Electronic Transport in Two-Dimensional Materials.

The isolation of graphene has sparked an unprecedented world wide research activity on the topic of two-dimensional (2D) materials. However, even though graphene has a high potential for various applications, its metallic character is a significant limitation for a large variety of electron devices, starting with the conventional field effect transistor. This limitation could be overcome in the near future by the use of layered van der Waals crystals such as black phosphorus (i.e., phosphorene, which is one to few layers of black phosphorus) and also by other semiconducting 2D materials such as transition metal dichalcogenides (TMDCs) of general formula MX2, where M is the metal and X the chalcogen (S, Se and Te). For instance, phosphorene exhibits a direct band gap varying between ~ 0.3 eV for bulk crystals to ~ 2 eV for a monolayer. In layered TMDC structures, each layer typically comprises 3 atomic planes (corresponding to a thickness below 8 Å), and consists of an hexagonally-organized atomic layer of metal atoms sandwiched between two planes of chalcogen atoms, also in hexagonal configurations. M–X bonds inside each layer are covalent; however, the sandwiching chalcogen layers are linked by weak van der Waals bonds, so that a bulk crystal is easy to cleave along the chalcogen planes. As with phosphorene, most semiconducting TMDC materials also see their band gap increasing as thickness decreases and they switch from indirect to direct band gap for a monolayer. To date, the most studied semiconductor TMDCs are MoS2 and WS2. Because they are atomically thin, 2D materials are particularly well suited for flexible electronics applications. After a brief description of the major features of phosphorene and TMDCs (crystal structures, relevant electronic properties as a function of thickness …), the talk will highlight recent results concerning field effect transistor characteristics. In particular, carrier mobility values as high as 5000 cm2/Vs have been measured in phosphorene films. Since local substitutional-type doping cannot be used in 2D materials, there is a general contacting problem (Schottky barrier formation), which is of paramount importance for device applications. In particular, two types of contacts can be formed: the top contact and the edge contact, which behave differently. Generally speaking, the semiconductor thickness is much smaller than the depletion length at the contact, which is an intriguing and novel situation. In the second part of the talk, various synthesis methods will be presented and discussed, particularly in view of their possible use in the field of large area electronics. For most devices fabricated so far, the 2D layers were obtained by exfoliation of bulk crystals. However, recent progress in synthesis include molecular beam epitaxy, controlled chemical vapour deposition and vapour phase transport. Those various approaches will be discussed, as well as the newly-introduced atomic layer deposition (ALD) process. As most 2D materials are grown on a separate substrate, the transfer problem will be highlighted, particularly concerning phosphorene which is highly hygroscopic, and sensitive to the presence of light. The presentation will be concluded by some perspectives concerning the industrialization of 2D materials.

Since the discovery of Dirac physics in graphene, research in 2D materials has exploded with the aim of finding new materials and harnessing their unique and tunable electronic and optical properties. The follow-on work on 2D dielectrics and semiconductors has led to the emergence and development of hexagonal boron nitride, black phosphorus, and transition metal disulfides. However, the spectrum of good insulating materials is still very narrow. Likewise, 2D materials exhibiting correlated phenomena such as superconductivity, magnetism, and ferroelectricity have yet to be developed or discovered. These properties will significantly enrich the spectrum of functional 2D materials, particularly in the case of high phase-transition temperatures. They will also advance a fascinating fundamental frontier of size and proximity effects on correlated ground states. Here, a broad family of layered metal thio(seleno)phosphate materials that are moderate- to wide-bandgap semiconductors with incipient ionic conductivity and a host of ferroic properties are reviewed. It is argued that this material class has the potential to merge the sought-after properties of complex oxides with electronic functions of 2D and quasi-2D electronic materials, as well as to create new avenues for both applied and fundamental materials research in structural and magnetic correlations.

