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D. Royer, Treatise Geochem. 6, 251 (2014). M. Huber, R. Caballero, Clim. Past 7, 603 (2011). G. S. Soreghan et al., Geology 36, 659 (2008). C. J. Poulsen, C. Tabor, J. D. White, Science 348, 1238 (2015). R. A. Berner, J. M. Vandenbrooks, P. D. Ward, Science 316, 557 (2007). P. G. Falkowski et al., Science 309, 2202 (2005). D. R. Greenwood et al., Geology 38, 15 (2010). I. P. Montañez, C. J. Poulsen, Annu. Rev. Earth Planet. Sci. 41, 629 (2013). I. J. Glasspool, A. C. Scott, Nat. Geosci. 3, 627 (2010). R. Tappert et al., Geochim. Cosmochim. Acta 121, 240 (2013). R. S. Arvidson, F. T. Mackenzie, M. W. Guidry, Chem. Geol. 362, 287 (2013). D. L. Royer, Y. Donnadieu, J. Park, J. Kowalczyk, Y. Goddéris, Am. J. Sci. 314, 1259 (2014). 10.1126/science.aac5264

NANOMATERIALS

Economical routes to colloidal nanocrystals A new precursor library yields high-quality quantum dots for device applications By Zeger Hens

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incorporation of the injected precursors, thiourea and a metal carboxylate. Instead, these precursors react to form the actual solute, and it is the solute supersaturation that initiates the nucleation and drives the growth of NCs (6). This two-step reaction mechanism has important consequences. First, the rate of solute formation sets the rate of solute consumption by nucleation of new nuclei or growth of existing nuclei into larger NCs (see the second figure, panel B). Second, hotinjection synthesis becomes a zero-sum game where solute consumption by growth

olloidal semiconductor nanocrystals (NCs) or “quantum dots” offer exquisite control of absorption and emission colors for device applications by tuning their size and shape (1). The proliferation of NC applications has been enabled by the “hot-injection” synthesis, which can tightly control the particle size to achieve pure colors (2). However, current hot-injection synthesis recipes are largely incompatible with low-cost, large-area remote phosphor applications of NCs. Reagents are too expensive, reaction mixtures too dilute, and conversion yields too low, especially if size tuning is accomplished by adjusting the reaction time. On page 1226 of this issue, Hendricks et al. (3) report on a radical improvement of the hot-injection synthesis economics of metal sulfide NCs by introducing a library of inexpensive thioureas as reagents. Diameter The hot-injection synthesis is a homogeneous nucleation and growth reaction (4) initiated by the rapid mixing of reagents at Color shift. Colloidal nanocrystals have size-dependent optical elevated temperatures in apolar properties. For example, photoluminescence of CdSe nanocrystals solvents. By stopping growth at shifts from blue to red with increasing diameter. a specific time, NCs of different sizes and thus different properties are readcan only increase at the expense of nucleily obtained (see the first figure). Hendricks ation. Faster solute formation benefits et al. introduce substituted thioureas as an nucleation, such that more NCs are formed alternative sulfur precursor for the formaby the time growth arrests nucleation (7). If tion of metal sulfide NCs, such as sulfides all precursors injected are transformed in of lead (PbS), cadmium, zinc, and copper, by more NCs, their size at the end of the rehot injection. As generic synthesis recipes action will be smaller, which makes for an for most of these NCs have been proposed effective mechanism for size tuning at full before (5), this result may not seem particuyield (8). larly groundbreaking. However, unlike other By proposing an easily implemented approaches, the authors focus on reactions chemical approach to adjust the rate of solthat have nearly quantitative chemical yield. ute formation, Hendricks et al. likely make They manage to control the final NC size the best possible use of this tuning scheme. by adapting the reactivity of the thioureas The vast possibilities for substitution make through subtle changes of their composifor an astonishing range of reactivities and tion (see the second figure, panel A). This thus a library of precursors fit to synthesize size-tuning scheme relies on the most recent understanding of the several steps that turn Physics and Chemistry of Nanostructures and Center for Nano injected precursors into NCs, especially the and Biophotonics, Ghent University, B9000 Ghent, Belgium. E-mail: [email protected] finding that NCs do not form by the separate

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cooling and reduced precipitation. Additional cooling may provide a mechanism for neartropical glaciation, because ice in the tropics cannot be modeled using radiative forcing from only CO2 (3). Both climate modelers and paleoclimate researchers most commonly express CO2 in terms of volume fraction (such as parts per million by volume, or ppmv). However, if total atmospheric pressure is changing, these CO2 estimates mean different things in terms of radiative forcing. For example, if the partial pressure of atmospheric CO2 remains constant but the total atmospheric pressure decreases because of a drop in the partial pressure of atmospheric O2, CO2 concentrations (expressed in ppmv) will increase, even though the CO2 radiative forcing will not have changed. Thus, if climate models prescribe CO2 ppmv estimates informed from proxies from a time period with a different atmospheric pressure from today’s (such as the Carboniferous or early Paleogene; see the figure), the modeled radiative forcing from CO2 may be incorrect. There would be great benefit to developing a common language for use in modeling and proxy work—for example, reporting whenever possible the sea-level partial pressure of CO2 along with the total atmospheric pressure of CO2. Additionally, because all CO2 proxies have been developed at modern atmospheric pressure, it will be crucial to test the sensitivity of CO2 proxies at differing atmospheric pressures. A critical limitation of exploring the climatic effects of Poulsen et al.’s model in the geologic past is that paleo-O2 records are uncertain (see the figure). During the Cenomanian and early Paleogene, O2 levels may have been lower or higher than today. New O2 proxies are in development (9, 10), but as with any new proxy system, they must be tested thoroughly. Poulsen et al.’s study provides a convincing argument for the importance of developing a robust history of atmospheric O2. ■

