L. Balicas

Atomic layers of hybridized boron nitride and graphene domains

Two-dimensional materials, such as graphene and monolayer hexagonal BN (h-BN), are attractive for demonstrating fundamental physics in materials and potential applications in next-generation electronics. Atomic sheets containing hybridized bonds involving elements B, N and C over wide compositional ranges could result in new materials with properties complementary to those of graphene and h-BN, enabling a rich variety of electronic structures, properties and applications. Here we report the synthesis and characterization of large-area atomic layers of h-BNC material, consisting of hybridized, randomly distributed domains of h-BN and C phases with compositions ranging from pure BN to pure graphene. Our studies reveal that their structural features and bandgap are distinct from those of graphene, doped graphene and h-BN. This new form of hybrid h-BNC material enables the development of bandgap-engineered applications in electronics and optics and properties that are distinct from those of graphene and h-BN.

Controlled synthesis and transfer of large-area WS2 sheets: from single layer to few layers.

The isolation of few-layered transition metal dichalcogenides has mainly been performed by mechanical and chemical exfoliation with very low yields. In this account, a controlled thermal reduction-sulfurization method is used to synthesize large-area (~1 cm(2)) WS2 sheets with thicknesses ranging from monolayers to a few layers. During synthesis, WOx thin films are first deposited on Si/SiO2 substrates, which are then sulfurized (under vacuum) at high temperatures (750-950 °C). An efficient route to transfer the synthesized WS2 films onto different substrates such as quartz and transmission electron microscopy (TEM) grids has been satisfactorily developed using concentrated HF. Samples with different thicknesses have been analyzed by Raman spectroscopy and TEM, and their photoluminescence properties have been evaluated. We demonstrated the presence of single-, bi-, and few-layered WS2 on as-grown samples. It is well known that the electronic structure of these materials is very sensitive to the number of layers, ranging from indirect band gap semiconductor in the bulk phase to direct band gap semiconductor in monolayers. This method has also proved successful in the synthesis of heterogeneous systems of MoS2 and WS2 layers, thus shedding light on the controlled production of heterolayered devices from transition metal chalcogenides.

Electron pockets in the Fermi surface of hole-doped high-Tc superconductors

High-temperature superconductivity in copper oxides occurs when the materials are chemically tuned to have a carrier concentration intermediate between their metallic state at high doping and their insulating state at zero doping. The underlying evolution of the electron system in the absence of superconductivity is still unclear, and a question of central importance is whether it involves any intermediate phase with broken symmetry. The Fermi surface of the electronic states in the underdoped ‘YBCO’ materials YBa2Cu3Oy and YBa2Cu4O8 was recently shown to include small pockets, in contrast with the large cylinder that characterizes the overdoped regime, pointing to a topological change in the Fermi surface. Here we report the observation of a negative Hall resistance in the magnetic-field-induced normal state of YBa2Cu3Oy and YBa2Cu4O8, which reveals that these pockets are electron-like rather than hole-like. We propose that these electron pockets most probably arise from a reconstruction of the Fermi surface caused by the onset of a density-wave phase, as is thought to occur in the electron-doped copper oxides near the onset of antiferromagnetic order. Comparison with materials of the La2CuO4 family that exhibit spin/charge density-wave order suggests that a Fermi surface reconstruction also occurs in those materials, pointing to a generic property of high-transition-temperature (Tc) superconductors.

Small anisotropy, weak thermal fluctuations, and high field superconductivity in Co-doped iron pnictide Ba(Fe1−xCox)2As2

We performed high-field magnetotransport and magnetization measurements on a single crystal of the 122-phase iron pnictide Ba(Fe1−xCox)2As2. Unlike the high-temperature superconductor cuprates and 1111-phase oxypnictides, Ba(Fe1−xCox)2As2 showed practically no broadening of the resistive transitions under magnetic fields up to 45 T. We report the temperature dependencies of the upper critical field Hc2 both parallel and perpendicular to the c-axis, the irreversibility field Hirrc(T), and a rather unusual symmetric volume pinning force curve Fp(H) suggestive of a strong pinning nanostructure. The anisotropy parameter γ=Hc2ab/Hc2c deduced from the slopes of dHc2ab/dT=4.9 T/K and dHc2c/dT=2.5 T/K decreases from ∼2 near Tc, to ∼1.5 at lower temperatures, much smaller than γ for 1111pnictides and high-Tc cuprates.

