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Nanotechnology is supposed to become one of the key enabling technologies of the 21st century. Its economic potential is forecast to be a market of several hundred billion Euros in the next decade. Therefore, nanotechnology has attracted the interest of many industry sectors and many companies redirecting internal activities to prepare themselves for this new challenge. At the same time governmental R&D decision makers all over the world are setting up new nanotechnology-specific research programmes aiming at putting their respective countries in a favourable position for the future. The aim of this paper is to use scientific and technological indicators to make predictions on economic development and to compare the situation in different countries.

This paper describes the new Nanomanufacturing programme at the National Science Foundation (NSF). This programme focuses on up-scaling fabrication and synthesis of functional nanoscale structures, devices and systems. It also emphasizes integration across multiple dimensional scales, multi-functionality across several energetic domains, and new design and manufacturing approaches such as biomimetics. The paper elaborates on key issues to this goal, including research in precursor nanomaterials and nanoparticles; fabrication, analysis and synthesis processes; instrumentation and equipment for characterization and processing; theory, modelling, simulation and control; design and integration of nanodevices and systems; and nanomanufacturing education and dissemination strategies. The discussion suggests a number of outstanding research issues and topics, and illustrates them with current research projects in nanomaterials processing, nanomachines and metrology, nanosystems and design etc supported by the NSF.

Clusters with closed electronic shells are highly stable, and this is a robust property with far-reaching implications for cluster abundance, cluster shapes, dynamical response to electric fields and cluster dissociation. Clusters with closed shells are also good candidates for the synthesis of novel materials. The density functional formalism is currently used to investigate the self-assembling of metallic clusters to yield cluster solids. A high intrinsic stability is not the only requirement for expecting a successful assembling, and we investigate additional conditions by performing computer simulations for selected examples. The study of the assembling of hydrogen-doped icosahedral aluminium clusters shows that an optimized relative orientation of each cluster unit with respect to all its neighbour clusters becomes a favourable condition. The analysis of the clustering effects observed in crystalline alloys formed by Pb and alkali metals (tetrahedral Pb₄ clusters surrounded by the alkali cations form in those alloys) provides further insight: coating passivates the clusters and helps to stabilize the assembled material.

We performed scanning tunnelling spectroscopy on insulating colloidal CdSe quantum dots attached to gold with a rigid self-assembled monolayer. By varying the tip-dot distance we change the relative rate of tunnelling into versus tunnelling out of the dot over a wide range. If the tip is retracted relatively far from the dot, tunnelling in is much slower than tunnelling out of the dot; electrons tunnel one at a time through the dot. The resonances in the conductance spectrum correspond to the single-particle energy levels of the CdSe quantum dot. When the tip is brought closer to the dot, tunnelling in can become as fast as tunnelling out of the dot. Up to three electrons can be present in the particle; the electron-electron Coulomb interactions lead to a more complex conductance spectrum.

We present results from a detailed simulation of InAs quantum dots embedded in an AlGaAs/GaAs heterostructure with a self-consistent three-dimensional solver of the Poisson-Schrödinger equation based on density functional theory and local density approximation. Single-electron effects in the structure are evaluated by computing the electrochemical potential by means of Slater's transition rule. We have evaluated the effect of strain in the InAs dot on the single-electron charging properties of the system, to assess the importance of including strain in the design of single-electron memories based on self-organized quantum dots.

We have simulated the electronic properties of silicon-germanium electron waveguides defined by selective etching on a SiGe heterostructure. In particular, we have investigated the dependence of quantum confinement and of one-dimensional subband separation on the waveguide width. Indeed, a larger subband separation means a larger dephasing length and larger electron mobility in the waveguide, and therefore increased possibilities of detecting mesoscopic transport effects. Accurate modelling of SiGe electron waveguides requires us to take into account the effect of strain in the SiGe heterostructure and of the interface states at the exposed SiGe surface, and to solve the Poisson-Schrödinger equation in two dimensions. Results are also shown for a structure in which a gate electrode is evaporated onto the SiGe waveguide, realizing a three-terminal device in which the gate voltage is used to control the number of propagating modes, and therefore the conductance of the channel.

