Size-dependent shape characteristics of 2D crystal blisters

Towards controlled synthesis of 2D crystals by chemical vapor deposition (CVD)

CONSPECTUS: After the discovery of graphene and the development of powerful exfoliation techniques, experimental preparation of two-dimensional (2D) crystals can be expected for any layered material that is known to chemistry. Besides graphene and hexagonal boron nitride (h-BN), transition metal chalcogenides (TMC) are among the most studied ultrathin materials. In particular, single-layer MoS2, a direct band gap semiconductor with ∼1.9 eV energy gap, is popular in physics and nanoelectronics, because it nicely complements semimetallic graphene and insulating h-BN monolayer as a construction component for flexible 2D electronics and because it was already successfully applied in the laboratory as basis material for transistors and other electronic and optoelectronic devices. Two-dimensional crystals are subject to significant quantum confinement: compared with their parent layered 3D material, they show different structural, electronic, and optical properties, such as spontaneous rippling as free-standing monolayer, significant changes of the electronic band structure, giant spin-orbit splitting, and enhanced photoluminescence. Most of those properties are intrinsic for the monolayer and already absent for two-layer stacks of the same 2D crystal. For example, single-layer MoS2 is a direct band gap semiconductor with spin-orbit splitting of 150 meV in the valence band, while the bilayer of the same material is an indirect band gap semiconductor without observable spin-orbit splitting. All these properties have been observed experimentally and are in excellent agreement with calculations based on density-functional theory. This Account reports theoretical studies of a subgroup of transition metal dichalcogenides with the composition MX2, with M = Mo, or W and X = Se or S, also referred to as "MoWSeS materials". Results on the electronic structure, quantum confinement, spin-orbit coupling, spontaneous monolayer rippling, and change of electronic properties in the presence of an external electric field are reported. While all materials of the MoWSeS family share the same qualitative properties, their individual values can differ strongly, for example, the spin-orbit splitting in WSe2 reaches the value of 428 meV, nearly three times that of MoS2. Further, we discuss the effect of strain on the electronic properties (straintronics). While MoWSeS single layers are very robust against external electric fields, bilayers show a linear reduction of the band gap, even reaching a semiconductor-metal phase transition, and an increase of the spin-orbit splitting from zero to the monolayer value at rather small fields. Strain is yet another possibility to control the band gap in a linear way, and MoWSeS monolayers become metallic at strain values of ∼10%. The density-functional based tight-binding model is a useful tool to investigate the electronic and structural properties, including electron conductance, of large MoS2 structures, which show spontaneous rippling in finite-temperature molecular dynamics simulations. Structural defects in MoS2 result in anisotropy of the electric conductivity. Finally, DFT predictions on the properties of noble metal dichalcogenides are presented. Most strikingly, 1T PdS2 is an indirect band gap semiconductor in its monolayer form but becomes metallic as a bilayer.

In this review, we survey the recent advances in nonlinear optics and the applications of two‐dimensional (2D) materials. We briefly cover the key developments pertaining to research in the nonlinear optics of graphene, the quintessential 2D material. Subsequently, we discuss the linear and nonlinear optical properties of several other 2D layered materials, including transition metal chalcogenides, black phosphorus, hexagonal boron nitride, perovskites, and topological insulators, as well as the recent progress in hybrid nanostructures containing 2D materials, such as composites with dyes, plasmonic particles, 2D crystals, and silicon integrated structures. Finally, we highlight a few representative current applications of 2D materials to photonic and optoelectronic devices.

Adoption of graphene and other 2D crystals in biomedicine is challenging — some guidelines to facilitate this process and avoid inflated expectations should be considered. Adoption of graphene and other 2D crystals in biomedicine is challenging — some guidelines to facilitate this process and avoid inflated expectations should be considered.

The solid progress in the study of a single two-dimensional (2D) material underpins the development for creating 2D material assemblies with various electronic and optoelectronic properties. We introduce an asymmetric structure by stacking monolayer semiconducting tungsten disulfide, metallic graphene, and insulating boron nitride to fabricate numerous red channel light-emitting devices (LEDs). All the 2D crystals were grown by chemical vapor deposition (CVD), which has great potential for future industrial scale-up. Our LEDs exhibit visibly observable electroluminescence (EL) at both 5.5 V forward and 7.0 V backward biasing, which correlates well with our asymmetric design. The red emission can last for at least several minutes, and the success rate of the working device that can emit detectable EL is up to 80%. In addition, we show that sample degradation is prone to happen when a continuing bias, much higher than the threshold voltage, is applied. Our success of using high-quality CVD-grown 2D materials for red light emitters is expected to provide the basis for flexible and transparent displays.

