Honeycomb RhI3 Flakes with High Environmental Stability for Optoelectronics

"V"-Shaped Changing Electronic Performance of Iodinene-Based Nanoflakes as a Function of Width

Special structures and prominent performance make 2D iodinene more appealing and valuable at the molecular level. Here, new-type electronic devices have been constructed with iodinene-based nanoflakes in different sizes and are theoretically studied for electronic transport properties. Our findings reveal that iodinene-based nanoflakes possess great electron transport suppression, achieving the same function as SiO2 on single molecule scale. Such transport suppression shows surprisingly nonlinear "V"-shaped trend with the width of the iodinene-based nanoflake. The medium-width iodinene-based nanoflake exhibits the strongest electron transport suppression, while the narrowest and widest ones display the largest electron transmission coefficients due to delocalized transmission eigenstates. Essentially, the weakest electron transport originates from an extremely small DOS and wide HOMO-LUMO gap. Specifically, increasing the width would diminish the extension of electronic states for the dominant transport orbitals, resulting in more butterfly-like electronic states. In non-equilibrium, negative differential resistance effect can be observed in iodinene-based devices, caused by the weakening and staying away from the Fermi level of transmission peaks influenced by the bias. Our findings provide insights into the relationship between the width of iodinene-based nanoflake and electronic transport properties, and lay a foundation in the device design and applications in molecular insulators and controllable-functional devices.

Low-Symmetry 2D t-InTe for Polarization-Sensitive UV-Vis-NIR Photodetection

Polarization-sensitive photodetection grounded on low-symmetry 2D materials has immense potential in improving detection accuracy, realizing intelligent detection, and enabling multidimensional visual perception, which has promising application prospects in bio-identification, optical communications, near-infrared imaging, radar, military, and security. However, the majority of the reported polarized photodetection are limited by UV-vis response range and low anisotropic photoresponsivity factor, limiting the achievement of high-performance anisotropic photodetection. Herein, 2D t-InTe crystal is introduced into anisotropic systems and developed to realize broadband-response and high-anisotropy-ratio polarized photodetection. Stemming from its narrow band gap and intrinsic low-symmetry lattice characteristic, 2D t-InTe-based photodetector exhibits a UV-vis-NIR broadband photoresponse and significant photoresponsivity anisotropy behavior, with an exceptional in-plane anisotropic factor of 1.81@808 nm laser, surpassing the performance of most reported 2D counterparts. This work expounds the anisotropic structure-activity relationship of 2D t-InTe crystal, and identifies 2D t-InTe as a prospective candidate for high-performance polarization-sensitive optoelectronics, laying the foundation for future multifunctional device applications.

Stacking Faults Enabled Second Harmonic Generation in Centrosymmetric van der Waals RhI3

Second harmonic generation (SHG) in van der Waals (vdW) materials has garnered significant attention due to its potential for integrated nonlinear optical and optoelectronic applications. Stacking faults in vdW materials are a typical kind of planar defect that introduces a degree of freedom to modulate the crystal symmetry and resultant SHG response. However, the physical origin and tunability of stacking-fault-governed SHG in vdW materials remain unclear. Here, taking the intrinsically centrosymmetric vdW RhI3 as an example, we theoretically reveal the origin of stacking-fault-governed SHG response, where the SHG response comes from the energetically favorable AC̅ stacking fault of which the electrical transitions along the high-symmetry paths Γ-M and Γ-K in the Brillion zone play the dominant role at 810 nm. Such a stacking-fault-governed SHG response is further confirmed via structural characterizations and SHG measurements. Furthermore, by applying hydrostatic pressure on RhI3, the correlation between structural evolution and SHG response is revealed with SHG enhancement up to 6.9 times, where the decreased electronic transition energies and higher momentum matrix elements due to the stronger interlayer interactions upon compression magnify the SHG susceptibility. This study develops a promising foundation for nonlinear nano-optics applications through the strategic design of stacking faults.

Tunable Electronic Transport of New-Type 2D Iodine Materials Affected by the Doping of Metal Elements

2D iodine structures under high pressures are more attractive and valuable due to their special structures and excellent properties. Here, electronic transport properties of such 2D iodine structures are theoretically studied by considering the influence of the metal-element doping. In equilibrium, metal elements in Group 1 can enhance the conductance dramatically and show a better enhancement effect. Around the Fermi level, the transmission probability exceeds 1 and can be improved by the metal-element doping for all devices. In particular, the device density of states explains well the distinctions between transmission coefficients originating from different doping methods. Contrary to the "big" site doping, the "small" site doping changes transmission eigenstates greatly, with pronounced electronic states around doped atoms. In non-equilibrium, the conductance of all devices is almost weaker than the equilibrium conductance, decreasing at low voltages and fluctuating at high voltages with various amplitudes. Under biases, K-big doping shows the optimal enhancement effect, and Mg-small doping exhibits the most effective attenuation effect on conductance. Contrastingly, the currents of all devices increase with bias linearly. The metal-element doping can boost current at low biases and weaken current at high voltages. These findings contribute much to understanding the effects of defects on electronic properties and provide solid support for the application of new-type 2D iodine materials in controllable electronics and sensors.

