Symposium EQ09—Cutting-Edge Plasma Processes for Next-Generation Materials Science Applications

The interactions between biomaterials and biological tissues and body fluids depend on the surface properties of the biomaterials and biological interactions and response. However, many types of current biomaterials with favorable bulk characteristics cannot produce the desirable biological and biochemical effects and surface modification is one of the effective ways to alter selectively the surface properties of biomaterials and biomedical implants. By means of plasma surface modification, predesigned surface properties such as biocompatibility and bacterial resistance can be endowed and at the same time, the inherent bulk properties such as mechanical strength can be maintained. Our research group has been conducting research on plasma immersion ion implantation and deposition (PIII&D) for more than 20 years. It is a non-line-of-sight technique and especially suitable for biomaterials and biomedical implants with a complex shape. Moreover, since the interface between biodegradable biomaterials and tissues and body fluids is dynamic, PIII&D offers the unique capability to optimize the interfacial physics and chemistry to achieve controlled and timely degradation and drug release. In this invited presentation, recent research activities in the Plasma Laboratory of City University of Hong Kong pertaining to surface treatment of biomaterials and biomedical devices, especially biodegradable and antibacterial materials, will be described.

Keywords: biomaterials, surface modification, plasma immersion ion implantation, antibacterial properties, osseointegration

Inguinal hernia occurs when an intestinal abdominal organ mostly part of small intestine protrudes into the inguinal canal through a weak spot in the abdominal muscles. Such weak spot results in the inner lining of the abdomen to pushes through the weakened area to form a balloon like sac. Up to date identified associated causes for inguinal hernia include increased pressure on the abdomen, pregnancy, obesity, heavy lifting and continuous coughing and sneezing. Recent data has shown that in the US, at least 10% of the population experiences some type of hernia that requires at least one million of abdominal hernia repair surgery each year. The hernia is repaired by an incision through the skin and then covered with a piece of surgical mesh. The most common material for such surgical meshes used in the US is polypropylene due to inert chemical property, light weight and mechanical strength that could withstand up to 170 mmHg of abdominal pressure. However, the main drawback with such polypropylene meshes (PoM) is bacteria growth induced infection after implanting the mesh in the body within the textile woven structure of the PoM. Such infection is mainly due to uneven topography of the mesh structure and is responsible for up to seven percent of post-operational complications that result in significant health risks and costs for prolonged patient treatment. Therefore, to address this issue, we have developed an innovative two-step process for fabrication of an antibacterial PoM via atmospheric plasma surface modification and functionalization and laser immobilization. First, the surface of the mesh was coated with 50 nm thick organosilicon by atmospheric plasma deposition system to enhance the hydrophilicity of the mesh surface and as a receptor layer for silver nanoparticles (AgNPs). Next, a thin layer of < 1µm AgNPs in the form of low viscosity ink was printed on the plasma deposited mesh via inkjet printing as an antibacterial agent. And then, non-destructive laser treatment technique was employed to selectively immobilize the deposited AgNPs onto the mesh. Such technique enables selective delivery of the laser thermal energy to the printed AgNP ink and fuse the particles together whereas the PoM is transparent to the laser wavelength and would not get damaged under laser beam irradiation. In addition, such fusion enhances sustained antibacterial properties of the mesh by delaying the release of the AgNPs immobilized layer from the PoM when implanted. A systematic study was performed against Staphylococcus aureus and Escherichia coli to assess the antimicrobial properties of the AgNPs immobilized PoM. Moreover, thermal, optical and mechanical characterizations evidenced no significant effect of plasma surface modification and functionalization and laser AgNPs immobilization on microstructure of the PoM.

Because of the COVID-19 pandemic, there is need to urgently advance a safe, sustainable, and highly efficient disinfection technologies to treat face masks and various surfaces that are regularly exposed to public use. The demand of masks is challenging especially for the societies in less-developed areas across the world. Billions of used masks are threatening the environment as a new source of plastic pollution. Creating a safe and reliable method for mask reuse would help alleviate the situation. To this extent we developed a corona discharge (CD) based disinfection method to efficiently disinfect and recharge face masks simultaneously. Electric field, ions and reactive species generated by CD are the key components causing DNA damage and protein denaturation to effectively disinfect the 1.8 mm thick, dielectric, porous N95 masks. Up to 99.99% reduction in E. coli can be easily achieved under 10 mins, consuming only 1.25 W of power. This makes it ideal for affordable and portable disinfection and reuse applications. Further, up to 99.99999% reduction can be achieved after 3 cycles of treatment with optimized parameters. Most importantly, CD can simultaneously restore electrostatic charges on the mask surface to prevent filtration efficiency deterioration. It also provides an efficient disinfection solution for shared surfaces, confined spaces, and possibly open air reduce application of disinfectants and to mitigate the spread of COVID-19. We are also developing a corona discharge (CD) based disinfection robot with for the purpose of automatic and non-contact disinfection. The advance of CD disinfection will help us get better prepared for future pandemics.

Hybrid non-thermal plasma catalysis has a significant potential to provide a low energy pathway to activate molecules and catalysts to enable processes to operate at lower temperatures than would occur if activated thermally. This talk will show how plasma activation can be utilised to promote the gas phase reactions for emsission control and clean hydrogen production. The mechanism of the reactions determined by kinetic analysis and in-situ diffuse reflectance infra-red Fourier transform spectroscopy (DRIFTS) will be shown and how this differs from the thermally activated reactions. A new cell was developed to enable the DRIFTS measurements to be performed in the presence of the plasma. A range of catalysts have been examined including the use of metal organic frameworks which show high levels of stability under plasma activation which opens up the possibility of using them as heterogeneous catalysts in the gas phase which is challenging under normal operating conditions in many cases.

Conductive polymers such as polypyrrole (PPy), polyaniline (PANI) and poly(3,4-ethylenedioxythiophene) (PEDOT) are extensively used as sensing materials and conductive electrodes in printed and flexible sensors. As sensing materials, conductive polymers provide an electrical response to changes in temperature, gas concentration, humidity and/or radiation exposure that easily couple to simple electronics to achieve real-time detection and wireless readings. Typical synthetic methods for the fabrication of conductive polymer films are complex chemical processes that involve several steps of processing and the use of toxic solvents such as sulfuric acid, N-methylpyrrolidinone, and acetonitrile. Conductive polymers prepared via these methods are required to be in the form of suspended solutions which eventually need to be deposited onto desired substrates through either printing or additive manufacturing process. In addition, the films produced by these methods often exhibit inferior properties due to challenges associated with solvent-substrate incompatibility, poor film adhesion, remnant of toxic solvents in the films and long drying times. In order to address such drawbacks an effective approach is to deposit such functional conductive polymers directly onto the targeted substrates of interest.

Recently, cold atmospheric plasma chemical vapor deposition (CAP-CVD) has become an attractive approach to synthesize and deposit different organic and inorganic functional materials directly onto the desired substrate in atmospheric conditions. In CAP-CVD, conductive polymers are synthesized by plasma polymerization where the precursor of interest is introduced into the plasma stream and polymerized on the substrate by the action of the reactive species present in the plasma glow. Thus, post processing operations like solvent drying or sintering are eliminated. The resultant conductive polymer films can act as active layers to produce printed gas and radiation sensors with high quality, good adhesion, and intrinsic conductivity. Moreover, since this process deposits high quality films at high production rates, it is highly compatible with roll-to-roll manufacturing techniques to produce low cost printed sensors.

As proof of concept, a systematic study on different plasma processing conditions including power, an ionizing gas mix composed of oxygen and argon, and introduction of oxidants such as bromine was carried out on the CAP-CVD polymerization of PANI, PPy and PEDOT. In this work we present our investigation on the optimum processing conditions to deposit high quality conductive polymer films by CAP-CVD as well as the characterization of plasma conductive polymers by using Raman Spectroscopy, ATR-FTIR, TGA and SEM techniques.

Ammonia synthesis from N₂ and H₂ through non-thermal plasmas has recently attracted great attention as an alternative to standard ammonia production methods, such as the Haber-Bosch process (HB process). Production of ammonia through the HB process has made a significant positive contribution via production of fertilizers directly linked to the high crop yields. However, the environmental problems associated with the HB process threatens the sustainable future of mankind with massive anthropogenic emissions, estimated at over 300 million metric tons of CO₂ annually [1]. Non-thermal plasmas could significantly reduce ammonia production associated greenhouse gas emissions as plasma-based reactors can be operated entirely by renewable energy resources. The synergistic effects between catalytic materials and plasmas in particular are being widely investigated to improve the yield and energy cost in plasma-driven ammonia production. One bottleneck in this effort is the lack of a microscopic-level understanding of the interaction between the low-temperature plasma and various materials. In this work, a non-thermal plasma was pulsed with different frequencies, respective to plasma residence time, in order to investigate effects on ammonia yield and related energy costs. The measurements were performed on an RF-driven plasma system including a quartz cylindrical plasma reactor whose diameter was 1”, power supply, and matching network. The reaction products were sampled downstream of the reaction zone through a 50 μm orifice into a Residual Gas Analyzer (RGA). We observed a weak dependence of ammonia yield on various metallic catalysts [2]. This is consistent with a reaction mechanism that is initiated by the interaction between plasma-produced atomic nitrogen and the hydrogen terminated surface, via an abstraction mechanism. This pathway is consistent with the results from Born-Oppenheimer Molecular Dynamics (BOMD) simulation, which suggest that the direct surface abstraction pathway is fast and proceeds with near 100% probability. We have also investigated approaches to reduce the energy cost of this process and found that plasma pulsing is an effective strategy to achieve that. Measurements on energy cost was conducted at 60 W with various fraction of nitrogen flow rate per total flow rate (5%~90%). Based on the geometry of stainless-steel cone mesh utilized as a catalytic material, the residence time in the plasma reaction zone is around 40 msec. By decreasing the plasma on-time to 2 msec, it was possible to decrease the energy cost by roughly a factor of four, compared to the continuous plasma-on case. We interpret this measurement according to the following mechanism: plasma-produced atomic nitrogen diffuses to the catalyst surface on a time scale of one millisecond and it is quickly converted to ammonia, but the plasma is then rapidly extinguished to prevent it from dissociating the ammonia as it diffuses back into the reactor volume. In other words, pulsing utilized the electrical power input more efficiently as it is not used to re-dissociate the desired reaction product.

Reference:
[1] Y. Tanabe et al., Developing More Sustainable Processes for Ammonia Synthesis. Coordination Chemistry Reviews, 257, 2551−2564 (2013).
[2] S. S. R. K. C. Yamijala et al., Harnessing Plasma Environments for Ammonia Catalysis: Mechanistic Insights from Experiments and Large-Scale Ab Initio Molecular Dynamics, The Journal of Physical Chemistry Letters, 11, 10469-10475 (2020).

Ammonia is essential for food security due to its use in fertilizers. Currently, ammonia synthesis at industrial scale proceeds via Haber-Bosch (HB) process typically at ~500 C and 500 bar, which is the most energy-consuming process in the chemical industry. Global ammonia production (~249.4 million tons) consumes ~1-2% of the world’s energy, 2-3% of the world’s natural gas output, and emits over 300 million metric tons of CO₂. With such energy requirements, the HB process is only economically viable at large-scale plants running continuously. Consequently, ammonia synthesis is currently centralized, hampering the access to affordable fertilizers in remote areas . The development of simplified alternatives to HB at milder conditions and compatible with intermittent electric power (e.g. from renewable energy sources) is a critical step toward small-scale, decentralized ammonia production.
During the past decade plasma catalytic ammonia synthesis has been driven by the cost-effective implementation of solar and wind sources, that would also reduce the carbon footprint of the process. The possible implementation of non-thermal plasma technology will be only feasible if there is an effective plasma catalytic system in terms of material selection and reactor design. However, to-date there is no fundamental basis for the rational selection of catalytic materials suitable for plasma systems. Selection of catalysts based on fundamental principles would help in developing new materials and further understanding the synergies in plasma catalytic processes.
One of our central hypotheses is that an optimal catalyst for plasma-assisted ammonia synthesis is one that delays the recombination of adsorbed hydrogen radicals (H*) into molecular hydrogen (H₂), allowing them to bind instead to the adsorbed nitrogen plasma activated species (N^activated) to form NH*. We postulated that a delay in recombination of hydrogen radicals can be achieved with catalysts that dissolve hydrogen more easily. Therefore, our interest in oxides stems from the possibility of facilitating hydrogen dissolution at mild reaction conditions. Moreover, oxides may promote NH* formation (an essential step in ammonia synthesis) by not binding strongly with nitrogen. An illustrative example is silica, where the N₂ adsorption obeys a peculiar mechanism governed by short-ranged forces.
Oxides, such as silica, are appealing materials for ammonia synthesis due to their: 1) lower electrical resistivity that can lead to more stable and uniform plasma discharges, a challenge in atmospheric plasmas; 2) readiness to dissolve hydrogen; 3) weakly bonding with nitrogen; 4) high chemical stability in the presence of water and some hydrocarbons, which are typical impurities in natural gas wells (hydrogen for ammonia production is typically obtained from steam reforming of methane), and 5) the presence of a porous structure with accessible pores for guest molecules diffusion. Herein we employ mesoporous silica SBA-15 with unimodal pore size of ~ 7 nm to assess the effect of ordered mesopores with and without being Ag impregnated. Interestingly, our data confirms the dominant presence of N₂ vibrational bands and a marginal benefit in the catalytic performance when using a metal i.e., Ag in this case. At specific discharge conditions SBA-15 performs better than the Ag impregnated mesoporous silica leading to the highest ammonia production of 11,000 ppm. Plasma on/off cycles demonstrate the desorption of ammonia when the plasma was turned off. Which can lead to future optimal reaction conditions for this reactor when adapted to intermittent renewable sources. We were able to measure a vibrational excitation temperature using the optical emission of the second positive N₂ vibrational band of 0.2±0.05 eV for SBA-15 and SBA-15-Ag.

The nucleation and growth of nanoparticles in low-temperature plasmas is recognized as a desirable powder synthesis technique. These systems impart a unipolar charge distribution onto particles dispersed within the ionized gas, slowing down growth via agglomeration and typically keeping the particle size well below 10 nm.[1] In addition, plasma-induced heating allows producing high quality single-crystal nanoparticles. While these properties are unique to these gas-phase processing technique, it can be argued that the full potential of low-temperature plasmas for nanoparticle synthesis and processing is still severely underexplored. This talk will present two applications of low-temperature plasmas for nanoparticle growth that in our opinion suggest that there is significant room for growth in this field.
The first study involves the plasma synthesis of alternative plasmonic materials. Our group has recently shown that titanium and zirconium nitride nanoparticles can be obtained from low-temperature plasma reactors using the corresponding metal chlorides and ammonia as precursor couple.[2, 3] These ceramic materials have high free charge carrier density, showing plasmonic response in the visible part of the spectrum. Compared to standard plasmonic materials such as gold and silver, they are abundant, inexpensive and possess a high melting point, enabling application in high temperature environments. Example of the application of plasma-produce nitride particle for photochemistry and for wavelength-selective thermal emitters will be discussed in details.[4, 5]
The second case to be discussed is centered on the use of low-temperature plasma not to produce high-quality crystalline particles, but rather for the production of amorphous nanoparticles. Simple tuning of process parameters such as plasma power allows producing silicon particles with an amorphous structure. In addition, for the case of silicon particle produced using silane as precursor, we have found that these particle are rich in hydrogen. The hydrogenated amorphous silicon particles are highly desirable for application in nanoenergetics, where they show superior performance compared to their crystalline counterpart in terms of reaction kinetics because of the rapid release of hydrogen from the nanomaterial upon ignition.

