SIMES Publications

Abstract

We explore adiabatic pumping in the presence of a periodic drive, finding a new phase in which the topologically quantized pumped quantity is energy rather than charge. The topological invariant is given by the winding number of the micromotion with respect to time within each cycle, momentum, and adiabatic tuning parameter. We show numerically that this pump is highly robust against both disorder and interactions, breaking down at large values of either in a manner identical to the Thouless charge pump. Finally, we suggest experimental protocols for measuring this phenomenon.

Abstract

Ultrathin transition metal dichalcogenides (TMDCs) have recently been extensively investigated to understand their electronic and optical properties. Here we study ultrathin Mo0.91W0.09Te2, a semiconducting alloy of MoTe2, using Raman, photoluminescence (PL), and optical absorption spectroscopy. Mo0.91W0.09Te2 transitions from an indirect to a direct optical band gap in the limit of monolayer thickness, exhibiting an optical gap of 1.10 eV, very close to its MoTe2 counterpart. We apply tensile strain, for the first time, to monolayer MoTe2 and Mo0.91W0.09Te2 to tune the band structure of these materials; we observe that their optical band gaps decrease by 70 meV at 2.3% uniaxial strain. The spectral widths of the PL peaks decrease with increasing strain, which we attribute to weaker exciton–phonon intervalley scattering. Strained MoTe2 and Mo0.91W0.09Te2extend the range of band gaps of TMDC monolayers further into the near-infrared, an important attribute for potential applications in optoelectronics.

Abstract

Doped semiconductors are the most important building elements for modern electronic devices¹. In silicon-based integrated circuits, facile and controllable fabrication and integration of these materials can be realized without introducing a high-resistance interface^2,3. Besides, the emergence of two-dimensional (2D) materials enables the realization of atomically thin integrated circuits^4,5,6,7,8,9. However, the 2D nature of these materials precludes the use of traditional ion implantation techniques for carrier doping and further hinders device development¹⁰. Here, we demonstrate a solvent-based intercalation method to achieve p-type, n-type and degenerately doped semiconductors in the same parent material at the atomically thin limit. In contrast to naturally grown n-type S-vacancy SnS₂, Cu intercalated bilayer SnS₂ obtained by this technique displays a hole field-effect mobility of ~40 cm² V⁻¹s⁻¹, and the obtained Co-SnS₂ exhibits a metal-like behaviour with sheet resistance comparable to that of few-layer graphene⁵. Combining this intercalation technique with lithography, an atomically seamless p–n–metal junction could be further realized with precise size and spatial control, which makes in-plane heterostructures practically applicable for integrated devices and other 2D materials. Therefore, the presented intercalation method can open a new avenue connecting the previously disparate worlds of integrated circuits and atomically thin materials.

Abstract

Abstract

Lithium−sulfur (Li−S) batteries are regarded as promising next-generation high energy density storage devices for both portable electronics and electric vehicles due to their high energy density, low cost, and environmental friendliness. However, there remain some issues yet to be fully addressed with the main challenges stemming from the ionically insulating nature of sulfur and the dissolution of polysulfides in electrolyte with subsequent parasitic reactions leading to low sulfur utilization and poor cycle life. The high flammability of sulfur is another serious safety concern which has hindered its further application. Herein, an aqueous inorganic polymer, ammonium polyphosphate (APP), has been developed as a novel multifunctional binder to address the above issues. The strong binding affinity of the main chain of APP with lithium polysulfides blocks diffusion of polysulfide anions and inhibits their shuttling effect. The coupling of APP with Li ion facilitates ion transfer and promotes the kinetics of the cathode reaction. Moreover, APP can serve as a flame retardant, thus significantly reducing the flammability of the sulfur cathode. In addition, the aqueous characteristic of the binder avoids the use of toxic organic solvents, thus significantly improving safety. As a result, a high rate capacity of 520 mAh g⁻¹ at 4 C and excellent cycling stability of ∼0.038% capacity decay per cycle at 0.5 C for 400 cycles are achieved based on this binder. This work offers a feasible and effective strategy for employing APP as an efficient multifunctional binder toward building nextgeneration high energy density Li−S batteries.

Abstract

The effects of swift heavy ion irradiation-induced disordering on the behavior of lanthanide zirconate compounds (Ln₂Zr₂O₇ where Ln = Sm, Er, or Nd) at high pressures are investigated. After irradiation with 2.2 GeV ¹⁹⁷Au ions, the initial ordered pyrochlore structure (Fd3¯m) transformed to a defect-fluorite structure (Fm3¯m) in Sm₂Zr₂O₇ and Nd₂Zr₂O₇. For irradiated Er₂Zr₂O₇, which has a defect-fluorite structure, ion irradiation induces local disordering by introducing Frenkel defects despite retention of the initial structure. When subjected to high pressures (>29 GPa) in the absence of irradiation, all of these compounds transform to a cotunnite-like (Pnma) phase, followed by sluggish amorphization with further compression. However, if these compounds are irradiated prior to compression, the high pressure cotunnite-like phase is not formed. Rather, they transform directly from their post-irradiation defectfluorite structure to an amorphous structure upon compression (>25 GPa). Defects and disordering induced by swift heavy ion irradiation alter the transformation pathways by raising the energetic barriers for the transformation to the high pressure cotunnite-like phase, rendering it inaccessible. As a result, the high pressure stability field of the amorphous phase is expanded to lower pressures when irradiation is coupled with compression. The responses of materials in the lanthanide zirconate system to irradiation and compression, both individually and in tandem, are strongly influenced by the specific lanthanide composition, which governs the defect energetics at extreme conditions.

