Mercouri G. Kanatzidis: Excellence and ... - ACS Publications

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Mercouri G. Kanatzidis: Excellence and Innovations in Inorganic and Solid-State Chemistry Indika U. Arachchige,† Gerasimos S. Armatas,‡ Kanishka Biswas,§ Kota S. Subrahmanyam,∥ Susan Latturner,⊥ Christos D. Malliakas,# Manolis J. Manos,¶ Youngtak Oh,∇ Kyriaki Polychronopoulou,○ Pierre. F. P. Poudeu,◆ Pantelis N. Trikalitis,$ Qichun Zhang,& Li-Dong Zhao,● and Sebastian C. Peter*,§ †

Department of Chemistry, Virginia Commonwealth University, Richmond, Virginia 23284-2006, United States Department of Materials Science and Technology, University of Crete, Vassilika Vouton, Heraklion 71003, Greece § New Chemistry Unit, Jawaharlal Nehru Centre for Advanced Scientific Research (JNCASR), Jakkur P.O., Bangalore 560064, India ∥ Centre for Nano and Soft Matter Sciences, Jalahalli, Bangalore 560013, India ⊥ Department of Chemistry and Biochemistry, Florida State University, Tallahassee, Florida 32308, United States # Department of Chemistry, Northwestern University, 2145 North Sheridan Road, Evanston, Illinois 60208, United States ¶ Department of Chemistry, University of Ioannina, GR-45110 Ioannina, Greece ∇ Center for Environment, Health, and Welfare Research, Korea Institute of Science and Technology, Seongbuk-gu, Seoul 136-791, Republic of Korea ○ Department of Mechanical Engineering, Khalifa University of Science, Technology, and Research, 127788 Abu Dhabi, United Arab Emirates ◆ Materials Science and Engineering, University of Michigan, Ann Arbor, Michigan 48109, United States $ Department of Chemistry, University of Crete, Voutes Campus, 71003 Heraklion, Greece & School of Materials Science and Engineering, Nanyang Technological University, Singapore 639798, Singapore ● School of Material Science and Engineering, Beihang University, Beijing 10091, China ‡

ABSTRACT: Over the last 3−4 decades, solid-state chemistry has emerged as the forefront of materials design and development. The field has revolutionized into a multidisciplinary subject and matured with a scope of new synthetic strategies, new challenges, and opportunities. Understanding the structure is very crucial in the design of appropriate materials for desired applications. Professor Mercouri G. Kanatzidis has encountered both challenges and opportunities during the course of the discovery of many novel materials. Throughout his scientific career, Mercouri and his group discovered several inorganic compounds and pioneered structure−property relationships. We, a few Ph.D. and postdoctoral students, celebrate his 60th birthday by providing a Viewpoint summarizing his contributions to inorganic solid-state chemistry. The topics discussed here are of significant interest to various scientific communities ranging from condensed matter to green energy production.

1. INTRODUCTION

subject with great success. Professor Mercouri G. Kanatzidis is one of them. Those that have worked with Professor M. Kanatzidis are well aware of his continuous search for novel materials with diverse properties and functionalities. The questions in the banner of his Web site at Northwestern University (“Where will the new materials come from?”, “there is a way to synthesize every material”, and “every material has

During the last 3−4 decades, solid-state chemistry has transformed into a significant force for the design of novel materials for various applications. Solid-state materials are targeted by combining the understanding of basic crystal structures, complex synthesis strategies, and structure−property relationships. Many researchers encounter both challenges and opportunities during the course of the discovery of many novel materials. However, only a few leaders adopt revolutionary pathways and innovative ideas for shaping and advancing the © 2017 American Chemical Society

Received: April 11, 2017 Published: June 27, 2017 7582

DOI: 10.1021/acs.inorgchem.7b00933 Inorg. Chem. 2017, 56, 7582−7597

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mesoporous structures with different pore sizes, shapes, and long-range periodicities were undeniably major breakthroughs in materials science. The beautiful transmission electron microscopy (TEM) images of the famous hexagonal (MCM41) and cubic (MCM-48) mesoporous silicates were indeed impressive, demonstrating the powerful role of soft molecules and their ordered aggregates in solution (micelles and their associated liquid crystals) in controlling the structure of hard inorganic materials.7 In the years after the Mobil publication, while an enormous worldwide research effort was devoted to this class of mesoporous oxidic materials and related solids, Mercouri, thinking “out of the box”, had the idea to apply this surfactant templating method to chalcogenide chemistry and the synthesis of mesoporous semiconductors. It was expected that these kinds of open-framework metal chalcogenide solids, with pore sizes on the nano- and mesoscale, would be of potentially broad technological interest in research areas ranging from optoelectonics to the physics of quantum confinement. In fact, the importance of porous metal chalcogenides was documented by Mercouri at the same time as Mobil’s publication, in his seminal paper published in Science in 1992 about crystalline openframework selenides.9 The surfactant-templated synthesis of mesostructured metal chalcogenides was a great challenge because, unlike silicates, metal chalcogenides do not have sol−gel chemistry. Therefore, he had to come up with novel synthetic approaches in order to make the nonoxidic analogues of the MCM-x family of solids. He decided that the best way is to use known metal chalcogenide clusters such as the adamantane [M4Q10]4−, dimeric [M2Q6]4−, or simple tetrahedral [MQ4]4− (M = Ge, Sn; Q = S, Se, Te) anions, which are soluble in polar solvents and could be linked together by transition-metal cations, in the presence of surfactants. Indeed, his group reported the first aqueous-mediated synthesis of mesostructured manganese germanium sulfide with hexagonal order, resembling the pore periodicity observed in MCM-41 silicates.10,11 At the same time, Ozin’s group reported the synthesis of similar solids made in formamide instead of water.12,13 A milestone in the work of Mercouri in this field was the synthesis of mesostructured semiconductors with varied pore organization using [SnSe4]4− anions linked by different transition-metal cations in the presence of pyridinium-based surfactants.14 These materials, indicated as (CP)4−2xMxSnSe4 (where 1.0 < x < 1.3; M = Mn2+, Fe2+, Co2+, Zn2+, Cd2+, and Hg2+ and CP = cetylpyridinium), showed uniform pore size and large diversity in pore organization, depending on the metal, and ranging from wormhole to hexagonal to cubic. That was also the first report on mesostructured chalcogenides with cubic pore symmetry, resembling the structure of MCM-48 mesoporous silica. The (CP)4−2xMxSnSe4 compounds showed well-defined, sharp optical absorptions associated with bandgap transitions in the energy range 1.4−2.5 eV.14 A very important work was also the development of the first mesostructured materials incorporating the biologically relevant Fe4S4 cluster.15 Accordingly, the supramolecular assembly of adamantine [Ge4Q10]4− (Q = S, Se) clusters with substitutionally labile [Fe4S4Cl4]2− clusters, in the presence of surfactants, resulted in the formation of novel mesophases containing [Fe4S4]2+ cores. The existence of the Fe4S4 core was confirmed with Mössbauer spectroscopy and also was independently determined by the excision method used to identify these clusters in proteins.

some property”) reflect his passion for innovation in materials science. In this Viewpoint, we celebrate the 60th birthday of Prof. Mercouri G. Kantzidis by reviewing the development of some of the areas of solid-state chemistry explored in his laboratory during the course of the last 3 decades. Mercouri G Kanatzidis was born in Thessaloniki, Greece, in 1957. He has a Bachelor of Science degree from Aristotle University in Greece. He received his Ph.D. degree in chemistry from the University of Iowa in 1984. He was a postdoctoral fellow at the University of Michigan and Northwestern University from 1985 to 1987. After beginning his independent academic career as a Professor at Michigan State University, he moved to Northwestern University in 2003, where he is currently the Charles E. and Emma H. Morrison Professor in Chemistry. Mercouri has received several awards during the last 3 decades on the basis of his scientific accomplishments. He has been named a Presidential Young Investigator by the National Science Foundation, an Alfred P. Sloan Fellow, a Beckman Young Investigator, a Camille and Henry Dreyfus Teaching Scholar, and a Guggenheim Fellow and in 2003 was awarded the Alexander von Humboldt Prize. In 2014, he received the Einstein Professor Award, Chinese Academy of Sciences Award, the International Thermoelectric Society Outstanding Achievement Award, the MRS Medal, and many more. The bulk of his research work is described in the more than 900 research publications and over 20 patents. His publications have been cited more than 53000 times, resulting in an h-index of 106. His breakthroughs in many subject areas are exemplified by highprofile publications in various fields over a period of years. In the last 3 decades, Mercouri and his group have discovered and explored a panorama of new inorganic materials, with pioneering accomplishments in complex synthesis approaches, structural characterizations, physical property measurements, and analyses of electronic structure.

2. MAJOR AREAS OF RESEARCH The group of Mercouri has carried out seminal work in synthetic chemistry and the development of new functional materials. He has been active in the field of new thermoelectric materials for the past 20 years. He also studies chalcogels, complex intermetallic phases, and superconductors. He has interests in the synthetic design and prediction of new phases, especially those that can cause disruptive changes in scientific thinking and in technology. In this Viewpoint, we review some of his major areas of research over the last 3 decades. This work has resulted in over 900 publications with more than 200 students across the globe, so it is impossible to include all of it. However, we address the key areas of his research with a few examples. 2.1. Chalcogenides. 2.1.1. Mesostructured and Mesoporous Metal Chalcogenides. The synthesis of polychalcogenide materials is a research topic that Mercouri’s group has been working on since the early stages of his career. Mercouri has contributed a lot of his effort and his vision to the polychalcogenide field and has provided unprecedented insight into the preparation of these materials as well as their potential applications.1−6 In the early 1990s, the seminal work of the Mobil group regarding the synthesis of templated mesoporous silicates, using surfactants as structure-directing agents, attracted enormous attention worldwide.7,8 The controlled polymerization of silicate species in the presence of soft organic molecules like surfactants and the resulting self-assembly into ordered 7583


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germanium-based mesoporous semiconductors with interesting optical and electronic properties. Indeed, the coordinative framework assembly of the coupling of [Ge9]4− with various linking metal ions (i.e., indium, antimony, tin, lead, cadmium) lends itself to producing a wide variety of well-ordered mesoporous semiconductors (NU-MGe-2, where M = Sb, In, Sn, Cd, and Pb);24 these compounds can be viewed as porous intermetallics, a class of materials that currently is undeveloped. Interestingly, the pore structure of these binary compound semiconductors was proven to be active, selective, and sensitive toward electron-transfer adsorbates. This work provided the first clue to the potential applications of these materials for sensor and field-effect transistor applications. For example, Mercouri’s research in this area demonstrated that the emission intensity of NU-InGe-2 decreased continuously when its mesopore surface interacted with increasing concentrations of electron-acceptor tetracyanoethylene (TCNE) molecules (Figure 2d). In stark contrast, the incorporation of molecules with electron-donor properties such as tetrathiafulvalene did not affect the photoluminescence (PL) emission properties of NUInGe-2. Upon using different reaction chemistries, Mercouri and his postdoc, Dr. Gerasimos Armatas, also demonstrated the synthesis of a new type of mesoporous germanium-rich chalcogenide frameworks. They utilized a redox coupling reaction of Zintl clusters [Ge9]4− (derived from the K2Ge9 compound) with chalcogen atoms (sulfur, selenium, and tellurium) in the presence of amphiphilic surfactant templates to construct high-surface-area mesoporous germanium-rich chalcogenides (NU-GeQ-N).25 This is Mercouri’s first significant work on well-ordered mesoporous metal chalcogenide semiconductors. The mesoporous NU-GeQ-N materials possess a germanium-rich chalcogenide framework perforated by a regular array of pore channels in hexagonal and cubic symmetry and exhibit high internal surface areas (ranging from 350 to 516 m2 g−1) and quite narrow pore-size distribution with pore diameters at ∼2.9−3.1 nm. Significantly, the GexQ-derived mesostructures feature a unique functionality that is the high polarizable internal pore surface; indeed, the surface polarizability arises from the heavier chalcogen atoms that comprise the framework. As a result, the NU-GeQ-N materials show promise for developing gas mixture separation processes based on surface polarizability rather than the size and shape of the pore. Gas adsorption and gas separation experiments clearly supported this hypothesis and showed excellent solubility separation of carbon dioxide (CO2) and methane (CH4) over hydrogen (H2) with respective separation factors of ∼88 and ∼70, in the low-pressure limit at 0 °C. Most importantly, the separation performance of these porous chalcogenides seems to be related to the strength of the interactions of the probe molecules with the porous surface following the range CO2 > CH4 > CO > H2; that is, the more polarizable molecules will interact more strongly. Such a gas separation process is relevant to hydrogen purification technology. 2.1.2. Porous Chalcogels. During the synthesis of some surfactant-templated mesostructured metal chalcogenides based on metathesis reactions and using Pt2+ or Pd2+ cations as linking metals between different chalcogenide clusters, gel formation was occasionally observed. That was called a “fortunate discovery” and led the way to the discovery of the very important family of porous chalcogels.26 Aerogels are a unique class of porous materials largely composed of randomly interconnected nanoparticles, possess-

