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High mobility WSe2 p- and n-type field-effect transistors contacted by highly doped graphene for low-resistance contacts.
Nano Lett.
PUBLISHED: 05-22-2014
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We report the fabrication of both n-type and p-type WSe2 field-effect transistors with hexagonal boron nitride passivated channels and ionic-liquid (IL)-gated graphene contacts. Our transport measurements reveal intrinsic channel properties including a metal-insulator transition at a characteristic conductivity close to the quantum conductance e(2)/h, a high ON/OFF ratio of >10(7) at 170 K, and large electron and hole mobility of ? ? 200 cm(2) V(-1 )s(-1) at 160 K. Decreasing the temperature to 77 K increases mobility of electrons to ?330 cm(2) V(-1) s(-1) and that of holes to ?270 cm(2) V(-1) s(-1). We attribute our ability to observe the intrinsic, phonon-limited conduction in both the electron and hole channels to the drastic reduction of the Schottky barriers between the channel and the graphene contact electrodes using IL gating. We elucidate this process by studying a Schottky diode consisting of a single graphene/WSe2 Schottky junction. Our results indicate the possibility to utilize chemically or electrostatically highly doped graphene for versatile, flexible, and transparent low-resistance ohmic contacts to a wide range of quasi-2D semiconductors.
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Mobility improvement and temperature dependence in MoSe2 field-effect transistors on parylene-C substrate.
ACS Nano
PUBLISHED: 04-16-2014
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We report low-temperature scanning tunneling microscopy characterization of MoSe2 crystals and the fabrication and electrical characterization of MoSe2 field-effect transistors on both SiO2 and parylene-C substrates. We find that the multilayer MoSe2 devices on parylene-C show a room-temperature mobility close to the mobility of bulk MoSe2 (100-160 cm(2) V(-1) s(-1)), which is significantly higher than that on SiO2 substrates (?50 cm(2) V(-1) s(-1)). The room-temperature mobility on both types of substrates are nearly thickness-independent. Our variable-temperature transport measurements reveal a metal-insulator transition at a characteristic conductivity of e(2)/h. The mobility of MoSe2 devices extracted from the metallic region on both SiO2 and parylene-C increases up to ?500 cm(2) V(-1) s(-1) as the temperature decreases to ?100 K, with the mobility of MoSe2 on SiO2 increasing more rapidly. In spite of the notable variation of charged impurities as indicated by the strongly sample-dependent low-temperature mobility, the mobility of all MoSe2 devices on SiO2 converges above 200 K, indicating that the high temperature (>200 K) mobility in these devices is nearly independent of the charged impurities. Our atomic force microscopy study of SiO2 and parylene-C substrates further rules out the surface roughness scattering as a major cause of the substrate-dependent mobility. We attribute the observed substrate dependence of MoSe2 mobility primarily to the surface polar optical phonon scattering originating from the SiO2 substrate, which is nearly absent in MoSe2 devices on parylene-C substrate.
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Flexible metallic nanowires with self-adaptive contacts to semiconducting transition-metal dichalcogenide monolayers.
Nat Nanotechnol
PUBLISHED: 03-20-2014
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In the pursuit of ultrasmall electronic components, monolayer electronic devices have recently been fabricated using transition-metal dichalcogenides. Monolayers of these materials are semiconducting, but nanowires with stoichiometry MX (M = Mo or W, X = S or Se) have been predicted to be metallic. Such nanowires have been chemically synthesized. However, the controlled connection of individual nanowires to monolayers, an important step in creating a two-dimensional integrated circuit, has so far remained elusive. In this work, by steering a focused electron beam, we directly fabricate MX nanowires that are less than a nanometre in width and Y junctions that connect designated points within a transition-metal dichalcogenide monolayer. In situ electrical measurements demonstrate that these nanowires are metallic, so they may serve as interconnects in future flexible nanocircuits fabricated entirely from the same monolayer. Sequential atom-resolved Z-contrast images reveal that the nanowires rotate and flex continuously under momentum transfer from the electron beam, while maintaining their structural integrity. They therefore exhibit self-adaptive connections to the monolayer from which they are sculpted. We find that the nanowires remain conductive while undergoing severe mechanical deformations, thus showing promise for mechanically robust flexible electronics. Density functional theory calculations further confirm the metallicity of the nanowires and account for their beam-induced mechanical behaviour. These results show that direct patterning of one-dimensional conducting nanowires in two-dimensional semiconducting materials with nanometre precision is possible using electron-beam-based techniques.
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Bond competition and phase evolution on the IrTe2 surface.
Nat Commun
PUBLISHED: 02-26-2014
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Compounds with incommensurate structural modulations have been extensively studied in last several decades. However, the relationship between structurally incommensurate/commensurate phases and associated electronic states remains enigmatic. Here we report the coexisting of complex incommensurate structures and highly unusual electronic roughness on the surface of in situ cleaved IrTe2 by using scanning tunnelling microscopy/spectroscopy, corroborated with extensive density-functional theory calculations. This behaviour is traced to structural instability, which induces a structural transition from a trigonal to a triclinic lattice below transition temperature, giving rise to the formation of unidirectional structural modulations with distinct wavelengths, accompanied by the opening of a 'pseudo'-gap in the surface layer. With further cooling the surface adopts a structure that reflects an ~6 × periodicity that is different from the bulk 5 × periodicity. Calculations show that the structure distortion is not associated with a charge density wave, but is rather associated with Te p-electron bonding.
