Sanghyo Lee

Stretchable Energy‐Harvesting Tactile Electronic Skin Capable of Differentiating Multiple Mechanical Stimuli Modes

The first stretchable energy-harvesting electronic-skin device capable of differentiating and generating energy from various mechanical stimuli, such as normal pressure, lateral strain, bending, and vibration, is presented. A pressure sensitivity of 0.7 kPa(-1) is achieved in the pressure region <1 kPa with power generation of tens of μW cm(-2) from a gentle finger touch.

Engineering of efficiency limiting free carriers and an interfacial energy barrier for an enhancing piezoelectric generation

The energy harvesting efficiency is of tremendous importance for the realization of a high output-power nanogenerator serving as the basis for self-powered electronics. Here we report that the device performance of a sound-driven piezoelectric energy nanogenerator (SPENG) is remarkably improved by controlling both the carrier density and the interfacial energy in a semiconducting ZnO nanowire (NW), thereby achieving its intrinsic efficiency limits. A SPENG with carrier-controlled ZnO NWs exhibits excellent energy harvesting characteristics with an average power density of 0.9 mW cm−3, as well as a near 50 fold increase in both output voltage and current compared to those of a conventional ZnO NW. In addition, we demonstrate for the first time that an optimized SPENG is large enough and very suitable to drive electrophoretic ink displays based on voltage-drive systems. This fundamental progress makes it possible to fabricate high performance nanogenerators for viable industrial applications in portable/wearable personal electronics such as electronic papers and smart identity cards.

p‐Type Conduction Characteristics of Lithium‐Doped ZnO Nanowires

Nanostructured electronic devices are expected to facilitate the continuing miniaturization of electronic devices and enable ultralow power device operation. In particular, 1D nanostructures such as nanowires (NW) and nanotubes have attracted great interest over the past decade because of their specifi c physical properties and their potential as building blocks for next-generation nanoelectronic devices. [ 1 , 2 ] Among various 1D materials, zinc oxide (ZnO), which has a direct and wide bandgap, is a promising candidate for light-emitting diodes, UV and gas sensors, transistor channels, and other devices that can utilize the unique vertical alignment characteristics and highly ordered single crystalline properties of NW structures. [ 3–6 ] However, because undoped ZnO NWs are intrinsically n-type, their use in practical devices has been hindered and much effort has been dedicated toward the development of p-type ZnO NWs. In particular, control and manipulation of the doping process is increasingly becoming a key approach for the realization of p-type ZnO NWs. To realize p-type ZnO NWs, the initial dopant candidates tested included group V elements to substitute for O and group III elements to substitute for Zn, despite the large size mismatches in both cases. Recently, group I species such as Li and Na have been used to synthesize p-type ZnO NWs based on the expectation that these elements would function as shallow acceptors in ZnO host materials. [ 7–9 ] Li has the smallest ionic radius (0.76 Å) of group I species, which is very close to that of Zn (0.74 Å). Furthermore, several reports of excited centers observed using electron paramagnetic resonance spectroscopy have indicated that Li atoms can act as shallow acceptors in substantial forms of Zn sites (Li Zn ). [ 10 , 11 ] In addition, it is well-known that Li has specifi c advantages over other dopant

High Performance PbS Quantum Dot/Graphene Hybrid Solar Cell with Efficient Charge Extraction

Hybrid colloidal quantum dot (CQD) solar cells are fabricated from multilayer stacks of lead sulfide (PbS) CQD and single layer graphene (SG). The inclusion of graphene interlayers is shown to increase power conversion efficiency by 9.18%. It is shown that the inclusion of conductive graphene enhances charge extraction in devices. Photoluminescence shows that graphene quenches emission from the quantum dot suggesting spontaneous charge transfer to graphene. CQD photodetectors exhibit increased photoresponse and improved transport properties. We propose that the CQD/SG hybrid structure is a route to make CQD thin films with improved charge extraction, therefore resulting in improved solar cell efficiency.

Surface energy-mediated construction of anisotropic semiconductor wires with selective crystallographic polarity

ZnO is a wide band-gap semiconductor with piezoelectric properties suitable for opto-electronics, sensors, and as an electrode material. Controlling the shape and crystallography of any semiconducting nanomaterial is a key step towards extending their use in applications. Whilst anisotropic ZnO wires have been routinely fabricated, precise control over the specific surface facets and tailoring of polar and non-polar growth directions still requires significant refinement. Manipulating the surface energy of crystal facets is a generic approach for the rational design and growth of one-dimensional (1D) building blocks1234. Although the surface energy is one basic factor for governing crystal nucleation and growth of anisotropic 1D structures, structural control based on surface energy minimization has not been yet demonstrated56789. Here, we report an electronic configuration scheme to rationally modulate surface electrostatic energies for crystallographic-selective growth of ZnO wires. The facets and orientations of ZnO wires are transformed between hexagonal and rectangular/diamond cross-sections with polar and non-polar growth directions, exhibiting different optical and piezoelectrical properties. Our novel synthetic route for ZnO wire fabrication provides new opportunities for future opto-electronics, piezoelectronics, and electronics, with new topological properties.