Despite the success of this methodology in making mesostructured semiconductors, the quality of these materials in terms of long-range pore organization was far behind that in the corresponding silicate analogues. The obvious problem was the very fast reaction kinetics taking place when the linking metal cation was added to the solution of the metal chalcogenide anions because of the formation of strong covalent metal−chalcogen bonds. In order to slow down the reaction kinetics, the use of the less labile Pt2+ as a linking cation proved to be a brilliant idea. Accordingly, the quality of the final products was remarkably improved, being comparable to the best mesoporous silicates.16−19 Excellent-quality mesostructured semiconductors with hexagonal MCM-41-type and cubic MCM-48-type (space group Ia3̅d) pore periodicity even with the formation of pseudo-single-crystal, cubosomelike, particle morphology were observed (see Figure 1). In these seminal publications, it was also reported that the pore space in these materials was indeed accessible via cation-exchange reactions.18

Figure 1. (a) Representative image of a large particle of C16PyPtGeSe showing hexagonal organization extending over its full body. Particle length >500 nm. Reproduced with permission from Mater. Res. Soc. Symp. Proc. 2002, 703). (b−c) Representative high-resolution scanning electron microscopy images of pseudo single crystals with rhombic dodecahedral morphology of high-quality mesostructured semiconductor c-C20PyPtSnSe, with cubic Ia3d̅ pore symmetry.18

The chemistry of surfactant-templated open-framework metal chalcogenides was further expanded by the introduction of metal clusters as building units. In particular, Mercouri’s group first demonstrated that appropriate germanide Zintl clusters in nonaqueous liquid-crystalline phases of cationic surfactants can assemble well-organized mesostructured and mesoporous germanium-based semiconductors. These include mesostructured cubic gyroidal20 and hexagonal mesoporous germanium.21,22 The proper precursors for the construction of these materials are negatively charged Zintl [Ge9]4− clusters and germanium(IV−) anions; the latter are derived from the Zintl Mg2Ge compound and give open frameworks when reacted with GeCl4 in the presence of a cationic surfactant. Figure 2 displays typical TEM images and N2 adsorption and desorption isotherms obtained from hexagonal mesoporous germanium (NU-Ge-1).12 A particularly interesting property of mesostructured germanium-based frameworks is the substantial blue shift in optical absorption (1.42−1.87 eV) relative to that of bulk germanium (0.66 eV). This large blue shift can be understood by considering the change in the density of the electronic energy states caused by the substantial dimensional reduction of the germanium structure from bulk germanium to ∼1−2 nm. In fact, this behavior is analogous to the well-known ability of nanocrystals to tune their optical properties through changes in the particle size.23 The paradigm of porous materials made from molecularbased Zintl compounds as building blocks represents a major advance, which allows the synthesis of new open-framework 7584


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Figure 2. (a and b) Typical TEM images showing a well-ordered arrangement of pores in hexagonal P6mm symmetry analogous to MCM-41. (c) N2 adsorption and desorption isotherms at 77 K (inset: nonlocal density functional theory pore-size distribution calculated from the adsorption branch) of mesoporous NU-Ge-1. Reproduced with permission from ref 22. Copyright 2008 Wiley-VCH. The N2 physisorption measurements indicate a Brunauer−Emmett−Teller surface area of 402 m2 g−1 and a quite narrow distribution of pore sizes with a peak maximum at ∼3.1 nm. (d) TCNE concentration dependence of the PL emission peak of mesoporous NU-InGe-2. Copyright 2008 American Chemical Society. The straight line fitting the data is given as a guide to the eye (R2 = 0.9938).

Bag et al. demonstrated the first metal chalcogenide aerogel successfully made from the metathesis route.26 Platinum is used as a linker metal to construct a porous network of chalcogenide clusters, [MxQ2x+2]4− (M = Sn, Ge; Q = S, Se). Various structural characterization methods such as IR spectroscopy, powder X-ray diffraction, pair distribution function, and smallangle X-ray scattering revealed that these chalcogels have a nonperiodic macrostructure with locally well-defined coordination between building blocks. The resulting platinum chalcogels possess highly porous structures indicated by high surface area (108−327 m2 g−1), large pore volume (0.21−0.85 cm3 g−1), and TEM images characteristic of porous materials. The highly porous structure of these chalcogels allows chalcogenide-rich surfaces to be easily accessible to the incoming species. As a result, the platinum chalcogels selectively attract soft heavymetal ions from the aqueous solution. These phenomena are a good example of hard soft acid base (HSAB) theory,50 where chalcogenide (e.g., S2−) acts as soft base attracting soft acids (e.g., Hg2+). This functional feature is highly suitable for use in aqueous heavy-metal waste remediation. For the metathesis reaction to succeed in providing a stabilized porous structure, the inorganic species should engage in a slow self-assembly process. Many transition-metal linkers such as Mn2+, Fe2+, and Cu2+ react too rapidly with

ing low density and high internal surface area and leading to potential applications such as catalysis,27−31 separations,32 sensing devices,33,34 and charge storage.35 Classical aerogel studies are mainly focused on oxide materials such as SiO2, Al2O3, and TiO2 or carbon-based materials,36 but there are exotic examples of metal chalcogenide aerogels featuring accessible open frameworks, high surface areas, surfaces with soft Lewis base properties, and more attractive electronic properties that are relevant for photocatalysis,37 photoconductivity, 38 photovoltaic cells, 39,40 electroluminescence,41−44 and water remediation.26,45 At the time that “chalcogels” were introduced by Mercouri, the field was relatively unexplored, and only limited studies were available, using methods such as thiolysis and nanoparticle condensation. For example, the thiolysis route was used to produce various binary metal chalcogenide aerogels such as LaSx,46 WSx,47 ZnS,48 and GeSx.49 However, this method is obliged to produce toxic H2S gas, and the synthetic process is difficult to manipulate. On the other hand, controlled oxidation of the nanoparticle route yielded various metal chalcogenide aerogel systems such as ZnS, CdS, CdSe, and PbS,41 but this route requires the delicate synthetic protocol of handling nanoparticles, which often results in failure of stabilized porous structure. 7585


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carrying out useful electrochemical and photochemical reactions.56 Liu et al. also demonstrated a nitrogenase-inspired biomimetic chalcogel system comprising double-cubane [Mo2Fe6S8(SPh)3] and single-cubane (Fe4S4) biomimetic clusters with photocatalytic functionality of N2 fixation and conversion to NH3 in ambient temperature and pressure conditions.57 These results suggest that redox-active iron sulfide containing materials can activate the N2 molecule upon visiblelight excitation, which can be reduced all the way to NH3 using protons and sacrificial electrons in an aqueous solution. These results contribute to ongoing efforts of mimicking nitrogenase in fixing N2 and point to a promising path in the development of catalysts for the reduction of N2 under ambient conditions. The Fe4S4 cubane cluster is not the only example of a chalcogenide unit studied for versatile functionalities. Starting from the work of Bag et al., Co(Ni)−Mo(W)−S network-based chalcogels with low-density spongelike structure are introduced that can absorb conjugated organic molecules and mercury ions and preferentially adsorb CO2 over H2.58 This work illustrates their high potential as gas-separation media and active hydrodesulfurization functionality toward thiophene. Islam et al. synthesized MoSx chalcogels with a high surface area of 353 m2 g−1 and a band gap of 1.5 eV through new molybdenum thiochlorides, α-MoSCl3 and β-MoSCl3.59 Subrahmanyam et al. also constructed ion-exchangeable molybdenum sulfide chalcogel through an oxidative coupling process, using (NH4)2MoS4 and iodine.60 X-ray photoelectron spectroscopy and pair distribution function analyses reveal that Mo6+ species undergo reduction during network assembly to produce Mo4+containing species where the chalcogel network consists of [Mo3S13] building blocks comprising triangular molybdenum metal clusters and S22− units. This chalcogel presents versatile functionalities of ion-exchange capability, high adsorption selectivity toward polarizable gases, and high affinity for iodine and mercury for environmental remediation. Banerjee et al. demonstrated chalcogels with [Mo 2 Fe 6 S8 (SPh) 3 ]3+ and [Sn2S6]4− double clusters possessing a photochemical reduction functionality of N2 to NH3 under white-light irradiation, in aqueous media, under ambient pressure and room temperature.61 These results demonstrate that light-driven nitrogen conversion to ammonia by MoFe sulfides is a viable process with implications in solar energy utilization and our understanding of primordial processes on Earth. One of the most distinctive features of chalcogels compared to other aerogel families is their activity in radioactive material capture. Recent works on chalcogels demonstrate their radioactive iodine-capture (129I and 131I) functionality. Riley et al. synthesized porous polyacrylonitrile−chalcogel hybrid material in pelletized form to capture high loadings of iodine (53.5 wt %) with greatly improved mechanical rigidity.62 Subrahmanyam et al. tested various chalcogel materials such as NiMoS4, CoMoS4, Sb4Sn3S12, Zn2Sn2S6, and K0.16CoSx (x = 4− 5) for iodine-capture application. These chalcogels showed high uptake, reaching up to 225 mass % (2.25 g g−1) of the final mass due to strong chemical and physical iodine−sulfide interactions. Interestingly, iodine-adsorbed chalcogels readily release iodine upon heating (75−220 °C) so that the host material can be reused. The latest work of Stazak-Jirkovsky et al. synthesized a compact and robust CoMoSx chalcogel structure to solve three fundamental problems of the electrochemical production of hydrogen: low efficiency, short lifetime, and high-cost materials. By combining the higher activity of CoSx building blocks with

chalcogenide clusters to form precipitates without extended porous networks that require stabilized gelation. In order to obtain a porous metal chalcogenide system, controlling the polymerization rate of the inorganic framework is crucial. Oh et al. demonstrated the significant role of bidentate ligands in a zinc metal precursor in reducing the kinetic rate of the metathesis reaction of divalent zinc ions and thiostannate cluster ions, forming a stabilized porous structure of Zn2+ coordinated with [SnS4]4−, [Sn2S6]4−, and [Sn4S10]4− clusters.51 The soft Lewis basic sulfidic surfaces provide high selectivity toward soft heavy-metal ions such as Hg2+ and high efficiency of removing those metal ions from aqueous media. The successful synthesis of aerogel using Zn2+ and [SnxS2x+2]4− (x = 1, 2, 4) demonstrates that modifying kinetic factors can affect the outcome of self-assembly reactions between inorganic building blocks. The first examples of chalcogels utilized three-dimensional tetrahedrally coordinated metal chalcogenide clusters, but lowdimensional chalcogenide building blocks are also a viable option for the versatile metathesis chemistry. Oh et al. synthesized the first chalcogel system with polysulfide backbones, endowing unique functionalities that arise when the twodimensional chalcogenide surface meets an accessible porous structure. The series of polysulfide chains [Sx]2− (x = 3, 4, 5, 6) forms a monolithic wet gel network with Pt2+ featuring ionexchangeable anionic networks (charge-balanced with potassium) and accessible S−S bonding sites capable of air-borne heavy-metal waste capture (Hg0). This is the first example of chalcogel demonstrating air-borne toxic mercury vapor remediation functionality in which conventional air pollution control techniques such as fabric filtering, electrostatic precipitation, and flue-gas desulfurization are not effective to remove.52,53 There are distinct examples of chalcogels that are composed of building blocks mimicking biological functional groups. Yuhas et al. presented porous chalcogenide frameworks that can contain both immobilized redox-active Fe4S4 clusters and light-harvesting photoredox dye molecules in close proximity.54 These multifunctional gels are shown to electrocatalytically reduce protons and carbon disulfide. In addition, the incorporation of a photoredox agent into the chalcogels is shown to photochemically produce hydrogen. The gels have a high degree of synthetic flexibility, which should allow for a wide range of light-driven processes relevant to the production of solar fuels. Yurina et al. further extended the work of Fe4S4cluster-based chalcogel chemistry by investigating the effects of [SnS2n+2]4− linking blocks ([SnS4]4−, [Sn2S6]4−, and [Sn4S10]4−) on the electrochemical and electrocatalytic properties of the chalcogels, as well as on the photophysical properties of incorporated light-harvesting dyes, tris(2,2′-bipyridyl)ruthenium(II) (Ru(bpy)32+).55 This chalcogel system shows enhancement of the electrochemical reduction of CS2 and the photochemical production of hydrogen and provides a novel avenue to controlling the function of multifunctional chalcogels for a wide range of electrochemical or photochemical processes relevant to solar fuels. In her following work, the incorporation of a third metal component (divalent platinum, zinc, cobalt, nickel, and tin) was found to alter the reduction potential of Fe4S4 in a favorable manner for photochemical hydrogen production. The ability to manipulate the properties of biomimetic chalcogels through synthetic control of the composition, while retaining both structural and functional properties, illustrates the chalcogels’ flexibility and potential in 7586