It is possible to discuss the propagation of an electronic current through certain layered nanostructures modelling them as a collection of random one-dimensional interfaces, through which a coherent signal can be transmitted or reflected while being scattered at each interface. We present a simple model in which a persistent random walk (the `t-r' model in one dimension) is used as a representation of the propagation of a signal in a medium with such random interfaces. In this model all the possible paths through the system leading to transmission or reflection can be enumerated in an expansion in the number of loops described by the path. This expansion allows us to conduct a statistical analysis of the length of the paths for different geometries and boundary conditions and to understand their scaling with the size of the system. By tuning the parameters of the model it is possible to interpolate smoothly between the ballistic and the diffusive regimes of propagation. An extension of this model to higher dimensions is presented. We show Monte Carlo simulations that support the theoretical results obtained.

Single-walled carbon nanotubes (SWNTs) are grown from a methane feedstock by thermal chemical vapour deposition (CVD). An ethylene-hydrogen plasma generated in an inductively coupled plasma reactor primarily yields multi-walled carbon nanotubes and thicker fibres. In both cases, an iron catalyst layer and an aluminium underlayer are deposited by ion beam sputtering onto silicon wafers for the growth of carbon nanotubes (CNTs). The plasma process provides well-aligned multi-walled nanofibres useful for fabrication of electrodes and sensors and further tip functionalization whereas thermal CVD produces a mat of SWNT ropes. In addition, CNTs grown at the tips of silicon cantilevers are demonstrated to be ideal for high-resolution imaging of biological samples and simulated Mars dust grains using atomic force microscopy.

Nanobiotechnology is a field that utilizes the techniques of nano- and microfabrication to study biosystems or to use biological material and principles to build new devices. As an example we discuss the development of a nanofabricated electrochemical detector array that reveals the spatio-temporal dynamics of exocytosis in single chromaffin cells. In a quantal release event a single vesicle fuses with the plasma membrane releasing its contents through the fusion pore. The time-resolved amperometric currents measured by the individual electrodes detecting different fractions of the released molecules allow determination of the time course as well as localization of quantal events. Such a device may be applicable to study the correlation of exocytotic events with signalling events that could be simultaneously monitored by fluorescence microscopy.

With this work we have assessed the minimum thickness and grain size realizable for a polysilicon (poly-Si) layer deposited with a low-pressure chemical vapour deposition technique. Three different approaches using pure silane in a standard horizontal reactor have been evaluated: (i) a direct poly-Si deposition, (ii) a hemispherical silicon grain deposition and (iii) an amorphous silicon (a-Si) deposition followed by crystallization thermal annealing. It has been demonstrated that the a-Si/crystallization process seems to be the best candidate for the deposition of ultra-thin poly-Si films. However, it involves process innovation in both the nucleation and annealing phases. With this method we have succeeded in the deposition of uniform 6 nm poly-Si layers with grain sizes around 10-20 nm.

In this paper, we investigate the effects of quantum confinement at the Si-SiO₂ interface on the properties of MOSFETs with a channel length of 50 nm. To this end, we have developed a three-dimensional Poisson-Schrödinger solver, based on an approximation which allows us to decouple the Schrödinger equation into an equation in the direction perpendicular to the channel and an equation in the plane of the channel. This code is able to provide the MOSFET transport properties for very small drain-to-source voltage. We have also evaluated the effects of the discrete distribution of dopants on the dispersion of threshold voltage, by simulating a large number of devices with uniform nominal doping profile but with different actual `atomistic' distributions of impurities.

We have performed an investigation of quantum point contacts (QPCs) defined by split gates on AlGaAs/GaAs heterostructures in order to understand the effects of surface states on the transport characteristics of mesoscopic devices. We compare results from transport measurements on a large number of QPCs with simulations using a Poisson-Schrödinger solver in which the effects of surface states are taken into account. We show that surface properties strongly affect the conductance of QPCs, and that a simple model for surface states allows us to reproduce with reasonable accuracy the pinch-off voltage of QPCs with different gate layouts.

