) method for maximizing band gaps in two-dimensional phononic crystals is developed in the paper. The band gaps of phononic crystals are computed by the NURBS-based isogeometric analysis, and the material distributions of two-dimensional phononic crystals are represented by the smooth, high-order continuity of the NURBS surface. The densities defined at each control point of the NURBS surface are employed as optimized design variables. The isogeometric analysis optimization using the same spline technique for both geometry and numerical analysis can benefit the optimization procedure and obtain smooth optimized structures, which can be easily used in 3D printing. The maximizing band gaps of phononic crystals for both out-of-plane and in-plane wave modes are obtained here using the present ITO approach. Numerical examples validated the effectiveness and reliability of the ITO method in broadening the band gaps and finding the optimal phononic crystals. And the numerical results show that the smooth optimized structures are obtained with fewer iterations.

Organic-Inorganic Hybrid Cuprous-Based Metal Halides with Unique Two-Dimensional Crystal Structure for White Light-Emitting Diodes.

Ternary cuprous (Cu+)-based metal halides,represented bycesium copper iodide (e.g., CsCu2I3and Cs3Cu2I5),are garnering increasing interest for light-emitting applications owing to their intrinsically high photoluminescence quantum yield and direct bandgap. Toward electrically driven light-emitting diodes (LEDs), it is highly desirable for the light emitters to have a high structural dimensionality as it may favor efficient electrical injection. However, unlike lead-based halide perovskites whose light-emitting units can be facilely arranged in three-dimensional (3D) ways, to date, nearly allternary Cu+-based metal halides crystallize into 0D or 1D networks of Cu-X (X =Cl, Br, I) polyhedra, whereas 3D and even 2D structures remain mostly uncharted. Here, by employing a fluorinated organic cation, we report a new kind of ternary Cu+-based metal halides, (DFPD)CuX2(DFPD+= 4,4-difluoropiperidinium), which exhibitsunique 2D layered crystal structure.Theoretical calculations reveal a highly dispersive conduction band of (DFPD)CuBr2, which is beneficial for charge carrier injection. It is also of particular significance to find that the 2D (DFPD)CuBr2crystals show appealing properties, including improved ambient stability and an efficient warm white-light emission, making it a promising candidate for single-component lighting and display applications.

Research on the effect of two-dimensional perovskite crystal structures on photocatalytic hydrogen production

Here, three kinds of two-dimensional (2D) perovskites, (3AP)PbI4, (4AP)PbI4 and (PEA)2PbI4, were used for photocatalytic hydrogen production. (PEA)2PbI4 produced the most hydrogen, as high as 1.41 mmol h−1 g−1. To further improve the photocatalytic effect, MoS2 was used to combine the crystals. With the assistance of MoS2, the (PEA)2PbI4@MoS2 compound produced more hydrogen than did the pure (PEA)2PbI4, with a production of 2.51 mmol h−1 g−1. The reason is that a high surface photovoltage was established between the surface of the 2D perovskite and MoS2, which promoted charge separation, resulting in high hydrogen production.

Preparation and characterization of Ti3C2 Mxene through back propagation neural network-genetic algorithm combined with surface method response

Mxene's distinctive two-dimensional (2D) layered structure and special surface chemistry make it highly versatile for adsorption, sensors, elecatalysis, and energy storage. However, it has always been a challenge to obtain Mxene with excellent performance. In this study, the process of Ti3C2 Mxene preparation by etching Ti3AlC2 with hydrofluoric acid was optimized using the back-propagation neural network-genetic algorithm (BPNN-GA) and response surface methodology (RSM). Compared to RSM, BPNN-GA predicted better accuracy for preparation conditions with higher values for R2 (0.9830), MSE (2.6188), RMSE (1.6183), MAE (0.8504), and MAPE (0.0089 %). The BPNN-GA optimized etching values for HF concentration, time, and temperature were 8.81 %, 7.01 h, and 45.07 ℃, respectively. Under these conditions, the Ti3C2 Mxene prepared achieved 99.39 % removal efficiency of methylene blue (MB). The preparation was validated by XRD, SEM, TEM, XPS, Zeta, and FTIR analyses, indicating Ti3C2 Mxene to be a two-dimensional crystal structure with a multilayer arrangement and surface termination groups. Zeta measurements indicated that its surface carried negative charges across a wide pH range and its suitability for cationic dye adsorption. Using BPNN-GA provides valuable guidance and a novel strategy for material preparation.

