A comparative study of the surface morphology, photoluminescence, and photoelectric spectra of heterostructures with InAs/GaAs quantum dots (QDs) grown on the surface, uncovered by etching away the cladding layer, and built in the GaAs matrix is reported. The red-shift of the ground transition energy in the surface QDs compared to the built-in ones has been shown to be related not only to relaxation of the elastic strain, but also to the differences in size, shape, and chemical composition of the nanoclusters. The method of photoelectric spectroscopy in a semiconductor/electrolyte system has been applied to monitor the process of etching of the cladding layer in situ. Using this method, controlled uncovering of the buried QDs was shown to be possible. In turn, this makes it possible to study the actual morphology of the buried quantum-size layers by atomic force microscopy.
We have studied the correlation between nitrogen composition of bulk GaAsN layers grown by molecular beam epitaxy using rf plasma cell and photoluminescence (PL) intensity. We have carried out careful optimization of the plasma cell aperture layout and plasma operation regimes as well as the growth condition of the GaAsN. We have demonstrated the same efficiency of PL from GaAsN layers with up to 1.5% of nitrogen as for GaAs analogues grown at the same temperature. The integrated PL intensity of the sample containing 2.5% drops only three times. Using post-growth annealing we eliminated defects related to low-temperature growth and thereby achieved the same radiative efficiency for GaAsN samples grown at 520 degreesC as for the reference layer of GaAs grown at 600 degreesC.
The controlled emission of ions by field desorption from nanotips is used to develop an atomic-size ion source. The source has been shown to emit Au and Ag ions from layers pre-deposited on W base tips with two characteristics important for applications: (1) the ion emission can be controlled such that it originates exclusively from the atomic-scale apex of a single nanotip positioned precisely on the main axis of the base tip and (2) the nanotip emits stable metallic ion currents in the pA range for at least tens of hours. Emission patterns, energy distributions and emitted ion currents versus time, voltage and temperature have been measured. These data permit a qualitative understanding of the source characteristics by considering their relation to the three essential source elements: the axial nanotip, the supply function and ionization by field desorption. Computer simulations are performed to explore the use of these sources to create nanometre-scale structures when coupled to electrostatic lenses or in a near-held configuration as well as the interactions of the ions in the range of 100-1000 eV with a Si substrate.
We proposed a method of implementing the Boltzmann machine neural network on electronic circuits by making use of the single-electron tunnelling phenomenon. The single-electron circuit shows stochastic behaviour in its operation because of the probabilistic nature of the electron tunnelling phenomenon. It can therefore be successfully used for implementing the stochastic neuron operation of the Boltzmann machine. The authors developed a single-electron neuron circuit that can produce the function required for the Boltzmann machine neuron. A method for constructing Boltzmann machine networks by combining the neuron circuits was also developed. The simulated-annealing operation can be performed easily by regulating an external control voltage for the network circuits. A sample network was designed that solves an instance of a combinatorial optimization problem. Computer simulation demonstrated that, through the simulated-annealing process, the sample network can converge to the global minimum energy state that represents the correct solution to the problem.
We show that regular arrays of pillars and helices, with repeat distance below 100 nm, can be grown by oblique deposition onto a patterned rotating substrate. The size limits for such structures are discussed.
We present a method for assessing an atomic force microscope's (AFM's) ability to reject externally applied vibrations. This method is demonstrated on one commercial and two prototype AFMs. For optimally functioning AFMs, we find that the response to externally applied vibrations obeys a 1/omega (2) frequency dependence. This 1/omega (2) frequency dependence can be understood by modelling the mechanical system which connects the AFM cantilever and the sample under test as a simple harmonic oscillator. According to such a model, the resonant frequency of the mechanical system which connects the AFM cantilever and the sample under test determines an AFM's ability to reject externally applied, low-frequency vibrations.
This paper gives a brief review of our recent work done in the area of nanometre-scale Coulomb blockade (CB) memory and logic devices, that enable us to realize future electron-number scalability by overcoming inherent problems to conventional semiconductor devices. We introduce multiple-tunnel junctions (MTJs), naturally formed in heavily doped semiconductor nanowires, as a key building block for our CB devices. For memory applications, the hybrid MTJ/MOS cell architecture is described, and its high-speed RAM operation is investigated. For logic applications the binary decision diagram logic is discussed as a suitable architecture for low-gain Mn transistors.
