Bio

Assistant Professor of Chemistry Hemamala Karunadasa works with colleagues in materials science, geology, and applied physics to drive the discovery of new materials with applications in clean energy. Using the tools of synthetic chemistry, her group designs hybrid materials that couple the structural tunability of organic molecules with the diverse electronic and optical properties of extended inorganic solids. This research targets materials such as sorbents for capturing environmental pollutants, electrodes for rechargeable batteries, phosphors for solid-state lighting, and absorbers for solar cells. They also design discrete molecular centers as catalysts for activating small molecules relevant to clean energy cycles.

Hemamala Karunadasa studied chemistry and materials science at Princeton University (A.B. with high honors 2003; Certificate in Materials Science and Engineering 2003), where her undergraduate thesis project with Professor Robert J. Cava examined geometric magnetic frustration in metal oxides. She moved from solid-state chemistry to solution-state chemistry for her doctoral studies in inorganic chemistry at the University of California, Berkeley (Ph.D. 2009) with Professor Jeffrey R. Long. Her thesis focused on heavy atom building units for magnetic molecules and molecular catalysts for generating hydrogen from water. She continued to study molecular electrocatalysts for water splitting during postdoctoral research with Berkeley Professors Christopher J. Chang and Jeffrey R. Long at the Lawrence Berkeley National Lab. She further explored molecular catalysts for hydrocarbon oxidation as a postdoc at the California Institute of Technology with Professor Harry B. Gray. She joined the Stanford Chemistry Department faculty in September 2012. Her research explores solution-state routes to new solid-state materials. She was recently awarded the NSF CAREER award and Alfred P. Sloan Foundation Fellowship, among other honors.

Professor Karunadasa’s lab at Stanford takes a molecular approach to extended solids. Lab members synthesize organic, inorganic and hybrid materials using solution- and solid-state techniques, including glovebox and Schlenk-line methods, and determine the structures of these materials using powder- and single-crystal x-ray diffraction. Lab tools also include a host of spectroscopic and electrochemical probes, imaging methods, and film deposition techniques. Group members further characterize their materials under extreme environments and in operating devices to tune new materials for diverse applications in renewable energy.

Please visit the lab website for more details and recent news.

Academic Appointments

Honors & Awards

  • Terman Faculty Fellowship, Stanford University (2015-2018)
  • Sloan Fellowship, Alfred P. Sloan Foundation (2015)
  • CAREER Award, National Science Foundation (2014)
  • ICCC41 Rising Star Award, 41st International Conference on Coordination Chemistry (2014)
  • Thieme Chemistry Journal Award, Thieme Chemistry Journal (2013)
  • Gabilan Junior Faculty Fellow, Stanford University (2012-2015)
  • BP Postdoctoral Fellowship, California Institute of Technology (2011-2012)
  • Graduate Fellowship, Tyco Electronics (2006-2007)
  • Outstanding Graduate Student Instructor Award, University of California, Berkeley (2006-2007)
  • Outstanding Undergraduate Thesis in Inorganic Chemistry, Princeton University (2003)

Boards, Advisory Committees, Professional Organizations

  • Editorial Advisory Board Member, Inorganic Chemistry (2016 - Present)

Professional Education

  • Postdoc, California Institute of Technology, Molecular catalysts for activating hydrocarbons (2011)
  • Postdoc, University of California, Berkeley and Lawrence Berkeley National Lab, Molecular catalysts for generating hydrogen from water (2010)
  • PhD, University of California, Berkeley, Inorganic Chemistry (2009)
  • AB, Princeton University, Chemistry (2003)
  • Certificate, Princeton University, Materials Science and Engineering (2003)

Patents

  • J.R. Long, C.J. Chang, H.I. Karunadasa, M. Majda. "United States Patent US2012217169-A1 Molecular metal-disulfide catalysts for generating hydrogen from water", Univ. California
  • J.R. Long, C.J. Chang, H.I. Karunadasa. "United States Patent US2012228152-A1 Molecular metal-oxo catalysts for generating hydrogen from water", Univ. California
  • H. I. Karunadasa, A. H. Slavney. "United States Patent 62273651 Bismuth-halide perovskite solar-cell absorbers having long carrier lifetimes", Leland Stanford Junior University, Jan 19, 2016
  • H. I. Karunadasa, D. Solis-Ibarra. "United States Patent PCT/US2014/054363 Reversible and irreversible chemisorption in nonporous, crystalline hybrid structures", Leland Stanford Junior University, Sep 5, 2014
  • H. I. Karunadasa, I. C. Smith, and M. D. McGehee. "United States Patent 20150357591 Solar cells comprising 2D perovskites", Leland Stanford Junior University, Jun 6, 2014
  • H. I. Karunadasa, E. R. Dohner. "United States Patent US2014/061946 Composition comprising a layered perovskite phosphor and method of formation", Leland Stanford Junior University, Oct 23, 2013

Contact

Additional Info

  • Mail Code: 5080
  • Other Names:
    Hema Karunadasa

Links

2018-19 Courses

All Publications

  • Carving Out Pores in Redox-Active One-Dimensional Coordination Polymers. Angewandte Chemie (International ed. in English) Clayman, N. E., Manumpil, M. A., Umeyama, D., Rudenko, A. E., Karunadasa, H. I., Waymouth, R. M. 2018

    Abstract

    Reduction of the insulating one-dimensional coordination polymer [Cu(abpy)PF6 ]n , 1a(PF6 ), (abpy=2,2'-azobispyridine) yields the conductive, porous polymer [Cu(abpy)]n , 2a. Pressed pellets of neutral 2a exhibit a conductivity of 0.093 Scm-1 at room temperature and a Brunauer-Emmett-Teller (BET) surface area of 56 m2 g-1 . Fine powders of 2a have a BET surface area of 90 m2 g-1 . Cyclic voltammetry shows that the reduction of 1a(PF6 ) to 2a is quasi-reversible, indicative of facile charge transfer through the bulk material. The BET surface area of the reduced polymer 2 can be controlled by changing the size of the counteranion X in the cationic [Cu(abpy)X]n . Reduction of [Cu(abpy)X]n with X=Br (2b) or BArF (2c; BArF =tetrakis(3,5-bis(trifluoromethyl)phenyl)), affords [Cu(abpy)]n polymers with surface areas of 60 and 200 m2 g-1 , respectively.

