Nanocables semiconductores que se comportan como uniones Josephson

La versatilidad de los materiales semiconductores está demostrada por la gran cantidad de nuevos dispositivos microelectrónicos que se logran día a día. Integrar estos materiales con metales superconductores abre una inmensa cantidad de posibilidades tanto en el campo de los dispositivos como en investigación en física básica, pero existen importantes problemas prácticos a la hora de combinarlos debido a sus diferentes características estructurales y electrónicas. Recientemente1 Jie Xiang y sus colaboradores han logrado superar estas dificultades.

Estos investigadores fabricaron un dispositivo electrónico (ver figura 1.a) hecho por un nanocable semiconductor, cuyo centro es de germanio y esta cubierto por silicio, con contactos de aluminio (superconductor debajo de 1.6K). Debido a sus dimensiones, la conducción en el nanocable esta cuantizada, con lo que el cable funciona como una válvula para la supercorriente.

En los metales superconductores, la corriente fluye sin disipación, aun en distancias macroscópicas. Hace mas de 40 años se predijo y comprobó experimentalmente, que una supercorriente puede fluir a través de una barrera no superconductora o unión Josephson. En el nuevo dispositivo, la unión Josephson que une ambos contactos de aluminio es un nanocable semiconductor. En este nanocable los electrones están confinados en un material con alta movilidad para los electrones como el germanio, mientras que el silicio produce un efecto de confinamiento en la dirección radial. Estos cables tienen además dos propiedades que los hacen óptimos para este tipo de dispositivos: los electrones pueden moverse distancias mayores a 100 nm sin sufrir procesos de dispersión, y es posible hacer buen contacto óhmico entre el aluminio y el nanocable.
En el cable solo puede haber electrones de determinadas energías, éstas se pueden sintonizar a través del voltaje de puerta, esta cuantización en las energías aparece como escalones en la conductancia del cable en función del voltaje de puerta. Estas medidas se realizan con temperaturas mayores a 1.6 K es decir por encima de la transición superconductora del aluminio. Cuando el dispositivo se enfría hasta el estado superconductor del aluminio, el voltaje de puerta permite controlar la corriente Josephson en el nanocable semiconductor.
Los electrones en un material superconductor se mueven formando pares (de Cooper) de carga 2e y están ligados por una energía 2D. Para que un electrón pase de la unión al superconductor y forme un par de Cooper, un hueco moviéndose en dirección opuesta debe quedar en la unión. Este proceso se conoce como reflexión Andreev, el proceso inverso, es decir un hueco reflejándose como un electrón, también es posible. Cada vez que un electrón o hueco se reflejan y se mueven a lo largo del cable ganarán una energía eV, donde V es el voltaje aplicado en la puerta. En este dispositivo se ha demostrado la existencia de múltiples reflexiones de Andreev. Esto sucede ya que el electrón solo podrá trasmitirse como electrón en estado normal al material superconductor cuando su energía sea mayor a 2D, es decir cuando debido a las múltiples reflexiones el electrón haya ganado suficiente energía. Esta condición es neV>2D, donde n es el numero de veces que el electrón atraviesa el cable.
En resumen con este dispositivo se han demostrado que es posible lograr uniones entre un superconductor y un nanocable semiconductor de muy alta calidad, que verifican las predicciones teóricas de transporte mesoscópico.

Hernandez Caballero Indiana M. CI: 15.242.745
Asignacion: EES
Fuente: http://images.google.co.ve/imgres?imgurl=http://blogs.creamoselfuturo.com/nano-tecnologia/wp-content/uploads/2007/04/flavioybfigure1.jpg&imgrefurl=http://blogs.creamoselfuturo.com/nano-tecnologia/2007/04/11/nanocables-semiconductores-que-se-comportan-como-uniones-josephson/&usg=__F4qqCp8kPxznrMQZ2Sq5zlYCcho=&h=355&w=841&sz=22&hl=es&start=16&itbs=1&tbnid=NURXgtvOjzk3MM:&tbnh=61&tbnw=145&prev=/images%3Fq%3DMetales%2Bpara%2Bsemiconductores%26hl%3Des%26gbv%3D2%26tbs%3Disch:1

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Nota sobre el desarrollo del concepto de semiconductores

En 1931, El afamado físico W. E Pauli afirmo: "Nadie debería investigar sobre semiconductores. Son una porquería. ¿Quien sabe si realmente existen los semiconductores?". Revisaremos la historia del concepto de los semiconductores para entender el contexto de estas palabras lapidantes.

Actualmente, ninguna de las tecnologías que han transformado el mundo sería concebible sin los semiconductores. Computadoras, teléfonos celulares y enorme número de productos están basados en la tecnología del semiconductor. Siendo tan importantes para nuestra sociedad, decenas de miles de investigadores en cientos de laboratorios investigan sobre las propiedades y aplicaciones de los semiconductores.

Entonces, ¿Por qué alguien tan inteligente como Pauli afirmó sus dudas sobre la existencia de los semiconductores? ¿Por qué él los consideraba una porquería y no valía la pena su investigación?

16 años antes de la realización práctica del primer transistor, la opinión de Pauli sobre los semiconductores era el reflejo de un estado de ánimo en la comunidad de físicos dedicados al estudio de los sólidos. Aun en 1938 muchos investigadores calificaban de suicidio científico la decisión de trabajar sobre el carburo de silicio. Entonces, en lugar de pensar en la falta visión de Pauli, debemos considerar que el desarrollo de la ciencia no es lineal. Revisemos el contexto del desarrollo de las ideas.




