Anomalous response of supported few-layer hexagonal boron nitride to DC electric fields: a confined water effect?

We use electric force microscopy (EFM) to study the response of supported few-layer hexagonal boron nitride (h-BN) to an electric field applied by the EFM tip. Our results show an anomalous behavior in the dielectric response of h-BN atop Si oxide for different bias polarities: for a positive bias applied to the tip, h-BN layers respond with a larger dielectric constant than the dielectric constant of the substrate, while for a negative bias, the h-BN dielectric constant appears to be smaller. Based on ab initio calculations, we propose that this behavior is due to a water layer confined between the Si oxide substrate and h-BN layers. This hypothesis was experimentally confirmed by sample annealing and also by a comparative analysis with h-BN on a non-polar substrate.

The use of a Ga+ focused ion beam to modify graphene for device applications.

In this work, we clarify the features of the lateral damage of line defects in single layer graphene. The line defects were produced through well-controlled etching of graphene using a Ga(+) focused ion beam. The lateral damage length was obtained from both the integrated intensity of the disorder induced Raman D band and the minimum ion fluence. Also, the line defects were characterized by polarized Raman spectroscopy. It was found that graphene is resilient under the etching conditions since the intensity of the defect induced Raman D peak exhibits a dependence on the direction of the lines relative to the crystalline lattice and also on the direction of the laser polarization relative to the lines. In addition, electrical measurements of the modified graphene were performed. Different ion fluences were used in order to obtain a completely insulating defect line in graphene, which was determined experimentally by means of charge injection and electric force microscopy measurements. These studies demonstrate that a Ga+ ion column combined with Raman spectroscopy is a powerful technique to produce and understand well-defined periodic arrays of defects in graphene, opening possibilities for better control of nanocarbon devices.

Charge injection on insulators via scanning probe microscopy techniques: towards data storage devices.

The idea of contact electrification aiming the development of nano-scale data storage devices has been explored through a careful investigation of charge injection on insulating films (SiO2 and PMMA) via scanning probe microscopy techniques. A complete route for data storage, showing simple and effective ways to write (inject the charge with an atomic force microscopy (AFM) tip), to read (detect the charge with electric force microscopy), to store (keep sample charge by changing ambient and surface conditions) and to erase the information (make the discharge process faster) is proposed and discussed. A detailed study of the influence of several parameters like AFM mode, bias voltage, relative humidity and surface hydrophobicity is also presented to optimize both charge injection and discharge processes. Results show that monitoring parameters such as ambient relative humidity and surface hydrophobic/hydrophilic character enable the control of pattern size, lateral dispersion, and storage time. The charge polarity is also dependent on the surface hydrophobicity and either positive or negative charges can become more appropriate for storage depending on the surface hydrophobic/hydrophilic character.

Electric force microscopy investigation of a MEH-PPV conjugated polymer blend: robustness or frailty?

Electric force microscopy (EFM) was employed in the electrical characterization of a blend of thermoplastic polyurethane (TPU) and poly(2-methoxy-5-(2'-ethyl-hexyloxy)-1,4-phenylenevinylene) (MEH-PPV) conjugated polymer. Although qualitative EFM interpretation is straightforward, its quantitative analysis always relies on approximated models. The extraction of physically reasonable parameters is normally assumed as a proof of validity of the theoretical model employed. In order to gather information about electric properties of this blend and to test the EFM technique itself, two distinct and discordant models were developed in this work to fit experimental EFM data. Even though MEH-PPV is regarded as a conductor in one model and as a dielectric in the other, both models yielded coherent and reasonable electrical properties for this blend. Such unexpected results are used to discuss the robustness or frailty of EFM in the analysis of complex materials.

Nanowires and nanoribbons formed by methylphosphonic acid.

The production and physical properties of nanowires and nanoribbons formed by methylphosphonic acid (MPA)--CH3PO(OH)2--were investigated. These structures are formed on an aluminum coated substrate when immersed in an ethanolic solution of MPA for several days. A careful investigation of the growth conditions resulted in a narrow window of solution concentrations and temperatures for the successful development of nanowires and nanoribbons. Several different techniques were employed to characterize these nanostructures: (1) Photoluminescence experiments showed a strong emission at 2.3 eV (green), which is visible to the naked eye; (2) X-ray diffraction experiments indicated a significant cristalinity, in agreement with atomic force microscopy (AFM) and transmission electron microscopy (TEM) morphology images, which show organized nano-scale wires and ribbons, (furthermore, AFM-Phase and TEM images also suggest that nanoribbons are formed by well-aligned nanowires); (3) Conductive-AFM experiments revealed an intermediary conductivity for these structures (10(-1)/Ohm x m), which is similar to some intrinsic semiconductors and; (4) finally, Infrared, Raman, and X-Ray Photoelectron Spectroscopies produced information about the contents, structure, and composition of both wires and ribbons.

