In this thesis density functional theory and density functional perturbation theory are employed ... more In this thesis density functional theory and density functional perturbation theory are employed to study structural, electronic, and vibrational properties of sp materials, in particular boron, lithium, and aluminum. We develop a theory that describes the properties of the recently discovered boron nanotubes. Our theory is based on a structure model of a broad boron sheet, being a single quasiplanar layer of boron. Based on the properties of that boron sheet, we propose a new route to achieve control over the atomic structure of nanotubes during their synthesis. Our results show that structure control can be accomplished by nanotubes which are rolled up from sheets with anisotropic in-plane mechanical properties. We further study the high-pressure phase diagram of various bulk structures of boron. In particular, we investigate layered boron materials, which are a new family of hypothetical bulk phases which we regard as stacked arrangement of different broad boron sheets. These met...
The nanoscale periodic potentials introduced by moiré patterns in semiconducting van der Waals (v... more The nanoscale periodic potentials introduced by moiré patterns in semiconducting van der Waals (vdW) heterostructures provide a new platform for designing exciton superlattices. To realize these applications, a thorough understanding of the localization and delocalization of interlayer excitons in the moiré potentials is necessary. Here, we investigated interlayer exciton dynamics and transport modulated by the moiré potentials in WS 2-WSe 2 heterobilayers in time, space, and momentum domains using transient absorption microscopy combined with firstprinciples calculations. Experimental results verified the theoretical prediction of energetically favorable K-Q interlayer excitons and unraveled exciton-population dynamics that was controlled by the twist-angle-dependent energy difference between the K-Q and K-K excitons. Spatially-and temporally-resolved exciton-population imaging directly visualizes exciton localization by twist-angle-dependent moiré potentials of ~100 meV. Exciton transport deviates significantly from normal diffusion due to the interplay between the moiré potentials and strong many-body interactions, leading to exciton-density-and twist-angle-dependent diffusion length. These results have important implications for designing vdW heterostructures for exciton and spin transport as well as for quantum communication applications.
In van der Waals heterostructures of 2D transition‐metal dichalcogenides (2D TMDCs) electron and ... more In van der Waals heterostructures of 2D transition‐metal dichalcogenides (2D TMDCs) electron and hole states are spatially localized in different layers forming long‐lived interlayer excitons. Here, the influence of additional electron or hole layers on the electronic properties of a MoS2/WSe2 heterobilayer (HBL), which is a direct bandgap material, is investigated from first principles. Additional layers modify the interlayer hybridization, mostly affecting the quasiparticle energy and real‐space extend of hole states at the Γ and electron states at the Q valleys. For a sufficient number of additional layers, the band edges move from K to Q or Γ, respectively. Adding electron layers to the HBL leads to more delocalized K and Q states, while Γ states do not extend much beyond the HBL, even when more hole layers are added. These results suggest a simple and yet powerful way to tune band edges and the real‐space extent of the electron and hole wave functions in TMDC heterostructures, ...
ABSTRACT The zone-folding method is a widely used technique in computing the electronic structure... more ABSTRACT The zone-folding method is a widely used technique in computing the electronic structure of carbon nanotubes. In this paper, curvature effects of boron and carbon nanotubes of different diameters and chiralities are systematically quantified using the density-functional-based tight-binding method. Here, the curvature effect in a nanotube is defined as the difference between the one-dimensional band structure calculated from the tubular atomic structure and the band structure calculated from the related two-dimensional sheet with the zone-folding method. For each nanotube, we quantify this difference by calculating the standard deviation of the band energies σE and the maximal relative deviation between the derived ballistic currents δImax. For all considered nanotubes with diameters d>2 nm, the standard deviation σE is below 60 meV and decreases only slowly, whereas δImax is still as large as 8% and does not tend to zero for large d.
