2D Layer Research Articles

Enhanced reversible reaction of hexagonal SnS2@rGO as anode materials for lithium-ion batteries: Analysis of the morphological change mechanism

Tin-based materials have attracted attention as a promising anode material for lithium-ion batteries due to their high theoretical capacity and alloy metal characteristics. However, the structure tends to easily collapse with repetitive charging and discharging processes, leading to volume expansion. The conversion step in the SnS2 reaction mechanism is also sometimes considered irreversible. To address these challenges, this study synthesized layered hexagonal 2D structure-type SnS2 using the oleylamine as surfactant through a wet chemical process. Subsequently, SnS2@rGO was synthesized after compounding with reduced graphene oxide (rGO), utilizing a smaller amount of nanomaterials compared to rGO, showing optimal efficiency. The morphological changes in SnS2 and SnS2@rGO during the charging and discharging processes was revealed that pulverization occurred more slowly in SnS2@rGO compared to SnS2. We quantified the volume expansion rates of SnS2 and SnS2@rGO electrodes, demonstrating that rGO effectively reduces volume expansion. Additionally, an analysis of lithiation mechanism through a comprehensive analysis method using CV, ex-situ XRD, and ex-situ TEM demonstrated that SnS2@rGO enhanced Li+ storage characteristics compared to SnS2, resulting in more reversible chemical changes. rGO and bare SnS2 show poor capacities 307.23 mAh/g and 87.17 mAh/g after 120 cycles, respectively. However bare SnS2 anchored on the surface of rGO exhibits increased electrical conductivity and improved performance (711 mAh/g after 120 cycles), confirming that rGO plays an important role. SnS2@rGO showed enhanced cycling stability and discharge capacity as shown in the electrochemical testing.

Covalent Bonding of MXene/COF Heterojunction for Ultralong Cycling Li-Ion Battery Electrodes.

Covalent organic frameworks (COFs) have emerged as promising renewable electrode materials for LIBs and gained significant attention, but their capacity has been limited by the densely packed 2D layer structures, low active site availability, and poor electronic conductivity. Combining COFs with high-conductivity MXenes is an effective strategy to enhance their electrochemical performance. Nevertheless, simply gluing them without conformal growth and covalent linkage restricts the number of redox-active sites and the structural stability of the composite. Therefore, in this study, a covalently assembled 3D COF on Ti3C2 MXenes (Ti3C2@COF) is synthesized and serves as an ultralong cycling electrode material for LIBs. Due to the covalent bonding between the COF and Ti3C2, the Ti3C2@COF composite exhibits excellent stability, good conductivity, and a unique 3D cavity structure that enables stable Li+ storage and rapid ion transport. As a result, the Ti3C2-supported 3D COF nanosheets deliver a high specific capacity of 490 mAh g-1 at 0.1 A g-1, along with an ultralong cyclability of 10,000 cycles at 1 A g-1. This work may inspire a wide range of 3D COF designs for high-performance electrode materials.

2D hybrid organic-inorganic perovskite displaying narrow-band violet-blue photoluminescence

Hybrid organic-inorganic perovskites represent a drastically expanding class of solid-state semiconducting materials for optoelectronics and photovoltaics. Thanks to the easy chemical tunability, this class of materials can offer emitters of very wide spectral range, however, violet-blue emitting hybrid perovskites still remain underrepresented. In this paper we report on a novel two-dimensional hybrid organic-inorganic perovskite (ethylammonium)2CdBr4. Crystal structure of the reported compound is composed of infinite inorganic 2D layers that are created by [CdBr6]4- octahedra connected in corner-by-corner manner. These layers are separated by organic ethyammonium cations which create hydrogen bonds with the inorganic part. Optical absorption measurements revealed the presence of a large band gap of 4.25 eV. Importantly, this material displays narrow photoluminescence band at 388 nm (full width at half maximum is 32 nm). Chromaticity coordinates of the perovskite emission are close to NTSC blue standard. Band structure and DOS calculations show that the states close to Fermi level predominantly originate from cadmium and bromide. Thus, here we offer a facile way towards violet-blue luminophore containing simple and widely available organic cation ethylammonium.

