Au‐Edged CuZnSe2 Heterostructured Nanosheets with Enhanced Electrochemical Performance

Two‐dimensional (2D) nanomaterials and heterostructured nanocrystals (NCs) are two hot topics in current nanoresearch. However, reports on heterostructured NCs with 2D features are still rare. In this work, we demonstrate a one‐pot colloidal chemistry route for synthesizing Au‐CuZnSe₂ heterostructures with spherical Au domains attached to the edge of a sheet of CuZnSe₂. This protocol involves the preferential formation of Au clusters and the seeded growth of CuZnSe₂ sheets because of the lattice matching of CuSe with Au. As an example to demonstrate the importance of such heterostructures, the electrochemical performance of Au‐CuZnSe₂ heterostructured nanosheets is compared with that of heterostructured nanorods, Au NCs, and CuZnSe₂ NCs. The heterostructured nanosheets exhibit the best electrochemical activity.     

Improving All‐Inorganic Perovskite Photodetectors by Preferred Orientation and Plasmonic Effect

All‐inorganic perovskites have high carrier mobility, long carrier diffusion length, excellent visible light absorption, and well overlapping with localized surface plasmon resonance (LSPR) of noble metal nanocrystals (NCs). The high‐performance photodetectors can be constructed by means of the intrinsic outstanding photoelectric properties, especially plasma coupling. Here, for the first time, inorganic perovskite photodetectors are demonstrated with synergetic effect of preferred‐orientation film and plasmonic with both high performance and solution process virtues, evidenced by 238% plasmonic enhancement factor and 10⁶ on/off ratio. The CsPbBr₃ and Au NC inks are assembled into high‐quality films by centrifugal‐casting and spin‐coating, respectively, which lead to the low cost and solution‐processed photodetectors. The remarkable near‐field enhancement effect induced by the coupling between Au LSPR and CsPbBr₃ photogenerated carriers is revealed by finite‐difference time‐domain simulations. The photodetector exhibits a light on/off ratio of more than 10⁶ under 532 nm laser illumination of 4.65 mW cm^?2. The photocurrent increases from 0.67 to 2.77 μA with centrifugal‐casting. Moreover, the photocurrent rises from 245.6 to 831.1 μA with Au NCs plasma enhancement, leading to an enhancement factor of 238%, which is the most optimal report among the LSPR‐enhanced photodetectors, to the best of our knowledge. The results of this study suggest that all‐inorganic perovskites are promising semiconductors for high‐performance solution‐processed photodetectors, which can be further enhanced by Au plasmonic effect, and hence have huge potentials in optical communication, safety monitoring, and biological sensing.     

Surface Plasmon‐Enhanced Photodetection in Few Layer MoS2 Phototransistors with Au Nanostructure Arrays

2D Molybdenum disulfide (MoS₂) is a promising candidate material for high‐speed and flexible optoelectronic devices, but only with low photoresponsivity. Here, a large enhancement of photocurrent response is obtained by coupling few‐layer MoS₂ with Au plasmonic nanostructure arrays. Au nanoparticles or nanoplates placed onto few‐layer MoS₂ surface can enhance the local optical field in the MoS₂ layer, due to the localized surface plasmon (LSP) resonance. After depositing 4 nm thick Au nanoparticles sparsely onto few‐layer MoS₂ phototransistors, a doubled increase in the photocurrent response is observed. The photocurrent of few‐layer MoS₂ phototransistors exhibits a threefold enhancement with periodic Au nanoarrays. The simulated optical field distribution confirms that light can be trapped and enhanced near the Au nanoplates. These findings offer an avenue for practical applications of high performance MoS₂‐based optoelectronic devices or systems in the future.     

Gap‐Mode Surface‐Plasmon‐Enhanced Photoluminescence and Photoresponse of MoS2

2D materials hold great potential for designing novel electronic and optoelectronic devices. However, 2D material can only absorb limited incident light. As a representative 2D semiconductor, monolayer MoS₂ can only absorb up to 10% of the incident light in the visible, which is not sufficient to achieve a high optical‐to‐electrical conversion efficiency. To overcome this shortcoming, a “gap‐mode” plasmon‐enhanced monolayer MoS₂ fluorescent emitter and photodetector is designed by squeezing the light‐field into Ag shell‐isolated nanoparticles–Au film gap, where the confined electromagnetic field can interact with monolayer MoS₂. With this gap‐mode plasmon‐enhanced configuration, a 110‐fold enhancement of photoluminescence intensity is achieved, exceeding values reached by other plasmon‐enhanced MoS₂ fluorescent emitters. In addition, a gap‐mode plasmon‐enhanced monolayer MoS₂ photodetector with an 880% enhancement in photocurrent and a responsivity of 287.5 A W^?1 is demonstrated, exceeding previously reported plasmon‐enhanced monolayer MoS₂ photodetectors.     