    View details for DOI 10.1002/anie.201807506

    View details for PubMedID 30230677

  • Small-Bandgap Halide Double Perovskites. Angewandte Chemie (International ed. in English) Slavney, A. H., Leppert, L., Saldivar Valdes, A., Bartesaghi, D., Savenije, T. J., Neaton, J. B., Karunadasa, H. 2018

    Abstract

    Despite their compositional versatility, most halide double perovskites feature large bandgaps. Herein, we describe a strategy for achieving small bandgaps in this family of materials. The new double perovskites Cs2AgTlX6 (X = Cl (1) and Br (2)) have direct bandgaps of 2.0 and 0.95 eV, respectively, which are ca. 1 eV lower than those of analogous perovskites. To our knowledge, 2 displays the lowest bandgap for any known halide perovskite. Unlike in AIBIIX3 perovskites, the bandgap transition in AI2BB'X6 double perovskites can show substantial metal-to-metal charge-transfer character. We demonstrate how this band-edge orbital composition can be used to achieve small bandgaps through the selection of energetically aligned B- and B'-site metal frontier orbitals. Calculations reveal a shallow, symmetry-forbidden region at the band edges for 1, which results in long (us) microwave conductivity lifetimes. We further describe a facile self-doping reaction in 2 through Br2 loss at ambient conditions.

    View details for DOI 10.1002/anie.201807421

    View details for PubMedID 30088309

  • Dynamically Disordered Lattice in a Layered Pb-I-SCN Perovskite Thin Film Probed by Two-Dimensional Infrared Spectroscopy. Journal of the American Chemical Society Nishida, J., Breen, J. P., Lindquist, K. P., Umeyama, D., Karunadasa, H. I., Fayer, M. D. 2018

    Abstract

    The dynamically flexible lattices in lead halide perovskites may play important roles in extending carrier recombination lifetime in 3D perovskite solar-cell absorbers and in exciton self-trapping in 2D perovskite white-light phosphors. Two-dimensional infrared (2D IR) spectroscopy was applied to study a recently reported Pb-I-SCN layered perovskite. The Pb-I-SCN perovskite was spin-coated on a SiO2 surface as a thin film, with a thickness of 100 nm, where the S12CN- anions were isotopically diluted with the ratio of S12CN:S13CN = 5:95 to avoid vibrational coupling and excitation transfer between adjacent SCN- anions. The 12CN stretch mode of the minor S12CN- component was the principal vibrational probe that reported on the structural evolution through 2D IR spectroscopy. Spectral diffusion was observed with a time constant of 4.1 ± 0.3 ps. Spectral diffusion arises from small structural changes that result in sampling of frequencies within the distribution of frequencies comprising the inhomogeneously broadened infrared absorption band. These transitions among discrete local structures are distinct from oscillatory phonon motions of the lattice. To accurately evaluate the structural dynamics through measurement of spectral diffusion, the vibrational coupling between adjacent SCN- anions had to be carefully treated. Although the inorganic layers of typical 2D perovskites are structurally isolated from each other, the 2D IR data demonstrated that the layers of the Pb-I-SCN perovskite are vibrationally coupled. When both S12CN- and S13CN- were pumped simultaneously, cross-peaks between S12CN and S13CN vibrations and an oscillating 2D band shape of the S12CN- vibration were observed. Both observables demonstrate vibrational coupling between the closest SCN- anions, which reside in different inorganic layers. The thin films and the isotopic dilution produced exceedingly small vibrational echo signal fields; measurements were made possible using the near-Brewster's angle reflection pump-probe geometry.

    View details for DOI 10.1021/jacs.8b03787

    View details for PubMedID 30024160

  • Layered Halide Double Perovskites: Dimensional Reduction of Cs2AgBiBr6 JOURNAL OF THE AMERICAN CHEMICAL SOCIETY Connor, B. A., Leppert, L., Smith, M. D., Neaton, J. B., Karunadasa, H. I. 2018; 140 (15): 5235–40

    Abstract

    We investigate the consequences of dimensional confinement on halide double perovskites by synthesizing two-dimensional analogues of the recently reported three-dimensional double perovskite Cs2AgBiBr6. The layered perovskites (BA)4AgBiBr8 (1) and (BA)2CsAgBiBr7 (2) (BA = CH3(CH2)3NH3+) feature metal-halide sheets of mono and bilayer thickness, respectively, where the ordered double-perovskite lattice is partitioned by organic cations. Electronic structure calculations indicate that the indirect bandgap of Cs2AgBiBr6 becomes direct when the infinitely thick inorganic lattice is reduced to monolayer thickness. Calculations on model systems allow us to separate the effects of dimensional reduction from those of the accompanying structural distortions in the inorganic sublattice. Detailed optical characterization shows that the photophysical properties of 1 and 2 are markedly different than those of their well-studied lead-halide analogs. Hybrid layered derivatives of double perovskites substantially expand on the substitutional flexibility of halide perovskites to encompass greater compositional and electronic diversity.