We report on the production of nanoscale patterning on Si substrates by low-energy ion-beam sputtering. The surface morphology and structure of the irradiated surface were studied by atomic force microscopy (AFM) and high-resolution transmission electron microscopy (HRTEM). Under ion irradiation at off-normal incidence angle (~50°), AFM images show the formation of both nanoripple and sawtooth-like structures for sputtering times longer than 20 min. The latter feature coarsens appreciably after 60 min of sputtering, inducing a large increase in the surface roughness. This behaviour is attributed to the preferential direction determined on the substrate by the ion beam for this incidence angle, leading to shadowing effects among surface features in the sputtering process. Under irradiation at normal incidence, the formation of an hexagonal array of nanodots is induced for irradiation times longer than 2 min. The shape and crystallinity of the nanodots were determined by HRTEM. At this incidence angle, the surface roughness is very low and remains largely unchanged even after 16 h of sputtering. For the two angle conditions studied, the formation of the corresponding surface structures can be understood as the interplay between an instability due to the sputtering yield dependence on the local surface curvature and surface smoothing processes such as surface diffusion.

Experimental measurements of the electrostatic properties of long molecules are presented in this work. It is shown that, in the case of carbon nanotubes, the electrostatic force between the scanning force microscopy tip and the sample enhances the network of nanotubes electrically connected to a metal electrode. On λ-DNA molecules the same kind of experiments result in essentially no measurable electrostatic force due to the molecules. From this we conclude that λ-DNA has a negligible electronic conductivity.

We have developed a set of working modes for scanning probe microscopy (SPM), which generalizes the usual method of acquiring data. We call these modes three-dimensional (3D) modes. Using these modes it is possible to measure typical SPM magnitudes, such as, for example, the tunnel current, the normal force and the amplitude or frequency of the cantilever oscillation, as a function of any other two magnitudes of the system: f(x₁,x₂). In this paper we present different examples of 3D modes. In particular, we have applied 3D modes to the study of the electrostatic interaction of co-adsorbed single walled carbon nanotubes and individual DNA molecules with a metallic scanning force microscopy tip. The data indicate that adsorbed DNA has a dielectric constant similar to that of the glass substrate.

The red-emitting nanophosphor Eu:Y₂O₃ was synthesized using the the microemulsions method. The microemulsion system was composed of petroleum ether (60-80 °C), nonionic surfactants NP₅/NP₉, aqueous yttrium nitrate/europium nitrate and ammonium hydroxide solution. The nanoparticles were studied by thermal analysis, x-ray diffraction, transmission electronic microscopy, scanning electron microscopy and photoluminescence. The size of the nanoparticles was in the range 10-100 nm, and showed a narrow size distribution, high crystallinity and special luminescent properties. Compared with the phosphors prepared by the conventional method, the quenching concentration of Eu was raised remarkably. For this type of nanophosphor, quenching starts at a Eu concentration of 10% (mol%), while a value of 6-8% was obtained for the conventional one (Tao Y 1996 Mater. Lett.28 137-40). Based on this study, we have successfully prepared some promising nanophosphors.

This work focuses on the analysis of atomic distances and deformations in an Au/Cu(001) metallic interface and on the calculation of the energy of this interface. We study the possible adaptation of the atomic distances at the interface of two crystals with a considerable difference between their lattice parameters, such as found in Au and Cu. These crystals have a misfit of 12.8% of such parameters. Hence, the growing thin film-substrate interface is strained. We show how the relaxation of different substrate-cluster structures (a few monolayers) takes place on an atomic scale. We find that pseudomorphic growth is only possible when the system is a Cu cluster on top of an Au substrate. In the opposite case, Au on a Cu substrate, the system relaxes generating a network of dislocations. In particular, mean changes in the lattice parameters at the interface are quantified. In addition, we carry out the energetic analysis of these systems, which is of great interest to describe local properties such as electrical conduction.