Generalized Scheme for High Performing Photodetectors with a p‐Type 2D Channel Layer and n‐Type Nanoparticles

A generalized scheme for the fabrication of high performance photodetectors consisting of a p‐type channel material and n‐type nanoparticles is proposed. The high performance of the proposed hybrid photodetector is achieved through enhanced photoabsorption and the photocurrent gain arising from its effective charge transfer mechanism. In this paper, the realization of this design is presented in a hybrid photodetector consisting of 2D p‐type black phosphorus (BP) and n‐type molybdenum disulfide nanoparticles (MoS₂ NPs), and it is demonstrated that it exhibits enhanced photoresponsivity and detectivity compared to pristine BP photodetectors. It is found that the performance of hybrid photodetector depends on the density of NPs on BP layer and that the response time can be reduced with increasing density of MoS₂ NPs. The rising and falling times of this photodetector are smaller than those of BP photodetectors without NPs. This proposed scheme is expected to work equally well for a photodetector with an n‐type channel material and p‐type nanoparticles.     

Stabilizing MoS2 Nanosheets through SnO2 Nanocrystal Decoration for High‐Performance Gas Sensing in Air

The unique properties of MoS₂ nanosheets make them a promising candidate for high‐performance room temperature sensing. However, the properties of pristine MoS₂ nanosheets are strongly influenced by the significant adsorption of oxygen in an air environment, which leads to instability of the MoS₂ sensing device, and all sensing results on MoS₂ reported to date were exclusively obtained in an inert atmosphere. This significantly limits the practical sensor application of MoS₂ in an air environment. Herein, a novel nanohybrid of SnO₂ nanocrystal (NC)‐decorated crumpled MoS₂ nanosheet (MoS₂/SnO₂) and its exciting air‐stable property for room temperature sensing of NO₂ are reported. Interestingly, the SnO₂ NCs serve as strong p‐type dopants for MoS₂, leading to p‐type channels in the MoS₂ nanosheets. The SnO₂ NCs also significantly enhance the stability of MoS₂ nanosheets in dry air. As a result, unlike other MoS₂ sensors operated in an inert gas (e.g. N₂), the nanohybrids exhibit high sensitivity, excellent selectivity, and repeatability to NO₂ under a practical dry air environment. This work suggests that NC decoration significantly tunes the properties of MoS₂ nanosheets for various applications.     

Plasmon‐Enhanced Photoelectrical Hydrogen Evolution on Monolayer MoS2 Decorated Cu1.75S‐Au Nanocrystals

Hydrogen production from water splitting through an efficient photoelectrochemical route requires photoinduced electron transfer from light harvesters to efficient electrocatalysts. Here, the plasmon‐enhanced photoelectrical nanocatalysts (NCs) have been successfully developed by coating a monolayer MoS₂ on the Cu_1.75S‐Au hetero‐nanoparticle for hydrogen evolution reaction (HER). The plasmonic NCs dramatically improve the HER, leading to 29.5‐fold increase of current under 650 nm excitation (1.0 W cm^?2). These NCs generate an exceptionally high current density of 200 mA cm^?2 at overpotential of 182.8 mV with a Tafel slope of 39 mV per decade and excellent stability, which is better than or comparable to the Pt‐free catalysts with carbon rod as counter electrode. The enhanced HER performance can be attributed to the significantly improved broad light absorption (400–3000 nm), more efficient charge separation and abundant active edge sites of monolayer MoS₂. The studies may provide a facile strategy for the fabrication of efficient plasmon‐enhanced photoelectrical NCs for HER.     

