Manufacturing Bits: April 15

Self-assembled nano-walls; CVD-enabled 2D materials; finding carbon nanotubes.

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Self-assembled nano-walls
Using a self-assembly process, Texas A&M University and the International Institute for Carbon-Neutral Energy Research have devised a new technology called “nano-walls.”

Researchers use common spray gun to create self-assembling nanoparticle films. (Source: Texas A&M).

Researchers use common spray gun to create self-assembling nanoparticle films. (Source: Texas A&M).

Researchers have developed an approach of applying a surface coating of thin, flat nanoplatelets using a common spray gun. When applied to a surface like a wall, the nanoplatelets self-assemble into nano-walls. The nano-walls act as rigid barriers that prevent oxygen gas from reaching the surface.

The technology could be used for anti-corrosion paints for metal surfaces. In the future, nanoplatelets could be used for gas separation membranes in industrial applications. Researchers are even interested in developing smart nano-walls, which are sensitive to magnetic fields.

The technology itself involves a large-scale, self-assembly of asymmetric colloidal particles, which, in turn, create fibers. The spray-coating method can be used to make thin, flexible and transparent epoxy films containing zirconium phosphate nanoplatelets. The nanoplatelets self-assembled into a lamellar arrangement aligned parallel to the substrate.

CVD-enabled 2D materials
The next big thing in the semiconductor industry could be molybdenum diselenide, a two-dimensional material that could be used to develop future field-effect transistors (FETs). This material has good electrostatics, a non-zero band gap, atomic scale thickness and pristine interfaces. The trouble is the material is complex and difficult to synthesize in the lab.

However, Rice University and Nanyang Technological University have devised a method for making one-atom-thick layers of molybdenum diselenide. Using chemical vapor deposition (CVD), researchers demonstrated the ability to grow uniform monolayers of molybdenum diselenide under ambient pressure, resulting in large single crystalline islands.

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This image from a scanning transmission electron microscope shows the individual atoms in a two-dimensional sheet of molybdenum diselenide. Credit: E. Ringe/Rice University

With the technology, researchers devised an FET. They also found the electronic properties and mobilities of molybdenum diselenide were better than those of molybdenum disulfide. Molybdenum diselenide and molybdenum disulfide belong to a class of materials known as transition metal dichalcogenides. Molybdenum disulfide is a similar material that has been studied for similar applications.

The photoluminescence intensity and peak position indicates a direct band gap of 1.5 eV for the molybdenum diselenide monolayers. A back-gated field effect transistor based on monolayer shows n-type channel behavior with average mobility of 50cm2 V–1 s–1.

“This new method will allow us to exploit the properties of molybdenum diselenide in a number of applications,” said Pulickel Ajayan, chair of Rice’s Department of Materials Science and NanoEngineering, on the university’s Web site. “Unlike graphene, which can now easily be made in large sheets, many interesting 2-D materials remain difficult to synthesize. Now that we have a stable, efficient way to produce 2-D molybdenum diselenide, we are planning to expand this robust procedure to other 2-D materials.”

Finding carbon nanotubes
Carbon nanotubes are carbon-based cylindrical structures. They are expected to be used in a plethora of applications. The problem is the structure and surface chemistry of individual carbon nanotubes are difficult to discern using current metrology techniques.

The Riken Center for Advanced Photonics has devised a high-resolution microscopy technique that can resolve individual carbon nanotubes. Researchers used STM-based, tip-enhanced Raman spectroscopy (STM-TERS) with 1.7nm spatial resolution in the ambient.

Raman spectroscopy involves exciting a material surface with a laser. The technology measures the change in laser energy after it is scattered from the surface.

Tip-enhanced Raman spectroscopy (TERS) is used to achieve close to molecular resolution. This is done by passing a metallic tip across the material surface to enhance the signals of nearby molecules. Using an atomic force microscope (AFM) tip, TERS has a resolution of around 10nm to 20nm.

Researchers replaced the AFM tip with a scanning tunneling microscope (STM) tip. The position of the metallic STM tip can be controlled more precisely than an AFM. This, in turn, makes it possible to scan a material with a gap between the tip and surface of less than 1nm, according to researchers.

Current STM-based techniques and STM-TERS methods require cryogenic temperatures and ultrahigh vacuums. Riken’s STM-TERS technique can be used with a compact chamber at ambient pressure and temperature. This broadens the range of materials that can be probed. “DNA sequencing, protein dynamics on biological membranes, and organic solar cells all require ambient conditions,” said Norihiko Hayazawa of Riken, on the organization’s Web site.

“With our STM-TERS system, we have achieved resolution of 1.7 nanometers, meaning that carbon nanotubes can be visualized at the dimensions of their diameter,” Hayazawa said. “This makes it possible for the first time to extract the local property of the carbon nanotubes optically without averaging.”