Abstract
Generating sustainable electricity from ambient humidity and natural evaporation has attracted tremendous interest recently as it requires no extra mechanical energy input and is deployable ...across all weather and geography conditions. Here, we present a device prototype for enhanced power generation from ambient humidity. This prototype uses both heterogenous materials assembled from a LiCl-loaded cellulon paper to facilitate moisture adsorption and a carbon-black-loaded cellulon paper to promote water evaporation. Exposing such a centimeter-sized device to ambient humidity can produce voltages of around 0.78 V and a current of around 7.5 μA, both of which can be sustained for more than 10 days. The enhanced electric output and durability are due to the continuous water flow that is directed by evaporation through numerous, negatively charged channels within the cellulon papers. The voltage and current exhibit an excellent scaling behavior upon device integration to sufficiently power commercial devices including even cell phones. The results open a promising prospect of sustainable electricity generation based on a synergy between spontaneous moisture adsorption and water evaporation.
The mechanical properties of 2D boron—borophene—are studied by first‐principles calculations. The recently synthesized borophene with a 1/6 concentration of hollow hexagons (HH) is shown to have ...in‐plane modulus C up to 210 N m−1 and bending stiffness as low as D = 0.39 eV. Thus, its Foppl–von Karman number per unit area, defined as C/D, reaches 568 nm−2, over twofold higher than graphene's value, establishing the borophene as one of the most flexible materials. Yet, the borophene has a specific modulus of 346 m2 s−2 and ideal strength of 16 N m−1, rivaling those (453 m2 s−2 and 34 N m−1) of graphene. In particular, its structural fluxionality enabled by delocalized multicenter chemical bonding favors structural phase transitions under tension, which result in exceptionally small breaking strains yet highly ductile breaking behavior. These mechanical properties can be further tailored by varying the HH concentration, and the boron sheet without HHs can even be stiffer than graphene against tension. The record high flexibility combined with excellent elasticity in boron sheets can be utilized for designing advanced composites and flexible devices.
Borophene is a graphene analog for boron. Here, it is predicted that borophene, albeit being lighter than graphene, combines unprecedented flexibility with excellent in‐plane elasticity, rendering it as an atomic membrane that is easiest to bend yet hard to stretch. Moreover, borophene can exceptionally resist considerably high loading even when extremely stretched, due to unique tension‐induced structural phase transitions.
Two‐dimensional (2D) materials, such as graphene and boron nitride, have specific lattice structures independent of external conditions. In contrast, the structure of 2D boron sensitively depends on ...metal substrate, as we show herein using the cluster expansion method and a newly developed surface structure‐search method, both based on first‐principles calculations. The preferred 2D boron on weaker interacting Au is nonplanar with significant buckling and numerous polymorphs as in vacuum, whereas on more reactive Ag, Cu, and Ni, the polymorphic energy degeneracy is lifted and a particular planar structure is found to be most stable. We also show that a layer composed of icosahedral B12 is unfavorable on Cu and Ni but unexpectedly becomes a possible minimum on Au and Ag. The substrate‐dependent 2D boron choices originate from a competition between the strain energy of buckling and chemical energy of electronic hybridization between boron and metal.
Concerto in B flat: 2D materials usually have a specific lattice structure that is independent of external conditions. In contrast, the structure of 2D boron (red, see picture) depends on the metal substrate. On weakly interacting metals, 2D boron shows significant off‐plane buckling but on more reactive metals, it strongly favors a planar structure.
Since the early nineteenth century, it has been known that an electric potential can be generated by driving an ionic liquid through fine channels or holes under a pressure gradient. More recently, ...it has been reported that carbon nanotubes can generate a voltage when immersed in flowing liquids, but the exact origin of these observations is unclear, and generating electricity without a pressure gradient remains a challenge. Here, we show that a voltage of a few millivolts can be produced by moving a droplet of sea water or ionic solution over a strip of monolayer graphene under ambient conditions. Through experiments and density functional theory calculations, we find that a pseudocapacitor is formed at the droplet/graphene interface, which is driven forward by the moving droplet, charging and discharging at the front and rear of the droplet. This gives rise to an electric potential that is proportional to the velocity and number of droplets. The potential is also found to be dependent on the concentration and ionic species of the droplet, and decreases sharply with an increasing number of graphene layers. We illustrate the potential of this electrokinetic phenomenon by using it to create a handwriting sensor and an energy-harvesting device.
Intrinsic semimetallicity of graphene and silicene largely limits their applications in functional devices. Mixing carbon and silicon atoms to form two-dimensional (2D) silicon carbide (Si x C1–x ) ...sheets is promising to overcome this issue. Using first-principles calculations combined with the cluster expansion method, we perform a comprehensive study on the thermodynamic stability and electronic properties of 2D Si x C1–x monolayers with 0 ≤ x ≤ 1. Upon varying the silicon concentration, the 2D Si x C1–x presents two distinct structural phases, a homogeneous phase with well dispersed Si (or C) atoms and an in-plane hybrid phase rich in SiC domains. While the in-plane hybrid structure shows uniform semiconducting properties with widely tunable band gap from 0 to 2.87 eV due to quantum confinement effect imposed by the SiC domains, the homogeneous structures can be semiconducting or remain semimetallic depending on a superlattice vector which dictates whether the sublattice symmetry is topologically broken. Moreover, we reveal a universal rule for describing the electronic properties of the homogeneous Si x C1–x structures. These findings suggest that the 2D Si x C1–x monolayers may present a new “family” of 2D materials, with a rich variety of properties for applications in electronics and optoelectronics.
