Two-dimensional (2D) materials are well-known to exhibit interesting phenomena due to quantum confinement. Here, we show that quantum confinement, together with structural anisotropy, result in an ...electric-field-tunable Dirac cone in 2D black phosphorus. Using density functional theory calculations, we find that an electric field, E ext, applied normal to a 2D black phosphorus thin film, can reduce the direct band gap of few-layer black phosphorus, resulting in an insulator-to-metal transition at a critical field, Ec. Increasing E ext beyond Ec can induce a Dirac cone in the system, provided the black phosphorus film is sufficiently thin. The electric field strength can tune the position of the Dirac cone and the Dirac-Fermi velocities, the latter being similar in magnitude to that in graphene. We show that the Dirac cone arises from an anisotropic interaction term between the frontier orbitals that are spatially separated due to the applied field, on different halves of the 2D slab. When this interaction term becomes vanishingly small for thicker films, the Dirac cone can no longer be induced. Spin-orbit coupling can gap out the Dirac cone at certain electric fields; however, a further increase in field strength reduces the spin-orbit-induced gap, eventually resulting in a topological-insulator-to-Dirac-semimetal transition.
Ab initio density functional theory calculations are performed to investigate the electronic structure of MoS2 armchair nanoribbons in the presence of an external static electric field. Such ...nanoribbons, which are nonmagnetic and semiconducting, exhibit a set of weakly interacting edge states whose energy position determines the band gap of the system. We show that, by applying an external transverse electric field, E ext, the nanoribbon band gap can be significantly reduced, leading to a metal–insulator transition beyond a certain critical value. Moreover, the presence of a sufficiently high density of states at the Fermi level in the vicinity of the metal–insulator transition leads to the onset of Stoner ferromagnetism that can be modulated, and even extinguished, by E ext. In the case of bilayer nanoribbons we further show that the band gap can be changed from indirect to direct by applying a transverse field, an effect that might be of significance for opto-electronics applications.
The recently discovered two-dimensional magnetic insulator CrI3 is an intriguing case for basic research and spintronic applications since it is a ferromagnet in the bulk but an antiferromagnet in ...bilayer form, with its magnetic ordering amenable to external manipulations. Using the first-principles quantum transport approach, we predict that injecting unpolarized charge current parallel to the interface of the bilayer-CrI3/monolayer-TaSe2 van der Waals (vdW) heterostructure will induce spin–orbit torque and thereby drive the dynamics of magnetization on the first monolayer of CrI3 in direct contact with TaSe2. By combining the calculated complex angular dependence of spin–orbit torque with the Landau-Lifshitz-Gilbert equation for classical dynamics of magnetization, we demonstrate that current pulses can switch the direction of magnetization on the first monolayer to become parallel to that of the second monolayer, thereby converting CrI3 from antiferromagnet to ferromagnet while not requiring any external magnetic field. We explain the mechanism of this reversible current-driven nonequilibrium phase transition by showing that first monolayer of CrI3 carries current due to evanescent wave functions injected by metallic transition metal dichalcogenide TaSe2, while concurrently acquiring strong spin–orbit coupling via such a proximity effect, whereas the second monolayer of CrI3 remains insulating. The transition can be detected by passing vertical read current through the vdW heterostructure, encapsulated by a bilayer of hexagonal boron nitride and sandwiched between graphite electrodes, where we find a tunneling magnetoresistance of ≃240%.
Two-dimensional (2D) layered transition metal dichalcogenides (TMDs) have been recently proposed as appealing candidate materials for spintronic applications owing to their distinctive atomic crystal ...structure and exotic physical properties arising from the large bonding anisotropy. Here we introduce the first MoS2-based spin-valves that employ monolayer MoS2 as the nonmagnetic spacer. In contrast with what is expected from the semiconducting band-structure of MoS2, the vertically sandwiched-MoS2 layers exhibit metallic behavior. This originates from their strong hybridization with the Ni and Fe atoms of the Permalloy (Py) electrode. The spin-valve effect is observed up to 240 K, with the highest magnetoresistance (MR) up to 0.73% at low temperatures. The experimental work is accompanied by the first principle electron transport calculations, which reveal an MR of ∼9% for an ideal Py/MoS2/Py junction. Our results clearly identify TMDs as a promising spacer compound in magnetic tunnel junctions and may open a new avenue for the TMDs-based spintronic applications.
