Bandwidth Control and Symmetry Breaking in a Mott-Hubbard Correlated Metal

In Mott materials strong electron correlation yields a spectrum of complex electronic structures. Recent synthesis advancements open realistic opportunities for harnessing Mott physics to design transformative devices. However, a major bottleneck in realizing such devices remains the lack of control over the electron correlation strength. This stems from the complexity of the electronic structure, which often veils the basic mechanisms underlying the correlation strength. This study presents control of the correlation strength by tuning the degree of orbital overlap using picometer-scale lattice engineering. This study illustrates how bandwidth control and concurrent symmetry breaking can govern the electronic structure of a correlated SrVO₃ model system. This study shows how tensile and compressive biaxial strain oppositely affect the SrVO₃ in-plane and out-of-plane orbital occupancy, resulting in the partial alleviation of the orbital degeneracy. The spectral weight redistribution under strain is derived and explained, which illustrates how high tensile strain drives the system toward a Mott insulating state. Implementation of such concepts can push correlated electron phenomena closer toward new solid-state devices and circuits. These findings therefore pave the way for understanding and controlling electron correlation in a broad range of functional materials, driving this powerful resource for novel electronics closer toward practical realization.     

Multiple Quantum States Induced in 1T-TaSe2 by Controlling the Stacking Order of Charge Density Waves

Van der Waals (vdW) materials afford unprecedented opportunities for control of electronic properties by utilizing the stacking degree of freedom. An intriguing frontier, largely unexplored, is the stacking of charge density wave (CDW) phases that is a broken-symmetry state with periodically modulated charge density and the atomic lattice. Employing density functional theory, it is uncovered that the stacking order can play a significant role in the quantum phase transitions of layered 1T-TaSe₂ with a striking 2D CDW order. By controlling the vertical stacking order of CDWs, bulk 1T-TaSe₂ can host various electronic phases including quasi-1D and 3D metals and band insulators. Particularly, the ground-state stacking configuration shows 3D metallicity due to the enhanced intralayer and interlayer electron hopping, and the second lowest energy configuration shows band insulating behavior via interlayer dimerization, implying potential metal-insulator transition. In ultrathin-layer 1T-TaSe₂, not only the stacking order but also the thickness dictate the electronic properties. While the monolayer is a Mott insulator, the bilayer (trilayer) is a band insulator (metal). More interestingly, the four-layer emerges as an insulator or a semimetal dependent on its stacking order. The wide-tunable electronic properties of 1T-TaSe₂ CDW compound will open a new pathway for designing novel quantum devices.     

Recent Advances in Twisted Structures of Flatland Materials and Crafting Moiré Superlattices

The past decade has witnessed the occurrence of novel 2D moiré patterns in nanoflatland materials. These visually beautiful moiré superlattices have become a playground on which exotic quantum phenomena can be observed. The state‐of‐the‐art experimental techniques that have been developed for crafting moiré superlattices of flatland materials are reviewed. Graphene and its heterostructure with boron nitride have now sparked new interlayer twists as a new degree of freedom for tuning several angle‐dependent physical properties, e.g., the appearance of van Hove singularities, tunable Mott insulator states, and the Hofstadter butterfly pattern. Moreover, the interplay of correlated insulating states and superconductivity is recently observed for a so‐called magic‐angle twisted bilayer graphene. Furthermore, beyond graphene, other 2D materials, such as silicene, phosphorene, and the recent black phosphorus /MoS₂ heterojunctions, which are 2D allotropes of bismuth and antimony grown on highly ordered pyrolytic graphite and MoS₂, are considered. Finally, the optically important exciton phenomenon, which depends on the moiré potential and has been observed for a moiré superlattice of transition metal dichalcogenides, is discussed. This overview aims to cover all the fascinating prospects that depend on the moiré superlattice, ranging from electronic structure to optical exotics among flatland materials.     

Electric-pulse-induced resistive switching and possible superconductivity in the Mott insulator GaTa4Se8

There is accumulated evidence today that an electric pulse can drastically modify the physical properties of correlated materials. An electric pulse was shown for example to induce an insulator-to-metal transition in manganites or in organic Mott insulators. We report here the first experimental evidence of a non-volatile electric pulse-induced insulator-to-metal transition and possible superconductivity in the Mott insulator GaTa₄Se₈. This resistive switching is concomitant to an electronic phase separation induced by the pulse. This phenomena most probably differs from the thermal, electronic injection or ionic diffusion processes explaining the resistive switching in materials foreseen for non-volatile memory (RRAM) applications.     

