WFU | Materials

Materials Synthesis

nanotube

Ag wires

PPV

The synthesis methods we employ are widely varied: CVD, Laser assisted CVD, PLD, RF Magnetron sputtering, Kratchmer Generator growth, chemical synthesis, and electro-chemical templating. We grow and characterize all the materials used in the various programs of the Center. Of particular interest in the center:

Electro-spun polymer fibers,
Polymer nanospheres and nanowires
Carbon Nanotubes, and their doped variants,
Metal Nanoparticles,
GaN, PbS, PbSe nanowires

Left: an assortment of the types of materials made and characterized at Nanotech. The top is an arc-grown SWCNT. The STM images shows the chiral nature of teh atomic structure of the nanotube. The tunneling spectroscopy and microscopy facilties at the Nanotech Center allow for imaging at very low current densities which opens the door to a wide range of macromolecular imaging without field-induced distortion.

The middle image is of metal nanoplates. We have extesnive experience in nanorod and nanoparticle synthesis in Ag and Au along with a few other metals. In the SEM shown the surfactant stabilizer on the surface (in this case CTAB) interacts across the surfaces of the particle from particle-to-particle. This leads to a form of sefl assembly. The scale overwhich these can be assembled seems to be dictated by nonequilibrium thermodynamic arguments.

The bottom image is of PPV nanotubes made through vapor phase polymerization in AAO templates. Notice the color chage as the tubes are bundled together. When separate, the tubes exhibit a blue-ish green, whereas after bundling they turn greenish-yellow. The self-absorption of singlet emission is well known in thick films but this demonstration at the nanosclae was unexpected.

A key aspect of any nanomaterials synthesis program is the characterization techniques utilized. We specialize in electron and scanning probe microscopies, scanning probe spectroscopies, and a number of optical techniques such as Raman, Luminscences, Pulse-Probe, Time-of-flight, and z-scan for nonlinear effects. We also have a number of long standing collaborations in Raman, HRTEM, EELS, and XPS-UPS, to further our characterization methods.

Materials Assembly

Once nanoscale building blocks have been created, what do you do with them. Usually, they must be assembled into a structure that makes use of their unusual properties at the macroscale. These assemblies can be a simple as a randomly places assortment of nanotubes into thin transparent conducting films. They can be as complex as multiple and distinct nano-elements assembled into crystalline-like registry such as a photonic crystal. At Nanotech we are approaching the complexity of order using several methods:

Directed chemical assembly

Entropic assembly

Biologically inspired biochemical assembly

Templated assembly

Chaotic advection and smart matrix assembly

Shown right are a few examples of our work. The top image is an SEM of an anodized alumina substrate. We have drmatically modified methods for the creation of such templates allowing broad and decoupled control over hole diameter and spacing. Through new chemical methods for management of the barrier layer, we have also shown that extraordinary control over aligned metal wires and tuning forks can be achieved. Through this work we have created metalens structures with negative refractive index and super-resolution.

In the middle image we show assmebly of a synthetic opal system. Through surface modification and interfacial forces, we have assembled photonic crystals with active emitters embedded in them, which we are investigating for potential uses in organic laser oscillators.

In the bottom image, is another area of research within the materials effort at Nanotech. This is an optically heterogeneous system or nanoantennae-composite system which is used to enhance the optical density of a polymer. Blending techniques yeilding structural hierarchy are of particular interest in this area of work such that the material exhibits properties across several length scales.