La investigación en metales

A finales del siglo XIX y principios del XX, fue notable el contraste entre los rápidos avances de la física de los metales y el desconcierto en la investigación sobre semiconductores.

Así, Sobre la naturaleza de la electricidad y las propiedades eléctricas de los metales, los resultados experimentales eran reproducibles y regulares, por lo que condujeron a rápidos y significativos avances:

* En 1840, Humphry Davy observó la dependencia inversa de la conductividad con la temperatura en un gran número de metales.

* Desde 1853 (Wiedemann y Franz) se conocía la relación entre las conductividades eléctrica y térmica de los metales.

* Ya en 1821 se contaba con resultados reproducibles sobre el poder termoeléctrico de los metales, es decir, se tenían datos sobre el efecto Seebeck.

* En 1879 se registraron observaciones del efecto Hall

* En 1897 Thomson descubre el electrón. Con toda esta gran masa de resultados experimentales, se presentaron casi inmediatamente los primeros modelos de trasporte eléctrico en los metales.

* En 1899 (Riecke) y en 1990 (Drude) se presentan modelos que interpretan y predicen todos estos resultados y orientan la investigación en el campo del trasporte eléctrico en metales. Pero las limitaciones de tales modelos constituyen un estimulo y dan lugar a nuevas experiencias científicas.

* En 1929, Sommerfeld presenta una teoría cuántica de los metales, la cual es inmediatamente reconocida y aceptada. Recordemos que al principio del siglo XX se dio un contexto revolucionario para la física, las ideas de Sommerfeld se basaron en los principios y formalismos de la mecánica cuántica.

Visto a posteriori, estos rápidos avances de la física de los metales se debe a dos razones principales:

* Por un lado, el milenario e importante rol tecnológico de los metales contribuyó a la obtención de muestras con un aceptable grado de pureza, los experimentos requerían esta clase de muestras puras.

* Por otro lado, basados en los datos experimentales, se contaba con teorías que funcionaban bien para los metales y eran pésimas para los semiconductores. El hecho de que la aproximación del electrón libre funcione tan bien para bandas anchas semillenas mantuvo durante años la ilusión de que los metales son más fáciles de entender que los semiconductores.

La investigación en semiconductores

En contraste con el gran número de leyes empíricas relativas a los metales, en lo que concierne a los semiconductores, no llegó a encontrarse por muchos años ninguna propiedad que mostrara un comportamiento reproducible y permitiese dar un contenido a la propia palabra "semiconductor".

* Como antecedente, en el siglo XVIII, Volta hablaba de materiales "de naturaleza semiconductora", la utilización del término fue puramente taxonómica durante casi siglo y medio, ya que se incluía en esa categoría a todos los materiales que no eran ni aislantes ni metales.

* Hittorf (1851) fue el primero en publicar resultados sobre la variación de la conductividad de los sulfuros de plata y cobre en función de la temperatura. La trasiscion de fase del sulfuro de plata a 170 ºC, hizo que los resultados pareciesen entonces más erráticos de lo que realmente eran (de hecho, si se traza el diagrama de Arrhenius a partir de la tabla de resultados de Hittorf para el Cu2S, se obtiene una recta, y la energía de activación resultante es próxima a la mitad de la banda prohibida de ese material). Hittorf creyó estar observando conducción electrolítica, y durante años muchos investigadores centraron sus esfuerzos en verificar si se cumplía la ley de Faraday. Se trataba de una vía muerta, pero, aunque las experiencias de Riecke en 1901 habían excluido concluyentemente esa hipótesis, se siguió barajando el modelo electrolítico para los semiconductores hasta bien entrados los años 30.

* En 1908 Königsberger propuso su teoría de la disociación, según la cual, los portadores de carga que se mueven libremente en un conductor resultan de la disociación de los átomos en electrones móviles e iones positivos fijos. La disociación era regulada por una energía de activación, que debe intervenir en la dependencia de la resistividad con la temperatura. Al intentar verificar su teoría, comparando los datos experimentales con sus predicciones, pudo establecer una clasificación de los sólidos en metales (en los que la energía de disociación era nula), aislantes (en los que era infinita) y "conductores variables" (en los que era finita).

* Weiss, en 1910, quien realizó numerosas experiencias para verificar ese modelo, fue el primer autor moderno en proponer el nombre de "semiconductor".

* Baedaker, en 1908, encontró un método de preparación de semiconductores en capas delgadas que le permitía cierto grado de control de las propiedades. La interpretación que hace de sus resultados ilustra el grado de desconcierto que existía y la falta de un concepto de semiconductor aceptado por todos los investigadores. Así, sus estudios sobre el ioduro de cobre le conducen a calificarlo de "conductor metálico con concentración de electrones variable". Al interpretar los resultados de efecto Hall, considera un sólo tipo de portadores, encontrando concentraciones sorprendentemente bajas. El modelo de Riecke para el efecto Hall, propuesto nueve años antes, que consideraba posible la existencia de portadores de carga positivos y negativos, le habría permitido interpretar correctamente sus resultados.

* En 1930 era aún aceptada de manera general la opinión de Gudden de que las propiedades semiconductoras son debidas a las impurezas y que ninguna sustancia pura puede ser semiconductora (la opinión de Gudden implicaba que toda sustancia pura sería, o bien aislante, o bien metálica).