Fabrication of Supported AuPt Alloy Nanocrystals with Enhanced Electrocatalytic Activity for Formic Acid Oxidation through Conversion Chemistry of Layer‐Deposited Pt2+ on Au Nanocrystals

The exploitation of nanoconfined conversion of Au‐ and Pt‐containing binary nanocrystals for developing a controllable synthesis of surfactant‐free AuPt nanocrystals with enhanced formic acid oxidation (FAO) activity is reported, which can be stably and evenly immobilized on various support materials to diversify and optimize their electrocatalytic performance. In this study, an atomic layer of Pt²⁺ species is discovered to be spontaneously deposited in situ on the Au nanocrystal generated from a reverse‐microemulsion solution. The resulting Au/Pt²⁺ nanocrystal thermally transforms into a reduced AuPt alloy nanocrystal during the subsequent solid‐state conversion process within the SiO₂ nanosphere. The alloy nanocrystals can be isolated from SiO₂ in a surfactant‐free form and then dispersedly loaded on the carbon sphere surface, allowing for the production of a supported electrocatalyst that exhibits much higher FAO activity than commercial Pt/C catalysts. Furthermore, by involving Fe₃O₄ nanocrystals in the conversion process, the AuPt alloy nanocrystals can be grown on the oxide surface, improving the durability of supported metal catalysts, and then uniformly loaded on a reduced graphene oxide (RGO) layer with high electroconductivity. This produces electrocatalytic AuPt/Fe₃O₄/RGO nanocomposites whose catalyst‐oxide‐graphene triple‐junction structure provides improved electrocatalytic properties in terms of both activity and durability in catalyzing FAO.     

A General and Straightforward Route to Noble Metal‐Decorated Mesoporous Transition‐Metal Oxides with Enhanced Gas Sensing Performance

Controllable and efficient synthesis of noble metal/transition‐metal oxide (TMO) composites with tailored nanostructures and precise components is essential for their application. Herein, a general mercaptosilane‐assisted one‐pot coassembly approach is developed to synthesize ordered mesoporous TMOs with agglomerated‐free noble metal nanoparticles, including Au/WO₃, Au/TiO₂, Au/NbO_x, and Pt/WO₃. 3‐mercaptopropyl trimethoxysilane is applied as a bridge agent to cohydrolyze with metal oxide precursors by alkoxysilane moieties and interact with the noble metal source (e.g., HAuCl₄ and H₂PtCl₄) by mercapto (? SH) groups, resulting in coassembly with poly(ethylene oxide)‐b‐polystyrene. The noble metal decorated TMO materials exhibit highly ordered mesoporous structure, large pore size (≈14–20 nm), high specific surface area (61–138 m² g^?1), and highly dispersed noble metal (e.g., Au and Pt) nanoparticles. In the system of Au/WO₃, in situ generated SiO₂ incorporation not only enhances their thermal stability but also induces the formation of ε‐phase WO₃ promoting gas sensing performance. Owning to its specific compositions and structure, the gas sensor based on Au/WO₃ materials possess enhanced ethanol sensing performance with a good response (R_air/R_gas = 36–50 ppm of ethanol), high selectivity, and excellent low‐concentration detection capability (down to 50 ppb) at low working temperature (200 °C).     

pH‐Dependent Galvanic Replacement of Supported and Colloidal Cu2O Nanocrystals with Gold and Palladium

Galvanic replacement reactions (GRRs) on nanoparticles (NPs) are typically performed between two metals, i.e., a solid metal NP and a replacing salt solution of a more noble metal. The solution pH in GRRs is commonly considered an irrelevant parameter. Yet, the solution pH plays a major role in GRRs involving metal oxide NPs. Here, Cu₂O nanocrystals (NCs) are studied as galvanic replacement (GR) precursors, undergoing replacement by gold and palladium, with the resulting nanostructures showing a strong dependence on the pH of the replacing metal salt solution. GRRs are reported for the first time on supported (chemically deposited) oxide NCs and the results are compared with those obtained with corresponding colloidal systems. Control of the pH enables production of different nanostructures, from metal‐decorated Cu₂O NCs to uniformly coated Cu₂O‐in‐metal (Cu₂O@Me) core–shell nanoarchitectures. Improved metal nucleation efficiencies at low pHs are attributed to changes in the Cu₂O surface charge resulting from protonation of the oxide surface. GR followed by etching of the Cu₂O cores provides metal nanocages that collapse upon drying; the latter is prevented using a sol–gel silica overlayer stabilizing the metal nanocages. Metal‐replaced Cu₂O NCs and their corresponding stabilized nanostructures may be useful as photocatalysts, electrocatalysts, and nanosensors.     

