Final Report Summary - CARBOTRON (Carbon-based nanoelectronics)
All research objectives have been duly addressed during the project. Additional objectives have been identified over the course of the project as can be expected in such a dynamic field of research; these are detailed after the original objectives. 14 publications were born of this work as detailed below (5 being first-author papers of the researcher), the majority of which were published in leading journals of the field, and two of which have generated an exceptionally high number of citations, indicative of the high impact of the research performed. The researcher has attended 5 conferences as listed below. We have made significant progress towards the study of carbon-based electronics as well as what's beyond it. Resources have been used as planned. Integration at the host is under way as the researcher has been moved to a new post-doctoral contract at the end of the fellowship, and the researcher will submit multiple applications for UK research grants with full support from the host. Knowledge transfer has been achieved through the researcher providing training in the use of the VASP software to students at the host. The researcher has also received training in the use of complex tight-binding models which was put to good use in one of the 2014 publications. Summary of the results for each objective is as follows:
Objective 1 (Nanoscale rectification in molecular electronics): A new type of mechanism was identified for carbon-based spintronics through 13C hyperfine interaction. While specific rectifiers were not studied as part of this project, the results obtained on the new mechanism are expected to lead to new advances in molecular electronics, including molecular scale rectification. Three papers have been published as detailed below.
Objective 2 (Single molecule sensing): So-called carbon nanobamboo, that is, junctions of carbon nanotubes of different diameter and chiral angle, can be grown from fullerenes inside larger diameter carbon nanotubes. We have shown that in the special case when the two nanotubes are mirror opposites, a simple symmetry-based classification can determine the stable structure. We have found that localised states appear at the junctions which can be the basis of their experimental analysis. Likewise, due to their localised nature, such states can be utilised for the detection of molecules that locally interact with them. This has been published in physica status solidi b.
Objective 3 (Electronic structure of carbonaceous materials: beyond density functional theory): One-dimensional carbon-based molecules have been studied using GW many-body methods which are able to provide very accurate band gaps. It has been demonstrated that the GW band gap is in good agreement with experiments. The results have been used to give a theoretical prediction of the attenuation factor in linear carbon chains which will be of essential use to experimentalists. One paper has been published in the Journal of Chemical Physics.
Looking ahead: Over the course of the project it became apparent that for many nanoscale electronics applications it is advised to look beyond carbon-based materials, hence the researcher looked at a number of other low-dimensional materials that are closely related to those in the original objectives above. These studies are a direct extension of Objectives 1 and 2. The materials in question are silicene, silicane and germanane, transition metal dichalcogendies, gallium chalcogenides, and indium chalcogendies. The research has shown that all of these materials are of tremendous use in nanoscale electronics due to their outstanding physical properties. Seven papers have been published as detailed below. In addition, related to Objective 3 we have looked at some of the vibrational properties of carbon-based materials which influence their nanoelectronics application. An original research paper and a review article was published as discussed below.
The most significant results are as follows:
1) We have shown in a combined work with experimentalists that the 13C hyperfine coupling in carbon nanotubes is as small as expected from the established literature, and that recent controversial measurements claiming that the hyperfine coupling in nanotube based double quantum dots is enhanced by 3 orders of magnitude is a result of misinterpretation. We have shown that the correct interpretation requires that the Tomonaga-Luttinger behavior of metallic nanotubes be taken into account. In addition we have shown that charged vacancies in carbon nanotubes undergo a significant hyperfine enhancement due to localization which can be exploited both in sample characterization and in spintronics. These results have been published in Physical Review Letters, Physical Review B, and Physica status solidi b.
2) Moving beyond the limitations of carbon-based electronics we have performed a theoretical study of a novel material called silicene, the silicon equivalent of graphene. We have shown that electrically gated silicene can be used in nanoelectronics as a semiconductor with a tunable band gap. These results have been published in Physical Review B and the paper has received over 100 citations.
3) We have shown that hydrogenated silicene, called silicane, and its germanium equivalent germanane, are stable two-dimensional semiconductors, one with an indirect and one with a direct band gap, offering a good range of nanoelectronics applications. We have also shown that the conduction band is largely influenced by second-nearest neighbour interaction. These results have been published in 2D Materials.
4) We have looked into the low-energy states of molybdenum disulphide and related transition metal dichalcogenides using a combination of first principles methods and have pointed out that the quantitatively correct description of spin-orbit splitting and trigonal warping of the valence and conduction bands are important for their potential applications as quantum dots and qubits. These results have been published in Physical Review B and Physical Review X, with the first having attracted 25 citations in the 10 months since publication.
5) We have shown that single-layer hexagonal gallium chalcogenides and indium chalcogenides are stable indirect semiconductors with a unique sombrero-shaped valence band which allows an engineering of Lifshitz transitions by hole-doping. We have pointed out that these materials exhibit very good optical absorption in the ultraviolet range, making them excellent materials for use in the detection of UV photons. In the case of gallium telluride we entered a collaboration with experimentalists to show that multilayer monoclinic gallium telluride can be used to build high-sensitivity photodetectors. These results have been published in ACS Nano and in 2 papers in Physical Review B.
6) We have used GW many-body theory to calculate the band structure of polyyne, the linear carbon chain. The calculations were fully converged and the resulting band gap was found to agree with measurements very well. The GW band structure was used to parametrise a tight-binding model to offer experimentalists a quantitative prediction for the attenuation coefficient. These results have been published in the Journal of Chemical Physics.
