Synthesis of Quasi-Freestanding Graphene Films Using Radical Species Formed in Cold Plasmas

For over a decade, the Stinespring laboratory has investigated scalable, plasma assisted synthesis (PAS) methods for the growth of graphene films on silicon carbide (SiC). These typically utilized CF4-based inductively coupled plasma (ICP) with reactive ion etching (RIE) to selectively etch silicon from the SiC lattice. This yielded a halogenated carbon-rich surface layer which was then annealed to produce the graphene layers. The thickness of the films was controlled by the plasma parameters, and overall, the process was readily scalable to the diameter of the SiC wafer. The PAS process reproducibly yielded two- to three-layer thick graphene films that were highly tethered to the underlying SiC substrate via an intermediate buffer layer. The buffer layer was compositionally similar to graphene. However, a significant number of graphene carbons were covalently bound to silicon atoms in the underlying substrate. This tethering lead to mixing of the film and substrate energy bands which degraded many of graphene’s most desirable electrical properties. The research described in this dissertation was aimed at improving graphene quality by reducing the extent of tethering using a fundamentally different plasma etching mechanism while maintaining scalability. In the ICP-RIE process, the etchant species include F and CFx (x = 1-3) radicals and their corresponding positive ions. These radicals are classified as “cold plasma species” in the sense that they are nominally in thermal equilibrium with the substrate and walls of the system. In contrast, the electrons exist at extremely high temperature (energy), and the ionic species are accelerated to energies on the order of several hundred electron volts by the plasma bias voltage that exists between the plasma and substrate. As a result, the ionic species create a directional, high rate etch that is dominated by physical etching characterized by energy and momentum transfer. In contrast, the neutral radicals chemically etch the surface at a much lower rate. In this work, the effects of physical etching due to high energy ions were eliminated by shielding the SiC substrate using a mask (e.g., quartz) supported by silicon posts. In this way, a microplasma consisting of chemically reactive cold plasma species was created in the small space between the substrate surface and the backside of the quartz mask. This process, referred to here as microplasma assisted synthesis (MPAS), was used to produce graphene films. A parametric investigation was conducted to determine the influence of MPAS operating parameters on graphene quality. The key parameters investigated included ICP power, RIE power, etch time, various mask materials, microreactor height, substrate cooling, initial surface morphology and SiC polytype. The resulting graphene films were characterized by x-ray photoelectron spectroscopy (XPS), Raman spectroscopy, and atomic force microscopy (AFM). Following optimization of the MPAS process, some tethering of the graphene films remained. However, films produced by MPAS consistently exhibited significantly less tethering than those produced using the PAS process. Moreover, both XPS and Raman spectroscopy indicated that these films were quasi-free standing, and, in some cases, they approached free standing graphene. From a wide view, the results of these studies demonstrate the potential of MPAS as a technique for realizing the controlled synthesis of high-quality, lightly tethered mono-, and few-layer graphene films directly on an insulating substrate. On a more fundamental level, the results of these studies provide insight into the surface chemistry of radical species

Microdischarge-Based Pressure Controlling Devices and their Applications to Chemical Sensing in Harsh Environments.

Microdischarges offer an alternative and often advantageous sensing and actuation method that has not been significantly exploited in microtransducers. This thesis explores the capabilities of microdischarges to address problems such as cavity pressure control, cavity pressure detection, and purity control of fill gases, which are relevant to microsystems. Microdischarge-based transducers have been developed for these purposes. One interesting aspect of microdischarge-based transducers is the wide latitude of operating temperatures, as they are advantageous for room and high temperature operation. On-chip sputter-ion pumps control the pressure and gas purity in cavities. They consist of thin-film titanium electrodes patterned on glass substrates. Microdischarges sputter the cathodes, resulting in the selective chemisorption of titanium-reactive gases. Using DC discharges, these devices have reduced the pressure by 168 Torr in an air-filled, hermetically sealed, 6.33 cm3 package. Starting at 200 Torr, the pressure reduction rate of air is 7.2 Torr/h; oxygen 11.5 Torr/h, and nitrogen 3.4 Torr/h. Relative humidity is reduced at 6%/h. The pumps do not remove helium, purifying gas environments by selectively removing contaminating nitrogen and oxygen. A theoretical model outlining the dependency of gas removal rates on microdischarge parameters is presented. Microdischarge-based pressure sensors operate by correlating the measured change in spatial current distribution of pulsed DC microdischarges with pressure. One sensor version uses three-dimensional arrays of horizontal bulk metal electrodes embedded in quartz substrates with electrode diameters of 1-2 mm and 50-100 µm inter-electrode spacing. These devices have been operated over 10-2,000 Torr, at temperatures as high as 1,000˚C. The maximum measured sensitivity is 5,420 ppm/Torr, while the minimum temperature coefficient of sensitivity is -550 ppm/K. Sensors of a second version use planar electrodes, with 0.13 mm2 active areas. To explore the utility of pressure controlling devices, these transducers are combined with an optical emission sensor to create a high temperature gas phase chemical detection microsystem. The microdischarge-based pressure sensor determines the sample and backfilling gas pressure while the microscale-sputter-ion pump purifies the gas environment. The contaminating nitrogen concentration has been reduced by 56.5x relative to helium and the spectral detection limit has been improved by 8x for carbon at 200°C.Ph.D.Electrical EngineeringUniversity of Michigan, Horace H. Rackham School of Graduate Studieshttp://deepblue.lib.umich.edu/bitstream/2027.42/62245/1/scottwri_1.pd

