We studied the cluster size distribution of dislocation-free InAs͞GaAs self-assembled quantum dots obtained by the Stranski-Krastanow mode of molecular beam epitaxy. The same scaling function was obtained over a wide range of dot density. The scaling function indicated that the cluster size fluctuation, normalized by the average size, is constant for all the quantum dot densities studied. The resemblance of the scaling function to that of the submonolayer homoepitaxial growth implies that the strain is not the essential factor determining the cluster size distribution of quantum dots.[S0031-9007(98)05602-6] PACS numbers: 68.55. 61.43.Hv, 82.20.Mj The growth of lattice-mismatch semiconductor systems such as Ge͞Si and InAs͞GaAs has been known to produce a dotlike structure by the Stranski-Krastanow (SK) mode. Recently this growth mode, especially of InAs͞GaAs dots (known as "self-assembled quantum dots" [1]) has drawn much attention as a method to obtain quantum dots which are damage-free and fabrication-free. Now practical applications of quantum dots are seriously sought for, and efforts are being made in fabricating quantum dot lasers with predicted high performance such as a high characteristic temperature and a low threshold current. However, the current InAs quantum dots have a problem in size uniformity which is commonly about 610%, and is insufficient for laser applications. It is intriguing to know if this common size fluctuation is essential or accidental.Not to mention practical applications, the scaling property of two-dimensional (2D) island size distribution is known for submonolayer coverage. According to the scaling assumption [2,3], the island distribution is given byHere N s is the number of islands (normalized by the number of lattice sites) which contain s atoms, u is the fractional surface coverage, ͗s͘ is the average number of atoms in an island, and f͑x͒ is the scaling function, which depends only on s͗͞s͘. This scaling assumption was confirmed experimentally in Fe homoepitaxy [4] and InAs͞ GaAs heteroepitaxy [5].It is interesting to know if something similar to Eq. (1) holds for three-dimensional (3D) island growth by the SK mode, especially because it was reported [6] that the total island density r shows a power law as a function of the coverage expressed as r r 0 ͑u 2 u c ͒ a (2) for u from 1.5 to 1.9 monolayer (ML), where r 0 is the proportionality coefficient, u c is the critical coverage (1.5 ML), and the critical exponent a is 1.76. Equation (2) implies the presence of the scaling region and consequently the scaling function.In this Letter, we present the 3D island size scaling which holds for a quite wide range of the total density of the dislocation-free InAs͞GaAs islands by the SK mode.The growth was done by the Riber 2300 molecular beam epitaxy (MBE) system on a nominally flat GaAs (001) substrate. After the growth of the GaAs buffer layer at a substrate temperature of 600 ± C, growth was interrupted for 3-5 min, to decrease the temperature to 490 ± C to obtain a c͑4 3 4͒ surf...
To slow the translocation of single-stranded DNA (ssDNA) through a solid-state nanopore, a nanopore was narrowed, and the effect of the narrowing on the DNA translocation speed was investigated. In order to accurately measure the speed, long (5.3 kb) ssDNA (namely, ss-poly(dA)) with uniform length (±0.4 kb) was synthesized. The diameters of nanopores fabricated by a transmission electron microscope were controlled by atomic-layer deposition. Reducing the nanopore diameter from 4.5 to 2.3 nm slowed down the translocation of ssDNA by more than 16 times (to 0.18 μs base(-1)) when 300 mV was applied across the nanopore. It is speculated that the interaction between the nanopore and the ssDNA dominates the translocation speed. Unexpectedly, the translocation speed of ssDNA through the 4.5 nm nanopore is more than two orders of magnitude higher than that of double-stranded DNA (dsDNA) through a nanopore of almost the same size. The cause of such a faster translocation of ssDNA can be explained by the weaker drag force inside the nanopore. Moreover, the measured translocation speeds of ssDNA and dsDNA agree well with those calculated by molecular-dynamics (MD) simulation. The MD simulation predicted that reducing the nanopore diameter to almost the same as that of ssDNA (i.e. 1.4 nm) decreases the translocation speed (to 1.4 μs base(-1)). Narrowing the nanopore is thus an effective approach for accomplishing nanopore DNA sequencing.
Abstract. The newly developed unifying discontinuous formulation named the correction procedure via reconstruction (CPR) for conservation laws is extended to solve the Navier-Stokes equations for 3D mixed grids. In the current development, tetrahedrons and triangular prisms are considered. The CPR method can unify several popular high order methods including the discontinuous Galerkin and the spectral volume methods into a more efficient differential form. By selecting the solution points to coincide with the flux points, solution reconstruction can be completely avoided. Accuracy studies confirmed that the optimal order of accuracy can be achieved with the method. Several benchmark test cases are computed by solving the Euler and compressible Navier-Stokes equations to demonstrate its performance.
The effects of various catalysts on the H 2 release characteristics of LiAlH 4 were studied. It was established that the catalytic activity defined by the H 2 desorption capacity by a mechanochemical reaction decreased in the series TiCl 3 > ZrCl 4 > VCl 3 > NiCl 2 > ZnCl 2 . Transition metal trialuminides Al 3 X (X = Ti, Zr, V, Ni) were prepared by the mechanochemical reaction of LiAlH 4 and the metal chlorides, while the reaction of LiAlH 4 and ZnCl 2 yields Zn. The high catalytic activity was attributed to the lower percentage d-character and the electronegativity of those transition metals. The catalytic activity of Ni was also improved with decreasing the Ni particle size. Those catalysts were classified to two groups (1: TiCl 3 , ZrCl 4 ; 2: VCl 3 , NiCl 2 , Ni, nano-Ni) from the viewpoint of the H 2 desorption mechanism.
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