We demonstrate a novel preparation method for the formation of metallic nanowires from gold, platinum, and silver using mesoporous silica SBA-15 as a template. SBA-15 controls the size and the growth direction of the nanowires and also prevents bulk aggregation of the metal. Nanowires can be isolated from the silica framework by treatment with HF.

Preparation of Noble Metal Nanowires Using Hexagonal Mesoporous Silica SBA-15

An astonishing assortment of structures have been described as "insulated molecular wires" (IMWs), thus illustrating the diversity of approaches to molecular-scale insulation. These systems demonstrate the scope of encapsulation in the molecular engineering of optoelectronic materials and organic semiconductors. This Review surveys the synthesis and structural characterization of IMWs, and highlights emerging structure-property relationships to determine how insulation can enhance the behavior of a molecular wire. We focus mainly on three IMW architectures: polyrotaxanes, polymer-wrapped pi systems, and dendronized polymers, and compare the properties of these systems with those of conjugated polymers threaded through mesoporous frameworks and zeolites. Encapsulation of molecular wires can enhance properties as diverse as luminescence, electrical transport, and chemical stability, which points to applications in electroluminescent displays, sensors, and the photochemical generation of hydrogen.

Insulated molecular wires.

In this paper, recent developments in magnetic tunnel junctions (MTJs) are reported with their potential impacts on integrated circuits. MTJs consist of two metal ferromagnets separated by a thin insulator and exhibit two resistances, low (Rp) or high (Rap) depending on the relative direction of ferromagnet magnetizations, parallel (P) or antiparallel (AP), respectively. Tunnel magnetoresistance (TMR) ratios, defined as (Rap $Rp)/Rp as high as 361%, have been obtained in MTJs with Co40Fe40B20 fixed and free layers made by sputtering with an industry-standard exchange-bias structure and post deposition annealing at Ta = 400 degC. The corresponding output voltage swing DeltaV is over 500 mV, which is five times greater than that of the conventional amorphous Al-O-barrier MTJs. The highest TMR ratio obtained so far is 500% in a pseudospin-valve MTJ annealed at Ta = 475 degC, showing a high potential of the current material system. In addition to this high-output voltage swing, current-induced magnetization switching (CIMS) takes place at the critical current densities (JCO) on the order of 106 A/cm2 in these MgO-barrier MTJs. Furthermore, high antiferromagnetic coupling between the two CoFeB layers in a synthetic ferrimagnetic free layer has been shown to result in a high thermal-stability factor with a reduced JCO compared to single free-layer MTJs. The high TMR ratio enabled by the MgO-barrier MTJs, together with the demonstration of CIMS at a low JCO, allows development of not only scalable magnetoresistive random-access memory with feature sizes below 90 nm but also new memory-in-logic CMOS circuits that can overcome a number of bottlenecks in the current integrated-circuit architecture

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Magnetic Tunnel Junctions for Spintronic Memories and Beyond

We discuss surface alloy phases and their stability based on surface phase diagrams constructed from the surface energy as a function of the surface composition. We show that in the simplest cases of pseudomorphic overlayers there are four generic classes of systems, characterized by the sign of the heat of segregation from the bulk and the sign of the excess interactions between the atoms in the surface (the surface mixing energy). We also consider the more complicated cases with ordered surface phases, nonpseudomorphic overlayers, second layer segregation, and multilayers. The discussion is based on density-functional calculations using the coherent-potential approximation and on effective-medium theory. We give self-consistent density-functional results for the segregation energy and surface mixing energy for all combinations of the transition and noble metals. Finally we discuss in detail the cases Ag/Cu(100), Pt/Cu(111), Ag/Pt(111), Co/Cu(111), Fe/Cu(111), and Pd/Cu(110) in connection with available experimental results.

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Phase diagrams for surface alloys

Spintronics of antiferromagnets is a new and rapidly developing field of the physics of magnetism. Even without macroscopic magnetization, antiferromagnets, similar to ferromagnetic materials are affected by spin-polarized current, and as in ferromagnets this phenomenon is based on a spin-dependent interaction between localized and free electrons. However, due to the nature of antiferromagnetic materials (complex magnetic structure, essential role of exchange interactions, absence of macroscopic magnetization) the study of possible spintronic effects requires new theoretical and experimental approaches. The purpose of this review is to systemize and describe recent developments in this area. After presenting the main features of structure and behavior of antiferromagnets various microscopic and phenomenological models for description of the current-induced phenomena in heterostructures containing ferro- and antiferromagnetic layers are considered. The questions related to an effect of antiferromagnetic ordering on an electric current, as well as the questions of possible creation of fully antiferromagnetic spin valves are discussed. In addition, we briefly discuss available experimental results and try to interpret them.

Spintronics of antiferromagnetic systems (Review Article)

Electronic properties of the $\mathrm{Si}(111)\ensuremath{-}(7\ifmmode\times\else\texttimes\fi{}7)$ surface are evaluated by scanning tunneling microscopy/spectroscopy (STM/STS) using artificially fabricated insulating trenches. When the surface is surrounded by a closed trench, the effect of the Schottky barrier naturally formed between the surface states and the bulk states is observed by STM. When a half-closed tape-shaped structure surrounded by the trench is fabricated, the current path is dominated by that through surface states. Its conductivity is estimated by measuring the voltage drop along the structure.

Electron Conduction through Surface States of the Si(111)-( 7×7) Surface

Chemisorption of atomic hydrogen on the Si(111) 7×7 surface was investigated using scanning tunneling microscopy. Contrary to the recent report by Golovchenko’s group that the monohydride phase is not stable even at room temperature, resulting in the formation of the trihydride phase, our scanning tunneling microscopy (STM) experiment has shown that the monohydride phase is indeed the stable phase on the clean Si(111) 7×7 surface.

Atomic hydrogen chemisorption on the Si(111) 7×7 surface

By evaporating Fe on to a water-terminated Si(100)(2×n) surface, we formed an Fe wire array reflecting the 2×n surface reconstruction. The average wire width was 2 nm and the period was 3 nm. The formation was caused by the deposited Fe atoms diffusing over the water-terminated flat area and being trapped at dimer vacancy lines. This array is applicable to magnetic devices.

Self-organized growth of Fe nanowire array on H2O/Si(100)(2×n)

The electronic states of silicon donors in a wide gap semiconductor, β-Ga2O3(100), have been studied using low-temperature scanning tunneling microscopy We observe one-dimensional rows along [010], as expected from the crystal structure In addition, substitutional Si donors are identified up to the fourth subsurface layer with clear spectroscopic features at the bottom of the conduction band The decay length of each subsurface Si donor is systematically measured, and reasonably agrees with a picture of the Si donor in bulk β-Ga2O3 These results strongly suggest that Si impurities are shallow donors and responsible for the high electrical conductivity of β-Ga2O3

T. Hashizume

Papers

Electron Conduction through Surface States of the Si(111)-( 7×7) Surface

Atomic hydrogen chemisorption on the Si(111) 7×7 surface

Self-organized growth of Fe nanowire array on H2O/Si(100)(2×n)

Atomically resolved silicon donor states of β-Ga2O3

Scanning tunneling microscopy/spectroscopy of dangling-bond wires fabricated on the Si(100)-2×1-H surface