[Show abstract][Hide abstract] ABSTRACT: Single- and few-layered transition metal dichalcogenides, such as MoS2 and WS2, are emerging two-dimensional materials exhibiting numerous and unusual physico-chemical properties that could be advantageous in the fabrication of unprecedented optoelectronic devices. Here we report a novel and alternative route to synthesize triangular monocrystals of MoS2 and Mo xW1-xS2 by annealing MoS2 and MoS2/WO3 precursors, respectively, in the presence of sulfur vapor. In particular, the Mo xW1-xS2 triangular monolayers show gradual concentration profiles of W and Mo whereby Mo concentrates in the islands’ center and W is more abundant on the outskirts of the triangular monocrystals. These observations were confirmed by atomic force microscopy, and high-resolution transmission electron microscopy, as well as Raman and photoluminescence spectroscopy. The presence of tunable PL signals depending on the Mo xW1-xS2 stoichiometries in 2D monocrystals opens up a wide range of applications in electronics and optoelectronics.
[Show abstract][Hide abstract] ABSTRACT: In this work, a study of resonance effects in the Raman spectra of twisted bilayer graphene (tBLG) is presented. The analysis takes into account the effect of the mismatch angle θ between the two layers, and also of the excitation laser energy on the frequency, linewidth, and intensity of the main Raman features, namely the rotationally induced R band, the G band, and the second-order G′ (or 2D) band. The resonance effects are explained based on the θ dependence of the tBLG electronic structure, as calculated by ab initio methodologies. The twist angle θ also defines the observation of a “D-like” band which obeys the double-resonance process, but relies on the superlattice along with long-range defects in order to fulfill momentum conservation. The study was possible due to the development of a route to produce and identify rotationally stacked bilayer graphene by means of atomic force microscopy (AFM).
Physical Review B 08/2013; 88(8):085401. · 3.66 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: This manuscript presents an experimental study on the optical visualization of single- and multi-walled carbon nanotubes. Optical micrographs of single-nanotubes and multi-walled carbon nanotubes sitting on SiO2/Si substrates are presented. Atomic force microscopy and Raman spectroscopy analysis provide morphological and structural characterization of the carbon nanotubes. Measurements taking into account different substrates, and also different values of wavelength of the incoming light, show that the optical contrast between the nanotubes and the SiO2 surface strongly depends on these two factors. A model based on interference effects explains the experimental results and establishes a route for substrate engineering that allows direct and fast observation of carbon nanotubes, as well as the measurement of their refractive indexes. Analysis on the polarization properties of the reflected light confirms the strong anisotropy on the optical absorption of carbon nanotubes.
Journal of Applied Physics 02/2013; 113(8). · 2.21 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: 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.
[Show abstract][Hide abstract] ABSTRACT: When two identical two-dimensional periodic structures are superposed, a mismatch rotation angle between the structures generates a superlattice. This effect is commonly observed in graphite, where the rotation between graphene layers generates Moiré patterns in scanning tunneling microscopy images. Here, a study of intravalley and intervalley double-resonance Raman processes mediated by static potentials in rotationally stacked bilayer graphene is presented. The peak properties depend on the mismatch rotation angle and can be used as an optical signature for superlattices in bilayer graphene. An atomic force microscopy system is used to produce and identify specific rotationally stacked bilayer graphenes that demonstrate the validity of our model.
[Show abstract][Hide abstract] ABSTRACT: The direct determination of the crystallographic orientation of graphene sheets was performed using lattice resolution atomic force microscopy images. A graphene sample, micromechanically exfoliated onto a SiO2 substrate showing well defined crystal edges, was imaged in lateral force mode. The lateral force images reveal the periodicity of the graphene hexagonal structure allowing the visualization of the lattice symmetries and determination of the crystal orientation. Crystal edges predominantly formed by zigzag or armchair directions were identified. The nature of the edges was confirmed by Raman spectroscopy.
Journal of Applied Physics 10/2011; 110(8):086101-086101-3. · 2.21 Impact Factor