Two-dimensional Crystals Research Articles

A high-haze liquid crystal grating device with asymmetric anchoring energies

We present the development of a novel two-dimensional (2-D) liquid crystal (LC) grating device with asymmetric surface anchoring. By employing a waffle-shaped electrode on one substrate, a 2-D electric field is generated. The introduction of weak anchoring energy on one of the substrates enables easier rotation of LC molecules between the electrodes upon voltage application in an asymmetrically anchored device. As a result, the proposed grating device exhibits a significant spatial phase difference in any direction, leading to a high-haze value in the opaque state (94.36%). Furthermore, it offers notable advantages such as enhanced fabricability, simplified driving mechanism, and reduced operating voltage requirements.

Dual-band photoluminescence mechanism of magnetic doped two-dimensional (PEA)2PbBr4 single crystals

Mn2+ doping effectively realizes white light emission in three-dimensional lead halide perovskite nanocrystals. Meanwhile, research on Mn-doped two-dimensional layered perovskite single crystal is limited. We report centimeter-scale Mn-doped PEA2PbBr4 (C6H5CH2CH2NH3+ and PEA+) single crystals prepared by a slow evaporation method. Mn2+ dopants mainly act as substitutional doping and exhibit paramagnetic properties in the crystal at low doping density, while interstitial doping of Mn2+ prevails and induces antiferromagnetic characteristics at high doping density. Mn:PEA2PbBr4 single crystals exhibit dual-band chromacity-tunable blue-orange photoluminescence originating from excitons and Mn2+ emission. The negative temperature quenching effect is achieved by Mn-doping defects for the temperature-dependent exciton photoluminescence. Upon testing in the low-pressure vacuum chamber, the Mn2+ peak of the single crystal shows a dramatic shift from 610 to 690 nm. These results indicate that Mn:PEA2PbBr4 single crystal can serve as a potential and promising luminescent device material that achieves color tunable properties by regulating the systematic changes in the intensity ratio of exciton emission and Mn2+ emission, which will be very helpful for exploring the application of perovskite in magneto-optical devices in the future.

Two-Dimensional Oxide Crystals for Device Applications: Challenges and Opportunities.

Atomically thin two-dimensional (2D) oxide crystals have garnered considerable attention because of their remarkable physical properties and potential for versatile applications. In recent years, significant advancements have been made in the design, preparation, and application of ultrathin 2D oxides, providing many opportunities for new-generation advanced technologies. This review focuses on the controllable preparation of 2D oxide crystals and their applications in electronic and optoelectronic devices. Based on their bonding nature, the various types of 2D oxide crystals are first summarized, including both layered and nonlayered crystals, as well as their current top-down and bottom-up synthetic approaches. Subsequently, in terms of the unique physical and electrical properties of 2D oxides, recent advances in device applications are emphasized, including photodetectors, field-effect transistors, dielectric layers, magnetic and ferroelectric devices, memories, and gas sensors. Finally, conclusions and future prospects of 2D oxide crystals are presented. It is hoped that this review will provide comprehensive and insightful guidance for the development of 2D oxide crystals and their device applications.

Multimode optomechanics with a two-dimensional optomechanical crystal

Chip-scale multimode optomechanical systems have unique benefits for sensing, metrology, and quantum technologies relative to their single-mode counterparts. Slot-mode optomechanical crystals enable sideband resolution and large optomechanical couplings of a single optical cavity to two microwave-frequency mechanical modes. Still, previous implementations have been limited to nanobeam geometries, whose effective quantum cooperativity at ultralow temperatures is limited by their low thermal conductance. In this work, we design and experimentally demonstrate a two-dimensional mechanical–optical–mechanical (MOM) platform that dispersively couples a slow-light slot-guided photonic-crystal waveguide mode and two slow-sound ∼ 7 GHz phononic wire modes localized in physically distinct regions. We first demonstrate optomechanical interactions in long waveguide sections, unveiling acoustic group velocities below 800 m/s, and then move on to mode-gap adiabatic heterostructure cavities with a tailored mechanical frequency difference. Through optomechanical spectroscopy, we demonstrate optical quality factors Q ∼ 105, vacuum optomechanical coupling rates, go/2π, of 1.5 MHz, and dynamical back-action effects beyond the single-mode picture. At a larger power and adequate laser-cavity detuning, we demonstrate regenerative optomechanical oscillations involving a single mechanical mode, extending to both mechanical modes through modulation of the input laser drive at their frequency difference. This work constitutes an important advance toward engineering MOM systems with nearly degenerate mechanical modes as part of hybrid multipartite quantum systems.

