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Understanding the chemical states of individual surface atoms and their arrangements is essential for addressing several current issues such as catalysis, energy stroage/conversion, and environmental protection. Here, we exploit a profile imaging technique to understand the correlation between surface atomic structures and the oxygen evolution reaction (OER) in Mn 3 O 4 nanoparticles. We image surface structures of Mn 3 O 4 nanoparticles and observe surface reconstructions in the (110) and (101) planes. Mn 3+ ions at the surface, which are commonly considered as the active sites in OER, disappear from the reconstructed planes, whereas Mn 3+ ions are still exposed at the edges of nanoparticles. Our observations suggest that surface reconstructions can deactivate low-index surfaces of Mn oxides in OER. These structural and chemical observations are further validated by density functional theory calculations. This work shows why atomic-scale characterization of surface structures is crucial for a molecular-level understanding of a chemical reaction in oxide nanoparticles.

Controlled phase conversion in polymorphic transition metal dichalcogenides (TMDs) provides a new synthetic route for realizing tunable nanomaterials. Most conversion methods from the stable 2H to metastable 1T phase are limited to kinetically slow cation insertion into atomically thin layered TMDs for charge transfer from intercalated ions. Here, we report that anion extraction by the selective reaction between carbon monoxide (CO) and chalcogen atoms enables predictive and scalable TMD polymorph control. Sulfur vacancy, induced by anion extraction, is a key factor in molybdenum disulfide (MoS 2 ) polymorph conversion without cation insertion. Thermodynamic MoS 2 -CO-CO 2 ternary phase diagram offers a processing window for efficient sulfur vacancy formation with precisely controlled MoS 2 structures from single layer to multilayer. To utilize our efficient phase conversion, we synthesize vertically stacked 1T-MoS 2 layers in carbon nanofibers, which exhibit highly efficient hydrogen evolution reaction catalytic activity. Anion extraction induces the polymorph conversion of tungsten disulfide (WS 2 ) from 2H to 1T. This reveals that our method can be utilized as a general polymorph control platform. The versatility of the gas-solid reaction-based polymorphic control will enable the engineering of metastable phases in 2D TMDs for further applications.

Solid-phase epitaxy (SPE), a solid-state phase transition of materials from an amorphous to a crystalline phase, is a convenient crystal growing technique. In particular, SPE can be used to grow α-Al 2 O 3 epitaxially with a novel structure that provides an effective substrate for improved performance of light-emitting diodes (LEDs). However, the inevitable two-step phase transformation through the γ-Al 2 O 3 phase hinders the expected improved crystallinity of α-Al 2 O 3 , and thereby further enhancement of LED performance. Herein, we provide a fundamental understanding of the SPE growth mechanism from amorphous to metastable γ-Al 2 O 3 using transmission electron microscopy (TEM) and density functional theory (DFT) calculations. The nanobeam precession electron diffraction technique enabled clear visualization of the double-positioning domain distribution in the SPE γ-Al 2 O 3 film and emphasized the need for careful selection of the viewing directions for any investigation of double-positioning domains. Void and stacking fault defects further investigated by high-resolution scanning TEM (STEM) analyses revealed how double-positioning domains and other SPE growth behaviors directly influence the crystallinity of SPE films. Additionally, DFT calculations revealed the origins of SPE growth behavior. The double-positioning γ-Al 2 O 3 domains randomly nucleate from the α-Al 2 O 3 substrate regardless of the α-Al 2 O 3 termination layer, but the large energy requirement for reversal of the γ-Al 2 O 3 stacking sequence prevents it from switching the domain type during the crystal growth. We expect that this study will be useful to improve the crystallinity of SPE γ- and α-Al 2 O 3 films.

We describe the direct preparation of crystalline Ni 3 S 2 thin films via atomic layer deposition (ALD) techniques at temperatures as low as 250 °C without postthermal treatments. A new ALD chemistry is proposed using bis(1-dimethylamino-2-methyl-2-butoxy) nickel(II) [Ni(dmamb) 2 ] and H 2 S as precursors. Homogeneous and conformal depositions of Ni 3 S 2 films were achieved on 4 in. wafers (both metal and oxide substrates, including Au and SiO 2 ). The resulting crystalline Ni 3 S 2 layers exhibited highly efficient and stable performance as electrocatalysts for both the hydrogen evolution reaction (HER) and the oxygen evolution reaction (OER) in alkaline solutions, with a low overpotential of 300 mV and a high turnover frequency for HER and an overpotential of 400 mV for OER (at a current density of 10 mA/cm 2 ). Using our Ni 3 S 2 films as both the cathode and the anode, two-electrode full-cell electrolyzers were constructed, which showed stable operation for 100 h at a current density of 10 mA/cm 2 . The proposed ALD electrocatalysts on planar surfaces exhibited the best performance among Ni 3 S 2 materials for overall water splitting recorded to date.

