Nanostructured Thermoelectric Chalcogenides

Thermoelectric materials are outstanding to transform temperature differences directly and reversibly into electrical voltage. Exploiting waste heat recovery as a source of power generation could help towards energy sustainability. Recently, the SnSe semiconductor was identified, in single-crystal form, as a mid-temperature thermoelectric material with record high figure of merit, high power factor and surprisingly low thermal conductivity. We describe the preparation of polycrystals of alloys of SnSe obtained by arc-melting; a rapid synthesis that results in strongly nanostructured samples with low thermal conductivity, advantageous for thermoelectricity, approaching the amorphous limit, around 0.3–0.5 W/mK. An initial screening of novel samples Sn1−xMxSe, by alloying with 3d and 4d transition metals such as M = Mn, Y, Ag, Mo, Cd or Au, provides for a means to optimize the power factor. M=Mo, Ag, with excellent values, are described in detail with characterization by x-ray powder diffraction (XRD), scanning electron microscopy (SEM), and electronic and thermal transport measurements. Rietveld analysis of XRD data demonstrates near-perfect stoichiometries of the above-mentioned alloys. SEM analysis shows stacking of nanosized sheets, with large surfaces parallel to layered slabs. An apparatus was developed for the simultaneous measurement of the Seebeck coefficient and electric conductivity at elevated temperatures

Impedance spectroscopy of encapsulated single graphene layers

In this work, we demonstrate the use of electrical impedance spectroscopy (EIS) for the disentanglement of several dielectric contributions in encapsulated single graphene layers. The dielectric data strongly vary qualitatively with the nominal graphene resistance. In the case of sufficiently low resistance of the graphene layers, the dielectric spectra are dominated by inductive contributions, which allow for disentanglement of the electrode/graphene interface resistance from the intrinsic graphene resistance by the application of an adequate equivalent circuit model. Higher resistance of the graphene layers leads to predominantly capacitive dielectric contributions, and the deconvolution is not feasible due to the experimental high frequency limit of the EIS technique

Ultra-broad spectral photo-response in FePS3 air-stable devices

Van der Waals materials with narrow energy gaps and efficient response over a broadband optical spectral range are key to widen the energy window of nanoscale optoelectronic devices. Here, we characterize FePS3 as an appealing narrow-gap p-type semiconductor with an efficient broadband photo-response, a high refractive index, and a remarkable resilience against air and light exposure. To enable fast prototyping, we provide a straightforward guideline to determine the thickness of few-layered FePS3 nanosheets extracted from the optical transmission characteristics of several flakes. The analysis of the electrical photo-response of FePS3 devices as a function of the excitation energy confirms a narrow gap suitable for near IR detection (1.23 eV) and, more interestingly, reveals a broad spectral responsivity up to the ultraviolet region. The experimental estimate for the gap energy is corroborated by ab-initio calculations. An analysis of photocurrent as a function of gate voltage and incident power reveals a photo-response dominated by photogating effects. Finally, aging studies of FePS3 nanosheets under ambient conditions show a limited reactivity of the outermost layers of flakes in long exposures to air

Nanostructured State-of-the-Art Thermoelectric Materials Prepared by Straight-Forward Arc-Melting Method

Thermoelectric materials constitute an alternative to harvest sustainable energy from waste heat. Among the most commonly utilized thermoelectric materials, we can mention Bi2Te3 (hole and electron conductivity type), PbTe and recently reported SnSe intermetallic alloys. We review recent results showing that all of them can be readily prepared in nanostructured form by arc-melting synthesis, yielding mechanically robust pellets of highly oriented polycrystals. These materials have been characterized by neutron powder diffraction (NPD), scanning electron microscopy (SEM) and electronic and thermal transport measurements. Analysis of NPD patterns demonstrates near-perfect stoichiometry of above-mentioned alloys and fair amount of anharmonicity of chemical bonds. SEM analysis shows stacking of nanosized sheets, each of them presumably single-crystalline, with large surfaces parallel to layered slabs. This nanostructuration affects notably thermoelectric properties, involving many surface boundaries (interfaces), which are responsible for large phonon scattering factors, yielding low thermal conductivity. Additionally, we describe homemade apparatus developed for the simultaneous measurement of Seebeck coefficient and electric conductivity at elevated temperatures

