Evgeniya H. Lock

Chemical Gradients on Graphene To Drive Droplet Motion

This work demonstrates the production of a well-controlled, chemical gradient on the surface of graphene. By inducing a gradient of oxygen functional groups, drops of water and dimethyl-methylphosphonate (a nerve agent simulant) are pulled in the direction of increasing oxygen content, while fluorine gradients push the droplet motion in the direction of decreasing fluorine content. The direction of motion is broadly attributed to increasing/decreasing hydrophilicity, which is correlated to high/low adhesion and binding energy. Such tunability in surface chemistry provides additional capabilities in device design for applications ranging from microfluidics to chemical sensing.

Surface composition, chemistry, and structure of polystyrene modified by electron-beam-generated plasma.

Polystyrene (PS) surfaces were treated by electron-beam-generated plasmas in argon/oxygen, argon/nitrogen, and argon/sulfur hexafluoride environments. The resulting modifications of the polymer surface energy, morphology, and chemical composition were analyzed by a suite of complementary analytical techniques: contact angle goniometry, atomic force microscopy (AFM), X-ray photoelectron spectroscopy (XPS), and reflection electron energy loss spectroscopy (REELS). The plasma treatments produced only minimal increases in the surface roughness while introducing the expected chemical modifications: oxygen-based after Ar/O(2) plasma, oxygen- and nitrogen-based after Ar/N(2) plasma, and fluorine-based after Ar/SF(6) plasma. Fluorinated PS surfaces became hydrophobic and did not significantly change their properties over time. In contrast, polymer treated in Ar/O(2) and Ar/N(2) plasmas initially became hydrophilic but underwent hydrophobic recovery after 28 days of aging. The aromatic carbon chemistry in the top 1 nm of these aged surfaces clearly indicated that the hydrophobic recovery was produced by reorientation/diffusion of undamaged aromatic polymer fragments from the bulk rather than by contamination. Nondestructive depth profiles of aged plasma-treated PS films were reconstructed from parallel angle-resolved XPS (ARXPS) measurements using a maximum-entropy algorithm. The salient features of reconstructed profiles were confirmed by sputter profiles obtained with 200 eV Ar ions. Both types of depth profiles showed that the electron-beam-generated plasma modifications are confined to the topmost 3-4 nm of the polymer surface, while valence band measurements and unsaturated carbon signatures in ARXPS and REELS data indicated that much of the PS structure was preserved below 9 nm.

Plasma-Based Surface Modification of Polystyrene Microtiter Plates for Covalent Immobilization of Biomolecules

In recent years, polymer surfaces have become increasingly popular for biomolecule attachment because of their relatively low cost and desirable bulk physicochemical characteristics. However, the chemical inertness of some polymer surfaces poses an obstacle to more expansive implementation of polymer materials in bioanalytical applications. We describe use of argon plasma to generate reactive hydroxyl moieties at the surface of polystyrene microtiter plates. The plates are then selectively functionalized with silanes and cross-linkers suitable for the covalent immobilization of biomolecules. This plasma-based method for microtiter plate functionalization was evaluated after each step by X-ray photoelectron spectroscopy, water contact angle analysis, atomic force microscopy, and bioimmobilization efficacy. We further demonstrate that the plasma treatment followed by silane derivatization supports direct, covalent immobilization of biomolecules on microtiter plates and thus overcomes challenging issues typically associated with simple physisorption. Importantly, biomolecules covalently immobilized onto microtiter plates using this plasma-based method retained functionality and demonstrated attachment efficiency comparable to commercial preactivated microtiter plates.

Effect of Physicochemical Anomalies of Soda-Lime Silicate Slides on Biomolecule Immobilization

Glass microscope slides are considered by many as the substrate of choice for microarray manufacturing due to their amenability to various surface chemistry modifications. The use of silanes to attach various functional groups onto glass slides has provided a versatile tool for the covalent immobilization of many diverse biomolecules of interest. We recently noted a dramatic reduction in biomolecule immobilization efficiency on standard microscope slides prepared using a well-characterized silanization method. A survey of commercial soda-lime slides yielded the surprising result that slides purchased prior to 2008 had superior immobilization efficiencies when compared to those purchased after 2008. Characterization of the slides by X-ray photoelectron spectroscopy (XPS), contact angle measurements, and atomic force microscopy (AFM), revealed a significant correlation (R > 0.9) between magnesium content, surface roughness, and bioimmobilization efficiency. High performance slides had higher magnesium content and higher root-mean-square (rms) roughness (P < 0.005) than slides with lower bioimmobilization efficiencies. Although the exact mechanism of how magnesium content and surface roughness affect silane deposition has not yet been defined, we show that recent changes in the chemical and physical properties of commercial soda-lime slides affect the ability of these slides to be covalently modified.

