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Plasma technology is actively used for nanoparticle synthesis and modification. All plasma techniques share the ambition of providing high quality, nanostructured materials with full control over their crystalline state and functional properties. Pulsed-DC physical/chemical vapour deposition, high power impulse magnetron sputtering, and pulsed cathodic arc are consolidated low-temperature plasma processes for the synthesis of high-quality nanocomposite films in vacuum environment. However, atmospheric arc discharge stands out thanks to the high throughput, wide variety, and excellent quality of obtained stand-alone nanomaterials, mainly core–shell nanoparticles, transition metal dichalcogenide monolayers, and carbon-based nanostructures, like graphene and carbon nanotubes. Unique capabilities of this arc technique are due to its flexibility and wide range of plasma parameters achievable by modulation of the frequency, duty cycle, and amplitude of pulse waveform. The many possibilities offered by pulsed arc discharges applied on synthesis of low-dimensional materials are reviewed here. Periodical variations in temperature and density of the pulsing arc plasma enable nanosynthesis with a more rational use of the supplied power. Parameters such as plasma composition, consumed power, process stability, material properties, and economical aspects, are discussed. Finally, a brief outlook towards future tendencies of nanomaterial preparation is proposed. Atmospheric pulsed arcs constitute promising, clean processes providing ecological and sustainable development in the production of nanomaterials both in industry and research laboratories.

Plasma is an ionized gas containing free electrons, ions, neutral species (atoms, molecules, radicals), photons from UV to infrared wavelengths. Its composition makes it a rich environment for reactive process, showing a collective response to external perturbations. Low temperature plasmas are hot electron gas (1-10 eV) which are not in thermal equilibrium with the other cold part of the atomic or molecular species (0.02 eV). These cold plasmas are then suitable for applications to delicate surfaces (skin) without damage, especially since RONS (Reactive Oxygen and Nitrogen Species) show healing properties. Biomedicine is facing new challenges like decontamination, wound healing, and oncotherapy. Plasma medical treatments are selective, instantaneous, tunable in comparison to lasers and chemicals. There is a big need for developing new plasma sources that can treat complex surfaces (with asperities, sharp edges, cavities, complex accessibility) For this purpose, two types of flexible Cold Atmospheric Plasma Jet (CAPJ) sources with different geometries (planar and radial) were made and characterized through: multijet length and flow regime, electrical parameters, Optical Emission Spectroscopy (OES), performance after bending the aerogel. He (99.995%) was used in both configurations. Finite Element Method simulations were run to predict and understand the electromagnetic process in this plasma jet configuration. In planar geometry, DBD (Dielectric Barrier Discharge) was constituted of a silica aerogel placed between two biased electrodes. Silica aerogel has a very low gas permeability and is mechanically stable (highly cross-linked fiber glass network). Plasma jets constituted of 3 aligned jets, a few mm long above the DBD region whose length is proportional to the He flow rate, flowing in laminar regime, were produced by Helium flow (flow rate: 1-4 slm) subjected to sinusoidal voltage of 5kV and 15kHz frequency. Higher flow rate produced turbulent flows. The ratio between gas velocity and discharge voltage can be adjusted: short jets (flow limited operation) are suitable for near surface treatments, longer jets (power limited operation) are appropriate for greater distance treatments. OES showed strong N2 emission. Planar DBD was studied in flat configuration then its flexibility was characterized in bent (concave, convex) configurations. Compliance of the plasma source is limited by aerogel size and thickness. There is possibility to extend the device from 1D to 2D (matrix of plasma jet lines). In radial geometry, DBD was constituted of a soft hollow cylinder (about 5 cm external diameter, 2.5 cm internal diameter and 10 cm long), made of polyethylene (PE) foam, material selected for its flexibility and low permeability. An aluminum/dielectric coated rod electrode was placed in the core of the cylinder, copper circular electrodes were disposed on the external cylinder wall at 15 mm distance from each other surrounding 1mm holes punched in the PE cylinder, where the CAPJs were formed. In this geometry, two different operations modes were apprehended: comb mode was represented by four equidistant axial CAPJs and round brush mode was experienced when three CAPJs were aligned at constant azimuthal angle. In this case, discharges were triggered at 15 kHz AC voltage and a lower voltage than in planar configuration (3.8 kV). The jets were uniform in length and intensity. OES were recorded in the active region (inside the PE housing) and in the afterglow region. In the afterglow region, the spectrum was similar to the one obtained in planar geometry. A preponderance of hydroxyl groups (OH), nitrogen molecules (N2, N2 +), He atoms was found. The source remains stable after deformation of the PE housing. The feasibility of these flexible plasma sources in radial and axial configurations is promising for treating any kind of surface with special topography.

