| 61. |
- Popok, Vladimir, 1966, et al.
(författare)
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Nanohillock formation by impact of small low-energy clusters with surfaces
- 2003
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Ingår i: Nuclear Instruments and Methods in Physics Research B. ; 207:2, s. 145-153
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Tidskriftsartikel (refereegranskat)abstract
- Results on nanoscale structuring of different substrates (silicon, pyrolytic graphite, indium-tin-oxide) using clusters from 20 to 100 atoms in size formed from gaseous precursors (O2, N2, Ar) at relatively low impact energy up to 15 keV are presented. Images of the substrate surfaces after cluster collisions obtained using atomic force microscopy (AFM) show the formation of hillocks from a few to 15 nm height with a basal diameter from 50 to 300 nm depending on implantation conditions. The shape and size of the structures are found to be a function of the cluster size and species, implantation energy, impact angle and the type of substrate. A model explaining the hillock formation is discussed.
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| 62. |
- Popok, Vladimir, 1966, et al.
(författare)
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Stopping of energetic cobalt clusters and formation of radiation damage in graphite
- 2009
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Ingår i: Physical Review B. ; 80:20, s. 205419(1-12)
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Tidskriftsartikel (refereegranskat)abstract
- The interaction of energetic (up to 200 eV/atom) size-selected Con clusters with HOPG is studied both experimentally and theoretically. Etching of the radiation damage areas introduced by cluster impacts provides a measure of the depth to which the collision cascades are developed and allowes a comparison of these data with the molecular dynamics simulations. Good agreement between the experimental results and modelling is obtained. It is shown that the projected range of the cluster constituents can be linearly scaled with the projected momentum (the cluster momentum divided by surface impact area). With decrease of cluster energies to ca. 10 eV/atom the transition from implantation to pinning is suggested. It is found that even after quite energetic impacts residual clusters remain intact in the shallow graphite layer. These clusters can catalyse reaction of atmospheric oxygen with damaged graphite areas under the thermal heating that leads to the formation of narrow (5-15 nm) random in shape surface channels (trenches) in the top few graphene layers. Thus, small imbedded Co nanoparticles can be used as a processing tool for graphene.
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| 63. |
- Popok, Vladimir, 1966, et al.
(författare)
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Surface nanostructuring by implantation of cluster ions
- 2004
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Ingår i: Vacuum. - : Elsevier BV. - 0042-207X. ; 76:2-3, s. 265-272
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Tidskriftsartikel (refereegranskat)abstract
- A brief state-of-the-art review in the field of cluster ion implantation is presented. Ionised cluster beams are considered as a controllable and versatile tool for modification and processing of surfaces and near-surface layers on an atomistic scale as an alternative to ion implantation and ion assisted deposition. The main effects occurring under cluster-surface collisions as well as advantages in the application of cluster ion beams are reviewed. The problem of surface erosion under impact of energetic cluster ions is emphasised in the paper. A model explaining crater and hillock formation on target surfaces with relation to the thermal-transfer effect and local target melting at the collision spot is discussed.
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| 64. |
- Prasalovich, Sergei, 1976, et al.
(författare)
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Experimental studies of complex crater formation under cluster implantation of solids
- 2005
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Ingår i: European Physical Journal D. - : Springer Science and Business Media LLC. - 1434-6060 .- 1434-6079. ; 36, s. 79-88
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Tidskriftsartikel (refereegranskat)abstract
- The results of a systematic study of surface defect formation after energetic Arn+ (n = 12, 22, 32, 54) and Xen+ (n = 4, 16) cluster ion implantation into silicon and sapphire are presented. Implantation energies vary from 3 to 18 keV/ion. Two cases of comparative studies are carried out: the same cluster species are implanted into two different substrates, i.e. Arn+ cluster ions into silicon and sapphire and two different cluster species Arn+ and Xen+ are implanted into the same kind of substrate (silicon). Atomic force, scanning electron and transmission electron microscopies (AFM, SEM and TEM) are used to study the implanted samples. The analysis reveals the formation of two types of surface erosion defects: simple and complex (with centrally positioned hillock) craters. It is found that the ratio of simple to complex crater formation as well as the hillock dimensions depend strongly on the cluster species, size and impact energy as well as on the type of substrate material. Qualitative models describing the two comparative cases of cluster implantation, the case of different cluster species and the case of different substrate materials, are proposed.
