A theoretical model for the electronic structure and transport properties of superlattice (SL) nanowires is presented, based on the electronic tunneling between quantum dots. Due to the periodic potential perturbation, SL nanowires exhibit unusual features in the electronic density of states that are absent in homogeneous nanowires. Transport property calculations of PbSe/PbS SL nanowires are presented, showing improved thermoelectric performance compared to homogeneous nanowires because of a lower lattice thermal conductivity and an enhanced Seebeck coefficient, indicating that SL nanowires are promising systems for thermoelectric applications.

The optical absorption for ? electrons as a function of the electron wavevector k is investigated by first order perturbation theory in graphite and single wall carbon nanotubes (SWNTs). The matrix element connecting two states in the valence and conduction bands is found to be significantly anisotropic in k -space and polarization dependent. In the case of graphite, the absorption shows a node around the equi-energy contour, and in the case of SWNTs we obtain selection rules that allow only transitions between certain pairs of subbands. The strength of the optical absorption is not only diameter dependent but also chirality dependent. The implications of the optical absorption matrix element on the resonant conditions are discussed.

Temperature-dependent resistance measurements of Bi-related nanowire arrays with different wire diameters and Sb concentrations are performed. The variation in the measured R(T) curves of these nanowires is closely related to the unique semimetal-semiconductor transition in Bi, and the results are explained by theoretical simulations. It is found that the special feature of the maximum in the resistance ratio R(10 K)/R(100 K) can be employed to experimentally identify the conditions for the semimetal-semiconductor transition.

The optical absorption from an indirect L-T point valence band transition in bismuth nanowires is simulated based on a surface sensitive term, which is very weak in bulk bismuth, but effectively couples the L and T point bands in bismuth nanowires. The experimental Fourier transform infrared absorption spectra in the energy range 0.08 to 0.5 eV are compared to theoretical simulations for absorption resulting from indirect interband transitions between subbands at the L point valence band to subbands at the T point valence band as the polarization, tellurium doping, and wire diameter are varied. The simulated absorption from this indirect transition reproduces many of the experimentally observed trends and allows us to conclude that the dominant feature in the experimental absorption spectra of Bi nanowires results from an indirect L-T point transition which is enhanced in Bi nanowires.

It is quite challenging to prepare subnanometer porous materials from traditional porous precursors, and use of supramolecules as carbon sources was seldom reported due to the complex preparation and purification processes. We explore a facile one-pot method to fabricate supramolecular coordination compounds as carbon sources. The resultant CB[6]-derived carbons (CBC) have a high N content of 7.0-22.0%, surface area of 552-861 m2 g-1, and subnano/mesopores. The CBC electrodes have a narrow size distribution at 5.9 Å, and the supercapacitor exhibits an energy density of 117.1 Wh kg-1 and a potential window of over 3.8 V in a two-electrode system in the ionic liquid (MMIMBF4) electrolyte with appropriate cationic (5.8 Å) and anionic (2.3 Å) diameter. This work presents the facile fabrication of novel supermolecule cucurbituril subnanoporous carbon materials and the smart design of "pores and balls" for high-performance energy storage systems.

The possible semiconducting device use of single wall carbon nanotubes (SWNTs) requires a technique for the determination of the exact structure of the nanotubes assembled in the device configuration. Raman spectroscopy has been established as a precise and non-destructive tool for the characterization of graphitic nanostructures. Double resonance theory, which is used to explain the dispersive nature of the Raman bands, has attracted much attention for its potential use for the characterization of the electronic and phonon spectra of these nanostructures. Dispersive features in the Raman spectra of low dimensional graphitic materials, such as carbon nanotubes, can be used to measure directly the anisotropy, or the trigonal warping effect, in the phonon dispersion relations about the hexagonal corner of the Brillouin zone (BZ) of graphite.

Electron spin resonance (ESR), magnetic susceptibility, and transport measurements were recently performed on a set of heat-treated polyparaphenylene (PPP)-based carbon samples, which are of significant interest as novel carbon-based anode electrodes in Li-ion batteries. Attention is focused on the evolution of the carbonaceous structures formed at low heat-treatment temperatures (THT) in the regime of 600° C ≤ THT ≤ 800° C, where percolative transport behavior is observed. At the percolation threshold, the coexistence of two spin centers with peak-to-peak Lorentzian linewidths of ΔHpp(300K) = 0.5 and 5.0 G is observed. The relatively high ratio of hydrogen. carbon (H/C) near is believed to influence the ESR results through an unresolved hyperfine interaction. Curie-Weiss temperaures are found from measurements of [Ipp(ΔHpp)2]-1, where Ipp is the peak-to-peak lineheight, yielding results that are in agreement with static susceptibility, χ(T), measurements. At low THT, PPP-based materials exhibit a large amount of disorder and this is evidenced by the high density of localized spins, Nc, which is obtained from a Curie-Weiss fit to χ(T) assuming a spin quantum number of $$S=\tfrac{1}{2}$$ . A model explaining the microstructure and high electrochemical doping capacity of PPP samples heat-treated to 700° C can be related to Li-ion battery performance.

