2D Crystals Research Articles

In-situ scanning tunneling microscopy observation of thickness-dependent air-sensitive layered materials and heterodevices

Quasi-two-dimensional (Quasi-2D) van der Waals (vdW) materials can be mechanically or chemically exfoliated down to monolayer because of their strong intralayer bonding and the weak interlayer vdW interaction. Thanks to this unique property, one can often find exotic thickness-dependent electronic properties from these quasi-2D vdW materials, which can lead to bandgap opening, emerging superconductivity, or enhanced charge density waves with decreasing thickness. Surface-sensitive scanning tunneling microscopy (STM) can provide direct observation of structural and electronic characteristics of such layered materials with atomic precision in real space. However, it is very challenging to preserve the intrinsic surfaces of air-sensitive quasi-2D materials between preparation and measurement. In addition, vdW 2D crystals after exfoliation are extremely hard to explore with a typical STM setup due to their small size (≤ 10 μm). Here, we present a straightforward method compatible with any STM setup having optical access: (1) exfoliating and/or stacking layered materials in a glove box, (2) transferring them to an ultra-high vacuum STM chamber using a suitcase without exposure to air, and (3) navigating surface to locate exfoliated vdW 2D flakes with different thicknesses. We successfully demonstrated that the clean surfaces of the air-sensitive $$\hbox {Fe}_3\hbox {GeTe}_2$$ can be effectively protected from unwanted oxidation during transfer. Furthermore, our method provides a simple but useful way to access a specific tiny stack of layered materials without any ex-situ fabrication processes for STM navigation. Our experimental improvement will open up a new way to investigate air-sensitive layered vdW materials with various thicknesses via surface-sensitive techniques including STM.

Interweaving Polar Charge Orders in a Layered Metallic Superatomic Crystal

A 3D crystal of superatoms is the first known antipolar metal and possesses the first polar metallic state hosted in superatomic units, opening a category of quantum materials with versatile layered nanostructures.

Importance of annexin V N-terminus for 2D crystal formation and quick purification protocol of recombinant annexin V.

Annexin V forms trimeric structures which further assemble into two-dimensional crystal (2D crystal) lattices on negatively charged phospholipid bilayer in a Ca2+-dependent manner. It is also known that annexin V 2D crystals show two types of symmetric patterns with six-fold symmetry (p6) and three-fold symmetry (p3). The p6 lattice also contains additional trimers in the gaps between the p6 axes, which are also referred to as non-p6 trimers because they do not participate in the formation of the p6 lattice. We here show that the annexin V N-terminal has significant influence on 2D crystal formation using high-speed atomic force microscopy (HS-AFM) observations. We also present a quick purification method to purify recombinant annexin V without any residual affinity tag after protein purification in ~3h.

Edge Reconstruction-Dependent Growth Kinetics of MoS2.

Understanding the growth mechanisms of multielement two-dimensional (2D) crystals is challenging because of the unbalanced stoichiometry and possible reconstruction of their edges. Here, we present a systematic theoretical study on the chemical vapor deposition (CVD) growth mechanism of MoS2. We found that the growth kinetics of MoS2 highly depends on its edge reconstruction determined by concentrations of Mo and S in the growth environment. Based on the calculated energies of nucleation and propagation of various MoS2 edges, we predicted the transition of a MoS2 island growth from a regime of a triangle enclosed by Mo-terminated zigzag edges that are passivated by 50% S (Mo-II edges), to a regime of continuous evolution within a triangle, hexagon, and inverted triangle with 75%-S-terminated edges (S-III edges) and Mo-II edges, and finally to a regime of triangles with Mo-terminated zigzag edges that are passivated by 100% S (Mo-III edges) by tuning the growth condition from Mo-rich to S-rich, which provides a reasonable explanation to many experimental observations. This study provides a general guideline on theoretical studies of 2D crystals' growth mechanisms, deepens our understanding on the growth mechanism of multielement 2D crystals, and is beneficial for the controllable synthesis of various 2D crystals.