    View details for DOI 10.1021/jacs.8b01543

    View details for Web of Science ID 000430642000046

    View details for PubMedID 29575889

  • Terahertz Emission from Hybrid Perovskites Driven by Ultrafast Charge Separation and Strong Electron-Phonon Coupling ADVANCED MATERIALS Guzelturk, B., Belisle, R. A., Smith, M. D., Bruening, K., Prasanna, R., Yuan, Y., Gopalan, V., Tassone, C. J., Karunadasa, H. I., McGehee, M. D., Lindenberg, A. M. 2018; 30 (11)

    Abstract

    Unusual photophysical properties of organic-inorganic hybrid perovskites have not only enabled exceptional performance in optoelectronic devices, but also led to debates on the nature of charge carriers in these materials. This study makes the first observation of intense terahertz (THz) emission from the hybrid perovskite methylammonium lead iodide (CH3 NH3 PbI3 ) following photoexcitation, enabling an ultrafast probe of charge separation, hot-carrier transport, and carrier-lattice coupling under 1-sun-equivalent illumination conditions. Using this approach, the initial charge separation/transport in the hybrid perovskites is shown to be driven by diffusion and not by surface fields or intrinsic ferroelectricity. Diffusivities of the hot and band-edge carriers along the surface normal direction are calculated by analyzing the emitted THz transients, with direct implications for hot-carrier device applications. Furthermore, photogenerated carriers are found to drive coherent terahertz-frequency lattice distortions, associated with reorganizations of the lead-iodide octahedra as well as coupled vibrations of the organic and inorganic sublattices. This strong and coherent carrier-lattice coupling is resolved on femtosecond timescales and found to be important both for resonant and far-above-gap photoexcitation. This study indicates that ultrafast lattice distortions play a key role in the initial processes associated with charge transport.

    View details for DOI 10.1002/adma.201704737

    View details for Web of Science ID 000427111300002

    View details for PubMedID 29359820

  • Charge Carrier Dynamics in Cs2AgBiBr6 Double Perovskite JOURNAL OF PHYSICAL CHEMISTRY C Bartesaghi, D., Slavney, A. H., Gelvez-Rueda, M. C., Connor, B. A., Grozema, F. C., Karunadasa, H. I., Savenije, T. J. 2018; 122 (9): 4809–16

    Abstract

    Double perovskites, comprising two different cations, are potential nontoxic alternatives to lead halide perovskites. Here, we characterized thin films and crystals of Cs2AgBiBr6 by time-resolved microwave conductance (TRMC), which probes formation and decay of mobile charges upon pulsed irradiation. Optical excitation of films results in the formation of charges with a yield times mobility product, φΣμ > 1 cm2/Vs. On excitation of millimeter-sized crystals, the TRMC signals show, apart from a fast decay, a long-lived tail. Interestingly, this tail is dominant when exciting close to the bandgap, implying the presence of mobile charges with microsecond lifetimes. From the temperature and intensity dependence of the TRMC signals, we deduce a shallow trap state density of around 1016/cm3 in the bulk of the crystal. Despite this high concentration, trap-assisted recombination of charges in the bulk appears to be slow, which is promising for photovoltaic applications.

    View details for DOI 10.1021/acs.jpcc.8b00572

    View details for Web of Science ID 000427331300008

    View details for PubMedID 29545908

    View details for PubMedCentralID PMC5846080

  • White-Light Emission from Layered Halide Perovskites ACCOUNTS OF CHEMICAL RESEARCH Smith, M. D., Karunadasa, H. I. 2018; 51 (3): 619–27

    Abstract

    With nearly 20% of global electricity consumed by lighting, more efficient illumination sources can enable massive energy savings. However, effectively creating the high-quality white light required for indoor illumination remains a challenge. To accurately represent color, the illumination source must provide photons with all the energies visible to our eye. Such a broad emission is difficult to achieve from a single material. In commercial white-light sources, one or more light-emitting diodes, coated by one or more phosphors, yield a combined emission that appears white. However, combining emitters leads to changes in the emission color over time due to the unequal degradation rates of the emitters and efficiency losses due to overlapping absorption and emission energies of the different components. A single material that emits broadband white light (a continuous emission spanning 400-700 nm) would obviate these problems. In 2014, we described broadband white-light emission upon near-UV excitation from three new layered perovskites. To date, nine white-light-emitting perovskites have been reported by us and others, making this a burgeoning field of study. This Account outlines our work on understanding how a bulk material, with no obvious emissive sites, can emit every color of the visible spectrum. Although the initial discoveries were fortuitous, our understanding of the emission mechanism and identification of structural parameters that correlate with the broad emission have now positioned us to design white-light emitters. Layered hybrid halide perovskites feature anionic layers of corner-sharing metal-halide octahedra partitioned by organic cations. The narrow, room-temperature photoluminescence of lead-halide perovskites has been studied for several decades, and attributed to the radiative recombination of free excitons (excited electron-hole pairs). We proposed that the broad white emission we observed primarily stems from exciton self-trapping. Here, the exciton couples strongly to the lattice, creating transient elastic lattice distortions that can be viewed as "excited-state defects". These deformations stabilize the exciton affording a broad emission with a large Stokes shift. Although material defects very likely contribute to the emission width, our mechanistic studies suggest that the emission mostly arises from the bulk material. Ultrafast spectroscopic measurements support self-trapping, with new, transient, electronic states appearing upon photoexcitation. Importantly, the broad emission appears common to layered Pb-Br and Pb-Cl perovskites, albeit with a strong temperature dependence. Although the emission is attributed to light-induced defects, it still reflects changes in the crystal structure. We find that greater out-of-plane octahedral tilting increases the propensity for the broad emission, enabling synthetic control over the broad emission. Many of these perovskites have color rendering abilities that exceed commercial requirements and mixing halides affords both "warm" and "cold" white light. The most efficient white-light-emitting perovskite has a quantum efficiency of 9%. Improving this value will make these phosphors attractive for solid-state lighting, particularly as large-area coatings that can be deposited inexpensively. The emission mechanism can also be extended to other low-dimensional systems. We hope this Account aids in expanding the phase space of white-light emitters and controlling their exciton dynamics by the synthetic, spectroscopic, theoretical, and engineering communities.

