S. A. Park

Large‐Scale Synthesis and Characterization of the Size‐Dependent Thermoelectric Properties of Uniformly Sized Bismuth Nanocrystals

Highly efficient thermoelectric materials have attracted tremendous attention because of various technological applications such as power generation from waste heat and environmentally friendly refrigeration. The efficiency of thermoelectric materials is generally evaluated in terms of thermoelectric figure of merit ZT= (sS/k)T, where s is the electrical conductivity, S is the Seebeck coefficient, k is the thermal conductivity, and T is the absolute temperature. Recently, various nanostructured thermoelectric materials have been reported to exhibit high ZT values. This increase in thermoelectric efficiency was attributed to the decrease of thermal conductivity caused by the increased interfaces to scatter phonons or the enhancement of power factor (sS) by quantum confinement effects. However, most of the highZT nanostructured materials were prepared by costly and complicated processes, making it very difficult to inexpensively synthesize a large quantity of nanostructured materials. More recently, several kinds of nanostructured bulk materials with high ZT values were fabricated in large quantity by a ball-milling process and subsequent hot-press process. Recently, colloidal chemical methods have been used to synthesize large quantities of uniform-sized nanocrystals. These chemical methods can synthesize uniform-sized nanocrystals in a size-controlled manner, allowing the characterization of size-dependent properties, which is very difficult to perform using top-down physical methods, such as the ballmilling process. Over the past few decades, intensive research has attempted to characterize the electrical properties of bulk bismuth (Bi), because it is semimetallic with a small band overlap and has high carrier mobility and extremely small carrier effective mass. Furthermore, thermoelectric properties of Bi nanocrystals were intensively studied, because theoretical calculations predicted that Bi nanocrystals can exhibit a ZT value as high as 10 at 77 K. Moreover, Bi costs around one tenth of the price of bismuth telluride, which is one of the most popular thermoelectric materials. However, a high ZT value has not yet been realized experimentally for Bi nanostructured materials. Although several chemical syntheses of Bi nanocrystals have been reported, the thermoelectric properties of spherical Bi nanocrystals have rarely been studied. Herein, we report a simple and largescale synthetic method to produce uniform-sized Bi nanocrystals with controlled sizes and characterized their sizedependent thermoelectric properties. The size-dependant electrical and thermal properties were clearly demonstrated using uniform Bi nanocrystals with controlled particle sizes. Interestingly, the ratio of electrical to thermal conductivity increased with decreasing particle size, which leads to the enhancement of the ZT values. Bi nanocrystals were synthesized by reducing bismuth dodecanethiolate, which was generated by the reaction of dodecanethiol and bismuth neodecanoate in octadecene. Bismuth thiolate was so reactive that Bi nanocrystals could be readily produced by injecting the mild reducing agent tri-noctylphosphine (TOP) into bismuth dodecanethiolate solution at a temperature as low as 80 8C. The sizes of Bi nanocrystals could be easily tuned by varying the aging temperature and time. Transmission electron microscopy (TEM) images (Figure 1a–f) show uniform-sized Bi nanocrystals with sizes ranging from 6 to 27 nm. All of the nanocrystals exhibited narrow size distribution with standard deviation of less than 10%. The electron diffraction patterns (Figure 1a–f, insets) revealed the highly crystalline nature of the Bi nanocrystals. The X-ray diffraction (XRD) patterns (Figure 1g) showed that all of the nanocrystals had a rhombohedral Bi structure (JCPDS 85-1331), while the peaks became broader as the size decreases. To demonstrate large-scale production, we performed the reaction with 20 mmol of bismuth precursor and obtained as much as [*] J. S. Son, K. Park, Prof. T. Hyeon National Creative Research Initiative Center for Oxide Nanocrystalline Materials World Class University (WCU) Program of Chemical Convergence for Energy & Environment (C2E2) School of Chemical and Biological Engineering Seoul National University, Seoul 151-744 (Korea) Fax: (+ 82)2-886-8457 E-mail: thyeon@snu.ac.kr

Pregabalin reduces post-operative pain after mastectomy: a double-blind, randomized, placebo-controlled study

Background: Pregabalin is used for the treatment of neuropathic pain and has shown analgesic efficacy in post‐operative pain. The aim of this randomized, double‐blinded, placebo‐controlled trial (Clinical Trials.gov ID NCT00938548) was to investigate the efficacy and safety of pregabalin for reducing post‐operative pain in patients after mastectomy.

Interfacial characteristics of N-incorporated HfAlO high-k thin films

The characteristics of N-incorporated HfO2–Al2O3 alloy films (HfAlO) were investigated by high-resolution x-ray photoelectron spectroscopy (XPS), near-edge x-ray absorption fine structure (NEXAFS), medium-energy ion scattering (MEIS), and capacitance–voltage measurements. The core-level energy states, Hf4f and Al2p peaks of a 15A thick film showed a shift to lower binding energy, resulting from the incorporation of nitrogen into the films. Absorption spectra of the OK edge of HfAlO were affected mainly by the Al2O3 in the film, and not by HfO2 after nitridation by NH3 annealing. The NEXAFS of NK edge and XPS data related to the chemical state suggested that the incorporated N atom is dominantly bonded to Al2O3, and not to HfO2. Moreover, MEIS results implied that there is a significant incorporation of N at the interface between the alloy film and Si. The incorporation of N effectively suppressed the leakage current without an increase in interfacial layer thickness, while the interfacial state of the N-i...