The design of a monomolecular barrow 1.6 nm×1.5 nm in dimension made of a central board, two rear legs and two front wheels, is discussed. This barrow is guided and driven by the tip apex of an STM which manipulates it from the rear legs. The central board has been chosen in such a way that the rotation of the front wheels can be deduced from the surface tunnelling current intensity measured during a sequence of manipulations of the barrow. Our calculations show that in UHV and without friction, the front wheels of the barrow that we designed will not rotate. This led us to design a new barrow including two ratchet molecular groups to compensate for the absence of friction.

The demand for increased miniaturization of integrated circuits has opened the way to the emerging field of molecular electronics. Recent experimental studies have established that single molecules or a finite ensemble of self-assembled molecules can perform the basic functions of conventional electronic components (i.e., transistors, wires and diodes). In particular, it has been demonstrated that molecular wires inserted into nanopores can be used as active elements for the fabrication of resonant tunnelling diodes (RTDs), whose I/V characteristics reveal a negative differential resistance (NDR) behaviour (i.e., a negative slope in the I/V curve). Here, we provide a detailed quantum-chemical description of a possible mechanism leading to NDR in polyphenylene-based molecular wires incorporating saturated spacers. This mechanism can be understood from the evolution of the molecular wire one-electron structure upon application of a static electric field aligned along the molecular axis, which simulates the driving voltage applied between the two electrodes in the RTD devices. The main parameters controlling the NDR behaviour can be fine tuned through molecular engineering of the wires.

In this paper we report on the fabrication and characterization of two-dimensional photonic crystals. They are realized as a hexagonal array of air holes etched in GaAs or InP based slab waveguide heterostructures. The fabrication steps are discussed, with a focus on the development of suitable dry etching processes. Holes with a depth of 1 µm on GaAs and 4 µm on InP were fabricated, which results in a good overlap of the guided mode with the photonic crystal patterns and low losses. The optical quality of the structures is assessed by transmission spectroscopy of photonic crystal blocks and photonic crystal-based waveguides.

We present a first-principles method for studying the electronic transport through nanoscale atomic systems under non-equilibrium conditions. The method is based on density functional theory, and allows the calculation of the response of the system to an applied finite potential difference. The potential drop profile and induced electronic current (and therefore the conductance) are obtained from first principles. The method takes into account the atomic structure of both the nanoscale structure and the semi-infinite electrodes through which the potential is applied. Non-equilibrium Green's function techniques are used to calculate the quantum conductance. Here we apply the method to the study of the electronic transport in wires of gold and silver with atomic thickness. We show the results of our calculations, and compare with some of the abundant experimental data on these systems.

Growth of ordered organic films of 3, 4, 9, 10-perylene-tetracarboxylic-dianhydride (PTCDA), on inorganic substrates of GaAs(001) is investigated by means of low-energy electron diffraction, scanning tunnelling microscopy and atomic force microscopy. The passivation of the sample can be achieved by exposing the substrate to sulphur (the SnS₂ compound) or wet-chemical etching using an S-containing etchant (S₂Cl₂). The sulphur-treated surfaces are less reactive and are suitable as substrates for the growth of epitaxial films. In this work we have compared the growth of organic molecules on substrates prepared by two different methods. The samples prepared by molecular beam epitaxy (MBE) present smoother surfaces compared with chemically treated samples. Therefore, an improvement of the PTCDA molecular order is observed for the MBE samples, which exhibit the formation of crystals.

The proposed nanometre-sized electronic devices are generally expected to show an increased probability of errors both in manufacturing and in service. Hence, there is a need to use fault-tolerant techniques in order to make reliable information processing systems out of those devices. Here we examine and compare four fault-tolerant techniques: R-fold multiple redundancy; cascaded triple modular redundancy; von Neumann's multiplexing method; and a reconfigurable computer technique. It is shown that the reconfiguration technique is the most effective technique, able to cope with manufacturing defect rates of the order of 0.01-0.1, but the technique requires enormous amounts of redundancy, of the order of 10³-10⁵. However, in the case of transient errors, multiple modular redundancy and multiplexing are the only feasible options.

