MoO3 Nanodots Decorated CdS Nanoribbons for High-Performance, Homojunction Photovoltaic Devices on Flexible Substrates

Highly efficient AgBr/h-MoO3 with charge separation tuning for photocatalytic degradation of trimethoprim: Mechanism insight and toxicity assessment

A highly solar active AgBr/h-MoO₃ composite was constructed by a facile precipitation method, and the charge separation tuning was achieved by photoreduction of AgBr. The photoreduced Ag⁰ on AgBr/h-MoO₃ acted as charge transfer bridge to form Z-scheme heterostructure, while the high degree of Ag reduction converted the material into type-II heterostructure. The synthesized optimal material promoted charge separation and visible light activity due to the incorporation of highly solar active AgBr, which showed ca. 2 times activity on trimethoprim (TMP) degradation than h-MoO₃. The contribution of reactive species on TMP degradation followed the order of O₂^- >¹O₂ > h⁺, which agree well with the proposed charge separation mechanism. The photocatalytic degradation mechanism of TMP was proposed based on the radical quenching, intermediate analysis and DFT calculation. The toxicity analysis based on QSAR calculation showed that part of the degradation intermediates are more toxic than TMP, thus sufficient mineralization are required to eliminate the potential risks of treated water. Moreover, the material showed high stability and activity after four reusing cycles, and it is applicable to treat contaminants in various water matrix. This work is expected to provide new insight into the charge separation tuning mechanism for the AgX based heterojunction, and rational design of highly efficient photocatalysts for organic contaminants degradation by solar irradiation.

Modulating the Electronic, Optical, and Transport Properties of CdTe and ZnTe Nanostructures with Organic Molecules: A Theoretical Investigation

In this paper, we systematically investigated the electronic, optical, and transport properties of CdTe and ZnTe nanostructures before and after adsorption with benzyl viologen (BV) and tetrafluoro-tetracyanoquinodimethane (F4-TCNQ) organic molecules based on the first principles calculation. First, the band gaps of CdTe and ZnTe nanostructures obviously decrease after BV and F4-TCNQ adsorptions. Interestingly, the electronic property calculation shows that BV and F4-TCNQ can donate/accept electrons to/from the surface of CdTe and ZnTe nanostructures, leading to an effective n-/p-type doping, respectively. Second, the optical absorption in a broad spectral range (from visible to near-infrared) of CdTe and ZnTe is significantly improved by adsorption of BV and F4-TCNQ molecules, offering great opportunities for the use of CdTe and ZnTe nanostructures in renewable energy fields. Lastly, the electrical transfer characteristics on CdTe and ZnTe nanostructure-based field-effect transistors clearly showed that the conduction of the nanostructures can be rationally tuned into n- and p-type conductivity with BV and F4-TCNQ adsorptions, respectively. Our work clearly demonstrates that the electronic, optical, and transport properties of CdTe and ZnTe nanostructures are effectively modulated by adsorption of BV and F4-TCNQ, which can be used to construct high-performance electronic and optoelectronic devices.

Theoretical study of a p-n homojunction SiGe field-effect transistor via covalent functionalization

p-n homojunctions are superior to p-n heterojunctions in constructing nanoscale functional devices, owing to the excellent crystallographic alignment. We tune the electronic properties of monolayer siligene (SiGe) into p/n-type via the covalent functionalization of electrophilic/nucleophilic dopants, using ab initio quantum transport calculations. It is found that the n-type doping effect of K atoms is stronger than that of benzyl viologen (BV) molecule on the surface of SiGe monolayer, owing to the strong covalent interaction. Both of p-type 2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane (F4TCNQ)-adsorbed and n-type 4 K-adsorbed SiGe systems show enhanced optical absorption in the infrared region, indicating their promising applications in infrared optoelectronic devices. By spatially adsorbing F4TCNQ molecule and K atoms on the source and drain leads, respectively, we designed a p-n homojunction SiGe field-effect transistor (FET). It is predicted that the built F4TCNQ-4K/SiGe FET can meet the requirements for high-performance (the high current density) and low-power (low subthreshold swing (SS)) applications, according to the International Technology Roadmap for Semiconductors in 2028. The present study gains some key insights into the importance of surface functionalization in constructing p-n homojunction electronic and optoelectronic devices based on monolayer SiGe.

Is Charge-Transfer Doping Possible at the Interfaces of Monolayer VSe2 with MoO3 and K?

