Wendy L. Mao

Stanford University, Palo Alto, California, United States

Are you Wendy L. Mao?

Claim your profile

Publications (160)832.71 Total impact

  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: Layered transition-metal dichalcogenides have emerged as exciting material systems with atomically thin geometries and unique electronic properties. Pressure is a powerful tool for continuously tuning their crystal and electronic structures away from the pristine states. Here, we systematically investigated the pressurized behavior of MoSe2 up to ∼60 GPa using multiple experimental techniques and ab-initio calculations. MoSe2 evolves from an anisotropic two-dimensional layered network to a three-dimensional structure without a structural transition, which is a complete contrast to MoS2. The role of the chalcogenide anions in stabilizing different layered patterns is underscored by our layer sliding calculations. MoSe2 possesses highly tunable transport properties under pressure, determined by the gradual narrowing of its band-gap followed by metallization. The continuous tuning of its electronic structure and band-gap in the range of visible light to infrared suggest possible energy-variable optoelectronics applications in pressurized transition-metal dichalcogenides.
  • Journal of Geophysical Research: Solid Earth 05/2015; DOI:10.1002/2015JB011901 · 3.44 Impact Factor
  • Source
    Physical Review B 05/2015; 91(18). DOI:10.1103/PhysRevB.91.184112 · 3.66 Impact Factor
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: Layered transition-metal dichalcogenides have emerged as exciting material systems with atomically thin geometries and unique electronic properties. Pressure is a powerful tool for continuously tuning their crystal and electronic structures away from the pristine states. Here, we systematically investigated the pressurized behavior of MoSe2 up to ~ 60 GPa using multiple experimental techniques and ab -initio calculations. MoSe2 evolves from an anisotropic two-dimensional layered network to a three-dimensional structure without a structural transition, which is a complete contrast to MoS2. The role of the chalcogenide anions in stabilizing different layered patterns is underscored by our layer sliding calculations. MoSe2 possesses highly tunable transport properties under pressure, determined by the gradual narrowing of its band-gap followed by metallization. The continuous tuning of its electronic structure and band-gap in the range of visible light to infrared suggest possible energy-variable optoelectronics applications in pressurized transition-metal dichalcogenides.
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: Li15Si4, the only crystalline phase that forms during lithiation of the Si anode in lithium-ion batteries, is found to undergo a structural transition to a new phase at 7 GPa. Despite the large unit cell of Li15Si4 (152 atoms in the unit cell), ab initio evolutionary metadynamics (using the USPEX code) successfully predicts the atomic structure of this new phase (β-Li15Si4), which has an orthorhombic structure with an Fdd2 space group. In the new β-Li15Si4 phase Si atoms are isolated by Li atoms analogous to the original cubic phase (α-Li15Si4), whereas the atomic packing is more efficient owing to the higher SiLi coordination number and shorter SiLi, LiLi bonds. β-Li15Si4 has substantially larger elastic moduli compared with α-Li15Si4, and has a good electrical conductivity. As a result, β-Li15Si4 has superior resistance to deformation and fracture under stress. The theoretical volume expansion of Si would decrease 25% if it transforms to β-Li15Si4, instead of α-Li15Si4, during lithiation. Moreover, β-Li15Si4 can be recovered back to ambient pressure, providing opportunities to further investigate its properties and potential applications.
    Advanced Energy Materials 04/2015; DOI:10.1002/aenm.201500214 · 14.39 Impact Factor
  • Wendy L Mao
    Nature Material 02/2015; 14(5). DOI:10.1038/nmat4245 · 36.43 Impact Factor
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: Carbonates are the main species that bring carbon deep into our planet through subduction. They are an important rock-forming mineral group, fundamentally distinct from silicates in the Earth’s crust in that carbon binds to three oxygen atoms, while silicon is bonded to four oxygens. Here we present experimental evidence that under the sufficiently high pressures and high temperatures existing in the lower mantle, ferromagnesian carbonates transform to a phase with tetrahedrally coordinated carbons. Above 80 GPa, in situ synchrotron infrared experiments show the unequivocal spectroscopic signature of the high-pressure phase of (Mg,Fe)CO3. Using ab-initio calculations, we assign the new infrared signature to C–O bands associated with tetrahedrally coordinated carbon with asymmetric C–O bonds. Tetrahedrally coordinated carbonates are expected to exhibit substantially different reactivity than low-pressure threefold coordinated carbonates, as well as different chemical properties in the liquid state. Hence, this may have significant implications for carbon reservoirs and fluxes, and the global geodynamic carbon cycle.
    Nature Communications 02/2015; DOI:10.1038/ncomms7311 · 10.74 Impact Factor
  • [Show abstract] [Hide abstract]
