Expression and analysis of presenilin 1 in a human neuronal system: localization in cell bodies and dendrites.

Mutations in the recently identified presenilin 1 gene on chromosome 14 cause early onset familial Alzheimer disease (FAD). Herein we describe the expression and analysis of the protein coded by presenilin 1 (PS1) in NT2N neurons, a human neuronal model system. PS1 was expressed using recombinant Semliki Forest virions and detected by introduced antigenic tags or antisera to PS1-derived peptides. Immunoprecipitation revealed two major PS1 bands of approximately 43 and 50 kDa, neither of which were N-glycosylated or O-glycosylated. Immunoreactive PS1 was detected in cell bodies and dendrites of NT2N neurons but not in axons or on the cell surface. PS1 was also detected in BHK cells, where it was also intracellular and colocalized with calnexin, a marker for the rough endoplasmic reticulum. A mutant form of PS1 linked to FAD did not differ from the wild-type protein at the light microscopic level. The model system described here will enable studies of the function of PS1 in human neurons and the role of mutant PS1 in FAD.

Differential subcellular mRNA targeting: deletion of a single nucleotide prevents the transport to axons but not to dendrites of rat hypothalamic magnocellular neurons.

It has previously been shown that mRNA encoding the arginine vasopressin (AVP) precursor is targeted to axons of rat magnocellular neurons of the hypothalamo-neurohypophyseal tract. In the homozygous Brattle-boro rat, which has a G nucleotide deletion in the coding region of the AVP gene, no such targeting is observed although the gene is transcribed. RNase protection and heteroduplex analyses demonstrate that, in heterozygous animals, which express both alleles of the AVP gene, the wild-type but not the mutant transcript is subject to axonal compartmentation. In contrast, wild-type and mutant AVP mRNAs are present in dendrites. These data suggest the existence of different mechanisms for mRNA targeting to the two subcellular compartments. Axonal mRNA localization appears to take place after protein synthesis; the mutant transcript is not available for axonal targeting because it lacks a stop codon preventing its release from ribosomes. Dendritic compartmentation, on the other hand, is likely to precede translation and, thus, would be unable to discriminate between the two mRNAs.

The chemistry of germanium ...

Thesis (Ph.D.)--Cornell university.

Concentration of germanium in the ash of American coals,

Mode of access: Internet.

G-factor and donor spin-lattice relaxation for electrons in germanium and silicon.

"This group report is based on an article submitted to the Physical review."

Germanium bibliography,

Mode of access: Internet.

Synaptology of the proximal segment of pyramidal cell basal dendrites

Pyramidal neurons are covered with dendritic spines, the main postsynaptic targets of excitatory (asymmetrical) synapses. However, the proximal portion of both the apical and basal dendrites is devoid of spines, suggesting a lack of excitatory inputs to this region. In the present study we used electron microscopy to analyse the proximal region of the basal dendrites of supra- and infragranular pyramidal cells to determine if this is the case. The proximal region of 80 basal dendrites sampled from the rat hindlimb representation in the primary somatosensory cortex was studied by electron microscopy A total of 317 synapses were found within this region of the dendrites, all of which were of the symmetrical type. These results suggest that glutamate receptors, although present in the cytoplasm, are not involved in synaptic junctions in the proximal portion of the dendrites. These data further support the idea that inhibitory terminals exclusively innervate the proximal region of basal dendrites.

On dendrites in Down syndrome and DS murine models: a spiny way to learn

Since the discovery in the 1970s that dendritic abnormalities in cortical pyramidal neurons are the most consistent pathologic correlate of mental retardation, research has focused on how dendritic alterations are related to reduced intellectual ability. Due in part to obvious ethical problems and in part to the lack of fruitful methods to study neuronal circuitry in the human cortex, there is little data about the microanatomical contribution to mental retardation. The recent identification of the genetic bases of some mental retardation associated alterations, coupled with the technology to create transgenic animal models and the introduction of powerful sophisticated tools in the field of microanatomy, has led to a growth in the studies of the alterations of pyramidal cell morphology in these disorders. Studies of individuals with Down syndrome, the most frequent genetic disorder leading to mental retardation, allow the analysis of the relationships between cognition, genotype and brain microanatomy. In Down syndrome the crucial question is to define the mechanisms by which an excess of normal gene products, in interaction with the environment, directs and constrains neural maturation, and how this abnormal development translates into cognition and behaviour. In the present article we discuss mainly Down syndrome-associated dendritic abnormalities and plasticity and the role of animal models in these studies. We believe that through the further development of such approaches, the study of the microanatomical substrates of mental retardation will contribute significantly to our understanding of the mechanisms underlying human brain disorders associated with mental retardation. (C) 2004 Elsevier Ltd. All rights reserved.