Polyparaphenylene (PPP)-based carbon powder is expected for materials of the negative electrode of the lithium ion secondary batteries. In this study, effects of heat treatment temperature on the PPP materials are investigated by means of electrical and structural measurement. The layer structure of the powder is analyzed in detail by the transmission electron microscope observation combined with digital image processing. Charge-discharge current characteristics, electrical resistivity under pressure and layer structure of the PPP-based carbon powder are measured and discussed.

Electron-phonon coupling in two-dimensional nanomaterials plays a fundamental role in determining their physical properties. Such interplay is particularly intriguing in semiconducting black phosphorus (BP) due to the highly anisotropic nature of its electronic structure and phonon dispersions. Here we report the direct observation of symmetry-dependent electron-phonon coupling in BP by performing the polarization-selective resonance Raman measurement in the visible and ultraviolet regimes, focusing on the out-of-plane Ag1 and in-plane Ag2 phonon modes. Their intrinsic resonance Raman excitation profiles (REPs) were extracted and quantitatively compared. The in-plane Ag2 mode exhibits remarkably strong resonance enhancement across the excitation wavelengths when the excitation polarization is parallel to the armchair (Ag2//AC) direction. In contrast, a dramatically weak resonance effect was observed for the same mode with the polarization parallel to zigzag (Ag2//ZZ) direction and for the out-of-plane Ag1 mode (Ag1//AC and Ag1//ZZ). Analysis on quantum perturbation theory and first-principles calculations on the anisotropic electron distributions in BP demonstrated that electron-phonon coupling considering the symmetry of the involved excited states and phonon vibration patterns is responsible for this phenomenon. Further analysis of the polarization-dependent REPs for Ag phonons allows us to resolve the existing controversies on the physical origin of Raman anomaly in BP and its dependence on excitation energy, sample thickness, phonon modes, and crystalline orientation. Our study gives deep insights into the underlying interplay between electrons and phonons in BP and paves the way for manipulating the electron-phonon coupling in anisotropic nanomaterials for future device applications.

Topological quantum and classical materials can exhibit robust properties that are protected against disorder, for example, for noninteracting particles and linear waves. Here, we demonstrate how to construct topologically protected states that arise from the combination of strong interactions and thermal fluctuations inherent to soft materials or miniaturized mechanical structures. Specifically, we consider fluctuating lines under tension (e.g., polymer or vortex lines), subject to a class of spatially modulated substrate potentials. At equilibrium, the lines acquire a collective tilt proportional to an integer topological invariant called the Chern number. This quantized tilt is robust against substrate disorder, as verified by classical Langevin dynamics simulations. This robustness arises because excitations in this system of thermally fluctuating lines are gapped by virtue of interline interactions. We establish the topological underpinning of this pattern via a mapping that we develop between the interacting-lines system and a hitherto unexplored generalization of Thouless pumping to imaginary time. Our work points to a new class of classical topological phenomena in which the topological signature manifests itself in a structural property observed at finite temperature rather than a transport measurement.

The mechanical and electronic properties of two-dimensional materials make them promising for use in flexible electronics1-3. Their atomic thickness and large-scale synthesis capability could enable the development of 'smart skin'1,3-5, which could transform ordinary objects into an intelligent distributed sensor network6. However, although many important components of such a distributed electronic system have already been demonstrated (for example, transistors, sensors and memory devices based on two-dimensional materials1,2,4,7), an efficient, flexible and always-on energy-harvesting solution, which is indispensable for self-powered systems, is still missing. Electromagnetic radiation from Wi-Fi systems operating at 2.4 and 5.9 gigahertz8 is becoming increasingly ubiquitous and would be ideal to harvest for powering future distributed electronics. However, the high frequencies used for Wi-Fi communications have remained elusive to radiofrequency harvesters (that is, rectennas) made of flexible semiconductors owing to their limited transport properties9-12. Here we demonstrate an atomically thin and flexible rectenna based on a MoS2 semiconducting-metallic-phase heterojunction with a cutoff frequency of 10 gigahertz, which represents an improvement in speed of roughly one order of magnitude compared with current state-of-the-art flexible rectifiers9-12. This flexible MoS2-based rectifier operates up to the X-band8 (8 to 12 gigahertz) and covers most of the unlicensed industrial, scientific and medical radio band, including the Wi-Fi channels. By integrating the ultrafast MoS2 rectifier with a flexible Wi-Fi-band antenna, we fabricate a fully flexible and integrated rectenna that achieves wireless energy harvesting of electromagnetic radiation in the Wi-Fi band with zero external bias (battery-free). Moreover, our MoS2 rectifier acts as a flexible mixer, realizing frequency conversion beyond 10 gigahertz. This work provides a universal energy-harvesting building block that can be integrated with various flexible electronic systems.