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Atomically resolved spectroscopic study of Sr2IrO4: experiment and theory.
Sci Rep
PUBLISHED: 04-11-2013
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Particularly in Sr2IrO4, the interplay between spin-orbit coupling, bandwidth and on-site Coulomb repulsion stabilizes a J(eff) = 1/2 spin-orbital entangled insulating state at low temperatures. Whether this insulating phase is Mott- or Slater-type, has been under intense debate. We address this issue via spatially resolved imaging and spectroscopic studies of the Sr2IrO4 surface using scanning tunneling microscopy/spectroscopy (STM/S). STS results clearly illustrate the opening of an insulating gap (150 ~ 250 meV) below the Néel temperature (TN), in qualitative agreement with our density-functional theory (DFT) calculations. More importantly, the temperature dependence of the gap is qualitatively consistent with our DFT + dynamical mean field theory (DMFT) results, both showing a continuous transition from a gapped insulating ground state to a non-gap phase as temperatures approach TN. These results indicate a significant Slater character of gap formation, thus suggesting that Sr2IrO4 is a uniquely correlated system, where Slater and Mott-Hubbard-type behaviors coexist.
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Optical generation of excitonic valley coherence in monolayer WSe2.
Nat Nanotechnol
PUBLISHED: 03-13-2013
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As a consequence of degeneracies arising from crystal symmetries, it is possible for electron states at band-edges (valleys) to have additional spin-like quantum numbers. An important question is whether coherent manipulation can be performed on such valley pseudospins, analogous to that implemented using true spin, in the quest for quantum technologies. Here, we show that valley coherence can be generated and detected. Because excitons in a single valley emit circularly polarized photons, linear polarization can only be generated through recombination of an exciton in a coherent superposition of the two valley states. Using monolayer semiconductor WSe2 devices, we first establish the circularly polarized optical selection rules for addressing individual valley excitons and trions. We then demonstrate coherence between valley excitons through the observation of linearly polarized luminescence, whose orientation coincides with that of the linearly polarized excitation, for any given polarization angle. In contrast, the corresponding photoluminescence from trions is not observed to be linearly polarized, consistent with the expectation that the emitted photon polarization is entangled with valley pseudospin. The ability to address coherence, in addition to valley polarization, is a step forward towards achieving quantum manipulation of the valley index necessary for coherent valleytronics.
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Electrical control of neutral and charged excitons in a monolayer semiconductor.
Nat Commun
PUBLISHED: 01-15-2013
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Monolayer group-VI transition metal dichalcogenides have recently emerged as semiconducting alternatives to graphene in which the true two-dimensionality is expected to illuminate new semiconducting physics. Here we investigate excitons and trions (their singly charged counterparts), which have thus far been challenging to generate and control in the ultimate two-dimensional limit. Utilizing high-quality monolayer molybdenum diselenide, we report the unambiguous observation and electrostatic tunability of charging effects in positively charged (X(+)), neutral (X(o)) and negatively charged (X(-)) excitons in field-effect transistors via photoluminescence. The trion charging energy is large (30?meV), enhanced by strong confinement and heavy effective masses, whereas the linewidth is narrow (5?meV) at temperatures <55?K. This is greater spectral contrast than in any known quasi-two-dimensional system. We also find the charging energies for X(+) and X(-) to be nearly identical implying the same effective mass for electrons and holes.
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Kinetically inhibited order in a diamond-lattice antiferromagnet.
Proc. Natl. Acad. Sci. U.S.A.
PUBLISHED: 09-06-2011
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Frustrated magnetic systems exhibit highly degenerate ground states and strong fluctuations, often leading to new physics. An intriguing example of current interest is the antiferromagnet on a diamond lattice, realized physically in A-site spinel materials. This is a prototypical system in three dimensions where frustration arises from competing interactions rather than purely geometric constraints, and theory suggests the possibility of unusual magnetic order at low temperature. Here, we present a comprehensive single-crystal neutron scattering study of CoAl(2)O(4), a highly frustrated A-site spinel. We observe strong diffuse scattering that peaks at wavevectors associated with Néel ordering. Below the temperature T(?) = 6.5 K, there is a dramatic change in the elastic scattering lineshape accompanied by the emergence of well-defined spin-wave excitations. T(?) had previously been associated with the onset of glassy behavior. Our new results suggest instead that T(?) signifies a first-order phase transition, but with true long-range order inhibited by the kinetic freezing of domain walls. This scenario might be expected to occur widely in frustrated systems containing first-order phase transitions and is a natural explanation for existing reports of anomalous glassy behavior in other materials.
