Molecular Beam Epitaxial Growth of III-V Semiconductor Nanostructures on Silicon Substrates

The main focus and concerns of this PhD thesis is the growth of III-V semiconductor nanostructures (Quantum dots (QDs) and quantum dashes) on silicon substrates using molecular beam epitaxy (MBE) technique. The investigation of influence of the major growth parameters on their basic properties (density, geometry, composition, size etc.) and the systematic characterization of their structural and optical properties are the core of the research work. The monolithic integration of III-V optoelectronic devices with silicon electronic circuits could bring enormous prospect for the existing semiconductor technology. Our challenging approach is to combine the superior passive optical properties of silicon with the superior optical emission properties of III-V material by reducing the amount of III-V materials to the very limit of the active region. Different heteroepitaxial integration approaches have been investigated to overcome the materials issues between III-V and Si. However, this include the self-assembled growth of InAs and InGaAs QDs in silicon and GaAx matrices directly on flat silicon substrate, sitecontrolled growth of (GaAs/In0,15Ga0,85As/GaAs) QDs on pre-patterned Si substrate and the direct growth of GaP on Si using migration enhanced epitaxy (MEE) and MBE growth modes. An efficient ex-situ-buffered HF (BHF) and in-situ surface cleaning sequence based on atomic hydrogen (AH) cleaning at 500 °C combined with thermal oxide desorption within a temperature range of 700-900 °C has been established. The removal of oxide desorption was confirmed by semicircular streaky reflection high energy electron diffraction (RHEED) patterns indicating a 2D smooth surface construction prior to the MBE growth. The evolution of size, density and shape of the QDs are ex-situ characterized by atomic-force microscopy (AFM) and transmission electron microscopy (TEM). The InAs QDs density is strongly increased from 108 to 1011 cm-2 at V/III ratios in the range of 15-35 (beam equivalent pressure values). InAs QD formations are not observed at temperatures of 500 °C and above. Growth experiments on (111) substrates show orientation dependent QD formation behaviour. A significant shape and size transition with elongated InAs quantum dots and dashes has been observed on (111) orientation and at higher Indium-growth rate of 0.3 ML/s. The 2D strain mapping derived from high-resolution TEM of InAs QDs embedded in silicon matrix confirmed semi-coherent and fully relaxed QDs embedded in defectfree silicon matrix. The strain relaxation is released by dislocation loops exclusively localized along the InAs/Si interfaces and partial dislocations with stacking faults inside the InAs clusters. The site controlled growth of GaAs/In0,15Ga0,85As/GaAs nanostructures has been demonstrated for the first time with 1 μm spacing and very low nominal deposition thicknesses, directly on pre-patterned Si without the use of SiO2 mask. Thin planar GaP layer was successfully grown through migration enhanced epitaxy (MEE) to initiate a planar GaP wetting layer at the polar/non-polar interface, which work as a virtual GaP substrate, for the GaP-MBE subsequently growth on the GaP-MEE layer with total thickness of 50 nm. The best root mean square (RMS) roughness value was as good as 1.3 nm. However, these results are highly encouraging for the realization of III-V optical devices on silicon for potential applications.

Room-temperature continuous-wave operation of GaInNAs/GaAs quantum dot laser with GaAsN barrier grown by solid source molecular beam epitaxy

We present the results of GaInNAs/GaAs quantum dot structures with GaAsN barrier layers grown by solid source molecular beam epitaxy. Extension of the emission wavelength of GaInNAs quantum dots by ~170nm was observed in samples with GaAsN barriers in place of GaAs. However, optimization of the GaAsN barrier layer thickness is necessary to avoid degradation in luminescence intensity and structural property of the GaInNAs dots. Lasers with GaInNAs quantum dots as active layer were fabricated and room-temperature continuous-wave lasing was observed for the first time. Lasing occurs via the ground state at ~1.2μm, with threshold current density of 2.1kA/cm[superscript 2] and maximum output power of 16mW. These results are significantly better than previously reported values for this quantum-dot system.

