Bulk Target Research Articles

Correlation of structural ordering with magnetic properties of pulsed laser deposited Co2FeGa Heusler alloy thin films

In the present contribution, structural and magnetic properties of Pulsed laser deposited (PLD) Co2FeGa (CFG) thin films have been studied as a function of substrate temperature. Structural investigation carried out by X-Ray Diffraction (XRD) measurement reveals mixed phase cubic structure of the films which changes from a nearly ordered to highly disordered phase with an increase in substrate temperature. Grazing Incidence X-Ray Diffraction (GIXRD) study shows the presence of disordered A2 phase similar to the bulk target. Grazing Incidence X-Ray Reflectivity (GIXR) measurement reveals the formation of bi-layer structure in the films with different density and thickness though the overall thickness remains same. Field Emission Scanning Electron Microscopy (FESEM) study shows the formation of a droplet-like morphology of the films. Extended X-ray Absorption Fine Structure (EXAFS) study serves a major role in complementing the XRD results and also shows a strong hybridization of Co with Ga maintaining the half metallicity. A novel approach in analyzing the EXAFS data additionally gives quantitative estimation of the different kinds of disorders present in the samples at the atomic level. In addition to this, magnetization result suggests that the films grown at lower substrate temperatures acts as proper ferromagnet following the well known spin wave theory with higher value of spontaneous magnetization in comparison to other samples. A lesser value of saturation magnetization in the films compared to bulk also supports the presence of antisite disorders.

Synthesis of bimetallic nanostructures by nanosecond laser ablation of multicomponent thin films in water

The paper presents results on nanosecond laser ablation of thin films immersed in a liquid. The thin films were prepared by consecutive deposition of layers of different metals by thermal evaporation (first layer) and classical on-axis pulsed laser deposition (second layer); Ni/Au, Ag/Au and Ni/Ag thin films were thus deposited on glass substrates. The as-prepared films were then placed at the bottom of a glass vessel filled with double distilled water and irradiated by nanosecond laser pulses delivered by a Nd:YAG laser system at λ = 355 nm. This resulted in the formation of colloids of the thin films’ material. We also compared the processes of ablation of a bulk target and a thin film in the liquid by irradiating a Au target and a Au thin film by the same laser wavelength and fluence (λ = 355 nm, F = 5 J/cm2). The optical properties of the colloids were evaluated by optical transmittance measurements in the UV– VIS spectral range. Transmission electron microscopy was employed to estimate the particles’ size distribution.

Enhanced electron emission from coated metal targets: Effect of surface thickness on performance

In this work, we establish an analytical formalism to address the temperature dependent electron emission from a metallic target with thin coating, operating at a finite temperature. Taking into account three dimensional parabolic energy dispersion for the target (base) material and suitable thickness dependent energy dispersion for the coating layer, Fermi Dirac statistics of electron energy distribution and Fowler’s mechanism of the electron emission, we discuss the dependence of the emission flux on the physical properties such as the Fermi level, work function, thickness of the coating material, and operating temperature. Our systematic estimation of how the thickness of coating affects the emission current demonstrates superior emission characteristics for thin coating layer at high temperature (above 1000 K), whereas in low temperature regime, a better response is expected from thicker coating layer. This underlying fundamental behavior appears to be essentially identical for all configurations when work function of the coating layer is lower than that of the bulk target work function. The analysis and predictions could be useful in designing new coated materials with suitable thickness for applications in the field of thin film devices and field emitters.