Beyond Gold: Spin‐Coated Ti3C2‐Based MXene Photodetectors

2D transition metal carbides, known as MXenes, are transparent when the samples are thin enough. They are also excellent electrical conductors with metal‐like carrier concentrations. Herein, these characteristics are exploited to replace gold (Au) in GaAs photodetectors. By simply spin‐coating transparent Ti₃C₂‐based MXene electrodes from aqueous suspensions onto GaAs patterned with a photoresist and lifted off with acetone, photodetectors that outperform more standard Au electrodes are fabricated. Both the Au‐ and MXene‐based devices show rectifying contacts with comparable Schottky barrier heights and internal electric fields. The latter, however, exhibit significantly higher responsivities and quantum efficiencies, with similar dark currents, hence showing better dynamic range and detectivity, and similar sub‐nanosecond response speeds compared to the Au‐based devices. The simple fabrication process is readily integratable into microelectronic, photonic‐integrated circuits and silicon photonics processes, with a wide range of applications from optical sensing to light detection and ranging and telecommunications.     

Solution‐Processed 3D RGO–MoS2/Pyramid Si Heterojunction for Ultrahigh Detectivity and Ultra‐Broadband Photodetection

Molybdenum disulfide (MoS₂), a typical 2D metal dichalcogenide (2DMD), has exhibited tremendous potential in optoelectronic device applications, especially in photodetection. However, due to the weak light absorption of planar mono‐/multilayers, limited cutoff wavelength edge, and lack of high‐quality junctions, most reported MoS₂‐based photodetectors show undesirable performance. Here, a structurized 3D heterojunction of RGO–MoS₂/pyramid Si is demonstrated via a simple solution‐processing method. Owing to the improved light absorption by the pyramid structure, the narrowed bandgap of the MoS₂ by the imperfect crystallinity, and the enhanced charge separation/transportation by the inserted reduced graphene oxide (RGO), the assembled photodetector exhibits excellent performance in terms of a large responsivity of 21.8 A W^?1, extremely high detectivity up to 3.8 × 10¹⁵ Jones (Jones = cm Hz^1/2 W^?1) and ultrabroad spectrum response ranging from 350 nm (ultraviolet) to 4.3 µm (midwave infrared). These device parameters represent the best results for MoS₂‐based self‐driven photodetectors, and the detectivity value sets a new record for the 2DMD‐based photodetectors reported thus far. Prospectively, the design of novel 3D heterojunction can be extended to other 2DMDs, opening up the opportunities for a host of high‐performance optoelectronic devices.     

Surfactant‐Concentration‐Dependent Shape Evolution of Au–Pd Alloy Nanocrystals from Rhombic Dodecahedron to Trisoctahedron and Hexoctahedron

The surface structure‐controlled synthesis of noble metal nanocrystals (NCs) bounded by high‐index facets has become a hot research topic due to their potential to significantly improve catalytic performance. This study reports the preparation of monodisperse Au–Pd alloy NCs with systematic shape evolution from rhombic dodecahedral (RD) to trisoctahedral (TOH), and hexoctahedral (HOH) structures by varying the concentration of surfactant in the surfactant‐mediated synthesis. The as‐prepared three kinds of alloy NCs possess almost the same size and composition as each other. It is suggested that the surfactant containing long‐chain octadecyltrimethyl ammonium (OTA⁺) ions plays a key role in the formation of high index facets, and the crystal growth kinetics may also have an effect on the formation of different nanocrystal morphologies. In addition, the catalytic activities of these NCs are evaluated by structure‐sensitive reactions, including ethanol electro‐oxidation and the catalytic reduction of 4‐nitrophenol (4‐NPh). These three types of Au–Pd alloy NCs exhibit different catalytic selectivities towards these two reactions. The catalytic activities toward electro‐oxidation of ethanol are in the order of HOH > RD > TOH, which follows the order of their corresponding surface energies. However, the activities toward catalytic reduction of 4‐NPh are in the order of RD > TOH > HOH, which should be related to the local structure of the surfaces.     

