Two-dimensional III-VIA semiconductors and their applications in optoelectronic devices

Tesis doctoral inédita leída en la Universidad Autónoma de Madrid, Facultad de Ciencias, Departamento de Física de la Materia Condensada. Fecha de lectura: 04-12-202

Black phosphorus: narrow gap, wide applications

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.Comment: Updated version of the perspective article about black phosphorus, including all the feedback received from arXiv users + reviewer

Matériaux 2D TMDC pour la génération d'hydrogène par photo-décomposition de l'eau

Le stockage de l'énergie solaire en énergie chimique est une approche hautement souhaitable pour résoudre le défi énergétique. Les cellules photo-électrochimiques combinent la collecte de l'énergie solaire et la décomposition de l'eau. Les nanofeuillets semiconducteur 2D de di-chalcogenures de métaux de transition (TMDC) sont considérés comme des matériaux attrayants pour l'élaboration de photocatalyseur efficace pour la conversion de l'énergie solaire en hydrogène. Malgré les propriétés optoélectroniques uniques des TMDC, la passivation de défauts de surface présents en concentration élevée est un défi important pour le développement de cette classe de matériaux. Dans ce contexte, le présent travail a concerné l'élaboration d'un photocatalyseur 2D TMDC pour la photo-décomposition de l'eau. Le développement de photocatalyseurs de haute performance a été examiné suivant deux axes principaux. Un premier axe de recherche consiste à passiver les défauts de surface des nanofeuillets 2D p-WSe2 à l'aide de complexes Mo-S pour diminuer la recombinaison des porteurs de charge photo-générés et améliorer l'activité photocatalytique. Nous avons démontré que des couches ultra minces de complexes thio et oxo-thio-Mo moléculaires représentent une classe idéale de catalyseurs, bien adaptée pour fonctionnaliser les matériaux 2D car ils sont stables dans des environnements aqueux, bon marché, respectueux de l'environnement. Des densités de courant de -2 mA cm-2 à -0.2 V par rapport à l'électrode d'hydrogène (NHE) ont été obtenues pour la nouvelle photocathode p-WSe2/ MoxSy. En plus d'offrir une activité électro-catalytique élevée, les films complexes Mo se sont révélés capables de guérir les défauts de surface. Les contributions respectives aux effets catalytiques et cicatrisants observées expérimentalement pour les divers complexes moléculaires de Mo impliquaient la forte adsorption sur les défauts ponctuels du substrat 2D WSe2 de complexes de Mo tels que (MoS4)2-, (MoOS3)2- et (Mo2S6O2)2-. Il a été démontré que ces couches de co-catalyseur Mo-S formés à un pH bien défini présentent un comportement n-semi-conducteur et l'ingénierie des bandes formées avec p-WSe2 s'est révélée appropriée pour assurer la séparation des charges et la migration efficace des électrons photo-induits pour la RDH, représentant un exemple de couche de passivation multi-composant avec de multiples propriétés. Un deuxième axe de travail concerne l'optimisation de la nanostructure du film de WSe2 comme objectif l'obtention d'une surface spécifique élevée et des parois de pores composées de quelques monofeuillets. Des films de WSe2 nanostructurés de haute surface et à bonne collecte de porteurs de charge ont été obtenus par co-assemblage des nanofeuillets de WSe2 et des nanofeuillets d'oxyde de graphène réduit (rGO) avec un rapport de nanofeuillets rGO/WSe2 optimal. Après dépôt de la couche ultra mince de co-catalyseur les nouvelles nanojunctions de rGO-WSe2/MoxSy ont présenté des photocourants jusqu'à -5 mA cm-2 à -0.2 V vs NHE. Des rendements quantiques d'environ 10% ont été obtenus pour des films de nanoplaquettes de WSe2 de 70 nm d'épaisseur en présence de rGO et catalyseur MoxSy. Le potentiel de notre procédé d'auto-assemblage en présence de nanofeuillets de TMDC et de rGO a été illustré avec l'obtention de microstructure optimisée de films de p-MoS2, p-WSe2 et p-WS2. Dans une perspective à plus long terme, les photoelectrodes basées sur des nanofeuillets de TMDC sont des candidats intéressants pour la conception de cellules tandem offrant des valeurs de band-gap (Eg) et présentant des potentiels de dégagement d'hydrogène modulables.Collecting and storing solar energy in chemical energy is a highly desirable approach to solve energy challenge. The great potential of a photoelectrochemical cell technology combines the harvesting of solar energy with the water splitting into a single device. 2D semiconducting nanosheets of Transition Metal Di-Chalcogenides (TMDC) are seen as an attractive material to design an efficient photocatalyst for the conversion of solar energy into hydrogen. Despite the unique optoelectronic properties of the TMDCs, the passivation of surface defects in high concentration is a remaining challenge for the development of this class of materials. In this context, the present work has aimed the elaboration of thin 2D TMDC photocatalyst for solar water splitting. The development of high performance photocatalysts was evaluated following two main axis. A first strategy consists in the surface defects passivation of 2D p-WSe2 nanosheets using Mo-S complexes to decrease the photogenerated charge carrier recombination and improve photocatalytic activity. We demonstrated these Mo thio and oxo-thio- molecular complexes films as an ideal class of catalysts, well-suited to functionalize 2D materials since they are stable in aqueous environments, cheap and environmentally benign. Current densities of -2 mA cm-2 at -0.2 V vs NHE electrode were obtained for the new p-WSe2/MoxSy photocathode. Besides developing high electro-catalytic activity, the Mo complexes films were shown to display ability to heal surface defects. The respective contributions in catalytic and healing effects observed experimentally for the various molecular Mo complexes involved strong adsorption on point defects of the 2D WSe2 substrate of Mo complexes such as (MoS4)2-, (MoOS3)2-and (Mo2S6O2)2-. The Mo complexes films spontaneously formed at well-defined pH were demonstrated to present n-semi-conducting behaviour and band engineering formed with p-WSe2 showed to be suitable for ensuring charge separation and efficient migration of the photo-induced electrons for the Hydrogen Evolution Reaction, thus representing an example of multicomponent passivation layer exhibiting multiple properties. A second strategy focus in the nanostructure optimization of WSe2 with high specific surface area and pore walls composed of few layers. Nanostructured WSe2 films of high surface area and good charge carrier collection were obtained by co-assembling WSe2 nanosheets and reduced graphene oxide (rGO) nanosheets with an optimal rGO/WSe2 nanosheet ratio. After deposition of co-catalyst thin layer, the new layered nanojunctions of rGO-WSe2/MoxSy exhibited photocurrents up to -5 mA cm-2 at -0.2V vs NHE. Incident-photon-to-current efficiency conversion of 10% were achieved for WSe2 nanoflakes of 70 nm thickness in presence of rGO and MoxSy co-catalyst. The potential of our self-assembly process in the presence of TMDC and rGO nanosheets has been illustrated with the obtention of optimized microstructures of p-MoS2, p-WSe2 and p-WS2. In a long-term perspective, photoelectrodes based on TMDC nanosheets are interesting candidates for the design of tandem cells offering modular band-gap (Eg) values and hydrogen onset potentials

