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- Hedman, Daniel, 1989-
(författare)
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Single-Walled Carbon Nanotubes : A theoretical study of stability, growth and properties
- 2019
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Since their discovery over 25 years ago, scientists have explored the remarkable properties of single-walled carbon nanotubes (SWCNTs) for use in high-tech materials and devices, such as strong light-weight composites, efficient electrical wires, supercapacitors and high-speed transistors. However, the mass production of such materials and devices is still limited by the capability of producing uniform high-quality SWCNTs. The properties of a SWCNT are determined by the orientation of the hexagonal grid of carbon atoms constituting the tube wall, this is known as the chirality of the SWCNT.Today's large-scale methods for producing SWCNTs, commonly known as growth, give products with a large spread of different chiralities. A mixture of chiralities give products with a mixture of different properties. This is one of the major obstacles preventing large-scale use of SWCNTs in future materials and devices. The goal is to achieve growth where the resulting product is uniform, meaning that all SWCNTs have the same chirality, a process termed chirality-specific growth. To achieve this requires a deep fundamental understanding of how SWCNTs grow, both from an experimental and a theoretical perspective.This work focuses on theoretical studies of SWCNTs and their growth mechanisms. With the goal of achieving a deeper understanding of how chirality arises during growth and how to control it. Thus, taking us ever closer to the ultimate goal of achieving chirality-specific growth. In this thesis, an introduction to the field is given and the current research questions are stated. Followed by chapters on carbon nanomaterials, SWCNTs and computational physics. A review of the state-of-the-art experimental and theoretical works relating to chirality specific growth is also given.The results presented in this thesis are obtained using first principle density functional theory calculations. Results show that the stability of short SWCNT-fragments can be linked to the products observed in experiments. In 84% of the investigate cases, the chirality of experimental products matches the chirality of the most stable SWCNT-fragments (within 0.2 eV). Further studies also reveal a previously unknown link between the stability of SWCNT-fragments and their length. Calculations show that at specific lengths the most stable chirality changes. Thus, introducing the concept of a switching length for SWCNT stabilities.This newly found property of SWCNTs is used in combination with previously published works to create a state-of-the-art analytical model to investigate growth of SWCNTs any temperature. Results from the model show that the most stable chirality obtained is dependent on the diameter, length of the SWCNT, the growth temperature and the composition of the catalyst. Finally, a detailed study on the ability of catalyst metals to sustain SWCNT growth points to Pt as an interesting candidate to achieve growth of rarely seen chiralities. The new knowledge gained from these results takes us even closer to achieving chirality-specific growth.
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| 2. |
- Löfgren, Robin
(författare)
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A Theoretical Investigation of the Nitrogen-Vacancy Center in Diamond as a Single Molecule Sensor and Qubit : Charging through Explicit Electron Donors
- 2021
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- The NV-center in diamond is one of the most well researched defects to date. Since its discovery in the 1960’s, a large body of experimental as well as theoretical work have been produced, investigating its properties and applications. The reason for the attention on this defect are its properties that are well suited for a number of applications. Some of those properties are: 1) photostable at room temperature; 2) long spin coherence time; 3) spin-flipping during the process of optical excitation and decay; 4) Optical readout of spin state. Some of the applications include qubits in quantum computers and sensors for single molecule properties. In order for the NV-center to function well, it is important to decouple its interaction with other defects in the diamond lattice or with the surface of the diamond, that could have a detrimental effect on the NV-center properties. In this work, we theoretically investigate how the NV-center properties are affected by some nearby defects. Those defects include: a nitrogen point defect in the diamond lattice, diamond surfaces, and an extended intrinsic stacking fault defect in the diamond lattice. It is the negative charge state of the NV-center that has the properties mentioned above, and therefore it is this charge state that is interesting for the applications. Here, we investigate our new theoretical method of charging the NV-center through an electron donor nitrogen in the diamond lattice. By instead charging with an explicit electron donor/acceptor, we avoid the complicated correction schemes associated with the tra-ditional theoretical method of introducing an artificial background charge density in a supercell for simulating charged defects. It can also be argued that our new method is a more physically correct method, as negatively charged NV-centers in diamond get their charge by accepting electrons from nearby nitrogens in the diamond lattice. In addition to the NV-center, we further test the method for other point defects in diamond.In this thesis, an introduction to the field is given and the current research questions are stated in chapter 1. Followed by chapters reviewing the current experimental and theoret-ical work regarding the NV-center, computational physics and density functional theory, and an overview of the software used in this work. The results presented in this thesis are obtained using density functional theory computations.Our results show that the method of charging the NV-center with a donor-nitrogen is viable for an NV-N distance of 7.5 ˚A or greater.When placing the NV-center in the vicinity to a terminated surface (F- H/O/OH- and N-terminated), its properties converge to bulk values already at 5 ˚A depth. This is great news when compared with the recent experimentally achieved distance of 1 nm, meaning that NV-centers could possibly be placed even closer to the surface without being affected. When placing the NV-center in the vicinity of an intrinsic stacking fault (ISF), our results show that the NV-center is not greatly affected down to a distance of 4.2 ˚A. However, when the NV-center is placed 3.8 ˚A or closer to the ISF, the ZPL is perturbed between 2.0 and 11.3 %. It is perturbed the most when placed inside the ISF glide plane. This is great news for the technical applications; some diamonds contain high densities of ISFs, and our results show that a NV-center can be placed really close to such an ISF without losing its sensitivity as a sensor of magnetic fields.We have also found that the excitation from the NV− ground state into donor-N+ (one-photon process) requires 2.31 eV and lead to a meta-stable NV0 and donor-N0 charge state, both of which are electron spin resonance (ESR) active and, thus, this transition could be investigated experimentally. The excitation to the neutral state can also be achieved through a two-photon process with the first step at 2.19 eV and the second step at 0.81 eV.When placing the NV-center in the vicinity to two nitrogens (one neutrally charged, and one positively charged acting as electron donor), we find that it is almost unaffected, with changes in the ZPL of 1-8 meV when the distance to the nitrogens is 9.40-12.52 ˚A. This means that a nearby nitrogen, whether it is neutral or positively charged does not affect the NV-center in a detrimental way.Our tests on charging defects through electron donors/acceptors reveals that our method also works for the following defect-donor/acceptor pairs: NV−-P+, NV−-B+, N+-B−, SiV−-N+, Be−-O+, Be2−-N+-N+.
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