Direct observation of localized defect States in semiconductor nanotube junctions

Photoinduced Charge Transfer in Single-Molecule p-n Junctions

We measured photoinduced charge separation in isolated individual C₆₀-tethered 2,5-dithienylpyrrole triad (C₆₀ triad) molecules with submolecular resolution using a custom-built laser-assisted scanning tunneling microscope. Laser illumination was introduced evanescently into the tunneling junction through total internal reflection, and the changes in tunneling current and electronic spectra caused by photoexcitation were measured and spatially resolved. Photoinduced charge separation was not detected for all C₆₀ triad molecules, indicating that the conformations of the molecules may affect the excitation probability, lifetime, and/or charge distribution. A photoinduced signal was not observed for dodecanethiol molecules in the surrounding matrix or for control molecules without C₆₀ moieties, as neither absorbs incident photons at this energy. This spectroscopic imaging technique has the potential to elucidate detailed photoinduced carrier dynamics, which are inaccessible via ensemble-scale (i.e., averaging) measurements, which can be used to direct the rational design and optimization of molecular p-n junctions and assemblies for energy harvesting.

The formation and stability of junctions in single-wall carbon nanotubes

The structure and stability of molecular junctions, which connect two single-wall carbon nanotubes (SWCNTs) of different diameters and chiral angles, (n ₁, m ₁)-(n ₂, m ₂), are systematically investigated by density functional tight binding calculations. More than 100 junctions, which connect well-aligned SWCNTs, were constructed and calculated. For a highly stable junction between two chiral (n ₁, m ₁) and (n ₂, m ₂) SWCNTs with opposite handedness, the number of pentagon-heptagon (5/7) pairs required to build the junction can be denoted as ∣∣n ₂ - n ₁∣ - ∣m ₂ - m ₁∣∣+min{∣n ₂ - n ₁∣, ∣m ₂ - m ₁∣} with (n ₂, m ₂) rotating π/3 angle or not. While for a junction connected by two zigzag, armchair or two chiral SWCNTs with the same handedness, the number of 5/7 pairs is equal to ∣n ₁ - n ₂∣ + ∣m ₁ - m ₂∣. Similar to the formation energies of grain boundaries in graphene, the curve of the formation energies vs. chiral angle difference present an 'M' shape indicating the preference of ∼30 degree junctions. Moreover, the formation energies of the zigzag-type and armchair-type junctions with zero misorientation angles are largely sensitive to the diameter difference of two sub-SWCNTs.

A novel route to synthesize carbon spheres and carbon nanotubes from carbon dioxide in a molten carbonate electrolyzer

Carbon dioxide is readily converted into carbon spheres (CSs) and carbon nanotubes (CNTs) in a molten carbonate electrolyzer.

Carbon nanotubes: properties, synthesis, purification, and medical applications

Current discoveries of different forms of carbon nanostructures have motivated research on their applications in various fields. They hold promise for applications in medicine, gene, and drug delivery areas. Many different production methods for carbon nanotubes (CNTs) have been introduced; functionalization, filling, doping, and chemical modification have been achieved, and characterization, separation, and manipulation of individual CNTs are now possible. Parameters such as structure, surface area, surface charge, size distribution, surface chemistry, and agglomeration state as well as purity of the samples have considerable impact on the reactivity of carbon nanotubes. Otherwise, the strength and flexibility of carbon nanotubes make them of potential use in controlling other nanoscale structures, which suggests they will have a significant role in nanotechnology engineering.

Diffusion and drift of graphene flake on graphite surface

Diffusion and drift of a graphene flake on a graphite surface are analyzed. A potential energy relief of the graphene flake is computed using ab initio and empirical calculations. Based on the analysis of this relief, different mechanisms of diffusion and drift of the graphene flake on the graphite surface are considered. A new mechanism of diffusion and drift of the flake is proposed. According to the proposed mechanism, rotational transition of the flake from commensurate to incommensurate state takes place with subsequent simultaneous rotation and translational motion until a commensurate state is reached again, and so on. Analytic expressions for the diffusion coefficient and mobility of the flake corresponding to different mechanisms are derived in wide ranges of temperatures and sizes of the flake. The molecular dynamics simulations and estimates based on ab initio and empirical calculations demonstrate that the proposed mechanism can be dominant under certain conditions. The influence of structural defects on the diffusion of the flake is examined on the basis of calculations of the potential energy relief and molecular dynamics simulations. The methods of control over the diffusion and drift of graphene components in nanoelectromechanical systems are discussed. The possibility to experimentally determine the barriers to relative motion of graphene layers based on the study of diffusion of a graphene flake is considered. The results obtained can also be applied to polycyclic aromatic molecules on graphene and should be qualitatively valid for a set of commensurate adsorbate-adsorbent systems.