Solvent Exfoliation of Electronic-Grade, Two-Dimensional Black Phosphorus

Solution dispersions of two-dimensional (2D) black phosphorus (BP), often referred to as phosphorene, are achieved by solvent exfoliation. These pristine, electronic-grade BP dispersions are produced with anhydrous, organic solvents in a sealed tip ultrasonication system, which circumvents BP degradation that would otherwise occur via solvated oxygen or water. Among conventional solvents, n-methyl-pyrrolidone (NMP) is found to provide stable, highly concentrated (~0.4 mg/mL) BP dispersions. Atomic force microscopy, scanning electron microscopy, transmission electron microscopy, Raman spectroscopy, and X-ray photoelectron spectroscopy show that the structure and chemistry of solvent-exfoliated BP nanosheets are comparable to mechanically exfoliated BP flakes. Additionally, residual NMP from the liquid-phase processing suppresses the rate of BP oxidation in ambient conditions. Solvent-exfoliated BP nanosheet field-effect transistors (FETs) exhibit ambipolar behavior with current on/off ratios and mobilities up to ~10000 and ~50 cm^2/(V*s), respectively. Overall, this study shows that stable, highly concentrated, electronic-grade 2D BP dispersions can be realized by scalable solvent exfoliation, thereby presenting opportunities for large-area, high-performance BP device applications.Comment: 6 figures, 31 pages, including supporting informatio

