Re: News: Molecular transistor discovery



Zigoteau wrote:
> Hi, Tim and Fred,
>
>
> > Some more details:
> > http://www.science.ualberta.ca/nav02.cfm?nav02=36033&nav01=11471
>
>
> If you hurry, the full text of the paper may be downloaded for free
> from the Nature site, www.nature.com. That's Piva et al., Nature 435
> (2005) 658.
>
> Although Wolkow claims it's a nanotransistor, it looks far further from
> a real technology than the nanotube transistor does. Nanotube
> transistors just lack the technology for producing large ordered arrays
> of the things. Wolkow's transistors lack a lot more.
>
> > I wonder if they observed quantized conductance.
>
> See for yourself, the answer is no. And why would you expect
> conductance to be quantized in a transistor? Conductance quantization
> occurs in nanowires, meaning nanostructures with a metallic
> bandstructure, i.e. a nonzero density of propagating modes at the Fermi
> level. The integer "quantum number" is the number of propagating modes
> in the energy range between the Fermi levels of the electrodes on
> either side. The assumption underlying Landauer's prediction of
> quantization is that these modes show 100% transmission, with zero
> coefficient of electron reflection at the interfaces with the two
> electrodes.
>
> Transistors are made from semiconductors. Their whole point is the
> possibility of control of current flow, and this can only be achieved
> if the coefficient of transmission of a given mode may vary. This is
> achieved by arranging for the modes to be evanescent under some bias
> conditions.
>
> Cheers,
>
> Zigoteau.

Experimentally, the paper did not report any I/V or dI/dV along the
length of the molecule. Looks like the set up was not there. Figure 5a
is the graph of tunnelling current vs tip bias, not exactly the same
thing.

Theoretically, the observation is attributed to a single molecule
conducting, which I'd think would correspond to a nanowire's
description (i.e., limit of mode number ->1). So an observation of
quantized conductance (or lack thereof) would be significant and
interesting. In fact, quantized conductance has been observed
experimentally in an atomic-scale conductor recently as well (Nature
Jan 6 2005, p. 47).

Regards,
Fred


.

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