Viewpoint pubs.acs.org/JPCL
Atomic Clusters: Opportunities in the Face of Challenges
A
tomic clusters composed of a few to a few hundred atoms are produced, mass isolated, and studied in the gas phase. Although this field has been evolving over the past 60 years, many of the advancements in atomic clusters have occurred over the past 40 years. The initial motivation for studying clusters was that one can learn how bulk properties evolve and at what size a cluster would mimic the properties of its crystal. Although some progress has been made toward this goal, no universal answer has emerged because it depends both upon the properties and the type of the clusters in question. Studies of gas-phase clusters, however, have allowed an unprecedented opportunity to study structure−property relationships of nanoscale materials, one atom and one electron at a time. In addition, clusters bridge many disciplines such as physics, chemistry, biology, and materials science and provide a unique platform for multidisciplinary research.1 One of the biggest challenges in atomic clusters is that it is nearly impossible to produce them in large enough quantities that are needed to synthesize new materials. Clusters are often reactive and when removed from the high-vacuum environment, they will be contaminated. When brought into the vicinity of each other, they will coalesce; when supported on a substrate, their geometry and properties will also change from that in the gas phase. In the face of these challenges one wonders if the knowledge gained from clusters in the gas phase can have any practical significance. In this Viewpoint, I discuss some of the opportunities that studies of gas-phase clusters offer by focusing on properties of complex materials that can be explained by knowing their structure−property relations as well as by focusing on the design and synthesis of a new class of cluster-assembled materials. I argue in general that scientific queries need not be limited to solve only those problems that seem to have immediate practical applications. Numerous examples exist in the literature to the contrary, and there are vast advancements in current technology that would not have occurred if scientific studies were not pursued solely for the sake of science. I begin with a discussion of magic numbers in clusters, which I believe has had the biggest impact on cluster science. In a pioneering paper in 1984, Knight and co-workers2 published the mass ion intensity distribution of Na clusters containing up to 100 atoms (Figure 1). The most important result of this paper, in my view, is that it linked the pounced peaks in the Na mass spectra at 2, 8, 20, 40, 58, and 92 atoms with clusters stabilized by electron shell closure rule, the so-called jellium rule. This novel insight helped to bridge two seemingly disparate fields, cluster physics and nuclear physics, where magic numbers in nuclei were explained to be due to nuclear shell closure in 1950 by Goeppert-Mayer.3 The jellium shell closure rule has now joined the rank of several other electron counting rules that have been developed in chemistry over a century to account for the stability of atoms and compounds. These include the octet (ns2 np6) shell closure rule4 that explains the inertness of noble gas atoms as well reactivity of halogens, the 18-electron rule that accounts for the stability of © 2015 American Chemical Society
Figure 1. (a) Mass spectrum of sodium clusters, N = 4−75. The inset corresponds to N = 75−100. (b) Calculated change in the electronic energy difference, Δ(N + 1) − Δ(N) vs N. The labels and the peaks correspond to the closed-shell orbitals [ref 2].
compounds containing transition metal atoms,5 the Hückel rule6 for aromatic organic molecules and the Wade−Mingos rule7 for boranes and Zintl ions. The discovery of C60 fullerene in the gas phase by Smalley and co-workers8 in 1985 revealed carbon’s unique chemistry and opened the door to a new era in carbon science (Figure 2). The bulk synthesis of C609 permitted experimentalists to confirm its structure and study its potential for technological applications, for instance as a catalyst,10 hydrogen storage material,11 contrast enhancement agent in magnetic resonance imaging,12 drug delivery,13 and so forth. The revolution in carbon science that began with the gas-phase discovery of C60 continues until today and the subsequent discovery of nanotubes,14 graphene,15 and many other novel carbon allotropes has led to development of materials with technological potential. Now, I will illustrate how the understanding of the stability of the gas-phase clusters, made possible through their electronic shell structure, can lead to the design and synthesis of new materials as well as explain some properties observed in bulk materials. In Scheme 1, I provide a schematic view of looking into the electronic shell structure of clusters with the hope of finding ways to use this information for practical applications. In 1992, Khanna and Jena16 proposed that the electron shell closure rules can be used to design stable magic clusters by Published: May 7, 2015 1549
DOI: 10.1021/acs.jpclett.5b00629 J. Phys. Chem. Lett. 2015, 6, 1549−1552
The Journal of Physical Chemistry Letters
Viewpoint