External fields are a powerful tool to probe optical excitations in a material. The linear energy... more External fields are a powerful tool to probe optical excitations in a material. The linear energy shift of an excitation in a magnetic field is quantified by its effective g-factor. Here we show how exciton g-factors and their sign can be determined by converged first principles calculations. We apply the method to monolayer excitons in semiconducting transition metal dichalcogenides and to interlayer excitons in MoSe 2 /WSe 2 heterobilayers and obtain good agreement with recent experimental data. The precision of our method allows to assign measured g-factors of optical peaks to specific transitions in the band structure and also to specific regions of the samples. This revealed the nature of various, previously measured interlayer exciton peaks. We further show that, due to specific optical selection rules, g-factors in van der Waals heterostructures are strongly spinand stacking-dependent. The calculation of orbital angular momenta requires the summation over hundreds of bands, indicating that for the considered two-dimensional materials the basis set size is a critical numerical issue. The presented approach can potentially be applied to a wide variety of semiconductors.
We report the experimental observation of radiative recombination from Rydberg excitons in a two-... more We report the experimental observation of radiative recombination from Rydberg excitons in a two-dimensional semiconductor, monolayer WSe2, encapsulated in hexagonal boron nitride. Excitonic emission up to the 4s excited state is directly observed in photoluminescence spectroscopy in an out-of-plane magnetic field up to 31 Tesla. We confirm the progressively larger exciton size for higher energy excited states through diamagnetic shift measurements. This also enables us to estimate the 1s exciton binding energy to be about 170 meV, which is significantly smaller than most previous reports. The Zeeman shift of the 1s to 3s states, from both luminescence and absorption measurements, exhibits a monotonic increase of-factor, reflecting nontrivial magnetic-dipole-moment differences between ground and excited exciton states. This systematic evolution of magnetic dipole moments is theoretically explained from the spreading of the Rydberg states in momentum space.
Correction for ‘Synthetic 2-D lead tin sulfide nanosheets with tuneable optoelectronic properties... more Correction for ‘Synthetic 2-D lead tin sulfide nanosheets with tuneable optoelectronic properties from a potentially scalable reaction pathway’ by Kane Norton et al., Chem. Sci., 2019, 10, 1035–1045.
Monolayers of transition-metal dichalcogenides feature exceptional optical properties that are do... more Monolayers of transition-metal dichalcogenides feature exceptional optical properties that are dominated by tightly bound electron-hole pairs, called excitons. Creating van der Waals heterostructures by deterministically stacking individual monolayers can tune various properties via the choice of materials 1 and the relative orientation of the layers 2,3. In these structures, a new type of exciton emerges where the electron and hole are spatially separated into different layers. These interlayer excitons 4-6 allow exploration of many-body quantum phenomena 7,8 and are ideally suited for valleytronic applications 9. A basic model of a fully spatially separated electron and hole stemming from the K valleys of the monolayer Brillouin zones is usually applied to describe such excitons. Here, we combine photoluminescence spectroscopy and first-principles calculations to expand the concept of interlayer excitons. We identify a partially charge-separated electron-hole pair in MoS 2 /WSe 2 heterostructures where the hole resides at the Γ point and the electron is located in a K valley. We control the emission energy of this new type of momentum-space indirect, yet strongly bound exciton by variation of the relative orientation of the layers. These findings represent a crucial step towards the understanding and control of excitonic effects in van der Waals heterostructures and devices. An optical micrograph of a representative MoS 2 /WSe 2 heterobilayer, which was fabricated by deterministic transfer and stacking 10 followed by an annealing procedure, is shown in Fig. 1a. All heterobilayer and isolated regions of the constituent monolayers were thoroughly studied by micro-photoluminescence spectroscopy, and typical spectra are shown in Fig. 1b. The monolayer regions display the well-known A exciton and trion peaks 11-14 near 1.65 and 1.9 eV for WSe 2 (green) and MoS 2 (blue), respectively. In the heterobilayer region the same two peaks are discernible, but they are slightly shifted in energy due to the modified dielectric environment 15,16. However, in addition, a new peak near 1.6 eV is observed, which is absent in the monolayer regions. We assign this peak to the interlayer exciton (ILE) 4,17. Now, we control the relative orientation of the transition-metal dichalcogenide (TMD) layers to reveal the k-space indirect nature of this ILE in MoS 2 /WSe 2 heterobilayers. The twist angle is measured with respect to the zigzag direction of each layer (green and
Thermolysis of molecular precursors followed by liquid phase exfoliation accesses 2-D IV–VI semic... more Thermolysis of molecular precursors followed by liquid phase exfoliation accesses 2-D IV–VI semiconductor nanomaterials.