* En esta etapa de desarrollo, Frenkel en 1931 muestra la teoría de los defectos puntuales en los cristales iónicos, que permitió sistematizar un gran número de resultados, al mostrar que las vacantes de anión dan lugar a conducción por electrones y las vacantes de catión a conducción por huecos. Este modelo permitía , por una parte, correlacionar claramente en muchas sustancias la concentración de defectos con la conductividad y, por otra, dar cuenta de la existencia de sólidos en los que el efecto Hall tiene signo positivo. Así, se realizaron gran cantidad de medidas sistemáticas en muchos semiconductores, que fueron clasificados como:

** "conductores por exceso" (con vacantes de anión),
** "conductores por defecto" (con vacantes de catión), y
** "conductores anfotéricos", que presentaban uno u otro comportamiento, según las condiciones de preparación.

Entretanto, estaban encontrándose las claves que permitirían establecer la teoría cuántica de los sólidos. Desde que von Laue (1912) descubrió la difracción de rayos X y Bragg (1913) determinó la estructura cristalina del ClNa, se sabía que los átomos en los sólidos se disponen siguiendo una estructura ordenada, triplemente periódica.

* Primero Strutt y luego Bloch, ambos en 1928, tratan el problema del electrón en el campo periódico. Bloch deduce las propiedades generales de los estados electrónicos, introduciendo las funciones de onda que llevan su nombre, pero no llega a ninguna conclusión sobre el origen del carácter metálico o aislante de un sólido.

* Por fin, Alan Wilson, en 1931, fue primer autor en extraer todas las consecuencias que la teoría de bandas implica para las propiedades del transporte, mostrando que una banda llena no contribuye al transporte de carga e introduciendo rigurosamente el concepto de banda prohibida , ademas introduce los conceptos de semiconductor intrínseco, semiconductor extrínseco (y por ende, impurezas dadoras y aceptoras). De nueva cuenta, el hecho de que se formularan rigurosamente los elementos básicos de una teoría, en este caso la teoría de semiconductores, no condujo a su aceptación inmediata por la comunidad científica.

* A mediados de los 40, los trabajos de Lark-Horovitz sobre el germanio y el silicio, permitió al concepto del semiconductor ser universalmente aceptado.

Hernandez Caballero Indiana M. CI: 15.242.745
Asignacion: EES
Fuente:http://images.google.co.ve/imgres?imgurl=http://www.electronicosonline.com/noticias/images/uploads/semiconductores2202.jpg&imgrefurl=http://vicente1064.blogspot.com/2008/01/nota-sobre-el-desarrollo-del-concepto.html&usg=__ovuwcA9aeOkIGwI7FHwBcX0CThw=&h=239&w=291&sz=10&hl=es&start=14&itbs=1&tbnid=qO7pumBWjP2s_M:&tbnh=94&tbnw=115&prev=/images%3Fq%3DMetales%2Bpara%2Bsemiconductores%26hl%3Des%26gbv%3D2%26tbs%3Disch:1

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CPUs frías de Intel

Abril 6, 2009 at 10:38 am (Tecnologia)

En un reciente artículo de su blog de avances tecnológicos, Intel ha anunciado un nuevo sistema de fabricación para procesadores que podría conseguir un consumo hasta diez veces menor que los procesadores actuales. Este avance podría suponer para la industria una importante mejora no solamente en la autonomía para dispositivos como ordenadores portátiles o teléfonos móviles sino una emisión de calor mucho más baja.
Una nueva tecnología de fabricación de microprocesadores podría suponer una reducción del voltaje de funcionamiento de un 50% y un consumo de energía que podría ser de hasta un décimo de los actuales transistores que forman los procesadores. Tal y como se describe en una entrada del blog sobre avances tecnológicos de Intel, se trata de un nuevo transistor de canal P construido sobre un nuevo sustrato de silicio que utililza compuestos semiconductores.

Son compuestos que se conocen como materiales III y V, por encontrarse en la tercera y quinta columna de la tabla periódica, justo al lado de la IV colummna que ocupa el silicio. Estas dos nuevas técnicas son las que podrían servir para fabricar circuitos lógicos CMOS que formarían parte de procesadores que consumirán menos y también que permanecerán más fríos en funcionamiento. Esto podría permitir mayores frecuencias de reloj para las CPUs.

Hernandez Caballero Indiana M. CI: 15.242.745
Asignacion: EES
Fuente: http://images.google.co.ve/imgres?imgurl=http://muycomputer.com/files/264-4476-FOTO/Intel%2520Dunnington%25200.jpg&imgrefurl=http://miranda23.wordpress.com/2009/04/06/&usg=__LbIfjL6Q7X5VMWm6cBMLPK-Y274=&h=614&w=860&sz=169&hl=es&start=1&um=1&itbs=1&tbnid=Pl62F4MMbbBn9M:&tbnh=104&tbnw=145&prev=/images%3Fq%3Davances%2Btecnologicos%2Bsemiconductores%2Bsilicio%26um%3D1%26hl%3Des%26client%3Dfirefox-a%26rls%3Dorg.mozilla:es-ES:official%26gbv%3D2%26tbs%3Disch:1

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An introduction to photonic band gaps

