Sticky Prices and Alternative Monetary Feedback Rules : How Robust is the Overshooting Phenomenon ?

Kempa, Bernd,Nelles, Michael 한국국제경제학회 1999 International Economic Journal Vol.13 No.3

Facet-Selective Growth on Nanowires Yields Multi-Component Nanostructures and Photonic Devices

Kempa, Thomas J.,Kim, Sun-Kyung,Day, Robert W.,Park, Hong-Gyu,Nocera, Daniel G.,Lieber, Charles M. American Chemical Society 2013 JOURNAL OF THE AMERICAN CHEMICAL SOCIETY - Vol.135 No.49

<P>Enhanced synthetic control of the morphology, crystal structure, and composition of nanostructures can drive advances in nanoscale devices. Axial and radial semiconductor nanowires are examples of nanostructures with one and two structural degrees of freedom, respectively, and their synthetically tuned and modulated properties have led to advances in nanotransistor, nanophotonic, and thermoelectric devices. Similarly, developing methods that allow for synthetic control of greater than two degrees of freedom could enable new opportunities for functional nanostructures. Here we demonstrate the first regioselective nanowire shell synthesis in studies of Ge and Si growth on faceted Si nanowire surfaces. The selectively deposited Ge is crystalline, and its facet position can be synthetically controlled <I>in situ</I>. We use this synthesis to prepare electrically addressable nanocavities into which solution soluble species such as Au nanoparticles can be incorporated. The method furnishes multicomponent nanostructures with unique photonic properties and presents a more sophisticated nanodevice platform for future applications in catalysis and photodetection.</P><P><B>Graphic Abstract</B> <IMG SRC='http://pubs.acs.org/appl/literatum/publisher/achs/journals/content/jacsat/2013/jacsat.2013.135.issue-49/ja411050r/production/images/medium/ja-2013-11050r_0005.gif'></P><P><A href='http://pubs.acs.org/doi/suppl/10.1021/ja411050r'>ACS Electronic Supporting Info</A></P>

Coaxial multishell nanowires with high-quality electronic interfaces and tunable optical cavities for ultrathin photovoltaics

Kempa, T.J.,Cahoon, J.F.,Kim, S.-K.,Day, R.W.,Bell, D.C.,Park, H.-G.,Lieber, C.M. National Academy of Sciences 2012 PROCEEDINGS OF THE NATIONAL ACADEMY OF SCIENCES OF Vol.109 No.5

High-throughput patterning of photonic structures with tunable periodicity

Kempa, Thomas J.,Bediako, D. Kwabena,Kim, Sun-Kyung,Park, Hong-Gyu,Nocera, Daniel G. National Academy of Sciences 2015 PROCEEDINGS OF THE NATIONAL ACADEMY OF SCIENCES OF Vol.112 No.17

<P><B>Significance</B></P><P>Patterning large substrate areas with arrays of submicrometer structures in a facile, reliable, and timely manner is important for fabrication of optical elements that capture, guide, and convert light. RIPPLE (reactive interface patterning promoted by lithographic electrochemistry) is an electrochemical patterning method that is demonstrated for the rapid fabrication of periodic arrays of metallic circular Bragg gratings over large substrate areas. The grating period can be tuned in situ over micrometer and submicrometer length scales in a high-throughput fashion. We have identified point-like and annular scattering modes at different planes above the structured surface, suggesting the potential to use such structures to control the propagation of light. The described methods may be useful for high-throughput fabrication of sensors and light-management elements for energy conversion applications.</P><P>A patterning method termed “RIPPLE” (reactive interface patterning promoted by lithographic electrochemistry) is applied to the fabrication of arrays of dielectric and metallic optical elements. This method uses cyclic voltammetry to impart patterns onto the working electrode of a standard three-electrode electrochemical setup. Using this technique and a template stripping process, periodic arrays of Ag circular Bragg gratings are patterned in a high-throughput fashion over large substrate areas. By varying the scan rate of the cyclically applied voltage ramps, the periodicity of the gratings can be tuned in situ over micrometer and submicrometer length scales. Characterization of the periodic arrays of periodic gratings identified point-like and annular scattering modes at different planes above the structured surface. Facile, reliable, and rapid patterning techniques like RIPPLE may enable the high-throughput and low-cost fabrication of photonic elements and metasurfaces for energy conversion and sensing applications.</P>

Global Macroeconomic Repercussions of US Trade Restrictions: Evidence from a GVAR Model

