We describe a novel quantum technology for possible ultra-fast, ultra-dense and ultra-low-power supercomputing. The technology utilizes single electrons as binary logic devices in which the spin of the electron encodes the bit information. Both two-dimensional cellular automata and random wired logic can be realized by laying out on a wafer specific geometric patterns of quantum dots each hosting a single electron. Various types of logic gates, combinational circuits for arithmetic logic units, and sequential circuits for memory have been designed. The technology has many advantages such as (1) the absence of physical interconnects between devices (inter-device interaction is provided by quantum mechanical spin-spin coupling between single electrons in adjacent quantum dots), (2) ultra-fast switching times of approximately 1 picosecond for individual devices, (3) extremely high bit density approaching 10 terabits cm-2, (4) non-volatile memory, (5) robustness and possible room-temperature operation with very high noise margin and reliability, (6) a very low power delay product (approximately 10(-20) J) for switching between logic levels, and (7) a very small power dissipation of a few tens of nanowatts per switching event. In spite of the above advantages, the technology also has some serious drawbacks in that the fan-out of individual logic devices may be small, wiring crossover is very problematic and the devices themselves have no inherent gain so that isolation between input and output is virtually non-existent. These are problems that plague all similar quantum technologies although they are seldom recognized as such. We will discuss these problems, and where possible, offer plausible solutions. In spite of these drawbacks, however, there are still enough attractive features of this technology to merit serious research. In this paper, we will describe how the spin-polarized single-electron logic devices work, along with the associated circuits and architecture. Finally, we will propose a new fabrication technique for realizing these chips which we believe is much more compatible with the demands of the technology than conventional nanofabrication methods.
A large-amplitude vibrating cantilever scanning force microscope is analyzed numerically using an integrated Lennard-Jones-type attractive force in combination with a repulsive indentation force. The calculation yields the motion of the vibrating tip and the time-dependent force it exerts on surfaces with different degrees of softness, as a function of tip-sample equilibrium distance.
We have applied the non-contact dynamic force microscopy method to investigate soft biological materials such as hexagonally-packed intermediate layers, DNA, and tobacco mosaic virus under ambient conditions. This method, where a stiff cantilever is oscillated close to its resonance frequency with an amplitude of 0.3-1.5 nm above the sample, allows highly reliable investigation of soft organic matter with minimized normal and lateral forces between tip and sample. The vertical and lateral resolution are determined to be < 1 angstrom and 1-2 nm, respectively, comparing favorably to established results from repulsive-mode scanning force microscopy experiments on absorbate covered surfaces in liquids. The interaction forces are found to be attractive, dominated by damping mechanisms and attractive force gradients of capillary and van der Waals interactions.
Living cells contain many molecules which can make simple decisions, such as whether to bind to a particular nucleotide sequence or not. A theory describing the practical limits of these molecular machines is reviewed. Level 0 theory uses Shannon's information theory to study genetic control systems. Level 1 theory uses Shannon's channel capacity theorem to explain how these biological molecules are able to make their decisions precisely in the face of the thermal maelstrom surrounding them. Level 2 theory shows how the Second Law of Thermodynamics defines the exact extent of the choices available to a molecular machine when it dissipates a given amount of energy. Even the famous Maxwell demon must obey this result. The theory also has implications for designing molecular computers.
We have investigated using the scanning tunneling microscope (STM) as a tool for surface modification of YBa2Cu3O7-delta (YBCO) thin films and have identified five distinct methods whereby the STM tip can modify the superconductor surface. (i) By lowering the tunneling resistance we make the tip scratch or 'mill' the sample surface mechanically. (ii) By increasing the bias voltage above about 4 V we can modify the surface by an apparent electron beam damaging process. (iii) By increasing the bias voltage above 10 V and raising the tunneling current, we can cause a more dramatic effect which is probably due to a thermal process. (iv) By operating the STM in a damp carbon dioxide atmosphere we can cause the STM tip to etch the surface electrochemically. (v) Finally, we have some preliminary data suggesting that the high field under an extremely sharp tip displaces the oxygen atoms in the YBCO lattice. Examples of each of these techniques are shown and discussed.
Since its first demonstration in 1986, the technique of employing surface-gate designs to define submicrometre-size geometries in the two-dimensional electron gas of AlGaAs/GaAs heterostructures has yielded a wide range of studies of semiconductor physics and novel modes of device operation. To have a future impact in this role, new gate architectures will be required to further develop the technique's ability to tune device geometries through the applied gate bias. Past and future aspects of this system are reviewed in terms of semiconductor physics, technology and device application, and the implications of recent approaches to gate designs are discussed.
The results of a nanolithography process for making co-planar tunnel junctions with a gap length lower than 30 nm and electrode width in the 100 nm range are presented. These electrodes are buried in the SiO2 substrate which makes the SiO2 gap surface accessible for atomic force microscopy characterization.