The stability of metallic nanowires is studied by analysing the shell- and supershell structures of total energy oscillations. Fully quantum-mechanical calculations are performed for Cs, Na and Al nanowires covering the whole range of valence electron densities in simple metals. Nanowires are modelled as infinite stabilized jellium cylinders and solving for their electronic structure self-consistently. The results are analysed within a semiclassical model. A comparison between the shell structures of nanowires and atomic clusters is made.

We propose a simple method for computing the single-particle eigenfunctions in nanostructures with three-dimensional confinement. The proposed procedure transfers the problem to the momentum space, solves an eigenvalue equation on a reduced wavevectors space and then transfers the solution back to the real space. We show that in such a way it is possible to obtain the eigenvectors and eigenvalues corresponding to lower energies with significant improvement in computing time and memory requirements with respect to numerical methods in the coordinate space. The method can be applied to structures with inhomogeneous effective mass and can easily include the full band structure. We have tested the code on typical confining potentials of nanostructures, in order to show the advantages and possible limitations of the proposed method.

We present a procedure for extracting the parameters of a simple model for localized states at the exposed surfaces of GaAs/AlGaAs heterostructures. Such a procedure is based on a comparison between experimental data on test structures and results from the solution of the Poisson-Schrödinger equation in the growth direction of the heterostructure. We show that a model of surface states based on just two parameters is sufficient for obtaining good agreement with experiments and significant improvement with respect to the assumption of Fermi level pinning. In addition, the procedure proposed allows the extraction of the donor concentration in the doped layer and the unintentional doping in the substrate.

We present a theoretical study of scanning tunnelling microscopy of a C₆₀ molecule adsorbed on Al and Au(111) surfaces and of electrical transport of short metallic nanotubes contacted in two ways to such surfaces. For the case of an adsorbed C₆₀ molecule, the change of substrate metal is reflected in a different charge transfer, in a different alignment of the Fermi level with respect to the molecular orbitals and, consequently, in a very different zero-bias transparency. For the case of metallic nanotubes with realistic contacts, the conductance turns out to be, at best, half the ideal value of 4e/h². We perform these studies within a recently developed ab initio scheme suitable for studying electrical transport in generic single-molecule junctions.

We have studied the effect of ultrasound on two different natural macroscopic vermiculite samples from Ojén and Santa Olalla (Spain). We describe the conditions for the preparation of nanometric particles by sonochemistry. Sonication produces not only a delamination effect in the [00l] direction, but also a breaking of layers in the other directions, while the crystalline character is retained. Some differences have been observed in the behaviour of both vermiculites. Thus, the Santa Olalla sample suffers an important decrease in crystallite size with up to 30 h treatment time; longer treatment times do not produce any effect on crystallite size. On the other hand, the Ojén sample suffers a decrease in its crystallite size with up to 80 h treatment time, remaining unchanged for longer treatments. The crystallite sizes in the [00l] direction are much smaller for the Ojén sample than for the Santa Olalla sample. In addition, the Santa Olalla sample maintains the typical laminar shape after sonication, while the Ojén sample suffers a morphological change. The different behaviours seem to be related to the different compositions and, therefore, the different layer charges of both vermiculite samples.

In this work, a conductive atomic force microscope (C-AFM) has been used to study the post-breakdown (BD) behaviour of ultrathin SiO₂ films at the nanometre scale. In particular, the post-BD conduction and BD propagation have been analysed. Although the post-BD switchings in the current-voltage curves measured with C-AFM demonstrate the localized nature of the phenomenon, it has been observed that it is electrically propagated to neighbouring areas.

The influence of magnetic anisotropy on magnetoresistance has been studied in magnetic-non-magnetic-magnetic sandwiches. A numerical model has been developed considering different anisotropies in each magnetic layer and different exchange couplings between them. Both contributions to the magnetoresistance are evaluated; anisotropic and giant magnetoresistance (GMR). Appropriate magnetic anisotropy configurations allow GMR without antiferromagnetic coupling between the magnetic layers. Sandwiches of permalloy-copper-permalloy have been obtained in a dc magnetron sputtering system with different thickness and different anisotropy distributions in order to make a comparison between the experimental magnetoresistance and the numerical results.

