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Photocatalytic use of the neat graphitic carbon nitride, g-C 3 N 4 , is usually restricted by unsatisfactory efficiency due to low solar light absorption and rapid recombination of photogenerated charge carriers. The introduction of metal ions into g-C 3 N 4 structure can effectively reduce the recombination rate of photogenerated charges, expand the light absorption range and in consequence improve overall photocatalytic efficiency. The series of graphitic carbon nitride based materials, doped with metals cations (iron, bismuth, cobalt, zinc), has been prepared. The doping-caused changes of band gap energy, flat band potential, photocurrent generation, and photoactivity range have been studied using spectroscopic and electrochemical techniques. Moreover, based on the photocatalytic activity tests the efficiency of change recombination has been estimated. Presented studies proved multifarious positive effects of doping on g-C 3 N 4 , such as extended light absorption, an improved yield of electron–hole pairs separation, suppressed recombination of charge carriers, reduced electron transport resistance, and improved photocatalytic efficiency of dye decomposition.

Strong electron correlations lie at the origin of high-temperature superconductivity. Its essence is believed to be captured by the Fermi-Hubbard model of repulsively interacting fermions on a lattice. Here we report on the site-resolved observation of charge and spin correlations in the two-dimensional (2D) Fermi-Hubbard model realized with ultracold atoms. Antiferromagnetic spin correlations are maximal at half-filling and weaken monotonically upon doping. At large doping, nearest-neighbor correlations between singly charged sites are negative, revealing the formation of a correlation hole, the suppressed probability of finding two fermions near each other. As the doping is reduced, the correlations become positive, signaling strong bunching of doublons and holes, in agreement with numerical calculations. The dynamics of the doublon-hole correlations should play an important role for transport in the Fermi-Hubbard model.

The chemical doping of molecular semiconductors is based on electron-transfer reactions between the semiconductor and dopant molecules; here, the redox potential of the dopant is key to control the Fermi level of the semiconductor 1 , 2 . The tunability and reproducibility of chemical doping are limited by the availability of dopant materials and the effects of impurities such as water. Here we focused on proton-coupled electron-transfer (PCET) reactions, which are widely used in biochemical processes 3 , 4 ; their redox potentials depend on an easily handled parameter, that is, proton activity. We immersed p-type organic semiconductor thin films in aqueous solutions with PCET-based redox pairs and hydrophobic molecular ions. Synergistic reactions of PCET and ion intercalation resulted in efficient chemical doping of crystalline organic semiconductor thin films under ambient conditions. In accordance with the Nernst equation, the Fermi levels of the semiconductors were controlled reproducibly with a high degree of precision—a thermal energy of about 25 millielectronvolts at room temperature and over a few hundred millielectronvolts around the band edge. A reference-electrode-free, resistive pH sensor based on this method is also proposed. A connection between semiconductor doping and proton activity, a widely used parameter in chemical and biochemical processes, may help create a platform for ambient semiconductor processes and biomolecular electronics. Proton-coupled electron-transfer reactions are used to achieve efficient chemical doping of organic semiconductor thin films under ambient conditions, and a reference-electrode-free, resistive pH sensor based on the method is proposed.

Electrolyte-gated organic transistors offer low bias operation facilitated by direct contact of the transistor channel with an electrolyte. Their operation mode is generally defined by the dimensionality of charge transport, where a field-effect transistor allows for electrostatic charge accumulation at the electrolyte/semiconductor interface, whereas an organic electrochemical transistor (OECT) facilitates penetration of ions into the bulk of the channel, considered a slow process, leading to volumetric doping and electronic transport. Conducting polymer OECTs allow for fast switching and high currents through incorporation of excess, hygroscopic ionic phases, but operate in depletion mode. Here, we show that the use of glycolated side chains on a thiophene backbone can result in accumulation mode OECTs with high currents, transconductance, and sharp subthreshold switching, while maintaining fast switching speeds. Compared with alkylated analogs of the same backbone, the triethylene glycol side chains shift the mode of operation of aqueous electrolyte-gated transistors from interfacial to bulk doping/transport and show complete and reversible electrochromism and high volumetric capacitance at low operating biases. We propose that the glycol side chains facilitate hydration and ion penetration, without compromising electronic mobility, and suggest that this synthetic approach can be used to guide the design of organic mixed conductors.

The efficiency with which polymeric semiconductors can be chemically doped—and the charge carrier densities that can thereby be achieved—is determined primarily by the electrochemical redox potential between the π-conjugated polymer and the dopant species 1 , 2 . Thus, matching the electron affinity of one with the ionization potential of the other can allow effective doping 3 , 4 . Here we describe a different process—which we term ‘anion exchange’—that might offer improved doping levels. This process is mediated by an ionic liquid solvent and can be pictured as the effective instantaneous exchange of a conventional small p-type dopant anion with a second anion provided by an ionic liquid. The introduction of optimized ionic salt (the ionic liquid solvent) into a conventional binary donor–acceptor system can overcome the redox potential limitations described by Marcus theory 5 , and allows an anion-exchange efficiency of nearly 100 per cent. As a result, doping levels of up to almost one charge per monomer unit can be achieved. This demonstration of increased doping levels, increased stability and excellent transport properties shows that anion-exchange doping, which can use an almost infinite selection of ionic salts, could be a powerful tool for the realization of advanced molecular electronics. The limitations of conventional chemical doping of polymeric semiconductors can be overcome by adding a second ionic species into the system, leading to enhanced doping, electrical conductivity and stability.