Strain engineering of 2D semiconductors

Departamento de Física de la Materia Condensada, Nanociencia y Biofísica (UAM).[EN] Strain engineering is a powerful tool that can be used to tune the optical and electronic properties of two-dimensional semiconductors such as TMDCs through the deformation of these materials. In this thesis I provide a whole description of the necessary details to perform optical measurements in function of the strain on two-dimensional materials like MoS2, MoSe2, WS2 or WSe2 by using experimental methods like micro-reflectance spectroscopy, photoluminescence and Raman spectroscopy. In the course of the thesis manuscript, I describe all the details that are needed for the calibration steps that will be useful for knowing the applied strain in the different experimental setups assembled in this work. Both for the method used for the application of uniaxial strain (by using the three-point flexural method on a polymer with a rectangular shape) and for the biaxial strain, thermal (through the thermal expansion of a polymer substrate) or mechanical (by bending a cruciform polymer substrate). In addition, I present a comparison to experimentally verify, as many theoretical works predict, that biaxial strain provides a more efficient way to tune the optical properties of MoS2 as compared with the uniaxial strain. In fact, we find that biaxial strain tunability or gauge factor is 2.3 times larger than the uniaxial strain gauge factor. I also show the behavior of the interlayer exciton in MoS2 flakes under the effect of the application of biaxial strain by using the thermal expansion method, finding that the gauge factor for this unconventional excitonic state is larger than the obtained for the A and B excitons. Moreover, the use of this method to apply strain will be crucial for the fabrication of microheaters that present a negligible thermal drift and a low time response. Some two-dimensional materials are air-sensitive materials and an inert atmosphere is necessary to manipulate them. That is the reason why I include an appendix in this thesis where I explore the use of a glove-less anaerobic chamber, a tool much more simple and convenient than a regular glove box for the effortless exfoliation and manipulation of the two-dimensional materials. Then I test the efficiency by comparing the stability of some air-sensitive materials such as black phosphorous or perovskites inside and outside the glove-less anaerobic chamber. Finally, I present another appendix where I show a method to fabricate large regions of atomically thin MoS2 layers through the sulfuration of MoO3. Areas of up to 300 × 300 μm2 with 2-4 layers in thickness are obtained. In addition, this layers are characterized by various techniques such as Raman spectroscopy, X-ray diffraction, transmission electron microscopy and electronic transport measurements, also finding a noticeable p-type behavior.[ES] La ingeniería de deformación es una potente herramienta que se puede emplear para modificar las propiedades tanto ópticas como electrónicas de semiconductores bidimensionales tal y como pueden ser los TMDCs por medio de la deformación de estos materiales. En esta tesis proporciono una descripción de todos los detalles para poder realizar medidas ópticas en función de la deformación, tanto uniaxial como biaxial, de materiales bidimensionales como el MoS2, MoSe2, WS2 o el WSe2, empleando métodos como la espectroscopía de micro-reflectancia, fotoluminiscencia y espectroscopía Raman. A lo largo de este manuscrito, también mostraré en detalle el método de calibración directa que nos indicará la cantidad de deformación aplicada para los distintos montajes experimentales que se han desarrollado a lo largo de esta tesis, tanto para el método empleado para la aplicación de deformación uniaxial (por medio del uso del método de flexión de tres puntos sobre un polímero con forma rectangular) como el usado para realizar deformación biaxial por expansión térmica (esta deformación se logra aprovechando la expansión térmica de un substrato de polímero al incrementar la temperatura) o mecánicamente (por medio de la deformación de un polímero que hace la función de substrato con forma de cruz). Además, realizo una comparativa con la que, tal y como muchos trabajos teóricos predicen, verifico experimentalmente la mayor eficiencia de la deformación biaxial mecánica frente a la deformación uniaxial con el fin de modificar la estructura de bandas del MoS2, encontrando un factor de calibre 2.3 veces mayor al aplicar deformación biaxial frente a la deformación uniaxial. También expongo cuál es el comportamiento del excitón de inter-capa al someter a copos de MoS2 a deformación biaxial por medio del método de expansión térmica, encontrando que el factor de calibre obtenido para este excitón es superior al de los excitones A y B. Además, el uso de este mismo método para la aplicación de deformación será clave para la fabricación de calefactores microscópicos con los que se obtiene una deriva térmica despreciable, así como una baja respuesta temporal. Algunos materiales bidimensionales son sensibles al aire y se necesita una atmósfera inerte para poder manipularlos. Por esta razón, en esta tesis también incluyo un anexo en el que exploro el uso de una cámara anaeróbica sin guantes, la cual es mucho más simple y conveniente que una cámara de guantes típica debido a que la exfoliación y la manipulación de estos materiales bidimensionales es mucho más sencilla con ella. Además, pruebo su eficacia por medio de la comparativa de materiales sensibles como el fósforo negro y las perovskitas tanto dentro como fuera de la cámara anaeróbica. Finalmente, añado un último anexo en el que muestro un método para fabricar grandes regiones de MoS2 por medio de la sulfuración de MoO3, obteniendo áreas de hasta 300 x 300 μm2 de 2 a 4 capas de espesor. Además, estas láminas delgadas de MoS2 se han caracterizado por medio de diversas técnicas como la espectroscopía Raman, la difracción de rayos X, microscopía electrónica de transmisión y medidas eléctricas de transporte, encontrando también un comportamiento tipo p destacable.Peer reviewe