1. Lopez, T. and L. Mangolini, On the nucleation and crystallization of nanoparticles in continuous-flow nonthermal plasma reactors. Journal of Vacuum Science & Technology B, 2014. 32(6): p. 061802.
2. Alvarez Barragan, A., N.V. Ilawe, L. Zhong, B.M. Wong, and L. Mangolini, A Non-Thermal Plasma Route to Plasmonic TiN Nanoparticles. The Journal of Physical Chemistry C, 2017. 121(4): p. 2316-2322.
3. Exarhos, S., A. Alvarez-Barragan, E. Aytan, A.A. Balandin, and L. Mangolini, Plasmonic Core–Shell Zirconium Nitride–Silicon Oxynitride Nanoparticles. ACS Energy Letters, 2018. 3(10): p. 2349-2356.
4. Barragan, A.A., S. Hanukovich, K. Bozhilov, S.S.R.K.C. Yamijala, B.M. Wong, P. Christopher, and L. Mangolini, Photochemistry of Plasmonic Titanium Nitride Nanocrystals. The Journal of Physical Chemistry C, 2019. 123(35): p. 21796-21804.
5. Berrospe Rodriguez, C., A. Alvarez Barragan, G. Nava, S. Exarhos, and L. Mangolini, Stabilizing the Plasmonic Response of Titanium Nitride Nanocrystals with a Silicon Oxynitride Shell: Implications for Refractory Optical Materials. ACS Applied Nano Materials, 2020. 3(5): p. 4504-4511.

Indium nitride is a direct bandgap semiconductor with a small bandgap of 0.7 eV. Properties such as high electron mobility and low electron effective mass make this material suitable for a variety of optoelectronic and electronic devices. Indium nitride can also be alloyed with gallium nitride to form a ternary compound whose bandgap can take a value from 0.7 to 3.4 eV allowing for use in applications that span from the infrared to the ultraviolet range of the spectrum. Here we present on our work to synthesize indium nitride nanoparticles. A low-pressure plasma reactor made from a glass tube was supplied with precursor gases of trimethylindium and ammonia along with a background gas of argon. A 13.56 MHz RF power supply was used to generate a nonthermal plasma in the reactor between three external cylindrical electrodes. During synthesis conditions were controlled to obtain nanoparticles with specific characteristics and properties. From image analysis of TEM data, we have been able to obtain indium nitride nanoparticles with a diameter smaller than the Bohr-exciton radius of this material. Average nanoparticle size as small as 3.6 nm with a standard deviation of 0.5 nm have been synthesized. XRD was used to characterize the crystallinity of nanoparticles, by controlling the power delivery to the plasma particle crystallinity was controlled. Primarily crystalline nanoparticles were synthesized at 75 W of forward power with a diffraction pattern consistent with hexagonal indium nitride. Surface analysis of the particles was conducted using FTIR which showed characteristic absorptions from bonds containing indium, nitrogen, oxygen, carbon, and hydrogen. Optical properties of the nanoparticles were probed with photoluminescence measurements and absorption spectroscopy.

Aluminum (Al)-based nanoenergetic materials have emerged as promising propellants and fuel additives in civilian and defense-related applications due to their superior gravimetric energy density, green combustion, and abundance on the earth. The native oxide layer on the surface of Al particles provides stability during storage, but adds weight, contributes no energy, and acts as a barrier to oxidation under combustion. For particles in the nanometer range, these limitations are serious because the oxide represents a significant fraction of the total mass. Hence, there is a need to eliminate the adverse effects of oxide on the energetic performance of Al nanoparticles. In this work, we demonstrate a nonthermal plasma-enhanced chemical vapor deposition (PECVD) to prepare core/shell nanostructures of Al with significantly improved energy release. We deposit thin fluorocarbon films (CF_x) onto the surface of Al nanoparticles and study their energy release under thermochemical analysis. We suggest that the perfluoro-based plasma films can improve the overall energy released from Al nanoparticles by providing F for the reduction of Al₂O₃ via an exothermic reaction to produce AlF₃, thereby utilizing the oxide to produce energy. We accomplish this by coating Al nanoparticles with F containing film via plasma deposition of a perfluorocarbon precursor. By depositing the right amount of coating, we aim to reduce the oxide in situ. As a result of the removal of the oxide barrier, we can derive maximum chemical energy due to improved contact between oxygen and metallic Al. We prepare the Al/CF_x core/shell nanoparticles using perfluorodecalin (C₁₀F₁₈) as a coating precursor in PECVD for different periods ranging from 30-165 min, corresponding to film thickness from 5 to 22 nm, as measured using high-resolution transmission electron microscopy (HRTEM). The thickness of the deposited films shows a linear relationship with deposition time with a rate of 0.15 nm/min. This allows for nanometer-level control of the film thickness and precise control of the amount of F on the surface of Al. The coated samples and the uncoated control were studied by thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC) to determine the effects of plasma films on their oxidation and energy release. All coated samples show increased weight gain, directly attributable to the conversion of Al into its oxide, and higher energy release relative to the uncoated control. In particular, at 55 min of plasma deposition, we obtain ~50% increase in the amount of energy released relative to the untreated sample. The composition of the plasma films was studied using high-angle annular dark-field (HAADF)-scanning transmission electron microscopy-(STEM)-energy dispersive spectroscopy (EDS), X-ray diffraction (XRD) analysis, and X-ray photoelectron spectroscopy (XPS). The results show the uniform distribution of F on Al nanoparticles and the presence of CF3, CF2, CF, and C-C bonds on the surface. The systematic increase of the energy released with deposition time is attributed to the increased amount of F in the coating. Optimum conditions are met when the amount of F matches the amount needed to react with the oxide on the particle surface. Above the optimum thickness, the gravimetric energy decreases because the coating adds more weight than the energy it extracts. In all cases, the energy of the treated particles is higher than that of the untreated control. This increase is further explained by the mechanism of interfacial reactions triggered due to the presence of perfluoro-based plasma films on the surface. XRD analysis along with SEM and TEM micrographs are used to support the claims in the explained mechanism. Thus, we conclude that the nonthermal plasma process has great potential in improving the performance of nanoenergetic materials by converting their native surface oxide into fuel.

Core-shell metallic nanoparticles such as nano-aluminum (nAl) are sought-after potential energetic additives due to the high heat of combustion of Al and potentially faster burning rate enabled by the high surface area to volume ratio. This work describes a recent effort conducted at US Army Combat Capabilities Development Command-Army Research Laboratory to increase the reactivity of commercial nAl by a two-step atmospheric pressure plasma surface treatment and coating using custom-made dielectric barrier discharge plasma reactors. The commercial nAl particles of nominal 40-60 nm average size were first treated with a helium (He) plasma for 30 min, followed by a subsequent coating of carbonaceous materials by diluting carbon monoxide (CO) in the He plasma for different durations. The He-plasma treated nAl has thin roughened alumina shell with its thickness more than 40% less than the as-received nAl with an intact single crystalline Al core. High resolution transmission electron micrographs (HRTEM) revealed sporadic crystalline deposits on nAl surfaces and in gaps among particles and short-ordered structure in the amorphous alumina shell for all plasma-treated samples, confirmed to be γ-alumina via electron energy loss spectroscopy (EELS) at the scanning TEM (STEM) mode. All plasma-coated samples after the second step possessed dispersive thin layers of carbonaceous coating, as evident by nanoscale chemical analyses with elemental mapping and X-ray energy dispersive spectra (XEDS) at STEM mode. The coating was determined as aluminum carboxylate via Fourier transform IR spectroscopy, a product from chemical reactions of Al and carboxylic acid functional groups coated on the He-plasma modified alumina surface. This work demonstrates the preliminary success of exploiting a new approach for mitigating the inactive alumina shell and further tailoring the superficial morphology and chemical properties of core-shell commercial nAl via a two-step atmospheric pressure plasma process. The efforts also emphasizes the significance of exploiting advanced materials characterization for a deeper understanding of the resultant nanoscale surface properties for ultimate optimization of plasma conditions.

Hydrophobic nanomaterials have drawn significant attention due to their applications in coatings, protein adsorption, nanomedicines, etc. For silicon nanomaterials, researchers have generally used external agents which react with the surface terminated Si-H bonds and render the surface hydrophobic. In the present work we investigate the hydrophobic nature of as-prepared silicon nanoparticles (SiNPs) produced in a nonthermal plasma reactor. SiNPs were synthesized in a flow-through reactor using silane with argon as the background gas. The total gas flowrate was 20 standard cubic centimeters per minute (sccm) and with a reactor pressure of 1.4 Torr. The plasma as generated using radiofrequency (rf) power of 10W at 13.56MHz is delivered via ring electrodes encircling the reactor. SiNPs formed in the plasma were collected on a stainless-steel mesh filter at the reactor exhaust. Further examination under Scanning Electron Microscopy (SEM) revealed the formation of flow-through aggregated wires of around 12 microns in length and 7-8 microns in diameter, comprised of primary SiNPs. Transmission electron microscopy showed the primary particle size to be in range of 5-7 nm, but no electron diffraction rings were detected, indicating that the Si NPs were amorphous.
Contact angle measurements were performed to test the hydrophobicity of the as-prepared SiNPs. A water contact angle of 146° was observed demonstrating the near-superhydrophobic (> 150°) nature of the aggregated SiNPs. The hydrophobic material was also successful in separating both vegetable cooking oil as well as heavy-duty pump oil from oil-water mixtures. Thermal experiments were also performed to illustrate the thermal stability of the hydrophobic nature. The nanomaterial was able to retain its hydrophobicity even at 450°C after 2h of heating showing robust stability. The material was also found to be stable under ambient air conditions as water contact angle did not change significantly even after a week of synthesis. These exciting properties signify the future prospects in coatings, biotechnology and more.

The combination of different materials in heterostructures often enables phenomena that are impossible to achieve in a single compound. A now-classic example is the interface-enhanced superconductivity of monolayer β-FeSe on (001)-oriented SrTiO₃ (STO). The observed superconducting gap in this system is substantially larger than in bulk β-FeSe due to a combination of electron doping and electron-phonon interactions at the FeSe/STO interface. Heterostructures derived from this FeSe/STO system are useful for the creation of layered quantum materials arrangements to study interface-enhanced processes, magnetic/superconducting proximity effects, and control of quantum properties via electrostatic gating. The plasma environment of pulsed laser deposition (PLD) is particularly suited for growing such heterostructures, either as stand-alone thin films on a variety of substrates or as designed multilayered and superlattice configurations. While characterizing the vacuum PLD environment in terms of the flux of depositing species, their kinetic energy, and the substrate temperature is generally considered sufficient for controlling film growth, in this work we explore how tuning more fundamental laser plasma parameters, such as the electron density and temperature, may alter the structure and interface of β-FeSe/STO heterostructures. We correlate measurements from Langmuir probe plasma diagnostics during FeSe growth with x-ray diffraction and x-ray reflectivity data from the epitaxial heterostructures obtained. Laser plasmas with electron density (n_e) below ∼5×10¹⁹ m^-3 and electron temperatures (T_e) of 0.1-0.2 eV lead to epitaxial c-axis oriented β-FeSe/STO with clear thickness fringes in dynamical x-ray diffraction (Pendellösung oscillations), indicating high crystal quality and sharp interface. On the other hand, plasmas with n_e ∼ 10²⁰ m^-3 and T_e ≈ 0.5-0.6 eV result in β-FeSe with a mixture of two epitaxial orientations, less defined interface, and a greater degree of mosaicity or defects. We will discuss how adjusting the laser plasma parameters may allow further control in plasma-mediated growth of β-FeSe/STO such as spatial modulation of nucleation sites, modification of island growth patterns, and activation of plasma-enhanced surface reactivity.

The forward peaking of the angular distribution of plasma species is one of the most distinctive characteristics of plasmas produced by pulsed nanosecond UV laser ablation of solids. It is the result of gas dynamics effects and it depends strongly on the photo-physics of the laser-plasma interaction. In pulsed laser deposition, where the plasma may comprise a variety of chemical species and ionization states, chemical heterogeneity in the angular distribution may affect the quality and properties of the resulting thin films. In processing applications, lack of control of the angular distribution may result in dissimilar effects in different locations of a single sample. In this work, we use a laser ablation/fluid plasma expansion model to simulate the angular distribution of single- and multi-species laser plasma expansions. We implement the model for plasmas produced by the ablation of elemental and compound metallic targets. The fluid simulation accounts for singly and doubly charged ions, as well as electrons and neutrals. The simulation is carried out over the centimeter distances required for proper evaluation of the angular distributions in a multidimensional setting using a state-of-the-art, open-source adaptive Cartesian mesh framework. The compressible solvers available in this framework (Riemann and all-Mach solvers) were adapted to include the equations of state of the plasma, which link local Saha equilibrium with augmented ideal gas expressions that include the internal degrees of freedom of the atomic species. Our implementation allows broad and detailed studies of the time-dependent angular behavior of all plasma constituents. This includes distributions of neutrals, ions, and electrons, as well as temperature and flow velocity profiles. As an example, the ablation of FeSe by the focused beam of a single KrF excimer laser pulse (248 nm, 25 ns, 10 mm² spot area), leads to a predicted angular distribution for peak number density of Fe⁰ with FWHM of ∼80° at a fluence of 2.0 J/cm². The FWHM decreases to ∼71° for 3.2 J/cm², in qualitative agreement with typical angular distributions observed in film growth. For the same conditions, the angular distribution for peak number density of Fe⁺ (i.e., singly charged ions) has FWHM of ∼40° and ∼22°, respectively. These results allow creation of spatially resolved maps of the degree of ionization during plasma irradiation of materials, enabling new types of gradients to induce materials transformations. We will discuss differences in elemental vs. compound metallic plumes and subtle variations in predicted angular distributions of different chemical species in compound plumes, which are critical for reproducible film synthesis and processing.

Pulsed laser deposition (PLD) is a versatile plasma-based technique for nonequilibrium growth of materials of complex stoichiometry. Kinetic manipulation of materials processing in PLD requires controlling the plasma parameters of its chemically intricate and transient plasma flows. In this research we compare trends in the plasma parameters measured for a laser plasma with predictions from a laser ablation/plasma fluid expansion simulation. Planar and cylindrical Langmuir probes were used to measure the ion and electron densities, as well as the electron temperature and Mach number of the plasma expansion. The laser plasma simulation couples laser-induced surface evaporation with fluid plasma expansion and is implemented in 2D-axisymmetric coordinates, effectively delivering predictions for the 3D temporal evolution of the corresponding plasma parameters. The simulation accounts for singly and doubly charged ions, as well as electrons and neutrals, and is performed in a multidimensional setting using a state-of-the-art, open-source adaptive Cartesian mesh (ACM) framework. The use of the dynamic ACM approach is essential for reliable evaluation of plasma parameters. This is because the initial (t < 30 ns) spatial resolution required near the target is tenths of microns, while the simulation domain is larger by multiple orders of magnitude. As the plasma plume expands (t >30-60 ns), the fine near-target resolution is no longer required for most of the simulation domain which allows the grid to be coarsened. However, proper resolution of the moving plasma front is still necessary throughout the entire plasma dynamics until the front reaches the anticipated substrate location. Only the very narrow plasma front region needs to be resolved for this purpose, while the remainder of the computational mesh can stay coarse. In this combined experimental and computational investigation, we analyzed a bimetallic plasma containing iron (Fe) and selenium (Se), obtained by ablating a solid pellet of FeSe with the 25 ns pulse of a KrF excimer laser (248 nm). For laser spot areas below 4.5 mm², the peak values of the experimental electron density, electron temperature, and Mach number of the expansion all increase gradually when laser fluence is in the 0.5-4.0 J/cm² range. The same trend is reproduced in the simulation. We will discuss how the measured time dependence of the plasma parameters can be used to constrain the simulation and test mechanisms of laser plasma formation that need to be invoked for accurate predictions of PLD plasmas containing multiple chemical species.