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Figure 3. (A) Capture of Hg2+ by the layered metal sulfide material K1.9Mn0.95Sn2.05S6. (B) Cs+ ion exchange by [CH2NH2]2Ga2Sb2S7·H2O. (C) Polysulfide-intercalated layered double hydroxide and the proposed binding of Hg2+ with the polysulfide unit. (D) Schematic for the selective sorption of heavy-metal ions by chalcogels.

conditions, the effectiveness of oxides for the sorption of heavymetal ions is reduced substantially because the hard protons strongly compete with the soft metal ions. In the case of MSIEs, the competition from protons on heavy-metal ion sorption is negligible and, thus, metal sulfides work very efficiently under acidic conditions. In addition, MSIEs are superior and probably more affordable than various sulfur-functionalized materials (e.g., thiol-functionalized mesoporous silica64) because they do not need any functionalization. Mercouri’s work showed that MSIEs are also excellent sorbents for cationic species not commonly thought of as soft metal ions. Thus, several MSIEs show high selectivity for the relatively “soft” Cs+ ion versus Na+ and K+ or for Sr2+ versus Ca2+ and Mg2+. The latter property is relevant to nuclear waste remediation because 137Cs+ and 90Sr2+ represent major biohazards in nuclear waste. Probably the most unexpected finding from Mercouri’s work on MSIEs was the exceptional selectivity of these materials for UO22+. Mercouri used to say, “UO22+ may not be as hard as people believe; uranium(VI) is bound to two oxo groups, thus UO22+ is large ion and may behave as Pb2+”. Indeed, Mercouri’s work revealed that the selectivity of MSIEs for UO22+ compares well with that for typical soft ions such as Hg2+ and Pb2+. Several papers from Mercouri’s group proved that MSIEs are capable of capturing uranium even from seawater, where the concentration of uranium is ≤10−8 M and several cations (Na+, Ca2+, and Mg2+) are in concentrations up to 0.5 M. Several MSIEs can now be prepared (via solid-state or hydrothermal synthesis) in a multigram scale and are stable in air over months or years. Mercouri’s group owns a number of patents on MSIEs, and his vision is for some of these materials to be commercialized over the next few years and provide an effective solution to the growing problem of the heavy-metal ion pollution of water resources. 2.1.4. Solution-Phase Synthesis of Nanoscale Chalcogenides. Going beyond extensive expertise in the production of nanostructured extended solids, Mercouri has significantly

the higher stability of MoSx units, CoMoSx chalcogels are designed to perform as a low-cost alternative to noble-metal catalysts for the efficient electrocatalytic production of hydrogen in both alkaline and acidic environments. This discovery refutes the common bias about chalcogenide compounds not being suitable for practical application because of their sensitivity toward the chemical environment and costly and demanding synthetic protocol. The work introduced herein demonstrates the great potential found in materials where versatile and dynamic chalcogenide chemistry meets the porous aerogel structure. Numerous potential metal chalcogel aerogels with a wide array of exotic functionalities are yet to be explored. 2.1.3. Advanced Ion-Exchange Materials for Environmental Remediation. In parallel with the development of porous chalcogels, the group of Mercouri was also exploring the synthesis of novel, crystalline open-framework and layered metal sulfides for the removal of heavy-metal ions and radionuclides (Figure 3). A recent review summarizes the most important results on metal sulfide ion exchangers (MSIEs).63 Prior to this work, very few thought that metal sulfides were feasible as ion-exchange materials. In a Gordon conference back in 2006, where some of us presented work on the ion-exchange properties of open-framework metal sulfides, some of the participants looking at our posters were surprised by the results and said “we thought only zeolites and clays show such ion exchange properties”. In fact, Mercouri proved through work described in a number of papers that MSIEs are far superior to zeolites and clays for the sorption of heavy metals (Hg2+, Pb2+, and Cd2+) and some radionuclides, with reasoning based on the HSAB theory. MSIEs containing soft S2− ligands exhibit innate selectivity for soft heavy-metal ions. Thus, a metal sulfide with loosely bond cations in its structure is capable of rapidly capturing soft metal ions (via ion exchange), even in the presence of a tremendous excess of several hard cations (e.g., Na+ and Ca2+) typically found in wastewater. Under similar conditions, the performance of oxidic sorbents is significantly hindered. Furthermore, under acidic 7587


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Inorganic Chemistry contributed to the field of nanoscience/chemistry via the development of innovative colloidal and wet-chemical syntheses for nanostructured thermoelectric materials. This contribution is highly significant because he mainly focused on the design and synthesis of more complex ternary and quaternary nanoscale materials, which have not been explored in the past. The binary nanocrystals of PbS, PbSe, PbTe, SnTe, and Ag2Te have been extensively studied and reported in the literature. However, reports on more complex ternary and quaternary nanocrystals are rare because of difficulties with phase separation, limitations of suitable precursors, and complications with the solubility of precursors in a common reaction medium. Nonetheless, Mercouri guided his students to successfully develop novel chemical routes for thermoelectrically relevant AgPb m SbTe m+2 , 65,66 Pb 1−x Sn x Te, 67 and PbmSb2nTem+3n68 nanocrystal systems. As described in section 2.3, lead chalcogenides are widely known as a class of materials with significant potential in energy production, conversion, and conservation. In comparison to binary lead chalcogenides, more complex ternary materials such as LAST-m (LAST = lead−antimony−silver−tellurium) exhibit high performance in thermoelectric applications (with a ZT of 1.2−1.7 at 700 K). The improved ZT is attributed to nanostructuring of silver and antimony in the binary PbTe solid matrix, which can potentially increase the boundary scattering of phonons, leading to very low thermal conductivities and higher ZT values. However, the nature of these silver- and antimony-rich nanostructures and their influence on high ZT performance is poorly understood. To address this issue, Mercouri has guided his students to produce nanocrystalline LAST-m materials using wet-chemical65 and colloidal66 syntheses. LAST-m nanocrystals produced via the coreduction of precursor salts under ambient conditions were highly polydisperse, which led to difficulties in structure− property elucidation. To solve this problem, a high-temperature colloidal synthesis was developed to produce high-quality quaternary nanocrystals with spherical shape and narrow size dispersity that exhibit absorption energy gaps in the mid-IR region (Eg = 0.43−0.45 eV for LAST-m where m = 1−18). The nanocrystals produced by both syntheses were metastable solid solutions (i.e., rock-salt crystals) that phase-transformed to AgSbTe2 and PbTe at 423−473 K. This research provided new insight into the synthesis of complex nanomaterials and their nanoscale phase stabilization, which greatly advanced the understanding of the atomic-scale nature of the bulk LAST-m materials. The contrast between the bulk LAST-m, which are largely nanosegregated, and the nanocrystalline LAST-m, which are metastable solid solutions under ambient conditions, provided new insight into the design and synthesis of more complex thermoelectric materials as both extended solids and nanostructures. Another thermoelectrically relevant material that exhibits unusual optical properties is lead−tin telluride.69 Ternary Pb1−xSnxTe alloys exhibit narrower energy gaps than their end members, PbTe (0.29 eV) and SnTe (0.18 eV), leading to an anomalous trend in the band energies as a function of the tin composition.70 This is explained by the band inversion model in which the valence and conduction bands are inverted (in SnTe) from those of PbTe. It is very difficult to precisely determine the band-edge structure of the intermediate compositions near and beyond the band inversion region because only high-carrier-concentration samples can be

obtained because of deviation from stoichiometry. To address this issue, the group of Mercouri reported an innovative method to synthesize Pb1−xSnxTe nanoalloys with sizes in the quantum confinement regime.67 As-prepared ternary nanocrystals exhibit qualitatively the same anomalous trend in energy-gap dependence with the tin concentration; however, the energy gap achieves only a minimum of 0.28 eV at x ∼ 0.67 but does not vanish at any tin concentration. This unique band structure obtained in Pb1−xSnxTe nanoalloys would be advantageous in the design and development of novel IR photodetectors, laser diodes, and thermophotovoltaic energy converters. An advantage in nanoscience/chemistry is its unique ability to produce colloidal nanoparticles of compounds that are inaccessible via conventional solid-state reactions such as wurtzite-type MnSe71 and Cu2SnSe372 and cubic Ge1−xSnx73−75 alloys. Nanoparticles of metastable compounds are produced either from inherent nanostabilization due to surface energies and size effects or from structural templating, which occurs when a precursor nanoparticle is chemically transformed into a derivative phase that retains targeted structural features. To this end, the Kanatzidis group reported the successful synthesis and nanoscale phase stabilization of the PbTe/Sb2Te3 system.68 Binary PbTe and Sb2Te3 are known as two immiscible phases of thermoelectrically relevant materials. The bulk PbTe/Sb2Te3 behaves as a pseudobinary system, which does not form solid solutions at or near room-temperature conditions.76 However, Kanatzidis et al.68 reported that when the PbTe/Sb2Te3 system is produced via a low-temperature colloidal route, a series of metastable PbmSb2nTem+3n nanocrystals that have no bulk-scale analogues can be produced. The ternary lead−antimony telluride nanoalloys exhibit energy gaps in the mid-IR region (0.43−0.45 eV) and undergo phase segregation into PbTe and Sb2Te3 above 300 °C. The contrast between the bulk materials of PbTe/Sb2Te3, which do not produce rock-salt ternary compounds, and the single-phase, quantum-confined nanocrystals, which are fully miscible solid solutions, can serve as an excellent example that can be used to better understand the thermodynamic limits of nonequilibrium material systems as the size is reduced to nanoscale regimes. 2.2. Intermetallics. 2.2.1. Exploratory Synthesis. Having had great success with the formation of new semiconducting materials from alkali polychalcogenide and chalcophosphate salt fluxes, the Kanatzidis group began to explore metal flux reactions in 1998. Metal flux synthesis involves the use of a lowmelting metal reactant that is present in large excess; when this reactant melts, it acts as a solvent for other reactants present. This effective solvation of refractory elements such as silicon, boron, and carbon and metals such as iridium and osmium allows these elements to be reactive at temperatures well below their melting points. Reactions can be carried out at lower temperatures than those typically used in solid-state synthesis, enabling the formation and isolation of metastable or kinetically stabilized products. The solution-state nature of metal flux reactions enables products to be grown as large crystals by the slow cooling of the molten mixture. This technique had been previously used to grow large crystals of known compounds and explored predominantly by the Canfield and Jeitschko groups to make binary and ternary intermetallics using lowmelting-flux metals such as tin.77−79 The Kanatzidis group’s initial forays into metal flux work were carried out to target the growth of complex silicides from molten aluminum. The aluminum/silicon phase diagram is 7588


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Figure 4. Lanthanide/actinides and late transition metals reacting with silicon in excess molten aluminum to form a wide variety of complex intermetallic silicides, growing from the flux as large crystals. Examples include (a) ErFe4Al9Si6, (b) Sm2Ni1.2Al4Si6.8, and (c) Th2Au3Al4Si2.

