Molecular Chemistry to the Fore: New Insights into the Fascinating

Department of Chemistry, Iowa State University, and Ames Laboratory, Ames, ... He was a postdoc with Rich Jordan at the University of Chicago from 200...
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Molecular Chemistry to the Fore: New Insights into the Fascinating World of Photoactive Colloidal Semiconductor Nanocrystals Javier Vela* Department of Chemistry, Iowa State University, and Ames Laboratory, Ames, Iowa 50011, United States ABSTRACT: Colloidal semiconductor nanocrystals possess unique properties that are unmatched by other chromophores such as organic dyes or transition-metal complexes. These versatile building blocks have generated much scientific interest and found applications in bioimaging, tracking, lighting, lasing, photovoltaics, photocatalysis, thermoelectrics, and spintronics. Despite these advances, important challenges remain, notably how to produce semiconductor nanostructures with predetermined architecture, how to produce metastable semiconductor nanostructures that are hard to isolate by conventional syntheses, and how to control the degree of surface loading or valence per nanocrystal. Molecular chemists are very familiar with these issues and can use their expertise to help solve these challenges. In this Perspective, we present our group’s recent work on bottom-up molecular control of nanoscale composition and morphology, low-temperature photochemical routes to semiconductor heterostructures and metastable phases, solar-to-chemical energy conversion with semiconductor-based photocatalysts, and controlled surface modification of colloidal semiconductors that bypasses ligand exchange.

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and help solve many of the remaining challenges and unknowns in the field of colloidal nanocrystals. Here, we describe recent efforts by our group aimed at achieving molecular-level (bottom-up) control of nanoscale composition and morphology, low-temperature photochemical synthesis of heterostructured nanocomposites and metastable low-dimensional phases, solar-to-chemical energy conversion of biorenewable feedstocks, and surface ligand doping, organization, and valence of semiconductor nanocrystal quantum dots. Molecular Programming: Bottom-Up Fabrication of Semiconductor Nanocrystals with Precisely Def ined Structure, Composition, and Morphology. Atomic composition gradients and phase segregation are ubiquitous features in many material preparations. The advent of colloidal methods has facilitated the synthesis and processing of low-dimensional nanomaterials and permitted fundamental investigations aimed at understanding the thermodynamic and kinetic reasons behind the generation of such features. Precursor decomposition is obviously affected by parameters such as reaction time and temperature, the presence of different surfactants, and the concentration of surfactants and precursors. To name a few recent examples, spheroidal (0D) PbSe nanocrystals result from the reaction between trioctylphosphine selenide (TOPSe) and lead (Pb) precursors over a wider temperature range (above ≥50 °C) compared to PbSe nanorods.43 Certain additives such as tris(diethylamino)phosphine accelerate PbSe growth by releasing amine in situ.43 The concentrations of TOPSe and TOP-telluride

olloidal semiconductor nanocrystals (quantum dots, rods, wires, platelets)1−11 have generated great scientific interest and found many applications in biological imaging,12−17 tracking,18−22 lighting,23,24 photovoltaics,25 photocatalysis,26 thermoelectrics,27 lasing,28 and spintronics.29 These highly versatile photoactive nanomaterials are well-known to benefit from size- and composition-tunable band gaps (300−4000 nm; 4.1−0.3 eV),1,30−32 broad and intense absorption (ε ≈ 106 L·mol−1·cm−1),33,34 long-lived excited states (up to 40 ns for CdSe, 500 ns for CuInS2, 1.8 μs for PbS),35−37 colloidal stability, and chemical and photostability.20,38−42 The optical properties of semiconductor nanocrystals are unmatched by other synthetic chromophores and fluorophores such as organic dyes or transition-metal complexes. Despite these advantages, and after a couple of decades of major scientific advances in this area, many challenges remain. Some of these challenges have to do with synthetic reproducibility and postsynthetic modification of these materials. Others relate to our ability to reliably and reproducibly control nanocrystal composition, architecture, and morphology (shape), how to selectively produce or assemble higher-order structures with specific configurations, and how to reliably dope chemical impurities into low-dimensional lattices. The best answer to these questions lies not on individual isolated examples but in our scientific ability to design distinct, powerful, and widely applicable synthetic strategies that span the continuum between the molecular, nano, meso, and bulk scales, thus enabling the effective processing and incorporation of new materials into energy conversion, catalytic, and tracking technologies. Molecular chemists are very familiar with and adept at dealing with many of these issues and can bring their expertise to the fore to address © 2013 American Chemical Society