Spherical quartz stones of around 1 cm in diameter have been exposed to anodic arc discharges in a helium atmosphere at 300 Torr. The arc current flowing between the graphite electrodes was set either in continuous DC mode (30–150 A) or in pulsed mode at 2 Hz (220 A peak). The ablation rate in each sample was systematically measured after several seconds of arc plasma treatment. Optical emission spectroscopy (OES) diagnostics and 2D fluid simulations of the arc discharge have shed light on the heat flux transport and the heating mechanisms of the quartz crystals. A linear correlation is found between the absorbed power density and the resulting rate of penetration, which yields a maximal value of 15 cm h−1 for approximately 150 W cm−2. The linear fit on the slope provides a specific energy of 40 kJ cm−3. The incident energy flux onto the sample surface promoted a phase transition from crystalline to glassy silica, as characterized via Raman spectroscopy. This study points out the strong potential of arc plasma technology for geothermal drilling applications.

Following up on a recent study describing a flexible plasma source operated in planar geometry, the performance of a cold atmospheric plasma jet (CAPJ) matrix emanating radially from a soft cylindrical surface in the open air is presented. The plasma device, which has a set of small outlets produced in its side surface, has a length of 12 cm and an outer diameter of 5.4 cm. The dielectric barrier discharge (DBD) sustaining the plasma jets is generated in helium flowing between two coaxial electrodes, which are separated by both an insulating tape and a cylindrical wall made of polymer foam. Two operation modes are considered: four equidistant CAPJs at the same axial position (round brush mode) and three aligned CAPJs at constant azimuthal angle (comb mode). All discharges, excited by 15 kHz-AC voltages at 3.8 kV in amplitude, have resulted in uniform lengths and intensities of the jets. Consumed discharge powers of between 0.1 and 1.5 W have been estimated from current-voltage measurements. Optical emission spectroscopy has shown the preponderance of hydroxyl groups, nitrogen molecules, and helium atoms in the active DBD region and the jet afterglow zone. This new design of CAPJ anticipates promising applications for treating the inner surface of hollow and delicate components for activation or healing purposes.

A new plasma source design that merges the main characteristics of capacitive dielectric barrier discharge (DBD) and cold atmospheric plasma jet (CAPJ) is discussed. The DBD system contains a flexible, porous matrix consisting of silica aerogel, which is comprised between two biased electrodes. The helium flow supply subjected to a sinusoidal voltage of around 5 kV in amplitude and 15 kHz in frequency provides a set of plasma jets that propagates more than 1 cm beyond the active DBD region. The studied plasma multi-jet system consists of an array of three aligned jets that flow in the laminar regime, and it is intended for treating the surfaces of 3D objects and large areas. CAPJ performance is discussed as a hypothetical morphing source in flat and bent configurations. Electrical characterization and optical emission spectroscopy diagnostics have provided current-voltage waveforms and the composition of the CAPJ through the aerogel layer, respectively. This novel source is promising for biomedical applications that require full adaptation of plasma parameters to delicate samples, such as wound healing and treatment of surgical margins in plasma-based cancer surgery.

The dynamics of nanoparticle growth in pulsed anodic arc discharge has been studied in time-resolved mode. To this end, a fast moving probe was employed to extract the material generated in a pulsed arc plasma held between two graphite electrodes. The probe motion was synchronized with the pulse phase and the exposure time to the plasma was set to 10 ms. The graphite anode was eroded in a helium atmosphere (300 Torr) by an arc plasma pulsed at 1 Hz with a 10% duty cycle and showing 250 A of peak current. The structure and morphology of the probe depositions were characterized by Raman spectroscopy, scanning electron microscopy, and focused ion beam. A maximal deposition rate of 260 μm/s was measured 5 mm away from the arc core during the active 0.1 s of the pulse. Such a rate yields a growth flux of 1.3 × 1021 cm−2 s−1, rich in carbon nanostructures (graphene platelets, nanotubes) with a characteristic aggregate size within 1–10 μm. The deposition during the inactive 0.9 s of the pulse was several orders of magnitude slower and consisted of amorphous carbon traces. Moreover, the nanoparticle distribution along the collecting probe is correlated with the pulse phase, thereby providing information on particle transport. Pulsed nanosynthesis can be modeled as a periodical growth process, where the volume and propagation velocity of the growth region can be adjusted through modulation of the pulse signal waveform. The proposed model constitutes a suitable framework to investigate the pulsed arc synthesis of nanomaterials with tailored physical and chemical properties.