    View details for DOI 10.1021/acs.accounts.7b00433

    View details for Web of Science ID 000428219800007

    View details for PubMedID 29461806

  • The Diversity of Layered Halide Perovskites ANNUAL REVIEW OF MATERIALS RESEARCH, VOL 48 Smith, M. D., Crace, E. J., Jaffe, A., Karunadasa, H. I., Clarke, D. R. 2018; 48: 111–36

    View details for DOI 10.1146/annurev-matsci-070317-124406

    View details for Web of Science ID 000438011300005

  • Pressure-Induced Metallization of the Halide Perovskite (CH3NH3)PbI3 JOURNAL OF THE AMERICAN CHEMICAL SOCIETY Jaffe, A., Lin, Y., Mao, W. L., Karunadasa, H. I. 2017; 139 (12): 4330-4333

    View details for DOI 10.1021/jacs.7b01162

    View details for Web of Science ID 000398247100024

  • Between the Sheets: Postsynthetic Transformations in Hybrid Perovskites CHEMISTRY OF MATERIALS Smith, I. C., Smith, M. D., Jaffe, A., Lin, Y., Karunadasa, H. I. 2017; 29 (5): 1868-1884

    View details for DOI 10.1021/acs.chemmater.6b05395

    View details for Web of Science ID 000396639400002

  • Chemical Approaches to Addressing the Instability and Toxicity of Lead-Halide Perovskite Absorbers INORGANIC CHEMISTRY Slayney, A. H., Smaha, R. W., Smith, I. C., Jaffe, A., Umeyama, D., Karunadasa, H. I. 2017; 56 (1): 46-55

    Abstract

    The impressive rise in efficiencies of solar cells employing the three-dimensional (3D) lead-iodide perovskite absorbers APbI3 (A = monovalent cation) has generated intense excitement. Although these perovskites have remarkable properties as solar-cell absorbers, their potential commercialization now requires a greater focus on the materials' inherent shortcomings and environmental impact. This creates a challenge and an opportunity for synthetic chemists to address these issues through the design of new materials. Synthetic chemistry offers powerful tools for manipulating the magnificent flexibility of the perovskite lattice to expand the number of functional analogues to APbI3. To highlight improvements that should be targeted in new materials, here we discuss the intrinsic instability and toxicity of 3D lead-halide perovskites. We consider possible sources of these instabilities and propose methods to overcome them through synthetic design. We also discuss new materials developed for realizing the exceptional photophysical properties of lead-halide perovskites in more environmentally benign materials. In this Forum Article, we provide a brief overview of the field with a focus on our group's contributions to identifying and addressing problems inherent to 3D lead-halide perovskites.

    View details for DOI 10.1021/acs.inorgchem.6b01336

    View details for Web of Science ID 000391248900007

    View details for PubMedID 27494338

  • Between the sheets: Post-synthetic transformations in hybrid perovskites Chemistry of Materials Smith, I. C., Smith, M. D., Jaffe, A., Lin, Y., Karunadasa, H. I. 2017; 29: 1868

    View details for DOI 10.1021/acs.chemmater.6b05395

  • Structural origins of broadband emission from layered Pb–Br hybrid perovskites Chemical Science Smith, M. D., Jaffe, A., Dohner, E. R., Lindenberg, A. M., Karunadasa, H. I. 2017

    View details for DOI 10.1039/C7SC01590A

  • Pressure-Induced Metallization of the Halide Perovskite (CH3NH3)PbI3 Journal of the American Chemical Society Jaffe, A., Lin, Y., Mao, W. L., Karunadasa, H. I. 2017; 139: 4330

    View details for DOI 10.1021/jacs.7b01162

  • Defect-Induced Band-Edge Reconstruction of a Bismuth-Halide Double Perovskite for Visible-Light Absorption Journal of the American Chemical Society Slavney, A. H., Leppart, L., Bartesaghi, D., Gold-Parker, A., Toney, M. F., Savenije, T. J., Neaton, J. B., Karunadasa, H. I. 2017; 139: 5015

    View details for DOI 10.1021/jacs.7b01629

  • Decreasing the electronic confinement in layered perovskites through intercalation CHEMICAL SCIENCE Smith, M. D., Pedesseau, L., Kepenekian, M., Smith, I. C., Katan, C., Even, J., Karunadasa, H. I. 2017; 8 (3): 1960-1968

