Kidong Park

Zn2GeO4 and Zn2SnO4 nanowires for high-capacity lithium- and sodium-ion batteries

Germanium (Ge) and tin (Sn) are considered to be the most promising alternatives to commercial carbon materials in lithium- and sodium-ion batteries. High-purity zinc germanium oxide (Zn2GeO4) and zinc tin oxide (Zn2SnO4) nanowires were synthesized using a hydrothermal method, and their electrochemical properties as anode materials in lithium- and sodium-ion batteries were comparatively investigated. The nanowires had a uniform morphology and consisted of single-crystalline rhombohedral (Zn2GeO4) and cubic (Zn2SnO4) phases. For lithium ion batteries, Zn2GeO4 and Zn2SnO4 showed an excellent cycling performance, with a capacity of 1220 and 983 mA h g−1 after 100 cycles, respectively. Their high capacities are attributed to a combination of the alloy formation reaction of Zn and Ge (or Sn) with Li, and the conversion reactions: ZnO + 2Li+ + 2e− ↔ Zn + Li2O and GeO2 (or SnO2) + 4Li+ + 4e− ↔ Ge (or Sn) + 2Li2O. For the first time, we examined the cycling performance of Zn2GeO4 and Zn2SnO4 in sodium ion batteries; their capacities were 342 mA h g−1 and 306 mA h g−1 after 100 cycles, respectively. The capacity of Zn2SnO4 is much higher than the theoretical capacity (100 mA h g−1), while that of Zn2SnO4 is close to the theoretical capacity (320 mA h g−1). We suggest a contribution of the conversion reaction in increasing the capacities, which is similar to the case of lithium ion batteries. The present systematic comparison between the lithiation and sodiation will provide valuable information for the development of high-performance lithium- and sodium-ion batteries.

Ultrasound synthesis of lead halide perovskite nanocrystals

We report the ultrasound-induced synthesis of APbX3 perovskite nanocrystals with a wide range of compositions, where A = CH3NH3, Cs, or HNCHNH3 (formamidinium), and X = Cl, Br, or I. Ultrasonic irradiation accelerates dissolution of the precursors (AX and PbX2) in toluene, and the dissolution rate determines the growth rate of the nanocrystals. We fabricated high-sensitivity photodetectors by homogenously spin coating the uniform size nanocrystals on large-area silicon oxide substrates.

In Situ Temperature-Dependent Transmission Electron Microscopy Studies of Pseudobinary mGeTe·Bi2Te3 (m = 3–8) Nanowires and First-Principles Calculations

Phase-change nanowires (NWs) have emerged as critical materials for fast-switching nonvolatile memory devices. In this study, we synthesized a series of mGeTe·Bi2Te3 (GBT) pseudobinary alloy NWs-Ge3Bi2Te6 (m = 3), Ge4Bi2Te7 (m = 4), Ge5Bi2Te8 (m = 5), Ge6Bi2Te9 (m = 6), and Ge8Bi2Te11 (m = 8)-and investigated their composition-dependent thermal stabilities and electrical properties. As m decreases, the phase of the NWs evolves from the cubic (C) to the hexagonal (H) phase, which produces unique superlattice structures that consist of periodic 2.2-3.8 nm slabs for m = 3-8. In situ temperature-dependent transmission electron microscopy reveals the higher thermal stability of the compositions with lower m values, and a phase transition from the H phase into the single-crystalline C phase at high temperatures (400 °C). First-principles calculations, performed for the superlattice structures (m = 1-8) of GBT and mGeTe·Sb2Te3 (GST), show an increasing stability of the H phase (versus the C phase) with decreasing m; the difference in stability being more marked for GBT than for GST. The calculations explain remarkably the phase evolution of the GBT and GST NWs as well as the composition-dependent thermal stabilities. Measurement of the current-voltage curves for individual GBT NWs shows that the resistivity is in the range 3-25 mΩ·cm, and the resistivity of the H phase is lower than that of the C phase, which has been supported by the calculations.

