Krzysztof Kempa

Superplastic carbon nanotubes.

The theoretical maximum tensile strain — that is, elongation — of a single-walled carbon nanotube is almost 20%, but in practice only 6% is achieved. Here we show that, at high temperatures, individual single-walled carbon nanotubes can undergo superplastic deformation, becoming nearly 280% longer and 15 times narrower before breaking. This superplastic deformation is the result of the nucleation and motion of kinks in the structure, and could prove useful in helping to strengthen and toughen ceramics and other nanocomposites at high temperatures.

Receiving and transmitting light-like radio waves: Antenna effect in arrays of aligned carbon nanotubes

We present optical measurements of random arrays of aligned carbon nanotubes, and show that the response is consistent with conventional radio antenna theory. We first demonstrate the polarization effect, the suppression of the reflected signal when the electric field of the incoming radiation is polarized perpendicular to the nanotube axis. Next, we observe the interference colors of the reflected light from an array, and show that they result from the length matching antenna effect. This antenna effect could be used in a variety of optoelectronic devices, including THz and IR detectors.

Metamaterial-plasmonic absorber structure for high efficiency amorphous silicon solar cells.

We show that a planar structure, consisting of an ultrathin semiconducting layer topped with a solid nanoscopically perforated metallic film and then a dielectric interference film, can highly absorb (superabsorb) electromagnetic radiation in the entire visible range, and thus can become a platform for high-efficiency solar cells. The perforated metallic film and the ultrathin absorber in this broadband superabsorber form a metamaterial effective film, which negatively refracts light in this broad frequency range. Our quantitative simulations confirm that the superabsorption bandwidth is maximized at the checkerboard pattern of the perforations. These simulations show also that the energy conversion efficiency of a single-junction amorphous silicon solar cell based on our optimized structure can exceed 12%.

Uniform self-forming metallic network as a high-performance transparent conductive electrode.

A transparent, conductive, and flexible electrode is demonstrated. It is based on an inexpensive and easily manufacturable metallic network formed by depositing metals onto a template film. This electrode shows excellent electro-optical properties, with the figure of merit ranging from 300 to 700, and transmittance from 82% (~4.3 Ω sq(-1) ) to 45% (~0.5 Ω sq(-1) ).

Foundations of Plasmonics

Plasma physics is a very mature field, studied extensively for well over a century. The cross-disciplinary field of plasmonics (electromagnetics of metallic nanostructures), on the other hand, with its potential for an extraordinary light control through novel class of materials and the resulting applications, has become very fashionable only recently. Inevitably, as a result of this rapid development, the deep connections with the mother discipline, the plasma physics, have sometimes been overlooked. The goal of this work is to review some of these basic connections, which are relevant, and ultimately helpful for researchers in the new field. We focus on the solid-state structured plasmas and address the issue of classical versus quantum treatments. We discuss the little known subtleties of the surface plasmons at metallic surfaces (e.g. multipole plasmons) and their consequences on plasmonics of the textured metallic films. Plasmonics of nanoparticles has been preceded by studies of plasma effects in metallic clusters and semiconducting quantum dots (QDs). In this context, we discuss the little known connection between the Mie resonance in metallic particles and the collective resonance in wide parabolic quantum wells (QWs) and QDs. Researchers dealing with plasmonics of thin films can benefit from earlier studies of plasmons in the semiconductor modulation doped heterojunctions and QWs, with its rich spectrum of intersubband and two-dimensional plasmons. In non-equilibrium plasmonic systems, generation of plasmons can be stimulated, leading to the exciting possibility of the plasmon instability. Extraordinarily complex is the plasmonics of carbon nanotubes and graphene, with its numerous van Hove, one- and three-dimensional plasmons, and we discuss how the plasmonics of metamaterials can benefit from this complexity. Finally, we discuss a few applications, which could directly benefit from plasmonics, including medical and the novel class of solar cells.

Subwavelength waveguide for visible light

The authors demonstrate transmission of visible light through metallic coaxial nanostructures many wavelengths in length, with coaxial electrode spacing much less than a wavelength. Since the light frequency is well below the plasma resonance in the metal of the electrodes, the propagating mode reduces to the well-known transverse electromagnetic mode of a coaxial waveguide. They have thus achieved a faithful analog of the conventional coaxial cable for visible light.