Received: December 17, 2012 Accepted: February 1, 2013 Published: February 1, 2013 653

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(TOPTe) affect the composition of ternary PbSeTe alloys.44 Relative Zn and Cd precursor concentrations45 and reaction temperature affect the composition of ZnxCd1−xSe alloys.46,47 In terms of reaction mechanisms, much has been learned about the atomic-level kinetics of solution-phase precursor decomposition by studying the preparation of cadmium chalcogenides.48,49 Adventitious impurities in widely used “technical grade” reagents lead to well-recognized and widespread irreproducibility issues. Famously, secondary phosphine and phosphinic acid impurities affect the nucleation of both II−VI50 and IV−VI51 nanocrystals.51 A recent study found that tertiary phosphine selenide is relatively unreactive against metal carboxylates compared to small quantities of secondary phosphines. While certain additives increase yields and mechanistic studies help understand such phenomena, a more general picture has recently started to emerge where rational precursor variations permit more predictable outcomes. For example, balancing bis(trimethylsilyl)chalcogenide (TMS2E, E = S, Se, Te) reactivity allows tuning of ternary PbExE′1−x alloy composition.52 Nevertheless, there is a lack of guiding principles in this area. Ef fect of Phosphine Chalcogenide Reactivity on Semiconductor Composition and Morphology. We recently reported that a single injection of a mixture containing trioctylphosphine sulfide and selenide (TOPS and TOPSe) into a solution of bis(octadecylphosphonate)cadmium in octadecene at 320 °C results in spontaneous formation of axially anisotropic CdS1−xSex nanorods having a thick “head” and a thin “tail” (Scheme 1).53 X-ray diffraction (XRD), high-resolution and

the much more stable nanocrystals, effectively a thermodynamic sink. This idea opens new avenues for achieving molecularlevel or “bottom-up” control of nanocrystal composition, morphology, and properties. By tuning the sterics and electronics and thus manipulating the chemical reactivity of molecular precursors, it is possible to fine-tune the nucleation rates of different nanocrystalline phases. To explore this idea, we used density functional theory (DFT) calculations and NMR experiments to study the factors that control precursor reactivity at the atomic level. We used computed energy differences between isolated phosphines (R3P) and phosphine chalcogenides (R3PE, E = S or Se) to estimate PE bond strengths.54 We also experimentally measured 31P and 77Se NMR spectra of several phosphines and phosphine chalcogenides (Figure 1). 31P resonances of all phosphine selenides

Scheme 1. Spontaneous Phase Segregation along the Axis of Ternary CdS1−xSex Nanorods

energy-filtered transmission electron microscopy (HRTEM and EFTEM), and energy-dispersive X-ray spectroscopy (EDS/ EDX) showed that these nanorods are compositionally graded single nanocrystals with wurtzite (hexagonal) structure. The head of the nanorods is CdSe-rich, whereas the tail of the nanorods is CdS-rich.53 Time evolution studies confirmed that these compositionally graded CdS1−xSex nanorods form sequentially, starting with quick ( DPPE > TBPE > TOPE > HPTE (E = S, Se). This trend explains the sequential formation of axially anisotropic CdS1−xSex nanorods. Because of a significantly weaker PE bond in TOPSe compared to that in TOPS (by 21 kcal/mol),54

Figure 2. Change in the CdS nanorod aspect ratio as a function of precursor reactivity. CdS nanorods made with TOPS (a), TBPS (b), and DPPS (c). Plot of CdS nanorod length (nm) and aspect ratio as a function of calculated PS bond strength (energy in kcal/mol) (d). Reprinted from ref 54. 655

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Figure 3. (a) High-temperature chalcogen exchange. XRD patterns (b) and Vegard’s plot (c) for CdS1−xSex dots obtained from different R3PS− R3PSe mixtures. Reprinted from ref 54.

