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A tough coat for silicon

March 23, 2017
The Agency for Science, Technology and Research (A*STAR)
Supercritical carbon dioxide delivers protective molecules to semiconductor surfaces, report researchers in a new article.

Supercritical carbon dioxide delivers protective molecules to semiconductor surfaces.

A simple, green method that applies a protective coating to semiconductors could help to develop these materials for many applications, from batteries to biosensors.

Silicon forms an oxide layer on its surface when exposed to air or moisture, which can detract from its electronic properties. Adding a 'skin' of molecules to the silicon can provide a physical barrier that prevents oxidation, but forming these monolayers can be tricky, requiring an inert atmosphere and long processing times, or demand the use of potentially harmful organic solvents.

Sreenivasa Reddy Puniredd of the Agency for Science, Technology and Research (A*STAR) Institute of Materials Research and Engineering, Singapore, and colleagues have now developed a new way to deliver the protective molecules using supercritical carbon dioxide (scCO2). Carbon dioxide is converted to scCO2 under high pressure, when it becomes a free-flowing liquid that is chemically inert, inexpensive, and more environmentally-friendly than traditional solvents.

The researchers used scCO2 to carry molecules called alkylthiols, which contain long carbon chains 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.

To apply the coating they used alkylthiols containing between seven and 18 carbon atoms to coat silicon, germanium, and silicon nanowires. Each procedure took a few hours, and produced monolayers between 1.6 nanometers and 2.3 nanometers thick that resisted wear and repelled water. The greatest effect was seen for the longest alkylthiol chains.

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. "The increase in stability was expected, but such long-term stability was a surprise," says Puniredd.

Silicon nanowires are being tested for a range of biological applications, including biosensors and antibacterial surfaces. Although fragile and easily damaged by other monolayer formation methods, the silicon nanowires were undamaged by the scCO2 process, allowing the researchers to test how they interacted 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. This is probably because the cells' proteins could not latch on to the monolayer's long carbon chains.

"This scCO2 technology can be adopted for many kinds of inorganic surface modification," says Puniredd. "The technology is not only scalable, but also enhances the quality and stability of the film. It can potentially replace billions of pounds of organic solvents used every year in thin-film fabrication and cleaning applications."

The A*STAR-affiliated researchers contributing to this research are from the Institute of Materials Research and Engineering. For more information about the team's research, please visit the Materials Processing & Characterisation department webpage (link below).

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Materials provided by The Agency for Science, Technology and Research (A*STAR). Note: Content may be edited for style and length.

Journal Reference:

  1. Bhavesh Bhartia, Sreenivasa Reddy Puniredd, Sundaramurthy Jayaraman, Chinnasamy Gandhimathi, Mohit Sharma, Yen-Chien Kuo, Chia-Hao Chen, Venugopal Jayarama Reddy, Cedric Troadec, Madapusi Palavedu Srinivasan. Highly Stable Bonding of Thiol Monolayers to Hydrogen-Terminated Si via Supercritical Carbon Dioxide: Toward a Super Hydrophobic and Bioresistant Surface. ACS Applied Materials & Interfaces, 2016; 8 (37): 24933 DOI: 10.1021/acsami.6b06018

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The Agency for Science, Technology and Research (A*STAR). "A tough coat for silicon." ScienceDaily. ScienceDaily, 23 March 2017. <>.
The Agency for Science, Technology and Research (A*STAR). (2017, March 23). A tough coat for silicon. ScienceDaily. Retrieved May 25, 2017 from
The Agency for Science, Technology and Research (A*STAR). "A tough coat for silicon." ScienceDaily. (accessed May 25, 2017).