Effects of catalyst surfaces on adsorption revealed by atomic force microscope force spectroscopy: photocatalytic degradation of diuron over zinc oxide.

Controlling adsorption of a heterogeneous catalyst requires a detailed understanding of the interactions between reactant molecules and the catalyst surface. Various characteristics relevant to adsorption have been theoretically predicted but have yet to be experimentally quantified. Here, we explore a model reaction based on diuron [3-(3,4-dichlorophenyl)-1,1-dimethylurea] photo-degradation over a ZnO particle catalyst. We used atomic force microscope (AFM)-based force spectroscopy under ambient conditions to investigate interactions between individual functional groups of diuron (NH2, Cl, and CH3) and surfaces of ZnO particles (polar Zn and O-terminated, and nonpolar Zn-O terminated). We were able to distinguish and identify the two polar surfaces of conventional ZnO particles and the nonpolar surface of ZnO nanorods based on force-distance curves of functionalized probe/surface pairs. We posit that the reaction involved physisorption and could be described in terms of Hamaker constants. These constants had an order-of-magnitude difference among the probe/surface interacting pairs based on polarity. Hence, we confirmed that van der Waals interactions determined the adsorption behavior. We interpreted the electronic distribution models of the probe-modifying molecules. The functional group configurations inferred the diuron adsorption configurations during contact with each ZnO facet. The adsorption affected characteristics of the reaction intermediates and the rate of degradation.

The biomechanical role of overall-shape transformation in a primitive multicellular organism: A case study of dimorphism in the filamentous cyanobacterium Arthrospira platensis.

Morphological transformations in primitive organisms have long been observed; however, its biomechanical roles are largely unexplored. In this study, we investigate the structural advantages of dimorphism in Arthrospira platensis, a filamentous multicellular cyanobacterium. We report that helical trichomes, the default shape, have a higher persistence length (Lp), indicating a higher resistance to bending or a large value of flexural rigidity (kf), the product of the local cell stiffness (E) and the moment of inertia of the trichomes' cross-section (I). Through Atomic Force Microscopy (AFM), we determined that the E of straight and helical trichomes were the same. In contrast, our computational model shows that I is greatly dependent on helical radii, implying that trichome morphology is the major contributor to kf variation. According to our estimation, increasing the helical radii alone can increase kf by 2 orders of magnitude. We also observe that straight trichomes have improved gliding ability, due to its structure and lower kf. Our study shows that dimorphism provides mechanical adjustability to the organism and may allow it to thrive in different environmental conditions. The higher kf provides helical trichomes a better nutrient uptake through advection in aquatic environments. On the other hand, the lower kf improves the gliding ability of straight trichomes in aquatic environments, enabling it to chemotactically relocate to more favorable territories when it encounters certain environmental stresses. When more optimal conditions are encountered, straight trichomes can revert to their original helical form. Our study is one of the first to highlight the biomechanical role of an overall-shape transformation in cyanobacteria.

Publisher's Note: Direct Imaging of Dynamic Glassy Behavior in a Strained Manganite Film [Phys. Rev. Lett. 115, 265701 (2015)].

This corrects the article DOI: 10.1103/PhysRevLett.115.265701.

Unexpected edge conduction in mercury telluride quantum wells under broken time-reversal symmetry.

The realization of quantum spin Hall effect in HgTe quantum wells is considered a milestone in the discovery of topological insulators. Quantum spin Hall states are predicted to allow current flow at the edges of an insulating bulk, as demonstrated in various experiments. A key prediction yet to be experimentally verified is the breakdown of the edge conduction under broken time-reversal symmetry. Here we first establish a systematic framework for the magnetic field dependence of electrostatically gated quantum spin Hall devices. We then study edge conduction of an inverted quantum well device under broken time-reversal symmetry using microwave impedance microscopy, and compare our findings to a non-inverted device. At zero magnetic field, only the inverted device shows clear edge conduction in its local conductivity profile, consistent with theory. Surprisingly, the edge conduction persists up to 9 T with little change. This indicates physics beyond simple quantum spin Hall model, including material-specific properties and possibly many-body effects.

Direct Imaging of Dynamic Glassy Behavior in a Strained Manganite Film.

Complex many-body interaction in perovskite manganites gives rise to a strong competition between ferromagnetic metallic and charge-ordered phases with nanoscale electronic inhomogeneity and glassy behaviors. Investigating this glassy state requires high-resolution imaging techniques with sufficient sensitivity and stability. Here, we present the results of a near-field microwave microscope imaging on the strain-driven glassy state in a manganite film. The high contrast between the two electrically distinct phases allows direct visualization of the phase separation. The low-temperature microscopic configurations differ upon cooling with different thermal histories. At sufficiently high temperatures, we observe switching between the two phases in either direction. The dynamic switching, however, stops below the glass transition temperature. Compared with the magnetization data, the phase separation was microscopically frozen, while spin relaxation was found in a short period of time.

Imaging of Coulomb-driven quantum Hall edge states.