An atomic force microscope was used to measure near-surface forces between individual metal-oxide polishing agent particles and planar optical glass surfaces immersed in aqueous fluids of carefully controlled chemistry. For a given polishing agent, the fluid pH was found to significantly influence the magnitude of the near-surface forces. Generally, if the fluid pH equals the isoelectric point of the polishing agent, then the attractive force between the polishing agent particle and the glass surface is maximized. The pH-driven modulation of near-surface forces was found to be completely reversible, indicating the need to control surface change during aqueous polishing of optical glass through manipulation of the slurry chemistry.
Measurements of micrometer and sub-micrometer surface features have been made using a stylus profiler, an STM, and AFM and a phase-measuring interferometric microscope. The differences between measurements of the same surface feature as obtained with the different instruments illustrate the problem of methods divergence. Measurements are compared in an effort to point out, and explain, the observed methods divergence.
Molecular nanotechnology requires computer-based design and analysis tools. Currently they are needed-as part of a computational approach to nanotechnology-to allow detailed, analysed designs to be presented and discussed and to dovetail with efforts to develop fabrication capabilities. In the longer term, as fabrication capabilities emerge, they will be needed for similar reasons as are needed in today's manufacturing systems: to allow investigation of design alternatives and to control manufacturing systems. Given the huge number of 'parts' which may be required in molecular machines the need for computer-based tools will, however, be even more primary for molecular nanotechnology than for today's manufacturing technology, surpassing the CAD requirements of VLSI. Whilst molecular CAD clearly shares some traits with existing CAD, a fundamental difference is that the position and bonding of every atom is of key interest in molecular manufacturing. This necessitates new approaches and techniques. Molecular manufacturing will eventually use diamondoid materials; materials which are crystalline. Molecular CAD tools for the design of diamondoid structures must incorporate techniques for manipulating crystalline materials. In this paper, we describe a molecular CAD tool-Crystal Clear-which allows the design of simple diamondoid structures in full atomic detail using some novel techniques based on the crystalline nature of diamondoid materials.
By employing the low-energy electron projection microscope for in situ visual control, it has been possible to attach or detach a fine manipulating tip to or from a carbon fiber network. In this way we were able to apply an electrical potential to a single nanometer-sized fiber while observing the electron projection images at TV rates. We also measured the maximal current through individual freestanding wires of diameter approximately 10 nm and lengths approximately 1 mum. It turned out that these carbon wires are able to transport a current density exceeding one million A cm-2 at room temperature.
In this letter the effect of active current limitation on the reliability and performance of cold-cathode emitters produced using nanometric fabrication techniques is described. MOSFET limiters were compared along with resistor-limiters and unlimited field emission devices. It is shown that active limiters have the ability to quench discharges before serious damage occurs.
Chemical vapor deposition (CVD) of polycrystalline films of diamond has improved dramatically during the past six years. During this time there have been parallel developments in modeling chemical processes at a diamond surface, with many of the first-principle studies being carried out within the density functional framework. We review our work on furthering the understanding of chemical processes that may be important in the growth process, which requires knowledge of first, the surfaces themselves and second, adsorption of hydrocarbon radicals to add more layers. Studies of both clean and hydrogenated (111), (100), and (110) surfaces of diamond show that clean surfaces tend to undergo strong relaxation and reconstruction while hydrogenation results in inert surfaces. Modeling interactions of various hydrocarbon radicals with diamond surfaces provides energies and geometries of viable processes while eliminating from further consideration processes that are found to be unfavorable. We also discuss briefly recent investigation of behavior at a step on a clean diamond surface, which may indicate a process leading to graphitization of the diamond surface.
The use of a scanning tunneling microscope (STM) for microroughness measurements of polished silicon wafers has been restricted to well cleaned surfaces. In this work it is shown that the water bound to the native oxide layer, not the oxide layer itself, causes the difficulties associated with STM work. Using prolonged scanning with a high bias voltage, we were able to remove bonded water and thus a wafer surface covered with the native oxide layer could be analysed. Prolonged scanning at a high voltage was also shown to be capable of partially removing the native oxide layer itself, thus giving a more correct value for the surface roughness of the silicon wafer surface. On the other hand, prolonged scanning using a lower imaging voltage increased the oxidation of the surface.
The development of microminiaturized biosensors requires techniques for immobilizing biomolecules on solid substrates, in an ordered fashion, and techniques for the subsequent visualization of these patterns. Scanning force microscopy (SFM) is a useful technique for visualizing ordered patterns, but it requires suitable substrates and attachment techniques. Here we present a photolithographic method which gives ordered patterns of biomolecules. Both SFM topographic and lateral force images of these patterns are shown and discussed.