Recently, black phosphorus (BP), an emerging layered two-dimensional (2D) material, has aroused much research interest. Distinguished from most of other 2D materials, BP is always a direct bandgap semiconductor regardless of the thickness, with the bandgap ranging from 0.3 eV (bulk) to 1.7 eV (monolayer), which is just fill the gap in the bandgap between graphene and transition metal dichalcogenides (TMDCs) in this frequency range. Besides, the BP exhibits many intriguing properties, such as high carrier mobility, highly tunable and anisotropic physical properties, which render the BP another star 2D material following graphene and TMDCs. In this review, we mainly focus on the advances in the optical properties of 2D BP, with the content covering the intrinsic optical properties and external perturbation effects on optical properties. Regarding the intrinsic optical properties, we introduce the anisotropic and layer-dependent optical absorption from interband transitions, the layer-dependent exciton binding energy and exciton absorption, visible-to-infrared photoluminescence, and stability of absorption and photoluminescence. As for external perturbation effects on optical properties, we introduce in-plane uniaxial and biaxial strain effects, gate-induced quantum confined Franz-Keldysh effect and Burstein-Moss effect. And finally we give a brief summary and outlook, pointing out some several interesting and important issues of BP, which need further studying urgently such as hyperbolic plasmons, intersubband transitions, optical properties in heterostructures and twist angle moiré superlattice and so on. This review gives an overview of the optical properties of BP and is expected to arouse the interest in further studying the BP.

The fascinating two-dimensional (2D) materials are being unconsciously applied in various fields from science to engineering, which is benefited from the glamorous physical and chemical properties of mechanics, optics, electronics, and magnetism. The representative 2D thermoelectric/piezoelectric materials can directly convert thermal/mechanical energy into electrical energy, which can resolve the energy issues and avoid further environmental deterioration. The thermoelectric or piezoelectric properties of various 2D materials, such as graphene, hexagonal boron nitride, black phosphorus, transition metal dichalcogenides (TMDs), arsenene, metal carbides and nitrides (MXenes), and so on, have been investigated in detail. Although tremendous progress has been achieved in the past few years, these properties still need to be improved for their practical application by designing new 2D materials, strain engineering, chemical functionalization, etc. In addition to this, in 2D materials, there are many other novel physical properties, such as magnetism, topology, valley, and so on. The combination of thermoelectricity/piezoelectricity with other unique properties may lead to novel device applications or scientific breakthroughs in new physics. Overall, the emergence of 2D thermoelectric and piezoelectric materials has expanded energy conversion research dramatically. By combing this new device concept with the novel 2D materials, original devices should have potential applications in energy harvesting.

Two-dimensional (2D) materials are crystals with one to a few layers of atoms and are being used in many fields such as optical modulator, photodetector, optical switch, and ultrafast lasers. Their exceptional optoelectronic and nonlinear optical properties make them as a suitable saturable absorber for laser cavities. This review focuses on the recent progress in ultrafast laser use 2D materials as a saturable absorber. 2D materials traditionally include graphene, topological insulators, transition metal dichalcogenides, as well as new materials such as black phosphorus, bismuthene, antimonene, and MXene. Material characteristics, fabrication techniques, and nonlinear properties are also introduced. Finally, future perspectives of ultrafast lasers based on 2D materials are also addressed.

A systematic review on 2D materials for volatile organic compound sensing

Thermoelectric (TE) energy conversion, which directly transforms waste heat into usable electricity, presents a crucial technology for sustainable power generation and energy efficiency enhancement. The emergence of two-dimensional (2D) materials has profoundly impacted this field by providing an atomically thin platform for unprecedented control over electronic and thermal transport properties. This comprehensive review critically analyzes the latest progress, persistent challenges, and future opportunities of 2D materials for advanced TE applications. We begin by systematically evaluating the synthesis and processing toolkit, correlating techniques from top-down exfoliation to bottom-up chemical vapor deposition with their specific impacts on microstructure and final device performance. Subsequently, we provide a critical assessment of the fundamental TE performance of key 2D families, including graphene, transition metal dichalcogenides (TMDs), MXenes, black phosphorus (BP), and hexagonal boron nitride (h-BN). The discussion details how advanced engineering strategies, such as strain modulation, layer number control, chemical doping, and heterostructure (HS) design, can dramatically enhance the power factor (PF) while simultaneously suppressing lattice thermal conductivity (κL). Finally, we showcase the successful translation of these materials into practical applications, encompassing flexible and wearable TE generators (WTEGs), self-powered sensors, and integrated energy harvesting systems.

This chapter describes the prospects of integration of Two-dimensional (2D) materials with photonic nanostructures to enable future active nanophotonic devices. It focuses on development of controlling light emission from transition metal dichalcogenides (TMD) by nanophotonic integration. The monolayer form demonstrates a range of novel optical effects, including strong excitonic effects, photo-luminescence , valley polarisation, single-photon emission and second-harmonic generation, which show great potential for fabricating novel “flat” active nanophotonic devices in the future. Integration with photonic nanostructures is a solution to such challenges, since photonic nanostructures such as plasmonic nanoparticles show great capability to manipulate light–matter interaction at the nanoscale. A greater understanding of the coupling between photonic nanostructures and 2D materials is required, thus potentially better controlling the properties of these materials. For example, non-linear coupling between 2D materials, especially second-order non-linearity, has attracted intense interest. Dielectric nanostructures, such as photonic crystal cavities, have also been proposed for enhancing the emission from TMD monolayers.