Copper-Based Two-Dimensional Metal-Organic Frameworks for Fenton-like Photocatalytic Degradation of Methylene Blue under UV and Sunlight Irradiation.

To efficiently degrade organic pollutants, photocatalysts must be effective under both ultraviolet (UV) radiation and sunlight. We synthesized a series of new metal-organic frameworks by using mild hydrothermal conditions. These frameworks incorporate three distinct bipyridyl ligands: pyrazine (pyr), 4,4'-bipyridine (bpy), and 1,2-bis(4-pyridyl)ethane (bpe). The resulting compounds are denoted as [Cu(pyz)(H2O)2MF6], [Cu(bpy)2(H2O)2]·MF6, and [Cu(bpe)2(H2O)2]·MF6·H2O [M = Zr (1, 3, and 5) and Hf (2, 4, and 6)]. All six compounds exhibited a two-dimensional crystal structure comprising infinitely nonintersecting linear chains. Compound 3 achieved 100% degradation of methylene blue (MB) after 8 min under UV irradiation and 100 min under natural sunlight in the presence of H2O2 as the electron acceptor. For compound 5, 100% MB degradation was achieved after 120 min under sunlight and 10 min under UV light. Moreover, reactive radical tests revealed that the dominant species involved in photocatalytic degradation are hydroxyl (•OH), superoxide radicals (•O2-), and photogenerated holes (h+). The photodegradation process followed pseudo-first-order kinetics, with photodegradation rate constants of 0.362 min-1 (0.039 min-1) for 3 and 0.316 min-1 (0.033 min-1) for 5 under UV (sunlight) irradiation. The developed photocatalysts with excellent activity and good recyclability are promising green catalysts for degrading organic pollutants during environmental decontamination.

High content pyridine nitrogen-doped carbon nanosheets derived from ZIF-L as anode materials of lithium-ion batteries with excellent capacity and rate performance

The precise control of nitrogen doping types in carbon materials is crucial for optimizing energy storage devices. In this study, we synthesize nitrogen-doped carbon nanosheets (NCS) using ZIF-L as a precursor. Comparatively, the two-dimensional crystal structure of ZIF-L provides several advantages over ZIF-8 as a precursor in terms of nitrogen doping efficiency, achieving a higher doping level (14.56 %) and an increased ratio of pyridinic nitrogen (53.60 %). Consequently, NCS display a greater capacity for pseudocapacitive lithium storage. Furthermore, by adjusting the carbonization temperature, we precisely tailor the graphite-nitrogen doping level of NCS within the range of 16.80–31.22 %. Remarkably, when employed as an anode material in lithium-ion batteries, NCS-800 demonstrates exceptional cycling stability, with a capacity retention of 1078 mAh g−1 after 500 cycles at a current density of 0.5 A g−1, as well as excellent rate capability (735 mAh g−1 at 5 A g−1). This study not only compares the impact of precursor crystal structure modifications on nitrogen doping and lithium storage performance but also introduces a novel synthesis methodology for highly nitrogen-doped carbon materials, offering new avenues for future research in this field.