    Abstract

    We show that post-synthetic small-molecule intercalation can significantly reduce the electronic confinement of 2D hybrid perovskites. Using a combined experimental and theoretical approach, we explain structural, optical, and electronic effects of intercalating highly polarizable molecules in layered perovskites designed to stabilize the intercalants. Polarizable molecules in the organic layers substantially alter the optical and electronic properties of the inorganic layers. By calculating the spatially resolved dielectric profiles of the organic and inorganic layers within the hybrid structure, we show that the intercalants afford organic layers that are more polarizable than the inorganic layers. This strategy reduces the confinement of excitons generated in the inorganic layers and affords the lowest exciton binding energy for an n = 1 perovskite of which we are aware. We also demonstrate a method for computationally evaluating the exciton's binding energy by solving the Bethe-Salpeter equation for the exciton, which includes an ab initio determination of the material's dielectric profile across organic and inorganic layers. This new semi-empirical method goes beyond the imprecise phenomenological approximation of abrupt dielectric-constant changes at the organic-inorganic interfaces. This work shows that incorporation of polarizable molecules in the organic layers, through intercalation or covalent attachment, is a viable strategy for tuning 2D perovskites towards mimicking the reduced electronic confinement and isotropic light absorption of 3D perovskites while maintaining the greater synthetic tunability of the layered architecture.

    View details for DOI 10.1039/c6sc02848a

    View details for Web of Science ID 000395906900032

    View details for PubMedID 28451311

  • Light-Induced Phase Segregation in Halide-Perovskite Absorbers ACS ENERGY LETTERS Slotcavage, D. J., Karunadasa, H. I., McGehee, M. D. 2016; 1 (6): 1199-1205

    View details for DOI 10.1021/acsenergylett.6b00495

    View details for Web of Science ID 000390086400021

  • Mechanism for Broadband White-Light Emission from Two-Dimensional (110) Hybrid Perovskites JOURNAL OF PHYSICAL CHEMISTRY LETTERS Hu, T., Smith, M. D., Dohner, E. R., Sher, M., Wu, X., Tuan Trinh, M., Fisher, A., Corbett, J., Zhu, X., Karunadasa, H. I., Lindenberg, A. M. 2016; 7 (12): 2258-2263

    Abstract

    The recently discovered phenomenon of broadband white-light emission at room temperature in the (110) two-dimensional organic-inorganic perovskite (N-MEDA)[PbBr4] (N-MEDA = N(1)-methylethane-1,2-diammonium) is promising for applications in solid-state lighting. However, the spectral broadening mechanism and, in particular, the processes and dynamics associated with the emissive species are still unclear. Herein, we apply a suite of ultrafast spectroscopic probes to measure the primary events directly following photoexcitation, which allows us to resolve the evolution of light-induced emissive states associated with white-light emission at femtosecond resolution. Terahertz spectra show fast free carrier trapping and transient absorption spectra show the formation of self-trapped excitons on femtosecond time-scales. Emission-wavelength-dependent dynamics of the self-trapped exciton luminescence are observed, indicative of an energy distribution of photogenerated emissive states in the perovskite. Our results are consistent with photogenerated carriers self-trapped in a deformable lattice due to strong electron-phonon coupling, where permanent lattice defects and correlated self-trapped states lend further inhomogeneity to the excited-state potential energy surface.

    View details for DOI 10.1021/acs.jpclett.6b00793

    View details for Web of Science ID 000378196000017

    View details for PubMedID 27246299

  • Red-to-Black Piezochromism in a Compressible Pb-l-SCN Layered Perovskite CHEMISTRY OF MATERIALS Umeyama, D., Lin, Y., Karunadasa, H. I. 2016; 28 (10): 3241-3244

    View details for DOI 10.1021/acs.chemmater.6b01147

    View details for Web of Science ID 000376825700003

  • A Bismuth-Halide Double Perovskite with Long Carrier Recombination Lifetime for Photovoltaic Applications. Journal of the American Chemical Society Slavney, A. H., Hu, T., Lindenberg, A. M., Karunadasa, H. I. 2016; 138 (7): 2138-2141

    Abstract

    Despite the remarkable rise in efficiencies of solar cells containing the lead-halide perovskite absorbers RPbX3 (R = organic cation; X = Br(-) or I(-)), the toxicity of lead remains a concern for the large-scale implementation of this technology. This has spurred the search for lead-free materials with similar optoelectronic properties. Here, we use the double-perovskite structure to incorporate nontoxic Bi(3+) into the perovskite lattice in Cs2AgBiBr6 (1). The solid shows a long room-temperature fundamental photoluminescence (PL) lifetime of ca. 660 ns, which is very encouraging for photovoltaic applications. Comparison between single-crystal and powder PL decay curves of 1 suggests inherently high defect tolerance. The material has an indirect bandgap of 1.95 eV, suited for a tandem solar cell. Furthermore, 1 is significantly more heat and moisture stable compared to (MA)PbI3. The extremely promising optical and physical properties of 1 shown here motivate further exploration of both inorganic and hybrid halide double perovskites for photovoltaics and other optoelectronics.

    View details for DOI 10.1021/jacs.5b13294

    View details for PubMedID 26853379

  • Chemical approaches to addressing the instability and toxicity of lead-halide perovskite absorbers Inorganic Chemistry Slavney, A. H., Smaha, R. W., Smith, I. C., Jaffe, A., Umeyama, D., Karunadasa, H. I. 2016

    View details for DOI 10.1021/acs.inorgchem.6b01336

  • High-pressure single-crystal structures of 3D lead-halide hybrid perovskites and pressure effects on their electronic and optical properties ACS Cent. Sci Jaffe, A., Lin, Y., Beavers, C. M., Voss, J., Mao, W. L., Karunadasa, H. I. 2016; 2: 201

    View details for DOI 10.1021/acscentsci.6b00055

  • Quinone-Functionalized Carbon Black Cathodes for Lithium Batteries with High Power Densities CHEMISTRY OF MATERIALS Jaffe, A., Valdes, A. S., Karunadasa, H. I. 2015; 27 (10): 3568-3571

