| 1. |
- Bulgakova, Nadya, 1956, et al.
(författare)
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Possible role of charge transport in enhanced carbon nanotube growth
- 2006
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Ingår i: Applied Physics A. - : Springer Science and Business Media LLC. - 0947-8396 .- 1432-0630. ; 85, s. 109-116
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Tidskriftsartikel (refereegranskat)abstract
- We consider the role of electric fields during metal-catalysed thermal chemical vapour deposition growth of carbon nanotubes and show that enhanced growth occurs from a negatively biased electrode. An electric field, applied externally to the growing tubes and/or generated as a result of electron emission or self-biasing, may strongly affect the carbon supply through the catalyst nanoparticle, enhancing the growth rate. Different aspects of the growth process are analysed: the nature of the nanoparticle catalysis, carbon dissolution kinetics, electron emission from the nanotube tips, charge transport in the nanotube-catalytic nanoparticle system and carbon drift and diffusion through the catalyst under the action of the electric field. A fundamental tenet for modelling of charge-transport dynamics during the nanotube growth process is proposed.
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| 2. |
- Bulgakova, Nadya, 1956, et al.
(författare)
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A general continuum approach to describe fast electronic transport in pulsed laser irradiated materials: the problem of Coulomb explosion
- 2005
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Ingår i: Applied Physics A. - : Springer Science and Business Media LLC. - 0947-8396 .- 1432-0630. ; 81, s. 345-356
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Tidskriftsartikel (refereegranskat)abstract
- We present a continuum model, based on a drift-diffusion approach, aimed at describing the dynamics of electronic excitation, heating, and charge-carrier transport in different materials (metals, semiconductors, and dielectrics) under femtosecond and nanosecond pulsed laser irradiation. The laser-induced charging of the targets is investigated at laser intensities above the material removal threshold. It is demonstrated that, for near-infrared femtosecond irradiation, charging of dielectric surfaces causes a sub-picosecond electrostatic rupture of the superficial layers, alternatively called Coulomb explosion (CE), while this effect is strongly inhibited for metals and semiconductors as a consequence of superior carrier transport properties. On the other hand, application of the model to UV nanosecond pulsed laser interaction with bulk silicon has pointed out the possibility of Coulomb explosion in semiconductors. For such regimes a simple analytical theory for the threshold laser fluence of CE has been developed, showing results in agreement with the experimental observations. Various related aspects concerning the possibility of CE depending on different irradiation parameters (fluence, wavelength and pulse duration) and material properties are discussed. This includes the temporal and spatial dynamics of charge-carrier generation in non-metallic targets and evolution of the reflection and absorption characteristics.
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| 3. |
- Bulgakova, Nadya, 1956, et al.
(författare)
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Electronic transport and consequences for material removal in ultrafst pulsed laser ablation of materials
- 2004
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Ingår i: Physical Review B. - 1098-0121. ; 69
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Tidskriftsartikel (refereegranskat)abstract
- Fast electronic transport is investigated theoretically based on a drift-diffusion approach for different classes of materials (metals, semiconductors, and dielectrics) under ultrafast, pulsed laser irradiation. The simulations are performed at intensities above the material removal threshold, characteristic for the ablation regime. The laser-induced charging of dielectric surfaces causes a subpicosecond electrostatic rupture of the superficial layers, an effect which, in comparison, is strongly inhibited for metals and semiconductors as a consequence of superior carrier transport properties.
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| 4. |
- Bulgakova, Nadya, 1956, et al.
(författare)
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Model description of surface charging during ultrafast pulsed laser ablationof materials
- 2004
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Ingår i: Applied Physics A. - : Springer Science and Business Media LLC. - 0947-8396 .- 1432-0630. ; 79, s. 1153-1155
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Tidskriftsartikel (refereegranskat)abstract
- We present a model describing the dynamical mechanisms responsible for generating fast ion ejection under ultra-short pulsed laser irradiation. The model is based on a simplified drift–diffusion approach describing the evolution of the laser-generated charge carriers, their transport, and the electric field generated as a result of quasi-neutrality breaking in the irradiated target. The importance of different processes in generating the non-thermal material-ejection mechanisms is discussed. A common frame is applied to dielectrics, semiconductors, and metals and different dynamical behaviour is observed. The modelling results are in good agreement with fs pump–probe studies and measurements of the velocity distributions of the emitted ions.
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| 5. |
- Bulgakova, Nadya, 1956, et al.
(författare)
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Surface charging under pulsed laser ablation of solids and its consequences: studies with a continuum approach
- 2005
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Ingår i: SPIE Proceedings. - : SPIE. - 0277-786X. ; 5714, s. 9-23
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Tidskriftsartikel (refereegranskat)abstract
- Dynamics of electronic excitation, heating and charge-carrier transport in different materials (metals, semiconductors, and dielectrics) under femtosecond pulsed laser irradiation is studied based on a unified continuum model. A simplified drift-diffusion approach is used to model the energy flow into the sample in the first hundreds of femtoseconds of the interaction. The laser-induced charging of the targets is investigated at laser intensities slightly above the material removal threshold. It is demonstrated that, under near-infrared femtosecond irradiation regimes, charging of dielectric surfaces causes a sub-picosecond electrostatic rupture of the superficial layers, alternatively called Coulomb explosion (CE), while this effect is strongly inhibited for metals and semiconductors as a consequence of superior carrier transport properties. Various related aspects concerning the possibility of CE for different irradiation parameters (fluence, wavelength and pulse duration) as well as the limitations of the model are discussed. These include the temporal and spatial dynamics of charge-carrier generation in non-metallic targets and evolution of the optical (reflection and absorption) characteristics. A controversial topic concerning CE probability in laser irradiated semiconductor targets is also a subject of this work.
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| 6. |
- 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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| 7. |
- Svensson, Johannes, 1978, et al.
(författare)
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Field emission induced deformations in SiO2 during CVD growth of carbon nanotubes
- 2006
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Ingår i: Physica status solidi. B, Basic research. - : Wiley. - 0370-1972 .- 1521-3951. ; 243, s. 3524-3527
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Tidskriftsartikel (refereegranskat)abstract
- The application of an electric field while growing carbon nanotubes with CVD can induce deformations in the SiO2 substrate. The effect is attributed to field emission from the tubes and Marangoni convection in a small molten SiO2 region underneath the tubes. Postgrowth deformation has been performed as well as large scale deformations using the collective effect of many field emitters. The porosity of one type of deformation is also examined and discussed.
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| 8. |
- Svensson, Johannes, 1978, et al.
(författare)
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Marangoni effect in SiO(2) during field-directed chemical vapor deposition growth of carbon nanotubes
- 2006
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Ingår i: Physical Review B. - 0163-1829. ; 73
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Tidskriftsartikel (refereegranskat)abstract
- Intense local heating of SiO2 is shown to occur during chemical vapor deposition growth of single-walled carbon nanotubes in the presence of an electric field. This gives rise to the Marangoni effect where strong convection currents are induced in the molten SiO2 layer. Nanoscale trenches and bumps are formed in the insulating layer directly below the growing nanotube.
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