A Droplet Method for Synthesis of Multiclass Ultrathin Metal Halides

2D metal halides have attracted increasing research attention in recent years; however, it is still challenging to synthesize them via liquid-phase methods. Here it is demonstrated that a droplet method is simple and efficient for the synthesis of multiclass 2D metal halides, including trivalent (BiI3 , SbI3 ), divalent (SnI2 , GeI2 ), and monovalent (CuI) ones. In particular, 2D SbI3 is first experimentally achieved, of which the thinnest thickness is ≈6 nm. The nucleation and growth of these metal halide nanosheets are mainly determined by the supersaturation of precursor solutions that are dynamically varying during the solution evaporation. After solution drying, the nanosheets can fall on the surface of many different substrates, which further enables the feasible fabrication of related heterostructures and devices. With SbI3 /WSe2 being a good demonstration, the photoluminescence intensity and photo responsivity of WSe2 is obviously enhanced after interfacing with SbI3 . The work opens a new pathway for 2D metal halides toward widespread investigation and applications.

2D SiP2/h-BN for a Gate-Controlled Phototransistor with Ultrahigh Sensitivity

Two-dimensional (2D) materials are extremely attractive for the construction of highly sensitive photodetectors due to their unique electronic and optical properties. However, developing 2D photodetectors with ultrahigh sensitivity for extremely low-light-level detection is still a challenge owing to the limitation of high dark current and low detectivity. Herein, a gate-controlled phototransistor based on 2D SiP₂/hexagonal boron nitride (h-BN) was rationally designed and demonstrated ultrahigh sensitivity for the first time. With a back-gate device geometry, the SiP₂/h-BN phototransistor exhibits an ultrahigh detectivity of 3.4 × 10¹³ Jones, which is one of the highest values among 2D material-based photodetectors. In addition, the phototransistor also shows a gate tunable responsivity of ≤43.5 A/W at a gate voltage of 30 V due to the photogating effect. The ultrahigh sensitivity of the SiP₂-based phototransistor is attributed to the extremely low dark current suppressed by the phototransistor configuration and the improved photocurrent by using h-BN as a substrate to reduce charge scattering. This work provides a facile strategy for improving the detectivity of photodetectors and validates the great potential of 2D SiP₂ phototransistors for ultrasensitive optoelectronic applications.

Atomically Unveiling an Atlas of Polytypes in Transition-Metal Trihalides

Transition-metal trihalides MX₃ (M = Cr, Ru; X = Cl, Br, and I) belong to a family of novel two-dimensional (2D) magnets that can exhibit topological magnons and electromagnetic properties, thus affording great promises in next-generation spintronic devices. Rich magnetic ground states observed in the MX₃ family are believed to be strongly correlated to the signature Kagome lattice and interlayer van der Waals coupling raised from distinct stacking orders. However, the intrinsic air instability of MX₃ makes their direct atomic-scale analysis challenging. Therefore, information on the stacking-registry-dependent magnetism for MX₃ remains elusive, which greatly hinders the engineering of desired phases. Here, we report a nondestructive transfer method and successfully realize an intact transfer of bilayer MX₃, as evidenced by scanning transmission electron microscopy (STEM). After surveying hundreds of MX₃ thin flakes, we provide a full spectrum of stacking orders in MX₃ with atomic precision and calculated their associated magnetic ground states, unveiled by combined STEM and density functional theory (DFT). In addition to well-documented phases, we discover a new monoclinic C2/c phase in the antiferromagnetic (AFM) structure widely existing in MX₃. Rich stacking polytypes, including C2/c, C2/m, R3̅, P3₁12, etc., provide rich and distinct magnetic ground states in MX₃. Besides, a high density of strain soliton boundaries is consistently found in all MX₃, combined with likely inverted structures, allowing AFM to ferromagnetic (FM) transitions in most MX₃. Therefore, our study sheds light on the structural basis of diverse magnetic orders in MX₃, paving the way for modulating magnetic couplings via stacking engineering.

