Metallic Spikes Research Articles

Spiked gold nanotriangles: formation, characterization and applications in surface-enhanced Raman spectroscopy and plasmon-enhanced catalysis.

We show the formation of metallic spikes on the surface of gold nanotriangles (AuNTs) by using the same reduction process which has been used for the synthesis of gold nanostars. We confirm that silver nitrate operates as a shape-directing agent in combination with ascorbic acid as the reducing agent and investigate the mechanism by dissecting the contribution of each component, i.e., anionic surfactant dioctyl sodium sulfosuccinate (AOT), ascorbic acid (AA), and AgNO3. Molecular dynamics (MD) simulations show that AA attaches to the AOT bilayer of nanotriangles, and covers the surface of gold clusters, which is of special relevance for the spike formation process at the AuNT surface. The surface modification goes hand in hand with a change of the optical properties. The increased thickness of the triangles and a sizeable fraction of silver atoms covering the spikes lead to a blue-shift of the intense near infrared absorption of the AuNTs. The sponge-like spiky surface increases both the surface enhanced Raman scattering (SERS) cross section of the particles and the photo-catalytic activity in comparison with the unmodified triangles, which is exemplified by the plasmon-driven dimerization of 4-nitrothiophenol (4-NTP) to 4,4′-dimercaptoazobenzene (DMAB).

Effects of particle size of aluminum powder in silver/aluminum paste on n-type solar cells

Silver paste, which mainly consists of silver metal, glass frit, and organics, has been used for contacting n + emitter of conventional p-type solar cells, whereas aluminum-added silver paste (silver/aluminum paste) has been used for p + emitter of n-type solar cells. It has been reported that the addition of aluminum powder to the silver paste decreases contact resistance between the paste electrodes and the p + emitter with increasing content of the aluminum powder in the paste, and in addition, that the silver/aluminum paste creates large and deep metallic spikes underneath the paste, which has been considered to decrease the contact resistance. However, how particle size of the aluminum powder affects the contact resistance and the electrical characteristics of the n-type solar cells has not been clear yet. In this study, the effects of the particle size of aluminum powder in the silver/aluminum paste on the contact resistance and the electrical characteristics are investigated. Our study demonstrates that the particle size of the aluminum powder strongly affects the contact resistance and the electrical characteristics. The contact resistance decreases not only with increasing the content of the aluminum powder in the silver/aluminum paste, but also with increasing the particle size of the powder. In addition, the optimization of the particle size of the aluminum powder can effectively increase FF of the n-type solar cells, resulting in increasing efficiency of the cells.

The electrical losses induced by silver paste in n-type silicon solar cells

Aluminum-added silver paste (Ag/Al paste) has been used for p+ emitter of n-type solar cells. The electrical losses due to shunting and recombination caused by the paste in the cells have been reported to originate from huge metallic spikes due to the aluminum. However, whether the aluminum actually induces the losses has not been clarified yet. In this study, the “floating contact method” is applied to aluminum-free silver (Al-free Ag) paste to investigate the effects of aluminum extraction from the Ag/Al paste and to understand how the aluminum principally induces the losses for the p+ emitter. Furthermore, the interfacial morphology between the Al-free Ag paste and p-type silicon is investigated. The Ag paste itself creates tiny crystallites for the p+ emitter, resulting in shunting and recombination. The result indicates that the aluminum addition to Ag paste is not the main reason for the electrical losses in the n-type solar cells.

Role of aluminum in silver paste contact to boron-doped silicon emitters

The addition of aluminum to silver metallization pastes has been found to lower the contact resistivity of a silver metallization on boron-doped silicon emitters for n-type Si solar cells. However, the addition of Al also induces more surface recombination and increases the Ag pattern′s line resistivity, both of which ultimately limit the cell efficiency. There is a need to develop a fundamental understanding of the role that Al plays in reducing the contact resistivity and to explore alternative additives. A fritless silver paste is used to allow direct analysis of the impact of Al on the Ag-Si interfacial microstructure and isolate the influence of Al on the electrical contact from the complicated Ag-Si interfacial glass layer. Electrical analysis shows that in a simplified system, Al decreases the contact resistivity by about three orders of magnitude. Detailed microstructural studies show that in the presence of Al, microscale metallic spikes of Al-Ag alloy and nanoscale metallic spikes of Ag-Si alloy penetrate the surface of the boron-doped Si emitters. These results demonstrate the role of Al in reducing the contact resistivity through the formation of micro- and nano-scale metallic spikes, allowing the direct contact to the emitters.

