Effective stiffness properties (D) of nanosized structural elements such as plates and beams differ from those predicted by standard continuum mechanics (D-c). These differences (D - D-c)/D-c depend on the size of the structural element. A simple model is constructed to predict this size dependence of the effective properties. The important length scale in the problem is identified to be the ratio of the surface elastic modulus to the elastic modulus of the bulk. In general, the non-dimensional difference in the elastic properties from continuum predictions (D - D-c)/D-c is found to scale as alpha S/Eh, where alpha is a constant which depends on the geometry of the structural element considered, S is a surface elastic constant, E is a bulk elastic modulus and h a length defining the size of the structural element. Thus, the quantity S/E is identified as a material length scale for elasticity of nanosized structures. The model is compared with direct atomistic simulations of nanoscale structures using the embedded atom method for FCC A1 and the Stillinger-Weber model of Si. Excellent agreement between the simulations and the model is found.
As the sizes of electronic and mechanical devices are decreased to the micron and nanometre level, it becomes particularly important to predict the thermal transport properties of the components. Using molecular level theories, such predictions are particularly important for modelling nano-electronic devices where scaling laws may change substantially but it is most difficult to accurately measure the properties. Hence, using the empirical bond order dependent force field, we have studied here the thermal conductivity of nanotubes' dependence on structure, defects and vacancies. The anisotropic character of the thermal conductivity of the graphite crystal is naturally reflected in the carbon nanotubes. We found that the carbon nanotubes have very high thermal conductivity comparable to diamond crystal and in-plane graphite sheet. In addition, nanotube bundles show very similar properties as graphite crystal in which dramatic difference in thermal conductivities along different crystal axis.
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, Delta, between the states \0 > and \1 >. The gap Delta 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.
This paper discusses the phase transformation of diamond cubic silicon under nano-indentation with the aid of molecular dynamics analysis using the Tersoff potential. By monitoring the positions of atoms within the model, the microstructural changes as silicon transforms from its diamond cubic structure to other phases were identified. The simulation showed that diamond cubic silicon transforms into a body-centred tetragonal form (beta-silicon) upon loading of the indentor. The change of structure is accomplished by the flattening of the tetrahedron structure in diamond cubic silicon. Upon unloading, the body-centred tetragonal form transforms into an amorphous phase accompanied by the loss of long-range order of the silicon atoms. By performing a second indentation on the amorphous zone, it was found that the body-centred-tetragonal-to-amorphous phase transformation could be a reversible process.
The possibility of modifying the electronic properties of nanotubes using gas molecule adsorption is investigated using the first-principles total energy density functional calculations. Detailed analysis of the electronic structures and energetics is performed for the semiconducting (10,0) single-walled carbon nanotube interacting with several representative gas molecules (NO2, NH3, CO, O-2, and H2O). The results elucidate the mechanisms of the adsorption-induced nanotube doping and illustrate an example of the simulation-based design characterization of nanoelectronic components.
The phenomena of dielectrophoresis and electrorotation, collectively referred to as AC electrokinetics. have been used for many years ro study, manipulate and separate particles on the cellular (1 mu m or more) scale. However, the technique has much to offer the expanding field of nanotechnology, that is the precise manipulation of particles on the nanometre scale. In this paper we present the principles of AC electrokinetics for particle manipulation, review the current state of AC electrokinetic techniques for the manipulation of particles on the nanometre scale, and consider how these principles may be applied to nanotechnology.
One potential application of molecular nanotechnology is the integration of molecular electronic function with advanced silicon technology. One step in this process is the tethering of individual molecules at specific locations on silicon surfaces. This paper reports the fabrication of arrays of individual organic molecules on H-passivated Si(100) surfaces patterned with an ultrahigh vacuum scanning tunnelling microscope (STM). Feedback controlled lithography (FCL) is used to create templates of individual silicon dangling bonds. Molecules introduced in the gas phase then spontaneously assemble onto these atomic templates. Norbornadiene (NBE), copper phthalocyanine (CuPc), and C-60 molecular arrays have been made by this technique and studied by STM imaging and spectroscopy. Both NBE and CuPc molecules appear as depressions in empty states images, whereas in filled states images they are nearly indistinguishable from Si dangling bonds. Furthermore, the fourfold symmetry and central copper atom of CuPc are clearly observed at positive sample bias. Spatial tunnelling conductance maps of CuPc illustrate charge transfer from the surrounding substrate when the molecule is bound to the surface via its central copper atom. On the other hand, when the CuPc molecule interacts with the substrate via an outer benzene ring, molecular rotation is observed. C-60 molecules display intramolecular structure in topographic images and spectroscopic data. The local density of states of C-60 clearly shows the location of the lowest unoccupied molecular orbital, which suggests that the highest occupied molecular orbital is located within 0.3 eV of the fermi level.