7) We have examined the spectral density of Raman scattering in graphene when emitting a pair of LA and LO phonons and found that the lineshape is asymmetric and largely determined by the kinematics of the fully-resonant two-phonon process involved. In addition we have compiled an overview of Raman spectroscopy in graphite and graphene, drawing attention to the practical significance of Raman spectroscopy for the study of multi-layers of graphene. These results have been published in Solid State Communications and physica status solidi b.
Objective 1 (Nanoscale rectification in molecular electronics): A new type of mechanism was identified for carbon-based spintronics through 13C hyperfine interaction. While specific rectifiers were not studied as part of this project, the results obtained on the new mechanism are expected to lead to new advances in molecular electronics, including molecular scale rectification. Three papers have been published as detailed below.
Objective 2 (Single molecule sensing): So-called carbon nanobamboo, that is, junctions of carbon nanotubes of different diameter and chiral angle, can be grown from fullerenes inside larger diameter carbon nanotubes. We have shown that in the special case when the two nanotubes are mirror opposites, a simple symmetry-based classification can determine the stable structure. We have found that localised states appear at the junctions which can be the basis of their experimental analysis. Likewise, due to their localised nature, such states can be utilised for the detection of molecules that locally interact with them. This has been published in physica status solidi b.
Objective 3 (Electronic structure of carbonaceous materials: beyond density functional theory): One-dimensional carbon-based molecules have been studied using GW many-body methods which are able to provide very accurate band gaps. It has been demonstrated that the GW band gap is in good agreement with experiments. The results have been used to give a theoretical prediction of the attenuation factor in linear carbon chains which will be of essential use to experimentalists. One paper has been published in the Journal of Chemical Physics.
Looking ahead: Over the course of the project it became apparent that for many nanoscale electronics applications it is advised to look beyond carbon-based materials, hence the researcher looked at a number of other low-dimensional materials that are closely related to those in the original objectives above. These studies are a direct extension of Objectives 1 and 2. The materials in question are silicene, silicane and germanane, transition metal dichalcogendies, gallium chalcogenides, and indium chalcogendies. The research has shown that all of these materials are of tremendous use in nanoscale electronics due to their outstanding physical properties. Seven papers have been published as detailed below. In addition, related to Objective 3 we have looked at some of the vibrational properties of carbon-based materials which influence their nanoelectronics application. An original research paper and a review article was published as discussed below.
The most significant results are as follows:
1) We have shown in a combined work with experimentalists that the 13C hyperfine coupling in carbon nanotubes is as small as expected from the established literature, and that recent controversial measurements claiming that the hyperfine coupling in nanotube based double quantum dots is enhanced by 3 orders of magnitude is a result of misinterpretation. We have shown that the correct interpretation requires that the Tomonaga-Luttinger behavior of metallic nanotubes be taken into account. In addition we have shown that charged vacancies in carbon nanotubes undergo a significant hyperfine enhancement due to localization which can be exploited both in sample characterization and in spintronics. These results have been published in Physical Review Letters, Physical Review B, and Physica status solidi b.
2) Moving beyond the limitations of carbon-based electronics we have performed a theoretical study of a novel material called silicene, the silicon equivalent of graphene. We have shown that electrically gated silicene can be used in nanoelectronics as a semiconductor with a tunable band gap. These results have been published in Physical Review B and the paper has received over 100 citations.
3) We have shown that hydrogenated silicene, called silicane, and its germanium equivalent germanane, are stable two-dimensional semiconductors, one with an indirect and one with a direct band gap, offering a good range of nanoelectronics applications. We have also shown that the conduction band is largely influenced by second-nearest neighbour interaction. These results have been published in 2D Materials.
4) We have looked into the low-energy states of molybdenum disulphide and related transition metal dichalcogenides using a combination of first principles methods and have pointed out that the quantitatively correct description of spin-orbit splitting and trigonal warping of the valence and conduction bands are important for their potential applications as quantum dots and qubits. These results have been published in Physical Review B and Physical Review X, with the first having attracted 25 citations in the 10 months since publication.
5) We have shown that single-layer hexagonal gallium chalcogenides and indium chalcogenides are stable indirect semiconductors with a unique sombrero-shaped valence band which allows an engineering of Lifshitz transitions by hole-doping. We have pointed out that these materials exhibit very good optical absorption in the ultraviolet range, making them excellent materials for use in the detection of UV photons. In the case of gallium telluride we entered a collaboration with experimentalists to show that multilayer monoclinic gallium telluride can be used to build high-sensitivity photodetectors. These results have been published in ACS Nano and in 2 papers in Physical Review B.
6) We have used GW many-body theory to calculate the band structure of polyyne, the linear carbon chain. The calculations were fully converged and the resulting band gap was found to agree with measurements very well. The GW band structure was used to parametrise a tight-binding model to offer experimentalists a quantitative prediction for the attenuation coefficient. These results have been published in the Journal of Chemical Physics.
7) We have examined the spectral density of Raman scattering in graphene when emitting a pair of LA and LO phonons and found that the lineshape is asymmetric and largely determined by the kinematics of the fully-resonant two-phonon process involved. In addition we have compiled an overview of Raman spectroscopy in graphite and graphene, drawing attention to the practical significance of Raman spectroscopy for the study of multi-layers of graphene. These results have been published in Solid State Communications and physica status solidi b.