Thickness-Dependent Differential Reflectance Spectra of Monolayer and Few-Layer MoS2, MoSe2, WS2 and WSe2

The research field of two dimensional (2D) materials strongly relies on optical microscopy characterization tools to identify atomically thin materials and to determine their number of layers. Moreover, optical microscopy-based techniques opened the door to study the optical properties of these nanomaterials. We presented a comprehensive study of the differential reflectance spectra of 2D semiconducting transition metal dichalcogenides (TMDCs), MoS2, MoSe2, WS2, and WSe2, with thickness ranging from one layer up to six layers. We analyzed the thickness-dependent energy of the different excitonic features, indicating the change in the band structure of the different TMDC materials with the number of layers. Our work provided a route to employ differential reflectance spectroscopy for determining the number of layers of MoS2, MoSe2, WS2, and WSe2.Comment: Main text (3 Figures) and Supp. Info. (23 Figures

Nanoscale Thermal Transport in 2D Nanostructures from Cryogenic to Room Temperature

Nanoscale scanning thermal microscopy (SThM) transport measurements from cryogenic to room temperature on 2D structures with sub 30 nm resolution are reported. This novel cryogenic operation of SThM, extending the temperature range of the sample down to 150 K, yields a clear insight into the nanothermal properties of the 2D nanostructures and supports the model of ballistic transport contribution at the edge of the detached areas of exfoliated graphene which leads to a size-dependent thermal resistance of the detached material. The thermal resistance of graphene on SiO2 is increased by one order of magnitude by the addition of a top layer of MoS2, over the temperature range of 150–300 K, providing pathways for increasing the efficiency of thermoelectric applications using van der Waals (vdW) materials. Density functional theory calculations demonstrate that this increase originates from the phonon transport filtering in the weak vdW coupling between the layers and the vibrational mismatch between MoS2 and graphene layers

Micro-reflectance and transmittance spectroscopy: a versatile and powerful tool to characterize 2D materials

Optical spectroscopy techniques such as differential reflectance and transmittance have proven to be very powerful techniques to study 2D materials. However, a thorough description of the experimental setups needed to carry out these measurements is lacking in the literature. We describe a versatile optical microscope setup to carry out differential reflectance and transmittance spectroscopy in 2D materials with a lateral resolution of ~1 micron in the visible and near-infrared part of the spectrum. We demonstrate the potential of the presented setup to determine the number of layers of 2D materials and to characterize their fundamental optical properties such as excitonic resonances. We illustrate its performance by studying mechanically exfoliated and chemical vapor-deposited transition metal dichalcogenide samples.Comment: 5 main text figures + 1 table with all the part numbers to replicate the experimental setup + 4 supp. info. figure

Highly responsive UV-photodetectors based on single electrospun TiO2 nanofibres

In this work we study the optoelectronic properties of individual TiO2 fibres produced through coupled sol-gel and electrospinning, by depositing them onto pre-patterned Ti/Au electrodes on SiO2/Si substrates. Transport measurements in the dark give a conductivity above 2*10^-5 S, which increases up to 8*10^-5 S in vacuum. Photocurrent measurements under UV-irradiation show high sensitivity (responsivity of 90 A/W for 375 nm wavelength) and a response time to illumination of ~ 5 s, which is superior to state-of-the-art TiO2-based UV photodetectors. Both responsivity and response speed are higher in air than in vacuum, due to oxygen adsorbed on the TiO2 surface which traps photoexcited free electrons in the conduction band, thus reducing the recombination processes. The photodetectors are sensitive to light polarization, with an anisotropy ratio of 12%. These results highlight the interesting combination of large surface area and low 1D transport resistance in electrospun TiO2 fibres. The simplicity of the sol-gel/electrospinning synthesis method, combined with a fast response and high responsivity makes them attractive candidates for UV-photodetection in ambient conditions. We anticipate their high (photo) conductance is also relevant for photocatalysis and dye-sensitized solar cells.Comment: 29 pages, 5 figures in the main text, 9 figures in the Supporting Information. in J. Mater. Chem. C, 201

