Hydrogen-free diamond-like carbon (DLC) thin films were prepared at different repetition rates of laser pulses by pulsed laser ablation of graphite target at room temperature. The microstructure of the thin films was characterized by Raman spectroscopy. Raman measurements showed that sp 3 bonded carbon fraction is reduced with increase of repetition rates of laser pulses from 5 to 20 Hz. Optical properties, namely photoluminescence (PL), optical absorption, extinction coefficient (k), and refractive index (n), were measured by fluorescence spectrophotometer, scanning spectrophotometer, and spectroscopic ellipsometer. By changing the repetition rates of laser pulses from 5 to 20 Hz, the PL and optical absorption were gradually increased, while the deposition rate and optical band gap of the films decreased strictly with the increase of repetition rate. The extinction coefficient (k) and refractive index (n) were found to be in the range of 0.468-0.938 and 1.92-2.27, respectively. These results indicate that repetition rate of laser pulses has a strong influence on the microstructure and optical properties of the films. Based on the experimental results, a possible causation about the effect of repetition rate on PL was proposed.

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The effect of nitrogen pressure on optical properties of hydrogen-free diamond-like carbon (DLC) films deposited by pulsed laser ablation graphite in different background pressures of nitrogen is reported. By varying nitrogen pressures from 0.05 to 15.00 Pa, the photoluminescence is gradually increased and optical transmittance is gradually decreased. Atomic force microscopy (AFM) is used to observe the surface morphology of the DLC films. The results indicate that the surface becomes unsmoothed and there are some globose particles on the films surface with the rise of nitrogen pressures. The microstructure of the films is characterized using Raman spectroscopy.Diamond-like carbon (DLC) is a very attractive material due to its unique properties, which resembles diamond while the synthetic conditions are easier compared with diamond [1][2][3][4] . Many physical and chemical methods have been adopted to synthesize DLC [4][5][6][7][8] . But most chemical vapor deposition (CVD) methods generate hydrogenated amorphous carbon which is not stable when temperature is high [9,10] . In addition, CVD techniques usually need high substrate temperature ( 800 o C) which is not favorable when the substrate is thermally sensitive. The pulsed laser deposition (PLD) has the unique ability to synthesize hydrogen-free DLC films at low deposition temperature. The deposition parameters (e.g. temperature, ambient gas pressure, laser intensity, etc) are important for PLD system. A wide range of DLC films growth conditions has been reported in the literature for the past several decades but there are few reports of nitrogen circumstances. In this letter, we report DLC films grown at different background pressures of nitrogen by PLD. DLC films are deposited by pulsed laser ablation of a high pure (99.999%) graphite target using a Tuilaser ThinFilms Star 20 excimer laser operating at 248 nm. The laser beam with energy of 120 mJ is focused with a spherical lens onto rotating target at an incidence angle of 45°. The repetition rate of laser pulses is set to 10 Hz. Before depositting the films, the quartz substrates are ultrasonically cleaned with acetone and alcohol for 10 minutes separately, then rinsed repeatedly in de-ionized water and blew dry and then placed in growth chamber immediately. The growth chamber is evacuated using a turbomolecular pump to 5.0 10 -6 Pa, and then backfilled with nitrogen to the required pressure. The DLC films are grown at different background nitrogen pressure ranging from 0.05 Pa to 15.00 Pa. The pulsed laser shots are maintained in the range of 10000-20000, in order to maintain the same film thickness for each deposition. The target to substrate distance is fixed at 60 mm. The films are deposited onto a rotating substrate which is wheeled by a motor in order to obtain uniform thin films. In order to avoid target drilling, the target is also rotated to provide each laser pulse a fresh target surface. Before deposition, the substrates are argon ion etched for residual impurities for 10 minutes.The microstruct...