Photonic band gap materials

It has long been known by diamantists, and used to check for authenticity, that dipping a gem in a solution with a matching index of refraction turns it invisible. This is due to the fact that what makes a homogeneous object's boundaries visible is the reflection and refraction of light therein. Therefore if light does not find difference in traversing a surface it will not be in any way scattered, as it is not in a homogeneous medium. The main feature of PXs is the periodic modulation of such property (dielectric constant) along one, two or three directions of space (see Fig. 1). In a composite formed by two dielectrics, we will consider the scattering centre that in which light propagates more slowly, i.e. that of higher e. If the scattering centres are regularly arranged in a medium, light is coherently scattered. In this case, interference will eventually cause that some frequencies will not be allowed to propagate, giving rise to forbidden and allowed bands. Under certain conditions that will be detailed later in this section, regions of frequency may appear that are forbidden regardless of the propagation direction in the PX (see Fig. 1). In such case, this PX is said to present a full PBG. On the contrary, if the forbidden photonic band varies with the propagation direction in the PX, a photonic pseudogap is spoken of. What is more, by introducing defects in the PX, we can introduce allowed energy levels in the gap, as occurs when a semiconductor is doped. All these facts permit to establish a parallelism between the formalism used for electrons in ordinary crystals and that for photons in PX.

Fig. 1.- Real space representation of a photonic crystal and reciprocal space regions for which propagation is shielded.

The Schrödinger equation for an electron of effective mass m in a crystal, in which the potential is V(r), can be written as:

(1)

where V(r) is a periodic function with the periodicity of the lattice, R:

V(r)=V(r+R)
(2)

The eigenstates of this equation are also periodic functions with period R. The dispersion relationship derived, E(k), will present a forbidden band for all energies E which have imaginary values. Similarly, in a medium in which a spatial modulation of the dielectric constant e(r) exists, photon propagation is governed by the classical wave equation for the magnetic field H(r):

(3)

In a PX, e(r) is a periodic function:

e(r)= e(r+R)
(4)

These equations show the parallelism between electrons in crystalline solids and photons in PX. In Fig. 2 it is shown how gaps are developed in both an electronic crystal and a PBG material.

Fig. 2.- Energy dispersion relations for free electron and electron in a (1D) solid and for a free photon and a photon in a PX.

The energy dispersion relation for an electron in vacuum is parabolic with no gaps. When a periodic potential is present gaps open and electrons with energies therein have localized (non-propagating) wavefunctions as opposed to those of electrons in allowed bands which have extended (propagating) wavefunctions. In a similar way, a periodic dielectric medium will present frequency regions where propagating photons are not allowed and will find it impossible to travel the crystal. One important difference between electrons and photons rests on the different nature of their associated waves. Electrons are scalar waves, while photons are vectorial ones. This implies that, in the latter case, polarization must be taken into account. This finally results in much more restrictive conditions for gap appearance that is the case for electrons. On the contrary, the electron wave equation is not scalable, since an intrinsic length measure is associated to the electron (de Broglie wavelength) whereby the potential periodicity can be gauged. This restriction does not apply for photons. The photon wave equation is scalable, hence if a PX presents a given periodicity length, it will show photonic bands in certain range of frequency and a scaling if it will result in a new system with exactly the same band scheme only accordingly scaled. If we halve the size we double the energies. The parameters on which the optical features of a PX will depend are indicated below:

-The type of symmetry of the structure.

-Dielectric constant contrast (e1/ e2).

-Filling factor, that is, the ratio between the volume occupied by each dielectric with respect to the total volume of the composite.

-The topology, which can be either cermet: scattering centres are isolated from each other; or network: scattering centres are connected between them.

-The shape of the scattering centres.

All these factors determine the photonic band structure of the PX and, therefore, its optical properties. Economou and Sigalas have published a general discussion about topologies in PBG theory and the generalization to other classical waves. Recently, it has been shown that a slight modification of either the symmetry or the shape of the scatterer can enlarge the gap.

Fig. 3.- Real and reciprocal space representation of triangular and square twodimensional crystals to highlight the formation of a full bandgap and a pseudogap.

In Fig. 3 it can be seen that high symmetry favours the photonic gap appearance. In this case the higher symmetry of the hexagonal lattice is reflected by the fact that the Brillouin zone is closer to circular than the square lattice: distances in reciprocal space to the principal points of the irreducible zone are similar. It is obvious that a high dielectric constant contrast produces that the optical bands significantly depart from the free photon behaviour (w=ck) producing wider gaps at the Bragg planes (Brillouin zone edges) in reciprocal space.

The concept of PBG is deeply rooted on that of Bragg diffraction. By virtue of coherent scattering each set of crystallographic planes may give rise to an X-ray diffraction peak at a certain frequency related to the interplanar distance. X-ray diffraction follows Bragg law:

2d(hkl)Sin q=ml; m =1,2,3,
(5)

where d(hkl) is the distance between atomic crystalline planes labelled by the Miller indices (hkl), q is the angle of the incident radiation, m is the diffraction order and l is the X-ray wavelength. As a consequence of the destructive interference, X-ray photons in the Bragg diffraction peak are not allowed to propagate through the crystal and are reflected. This effect reveals the absence of photonic states for the selected direction with the frequency determined by Bragg law. Bragg diffraction peaks, in ordinary crystals, appear at the X-ray region since the lattice parameters are of the order of several Angstroms. An entirely similar effect takes place in PXs. Due to the existence of crystalline planes in the PX, some frequency regions will be diffracted according to the Bragg law for the optical region:

(6)

where lc is the wavelength of the EM wave, d(hkl) the interplanar distance for the (hkl) crystallographic direction, áeñ the average dielectric constant of the PX
and q(hkl) the angle between the incident radiation and the normal to the set of crystalline planes determined by the (hkl) indices. An important difference between X-ray diffraction in solids and diffraction in PXs is the bandwidth of the Bragg peaks. X-ray diffraction peaks are extremely narrow (Dl/l» 10-6) and mainly instrumentally broadened. In PXs, the diffraction condition for a given direction of the wave vector, is met for a larger range of frequencies (Dl/l » 10-2). This originates mainly in the different contrast of dielectric constants occurring for these two well apart wavelengths: indices of refraction for X-ray radiation scarcely differ from unity whereas at optical wavelengths they are rather larger. Eventually, Bragg peaks in PXs become so broad that can overlap others coming from different crystallographic planes. This is schematically depicted in Fig. 3 where we show that this occurs in a hexagonal symmetry as opposed to the square symmetry in which the gaps do not overlap. Consequently, it could be possible to find a certain frequency region where the photon is not allowed to propagate regardless of direction. A material with these properties is called a PX with a full band gap.

Hernandez Caballero Indiana M. CI: 15.242.745
Asignacion: EES
Fuente:http://images.google.co.ve/imgres?imgurl=http://luxrerum.icmm.csic.es/Imagenes/bands.gif&imgrefurl=http://luxrerum.icmm.csic.es/%3Fq%3Dnode/research/PCintro&usg=__gGCtRNMV8_MAI_OsNwiXVMrbUI0=&h=1110&w=1297&sz=43&hl=es&start=3&um=1&itbs=1&tbnid=V6PJJjdedDzc5M:&tbnh=128&tbnw=150&prev=/images%3Fq%3DBragg%2Bscattering%2Bof%2Belectron%2Bin%2B1D%26um%3D1%26hl%3Des%26client%3Dfirefox-a%26rls%3Dorg.mozilla:es-ES:official%26gbv%3D2%26tbs%3Disch:1

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Metal Information: What is a Metal?

Our common experience of metals is that they are shiny, hard and conduct electricity. In order to understand how this makes metals useful in batteries, we need to know about the structure of metals and how this gives them the properties we are familiar with. This discussion is restricted to pure metals such as lead, gold, copper and so on; alloys will be addressed elsewhere.

Metal Information: Definition of a Metal

A metal is defined as any element that loses its outer shell electrons to become stable. Since neutral atoms have equal numbers of electrons (- charge) and protons (+ charge). Therefore when a metal atom loses electrons it becomes positively charged. This charged particle is called an ion

This is the state virtually all metal atoms are in when encountered in the real world. These positive ions are almost always combined with non-metal negative ions like chloride and fluoride to make ionic compounds. These are everywhere; Sodium Chloride is sodium metal and chloride non metal. Rust is some combination of Iron metal and oxide non metal, such as Fe2O3. Virtually everything that is not organic is an ionic compound.

Metal Information: How Are Pure Metals Made?

There are various techniques for separating the metal ions from the non-metal ions in a substance. Whether by heating or dissolving in a solution and using an electric current to isolate them, the effect is the same - to isolate the positive metal ions. These processes also involve the supply of electricity.

When the positive metal ions encounter free electrons, and there is nothing nearby to remove the electrons, they reform into complete metal atoms. These metal atoms adhere to each other and so metal crystals grow. Once in a metal crystal, a metal atom still has a very weak hold of its outer shell, or valence, electrons. These electrons tend to drop off and float around the now once more positively charged metal ion. These are called delocalised electrons.

The result of many millions of atoms doing this is that there are layers of positive ions surrounded by a "sea" of delocalised electrons. The negative charges of the electrons hold the positive ions in place, and vice versa.

Metal Information: The Properties of Metals This model for the structure of metals explains the properties we are familiar with.
Conductivity The free electrons can move, and so when electrons are pushed in at one end of a piece of metal others come out at the other end. There are no empty spaces in the metal. The easiest way to visualise this is as a hose filled with marbles; when one goes in another comes out immediately at the other end. Thus current can flow through metals. It is this presence of available electrons in metals that enable them to be used to generate current in batteries.
Lustre The lustre, or shine, of metals is caused by the electrons reflecting light. All pure metals reflect well. Metals that do not seem to do so, like lead, are coated with a thin layer of oxide (rust). If this layer is scraped off, the reflective metal can be seen underneath.
Strength The pull between the layers of positive ions and negative electrons is very strong and holds the layers tightly together. This is not immediately obvious from this model of bonding.
Being Prone to Corrosion The free electrons can be absorbed by other substances, such as oxygen gas in the atmosphere. When this happens there is an imbalance of electrons and positive ions in the metal, and the unbalanced metal ions will form ionic bonds with other chemicals, and rust is produced.

Hernandez Caballero Indiana M. CI: 15.242.745
Asignacion: EES
Fuente: http://www.green-planet-solar-energy.com/metal-information.html

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A Quantum Contribution to techonlogy

AT the nanoscale, computer simulations are often the only way that researchers can learn about materials. Imagine the shaft of a human hair sliced about 50,000 times. One slice is about a nanometer, or one-billionth of a meter—a distance that can be spanned by just 3 to 10 atoms. This minute size range is the realm of nanoscale science, where materials typically measure between 1 and 100 nanometers (nm) across.

Accurate descriptions of nanoscale materials must account for the behavior of individual atoms and electrons: how they move, how they form bonds, and how those bonds break. In 1999, a Livermore simulation of such quantum behavior revealed the secrets of hydrogen fluoride mixing with water at high temperatures and pressures. The motion being modeled lasted just 1 picosecond (a trillionth of a second), yet the calculation required 15 days and the entire resources of Blue Pacific, which at the time was Livermore's fastest supercomputer. (See S&TR, July/August 1999, Quantum Molecular Virtual Library.) In the years since, computers have grown far more powerful, imaging devices can record even smaller features, and nanoscale science is thriving.

