Stacking and Registry Effects in Layered Materials: The Case of Hexagonal Boron Nitride

The interlayer sliding energy landscape of hexagonal boron nitride (h-BN) is investigated via a van der Waals corrected density functional theory approach. It is found that the main role of the van der Waals forces is to "anchor" the layers at a fixed distance, whereas the electrostatic forces dictate the optimal stacking mode and the interlayer sliding energy. A nearly free-sliding path is identified, along which bandgap modulations of ~0.6 eV are obtained. We propose a simple geometrical model that quantifies the registry matching between the layers and captures the essence of the corrugated h-BN interlayer energy landscape. The simplicity of this phenomenological model opens the way to the modeling of complex layered structures, such as carbon and boron nitride nanotubes.Comment: 4 Pages, 3 Figure

Covalently Functionalized Nanotubes as Nanometer-Sized Probes in Chemistry and Biology

Carbon nanotubes combine a range of properties that make them well suited for use as probe tips in applications such as atomic force microscopy (AFM)1, 2, 3. Their high aspect ratio, for example, opens up the possibility of probing the deep crevices4 that occur in microelectronic circuits, and the small effective radius of nanotube tips significantly improves the lateral resolution beyond what can be achieved using commercial silicon tips5. Another characteristic feature of nanotubes is their ability to buckle elastically4, 6, which makes them very robust while limiting the maximum force that is applied to delicate organic and biological samples. Earlier investigations into the performance of nanotubes as scanning probe microscopy tips have focused on topographical imaging, but a potentially more significant issue is the question of whether nanotubes can be modified to create probes that can sense and manipulate matter at the molecular level7. Here we demonstrate that nanotube tips with the capability of chemical and biological discrimination can be created with acidic functionality and by coupling basic or hydrophobic functionalities or biomolecular probes to the carboxyl groups that are present at the open tip ends. We have used these modified nanotubes as AFM tips to titrate the acid and base groups, to image patterned samples based on molecular interactions, and to measure the binding force between single protein ligand pairs. As carboxyl groups are readily derivatized by a variety of reactions8, the preparation of a wide range of functionalized nanotube tips should be possible, thus creating molecular probes with potential applications in many areas of chemistry and biology.Chemistry and Chemical Biolog

Covalently Functionalized Nanotubes as Nanometer-Sized Probes in Chemistry and Biology

Carbon nanotubes combine a range of properties that make them well suited for use as probe tips in applications such as atomic force microscopy (AFM)1, 2, 3. Their high aspect ratio, for example, opens up the possibility of probing the deep crevices4 that occur in microelectronic circuits, and the small effective radius of nanotube tips significantly improves the lateral resolution beyond what can be achieved using commercial silicon tips5. Another characteristic feature of nanotubes is their ability to buckle elastically4, 6, which makes them very robust while limiting the maximum force that is applied to delicate organic and biological samples. Earlier investigations into the performance of nanotubes as scanning probe microscopy tips have focused on topographical imaging, but a potentially more significant issue is the question of whether nanotubes can be modified to create probes that can sense and manipulate matter at the molecular level7. Here we demonstrate that nanotube tips with the capability of chemical and biological discrimination can be created with acidic functionality and by coupling basic or hydrophobic functionalities or biomolecular probes to the carboxyl groups that are present at the open tip ends. We have used these modified nanotubes as AFM tips to titrate the acid and base groups, to image patterned samples based on molecular interactions, and to measure the binding force between single protein–ligand pairs. As carboxyl groups are readily derivatized by a variety of reactions8, the preparation of a wide range of functionalized nanotube tips should be possible, thus creating molecular probes with potential applications in many areas of chemistry and biology

Carbon Nanotube-Based Nonvolatile Random Access Memory for Molecular Computing

A concept for molecular electronics exploiting carbon nanotubes as both molecular device elements and molecular wires for reading and writing information was developed. Each device element is based on a suspended, crossed nanotube geometry that leads to bistable, electrostatically switchable ON/OFF states. The device elements are naturally addressable in large arrays by the carbon nanotube molecular wires making up the devices. These reversible, bistable device elements could be used to construct nonvolatile random access memory and logic function tables at an integration level approaching 1012 elements per square centimeter and an element operation frequency in excess of 100 gigahertz. The viability of this concept is demonstrated by detailed calculations and by the experimental realization of a reversible, bistable nanotube-based bit

Defect-Free Carbon Nanotube Coils

Carbon nanotubes are promising building blocks for various nanoelectronic components. A highly desirable geometry for such applications is a coil. However, coiled nanotube structures reported so far were inherently defective or had no free ends accessible for contacting. Here we demonstrate the spontaneous self-coiling of single-wall carbon nanotubes into defect-free coils of up to more than 70 turns with identical diameter and chirality, and free ends. We characterize the structure, formation mechanism, and electrical properties of these coils by different microscopies, molecular dynamics simulations, Raman spectroscopy, and electrical and magnetic measurements. The coils are highly conductive, as expected for defect-free carbon nanotubes, but adjacent nanotube segments in the coil are more highly coupled than in regular bundles of single-wall carbon nanotubes, owing to their perfect crystal momentum matching, which enables tunneling between the turns. Although this behavior does not yet enable the performance of these nanotube coils as inductive devices, it does point a clear path for their realization. Hence, this study represents a major step toward the production of many different nanotube coil devices, including inductors, electromagnets, transformers, and dynamos

Guided Growth of Horizontal GaN Nanowires on Spinel with Orientation-Controlled Morphologies

We report the guided growth of horizontal GaN nanowires (NWs) on spinel (MgAl₂O₄) substrates with three different orientations: (111), (100), and (110). The NWs form ordered arrays with distinct morphologies on the surface of the substrates, controlled by the interaction with the substrate. The geometry of the NWs matches the symmetry of the spinel surfaces: on MgAl₂O₄(111), (100), and (110) the NWs grow in six, four, and two directions, respectively. The epitaxial relations and morphologies of the NW–substrate interface were characterized by cross-sectional transmission electron microscopy. The substrate was found to be mobilized during the growth and either climb up or recede on/under one or two sides of the NW, depending on the substrate orientation. Possible reasons for the similarity and differences between the orientations of the NWs and thin GaN films grown on MgAl₂O₄ are proposed. These results demonstrate the generality and flexibility of the guided growth phenomenon in NWs and specifically show that MgAl₂O₄(111) could be a low-mismatch substrate for the growth of high-quality GaN layers and NWs

Strain relaxation mechanisms in ZnSe@ZnTe core-shell nanowires grown horizontally from a guided growth approach

Resumen del trabajo presentado al VIII International Congress on Analytical Nanoscience and Nanotechnology, celebrado en Barcelona (España) del 3 al 5 de julio de 2017.The organization of nanowires on surfaces remains a major obstacle towards their large scale integration into functional devices. In order to overcome these issues, aligned arrays of heterostructured horizontal planar core-shell ZnSe@ZnTe nanowires were grown exploiting the epitaxial relations with the substrate in a guided growth approach to form well organized assemblies. We exploit the directional control of the guided growth for the parallel production of multiple radial p-n heterojunctions. The formed arrays exhibit great optoelectronic properties, with dark currents below the detection limit and upon illumination a rectifying behavior with photovoltaic characteristics. By the use of atomic resolution (S)TEM together with Geometric Phase Analysis (GPA), a deep understanding of the strain fields on the different nanostructures can be obtained. In that framework, we perform a study of the relaxation mechanisms taking place in the structure and how are they affected by the core morphology and substrate orientation with the aim of being able to exploit the strain on them to optimize the electronic behavior of the nanostructures.Peer reviewe