Figure 5. (a) Structural view of temperature-dependent changes in the distances and angles in Yb4TGe8. The solid bonds and dashed points represent the structure at 10 and 300 K, respectively. Arrows indicate the direction of atomic motion with temperature. (b) Five-coordinate squarepyramidal T center with four basal T−Ge(2) bonds and one axial T−Ge(1) bond. (c) Decreasing Ge(2)−T−Ge(2) angles of the basal bonds (φ1 and φ2) with rising temperature. (d) Increasing Ge(2)−T−Ge(1) angles of the axial bonds (φ3 and φ4) with rising temperature.

yields GdCo1−xGa3Ge, which exhibits charge-density wave behavior, and an indium flux reaction produces a rare intermetallic semiconductor, EuIr4In2Ge4,84,85 as well as zero thermal expansion compounds RE4TGe8 (RE = Gd, Yb; T = Cr−Ni, Ag).86 Reactions of ytterbium in an indium flux often yield mixed-valent compounds, whether the indium is incorporated (Yb 7 T 4 InGe 12 and Yb 3 AuGe 2 In 3 ) or not (Yb2CuGe6).87−89 The metal flux technique often produces compounds with novel structure types as in the case of Yb6.6Ir6Sn16,90 YbGe2.83,91 YbMn0.17Si1.88.92 More recently, the Kanatzidis group has explored the metal flux chemistry of heavier elements such as tin and bismuth. Reactions in these melts have produced several high-temperature soft ferromagnetic stannides in the hafnium/iron/tin system and the new bismuthide superconductor LaPd1−xBi2.93,94 In addition to the discovery and crystal growth of new materials, the exploration of various metal fluxes by the Kanatzidis group has enabled reactivity trends to be discerned (for instance, the flux reactivity of the triels decreases down the column, with aluminum always incorporated into products and

amenable to this, given the lack of competing aluminum/silicon binary phases and the fact that aluminum-rich melts are excellent solvents for the majority of the periodic table. Reactions of a rare earth and a transition metal with silicon in molten aluminum yielded a wide variety of compounds: from the initial report in 1998 of the synthesis of Sm2Ni(NixSi1−x)Al4Si6, which exhibits several low-temperature magnetic ordering transitions, to the growth of oxidation-resistant REFe4Al9Si6, to the isolation of RE8Ru12Al49Si9(AlxSi12−x) with a highly complex cubic structure that required singlecrystal neutron diffraction to determine aluminum and silicon siting.80−82 This chemistry can be extended to the actinides, with the strongly reducing nature of the aluminum flux allowing for the use of ThO2 as a reactant to form Th2(AuxSi1−x)[AuAl2]nSi2 (n = 1, 2, 4), a homologous series of three structurally related intermetallics (Figure 4).83 The exploration of metal flux chemistry of the triels readily expanded to the use of gallium and indium fluxes. These elements were found to be particularly fruitful growth media for germanide intermetallics. For instance, a gallium flux reaction 7589


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conductivities), and absolute temperature, respectively. Although it is quite difficult to control the above parameters independently because of their complex interrelationships, thermoelectric performance records have been broken continuously in the past several decades. Undoubtedly, Mercouri significantly contributed to the tremendous progress in the field of thermoelectrics over the past 2 decades. One of Mercouri’s major contributions in the field of thermoelectrics is the introduction and development of the concept of nanostructuring as a means to improve the ZT values of bulk semiconductors. In addition, Mecouri contributed new inorganic solids and a new material design concept for the development of high-performance thermoelectrics and provided a deeper fundamental understanding of the structure−property correlation in solid-state materials. Some of his most notable contributions have been achieved in lead or tin chalcogenides and their nanostructures. Mercouri’s significant leadership in the advancement of the field of thermoelectricity has been recognized by several important awards, such as the 2016 Samson Prime Minister’s Prize for Innovation in Alternative Fuels for Transportation, the 2015 Renewable Energy Prize, Eni award, the 2015 De Gennes Prize from Royal Society of Chemistry, the 2014 MRS medals award, etc. Mercouri’s first significant work on thermoelectrics was the discovery in 2000 by Chung et al. of CsBi4Te6,110 a new material that exhibited a high thermoelectric figure of merit below room temperature (ZTmax ∼ 0.8 at 225 K). At cryogenic temperatures, the thermoelectric performance of CsBi4Te6 appears to exceed those of Bi2−xSbxTe3−ySey alloys. This highly cited work was the launch point of Mercouri’s research on thermoelectric materials. Thanks to a Multidisciplinary University Research Initiative program from the Office of Naval Research, Mercouri and his collaborators initiated work toward optimization of the thermoelectric properties of PbTe using various dopants. This early effort led to the report in 2004 by Hsu et al. of high-performance n-type thermoelectrics based on AgPbmSbTe2+m (LAST) compositions.111 In the temperature range 600−900 K, the LAST system outperformed all reported bulk thermoelectrics at that time. The high ZT observed in these materials mainly came from the drastic reduction in the total thermal conductivity. Although the LAST materials were initially believed to be a uniform solid solution, further careful investigations by Quarez et al. revealed the presence of several heterogeneities that are silver- and antimony-rich, demonstrating that the LAST materials are, in fact, nanostructured.112 These initial observations provided solid foundations for the introduction of the new concept of phonon scattering by the small nanoprecipitates embedded on the bulk thermoelectric matrix as a means to drastically reduce the lattice thermal conductivity of bulk semiconductors. These reports have triggered broad-based enthusiasm especially for the PbTe system in the field of thermoelectrics. Therefore, Mercouri and his group at Michigan State University and later at Northwestern University continued research toward the ptype counterpart of LAST and also the development of compositional design and synthesis approaches to intentionally create desired nanoscale features in bulk lead chalcogenide matrixes. This early work in the development of PbTe-based materials resulted in ground-breaking results such as the discovery by Poudeu et al. in 2006 of Na1−xPbmSbyTe2+m (SALT = sodium−antimony−lead−tellurium),113 the first high-performance p-type counterpart of LAST with a ZT

gallium and indium less so). This increased understanding of flux chemistry may eventually enable researchers to tailor flux reactions to target specific structures and compositions. At the current time, metal flux reactions are still largely exploratory but highly productive, yielding complex materials with a wide range of physical properties. Many other research groups have been inspired to make use of this synthetic technique, evidenced by the fact that the excellent review article on the metal flux synthesis written in 2005 by Mercouri, Rainer, and Wolfgang has been cited hundreds of times.79 2.2.2. Solid-State Physics. The discovery of a vast range of novel materials over the past 3 decades has motivated Mercouri to explore the intersection of complex physics and chemistry to explain many of the unusual properties seen in these compounds. Although he is by practice a solid-state chemist, he has expanded his research interest toward solid-state physics. He has contributed materials of great interest to the condensed matter physics community. Special attention must be given to the compounds containing rare earths especially cerium, europium, and ytterbium because of the presence of an unstable electronic 4f shell. The discovery of complex physical effects in these compounds, such as intermediate valence, Kondo effect, valence fluctuation, heavy Fermion, superconductivity, structural transitions, zero thermal expansion etc., has stimulated much interest in these materials. Many of the new compounds discovered in Mercouri’s group present novel examples of the physical properties of the type mentioned above. The notable examples are zero thermal expansion in YbGaGe95 and RE4TGe8 (RE = Gd, Yb; T = Cr− Ni, Ag)86 within the temperature range 100−300 K (Figure 5), antiferromagnetic Kondo lattice in the layered compound CePd1−xBi2,96 Dirac Fermions and superconductivity in the structures (AgxPb1−xSe)5(Bi2Se3)3m (m = 1,2), superconductivity in the intermetallic pnictide compound Ca11Bi10−x,97 Dresselhaus spin splitting observed in the very unusual new semiconductor intermetallic compound EuIr4In4Ge2,98 lowtemperature Fermi-liquid regime in Yb3Ga7Ge3,99 mixed-valent behavior in Yb2Au3In5,100 and many more. Mercouri has always had the thirst to contribute his knowledge and wisdom to expand the boundaries of current knowledge. As a postdoctoral fellow in the group of Tobin Marks at Northwestern University, he discovered the hightemperature superconductor EuBa2Cu3O7−x101 during the height of the copper oxide superconductor era. He has extended the knowledge in superconductors as an independent researcher, and when iron-based superconductors were discovered in 2006, Mercouri and co-workers focused on this area and developed several novel materials as superconductors.102−109 2.3. Thermoelectric Materials. Thermoelectric technology is an environmentally friendly power source that can use low-grade heat energy such as solar heat, geothermal energy, and industrial waste heat to generate electricity. Thermoelectric generators are solid-state devices that can directly convert thermal energy to electricity, have the advantages of simple structure, reliability, no moving parts, and being friendly to the environment, can be widely used in the aerospace and military fields, and have broad prospects in the application of recovery of industrial waste heat. The efficiency of thermoelectric materials is determined by the dimensionless figure of merit (ZT), defined as ZT = (S2σ/κ)T, where S, σ, κ, and T are the Seebeck coefficient, electrical conductivity, total thermal conductivity (a sum of the electron κele and lattice κlat thermal 7590


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Figure 6. (a and b) Through the combination of the effects of atomic-scale alloy doping, endotaxial nanostructuring, and mesoscale grain-boundary control, maximum phonon scattering has been achieved at high temperatures and ZT has been increased beyond the value possible with nanostructuring alone in PbTe. Inset in part b shows the mesoscale grains (TEM image) in PbTe formed by SPS processing. Reproduced with permission from ref 117. Copyright 2012 Nature Publishing Group. (c) SnSe crystal structure along the a, b, and c axes along with a highly distorted SnSe7 coordination polyhedron. Reproduced with permission from ref 119. Copyright 2012 Nature Publishing Group. (d) ZT values of undoped and hole-doped SnSe along different axial directions. Reproduced with permission from ref 120. Copyright 2012 Nature Publishing Group.

realized in a simple compound such as SnSe because it does not have high molecular weight, a complex crystal structure, or a large unit cell. An ultrahigh power factor can also be achieved in a not-so-narrow band-gap semiconductor of only orthorhombic crystal symmetry. The multiple valence band extrema lying close in energy are the key to this performance, which persists over a wide temperature plateau from 300 to 773 K and perhaps even wider. The discovery of exceptional physical properties in SnSe clearly points to new directions in thermoelectric science in terms of what materials systems we might pursue as superior thermoelectrics. Thermoelectrics is a major field in Mercouri’s group. Therefore, in addition to the few examples highlighted in this paper, credit should be given to numerous dedicated graduate students and postdoctoral fellows who have contributed to yjr thermoelectric efforts in the Kanatzidis group over the past 2 decades. Their names appear in the various important publications highlighting Mercouri’s thermoelectric achievements. 2.4. Charge Density Wave (CDW). One remarkable property of polychalcogenides and especially polytellurides is the appearance of CDW states. CDWs in general arise through electronic instabilities in metallic systems, and CDWs are created because of strong phonon−electron interactions. CDWs and superconductivity have a lot of similarities, and the origin of both properties can be seen as the two different sides of the same coin where similar laws of physics can be applied. Over the last 15 years, Mercouri’s group has synthesized and properly characterized polytelluride compounds121 that nowadays are considered prototypical CDW systems.122 In contrast to superconductivity, CDW can exist well above room temperature, and several complex ternary and quaternary CDW materials synthesized by the Kanatzidis group have CDW transitions at high temperature. Although CDW materials have been known since 1800, the main difficulty and challenge for their characterization is the often-observed incommensurate-modulated CDW ordering with respect to the underlining crystal lattice. Crystallographic characterization

value of 1.6 at 700 K, and the development of synthesis approaches such as spinodal decomposition by Androulaki et al.114and matrix encapsulation by Sootsman et al.115 for the fabrication of high-performance n- and p-type nanostructured PbTe-based thermoelectric materials. However, the drawback of the process is that the nanoprecipitates embedded in the lead chalcogenide matrixes can inhibit the carrier flow, thus decreasing the carrier mobility. In 2011, Biswas et al. developed strained endotaxial nanostructures based on PbTe/SrTe doped with sodium, which can selectively scatter phonons but allow the carrier to flow across the interfaces. Therefore, these engineered nanostructures keep the carrier mobility high.116 This approach provided a ZT value of 1.7 at 875 K in p-type PbTe. Following this work, Mercouri and Biswas et al. demonstrated in 2012 a panoscopic approach to the scattering of heat-carrying phonons across integrated length scales by the development of hierarchical nano/mesoarchitectures in bulk inorganic material.117 This approach goes beyond nanostructuring and produces a ZT value of ∼2.2 at 915 K in ptype PbTe/SrTe (4 mol %) doped with 2 mol % sodium (Figure 6a,b). Achieving a ZT value of over 2 demonstrates a paradigm shift in the field of thermoelectrics. The increase in ZT beyond the threshold of 2 highlights the role of, and need for, multiscale hierarchical architecture in phonon scattering of bulk thermoelectrics and offers a realistic prospect for the recovery of a significant portion of waste heat. Advances in Thermoelectrics: From Single Phases to Hierarchical Nanostructures and Back.118 In 2014 and 2016, Zhao et al. achieved a maximum ZT value of 2.6 at 923 K in layered SnSe crystals (Figure 6c,d), and remarkably high ZT values could even be achieved over a wide range of temperatures of 300−773 K, which is important to realize a high thermoelectric conversion efficiency.119,120 The high thermoelectric performance of SnSe crystals suggests that single-phase materials with strongly anharmonic bonding, layered structure, and low thermal conductivity are promising candidates for the development of high-performance thermoelectrics. It is remarkable that a low thermal conductivity can be 7591