Moiré‐Pattern‐Tuned Electronic Structures of van der Waals Heterostructures

Moiré patterns are quasi‐periodic geometric patterns generated by the incommensurate stacking between two monolayers; they have rapidly attracted enormous attention due to their profound ability to modulate the electronic properties of 2D materials. For instance, the Bloch band of the Moiré superlattice, which is known as the Moiré band, can become flat at a specific series of discrete angles, and these flat bands are capable of exhibiting strong correlation behaviors such as the high‐temperature superconductivity reported recently. Moiré patterns can alter electronic properties, while surface reconstruction can modify Moiré patterns. In this review, the fundamental geometry is discussed and the basic electronic structure modification is summarized. Surface reconstruction is a method of tuning the electronic properties of a Moiré superlattice. Strong correlation phenomena, such as superconductivity, superfluidity, and magnetism induced by the flat bands, have been confirmed experimentally in recent years, which will be discussed in detail. Some possible application opportunities based on the fascinating characteristics of the Moiré pattern will also be presented. Because of the growing interest in Moiré patterns and related physical phenomena, it is anticipated that a deeper understanding of the fundamental physics of Moiré systems and further progress in the investigation of strong correlation phenomena are forthcoming.     

Recent progress in synthesis of two-dimensional hexagonal boron nitride

Two-dimensional (2D) materials have recently received a great deal of attention due to their unique structures and fascinating properties, as well as their potential applications. 2D hexagonal boron nitride (2D h-BN), an insulator with excellent thermal stability, chemical inertness, and unique electronic and optical properties, and a band gap of 5.97 eV, is considered to be an ideal candidate for integration with other 2D materials. Nevertheless, the controllable growth of high-quality 2D h-BN is still a great challenge. A comprehensive overview of the progress that has been made in the synthesis of 2D h-BN is presented, highlighting the advantages and disadvantages of various synthesis approaches. In addition, the electronic, optical, thermal, and mechanical properties, heterostructures, and related applications of 2D h-BN are discussed.     

Manifestation of Strongly Correlated Electrons in a 2D Kagome Metal–Organic Framework

2D and layered electronic materials characterized by a kagome lattice, whose valence band structure includes two Dirac bands and one flat band, can host a wide range of tunable topological and strongly correlated electronic phases. While strong electron correlations have been observed in inorganic kagome crystals, they remain elusive in organic systems, which benefit from versatile synthesis protocols via molecular self-assembly and metal-ligand coordination. Here, direct experimental evidence of local magnetic moments resulting from strong electron–electron Coulomb interactions in a 2D metal–organic framework (MOF) is reported. The latter consists of di-cyano-anthracene (DCA) molecules arranged in a kagome structure via coordination with copper (Cu) atoms on a silver surface [Ag(111)]. Temperature-dependent scanning tunneling spectroscopy reveals magnetic moments spatially confined to DCA and Cu sites of the MOF, and Kondo screened by the Ag(111) conduction electrons. By density functional theory and mean-field Hubbard modeling, it is shown that these magnetic moments are the direct consequence of strong Coulomb interactions between electrons within the kagome MOF. The findings pave the way for nanoelectronics and spintronics technologies based on controllable correlated electron phases in 2D organic materials.     

Reversible Ferromagnetic Phase Transition in Electrode‐Gated Manganites

The electronic phase transition has been considered as a dominant factor in the phenomena of colossal magnetoresistance, metal‐insulator transition, and exchange bias in correlated electron systems. However, the effective manipulation of the electronic phase transition has remained a challenging issue. Here, the reversible control of ferromagnetic phase transition in manganite films through ionic liquid gating is reported. Under different gate voltages, the formation and annihilation of an insulating and magnetically hard phase in the magnetically soft matrix, which randomly nucleates and grows across the film instead of initiating at the surface and spreading to the bottom, is directly observed. This discovery provides a conceptually novel vision for the electric‐field tuning of phase transition in correlated oxides. In addition to its fundamental significance, the realization of a reversible metal‐insulator transition in colossal magnetoresistance materials will also further the development of four‐state memories, which can be manipulated by a combination of electrode gating and the application of a magnetic field.     