Alchemy‐Inspired Green Paper for Spontaneous Recovery of Noble Metals

Recycling of noble metal from waste materials, namely from electronic wastes (e‐waste), spent catalyst, and industrial wastewater, is attracting growing attention due to the scarcity, economic importance, and criticality of those noble metals. Traditional techniques reported to date require toxic reagent and strict extraction conditions, which deeply hinders the development of precious metal recovery in complex environments. Here, an approach is proposed that uses flexible metallic transition metal dichalcogenide (TMD) paper, which provides abundant active sites for spontaneous adsorption and reduction of noble metal ions, as an Alchemy‐inspired template to recover noble metal in an efficient and green way without the aid of reductant and heating. The metallic TMD (MoS₂, WS₂) paper is shown to rapidly extract five noble metal ions (Au, Pd, Pt, Ag, and Ru) from complex samples containing various interferents. This unique property endows the metallic TMD paper with gifted ability in extracting gold from e‐waste, and recovering platinum group metals (palladium and platinum) from spent catalysts, which provides a blueprint for the design of next‐generation green platforms for noble metal regeneration.     

Enhanced Performance of MoS2 Photodetectors by Inserting an ALD‐Processed TiO2 Interlayer

2D molybdenum disulfide (MoS₂) possesses excellent optoelectronic properties that make it a promising candidate for use in high‐performance photodetectors. Yet, to meet the growing demand for practical and reliable MoS₂ photodetectors, the critical issue of defect introduction to the interface between the exfoliated MoS₂ and the electrode metal during fabrication must be addressed, because defects deteriorate the device performance. To achieve this objective, the use of an atomic layer‐deposited TiO₂ interlayer (between exfoliated MoS₂ and electrode) is reported in this work, for the first time, to enhance the performance of MoS₂ photodetectors. The TiO₂ interlayer is inserted through 20 atomic layer deposition cycles before depositing the electrode metal on MoS₂/SiO₂ substrate, leading to significantly enhanced photoresponsivity and response speed. These results pave the way for practical applications and provide a novel direction for optimizing the interlayer material.     

Electrically Pumped White‐Light‐Emitting Diodes Based on Histidine‐Doped MoS2 Quantum Dots

MoS₂ quantum dots (QDs)‐based white‐light‐emitting diodes (QD‐WLEDs) are designed, fabricated, and demonstrated. The highly luminescent, histidine‐doped MoS₂ QDs synthesized by microwave induced fragmentation of 2D MoS₂ nanoflakes possess a wide distribution of available electronic states as inferred from the pronounced excitation‐wavelength‐dependent emission properties. Notably, the histidine‐doped MoS₂ QDs show a very strong emission intensity, which exceeds seven times of magnitude larger than that of pristine MoS₂ QDs. The strongly enhanced emission is mainly attributed to nitrogen acceptor bound excitons and passivation of defects by histidine‐doping, which can enhance the radiative recombination drastically. The enabled electroluminescence (EL) spectra of the QD‐WLEDs with the main peak around 500 nm are found to be consistent with the photoluminescence spectra of the histidine‐doped MoS₂ QDs. The enhanced intensity of EL spectra with the current increase shows the stability of histidine‐doped MoS₂ based QD‐WLEDs. The typical EL spectrum of the novel QD‐WLEDs has a Commission Internationale de l'Eclairage chromaticity coordinate of (0.30, 0.36) exhibiting an intrinsic broadband white‐light emission. The unprecedented and low‐toxicity QD‐WLEDs based on a single light‐emitting material can serve as an excellent alternative for using transition metal dichalcogenides QDs as next generation optoelectronic devices.     

Microlandscaping of Au Nanoparticles on Few‐Layer MoS2 Films for Chemical Sensing

Surface modification or decoration of ultrathin MoS₂ films with chemical moieties is appealing since nanointerfacing can functionalize MoS₂ films with bonus potentials. In this work, a facile and effective method for microlandscaping of Au nanoparticles (NPs) on few‐layer MoS₂ films is developed. This approach first employs a focused laser beam to premodify the MoS₂ films to achieve active surface domains with unbound sulfur. When the activated surface is subsequently immersed in AuCl₃ solution, Au NPs are found to preferentially decorate onto the modified regions. As a result, Au NPs can be selectively and locally anchored onto designated regions on the MoS₂ surface. With a scanning laser beam, microlandscapes comprising of Au NPs decorated on laser‐defined micropatterns are constructed. By varying the laser power, reaction time and thickness of the MoS₂ films, the size and density of the NPs can be controlled. The resulting hybrid materials are demonstrated as efficient Raman active surfaces for the detection of aromatic molecules with high sensitivity.     