that ligands can be effectively used to design a metallic core consistent with electronic shell closure and thus be very stable. Bulk quantities of several cluster assembled materials composed of coated gold and silver clusters have now been synthesized and characterized.28,29 Superhalogens are another class of superatoms. Coined by Gutsev and Boldyrev in the early 1980s30 and experimentally verified by Wang and co-workers31 in 1999, superhalogens are composed of a metal atom at the core and surrounded by halogen atoms whose number exceed the maximal valence of the metal atom by one. Not only do these clusters mimic the chemistry of halogen atoms but their electron affinities also far exceed those of halogen atoms. Work over the past decade has considerably expanded the scope of superhalogens, which can now be formed without the benefit of a metal atom or a halogen atom, or both.32,33 A new species, termed hyperhalogens,34 was shown to form when a metal atom is surrounded by superhalogens instead of halogens. They are found to be even more electronegative than their superhalogen building blocks. Due to their strong oxidizing properties, superand hyperhalogens can be used to promote unusual chemistry, enabling noble gas atoms to form chemical bonds,35 and to access higher oxidation states of metals.36 Lessons learned from super- and hyperhalogens in the gas phase are now enabling researchers to design and synthesize a new class of salts referred to as super- and hypersalts, where the anionic components of these salts are super- and hyperhalogens, respectively. A typical example of a supersalt is Cs(AuF6) where AuF6 is a superhalogen anion that was used to ionize Xe, a noble gas atom.37 Similarly, consider Al(BH4)3, which can store about 17 wt % hydrogen. Unfortunately, it is unsafe; it is highly volatile and pyrophoric. However, it can be made much safer by first converting it into a hyperhalogen Al(BH4)4 and then combining it with K+ cation to form KAl(BH4)4 hypersalt.38 Superhalogens can also address some of the challenges in hydrogen storage materials such as metal borohydrides [M(BH4)x, x = valence of M]. Although these complex hydrides can store large amounts of hydrogen to meet the needs of the mobile industry, they exhibit intermediate phases during dehydrogenation that make them irreversible. They are found to contain B3H8− and B12H122− anions.39−42 It has been shown that like BH4, B3H8 and B12H12 are superhalogens. Because their electron affinities are larger than that of BH4, they bind strongly with the metal cations, thus accounting for their appearance as intermediate products during dehydrogenation.43 Another example where studies of superhalogens in the gas phase can have impact on materials design is in environmentally friendly Li-ion batteries. The electrolytes currently used in these batteries are salts composed of Li+ cations and halogencontaining complex anions. Because halogens are toxic, there is a need to find halogen-free electrolytes. Realizing that these complex anions are all superhalogens that can also be formed without using a single halogen atom, Giri et al.44 have suggested that environmentally safe halogen-free electrolytes in Li-ion batteries can be synthesized. They have identified Li+(CB11H12)− as a potential environmentally safe electrolyte. LiBH4 has already been demonstrated45 to be a halogen-free solid state electrolyte. In a more recent work, BH4− anions have also been used to synthesize hydride-based perovskites.46 Although most perovskites have the composition ABO3 (A and B are metal cations), hydride-based perovskites are rarely known. The fact that BH4−
Figure 2. Time-of-flight mass spectra of carbon clusters prepared by laser vaporization of graphite and cooled in a supersonic beam. The three spectra shown differ in the extent of helium collisons occurring in the supersonic nozzle [ref 8].
Scheme 1. Schematic View of Looking at the Shell Structure in Clusters and Examining Their Potential for Materials Synthesis
tailoring their size, composition, and charge state and suggested that such clusters can then be used as building blocks of a new class of cluster assembled materials with unusual properties. The first example of such a crystal is fulleride,17 which is composed of C60 clusters as building blocks. The fact that the properties of fulleride are very different from those of graphite, diamond, or graphene demonstrates the opportunities in synthesizing materials where clusters instead of atoms form the building blocks. In subsequent publications,18,19 the authors coined the word “superatoms” to describe clusters that mimic the chemistry of atoms and further suggested that a threedimensional periodic table can be constructed where the “superatoms” constitute the third dimension. A considerable amount of work20−25 has since been done, and cluster assembled materials have been predicted and synthesized. One particular class of materials that show great promise are coated metal clusters where the ligands protect the metallic core from coalescing.26 Hakkinen and co-workers27 showed 1550
DOI: 10.1021/acs.jpclett.5b00629 J. Phys. Chem. Lett. 2015, 6, 1549−1552
The Journal of Physical Chemistry Letters
■
ACKNOWLEDGMENTS This work is partially supported by the U.S. Department of Energy, Office of Basic Energy Sciences, Division of Materials Sciences and Engineering under Award # DE-FG0296ER45579.