Abstract

A self‐healing polymer (SHP) with abundant hydrogen bonds, appropriate viscoelasticity, and stretchability is a promising binder to improve cycle performance of Si microparticle anodes in lithium (Li) ion batteries. Besides high capacity and long cycle life, efficient rate performance is strongly desirable for practical Si anode implementation. Here, polyethylene glycol (PEG) groups are incorporated into the SHP, facilitating Li ionic conduction within the binder. The concept of the SHP‐PEG binder involves improving the interface between Si microparticles and electrolytes after cycling based on the combination of self‐healing ability and fast Li ionic conduction. Through the systematic study of mixing PEG Mw and ratio, the polymeric binder combining SHP and PEG with M_w 750 in an optimal ratio of 60:40 (mol%) achieves a high discharging capacity of ≈2600 mA h g⁻¹, reasonable rate performance especially when >1C and maintains 80% of their initial capacity even after ≈150 cycles at 0.5C. The described concept for the polymeric binder, embedding both self‐healing ability and high Li ionic conductivity, should be equally useful for next generation batteries utilizing high capacity materials which suffer from huge volume change during cycling.

Abstract

The structural evolution of lanthanide A₂TiO₅ (A = Dy, Gd, Yb, Er) at high pressure is investigated using synchrotron X-ray diffraction. The effects of A-site cation size and of the initial structure are systematically examined by varying the composition of the isostructural lanthanide titanates and the structure of dysprosium titanate polymorphs (orthorhombic, hexagonal, and cubic), respectively. All samples undergo irreversible high-pressure phase transformations, but with different onset pressures depending on the initial structure. While each individual phase exhibits different phase transformation histories, all samples commonly experience a sluggish transformation to a defect cotunnite-like (Pnma) phase for a certain pressure range. Orthorhombic Dy₂TiO₅ and Gd₂TiO₅ form P2₁am at pressures below 9 GPa and Pnma above 13 GPa. Pyrochlore-type Dy₂TiO₅ and Er₂TiO₅ as well as defect-fluorite-type Yb₂TiO₅ form Pnma at ∼21 GPa, followed by Im3̅m. Hexagonal Dy₂TiO₅ forms Pnma directly, although a small amount of remnants of hexagonal Dy₂TiO₅ is observed even at the highest pressure (∼55 GPa) reached, indicating kinetic limitations in the hexagonal Dy₂TiO₅ phase transformations at high pressure. Decompression of these materials leads to different metastable phases. Most interestingly, a high-pressure cubic X-type phase (Im3̅m) is confirmed using high-resolution transmission electron microscopy on recovered pyrochlore-type Er₂TiO₅. The kinetic constraints on this metastable phase yield a mixture of both the X-type phase and amorphous domains upon pressure release. This is the first observation of an X-type phase for an A₂BO₅ composition at high pressure.

Abstract

We present an efficient implementation of the Bethe–Salpeter equation (BSE) method for obtaining core-level spectra including X-ray absorption (XAS), X-ray emission (XES), and both resonant and non-resonant inelastic X-ray scattering spectra (N/RIXS). Calculations are based on density functional theory (DFT) electronic structures generated either by abinit or Quantumespresso, both plane-wave basis, pseudopotential codes. This electronic structure is improved through the inclusion of a GW self energy. The projector augmented wave technique is used to evaluate transition matrix elements between core-level and band states. Final two-particle scattering states are obtained with the NIST core-level BSE solver (NBSE). We have previously reported this implementation, which we refer to as ocean(Obtaining Core Excitations from Ab initio electronic structure and NBSE) (Vinson et al., 2011). Here, we present additional efficiencies that enable us to evaluate spectra for systems ten times larger than previously possible; containing up to a few thousand electrons. These improvements include the implementation of optimal basis functions that reduce the cost of the initial DFT calculations, more complete parallelization of the screening calculation and of the action of the BSE Hamiltonian, and various memory reductions. Scaling is demonstrated on supercells of SrTiO3 and example spectra for the organic light emitting molecule Tris-(8-hydroxyquinoline)aluminum (Alq3) are presented. The ability to perform large-scale spectral calculations is particularly advantageous for investigating dilute or non-periodic systems such as doped materials, amorphous systems, or complex nano-structures.

Abstract

Global warming and energy crises severely limit the ability of human civilization to develop along a sustainable path. Increasing renewable energy sources and decreasing energy consumption are fundamental steps to achieve sustainability. Technological innovations that allow energy-saving behaviour can support sustainable development pathways. Energy-saving fabrics with a superior cooling effect and satisfactory wearability properties provide a novel way of saving the energy used by indoor cooling systems. Here, we report the large-scale extrusion of uniform and continuous nanoporous polyethylene (nanoPE) microfibres with cotton-like softness for industrial fabric production. The nanopores embedded in the fibre effectively scatter visible light to make it opaque without compromising the mid-infrared transparency. Moreover, using industrial machines, the nanoPE microfibres are utilized to mass produce fabrics. Compared with commercial cotton fabric of the same thickness, the nanoPE fabric exhibits a great cooling power, lowering the human skin temperature by 2.3 °C, which corresponds to a greater than 20% saving on indoor cooling energy. Besides the superior cooling effect, the nanoPE fabric also displays impressive wearability and durability. As a result, nanoPE microfibres represent basic building blocks to revolutionize fabrics for human body cooling and pave an innovative way to sustainable energy.