Plasma-Driven Tuning of Dielectric Permittivity in Graphene.

Materials with tunable negative electromagnetic performance, i.e., where dielectric permittivity becomes negative, have long been pursued in materials research due to their peculiar electromagnetic (EM) characteristics. Here, this promising feature is reported in materials on the case of plasma-synthesized nitrogen-doped graphene sheets with tunable permittivity over a wide (1-40 GHz) frequency range. Selectively incorporated nitrogen atoms in a graphene scaffold tailor the electronic structure in a way that provides an ultra-low energy (0.5-2 eV) 2D surface plasmon excitation, leading to subunitary and negative dielectric constant values in the Ka-band, from 30 up to 40 GHz. By allowing the tailoring of structures at atomic scale, this novel plasma-based approach creates a new paradigm for designing 2D nanomaterials like nanocarbons with controllable and tunable permittivity, opening a path to the next generation of 2D metamaterials.

Low-temperature low-power PECVD synthesis of vertically aligned graphene.

The need for 2D vertical graphene nanosheets (VGNs) is driven by its great potential in diverse energy, electronics, and sensor applications, wherein many cases a low-temperature synthesis is preferred due to requirements of the manufacturing process. Unfortunately, most of today's known methods, including plasma, require either relatively high temperatures or high plasma powers. Herein, we report on a controllable synthesis of VGNs at a pushed down low-temperature boundary for synthesis, the low temperatures (450 °C) and low plasma powers (30 W) using capacitively coupled plasma (CCP) driven by radio-frequency power at 13.56 MHz. The strategies implemented also include unrevealing the role of Nickel (Ni) catalyst thin film on the substrates (Si/Al). It was found that the Ni catalyst on Si/Al initiates the nucleation/growth of VGNs at 450 °C in comparison to the substrates without Ni catalyst. With increasing temperature, the graphene nanosheets become bigger in size, well-structured and well separated. The role of Ni catalysts is hence to boost the growth rate, density, and quality of the growing VGNs. Furthermore, this CCP method can be used to synthesize VGNs at the lowest temperatures possible so far on a variety of substrates and provide new opportunities in the practical application of VGNs.

Janus nanocellulose membrane by nitrogen plasma: Hydrophilicity to hydrophobicity selective switch.

Cellulose nanofibrils are one of the keystone materials for sustainable future, yet their poor water repellency hinders their push into industrial applications. Due to complexity and poor economical outcome and/or processing toxicity of the existing hydrophobization methods, nanocellulose loses against its antagonist plastic in medical and food industries. Herein, we demonstrate for the first time the "one-side selective water-repellency activation" in nanocellulose membranes by the means of mild N2-plasma treatment, exhibiting lowest wettability after 20 s of treatment. Hydrophobicity and accompanying Janus character were justified by the topological, chemical and structural reorganizations in cellulose nanofibrils. The findings suggest that the mechanism behind the hydrophilic/hydrophobic change primarily relies on the interplay between OH removal and appearance of SiCH3, originating from the polysiloxanes-based substrate, as well as complementary CNH2 groups formation. First-principles calculations show that NH2 groups moderately increase hydrophobicity, while various SiCH3 substitutions wholly change the character of the surface to repel water. Using nitrogen is shown to be crucial, as N(H)Si(CH3)3 groups induce greater hydrophobicity than simple OSi(CH3)3. Finally, the obtained materials absorb water on the hydrophilic side, while remaining hydrophobic on the other, exhibit high tensile strength, and protection against UV light, demonstrating applicability over wide range of applications.

Advanced Carbon-Nickel Sulfide Hybrid Nanostructures: Extending the Limits of Battery-Type Electrodes for Redox-Based Supercapacitor Applications.