Acoustic 4 × 2 encoder based on linear waveguides in two-dimensional solid-solid phononic crystals

The paper proposes a design for an acoustic 4 × 2 encoder that incorporates acoustic waveguides within a two-dimensional square lattice phononic crystal structure. The encoder's structure comprises of a nylon matrix embedded with a 33 × 30 molybdenum rod array, featuring four input ports and two output ports. The simulation results demonstrate that the proposed structure can effectively achieve the acoustic 4 × 2 encoder functionality. To evaluate the performance of the acoustic 4 × 2 encoder, we analyzed the transmission spectrum using the finite element method (FEM).

High-Throughput Search for Triplet Point Defects with Narrow Emission Lines in 2D Materials.

We employ a first-principles computational workflow to screen for optically accessible, high-spin point defects in wide band gap, two-dimensional (2D) crystals. Starting from an initial set of 5388 point defects, comprising both native and extrinsic, single and double defects in ten previously synthesized 2D host materials, we identify 596 defects with a triplet ground state. For these defects, we calculate the defect formation energy, hyperfine (HF) coupling, and zero-field splitting (ZFS) tensors. For 39 triplet transitions exhibiting particularly low Huang-Rhys factors, we calculate the full photoluminescence (PL) spectrum. Our approach reveals many spin defects with narrow PL line shapes and emission frequencies covering a broad spectral range. Most of the defects are hosted in hexagonal BN (hBN), which we ascribe to its high stiffness, but some are also found in MgI2, MoS2, MgBr2 and CaI2. As specific examples, we propose the defects vSMoS0 and NiSMoS0 in MoS2 as interesting candidates with potential applications to magnetic field sensors and quantum information technology.

Orientational ordering of polydisperse nanorods on a flat surface

The use of two-dimensional liquid crystals (2D LCs) made of carbon nanorods has become increasingly popular in electronic, optical, and energy applications due to their unique properties. However, the orientational ordering of such liquid crystals remains poorly understood. Here, we model of 2D LCs using the length polydisperse hard needle model and apply theOnsager theory to explore the impact of length polydispersity on nematic ordering. Our results show that length polydispersity plays a crucial rolein stabilizing the nematic phase over the isotropic phase, albeit at the expense of weaker orientational ordering. Longer rods, in particular, contribute to the stabilization of the nematic phase, whereas shorter rods result in weaker nematic ordering. To achieve a highly ordered 2D nematic phase, the polydispersity of the nanorods must be reduced.

Roses in the nonperturbative current response of artificial crystals.

In two-dimensional artificial crystals with large real-space periodicity, the nonlinear current response to a large applied electric field can feature a strong angular dependence, which encodes information about the band dispersion and Berry curvature of isolated electronic Bloch minibands. Within the relaxation-time approximation, we obtain analytic expressions up to infinite order in the driving field for the current in a band-projected theory with time-reversal and trigonal symmetry. For a fixed field strength, the dependence of the current on the direction of the applied field is given by rose curves whose petal structure is symmetry constrained and is obtained from an expansion in real-space translation vectors. We illustrate our theory with calculations on periodically buckled graphene and twisted double bilayer graphene, wherein the discussed physics can be accessed at experimentally relevant field strengths.

