A. Karacs

Pressure dependent formation of small Cu and Ag particles during laser ablation

Abstract In this paper the possibility of application of laser ablation in inert gas atmosphere for deposition of nanosized particles is examined. Cu and Ag were deposited by laser ablation in 0–10 mbar Ar atmosphere. The samples were characterized by TEM and XPS. It was found that below 1 mbar for Cu and 2 mbar for Ag, island like thin film formation occurs. In Cu above 1 mbar lonely nanoparticles with diameters of 3–4 nm as well as large cluster aggregates were found, instead of the islands. This morphology remained unchanged up to 10 mbar. On the contrary, in Ag at 5 mbar individual particles with sizes of 4–6 nm were observed instead of the aggregates or islands. At 10 mbar the number of the 4–6 nm particles strongly decreased, and particles in the size range of 10 nm became dominant. The observed results may be related to the different decelerating rate of the evaporated Cu and Ag atoms in the Ar atmosphere.

Laser ablation induced formation of nanoparticles and nanocrystal networks

Using experimental data on morphology of nanophase materials prepared by pulsed laser ablation in an inert gas atmosphere, we present a phenomenological description of their condensation process. According to our idea, in high enough background pressure a shock wave is initiated by collisions between gas and target atoms, which slows down and spatially confines the plume, while it is effectively cooled by further collisions. Thus, the plume becomes highly supersaturated and the condensed phase of the target material starts to develop via homogeneous nucleation. Later on, still in a very limited volume, nanoparticles grow via complete or incomplete coalescence forming compact objects or large networks. The pressure threshold for gas phase condensation is intimately related to the collisional energy loss characteristics of the particular gas-target combination as well as to the thermophysical properties of the target.

Nanostructuring of a silicon surface by laser redeposition of Si vapor

We report on the surface nanostructuring of silicon wafer by self-organization of redeposited Si nanoparticles, at various energy levels, in the vaporization regime of laser-matter interaction. By using the semiconfined configuration, a quasi-two-dimensional turbulent Si vapor field with gradients of pressure and temperature is formed. The turbulent field evolves into point vortices which condense into Si nanodroplets. At a low laser energy of ∼1.2 J (0.23 GW/cm2), the inertial instability of nanodroplets under gradients of pressure and temperature, cause their intermittent accumulation in the low-pressure regions of turbulent field. The solidification of Si nanodroplets into particles and their redeposition, cause a simple two-dimensional low density nanostructuring of Si wafer in the near periphery region, and a high density nanostructuring in the periphery region of the spot. The pattern of redeposited Si nanoparticles in these regions is equivalent to the pattern of point vortices in a two-dimensional...

Flow instability of fluid-metal layer generated by laser pulse on an inclined metal surface: Experiments and simulation

The authors have shown that nanosecond laser-matter interaction with metal surface under an angle causes the formation of nonlinear micron-scale waves and localized structures resembling the gravitationally caused flow structures on the inclined plate. Juxtaposition of experimentally generated and numerically simulated structures on the basis of the equation derived by Frenkel and Indireshkumar [Phys. Rev. E 55, 1174 (1997)] shows a very good qualitative agreement; the experiments confirmed the dependence of the structure evolution on the laser power profile and on the irradiation angle. Dispersion of ordered long-range structures into chaotic ones was observed for the irradiation angle of θ=20°.

Combination of CVD Diamond and DLC Film Growth with Pulsed Laser Deposition to Enhance the Corrosive Protection of Diamond Layers

CVD diamond layers are often used as protective layers. One of the most important of these applications requires pinhole-free layers to protect against fluid materials, such as found in chemically aggressive environment. These pinholes are present even in very good quality CVD diamond films. In this work we combined the Pulsed Laser Deposition (PLD) technique with Microwave assisted Chemical Vapor Deposition (MW-CVD). We used CVD diamond films prepared under different conditions and layer thicknesses. Both of these proceses produced inperfect protective layers, but we proved that a PLD DLC film over the diamond layer does reduce the number of pinholes in the coating. We used special chemical alcaline etching to detect the remaining pinholes, and to test the corrosion protective properties of the layers. As a result we were able to prepare samples of 1 x 1cm2 with only 0.2 micron thickness without any pinholes, while in CVD diamond layers a thickness of 2,5 micron was needed for the same level of compactness.