Pulsed anodic arc discharge is a novel synthesis method of nanomaterials by means of low-temperature atmospheric plasma. The technique consists in periodically supplying DC power to two vertically aligned electrodes in the form of short unipolar pulses with peak currents of a few hundred Amperes in a helium atmosphere. The pulsed arc plasmas are sustained at frequencies on the order of 1 Hz with around 10% of duty cycle. It constitutes a versatile technique thanks to a series of advantages compared to continuous DC arc processes, in particular, flexibility in the experimental conditions, process stability and repeatability, better utilization of ablating anode material, lower production of macroparticles, and lower thermal loads. Such features are discussed in this article. A brief overview concerning the recent accomplishments of pulsed arc discharge on deposition of carbon nanostructures (graphene and carbon nanotubes) and few-layer flakes of molybdenum disulphide and an outlook on future applications of this method for the discovery of new materials with tailored functional properties are provided.

Here, the synthesis of Molybdenum Disulphide (MoS2) flakes by means of anodic atmospheric arc discharge is reported for the first time. The vertical electrode configuration consisted of a compound anode (hollow graphite anode filled with MoS2 powder) and a solid graphite cathode placed just above of the compound anode. Arc processes were operated in pulsed mode to preferentially evaporate the powder component from the anode and to minimize Carbon ablation. Pulsed anodic arc discharges were conducted at 2 Hz and 10% duty cycle in 300 Torr of Helium with a peak current of 250-300 A and a peak voltage of 35 V. A probe made of Tungsten wire was placed in the vicinity of the arc column to collect the evaporated material. The measured thickness profile was correlated to the particle flux distribution and it was fitted by a simple model of plasma expansion. During pulse phase, electron density was estimated around 5E22 m-3 or higher, and ion current density was of the order of 10 A/mm2. Morphology, structure and composition of the samples were characterized by Raman spectroscopy, atomic force microscopy (AFM), scanning electron microscopy (SEM), transmission electron microscopy (TEM), and X-ray diffraction (XRD). The study shows that pulsed arc discharge of the compound anode leads to moderate C deposition combined with MoS2 deposition in the form of fragmented nanocrystals and few atomic monolayers of MoS2. Such synthesis technique is promising to produce new 2D nanomaterials with tailored structure and functionality thanks to the flexibility of pulsed power.

Anodic arc discharge operated with a periodic voltage signal shows better performance in nanomaterial synthesis compared to steady DC arc discharge. The main advantages of pulsed arcs consist of improved process control, high arc stability, tailoring of material properties, and growth of high-quality materials thanks to reduced production of macroparticles. This study shows the relevance of pulse frequency (1-5 Hz), duty cycle (10-50%), peak arc voltage (~50 V) and peak arc current (~200 A) on determining arc discharge properties. All arc processes are performed in a helium atmosphere at around 300 Torr. Pulsed arc discharges show ablation rates and average power consumption values of the order of 1 mg/s and 1 kW, respectively, which are competitive with typical values of standard DC anodic arcs. Pulsed arc syntheses of low-dimensional carbon (graphene, nanotubes) and molybdenum disulfide nanomaterials are treated as examples for this new deposition method. Plasma parameters have been estimated by optical emission spectroscopy and fast Langmuir probe diagnostics. Characterization techniques such as Raman spectroscopy, scanning electron microscopy and x-ray diffraction have provided information on structural and morphological properties of the cathode deposit. The basic physical processes during arc discharge are described in terms of a global model that considers the total pressure evolution of the system. Such model provides a scenario for the observed erosion dynamics and optical emission patterns associated to the different arc discharge phases. The electrical properties of pulsed arc discharges have been characterized and related to basic plasma structure. In conclusion, pulsed anodic arc plasmas are promising for the synthesis of nanomaterials with special physical and surface properties suitable for electrical, mechanical and optical applications.

