3D Insulator Research Articles

Cavity Born-Oppenheimer approximation for molecules and materials via electric field response.

We present an abinitio method for computing vibro-polariton and phonon-polariton spectra of molecules and solids coupled to the photon modes of optical cavities. We demonstrate that if interactions of cavity photon modes with both nuclear and electronic degrees of freedom are treated on the level of the cavity Born-Oppenheimer approximation, spectra can be expressed in terms of the matter response to electric fields and nuclear displacements, which are readily available in standard density functional perturbation theory implementations. In this framework, results over a range of cavity parameters can be obtained without the need for additional electronic structure calculations, enabling efficient calculations on a wide range of parameters. Furthermore, this approach enables results to be more readily interpreted in terms of the more familiar cavity-independent molecular electric field response properties, such as polarizability and Born effective charges, which enter into the vibro-polariton calculation. Using corresponding electric field response properties of bulk insulating systems, we are also able to obtain the Γ point phonon-polariton spectra of two dimensional (2D) insulators. Results for a selection of cavity-coupled molecular and 2D crystal systems are presented to demonstrate the method.

Understanding vapor phase growth of hexagonal boron nitride.

Hexagonal boron nitride (hBN), with its atomically flat structure, excellent chemical stability, and large band gap energy (∼6 eV), serves as an exemplary 2D insulator in electronics. Additionally, it offers exceptional attributes for the growth and encapsulation of semiconductor transition metal dichalcogenides (TMDCs). Current methodologies for producing hBN thin films primarily involve exfoliating multi-layer or bulk crystals and thin film growth via chemical vapor deposition (CVD), which entails the thermal decomposition and surface reaction of molecular precursors like ammonia boranes (NH3BH3) and borazine (B3N3H6). These molecular precursors contain pre-existing B-N bonds, thus promoting the nucleation of BN. However, the quality and phase purity of resulting BN films are greatly influenced by the film preparation and deposition process conditions that remain a substantial concern. This study aims to comprehensively investigate the impact of varied CVD systems, parameters, and precursor chemistry on the synthesis of high-quality, large scale hBN on both catalytic and non-catalytic substrates. The comparative analysis provided new insights into most effective approaches concerning both quality and scalability of vapor phase grown hBN films.

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.

Local gate control of Mott metal-insulator transition in a 2D metal-organic framework

Electron-electron interactions in materials lead to exotic many-body quantum phenomena, including Mott metal-insulator transitions (MITs), magnetism, quantum spin liquids, and superconductivity. These phases depend on electronic band occupation and can be controlled via the chemical potential. Flat bands in two-dimensional (2D) and layered materials with a kagome lattice enhance electronic correlations. Although theoretically predicted, correlated-electron Mott insulating phases in monolayer 2D metal-organic frameworks (MOFs) with a kagome structure have not yet been realised experimentally. Here, we synthesise a 2D kagome MOF on a 2D insulator. Scanning tunnelling microscopy (STM) and spectroscopy reveal a MOF electronic energy gap of ∼200 meV, consistent with dynamical mean-field theory predictions of a Mott insulator. Combining template-induced (via work function variations of the substrate) and STM probe-induced gating, we locally tune the electron population of the MOF kagome bands and induce Mott MITs. These findings enable technologies based on electrostatic control of many-body quantum phases in 2D MOFs.

Controllable Synthesis of Transferable Ultrathin Bi2Ge(Si)O5 Dielectric Alloys with Composition-Tunable High-κ Properties.

Two-dimensional (2D) alloys hold great promise to serve as important components of 2D transistors, since their properties allow continuous regulation by varying their compositions. However, previous studies are mainly limited to the metallic/semiconducting ones as contact/channel materials, but very few are related to the insulating dielectrics. Here, we use a facile one-step chemical vapor deposition (CVD) method to synthesize ultrathin Bi2SixGe1-xO5 dielectric alloys, whose composition is tunable over the full range of x just by changing the relative ratios of the GeO2/SiO2 precursors. Moreover, their dielectric properties are highly composition-tunable, showing a record-high dielectric constant of >40 among CVD-grown 2D insulators. The vertically grown nature of Bi2GeO5 and Bi2SixGe1-xO5 enables polymer-free transfer and subsequent clean van der Waals integration as the high-κ encapsulation layer to enhance the mobility of 2D semiconductors. Besides, the MoS2 transistors using Bi2SixGe1-xO5 alloy as gate dielectrics exhibit a large Ion/Ioff (>108), ideal subthreshold swing of ∼61 mV/decade, and a small gate hysteresis (∼5 mV). Our work not only gives very few examples on controlled CVD growth of insulating dielectric alloys but also expands the family of 2D single-crystalline high-κ dielectrics.

