2D Rare Earth Material (EuOCl) with Ultra-Narrow Photoluminescence at Room Temperature.

High color purity and color rendition of 2D luminescent materials have long been pursued for applications in low-dimensional lighting, display, biolabeling, and laser. However, the reported photoluminescence (PL) linewidth of most 2D luminescent materials is about dozens of meV. Herein, a brand-new luminescent system of 2D rare earth (RE) material EuOCl (1.1 nm) with ultra-narrow linewidth (1.2 meV) at room temperature is successfully synthesized via chemical vapor deposition (CVD). The linewidth of EuOCl flakes at room temperature is even narrower than most 2D luminescent materials and heterostructures detected at below 10 K. Impressively, the as-synthesized EuOCl flakes show abnormal temperature-dependent photoluminescent properties, which is absolutely different from the relatively stable 4f-4f transitions in RE owing to shielding from outer shell electrons. J-mixing effect has been successfully applied for this phenomenon. Undoubtedly, luminescent 2D EuOCl flakes will open new territory for the applications of 2D RE materials in the 2D luminescent areas, especially for the applications at room temperature.

Temperature dependence of Raman responses of few-layer PtS2.

Platinum disulfide (PtS2) is a newly emerging 2D material, which possesses relatively high carrier mobility, a widely tunable band gap from 0.25 to 1.6 eV, and ultra-high air stability, showing a potential in electronics and optoelectronics. Here, for the first time, we study the temperature-dependent Raman spectra on PtS2 with different thicknesses. It was found that with the temperature increase from 80 to 298 K, the [Formula: see text] and [Formula: see text] modes of all samples show linear softening. Moreover, the linear softening with temperature of PtS2 is much smaller than other 2D transition metal dichalcogenides, which could be attributed to the stronger interlayer coupling in PtS2. Our work gives fundamental temperature-dependent vibrational information of PtS2, which will be useful in future PtS2-based electronic devices.

Universal p-Type Doping via Lewis Acid for 2D Transition-Metal Dichalcogenides.

Developing spatially controlled and universal p-type doping of transition-metal dichalcogenides (TMDs) is critical for optoelectronics. Here, a facile and universal p-doping strategy via Sn4+ ions exchanging is proposed and the p-doping of PdSe2 is demonstrated systematically as the example. The polarity of PdSe2 can be modulated from n-type to bipolar and p-type precisely by changing the concentration of SnCl4 solution. The modulation effectively reduces the electron concentration and improves the work function by ∼72 meV. In addition, the solution-processable route makes the spatially controlled doping possible, which is demonstrated by constructing the lateral PdSe2 p-n homojunction with rectification behavior and photovoltaic effect. This p-doping method has been further proved in modulating various TMDs including WSe2, WS2, ReSe2, MoSe2, MoTe2, and PtSe2. This spatially controlled and universal method based on Sn atoms substitution realizes p-type doping of TMDs.

High-TC Two-Dimensional Ferroelectric CuCrS2 Grown via Chemical Vapor Deposition.

Two-dimensional (2D) ferroelectrics have attracted intensive attention. However, the 2D ferroelectrics remain rare, and especially few of them represent high ferroelectric transition temperature (TC), which is important for the usability of ferroelectrics. Herein, CuCrS2 nanoflakes are synthesized by salt-assisted chemical vapor deposition and exhibit switchable ferroelectric polarization even when the thickness is downscaled to 6 nm. On the contrary, a CuCrS2 nanoflake shows a TC as high as ∼700 K, which can be attributed to the robust tetrahedral bonding configurations of Cu cations. Such robustness can be further clarified by a theoretically predicted high order-disorder transition barrier and structure evolution from 600 to 800 K. Additionally, the interlocked out-of-plane (OOP) and in-plane (IP) ferroelectric domains are observed and two kinds of devices based on OOP and IP polarizations are demonstrated.

Intercalation Strategy in 2D Materials for Electronics and Optoelectronics.

Intercalation is an effective approach to tune the physical and chemical properties of 2D materials due to their abundant van der Waals gaps that can host high-density intercalated guest matters. This approach has been widely employed to modulate the optical, electrical, and photoelectrical properties of 2D materials for their applications in electronic and optoelectronic devices. Thus it is necessary to review the recent progress of the intercalation strategy in 2D materials and their applications in devices. Herein, various intercalation strategies and the novel properties of the intercalated 2D materials as well as their applications in electronics and optoelectronics are summarized. In the end, the development tendency of this promising approach for 2D materials is also outlined.