Strategies for Developing Countries to Expand Their Shares in the Global Construction Market: Phase-Based SWOT and AAA Analyses of Korea

Koreas level of competitiveness in the global construction market has significantly improved over the years; from being an underdeveloped country in the 1960s, Korea is currently one of the top ten countries in terms of revenue in the global construction market. In general, an international construction business entails various risks and requires advanced financing and managerial and technical ability to become competitive. Therefore, it is challenging for developing countries to expand their share in the global construction market. In this regard, the case of the Korean international construction business (KICB) may serve as a benchmark for other countries aspiring to enter or reinforce their status in the global construction market. To this end, the strategies to achieve and maintain competitiveness need to be analyzed chronologically rather than focusing only on a particular period, especially the current status. This paper details the history of KICB, showing its evolution over the past 40 years, divided into four major phases that correspond with significant events. Moreover, the important aspects for securing competitive advantage in the market are clarified using four sets of strength, weakness, opportunity, and threat (SWOT) analyses. In addition, the strategic evolution over the four major phases is explored by introducing the adaptation, aggregation, and arbitrage (AAA) framework. The research findings indicate that the strategies adopted by KICB for achieving competitiveness included presenting a memo- rable performance in a primary country of a particular region (adaptation) during the initial phase, achieving economies of scale in a few regions during the development phase (aggregation), developing international specialization during the depression phase (arbitrage), and adapting to receive orders in a severely competitive market while coping with the local industry protection policy during the resurrection phase (adaptation). DOI: 10.1061/(ASCE)CO.1943-7862.0000316.

Strain-mediated interlayer coupling effects on the excitonic behaviors in an epitaxially-grown MoS2/WS2 van der Waals heterobilayer

van der Waals heterostructures composed of two different monolayer crystals have recently attracted attention as a powerful and versatile platform for studying fundamental physics, as well as having great potential in future functional devices because of the diversity in the band alignments and the unique interlayer coupling that occurs at the heterojunction interface. However, despite these attractive features, a fundamental understanding of the underlying physics accounting for the effect of interlayer coupling on the interactions between electrons, photons, and phonons in the stacked heterobilayer is still lacking. Here, we demonstrate a detailed analysis of the strain-dependent excitonic behavior of an epitaxially grown MoS2/WS2 vertical heterostructure under uniaxial tensile and compressive strain that enables the interlayer interactions to be modulated along with the electronic band structure. We find that the strain-modulated interlayer coupling directly affects the characteristic combined vibrational and excitonic properties of each monolayer in the heterobilayer. It is further revealed that the relative photoluminescence intensity ratio of WS2 to MoS2 in our heterobilayer increases monotonically with tensile strain and decreases with compressive strain. We attribute the strain-dependent emission behavior of the heterobilayer to the modulation of the band structure for each monolayer, which is dictated by the alterations in the band gap transitions. These findings present an important pathway toward designing heterostructures and flexible devices.

Solution processed vertically stacked ZnO sheet-like nanorod p–n homojunctions and their application as UV photodetectors

One long-standing goal in the development of one-dimensional nanostructured electronic devices is to facilitate the ongoing trend of miniaturization so as to enable ultralow power operation. Zinc oxide (ZnO) nanostructures, which have a direct and wide bandgap, are a central component in numerous electronic and optoelectronic applications. Here, we address vertically stacked ZnO sheet-like nanorod (SLNR) p–n homojunctions composed of single-crystalline undoped (n-type) and Li-doped (p-type) ZnO SLNRs by a multi-step solution-based hydrothermal route. Precise control of the molar concentration represented one of the basic factors in ensuring that the p–n homojunctions possessed appropriate densities and suitable morphologies. An extensive analysis of the luminescence features was carried out in order to identify p-type conduction in the Li-doped ZnO SLNRs. In addition, the SLNR-based p–n homojunctions exhibited distinct electrical features that validated their potential use as ultraviolet photodetectors, thereby spurring progress in the development of practical optoelectronics.

A Financing Model to Solve Financial Barriers for Implementing Green Building Projects

Along with the growing interest in greenhouse gas reduction, the effect of greenhouse gas energy reduction from implementing green buildings is gaining attention. The government of the Republic of Korea has set green growth as its paradigm for national development, and there is a growing interest in energy saving for green buildings. However, green buildings may have financial barriers that have high initial construction costs and uncertainties about future project value. Under the circumstances, governmental support to attract private funding is necessary to implement green building projects. The objective of this study is to suggest a financing model for facilitating green building projects with a governmental guarantee based on Certified Emission Reduction (CER). In this model, the government provides a guarantee for the increased costs of a green building project in return for CER. And this study presents the validation of the model as well as feasibility for implementing green building project. In addition, the suggested model assumed governmental guarantees for the increased cost, but private guarantees seem to be feasible as well because of the promising value of the guarantee from CER. To do this, certification of Clean Development Mechanisms (CDMs) for green buildings must be obtained.

Relationship between the financial crisis of Korean construction firms and macroeconomic fluctuations

Purpose – This study aims to analyze the relationship between the financial crisis of Korean construction firms and macroeconomic fluctuations.Design/methodology/approach – In this study, current ratio has been used an acting variable for liquidity ratio, and debt ratio for leverage ratio. GNI (Gross National Income), L (index of Liquidity), exchange rate, interest, and CPI (Consumer Price Index) were used for the macroeconomic variables. VECM consisted of Crt model and Drt model to analyze the relationship between current ratio and macroeconomic variables, and between debt ratio and macroeconomic variables, in order to analyze each model through variance decomposition and impulse response function.Findings – In Crt model, L is revealed as highly influencing current ratio. In other words, most fundraising is focused on highly capable financial institutes, investment corporations and public funds, since the scale of construction project funds is huge. Such financial sources actually belong to index L (inde...