Origin of novel diffusions of Cu and Ag in semiconductors: the case of CdTe

Enhanced environmental adaptability of sandwich-like MoS2/Ag/WC nanomultilayer films via Ag nanoparticle diffusion-dominated defect repair

Robustness and environmental adaptability are crucial for molybdenum disulfide (MoS2) films to minimize friction and wear in industrial applications. However, current sputtered MoS2 films suffer from inherent defects, including insufficient hardness, poor crystallinity, and susceptibility to oxidation, thereby limiting their longevity and reliability. Here, we present a sandwich-like nanomultilayer architecture comprising alternating MoS2 and tungsten carbide (WC) layers integrated with Ag nanoparticles. This architecture demonstrates robust corrosion resistance, effectively protecting the MoS2 within the film for over 18 months of air exposure and exhibiting minimal corrosion during 21 days of salt spray tests. The remarkable environmental stability of the sandwich-like MoS2/Ag/WC nanomultilayer film is attributed to the creation of numerous heterogeneous interfaces and the spontaneous diffusion and repair of Ag atoms through defect channels of the film, impeding the penetration of corrosive agents. Furthermore, during the frictional process, Ag, characterized by its inherent high mobility and ductility, facilitates the formation of a dense tribofilm on its counterpart ball, encapsulating formed metal oxides to prevent adhesive wear. As a result, the film exhibits a significantly reduced wear rate (1.25 × 10-7 mm3 N-1 m-1) even after long-term salt spray corrosion and air exposure. This study offers a general route for designing MoS2-based materials toward long-lifetime and environmental adaptability via self-repair mechanisms.

Dual-Driven Ion/Electron Migration and Sodium Storage by In Situ Introduction of Copper Ions and a Carbon-Conductive Framework in a Tin-Based Sulfide Anode

Tin sulfide (SnS) has emerged as a promising anode material for sodium ion batteries (SIBs) due to its high theoretical capacity and large interlayer spacing. However, several challenges, such as severe insufficient electrochemical reactivity, rapid capacity degradation, and poor rate performance, still hinder its application in SIBs. In this study, in situ introduction of copper ions and a carbon conductive framework to form SnS nanocrystals embedded in a Cu2SnS3 lamellar structure heterojunction composite (SnS/Cu2SnS3/RGO) with graphene as the supporting material is proposed to achieve dual-driven sodium ion/electron migration during the continuous electrochemical process. The designed structure facilitates the preferential electrochemical reduction of copper ions into copper nanocrystals during the discharge process and functions as a catalytically active center to promote multivalence tin sodiation reaction. Furthermore, during the charging process, the presence of copper nanocrystals also facilitates efficient desodiation of NaxSn and further activates to form higher valence state sulfides. As a result, the SnS/Cu2SnS3/RGO composite demonstrates high cycling stability with a high reversible capacity of 395 mAh g-1 at 5A g-1 after 500 cycles with a capacity retention of 85.6%. In addition, the assembled Na3V2(PO4)3∥SnS/Cu2SnS3/RGO sodium ion full cell achieves 93.7% capacity retention after 80 cycles at 0.5 A g-1.

General synthesis of ionic-electronic coupled two-dimensional materials

Two-dimensional (2D) AMX2 compounds are a family of mixed ionic and electronic conductors (where A is a monovalent metal ion, M is a trivalent metal, and X is a chalcogen) that offer a fascinating platform to explore intrinsic coupled ionic-electronic properties. However, the synthesis of 2D AMX2 compounds remains challenging due to their multielement characteristics and various by-products. Here, we report a separated-precursor-supply chemical vapor deposition strategy to manipulate the chemical reactions and evaporation of precursors, facilitating the successful fabrication of 20 types of 2D AMX2 flakes. Notably, a 10.4 nm-thick AgCrS2 flake shows superionic behavior at room temperature, with an ionic conductivity of 192.8 mS/cm. Room temperature ferroelectricity and reconfigurable positive/negative photovoltaic currents have been observed in CuScS2 flakes. This study not only provides an effective approach for the synthesis of multielement 2D materials with unique properties, but also lays the foundation for the exploration of 2D AMX2 compounds in electronic, optoelectronic, and neuromorphic devices.

Enhanced Anharmonicity by Forming Low-Symmetry Off-Center Phase: The Case of Two-Dimensional Group-IB Chalcogenides

Enhanced anharmonicity is required to achieve many interesting phenomena in thermoelectricity, superconductivity, ferroelectricity, etc. Here, we propose a novel mechanism for enhancing anharmonicity by forming the low-symmetry off-center ground state, such as the s(II) phase, in two-dimensional A^IB₂X chalcogenides (A^IB = Cu, Ag and Au; X = S, Se, and Te). In this system, the in-plane rotational phonon mode introduces a much stronger anharmonicity in the distorted s(II) phase than in the nondistorted s(I) phase. We show that the stabilities of the s(I) and s(II) phases arise from the ionicity and the ionic size; for example, the low ionicity and the small ionic size favor the s(II) phase. We further demonstrate that the anharmonicity can be tuned by controlling the strain-induced s(II)-to-s(I) phase transition, which explains the anomalous lattice thermal conductivity. Our work relates anharmonicity to symmetry-breaking structural distortion and widens the ways to design excellent thermoelectric materials.