General Synthesis of 2D Magnetic Transition Metal Dihalides via Trihalide Reduction

Two-dimensional (2D) transition metal dihalides (TMDHs) have been receiving extensive attention due to their diversified magnetic properties and promising applications in spintronics. However, controlled growth of 2D TMDHs remains challenging owing to their extreme sensitivity to atmospheric moisture. Herein, using a home-built nitrogen-filled interconnected glovebox system, a universal chemical vapor deposition synthesis route of high-quality 2D TMDH flakes (1T-FeCl₂, FeBr₂, VCl₂, and VBr₂) by reduction of their trihalide counterparts is developed. Representatively, ultrathin (∼8.6 nm) FeCl₂ flakes are synthesized on SiO₂/Si, while on graphene/Cu foil the thickness can be down to monolayer (1L). Reflective magnetic circular dichroism spectroscopy shows an interlayer antiferromagnetic ordering of FeCl₂ with a Neel temperature at ∼17 K. Scanning tunneling microscopy and spectroscopy further identify the atomic-scale structures and band features of 1L and bilayer FeCl₂ on graphene/Cu foil.

Spin Ordering Induced Broadband Photodetection Based on Two-Dimensional Magnetic Semiconductor α-MnSe

Two-dimensional (2D) magnetic semiconductors are considered to have great application prospects in spintronic logic devices, memory devices, and photodetectors, due to their unique structures and outstanding physical properties in 2D confinement. Understanding the influence of magnetism on optical/optoelectronic properties of 2D magnetic semiconductors is a significant issue for constructing multifunctional electronic devices and implementing sophisticated functions. Herein, the influence of spin ordering and magnons on the optical/optoelectronic properties of 2D magnetic semiconductor α-MnSe synthesized by space-confined chemical vapor deposition (CVD) is explored systematically. The spin-ordering-induced magnetic phase transition triggers temperature-dependent photoluminescence spectra to produce a huge transition at Néel temperature (TN  ≈ 160 K). The magnons- and defects-induced emissions are the primary luminescence path below TN and direct internal 4 a T1g →6 A1g transition-induced emissions are the main luminescence path above TN . Additionally, the magnons and defect structures endow 2D α-MnSe with a broadband luminescence from 550 to 880 nm, and an ultraviolet-near-infrared photoresponse from 365 to 808 nm. Moreover, the device also demonstrates improved photodetection performance at 80 K, possibly influenced by spin ordering and trap states associated with defects. These above findings indicate that 2D magnetic semiconductor α-MnSe provides an excellent platform for magneto-optical and magneto-optoelectronic research.

Two-Dimensional Field-Effect Transistor Sensors: The Road toward Commercialization

The evolutionary success in information technology has been sustained by the rapid growth of sensor technology. Recently, advances in sensor technology have promoted the ambitious requirement to build intelligent systems that can be controlled by external stimuli along with independent operation, adaptivity, and low energy expenditure. Among various sensing techniques, field-effect transistors (FETs) with channels made of two-dimensional (2D) materials attract increasing attention for advantages such as label-free detection, fast response, easy operation, and capability of integration. With atomic thickness, 2D materials restrict the carrier flow within the material surface and expose it directly to the external environment, leading to efficient signal acquisition and conversion. This review summarizes the latest advances of 2D-materials-based FET (2D FET) sensors in a comprehensive manner that contains the material, operating principles, fabrication technologies, proof-of-concept applications, and prototypes. First, a brief description of the background and fundamentals is provided. The subsequent contents summarize physical, chemical, and biological 2D FET sensors and their applications. Then, we highlight the challenges of their commercialization and discuss corresponding solution techniques. The following section presents a systematic survey of recent progress in developing commercial prototypes. Lastly, we summarize the long-standing efforts and prospective future development of 2D FET-based sensing systems toward commercialization.

2D Layered Material Alloys: Synthesis and Application in Electronic and Optoelectronic Devices

2D layered materials (2DLMs) have come under the limelight of scientific and engineering research and broke new ground across a broad range of disciplines in the past decade. Nevertheless, the members of stoichiometric 2DLMs are relatively limited. This renders them incompetent to fulfill the multitudinous scenarios across the breadth of electronic and optoelectronic applications since the characteristics exhibited by a specific material are relatively monotonous and limited. Inspiringly, alloying of 2DLMs can markedly broaden the 2D family through composition modulation and it has ushered a whole new research domain: 2DLM alloy nano-electronics and nano-optoelectronics. This review begins with a comprehensive survey on synthetic technologies for the production of 2DLM alloys, which include chemical vapor transport, chemical vapor deposition, pulsed-laser deposition, and molecular beam epitaxy, spanning their development, as well as, advantages and disadvantages. Then, the up-to-date advances of 2DLM alloys in electronic devices are summarized. Subsequently, the up-to-date advances of 2DLM alloys in optoelectronic devices are summarized. In the end, the ongoing challenges of this emerging field are highlighted and the future opportunities are envisioned, which aim to navigate the coming exploration and fully exert the pivotal role of 2DLMs toward the next generation of electronic and optoelectronic devices.