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| 65. |
- Prasalovich, Sergei, 1976, et al.
(författare)
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Surface entropy of rare-gas clusters
- 2005
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Ingår i: J. Chem. Phys.. - : AIP Publishing. - 0021-9606. ; 123
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Tidskriftsartikel (refereegranskat)abstract
- Abundances of Ar and Xe clusters produced in a supersonic expansion source are inverted to find relative dissociation energies. The values around the shell and subshell closings at N=55, 71, and 147 differ from theoretical values derived from ground-state energies of Lennard-Jones clusters. A significant part of the difference can be accounted for by the conformational entropies of surface atoms and vacancies.
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| 66. |
- Samela, J., et al.
(författare)
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Argon cluster impacts on layered silicon, silica, and graphite surfaces
- 2007
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Ingår i: European Physical Journal D. ; 43, s. 181-184
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Tidskriftsartikel (refereegranskat)abstract
- Seven structures of covalently bonded materials are used as targets of 6 keV Ar12 cluster bombardment in classical molecular dynamics simulations. Energy deposition, cratering and Ar ranges are compared and remarkable differences are found between the structures. In particular, bombardment of a thin 2 nm silica layer on top of the Si(111) surface is shown to behave quite differently from bombardment of pure Si.
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| 67. |
- Samela, J., et al.
(författare)
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Origin of complex impact craters on native oxide coated silicon surfaces
- 2008
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Ingår i: Physical Review B. ; 77, s. 075309(1-15)
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Tidskriftsartikel (refereegranskat)abstract
- Crater structures induced by impact of keV-energy Ar cluster ions on silicon surfaces are measured with atomic force microscopy. Complex crater structures consisting of a central hillock and outer rim are observed more often on targets covered with a native silicon oxide layer than on targets without the oxide layer. To explain the formation of these complex crater structures, classical molecular dynamics simulations of Ar cluster impacts on oxide coated silicon surfaces, as well as on bulk amorphous silica, amorphous Si, and crystalline Si substrates, are carried out. The diameter of the simulated hillock structures in the silicon oxide layer is in agreement with the experimental results, but the simulations cannot directly explain the height of hillocks and the outer rim structures when the oxide coated silicon substrate is free of defects. However, in simulations of 5 keV/atom Ar12 cluster impacts, transient displacements of the amorphous silicon or silicon oxide substrate surfaces are induced in an approximately 50 nm wide area surrounding the impact point. In silicon oxide, the transient displacements induce small topographical changes on the surface in the vicinity of the central hillock. The comparison of cluster stopping mechanisms in the various silicon oxide and silicon structures shows that the largest lateral momentum is induced in the silicon oxide layer during the impact; thus, the transient displacements on the surface are stronger than in the other substrates. This can be a reason for the higher frequency of occurrence of the complex craters on oxide coated silicon.
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| 68. |
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| 69. |
- Stoian, Razvan, et al.