Bernd Kempa,Nazmus Sadat Khan 한국국제경제학회 2019 International Economic Journal Vol.33 No.4

We employ a global vector autoregression (GVAR) model to analyze international spillover effects of US trade restrictions, modeled as a reduction of US imports. Our sample consists of the US and 25 countries in the rest of the world, grouped into larger regions comprising European nations, non-European industrial countries and emerging economies. We find US trade restrictions to reduce trade volumes and income levels in the rest of the world as well as in the US. The trade balance deteriorates across all world regions except in the US, where it is unaffected by the trade restrictions. We also model the effects of a trade war in which the rest of the world responds in equal measure to the trade restrictions imposed by the US. We again find that export and import activity recedes both in the US and in the rest of the world, although the resulting effects are now strongest in the short run. The trade balance improves in the rest of the world but deteriorates in the US. In terms of the GDP response, the rest of the world is initially much harder hit by the imposition of the retaliatory trade measure than is the US.

Quantum oscillations in the surface excitations of ultrathin Mg(0001) films

Teng, Ao,Kempa, Krzysztof,Ö,zer, Mustafa M.,Hus, Saban M.,Snijders, Paul C.,Lee, Geunseop,Weitering, Hanno H. American Physical Society 2014 Physical review. B, Condensed matter and materials Vol.90 No.11

Laterally assembled nanowires for ultrathin broadband solar absorbers.

Song, Kyung-Deok,Kempa, Thomas J,Park, Hong-Gyu,Kim, Sun-Kyung Optical Society of America 2014 Optics express Vol.22 No.suppl3

<P>We studied optical resonances in laterally oriented Si nanowire arrays by conducting finite-difference time-domain simulations. Localized Fabry-Perot and whispering-gallery modes are supported within the cross section of each nanowire in the array and result in broadband light absorption. Comparison of a nanowire array with a single nanowire shows that the current density (J(SC)) is preserved for a range of nanowire morphologies. The J(SC) of a nanowire array depends on the spacing of its constituent nanowires, which indicates that both diffraction and optical antenna effects contribute to light absorption. Furthermore, a vertically stacked nanowire array exhibits significantly enhanced light absorption because of the emergence of coupled cavity-waveguide modes and the mitigation of a screening effect. With the assumption of unity internal quantum efficiency, the J(SC) of an 800-nm-thick cross-stacked nanowire array is 14.0 mA/cm2, which yields a ~60% enhancement compared with an equivalent bulk film absorber. These numerical results underpin a rational design strategy for ultrathin solar absorbers based on assembled nanowire cavities.</P>

Asymmetric Transmission of Monetary Policy in Europe: a Markov-switching Approach

( Torben W. Hendricks ),( Bernd Kempa ) 세종대학교 경제통합연구소 (구 세종대학교 국제경제연구소) 2008 Journal of Economic Integration Vol.23 No.4

This paper addresses the question of whether the common monetary policy of the European Central Bank is transmitted asymmetrically within the Euro area. To this end, we employ a Markov switching model to identify a credit channel of monetary policy transmission and investigate whether the credit channel is active or passive across four Euro-area countries (France, Germany, Italy and the Netherlands) as well as the United Kingdom at any given point in time. We find the timing and the duration of the regime switches into and out of the credit channel regime to be fairly synchronous for France, Germany and Italy, while being more heterogeneous for both the Netherlands and the United Kingdom. The asymmetry of the credit channel documented in this paper suggests that the heterogeneity of the monetary transmission mechanism is still very much present in the Euro area. This heterogeneity poses serious policy problems for the European Central Bank (ECB), as monetary policy continues to exert differential impacts across individual member countries of the monetary union. Also, it constitutes a convincing reason of why the ECB, the European Commission and a number of national governments actively push for further liberalization of financial markets in Europe.

Design of Nanowire Optical Cavities as Efficient Photon Absorbers

Kim, Sun-Kyung,Song, Kyung-Deok,Kempa, Thomas J.,Day, Robert W.,Lieber, Charles M.,Park, Hong-Gyu American Chemical Society 2014 ACS NANO Vol.8 No.4