Transition metal dichalcogenides have a laminar structure, with strongly covalently bonded layers weakly interacting through van der Waals forces. They are of special interest also because of their ...unique properties once exfoliated in nanoflakes. We analyze the microstructure of oxidized TiS2 nanoflakes with atomically resolved scanning transmission electron microscopy and propose a comprehensive model for their reactivity by means of first-principles simulations. In particular we find that reaction to water proceeds from the edges of the flake, while it is thermodynamically possible but kinetically hindered in the middle, unless it is initiated by the presence of a surface vacancy. Importantly O substitution for S allows fine-tuning control of the flake bandgap, paving the way for the use of TiS2‑x O x alloys as surface catalysts and photovoltaic materials.
Liquid-phase exfoliation of layered materials offers a large-scale approach toward the synthesis of 2D nanostructures. Structural properties of materials can however change during transition from ...bulk to the 2D state. Any such changes must be examined and understood for successful implementation of 2D nanostructures. In this work, we demonstrate nonbulk stacking sequences in the few-layer MoS2 and WS2 nanoflakes produced by liquid-phase exfoliation. Our analysis shows that nonbulk stacking sequences can be derived from its bulk counterparts by translational shifts of the layers. No structural changes within the layers were observed. Twenty-seven MoS2 and five WS2 nanoflakes were imaged and analyzed. Nine MoS2 and four WS2 nanoflakes displayed nonbulk stacking. Such dominance of the nonbulk stacking suggests high possibility of unusual stacking sequences in other 2D nanostructures. Notably, the electronic structure of some non bulk stacked bilayers presents characteristics which are uncommon to either the bulk phase or the single monolayer, for instance, a spin-split conduction band bottom. Our main characterization technique was annular dark-field scanning transmission electron microscopy, which offers direct and reliable imaging of atomic columns. The stacking characterization approach employed here can be readily applied toward other few-layer transition metal chalcogenides and oxides.
Ultrathin ferroelectric semiconductors with high charge carrier mobility are much coveted systems for the advancement of various electronic and optoelectronic devices. However, in traditional oxide ...ferroelectric insulators, the ferroelectric transition temperature decreases drastically with decreasing material thickness and ceases to exist below certain critical thickness owing to depolarizing fields. Herein, we show the emergence of an ordered ferroelectric ground state in ultrathin (∼2 nm) single crystalline nanosheets of Bi2O2Se at room temperature. Free-standing ferroelectric nanosheets, in which oppositely charged alternating layers are self-assembled together by electrostatic interactions, are synthesized by a simple, rapid, and scalable wet chemical procedure at room temperature. The existence of ferroelectricity in Bi2O2Se nanosheets is confirmed by dielectric measurements and piezoresponse force spectroscopy. The spontaneous orthorhombic distortion in the ultrathin nanosheets breaks the local inversion symmetry, thereby resulting in ferroelectricity. The local structural distortion and the formation of spontaneous dipole moment were directly probed by atomic resolution scanning transmission electron microscopy and density functional theory calculations.
GeTe is among the most fascinating inorganic compounds for thermoelectric (TE) conversion of waste heat into electricity. However, the TE performance in its ambient rhombohedral phase is strongly ...impeded by natural excessive Ge vacancies resulting in high hole concentration, and the rhombohedral to cubic phase transition at high temperature (
T
∼ 700 K) deteriorates its mechanical robustness. Thus, stabilization of the high
T
cubic phase near ambient conditions would resolve many of these unwarranted challenges. Importantly, the higher symmetric cubic phase is beneficial for large Seebeck coefficient (
S
) due to its higher valence band (VB) degeneracy. Here, we show a simple innovative strategy of using high energy ball-milling (BM) and spark plasma sintering (SPS) to promote the crystal symmetry in Sb doped GeTe, which stabilizes in a near-cubic phase under ambient conditions. Consequently, the energy gap between the primary and secondary VBs drastically decreases to ∼0.06 eV and the band degeneracy enhances, leading to high
S
. BM followed by SPS simultaneously lead to the formation of hierarchical nano/meso architectures comprising solid solution point defects, Ge and GeSb
4
Te
7
nanoprecipitates and nano/mesoscale grains, which efficiently scatter broad length scales (few Å-200 nm) of phonons responsible for thermal transport. As a result, the lattice thermal conductivity (
κ
lat
) is suppressed to ∼0.59 W m
−1
K
−1
. This combined effect of VB convergence due to enhanced crystal symmetry and ultra-low
κ
lat
via
hierarchical nanostructuring results in an ultra-high TE figure of merit (
zT
) ∼2.5 at 662 K in Ge
0.9
Sb
0.1
Te-BM + SPS. Furthermore, the fabricated double leg thermoelectric device shows promising output power density of ∼570 mW cm
−2
for a Δ
T
of 442 K.
Extreme electronic band convergence and nano/meso-structured phonon scattering leading to ultra-high thermoelectric performance in the near cubic Sb doped GeTe.
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