Electrical Polarity Modulation in V-Doped Monolayer WS2 for Homogeneous CMOS Inverters.

As demand for higher integration density and smaller devices grows, silicon-based complementary metal-oxide-semiconductor (CMOS) devices will soon reach their ultimate limits. 2D transition metal dichalcogenides (TMDs) semiconductors, known for excellent electrical performance and stable atomic structure, are seen as promising materials for future integrated circuits. However, controlled and reliable doping of 2D TMDs, a key step for creating homogeneous CMOS logic components, remains a challenge. In this study, a continuous electrical polarity modulation of monolayer WS2 from intrinsic n-type to ambipolar, then to p-type, and ultimately to a quasi-metallic state is achieved simply by introducing controllable amounts of vanadium (V) atoms into the WS2 lattice as p-type dopants during chemical vapor deposition (CVD). The achievement of purely p-type field-effect transistors (FETs) is particularly noteworthy based on the 4.7 at% V-doped monolayer WS2, demonstrating a remarkable on/off current ratio of 105. Expanding on this triumph, the first initial prototype of ultrathin homogeneous CMOS inverters based on monolayer WS2 is being constructed. These outcomes validate the feasibility of constructing homogeneous CMOS devices through the atomic doping process of 2D materials, marking a significant milestone for the future development of integrated circuits.

An inorganic-blended p-type semiconductor with robust electrical and mechanical properties.

Inorganic semiconductors typically have limited p-type behavior due to the scarcity of holes and the localized valence band maximum, hindering the progress of complementary devices and circuits. In this work, we propose an inorganic blending strategy to activate the hole-transporting character in an inorganic semiconductor compound, namely tellurium-selenium-oxygen (TeSeO). By rationally combining intrinsic p-type semimetal, semiconductor, and wide-bandgap semiconductor into a single compound, the TeSeO system displays tunable bandgaps ranging from 0.7 to 2.2 eV. Wafer-scale ultrathin TeSeO films, which can be deposited at room temperature, display high hole field-effect mobility of 48.5 cm2/(Vs) and robust hole transport properties, facilitated by Te-Te (Se) portions and O-Te-O portions, respectively. The nanosphere lithography process is employed to create nanopatterned honeycomb TeSeO broadband photodetectors, demonstrating a high responsibility of 603 A/W, an ultrafast response of 5 µs, and superior mechanical flexibility. The p-type TeSeO system is highly adaptable, scalable, and reliable, which can address emerging technological needs that current semiconductor solutions may not fulfill.

Pulse irradiation synthesis of metal chalcogenides on flexible substrates for enhanced photothermoelectric performance.

High synthesis temperatures and specific growth substrates are typically required to obtain crystalline or oriented inorganic functional thin films, posing a significant challenge for their utilization in large-scale, low-cost (opto-)electronic applications on conventional flexible substrates. Here, we explore a pulse irradiation synthesis (PIS) to prepare thermoelectric metal chalcogenide (e.g., Bi2Se3, SnSe2, and Bi2Te3) films on multiple polymeric substrates. The self-propagating combustion process enables PIS to achieve a synthesis temperature as low as 150 °C, with an ultrafast reaction completed within one second. Beyond the photothermoelectric (PTE) property, the thermal coupling between polymeric substrates and bismuth selenide films is also examined to enhance the PTE performance, resulting in a responsivity of 71.9 V/W and a response time of less than 50 ms at 1550 nm, surpassing most of its counterparts. This PIS platform offers a promising route for realizing flexible PTE or thermoelectric devices in an energy-, time-, and cost-efficient manner.

Contact Engineering of Halide Perovskites: Gold is Not Good Enough; Metalloid is Better.