Titanium alloys are used in many industrial sectors including aeronautics and biomedical sectors, due to their low density combined with excellent mechanical properties (high strength and Young's modulus, corrosion resistance, etc.). Surface properties are also of paramount importance, for example to ensure good resistance to wear or to increase biocompatibility, main properties sought after in the biomedical field. Thus, many surface treatments are carried out, among which oxidation treatments, which in addition to the properties mentioned above, affect to varying degrees the fatigue resistance of the part treated depending on the degree of roughness, porosity and density adhesion of the formed layers. In the present work, we presents an original oxidation treatment based on the coupled use of laser shock and microwave plasma oxygen implantation (MWPO) in order to improve in the first place the adhesion of the oxide layers, while inducing new functionalities. The process is versatile enough allowing (i) to obtain adherent oxide layers without adversely modifying the substrate, and (ii) to provide additional functions to the material treated, we propose here to explore a new combination of the two MWPO and ceramic conversion (CC) treatments, completed by preliminary mechanical hardening by laser shock. Here, the CC treatment consists of 2 successive stages of high temperature thermal treatment of oxidation then diffusion. The implementation of this process has been carried out initially on single-phase pure titanium. The treatment made up of 3 stages: (i) a preliminary surface treatment by laser shock, in order to create special grain boundaries on the surface making it possible to improve the diffusion of oxygen during the following two stages, (ii) an oxygen implantation step by MWPO process to produce more or less thick oxide layers, depending on the process parameters without modifying the substrate and (iii) a thermal step (of CC type) to allow more partial diffusion of oxygen and thus improve the adhesion of the layer. In order to keep the benefit of the previous steps and to simplify the process, this 3rd step has been carried in the plasma reactor at a lower temperature than the standard CC treatment. The characterization of the thickness and adhesion of the layers, the observation of the substrate microstructure by SEM/EBSD, the characterization of roughness and microhardness and the characterization of fatigue behavior have been performed.

Applications of pure nanoscale materials have rapidly expanded in recent years and with the development of non-thermal plasma as a synthesis method, these materials can be produced with high purity and narrow size distribution compared to standard thermal or wet production methods. Unfortunately, non-thermal plasma synthesis has been limited to structures below 10 nm in diameter due to unipolar charging effects inhibiting particle growth, thus restricting potential applications [1]. For instance, current efforts in lithium-ion battery development are focused on integrating nanoscale silicon into the anode with a theoretically optimal size near 100 nm in diameter [2]. The relaxation of particle net charge outside of a plasma and re-initiation of the plasma-instigated growth process merits investigation as the reduction of net charge may allow further particle growth through increased particle collision.

To perform this investigation, an RF capacitive plasma was simply pulsed at different frequencies with respect to a particle’s residence time within the plasma reactor. Silicon particles were produced through the decomposition of silane gas in the plasma flowing through a cylindrical region top to bottom and collected onto a stainless steel mesh. Ex-situ characterization was performed via Transmission Electron Microscopy (TEM) and Scherrer’s Analysis through X-Ray Diffraction (XRD). Effects of reactor pressure and length were tested and accounted for in the produced materials, showing generally understood trends. Additionally, pulsing frequency with respect to particle residence time, plasma exposure time (different pulsed frequencies with identical amounts of time in plasma), and duty cycle effects were tested. These pulsing trails enabled production of larger particle diameters in comparison to their non-pulsed counterparts. A constant plasma condition with a reactor pressure at 8 Torr produces nanopowder with average particle size of 8.0 nm; however, pulsing conditions were able to achieve an average particle size of 22.6 nm. Trends relating to crystallinity and new particle size limitations were also observed.

Materials produced at varying pulse rates were coated with graphitic carbon and tested in lithium-ion batteries to observe cycling stability and associated charge capacity. Finally, numerical simulations were performed to explain the altered charging and growth mechanics of particles repeatedly exposed to non-thermal plasma conditions.

[1] Lopez, T., Mangolini, L., On the nucleation and crystallization of nanoparticles in continuous-flow nonthermal plasma reactors. J. Vac. Sci. Tech. B 32(6), 061802 (2014)

[2] Zhu, G., Wang, Y., Yang, S., Qu, Q., Zheng, H., Correlation between the physical parameters and the electrochemical performance of a silicon anode in lithium-ion batteries. J. Materiomics 5(2), 163-175 (2019)

In recent years, carbon nanodots (CDs) have been of great interest for their applicability in areas such as solar energy harvesting, nano-sensing, photocatalysis, etc. Traditional routes for synthesis for CDs involve both top-down methods like laser ablation, electrochemical synthesis, and chemical oxidation, as well as bottom-up techniques like hydrothermal synthesis, microwave pyrolysis and others. In this work we report the synthesis of CDs in a nonthermal radiofrequency plasma reactor using methane as the carbon precursor. Nonthermal plasma synthesis offers several advantages over traditional routes in terms of simplicity, solvent free synthesis and near room temperature growth of nanoparticles. Radiofrequency power at 13.56 MHz was supplied through ring electrodes to a flow-through reactor. The methane precursor was diluted with argon and hydrogen (H₂), in varying concentrations, for a total flowrate of ~100 sccm. The reactor pressure was kept at 2.9 Torr. The CDs formed in the plasma were deposited onto arbitrary substrates via inertial impaction. The synthesized material was characterized using Transmission Electron Microscopy (TEM), Scanning Electron Microscopy (SEM), Raman spectroscopy, and UV-Vis optical absorption. Raman spectroscopy exhibited two significant features: the D band around 1330 cm^-1 and the G band around 1590 cm^-1. While Raman spectroscopy did not reveal significant modifications in the bonding configuration upon changing synthesis parameters, by controlling the hydrogen content in the plasma via introduction of H₂ gas, there arose a dramatic shift in the visual appearance, nanostructure, and UV-Vis optical absorption. Under hydrogen-poor conditions, the CDs were black and accompanied by optical absorption across the visible range, while hydrogen-rich conditions led to white-appearing CD powders with an onset of optical absorption in the UV. TEM image analysis indicated that the average particle size was around 6nm under hydrogen-poor conditions, and much larger at >40nm for hydrogen-rich conditions. Additionally, the black CDs appeared to be in nano-onion-like morphologies, as evidenced with TEM imaging, while the white CDs appeared entirely amorphous with no layered structure within the primary particles, consistent with highly diversified sp² and sp³ bonding within the CDs. Our future work will quantify the degree of sp³ hybridization for CDs of both varieties, as well as measuring the effect of hydrogen during synthesis on the electrical conductivity of CD films created by inertial impaction directly from the reactor.

Nonthermal plasmas have attracted increasing attention for the synthesis of groups IV and III-V semiconducting nanocrystals owing to their nonequilibrium environment which enables the nucleation, growth, and crystallization of materials with high melting point from gas phase precursors at room temperature. The resulting negatively charged nanocrystals with a narrow size distribution can be accelerated to a high velocity through a nozzle to form thin films via impaction onto a substrate. The scalability of this single step, ligand-free, amenable to doping, and high yield deposition technique has recently been demonstrated by depositing uniform nanoparticle films of tunable thickness and porosity on 125 mm-diameter silicon wafers. Despite impressive applications in electronic and light-emitting devices, plasma-synthesized thin films have come short in supporting optically-induced magnetic resonances (OMR) due to the small size of their constituent nanoparticles. This is mainly because of the short nanoparticle residence time in dusty plasma reactors with a uniform diffuse discharge resulting in particle sizes in the range of 2 to 10 nm. Synthesis attempts aimed for larger nanocrystals in diffuse discharges via changing the operating conditions, including the flow rates, operating pressure, tube size, and plasma power usually lead to either polycrystalline or amorphous nanoparticles, or nanocrystals with a wide size distribution. Here, we report the synthesis of monodisperse Si nanocrystals of over 80 nm with a standard deviation of 1.0 nm by intentionally operating the plasma in a regime where the discharge is constricted. In this regime, the plasma consists of two regions: a diffuse region that extends a few centimeters upstream of the RF electrode, and a striated high-luminosity plasma filament that rotates close to the wall of the tube between the RF electrode and the lower flange. Previous laser scattering studies have shown that the particle growth begins in the diffuse plasma and particles are trapped in a region upstream of the electrode. The filamentary plasma then most likely sinters the particles by annealing them to temperatures much higher than the gas temperature resulting in formation of single crystals. Using this method, we deposited a 550 nm thick fractal-like random film of resonant Si nanocrystals in an air host matrix with a 23% volume fill fraction on a glass substrate. Angle, polarization, and wavelength resolved reflection and transmission measurements of the film were performed. The film shows an absorption peak of close to an ideal blackbody at the particles’ OMR with a broadened absorption tail emanating from the OMR and progressing out to longer wavelengths. Overall, the particle film is over 38% more absorbing compared to an antireflective-coated crystalline silicon slab of the same thickness using 77% less material. The particle film has antireflective properties regardless of the form of the incident excitation as evident by the fact that transmission ≈ 1− absorption. Similar to a blackbody, there is a strong angle insensitivity of absorption up to angles of incidence as steep as 70 degrees at the OMR spectral location. The film’s optical response also shows significant polarization independence. This study paves the way for synthesis of resonant Si nanoparticles using nonthermal plasmas and displays the unique optical properties of 3D films fabricated by this single-step technique.

Acknowledgements: This work is supported by the Army Research Office under MURI project under W911NF-18-1-0240. Portions of this work were conducted in the Minnesota Nano Center, which is supported by the National Science Foundation through the National Nanotechnology Coordinated Infrastructure (NNCI) under Award Number ECCS-2025124.

2D materials have been the focus of intense research in the last decade due to their unique physical and chemical properties. This presentation will highlight our recent progress on the synthesis of two-dimensional transition metal di- and tri-chalcogenides (2DTMCs) for nanoelectronics and catalysis applications using atomic layer deposition (ALD). ALD-grown 2DTMC films typically exhibit a high density of out-of-plane 3D structures in addition to 2D horizontal layers. While the out-of-plane 3D structures are ideal for catalysis applications, the presence of such 3D structures can hinder charge transport, which hampers device applications. In this presentation I will show how we used mechanistic insight obtained by HRTEM and DFT simulations to tune the shape and density of the 3D structures during plasma-enhanced ALD. The obtained morphology control was further confirmed by electrocatalysis and electrical measurements.
In the second part of my talk I will demonstrate that we also can modulate the crystal phase of transition metal chalcogenides through plasma-enhanced ALD. Phase-control between the metallic TiS₂ and semiconducting TiS₃ phases was achieved by carefully tuning the co-reactant (H₂S gas vs plasma) and deposition temperature during ALD. These two material phases were differentiated using a variety of characterization techniques.
This work sets the foundation for achieving electrical and catalytic property modulation through phase and morphology control in low-dimensional materials using plasma-enhanced ALD.

Low-temperature plasmas (LTPs) have long been recognized as effective in enhancing physical and chemical processes during materials synthesis. The emergence of two-dimensional (2D) materials has renewed the interest in using LTPs for engineering these systems at the atomic scale. LTPs generated by laser ablation are of particular interest because of their rich chemistry and spatiotemporal phenomena. These plasmas can produce reactivity, temperature, and stress profiles to induce chemo-thermo-mechanical materials transformations that are not accessible by other approaches. The kinetic energy of ions can be tuned in the ∼1-400 eV range and ion densities controlled between 10⁹ and 10¹⁵ cm^-3. Compressive stress ranging from kPa to tens of GPa, due to energetic bombardment, and temperature pulses between hundreds and thousands of kelvin can be delivered by the leading edge of the plasma expansion. The sequence and timing of these conditions are adjustable, allowing new types of nonequilibrium processing. Despite their use in solid state physics for decades, there is a vast domain of unexplored plasma regimes with potential for breakthroughs in 2D materials research. In this talk, I will review the state of the art in plasma synthesis and processing of 2D materials, such as graphene, transition metal dichalcogenides, and atomic-monolayer epitaxial quantum materials. In particular, I will show how Langmuir probe, optical emission spectroscopy, gated-intensified CCD imaging, and laser-induced fluorescence spectroscopy diagnostics are being used in conjunction with particle-in-a-cell and fluid simulations to control plasma parameters and effect materials transformation in 2D and quantum materials.

The low-temperature plasma synthesis of nanoparticles has many benefits over conventional powder production methods. Some of these include the ability to produce high purity semiconductor and ceramic nanoparticles with narrow size distributions[1]. By varying the ratio of precursors, the final composition of the materials can be finely controlled. In addition, the relatively low temperatures of non-thermal plasmas enable the production of amorphous nanoparticles, providing opportunities and potential applications that are still severely underexplored by the plasma community. This study focuses on the plasma processing of structural materials, with particular attention to amorphous ceramics. For this work, the silicon-carbo-nitride system (SiCN) was chosen because of its high temperature structural properties. SiCN has been shown to have good creep resistance and oxidation resistance[2] as well as high hardness[3] at temperatures past 1000°C. However, SiCN is primarily produced polymer pyrolysis and ceramisation of organosilicon precursors, which can make it difficult to control the final composition of the material and typically crystallizes durring sintering. We have successfully produced bulk amorphous SiCN samples by sintering plasma-produced nanoparticles. The resulting hardness, fracture toughness, and microstructure of the samples will be discussed in detail.

[1] Mangolini, “Synthesis, Properties, and Applications of Silicon Nanocrystals.”
[2] Raj et al., “Oxidation Kinetics of an Amorphous Silicon Carbonitride Ceramic.”
[3] Herrmann et al., “Silicon Nitride/Silicon Carbide Nanocomposite Materials.”

All-solid-state potentiometric sensors based on printed solid-contact ion-selective electrodes have shown great potential for point of care and portable diagnostic systems due to their simple integration, cost-effectiveness, and scalability. Despite the low cost such printed electrochemical sensors, they are often unsuitable for long-term monitoring application due to the sensor drift. One of the main causes of drift in printed ISEs is the water-layer-formation between the ion selective membrane (ISM) and the electrode resulting in an interfacial potential change overtime. To date, significant efforts have been undertaken to improve the electrochemical performance and stability of the solid contact- ISM in order to modify the interfacial properties of ISM and electrode. However, their added fabrication steps, increased cost, and need of various chemical agents have restricted their wide application. In this study, we have attempted to address the current challenges by employing a simple, scalable, and novel atmospheric plasma surface treatment to modify the screen-printed carbon electrode surface and improve interfacial boding between ISM and carbon electrode. As a proof of concept, the plasma surface modification effect was assessed with nitrate ISEs. Our results showed that plasma-treated electrodes had a significantly lower potential drift as compared to ISEs that were not surface treated with plasma. In addition, plasma treated ISEs demonstrated an improved reproducibility with less potential variation between fabricated sensors as compared to untreated electrodes. The enhanced electrochemical performance of plasma-treated ISE can be explained by the improved interfacial bonding and reduced water-layer, which was also confirmed by standard mechanical peel tests. Raman spectroscopy analysis of the carbon surfaces revealed a highly organized graphenic structure on the printed carbon surface originated by atmospheric plasma treatment. Effect of plasma treatment on reduced water uptake and generated graphene structure were further confirmed by electrochemical impedance spectroscopy with decreased charge transfer resistance for the plasma treated ISEs. Delamination of ISM for the untreated electrodes, as characterized by scanning electron microscopy (SEM), indicated the ingression of water at the interface for sensors with the untreated electrode while the plasma-treated electrodes showed a firmly packed configuration.

The synthesis of pure metal particles is of great interest for many applications including catalysis and metal additive manufacturing. For metal particle synthesis with nonthermal plasmas, metalorganic precursors with suitable vapor pressures are widely available. However, with many of these precursors it is difficult to avoid the incorporation of carbon contaminations into the metal particles. Metal chlorides are another group of attractive precursors. For many metals, chlorides are often produced as intermediates early in the extraction process or they are easily derived from other low-refined metal compounds. Most metal chlorides are liquid or solid at room temperature but many have attractive vapor pressures for low pressure synthesis. Here, we demonstrate the synthesis of aluminum (Al) nanoparticles from aluminum trichloride (AlCl₃). A vapor of AlCl₃ carried by an excess flow of argon is fed into a tubular low pressure nonthermal plasma reactor with one inch outer diameter operating at pressures around 5 Torr. Radiofrequency power between 80-200 W (nominal) is applied through inductive coupling. Hydrogen is added as a scavenger for chlorine. Aluminum nanoparticles with average diameters between 10-20 nm are produced with minimal chlorine contamination. While the nanoparticles oxidize rapidly when removed from the synthesis reactor, X-ray diffraction and transmission electron microscopy provide clear evidence for the presence of Al nanoparticles.

This work was primarily supported through the US Army Combat Capabilities Development Command-Army Research Laboratory under Cooperative Agreement W911NF-19-2-0283.

Many approaches to graphene synthesis can require high temperature, strong/toxic reducing agents, or are expensive. The microplasma-electrochemical reactor (MEC), composed of an atmospheric-pressure microplasma with an aqueous solution as an electrode, may overcome these difficulties by providing physico-chemical conditions that are difficult to achieve otherwise. They can initiate non-equilibrium electrochemistry and nucleation in solution without additional heating or reducing agents, improving the efficiency of synthesis. In addition, the technological barrier to implementing MECs is low. MECs have successfully synthesized graphene quantum dot (GQD) synthesis in aqueous solution [1,2].