Giant piezoresistance in silicon-germanium alloys

We use first-principles electronic structure methods to show that the piezoresistive strain gauge factor of single-crystalline bulk n-type silicon-germanium alloys at carefully controlled composition can reach values of G = 500, three times larger than that of silicon, the most sensitive such material used in industry today. At cryogenic temperatures of 4 K we find gauge factors of G = 135 000, 13 times larger than that observed in Si whiskers. The improved piezoresistance is achieved by tuning the scattering of carriers between different (Delta and L) conduction band valleys by controlling the alloy composition and strain configuration.

Giant enhancement of n-type carrier mobility in highly strained germanium nanostructures

First-principles electronic structure methods are used to predict the rate of n-type carrier scattering due to phonons in highly-strained Ge. We show that strains achievable in nanoscale structures, where Ge becomes a direct bandgap semiconductor, cause the phonon-limited mobility to be enhanced by hundreds of times that of unstrained Ge, and over a thousand times that of Si. This makes highly tensile strained Ge a most promising material for the construction of channels in CMOS devices, as well as for Si-based photonic applications. Biaxial (001) strain achieves mobility enhancements of 100 to 1000 with strains over 2%. Low temperature mobility can be increased by even larger factors. Second order terms in the deformation potential of the Gamma valley are found to be important in this mobility enhancement. Although they are modified by shifts in the conduction band valleys, which are caused by carrier quantum confinement, these mobility enhancements persist in strained nanostructures down to sizes of 20 nm.

Non-equilibrium induction of tin in germanium: towards direct bandgap Ge1-xSnx nanowires

The development of non-equilibrium group IV nanoscale alloys is critical to achieving new functionalities, such as the formation of a direct bandgap in a conventional indirect bandgap elemental semiconductor. Here, we describe the fabrication of uniform diameter, direct bandgap Ge1-xSnx alloy nanowires, with a Sn incorporation up to 9.2[thinsp]at.%, far in excess of the equilibrium solubility of Sn in bulk Ge, through a conventional catalytic bottom-up growth paradigm using noble metal and metal alloy catalysts. Metal alloy catalysts permitted a greater inclusion of Sn in Ge nanowires compared with conventional Au catalysts, when used during vapour-liquid-solid growth. The addition of an annealing step close to the Ge-Sn eutectic temperature (230[thinsp][deg]C) during cool-down, further facilitated the excessive dissolution of Sn in the nanowires. Sn was distributed throughout the Ge nanowire lattice with no metallic Sn segregation or precipitation at the surface or within the bulk of the nanowires. The non-equilibrium incorporation of Sn into the Ge nanowires can be understood in terms of a kinetic trapping model for impurity incorporation at the triple-phase boundary during growth.