The operation stability of halide perovskite devices is the critical issue that impedes their commercialization. The main reasons are that the ambient H2 O molecules can easily deteriorate the perovskites, while the metal electrodes react in different degrees with the perovskites. Herein, one kind of new electrode, the metalloids, is reported, which are much more stable than the conventional noble metals as electrical contacts for halide perovskites. The degradation mechanism of halide perovskites with noble metal electrodes is carefully studied and compared with the metalloid electrodes. It is found that the iodide ions can easily halogenate Cu and Ag in halide perovskites. Although Au is almost not halogenated, it can also decompose the perovskite film. On the contrary, after long-term storage, the metalloid electrodes remain intact on the perovskite film without any degradation. In addition, the long-time operation stability of the perovskite devices with metalloid electrodes is much higher than that of noble metals. First-principles calculations confirm the exceptional stability of the metalloid electrodes.This work explores the ultra-stable electrodes for halide perovskites, paving the way to the large-scale deployment of perovskite-based electronic devices.

Van der Waals nanomesh electronics on arbitrary surfaces.

Chemical bonds, including covalent and ionic bonds, endow semiconductors with stable electronic configurations but also impose constraints on their synthesis and lattice-mismatched heteroepitaxy. Here, the unique multi-scale van der Waals (vdWs) interactions are explored in one-dimensional tellurium (Te) systems to overcome these restrictions, enabled by the vdWs bonds between Te atomic chains and the spontaneous misfit relaxation at quasi-vdWs interfaces. Wafer-scale Te vdWs nanomeshes composed of self-welding Te nanowires are laterally vapor grown on arbitrary surfaces at a low temperature of 100 °C, bringing greater integration freedoms for enhanced device functionality and broad applicability. The prepared Te vdWs nanomeshes can be patterned at the microscale and exhibit high field-effect hole mobility of 145 cm2/Vs, ultrafast photoresponse below 3 µs in paper-based infrared photodetectors, as well as controllable electronic structure in mixed-dimensional heterojunctions. All these device metrics of Te vdWs nanomesh electronics are promising to meet emerging technological demands.

Electrically Switchable Polarization in Bi2 O2 Se Ferroelectric Semiconductors.

Atomically 2D layered ferroelectric semiconductors, in which the polarization switching process occurs within the channel material itself, offer a new material platform that can drive electronic components toward structural simplification and high-density integration. Here, a room-temperature 2D layered ferroelectric semiconductor, bismuth oxychalcogenides (Bi2 O2 Se), is investigated with a thickness down to 7.3 nm (≈12 layers) and piezoelectric coefficient (d33 ) of 4.4 ± 0.1 pm V-1 . The random orientations and electrically dependent polarization of the dipoles in Bi2 O2 Se are separately uncovered owing to the structural symmetry-breaking at room temperature. Specifically, the interplay between ferroelectricity and semiconducting characteristics of Bi2 O2 Se is explored on device-level operation, revealing the hysteresis behavior and memory window (MW) formation. Leveraging the ferroelectric polarization originating from Bi2 O2 Se, the fabricated device exhibits "smart" photoresponse tunability and excellent electronic characteristics, e.g., a high on/off current ratio > 104 and a large MW to the sweeping range of 47% at VGS  = ±5 V. These results demonstrate the synergistic combination of ferroelectricity with semiconducting characteristics in Bi2 O2 Se, laying the foundation for integrating sensing, logic, and memory functions into a single material system that can overcome the bottlenecks in von Neumann architecture.

Au-Seeded CsPbI3 Nanowire Optoelectronics via Exothermic Nucleation.

Converting vapor precursors to solid nanostructures via a liquid noble-metal seed is a common vapor deposition principle. However, such a noble-metal-seeded process is excluded from the crystalline halide perovskite synthesis, mainly hindered by the growth mechanism shortness. Herein, powered by a spontaneous exothermic nucleation process (ΔH < 0), the Au-seeded CsPbI3 nanowires (NWs) growth is realized based on a vapor-liquid-solid (VLS) growth mode. It is energetically favored that the Au seeds are reacted with a Pb vapor precursor to form molten Au-Pb droplets at temperatures down to 212 °C, further triggering the low-temperature VLS growth of CsPbI3 NWs. More importantly, this Au-seeded process reduces in-bandgap trap states and consequently avoids Shockley-Read-Hall recombination, contributing to outstanding photodetector performances. Our work extends the powerful Au-seeded VLS growth mode to the emerging halide perovskites, which will facilitate their nanostructures with tailored material properties.

Mixed-Dimensional Anti-ambipolar Phototransistors Based on 1D GaAsSb/2D MoS2 Heterojunctions.

