Spin-optotronic Properties of Organometal Halide Perovskites - The

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Cite This: J. Phys. Chem. Lett. 2018, 9, 6103−6111

Spin-optotronic Properties of Organometal Halide Perovskites Yuan Ping* and Jin Zhong Zhang*

J. Phys. Chem. Lett. 2018.9:6103-6111. Downloaded from pubs.acs.org by UNIV OF SUNDERLAND on 10/18/18. For personal use only.

Department of Chemistry and Biochemistry, University of California, Santa Cruz, California 95064, United States ABSTRACT: Spin is an intrinsic quantum mechanical property of fundamental particles including the electron. The spin property is intimately related to electronic and optical properties of molecules and materials. The combination of spin (magnetic), electronic, and optical properties of materials, such as organometal halide perovskites (OMHP), has attracted increasing attention, which has led to a new field termed spin-optotronics based on all three key properties. This growing field has implications in emerging technological applications across disciplines, including photonics, electronics, spintronics, quantum computation, and information storage. This Perspective provides a brief introduction to this field from both experimental and computational aspects, with a focus on the effect of spin on charge carrier dynamics in OMHP, a class of materials with novel properties and promising applications in a number of fields. For instance, recent studies have demonstrated the use of ultrafast laser techniques in probing the fundamental charge carrier dynamics in relation to spin properties. Because of strong spin−orbit coupling (SOC) and broken inversion symmetry that result in Rashba and Dresselhaus effects, OMHP are considered ideal for manipulating spin states for spin-optotronics applications. In the meantime, on the basis of first-principles calculations and effective model Hamiltonians, the Rashba splitting in locally polarized domains can result in spin-forbidden recombination with significantly slow transition rate due to the mismatch of spin and momentum. We summarize the state-of-the-art first-principles methods and their current limitations for ultrafast charge and spin dynamics for realistic solid-state systems in general. To conclude, we note some promising future research and development directions for both experimental and theoretical ultrafast spin dynamics studies of OMHP.

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ombining spin (or magnetic) and optical properties of materials to enable emerging spin-optotronics or magneto-optic technologies has recently attracted increasing attention because of potential applications in quantum information technology, photovoltaics (PV), and light-emitting diodes (LEDs).1−4 Core to these research areas is the spin character of electrons, which is responsible for magnetic properties and also strongly influences optical properties of materials. For example, in most cases, at least those involving small or light molecules, spin must be conserved during optical (dipole-allowed) transitions, including absorption or emission of light. However, in other cases, spin does not have to be conserved, for example in situations where there are strong spin−orbit couplings (SOC), for molecules and solids.6−8 Therefore, spin and optical properties are intimately connected. One can potentially use spin to control optical properties of charge carriers, including both intrinsic spin, for example, SOC, or extrinsic spin, for example, chemical doping or surface dangling bonds containing unpaired electrons. For example, SOC can split energy levels of molecules as well as semiconductors, such as CdSe nanocrystals.9 Likewise, Mn2+doping of ZnO results in charge transfer between the magnetic dopant and nonmagnetic host and induces room-temperature ferromagnetism, demonstrating the significant impact that dopants have on magnetic properties of the host.10,11 Similarly, optically addressable spin states associated with Si vacancies in SiC are considered promising for quantum information technologies.4 However, in most cases, charge carrier dynamics associated with such spin states are rarely investigated, partly © 2018 American Chemical Society

Recent studies have shown that spin affects the charge carrier dynamics related to the optical and magnetic properties of organometal halide perovskites, thereby suggesting potential applications in LEDs and spin-optotronics. because spin states tend to be element-specific and most optical techniques lack elemental specificity. Organometal halide perovskites (OMHP), for example, CH3NH3PbX3 (X = Cl, Br, I), are a class of materials that have shown great potential for PV and LED applications because of their unique optical and electronic properties.12−14 In addition, recent studies have revealed unique spin properties of the OMHP, showing promise for spintronics and spin-optotronics applications.1,15,16 For example, a Rashba spin splitting has been predicted in methylammonium-based perovskites CH3NH3MX3.17−19 Rashba and Dresselhaus spin splittings are fundamentally related to SOC and inversion asymmetry of the systems. Taking an example of a quasi-2D system in C2v symmetry for simplicity,15 the Rashba−Dresselhaus HamilReceived: August 14, 2018 Accepted: October 3, 2018 Published: October 3, 2018 6103