Modifying germanium nanowires for future devices: an in situ TEM study

Germanium was of great interest in the 1950’s when it was used for the first transistor device. However, due to the water soluble and unstable oxide it was surpassed by silicon. Today, as device dimensions are shrinking the silicon oxide is no longer suitable due to gate leakage and other low-κ dielectrics such as Al2O3 and HfO2 are being used. Germanium (Ge) is a promising material to replace or integrate with silicon (Si) to continue the trend of Moore’s law. Germanium has better intrinsic mobilities than silicon and is also silicon fab compatible so it would be an ideal material choice to integrate into silicon-based technologies. The progression towards nanoelectronics requires a lot of in depth studies. Dynamic TEM studies allow observations of reactions to allow a better understanding of mechanisms and how an external stimulus may affect a material/structure. This thesis details in situ TEM experiments to investigate some essential processes for germanium nanowire (NW) integration into nanoelectronic devices; i.e. doping and Ohmic contact formation. Chapter 1 reviews recent advances in dynamic TEM studies on semiconductor (namely silicon and germanium) nanostructures. The areas included are nanowire/crystal growth, germanide/silicide formation, irradiation, electrical biasing, batteries and strain. Chapter 2 details the study of ion irradiation and the damage incurred in germanium nanowires. An experimental set-up is described to allow for concurrent observation in the TEM of a nanowire following sequential ion implantation steps. Grown nanowires were deposited on a FIB labelled SiN membrane grid which facilitated HRTEM imaging and facile navigation to a specific nanowire. Cross sections of irradiated nanowires were also performed to evaluate the damage across the nanowire diameter. Experiments were conducted at 30 kV and 5 kV ion energies to study the effect of beam energy on nanowires of varied diameters. The results on nanowires were also compared to the damage profile in bulk germanium with both 30 kV and 5 kV ion beam energies. Chapter 3 extends the work from chapter 2 whereby nanowires are annealed post ion irradiation. In situ thermal annealing experiments were conducted to observe the recrystallization of the nanowires. A method to promote solid phase epitaxial growth is investigated by irradiating only small areas of a nanowire to maintain a seed from which the epitaxial growth can initiate. It was also found that strain in the nanowire greatly effects defect formation and random nucleation and growth. To obtain full recovery of the crystal structure of a nanowire, a stable support which reduces strain in the nanowire is essential as well as containing a seed from which solid phase epitaxial growth can initiate. Chapter 4 details the study of nickel germanide formation in germanium nanostructures. Rows of EBL (electron beam lithography) defined Ni-capped germanium nanopillars were extracted in FIB cross sections and annealed in situ to observe the germanide formation. Chapter 5 summarizes the key conclusions of each chapter and discusses an outlook on the future of germanium nanowire studies to facilitate their future incorporation into nanodevices.

Diameter-driven crossover in resistive behaviour of heavily doped self-seeded germanium nanowires

The dependence of the resistivity with changing diameter of heavily-doped self-seeded germanium nanowires was studied for the diameter range 40 to 11 nm. The experimental data reveal an initial strong reduction of the resistivity with diameter decrease. At about 20 nm a region of slowly varying resistivity emerges with a peak feature around 14 nm. For diameters above 20 nm, nanowires were found to be describable by classical means. For smaller diameters a quantum-based approach was required where we employed the 1D Kubo–Greenwood framework and also revealed the dominant charge carriers to be heavy holes. For both regimes the theoretical results and experimental data agree qualitatively well assuming a spatial spreading of the free holes towards the nanowire centre upon diameter reduction.

Germanium content in marine sediments

In the geosphere, germanium (Ge) has a chemical behavior close to that of silicon (Si), and Ge commonly substitutes for Si (in small proportions) in silicates. Studying the evolution of the respective proportions of Ge and Si through time allows us to better constrain the global Si cycle. The marine inventory of Ge present as dissolved germanic acid is facing two main sinks known through the study of present sediments: 1) incorporation into diatom frustules and transfer to sediments by these "shuttles", 2) capture of Ge released to pore water through frustule dissolution by authigenic mineral phases forming within reducing sediments. Our goals are to determine whether such a bio-induced transfer of Ge is also achieved by radiolarian and whether Ge could be trapped directly from seawater into authigenic phases with no intervention of opal-secreting organisms (shuttles). To this end, we studied two Paleozoic radiolarite formations and geological formations dated of Devonian, Jurassic and Cretaceous, deposited under more or less drastic redox conditions. Our results show that the Ge/Si values observed for these radiolarites are close to (slightly above) those measured from modern diatoms and sponges. In addition, our results confirm what is observed with some present-day reducing sediments: the ancient sediments that underwent reducing depositional conditions are authigenically enriched in Ge. Furthermore, it is probable that at least a part of the authigenic Ge came directly from seawater. The recurrence and extent (through time and space) of anoxic conditions affecting sea bottoms have been quite important through the geological times; consequently, the capture of Ge by reducing sediments must have impacted Ge distribution and in turn, the evolution of the seawater Ge/Si ratio.