The incapability of modulating the photoresponse of assembled heterostructure devices has remained a challenge for the development of optoelectronics with multifunctionality. Here, a gate-tunable and anti-ambipolar phototransistor is reported based on 1D GaAsSb nanowire/2D MoS2 nanoflake mixed-dimensional van der Waals heterojunctions. The resulting heterojunction shows apparently asymmetric control over the anti-ambipolar transfer characteristics, possessing potential to implement electronic functions in logic circuits. Meanwhile, such an anti-ambipolar device allows the synchronous adjustment of band slope and depletion regions by gating in both components, thereby giving rise to the gate-tunability of the photoresponse. Coupled with the synergistic effect of the materials in different dimensionality, the hybrid heterojunction can be readily modulated by the external gate to achieve a high-performance photodetector exhibiting a large on/off current ratio of 4 × 104, fast response of 50 µs, and high detectivity of 1.64 × 1011 Jones. Due to the formation of type-II band alignment and strong interfacial coupling, a prominent photovoltaic response is explored in the heterojunction as well. Finally, a visible image sensor based on this hybrid device is demonstrated with good imaging capability, suggesting the promising application prospect in future optoelectronic systems.

Two-Step Chemical Vapor Deposition-Synthesized Lead-Free All-Inorganic Cs3Sb2Br9 Perovskite Microplates for Optoelectronic Applications.

Lead-based halide perovskites (APbX3, where A = organic or inorganic cation, X = Cl, Br, I) are suitable materials for many optoelectronic devices due to their many attractive properties. However, the concern of lead toxicity and the poor ambient and operational stability of the organic cation group greatly limit their practical utilization. Therefore, there has recently been great interest in lead-free, environment-friendly all-inorganic halide perovskites (IHPs). Sb and Sn are common species suggested to replace Pb for Pb-free IHPs. However, the large difference in the melting points of the precursor materials (e.g., CsBr and SbBr3 precursors for Cs3Sb2Br9) makes the chemical vapor deposition (CVD) growth of high-quality Pb-free IHPs a very challenging task. In this work, we developed a two-step CVD method to overcome this challenge and successfully synthesized Pb-free Cs3Sb2Br9 perovskite microplates. Cs3Sb2Br9 microplates ∼25 µm in size with the exciton absorption peak at ∼2.8 eV and a band gap of ∼2.85 eV were obtained. The microplates have a smooth hexagonal morphology and show a large Stokes shift of ∼450 meV and exciton binding energy of ∼200 meV. To demonstrate the applications of these microplates in optoelectronics, simple photoconductive devices were fabricated. These photodetectors exhibit a current on/off ratio of 2.36 × 102, a responsivity of 36.9 mA/W, and a detectivity of 1.0 × 1010 Jones with a fast response of rise and decay time of 61.5 and 24 ms, respectively, upon 450 nm photon irradiation. Finally, the Cs3Sb2Br9 microplates also show good stability in ambient air without encapsulation. These results demonstrate that the 2-step CVD process is an effective approach to synthesize high-quality all-inorganic lead-free Cs3Sb2Br9 perovskite microplates that have the potential for future high-performance optoelectronic device applications.

High-Performance Flexible Self-Powered Photodetectors Utilizing Spontaneous Electron and Hole Separation in Quasi-2D Halide Perovskites.

Although there are recent advances in many areas of quasi-2D halide perovskites, photodetectors based on these materials still cannot achieve satisfactory performance for practical applications where high responsivity, fast response, self-powered nature, and excellent mechanical flexibility are urgently desired. Herein, utilizing one-step spin-coating method, self-assemble quasi-2D perovskite films with graded phase distribution in the order of increasing number of metal halide octahedral layers are successfully prepared. Gradient type-II band alignments along the out-of-plane direction of perovskites with spontaneous separation of photo-generated electrons and holes are obtained and then employed to construct self-powered vertical-structure photodetectors for the first time. Without any driving voltage, the device exhibits impressive performance with the responsivity up to 444 mA W-1 and ultrashort response time down to 52 µs. With a bias voltage of 1.5 V, the device responsivity becomes 3463 mA W-1 with the response speed as fast as 24 µs. Importantly, the device's mechanical flexibility is greatly enhanced since the photocurrent prefers flowing through the metal halide octahedral layers between the top and bottom contact electrodes in the vertical device structure, being more tolerant to film damage. These results evidently indicate the potential of graded quasi-2D perovskite phases for next-generation optoelectronic devices.

