High-efficiency clean EUV plasma source at 10–30 nm, driven by a long-pulse-width excimer laser

A long-pulse-width high-output energy (120 ns FWHM, 7 J) XeCl laser has been focused on thin tape targets (Cu and Ta) to generate more than 100-ns-long (FWHM) EUV pulses in the 10–30 nm spectral region, suitable for projection microlithography. The conversion efficiency was more than 20% over a 2π solid angle. We observed debris emission using a gated CCD camera, and measured the debris speed for different irradiation conditions. We found irradiation conditions such that the measured velocities were low enough that simple mechanical devices combined with krypton at low-pressure could efficiently stop both ionic debris and cluster debris. Our results show that a suitable combination of driving-laser characteristics, target material and thickness, environment gas and mechanical choppers can make clean and increase the power of EUV solid-target laser-plasma sources. Received: 21 October 2002 / Published online: 26 February 2003 RID="*" ID="*"Corresponding author. Fax: +39-06/9400-5334, E-mail: bollanti@frascati.enea.it     

Silicon nanowires grown from Au-coated Si substrate

Amorphous Si nanowires were grown on an Au-coated Si substrate by heat treatment at 1000 °C under an H₂ atmosphere. The nanowires have a length of several tens of a micron and a diameter of 10–20 nm. The growth mechanism of the nanowires was investigated and explained with a solid–liquid–solid model. Received: 11 July 2002 / Accepted: 7 July 2002 / Published online: 4 December 2002 RID="*" ID="*"Corresponding author. Fax: +86/10-62751615, E-mail: yudp@pku.edu.cn     

Numerical analysis of nanocluster formation within ns-laser ablation plume

The CIP method is used to calculate macroscopic plume expansion combined with the Zeldovich–Raiser theory for the cluster-formation process such as nucleation and growth. The effect of background gas and latent heat is examined in the one-dimensional case. The latent heats keep the plume temperature at 2500 K for a long period and this explains the delayed photoluminescence. Two contradicting experiments on the size dependence on ambient pressure are clearly explained and are attributed to the difference of laser energy. In two-dimensional simulation, a ‘mushroom-like’ plume shape is replicated, consistent with experimental results. Received: 17 January 2003 / Accepted: 8 February 2003 / Published online: 28 May 2003 RID="*" ID="*"Corresponding author. Fax: +81-3-5237-2860, E-mail: tohkubo@es.titech.ac.jp     

Energetics of Brownian motors: a review

We review the literature on the energetics of Brownian motors, distinguishing between forced ratchets, chemical motors – driven out of equilibrium by differences of chemical potential, and thermal motors – driven by temperature differences. The discussion is focused on the definition of efficiency and the compatibility between the models and the laws of thermodynamics. Received: 13 November 2001 / Accepted: 10 January 2002 / Published online: 22 April 2002     

Force-feedback joystick as a low-cost haptic interface for an atomic-force-microscopy nanomanipulator

In order to manipulate materials at the nanometer scale, new methods and devices have to be developed. A nanomanipulator interface was designed and implemented in a commercial atomic-force-microscope (AFM) system. With the aid of a positioning joystick, direct positioning of the AFM probe with nanometer precision was possible. A commercial force-feedback joystick served as a haptic interface and provided the user with real-time feeling of the tip–sample interactions. Due to the open design, the manipulator interface could be used with other microscopes of the SPM family. In addition, the nanomanipulator and an UV-laser microbeam for photoablation were combined on an inverted optical microscope. To test the nanomanipulator, human metaphase chromosomes were dissected using both photoablation and mechanical AFM manipulation. The experimental results show that by combining both methods, biological material can be manipulated on different size scales in one integrated instrument. The effects of manipulation on the chromosome were studied in detail by AFM. Sub-400 nm cuts were achieved by photonic ablation. Chromosomal fragments of a size less than of 500 nm could be isolated. By means of mechanical microdissection, different cut sizes ranging from 80 nm to 500 nm could be easily obtained by applying different load forces. Received: 2 September 2002 / Accepted: 2 September 2002 / Published online: 5 March 2003 RID="*" ID="*"Current affiliation: Nanotechnology Group, ETH Zurich, Switzerland RID="**" ID="**"Corresponding author. Fax: +49-89/2180-4331, E-mail: heckl@lmu.de     