The edges of a two-dimensional electron gas (2DEG) in the quantum Hall effect (QHE) regime are divided into alternating metallic and insulating strips, with their widths determined by the energy gaps of the QHE states and the electrostatic Coulomb interaction. Local probing of these submicrometer features, however, is challenging due to the buried 2DEG structures. Using a newly developed microwave impedance microscope, we demonstrate the real-space conductivity mapping of the edge and bulk states. The sizes, positions, and field dependence of the edge strips around the sample perimeter agree quantitatively with the self-consistent electrostatic picture. The evolution of microwave images as a function of magnetic fields provides rich microscopic information around the ν=2 QHE state.

Cryogenic microwave imaging of metal-insulator transition in doped silicon.

We report the instrumentation and experimental results of a cryogenic scanning microwave impedance microscope. The microwave probe and the scanning stage are located inside the variable temperature insert of a helium cryostat. Microwave signals in the distance modulation mode are used for monitoring the tip-sample distance and adjusting the phase of the two output channels. The ability to spatially resolve the metal-insulator transition in a doped silicon sample is demonstrated. The data agree with a semiquantitative finite element simulation. Effects of the thermal energy and electric fields on local charge carriers can be seen in the images taken at different temperatures and dc biases.

Ultrathin topological insulator Bi2Se3 nanoribbons exfoliated by atomic force microscopy.

Ultrathin topological insulator nanostructures, in which coupling between top and bottom surface states takes place, are of great intellectual and practical importance. Due to the weak van der Waals interaction between adjacent quintuple layers (QLs), the layered bismuth selenide (Bi(2)Se(3)), a single Dirac-cone topological insulator with a large bulk gap, can be exfoliated down to a few QLs. In this paper, we report the first controlled mechanical exfoliation of Bi(2)Se(3) nanoribbons (>50 QLs) by an atomic force microscope (AFM) tip down to a single QL. Microwave impedance microscopy is employed to map out the local conductivity of such ultrathin nanoribbons, showing drastic difference in sheet resistance between 1-2 QLs and 4-5 QLs. Transport measurement carried out on an exfoliated (50 QLs) ribbons. These AFM-exfoliated thin nanoribbons afford interesting candidates for studying the transition from quantum spin Hall surface to edge states.

Mesoscopic percolating resistance network in a strained manganite thin film.

Many unusual behaviors in complex oxides are deeply associated with the spontaneous emergence of microscopic phase separation. Depending on the underlying mechanism, the competing phases can form ordered or random patterns at vastly different length scales. By using a microwave impedance microscope, we observed an orientation-ordered percolating network in strained Nd(1/2)Sr(1/2)MnO3 thin films with a large period of 100 nanometers. The filamentary metallic domains align preferentially along certain crystal axes of the substrate, suggesting the anisotropic elastic strain as the key interaction in this system. The local impedance maps provide microscopic electrical information of the hysteretic behavior in strained thin film manganites, suggesting close connection between the glassy order and the colossal magnetoresistance effects at low temperatures.

Hierarchy of electronic properties of chemically derived and pristine graphene probed by microwave imaging.

Local electrical imaging using microwave impedance microscope is performed on graphene in different modalities, yielding a rich hierarchy of the local conductivity. The low-conductivity graphite oxide and its derivatives show significant electronic inhomogeneity. For the conductive chemical graphene, the residual defects lead to a systematic reduction of the microwave signals. In contrast, the signals on pristine graphene agree well with a lumped-element circuit model. The local impedance information can also be used to verify the electrical contact between overlapped graphene pieces.

Nanoscale Electronic Inhomogeneity in In2Se3 Nanoribbons Revealed by Microwave Impedance Microscopy.

Driven by interactions due to the charge, spin, orbital, and lattice degrees of freedom, nanoscale inhomogeneity has emerged as a new theme for materials with novel properties near multiphase boundaries. As vividly demonstrated in complex metal oxides (see refs 1-5) and chalcogenides (see refs 6 and 7), these microscopic phases are of great scientific and technological importance for research in high-temperature superconductors (see refs 1 and 2), colossal magnetoresistance effect (see ref 4), phase-change memories (see refs 5 and 6), and domain switching operations (see refs 7-9). Direct imaging on dielectric properties of these local phases, however, presents a big challenge for existing scanning probe techniques. Here, we report the observation of electronic inhomogeneity in indium selenide (In(2)Se(3)) nanoribbons (see ref 10) by near-field scanning microwave impedance microscopy (see refs 11-13). Multiple phases with local resistivity spanning 6 orders of magnitude are identified as the coexistence of superlattice, simple hexagonal lattice and amorphous structures with approximately 100 nm inhomogeneous length scale, consistent with high-resolution transmission electron microscope studies. The atomic-force-microscope-compatible microwave probe is able to perform a quantitative subsurface electrical study in a noninvasive manner. Finally, the phase change memory function in In(2)Se(3) nanoribbon devices can be locally recorded with big signals of opposite signs.