The influence of the demagnetizing field on the concentration of spin wave energy in two-dimensional magnonic crystals

We use the Plane Wave Method to theoretically study thin-film magnonic crystals (MCs) composed of two very common magnetic materials: cobalt and permalloy. In both cases, we consider Co inclusions in the Py matrix and Py inclusions in the Co matrix. An external magnetic field is applied in the plane of the structure, leading to the formation of a demagnetizing field at the interface between the inclusions and matrix. Previous studies have shown that this field strongly affects the spectrum of spin waves, including the position and width of bandgaps. In this study, we exploit the in-plane squeezing of the MC structure to enhance the demagnetizing field. This results in the transfer of low-frequency spin waves from Py to Co, affecting the energy distribution (i.e., the spin-wave profile). The change in the concentration of spin-wave profiles leads to certain peculiarities in the spin-wave frequency spectrum. These include modes repulsion caused by hybridization, which in turn leads to the reordering of modes in the spectrum.

Emergent Intrinsic Ferromagnetism in Two-Dimensional Trigonal Rhodium Oxide.

Two-dimensional (2D) van der Waals single crystals with long-range magnetic order are the precondition and urgent task for developing a 2D spintronics device. In contrast to graphene and transition metal dichalcogenides, the study of 2D single-crystal metal oxides with intrinsic ferromagnetic properties remains a huge challenge. Here, we report a large-size trigonal single-crystal rhodium oxide (SC-Tri-RhO2), with crystal parameters of a = b = 3.074 Å, c = 6.116 Å, and a space group of P3̅m1 (164), exhibiting strong ferromagnetism (FM) at a rather high temperature. Furthermore, theoretical calculations suggest that the ferromagnetism in SC-Tri-RhO2 originates from spin splitting near the Fermi level, and the total magnetic moment is contributed mainly by the Rh atom.

Stamped production of single-crystal hexagonal boron nitride monolayers on various insulating substrates

Controllable growth of two-dimensional (2D) single crystals on insulating substrates is the ultimate pursuit for realizing high-end applications in electronics and optoelectronics. However, for the most typical 2D insulator, hexagonal boron nitride (hBN), the production of a single-crystal monolayer on insulating substrates remains challenging. Here, we propose a methodology to realize the facile production of inch-sized single-crystal hBN monolayers on various insulating substrates by an atomic-scale stamp-like technique. The single-crystal Cu foils grown with hBN films can stick tightly (within 0.35 nm) to the insulating substrate at sub-melting temperature of Cu and extrude the hBN grown on the metallic surface onto the insulating substrate. Single-crystal hBN films can then be obtained by removing the Cu foil similar to the stamp process, regardless of the type or crystallinity of the insulating substrates. Our work will likely promote the manufacturing process of fully single-crystal 2D material-based devices and their applications.

Nonperturbative indirect exchange in spin valley coupled two-dimensional crystals

Nonperturbative indirect exchange in spin valley coupled two-dimensional crystals

Exciton-Sensitized Second-Harmonic Generation in 2D Heterostructures.

The efficient optical second-harmonic generation (SHG) of two-dimensional (2D) crystals, coupled with their atomic thickness, which circumvents the phase-match problem, has garnered considerable attention. While various 2D heterostructures have shown promising applications in photodetectors, switching electronics, and photovoltaics, the modulation of nonlinear optical properties in such heterosystems remains unexplored. In this study, we investigate exciton-sensitized SHG in heterobilayers of transition metal dichalcogenides (TMDs), where photoexcitation of one donor layer enhances the SHG response of the other as an acceptor. We utilize polarization-resolved interferometry to detect the SHG intensity and phase of each individual layer, revealing the energetic match between the excitonic resonances of donors and the SHG enhancement of acceptors for four TMD combinations. Our results also uncover the dynamic nature of interlayer coupling, as made evident by the dependence of sensitization on interlayer gap spacing and the average power of the fundamental beam. This work provides insights into how the interlayer coupling of two different layers can modify nonlinear optical phenomena in 2D heterostructures.

Growth and Local Structures of Single Crystalline Flakes of Three-Dimensional Covalent Organic Frameworks in Water.