DOI: 10.1021/acs.jpclett.8b02498 J. Phys. Chem. Lett. 2018, 9, 6103−6111

The Journal of Physical Chemistry Letters

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tonian is HRD(k∥) = λR(kxσy − kyσx) + λD(kxσx − kyσy), where σ = (σx, σy, σz) is the vector of Pauli spin matrices and k is the wavevector in the first Brillouin zone, with k∥ = (kx, ky) and k⊥ = kz. When λD = 0, one obtains the pure Rashba effect that relates to the site inversion asymmetry. On the other hand, when λR = 0, the remaining term is the Dresselhaus effect, related to the bulk inversion asymmetry (commonly found in zinc blende structures). An example of Rashba effect on the spin splitting and spin texture of OMHP is shown in Figure 1,

spin or magnetic properties or vice versa in magneto-optical technologies.2 The spin states also play an important role in the dynamics of charge carriers.25 In some cases, the organic component is replaced with an inorganic metal such as Cs. For example, bright triplet excitons are observed in CsPbX3 (X = I, Br, Cl), which is attributed to the favorable effect of SOC in conjunction with broken inversion symmetry.8 Relevant crystal structure, electronic band structure, energy levels associated with electron−hole exchange and Rashba effect, and a representative transmission electron microcsopy image are shown in Figure 2. The CsPbX3 perovskites are expected to exhibit a large Rashba effect, which occurs in semiconductors with strong SOC and broken inversion symmetry and should alter the fine electronic structure. This leads to the suggestion that the bright triplet exciton can have an energy below the dark singlet exciton (as shown in Figure 2c), which has been confirmed experimentally. Similarly, high spin polarization and ultralarge photoinduced magnetization of CH3NH3PbI3 (MAPbI3) is achieved when circularly polarized light is used to excite the sample.25 Figure 3a shows the energy bands of MAPbI3 at the R-point (point group symmetry representation) with their respective levels from vacuum (experimental). The dashed box indicates the bands of interest with SOC. As shown in Figure 3c, spin relaxation lifetimes of 7 ps for the electron (τe) and 1 ps for the hole (τh) were observed, making the materials potentially useful for ultrafast spin switch applications. Figure 3b gives a detailed explanation of these spin relaxation times or J relaxation times (intraband interstate transfer time or “J-flip” corresponding to spin flip).

Figure 1. Diagram of Rashba bands and the electron transport path. The cyan and orange arrows indicate the directions of the spins. The spin texture χ indicates spin vortex direction with its signs characterizing spin rotation “clockwise” (χ = −1) and “counterclockwise” (χ = +1). After absorbing the photons, the excited electrons on conduction bands Cχ=+1 and Cχ=−1 will quickly relax to Cχ=−1 band minimum due to the inelastic phonon scattering. Similarly, the holes will quickly relax to the Vχ=+1 band maximum. However, the radiative recombination of Cχ=−1 → Vχ=+1 is a spinforbidden process because of the opposite spin states they have. Moreover, the minimum of the Cχ=−1 band and the maximum of the Vχ=+1 band are located in different positions in the Brillouin zone. This creates an indirect band gap for recombination, which further slows the recombination process. Reproduced from ref 20. Copyright 2015 American Chemical Society.

This work demonstrates the power of ultrafast studies in probing the fundamental charge carrier dynamics in relation to spin properties.

