Thermoelectric materials generate electric energy from waste heat, with conversion efficiency governed by the dimensionless figure of merit, ZT. Single-crystal tin selenide (SnSe) was discovered to exhibit a high ZT of roughly 2.2–2.6 at 913 K, but more practical and deployable polycrystal versions of the same compound suffer from much poorer overall ZT, thereby thwarting prospects for cost-effective lead-free thermoelectrics. The poor polycrystal bulk performance is attributed to traces of tin oxides covering the surface of SnSe powders, which increases thermal conductivity, reduces electrical conductivity and thereby reduces ZT. Here, we report that hole-doped SnSe polycrystalline samples with reagents carefully purified and tin oxides removed exhibit an ZT of roughly 3.1 at 783 K. Its lattice thermal conductivity is ultralow at roughly 0.07 W m–1 K–1 at 783 K, lower than the single crystals. The path to ultrahigh thermoelectric performance in polycrystalline samples is the proper removal of the deleterious thermally conductive oxides from the surface of SnSe grains. These results could open an era of high-performance practical thermoelectrics from this high-performance material.

Nearly intrinsic charge and thermal transport properties of polycrystalline SnSe materials are unveiled for the first time. We confirm that tin oxide layers on the surface of SnSe powder are the origin of paradoxically higher apparent thermal conductivity in polycrystalline samples over single crystal. Our surface oxide removing strategy on polycrystalline SnSe materials reveals even lower thermal conductivity than single-crystal samples and simultaneously enhances the hole mobility, electrical conductivity, and Seebeck coefficient, leading to an extraordinarily high thermoelectric figure of merit (ZT) of 2.5.

Graphene quantum dots (GQDs) have been considered as a novel material because their electronic and optoelectronic properties can be tuned by controlling the size and the functional groups of GQDs. Here we report the synthesis of reduction-controlled GQDs and their application to bulk heterojunction (BHJ) solar cells with enhanced power conversion efficiency (PCE). Three different types of GQDs--graphene oxide quantum dots (GOQDs), 5 h reduced GQDs, and 10 h reduced GQDs--were tested in BHJ solar cells, and the results indicate that GQDs play an important role in increasing optical absorptivity and charge carrier extraction of the BHJ solar cells. The enhanced optical absorptivity by rich functional groups in GOQDs increases short-circuit current, while the improved conductivity of reduced GQDs leads to the increase of fill factors. Thus, the reduction level of GQDs needs to be intermediate to balance the absorptivity and conductivity. Indeed, the partially reduced GQDs yielded the outstandingly improved PCE of 7.60% in BHJ devices compared to a reference device without GQDs (6.70%).

Lithium-sulfur (Li-S) batteries are expected to overcome the limit of current energy storage devices by delivering high specific energy with low material cost. However, the potential of Li-S batteries has not yet been realized because of several technical barriers. Poor electrochemical performance is mainly attributed to the low electrical conductivity of the fully charged and discharged species, the irreversible loss of polysulfide anions and the decrease in the number of electrochemically active reaction sites during battery operation. Here, we report that the introduction of graphene quantum dots (GQDs) into the sulfur cathode dramatically enhanced sulfur/sulfide utilization, yielding high performance. In addition, the GQDs induced structural integrity of the sulfur-carbon electrode composite by oxygen-rich functional groups. This hierarchical architecture enabled fast charge transfer while minimizing the loss of lithium polysulfides, which is attributed to the physicochemical properties of GQDs. The mechanisms through which excellent cycling and rate performance are achieved were thoroughly studied by analyzing capacity versus voltage profiles. Furthermore, experimental observations and theoretical calculations further clarified the role played by GQDs by proving that C-S bonding occurs. Thus, the introduction of GQDs into Li-S batteries will provide an important breakthrough allowing their use as high-performance and low-cost batteries for next-generation energy storage systems. INTRODUCTIONRechargeable lithium-ion batteries are widely used in various applications, such as portable devices, bio-medical implants and electric vehicles, because of their high energy and power density. 1,2 However, current lithium-ion batteries based on the graphite and transition metal oxide couple have nearly reached their ceiling with respect to storage capability because of the limitations associated with their electrical properties and crystal structure. Therefore, breakthroughs in new energy storage systems that can surpass the current performance barrier of lithium-ion batteries should be brought about in a timely manner. Recently, Li-S batteries that can operate by the reversible electrochemical transformation between sulfur (S 8 ) and dilithium sulfide (Li 2 S) have attracted great attention because they can deliver high energy with a moderate voltage owing to the direct use of