Manipulating the Formation of 2D/3D Heterostructure in Stable High-Performance Printable CsPbI3 Perovskite Solar Cells.

Manipulating the formation process of the 2D/3D perovskite heterostructure, including its nucleation/growth dynamics and phase transition pathway, plays a critical role in controlling the charge transport between 2D and 3D crystals, and consequently, the scalable fabrication of efficient and stable perovskite solar cells. Herein, the structural evolution and phase transition pathways of the ligand-dependent 2D perovskite atop the 3D surface are revealed using time-resolved X-ray scattering. The results show that the ligand size and shape have a critical influence on the final 2D structure. In particular, ligands with smaller sizes and more reactive sites tend to form the n= 1 phase. Increasing the ligand size and decreasing the reactive sites promote the transformation from 3D to n= 3 and n< 3 phases. These findings are useful for the rational design of the phase distribution in 2D perovskites to balance the charge transport and stability of the perovskite films. Finally, solar cells based on ambient-printed CsPbI3 with n-butylammonium iodide treatment achieve an improved efficiency of 20.33%, which is the highest reported value for printed inorganic perovskite solar cells.

Lattice Chern-Simons model for FQHE

Motivated by [60][61] and using topological invariance and cycle homology, we construct an effective gauge invariant lattice Chern-Simons (CS) model for the fractional quantum Hall effect (FQHE) with applications to (i) the Laughlin state on 3D crystals, (ii) Haldane hierarchy, and (iii) composite fermions. First, we give a new derivation of the lattice CS action and demonstrate its gauge symmetry and invariance under lattice duality. Then, we use this lattice construction to study the filling factors of the above mentioned QH states. We show amongst others that the νlattL of Laughlin state on lattice is given by the ratio Φ0/ΦWL with ΦWL designating the flux of CS link variables through Wilson loops WL. For composite fermions, we use the adb formalism to show that the factor νlattCF is given by the charge Qb of the dumbbell field under the dual gauge link variables. Our model gives explicitly the effective filling fraction of composite fermions on the lattice as νlatt⁎=νlattCF/(1−2nνlattCF) with νlattCF being the bare lattice filling factor in agreement with the continuum limit result.

Enantioselective sensing by collective circular dichroism.

Quantitative determination and in situ monitoring of molecular chirality at extremely low concentrations is still challenging with simple optics because of the molecular-scale mismatch with the incident light wavelength. Advances in spectroscopy1-4 and nanophotonics have successfully lowered the detection limit in enantioselective sensing, as it can bring the microscopic chiral characteristics of molecules into the macroscopic scale5-7 or squeeze the chiral light into the subwavelength scale8-17. Conventional nanophotonic approaches depend mainly on the optical helicity density8,9 by localized resonances within an individual structure, such as localized surface plasmon resonances (LSPRs)10-16 or dielectric Mie resonances17. These approaches use the local chiral hotspots in the immediate vicinity of the structure, whereas the handedness of these hotspots varies spatially. As such, these localized resonance modes tend to be error-prone to the stochasticity of the target molecular orientations, vibrations and local concentrations18,19. Here we identified enantioselective characteristics of collective resonances (CRs)20 arising from assembled 2D crystals of isotropic, 432-symmetric chiral gold nanoparticles (helicoids)21,22. The CRs exhibit a strong and uniform chiral near field over a large volume above the 2D crystal plane, resulting from the collectively spinning, optically induced dipoles at each helicoid. Thus, energy redistribution by molecular back action on the chiral near field shifts the CRs in opposite directions, depending on the handedness of the analyte, maximizing the modulation of the collective circular dichroism (CD).