2D Cu9 S5 /PtS2 /WSe2 Double Heterojunction Bipolar Transistor with High Current Gain.

Bipolar junction transistor (BJT) as one important circuit element is now widely used in high-speed computation and communication for its capability of high-power signal amplification. 2D materials and their heterostructures are promising in building high-amplification and high-frequency BJTs because they can be naturally thin and highly designable in tailoring components properties. However, currently the low emitter injection efficiency results in only moderate current gain achieved in the pioneer researches, severely restraining its future development. Herein, it is shown that an elaborately designed double heterojunction bipolar transistor (DHBT) can greatly promote the injection efficiency, improving the current gain by order of magnitude. In this DHBT high-doping-density wide-bandgap 2D Cu9 S5 is used as emitter and narrow-bandgap PtS2 as base. This heterostructure efficiently suppresses the reverse electron flux from base and increase the injection efficiency. Consequently, the DHBT achieves an excellent current gain (ß ≈ 910). This work systematically explores the electrical behavior of 2D materials based DHBT, and provides deep insight of the architecture design for building high gain DHBT, which may promote the applications of 2Dheterojunctions in the fields of integrated circuits.

Room-Temperature Ferroelectricity in 2D Metal-Tellurium-Oxyhalide Cd7Te7Cl8O17 via Selenium-Induced Selective-Bonding Growth.

Two-dimensional (2D) ferroelectric materials have attracted increasing interest due to meeting the requirements of integration, miniaturization, and multifunction of devices. However, the exploration of intrinsic 2D ferroelectric materials is still in the early stage, for which the related reports are still limited, especially fewer ones prepared by chemical vapor deposition (CVD). Here, the ultrathin metal-tellurium-oxyhalide Cd7Te7Cl8O17 (CTCO) flakes as thin as 3.8 nm are realized via the selenium-induced selective-bonding CVD method. The growth mechanism has been confirmed by experiments and theoretical calculations, which can be ascribed to the induction of selective bonding of a hydrogen atom in H2O molecules by the introduction of selenium, leading to the generation of strong oxidants. Excitingly, switchable out-of-plane ferroelectric polarization was observed in CTCO flakes down to 6 nm at room temperature, which may be caused by mobile Cl vacancies. This work has implications for the synthesis and applications of 2D ferroelectric materials.

Tunneling Diode Based on WSe2 /SnS2 Heterostructure Incorporating High Detectivity and Responsivity.

van der Waals (vdW) heterostructures based on atomically thin 2D materials have led to a new era in next-generation optoelectronics due to their tailored energy band alignments and ultrathin morphological features, especially in photodetectors. However, these photodetectors often show an inevitable compromise between photodetectivity and photoresponsivity with one high and the other low. Herein, a highly sensitive WSe2 /SnS2 photodiode is constructed on BN thin film by exfoliating each material and manually stacking them. The WSe2 /SnS2 vdW heterostructure shows ultralow dark currents resulting from the depletion region at the junction and high direct tunneling current when illuminated, which is confirmed by the energy band structures and electrical characteristics fitted with direct tunneling. Thus, the distinctive WSe2 /SnS2 vdW heterostructure exhibits both ultrahigh photodetectivity of 1.29 × 1013 Jones (Iph /Idark ratio of ≈106 ) and photoresponsivity of 244 A W-1 at a reverse bias under the illumination of 550 nm light (3.77 mW cm-2 ).

Strong In-Plane Anisotropies of Optical and Electrical Response in Layered Dimetal Chalcogenide.

An interesting in-plane anisotropic layered dimetal chalcogenide Ta2NiS5 is introduced, and the optical and electrical properties with respect to its in-plane anisotropy are systematically studied. The Raman vibration modes have been identified by Raman spectra measurements combined with calculations of phonon-related properties. Importantly, the Ta2NiS5 flakes exhibit strong anisotropic Raman response under the angle-resolved polarized Raman spectroscopy measurements. We found that Raman intensities of the Ag mode not only depend on rotation angle but are also related to the sample thickness. In contrast, the infrared absorption with light polarized along the a axis direction is always larger than that in the c axis direction regardless of thickness under the polarization-resolved infrared spectroscopy measurements. Remarkably, the first-principles calculations combined with angle-resolved conductance measurements indicate strong anisotropic conductivity of Ta2NiS5. Our results not only prove Ta2NiS5 is a promising in-plane anisotropic 2D material but also provide an interesting platform for future functionalized electronic devices.