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Inorganic Chemistry of incommensurate-modulated systems is a very challenging task, and the Kanatzidis group’s experience with advanced crystallographic techniques (such as multidimensional superspace crystallography) was vital to their significant contributions to the understanding of CDW materials. For example, the textbook standard CDW system 2H-NbSe2 has been studied in more than 2000 papers in the last 50 years, but its modulated structure was unknown and recently reported by the group.123 More than 15 different CDW systems are reported by the Kanatzidis group that span beyond chalcogenides and include classes of germanides and gallides. Using the know-how of working with electronic instabilities in CDW systems and thermoelectrics, the group started working on superconductors by tailoring the properties of narrow-gap semiconductors to favor superconductivity and create a series of novel superconductors. 2.5. Radiation Detector Materials. Recently, Mercouri and his group have developed several new materials that can detect hard radiation such as X-ray and γ-rays. This technology is required to counteract the threat of nuclear weapons and/or dirty bombs in the form of small devices that could be transported in a suitcase. In Mercouri’s words, “The terrorist attacks of 9/11 heightened interest in this area of security, but the problem remains a real challenge”. Having a strong background in the field of solid-state chemistry, Mercouri and his team focused their efforts on the development of radiation detector materials. To design an effective detector, Mercouri focused on the heavy element part of the periodic table based on the chemical requirements such as band gap, carrier mobility, density, etc. He also developed a design concept of dimensional reduction (Figure 7) to make new semiconductor materials of heavy elements in which most of the compound’s electrons are bound up and not mobile.124 Heavy elements, such as mercury, thallium, selenium, and cesium, have been exploited to develop the materials because they are very dense and absorb γ-rays very well. In most materials, γ-rays emitted by nuclear materials would just pass right through, making them undetectable. On the other hand, when γ-rays enter a heavy-metal-based compound, they excite the electrons, making them mobile and thus detectable. Because each element has a particular spectrum, the signal identifies the detected material very precisely. In Mercouri’s words, “It’s like having a bucket of water and adding one dropthe change is negligible, and we needed a heavy element material without a lot of electrons. This doesn’t exist naturally so we had to design a new material.” The challenges of designing a heavy-metal-based compound without any mobile electrons has led to the development of a series of interesting materials, for example, Cs2Hg6 S7 ,125−129 Cs2 Cd3 Te 4,124 Cs 2 Hg 3 Se 4 , 1 2 4 SbSeI, 1 3 0 Pb 2 P 2 Se 6 , 1 3 1 , 1 3 2 Tl 6 SI 4 , 1 3 3 CsPbBr3,134,135 Tl6SeI4,136 and TlGaSe2,137 as effective radiation detectors particularly for γ-rays. Interestingly, all of these materials are functional at room temperature and scalable as well. 2.6. Perovskite Halide Based Solar Cell. As technically and economically possible alternatives to conventional solidstate devices such as Si, CdTe, and CuIn1−xGaxSe2, dyesensitized solar cells (DSCs) are attractive and promising to many scientists. Despite relatively high conversion efficiencies for solar energy, typical DSCs suffer from durability issues that result from organic liquid electrolytes containing the I−/I3− redox couple, causing serious problems such as electrode corrosion and electrolyte leakage. Although a lot of effort has

Figure 7. Dimensional reduction as a concept illustrated schematically. The dimensionality of a semiconductor, MQ, decreases, and the energy gap increases with the introduction of A2Q in its structure, while the elemental composition retains its heavy-atom character. Implementation of the chemical concept is achieved when appropriate alkali-metal chalcogenides are reacted with binary metal chalcogenides. (Alkali metals are depicted as green spheres, metal elements are shown as red spheres, and the chalcogen elements are yellow spheres.) Typical examples shown include Cs2Hg6S7 as an open-framework three-dimensional tetragonal structure, Cs2Cd3Te4 and Cs2Hg3Se4 as two-dimensional layered compounds, K2Hg3S4 as a compound that exhibits quasi-one-dimensional Hg−S chains (here depicted along the b axis), and K6HgS4 in which the increased S/Hg ratio forces the dimensionality to 0, i.e., the appearance of isolated Hg−S pyramidal structures (molecular salt). The above structures exhibit a blue shift in their band gap with respect to the metal binary basis. The experimentally determined blue shift is commensurate with the lattice dimensionality reduction.

been put into replacing the liquid electrolytes to avoid practical problems with sealing, the efficiency has remained low at around 7%.138 Very recently, Grätzel’s work showed ∼12% efficiency utilizing a cobalt-based redox liquid electrolyte.139 However, this cell still contains a highly volatile liquid electrolyte, which is highly corrosive, volatile, and photoreactive. Clearly, traditional DSCs (Grätzel cell) face big challenges in reaching real applications. Now, these dilemmas have been broken down by a recent publication in Nature.140 On p 486 in volume 485, Kanatzidis et al. first showed that a new type of lead-free perovskite DSC, purposely created using solid-state p-type CsSnI3 with remarkably high hole mobility (μh = 585 cm2 V−1 s−1) to replace the traditional liquid electrolytes containing an I−/I3− redox couple, can reach a conversion efficiency of ∼10.2%. Most importantly, this cell is made of inexpensive elements and can be solution-processable. These factors will make this cell much cheaper and more practical. The lead-free perovskite solar cell is different from the Grätzel cell (Figure 8a). The Grätzel cell can be identified as a photoelectrochemical DSC, where redox couples such as I−/I3− and cobalt-based species are required. The nature of charge transport in Grätzel cells is ionic transport, whose speed is controlled by diffusion. The mechanism of the Grätzel cell is as follows: (1) dye molecules absorb photons, forming excited states; (2) dye molecules inject electrons into semiconducting TiO2 and leave holes on dye molecules; (3) the hole is filled by an electron from an iodide ion (the reaction is 2dye*+ + 3I−→ 7592


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Figure 8. Diagrams of two types of DSCs: (a) Grätzel cell; (b) lead-free perovskite solar cell.

2dye + I3−); (4) electrons are collected from TiO2 at the cathode; (5) the anode injects electrons into the cell, regenerating the iodide. The lead-free perovskite solar cell is a new type of solar cell, which can be defined as a solid-state DSC because this cell is an all-solid-state DSC and has no electrochemistry (Figure 8b). The nature of charge transport in a lead-free perovskite solar cell is hole transport, which is influenced by conductivity and charge-transport mobility. In addition, the hole semiconductor CsSnI3 can also absorb its own light, possibly contributing to electron−hole pairs. In a lead-free perovskite solar cell, the photon absorption mechanism and the electrons’ pathway from excited dye molecules to the cathode via TiO2 are similar to those of the Grätzel cell. The big difference in both cells is the mechanism of the holes’ recovery in dye molecules. In a lead-free perovskite solar cell, the holes in dye molecules will be quickly transported into the anode through highly conductive p-type CsSnI3, while the holes in dye molecules from the Grätzel cell have to be refilled by the electrons from a redox couple such as I−/I3− and a cobalt-based species. Clearly, it is a paradigm shift. More importantly, the CsSnI3−xFx compounds truly enable these types of cells, eliminating the liquid electrolyte, which has frustrated researchers for so long. A lead-free perovskite solar cell is a big fundamental advance in DSCs to push the field forward and speed up real applications. One should note that much higher efficiencies are likely to be achieved with further optimization, the employment of new dyes, and the replacement of solid-state p-type semiconducting materials with much higher hole mobility. This research, along with a paper by Snaith et al.141 and another by Park et al. in Korea,142 which appeared after this paper (lead-free perovskite solar cells), helped to launch the perovskite revolution. More recently, Stoumpos et al. reported a detailed work on the synthesis, structure, and properties of tin and lead perovskites,143 which gave a tremendous push in the field. This paper is considered to be one of the most cited papers in the past decade in Inorganic Chemistry. Obviously, these researches are believed to have opened up the possibility of semiconducting solid materials breaking through the current state and promoting much higher efficiencies than were ever possible with unconventional DSCs.

overview clearly establishes the tremendous breadth and creativity of Mercouri’s research in the exploratory and/or targeted synthesis and development of a wide range of novel materials over the past 30 years. Interestingly, he has used almost the entire periodic table of the elements. Given the enormity and diversity of his contributions, this Viewpoint is necessarily incomplete. However, we do hope that the paper will provide at least a glimpse into Mercouri’s contributions to the field of inorganic materials. It must be mentioned that his love of fundamental science, particularly in chemistry, inspired many of his colleagues and students. Among the 200-plus students mentored by Mercouri, most of them have gone on to successful careers in materials or solid-state chemistry, spanning industrial, academic, and government sectors around the world. The future of solid-state chemistry is being shaped by his former students and co-workers. We former students dedicate this Viewpoint to celebrate his 60th birthday and 3 decades of scientific research as a humble reward for his ample accomplishments and a thank you for his guidance.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] (S.C.P.). Phone: 08022082298. Fax: 080-22082627. ORCID

Indika U. Arachchige: 0000-0001-6025-5011 Kanishka Biswas: 0000-0001-9119-2455 Pierre. F. P. Poudeu: 0000-0002-2422-9550 Li-Dong Zhao: 0000-0003-1247-4345 Sebastian C. Peter: 0000-0002-5211-446X Notes

The authors declare no competing financial interest.

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REFERENCES

(1) Kanatzidis, M. G. Molten alkali-metal polychalcogenides as reagents and solvents for the synthesis of new chalcogenide materials. Chem. Mater. 1990, 2, 353−363. (2) Kanatzidis, M. G.; Huang, S.-P. Coordination chemistry of heavy polychalcogenide ligands. Coord. Chem. Rev. 1994, 130, 509−621. (3) Kanatzidis, M. G.; Pottgen, R.; Jeitschko, W. The metal flux: A preparative tool for the exploration of intermetallic compounds. Angew. Chem., Int. Ed. 2005, 44, 6996−7023. (4) Stoumpos, C. C.; Kanatzidis, M. G. The renaissance of halide perovskites and their evolution as emerging semiconductors. Acc. Chem. Res. 2015, 48, 2791−2802.

3. CONCLUSIONS In this Viewpoint, we attempted to highlight a few of the remarkable contributions of Mercouri Kanatzidis to chemistry with a special focus on inorganic solid-state materials. This 7593


