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Photoluminescent molecules and nanomaterials have potential applications as active waveguides, but have often been limited by high optical losses and complex fabrication processes. We explored ligand-protected metal nanoclusters, which can have strong, stable and tunable emission, as waveguides. Two alloy ligand-protected metal nanoclusters, Pt 1 Ag 18 and Au x Ag 19-x (7 ≤ x ≤ 9), were synthesized and structurally determined. Crystals of both nanoclusters exhibit excellent optical waveguide performance, with optical-loss coefficients of 5.26 × 10 −3 and 7.77 × 10 −3 decibels per micrometer, dB μm −1 , which are lower than those demonstrated by most inorganic, organic, and hybrid materials. The crystal packing and molecular orientation of the Pt 1 Ag 18 compound led to an extremely high polarization ratio of 0.91. Aggregation enhanced the quantum yields of Pt 1 Ag 18 and Au x Ag 19-x nanoclusters by 115- and 1.5-fold, respectively. This photonic cluster with low loss and high polarization provides a generalizable and versatile platform for active waveguides and polarizable materials.

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With intense urbanization and sustained population growth, securing food production with limited land sources has increasingly become a pressing issue. Based on an analysis of international cereal (i.e., barley, buckwheat, maize, oats, rice, rye, sorghum, soybean, and wheat) trade and differences in yields of the cereal between export and import countries over the period of 2007 to 2011, we explore the great potential of land saving through the international cereal trade. By 'land saving', we refer to the reduced global total of lands required to produce a necessary amount of cereal when cereal is exported from a country with relatively large yield of the cereal to a country with relatively small yield of the cereal. Our scenario analysis suggests that international cereal trade would help mitigate the shortage of domestic arable land for many island countries (e.g., Japan) and countries in the arid Middle East and North Africa (e.g., Syria and Morocco). Furthermore, international cereal trade has the potential to generate 'land saving' of 50,092,284 ha of land per year, which is roughly the size of Spain. Drawing upon the definition of a similar concept-virtual water (Hoekstra and Hung 2002), we define virtual land as the area of land resources used for the production of goods. Through introducing the concept of virtual land, we believe land resources that are traditionally considered as stationary resources can flow with anthropogenic socioeconomic activities. The largest virtual-land flows (> 3,000,000 ha/year) exist between the United States (US) to China, Brazil to China, the US to Japan, the US to Mexico, and Argentina to China. However, not all virtual-land flows necessarily result in land saving. Thus, more endeavors are needed to plan the virtual-land flows for a larger land saving at the global scale.

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