Chemical bonding in epitaxial ZrB2 studied by X-ray spectroscopy

The chemical bonding in an epitaxial ZrB2 film is investigated by Zr K-edge (1s) X-ray absorption near-edge structure (XANES) and extended X-ray absorption fine structure (EXAFS) spectroscopies and compared to the ZrB2 compound target from which the film was synthesized as well as a bulk α-Zr reference. Quantitative analysis of X-ray photoelectron spectroscopy spectra reveals at the surface: ~5% O in the epitaxial ZrB2 film, ~19% O in the ZrB2 compound target and ~22% O in the bulk α-Zr reference after completed sputter cleaning. For the ZrB2 compound target, X-ray diffraction (XRD) shows weak but visible 1¯11, 111, and 220 peaks from monoclinic ZrO2 together with peaks from ZrB2 and where the intensity distribution for the ZrB2 peaks show a randomly oriented target material. For the bulk α-Zr reference no peaks from any crystalline oxide were visible in the diffractogram recorded from the metal with preferred 0001-orientation. The Zr K-edge absorption from the two ZrB2 samples demonstrate more pronounced oscillations for the epitaxial ZrB2 film than in the bulk ZrB2 attributed to the high atomic ordering within the columns of the film. The XANES exhibits no pre-peak due to lack of p-d hybridization in ZrB2, but with a chemical shift towards higher energy of 4 eV in the film and 6 eV for the bulk compared to α-Zr (17.993 keV) from the charge-transfer from Zr to B. The 2 eV larger shift in bulk ZrB2 material suggests higher oxygen content than in the epitaxial film, which is supported by XPS. In EXAFS, the modeled cell-edge in ZrB2 is slightly smaller for the film (a = 3.165 Å, c = 3.520 Å) when compared to the bulk target material (a = 3.175 Å, c = 3.540 Å) and for the hexagonal closest-packed metal (α-phase, a = 3.254 Å, c = 5.147 Å). The modeled coordination numbers show that the EXAFS spectra of the epitaxial ZrB2 film is highly anisotropic with strong in-plane contribution, while the bulk target material is more isotropic. The Zr-B distance in the film of 2.539 Å is in agreement with the calculated value from XRD data of 2.542 Å. This is slightly shorter compared to that in the ZrB2 compound target 2.599 Å, supporting the XANES results of a higher atomic order within the columns of the film compared to bulk ZrB2.

Simulation of ultrafast bursts of subpicosecond pulses: In pursuit of efficiency

Using a hydrodynamic two-temperature model, we simulate multi-pulse laser ablation of an aluminum bulk target. The results of modeling demonstrate that the effectiveness of the multi-pulse ablation increases an order of magnitude in comparison to a single-pulse ablation of the same energy, while the repetition rate grows up to several GHz because the material surface does not cool down substantially between successive pulses. To prevent the shielding and suppression effects, the fluence of each pulse in the burst should have a subthreshold value to avoid the generation of slow moving ablated condensed-phase nanolayers. The obtained results are consistent with recent experiments on ablation by ultrafast bursts of ultrashort pulses.

Crystallographic characterization of laser-generated, polymer-stabilized 4 nm silver-gold alloyed nanoparticles

Monometallic silver and gold nanoparticles and bimetallic silver-gold (AgAu) nanoparticles were prepared by laser ablation in liquids in the atomic composition range of Ag:Au from 0:100 to 100:0 with steps of 10 at% and colloidally stabilized with poly(N-vinylpyrrolidone) (PVP). As metallic bulk targets for laser ablation, pure silver, pure gold, and alloyed AgAu foils with the desired composition were used. Size separation by centrifugation and freeze-drying gave monodisperse spherical nanoparticles with a diameter of 4 nm as determined by differential centrifugal sedimentation (DCS) and transmission electron microscopy (TEM). A crystallographic characterization of the nanoparticles was carried out by X-ray powder diffraction (XRD) and Rietveld refinement, leading to highly precise cubic lattice parameters (fcc crystal system) and crystallite sizes. For comparison, the same analysis including the determination of the microstrain was carried out for the bulk target materials (AgAu alloys in the full concentration range). Both nanoparticles and bulk target materials obeyed Vegard's rule, with only slight deviations. The fact that the crystallite size as determined by XRD was identical to the hydrodynamic diameter by DCS and the Feret diameter by TEM indicates that the particles consist of only one domain, i.e. they are single crystals. The combination of UV-vis spectroscopy with energy-dispersive X-ray spectroscopy (EDX) as line scan along the nanoparticle showed a homogenous distribution of the gold and silver inside the nanoparticles, indicating solid solution alloys, in contrast to what was observed earlier for chemically prepared AgAu nanoparticles by reduction of metal ions in water.