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(25) Armatas, G. S.; Kanatzidis, M. G. Mesoporous germanium-rich chalcogenido frameworks with highly polarizable surfaces and relevance to gas separation. Nat. Mater. 2009, 8, 217−222. (26) Bag, S.; Trikalitis, P. N.; Chupas, P. J.; Armatas, G. S.; Kanatzidis, M. G. Porous semiconducting gels and aerogels from chalcogenide clusters. Science 2007, 317, 490−493. (27) Rolison, D. R. Catalytic nanoarchitectures - The importance of nothing and the unimportance of periodicity. Science 2003, 299, 1698− 1701. (28) Pajonk, G. M. Aerogel catalysts. Appl. Catal. 1991, 72, 217−266. (29) Schneider, M.; Baiker, A. Titania-based aerogels. Catal. Today 1997, 35, 339−365. (30) Husing, N.; Schubert, U.; Misof, K.; Fratzl, P. Formation and structure of porous gel networks from Si(OMe)4 in the presence of A(CH2)nSi(OR)3 (A = functional group). Chem. Mater. 1998, 10, 3024−3032. (31) Ko, E. I. Aerogels as catalysts and catalyst supports. CHEMTECH 1993, 23, 31−36. (32) Sui, R.; Liu, S.; Lajoie, G. A.; Charpentier, P. A. Preparing titania aerogel monolithic chromatography columns using supercritical carbon dioxide. J. Sep. Sci. 2010, 33, 1604−1609. (33) Delaizir, G.; Lucas, P.; Zhang, X. H.; Ma, H. L.; Bureau, B.; Lucas, J. Infrared glass-ceramics with fine porous surfaces for optical sensor applications. J. Am. Ceram. Soc. 2007, 90, 2073−2077. (34) Leventis, N.; Elder, I. A.; Rolison, D. R.; Anderson, M. L.; Merzbacher, C. I. Durable modification of silica aerogel monoliths with fluorescent 2,7-diazapyrenium moieties. Sensing oxygen near the speed of open-air diffusion. Chem. Mater. 1999, 11, 2837−2845. (35) Rolison, D. R.; Dunn, B. Electrically conductive oxide aerogels: new materials in electrochemistry. J. Mater. Chem. 2001, 11, 963−980. (36) Husing, N.; Schubert, U. Aerogels airy materials: Chemistry, structure, and properties. Angew. Chem., Int. Ed. 1998, 37, 22−45. (37) Mills, A.; Le Hunte, S. L. An overview of semiconductor photocatalysis. J. Photochem. Photobiol., A 1997, 108, 1−35. (38) Auborn, J. J.; Barberio, Y. L.; Hanson, K. J.; Schleich, D. M.; Martin, M. J. Amorphous molybdenum sulfide electrodes for nonaqueous electrochemical cells. J. Electrochem. Soc. 1987, 134, 580−586. (39) Lokhande, C. D.; Ennaoui, A.; Patil, P. S.; Giersig, M.; Diesner, K.; Muller, M.; Tributsch, H. Chemical bath deposition of indium sulphide thin films: preparation and characterization. Thin Solid Films 1999, 340, 18−23. (40) Grasso, C.; Nanu, M.; Goossens, A.; Burgelman, M. Electron transport in CuInS2-based nanostructured solar cells. Thin Solid Films 2005, 480-481, 87−91. (41) Mohanan, J. L.; Arachchige, I. U.; Brock, S. L. Porous semiconductor chalcogenide aerogels. Science 2005, 307, 397−400. (42) Husing, N. Sol−gel methods for materials processing: focusing on materials for pollution control, water purification, and soil remediation; Springer: Amsterdam, The Netherlands, 2008; pp 91−104. (43) Arachchige, I. U.; Brock, S. L. Sol-gel methods for the assembly of metal chalcogenide quantum dots. Acc. Chem. Res. 2007, 40, 801− 809. (44) Shriver, D. F.; Atkins, P. W.; Overton, T. L.; Rourke, J. P.; Weller, M. T.; Armstrong, F. A. Shriver & Atkins Inorganic Chemistry; W. H. Freeman and Company: New York, 2006. (45) Bag, S.; Arachchige, I. U.; Kanatzidis, M. G. Aerogels from metal chalcogenides and their emerging unique properties. J. Mater. Chem. 2008, 18, 3628−3632. (46) Dunleavy, M.; Allen, G. C.; Paul, M. Characterization of lanthanum sulfides. Adv. Mater. 1992, 4, 424−427. (47) Stanic, V.; Pierre, A. C.; Etsell, T. H.; Mikula, R. J. Preparation of tungsten sulfides by sol-gel processing. J. Non-Cryst. Solids 1997, 220, 58−62. (48) Stanic, V.; Etsell, T. H.; Pierre, A. C.; Mikula, R. J. Sol-gel processing of ZnS. Mater. Lett. 1997, 31, 35−38. (49) Stanic, V.; Pierre, A. C.; Etsell, T. H.; Mikula, R. J. Influence of reaction parameters on the microstructure of the germanium disulfide gel. J. Am. Ceram. Soc. 2000, 83, 1790−1796.

(5) Tan, G. J.; Zhao, L.-D.; Kanatzidis, M. G. Rationally designing high-performance bulk thermoelectric materials. Chem. Rev. 2016, 116, 12123−12149. (6) Kanatzidis, M. G. Discovery-Synthesis, Design, and Prediction of Chalcogenide Phases. Inorg. Chem. 2017, 56, 3158−3173. (7) Beck, J. S.; Vartuli, J. C.; Roth, W. J.; Leonowicz, M. E.; Kresge, C. T.; Schmitt, K. D.; Chu, C. T. W.; Olson, D. H.; Sheppard, E. W.; McCullen, S. B.; Higgins, J. B.; Schlenker, J. L. A new family of mesoporous molecular-sieves prepared with liquid-crystal templates. J. Am. Chem. Soc. 1992, 114, 10834−10843. (8) Kresge, C. T.; Leonowicz, M. E.; Roth, W. J.; Vartuli, J. C.; Beck, J. S. Ordered mesoporous molecular sieves synthesized by a liquidcrystal template mechanism. Nature 1992, 359, 710−712. (9) Dhingra, S.; Kanatzidis, M. G. Open framework structures based on Sex2‑ fragments: synthesis of (Ph4P)[M(Se6)2] (M = Ga, In, TI) in molten (Ph4P)2Sex. Science 1992, 258, 1769−1772. (10) Rangan, K. K.; Billinge, S. J. L.; Petkov, V.; Heising, J.; Kanatzidis, M. G. Aqueous mediated synthesis of mesostructured manganese germanium sulfide with hexagonal order. Chem. Mater. 1999, 11, 2629−2632. (11) Wachhold, M.; Rangan, K. K.; Billinge, S. J. L.; Petkov, V.; Heising, J.; Kanatzidis, M. G. Mesostructured non-oxidic solids with adjustable worm-hole shaped pores: M-Ge-Q (Q = S, Se) frameworks based on tetrahedral [Ge4Q10]4‑ clusters. Adv. Mater. 2000, 12, 85−91. (12) MacLachlan, M. J.; Coombs, N.; Bedard, R. L.; White, S.; Thompson, L. K.; Ozin, G. A. Mesostructured metal germanium sulfides. J. Am. Chem. Soc. 1999, 121, 12005−12017. (13) Ozin, G. A.; MacLachlan, M. J.; Coombs, N. Non-aqueous supramolecular assembly of mesostructured metal germanium sulphides from (Ge4S10)4‑ clusters. Nature 1999, 397, 681−684. (14) Trikalitis, P. N.; Rangan, K. K.; Bakas, T.; Kanatzidis, M. G. Varied pore organization in mesostructured semiconductors based on the [SnSe4]4‑ anion. Nature 2001, 410, 671−675. (15) Trikalitis, P. N.; Bakas, T.; Papaefthymiou, V.; Kanatzidis, M. G. Supramolecular assembly of hexagonal mesostructured germanium sulfide and selenide nanocomposites incorporating the biologically relevant Fe4S4 cluster. Angew. Chem., Int. Ed. 2000, 39, 4558−4562. (16) Trikalitis, P. N.; Rangan, K. K.; Kanatzidis, M. G. Platinum chalcogenido MCM-41 analogues. High hexagonal order in mesostructured semiconductors based on Pt2+ and [Ge4Q10]4‑ (Q = S, Se) and [Sn4Se10]4‑ adamantane clusters. J. Am. Chem. Soc. 2002, 124, 2604−2613. (17) Trikalitis, P. N.; Rangan, K. K.; Bakas, T.; Kanatzidis, M. G. Single-crystal mesostructured semiconductors with cubic Ia-3d symmetry and ion-exchange properties. J. Am. Chem. Soc. 2002, 124, 12255−12260. (18) Trikalitis, P. N.; Ding, N.; Malliakas, C.; Billinge, S. J. L.; Kanatzidis, M. G. Mesostructured selenides with cubic MCM-48 type symmetry: Large framework elasticity and uncommon resiliency to strong acids. J. Am. Chem. Soc. 2004, 126, 15326−15327. (19) Trikalitis, P. N.; Bakas, T.; Kanatzidis, M. G. Periodic hexagonal mesostructured chalcogenides based on platinum and [SnSe4]4‑ and [SnTe4]4‑ precursors. Solvent dependence of nanopore and wall organization. J. Am. Chem. Soc. 2005, 127, 3910−3920. (20) Armatas, G. S.; Kanatzidis, M. G. Mesostructured germanium with cubic pore symmetry. Nature 2006, 441, 1122−1125. (21) Armatas, G. S.; Kanatzidis, M. G. Hexagonal mesoporous germanium. Science 2006, 313, 817−820. (22) Armatas, G. S.; Kanatzidis, M. G. High-surface-area mesoporous germanium from oxidative polymerization of the deltahedral [Ge9]4‑ cluster: Electronic structure modulation with donor and acceptor molecules. Adv. Mater. 2008, 20, 546−550. (23) Klimov, V. I.; Mikhailovsky, A. A.; Xu, S.; Malko, A.; Hollingsworth, J. A.; Leatherdale, C. A.; Eisler, H.-J.; Bawendi, M. G. Optical gain and stimulated emission in nanocrystal quantum dots. Science 2000, 290, 314−317. (24) Armatas, G. S.; Kanatzidis, M. G. Mesoporous compound semiconductors from the reaction of metal ions with deltahedral [Ge9]4‑ clusters. J. Am. Chem. Soc. 2008, 130, 11430−11436. 7594


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Inorganic Chemistry (50) Miessler, G. L.; Tarr, D. A. Inorganic Chemistry; Pearson Prentice Hall: Upper Saddle River, NJ, 2004. (51) Oh, Y.; Bag, S.; Malliakas, C. D.; Kanatzidis, M. G. Selective surfaces: High-surface-area zinc tin sulfide chalcogels. Chem. Mater. 2011, 23, 2447−2456. (52) Chu, P.; Porcella, D. B. Mercury stack emissions from us electric utility power-plants. Water, Air, Soil Pollut. 1995, 80, 135−144. (53) Meij, R. The fate of mercury in coal-fired power-plants and the influence of wet flue-gas desulfurization. Water, Air, Soil Pollut. 1991, 56, 21−33. (54) Yuhas, B. D.; Smeigh, A. L.; Samuel, A. P. S.; Shim, Y.; Bag, S.; Douvalis, A. P.; Wasielewski, M. R.; Kanatzidis, M. G. Biomimetic multifunctional porous chalcogels as solar fuel catalysts. J. Am. Chem. Soc. 2011, 133, 7252−7255. (55) Shim, Y.; Yuhas, B. D.; Dyar, S. M.; Smeigh, A. L.; Douvalis, A. P.; Wasielewski, M. R.; Kanatzidis, M. G. Tunable biomimetic chalcogels with Fe4S4 cores and [SnnS2n+2]4− (n = 1, 2, 4) building blocks for solar fuel catalysis. J. Am. Chem. Soc. 2013, 135, 2330−2337. (56) Shim, Y.; Young, R. M.; Douvalis, A. P.; Dyar, S. M.; Yuhas, B. D.; Bakas, T.; Wasielewski, M. R.; Kanatzidis, M. G. Enhanced photochemical hydrogen evolution from Fe4S4-based biomimetic chalcogels containing M2+ (M = Pt, Zn, Co, Ni, Sn) centers. J. Am. Chem. Soc. 2014, 136, 13371−13380. (57) Liu, J.; Kelley, M. S.; Wu, W.; Banerjee, A.; Douvalis, A. P.; Wu, J.; Zhang, Y.; Schatz, G. C.; Kanatzidis, M. G. Nitrogenase-mimic ironcontaining chalcogels for photochemical reduction of dinitrogen to ammonia. Proc. Natl. Acad. Sci. U. S. A. 2016, 113, 5530−5535. (58) Bag, S.; Gaudette, A. F.; Bussell, M. E.; Kanatzidis, M. G. Spongy chalcogels of non-platinum metals act as effective hydrodesulfurization catalysts. Nat. Chem. 2009, 1, 217−224. (59) Islam, S. M.; Subrahmanyam, K. S.; Malliakas, C. D.; Kanatzidis, M. G. One-dimensional molybdenum thiochlorides and their use in high surface area MoSx chalcogels. Chem. Mater. 2014, 26, 5151−5160. (60) Subrahmanyam, K. S.; Malliakas, C. D.; Sarma, D.; Armatas, G. S.; Wu, J.; Kanatzidis, M. G. Ion-exchangeable molybdenum sulfide porous chalcogel: Gas adsorption and capture of iodine and mercury. J. Am. Chem. Soc. 2015, 137, 13943−13948. (61) Banerjee, A.; Yuhas, B. D.; Margulies, E. A.; Zhang, Y.; Shim, Y.; Wasielewski, M. R.; Kanatzidis, M. G. Photochemical nitrogen conversion to ammonia in ambient conditions with FeMoS-chalcogels. J. Am. Chem. Soc. 2015, 137, 2030−2034. (62) Riley, B. J.; Pierce, D. A.; Chun, J.; Matyás,̌ J.; Lepry, W. C.; Garn, T. G.; Law, J. D.; Kanatzidis, M. G. Polyacrylonitrile-chalcogel hybrid sorbents for radioiodine capture. Environ. Sci. Technol. 2014, 48, 5832−5839. (63) Manos, M. J.; Kanatzidis, M. G. Metal sulfide ion exchangers: superior sorbents for the capture of toxic and nuclear waste-related metal ions. Chem. Sci. 2016, 7, 4804−4824. (64) Feng, X.; Fryxell, G. E.; Wang, L.-Q.; Kim, A. Y.; Liu, J.; Kemner, K. M. Functionalized monolayers on ordered mesoporous supports. Science 1997, 276, 923−926. (65) Karkamkar, A. J.; Kanatzidis, M. G. Chemical routes to nanocrystalline thermoelectrically relevant AgPbmSbTem+2 materials. J. Am. Chem. Soc. 2006, 128, 6002−6003. (66) Arachchige, I. U.; Wu, J.; Dravid, V. P.; Kanatzidis, M. G. Nanocrystals of the quaternary thermoelectric materials: AgPbmSbTem+2 (m = 1−18): Phase-segregated or solid solutions? Adv. Mater. 2008, 20, 3638−3642. (67) Arachchige, I. U.; Kanatzidis, M. G. Anomalous band gap evolution from band inversion in Pb1‑xSnxTe nanocrystals. Nano Lett. 2009, 9, 1583−1587. (68) Soriano, R. B.; Arachchige, I. U.; Malliakas, C. D.; Wu, J.; Kanatzidis, M. G. Nanoscale stabilization of new phases in the PbTe− Sb2Te3 system: PbmSb2nTem+3n nanocrystals. J. Am. Chem. Soc. 2013, 135, 768−774. (69) Dimmock, J. O.; Melngailis, I.; Strauss, A. J. Band structure and laser action in PbxSn1−xTe. Phys. Rev. Lett. 1966, 16, 1193−1196.