Pulsed arc discharges can improve arc control and tailor the ablation process in the production of 1D and 2D nanostructures from carbon anodes. In this work, low-dimensional carbon nanoparticles have been generated by means of anodic arc discharge in helium atmosphere excited with a square-wave modulated signal (1–5 Hz, 10% duty cycle). The discharges were performed between two graphite electrodes with maximal peak current of 250 A and maximal voltage of 65 V. The erosion rates and conversion efficiency of the ablated anode are compared to reference samples grown in DC steady arc mode. Ablation rates in pulsed arcs are typically of the order of 1 mg s−1. Combination of fast Langmuir probe diagnostics and optical emission spectroscopy provided plasma parameters of the discharges at the arc column. Ranges of 1016–1017 m−3 for electron density and 0.5–2.0 eV for electron temperature are estimated. The obtained samples were characterized with Raman spectroscopy and scanning electron microscopy. The deposit on the cathode after pulsed arc consisted of carbon nanostructures such as graphene nano-platelets and carbon nanotubes. Erosion dynamics of pulsed arc discharge has been described in terms of a global model and compared to steady arc discharge. A correlation is identified among discharge regimes, optical emission patterns and ablation modes. In conclusion, pulsed anodic arc discharge is a very efficient source of carbon nanomaterials. The large control of the discharge characteristics will permit to tailor accurately the production and the properties of carbon nanotubes and graphene. This deposition method is promising for the fabrication of semiconducting nanomaterials with tuneable electrical and optical properties.

We studied the optical properties of a two-dimensional (2D) photonic crystal monolayer made up of silica submicrospheres, partly covered by a metallic coating. The silica particles, synthesized by the Stober method, had a diameter of about 300 nm, with little size dispersion. They were deposited on glass in a hexagonal pattern by the Langmuir–Blodgett process. The resulting film consisted of a single monolayer of particles. The final step of sample preparation consisted of depositing a discontinuous gold layer on top of the 2D crystal. In order to evaluate the effects of small compositional and structural changes on the optical properties of the 2D crystal, the gold layer was applied to a thickness of less than 2 nm. Anisotropy in the metallic coating was induced by tilting the deposition angle by 10° from the vertical: a discontinuous layer was obtained on top of the particles, with elongated shapes pointing towards the source of the evaporation. The shape of the particles and the film structure were characterized by scanning electron microscopy. Ellipsometric measurements were performed in transmission mode in order to assess the sample anisotropy, and the plasmon response of the samples was determined by extinction measurements. The original 2D crystal presented unexpected optical and uniaxial anisotropy which was increased by a factor of 2 after gold coating.

Erbium-doped photonic materials and structures have been prepared by sol-gel, in the form of silica-hafnia based optical planar waveguides and 1-D photonic bandgap structures consisting of alternating silica and titania layers. The Er 3+ ions were found to be sensitive probes of the waveguide glass matrix structure, especially when hafnium-containing nanocrystallites were present, which narrowed and resolved different Stark components in the photoluminescence peaks. In 1-D Fabry-Perot microcavities, on the other hand, efficient energy transfer was observed from Yb 3+ to Er 3+ ions when these were present simultaneously in the same defect layer, but not when the two types of ions were isolated in separate defect layers of coupled microcavities.

SiO2, TiO2 and mixed SiO2–TiO2 thin films were prepared, via sol-gel processing, by spin-coating on silicon wafer substrates. They were either dried at room temperature, or subjected to different heat treatments, from 600 °C up to 950 °C. Infrared spectroscopy was performed on these films, in transmission mode, using variable incidence, light, either natural or polarized. The longitudinal optical (LO) components of the rocking and asymmetric stretching Si–O–Si vibrations become more visible and the position can be more accurately determinated, respectively, with increasing incidence angles away from normal. These peak positions shift to higher frequencies with thermal treatment, as the films undergo a transition from the gel to the glassy state. In titania films, the detection of LO vibrations is less evident than in silica based-films, even at oblique incidence. (© 2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