Excellent Thermoelectric Performance of 2D CuMN2 (M = Sb, Bi; N = S, Se) at Room Temperature

2D copper-based semiconductors generally possess low lattice thermal conductivity due to their strong anharmonic scattering and quantum confinement effect, making them promising candidate materials in the field of high-performance thermoelectric devices. In this work, we proposed four 2D copper-based materials, namely CuSbS₂, CuSbSe₂, CuBiS₂, and CuBiSe₂. Based on the framework of density functional theory and Boltzmann transport equation, we revealed that the monolayers possess high stability and narrow band gaps of 0.57~1.10 eV. Moreover, the high carrier mobilities (10²~10³ cm²·V^-1·s^-1) of these monolayers lead to high conductivities (10⁶~10⁷ Ω^-1·m^-1) and high-power factors (18.04~47.34 mW/mK²). Besides, as the strong phonon-phonon anharmonic scattering, the monolayers also show ultra-low lattice thermal conductivities of 0.23~3.30 W/mK at 300 K. As results show, all the monolayers for both p-type and n-type simultaneously show high thermoelectric figure of merit (ZT) of about 0.91~1.53 at room temperature.

Intermediate range structure of amorphous Cu2GeTe3: ab initio molecular dynamics study

We have investigated the intermediate range structure of amorphous Cu₂GeTe₃ based on ab initio molecular dynamics simulations. The highest population of ring size is three, which makes the triangle structure. This ring consists of mainly Cu₂Te. Rings may also consist of CuCuCu, Cu₂Ge, and CuGeTe, where approximately 88% of Cu atoms in the system are related with the three-membered ring. The second highest population of ring size is five. Three- and five-membered rings in the amorphous phase originate from six-membered ring in the crystalline phase. This situation can enhance the phase transition between crystalline and amorphous phases. In the phase change process, Cu atoms may diffuse in the amorphous state with changing bonds. The diffusion coefficient of Cu D _Cu is estimated to be approximately 0.12 × 10^-9 m² s^-1. Such high diffusion coefficient of Cu atoms is contributed from only 10% of Cu atoms in the amorphous phase.

The influence of electrode for electroluminescence devices based on all-inorganic halide perovskite CsPbBr3

Electroluminescence devices based on all-inorganic halide perovskite material with excellent luminescence performance have been studied extensively in recent years. However, the important role for the electrodes of electroluminescence devices is payed few attention by theoretical and experimental studies. Appropriate electrodes can reduce the Schottky barrier height to decrease the energy loss, and prevent the metal impurities from diffusing into the perovskite material to generate deep traps levels, which improves the luminous efficiency and lifetime of devices. In this paper, not only the interface effects between CsPbBr₃ and common metal electrode (Ag, Au, Ni, Cu and Pt) are studied by first-principle calculations, but also the diffusion effects of metal electrode atom into the CsPbBr₃ layer are also explored by nudged elastic band calculations. The calculated results show the metal Ag is more suitable for the cathode for CsPbBr₃ electroluminescence devices, while the metal Pt is more applicable for the anode. Based on the overall consideration about the interface effects and diffusion effects of the CsPbBr₃-metal electrode junctions, the essential principle is analyzed. The work provides theoretical guidance for how to select the right electrode for the electroluminescence performance of all-inorganic halide perovskite. The critical factor of Schottky barrier height between the electrode and the light-emitting semiconductor, and transition level generated by metal impurities also provide a valuable reference how to select the suitable electrodes for other electroluminescence devices.

Origin of the Distinct Diffusion Behaviors of Cu and Ag in Covalent and Ionic Semiconductors

It is well known that Cu diffuses faster than Ag in covalent semiconductors such as Si, which has prevented the replacement of Ag by Cu as a contact material in Si solar cells for reducing the cost. Surprisingly, in more ionic materials such as CdTe, Ag diffuses faster than Cu despite that it is larger than Cu, which has prevented the replacement of Cu by Ag in CdTe solar cells to improve the performance. But, so far, the mechanisms behind these distinct diffusion behaviors of Cu and Ag in covalent and ionic semiconductors have not been addressed. Here we reveal the underlying mechanisms by combining the first-principles calculations and group theory analysis. We find that the symmetry controlled s-d coupling plays a critical role in determining the diffusion behaviors. The s-d coupling is absent in pure covalent semiconductors but increases with the ionicity of the zinc blende semiconductors, and is larger for Cu than for Ag, owing to its higher d orbital energy. In conjunction with Coulomb interaction and strain energy, the s-d coupling is able to explain all the diffusion behaviors from Cu to Ag and from covalent to ionic hosts. This in-depth understanding enables us to engineer the diffusion of impurities in various semiconductors.

Synthesis and enhanced fluorescence of Ag doped CdTe semiconductor quantum dots

Doping with intentional impurities is an intriguing way to tune the properties of semiconductor nanocrystals. However, the synthesis of some specific doped semiconductor nanocrystals remains a challenge and the doping mechanism in this strongly confined system is still not clearly understood. In this work, we report, for the first time, the synthesis of stable and water-soluble Ag-doped CdTe semiconductor quantum dots (SQDs) via a facile aqueous approach. Experimental characterization demonstrated the efficient doping of the Ag impurities into the CdTe SQDs with an appropriate reaction time. By doping 0.3% Ag impurities, the Stokes shift is decreased by 120 meV, the fluorescence intensity is enhanced more than 3 times, the radiative rate is enhanced 4.2 times, and the non-radiative rate is efficiently suppressed. These observations reveal that the fluorescence enhancement in Ag-doped CdTe SQDs is mainly attributed to the minimization of surface defects, filling of the trap states, and the enhancement of the radiative rate by the silver dopants. Our results suggest that the silver doping is an efficient method for tuning the optical properties of the CdTe SQDs.