Graphene's Potential in Electronic Devices: Characteristics, Applications, Challenges, And Solutions

This comprehensive overview explores the multifaceted world of graphene, a two-dimensional crystal structure composed of a single layer of carbon atoms, and its applications in electronic devices. The discussion begins with an introduction highlighting the remarkable potential of graphene in the field of electronics. Section 2 delves into the basic characteristics of graphene, including its atomic structure, excellent electronic properties, optical and thermal attributes, high conductivity, and mobility, as well as its mechanical strength and flexibility. It also briefly outlines the fundamental methods of graphene preparation. Section 3 delves into the application of graphene in electronic devices, covering topics such as graphene's role in Field Effect Transistors (FETs), including carrier control characteristics and the basic principles of FETs. Additionally, it explores the prospects of graphene in FET technology, its application in flexible electronic devices, and its integration into optoelectronic devices. Section 4 addresses the dilemmas and solutions faced by graphene electronic devices, such as issues related to the preparation process and stability, the impact of defects and impurities, and current solutions to overcome these challenges. Finally, the conclusion underscores the immense potential of graphene in revolutionizing electronic devices while recognizing the ongoing efforts for future innovations in this field.

Prediction of a two-dimensional high Curie temperature Weyl nodal line kagome semimetal.

Kagome lattices may have numerous exotic physical properties, such as stable ferromagnetism and topological states. Herein, combining the particle swarm structure search method with first-principles calculations, we identify a two-dimensional (2D) kagome Mo2Se3 crystal structure with space group P6/mmm. The results show that 2D kagome Mo2Se3 is a 100% spin-polarized topological nodal line semimetal and exhibits excellent ambient stability. The band crossing points form two nodal loops around the high-symmetry points Γ and K. On the other hand, Mo2Se3 shows intrinsic ferromagnetism with a large magnetic moment of 3.05 μB per Mo atom and magnetic anisotropy energy (MAE) of 4.78 meV. Monte Carlo simulations estimate that Mo2Se3 possesses a high Curie temperature of about 673 K. In addition, its ferromagnetic ground state can be well preserved under external strain, and the MAE can be improved by increasing the strain. More importantly, the position of each nodal line can be adjusted to the Fermi level through hole doping. This multifunctional 2D magnetic material that combines spin and topology has great potential in the field of nanoscale spintronic devices.

A review of methods of achieving the optimal band gap in two-dimensional phononic crystal structures

A review of methods of achieving the optimal band gap in two-dimensional phononic crystal structures

Multipole groups and fracton phenomena on arbitrary crystalline lattices

Multipole symmetries are of interest in multiple contexts, from the study of fracton phases, to nonergodic quantum dynamics, to the exploration of new hydrodynamic universality classes. However, prior explorations have focused on continuum systems or hypercubic lattices. In this work, we systematically explore multipole symmetries on arbitrary crystal lattices. We explain how, given a crystal structure (specified by a space group and the occupied Wyckoff positions), one may systematically construct all consistent multipole groups. We focus on two-dimensional crystal structures for simplicity, although our methods are general and extend straightforwardly to three dimensions. We classify the possible multipole groups on all two-dimensional Bravais lattices, and on the Kagome and breathing Kagome crystal structures to illustrate the procedure on general crystal lattices. Using Wyckoff positions, we provide an in-principle classification of all possible multipole groups in any space group. We explain how, given a valid multipole group, one may construct a consistent lattice Hamiltonian and a low-energy field theory. We then explore the physical consequences, beginning by generalizing certain results originally obtained on hypercubic lattices to arbitrary crystal structures. Next, we identify two apparently novel phenomena: an emergent, robust subsystem symmetry on the triangular lattice, and an exact multipolar symmetry on the breathing Kagome lattice that does not include conservation of charge (monopole), but instead conserves a vector charge. This makes clear that there is new physics to be found by exploring the consequences of multipolar symmetries on arbitrary lattices, and this work provides the map for the exploration thereof, as well as guiding the search for emergent multipolar symmetries and the attendant exotic phenomena in real materials based on nonhypercubic lattices.

