Science of 2.5 dimensional materials: paradigm shift of materials science toward future social innovation.

The past decades of materials science discoveries are the basis of our present society - from the foundation of semiconductor devices to the recent development of internet of things (IoT) technologies. These materials science developments have depended mainly on control of rigid chemical bonds, such as covalent and ionic bonds, in organic molecules and polymers, inorganic crystals and thin films. The recent discovery of graphene and other two-dimensional (2D) materials offers a novel approach to synthesizing materials by controlling their weak out-of-plane van der Waals (vdW) interactions. Artificial stacks of different types of 2D materials are a novel concept in materials synthesis, with the stacks not limited by rigid chemical bonds nor by lattice constants. This offers plenty of opportunities to explore new physics, chemistry, and engineering. An often-overlooked characteristic of vdW stacks is the well-defined 2D nanospace between the layers, which provides unique physical phenomena and a rich field for synthesis of novel materials. Applying the science of intercalation compounds to 2D materials provides new insights and expectations about the use of the vdW nanospace. We call this nascent field of science '2.5 dimensional (2.5D) materials,' to acknowledge the important extra degree of freedom beyond 2D materials. 2.5D materials not only offer a new field of scientific research, but also contribute to the development of practical applications, and will lead to future social innovation. In this paper, we introduce the new scientific concept of this science of '2.5D materials' and review recent research developments based on this new scientific concept.

Coupling and Decoupling of Bilayer Graphene Monitored by Electron Energy Loss Spectroscopy.

We studied the interlayer coupling and decoupling of bilayer graphene (BLG) using spatially resolved electron energy loss spectroscopy with a monochromated electron source. We correlated the twist-angle-dependent energy band hybridization with Moiré superlattices and the corresponding optical absorption peaks. The optical absorption peak originates from the excitonic transition between the hybridized van Hove singularities (vHSs), which shifts systematically with the twist angle. We then proved that the BLG decouples when a monolayer of metal chloride is intercalated in its van der Waals gap and results in the elimination of the vHS peak.

Pinning in a Contact and Noncontact Manner: Direct Observation of a Three-Phase Contact Line Using Graphene Liquid Cells.

Pinning of a three-phase contact line at the nanoscale cannot be explained by conventional macroscale theories and thus requires an experimental insight to understand this phenomenon. We performed in-situ transmission electron microscopy observation of the three-phase contact lines of bubbles inside graphene liquid cells to experimentally investigate the causes of nanoscale pinning. In our observations, the three-phase contact line was not affected by the 0.6 nm-thick inhomogeneity of the graphene surface, but thicker metal nanoparticles with diameters of 2-10 nm and nanoflakes caused pinning of the gas-liquid interface. Notably, we found that flake-like objects can cause pinning that prevents the bubble overcome the flake object in a noncontact state, with a 2 nm-thick liquid film between them and the bubble. This phenomenon can be explained by the repulsive force obtained using the Derjaguin, Landau, Verwey, and Overbeek theory. We also observed that the flake temporally prevented the gas-liquid interface moving away from the flake. We discussed the physical mechanism of the attractive force-like phenomenon by considering the nanoconfinement effect of the liquid sandwiched by two graphene sheets and the hydration layer formed near the solid surface.

One-step vapour phase growth of two-dimensional formamidinium-based perovskite and its hot carrier dynamics.

Formamidinum lead iodide perovskite is one of the most promising materials for application in solar cells due to its narrow band gap and higher thermal stability. In this work, we demonstrate the facile synthesis of square-shaped formamidinium lead iodide single crystals on indium tin oxide (ITO) substrates using a one-step vapour phase deposition method. Formamidinium lead iodide-based two-dimensional layered perovskite crystals were successfully synthesized by controlling the deposition conditions. These crystals exhibited a blue-shifted photoluminescence (PL) compared to the conventional formaminium lead iodide perovskite crystals. Power law fittings of the excitation power dependent PL spectra revealed that Auger heating becomes dominant at high excitation densities. In addition, we observed an asymmetric broadening of the PL peak tail at the high energy side, indicating light emission from hot carriers even under steady-state illumination conditions. Phonon-bottleneck effect and Auger heating were considered as the main mechanisms for retardation of hot carrier cooling. Further analysis of the high energy tails using Maxwell-Boltzmann fitting revealed hot-carrier temperatures as high as 690 K. Our findings provide an important aspect of the synthetic approach of perovskites for their potential application in hot carrier solar cells.