All-Inorganic Metal Halide Perovskite Nanocrystals: Opportunities and Challenges.

The past decade has witnessed the growing interest in metal halide perovskites as driven by their promising applications in diverse fields. The low intrinsic stability of the early developed organic versions has however hampered their widespread applications. Very recently, all-inorganic perovskite nanocrystals have emerged as a new class of materials that hold great promise for the practical applications in solar cells, photodetectors, light-emitting diodes, and lasers, among others. In this Outlook, we first discuss the recent developments in the preparation, properties, and applications of all-inorganic metal halide perovskite nanocrystals, with a particular focus on CsPbX3, and then provide our view of current challenges and future directions in this emerging area. Our goal is to introduce the current status of this type of new materials to researchers from different areas and motivate them to explore all the potentials

Light Generation and Harvesting in a Van der Waals Heterostructure

Two-dimensional (2D) materials are a new type of materials under intense study because of their interesting physical properties and wide range of potential applications from nanoelectronics to sensing and photonics. Monolayers of semiconducting transition metal dichalcogenides MoS2 or WSe2 have been proposed as promising channel materials for field-effect transistors (FETs). Their high mechanical flexibility, stability and quality coupled with potentially inexpensive production methods offer potential advantages compared to organic and crystalline bulk semiconductors. Due to quantum mechanical confinement, the band gap in monolayer MoS2 is direct in nature, leading to a strong interaction with light that can be exploited for building phototransistors and ultrasensitive photodetectors. Here, we report on the realization of light-emitting diodes based on vertical heterojunctions composed of n-type monolayer MoS2 and p-type silicon. Careful interface engineering allows us to realize diodes showing rectification and light emission from the entire surface of the heterojunction. Electroluminescence spectra show clear signs of direct excitons related to the optical transitions between the conduction and valence bands. Our pn diodes can also operate as solar cells, with typical external quantum efficiency exceeding 4%. Our work opens up the way to more sophisticated optoelectronic devices such as lasers and heterostructure solar cells based on hybrids of two-dimensional (2D) semiconductors and silicon.Comment: Submitted versio

Two-Dimensional Halide Perovskites for Emerging New- Generation Photodetectors

Compared to their conventional three-dimensional (3D) counterparts, two-dimensional (2D) halide perovskites have attracted more interests recently in a variety of areas related to optoelectronics because of their unique structural characteristics and enhanced performances. In general, there are two distinct types of 2D halide perovskites. One represents those perovskites with an intrinsic layered crystal structure (i.e. MX6 layers, M = metal and X = Cl, Br, I), the other defines the perovskites with a 2D nanostructured morphology such as nanoplatelets and nanosheets. Recent studies have shown that 2D halide perovskites hold promising potential for the development of new-generation photodetectors, mainly arising from their highly efficient photoluminescence and absorbance, color tunability in the visible-light range and relatively high stability. In this chapter, we present the summary and highlights of latest researches on these two types of 2D halide perovskites for developing photodetectors, with an emphasis on synthesis methods, structural characterization, optoelectronic properties, and theoretical analysis and simulations. We also discuss the current challenging issues and future perspective. We hope this chapter would add new elements for understanding halide perovskite-based 2D materials and for developing their more efficient optoelectronic devices

Surface Chemistry Control of 2D Nanomaterial Morphologies, Optoelectronic Responses, and Physicochemical Properties

