Monthly Archives: February 2010

NPs dividing lines

Nanoparticles feature size (nm) at which changes may be expected :

Catalytic activity  <5 nm

Making hard magnetic materials soft <20

Producing refractive index changes  <50

Producing super paprmagnetism and others  <100

Electromagnetic phenomena

Producing strengthening and toughening  <100

Modifying hardness and plasticity     <100

Source: Camargo P H, Satyanarayana K G, Wypych F, Nanocomposites: senthesis, structure, properties, and new application opportunities, Materials Research, Vol 12, No 1, Sao Carls, (2009)

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Raman Scattering- spectroscopy

Vibrational spectroscopy involves photons that induce transitions between vibrational states in molecules and solids, typically in the infrared IR frequency range from 2 to 12 X 10^13 Hz.

Two cases of frequency difference is given by  V inc – V emit  = Δ N  Vº

Δ N = ± 1  is obeyed since the same IR selection rule

V inc > V emit corresponding to a Stokes line

V inc < V emit for an anti-Stokes line.

Infrared active vibrational modes arise from a change in the electric dipole moment µ of the molecule, while Raman active vibrational modes involve a change in the polarizability P = µ ind / E, where the electric vector E of the incident light induces the dipole moment µ ind in the sample. Some vibrational modes are IR active, that is measurable by IR spectroscopy, and some are Raman active.

FTIR and Raman spectroscopy measure the absorption of radiation by high-frequency (ie optical branch) phonon vibrations, and are also sensitive to the presence of particular chemical groups such as hydroxyl (-OH) methyl (-CH3) imido (-NH) and amino (-NH2). Each of these groups absorbs IR radiation at a characteristic frequency, and the actual frequency of absorption varies somewhat with the environment.

What is traditionally considered as Raman scattering of light or Raman spectroscopy, is spectroscopy in which the phonon vibration corresponding to the energy difference of incident light and emitted light, is an optical phonon of the type with a frequency of vibration in the IR region of the spectrum, corresponding to about ~ 400 cm-1, or a frequency of ~ 1.2 x 10^13 Hz. When a low frequency acoustical phonon is involved in the scattering of Raman type, then the process is referred to as Brillon scattering. Acoustic phonons can have frequencies of vibration or energies that are a factor of 1000 less than those of optical phonons. Typical values are ~ 1.5 x 10^10 Hz or ~ 0.5 cm-1. Brillon spectroscopy involves both Stokes and anti-Stokes lines, as does Raman spectroscopy.

For an infinitely long linear NACL lattice each atom vibrates with the same frequency (equation 7.3 page 198). However this is not true for a short chain. The results of calculation of the force constant of the Cl-  ion as a function of its distance from the centre of the chain to the end for a 20-ion chain show that except for the end ion, the force constant gradually increases as the ions get closer to the end of the chain. This means the frequencies increase and the amplitudes of vibration decrease as the ions get closer to the end of the linear lattice. Each Cl- ion in the chain has a different force constant and therefore a different vibrational frequency (as do the Na+ ions), unlike the infinite chain. A consequence of this is that a spectroscopic measurement such as by Raman or IR will have broader lines in the nanosized materials compared to the bulk materials (below around 15 nm). The broadening is a result of overlap of lines from the slightly different frequencies of the different atoms or molecules in the materials having nanometer dimensions.

There is an important difference between a one dimensional nanostructure and a two or three dimensional structure. In the one dimensional case the number of atoms or ions at the end of the chain does not change as the chain gets shorter whereas in the higher dimensional nanostructures the  number of atoms on the surface increases as the material  becomes smaller in the nanometer regime ~ below 10 nm.

Source: Owen F J, Poole C P, The physics and chemistry of nanosolids, pp 66-72, and pp 197-207, WILEY, (2008)

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Affinity Biosensors: Self Assembled Monolayers

Two model systems are common, one is based on thiols coupling (general formula R-SH) on Au(111) thin film with n-archetypal case of alkanethiols (both long and short-chain), and another on irreversible silane coupling to SiO2 thin films grown on Si substrates (Myhra 2004, Schreiber 2004).

Myhra S, Crossley A, Barsoum M W, Crystal chemistry from XPS analysis of carbide derived Mn+1 AXn (n=1) nano laminate compounds, Journal of physics and chemistry of solids Vol 63, pp 2063-2068 (2002)

Myhra S, A review of enabling technologies based on scanning probe microscopy relevant to bioanalysis, Biosensors and Bioelectronics Vol 19, pp 1345–1354 (2004)

Schreiber F, Structure and growth of self-assembling monolayers, Progress in Surface Science, Vol 65, Issues 5-8, pp 151-257 (2000)

Schreiber, F. Structure of thiol-based self-assembled monolayers, in Encyclopedia of Materials: Science and Technology, Elsevier, Oxford, pp.9323–9332 (2001)

Schreiber F, (Oxford Univ); Self assembled monolayers: from simple model system to biofunctionalized interfaces, J. of Physics: Condensed. Matter 16; R881–R900 (2004)

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Atomic force microscopy: first image of molecule showing chemical bonds

Pentacene: Image IBM and Science












Atomic force microscopy (AFM), measures the attractive force between atoms in the probe and the target. The image is created by bumping the probe over the atoms of the molecule – much in the way we might feel our way around in a dark bedroom. Another method Scanning tunneling microscopy STM uses such a probe to measure the charge density associated with individual atoms.

Both methods build up a picture of a target’s surface and should be suitable for imaging individual molecules. But they have not been able to approach the detail of TEM.

Atomic force microscope was used to image the molecule in unprecedented resolution, published for the first time in New Scientist issue of 28 August 2009.

The portrait of pentacene, an organic molecule consisting of five benzene rings, shows off the chemical bonds between the carbon and hydrogen atoms. It may seem a somewhat surprising first, since atoms have been imaged for decades.

The earliest pictures of individual atoms were captured in the 1970s by blasting a target – typically a chunk of metal – with a beam of electrons, a technique known as transmission electron microscopy (TEM)…. But strange though it might seem, imaging larger molecules at the same level of detail has not been possible – atoms are robust enough to withstand existing tools, but the structures of molecules are not.

Rather than relying on an optical system to produce pictures, atomic force microscopes use a probe that narrows to an atomic-scale tip, and measures the forces of attraction between the tip and the molecule’s components. Lead researcher Leo Gross was able to get the shot of pentacene because he stuck a molecule of carbon monoxide (CO) on the tip of the probe.

Though, this is not the first-ever image of a single molecule, that has been possible for quite some time. This IS, however, the first ever AFM image in which the chemical structure of a molecule is visible (you can see where the atoms are). There are many STM images with equally good resolution, but they map electron density of states so you get images of orbitals, whereas this is a map of where chemical bonds would form (where the sticks would attach in a ball and stick model). At higher temperatures, molecules wiggle around more so it’s harder to image them with such clarity. The sensor has to be focused in order to raster scan the surface (it’s like reading braille rather than taking a picture).





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