Controlled scratches on octadecylphosphonic acid self-assembled monolayers were made using atomic force microscopy tips as indenters. Scratch morphological evolution was followed as a function of time, at room temperature, for samples prepared by drip coating and crystal melting methods. Self-healing, ranging from partial to complete, was observed on drip coated samples. However, no substantial healing was observed on crystal melted samples. Such different behaviour is discussed in terms of the scratching mechanism on both sample types.
Semiconducting iron disilicide (beta-FeSi2) precipitate layers were synthesized by means of Fe+ implantation into Si(100) at an energy of 40 keV and a dose of 1 X 10(16) cm(-2) followed by nanosecond pulsed ion-beam treatment of the implanted Si layers. Glancing incidence x-ray diffraction (GIXRD) and atomic force microscopy (AFM) were employed for the structural characterization, and optical absorption and photoluminescence (PL) spectroscopies were used for the optical characterization of the precipitate layers formed. The GIXRD results indicate the formation of oriented beta-FeSi2, precipitates surrounded by a polycrystalline Si matrix. AFM data show the precipitate sizes to be in the range of 25-90 nm. The results of measuring the optical absorption indicate that the formed precipitates have a direct-band structure with an energy gap of 0.83 eV. It is shown that the 1.5 mum PL signal of beta-FeSi2 precipitates is observed up to a temperature of 210 K and does not saturate up to the pump power of 250 mW.
Ultrasonic vibration can be nonlinearly detected by means of an atomic force microscopy cantilever when the tip is in contact with a sample surface owing to the so-called (sample-induced) ultrasonic force. The procedure has been developed as a novel technique, ultrasonic force microscopy (UFM), that provides information about the nanoscale elastic and adhesive properties of surfaces. Here, we compare differences in the UFM signal when ultrasound is excited from the back of the sample (sample UFM) and from the cantilever base (waveguide UFM). UFM relies on the nonlinear ultrasound-induced cantilever displacement (due to the aforementioned ultrasonic force), and does not monitor the linear high-frequency vibration of the cantilever. In this paper, we discuss the influence of a linear high-frequency cantilever response in the UFM measurements and provide experimental evidence of the feasibility of nonlinearly detecting the free ultrasonic cantilever vibration when the tip is out of contact with the sample surface using the typical laser-beam deflection method for monitoring cantilever displacements.
Two different mechanisms of homogeneous broadening of the zero-optical-phonon spectral line in CdSe nanocrystal quantum dots are analysed. The first mechanism is due to modulation of the optical-phonon mode frequencies. The second one is caused by multiple-acoustic-phonon-assisted transitions. We show that homogeneous broadening due to acoustic phonons dominates at low temperatures and for small nanocrystal radii. For large nanocrystal radii and at room temperature, the two mechanisms become comparable.
We study experimentally and numerically the statistical conductance properties of Al breaking nanocontacts at room temperature. The measured conductance histogram (CH) from hundreds of consecutive experiments exhibits peaks close to integer values in units of 2e(2)/h. The results are compared with minimum-cross-section histograms obtained from molecular dynamics calculations. The agreement between these atomistic simulations and experiments provides evidence that the CH is a signature of favourable atomic configurations.
The formation of gold nanoparticle assemblies in a patterned manner on suitable substrates is described. The protocol for realizing such structures comprises the following steps. In the first step, patterned films of a fatty amine are thermally evaporated onto solid supports using suitable masks (e.g. a TEM grid). Thereafter, the fatty amine film is immersed in chloroauric acid solution and chloroaurate (AuCl4-) ions entrapped in the lipid matrix by electrostatic complexation with the ammonium ions of the fatty amine molecules. The final step involves the reduction of the AuCl4- ions in situ thus leading to the formation of gold nanoparticles within the patterned lipid matrix. The process of metal ion incorporation and reduction may be repeated a number of times to increase the nanoparticle density in the lipid matrix. AuCl4- ion entrapment and formation of gold nanoparticles within the patterned lipid matrix has been followed by quartz crystal microgravimetry, UV-vis spectroscopy, Fourier transform infrared spectroscopy, scanning electron microscopy and energy dispersive analysis of x-ray measurements. The protocol described shows immense potential for extension to assemblies of nanoparticles in more intricate patterns as well as to the growth of semiconductor quantum dots in such patterns.