Being a metallic transition-metal dichalcogenide, monolayer vanadium diselenide (VSe₂) exhibits many novel properties, such as charge density waves and magnetism. Its interfaces with other materials can potentially be used in device applications as well as for manipulating its intrinsic properties. Here, we present a scanning tunneling microscopy and synchrotron-based X-ray photoemission spectroscopy study of the surface charge-transfer doping using efficient electron-withdrawing and electron-donating materials, that is, molybdenum trioxide (MoO₃) and potassium (K), on the molecular beam epitaxy-grown monolayer VSe₂ on highly oriented pyrolytic graphite (HOPG). We demonstrate that monolayer VSe₂ is immune to MoO₃- and K-doping effects. However, at the monolayer edges where the local chemical reactivity is higher because of Se deficiency, MoO₃ is seen to react with VSe₂ to form molybdenum dioxide (MoO₂) and vanadium dioxide (VO₂). Compared to the obvious charge-transfer doping effects of MoO₃ and K on HOPG, the electronic structure of monolayer VSe₂ is barely perturbed. This is attributed to the large density of states at the Fermi level of monolayer VSe₂ carrying the metallic character. This work provides new insights into the chemical and electronic properties of monolayer VSe₂, important for future VSe₂-based electronic device design.

High-Performance Nanofloating Gate Memory Based on Lead Halide Perovskite Nanocrystals

Lead halide perovskites have been extensively investigated in a host of optoelectronic devices, such as solar cells, light-emitting diodes, and photodetectors. The halogen vacancy defects arising from the halogen-poor growth environment are normally regarded as an unfavorable factor to restrict the device performance. Here, for the first time, we demonstrate the utilization of the vacancy defects in lead halide perovskite nanostructures for achieving high-performance nanofloating gate memories (NFGMs). CH₃NH₃PbBr₃ nanocrystals (NCs) were uniformly decorated on the CdS nanoribbon (NR) surface via a facile dip-coating process, forming a CdS NR/CH₃NH₃PbBr₃ NC core-shell structure. Significantly, owing to the existence of sufficient carrier trapping states in CH₃NH₃PbBr₃ NCs, the hybrid device possessed an ultralarge memory window up to 77.4 V, a long retention time of 12 000 s, a high current ON/OFF ratio of 7 × 10⁷, and a long-term air stability for 50 days. The memory window of the device is among the highest for the low-dimensional nanostructure-based NFGMs. Also, this strategy shows good universality and can be extended to other perovskite nanostructures for the construction of high-performance NFGMs. This work paves the way toward the fabrication of new-generation, high-capacity nonvolatile memories using lead halide perovskite nanostructures.

Charge carrier-selective contacts for nanowire solar cells

Charge carrier-selective contacts transform a light-absorbing semiconductor into a photovoltaic device. Current record efficiency solar cells nearly all use advanced heterojunction contacts that simultaneously provide carrier selectivity and contact passivation. One remaining challenge with heterojunction contacts is the tradeoff between better carrier selectivity/contact passivation (thicker layers) and better carrier extraction (thinner layers). Here we demonstrate that the nanowire geometry can remove this tradeoff by utilizing a permanent local gate (molybdenum oxide surface layer) to control the carrier selectivity of an adjacent ohmic metal contact. We show an open-circuit voltage increase for single indium phosphide nanowire solar cells by up to 335 mV, ultimately reaching 835 mV, and a reduction in open-circuit voltage spread from 303 to 105 mV after application of the surface gate. Importantly, reference experiments show that the carriers are not extracted via the molybdenum oxide but the ohmic metal contacts at the wire ends.

Balancing the carrier selectivity and extraction by the selective contacts is of vital importance to the performance of the nanowire solar cells. Here Oener et al. employ a permanent local gate to overcome this tradeoff and substantially increase the open-circuit voltage by 335 mV.

Surface Charge Transfer Doping of Low-Dimensional Nanostructures toward High-Performance Nanodevices

Device applications of low-dimensional semiconductor nanostructures rely on the ability to rationally tune their electronic properties. However, the conventional doping method by introducing impurities into the nanostructures suffers from the low efficiency, poor reliability, and damage to the host lattices. Alternatively, surface charge transfer doping (SCTD) is emerging as a simple yet efficient technique to achieve reliable doping in a nondestructive manner, which can modulate the carrier concentration by injecting or extracting the carrier charges between the surface dopant and semiconductor due to the work-function difference. SCTD is particularly useful for low-dimensional nanostructures that possess high surface area and single-crystalline structure. The high reproducibility, as well as the high spatial selectivity, makes SCTD a promising technique to construct high-performance nanodevices based on low-dimensional nanostructures. Here, recent advances of SCTD are summarized systematically and critically, focusing on its potential applications in one- and two-dimensional nanostructures. Mechanisms as well as characterization techniques for the surface charge transfer are analyzed. We also highlight the progress in the construction of novel nanoelectronic and nano-optoelectronic devices via SCTD. Finally, the challenges and future research opportunities of the SCTD method are prospected.