New robust 2D cobalt (II) coordination polymer as an effective turn-on luminescent sensor to detect Tb3+ and pefloxacin

It is necessary to employ highly sensitive and rapid detection of antibiotics and heavy ions in the environment. A mixed-ligands ternary Co (II) coordination polymer containing new L ligands, {[Co(L)(mip)(H2O)]·2H2O}n (1), (L = 2,2′-bis-(2-pyridin-3-yl-vinyl)-3H,3′H-[5,5]bibenzoimidazole, H2mip= 5-methylisophthalic acid) was synthesized under solvothermal condition. Compound 1 displays a sql 2D layer with the point symbol of {44.62}. Compound 1 owns excellent thermal and pH stability and can effectively sensitize the luminescence of Tb3+ ions through the “antenna effect”. Compound 1 can serve as luminescent probe for detecting pefloxacin and Tb3+ with low detection limits. Especially Compound 1 is the first turn-on sensor for the detecting Tb3+ ions and pefloxacin via fluorescence enhancement. In addition, the sensing mechanism is discussed in detail.

Boosting Selective Nitrogen Oxidation to Nitric Acid by Synergizing Cobalt Phthalocyanine on Carbon Nitride Surface

AbstractThe Ostwald process, which is producing HNO3 for commercial use, involves the catalytic oxidation of NH3 and a series of chemical reactions conducted under severe operating conditions. Due to their energy‐intensive nature, these activities play a major role in greenhouse gas emissions and global energy consumption. In response to the urgent requirements of the global energy and environmental sectors, there is an increasingly critical need to develop novel, highly efficient, and environmentally sustainable methods. Herein, CoPc/C3N4 electrocatalyst, integrating CoPc nanotubes with C3N4 nanosheets, is shown. The CoPc/C3N4 electrocatalyst demonstrates yield rate of 871.8 µmol h−1 gcat−1 at 2.2 V, with corresponding Faradaic efficiency (FE) of 46.4% at 2.1 V, which notably surpasses that of CoPc. Through a combination of experimental investigations and density functional theory (DFT) calculations, this study shows that CoPc anchored on C3N4 effectively simplifies the adsorption and activation of chemically inactive nitrogen molecules. The improved catalytic activity for composite system may be the reason of re‐distribution of charges over the CoPc, tuning the valence orbital of Co center due to the presence of 2D layer of C3N4. This mechanism significantly lowers the energy barrier required for critical breaking of inert N2, ultimately leading to a significant improvement in N2 oxidation efficiency.

Controlled dion-jacobson low-dimensional surface phase enables highly efficient and stable perovskite solar cells

The 2D/3D heterojunction structure emerges as a viable approach for enhancing the efficiency and stability of perovskite photovoltaics. However, the formation of an accumulative low-dimensional 2D perovskite (n=1) cladding layer often impedes carrier transport due to the insulating nature and high quantum confinement, and there is a paucity of detailed understanding regarding its surface phase control. This study introduces a Dion-Jacobson (DJ) phase 2D perovskite, employing decane-1,10-diammonium diiodide (DDADI) to interface with 3D perovskite, leveraging long-chain diammonium cations for structural stability and defect passivation on the 3D FAPbI3 perovskite surface. In addition, a novel PbI2-assisted phase control (PAPC) technique is proposed to mitigate the quantum confinement effects of the 2D layer, especially reducing the formation of the highly confined insulating n=1 phase. X-ray scattering analysis confirms the method's efficacy in promoting the formation of an n=2 phase, facilitating cascading HOMO levels and improving hole carrier transport. The optimized 2D/3D perovskite solar cell (PSC) achieve an exemplary efficiency of 25.16 %, with a notable open-circuit voltage of 1.192 V, and retain 92.9 % of its initial efficiency after 1000 hours in a nitrogen atmosphere, signifying a strategic advancement in 2D/3D PSC construction.