Recent Experimental Breakthroughs on 2D Transistors: Approaching the Theoretical Limit

AbstractSince Si‐based Moore's law is physically limited, 2D semiconductors are proposed as successors to continue shrinking the transistor size for more Moore electronics. However, limited by experimental technology bottlenecks, the theoretical predicted superiorities of the 2D transistors over the state‐of‐the‐art Si transistors have been lacking concrete evidence for a decade. In this review, recent exciting experimental breakthroughs for 2D transistors are presented, including gate length miniaturization to a sub‐1 nm limit, electrode contact optimization to the resistance quantum limit, high‐quality dielectric fabrication with an equivalent oxide thickness to sub‐0.5 nm, novel architecture form (2D fin field‐effect transistor), and back‐end‐of‐line integration of directly grown 2D materials on Si complementary metal‐oxide‐semiconductor circuits. Remarkably, an ultrashort channel, Ohmic contact, ballistic transport, and ultrathin dielectric layer are simultaneously satisfied in the 2D InSe transistor, and device performances approaching the theoretical limit are observed. The measured key figures of merit of the ideal 2D InSe transistor are comparable to or even surpass those of the Si transistors. Finally, the challenges and outlook on more Moore electronics based on 2D transistors are highlighted.

Design and Simulation of Tunneling Diodes with 2D Insulators for Rectenna Switches.

Rectenna is the key component in radio-frequency circuits for receiving and converting electromagnetic waves into direct current. However, it is very challenging for the conventional semiconductor diode switches to rectify high-frequency signals for 6G telecommunication (>100 GHz), medical detection (>THz), and rectenna solar cells (optical frequencies). Such a major challenge can be resolved by replacing the conventional semiconductor diodes with tunneling diodes as the rectenna switches. In this work, metal-insulator-metal (MIM) tunneling diodes based on 2D insulating materials were designed, and their performance was evaluated using a comprehensive simulation approach which includes a density-function theory simulation of 2D insulator materials, the modeling of the electrical characteristics of tunneling diodes, and circuit simulation for rectifiers. It is found that novel 2D insulators such as monolayer TiO2 can be obtained by oxidizing sulfur-metal layered materials. The MIM diodes based on such insulators exhibit fast tunneling and excellent current rectifying properties. Such tunneling diodes effectively convert the received high-frequency electromagnetic waves into direct current.

Experimental study and modeling of the wettability of fluorine graphene. Corrosion behavior of the fluorine graphene layer on a copper substrate

Fluorographene (FG) is a high-quality 2D insulator, which is effectively used as the atomically thin tunnel barrier. To date, the most common idea is that the complete fluorination of graphene (G) leads to an increase in hydrophobicity. We have shown that in reality fluorination leads to superhydrophilicity and to a significant increase in the defectiveness of the graphene layer. Without hydrocarbons, as well as due to the high defectiveness and superhydrophilicity, the corrosion current of FG is an order of magnitude higher than for pure copper and two orders of magnitude higher than for G synthesized on copper. The simulation results confirm experimental data on FG superhydrophilicity after prolonged fluorination. Thus, in order to achieve high hydrophilicity, it is not necessary to create hydroxyl groups on the surface of FG. A higher concentration of fluorine (the C/F composition ratio) is realized on a thicker copper plate with a graphene layer, compared to thin copper foil. Different fluorine concentrations are associated with different sizes of crystal grains, their orientation, as well as defectiveness and the number of graphene layers. The corrosive effect leads to a decrease in roughness and a multiple decrease in the amount of F on the FG.

Designing van der Waals magnetic tunnel junctions with high tunnel magnetoresistance via Brillouin zone filtering.