Two-step synthesis and characterization of vertically stacked SnS-WS2 and SnS-MoS2 p-n heterojunctions.

We demonstrate the synthesis of unique heterostructures consisting of SnS and WS2 (or SnS and MoS2) by two-step chemical vapor deposition (CVD). After the first CVD growth of triangular WS2 (MoS2) grains, the second CVD step was performed to grow square SnS grains on the same substrate. We found that these SnS grains can be grown at very low temperature with the substrate temperature of 200 °C. Most of the SnS grains nucleated from the side edges of WS2 (MoS2) grains, resulting in the formation of partly stacked heterostructures with a large overlapping area. The SnS grains showed doped p-type transfer character with a hole mobility of 15 cm2 V-1 s-1, while the WS2 and MoS2 grains displayed n-type character with a high on/off ratio of >106. The SnS-WS2 and SnS-MoS2 heterostructures exhibited clear rectifying behavior, signifying the formation of p-n junctions at their interfaces. This heterostructure growth combined with the low temperature SnS growth will provide a promising means to exploit two-dimensional heterostructures by avoiding possible damage to the first material.

van der Waals interaction-induced photoluminescence weakening and multilayer growth in epitaxially aligned WS2.

Recently, transition metal dichalcogenides (TMDCs) have attracted great interest due to their unique electronic and optical properties. Chemical vapor deposition (CVD) has been regarded as the most promising method for the synthesis of large-area TMDCs with high reproducibility. Having similar hexagonal crystal structures with many TMDCs, c-plane sapphire is commonly used as a growth substrate in CVD. However, few studies have been reported on the influence of the sapphire substrate on the growth behavior and physical properties of TMDCs. In this work, we demonstrate that higher strain is induced in epitaxially grown WS2 grains via van der Waals interactions with sapphire as compared with misaligned WS2 grains. In addition, this strain was found to enhance overlayer deposition on monolayer WS2, while multilayer growth was not observed in non-epitaxial WS2. Photoluminescence (PL) of the epitaxially grown WS2 grains was reduced, reflecting the effective van der Waals interaction with sapphire. Moreover, low-temperature PL measurements revealed strong influence of the c-plane sapphire surface on the optical properties of WS2. Density functional theory (DFT) calculation supports that the aligned WS2 grains are more strongly bound to the sapphire surface, as compared with misaligned WS2. Our work offers a new insight into the understanding of the influence of the substrate on the CVD-grown TMDC materials.

Synthesis, structure and applications of graphene-based 2D heterostructures.

With the profuse amount of two-dimensional (2D) materials discovered and the improvements in their synthesis and handling, the field of 2D heterostructures has gained increased interest in recent years. Such heterostructures not only overcome the inherent limitations of each of the materials, but also allow the realization of novel properties by their proper combination. The physical and mechanical properties of graphene mean it has a prominent place in the area of 2D heterostructures. In this review, we will discuss the evolution and current state in the synthesis and applications of graphene-based 2D heterostructures. In addition to stacked and in-plane heterostructures with other 2D materials and their potential applications, we will also cover heterostructures realized with lower dimensionality materials, along with intercalation in few-layer graphene as a special case of a heterostructure. Finally, graphene heterostructures produced using liquid phase exfoliation techniques and their applications to energy storage will be reviewed.

Epitaxial chemical vapour deposition growth of monolayer hexagonal boron nitride on a Cu(111)/sapphire substrate.

