G. Andrew D. Briggs

Structural transformations in graphene studied with high spatial and temporal resolution.

Graphene has remarkable electronic properties, such as ballistic transport and quantum Hall effects, and has also been used as a support for samples in high-resolution transmission electron microscopy and as a transparent electrode in photovoltaic devices. There is now a demand for techniques that can manipulate the structural and physical properties of graphene, in conjunction with the facility to monitor the changes in situ with atomic precision. Here, we show that irradiation with an 80 kV electron beam can selectively remove monolayers in few-layer graphene sheets by means of electron-beam-induced sputtering. Aberration-corrected, low-voltage, high-resolution transmission electron microscopy with sub-ångström resolution is used to examine the structural reconstruction occurring at the single atomic level. We find preferential termination for graphene layers along the zigzag orientation for large hole sizes. The temporal resolution can also be reduced to 80 ms, enabling real-time observation of the reconstruction of carbon atoms during the sputtering process. We also report electron-beam-induced rapid displacement of monolayers, fast elastic distortions and flexible bending at the edges of graphene sheets. These results reveal how energy transfer from the electron beam to few-layer graphene sheets leads to unique structural transformations.

Direct imaging of rotational stacking faults in few layer graphene.

Few layer graphene nanostructures are directly imaged using aberration corrected high-resolution transmission electron microscopy with an electron accelerating voltage of 80 kV. We observe rotational stacking faults in the HRTEM images of 2-6 layers of graphene sheets, giving rise to Moir patterns. By filtering in the frequency domain using a Fourier transform, we reconstruct the graphene lattice of each sheet and determine the packing structure and relative orientations of up to six separate sets. Direct evidence is obtained for few layer graphene sheets with packing that is different to the standard AB Bernal packing of bulk graphite. This has implications toward bilayer and few layer graphene electronic devices and the determination of their intrinsic structure.

InGaN quantum dots grown by metalorganic vapor phase epitaxy employing a post-growth nitrogen anneal

We describe the growth of InGaN quantum dots (QDs) by metalorganic vapor phase epitaxy. A thin InGaN epilayer is grown on a GaN buffer layer and then annealed at the growth temperature in molecular nitrogen inducing quantum dot formation. Microphotoluminescence studies of these QDs reveal sharp peaks with typical linewidths of ∼700 μeV at 4.2 K, the linewidth being limited by the spectral resolution. Time-resolved photoluminescence suggests that the excitons in these structures have lifetimes in excess of 2 ns at 4.2 K.

A Cyclic Porphyrin Trimer as a Receptor for Fullerenes

A cyclic porphyrin trimer has been synthesized which has a high affinity for fullerenes. It forms 1:1 complexes with C(60) and C(70) with association constants of 2 x 10(6) and 2 x 10(8) M(-1), respectively, in toluene. Its affinities for C(86) and La@C(82) are too strong to measure by fluorescence titration. The solvent dependence of the association constants shows that solvation of both the guest and the host influence the binding strength.

Growth modes in heteroepitaxy of InGaN on GaN

The morphology of InGaN epilayers grown by metal-organic vapor phase epitaxy on GaN pseudosubstrates has been examined by atomic force microscopy. The composition of the epilayers has been measured using a combination of secondary ion mass spectrometry and x-ray photoelectron spectroscopy. The dependence of the growth mode on the growth conditions has been investigated. At the lowest temperatures and NH3 fluxes, a two-dimensional island nucleation growth mode is described, in which flat islands form stacks which align along underlying GaN terraces. As the growth temperature is increased a transition to a step-flow growth mode is observed. A transition from two-dimensional island nucleation to step-flow growth may also be achieved by increasing the NH3 flux, or by decreasing the trimethylindium flux. Each transition is discussed in terms of both surface kinetics and indium incorporation into the growing film. A transition from two-dimensional to three-dimensional growth may be induced by an increase in the...

Optical Schemes for Quantum Computation in Quantum Dot Molecules

We give three methods for entangling quantum states in quantum dots. We do this by showing how to tailor the resonant energy (Forster-Dexter) transfer mechanisms and the biexciton binding energy in a quantum dot molecule. We calculate the magnitude of these two electrostatic interactions as a function of dot size, interdot separation, material composition, confinement potential, and applied electric field by using an envelope function approximation in a two-cuboid dot molecule. In the first implementation, we show that it is desirable to suppress the Forster coupling and to create entanglement by using the biexciton energy alone. We show how to perform universal quantum logic in a second implementation which uses the biexciton energy together with appropriately tuned laser pulses: by selecting appropriate material parameters high-fidelity logic can be achieved. The third implementation proposes generating quantum entanglement by switching the Forster interaction itself. We show that the energy transfer can be fast enough in certain dot structures that switching can occur on a time scale which is much less than the typical decoherence times.