Initial evaluation and comparison of plasma damage to atomic layer carbon materials using conventional and low Te plasma sources

The ability to achieve atomic layer precision is the utmost goal in the implementation of atomic layer etch technology. Carbon-based materials such as carbon nanotubes (CNTs) and graphene are single atomic layers of carbon with unique properties and, as such, represent the ultimate candidates to study the ability to process with atomic layer precision and assess impact of plasma damage to atomic layer materials. In this work, the authors use these materials to evaluate the atomic layer processing capabilities of electron beam generated plasmas. First, the authors evaluate damage to semiconducting CNTs when exposed to beam-generated plasmas and compare these results against the results using typical plasma used in semiconductor processing. The authors find that the beam generated plasma resulted in significantly lower current degradation in comparison to typical plasmas. Next, the authors evaluated the use of electron beam generated plasmas to process graphene-based devices by functionalizing graphene with...

Electron Beam-Produced Plasmas

High-energy electron beams offer a unique alternative to discharges as plasma sources for material processing. In this paper, we present several images of sheetlike beams produced in a variety of gases and in different configurations. In addition to their aesthetic value, the images provide insight into the plasma and system properties.

Reply on the Comments on &#8220;Initiation of Pulsed Corona Discharge Under Supercritical Conditions&#8221; by C. H. Zhang, J. M. K. MacAlpine, and H. Akiyama

Generation of plasma under supercritical conditions is of fundamental and applied interest. In this paper, the reduced electric fields required for breakdown of gaseous and supercritical carbon dioxide are comparatively analyzed for planar and coaxial cylindrical geometries. The indirect comparison of measured breakdown voltages suggests an essential change in the ionization mechanism both for uniform and nonuniform fields

Generation of Pulse Corona Discharge in Point-to-Plane Geometry Under Supercritical Conditions

Summary form only given. Generation of plasma under supercritical conditions is an unexplored area in plasma science. The known media of plasma applications include gas and liquid states with pressures from vacuum to atmospheric and above. Discharge development depends on the E/n ratio and the density of the media. In gaseous phase, the distance between the molecules is relatively large and electrical discharge can be achieved by using relatively low voltage. Concurrently, in liquid phase due to the densely packed molecules the required voltage for plasma generation is high. Some studies on plasma application for pollution control showed that small amount of dispersed liquid droplets increase the efficiency of the chemical utilization of the high energy electrons and reduce the required voltage at the same time. Supercritical fluid offers the unique advantage of having areas of low and high density that coexist in the fluid. This means that the discharge generated under these conditions starts in the low density gas like area where the required voltage for discharge initiation is low. Then the streamers rapidly propagate to the high density liquid like clusters, where the discharge propagation is controlled by mechanisms typical for the liquids. Our previous results on plasma generation under supercritical CO2 conditions in wire to cylinder pulsed power reactor suggest significant reduction of the breakdown voltage. At the same time the experiments conducted for point-to-plane configuration showed that the possibility for plasma generation extends far beyond the critical conditions (T=304 K, P=73.8 bar). In this paper we extend the knowledge on the dependence of the voltage required for plasma initiation on the pressure, temperature and density of the supercritical media. Furthermore, three different point-to-plane geometries are investigated to provide insight of the configuration influence on the plasma generation

Corona discharge under supercritical conditions

Summary form only given. Generation of non-thermal discharge in supercritical fluid environment is considered difficult due to high pressures and temperatures of the critical points of the fluids. This arises form Paschens law that suggests that increasing the process pressure leads to increase in the voltage required for plasma generation and, therefore, to enhanced operational cost. We have studied generation of corona discharge under supercritical fluid conditions near the critical point. The required voltage for this process is three times lower than the Paschens law prediction due to extensive cluster formation under the investigated conditions. However, there is a need of further investigation of the dependence of the breakdown voltage on the temperature and pressure of the supercritical fluid, so that this new process-generation of plasma under supercritical fluid conditions can be used for material science and pollutant removal applications. This promising new technology offers combination of the advantages of supercritical fluids as a unique reaction media due to its heterogeneous chemistry, enhanced heat and mass transfer with the benefits of the high energetic plasma state that is characterized with fast chemical reactions and high selectivity.