Self-Anti-Stacking 2D Metal Phosphide Loop-Sheet Heterostructures by Edge-Topological Regulation for Highly Efficient Water Oxidation.

2D metal phosphide loop-sheet heterostructures are controllably synthesized by edge-topological regulation, where Ni2 P nanosheets are edge-confined by the N-doped carbon loop, containing ultrafine NiFeP nanocrystals (denoted as NiFeP@NC/Ni2 P). This loop-sheet feature with lifted-edges prevents the stacking of nanosheets and induces accessible open channels for catalytic site exposure and gas bubble release. Importantly, these NiFeP@NC/Ni2 P hybrids exhibit a remarkable oxygen evolution activity with an overpotential of 223 mV at 20 mA cm-2 and a Tafel slope of 46.1 mV dec-1 , constituting the record-high performance among reported metal phosphide electrocatalysts. The NiFeP@NC/Ni2 P hybrids are also employed as both anode and cathode to achieve an alkaline electrolyzer for overall water splitting, delivering a current density of 10 mA cm-2 with a voltage of 1.57 V, comparable to that of the commercial Pt/C||RuO2 couple (1.56 V). Moreover, a photovoltaic-electrolysis coupling system can as well be effectively established for robust overall water splitting. Evidently, this ingenious protocol would expand the toolbox for designing efficient 2D nanomaterials for practical applications.

Perovskite Core-Shell Nanowire Transistors: Interfacial Transfer Doping and Surface Passivation.

While halide perovskite electronics are rapidly developing, they are greatly limited by the inferior charge transport and poor stability. In this work, effective surface charge transfer doping of vapor-liquid-solid (VLS)-grown single-crystalline cesium lead bromide perovskite (CsPbBr3) nanowires (NWs) via molybdenum trioxide (MoO3) surface functionalization is achieved. Once fabricated into NW devices, due to the efficient interfacial charge transfer and reduced impurity scattering, a 15× increase in the field-effect hole mobility (µh) from 1.5 to 23.3 cm2/(V s) is accomplished after depositing the 10 nm thick MoO3 shell. This enhanced mobility is already better than any mobility value reported for perovskite field-effect transistors (FETs) to date. The photodetection performance of these CsPbBr3/MoO3 core-shell NWs is also investigated to yield a superior responsivity (R) up to 2.36 × 103 A/W and an external quantum efficiency (EQE) of over 5.48 × 105% toward the 532 nm regime. Importantly, the MoO3 shell can provide excellent surface passivation to the CsPbBr3 NW core that minimizes the diffusion of detrimental water and oxygen molecules, improving the air stability of CsPbBr3/MoO3 core-shell NW devices. All these findings evidently demonstrate the surface doping as an enabling technology to realize high-mobility and air-stable low-dimensional halide perovskite devices.

Bication-Mediated Quasi-2D Halide Perovskites for High-Performance Flexible Photodetectors: From Ruddlesden-Popper Type to Dion-Jacobson Type.

Quasi-2D halide perovskites, especially the Ruddlesden-Popper perovskites (RPPs), have attracted great attention because of their promising properties for optoelectronics; however, there are still serious drawbacks, such as inefficient charge transport, poor stability, and unsatisfactory mechanical flexibility, restricting further utilization in advanced technologies. Herein, high-quality quasi-2D halide perovskite thin films are successfully synthesized with the introduction of the unique bication ethylenediammonium (EDA) via a one-step spin-coating method. This bication EDA, with short alkyl chain length, can not only substitute the typically bulky and weakly van der Waals-interacted organic bilayer spacer cations forming the novel Dion-Jacobson phase to enhance the mechanical flexibility of the quasi-2D perovskite (e.g., EDA(MA)n-1PbnI3n+1; MA = CH3NH3+) but also serve as a normal cation to achieve the more intact films (e.g., (iBA)2(MA)3-2x(EDA)xPb4I13). When fabricated into photodetectors, these optimized EDA-based perovskites deliver an excellent responsivity of 125 mA/W and a fast response time down to 380 µs under 532 nm irradiation. More importantly, the device with the Dion-Jacobson phase perovskite can be bent down to a radius of 2 mm and processed with 10,000 cycles of the bending test without any noticeable performance degradation because of its superior structure to RPPs. Besides, these films do not exhibit any material deterioration after ambient storage for 30 days. All these performance parameters are already comparable or even better than those of the state-of-the-art RPPs recently reported. This work provides valuable design guidelines of the quasi-2D perovskites to obtain high-performance flexible photodetectors for next-generation optoelectronics.