Recent discoveries of supposedly pure α-tetragonal boron require to revisit its structure. The sy... more Recent discoveries of supposedly pure α-tetragonal boron require to revisit its structure. The system is also interesting with respect to a new type of geometrical frustration in elemental crystals, which was found in β-rhombohedral boron. Based on density functional theory calculations, the present study has resolved the structural and thermodynamic characteristics of pure α-tetragonal boron. Different from β-rhombohedral boron, the conditions for stable covalent bonding (a band gap and completely filled valence bands) are almost fulfilled at a composition B 52 with two 4c interstitial sites occupied. This indicates that the ground state of pure α-tetragonal boron is stoichiometric. However, the covalent condition is not perfectly fulfilled because non-bonding ingap states exist that cannot be eliminated. The half occupation of the 4c sites yields a macroscopic amount of residual entropy, which is as large as that of β-rhombohedral boron. Therefore, αtetragonal boron can be classified as an elemental crystal with geometrical frustration. Deviations from stoichiometry can occur only at finite temperatures. Thermodynamic considerations show that deviations δ from the stoichiometric composition (B 52+δ) are small and positive. For reported high-pressure syntheses conditions δ is predicted to be about 0.1 to 0.2. An important difference between pure and C-or N-containing α-tetragonal boron is found in the occupation of interstitial sites: the pure form prefers to occupy the 4c sites, whereas in C-or N-containing forms a mixture of 2a, 8h, and 8i sites are occupied. The present article provides relations of site occupation, δ values, and lattice parameters, which enable us to identify pure α-tetragonal and distinguish the pure form from other ones.
many-We report about a recent ab initio study of boron nanotubes (BNTs). The latter were first pr... more many-We report about a recent ab initio study of boron nanotubes (BNTs). The latter were first predicted by theory, and have recently been synthesized experimentally. To understand the basic properties of BNTs, we have derived a structure model for an extended boron sheet (i.e. a boron analogue of a single graphene sheet) as a possible structural precursor of boron nanotubes. This sheet has a puckered structure, high stiffness, and anisotropic bonding properties. Puckering turns out to be the key mechanism for stabilizing sp sigma bonds lying along the armchair direction of the sheet. The BNTs have puckered surfaces as well, and their chiral angles are defined in a range from 0 to 90 degree. We show that all ideal boron nanotubes are metallic, irrespective of their radii and chiral angles, and discuss the possible existence of helical currents in chiral BNTs. Furthermore we show that strain energies of BNTs depend on their radii AND on their chiral angles. This unique property could be the basis of a new structure control mechanism in nanotechnology that permits to make nanotubes of a specified type, only. Zigzag nanotubes for example seem to have very little strain energy, if any. Therefore they should be highly susceptible to structural collapses, and might not exist at all.
The catalyst-assisted nucleation and growth mechanisms for many kinds of nanowires and nanotubes ... more The catalyst-assisted nucleation and growth mechanisms for many kinds of nanowires and nanotubes are pretty well understood. At times, though, 1D nanostructures form without a catalyst and the argued growth modes have inconsistencies. One such example is the catalyst-free growth of aluminium borate nanowires. Here we develop an in-situ catalyst-free room temperature growth route for aluminium nanowires using the electron beam in a transmission electron microscope. We provide strong experimental evidence that supports a formation process that can be viewed as a phase transition in which the generation of free-volume induced by the electron beam irradiation enhances the atomic mobility within the precursor material. The enhanced atomic mobility and specific features of the crystal structure of Al5BO9 drive the atomic rearrangement that results in the large scale formation of highly crystalline aluminium borate nanowires. The whole formation process can be completed within fractions of...