Figure 4. (a) Sulfur-to-selenium ratio (S/Se) in CdS1−xSex dots as a function of relative precursor concentration and reactivity: {([R3PS]/[R3PSe]) [(ΔGPSe)/(ΔGPS)]}, slope k = 0.00688. (b) Size-corrected band gap of CdS1−xSex dots obtained from different R3PS−R3PSe mixtures. Faster CdSe compared to CdS nucleation with TOPSe−TOPS leads to CdSe/CdS core/shells, whereas concomitant CdSe and CdS nucleation with TOPSe−DPPS leads to CdS1−xSex alloys. TEM images (c,d) and EDS line scans (e−h) of axially anisotropic CdS0.42Se0.58 nanorods made with a 9:1 TOPS−TOPSe precursor mixture (c,e,g) and regular CdS0.34Se0.66 nanorods made with a 9:1 TBPS−TBPSe precursor mixture (d,f,h). Arrows represent 50 nm in (e) and 17.5 nm in (f). Other conditions: Cd(ODPA)2, 320 °C, 85 min. Reprinted from ref 54.

underutilized technique for studying radial composition gradients in spherical nanoparticles (Figure 5). APT was successfully used

concentration control the overall composition of ternary CdS1−xSex quantum dots (Figure 4a), their exact microstructure strongly depends on the relative order of nucleation and growth of their constituent phases.47,54 Two R3PS and R3PSe precursors with very different reactivity lead to sequential formation of CdSe/CdS core/shell heterostructures, whereas two precursors with similar reactivity nucleate concomitantly and lead to true CdS1−xSex alloys (Figure 4b),54 thus suppressing axial phase segregation. Similarly, two R3PS and R3PSe precursors with very different reactivity produce axially anisotropic, compositionally graded CdS1−xSex nanorods (Figure 4c,e,g), whereas two precursors with similar reactivity produce axially alloyed CdS1−xSex nanorods with a more consistent S/Se ratio along their axis (Figure 4d,f,h).54 These compositionally graded nanocrystals highlight an important and currently underestimated challenge in the field of colloidal semiconductors. X-ray diffraction (XRD) and electron-microscopy-based elemental mapping techniques alone are often not enough to identify and quantify radial composition gradients in small ( 0) but require much less energy than water splitting (Scheme 4b). We used semiconductor−metal hybrid heterostructures for sunlightdriven dehydrogenation and hydrogenolysis of benzyl alcohol as a model substrate. The heterostructure’s precise composition exquisitely dictates the turnover numbers and product distribution (Figure 7). A few metal (Pt, Pd) islands on the

Figure 7. Selective alcohol dehydrogenation (A, CdS−Pt) and hydrogenolysis (B, CdS0.4Se0.6−Pd) of benzyl alcohol with heterostructured photocatalysts. Reprinted from ref 151.

Scheme 6. Thiol−Ligand Exchangea semiconductor surface greatly enhance activity and selectivity and even help prevent photocatalyst etching and degradation (below). CdS−Pt favors H2 (dehydrogenation) over toluene (hydrogenolysis) 8:1 (TON = 1240 mol of H2·mol of catalyst−1·h−1), whereas CdS0.4Se0.6−Pd favors toluene over H2 3:1 (TON = 490 mol of toluene·mol of catalyst−1·h−1). This demonstrates the feasibility of achieving room-temperature, sunlight-driven conversions of alcohols and other biomassderived substrates into fuels and chemicals using finely heterostructured, photocatalytic nanocomposites. During our exploratory studies, we observed that the presence of surface-bound metal particles (islands) not only increases the activity and affects the selectivity of the photocatalyst but also greatly increases its stability and lifetime (Figure 8).