(70) Abramof, E.; Ferreira, S. O.; Rappl, P. H. O.; Closs, H.; Bandeira, I. N. Electrical properties of Pb1‑xSnxTe layers with 0 ≤ x ≤ 1 grown by molecular beam epitaxy. J. Appl. Phys. 1997, 82, 2405−2410. (71) Sines, I. T.; Misra, R.; Schiffer, P.; Schaak, R. E. Colloidal synthesis of non-equilibrium wurtzite-type MnSe. Angew. Chem., Int. Ed. 2010, 49, 4638−4640. (72) Norako, M. E.; Greaney, M. J.; Brutchey, R. L. Synthesis of characterization of wurtzite-phase copper tin selenide nanocrystals. J. Am. Chem. Soc. 2012, 134, 23−26. (73) Esteves, R. J. A.; Ho, M. Q.; Arachchige, I. U. Nanocrystalline group IV alloy semiconductors: Synthesis and characterization of Ge1−xSnx quantum dots for tunable bandgaps. Chem. Mater. 2015, 27, 1559−1568. (74) Esteves, R. J. A.; Hafiz, S.; Demchenko, D. O.; Ö zgür, Ü .; Arachchige, I. U. Ultra-small Ge1‑xSnx quantum dots with visible photoluminescence. Chem. Commun. 2016, 52, 11665−11668. (75) Hafiz, S. A.; Esteves, R. J.; Demchenko, D. O.; Arachchige, I. U.; Ö zgür, Ü . Energy gap tuning and carrier dynamics in colloidal Ge1−xSnx quantum dots. J. Phys. Chem. Lett. 2016, 7, 3295−3301. (76) Zhu, P.; Imai, Y.; Isoda, Y.; Shinohara, Y.; Jia, X.; Zou, G. Enhanced thermoelectric properties of PbTe alloyed with Sb2Te3. J. Phys.: Condens. Matter 2005, 17, 7319−7327. (77) Elwell, D.; Scheel, H. J. Crystal growth from high-temperature solutions; Academic Press: New York, 1975. (78) Canfield, P. C.; Fisk, Z. Growth of single crystals from metallic fluxes. Philos. Mag. B 1992, 65, 1117−1123. (79) Kanatzidis, M. G.; Pottgen, R.; Jeitschko, W. The metal flux: a preparative tool for the exploration of intermetallic compounds. Angew. Chem., Int. Ed. 2005, 44, 6996−7023. (80) Chen, X. Z.; Sportouch, S.; Sieve, B.; Brazis, P.; Kannewurf, C. R.; Cowen, J. A.; Patschke, R.; Kanatzidis, M. G. Exploratory Synthesis with Molten Aluminum as a Solvent and Routes to Multinary Aluminum Silicides. Sm2Ni(NixSi1‑x)Al4Si6 (x = 0.18−0.27): A New Silicide with a Ferromagnetic Transition at 17.5 K. Chem. Mater. 1998, 10, 3202−3211. (81) Sieve, B.; Gray, D. L.; Henning, R.; Bakas, T.; Schultz, A. J.; Kanatzidis, M. G. Al flux synthesis of the oxidation-resistant quaternary phase REFe4Al9Si6 (RE = Tb, Er). Chem. Mater. 2008, 20, 6107−6115. (82) Sieve, B.; Chen, X. Z.; Henning, R.; Brazis, P.; Kannewurf, C. R.; Cowen, J. A.; Schultz, A. J.; Kanatzidis, M. G. Cubic aluminum silicides RE8Ru12Al49Si9(AlxSi12‑x) (RE = Pr, Sm) from liquid aluminum. empty (Si,Al)12 cuboctahedral clusters and assignment of the Al/Si distribution with neutron diffraction. J. Am. Chem. Soc. 2001, 123, 7040−7047. (83) Latturner, S. E.; Bilc, D.; Mahanti, S. D.; Kanatzidis, M. G. Quaternary intermetallics grown from molten aluminum: The homologous series Th2(AuxSi1‑x)[AuAl2]nSi2 (n = 1, 2, 4). Chem. Mater. 2002, 14, 1695−1705. (84) Zhuravleva, M. A.; Evain, M.; Petricek, V.; Kanatzidis, M. G. GdCo1‑xGa3Ge: Charge density wave in a Ga square net. J. Am. Chem. Soc. 2007, 129, 3082−3083. (85) Calta, N. P.; Im, J.; Rodriguez, A. P.; Fang, L.; Bugaris, D. E.; Chasapis, T. C.; Freeman, A. J.; Kanatzidis, M. G. Hybridization Gap and Dresselhaus Spin Splitting in EuIr4In2Ge4. Angew. Chem., Int. Ed. 2015, 54, 9186−9191. (86) Peter, S. C.; Chondroudi, M.; Malliakas, C. D.; Balasubramanian, M.; Kanatzidis, M. G. Anomalous thermal expansion in the square-net compounds RE4TGe8 (RE = Yb, Gd; T = Cr−Ni, Ag). J. Am. Chem. Soc. 2011, 133, 13840−13843. (87) Chondroudi, M.; Balasubramanian, M.; Welp, U.; Kwok, W. K.; Kanatzidis, M. G. Mixed valency in Yb7Co4InGe12: A novel intermetallic compound stabilized in liquid indium. Chem. Mater. 2007, 19, 4769−4775. (88) Peter, S. C.; Subbarao, U.; Sarkar, S.; Vaitheeswaran, G.; Svane, A.; Kanatzidis, M. G. Crystal structure of Yb2CuGe6 and Yb3Cu4Ge4 and the valency of ytterbium. J. Alloys Compd. 2014, 589, 405−411. (89) Chondroudi, M.; Peter, S. C.; Malliakas, C. D.; Balasubramanian, M.; Li, Q.; Kanatzidis, M. G. Yb3AuGe2In3: An 7595


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conducting properties of Ba1−xNaxFe2As2. Phys. Rev. B: Condens. Matter Mater. Phys. 2013, 88, 094510. (107) Bud’ko, S.; Sturza, M.; Chung, D. Y.; Kanatzidis, M. G.; Canfield, P. C. Heat Capacity Jump at Tc and Pressure Derivatives of Superconducting Transition Temperature in the Ba1‑xKxFe2As2 (0.2 ≤ x ≤ 1.0) Series. Phys. Rev. B 2013, 87, 100509-1−100509-5. (108) Shoemaker, D. P.; Chung, D. Y.; Claus, H.; Francisco, M. C.; Avci, S.; Llobet, A.; Kanatzidis, M. G. Phase relations in KxFe2‑ySe2 and the structure of superconducting KxFe2Se2 via high-resolution synchrotron diffraction. Phys. Rev. B: Condens. Matter Mater. Phys. 2012, 86, 184511. (109) Todorov, I.; Chung, D. Y.; Claus, H.; Malliakas, C. D.; Douvalis, A. P.; Bakas, T.; He, J. Q.; Dravid, V. P.; Kanatzidis, M. G. Topotactic Redox Chemistry of NaFeAs in Water and Air and Superconducting Behavior with Stoichiometry Change Chemistry of Materials. Chem. Mater. 2010, 22, 3916−3925. (110) Chung, D. Y.; Hogan, T.; Brazis, P.; Rocci-Lane, M.; Kannewurf, C.; Bastea, M.; Uher, C.; Kanatzidis, M. G. CsBi4Te6: A high-performance thermoelectric material for low-temperature applications. Science 2000, 287, 1024−1027. (111) Hsu, K. F.; Loo, S.; Guo, F.; Chen, W.; Dyck, J. S.; Uher, C.; Hogan, T.; Polychroniadis, E. K.; Kanatzidis, M. G. Cubic AgPbmSbTe2+m: Bulk thermoelectric materials with high figure of merit. Science 2004, 303, 818−821. (112) Quarez, E.; Hsu, K. F.; Pcionek, R.; Frangis, N.; Polychroniadis, E. K.; Kanatzidis, M. G. Nanostructuring, compositional fluctuations, and atomic ordering in the thermoelectric materials AgPbmSbTe2+m. The myth of solid solutions. J. Am. Chem. Soc. 2005, 127, 9177−9190. (113) Poudeu, P. F. R.; D’Angelo, J.; Downey, A. D.; Short, J. L.; Hogan, T. P.; Kanatzidis, M. G. High thermoelectric figure of merit and nanostructuring in bulk p-type Na1‑xPbmSbyTem+2. Angew. Chem., Int. Ed. 2006, 45, 3835−3839. (114) Androulakis, J.; Lin, C.-H.; Kong, H.-J.; Uher, C.; Wu, C.-I.; Hogan, T.; Cook, B. A.; Caillat, T.; Paraskevopoulos, K. M.; Kanatzidis, M. G. Spinodal decomposition and nucleation and growth as a means to bulk nanostructured thermoelectrics: Enhanced performance in Pb1‑xSnxTe-PbS. J. Am. Chem. Soc. 2007, 129, 9780− 9788. (115) Sootsman, J. R.; Chung, D. Y.; Kanatzidis, M. G. New and old concepts in thermoelectric materials. Angew. Chem., Int. Ed. 2009, 48, 8616−8639. (116) Biswas, K.; He, J. Q.; Zhang, Q. C.; Wang, G. Y.; Uher, C.; Dravid, V. P.; Kanatzidis, M. G. Strained endotaxial nanostructures with high thermoelectric figure of merit. Nat. Chem. 2011, 3, 160−166. (117) Biswas, K.; He, J. Q.; Blum, I. D.; Wu, C. I.; Hogan, T. P.; Seidman, D. N.; Dravid, V. P.; Kanatzidis, M. G. High-performance bulk thermoelectrics with all-scale hierarchical architectures. Nature 2012, 489, 414−418. (118) Kanatzidis, M. G. Advances in thermoelectrics: From single phases to hierarchical nanostructures and back. MRS Bull. 2015, 40, 687−694. (119) Zhao, L.-D.; Lo, S.-H.; Zhang, Y.; Sun, H.; Tan, G.; Uher, C.; Wolverton, C.; Dravid, V. P.; Kanatzidis, M. G. Ultralow thermal conductivity and high thermoelectric figure of merit in SnSe crystals. Nature 2014, 508, 373−377. (120) Zhao, L. D.; Tan, G. J.; Hao, S. Q.; He, J. Q.; Pei, Y. L.; Chi, H.; Wang, H.; Gong, S. K.; Xu, H. B.; Dravid, V. P.; Uher, C.; Snyder, G. J.; Wolverton, C.; Kanatzidis, M. G. Ultrahigh power factor and thermoelectric performance in hole-doped single-crystal SnSe. Science 2016, 351, 141−144. (121) Patschke, R.; Kanatzidis, M. G. Polytelluride compounds containing distorted nets of tellurium. Phys. Chem. Chem. Phys. 2002, 4, 3266−3281. (122) Malliakas, C.; Billinge, S. J. L.; Kim, H. J.; Kanatzidis, M. G. Square nets of tellurium: Rare-earth dependent variation in the chargedensity wave of RETe3 (RE = Rare-earth element). J. Am. Chem. Soc. 2005, 127, 6510−6511.

