Origin of the Bright Photoluminescence of Thiolate-protected Gold Nanoclusters: Confined Structural Water Molecules as Real Emitters

The availability of a range of excited states has enriched zero-, one- and two- dimensional quantum nanomaterials with interesting luminescence properties, in particular for noble metal nanoclusters (NCs) as typical examples. But, the elucidation and origin of optoelectronic properties remains elusive. In this report, using widely used Au(I)-alkanethiolate complex (Au(I)-SRs, R = -(CH2)12H) with AIE characteristics as a model system, by judiciously manipulating the delicate surface ligand interactions at the nanoscale interface, together with a careful spectral investigations and an isotope diagnostic experiment of heavy water (D2O), we evidenced that the structural water molecules (SWs) confined in the nanoscale interface or space are real emitter centers for photoluminescence (PL) of metal NCs and the aggregate of Au(I)-SRs complexes, instead of well-organized metal core dominated by quantum confinement mechanics. Interestingly, the aggregation of Au(I)-SRs generated dual fluorescence-phosphorescence emission and the photoluminescence intensity was independent on the degree of aggregation but showed strong dependency on the content and state of structural water molecules (SWs) confined in the aggregates. SWs are different from traditional hydrogen bonded water molecules, wherein, due to interfacial adsorption or spatial confinement, the p orbitals of two O atoms in SWs can form a weak electron interaction through spatial overlapping, which concomitantly constructs a group of interfacial states with π bond characteristics, consequently providing some alternative channels (or pathways) to the radiation and/or non-radiation relaxation of electrons. Our results provide completely new insights to understand the fascinating properties (including photoluminescence, catalysis and chirality, etc.) of other low-dimension quantum dots and even for aggregation-induced emission luminophores (AIEgens). This also answers the century old debate on whether and how water molecules emit bright color.

Tunable Metallic-Like Transport in Polypyrrole

Conjugated polymers (CPs), organic macromolecules with linear backbone of alternating C–C and C=C bonds, possess unique semiconductive properties, providing new opportunities for organic electronics, photonics, information, and energy devices. Seeking the metallic or metallic-like, even superconducting properties beyond semiconductivity in CPs is always one of the ultimate goals in polymer science and condensed matter. Only two metallic and semi-metallic transport cases – aniline-derived polyaniline (PANI) and thiophene-derived poly(3,4-ethylenedioxythiophene) (PEDOT) – have been reported since the development of CPs for four decades. Controllable synthesis is a key challenge in discovering more cases. Here we report the metallic-like transport behavior of another CP, polypyrrole (PPy). We observe that the transport behavior of PPy changes from semiconductor to insulator-metal transition, and gradually realizes metallic-like performance when the crystalline degree increases. Using a generalized Einstein relation model, we rationalized the mechanism behind the observation. The metallic-like transport in PPy demonstrates electron strong correlation and phonon-electron interaction in soft condensation matter, and may find practical applications of CPs in electrics and spintronics.

A First-Principles Study of the Cu-Containing β″ Precipitates in Al-Mg-Si-Cu Alloy

The nanostructured β″ precipitates are critical for the strength of Al-Mg-Si-(Cu) aluminum alloys. However, there are still controversial reports about the composition of Cu-containing β″ phases. In this work, first-principles calculations based on density functional theory were used to investigate the composition, mechanical properties, and electronic structure of Cu-containing β″ phases. The results predict that the Cu-containing β″ precipitates with a stoichiometry of Mg4+xAl2−xCuSi4 (x = 0, 1) are energetically favorable. As the concentration of Cu atoms increases, Cu-containing β″ phases with different compositions will appear, such as Mg4AlCu2Si4 and Mg4Cu3Si4. The replacement order of Cu atoms in β″ phases can be summarized as one Si3/Al site → two Si3/Al sites → two Si3/Al sites and one Mg1 site. The calculated elastic constants of the considered β″ phases suggest that they are all mechanically stable, and all β″ phases are ductile. When Cu atoms replace Al atoms at Si3/Al sites in β″ phases, the values of bulk modulus (B), shear modulus (G), and Young’s modulus (E) all increase. The calculation of the phonon spectrum shows that Mg4+xAl2−xCuSi4 (x = 0, 1) are also dynamically stable. The electronic structure analysis shows that the bond between the Si atom and the Cu atom has a covalent like property. The incorporation of the Cu atom enhances the electron interaction between the Mg2 and the Si3 atom so that the Mg2 atom also joins the Si network, which may be one of the reasons why Cu atoms increase the structure stability of the β″ phases.