a

Reprinted from ref 189.

modified through amidation or layer-by-layer ionic pairing. This procedure is well-established for soft nanocrystal surfaces with high affinity toward soft “polarizable” thiol ligands.124,154 Its use

Figure 8. The presence of surface-bound metal (Pt) islands greatly stabilizes semiconductor nanorods (CdS) against photobleaching, etching, and degradation. UV−vis absorption (a,d) and TEM (b,c,e,f) of CdS nanorods (top) and CdS−Pt heterostructures (bottom) before and after photocatalysis. Reprinted from ref 151. 659

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single monolayer perpendicular to the nanocrystal surface. The relative population of the different surface ligands is proportional to the relative ligand concentrations used during nanocrystal synthesis (Figure 9). 1H NMR resonances from surface-bound ligands are characteristically broadened and show faster T1 relaxation times compared to those from free ligands. We believe this is due to the close proximity of surface-bound ligand protons to unpaired electrons localized on surface trap states. As noted above, semiconductor nanocrystals are strong light absorbers (ε ≈ 106 L·mol−1·cm−1),33,34 become redox-active under ambient light, and remain redox-active after dark storage for several hours.129 Photogenerated carriers are known to localize on surface defects or “dangling bonds” (unpassivated ions) on the nanocrystal surface. Thus, 1H NMR peak broadening does not simply arise from fast equilibration between surface-bound and free ligands,168−172 but also from surfacetrapped electrons behaving as paramagnetic impurities at the nanocrystal inorganic−organic ligand interface. Diffusion ordered spectroscopy (DOSY) shows that resonances from surfacebound ligands have very slow diffusion rates that are about 1 order of magnitude slower compared to resonances from free ligands and trace solvent.173 More detailed NMR and EPR measurements are needed to measure the surface concentration of paramagnetic impurities and defects in colloidal semiconductor nanocrystals.171,174−176 Within a single layer of surface ligands, two (or more) ligands can assemble to form islands or rafts of identical composition,177−181 or they can mix together and distribute homogeneously (at random) on the nanocrystal surface. This question has been the subject of intense research and debate for plasmonic gold nanoparticles,182−184 but it was not addressed for semiconducting nanocrystals until recently. To distinguish between these two scenarios (rafts versus mixing), we used rotating-frame Overhauser effect spectroscopy (ROESY). A variant of nuclear Overhauser effect spectroscopy (NOESY), ROESY is used for intermediate to high molecular weight species (>1−3 kDa).185−188 We observed strong through-space heterocorrelations between dissimilar ligands for all mixed ligand samples that we studied. For example, the ROESY spectrum of a 50%/50% methyl/vinyl-capped CdS quantum dot sample clearly shows through-space heteroligand coupling between the protons on the methyl end group (C(10)H3) of the saturated ligand and the protons on the vinyl end group (C(10)H2, C(9)H) as well as on the allylic position (C(8)H2) of the unsaturated ligand (blue arrows in Figure 10). We recorded similar observations from the ROESY spectrum of a 50%/50% vinyl/azide-capped CdS quantum dot sample, namely, through-space heteroligand coupling between the protons on the last methylene (C(10)H2) of the azide ligand and the protons on the vinyl end group (C(10)H2, C(9)H) as well as on the allylic position (C(8)H2) of the unsaturated ligand. The experimentally observed through-space (NOE) heteroligand correlations are either stronger or at least as strong as the homoligand correlations, which can arise from within individual ligands. We concluded that mixtures of ligands with similar chain lengths homogeneously distribute themselves on the nanocrystal surface.189 Differential scanning calorimetry (DSC) may help quantify the presence of highly crystalline raftlike domains within the surface SAMs of nanocrystals containing other, more dissimilar ligand mixtures.190−192 High-Resolution scanning transmission electron microscopy (STEM) may further allow the identification of specific patterns accompanying surface ligand phase segregation.182−184