can serve as a replacement for O is offering opportunities to explore a new class of cluster-based perovskites with potential applications in ferroelectric and solar cell materials. Thus, merging the concepts of molecular chemistry with ceramic host lattices can lead to the design and synthesis of an unusual class of complex hydride perovskite materials. Knowledge gained from the interaction of hydrogen with metal atoms and their cations in the gas phase is also enabling the design and synthesis of a new class of hydrogen storage materials. Kubas47 had demonstrated that hydrogen can bind to a transition metal atom in quasimolecular form with binding energies intermediate between physisorption and chemisorption. This is what is required of hydrogen storage materials so that hydrogen can be released under ambient conditions and be used as a fuel in the transportation industry. Later, Niu et al.48 had demonstrated that a metal cation can similarly trap hydrogen in quasimolecular form through a charge polarization mechanism. These mechanisms are now being used to trap hydrogen in quasimolecular form by doping porous materials with metal atoms. In summary, the challenge with most clusters studied in the gas phase is that they are reactive and are likely to lose their identity when assembled or supported on a surface. In addition, it is impossible to produce these clusters in large enough quantity to be useful for materials synthesis. Here, I show that there are opportunities in the face of these challenges. One such opportunity I have focused on is the impact an understanding of their stability can have on materials science. I have tried to illustrate how the knowledge gained from the studies of magic clusters in the gas phase can open new pathways for the design and synthesis of unique materials where these clusters serve as building blocks. I have also shown that complex metal hydrides can be regarded as salts where the negative ions are clusters mimicking the chemistry of halogen atoms. They can be identified as superhalogens and explain the origin of intermediate phases that make these hydrides irreversible. Studies of nontraditional superhalogens in the past decade are also enabling the design of halogen-free electrolytes in Li-ion batteries. Although I have limited the above discussion to magic clusters, there are examples where studies of gas-phase clusters have been useful to gain a fundamental understanding of catalysis, magnetism, and optics. For example, it is possible to create a magnetic particle from an otherwise nonmagnetic material by controlling its size and composition.49 Chemically inert elements such as gold can exhibit unusual catalytic properties at the subnano scale.50 Size and shape can also be used as an effective tool to control the “band-gap” of nanoclusters for application in optical devices. The challenges still remain, including how to keep the identity of these clusters intact after assembly. The benefit of focusing on basic science of gas-phase clusters, however, is that one does not know when and how knowledge gained can be useful to solve societal problems. It is this “unknown” that is science’s richest reward.
■
REFERENCES
