Propriedades eletromecânicas de nanoestruturas por microscopia de varredura por sonda

Abstract

This work shows studies regarding electromechanical properties of single wall carbon nanotubes, some solid lubricants (graphene, molybdenum disulfide, talc) and two other layered materials: bismuth selenide and mica. During all work, carried out at Laboratório de Nanoscopia of the Departamento de Física (DF-UFMG), we had anintense collaboration with the Laboratório de Estrutura Eletrônica (DF/UFMG), Laboratório de Espectroscopia e Imagens Espectroscópicas de Nano-Materiais (DFUFMG) and Laboratórios de Espectroscopia Raman (at DF/UFMG and at Dimat/Inmetro/RJ). The behavior of single-wall carbon nanotubes (SWNTs) to radial compressionwas investigated via Atomic Force Microscopy (AFM) measurements. We found that the response of a nanotube (with diameter d) to a force F, applied by an AFM tip (with radius R) rescaled through the quantity Fd3/2(2R)-1/2, falls into a universal curve as a function of the compressive strain. The application of this model to the radial Youngsmodulus, Er, leads to a further universal-type behavior, which explains the large variations of nanotube Er reported in the literature. In another work with SWNTs, we present a new approach for their electricalcharacterization, where we used Electric Force Microscopy (EFM) images. The differences of the electric response of metallic and semiconducting carbon nanotubes to a DC electric field applied by the tip of a Scanning Probe Microscope (SPM) are observed accurately. Not only the magnitudes of their responses are different, but, more important, the EFM line profile across a nanotube is qualitatively distinct for metallic orsemiconducting tubes. Following this previous result, we propose a new perspective to control the electrical behavior of semiconducting SWNTs through their surface contact. Several nanotubes (metallic and semiconducting) were put in contact with different materials (via a SPM tip) during electric charging experiments. The experimental data show that semiconducting nanotubes present two distinct behaviors depending on the tip material: for bare Si and metal-covered tips, they present their characteristic response. However, upon contact with a diamond-coated tip, semiconducting nanotubes actually portray a metallic-type electric response. Metallic nanotubes, nonetheless, show their typical behavior, independent of the probe nature. Ab initio calculations suggest that the metallic behavior of semiconducting SWNTs is induced by a strong doping of the tubes when in contact with diamond surfaces. On graphene samples, we got theoretical and experimental evidences of compression-induced surface diamondization of few-layer graphene, which originates a new 2D material. When two or more graphene layers are compressed in the presence of water, ab initio calculations predict that hydroxyl groups stabilize the sp3 hybridization of the two topmost carbon layers, creating a single layer of hydroxylated diamond, or Diamondol, which is a ferromagnetic insulator. In the pursuit for this new material, EFM, Magnetic Force Microscopy (MFM) and Raman spectroscopy experiments were performed. The EFM data show a compression-induced charging inhibition of bilayer and fewlayer graphene, which is reversible in most of the cases, water-dependent, and it is absent in single-layer graphene. Magnetic measurements reveal the possibility of obtaining permanent Diamondol. Raman results, in the permanent samples, indicate that this is a promising way to confirm the re-hibridization. Finally, we report mechanical response of few-layer graphene, h-BN (Hexagonal Boron Nitride), talc (soapstone), mica, Bi2Se3 (Bismuth Selenide) and MoS2 (Molybdenun Disulfide) to the simultaneous compression and shear by an AFM tip. The response is characterized by the vertical expansion of the two-dimensional (2D) solidlubricant materials upon compression. Such effect is proportional to the applied load, it is null in the absence of shear, increases with tip velocity, and it is anisotropic and reversible. It also has similar magnitudes in few-layer graphene, h-BN and MoS2, but it is absent in single-layer graphene and in few-layer mica and Bi2Se3. We propose a physical mechanism for the effect where the combined compressive and shear stresses from the tip induce dynamical wrinkling on the upper material layers, leading to the observed flake thickening.