One of the main challenges going forward is to develop a detailed mechanism of GQD growth, which is a general difficulty in plasma-based nanomaterials synthesis due to the complexity of non-equilibrium plasma chemistry and interactions with surfaces. So far the relevant species and reactions have mainly been inferred from ex situ or macroscopic effects, with a limited degree of detail and certainty. The liquid-phase diagnostics of the plasma-water interfacial region can provide new insight into how the MEC transforms the precursor into GQDs. Current experimental techniques used for the analysis of liquid chemistry suffer from a lack of selectivity and/or degradation of dyes, chemical probes, or spin traps/probes introduced into the liquid. The spatial distribution of species is not often accessible. Most importantly, the majority of the diagnostics must be performed ex situ, removed from the plasma reactor and after treatment.

To address this challenge, we have developed an in situ multi-diagnostics approach to encompass a wide range of physical and chemical properties at the plasma-water interface. The centerpiece of this platform is in situ spontaneous Raman microspectroscopy, which offers several important advantages over the aforementioned diagnostic tools: non-intrusiveness, selectivity, versatility, and straightforward calibration. By developing a light-sheet technique, we have experimentally investigated the interfacial region with a spatial resolution as high as several tens of microns. To gain insight into the physical state of the solvent, we tracked the Raman spectrum of water. In particular, changes to the –OH stretch band shape indicate a weakening of the hydrogen bonding network of water with. These changes to the Raman spectra over the course of plasma treatment occur at both fast and slow time scales and become more pronounced as the detection volume approaches the interface. We also tracked the aqueous species H₂O₂ and NO₃^-, whose concentrations both increase when approaching the interface to within several tens of µm [3]. An interfacial layer of excess NO₃^- concentration was found to extend 28 µm in depth. Similar interfacial layers have been modeled for transient species such as OH radicals but not for NO₃^-, a stable product of plasma-activated water.

Raman spectroscopy of the liquid environment was complemented by in situ photoluminescence (PL) spectroscopy to track the appearance of GQDs in real time. The PL spectrum evolves differently according to the depth of detection in the liquid, and so particle image velocimetry of the liquid flow field was performed to gain an understanding of possible transport mechanisms. These measurements of the liquid phase were complemented by optical emission spectroscopy of plasma properties such as the electron number density and presence of excited species as a function of the distance from the interface. Together, these experimental results represent the fullest description to date of the physico-chemical environment enabling GQD synthesis and mark an important step towards the discovery of the reaction mechanism.

The expansion of HiPIMS as both a research and production technique has demonstrated the need to better understand transient events inherent to the discharge, specifically the Positive Kick^TM pulse, first commercialized by Starfire Industries, responsible for imparting ion energy to the workpiece at low gas temperatures. Previous research has shown that sputtered metal ions arrive at the workpiece with tunable ion energies depending on the conditions of the discharge, while limiting ion energies of working gases. Much remains to be understood about the nature of the plasma responsible for this.

In the current work, a Starfire Industries 2-2 Impulse^® with Positive Kick^TM was utilized in conjunction with an Impedans Semion gridded energy analyzer to determine the transient nature of the ion energy distribution function (IEDF) during the positive kick pulse. Simultaneously, time resolved Langmuir probe data allowed for the plasma potential, electron energy-distribution function (EEDF), and normalized plasma density to all be resolved with µs-resolution. An unbalanced magnetron was characterized for a variety of gas pressures, pulse lengths, kick pulse lengths and amplitudes.

Ion energies within 5% of the kick pulse bias voltage are consistently observed at the detector within several μs of the beginning of the kick pulse indicating rapid transit times for the plasma across the chamber. Plasma transit times were mild functions of pressure, but not of pulse lengths or amplitudes, as measured both by the IEDF and plasma potential. Shifts in the IEDF early in the kick pulse suggest a transition between mechanisms for ion transport as the plasma sheath forms. Additionally, decay times for ion energy distributions occurred on similar time scales suggesting the plasma expansion enabled by the kick pulse is short lived. Ion energies above the kick pulse bias voltage are consistently absent over the conditions tested (Semion detection maximum for this configuration is 150 eV) indicating that the pulse conditions may be suitable for the minimization of instabilities that may be deleterious to both film quality and uniformity of deposition. These and other results will be discussed in conjunction with time resolved electron energy distribution measurements to provide an understanding of plasma dynamics and the effect of the magnetron’s magnetic trap.

Plasmas generated by laser ablation are of interest for materials processing because of their rich chemistry and spatiotemporal phenomena. The chemical diversity, variety of gas backgrounds, and shock wave characteristics of laser-produced plasmas are particularly conducive to kinetic control of materials synthesis. Measurements of laser plasma parameters using Langmuir probes, TOF mass spectroscopy, optical emission spectroscopy, laser-induced fluorescence, as well as photography and imaging techniques have established numerous correlations between plasma parameters and processing outcomes. These correlations represent a substantial body of knowledge, but do not provide an exhaustive picture of the laser plasma processing environment. This is due in part to the difficulty in gathering direct experimental information on the strongly time and space dependent plasma parameters at the plasma-surface boundary. Moreover, processing of 2D materials has raised new questions about the ion densities, kinetic energies, and plasma temperatures that are useful for manipulation of these atomically thin systems. In this context, laser plasma simulations may offer insights into the plasma characteristics in the vicinity of the processing area which, in conjunction with experiments, could reveal plasma conditions for controlling processes such as doping, defect annealing, and kinetically-driven phase transformations. The complexity and broad technical applications of laser-generated plasmas have for decades been motivators for their study via numerical simulations. Modern studies based on a variety of fluid, kinetic, and hybrid models are well described in the literature. The majority of investigations have focused on one-dimensional (1D) descriptions of single-element plumes, very close to the ablation target surface (<1 mm) and over short timescales (<1 µs). Detailed analysis of this domain is valuable in applications dominated by near-target processes, such as micromachining, laser-induced breakdown spectroscopy, and laser sintering/additive manufacturing. Simulations up to several centimeters in length and times >10 μs, have received much less attention but are needed to guide materials synthesis and modification approaches, where substrates are placed several centimeters away from ablation targets. Simulations over long distances are computationally costly and must include three-dimensional (3D) effects. They also need to account for the thermo-physical properties of specific chemical species. While embedding detailed mechanisms is desirable, simulations of practical utility need to be fast and meet predictive benchmarks. In this work we balance these demands using an effective model that incorporates standard features of UV nanosecond laser ablation plasmas: thermal evaporation, bremsstrahlung emission from plasma electrons, as well as photoionization and free-free laser absorption by the plasma. An additional effective plasma absorption coefficient is introduced as a stand-in term for other mechanisms that are impractical to account for directly—due to computational cost or unavailability of physical parameters. The effective model is constrained by Langmuir probe experimental data. The simulations are set up in a 2D-axisymmetric geometry using a solver with an adaptive Cartesian mesh. We simulate plasmas of selenium (Se) and tellurium (Te), which are of current interest in synthesis of transition metal chalcogenide materials. Predictions of Se and Te plasmas produced by 248-nm, pulsed nanosecond ablation of metal targets (4 J/cm² laser fluence and 1.8 mm² laser spot area) show chalcogen plumes with spatial gradients of plasma density that are steeper than those for more commonly studied copper (Cu) plumes by up to three orders of magnitude. Their spatial ion distributions have central bulges, in contrast to the edge-only ionization of Cu. The range of plasma temperatures for Se and Te is higher than for Cu by more than 0.5 eV.

A novel plasma enhanced low temperature (around 400°C) process was developed for CMOS device dopant activation, post integrated circuits (ICs) fabrication processing, and for low temperature processing of organic and other materials needing low temperature conditioning. The main challenge for CMOS device processing is to maintain low thermal budget for source/drain’s dopant activation while limiting dopant diffusion within the source/drain thickness layers, to produce optimal transistor performance. Low temperature processing of ICs is critical to eliminate undesirable materials alloying and structural changes in metals used in ICs’ interconnect layers, and to limit diffusion-based doping profile changes in the CMOS source/drain regions, which degrade transistor performance and reliability.

Most frequently used annealing processes are furnace annealing, rapid thermal processing (RTP), arc flash annealing, laser annealing, and microwave annealing (MWA). Recent publications show that MWA produce dopant activation at temperatures of 500-600°C, which are low compared to other processes, but still higher than the post IC processing target of 400°C.

An innovative plasma-based low temperature annealing process will be discussed, with plasma energy providing enhanced dopant activation at relatively low temperature with controlled nanoscale depth dopant diffusion. Preliminary experiments demonstrated good activation of phosphorus (P) dopant at 416 °C while maintaining a relatively uniform junction depth of ~ 20 nm.

Experiments involved using Si wafers implanted with 60 keV P ions with a dose of 5x10¹⁵ ions/cm². The Si surface temperature, read by a pyrometer, was around 416 °C during the annealing process. Four-point probe measurement revealed sheet resistance of the Si surface wafer before (77-163 KW/sq) and after (42-46 KW/sq) annealing, indicating very good activation of the P atoms at 416 °C.

Following annealing, the P-doped Si wafer was analyzed using X-ray photoelectron spectroscopy (XPS) depth profiling, to explore the distribution of P atoms from the surface into the bulk of the activated Si wafer. Prior to depth profiling, the sample was cleaned with low energy (1 kV) Ar ions bombardment for 2 minutes to remove adventitious carbon and oxygen atoms adsorbed on the surface of materials exposed to atmospheric conditions. The P-activated Si wafer surface cleaning was confirmed by the disappearance of the XPS C / O peaks. P atoms-depth profiling was performed by bombarding the Si surface with 5 keV Ar⁺ ions at 5 minutes interval plus XPS analysis, which revealed that the P 2p and Si 2p XPS peaks remain stable in intensity until full 40 minutes bombardment time. Measurement of the step between the original and etched Si surface, across the etched Si surface edge, revealed 20 nm depth. This value, together with the stable intensity of the XPS P 2p peak after etching indicates that the P dopant concentration is fairly uniform in the first 20 nm of the plasma-enhanced annealing activated Si wafer. Further longer time analysis will determine the P-atoms distribution throughout the entire junction depth.


The preliminary results reported here show that plasma enhanced low temperature processing can induce dopant activation at ~ 400 °C while maintaining shallow junction around 20 nm for fabrication of high-performance Si-based CMOS devices, as well as post IC processing. The process can also be used to condition organic and other materials requiring low temperature conditioning.

Atmospheric pressure non-equilibrium plasmas continue to attract considerable attention in surface engineering [1,2]. By providing unique reactive environments at moderate gas temperatures and atmospheric pressure, they have opened new opportunities to tailor the surface properties of materials as well as to prepare a variety of thin films with different chemical compositions, structures and morphologies. In this context, dielectric barrier discharges (DBDs) have played a major role due to their simple design and versatility. A plethora of DBD-based apparatuses and processes has been therefore devised to meet the ever-increasing demand for atmospheric pressure plasma technologies in various surface engineering applications. In this contribution we will present our recent advances in surface modification by DBDs with a special focus on the development of materials for environmental protection and remediation. The presented examples will include the design of surfaces for enhanced water condensation and recovery [3,4], the preparation of recyclable sorbents for the separation of hydrocarbon liquids from water [5], the deposition of photocatalytic thin films on macroporous supports for wastewater treatment [6], the plasma-assisted exsolution of nanoparticles on the surface of perovskite oxides as a promising approach towards the preparation of nanocatalysts for sustainable chemical production.

[1] F. Massines, C. Sarra-Bournet, F. Fanelli, N. Naudé, N. Gherardi Plasma Processes and Polymers 9 (2012) 1041.
[2] F. Fanelli, F. Fracassi, Surface and Coatings Technology 322 (2017) 174.
[3] F. Fanelli, A. M. Mastrangelo, F. Fracassi, Langmuir 30 (2014) 857.
[4] F. Fanelli, A. M. Mastrangelo, G. Caputo, F. Fracassi, Surface and Coatings Technology 358 (2019) 67
[5] F. Fanelli, F. Fracassi, Plasma Processes and Polymers 13 (2016) 470
[6] A. Uricchio, E. Nadal, B. Plujat, G. Plantard, F. Massines, F. Fanelli, Applied Surface Science 561 (2021) 150014.

New devices and applications are increasingly requiring tailored material properties which necessitates the development of advanced materials and novel nanocomposites to drive these new technologies. In addition to the material-specific properties there are two other key requirements to the success of new technologies, low-cost to performance ratio and a relative abundance of the materials. Two materials that can satisfy these requirements are carbon and zinc, with their derivative forms such as oxides or sulfides as well as micro- or nano-geometries widening the research impact to a myriad of applications.
Blending these materials can lead to composite materials that greatly outperform each individual component material in energy storage applications. ZnO sites can act as pseudocapacitors to enhance the total capacitance, which when combined with a supportive carbon matrix leads to a highly reversible supercapacitor device.¹ Additionally, both ZnO and ZnS have high theoretical capacities in the application of lithium-ion battery anodes, and are relatively abundant, non-toxic, low-cost and environmentally friendly.^2,3 Mechanical failure due to volume expansion and contraction during the charge and discharge can be alleviated by the usage of highly conductive, flexible and strong carbon-based supports can lead to a range of devices with enhanced performance.⁴
It is obvious that a wide range of applications and benefits are available from the combination of zinc oxides and sulfides with nanocarbons, though the question remains; how can this be achieved simply, at minimum cost and on a larger scale? Significant progress has been made in forming macroscopic assemblies of CNTs by direct spinning CNT aerogels in a floating-catalyst chemical vapour deposition (FC-CVD) into a fibre or mat,^5,6 which would represent a strong candidate for the supportive and conductive base. To create a composite material with the FC-CVD system as-is, these fibres and mats would have to be post-treated, diminishing the overall production rate. In this work, we detail the integration of an atmospheric-pressure plasma system with a FC-CVD system where we can successfully obtain a CNT-Zn composite material in a single step. XPS depth profiling measurements demonstrate that the zinc materials penetrate further into the composite when treated in-line compared with an off-line post-treatment. Application-related measurements are reported utilising the in-line produced composite material.
References
(1) Sankapal, B. R.; Gajare, H. B.; Karade, S. S.; Salunkhe, R. R.; Dubal, D. P. Zinc Oxide Encapsulated Carbon Nanotube Thin Films for Energy Storage Applications. Electrochim. Acta 2016, 192, 377–384.
(2) Zhang, Y.; Wei, Y.; Li, H.; Zhao, Y.; Yin, F.; Wang, X. Simple Fabrication of Free-Standing ZnO/Graphene/Carbon Nanotube Composite Anode for Lithium-Ion Batteries. Mater. Lett. 2016, 184, 235–238.
(3) Zhang, W.; Huang, Z.; Zhou, H.; Li, S.; Wang, C.; Li, H.; Yan, Z.; Wang, F.; Kuang, Y. Facile Synthesis of ZnS Nanoparticles Decorated on Defective CNTs with Excellent Performances for Lithium-Ion Batteries Anode Material. J. Alloys Compd. 2020, 816, 152633.
(4) Zhang, H.; Wang, Y.; Zhao, W.; Zou, M.; Chen, Y.; Yang, L.; Xu, L.; Wu, H.; Cao, A. MOF-Derived ZnO Nanoparticles Covered by N-Doped Carbon Layers and Hybridized on Carbon Nanotubes for Lithium-Ion Battery Anodes. ACS Appl. Mater. Interfaces 2017, 9 (43), 37813–37822.
(5) Li, Y. L.; Kinloch, I. A.; Windle, A. H. Direct Spinning of Carbon Nanotube Fibers from Chemical Vapor Deposition Synthesis. Science (80-. ). 2004, 304 (5668), 276–278.
(6) Gspann, T. S.; Smail, F. R.; Windle, A. H. Spinning of Carbon Nanotube Fibres Using the Floating Catalyst High Temperature Route: Purity Issues and the Critical Role of Sulphur. Faraday Discuss. 2014, 173, 47–65.

