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

A 3D Graphene? Discoveries of materials with exciting electronic properties have propelled condensed matter physics over the past decade. Two of the best-known examples, graphene and topological insulators, have something in common: a linear energy-momentum relationship—the Dirac dispersion—in their two-dimensional (2D) electronic states. Topological insulators also have a more mundane aspect of their electronic structure, characterized by a band gap. Another class of materials, topological Dirac semimetals, has been proposed that has a linear dispersion along all three momentum directions—a bulk Dirac cone; these materials are predicted to have intriguing electronic properties and to be related to other exotic states through quantum phase transitions. Liu et al. (p. 864, published online 16 January) detected such a state in the compound Na3Bi by using photoemission spectroscopy. Angle-resolved photoemission spectroscopy is used to detect bulk Dirac cones in a three-dimensional analog of graphene. Three-dimensional (3D) topological Dirac semimetals (TDSs) represent an unusual state of quantum matter that can be viewed as “3D graphene.” In contrast to 2D Dirac fermions in graphene or on the surface of 3D topological insulators, TDSs possess 3D Dirac fermions in the bulk. By investigating the electronic structure of Na3Bi with angle-resolved photoemission spectroscopy, we detected 3D Dirac fermions with linear dispersions along all momentum directions. Furthermore, we demonstrated the robustness of 3D Dirac fermions in Na3Bi against in situ surface doping. Our results establish Na3Bi as a model system for 3D TDSs, which can serve as an ideal platform for the systematic study of quantum phase transitions between rich topological quantum states.

Discovery of a Three-dimensional Topological Dirac Semimetal, Na3Bi

DOI: 10.1002/admi.201600121 degradation caused by ambient medium. By performing simultaneous topographic and optical time series imaging over several months, we show that a nanolayer BP exposed to ambient environment degrades at a steadily increasing rate until saturation begins, so that the degraded area and volume of degraded regions as functions of time follow the well-known S-shaped growth curve (sigmoid growth curve).[12] Phenomenological modeling of experimental results suggests a strong influence of degraded areas on adjacent BP. Our model is advantageous since it is based on elementary probabilities that can be related to the O2 and H2O content in the ambient medium, as well as to the chemical reaction processes that result in oxidized phosphorus species. Therefore, our phenomenological model can, in principle, be used to predict the degradation time under different environmental conditions. Our work incorporates experimentally measured geometric patterns of BP degradation with model prediction allowing us to determine the growth saturation value, steepness of the growth, and the mid-point of the growth, and the relative importance of neighboring interactions. Mechanical exfoliation was used for BP sample preparation on an Si substrate. After the exfoliation, the substrates were loaded in to ALD chamber (Cambridge NanoTech, Savannah 200) immediately for the Al2O3 deposition with TMA (trimethyl aluminum) and water as precursors. For the first three rounds, only TMA is flowed through the chamber to prevent oxidization of the BP flakes. The ALD was performed at 200 °C. Various thicknesses (1, 5, 10, 20, 50, and 100 nm) of Al2O3 were coated on the BP flakes using ALD. Topographic and near-field optical images were acquired using a commercial s-SNOM system (neaspec.com). An AFM tip coated with PtIr, oscillating at resonance frequency of f ≈ 280 kHz, is irradiated by a focused quantum cascade laser (QCL) laser at 45° with respect to the sample surface (Figure 1a). The scattered signal from the tip– sample interface region is demodulated at high harmonics of the tip resonance frequency (nf, n > 1) and detected by phasemodulation (pseudo-heterodyne) interferometry, producing simultaneous topography, optical amplitude, and phase images. We performed infrared (IR) optical amplitude and phase resolved near-field spectroscopy imaging of unencapsulated BP flake left to degrade in order to chemically identify the degraded region in BP in the laser wavelength range, λ = 5.5 – 10.8 μm. Figure 1b shows a topography image and Figure 1c simultaneously recorded IR third harmonic amplitude and phase optical images of a 10 nm thick BP flake left to degrade for some time in ambient environment. Although the various AFM height distributions shown in the topography and line profile images in Figure 1b are not sufficient to identify the chemical composition of nano-islands, taller topographic protrusions on BP samples exposed to ambient environment have been used to Among numerous van der Waals materials that have been identified lately, black phosphorus (BP) has generated intense interest for various optoelectronic applications due to its outstanding physical properties. Its tunable, direct, and narrow band gap makes it an attractive semiconductor for nanoelectronics and nanophotonics applications.[1–3] Similar to other 2D layered materials, BP can be prepared by simple mechanical exfoliation with additional exotic features such as its extraordinary in-plane anisotropic electrical, optical, and vibrational properties.[4–7] The major impediment to research and prospective application of single/few-layer BP is its chemical degradation under ambient conditions.[8] An extensive understanding of the degradation process and reliable ways to achieve stable and scalable degradation protection is necessary in order to realize the full potential of this material in its singleand few-layer forms. Recent studies have shed light on the underlying mechanism and speed of degradation by atomic force microscopy (AFM) and spectroscopic techniques.[8,9] Few passivation methods have also been investigated with various degree of success to slow down and prevent degradation.[3] Breakthroughs in developing powerful passivation methods to overcome this degradation issue require deeper experimental understanding and theoretical modeling of the degradation process as well as the effectiveness of passivation techniques. We report the first experimental quantification of geometric properties and theoretical modeling of the chemical degradation process of BP and investigate the effectiveness of passivation coatings using infrared scattering type scanning near-field microscopy (s-SNOM).[10,11] We chemically identify oxidized phosphorus species locally at the onset of degradation by nanoscale spectroscopic imaging at mid-infrared frequencies. We found that these species can form underneath 5 nm thick Al2O3 coating deposited by atomic layer deposition (ALD) indicating that thin coating is insufficient to protect BP against

