Sven Stauss

Synthesis of higher diamondoids by pulsed laser ablation plasmas in supercritical CO2

Pulsed laser ablation (wavelength 532 nm; fluence 18 J/cm2; pulse width 7 ns; repetition rate 10 Hz) of highly oriented pyrolytic graphite was conducted in adamantane-dissolved supercritical CO2 with and without cyclohexane as a cosolvent. Micro-Raman spectroscopy of the products revealed the presence of hydrocarbons possessing sp3-hybridized carbons similar to diamond structures. The synthesis of diamantane and other possible diamondoids consisting of up to 12 cages was confirmed by gas chromatography–mass spectrometry. Furthermore, gas chromatography–mass spectrometry measurements of samples before and after pyrolysis treatment indicate the synthesis of the most compact decamantane, namely, superadamantane. It is thought that oxidant species originating from CO2 during pulsed laser ablation might lead to the selective dissociation of C-H bonds, enabling the synthesis of low H/C ratio molecules. Therefore, laser ablation in supercritical CO2 is proposed as a practical method for synthesizing diamondoids.

The importance of spinning speed in fabrication of spin-coated organic thin film transistors: Film morphology and field effect mobility

We have investigated the film morphology and the field effect mobility of 2,7-dioctyl[1]benzothieno[3,2-b][1]benzothiophene (C8-BTBT) thin films which were formed by spin coating on the SiO2 substrate with solution-processed graphene electrodes. The domain size and the density of aggregates in the C8-BTBT film showed the same dependence on the spinning speed. These competitive two factors (domain size and density of aggregates) give an optimum spinning speed, at which the field effect mobility of C8-BTBT transistor showed a maximum (2.6 cm2/V s). This result indicates the importance of spinning speed in the fabrication of solution processed organic thin film transistors by spin coating.

Plasma microreactor in supercritical xenon and its application to diamondoid synthesis

The generation of plasmas in a microreactor is demonstrated in xenon from atmospheric pressure up to supercritical conditions. Ac high voltage at a frequency of 15 kHz was applied across a 25-µm discharge gap between a tungsten wire and a fused silica micro-capillary tube in a coaxial configuration. Using this continuous flow supercritical fluid microreactor, it was possible to synthesize diamantane and other diamondoids up to nonamantane, using adamantane as a precursor and seed. It is anticipated that plasmas generated in supercritical fluid microreactors may not only allow faster fabrication of diamondoids, but also offer opportunities for the fabrication of other nanomaterials.

Pulsed Laser Ablation Synthesis of Diamond Molecules in Supercritical Fluids

Nanocarbon materials have been synthesized by pulsed laser ablation (532 nm; 52 J/cm2; 7 ns; 10 Hz) of highly oriented pyrolytic graphite in adamantane-dissolved supercritical xenon at a temperature T = 290.2 K and pressure p = 5.86 MPa. Micro-Raman spectroscopy of the products revealed the presence of hydrocarbons possessing sp3 hybridized bonds also found in diamond structures. The synthesis of diamantane was confirmed by gas chromatography-mass spectrometry. The same measurements also indicate the possible synthesis of other diamondoids up to octamantane. Thus, laser ablation in supercritical fluids is proposed as one practical method of synthesizing diamondoids.

Pulsed laser ablation plasmas generated in CO2 under high-pressure conditions up to supercritical fluid

Pulsed laser ablation of solids in supercritical media has a large potential for nanomaterials fabrication. We investigated plasmas generated by pulsed laser ablation of Ni targets in CO2 at pressures ranging from 0.1 to 16 MPa at 304.5 K. Plasma species were characterized by optical emission spectroscopy, and the evolution of cavitation bubbles and shockwaves were observed by time-resolved shadowgraph imaging. Ni and O atomic emissions decreased with increasing gas pressure; however, near the critical point the intensities reached local maxima, probably due to the enhancement of the plasma excitation and effective quenching resulting from the large density fluctuation.

