Cuprous halides semiconductors as a new means for highly efficient light-emitting diodes

The Boundary between Two Modes of Gas Evolution: Oscillatory (H2 and O2) and Conventional Redox (O2 Only), in the Hydrocarbon/H2O2/Cu(II)/CH3CN System

During the oxidation of hydrocarbons using hydrogen peroxide solutions, the evolution of gaseous oxygen is a side and undesirable process, in which the consumption of the oxidizer is not associated with the formation of target products. Therefore, no attention is paid to the systematic study of the chemical composition of the gas and the mechanisms of its formation. Filling this gap, the authors discovered a number of new, previously unidentified, interesting facts concerning both gas evolution and the oxidation of hydrocarbons. In a 33% H2O2/Cu2Cl4·2DMG/CH3CN system, where DMG is dimethylglyoxime (Butane-2,3-dione dioxime), and is at 50 °C, evidence of significant evolution of gaseous hydrogen, along with the evolution of gaseous oxygen was found. In the authors’ opinion, which requires additional verification, the ratio of gaseous hydrogen and oxygen in the discussed catalytic system can reach up to 1:1. The conditions in which only gaseous oxygen is formed are selected. Using a number of oxidizable hydrocarbons with the first adiabatic ionization potentials (AIPs) of a wide range of values, it was found that the first stage of such a process of evolving only gaseous oxygen was the single electron transfer from hydrogen peroxide molecules to trinuclear copper clusters with the formation, respectively, of hydrogen peroxide radical cations H2O2•+ and radical anions Cu3Cl5•− (AIP = 5 eV). When the conditions for the implementation of such a single electron transfer mechanism are exhausted, the channel of decomposition of hydrogen peroxide molecules into gaseous hydrogen and oxygen is switched on, which is accompanied by the transition of the system to an oscillatory mode of gas evolution. In some cases, the formation of additional amounts of gaseous products is provided by the catalytically activated decomposition of water molecules into hydrogen and oxygen after the complete consumption of hydrogen peroxide molecules in the reaction of gaseous oxygen evolution. The adiabatic electron affinity of various forms of copper molecules involved in chemical processes is calculated by the density functional theory method.

AACVD of Cu3N on Al2O3 Using CuCl2 and NH3

Cu₃N has been grown on m-Al₂O₃ by aerosol-assisted chemical vapor deposition using 0.1 M CuCl₂ in CH₃CH₂OH under an excess of NH₃ at 600 °C, which led to the deposition of Cu that was subsequently converted into Cu₃N under NH₃: O₂ at 400 °C in a two-step process without exposure to the ambient. The reaction of CuCl₂ with an excess of NH₃ did not lead to the growth of Cu₃N, which is different to the case of halide vapor phase epitaxy of III-V semiconductors. The Cu₃N layers obtained in this way had an anti-ReO₃ cubic crystal structure with a lattice constant of 3.8 Å and were found to be persistently n-type, with a room temperature carrier density of n = 2 × 10¹⁶ cm^-3 and mobility of µ_n = 32 cm²/Vs. The surface depletion, calculated in the effective mass approximation, was found to extend over ~0.15 µm by considering a surface barrier height of ϕ_B = 0.4 eV related to the formation of native Cu₂O.

Toward Exotic Layered Materials: 2D Cuprous Iodide

Heterostructures composed of 2D materials are already opening many new possibilities in such fields of technology as electronics and magnonics, but far more could be achieved if the number and diversity of 2D materials were increased. So far, only a few dozen 2D crystals have been extracted from materials that exhibit a layered phase in ambient conditions, omitting entirely the large number of layered materials that may exist at other temperatures and pressures. This work demonstrates how such structures can be stabilized in 2D van der Waals (vdw) stacks under room temperature via growing them directly in graphene encapsulation by using graphene oxide as the template material. Specifically, an ambient stable 2D structure of copper and iodine, a material that normally only occurs in layered form at elevated temperatures between 645 and 675 K, is produced. The results establish a simple route to the production of more exotic phases of materials that would otherwise be difficult or impossible to stabilize for experiments in ambient.

Engineering Copper Iodide (CuI) for Multifunctional p-Type Transparent Semiconductors and Conductors

Developing transparent p-type semiconductors and conductors has attracted significant interest in both academia and industry because metal oxides only show efficient n-type characteristics at room temperature. Among the different candidates, copper iodide (CuI) is one of the most promising p-type materials because of its widely adjustable conductivity from transparent electrodes to semiconducting layers in transistors. CuI can form thin films with high transparency in the visible light region using various low-temperature deposition techniques. This progress report aims to provide a basic understanding of CuI-based materials and recent progress in the development of various devices. The first section provides a brief introduction to CuI with respect to electronic structure, defect states, charge transport physics, and overviews the CuI film deposition methods. The material design concepts through doping/alloying approaches to adjust the optoelectrical properties are also discussed in the first section. The following section presents recent advances in state-of-the-art CuI-based devices, including transparent electrodes, thermoelectric devices, p-n diodes, p-channel transistors, light emitting diodes, and solar cells. In conclusion, current challenges and perspective opportunities are highlighted.