Livermore's Quantum Simulations Group in the Physics and Advanced Technologies Directorate is a leader in modeling material processes using quantum molecular dynamics methods. The group's early projects examined basic but poorly understood phenomena such as water under extreme pressure. (See S&TR, April 2002, Quantum Simulations Tell the Atomic-Level Story.) More recently, quantum simulations revealed a new melt curve of hydrogen at extremely high pressures. (See S&TR, January/February 2005, Experiment and Theory Have a New Partner: Simulation.) In 2006, a quantum simulation run on Livermore's BlueGene/L platform won the Gordon Bell Prize for Peak Performance.

More recently, the Livermore group has begun working on simulations for a diverse group of technological applications. For example, nanoscale materials could improve cooling technologies in military equipment and reduce the size of gamma radiation detectors being developed for homeland security. The Department of Energy (DOE) is funding research to dramatically improve storage systems for hydrogen fuel on vehicles. In addition, computer chip manufacturers must ensure that their quality-control tools can detect defects in chips as their size continues to shrink. All of these applications require exploring materials at the nanoscale, a regime where simulations are often the most effective approach. "Nanoscale experiments are expensive," says computational scientist Andrew Williamson, a project leader in the Quantum Simulations Group. "At this scale, simulations can be more cost effective."

The computer codes for modeling dynamics at the molecular level are density functional theory and quantum Monte Carlo. Both types of code start from first principles—that is, with no laws other than quantum mechanics characterizing the system being studied. Density functional theory in quantum mechanics describes the electronic density of a molecular or condensed system. It can model atomic motion and the complex dynamics of material interactions. Quantum Monte Carlo also simulates these behaviors, but it uses a different technique. As the code's name implies, the computer essentially "throws the dice" millions of times to select possible answers.

Quantum Monte Carlo codes are more accurate than density functional theory codes, but they can be extremely demanding of computational resources. Williamson and his colleagues have developed a linear scaling technique that greatly reduces the computing time for quantum Monte Carlo calculations. Still, for most problems, density functional theory is the first choice.

Refrigeration with Nanowires In one project, researchers in the Quantum Simulations Group are evaluating new materials to provide cooling for military applications. Their simulations, which are funded by the Defense Advanced Research Projects Agency (DARPA), indicate that a highly efficient thermoelectric material may be achievable using silicon germanium (SiGe) nanowires. Thermoelectric materials convert heat into electricity and vice versa. They have no moving parts and release no pollutants into the environment. A few niche markets have used them for decades to cool electrical parts or generate power. Researchers have considered using thermoelectric-based refrigerators to replace current heat-pump-based refrigerators that compress and expand a refrigerant such as Freon. However, despite extensive research, the efficiency of these materials has remained low. A highly efficient thermoelectric material must exhibit a combination of properties that do not coexist in conventional materials. It must have the high thermoelectric power of semiconductors, the high electrical conductivity of metals, and the low thermal conductivity of insulators. By measuring these features, scientists can determine a material's efficiency or figure of merit, which is known as its ZT. The highest ZT achieved in the past 40 years is 1. A thermoelectric material designed to replace a conventional Freon-gas refrigerator must have a ZT of at least 3. A semiconductor nanowire is an ideal thermoelectric material. Nanowires are so thin they are often considered to have only one dimension: their length. This extreme thinness restricts electrons and holes in a process called quantum confinement, which increases electrical conductivity. A nanowire's small size also increases the influence of its surfaces, reducing thermal conductivity. To date, the best thermoelectric materials are superlattice nanowires with a ZT of 2.5 to 3. For the DARPA project, Livermore scientist Trinh Vo, a postdoctoral researcher, developed simulations to compare the growth direction, surface structure, and size of silicon nanowires and determine the optimal properties for electrical conductivity. Vo studied silicon with lattices grown in directions known as [001], [011], and [111], and with symmetric, canted, and reconstructed surfaces. Starting with bulk silicon, she computationally constructed 1-, 2-, and 3-nm cylinders of silicon "terminated" with hydrogen on their surfaces. She then optimized their atomic structure using a density functional code called QBox. The [011] growth direction showed the highest electrical conductivity and thermoelectric power, two parameters that increase ZT. The effect of the wire's size was mixed. For wires with canted surfaces grown in the [001] and [111] directions, effective mass increased as the wire's diameter decreased. (As effective mass decreases, electrical conductivity increases and, thus, improves ZT.) However, for the [011] growth direction, where straight channels allow easy electron transport along the wire, effective mass remained the same regardless of the wire's dimension. "These findings indicate that we can tune the electron mass and mobility to optimize a wire's electronic conductivity," says Vo. Canted nanowires grown in the [001] direction can achieve a ZT of 3.5 but require considerable doping with either phosphorus or boron. "I doubt that the wires could be doped strongly enough for this surface to work," says Vo. "Wires grown in the [011] direction will probably be the best compromise." Although the low effective mass of silicon increases electrical conductivity, it also contributes to a high thermal conductivity. Thermal conductivity must be low for a thermoelectric material to be efficient. One solution is to change the material used for the wires. Vo's simulations indicate that a SiGe combination will reduce lattice thermal conductivity by as much as five times without affecting electrical conductivity. She is now working with Livermore scientist John Reed, who also is a postdoctoral researcher, to optimize wires made of silicon and germanium. In collaboration with colleagues at the Massachusetts Institute of Technology (MIT), Reed is using classical molecular dynamics techniques to calculate the thermal conductivity of wires with various configurations of silicon and germanium atoms. The goal is to create a SiGe wire with the lowest possible thermal conductivity. Optimizing the SiGe wire involves an iterative scheme. Reed extracts fluctuations in heat current from the results of his molecular dynamics calculations, and the MIT team inputs these data into a cluster-expansion-based optimization method. The cluster-expansion algorithm produces candidate structures. The thermal conductivities of these structures are then calculated by Reed's code and plugged back into the MIT optimization calculation. This iterative process can also be used to optimize the thermal properties of semiconductors similar to silicon and germanium. "The cluster-expansion method could propose a configuration for silicon and germanium that is impossible to fabricate," says Reed. Consequently, potential configurations must be evaluated to ascertain whether they can be fabricated and doped appropriately, and whether they are stable.