Molybdenum disulfide bilayers with well-defined interlayer twist angle were constructed by stacki... more Molybdenum disulfide bilayers with well-defined interlayer twist angle were constructed by stacking single-crystal monolayers. Varying interlayer twist angle results in strong tuning of the indirect optical transition energy and second-harmonic generation and weak tuning of direct optical transition energies and Raman mode frequencies. Electronic structure calculations show the interlayer separation changes with twist due to repulsion between sulfur atoms, resulting in shifts of the indirect optical transition energies. These results show that interlayer alignment is a crucial variable in tailoring the properties of two-dimensional heterostructures.
A well-defined cluster containing 12 equivalent platinum atoms was prepared by ion exchange of an... more A well-defined cluster containing 12 equivalent platinum atoms was prepared by ion exchange of an NaY zeolite, followed by hydrogen reduction. It was characterized by electron paramagnetic resonance (EPR) spectroscopy, hyperfine sublevel correlation (HYSCORE), and theoretical calculations. Combing the results of the experiments with density functional calculations, the likely structure of this cluster is icosahedral Pt 13 H m , possibly with a low positive charge. The adsorbed H/D on the Pt cluster surface can be exchanged reversibly at room temperature. From H/D desorption experiments, an H 2 binding energy of 1.36 eV is derived, in reasonable agreement with the calculated value but clearly larger than that for a (111) Pt single-crystal surface, revealing a finite size effect. While the hydrogen-covered cluster should clearly be regarded as a molecule, it is conceivable that the cluster adopts metallic character upon hydrogen desorption. It is likely that up to m) 30 H atoms bind to this cluster with 12 surface atoms, which has important implications for the determination of the dispersion of small Pt catalyst particles by hydrogen chemisorption. Calculations as well as experiments give evidence of an interesting magnetic behavior with high-spin states playing a prominent role. There are strong indications that a reservoir of EPR silent but structurally similar clusters exists which can partly be converted to EPR visible species by H/D exchange or by gas adsorption.
The success of future nanotechnologies will strongly depend on our ability to control the structu... more The success of future nanotechnologies will strongly depend on our ability to control the structure of materials on the atomic scale. For carbon nanotubes it turns out that one of their structural parameters-the chirality-may not be controlled during synthesis. We explain the basic reason for this defect and show that novel classes of nanotubes like boron nanotubes, which are related to sheets with anisotropic in-plane mechanical properties, could actually overcome these problems. Our results further suggest that extended searches for nanomaterials similar to pure boron might allow for one of the simplest and most direct ways to achieve structural control within nanotechnology.
We present a model system that might serve as a blueprint for the controlled layout of graphene b... more We present a model system that might serve as a blueprint for the controlled layout of graphene based nanodevices. The systems consists of chains of B 7 clusters implanted in a graphene matrix, where the boron clusters are not directly connected. We show that the graphene matrix easily accepts these alternating boron chains, and that the implanted boron components may dramatically modify the electronic properties of graphene based nanomaterials. This suggests a functionalization of graphene nanomaterials, where the semiconducting properties might be supplemented by parts of the graphene matrix itself, but the basic wiring will be provided by alternating chains of implanted boron clusters that connect these areas.
changes to the manuscript text and/or graphics which may affect the content, and all legal discla... more changes to the manuscript text and/or graphics which may affect the content, and all legal disclaimers that apply to the journal pertain. In no event shall TUP be held responsible for errors or consequences arising from the use of any information contained in these "Just Accepted" manuscripts. To cite this manuscript please use its Digital Object Identifier (DOI®), which is identical for all formats of publication.