α-MnO 2 -structured materials are generally classified as semiconductors; thus, we present a strategy to increase electrochemical utilization through the design of a conductive material interface. Surface treatment of silver hollandite (Ag x Mn 8 O 16 ) with Ag + (Ag 2 O) provides significant benefits to the resultant electrochemistry, including a decreased charge-transfer resistance and a 2-fold increase in deliverable energy density at a high rate. The improved function of this designed interface relative to conventional electrode fabrication strategies is highlighted.

To enhance the photoluminescence of lanthanide oxide, a clear understanding of its defect chemistry is necessary. In particular, when yttrium oxide, a widely used phosphor, undergoes doping, several of its atomic structures may be coupled with point defects that are difficult to understand through experimental results alone. Here, we report the strong enhancement of the photoluminescence (PL) of Y 2 O 3 :Bi 3+ via codoping with Li + ions and suggest a plausible mechanism for that enhancement using both experimental and computational studies. The codoping of Li + ions into the Y 2 O 3 :Bi 3+ phosphor was found to cause significant changes in its structural and optical properties. Interestingly, unlike previous reports on Li + codoping with several other phosphors, we found that Li + ions preferentially occupy interstitial sites of the Y 2 O 3 :Bi 3+ phosphor. Computational insights based on density functional theory calculations also indicate that Li + is energetically more stable in the interstitial sites than in the substitutional sites. In addition, interstitially doped Li + was found to favor the vicinity of Bi 3+ by an energy difference of 0.40 eV in comparison to isolated sites. The calculated DOS showed the formation of a shallow level directly above the unoccupied 6p orbital of Bi 3+ as the result of interstitial Li + doping, which may be responsible for the enhanced PL. Although the crystallinity of the host materials increased with the addition of Li salts, the degree of increase was minimal when the Li + content was low (<1 mol %) where major PL enhancement was observed. Therefore, we reason that the enhanced PL mainly results from the shallow levels created by the interstitial Li + .

Combinatorial optical screening of aprotic electrocatalysts has not yet been achieved primarily due to H + -associated mechanisms of fluorophore modulation. We have overcome this problem by using fluorophore metal-organic complexes. In particular, eosin Y and quinine can be coordinated with various metallic cations (e.g., Li + , Na + , Mg 2+ , Zn 2+ , and Al 3+ ) in aprotic solvents, triggering changes in their fluorescent properties. These interactions have been used in a reliable screening method to determine oxygen reduction/evolution reaction activities of 100 Mn-based binary catalysts for the aprotic Li-air battery.

Manganese oxides are considered attractive cathode materials for rechargeable batteries due to the high abundance and environmental friendliness of manganese. In particular, cryptomelane and hollandite are desirable due to their ability to host cations within their octahedral molecular sieve (OMS-2) α-MnO 2 structure. In this work, we investigate silver containing α-MnO 2 structured materials (Ag x Mn 8 O 16 , x = 1.22, L-Ag-OMS-2 or 1.66, H-Ag-OMS-2) as host materials for Li ion and Na ion insertion/deinsertion. The results indicate a significant difference in the lithiation versus sodiation process of the OMS-2 materials. Initial reduction of Ag 1.22 Mn 8 O 16 to 1.0 V delivered ∼370 mAh/g. Cycling of Ag 1.22 Mn 8 O 16 between voltage ranges of 3.8-1.7 V and 3.8-1.3 V in a Na battery delivered initial capacities of 113 and 247 mAh/g, respectively. In contrast, Ag 1.66 Mn 8 O 16 delivered only 15 mAh/g, ∼ 0.5 electron equivalents, to 1.7 and 1.3 V. Study of the system by electrochemical impedance spectroscopy (EIS) showed a significant decrease in charge transfer resistance from 2029 Ω to 594 Ω after 1.5 electron equivalents per Ag 1.22 Mn 8 O 16 formula unit of Na ion insertion. In contrast, both Ag 1.22 Mn 8 O 16 and Ag 1.66 Mn 8 O 16 exhibited gradual impedance increases during lithiation. The formation of silver metal could be detected only in the sodiated material by X-ray diffraction (XRD). Thus, the impedance of Ag-OMS-2 decreases upon sodiation coincident with the formation of silver metal during the discharge process, consistent with the more favorable formation of silver metal during the sodiation process relative to the lithation process.

