Lisha Fan

Highly efficient and recyclable carbon soot sponge for oil cleanup.

Carbon soot (CS) has the advantages of cost-effectiveness and production scalability over other carbons (i.e., graphene, CNTs) in their synthesis. However, little research has been conducted to explore the potential applications of CS. In this study, we demonstrated that a common daily waste-CS-can be used for developing a cost-effective absorbent (CS-sponge) to remove oil contaminants from water. The CS was synthesized by an ethylene-oxygen combustion flame. The CS-sponge was prepared via a dip-coating method. Without further surface modification and pretreatments, the CS-sponge demonstrates high absorption capacities (up to 80 times its own weight) for a broad spectrum of oils and organic solvents with a recyclability of more than 10 times. These research results show evidence that the CS-sponge is promising in environmental remediation for large-scale, low-cost removal of oils from water.

Direct writing of graphene patterns on insulating substrates under ambient conditions

To unleash the full potential of graphene in electronics and optoelectronics, high-quality graphene patterns on insulating substrates are required. However, existing methods generally follow a “synthesis + patterning” strategy, which are time consuming and costly for fabricating high-quality graphene patterns on desired substrates. We developed a nanofabrication process to deposit high-quality graphene patterns directly on insulating substrates via a solid-phase laser direct writing (LDW) process. Open-air and room-temperature fabrication of graphene patterns on insulating substrates has been achieved via a femtosecond LDW process without graphene transfer and patterning. Various graphene patterns, including texts, spirals, line arrays, and integrated circuit patterns, with a feature line width of 800 nm and a low sheet resistance of 205 ohm/sq, were fabricated. The LDW method provides a facile and cost-effective way to fabricate complex and high-quality graphene patterns directly on target substrates, which opens a door for fabricating various advanced functional devices.

Flame-enhanced laser-induced breakdown spectroscopy

Flame-enhanced laser-induced breakdown spectroscopy (LIBS) was investigated to improve the sensitivity of LIBS. It was realized by generating laser-induced plasmas in the blue outer envelope of a neutral oxy-acetylene flame. Fast imaging and temporally resolved spectroscopy of the plasmas were carried out. Enhanced intensity of up to 4 times and narrowed full width at half maximum (FWHM) down to 60% for emission lines were observed. Electron temperatures and densities were calculated to investigate the flame effects on plasma evolution. These calculated electron temperatures and densities showed that high-temperature and low-density plasmas were achieved before 4 µs in the flame environment, which has the potential to improve LIBS sensitivity and spectral resolution.

High-performance flexible solid-state supercapacitors based on MnO2-decorated nanocarbon electrodes

Flexible energy storage units are highly desired to meet the ever-increasing demands for flexible electronics. In this paper, highly flexible solid-state supercapacitors are fabricated using MnO2-decorated nanocarbon electrodes and 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide–poly(vinylidene fluoride)-hexafluoropropylene ([EMIM][NTf2]–PVdF(HFP)) gel electrolytes. The flexible electrodes are prepared by electrodepositing MnO2 onto the carbon nanotube/carbon nanoonion (CNT/CNO) films. CNT/CNO films have a large surface area for MnO2 deposition and work as mechanical supports with high flexibility and light weight. CNOs act as spacers to separate CNTs, introducing mesopores inside the CNT/CNO films for preventing pore blocking during MnO2 deposition. The supercapacitor exhibits enhanced electrochemical performance with an energy density of 16.4 W h−1 kg−1 at a power density of 33.3 kW kg−1 by using the [EMIM][NTf2]–PVdF(HFP) gel electrolyte. Moreover, the supercapacitors can exhibit high electrochemical performance under large mechanical stress, making the devices suitable for flexible electronics.

Resonant vibrational excitation of ethylene molecules in laser-assisted diamond deposition

The influence of resonant vibrational excitation of ethylene molecules in combustion chemical vapor deposition of diamond was investigated. Resonant vibrational excitation of the CH2-wagging mode (a type c fundamental band, υ7, at 949.3 cm−1) in ethylene molecules was achieved by using a wavelength-tunable CO2 laser with a matching wavelength at 10.532 µm. By comparing to laser irradiation at off-resonance wavelengths, an on-resonance vibrational excitation is more efficient in energy coupling, increasing flame temperatures, accelerating the combustion reactions, and promoting diamond deposition. An enhanced rate of 5.7 was achieved in terms of the diamond growth rate with an improved diamond quality index at a high flame temperature under a resonant excitation of the CH2-wagging mode. This study demonstrates that a resonant vibrational excitation is an effective route for coupling energy into the gas phase reactions and promoting the diamond synthesis process.