Local inhomogeneity in gate hysteresis of carbon nanotube field-effect transistors investigated by scanning gate microscopy

Local nature of gate hysteresis in a carbon nanotube field-effect transistor (CNFET) was studied using scanning gate microscopy (SGM). A sequential set of SGM images of the CNFET fabricated on a SiO(2)/Si substrate was obtained at a low temperature under an ultra-high vacuum. Comparisons of the SGM images obtained at decreasing and increasing gate voltage steps revealed that the order of appearance of SGM defects could not be accounted for by a uniform distribution of hysteretic gate screening along the carbon nanotube (CNT) channel. It was concluded that the gate hysteresis in the CNFET had substantial local variations along the CNT. The local inhomogeneity in gate hysteresis was attributed to inhomogeneous distribution of screening charge traps or sources on the SiO(2) surface.

Cesium-filled single wall carbon nanotubes as conducting nanowires: scanning tunneling spectroscopy study

Metal-filled single wall carbon nanotubes (SWCNTs) are examined for possible application to conducting wires in nanoelectronics architecture. The local electronic structure of SWCNTs partially filled with cesium atoms is studied with scanning tunneling spectroscopy. The conduction and valence bands are shifted downward with two localized states in the gap at the location where the Cs atoms are filled. From a first-principles calculation, we confirm that these two gap states are bound states originating from the two lowest conduction bands.

Raman spectroscopy and imaging of ultralong carbon nanotubes

Raman spectroscopy and confocal Raman imaging with 514 nm excitation was performed on recently developed ultralong carbon nanotubes grown by the "fast-heating" chemical vapor deposition (CVD) method. The ultralong nanotubes are found to consist of both semiconducting and metallic types, with spectra that are consistent with the nanotubes being single walled. Characterization of nanotube diameters shows that short nanotubes appearing near the sample catalyst region have a broader distribution than is observed for the ultralong nanotubes. The narrow diameter distribution is determined by uniformity of catalyst particle size and gives additional evidence for the proposed "kite" mechanism for long nanotube growth. Raman imaging was performed over large length scales (up to 140 microm). Imaging reveals the ultralong nanotubes to be of high quality, with a very low defect density. Variations in G-band frequencies and intensity demonstrate the occurrence of minor structural changes and variations in nanotube-substrate interaction along the length of the nanotubes. Evidence also demonstrates that larger structural changes resulting in a full chirality change can occur in these nanotube types to produce a metal-to-semiconductor intramolecular junction.

Electronic properties and reactivity of Pt-doped carbon nanotubes

The structures of the (5,5) single-walled carbon nanotube (SWCNT) segments with hemispheric carbon cages capped at the ends (SWCNT rod) and the Pt-doped SWCNT rods have been studied within density functional theory. Our theoretical studies find that the hemispheric cages introduce localized states on the caps. The cap-Pt-doped SWCNT rods can be utilized as sensors because of the sensitivity of the doped Pt atom. The Pt-doped SWCNT rods can also be used as catalysts, where the doped Pt atom serves as the enhanced and localized active center on the SWCNT. The adsorptions of C(2)H(4) and H(2) on the Pt atom in the Pt-doped SWCNT rods reveal different adsorption characteristics. The adsorption of C(2)H(4) on the Pt atom in all of the three Pt-doped SWCNT rods studied (cap-end-doped, cap-doped, and wall-doped) is physisorption with the strongest interaction occurring in the middle of the sidewall of the SWCNT. On the other hand, the adsorption of H(2) on the Pt atom at the sidewall of the SWCNT is chemisorption resulting in the decomposition of H(2), and the adsorption of H(2) at the hemispheric caps is physisorption.

Identifying and counting point defects in carbon nanotubes

The prevailing conception of carbon nanotubes and particularly single-walled carbon nanotubes (SWNTs) continues to be one of perfectly crystalline wires. Here, we demonstrate a selective electrochemical method that labels point defects and makes them easily visible for quantitative analysis. High-quality SWNTs are confirmed to contain one defect per 4 microm on average, with a distribution weighted towards areas of SWNT curvature. Although this defect density compares favourably to high-quality, silicon single-crystals, the presence of a single defect can have tremendous electronic effects in one-dimensional conductors such as SWNTs. We demonstrate a one-to-one correspondence between chemically active point defects and sites of local electronic sensitivity in SWNT circuits, confirming the expectation that individual defects may be critical to understanding and controlling variability, noise and chemical sensitivity in SWNT electronic devices. By varying the SWNT synthesis technique, we further show that the defect spacing can be varied over orders of magnitude. The ability to detect and analyse point defects, especially at very low concentrations, indicates the promise of this technique for quantitative process analysis, especially in nanoelectronics development.