A new type of organic semiconductor, based on self-assembled modified DNA nucleosides (deoxyguanosine lipophilic derivative), is employed to fabricate hybrid molecular devices. Two different modifications of the guanosine are synthesized in order to change the oxidation potential of the active molecular system. The influence of the solvent on the self-assembly process and on the carrier transport is investigated by atomic force microscopy and electrical measurements, respectively. Our results demonstrate that upon appropriate engineering of the molecule and choice of the solvent, self-organization of molecules in ordered structures is achieved, resulting in high conductivity and electronic rectification in solid-state, planar molecular devices. The transport properties depend strongly on the molecular structure and on the arrangement of the molecules into the hybrid devices, thus opening the route to molecular transport engineering in hybrid molecular electronics.

We have studied the atomic arrangement and defect formation in metal nanowires (NWs) generated by mechanical elongation using in situ high resolution transmission electron microscopy. It has been observed that the narrowest constriction of gold and platinum NWs is crystalline and defect-free; in particular, gold NWs adopt only three kinds of atomic arrangement. A model correlating these gold structures and the quantum conductance behaviour is proposed, which showed a remarkable agreement with ultrahigh-vacuum mechanically controllable break junction electrical transport measurements.

The transverse dipole spin response of elliptic quantum dots is analysed as a function of an applied static magnetic field and dot deformation. Interaction effects are numerically studied within the time-dependent local-spin-density theory using a two-component spinorial formalism.

The methyltransferase-directed addressing of fusion proteins to DNA scaffolds offers an approach to the construction of protein/nucleic acid biostructures with potential in a variety of applications. The technology is currently only limited by the yield of high occupancy structures. However, current evidence shows that DNA scaffolds that contain three or four targeted proteins can be reliably constructed. This permits a variety of macromolecular designs, several of which are given in this paper. Designs for open and closed two-dimensional and three-dimensional assemblies and a design for a molecular switch are discussed. The closed two-dimensional assembly takes the form of a square, and could find application as a component of other systems including a macromolecular rotaxane. The closed three-dimensional system takes the form of a trigonal bipyramid and could find application as a macromolecular carcerand. The molecular switch could find application as a peptide biosensor. Guidelines for the construction and structural verification of these designs are reported.

An overview of the theoretical and experimental efforts in obtaining a photonic bandgap, a frequency band in three-dimensional dielectric structures in which electromagnetic (EM) waves are forbidden, is presented. Photonic crystals offer unique ways to tailor light and the propagation of EM waves.

The possibilities for the design of larger diode logic circuits such as a one-bit half-adder inside a molecule are investigated, based on a recent extension of the elastic scattering quantum chemistry technique. Since any diode logic circuit for an adder needs OR-gates and AND-gates as basic components and the properties of OR-gates have already been discussed in the ballistic and tunnelling electron transport regime, we focus on the more complicated AND-gates in the present work. For this case the output current, calculated from the transmission coefficients by using the Landauer-Büttiker formula, shows four different logical levels instead of two. The origin of this level variety is analysed in detail. The concept of programmable gate logic arrays is also addressed, where for intra-molecular circuits distinct deviations from earlier macroscopic or mesoscopic implementations of this scheme are found.

The transmission of light through an aperture in a metal film is extremely small when the aperture diameter is much smaller than the optical wavelength. But when the metal surface surrounding the subwavelength hole is corrugated, the incident light can couple to surface plasmons (SP), excitation modes on the metal surface. A resonant interaction leads to an enhanced transmission at wavelengths determined by the corrugation pitch. We discuss applications of the SP enhanced transmission in near-field scanning optical microscopy and in high-density optical data storage.

