Table of contents

This special issue of Nanotechnology contains contributions presented at the Trends in Nanotechnology (TNT2000) conference, held in Toledo (Spain) from October 12-16, 2000. More than 160 scientists from Europe, United States and Japan attended this meeting and contributed with talks and stimulating discussions about their recent research.

The rapidly emerging areas of nanoscale science and technology are focused on the design, fabrication and characterization of functional objects having dimensions at the nanometre length scale. New advances in this emerging area are expected to have long range implications in a wide variety of different scientific and engineering disciplines. The importance of nanoscale science is growing worldwide and it is now widely recognized as a critical component to the future growth of the world economy. In response to the growing awareness of the importance of nanotechnology, the international TNT conference was organized to discuss the latest advances. This event was particularly effective in transmitting information and establishing contacts among workers in this field.

The aim of this conference was to focus on the applications of nanotechnology and to bring together in a scientific forum various groups throughout the world working in this field. The scientific program covered a wide spectrum of nanotechnology research (nanotubes, nanomagnetism, quantum computing, nanodevices and nanomachines, nanoscale integration, nanobiology, molecular nanotechnology, computational methods for nanotechnology, nanomaterials, etc.) including keynote lectures, oral presentations and posters.

We would like to thank all the participants for their assistance as well as the authors for their written contributions. We are indebted to the following scientific institutions, companies and government agencies for their help and financial support: Universidad Autónoma de Madrid, Consejo Superior de Investigaciones Científicas, Universidad de Castilla La Mancha, University of Cambridge, Universidad Complutense de Madrid, IBM, The Institute of Nanotechnology (IoN), World Scientific, MATS UK and the Ministerio de Ciencia y Tecnología. We would also like to thank the following companies for their participation: NanoTec, MTS, FEI and Obducat.

Guest Editors

Antonio Correia CMP Cientifica Spain Mark Welland University of Cambridge UK Pedro A Serena ICMM/CSIC Spain

In 1996 the Esprit programme of the European Commission launched a research initiative on nanoscale integrated circuits which is currently being followed up by the IST programme. This paper summarizes the motivation for launching this initiative and summarizes some of the tendencies which seem to emerge from the point of view of the author.

It is now accepted that nanotechnology is one of the key enabling technologies for sustainable and competitive growth in Europe. Nanoelectronics is certainly the branch with the most significant commercial impact and covers a huge range of interdisciplinary areas of research and development such as molecular electronics, bioelectronics, spintronics, nanoimprint, nanoscale optics, lithography, architecture and nanoprobes. It is also accepted that a significant investment will be required to ensure Europe's competitiveness in nanotechnology. At this stage it is impossible to predict the exact course that the nanoelectronics revolution will take and, therefore, its effect on our daily lives. We can, however, be resonably sure that nanotechnology will have a profound impact on the future development of many commercial sectors. The greatest impact is likely to be in the electronics sector, where the demand for technologies permitting faster processing of data at lower costs will remain undiminished. In order to avoid European industry and R & D being left behind the United States and Japan in this fast emerging nanoelectronics field, the PHANTOMS Network Scheme will promote European science and research through a pluri-national networking action, put together research capacities present in the various European regions and stimulate commercial nanoelectronic applications.

We review the salient aspects of nanoimprint lithography and consider the challenges it faces in becoming a standard fabrication technique, such as costs and throughput. We discuss material issues such as visco-elasticity and functionality of the printed material. By way of an illustration, we present printing results of 50 nm features over a 2×2 cm² area which are reproducible with high fidelity. Data of printing 15 nm features in PMMA using a Cr stamp was obtained.

We have fabricated an aluminium single-electron transistor (SET) and characterized it at frequencies up to 8 MHz by measuring the reflected signal from a 331 MHz resonant tank in which the transistor is embedded. Within a bandwidth of 8 MHz, we measured the charge sensitivity of this radio-frequency SET, i.e. rf-SET, to 6.3×10^-6 e/(Hz)^1/2, which corresponds to the uncoupled energy sensitivity of 13ℏ.

A simple linear electromechanical model for an electrostatically driven resonating cantilever is derived. The model has been developed in order to determine dynamic quantities such as the capacitive current flowing through the cantilever-driver system at the resonance frequency, and it allows us to calculate static magnitudes such as position and voltage of collapse or the voltage versus deflection characteristic. The model is used to demonstrate the theoretical sensitivity on the attogram scale of a mass sensor based on a nanometre-scale cantilever, and to analyse the effect of an extra feedback loop in the control circuit to increase the Q factor.

We demonstrate the nonlinear optical activity of large Na and K clusters, grown on Au metal film-supported alkane thiol monolayers of different chain lengths. Both the total optical second-harmonic signal intensity as well as its dependence on the angle of incidence depend on the surface coverage with the alkali metal, thus demonstrating the sensitivity of second-harmonic generation to morphological changes in these complex systems. Simultaneously performed extinction and scanning force microscopy measurements under ambient air conditions allow us to obtain independent information on the surface coverage and cluster morphology. The latter measurements also show that the optical activity of sodium clusters grown on alkane thiols at 150 K and annealed to 300 K is conserved on dosing with oxygen.

