Transcript Silver (Ag)
Silver (Ag) Jeanne Bonner PHYS 275 December 3, 2007 Outline • • • • • Why silver? Properties Uses SERS and silver Sources Why silver? • SERS scattering strongest on silver. • This left me wondering why. General Properties • • • • Chemical Name - Silver Chemical Symbol - Ag Atomic Number - 47 Chemical Series – Transition Metal • Group 11, Period 5, Block D • Appearance – lustrous white metal General Properties • Standard Atomic Weight – 107.8682 g/mol • Electronic Configuration – 1s22s22p63s23p63d104s24p64d105s1 • Electrons per shell – 2, 8, 18, 18, 1 • Silver is a little harder than gold and is very ductile (can be pulled out into wires) and malleable (can be beaten into sheets), being exceeded only by gold and perhaps palladium. Physical Properties • Color – silver • Phase – solid • Density (near r.t.) – 10.49 g·cm−3 • Liquid Density (at m.p.) – 9.320 g·cm−3 • Melting Point – 961.78 °C (1234.93 K, 1763.2 °F) • Boiling Point – 2162 °C (2435 K, 3924 °F) Physical Properties • Heat of fusion – 11.28 kJ·mol-1 • Heat of vaporization – 250.58 kJ·mol-1 • Heat capacity – (25 °C) 25.350 J·mol−1·K−1 Atomic Properties • Crystal Structure – fcc • Lattice constant – a = 4.09 Ǻ • Coordination Number – 12 • Atomic radius – 160 pm Miscellaneous Properties • Magnetic ordering – diamagnetic • Electrical resistivity (lowest of all metals) – (20 °C) 15.87 nΩ·m – ρ = R·(A/l) = E/J = 1/σ – Silver’s greater cost and tarnishability has prevented it from being widely used in place of copper for electrical purposes. – Stable in pure air and water. – Tarnishes when exposed to ozone, hydrogen sulfide, or air containing sulfur. Miscellaneous Properties • Thermal conductivity (highest of all metals) – (300 K) 429 W·m−1·K−1 ) – k = (ΔQ/Δt)(L/(A·ΔT)) – Only non-metal diamond has higher thermal conductivity • Silver is the best reflector of visible light known. – Highest optical reflectivity of the metals; although, aluminum outdoes it in parts of the visible spectrum • Silver is a poor reflector of UV light. Uses • • • • • Jewelry and silverware Amalgams for fittings and fillings Photography Has germicidal effects (not well understood) Electrical and electronic products – printed circuits using silver paints – silver electrical contacts for computer keyboards • Mirrors • Catalyst in oxidation reactions Uses • Coined to produce money • Before the advent of antibiotics, silver was used to prevent infection • Renewed interest recently in using silver as a broad spectrum antimicrobial – Being used with alginate to prevent infections as part of wound management procedures, particularly applicable to burn victims SERS and Silver • Nanoscience deals with the behavior of matter on length scales where a large number of atoms play a role, but where the system is still small enough that the material does not behave like bulk matter. – e.g. a 5-nm gold particle, which contains on the order of 105 atoms, absorbs light strongly at 520 nm, whereas bulk gold is reflective at this wavelength and small clusters of gold atoms have absorption at shorter wavelengths. SERS and Silver • Triumph of classical physics is the intensity and wavelength of the plasmon excitation in nanoparticles is explained with high precision by classical electromagnetic theory. – i.e., solving Maxwell’s equations for light scattering from the appropriate particle structure, with the only materials parameters needed being the frequency dependent dielectric constants of the metal and surrounding material. SERS and Silver • Mie presented a detailed solution for light scattering from a sphere that is very commonly used, even for nonspherical particles. • Quantitatively, the extinction cross section of metal nanoparticles relates to the complex dielectric function of metals and is predicted by Mie theory to be – – – – – – σ = ( 24 π2 R3 εm(3/2)/ λ )( ε” /[ε’ + 2 εm ]2 + ε”2 ) ε’ real and ε” imaginary dielectric constant ε’ negative in visible, when ε’ = - 2 