The interacting induced dipoles polarization model implemented in the program POLAR is used for the calculation of the molecular dipole-dipole polarizability . The method is tested with Sc₁-Sc₇, Sc₁₂, Sc₁₇, Sc₇₄, C, C₁₂, C₆₀, C₇₀, C₈₂-fullerene, Sc@C₆₀, Sc@C₈₂, Sc₂@C₈₂, Sc₃@C₈₂, C₁-C₆, C₁₀, C₁₃, C₁₆, C₁₉, C₂₂, C₂₄, C₄₂, C₅₄, C₈₄ and C₉₆-graphite clusters. The polarizability is an important quantity for the identification of clusters with different numbers of atoms and even for the separation of isomers. The results for the polarizability are of the same order of magnitude as reference calculations performed with another program based on the same formulation. The bulk limit for the polarizability is estimated from the Clausius-Mossotti relationship. The polarizability trend for these clusters as a function of size is different from what one might have expected. The clusters are more polarizable than one might have inferred from the bulk. Previous theoretical work has yielded the same trend for small Si_n, Ge_n and Ga_nAs_m clusters. However, previous experimental work has yielded the opposite trend for larger Si_n, Ga_nAs_m and Ge_nTe_m clusters. At present, the origin of this difference is problematic. One might argue that smaller clusters need not behave like those of intermediate size. The high polarizability of small clusters is attributed to dangling bonds at the surface of the cluster, which resembles a metallic cluster.

The electrical and optical response of thin films of surface functionalized nanoparticles upon exposure to various chemical vapours has been studied. It was found that the electrical response to chemical vapours adsorbed on the various nanoparticle films varied markedly and was determined by the surface functional groups. Ellipsometric studies revealed that the film thickness increased during exposure to the chemical vapours. These thickness changes of the films correlate with the changes in electrical conductivity. Two physical effects are believed to play a role in determining these conductivity changes. Under high partial pressure, the change in nanoparticle core-core separation is the main contribution to the change in conductivity and generally leads to a reduction in the conductivity. However, for relatively low partial pressures the adsorption of vapour molecules leads to permittivity changes that tend to increase the conductivity.

This special issue of Nanotechnology contains contributions presented at the second `Trends in Nanotechnology' (TNT2001) international conference, held in Segovia (Spain), 3–7 September 2001. More than 250 scientists from Europe, the United States, Japan and other countries worldwide attended this meeting and contributed with talks, posters and stimulating discussions about their recent research.

The importance of nanoscale science is growing worldwide and it is now widely recognized as a critical component to the future growth of the economy. Therefore, global investment in nanotechnology has been growing annually during the last decade and in the last 12 months not only from programmes funded by public institutions but also from venture capitalists. In response to the growing awareness of the importance of nanotechnology, many conferences are being organized to discuss the latest advances. Among these, the series of conferences `Trends in Nanotechnology' have become key meetings in the field since they bring together top-level speakers and fresh ideas in an environment that is suitable for stimulating discussions, the exchange of ideas and building scientific and personal relations among participants.

The aim of this conference was to focus on the applications of nanotechnology and to bring together, in a scientific forum, various groups from throughout the world. TNT2001 was particularly effective in transmitting information and establishing contacts among workers in this field. Graduate students attending such conferences quickly learn the importance of interdisciplinary skills and become more effective in their future research. Last year, almost 50 graduate students received a grant to assist TNT2001 and presented their work through a poster. The scientific programme, without parallel sessions, covered a wide spectrum of nanotechnology research such as nanotubes, nanomagnetism, nanodevices, nanoscale integration, nanobiology, molecular electronics, computational methods for nanotechnology and nanomaterials.

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, CMP Cientifica, University of Cambridge, University of Purdue, Universidad Complutense de Madrid, Universidad Carlos III de Madrid, Universidad SEK, PHANTOMS Network (funded by the EU through its IST programme), National Science Foundation, NASA, World Scientific, Motorola, Institute of Physics Publishing, Diputación Provincial de Segovia and the Ministerio Español de Ciencia y Tecnología. We would like also to thank the following companies for their participation: NanoTec, FEI, Raith, Schaefer and ScienTec.

We invite readers of this special issue of Nanotechnology to join us in the `Trends in Nanotechnology' conference series. The next conference (TNT2002) is being held in Santiago de Compostela (Spain), 9–13 September 2002.

Guest EditorsAntonio CorreiaCMP Cientifica, SpainPedro A SerenaICMM/CSIC, SpainMark WellandUniversity of Cambridge, UKRon ReifenbergerPurdue University, USA