Search

Much effort has been made recently to develop future devices or sensors comprising nanoscale building blocks such as nanowires, carbon nanotubes, DNA, proteins, and nanoparticles. One of the key requirements to realize these devices/sensors is the capability to place individual nanoscale building blocks onto exact substrate locations. This article will review recent progress in various approaches exploited to place or assemble nanoscale building blocks in a controlled way.

We demonstrate a large-scale placement of nanoparticles through a scheme named "electrostatic funneling", in which charged nanoparticles are guided by an electrostatic potential energy gradient and placed on targeted locations with nanoscale precision. The guiding electrostatic structures are defined using current CMOS fabrication technology. The effectiveness of this scheme is demonstrated for a variety of geometries including one-dimensional and zero-dimensional patterns as well as three-dimensional step structures. Placement precision of 6 nm has been demonstrated using a one-dimensional guiding structure comprising alternatively charged lines with line width of approximately 100 nm. Detailed calculations using DLVO theory agree well with the observed long-range interactions and also estimate lateral forces as strong as (1-3) x 10(-7) dyn, which well explains the observed guided placement of Au nanoparticles.

Fermi-Dirac electron thermal excitation is an intrinsic phenomenon that limits functionality of various electron systems. Efforts to manipulate electron thermal excitation have been successful when the entire system is cooled to cryogenic temperatures, typically, Electrons can behave as if they are at a temperature different from that of the solid in which they are embedded. Here, the authors demonstrate a room temperature device that can generate electrons with an effective temperature of 45 K by using quantum wells to filter out energetic particles.

Ostwald ripening, in which large clusters grow at the expense of smaller ones, is driven by differences in the chemical potentials of these clusters. We demonstrate that ripening can also occur without chemical potential differences between small and large particles, by observing the exchange of single atoms between linear chains of Ir atoms; on a W(110) surface these dissociate at a rate independent of length. Ripening takes place because of the stochastic nature of individual atomic events, and is characterized by large fluctuations in the number of exchanges for a chain to disappear. This mechanism should be generally applicable in the ripening of one-dimensional chains, formed on metals as well as semiconductors, and for which the dissociation energy is insensitive to length.

In order to explore cohesion in surface clusters, interactions between Pd atoms adsorbed on W(110) have been characterized in observations of individual adatoms using a helium-cooled field ion microscope. The free energies of pair interactions F({bold R}) have been derived from detailed measurements of the probability of finding two atoms on an otherwise empty plane at a vector separation {bold R} from each other. For Pd, pair free energies are similar to previous results for Ir atoms: interactions extend over distances larger than 13 {Angstrom}, and show a strong directional anisotropy. Along the close-packed [1{bar 1}1] direction, interactions between two Pd atoms are very close to those for Ir adatoms, even though the binding energy of Ir atoms to the substrate is more than twice that of Pd. With quantitative pair free energies available, the contributions from many-atom effects to the cohesion of clusters have been evaluated by examining the transformation from chain structures to two-dimensional islands, which takes place with both Pd and Ir on W(110). Many-atom effects are parametrized in terms of the trio interactions F{sub 3lin} in linear and F{sub 3tri} in triangular configurations. It is found that many-atom effects, rather than amounting to small perturbations ofmore » the pair contributions, are the {ital major} contributors to the cohesion of two-dimensional clusters for both Pd and Ir on W(110). {copyright} {ital 1999} {ital The American Physical Society}« less

Contributions from many-atom effects to the cohesion of Ir clusters on W(110) have been evaluated by relying on directly measured pair free energies. Strong trio interactions must be invoked to account for the structural change from linear chains to two-dimensional islands which takes place when the number of Ir atoms in a cluster increases above 14. Many-atom effects rather than pair interactions are found to be entirely responsible for the cohesion of 2-D Ir islands; pair contributions are actually repulsive. In Ir chains, pair interactions are, at most, comparable to trio contributions.

Although single-electron devices have many advantages over conventional electronic devices and are therefore expected to have important applications for military, space, and commercial use, many fabrication challenges associated with nanoscale geometrical control have limited their implementation for practical use. The aim of this project was to create new single-electron device architecture and its associated fabrication techniques to realize single-electron device fabrication on a large scale, thereby enabling their implementation for practical applications. We demonstrated 1) chip-level fabrication of single-electron transistors, 2) that they can be fabricated in completely parallel processing, with each device individually addressable, 3) clear I-V characteristics of Coulomb blockade/staircase and Coulomb oscillations, and 4) that they can operate at room temperature. These results show that fabrication of integrated systems of room-temperature single-electron devices is now possible, paving a pathway toward practical use of single-electron devices.

