A combined laser-AFM-Raman setup is applied to study nanoscale surface modifications in DLC (a-C:H) films caused by multipulse laser irradiation at low (sub-threshold) fuences. The beam of a pulsed Nd:YAG laser (wavelength lambda = 532 nm, pulse duration tau = 7 ns, pulse repetition rate f = 1-2.5 kHz) is introduced into the optical path of an atomic force microscope (AFM), matched with that of a Raman confocal microscope. Three laser-induced processes that determine nanoscale surface modifications are found to occur in laser spots: (i) annealing - resulting in subnanometer surface deepening at the spot edges, (ii) graphitization - leading to surface swelling of 5-10 nm height, and (iii) nanoablation (oxidation) - resulting in formation of microcraters of few nm to hundred nm depth in the spot center. Raman spectra confirm the surface graphitization in the spots during formation of nm-height hillocks and microcraters, evidencing a decrease of the graphitized layer thickness at lower fluences. The nanoablation rates of 4 x 10(-5)-4 x 10(-3) nm/pulse are determined for multipulse irradiation of a-C:H films at the fluences of E = 0.045-0.1 J/cm(2). The extremely low rates of laser ablation of the DLC surface are characteristic of pulsed laser-assisted oxidation with the activation energy of similar to 6.3 kcal/mol estimated from the Arrhenius plot of the ablation rate-on-fluence dependence.

Abstract—Influence of nonlinear hydrodynamic effects on formation of pressure pulses investigated in condensed media under pulsed actions leading to fast local variation in their density. In contrast to the ordinary thermoacoustic mechanism where density variation is caused by absorption of the incident radiation energy, models with the given space–time perturbation of density in an incompressible fluid are considered. It is shown how hydrodynamic nonlinearities give rise to transformation of a pressure pulse generated at different perturbation amplitudes and durations.

A possibility of laser printing of graphene nanoribbon pixels while preserving the integrity of the structure and shape on the silicon substrate in accordance with the irradiated laser spot is demonstrated. To provide the transfer, a target consisting of a transparent sapphire plate and an absorbing thin titanium film (500 nm thick) coated with a film consisting of graphene nanoribbons is irradiated with a KrF excimer laser (λ=248 nm, τ=20 ns). Optimal conditions for laser irradiation are determined and a technique is developed for transferring a carbon nanomaterial from a growth nickel surface to a titanium film aimed at creating stronger bonds between the nanoribbons. Raman spectroscopy confirms the preservation of structural features of the synthesized atomically precise 7-atoms-wide graphene nanoribbons with an armchair edge during laser transfer.

The behavior of pressure pulses excited in a metal ablated with picosecond laser pulses is analyzed. It is shown that with this effect, the contribution from the thermoacoustic mechanism can exceed the evaporation pressure even when a stationary evaporation regime is established. This pressure behavior differs markedly from that characteristic of the nanosecond regime and is consistent with the obtained experimental data on the monitoring of the recoil pressure under picosecond laser irradiation of lead and aluminum.

The quasi-stationary regime of laser metal ablation is investigated within a model with two density variation fronts arising from the density decrease in the bleaching wave with the preceding compression shock wave. The investigation allows qualitatively describing features of recoil pressure behavior recently observed in laser ablation of mercury, which are associated with occurrence of an additional pressure peak and a decrease in the time of arrival of the pressure signal at the sensor from the irradiated target. It is shown that the results of this approach are not in conflict with the available data on the shock compression of mercury.

Chemical vapor deposition synthesis of graphene on polycrystalline copper substrates from methane is a promising technique for industrial production and application. However, the quality of grown graphene can be improved by using single-crystal copper (111). In this paper, we propose to synthesize graphene on epitaxial single-crystal Cu film deposited and recrystallized on a basal-plane sapphire substrate. The effect of film thickness, temperature, and time of annealing on the size of copper grains and their orientation is demonstrated. Under optimized conditions, the copper grains with the (111) orientation and a record size of several millimeters are obtained, and the single-crystal graphene is grown over their entire area. The high quality of synthesized graphene has been confirmed by Raman spectroscopy, scanning electron microscopy, and the sheet resistance measurements by the four point probe method.

The transfer of synthesized graphene nanoribbons with the initial characteristics obtained on the growth surface is an urgent and complex problem. Laser methods proved themselves well as a delicate and selective tool for the transfer of carbon nanomaterials. The simplicity of implementation of laser methods reduces the number of intermediate manipulations with the transferred material, increasing the safety of its structure. Here, we studied and implemented laser-induced blister forward transfer of high-quality graphene nanoribbons from a metal surface to a SiO2/Si substrate. We also studied the effect of the growth parameters and the transfer method on the structure of transferred carbon sites. The retention of the initial crystal structure of the transferred atomically precise graphene nanoribbons was confirmed by Raman spectroscopy.