Electric-field-driven non-volatile multi-state switching of individual skyrmions in a multiferroic heterostructure.

Electrical manipulation of skyrmions attracts considerable attention for its rich physics and promising applications. To date, such a manipulation is realized mainly via spin-polarized current based on spin-transfer torque or spin-orbital torque effect. However, this scheme is energy consuming and may produce massive Joule heating. To reduce energy dissipation and risk of heightened temperatures of skyrmion-based devices, an effective solution is to use electric field instead of current as stimulus. Here, we realize an electric-field manipulation of skyrmions in a nanostructured ferromagnetic/ferroelectrical heterostructure at room temperature via an inverse magneto-mechanical effect. Intriguingly, such a manipulation is non-volatile and exhibits a multistate feature. Numerical simulations indicate that the electric-field manipulation of skyrmions originates from strain-mediated modification of effective magnetic anisotropy and Dzyaloshinskii-Moriya interaction. Our results open a direction for constructing low-energy-dissipation, non-volatile, and multistate skyrmion-based spintronic devices.

Bending Strain-Tailored Magnetic and Electronic Transport Properties of Reactively Sputtered γ'-Fe4N/Muscovite Epitaxial Heterostructures toward Flexible Spintronics.

The strain modulation on the magnetic and electronic transport properties of the ferromagnetic films is one of the hot topics due to the practical applications in flexible and wearable spintronic devices. However, the large strain-induced saturation magnetization and resistance change is not easy to achieve because most of the ferromagnetic films deposited on flexible substrates are polycrystalline or amorphous. Here, the flexible epitaxial γ'-Fe4N/mica films are fabricated by facing-target reactive sputtering. At a tensile strain with a radius of curvature (ROC) of 3 mm, the saturation magnetization (Ms) of the γ'-Fe4N/mica film is tailored significantly with a maximal variation of 210%. Meanwhile, the magnetic anisotropy was broadly tunable at different strains, where the out-of-plane Mr/Ms at a tensile strain of ROC = 2 mm is six times larger than that at the unbent state. Besides, the strain-tailored longitudinal resistance Rxx and anomalous Hall resistivity ρxy appear where the drop of Rxx (ρxy) reaches 5% (22%) at a tensile strain of ROC = 3 mm. The shift of the nitrogen position in the γ'-Fe4N unit cell at different bending strains plays a key role in the strain-tailored magnetic and electronic transport properties. The flexible epitaxial γ'-Fe4N films have the potential applications in magneto- and electromechanical wearable spintronic devices.

Atomic origin of spin-valve magnetoresistance at the SrRuO3 grain boundary.

Defects exist ubiquitously in crystal materials, and usually exhibit a very different nature from the bulk matrix. Hence, their presence can have significant impacts on the properties of devices. Although it is well accepted that the properties of defects are determined by their unique atomic environments, the precise knowledge of such relationships is far from clear for most oxides because of the complexity of defects and difficulties in characterization. Here, we fabricate a 36.8° SrRuO3 grain boundary of which the transport measurements show a spin-valve magnetoresistance. We identify its atomic arrangement, including oxygen, using scanning transmission electron microscopy and spectroscopy. Based on the as-obtained atomic structure, the density functional theory calculations suggest that the spin-valve magnetoresistance occurs because of dramatically reduced magnetic moments at the boundary. The ability to manipulate magnetic properties at the nanometer scale via defect control allows new strategies to design magnetic/electronic devices with low-dimensional magnetic order.