Ordered Variant of the YbAuIn Structure Exhibiting Mixed-Valent Yb Behavior. Inorg. Chem. 2011, 50, 1184−1193. (90) Peter, S. C.; Subbarao, U.; Rayaprol, S.; Martin, J. B.; Balasubramanian, M.; Malliakas, C. D.; Kanatzidis, M. G. Flux Growth of Yb6.6Ir6Sn16 Having Mixed-Valent Ytterbium. Inorg. Chem. 2014, 53, 6615−6623. (91) Peter, S. C.; Kanatzidis, M. G. The new binary intermetallic YbGe2.83. J. Solid State Chem. 2010, 183, 2077−2081. (92) Peter, S. C.; Malliakas, C. D.; Kanatzidis, M. G. Structure and Unusual Magnetic Properties of YbMn0.17Si1.88. Inorg. Chem. 2013, 52, 4909−4915. (93) Calta, N. P.; Francisco, M. C.; Malliakas, C. D.; Schlueter, J. A.; Kanatzidis, M. G. Four high-temperature ferromagnets in the Hf−Fe− Sn system. Chem. Mater. 2014, 26, 6827−6837. (94) Han, F.; Malliakas, C. D.; Stoumpos, C. C.; Sturza, M.; Claus, H.; Chung, D. Y.; Kanatzidis, M. G. Superconductivity and strong intrinsic defects in LaPd1−xBi2. Phys. Rev. B: Condens. Matter Mater. Phys. 2013, 88, 144511. (95) Salvador, J. R.; Guo, F.; Hogan, T.; Kanatzidis, M. G. Zero thermal expansion in YbGaGe due to an electronic valence transition. Nature 2003, 425, 702−705. (96) Han, F.; Wan, X.; Phelan, D.; Stoumpos, C. C.; Sturza, M.; Malliakas, C. D.; Li, Q.; Han, T.-H.; Zhao, Q.; Chung, D. Y.; Kanatzidis, M. G. Antiferromagnetic Kondo lattice in the layered compound CePd1−xBi2 and comparison to the superconductor LaPd1−xBi2. Phys. Rev. B: Condens. Matter Mater. Phys. 2015, 92, 045112. (97) Sturza, M.; Han, F.; Malliakas, C. D.; Chung, D. Y.; Claus, H.; Kanatzidis, M. G. Superconductivity in the intermetallic pnictide compound Ca11Bi10−x. Phys. Rev. B: Condens. Matter Mater. Phys. 2014, 89, 054512. (98) Calta, N. P.; Im, J.; Rodriguez, A. P.; Fang, L.; Bugaris, D. E.; Chasapis, T. C.; Freeman, A. J.; Kanatzidis, M. G. Hybridization Gap and Dresselhaus Spin Splitting in EuIr4In2Ge4. Angew. Chem. 2015, 127, 9318−9323. (99) Peter, S. C.; Malliakas, C. D.; Nakotte, H.; Kothapilli, K.; Rayaprol, S.; Schultz, A. J.; Kanatzidis, M. G. The polygallides: Yb3Ga7Ge3 and YbGa4Ge2. J. Solid State Chem. 2012, 187, 200−207. (100) Sebastian, C. P.; Salvador, J.; Martin, J. B.; Kanatzidis, M. G. New Intermetallics YbAu2In4 and Yb2Au3In5. Inorg. Chem. 2010, 49, 10468−10474. (101) Kanatzidis, M. G.; Marks, T. J.; Marcy, H. O.; McCarthy, W. J.; Kannewurf, C. R. Structure and Electronic Anisotropy in Polycrystalline Compactions of the High Tc Superconductor EuBa2Cu3O7‑x. Solid State Commun. 1988, 65, 1333−1337. (102) Allred, J. M.; Taddei, K. M.; Bugaris, D. E.; Krogstad, M. J.; Lapidus, S. H.; Chung, D. Y.; Claus, H.; Kanatzidis, M. G.; Brown, D. E.; Kang, J.; Fernandes, R. M.; Eremin, I.; Rosenkranz, S.; Chmaissem, O.; Osborn, R. Double-Q Spin-Density Wave in Iron Arsenide Superconductors. Nat. Phys. 2016, 12, 493. (103) Arham, H. Z.; Bugaris, D. E.; Chung, D. Y.; Kanatzidis, M. G.; H, G. L. Point contact spectroscopy in the superconducting and normal state of NaFe1−xCoxAs. arXiv 2014, 1406, 0038. (104) Avci, S.; Chmaissem, O.; Allred, J. M.; Rosenkranz, S.; Eremin, I.; Chubukov, A. V.; Bugaris, D. E.; Chung, D. Y.; Kanatzidis, M. G.; Castellan, J.-P.; Schlueter, J. A.; Claus, H.; Khalyavin, D. D.; Manuel, P.; Daoud-Aladine, A.; Osborn, R. Magnetically driven suppression of nematic order in an iron-based superconductor. Nat. Commun. 2014, 5, 3845. (105) Bud’ko, S. L.; Chung, D. Y.; Bugaris, D.; Claus, H.; Kanatzidis, M. G.; Canfield, P. C. Heat capacity jump at Tc and pressure derivatives of superconducting transition temperature in the Ba1−xNaxFe2As2 (0.1 × 0.9) series. Phys. Rev. B: Condens. Matter Mater. Phys. 2014, 89, 014510. (106) Avci, S.; Allred, J. M.; Chmaissem, O.; Chung, D. Y.; Rosenkranz, S.; Schlueter, J. A.; Claus, H.; Daoud-Aladine, A.; Khalyavin, D. D.; Manuel, P.; Llobet, A.; Suchomel, M. R.; Kanatzidis, M. G.; Osborn, R. Structural, Magnetic, and Super7596


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Inorganic Chemistry (123) Malliakas, C. D.; Kanatzidis, M. G. Nb−Nb interactions define the charge density wave structure of 2H-NbSe2. J. Am. Chem. Soc. 2013, 135, 1719−1722. (124) Androulakis, J.; Peter, S. C.; Li, H.; Malliakas, C. D.; Peters, J. A.; Liu, Z.; Wessels, B. W.; Song, J.-H.; Jin, H.; Freeman, A. J.; Kanatzidis, M. G. Dimensional reduction: A design tool for new radiation detection materials. Adv. Mater. 2011, 23, 4163−4167. (125) Liu, Z.; Peters, J. A.; Li, H.; Kanatzidis, M. G.; Im, J.; Jin, H.; Freeman, A. J.; Wessels, B. W. Photo-Induced Current Transient Spectroscopy of Semi-insulating Single Crystal Cs2Hg6S7. J. Electron. Mater. 2015, 44, 222−226. (126) Li, H.; Malliakas, C. D.; Liu, Z.; Peters, J. A.; Sebastian, M.; Zhao, L.; Chung, D. Y.; Wessels, B. W.; Kanatzidis, M. G. Investigation of Semi-Insulating Cs2Hg6S7 and Cs2Hg6‑xCdxS7 Alloy for Hard Radiation Detection. Cryst. Growth Des. 2014, 14, 5949−5956. (127) Peters, J. A.; Cho, N. K.; Liu, Z. F.; Wessels, B. W.; Li, H.; Androulakis, J.; Kanatzidis, M. G. Investigation of defect levels in Cs2Hg6S7 single crystals by photoconductivity and photoluminescence spectroscopie. J. Appl. Phys. 2012, 112, 063702. (128) Im, J.; Jin, H.; Li, H.; Peters, J. A.; Liu, Z.; Wessels, B. W.; Kanatzidis, M. G.; Freeman, A. J. Formation of Native Defects in the X-ray Detector Material Cs2Hg6S7. Appl. Phys. Lett. 2012, 101, 2021031−202103-4. (129) Li, H.; Peters, J. A.; Liu, Z.; Sebastian, M.; Malliakas, C. D.; Androulakis, J.; Zhao, L.; Chung, I.; Nguyen, S. L.; Johnsen, S.; Wessels, B. W.; Kanatzidis, M. G. Crystal Growth and Characterization of the X-ray and γ-ray Detector Material Cs2Hg6S7. Cryst. Growth Des. 2012, 12, 3250−3256. (130) Wibowo, A. C.; Malliakas, C. D.; Liu, Z.; Peters, J. A.; Sebastian, M.; Chung, D. Y.; Wessels, B. W.; Kanatzidis, M. G. Photoconductivity in the Chalcohalide Semiconductor, SbSeI: a New Candidate for Hard Radiation Detection. Inorg. Chem. 2013, 52, 7045−7050. (131) Wang, P. L.; Kostina, S. S.; Meng, F.; Kontsevoi, O. Y.; Liu, Z.; Chen, P.; Peters, J. A.; Hanson, M.; He, Y.; Chung, D. Y.; Freeman, A. J.; Wessels, B. W.; Kanatzidis, M. G. Refined Synthesis and Crystal Growth of Pb2P2Se6 for Hard Radiation Detectors. Cryst. Growth Des. 2016, 16, 5100−5109. (132) Wang, P. L.; Liu, Z. F.; Chen, P.; Peters, J. A.; Tan, G. J.; Im, J.; Lin, W. W.; Freeman, A. J.; Wessels, B. W.; Kanatzidis, M. G. Hard Radiation Detection from the Selenophosphate Pb2P2Se6. Adv. Funct. Mater. 2015, 25, 4874−4881. (133) Nguyen, S. L.; Malliakas, C. D.; Peters, J. A.; Liu, Z.; Im, J.; Zhao, L.-D.; Sebastian, M.; Jin, H.; Li, H.; Johnsen, S.; Wessels, B. W.; Freeman, A. J.; Kanatzidis, M. G. Photoconductivity in Tl6SI4: A Novel Semiconductor for Hard Radiation Detection. Chem. Mater. 2013, 25, 2868−2877. (134) Clark, D. J.; Stoumpos, C. C.; Saouma, F. O.; Kanatzidis, M. G.; Jang, J. I. Polarization three-photon absorption and subsequent photoluminescence in CsPbBr3 single crystal at room temperature. Phys. Rev. B: Condens. Matter Mater. Phys. 2016, 93, 195202. (135) Stoumpos, C. C.; Malliakas, C. D.; Peters, J. A.; Liu, Z.; Sebastian, M.; Im, J.; Chasapis, T. C.; Wibowo, A. C.; Chung, D. Y.; Freeman, A. J.; Wessels, B. W.; Kanatzidis, M. G. Crystal Growth of the Perovskite Semiconductor CsPbBr3: A New Material for HighEnergy Radiation Detection. Cryst. Growth Des. 2013, 13, 2722−2727. (136) Kostina, S. S.; Peters, J. A.; Lin, W.; Chen, P.; Liu, Z.; Wang, P. L.; Kanatzidis, M. G.; Wessels, B. W. Photoluminescense Fatigue and Inhomogeneous Line Broadening in Semi-insulating Tl6SeI4 single crystals. Semicond. Sci. Technol. 2016, 31, 065009. (137) Johnsen, S.; Liu, Z.; Peters, J. A.; Song, J.-H.; Peter, S. C.; Malliakas, C. D.; Cho, N. K.; Jin, H.; Freeman, A. J.; Wessels, B. W.; Kanatzidis, M. G. Thallium Chalcogenide-Based Wide-Band-Gap Semiconductors: TlGaSe2 for Radiation Detectors. Chem. Mater. 2011, 23, 3120−3128. (138) Daeneke, T.; Kwon, T.-H.; Holmes, A. B.; Duffy, N. W.; Bach, U.; Spiccia, L. High-efficiency dye-sensitized solar cells with ferrocenebased electrolytes. Nat. Chem. 2011, 3, 213−215.

(139) Yella, A.; Lee, H.-W.; Tsao, H. N.; Yi, C.; Chandiran, A. K.; Nazeeruddin, M. K.; Diau, E. W.-G.; Yeh, C.-Y.; Zakeeruddin, S. M.; Grätzel, M. Porphyrin-Sensitized Solar Cells with Cobalt (II/III)− Based Redox Electrolyte Exceed 12% Efficiency. Science 2011, 334, 629−634. (140) Chung, I.; Lee, B.; He, J.; Chang, R. P. H.; Kanatzidis, M. G. All-solid-state dye-sensitized solar cells with high efficiency. Nature 2012, 485, 486−489. (141) Lee, M. M.; Teuscher, J.; Miyasaka, T.; Murakami, T. N.; Snaith, H. J. Efficient Hybrid Solar Cells Based on Meso-Superstructured Organometal Halide Perovskites. Science 2012, 338, 643− 647. (142) Kim, H.-S.; Lee, C.-R.; Im, J.-H.; Lee, K.-B.; Moehl, T.; Marchioro, A.; Moon, S.-J.; Humphry-Baker, R.; Yum, J.-H.; Moser, J. E.; Grätzel, M.; Park, N.-G. Lead Iodide Perovskite Sensitized AllSolid-State Submicron Thin Film Mesoscopic Solar Cell with Efficiency Exceeding 9%. Sci. Rep. 2012, 2, 591. (143) Stoumpos, C. C.; Malliakas, C. D.; Kanatzidis, M. G. Semiconducting Tin and Lead Iodide Perovskites with Organic Cations: Phase Transitions, High Mobilities, and Near-Infrared Photoluminescent Properties. Inorg. Chem. 2013, 52, 9019−9038.

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