Hydrogen plasma treatment of thin films and bulk silica is important process for surface cleaning, smoothing and patterning. We studied etching of SiO2 plates in H2 + O2 microwave plasma at moderate pressures at high temperatures in the range of 790-1300 degrees C. A low-coherent optical interferometer was used for simultaneous in situ measurement of etch rate (ER) and the SiO2 temperature. We show that even a small addition of O2 (<1%) in microwave H2 plasma can effectively reduce the SiO2 ER due to redox reaction. Using an optical profilometry and atomic force microscopy we observed not only a strong etching inhibition by O2 addition in gas, but a signifi-cantly smoother etched surface as well. The activation energies Ea = 128 +/- 8 kJ/mol and 276 +/- 23 kJ/mol are found for the etching in pure H2 and H2 + 0.4 %O2 plasmas, respectively. An exponential dependence of ER on O2 content in gas is established, particularly, the O2 concentration as small as 2.6 % is sufficient to reduce the ER by 100 times. The effect of oxygen induced etching slow-down is supported by thermodynamic consideration of the surface reactions involved. The experiments were complimented with the plasma diagnostics by optical emission plasma spectroscopy to detect volatile etching products, and other species such as OH, and correlate the Si emission intensity with the measured ER. Our results confirm the efficiency of oxygen to control in a wide range the etch rate and surface topography upon atomic hydrogen etching in a microwave plasma.

Germanium-Vacancy (Ge-V) color centers in diamond have narrow-band photoluminescence (PL) at room temperature (RT) and attract considerable attention due to their possible application in quantum information technologies, biomedicine, and local optical thermometry. In this work, we used microwave plasma chemical vapor deposition (MPCVD) to synthesize Ge-doped polycrystalline diamond (PCD) films and single-crystal diamond (SCD) epitaxial layers in hydrogen-methane-germane gas mixtures. Thick (>100 & mu;m) layers of both types were grown to analyze their structure and luminescence characteristics. The first demonstration of the absorption of Ge-V centers in both PCD and SCD materials at low temperatures is presented. The sufficient narrowing of the Ge-V PL line for the SCD sample in comparison with the PCD sample was observed. On the other hand, the intensity of the Ge-V signal was an order of magnitude higher in the PCD sample compared to the SCD sample. A set of single Ge-V centers formed during CVD has been demonstrated. The results obtained can be used to manufacture and design various photonic devices based on Ge-V color centers.

We report the growth of Ge-doped homoepitaxial diamond films by microwave plasma CVD in GeH4-CH4-H2 gas mixtures at moderate pressures (70-100 Torr). Optical emission spectroscopy was used to monitor Ge, H, and C2 species in the plasma at different process parameters, and trends for intensities of those radicals, gas temperature, and excitation temperature, with variations of GeH4 or CH4 precursor concentrations, were investigated. The film deposited on (111)-oriented single crystal diamond substrates in a high growth rate regime revealed a strong emission of a germanium-vacancy (GeV) color center with a zero-phonon line at ≈604 nm wavelength in photoluminescence (PL) spectra, confirming the successful doping. The observed PL shift for the GeV defect is caused by stress in the films, as evidenced and quantified by Raman spectra. These results suggest that in situ doping with Ge using a GeH4 precursor is a convenient method of controlling the formation of GeV centers in epitaxial diamond films for photonic applications.

A new approach to the fabrication of graphene field emitters on a variety of substrates at room temperature and in an ambient environment is demonstrated. The required shape and orientation of the graphene flakes along the field are created by the blister-based laser-induced forward transfer of CVD high-quality single-layer graphene. The proposed technique allows the formation of emitting crumpled graphene patterns without losing the quality of the initially synthesized graphene, as shown by Raman spectroscopy. The electron field emission properties of crumpled graphene imprints 1 × 1 mm2 in size were studied. The transferred graphene flakes demonstrated good adhesion and emission characteristics.