We have synthesized a single crystalline Y 2 C electride of centimeter-scale by floating-zone method and successfully characterized its anisotropic electrical and magnetic properties. In-plane resistivity upturn at low temperature together with anisotropic behavior of negative magnetoresistance is ascribed to the stronger suppression of spin fluctuation along in-plane than that along the c-axis, verifying the existence of magnetic moments preferred for the c-axis. A superior magnetic moment along the c-axis to that along the in-plane direction strongly demonstrates the anisotropic magnetism of Y 2 C electride containing a magnetically easy axis. It is clarified from the theoretical calculations that the anisotropic nature of the Y 2 C electride originates from strongly localized anionic electrons with an inherent magnetic anisotropy in the interlayer spaces.

Due to the numerous advantages of nanofibers, there is a strong demand in various fields for nanofibrous structures fabricated by electrospinning. However, the process is currently beset by troublesome limitations with respect to geometric and morphological control of electrospun nanofibrous mats. This study presents a direct-write electrospinning process and apparatus with improved focusing and scanning functionalities for the fabrication of various patterned thick mats and nanofibrous patterns with high geometric fidelity, supported by a number of experimental results. Consequently, various patterned nanofibrous mats were fabricated using the developed method. Additionally, the fabricated mat was successfully used for cell patterning as a bioengineering application. The proposed method is expected to significantly improve the properties and functionalities of nanofibrous mats in a variety of applications.

The SERS phenomenon was studied using a large set of silver nanocube dimers programmed to self-assemble in preset locations of a patterned substrate. This SERS substrate made it possible to demonstrate the dependence of the SERS enhancement on the geometry of the silver nanocube dimers and to quantify the dispersion in the SERS enhancement obtained in an ensemble of dimers. In addition to the effects of the gap distance of the dimer and the orientation of the dimer axis relative to the laser polarization on SERS enhancement, the data reveal an interesting dependence of the site-to-site variations of the enhancement on the relative orientation of the nanocubes in the dimer. We observed the highest heterogeneity in the SERS signal intensity with face-to-face dimers and a more robust SERS enhancement with face-to-edge dimers. Numerical calculations indicate that the plasmon resonance frequencies of face-to-face dimers shift considerably with small changes in gap distance. The resonance frequency shifts make it less likely for many of the dimers to satisfy the matching condition between the photon frequencies and the plasmon resonance frequency, offering an explanation for the large site-to-site variations in SERS signal intensity. These results indicate that plasmonic nanostructure designs for SERS substrates for real-world applications should be selected not only to maximize the signal enhancement potential but also to minimize the heterogeneity of the substrate with respect to signal enhancement. The latter criterion poses new challenges to experimentalists and theorists alike.

Silicon nanowires (SiNWs) offer promising inorganic nanostructures for biomedical application. Here, we report the development of a novel SiNW array designed for isolating primary CD4(+) T lymphocytes from the heterogeneous mixture of cell populations. Our system employed the specific high-affinity binding features of streptavidin (STR)-functionalized SiNW with biotin-labeled CD4(+) T lymphocytes. Fabricated SiNW arrays easily separated the CD4(+) T lymphocytes from the mouse whole splenocytes with over approximately 88% purity and demonstrated tight attachment to CD4(+) T lymphocytes by scanning electron microscopy. Thus, our STR-SiNW arrays provide a potential tool for specific cell separation and further present a possibility to be applied to the other area of biomedical applications.

Nylon 6 fibers are used, for the first time, in dye-sensitized solar cells (DSSCs). The overall energy conversion efficiency obtained with 0.18 M nylon 6 reaches 6.2%, which is comparable to that (6.7%) obtained without adding nylon 6 on the day of cell fabrication. However, it is found that the long-term stability of the DSSCs with nylon 6 is superior to that of a reference electrolyte as a result of the complexation of nylon 6 with I(3)(-). Furthermore, nylon 6 is found to be a corrosion inhibitor for silver metal in the electrolyte containing I(3)(-).