The 2022 Plasma Roadmap: low temperature plasma science and technology

Documento escrito por un elevado número de autores/as, solo se referencia el/la que aparece en primer lugar y los/as autores/as pertenecientes a la UC3M.The 2022 Roadmap is the next update in the series of Plasma Roadmaps published by Journal of Physics D with the intent to identify important outstanding challenges in the field of low-temperature plasma (LTP) physics and technology. The format of the Roadmap is the same as the previous Roadmaps representing the visions of 41 leading experts representing 21 countries and five continents in the various sub-fields of LTP science and technology. In recognition of the evolution in the field, several new topics have been introduced or given more prominence. These new topics and emphasis highlight increased interests in plasma-enabled additive manufacturing, soft materials, electrification of chemical conversions, plasma propulsion, extreme plasma regimes, plasmas in hypersonics and data-driven plasma science.Cristina Canal acknowledges PID2019-103892RB-I00/AEI/10.13039/501100011033 Project (AEI) and the Generalitat de Catalunya for the ICREA Academia Award and SGR2017-1165. The research by Annemie Bogaerts was funded by the European Research Council (ERC) under the European Union's Horizon 2020 research and innovation programme (ERC Synergy Grant 810182 SCOPE). Eduardo Ahedo was funded by Spain's Agencia Estatal de Investigación, under Grant No. PID2019-108034RB-I00 (ESPEOS Project)

The 2022 Plasma Roadmap: low temperature plasma science and technology

The 2022 Roadmap is the next update in the series of Plasma Roadmaps published by Journal of Physics D with the intent to identify important outstanding challenges in the field of low-temperature plasma (LTP) physics and technology. The format of the Roadmap is the same as the previous Roadmaps representing the visions of 41 leading experts representing 21 countries and five continents in the various sub-fields of LTP science and technology. In recognition of the evolution in the field, several new topics have been introduced or given more prominence. These new topics and emphasis highlight increased interests in plasma-enabled additive manufacturing, soft materials, electrification of chemical conversions, plasma propulsion, extreme plasma regimes, plasmas in hypersonics, data-driven plasma science and technology and the contribution of LTP to combat COVID-19. In the last few decades, LTP science and technology has made a tremendously positive impact on our society. It is our hope that this roadmap will help continue this excellent track record over the next 5–10 years.Peer ReviewedPostprint (published version

Simulation de profils de gravure et de dépôt à l’échelle du motif pour l’étude des procédés de microfabrication utilisant une source plasma de haute densité à basse pression

En lien avec l’avancée rapide de la réduction de la taille des motifs en microfabrication, des processus physiques négligeables à plus grande échelle deviennent dominants lorsque cette taille s’approche de l’échelle nanométrique. L’identification et une meilleure compréhension de ces différents processus sont essentielles pour améliorer le contrôle des procédés et poursuivre la «nanométrisation» des composantes électroniques. Un simulateur cellulaire à l’échelle du motif en deux dimensions s’appuyant sur les méthodes Monte-Carlo a été développé pour étudier l’évolution du profil lors de procédés de microfabrication. Le domaine de gravure est discrétisé en cellules carrées représentant la géométrie initiale du système masque-substrat. On insère les particules neutres et ioniques à l’interface du domaine de simulation en prenant compte des fonctions de distribution en énergie et en angle respectives de chacune des espèces. Le transport des particules est effectué jusqu’à la surface en tenant compte des probabilités de réflexion des ions énergétiques sur les parois ou de la réémission des particules neutres. Le modèle d’interaction particule-surface tient compte des différents mécanismes de gravure sèche telle que la pulvérisation, la gravure chimique réactive et la gravure réactive ionique. Le transport des produits de gravure est pris en compte ainsi que le dépôt menant à la croissance d’une couche mince. La validité du simulateur est vérifiée par comparaison entre les profils simulés et les observations expérimentales issues de la gravure par pulvérisation du platine par une source de plasma d’argon.With the reduction of feature dimensions, otherwise negligible processes are becoming dominant in microfabricated profile evolution. Improved understanding of these different processes is essential to improve the control of the microfabrication processes and to further decrease of the feature size. To help attaining such control, a 2D feature scale cellular simulator using Monte-Carlo techniques was developed. The calculation domain is discretized in square cells representing empty space, substrate or mask of the initial system. Neutral and ion species are inserted at simulation interface from their respective angular and energy distributions functions. Particles transport to the feature surface is calculated while taking into account ion reflection on sidewall and neutral reemission. The particles-surface interaction model includes the different etching mechanisms such as sputtering, reactive etching and reactive ion etching. Etch product transport is also taken into account as is their deposition leading to thin film growth. Simulation validity is confirmed by comparison between simulated profiles and experimental observations issued from sputtering of platinum in argon plasma source