Supramolecular architectures and nanostructures at metal surfaces

The controlled formation of non-covalent bonds (H-bonding, metal–ligand interactions) is the key ingredient for the fabrication of supramolecular architectures and nanostructures. Upon deposition of molecular building blocks at well-defined surfaces, this issue can be directly addressed. Scanning tunneling microscopy observations are presented, which provide insight into the interaction of functional groups on metal substrates at the molecular level. In particular, carboxylic acids were employed: (4-[(pyrid-4-yl-ethynyl)]-benzoic acid (PEBA), 4-[trans-2-(pyrid-4-yl-vinyl)]-benzoic acid (PVBA) and trimesic acid (1,3,5-benzenetricarboxylic acid, TMA), which could be stabilized in a flat geometry at the surface. By choosing the appropriate substrate material and symmetry, the sensitive balance of intermolecular and molecule–substrate interactions can be tuned to obtain well-defined supramolecular architectures and nanostructures. The head-to-tail hydrogen bonding of the related rod-like species PEBA and PVBA stabilizes molecular rows on Ag(111). The subtle difference in the molecular geometries is reflected in the lateral ordering: While 2-D islanding is encountered with PEBA, 1-D nanogratings of supramolecular chiral H-bonded twin chains evolve for PVBA. The threefold symmetry of TMA in conjunction with the self-complementarity of its exodentate groups accounts for the formation of H-bonded honeycomb networks on Cu(100) at low temperatures. Metal–ligand interactions were probed with PVBA and TMA at Cu surfaces at ambient temperature. Deprotonation of the carboxyl moiety takes place, which readily interacts with Cu adatoms evaporated from step edges. This leads to a head-to-head pairing of PVBA on Cu(111) and cloverleaf-shaped Cu–TMA coordination compounds on Cu(001). Received: 4 June 2002 / Accepted: 2 October 2002 / Published online: 5 February 2003 RID="*" ID="*"Corresponding author. Fax: +41-21/693-3604, E-mail: johannes.barth@epfl.ch     

Femtosecond study of surface structure and composition and time-resolved spectroscopy in metals

Surface structuring and compositioning in aluminum alloy 2024-T3 were demonstrated using a femtosecond pulse laser. Surface nanostructuring was developed as a function of laser parameters and the surface micrographs of the scanning electron microscopy were characterized as a function of incident laser fluence. Surface compositioning was performed by selectively removing the elements on the surface of the sample. Femtosecond studies of highly excited electrons were performed by a pump–probe technique, and the thermalization time was found to be in a range of 1.5–3 ps, increasing with incident fluence. The time-resolved measurement is well matched to the numerical calculation. Received: 6 September 2001 / Accepted: 18 July 2002 / Published online: 25 October 2002 RID="*" ID="*"Corresponding author. Fax: +1-405/744-6811, E-mail: dou@okstate.edu     

A simple wet-chemical synthesis and characterization of CuO nanorods

Using a simple wet-chemical route, we synthesized CuO nanorods with diameters of ca. 5–15 nm and lengths of up to 400 nm. The purity, crystallinity, morphology, structure features, and chemical composition of the as-prepared CuO nanorods were investigated by powder X-ray diffraction, transmission electron microscopy, high-resolution transmission electron microscopy, X-ray photoelectron spectroscopy, and Raman spectroscopy. Received: 22 March 2002 / Accepted: 12 June 2002 / Published online: 28 October 2002 RID="*" ID="*"Corresponding author. Fax: +86-25/359-5535, E-mail: wangqun@nju.edu.cn     

Ratchets, power strokes, and molecular motors

We present a method for determining the effective driving potential for a molecular motor from measurements of its stochastic position versus time. In developing the method we can make precise the previously vague notions of ‘Brownian ratchet’ and ‘power stroke’, and suggest means to experimentally distinguish between the two. In particular, we distinguish between two kinds of ratchets: ratchets that rectify large fluctuations and ratchets that bias small fluctuations. Received: 24 October 2001 / Accepted: 11 February 2002 / Published online: 22 April 2002     

Magnetic properties of Ni:SiO<Subscript>2</Subscript> nanocomposites synthesized by a modified sol–gel method