Synthesis of nitrogen-doped diamond films using vibrational excitation of ammonia molecules in laser-assisted combustion flames

Nitrogen-doped diamond was synthesized in open air using laser-assisted combustion flame method. A wavelength-tunable CO2 laser was used to resonantly excite the vibration modes of ammonia molecules, which were added into the diamond forming combustion flame. The wavelength of the CO2 laser was tuned to match frequencies of the NH wagging mode of the ammonia molecules. High efficiency energy coupling was achieved at laser wavelengths of 9.219, 10.35, and 10.719 μm, which are related to a rotational–vibrational transition (1084.63 cm−1), and splitting of the NH wagging mode (υ2+, 932.51 cm−1 and υ2−, 968.32 cm−1). Vibrational excitations of the ammonia molecules under these wavelengths actively intervenes the reaction courses, which steers the chemical reaction in the combustion flame and eventually promotes nitrogen concentration in the deposited diamond films. Concentration of the doped nitrogen atoms reaches up to 1.5 × 1020 atoms/cm3 in the diamond films deposited with a laser wavelength of 9.219 μm. O...

Ultraviolet laser photolysis of hydrocarbons for nondiamond carbon suppression in chemical vapor deposition of diamond films

In this work, we demonstrate that ultraviolet (UV) laser photolysis of hydrocarbon species alters the flame chemistry such that it promotes the diamond growth rate and film quality. Optical emission spectroscopy and laser-induced fluorescence demonstrate that direct UV laser irradiation of a diamond-forming combustion flame produces a large amount of reactive species that play critical roles in diamond growth, thereby leading to enhanced diamond growth. The diamond growth rate is more than doubled, and diamond quality is improved by 4.2%. Investigation of the diamond nucleation process suggests that the diamond nucleation time is significantly shortened and nondiamond carbon accumulation is greatly suppressed with UV laser irradiation of the combustion flame in a laser-parallel-to-substrate geometry. A narrow amorphous carbon transition zone, averaging 4 nm in thickness, is identified at the film–substrate interface area using transmission electron microscopy, confirming the suppression effect of UV laser irradiation on nondiamond carbon formation. The discovery of the advantages of UV photochemistry in diamond growth is of great significance for vastly improving the synthesis of a broad range of technically important materials.

EFFECT OF LASER-ASSISTED RESONANT EXCITATION ON THE GROWTH OF GaN FILMS

Gallium nitride (GaN) films were grown using laser-assisted metal organic chemical vapor deposition (LMOCVD). The vibrational mode (1084.63 cm−1) of ammonia (NH3) molecules was resonantly excited using a wavelength-tunable CO2 laser at a wavelength of 9.219 µm due to its high absorption cross-section. Through wavelength-matched resonant excitation of the NH3 molecules, highly c-axis oriented GaN films were successfully deposited on sapphire (α-Al2O3) substrates at low temperatures (250 to 600 °C). The strong (0001) GaN peak in X-ray diffraction spectra confirmed the good crystalline quality of GaN films. Additionally, the resonant vibrational excitation of NH3 in LMOCVD promoted the GaN growth rate considerably compared to that synthesized by MOCVD without resonant vibrational excitation of NH3 molecules.Gallium nitride (GaN) films were grown using laser-assisted metal organic chemical vapor deposition (LMOCVD). The vibrational mode (1084.63 cm−1) of ammonia (NH3) molecules was resonantly excited using a wavelength-tunable CO2 laser at a wavelength of 9.219 µm due to its high absorption cross-section. Through wavelength-matched resonant excitation of the NH3 molecules, highly c-axis oriented GaN films were successfully deposited on sapphire (α-Al2O3) substrates at low temperatures (250 to 600 °C). The strong (0001) GaN peak in X-ray diffraction spectra confirmed the good crystalline quality of GaN films. Additionally, the resonant vibrational excitation of NH3 in LMOCVD promoted the GaN growth rate considerably compared to that synthesized by MOCVD without resonant vibrational excitation of NH3 molecules.

Thermal characterization of diamond films through modulated photothermal radiometry.

Diamond (Dia) films are promising heat-dissipative materials for electronic packages because they combine high thermal conductivity with high electrical resistivity. However, precise knowledge of the thermal properties of the diamond films is crucial to their potential application as passive thermal management substrates in electronics. In this study, modulated photothermal radiometry in a front-face configuration was employed to thermally characterize polycrystalline diamond films deposited onto silicon (Si) substrates through laser-assisted combustion synthesis. The intrinsic thermal conductivity of diamond films and the thermal boundary resistance at the interface between the diamond film and the Si substrate were investigated. The results enlighten the correlation between the deposition process, film purity, film transverse thermal conductivity, and interface thermal resistance.

Laser-assisted solid-state synthesis of carbon nanotube/silicon core/shell structures

A single-step solid-state synthetic approach was developed for the synthesis of silicon-coated carbon nanotube (CNT) core/shell structures. This was achieved through laser-induced melting and evaporation of CNT-deposited Si substrates using a continuous wavelength CO2 laser. The synthesis location of the CNT/Si structures was defined by the laser-irradiated spots. The thickness of the coating was controlled by tuning the laser power and synthesis time during the coating process. This laser-based synthetic technique provides a convenient approach for solid-state, controllable, gas-free, simple and cost-effective fabrication of CNT/Si core/shell structures.