Structural Characterization of Alloyed Al/Ti and Ti Contacts on SiC

Al-rich Al/Ti contacts on p-type SiC show good ohmic behaviour for a wide range of doping density after high temperature (> 950°C) post deposition annealing independently of the 4Hand 6H-SiC polytype [1, 2] employed. This ohmic contact behaviour appears to be closely related to the high Al content of the deposited metal layers. Several hypotheses have been proposed to explain the role of Al in such Al/Ti alloyed contacts mainly based on the doping effect of this element in SiC. However, Al/Ti-SiC alloyed contacts constitute a complex quaternary system where the formation and spatial distribution of different phases as well as loss of volatile Al could strongly affect the contact electrical characteristics. From recent structural investigations, two main experimental results deserve attention. On one hand, some of the phases expected from quaternary phase diagram based predictions have been identified in Al-rich Al/Ti-SiC alloyed contacts [3]. On the other hand, metallization spikes have been detected in this system. A high density of these metallic intrusions in the semiconductor is apparently bound to the ohmic behaviour and to the good contact-to-contact reproducibility observed [4]. Despite these experimental evidences several questions concerning the composition and the electrical behaviour of these contact regions are still open. Among these questions are the Al behaviour during the reaction that is not understood and the composition of the observed metallic spikes, suspected to enhance field emission, that is unknown. Additional structural investigations are thus needed to shed light upon these points. In this way, we report in this work a structural and chemical investigation of Al-rich Al/Ti-SiC alloyed contacts based on the Rutherford Back Scattering technique in the channeling mode (RBS-C) and Cross-Sectional Transmission Electron Microscopy (XTEM) in various analysis modes. These results are compared with those obtained from a preliminary structural investigation of Ti-SiC contacts.

New U/Pu metallic spikes for safeguards accountancy measurements at reprocessing plants

A new generation of metallic uranium/plutonium spikes has been developed for U and Pu determinations in undiluted input solutions of reprocessing plants. The Pu spike is in the form of a Gd-6% Pu-0.16% Nd (mass%) alloy disc prepared by levitation melting. The Gd provides a ductile matrix in which the Pu is present in solid solution. The alloy has good oxidation resistance, but is readily soluble in the nitric acid medium of the input solution. The Nd addition is to permit a determination of input tank volume via the measurement of 142 143 Nd ratios. The U spike is a metal disc with a 235U enrichment of 19.9 amount%. A γ-counting method of plutonium homogeneity control was developed using a Monte-Carlo simulation to correct for γ-ray self-absorption. The spikes were certified for uranium and plutonium mass fractions and amount ratios as Isotopic Reference Material (Spike) IRMM-1030.

Can Ohmic Spikes Define Quantum Systems?

We investigate a system defined by an array of metallic spikes which extend down from an annealed Ni–Au–Ge metallisation deposited on the surface of an AlGaAs/GaAs heterostructure. For the case where the spikes penetrate the two dimensional gas (2DEG), we show that electrons injected from the spikes into the 2DEG maintain phase coherence at low temperatures, resulting in quantum interference processes determined by the spike distribution. A unique device which induces a topological transition from a Corbino disc to a Hall bar is employed to explore this novel phenomenon. We discuss refinements to the spiking process which will facilitate the incorporation of these metallic spikes into nanostructure designs.

Physical and electrical investigation of ohmic contacts to AlGaAs/GaAs heterostructures

The mechanism by which Ni-Au-Ge metallizations establish electrical contact to the two-dimensional electron gas (2DEG) in modulation-doped AlGaAs/GaAs heterostructures is investigated. Transmission electron microscopy was used to examine samples after electrical characterization by magnetoresistance measurements at cryogenic temperatures. We present a picture in which a 2DEG of reduced electron density exists under the deposited metallization. The success of the contacting procedure is described in terms of the magnitude of this density and the size, areal density, and penetration depth of a series of metallic spikes which establish the electrical link to the 2DEG. We suggest that the electrical behavior is not dominated by the current injection process at the spike/2DEG interface but is instead dictated by scattering from the array of antidots formed by the spikes and by a dependence of the 2DEG density on the size of the metallic pad. The implications of this picture for future nanostructure devices, featuring patterned ohmic metallization smaller than a micron, are discussed and preliminary results are reported.

