Multifunctional Applications Enabled by Fluorination of Hexagonal Boron Nitride.

2D materials exhibit exceptional properties as compared to their macroscopic counterparts, with promising applications in nearly every area of science and technology. To unlock further functionality, the chemical functionalization of 2D structures is a powerful technique that enables tunability and new properties within these materials. Here, the successful effort to chemically functionalize hexagonal boron nitride (hBN), a chemically inert 2D ceramic with weak interlayer forces, using a gas-phase fluorination process is exploited. The fluorine functionalization guides interlayer expansion and increased polar surface charges on the hBN sheets resulting in a number of vastly improved applications. Specifically, the F-hBN exhibits enhanced dispersibility and thermal conductivity at higher temperatures by more than 75% offering exceptional performance as a thermofluid additive. Dispersion of low volumes of F-hBN in lubricating oils also offers marked improvements in lubrication and wear resistance for steel tribological contacts decreasing friction by 31% and wear by 71%. Additionally, incorporating numerous negatively charged fluorine atoms on hBN induces a permanent dipole moment, demonstrating its applicability in microelectronic device applications. The findings suggest that anchoring chemical functionalities to hBN moieties improves a variety of properties for h-BN, making it suitable for numerous other applications such as fillers or reinforcement agents and developing high-performance composite structures.

Tunable 2D Conjugated Porous Organic Polymer Films for Precise Molecular Nanofiltration and Optoelectronics.

Structural design of 2D conjugated porous organic polymer films (2D CPOPs), by tuning linkage chemistries and pore sizes, provides great adaptability for various applications, including membrane separation. Here, four free-standing 2D CPOP films of imine- or hydrazone-linked polymers (ILP/HLP) in combination with benzene (B-ILP/HLP) and triphenylbenzene (TPB-ILP/HLP) aromatic cores are synthesized. The anisotropic disordered films, composed of polymeric layered structures, can be exfoliated into ultrathin 2D-nanosheets with layer-dependent electrical properties. The bulk CPOP films exhibit structure-dependent optical properties, triboelectric nanogenerator output, and robust mechanical properties, rivaling previously reported 2D polymers and porous materials. The exfoliation energies of the 2D CPOPs and their mechanical behavior at the molecular level are investigated using density function theory (DFT) and molecular dynamics (MD) simulations, respectively. Exploiting the structural tunability, the comparative organic solvent nanofiltration (OSN) performance of six membranes having different pore sizes and linkages to yield valuable trends in molecular weight selectivity is investigated. Interestingly, the OSN performances follow the predicted transport modeling values based on theoretical pore size calculations, signifying the existence of permanent porosity in these materials. The membranes exhibit excellent stability in organic solvents at high pressures devoid of any structural deformations, revealing their potential in practical OSN applications.

Imine-Linked 2D Conjugated Porous Organic Polymer Films for Tunable Acid Vapor Sensing.

Two-dimensional (2D) films of conjugated porous organic polymers (C-POPs) can translate the rich in-plane functionalities of conjugated frameworks into diverse optical and electronic applications while addressing the processability issues of their crystalline analogs for adaptable device architectures. However, the lack of facile single-step synthetic routes to obtain large-area high-quality films of 2D-C-POPs has limited their application possibilities so far. Here, we report the synthesis of four mechanically robust imine-linked 2D-C-POP free-standing films using a single-step fast condensation route that is scalable and tunable. The rigid covalently bonded 2D structures of the C-POP films offer high stability for volatile gas sensing in harsh environments while simultaneously enhancing site accessibility for gas molecules due to mesoporosity by structural design. Structurally, all films were composed of exfoliable layers of 2D polymeric nanosheets (NSs) that displayed anisotropy from disordered stacking, evinced by out-of-plane birefringent properties. The tunable in-plane conjugation, different nitrogen centers, and porous structures allow the films to act as ultraresponsive colorimetric sensors for acid sensing via reversible imine bond protonation. All the films could detect hydrogen chloride (HCl) gas down to 0.05 ppm, far exceeding the Occupational Safety and Health Administration's permissible exposure limit of 5 ppm with fast response time and good recyclability. Computational insights elucidated the effect of conjugation and tertiary nitrogen in the structures on the sensitivity and response time of the films. Furthermore, we exploited the exfoliated large 2D NSs and anisotropic optoelectronic properties of the films to adapt them into micro-optical and triboelectric devices to demonstrate their real-time sensing capabilities.

