Synthesis method expands material possibilities

This study is about the beginning of civilization, humans have exploited new materials to improve their lives, from the prehistoric Stone Age, Bronze Age, and Iron Age to the modern Silicon Age. With each period came technological breakthroughs that transformed the way we live. Consider the 1961 invention of the silicon chip, which paved the way for the digital revolution. Without this tiny electronic component, we’d have no laptops or cell phones.

Addressing today’s challenges will similarly require material advances. For example, how do we make solar panels that convert sunlight into electricity more efficiently? Batteries that last longer? Ever-smaller electronic devices? Scientists are seeking solutions to these very questions through material science  and engineering. They’re both improving the performance of existing materials and creating brand-new material with unparalleled properties.

Over the past decade, scientists at the Center for Functional Nanomaterials (CFN) at the U.S. Department of Energy (DOE)’s Brookhaven National Laboratory have established themselves as leaders in this area. In particular, they are developing a new method for making materials: infiltration synthesis.

As its name suggests, infiltration synthesis involves infiltrating, or infusing, one material into another. By infusing an inorganic (non-carbon-containing) material in an organic (carbon-containing) material, one can generate a “hybrid” material with properties not seen in either of the starting components. The organic species could be polymer thin films, polymers patterned in a particular geometrical shape using a light source or electron beam (a technique known as lithography), polymers self-assembled from two or more chemically distinct “blocks” (block copolymer), or even self-assembled DNA structures. Infiltration occurs as the organic matrix is exposed to inorganic-containing gas or liquid precursors (starting materials) in an alternating order.

Synthesis method expands material possibilities

Fig: Scanning electron and optical microscope images of a zinc oxide nanowire array, nanowire array transistor, and nanowire array photodetector of ultraviolet (UV) light (top). The scientists combined infiltration synthesis and lithography to fabricate precisely aligned nanowire arrays and integrate them into devices

By placing the hybrid material under oxygen plasma (an electrically charged gas) or in a high-temperature oxygen environment, scientists can also selectively remove the organic component. The inorganic part remains behind and inherits the organic “template” pattern, which is useful for creating inorganic nanostructures and integrating them into electronic devices.

“Conventional pure chemistry-based approaches like chemical synthesis are complex,” explained Chang-Yong Nam, a scientist in the CFN Electronic Nanomaterials Group who is leading the infiltration synthesis research. “There’s no guarantee you’ll end up with the properties you targeted. And creating very small features—which are important for making electronic devices—is difficult. Infiltration synthesis addresses these issues, and the required tools are readily available in any nanofabrication facility.”

Nam, CFN colleagues, and external collaborators have been demonstrating how infiltration synthesis can be used to create a host of new functional materials, enabling a wide variety of applications.

In 2015, they used infiltration synthesis and lithography to pattern inorganic nanowires—wire-shaped structures with a width on the order of billionths of a meter—into a transistor. This study was the first to show that the technique could be used to pattern an electronic device. Extending this initial concept, they made arrays of perfectly aligned nanowires into highly sensitive photodetectors of ultraviolet (UV) light. To increase sensitivity even further, they converted stacked self-assembling block copolymer patterns into a 3D “nanomesh” architecture. The large surface area and pores enabled by this 3D layered geometry allowed for the placement of many more nanowire sensing elements.

Synthesis method expands material possibilities

Fig: (Left) Top- and side-view scanning electron microscope images of a ZnO nanomesh. (Right) A nanomesh device with electrodes (yellow) patterned by lithography. As shown in the graph, the device with six layers absorbed the most ultraviolet light, leading to the highest electrical currents.

The team is also making a hybrid-based neuromorphic switching device, which models the way the brain computes and transmits information. In initial demonstrations, their hybrid structure showed potential in mimicking the action of brain synapses, or the connections between neurons. They also found that the hybridization significantly reduced device-to-device performance variability, which is critical for creating practical, large-scale neuromorphic device arrays. Such brain-inspired computing would offer significant leaps in energy efficiency and processing speed for artificial intelligence tasks such as learning, searching, and sensing.

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