Osteomielitis esternal: utilidad de las pruebas de diagnóstico por imágenes con enfoque en medicina nuclear

Introducción: El diagnóstico de osteomielitis esternal post-esternotomía resulta difícil empleando síntomas clínicos o de laboratorio y las imágenes morfológicas orientan a sospecha más que al diagnóstico. Un diagnóstico precoz ofrece calidad de vida y el mejor tratamiento para reducir una mortalidad que oscila entre 14% y 47%. La gammagrafía con leucocitos marcados ofrece el mejor rendimiento diagnóstico para infecciones y se destaca como el patrón de oro diagnóstico. Objetivo: Identificar el desempeño y utilidad de la gammagrafía con leucocitos autólogos marcados con 99mTc-HMPAO en los estudios realizados para la evaluación de osteomielitis esternal. Materiales y métodos: Se realizó un estudio descriptivo, retrospectivo de prueba diagnóstica en la Fundación Cardioinfantil de Bogotá entre enero/2010 y mayo/2015 evaluando gammagrafías con leucocitos marcados ante la sospecha de osteomielitis posterior a esternotomía. Resultados: Se evaluaron 52 pacientes, en los que la gammagrafía con leucocitos mostró 23 pacientes (44,2%) con osteomielitis esternal, logrando una sensibilidad y especificidad del 88,46% y 100% respectivamente. El valor predictivo positivo fue de 100%, y el valor predictivo negativo fue de 89,66%. El impacto de una prueba negativa no modificó el manejo médico inicial en el 93% de los casos mientras que una prueba positiva lo modificó en el 83%. Conclusiones: La gammagrafía con leucocitos autólogos radiomarcados con 99mTc-HMPAO continúa siendo el patrón de oro de referencia no invasiva para el diagnóstico de osteomielitis, y en el caso de osteomielitis esternal se convierte en la prueba de elección pertinente en la selección de pacientes que ameritan una re-intervención quirúrgica.

Determination of the direct band-gap energy of InAlAs matched to InP by photoluminescence excitation spectroscopy

A series of InxAl1-xAs samples (0.51≪x≪0.55)coherently grown on InP was studied in order to measure the band-gap energy of the lattice matched composition. As the substrate is opaque to the relevant photon energies, a method is developed to calculate the optical absorption coefficient from the photoluminescence excitation spectra. The effect of strain on the band-gap energy has been taken into account. For x=0.532, at 14 K we have obtained Eg0=1549±6 meV

Atomic diffusion induced by stress relaxation in InGaAs/GaAs epitaxial layers

The origin of the microscopic inhomogeneities in InxGa1-xAs layers grown on GaAs by molecular beam epitaxy is analyzed through the optical absorption spectra near the band gap. It is seen that, for relaxed thick layers of about 2.8μm, composition inhomogeneities are responsible for the band edge smoothing into the whole compositional range (0.05

Knowledge, attitudes and practices of farmers on rodent pests and their management in the lowlands of the Sierra Madre Biodiversity Corridor, Philippines

A survey of the knowledge, attitudes and practices (KAP) of 100 rice farmers and 50 coconut farmers was conducted in the coastal lowland agro-ecosystems of the Sierra Madre Biodiversity Corridor, Luzon, Philippines to identify current rodent management practices and to understand the extent of rat damage and the attitudes of farmers to community actions for rodent management. Pests were most commonly listed as one of the three most important rice and coconut production constraints. Other major crop production constraints were typhoons and insufficient water. Farmers consider rats to be the major pest of coconut and of rice during the wet season rice crop, with average yield losses of 3.0% and 13.2%, respectively. Rice and coconut farmers practised a wide range of rodent management techniques. These included scrub clearance, hunting and trapping. Of the 42 rice farmers and 3 coconut farmers that applied rodenticides to control rodents, all used the acute rodenticide, zinc phosphide. However, only ten rice farmers (23.8%) applied rodenticides prior to the booting stage and only seven farmers (15.6%) conducted pre-baiting before applying zinc phosphide. The majority of farmers belonged to farmer organisations and believed that rat control can only be done by farmers working together. However, during the last cropping season, less than a third of rice farmers (31.2%) applied rodent management as a group. In order to reduce the impact of rodents on the farmers of the coastal lowlands of the Sierra Madre Biodiversity Corridor, integrated management strategies need to be developed that specifically target the pest rodents in a sustainable manner, and community actions for rodent management should be promoted.

The interplay of metal-atom ordering, Fermi leveltuning and thermoelectric properties in cobalt shandites Co3M2S2 (M = Sn, In)

