Yigal Lilach

Gas Sensor Based on Metal−Insulator Transition in VO2 Nanowire Thermistor

Using temperature driven sharp metal-insulator phase transition in single crystal VO(2) nanowires, the realization of a novel gas sensing concept has been tested. Varying the temperature of the nanowire close to the transition edge, the conductance of the nanowire becomes extremely responsive to the tiny changes in molecular composition, pressure, and temperature of the ambient gas environment. This gas sensing analog of the transition edge sensor radiometry used in astrophysics opens new opportunities in gas sensorics.

Generation of electron Airy beams

We report the first experimental generation and observation of Airy beams of free electrons. The electron Airy beams are generated by diffraction of electrons through a nanoscale hologram, that imprints a cubic phase modulation on the beams’ transverse plane. We observed the spatial evolution dynamics of an arc-shaped, self accelerating and shape preserving electron Airy beams. We directly observed the ability of electrons to self-heal, restoring their original shape after passing an obstacle. This electromagnetic method opens up new avenues for steering electrons, like their photonic counterparts, since their wave packets can be imprinted with arbitrary shapes or trajectories. Furthermore, these beams can be easily manipulated using magnetic or electric potentials. It is also possible to efficiently self mix narrow beams having opposite signs of acceleration, hence obtaining a new type of electron interferometer.Within the framework of quantum mechanics, a unique particle wave packet exists in the form of the Airy function. Its counterintuitive properties are revealed as it propagates in time or space: the quantum probability wave packet preserves its shape despite dispersion or diffraction and propagates along a parabolic caustic trajectory, even though no force is applied. This does not contradict Newton’s laws of motion, because the wave packet centroid propagates along a straight line. Nearly 30 years later, this wave packet, known as an accelerating Airy beam, was realized in the optical domain; later it was generalized to an orthogonal and complete family of beams that propagate along parabolic trajectories, as well as to beams that propagate along arbitrary convex trajectories. Here we report the experimental generation and observation of the Airy beams of free electrons. These electron Airy beams were generated by diffraction of electrons through a nanoscale hologram, which imprinted on the electrons’ wavefunction a cubic phase modulation in the transverse plane. The highest-intensity lobes of the generated beams indeed followed parabolic trajectories. We directly observed a non-spreading electron wavefunction that self-heals, restoring its original shape after passing an obstacle. This holographic generation of electron Airy beams opens up new avenues for steering electronic wave packets like their photonic counterparts, because the wave packets can be imprinted with arbitrary shapes or trajectories.

Toward Two-Dimensional All-Carbon Heterostructures via Ion Beam Patterning of Single-Layer Graphene.

Graphene has many claims to fame: it is the thinnest possible membrane, it has unique electronic and excellent mechanical properties, and it provides the perfect model structure for studying materials science at the atomic level. However, for many practical studies and applications the ordered hexagon arrangement of carbon atoms in graphene is not directly suitable. Here, we show that the atoms can be locally either removed or rearranged into a random pattern of polygons using a focused ion beam (FIB). The atomic structure of the disordered regions is confirmed with atomic-resolution scanning transmission electron microscopy images. These structural modifications can be made on macroscopic scales with a spatial resolution determined only by the size of the ion beam. With just one processing step, three types of structures can be defined within a graphene layer: chemically inert graphene, chemically active amorphous 2D carbon, and empty areas. This, along with the changes in properties, gives promise that FIB patterning of graphene will open the way for creating all-carbon heterostructures to be used in fields ranging from nanoelectronics and chemical sensing to composite materials.

Sculpturing the electron wave function using nanoscale phase masks.

Electron beams are extensively used in lithography, microscopy, material studies and electronic chip inspection. Today, beams are mainly shaped using magnetic or electric forces, enabling only simple shaping tasks such as focusing or scanning. Recently, binary amplitude gratings achieved complex shapes. These, however, generate multiple diffraction orders, hence the desired shape, appearing only in one order, retains little of the beam energy. Here we demonstrate a method in electron-optics for arbitrarily shaping electron beams into a single desired shape, by precise patterning of a thin-membrane. It is conceptually similar to shaping light beams using refractive or diffractive glass elements such as lenses or holograms - rather than applying electromagnetic forces, the beam is controlled by spatially modulating its wavefront. Our method allows for nearly-maximal energy transference to the designed shape, and may avoid physical damage and charging effects that are the scorn of commonly-used (e.g. Zernike and Hilbert) phase-plates. The experimental demonstrations presented here - on-axis Hermite-Gauss and Laguerre-Gauss (vortex) beams, and computer-generated holograms - are a first example of nearly-arbitrary manipulation of electron beams. Our results herald exciting prospects for microscopic material studies, enables electron lithography with fixed sample and beam and high resolution electronic chip inspection by structured electron illumination.