Hexagonal boron nitride (h-BN), an atomically thin insulating material, shows a large band gap, mechanical flexibility, and optical transparency. It can be stacked with other two-dimensional (2D) materials through van der Waals interactions to form layered heterostructures. These properties promise its application as an insulating layer of novel 2D electronic devices due to its atomically smooth surface with a large band gap. Herein, we demonstrated the ambient-pressure chemical vapour deposition (CVD) growth of high-quality, large-area monolayer h-BN on a Cu(111) thin film deposited on a c-plane sapphire using ammonia borane (BH3NH3) as the feedstock. Highly oriented triangular h-BN grains grow on Cu(111), which finally coalescence to cover the entire Cu surface. Low-energy electron diffraction (LEED) measurements indicated that the hexagonal lattice of the monolayer h-BN is well-oriented along the underlying Cu(111) lattice, thus implying the epitaxial growth of h-BN, which can be applied in various 2D electronic devices.

Vertical heterostructures of MoS2 and graphene nanoribbons grown by two-step chemical vapor deposition for high-gain photodetectors.

Heterostructures of two-dimensional (2D) layered materials have attracted growing interest due to their unique properties and possible applications in electronics, photonics, and energy. Reduction of the dimensionality from 2D to one-dimensional (1D), such as graphene nanoribbons (GNRs), is also interesting due to the electron confinement effect and unique edge effects. Here, we demonstrate a bottom-up approach to grow vertical heterostructures of MoS2 and GNRs by a two-step chemical vapor deposition (CVD) method. Single-layer GNRs were first grown by ambient pressure CVD on an epitaxial Cu(100) film, followed by the second CVD process to grow MoS2 over the GNRs. The MoS2 layer was found to grow preferentially on the GNR surface, while the coverage could be further tuned by adjusting the growth conditions. The MoS2/GNR nanostructures show clear photosensitivity to visible light with an optical response much higher than that of a 2D MoS2/graphene heterostructure. The ability to grow a novel 1D heterostructure of layered materials by a bottom-up CVD approach will open up a new avenue to expand the dimensionality of the material synthesis and applications.

Strain engineering the properties of graphene and other two-dimensional crystals.

Graphene has been widely studied for its many extraordinary properties, and other two-dimensional layered materials are now gaining increased interest. These excellent properties make thin layer materials very attractive for integration into a wide variety of technologies, particularly in flexible optoelectronic devices. Therefore, gaining control over these properties will allow for a more focused design and optimisation of these possible technologies. Through the application of mechanical strain it is possible to alter the electronic structures of two-dimensional crystals, such as graphene and transition metal dichalcogenides (e.g. MoS2), and these changes in electronic structure can alter their behaviour. In this perspective we discuss recent advances in the strain engineering of thin layer materials, with a focus on using Raman spectroscopy and electrical transport to investigate the effect of strain as well as the effect of strain on the chemical functionalisation of graphene.

Synthesis of Few-Layer Hexagonal Boron Nitride for Magnetic Tunnel Junction Application.

Hexagonal boron nitride (hBN), a wide-gap two-dimensional (2D) insulator, is an ideal tunneling barrier for many applications because of the atomically flat surface, high crystalline quality, and high stability. Few-layer hBN with a thickness of 1-2 nm is an effective barrier for electron tunneling, but the preparation of few-layer hBN relies on mechanical exfoliation from bulk hBN crystals. Here, we report the large-area growth of few-layer hBN by chemical vapor deposition on ferromagnetic Ni-Fe thin films and its application to tunnel barriers of magnetic tunnel junction (MTJ) devices. Few-layer hBN sheets mainly consisting of two to three layers have been successfully synthesized on a Ni-Fe catalyst at a high growth temperature of 1200 °C. The MTJ devices were fabricated on as-grown hBN by using the Ni-Fe film as the bottom ferromagnetic electrode to avoid contamination and surface oxidation. We found that trilayer hBN gives a higher tunneling magnetoresistance (TMR) ratio than bilayer hBN, resulting in a high TMR ratio up to 10% at ∼10 K.

Alkali metal bilayer intercalation in graphene.