Biotene: Earth-Abundant 2D Material as Sustainable Anode for Li/Na-Ion Battery.

Natural ores are abundant, cost-effective, and environmentally friendly. Ultrathin (2D) layers of a naturally abundant van der Waals mineral, Biotite, have been prepared in bulk via exfoliation. We report here that this 2D Biotene material has shown extraordinary Li-Na-ion battery anode properties with ultralong cycling stability. Biotene shows 302 and 141 mAh g-1 first cycle-specific charge capacity for Li- and Na-ion battery applications with ∼90% initial Coulombic efficiency. The electrode exhibits significantly extended cycling stability with ∼75% capacity retention after 4000 cycles even at higher current densities (500-2000 mA g-1). Further, density functional theory studies show the possible Li intercalation mechanism between the 2D Biotene layers. Our work brings new directions toward designing the next generation of metal-ion battery anodes.

Non-Linear Optics at Twist Interfaces in h-BN/SiC Heterostructures.

Understanding the emergent electronic structure in twisted atomically thin layers has led to the exciting field of twistronics. However, practical applications of such systems are challenging since the specific angular correlations between the layers must be precisely controlled and the layers have to be single crystalline with uniform atomic ordering. Here, an alternative, simple, and scalable approach is suggested, where nanocrystallinetwo-dimensional (2D) film on 3D substrates yields twisted-interface-dependent properties. Ultrawide-bandgap hexagonal boron nitride (h-BN) thin films are directly grown on high in-plane lattice mismatched wide-bandgap silicon carbide (4H-SiC) substrates to explore the twist-dependent structure-property correlations. Concurrently, nanocrystalline h-BN thin film shows strong non-linear second-harmonic generation and ultra-low cross-plane thermal conductivity at room temperature, which are attributed to the twisted domain edges between van der Waals stacked nanocrystals with random in-plane orientations. First-principles calculations based on time-dependent density functional theory manifest strong even-order optical nonlinearity in twisted h-BN layers. This work unveils that directly deposited 2D nanocrystalline thin film on 3D substrates could provide easily accessible twist-interfaces, therefore enabling a simple and scalable approach to utilize the 2D-twistronics integrated in 3D material devices for next-generation nanotechnology.

Heteroatom Functionalization of H-Terminated Diamond Surfaces.

Diamond surface functionalization has received significant research interest recently. Specifically, H-termination has been widely adopted because it endows the diamond surface with negative electron affinity and the hole carrier is injected in the presence of surface transfer dopants. Exploring different functional groups' attachment on diamond surfaces and their impact on the electronic structure, using wet and dry chemical approaches, would hence be of interest in engineering diamond as a semiconductor. Here, we report the functionalization of the H-terminated diamond surface with nitrogen and sulfur heteroatoms. Surface characterization of functionalized diamond surfaces shows that these groups are well-distributed and covalently bonded to diamonds. Four chemical functional groups (-SH, -S-S-, -S-O, and -S=O) were found on the sulfurized diamond surface, and two groups (-NH2 and =NH) upon amination. We also report co-functionalization of surface with N and S (N-S), where sulfurization promotes sequential amination efficiency with reduced exposure time. Electrical measurement shows that heteroatom-modified diamond surfaces possess higher conductivity than H-terminated diamonds. Density functional theory (DFT) shows that upon functionalization with various N/S ratios, the conduction band minimum and valence band maximum downshift, which lowers the bandgap in comparison to an H-terminated diamond. These observations suggest the possibility of heteroatom functionalizations with enhanced surface electrical conductivity on the diamond that are useful for various electronic applications.

Phase Stability of Hexagonal/Cubic Boron Nitride Nanocomposites.

Boron nitride (BN) is an exceptional material, and among its polymorphs, two-dimensional (2D) hexagonal and three-dimensional (3D) cubic BN (h-BN and c-BN) phases are most common. The phase stability regimes of these BN phases are still under debate, and phase transformations of h-BN/c-BN remain a topic of interest. Here, we investigate the phase stability of 2D/3D h-BN/c-BN nanocomposites and show that the coexistence of two phases can lead to strong nonlinear optical properties and low thermal conductivity at room temperature. Furthermore, spark-plasma sintering of the nanocomposite shows complete phase transformation to 2D h-BN with improved crystalline quality, where 3D c-BN possibly governs the nucleation and growth kinetics. Our demonstration might be insightful in phase engineering of BN polymorph-based nanocomposites with desirable properties for optoelectronics and thermal energy management applications.