Indiana University-Purdue University Indianapolis (IUPUI)The field of two-dimensional (2D) nanomaterials first began in earnest with the discovery of graphene in 2004 due to their unique shape-dependent optical, electronic, and mechanical properties. These properties arise due to their one-dimensional confinement and are further influenced by the elemental composition of the inorganic crystal lattice. There has been an intense focus on developing new compositions of 2D nanomaterials to take advantage of their intrinsic beneficial properties in a variety of applications including catalysis, energy storage and harvesting, sensing, and polymer nanocomposites. However, compared to the field of bulk materials, the influence of surface chemistry on 2D nanomaterials is still underdeveloped. 2D nanomaterials are considered an “all-surface” atomic structure with heights of a single to few layers of atoms. The synthetic methods used to produce 2D materials include bottom-up colloidal methods and top-down exfoliation related techniques. Both cases result in poorly controlled surface chemistry with many undercoordinated surface atoms and/or undesirable molecules bound to the surface. Considering the importance surfaces play in most applications (i.e., catalysis and polymer processing) it is imperative to better understand how to manipulate the surface of 2D nanomaterials to unlock their full technological potential. Through a focus of the ligand-surface atom bonding in addition to the overall ligand structure we highlight the ability to direct morphological outcomes in lead free halide perovskites, maximize optoelectronic responses in substoichiometric tungsten oxide, and alter physicochemical properties titanium carbide MXenes. The careful control of precursor materials including poly(ethylene glycol) (PEG) surface ligands during the synthesis of bismuth halide perovskites resulted in the formation of 2D quasi-Ruddlesden-Popper phase nanomaterials. Through small angle X-ray scattering (SAXS) and in conjunction with X-ray photoelectron spectroscopy (XPS) we were able to conclude that an in-situ formation of an amino functional group on our PEG-amine ligand was inserted into the perovskite crystal lattice enabling 2D morphology formation. Additionally, through UV-vis absorption and ultraviolet photoelectron spectroscopies we were able to develop a complete electronic band structure of materials containing varying halides (i.e., Cl, Br, and I). Furthermore, through the increased solubility profile of the PEG ligands we observed solvent controlled assemblies of varying mesostructures. We developed an ex-situ ligand treatment to manipulate the localized surface plasmon resonance (LSPR) response of anion vacancy doped tungsten oxide (WO3-x) nanoplatelets (NPLs). Upon ligand treatment to alter the surface passivating ligand from carboxylic acid containing myristic acid (MA) to tetradecylphosphonic acid (TDPA) we observed a >100 nm blue shift in the LSPR response. Using Fourier transform infrared (FTIR) and Raman spectroscopies in conjunction with DFT calculated Raman spectra we were able to conclude this shift was due to the formation of tridentate phosphonate bonds on the NPLs surface. Phosphonate bonding allows for an increase in surface passivation per ligand decreasing surface trapped electrons. These previously trapped electrons were then able to participate as free electrons in the LSPR response. Electron paramagnetic spectroscopy (EPR) further supported this decrease in surface traps through a decrease and shift of the EPR signal related to metal oxide surface trapped electrons. Lastly, using our knowledge of PEG ligands we were able to modify esterification chemistry to covalently attach PEG ligands to a MXene surface. The successful formation of an ester bond between a carboxylic acid containing PEG ligand and hydroxyl terminating group on the MXene surface was supported by FTIR spectroscopy and thermogravimetric analysis. The attachment of PEG resulted in a drastic change in the hydrophilicity of the MXene surface. Where MXenes were previously only processed in extremely polar solvents the PEG attachment allowed for high dispersibility in a wide range of polar and non-polar organic solvents, effectively increasing their processability. Further, this chemistry was modified to include an additional functional group on the PEG ligand to increase the valency of the post-modification MXene nanoflakes. Overall, work presented in this dissertation represents the development and application of surface chemistry to relatively new 2D nanomaterials. We believe our work significantly increases the knowledge of 2D halide perovskite formation, manipulation of LSPR active metal oxide materials, and the future processing of MXene materials