Chemical vapor deposition synthesis of graphene on copper foil from methane is the most promising technology for industrial production. However, an important problem of the formation of the additional graphene layers during synthesis arises due to the strong roughness of the initial copper foil. In this paper, various approaches are demonstrated to form a smooth copper surface before graphene synthesis to reduce the amount of few layer graphene islands. Six methods of surface processing of copper foils are studied and the decrease of the roughness from 250 to as low as 80 nm is achieved. The correlation between foil roughness and the formation of the additional layer is demonstrated. Under optimized conditions of surface treatment, the content of the additional graphene layer drops from 9 to 2.1%. The quality and the number of layers of synthesized graphene are analyzed by Raman spectroscopy, scanning electron microscopy and measurements of charge mobility.

Two waves model where shock wave is combined with rarefaction wave appearing in laser ablation due to metal-nonmetal transition effect is investigated using conservation laws for mass and momentum fluxes for the steady-state regime of the process. This approach permits to obtain the relation between front velocities of the waves which shows that the rarefaction wave can be rather slow compared with the generated shock wave.

Behavior of liquid mercury exposed to 25-ns laser pulses is investigated using acoustic and optical diagnostics. It is found that when pressure pulses generated in the target change, an additional peak appears as the laser intensity increases, which can be due to the motion of the metal–nonmetal transition front. This assumption agrees with a decrease in the reflected laser pulse and with the behavior of the pressure pulses in the case of free and loaded irradiated surfaces.

The patterning and transfer of a two-dimensional graphene film without damaging its original structure is an urgent and difficult task. For this purpose, we propose the use of the blister-based laser-induced forward transfer (BB-LIFT), which has proven itself in the transfer of such delicate materials. The ease of implementation of laser techniques reduces the number of intermediate manipulations with a graphene film, increasing its safety. The work demonstrates the promise of BB-LIFT of single-layer graphene from a metal surface to a SiO2/Si substrate. The effect of the parameters of this method on the structure of transferred graphene islands is investigated. The relevance of reducing the distance between irradiating and receiving substrates for the transfer of free-lying graphene is demonstrated. The reasons for the damage to the integrity of the carbon film observed in the experiments are discussed. The preservation of the original crystal structure of transferred graphene is confirmed by Raman spectroscopy.

We have examined the effect of focused nanosecond laser pulses on the formation and tribological properties of microstructures in the form of a matrix of craters produced in a – C : H : Si : O diamond-like films (DLFs), both undoped and doped with tungsten. The studies are performed by scanning probe microscopy (SPM) using SPM probes of standard design with a tip and probes without a tip (tipless-type probes). A special procedure is developed for assessing the wettability (contact angle) of individual sections of the laser-induced microstructure by the local adhesion force. According to estimates, the material inside the crater becomes softer and more hydrophobic as a result of laser irradiation. This is manifested to the greatest extent in tungsten-doped DLFs, where the crater walls are arranged like terraces formed by ‘nanoflakes’, i.e. grains of the substance. Scanning of microstructures with a tipless probe in the regime of lateral force microscopy demonstrates a decrease in friction in the crater area.

Pressure transducer calibration is modeled in the regime where calculated thermoacoustic signal and experimental information about normal boiling temperature attainment due to nanosecond laser pulses irradiation are used. It is shown that the regime demonstrated recently for 30 ns laser action is not straightforwardly applicable for shorter pulses due to strong thermoacoustic signal enhancement compared with vaporization signal near normal boiling point. For subnanosecond laser irradiation two pulses action is suggested where shorter and longer pulses are used simultaneously.

The possibility of laser printing of single-walled carbon nanotubes (SWCNTs) and graphene via blister-based laser-induced forward transfer technique is demonstrated. Laser radiation with optimized fluence, that is absorbed in a thin aluminium film, causes its local evaporation and blistering without rupture, which leads to the ejection of carbon nanomaterial from the donor to the receiving substrate. The use of preliminary cutting of the donor layer into square pixels allows printing SWCNTs while maintaining their shape. Raman analysis indicates the transfer of carbon nanomaterials without significant degradation.

Chemical vapor deposition synthesis of graphene on copper foil from methane is the most promising technology for industrial production. However, an important problem of the formation of the second and subsequent graphene layers during synthesis arises due to the strong roughness of the initial copper foil. Here we demonstrate the various approaches to prepare a smooth copper surface before graphene synthesis to reduce the formation of multi-layer graphene islands. Six methods of surface processing of copper foils are studied, and the decrease of the roughness from 250 to as low as 80 nm is achieved. The correlation between roughness and the formation of multi-layer graphene is demonstrated. Under optimized conditions of surface treatment, the content of the multi-layer graphene islands drops from 9% to 2.1%. The quality and the number of layers of synthesized graphene are analyzed by Raman spectroscopy, scanning electron microscopy, and measurements of charge mobility.