Alkali metal (AM) intercalation between graphene layers holds promise for electronic manipulation and energy storage, yet the underlying mechanism remains challenging to fully comprehend despite extensive research. In this study, we employ low-voltage scanning transmission electron microscopy (LV-STEM) to visualize the atomic structure of intercalated AMs (potassium, rubidium, and cesium) in bilayer graphene (BLG). Our findings reveal that the intercalated AMs adopt bilayer structures with hcp stacking, and specifically a C6M2C6 composition. These structures closely resemble the bilayer form of fcc (111) structure observed in AMs under high-pressure conditions. A negative charge transferred from bilayer AMs to graphene layers of approximately 1~1.5×1014 e-/cm-2 was determined by electron energy loss spectroscopy (EELS), Raman, and electrical transport. The bilayer AM is stable in BLG and graphite superficial layers but absent in the graphite interior, primarily dominated by single-layer AM intercalation. This hints at enhancing AM intercalation capacity by thinning the graphite material.

Random but limited pressure of graphene liquid cells.

Even though many researchers have used graphene liquid cells for atomic-resolution observation of liquid samples in the last decade, no one has yet simultaneously measured their three-dimensional shape and pressure. In this study, we have done so with an atomic force microscope, for cells with base radii of 20-134 nm and height of 3.9-21.2 nm. Their inner pressure ranged from 1.0 to 63 MPa but the maximum value decreased as the base radius increased. We discuss the mechanism that results in this inverse relationship by introducing an adhesive force between the graphene membranes. Also, the sample preparation procedure used in this experiment is highly reproducible and transferable to a wide variety of substrates.

Molybdenum Chloride Nanostructures with Giant Lattice Distortions Intercalated into Bilayer Graphene.

The nanospace of the van der Waals (vdW) gap between structural units of two-dimensional (2D) materials serves as a platform for growing unusual 2D systems through intercalation and studying their properties. Various kinds of metal chlorides have previously been intercalated for tuning the properties of host layered materials, but the atomic structure of the intercalants remains still unidentified. In this study, we investigate the atomic structural transformation of molybdenum(V) chloride (MoCl5) after intercalation into bilayer graphene (BLG). Using scanning transmission electron microscopy, we found that the intercalated material represents MoCl3 networks, MoCl2 chains, and Mo5Cl10 rings. Giant lattice distortions and frequent structural transitions occur in the 2D MoClx that have never been observed in metal chloride systems. The trend of symmetric to nonsymmetric structural transformations can cause additional charge transfer from BLG to the intercalated MoClx, as suggested by our density functional theory calculations. Our study deepens the understanding of the behavior of matter in the confined space of the vdW gap in BLG and provides hints at a more efficient tuning of material properties by intercalation for potential applications, including transparent conductive films, optoelectronics, and energy storage.

Surface etching and edge control of hexagonal boron nitride assisted by triangular Sn nanoplates.

Hexagonal boron nitride (hBN) is an ideal insulating substrate and template for other two-dimensional (2D) materials. The combination of hBN and 2D materials of group IV atoms, such as graphene, is interesting, because it can offer attractive physical properties and promising applications. Here, we demonstrate the unique behavior of tin (Sn), one of the group IV elements, on multilayer hBN which was grown by chemical vapor deposition (CVD). At high temperatures, triangular nanoplates formed after thermal deposition of Sn on the hBN surface, with their orientations determined by the hBN lattice. The triangular Sn nanoplates moved on the hBN surface, leaving monolayer-deep nanotrenches. Low-energy electron microscopy (LEEM) revealed that the nanotrenches are aligned in the armchair directions of the hBN. Furthermore, an additional Ar annealing without supplying Sn vapor induced the structural change of the linear trenches to triangular pits, indicating the preferential formation of zigzag edges in the absence of Sn. Our work highlights the unique behavior of Sn on hBN and offers a novel route to engineer the hBN surface.

Twist Angle-Dependent Molecular Intercalation and Sheet Resistance in Bilayer Graphene.