The field of valleytronics, which exploits the valley degree of freedom in two-dimensional (2D) materials, has emerged as a promising frontier in next-generation electronic and photonic devices. In this context, a valley refers to an energy extremum in the electronic band structure of a material, which can serve as a discrete quantum number — akin to charge or spin — for information encoding. Because many 2D materials possess two inequivalent valleys in momentum space, they can naturally represent binary logic states, making them attractive for low-power electronics and quantum computing. This topical review explores the valley-dependent Berry phase effects and their significant role in various valleytronic applications. We begin by introducing the fundamentals of topology in solid-state physics, emphasizing the Berry phase — a geometric phase with profound quantum mechanical implications — and its associated Berry curvature, which acts as an effective magnetic field in momentum space. Next, we review key 2D materials, including monolayer and bilayer graphene as well as transition metal dichalcogenides, to illustrate how valley-dependent Berry curvature gives rise to orbital magnetic moments and the valley Hall effect. Advanced concepts, such as Chern numbers and valley Chern numbers, are discussed to quantify the topological order in these systems. The review also highlights the emergence of topological kink states and domain wall states in graphene, which are linked to robust edge conduction. Furthermore, we examine the valley Zeeman effect and the mechanisms underlying valley polarization (VP), wherein an imbalance of electrons between distinct momentum valleys enables novel device functionalities. Recent experimental breakthroughs — such as observations of the exciton Hall effect — demonstrate the technological potential of Berry phase effects in controlling VP and current. Finally, we underscore the critical role of these geometric and topological phenomena in driving future valleytronic innovations in 2D materials.

Black phosphorus (bP) is a promising two-dimensional (2D) material for opto-electronic applications. Strongly bound excitons with binding energies up to 0.3 eV and remarkably large trion binding energies up to 100 meV have been observed for supported monolayer bP. Surprisingly, this trion binding energy is significantly larger than those found in other 2D materials (e.g. about 30 meV in transition metal dichalcogenides). This has previously been ascribed to the quasi-1D nature of bP. In this work we show, using first principles calculations, that the trion binding energy of bP is indeed large (80 meV) when referenced to the lowest bright exciton but only 30 meV when its energy is measured relative to the lowest dark exciton. Our analysis thus shows that the trion binding energy in bP is not larger than in other 2D materials, and the previous conclusions have to be understood incorporating the large splitting between the dark and bright excitons in bP. We also explore the effect of substrate and in-plane strain of the exciton and trion binding energies and show that these effects do not change the main conclusions. Our results correct the misconception that trion binding energies in monolayer bP are particularly large due to its quasi-1D structure and contribute to the establishment of more a detailed understanding of optical properties of atomically thin semiconductors.

The rapid evolution of wearable and portable electronics has driven an unprecedented demand for flexible, efficient, and durable energy storage solutions. Two-dimensional (2D) materials have captured significant interest due to their unique physical, chemical, and electrochemical characteristics and their ability to create heterostructures with other nanomaterials, enhancing both mechanical strength and electrochemical performance. This chapter delves into the current state-of-the-art knowledge surrounding these advanced flexible 2D material-based flexible lithium-ion batteries (LIBs), highlighting the unique properties of 2D materials that make them ideal for flexible energy storage systems. 2D materials, such as graphene, transition metal dichalcogenides (TMDs), MXenes, and black phosphorus (BP), exhibit exceptional electrical conductivity, large surface area, and mechanical flexibility, and are discussed here. These properties enable the fabrication of LIBs, which offer high energy and power densities and maintain performance under mechanical deformation. This chapter begins with an overview of 2D materials and then delves into recent advancements in 2D material-based flexible LIBs utilizing these materials. There is an emphasis on intricate relationships between the structure and properties of flexible electrodes, particularly focusing on their role in free-standing films. The chapter highlights the integration of these 2D materials into flexible architectures, such as fiber-based devices and wearable electronics, with the potential to revolutionize energy storage by enabling lightweight, flexible, and high-performance devices that conform to various shapes and applications. Fabrication techniques, including inkjet printing, chemical vapor deposition (CVD), and layer-by-layer assembly, facilitate the precise engineering of 2D material-based electrodes and flexible substrates. This chapter addresses the origins of quantum capacitance ( CQ ), its contribution to boosting overall capacitance, and various strategies to enhance CQ . It also reviews the current advancements in CQ for electrode materials, including carbon, 2D materials, and their composites. The concluding section addresses the existing challenges and the prospects for research on flexible LIB devices.

