Atomic Layer Materials Research Articles

Growth of copper single crystal using cone die Czochralski method

Copper (Cu) and other metal single crystals are useful as substrates for the deposition of atomic layer materials for many electronic applications, but the growth of large-sized single crystals is difficult to achieve. Characteristics of the metal material, namely seed elongation and intense cooling radiation at high temperatures during crystal growth, are the main challenges encountered when growing ingots with large diameters. These problems can be resolved by optimizing the crystal growth parameters. By adjusting the shoulder formation angle of the ingot shape to approximately 20° to 40°, we are able to grow a large (1-inch diameter, 30 mm length) single crystal of metal Cu using the Czochralski (CZ) method. However, the generation of suspended solids and film impurities such as reactants and precipitates and their nucleation, growth, and solidification, limited the further increase in size of the Cu crystal. Using the cone-shape die CZ (CD-CZ) method solves this problem and a 2-inch diameter Cu single crystal is successfully grown. This is the world’s largest single crystal metal grown using this method and it paves the way for the growth of other metal crystals.

Advances in Engineering Toolkits for Construction of Ultralow Disordered Van der Waals Heterostructures

AbstractThe exploration of emerging quantum phenomena by stacking dissimilar atomic layered materials into van der Waals (vdW) heterostructures has driven the development of layer assembly techniques. Achieving ultralow disorder within these heterostructures is crucial for unlocking their novel physical properties. However, current fabrication methods for designer heterostructures have limitations in throughput, yield, and scalability. Over the past decade, engineering toolkits have evolved to address some of these challenges, but their adoption for fabricating designer heterostructures remains limited. In this review, an overview of these emerging engineering toolkits is provided, and examine their utility and limitations in achieving ultralow disordered heterostructures. It is hoped that the insights from this review article can help guide future research directions on advancing the fabrication process of designer heterostructures.

Thermoelectric effects in ballistic junctions of 2D-Weyl semimetals

Two dimensional Weyl semimetals are a variant of topological materials in two dimensions, which are realized in three atomic layer materials. These materials possess isotropic or anisotropic dispersions around Weyl nodes and present some common physical properties with their 3D counterparts. We study the thermoelectric effects in a ballistic junction of 2D-Weyl semimetal with isotropic or anisotropic dispersions around the Weyl points. We demonstrate that these two types of 2D-Weyl semimetals exhibit different thermoelectric responses at low chemical potentials of the leads. The Weyl semimetal layer’s physical characteristics can significantly affect these junctions’ thermoelectric properties. Moreover, significant Seebeck coefficient and thermoelectric figure of merit of these junctions are only observed at the low chemical potential of the leads. In particular, we unveil that thermoelectric effects in the ballistic junction of 2D-Weyl semimetals are not as efficient as their 3D counterparts.

A Self‐Powered Photodetector Based on Graphene Enhanced WSe2/PtSe2 Heterodiode with Fast Speed and Broadband Response

AbstractNarrow bandgap 2D layered material platinum selenide (PtSe2) with good environmental stability, high carrier mobility, and high light absorption, has been widely investigated for uncooled midwave‐infrared (MWIR) photodetection. However, the phototransistor based on the PtSe2 operation at room temperature suffered from the high dark current and background noise. Here, a graphene (G)‐enhanced G‐WSe2/PtSe2 hetero‐diode placed on a metal electrode is reported. To enhance the photogain, a graphene layer placed on the WSe2/PtSe2 heterodiode as a local gating layer is designed. The device exhibits an ultra‐high light on/off ratio of up to 108 and ultra‐fast photoresponse speed with raising time τr = 0.9 µs and decay time of τd = 1.5 µs in the visible spectra range. Notably, ultrabroad band photoresponse from 405 to 3366 nm is demonstrated under the self‐power model. Notably, the device presented a competitive photovoltaic effect with a high energy conversion efficiency (PCE) of 3.45%. The results pave the way toward a new approach to tuning the performance of the atomic thin layered materials photodetector.

Prominently enhanced luminescence from a continuous monolayer of transition metal dichalcogenide on all-dielectric metasurfaces.