The reflection properties of 300 nm periodically structured silicon surfaces with depth varying between 35 and 190 nm, prepared by interference lithography, were examined in the range 200 nm < lambda < 3000 nm. A decrease in the reflectivity that becomes stronger with increasing structure depth is observed below 1000 nm. This broad-band reduction is caused by diffraction effects at short wavelengths and by the 'moth-eye effect' at long wavelengths. The results show a universal behaviour in the optical-path to wavelength ratio dependence of the reflectivity and are in good agreement with the results obtained for the 'moth-eye effect' from the effective medium theory.
A three-dimensional model of molecular dynamics (MD) is proposed to study the effects of tool geometry and processing resistance on the atomic-scale cutting mechanism. The model includes the utilization of the Morse potential function to simulate the interatomic force between the workpiece and a tool. The results show that the cutting resistance increases with the angle of the pin tool and the depth of cut, and the cutting force is essentially constant over the range of velocities simulated. In addition, the obtained cutting resistance of present MD simulation exhibits an evident relationship to the ratio of the vertical and the horizontal contact area between the tool and the workpiece within the range of a pin angle of 90-150 degrees. Finally, work hardening and stick-slip phenomena during the process are also observed.
The machining characteristics of nano-lithography are studied using atomic force microscopy (AFM). Scribing (scratching) experiments containing reciprocal single line furrows and multiple furrows are conducted to investigate the influence of the working parameters on the machined surface's properties, and upon the machining efficiency. The influence of the working parameters, including the applied load on the cantilever, scribing cycles, scribing speed and scribing feed, on the surface roughness, surface depth and material removal rate can then be accessed. Results indicate that the applied load is more significant than the scribing cycles on the groove depth. However, rougher surfaces are produced at larger loads. In multiple furrows produced with larger applied loads in order to obtain deeper furrows, surface roughness is improved by adjusting the scribing feed to a small value.
Tip-derived artifacts remain one of the chief limitations of atomic force microscopy (AFM) when attempting to measure sub-nanometre structures. Carbon nanotubes represent ideal structures for use as AFM tips because of their small diameter, high aspect ratio and high strength. We attached single carbon nanotube AFM tips using a novel are discharge method. Using these modified tips, we successfully imaged a protein filament found in sponge spicules of Tethya aurantia. We report a modular stave-like structure for the protein filament that was previously unobservable with conventional AFM cantilevers.
Kinesin is a microtubule-associated protein, converting chemical into mechanical energy. Based on its ability to also work outside cells, it has recently been shown that this biological machinery might be usable for nanotechnological developments. Possible applications of the kinesin-based motor system require the solution of numerous methodological and technical problems, including the orientation of force generation into a desired direction and the determination of the tolerable roughness of the surfaces used, the minimal free vertical space still enabling force-generating activity, and the temporal stability of the system. This paper reports on the example of microtubules gliding across kinesin-coated surfaces and shows that the force-generating system needs a minimal foe working space of about 100 nm height and works up to 3 h with nearly constant velocity. Individual microtubules were observed to cover distances of at least 1 mm without being detached from the surface and to overcome steps of up to 286 nm height. In addition, mechanically induced how fields were shown to force gliding microtubules to move in one and the same direction. This result is regarded as being an essential step towards future developments of kinesin-based microdevices as this approach avoids neutralization of single forces acting in opposite directions.
