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John O | March 2017

Researchers add coating to silicon semiconductors to enhance conductivity


a team of researchers from the agency for science, technology and research (a*star) institute of materials research and engineering developed a monolayer coating using supercritical carbon dioxide (scco2) that prevents an oxide layer forming on semiconductors, which impacts its electrical conductivity.

 

astar_600

a*star reserachers prevented oxide layers on semiconducters with supercritical carbon dioxide.
(wikimedia commons)

 

according to a report on the a*star website, srco2 is formed when carbon dioxide is placed under high pressure and it “becomes a free-flowing liquid that is chemically inert, inexpensive, and more environmentally-friendly than traditional solvents.”

 

the srco2 carries alkylthiols, molecules with long carbon chains and with a sulfur atom at one end. “sulfur forms a stable bond with silicon, while the water-repelling carbon chains make a tightly-packed skin on silicon’s surface,” the article explained.

 

the researchers applied the alkythiols to silicon, geranium, and silicon nanowires to produce monolayers ranging from 1.6-2.3 nanometers thick that were able to resist wear and repel water. the longer the chain of carbon was then the greater the effect of the coating.

 

according to the researchers, “the monolayers also protected the surface from oxygen for more than 50 days; those prepared using conventional solvents were typically stable for less than seven days.”

 

silicon nanowires, in particular, are being tested for a variety of appications, including biosensors and antibacterial surfaces. creating nanowires is a delicate process to begin with and adding the monolayers through the srco2 process proved more successful than previous methods that only increased the damage to the nanowires. this allowed the researchers to test the interaction of nanowires with human liver cells.

 

“those protected by the 18-carbon alkylthiol significantly reduced cell growth on the nanowires, compared with unprotected nanowires or a flat silicon surface,” the article said. “this is probably because the cells’ proteins could not latch on to the monolayer’s long carbon chains.”

 

the research was recently published in applied materials & interfaces. the abstract from the report read:

 

“oxide-free silicon chemistry has been widely studied using wet-chemistry methods, but for emerging applications such as molecular electronics on silicon, nanowire-based sensors, and biochips, these methods may not be suitable as they can give rise to defects due to surface contamination, residual solvents, which in turn can affect the grafted monolayer devices for practical applications.

 

“therefore, there is a need for a cleaner, reproducible, scalable, and environmentally benign monolayer grafting process. in this work, monolayers of alkylthiols were deposited on oxide-free semiconductor surfaces using supercritical carbon dioxide (scco2) as a carrier fluid owing to its favorable physical properties.

 

“the identity of grafted monolayers was monitored with fourier transform infrared (ftir) spectroscopy, high-resolution x-ray photoelectron spectroscopy (hrxps), xps, atomic force microscopy (afm), contact angle measurements, and ellipsometry. monolayers on oxide-free silicon were able to passivate the surface for more than 50 days (10 times than the conventional methods) without any oxide formation in ambient atmosphere.

 

“application of the scco2 process was further extended by depositing alkylthiol monolayers on fragile and brittle 1d silicon nanowires (sinws) and 2d germanium substrates. with the recent interest in sinws for biological applications, the thiol-passivated oxide-free silicon nanowire surfaces were also studied for their biological response.

 

“alkylthiol-functionalized sinws showed a significant decrease in cell proliferation owing to their superhydrophobicity combined with the rough surface morphology. furthermore, tribological studies showed a sharp decrease in the coefficient of friction, which was found to be dependent on the alkyl chain length and surface bond.

 

“these studies can be used for the development of cost-effective and highly stable monolayers for practical applications such as solar cells, biosensors, molecular electronics, micro- and nano- electromechanical systems, antifouling agents, and drug delivery.”

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