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Anomalous compressibility effects and superconductivity of EuFe2As2 under high pressures.
J Phys Condens Matter
PUBLISHED: 06-29-2010
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The crystal structure and electrical resistance of structurally layered EuFe(2)As(2) have been studied up to 70 GPa and down to a temperature of 10 K, using a synchrotron x-ray source and designer diamond anvils. The room temperature compression of the tetragonal phase of EuFe(2)As(2) (I4/mmm) results in an increase in the a-axis length and a rapid decrease in the c-axis length with increasing pressure. This anomalous compression reaches a maximum at 8 GPa and the tetragonal lattice behaves normally above 10 GPa, with a nearly constant c/a axial ratio. The rapid rise in the superconducting transition temperature (T(c)) to 41 K with increasing pressure is correlated with this anomalous compression, and a decrease in T(c) is observed above 10 GPa. We present P-V data or the equation of state for EuFe(2)As(2) both in the ambient tetragonal phase and in the high pressure collapsed tetragonal phase up to 70 GPa.
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Structural and magnetic phase transitions in NdCoAsO under high pressures.
J Phys Condens Matter
PUBLISHED: 04-15-2010
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We have investigated structural and magnetic phase transitions under high pressures in a quaternary rare-earth transition-metal arsenide oxide NdCoAsO compound that is isostructural to the high temperature superconductor parent phase NdFeAsO. The four-probe electrical resistance measurements carried out in a designer diamond anvil cell show that the ferromagnetic Curie temperature and antiferromagnetic Néel temperature increase with an increase in pressure. High pressure x-ray diffraction studies using a synchrotron source show a structural phase transition from a tetragonal phase to a new crystallographic phase at a pressure of 23 GPa at 300 K. The NdCoAsO sample remained antiferromagnetic and non-superconducting down to 10 K and up to the highest pressure achieved in this experiment, 53 GPa. A P-T phase diagram for NdCoAsO is presented from ambient conditions to P = 53 GPa and T = 10 K.
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Spin polarons in the correlated metallic pyrochlore Cd2Re2O7.
Phys. Rev. Lett.
PUBLISHED: 01-14-2010
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Muon spin rotation spectroscopy reveals localized electron states in the geometrically frustrated metallic pyrochlore Cd2Re2O7 at temperatures from 2 to 300 K in transverse magnetic fields up to 7 T. Two distinctive types of localized states, with characteristic radii of about 0.5 and 0.15 nm, are detected at high and low temperature, respectively. These states may be spin polarons, formed due to strong exchange interaction between itinerant electrons and the magnetic 5d electrons of Re ions, which may determine the peculiar electronic and magnetic properties of Cd2Re2O7.
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Resonant ultrasound studies of the layered perovskite system Ca2-xSrxRuO4.
J. Acoust. Soc. Am.
PUBLISHED: 12-17-2009
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The elastic response of the layered perovskite system Ca(2-x)Sr(x)RuO(4) (0.2< or =x< or =2) has been studied as a function of temperature and doping concentration x using resonant ultrasound spectroscopy. The elastic constants c(11) and c(44) have been obtained for three polycrystalline samples (x=1.0, 0.5, and 0.3) and show a softening trend with increasing Ca-content. In addition, the temperature-dependence of the elastic response of five single-crystals (x=2.0, 1.9, 0.5, 0.3, and 0.2) has been measured. For 2.0> or =x> or =0.5, a dramatic softening over a wide temperature range is observed upon cooling, which is attributed to the rotational instability of RuO(6) octahedra (for x=2.0 and 1.9) and the static rotation of the octahedra (for x=0.5). For the Ca-rich samples (x=0.3 and 0.2), the softening occurs in a very narrow temperature range, corresponding to the structural phase transition from high-temperature tetragonal to low-temperature orthorhombic symmetry.
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The performance of in situ grown Schottky-barrier single wall carbon nanotube field-effect transistors.
Nanotechnology
PUBLISHED: 02-03-2009
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Electrical transport measurements were used to study device behavior that results from the interplay of defects and inadvertent contact variance that develops in as-grown semiconducting single wall carbon nanotube devices with nominally identical Au contacts. The transport measurements reveal that as-grown nanotubes contain defects that limit the performance of field-effect transistors with ohmic contacts. In Schottky-barrier field-effect transistors the device performance is dominated by the Schottky barrier and the nanotube defects have little effect. We also observed strong rectifying behavior attributed to extreme contact asymmetry due to the different nanoscale roughness of the gold contacts formed during nanotube growth.
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JoVE Visualize is a tool created to match the last 5 years of PubMed publications to methods in JoVE's video library.

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In developing our video relationships, we compare around 5 million PubMed articles to our library of over 4,500 methods videos. In some cases the language used in the PubMed abstracts makes matching that content to a JoVE video difficult. In other cases, there happens not to be any content in our video library that is relevant to the topic of a given abstract. In these cases, our algorithms are trying their best to display videos with relevant content, which can sometimes result in matched videos with only a slight relation.