Suspended monolayer transition metal dichalcogenides (TMD) aremembranes that combine ultralow mass and exceptional optical prop-erties, making them intriguing materials for opto-mechanical applica-tions. However, the low measured quality factor of TMD resonatorshas been a roadblock so far. In this thesis, we first show an ultra-sensitive optical readout of monolayer TMD resonators that allows usto reveal their mechanical properties at cryogenic temperatures. Wefind that the quality factor of monolayer WSe2resonators greatly in-creases below room temperature, reaching values as high as 16000 at liquid nitrogen temperature and 47000 at liquid helium temper-ature. This surpasses the quality factor of monolayer graphene res-onators with similar surface areas. Upon cooling the resonator, the res-onant frequency increases significantly due to the thermal contractionof the WSe2lattice. These measurements allow us to experimentallystudy the thermal expansion coefficient of WSe2 monolayers for thefirst time. High Q-factors are also found in resonators based on MoS2 and MoSe2 monolayers. The high quality-factor found in this workopens new possibilities for coupling mechanical vibrational states totwo-dimensional excitons, valley pseudospins, and single quantumemitters and for quantum opto-mechanical experiments based on theCasimir interaction.The sensing capabilities offered by these high Q-factor nanome-chanical oscillators are also of interest for studying thermodynamicproperties in condensed matter regimes that are difficult to access. Inthe second part of the thesis, we use optomechanical systems basedon a MoSe2 monolayer to probe the thermal properties of phononsin two-dimensional lattices. We measure the thermal conductivityand the specific heat capacity down to cryogenic temperature. Thephonon transport crossovers from the diffusive to the ballistic regimewhen lowering the temperature below~100 K. The temperature de-pendence of the specific heat capacity approaches a quadratic depen-dence, the signature of two-dimensional lattices. Both the thermalconductivity and the specific heat capacity measurements are consis-tent with predictions based on first-principles. Our result establishes anew strategy to investigate thermal transport in two-dimensional ma-terials, and allows for exploring the phonon hydrodynamic regime,the anomalous heat conduction, and the phase transitions of electronicmany-body collective phenomena in monolayers Los dicalcogenuros de metal de transición (TMD) monocapa suspendidos combinan una masa ultrabaja y propiedades ópticas excepcionales, lo que los convierte en materiales intrigantes para aplicaciones opto-mecánicas. Sin embargo, el bajo factor de calidad Q medido en los resonadores de TMD ha sido un obstáculo hasta ahora. En esta tesis, primero mostramos una lectura óptica ultra sensible de resonadores TMD de monocapa que nos permite revelar sus propiedades mecánicas a temperaturas criogénicas. Encontramos que el factor de calidad de los resonadores WSe2 monocapa aumenta considerablemente por debajo de la temperatura ambiente, alcanzando valores tan altos como 1.6 x 104 en temperatura de nitrógeno líquido y 4.7 x 104 en temperatura de helio líquido. Esto supera el factor de calidad de los resonadores de grafeno monocapa con áreas de superficie similares. Al enfriar el resonador, la frecuencia de resonancia aumenta significativamente debido a la contracción térmica la red del cristal de WSe2. Estas mediciones nos permiten estudiar experimentalmente el coeficiente de expansión térmica de las monocapas de WSe2 por primera vez. Los altos factores Q también se encuentran en los resonadores basados en las monocapas de MoS2 y MoSe2. El alto factor de calidad que se encuentra en este trabajo abre nuevas posibilidades para acoplar estados vibracionales mecánicos a excitones bidimensionales, valley pseudo-spins y emisores cuánticos únicos y para experimentos opto-mecánicos cuánticos basados en la interacción de Casimir. Las capacidades de detección ofrecidas por este nano-resonador mecánico de alto factor Q también son interesantes para estudiar propiedades termodinámicas en regímenes de la materia condensada a los que es difícil acceder. En la segunda parte de la tesis, utilizamos sistemas optomecánicos basados en una monocapa de MoSe2 para probar las propiedades térmicas de los fonones en redes de cristales bidimensionales. Medimos la conductividad térmica y la capacidad calorífica específica hasta temperaturas criogénicas. Los régimenes de transporte de fonones pasan de el difuso al balístico al bajar la temperatura por debajo de 100 K. La dependencia de la temperatura de la capacidad calorífica específica se aproxima a una dependencia cuadrática, lo cual es la firma de las redes bidimensionales. Tanto la conductividad térmica como las mediciones de la capacidad calorífica específica son coherentes con las predicciones basadas en primeros principios. Nuestro resultado establece una nueva estrategia para investigar el transporte térmico en materiales bidimensionales y permite explorar el régimen hidrodinámico de fonones, la conducción de calor anómala y las transiciones de fase de los fenómenos colectivos de cuerpos electrónicos en monocapas.

