arXiv subjects

Andres Castellanos-Gomez

Publications and source records attributed to Andres Castellanos-Gomez.

At least 73 records · Page 4Linked to original sources

arXiv2019

A strain tunable single-layer MoS2 photodetector

Strain engineering, which aims to tune the bandgap of a semiconductor by the application of strain, has emerged as an interesting way to control the electrical and optical properties of two-dimensional (2D) materials. Apart from the changes in the intrinsic properties of 2D materials, the application of strain can be also used to modify the characteristics of devices based on them. In this work, we study flexible and transparent photodetectors based on single-layer MoS2 under the application of biaxial strain. We find that by controlling the level of strain, we can tune the photoresponsivity (by 2-3 orders of magnitude), the response time (from <80 ms to 1.5 s) and the spectral bandwidth (with a gauge factor of 135 meV/% or 58 nm/%) of the device.

physics.app-ph↗

arXiv2019

Anisotropic buckling of few-layer black phosphorus

When a two-dimensional material, adhered onto a compliant substrate, is subjected to compression it can undertake a buckling instability yielding to a periodic rippling. Interestingly, when black phosphorus flakes are compressed along the zig-zag crystal direction the flake buckles forming ripples with a 40% longer period than that obtained when the compression is applied along the armchair direction. This anisotropic buckling stems from the puckered honeycomb crystal structure of black phosphorus and a quantitative analysis of the ripple period allows us to determine the Youngs's modulus of few-layer black phosphorus along the armchair direction (EbP_AC = 35.1 +- 6.3 GPa) and the zig-zag direction (EbP_ZZ = 93.3 +- 21.8 GPa).

physics.app-ph↗

arXiv2019

Mechanical and liquid phase exfoliation of cylindrite: a natural van der Waals superlattice with intrinsic magnetic interactions

We report the isolation of thin flakes of cylindrite, a naturally occurring van der Waals superlattice, by means of mechanical and liquid phase exfoliation. We find that this material is a heavily doped p-type semiconductor with a narrow gap (<0.85 eV) with intrinsic magnetic interactions that are preserved even in the exfoliated nanosheets. Due to its environmental stability and high electrical conductivity, cylindrite can be an interesting alternative to the existing two-dimensional magnetic materials.

physics.app-ph↗

arXiv2019

Fast Yet Quantum-Efficient Few-Layer Vertical MoS2 Photodetectors

Semiconducting 2D materials, such as molybdenum disulfide (MoS2) and other members of the transition metal dichalcogenide family, have emerged as promising materials for applications in high performance nanoelectronics that exhibit excellent electrical and optical properties. Highly efficient photocurrent (PC) generation is reported in vertical few layer MoS2 devices contacted with semitransparent metallic electrodes. The light absorption of the device can be improved by fabricating vertical photodevices using few layer flakes, achieving a photoresponse of up to 0.11 A/W and an external quantum efficiency (EQE) of up to 30%. Because of the vertical design, the distance between electrodes can be kept in the range of a few nanometers, thus substantially reducing the collection time of photogenerated carriers and increasing the efficiency of the devices. The wavelength dependent PC, photoresponsivity, and EQE are measured over a photon energy range from 1.24 to 2.58 eV. Compared to previous in plane and vertical devices, these vertical few layer MoS2 photodevices exhibit very short response time, 60 ns, and a cutoff frequency of 5.5 MHz, while maintaining high photoresponsivity.

physics.app-ph↗

arXiv2018

Moiré Intralayer Excitons in a MoSe$_2$/MoS$_2$ Heterostructure

Spatially periodic structures with a long range period, referred to as moiré pattern, can be obtained in van der Waals bilayers in the presence of a small stacking angle or of lattice mismatch between the monolayers. Theoretical predictions suggest that the resulting spatially periodic variation of the band structure modifies the optical properties of both intra and interlayer excitons of transition metal dichalcogenides heterostructures. Here, we report on the impact of the moiré pattern formed in a MoSe$_2$/MoS$_2$ heterobilayer encapsulated in hexagonal boron nitride. The periodic in-plane potential results in a splitting of the MoSe$_2$ exciton and trion in both emission and absorption spectra. The observed energy difference between the split peaks is fully consistent with theoretical predictions.

cond-mat.mes-hall↗

arXiv2018

Towards Air Stability of Ultra-Thin GaSe Devices: Avoiding Environmental and Laser-Induced Degradation by Encapsulation

Gallium selenide (GaSe) is a novel two-dimensional material, which belongs to the layered III-VIA semiconductors family and attracted interest recently as it displays single-photon emitters at room temperature and strong optical non-linearity. Nonetheless, few-layer GaSe is not stable under ambient conditions and it tends to degrade over time. Here we combine atomic force microscopy, Raman spectroscopy and optoelectronic measurements in photodetectors based on thin GaSe to study its long-term stability. We found that the GaSe flakes exposed to air tend to decompose forming firstly amorphous selenium and Ga2Se3 and subsequently Ga2O3. While the first stage is accompanied by an increase in photocurrent, in the second stage we observe a decrease in photocurrent which leads to the final failure of GaSe photodetectors. Additionally, we found that the encapsulation of the GaSe photodetectors with hexagonal boron nitride (h-BN) can protect the GaSe from degradation and can help to achieve long-term stability of the devices.

cond-mat.mtrl-sci↗

arXiv2018

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 in a broad range of the visible spectrum through a fit of the acquired spectra to a Fresnel law based model.

cond-mat.mtrl-sci↗

arXiv2018

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.

