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- Iwata, H., et al.
(författare)
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A new type of quantum wells : Stacking faults in silicon carbide
- 2003
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Ingår i: Microelectronics Journal. - 0026-2692. ; 34:5, s. 371-374
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Konferensbidrag (övrigt vetenskapligt/konstnärligt)abstract
- We report on a new type of quantum wells with the width as thin as 10Å, which are composed of SiC only, and consequently have ideal interfaces. These quantum wells are actually stacking faults in SiC. Certain types of stacking faults in SiC polytypes create small 3C-like regions, where the stacking sequences along the c-axis become locally cubic in the hexagonal host crystals. Since the conduction band offsets between the cubic and hexagonal polytypes are very large with the conduction band minima of 3C-SiC lower than that of the other polytypes, such thin 3C inclusions can introduce locally lower conduction bands, thus acting as quantum films perpendicular to the c-axis. One mechanism for the occurrence of stacking faults in the perfect SiC single crystals is the motion of partial dislocations in the basal planes, the partial dislocations leaving behind stacking fault regions. © 2003 Elsevier Science Ltd. All rights reserved.
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| 4. |
- Goss, J.P., et al.
(författare)
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A first principles study of interstitial Si in diamond
- 1997
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Ingår i: Defects in Semiconductors. - : Trans Tech Publications Inc.. ; , s. 781-786
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Konferensbidrag (övrigt vetenskapligt/konstnärligt)abstract
- The results of first principles calculations using AIMPRO (a local-density-functional code) are presented to describe the stability of interstitial Si in diamond. It is found that the 〈100〉 and 〈110〉 split-interstitial configurations are only local minima in the total energy surface. Td interstitial Si reconstructs to form a substitutional Si site close to a self-interstitial. The difference in total energy between the split configuration and the substitutional-Si-self-interstitial complex is more than 6 eV. It is concluded that Si would not adopt an interstitial location in diamond as has been previously suggested from experimental evidence. Interestingly the self-interstitial remains bound to the substitutional Si with a binding energy around 1 eV.
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| 5. |
- Gowda, Vasantha, et al.
(författare)
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Structural investigations of rare earth dialkyl dithiocarbamate complexes: solid-state NMR, X-ray diffraction and DFT calculation studies
- 2013
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Konferensbidrag (övrigt vetenskapligt/konstnärligt)abstract
- In this study, we made an attempt to qualitatively study the structures of few rare earth metal complexes by employing solid state NMR, X-Ray Diffraction, and preliminary DFT calculations. High resolution 13C and 15N solid state CP/MAS NMR spectra were recorded for six diamagnetic polycrystalline rare earth dialkyldithiocarbamates of the general formula [(RE2S2CNR2)3 PHEN] (where RE=La or Y, R=C2H5, C3H7, and i-C3H7) [1]. Different isotropic 13C and 15N chemical shifts for the three dialkyldithiocarbamato groups were observed. Regulacio et al. (2005) inferred that irrespective of the alkyl chains, rare earth complexes of dialkyldithiocarbamates and phenanthroline (3:1) ligands always crystallize in a monoclinic system with a space P21/c group. However, comparative analysis of solid state 13C/15N CPMAS spectra of polycrystalline yttrium and lanthanum diethyldithiocarbamate complexes shows the presence of significant differences, indicating structural variations of these complexes. Also, quite different X-Ray diffraction powder pattern was observed for the above two complexes. Finally, the computational geometry optimization of Y and La complexes, followed by the preliminary calculation of 13C and 15N chemical shifts and shielding contributions with the ADF program [2], found to be very near to the experimental results.
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| 6. |
- Hedman, Daniel, 1989-
(författare)
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A Theoretical Study: The Connection between Stability of Single-Walled Carbon Nanotubes and Observed Products
- 2017
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Licentiatavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Over the past 20 years’ researchers have tried to utilize the remarkable properties of single-walled carbon nanotubes (SWCNTs) to create new high-tech materials and devices, such as strong light-weight composites, efficient electrical wires and super-fast transistors. But the mass production of these materials and devices are still hampered by the poor uniformity of the produced SWCNTs. These are hollow cylindrical tubes of carbon where the atomic structure of the tube wall consists of just a single atomic layer of carbon atoms arranged in a hexagonal grid. For a SWCNT the orientation of the hexagonal grid making up the tube wall is what determines its properties, this orientation is known as the chirality of a SWCNT. As an example, tubes with certain chiralities will be electrically conductive while others having different chiralities will be semiconducting.Today’s large scale methods for producing SWCNTs, commonly known as growth of SWCNTs, gives products with a large spread of different chiralities. A mixture of chiralities will give products with a mixture of different properties. This is one of the major problems holding back the use of SWCNTs in future materials and devices. The ultimate goal is to achieve growth where the resulting product is uniform, meaning that all of the SWCNTs have the same chirality, a process termed chirality-specific growth. To achieve chirality-specific growth of SWCNTs requires us to obtain a better fundamental understanding about how they grow, both from an experimental and a theoretical point of view.This work focuses on theoretical studies of SWCNT properties and how they relate to the growth process, thereby giving us vital new information about how SWCNTs grow and taking us ever closer to achieving the ultimate goal of chirality-specific growth. In this thesis, an introduction to the field is given and the current state of the art experiments focusing on chirality-specific growth of SWCNTs are presented. A brief review of the current theoretical works and computer simulations related to growth of SWCNTs is also presented. The results presented in this thesis are obtained using first principle density functional theory. The first study shows a correlation between the stability of SWCNT-fragments and the observed products from experiments. Calculations confirm that in 84% of the investigated cases the chirality of experimental products matches the chirality of the most stable SWCNT-fragments (within 0.2 eV). Further theoretical calculations also reveal a previously unknown link between the stability of SWCNT-fragments and their length. The calculations show that at specific SWCNT-fragment lengths the most stable chirality changes. Thus, introducing the concept of a switching length for SWCNT stability. How these new results link to the existing understanding of SWCNT growth is discussed at the end of the thesis.
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| 7. |
- 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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| 8. |
- 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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