David Perez de Lara

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−1 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.

Gate tunable photovoltaic effect in MoS2 vertical p–n homostructures

p–n junctions based on vertically stacked single or few-layer transition metal dichalcogenides (TMDCs) have attracted substantial scientific interest. Due to the propensity of TMDCs to show exclusively one type of conductivity, n- or p-type, heterojunctions of different materials are typically fabricated to produce diode-like current rectification and photovoltaic response. Recently, artificial, stable and substitutional doping of MoS2 into n- and p-type materials has been demonstrated. MoS2 is an interesting material for use in optoelectronic applications due to its potential of low-cost production in large quantities, strong light–matter interactions and chemical stability. Here we report the characterization of the optoelectronic properties of vertical homojunctions made by stacking few-layer flakes of MoS2:Fe (n-type) and MoS2:Nb (p-type). The junctions exhibit a peak external quantum efficiency of 4.7% and a maximum open circuit voltage of 0.51 V; they are stable in air; and their rectification characteristics and photovoltaic response are in excellent agreement with the Shockley diode model. The gate-tunability of the maximum output power, the ideality factor and the shunt resistance indicate that the dark current is dominated by trap-assisted recombination and that the photocurrent collection depends strongly on the spatial extent of the space charge region. We demonstrate a response time faster than 80 ms and highlight the potential to integrate such devices into quasi-transparent and flexible optoelectronics.

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 for studying 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 for carrying out differential reflectance and transmittance spectroscopy in 2D materials with a lateral resolution of ~1 µm 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 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.

Biaxial strain tuning of the optical properties of single-layer transition metal dichalcogenides

Since their discovery, single-layer semiconducting transition metal dichalcogenides have attracted much attention, thanks to their outstanding optical and mechanical properties. Strain engineering in these two-dimensional materials aims to tune their bandgap energy and to modify their optoelectronic properties by the application of external strain. In this paper, we demonstrate that biaxial strain, both tensile and compressive, can be applied and released in a timescale of a few seconds in a reproducible way on transition metal dichalcogenides monolayers deposited on polymeric substrates. We can control the amount of biaxial strain applied by letting the substrate expand or compress. To do this, we change the substrate temperature and choose materials with a large thermal expansion coefficient. After the investigation of the substrate-dependent strain transfer, we performed micro-differential spectroscopy of four transition metal dichalcogenides monolayers (MoS2, MoSe2, WS2, WSe2) under the application of biaxial strain and measured their optical properties. For tensile strain, we observe a redshift of the bandgap that reaches a value as large as 95 meV/% in the case of single-layer WS2 deposited on polypropylene. The observed bandgap shifts as a function of substrate extension/compression follow the order MoSe2 < MoS2 < WSe2 < WS2. Theoretical calculations of these four materials under biaxial strain predict the same trend for the material-dependent rates of the shift and reproduce well the features observed in the measured reflectance spectra.Strain engineering: Tuning the bandgap of 2D materialsThe bandgap of two-dimensional semiconducting materials can be easily tuned in real time by stretching or compressing them. An international team of researcher led by Dr. Andres Castellanos-Gomez at IMDEA Nanoscience, Spain, studied the optical properties of single-atom thick two-dimensional semiconductors under the application of tensile or compressive biaxial strain. In order to apply the strain the researchers exploited the thermal expansion or compression of the different substrates carrying the atomically thin materials and then compared their results to atomistic simulations. This strain method can be applied in a fast and reversible way and it leads to large changes in the band structure of these semiconducting materials. Research into strain engineering two-dimensional materials may help us in fabricating novel devices like color-changing light emitters or novel and more efficient solar cells.

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.Two-dimensional semiconducting materials are particularly appealing for many applications. Although theory predicts a large number of two-dimensional 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.

High Throughput Characterization of Epitaxially Grown Single-Layer MoS2

The growth of single-layer MoS2 with chemical vapor deposition is an established method that can produce large-area and high quality samples. In this article, we investigate the geometrical and optical properties of hundreds of individual single-layer MoS2 crystallites grown on a highly-polished sapphire substrate. Most of the crystallites are oriented along the terraces of the sapphire substrate and have an area comprised between 10 µm2 and 60 µm2. Differential reflectance measurements performed on these crystallites show that the area of the MoS2 crystallites has an influence on the position and broadening of the B exciton while the orientation does not influence the A and B excitons of MoS2. These measurements demonstrate that differential reflectance measurements have the potential to be used to characterize the homogeneity of large-area chemical vapor deposition (CVD)-grown samples.

Lithography-free electrical transport measurements on 2D materials by direct microprobing

We present a method to carry out electrical and optoelectronic measurements on 2D materials using carbon fiber microprobes to directly make electrical contacts with the 2D materials without damaging them. The working principle of this microprobing method is illustrated by measuring transport in MoS2 flakes in vertical (transport in the out-of-plane direction) and lateral (transport within the crystal plane) configurations, finding performances comparable to those reported for MoS2 devices fabricated by the conventional lithographic process. We also show that this method can be used with other 2D materials.

Optical contrast and refractive index of natural van der Waals heterostructure nanosheets of franckeite

We study mechanically exfoliated nanosheets of franckeite by quantitative optical microscopy. The analysis of transmission-mode and epi-illumination-mode optical microscopy images provides a rapid method to estimate the thickness of the exfoliated flakes at first glance. A quantitative analysis of the optical contrast spectra by means of micro-reflectance allows one to determine the refractive index of franckeite over a broad range of the visible spectrum through a fit of the acquired spectra to a model based on the Fresnel law.

Photodiodes based in La0.7Sr0.3MnO3/single layer MoS2 hybrid vertical heterostructures

The fabrication of artificial materials by stacking of individual two-dimensional (2D) materials is amongst one of the most promising research avenues in the field of 2D materials. Moreover, this strategy to fabricate new man-made materials can be further extended by fabricating hybrid stacks between 2D materials and other functional materials with different dimensionality making the potential number of combinations almost infinite. Among all these possible combinations, mixing 2D materials with transition metal oxides can result especially useful because of the large amount of interesting physical phenomena displayed separately by these two material families. We present a hybrid device based on the stacking of a single layer MoS2 onto a lanthanum strontium manganite (La0.7Sr0.3MnO3) thin film, creating an atomically thin device. It shows a rectifying electrical transport with a ratio of 103, and a photovoltaic effect with V oc up to 0.4 V. The photodiode behaviour arises as a consequence of the different doping character of these two materials. This result paves the way towards combining the efforts of these two large materials science communities.

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.