A key actor in the conventional theory of superconductivity is the induced interaction between electrons mediated by the exchange of virtual collective fluctuations (phonons in the case of conventional s-wave superconductors). Other collective modes that can play the same role, especially spin fluctuations, have been widely discussed in the context of high-temperature and heavy Fermion superconductors. The strength of such collective fluctuations is measured by the associated susceptibility. Here we use differential elastoresistance measurements from five optimally doped iron-based superconductors to show that divergent nematic susceptibility appears to be a generic feature in the optimal doping regime of these materials. This observation motivates consideration of the effects of nematic fluctuations on the superconducting pairing interaction in this family of compounds and possibly beyond.

Since the demonstration of p-type gallium nitride (GaN) through doping with substitutional magnesium (Mg) atoms 1 , 2 , rapid and comprehensive developments, such as blue light-emitting diodes, have considerably shaped our modern lives and contributed to a more carbon-neutral society 3 – 5 . However, the details of the interplay between GaN and Mg have remained largely unknown 6 – 11 . Here we observe that Mg-intercalated GaN superlattices can form spontaneously by annealing a metallic Mg film on GaN at atmospheric pressure. To our knowledge, this marks the first instance of a two-dimensional metal intercalated into a bulk semiconductor, with each Mg monolayer being intricately inserted between several monolayers of hexagonal GaN. Characterized as an interstitial intercalation, this process induces substantial uniaxial compressive strain perpendicular to the interstitial layers. Consequently, the GaN layers in the Mg-intercalated GaN superlattices exhibit an exceptional elastic strain exceeding −10% (equivalent to a stress of more than 20 GPa), among the highest recorded for thin-film materials 12 . The strain alters the electronic band structure and greatly enhances hole transport along the compression direction. Furthermore, the Mg sheets induce a unique periodic transition in GaN polarity, generating polarization-field-induced net charges. These characteristics offer fresh insights into semiconductor doping and conductivity enhancement, as well as into elastic strain engineering of nanomaterials and metal–semiconductor superlattices 13 . The spontaneous formation of magnesium-intercalated gallium nitride superlattices by the interstitial intercalation of two-dimensional magnesium results in considerable compressive strain perpendicular to the layers, leading to enhanced hole transport.

2D semiconductors, particularly transition metal dichalcogenides (TMDs), have emerged as highly promising for new electronic technologies. However, a key challenge in fabricating devices out of 2D semiconductors is the need for ultra-clean contacts with resistances approaching the quantum limit. The lack of high-quality, low-contact-resistance P-type and N-type contacts on 2D TMDs has limited progress towards the next generation of low-power devices, such as the tunnel field-effect transistors. In this Expert Recommendation, we summarize strategies and provide guidance for making clean van der Waals contacts on monolayered TMD semiconductors. We also discuss the physics of contacts in 2D semiconductors and prospects for achieving quantum conductance.A key challenge in fabricating devices out of 2D materials is in making good electrical contact. This Expert Recommendation discusses the physics of electrical contacts and provides tips on improving contact quality.

The anomalous Hall effect is a fundamental transport process in solids arising from the spin-orbit coupling. In a quantum anomalous Hall insulator, spontaneous magnetic moments and spin-orbit coupling combine to give rise to a topologically nontrivial electronic structure, leading to the quantized Hall effect without an external magnetic field. Based on first-principles calculations, we predict that the tetradymite semiconductors Bi₂Te₃, Bi₂Se₃, and Sb₂Te₃ form magnetically ordered insulators when doped with transition metal elements (Cr or Fe), in contrast to conventional dilute magnetic semiconductors where free carriers are necessary to mediate the magnetic coupling. In two-dimensional thin films, this magnetic order gives rise to a topological electronic structure characterized by a finite Chern number, with the Hall conductance quantized in units of e²/h (where e is the charge of an electron and h is Planck's constant).

The conversion efficiency of heat to electricity in thermoelectric materials depends on both their thermopower and electrical conductivity. It is now reported that, unlike their inorganic counterparts, organic thermoelectric materials show an improvement in both these parameters when the volume of dopant elements is minimized; furthermore, a high conversion efficiency is achieved in PEDOT:PSS blends. Significant improvements to the thermoelectric figure of merit ZT have emerged in recent years, primarily due to the engineering of material composition and nanostructure in inorganic semiconductors 1 (ISCs). However, many present high- ZT materials are based on low-abundance elements that pose challenges for scale-up, as they entail high material costs in addition to brittleness and difficulty in large-area deposition. Here we demonstrate a strategy to improve ZT in conductive polymers and other organic semiconductors (OSCs) for which the base elements are earth-abundant. By minimizing total dopant volume, we show that all three parameters constituting ZT vary in a manner so that ZT increases; this stands in sharp contrast to ISCs, for which these parameters have trade-offs. Reducing dopant volume is found to be as important as optimizing carrier concentration when maximizing ZT in OSCs. Implementing this strategy with the dopant poly(styrenesulphonate) in poly(3,4-ethylenedioxythiophene), we achieve Z T = 0.42 at room temperature.