Ternary alloy nanocrystals of tin and germanium chalcogenides

Tin (Sn) and germanium (Ge) chalcogenides have recently gained popularity as potential alternatives to the toxic lead chalcogenides, due in part to their relatively higher chemical and environmental stability, for use in photovoltaic devices. Herein, we report the synthesis of ternary composition chalcogenide, i.e., SnxGe1−xS, SnxGe1−xSe, GeSxSe1−x, and SnSxSe1−x, nanocrystals (NCs) by novel gas-phase laser photolysis. A full series of completely miscible alloy nanocrystals, the band gaps of which could be tuned to cover a wide range (0.9–1.6 eV), were obtained by facile composition tuning through adept control of the relative pressure of the precursors. Two cation alloy (SnxGe1−xS and SnxGe1−xSe) NCs exhibited unique anisotropic bowing phenomena of the orthorhombic phase lattice constants, but the anion alloy (GeSxSe1−x and SnSxSe1−x) NCs did not. The cation alloy NCs showed optical bowing, which is well correlated with the bowing of the lattice parameters. The SnxGe1−xS and SnSxSe1−x NCs showed remarkably higher photoconversion efficiency in photovoltaic and photodetector devices when compared with that of end members.

Schottky nanocontact of one-dimensional semiconductor nanostructures probed by using conductive atomic force microscopy.

To develop the advanced electronic devices, the surface/interface of each component must be carefully considered. Here, we investigate the electrical properties of metal-semiconductor nanoscale junction using conductive atomic force microscopy (C-AFM). Single-crystalline CdS, CdSe, and ZnO one-dimensional nanostructures are synthesized via chemical vapor transport, and individual nanobelts (or nanowires) are used to fabricate nanojunction electrodes. The current-voltage (I -V) curves are obtained by placing a C-AFM metal (PtIr) tip as a movable contact on the nanobelt (or nanowire), and often exhibit a resistive switching behavior that is rationalized by the Schottky (high resistance state) and ohmic (low resistance state) contacts between the metal and semiconductor. We obtain the Schottky barrier height and the ideality factor through fitting analysis of the I-V curves. The present nanojunction devices exhibit a lower Schottky barrier height and a higher ideality factor than those of the bulk materials, which is consistent with the findings of previous works on nanostructures. It is shown that C-AFM is a powerful tool for characterization of the Schottky contact of conducting channels between semiconductor nanostructures and metal electrodes.

Bent Polytypic ZnSe and CdSe Nanowires Probed by Photoluminescence

Nanowires (NWs) have witnessed tremendous development over the past two decades owing to their varying potential applications. Semiconductor NWs often contain stacking faults due to the presence of coexisting phases, which frequently hampers their use. Herein, it is investigated how stacking faults affect the optical properties of bent ZnSe and CdSe NWs, which are synthesized using the vapor transport method. Polytypic zinc blende-wurtzite structures are produced for both these NWs by altering the growth conditions. The NWs are bent by the mechanical buckling of poly(dimethylsilioxane), and micro-photoluminescence (PL) spectra were then collected for individual NWs with various bending strains (0-2%). The PL measurements show peak broadening and red shifts of the near-band-edge emission as the bending strain increases, indicating that the bandgap decreases with increasing the bending strain. Remarkably, the bandgap decrease is more significant for the polytypic NWs than for the single phase NWs. This work provides insights into flexible electronic devices of 1D nanostructures by engineering the polytypic structures.

Quantum Dots Formed in Three-dimensional Dirac Semimetal Cd3As2 Nanowires

We demonstrate quantum dot (QD) formation in three-dimensional Dirac semimetal Cd3As2 nanowires using two electrostatically tuned p-n junctions with a gate and magnetic fields. The linear conductance measured as a function of gate voltage under high magnetic fields is strongly suppressed at the Dirac point close to zero conductance, showing strong conductance oscillations. Remarkably, in this regime, the Cd3As2 nanowire device exhibits Coulomb diamond features, indicating that a clean single QD forms in the Dirac semimetal nanowire. Our results show that a p-type QD can be formed between two n-type leads underneath metal contacts in the nanowire by applying gate voltages under strong magnetic fields. Analysis of the quantum confinement in the gapless band structure confirms that p-n junctions formed between the p-type QD and two neighboring n-type leads under high magnetic fields behave as resistive tunnel barriers due to cyclotron motion, resulting in the suppression of Klein tunneling. The p-type QD with magnetic field-induced confinement shows a single hole filling. Our results will open up a route to quantum devices such as QDs or quantum point contacts based on Dirac and Weyl semimetals.