Optical and Electrical Diagnosis of Atmospheric Pressure Plasma Jets

Radio frequency atmospheric-pressure plasma jets have gained popularity in recent years, both in academia and industry, due to their ability to produce reactive chemical species at relatively cold gas temperatures. Operating at atmospheric-pressure allows for greater scalability than low-pressure plasma discharges, that are confined to operate in a vacuum chamber, offering advantages over current manufacturing techniques. Operation at atmospheric-pressure has also resulted in growing research into use of plasmas for therapeutic applications in biomedicine. Atmospheric-pressure plasma devices are beginning to be certified as medical devices in clinical settings, utilising their efficient production of reactive species in a cold, dry environment. The underlying mechanisms behind these processes are poorly understood, especially in the highly complex chemical conditions surrounding biomedical applications. Researchers require knowledge of the plasma chemistry to infer what subsequent interactions are taking place. Once a particular mechanism has been established, the plasma chemistry in atmospheric-pressure plasma devices can be tailored and optimised for a particular application. To achieve this, investigators require not only identification of which species are present, but also their concentrations, and how species can be maximised or minimised to yield the best therapeutic effect. Diagnostics are required which can measure reactive species in ambient air, but also identify the underlying plasma dynamics responsible. To this end, novel picosecond two-photon absorption laser induced fluorescence is implemented, allowing for the first time, spatially resolved measurement of plasma produced atomic species in ambient air. Production of atomic nitrogen and atomic oxygen is linked to various plasma parameters such as: molecular admixture, voltage, and operating frequency. A new methodology for measuring plasma power in small radio frequency atmospheric-pressure plasma devices is presented, and has allowed for better understanding of the plasma dynamics. This has identified how reactive species can be maximised through increased plasma electron density, but also how they can be produced most efficiently. Furthermore, this methodology has allowed for the confirmation of different operating modes inside the plasma, in agreement with phase resolved optical emission spectroscopy. With the knowledge gained from how plasma dynamics and plasma chemistry changes with input parameter variations, it has been possible to identify key reactive species in industrial scenarios, such as the case study of photoresist removal at atmospheric-pressure

The 2022 plasma roadmap: low temperature plasma science and technology

The 2022 Roadmap is the next update in the series of Plasma Roadmaps published by Journal of Physics D with the intent to identify important outstanding challenges in the field of low-temperature plasma (LTP) physics and technology. The format of the Roadmap is the same as the previous Roadmaps representing the visions of 41 leading experts representing 21 countries and five continents in the various sub-fields of LTP science and technology. In recognition of the evolution in the field, several new topics have been introduced or given more prominence. These new topics and emphasis highlight increased interests in plasma-enabled additive manufacturing, soft materials, electrification of chemical conversions, plasma propulsion, extreme plasma regimes, plasmas in hypersonics, data-driven plasma science and technology and the contribution of LTP to combat COVID-19. In the last few decades, LTP science and technology has made a tremendously positive impact on our society. It is our hope that this roadmap will help continue this excellent track record over the next 5–10 years

Localized surface functionalization with atmospheric-pressure microplasma jet for cell-on-a-chip applications

Surface properties of biopolymers are crucial for providing topographical and chemical cues to affect cellular behaviors, such as attachment, spreading, viability, proliferation, and differentiation. As an effective surface modification technique, plasma treatment is often applied to enhance surface wettability, adhesion, and biocompatibility of polymers. This study concentrates on developing technical platforms, experimental procedures, and computational-statistical models to manipulate and control the cellular functions on specifically modified polymer surfaces. A novel freeform microplasma-generated maskless surface patterning process was developed to create spatially defined topological and chemical features on biopolymer surface. Global and localized plasma functionalization was performed on polycaprolactone (PCL) samples to introduce biophysical, biochemical, biological and structural cues to enhance cellular response including attachment, proliferation and differentiation. A plasma computational-statistical model was developed to predict the changes in biopolymer surface physicochemical properties following the oxygen based plasma surface functionalization. Furthermore, an integrated system including localized plasma functionalization was specifically designed for the development of biologically inspired devices. The capabilities, benefits, and challenges of the integrated multifunctional biofabrication system to develop cell-on-a-chip device were also illustrated. The objective of this thesis is to contribute scientific and engineering knowledge to the utilization of plasma chemistry to enhance surface functionalization, development of an engineering model for local plasma treatment, and integration of biofabrication processes to assemble cell-on-a-chip devices.Ph.D., Mechanical Engineering and Mechanics -- Drexel University, 201

Exploring gas-phase plasma chemistry and plasma-surface interactions: progress in plasma-assisted catalysis

2020 Spring.Includes bibliographical references.To view the abstract, please see the full text of the document