Properties of high-density amorphous carbon films deposited by laser ablation

The properties of carbon films deposited by high intensity pulsed Nd:G lass laser are reported. Different measurements like TEM, SIMS, EELS, XPS and A FM showed the formation of a high-density amorphous carbon layer with good protection against che mically aggressive alkaline solutions. We demonstrated that instead of excimer lase rs, Nd-based solid state lasers can be used successfully for preparation of high quality amorphous carbon films. Introduction Carbon layers have growing importance because of the wide range of applications. The wide variety of carbon materials based on the unique bonding property of t he carbon atoms. Carbon has 4 valence electrons (2s 2 p). Chemical bonds may include both π or σ bonds. In case of ideal diamond crystal all 4 bonds are σ, which is known as sp 3 hybridization. In this form the carbon crystal has extreme physical and chemical par meters. In case of graphite crystal 3 σ and 1 π bonds exist (it is called sp 2 hybridization). The difference in only one bond changes the properties of the material drastically. In amorphous carbon structures both of the two hybrid states (sp 2 and sp) can exist. Therefore a wide scale of properties of the a morphous carbon materials can be achieved. Preparation of sp 3 rich diamond-like carbon (DLC) or sp 2 rich graphite-like carbon (GLC) films in high quality is a great challenge now adays. Multi-technique investigation of these layers can result in enhancing their qualit y. The commonly used methods for producing DLC layers are PVD [1], plasma CVD [2-4] ion beam de position [5,6], sputtering [7] and pulsed laser deposition (PLD) [8-14]. Many of the PLD studies concentrate on excimer laser deposition techniques. Our aim was to produce good quality carbon f ilms for protective purpose with a Nd:Glass laser, which doesn’t use any toxic gase s in contrast to the excimer lasers. It may be a very important fact in environmentally friendly industri al applications. Experiment The carbon thin films were deposited on both silicon and silicon dioxide s ubstrates at room temperature in 10 -6 mbar pressure. The laser wavelength was 1054nm with 1Hz repetiti on rate. The 40ns long laser pulses were focused onto a spot of 5mm , which resulted in 10 W/cm power density. Using an excimer laser at so high energies w ll lead to formation of a predominantly graphitic layer [15] due to the high level of multiphoton i onization and inverse bremsstrahlung heating in the plasma. The dimensions of the samples were 1cm x 5cm, but larger areas can also be covered using this method. Materials Science Forum Online: 2003-01-15 ISSN: 1662-9752, Vols. 414-415, pp 127-132 doi:10.4028/www.scientific.net/MSF.414-415.127

Thin film carbon layers with continously changing bonding properties

Polycrystalline diamond and diamond-like carbon (DLC) films were deposited by microwave chemical vapor deposition (MW-CVD) and by pulsed laser deposition (PLD) respectively. Ar ion bombardment was used to change the properties of these layers. The sp2 bonds were determined directly by reflected electron energy loss spectroscopy (REELS) and further characterization was made by Raman scattering. The polycrystalline diamond showed only very slight π-π* transition at 6.5 eV, but after Ar ion bombardment strong peak was formed but definitely shifted to lower energy compared to the well known π-π* transition of graphite. The as deposited PLD carbon films showed broad peak around 5eV clearly different than the π-π* transition (6.5eV). After Ar+ ion bombardment the peak was shifted also to lower energy range (4-5eV) with a remaining part at 6.5eV. The lower energy part of the peak can be correlated to the transition of sp3 sites, while this change in peak position was not detectable after ion bombardment of the reference HOPG sample, which does not contain sp3 hybridized carbon atoms.

Formation and characterization of electric contacts on CVD diamond films prepared by ion implantation

Diamond layers have a potential application as the highest band-gap semiconductor for electronic devices. One of the major problems is to form electric contact on the diamond surface useful for an electronic device. This paper shows the properties of the contacts formed by the very promising ion implantation technique. The diamond layers were deposited with Microwave Assisted Chemical Vapor Deposition (MW-CVD) equipped with special extra features like High Voltage Bias and Heated Substrate Holder [1]. Phosphoruos ion implantation and gold deposition were used for the contact formation. This technique resulted graphitization the top of the diamond film and intermixing of gold with the graphite or diamond surface. The properties of the contacts were tested with surface conduction characterization methods, and the properties of the contact to diamond interface was investigated with SIMS (Secondary Ion Mass Spectroscopy ) and XPS (X-ray Photoelectron Spectroscopy).