    View details for DOI 10.1021/acs.chemmater.5b00990

    View details for Web of Science ID 000355382700005

  • Pressure-Induced Conductivity and Yellow-to-Black Piezochromism in a Layered Cu-Cl Hybrid Perovskite. Journal of the American Chemical Society Jaffe, A., Lin, Y., Mao, W. L., Karunadasa, H. I. 2015; 137 (4): 1673-1678

    Abstract

    Pressure-induced changes in the electronic structure of two-dimensional Cu-based materials have been a subject of intense study. In particular, the possibility of suppressing the Jahn-Teller distortion of d(9) Cu centers with applied pressure has been debated over a number of decades. We studied the structural and electronic changes resulting from the application of pressures up to ca. 60 GPa on a two-dimensional copper(II)-chloride perovskite using diamond anvil cells (DACs), through a combination of in situ powder X-ray diffraction, electronic absorption and vibrational spectroscopy, dc resistivity measurements, and optical observations. Our measurements show that compression of this charge-transfer insulator initially yields a first-order structural phase transition at ca. 4 GPa similar to previous reports on other Cu(II)-Cl perovskites, during which the originally translucent yellow solid turns red. Further compression induces a previously unreported phase transition at ca. 8 GPa and dramatic piezochromism from translucent red-orange to opaque black. Two-probe dc resistivity measurements conducted within the DAC show the first instance of appreciable conductivity in Cu(II)-Cl perovskites. The conductivity increases by 5 orders of magnitude between 7 and 50 GPa, with a maximum measured conductivity of 2.9 × 10(-4) S·cm(-1) at 51.4 GPa. Electronic absorption spectroscopy and variable-temperature conductivity measurements indicate that the perovskite behaves as a 1.0 eV band-gap semiconductor at 39.7 GPa and has an activation energy for electronic conduction of 0.232(1) eV at 40.2 GPa. Remarkably, all these changes are reversible: the material reverts to a translucent yellow solid upon decompression, and ambient pressure powder X-ray diffraction data taken before and after compression up to 60 GPa show that the original structure is maintained with minimal hysteresis.

    View details for DOI 10.1021/ja512396m

    View details for PubMedID 25580620

  • Reversible photo-induced trap formation in mixed-halide hybrid perovskites for photovoltaics CHEMICAL SCIENCE Hoke, E. T., Slotcavage, D. J., Dohner, E. R., Bowring, A. R., Karunadasa, H. I., McGehee, M. D. 2015; 6 (1): 613-617

    View details for DOI 10.1039/c4sc03141e

    View details for Web of Science ID 000345901600072

  • CH3NH3PbI3 perovskite single crystals: surface photophysics and their interaction with the environment CHEMICAL SCIENCE Grancini, G., D'Innocenzo, V., DOHNER, E. R., Martino, N., Kandada, A. R., Mosconi, E., De Angelis, F., Karunadasa, H. I., Hoke, E. T., Petrozza, A. 2015; 6 (12): 7305-7310

    View details for DOI 10.1039/c5sc02542g

    View details for Web of Science ID 000365225300074

  • Post-synthetic halide conversion and selective halogen capture in hybrid perovskites CHEMICAL SCIENCE Solis-Ibarra, D., Smith, I. C., Karunadasa, H. I. 2015; 6 (7): 4054-4059

    View details for DOI 10.1039/c5sc01135c

    View details for Web of Science ID 000356176200048

  • A Layered Hybrid Perovskite Solar-Cell Absorber with Enhanced Moisture Stability ANGEWANDTE CHEMIE-INTERNATIONAL EDITION Smith, I. C., Hoke, E. T., Solis-Ibarra, D., McGehee, M. D., Karunadasa, H. I. 2014; 53 (42): 11232-11235

    View details for DOI 10.1002/anie.201406466

    View details for Web of Science ID 000343751100020

  • Intrinsic White-Light Emission from Layered Hybrid Perovskites JOURNAL OF THE AMERICAN CHEMICAL SOCIETY Dohner, E. R., Jaffe, A., Bradshaw, L. R., Karunadasa, H. I. 2014; 136 (38): 13154-13157

    View details for DOI 10.1021/ja507086b

    View details for Web of Science ID 000342328200021

  • Lithium cycling in a self-assembled copper chloride-polyether hybrid electrode. Inorganic chemistry Jaffe, A., Karunadasa, H. I. 2014; 53 (13): 6494-6496

    Abstract

    Atomic-scale integration of polyether molecules and copper(II) chloride layers in a two-dimensional perovskite affords, to the best of our knowledge, the first example of extended Li(+) cycling in a metal chloride electrode. The hybrid can cycle over 200 times as a cathode in a lithium battery with an open-circuit voltage of 3.2 V. In contrast, CuCl2 alone or the precursors to the hybrid cannot be cycled in a lithium battery, demonstrating the importance of the layered, organic-inorganic architecture. This work shows that appropriate organic groups can enable Li(+) cycling in inexpensive, nontoxic, metal halide electrodes, which is promising for large-scale applications.