Magnetic tuning in a novel half-metallic Ir2TeI2 monolayer

A two-dimensional (2D) high-temperature ferromagnetic half-metal whose magnetic and electronic properties can be flexibly tuned is required for the application of new spintronics devices. In this paper, we predict a stable Ir₂TeI₂ monolayer with half-metallicity by systematical first-principles calculations. Its ground state is found to exhibit inherent ferromagnetism and strong out-of-plane magnetic anisotropy of up to 1.024 meV per unit cell. The Curie temperature is estimated to be 293 K based on Monte Carlo simulation. Interestingly, a switch of magnetic axis between in-plane and out-of-plane is achievable under hole and electron doping, which allows for the effective control of spin injection/detection in such 2D systems. Furthermore, the employment of biaxial strain can realize the transition between ferromagnetic and antiferromagnetic states. These findings not only broaden the scope of 2D half-metal materials but they also provide an ideal platform for future applications of multifunctional spintronic devices.     

Excitons and Electron–Hole Liquid State in 2D γ‐Phase Group‐IV Monochalcogenides

Different dispersion near the electronic band edge of a semiconductor can have great influence on its transport, thermoelectric, and optical properties. Using first‐principles calculations, it is demonstrated that a new phase of group‐IV monochalcogenides (γ‐MX, M = Ge, Sn; X = S, Se, or Te) can be stabilized in monolayer limit. γ‐MXs are shown to possess a unique band dispersion—that is, camel's back like structure—in the top valence band. The band nesting effect near the camel's back region induces a large excitonic absorbance and significantly different exciton behaviors from other 2D materials. Importantly, the small effective mass and the indirect characteristics of lowest‐energy exciton render it advantageous for the generation of electron–hole liquid state. After careful evaluation of the electron–hole dissociation temperature and the Mott critical density, it is predicted that a high‐temperature exciton gas to electron–hole liquid phase transition can be achieved in these materials with a low excitation power density. The findings open up new opportunities for both the fundamental research on exciton physics and design of excitonic devices based on 2D materials with distinct band dispersion.     

Cover Picture: Excitonic and Vibrational Properties of Single‐Walled Semiconducting Carbon Nanotubes (Adv. Funct. Mater. 17/2007)

The cover picture illustrates excited state dynamics of semiconducting single‐walled carbon nanotubes studied theoretically. Sergai Tretiak and Svetlana Kilina report on p. 3405 that absorption of the light quantum leads to spatially delocalized photoexcitation, which can be described as tightly bound excitons and characterized by 2D plots of transition density. The photoexcitation is coupled to the vibrational degrees of freedom, leading to complex exciton‐phonon dynamics, which can be monitored experimentally using ultrafast spectroscopic probes. We review quantum‐chemical studies of the excited‐state electronic structure of finite‐size semiconducting single‐walled carbon nanotubes (SWCNTs) using methodologies previously successfully applied to describe conjugated polymers and other organic molecular materials. The results of our simulations are in quantitative agreement with available spectroscopic data and show intricate details of excited‐state properties and photoinduced vibrational dynamics in carbon nanotubes. We analyze in detail the nature of strongly bound first and second excitons in SWCNTs for a number of different tubes, emphasizing emerging size‐scaling laws. Characteristic delocalization properties of excited states are identified by the underlying photoinduced changes in charge densities and bond orders. Due to the rigid structure, exciton–phonon coupling is much weaker in SWCNTs compared to typical molecular materials. Yet we find that, in the ground state, a SWCNT's surface experiences the corrugation associated with electron–phonon interactions. Vibrational relaxation following photoexcitation reduces this corrugation, leading to a local distortion of the tube surface, which is similar to the formation of self‐trapped excitons in conjugated polymers. The calculated associated Stokes shift increases with enlargement of the tube diameters. Such exciton vibrational phenomena are possible to detect experimentally, allowing for better understanding of photoinduced electronic dynamics in nanotube materials.     

Charge Disproportionation and Voltage‐Induced Metal–Insulator Transitions Evidenced in β‐PbxV2O5 Nanowires

The roster of materials exhibiting metal–insulator transitions with sharply discontinuous switching of electrical conductivity close to room temperature remains rather sparse, despite the fundamental interest in the electronic instabilities manifested in such materials and the plethora of potential technological applications ranging from frequency‐agile metamaterials to electrochromic coatings and Mott field‐effect transistors. Here, unprecedented, pronounced metal‐insulator transitions induced by application of a voltage are demonstrated for nanowires of a vanadium oxide bronze with intercalated divalent cations, β‐Pb_xV₂O₅ (x ≈ 0.33). The induction of the phase transition through application of an electric field at room temperature makes this system particularly attractive and viable for technological applications. A mechanistic basis for the phase transition is proposed based on charge disproportionation evidenced at room temperature in near‐edge X‐ray absorption fine structure (NEXAFS) spectroscopy measurements, ab initio density functional theory calculations of the band structure, and electrical transport data, suggesting that transformation to the metallic state is induced by melting of specific charge localization and ordering motifs extant in these materials.     

