Thermoelectric technology enables the harvest of waste heat and its direct conversion into electricity. The conversion efficiency is determined by the materials figure of merit Here we show a maximum of ~2.8 ± 0.5 at 773 kelvin in n-type tin selenide (SnSe) crystals out of plane. The thermal conductivity in layered SnSe crystals is the lowest in the out-of-plane direction [two-dimensional (2D) phonon transport]. We doped SnSe with bromine to make n-type SnSe crystals with the overlapping interlayer charge density (3D charge transport). A continuous phase transition increases the symmetry and diverges two converged conduction bands. These two factors improve carrier mobility, while preserving a large Seebeck coefficient. Our findings can be applied in 2D layered materials and provide a new strategy to enhance out-of-plane electrical transport properties without degrading thermal properties.

Symmetric sodium-ion batteries possess promising features such as low cost, easy manufacturing process, and facile recycling post-process, which are suitable for the application of large-scale stationary energy storage. Herein, we proposed a symmetric sodium-ion battery based on dual-electron reactions of a NASICON-structured Na3MnTi­(PO4)3 material. The Na3MnTi­(PO4)3 electrode can deliver a stable capacity of up to 160 mAh g–1 with a Coulombic efficiency of 97% at 0.1 C by utilizing the redox reactions of Ti3+/4+, Mn2+/3+, and Mn3+/4+. This is the first time to investigate the symmetric sodium-ion full cell using Na3MnTi­(PO4)3 as both cathode and anode in the organic electrolyte, demonstrating excellent reversibility and cycling performance with voltage plateaus of about 1.4 and 1.9 V. The full cell exhibits a reversible capacity of 75 mAh g–1 at 0.1 C and an energy density of 52 Wh kg–1. In addition, both ex situ X-ray diffraction (XRD) analysis and first-principles calculations are employed to investigate the sodiation mechanism and structural evolution. The current research provides a feasible strategy for the symmetric sodium-ion batteries to achieve high energy density.

By confining a diffusion atom in a nanometer region using surface potential heterogeneity, we have successfully employed a time-dependent tunneling spectroscopy to quantitatively study its random motion. A hopping rate in the range of 1-10 4 Hz, 3 orders of magnitude faster than those accessible by the existing diffusion methods based on scanning tunneling microscopy, was demonstrated for single Cu atoms diffusing in the faulted half unit cell of Si111-7 7. Our technique is potentially useful to detect fast diffusion processes such as H quantum diffusion at atomic scale. DOI: 10.1103/PhysRevLett.94.036103 PACS numbers: 68.35.Fx, 68.35.Md, 68.37.Ef, 68.47.Fg Surface diffusion is an important subject of study in physics, chemistry, biology, and materials science. It plays a vital role in chemical and surface catalytic reactions, selfassembly, crystal growth, and thin film epitaxy [1][2][3][4][5]. Observation and understanding of surface diffusion phenomenon in the past largely depend on the development of techniques [1,4]. Among the techniques, scanning tunneling microscopy (STM) [6] is advantageous in directly providing the atomistic mechanisms of diffusion [3][4][5][7][8][9][10] but is severely limited to slow diffusion rates from 10 ÿ19 to 10 ÿ14 cm 2 =s, as compared to the wide range from 10 ÿ19 to 10 ÿ5 cm 2 =s covered by the collection of different techniques [4]. Thus, processes in quantum diffusion of light atoms and in many other systems of practical interest with fast diffusion rates remain inaccessible by the STM technique.Limited by the electronic feedback loop response, the diffusion rate that can be probed by the widely used frameby-frame imaging [11] and atom-tracking [12] methods is in the slow regime. In atom tracking for which no full frame imaging is required, the fastest measurable hopping rate is about 10 Hz. With a state-of-art design [7,9,10], STM imaging can now be performed at a rate of 10 frame=s, again leading to observation of a maximum hopping rate of only 10 Hz (equivalent to 10 ÿ14 cm 2 =s for nearest neighbor site hopping). Using a new concept of measuring diffusion by the bypass of the feedback loop, we demonstrate in this Letter a new STM-based method that can expand the range by at least 3 orders of magnitude faster, to a hopping rate on the order of 10 4 Hz (equivalent to 10 ÿ11 cm 2 =s). Further expansion of our method will become possible if faster preamplifier and data acquisition systems are used. This leads to significant overlaps in measurable diffusion rates with other techniques while retaining the atomic resolution. The extension will not only enable STM study of diffusion in the fast regime to facilitate direct comparison with results obtained by other techniques, but also enable the study of processes such as H quantum diffusion for which cooling never slows it down [1]. As shown by Gomer and his co-workers with field emission microscopy, the H quantum diffusion measured for a large number of systems [13][14][15][16] falls in the range of 10 ÿ13 to 10 ÿ10 cm 2 =s...