Chemical synthesis of two-dimensional atomic crystals, heterostructures and superlattices

Ultralow thermal conductivity of turbostratically disordered MoSe2 ultra-thin films and implications for heterostructures

Films containing 8, 16, 24, 32 and 64 MoSe₂ layers were synthesized using the modulated elemental reactants method. X-ray reflectivity patterns showed that the annealed films were the targeted number of MoSe₂ layers thick with atomically smooth interfaces. In-plane x-ray diffraction (XRD) scans contained only hk0 reflections for crystalline MoSe₂ monolayers. Specular XRD patterns contained only 00l reflections, also indicating that the hk0 plane of the MoSe₂ layers are parallel to the substrate. Both XRD and electron microscopy techniques indicated that the hk0 planes are rotationally disordered with respect to one another, with all orientations equally probable for large areas. The rotational disorder between MoSe₂ layers is present even when analyzed spots are within 10 nm of one another. Cross-plane thermal conductivities of 0.07-0.09 W m^-1 K^-1 were measured by time domain thermoreflectance, with the thinnest films exhibiting the lowest conductivity. The structural analysis suggests that the ultralow thermal conductivity is a consequence of rotational disorder, which increases the separation between MoSe₂ layers. The bonding environment of the Se atoms also becomes significantly distorted from C _3v symmetry due to the rotational disorder between layers. This structural disorder efficiently reduces the group velocity of the transverse phonon modes but not that of longitudinal modes. Since rotational disorder between adjacent layers in heterostructures is expected if the constituents have incommensurate lattices, this study indicates that these heterostructures will have very low cross-plane thermal conductivity.

Recent Advances in 2D Lateral Heterostructures

Recent developments in synthesis and nanofabrication technologies offer the tantalizing prospect of realizing various applications from two-dimensional (2D) materials. A revolutionary development is to flexibly construct many different kinds of heterostructures with a diversity of 2D materials. These 2D heterostructures play an important role in semiconductor and condensed matter physics studies and are promising candidates for new device designs in the fields of integrated circuits and quantum sciences. Theoretical and experimental studies have focused on both vertical and lateral 2D heterostructures; the lateral heterostructures are considered to be easier for planner integration and exhibit unique electronic and photoelectronic properties. In this review, we give a summary of the properties of lateral heterostructures with homogeneous junction and heterogeneous junction, where the homogeneous junctions have the same host materials and the heterogeneous junctions are combined with different materials. Afterward, we discuss the applications and experimental synthesis of lateral 2D heterostructures. Moreover, a perspective on lateral 2D heterostructures is given at the end.

Lateral heterostructures and one-dimensional interfaces in 2D transition metal dichalcogenides

The growth and exfoliation of two-dimensional (2D) materials have led to the creation of edges and novel interfacial states at the juncture between crystals with different composition or phases. These hybrid heterostructures (HSs) can be built as vertical van der Waals stacks, resulting in a 2D interface, or as stitched adjacent monolayer crystals, resulting in one-dimensional (1D) interfaces. Although most attention has been focused on vertical HSs, increasing theoretical and experimental interest in 1D interfaces is evident. In-plane interfacial states between different 2D materials inherit properties from both crystals, giving rise to robust states with unique 1D non-parabolic dispersion and strong spin-orbit effects. With such unique characteristics, these states provide an exciting platform for realizing 1D physics. Here, we review and discuss advances in 1D heterojunctions, with emphasis on theoretical approaches for describing those between semiconducting transition metal dichalcogenides MX ₂ (with M  =  Mo, W and X  =  S, Se, Te), and how the interfacial states can be characterized and utilized. We also address how the interfaces depend on edge geometries (such as zigzag and armchair) or strain, as lattice parameters differ across the interface, and how these features affect excitonic/optical response. This review is intended to serve as a resource for promoting theoretical and experimental studies in this rapidly evolving field.

Electrical Properties of Two-Dimensional Materials Used in Gas Sensors

In the search for gas sensing materials, two-dimensional materials offer the possibility of designing sensors capable of tuning the electronic band structure by controlling their thickness, quantity of dopants, alloying between different materials, vertical stacking, and the presence of gases. Through materials engineering it is feasible to study the electrical properties of two-dimensional materials which are directly related to their crystalline structure, first Brillouin zone, and dispersion energy, the latter estimated through the tight-binding model. A review of the electrical properties directly related to the crystalline structure of these materials is made in this article for the two-dimensional materials used in the design of gas sensors. It was found that most 2D sensing materials have a hexagonal crystalline structure, although some materials have monoclinic, orthorhombic and triclinic structures. Through the simulation of the mathematical models of the dispersion energy, two-dimensional and three-dimensional electronic band structures were predicted for graphene, hexagonal boron nitride (h-BN) and silicene, which must be known before designing a gas sensor.

Tuning orbital orientation endows molybdenum disulfide with exceptional alkaline hydrogen evolution capability

Molybdenum disulfide is naturally inert for alkaline hydrogen evolution catalysis, due to its unfavorable water adsorption and dissociation feature originated from the unsuitable orbital orientation. Herein, we successfully endow molybdenum disulfide with exceptional alkaline hydrogen evolution capability by carbon-induced orbital modulation. The prepared carbon doped molybdenum disulfide displays an unprecedented overpotential of 45 mV at 10 mA cm^-2, which is substantially lower than 228 mV of the molybdenum disulfide and also represents the best alkaline hydrogen evolution catalytic activity among the ever-reported molybdenum disulfide catalysts. Fine structural analysis indicates the electronic and coordination structures of molybdenum disulfide have been significantly changed with carbon incorporation. Moreover, theoretical calculation further reveals carbon doping could create empty 2p orbitals perpendicular to the basal plane, enabling energetically favorable water adsorption and dissociation. The concept of orbital modulation could offer a unique approach for the rational design of hydrogen evolution catalysts and beyond.