The basic principle behind the above dynamics study is based on interaction of electronic spin states with light with specific polarization.25,26 In other words, the charge carrier dynamics depends on the spin character of the electronic states as well as the polarization of the pump and probe light used to excite and probe the charge carriers. The spin states are strongly dependent on the chemical nature of the materials (composition and crystal symmetry as well as electronic structure). Using MAPbI3 as an example, density functional theory (DFT) calculations of its electronic band structure show that the R-point is where the band gap locates.27 The valence band (VB) edge contains contributions mainly from the Pb(6s)I(5p) atomic orbitals, while the conduction band (CB) edge arises mainly from Pb(6p) orbitals. Because of SOC, the CB with orbital angular momentum L = 1 and spin angular momentum S = 1/2 is split into two states with different total angular momentum, J = 1/2 and J = 3/2, whereas the VB with L = 0 is almost unaffected. For optical transitions and charge carrier dynamics near the band edges, the topmost VB (J = S = 1/2) and the bottom-most CB (J = 1/ 2, S = 1/2), both of which are doubly degenerate (mj = ±1/2, which is the projection of J in the z-direction), are relevant and thus of interest.

where the radiative recombination at band edges is a spinforbidden process due to the opposite spin states they have [“clockwise” spin rotation at the conduction band minimum (CBM) and “counterclockwise” spin rotation at the valence band maximum (VBM)], resulting in long recombination lifetime.20 Previous work also showed tuning the symmetry of OMHP, for example simple translation of the central ion along the z axis or ferroelectric switching, can manipulate the Rashba splitting.17 Compared to bulk perovskites, nanostructured perovskites, for example, perovskite quantum dots (PQDs) or perovskite nanocrystals (PNCs), provide additional advantages, such as high color purity, tunable emission, high photoluminescence (PL) quantum yield (up to 90%), and solution processability.21 These outstanding characteristics make them highly promising for high-efficiency LED applications.22 Moreover, their optical and magnetic properties can be manipulated by chemical doping.23 For example, magnetic dopants such as Mn2+, Co2+, Ni2+, and Cu2+ have highly interesting and important optical properties in applications such as electroluminescence, spintronics, and LEDs.24 Their optical properties are dependent on the spin properties of such dopants; thus, one can exploit their interplay to control optical properties by altering 6104



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Figure 2. Crystal and electronic structure for CsPbBr3 perovskite. (a) Orthorhombic crystal structure of CsPbBr3 (Pnma space group, unit cell shown as a frame), which differs from the idealized cubic perovskite by an octahedral tilting. (b) Calculated band structure of cubic CsPbBr3 perovskite. The inset shows the first Brillouin zone of the cubic crystal lattice. The electronic band gap is indicated in the band structure at the R point. The valence (conduction) band maximum (minimum) has R6+ (R6−) symmetry. (c) The expected fine structure of the band-edge exciton considering short-range electron−hole exchange (middle) and then including the Rashba effect (right) under orthorhombic symmetry. The latter splits the exciton into three bright states with transition dipoles oriented along the orthorhombic symmetry axes (labeled x, y, and z) and a higherenergy dark state (labeled “d”). The energetic order of the three lowest sublevels is determined by the orthorhombic distortion. The orthorhombic unit cell (bottom) and the resulting sublevel order is shown for CsPbBr3. (d) Transmission electron micrograph of an individual CsPbBr3 nanocrystal with an edge length of L = 14 nm. Reproduced with permission from ref 8. Copyright 2018 Nature Publishing Group.