Defocusing microscopy.

Transparent objects (phase objects) are not visible in a standard brightfield optical microscope. In order to see such objects the most used technique is phase-contrast microscopy. In phase-contrast microscopy the contrast observed is proportional to the optical path difference introduced by the object. If the index of refraction is uniform, phase-contrast microscopy then yields a measure of the thickness profile of phase objects. We show that by slightly defocusing an optical microscope operating in brightfield, phase objects become visible. We modeled such an effect and show that the image contrast of a phase object is proportional to the amount of defocusing and proportional to the two-dimensional Laplacian of the optical path difference introduced by the object. For uniform index of refraction, defocusing microscopy then yields a measure of the curvature profile of phase objects. We extended our previous model for thin objects to thick objects. To check our theoretical model, we use as phase objects polystyrene spherical caps and compare their curvature radii obtained by defocusing microscopy (DM) to those obtained with atomic force microscopy (AFM). We also show that for thick curved phase objects one can reconstruct their thickness profiles from DM images. We illustrate the utility of defocusing microscopy in biological systems to study cell motility. In particular, we visualize and quantitatively measure real-time cytoskeleton curvature fluctuations of macrophages (a cell of the innate immune system). The study of such fluctuations might be important for a better understanding of the engulfment process of pathogens during phagocytosis.

Universal response of single-wall carbon nanotubes to radial compression.

The mechanical response of single-wall carbon nanotubes to radial compression is investigated via atomic force microscopy (AFM). We find that the force F applied by an AFM tip (with radius R) onto a nanotube (with diameter d), rescaled through the quantity Fd;{3/2}(2R);{-1/2}, falls into a universal curve as a function of the compressive strain. Such universality is reproduced analytically in a model where the graphene bending modulus is the only fitting parameter. The application of this model to the radial Young's modulus E_{r} leads to a further universal-type behavior which explains the large variations of nanotube E_{r} reported in the literature.

Deformation induced semiconductor-metal transition in single wall carbon nanotubes probed by electric force microscopy.

We report the direct experimental observation of the semiconductor-metal transition in single-wall carbon nanotubes (SWNTs) induced by compression with the tip of an atomic force microscope. This transition is probed via electric force microscopy by monitoring SWNT charge storage. Experimental data show that such charge storage is different for metallic and semiconducting SWNTs, with the latter presenting a strong dependence on the tip-SWNT force during injection. Ab initio calculations corroborate experimental observations and their interpretation.

Effects of substrate polarity and chain length on conformational and thermal properties of phosphonic acid self-assembled bilayers.

Conformational and thermal behavior of self-assembled structures of three phosphonic acids, OPA (octadecylphosphonic acid), TPA (tetradecylphosphonic acid), and OcPA (octylphosphonic acid), with different alkyl chain lengths are investigated and compared. The orientation of self-assembled bilayers depends on whether the substrate is nonpolar (graphite) or polar (mica). For nonpolar substrates, bilayers lay parallel to the surface, and for polar substrates, bilayers lay perpendicular to the surface. Thermal behaviors of these structures on mica and graphite are also investigated, showing that, depending on the temperature they are submitted to, molecules stack, unstack, or agglomerate on mica and form larger domains on graphite.

Influence of the atomic structure on the Raman spectra of graphite edges.

A study of step edges in graphite with different atomic structures combining Raman spectroscopy and scanning probe microscopy is presented. The orientation of the carbon hexagons with respect to the edge axis, in the so-called armchair or zigzag arrangements, is distinguished spectroscopically by the intensity of a disorder-induced Raman peak. This effect is explained by applying the double resonance theory to a semi-infinite graphite crystal and by considering the one-dimensional character of the defect.

Anisotropy of the Raman spectra of nanographite ribbons.

A polarized Raman study of nanographite ribbons on a highly oriented pyrolytic graphite substrate is reported. The Raman peak of the nanographite ribbons exhibits an intensity dependence on the light polarization direction relative to the nanographite ribbon axis. This result is due to the quantum confinement of the electrons in the 1D band structure of the nanographite ribbons, combined with the anisotropy of the light absorption in 2D graphite, in agreement with theoretical predictions.