Triptycene Based 3D Covalent Organic Frameworks (COFs)—An Emerging Class of 3D Structures

Covalent Organic Frameworks (COFs) are a newly emerged class of porous materials consisting of organic building blocks linked by strong covalent bonds. The physical and chemical properties of COFs, i.e., modularity, porosity, well-developed specific surface area, crystallinity, and chemical-thermal stability, make them a good application material, especially in the aspects of adsorption and gas separation. The organic compositions of their building blocks also render them with biocompatible properties; therefore, they also have potential in biomedical applications. Depending on the symmetry of the building blocks, COF materials form two-dimensional (2D COF) or three-dimensional (3D COF) crystal structures. 3D COF structures have a higher specific surface area, they are much lighter due to their low density, and they have a larger volume than 2D COF crystals, but, unlike the latter, 3D COF crystals are less frequently obtained and studied. Selecting and obtaining suitable building blocks to form a stable 3D COF crystal structure is challenging and therefore of interest to the chemical community. Triptycene, due to its 3D structure, is a versatile building block for the synthesis of 3D COFs. Polymeric materials containing triptycene fragments show good thermal stability parameters and have a very well-developed surface area. They often tend to be characterized by more than one type of porosity and exhibit impressive gas adsorption properties. The introduction of a triptycene backbone into the structure of 3D COFs is a relatively new procedure, the results of which only began to be published in 2020. Triptycene-based 3D COFs show interesting physicochemical properties, i.e., high physical stability and high specific surface area. In addition, they have variable porosities with different pore diameters, capable of adsorbing both gases and large biological molecules. These promising parameters, guaranteed by the addition of a triptycene backbone to the 3D structure of COFs, may create new opportunities for the application of such materials in many industrial and biomedical areas. This review aims to draw attention to the symmetry of the building blocks used for COF synthesis. In particular, we discussed triptycene as a building block for the synthesis of 3D COFs and we present the latest results in this area.

Spin Coating of Silica Nanocolloids on Mica: Self-Assembly of Two-Dimensional Colloid Crystal Structures and Thin Films

The viability of spin-coating methods for the self-assembly of 150 nm diameter silica nanocolloids into large crystal structures on mica was investigated using different colloidal concentrations, accelerations, and rotational speeds. The samples were imaged by atomic force microscopy (AFM) in intermittent contact mode. Low colloidal concentration led to a size-dependent ordering configuration. The largest nanocolloidal particles formed crystalline close-packed structures that were surrounded by increasingly smaller nanocolloids configured into more polycrystalline or amorphous formations. This phenomenon became increasingly suppressed by increasing colloidal concentration. Two dimensional-fast Fourier transform (2D-FFT) radially averaged profiles of the topography images revealed increasing interparticle spacing with increasing rotational acceleration, from close-packed structuring at low accelerations to increasingly spaced packing at high acceleration (>800 rpm/s). This behaviour is attributed to rapid liquid shedding from the increased acceleration. Analysis with radial distribution functions quantified the extent of ordering and revealed an optimum spin speed that caused the formation of large, highly crystalline structures. This optimum spin speed is governed by the relationship between the rotational speed and the liquid film thickness that affect the uniformity of the film and the magnitude of the capillary forces generated.

4,5-Bisnitratomethyl and Similar Representatives of the Rare Class of 2-Nitro-1,2,3-triazoles.

A [3+2]-cycloaddition toward bishydroxymethyl-1,2,3-triazole makes the title compound available through selective nitration. The obtained sensitive 2-nitrotriazole was shown to have a high density of 1.764 g cm-3 and a detonation velocity of 8590 m s-1. It can also be classified as an oxidizer with an oxygen balance toward CO of 12%. Further representatives of this rare class of 2-nitro-1,2,3-triazoles were synthesized. One- and two-dimensional 15N NMR spectroscopy, crystal structure, and elemental analysis were performed.

Continuous chiral distances for two-dimensional lattices.

Chirality was traditionally considered a binary property of periodic lattices and crystals. However, the classes of two-dimensional lattices modulo rigid motion form a continuous space, which was recently parametrized by three geographic-style coordinates. The four non-oblique Bravais classes of two-dimensional lattices form low-dimensional singular subspaces in the full continuous space. Now, the deviations of a lattice from its higher symmetry neighbors can be continuously quantified by real-valued distances satisfying metric axioms. This article analyzes these and newer G-chiral distances for millions of two-dimensional lattices that are extracted from thousands of available two-dimensional materials and real crystal structures in the Cambridge Structural Database.