For the VB, the two states with mj = 1/2 are given by

The contribution of spin states can be predicted using the Clebsch−Gordan (CG) coefficients: L , S , J , mj⟩ =

∑ ml∑ ms L , S , ml , ms⟩⟨L , S , ml , ms|L , S , J , ms⟩

0,

1 1 1 , ,+ 2 2 2

= 1,

1 1 , 0, + 2 2

0,

1 1 1 , ,− 2 2 2

= 1,

1 1 , 0, + 2 2

(1)

where S = 1/2 and L = 1 for the CB and L = 0 for the VB. If mj ≠ ml + ms, the CG coefficient is zero. The nonzero CG coefficients can be obtained from the CG table for the addition of angular momentum. For the CB, the two states with mj = ±1/2 are given by 1 1 1 , ,+ 2 2 2 1 1 1 = 1, , 0, + 2 2 3

where ms = +1/2 and −1/2 represent the spin-up and spindown states, respectively. When the square module of the wave function is acquired for probability, the J-up (mj = +1/2) state consists of 1/3 spin-up and 2/3 spin-down electrons, while the J-down (mj = −1/2) state consists of 2/3 spin-up and 1/3 spindown electrons.25 To probe the charge carrier dynamics with spin polarization, circularly polarized pump light will be used, as shown in Figure 4, instead of the linearly polarized light used in typical degenerate pump−probe studies. The photon with positive helicity (σ+) has spin of +ℏ along its direction of propagation and is defined as left circularly polarized based on the convention of detector’s point of view (counterclockwise) and the opposite (σ−) for right circularly polarized (clockwise). The probe beam polarization is set to linear (s-polarized), which consists of two equal components of left and right

1,

1 1 1 , ,− 2 2 2 1 1 2 = 1, , − 1, + 2 2 3 1 1 1, , + 1, − 2 2

1 1 2 1, , + 1, − 2 2 3

+

1,

+

(3)

1 3 (2) 6105



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Figure 3. (a) Energy bands of CH3NH3PbI3 at R-point (point group symmetry representation)5 with their respective levels from vacuum (experimental).2,3 Dashed box indicates the bands of interest. (b) Model of near band-edge photoexcitation by σ+ photon and J-states dynamics of CH3NH3PbI3. The state notation is written as |J, mJ⟩ where J = 1/2 is the electron’s total angular momentum quantum number and mJ = ± 1/2 is its projection in the z-axis. Absorption of σ+ pump will raise the angular momentum by +ℏ (Δmj = +1). (c) Normalized circular pump−probe decay transients with 19 μJ/cm2 σ+ pump and σ+ probe (blue), σ− probe (red), and their total (magenta) at 293 K (top) and 77 K (bottom). The experimental data is globally fitted for σ+ probe, σ− probe, and their sum. Reproduced from ref 25. Copyright 2015 American Chemical Society.

Other studies have been conducted to probe charge carrier dynamics of OMHP.29−31 Interpretation of dynamics data has been complicated by the multiple processes involved and the intrinsic multicomponent nature of the material, including defects.21,32 Spin dependence is usually buried in the complex dynamics observed. It is also challenging to associate a specific dynamic feature to individual elements, as optical techniques cannot provide element-specific information. Compared to optical pump/optical probe, optical pump/Xray probe techniques can directly connect local spin properties of individual elements, including intentionally introduced magnetic dopants, to specific dynamic features of the charge carriers, because the X-ray probe is element-specific and highly sensitive to the local structure of individual atoms or ions.1 Figure 5 shows a schematic illustration of an optical pump/Xray probe experiment, where the X-ray can be tuned to prove a specific element, for example Pb, in this case. With σ+ for the pump pulse with above band gap excitation, the electron will be created in the conduction band (CB) with an up spin (mj = 1/2), while the hole will be created in the valence band (VB) with a down spin (mj = −1/2). Very recently, the first elementselective study was reported on charge carrier dynamics in photoexcited CsPbBr3 and CsPb(ClBr)3 PNCs, using timeresolved X-ray absorption spectroscopy with 80 ps time resolution.33 The holes in the valence band were determined to be localized at Br atoms, forming small polarons, while electrons delocalized in the conduction band, shedding light on the physical nature of modest charge carrier mobilities. However, the spin selectivity has not been explored in this study, which should be an interesting area for future research.