Grain boundaries (GBs) are structural imperfections that typically degrade the performance of materials. Here we show that dislocations and GBs in two-dimensional (2D) metal dichalcogenides MX2 (M = ...Mo, W; X = S, Se) can actually improve the material by giving it a qualitatively new physical property: magnetism. The dislocations studied all display a substantial magnetic moment of ∼1 Bohr magneton. In contrast, dislocations in other well-studied 2D materials are typically nonmagnetic. GBs composed of pentagon–heptagon pairs interact ferromagnetically and transition from semiconductor to half-metal or metal as a function of tilt angle and/or doping level. When the tilt angle exceeds 47°, the structural energetics favor square–octagon pairs and the GB becomes an antiferromagnetic semiconductor. These exceptional magnetic properties arise from interplay of dislocation-induced localized states, doping, and locally unbalanced stoichiometry. Purposeful engineering of topological GBs may be able to convert MX2 into a promising 2D magnetic semiconductor.
Two-dimensional (2D) boron (that is, borophene) was recently synthesized following theoretical predictions
. Its metallic nature and high in-plane anisotropy combine many of the desirable attributes ...of graphene
and monolayer black phosphorus
. As a synthetic 2D material, its structural properties cannot be deduced from bulk boron, which implies that the intrinsic defects of borophene remain unexplored. Here we investigate borophene line defects at the atomic scale with ultrahigh vacuum (UHV) scanning tunnelling microscopy/spectroscopy (STM/STS) and density functional theory (DFT). Under suitable growth conditions, borophene phases that correspond to the v
and v
models are found to intermix and accommodate line defects in each other with structures that match the constituent units of the other phase. These line defects energetically favour spatially periodic self-assembly that gives rise to new borophene phases, which ultimately blurs the distinction between borophene crystals and defects. This phenomenon is unique to borophene as a result of its high in-plane anisotropy and energetically and structurally similar polymorphs. Low-temperature measurements further reveal subtle electronic features that are consistent with a charge density wave (CDW), which are modulated by line defects. This atomic-level understanding is likely to inform ongoing efforts to devise and realize applications based on borophene.
Hexagonal boron nitride (h-BN) sheet is a structural analogue of graphene, yet its growth mechanism has been rarely studied, as complicated by its binary composition. Here, we reveal an atomistic ...growth mechanism for the h-BN islands by combining crystal growth theory with comprehensive first-principles calculations. The island shapes preferred by edge equilibrium are found to be inconsistent with experimental facts, which is in contrast to previous common views. Then the growth kinetics is explored by analyzing the diffusion and docking of boron and nitrogen atoms at the edges in a step-by-step manner of the nanoreactor approach. The determined sequence of atom-by-atom accretion reveals a strong kinetic anisotropy of growth. Depending on the chemical potential of constituent elements, it yields the h-BN shapes as equilateral triangles or hexagons, explaining a number of experimental observations and opening a way for the synthesis of quality h-BN with controlled morphology. The richer growth kinetics of h-BN compared to graphene is further extendable to other binary two-dimensional materials, notably metal dichalcogenides.
Boron nitride (BN) structures are featured by their excellent thermal and chemical stability and unique electronic and optical properties. However, the lack of controlled synthesis of quality samples ...and the electrically insulating property largely prevent realizing the full potential of BN nanostructures. A comprehensive overview of the current status of the synthesis of two‐dimensional hexagonal BN sheets, three dimensional porous hexagonal BN materials and BN‐involved heterostructures is provided, highlighting the advantages of different synthetic methods. In addition, structural characterization, functionalizations and prospective applications of hexagonal BN sheets are intensively discussed. One‐dimensional BN nanoribbons and nanotubes are then discussed in terms of structure, fabrication and functionality. In particular, the existing routes in pursuit of tunable electronic and magnetic properties in various BN structures are surveyed, calling upon synergetic experimental and theoretical efforts to address the challenges for pioneering the applications of BN into functional devices. Finally, the progress in BN superstructures and novel B/N nanostructures is also briefly introduced.
Boron nitride nanostructures have attracted intense attention for their excellent properties. Controlled synthesis of quality samples and the electrically insulating property largely prevents the realization of the full potential of BN nanostructures. The current status of the synthesis and applications of two dimensional h‐BN are reviewed, as well as versatile strategies in pursuit of tunable electronic and magnetic properties in BN nanostructures.
Inspired by rich physics and functionalities of graphenes, scientists have taken an intensive interest in two-dimensional (2D) crystals of h-BN (analogue of graphite, so-called “white” graphite). ...Recent calculations have predicted the exciting potentials of BN nanoribbons in spintronics due to tunable magnetic and electrical properties; however no experimental evidence has been provided since fabrication of such ribbons remains a challenge. Here, we show that few- and single-layered BN nanoribbons, mostly terminated with zigzag edges, can be produced under unwrapping multiwalled BN nanotubes through plasma etching. The interesting stepwise unwrapping and intermediate states were observed and analyzed. Opposed to insulating primal tubes, the nanoribbons become semiconducting due to doping-like conducting edge states and vacancy defects, as revealed by structural analyses and ab initio simulations. This study paves the way for BN nanoribbon production and usage as functional semiconductors with a wide range of applications in optoelectronics and spintronics.
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