New Aspects of (Semi-Insulating) GaP:Cu

buried Schottky contact model for the complex behavior of Cu in GaP. PIXE studies show that Cu can reach very high concentrations in that material. Channeling indicates that a part of the Cu forms precipitates. Hall measurements on sets of samples were made to examine the electrical trans port coefficients. The mobilities of the samples with the most abundant concentration of Cu were found to be dominated by a term ocT0·5 which was identified as a component induced by metallic spikes with non-overlapping depl etion zones. We come to the conclusion that Cu introduces an acceptor like defect as well as precipitates. For high initial carrier concentrations,

Uranium-plutonium metallic spikes for IDMS accountancy measurements: preparation and characterization

Alloys containing uranium and plutonium have been prepared as candidates for use as metallic spikes for IDMS determinations of uranium and plutonium in spent fuel solutions. Three compositions, U-0.5%Pu-50%Ti, U-0.5%Pu-25%Nb-25%Zr and U-1%Pu-2.3%Nb #1 #1 All the percentages indicated in this work are weight percentages. , prepared by crucible-less induction melting, were selected because of good cold formability. The plutonium homogeneity was estimated to be 0.1–0.4% RSD for samples containing 100 mg uranium and 1 mg plutonium by a γ-spectrometric method using 235U in the spike as an internal standard to correct for self-absorption. Metallography was carried out on buttons and foils of each of the alloys and revealed fine, homogeneous structures.

Analysis of a rhenium-osmium solid-solution spike by inductively coupled plasma mass spectrometry

The rhenium-osmium decay scheme (187Re →187Os) offers a unique opportunity to investigate the genesis of, and directly date, ultramafic rock, sulphide and platinum ore deposits. Inductively coupled plasma mass spectrometry (ICP-MS) is a viable method for Os isotopic analysis as it provides the high temperatures necessary to ionise Os. The sample can be introduced into the ICP mass spectrometer either by conventional nebulisation or by distillation using an Os “generator.” Generator-mode analysis is superior to nebulisation because (i) the total number of counts is two orders of magnitude higher for a given sample size and (ii) Os oxidises readily to volatile OsO4, which has a boiling-point lower than most Re compounds, so that Os can be efficiently separated from 187Re. The importance of Os loss during sample preparation was minimised by combining the sample powder with a powdered Re-Os isotopic spike prior to any chemical treatment. The spike is in a nickel sulphide matrix and was made by a thioacetamide co-precipitation of Os, Re and Ni followed by a fire assay. The 190Os/192Os ratio in this preparation is 51.5 ± 0.9, the 187Os/188Os ratio is 0.063 ± 0.006 and the 185Re/187Re ratio is 17.04 ± 0.41. These values are within the error limits quoted by the Oak Ridge National Laboratory and Techsnabexport, the suppliers of the metallic spikes. Parameters that significantly influence the Os analyses include the oxidising agent and the run temperature. The oxidising agent H5IO6 is preferred to H2O2 or HNO3 as it has the highest electrode potential, provides a prolonged, consistent reaction and is more stable when stored. Chilling the sample and the H5IO6, initially retards uncontrolled OsO4 emission. Heat applied later in the run releases OsO4 and helps maintain a high count rate. Thus, OsO4 can be generated in a steady, controllable and reproducible manner.

Neutron-Induced Metallic Spike Zones in GaAs

The metallic spike model for neutron damage has been shown to account for the observed anomalous infrared absorption in GaAs. In this paper, the electrical properties of semiconductors containing metallic spikes are explored. The metallic zones are shown to act as deep potential wells which trap carriers from the host semiconductor energy bands. The component of mobility associated with carrier scattering from the depletion region surrounding charged spikes is estimated as a function of temperature, including the temperature dependence of the trapped charge. Hall measurement data taken before and after neutron irradiation of n-type GaAs are compared with theory and good agreement is obtained. It is proposed that the high field trapping and slow release of electrons observed in neutronirradiated Gunn diodes is associated with the presence of metallic spikes. Hot electrons in high field domains penetrate the electrostatic barrier and are trapped within the spikes. When the low field condition is restored, excess electrons return to the host semiconductor matrix. The rate of escape of excess electrons is estimated from considerations of the processes of emission over and tunneling through the electrostatic barrier. Measurements of the temperature - dependent decay rates of the excess charge are obtained from neutron-irradiated Gunn diodes. Two decay rates were obtained at each temperature. The shorter decay time shows a temperature dependence consistent with a quantum tunneling mechanism. The longer decay time shows a stronger temperature dependence which is in qualitative agreement with emission of electrons over the barrier.