Surfactant Ligand Removal and Rational Fabrication of Inorganically Connected Quantum Dots

http://dx.doi.org/10.1021/nl202892p

A novel method is reported to create inorganically connected nanocrystal (NC) assemblies for both II–VI and IV–VI semiconductors by removing surfactant ligands using (NH₄)₂S. This surface modification process differs from ligand exchange methods in that no new surfactant ligands are introduced and the post-treated NC surfaces are nearly bare. The detailed mechanism study shows that the high reactivity between (NH₄)₂S and metal–surfactant ligand complexes enables the complete removal of surfactant ligands in seconds and converts the NC metal-rich shells into metal sulfides. The post-treated NCs are connected through metal–sulfide bonding and form a larger NCs film assembly, while still maintaining quantum confinement. Such “connected but confined” NC assemblies are promising new materials for electronic and optoelectronic devices.

DOI: 10.1021/nl202892p

Publication Date (Web): October 19, 2011

Value Density of US Coins

A coin machine breaks open and you have to fill your pockets with coins. There are piles of quarters, dimes, nickels, and pennies. Which coins do you start grabbing? Well, assuming you have a good belt to keep your pants up, you want to pick the coin with the largest value density. What I mean by that is, you want the coin that has the most value for its given volume. That way you can fit the most money into your pockets. Although I suspected the dime and the quarter were great candidates, I wasn't sure which coin would win. So I ran the numbers.

The dimensions of the coins are readily available, and assuming they are cylinders, the volumes are easily calculated to be: quarter - 4.74, dime - 2.52, nickel - 3.53, and penny 2.85 (cm^3). The value densities are therefore: quarter - 5.28, dime - 3.97, nickel - 1.42 and penny - 0.35 (cents / cm^3).

So in conclusion, fill your pockets with quarters. You will be able to hold 33% more money than the guy grabbing dimes. And the guy grabbing pennies is completely out of his mind, unless the quarters are pre-1982 and he fully understands the value of copper - I still recommend quarters.

If we have the option taking half dollar or the current dollar coins, then grab those. Their volumes are: dollar - 5.51 and half dollar - 7.36 (cm^3). Their value densities are: dollar - 18.14 and 6.80 - half dollar (cents / cm^3). However since these two coins are not that common, we will consider such a hypothetical opportunity to be unrealistic.