Ni nanoparticles embedded in an amorphous SiO₂ matrix were produced by a modified sol–gel method. This method resulted in nanocomposites with a controlled size distribution and good dispersion of the metallic particles. The particle-size distributions were found to have an average radius of ∼3 nm, as inferred from transmission electron microscopy, X-ray-diffraction analysis, and magnetic measurements. Magnetic characterizations revealed that samples exhibit superparamagnetic behavior above the blocking temperature T_B, 20 K≤T_B≤40 K, and absence of a shift along the field axis on hysteresis loops measured at T≤T_B, indicating that the metallic nanoparticles are also free from an oxide layer. Received: 7 October 2002 / Accepted: 9 October 2002 / Published online: 8 January 2003 RID="*" ID="*"Corresponding author. Fax: +55-11/3091-6984, E-mail: rjardim@if.usp.br     

Nanostructures fabricated on metal surfaces assisted by laser with optical near-field effects

Nanostructures on metal film surfaces have been written directly using a pulsed ultraviolet laser. The optical near-field effects of the laser were investigated. Spherical silica particles (500–1000 nm in diameter) were placed on metal films. After laser illumination with a single laser shot, nanoholes were obtained at the original position of the particles. The mechanism for the formation of the nanostructure patterns was investigated and found to be the near-field optical resonance effect induced by the particles on the surface. The size of the nanohole was studied as a function of laser fluence and silica particle size. The experimental results show a good agreement with those of the relevant theoretical calculations of the near-field light intensity distribution. The method of particle-enhanced laser irradiation allows the study of field enhancement effects as well as its potentialapplications for nanolithography. Received: 10 December 2002 / Accepted: 20 January 2003 / Published online: 28 May 2003 RID="*" ID="*"Corresponding author. Fax: +65-777/1349, E-mail: HUANG_Sumei@dsi.a-star.edu.sg     

Electron ratchet effect in semiconductor devices and artificial materials with broken centrosymmetry

Studies on nonlinear electron transport in nanometer-sized semiconductor devices with broken centrosymmetry are reviewed. In these devices, an applied alternating (rocking) electric field induces a net flow of electrons in the direction perpendicular to that of the applied field. Such an electron ratchet effect has been observed in a number of differently designed devices, fabricated from two types of semiconductor material systems. The functionality is interpreted with an extended Büttiker–Landauer formula. We show that the devices operate at both cryogenic and room temperatures and at frequencies up to at least 50 GHz. Based on a similar microscopic mechanism, we have also constructed, to the best of our knowledge, the first artificial electronic nanomaterial that operates at room temperature. The promising possibilities for practical applications, such as rectification, microwave detection, second-harmonic generation, etc., are also discussed. Received: 16 January 2002 / Accepted: 11 February 2002 / Published online: 22 April 2002     

Quantum ratchets and quantum heat pumps

Quantum ratchets are Brownian motors in which the quantum dynamics of particles induces qualitatively new behavior. We review a series of experiments in which asymmetric semiconductor devices of sub-micron dimensions are used to study quantum ratchets for electrons. In rocked quantum-dot ratchets electron-wave interference is used to create a non-linear voltage response, leading to a ratchet effect. The direction of the net ratchet current in this type of device can be sensitively controlled by changing one of the following experimental variables: a small external magnetic field, the amplitude of the rocking force, or the Fermi energy. We also describe a tunneling ratchet in which the current direction depends on temperature. In our discussion of the tunneling ratchet we distinguish between three contributions to the non-linear current–voltage characteristics that lead to the ratchet effect: thermal excitation over energy barriers, tunneling through barriers, and wave reflection from barriers. Finally, we discuss the operation of adiabatically rocked tunneling ratchets as heat pumps. Received: 8 February 2002 / Accepted: 11 February 2002 / Published online: 22 April 2002     

Stochastic mechano-chemical kinetics of molecular motors: A multidisciplinary enterprise from a physicist’s perspective