is widespread for II−VI and IV−VI semiconductors such as zinc, cadmium, and mercury chalcogenides. Biological building blocks have been explored extensively as alternatives for nanocrystal surface modification. The most-often-used procedure exploits strong binding between streptavidin and biotin.155−158 One end of the streptavidin−biotin pair is attached to the nanocrystal surface, and the other end is covalently or noncovalently attached to a molecule or material of interest. Inorganic modification methods were more recently introduced to incorporate functionality onto nanocrystal surfaces. Inorganic ligand exchange with complex anions such as Sn2S64− produces compact nanocrystals with improved carrier mobility or “hopping” across nanocrystal solids.159−161 These small ligands improve interparticle coupling by removing insulating organic ligands.160,162 While powerful and widespread, these known methods have limitations. Thiol−ligand exchange does not always work for transition-metal oxides, nitrides, or other “hard” nanocrystal surfaces.154 More importantly, ligand exchange removes the nanocrystal’s native ligands, that is, those that originate from its synthesis; this can cause etching and introduce surface defects, affecting the nanocrystal’s optical properties.163 Biological building blocks require milder reaction conditions but often involve lengthy multistep syntheses on tiny amounts of expensive materials, and their large footprint and thermal instability result in low surface coverage and chemical incompatibility. Critically, the most important limitation of these methods is their inability to control the extent of surface modification or number of functional groups or “valence” introduced per nanocrystal. Surface Doping and Valence Control in Colloidal Semiconductors. In view of the problems and limitations associated with quantum dot surface modification by well-established methods, we recently reported an alternative surface ligand “doping” approach to control the number of functional groups (valence) per nanocrystal that bypasses ligand exchange. We showed that “surface-doped” nanocrystals can be made not only by ligand exchange, as has been done routinely in the past, but also by direct nanocrystal synthesis in the presence of two (or more) different capping ligands. At least one ligand contains a chemically active, terminally unsaturated group such as a vinyl (−CHCH2) or azide (−N3); another ligand is completely saturated (aliphatic) and ends with a methyl (−CH3) (Scheme 7). Scheme 7. Synthesis and Direct Modification of Quantum Dots Capped with Chemically Active Native Ligands (A)a

a

Reprinted from ref 189.

Simply varying the active-to-inactive ligand ratio used during nanocrystal synthesis (x-to-y in Scheme 7) allows control of the number of chemically active groups and thus the amount or degree of functionality (valence) that can be introduced per nanocrystal. Surface modification can be easily accomplished by direct reaction of the chemically active unsaturated groups without resorting to ligand exchange152,164−167 (A-to-C transformation in Scheme 7). IR and NMR spectroscopies showed that carboxylate ligands bind to the nanocrystal surface in a bidentate fashion, forming a 660

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Figure 9. 1H NMR spectra of CdS nanocrystal quantum dots capped with mixtures of methyl (−CH3)-, vinyl (−CHCH2)-, and azide (−N3)terminated C10 carboxylate ligands. Surface ligand populations are proportional to the relative ligand concentrations used during nanocrystal synthesis. Reprinted from ref 189.

Figure 10. (a) Vinyl/methyl-capped, mixed ligand CdS nanocrystal quantum dots, representative DOSY (b) and ROESY (c) spectra, and valence control via cross metathesis (d). Reprinted from ref 189.

