Stephanie A. Getty

Near-perfect conduction through a ferrocene-based molecular wire

Here we describe the design, single-molecule transport measurements, and theoretical modeling of a ferrocene-based organometallic molecular wire, whose bias-dependent conductance shows a clear Lorentzian form with magnitude exceeding 70% of the conductance quantum

Compact two‐step laser time‐of‐flight mass spectrometer for in situ analyses of aromatic organics on planetary missions

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Performance of a carbon nanotube field emission electron gun

. We attribute this unprecedented level of single-molecule conductance to a manifestation of the low-lying molecular resonance and extended orbital network long predicted for a conjugated organic system. A similar-in-length, all-organic conjugated phenylethynyl oligomer molecular framework shows much lower conductance.

Multiwalled carbon nanotubes for stray light suppression in space flight instruments

RATIONALE A miniature time-of-flight mass spectrometer measuring 20 cm in length has been adapted to demonstrate two-step laser desorption/ionization (LDI) in a compact instrument package for enhanced organics detection. Two-step LDI decouples the desorption and ionization processes, relative to traditional LDI, in order to produce low-fragmentation mass spectra of organic analytes. Tuning the UV ionization laser energy would allow control of the degree of fragmentation, which might enable better identification of constituent species. METHODS A reflectron time-of-flight mass spectrometer prototype was modified to allow a two-laser configuration, with IR (1064 nm) desorption followed by UV (266 nm) postionization. A relatively low ion extraction voltage of 5 kV was applied at the sample inlet. RESULTS The instrument capabilities and performance were demonstrated with analysis of a model polycyclic aromatic hydrocarbon, representing a class of compounds important to the fields of Earth and planetary science. Two-step laser mass spectrometry (L2MS) analysis of a model PAH, pyrene, was demonstrated, including molecular ion identification and the onset of tunable fragmentation as a function of ionizing laser energy. Mass resolution m/Δm = 380 at full width at half-maximum was achieved for gas-phase postionization of desorbed neutrals in this highly compact mass analyzer. CONCLUSIONS Achieving L2MS in a highly miniaturized instrument enables a powerful approach to the detection and characterization of aromatic organics in remote terrestrial and planetary applications. Tunable detection of molecular and fragment ions with high mass resolution, diagnostic of molecular structure, is possible on such a compact L2MS instrument. The selectivity of L2MS against low-mass inorganic salt interferences is a key advantage when working with unprocessed, natural samples, and a mechanism for the observed selectivity is proposed.

Characterization of Nitrogen-Incorporated Ultrananocrystalline Diamond as a Robust Cold Cathode Material

A cold cathode field emission electron gun (e-gun) based on a patterned carbon nanotube (CNT) film has been fabricated for use in a miniaturized reflectron time-of-flight mass spectrometer (RTOF MS), with future applications in other charged particle spectrometers, and performance of the CNT e-gun has been evaluated. A thermionic electron gun has also been fabricated and evaluated in parallel and its performance is used as a benchmark in the evaluation of our CNT e-gun. Implications for future improvements and integration into the RTOF MS are discussed.

Simulation of a Miniature, Low-Power Time-of-Flight Mass Spectrometer for In Situ Analysis of Planetary Atmospheres

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.

Hemispherical reflectance and emittance properties of carbon nanotubes coatings at infrared wavelengths

Carbon materials, including carbon nanotubes and nanostructured diamond, have been investigated for over a decade for application to electron field emission devices. In particular, they have been investigated because of their low power consumption, potential for miniaturization, and robustness as field emission materials, all properties that make nanocarbon materials strong candidates for applications as long life electron sources for mass spectrometers for space exploration, where electron sources are exposed to harsh environments, .A miniaturized mass spectrometer under development for in situ chemical analysis on the moon and other planetary environments requires a robust, long-lived electron source, to generate ions from gaseous sample using electron impact ionization. To this end, we have explored the field emission properties and lifetime of nitrogen-incorporated ultrananocrystalline diamond films. We will present recent results revealing that UNCD films with nitrogen incorporation during growth (N-UNCD) yield stable/high fieldinduced electron emission in high vacuum for up to 1000 hours.

A miniature MEMS and NEMS enabled time-of-flight mass spectrometer for investigations in planetary science

We are implementing nano- and micro-technologies to develop a miniaturized electron impact ionization mass spectrometer for planetary science. Microfabrication technology is used to fabricate the ion and electron optics, and a carbon nanotube (CNT) cathode is used to generate the ionizing electron beam. Future NASA planetary science missions demand miniaturized, low power mass spectrometers that exhibit high resolution and sensitivity to search for evidence of past and present habitability on the surface and in the atmosphere of priority targets such as Mars, Titan, Enceladus, Venus, Europa, and short-period comets. Toward this objective, we are developing a miniature, high resolution reflectron time-of-flight mass spectrometer (Mini TOF-MS) that features a low-power CNT field emission electron impact ionization source and microfabricated ion optics and reflectron mass analyzer in a parallel-plate geometry that is scalable. Charged particle electrodynamic modeling (SIMION 8.0.4) is employed to guide the iterative design of electron and ion optic components and to characterize the overall performance of the Mini TOF-MS device via simulation. Miniature (< 1000 cm3) TOF-MS designs (ion source, mass analyzer, detector only) demonstrate simulated mass resolutions > 600 at sensitivity levels on the order of 10-3 cps/molecule N2/cc while consuming 1.3 W of power and are comparable to current spaceflight mass spectrometers. Higher performance designs have also been simulated and indicate mass resolutions ~1000, though at the expense of sensitivity and instrument volume.

MOMA: The Challenge to Search for Organics and Biosignatures on Mars

Recent visible wavelength observations of Multiwalled Carbon Nanotubes (MWCNT) coatings have revealed that they represent the blackest materials known in nature with a Total Hemispherical Reflectance (THR) of less than 0.25%. This makes them exceptionally good as absorbers, with the potential to provide order-ofmagnitude improvement in stray-light suppression over current black surface treatments when used in an optical system. Here we extend the characterization of this class of materials into the infrared spectral region to further evaluate their potential for use on instrument baffles for stray-light suppression and to manage spacecraft thermal properties through radiant heat transfer process. These characterizations will include the wavelength-dependent Total Hemispherical Reflectance (THR) properties in the mid- and far-infrared spectral regions (2-110 μm). Determination of the temperature-dependent emittance will be investigated in the temperature range of 40 to 300 K. These results will be compared with other more conventional black coatings such as Acktar Fractal Black or Z306 coatings among others.

An AOTF-LDTOF spectrometer suite for in situ organic detection and characterization

Solar system exploration and the anticipated discovery of biomarker molecules is driving the development of a new miniature time-of-flight (TOF) mass spectrometer (MS). Space flight science investigations become more feasible through instrument miniaturization, which reduces size, mass, and power consumption. However, miniaturization of space flight mass spectrometers is increasingly difficult using current component technology. Micro electro mechanical systems (MEMS) and nano electro mechanical systems (NEMS) technologies offer the potential of reducing size by orders of magnitude, providing significant system requirement benefits as well. Historically, TOF mass spectrometry has been limited to large separation distances as ion mass analysis depends upon the ion flight path. Increased TOF MS system miniaturization may be realized employing newly available high speed computing electronics, coupled with MEMS and NEMS components. Recent efforts at NASA Goddard Space Flight Center in the development of a miniaturized TOF mass spectrometer with integral MEMS and NEMS components are presented. A systems overview, design and prototype, MEMS silicon ion lenses, a carbon nanotube electron gun, ionization methods, as well as performance data and relevant applications are discussed.