The synthesis of nanoparticles (NPs) and films with non-thermal plasma assisted methods offers unique opportunities in the fields of bio-imaging, drug delivery, photovoltaics, microelectronics manufacturing, as well as in renewable energy efforts. The unique conditions of non-thermal plasma systems make them suitable for the synthesis of nanoparticles of materials with high melting points or with properties that are highly dependent on the NPs size distribution (luminescence, hardness). Plasmas also enable synthesis of compound nanoparticles such as group III-V semiconducting materials and metal oxides that require covalent bonding rather than the ionic bonding promoted by liquid phase synthesis.
Understanding the relationship between plasma operating parameters and particle growth, morphology, and composition can lead to the development of tailored nanomaterials and optimization of the conditions for the production of new and novel NPs. These relationships, however, are complicated by the variety of phenomena that occur in plasma and that dominate different stages of the particle growth. One of the key phenomena that controls the early stages of NP growth is the deposition of small species on the surface of incipient nanoparticles. This process is highly dependent on the shape and chemistry of both the small fragments and the surface of clusters, which makes its understanding and modelling still quite difficult.
In this work, we report on results from a computational investigation of silicon NPs with the goals of clarifying how operating conditions (temperature), size and surface of clusters influence the collision events and the dynamics of the surface growth mechanism. Specifically, we focus on the probability to form a bond between small silane fragments (SiH_x, x = 1-4) and large silanes Si4, Si2H6, and Si29H36 molecules after a collision has taken place (sticking probability). To reduce the parameter space, we investigate the effect of temperature, size, and hydrogen surface coverage.
The results, obtained by using classical reactive molecular dynamics simulations, show that the sticking probability is the result of the interplay between two phenomena: physi- and chemi-sorption. While some general trends are common to all types of collisions, like a decrease in the sticking probability with increasing temperature and hydrogen coverage, it is possible to observe a complicated dependence on the colliding species characteristics especially for temperatures below 400 K. This effect is the results of the different reaction pathways and the corresponding surface embedding that are possible for the small fragments, as well as the competing effect between desorption and physisorption to chemisorption conversion. Although the resulting sticking coefficients are not easily expressed in terms of the numerous factors that affect them, they constitute a good dataset for supervised learning models, which we used to quantify both the amount of potential computational effort that can be saved when analyzing similar systems, and to perform a regression of the sticking coefficients, when species that were not directly modelled are required.

Solar thermal collectors are one of the technologies used for harnessing and utilizing solar energy, in which the incident radiation is absorbed and converted into thermal energy for use in a wide variety of applications ¹. However, conventional collectors, which rely on solid surface absorbers and use traditional heat transfer fluids, exhibit deficiencies due to large heat losses² and poor solar-thermal conversion (STC) performance.³ Direct absorption solar collectors (DASCs), therefore, were introduced to volumetrically absorb and convert solar radiation into heat,⁴ in which nanofluids (NFs) with their exceptional optical and thermal properties can significantly improve the STC efficiency.^5,6
In this work, copper oxides (CuO_x) were chosen due to their high thermal conductivity,⁷ broad absorption,⁸ high physical and thermal stability ⁹, abundancy at low cost and limited environmental impact with low toxicity.¹⁰ The synthesis of our surfactant-free CuO_x nanoparticles (sf-CuO_x NPs) was carried out with a custom-built system based on plasma-induced non-equilibrium electrochemistry (PiNE) which has recently demonstrated exceptional versatility for the production of a wide range of sf-NPs including oxides.¹¹ Furthermore, post-synthesis heat treatment was performed to determine the impacts on the size, morphology, surface structure, and chemical composition of the NPs.
Following, we re-dispersed sf-CuO_x NPs in ethylene glycol (EG) as a base fluid at different volume fractions up to 0.01%. After determining the solar absorption and scattering behaviours of the NFs using measurements from ultraviolet/visible/near-infrared spectroscopy over the wavelength range 280-2500 nm, the NFs were then exposed to simulated solar radiation of one sun (∼1000 W/m²) and the temperature change was recorded. Temperature measurements were then used to validate a 3-D numerical model employed in Ansys Fluent. The model results revealed that the STC efficiency reached a remarkable value of 81.92% at a volume fraction of as low as 0.01%, indicating that our sf-CuO_x NPs prepared by plasma-liquid interactions represent a promising candidate for NF-based DASCs.

References

1. Duffie, J. A. & Beckman, W. A. Solar engineering of thermal processes. (John Wiley & Sons, 2013).
2. Rasih, R. A., Sidik, N. A. C. & Samion, S. Numerical investigation of direct absorption solar collector using nanofluids: A review. in IOP Conference Series: Materials Science and Engineering vol. 469 12059 (IOP Publishing, 2019).
3. Chamsa-Ard, W., Brundavanam, S., Fung, C. C., Fawcett, D. & Poinern, G. %J N. Nanofluid types, their synthesis, properties and incorporation in direct solar thermal collectors: A review. 7, 131 (2017).
4. Minardi, J. E. & Chuang, H. N. Performance of a “black” liquid flat-plate solar collector. Sol. Energy 17, 179–183 (1975).
5. Otanicar, T. P., Phelan, P. E. & Golden, J. S. %J S. E. Optical properties of liquids for direct absorption solar thermal energy systems. 83, 969–977 (2009).
6. Choi, S. U. S. & Eastman, J. A. Enhancing thermal conductivity of fluids with nanoparticles. (1995).
7. Ghosh, S. et al. Atmospheric-pressure dielectric barrier discharge with capillary injection for gas-phase nanoparticle synthesis. 48, 314003 (2015).
8. Yang, Y., Xu, D., Wu, Q. & Diao, P. Cu 2 O/CuO bilayered composite as a high-efficiency photocathode for photoelectrochemical hydrogen evolution reaction. Sci. Rep. 6, 35158 (2016).
9. Qiang, A., Zhao, L., Xu, C. & Zhou, M. Effect of Dispersant on the Colloidal Stability of Nanosized CuO Suspension. J. Dispers. Sci. Technol. 28, 1004–1007 (2007).
10. Zhang, L. et al. In situ study of thermal stability of copper oxide nanowires at anaerobic environment. J. Nanomater. 2014, (2014).
11. Saito, G. & Akiyama, T. Nanomaterial Synthesis Using Plasma Generation in Liquid. J. Nanomater. 2015, 123696 (2015).

Structural design has always been considered as one of the approaches to obtain and optimize material properties, realizing their value in applications. The same material will display different macro performances through the construction of different structures. One-dimensional and two-dimensional nanomaterials, such as carbon nanotubes, graphene, etc., have a series of excellent physical properties that other traditional materials cannot reach, and have become the preferred research materials in many application fields. Taking them as the "Building Blocks", the final performance of their assembly also depends on the structural design and construction. This time, we will show how to apply the concept of structural design to these nanostructured "building blocks" such as carbon nanotubes and graphene to achieve new breakthroughs in adhesion, sensing, energy storage in extreme environments [1-5].

References
[1] M. Xu et al. Science, 330, 6009 (2010)
[2] M. Xu et al. Nat. Commun., 7, 13450 (2016)
[3] X. Wan et al, Nat. Catalysis, 2, 259 (2019)
[4] M. Zhang et al., Adv. Func. Mat., 30, 2004564 (2020)
[5] X. Gao et al., Nat. Commun., 11, 6160 (2020)

Microscopic liquid droplets are gaining attention as efficient microreactors for new chemical and nanomaterials processing techniques. Typically, aqueous droplets are introduced into a carrier fluid in a microfluidic channel. However, charged microdroplets, especially in the gas phase e.g. from electrospray, have exhibited interesting effects with regard to enhanced nanomaterials synthesis or accelerated reactions and, as yet, the mechanisms involved are unclear. When a low temperature plasma is added to the gas – liquid interface at a droplet surface, the plasma provides a much greater source of charge flux than other techniques. When the droplets travel through a low temperature plasma at atmospheric pressure a number of remarkable and unexpected effects have been observed. For flight times less than 1 millisecond, certain plasma-induced chemical reactions proceed significantly faster than in plasma – bulk liquid configurations and many orders of magnitude faster than in standard bulk chemistry. The plasma also delivers chemical radicals, photons and ions to the surface and the complex interplay between arriving flux, droplet surface charge, internal and external electric fields as well as the restricted volume and high surface to volume ratio, offers new avenues for microreactor development.

There is a large potential for new plasma-liquid processes in medical, chemical, biological, environmental and materials applications, and the plasma – microdroplet system has unique features that provide opportunities for exploitation, including a controlled gas environment, a large surface area to volume ratio for surface reactions, small volume for rapid mixing, and low droplet temperature which allows for a wide range of liquids, additives and precursors. Chemical synthesis and encapsulation in flight enables on-demand supply of materials and chemicals and also remote delivery, without intermediate filtration or purification stages. However synthesis rates need to be rapid i.e. closer to microseconds than minutes. Applications requiring instantaneous and remote chemical species and/or nanoparticles include, for example, plasma-medicine, agriculture and microreaction.

We have demonstrated the use of droplet streams where each individual droplet acts as a chemical microreactors to produce nanoparticles in flight, many orders of magnitude faster than reported by high energy radiolysis or chemical synthesis. The nature of the charge flux and the formation of a charged surface layer composed of solvated electrons is a critical aspect of this system but little explored or understood. Current theories of microparticle charging in a collisional plasma environment are very limited. While inflight charge measurements represent a significant challenge, in the high noise environment of a plasma, the relatively large size of the droplet (10 – 20 μm diameter) and the limited evaporation over the flight time, offer the prospect of using droplets as a spherical probe to develop enhanced collisional probe theories. We have measured the average charge of microdroplets of ~1E6 electrons, close to the Rayleigh limit and considerably higher than that obtained by other charging methods. Analytical – numerical and finite element simulations, in tandem with charge measurements, are being developed to better understand the droplet electrical environment and ultimately to link chemistry and charge in a consistent framework.

Plasmas are widely used industrially for a wide range of processes including for surface cleaning, inducing functionality to the surfaces, chemical reduction of metal oxides and polymerization etc. Here we report the use of an atmospheric pressure plasma in a dielectric-barrier discharge (DBD) configuration for exsolving nanoparticles (NPs) from within the oxide matrix. Usually, the exsolution of NPs in perovskites oxides follows a thermal-reduction process in the presence of hydrogen at temperatures higher than 800 ^oC for more than 12 h. Surprisingly, DBD treatments of perovskites oxides (La_0.43Ca_0.37Ni_0.06Ti_0.94O_2.955, LCTN_{A-site deficient}) resulted in the exsolution of NPs within few minutes at ambient-temperature even without the need for hydrogen. The changes in the size and distribution of NPs in He-fed DBD at 1.2 kV and at 1 kV are compared. Furthermore, the role of hydrogen-containing DBD on the nature of exsolution has also been studied and compared with pure He DBD. The DBD not only induces oxygen vacancies but also provides free electrons that are sufficient for driving the exsolution of NPs from an estimated exsolution depth ranging from 15-45 nm. Such exsolved NPs are firmly socketed within the oxide surface and hence induced excellent catalytic activities as discussed in this contribution. The concept of exsolution on a variety of perovskites oxides can be demonstrated with our DBD system that can even directly be applied for an in-situ energy application [1-8].

References:

[1] M. Emre Sener, Sanjayan Sathasivam, Robert Palgrave, Raul Quesada Cabrera and Daren J. Caruana, Green Chem., 2020,22, 1406-1413 https://doi.org/10.1039/D0GC00080A
[2] Ding-Xin Liu, Felipe Iza, Xiao-Hua Wang, Zhi-Zhen Ma, Ming-Zhe Rong, Michael G Kong, A theoretical insight into low-temperature atmospheric-pressure He+H2 plasmas 2013 Plasma Sources Sci. Technol. 22 055016 https://doi.org/10.1088/0963-0252/22/5/055016
[3] Islam, Q. A., Paydar, S., Akbar, N., Zhu, B., Wu, Y., Nanoparticle exsolution in perovskite oxide and its sustainable electrochemical energy systems, Journal of Power Sources 2021, 492, 229626, https://doi.org/10.1016/j.jpowsour.2021.229626.
[4] Sun, X., Chen, H., Yin, Y., Curnan, M. T., Han, J. W., Chen, Y., Ma, Z., Progress of Exsolved Metal Nanoparticles on Oxides as High Performance (Electro)Catalysts for the Conversion of Small Molecules. Small 2021, 17, 2005383. https://doi.org/10.1002/smll.202005383
[5] Kwon, O., Joo,S., Choi, S., Sengodan, S., and Kim,G., Review on exsolution and its driving forces in perovskites. J. Phys. Energy 2020, 2, 032001. https://doi.org/10.1088/2515-7655/ab8c1f
[6] Zhu, T., Troiani, H., Mogni, L. V., Santaya, M., Han, M., Barnett, S.A., Exsolution and electrochemistry in perovskite solid oxide fuel cell anodes: Role of stoichiometry in Sr(Ti,Fe,Ni)O3, Journal of Power Sources 2019, 439, 227077. https://doi.org/10.1016/j.jpowsour.2019.227077
[7] V. Kyriakou, D. Neagu, E.I. Papaioannou, I.S. Metcalfe, M.C.M. van de Sanden, M.N. Tsampas, Co-electrolysis of H₂O and CO₂ on exsolved Ni nanoparticles for efficient syngas generation at controllable H₂/CO ratios, Applied Catalysis B: Environmental 2019, 258, 117950. https://doi.org/10.1016/j.apcatb.2019.117950
[8] Myung, Jh., Neagu, D., Miller, D., Irvine, J. T. S. Switching on electrocatalytic activity in solid oxide cells, Nature 2016, 537, 528–531. https://doi.org/10.1038/nature19090

Fluoropolymers are suitable for many applications, including textile, building, and biomedical materials. They are mainly used for their chemical inertness and low friction coefficient. However, for many applications, their low surface energy leads to poor adhesion during the assembly with other materials. Among the different techniques used to enhance the adhesion of polymers, atmospheric pressure discharges (APD) remain fast and cheap methods that are suitable for industrial applications. The highly energetic species present in the ionized gas allow the breaking of molecular bonds at the film surface and favor the recombination with polar free radicals from the discharge. Although this approach has been proven to be efficient, the understanding of the chemical and physical processes leading to these results remains not yet fully understood. In this study, we performed the modification of fluoropolymer surfaces at atmospheric pressure by using an inert plasma treatment equipped with a roll-to-roll system. The effect of the discharge on different fluoropolymers was studied through a detailed surface analysis before and after each treatment. The characterization of the extreme surface was carried out by X-ray photoelectron spectroscopy (XPS) in both survey and C1s high resolution. In addition, infrared spectroscopy in the attenuated total reflectance mode (IR-ATR) was used to evaluate the modifications observed on the first top micrometers of the samples. All results were correlated with the modification of the surface energy obtained from the static contact angle analysis measured using both water and diiodomethane. The dynamic contact angle was used to evaluate the affinity of the different functional groups deposited with water. The obtained results highlight the possibility to create new hydrophilic functionalities bonded on the fluoropolymer that is likely to improve the adhesion of these polymers in composite assemblies. These promising results constitute the first step toward a better understanding of the fundamental chemical modifications induced by using this relatively new approach.
Keywords: Atmospheric pressure plasma, fluoropolymer, power, organic precursor, duty cycle.

Luminescent silicon (Si) nanocrystals are intriguing nanomaterials for biomedical application due to their unique optical properties and biocompatibility. Compared with cadmium containing fluorophores, Si nanocrystals are relatively non-toxic, abundant, and biocompatible. Synthesis of water-soluble Si nanocrystals is challenging as they are hydrophobic and mainly dispersible in non-polar organic solvents through hydrosilylation with non-polar alkyl ligands. A variety of approaches have been used to synthesize water-soluble Si nanocrystals through post synthetic surface modification with hydrophilic ligands including carboxylic acids, amines, hydroxyl groups, and hydrophilic polymers. Here, we demonstrate two approaches to synthesize water-soluble Si nanocrystals: A single ligand colloidal surface modification with polyethylene glycol (PEG) ligands and a two-step method of mixed-ligand surface modification through inflight surface functionalization with acrylic acid ligands coupled with colloidal PEGylation. Silicon nanocrystals were synthesized in a continuous-flow, low pressure capacitively coupled plasma reactor operating at pressure around 2 torr. In contrast to pristine hydrophobic Si particles, surface functionalized Si nanocrystals were dispersible in both water and biological buffer solutions and colloidally stable for about 24 h. Their surface properties were evaluated using X-ray photon spectroscopy and Fourier transform infrared spectroscopy. Transmission electron microscopy images provided evidence for both individual passivation and encapsulation of multiple nanoparticles together. The acrylic acid grafted-PEGylated Si nanocrystals were evaluated with MDA-MB-231 cell line and found that the cells are tolerant to these nanocrystals up to 30 μg/mL for 72 hours.