INSIGHTS | P E R S P E C T I V E S

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Visual tracking in the dead of night Even in dim light, hawkmoths can track the motions of wind-tossed flowers By Eric Warrant

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The reactivity of substituted thioureas can be tweaked by changing functional groups indicated in (A) as red (low reactivity) and green (high reactivity). In (B), the decomposition rates of the two precursors shown in (A) (the bold red and green curves) dictate the rates of monomer consumption by nucleation (blue curves) and growth (dashed orange curves). These processes are exponentially fast, so rates are plotted against the logarithm of time. When growth overtakes nucleation, the number of final NCs that will be formed from the nuclei (shown in blue) is now fixed; there are fewer nuclei for the slow reaction. At the end of the reaction, the less reactive precursor forms a smaller number of larger NCs (red), whereas the more reactive precursor forms a larger number of NCs that are smaller in size (green).

metal sulfide NCs with a predefined size starting from common chemicals. In this way, the work sets a standard for synthesis schemes of other NC material classes such as metal selenides, phosphides, and arsenides that can make possible large-area, low-cost NC applications such as remote phosphor lighting or solar concentrators (9, 10), thus stressing the importance of finding similarly versatile precursors for these NCs. Especially for their PbS synthesis, Hendricks et al. show that the reaction rate gives them full control over the NC size while maintaining extremely narrow size dispersions (the NCs are all nearly the same size). Thus, the size dispersion is not tightly linked to the reaction rate. Nor is the use of thioureas in itself sufficient to obtain a tight size dispersion. The thiourea-based synthesis of CuxS and Cu2ZnSnS4 (CZTS), for example, yields quite polydisperse NCs. These findings indicate that, unlike the case for size control, the question as to what factors really determine size dispersion in a hot-injection 1212

synthesis remains to be answered. Hence, following Hendricks et al. in their pursuit of insight-enabled synthesis economics, understanding and controlling size dispersion in the hot-injection synthesis appears to be an essential next step on the road to affordable, high-quality NCs. ■ REFERENCES

1. M. V. Kovalenko et al., ACS Nano 9, 1012 (2015). 2. C. B. Murray, D. J. Norris, M. G. Bawendi, J. Am. Chem. Soc. 115, 8706 (1993). 3. M. P. Hendricks, M. P. Campos, G. T. Cleveland, I. Jen-La Plante, J. S. Owen, Science 348, 1226 (2015). 4. V. K. LaMer, R. H. Dinegar, J. Am. Chem. Soc. 72, 4847 (1950). 5. H. Zhang, B.-R. Hyun, F. W. Wise, R. D. Robinson, Nano Lett. 12, 5856 (2012). 6. H. Liu, J. S. Owen, A. P. Alivisatos, J. Am. Chem. Soc. 129, 305 (2007). 7. J. S. Owen, E. M. Chan, H. Liu, A. P. Alivisatos, J. Am. Chem. Soc. 132, 18206 (2010). 8. S. Abe, R. K. Čapek, B. De Geyter, Z. Hens, ACS Nano 6, 42 (2012). 9. Y. Shirasaki, G. J. Supran, M. G. Bawendi, V. Bulovic, Nat. Photonics 7, 13 (2013). 10. C. S. Erickson et al., ACS Nano 8, 3461 (2014). 10.1126/science.aab0866

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octurnal insects live in a dim world. They have brains smaller than a grain of rice, and eyes that are even smaller. Yet, they have remarkable visual abilities, many of which seem to defy what is physically possible (1). On page 1245 of this issue, Sponberg et al. reveal how one species, the hawkmoth Manduca sexta, is able to accurately track wind-tossed flowers in near darkness and remain stationary while hovering and feeding (2). Examples of remarkable visual abilities include the nocturnal central American sweat bee Megalopta genalis, which can use learned visual landmarks to navigate from its nest—an inconspicuous hollowed-out twig hanging in the tangled undergrowth— through a dark and complex rainforest to a distant source of nocturnal flowers, and then return (3). The nocturnal Australian bull ant Myrmecia pyriformis manages similar navigational feats on foot (4). Nocturnal South African dung beetles can use the dim celestial pattern of polarized light around the moon (5) or the bright band of light in the Milky Way (6) as a visual compass to trace out a beeline when rolling dung balls. Some nocturnal insects, like the elephant hawkmoth Deilephila elpenor, even have trichromatic color vision (7). These impressive behavioral feats are likely to depend on specialized neural mechanisms in the brain that sum photons over space and time (8, 9), thus greatly strengthening the visual impression even though each photoreceptor absorbs photons at extremely low rates—in the American cockroach, as few as one photon every 10 s (10). Sponberg et al. now quantify the extent of these summation strategies for the detection of a moving target: a wind-tossed flower visually tracked by a dusk- and dawnsciencemag.org SCIENCE

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Economical routes to colloidal nanocrystals Zeger Hens Science 348, 1211 (2015); DOI: 10.1126/science.aab0866

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