Two-dimensional crystals have emerged as a class of materials that may impact future electronic technologies. Experimentally identifying and characterizing new functional two-dimensional materials is challenging, but also potentially rewarding. Here, we fabricate field-effect transistors based on few-layer black phosphorus crystals with thickness down to a few nanometres. Reliable transistor performance is achieved at room temperature in samples thinner than 7.5 nm, with drain current modulation on the order of 10(5) and well-developed current saturation in the I-V characteristics. The charge-carrier mobility is found to be thickness-dependent, with the highest values up to ∼ 1,000 cm(2) V(-1) s(-1) obtained for a thickness of ∼ 10 nm. Our results demonstrate the potential of black phosphorus thin crystals as a new two-dimensional material for applications in nanoelectronic devices.

http://absimage.aps.org/image/MAR14/MWS_MAR14-2013-005031.pdf

Black Phosphorus Field-effect Transistors

Recently synthesized atomically thin boron sheets (that is, borophene) provide a fascinating template for new material property discovery. Here, we report findings of an extraordinary combination of unusual mechanical and electronic properties in hydrogenated borophene, known as borophane, from first-principles calculations. This novel 2D material has been shown to exhibit robust Dirac transport physics. Our study unveils that borophane is auxetic with a surprising negative Poisson’s ratio stemming from its unique puckered triangle hinge structure and the associated hinge dihedral angle variation under a tensile strain in the armchair direction. Our results also identify borophane to be ferroelastic with a stress-driven 90° lattice rotation in the boron layer, accompanied by a remarkable orientation switch of the anisotropic Dirac transport channels. These outstanding strain-engineered properties make borophane a highly versatile and promising 2D material for innovative applications in microelectromechanical and nanoelectronic devices.

Auxetic and ferroelastic borophane: A novel 2D material with negative Possion’s ratio and switchable Dirac transport channels

2D carbides and nitrides of transition metals, also known as MXenes, are an emerging class of 2D nanomaterials that have shown excellent performances and broad application prospects in the fields of energy storage, catalysis, sensing, electromagnetic shielding, electronics and photonics, and life sciences. This unusual diversity of applications is due to their superior hydrophilicity and conductivity, high carrier concentration, ultra-high volumetric capacitance, rich surface chemistry, and large specific surface area. However, it is difficult to make MXenes with the desired surface functional groups that deliver high reactivity and high stability, because most MXenes are extracted from ceramics (MAX phase) by an etching process, where a large number of metal atoms are inevitably exposed on the surface, with other anions and cations embedded uncontrollably. The exposed metal atoms and implanted ions are thermodynamically unstable and readily react with trace oxygen or oxygen-containing groups to form the corresponding metal oxides or degrade chemically, resulting in a sharp decline in activity and loss of excellent physicochemical properties. The addition of certain synergistic additives during the intercalation and chemical modification of surface functional groups under non-hazardous conditions can result in stable and efficient MXene-based materials with exceptional optical, electrical, and magnetic properties. This review discusses several such methods, mainly additive-mediated intercalation and chemical modification of the surface functional groups of MXene-based materials, followed by their potential applications. Finally, perspectives are given to discuss the future challenges and promising opportunities of this exciting field.

Additive-mediated intercalation and surface modification of MXenes.

Two-dimensional (2D) materials with a moderate bandgap and high carrier mobility are useful for applications in optoelectronics. In this work, we present a systematic investigation of the mechanical, electronic and optical properties of a B2P6 monolayer using first-principles calculations. Monolayer B2P6 was estimated to be an anisotropic material from direction-dependent in-plane Young's moduli and Poisson's ratios. Also, B2P6 exhibits an ultrahigh electron mobility of ∼5888 cm2 V−1 s−1, showing advantages for application in high-speed optoelectronic devices. More importantly, for the B2P6 monolayer, a desirable transformation from an indirect to direct band gap was observed at a biaxial tensile strain of ∼4%. Increasing the biaxial strain reduces the gap and preserves the suitable band edge positions for photocatalytic water splitting in the observed strain range of 1–8%. The decreased gap also enhances the visible light absorption of the B2P6 monolayer. These findings indicate that the B2P6 monolayer has promising applications in photocatalytic and photovoltaic devices.

Mechanical, electronic and optical properties of a novel B2P6 monolayer: ultrahigh carrier mobility and strong optical absorption.

The polarization of a material and its response to applied electric and magnetic fields are key solid-state properties with a long history in insulators, although a satisfactory theory required new concepts such as Berry-phase gauge fields. In metals, quantities such as static polarization and the magnetoelectric θ term cease to be well defined. In polar metals, there can be analogous dynamical current responses, which we study in a common theoretical framework. We find that current responses to dynamical strain in polar metals depend on both the first and second Chern forms, related to polarization and magnetoelectricity in insulators as well as the orbital magnetization on the Fermi surface. We provide realistic estimates that predict that the latter contribution will dominate, and we investigate the feasibility of experimental detection of this effect.