The development and commercialization of novel hybrid intelligent systems with rich functionaliti... more The development and commercialization of novel hybrid intelligent systems with rich functionalities, providing strong benefit in the area of health-care, and electronic applications are among the most crucial topics of the scientific community and industrial players. Nanowires can provide excellent building blocks for hybrid nanoelectronics, due to their efficient charge transport characteristics and
In this thesis density functional theory and density functional perturbation theory are employed ... more In this thesis density functional theory and density functional perturbation theory are employed to study structural, electronic, and vibrational properties of sp materials, in particular boron, lithium, and aluminum. We develop a theory that describes the properties of the recently discovered boron nanotubes. Our theory is based on a structure model of a broad boron sheet, being a single quasiplanar layer of boron. Based on the properties of that boron sheet, we propose a new route to achieve control over the atomic structure of nanotubes during their synthesis. Our results show that structure control can be accomplished by nanotubes which are rolled up from sheets with anisotropic in-plane mechanical properties. We further study the high-pressure phase diagram of various bulk structures of boron. In particular, we investigate layered boron materials, which are a new family of hypothetical bulk phases which we regard as stacked arrangement of different broad boron sheets. These met...
The nanoscale periodic potentials introduced by moiré patterns in semiconducting van der Waals (v... more The nanoscale periodic potentials introduced by moiré patterns in semiconducting van der Waals (vdW) heterostructures provide a new platform for designing exciton superlattices. To realize these applications, a thorough understanding of the localization and delocalization of interlayer excitons in the moiré potentials is necessary. Here, we investigated interlayer exciton dynamics and transport modulated by the moiré potentials in WS 2-WSe 2 heterobilayers in time, space, and momentum domains using transient absorption microscopy combined with firstprinciples calculations. Experimental results verified the theoretical prediction of energetically favorable K-Q interlayer excitons and unraveled exciton-population dynamics that was controlled by the twist-angle-dependent energy difference between the K-Q and K-K excitons. Spatially-and temporally-resolved exciton-population imaging directly visualizes exciton localization by twist-angle-dependent moiré potentials of ~100 meV. Exciton transport deviates significantly from normal diffusion due to the interplay between the moiré potentials and strong many-body interactions, leading to exciton-density-and twist-angle-dependent diffusion length. These results have important implications for designing vdW heterostructures for exciton and spin transport as well as for quantum communication applications.
In van der Waals heterostructures of 2D transition‐metal dichalcogenides (2D TMDCs) electron and ... more In van der Waals heterostructures of 2D transition‐metal dichalcogenides (2D TMDCs) electron and hole states are spatially localized in different layers forming long‐lived interlayer excitons. Here, the influence of additional electron or hole layers on the electronic properties of a MoS2/WSe2 heterobilayer (HBL), which is a direct bandgap material, is investigated from first principles. Additional layers modify the interlayer hybridization, mostly affecting the quasiparticle energy and real‐space extend of hole states at the Γ and electron states at the Q valleys. For a sufficient number of additional layers, the band edges move from K to Q or Γ, respectively. Adding electron layers to the HBL leads to more delocalized K and Q states, while Γ states do not extend much beyond the HBL, even when more hole layers are added. These results suggest a simple and yet powerful way to tune band edges and the real‐space extent of the electron and hole wave functions in TMDC heterostructures, ...
ABSTRACT The zone-folding method is a widely used technique in computing the electronic structure... more ABSTRACT The zone-folding method is a widely used technique in computing the electronic structure of carbon nanotubes. In this paper, curvature effects of boron and carbon nanotubes of different diameters and chiralities are systematically quantified using the density-functional-based tight-binding method. Here, the curvature effect in a nanotube is defined as the difference between the one-dimensional band structure calculated from the tubular atomic structure and the band structure calculated from the related two-dimensional sheet with the zone-folding method. For each nanotube, we quantify this difference by calculating the standard deviation of the band energies σE and the maximal relative deviation between the derived ballistic currents δImax. For all considered nanotubes with diameters d>2 nm, the standard deviation σE is below 60 meV and decreases only slowly, whereas δImax is still as large as 8% and does not tend to zero for large d.