Surface Charge Transfer Doping via Transition Metal Oxides for Efficient p-Type Doping of II-VI Nanostructures

Wide band gap II-VI nanostructures are important building blocks for new-generation electronic and optoelectronic devices. However, the difficulty of realizing p-type conductivity in these materials via conventional doping methods has severely handicapped the fabrication of p-n homojunctions and complementary circuits, which are the fundamental components for high-performance devices. Herein, by using first-principles density functional theory calculations, we demonstrated a simple yet efficient way to achieve controlled p-type doping on II-VI nanostructures via surface charge transfer doping (SCTD) using high work function transition metal oxides such as MoO₃, WO₃, CrO₃, and V₂O₅ as dopants. Our calculations revealed that these oxides were capable of drawing electrons from II-VI nanostructures, leading to accumulation of positive charges (holes injection) in the II-VI nanostructures. As a result, Fermi levels of the II-VI nanostructures were shifted toward the valence band regions after surface modifications, along with the large enhancement of work functions. In situ ultraviolet photoelectron spectroscopy and X-ray photoelectron spectroscopy characterizations verified the significant interfacial charge transfer between II-VI nanostructures and surface dopants. Both theoretical calculations and electrical transfer measurements on the II-VI nanostructure-based field-effect transistors clearly showed the p-type conductivity of the nanostructures after surface modifications. Strikingly, II-VI nanowires could undergo semiconductor-to-metal transition by further increasing the SCTD level. SCTD offers the possibility to create a variety of electronic and optoelectronic devices from the II-VI nanostructures via realization of complementary doping.

Symmetry breaking in semiconductor nanocrystals via kinetic-controlled surface diffusion: a strategy for manipulating the junction structure

The synthesis of semiconductor nanocrystals is usually limited to high-level symmetry, as constrained by the inherent, for example, face-centered cubic or hexagonal close-packed lattices of the crystals. Herein, we report a robust approach for breaking the symmetry of the CdS lattice and obtaining high-quality CdS ultrathin monopods, bipods, tripods, and tetrapods. The success relies on manipulating reaction kinetics by dropwise addition of a precursor solution, which permits deterministic control over the number of CdS monomers in the reaction solution. With rapid monomer supply by fast precursor injection, growth was restricted to only one {111} facet of the nascent CdS tetrahedron to produce an asymmetric ultrathin monopod (a zinc-blende tip with a wurtzite arm). Otherwise, growth monomers could access adjacent {111} facets through surface diffusion and thus lead to the switch of the growth pattern from asymmetric to symmetric to generate an ultrathin multipod (a zinc-blende tip/core with multi-wurtzite arms). These symmetry-controlled photocatalysts were characterized by a fine-tuned zinc blende-wurtzite intergrowth type-II homojunction. After evaluating their structure-dependent solar-hydrogen-production properties, the CdS ultrathin monopod with an appropriate length for controllable charge transportation showed the highest photocatalytic activity.

Surface Charge Transfer Doping of Monolayer Phosphorene via Molecular Adsorption

Monolayer phosphorene has attracted much attention owing to its extraordinary electronic, optical, and structural properties. Rationally tuning the electrical transport characteristics of monolayer phosphorene is essential to its applications in electronic and optoelectronic devices. Herein, we study the electronic transport behaviors of monolayer phosphorene with surface charge transfer doping of electrophilic molecules, including 2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane (F4TCNQ), NO2, and MoO3, using density functional theory combined with the nonequilibrium Green's function formalism. F4TCNQ shows optimal performance in enhancing the p-type conductance of monolayer phosphorene. Static electronic properties indicate that the enhancement is originated from the charge transfer between adsorbed molecule and phosphorene layer. Dynamic transport behaviors demonstrate that additional channels for hole transport in host monolayer phosphorene were generated upon the adsorption of molecule. Our work unveils the great potential of surface charge transfer doping in tuning the electronic properties of monolayer phosphorene and is of significance to its application in high-performance devices.