Sarah A. Burke

Strain-induced Pseudo-Magnetic Fields Greater Than 300 Tesla in Graphene Nanobubbles

Straining Graphenes Electronic States The conduction electrons in graphene, single sheets of graphite, can have very high mobilities. Under the influence of an applied magnetic field, a series of energy steps, or Landau levels, can be observed that correspond to the conduction electrons traveling in cyclotron orbits. Recent theoretical work has indicated that if graphene layers are strained, the strain field creates a pseudomagnetic field that should also lead to observable Landau levels. Levy et al. (p. 544) used scanning tunneling microscopy to probe the energy levels of graphene grown on a platinum surface, which forms highly strained “nanobubbles.” The strain is equivalent to applying very high magnetic fields (in excess of 300 tesla). Thus, the electronic properties of graphene can indeed be modified using applied strain. Strain creates energy levels in graphene that are similar to those seen in very high applied magnetic fields. Recent theoretical proposals suggest that strain can be used to engineer graphene electronic states through the creation of a pseudo–magnetic field. This effect is unique to graphene because of its massless Dirac fermion-like band structure and particular lattice symmetry (C3v). Here, we present experimental spectroscopic measurements by scanning tunneling microscopy of highly strained nanobubbles that form when graphene is grown on a platinum (111) surface. The nanobubbles exhibit Landau levels that form in the presence of strain-induced pseudo–magnetic fields greater than 300 tesla. This demonstration of enormous pseudo–magnetic fields opens the door to both the study of charge carriers in previously inaccessible high magnetic field regimes and deliberate mechanical control over electronic structure in graphene or so-called “strain engineering.”

Atomic force microscopy and theoretical considerations of surface properties and turgor pressures of bacteria

The properties of viable bacteria were investigated with the Atomic Force Microscope (AFM). By depositing bacteria on aluminum oxide filters, the adhesion of Si3N4 tips to the surfaces of Gram-negative bacterial strains possessing different lipopolysacharides (LPS) (i.e. Pseudomonas aeruginosa PAO1 and its isogenic mutants) was investigated without the use of surface modifying or bonding agents to adhere cells to the filter. Our measurements suggest that adhesion forces for Si3N4 to these bacteria were below our detection limit of 50–100 pN. Turgor pressures were also investigated for a spherical Gram-positive bacterium (Enterococcus hirae) as well as the rod-shaped Gram-negative P. aeruginosa. A simple relationship between bacterial indentation depth and turgor pressure for the spherical bacterium was first derived and gave a turgor pressure for E. hirae in deionized water of 4–6×10 5 Pa. This is the first such measurement for a spherical Gram-positive bacterium. AFM deformations of the cell envelope of P. aeruginosa gave turgor pressures in the range 0.1–0.2 × 10 5 Pa in growth medium and 1.5–4 × 10 5 Pa in distilled water. These pressure ranges compared well with previously published values derived by other means for Gram-negative rods. The imaging of bacteria under growth medium was only possible on aluminum-oxide filters. It is proposed that the 20 nm diameter pores of these filters might facilitate the attachment of bacteria. A Monte-Carlo study was carried out which showed that bacterial adhesion will be both encouraged and stronger if hydrogen bonding takes place between LPS O-sidechains and the inside surface of the filter’s pores.

Scanning tunneling spectroscopy of superconducting LiFeAs single crystals: evidence for two nodeless energy gaps and coupling to a bosonic mode.

The superconducting compound LiFeAs is studied by scanning tunneling microscopy and spectroscopy. A gap map of the unreconstructed surface indicates a high degree of homogeneity in this system. Spectra at 2 K show two nodeless superconducting gaps with Δ(1)=5.3±0.1 meV and Δ(2)=2.5±0.2 meV. The gaps close as the temperature is increased to the bulk T(c), indicating that the surface accurately represents the bulk. A dip-hump structure is observed below T(c) with an energy scale consistent with a magnetic resonance recently reported by inelastic neutron scattering.