Structural and Optical Characterization of III-V Nanostructures Monolithically Grown on Si Substrates

Group III-V semiconductor nanostructures have emerged as an important material platform over the past decades for wide-range device implementation in the field of electronics and optoelectronics. Among them, nanowires (NWs) are particularly attractive owing to the elastic strain relaxation through their sidewall facets which allows for the combination of lattice mismatched materials. Hence, optically active III-V materials become compatible with the mature and prevalent Si platform. Moreover, NWs are ideal for hosting quantum dots (QDs) ensuring their deterministic positioning and uniformity. This configuration opens the route for sophisticated applications including single photon emission, a crucial function in quantum information processing. In addition, another type of nanostructures that has attracted attention is the two-dimensional nanosheets, whose principal benefit is the band structure tuning from bulk to 2D by modulation of their thickness. Consequently, they are established as promising blocks for various optoelectronic devices and applications. In the current thesis, we reported the growth of self-catalysed AlGaAs NWs monolithically on Si (111) substrates via solid-source molecular beam epitaxy (MBE). The self-formation of an Al-rich shell is exhibited, which is thicker at the base and thins down towards the NW tip, while it demonstrates wide alloy fluctuations. The predominantly ZB structure presents twin defects and occasional WZ insertions, further increasing the intricacy of the NWs. The optical probing via photoluminescence reveals fully tuneable emission with the Al content of the alloy. Among the morphological variations of AlGaAs NWs, the branched NWs are of unique interest. The branching events increase with Al content, while the branches are confirmed to grow on the NW trunks epitaxially. In addition, complex compositional distribution in the branches is presented, as Ga-rich stripes along the growth direction of the branches, attributed to the different nucleation energies on different faces at the liquid/solid interface of the branch, intersect with Ga-rich stripes perpendicular to them, deriving from the rotation of the sample during growth. Moreover, self-catalysed, single GaAs/AlGaAs dot-in-wire structures have been designed and grown by inserting a short GaAs segment in each AlGaAs NW. The exhaustive optical probing reveals centrally localized peaks, with a decently narrow linewidth of roughly 490 μeV. The QD emission is comprised of an exciton and a biexciton transition, while a high degree of polarization is noticed when compared to the AlGaAs NW-related emission. The above characteristics are important steps towards achieving single photon emission. Finally, we optically inspect InAs nanosheets grown via MBE via photoluminescence measurements. Pristine nanosheets exhibit surface charge via carrier trapping mechanisms at the surface states, which is suggestive of the “memory effect”. The impact of sulphur passivation and core/shell configuration on the optical response of the nanosheets is evaluated. In addition, we fabricated an optoelectronic memory unit based on pristine InAs nanosheets, adopting a field-effect transistor configuration, which demonstrates negative photoresponse with good reproducibility and ultra-low power consumption

Anisotropic Dielectric Breakdown Strength of Single Crystal Hexagonal Boron Nitride

Dielectric breakdown has historically been of great interest from the perspectives of fundamental physics and electrical reliability. However, to date, the anisotropy in the dielectric breakdown has not been discussed. Here, we report an anisotropic dielectric breakdown strength (EBD) for h-BN, which is used as an ideal substrate for two-dimensional (2D) material devices. Under a well-controlled relative humidity, EBD values in the directions both normal and parallel to the c axis (EBD+c & EBD//c) were measured to be 3 and 12 MV/cm, respectively. When the crystal structure is changed from sp3 of cubic-BN (c-BN) to sp2 of h-BN, EBD+c for h-BN becomes smaller than that for c-BN, while EBD//c for h-BN drastically increases. Therefore, h-BN can possess a relatively high EBD concentrated only in the direction parallel to the c axis by conceding a weak bonding direction in the highly anisotropic crystal structure. This explains why the EBD//c for h-BN is higher than that for diamond. Moreover, the presented EBD value obtained from the high quality bulk h-BN crystal can be regarded as the standard for qualifying the crystallinity of h-BN layers grown via chemical vapor deposition for future electronic applications