Quasi-2D Cu2 S crystals on graphene: in-situ growth and ab-initio calculations

Tunable Intrinsic Phonon Mode versus Anomalous Thermal Transport in Two-Dimensional Strongly Anharmonic Group IB Chalcogenides A2IBSe1/2Te1/2 (AIB = Cu, Ag, or Au)

Group IB chalcogenides, as promising thermoelectric materials, have ultralow thermal transport. Here, we propose a peculiar intrinsic B2 phonon mode that includes the in-plane rotational and stretching vibrations of metal atoms in two-dimensional A2IBSe1/2Te1/2 (AIB = Cu, Ag, or Au). The B2 mode is sensitive to the metal-atom mass, temperature, and strain for effectively tuning the lattice thermal conductivity. The in-plane stretching vibration leads to an unexpected increase in the lattice thermal conductivity from Cu to Ag and to Au systems, in contrast to Keyes' theory. The s(I) phase can be stabilized by the temperature-hardened B2 mode to reduce the lattice thermal conductivity, following the ∼T-0.59 instead of the traditional ∼T-1 trend. The s(II)-to-s(I) phase transition is driven by the strain-softened B2 mode to greatly enhance thermal transport via weakening the anharmonicity. Our work establishes the relationship of tunable intrinsic phonon mode versus thermal transport in two-dimensional group IB chalcogenides.

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.

Selective visible-light driven highly efficient photocatalytic reduction of CO2 to C2H5OH by two-dimensional Cu2S monolayers

Solar-driven highly-efficient photocatalytic reduction of CO2 into value-added fuels has been regarded as a promising strategy to assuage the current global warming and energy crisis, but developing highly product-selective, long-term stable and low-cost photocatalysts for C2 production remains a grand challenge. Herein, we demonstrate that two-dimensional β- and δ-phase Cu2S monolayers are promising photocatalysts for the reduction of CO2 into C2H5OH. The calculated potential-limiting steps for the CO2 reduction reaction (CO2RR) are less than 0.50 eV, while those for the hydrogen evolution reaction are as high as 1.53 and 0.87 eV. Most strikingly, the C-C coupling only needs to overcome an ultra-low kinetic barrier of ∼0.30 eV, half of that on the Cu surface, indicating that they can boost the C2H5OH conversion efficiency greatly. Besides, these catalysts also exhibit satisfactory band edge positions and suitable visible light absorption, rendering them ideal for the visible light driven CO2RR. Our work not only provides a promising photocatalyst for achieving the efficient and selective CO2RR, but also brings new opportunities for advanced sustainable C2H5OH product.

Localized surface electromagnetic waves in CrI3-based magnetophotonic structures

Resulting from strong magnetic anisotropy two-dimensional ferromagnetism was recently shown to be stabilized in chromium triiodide, CrI₃, in the monolayer limit. While its properties remain largely unexplored, it provides a unique material-specific platform to unveil its electromagnetic properties associated with coupling of modes. Indeed, trigonal symmetry in the presence of out-of-plane magnetization results in a non-trivial structure of the conductivity tensor, including the off-diagonal terms. In this paper, we study the surface electromagnetic waves localized in a CrI₃-based structure using the results of ab initio calculations for the CrI₃ conductivity tensor. In particular, we provide an estimate for the critical angle corresponding to the surface plasmon polariton generation in the Kretschmann-Raether configuration by a detailed investigation of reflectance spectrum as well as the magnetic field distribution for different CrI₃ layer thicknesses. We also study the bilayer structure formed by two CrI₃ layers separated by a SiO₂ spacer and show that the surface plasmon resonance can be achieved at the interface between CrI₃ and air depending on the spacer thickness.

Two-Dimensional Square-A2B (A = Cu, Ag, Au, and B = S, Se): Auxetic Semiconductors with High Carrier Mobilities and Unusually Low Lattice Thermal Conductivities

Using evolutionary structure search combined with ab initio theory, we investigate the electronic, thermal, and mechanical properties of two-dimensional (2D) A₂B (A = Cu, Ag, Au, and B = S, Se) auxetic semiconductors. Two types of structures are found to have low energy, namely, s(I/II)-A₂B, which have direct bandgaps in the range 1.09-2.60 eV and high electron mobilities. Among these semiconductors, Cu₂B and Ag₂B have light holes with 2 orders of magnitude larger mobility than the heavy holes, up to 9.51 × 10⁴ cm² V^-1 s^-1, giving the possibility of achieving highly anisotropic hole transport with the application of a uniaxial strain. Due to the ionic bonding nature, s-A₂B structures have unusually low lattice thermal conductivities down to 1.5 W m^-1 K^-1 at 300 K, which are quite promising for new generation thermoelectric devices. Besides, s-A₂B structures show extraordinary flexibility with ultralow Young's moduli (down to 20 N/m), which are lower than most previously reported 2D materials. Moreover, under strain along the diagonal direction, five of the structures have in-plane negative Poisson's ratios.