Influence of target structure on laser plasma generation efficiency

Spatial restrictions on heat transfer imposed in foil and sintered powder titanium targets have resulted in sufficient increase of efficiency of laser plasma generation as compared to bulk Ti targets. Especially at low laser fluences, where target material input in plasma is higher than ambient air’s. Also, the pattern of momentum coupling coefficient Cm dependency on laser fluence F has changed from convex to concave. Minimum difference in Cm values for bulk and foil targets was 1.68 times, maximum – 1.5 orders of magnitude (always higher for foil). At the impact on sintered porous targets momentum coupling coefficient was lower than for foil, but normalised by mass density, the results were about equal. To our mind, obtained results show that suppression of heat dissipation in porous targets can be same efficient as in foils, but with more benefits for feeding systems and energy efficiency of laser plasma generators.

2,4-dinitrotoluene detected using portable Raman spectrometer and femtosecond laser fabricated Au–Ag nanoparticles and nanostructures

We report results from our studies on the fabrication of bimetallic nanoparticles (NPs) and nanostructures (NSs) using femtosecond (∼50 fs) laser ablation technique. The NPs and NSs were achieved by immersing bulk targets with different Au–Ag compositions in acetone followed by ablation. We demonstrate their application towards detection of an explosive molecule of 2, 4-Dinitrotoluene, DNT (25 μM) using a portable Raman spectrometer (785 nm). A tuneable surface plasmon peak observed in the UV-Visible absorption spectra with varying Au proportions confirmed the formation of bimetallic NPs. The size, shape and crystallographic phases were investigated by transmission electron microscope (TEM) and selected area diffraction pattern (SAED). The surface morphology of fabricated NSs was characterized using field emission scanning electron microscope (FESEM). Both NPs and NSs were employed as Surface Enhanced Raman Scattering (SERS) substrates for sensing Methylene Blue (MB) and DNT. From the results obtained in this investigation we concluded that Au70Ag30 NPs/NSs exhibited superior SERS performance compared to pure Ag, Au and other compositions. A detection limit of ∼10−9 M for MB and ∼10−6 M for DNT was achieved with corresponding enhancement factors of ∼107 and ∼104. Furthermore, we have also observed a ‘factor of 3’ increase in the SERS intensity by simply drop casting Ag NPs on the gaps of Au70Ag30 NSs. This clearly demonstrates that individual NPs, NSs and NPs on NSs (hybrid SERS targets) can be achieved in a single experiment and in combination provide efficient means of detecting trace quantities of explosive molecules.

Laser synthesized magnetically recyclable titanium ferrite nanoparticles for photodegradation of dyes

In this work, contaminated free titanium ferrite nanoparticles were successfully prepared through laser ablation approach in liquid media. The synthesized nanoparticles were characterized in terms of the structural, bonding nature, chemical composition using X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), Raman spectroscopy and Energy-dispersive X-ray spectroscopy (EDX) techniques. The X-ray diffraction (XRD) pattern confirmed multiphase structure of titanium ferrite nanoparticles and revealed a vast difference between the laser ablated nanoparticles and its mother target. The XRD, FTIR and Raman measurements showed the formation of magnetic titanium ferrite nanoparticles as compared to bulk target. EDX spectrum also confirmed the formation of titanium ferrite nanoparticles. The photo catalytic activity performance of synthesized nanoparticles was tested to Rhodamine 6 G which was studied through time based absorption variations noted by ultra violet visible spectroscopy. The photo catalytic degradation efficiency of the sample retained nearly 80% of its initial value after two cycle of operation. These magnetic titanium ferrite nanoparticles exhibited good photocatalytic activities under UV light irradiation and can be recycled many times by magnetic separation without major loss of activity. Present studies indicate that titanium ferrite nanoparticles are a promising material for contaminated groundwater treatment and might open a new avenue in the designing of a cheaper and efficient large scale water purification plants.