Unravelling the room temperature growth of two-dimensional h-BN nanosheets for multifunctional applications.

The room temperature growth of two-dimensional van der Waals (2D-vdW) materials is indispensable for state-of-the-art nanotechnology. Low temperature growth supersedes the requirement of elevated growth temperatures accompanied with high thermal budgets. Moreover, for electronic applications, low or room temperature growth reduces the possibility of intrinsic film-substrate interfacial thermal diffusion related deterioration of the functional properties and the consequent deterioration of the device performance. Here, we demonstrated the growth of ultrawide-bandgap boron nitride (BN) at room temperature by using the pulsed laser deposition (PLD) process, which exhibited various functional properties for potential applications. Comprehensive chemical, spectroscopic and microscopic characterizations confirmed the growth of ordered nanosheet-like hexagonal BN (h-BN). Functionally, the nanosheets show hydrophobicity, high lubricity (low coefficient of friction), and a low refractive index within the visible to near-infrared wavelength range, and room temperature single-photon quantum emission. Our work unveils an important step that brings a plethora of potential applications for these room temperature grown h-BN nanosheets as the synthesis can be feasible on any given substrate, thus creating a scenario for "h-BN on demand" under a frugal thermal budget.

Fluorinated Multi-Walled Carbon Nanotubes Coated Separator Mitigates Polysulfide Shuttle in Lithium-Sulfur Batteries.

Li-S batteries still suffer from two of the major challenges: polysulfide shuttle and low inherent conductivity of sulfur. Here, we report a facile way to develop a bifunctional separator coated with fluorinated multiwalled carbon nanotubes. Mild fluorination does not affect the inherent graphitic structure of carbon nanotubes as shown by transmission electron microscopy. Fluorinated carbon nanotubes show an improved capacity retention by trapping/repelling lithium polysulfides at the cathode, while simultaneously acting as the "second current collector". Moreover, reduced charge-transfer resistance and enhanced electrochemical performance at the cathode-separator interface result in a high gravimetric capacity of around 670 mAh g-1 at 4C. Unique chemical interactions between fluorine and carbon at the separator and the polysulfides, studied using DFT calculations, establish a new direction of utilizing highly electronegative fluorine moieties and absorption-based porous carbons for mitigation of polysulfide shuttle in Li-S batteries.

Piezoelectricity across 2D Phase Boundaries.

Piezoelectricity in low-dimensional materials and metal-semiconductor junctions has attracted recent attention. Herein, a 2D in-plane metal-semiconductor junction made of multilayer 2H and 1T' phases of molybdenum(IV) telluride (MoTe2 ) is investigated. Strong piezoelectric response is observed using piezoresponse force microscopy at the 2H-1T' junction, despite that the multilayers of each individual phase are weakly piezoelectric. The experimental results and density functional theory calculations suggest that the amplified piezoelectric response observed at the junction is due to the charge transfer across the semiconducting and metallic junctions resulting in the formation of dipoles and excess charge density, allowing the engineering of piezoelectric response in atomically thin materials.

K-point longitudinal acoustic phonons are responsible for ultrafast intervalley scattering in monolayer MoSe2.

In transition metal dichalcogenides, valley depolarization through intervalley carrier scattering by zone-edge phonons is often unavoidable. Although valley depolarization processes related to various acoustic phonons have been suggested, their optical verification is still vague due to nearly degenerate phonon frequencies on acoustic phonon branches at zone-edge momentums. Here we report an unambiguous phonon momentum determination of the longitudinal acoustic (LA) phonons at the K point, which are responsible for the ultrafast valley depolarization in monolayer MoSe2. Using sub-10-fs-resolution pump-probe spectroscopy, we observed coherent phonons signals at both even and odd-orders of zone-edge LA mode involved in intervalley carrier scattering process. Our phonon-symmetry analysis and first-principles calculations reveal that only the LA phonon at the K point, as opposed to the M point, can produce experimental odd-order LA phonon signals from its nonlinear optical modulation. This work will provide momentum-resolved descriptions of phonon-carrier intervalley scattering processes in valleytronic materials.

Amine-Functionalized Carbon Nanodot Electrocatalysts Converting Carbon Dioxide to Methane.