2D materials such as transition metal dichalcogenides (TMDCs) are a new class of atomic-layer materials possessing optical and electric properties that significantly depend on the number of layers. Electronic transitions can be manipulated in artificial resonant electromagnetic (EM) fields using metasurfaces and other designed nanostructures. Here, we demonstrate prominently resonant enhancement in the photoluminescence (PL) of atomic monolayer, WS2, doped with a small quantity of Mo. The excitonic PL showed a strong enhancement effect on a higher-order magnetic resonance of all-dielectric metasurfaces consisting of periodic arrays of Si nanopellets. The PL intensity witnessed a 300-fold enhancement compared to the reference PL intensity on a flat silicon dioxide (SiO2) layer, which suggests a drastic change in the dynamics of photoexcited states. Confocal PL microscopy and the analysis revealed that the single photons were coherently emitted from the TMDC monolayer on the metasurface. Furthermore, examining the PL lifetime in the ps and ns timescales clarified two exponential components at the prominent exciton PL: a short-time component decaying in 22 ps and a long-time component lasting over 10 ns. Therefore, we can infer that the radiative components were significantly activated in the TMDC monolayer on the metasurfaces in comparison to the reference monolayer on a flat SiO2 layer.

Raman scattering monitoring of thin film materials for atomic layer etching/deposition in the nano-semiconductor process integration

According to Moore's law, the semiconductor industry is experiencing certain challenges in terms of adapting to highly sophisticated integrated technology. Therefore, controlling materials at the atomic scale is considered a mandatory requirement for further development. To this end, atomic layer deposition and etching skills are being increasingly researched as potential solutions. However, several considerations exist for adopting atomic technology with respect to surface analysis. This review primarily focuses on the use of Raman scattering for evaluating atomic-layered materials. Raman scattering analysis is expected to gradually expand as a semiconductor process and mass-production monitoring technology. As this can enhance the applications of this method, our review can form the basis for establishing Raman scattering analysis as a new trend for atomic-scale monitoring.

Studying the Adsorption of Gas Molecules and Defects on Modulating the Electronic Transport Characteristics of Monolayer Penta-BN2-Based Devices.

Two-dimensional atomic layer materials, as an important part of the post-Moore era, have recently become an ideal choice for the preparation of high-efficiency, low-power, and miniaturized gas sensors. In this work, our study utilized density functional theory and the nonequilibrium Green's function method to investigate the electronic properties of the pentagonal BN2 (P-BN2) monolayer, as well as its gas-sensing properties for organic and inorganic gases. We also investigated how defects affect the quantum transport properties of the P-BN2-based device. Our findings demonstrate that the CO, H2S, NH3, SO2, C2H5OH, C3H6OH, CH3OH, and CH4 undergo physisorption on the P-BN2 monolayer, while NO, NO2, C2H2, C2H4, and HCHO undergo chemisorption. Then, we analyzed the impact of gas molecules chemisorbed on the P-BN2 monolayer on the electronic transport properties of the P-BN2-based gas sensor. When these five gas molecules are adsorbed, the current of the P-BN2-based gas sensor is greatly reduced. In addition, the effect of defects on the quantum transport properties of the P-BN2-based device is investigated. The results indicate that defects of N, B, and BN atoms lead to a decrease in the current of P-BN2-based nanodevices. Moreover, both the adsorption of gas molecules and the formation of vacancy defects leading to a decrease in device current can be revealed by the local device density of states near the zero-bias Fermi level, elucidating their microscopic mechanisms. Finally, gas molecules can also cause a decrease in the current of defect systems. These theoretical studies are of great significance for exploring two-dimensional atomic layer materials as high-efficiency gas sensors.

Electrochemical production of two-dimensional atomic layer materials and their application for energy storage devices

Two-dimensional (2D) atomic layer materials have attracted a great deal of attention due to their superior chemical, physical, and electronic properties, and have demonstrated excellent performance in various applications such as energy storage devices, catalysts, sensors, and transistors. Nevertheless, the cost-effective and large-scale production of high-quality 2D materials is critical for practical applications and progressive development in the industry. Electrochemical exfoliation is a recently introduced technique for the facile, environmentally friendly, fast, large-scale production of 2D materials. In this review, we summarize recent advances in different types of electrochemical exfoliation methods for efficiently preparing 2D materials, along with the characteristics of each method, and then introduce their applications as electrode materials for energy storage devices. Finally, the remaining challenges and prospects for developing the electrochemical exfoliation process of 2D materials for energy storage devices are discussed.