Characterization of highly crystalline lead iodide nanosheets prepared by room-temperature solution processing

Two-dimensional (2D) semiconducting materials are particularly appealing for many applications. Although theory predicts a large number of 2D materials, experimentally only a few of these materials have been identified and characterized comprehensively in the ultrathin limit. Lead iodide, which belongs to the transition metal halides family and has a direct bandgap in the visible spectrum, has been known for a long time and has been well characterized in its bulk form. Nevertheless, studies of this material in the nanometer thickness regime are rather scarce. In this article we demonstrate an easy way to synthesize ultrathin, highly crystalline flakes of PbI2 by precipitation from a solution in water. We thoroughly characterize the produced thin flakes with different techniques ranging from optical and Raman spectroscopy to temperature-dependent photoluminescence and electron microscopy. We compare the results to ab initio calculations of the band structure of the material. Finally, we fabricate photodetectors based on PbI2 and study their optoelectronic properties.We acknowledge financial support from the European Commission under the Graphene Flagship (CNECTICT-604391), and European Research Council (ERC-StG-MINT 307609), the MINECO, the Comunidad de Madrid, the Netherlands Organisation for Scientific Research (NWO), and the German Science Foundation (DFG). JLL and JFR acknowledge financial support by Marie-Curie-ITN 607904-SPINOGRAPH. JFR acknowledges financial support from MEC-Spain (MAT2016-78625-C2)

Low-dimensional semiconductors: synthesis, properties and devices

Tesis Doctoral inédita leída en la Universidad Autónoma de Madrid, Facultad de Ciencias, Departamento de Física de la Materia Condensada. Fecha de lectura: 24-10-201

Atomically thin p-n junctions based on two-dimensional materials

Recent research in two-dimensional (2D) materials has boosted a renovated interest in the p–n junction, one of the oldest electrical components which can be used in electronics and optoelectronics. 2D materials offer remarkable flexibility to design novel p–n junction device architectures, not possible with conventional bulk semiconductors. In this Review we thoroughly describe the different 2D p–n junction geometries studied so far, focusing on vertical (out-of-plane) and lateral (in-plane) 2D junctions and on mixed-dimensional junctions. We discuss the assembly methods developed to fabricate 2D p–n junctions making a distinction between top-down and bottom-up approaches. We also revise the literature studying the different applications of these atomically thin p–n junctions in electronic and optoelectronic devices. We discuss experiments on 2D p–n junctions used as current rectifiers, photodetectors, solar cells and light emitting devices. The important electronics and optoelectronics parameters of the discussed devices are listed in a table to facilitate their comparison. We conclude the Review with a critical discussion about the future outlook and challenges of this incipient research field.QN/van der Zant La

Low Thermal Conductivity in Franckeite Heterostructures

Layered crystals are known to be good candidates for bulk thermoelectric applications as they open new ways to realise highly efficient devices. Two dimensional materials, isolated from layered materials, and their stacking into heterostructures have attracted intense research attention for nanoscale applications due to their high Seebeck coefficient and possibilities to engineer their thermoelectric properties. However, integration to thermoelectric devices is problematic due to their usually high thermal conductivities. Reporting on thermal transport studies between 150 and 300 K, we show that franckeite, a naturally occurring 2D heterostructure, exhibits a very low thermal conductivity which combined with its previously reported high Seebeck coefficient and electrical conductance make it a promising candidate for low dimensional thermoelectric applications. We find cross- and in-plane thermal conductivity values at room temperature of 0.70 and 0.88 W m−1 K−1, respectively, which is one of the lowest values reported today for 2D-materials. Interestingly, a 1.77 nm thick layer of franckeite shows very low thermal conductivity similar to one of the most widely used thermoelectric material Bi2Te3 with the thickness of 10–20 nm. We show that this is due to the low Debye frequency of franckeite and scattering of phonon transport through van der Waals interface between different layers. This observation open new routes for high efficient ultra-thin thermoelectric applications