Since the discovery of carbon nanotubes in 1991 (ref. 1), there have been significant research efforts to synthesize nanometre-scale tubular forms of various solids. The formation of tubular ...nanostructure generally requires a layered or anisotropic crystal structure. There are reports of nanotubes made from silica, alumina, silicon and metals that do not have a layered crystal structure; they are synthesized by using carbon nanotubes and porous membranes as templates, or by thin-film rolling. These nanotubes, however, are either amorphous, polycrystalline or exist only in ultrahigh vacuum. The growth of single-crystal semiconductor hollow nanotubes would be advantageous in potential nanoscale electronics, optoelectronics and biochemical-sensing applications. Here we report an 'epitaxial casting' approach for the synthesis of single-crystal GaN nanotubes with inner diameters of 30-200 nm and wall thicknesses of 5-50 nm. Hexagonal ZnO nanowires were used as templates for the epitaxial overgrowth of thin GaN layers in a chemical vapour deposition system. The ZnO nanowire templates were subsequently removed by thermal reduction and evaporation, resulting in ordered arrays of GaN nanotubes on the substrates. This templating process should be applicable to many other semiconductor systems.
High-quality gallium nitride nanowires have been synthesized via metal-initiated metalorganic chemical vapor deposition for the first time. Excellent substrate coverage was observed for wires ...prepared on silicon, c-plane, and a-plane sapphire substrates. The wires were formed via the vapor−liquid−solid mechanism with gold, iron, or nickel as growth initiators and were found to have widths of 15-200 nm. Transmission electron microscopy confirmed that the wires were single-crystalline and were oriented predominantly along the 210 or 110 direction. Wires growing along the 210 orientation were found to have triangular cross-sections. Transport measurements confirmed that the wires were n-type and had electron mobilities of ∼65 cm2/V·s. Photoluminescence measurements showed band edge emission at 3.35 eV (at 5 K), with a marked absence of low-energy emission from impurity defects.
In order to maintain the provision of high-quality ecosystem services in wetlands, it is important to protect the ecosystems through the designation of protected areas. However, the process of ...designating protected areas can potentially give rise to social conflicts or problems by the acquisition of private lands. As an alternative, the institution of payments for ecosystem services (PES) can be a more viable solution. This study intends to propose reasonable contract standards for PES that consider the preferences of both beneficiaries and providers, which are necessary for the successful introduction of PES in wetland protection areas in Korea. In doing so, we employed choice experiments to estimate the willingness to pay (WTP) and willingness to accept (WTA) of different stakeholders. Our findings indicate that both beneficiaries and providers had a positive perception of PES contract terms. Moreover, the WTP and WTA values were comparable, suggesting that the unit price of PES could be determined within a reasonable range. These results can serve as a foundation for acquiring additional funds required for the introduction of PES in wetland protected areas.
Protected areas are places that provide diverse ecosystem services, including cultural ecosystem services. At the same time, the development and unbalanced use of natural resources in protected areas ...often create environmental threats and social conflicts. This study estimates the economic value of environmental consequences derived from the construction of an airport in a national park in Korea. We employ a discrete choice experiment to assess economic values and consider a key part of the tradeoffs derived from the airport construction at Heuksan Island of Dadohae Marine National Park in Korea. The results show that social benefits related to improved accessibility and tourism opportunities would be generated but at substantial costs resulting from environmental degradation and the reduction in bird populations. A segmentation analysis also reveals that heterogeneous preferences exist based on variables reflecting individuals’ visit experiences and trust in the government. The findings indicate that considering environmental impacts is important when evaluating the feasibility of a development project in a protected area.
The demands on higher bandwidth with reduced power consumption in mobile market are driving mobile DRAM with advanced design techniques. Proposed LPDDR4 in this paper achieves over 39% improvement in ...power efficiency and over 4.3 Gbps data rate with 1.1 V supply voltage. These are challenging targets compared with those of LPDDR3. This work describes design schemes employed in LPDDR4 to satisfy these requirements, such as multi-channel-per-die architecture, multiple training modes, low-swing interface, DQS and clock frequency dividing, and internal reference for data and command-address signals. This chip was fabricated in a 3-metal 2y-nm DRAM CMOS process.
Reversible posterior leukoencephalopathy syndrome is a recently recognized disorder with characteristic radiologic findings that mainly involve the white/gray matter of the parieto-occipital lobes. ...This complex syndrome is associated with cyclosporine A therapy or a variety of other conditions in which blood pressure rises acutely. Twelve patients from a variety of conditions who met the diagnostic criteria for this syndrome were studied. Interestingly, three of these patients had intra-abdominal neurogenic tumors, which have rarely been reported. Initial cranial magnetic resonance imaging scans revealed fairly symmetric areas of increased T
2 signal involving both white and gray matter of parieto-occipital lobes in the majority of the patients. However, the lesions were often located outside the parieto-occipital regions. Four patients had occipital region magnetic resonance spectroscopy during the acute phase, which revealed high lactate peak and normal
N-acetyl aspartate/creatine and choline peaks. With appropriate treatment, most patients recovered from this syndrome and experienced almost complete resolution of brain lesions on follow-up magnetic resonance imaging. One patient, however, continued to have small residual hemosiderin deposits on a follow-up magnetic resonance imaging with neurologic sequellae. In conclusion, a better understanding of this complex syndrome may obviate unnecessary investigations and allow management of the associated problems in prompt and appropriate ways.
This study examines the determinants of income inequality in nonmetropolitan tourism- and recreation-dependent, manufacturing-dependent, and farming-dependent communities. Earnings in tourism and ...recreation industry, race, and region show a positive relationship with income inequality. Median household income and manufacturing employment are negatively related to income inequality. Tourism- and recreation-related variables (tourism and recreation employment, and tourism and recreation earnings) are identified as factors that contribute to income inequality. Race dualism and tourism- and recreation-dependent communities also contribute to increases in income inequality. Results of this study show that income inequality in selected U.S. nonmetropolitan communities increased during 1990 and 2000. The need for additional research on income inequality associated with tourism and recreation development is discussed. Characteristics of the labor market in the tourism and recreation industry and economic structures of the communities will be main issues in investigating the relationship between tourism and recreation development and income inequality.
PDF