Simulations indicate that when canted nanowires have lattices grown in the (a) [001] and (b) [111] directions, the wire's effective mass increases as its diameter decreases. These configurations will increase electrical conductivity in thermoelectric materials. (c) When silicon nanowires are grown in the [011] direction, electron states are oriented along the wire. In this configuration, effective mass does not change when the wire's diameter changes.

A Better Radiation Detector Quantum simulations are also helping researchers develop a lightweight, high-resolution gamma radiation detector. A portable detector that can identify specific threat agents while ignoring the many legitimate sources of radiation has been a long-term goal to enhance security in the U.S. and worldwide. Such units would allow security personnel at cargo ports, airport terminals, and border crossings to quickly and easily detect threat agents before they enter the country. Improvements in the detection of weapons-grade nuclear materials are also critical to the effectiveness of the U.S. nonproliferation program. The challenge in designing a portable gamma radiation detector is that high-purity germanium, the best material to date, cannot be used at room temperature. It must be cooled to remove its inherent background noise so the detector can read the signal emitted by gamma rays. Because of the cooling required with current technology, a high-resolution germanium-based radiation detector is typically a heavy, fragile unit. Researchers have proposed about 20 semiconductor elements and alloys as substitutes for germanium. Unfortunately, many of these materials have not performed as well as expected. Experimental investigations of every possible material would be prohibitively expensive. Now that computers can accurately predict material properties, computer scientists have joined the search for a new detector material. When a semiconductor material interacts with gamma radiation, it produces electron–hole pairs that are detected as an electrical signal. A candidate material should therefore have highly mobile electrons (and holes) and long electron–hole recombination times to maximize the signal from each absorbed gamma ray. For use at room temperature, the material must also have an energy band gap large enough to preclude thermal excitations. These features are controlled both by the intrinsic electronic properties of the semiconductor, such as its band structure and effective masses, and by the purity of the material. Structural defects in the material can trap electrons, reducing their mobility and increasing the probability of recombination, which in turn reduces the resolution of the detector. Lawrence Fellow Vincenzo Lordi is performing first-principles studies to characterize the microscopic properties of materials and determine which ones are the best for semiconductor alloys. "Our research focuses on material impurities and ways to eliminate them," says Lordi. His simulations first provide an atomistic view of potential detector materials such as bulk gallium telluride and aluminum antimonide. Using density functional theory, he models the microscopic mechanisms by which defects degrade mobility. He then can calculate the intrinsic limits of mobility. Defects may be either native to the material or nonnative, for example, from a dopant. Kuang Jen Wu, a scientist in the Chemistry, Materials, and Life Sciences Directorate, is experimenting with aluminum antimonide, but the crystals produced to date have not been pure enough for use in a detector. Wu has tried annealing the crystals to repair some of the defects and is using Lordi's recent calculations to guide the annealing procedures. Lordi and Williamson have also begun to develop a first-principles computational toolkit that will predict the structural, electronic, and transport properties of different semiconductor detector materials. The toolkit will evaluate a candidate material, determine the formation energies for a range of structural defects and dopants, and identify the most commonly formed defects. It will then predict how the concentration and distribution of defects will affect the material's electronic band structure, effective mass, and charge carrier mobility, lifetime, and scattering rate. Ultimately, the toolkit will be used to create a database of fully characterized candidate materials for semiconductor detectors. With extensive information showing how the sensitivity of transport properties is affected by imperfections in a material's structure, the team can more easily identify promising materials. "The database will also allow us to evaluate methods to improve a material's performance by modifying the synthesis process," says Lordi. A high-resolution room-temperature radiation detector has other potential applications in addition to homeland security and nonproliferation. For example, astrophysicists are interested in using these detectors to better study gamma-ray bursts, the most luminous events to occur since the big bang. Orbiting satellites now detect a gamma-ray burst somewhere in the universe about once a day.
Squeezing Hydrogen from a Sponge Another quantum simulation project is looking at material optimization, this time to allow auto manufacturers to scrap the internal combustion engine and make the move to hydrogen-fueled vehicles. "Fuel storage is a major stumbling block to further development of hydrogen vehicles," says Williamson. "With current storage technology, a tank holding enough hydrogen to travel 480 kilometers would be much too big and heavy for a car." Scientists have been trying to solve this conundrum for more than 30 years. Williamson is working with researchers Julie Herberg and Ted Baumann from the Chemistry, Materials, and Life Sciences Directorate to determine if spongelike materials made of carbon can be used to soak up hydrogen and efficiently store it on cars. This project, which is funded by DOE's Office of Science, also includes computer scientists from the National Renewable Energy Laboratory (NREL) in Boulder, Colorado. Williamson likens the team's research to Thomas Edison's hunt for the best light bulb filament. Edison experimented with thousands of materials before settling on carbon. The ideal storage material cannot react with hydrogen but must weakly bind to it so that hydrogen can be easily drawn off when a vehicle needs more power. In other words, the binding must be reversible. The optimal storage material will be very light so that more of the weight of the full tank is taken up by hydrogen, rather than by the tank itself. Before Williamson joined the project, the NREL scientists conducted experiments with boron and boron-doped carbon (C35B) fullerenes. Their research showed that pure boron would have to be heated to release hydrogen, but C35B remains a possible choice. These materials absorb hydrogen by the Van der Waals intermolecular force. This force, which is much weaker than a chemical bond, arises when molecules polarize into dipoles. These intermolecular forces may be feeble, but life as we know it would not exist without them. For example, the Van der Waals force provides just enough attraction to hold water molecules together in the liquid state. Density functional theory does not capture Van der Waals forces. Instead, Williamson and his colleagues at NREL are using diffusion quantum Monte Carlo. By also incorporating Livermore's faster, linearly scaled version of quantum Monte Carlo, they have performed the first highly accurate quantum Monte Carlo studies of potential hydrogen storage materials. Calculations of the Van der Waals binding energy for hydrogen and carbon fullerenes doped with either boron or beryllium showed both materials to be adequate for reversible hydrogen storage. The team is now investigating other possible carbon-based storage materials, such as calcium?]intercalated graphite. These preliminary results will be augmented by a new coding capability. Williamson and the NREL researchers are modifying density functional theory to explicitly include the nonlocal correlation effect particularly tailored for the Van der Waals interactions. This new tool and diffusion quantum Monte Carlo complement one another and should firmly establish the binding energy and reversibility of hydrogen in candidate materials. Having these data is crucial because DOE is considering whether to continue research on carbon-based storage materials for hydrogen-fueled vehicles.