Accuracy trade-off between one-electron and excitonic spectra of cuprous halides in first-principles calculations

Because of the sophisticated error cancellation in the density functional theory (DFT)-based calculations, a theoretically more accurate input would not guarantee a better output. In this work, our first-principles GW plus Bethe-Salpeter equation calculations using pseudopotentials show that cuprous halides (CuCl and CuBr) are such extreme cases for which a better one-electron band is not accompanied with a better exciton binding energy. Moreover, we find that the exchange interaction of Cu core electrons plays a crucial role in their ground-state electronic properties, especially in the energy gap and macroscopic dielectric constant. Our work provides new insights into the understanding of the electronic structure of cuprous halides from the DFT perspective.

Intrinsically p-type cuprous iodide semiconductor for hybrid light-emitting diodes

Cuprous halides, characterized by a direct wide band-gap and a good lattice matching with Si, is an intrinsic p-type I-VII compound semiconductor. It shows remarkable optoelectronic properties, including a large exciton binding energy at room temperature and a very small piezoelectric coefficient. The major obstacle to its application is the difficulty in growing a single-crystal epitaxial film of cuprous halides. We first demonstrate the single crystal epitaxy of high quality cuprous iodide (CuI) film grown on Si and sapphire substrates by molecular beam epitaxy. Enhanced photoluminescence on the order of magnitude larger than that of GaN and continuous-wave optically pumped lasing were found in MBE grown CuI film. The intrinsic p-type characteristics of CuI were confirmed using an n-AlGaN/p-CuI junction that emits blue light. The discovery will provide an alternative way towards highly efficient optoelectronic devices compatible with both Si and III-nitride technologies.

Violet Light-Emitting Diodes Based on p-CuI Thin Film/n-MgZnO Quantum Dot Heterojunction

As the lighting technology evolves, the need for violet light-emitting diodes (LEDs) is growing for high color rendering index lighting. The present technology for violet LEDs is based on the high-cost GaN materials and metal-organic chemical vapor deposition process; therefore, there have recently been intensive studies on developing low-cost alternative materials and processes. In this study, for the first time, we demonstrated violet LEDs based on low-cost materials and processes using a p-CuI thin film/n-MgZnO quantum dot (QD) heterojunction. The p-CuI thin film layer was prepared by an iodination process of Cu films, and the n-MgZnO layer was deposited by spin-coating presynthesized n-MgZnO QDs. To maximize the performance of the violet LED, an optimizing process was performed for each layer of p- and n-type materials. The optimized LED with 1 × 1 mm²-area pixel fabricated using the p-CuI thin film at the iodination temperature of 15 °C and the n-MgZnO QDs at the Mg alloying concentration of 2.7 at. % exhibited the strongest violet emissions at 6 V.

Introduction of TiO2 in CuI for Its Improved Performance as a p-Type Transparent Conductor

The challenges of making high-performance, low-temperature processed, p-type transparent conductors (TCs) have been the main bottleneck for the development of flexible transparent electronics. Though a few p-type transparent conducting oxides (TCOs) have shown promising results, they need high processing temperature to achieve the required conductivity which makes them unsuitable for organic and flexible electronic applications. Copper iodide is a wide band gap p-type semiconductor that can be heavily doped at low temperature (<100 °C) to achieve conductivity comparable or higher than many of the well-established p-type TCOs. However, as-processed CuI loses its transparency and conductivity with time in an ambient condition which makes them unsuitable for long-term applications. Herein, we propose CuI-TiO₂ composite thin films as a replacement of pure CuI. We show that the introduction of TiO₂ in CuI makes it more stable in ambient conditions while also improving its conductivity and transparency. A detailed comparative analysis between CuI and CuI-TiO₂ composite thin films has been performed to understand the reasons for improved conductivity, transparency, and stability of CuI-TiO₂ samples in comparison to pure CuI samples. The enhanced conductivity in CuI-TiO₂ stems from the highly conductive space-charge layer formation at the CuI-TiO₂ interface, whereas the improved transparency is due to reduced CuI grain growth mobility in the presence of TiO₂. The improved stability of CuI-TiO₂ in comparison to pure CuI is a result of inhibited recrystallization and grain growth, reduced loss of iodine, and limited oxidation of the CuI phase in the presence of TiO₂. For optimized fraction of TiO₂, an average transparency of ∼78% (in 450-800 nm region) and a resistivity of 14 mΩ·cm are achieved, while maintaining a relatively high mobility of ∼3.5 cm² V^-1 s^-1 with hole concentration reaching as high as 1.3 × 10²⁰ cm^-3. Most importantly, this work opens up the possibility to design a new range of p-type transparent conducting materials using the CuI/insulator composite system such as CuI/SiO₂, CuI/Al₂O₃, CuI/SiN_x, and so forth.

Low Cost Fabrication of Si NWs/CuI Heterostructures

In this paper, we present the realization by a low cost approach compatible with silicon technology of new nanostructures, characterized by the presence of different materials, such as copper iodide (CuI) and silicon nanowires (Si NWs). Silicon is the principal material of the microelectronics field for its low cost, easy manufacturing and market stability. In particular, Si NWs emerged in the literature as the key materials for modern nanodevices. Copper iodide is a direct wide bandgap p-type semiconductor used for several applications as a transparent hole conducting layers for dye-sensitized solar cells, light emitting diodes and for environmental purification. We demonstrated the preparation of a solid system in which Si NWs are embedded in CuI material and the structural, electrical and optical characterization is presented. These new combined Si NWs/CuI systems have strong potentiality to obtain new nanostructures characterized by different doping, that is strategic for the possibility to realize p-n junction device. Moreover, the combination of these different materials opens the route to obtain multifunction devices characterized by promising absorption, light emission, and electrical conduction.