Papers by Andres Castellanos-Gomez
We explore the use of Si 3 N 4 /Si substrates as a substitute of the standard SiO 2 /Si substrate... more We explore the use of Si 3 N 4 /Si substrates as a substitute of the standard SiO 2 /Si substrates employed nowadays to fabricate nanodevices based on 2D materials. We systematically study the visibility of several 2D semiconducting materials that are attracting a great deal of interest in nanoelectronics and optoelectronics: MoS 2 , MoSe 2 , WSe 2 and black-phosphorus. We find that the use of Si 3 N 4 /Si substrates provides an increase of the optical contrast up to a 50%–100% and also the maximum contrast shifts towards wavelength values optimal for human eye detection, making optical identification of 2D semiconductors easier.
The possibility of spatially resolving the optical properties of atomically thin materials is esp... more The possibility of spatially resolving the optical properties of atomically thin materials is especially appealing as they can be modulated at the micro-and nanoscale by reducing their thickness, changing the doping level or applying a mechanical deformation. Therefore, optical spectroscopy techniques with high spatial resolution are necessary to get a deeper insight into the properties of two-dimensional materials. Here we study the optical absorption of single-and few-layer molybdenum disulfide (MoS 2) in the spectral range from 1.24 eV to 3.22 eV (385 nm to 1000 nm) by developing a hyperspectral imaging technique that allows one to probe the optical properties with diffraction limited spatial resolution. We find hyperspectral imaging very suited to study indirect bandgap semiconductors, unlike photoluminescence that only provides high luminescence yield for direct gap semiconductors. Moreover, this work opens the door to study the spatial variation of the optical properties of other two-dimensional systems, including non-semiconducting materials where scanning photoluminescence cannot be employed.
Titanium trisulfide (TiS3) has recently attracted the interest of the 2D community as it presents... more Titanium trisulfide (TiS3) has recently attracted the interest of the 2D community as it presents a direct bandgap of ~1.0 eV, shows remarkable photoresponse, and has a predicted carrier mobility up to 10000 cm 2 V-1 s-1. However, a study of the vibrational properties of TiS3, relevant to understanding the electron-phonon interaction which can be the main mechanism limiting the charge carrier mobility, is still lacking. In this work, we take the first steps to study the vibrational properties of TiS3 through temperature dependent Raman spectroscopy measurements of TiS3 nanoribbons and nanosheets. Our investigation shows that all the Raman modes linearly soften (red shift) as the temperature increases from 88 K to 570 K, due to the anharmonic vibrations of the lattice which also includes contributions from the lattice thermal expansion. This softening with the temperature of the TiS3 modes is more pronounced than that observed in other 2D semiconductors such as MoS2, MoSe2, WSe2 or black phosphorus (BP). This marked temperature dependence of the Raman could be exploited to determine the temperature of TiS3 nanodevices by using Raman spectroscopy as a non-invasive and local thermal probe. Interestingly, the TiS3 nanosheets show a stronger temperature dependence of the Raman modes than the nanoribbons, which we attribute to a lower interlayer coupling in the nanosheets. This is the post-peer reviewed version of the following article: A.S. Pawbake et al. " Temperature dependent Raman spectroscopy of titanium trisulfide (TiS3) nanoribbons and nanosheets " ACS Applied Materials and Interfaces (2015)
We present a study of the electronic and optical bandgap in layered TiS 3 , an almost unexplored ... more We present a study of the electronic and optical bandgap in layered TiS 3 , an almost unexplored semiconductor that has attracted recent attention because of its large carrier mobility and inplane anisotropic properties, to determine its exciton binding energy. We This is the post-peer reviewed version of the following article: A.J. Molina-Mendoza et al. " Electronic bandgap and exciton binding energy of layered semiconductor TiS3 " Advanced Electronic Materials vol. 1, issue 9 (2015) 2 combine scanning tunneling spectroscopy and photoelectrochemical measurements with random phase approximation and Bethe-Salpeter equation calculations to obtain the electronic and optical bandgaps and thus the exciton binding energy. We find experimental values for the electronic bandgap, optical bandgap and exciton binding energy of 1.2 eV, 1.07 eV and 130 meV, respectively, and 1.15 eV, 1.05 eV and 100 meV for the corresponding theoretical results. The exciton binding energy is orders of magnitude larger than that of common semiconductors and comparable to bulk transition metal dichalcogenides, making TiS 3 ribbons a highly interesting material for optoelectronic applications and for studying excitonic phenomena even at room temperature.
ABSTRACT: Hybrid devices combining 2Ds with other nanomaterials have demonstrated high performanc... more ABSTRACT: Hybrid devices combining 2Ds with other nanomaterials have demonstrated high performance in optoelec-tronic applications, but these devices are limited to combinations between 2D materials and colloidal quantum dots so far. We present an easy drop-casting based functionalization of MoS 2-based photodetectors that results in an enhancement of the photoresponse of about four orders of magnitude, reaching responsivities up to 100 A·W-1. The functionaliza-tion is technologically trivial, air-stable, fully reversible and reproducible, and opens the door to the combination of 2D-materials with molecular dyes for the development of high performance photodetectors.
The fabrication of artificial materials by stacking of individual two-dimensional (2D) materials ... more 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 10 3 , 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.
P-n junctions based on vertically stacked single or few layer transition metal dichalcogenides (T... more 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, nor 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 MoS 2 into n-and p-type has been demonstrated. MoS 2 is an interesting material to use for optoelectronic applications due to the 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 MoS 2 :Fe (n-type) and MoS 2 :Nb (p-type). The junctions exhibit a peak external quantum efficiency of 4.7 %, 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 to 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.