Epitaxial Atomic Substitution for MoS2–MoN Heterostructure Synthesis

Integrating different two-dimensional (2D) crystals is highly demanded for advancing their application in next-generation electronics. 2D transition metal carbides, nitrides, and carbonitrides (MXenes), as new members in the 2D family, are promising candidates for 2D electrodes because of their high conductivity and stability. However, integrating MXenes with other 2D semiconductors has been underdeveloped due to the limitation of top-down etching synthesis of MXenes. Our recent development of atomic substitution synthesis achieved ultrathin non-van der Waals (non-vdW) transition metal nitrides (TMNs) through the conversion of vdW transition metal dichalcogenides (TMDs), opening opportunities of combining TMDs with TMNs via controllable partial conversion. Here, we perform an in-depth study of the atomic substitution process from semiconducting MoS2 to metallic MoN and realize both lateral and vertical MoN-MoS2 heterostructures via edge and surface epitaxial conversion, respectively. The structural evolution investigation from MoS2 to MoN using high-resolution transmission electron microscopy suggests atomically bonded interface for lateral heterostructures and moiré pattern in vertical heterostructures. Moreover, mask-assisted atomic substitution is applied to create patterned MoN-MoS2-MoN lateral heterostructures. Electrical measurements reveal a Schottky barrier height of meV for a three-layer MoS2-MoN interface, showcasing the potential of atomically bonded lateral heterostructures for MoS2 electronics with MoN as contact electrodes.

A highly ordered 8/1 helical pyramidal column self-organized from the crown conformation of achiral hexa(butyloxy)triphenylene

Liquid crystals of disc-like molecules are known for 45 years. 2,3,6,7,10,11-Hexakis(alkyloxy)triphenylene (HATn) containing n = 4 to 11 carbons in its achiral alkyl groups, represents one of the classic families of disc-like molecules. X-ray analysis suggested that disc-like molecules stack on top of each other at irregular spacing in columns distributed in the melt of their alkyl groups forming discotic or columnar or discotic nematic liquid crystals. Interest in discotic liquid crystals comes mostly from their conducting and photoconducting properties. While writing a recent perspective on the 45th anniversary of discotic liquid crystals we realized that the molecular structures of the 3D crystal and 2D liquid crystal phases of discotic molecules were never determined at the molecular level, in spite of the great interest in their physical properties. Since structure determines function, we decided to perform a detailed investigation of the molecular structure of HATn self-organizations by reconstructing their oriented fiber X-ray diffractograms both in the crystal and liquid crystal states. This communication reports the synthesis, analysis by differential scanning calorimetry of HATn containing n = 4 to 12 carbons in its achiral alkyl groups and the molecular structure of HAT4 in its crystal and liquid crystal states. Unexpectedly, a highly ordered 8/1 helical pyramidal crystalline column assembled from a crown-like conformation of the HAT4 and a non-helical column of the discotic liquid crystal state, the last similar to the original suggested supramolecular structure, were resolved for the first time at the molecular level. These preliminary results raise a lot of fundamental issues in the area of discotic and columnar self-organizations and of related helical columnar self-organizations generated with different building blocks and belonging to different disciplines. A comprehensive mechanistic elucidation of the molecular details of helical and non-helical self-organizations in synthetic complex systems may become as influential in the field of soft condensed matter as that of helical and non-helical self-organizations in the field of molecular biology.

Electrically Tunable MXene Nanomechanical Resonators Vibrating at Very High Frequencies

As an emerging class of two-dimensional (2D) layered nanomaterial, MXene exhibits a number of intriguing properties, such as good electrical conductivity and high elastic modulus, and has witnessed continued growth in related device research. However, nanoscale MXene devices which leverage both the intrinsic electrical and mechanical properties of these 2D crystals have not been experimentally studied. Here we demonstrate nanoelectromechanical resonators based on 2D MXene crystals, where Ti3C2Tx drumheads with a wide range of thickness, from more than 50 layers all the way down to a monolayer, exhibit robust nanomechanical vibrations with fundamental-mode frequency f0 up to >70 MHz in the very high frequency (VHF) band, a displacement noise density down to 52 fm/Hz1/2, and a fundamental-mode frequency-quality factor product up to f0 × Q ≈ 6.85 × 109 Hz. By combining experimental results with theoretical calculations, we independently derive the Young's modulus of 2D Ti3C2Tx crystals to be 270-360 GPa, in excellent agreement with nanoindentation measurements, based on which we elucidate frequency scaling pathways toward microwave frequencies. We further demonstrate electrical tuning of resonance frequency in MXene resonators and frequency-shift-based MXene vacuum gauges with responsivity of 736%/Torr and detection range down to 10-4 Torr. Our study can lead to the design and creation of nanoscale vibratory devices that exploit the intrinsic electrical and mechanical properties in 2D MXene crystals.