    View details for DOI 10.1021/ic500860t

    View details for PubMedID 24917248

  • Self-Assembly of Broadband White-Light Emitters JOURNAL OF THE AMERICAN CHEMICAL SOCIETY Dohner, E. R., Hoke, E. T., Karunadasa, H. I. 2014; 136 (5): 1718-1721

    Abstract

    We use organic cations to template the solution-state assembly of corrugated lead halide layers in bulk crystalline materials. These layered hybrids emit radiation across the entire visible spectrum upon ultraviolet excitation. They are promising as single-source white-light phosphors for use with ultraviolet light-emitting diodes in solid-state lighting devices. The broadband emission provides high color rendition and the chromaticity coordinates of the emission can be tuned through halide substitution. We have isolated materials that emit the "warm" white light sought for many indoor lighting applications as well as "cold" white light that approximates the visible region of the solar spectrum. Material syntheses are inexpensive and scalable and binding agents are not required for film deposition, eliminating problems of binder photodegradation. These well-defined and tunable structures provide a flexible platform for studying the rare phenomenon of intrinsic broadband emission from bulk materials.

    View details for DOI 10.1021/ja411045r

    View details for Web of Science ID 000331493700010

    View details for PubMedID 24422494

  • Reversible and Irreversible Chemisorption in Nonporous-Crystalline Hybrids ANGEWANDTE CHEMIE-INTERNATIONAL EDITION Solis-Ibarra, D., Karunadasa, H. I. 2014; 53 (4): 1039-1042

    Abstract

    The tools of synthetic chemistry allow us to fine-tune the reactivity of molecules at a level of precision not yet accessible with inorganic solids. We have investigated hybrids that couple molecules to the superior thermal and mechanical properties of solids. Herein we present, to the best of our knowledge, the first demonstration of reactivity between hybrid perovskites and substrates. Reaction with iodine vapor results in a remarkable expansion of these materials (up to 36 % in volume) where new covalent CI bonds are formed with retention of crystallinity. These hybrids also show unusual examples of reversible chemisorption. Here, solid-state interactions extend the lifetime of molecules that cannot be isolated in solution. We have tuned the half-lives of the iodinated structures from 3 h to 3 days. These nonporous hybrids drive substrate capture and controlled release through chemical reactivity. We illustrate the strengths of the hybrid by considering radioactive iodine capture.

    View details for DOI 10.1002/anie.201309786

    View details for Web of Science ID 000329879500022

    View details for PubMedID 24311056

  • Low-Spin Hexacoordinate Mn(III): Synthesis and Spectroscopic Investigation of Homoleptic Tris(pyrazolyl)borate and Tris(carbene)borate Complexes INORGANIC CHEMISTRY Forshaw, A. P., Smith, J. M., Ozarowski, A., Krzystek, J., Smirnov, D., Zvyagin, S. A., Harris, T. D., Karunadasa, H. I., Zadrozny, J. M., Schnegg, A., Holldack, K., Jackson, T. A., Alamiri, A., Barnes, D. M., Telser, J. 2013; 52 (1): 144-159

    Abstract

    Three complexes of Mn(III) with "scorpionate" type ligands have been investigated by a variety of physical techniques. The complexes are [Tp(2)Mn]SbF(6) (1), [Tp(2)*Mn]SbF(6) (2), and [{PhB(MeIm)(3)}(2)Mn](CF(3)SO(3)) (3a), where Tp(-) = hydrotris(pyrazolyl)borate anion, Tp*(-) = hydrotris(3,5-dimethylpyrazolyl)borate anion, and PhB(MeIm)(3)(-) = phenyltris(3-methylimidazol-2-yl)borate anion. The crystal structure of 3a is reported; the structures of 1 and 2 have been previously reported, but were reconfirmed in this work. The synthesis and characterization of [{PhB(MeIm)(3)}(2)Mn]Cl (3b) are also described. These complexes are of interest in that, in contrast to many hexacoordinate (pseudo-octahedral) complexes of Mn(III), they exhibit a low-spin (triplet) ground state, rather than the high-spin (quintet) ground state. Solid-state electronic absorption spectroscopy, SQUID magnetometry, and high-frequency and -field electron paramagnetic resonance (HFEPR) spectroscopy were applied. HFEPR, in particular, was useful in characterizing the S = 1 spin Hamiltonian parameters for complex 1, D = +19.97(1), E = 0.42(2) cm(-1), and for 2, D = +15.89(2), E = 0.04(1) cm(-1). In addition, frequency domain Fourier-transform THz-EPR spectroscopy, using coherent synchrotron radiation, was applied to 1 only and gave results in good agreement with HFEPR. Variable-temperature dc magnetic susceptibility measurements of 1 and 2 were also in good agreement with the HFEPR results. This magnitude of zero-field splitting (zfs) is over 4 times larger than that in comparable hexacoordinate Mn(III) systems with S = 2 ground states. Complexes 3a and 3b (i.e., regardless of counteranion) have a yet much larger magnitude zfs, which may be the result of unquenched orbital angular momentum so that the spin Hamiltonian model is not appropriate. The triplet ground state is rationalized in each complex by ligand-field theory (LFT) and by quantum chemistry theory, both density functional theory and unrestricted Hartree-Fock methods. This analysis also shows that spin-crossover behavior is not thermally accessible for these complexes as solids. The donor properties of the three different scorpionate ligands were further characterized using the LFT model that suggests that the tris(carbene)borate is a strong σ-donor with little or no π-bonding.

