Stergios J. Papadakis

Wafer-level assembly of carbon nanotube networks using dielectrophoresis

We use dielectrophoresis (DEP) to controllably and simultaneously assemble multiple carbon nanotube (CNT) networks at the wafer level. By an appropriate choice of electrode dimensions and geometry, an electric field is generated that captures CNTs from a sizable volume of suspension, resulting in good CNT network uniformity and alignment. During the DEP process, the electrical characteristics of the CNT network are measured and correlated with the network morphology. These experiments give novel insight into the physics of DEP assembly of CNT networks, and demonstrate the scalability of DEP for future device applications.

Dielectrophoretic assembly of reversible and irreversible metal nanowire networks and vertically aligned arrays

We demonstrate the dielectrophoretic control of metallic nanowires (NWs) in liquid suspensions. By varying a range of parameters including the magnitude and frequency of the applied electric field, the liquid suspending the NWs, and the flow conditions, we demonstrate control over NW network formation and dissolution, as well as ordering of NWs into vertically aligned arrays. These results suggest a straightforward strategy for NW assembly and integration in devices.

Multiwalled carbon nanotubes for stray light suppression in space flight instruments

Observations of the Earth are extremely challenging; its large angular extent floods scientific instruments with high flux within and adjacent to the desired field of view. This bright light diffracts from instrument structures, rattles around and invariably contaminates measurements. Astrophysical observations also are impacted by stray light that obscures very dim objects and degrades signal to noise in spectroscopic measurements. Stray light is controlled by utilizing low reflectance structural surface treatments and by using baffles and stops to limit this background noise. In 2007 GSFC researchers discovered that Multiwalled Carbon Nanotubes (MWCNTs) are exceptionally good absorbers, with potential to provide order-of-magnitude improvement over current surface treatments and a resulting factor of 10,000 reduction in stray light when applied to an entire optical train. Development of this technology will provide numerous benefits including: a.) simplification of instrument stray light controls to achieve equivalent performance, b.) increasing observational efficiencies by recovering currently unusable scenes in high contrast regions, and c.) enabling low-noise observations that are beyond current capabilities. Our objective was to develop and apply MWCNTs to instrument components to realize these benefits. We have addressed the technical challenges to advance the technology by tuning the MWCNT geometry using a variety of methods to provide a factor of 10 improvement over current surface treatments used in space flight hardware. Techniques are being developed to apply the optimized geometry to typical instrument components such as spiders, baffles and tubes. Application of the nanostructures to alternate materials (or by contact transfer) is also being investigated. In addition, candidate geometries have been tested and optimized for robustness to survive integration, testing, launch and operations associated with space flight hardware. The benefits of this technology extend to space science where observations of extremely dim objects require suppression of stray light.

A flat laser array aperture

We describe a design concept for a flat (or conformal) thin-plate laser phased-array aperture. The aperture consists of a substrate supporting a grid of single-mode optical waveguides fabricated from a linear electro-optic material. The waveguides are coupled to a single laser source or detector. An arrangement of electrodes provides for two-dimensional beam steering by controlling the phase of the light entering the grid. The electrodes can also be modulated to simultaneously provide atmospheric turbulence modulation for long-range free-space optical communication. An approach for fabrication is also outlined.

Multifunction array lidar network for intruder detection, tracking, and identification

This paper describes a novel lidar sensor network based on a small, flat, optical phase array laser aperture. The network of array lidars provides collaborative automatic detection, tracking, acquisition cueing, and intruder type identification, as well as free space optics communication. The lidar consists of four optical phased arrays, each with about 1 million radiating elements in a 1-cm2 aperture that enable electronic beam steering analogous to microwave array antennas. The four arrays are mounted on four sides of a stalk and have a full 360° field of view. A small number of such stalks can perform target detection over a large area.

Scanning surface-enhanced Raman spectroscopy of silver nanowires

We report results of scanning micro-Raman spectroscopy obtained on isolated nanowires and networks of nanowires with different geometries and surface morphologies. We measured a strong, relatively homogeneous, surface enhancement of the Raman response from nanowires with a rough surface morphology, and detected a more sporadic enhanced response detected from smooth nanowires. These results provide the first steps towards the development of selective sensors for hazardous bio- and chemical-agent detection that rely on a combination of electronic conductance measurements and Raman spectroscopic measurements from metallic nanowire networks.

A micro-fabricated sheet-beam Orotron THz source

We describe the fabrication of an Orotron driven by a sheet beam of electrons. The sheet beam is generated by a carbon nanotube field emission electron gun, which is less than 2 mm in total thickness. The orotron cavity is 2 cm long and 1 cm wide, and houses a microfabricated Smith-Purcell grating which generates the THz radiation. The sheet beam is 5 μm thick and 6 mm wide, and it travels within 15 μm of the top surface of the Smith-Purcell grating for the length of the cavity. The Orotron is discretely tunable, which means that there are a number of cavity resonances that can be driven by changing the energy of the beam such that for the period of the Smith-Purcell grating the cavity is driven on one of the resonances. For this work, a target frequency of 0.5 THz, corresponding to a beam energy of 3 keV, was used.

The RAVAN CubeSat mission: Advancing technologies for climate observation

The Radiometer Assessment using Vertically Aligned Nanotubes (RAVAN) CubeSat mission demonstrates an affordable, accurate radiometer that directly measures Earth-leaving fluxes of total and solar-reflected radiation. The radiometer exploits two key technologies: vertically aligned carbon nanotubes used as the radiometer absorber and a gallium fixed-point blackbody as an internal calibration source. RAVAN will fly on a 3U CubeSat, with a launch likely at the end of 2016. Our ability to understand and predict future climate is limited by our ability to track energy within the Earth system. RAVAN will enable the development of an Earth radiation budget constellation that could provide the global, diurnal measurements needed to significantly advance our understanding of ongoing and future climate change.

The power of inexpensive satellite constellations

Two thematic drivers are motivating the science community towards constellations of small satellites, the revelation that many next generation system science questions are uniquely addressed with sufficient numbers of simultaneous space based measurements, and the realization that space is historically expensive, and in an environment of constrained costs, we must innovate to ―do more with less‖. We present analysis that answers many of the key questions surrounding constellations of scientific satellites, including research that resulted from the GEOScan community based effort originally intended as hosted payloads on Iridium NEXT. We present analysis that answers the question how many satellites does global system science require? Perhaps serendipitously, the analyses show that many of the key science questions independently converge towards similar results, i.e. that approximately 60+ satellites are needed for transformative, as opposed to incremental capability in system science. The current challenge is how to effectively transition products from design to mass production for space based instruments and vehicles. Ideally, the lesson learned from past designs and builds of various space products should pave the way toward a better manufacturing plan that utilizes just a fraction of the prototype‘s cost. Using the commercial products industry implementations of mass customization as an example, we will discuss about the benefits of standardization in design requirements for space instruments and vehicles. For example, the instruments (payloads) are designed to have standardized elements, components, or modules that interchangeably work together within a linkage system. We conclude with a discussion on implementation plans and the new paradigms for community and international cooperation enabled by small satellite constellations.

Scanning surface-enhanced Raman spectroscopy (SERS) of chemical agent simulants on templated Au-Ag nanowire substrates

We report the results of scanning micro-Raman spectroscopy obtained on Au-Ag nanowires for a variety of chemical warfare agent simulants. Rough silver segments embedded in gold nanowires showed enhancement of 105 - 107 and allowed unique identification of 3 of 4 chemical agent simulants tested. These results suggest a promising method for detection of compounds significant for security applications, leading to sensors that are compact and selective.