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Dynamical piezoelectric and magnetopiezoelectric effects in polar metals from Berry phases and orbital moments

Out-of-plane ferroelectricity in the two-dimensional (2D) regime shows great promise for applications in next-generation electronic devices but has been rarely reported. Herein, by using high-throughput search and density functional theory (DFT) calculations, three types of ferroelectric MXene phases (type-I: Nb2CS2 and Ta2CS2; type-II: Sc2CO2 and Y2CO2; and type-III: Sc2CS2 and Y2CS2) are for the first time predicted to be achievable by surface functionalization of 2D MXenes. The identified 2D ferroelectric MXenes not only demonstrate remarkable and reversible spontaneous electric polarization along both the out-of-plane and in-plane orientations but also exhibit giant out-of-plane and in-plane piezoelectric response to the external strain. More excitingly, the type-III ferroelectric MXenes possess rare out-of-plane auxeticity that can be triggered by the tensile strain along both the zig-zag and the arm-chair directions. The intriguing ferroelectricity, large piezoelectric response, and auxeticity render the ferroelectric MXenes versatile candidates for nanoscale electronic and mechanic devices.

First-principles screening of novel ferroelectric MXene phases with a large piezoelectric response and unusual auxeticity.

Two-dimensional (2D) materials integrated with anisotropy and ferroelasticity are highly desired for controllable polarization-sensitive devices and ferroelastic memorizers but are rarely reported. Herein, using the particle swap optimization (PSO) method and first-principles calculations, we theoretically predicted the coexistence of single-direction auxeticity, anisotropic carrier mobility, and intrinsic ferroelasticity in 2D silver oxide (AgO). Linear and square–planar Ag–O ligands align vertically in 2D AgO, leading to remarkable in-plane anisotropy. 2D AgO exhibits a large out-of-plane negative Poisson's ratio (NPR) of −0.47 which can only be triggered by the uniaxial strain along the y-direction. The electrons/holes favor the transport along the x-direction with mobilities of up to ∼6000/4000 cm2 V−1 s−1, which is around 10/17 times higher than that along the y-direction. Moreover, a strain-driven 90° lattice rotation is found in 2D AgO with a record high reversal strain of 107.6%. Such integration allows us to tune the direction of the anisotropic properties, and at the same time enables the efficient identification of the bi-stable states during the ferroelastic transition, thus promising the versatile applications of 2D AgO in controllable mechanic/electronic devices and non-volatile information storage.

First-principles prediction of ferroelasticity tuned anisotropic auxeticity and carrier mobility in two-dimensional AgO

Solvent plays an important role in the surface interaction of molecules. In this study, we use "chlorophyll a", an archetypical molecule, to investigate its supramolecular self-assembly with chemically reduced graphene oxide in three different types of solvents: polar protic, polar aprotic, and non-polar. It was observed that only a polar protic solvent that can donate protons facilitates the hydrogen bonding between chlorophyll a and chemically reduced graphene oxide nanosheets in a hybrid system. The formation of hydrogen bonds further initiates the other non-covalent interactions such as π-π stacking and hydrophobic interaction, which altogether play a key driving force for supramolecular self-assembly of chlorophylls on chemically reduced graphene oxides. The experimental results are strongly supported by density functional theory calculations, which show robust electron coupling between chlorophylls and chemically reduced graphene oxide.

Solvent Effect on Supramolecular Self-Assembly of Chlorophylls a on Chemically Reduced Graphene Oxide.

Polar half-metals, possessing both polarity and half-metallicity, may have potential as media in novel spintronic quantum devices, but they have never been reported in the two-dimensional (2D) form. By combining the particle swarm optimization with first-principles calculations, we predict a stable 2D polar half-metal, quintuple layered Co2Se3 monolayer. This is an XY magnet where its magnetism is governed by the interplay between super and double exchange along the Co–Se–Co motifs. Notably, 2D Co2Se3 also exhibits out-of-plane piezoelectricity. We have, thus, discovered a multiferroic 2D compound with robust half-metallicity, rendering it great potential in advanced multiferroic applications.

Chunmei Zhang

Papers

First-principles screening of novel ferroelectric MXene phases with a large piezoelectric response and unusual auxeticity.

First-principles prediction of ferroelasticity tuned anisotropic auxeticity and carrier mobility in two-dimensional AgO

Solvent Effect on Supramolecular Self-Assembly of Chlorophylls a on Chemically Reduced Graphene Oxide.

First-principles prediction of polar half-metallicity and out-of-plane piezoelectricity in two-dimensional quintuple layered cobalt selenide