Photoluminescence (PL) of porous InP formed by electrochemical etching techniques is measured at room temperature. Compared to the PL of the bulk InP wafer, a blueshifted PL emission is observed. It is found that the amount of the blueshifted energy depends on the microstructure of porous InP, which in turn strongly depends on the potential voltage applied during the electrochemical etching. Time-resolved PL study of the sample shows that surface nonradiative recombination is dominant.
Graphite is an important material for use as a solid lubricant that works even at high temperatures. Generally, the reason for the lubricity is that carbon layers easily slide against each other due to the layered structure with weak interlayer interaction. However, the atomic nature of the interlayer interaction is still not fully understood. To improve this understanding, we applied ultrasonic atomic force microscopy to highly oriented pyrolytic graphite and observed edge dislocations accompanied by extra half-planes. Through observation of the dislocation behaviour under different loads, we found that the dislocation moved laterally by 20 nm as the load increased by 80 nN, and it returned to the original position as the load decreased. To explain this result, we propose a model for the lateral motion of the dislocation, which includes a spring and pinning point. This finding of the large lateral motion confirms the extraordinarily easy sliding between carbon layers, which is relevant to the performance as a solid lubricant. It may also be relevant to the significant material transport in graphite intercalation compounds and in carbon nanotubes.
Low-temperature (T approximate to 2 K) photoluminescence (PL) and photoluminescence excitation (PLE) spectra of GaAs/AlGaAs (x = 0.05) structures with shallow quantum wells (QWs) were investigated. It was found that the PLE spectra exhibit a number of broad bands in the above-barrier energy region; these bands alternate 'in opposite phases' in the spectra of free excitons and excitonic complexes (trions) (i.e. an increase in the exciton luminescence intensity is accompanied by a decrease in the luminescence intensity of the complexes). Effects originating from simultaneous irradiation of the sample by two laser beams of different wavelengths were studied. In the case where the photon energy of the Ti-sapphire laser is tuned to excite only the QW states, additional pumping by a He-Ne or Ar-ion laser results in the shift of the equilibrium in the exciton-trion system towards an increase in the concentration of the latter species. On the other hand, upon excitation into certain barrier states with energies both below and above the barrier bandgap, additional pump shifts the equilibrium in the opposite direction.
The prominent role of software in nanotechnology research and development suggests that open source development methods might offer advantages in improving reliability, performance and accessibility. Open source approaches have shown new opportunities for voluntary cooperation to create and improve complex software. Suitable software licences could be used to promote access, compatibility and sharing of improvements. Many companies currently associated with nanotechnology produce materials, equipment and research and development services, all of which could support open source business models; however, no company yet emphasizes an open source strategy. Some molecular modelling software is already open source or public domain. Software for molecular engineering constitutes an important opportunity for open sourcing, especially if systems architectures encouraging collaboration can be further developed. Analysis suggests that the net impact of open sourcing would be to enhance safety. Initiatives for open sourcing of molecular nanotechnology could be strengthened by coalition building, and appropriate strategies for open source licensing of copyrights and patents.
Two bola-amphiphilic alpha, omega -diboronic acids separated by a (CH2)(11) or (CH2)(12) group were synthesized. Complexation with chiral diols readily gave new amphiphiles end-capped with the chiral substituents. Some of these acted as good gelators of organic solvents. Transmission electron microscope and scanning electron microscope observations established that a variety of super-structures are created in the organogels, depending on the solvents and the structure of the chiral end-cap groups. In most cases, the fibrous aggregates, the network structure which is the driving-force for gelation, showed a helical higher-order structure reflecting the chirality of the end-cap groups. The results indicate that the combinatorial approach utilizing boronic acid functions and diol compounds is useful in creating a variety of new super-structures in the gel phase.
In order to establish whether atomic force microscope (AFM) grown SiO2 is appropriate for use as a gate oxide in nanoelectronics, a characterization of these films needs to be performed. In this paper results on AFM fabrication and topographical characterization of large-area SiO2 patterns are presented. This paper is centred around the SiO2 surface and SiO2-Si interface roughness, due to its importance in relation to the quality of ultrathin dielectrics. Our results show quite similar values to those obtained for thermal oxides and thus we suggest that AFM-grown SiO2 is a suitable candidate for gate oxide applications in nanodevices.
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).