Manufacturing Bits: Aug. 27

Growing tubes
Single-wall carbon nanotubes could one day be used in electronics, optoelectronics, biomedical imaging and other applications. But the synthesis of nanotubes with defined chiralities has been a stumbling block. A chiral molecule is a molecule that has a non-superposable mirror image.

The University of Southern California has shown that chirality-pure short nanotubes can be used as seeds to grow longer nanotubes. This was accomplished by using vapor-phase epitaxial technology. This, in turn, opens up a new route towards carbon nanotube synthesis.

The yield of vapor-phase epitaxial growth is limited, due to the lack of understanding of the process, according to researchers. Still, researchers developed a chirality-dependent growth and termination mechanism. In doing so, researchers developed seven single-chirality nanotubes. This included near zigzag, medium chiral angle, and near armchair semiconductors, as well as armchair metallic nanotubes.

Researchers revealed that the growth rates of nanotubes increase with their chiral angles. “Previously it was very difficult to control the chirality, or atomic structure, of nanotubes, particularly when using metal nanoparticles,” said Bilu Liu, a postdoctoral research associate at the USC Viterbi School of Engineering, on the Nanowerk Web site. “The structures may look quite similar, but the properties are very different. In this paper we decode the atomic structure of nanotubes and show how to control precisely that atomic structure.”

3D metrology
Empa, a research institute within the ETH Domain, has devised an instrument that maps the physical and chemical surfaces of materials down to the atomic level and in 3D.

The so-called 3D NanoChemiscope combines a scanning force microscope (SFM) and a mass spectrometer. The SFM scans the surface with a tiny tip. The time-of-flight secondary ion mass spectrometer (ToF-SIMS) determines the composition of a material.

The system solves a major problem. In a structure, SFM technology enables lateral and vertical resolution down to the atomic scale. But the SFM is unable to provide the necessary compositional analysis of a structure. In fact, the ability to obtain the required chemical information on a structure in 3D is lacking with existing technologies.

The ToF-SIMS technology solves the problem by providing the elemental and molecular composition on surfaces with high sensitivity. The technology makes use of a pulsed primary ion beam technique. “Measuring the flight time for each ion over a fixed distance allows the determination of its mass,” according to Empa.

Images can be acquired for all atoms and molecules. The SFM has demonstrated the ability to show images at 12µm x 12µm in size. In one application, the TOF-SIMS can identify where the different materials or polymers in the polymer blend are located on the surface.

The result of a combined three-dimensional ToF-SIMS-/SFM surface analysis of a PCBM/CyI-polymer blend used by Empa’s Functional Polymers Laboratory to produce organic solar cells. (Source: EMPA)

The prototype instrument measures one meter long, 70 centimeter wide and 1.7 meters tall. The system is equipped with a transport system. It makes use of piezomotors to move the sample back and forth on tracks. The tracks are coated with a diamond-like carbon layer. The sample holder can move along five axes. This allows the sample to be analyzed from any angle.

Making waves
The University of Southampton has devised an ultra-violet (UV) direct write technology for use in fabricating planar waveguides or planar lightwave circuits (PLCs).

PLCs are a hybrid between optical fibers and chips. Waveguides are components that guide electromagnetic waves in a system, such as long-haul optical communications networks. Today’s PLC devices are made with slow and expensive lithography and etch techniques. More recently, laser direct writing is being used as a viable technique to make waveguides, according to researchers.

Researchers from Southampton have devised an approach that makes use of two 244nm interfering beams from an interferometer. In this work, researchers have demonstrated a novel phase modulated method using an electro-optical modulator for planar Bragg grating fabrication. This, in turn, enables the tool to imprint the desired Bragg grating structure and waveguide.

During the fabrication process, the sample is transported to the stage. The beams are focused into the substrate. The beam’s electric field allows the precise control of the phase and amplitude of the resulting Bragg grating structures, according to researchers.

With the technology, researchers produced Bragg gratings over a 770–1670nm wavelength range. It also fabricated optical Hilbert transformers for single-sideband filters and on-chip single-photon-number-resolving detectors for telecommunication wavelengths.

–Mark LaPedus