Detergent Triggers Self Assembly of Two-Dimensional Zinc Oxide

A wide range of materials for which 2-D versions were unavailable can now be made atomically thin with new technique

2 min read
Detergent Triggers Self Assembly of Two-Dimensional Zinc Oxide
Illustration: University of Wisconsin—Madison

There is a fairly large number of materials that might have some pretty attractive properties if they could be made into monolayer, two-dimensional (2-D) sheets. Unfortunately, unlike graphene, which is fabricated by peeling away layers from bulk graphite, these other materials don’t have a multi-layered source. But now the fabrication of these materials has become a possibility through a novel “bottom-up” production technique whose development may have just changed the future of 2-D materials in electronics.

Researchers at the University of Wisconsin-Madison (UW-Madison) have developed a technique in which a zinc oxide monolayer self assembles in a liquid with the help of a surfactant.  After six years of trial-and-error testing with different surfacants, the UW-Madison researchers believe they have found the right mix.

In research explained in a paper published in the journal Nature Communications, the researchers discovered that when a surfactant—essentially a detergent—containing sulfate ions was placed in a liquid containing zinc ions, it would trigger the self-assembly of zinc oxide nanosheets. The negatively charged sulfate ions in the surfactant attract the positively charge zinc ions, and within a couple of hours, the 2-D zinc oxide is formed.

The idea for this approach came to Xudong Wong, one of the authors of the paper, while teaching a class on nanotechnology back in 2009.

“Under the correct conditions, a surfactant will self-assemble to form a monolayer,” said Wong, in a press release. “This is a well-known process that I teach in class. So while teaching this I wondered why we wouldn't be able to reverse this method and use the surfactant monolayer first to grow the crystalline face.”

In tests, the researchers discovered that the zinc oxide monolayers are able to function as p-type semiconductor transistors in which holes represent the majority of carriers and electrons are the minority. Just to give you a sense of how making zinc oxide down a 2-D material changes its properties, it’s important to know that in its bulk form, zinc oxide is an n-type semiconductor. Producing p-type semiconductors from zinc oxide has been the aim of much research.

Despite the breakthrough for zinc oxide production in two-dimensions, the real impact of this research may be that it opens the door for the production of fairly wide range of other 2-D materials that were not possible previously. To this end, the researchers are already looking at using their bottom-up surfactant technique to produce 2-D versions of gold and palladium. Wang added, in the press release: “It brings a lot of new functional material to this 2-D material category.”

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3 Ways 3D Chip Tech Is Upending Computing

AMD, Graphcore, and Intel show why the industry’s leading edge is going vertical

8 min read
A stack of 3 images.  One of a chip, another is a group of chips and a single grey chip.
Intel; Graphcore; AMD

A crop of high-performance processors is showing that the new direction for continuing Moore’s Law is all about up. Each generation of processor needs to perform better than the last, and, at its most basic, that means integrating more logic onto the silicon. But there are two problems: One is that our ability to shrink transistors and the logic and memory blocks they make up is slowing down. The other is that chips have reached their size limits. Photolithography tools can pattern only an area of about 850 square millimeters, which is about the size of a top-of-the-line Nvidia GPU.

For a few years now, developers of systems-on-chips have begun to break up their ever-larger designs into smaller chiplets and link them together inside the same package to effectively increase the silicon area, among other advantages. In CPUs, these links have mostly been so-called 2.5D, where the chiplets are set beside each other and connected using short, dense interconnects. Momentum for this type of integration will likely only grow now that most of the major manufacturers have agreed on a 2.5D chiplet-to-chiplet communications standard.

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