K. Kempa

Highly efficient molecular delivery into mammalian cells using carbon nanotube spearing.

Introduction of exogenous DNA into mammalian cells represents a powerful approach for manipulating signal transduction. The available techniques, however, are limited by low transduction efficiency and low cell viability after transduction. Here we report a highly efficient molecular delivery technique, named nanotube spearing, based on the penetration of nickel-embedded nanotubes into cell membranes by magnetic field driving. DNA plasmids containing the enhanced green fluorescent protein (EGFP) sequence were immobilized onto the nanotubes, and subsequently speared into targeted cells. We have achieved an unprecedented high transduction efficiency in Bal17 B-lymphoma, ex vivo B cells and primary neurons with high viability after transduction. This technique may provide a powerful tool for highly efficient gene transfer into a variety of cells, especially the hard-to-transfect cells.

Growth of large periodic arrays of carbon nanotubes

Large periodic arrays of carbon nanotubes have been grown by plasma-enhanced hot filament chemical vapor deposition on periodic arrays of nickel dots that were prepared by polystyrene nanosphere lithography. A single layer of self-assembled polystyrene spheres was first uniformly deposited on a silicon wafer as a mask, and then electron beam vaporization was used to deposit a nickel layer through the mask. The size of and spacing between the nickel dots are tunable by varying the diameter of the polystyrene spheres, which consequently determines the diameter and site density of carbon nanotubes. The technique can be scaled up at much lower cost than electron beam lithography.

Unrestricted superlensing in a triangular two dimensional photonic crystal.

We demonstrate unrestricted superlensing in a triangular twodimensional hotonic crystal. We investigate simple two-point light sources maged by a slab lenses made of this photonic crystal, and show that the efraction of light follows simple rules of geometric optics with the Snellslaw efraction at each interface, and an effective isotropic refractive index n= -1 for light propagating inside the crystal. We contrast this behavior with that of a square two-dimensional photonic crystal in the first photonic band, where the effective dielectric response is anisotropic. This leads to a restricted superlensing, which does not follow the geometric optics.

Field emission of carbon nanotubes grown on carbon cloth

Field emission from carbon nanotubes grown on carbon cloth has been studied. An extremely low electric field of less than 0.4V∕μm is required to reach an emission current density of 1mA∕cm2. This ultralow operating electric field of carbon nanotubes grown on carbon cloth is mainly due to a very high field enhancement factor of 1.882×104, which is the result of geometrical configuration of the carbon nanotubes and the substrate. In addition to the field enhancement, the highly disordered microstructure of carbon nanotubes grown on carbon cloth plays an important role to field emission. This unexpected result indicates that the roughness of the substrates on which carbon nanotubes grow is very important. This result also brings us significantly closer to practical applications such as highly efficient lamps, field emission displays, micro vacuum electron sources, etc.

Giant field enhancement at carbon nanotube tips induced by multistage effect

Recently, we have reported an extremely strong field emission from carbon nanotubes grown on carbon cloth, with the field-enhancement factor of up to 18 800. In this paper, we study the origins of this effect, by investigating field emission from individual carbon nanotubes, in a transmission electron microscope equipped with a piezo manipulator. Microscopic analysis reveals a multistage structure of some of the nanotubes, characterized by an order of magnitude smaller nanotubes branching off the tips of bigger nanotubes (or carbon fibers). The multistage structure causes a macroscopic enhancement of the electric field, which can match that of a single macroscopically long nanotube with length equal to the combined length of all stages, and the tip radius equal to that of the thinnest nanotube in the structure. This not only explains the observed giant field enhancement, but also provides important clues for the design of nanotube emitters for electronic applications.

Lattice thermal conductivity of wires

The lattice thermal conductivity of free standing wires is calculated within a Boltzmann equation approach. Diffusive and specular phonon scattering at the wire surfaces are included by appropriate boundary conditions on the phonon distribution. The wire thermal conductivity is found to decrease markedly below the bulk value for narrow wires. We show that this decrease in wires is larger than that which occurs in free standing wells of comparable size.

Periodicity and alignment of large-scale carbon nanotubes arrays

Intensive studies have been carried out on controlling the periodicity and alignment of large-scale periodic arrays of carbon nanotubes (CNTs) using plasma-enhanced chemical vapor deposition. Catalytic dots are first prepared by self-assembly of polystyrene spheres on chromium-coated silicon substrates. Preparation parameters for CNTs growth including temperature, thickness of catalytic dots, plasma current intensity, and pregrowth plasma etching time are fine tuned and analyzed to achieve optimal combinations. High-quality aligned CNTs arrays with long-range periodicity and controlled diameters have been achieved. The good periodicity and alignment are critical for their applications such as photonic crystals, negative index of refraction, etc.

Spontaneous polarization of electrons in quantum dashes

We investigate the possibility of generating a spontaneous anti‐ferroelectric polarization in an array of elongated quantum dots, which we call ‘‘quantum dashes.’’ We suggest ways in which this phenomenon might be observed and point out possible technological applications resulting from this phenomenon.

High-bias-induced structure and the corresponding electronic property changes in carbon nanotubes

Atomic-scale microstructure changes of carbon nanotubes under high bias∕high current conditions were studied by in situ high-resolution transmission electron microscopy. We found that high bias voltage caused significant structure changes, such as crystallization and elimination of amorphous coating on the surface of nanotube walls, removal of nanotube walls, and formation of atomic-scale kinks. These structural changes are attributed to high temperatures induced by high bias resistive heating on the nanotubes. These structural changes cause dramatic electronic property changes of the nanotubes correspondingly. The results provide an efficient route to tailor the electronic properties of nanotubes by appropriate structural modifications.

Visible light diffraction studies on periodically aligned arrays of carbon nanotubes : Experimental and theoretical comparison

We have investigated visible light diffraction on honeycomb arrays of aligned carbon nanotubes grown on nickel nanoparticles prepared using the nanosphere lithography. A monolayer of 980nm polystyrene spheres was used as the mask for the deposition of nickel nanoparticles from which carbon nanotubes of 100nm in diameter and up to a couple of microns in length were grown. We show that a standard theory of diffraction from point scatterers explains all the observed diffraction features including Bragg’s law and the strong enhancement of the second and fifth order diffraction spots.