Livermore researchers calculated the binding energy of a boron-doped carbon fullerene to determine if it is a suitable material for hydrogen storage systems in vehicles.

Quality Control for Chip Manufacture Each year, computers become faster and more powerful because chip manufacturers can fit more features on a silicon wafer. As silicon chips get smaller, they become more difficult to make. Manufactured chips now have features measuring 65 nm, and in the laboratory, features can be made as small as 25 nm. Experts predict that features will be less than 10 nm by 2015. Silicon at 10 nm may behave differently than it does at 50 or 100 nm. At the larger, bulk scale, silicon's behavior follows the rules of classical molecular dynamics. At smaller scales, however, quantum mechanics rules behavior. The Quantum Simulations Group is working to better understand the optical properties of this important material. Postdoctoral researcher Sebastien Hamel is using quantum simulations in a project funded by Intel Corporation and KLA-Tencor to determine the transition point between bulk and nanoscale behavior. Semiconductor manufacturers such as Intel use optical scatterometry equipment made by KLA-Tencor to control the quality of their silicon wafers. In scatterometry, light shone on a wafer at a specific angle bounces off the wafer's features. Measurements of the scattered light reveal critical structural parameters of the wafer's nanoscale features. As feature size shrinks, KLA-Tencor must adjust its algorithms to account for the changed properties of silicon. At 65 nm, the refractive index of silicon is the same as that of bulk silicon, but at 10 nm, its refractive index is unknown. "We know something about the properties of silicon nanowires and have plenty of information on bulk silicon, but we don't know much at all about silicon at dimensions in between," says Hamel. "We need to determine how characteristics such as width, height, and rounded corners will affect the material's properties. The big question is how small can a feature be and still behave like bulk silicon? Or at what point does quantum behavior kick in?" In Hamel's simulations using density functional theory, he looked for the distribution of electrons and the material's dielectric response. He found that a slab of silicon only 2.5 nm thick responds the same as bulk silicon. Silicon nanowires are different because they have so much surface area. For them, 5 nm is the limit for bulklike behavior. In the Intel laboratories, researchers have produced features as small as 5 nm by 25 nm, which Hamel predicts will have a dielectric response the same as bulk material. His next research effort will examine the frequency dependence of the dielectric response—or absorption spectrum—of silicon nanostructures. The KLA-Tencor equipment uses a broad spectrum of light for the optical scatterometry experiments, but some parts of the absorption spectrum may be more sensitive to size than others. "KLA-Tencor wants to determine how long the scatterometry technology will be effective," says Hamel. "At what point will chip manufacturers need a new quality-control technology?"

Manufacturers use optical scatterometry as a quality-control tool to ensure that semiconductor chips are free of defects. (a) A silicon wafer 30 centimeters in diameter is covered with thousands of minute features. (b) Scatterometry measures light as it bounces off a wafer's features. The scattering pattern indicates if a chip has been manufactured precisely as designed.

Quantum Coming of Age The University of California recently selected Williamson as an Executive Management Discovery Fellow. Each year, the university chooses one fellow per campus and offers resources for these people to establish strategic partnerships and collaborations with industry, particularly small businesses, to spur the California economy. That a quantum simulations expert was selected as the Laboratory's Discovery Fellow reflects the growing importance of the quantum world in the private sector.

Hernandez Caballero Indiana M. CI: 15.242.745
Asignacion: EES
Fuente: https://www.llnl.gov/str/May07/Williamson.htm

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