Towards molecular electronics with large-area molecular junctions

 

Hylke B. Akkerman, Paul W. M. Blom, Dago M. de Leeuw2& Bert de Boer

 

NATURE, Vol 441, 4 May 2006 (69-71)

 

Purpose of Study

 

The paper demonstrates a high yield method to manufacture stable molecular junctions with diameters up to 100 µm. The technique is simple, potentially low-cost, and can be scaled up easily, making the process a potential stepping stone to practical molecular electronics.

 

Methods

 

To create the junctions, gold electrodes are first vapor-deposited on silicon. A photoresist layer is then added by spin-coating, which allows holes (10-100 µm diameter) to be created by photolithography. The substrate is then submerged in an alkane dithiol solution, where the thiol molecules can reach the gold electrode through the photoresist holes and creates circular regions of SAMs on top of the gold electrode. After the self-asssembly, a conductive polymer, poly(3,4-ethylenedioxythiophene) stabilized with poly(4-styrenesulphonic acid) (PEDOT:PSS), is spin coated on top of the SAM, creating a ~ 90 nm thick coating. The junction is then complete once the top gold electrode is vapor-deposited through a shadow mask. The final step during fabrication is then to simply etch away the exposed PEDOT:PSS.

 

To examine the performance of the junctions, current vs. applied voltage (I-V) is measured and current density and current per molecule is calculated.

 

Key Findings

 

·         The yield of functional molecular junctions is larger than 95%  for all hole diameters. The high yield is attributed to the large difference in surface tension between the PEDOT:PSS and the alkane thiols.

 

·         Monothiol-based molecular junctions can be created and exhibit similar I-V behavior, but they are more difficult to use because their hydrophobic methyl groups.

 

·         The current density decreases with increased chain length of the alkane dithiol molecule, which demonstrates the transport mechanism is through bond tunneling.

 

·         The current density dependence of the molecular junctions on voltage and temperature is identical to that of nanopore devices.

 

·         The current per molecule in the large-are molecular junctions are comparable to nanopore junctions.

 

·         The junctions showed no degradation of performance after 75 days.

 

 

Posted by Steve Vilt