Ingeniería de deformación de semiconductores 2D

Tesis Doctoral inédita leída en la Universidad Autónoma de Madrid, Facultad de Ciencias, Departamento de Física de la Materia Condensada. Fecha de Lectura: 26-04-202

Anisotropic Tunability of Vibrational Modes in Black Phosphorus Under Uniaxial Compressive/Tensile Strain

Abstract Strain engineering is a powerful strategy for tuning the optical, electrical, vibrational properties of 2D nanomaterials. In this work, a four‐point bending apparatus is constructed to apply both compressive and tensile strain on 2D anisotropic black phosphorus flake. Further polarized Raman spectroscopy is used to study the vibrational modes of black phosphorus flakes under uniaxial strain applied along various crystalline orientations. Here, a strong anisotropic blue/redshift of A1g, B2g, and A2g modes is found under compressive/tensile strain, respectively. Interestingly, mode A1g exhibits the maximum/minimum shift while mode B2g and mode A2g present the minimum/maximum shift when the strain is applied along armchair/zigzag direction. Density functional theory calculations are carried out to investigate the anisotropic strain response mechanism, finding that the strain‐induced regulation of the P─P bond angle, bond length, and especially interlayer interaction has a giant influence on the Raman shift

Impedance Spectroscopy of Encapsulated Single Graphene Layers

[EN] In this work, we demonstrate the use of electrical impedance spectroscopy (EIS) for the disentanglement of several dielectric contributions in encapsulated single graphene layers. The dielectric data strongly vary qualitatively with the nominal graphene resistance. In the case of sufficiently low resistance of the graphene layers, the dielectric spectra are dominated by inductive contributions, which allow for disentanglement of the electrode/graphene interface resistance from the intrinsic graphene resistance by the application of an adequate equivalent circuit model. Higher resistance of the graphene layers leads to predominantly capacitive dielectric contributions, and the deconvolution is not feasible due to the experimental high frequency limit of the EIS technique.This research was funded by the EU graphene flagship (grant Graphene Core3 881603), the EU FLAG-ERA project To2Dox (JTC-2019-009) and the Spanish MINECO (grant PID2020-118078RBI00).Peer reviewe

Impedance spectroscopy of encapsulated single graphene layers

In this work, we demonstrate the use of electrical impedance spectroscopy (EIS) for the disentanglement of several dielectric contributions in encapsulated single graphene layers. The dielectric data strongly vary qualitatively with the nominal graphene resistance. In the case of sufficiently low resistance of the graphene layers, the dielectric spectra are dominated by inductive contributions, which allow for disentanglement of the electrode/graphene interface resistance from the intrinsic graphene resistance by the application of an adequate equivalent circuit model. Higher resistance of the graphene layers leads to predominantly capacitive dielectric contributions, and the deconvolution is not feasible due to the experimental high frequency limit of the EIS technique.EU graphene flagshipMinisterio de Economía y Competitividad (España)Depto. de Física de MaterialesFac. de Ciencias FísicasTRUEpu

Eco-Friendly Disposable WS2 Paper Sensor for Sub-ppm NO2 Detection at Room Temperature

12 páginas, 8 figurasWe developed inexpensive and disposable gas sensors with a low environmental footprint. This approach is based on a biodegradable substrate, paper, and features safe and nontoxic electronic materials. We show that abrasion-induced deposited WS nanoplatelets on paper can be employed as a successful sensing layer to develop high-sensitivity and selective sensors, which operate even at room temperature. Its performance is investigated, at room temperature, against NO exposure, finding that the electrical resistance of the device drops dramatically upon NO adsorption, decreasing by ~42% (~31% half a year later) for 0.8 ppm concentration, and establishing a detection limit around~2 ppb (~3 ppb half a year later). The sensor is highly selective towards NO gas with respect to the interferents NH and CO, whose responses were only 1.8% (obtained for 30 ppm) and 1.5% (obtained for 8 ppm), respectively. Interestingly, an improved response of the developed sensor under humid conditions was observed (tested for 25% relative humidity at 23 C). The high-performance, in conjunction with its small dimensions, low cost, operation at room temperature, and the possibility of using it as a portable system, makes this sensor a promising candidate for continuous monitoring of NO on-site.This project has received funding from the European Research Council (ERC) under the European Union’s Horizon 2020 research and innovation program (grant agreement n◦755655, ERC-StG 2017 project 2 D-TOPSENSE) and the Ministry of Science and Innovation (Spain) through the project PID2020-115566 RB-I00 and RTI2018-095856-B-C22 (AEI/FEDER). The authors extend their sincere appreciation to the Distinguished Scientist Fellowship Program (DSFP) at King Saud University for funding of this work. D. M. acknowledges the financial support from the Fundación General CSIC via Programa ComFuturo