εm plasmon peak occurs εm dielectric constant surrounding medium R nanoparticle radius λ illuminating wavelength SERS and silver • Important developments in nanoscience methods for making nanoparticles has motivated the implementation of computational methods that can describe light scattering from particles of arbitrary shape. • Current numerical methods are capable of doing this for structures several micrometers in size. SERS and Silver • • • • Discrete dipole approximation (DDA) method Intense absorption and scattering exhibited as a result of plasmon excitation Excitation wavelength depends on shape and size of nanoparticle Hot spots responsible for most of dielectric shift that leads to redshifted plasmon wavelengths Fig. 6. Optical properties of silver nanoparticles from DDA calculations. (a) Extinction spectra of silver particles of different shapes (sphere, cylinder, cube, triangular prism, and tetrahedron), all having the same volume as that of an R=50-nm sphere. Contours of the local field |E|2 for sphere (b), cube (c), and tetrahedron (d). These results are adapted from ref. 75. SERS and Silver • Classical electromagnetic theory makes important errors in determining the near-field behavior very close to the nanoparticle’s surface • Ability to quantitatively explain SERS and other measurements is still uncertain. • Qualitative predictions have been verified by experiment and good correspondence between theory and experiment have been found. SERS and Silver • Classical electrodynamics is a continuum approximation that replaces the response of the atoms in a solid to an applied electromagnetic field by the response of a continuous object that is characterized by a dielectric function. • This approximation can break down at the interface between two materials where abrupt dielectric function changes occur. • This is important in the interpretation of SERS measurements. SERS and Silver • In particles that are sufficiently small there can be size dependent dielectric functions – as in quantum dots • Ultimately it will be important to combine quantum mechanics and electrodynamics to describe many optical properties in nanoscale systems – particulary near-field behavior associated with nanoparticles Sources • George C. Schatz, “Using theory and computation to model nanoscale properties,” Department of Chemistry, Northwestern University, Evanston, IL 60208-3113. Article contributed on March 9, 2007 as part of the special series of Inaugural Articles by members of the National Academy of Sciences elected on May 3, 2005. • Richard J. C. Brown, Jian Wang, and Martin J. T. Milton, “Electromagnetic Modelling of Raman Enhancement from Nanoscale Structures as a Means to Predict the Efficacy of SERS Substrates,” Journal of Nanomaterials, July 31, 2007. • Neil W. Ashcroft and N. David Mermin, Solid State Physics, Thomson Learning Inc., 1976. • Raman Spectroscopy Solutions [Internet]. [cited 2007 Nov 28]. Available from: http://www.andor.com/chemistry/?app=64 • G. P. Wiederrecht, “Near-field optical imaging of noble metal nanoparticle,” Eur. Phys. J. Appl. Phys, volume 28, pages 3-18, 2004. Sources • Wikipedia contributors. Silver [Internet]. Wikipedia, The Free Encyclopedia; [cited 2007 Nov 28]. Available from: http://en.wikipedia.org/wiki/Silver . • Wikipedia contributors. Resistivity [Internet]. Wikipedia, The Free Encyclopedia; [cited 2007 Nov 28]. Available from: http://en.wikipedia.org/wiki/Electrical_resistivity . • Wikipedia contributors. Electrical conductivity [Internet]. Wikipedia, The Free Encyclopedia; [cited 2007 Nov 28]. Available from: http://en.wikipedia.org/wiki/Electrical_conductivity . • Wikipedia contributors. Thermal conductivity [Internet]. Wikipedia, The Free Encyclopedia; [cited 2007 Nov 28]. Available from: http://en.wikipedia.org/wiki/Thermal_conductivity • Metallic Structures [Internet]. [cited 2007 Nov 28]. Available from: http://www.chemguide.co.uk/atoms/structures/metals.html .