Photocatalytic performance of few-layer graphitic C3N4: enhanced by interlayer coupling

For atomically thin two-dimensional materials, van der Waals interlayer coupling is a crucial factor to tune or produce novel physicochemical properties. In terms of photocatalysis, however, researching into the interlayer coupling effect is still in its infancy, especially that involving excited state dynamics. Here, by performing many-body perturbation theory and ab initio nonadiabatic molecular dynamics, we find that metal-free few-layer graphitic C₃N₄ (g-C₃N₄) possesses a better photocatalytic hydrogen evolution performance due to interlayer coupling compared with ultrathin monolayer g-C₃N₄. Specifically, few-layer g-C₃N₄ activates the electronic transition channel around the Fermi level and transforms dark excitation to bright excitation, which broadens the solar light absorption region. Meanwhile, few-layer g-C₃N₄ can effectively weaken the strong binding energy between nitrogen and hydrogen by means of intralayer charge transfer, and can enhance the activity of hydrogen evolution reactive sites. Furthermore, the interlayer coupling tends to localize photogenerated electrons at the reactive sites, which can provide more active electrons to participate in the catalytic reaction.

Na-assisted fast growth of large single-crystal MoS2 on sapphire

Monolayer molybdenum sulfide (MoS₂), a typical semiconducting transition metal dichalcogenide, has emerged as a perfect platform for next-generation electronics and optoelectronics due to its sizeable band gap and strong light-matter interactions. Nevertheless, the controlled growth of a monolayer MoS₂ single-crystal with a large-domain size and high crystal quality still faces great challenges. Herein, we demonstrate the fast growth of a large-domain monolayer MoS₂ on the c-plane sapphire substrate with the assistance of sodium chloride (NaCl) crystals as the intermediate promoter. Particularly, the volatilization temperature of the NaCl crystal and the growth temperature of MoS₂ are established to be the key parameters that influence the growth efficiency of MoS₂ at an optimized growth condition. Monolayer triangular MoS₂ domain with an edge length ∼300 μm is obtained within 1 min, featured with a growth rate ∼5 μm s^-1. The Na element from the NaCl crystal is found to be able to facilitate the two dimensional growth of monolayer MoS₂. This work thus offers novel insights into the high-efficiency production of large-domain monolayer MoS₂ on insulating growth substrates.

Synergistic additive-mediated CVD growth and chemical modification of 2D materials

This review summarizes significant advances in the use of typical synergistic additives in growth of 2D materials with chemical vapor deposition, and the corresponding performance improvement of field effect transistors and photodetectors.

Two-dimensional plumbum-doped tin diselenide monolayer transistor with high on/off ratio

Doping can effectively regulate the electrical and optical properties of two-dimensional semiconductors. Here, we present high-quality Pb-doped SnSe₂ monolayer exfoliated using a micromechanical cleavage method. X-ray photoelectron spectroscopy measurement demonstrates that Pb content of the doped sample is ∼3.6% and doping induces the downward shift of the Fermi level with respect to the pure SnSe₂. Transmission electron microscopy characterization exhibits that Pb_0.036Sn_0.964Se₂ nanosheets have a high-quality hexagonal symmetry structure and Pb element is uniformly distributed in the nanosheets. The current of the SnSe₂ field effect transistors (FETs) was found to be very difficult to turn off due to the high electron density. The FETs based on the Pb_0.036Sn_0.964Se₂ monolayer show n-type behavior with a high on/off ratio of 10⁶ which is higher than any values of SnSe₂ FETs reported at the moment. The estimated carrier concentration of Pb_0.036Sn_0.964Se₂ is approximately six times lower than that of SnSe₂. The results suggest that the method of reducing carrier concentration by doping to achieve high on/off ratio is effective, and Pb-doped SnSe₂ monolayer has significant potential in future nanoelectronic and optoelectronic applications.

Halide-Induced Self-Limited Growth of Ultrathin Nonlayered Ge Flakes for High-Performance Phototransistors

2D nonlayered materials have attracted intensive attention due to their unique surface structure and novel physical properties. However, it is still a great challenge to realize the 2D planar structures of nonlayered materials owing to the naturally intrinsic covalent bonds. Ge is one of them with cubic structure impeding its 2D anisotropic growth. Here, the ultrathin single-crystalline Ge flakes as thin as 8.5 nm were realized via halide-assisted self-limited CVD growth. The growth mechanism has been confirmed by experiments and theoretical calculations, which can be attributed to the preferential growth of the (111) plane with the lowest formation energy and the giant interface distortion effect of the Cl-Ge motif. Excitingly, a Ge flake-based phototransistor shows excellent performances such as a high hole mobility of ∼263 cm² V^-1 s^-1, a high responsivity of ∼200 A/W, and fast response rates (τ_rise = 70 ms, τ_decay = 6 ms), suggesting its great potential in the applications of electronics and optoelectronics.

Two-dimensional light-emitting materials: preparation, properties and applications

We review the recent development in two-dimensional (2D) light-emitting materials and describe their preparation methods, optical/optoelectronic properties and applications.