A combination of structural, physical and computational techniques including powder X-ray and neutron diffraction, SQUID magnetometry, electrical and thermal transport measurements, DFT calculations and 119Sn Mössbauer and X-ray photoelec-tron spectroscopies has been applied to Co3Sn2-xInxS2 (0 ≤ x ≤ 2) in an effort to understand the relationship between metal-atom ordering and physical properties as the Fermi level is systematically varied. Whilst solid solution behavior is found throughout the composition region, powder neutron diffraction reveals that indium preferentially occupies an inter-layer site over an alternative kagome-like intra-layer site. DFT calculations indicate that this ordering, which leads to a lowering of energy, is related to the dif-fering bonding properties of tin and indium. Spectroscopic data suggest that throughout the composition range 0 ≤ x ≤ 2, all ele-ments adopt oxidation states that are significantly reduced from expectations based on formal charges. Chemical substitution ena-bles the electrical transport properties to be controlled through tuning of the Fermi level within a region of the density of states, which comprises narrow bands of predominantly Co d-character. This leads to a compositionally-induced double metal-to-semiconductor-to-metal transition. The marked increase in the Seebeck coefficient as the semiconducting region is approached leads to a substantial improvement in the thermoelectric figure of merit, ZT, which exhibits a maximum of ZT = 0.32 at 673 K. At 425 K, the figure of merit for phases in the region 0.8 ≤ x ≤ 0.85 is amongst the highest reported for sulphide phases, suggesting these materials may have applications in low-grade waste heat recovery.

The current status of anticoagulant resistance in rats and mice in the UK

Introduction: Resistance to anticoagulants in Norway rats (Rattus norvegicus) and house mice (Mus domesticus) has been studied in the UK since the early 1960s. In no other country in the world is our understanding of resistance phenomena so extensive and profound. Almost every aspect of resistance in the key rodent target species has been examined in laboratory and field trials and results obtained by independent researchers have been published. It is the principal purpose of this document to present a short synopsis of this information. More recently, however, the development of genetical techniques has provided a definitive means of detection of resistant genotypes among pest rodent populations. Preliminary information from a number of such surveys will also be presented. Resistance in Norway rats: A total of nine different anticoagulant resistance mutations (single nucleotide polymorphisms or SNPs) are found among Norway rats in the UK. In no other country worldwide are present so many different forms of Norway rat resistance. Among these nine SNPs, five are known to confer on rats that carry them a significant degree of resistance to anticoagulant rodenticides. These mutations are: L128Q, Y139S, L120Q, Y139C and Y139F. The latter three mutations confer, to varying degrees, practical resistance to bromadiolone and difenacoum, the two second-generation anticoagulants in predominant use in the UK. It is the recommendation of RRAG that bromadiolone and difenacoum should not be used against rats carrying the L120Q, Y139C and Y139F mutations because this will promote the spread of resistance and jeopardise the long-term efficacy of anticoagulants. Brodifacoum, flocoumafen and difethialone are effective against these three genotypes but cannot presently be used because of the regulatory restriction that they can only be applied against rats that are living and feeding predominantly indoors. Our understanding of the geographical distribution of Norway rat resistance in incomplete but is rapidly increasing. In particular, the mapping of the focus of L120Q Norway rat resistance in central-southern England by DNA sequencing is well advanced. We now know that rats carrying this resistance mutation are present across a large part of the counties of Hampshire, Berkshire and Wiltshire, and the resistance spreads into Avon, Oxfordshire and Surrey. It is also found, perhaps as outlier foci, in south-west Scotland and East Sussex. L120Q is currently the most severe form of anticoagulant resistance found in Norway rats and is prevalent over a considerable part of central-southern England. A second form of advanced Norway rat resistance is conferred by the Y139C mutation. This is noteworthy because it occurs in at least four different foci that are widely geographically dispersed, namely in Dumfries and Galloway, Gloucestershire, Yorkshire and Norfolk. Once again, bromadiolone and difenacoum are not recommended for use against rats carrying this genotype and a concern of RRAG is that continued applications of resisted active substances may result in Y139C becoming more or less ubiquitous across much of the UK. Another type of advanced resistance, the Y139F mutation, is present in Kent and Sussex. This means that Norway rats, carrying some degree of resistance to bromadiolone and difenacoum, are now found from the south coast of Kent, west into the city of Bristol, to Yorkshire in the north-east and to the south-west of Scotland. This difficult situation can only deteriorate further where these three genotypes exist and resisted anticoagulants are predominantly used against them. Resistance in house mice: House mouse is not so well understood but the presence in the UK of two resistant genotypes, L128S and Y139C, is confirmed. House mice are naturally tolerant to anticoagulants and such is the nature of this tolerance, and the presence of genetical resistance, that house mice resistant to the first-generation anticoagulants are considered to be widespread in the UK. Consequently, baits containing warfarin, sodium warfarin, chlorophacinone and coumatetralyl are not approved for use against mice. This regulatory position is endorsed by RRAG. Baits containing brodifacoum, flocoumafen and difethialone are effective against house mice and may be applied in practice because house mouse infestations are predominantly indoors. There are some reports of resistance among mice in some areas to the second-generation anticoagulant bromadiolone, while difenacoum remains largely efficacious. Alternatives to anticoagulants: The use of habitat manipulation, that is the removal of harbourage, denial of the availability of food and the prevention of ingress to structures, is an essential component of sustainable rodent pest management. All are of importance in the management of resistant rodents and have the advantage of not selecting for resistant genotypes. The use of these techniques may be particularly valuable in preventing the build-up of rat infestations. However, none can be used to remove any sizeable extant rat infestation and for practical reasons their use against house mice is problematic. Few alternative chemical interventions are available in the European Union because of the removal from the market of zinc phosphide, calciferol and bromethalin. Our virtual complete reliance on the use of anticoagulants for the chemical control of rodents in the UK, and more widely in the EU, calls for improved schemes for resistance management. Of course, these might involve the use of alternatives to anticoagulant rodenticides. Also important is an increasing knowledge of the distribution of resistance mutations in rats and mice and the use of only fully effective anticoagulants against them.