New First Order Raman-active Modes in Few Layered Transition Metal Dichalcogenides

Although the main Raman features of semiconducting transition metal dichalcogenides are well known for the monolayer and bulk, there are important differences exhibited by few layered systems which have not been fully addressed. WSe2 samples were synthesized and ab-initio calculations carried out. We calculated phonon dispersions and Raman-active modes in layered systems: WSe2, MoSe2, WS2 and MoS2 ranging from monolayers to five-layers and the bulk. First, we confirmed that as the number of layers increase, the E′, E″ and E2g modes shift to lower frequencies, and the A′1 and A1g modes shift to higher frequencies. Second, new high frequency first order A′1 and A1g modes appear, explaining recently reported experimental data for WSe2, MoSe2 and MoS2. Third, splitting of modes around A′1 and A1g is found which explains those observed in MoSe2. Finally, exterior and interior layers possess different vibrational frequencies. Therefore, it is now possible to precisely identify few-layered STMD.

Field-Effect Transistors Based on Few-Layered α-MoTe2

Here we report the properties of field-effect transistors based on a few layers of chemical vapor transport grown α-MoTe2 crystals mechanically exfoliated onto SiO2. We performed field-effect and Hall mobility measurements, as well as Raman scattering and transmission electron microscopy. In contrast to both MoS2 and MoSe2, our MoTe2 field-effect transistors are observed to be hole-doped, displaying on/off ratios surpassing 10(6) and typical subthreshold swings of ∼140 mV per decade. Both field-effect and Hall mobilities indicate maximum values approaching or surpassing 10 cm(2)/(V s), which are comparable to figures previously reported for single or bilayered MoS2 and/or for MoSe2 exfoliated onto SiO2 at room temperature and without the use of dielectric engineering. Raman scattering reveals sharp modes in agreement with previous reports, whose frequencies are found to display little or no dependence on the number of layers. Given that MoS2 is electron-doped, the stacking of MoTe2 onto MoS2 could produce ambipolar field-effect transistors and a gap modulation. Although the overall electronic performance of MoTe2 is comparable to those of MoS2 and MoSe2, the heavier element Te leads to a stronger spin-orbit coupling and possibly to concomitantly longer decoherence times for exciton valley and spin indexes.

Upper critical fields and thermally-activated transport of NdFeAsO(0.7)F(0.3) single crystal

We present detailed measurements of the longitudinal resistivity rho(xx)(T,H) and the upper critical field H(c2) of NdFeAsO(0.7)F(0.3) single crystals in strong dc and pulsed magnetic fields up to 45 and 60 T, respectively. We found that the field scale of H(c2) is comparable to H(c2)similar to 100 T of high-T(c) cuprates. H(c2)(T) parallel to the c axis exhibits a pronounced upward curvature similar to what was extracted from earlier measurements on polycrystalline LaFeAs(O,F), NdFeAs(O,F), and SmFeAs(O,F) samples. Thus, this behavior of H(c2)(perpendicular to)(T) is indeed an intrinsic feature of oxypnictides rather than manifestation of vortex lattice melting or granularity. The orientational dependence of H(c2)(theta) as a function of the angle theta between H and the c axis shows deviations from the one-band Ginzburg-Landau scaling. The mass anisotropy parameter gamma(T)=(m(c)/m(ab))(1/2)=H(c2)(parallel to)/H(c2)(perpendicular to) obtained from these measurements decreases as temperature decreases from gamma similar or equal to 9.2 at 44 K to gamma similar or equal to 5 at 34 K, where parallel to and perpendicular to correspond to H parallel and perpendicular to the ab planes, respectively. Spin-dependent magnetoresistance and nonlinearities in the Hall coefficient suggest contribution to the conductivity from electron-electron interactions modified by disorder reminiscent of that in diluted magnetic semiconductors. The Ohmic resistivity rho(xx)(T,H) measured below T(c) but above the irreversibility field exhibits a clear Arrhenius thermally-activated behavior rho=rho(0) exp[-E(a)(T,H)/T] over 4-5 decades of rho(xx). The activation energy E(a)(T,H) has very different field dependencies for H parallel to ab and H perpendicular to ab varying from 4x10(3) K at H=0.2 T to similar to 200 K at H=35 T. We discuss to what extent different pairing scenarios suggested in the literature can manifest themselves in the observed behavior of H(c2), using the two-band model of superconductivity in oxypnictides. The results indicate the importance of paramagnetic effects on H(c2)(T) in oxypnictides, which may significantly reduce H(c2)(0) as compared to H(c2)(0)similar to 200-300 T based on extrapolations of H(c2)(T) near T(c) down to low temperatures.