Due to the enormous chemical and structural diversities and designable properties and functionalities, covalent organic frameworks (COFs) hold great promise as tailored materials for industrial applications in electronics, biology, and energy technologies. They were typically obtained as partially crystalline materials, although a few single-crystal three-dimensional (3D) COFs have been obtained recently with structures probed by diffraction techniques. However, it remains challenging to grow single-crystal COFs with controlled morphology and to elucidate the local structures of 3D COFs, imposing severe limitations on the applications and understanding of the local structure-property correlations. Herein, we develop a method for designed growth of five types of single crystalline flakes of 3D COFs with controlled morphology, front crystal facets, and defined edge structures as well as surface chemistry using surfactants that can be self-assembled into layered structures to confine crystal growth in water. The flakes enable direct observation of local structures including monomer units, pore structure, edge structure, grain boundary, and lattice distortion of 3D COFs as well as gradually curved surfaces in kinked but single crystalline 3D COFs with a resolution of up to ∼1.7 Å. In comparison with flakes of two-dimensional crystals, the synthesized flakes show much higher chemical, mechanical, and thermal stability.

Formation of binary magnon polaron in a two-dimensional artificial magneto-elastic crystal

We observed strong tripartite magnon-phonon-magnon coupling in a two-dimensional periodic array of magnetostrictive nanomagnets deposited on a piezoelectric substrate, forming a 2D magnetoelastic “crystal”; the coupling occurred between two Kittel-type spin wave (magnon) modes and a (non-Kittel) magnetoelastic spin wave mode caused by a surface acoustic wave (SAW) (phonons). The strongest coupling occurred when the frequencies and wavevectors of the three modes matched, leading to perfect phase matching. We achieved this condition by carefully engineering the frequency of the SAW, the nanomagnet dimensions and the bias magnetic field that determined the frequencies of the two Kittel-type modes. The strong coupling (cooperativity factor exceeding unity) led to the formation of a new quasi-particle, called a binary magnon-polaron, accompanied by nearly complete (~100%) transfer of energy from the magnetoelastic mode to the two Kittel-type modes. This coupling phenomenon exhibited significant anisotropy since the array did not have rotational symmetry in space. The experimental observations were in good agreement with the theoretical simulations.

Electron Transport Properties of Graphene/WS2 Van Der Waals Heterojunctions.

Van der Waals heterojunctions of two-dimensional atomic crystals are widely used to build functional devices due to their excellent optoelectronic properties, which are attracting more and more attention, and various methods have been developed to study their structure and properties. Here, density functional theory combined with the nonequilibrium Green's function technique has been used to calculate the transport properties of graphene/WS2 heterojunctions. It is observed that the formation of heterojunctions does not lead to the opening of the Dirac point of graphene. Instead, the respective band structures of both graphene and WS2 are preserved. Therefore, the heterojunction follows a unique Ohm's law at low bias voltages, despite the presence of a certain rotation angle between the two surfaces within the heterojunction. The transmission spectra, the density of states, and the transmission eigenstate are used to investigate the origin and mechanism of unique linear I-V characteristics. This study provides a theoretical framework for designing mixed-dimensional heterojunction nanoelectronic devices.

Peculiarities of Structuring of Ultrafine hBN Particles on the Surfaces of Polyamide Filaments

Purpose. Establishment of the mechanism of formation of nanofilms from ultrafine two-dimensional crystals of hexagonal boron nitride.Methods. Film structures from ultrafine two-dimensional crystals of hexagonal boron nitride were created both on the surface of a filament separated from a PA-6 polyamide yarn and on a silicon substrate. Ultra sonication was used to fix UC hBN from an aqueous colloidal system on surfaces. The characterization of UC hBN and films made from them was performed by the following methods: сщт scanning electron with energy dispersive elemental analysis, probe atomic force microscopy, vibrational Fourier IR spectroscopy (and Raman (Raman) scattering, as well as fluorescence spectroscopy, X-ray diffractometry and X-ray phase analysis, small-angle X-ray scattering.Results. The dependence of the intensity of the E2g line (I = 1362.8 cm–1) in the RS spectrum of a film structure deposited on the surface of aqueous CS UC hBN filaments on the time UST – tUST has been studied. Based on the results of the analysis of confocal, SEM and AFM images, RS and FS spectroscopy, the multilayer nature of the UC hBN film structures on the surface of the filaments and the silicon wafer was proved. The FS spectrum contains excitations on lines lying inside the band gap.Conclusion. The formation of structures on the surfaces of filaments and a silicon plate from an aqueous CS of UC hBN particles after UST occurs due to either covalent bonds in the plane of hexagons with abnormal sizes up to 1 μm, or van der Waals and ionic-covalent bonds with the formation of multilayer structures with heights from 3.6 to 340 nm.