Polymorphism and optical properties in [NH4][InSe2]

The solvothermal synthesis and characterization of two indium selenides with stoichiometry [NH4][InSe2] is described. Yellow [NH4][InSe2] (1), which exhibits a layered structure, was initially prepared in an aqueous solution of trans-1,4-diaminocyclohexane, and subsequently using a concentrated ammonia solution. A red polymorph of one-dimensional character, [NH4][InSe2] (2), was obtained using 3,5-dimethylpyridine as solvent. [NH4][InSe2] (1) crystallizes in the non-centrosymmetric space group Cc (a=11.5147(6), b=11.3242(6), c=15.9969(9) Å and β=100.354(3)°). The structural motif of the layers is the In4Se10 adamantane unit, composed of four corner-linked InSe4 tetrahedra. These units are linked by their corners, forming [InSe2]− layers which are stacked back to back along the c-direction, and interspaced by [NH4]+cations. The one-dimensional polymorph, (2), crystallizes in the tetragonal space group, I4/mcm (a=8.2519(16), c=6.9059 (14) Å). This structure contains infinite chains of edge-sharing InSe4 tetrahedra separated by [NH4]+ cations.

Ion beam assisted deposition of an organic light emitting diode electrode

This work presents the electro-optical characterization of metal-organic interfaces prepared by the Ion Beam Assisted Deposition (IBAD) method. IBAD applied in this work combines simultaneously metallic film deposition and bombardment with an independently controlled ion beam, allowing different penetration of the ions and the evaporated metallic elements into the polymer. The result is a hybrid, non-abrupt interface, where polymer, metal and ion coexists. We used an organic light emitting diode, which has a typical vertical-architecture, for the interface characterization: Glass/Indium Tin Oxide (ITO)/Poly[ethylene-dioxythiophene/poly{styrenesulfonicacid}]) (PEDOT:PSS) /Emitting Polymer/Metal. The emitting polymer layer comprised of the Poly[(9,9-dioctyl-2,7-divinylenefluorenylene)-alt-co-{2-methoxy-5-(2-ethylhexyloxy)-1,4-phenylene}] (PFO) and the metal layer of aluminum prepared with different Ar(+) ion energies varying in the range from 0 to 1000 eV. Photoluminescence, Current-Voltage and Electroluminescence measurements were used to study the emission and electron injection properties. Changes of these properties were related with the damage caused by the energetic ions and the metal penetration into the polymer. Computer simulations of hybrid interface damage and metal penetration were confronted with experimental data. (C) 2010 Elsevier B.V. All rights reserved.

Enzyme immobilisation on electroactive nanostructured membranes (ENM): Optimised architectures for biosensing

Electroactive nanostructured membranes have been produced by the layer-by-layer (LbL) technique, and used to make electrochemical enzyme biosensors for glucose by modification with cobalt hexacyanoferrate redox mediator and immobilisation of glucose oxidase enzyme. Indium tin oxide (ITO) glass electrodes were modified with up to three bilayers of polyamidoamine (PAMAM) dendrimers containing gold nanoparticles and poly(vinylsulfonate) (PVS). The gold nanoparticles were covered with cobalt hexacyanoferrate that functioned as a redox mediator, allowing the modified electrode to be used to detect H(2)O(2), the product of the oxidase enzymatic reaction, at 0.0 V vs. SCE. Enzyme was then immobilised by cross-linking with glutaraldehyde. Several parameters for optimisation of the glucose biosensor were investigated, including the number of deposited bilayers, the enzyme immobilisation protocol and the concentrations of immobilised enzyme and of the protein that was crosslinked with PAMAM. The latter was used to provide glucose oxidase with a friendly environment, in order to preserve its bioactivity. The optimised biosensor, with three bilayers, has high sensitivity and operational stability, with a detection limit of 6.1 mu M and an apparent Michaelis-Menten constant of 0.20 mM. It showed good selectivity against interferents and is suitable for glucose measurements in natural samples. (C) 2008 Elsevier B.V. All rights reserved.