External fields are a powerful tool to probe optical excitations in a material. The linear energy... more External fields are a powerful tool to probe optical excitations in a material. The linear energy shift of an excitation in a magnetic field is quantified by its effective g-factor. Here we show how exciton g-factors and their sign can be determined by converged first principles calculations. We apply the method to monolayer excitons in semiconducting transition metal dichalcogenides and to interlayer excitons in MoSe 2 /WSe 2 heterobilayers and obtain good agreement with recent experimental data. The precision of our method allows to assign measured g-factors of optical peaks to specific transitions in the band structure and also to specific regions of the samples. This revealed the nature of various, previously measured interlayer exciton peaks. We further show that, due to specific optical selection rules, g-factors in van der Waals heterostructures are strongly spinand stacking-dependent. The calculation of orbital angular momenta requires the summation over hundreds of bands, indicating that for the considered two-dimensional materials the basis set size is a critical numerical issue. The presented approach can potentially be applied to a wide variety of semiconductors.
We report the experimental observation of radiative recombination from Rydberg excitons in a two-... more We report the experimental observation of radiative recombination from Rydberg excitons in a two-dimensional semiconductor, monolayer WSe2, encapsulated in hexagonal boron nitride. Excitonic emission up to the 4s excited state is directly observed in photoluminescence spectroscopy in an out-of-plane magnetic field up to 31 Tesla. We confirm the progressively larger exciton size for higher energy excited states through diamagnetic shift measurements. This also enables us to estimate the 1s exciton binding energy to be about 170 meV, which is significantly smaller than most previous reports. The Zeeman shift of the 1s to 3s states, from both luminescence and absorption measurements, exhibits a monotonic increase of-factor, reflecting nontrivial magnetic-dipole-moment differences between ground and excited exciton states. This systematic evolution of magnetic dipole moments is theoretically explained from the spreading of the Rydberg states in momentum space.
Correction for ‘Synthetic 2-D lead tin sulfide nanosheets with tuneable optoelectronic properties... more Correction for ‘Synthetic 2-D lead tin sulfide nanosheets with tuneable optoelectronic properties from a potentially scalable reaction pathway’ by Kane Norton et al., Chem. Sci., 2019, 10, 1035–1045.
Monolayers of transition-metal dichalcogenides feature exceptional optical properties that are do... more Monolayers of transition-metal dichalcogenides feature exceptional optical properties that are dominated by tightly bound electron-hole pairs, called excitons. Creating van der Waals heterostructures by deterministically stacking individual monolayers can tune various properties via the choice of materials 1 and the relative orientation of the layers 2,3. In these structures, a new type of exciton emerges where the electron and hole are spatially separated into different layers. These interlayer excitons 4-6 allow exploration of many-body quantum phenomena 7,8 and are ideally suited for valleytronic applications 9. A basic model of a fully spatially separated electron and hole stemming from the K valleys of the monolayer Brillouin zones is usually applied to describe such excitons. Here, we combine photoluminescence spectroscopy and first-principles calculations to expand the concept of interlayer excitons. We identify a partially charge-separated electron-hole pair in MoS 2 /WSe 2 heterostructures where the hole resides at the Γ point and the electron is located in a K valley. We control the emission energy of this new type of momentum-space indirect, yet strongly bound exciton by variation of the relative orientation of the layers. These findings represent a crucial step towards the understanding and control of excitonic effects in van der Waals heterostructures and devices. An optical micrograph of a representative MoS 2 /WSe 2 heterobilayer, which was fabricated by deterministic transfer and stacking 10 followed by an annealing procedure, is shown in Fig. 1a. All heterobilayer and isolated regions of the constituent monolayers were thoroughly studied by micro-photoluminescence spectroscopy, and typical spectra are shown in Fig. 1b. The monolayer regions display the well-known A exciton and trion peaks 11-14 near 1.65 and 1.9 eV for WSe 2 (green) and MoS 2 (blue), respectively. In the heterobilayer region the same two peaks are discernible, but they are slightly shifted in energy due to the modified dielectric environment 15,16. However, in addition, a new peak near 1.6 eV is observed, which is absent in the monolayer regions. We assign this peak to the interlayer exciton (ILE) 4,17. Now, we control the relative orientation of the transition-metal dichalcogenide (TMD) layers to reveal the k-space indirect nature of this ILE in MoS 2 /WSe 2 heterobilayers. The twist angle is measured with respect to the zigzag direction of each layer (green and
Thermolysis of molecular precursors followed by liquid phase exfoliation accesses 2-D IV–VI semic... more Thermolysis of molecular precursors followed by liquid phase exfoliation accesses 2-D IV–VI semiconductor nanomaterials.