Magnetic tunnel junctions (MTJs) consisting of two-dimensional (2D) van der Waals heterostructures have no inter-layer chemical bonds; therefore, their spin tunneling is determined solely by the Brillouin zone (BZ) filtering effect. To obtain high tunnel magnetoresistance (TMR), they should possess transversal momentum-resolved conduction channels for the electrodes and transmission channels for the barriers. Here, we investigate 2D magnets as electrodes whose Curie temperatures approach room temperature and also hexagonal 2D insulators as the barrier. Iron-based compounds such as FexGeTe2 (x = 3 and 4) are calculated to have high transmission coefficients over the entire in-plane BZ for the majority spin channel, while this should only happen around Γ for the minority spin channel. Correspondingly, various 2H-type transition metal dichalcogenides (TMDs) are found to function effectively as spin barriers, where electrons are only allowed to tunnel through them around the K and M points. BZ spin filtering is confirmed to be the major mechanism of the TMR effect by the MTJ transport calculation using the non-equilibrium Green function method. Furthermore, the TMR is calculated to be nearly independent of the barrier layer thickness as the BZ filtering is an interfacial effect. This work sheds light on material selection procedures and designing ultra-thin and robust van der Waals MTJs.

The influence of various factors on the transition of the fluorinated graphene wettability from superhydrophilicity to high hydrophobicity

The wettability of fluorographene has been experimentally and theoretically investigated. The obtained results of modeling the contact angle using molecular dynamics are consistent with the experiment. Fluorographene is a high-quality insulator. The 2D insulator may be used as the atomically thin tunnel barrier. The interaction of fluorographene with water results in the formation of hydroxyl groups. Properties of hydroxofluorographene G(OH)F or Cn(OH)yFx imply temperature antiferromagnetic ordering. Previously it was assumed that complete fluorination leads to high hydrophobicity. The authors of this paper have shown that prolonged fluorination, on the contrary, induces superhydrophilicity. The adsorption of hydrocarbons from the ambient air onto the fluorographene surface results in high hydrophobicity. The combined effect of adsorbed hydrocarbons and nano-micro texturing of the surface due to fluorination allows for the transition to superhydrophobicity. After long-term storage of fluorographene in the air for 6 months, the fluorine content (in contrast to other works) has decreased slightly (by less than 10 %). Fluorine is present at a temperature of 250–300 °C in a hydrogen atmosphere. Stable C1F8.5 complexes have been obtained for the first time as a result of the self-organization of dome-shaped surface structures during prolonged fluorination. It is assumed that part of the fluorine (C1F1) has a covalent or ionic bond, and the rest of the fluorine molecules are arranged in a closed framework surrounded by C atoms. Usually, complete fluorination of graphene leads to a ratio of C1Fn less than 1 (maximum value n = 1–1.1). Dome-shaped structures with an external carbon backbone may be used for gas storage. It is possible to control the properties of fluorographene and ensure the transition from superhydrophilicity to superhydrophobicity.

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.

Probing Chern number by opacity and topological phase transition by a nonlocal Chern marker

In 2D semiconductors and insulators, the Chern number of the valence band Bloch state is an important quantity that has been linked to various material properties, such as the topological order. We elaborate that the opacity of 2D materials to circularly polarized light over a wide range of frequencies, measured in units of the fine structure constant, can be used to extract a spectral function that frequency-integrates to the Chern number, offering a simple optical experiment to measure it. This method is subsequently generalized to finite temperature and locally on every lattice site by a linear response theory, which helps to extract the Chern marker that maps the Chern number to lattice sites. The long range response in our theory corresponds to a Chern correlator that acts like the internal fluctuation of the Chern marker, and is found to be enhanced in the topologically nontrivial phase. Finally, from the Fourier transform of the valence band Berry curvature, a nonlocal Chern marker is further introduced, whose decay length diverges at topological phase transitions and therefore serves as a faithful indicator of the transitions, and moreover can be interpreted as a Wannier state correlation function. The concepts discussed in this work explore multi-faceted aspects of topology and should help address the impact of system inhomogeneities.

Coherent-Error Threshold for Surface Codes from Majorana Delocalization.