Ultralow lattice thermal conductivity induced high thermoelectric performance in the δ-Cu2S monolayer

Motivated by the recent experimental exfoliation of β-Cu2S thin films and the theoretical finding of a new phase labeled the δ-Cu2S monolayer, we carried out extensive studies on thermal conductivity and thermoelectric properties of the new phase using first principles combined with Boltzmann transport theory, focusing on the analysis of group velocities, Gruneisen parameters, three-phonon scattering rates, and the scattering phase space. Our results show that the δ-Cu2S monolayer exhibits an intrinsically ultralow lattice thermal conductivity of 0.10 W m-1 K-1 at 800 K. Such an ultralow lattice thermal conductivity leads to a high thermoelectric figure of merit ZT = 1.33 at 800 K in an optimum p-type doping concentration, which is not only larger than the value of 1.23 in In2S3 doped Cu2S at 850 K but also comparable with the value of 1.7 in Cu1.97S at 1000 K, exhibiting good potential in thermoelectric applications.

Copper(i) sulfide: a two-dimensional semiconductor with superior oxidation resistance and high carrier mobility

Two-dimensional (2D) semiconductors with suitable direct band gaps, high carrier mobility, and excellent open-air stability are especially desirable for material applications. Herein, we show theoretical evidence of a new phase of a copper(i) sulfide (Cu₂S) monolayer, denoted δ-Cu₂S, with both novel electronic properties and superior oxidation resistance. We find that both monolayer and bilayer δ-Cu₂S have much lower formation energy than the known β-Cu₂S phase. Given that β-Cu₂S sheets have been recently synthesized in the laboratory (Adv. Mater.2016, 28, 8271), the higher stability of δ-Cu₂S than that of β-Cu₂S sheets suggests a high possibility of experimental realization of δ-Cu₂S. Stability analysis indicates that δ-Cu₂S is dynamically and thermally stable. Notably, δ-Cu₂S exhibits superior oxidation resistance, due to the high activation energy of 1.98 eV for the chemisorption of O₂ on δ-Cu₂S. On its electronic properties, δ-Cu₂S is a semiconductor with a modest direct band gap (1.26 eV) and an ultrahigh electron mobility of up to 6880 cm² V^-1 s^-1, about 27 times that (246 cm² V^-1 s^-1) of the β-Cu₂S bilayer. The marked difference between the electron and hole mobilities of δ-Cu₂S suggests easy separation of electrons and holes for solar energy conversion. Combination of these novel properties makes δ-Cu₂S a promising 2D material for future applications in electronics and optoelectronics with high thermal and chemical stability.

Two-dimensional materials from high-throughput computational exfoliation of experimentally known compounds

Two-dimensional (2D) materials have emerged as promising candidates for next-generation electronic and optoelectronic applications. Yet, only a few dozen 2D materials have been successfully synthesized or exfoliated. Here, we search for 2D materials that can be easily exfoliated from their parent compounds. Starting from 108,423 unique, experimentally known 3D compounds, we identify a subset of 5,619 compounds that appear layered according to robust geometric and bonding criteria. High-throughput calculations using van der Waals density functional theory, validated against experimental structural data and calculated random phase approximation binding energies, further allowed the identification of 1,825 compounds that are either easily or potentially exfoliable. In particular, the subset of 1,036 easily exfoliable cases provides novel structural prototypes and simple ternary compounds as well as a large portfolio of materials to search from for optimal properties. For a subset of 258 compounds, we explore vibrational, electronic, magnetic and topological properties, identifying 56 ferromagnetic and antiferromagnetic systems, including half-metals and half-semiconductors.

Observation of photoluminescence from large-scale layer-controlled 2D ß-Cu2S synthesized by the vapor-phase sulfurization of copper thin films

Two-dimensional (2D) copper chalcogenides (Cu_2-x X where X = S, Se, Te) have had much attention regarding various applications due to their remarkable optical and electrical properties, abundance, and environmentally friendly natures. This work indicates that highly uniform Cu_2-x S (where 0 < x < 1) nanosheets can be obtained by the two-step method of Cu deposition by sputtering with precisely controlled and extremely low growth rate followed by vapor-phase sulfurization. The phase transformations of thin Cu_2-x S films upon the Cu seed layer thickness are investigated. A unique thickness-constrained synthesis process using vapor-phase sulfurization is employed here, which evolves from a vertical to lateral growth mechanism based on the optimization of the Cu seed layer thickness. Atomically thin 2D β-Cu₂S film was successfully synthesized using the thinnest Cu seed film. We have systematically investigated the phase- and thickness-dependent optical properties of Cu_2-x S films at room temperature. Micro-photoluminescence (PL) spectroscopy reveals that the 2D β-Cu₂S film possesses a direct band gap with an energy of 1.1 eV while the PL intensities are greatly suppressed in the multilayer Cu_2-x S (where 0 ≤ x < 1).

Large-Size 2D β-Cu2 S Nanosheets with Giant Phase Transition Temperature Lowering (120 K) Synthesized by a Novel Method of Super-Cooling Chemical-Vapor-Deposition

2D triangular β-Cu₂ S nanosheets with large size and high quality are synthesized by a novel method of super-cooling chemical-vapor-deposition. The phase transition of this 2D material from β-Cu₂ S to γ-Cu₂ S occurs at 258 K (-15 °C), and such transition temperature is 120 K lower than that of its bulk counterpart (about 378 K).