Rechargeable calcium batteries have attracted increasing attention as promising multivalent ion battery systems due to the high abundance of calcium. However, the development has been hampered by the lack of suitable cathodes to accommodate the large and divalent Ca2+ ions at a high redox potential with sufficiently fast ionic conduction. Herein, we report a new intercalation host which presents 500 cycles with a capacity retention of 90% and a remarkable power capability at ~3.2 V (vs. Ca/Ca2+) in a calcium battery. The cathode material derived from Na0.5VPO4.8F0.7 is demonstrated to reversibly accommodate a large amount of Ca2+ ions, forming a series of CaxNa0.5VPO4.8F0.7 (0 < x < 0.5) phases without any noticeable structural degradation. The robust framework enables one of the smallest volume changes (1.4%) and the lowest diffusion barriers for Ca2+ among the cathodes reported to date, offering the basis for the outstanding cycle life and power capability.

Graphene quantum dots (GQDs), a newly emerging 0-dimensional graphene based material, have been widely exploited in optoelectronic devices due to their tunable optical and electronic properties depending on their functional groups. Moreover, the dispersibility of GQDs in common solvents depending on hydrophobicity or hydrophilicity can be controlled by chemical functionalization, which is particularly important for homogeneous incorporation into various polymer layers. Here we report that a surface-engineered GQD-incorporated polymer photovoltaic device shows enhanced power conversion efficiency (PCE), where the oxygen-related functionalization of GQDs enabled good dispersity in a PEDOT:PSS hole extraction layer, leading to significantly improved short circuit current density (Jsc) value. To maximize the PCE of the device, hydrophobic GQDs that are hydrothermally reduced (rGQD) were additionally incorporated in a bulk-heterojunction layer, which is found to promote a synergistic effect with the GQD-incorporated hole extraction layer.

In electron microscopy, charging of non-conductive biological samples by focused electron beams hinders their high-resolution imaging. Gold or platinum coatings have been commonly used to prevent such sample charging, but it disables further quantitative and qualitative chemical analyses by energy dispersive spectroscopy (EDS). Here we report that graphene-coating on biological samples enables nondestructive high-resolution imaging by scanning electron microscopy (SEM) as well as chemical analysis by EDS, utilizing graphene's transparency to electron beams, high conductivity, outstanding mechanical strength, and flexibility. We believe that the graphene-coated imaging and analysis would provide us a new opportunity to explore various biological phenomena unseen before due to the limitation in sample preparation and image resolution, which will broaden our understanding on the life mechanism of various living organisms.Comprehensive understanding of biological objectstheir chemical, physiochemical and biological characteristicscan be effectively achieved through electron microscopy (EM) analysis 1-4 , preferably without any fixation or auxiliary surface treatment. Scanning electron microscope (SEM) and transmission electron microscope (TEM) are increasingly more employed as they provide direct imaging of specimen's morphological structures 5 with high-resolution. In addition, the unique interaction between electron beams and specimen enables various physical and chemical analyses such as energy dispersive spectroscopy (EDS), electron probe micro analysis (EPMA), and electron energy loss spectroscopy (EELS) 6,7 . Nevertheless, charge accumulation on non-conductive surface by electron beams has always hindered EMmediated biological studies as it distorts the morphological and chemical characteristics of the specimens 8-12 . Thus, various metal deposition methods have been employed to dissipate the charge in the non-conducting specimens [13][14][15] . However, the relatively thick metal coatings hamper from studying sample's fine surface structure at nanometer scale, because the surface features smaller than metal grain size (~10 nm) cannot be imaged 3 properly. In addition, X-ray fluorescence signals required for EDS analysis are screened by metal layers 16 . Furthermore, it is usually difficult to use the metal-coated samples for further analyses such as TEM that requires electron-transparency. Here we report that graphene-coating on biological samples enables non-destructive high-resolution imaging by scanning electron microscopy (SEM) as well as chemical analysis by EDS, utilizing graphene's transparency to electron beams, high conductivity, outstanding mechanical strength, and flexibility [17][18][19][20] . We believe that the graphene-coated imaging and analysis would provide us a new opportunity to explore various biological phenomena unseen before due to the limitation in sample preparation and image resolution, which will broaden our understanding on the life mechanism of various living organisms.A recent pr...

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