Graphene Nanoplatelets-Based Textured Polymeric Fibrous Fabrics for the Next-Generation Devices.

Graphene is a 2D crystal composed of carbon atoms in a hexagonal arrangement. From their isolation, graphene nanoplatelets (nCD) have revolutionized material science due to their unique properties, and, nowadays, there are countless applications, including drug delivery, biosensors, energy storage, and tissue engineering. Within this work, nCD were combined with PLA, a widely used and clinically relevant thermoplastic polymer, to produce advanced composite texturized electrospun fabric for the next-generation devices. The electrospinning manufacturing process was set-up by virtue of a proper characterization of the composite raw material and its solution. From the morphological point of view, the nCD addition permitted the reduction of the fiber diameter while the texture allowed more aligned fibers. After that, mechanical features of fabrics were tested at RT and upon heating (40 °C, 69 °C), showing the reinforcement action of nCD mainly in the texturized mats at 40 °C. Finally, mats' degradation in simulated physiological fluid was minimal up to 30 d, even if composite mats revealed excellent fluid-handling capability. Moreover, no toxic impurities and degradation products were pointed out during the incubation. This work gains insight on the effects of the combination of composite carbon-based material and texturized fibers to reach highly performing fabrics.

Phase control of two-dimensional tin sulfide compounds deposited via atomic layer deposition

Two-dimensional (2D) semiconductor materials are emerging as potential candidates for emerging electronic device applications due to their excellent electrical properties and unique properties such as flexibility and transparency. Among the various potential 2D semiconductor materials, tin sulfide compounds can form polymorphs of different 2D crystals: 2D layered-SnS2 with hexagonal symmetry and 2D layered SnS with orthorhombic symmetry. In addition, since they each have different electrical and optical properties, their controlled growth is important for appropriate target applications. Therefore, we investigated the phase control of tin sulfide compounds deposited via atomic layer deposition according to their deposition temperature and post-annealing conditions. A strategy for controlling the growth of SnS2 or SnS was proposed by investigating phase transition tendencies according to deposition temperature and annealing conditions (temperature, pressure, atmosphere). As a result, it was possible to selectively deposit SnS2 and SnS thin films based on a phase transition study; it was confirmed that SnS2 and SnS thin films exhibited n-type and p-type semiconductor characteristics, respectively.

Two-dimensional ferromagnetic materials: From materials to devices

The magnetic anisotropy can stabilize long-range ferromagnetic order in pristine two-dimensional (2D) crystals, which inspires the research of fundamental physics and spintronic devices based on 2D ferromagnetic materials. 2D ferromagnetic materials with intriguing magnetic properties offer an excellent platform for studying magnetism at 2D limit and exploring advanced spintronic devices. As the dimensionality decreases to 2D scale, 2D ferromagnetic materials exhibit distinctive magnetic properties compared with their bulk counterparts. In this Perspective, the recent progress and prospects of 2D ferromagnetic materials in magnetism, manipulation, and device applications are highlighted.