Development of a dielectric barrier discharge (DBD) cryo-microplasma : generation and diagnostics

We developed a cryo-microplasma, which can continuously control gas temperature below room temperature and below the freezing point of water. To develop the cryo-microplasma, we first developed an atmospheric-pressure low-temperature microplasma that can suppress the increase in its gas temperature. Helium gas was employed, which was generated in open air. The average estimated electron density and temperature were 108–109 cm−3 and 4–5 eV, respectively, independent of the applied voltage. Then, helium gas, which was the working gas of the atmospheric-pressure low-temperature microplasma, was cooled by liquid nitrogen to generate an atmospheric-pressure cryo-microplasma in open air. We observed the generation of frost around the quartz tube in which the plasma was generated and an increase in atomic oxygen emission by optical emission spectroscopy. Finally, to avoid the generation of frost, a cryo-microplasma was generated in a reactor chamber separated from open air. Helium, nitrogen and oxygen were employed as working gases. Using thermocouples and by estimation from the nitrogen rotational temperature, we verified that the gas temperature of the cryo-microplasma was much lower (Tg ≈ 180–300 K) than that of the conventional atmospheric-pressure low-temperature plasma (above 300 K).

Synthesis of the Higher-Order Diamondoid Hexamantane Using Low-Temperature Plasmas Generated in Supercritical Xenon

Diamondoid molecules were synthesized from adamantane (C10H16) using low-temperature plasmas generated in supercritical xenon. The carbon content of the synthesized materials was verified by energy dispersive X-ray spectroscopy, while micro-Raman spectroscopy measurements confirmed that the synthesized materials contained sp3 bonds, the features in the Raman spectra being similar to those found in the Raman spectra of higher order diamondoids. Mass peaks at m/z = 396 were most abundant and might be attributed to C30H36 isomers of hexamantane. The synthesis of this particular type of diamondoid is explained by the fewer necessary cleavages of C–C bonds or C–H occurring to form the diamondoid.

Pure air–plasma bullets propagating inside microcapillaries and in ambient air

This paper reports on the characterization of air–plasma bullets in microcapillary tubes and in ambient air, obtained without the use of inert or noble gases. The bullets were produced by nanosecond repetitively pulsed discharges, applied in a dielectric barrier discharge configuration. The anode was a tungsten wire with a diameter of 50 µm, centered in the microcapillary, while the cathode was a silver ring, fixed on the outer surface of the fused silica tube. The effects of the applied voltage and the inner diameter of the microcapillary tube on the plasma behavior were investigated. Inside the tubes, while the topology of the bullets seems to be strongly dependent on the diameter, their velocity is only a function of the amplitude of the applied voltage. In ambient air, the propagation of air bullets with a velocity of about 1.25 × 105 m s−1 is observed.

Synthesis of Diamondoids by Supercritical Xenon Discharge Plasma

Diamondoids were synthesized by dielectric barrier discharges in supercritical xenon containing dissolved adamantane, which served as a precursor. The synthesis of diamantane was confirmed by gas chromatography mass spectrometry measurements, in addition to energy-dispersive X-ray spectroscopy and micro-Raman spectroscopy measurements. Moreover, the possible synthesis of two higher-order diamondoids, pentamantane and decamantane, with molecular weights of 330 and 456, respectively, is indicated from the selective ion monitoring mode. The largest production yield was obtained in the vicinity of the critical point.

Synthesis and investigation of reaction mechanisms of diamondoids produced using plasmas generated inside microcapillaries in supercritical xenon

We have synthesized diamondoids using dielectric barrier discharge microplasmas generated inside a microcapillary reactor in supercritical xenon. The plasmas were generated near the critical temperature () and pressure () of xenon in the ranges of and under both batch-type and continuous flow conditions with gas flow rates of 0.01?0.5 mL min?1. Micro-Raman spectra of the synthesized particles showed features characteristic of diamondoids, while gas chromatography?mass spectrometry measurements revealed that diamondoids up to undecamantane were possibly synthesized. Further, the amount of obtained diamantane was greater than those obtained using previously reported diamondoid synthesis processes that involve plasmas in supercritical fluids. This increase is attributed to the higher solubility of the supercritical medium, i.e., xenon, and the higher efficiency of the microreactor. A detailed gas chromatography?mass spectrometry analysis showed that higher diamondoids grow in a stepwise manner via the alternate removal of hydrogen atoms and the addition of methyl groups.