Calculations using an analytic potential show that carbon nanocones can exhibit conventional cone shapes or can form concentric wave-like metastable structures, depending on the nanocone radius. Single nanocones can be assembled into extended two-dimensional structures arranged in a self-similar fashion with fivefold symmetry as system size is increased. Calculations of the electronic properties of nanocones indicate that a pentagon in the centre of a cone is the most probable spot for emitting tunnelling electrons in the presence of an external field. This implies that nanocone assemblies, if practically accessible, could be used as highly localized electron sources for templating at scales below more traditional lithographies. (Some figures in this article are in colour only in the electronic version).
Non-equilibrium properties of quantum dots forming type-II heterostructures with the matrix material are analysed theoretically. Their main distinction from standard type-I dots results from the separation of non-equilibrium carriers in a staggered band diagram and, hence, a decisive role of Coulomb potentials creating potential barriers for recombination. Depending on the material parameters and temperature, these barriers are overcome either by activation or by tunnelling. For both recombination mechanisms, the amplitude of potential, the quantum dot charge, the recombination time, the luminescence spectrum and some other non-equilibrium characteristics are calculated for different intensities of optical excitation.
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.
The temperature dependence of the electrical conductivity of a cylindrical nanowire is investigated. The calculations are performed for thin wires with an ellipsoidal Fermi surface, based on the Fuchs-Sondheimer boundary scattering theory given for thin metallic films with spherical Fermi surfaces. It is seen that the conductivity depends on the process of scattering of the carrier on the surface, and on the thickness of the wire for wires with diameters smaller than the carrier's mean free path. The temperature dependence of the electrical resistivity of a Bi nanowire is calculated and compared with the experimental results reported by Gurvitch, where good agreement is observed.
A recent proposal, in which 1-bit memory cells and simple logic gates such as NOT and NOR gates were based on C-60 molecules in an electromechanical grid acting as transistors, is extended to larger architectures. In order to meet the requirements of standard digital circuit architectures, some modifications have to be made compared to the original model. For example, the number of transistors has to be increased from two to thirteen for a single NOR gate to guarantee balanced logical levels. In the scheme employed to achieve this in the current work, all two-input gates, namely OR, AND and XOR gates, can be easily constructed using the same concept. These gates are then used to design a 1-bit full-adder and a clocked D-latch, which are then combined with the earlier proposed 1-bit memory cell as the basic constituents of a memory/adder model. Clocked signal transmissions, corresponding to the read process of two 2-bit words from memory cells, their movement through registers and finally their addition and passing the output through another register, are simulated using the electrical circuit software SPICE. For the design of this memory/adder circuit, 464 single C-60 transistors are used.
We present results to show the effects of device scaling on the relative performance of three device technologies, namely resonant tunnelling diodes (RTD), electronic quantum cellular automata (QCA) and complementary metal-oxide-semiconductor (CMOS) transistor technology. The minimum feature size (MOSFET or HFET gate lengths lambda) is scaled from 250 to 50 nm, while QCA inter-dot separations of 20 to 2 nm are examined. The comparison is made using a standard digital circuit architecture, namely a memory-adder model. Our aim is to compare these device technologies on the system level rather than individual device level. The results show that the RTDs offer speed advantages over CMOS, but improvements in the circuit density are limited. The electronic QCAs will suffer both from effectively low packing density and low operating speeds in comparison to CMOS if conventional designs and a 2D architecture are used.
A new scheme for the generation of coherent radiation on the intersubband transition without population inversion between subbands is presented. The scheme is based on the resonant nonlinear mixing of the optical laser fields on the two interband transitions that are intracavity generated in the same active region. The two-wavelength lasing on the interband transitions can be achieved at a substantially lower threshold current than gain on the intersubband transition. This may ensure cw room-temperature operation. Due to the parametric, inversionless nature of generation, the lasers proposed are especially promising for long-wave length operation above 20 mum.
A new simple variational wavefunction with a few variational parameters for two-dimensional X+ and X- trions is suggested. The function gives accurate results for the singlet and triplet state energies of X+ and X- trions in the whole range of electron-to-hole mass ratio. The mass ratio range where the triplet state exists has been found, and the behaviour of the triplet state energy has been examined near the critical mass ratio. The other excited states of the trion are analysed in the vicinity of their critical mass ratios.
Gold nanoclusters, chemically passivated with decanethiol and dissolved in toluene, have been deposited from solution onto selected regions of oxidized silicon (100) surfaces patterned either with photoresist or an etched step. When the perimeter of a droplet crosses the boundary between the resist and the silicon surface, we observe transport of cluster solution along such discontinuities, outside of the droplet. Such guided flow can extend for over 600 mum across the surface, producing cluster chains as narrow as similar to 120 nm, once the toluene has evaporated. The same experiment with an etched step produces no transport of clusters, but rather selective deposition and growth around the discontinuity. These different responses are attributed to the step/boundary material-principally its interaction with the toluene solvent during evaporation.
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.
We have performed density functional theory calculations to show how a tungsten scanning probe can mediate the interactions between bistable Si(100) surface dimers. Interpreting the state of each dimer as a bit of information, we demonstrate the use of this mediated interaction to construct a NOR logic gate.