Quantum Dot Functionalization without Ligand Exchange. Surface-doped quantum dots capped with chemically active native ligands allow control over the extent of surface modification by simple organic reactions. Using an excess of small fluorinated molecules as model substrates, we reacted vinyl- and azide-capped quantum dots via cross-metathesis193,194 and “click” (Huisgen’s [3 + 2]-cycloaddition)195−200 conditions, respectively, and followed the outcome by UV−vis, PL, 1H and 19 F NMR, XRD, TEM, and EDX. Both metathesis and click

reactions proceed without affecting the particle size or optical properties and without undesirable dendronization and crosslinking.165,166,201−203 Critically, EDX confirmed that the final loading of fluorinated groups per quantum dot is proportional to the original population of surface-active native (vinyl or azide) ligands in the original nanocrystals (Figure 9d). To the best of our knowledge, this was one of only a few of examples of valence control on semiconductor nanocrystal quantum dots204−206 and the only one that bypassed ligand exchange. 661

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semiconductor surface significantly enhance activity and selectivity and also greatly stabilize the semiconductor against photoinduced etching and degradation. We also reported a fast, low-temperature, and scalable photochemical route to synthesize very small (3 nm) monodisperse cobalt oxyhydroxide (Co(O)OH) nanocrystals. The alternative dark thermal reaction proceeds much more slowly and produces much larger (250 nm) polydisperse Co(O)OH aggregates. Photochemical methods such as these facilitate the generation, isolation, and study of metastable nanomaterials having unusual size, composition, and morphology. These harder-to-isolate and highly reactive phases, inaccessible using conventional hightemperature pyrolysis, are likely to possess enhanced and unprecedented chemical, electromagnetic, and catalytic properties. We have also investigated fundamentally new surface ligand modification or doping strategies to control the degree of functionalization or valence per nanocrystal while retaining the nanocrystal’s original colloidal and photostability. To achieve this goal, it was not only necessary to develop conceptually new surface modification strategies but also to investigate the changes in ligand organization, surface chemistry, and overall nanocrystal properties that arise as a result of such strategies. For example, 2D NMR experiments (DOSY, ROESY) helped us address ongoing questions related to ligand assembly into rafts on nanocrystal surfaces. These techniques could also help us investigate how small molecules such as organic dyes or metal complexes interact with nanocrystal surfaces, for example, by becoming entangled with surface-bound ligands. In energy and materials science, the ability to fine-tune the number and relative configuration of energy- and charge-transfer donors and acceptors will provide unprecedented control over quantum dot exciton decay and chemical reaction pathways across the inorganic(crystal)−organic(ligand)−solvent(medium) interface. In biology, the extent of surface coverage by a particular functional group can have a large impact on a nanocrystal’s affinity and permeability to a variety of biological structures and thus on its ability to localize, penetrate, and be transported across specific tissues and cellular and subcellular structures.

“Surface-doped” nanocrystals can be made not only by ligand exchange, as has been done routinely in the past, but also by direct nanocrystal synthesis in the presence of two (or more) different capping ligands. Simply varying the active-to-inactive ligand ratio used during nanocrystal synthesis allows control of the number of chemically active groups and thus the amount or degree of functionality (valence) that can be introduced per nanocrystal. In this Perspective, we have highlighted our use of principles (Hammond’s postulate, chemical reactivity tuning via substitution) and techniques (multinuclei and multidimensional NMR, DFT) borrowed from molecular chemistry to improve our ability to control and understand the synthesis, activation, and functionalization of colloidal semiconductor nanocrystals. Building on our observations on the spontaneous formation of compositionally graded CdS1−xSex nanorods, we used a systematic approach to study chemical reactivity across a homologous series of soluble phosphine chalcogenide precursors, allowing us to predict how closely related molecular reactants affect the nucleation rates and selectivities for phase segregation and anisotropic growth of their nanocrystalline products. We demonstrated that molecular reactivity is useful in tuning not only the relative rates of homogeneous nucleation but also the rates of heterogeneous nucleation of new crystalline material epitaxially grown on top of existing nuclei. This chemistry, facilitated by the addition of secondary amines, allowed us to solve some of the unique challenges that accompany the synthesis of thick-shell, nonblinking CdSe/ nCdS (n > 10) quantum dots, enabling the application of these outstanding materials in advanced biological imaging and tracking. Research into modulating the chemical reactivity of not only anionic (chalcogenide, pnictide) but also cationic precursors can and will positively impact the synthesis of many other complex, technologically relevant materials such as Cu2ZnSnS4, TaON, Yb14MnSb11, and PtxBiyPbz. Seeking better routes to build charge-separating semiconductor−metal hybrid heterostructures, we demonstrated distinct thermal and photochemical pathways for the formation of metal nanoparticles on colloidal semiconductors. Careful selection of synthetic conditions allowed us to deposit of Pt and Pd particles on CdS and CdS0.4Se0.6 nanorods with a high degree of selectivity for surface-bound (photochemically) versus freestanding metal particles (thermally). The ability to tune the spatial composition of hybrid heterostructures will advance our ability to engineer and direct energy flows at the nanoscale. We successfully used semiconductor−metal heterostructures for sunlight-driven dehydrogenation and hydrogenolysis of benzyl alcohol as a model of biomass-derived feedstocks. The heterostructure composition dictates activity, product distribution, and turnovers. A few metal islands on the