The stacking and bending of graphene are trivial but extremely powerful agents of control over graphene’s manifold physics. By changing the twist angle, one can drive the system over a plethora of exotic states via strong electron correlation, thanks to the moiré superlattice potentials, while the periodic or triaxial strains induce discretization of the band structure into Landau levels without the need for an external magnetic field. We fabricated a hybrid system comprising both the stacking and bending tuning knobs. We have grown the graphene monolayers by chemical vapor deposition, using 12C and 13C precursors, which enabled us to individually address the layers through Raman spectroscopy mapping. We achieved the long-range spatial modulation by sculpturing the top layer (13C) over uniform magnetic nanoparticles (NPs) deposited on the bottom layer (12C). An atomic force microscopy study revealed that the top layer tends to relax into pyramidal corrugations with C3 axial symmetry at the position of the NPs, which have been widely reported as a source of large pseudomagnetic fields (PMFs) in graphene monolayers. The modulated graphene bilayer (MGBL) also contains a few micrometer large domains, with the twist angle ∼10°, which were identified via extreme enhancement of the Raman intensity of the G-mode due to formation of van Hove singularities (VHSs). We thereby conclude that the twist-induced VHSs coexist with the PMFs generated in the strained pyramidal objects without mutual disturbance. The graphene bilayer modulated with magnetic NPs is a non-trivial hybrid system that accommodates features of twist-induced VHSs and PMFs in environs of giant classical spins.

The preparation of green, facile, and cost-effective energy storage materials remains a big challenge. In this paper, a cobalt sulfide/porous carbon (Co4S3/PC) composite electrode is facilely prepared using the natural eggshell membrane (ESM) as a basal substrate. Under hydrothermal conditions, Co4S3 is grown on the ESM to form Co4S3/ESM and carbonized to form Co4S3/PC. The as-synthesized Co4S3/PC composite is used as an electrode material. The carbide from the ESM shows a porous structure and high specific surface area, which provides large space for Co4S3 attaching and ion migrating. Co4S3/PC shows much higher specific capacitance values than the sum of Co4S3 and PC electrodes, indicating a significant synergistic effect. More importantly, the Co4S3 is a typical faradic material, which exchanges Faraday charge with an electrolyte and subsequently transmits an electron to the whole electrode due to the high conductivity of the carbonized ESM. Such a synergistic effect offers the as-synthesized Co4S3/PC electrode a significant improvement in performance over both the ESM-derived carbon and the original Co4S3. Besides, the Co4S3/PC electrode shows a wide potential window, low resistance, and high specific capacitance. After 1000 cycles, the electrode retains a high cycling capacity. This study provides a novel insight for high-performance biomass-derived carbon preparation for pseudocapacitors and other electrochemical devices.

The massless Dirac electron transport in graphene has led to a variety of unique light-matter interaction phenomena, which promise many novel optoelectronic applications. Most of the effects are only accessible by breaking the spatial symmetry, through introducing edges, p-n junctions, or heterogeneous interfaces. The recent development of direct synthesis of lateral heterostructures offers new opportunities to achieve the desired asymmetry. As a proof of concept, we study the photothermoelectric effect in an asymmetric lateral heterojunction between the Dirac semimetallic monolayer graphene and the parabolic semiconducting monolayer MoS2. Very different hot-carrier cooling mechanisms on the graphene and the MoS2 sides allow us to resolve the asymmetric thermalization pathways of photoinduced hot carriers spatially with electrostatic gate tunability. We also demonstrate the potential of graphene-2D semiconductor lateral heterojunctions as broadband infrared photodetectors. The proposed structure shows an extreme in-plane asymmetry and provides a new platform to study light-matter interactions in low-dimensional systems.

The electrons in solids not only conduct electricity but also conduct heat, as they transfer energy from a hot junctionHot junction to a cold junctionCold junction . Just as the electrical conductivity characterizes the response of a material to an applied voltage, the thermal conductivity likewise characterizes the material with regard to heat flow in the presence of a temperature gradient. In fact, the electrical conductivityElectrical conductivity and thermal conductivity are coupled, since thermal conduction also transports charge and electrical conduction also transports energyEnergy .