Compressive solitary waves in black phosphorene

Two-dimensional (2D) atomic crystals are considered to be very promising for nanoelectronics and spintronics applications. It provides a large class of materials proposed to be important for long distance spin transport, spin polarized tunneling and large spin-orbit coupling. Here I will discuss spintronic aspects of these 2D materials and their heterostructures. Graphene is considered to be an ideal material for spin transport due to the high mobility and long spin lifetime of the carriers. Recently, we demonstrated spin transport over distances of 16 µm and spin lifetimes up to 1.2 ns in large area CVD graphene on SiO 2 /Si substrate at room temperature [1]. Subsequently, using insulating h-BN as a tunnel barrier on graphene, we observe an enhancement in spin polarized tunneling [2], and its negative sign for thicker h-BN layers [3]. These signatures provide an experimental evidence of the spin filtering across the ferromagnet/hBN-graphene van der Waals heterostructures. We also employed 2D materials such as h-BN and MoS 2 in ferromagnetic tunnel junctions for observation of tunnel magnetoresistance up to room temperature [4]. We further aim to address the issue of spin manipulation in graphene by employing heterostructures with other 2D semiconductors [5], topological insulators [6] and materials with novel spin textures. I will present both electronic and spintronic properties of these 2D materials and their heterostructures. These findings open a platform for exploring novel spin functionalities in 2D crystals and understanding the basic phenomenon that control their behavior.

2D stacking presents a promising avenue for creating periodic superstructures that unveil novel physical phenomena. While extensive research has focused on lateral 2D material superstructures formed through composition modulation and twisted moiré structures, the exploration of vertical periodicity in 2D material superstructures remains limited. Although weak van der Waals interfaces enable layer-by-layer vertical stacking, traditional methods struggle to control in-plane crystal orientation over large areas, and the vertical dimension is constrained by unscalable, low-throughput processes, preventing the achievement of global order structures. In this study, a supercell multiplying approach is introduced that enables high-throughput construction of 3D superstructures on a macroscopic scale, utilizing artificially stacked single-crystalline 2D multilayers as foundational repeating units. By employing wafer-scale single-crystalline 2D materials and referencing the crystal orientation of substrates, the method ensures precise alignment of crystal orientation within and across each supercell, thereby achieving controllable periodicity along all three axes. A centimeter-scale 3R-MoS₂ crystal is successfully constructed, comprising over 200 monolayers of single-crystalline MoS₂, through a bottom-up stacking process. Additionally, the approach accommodates the integration of amorphous oxide, enabling the assembly of 3D non-linear optical crystals with quasi-phase matching. This method paves the way for the bottom-up construction of macroscopic artificial 3D crystals with atomic plane precision, enabling tailored optical, electrical, and thermal properties and advancing the development of novel artificial materials and high-performance applications.