Rapid Modeling of Photopolymerization in Projection Two-Photon Lithography via an Operator Splitting Finite Difference Method

Abstract Two-photon lithography (TPL) is an attractive technique for nanoscale additive manufacturing of functional 3D structures due to its ability to print sub-diffraction features with light. Despite its advantages, it has not been widely adopted due to its slow point-by-point writing mechanism. Projection TPL (P-TPL) is a high-throughput variant that overcomes this limitation by enabling the printing of entire 2D layers at once. However, printing the desired 3D structures is challenging due to the lack of fast and accurate process models. Here, we present a fast and accurate physics-based model of P-TPL to predict the printed geometry and the degree of curing. Our model implements a finite difference method enabled by operator splitting to solve the reaction-diffusion rate equations that govern photopolymerization. When compared with finite element simulations, our model is at least a hundred times faster and its predictions lie within 5% of the predictions of the finite element simulations. This rapid modeling capability enabled performing high-fidelity simulations of printing of arbitrarily complex 3D structures for the first time. We demonstrate how these 3D simulations can predict those aspects of the 3D printing behavior that cannot be captured by simulating the printing of individual 2D layers. Thus, our models provide a resource-efficient and knowledge-based predictive capability that can significantly reduce the need for guesswork-based iterations during process planning and optimization.

Dimensional Regulation in Metal‐Free Perovskites by Compositional Engineering to Achieve Record Low X‐Ray Detection Limits

AbstractUtilizing the manipulation of perovskite dimensions has been proven as an effective approach in regulating perovskite properties. Nevertheless, achieving precise control over the dimensions of perovskites within the same system poses a significant challenge. In this study, we introduce a sophisticated method to attain precise dimensional control in metal‐free perovskites (MFPs), specifically through the process of octahedron tailoring by compositional engineering. Accordingly, we successfully instigated a transition from HPIP−NH4I3 ⋅ H2O (3D), HPIP2−NH4I5 (2D) and HPIP3−NH4I7 (1D) structures. Notably, HPIP2−NH4I5 is the first 2D MFP. As anticipated, these perovskites exhibited completely distinct fluorescence and X‐ray detection capabilities due to their differing dimensions. Remarkably, the 2D HPIP2−NH4I5 device effectively hindered ion migration perpendicular to the 2D layers, achieving the lowest detection limit of 12.2 nGyair s−1 among metal‐free single crystals‐based detectors. This study expands the dimensionality control strategies for MFPs and introduces, for the first time, the potential of 2D MFPs as high‐performance X‐ray detectors, thereby enriching the diversity of the MFPs family.

Dimensional Regulation in Metal-Free Perovskites by Compositional Engineering to Achieve Record Low X-Ray Detection Limits.

Utilizing the manipulation of perovskite dimensions has been proven as an effective approach in regulating perovskite properties. Nevertheless, achieving precise control over the dimensions of perovskites within the same system poses a significant challenge. In this study, we introduce a sophisticated method to attain precise dimensional control in metal-free perovskites (MFPs), specifically through the process of octahedron tailoring by compositional engineering. Accordingly, we successfully instigated a transition from HPIP-NH4I3 ⋅ H2O (3D), HPIP2-NH4I5 (2D) and HPIP3-NH4I7 (1D) structures. Notably, HPIP2-NH4I5 is the first 2D MFP. As anticipated, these perovskites exhibited completely distinct fluorescence and X-ray detection capabilities due to their differing dimensions. Remarkably, the 2D HPIP2-NH4I5 device effectively hindered ion migration perpendicular to the 2D layers, achieving the lowest detection limit of 12.2 nGyair s-1 among metal-free single crystals-based detectors. This study expands the dimensionality control strategies for MFPs and introduces, for the first time, the potential of 2D MFPs as high-performance X-ray detectors, thereby enriching the diversity of the MFPs family.

2D organic-inorganic lead perovskite: Advancing X-ray detection capability

Two-dimensional (2D) perovskite single crystals have risen to be prominent in the domain of optoelectronic materials due to their exceptional characteristics including notably low ion migration and defect density. A distinguishing feature is the strategic inclusion of larger organic amine spacer, alternately inserted between inorganic lead halide octahedral layers. This spacer effectively mitigates ion migration, even under higher bias voltages, which is particularly advantageous for X-ray detection. However, the simultaneous realization of multiple functions within this realm remains a formidable challenge. In this work, we have synthesized a high-quality organic spacer-incorporated single layered 2D perovskite single crystal of [CYP]PbBr4 (CYP = 1-cyclohexylpiperazine). Our findings reveal the impressive potential of [CYP]PbBr4 in the domain of X-ray detection, as evidenced by the ultrahigh X-ray sensitivity, long-term stability, and astonishingly low detection limit of 1.72 nGyairs-1. Notably, [CYP]PbBr4 also emits bright broadband yellow light with quantum yield of 30.3%, and the fabricated white LED achieves a high Color Rendering Index of 94.1, which surpasses that of commercially available white LED based on YAG: Ce3+ (< 80).