Energy confinement and thermal boundary conductance effects on short-pulsed thermal ablation thresholds in thin films

For this paper, single-pulse ablation mechanisms of ultrafast laser pulses ($25\phantom{\rule{0.222222em}{0ex}}\mathrm{ps}$) were studied for thin gold films ($65\phantom{\rule{0.222222em}{0ex}}\mathrm{nm}$) on an array of substrates with varying physical properties. Using time-domain thermoreflectance, the interfacial properties of the thin-film systems are measured: in particular, the thermal boundary conductance. We find that an often used, and widely accepted relation describing threshold fluences of homogeneous bulk targets breaks down at the nanoscale. Rather than relying solely on the properties of the ablated Au film, the ablation threshold of these Au/substrate systems is found to be dependent on the measured thermal boundary conductance; we additionally find no discernible trend between the damage threshold and properties of the underlying substrate. These results are discussed in terms of diffusive thermal transport and the interfacial bond strength.

Studies of the Cr-CrN coating characteristics formed by means of the magnetron sputtering method from bulk target

The paper presents the study’s results of ion-plasma chromium based coating characteristics produced on blade steel samples 12Kh13 and EI961 by means of the magnetron sputtering method from the bulk “hot” target. A set of metallographic studies and erosion tests of coatings were carried out using the research equipment URI (unique research installation) “Hydroshock rig Erosion-M” of NRU “MPEI”. Cr-CrN based coatings have a layered structure; thickness of intermediate Cr layers ranges from 0.7 to 1.7 μm, thickness of nitride layers CrN ranges from 1.5 to 4 μm, while the overall coating thickness is 17.0–21.5 μm; coating microhardness is 1830–1880 HV0.05. The resulting coatings are found to increase 1.5 times the incubation period duration of erosion wear for steels 12Kh13 and EI961; they reduce the maximum erosion rate 1.3 times, and the steady erosion rate – 1.5 times.

Au-Pt alloy nanoparticles obtained by nanosecond laser irradiation of gold and platinum bulk targets in an ethylene glycol solution

Au-Pt alloy nanoparticles (NPs) of different compositions ( Au0Pt100 , Au30Pt70 , Au50Pt50 , Au70Pt30 , and Au100Pt0 were obtained using the nanosecond laser ablation of gold and platinum bulk targets in ethylene glycol, followed by mixing highly monodisperse Au and Pt nanocolloids, for the first time. UV-vis absorption spectra of NPs showed that by increasing the Au content in the Au-Pt NPs, the surface plasmon resonance (SPR) peak red-shifted, from 260 to 573nm in a nonlinear way. In addition, the mean crystalline size, crystal structure, d-spacing, and lattice parameters of NPs were estimated from the XRD spectra. Microscopy studies revealed the most NPs have a spherical or near-spherical shape, and the average sizes of Au0Pt100 , Au30Pt70 , Au50Pt50 , Au70Pt30 , and Au100Pt0 NPs were calculated to be 12.50, 14.15, 18.53, 19.29, and 26.38nm, respectively. Also, the chemical identity of the molecules adhering to the NPs surface was considered by Raman and FT-IR spectroscopy techniques. Among different synthesis methods, the demonstrated technique allows easy synthesis of alloy NPs in aqueous media at room temperature with no formation of by-products.

High ionization states observed in soft x-ray spectra from plasmas of second row transition elements produced by femtosecond laser pulses

The soft x-ray emission spectra of femtosecond-laser-produced plasmas from 2nd row transition elements from yttrium (Z = 39) to palladium (Z = 46), with the exception of technetium (Z = 43), were measured in the 1–5 nm region. Plasmas were produced by shining pulses from a titanium–sapphire laser with 65 fs pulse duration and an energy per pulse of 4.5 mJ focused to an intensity of 3 × 1015 W cm−2 onto bulk targets. While the emission spectra from yttrium to molybdenum (Z = 42) contain only the unresolved transition arrays (UTA) already observed in nanosecond and picosecond laser-target interactions described in our previous paper (Lokasani et al 2015 J. Phys. B: At. Mol. Opt. Phys. 48 245009), transitions from higher ionization states are clearly demonstrated in the spectra emitted from ruthenium (Z = 44), rhodium (Z = 45) and palladium targets heated by the femtosecond laser. These UTAs are interpreted by comparing the experimental spectra to calculated results, obtained using the Cowan suite of codes, the flexible atomic code as well as previous predictions of isoelectronic trends. The 3d–4f transitions arrays emitted from Ru XXI-XXIII, Rh XXI-XXIII and Pd XXI-XXIII ions are clearly seen in the observed spectra. To our knowledge, such high ionization states are demonstrated for the first time at such moderate laser energies.