Figure 4. Illustration of pump−probe setup with controlled polarizations (e.g., σ+ for pump and linear for probe).

circularly polarized light. A quarter wave plate is used to convert the left- and right-circular component into s- and ppolarized light, respectively, which are then split by a Wollaston prism for separate detection by two photodetectors. By this setup, we can excite the sample with circular polarized light (e.g., σ+) and monitor the probe light in different polarizations such as σ+, σ−, or their combination (linear) to provide spin-sensitive charge carrier dynamics. As illustrated in Figure 3 and using |J, mj⟩ to label the spin states, absorption of σ+ pump beam raises the angular momentum by +ℏ (Δmj = +1) and results in transition from the |1/2, −1/2⟩ to the |1/2, +1/2⟩ spin state. The excited electron can recombine directly to return to the original state or can transfer to the |1/2, −1/2⟩ excited state and then recombine to end in the |1/2, +1/2⟩ ground state.25 The charge carrier dynamics needs to be fit with mathematical functions and analyzed carefully using kinetic models to account for the major decay pathways.28 In the description above, there is no account of any trap states in the band gap, including those introduced intentionally from dopants, which can contribute to the electron relaxation and recombination. 6106



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tributions and dynamical correlations. However, only small systems such as atoms46 and small molecules47 have been practical for ultrafast charge carrier dynamics simulations, and further development is needed to include nuclear motions through Ehrenfest approximation or MBPT.47 For the Boltzmann equation (BE) with MBPT, it includes dynamical correlations through the GW self-energy for electron−electron scattering48 and Fan self-energy for electron−phonon scattering54 and is practical and suitable for solids compared with other methods. In particular, the first-principles determined parameters for BE need to be computed only once and then can be applied to each pump wavelength, pump power, and probe wavelength combination,53 unlike TDDFT with NAMD. However, approximations are still included in BE with MBPT, such as using first-order perturbation theory for electron− electron and electron−phonon scatterings and the scattering rates are computed at an equilibrium carrier distribution. A recent effort has been made to include electron−phonon and electron−electron scattering rates at nonequilibrium carrier distributions and eliminate the effective electron temperature assumption27,49,55−57 in first-principles simulations of ultrafast dynamics in plasmonic metal nanoparticles.53 Extending this methodology53 to complex semiconductors such as OMHP still requires further justification and development. In addition, the calculations of electron−electron scattering rates from GW self-energy can be significantly sped up with efficient numerical implementations that avoid explicit empty states58,59 for largescale systems like OMHP with defects and dopants. Compared to ultrafast carrier dynamics, first-principles study of ultrafast spin dynamics is less developed, and the published work has been mostly applied to simple ferromagnetic metals.60 More spin dynamics studies have been performed for semiconductor quantum wells with model Hamiltonians through a many-body kinetic spin Bloch equation approach61 or equation of motion of spin density matrix,62 which pave the theoretical foundation for future first-principles ultrafast spin dynamics development. Similarly, spin dynamics in OMHP also have been studied using only model Hamiltonians (e.g., the Rashba Hamiltonian, including SOC and broken inversion symmetry)2,8,17 and lack a complete description of electronic structure and spin−charge−lattice interaction from first principles. Model Hamiltonians may be qualitatively reasonable for pristine OMHP, particularly elucidating the role of SOC and its interaction with circularly polarized light.2,25,63 However, in the presence of defects and surface states in any realistic materials, first-principles calculations are necessary to describe the local structures and chemical bonding accurately. Therefore, along with the ultrafast measurements, a firstprinciples framework of ultrafast spin dynamics needs to be developed and applied to understand the Rashba effect on spin dynamics of OMHP. It will provide enormous physical insights for interpreting spin-dependent pump−probe experiments and understanding the interplay between spin, charge, lattice, and incident photons. Besides the rich intrinsic spin properties in OMHP, interaction between intrinsic and extrinsic spins can provide intriguing spin-dependent dynamics, as illustrated in Figure 6. The charge carrier lifetime is expected to be strongly dependent on the nature of the spin, where a spin-conserved relaxation from the conduction band edge to the dopant or surface radical ligand-induced gap state is a fast process and a spin-forbidden relaxation is a slow process (Figure 6a), which can be probed experimentally using circularly polarized light

Figure 5. Illustration of an optical pump/X-ray probe setup with controlled polarizations (e.g., σ+ for pump and linear for probe). The electron is initially created in a spin-up state (mj = +1/2) while the hole is in a spin-down state (mj = −1/2).

