Imaging ‘Invisible' Dopant Atoms in Semiconductor Nanocrystals

Extended abstract of a paper presented at Microscopy and Microanalysis 2012 in Phoenix, Arizona, USA, July 29 - August 2, 201

Stranski-Krastanow growth of (Si)Ge/Si(001): transmission electron microscopy compared with segregation theory

We performed transmission electron microscopy of SiGe/Si(001) and Ge/Si(001) samples that undergo the Stranski–Krastanov transition from flat layer to island growth. With the help of quantitative X-ray maps of those layers, we have determined the total amount of deposited germanium at which islanding commences. The maximum amount of Ge buried in a flat layer amounts to 2.3 monolayers. We show by modelling that it is the strain due to the total amount of Ge atoms deposited that drives the islanding process. At 600°C [400°C], 1.62 [1.74] monolayers of Ge are expected from simulations to segregate towards the surface, the strain of which is sufficient to trigger plastic relaxation by islanding, in agreement with our electron microscope observations

Qac genes and biocide tolerance in clinical veterinary methicillin-resistant and methicillin-susceptible Staphylococcus aureus and Staphylococcus pseudintermedius

Qac genes are associated with increased tolerance to quaternary ammonium compounds and other cationic biocides such as chlorhexidine. This study aimed to determine whether qac genes and increased biocide tolerance were present in 125 clinical methicillin-resistant and susceptible veterinary staphylococci. A total of 125 methicillin-resistant Staphylococcus aureus (MRSA) and methicillin-resistant and -susceptible Staphylococcus pseudintermedius (MRSP and MSSP) from three archived Australian veterinary staphylococci collections underwent whole genome sequencing, multilocus sequence typing and qac gene screening. Two MRSA isolates (12%) harboured qacA/B genes; both isolates were ST8 from horses. QacJ, qacG and smr genes were identified in 28/90 (31%) MRSP and 1/18 (6%) MSSP isolates. ST71 MRSP was significantly more likely to harbour qac genes than other MRSP clones (p < 0.05). A random subset of 31 isolates underwent minimum bactericidal concentration (MBC) testing against F10SCTM (benzalkonium chloride and biguanide), and HexaconTM (chlorhexidine gluconate), with and without the addition of bovine serum albumin (BSA) as an in vitro substitute for organic matter contamination. Qac genes were not associated with increased phenotypic biocide tolerance but biocide efficacy was significantly affected by the presence of BSA. In the absence of BSA, all MBC values were well below the recommended usage concentration. When BSA was present, regardless of qac gene presence, 50% of MRSA and 43% of MRSP had an F10SCTM MBC above the recommended concentration for general disinfection. Qac genes did not confer increased in vitro biocide tolerance to veterinary staphylococci. Organic matter contamination must be minimized to ensure the efficacy of biocides against MRSA and MRSP

Dynamical energy analysis on mesh grids: a new tool for describing the vibro-acoustic response of complex mechanical structures

We present a new approach for modelling noise and vibration in complex mechanical structures in the mid-to-high frequency regime. It is based on a dynamical energy analysis (DEA) formulation which extends standard techniques such as statistical energy analysis (SEA) towards non-diffusive wave fields. DEA takes into account the full directionality of the wave field and makes sub-structuring obsolete. It can thus be implemented on mesh grids commonly used, for example, in the finite element method (FEM). The resulting mesh based formulation of DEA can be implemented very efficiently using discrete flow mapping (DFM) as detailed in [1] and described here for applications in vibro-acoustics

The structure of typical clusters in large sparse random configurations

The initial purpose of this work is to provide a probabilistic explanation of a recent result on a version of Smoluchowski's coagulation equations in which the number of aggregations is limited. The latter models the deterministic evolution of concentrations of particles in a medium where particles coalesce pairwise as time passes and each particle can only perform a given number of aggregations. Under appropriate assumptions, the concentrations of particles converge as time tends to infinity to some measure which bears a striking resemblance with the distribution of the total population of a Galton-Watson process started from two ancestors. Roughly speaking, the configuration model is a stochastic construction which aims at producing a typical graph on a set of vertices with pre-described degrees. Specifically, one attaches to each vertex a certain number of stubs, and then join pairwise the stubs uniformly at random to create edges between vertices. In this work, we use the configuration model as the stochastic counterpart of Smoluchowski's coagulation equations with limited aggregations. We establish a hydrodynamical type limit theorem for the empirical measure of the shapes of clusters in the configuration model when the number of vertices tends to $\infty$. The limit is given in terms of the distribution of a Galton-Watson process started with two ancestors