A molecular motor is made of either a single macromolecule or a macromolecular complex. Just like their macroscopic counterparts, molecular motors “transduce” input energy into mechanical work. All the nano-motors considered here operate under isothermal conditions far from equilibrium. Moreover, one of the possible mechanisms of energy transduction, called Brownian ratchet, does not even have any macroscopic counterpart. But, molecular motor is not synonymous with Brownian ratchet; a large number of molecular motors execute a noisy power stroke, rather than operating as Brownian ratchet. We review not only the structural design and stochastic kinetics of individual single motors, but also their coordination, cooperation and competition as well as the assembly of multi-module motors in various intracellular kinetic processes. Although all the motors considered here execute mechanical movements, efficiency and power output are not necessarily good measures of performance of some motors. Among the intracellular nano-motors, we consider the porters, sliders and rowers, pistons and hooks, exporters, importers, packers and movers as well as those that also synthesize, manipulate and degrade “macromolecules of life”. We review mostly the quantitative models for the kinetics of these motors. We also describe several of those motor-driven intracellular stochastic processes for which quantitative models are yet to be developed. In part I, we discuss mainly the methodology and the generic models of various important classes of molecular motors. In part II, we review many specific examples emphasizing the unity of the basic mechanisms as well as diversity of operations arising from the differences in their detailed structure and kinetics. Multi-disciplinary research is presented here from the perspective of physicists.     

Flexible ceramic–polymer composite films with temperature-insensitive and tunable dielectric permittivity

BaTiO₃–polymer composite layers have been produced by the spin-on technique (thickness 3–10 μm). The dielectric permittivity of the layers at room temperature can be tuned from 2.8 to approximately 33 by varying the ceramic filling from 0 to 60% by volume. The dielectric properties of the films are almost insensitive to temperature variations in the range 20–180 °C. Free-standing composite layers with ceramic content ≤50% are flexible without noticeable change of permittivity after repeated mechanical bending. Received: 22 November 2001 / Accepted: 24 November 2001 / Published online: 23 January 2002     

Electromagnetically induced transparency mediated by phonons in strongly coupled exciton–phonon systems

It is shown theoretically that electromagnetically induced transparency (EIT) due to strong exciton–phonon coupling can occur in strongly coupled exciton–phonon systems such as polymers and organic semiconductors and lead to ultra-slow light effects. The results indicate that the strong coupling of excitons and phonons is important, but the exciton– exciton interaction plays a small role in the generation of the EIT. Numerical results for polydiacetylene–toluene sulfonate are also presented. This EIT in a solid-state medium might be utilized for efficient multiwave mixing and quantum nondemolition measurements, as well as for novel acousto-optical devices. Received: 21 August 2002 / Published online: 20 December 2002 RID="*" ID="*"Corresponding author. E-mail: zhukadi@yahoo.com     

Group-IV nanocluster formation by ion-beam synthesis

A short review of our investigations devoted to the use of ion-beam-synthesized nanoclusters for silicon-based light emission and nonvolatile memory effects is presented. Blue-violet light emission is demonstrated based on Ge-implanted silicon dioxide layers thermally grown on silicon substrates. This version of silicon-based light emission relies on Ge-related defects in the amorphous ≡Si–O–Si≡ network. The photoluminescence and electroluminescence are excited by a singlet S₀–S₁ transition of a neutral oxygen vacancy and by electron injection from the silicon substrate into the silicon dioxide layer, respectively. Whereas the photoluminescence excitation is a well-known mechanism, for the case of electroluminescence an interpretation was performed for the first time in the course of our studies. It was found that the most probable way to excite luminescence centers is the impact excitation by hot electrons. Whereas the injection is explained by trap-assisted tunneling of electrons from the substrate into the oxide, the electrons will be transported via traps or in the SiO₂ conduction band. The application of the silicon-based light-emitting devices for an integrated optocoupler arrangement is described. Another application of nanoclusters is based on the investigation of thin Si-implanted silicon dioxide layers for nonvolatile memory devices. First promising results demonstrate that the observed programming window can reach several volts and the devices exhibit excellent retention behavior. A 256 K-nv-SRAM is demonstrated showing a programming window of >1 V for write pulses of 12 V/8 ms. Received: 21 August 2002 / Accepted: 21 August 2002 / Published online: 12 February 2003 RID="*" ID="*"Corresponding author. Fax: +49-351/260-3411, E-mail: w.skorupa@fz-rossendorf.de     

Potential for micromachined actuation of ultra-wide continuously tunable optoelectronic devices