Devices in which the transport and storage of single electrons are systematically controlled could lead to a new generation of nanoscale devices and sensors. The attractive features of these devices include operation at extremely low power, scalability to the sub-nanometre regime and extremely high charge sensitivity. However, the fabrication of single-electron devices requires nanoscale geometrical control, which has limited their fabrication to small numbers of devices at a time, significantly restricting their implementation in practical devices. Here we report the parallel fabrication of single-electron devices, which results in multiple, individually addressable, single-electron devices that operate at room temperature. This was made possible using CMOS fabrication technology and implementing self-alignment of the source and drain electrodes, which are vertically separated by thin dielectric films. We demonstrate clear Coulomb staircase/blockade and Coulomb oscillations at room temperature and also at low temperatures.

The capability of placing individual nanoscale building blocks on exact substrate locations in a controlled manner is one of the key requirements to realize future electronic, optical, and magnetic devices and sensors that are composed of such blocks. This article reviews some important advances in the strategies for controlled placement of nanoscale building blocks. In particular, we will overview template assisted placement that utilizes physical, molecular, or electrostatic templates, DNA-programmed assembly, placement using dielectrophoresis, approaches for non-close-packed assembly of spherical particles, and recent development of focused placement schemes including electrostatic funneling, focused placement via molecular gradient patterns, electrodynamic focusing of charged aerosols, and others.

Although surface diffusion has been studied on the atomic level for decades, little is known about jump processes in self-diffusion. The diffusion of W atoms on the densely packed W(110) plane has therefore been examined,using the field ion microscope to probe contributions of jumps other than between nearest-neighbor sites. Measurements of the mean-square displacements of atoms along the x and y axes ( and , respectively) and of the detailed spatial distribution of atomic displacements serve to define the nature of the jumps important in diffusion. It is shown that, in addition to nearest-neighbor and second-nearest-neighbor transitions along the close-packed , atoms also carry out jumps along (001) and , and that the contribution from these processes increases rapidly with temperature.

A variety of jumps has in the past been identified in diffusion of atoms on 1D channeled surfaces. To establish the jump processes important in diffusion on a 2D surface, the movement of individual Pd atoms has been examined on W(110). From the distribution of displacements of Pd at high temperatures, double jumps are found along the close-packed111. For the first time, sizable differences are also observed between the mean-square displacements along x and y, which demonstrate unexpected contributions from jumps along110, but not along001. These jumps proceed over activation barriers higher than for single jumps, under conditions predicted from previous work with Pd on the channeled W(211).

Self-assembly of one-dimensional surface structures is examined by tracking single Ir and Pd atoms on W(110) as they incorporate into chains of Ir and Pd, respectively. Ir adatoms move parallel to the chains, but do not come close to the chain sides; incorporation occurs only at the ends. Pd adatoms also migrate parallel to Pd chains, and incorporate at the ends. Occasionally, in the end region, they do jump to the side of a chain, and migrate there until they find an end site. Incorporation behavior for both Ir and Pd is driven by highly anisotropic, long-ranged interactions between atoms on the surface.

Interactions dictating the shape and stability of surface islands have been examined for a simple model system, palladium clusters on the W(110) plane. Observations in a field ion microscope of the distribution of two Pd adatoms over the surface yield quantitative values for pair interactions. These are found to be complex, extending over distances longer than 10 A and to vary strongly with the orientation of the pair axis on the (110) surface. Using the measured pair energies it has been possible to infer the magnitude of many-atom effects necessary to account for the equilibrium shape of the clusters. Many-atom interactions turn out to make by far the largest contribution to cluster cohesion; modeling of growth phenomena in terms of nearest-neighbor bonds only is clearly problematic.

In VLSI technology, there is interest in monitoring the sequence of phase formation of TiSi2 (c-Ti ⇒ a-TiSi ⇒ C49 TiSi2 ⇒ C54 TiSi2), with the prospect of reducing the temperature of formation of the stable C54 TiSi2 phase. In this study, phase formation characteristics of TiSi2 during rapid thermal annealing(RTA) of Ti-Si bilayers are investigated by means of in situ four point probe resistance measurements. Ex situ X-ray diffraction(XRD) was used for phase identification and characterization. Results indicate that the same multi-step sequence of transformations precede the formation of the C54 TiSi2 phase for heating rates from 1°C/s to 100°C/s. Also, all intermediate and metastable phases which occur at l°C/s also occur at 100°C/s. Temperature dependence and kinetics of the C49 TiSi2 and the C54 TiSi2 phase formation were studied over a wide range of heating rates. Activation energies estimated for the two processes were ∼2eV and ∼5eV respectively. Finally, a new Electrical Thermal Annealing(ETA) technique for heating at rates up to 30000°C/s is introduced. Preliminary in situ resistivity measurement results of TiSi2 formation at these high heating rates are also presented.

This letter reports single-particle placement in which exactly one nanoparticle is electrostatically guided and placed onto a target location. Using an ∼20 nm Au nanoparticle colloid as a model system, we demonstrate that self-limiting interactions between a charged nanoparticle and a charged substrate surface are extremely effective in positioning a single Au nanoparticle on each target location. Detailed theoretical calculations revealed that the self-limiting capability in the nanoparticle positioning is due to an increase in the free energy barrier after the first nanoparticle lands on a target position, effectively blocking the approach of other nanoparticles.