An antiferromagnetic two-dimensional material: Chromium diiodides monolayer

The two-dimensional (2D) ferromagnetic materials and the related van der Waals homostructures have attracted considerable interest, while the 2D antiferromagnetic material has not yet been reported. Based on first-principles calculations, we investigate both electronic structures and magnetic orderings of bulk and monolayer of chromium diiodides (CrI₂). We demonstrate a counter-intuitive fact that the ground state of the free-standing monolayer of CrI₂ is antiferromagnetic though the bulk possesses macroscopic ferromagnetic ordering. The interlayer interaction remains antiferromagnetic up to few-layer scenarios. The unique feature of CrI₂ makes it an ideal workbench to investigate the relation between magnetic couplings and interlayer van der Waals interactions, and may offer an opportunity to 2D antiferromagnetic spintronic devices.     

Interfacial Strain: The Driving Force for Selective Orbital Occupancy in Manganite Thin Films

Complex oxides with perovskite structure are the ideal arena to study a plethora of physical properties including superconductivity, ferromagnetism, ferroelectricity, piezoelectricity and more. Among them, transition metal oxides are especially relevant since they present large electronic correlations leading to a strong competition between lattice, charge, spin, and orbital degrees of freedom. In particular, manganese perovskites oxides exhibit half‐metallic character and colossal magnetoresistive response rendering them as the ideal materials to develop novel concepts of oxide‐electronic devices and for the study of fundamental physical interactions. Due to the close similarity between kinetic energy of charge carriers and Coulomb repulsion, tiny perturbations caused by small changes in temperature, magnetic or electric fields, strain and so forth may drastically modify the magnetic and transport properties of these materials. In particular clarifying the role of interfacial strain in manganite thin films is interesting not only for device applications but also for basic understanding of physical interactions. A better comprehension of such strongly correlated systems might lead to control the different degrees of freedom in a near future contributing to the development of the so called orbitronics, i.e. controlling and modifying at will the orbital orientation of the 3d levels in transition metals. Here we reveal the importance of interfacial strain in high quality epitaxial thin films of La_2/3Ca_1/3MnO₃ (LCMO), grown on top of SrTiO₃ (STO) and NdGaO₃ (NGO) (001)‐oriented substrates. We show that in such systems interfacial strain due to lattice mismatch lifts the degeneracy of the e_g and t_2g orbitals close to the film/substrate interface inducing Jahn‐Teller like distortions and promoting selective orbital occupancy and the appearance of an orbital glass insulating state in an otherwise ferromagnetic metallic material. These results highlight the role of strain and identify it as a key parameter in orbital control.     

Adhesion evaluation on low-cost alternatives to thermosetting epoxy encapsulants

The thermosetting epoxy curing systems have been widely used as encapsulants in the electronic packaging industry. With the continual evolving of electronic product markets, material suppliers have been challenged to provide more options to meet the requirements of advanced, yet cost effective, packaging solutions. In this paper, two low-cost alternative materials have been investigated experimentally regarding their adhesion and reliability performance, and these have then been compared with the thermosetting epoxy systems. One of the materials is thermoplastic bisphenol A epoxy/phenoxy resin, and the other is an interpenetrating polymer network composed of an epoxy curing component and a free radical polymerizable component. Some formulations of the materials being studied could exhibit excellent adhesion, durability and application reliability. While reworkability is expected for these materials, they are promising as cost effective encapsulants for electronic packaging, and may be applied with appropriate processing techniques.     

Silicene Nanoribbons on an Insulating Thin Film

Silicene, a new 2D material has attracted intense research because of the ubiquitous use of silicon in modern technology. However, producing free-standing silicene has proved to be a huge challenge. Until now, silicene could be synthesized only on metal surfaces where it naturally forms strong interactions with the metal substrate that modify its electronic properties. Here, the authors report the first experimental evidence of silicene nanoribbons on an insulating NaCl thin film. This work represents a major breakthrough, for the study of the intrinsic properties of silicene, and by extension to other 2D materials that have so far only been grown on metal surfaces.     