This is the post-peer reviewed version of the following article: J. Quereda et al. " Strong quant... more This is the post-peer reviewed version of the following article: J. Quereda et al. " Strong quantum confinement effect in the optical properties of ultrathin α-In2Se3 " .
The fabrication of van der Waals heterostructures, artificial materials assembled by individually... more The fabrication of van der Waals heterostructures, artificial materials assembled by individually stacking atomically thin (2D) materials, is one of the most promising directions in 2D materials research. Until now, the most widespread approach to stack 2D layers relies on deterministic placement methods which are cumbersome when fabricating multilayered stacks. Moreover, they tend to suffer from poor control over the lattice orientations and the presence of unwanted adsorbates between the stacked layers. Here, we present a different approach to fabricate ultrathin heterostructures by exfoliation of bulk franckeite which is a naturally occurring and air stable van der Waals heterostructure (composed of alternating SnS 2 -like and PbS-like layers stacked on top of each other). Presenting both an attractive narrow bandgap (<0.7 eV) and ptype doping, we find that the material can be exfoliated both mechanically and chemically down to few-layer thicknesses. We present extensive theoretical and experimental characterizations of the material's electronic properties and crystal structure, and explore applications for near-infrared photodetectors (exploiting its narrow bandgap) and for p-n junctions based on the stacking of MoS 2 (n-doped) and franckeite (p-doped).
The recent isolation of atomically thin black phosphorus by mechanical exfoliation of bulk layere... more The recent isolation of atomically thin black phosphorus by mechanical exfoliation of bulk layered crystals has triggered an unprecedented interest, even higher than that raised by the first works on graphene and other two-dimensional, in the nanoscience and nanotechnology community. In this Perspective we critically analyze the reasons behind the surge of experimental and theoretical works on this novel two-dimensional material. We believe that the fact that black phosphorus band gap value spans over a wide range of the electromagnetic spectrum that was not covered by any other two-dimensional material isolated to date (with remarkable industrial interest such as thermal imaging, thermoelectrics, fiber optics communication, photovoltaics, etc), its high carrier mobility, its ambipolar field-effect and its rather unusual in-plane anisotropy drew the attention of the scientific community towards this two-dimensional material. Here we also review the current advances, the future directions and the challenges in this young research field.
The isolation of mechanically exfoliated graphene in 2004 1 sparked the research on two dimension... more The isolation of mechanically exfoliated graphene in 2004 1 sparked the research on two dimensional materials which keeps growing at a tremendous rate. It is often argued that the success of this research field is sustained by the potential of these materials to have a disruptive impact in different technological areas such as electronics and optoelectronics. However, at this stage most of the works on these systems are still focused on addressing fundamental questions and thus we might fall in the risk of selling empty promises and creating expectations in the society that might not be fulfilled in a near future. One of the real key factors behind the rise of this research field, on the other hand, is that mechanical exfoliation has democratized material science as high quality samples, showing an interesting plethora of physical phenomena, can be prepared in almost any laboratory even without specialized and expensive equipment. Just with a piece of a bulk layered material, a roll of tape and an optical microscope any trained researcher can isolate atomically thin layers of many different 2D materials ranging from wide band gap insulators to superconductors. Since 2010/2011 this relatively young field has experienced a new boost, originating from the works on the semiconducting 'cousins' of graphene, i.e. atomically thin, or, two dimensional, semiconductors). 2-4 Are we just entering new 'hype' phase, or are there intrinsic and profound reasons to justify this excitement of the scientific community.
We present a method to carry out electrical and opto-electronic measurements on 2D materials usin... more 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. Isolation of 2D materials by mechanical exfoliation of bulk layered crystals 1 has changed the paradigm of materials science, where typically extremely high quality samples are required to observe intriguing physical phenomena and to understand their origin 2–4. Until the isolation of graphene 5 and other 2D materials 6–9 , those high quality samples were grown by quite sophisticated methods (molecular beam epitaxy, pulsed laser deposition, etc), accessible to only few well-equipped and highly experienced laboratories
The potential of bulk black-phosphorus, a layered semiconducting material with a direct band gap ... more The potential of bulk black-phosphorus, a layered semiconducting material with a direct band gap of ~0.3 eV, for thermoelectric applications has been experimentally studied. The Seebeck Coefficient (S) has been measured in the temperature range from 300 K to 385 K, finding a value of S = +335 ± 10 µV/K at room temperature (indicating a naturally occurring p-type conductivity). S increases with temperature, as expected for p-type semiconductors, which can be attributed to an increase of the charge carrier density. The electrical resistance drops up to a 40 % while heating in the studied temperature range. As a consequence, the power factor at 385 K is 2.7 times higher than that at room temperature. This work indicates the prospective use of black-phosphorus in thermoelectric applications such as thermal energy scavenging, which typically require devices with high performance at temperatures near room temperature.
and (A.C-G.) [email protected] KEYWORDS. Black phosphorus, strain engineering, uniaxia... more and (A.C-G.) [email protected] KEYWORDS. Black phosphorus, strain engineering, uniaxial strain, local strain, periodic deformation, quantum confinement, optical absorption. This is the post-peer reviewed version of the following article: J. Quereda et al. "Strong modulation of optical properties in black phosphorus through strain-engineered rippling"
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Papers by Andres Castellanos-Gomez