Tag Archives: thin film transistors

Transmetalation, substituting one set of metal atoms for another set

Leave a reply

Transmetalation bears a resemblance of sorts to transmutation. While the chemists from the University of Oregon aren’t turning lead to gold through an alchemical process they are switching out individual metal atoms, aluminum for indium. From a July 21, 2014 news item on ScienceDaily,

The yield so far is small, but chemists at the University of Oregon have developed a low-energy, solution-based mineral substitution process to make a precursor to transparent thin films that could find use in electronics and alternative energy devices.

A paper describing the approach is highlighted on the cover of the July 21 [2014] issue of the journal Inorganic Chemistry, which draws the most citations of research in the inorganic and nuclear chemistry fields. [emphasis mine] The paper was chosen by the American Chemical Society journal as an ACS Editor’s Choice for its potential scientific and broad public interest when it initially published online.

One observation unrelated to the research, the competition amongst universities seems to be heating up. While journals often tout their impact factor, it’s usually more discreetly than in what amounts to a citation in the second paragraph of the university news release, which originated the news item.

The July 21, 2014 University of Oregon news release (also on EurekAlert), describes the work in more detail,

The process described in the paper represents a new approach to transmetalation, in which individual atoms of one metal complex — a cluster in this case — are individually substituted in water. For this study, Maisha K. Kamunde-Devonish and Milton N. Jackson Jr., doctoral students in the Department of Chemistry and Biochemistry, replaced aluminum atoms with indium atoms.

The goal is to develop inorganic clusters as precursors that result in dense thin films with negligible defects, resulting in new functional materials and thin-film metal oxides. The latter would have wide application in a variety of electronic devices.

“Since the numbers of compounds that fit this bill is small, we are looking at transmetelation as a method for creating new precursors with new combinations of metals that would circumvent barriers to performance,” Kamunde-Devonish said.

Components in these devices now use deposition techniques that require a lot of energy in the form of pressure or temperature. Doing so in a more green way — reducing chemical waste during preparation — could reduce manufacturing costs and allow for larger-scale materials, she said.

“In essence,” said co-author Darren W. Johnson, a professor of chemistry, “we can prepare one type of nanoscale cluster compound, and then step-by-step substitute out the individual metal atoms to make new clusters that cannot be made by direct methods. The cluster we report in this paper serves as an excellent solution precursor to make very smooth thin films of amorphous aluminum indium oxide, a semiconductor material that can be used in transparent thin-film transistors.”

Transmetalation normally involves a reaction done in organic chemistry in which the substitution of metal ions generates new metal-carbon bonds for use in catalytic systems and to synthesize new metal complexes.

“This is a new way to use the process,” Kamunde-Devonish said, “Usually you take smaller building blocks and put them together to form a mix of your basic two or three metals. Instead of building a house from the ground up, we’re doing some remodeling. In everyday life that happens regularly, but in chemistry it doesn’t happen very often. We’ve been trying to make materials, compounds, anything that can be useful to improve the processes to make thin films that find application in a variety of electronic devices.”

The process, she added, could be turned into a toolbox that allows for precise substitutions to generate specifically desired properties. “Currently, we can only make small amounts,” she said, “but the fact that we can do this will allow us to get a fundamental understanding of how this process happens. The technology is possible already. It’s just a matter of determining if this type of material we’ve produced is the best for the process.”

Here’s a citation for and a link to the paper,

Transmetalation of Aqueous Inorganic Clusters: A Useful Route to the Synthesis of Heterometallic Aluminum and Indium Hydroxo—Aquo Clusters by Maisha K. Kamunde-Devonish, Milton N. Jackson, Jr., Zachary L. Mensinger, Lev N. Zakharov, and Darren W. Johnson. Inorg. Chem., 2014, 53 (14), pp 7101–7105 DOI: 10.1021/ic403121r Publication Date (Web): April 18, 2014

Copyright © 2014 American Chemical Society

This paper appears to be open access (I was able to view the HTML version when I clicked).