Bio-inspired networks for optoelectronic applications

Modern optoelectronics needs development of new materials characterized not only by high optical transparency and electrical conductivity, but also by mechanical strength, and flexibility. Recent advances employ grids of metallic micro- and nanowires, but the overall performance of the resulting material composites remains unsatisfactory. In this work, we propose a new strategy: application of natural scaffoldings perfected by evolution. In this context, we study two bio-inspired networks for two specific optoelectronic applications. The first network, intended for solar cells, light sources and similar devices, has a quasi-fractal structure and is derived directly from a chemically extracted leaf venation system. The second network is intended for touch screens and flexible displays, and is obtained by metalizing a spiders silk web. We demonstrate that each of these networks attain an exceptional optoelectonic and mechanical performance for its intended purpose, providing a promising direction in the development of more efficient optoelectronic devices.

Interaction between carbon nanotubes and mammalian cells: characterization by flow cytometry and application

We show herein that CNT-cell complexes are formed in the presence of a magnetic field. The complexes were analyzed by flow cytometry as a quantitative method for monitoring the physical interactions between CNTs and cells. We observed an increase in side scattering signals, where the amplitude was proportional to the amount of CNTs that are associated with cells. Even after the formation of CNT-cell complexes, cell viability was not significantly decreased. The association between CNTs and cells was strong enough to be used for manipulating the complexes and thereby conducting cell separation with magnetic force. In addition, the CNT-cell complexes were also utilized to facilitate electroporation. We observed a time constant from CNT-cell complexes but not from cells alone, indicating a high level of pore formation in cell membranes. Experimentally, we achieved the expression of enhanced green fluorescence protein by using a low electroporation voltage after the formation of CNT-cell complexes. These results suggest that higher transfection efficiency, lower electroporation voltage, and miniaturized setup dimension of electroporation may be accomplished through the CNT strategy outlined herein.

Roadmap on optical energy conversion

For decades, progress in the field of optical (including solar) energy conversion was dominated by advances in the conventional concentrating optics and materials design. In recent years, however, conceptual and technological breakthroughs in the fields of nanophotonics and plasmonics combined with a better understanding of the thermodynamics of the photon energy-conversion processes reshaped the landscape of energy-conversion schemes and devices. Nanostructured devices and materials that make use of size quantization effects to manipulate photon density of states offer a way to overcome the conventional light absorption limits. Novel optical spectrum splitting and photon-recycling schemes reduce the entropy production in the optical energy-conversion platforms and boost their efficiencies. Optical design concepts are rapidly expanding into the infrared energy band, offering new approaches to harvest waste heat, to reduce the thermal emission losses, and to achieve noncontact radiative cooling of solar cells as well as of optical and electronic circuitries. Light–matter interaction enabled by nanophotonics and plasmonics underlie the performance of the third- and fourth-generation energy-conversion devices, including up- and down-conversion of photon energy, near-field radiative energy transfer, and hot electron generation and harvesting. Finally, the increased market penetration of alternative solar energy-conversion technologies amplifies the role of cost-driven and environmental considerations. This roadmap on optical energy conversion provides a snapshot of the state of the art in optical energy conversion, remaining challenges, and most promising approaches to address these challenges. Leading experts authored 19 focused short sections of the roadmap where they share their vision on a specific aspect of this burgeoning research field. The roadmap opens up with a tutorial section, which introduces major concepts and terminology. It is our hope that the roadmap will serve as an important resource for the scientific community, new generations of researchers, funding agencies, industry experts, and investors.

Hot electron effect in nanoscopically thin photovoltaic junctions

The open circuit voltage in ultrathin amorphous silicon solar cells is found to increase with light energy (frequency), due to extraction of hot electrons. The ultrathin nature of these junctions also leads to large internal electric fields, yielding reduced recombination and increased current. A simple phenomenological argument provides a qualitative understanding of these effects and gives guidelines for designing future, high-efficiency, hot electron solar cells.