Phosphorus‐Rich Colloidal Cobalt Diphosphide (CoP2) Nanocrystals for Electrochemical and Photoelectrochemical Hydrogen Evolution

Developing earth‐abundant and efficient electrocatalysts for photoelectrochemical water splitting is critical to realizing a high‐performance solar‐to‐hydrogen energy conversion process. Herein, phosphorus‐rich colloidal cobalt diphosphide nanocrystals (CoP₂ NCs) are synthesized via hot injection. The CoP₂ NCs show a Pt‐like hydrogen evolution reaction (HER) electrocatalytic activity in acidic solution with a small overpotential of 39 mV to achieve ?10 mA cm^?2 and a very low Tafel slope of 32 mV dec^?1. Density functional theory (DFT) calculations reveal that the high P content both physically separates Co atoms to prevent H from over binding to multiple Co atoms, while simultaneously stabilizing H adsorbed to single Co atoms. The catalytic performance of the CoP₂ NCs is further demonstrated in a metal–insulator–semiconductor photoelectrochemical device consisting of bottom p‐Si light absorber, atomic layer deposition Al–ZnO passivation layers, and the CoP₂ cocatalyst. The p‐Si/AZO/TiO₂/CoP₂ photocathode shows a photocurrent density of ?16.7 mA cm^?2 at 0 V versus reversible hydrogen electrode (RHE) and an output photovoltage of 0.54 V. The high performance and stability are attributed to the junction between p‐Si and AZO, the corrosion‐resistance of the pinhole‐free TiO₂ protective layer, and the fast HER kinetics of the CoP₂ NCs.     

Giant Enhancements of Perpendicular Magnetic Anisotropy and Spin‐Orbit Torque by a MoS2 Layer

2D transition metal dichalcogenides have attracted much attention in the field of spintronics due to their rich spin‐dependent properties. The promise of highly compact and low‐energy‐consumption spin‐orbit torque (SOT) devices motivates the search for structures and materials that can satisfy the requirements of giant perpendicular magnetic anisotropy (PMA) and large SOT simultaneously in SOT‐based magnetic memory. Here, it is demonstrated that PMA and SOT in a heavy metal/transition metal ferromagnet structure, Pt/[Co/Ni]₂, can be greatly enhanced by introducing a molybdenum disulfide (MoS₂) underlayer. According to first‐principles calculation and X‐ray absorption spectroscopy (XAS), the enhancement of the PMA is ascribed to the modification of the orbital hybridization at the interface of Pt/Co due to MoS₂. The enhancement of SOT by the role played by MoS₂ is explained, which is strongly supported by the identical behavior of SOT and PMA as a function of Pt thickness. This work provides new possibilities to integrate 2D materials into promising spintronics devices.     

A Fermi‐Level‐Pinning‐Free 1D Electrical Contact at the Intrinsic 2D MoS2–Metal Junction

Currently 2D crystals are being studied intensively for use in future nanoelectronics, as conventional semiconductor devices face challenges in high power consumption and short channel effects when scaled to the quantum limit. Toward this end, achieving barrier‐free contact to 2D semiconductors has emerged as a major roadblock. In conventional contacts to bulk metals, the 2D semiconductor Fermi levels become pinned inside the bandgap, deviating from the ideal Schottky–Mott rule and resulting in significant suppression of carrier transport in the device. Here, MoS₂ polarity control is realized without extrinsic doping by employing a 1D elemental metal contact scheme. The use of high‐work‐function palladium (Pd) or gold (Au) enables a high‐quality p‐type dominant contact to intrinsic MoS₂, realizing Fermi level depinning. Field‐effect transistors (FETs) with Pd edge contact and Au edge contact show high performance with the highest hole mobility reaching 330 and 432 cm² V^?1 s^?1 at 300 K, respectively. The ideal Fermi level alignment allows creation of p‐ and n‐type FETs on the same intrinsic MoS₂ flake using Pd and low‐work‐function molybdenum (Mo) contacts, respectively. This device acts as an efficient inverter, a basic building block for semiconductor integrated circuits, with gain reaching 15 at V_D = 5 V.