/pdf/nanoscopy-of-black-phosphorus-degradation-1hk3xem7lw.pdf

Nanoscopy of Black Phosphorus Degradation

Fermions--elementary particles such as electrons--are classified as Dirac, Majorana or Weyl. Majorana and Weyl fermions had not been observed experimentally until the recent discovery of condensed matter systems such as topological superconductors and semimetals, in which they arise as low-energy excitations. Here we propose the existence of a previously overlooked type of Weyl fermion that emerges at the boundary between electron and hole pockets in a new phase of matter. This particle was missed by Weyl because it breaks the stringent Lorentz symmetry in high-energy physics. Lorentz invariance, however, is not present in condensed matter physics, and by generalizing the Dirac equation, we find the new type of Weyl fermion. In particular, whereas Weyl semimetals--materials hosting Weyl fermions--were previously thought to have standard Weyl points with a point-like Fermi surface (which we refer to as type-I), we discover a type-II Weyl point, which is still a protected crossing, but appears at the contact of electron and hole pockets in type-II Weyl semimetals. We predict that WTe2 is an example of a topological semimetal hosting the new particle as a low-energy excitation around such a type-II Weyl point. The existence of type-II Weyl points in WTe2 means that many of its physical properties are very different to those of standard Weyl semimetals with point-like Fermi surfaces.

/pdf/type-ii-weyl-semimetals-3b84bm463g.pdf

Type-II Weyl semimetals

Tungsten disulfide (WS2) has attracted great attention for use in optoelectronics due to its suitable bandgap and adjustable properties. However, the structure and photoelectric properties of bulk WS2 are not well understood. The first-principles method is applied herein to study the structural stability and electronic and optical properties of WS2. Two phases, viz. hexagonal and rhombohedral, are considered. The results reveal that the two bulk WS2 phases are thermodynamically and dynamically stable based on the enthalpy of formation and phonon dispersion. The structural stability of WS2 is attributed to the S–W–S sandwich structure. The calculated bandgap of hexagonal and rhombohedral WS2 is 1.552 eV and 1.488 eV, respectively, indicating that WS2 is semiconducting. The calculated optical properties show that WS2 exhibits excellent adsorption capacity for ultraviolet light, whether in the hexagonal or rhombohedral structure.

Structural Stability and Electronic and Optical Properties of Bulk WS 2 from First-Principles Investigations

A van der Waals heterostructure combined with intrinsic magnetism and topological orders have recently paved attractive avenues to realize quantum anomalous Hall effects. In this work, using first-principles calculations and effective model analysis, we propose that the robust quantum anomalous Hall states with sizable band gaps emerge in the van der Waals heterostructure of germanene/${\mathrm{Cr}}_{2}{\mathrm{Ge}}_{2}{\mathrm{Te}}_{6}$. This heterostructure possesses high thermodynamic stability, thus facilitating its experimental fabrication. Furthermore, we uncover that the proximity effect enhances the coupling between the germanene and ${\mathrm{Cr}}_{2}{\mathrm{Ge}}_{2}{\mathrm{Te}}_{6}$ layers, inducing the nontrivial band gaps in a wide range from 29 to 72 meV. The chiral edge states inside the band gap, leading to Hall conductance quantized to $\ensuremath{-}{e}^{2}/h$, are clearly visible. These findings provide an ideal candidate to detect the quantum anomalous Hall states and realize further applications to nontrivial quantum transport at a high temperature.