Multi-beam ultrafast laser processing of free-standing nanofilms

In this study, femtosecond laser-based multi-beam interference laser processing on nanofilms with nanometer thicknesses was demonstrated. The resulting multi-hole, two-dimensional lattice pattern reflected a laser interference fringe formed on the surface of the nanofilm, with no breaks or cracks. In anticipation of the actual nanostructure fabrication, additional laser processing was performed to drill additional holes in the spaces between the existing holes, resulting in high-density multi-point hole drilling beyond the interference fringe pitch. Notably, processing materials with thicknesses close to 100 nm or less is difficult even with a state-of-the-art focused-ion-beam system. The presented method, in contrast, allows instantaneous, submicrometer-scale multi-point hole drilling of nanofilms over a large area, opening up a new frontier of nanoengineering. Future applications will include the fabrication of electron phase plates, membrane-based optomechanical devices, microelectromechanical systems, and engineering of atomic layer materials.

Electronic, Thermodynamic Stability, and Band Alignment Behavior of the CoVSi/NaCl Heterojunction

We report the band discontinuity of the CoVSi/NaCl heterointerface. First principle calculations based on density functional theory using GGA, GGA + U, and GGA + mbJ approximations were applied to study the structural, electronic, and band alignment properties. Structural and thermodynamic stability studies indicate that this semiconductor - dielectric heterojunction can be synthesized experimentally in thermodynamic equilibrium conditions. The valence and conduction band offset values (VBO and CBO) were 0.74 and 3.02 eV, respectively. Also, the effective electron affinity parameter (χ e) for both CoVSi and NaCl were calculated as ∼1.51 and ∼0.769 eV, respectively, using Anderson’s law. The study of the electronic structure expresses the occurrence of half-metallic ferromagnetic behavior with a narrow band gap of about 0.09 eV. In this heterojunction, electrons and holes were confined to the CoVSi layers, and conduction band minimum and valence band minimum were replaced in the CoVSi layers. This restriction, applied to load carriers on one side of the interface, significantly increases the light-material interaction in light-emission programs. Therefore, this heterojunction can be recommended for light-emitting applications and thin atomic layer materials with quantum confinement of charge carriers.

Substrate-Mediated Borophane Polymorphs through Hydrogenation of Two-Dimensional Boron Sheets.

The two-dimensional boron monolayer (borophene) stands out from the two-dimensional atomic layered materials due to its structural flexibility and tunable electronic and mechanical properties from a large number of allotropic materials. The stability of pristine borophene polymorphs could possibly be improved via hydrogenation with atomic hydrogen (referred to as borophane). However, the precise adsorption structures and the underlying mechanism are still elusive. Employing first-principles calculations, we demonstrate the optimal configurations of freestanding borophanes and the ones grown on metallic substrates. For freestanding borophenes, the energetically favored hydrogen adsorption sites are sensitive to the polymorphs and corresponding coordination numbers of boron atoms. With various metal substrates, the hydrogenation configurations of borophenes are modulated significantly, attributed to the overlap between B pz and H s orbitals. These findings provide a deep insight into the hydrogenating borophenes and facilitate the stabilization of two-dimensional boron polymorphs by engineering hydrogen adsorption sites and concentrations.

Graphene, phosphorene and silicene coatings on the (0001) surfaces of hcp metals: Structural stability and hydrophobicity

Single atomic layer or monolayer (ML) 2D materials have unique features that can be relevant for protection and functionalization of high-performance surfaces. Here we report a combined density functional theory and ab initio molecular dynamics study on the stability of graphene (Gr), phosphorene (Pp) and silicene (Sl) coatings at the most stable (0001) surface of the metals that have hcp structure at room temperature: Hf, Mg, Os, Re, Ru, Sc, Ti, Zn and Zr. We found that the three 2D materials are stable at the surfaces of many of these metals: Gr is stable at Os, Re, Ti, Zn and Zr; Pp is stable in the blue form at Hf, Mg, Ti and Zr; Sl is stable in the 2D zigzag structure at Hf, Mg and Ti and in the planar form at the Zr surface. For the remaining metals Gr, Pp and Sl do not form van der Waals bonded MLs. In those cases we found structures such as 2D arrangements of mixed hexagons and pentagons, trigonal pyramidal structures and 2 half-monolayers. Many of these structures are very stable coatings covalently bound to the surfaces with considerable adsorption energies that can reach − 2 eV per atom of the coating. Gr imparts hydrophobicity to the surfaces of all materials here studied and increases the distance between these and the first H2O layer by as much as 75 % for Gr coated Ti. These H2O layers have increased rigidity despite being located further apart from the surfaces. These effects could be explored for the protection or functionalization of high-performance systems such as metallic electrodes for example.