Shaping plasmonic light beams with near-field plasmonic holograms

We introduce a new class of plasmonic holograms for the near field. These holograms provide complete control of the amplitude and phase of surface plasmon polaritons (SPPs), thereby enabling the generation of any desired plasmonic light beam. The scheme is based on a two-dimensional near-field plasmonic hologram, which couples the SPP from free space into the metal–dielectric interface, and also sets the attributes of the plasmonic beam. We demonstrate the concept for a wide variety of plasmonic beams with different qualities—in particular, “self-similar” Hermite–Gauss beams, “nondiffracting” cosine-Gauss beams, and “self-accelerating” Airy beams.

Anomalous Temperature Dependent Transport through Single Colloidal Nanorods Strongly Coupled to Metallic Leads

We report wiring of individual colloidal nanorods (NRs), 30-60 nm long by 3.5-5 nm diameter. Strong electrical coupling is achieved by electron beam induced deposition (EBID) of metallic lines targeting NR tips with nanometric precision. At T = 4 K many devices exhibit smooth I(V) curves with no sharp onset features, which remarkably fit a Fowler-Nordheim tunneling model. All devices exhibit an anomalous exponential temperature dependence of the form I approximately exp(T/T(0)). This irregular behavior cannot be explained by any hopping or activation model and is interpreted by accounting for the lowering of the NR conduction band due to lattice dilation and phonon coupling.

Compression and caging of CD3Cl by H2O layers on Ru(001)

The interaction of two similar coadsorbed dipolar molecules H2O and CD3Cl has been studied as a function of coverage over Ru(001) under ultra high vacuum conditions. The complementary techniques of temperature-programmed desorption mass spectrometry (ΔP-TPD) and work function change in a Δφ-TPD mode were employed. Adsorption of water on top of CD3Cl reveals two major trends: At submonolayer methyl chloride coverage, post-deposited water compresses the methyl chloride molecules and forces them to flip over to the methyl down configuration at the second layer, leading eventually to three-dimensional islands. This is indicated by both CD3Cl ΔP-TPD and differential work function [d(Δφ)/dT] data. Higher water coverage [θ(H2O)>1.2 bilayers (BL)] causes full detachment of the CD3Cl molecules from the ruthenium surface, to be encapsulated within the amorphous solid water (ASW) layer that is formed. At even higher water coverage [θ(H2O)>5 BL], methyl chloride desorbs in an explosivelike mode at 165 K. The caged me...

Dipole–dipole interactions among CH3Cl molecules on Ru(001): Correlation between work function change and thermal desorption studies

Work function change measurements (ΔΦ) combined with temperature programmed desorption (TPD) were employed to study layer growth mechanism and the CH3Cl dipole–dipole interactions on Ru(001), over the temperature range of 97 K–230 K. The activation energy for desorption (Ea) and the molecular dipole moment (μ) both decrease from 55.9 kJ/mol and 2.44 D, at the zero coverage limit, to 38.6 kJ/mol and 1.27 D, at one monolayer. This coverage dependence originates from strong dipolar lateral repulsion among neighbor CH3Cl molecules. Using a model introduced by Maschhoff and Cowin (MC) [J. Chem. Phys. 101, 8138 (1994)], the isolated adsorbed molecule’s dipole moment, μ0 (2.35 D) and polarizability α(8.1×10−24 cm3), were extracted from TPD data. These values agree very well with μ0 (2.12 D) and α(9.2×10−24 cm3) obtained from work function change measurements by employing the same MC model. The ability to simulate both TPD and work function change data over a wide coverage range within the framework of a single e...

Demonstration of spatially inhomogeneous vector beams with elliptical symmetry

We experimentally demonstrate vector beams having an elliptical symmetry of polarization, breaking the cylindrical symmetry of vector beams (e.g., radially polarized beams). Applications of such beams vary from material processing, lithography, and optical memories to excitation of elliptically shaped nanoparticles and plasmonic structures.

Photochemistry of caged molecules: CD3Cl@Ice

Hydrocarbons formation following UV photo-induced dissociation of CD3Cl trapped and caged inside thin amorphous solid water (ASW) layers on Ru(001) has been measured for the first time under well-defined UHV conditions. Stable products such as C2D6, CHD3, CD3CD2Cl, CD3OH were detected via post-irradiation temperature programmed desorption. Specific reactivity pathways for the various photo-products were identified based on excitation wavelengths, ASW layer thickness, and parent molecules initial coverage dependence. Cross sections of (1–6)×10−19 cm2 and (1–3)×10−20 cm2 at 193 nm and 248, respectively, were measured. These photo-induced phenomena of caged molecules are discussed as a possible mechanism for the formation of hydrocarbons in interstellar space.