The electrochemical conversion of carbon dioxide (CO2 ) to methane (CH4 ), which can be used not only as fuel but also as a hydrogen carrier, has drawn great attention for use in supporting carbon capture and utilization. The design of active and selective electrocatalysts with exceptional CO2 -to-CH4 conversion efficiency is highly desirable; however, it remains a challenge. Here a molecular tuning strategy-in situ amine functionalization of nitrogen-doped graphene quantum dots (GQDs) for highly efficient CO2 -to-CH4 conversion is presented. Amine functionalized nitrogen-doped GQDs achieve a CH4 Faradic efficiency (FE) of 63% and 46%, respectively, at CH4 partial current densities of 170 and 258 mA cm-2 , approximating to or even outperforming state-of-the-art Cu-based electrocatalysts. These GQDs also convert CO2 to C2 products mainly including C2 H4 and C2 H5 OH with a maximum FE of ≈10%. A systematic analysis reveals that the CH4 yield varies linearly with amine group content, whereas the C2 production rate is positively dependent on pyridinic N dopant content. This work provides insight into the rational design of carbon catalysts with CO2 -to-CH4 conversion efficiency at the industrially relevant level.

Gas-Phase Fluorination of Hexagonal Boron Nitride.

Hexagonal boron nitride (hBN) has received much attention in recent years as a 2D dielectric material with potential applications ranging from catalysts to electronics. hBN is a stable covalent compound with a planar hexagonal lattice and is relatively unreactive to most chemical environments, making the chemical functionalization of hBN challenging. Here, a simple, scalable strategy to fluorinate hBN using a direct gas-phase fluorination technique is reported. The nature of fluorine bonding to the hBN lattice and their chemical coordination are described based on various characterization studies and theoretical models. The fluorine functionalized hBN shows a bandgap reduction and displays a semiconducting behavior due to the fluorination process. Additionally, the fluorinated hBN shows significant improvement in its thermal and friction properties, which could be substantial in applications such as lubricants and thermal fluids. Theory and simulations reveal that the enhanced friction properties of fluorinated hBN result from reduced inter-planar interaction energy by electrostatic repulsion of intercalated fluorine atoms between hBN layers without significant disruption of the in-plane lattice. This technique paves the way for the fluorination of several other 2D structures for various applications such as magnetism and functional nanoscale electronic devices.

Structure, Properties and Applications of Two-Dimensional Hexagonal Boron Nitride.

Hexagonal boron nitride (h-BN) has emerged as a strong candidate for two-dimensional (2D) material owing to its exciting optoelectrical properties combined with mechanical robustness, thermal stability, and chemical inertness. Super-thin h-BN layers have gained significant attention from the scientific community for many applications, including nanoelectronics, photonics, biomedical, anti-corrosion, and catalysis, among others. This review provides a systematic elaboration of the structural, electrical, mechanical, optical, and thermal properties of h-BN followed by a comprehensive account of state-of-the-art synthesis strategies for 2D h-BN, including chemical exfoliation, chemical, and physical vapor deposition, and other methods that have been successfully developed in recent years. It further elaborates a wide variety of processing routes developed for doping, substitution, functionalization, and combination with other materials to form heterostructures. Based on the extraordinary properties and thermal-mechanical-chemical stability of 2D h-BN, various potential applications of these structures are described.

Damage-tolerant 3D-printed ceramics via conformal coating.

Ceramic materials, despite their high strength and modulus, are limited in many structural applications due to inherent brittleness and low toughness. Nevertheless, ceramic-based structures, in nature, overcome this limitation using bottom-up complex hierarchical assembly of hard ceramic and soft polymer, where ceramics are packaged with tiny fraction of polymers in an internalized fashion. Here, we propose a far simpler approach of entirely externalizing the soft phase via conformal polymer coating over architected ceramic structures, leading to damage tolerance. Architected structures are printed using silica-filled preceramic polymer, pyrolyzed to stabilize the ceramic scaffolds, and then dip-coated conformally with a thin, flexible epoxy polymer. The polymer-coated architected structures show multifold improvement in compressive strength and toughness while resisting catastrophic failure through a considerable delay of the damage propagation. This surface modification approach allows a simple strategy to build complex ceramic parts that are far more damage-tolerant than their traditional counterparts.

Substitution of copper atoms into defect-rich molybdenum sulfides and their electrocatalytic activity.