Graphene Oxide‐Based Adsorbents for Organic‐Dyes Removal from Contaminated Water: A Review

AbstractPolluted organic dyes in water are considered as some of the most serious problems affecting humans. Adsorption techniques using traditional adsorbents such as clay, zeolite, silica, active carbon, and polymers are low cost, safe, and efficient for the treatment of organic dyes in contaminated water. However, traditional adsorbents usually have low adsorption capacity (qmax). To improve qmax, graphene oxide (GO) is considered as an excellent adsorption enhancer owing to its two‐dimensional single atomic layer material of sp2‐hybridised carbon atoms arranged in a honeycomb lattice structure with high surface area, high solubility, good chemical stability, strong mechanical strength, and excellent thermal stability. In this review, the synthesis and advantage results on improving the adsorption efficiency and qmax when applying GO‐based nanocomposites as nanosorbents for organic‐dye removal are systematised and discussed in detail.

Intrinsic spin-valley locking for conducting electrons in metal-semiconductor-metal lateral heterostructures of 1H -transition-metal dichalcogenides

Lateral heterostructures of atomic layered materials alter the electronic properties of pristine crystals and provide a possibility to produce useful monolayer materials. We reveal that metal-semiconductor-metal lateral heterojunctions of $1H$-transition-metal dichalcogenides intrinsically possess conducting channels of electrons with spin-valley locking without any extrinsic fabrication breaking the crystal symmetry, e.g., gate electrodes. We theoretically investigate the electronic structure and transport properties of lateral heterojunctions and show that the heterostructure produces conducting channels through the $K$ and ${K}^{\ensuremath{'}}$ valleys in the semiconducting transition-metal dichalcogenides and restricts the spin of the conducting electrons in each valley due to the valley-dependent charge transfer effect. Moreover, the theoretical investigation shows that the heterojunction of ${\mathrm{WSe}}_{2}$ realizes a high transmission probability for valley-spin locked electrons even in a long semiconducting region. The heterojunction also provides a useful electronic transport property, a steplike I-V characteristic.

Visualizing Line Defects in non-van der Waals Bi2O2Se Using Raman Spectroscopy.

Atomic-layered materials, such as high-quality bismuth oxychalcogenides, which are composed of oppositely charged alternate layers grown using chemical vapor deposition, have attracted considerable attention. Their physical properties are well-suited for high-speed, low-power-consumption optoelectronic devices, and the rapid determination of their crystallographic characteristics is crucial for scalability and integration. In this study, we introduce how the crystallographic structure and quality of such materials can be projected through Raman spectroscopy analysis. Frequency modes at ∼55, ∼78, ∼360, and ∼434 cm-1 were detected, bearing out theoretical calculations from the literature. The low-frequency modes positioned at 55 and 78 cm-1 were activated by structural defects, such as grain boundaries and O-rich edges in the Bi2O2Se crystals, accompanied by sensitivity to the excitation energy. Furthermore, the line defects at ∼55 cm-1 exhibited a strong 2-fold polarization dependence, similar to graphene/graphite edges. Our results can help illuminate the mechanism for activating the Raman-active mode from the infrared active mode by defects, as well as the electronic structures of these two-dimensional layered materials. We also suggest that the nanoscale width line defects in Bi2O2Se can be visualized using Raman spectroscopy.

Magnetism, symmetry and spin transport in van der Waals layered systems

The discovery of an ever-increasing family of atomic layered magnetic materials, together with the already established vast catalogue of strong spin–orbit coupling and topological systems, calls for some guiding principles to tailor and optimize novel spin transport and optical properties at their interfaces. Here, we focus on the latest developments in both fields that have brought them closer together and make them ripe for future fruitful synergy. After outlining fundamentals on van der Waals magnetism and spin–orbit coupling effects, we discuss how their coexistence, manipulation and competition could ultimately establish new ways to engineer robust spin textures and drive the generation and dynamics of spin current and magnetization switching in 2D-materials-based van der Waals heterostructures. Grounding our analysis on existing experimental results and theoretical considerations, we draw a prospective analysis about how intertwined magnetism and spin–orbit torque phenomena combine at interfaces with well-defined symmetries and how this dictates the nature and figures of merit of spin–orbit torque and angular momentum transfer. This will serve as a guiding role in designing future non-volatile memory devices that utilize the unique properties of 2D materials with the spin degree of freedom.