We study the effect of the interactions of electrons and holes on the optical properties of Ge/Si type-II quantum dots (QDs). In contrast to type-I systems, the excitonic absorption is found to be blueshifted when exciton-hole and exciton-exciton complexes are formed. For a positively charged dot, we argue that this is the consequence of dominance of the hole-hole interaction compared with the electron-hole interaction due to the spatial separation of the electron and hole. When two excitons are excited in the dot, the electrons are found to be spatially separated and have different quantization energies. This is the reason why the biexciton absorption is blueshifted as compared with a single exciton. The spatial separation of electrons makes it possible for a dot to trap more electrons than there are holes. As a result, the conductivity of stacked arrays of Ge/n-Si QDs is found to decrease under interband optical excitation. The negative photoeffect is explained by the trapping of mobile electrons in the quantum well created by the Hartree potential of holes photoexcited in the dots.
Self-organized microcrystals and nanocrystals (quantum dots) of AgCl and AgBr embedded in KCl and KBr crystalline matrices and retaining the orientation of the host lattice were studied by optically detected magnetic resonance, It was unambiguously shown that self-organized microcrystalline silver halides can be grown inside alkali halide crystals with the properties of bulk crystals since the optically detected magnetic resonance spectra of the embedded microcrystals were practically the same or close to those in bulk AgCl and AgBr and could be used as a 'fingerprint' for AgCl and AgBr. For AgCl nanocrystals in a KCl matrix the anisotropy of the K-factor both for isolated self-trapped holes and for self-trapped holes forming self-trapped excitons was found to be substantially reduced compared with those of bulk AgCl crystals. This implies a considerable suppression of the Jahn-Teller (JT) effect in nanoparticles. A rather general mechanism of the suppression of the JT effect in nanocrystals is proposed, taking into account the additional deformation field appearing because of the strong vibronic interaction at the interface. It was concluded that the distribution of exchange interactions for electron-hole pairs and triplet excitons in the KBr:AgBr system is due to a distribution of AgBr crystal sizes. The holes seem to be self-trapped in the AgBr because of the dynamical JT effect. The exchange splitting increases for distant electron-hole pairs with a decrease of AgBr size. The spectra with exchange splitting larger than that in bulk AgBr (1.9 cm(-1)) seem to belong to AgBr nanocrystals. In contrast to AgCl the wavelength of the luminescence in AgBr micro- and nanocrystals embedded in a KBr matrix decreases with the decrease of AgBr crystal size.
For the very first time ensembles of Ge quantum dots have been formed in an unstrained heterosystem of GaAs/ZnSe/Ge. The geometric properties (dimensional characteristics) and electronic structure of the islands were studied using scanning tunnelling microscopy, Raman spectroscopy and photoluminescence.
Dynamic properties of the resonant tunnelling structures (RTS) as an example of the dynamic nature of nanoelements are studied. Hysteresis and 'plateaulike' behaviours of the time-averaged current-voltage (I-V) curve of resonant tunnelling structures may be obtained through analysing the high-frequency current oscillations in such structures. Our study shows that 'soft' generation of oscillations leads to the characteristic 'plateaulike' I-V form and 'rigid' generation of oscillations leads to 'hysteresis' I-V behaviour. The analysis is obtained by investigation of the limit cycles of the dynamical system corresponding to the modified equivalent circuit of the RTS developed by Buot and Jensen. In this paper it is clarified that RTS may be used not only as a device for generation of THz oscillations but it may also be used as the dynamic trigger which has two stable states: the stable limit cycle and stable static state.
Time-resolved photoluminescence (PL) was studied to determine the radiative recombination lifetimes in InAs/GaAs quantum dots (QDs) fabricated on misoriented substrates by the Stransky-Krastanov method. The intensity of PL from the ground state, excited state and wetting layer of QD was also studied as a function of the excitation density. Measured lifetimes were as high as 3.1 ns. The measurements have clearly demonstrated the band state filling effect: it also provides the possibility of estimating the excitation power density corresponding to the manifestation of this effect at the excitation power of 30 A cm(-2).
A replicative assembly methodology may be based on assembly stations, each with two degrees of rotational freedom. These stations share translational degrees of freedom in the three Cartesian axes by using a common translating mechanism. The methodology provides replication of the assembly stations, but due to the common translating mechanism and control systems, it cannot be termed self-replicating. The term 'exponential assembly' is proposed to differentiate this from self-replication. The exponential assembly architecture can use parts made from many manufacturing methods, provided that parts of considerable complexity can be produced. Because integrated circuit manufacturing methods used for micro-electromechanical systems allow large numbers of complex components to be produced, it is one possible method for pursuing such an approach to manufacturing. The methodology is scalable and therefore useful for producing assembly stations which might in turn produce other devices at ever-decreasing length scales as part of a top-down approach to nanotechnology.
We study p-i-n diodes incorporating InAs/AlAs self-assembled quantum dots (QDs) to probe the electron and hole levels of the dots. A comparative analysis of capacitance-voltage, current-voltage and electroluminescence measurements shows that p-i-n structures could be successfully used as QD spectrometers.
We investigate the transport properties of magnetic edge states in the presence of a perpendicular non-homogeneous magnetic field in a quantum wire. Systems are studied where the magnetic Field exhibits a discontinuous jump in the transverse direction. The energy spectra and wavefunctions of these systems are calculated. The resistance of the quantum wire in the presence of such a magnetic interface is obtained as a function of the Fermi energy and of the homogeneous background magnetic field.