Recent discoveries of supposedly pure α-tetragonal boron require to revisit its structure. The sy... more Recent discoveries of supposedly pure α-tetragonal boron require to revisit its structure. The system is also interesting with respect to a new type of geometrical frustration in elemental crystals, which was found in β-rhombohedral boron. Based on density functional theory calculations, the present study has resolved the structural and thermodynamic characteristics of pure α-tetragonal boron. Different from β-rhombohedral boron, the conditions for stable covalent bonding (a band gap and completely filled valence bands) are almost fulfilled at a composition B 52 with two 4c interstitial sites occupied. This indicates that the ground state of pure α-tetragonal boron is stoichiometric. However, the covalent condition is not perfectly fulfilled because non-bonding ingap states exist that cannot be eliminated. The half occupation of the 4c sites yields a macroscopic amount of residual entropy, which is as large as that of β-rhombohedral boron. Therefore, αtetragonal boron can be classified as an elemental crystal with geometrical frustration. Deviations from stoichiometry can occur only at finite temperatures. Thermodynamic considerations show that deviations δ from the stoichiometric composition (B 52+δ) are small and positive. For reported high-pressure syntheses conditions δ is predicted to be about 0.1 to 0.2. An important difference between pure and C-or N-containing α-tetragonal boron is found in the occupation of interstitial sites: the pure form prefers to occupy the 4c sites, whereas in C-or N-containing forms a mixture of 2a, 8h, and 8i sites are occupied. The present article provides relations of site occupation, δ values, and lattice parameters, which enable us to identify pure α-tetragonal and distinguish the pure form from other ones.
many-We report about a recent ab initio study of boron nanotubes (BNTs). The latter were first pr... more many-We report about a recent ab initio study of boron nanotubes (BNTs). The latter were first predicted by theory, and have recently been synthesized experimentally. To understand the basic properties of BNTs, we have derived a structure model for an extended boron sheet (i.e. a boron analogue of a single graphene sheet) as a possible structural precursor of boron nanotubes. This sheet has a puckered structure, high stiffness, and anisotropic bonding properties. Puckering turns out to be the key mechanism for stabilizing sp sigma bonds lying along the armchair direction of the sheet. The BNTs have puckered surfaces as well, and their chiral angles are defined in a range from 0 to 90 degree. We show that all ideal boron nanotubes are metallic, irrespective of their radii and chiral angles, and discuss the possible existence of helical currents in chiral BNTs. Furthermore we show that strain energies of BNTs depend on their radii AND on their chiral angles. This unique property could be the basis of a new structure control mechanism in nanotechnology that permits to make nanotubes of a specified type, only. Zigzag nanotubes for example seem to have very little strain energy, if any. Therefore they should be highly susceptible to structural collapses, and might not exist at all.
The catalyst-assisted nucleation and growth mechanisms for many kinds of nanowires and nanotubes ... more The catalyst-assisted nucleation and growth mechanisms for many kinds of nanowires and nanotubes are pretty well understood. At times, though, 1D nanostructures form without a catalyst and the argued growth modes have inconsistencies. One such example is the catalyst-free growth of aluminium borate nanowires. Here we develop an in-situ catalyst-free room temperature growth route for aluminium nanowires using the electron beam in a transmission electron microscope. We provide strong experimental evidence that supports a formation process that can be viewed as a phase transition in which the generation of free-volume induced by the electron beam irradiation enhances the atomic mobility within the precursor material. The enhanced atomic mobility and specific features of the crystal structure of Al5BO9 drive the atomic rearrangement that results in the large scale formation of highly crystalline aluminium borate nanowires. The whole formation process can be completed within fractions of...