This work was primarily supported through the National Institute of Health under award R01DA045549.

Perovskite solar cells are considered as “The Next Big Thing” in photovoltaics research as within only ten years the power conversion efficiency (PCE) has reached from 3.8% to 23%.^[1] Researchers worldwide are developing various procedures for mass scale fabrication of perovskite devices. The three main components of fabricating any types of solar cell devices are: absorbing layer to absorb photons followed by exciton formation, transport layers to conduct the charge carriers and metal contacts to transfer the carriers to the circuit. Transport layers are divided in two main categories electron transport layer (ETL) and hole transport layer (HTL) and metal oxides are largely favoured for both types of transport layers. Microplasma-based synthesis of metal-oxide transport layer is an emerging technology and a potential alternative to other chemical and physical synthesis methods.^[2] The one-step synthesis procedure is simple, less time consuming than other chemical method, can be performed in atmospheric condition and does not require any harsh or toxic chemicals. The transport layers can also be deposited easily with flexible substrates and do not need high temperature annealing which reduce the conductivity of the transparent conducting oxide substrates. In this contribution, we have synthesized HTL (NiO, CuO) and ETL (ZnO) through microplasma methods and directly deposited onto the device structure. From in-depth characterization techniques we observed that the as-synthesized metal-oxides are highly pure, stable and do not require any purification or annealing after the deposition. The absolute position of the band-edges can be tailored to match, for instance, with that of methyl ammonium lead iodide (MAPI) perovskite and the metal-oxides. The as-fabricated perovskite devices show promising photon to electron conversion efficiency.

Plasma-induced solution chemistry can be driven by electrons, ions, photons and radicals. In thermal catalytic, electrocatalytic, conventional electrolysis and plasmon-driven chemical transformations, metal surfaces play key roles. In contrast, in plasma-driven processes, electrons are produced in a gas phase plasma and injected into the interfacing liquid without the need for a solid electrode. The high-power density in plasmas enables exceptionally large yields of electrons, some having high energies up to 10 eV or more, leading to a high concentration of electrons in a near plasma-liquid interfacial region with a thickness up to a few tens of nm. These conditions enable unique chemical transformations including the synthesis of metallic nanomaterials enabled by the reduction of salt ion precursors. This presentation will provide an overview of our current understanding of the processes underpinning material synthesis at the plasma liquid interface and an outlook on opportunities to control such processes.
This work was supported by the Army Research Office under Grant Number W911NF-20-1-0105.

pH is an essential parameter most commonly used to evaluate and control the conditions of various chemical and biological processes as well as to maintain the quality of products in many industries ranging from health care, food, environmental, chemical, and pharmaceutical industries. Different techniques including potentiometric, chemiresistive, capacitive, and optical methods have been developed for measuring pH. Among these methods, potentiometric sensing technique has gained considerable interest because of simple and low-cost fabrication process. Various functional materials including glass, metal oxides, and conductive polymers have been investigated for the fabrication of potentiometric based pH sensors. Generally, sensing performance of a pH sensor is evaluated in terms of its sensitivity, selectivity, working range, and repeatability. Most of the commercially available pH sensors consist of glass membrane as the sensing component because of several advantages of glass-based electrodes such as long service life, high accuracy, and excellent stability in various solutions. However, due to their bulkiness and poor mechanical properties, conventional glass-based sensors are not ideally suitable for wearable or flexible applications.

In recent years, cold atmospheric plasma (CAP) assisted deposition technique has gained much attention because of its feasibility in producing thin films with high quality and consistent properties in large scale onto desired substrates in atmospheric conditions. The CAP deposition technique eliminates the requirement of conventional slow vacuum deposition processes (e.g., CVD and PVD) while producing superior thin film deposition results compared to the conventional thermal spray coating technologies. In this work, we have demonstrated the use of CAP thin film glass deposition to fabricate flexible solid-state glass-based pH sensor using siloxane precursors in a simple and facile approach. Our sensitivity test results indicated that these sensors exhibit excellent sensitivity and selectivity over a wide physiologically relevant pH range. Additionally, in this study we investigated the morphology and chemical composition using SEM and FTIR and correlated with the sensing properties of the demonstrated all solid-state pH sensor.

Exsolution of nanoparticles (NPs) for oxide support is considered as highly effective and efficient process for the synthesis of functional nanomaterial to be applied in catalysis and energy conversion applications. Unlike other nanoparticles decoration techniques (for instance deposition or impregnation), exsolution offers great control on the size and distribution of the particles and offers anchorage with the host matrix making them resilient to agglomeration. Conventionally, exsolution is performed in mild reducing atmosphere at elevated temperature, making them a relatively lengthy (10-30 hours) and energy demanding process. Herein a new dimension in exsolution is explored and NPs exsolution is demonstrated using low pressure plasmas, making exsolution to occur in minutes rather than hours. A non-stoichiometric (A-site deficient) perovskite oxide (POx) having a composition of La_0.43Ca_0.37Ti_0.94Ni_0.06O_2.955(LCTN) treated in low pressure plasma in argon atmosphere results in the exsolution of nanoparticles. Particle size and density of the exsolved nanoparticles were optimized by tuning the treatment time and plasma power, which also helps in understanding the mechanism involved in plasma enhanced exsolution. The average particle size does not change drastically and lies within the range of 19-22 nm for different plasma treatment times. However, the plasma density changes from 60 particles/µm² to ~500 particles/µm² when process time is increased from 10 minutes to 15 minutes. The creation of oxygen vacancies and a negative surface charge density are considered the main driving forces for plasma-induced exsolution. The effect of exsolved nanoparticles was analysed by employing CO catalytic testing to access CO₂ production rate. The catalytic reactivity of the plasma exsolved LCTN samples have been tested and present encouraging results. This makes plasma exsolution an interesting approach which will open new dimension in the field exsolution and provides future solutions in catalysis and energy conversion.

Today, atmospheric pressure dielectric barrier discharges (DBD) have the potential to make the difference for the transition toward a cleaner and prosperous manufacturing processes. Compared to low pressure plasma, atmospheric pressure plasma does not require expensive pumping systems or closed reactors. For this reason, DBD processes are more suitable for industrial applications where large surfaces must be treated. Also, the possibility to operate surface modifications at room temperature makes DBDs particularly suitable for the modification of thermosensitive materials such as composite polymers.
In the last years, researchers have demonstrated that discharges at atmospheric pressure are very sensitive to the gas composition, the electrodes geometry, or the electrical excitation used. As an example, in nitrogen operating plasma, few tenth of ppm added can stabilize the discharge in a homogenous physical regime, while few hundreds of ppm can create a filamentary mode. These different physical regimes can differently affect the surface, provoke a change in the chemical reactions and finally lead to different surface proprieties. One can easily imagine that a small modification of the physical regime during an industrial process could provoke a strong modification of the performance of the final products.

Through this work, we aim to highlight the current understanding of the chemical reactions involved in a DBD process during the modification of a large polymer area. The remaining scientific challenges to control the physical regimes in an industrial reactor will also be presented. To gain insight into the physics of the discharge, optical emission spectroscopy (OES) and electrical characterization have been performed in real time. Measurements were performed at atmospheric pressure in flowing N₂. N₂O was also admixed to understand the influence of impurities on the behavior and the species present in the discharge. Results show that in pure N₂, the main feature of the spectra corresponds to transition of the N₂ second positive system (C-B). Several peaks of the NO (A-X) transition, as well as the CN (B-X) have been also observed. When a small amount of N₂O is added to the discharge, a complete disappearance of the CN and NO is denoted. The N₂ level and NO being excited mainly by direct electron impact, the absence of NO and the slight decrease of N₂ intensities have been related to the reduction of the electron temperature. In comparison, a decrease of CN emissions with the increase of N₂O has been related to lowered production of C or CN from the etching of fluoropolymer during the process. To confirm this, different electrical conditions have been compared. A modulated voltage has been created to control the ON time of the discharge. The results suggest that, by changing the duty cycle while maintaining the same mean power, optical emissions of the CN were less intense at lower voltage and higher duty cycle.

These results were correlated with XPS analysis of the surface. The analysis indicates that the lower the voltage, the higher the amount of CN present on the surface, which was correlated to the lesser CN concentration measured in the gas phase. This means that impurities and/or the electrical excitation of the discharge play an important role in the amount of CN on the surface, and hence on the adhesion improvements of the polymer. In conclusion, the real-time analysis of the CN emissions by OES seems a very promising method to control the process and ensure the good reproducibility of the surface properties.

Plasma processes operated at atmospheric pressure have been shown to be reliable and cost efficient methods¹ for material synthesis and surface engineering for micro/nanoscale fabrication. More specifically, atmospheric pressure microplasmas are of great interest for metal nanoparticle synthesis from bulk solid materials, by-passing conventional processes that require hazardous chemicals, numerous processing steps and/or expensive vacuum equipment.² Metal nanoparticles have shown interesting optical and electronic properties. At the nanoscale, metal nanoparticles exhibit a transition into a semiconductor behaviour and bandgap widening due to quantum confinement.^3,4 Especially bismuth metal, owing to its large exciton Bohr’s radius, starts exhibiting quantum confinement at diameter of ~50nm.⁴ This bismuth semimetal behaviour is highly desirable for optoelectronics, energy storage and thermoelectric applications.
Here we present work on the synthesis of bismuth quantum dots (Bi-QDs) by a one-step atmospheric pressure plasma process from a bismuth wire, either for direct deposition in the form of films or directly dispersed in liquid. Various plasma reactor architectures are examined to study the possible deposition conditions. The synthesised Bi-QDs are studied for their morphological, chemical, optical and electronic properties. The mean diameter of these QDs is in the range of 2-3 nm depending on the gas flow. XPS analysis shows the presence of oxidation peaks however XRD analysis show metallic peaks only which indicates the presence of surface oxidation which may have been caused by atmospheric interactions following the synthesis process.
Reference:
1. Velusamy, T. et al. Ultra-small CuO nanoparticles with tailored energy-band diagram synthesized by a hybrid plasma-liquid process. Plasma Process. Polym. 14, 1–8 (2017).
2. Yao, D. et al. In Situ Fragmented Bismuth Nanoparticles for Electrocatalytic Nitrogen Reduction. Adv. Energy Mater. 10, (2020).
3. El-Sayed, M. A. Some interesting properties of metals confined in time and nanometer space of different shapes. Acc. Chem. Res. 34, 257–264 (2001).
4. Zhou, G., Li, L. & Li, G. H. Semimetal to semiconductor transition and thermoelectric properties of bismuth nanotubes. J. Appl. Phys. 109, (2011).

Microplasma techniques in recent times achieved attractive performance in nanomaterial processing. Microplasmas are special kind of sub-millimetre plasmas that are simple and easy to establish for different applications.^1,2 With this technique several kinds of nanomaterials such as metal, transition metal oxides etc. were demonstrated with flexibility over the choice of precursors.³ Nickel oxide (NiO) is an excellent p-type semiconductor, which is widely employed as electrode material in many applications like supercapacitors, batteries, solar cells.^4,5,6 NiO can be produced through chemical and physical deposition methods, however, most of current methods rely on toxic precursors and time consuming post-process steps.
In this work, we demonstrate the synthesis of nickel oxide nanoparticle thin film using a gas phase microplasma technique. The characteristics of the films were studied for different oxygen precursor along with He carrier gas and film deposition conditions. It was observed that at a given He/O₂ gas mixture, the films appear with different macroscopic morphologies including columnar or planar features depending on the deposition condition. The nanoparticles deposited however exhibit diameters well below 10 nm. Surface chemical analysis and the investigation of the crystal structure show that the nanoparticle are composed of cubic NiO phase. We present the initial application of this process for transport layer in solar cell devices.

Reference:
(1) Chiang, W.; Mariotti, D.; Sankaran, R. M.; Eden, J. G.; Ostrikov, K. (Ken). Adv. Mater. 2019, 32 (18), 1905508.
(2) Mariotti, D.; Belmonte, T.; Benedikt, J.; Velusamy, T.; Jain, G.; Švrček, V. Plasma Process. Polym. 2016, 13 (1), 70–90.
(3) Mariotti, D.; Sankaran, R. M. J. Phys. D. Appl. Phys. 2010, 43 (32), 323001.
(4) Mamak, M.; Coombs, N.; Ozin, G. A. Adv. Funct. Mater. 2001, 11 (1), 59–63.
(5) Jeng, J.-Y.; Chen, K.-C.; Chiang, T.-Y.; Lin, P.-Y.; Tsai, T.-D.; Chang, Y.-C.; Guo, T.-F.; Chen, P.; Wen, T.-C.; Hsu, Y.-J. Adv. Mater. 2014, 26 (24), 4107–4113.
(6) Nelson, P. A.; Elliott, J. M.; Attard, G. S.; Owen, J. R. Chem. Mater. 2002, 14 (2), 524–529.

With recent development of atmospheric-pressure technologies, various plasma applications with liquid/solution have been extensively studied. In this presentation, we are demonstrating pattern drawing as well as monodispersed spherical particle synthesis via atmospheric-pressure plasma using controlled microdroplets. Such microdroplets were ejected from an inkjet device with high reproducibility in place and size.
The first part of the presentation is about development of plasma-assisted inkjet printing, where inks during flight and after landing on a substrate are irradiated by atmospheric-pressure nonequilibrium plasma. Nanosecond pulsed discharge plasmas were applied here. When the ejected ink was exposed to plasma, reactive species from the plasma as well as rapid and local heating are expected to promote various reactions. Thus plasma-assisted inkjet printing could provide various advantages to inkjet printing, such as less pre- or post-treatments, low-temperature and rapid sintering/reduction, narrower pattern width, and/or on-site polymerization. Such advantages will be demonstrated with silver patterning using silver-nanoparticle-dispersed ink [1], poly(3,4-ethylenedioxythiophene) (PEDOT) pattern fabrication using 3,4-ethylenedioxythiophene (EDOT) monomer stock solution ink [2], and gold patterning using particle-free aqueous chloroauric acid solution ink [3].
The later part will be about monodispersed sub-micrometer spherical gold particles synthesis from chloroauric acid (HAuCl₄) solution [4]. While our previous study demonstrated that size-controlled particle synthesis with microdroplets generated by a mist atomizer, the particle sizes were distributed widely, originating from wide size distribution of the mist [5]. Here, by using an inkjet device, microdroplets were generated with high reproducibility. The plasma applied here was radio-frequency argon plasma. The synthesized sub-micrometer gold particles have a narrow size distribution (3%–9% standard deviation), and their diameters can be controlled at least in the 0.3–0.6 μm range by adjusting the concentration of the solution. Further details will be presented in the symposium.

[1] M. Tsumaki, K. Nitta, S. Jeon, K. Terashima, T. Ito, J. Phys. D: Appl. Phys. 51, 30LT01 (2018).
[2] K. Nitta, M. Tsumaki, T. Kawano, K. Terashima, T. Ito, J. Phys. D: Appl. Phys. 52, 315202 (2019).
[3] K. Nitta, K. Ishizumi, Y. Shimizu, K. Terashima, T. Ito, Materials Chemistry and Physics 258, 123836 (2021).
[4] K. Nitta, Y. Shimizu, K. Terashima, T. Ito, J. Phys. D: Appl. Phys. 54, 33LT01 (2021).
[5] M. Tsumaki, Y. Shimizu, T. Ito, Materials Letters 166, 81 (2016).

