Page numbers please

If you are writing a paper and you reference a book, with regard to a specific concept or result, you should give the relevant page number of the book, so readers can find the relevant pages in the book.

I concede that certain times this may be unnecessary because one can find the relevant section using the table of contents or index.

However, I often get frustrated when I can not find it.

Gold may rust (and increase its value)

G.U. Kulkarni gave a really nice talk at the conference from which I learnt a few fascinating things about gold nanoparticles. He is co-author of a nice concise review, Size Dependent Chemistry: Properties of Nanocrystals.

Bulk gold surfaces are inert because the dissociative chemisorption energy of oxygen is positive unlike all other metallic elements [Nolan, Accounts Chem. Res. 1998]. However, this energy can be negative for nanoparticles.

Another key property of gold nano-particles is that there is a plasmon collective mode associated with the surface electrons and lies in the visible.

Heterogeneous catalysis with nanoparticles can be enhanced because of the large fraction of atoms on the surface. e.g., for 100 atoms, 50% are on the surface.

Pollution control in cars is achieveed by a catalytic converter which converts CO to CO2 using Pt-Pd, Pt-Rh catalysts.

Problems: expensive materials and bed has to be heated, leading to a search for alternative catalysts.

Au/TiO2 [gold nanoparticles on titania surfaces] is a good catalyst. Hurata (1984) made this revolutionary discovery, which was unexpected because gold was expected to be inert.

25 years later, it still seems anatase TiO2 is the best substrate. A key property is multiple oxidation states of Ti that are possible in titania.

Another key is the strong coupling of delocalised electrons on metal nano-particle surface and oxygen atom on titania surface. [see DFT paper, ref?]

It seems optimum size of gold is about 2-3nm.

Why does it work? Not just high surface area but also electronic structure of gold nano-particle which can undergo a metal-insulator transition as a function of size.

Effective Hamiltonians for methine dyes

Here are the slides of the talk I gave today at the conference at JNCASR in Bangalore.

Deconstructing charge transport in complex materials III

Previously I have written posts about the important issue of understanding (and enhancing) the charge mobility of molecular materials. Key questions include:

What determines the relative magnitude of electron and hole mobilities?

How does mobility depend on the intermolecular separation and relative orientation?

I have tried to emphasize the charge transport is largely incoherent and that consequently a quantitative and qualitative understanding can be achieved via Marcus-Hush electron transfer theory [which is essentially equivalent to Holstein's small polaron theory]. I really don't think these points are appreciated enough (or at all) by people working on these materials.

This week at the conference I was delighted to become aware of the nice work of the group of Swapan Pati (one of the organisers) on this problem. [Their work precedes my rantings on this blog].

In this 2007 J. Chem. Phys. paper they take such approach and perform electronic structure calculations to calculate the two key physical quantities H_DA, (the matrix element for charge transfer = the Huckel parameter t) and the reorganisation energy for both electron and hole transport in different single crystal polymorphs of benzene and napthalene. H_DA falls off rapidly with distance and can vary significantly with the relative orientation and position of aromatic rings. The relative mobility of electrons and holes is determined by the relative magnitude of both H_DA and the reorganisation energy. Contrary to the standard dogma there are situations where the electron mobility is larger than the hole mobility.

Mohakud and Pati also have a J. Materials Chemistry paper applying a similar approach to octathio[8]circulene.

To me, important open questions include:

• How will these results be modified by the screening and polarisation associated with bulk crystal? [I suspect H_DA may not change much but the reorganisation energy may increase significantly].
• How does the experimental activation energy for the mobility compare to 1/4 of the reorganisation energy?
• Can this approach be extended to describe the observed field-dependent mobility? [see for example this 2008 PRL by Emin].

A simple model for solvent effects in chemically complex charge transfer dyes

Anna Painelli (Parma, Italy) gave a really nice talk,

How molecular functional materials respond to the environment: from solvation to cooperativity.

She illustrated how simple model Hamiltonians can capture essential photophysical properties of complex molecular systems. The first system was a donor-acceptor (D-A) molecule in a solvent.

The simple model describes two electronic states (D-A and D+-A-), one vibrational mode, and the solvent which is described by one classical co-ordinate and a key parameter (the polarisability).

This nice paper from J. Phys. Chem A. in 2002 shows how this model gives a good quantitative description of the absorption and emission spectra for several different dyes in three different polar solvents.

This 2006 JACS paper extends the model to describing "quadrupolar" molecule, which has the structure DAD . The model involves 3 electronic states, 2 vibrations, plus the solvent polarisation.

I am keen to apply this model for solvent effects to methine dyes, such as the Green Flourescent Protein chromophore, which Seth Olsen and I have argued is nicely described by a three state model.

Estimating the conductivity of DNA

Rosa Di Felice gave an interesting talk about computational studies of the electronic structure of DNA based systems. A combined experimental-theoretical review is here.

A key quantity for calculating the electron transfer rate (and conductivity) is the transfer integral (tight binding hopping integral). Rosa mentioned this nice methodological paper from J. Chem. Phys. which shows how to extract this parameter from methods such as DFT.

A basic unsolved problem about phase transition kinetics

It is always interesting to learn about a basic science problem which is unsolved.

Biman Bagchi gave a really nice talk yesterday, Nucleation, growth and metastability at large supersaturations.

Pure water remains liquid to -40 degrees celcius. This is supercooling. The key to formation of the solid phase is the presence of nucleation centres. At what rate will they grow?

Formation of the bulk phase is favoured thermodynamically but formations of small regions of solid are unfavourable because of their surface free energy.

Classical nucleation growth theory predicts a rate that is too small by as much as 10 orders of magnitude, compared to experiment!

["This is an embarrassment to natural science"]

Most textbooks overlook this major shortcoming.

Nuclei larger than a critical radius will grow, but there is an energy barrier to formation of these larger nuclei. The classical theory grossly overestimates this barrier. The classical theory is a mean-field theory.

This PRL by Bagchi's group gives a nice discussion of problem and one of his contributions. It includes the figure below. A key thing it shows contradicts the classical model for which the growth of the liquid is assumed to occur through the growth of a single cluster.