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest. Biography Javier Vela, assistant professor of chemistry at Iowa State University, received his B.S. from UNAM in 2001 and his Ph.D. with Pat Holland at the University of Rochester in 2005. He was a postdoc with Rich Jordan at the University of Chicago from 2005 to 2006 and a Director’s Fellow with Jennifer Hollingsworth and Victor Klimov at Los Alamos National Laboratory from 2006 to 2009. He was named ACS Inorganic Young Investigator in 2006 and Hispanic Engineering Top-40-Under-40 in 2011. He joined ISU in 2009 (www.chem.iastate. edu/faculty/Javier_Vela).

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ACKNOWLEDGMENTS J.V. gratefully acknowledges the talented students and collaborators who contributed to this work, some of whom are listed as coauthors in the references. Special thanks go to Purnima Ruberu, Yijun Guo, Sam Alvarado (who also helped with graphics), Elham Tavasoli, Ning Fang, Hua-Jun Fan, Andreja Bakac, Emily Smith, and Jake Petrich. J.V. also thanks Iowa State University, IPRT, Ames Lab, and Plant Sciences 662

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tional Donor−Acceptor Combinations. Angew. Chem., Int. Ed. 2006, 45, 4562−4588. (20) Vela, J.; Htoon, H.; Chen, Y.; Park, Y.-S.; Ghosh, Y.; Goodwin, P. M.; Werner, J. H.; Wells, N. P.; Casson, J. L.; Hollingsworth, J. A. Effect of Shell Thickness and Composition on Blinking Suppression and the Blinking Mechanism in ‘Giant’ CdSe/CdS Nanocrystal Quantum Dots. J. Biophoton. 2010, 3, 706−717. (21) Algar, W. R.; Susumu, K.; Delehanty, J. B.; Medintz, I. L. Semiconductor Quantum Dots in Bioanalysis: Crossing the Valley of Death. Anal. Chem. 2011, 83, 8826−8837. (22) Medintz, I. L.; Mattoussi, H. Quantum Dot-Based Resonance Energy Transfer and Its Growing Application in Biology. Phys. Chem. Chem. Phys. 2009, 11, 17−45. (23) Shields, A. J. Semiconductor Quantum Light Sources. Nat. Photonics 2007, 1, 215−223. (24) Bulovic, V.; Anikeeva, P. O.; Halpert, J. E.; Bawendi, M. G. Quantum Dot Light-Emitting Devices with Electroluminescence Tunable over the Entire Visible Spectrum. Nano Lett. 2009, 9, 2532−2536. (25) Kamat, P. V. Meeting the Clean Energy Demand: Nanostructure Architectures for Solar Energy Conversion. J. Phys. Chem. C 2007, 111, 2834−2860. (26) Kamat, P. V. Quantum Dot Solar Cells. Semiconductor Nanocrystals as Light Harvesters. J. Phys. Chem. C 2008, 112, 18737−18753. (27) Dresselhaus, M. S.; Chen, G.; Tang, M. Y.; Yang, R. G.; Lee, H.; Wang, D. Z.; Ren, Z. F.; Fleurial, J. P.; Gogna, P. New Directions for Low-Dimensional Thermoelectric Materials. Adv. Mater. 