Tailored scaling represents a principle of success that, both in nature and in technology, allows the effectiveness of physical effects to be enhanced. Mutation and selection in nature are imitated in technology, e.g. by model calculation and design. Proper scaling of dimensions in natural photonic crystals and our fabricated artificial 1D photonic crystals (DBRs, distributed Bragg reflectors) enable efficient diffractive interaction in a specific spectral range. For our optical microsystems we illustrate that tailored miniaturization may also increase the mechanical stability and the effectiveness of spectral tuning by thermal and electrostatic actuation, since the relative significance of the fundamental physical forces involved considerably changes with scaling. These basic physical principles are rigorously applied in micromachined 1.55-μm vertical-resonator-based devices. We modeled, implemented and characterized 1.55-μm micromachined optical filters and vertical-cavity surface-emitting laser devices capable of wide, monotonic and kink-free tuning by a single control parameter. Tuning is achieved by mechanical actuation of one or several air-gaps that are part of the vertical resonator including two ultra-highly reflective DBR mirrors of strong refractive index contrast: (i) Δn=2.17 for InP/air-gap DBRs (3.5 periods) using GaInAs sacrificial layers and (ii) Δn=0.5 for Si₃N₄/SiO₂ DBRs (12 periods) with a polymer sacrificial layer to implement the air-cavity. In semiconductor multiple air-gap filters, a continuous tuning of >8% of the absolute wavelength is obtained. Varying the reverse voltage (U=0–5 V) between the membranes (electrostatic actuation), a tuning range of >110 nm was obtained for a large number of devices. The correlation of the wavelength and the applied voltage is accurately reproducible without any hysteresis. In two filters, tuning of 127 and 130 nm was observed for about ΔU=7 V. The extremely wide tuning range and the very small voltage required are record values to the best of our knowledge. For thermally actuated dielectric filters based on polymer sacrificial layers, Δλ/ΔU=-7 nm/V is found. Received: 10 May 2002 / Published online: 8 August 2002     

Nanoscale effects in focused ion beam processing

Focused ion beams with diameters of 8 to 50 nm are used for material processing in the nanoscale regime. In this paper, effects of the ion beam–solid interaction determining the formation of small structures by ion-beam sputtering and chemically assisted material deposition and etching are investigated. In the case of decreasing feature size, angle-dependent sputtering, a non-constant sputter rate, and scattered ions play an important role. The impact on side-wall angle, aspect ratio, and shape of the bottom of the etched structures is discussed. In beam tail regions, these effects will be especially pronounced, leading to material swelling instead of material removal. Ion beam assisted etching and deposition will face additional effects. For small structures, gas depletion becomes a significant drawback. The impact on gas depletion and the competition with sputtering are discussed. Received: 21 August 2002 / Accepted: 21 August 2002 / Published online: 12 February 2003 RID="*" ID="*"Corresponding author. Fax: +49-9131/761360, E-mail: frey@iis-b.fhg.de     

Modification of thin-layer systems by swift heavy ions

The electronic energy loss of swift heavy ions (MeV/amu) within a solid results in a highly excited cylindrical zone of some nm in diameter, within which all atoms may be in motion for some tens of ps (transient local melting). After cooling down, a defect-rich or even amorphous latent track is left in many cases, especially in insulating materials. The resulting property alterations (density, micro-structure, morphology, phase composition, etc.) have been investigated for many bulk materials, while only very few experiments have been carried out with thin-film systems. In the present paper, a summary will be given of our studies on the transport of matter in thin-film packages induced by irradiation with high-energy ions. These is, on the one hand, atomic mixing at the interfaces, which is especially pronounced in ceramic systems and which seems to occur by interdiffusion in the molten ion track. On the other hand, we have discovered a self-organisation phenomenon in swift-heavy-ion-irradiated NiO layers, which at low fluences first showed periodic cracking perpendicular to the projected beam direction. After application of high fluences, the NiO layer was reorganised in 100-nm-thick and 1-μm-high NiO lamellae of the same separation distance (1–3 μm) and orientation as found for the cracks. Both effects can be attributed to transient melting of the material surrounding the ion trajectory. Received: 11 November 2002 / Accepted: 12 November 2002 / Published online: 4 April 2003 RID="*" ID="*"Corresponding author. Fax: +49-711/6853-866, E-mail: bolse@ifs.physik.uni-stuttgart.de