Phase separation of a Ge2Sb2Te5 alloy in the transition from an amorphous structure to crystalline structures

Changes in the structural and electrical properties of a Ge2Sb2Te5 alloy thin film induced by phase transition were investigated using various analytical techniques. X-ray diffraction and scanning photoelectron microscopy showed that the phase separation occurred in a local area of the film during a phase transition when the amorphous structure was being transformed into crystalline structures. It was found that the heterogeneous distribution of Sb atoms that diffused during the phase transition accompanied the phase separation. Atomic force microscopy was used to examine the changes in surface morphology and roughness. The electrical conductance of the film was dramatically improved after the phase transition from an amorphous structure to crystalline structures as evidenced by the sheet resistance measurements. The sheet resistance changed from ∼109to∼102Ω∕sq. during the phase transition. Differential scanning calorimetry was used to determine the exact phase transition temperature (160–170°C) and the e...

Evolution of tungsten-oxide whiskers synthesized by a rapid thermal-annealing treatment

Tungsten oxide whiskers were prepared on a tungsten thin film by oxidation with H2O and a subsequent annealing treatment at a temperature of over 900 °C in a vacuum. The tungsten oxide formed by oxidation was transformed into smooth, straight whiskers with a monoclinic-crystalline structure after the vacuum annealing treatment. The whiskers showed an oxygen-deficient stoichiometry and a crystalline structure consistent with W18O49, which was dependent on the annealing temperature and vacuum used. The competition between the whisker growth and the dissociation of W oxide has a significant effect on the crystal shape, as well as the size of the whiskers. A change in the binding state during whisker formation indicates that some of the dissociated W oxide contributes to whisker formation and that crystalline whiskers are grown at nucleation sites through this process.

Interfacial reaction depending on the stack structure of Al2O3 and HfO2 during film growth and postannealing

Interfacial reactions as a function of the stack structure of Al2O3 and HfO2 grown on Si by atomic-layer deposition were examined by various physical and electrical measurements. In the case of an Al2O3 film with a buffer layer of HfO2, reactions between the Al2O3 and Si layers were suppressed, while a HfO2 film with an Al2O3 buffer layer on the Si readily interacted with Si, forming a Hf–Al–Si–O compound. The thickness of the interfacial layer increased dramatically after an annealing treatment in which a buffer layer of Al2O3 was used, while no change in thickness was observed in the film in which a HfO2 buffer layer was used. Moreover, the stoichiometric change caused by a different reaction process altered the chemical state of the films, which affected charge trapping and the interfacial trap density.

Wave propagation characteristics of a figure-eight shaped nanoaperture

Wave propagation characteristics of apertures were analyzed to explain the light transmission of metallic nanoapertures. Based on Maxwell’s equations, the wave dispersion relations of wave propagation modes in nanoapertures were derived. The resonance frequency shift of the aperture and the variation of the spot size are explained with the dispersion relations. The relationship between near-field and far-field light transmission power throughput and spot size is also shown with the wave mode change predicted by the dispersion relations.

Enhanced thermal stability of high-dielectric Gd2O3 films using ZrO2 incorporation

The thermal stability of Gd2O3 films, containing added Zr, was investigated using various techniques. The structural characteristics of epitaxial Gd2O3 are maintained on the Si(111) substrate when the Zr is codeposited along with Gd. The incorporation of ZrO2 into Gd2O3 improves the crystallinity of the film and no interfacial layers were observed. In particular, the structural stability with no deformation is greatly enhanced, and silicate formation is drastically suppressed, up to an annealing temperature of 800 °C. The interfacial defects caused by extensive interactions between Gd and Si are also minimized.

Comparison of thermal conductivity in nanodot nanocomposites and nanograined nanocomposites

Most recent increases in thermoelectric performance have come by reducing thermal conductivity through nanostructuring. Therefore, current research efforts focus mainly on bulk nanocomposites. We simulated the thermal conductivities of two types of nanocomposites. We nanostructured Tl0.02Pb0.98Te by (i) embedding InSb nanodots in it, creating a nanodot nanocomposite, and (ii) polycrystallizing it, creating a nanograined nanocomposite. The nanograined nanocomposite achieved lower thermal conductivity than did the nanodot nanocomposite due to the ability of the nanosized grains in nanograined nanocomposites to effectively scatter phonons over a wide range of frequencies, as long as the nanograined nanocomposite has sufficiently small grain size.

Effect of ZrO2 incorporation into high dielectric Gd2O3 film grown on Si(111)

Gd2O3 films, in which ZrO2 was incorporated, were epitaxially grown on Si(111) using an electron-beam evaporation and effusion method. The crystalline structure and morphological characteristics were investigated by various measurements. A silicide layer was locally formed during the initial growth stage due to interactions between elemental Gd and Si in the Gd2O3 film, resulting in poor interfacial characteristics and extensive destruction of the crystalline structure. However, the incorporation of ZrO2 influenced the unit-cell structure of Gd2O3, which contains oxygen vacancies that is located diagonally, enhancing the structural stability owing to the effective suppression of the interfacial layer. The effect on the initial growth stage as the result of incorporation improves the crystalline quality of the epitaxial Gd2O3 film and structural coherence between the film and substrate.