(författare)
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Comment on Coulomb explosion in femtosecond laser ablation of Si(111)
- 2004
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Ingår i: Applied Physics Letters. - : AIP Publishing. - 0003-6951 .- 1077-3118. ; 85, s. 694-695
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Tidskriftsartikel (refereegranskat)abstract
- In a recent letter Roeterdink 1 report on the occurrence of an electrostatic form of material removal from solid silicon samples irradiated with high-intensity ultrashort laser pulses. The arguments are essentially derived from time-of-flight observations of the emitted single- and double-ionized silicon atoms and considerations related to momentum conservation during the excitation and expansion phase. A linear scaling of the velocity of species with different charge states is considered as an argument for Coulomb explosion being responsible for ion emission from the irradiated silicon sample in a high fluence regime. A similar effect derived directly from momentum conservation upon particle ejection has been determined before as a proof for the occurrence of Coulomb explosion from laser irradiated dielectrics,2,3 but a similar process for semiconductors was not observed at intensities just above the ablation threshold.3 The absence of Coulomb explosion from semiconductors within the range of investigated fluences (up to 1 J/cm2) was related to efficient electronic transport able to counterbalance the laser-induced charge deficiency. The "discrepancy" discussed in the letter, involving a threshold criterion for the surface Coulombic explosion, is based on a misinterpretation of the data presented by Stoian 3 Usually the mechanism of Coulomb explosion is explained as being driven by positively charged superficial layers that will electrostatically repel each other, assisted in some cases by the pulling force exercised by the photo-emitted charge cloud close to the ionized surface. Recent calculations4 regarding the mechanisms for surface electrostatic disruption have shown that Coulomb explosion may occur even without the additional electron pull. Provided that substantial photoelectron emission occurs and carrier mobility is intrinsically low or lowered during the excitation, the electron transport from the excited bulk region cannot compensate the photoelectric flow, and, therefore, the surface neutrality will be broken. A significant uncompensated remnant positive charge will be localized within the first surface layers and the ions within this region will be mutually repelled. The authors have overlooked, when referring to the results of Stoian ,3 the fact that in subsurface regions where photoemission is less effective one can also obtain a high density of carriers. The discussion and arguments in Ref. 3 refer to the net charge (the absolute difference between the ion and electron density) in the surface layers, with no a priori restrictions for the absolute carrier density to reach supercritical values at the irradiation wavelength. According to the study of Roeterdink , the high carrier density induced by the ultrafast laser excitation, in excess of 1022 cm–3 will destabilize the lattice, leading to the surface electrostatic disruption. A fractional charge of more than 0.3 excited electrons per silicon atom is calculated to generate electric fields matching the observed momentum transfer, but the local neutrality of the sample appears not to be disturbed. Lattice destabilization at high carrier density is well documented in the literature5 but it does not involve any neutrality breakdown that may cause electrostatic material ejection. It is thus unclear how high carrier densities alone, even close to the solid density, may lead to high electrostatic fields and Coulomb explosion of the region, without any deviation from the electric neutrality. One may invoke either strong photoelectron emission or charge separation caused by nonequilibrium transport, but this is not clearly discussed in the letter. If the authors imply that high excitation simultaneously means high photoelectron yields, a net, uncompensated charge of more than 0.3 missing electrons per atom can be reached in the present case only if one assumes that all the excited electrons have been removed from the superficial layers, without any other forms of electronic supply during the emission time. If one considers that the excitation depth in silicon is several hundreds of nanometers and the electrons can be removed with a certain probability only from a narrow region beneath the surface, the uncompensated region will be rapidly neutralized by bulk electronic transport, keeping the net charge below the critical value for Coulomb explosion. Though the possibility to efficiently charge the Si surface layers beyond the threshold for bond-breaking and macroscopic rupture of the surface may conceivably appear under extreme irradiation conditions involving high intensities where high carrier densities are generated on the leading edge of the pulse and electronic transport is strongly perturbed in the destablized lattice, the experimental results are usually embedded in a series of additional effects and artifacts in the laser-generated plume that may obscure a clear interpretation of the ejection mechanisms. Moreover, one has to carefully consider the changes that occur in the irradiation geometry once high fluences and high irradiation doses are used. Here, one of the consequences is indicated by the authors in the appearance of a "second ejection channel." We suggest a possible alternative explanation for the experimental facts observed by the authors. Under the experimental conditions reported, i.e., fluence in the range of 3–10 J/cm2, several times higher than the ablation threshold, the material removal rates are considerable. An immediate effect, accentuated by the production of a crater at the surface (as acknowledged by the authors) after several laser pulses, is a highly collisional, dense plume, initially confined in the crater before expanding. Consequently charge separation may occur in the gas phase,6 leading to a similar experimental signature, namely a linear scale of the velocity of different ionized species with their intrinsic charge. More explicitly, the appearance of the so-called double layer will accelerate multiple charges and the same momentum regulations occur.6
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| 70. |
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