Bilayer graphene (BLG) has a two-dimensional (2D) interlayer nanospace that can be used to intercalate molecules and ions, resulting in a significant change of its electronic and magnetic properties. Intercalation of BLG with different materials, such as FeCl3, MoCl5, Li ions, and Ca ions, has been demonstrated. However, little is known about how the twist angle of the BLG host affects intercalation. Here, by using artificially stacked BLG with controlled twist angles, we systematically investigated the twist angle dependence of intercalation of metal chlorides. We discovered that BLG with high twist angles of >15° is more favorable for intercalation than BLG with low twist angles. Density functional theory calculations suggested that the weaker interlayer coupling in high twist angle BLG is the key for effective intercalation. Scanning transmission electron microscope observations revealed that co-intercalation of AlCl3 and CuCl2 molecules into BLG gives various 2D structures in the confined interlayer nanospace. Moreover, before intercalation we observed a significantly lower sheet resistance in BLG with high twist angles (281 ± 98 Ω/□) than that in AB stacked BLG (580 ± 124 Ω/□). Intercalation further decreased the sheet resistance, reaching values as low as 48 Ω/□, which is the lowest value reported so far for BLG. This work provides a twist angle-dependent phenomenon, in which enhanced intercalation and drastic changes of the electrical properties can be realized by controlling the stacking angle of adjacent graphene layers.

Polymorphic Phases of Metal Chlorides in the Confined 2D Space of Bilayer Graphene.

Unprecedented 2D metal chloride structures are grown between sheets of bilayer graphene through intercalation of metal and chlorine atoms. Numerous spatially confined 2D phases of AlCl3 and CuCl2 distinct from their typical bulk forms are found, and the transformations between these new phases under the electron beam are directly observed by in situ scanning transmission electron microscopy (STEM). The density functional theory calculations confirm the metastability of the atomic structures derived from the STEM experiments and provide insights into the electronic properties of the phases, which range from insulators to semimetals. Additionally, the co-intercalation of different metal chlorides is found to create completely new hybrid systems; in-plane quasi-1D AlCl3 /CuCl2 heterostructures are obtained. The existence of polymorphic phases hints at the unique possibilities for fabricating new types of 2D materials with diverse electronic properties confined between graphene sheets.

Patterned growth of graphene over epitaxial catalyst.

Rectangle- and triangle-shaped microscale graphene films are grown on epitaxial Co films deposited on single-crystal MgO substrates with (001) and (111) planes, respectively. A thin film of Co or Ni metal is epitaxially deposited on a MgO substrate by sputtering while heating the substrate. Thermal decomposition of polystyrene over this epitaxial metal film in vacuum gives rectangular or triangular pit structures whose orientation and shape are strongly dependent on the crystallographic orientation of the MgO substrate. Raman mapping measurements indicate preferential formation of few-layer graphene films inside these pits. The rectangular graphene films are transferred onto a SiO(2)/Si substrate while maintaining the original shape and field-effect transistors are fabricated using the transferred films. These findings on the formation of rectangular/triangular graphene give new insights on the formation mechanism of graphene and can be applied for more advanced/controlled graphene growth.

Effective patterning of metal nanoparticles on sapphire surface for aligned growth of single-walled carbon nanotubes.

We studied patterning of metal nanoparticle catalyst on sapphire (alpha-Al2O3) substrates to grow horizontally-aligned single-walled carbon nanotubes (SWNTs) from defined positions. Various modifications of sapphire surface were investigated for effective adsorption of iron oxide nanoparticles. We found that phosphoric acids bind to sapphire surface and enhance the adsorption of the nanoparticles. This surface modification enabled effective patterning of nanoparticle via electron beam lithography. The small nanoparticles with a mean diameter of 5 nm tended to diffuse at high temperature during the nanotube growth, while the large particles with 12 nm mean diameter mostly stayed at the same position giving aligned SWNTs grown from the catalyst pattern.

Nanoscale Bubble Dynamics Induced by Damage of Graphene Liquid Cells.

Graphene liquid cells provide the highest possible spatial resolution for liquid-phase transmission electron microscopy. Here, in graphene liquid cells (GLCs), we studied the nanoscale dynamics of bubbles induced by controllable damage in graphene. The extent of damage depended on the electron dose rate and the presence of bubbles in the cell. After graphene was damaged, air leaked from the bubbles into the water. We also observed the unexpected directional nucleation of new bubbles, which is beyond the explanation of conventional diffusion theory. We attributed this to the effect of nanoscale confinement. These findings provide new insights into complex fluid phenomena under nanoscale confinement.