(1) NanoclustersA Bridge Across Disciplines; Jena, P., Castleman, A. W., Jr., Eds.; Elsevier: Amsterdam,2010. (2) Knight, W. D.; Clemenger, K.; de Heer, W. A.; Suanders, W. A.; Chou, M. Y.; Cohen, M. L. Electronic Shell Structure and Abundances of Sodium Clusters. Phys. Rev. Lett. 1984, 52, 2141−2144. (3) Goeppert-Mayer, M. Nuclear Configurations in the Spin-Orbit Coupling Model. I. Empirical Evidence. Phys. Rev. 1950, 78, 16−21. Goeppert-Mayer, M. Nuclear Configurations in the Spin-Orbit Coupling Model. II. Theoretical Considerations. Phys. Rev. 1950, 78, 22−23. (4) Lewis, G. N. The Atom and the Molecule. J. Am. Chem. Soc. 1916, 38, 762−785. (5) Langmuir, I. Types of Valence. Science 1921, 54, 59−67. (6) Hückel, E. Quantentheoretische Beiträge zum Benzolproblem. Z. Phys. 1931, 70, 204−86. (7) Wade, K. The Structural Significance of the Number of Skeletal Bonding Electron-Pairs in Carboranes, the Higher Boranes and Borane Anions, and Various Transition-Metal Carbonyl Cluster Compounds. J. Chem. Soc. D 1971, 792−793. Wade, K. Structural and Bonding Patterns in Cluster Chemistry. Adv. Inorg. Chem. Radiochem. 1976, 18, 1−66. Mingos, D. M. P. Polyhedral Skeletal Electron Pair Approach. Acc. Chem. Res. 1984, 17, 311−319. Mingos, D. M. P.; Johnston, R. L. Theoretical Models of Cluster Bonding. Struct. Bonding (Berlin, Ger.) 1987, 68, 29−87. (8) Kroto, H. W.; Heath, J. R.; O’Brien, S. C.; Curl, R. F.; Smalley, R. E. C60: Buckminsterfullerene. Nature 1985, 318, 162−163. (9) Kratschmer, W.; Lamb, L. D.; Fostiropoulos, K.; Huffman, D. R. Solid C60: A New Form of Carbon. Nature 1990, 347, 354−358. (10) Berseth, P. A.; Harter, A. G.; Zidan, R.; Blomqvist, A.; Araujo, C. M.; Scheicher, R. H.; Ahuja, A.; Jena, P. Carbon Nanomaterials as Catalysts for Hydrogen Uptake and Release in NaAlH4. Nano Lett. 2009, 9, 1501. (11) Teprovich, J. A., Jr.; Wellons, M. S.; Lascola, R.; Hwang, S.-J.; Ward, P. A.; Compton, R. N.; Zidan, R. Synthesis and Characterization of a Lithium-Doped Fullerane (Lix−C60−Hy) for Reversible Hydrogen Storage. Nano Lett. 2012, 12, 582−589. (12) Bolskar, R. D. Gadofullerene MRI Contrast Agents. Nanomedicine (London, U.K.) 2008, 3, 201−213. (13) Chun, Ke P.; Qiao, R. Carbon Nanomaterials in Biological Systems. J. Phys.: Condens. Matter 2007, 19, 373101. (14) Iijima, S. Helical Microtubules of Graphitic Carbon. Nature 1991, 354, 56−58. (15) Novoselov, K. S.; Geim, A. K.; Morozov, S. V.; Jiang, D.; Zhang, Y.; Dubonos, S. V.; Grigorieva, I. V.; Firsov, A. A. Electric Field Effect in Atomically Thin Carbon Films. Science 2004, 306, 666−669. (16) Khanna, S. N.; Jena, P. Assembling Crystals from Clusters. Phys. Rev. Lett. 1992, 69, 1664−1667. (17) Holczer, K.; Klein, O.; Huang, S.; Kaner, R. B.; Fu, K.; Whetten, R. L.; Diederich, F. Alkali-Fulleride Superconductors: Synthesis, Composition, and Diamagnetic Shielding. Science 1991, 252, 1154− 1157. Hebard, A. F.; Rosseinsky, M. J.; Haddon, R. C.; Murphy, D. W.; Glarum, S. H.; T. T. Palstra, M.; Ramirez, A. P.; Kortan, A. R. Superconductivity at 18 K in Potassium-Doped C60. Nature 1991, 350, 600−601. Rosseinsky, M. J.; Ramirez, A. P.; Glarum, S. H.; Murphy, D. W.; Haddon, R. C.; Hebard, A. F.; Palstra, T. T. M.; Kortan, A. R.; Zahurak, S. M.; Makhija, A. V. Superconductivity at 28 K in RbxC60. Phys. Rev. Lett. 1991, 66, 2830−2832. (18) Khanna, S. N.; Jena, P. Designing Ionic Solids from Metallic Clusters. Chem. Phys. Lett. 1994, 219, 479−483. (19) Khanna, S. N.; Jena, P. Atomic Clusters: Building Blocks for a Class of Solids. Phys. Rev. B 1995, 51, 13705−13716.