<P>Recent investigations of semiconductor nanowires have provided strong evidence for enhanced light absorption, which has been attributed to nanowire structures functioning as optical cavities. Precise synthetic control of nanowire parameters including chemical composition and morphology has also led to dramatic modulation of absorption properties. Here we report finite-difference time-domain (FDTD) simulations for silicon (Si) nanowire cavities to elucidate the key factors that determine enhanced light absorption. The FDTD simulations revealed that a crystalline Si nanowire with an embedded 20-nm-thick amorphous Si shell yields 40% enhancement of absorption as compared to a homogeneous crystalline Si nanowire, under air-mass 1.5 global solar spectrum for wavelengths between 280 and 1000 nm. Such a large enhancement in absorption results from localization of several resonant modes within the amorphous Si shell. A nanowire with a rectangular cross section exhibited enhanced absorption at specific wavelengths with respect to a hexagonal nanowire. The pronounced absorption peaks were assigned to resonant modes with a high symmetry that red-shifted with increasing size of the rectangular nanowire. We extended our studies to investigate the optical properties of single- and multilayer arrays of these horizontally oriented nanowire building blocks. The absorption efficiency of a nanowire stack increases with the number of nanowire layers and was found to be greater than that of a bulk structure or even a single nanowire of equivalent thickness. Lastly, we found that a single-layer nanowire array preserves the structured absorption spectrum of a single nanowire and ascribed this result to a diffraction effect of the periodic nanowire array. The results from these provide insight into the design of nanowire optical cavities with tunable and enhanced light absorption and thus, could help enable the development of ultrathin solar cells and other nanoscale optoelectronic devices.</P><P><B>Graphic Abstract</B> <IMG SRC='http://pubs.acs.org/appl/literatum/publisher/achs/journals/content/ancac3/2014/ancac3.2014.8.issue-4/nn5003776/production/images/medium/nn-2014-003776_0008.gif'></P><P><A href='http://pubs.acs.org/doi/suppl/10.1021/nn5003776'>ACS Electronic Supporting Info</A></P>

Tuning Light Absorption in Core/Shell Silicon Nanowire Photovoltaic Devices through Morphological Design

Kim, Sun-Kyung,Day, Robert W.,Cahoon, James F.,Kempa, Thomas J.,Song, Kyung-Deok,Park, Hong-Gyu,Lieber, Charles M. American Chemical Society 2012 Nano letters Vol.12 No.9

<P>Subwavelength diameter semiconductor nanowires can support optical resonances with anomalously large absorption cross sections, and thus tailoring these resonances to specific frequencies could enable a number of nanophotonic applications. Here, we report the design and synthesis of core/shell p-type/intrinsic/n-type (p/i/n) Si nanowires (NWs) with different sizes and cross-sectional morphologies as well as measurement and simulation of photocurrent spectra from single-NW devices fabricated from these NW building blocks. Approximately hexagonal cross-section p/i/n coaxial NWs of various diameters (170–380 nm) were controllably synthesized by changing the Au catalyst diameter, which determines core diameter, as well as shell deposition time, which determines shell thickness. Measured polarization-resolved photocurrent spectra exhibit well-defined diameter-dependent peaks. The corresponding external quantum efficiency (EQE) spectra calculated from these data show good quantitative agreement with finite-difference time-domain (FDTD) simulations and allow assignment of the observed peaks to Fabry–Perot, whispering-gallery, and complex high-order resonant absorption modes. This comparison revealed a systematic red-shift of equivalent modes as a function of increasing NW diameter and a progressive increase in the number of resonances. In addition, tuning shell synthetic conditions to enable enhanced growth on select facets yielded NWs with approximately rectangular cross sections; analysis of transmission electron microscopy and scanning electron microscopy images demonstrate that growth of the n-type shell at 860 °C in the presence of phosphine leads to enhanced relative Si growth rates on the four {113} facets. Notably, polarization-resolved photocurrent spectra demonstrate that at longer wavelengths the rectangular cross-section NWs have narrow and significantly larger amplitude peaks with respect to similar size hexagonal NWs. A rectangular NW with a diameter of 260 nm yields a dominant mode centered at 570 nm with near-unity EQE in the transverse-electric polarized spectrum. Quantitative comparisons with FDTD simulations demonstrate that these new peaks arise from cavity modes with high symmetry that conform to the cross-sectional morphology of the rectangular NW, resulting in low optical loss of the mode. The ability to modulate absorption with changes in nanoscale morphology by controlled synthesis represents a promising route for developing new photovoltaic and optoelectronic devices.</P><P><B>Graphic Abstract</B> <IMG SRC='http://pubs.acs.org/appl/literatum/publisher/achs/journals/content/nalefd/2012/nalefd.2012.12.issue-9/nl302578z/production/images/medium/nl-2012-02578z_0005.gif'></P><P><A href='http://pubs.acs.org/doi/suppl/10.1021/nl302578z'>ACS Electronic Supporting Info</A></P>