    View details for DOI 10.1021/ic301630d

    View details for Web of Science ID 000313220500019

    View details for PubMedID 23259486

  • Electrochemical generation of hydrogen from acetic acid using a molecular molybdenum-oxo catalyst ENERGY & ENVIRONMENTAL SCIENCE Thoi, V. S., Karunadasa, H. I., Surendranath, Y., Long, J. R., Chang, C. J. 2012; 5 (7): 7762-7770

    View details for DOI 10.1039/c2ee21519e

    View details for Web of Science ID 000305530900010

  • A molecular MoS2 edge site for catalytic hydrogen production Science Karunadasa, H. I., Montalvo, E., Sun, Y., Majda, M., Majda, J. R., Chang, C. J. 2012; 335 (698)
  • A computational and experimental study of the mechanism of hydrogen generation from water by a molecular molybdenum-oxo electro catalyst J. Am. Chem. Soc Sundstrom, E. J., Yang, X., Thoi, V. S., Karunadasa, H. I., Chang, C. J., Long, J. R., Head-Gordon, M. 2012; 134 (5233)
  • A molecular molybdenum-oxo catalyst for generating hydrogen from water NATURE Karunadasa, H. I., Chang, C. J., Long, J. R. 2010; 464 (7293): 1329-1333

    Abstract

    A growing awareness of issues related to anthropogenic climate change and an increase in global energy demand have made the search for viable carbon-neutral sources of renewable energy one of the most important challenges in science today. The chemical community is therefore seeking efficient and inexpensive catalysts that can produce large quantities of hydrogen gas from water. Here we identify a molybdenum-oxo complex that can catalytically generate gaseous hydrogen either from water at neutral pH or from sea water. This work shows that high-valency metal-oxo species can be used to create reduction catalysts that are robust and functional in water, a concept that has broad implications for the design of 'green' and sustainable chemistry cycles.

    View details for DOI 10.1038/nature08969

    View details for Web of Science ID 000277149000042

    View details for PubMedID 20428167

  • Magnetic properties of Ba2HoSbO6 with a frustrated lattice geometry PHYSICAL REVIEW B Calder, S., Ke, X., Bert, F., Amato, A., Baines, C., Carboni, C., Cava, R. J., Daoud-Aladine, A., Deen, P., Fennell, T., Hillier, A. D., Karunadasa, H., Taylor, J. W., Mendels, P., Schiffer, P., Bramwell, S. T. 2010; 81 (6)

    View details for DOI 10.1103/PhysRevB.81.064425

    View details for Web of Science ID 000274998100076

  • Enhancing the magnetic anisotropy of cyano-ligated Cr(II) and Cr(III) complexes via heavy-halide ligand effects Inorg. Chem. Karudanasa, H. I., Arquero, K. D., Berben, L. A., Long, J. R. 2010; 49 (4738)
  • Synthesis and Redox-Induced Structural Isomerization of the Pentagonal Bipyramidal Complexes [W(CN)(5)(CO)(2)](3-) and [W(CN)(5)(CO)(2)](2-) ANGEWANDTE CHEMIE-INTERNATIONAL EDITION Karunadasa, H. I., Long, J. R. 2009; 48 (4): 738-741

    View details for DOI 10.1002/anie.200804199

    View details for Web of Science ID 000262676000010

    View details for PubMedID 19072955

  • Honeycombs of triangles and magnetic frustration in SrLn2O4 (Ln = Gd, Dy, Ho, Er, Tm, and Yb) Phys. Rev. B Karudanasa, H. I., Huang, Q., Ueland, B. G., Schiffer, P., Cava, R. J. 2005; 71 (144414)
  • Quantum and thermal spin relaxation in the diluted spin ice Dy2-xMxTi2O7 (M=Lu,Y) PHYSICAL REVIEW B Snyder, J., Ueland, B. G., Mizel, A., Slusky, J. S., Karunadasa, H., Cava, R. J., Schiffer, P. 2004; 70 (18)

    View details for DOI 10.1103/PhysRevB.70.184431

    View details for Web of Science ID 000225477300083

  • 2,2 '-dibromo-3,3 ',4,4 ',5,5 ',6,6 '-octamethyl-1,1 '-biphenyl ACTA CRYSTALLOGRAPHICA SECTION E-STRUCTURE REPORTS ONLINE Karunadasa, H., Leggett, C., Wong, S. 2004; 60: O1499-O1500

    View details for DOI 10.1107/S160053680401709X

    View details for Web of Science ID 000223624500082

  • Low temperature spin freezing in Dy2Ti2O7 spin ice Phys. Rev. B Snyder, J., Ueland, B. G., Slusky, J. S., Karunadasa, H. I., Cava, R. J., Schiffer, P. 2004; 69 (064414)
  • Quantum-classical reentrant relaxation crossover in DY2Ti2O7 spin ice PHYSICAL REVIEW LETTERS Snyder, J., Ueland, B. G., Slusky, J. S., Karunadasa, H., Cava, R. J., Mizel, A., Schiffer, P. 2003; 91 (10)

    Abstract

    We have studied spin relaxation in the spin ice compound Dy2Ti2O7 through measurements of the ac magnetic susceptibility. While the characteristic spin-relaxation time (tau) is thermally activated at high temperatures, it becomes almost temperature independent below T(cross) approximately 13 K. This behavior, combined with nonmonotonic magnetic field dependence of tau, indicates that quantum tunneling dominates the relaxational process below that temperature. As the low-entropy spin ice state develops below T(ice) approximately 4 K, tau increases sharply with decreasing temperature, suggesting the emergence of a collective degree of freedom for which thermal relaxation processes again become important as the spins become strongly correlated.

    View details for DOI 10.1103/PhysRevLett.91.107201

    View details for Web of Science ID 000185485700035

    View details for PubMedID 14525500

  • Ba2LnSbO6 and Sr2LnSbO6 (Ln = Dy, Ho, Gd) double perovskites: lanthanides in the geometrically frustrating fcc lattice Proc. Natl. Acad. Sci. Karunadasa, H. I., Huang, Q., Ueland, B. G., Schiffer, P., Cava, R. J., 2000; 100: 8097