Two-dimensional hybrid perovskite crystals for highly sensitive and stable UV light detector

Two-dimensional metal halide perovskites (2D MHPs) are promising candidates for ultraviolet (UV) light detection due to their narrow band gap and high stability in the air atmosphere. MHP crystals, with fewer defects and grain boundaries than polycrystalline films, are more suitable for photodetectors and other advanced semiconductor devices. In this paper, aimed to fabricate high-performance UV light detector, a series of high-quality and large-sized 2D perovskite crystals with the composition of (BA)2Pb(ClxBr1-x)4 were prepared and the best UV responsibility was found in the (BA)2Pb(Cl0.1Br0.9)4 crystal. Despite the detector was fabricated by the simple silver brushing method, it exhibits high responsivity of 3.19 mA/W, high detection rate of 3.87 × 1011 Jones, and fast response speed to UV light of 340 nm under the bias of 20 V. Moreover, the detector shows remarkable light sensitivity even under extremely weak UV illumination of 5.4 μW/cm2 and satisfactory stability during a long-duration switching operation. The excellent light response performance of the (BA)2Pb(Cl0.1Br0.9)4 detector was ascribed to the very low dark current (0.84 pA) of the detector, which highly relies on the crystal quality of the light-absorbing perovskites. The relationship between the structural and the compositional characteristics of (BA)2Pb(Cl0.1Br0.9)4 and its light response property was analyzed. This work provides a promising candidate material for sensitive UV detection and high-performance photodetectors.

Pushing the limits of complete omnidirectional bandgaps in 2D nonsymmorphic single-phase phononic crystals

We expand the limits of complete omnidirectional bandgaps (neither P- nor S-waves can propagate) for high symmetry two-dimensional phononic crystal (PnC) designs. We reveal an extremely large 124% complete omnidirectional bandgap and demonstrate the possibility of creating custom, mechanically robust PnCs with improved characteristics via simple geometric changes to known PnC designs. The findings are experimentally validated, proving that it is feasible to achieve extreme ultrasonic attenuation using the nonsymmorphic p4gm symmetry group design, for both P- and S-waves, which significantly outperforms symmorphic p4mm-group PnC designs. We shed light on the high attenuation properties of the p4gm PnC for S-waves (which is rarely explored experimentally), revealing differences between the mode types. Practical insight into the design of PnCs with improved acoustic properties for potential applications in the field of vibration isolation, most notably when S-wave elimination is vital, is discussed.

Non-covalent Supramolecular 1D Alternating Copolymer in Crystal toward 2D Anisotropic Photon Transport.

To realize organic integrated optoelectronic circuits, there is a need for anisotropic optical waveguides at the micro/nanoscale. Anisotropic alignment of one-dimensional (1D)-ordered supramolecular structures composed of light-emissive π-conjugated molecules in a crystal may meet the requirements of such waveguides. Here, we designed a bipyridyl-appended acrylonitrile-based π-conjugated molecule 1, which produced a 1D supramolecular polymer constructed through non-covalent bonding between a lone pair in 1 and a σ-hole in 1,4-diiodo-2,3,5,6-tetrafluorobenzene 2. The 1D copolymer of 1 and 2 is aligned horizontally with the two-dimensional (2D) crystal surface because of the angle-controlled supramolecular synthons. As a result of control over the non-covalent bonding direction, anisotropic photoluminescence and photon transport (optical waveguiding) characteristics are realized by orienting the transition dipole moment horizontally with respect to the 2D surface. Compared with the loss coefficient αL =52dBcm-1 for the long-axis direction of the 2D platelet cocrystal, a very large difference of αS =2111dBcm-1 is present in the crystal short-axis direction. The anisotropic waveguiding ability, αL/αS, is estimated to be 41, which is more than an order of magnitude greater than previously reported 2D platelet crystal waveguides. This supramolecular synthon provides an approach to designing anisotropic photon transporters and highly regulated optical logic circuits.