Stable, porous, light-emitting post-modification covalent organic frameworks conjugated molecularly imprinted polymers for selective detection of pyrraline in salami products

Molecular imprinting is one of the most frequently occurring post-modification in the preparation of covalent organic frameworks (COFs) to enhance selectivity and specificity. In this study, we prepared a 2D layer structure of methoxy-conjugated COFs with the modification of azide (4-azido-L-phenylalanine), named [4-ALP]0.17-COFs, which exhibited a large specific surface area of 827.6 m2/g, good stability of water, polar solvents, chemistry, and thermodynamics. Fluorescent COF nanosheets ([4-ALP]0.17-CONs) obtained by liquid-assisted ultrasonic stripping have excellent blue luminescence properties and ultra-high absolute fluorescence quantum yield of 33.34 %. The modifiable functional groups in the surface of [4-ALP]0.17-CONs interacted with the targets and functional monomers of molecularly imprinted polymers (MIPs) through hydrogen bonding interactions, to form the 3D holes with recognition sites. The quantitative detection of pyrraline (PRL) could be achieved in the concentration range of 0.05–4 μg/L with the LOD was 34.81 ng/L. The spiked recovery of PRL in meat products was 88.01–106.00 %. The [4-ALP]0.17-CONs@MIPs sensing system showed excellent stability, reliability, reusability, and practicability, promising its potential for targeted monitoring of trace molecules in real matrices.

Construction of coordination polymer [Ni(ppda) (tib) (H2O)]·H2O and photocatalytic degradation of indigo carmine

Semiconductor coordination polymers have garnered significant attention as photocatalysts for wastewater treatment owing to their adaptable structure, expansive specific surface area, and outstanding selectivity, stability, and sensitivity. In this study, coordination polymer [Ni(ppda) (tib) (H2O)]·H2O was synthesized utilizing flexible phenyldiacetic acid (H2ppda) and rigid 1,3,5-tris(1-imidazolyl)benzene (tib). Crystal structure analysis revealed that the three-linked tib ligands and the central Ni(II) ion create a wavy 2D surface. Additionally, cis-ppda2- ligands form a 1D wave chain alongside central Ni(II), which traverse through a 2D layer. Adjacent layers are interlaced via ppda2−, resulting in a complex 3D supramolecular structure. The fundamental framework of [Ni(ppda) (tib) (H2O)]·H2O exhibits stability up to 316 °C and remains stable over 5 cycles, with a band gap energy of 2.9 eV, rendering it a viable candidate for catalytic applications. In aqueous systems, [Ni(ppda) (tib) (H2O)]·H2O demonstrates efficient photocatalytic degradation of Indigo Carmine (IC), achieving a degradation rate of 98.5% and a reaction rate of 0.027 min−1, showcasing commendable selectivity and semiconductor properties. The primary photocatalytic degradation mechanism involves the synergistic formation of ·OH and ·O2- active species through the interaction between [Ni(ppda) (tib) (H2O)]·H2O and H2O2, facilitating the oxidative degradation of dye molecules.

Ultra-flat bands at large twist angles in group-V twisted bilayer materials.

Flat bands in 2D twisted materials are key to the realization of correlation-related exotic phenomena. However, a flat band often was achieved in the large system with a very small twist angle, which enormously increases the computational and experimental complexity. In this work, we proposed group-V twisted bilayer materials, including P, As, and Sb in the β phase with large twist angles. The band structure of twisted bilayer materials up to 2524 atoms has been investigated by a deep learning method DeepH, which significantly reduces the computational time. Our results show that the bandgap and the flat bandwidth of twisted bilayer β-P, β-As, and β-Sb reduce gradually with the decreasing of twist angle, and the ultra-flat band with bandwidth approaching 0eV is achieved. Interestingly, we found that a twist angle of 9.43° is sufficient to achieve the band flatness for β-As comparable to that of twist bilayer graphene at the magic angle of 1.08°. Moreover, we also find that the bandgap reduces with decreasing interlayer distance while the flat band is still preserved, which suggests interlayer distance as an effective routine to tune the bandgap of flat band systems. Our research provides a feasible platform for exploring physical phenomena related to flat bands in twisted layered 2D materials.