cond-mat.mtrl-sci↗

arXiv2018

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.

cond-mat.mes-hall↗

arXiv2018

Simultaneous Assembly of van der Waals Heterostructures into Multiple Nanodevices

Van der Waals heterostructures (vdWH) are made of different two-dimensional (2D) layers stacked on top of each other, forming a single material with unique properties that differ from those of the individual 2D constituent layers, and that can be modulated through the interlayer interaction. These hetero-materials can be artificially made by mechanical stamping, solution processing or epitaxial growth. Alternatively, franckeite has been recently described as an example of a naturally-occurring vdWH that can be exfoliated down to nanometer thicknesses. Research on vdWHs has so far been limited to manually exfoliated and stamped individual devices. Here, a scalable and fast method to fabricate vdWH nanodevices from liquid phase exfoliated nanoflakes is reported. The transport and positioning of the flakes into localized submicrometer structures is achieved simultaneously in multiple devices via a dielectrophoretic process. The complex vdWH is preserved after dielectrophoresis and the properties of the resulting field-effect transistors are equivalent to those fabricated via mechanical exfoliation and stamping. The combination of liquid phase exfoliation and dielectrophoretic assembly is particularly suited for the study of vdWHs and applications where large-scale fabrication is required.

cond-mat.mtrl-sci↗

arXiv2018

Biaxial strain in atomically thin transition metal dichalcogenides

Strain engineering in single-layer semiconducting transition metal dichalcogenides aims to tune their bandgap energy and to modify their optoelectronic properties by the application of external strain. In this paper we study transition metal dichalcogenides monolayers deposited on polymeric substrates under the application of biaxial strain, both tensile and compressive. We can control the amount of biaxial strain applied by letting the substrate thermally expand or compress by changing the substrate temperature. After modelling the substrate-dependent strain transfer process with a finite elements simulation, 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 94 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 WS2 > WSe2 > MoS2 > MoSe2.

cond-mat.mtrl-sci↗

arXiv2018

Elucidating the Methylammonium (MA) Conformation in MAPbBr3 Perovskite with Application in Solar Cells

Hybrid organic-inorganic perovskites, MAPbX3 (X= halogen), containing methylammonium (MA: CH3-NH3+) in the large voids conformed by the PbX6 octahedral network, are the active absorption materials in the new generation of solar cells. CH3NH3PbBr3 is a promising alternative with a large band-gap that gives rise to a high open circuit voltage. A deep knowledge of the crystal structure and, in particular, the MA conformation inside the perovskite cage across the phase transitions undergone below room temperature, seems essential to establish structure-property correlations that may drive to further improvements. The presence of protons requires the use of neutrons, combined with synchrotron XRD data that help to depict subtle symmetry changes undergone upon cooling. We present a consistent picture of the structural features of this fascinating material, in complement with photocurrent measurements from a photodetector device, demonstrating the potential of MAPbBr3 in optoelectronics.

cond-mat.mtrl-sci↗

arXiv2017

Black phosphorus: A new bandgap tuning knob

Modern electronics rely on devices whose functionality can be adjusted by the end-user with an external 'knob'. A new tuning knob to modify the band gap of black phosphorus has been experimentally demonstrated.

cond-mat.mes-hall↗

arXiv2017

Recent progress in the assembly of nanodevices and van der Waals heterostructures by deterministic placement of 2D materials

Designer heterostructures can now be assembled layer-by-layer with unmatched precision thanks to the recently developed deterministic placement methods to transfer two-dimensional (2D) materials. This possibility constitutes the birth of a very active research field on the so-called van der Waals heterostructures. Moreover, these deterministic placement methods also open the door to fabricate complex devices, which would be otherwise very difficult to achieve by conventional bottom-up nanofabrication approaches, and to fabricate fully-encapsulated devices with exquisite electronic properties. The integration of 2D materials with existing technologies such as photonic and superconducting waveguides and fiber optics is another exciting possibility. Here, we review the state-of-the-art of the deterministic placement methods, describing and comparing the different alternative methods available in the literature and we illustrate their potential to fabricate van der Waals heterostructures, to integrate 2D materials into complex devices and to fabricate artificial bilayer structures where the layers present a user-defined rotational twisting angle.

physics.app-ph↗

arXiv2017

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

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 spectros-copy 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.

cond-mat.mes-hall↗

arXiv2017

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 Voc 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.

cond-mat.mes-hall↗

arXiv2017

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

We present a method to carry out electrical and opto-electronic measurements on 2D materials using carbon fiber microprobes to directly make electrical contacts to 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 conventional lithographic process. We also show that this method can be used with other 2D materials.

physics.ins-det↗

arXiv2017

A versatile scanning photocurrent mapping system to characterize optoelectronic devices based on 2D materials

The investigation of optoelectronic devices based on two-dimensional materials and their heterostructures is a very active area of investigation with both fundamental and applied aspects involved. We present a description of a home-built scanning photocurrent microscope that we have designed and developed to perform electronic transport and optical measurements of two-dimensional materials based devices. The complete system is rather inexpensive (<10000 EUR) and it can be easily replicated in any laboratory. To illustrate the setup we measure current-voltage characteristics, in dark and under global illumination, of an ultra-thin PN junction formed by the stacking of an n-doped few-layer MoS2 flake onto a p-type MoS2 flake. We then acquire scanning photocurrent maps and by mapping the short circuit current generated in the device under local illumination we find that at zero bias the photocurrent is generated mostly in the region of overlap between the n-type and p-type flakes.

cond-mat.mes-hall↗