Artificially stacked atomic layers: Toward new van der Waals solids

Strong in-plane bonding and weak van der Waals interplanar interactions characterize a large number of layered materials, as epitomized by graphite. The advent of graphene (G), individual layers from graphite, and atomic layers isolated from a few other van der Waals bonded layered compounds has enabled the ability to pick, place, and stack atomic layers of arbitrary compositions and build unique layered materials, which would be otherwise impossible to synthesize via other known techniques. Here we demonstrate this concept for solids consisting of randomly stacked layers of graphene and hexagonal boron nitride (h-BN). Dispersions of exfoliated h-BN layers and graphene have been prepared by liquid phase exfoliation methods and mixed, in various concentrations, to create artificially stacked h-BN/G solids. These van der Waals stacked hybrid solid materials show interesting electrical, mechanical, and optical properties distinctly different from their starting parent layers. From extensive first principle calculations we identify (i) a novel approach to control the dipole at the h-BN/G interface by properly sandwiching or sliding layers of h-BN and graphene, and (ii) a way to inject carriers in graphene upon UV excitations of the Frenkell-like excitons of the h-BN layer(s). Our combined approach could be used to create artificial materials, made predominantly from inter planar van der Waals stacking of robust bond saturated atomic layers of different solids with vastly different properties.

A coherent three-dimensional Fermi surface in a high-transition-temperature superconductor

All conventional metals are known to possess a three-dimensional Fermi surface, which is the locus in reciprocal space of the long-lived electronic excitations that govern their electronic properties at low temperatures. These excitations should have well-defined momenta with components in all three dimensions. The high-transition-temperature (high-Tc) copper oxide superconductors have unusual, highly two-dimensional properties above the superconducting transition. This, coupled with a lack of unambiguous evidence for a three-dimensional Fermi surface, has led to many new and exotic models for the underlying electronic ground state. Here we report the observation of polar angular magnetoresistance oscillations in the overdoped superconductor Tl2Ba2CuO6+δ in high magnetic fields, which firmly establishes the existence of a coherent three-dimensional Fermi surface. Analysis of the oscillations reveals that at certain symmetry points, however, this surface is strictly two-dimensional. This striking form of the Fermi surface topography, long-predicted by electronic band structure calculations, provides a natural explanation for a wide range of anisotropic properties both in the normal and superconducting states. Our data reveal that, despite their extreme electrical anisotropy, the high-Tc materials at high doping levels can be understood within a framework of conventional three-dimensional metal physics.

CVD-grown monolayered MoS2 as an effective photosensor operating at low-voltage

We report the fabrication of a photosensor based on as-grown single crystal monolayers of MoS2 synthesized by chemical vapor deposition (CVD). The measurements were performed using Au/Ti leads in a two terminal configuration on CVD-grown MoS2 on a SiO2/Si substrate. The device was operated in air at room temperature at low bias voltages ranging from −2 V to 2 V and its sensing capabilities were tested for two different excitation wavelengths (514.5 nm and 488 nm). The responsivity reached 1.1 mA W−1 when excited with a 514.5 nm laser at a bias of 1.5 V. This responsivity is one order of magnitude larger than that reported from photo devices fabricated using CVD-grown multilayered WS2. A rectifying-effect was observed for the optically excited current, which was four times larger in the direct polarization bias when compared to the reverse bias photocurrent. Such rectifying behavior can be attributed to the asymmetric electrode placement on the triangular MoS2 monocrystal. It is envisioned that these components could eventually be used as efficient and low cost photosensors based on CVD-grown transition metal dichalcogenide monolayers.