Effect of Strong Intermolecular Interaction in 2D Inorganic Molecular Crystals

Strong intermolecular interactions in 2D organic molecular crystals arising from π-π stacking have been widely explored to achieve high thermal stability, high carrier mobility, and novel physical properties, which have already produced phenomenal progress. However, strong intermolecular interactions in 2D inorganic molecular crystals (2DIMCs) have rarely been investigated, severely limiting both the fundamental research in molecular physics and the potential applications of 2DIMCs for optoelectronics. Here, the effect of strong intermolecular interactions induced by unique short intermolecular Se-Se and P-Se contacts in 2D α-P₄Se₃ nanoflakes is reported. On the basis of theoretical calculations of the charge density distribution and an analysis of the thermal expansion and plastic-crystal transition, the physical picture of strong intermolecular interactions can be elucidated as a higher charge density between adjacent P₄Se₃ molecules, arising from an orderly and close packing of P₄Se₃ molecules. More importantly, encouraged by the strong intermolecular coupling, the in-plane mobility of α-P₄Se₃ nanoflakes is first calculated with a quantum nuclear tunneling model, and a competitive hole mobility of 0.4 cm² V^-1 s^-1 is obtained. Our work sheds new light on the intermolecular interactions in 2D inorganic molecular crystals and is highly significant for promoting the development of molecular physics and optoelectronics.

2D Bi2Se3 materials for optoelectronics

2D layered materials with diverse exciting properties have recently attracted tremendous interest in the scientific community. Layered topological insulator Bi2Se3 comes into the spotlight as an exotic state of quantum matter with insulating bulk states and metallic Dirac-like surface states. Its unique crystal and electronic structure offer attractive features such as broadband optical absorption, thickness-dependent surface bandgap and polarization-sensitive photoresponse, which enable 2D Bi2Se3 to be a promising candidate for optoelectronic applications. Herein, we present a comprehensive summary on the recent advances of 2D Bi2Se3 materials. The structure and inherent properties of Bi2Se3 are firstly described and its preparation approaches (i.e., solution synthesis and van der Waals epitaxy growth) are then introduced. Moreover, the optoelectronic applications of 2D Bi2Se3 materials in visible-infrared detection, terahertz detection, and opto-spintronic device are discussed in detail. Finally, the challenges and prospects in this field are expounded on the basis of current development.

2D NbOI2 : A Chiral Semiconductor with Highly In-Plane Anisotropic Electrical and Optical Properties

Exploring in-plane anisotropic 2D materials is of great significance to the fundamental studies and further development of polarizationsensitive optoelectronics. Herein, chiral niobium oxide diiodide (NbOI₂ ) is introduced into the intriguing anisotropic 2D family with the experimental demonstration of anisotropic optical and electrical properties. 2D NbOI₂ crystals exhibit highly anisotropic dispersed band structures around the Fermi surface and strong in-plane anisotropy of phonon vibrations owing to the different bonding modes of Nb atoms along the b- and c-axes. Consequently, the anisotropic factors of optical absorbance and photoresponsivity in 2D NbOI₂ crystals reach up to 1.75 and 1.7, respectively. These anisotropic properties make 2D NbOI₂ an interesting platform for novel polarization-sensitive optoelectronic applications.

Polymorphism in Post-Dichalcogenide Two-Dimensional Materials

Two-dimensional (2D) materials exhibit a wide range of atomic structures, compositions, and associated versatility of properties. Furthermore, for a given composition, a variety of different crystal structures (i.e., polymorphs) can be observed. Polymorphism in 2D materials presents a fertile landscape for designing novel architectures and imparting new functionalities. The objective of this Review is to identify the polymorphs of emerging 2D materials, describe their polymorph-dependent properties, and outline methods used for polymorph control. Since traditional 2D materials (e.g., graphene, hexagonal boron nitride, and transition metal dichalcogenides) have already been studied extensively, the focus here is on polymorphism in post-dichalcogenide 2D materials including group III, IV, and V elemental 2D materials, layered group III, IV, and V metal chalcogenides, and 2D transition metal halides. In addition to providing a comprehensive survey of recent experimental and theoretical literature, this Review identifies the most promising opportunities for future research including how 2D polymorph engineering can provide a pathway to materials by design.