In order to establish whether atomic force microscope (AFM) grown SiO₂ is appropriate for use as a gate oxide in nanoelectronics, a characterization of these films needs to be performed. In this paper results on AFM fabrication and topographical characterization of large-area SiO₂ patterns are presented. This paper is centred around the SiO₂ surface and SiO₂-Si interface roughness, due to its importance in relation to the quality of ultrathin dielectrics. Our results show quite similar values to those obtained for thermal oxides and thus we suggest that AFM-grown SiO₂ is a suitable candidate for gate oxide applications in nanodevices.

In this paper two ways of measuring current maps on gold micro-stripes on a nanometric scale are shown. In the first one, an alternating current is applied to the stripe and the thermal expansion of the tip-sample system is registered by the cantilever. In the second one, maps of adhesion of the surface are acquired simultaneously with the topography. The adhesion force map varies with the temperature because it depends mainly on the water meniscus between tip and sample, and the size of the meniscus depends on the temperature.

We study experimentally and numerically the statistical conductance properties of Al breaking nanocontacts at room temperature. The measured conductance histogram (CH) from hundreds of consecutive experiments exhibits peaks close to integer values in units of 2e²/h. The results are compared with minimum-cross-section histograms obtained from molecular dynamics calculations. The agreement between these atomistic simulations and experiments provides evidence that the CH is a signature of favourable atomic configurations.

We present an account of how the conductance distribution varies with the Fermi energy in quasi-one-dimensional disordered systems. Different amounts of disorder have been considered to analyse different conduction regimes (from the ballistic to the diffusive). We also show the evolution of the transmission probability distribution for each propagating mode. For moderate disorder (transition between ballistic and diffusive regimes) we find that when the Fermi energy value allows n>1 open modes, there are (in general) two clear conductance distribution maxima close to (n-1)G₀ and nG₀, independent of the Fermi energy value. These results demonstrate that the conductance distributions have a complex structure with large variances. These features found in the conductance distributions illustrate the importance of the role attributed to disorder in the design of nanometric devices.

The controlled assembly of metal nanoparticles into macroscopic materials using DNA oligonucleotides has opened new directions of research in nanoscience and nanotechnology. Here, we describe recent ab initio calculations on structural and electronic properties of the subsystems forming these materials: bare and thiol-passivated gold nanoclusters, gold nanowires and fragments of DNA chains. Our results indicate that gold nanoclusters are distorted dramatically by a passivating methylthiol monolayer, that monatomic gold chains are stable in zigzag geometries and that dry acidic λ-DNA is a good insulator. These results provide useful insights towards the complete understanding, design and proper utilization of hybrid DNA-gold nanostructured materials.

Transport characterizations of wires obtained by electron beam lithography and etching of (100) Si/SiGe heterostructures with a high-mobility two-dimensional electron gas are reported. Depending on the wire width, two different regimes for the electrical transport are found. Wires with a width larger than ~200 nm exhibit metallic behaviour in the quasi-ballistic regime. The conductance dependence on the wire width reveals the presence of a depletion layer, ~100 nm thick, on each etched side of the wire. The wires of width smaller than 200 nm have very large resistance and two different behaviours. The first kind of wires exhibit a zero-current region, compatible with a Coulomb blockade effect involving multiple tunnel junctions or with a space-charge limited current. Other wires are insulating up to applied voltages larger than 5-6 V and their I-V characteristics can be fitted by the functional dependence of voltage-induced tunnelling of Fowler-Nordheim type.

During the meetings of the theory and modelling working group, within the MEL-ARI (Microelectronics Advanced Research Initiative) and NID-FET (Nanotechnology Information Devices-Future and Emerging Technologies) initiatives of the European Commission, we have been discussing the current status and the future perspectives of nanoscale device modelling. The outcome of such a discussion is summarized in the present paper, outlining the major challenges for the future, such as the integration of nonequilibrium phenomena and of molecular-scale properties. We believe that modelling has a growing importance in the development of nanoelectronic devices and must therefore make a move from physics to engineering, providing valid design tools, with quantitative predictive capabilities.

Monte Carlo numerical simulations of Ising thin films have been used in an effort to characterize the dimensional crossover of the phase transitions in nanoscopic layered systems with a thickness of a few atoms. In particular, we have determined the behaviour of the critical temperature, of the effective critical exponents and of the crossover temperature. We have also compared our results with experimental data for FeCo using a proper extrapolation.