Besides experimental studies, there have been significant efforts on theoretical and computational studies of OMHP, with a focus on electronic structure, optical excitations, defects and polaron formation, and charge carrier dynamics.34−40 As one of the most promising photovoltaic materials, the long carrier lifetime and diffusion length have made OMHP extremely attractive. The recent work shows the long carrier recombination lifetime is directly related to its spin-forbidden transition at the band edges.20 This provides important insights and qualitative explanation of the Rashba effect on carrier recombination lifetime without an explicit first-principles ultrafast carrier or spin dynamics simulation.

Using first-principles calculations and an effective model Hamiltonian, it was found that the Rashba splitting arising from SOC in locally polarized domains can result in spin-forbidden recombination that has a significantly slower transition rate due to the mismatch of spin and momentum. Modeling of ultrafast charge carrier dynamics and pump− probe measurements from first principles has made significant progress in recent years, mainly through three different approaches: time-dependent DFT (TDDFT) with nonadiabatic molecular dynamics (NAMD),41−45 nonequilibrium Green’s function (NEGF),46,47 and Boltzmann equation coupled with many body perturbation theory (MBPT).48−53 All three methods have their advantages and disadvantages as briefly summarized below. For example, for TDDFT with NAMD, it can include nuclear dynamics and nonlinear optical response with the real-time propagation. However, it is fundamentally a mean-field theory, lacks dynamical and nonlocal correlations, and has been mainly applied to molecule systems.41,42 For NEGF, in principle it accurately describes electron−electron scatterings at nonequilibrium carrier dis6107



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Figure 7. Illustration of possible formation of magnetic polarons in doped OMHP due to interactions between intrinsic defect, magnetic dopants, and lattice (top). Left: schematic band structure of OMHP [valence band, Pb(6s) I(5p); conduction band, Pb(6p) with 3d impurities and a spin split donor impurity band]. Right: Representation of magnetic polarons (big circles). A donor electron in its hydrogenic orbit couples with its spin antiparallel to impurities with a 3d shell that is half-full or more than half-full. Cation sites are represented by small circles. Oxygen is not shown; the unoccupied oxygen sites are represented by squares. Exciton magnetic polarons form because of spin, exciton, and LO phonon interactions (bottom). Adapted with permission from refs 9 and 74. Copyright 1996 and 2017 IOPScience.

Figure 6. (a) Spin dynamics for OMHP in the presence of magnetic dopants and surface ligands, where a spin-conserved relaxation from the conduction band edge to the dopant or surface radical ligand induced gap state is a fast process and a spin-forbidden relaxation is a slow process; (b) ultrafast spin dynamics with circularly polarized light, where σ+ and σ− denote two spin polarization directions.