Epitaxial Growth of GaN/AlN on h-BN/Si(111) by Metal–Organic Chemical Vapor Deposition: An Interface Analysis

Among III-nitride semiconductors, hexagonal boron nitride (h-BN) has a two-dimensional crystal structure and can be used as an insulator for nanotransistors or a substrate release layer for flexible electronics, which heavily rely on its interface properties. In this study, h-BN films were successfully grown on 150 mm Si(111) substrates by metal–organic chemical vapor deposition using AlN as a nucleation layer, and van der Waals epitaxy of GaN on the h-BN films was investigated, including the challenges of exfoliation of epilayers. As a result of the accumulated stress in GaN films during growth, self-exfoliation of GaN films was observed on GaN of thickness 1 μm and more. This issue was resolved by decreasing the h-BN thickness from 8 to 3 nm and lower, which results in improved crystal quality as evidenced by the smaller linewidth of the (0002) GaN X-ray rocking curves. Based on the exfoliation behavior of a series of GaN grown with different thicknesses, a self-exfoliation mechanism was proposed and an interface binding energy of 54 meV was estimated for the h-BN/AlN interface.

Large-scale piezoelectric ultrasonic transducers with tubular near-period phononic crystal point defect structure

<sec>The coupling vibration of large-scale piezoelectric ultrasonic transducer will make the average value of the longitudinal displacement amplitude of its radiation surface small and the amplitude distribution uneven, which seriously affects the performance and reliability of the system. In order to improve the performance of large-scale ultrasonic vibration system, a two-dimensional hole/slot near-periodic phononic crystal structure is used to suppress the transverse vibration, but the structure will in turn affect the mechanical strength of the transducer while achieving the suppression of the transverse vibration. The working bandwidth and other performance parameters have adverse effects. Based on this, a new idea of optimizing the large-scale sandwich longitudinal vibration piezoelectric ceramic transducer by using the tubular near-periodic phononic crystal point defect structure is proposed. This method can not only use the point defect mode of the constructed solid/gas two-dimensional near-periodic phononic crystal structure to obtain extremely low energy loss, but also effectively improve the longitudinal displacement amplitude and amplitude distribution uniformity of the radiation surface of the system. The double annular holes in the pipe string structure can also be used to enhance the multiple scattering of sound waves, so that the transducer can also produce a band gap under the low conditions of the pipe string, effectively suppressing the transverse vibration, at the same time, significantly broadening the working bandwidth of the transducer system, enhancing the stability and mechanical strength of the system, and reducing the processing cost. Simulation results and experimental processing test results also prove the effectiveness of the optimization.</sec><sec>In order to find the best parameters for the performance of the large-scale longitudinal vibration piezoelectric ultrasonic transducer, in the paper the finite element analysis software is used to study the influence of the inner radius <i>r</i><sub>1</sub> of the pipe string, the width <i>r</i> of the pipe string ring, the radius <i>R</i> of the outermost air cylinder hole, and the height <i>h</i><sub>2</sub> of the pipe string at the longitudinal resonance frequency of the transducer performance, the longitudinal displacement amplitude distribution uniformity of the radiation surface, and the average longitudinal displacement amplitude. In the research is finally found the range of parameters that can make the performance of the transducer reach a relatively ideal state. The simulation results show that the tubular near-periodic phononic crystal point defect structure can improve the performance of large-scale longitudinal vibration piezoelectric ultrasonic transducer.</sec>