In this paper we report on the production of single-wall carbon nanotubes (SWNT) by the laser-ablation technique using graphite, pitch and coke as carbonaceous feedstock materials. Using a continuous-wave CO₂-laser a wide range of experimental conditions have been studied by systematically varying the nature and the concentration of the metal catalyst, the type and pressure of the buffer-gas as well as the laser conditions. Employing graphite as a precursor material, the experimental parameters have been optimized in order to synthesize high-density SWNT material. On the other hand, using pitch and coke as carbonaceous feedstock materials can lead to the formation of SWNT. However, the amount of SWNT material obtained is much lower than in the case of graphite. We qualitatively analyse the whole synthesis process and show that the formation of SWNT is closely related to the use of an appropriate feedstock material as well as to favourable local temperature conditions experienced by the evaporated species.

Nanoclub Lateral (NCL) acts as a German scientific center for coordinating the transfer of research and technology between 70 university groups and industrial laboratories in the field of lateral functional nanostructures. This large number of competent members and attractive research and development topics provides the critical mass for synergy but requires efficient structures for its coordination.

The main mission of NCL is the efficient and accelerated transfer of information from the university area to individual laboratories with a specific focus on components for future information technology applications.

This paper gives a brief review of our recent work done in the area of nanometre-scale Coulomb blockade (CB) memory and logic devices, that enable us to realize future electron-number scalability by overcoming inherent problems to conventional semiconductor devices. We introduce multiple-tunnel junctions (MTJs), naturally formed in heavily doped semiconductor nanowires, as a key building block for our CB devices. For memory applications, the hybrid MTJ/MOS cell architecture is described, and its high-speed RAM operation is investigated. For logic applications the binary decision diagram logic is discussed as a suitable architecture for low-gain MTJ transistors.

Building upon traditional ab initio quantum chemistry calculations, we present a theoretical study of the transport properties of C₆₀ molecules connected in realistic ways to Al metallic electrodes. A Green function technique that combines standard density functional calculations with an effective tight-binding model allows us to calculate the ab initio electrical transport properties of the fullerenes in contact with the electrodes. Our results are relevant for the correct interpretation of scanning tunnelling microscope visualization of these and related molecules adsorbed on different substrates as well as for predicting the electrical conduction properties of molecular devices currently under study.

Ultrathin SiO₂ films (3-6 nm) have been electrically characterized with a conductive atomic force microscope. This technique allows the electrical characterization of areas of ~100 nm², which are comparable to the area of breakdown spots. Sequences of voltage ramps on fixed oxide locations have been applied to induce the oxide degradation and its conduction properties have been analysed within the nanometre scale range. In particular, on-off fluctuations before and after breakdown are reported on single breakdown spots.

The controlled emission of ions by field desorption from nanotips is used to develop an atomic-size ion source. The source has been shown to emit Au and Ag ions from layers pre-deposited on W base tips with two characteristics important for applications: (1) the ion emission can be controlled such that it originates exclusively from the atomic-scale apex of a single nanotip positioned precisely on the main axis of the base tip and (2) the nanotip emits stable metallic ion currents in the pA range for at least tens of hours. Emission patterns, energy distributions and emitted ion currents versus time, voltage and temperature have been measured. These data permit a qualitative understanding of the source characteristics by considering their relation to the three essential source elements: the axial nanotip, the supply function and ionization by field desorption. Computer simulations are performed to explore the use of these sources to create nanometre-scale structures when coupled to electrostatic lenses or in a near-field configuration as well as the interactions of the ions in the range of 100-1000 eV with a Si substrate.

We have investigated pattern formation of thin PMMA films during both hot embossing and demoulding of micro- and nanostructures. During filling of the stamp cavities, compressive and capillary effects were observed, and under certain conditions periodic patterns with characteristic length scales formed. In unstructured stamp regions the form of these patterns is affected by local differences in the pressure. For structured stamps, the self-assembly is strongly influenced by the size and the shape of the stamp cavities. Rapid expansion of trapped air resulted in viscous fingering patterns and a dewetting behaviour of the polymer could be observed.

We have theoretically studied the operation principle of a memory cell using the parallel coupled arrays. This makes use of the unique property of the coupled arrays that is the Coulomb blockade by electron-hole pairs, rather than the Coulomb blockade by electrons in other single-electron memory cells. The main feature, that the binding strength of the electron-hole pairs increases with the coupling strength between the two arrays, can lead to the enhanced stability of stored charges in the memory device.

We propose two potential realizations for quantum bits based on nanometre-scale magnetic particles of large spin S and high-anisotropy molecular clusters. In case (1) the bit-value basis states |0⟩ and |1⟩ are the ground and first excited spin states S_z = S and S-1, separated by an energy gap given by the ferromagnetic resonance frequency. In case (2), when there is significant tunnelling through the anisotropy barrier, the qubit states correspond to the symmetric, |0⟩, and antisymmetric, |1⟩, combinations of the twofold degenerate ground state S_z = ±S. In each case the temperature of operation must be low compared to the energy gap, Δ, between the states |0⟩ and |1⟩. The gap Δ in case (2) can be controlled with an external magnetic field perpendicular to the easy axis of the molecular cluster. The states of different molecular clusters and magnetic particles may be entangled by connecting them by superconducting lines with Josephson switches, leading to the potential for quantum computing hardware.