Stretchable photodetectors based on 2D materials: materials synthesis, fabrications and applications

The layered two-dimensional (2D) materials, e.g., the well-known graphene, transition metal dichalcogenides, and topological insulators, have attracted great interest of researchers from fundamental research as well as industries owing to their intriguing properties in a number of aspects. In practical applications, the efficient modification of the features of 2D materials is compelling and essential to achieve desired functionalities. Ion implantation has been successfully applied to synthesize graphene. The most exotic advantage of ion beam technology is that it offers distinct options of energetic beams, which has recently shown the unique capability to modify and tailor the properties of versatile 2D materials. To name a few, the energetic ion beams could implement the surface morphology or layer-to-layer structural engineering of 2D materials. At the microcosmic level, the introduction of ion beam induced defects and intentional doping of specific ions are the basis of tailoring properties of 2D materials. By manipulating the parameters of ion beams (energies, species, fluences, incident angles, etc.), the modified 2D materials may possess novel properties, which are unprecedented in pristine ones. Promising applications based on these 2D materials with ion beam tailored features have been realized in a broad range of fields. In this work, we systematically review the latest research progress on the deployed techniques and property modifications of graphene and other 2D materials under the treatment of various ion beams. A few selected applications are presented to indicate the practical potentials of ion beam modified 2D materials in distinct areas. Perspectives on future developments are also provided by focusing on several promising topics.

Two-dimensional (2D) materials have attracted increasing interest in the last few years due to their unique morphology and properties and their use in a variety of applications, ranging from electronics to energy storage or catalysis, to name a few. Apart from various well-established 2D materials, such as graphene, h-BN, or MoS2, black phosphorus (BP) has received considerable attention over the last two years because its direct bandgap and his appealing properties for electronic and ultrafast optoelectronic applications. However, its lack of environmental stability severely limits its synthesis and processing. We have recently demonstrated that high-quality, few-layer BP nanosheets, with controllable size and observable photoluminescence, can be produced in large quantities by liquid phase exfoliation under ambient conditions.1 Beyond this success, a more sophisticated chemical modification is highly desirable, like the molecular doping, that is an effective and flexible method towards modulating the electronic properties of 2D materials. Herein, we have developed a chemical route for the non-covalent functionalization of BP with different moieties, including electron-withdrawing organic molecules or tailor-made surfactants.2 Moreover we have extended these concepts towards a new 2D material: antimonene, creating stable suspensions in water and developing its non-covalent functionalization.3 1) D. Hanlon, C. Backes, E. Doherty, C. S. Cucinotta, N. C. Berner, C. Boland, K. Lee, A. Harvey, P. Lynch, Z. Gholamvand, S. F. Zhang, K. P. Wang, G. Moynihan, A. Pokle, Q. M. Ramasse, N. McEvoy, W. J. Blau, J. Wang, G. Abellan, F. Hauke, A. Hirsch, S. Sanvito, D. D. O’Regan, G. S. Duesberg, V. Nicolosi, J. N. Coleman, Nat. Commun. 2015, 6, 8563. 2) G. Abellán, V. Lloret, U. Mundloch, M. Marcia, C. Neiss, A. Görling, M. Varela, F. Hauke, A. Hirsch, Angew. Chem. In. Ed., 2016, 55 (47), 14557. 3) C. Gibaja, D. Rodriguez-San-Miguel, P. Ares, J. Gómez-Herrero, M. Varela, R. Gillen, J. Maultzsch, F. Hauke, A. Hirsch, G. Abellán, F. Zamora, Angew. Chem. In. Ed., 2016, 55 (46), 14345.