Statistical mechanics mappings provide key insights on quantum error correction. However, existing mappings assume incoherent noise, thus ignoring coherent errors due to, e.g., spurious gate rotations. We map the surface code with coherent errors, taken as X or Z rotations (replacing bit or phase flips), to a two-dimensional (2D) Ising model with complex couplings, and further to a 2D Majorana scattering network. Our mappings reveal both commonalities and qualitative differences in correcting coherent and incoherent errors. For both, the error-correcting phase maps, as we explicitly show by linking 2D networks to 1D fermions, to a Z_{2}-nontrivial 2D insulator. However, beyond a rotation angle ϕ_{th}, instead of a Z_{2}-trivial insulator as for incoherent errors, coherent errors map to a Majorana metal. This ϕ_{th} is the theoretically achievable storage threshold. We numerically find ϕ_{th}≈0.14π. The corresponding bit-flip rate sin^{2}(ϕ_{th})≈0.18 exceeds the known incoherent threshold p_{th}≈0.11.

Polaritonic Probe of an Emergent 2D Dipole Interface.

The use of work-function-mediated charge transfer has recently emerged as a reliable route toward nanoscale electrostatic control of individual atomic layers. Using α-RuCl3 as a 2D electron acceptor, we are able to induce emergent nano-optical behavior in hexagonal boron nitride (hBN) that arises due to interlayer charge polarization. Using scattering-type scanning near-field optical microscopy (s-SNOM), we find that a thin layer of α-RuCl3 adjacent to an hBN slab reduces the propagation length of hBN phonon polaritons (PhPs) in significant excess of what can be attributed to intrinsic optical losses. Concomitant nano-optical spectroscopy experiments reveal a novel resonance that aligns energetically with the region of excess PhP losses. These experimental observations are elucidated by first-principles density-functional theory and near-field model calculations, which show that the formation of a large interfacial dipole suppresses out-of-plane PhP propagation. Our results demonstrate the potential utility of charge-transfer heterostructures for tailoring optoelectronic properties of 2D insulators.

Vertically grown ultrathin Bi2SiO5 as high-κ single-crystalline gate dielectric

Single-crystalline high-κ dielectric materials are desired for the development of future two-dimensional (2D) electronic devices. However, curent 2D gate insulators still face challenges, such as insufficient dielectric constant and difficult to obtain free-standing and transferrable ultrathin films. Here, we demonstrate that ultrathin Bi2SiO5 crystals grown by chemical vapor deposition (CVD) can serve as excellent gate dielectric layers for 2D semiconductors, showing a high dielectric constant (>30) and large band gap (~3.8 eV). Unlike other 2D insulators synthesized via in-plane CVD on substrates, vertically grown Bi2SiO5 can be easily transferred onto other substrates by polymer-free mechanical pressing, which greatly facilitates its ideal van der Waals integration with few-layer MoS2 as high-κ dielectrics and screening layers. The Bi2SiO5 gated MoS2 field-effect transistors exhibit an ignorable hysteresis (~3 mV) and low drain induced barrier lowering (~5 mV/V). Our work suggests vertically grown Bi2SiO5 nanoflakes as promising candidates to improve the performance of 2D electronic devices.

Organic Molecules as Origin of Visible-Range Single Photon Emission from Hexagonal Boron Nitride and Mica.

The discovery of room-temperature single-photon emitters (SPEs) hosted by two-dimensional hexagonal boron nitride (2D hBN) has sparked intense research interest. Although emitters in the vicinity of 2 eV have been studied extensively, their microscopic identity has remained elusive. The discussion of this class of SPEs has centered on point defects in the hBN crystal lattice, but none of the candidate defect structures have been able to capture the great heterogeneity in emitter properties that is observed experimentally. Employing a widely used sample preparation protocol but disentangling several confounding factors, we demonstrate conclusively that heterogeneous single-photon emission at ∼2 eV associated with hBN originates from organic molecules, presumably aromatic fluorophores. The appearance of those SPEs depends critically on the presence of organic processing residues during sample preparation, and emitters formed during heat treatment are not located within the hBN crystal as previously thought, but at the hBN/substrate interface. We further demonstrate that the same class of SPEs can be observed in a different 2D insulator, fluorophlogopite mica.

Integration of Self-Passivated Topological Electrodes for Advanced 2D Optoelectronic Devices.

