Herein we describe the morphology-controlled synthesis of three-dimensional (3D) ZnO nanoarchitectures via a facile hydrothermal route. In this approach, vertical and in-plane crystallographic orientations of ZnO crystals were tuned by appropriate patterning of growth masks and seed layer control, which enabled the formation of three types of ZnO nanoarchitectures. If polycrystalline ZnO layers with poor c-axis orientation were used as a seed, flower-like ZnO nanoarchitectures composed of radially oriented ZnO nanorods were achieved. In the case of c-axis oriented ZnO seeds, polygonal ZnO pillars grew vertically at the center of the growth holes. Even in the latter case, multidomain columnar–joint structures or single domain crystal structures with well-defined hexagonal facets were achieved based on the existence of six-fold in-plane symmetry of the ZnO seed layers. These morphology-controlled ZnO nanoarchitectures exhibited clear differences in light propagation characteristics, which could be ascribed to strong light guiding in the one-dimensional nanostructures.

Among the various types of fuel cells, the polymer electrolyte fuel cell (PEFC) is one of the prospective power sources for automotive applications, stationary cogeneration systems, and mobile electronic devices. The PEFC is very sensitive to the high temperature environment inside the fuel cell, and non-uniform temperature distribution reduces its performance. In this study, the performance of cooling plates for the PEFC was investigated by using threedimensional computational fluid dynamics with commercial software. Six cooling plates were designed with different channel configurations. Models 1 and 4 had typical serpentine and parallel configurations, respectively. Models 2 and 3 had modified serpentine structures from Model 1, while Models 5 and 6 had modified parallel structures from Model 4. Models 1 and 2 showed relatively high temperatures around the outlet and the inlet area of the channel, respectively. Cooling performance of Models 4 and 5 was lower than that of Model 6 due to non-uniform fluid flow and temperature distributions. Models 3 and 6 showed higher cooling performance than serpentine type models and parallel type models, respectively. The performance of Model 3 was superior to that of Model 6 with respect to the control of the maximum surface temperature and uniformity. The thermal performance of Model 3 improved over Model 6 with the increase of heat flux. However, the pressure drop of Model 3 was higher than that of Model 6 because Model 3 had relatively high flow velocity through its channel and greater number of bends than Model 6.

Active nanophotonic devices are attractive due to their low-power consumption, ultrafast modulation speed and high-density integration. Although electrical operation is required for practical implementation of these devices, it is not straightforward to introduce a proper current path into such a wavelength-scale nanostructure without affecting the optical properties. For example, to demonstrate electrically driven nanolasers, complicated fabrication techniques have been used thus far. Here we report an electrically driven microdisk laser using a transparent graphene electrode. Current is injected efficiently through the graphene sheet covering the top surface of the microdisk cavity, and, for the first time, lasing operation was achieved with a low-threshold current of ~300 μA at room temperature. In addition, we measured significant electroluminescence from a graphene-contact subwavelength-scale single nanopillar structure. This work represents a new paradigm for the practical applications of integrated photonic systems, by conformally mounting graphene on the complex surfaces of non-planar three-dimensional nanostructures.

We investigated surface-roughness-dependent optical loss in a plasmonic cavity consisting of a semiconductor nanodisk/silver nanopan structure. Numerical simulations show that the quality factors of plasmonic resonant modes significantly depend on the surface roughness of the dielectric-metal interface in the cavity structure. In the transverse-magnetic-like whispering-gallery plasmonic mode excited in a structure with disk diameter of 1000 nm, the total quality factor decreased from 260 to 130 with increasing root-mean-square (rms) surface roughness from 0 to 5 nm. This quantitative theoretical study shows that the smooth metal surface plays a critical role in high-performance plasmonic devices.