Conductance peaks and gaps in single-electron device with the presence of electron-electron interaction - “Nonequilibrium green function approach”

Consider a single-electron transistor (SET) with a small size quantum dot (QD), where confined energy and the Coulomb interaction control the charges adding to QD. In this paper, a theoretical analysis of the relation between source-drain voltage and gate voltage has been done to define quantum-Coulomb blocked (and unblocked) diamonds for QD that has N electrons. An analytical equation for the conductance has been derived using the non-equilibrium Green function technique (NEGFT). Further, the effect of QD size and the tunnelling rate on conductance peaks and gaps have been investigated. Finally, the effect of gate voltage on conductance peaks and gaps with respect the quantum-Coulomb blocked regions has been analysed.

Higher-order topological superconductivity from repulsive interactions in kagome and honeycomb systems

We discuss a pairing mechanism in interacting two-dimensional multipartite lattices that intrinsically leads to a second order topological superconducting state with a spatially modulated gap. When the chemical potential is close to Dirac points, oppositely moving electrons on the Fermi surface undergo an interference phenomenon in which the Berry phase converts a repulsive electron-electron interaction into an effective attraction. The topology of the superconducting phase manifests as gapped edge modes in the quasiparticle spectrum and Majorana Kramers pairs at the corners. We present symmetry arguments which constrain the possible form of the electron-electron interactions in these systems and classify the possible superconducting phases which result. Exact diagonalization of the Bogoliubov-de Gennes Hamiltonian confirms the existence of gapped edge states and Majorana corner states, which strongly depend on the spatial structure of the gap. Possible applications to vanadium-based superconducting kagome metals AV$_3$Sb$_3$ (A=K,Rb,Cs) are discussed.

Electronic properties of semiconductor quantum wires for shallow symmetric and asymmetric confinements

Quantum wires (QWs) and quantum point contacts (QPCs) have been realized in GaAs/AlGaAs heterostructures in which a two-dimensional electron gas (2DEG) resides at the interface between GaAs and AlGaAs layered semiconductors. The electron transport in these structures has previously been studied experimentally and theoretically, and a 0.7 conductance anomaly has been discovered. The present paper is motivated by experiments with a QW in shallow symmetric and asymmetric confinements that have shown additional conductance anomalies at zero magnetic field. The proposed device consists of a QPC that is formed by split gates and a top gate between two large electron reservoirs. This paper is focused on the theoretical study of electron transport through a wide top-gated QPC in a low-density regime and is based on density functional theory. The electron-electron interaction and shallow confinement make the splitting of the conduction channel into two channels possible. Each of them becomes spin-polarized at certain split and top gates voltages and may contribute to conductance giving rise to additional conductance anomalies. For symmetrically loaded split gates two conduction channels contribute equally to conductance. For the case of asymmetrically applied voltage between split gates conductance anomalies may occur between values of 0.25(2e2/h) and 0.7(2e2/h) depending on the increased asymmetry in split gates voltages. This corresponds to different degrees of spin-polarization in the two conduction channels that contribute differently to conductance. In the case of a strong asymmetry in split gates voltages one channel of conduction is pinched off and just the one remaining channel contributes to conductance. We have found that on the perimeter of the anti-dot there are spin-polarized states. These states may also contribute to conductance if the radius of the anti-dot is small enough and tunnelling between these states may occur. The spin-polarized states in the QPC with shallow confinement tuned by electric means may be used for the purposes of quantum technology.

Impedance matching to axion dark matter: considerations of the photon-electron interaction

We introduce the concept of impedance matching to axion dark matter by posing the question of why axion detection is difficult, even though there is enough power in each square meter of incident dark-matter flux to energize a LED light bulb. By quantifying backreaction on the axion field, we show that a small axion-photon coupling does not by itself prevent an order-unity fraction of the dark matter from being absorbed through optimal impedance match. We further show, in contrast, that the electromagnetic charges and the self-impedance of their coupling to photons provide the principal constraint on power absorption integrated across a search band. Using the equations of axion electrodynamics, we demonstrate stringent limitations on absorbed power in linear, time-invariant, passive receivers. Our results yield fundamental constraints, arising from the photon-electron interaction, on improving integrated power absorption beyond the cavity haloscope technique. The analysis also has significant practical implications, showing apparent tension with the sensitivity projections for a number of planned axion searches. We additionally provide a basis for more accurate signal power calculations and calibration models, especially for receivers using multi-wavelength open configurations such as dish antennas and dielectric haloscopes.