Structural Regulations of Layered Borates: From Centric Layers to Chiral Porous Layers.

Three pentaborates were made by precise structural regulations under hydrothermal conditions, namely, K2Cs[B5O8(OH)]·0.5CO3 (1), KNa4Cs[B5O8(OH)]2·2OH (2), and NaBa[B5O8.5(OH)] (3). Compound 1 features the typical 2D layers constructed from the four-connected B5O10(OH) clusters. By adjustment of the reactants and pH values of the systems to remove interlayered CO3 2- groups, the centric layers in 1 were transformed to the chiral layers of 2. By further adjusting cationic templates based on host-guest charge matching, the B5O10(OH) cluster unit was transformed to B5O11(OH), which built the chiral porous layers of 3. The chiral compounds 2 and 3 exhibit moderate second harmonic generation (SHG) responses of 1.1 and 1.7 times that of KDP (KH2PO4). The structural regulations actualize the evolutions on both structural symmetries and dimensions.

(Invited) Bulk Traps in Layered 2D Gate Dielectrics

In this work, we synthesize new 2D layered dielectrics and fabricate metal-insulator-metal (MIM) capacitors to determine their viability for scaled gate dielectrics (ZrNCl, HfNCl, BiOCl, and Mg(OH)2) in transition metal dichalcogenide-based transistors. While successful synthesis and fabrication was demonstrated, the properties of the dielectrics were decidedly underwhelming for device applications. The dielectric constants, in most cases, were only marginally better than SiO2 (k = 4-6), the leakage currents were too high due to poor band offsets, and most importantly, the bulk trap density, as seen previously in III-V devices, was very high. Overall, there still is no viable 2D gate dielectric for scaled field effect transistors.

Integrating spin-dependent emission and dielectric switching in FeII catenated metal-organic frameworks

Mechanically interlocked molecules (MIMs) including famous catenanes show switchable physical properties and attract continuous research interest due to their potential application in molecular devices. The advantages of using spin crossover (SCO) materials here are enormous, allowing for control through diverse stimuli and highly specific functions, and enabling the transfer of the internal dynamics of MIMs from solution to solid state, leading to macroscopic applications. Herein, we report the efficient self-assembly of catenated metal-organic frameworks (termed catena-MOFs) induced by stacking interactions, through the combination of rationally selected flexible and conjugated naphthalene diimide-based bis-pyridyl ligand (BPND), [MI(CN)2]− (M = Ag or Au) and Fe2+ in a one-step strategy. The obtained bimetallic Hofmann-type SCO-MOFs [FeII(BPND){Ag(CN)2}2]·3CHCl3 (1Ag) and [FeII(BPND{Au(CN)2}2]·2CHCl3·2H2O (1Au) possess a unique three-dimensional (3D) catena-MOF constructed from the polycatenation of two-dimensional (2D) layers with hxl topology. Both complexes undergo thermal- and light-induced SCO. Significantly, abnormal increases in the maximum emission intensity and dielectric constant can be detected simultaneously with the switching of spin states. This research opens up SCO-actuated bistable MIMs that afford dual functionality of coupled fluorescence emission and dielectricity.

A Fluorine-Functionalized 3D Covalent Organic Framework with Entangled 2D Layers

A Fluorine-Functionalized 3D Covalent Organic Framework with Entangled 2D Layers

Near-Field Radiative Heat Transfer between Layered β-GeSe Slabs: First-Principles Approach.