Puru Jena
■
Viewpoint
Physics Department, Virginia Commonwealth University, Richmond, Virginia 23284-2000, United States
AUTHOR INFORMATION
Notes
The authors declare no competing financial interest. 1551
DOI: 10.1021/acs.jpclett.5b00629 J. Phys. Chem. Lett. 2015, 6, 1549−1552
The Journal of Physical Chemistry Letters
Viewpoint
Confirmation for Formation of [B12H12]2− Complexes During Hydrogen Desorption from Metal Borohydrides. J. Phys. Chem. C 2008, 112, 3164−3169. (41) Soloveichik, G. L.; Gao, Y.; Rijssenbeek, J.; Andrus, M.; Kniajanski, S.; Bowman, R. C., Jr.; Hwang, S. J.; Zhao, J. C. Magnesium Borohydride as a Hydrogen Storage Material: Properties and Dehydrogenation Pathway of Unsolvated Mg(BH4)2. Int. J. Hydrogen Energy 2009, 34, 916−928. (42) Wang, L. L.; Graham, D. D.; Robertson, I. M.; Johnson, D. D. On the Reversibility of Hydrogen-Storage Reactions in Ca(BH4)2: Characterization via Experiment and Theory. J. Phys. Chem. C 2009, 113, 20088−20096. (43) Liu, Y.; Giri, S.; Zhou, J.; Jena, P. Intermediate Phases During Decomposition of Metal Borohydrides, M(BH4)n (M = Na, Mg, Y). J. Phys. Chem. C 2014, 118, 28456−28461. (44) Giri, S.; Behera, S.; Jena, P. Superhalogens As Building Blocks of Halogen-Free Electrolytes in Li-Ion Batteries. Angew. Chem., Int. Ed. 2014, 53, 13916−13919. (45) Unemoto, A.; Matsuo, M.; Orimo, S. Complex Hydrides for Electrochemical Energy Storage. Adv. Funct. Mater. 2014, 24, 2267− 2279. (46) Schouwink, P.; Ley, M. B.; Tissot, A.; Hagemann, H.; Jensen, T. R.; Smrcok, L.; Cerny, R. Structure and Properties of Complex Hydride Perovskite Materials. Nat. Commun. 2014, 5, 5706. (47) Kubas, G. Metal-Dihydrogen and σ-Bond Coordination: The Consummate Extension of the Dewar−Chatt−Duncanson Model for Metal-Olefin π Bonding. J. Organomet. Chem. 2001, 635, 37−68. (48) Niu, J.; Rao, B. K.; Jena, P. Binding of Hydrogen Molecules by a Transition-Metal Ion. Phys. Rev. Lett. 1992, 68, 2277−2280. (49) Wu, M.; Jena, P. Magnetic Hollow Cages with Colossal Moments. J. Chem. Phys. 2013, 139, 044301. (50) Haruta, M. When Gold Is Not Noble: Catalysis by Nanoparticles. Chem. Rec. 2003, 3, 75−87.
(20) Castleman, A. W., Jr.; Khanna, S. N. Clusters, Superatoms, and Building Blocks of New Materials. J. Phys. Chem. C 2009, 113, 2664− 2675. (21) Bergeron, D. E.; Castleman, A. W.; Morisato, T.; Khanna, S. N. Formation of Al13I−: Evidence for the Superhalogen Character of Al−13. Science 2004, 304, 84−87. (22) Claridge, S. A.; Castleman, A. W.; Khanna, S. N.; Murray, C. B.; Sen, A.; Weiss, P. S. Cluster-Assembled Materials. ACS Nano 2009, 3, 244−255. (23) Kasuya, A.; Sivamohan, R.; Barnakov, Y.; Dmitruk, I. G.; Nirasawa, T.; Romanyuk, V. R.; Kumar, V.; Mamykin, S. V.; Tohji, K.; Jeyadevan, B.; Shinoda, K.; Kudo, T.; Terasaki, O.; Liu, Z.; Belosludov, R.; Sundararajan, V.; Kawazoe, Y. Ultra-Stable Nanoparticles of CdSe Revealed from Mass Spectrometry. Nat. Mater. 