Transition-metal sulfides combined with conductive carbon nanostructures are considered promising electrode materials for redox-based supercapacitors due to their high specific capacity. However, the low rate capability of these electrodes, still considered "battery-type" electrodes, presents an obstacle for general use. In this work, we demonstrate a successful and fast fabrication process of metal sulfide-carbon nanostructures ideal for charge-storage electrodes with ultra-high capacity and outstanding rate capability. The novel hybrid binder-free electrode material consists of a vertically aligned carbon nanotube (VCN), terminated by a nanosized single-crystal metallic Ni grain; Ni is covered by a nickel nitride (Ni3N) interlayer and topped by trinickel disulfide (Ni3S2, heazlewoodite). Thus, the electrode is formed by a Ni3S2/Ni3N/Ni@NVCN architecture with a unique broccoli-like morphology. Electrochemical measurements show that these hybrid binder-free electrodes exhibit one of the best electrochemical performances compared to the other reported Ni3S2-based electrodes, evidencing an ultra-high specific capacity (856.3 C g-1 at 3 A g-1), outstanding rate capability (77.2% retention at 13 A g-1), and excellent cycling stability (83% retention after 4000 cycles at 13 A g-1). The remarkable electrochemical performance of the binder-free Ni3S2/Ni3N/Ni@NVCN electrodes is a significant step forward, improving rate capability and capacity for redox-based supercapacitor applications.

Oriented Carbon Nanostructures from Plasma Reformed Resorcinol-Formaldehyde Polymer Gels for Gas Sensor Applications.

Oriented carbon nanostructures (OCNs) with dominant graphitic characteristics have attracted research interest for various applications due to the excellent electrical and optical properties owing to their vertical orientation, interconnected structures, electronic properties, and large surface area. Plasma enhanced chemical vapor deposition (PECVD) is considered as a promising method for the large-scale synthesis of OCNs. Alternatively, structural reformation of natural carbon precursor or phenol-based polymers using plasma-assisted surface treatment is also considered for the fabrication of OCNs. In this work, we have demonstrated a fast technique for the synthesis of OCNs by plasma-assisted structure reformation of resorcinol-formaldehyde (RF) polymer gels using radio-frequency inductively coupled plasma (rf-ICP). A thin layer of RF polymer gel cast on a glass substrate was used as the carbon source and treated with rf plasma under different plasma discharge conditions. Argon and hydrogen gases were used in surface treatment, and the growth of carbon nanostructures at different discharge parameters was systematically examined. This study explored the influence of the gas flow rate, the plasma power, and the treatment time on the structural reformation of polymer gel to produce OCNs. Moreover, the gas-sensing properties of as-prepared OCNs towards ethanol at atmospheric conditions were also investigated.

Oriented Carbon Nanostructures by Plasma Processing: Recent Advances and Future Challenges.

Carbon, one of the most abundant materials, is very attractive for many applications because it exists in a variety of forms based on dimensions, such as zero-dimensional (0D), one-dimensional (1D), two-dimensional (2D), and-three dimensional (3D). Carbon nanowall (CNW) is a vertically-oriented 2D form of a graphene-like structure with open boundaries, sharp edges, nonstacking morphology, large interlayer spacing, and a huge surface area. Plasma-enhanced chemical vapor deposition (PECVD) is widely used for the large-scale synthesis and functionalization of carbon nanowalls (CNWs) with different types of plasma activation. Plasma-enhanced techniques open up possibilities to improve the structure and morphology of CNWs by controlling the plasma discharge parameters. Plasma-assisted surface treatment on CNWs improves their stability against structural degradation and surface chemistry with enhanced electrical and chemical properties. These advantages broaden the applications of CNWs in electrochemical energy storage devices, catalysis, and electronic devices and sensing devices to extremely thin black body coatings. However, the controlled growth of CNWs for specific applications remains a challenge. In these aspects, this review discusses the growth of CNWs using different plasma activation, the influence of various plasma-discharge parameters, and plasma-assisted surface treatment techniques for tailoring the properties of CNWs. The challenges and possibilities of CNW-related research are also discussed.