Quantum Sizes Research Articles

Size effect of quantum conductance in single-walled carbon nanotube quantum dots

The quantum conductance of two kinds of carbon nanotube quantum dots (CNQD) composed of (5,5) and (10,0) tubes, namely (10,0)/(5,5)/(10,0) and (5,5)/(10,0)/(5,5) with different quantum sizes, are calculated. It is shown that for (10,0)/(5,5)/(10,0) CNQD, one on-resonant peak at the Fermi energy exists only for special QD sizes, and the width of the conductance gap increases from 1.0 eV to 3.2 eV with the increase of size. The positions of peaks around the Fermi energy are obtained by the electronic structure of individual finite (5,5) tubes. We also find that the (5,5)/(10,0)/(5,5) CNQDs behave as a quantum dot, and its localized QD states are different from that of the former CNQD because of the existence of the interface states between (5,5)/(10,0) junctions. For (5,5)/(10,0)/(5,5) CNQD, there is no conductance gap with QD’s size smaller than 7 layers, and the conductance peak around the interface quasilocalized state -0.26 eV disappears with QD sizes larger than 23 layers. In addition, for the (5,5)/(10,0)/(5,5) CNQD, the connection method can change the degree of electronic localization of intermediate (10,0) tube.

Tight-binding theory of quantum-dot quantum wells: Single-particle effects and near-band-edge structure

Electron and hole states of multishell CdS/HgS/CdS quantum-dot quantum-well nanocrystals are determined by use of atomistic tight-binding theory. Single-particle energies, symmetries and charge densities, trapping in the internal quantum well, transition energies, and optical spectra are determined. Comparison with experiment shows that tight-binding theory provides a good description of nanosystems with monolayer variations in composition. Tight-binding theory correctly predicts the Stokes shift between the dim ground state and the lowest optically active transition. The energy splitting between the lowest two optically active transitions is also correctly described. The Stokes shift is determined by the level splitting between the lowest two hole states. The splitting between the lowest two optically active transitions is determined by the Stokes shift and the splitting between the lowest two electron states. Comparison with multiband effective mass theories shows that both theories provide a similar picture for the single-particle states but that the tight-binding theory provides a better description of observed transition energies. Calculations are done for different nanocrystal shapes, different positions and sizes of the internal quantum well, different values for the surface passivation, the spin-orbit coupling, the HgS bulk band gap, and the band offset. Results are robust to these variations indicating that the physical character of states in quantum-dot quantum wells is determined by the effects of global confinement in the dot and local confinement in the internal well rather than by the specific details of the quantum-dot quantum-well size, shape, or geometry or by special, fortuitous choices for material parameters in the models.

The influence of the size of silver ultra-fine particles and an external field voltage on the theoretical spectral distribution of the long-wavelength quantum yield for an Ag–CsO2thin film

The dependence of the spectral distribution of the long-wavelength quantum yield for an Ag–CsO2 thin film on the size of silver ultra-fine particles embedded in a CsO2 semiconductor and on external voltages are predicted and discussed theoretically. The theoretical results show that an increase in the size of the silver ultra-fine particles, obviously causes the quantum yield values of this thin film to increase, and makes the full widths at half maximum become narrow, and leads to the emergence of an exactly Gaussian form. It is also pointed out that the critical diameters of the silver ultra-fine particles between discrete electron eigenstates (quantum sizes region) and quasicontinuous energy bands for the energy band of silver ultra-fine particles in CsO2 are less than 3.1 nm. Quantificational explanations of the observed quantum yield in the visible and near-infrared regions are given. Without the external field voltage, it is found that the theoretical curves for 6.2 and 3.1 nm silver ultra-fine particles are in good agreement with the experimental results reported by other researchers.

Quantum size effect and temperature effect on spin-polarized transport in ZnSe/Zn 1– xMn xSe multilayers

We present tight-binding Green's function investigations of the influence of quantum sizes and the temperature on spin tunneling transport through a tunable ZnSe/Zn 1– x Mn x Se multilayers under an external electric field. It is found that the degree of polarization can be significantly induced by the size of the corresponding structure. As the size of the multilayers is increasing, both the transmission spectra and the current density spectra become more complicated and the degree of spin polarization is higher. It is also found that the degree of the spin polarization is drastically reduced as the temperature is increasing.