Structures of sputter-deposited films have been mostly determined in a way shown in the Thornton diagram, in which there are two key parameters: substrate temperature and sputtering pressure. Here we add another parameter, impurity, that adsorbs on, migrates on, and eventually desorb from the surfaces, and thus modify the film structures.
The first example is sputter deposition of single crystalline ZnO films on 18%-lattice mismatched sapphire substrates using N atoms as impurities. Generally, growth of single crystalline films on such large lattice mismatched substrates is impossible because the crystal growth ends up in the formation of relatively large three-dimensional (3D) islands (∼100 nm in diameter) with large number of dislocations. Interestingly, however, we have observed that control of adsorption/desorption behavior of N atoms during deposition enables us to grow single crystalline ZnO films even on the sapphire substrates [1]. Here, buffer layers consisting of nano-sized 3D islands are initially grown by making N atoms adsorb on the surfaces. Next, ZnO films are fabricated on the 10-nm-thick buffer layers without N atoms. The crystal grains start to grow originating from the 3D islands, but in a short time, they coalesce to form two-dimensional (2D) layers. Eventually, the films grow in 2D mode having atomically flat surfaces. We consider that the desorption/adsorption behaviour of N atoms influences the surface energy and brings this interesting growth mode. During the buffer layer deposition, adsorbed N atoms lower the surface energy and lead to the formation of nano-sized grains. Such grans with high surface-to-volume ratio relieve the strain efficiently at the surface, resulting in the low dislocation density. On the other hand, N-atom desorption after the buffer layer deposition causes a drastic increase in the surface energy, which provides a driving force for coalescence of crystal grains and 2D-layer formation. In fact, we observed from vacuum ultraviolet absorption spectroscopy (VUVAS) and secondary ion mass spectroscopy that N solubility in ZnO is low of a few atomic ppm. This property makes N atoms segregate to the surface as well as to the grain boundaries, and thus reduces the surface free energy.
The next example is sputter deposition of amorphous In₂O₃:Sn (a-ITO) films on glass substrates. ITO films are essential for transparent electrodes, and a-ITO films have recently attracted attention due to the advantages such as surface smoothness, good short-range uniformity, low internal stress, and high etching rate. However, the mobility of conventional a-ITO films, fabricated below crystallization temperature, is quite low of 20–30 cm²/Vs. Furthermore, the amorphous phases are unstable and easily crystallized around 150°C, limiting the use of a-ITO films in practical devices. Here we show that impurities can solve the problems again. We have succeeded in fabrication of a-ITO films at high temperature of 150°C using N atoms, where the films maintain the amorphous structures even after annealing at 300°C . Both VUVAS and x-ray fluorescence results indicate that the amorphization is caused not only by the incorporated N atoms into the films that disorder the bixbyite In₂O₃ structure, but by the adsorbed N atoms inhibiting the crystal nucleation, which are eventually desorbed from the surfaces. The most remarkable feature of the a-ITO films is the high carrier mobility over 50 cm²/Vs, attributed to the high-temperature deposition leading both to increased film density and to enhanced short- and mid-range ordering in the amorphous structures.
We believe that sputtering deposition with “impurities” offer new opportunities for designing materials with unprecedented structures.
This work was supported by JSPS KAKENHI Grant Numbers JP21H01372, NTT collaborative research, and Toyota Riken Scholar.
[1] N. Itagaki, et al., Sci. Rep., 10, 46691 (2020).
[2] T. Takasaki, et. al., Proc. 9th ICRP, 60, GT1.150 (2015).

Plasmas in solution provide attractive processing environments for surface modification of materials. The chemical species such as oxygen and hydroxyl radicals in plasma introduce functional groups on surface of materials [1,2], and such surface modifications on carbon nanotube or hexagonal boron nitride (hBN) were reported to help their dispersion in polymers for developments of composite materials [3,4]. Our group revealed that surface modification via plasmas in solution was effective to achieve tough composites with high filler contents [4]. However, the mechanism of dispersion via such plasma modification is not fully understood and the methods to evaluate the surface modification is still insufficient.
In this study, zeta potential was used as the indicator of surface modification of hBN particles for developing its composite elastomers with polyrotaxane. Our previous study demonstrated that a plasma in solution with hydroquinone additive could produce water-dispersible hBN with higher zeta potential than hBN modified by a plasma in solution without additives [5]. Here, such water-dispersible hBN was dispersed into polyrotaxane as composite materials. Polyrotaxane is a unique polymer with movable crosslinking points allowing homogeneous deformation which agrees with ideal rubber elasticity [6]. The prepared hBN/polyrotaxane composites were observed with X-ray computed tomography (X-CT), and their tensile properties were measured.
In this experiment, plasmas were generated in flowing solution with cavitation bubbles dispersing hBN particles having a diameter of 0.2 μm by applying a bi-polar pulsed voltage [7]. Plasma processing in hydroquinone solution could produce water-dispersible hBN particles named as “HQpBN”, and hBN particles modified via plasma processing in pure water were named as “pBN”. The zeta potential measurement (ZetaView PMX 110, Particle-Metrix) confirmed that HQpBN had zeta potential of -43 mV at pH 7, slightly higher than -37 mV of pBN. Polyrotaxane composites with hBN concentration of 30 wt% were prepared with the pBN and HQpBN. X-CT (SkyScan1278, Bruker Inc.) observation revealed that pBN/polyrotaxane composite had pBN aggregate with size of 1–10 μm inside the composite, whereas HQpBN/polyrotaxane composite had no observable aggregates with a resolution of 0.5 μm/pixel. The pBN/polyrotaxane composite had tensile strength of 3.7 MPa and extension ratio at break of 1.6, whereas the HQpBN/polyrotaxane composite achieved tensile strength of 6.5 MPa and extension ratio at break of 2.0.
It was demonstrated that hBN with enlarged zeta potential showed uniform dispersion in composite and achieved high tensile properties of composite. These results suggest that the zeta potential can be an indicator of surface modification via plasma in solution with the purpose for preparing uniformly dispersed composites.

[1] J. P. Boudou, J. I. Paredes, A. Cuesta, A. Martínez-Alonso, and J. M. D. Tascón, Carbon N. Y. 41, 41 (2003).
[2] N. Sakakibara, K. Inoue, S. Takahashi, T. Goto, T. Ito, K. Akada, J. Miyawaki, Y. Hakuta, K. Terashima, and Y. Harada, Phys. Chem. Chem. Phys. 23, 10468 (2021).
[3] T. Shirafuji, Y. Noguchi, T. Yamamoto, J. Hieda, N. Saito, O. Takai, A. Tshchimoto, K. Nojima, and Y. Okabe, Jpn. J. Appl. Phys. 52, 125101 (2013).
[4] T. Goto, M. Iida, H. Tan, C. Liu, K. Mayumi, R. Maeda, K. Kitahara, K. Hatakeyama, T. Ito, Y. Shimizu, H. Yokoyama, K. Kimura, K. Ito, Y. Hakuta, and K. Terashima, Appl. Phys. Lett. 112, 101901 (2018).
[5] K. Inoue, T. Goto, M. Iida, T. Ito, Y. Shimizu, Y. Hakuta, and K. Terashima, J. Phys. D. Appl. Phys. 53, 42LT01 (2020).
[6] K. Minato, K. Mayumi, R. Maeda, K. Kato, H. Yokoyama, and K. Ito, Polymer. 128, 386 (2017).
[7] Y. Oka, K. Ohnishi, K. Asami, M. Suyama, Y. Nishimura, T. Hashimoto, K. Yonezawa, T. Nakamura, and M. Yatsuzuka, Vacuum 136, 209 (2017).

Assembly of functional nanostructures on substrates are useful for chemical and biomolecular sensing,optoelectronics, catalysis, and energy conversion and generation. However, conventional methods to produce assembly of nanostructures on substrates are usually time-consuming, complicated, and involving toxic and expensive chemicals. Here we report the fabrication method of microplasma-engineered nanoassembly (MEN) of plasmonic Au-Ag core-shell nanoparticle for ultrasensitive flexible surface-enhanced Raman scattering (SERS) substrates. Plasmonic Au- Ag core-shell NPs with tuned LSPR properties can be synthesized
and deposited on cellulose papers in one step with ambient condition using microplasmas[1]. The synthesized Au-Ag core-shell NPs are composed with crystalline-twinned Au cores coated by Ag shells via epitaxial growth under microplsamas, providing enhanced charge transportation during Raman scattering. 3D confocal microRaman scattering study shows that a large SERS volume was formed in the as- fabricated MEN substrates, leading significant SERS properties of 1 fM LoD and ~10 ¹² enhanced factor (EF) with the Rhodamine 6G (R6G) as the Raman probe. However, However, we also demonstrated the different kinds of Raman probe including Ketoprofen (KP), folic acid (FA), and Salicylic acid (SA). They are one of the important molecules of the human body. The limit of detection (LOD) of KP , FA and SA by Au@Ag SERS paper-based substrate can be low to 10^-10M,10^-12M and 10^-8M. To sum up, Au@Ag material on paper-based substrate could provide SERS performance as sensitivities technique for the detection of human body molecular. In the future, our work demonstrates a scalable method to synthesize plasmonic nanoassenblies under ambient condition for emerging applications including nanocataysis, sustainable energy, and biomedical imaging.

Graphene quantum dots (GQDs), one of the newest types of carbon nanoparticles (CNPs) with size <20 nm, have attracted many research attentions due to the highly tunable photoluminescence (PL) properties, biocompatibility, chemical and photo stability, and low toxicity. Doping GQDs with heteroatoms, such as nitrogen, can increase the PL quantum yield and has strong electron withdrawing ability. Therefore, nitrogen-doped GQDs as fluorescent pH monitor excels in sensitivity, selectivity, rapidity, and applicability comparing with the conventional electrochemical method, especially for bio-related and medical applications. Good biocompatibility and low toxicity also make NGQDs more preferable to semiconductor quantum dots, organic fluorescent dyes, and noble-metal nanoparticles. As a matter of fact, pH value plays a very important and crucial role in all life forms. A small variation in pH not only endangers the lives of plants and animal but also human beings as it will mainly pollute our drinking water, which may cause some health issues, like physiological dysfunction or diseases. Moreover, with the fact the extracellular pH value of cancerous cell is lower than the intracellular pH value, early cancer diagnostic can be achieved.

Most of the approaches for synthesizing NGQDs are complex, time-consuming, in need of harsh reaction condition, and costly. Hereby, microplasma emerges as one of the most promising approaches for nanomaterials processing due to its facile, stable, efficient, and relatively low cost. The high electron density provided are very energetic (~10eV), which allows non-thermal dissociation of molecular gases to form high concentrations of reactive radical species, e.g. OH^●, O^●, O₂^-●, and NO^●. These reactive radical species along with electrons are brought into the precursor solutions and will interact with the solution chemical species to form GQDs in a cascaded chemical reactions.

Here we report a novel and effective synthesis of the emission-tuneable NGQDs by nanoengineering the N dopant configuration and surface functionalities of GQD surfaces under ambient conditions using microplasmas. The emission-tuneable NGQDs was explored by precisely controlling the chemistry conditions of the reactions during the synthesis This synthetic method leads to highly crystalline particles with easily tunable properties in high reproducibility from a simple chitosan biomass treated in different dilute acid solutions. Based on protonation and deprotonation of NGQDs surface functional groups, it is believed that the distinct proportions of surface functionalities can greatly influence the pH sensing performance. An optimum PL based pH probe with broad linear range of 1.78-13.56 and high accuracy can be obtained from acetic acid treated chitosan with high quantum yield up to ~30%. Moreover, the linear relationship between the UV absorption and the pH values are also obtained, covering from pH 1.25-10.17 and pH 10.17-13.24. The dual optical sensing characteristics allow for almost covering the entire pH range and better accuracy. Our work offers a promising strategy to engineer the functionalities of NGQDs applicable not only for optimal pH sensors but also for other applications with great potential.

Atmospheric pressure plasmas have drawn great attention in a wide variety of fields, such as surface engineering, material processing, biomedicine and agriculture. Possessing potential economic benefits in comparison with low-pressure plasmas, atmospheric pressure plasmas, without the requirement of vacuum technologies, are particularly attractive for processing large-area low-cost photovoltaic devices. Here we demonstrate examples of performance enhancement of n-i-p lead halide perovskite solar cells with the assistance of atmospheric pressure plasma treatments. In the first case, a nitrogen DC-pulse atmospheric-pressure plasma jet (APPJ) was applied to pre-treat the fluorine-doped tin oxide (FTO) glass substrates for perovskite solar cells. In comparison with a conventional 15-min UV-ozone treatment, a 10-s APPJ treatment can effectively decontaminate and enhance the wettability of FTO substrates and thus improve the cell efficiency by 27%. In the second case, a planar-type dielectric barrier discharge apparatus (DBD) was used to treat the absorber layers of perovskite solar cells. The perovskite absorber layer was formed by spin-coating a dimethylformamide solution containing CH₃NH₃I and PbI₂, followed by a soft-bake. Prior to the deposition of the spiro-OMeTAD hole transport layer, the DBD treatment was carried out in the nitrogen ambient. The result shows that a 20-s treatment can introduce the proper surface modification and grain growth of the perovskite absorber and thus improve the cell efficiency by 38%. In the third case, a low-temperature atmospheric-pressure dielectric-barrier-discharge-jet (DBDjet) was applied to post-treat the jet-sprayed silver nanowires (AgNWs) counter electrodes for fully solution-processed perovskite solar cells. The DBDjet treatment can remove the polyvinylpyrrolidone on the surface of AgNWs and thus improve the interfacial contacts among AgNWs and between the AgNWs layer and hole transport layer. A 33% enhancement in the cell efficiency was achieved when the AgNWs electrode was treated by the DBDjet for 10 times at a scan rate of 0.5 cm/s.

Metal-organic frameworks (MOFs) or supramolecular coordination polymers are porous materials composed of metal ions and organic ligands. Due to their high surface area and designability, they are expected to be applied to catalyst, gas storage, and drug delivery. Moreover, they also serve as solid precursors for metals and metal oxide. Many porous transition metal oxides, such as CuO and Cu₂O, have been obtained by pyrolysis of MOFs under controlled atmospheres. Therefore, the control of the morphology of MOFs has recently attracted much attention in terms of both direct applications of MOFs and their application to intermediate materials for various metal or metal oxide materials synthesis. Here, we focused on Cu(HBTC)(H₂O), a MOF with Cu²⁺ ions and organic ligands, 1,3,5-tricarboxylbenzene (H₃BTC, trimesic acid), as raw materials. Cu(HBTC)(H₂O)₃ has been studied as an antibacterial material, a support agent for catalysts with metal nanoparticles, and intermediate material for Fe-BTC, which is a highly efficient nanozyme. Cu(HBTC)(H₂O)₃ is known as a one-dimensional material, but in recent years, the synthesis of two-dimensional nanosheets has been reported. In this study, using plasma-in-water treatment solution of H₃BTC, three-dimensional cuboid Cu(HBTC)(H₂O)₃ was synthesized [1]. It was also observed that the morphology changed with plasma treatment time. Plasma in liquid has been studied for applications in materials synthesis and surface modification, including the synthesis of MOFs [2]. However, the effect of plasma in liquid on organic ligands has not yet been clarified, and in this study, we observed the effect by treating only organic ligands with plasma in water. The plasma-treated solution of H₃BTC was also analyzed to study the effect of plasma-in-water treatment on the organic ligands by liquid chromatography, fluorescent emission spectra, and UV-Vis spectra. These results suggest that hydroxy groups are functionalized on the organic ligands by plasma-in-water treatment. Partial plasma treatment of raw materials, demonstrated here, could be useful in future studies on complex synthesis.
[1] M. Kanno, T. Ito, Y. Shimizu, and K. Terashima (submitted to Plasma Processes and Polymers)
[2] M. Kanno, T. Kitao, T. Ito, and K. Terashima, RSC Advances (in press)

Quantum confinement of graphene quantum dots (GQDs) yields a unique photoluminescence (PL) properties which has been widely used for many applications including bioimaging, chemical and biological sensing, LED, optoelectronic devices, and energy related applications. This performance can be further enhanced by introducing heteroatom, such as nitrogen (N) into the sp² carbon core. The high electronegativity of N atom has been proven to provide better electron transfer for improving both sensing and catalytic performances. Despite the promising facts, the applicability of NGQDs have been hampered by their synthesis methods. Postgrowth N doping has been regarded as time consuming and requiring harsh reaction conditions, while the direct synthesis via top-down or bottom-up approach also suffers from uncontrolled NGQDs properties, long reaction time, laborious synthetic procedures, harsh reaction conditions, and scalability.

