Victor Carozo

Facile synthesis of MoS2 and MoxW1-xS2 triangular monolayers

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 MoxW1-xS2 by annealing MoS2 and MoS2/WO3 precursors, respectively, in the presence of sulfur vapor. In particular, the MoxW1-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 MoxW1-xS2 stoichiometries in 2D monocrystals opens up a wide range of applications in electronics and optoelectronics.

Optical identification of sulfur vacancies: Bound excitons at the edges of monolayer tungsten disulfide

Bound exciton is a signature of sulfur vacancies, and thus, it can be used to investigate defects in atomically thin materials. Defects play a significant role in tailoring the optical properties of two-dimensional materials. Optical signatures of defect-bound excitons are important tools to probe defective regions and thus interrogate the optical quality of as-grown semiconducting monolayer materials. We have performed a systematic study of defect-bound excitons using photoluminescence (PL) spectroscopy combined with atomically resolved scanning electron microscopy and first-principles calculations. Spatially resolved PL spectroscopy at low temperatures revealed bound excitons that were present only on the edges of monolayer tungsten disulfide and not in the interior. Optical pumping of the bound excitons was sublinear, confirming their bound nature. Atomic-resolution images reveal that the areal density of monosulfur vacancies is much larger near the edges (0.92 ± 0.45 nm−2) than in the interior (0.33 ± 0.11 nm−2). Temperature-dependent PL measurements found a thermal activation energy of ~36 meV; surprisingly, this is much smaller than the bound-exciton binding energy of ~300 meV. We show that this apparent inconsistency is related to a thermal dissociation of the bound exciton that liberates the neutral excitons from negatively charged point defects. First-principles calculations confirm that sulfur monovacancies introduce midgap states that host optical transitions with finite matrix elements, with emission energies ranging from 200 to 400 meV below the neutral-exciton emission line. These results demonstrate that bound-exciton emission induced by monosulfur vacancies is concentrated near the edges of as-grown monolayer tungsten disulfide.

Fabrication and characterization of ultraviolet photosensors from ZnO nanowires prepared using chemical bath deposition method

Highly crystalline zinc oxide (ZnO) nanowires (NWs) were synthesized through chemical bath deposition (CBD) method by using a simple seeding technique. The process includes dispersion of commercially available ZnO nanoparticles through spraying on a desired substrate prior to the CBD growth. A typical growth period of 16 h produced ZnO NW assemblies with an average diameter of ∼45 nm and lengths of 1–1.3 μm, with an optical band gap of ∼3.61 eV. The as-prepared ZnO NWs were photoactive under ultra violet (UV) illumination. Photodetector devices fabricated using these NW assemblies demonstrated a high photoresponse factor of ∼40 and 120 at room temperature under moderate UV illumination power of ∼250 μW/cm2. These findings indicate the possibility of using ZnO NWs, grown using the simple method discussed in this paper, for various opto-electronic applications.

Aligned carbon nanotube/zinc oxide nanowire hybrids as high performance electrodes for supercapacitor applications

Carbon nanotube/metal oxide based hybrids are envisioned as high performance electrochemical energy storage electrodes since these systems can provide improved performances utilizing an electric double layer coupled with fast faradaic pseudocapacitive charge storage mechanisms. In this work, we show that high performance supercapacitor electrodes with a specific capacitance of ∼192 F/g along with a maximum energy density of ∼3.8 W h/kg and a power density of ∼28 kW/kg can be achieved by synthesizing zinc oxide nanowires (ZnO NWs) directly on top of aligned multi-walled carbon nanotubes (MWCNTs). In comparison to pristine MWCNTs, these constitute a 12-fold of increase in specific capacitance as well as corresponding power and energy density values. These electrodes also possess high cycling stability and were able to retain ∼99% of their specific capacitance value over 2000 charging discharging cycles. These findings indicate potential use of a MWCNT/ZnO NW hybrid material for future electrochemical energy...

High flex cycle testing of CVD monolayer WS2 TFTs on thin flexible polyimide

Two-dimensional transition metal dichalcogenides are potential candidates for high-performance flexible electronics. In this paper, we report thin film transistors (TFTs) fabricated on ~5 μm thick solution-cast polyimide substrates using chemical vapor deposition (CVD) synthesized single layer WS2 as the active layer. The linear region field effect mobility ranges from 2 to 10 cm2 V−1 s−1, with current on–off ratio exceeding 106. By using a thin polyimide substrate, the bending induced tensile stress on our TFTs is relatively small when compared to devices fabricated on thicker flexible substrates. Static bending and up to 50 000 bending/flattening cycles were employed to investigate the reliability of these TFTs for potential flexible electronic applications. Our results demonstrate that CVD grown WS2 TFTs fabricated on thin polyimide have good performance stability for up to 2 mm radius bending and 50 000 bending cycles. It is therefore clear that thin polymeric substrates provide a simple approach for reliable, scalable, and high-performance 2D-TMD-based flexible electronics.

The role of interference and polarization effects in the optical visualization of carbon nanotubes

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.