Paired gap states in a semiconducting carbon nanotube: deep and shallow levels

Several paired, localized gap states were observed in semiconducting single-wall carbon nanotubes using spatially resolved scanning tunneling spectroscopy. A pair of gap states is found far from the band edges, forming deep levels, while the other pair is located near the band edges, forming shallow levels. With the help of a first-principles study, the former is explained by a vacancy-adatom complex while the latter is explained by a pentagon-heptagon structure. Our experimental observation indicates that the presence of the gap states provides a means to perform local band-gap engineering as well as doping without impurity substitution.

Raman spectral imaging of a carbon nanotube intramolecular junction

Confocal Raman spectral imaging results are presented for a metal-to-semiconductor intramolecular junction (IMJ) on an isolated carbon nanotube. Spectra observed at the junction are consistent with the symmetry lowering expected from the occurrence of pentagon-heptagon defects to produce the chirality shift. The IMJ transition zone is sharp and preserves the nanotube diameter. These results have significant implications for the future use of IMJs as electronic devices, including how prevalent these structures are and how their growth may be rationally targeted. Raman imaging has been demonstrated to be a powerful tool for IMJ studies and provides a more accessible method for further studies of IMJ structure and growth.

Identifying defects in nanoscale materials

We have developed a novel iterative experimental-theoretical technique which can identify the atomic structure of defects in many-atom nanoscale materials from scanning tunneling microscopy and spectroscopy data. A given model for a defect structure is iteratively improved until calculated microscopy and spectroscopy data based on the model converge on the experimental results. We use the technique to identify a defect responsible for the electronic properties of a carbon nanotube intramolecular junction. Our technique can be extended for analysis of defect structures in nanoscale materials in general.

Scanning tunnelling microscopy of carbon nanotubes

This paper briefly reviews how scanning tunnelling microscopy (STM) and spectroscopy (STS) are used to analyse the atomic structure and the electronic properties of individual single-wall carbon nanotubes. In this area, the progress accomplished over the past several years has been spectacular. As this paper demonstrates, all the effects predicted by theory have been verified experimentally. Geometrical and electronic effects specific to carbon nanotubes are illustrated by analysing a series of STM images and STS spectra computed using a tight-binding theory. The simulations include a catalogue of images of 27 single-wall nanotubes, Stone-Wales defects in semiconducting nanotubes, and a symmetric Y-junction.

Phonon spectromicroscopy of carbon nanostructures with atomic resolution

The vibrational properties of single-wall carbon nanotubes have been probed locally with atomic-scale resolution by inelastic electron tunneling spectroscopy with a low-temperature scanning tunneling microscope. The high spatial resolution has allowed the unraveling of changes in the local phonon spectrum related to topological defects. We demonstrated that the radial breathing mode is suppressed within tube segments of lengths below approximately 3 nm, and that in the cap region phonon modes characteristic of the fullerene hemisphere are emerging. Phonon spectromicroscopy should lead to a better understanding of the mechanisms that limit the transport of heat or electrical charge inside nanostructured carbon materials.

CoNTub: An Algorithm for Connecting Two Arbitrary Carbon Nanotubes

We have developed the first computer program for determining the coordinates of heterojunctions between two arbitrary carbon nanotubes. This software implements the topological algebra based on the concept of strip, a continuous subset of carbon rings containing all the topological defects (nonhexagonal carbon rings). The user easily generates any heterojunction by merely introducing the indices (i,j) and length of the two nanotubes to be connected. The resulting structure is immediately visualized and can be exported in the protein-data-bank (PDB) format. Two classes of heterojunctions are distinguished depending on whether a cone between the connected nanotubes is required. This method is applicable to all kinds of two nanotube heterojunctions, including Dunlap's knees and others related. In addition, this program also generates single- and multiwalled carbon nanotubes (SWNT and MWNT). This application has been implemented as a Java applet, and it is freely available at the following web address: http://www.ugr.es/local/gmdm/java/contub/contub.html

Unveiling metal-cage hybrid states in a single endohedral metallofullerene

The local structural and electronic properties of individual metallofullerenes are studied using scanning tunneling microscopy, scanning tunneling spectroscopy, and theoretical simulations. The energy-resolved metal-cage hybrid states of a single endohedral metallofullerene Dy@C82 isomer I have been spatially mapped, supporting a complex picture consisting of the orbital hybridization and charge transfer for the interaction between the cage and the metal atom. The relative position of the encapsulated Dy atom inside the cage and the molecular orientation on the surface have been inferred by comparing the experimental results with theoretical simulations. The combined technique provides promising applications in the fields of in situ characterization and diagnostics of metallofullerene-based nanodevices.