Interface optical phonons in spheroidal dots: Raman selection rules

The contribution of interface phonons to the first order Raman scattering in nanocrystals with non spherical geometry is analyzed. Interface optical phonons in the spheroidal geometry are discussed and the corresponding Frohlich-like electron-phonon interaction is reported in the framework of the dielectric continuum approach. It is shown that the interface phonon modes are strongly dependent on the nanocrystal geometry, particularly on the ellipsoid's semi-axis ratio. The new Raman selection rules have revealed that solely interface phonon modes with even angular momentum are allowed to contribute to the first order phonon-assisted scattering of light. On this basis we are able to give an explanation for the observed low frequency shoulders present in the Raman cross-section of several II-VI semiconductor nanostructures.Comment: 8 pages, 2 figure

Theory of Luminescent Emission in Nanocrystal ZnS:Mn with an Extra Electron

We consider the effect of an extra electron injected into a doped quantum dot $ZnS:Mn^{2+}$. The Coulomb interaction and the exchange interaction between the extra electron and the states of the Mn ion will mix the wavefunctions, split the impurity energy levels, break the previous selection rules and change the transition probabilities. Using this model of an extra electron in the doped quantum dot, we calculated the energy and the wavefunctions, the luminescence probability and the transition lifetime and compare with the experiments. Our calculation shows that two orders of magnitudes of lifetime shortening can occur in the transition $^4T_1-^6A_1$ when an extra electron is present.Comment: 15 pages, 2 Figs No change in Fig

The scaling attractor and ultimate dynamics for Smoluchowski's coagulation equations

We describe a basic framework for studying dynamic scaling that has roots in dynamical systems and probability theory. Within this framework, we study Smoluchowski's coagulation equation for the three simplest rate kernels $K(x,y)=2$, $x+y$ and $xy$. In another work, we classified all self-similar solutions and all universality classes (domains of attraction) for scaling limits under weak convergence (Comm. Pure Appl. Math 57 (2004)1197-1232). Here we add to this a complete description of the set of all limit points of solutions modulo scaling (the scaling attractor) and the dynamics on this limit set (the ultimate dynamics). The main tool is Bertoin's L\'{e}vy-Khintchine representation formula for eternal solutions of Smoluchowski's equation (Adv. Appl. Prob. 12 (2002) 547--64). This representation linearizes the dynamics on the scaling attractor, revealing these dynamics to be conjugate to a continuous dilation, and chaotic in a classical sense. Furthermore, our study of scaling limits explains how Smoluchowski dynamics ``compactifies'' in a natural way that accounts for clusters of zero and infinite size (dust and gel)

Characterization of staphylococcal cassette chromosome mec elements from methicillin-resistant Staphylococcus pseudintermedius infections in Australian animals

We examined the oxacillin resistance phenotype and genomic structure of staphylococcal cassette chromosome mec (SCCmec) elements from 77 veterinary methicillin-resistant Staphylococcus pseudintermedius (MRSP) isolates. Isolates were characterized by oxacillin broth microdilution, whole-genome sequencing, and bioformatics analysis. Five previously described SCCmec elements, and a sixth novel element, were identified: SCCmec III (also known as II-III), ΨSCCmec57395, and SCCmecNA45 (a SCCmec VII variant), all previously described in MRSP, and SCCmec IVg and SCCmec VT, previously described in both methicillin-resistant Staphylococcus aureus (MRSA) and MRSP. The sixth element was novel and found among nine geographically clustered isolates. This novel pseudostaphylococcal cassette chromosome (ΨSCCmecKW21) contained a class A mec gene complex but lacked ccr genes. It also harbored heavy metal (cadmium) resistance determinants. The median oxacillin MIC values among ΨSCCmecKW21, SCCmec III, and SCCmec VT isolates were significantly higher than those determined for the SCCmecNA45 VII variant isolates and ΨSCCmec57395 and SCCmec IVg isolates. ΨSCCmecKW21 was found exclusively in sequence type 497 (ST497), an MRSP clone that is locally successful in Victoria, Australia. Future studies are necessary to determine if this clone has disseminated further afield and if ΨSCCmecKW21 has moved into other MRSP lineages or staphylococcal species. IMPORTANCE Staphylococcus pseudintermedius is a significant veterinary pathogen and occasional cause of infections in humans. β-Lactams are an important group of antimicrobials used to treat staphylococcal infections in humans and animals. However, when staphylococci become methicillin resistant via the acquisition of a mobile genetic element called staphylococcal cassette chromosome mec (SCCmec), they become resistant to all β-lactams. This study detected a novel SCCmec element among a cluster of methicillin-resistant S. pseudintermedius isolates from animals in Australia. It also detected SCCmec elements in S. pseudintermedius that had high similarity to those identified in methicillin-resistant Staphylococcus aureus, demonstrating how human and animal pathogens can share the same resistance determinants