2004, 3, 99−102. (24) Kumar, V.; Kawazoe, Y. Metal-Encapsulated Fullerene-like and Cubic Caged Clusters of Silicon. Phys. Rev. Lett. 2001, 87, 045503− 045506. Kumar, V.; Kawazoe, Y. Metal-Encapsulated Caged Clusters of Germanium with Large Gaps and Different Growth Behavior than Silicon. Phys. Rev. Lett. 2002, 88, 235504−235507. (25) Koyasu, K.; Akutsu, M.; Mitsui, M.; Nakajima, A. Selective Formation of MSi16 (M = Sc, Ti, V). J. Am. Chem. Soc. 2005, 127, 4998−4999. (26) Negishi, Y.; Chaki, N. K.; Shichibu, Y.; Whetten, R. L.; Tsukuda, T. Origin of Magic Stability of Thiolated Gold Clusters: A Case Study on Au25(SC6H13)18. J. Am. Chem. Soc. 2007, 129, 11322−11323. (27) Walter, M.; Akola, J.; Lopez-Acevedo, O.; Jadzinsky, P. D.; Calero, G.; Ackerson, C. J.; Whetten, R. L.; Gronbeck, H.; Hakkinen, H. A Unified View of Ligand-Protected Gold Clusters As Superatom Complexes. Proc. Nat. Acad. Sci. U.S.A. 2008, 105, 9157−9162. (28) Yang, H.; Wang, Y.; Huang, H.; Gell, L.; Lehtovaara, L.; Malola, S.; Häkkinen, H.; Zheng, N. All-Thiol-Stabilized Ag44 and Au12Ag32 Nanoparticles with Single-Crystal Structures. Nat. Commun. 2013, 4, 2422. (29) Desireddy, A.; Conn, B. E.; Guo, J.; Yoon, B.; Barnett, R. N.; Monahan, B. M.; Kirschbaum, K.; Griffith, W. P.; Whetten, R. L.; Landman, U.; Bigioni, T. P. Ultrastable Silver Nanoparticles. Nature 2013, 501, 399−402. (30) Gutsev, G.; Boldyrev, A. I. DVM-Xα Calculations on the Ionization Potentials of MXk+1− Complex Anions and the Electron Affinities of MXk+1 “Superhalogens”. Chem. Phys. 1981, 56, 277−283. (31) Wang, X. B.; Ding, C. F.; Wang, L. S.; Boldyrev, A. I.; Simmons, J. First Experimental Photoelectron Spectra of Superhalogens and their Theoretical Interpretations. J. Chem. Phys. 1999, 110, 4763−4771. (32) Samanta, D.; Wu, M. M.; Jena, P. Au(CN)n Complexes: Superhalogens with Pesudohalogens As Building Blocks. J. Inorg. Chem. 2011, 50, 8918−8925. (33) Pathak, B.; Samanta, D.; Ahuja, R.; Jena, P. Borane Derivatives: A New Class of Super and Hyper Halogens. ChemPhysChem 2011, 12, 2423−2428. (34) Willis, M.; Gotz, M.; Kandalam, A. K.; Gantefor, G. F.; Jena, P. Hyperhalogens: Discovery of a New Class of Electronegative Species. Angew. Chem., Int. Ed. 2010, 49, 8966−8970. (35) Samanta, D. Prediction of Superhalogen-Stabilized Noble Gas Compounds. J. Phys. Chem. Lett. 2014, 5, 3151−3156. (36) Samanta, D.; Jena, P. Zn in +III Oxidation State. J. Am. Chem. Soc. 2012, 134, 8400−8403. (37) Bartlett, N.; Lohmann, D. H. Dioxygenyl Hexafluoroplatinate(v) O2+ [PtF6]−. Proc. Chem. Soc. 1962, 115−116. Bartlett, N. Xenon Hexafluoroplatinate(v) Xe+[PtF6]−. Proc. Chem. Soc. 1962, 218. (38) Knight, D. A.; Zidan, R.; Lascola, R.; Mohtadi, R.; Ling, C.; Sivasubramaniam, P. K.; Kaduk, J. A.; Hwang, S.-J.; Samanta, D.; Jena, P. Stabilization of Hydrogen Rich, Yet Highly Pyrophoric Al(BH4)3 via the Synthesis of the Hypersalt K[Al(BH4)4]. J. Phys. Chem. C 2013, 117, 19905−19915. (39) Orimo, S.; Nakamori, Y.; Ohba, N.; Miwa, K.; Aoki, M.; Towata, S.; Züttel, A. Experimental Studies on Intermediate Compound of LiBH4. Appl. Phys. Lett. 2006, 89, 021920. (40) Hwang, S. J.; Bowman, R. C.; Reiter, J. W., Jr.; Rijssenbeek, J.; Soloveichik, G. L.; Zhao, J. C.; Kabbour, H.; Ahn, C. C. NMR 1552
DOI: 10.1021/acs.jpclett.5b00629 J. Phys. Chem. Lett. 2015, 6, 1549−1552