Integration of topological insulator nanogap with atomic single layer for boosting photoluminescence

As a new class of nanomaterials, topological insulators with the Dirac surface conducting state and insulating bulk state offer a promising platform for plasmonics with breaking through the frequency and integration limitation of traditional materials. The combination of topological insulators with atomic-layer materials will be significant for the realization of integrated plasmonic functional devices. Herein, we experimentally report the plasmon-enhanced photoluminescence (PL) emission behavior of molybdenum disulfide (MoS2) single layer integrated with an antimony telluride (Sb2Te3) topological insulator nanogap. It is found that the PL response of MoS2 on the nanogap is dependent on the polarization of incident light. The MoS2 PL intensity can be distinctly improved with plasmonic excitation on Sb2Te3 nanogap. These results will contribute to exploring plasmonic behaviors of topological insulators and their applications in enhanced light-matter interaction for nanoscale light emission and harvesting devices.

Observation of magnetoresistance in CrI3/graphene van derWaals heterostructures* *Project supported by the National Natural Science Foundation of China (Grant No. 51872039) and Science and Technology Program of Sichuan, China (Grant No. M112018JY0025).

Two-dimensional ferromagnetic van der Waals (2D vdW) heterostructures have opened new avenues for creating artificial materials with unprecedented electrical and optical functions beyond the reach of isolated 2D atomic layered materials, and for manipulating spin degree of freedom at the limit of few atomic layers, which empower next-generation spintronic and memory devices. However, to date, the electronic properties of 2D ferromagnetic heterostructures still remain elusive. Here, we report an unambiguous magnetoresistance behavior in CrI3/graphene heterostructures, with a maximum magnetoresistance ratio of 2.8%. The magnetoresistance increases with increasing magnetic field, which leads to decreasing carrier densities through Lorentz force, and decreases with the increase of the bias voltage. This work highlights the feasibilities of applying two-dimensional ferromagnetic vdW heterostructures in spintronic and memory devices.

Analysis of optical transition rules in buckled and puckered bismuthene monolayers based on electron-photon dipole vectors

When a material absorbs incident light, electrons can undergo an optical transition between valence and conduction bands depending on the frequency and polarization of light. In atomic-layered materials such as bismuthene, even a small change in its geometrical structure, e.g. from the buckled to puckered lattices, completely alters its electronic properties and, accordingly, its possible optical transitions. Since the strength and selection rule of optical transitions can be understood by analyzing the electron-photon dipole vectors, we calculate dipole vectors as a function of electronic wave vector between two quantum states in buckled and puckered bismuthene monolayers. We find that each of the bismuthene structures has its own, unique dipole vectors, implying a certain geometrical dependence of the dipole vectors. These materials are expected as a potential candidate for optoelectronics applications. Furthermore, one would consider applying various light polarization in these materials when the optical transition rules are well understood.

Epitaxial Growth of Two-Dimensional Insulator Monolayer Honeycomb BeO.

The emergence of two-dimensional (2D) materials launched a fascinating frontier of flatland electronics. Most crystalline atomic layer materials are based on layered van der Waals materials with weak interlayer bonding, which naturally leads to thermodynamically stable monolayers. We report the synthesis of a 2D insulator composed of a single atomic sheet of honeycomb structure BeO (h-BeO), although its bulk counterpart has a wurtzite structure. The h-BeO is grown by molecular beam epitaxy (MBE) on Ag(111) thin films that are also epitaxially grown on Si(111) wafers. Using scanning tunneling microscopy and spectroscopy (STM/S), the honeycomb BeO lattice constant is determined to be 2.65 Å with an insulating band gap of 6 eV. Our low-energy electron diffraction measurements indicate that the h-BeO forms a continuous layer with good crystallinity at the millimeter scale. Moiré pattern analysis shows the BeO honeycomb structure maintains long-range phase coherence in atomic registry even across Ag steps. We find that the interaction between the h-BeO layer and the Ag(111) substrate is weak by using STS and complementary density functional theory calculations. We not only demonstrate the feasibility of growing h-BeO monolayers by MBE, but also illustrate that the large-scale growth, weak substrate interactions, and long-range crystallinity make h-BeO an attractive candidate for future technological applications. More significantly, the ability to create a stable single-crystalline atomic sheet without a bulk layered counterpart is an intriguing approach to tailoring 2D electronic materials.