Two-dimensional (2D) materials have opened up a field for developing the next generation of optoelectronic devices thanks to their novel properties such as ultrafast charge injection/extraction, strong light-matter interactions (despite being atomically thin) and quantum confinement(1). In addition, recent advancements in liquid phase exfoliation has revealed a new scalable route to investigate these materials(2). A good example is the fabrication of graphene oxide (GO) where the high resistive precursor of graphene that has to be reduced to recover its favorable optoelectronic properties. In most cases, ultra-high temperature (>1000ºC) and toxic chemicals (hydrazine) have to be used. In this talk, we will present a novel solution-processed film containing T3C2Tx monolayers, a 2D crystal from the recently discovered family of MXenes(3). This new MXene 2D family can be synthetized by selectively etching the A element of the MAX phase and formed into stacks of transition metal carbides and carbonitrides. Indeed, MXenes have recently gained attention due their a hydrophilic behavior, high metallic conductivity and excellent mechanical properties(4). Herein, we explore the use of freestanding nanometer-thin films of T3C2Tx for solution-processed transparent conductive electrodes. We demonstrate that our T3C2Tx films possess half of the sheet resistance (437 Ohm/sq) of the best reduced graphene oxide by produced solution-processed methods(5) while preserving an 80% of its transmission. In this talk, we will show that by using a simple spin-coating technique we can control the number of flakes forming the film (Figure 1a) and optimize the optoelectronic properties of our films so that they exhibit a figure of merit twice as large as reduced-GO (Figure 1b). The novelty of our approach is our in deep understanding of the limiting factors in the resistivity of the films such as heat treatments and functional end-groups. In addition, we will illustrate their metallic behavior by on-chip measurements and report for the first time the experimental measurement of their work function, which is crucial for advanced device design. This demonstration of a novel metallic and solution-processed 2D material with high conductivity using T3C2Txflakes with low-temperature fabrication provides a new possible pathway towards scalable manufacturing of ultrathin film devices. 1. F. Bonaccorso et al., Graphene, related two-dimensional crystals, and hybrid systems for energy conversion and storage. Science 347, 1246501 (2015). 2. V. Nicolosi, M. Chhowalla, M. G. Kanatzidis, M. S. Strano, J. N. Coleman, Liquid Exfoliation of Layered Materials. Science 340, 1226419 (2013). 3. M. Naguib, V. N. Mochalin, M. W. Barsoum, Y. Gogotsi, 25th Anniversary Article: MXenes: A New Family of Two-Dimensional Materials. Advanced Materials 26, 992-1005 (2014). 4. M. Naguib, Y. Gogotsi, Synthesis of Two-Dimensional Materials by Selective Extraction. Accounts of Chemical Research 48, 128-135 (2015). 5. H. A. Becerril et al., Evaluation of Solution-Processed Reduced Graphene Oxide Films as Transparent Conductors. ACS Nano 2, 463-470 (2008). Figure 1

Metamaterials exhibiting auxeticity, defined as positive strain normal to an applied strain, have been receiving tremendous interest in recent years as they have found unique applications as shock absorbers, for thermal expansion compensation, and in smart interfaces.[1] This highly unusual behavior can quantified in terms of Poisson's ratio (ν) defined as the amount of the response strain over applied strain, where ν > 0 in most typical materials, while ν < 0 in auxetics. Molecular auxetic materials are crystalline solids which unit cell is auxetic.[2] A number of two-dimensional (2D) materials, including but not limited to single-layer and few-layer graphene,[3] are weakly auxetic due to their expansion along both in-plane longitudinal and transversal directions, caused by their thinning along the z-axis. Nonetheless, the achievement of a significant degree of auxeticity beyond the mesoscopic level has been theoretically predicted to require the modification of 2D materials through the engineering of periodic arrays of defects,[3] which is hardly feasible with commonly available nanofabrication techniques, and has never been demonstrated in practice. An alternative approach towards achieving a significant degree of auxeticity rests on the design of novel 2D crystals with specific electronic structure and unit cells, with 2D carbides and semicarbides (a sub-class of 2D M-Xenes) at the forefront of such efforts.The term giant auxeticity refers to molecular auxetic metals in which the density-of-electronic states at the Fermi level increases upon stretching thus leading to an increase in electrical and thermal conductivity in both longitudinal and transversal directions. Theoretical studies [4] seem to indicate that the root cause for giant auxeticity in 2D (semi)carbides resides on the increasing delocalization of the coupling between p-electrons (from C atoms) and d-electrons (from metal atoms) which causes a strong planarization and elongation of the metal-carbon bonds. For example, tungsten semicarbide (2D-W2C) was theoretically predicted to combine very high negative Poisson's ratio (ν < -0.4) with giant auxeticity.[4]In the initial part of our presentation, we will report on a specially designed dual-zone remote plasma system conceived to grow giant-auxetic 2D semicarbides out of thermodynamic equilibrium with a well-tuned ratio of precursors.[1] Although 2D-W2C was theoretically predicted to exhibit giant auxeticity, it had yet to be synthesized before the use of our and other out-of-equilibrium techniques, as the full carbide is energetically favored under thermodynamic equilibrium synthesis processes, such as dual-zone furnace-based chemical vapor deposition (CVD), and this is a common issue of multiple semicarbides. We will report on the specific conditions under which dual-zone, remote-plasma enhanced CVD allowed for the synthesis of flakes of few-layer W2C which are 2D in nature due to retained periodicity at the mesoscopic level in a Stranski–Krastanov growth process.[5]In the next part of our presentation, we will show how nanooptical dilatometry techniques based on pump-probe scanning near-field optical microscopy (SNOM) [6] are essential nanophotonics imaging tools towards the identification of 2D molecular auxetic materials, and the quantification of their negative Poisson ratio. To date, there are very few experimental techniques, if any, that are suitable for the purpose of acquiring quantitative maps of the expansivity of flakes of 2D materials with nanoscale lateral resolution in spite of huge demand, for example in the characterization of auxetics. Our group for has pioneered near-field thermoreflectance techniques as nanophotonic tools for precisely mapping the thermal and mechanical properties at the nanoscale. In these pump-probe experiments, samples are periodically heated by a pump laser beam from an inverted optical microscope, while periodic changes in the sample's local reflectivity are measured, in phase and amplitude, through an upright aperture-type SNOM operating in reflection mode. Here we will show that ω-2ω near-field thermoreflectance experiments, in which the probe is locked in for detection at double frequency (2ω) as the pump, are spectacularly sensitive to the local expansivity of surfaces, and can thus be successfully used as nanophotonics dilatometers to characterize a wide class of auxetic materials.Collectively, our presentation focuses on potental applications of 2D giant auxetics in photonics and energy, which include robust signalk detection and the photomodulation of charge collection, for example in electrodes for stretchable solar cells and phototransducers.[1] Ren et al Smart Mater Struct 27 (2018) 023001[2] Grima et al, Advanced Mater 27 (2014) 1455[3] Grima et al, Ann Phys 530 (2018) 1700330[4] Wu et al, Phys Chem Chem Phys 20 (2018) 18893[5] Stocek, Ullah and Fanchini, Mater Horizons 11 (2024) 3066[6] Wong, Ezugwu and Fanchini, Adv Mater Interfaces 11 (2024) 2300806 Figure 1