Strain Mapping and Raman Spectroscopy of Bent GaP and GaAs Nanowires

Strain engineering of nanowires (NWs) has been recognized as a powerful strategy for tuning the optical and electronic properties of nanoscale semiconductors. Therefore, the characterization of the strains with nanometer-scale spatial resolution is of great importance for various promising applications. In the present work, we synthesized single-crystalline zinc blende phase GaP and GaAs NWs using the chemical vapor transport method and visualized their bending strains (up to 3%) with high precision using the nanobeam electron diffraction technique. The strain mapping at all crystallographic axes revealed that (i) maximum strain exists along the growth direction ([111]) with the tensile and compressive strains at the outer and inner parts, respectively; (ii) the opposite strains appeared along the perpendicular direction ([2̅11]); and (iii) the tensile strain was larger than the coexisting compressive strain at all axes. The Raman spectrum collected for individual bent NWs showed the peak broadening and red shift of the transverse optical modes that were well-correlated with the strain maps. These results are consistent with the larger mechanical modulus of GaP than that of GaAs. Our work provides new insight into the bending strain of III–V semiconductors, which is of paramount importance in the performance of flexible or bendable electronics.

Orthorhombic NiSe2 Nanocrystals on Si Nanowires for Efficient Photoelectrochemical Water Splitting

Photocatalytic water splitting is a vital technology for clean renewable energy. Despite enormous progress, the search for earth-abundant photocatalysts with long-term stability and high catalytic activity is still an important issue. We report three possible polymorphs of nickel selenide (orthorhombic phase NiSe2, cubic phase NiSe2, and hexagonal phase NiSe) as bifunctional catalysts for water-splitting photoelectrochemical (PEC) cells. Photocathodes or photoanodes were fabricated by depositing the nickel selenide nanocrystals (NCs) onto p- or n-type Si nanowire arrays. Detailed structural analysis reveals that compared to the other two types, the orthorhombic NiSe2 NCs are more metallic and form less surface oxides. As a result, the orthorhombic NiSe2 NCs significantly enhanced the performance of water-splitting PEC cells by increasing the photocurrents and shifting the onset potentials. The high photocurrent is ascribed to the excellent catalytic activity toward water splitting, resulting in a low charge-transfer resistance. The onset potential shift can be determined by the shift of the flat-band potential. A large band bending occurs at the electrolyte interface, so that photoelectrons or photoholes are efficiently generated to accelerate the photocatalytic reaction at the active sites of orthorhombic NiSe2. The remarkable bifunctional photocatalytic activity of orthorhombic NiSe2 promises efficient PEC water splitting.

Remote impurity scattering in Si MOSFETs with thin gate oxides ‐ possible screening effect by mobile charges in the gate electrodes ‐

We have systematically investigated the electron mobilities, μ, in n‐Si MOSFETs with various gate oxide thicknesses (toxu2009=u20094u2009nm∼25u2009nm). It was found that μ is reduced with decreasing the thickness of the gate oxide in low electron density (Ns)/temperature (T) regions. Furthermore, from comparison between experiment and theory, we have found that the measured mobility for toxu2009=u20095u2009nm is much higher than the calculated one, while the measured μ for toxu2009=u200925u2009nm is in reasonable agreement with theory. Although the mechanism for the reduction in the scattering strength by remote impurities when toxu2009=u20095u2009nm is not clear at present, one plausible explanation is the screening effect by mobile electrons in the gate electrodes.