Enhanced laser absorption from radiation pressure in intense laser plasma interactions

The reflectivity of a short-pulse laser at intensities of with ultra-high contrast () on sub-micrometer silicon nitride foils was studied experimentally using varying polarizations and target thicknesses. The reflected intensity and beam quality were found to be relatively constant with respect to intensity for bulk targets. For submicron targets, the measured reflectivity drops substantially without a corresponding increase in transmission, indicating increased conversion of fundamental to other wavelengths and particle heating. Experimental results and trends observed in 3D particle-in-cell simulations emphasize the critical role of ion motion due to radiation pressure on the absorption process. Ion motion during ultra-short pulses enhances the electron heating, which subsequently transfers more energy to the ions.

Investigating target versus impactor influences on Martian crater morphology at the simple‐complex transition

AbstractComparing craters of identical diameter on a planet is an empirical method of studying the effects of different target and impactor properties while holding total impact energy nearly constant. We have analyzed the Martian crater population within a narrow diameter range (7 km < crater diameter < 9 km) at the simple‐complex crater transition using three approaches. We looked for correlations of morphology with surface geology using a global crater database and global geologic map. We examined selected regions in detail with high‐resolution images to further understand the relationship between crater morphology and bulk target properties. Finally, we examined craters in close proximity to each other in order to hold target properties constant, so that we could isolate impactor effects on crater morphology. We found a strong correlation between target properties and interior crater morphology, and we found little evidence that impactor properties (other than impact angle) affect crater appearance. Central uplift and wall slumping are enhanced for less consolidated targets. Layered targets affected both the excavation and modification stages of complex crater formation; the resulting craters have pseudoterraces, flat floors, and central pits.

Cover Picture: Opportunities and Challenges for Laser Synthesis of Colloids (ChemPhysChem 9/2017)

The Front Cover picture visualizes one of the main approaches in laser synthesis and processing of colloids, namely laser ablation in liquid (LAL). In LAL, nanoparticles are obtained by laser ablation synthesis in solution (LASiS) of a bulk target. More information can be found in the Editorial by B. Gökce, V. Amendola, and S. Barcikowski on page 983 in Issue 9, 2017 (DOI: 10.1002/cphc.201700310).

Laser ablated lead free (Na, K) NbO3 thin films with excess alkali-content

Lead free (Na0.5K0.5) NbO3 (NKN) being hygroscopic in nature is very difficult to be fabricated with enhanced properties in thin films. To maintain stoichiometry 0, 5 and 10% mole excess of Na on A-site were added on NKN bulk targets. The perovskite ABO3 crystal structure incorporating 0, 5 and 10% excess alkali ions were grown on Pt/Ti/SiO2/Si substrate by pulsed laser deposition, using stoichiometric high density ceramic targets. X-ray diffraction peaks and Raman scattering spectra suggest the formation of single phase of the film in monoclinic phase. The dielectric properties and leakage current is improved with increasing amount of Na concentration. The reduction in leakage current with addition of excess Na concentration may be due to increase in grain size decrease in the connection of grain boundaries with grains. The impedance spectra of the film shows single dielectric relaxation which is non-debye type and the relaxation frequency is shifted to higher side at higher frequency.