Kinesin motor proteins and the microtubule cytoskeleton function as an intracellular railroad system-a railroad with nanometre-scale engines running on nanometre-scale tracks. Our long-term objective is to rake this molecular transport machinery and integrate it into kinesin-powered microdevices. As a step toward this objective, we have coupled kinesin to microscale silicon chips that were patterned photolithographically and etched from silicon membranes. The microchips were observed by light microscopy to move on microtubules aligned and immobilized on the surface of a microscope flowchamber. The microchips translated, rotated, and flipped over. From these examples of microchip movements, it is conceivable that this technology can be extended to moving more elaborate microparts, like gears, or rotors, or levers using kinesin motors. This will allow kinesin forces to be coupled to a useful action in a microdevice. For example, a microrotor turned by kinesin could demonstrate the feasibility of creating a kinesin-powered microgenerator or micropump.
We investigated the use of quantum bits (qubits)-semiconductor quantum dots containing one electron and each consisting of two tunnel-connected parts-as basic elements of the quantum computer. The numerical solution of a Schrodinger equation taking account of the Coulomb field of adjacent electrons shows that in such structures the realization of a full set of basic logic operations, which are necessary for fulfillment of quantum computations, is possible. Durations of one- and two-qubit operations versus qubit geometry are obtained. Decoherence rates due to spontaneous emission of phonons and acoustic phonons (both piezoelectric and deformation) are evaluated. Analysis of these rates shows the proposed qubit to be coherent enough to work for an unlimited time.
The phase diagrams of small particles (with diameters in the nm range) are studied theoretically. In the Limit where thermodynamical arguments remain valid, it is deduced that the phase diagram of small particles is a function of their size. For the case of ideal solutions, it is shown that the lens-shaped solidus-liquidus curves are shifted to lower temperatures when the dimensions of the particle decrease. Additionally, at fixed temperatures between the highest bulk melting point and the lowest melting point of the particle, the relative concentrations of the solid and Liquid phases are different in the particle and bulk material.
We report measurements of the diffraction pattern of a two-dimensional photonic quasicrystal structure and use the set of plane waves defined by the diffraction pattern as the basis of a theoretical approach to calculate the photonic band structure of the system. An important feature of the model is that it retains the essence of the rotational and inflational properties of the quasicrystal at all levels of approximation: properties lost in approximate models which artificially introduce elements of periodicity. The calculated density of modes of the quasicrystals is found to display a weakly depleted region analogous to the bandgap that occurs in a periodic system. The calculated transmission spectra for different polarizations and directions of propagation show features that correlate with the behaviour of the density of modes.
In this paper, a new scanning moire method is developed to measure the in-plane deformation of mica using an atomic force microscope (AFM). Moire patterns are formed by the scanning line of the CRT in the AFM system, and the atomic lattice of the mica or high-orientated pyrolytic graphite (HOPG). The measurement principle and the techniques employed for grating preparation are described in detail. This new method is used to measure the residual deformation of a mica plate after irradiation by a Nd-YAG laser, and to determine the residual strain of HOPG under a tensile load. Some interesting results are obtained. The successful results verify the feasibility of this method for measuring deformation in the nanometre range using the lattice of the material as the model grid.
Micromechanical cantilevers used in atomic force microscopy are characterized by the geometry, the elastic modulus E and the quality factor Q. The sensor can be regarded as a rectangular bar clamped on one side and free on the other. In contrast to a simple harmonic oscillator a cantilever has different eigenfrequencies omega(n) and a mode-dependent spring constant D-n. Using the fluctuation-dissipation theorem we developed a simple model to calculate the thermal noise on each eigenmode for a free cantilever. With this result we can decide whether measuring on higher eigenmodes increases the force sensitivity.
Dendrimers are well defined, highly branched macromolecules that radiate from a central core and are synthesized through a stepwise, repetitive reaction sequence that guarantees complete shells for each generation, leading to polymers that are monodisperse. The synthetic procedures developed for dendrimer preparation permit nearly complete control over the critical molecular design parameters, such as size, shape, surface/interior chemistry, flexibility, and topology. Recent results suggest that dendritic polymers may provide the key to developing a reliable and economical fabrication and manufacturing route to functional nanoscale materials that would have unique properties (electronic, optical, opto-electronic, magnetic, chemical, or biological). In turn, these could be used in designing new nanoscale devices. In this paper, we determine the 3D molecular structure of various dendrimers with continuous configurational Boltzmann biased direct Monte Carlo method and study their energetic and structural properties using molecular dynamics after annealing these molecular representations.