2007, 19, 1043−1053. (28) Klimov, V. I.; Ivanov, S. A.; Nanda, J.; Achermann, M.; Bezel, I.; McGuire, J. A.; Piryatinski, A. Single-Exciton Optical Gain in Semiconductor Nanocrystals. Nature 2007, 447, 441−446. (29) Felser, C.; Fecher, G. H.; Balke, B. Spintronics: A Challenge for Materials Science and Solid-State Chemistry. Angew. Chem., Int. Ed. 2007, 46, 668−699. (30) Brus, L. E. A Simple-Model for the Ionization-Potential, Electron-Affinity, and Aqueous Redox Potentials of Small Semiconductor Crystallites. J. Chem. Phys. 1983, 79, 5566−5571. (31) Efros, A. L.; Rosen, M. The Electronic Structure of Semiconductor Nanocrystals. Annu. Rev. Mater. Sci. 2000, 30, 475− 521. (32) Rogach, A. L.; Eychmuller, A.; Hickey, S. G.; Kershaw, S. V. Infrared-Emitting Colloidal Nanocrystals: Synthesis, Assembly, Spectroscopy, and Applications. Small 2007, 3, 536−557. (33) Yu, W. W.; Qu, L. H.; Guo, W. Z.; Peng, X. G. Experimental Determination of the Extinction Coefficient of CdTe, CdSe, and CdS Nanocrystals. Chem. Mater. 2003, 15, 2854−2860. (34) Mulvaney, P.; Jasieniak, J.; Smith, L.; van Embden, J.; Califano, M. Re-Examination of the Size-Dependent Absorption Properties of CdSe Quantum Dots. J. Phys. Chem. C 2009, 113, 19468−19474. (35) Garcia-Santamaria, F.; Chen, Y.; Vela, J.; Schaller, R. D.; Hollingsworth, J. A.; Klimov, V. I. Suppressed Auger Recombination in “Giant” Nanocrystals Boosts Optical Gain Performance. Nano Lett. 2009, 9, 3482−3488. (36) Klimov, V. I.; Li, L. A.; Pandey, A.; Werder, D. J.; Khanal, B. P.; Pietryga, J. M. Efficient Synthesis of Highly Luminescent Copper Indium Sulfide-Based Core/Shell Nanocrystals with Surprisingly Long-Lived Emission. J. Am. Chem. Soc. 2011, 133, 1176−1179. (37) Cademartiri, L.; Bertolotti, J.; Sapienza, R.; Wiersma, D. S.; von Freymann, G.; Ozin, G. A. Multigram Scale, Solventless, and DiffusionControlled Route to Highly Monodisperse PbS Nanocrystals. J. Phys. Chem. B 2006, 110, 671−673. (38) Dubertret, B.; Spinicelli, P.; Mahler, B.; Buil, S.; Quelin, X.; Hermier, J. P. Non-Blinking Semiconductor Colloidal Quantum Dots for Biology, Optoelectronics and Quantum Optics. ChemPhysChem 2009, 10, 879−882. (39) Hermier, J. P.; Mahler, B.; Spinicelli, P.; Buil, S.; Quelin, X.; Dubertret, B. Towards Non-Blinking Colloidal Quantum Dots. Nat. Mater. 2008, 7, 659−664.

Institute for startup funds. Work on photocatalytic deoxygenation reactions was supported by the U.S. Department of Energy, Office of Basic Energy Sciences, Division of Chemical Sciences, Geosciences, and Biosciences through the Ames Laboratory. The Ames Laboratory is operated for the U.S. Department of Energy by Iowa State University under Contract DE-AC02-07CH11358. Work on hydrogen production from biomass was supported by Phillips66 (formerly Conoco Phillips) through a partnership with the Bioeconomy Institute.

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