Nanotechnology-enabled robot skin

2 Replies

We take it for granted most of the time. The ability to sense pressure and respond to appropriately doesn’t seem like any great gift but without it, you’d crush fragile objects or be unable to hold onto the heavy ones.

It’s this ability to sense pressure that’s a stumbling block for robotmakers who want to move robots into jobs that require some dexterity, e.g., one that could clean yours windows and your walls without damaging one or failing to clean the other.

Two research teams have recently published papers about their work on solving the ‘pressure problem’. From the article by Jason Palmer for BBC News,

The materials, which can sense pressure as sensitively and quickly as human skin, have been outlined by two groups reporting in [the journal] Nature Materials.

The skins are arrays of small pressure sensors that convert tiny changes in pressure into electrical signals.

The arrays are built into or under flexible rubber sheets that could be stretched into a variety of shapes.

The materials could be used to sheath artificial limbs or to create robots that can pick up and hold fragile objects. They could also be used to improve tools for minimally-invasive surgery.

One team is located at the University of California, Berkeley and the other at Stanford University. The Berkeley team headed by Ali Javey, associate professor of electrical engineering and computer sciences has named their artificial skin ‘e-skin’. From the article by Dan Nosowitz on the Fast Company website,

Researchers at the University of California at Berkeley, backed by DARPA funding, have come up with a thin prototype material that’s getting science nerds all in a tizzy about the future of robotics.

This material is made from germanium and silicon nanowires grown on a cylinder, then rolled around a sticky polyimide substrate. What does that get you? As CNet says, “The result was a shiny, thin, and flexible electronic material organized into a matrix of transistors, each of which with hundreds of semiconductor nanowires.”

But what takes the material to the next level is the thin layer of pressure-sensitive rubber added to the prototype’s surface, capable of measuring pressures between zero and 15 kilopascals–about the normal range of pressure for a low-intensity human activity, like, say, writing a blog post. Basically, this rubber layer turns the nanowire material into a sort of artificial skin, which is being played up as a miracle material.

As Nosowitz points out, this is a remarkable achievement and it is a first step since skin registers pressure, pain, temperature, wetness, and more. Here’s an illustration of Berkeley’s e-skin (Source: University of California Berkeley, accessed from  http://berkeley.edu/news/media/releases/2010/09/12_eskin.shtml Sept. 14, 2010),

An artist’s illustration of an artificial e-skin with nanowire active matrix circuitry covering a hand. The fragile egg illustrates the functionality of the e-skin device for prosthetic and robotic applications.

The Stanford team’s approach has some similarities to the Berkeley’s (from Jason Palmer’s BBC article),

“Javey’s work is a nice demonstration of their capability in making a large array of nanowire TFTs [this film transistor],” said Zhenan Bao of Stanford University, whose group demonstrated the second approach.

The heart of Professor Bao’s devices is micro-structured rubber sheet in the middle of the TFT – effectively re-creating the functionality of the Berkeley group’s skins with less layers.

“Instead of laminating a pressure-sensitive resistor array on top of a nanowire TFT array, we made our transistors to be pressure sensitive,” Professor Bao explained to BBC News.

Here’s a short video about the Stanford team’s work (Source: Stanford University, accessed from http://news.stanford.edu/news/2010/september/sensitive-artificial-skin-091210.html Sept. 14, 2010),

Both approaches to the ‘pressure problem’ have at least one shortcoming. The Berkeley’s team’s e-skin has less sensitivity than Stanford’s while the Stanford team’s artificial skin is less flexible than e-skin as per Palmer’s BBC article. Also, I noticed that the Berkeley team at least is being funded by DARPA ([US Dept. of Defense] Defense Advanced Research Projects Agency) so I’m assuming a fair degree of military interest, which always gives me pause. Nonetheless, bravo to both teams.