Electron spin relaxation of N@C60 in CS2

We examine the temperature dependence of the electron spin relaxation times of the molecules N@C60 and N@C70 (which comprise atomic nitrogen trapped within a carbon cage) in liquid CS2 solution. The results are inconsistent with the fluctuating zero-field splitting (ZFS) mechanism, which is commonly invoked to explain electron spin relaxation for S> or =1 spins in liquid solution, and is the mechanism postulated in the literature for these systems. Instead, we find an Arrhenius temperature dependence for N@C60 , indicating the spin relaxation is driven primarily by an Orbach process. For the asymmetric N@C70 molecule, which has a permanent ZFS, we resolve an additional relaxation mechanism caused by the rapid reorientation of its ZFS. We also report the longest coherence time (T2) ever observed for a molecular electron spin, being 0.25 ms at 170 K.

Investigating the Diameter-Dependent Stability of Single-Walled Carbon Nanotubes

We investigate the long-standing question of whether electrons accelerated at 80 kV are below the knock-on damage threshold for single-walled carbon nanotubes (SWNTs). Aberration-corrected high-resolution transmission electron microscopy is used to directly image the atomic structure of the SWNTs and provides in situ monitoring of the structural modification induced by electron beam irradiation at 80 kV. We find that SWNTs with small diameters of 1 nm are damaged by the electron beam, and defects are produced in the side walls that can lead to their destruction. SWNTs with diameters of 1.3 nm and larger are more stable against degradation, and stability increases with diameter. The effect of diameter, defects, and exterior contamination on the inherent stability of SWNTs under electron beam irradiation is investigated.

Low temperature assembly of fullerene arrays in single-walled carbon nanotubes using supercritical fluids

Molecules assembled inside nanotubes to form 1D arrays exhibit functional properties different to the bulk crystal and have been proposed for many applications ranging from catalysis to quantum computing. We have discovered that single-walled carbon nanotubes can be efficiently filled with fullerenes in supercritical fluids at temperatures as low as 30–50 °C. Despite the low solubility of fullerenes in supercritical fluids, the nanotube filling was particularly effective in supercritical carbon dioxide producing Cn@SWNT structures in 70% yield at 50 °C. This method was also applied for functionalized and endohedral fullerenes and allows insertion of thermally unstable molecules which would be impossible to insert in nanotubes using standard techniques. We discuss the advantages of using supercritical fluids as compared to conventional solvents and propose mechanisms for fullerene encapsulation at low temperatures.

Conductance enlargement in picoscale electroburnt graphene nanojunctions

Significance Continuation of Moore’s law to the sub–10-nm scale requires the development of new technologies for creating electrode nanogaps, in architectures which allow a third electrostatic gate. Electroburnt graphene junctions (EGNs) have the potential to fulfill this need, provided their properties at the moment of gap formation can be understood and controlled. In contrast with mechanically controlled break junctions, whose conductance decreases monotonically as the junction approaches rupture, we show that EGNs exhibit a surprising conductance enlargement just before breaking, which signals the formation of a picoscale current path formed from a single sp2 bond. Just as Schottky barriers are a common feature of semiconductor interfaces, conductance enlargement is a common property of EGNs and will be unavoidably encountered by all research groups working on the development of this new technology. Provided the electrical properties of electroburnt graphene junctions can be understood and controlled, they have the potential to underpin the development of a wide range of future sub-10-nm electrical devices. We examine both theoretically and experimentally the electrical conductance of electroburnt graphene junctions at the last stages of nanogap formation. We account for the appearance of a counterintuitive increase in electrical conductance just before the gap forms. This is a manifestation of room-temperature quantum interference and arises from a combination of the semimetallic band structure of graphene and a cross-over from electrodes with multiple-path connectivity to single-path connectivity just before breaking. Therefore, our results suggest that conductance enlargement before junction rupture is a signal of the formation of electroburnt junctions, with a picoscale current path formed from a single sp2 bond.