In this work, one-dimensional multilayer structures of periodically alternating low refractive index (SiO2) and high index (TiO2) materials have been deposited by sol–gel spin-coating, in the form of dielectric mirrors (distributed Bragg reflectors) and Fabry-Perot microcavities. Some of the microcavities were doped with europium. These structures have been characterized structurally (by X-ray diffraction, Raman spectroscopy and field emission-scanning electron microscopy) and optically (by ellipsometry and visible–near IR spectroscopy); their optical properties, namely the stop band of high reflectivity and, in the case of the microcavities, the cavity mode and the photoluminescence behavior of the Eu3+ ions inserted in the cavity layer, were assessed. In addition, three-dimensional-like structures have also been synthesized by a colloidal/sol–gel route, starting with the self-organization of a colloidal dispersion of polystyrene microspheres, by dip-coating or convective self-assembly, followed by infiltration of the interstices with titania and removal of the polymeric template. The structural and optical properties of the inverse opals prepared by this method have also been studied, by scanning electron microscopy and by visible reflection spectroscopy, in order to assess their photonic bandgap properties.

Since the launching of the photonic bandgap concept in 1987, the development of corresponding structures has expanded very rapidly, in particular two-dimensional semiconductor-based structures. In the case of sol–gel derived materials, the main emphasis for the past year has been on one-dimensional multilayer stacks and, in particular, on three-dimensional structures of the opal and inverse opal type.

The use of large nanopatterned surfaces areas arouses a growing interest for the fabrication of templates, thin membranes, biomedical sensors and large-area photonic devices. Langmuir-Blodgett (LB) technique offers the possibility to deposit self-assembled monolayers of molecules and nanometric particles on surfaces exhibiting a structural order and uniformity at large scale. In this work, we studied the characteristics and properties of silica nanoparticles, produced by sol-gel process, for self-assembly in close-packed 2D crystal monolayers, deposited on glass and silicon substrates using the LB technique. The resulting monolayers were characterized by SEM (Scanning Electron Microscopy), AFM (Atomic Force Microscopy), optical measurements, transmittance and contact angle measurements.

The effects of environmental conditions and fluorine content on friction of diamond-like carbon (DLC) films are determined and discussed. Relative humidity (RH) has a considerable effect on friction values, modifying them up to a factor 2. Two regions of RH are found, depending on its effect over friction. From 20% to 60% of RH, friction decreases probably due to a better substitution of superficial atoms by hydrogen. From 60% to 80% friction remains stable and equal for almost all samples. A possible explanation would be the formation of a physisorbed layer of water on the surface, which would mask the relatively small chemical and structural variations of the films. Moreover, the different components of surface free energy (SFE) are determined using Van Oss–Chaudury–Good model. The results clearly indicate the importance of Lifshitz–Van der Waals component on SFE and the small correction that supposes acid-base interactions. According to this model, the studied DLC is a monopolar basic material. The basic component is reduced when F content is increased.

This paper deals with glass-based photonic structures, used to control and modify the optical and spectroscopic properties of rare earth ions. The spectroscopic assessment of sol–gel-derived planar waveguides and 1D photonic band gap structures is reported. The spectroscopic, optical, and structural properties of planar waveguides with (100 − x)SiO2–xHfO2–yErO1.5 with y = 0.3, 0.01; and x = 10, 20, 30, 40 have been investigated by photoluminescence and Raman spectroscopy. The radiative quantum efficiency of the 4I13/2 metastable state of Er3+ ions is between 84 and 88% depending on the Si/Hf molar ratio. The sol–gel-derived one-dimensional cavity was realized by a Eu3+-activated dielectric layer placed between distributed Bragg reflectors (DBRs).

Since the launching of the photonic bandgap concept in 1987, the development of the corresponding structures has expanded very rapidly, in particular for two-dimensional semiconductor-based structures. In the case of sol-gel derived materials, the main emphasis has been on one-dimensional mutilayer stacks and, in particular, on three-dimensional structures of the opal and inverse opal type. In this work, one-dimensional multilayer stacks of periodically alternating low refractive index (SiO 2 ) and high index (TiO 2 ) materials have been deposited by spin-coating onto silica or single crystal Si substrates, in the form of dielectric mirrors (distributed Bragg reflectors) and Fabry-Perot microcavities, both single and coupled (double). Some of the microcavities were doped with rare-earth elements (Er and Yb). These structures have been characterized by X-ray diffraction, infrared spectroscopy and field emission-scanning electron microscopy; their optical properties have also been measured, namely the stop band of high reflectivity and, in the case of the microcavities, the cavity modes and the photoluminescence behavior of the rare-earth ions inserted in the cavity layers. The presence of Er in both cavity layers of coupled microcavities was found to lead to a substantial increase in photoluminescence signal intensity and width, compared to the cases where Er was present only in one of the cavities. The possibility of an antenna effect between Yb 3+ and Er 3+ ions is also examined.