with σ+ and σ− spin polarization directions (Figure 6b), as explained earlier. Furthermore, similar to diluted magnetic semiconductors (DMSs), OMHP have large electron−phonon interactions, with large polarons often forming.64−66 A polaron by definition is a quasiparticle of self-trapped electrons accompanied by lattice distortions as a whole. Depending on the size of this quasiparticle, one can define small polarons or large polarons.67 In the meantime, most intrinsic defects are shallow impurities with low formation energies,40,68−73 both of which were considered as important factors for the formation of magnetic polarons in DMSs. One intriguing possibility for doped OMHP with magnetic impurities is illustrated in Figure 7 (top), where the magnetic polaron formation induces large magnetic polarization (denoted by the large blue circle in Figure 7, top right) in the neighbors of magnetic dopants (denoted by arrows in Figure 7, top right) that are observed in lightly doped ZnO10 because of the interaction among intrinsic defects (oxygen vacancy, denoted by an empty square), magnetic dopants, and lattice vibrations. This could lead to emergent magnetic properties, such as room-temperature ferromagnetism. In addition, a collective exciton magnetic polaron (EMP), which is a coupling composite of free exciton, coupled spins, and LO phonon in polar semiconductors, may form in Mn2+-doped CH3NH3PbBr3 microrods,74 as illustrated in Figure 7 (bottom), which is very different from the standard view that in OMHP electrons and holes are loosely bounded and dissociate rapidly at room temperature. In summary, OMHP exhibit intriguing spin-optotronic properties that are potentially useful for a variety of new technological applications, including PV solar cells, LEDs, lasers, transistors, and sensors. Fundamental understanding of these properties is still at the early stages and requires further research, especially in terms of the spin-dependent carrier dynamics and spin relaxation. It is both interesting and

important to control the spin-states and properties of charge carriers in them to achieve desired functionality for specific applications. Both experimental and theoretical/computational efforts are necessary to gain deep understanding at the atomic level and on ultrafast time scales. Importantly, the interplay among spin−orbit interaction, ferroelectricity, spin−lattice interaction, and magnetic coupling in the presence of magnetic dopants in OMHP can create enormous emergent phenomena, which may open new research directions that extend to other materials, where first-principles calculations could provide extremely valuable insights and guidance.75−77 In particular, methodology development on first-principles ultrafast spin dynamics including electron−electron, electron−phonon, and electron−hole interactions at nonequilibrium carrier distributions for realistic materials is urgently needed in order to reliably interpret and guide experimental studies. Technological applications need to be further explored as well, including fields of quantum computation, information storage, sensing, and energy.

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. ORCID

Yuan Ping: 0000-0002-0123-3389 Jin Zhong Zhang: 0000-0003-3437-912X 6108



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Notes

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The authors declare no competing financial interest. Biographies Yuan Ping received her B.Sc. degree in Chemical Physics from University of Science and Technology of China, China, in 2007 and her Ph.D. in physical chemistry from University of California, Davis in 2013. She was a materials postdoctoral fellow in the DOE energy hubJoint Center for Artificial Photosynthesis at California Institute of Technology from 2013 to 2016. In 2016, she joined the faculty at UC Santa Cruz as an assistant professor. Ping’s recent research interests focus on first-principles methodology development on excited-state dynamics for solids, in particular, from many-body perturbation theory with improved numerical efficiency and accuracy, and charged defect properties as color centers in low-dimensional systems and dopants’ effect on carrier transport in polaronic oxides. Ping has authored over 20 peer-reviewed publications, and she is a recipient of a Hellman Fellowship 2018. Jin Zhong Zhang received his B.Sc. degree in Chemistry from Fudan University, Shanghai, China, in 1983 and his Ph.D. in physical chemistry from University of Washington, Seattle in 1989. He was a postdoctoral research fellow at University of California Berkeley from 1989 to 1992. In 1992, he joined the faculty at UC Santa Cruz, where he is currently full professor of chemistry and biochemistry. Zhang’s recent research interests focus on design, synthesis, characterization, and applications of advanced materials, including semiconductor, metal, and metal oxide nanomaterials, particularly in the areas of solar energy conversion, solid-state lighting, sensing, and biomedical detection/therapy. He has authored over 320 publications and three books. Zhang has been serving as a senior editor for JPCL published by ACS since 2004. He is a Fellow of AAAS, APS, and ACS. He is the recipient of the 2014 Richard A. Glenn Award of the ACS Energy and Fuel Division.

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ACKNOWLEDGMENTS We acknowledge financial support from NASA through MACES (NNX15AQ01A) and UCSC Committee on Research Special Research Grant. Y.P. acknowledges financial support from NSF DMR-1760260 and Hellman Fellowship. We are grateful to Ravishankar Sundararaman for helpful discussions.

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