Versatile Crystal Structures and (Opto)electronic Applications of the 2D Metal Mono‐, Di‐, and Tri‐Chalcogenide Nanosheets

Emerging 2D metal chalcogenides present excellent performance for electronic and optoelectronic applications. In contrast to graphene and other 2D materials, 2D metal chalcogenides possess intrinsic bandgaps, versatile band structures, and superior atmospheric stability. The many categories of 2D metal chalcogenides ensure that they can be applied to various practical scenarios. 2D metal monochalcogenides, dichalcogenides, and trichalcogenides are the three main categories of these materials. They have distinct crystal structures resulting in different characteristics. Some basic device characteristics, such as the charge carrier characteristics, scattering mechanisms, interfacial contacts, and band alignments of heterojunctions, are vital factors for practical device applications that ensure that the desired properties can be achieved. Various electronic, optoelectronic, and photonic applications based on 2D metal chalcogenides have been extensively investigated. 2D metal chalcogenides are considered as competitive candidates for future electronic and optoelectronic applications.     

2D Nanomaterial Arrays for Electronics and Optoelectronics

Two‐dimensional (2D) materials, benefitting from their unique planar structure and various appealing electronic properties, have attracted much attention for novel electronic and optoelectronic applications. As a basis for practical devices, the study of micro/nano‐2D material arrays based on coupling effects and synergistic effects is critical to the functionalization and integration of 2D materials. Moreover, micro/nano‐2D material arrays are compatible with traditional complementary metal oxide semiconductor (CMOS) electronics, catering well to high‐integration, high‐sensitivity, and low‐cost sensing and imaging systems. This review presents some recent studies on 2D material arrays in sequence from their novel preparations to high‐integration applications as well as explorations on dimension tuning. A first focus is on various typical fabrication methods for 2D material arrays, including photolithography, 2D printing, seeded growth, van der Waals epitaxial growth, and self‐assembly. Then, the applications of 2D material arrays, such as field effect transistors, photodetectors, pressure sensors, as well as flexible electronic devices of photodetectors and strain sensors, are elaborately introduced. Furthermore, the recent burgeoning exploration of mixed‐dimensional heterostructure arrays including 0D/2D, 1D/2D, and 3D/2D is discussed. Ultimately, conclusions and an outlook based on the current developments in this promising field are presented.     

Layer Sliding And Twisting Induced Electronic Transitions In Correlated Magnetic 1t-Nbse2 Bilayers

Correlated 2D layers, like 1T-phases of TaS₂, TaSe₂, and NbSe₂, exhibit rich tunability through varying interlayer couplings, which promotes the understanding of electron correlation in the 2D limit. However, the coupling mechanism is, so far, poorly understood and is tentatively ascribed to interactions among the ${\mathrm{d}}_{{\mathrm{z}}^2}$orbitals of Ta or Nb atoms. Here, it is theoretically shown that the interlayer hybridization and localization strength of interfacial Se p_z orbitals, rather than Nb ${\mathrm{d}}_{z^2}$orbitals, govern the variation of electron-correlated properties upon interlayer sliding or twisting in correlated magnetic 1T-NbSe₂ bilayers. Each of the layers is in a star-of-David (SOD) charge-density-wave phase. Geometric and electronic structures and magnetic properties of 28 different stacking configurations are examined and analyzed using density-functional-theory calculations. It is found that the SOD contains a localized region, in which interlayer Se p_z hybridization plays a paramount role in varying the energy levels of the two Hubbard bands. These variations lead to three electronic transitions among four insulating states, which demonstrate the effectiveness of interlayer interactions to modulate correlated magnetic properties in a prototypical correlated magnetic insulator.     

Opportunities and Challenges in Precise Synthesis of Transition Metal Single-Atom Supported by 2D Materials as Catalysts toward Oxygen Reduction Reaction

Transition metal single-atom catalysts (SACs) are currently a hot area of research in the field of electrocatalytic oxygen reduction reaction (ORR). In this review, the recent advances in transition metal single-atom supported by 2D materials as catalysts for ORR with high performance are reported. Due to their large surface area, uniformly exposed lattice plane, and adjustable electronic state, 2D materials are ideal supporting materials for exploring ORR active sites and surface reactions. The rational design principles and synthetic strategies of transition metal SACs supported by 2D materials are systematically introduced while the identification of active sites, their possible catalytic mechanisms as well as the perspectives on the future of transition metal SACs supported by 2D materials for ORR applications are discussed. Finally, according to the current development trend of ORR catalysts, the future opportunities and challenges of transition metal SACs supported by 2D materials are summarized.