Moreover, the practicality of NGQDs can be further enhanced by incorporating the NGQDs in appropriate 3D matrices. Materials with 3D architectures like hydrogels and sponges have shown great potential in relevant applications due to their own merits. With regards to the presence of surface functional groups, the NGQDs not only able to act as stimuli response probe to the matrices, but also as nanoscale crosslinker via covalent or non-covalent interaction. As a result, the resulting composites exhibit better mechanical properties than the control groups. It is also envisaged that this nanoscale crosslinker is capable of replacing the conventional toxic crosslinker (e.g. EDC-NHS, glutaraldehyde, etc.), which further boosts the applicability either in biomedical or environmental field.
Water contamination has undoubtedly been one of the most concerning problem faced by many people. The contamination generally involves heavy metal ions (Hg²⁺, As³⁺, Cd²⁺, etc.), organic dyes (remazol brilliant blue R (RBBR), rhodamine B, congo red, etc.), and organic pollutants (nitroarenes family) which are highly toxic and dangerous to human being. Among other available methods, adsorption has been one of the most promising method to remove those contaminations from water owing to its effectiveness, efficiency, low cost, and simplicity.

Microplasma as gaseous discharge with one dimension geometrically confined to <1 mm emerges as an alternative way to promote crosslinking and polymerization process with the absence of toxic chemical crosslinker at ambient conditions. Besides, microplasma is also capable of cleaving polymeric chain to generate porous structures in the resulting 3D materials, resulting in enhanced pore size, surface area, hydrophilicity, and wettability. Herein, we utilized a microplasma system to synthesize NGQDs from chitosan at ambient conditions. The post-plasma solution was converted into NGQDs gel by neutralizing the unreacted chitosan with sodium hydroxide (NaOH) and subjected to another microplasma treatment for crosslinking process to generate NGQDs hydrogel. The fabricated NGQDs hydrogel can be further converted to NGQDs sponge by subsequent freeze-thawing method, showing remarkable water uptake and dye adsorption ability for RBBR. Our work provides an insight into the green and sustainable development of NGQDs sponge applicable for water environmental application.

Recently experimental and theoretical works have reported that graphene quantum dots (GQDs), a unique form of zero dimensional nanostructure, and their exceptional properties make them promising in biosensing applications. Surface-enhanced Raman scattering(SERS) is an ultra-sensitive analytical technique for bio-molecules detection. While the potential of surface plasmon resonance(SPR) metals (e.g. Au and Ag) and graphene for SERS has been demonstrated, but the work of GQDs applied as SERS substrates is still lacking. Here we reported the rational design to develop GQD-based SERS active substrate.
Furthermore, modified GQD with metal nanostructures will lead to important advance for SERS-based detection. Here we demonstrate a facile synthesis of NGQD/AgNP nanohybrids by the atmospheric-pressure microplasma-assisted electrochemistry. Detailed nanomaterial characterizations including transmission electron microscopy, UV/Vis spectroscopy show that the microplasma-assisted electrochemical reaction can successfully grow Ag nanoparticles (AgNP) onto the NGQD surfaces to form the Ag@NGQD NP nanohybrids with heterodimeric nanostructures within minute scale. Besides, the photoluminescence (PL) optical study of NGQDs and AgNP@NGQD indicated that the non-radiative fluorescence resonance energy transfer(FRET) involved in the AgNP@NGQD nanohybrids.In the systematic Raman study, Rhodamine 6G(R6G) and B-carotene are selected as the Raman probe molecules. First we compare the SERS property of four kinds of Ag@NGQDs with different photoluminescence property (e.g. different emission wavelength), Raman results show that SERS performance of [email protected] is highly influenced by overlap area in PL-UV image. The as-produced AgNP@NGQD nanohybrids shows superior SERS performance with high enhancement factor (EF) around 1x10⁸. We further studied the NGQD-AgNP nanohybrids with different FRET efficiency. The results revealed that FRET of the as-produced AgNP@NGQD nanohybrids is the dominant factor to SERS properties in our
study.
On the other hand , the asprepared Ag@NGQD exhibited excitation-independent behavior and high optical stability. Furthermore, based on the complexation between As3+ and Ag@NGQD, the fluorescence intensity of the Ag@NGQD could be greatly quenched by the addition of a small amount of As 3+ ions. The linearity range is 0.1–1000 μM with a detection limit of 0.7 nM. For this reason, the proposed method was demonstrated to be selective and suitable for As 3+ analysis in natural water samples. And, Ag@NGQD with the biosafety have the potentials for intracellular As3+ detection.

Plasma technology plays a vital role in the manufacturing of materials for a wide range of industries. This talk presents a plasma-fabricated graphene nanostructure and its applications for solar-thermal conversion. In the plasma-enhanced chemical vapor deposition process, an electric field is produced by gas discharge and the graphene planes are grown along the electric field lines that converge normally onto the surface of substrates, forming vertically-arranged graphene nanosheets (VGs). With vertical orientation and open channels, VGs show the potential of constructing light traps that enable a high absorptance of 98% towards solar irradiation. Besides, plasma-enabled in-situ nitrogen doping during the graphene growth can mediate the surface wettability of the VGs from hydrophobic to hydrophilic. In particular, the hydrophilic N-doped VGs can serve as capillary pumps to provide sufficient water supply for solar-driven interfacial evaporation, and thus establish a high evaporation rate and high energy conversion efficiency in solar desalination applications.


Reference:
[1] Shenghao Wu, Guoping Xiong, Huachao Yang, Biyao Gong, Yikuan Tian, Chenxuan Xu, Yan Wang, Timothy Fisher, Jianhua Yan, Kefa Cen, Tengfei Luo, Xin Tu, Zheng Bo*, Kostya (Ken) Ostrikov. Multifunctional solar waterways: plasma-enabled self-cleaning nanoarchitectures for energy-efficient desalination. Advanced Energy Materials, 2019, 9, 1901286, https://doi.org/10.1002/aenm.201901286.
[2] Shenghao Wu, Guoping Xiong, Huachao Yang, Yikuan Tian, Biyao Gong, Huiwen Wan, Yan Wang, Timothy S. Fisher, Jianhua Yan, Kefa Cen, Zheng Bo*, Kostya (Ken) Ostrikov. Scalable production of integrated graphene nanoarchitectures for ultrafast solar-thermal conversion and vapor generation. Matter, 2019, 1, 1017−1032, https://doi.org/10.1016/j.matt.2019.06.010.
[3] Biyao Gong, Huachao Yang, Shenghao Wu, Guoping Xiong, Jianhua Yan, Kefa Cen, Zheng Bo*, Kostya (Ken) Ostrikov. Graphene array–based ant–fouling solar vapour gap membrane distillation with high energy efficiency. Nano-Micro Letters, 2019, 11:51, https://doi.org/10.1007/s40820-019-0281-1.

Zn_1-xMg_xO are promising materials for optoelectronic/excitonic devices because of the large exciton binding energy (60 meV) and the potentially tunable band gaps [1]. Ohtomo et. al. have recently demonstrated the bandgap tuning of wurtzite Zn_1-xMg_xO films in the range 3.4–4.15 eV [2], where the films have been grown by pulsed laser deposition (PLD). However, the large lattice mismatch between Zn_1-xMg_xO films and commonly used substrates, such as sapphire, often results in poor crystal quality. Aiming to overcome this challenge, we here perform epitaxial growth of Zn_1-xMg_xO films on sapphire substrates using inverted Stranski-Krastanov (inverted SK) mode [3]. In this mode, strain-relaxed nano-sized 3D islands (buffer layers) are initially grown by using impurities to control the surface energy. Then the islands coalesce to form 2D layer after the desorption of impurities, and eventually, films grow in 2D mode and form single crystals. This mode has enabled us to grow single crystalline films on large lattice mismatched substrates [3]. Here we demonstrate inverted SK growth of Zn_1-xMg_xO films using N atoms as impurities, where sputtering method is employed for film deposition taking the advantages of the high dissociation degree of N₂.
All the films were fabricated by radio frequency magnetron sputtering. First, 10-nm thick buffer layers consisting of 3D islands were deposited on sapphire substrates at 780°C using N₂/Ar gas. Next, wurtzite Zn_1-xMg_xO films were fabricated on the buffer layers at 800°C using O₂/Ar gas, and the films were post annealed in O₂ atmosphere at 880°C for 10 hours.
We have observed inverted SK growth and thus obtained high-quality single-crystalline Zn_1-xMg_xO films of x = 0.05–0.33. Atomic force microscopy measurement revealed that the films have atomically flat surfaces with root mean square roughness of 0.41–0.75 nm. The high crystal qualities of the films even with large Mg contents were also proved by x-ray diffraction analysis.The full widths at half maximum of the (0002) rocking curves are in the range between 0.05° (x = 0.33) and 0.07° (x = 0.14). These values are comparable to that of bulk single-crystal ZnO. Furthermore, we observed that the optical absorption edge shifts continuously toward the shorter wavelength with increasing x, and the band gap has been tuned in the range 3.37–4.03 eV. These results allow us to conclude that the combination of inverted SK growth and magnetron sputtering is a powerful method of growth of high-quality single-crystalline Zn_1-xMg_xO films on lattice mismatched substrates.
This work was supported by JSPS KAKENHI Grant Numbers JP21H01372, NTT collaborative research, and Toyota Riken Scholar.
[1] K. Nakahara, et al., Appl. Phys. Lett. 97, 013501 (2010)
[2] A. Ohtomo, et al., Appl. Phys. Lett., 72, 2466 (1998).
[3] N. Itagaki, et al., Sci. Rep., 10, 4669 (2020).

We have developed novel semiconductors, (ZnO)_x(InN)_1-x (called “ZION” hereinafter), which are pseudo-binary alloys of ZnO and InN. ZION have tunable band gaps across the entire visible spectrum, high exciton binding energy of 30–60 meV, and high optical absorption coefficients of 10⁵ cm^-1, making ZION promising materials for optoelectronic and excitonic devices [1, 2]. Recently, we have succeeded in the heteroepitaxial growth of ZION films on sapphire, ZnO and GaN substrates by radio-frequency (rf) magnetron sputtering, where the temperature has been kept below 550°C so that the thermal dissociation of In–N bond is reduced. Furthermore, we found that under such conditions, “skewness”, the third moment of the surface height distribution of the substrates, is a key parameter for enhancement of adatom migration on the surface and thus for growth of high quality ZION films [3]. Beside the morphology, the surface polarity of the substrates might also play an important role in adatom migration since it affects the sticking coefficient of adatoms [4]. We here demonstrate epitaxial growth of ZION films on both O-polar and Zn-polar ZnO substrates and discuss the impacts on the migration as well as the crystal growth of ZION films.
ZION films were deposited on ZnO substrates by rf magnetron sputtering where the lattice mismatch between the film and the substrate is 1.6%. O₂, N₂ and Ar gases were used, and the gas flow rates were 1.8, 24.0, and 30.2 sccm, respectively. The total pressure was 0.50 Pa. ZnO and In targets with purity of 99.99% were used. The substrate temperature was 450°C. The deposition time was 220 seconds, and the film thickness on the O-polar and Zn-polar substrates were 25 and 35 nm, respectively. The chemical composition ratio was (ZnO)_0.85(InN)_0.15, confirmed by x-ray fluorescence spectrometry.
The heteroepitaxial growth of ZION films on both surfaces were confirmed by high-resolution transmission electron microscopy (HRTEM), where all the films have atomically sharp interfaces with the substrates. Interestingly, however, the HRTEM images show a significant difference in the lattice relaxation process between the films. On Zn-polar ZnO surfaces, relaxation occurs at the beginning of the film growth, whereas on O-polar surfaces, ZION films are fully coherent with the 1.6%-lattice-mismatched ZnO substrates at least for 15 mono layers. We also found that the difference in the depth profile of the chemical composition. Rutherford back-scattering spectra show that on Zn-polar surfaces, there is no compositional change in the depth direction, indicating the particle flux toward the substrate mainly governs the film growth. While on O-polar surfaces, In and N contents decrease toward the surfaces, where adatoms tend to be incorporated into thermodynamically favored lattice positions. We consider that these differences are brought by the differences in the adatom migration, that is, O-polar surfaces provide smaller sticking coefficients and thus longer migration length of Zn/In atoms due to the smaller number of back bonds of Zn/In atoms on the surfaces [4]. This hypothesis is consistent with the facts that O-polar surfaces lead to lower deposition rate and to higher crystal quality of ZION films where the full-width at half maximum of (002) x-ray rocking curves is about 0.2°.
This work was supported by JSPS KAKENHI Grant Numbers JP21H01372, NTT collaborative research, Toyota Riken Scholar.

[1] N. Itagaki, et al., Mater. Res. Express, 1, 36405 (2014).
[2] K. Matsushima et al., Jpn. J. Appl. Phys., 52, 11NM06 (2013).
[3] R. Narishige et al., Jpn. J. Appl. Phys., 60, SAAB02 (2021).
[4] H. Kato, et al., J. Crystal Growth, 265, 375 (2004).

In₂O₃:Sn (ITO) films are essential for transparent electrodes in flat panel displays, touch screens, and so on. Recently, amorphous ITO(a-ITO) has attracted attention as an alternative to polycrystalline ITO due to the advantages such as surface smoothness, high etching rate and low internal stress. However, the mobility of conventional a-ITO films, deposited by sputtering below crystallization temperature of In2O3 (~150°C) is quite low of 20–30 cm²/Vs. Furthermore, the amorphous phases are unstable and they easily crystallize around 150°C, limiting the use of a-ITO films in practical devices. We have recently clarified that beside the substrate temperature, impurity behavior on the surfaces during the growth affects the film structures [1], and have succeeded in fabricating a-ITO films at 150°C using nitrogen as impurity, where the mobility is high of 50 cm²/Vs [2]. Here we demonstrate sputtering synthesis of a-ITO films at 350°C using nitrogen and see what happens if we increase the temperature far above the crystallization temperature of In₂O₃.
ITO films were deposited by radio-frequency magnetron sputtering on quartz grass substrate at 350°C. Ar and N₂ gases were used and the total pressure was 0.9 Pa. The gas flow rate ratios of N₂ was 0–20%. In₂O₃:Sn (10wt.%) targets were used. The film thickness was 50 nm. N content in the films and the absolute N atom density in the plasma were measured by x-ray fluorescence (XRF) and vacuum ultraviolet absorption spectroscopy (VUVAS) [3], respectively.
X-ray diffraction (XRD) measurements revealed that a-ITO films are fabricated even at high temperature of 350°C using nitrogen. For the ITO films fabricated at N₂ flow rate ratios of 0–7%, diffraction peaks from In₂O₃ (222) plane are observed at 30.5°, while, no peaks are observed for the films fabricated at N₂ = 10–20%, indicating the films have amorphous structure. We found from VUVAS and XRF measurements that despite the high density of N atom in the plasma of 10¹¹ cm^-3, being one order of magnitude higher than O atom density, N contents in the films are only a few at.%. These results suggest that the amorphization is caused not only by the N atoms incorporated into the films that disorder the bixbyite In₂O₃ structure, but by the adsorbed N atoms that inhibiting the crystal nucleation, which are eventually desorbed from the surfaces. Owing to the amorphous structure, the a-ITO films have smooth surface with the root mean square (RMS) roughness of 0.8 nm, which is half of that of films fabricated without N₂. The most remarkable feature of the a-ITO films is the high carrier mobility of 66 cm²/Vs. We consider that the high-temperature deposition leads both to increased film density and to enhanced short- and mid-range ordering in the amorphous structures, resulting in such high mobility.
This work was supported by JSPS KAKENHI Grant Number JP21H01372, NTT collaborative research, and Toyota Riken Scholar.
[1] N. Itagaki, et al., Sci. Rep., 10, 4669 (2020).
[2] T. Takasaki, et. al., Proc. 9th ICRP, 60, GT1.150 (2015).
[3] S. Tada, et al., J. Appl. Phys. 88, 1756 (2000).