Comparative study of shock-wave hardening and substructure evolution of 304L and Hadfield steels irradiated with a nanosecond relativistic high-current electron beam

We present the results of a comparative study of the shock-wave hardening regularities and mechanisms revealed for bulk (thickness h = 6 and 9.3 mm) targets made of austenitic 304L stainless steel and Hadfield steel. A high-current relativistic electron beam (45 ns, 1.35 MeV, 34 GW/cm2) produced by the SINUS-7 accelerator was used for generation of a shock wave. It is revealed by 2D-computer simulation for type 304 steel that the direct ablation of the target material leads to generation of shock wave with duration of ∼0.1 μs and amplitude of ∼20 GPa, and the strain rate during its direct propagation and reflection from the free rear surface decreases from ∼2 down to ∼0.4 μs−1. It is found experimentally that in the absence of a rear spall (h = 9.3 mm) the shock-wave loading of both steels leads to formation of three hardened layers: a front layer with a maximum microhardness at a depth of 0.5–1 mm from the bottom of ablation hole, which is in a reasonable agreement with the predictions of the heat-transfer calculations, as well as intermediate and rear-side layers. In case of 304L stainless steel, the depth distributions of microhardness and fraction of twinned grains are consistent with each other, while in the Hadfield steel, the correlation is within the front and intermediate hardened layers only. It is shown by microstructural characterization and analysis of hardening mechanisms that in the case of 304L stainless steel, both front and rear-side hardening are significantly associated with the formation of new intra-phase boundaries by deformation twinning. In the Hadfield steel, unlike the 304L stainless steel, the unusual rear-side hardening is mainly due to increasing the dislocation density under submicrosecond single cycle of compression followed by tension with peak stress of ∼3 GPa.

Nanoparticles alloying in liquids: Laser-ablation-generated Ag or Pd nanoparticles and laser irradiation-induced AgPd nanoparticle alloying

Laser irradiation of a mixture of single-element micro/nanomaterials may lead to their alloying and fabrication of multi-element structures. In addition to the laser induced alloying of particulates in the form of micro/nanopowders in ambient atmosphere (which forms the basis of the field of additive manufacturing technology), another interesting problem is the laser-induced alloying of a mixture of single-element nanoparticles in liquids since this process may lead to the direct fabrication of alloyed-nanoparticle colloidal solutions. In this work, bare-surface ligand-free Ag and Pd nanoparticles in solution were prepared by laser ablation of the corresponding bulk target materials, separately in water. The two solutions were mixed and the mixed solution was laser irradiated for different time durations in order to investigate the laser-induced nanoparticles alloying in liquid. Nanoparticles alloying and the formation of AgPd alloyed nanoparticles takes place with a decrease of the intensity of the surface-plasmon resonance peak of the Ag nanoparticles (at ∼405 nm) with the irradiation time while the low wavelength interband absorption peaks of either Ag or Pd nanoparticles remain unaffected by the irradiation for a time duration even as long as 30 min. The nanoalloys have lattice constants with values between those of the pure metals, which indicates that they consist of Ag and Pd in an approximately 1:1 ratio similar to the atomic composition of the starting mixed-nanoparticle solution. Formation of nanoparticle networks consisting of bimetallic alloyed nanoparticles and nanoparticles that remain as single elements (even after the end of the irradiation), joining together, are also formed. The binding energies of the 3d core electrons of both Ag and Pd nanoparticles shift to lower energies with the irradiation time, which is also a typical characteristic of AgPd alloyed nanoparticles. The mechanisms of nanoparticles alloying and network formation are also discussed.

Influence of a few-atomic silver molecules on the high-order harmonic generation in the laser-produced plasmas

The plasmas containing small molecules of silver produced during ablation of the bulk silver and silver nanoparticles (NPs) at the conditions suitable for efficient harmonic generation of the ultrashort pulses propagating through the laser-produced plasmas (LPPs) were studied. The time-of-flight mass-spectroscopy studies of plasmas confirmed the presence of these species in the plasmas. We suggest that the harmonics generated during the ablation of nanoparticle and bulk targets could be enhanced due to the appearance of these small species (Ag2, Ag3) at the moment of propagation of the ultrashort laser pulses through such plasmas.