Efficient preparation and characterization of graphene based on ball milling

Graphene has a unique two-dimensional crystal structure and excellent physical and chemical properties. It is one of the new strategic materials developed by countries around the world in the 21st century. In recent years, the method of preparing graphene by mechanical stripping is becoming mature, including oxidation reduction method, mechanical stripping method, SiC epitaxial growth method, chemical vapor deposition method, etc. Based on the ball milling method, graphene is prepared in this study using a uniform design test method, scaled graphite as the raw material, and metallic aluminum as the grinding aid while considering the influence of ball ratio, grinding time, and rotation rate. A mathematical model was established based on partial least squares regression analysis and stepwise regression analysis. The best experimental scheme was obtained through the selection and optimization of the mathematical model, and the process of efficiently preparing graphene in the laboratory was explored. The prepared products were characterized by X-ray diffractometer, automatic specific surface area and porosity analyzer, and laser confocal Raman spectrometer. The results indicate that using aluminum powder as a grinding aid can significantly improve the efficiency of ball milling, the specific surface area of graphene produced is 542.598 m2/g, with the number of layers ranging from 2 to 5 layers. It is proved that the proposed graphene preparation method can obtain graphene with better quality. The study's findings serve as a guide for the production of graphene.

Simultaneous exfoliation and functionalization of black phosphorus by sucrose-assisted ball milling with NMP intercalating and preparation of flame retardant polyvinyl alcohol film

Recently, layered black phosphorus (BP) has become a rising star in two-dimensional materials because of high thermal stability, excellent mechanical properties, and size effect. Layered BP can be used as an ideal filler for the preparation of polymer-based composites with high performance. Realizing the exfoliation and functionalization of BP has become the key to BP's application. Under the protection of N-methylpyrrolidone (NMP) and the assistance of sucrose, we completed the preparation and functionalization of layered BP by ball milling. Surface grafting of oxygen-containing functional groups can realize the uniform dispersion of BP in matrix and improve its stability in air. As expected, the nanocomposites prepared by adding sucrose grafted BP into polyvinyl alcohol (PVA) matrix have significantly improved flame retardancy and mechanical strength. When the addition amount of BP-sucrose(N) was 5 wt%, compared with pure PVA, the decrease of the peak heat release rate (PHRR) and total heat release (THR) reached 52.5% and 32.8%, and the tensile strength increased by 131.2%. Moreover, the addition of NMP can avoid the excessive destruction of the two-dimensional crystal structure of BP, which can also improve the performance of nanocomposites. Due to the encapsulation effect of sucrose, the nanocomposites have excellent air stability. The Ag1/Ag2 value only decreases from 0.52 to 0.49 even after three months of exposure to environmental conditions, which proves that the degree of oxidation is low. This work has provided an efficient way to exfoliate and functionalize BP simultaneously, and expanded the application of BP in flame retardant nanocomposites.

New 2D Material: Two-Dimensional Black Phosphorus (2D BP)

In this article, I want to show some properties of black phosphorus (BP) and some of its applications. In particular, in Sec. 1, I give an introduction to the topic and some historical notes, in Sec. 2, the two-dimensional crystal structure of the BP is explained, in Sec. 3, the optical and electronic properties of the BP are shown, in Sec. 4, the biomedical applications of the BP are listed and finally in Sec. 5, there are conclusions.

Acousto-optical coupling of the side surface acoustic and optical waves in a two-dimensional phoxonic crystal slab hetero-structure cavity

Phoxonic crystal is a kind of artificial periodic structure which can manipulate light and sound in the same time and space domain. It has good application prospect in optical communication system and opto-mechnical sensor. In this paper, a two-dimensional silicon phoxonic crystal slab structure is proposed, which can exhibit large phononic and photonic bandgaps simultaneously. Then a periodic cylindrical structure is introduced on the side surface of the slab. The effects of the height and radius of the cylinder on the side surface wave modes are analyzed. On this basis, a phoxonic crystal hetero-structure cavity composed of cylinders with different radii and heights is constructed, which can simultaneously confine the acoustic and optical modes because of mode gap effect. Seven phononic cavity modes and two photonic cavity modes are obtained. The two photonic cavity modes are called dielectric mode and air mode respectively. Further, the acousto-optical coupling rates between the seven phononic cavity modes and dielectric mode are calculated. Finally, a gap is etched on each cylinder in hetero-structure cavity, which can increase the surface area of the cavity by 23%. Then two phononic cavity modes and photonic dielectric mode with higher coupling rates are considered. The results show that etching the z-direction gap in the center of the cylinder can effectively increase the acousto-optical coupling rates.