Molybdenum disulfide bilayers with well-defined interlayer twist angle were constructed by stacki... more Molybdenum disulfide bilayers with well-defined interlayer twist angle were constructed by stacking single-crystal monolayers. Varying interlayer twist angle results in strong tuning of the indirect optical transition energy and second-harmonic generation and weak tuning of direct optical transition energies and Raman mode frequencies. Electronic structure calculations show the interlayer separation changes with twist due to repulsion between sulfur atoms, resulting in shifts of the indirect optical transition energies. These results show that interlayer alignment is a crucial variable in tailoring the properties of two-dimensional heterostructures.
A well-defined cluster containing 12 equivalent platinum atoms was prepared by ion exchange of an... more A well-defined cluster containing 12 equivalent platinum atoms was prepared by ion exchange of an NaY zeolite, followed by hydrogen reduction. It was characterized by electron paramagnetic resonance (EPR) spectroscopy, hyperfine sublevel correlation (HYSCORE), and theoretical calculations. Combing the results of the experiments with density functional calculations, the likely structure of this cluster is icosahedral Pt 13 H m , possibly with a low positive charge. The adsorbed H/D on the Pt cluster surface can be exchanged reversibly at room temperature. From H/D desorption experiments, an H 2 binding energy of 1.36 eV is derived, in reasonable agreement with the calculated value but clearly larger than that for a (111) Pt single-crystal surface, revealing a finite size effect. While the hydrogen-covered cluster should clearly be regarded as a molecule, it is conceivable that the cluster adopts metallic character upon hydrogen desorption. It is likely that up to m) 30 H atoms bind to this cluster with 12 surface atoms, which has important implications for the determination of the dispersion of small Pt catalyst particles by hydrogen chemisorption. Calculations as well as experiments give evidence of an interesting magnetic behavior with high-spin states playing a prominent role. There are strong indications that a reservoir of EPR silent but structurally similar clusters exists which can partly be converted to EPR visible species by H/D exchange or by gas adsorption.
The success of future nanotechnologies will strongly depend on our ability to control the structu... more The success of future nanotechnologies will strongly depend on our ability to control the structure of materials on the atomic scale. For carbon nanotubes it turns out that one of their structural parameters-the chirality-may not be controlled during synthesis. We explain the basic reason for this defect and show that novel classes of nanotubes like boron nanotubes, which are related to sheets with anisotropic in-plane mechanical properties, could actually overcome these problems. Our results further suggest that extended searches for nanomaterials similar to pure boron might allow for one of the simplest and most direct ways to achieve structural control within nanotechnology.
We present a model system that might serve as a blueprint for the controlled layout of graphene b... more We present a model system that might serve as a blueprint for the controlled layout of graphene based nanodevices. The systems consists of chains of B 7 clusters implanted in a graphene matrix, where the boron clusters are not directly connected. We show that the graphene matrix easily accepts these alternating boron chains, and that the implanted boron components may dramatically modify the electronic properties of graphene based nanomaterials. This suggests a functionalization of graphene nanomaterials, where the semiconducting properties might be supplemented by parts of the graphene matrix itself, but the basic wiring will be provided by alternating chains of implanted boron clusters that connect these areas.
changes to the manuscript text and/or graphics which may affect the content, and all legal discla... more changes to the manuscript text and/or graphics which may affect the content, and all legal disclaimers that apply to the journal pertain. In no event shall TUP be held responsible for errors or consequences arising from the use of any information contained in these "Just Accepted" manuscripts. To cite this manuscript please use its Digital Object Identifier (DOI®), which is identical for all formats of publication.
The development and commercialization of novel hybrid intelligent systems with rich functionaliti... more The development and commercialization of novel hybrid intelligent systems with rich functionalities, providing strong benefit in the area of health-care, and electronic applications are among the most crucial topics of the scientific community and industrial players. Nanowires can provide excellent building blocks for hybrid nanoelectronics, due to their efficient charge transport characteristics and
Uploads
Papers by Jens Kunstmann