Impact of surface free energy on two-dimensional crystallization

Two-dimensional (2D) molecular crystals are emerging materials with fascinating optoelectronic properties. However, growing 2D organic single crystals is challenging because little is known about the factors that determine 2D crystallization. Herein, we investigated the relationship between 2D crystallization and the surface free energy of organic semiconductors. We proposed a term called surface free energy anisotropy factor (g) to measure the tendency of 2D crystallization. Taking two [1]benzothieno[3,2-b][1]benzothiophene derivatives as examples, we identified that the one with a larger g factor showed a higher tendency for 2D crystallization, irrespective of the external factors. This work provides a guideline for the rational design of organic semiconductors toward 2D crystallization.

Quantum Emitters in Hexagonal Boron Nitride.

Hexagonal boron nitride (hBN) has emerged as a fascinating platform to explore quantum emitters and their applications. Beyond being a wide-bandgap material, it is also a van der Waals crystal, enabling direct exfoliation of atomically thin layers─a combination which offers unique advantages over bulk, 3D crystals. In this Mini Review we discuss the unique properties of hBN quantum emitters and highlight progress toward their future implementation in practical devices. We focus on engineering and integration of the emitters with scalable photonic resonators. We also highlight recently discovered spin defects in hBN and discuss their potential utility for quantum sensing. All in all, hBN has become a front runner in explorations of solid-state quantum science with promising future prospects.

Theoretical prediction of two-dimensional WSi2N4 materials for photocatalytic water splitting

Recently, novel two-dimensional (2D) crystals, MSi2N4 (M = Mo, W) materials, have been successfully synthesized experimentally and have comparable excellent catalytic properties as that of MoS2. The suitability of MA2Z4 family materials in photocatalytic water splitting can't be fully determined by whether the bandgap edge of the material cross the standard redox potential of water. Photoelectric properties and electron–hole separation are also critical factors to be considered. We investigated the bandgap edge positions and the photoelectric and the electron–hole excitation properties of 2D MoSi2N4 and its family of materials (CrSi2N4, WSi2N4) in water by first-principles calculations, and the results indicate that WSi2N4 may be a relatively high-performing photocatalyst. Relative to the MoSi2N4 bandgap (1.74 eV), the bandgap of WSi2N4 is 2.06 eV, and the conduction-band minimum edge band potential (−0.42 eV) is close to the hydrogen precipitation potential in water at pH = 7. The bandgaps of the MSi2N4 (M = Mo, W) materials cross the water redox potential (1.23 eV), and both have favorable adsorption for H2O molecules. However, compared with the absorption spectrum and excited states of MoSi2N4 in water, WSi2N4 exhibits a broader and more enhanced visible light absorption range and intensity as well as a higher electron–hole separation. 2D WSi2N4 could achieve the half-reaction of photocatalytic water splitting under visible light irradiation, and the photogenerated electrons in the conduction band can spontaneously reduce H+ ions to hydrogen, suggesting that WSi2N4 might be composed of a heterogeneous structure with other photocatalysts to accomplish the redox of water.

Highly Symmetric, Self-Assembling 3D DNA Crystals with Cubic and Trigonal Lattices.

The rational design of nanoscopic DNA tiles has yielded highly ordered crystalline matter in 2D and 3D. The most well-studied 3D tile is the DNA tensegrity triangle, which is known to self-assemble into macroscopic crystals. However, contemporary rational design parameters for 3D DNA crystals nearly universally invoke integer numbers of DNA helical turns and Watson-Crick (WC) base pairs. In this study, 24-bp edges are substituted into a previously 21-bp (two helical turns of DNA) tensegrity triangle motif to explore whether such unconventional motif can self-assemble into 3D crystals. The use of noncanonical base pairs in the sticky ends results in a cubic arrangement of tensegrity triangles with exceedingly high symmetry, assembling a lattice from winding helical axes and diamond-like tessellation patterns. Reverting this motif to sticky ends with Watson-Crick pairs results in a trigonal hexagonal arrangement, replicating this diamond arrangement in a hexagonal context. These results showcase that the authors can generate unexpected, highly complex, pathways for materials design by testing modifications to 3D tiles without prior knowledge of the ensuing symmetry. This study expands the rational design toolbox for DNA nanotechnology; and it further illustrates the existence of yet-unexplored arrangements of crystalline soft matter.