With the rapid development of two-dimensional semiconductor technology, the inevitable chemical disorder at a typical metal-semiconductor interface has become an increasingly serious problem that degrades the performance of 2D semiconductor optoelectronic devices. Herein, defect-free van der Waals contacts have been achieved by utilizing topological Bi2 Se3 as the electrodes. Such clean and atomically sharp contacts avoid the consumption of photogenerated carriers at the interface, enabling a markedly boosted sensitivity as compared to counterpart devices with directly deposited metal electrodes. Typically, the device with 2D WSe2 channel realizes a high responsivity of 20.5AW-1 , an excellent detectivity of 2.18×1012 Jones, and a fast rise/decay time of 41.66/38.81ms. Furthermore, high-resolution visible-light imaging capability of the WSe2 device is demonstrated, indicating its promising application prospect in future optoelectronic systems. More inspiringly, the topological electrodes are universally applicable to other 2D semiconductor channels, including WS2 and InSe, suggesting its broad applicability. These results open fascinating opportunities for the development of high-performance electronics and optoelectronics.

Mesoscopic 2D molecular self-assembly on an insulator

Two-dimensional (2D) nanostructures and nanomaterials offer potential for a wide range of technological applications in electronics, optoelectronics, data storage, sensing and catalysis. On-surface molecular self-assembly—where organic molecules act as building blocks and where surfaces play the role of supporting templates—allows for the bottom-up synthesis of such 2D systems with tuneable atomically precise morphologies and tailored electronic properties. These self-assembly protocols are well established on metal surfaces, but remain limited on electronically gapped substrates (insulators, semiconductors). The latter are useful for preventing electronic coupling (that is, hybridization between molecular assembly and underlying surface) and for avoiding quenching of optical processes, necessary for prospective electronic and optoelectronic applications. In particular, molecular self-assembly on surfaces other than weakly interacting metals can be challenging due to substrate reactivity, defects and inhomogeneities, resulting in intricate energy landscapes that limit the growth kinetically and hampers the synthesis of large-area defect-free 2D systems. Here, we demonstrate the self-assembly of a 2D, atomically thin organic molecular film on a model wide bandgap 2D insulator, single-layer hexagonal boron nitride (hBN) on Cu(111). The molecular film consists of flat, aromatic 9,10-di-cyano-anthracene (DCA) molecules. Our low-temperature scanning tunnelling microscopy and spectroscopy measurements revealed mesoscopic (>100 × 100 nm2), topographically homogeneous crystalline molecular domains resulting from flat molecular adsorption and noncovalent in-plane cyano-ring bonding, with electronically decoupled molecular orbitals (MOs) lying within the hBN electronic gap. These MOs exhibit an energy level spatial modulation (∼300 meV) that follows the moiré work function variation of hBN on Cu(111). These findings pave the way for large-area, atomically precise, highly crystalline 2D organic (and metal-organic) nanomaterials on electronically functional wide bandgap insulators.

Transition to an excitonic insulator from a two-dimensional conventional insulator

In this paper, first, we present a general formulation to investigate the ground-state and elementary excitations of an excitonic insulator (EI) in real materials. In addition, we discuss the out-of-equilibrium state induced (albeit transiently) by high-intensity light illumination of a conventional two-dimensional (2D) insulator. We then present various band structure models which allow us to study the transition from a conventional insulator to an EI in 2D materials as a function of the dielectric constant, the conventional-insulator gap (and chemical potential), the bandwidths of the conduction and valence bands, and the Bravais lattice unit-cell size. One of the goals of this investigation is to determine which range of these experimentally determined parameters to consider in order to find the best candidate materials to realize the excitonic insulator. The numerical solution to the EI gap equation for various band structures shows a significant and interesting momentum dependence of the EI gap function $\mathrm{\ensuremath{\Delta}}(\stackrel{P\vec}{k})$ and of the zero-temperature electron and hole momentum distribution across the Brillouin zone. Last, we discuss the fact that these features can be detected by tunneling microscopy.

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AbstractThis cover (DOI: 10.1002/inf2.12398) shows a 2D InSe FET for fast and direct identification of SARS‐CoV‐2 variants. With a sub‐fM detection limit, such a device can directly recognize the difference between complementary and mutant nucleic acid sequences with diverse SNV types and positions in 15 min. This capability is due to the trap states in oxidized 2D InSe (InSe1‐xOx) after ALD Al2O3, making the carrier transport more sensitive to the EF shift from the negative charges of nucleic acid sequences. image