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

Fabrication of novel MEMS-based

polymer electrolyte fuel cell architectures with catalytic electrodes supported on porous SiO2

 

 

Kyong-Bok Min, Shuji Tanaka and Masayoshi Esashi

J. Micromech. Microeng. 16 (2006) 505–511

 

Purpose

Two types of micro polymer electrolyte fuel cells (PEFC) were fabricated on silicon and glass substrates using MEMS technology. The study hoped to confirm that the fabricated PEFC’s could provide a low cost and easily manufactured power source for low power (1-100 mW) applications such as micro-sensors.

 

Methods and Procedures

·         Two fuel cell structures were proposed: ‘alternatively inverted configuration’ and ‘coplanar configuration’. Both structures consist of  silicon substrates (containing porousSiO2 layers, platinum-based catalytic electrodes and gas feedHoles), glass substrates (containing micro-gas channels and gas ports)  and  a PEM.

 

·         The silicon and glass substrates were created with MEMS technologies such as, plasma etching, deep reactive ion etching, and photolithography.

 

·         The catalytic electrode was made out of Pt/Ti which was sputter deposited on top of porous SiO2.

Key Findings

·         The alternatively inverted configuration fuel cells obtained a maximum power density of 90 μW cm−2 and then gradually deteriorated over time. This drop off was due to the PEM absorbing water near the cathode and causing the PEM to peel off from the catalytic electrodes. It is vital to fuel cell performance to keep stable contact between the PEM and the catalytic electrodes.

 

·         The coplanar configuration fuel cells obtained a maximum density of 40 μW cm−2. This configuration also had deterioration over time and again was attributed to the peeling of the PEM.

 

·         A CV of a Pt/Ti film on top of porous SiO2 showed extremely weak hydrogen adsorption and desorption peaks. This would strongly suggest that the Pt/Ti electrodes have low catalytic activity and prevents higher power densities from being achieved in the fuel cells.

 

Definitions

CV (Cyclic voltammetry): a potentiodynamic electrochemical measurement used to study electrochemical properties.

 

Direct synthesis of high-quality single-walled carbon

nanotubes on silicon and quartz substrates

Yoichi Murakami, Yuhei Miyauchi, Shohei Chiashi, Shigeo Maruyama

Chemical Physics Letters 377 (2003) 49–54

Purpose

A new dip-coating technique for synthesizing high-quality SWNT’s on silicon and quartz substrates has been developed. This new method does not require support/underlayer material and is more cost effective and easier to implement than previous techniques.

Methods

Either n-type Silicon or fused quartz was submerged into a prepared metallic acetate solution for 10 min. The substrate was then dried, placed in a 400°C furnace for 5 min., and finally underwent the CVD process. The CVD process was performed at varying temperatures and used the reaction of ethanol vapor to synthesize SWNT’s on the substrate surface. The synthesized SWNTs were characterized by FE-SEM and micro-Raman scattering measurements.

 

Key Findings

·         The amount of synthesized SWNT’s was quantified by using the height ratio of the G-band to the Si-derived peak for Raman spectra. This analysis showed that the optimal temperature for SWNT synthesis was 800°C, as this is where the largest ratio occurred.

 

·         The high quality of the SWNT’s was confirmed by the high G/D ratio in the Raman spectra.

 

·         The diameter of the SWNT’s was estimated from the RBM Raman shift and found to be between 1.1 to 1.7 nm.

 

·         SEM images show a web-like layer of SWNT’s atop the substrate surface.

 

·         TEM images show that the SWNT are free of metal debris and multi-walled nanotubes.

 

 

RBM: Radial Breathing mode – part of the Raman spectra that can characterize certain features of SWNT’s (such as diameter).

D: disorder induces mode – part of the Raman spectra

Bioadhesion of Polymers for BioMEMS

Dharma R. Tokachichu and Bharat Bhushan

IEEE TRANSACTIONS ON NANOTECHNOLOGY, VOL. 5, NO. 3, MAY 2006

Purpose of Study

Determine the affect a perfluorodecyltriethoxysilane (PFTDES) monolayer coating has on bioadhesion.

 

Methods

Polymer surfaces of poly(methylmethacrylate) (PMMA) and poly(dimethylsiloxane) (PDMS) were oxygen plasma treated and then coated with a self-assembled monolayer of perfluorodecyltriethoxysilane. Water contact angle measurements were taken with a contact angle goniometer and adhesion measurements were taken with a commerical AFM. Adhesion measurements were taken in ambient or in the presence of PBS with either a bare silicon-nitride tip or a silicion-nitride tip dipped in FBS.

Results

• The perfluorodecyltriethoxysilane monolayers resulted in a higher contact angle for water.
• Virgin PDMS, due to its higher electrostatic charge, has higher adhesion than virgin PMMA in ambient.
• The adhesion values of the PFDTES-coated surfaces were the same for both PMMA and PDMS for all tests, meaning surface chemistry is not affected by the substrate used.
• PFDTES-coated surfaces have very low bioadhesion.

Oxygen Plasma Treatment- O2 is split into radicals and ions, which are effective at removing organic contaminants on your surface.

FBS=Fetal Bovine Serum-Contains a diverse amount of proteins and other essential biological compounds for cell growth.  Is the most commonly used serum for culturing cells.