| 1. |
- Axelsson, S., et al.
(author)
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Theoretical and experimental investigations of three-terminal carbon nanotube nanorelays
- 2005
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In: New J. Phys. - : IOP Publishing. - 1367-2630. ; 7
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Journal article (peer-reviewed)abstract
- We present theoretical and experimental investigations of three-terminal nanoelectromechanical relays based on suspended carbon nanotubes. A charge is induced in the nanotube by applying a voltage to an underlying gate electrode thus inducing the nanotube to bend and make contact with a drain electrode. Such devices have potential applications as fast switches, logic devices, memory elements and pulse generators. We describe two modes of operation: a contact mode where the nanotube makes physical contact with the drain electrode and a non-contact mode where electrical contact between the nanotube and the drain electrode is made via a field emission current.
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| 2. |
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| 3. |
- Boyle, Mark, et al.
(author)
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Excitation dynamics of Rydberg states in C60
- 2005
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In: European Physical Journal D. - : Springer Science and Business Media LLC. - 1434-6060 .- 1434-6079. ; 36, s. 339-351
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Journal article (peer-reviewed)abstract
- The electron and nuclear dynamics of C60 fullerenes irradiated with femtosecond laser pulses are investigated with photoelectron and photoion spectroscopy. The focus of this work is the detailed exploration of the population mechanism of Rydberg levels within the excitation process of neutral C60. The effect of excitation wavelength, intensity, chirp, and polarization on the kinetic energy distribution of photoelectrons in single-pulse experiments gives first insight into the underlying processes. In combination with time-resolved two-color pump-probe spectroscopy depending on either pump, or probe pulse intensity, a more complete picture of the interaction can be drawn. The results point towards a very interesting but nevertheless complex behavior including four steps: (i) non-adiabatic multielectron excitation of the HOMO (hu) → LUMO+1 (t1g) transition; (ii) thermalization within the hot electron cloud on a time scale below 100fs, followed by a coupling of energy to vibrational modes of the molecule via doorway state(s); (iii) population of electronically excited Rydberg states by multiphoton absorption, and (iv) single photon ionization from the excited Rydberg states. This excitation process results in a characteristic sequence of photoelectron lines in the photoemission spectra. The comparison of the experimental results with recent theoretical work gives convincing evidence that non-adiabatic multielectron dynamics (NMED) plays a key role for the understanding of the response of C60 to short-pulse laser radiation.
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| 4. |
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| 5. |
- Bulgakova, Nadya, 1956, et al.
(author)
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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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In: Applied Physics A. - : Springer Science and Business Media LLC. - 0947-8396 .- 1432-0630. ; 81, s. 345-356
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Journal article (peer-reviewed)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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| 6. |
- Bulgakova, Nadya, 1956, et al.
(author)
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Possible role of charge transport in enhanced carbon nanotube growth
- 2006
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In: Applied Physics A. - : Springer Science and Business Media LLC. - 0947-8396 .- 1432-0630. ; 85, s. 109-116
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Journal article (peer-reviewed)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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| 7. |
- Bulgakova, Nadya, 1956, et al.
(author)
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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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In: SPIE Proceedings. - : SPIE. - 0277-786X. ; 5714, s. 9-23
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Journal article (peer-reviewed)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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| 8. |
- Campbell, Eleanor E B, 1960, et al.
(author)
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Ionisation of Fullerenes and Fullerene Clusters using Ultrashort Laser Pulses
- 2006
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In: Photochem. Photobiol. Sci.. - : Springer Science and Business Media LLC. - 1474-905X. ; 5, s. 1183-1189
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Journal article (peer-reviewed)abstract
- We give a brief review of the literature concerning the ultra-short pulse ionisation of fullerenes in the gas phase. Emphasis is placed on the excitation time dependence of different ionisation regimes as manifested by photoelectron spectroscopy. The ionisation rates are modelled for the intermediate situation where the excitation energy is equilibrated between electronic degrees of freedom but not yet coupled to vibrational degrees of freedom. The model is shown to describe many aspects of the experiments. New results are presented on the intra-cluster molecular fusion of fullerene molecules when van der Waals bound clusters of fullerenes are exposed to ultra-short laser pulses. Pump–probe measurements give a decay time constant for the intra-cluster fusion reaction of 520 ± 55 fs. A comparison with monomer ionisation results suggests that the time window for the fusion reaction is influenced by the coupling of the electronic excitation energy to vibrational degrees of freedom of the molecules in the cluster.
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| 9. |
- Ding, Feng, 1970, et al.
(author)
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Graphitic encapsulation of catalyst particles in carbon nanotube production
- 2006
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In: Journal of Physical Chemistry B. - : American Chemical Society (ACS). - 1520-5207 .- 1520-6106. ; 110:15, s. 7666-7670
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Journal article (peer-reviewed)abstract
- A new model is proposed for the encapsulation of catalyst metal particles by graphite layers that are obtained, for example, in low-temperature chemical vapor deposition production of carbon nanotubes (CNTs). In this model graphite layers are primarily formed from the dissolved carbon atoms in the metal-carbide particle when the particle cools. This mechanism is in good agreement with molecular dynamics simulations (which show that precipitated carbon atoms preferentially form graphite sheets instead of CNTs at low temperatures) and experimental results (e.g., encapsulated metal particles are found in low-temperature zones and CNTs in high-temperature regions of production apparatus, very small catalyst particles are generally not encapsulated, and the ratio of the number of graphitic layers to the diameter of the catalyst particle is typically 0.25 nm(-1)).
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| 10. |
- Dittmer, Staffan, 1970, et al.
(author)
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In situ Raman studies of single-walled carbon nanotubes grown by local
- 2008
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In: Chemical Physics Letters. - : Elsevier BV. - 0009-2614 .- 1873-4448. ; 457
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Journal article (peer-reviewed)abstract
- Using in situ Raman spectroscopy we investigate single wall carbon nanotube growth on Mo electrodes, using a highly localized resistive heating technique. Small diameter semiconducting single wall nanotubes grow very rapidly when the catalyst support is heated to a temperature of 800 C. The G/D ratio shows an interesting time-dependent behaviour. It first decreases, indicating the presence of amorphous carbon and then significantly increases again after ca. 5 min growth while retaining the position and shape expected for predominantly semiconducting carbon nanotubes.
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