Intrinsic quantum anomalous Hall phase induced by proximity in the van der Waals heterostructure germanene/ Cr 2 Ge 2 Te 6

Hourglass dispersion is generally believed to be solely protected by nonsymmorphic symmetries, because these symmetries can introduce high-dimensional projective representations. Here, based on symmetry arguments, we propose that the hourglass dispersion can be jointly protected by symmorphic and nonsymmorphic symmetries, instead of the conventional nonsymmorphic symmetry. Moreover, using first-principles calculations, we realize our proposal in the phonon spectra of realistic materials that share an antiperovskite structure with space group $P4$/nmm. Importantly, the neck points of these hourglass dispersions trace out two nodal rings tangential to four nodal lines, forming a unique hourglass nodal cage in the bulk Brillouin zone. The Berry phase analysis reveals the nontrivial topology of these nodal rings and nodal lines. Furthermore, nontrivial surface states and isofrequency surface arcs are visible, facilitating their experimental confirmation of such exotic quasiparticles. work not only offers different insights into the hourglass dispersion, but also expands aspects for studying promising topological quasiparticles in condensed-matter systems.

Hourglass phonons jointly protected by symmorphic and nonsymmorphic symmetries

The interplay between light and matter attracts tremendous interest for exploring novel topological quantum states and their phase transitions. Here we show by first-principles calculations and the Floquet theorem that a carbon allotrope body-centered tetragonal ${\text{C}}_{16}$ $(\mathrm{bct}\text{\ensuremath{-}}{\mathrm{C}}_{16})$, a typical nodal-line semimetal, exhibits exotic photoinduced Floquet mixed-Weyl semimetallic features. Under the irradiation of a linearly polarized light, $\mathrm{bct}\text{\ensuremath{-}}{\mathrm{C}}_{16}$ undergoes a topological phase transition from a nodal-line semimetal to a Weyl semimetal with two pairs of tunable Weyl points. With increasing the light intensity, left-handed Weyl points evolve from type I into type II while right-handed ones are always preserved to be type I, giving rise to light-induced unconventional Weyl pairs composed of distinct types of Weyl points. Importantly, a special Weyl pair formed by type-I and type-III Weyl points is present at the critical transition point. The photon-dressed Fermi arcs connecting the projections of two different types of Weyl points are clearly visible, further revealing their unique topological features. Our work not only realizes promising unconventional Weyl pairs but also paves a reliable avenue for investigating light-induced topological phase transitions.

Photoinduced Floquet mixed-Weyl semimetallic phase in a carbon allotrope

The exploration of carbon phases with intact massless Dirac fermions in the presence of defects is critical for practical applications to nanoelectronics. Here, we identify by first-principles calculations that the Dirac cones can exist in graphene with stacking fault (SF) induced periodic line defects. These structures are width (n)-dependent to graphene nanoribbon and are thus termed as (SF)n-graphene. The electronic properties reveal that the semimetallic features with Dirac cones occur in (SF)n-graphene with n = 3m + 1, where m is a positive integer, and then lead to a quasi-one-dimensional conducting channel. Importantly, it is found that the twisted Dirac cone in the (SF)4-graphene is tunable among type-I, type-II, and type-III through a small uniaxial strain. The further stability analysis shows that (SF)n-graphene is thermodynamic stable. Our findings provide an artificial avenue for exploring Dirac Ffermions in carbon-allotropic structures in the presence of defects.

Dirac Fermions in Graphene with Stacking Fault Induced Periodic Line Defects.

Vacancies in phosphorene play important roles on understanding of fundamental properties and further applications in nanoelectronics and optoelectronics. In this paper, we have systematically investigated charged states and related surface oxidation induced by the single and double vacancies in phosphorene using first-principles calculations. We find that these defects can both achieve stable −1 charge states, which significantly affect the geometries and electronic properties. The reaction pathway calculations reveal that vacancy defects can remarkably lower the reaction barrier for O2 dissociation. More importantly, we propose a unique bioxidation mechanism in the presence of the double vacancy, resulting in the ultralow reaction barrier only 0.26 eV, which matches the rapid degradation of phosphorene in experiments. In addition, oxidation reaction can also impact on charged vacancy defects, providing a possible mechanism to explain the decrease of p-type doping and modest increased band gap. This work ...

Fangyang Zhan

Papers

Intrinsic quantum anomalous Hall phase induced by proximity in the van der Waals heterostructure germanene/ Cr 2 Ge 2 Te 6

Hourglass phonons jointly protected by symmorphic and nonsymmorphic symmetries

Photoinduced Floquet mixed-Weyl semimetallic phase in a carbon allotrope

Dirac Fermions in Graphene with Stacking Fault Induced Periodic Line Defects.

Interplay of Charged States and Oxygen Dissociation Induced by Vacancies in Phosphorene