Phytase immobilization on modified electrodes for amperometric biosensing

Phytase (myo-inositol hexaphosphate phosphohydrolase) and phytic acid (myo-inositol hexaphosphate) play an important environmental role, in addition to being a health issue in food industry. Phytic acid is antinutritional due to its ability to chelate metal ions and may also react with proteins decreasing their bioavailability. In this work, we produced biosensors with phytase immobilized in Layer-by-Layer (LbL) films, which could detect phytic acid with a detection limit of 0.19 mmol L-1, which is sufficient to detect phytic acid in seeds of grains and vegetables. The biosensosrs consisted of LbL films containing up to eight bilayers of phytase alternated with poly(allylamine) hydrochloride (PAH) deposited onto an indium-tin oxide (ITO) substrate modified with Prussian Blue. Amperometric detection was conducted in an acetate buffer solution (at pH 5.5) at room temperature, with the biosensor response attributed to the formation of phosphate ions. In subsidiary experiments with the currents measured at 0.0 V (vs. SCE), we demonstrated the absence of effects from some interferents, pointing to a good selectivity of the biosensor. (c) 2007 Elsevier B.V. All rights reserved.

Using phospholipid Langmuir and Langmuir-Blodgett films as matrix for urease immobilization

The immobilization of enzymes in organized two-dimensional matrices is a key requirement for many biotechnological applications. In this paper, we used the Langmuir-Blodgett (LB) technique to obtain controlled architectures of urease immobilized in solid supports, whose physicochemical properties were investigated in detail. Urease molecules were adsorbed at the air-water interface and incorporated into Langmuir monolayers of the phospholipid dipalmitoyl phosphatidyl glycerol (DPPG). Incorporation of urease made DPPG monolayers more flexible and caused the reduction of the equilibrium and dynamic elasticity of the film. Urease and DPPG-urease mixed monolayers could be transferred onto solid substrates, forming LB films. A close packing arrangement of urease was obtained, especially in the mixed LB films, which was inferred with nanogravimetry and electrochemistry measurements. From the blocking effect of the LB films deposited onto indium tin oxide (ITO) substrates, the electrochemical properties of the LB films pointed to a charge transport controlled by the lipid architecture. (c) 2007 Elsevier Inc. All rights reserved.

Adsorption kinetics and charge inversion in layer-by-layer films from nickel tetrasulfonated phthalocyanine and poly(allylamine hydrochloride)

Layer-by-layer (LBL) films of nickel tetrasulfonated phthalocyanine (NiTsPc) alternated with poly(allylamine hydrochloride) (PAH) have been prepared, whose surface charge has been evaluated using surface potential measurements. From adsorption kinetics results, we obtained the immersion time of similar to 40 s, which was used to assemble layers of NiTsPc. The effect of gold (Au) and aluminum (Al) electrodes on the charge behavior was examined. We found that the surface potential (i.e. surface charge) was inverted each time a layer of PAH was alternated with another of NiTsPc molecules for the two types of electrodes, which was attributed to charge overcompensation between positive charges of PAH molecules, and negative charges from NiTsPc molecules. (C) 2009 Elsevier B.V. All rights reserved.

Immobilization of Poly(propylene imine) Dendrimer/Nickel Phtalocyanine as Nanostructured Multilayer Films To Be Used as Gate Membranes for SEGFET pH Sensors

We describe the assembly of layer-by-layer films based on the poly(propylene imine) dendrimer (PPID) generation 3 and nickel tetrasulfonated phthalocyanine (NiTsPc) for application as chemically sensitive membranes in sepal alive extended-gate field effect transistor (SEGFET) pH sensors PPID/NiTsPc films wet e adsorbed on quartz, glass. indium tin oxide. or gold (Au)-covered glass substrates Multilayer formation was monitored via UV-vis absorption upon following the increment in the Q-band intensity (615 nm) of NiTsPc The nanostructured membranes were very stable in a pH range of 4-10 and displayed a good sensitivity toward H(+), ca 30 mV/pH for PPID/N(1)TsPc films deposited on Au-covered substrates For films deposited on ITO, the sensitivity was ca 52 4 mV/pH. close to the expected theoretical value for ton-sensitive membranes. The use of chemically stable PPID/NiTsPc films as gate membranes in SEGFETs, as introduced here, may represent an alternative for the fabrication of nanostructured, porous platforms for enzyme immobilization to be used in enzymatic biosensors.