Studies on intercalation or substitution of atoms into layered two-dimensional (2D) materials are rapidly expanding and gaining significant consideration due to their importance in electronics, catalysts, batteries, sensors, etc. In this manuscript, we report a straightforward method to create sulphur (S) deficient molybdenum (Mo) sulfide (MoS2-x ) structures and substitute them with zerovalent copper (Cu) atoms using a colloidal synthesis method. The synthesized materials were studied using several techniques to understand the proportion and position of copper atoms and the effect of copper functionalization. Specifically, the impact of change in the ratio of Cu : S and the hydrogen evolution reaction (HER) activity of the derived materials were evaluated. This technique paves the way for the synthesis of various functionalized 2D materials with a significant impact on their physical and chemical behavior making them potential candidates for catalysis and several other applications such as energy storage and the development of numerous functional devices.

Full-color fluorescent carbon quantum dots.

Quantum dots have innate advantages as the key component of optoelectronic devices. For white light-emitting diodes (WLEDs), the modulation of the spectrum and color of the device often involves various quantum dots of different emission wavelengths. Here, we fabricate a series of carbon quantum dots (CQDs) through a scalable acid reagent engineering strategy. The growing electron-withdrawing groups on the surface of CQDs that originated from acid reagents boost their photoluminescence wavelength red shift and raise their particle sizes, elucidating the quantum size effect. These CQDs emit bright and remarkably stable full-color fluorescence ranging from blue to red light and even white light. Full-color emissive polymer films and all types of high-color rendering index WLEDs are synthesized by mixing multiple kinds of CQDs in appropriate ratios. The universal electron-donating/withdrawing group engineering approach for synthesizing tunable emissive CQDs will facilitate the progress of carbon-based luminescent materials for manufacturing forward-looking films and devices.

Scale-Enhanced Magnetism in Exfoliated Atomically Thin Magnetite Sheets.

The discovery of ferromagnetism in atomically thin layers at room temperature widens the prospects of 2D materials for device applications. Recently, two independent experiments demonstrated magnetic ordering in two dissimilar 2D systems, CrI3 and Cr2 Ge2 Te6 , at low temperatures and in VSe2 at room temperature, but observation of intrinsic room-temperature magnetism in 2D materials is still a challenge. Here a transition at room temperature that increases the magnetization in magnetite while thinning down the bulk material to a few atom-thick sheets is reported. DC magnetization measurements prove ferrimagnetic ordering with increased magnetization and density functional theory calculations ascribe their origin to the low dimensionality of the magnetite layers. In addition, surface energy calculations for different cleavage planes in passivated magnetite crystal agree with the experimental observations of obtaining 2D sheets from non-van der Waals crystals.

Flexible planar supercapacitors by straightforward filtration and laser processing steps.

There is ever increasing demand for flexible energy storage devices due to the development of wearable electronics and other small electronic devices. The electrode flexibility is best provided by a special set of nanomaterials, but the required methodology typically consists of multiple steps and are designed just for the specific materials. Here, a facile and scalable method of making flexible and mechanically robust planar supercapacitors with interdigital electrode structure made of commercial carbon nanomaterials and silver nanowires is presented. The capacitor structure is achieved with vacuum filtration through a micropatterned contact mask and finished with simple laser processing steps. A maximum specific capacitance of 4 F cm-3 was measured with cyclic voltammetry at scan rate of 5 mV s-1. The reliability and charge transfer properties of devices were further investigated with galvanostatic charge-discharge measurements and electrochemical impedance spectroscopy, respectively. Furthermore, mechanical bending tests confirmed the devices have excellent mechanical integrity, and the deformations have no adverse effects on the electrochemical charge-discharge behavior and stability.

Extraction of Two-Dimensional Aluminum Alloys from Decagonal Quasicrystals.

Atomically thin metallic alloys are receiving increased attention due to their prospective applications as interconnects/contacts in two-dimensional (2D) circuits, sensors, and catalysts, among others. In this work, we demonstrate an easily scalable technique for the synthesis of 2D metallic alloys from their 3D quasicrystalline precursors. We have used aluminum (Al)-based single-phase decagonal quasicrystal Al66Co17Cu17 alloy to extract the corresponding 2D alloy structure. The 2D layered Al alloy possesses 2-fold decagonal quasicrystalline symmetry and consists of two- or three-layer-thick sheets with a lateral dimension of microns. These 2D metallic layers were combined with the atomic layers of tungsten disulfide to form the stacked heterostructures, which is demonstrated to be a stable and efficient catalyst for hydrogen evolution reaction.