David D. Allred received Alumni Professorship Award

Magnesium fluoride on xenon difluoride passivated aluminum (Al+XeMgF₂) mirrors have high reflectance encompassing the H Lyman-α at 121.6 nm. Al+XeMgF₂ is a key candidate for space telescopes and satellites that demand far-UV (FUV) measurements coupled with high reflectance at longer wavelengths. Contamination can significantly reduce FUV reflectance, so Al+XeMgF₂ mirrors must be as clean as possible. Protecting the surfaces while in storage is also desirable. We investigated the suitability of four different formulations of Photonic Cleaning Technologies' First Contact Polymer for cleaning and protecting Al+XeMgF₂ coatings by repeatedly cleaning test samples. These were cleaved from a silicon wafer coated with 300 nm of chemical vapor deposited (CVD) silicon nitride (Si₃N₄). All the formulations could clean samples at least once. Using Variable-Angle, Spectroscopic Ellipsometry (VASE), we determined that two (S2 and S3) of the four tested formulations were able to clean and protect the Al+XeMgF₂ surfaces multiple times (>20) over 5 months without detectable alumina growth on the Al in a low humidity environment. There were also no changes to the thickness of 'apparent' MgF₂. Apparent MgF₂ includes the deposited MgF₂, the 2–3 nm AlF₃ layer produced by the XeF₂ passivation step, and contributions from surface roughening. There was also no detectable alumina growth for the controls. The fact that the samples were stored between tests in a desiccator with their First Contact overcoat provides evidence that Al+XeMgF₂ samples can successfully be protected and stored under some First Contact formulations for at least five months in a dry environment. Far-ultraviolet reflectance is not reported here.

We investigated the growth of carbon nanotubes (CNTs) directly on stainless steel substrates. The CNTs were grown using a two-step process: oxidation of the stainless steel surface and CNT growth. The samples were oxidized in an 800 °C furnace fed with a flow of air for 4 min. CNTs were grown by switching the flow to ethylene, which both reduces the oxide and initializes CNT growth. The time of CNT growth was varied to understand how the samples evolved over time. To better understand the growth mechanisms, we isolated cross-sections of the CNT-substrate interface using a focused ion beam. These cross-sections were investigated with transmission electron microscopy and energy dispersive X-ray spectroscopy. CNTs were seen to grow from iron-rich nanoparticles embedded in the oxide layer. The oxide layer was also seen to lose iron over time, suggesting that these iron nanoparticles were reduced out of the oxide. The base particles were embedded in the oxide layer, leaving cavities when the CNTs were removed. The diameters of the nanotubes were also seen to grow over time as a result of carbon infiltration. The effects of the embedded particle and infiltration quickly isolate the catalyst, leading to short CNTs (1–10 µm).

The effects of physiologically relevant glucose concentrations on the optical properties of whole blood were measured in-vitro. A concentration increase of +400 mg/dL caused a decrease in the scattering coefficient by 10% over all wavelengths studied. To determine potential mechanisms for the change in the scattering coefficient, we employed optical microscopy to quantify the change in erythrocyte geometry. A 15% change in cell thickness was observed following a glucose increase of +500 mg/dL.

Carbon nanotubes (CNTs) possess many unique properties that make them ideal for field emission. However, screening due to high density and poor substrate adhesion limits their application. We tested the field emission of various patterned vertically aligned carbon nanotube (VACNT) arrays adhered to copper substrates using carbon paste. After many fabrication steps to improve uniformity, we found that the field emission was dominated by individual CNTs that were taller than the bulk VACNT arrays. After testing a sample with silver epoxy as the binder, we found that the failure mechanism was adhesion to the substrate. Using energy dispersive xray spectroscopy (EDX), we found that the carbon paste migrated into the VACNT bulk volume while the silver epoxy did not. The migration of carbon paste into the volume may explain why the carbon paste had greater adhesion than the silver epoxy.

Windows for vacuum ultraviolet (VUV) sources are valuable for many applications but difficult to fabricate due to most materials being too absorptive at VUV wavelengths. We have designed, fabricated, and characterized a carbon nanotube (CNT) collimator as a window with high (VUV) transmission and significant differential pumping. The CNT collimators are arrays of square channels of various dimensions and height with sidewalls composed of vertically aligned CNT forests. The CNT collimators in this work exhibited peak intensity transmissions for VUV light (58.4 nm) of 18%–37% of that reported for the same system without a collimator present [S. Olsen, D. Allred, and R. Vanfleet, J. Vac. Sci. Technol. A (2024)]. Further analysis found that the peak intensity transmissions were lowered due to carbon deposition on the phosphor viewing screen from contaminants. The CNT collimator also had significant sidewall reflection in the VUV range (⁠R = 0.21 +/- 0.08) in the VUV range for angles 15.6 degrees and below). Pressure ratios (low pressure over high pressure) in the VUV transmission experiment were dominated by leaks in the alignment mechanism. Additional experiments demonstrated the CNT collimator’s reflection and superior differential pumping with pressure ratios less than 0.001.

Hollow cathodes are a common type of vacuum ultraviolet (VUV) light source with a wide range of design and application. We determined the VUV (58.4 nm) intensity distribution of a hollow cathode as a function of current and pressure. Our model describes the intensity distribution of a McPherson 629-like hollow cathode helium plasma within the range of 0.50–1.00 A and 0.50–1.00 Torr as a ring with a center peak. We found that for all pressures and currents considered, the ring emits more VUV light than the center peak. We also found that the center peak has a minimum VUV light emission near 0.9 Torr.