The group-IV monochalcogenide monolayers, GeSe, are interesting and novel two-dimensional (2D) semiconductor materials due to their highly anisotropic physical properties. Monolayers of the different GeSe polymorphs have already had their physical properties and potential applications extensively investigated. However, few-layer homostructures, which can also be approximated as 2D systems in many cases, have not received the same attention. For this reason, in this work, we investigate the optical properties of a free-standing few-layer β-GeSe system and use this information to investigate their performance in the near-field radiative heat transfer (NFRHT). The required optical conductivity of the few-layer 2D material is calculated by using density functional theory (DFT), including spin-orbit coupling. The band structure is investigated for up to five layers, and the effective electron masses are calculated correspondingly. Using this information, both the intraband transitions due to the presence of free electrons introduced by doping and the interband transitions are considered. The contribution of the ionic vibrations is also included in calculating the optical properties because of its relevance to NFRHT through the resulting active optical phonons. With all these contributions included, more realistic predictions of the NFRHT between the layered 2D β-GeSe materials can be obtained. It is found that the heat transfer attainable with the layered system is similar to that of a single layer of β-GeSe we have obtained previously.

Gas Adsorption Mechanism on 2D Materials: The Hyperpolarizability Evolution Analyzed by Nonlinear Optics

AbstractWhile understanding the competitive adsorption behavior of gas sensor is important, it is yet to be unraveled. Especially for the influence of water molecules to the gas adsorbed on 2D materials. This study explores the potential of layered 2D materials as a candidate material for gas sensing, employing non‐destructive measurement, and second harmonic generation (SHG). The investigation focuses on analyzing oxygen, ammonia, and water vapor adsorbed on a WS2 surface by studying the evolutions in electric dipole and electric field. Leveraging the simplified bond hyperpolarizability model (SBHM), a foundation is established for gas sensors utilizing high‐quality 2D materials. This approach facilitates the detection of material modifications in response to environmental influences, including the inevitable water molecules. The obtained hyperpolarizability from SBHM exhibits remarkable consistency with Langmuir's adsorption model, confirming the physical adsorption in the system. In addition, the competitive effects between gases are explored by comparing experimental results with theoretical predictions based on Boltzmann distribution and density functional theory (DFT) calculations. This highlights the effectiveness of SHG and SBHM in studying gas adsorption on layered van der Waals materials.

Synthesis, structure and photoluminescence properties of heterometallic-based coordination polymers of trimesic acid.

Reacting trimesic acid (H3TMA, C9H6O6) with CaCl2 and MCl2 at 110 °C under hydrothermal conditions gave the isostructural heterobimetallic coordination polymers (CPs) catena-poly[[tetraaquazinc(II)]-μ-5-carboxybenzene-1,3-dicarboxylato-[tetraaquacalcium(II)]-μ-5-carboxybenzene-1,3-dicarboxylato], [CaZn(HTMA)2(H2O)8]n, 1, and catena-poly[[tetraaquacobalt(II)]-μ-5-carboxybenzene-1,3-dicarboxylato-[tetraaquacalcium(II)]-μ-5-carboxybenzene-1,3-dicarboxylato], [CaCo(HTMA)2(H2O)8]n, 2. Compounds 1 and 2 crystallize in the monoclinic space group C2/c. The solid-state structures consist of eight-coordinate CaII ions and six-coordinate MII ions. These ions are connected by a doubly deprotonated HTMA2- ligand to create a one-dimensional (1D) zigzag chain. Poly[[decaaquabis(μ3-benzene-1,3,5-tricarboxylato)calcium(II)dizinc(II)] dihydrate], {[CaZn2(TMA)2(H2O)10]·2H2O}n, 3, was found incidentally as a minor by-product during the synthesis of 1 at a temperature of 140 °C. It forms crystals in the orthorhombic space group Ccce. The structure of 3 consists of a two-dimensional (2D) layer composed of [Zn(TMA)] chains that are interconnected by CaII ions. The presence of aromatic carboxylic acid ligands and water molecules, which can form numerous hydrogen bonds and π-π interactions, increases the stability of the three-dimensional (3D) supramolecular architecture of these CPs. Compounds 1 and 2 exhibit thermal stability up to 420 °C, as indicated by the thermogravimetric analysis (TGA) curves. The powder X-ray diffraction (PXRD) data reveal the formation of unidentified phases in methanol and dimethyl sulfoxide, while 1 exhibits chemical stability in a wide range of solvents. The luminescence properties of 1 dispersed in various low molecular weight organic solvents was also examined. The results demonstrate excellent selectivity, sensitivity and recyclability for detecting acetone molecules in aqueous media. Additionally, a possible sensing mechanism is also outlined.