This paper deals with glass-based photonic structures, which allow to control and modify the optical and spectroscopic properties of rare earth ions. The spectroscopic assessment of sol gel-derived planar waveguides and 1-D photonic band gap structures is reported. The spectroscopic, optical, and structural properties of (100-x)SiO2 - xHfO2 -yErO1.5 (y = 0.3, 0.01; x = 10, 20, 30, 40 ) planar waveguides have been investigated by using photoluminescence and Raman spectroscopy. The radiative quantum efficiency of the 4I13/2 metastable state of Er3+ ions was estimated between 84 % and 88 % depending on the Si/Hf molar ratio. The sol gel-derived one-dimensional cavity was realized by a Eu3+-activated dielectric layer placed between two distributed Bragg reflectors (DBRs).

The ion fluxes and ion energy distributions (IED) corresponding to discharges in methane (CH4) were measured in time-averaged mode with a compact retarding field energy analyser (RFEA). The RFEA was placed on a biased electrode at room temperature, which was powered by either radiofrequency (13.56 MHz) or asymmetric bipolar pulsed-dc (250 kHz) signals. The shape of the resulting IED showed the relevant populations of ions bombarding the cathode at discharge parameters typical in the material processing technology: working pressures ranging from 1 to 10 Pa and cathode bias voltages between 100 and 200 V. High-energy peaks in the IED were detected at low pressures, whereas low-energy populations became progressively dominant at higher pressures. This effect is attributed to the transition from collisionless to collisional regimes of the cathode sheath as the pressure increases. On the other hand, pulsed-dc plasmas showed broader IED than RF discharges. This fact is connected to the different working frequencies and the intense peak voltages (up to 450 V) driven by the pulsed power supply. This work improves our understanding in plasma processes at the cathode level, which are of crucial importance for the growth and processing of materials requiring controlled ion bombardment. Examples of industrial applications with these requirements are plasma cleaning, ion etching processes during fabrication of microelectronic devices and plasma-enhanced chemical vapour deposition of hard coatings (diamond-like carbon, carbides and nitrides).

By combining colloidal lithography and pulsed-DC plasma deposition of diamond-like amorphous carbon films, we have fabricated sub-microstructures that modulate the surface properties of this material. A self-assembled monolayer of silica sub-micron spheres was deposited on monocrystalline silicon, which was patterned either by thermal annealing of the particles or by hole-mask lithography via ion beam etching with argon. The samples showed hemispherical close-packed topographies and micro-pillar networks. The coefficient of friction, measured with a nanotribometer, and the wettability were controlled through this special surface patterning. These structures are suitable for applications requiring protective coatings with large area showing hydrophobic properties.

Monodispersed colloidal crystals based on silica sub-micrometric particles were synthesized using the Stöber-Fink-Bohn process. The control of nucleation and coalescence result in improved characteristics such as high sphericity and very low size dispersion. The resulting silica particles show characteristics suitable for self-assembling across large areas of closely-packed 2D crystal monolayers by an accurate Langmuir-Blodgett deposition process on glass, fused silica and silicon substrates. Due to their special optical properties, colloidal films have potential applications in fields including photonics, electronics, electro-optics, medicine (detectors and sensors), membrane filters and surface devices. The deposited monolayers of silica particles were characterized by means of FESEM, AFM and optical transmittance measurements in order to analyze their specific properties and characteristics. We propose a theoretical calculation for the photonic band gaps in 2D systems using an extrapolation of the photonic behavior of the crystal from 3D to 2D. In this work we show that the methodology used and the conditions in self-assembly processes are decisive for producing high-quality two-dimensional colloidal crystals by the Langmuir-Blodgett technique.