We here present a planarized solid-state chemical reaction that can produce transition metal monochalcogenide (TMMC) 2D crystals with large lateral extent and finely controllable thickness down to individual layers. The enhanced lateral diffusion of a gaseous reactant at the interface between a solid precursor and graphene was found to provide a universal route towards layered TMMCs of different compositions. A unique layer-by-layer growth mechanism yields atomically abrupt crystal interfaces and kinetically controllable thickness down to a single TMMC layer. Our approach stabilizes 2D crystals with commonly unattainable thermodynamic phases, such as β-Cu2S and γ-CuSe, and spectroscopic characterization reveals ultra-large phase transition depression and interesting electronic properties. The presented ability to produce large-scale 2D crystals with high environmental stability was applied to highly sensitive and fast optoelectronic sensors. Our approach extends the morphological, compositional, and thermodynamic complexity of 2D materials.

Geometry and chiral symmetry breaking of ripple junctions in 2D materials

Graphene and other 2D atomic crystals are of considerable interest in catalysis because of their unique structural and electronic properties. Over the past decade, the materials have been used in a variety of reactions, including the oxygen reduction reaction, water splitting and CO2 activation, and have been shown to exhibit a range of catalytic mechanisms. Here, we review recent advances in the use of graphene and other 2D materials in catalytic applications, focusing in particular on the catalytic activity of heterogeneous systems such as van der Waals heterostructures (stacks of several 2D crystals). We discuss the advantages of these materials for catalysis and the different routes available to tune their electronic states and active sites. We also explore the future opportunities of these catalytic materials and the challenges they face in terms of both fundamental understanding and the development of industrial applications.

The new paradigm of heterostructures based on two-dimensional (2D) atomic crystals has already led to the observation of exciting physical phenomena and creation of novel devices. The possibility of combining layers of different 2D materials in one stack allows unprecedented control over the electronic and optical properties of the resulting material. Still, the current method of mechanical transfer of individual 2D crystals, though allowing exceptional control over the quality of such structures and interfaces, is not scalable. Here we show that such heterostructures can be assembled from chemically exfoliated 2D crystals, allowing for low-cost and scalable methods to be used in device fabrication.

A series of novel two-dimensional materials inspired from a 4D polytope, tesseract, have been proposed by density functional theory (DFT) based computations. Both C24X12 and C16X16 (X = O, S and Se) are found to have great thermodynamic and dynamic stabilities, and C24X12 exhibited excellent thermal stability up to 1000 K. All these 2D crystals are semiconductors with 2.17 eV to 3.35 eV band gaps at the HSE06 theoretical level, except for C24S12 (4.14 eV energy gap). Moreover, the intrinsic pore sizes of C24Se12 are suitable to sieve He from the He/CH4 mixture, with over 80% separation ratio and nearly 100% selectivity. Our findings not only enlarged the boundary of the 2D family, but also offered another potential method to recover helium from natural gas at ambient conditions.