Determination of the local contact potential difference of PTCDA on NaCl: a comparison of techniques

There has been increasing focus on the use of Kelvin probe force microscopy (KPFM) for the determination of local electronic structure in recent years, especially in systems where other methods, such as scanning tunnelling microscopy/spectroscopy, may be intractable. We have examined three methods for determining the local apparent contact potential difference (CPD): frequency modulation KPFM (FM-KPFM), amplitude modulation KPFM (AM-KPFM), and frequency shift-bias spectroscopy, on a test system of 3,4,9,10-perylene tetracarboxylic dianhydride (PTCDA) on NaCl, an example of an organic semiconductor on a bulk insulating substrate. We will discuss the influence of the bias modulation on the apparent CPD measurement by FM-KPFM compared to the DC-bias spectroscopy method, and provide a comparison of AM-KPFM, AM-slope detection KPFM and FM-KPFM imaging resolution and accuracy. We will also discuss the distance dependence of the CPD as measured by FM-KPFM for both the PTCDA organic deposit and the NaCl substrate.

Templated growth of 3,4,9,10-perylenetetracarboxylic dianhydride molecules on a nanostructured insulator

Nanometre-scale 3,4,9,10-perylenetetracarboxylic dianhydride (PTCDA) crystallites were produced by trapping the molecules inside monolayer-deep rectangular pits on an alkali halide surface. Noncontact atomic force microscopy was used to measure the crystallite dimensions and lattice structure with molecular resolution. The molecule‐substrate lattice mismatch and island heights, typically three to four PTCDA layers, indicate a stress in the first two layers. One- and two-layer crystallites were only observed in pits with side lengths smaller than 10 nm. (Some figures in this article are in colour only in the electronic version)

Bound states of defects in superconducting LiFeAs studied by scanning tunneling spectroscopy

Defects in LiFeAs are studied by scanning tunneling microscopy (STM) and spectroscopy (STS). Topographic images of the five predominant defects allow the identification of their position within the lattice. The most commonly observed defect is associated with an Fe site and does not break the local lattice symmetry, exhibiting a bound state near the edge of the smaller gap in this multi-gap superconductor. Three other common defects, including one also on an Fe site, are observed to break local lattice symmetry and are pair-breaking indicated by clear in-gap bound states, in addition to states near the smaller gap edge. STS maps reveal complex, extended real-space bound state patterns, including one with a chiral distribution of the local density of states (LDOS). The multiple bound state resonances observed within the gaps and at the inner gap edge are consistent with theoretical predictions for s

Molecular resolution imaging of C60 on Au(111) by non-contact atomic force microscopy

^{\pm}

Sign inversion in the superconducting order parameter of LiFeAs inferred from Bogoliubov quasiparticle interference

gap symmetry proposed for LiFeAs and other iron pnictides.

Use of an electron-beam evaporator for the creation of nanostructured pits in an insulating surface

Non-contact atomic force microscopy (NC-AFM) was used to study thin films of C60 on Au(111). After observing the Au(111) reconstruction, 2–3 monolayers of C60 were deposited onto the Au surface. The close-packed C60 surface was imaged by NC-AFM with molecular resolution. Enhanced corrugation and a stretching of the C60 lattice were observed at step edges. Based on a calculation of the force required to displace an edge molecule, it is proposed that the edge effects are a result of tip-induced displacements of edge molecules. While imaging small clusters of C60, some molecules were removed, leading to structural rearrangements of the clusters.

Pronounced polarization-induced energy level shifts at boundaries of organic semiconductor nanostructures.

Quasiparticle interference (QPI) by means of scanning tunneling microscopy/spectroscopy (STM/STS), angle resolved photoemission spectroscopy (ARPES), and multi-orbital tight bind- ing calculations are used to investigate the band structure and superconducting order parameter of LiFeAs. Using this combination we identify intra- and interband scattering vectors between the hole (h) and electron (e) bands in the QPI maps. Discrepancies in the band dispersions inferred from previous ARPES and STM/STS are reconciled by recognizing a difference in the