A solution to mean delay in the ∑ Mc/ Gck/1 cyclic priority queue with cycle ( k) and class ( c) dependent feedback and service times

In this paper we develop an original solution to mean delay in a general ∑ M c / G ck /1 cyclic priority queue with class ( c) and cycle ( k)-dependent service time and feedback. Each class has its priority assigned. There may be one or more classes with the same priority. This model is suitable for performance analysis of round robin processor sharing policies. The analysis may be used to examine the effects of quantum sizes in round robin scheduling used in operating systems. Each customer, upon entering the system, requires a number of service cycles before it leaves the system. Each service cycle is characterized by its service-time distribution, and the probability of having at least one more cycle before the customer leaves the system. These characteristics of cycles may be different for different cycles of a service process. The solution is represented as a system of linear equations. It may be efficiently solved using the Gauss–Seidel iterative procedure. A general solution is developed of which, the two special cases are a non-priority (single-priority) and a one-class-per-priority non-preemptive queues. Computational complexity of the numerical procedure is between computational complexity of the two special cases, O( C 3 K 3) and O( CK 3), respectively. C stands for the number of classes, and K stands for the maximum number of cycles being numerically considered.

Broad spectral bandwidth semiconductor lasers

Wavelength-tuned semiconductor lasers have been realised in a GaAs/AlGaAs structure in which the active layer contains three different sizes of quantum well. Extended cavity operation with an external grating has enabled a useful tuning range of 88 nm (778 – 866 nm) to be demonstrated while the threshold current varied by no more than 20 mA about a nominal value of 100 mA across the tuning range. Alternatively, the wide spectral bandwidth of the structure could be used in very short-pulse modelocked lasers.

Electroluminescence of Heavily Doped p‐Type Porous Silicon under Electrochemical Oxidation in Galvanostatic Regime

Visible light emission is obtained during anodic oxidation of heavily doped p‐type (P+) porous silicon layers. Similar characteristics are observed for the electroluminescence (EL) on lightly doped substrates (P−). This indicates that the coarser structure of the heavily doped samples also presents a thin structure, with crystallites of quantum sizes responsible for the emission. For the first time the EL and photoluminescence phenomena of this type of material are studied as a function of the oxidation level. So it is possible to separate the passivation effect due to the oxide growth and the carrier injection process into the crystallites. The main characteristics of the light emission are discussed with a simple model which underlines the key role of the injection of carriers (for the EL) and its significance for the oxidation process. The vanishing of the EL, which is observed under anodic oxidation, seems to be related in the higher anodizing potential to some depassivation of the porous silicon surface.

Electroluminescence of heavily doped p-type porous silicon under electrochemical oxidation in the potentiostatic regime

Visible light emission is obtained during the anodic oxidation of heavily doped p-type porous silicon layers, with similar characteristics to the electroluminescence observed on lightly doped substrates. This indicates that the coarser structure of the heavily doped layers also presents a thinner structure, with crystallites of quantum sizes responsible for the emission. In this paper, the electroluminescence is studied in potentiostatic conditions, under either fixed or scanned potentials. Some reversibility in the spectral changes versus potential variations can be evidenced by selecting correctly the scanning conditions and the potential values. The results allow one to conclude that the electroluminescence spectral evolution is mostly determined by the applied polarisation and can be attributed to the selectivity of the carriers injection into the quantum-size crystallites of the material. The electroluminescence vanishing which is observed in the second part of the anodic oxidation regime seems related in the higher potential range to some depassivation of the porous silicon surface. For the lower potential range, owing to the oxide growth, the emission stops when the polarisation is no longer enough to inject holes into the confined crystallites.

M/G/1 round robin discipline

This paper analyzes a round robin M/G/1 queuing discipline with quantum sizes that depend on the number of circuits a job has made. Tight bounds on the steady state mean response conditional on job size are found. Results show that the round robin discipline is relatively insensitive to changes in workload and follows the processor sharing response curves surprisingly closely, even for large quantum sizes. The analysis allows determination of a proper cutoff point, the point at which a job would be sent to a lower priority queue. Also derived is the Laplace-Stieltjes transform of the response of a job that arrives to find a given workload. The transient mean response is calculated for this case.