Topological Superconductivity Based on Antisymmetric Spin-Orbit Coupling.

Topological superconductivity (TSC) has drawn much attention for its fundamental interest and application in quantum computation. An outstanding challenge is the lack of intrinsic TSC materials with a p-wave pairing gap, which has led to the development of an effective p-wave theory of coupling s-wave gap with Rashba spin-orbit coupling (RSOC). However, the RSOC-strict mechanism and materials pose still both fundamental and practical limitations. Here, we generalize this theory to antisymmetric SOC (ASOC). Using k·p perturbation theory, we demonstrate that 2D crystals, with point groups of C2, C4, C6, C2v, C4v, C6v, D2, D4, D6, S4, or D2d, can all facilitate the desired ASOC. Remarkably, this enables us to discover 314 TSC candidates by screening 2D material databases, which are further confirmed by first-principles calculations of Majorana boundary modes and the topological invariant of the superconducting gap. Our work fundamentally enriches TSC theory and greatly expands the classes of TSC materials for experimental exploration.

Ultra-bright, efficient and stable perovskite light-emitting diodes.

Metal halide perovskites are attracting a lot of attention as next-generation light-emitting materials owing to their excellent emission properties, with narrow band emission1-4. However, perovskite light-emitting diodes (PeLEDs), irrespective of their material type (polycrystals or nanocrystals), have not realized high luminance, high efficiency and long lifetime simultaneously, as they are influenced by intrinsic limitations related to the trade-off of properties between charge transport and confinement in each type of perovskite material5-8. Here, we report an ultra-bright, efficient and stable PeLED made of core/shell perovskite nanocrystals with a size of approximately 10 nm, obtained using a simple in situ reaction of benzylphosphonic acid (BPA) additive with three-dimensional (3D) polycrystalline perovskite films, without separate synthesis processes. During the reaction, large 3D crystals are split into nanocrystals and the BPA surrounds the nanocrystals, achieving strong carrier confinement. The BPA shell passivates the undercoordinated lead atoms by forming covalent bonds, and thereby greatly reduces the trap density while maintaining good charge-transport properties for the 3D perovskites. We demonstrate simultaneously efficient, bright and stable PeLEDs that have a maximum brightness of approximately 470,000 cd m-2, maximum external quantum efficiency of 28.9% (average = 25.2 ± 1.6% over 40 devices), maximum current efficiency of 151 cd A-1 and half-lifetime of 520 h at 1,000 cd m-2 (estimated half-lifetime >30,000 h at 100 cd m-2). Our work sheds light on the possibility that PeLEDs can be commercialized in the future display industry.

Self-organized quantum dots in marginally twisted MoSe2/WSe2 and MoS2/WS2 bilayers

Moiré superlattices in twistronic heterostructures are a powerful tool for materials engineering. In marginally twisted (small misalignment angle, θ) bilayers of nearly lattice-matched two-dimensional (2D) crystals moiré patterns take the form of domains of commensurate stacking, separated by a network of domain walls (NoDW) with strain hot spots at the NoDW nodes. Here, we show that, for type-II transition metal dichalcogenide bilayers MoX2/WX2 (X=S, Se), the hydrostatic strain component in these hot spots creates quantum dots for electrons and holes. We investigate the electron/hole states bound by such objects, discussing their manifestations via the intralayer intraband infrared transitions. The electron/hole confinement, which is strongest for θ < 0.5°, leads to a red-shift of their recombination line producing single-photon emitters (SPE) broadly tuneable around 1 eV by misalignment angle. These self-organized dots can form in bilayers with both aligned and inverted MoX2 and WX2 unit cells, emitting photons with different polarizations. We also find that the hot spots of strain reduce the intralayer MoX2 A-exciton energy, enabling selective population of the quantum dot states.