The Blechle Research Group investigates the chemical behavior of artificial plasmas and the way they interact with the surface of materials. The unique environment of low-temperature, non-thermal plasmas (NTPs) can be utilized for a variety of surface modification applications (including the design of semiconductors, biomaterials, catalysts, and much more). However, despite the wide-spread influence, much of their underlying chemistry remains unexplored. As such, we are investigating the synergy of the plasma-surface interface to provide insight into the type of reactions that occur and how we can control plasma conditions to achieve desired surface outcomes. The approach is two-fold: (1) understand the fundamental chemistry of gas-phase species produced in NTPs by different precursor gas sources and the role that plasma parameters (applied power, precursor flow rate, etc.) have on gas-phase composition; and (2) investigate the behavior of these species when in contact with substrates to establish trends in surface-mediated reactions and the means by which they occur. To fully understand these environments, we must investigate the distinct regions of the system highlighted in Figure 1 (below):
Fig. 1. A representation of a plasma reactor cross-section, highlighting the three distinct regions of interest for this work: the plasma bulk, the substrate, and the region in which these two distinct systems interact with one another.
NTP systems have become a ubiquitous part of materials modification processes. They require less energy, generate less waste, and provide a greater degree of surface control compared to traditional wet-chemical methods. Additionally, as seen in Figure 2, plasmas can be tuned to produce a variety of outcomes including deposition, etching, functionalization, and activation – or a combination thereof. Optimizing these systems to achieve a specific result, however, is generally a taxing process of trial-and-error, given the complex chemical nature of the NTP itself. Then, even when optimal conditions are found, reproducing these results in other reactor systems can be difficult due to the vast parameter space. Thus, by exploring the chemistry of the plasma-surface interface, we hope to gain further insight into the mechanisms that govern the resulting material. Such knowledge would allow for reliable and tunable surface outcomes across a range of NTP systems.
Fig. 2. A representation of possible outcomes that can be achieved via plasma-surface interactions.
Our home-built NTP systems utilize inductively-coupled radio frequency. These reactors are highly customizable, depending on the project being studied, but a general schematic can be seen below in Figure 3. Our primary diagnostic tool is optical emission spectroscopy (OES) which can be used for in situ monitoring of excited state species within the plasma. This allows for the determination of both species identity and density, with and without a material present. Additionally, there are a number of characteristic energies that can be monitored within plasmas, including translational, rotational and vibrational temperature (TT, TR and TV, respectively) of the non-ionized species. Evaluating these temperatures can also be accomplished using OES, and they provide insight into the complex energetic environment of the system. A direct comparison of these energetic parameters in the presence of a substrate can be used to elucidate the mechanisms of species formation and evaluation of the system's non-equilibrium nature. Lastly, with time-resolved OES, we can monitor the kinetics of key reactive species providing further insight into the plasma-surface interface.
Fig. 3. A schematic of a standard NTP reactor utilized within the Blechle Research Group.
Material outcomes can be influenced by the plasma parameter space and the location of the material within the reactor. Whether these species are involved in adsorption/desorption mechanisms, molecule-surface quenching, or a combination thereof, is of significant importance. Each of the measurable quantities of bulk plasmas (TT, TR, TV and reaction rate) can be correlated to surface outcomes. By evaluating the resulting material via water contact angle (WCA) goniometry and IR spectroscopy, we can further elucidate the synergy of the plasma-surface interface. One such example can be seen in Figure 4, which compares the rotational temperatures of both ground and excited state nitric oxide with and without a silicon substrate present.
Fig. 4. Data points represent internal temperatures determined for NO in the presence of a silicon substrate as a function of applied power. Ground state (purple) and excited state (orange) species were formed in a 100 mTorr NO plasma. Dashed lines represent lines of best fit for data collected under identical conditions, without the presence of a substrate.