    ABSTRACT: Carbonates are the main species that bring carbon deep into our planet through subduction. They are an important rock-forming mineral group, fundamentally distinct from silicates in the Earth’s crust in that carbon binds to three oxygen atoms, while silicon is bonded to four oxygens. Here we present experimental evidence that under the sufficiently high pressures and high temperatures existing in the lower mantle, ferromagnesian carbonates transform to a phase with tetrahedrally coordinated carbons. Above 80 GPa, in situ synchrotron infrared experiments show the unequivocal spectroscopic signature of the high-pressure phase of (Mg,Fe)CO3. Using ab-initio calculations, we assign the new infrared signature to C–O bands associated with tetrahedrally coordinated carbon with asymmetric C–O bonds. Tetrahedrally coordinated carbonates are expected to exhibit substantially different reactivity than low-pressure threefold coordinated carbonates, as well as different chemical properties in the liquid state. Hence, this may have significant implications for carbon reservoirs and fluxes, and the global geodynamic carbon cycle.
    Nature Communications 02/2015; · 10.74 Impact Factor
  • [Show abstract] [Hide abstract]
    ABSTRACT: NaYF4:Yb(3+),Er(3+) nanoparticle upconverters are hindered by low quantum efficiencies arising in large part from the parity-forbidden nature of their optical transitions and the non-optimal spatial separations between lanthanide ions. Here, we use pressure-induced lattice distortion to systematically modify both parameters. While hexagonal-phase nanoparticles exhibit a monotonic decrease in upconversion emission, cubic-phase particles experience a nearly twofold increase in efficiency. In-situ x-ray diffraction indicates that these emission changes require only a 1% reduction in lattice constant. Our work highlights the intricate relationship between upconversion efficiency and lattice geometry and provides a promising approach to modifying the quantum efficiency of any lanthanide upconverter.
    Nano Letters 02/2015; 15(3). DOI:10.1021/nl504738k · 12.94 Impact Factor
  • [Show abstract] [Hide abstract]
    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 d9 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 conductivity 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 CuII-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 CuII-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 bandgap 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.
    Journal of the American Chemical Society 01/2015; 137(4). DOI:10.1021/ja512396m · 11.44 Impact Factor
  • Yu Lin, Wendy L. Mao
    Chinese Science Bulletin 12/2014; 59(36):5235-5240. DOI:10.1007/s11434-014-0624-8 · 1.37 Impact Factor
  • [Show abstract] [Hide abstract]
    ABSTRACT: The high-pressure behavior of diamantane was investigated using angle-dispersive synchrotron x-ray diffraction (XRD) and Raman spectroscopy in diamond anvil cells. Our experiments revealed that the structural transitions in diamantane were extremely sensitive to deviatoric stress. Under non-hydrostatic conditions, diamantane underwent a cubic (space group Pa3) to a monoclinic phase transition at below 0.15 GPa, the lowest pressure we were able to measure. Upon further compression to 3.5 GPa, this monoclinic phase transformed into another high-pressure monoclinic phase which persisted to 32 GPa, the highest pressure studied in our experiments. However, under more hydrostatic conditions using silicone oil as a pressure medium, the transition pressure to the first high-pressure monoclinic phase was elevated to 7-10 GPa, which coincided with the hydrostatic limit of silicone oil. In another experiment using helium as a pressure medium, no phase transitions were observed to the highest pressure we reached (13 GPa). In addition, large hysteresis and sluggish transition kinetics were observed upon decompression. Over the pressure range where phase transitions were confirmed by XRD, only continuous changes in the Raman spectra were observed. This suggests that these phase transitions are associated with unit cell distortions and modifications in molecular packing rather than the formation of new carbon-carbon bonds under pressure.
    The Journal of Chemical Physics 10/2014; 141(15):154305. DOI:10.1063/1.4897252 · 3.12 Impact Factor
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: Mn$_3$O$_4$ is a spin frustrated magnet that adopts a tetragonally distorted spinel structure at ambient conditions and a CaMn$_2$O$_4$-type postspinel structure at high pressure. We conducted both optical measurements and \emph{ab} \emph{initio} calculations, and systematically studied the electronic band structures of both the spinel and postspinel Mn$_3$O$_4$ phases. For both phases, theoretical electronic structures are consistent with the optical absorption spectra, and display characteristic band-splitting of the conduction band. The band gap obtained from the absorption spectra is 1.91(6) eV for the spinel phase, and 0.94(2) eV for the postspinel phase. Both phases are charge-transfer type insulators. The Mn 3\emph{d} $t_2$$_g$ and O 2\emph{p} form antibonding orbitals situated at the conduction band with higher energy.
  • [Show abstract] [Hide abstract]
    ABSTRACT: Results of high-pressure infrared (IR) and Raman spectroscopy measurements are presented for the mixed valence compound CsAuI3, where Au adopts Au-I and Au-III valency. Raman spectroscopy shows softening with pressure of the vibration modes in the Au-III-I-4 square planar units in the tetragonal phase, indicating a similar pressure-induced lattice distortion as found for the closely related compounds CsAuCl3 and CsAuBr3. Multiple features in the higher pressure spectra confirm that the high-pressure phase has a lower symmetry than the ambient pressure tetragonal structure, consistent with an orthorhombic structure discovered recently by x-ray diffraction measurements. From IR spectroscopy, we observed rapid bandgap closure at a rate of 0.2 eV/GPa in the tetragonal phase of CsAuI3, close to the tetragonal-orthorhombic phase transition. The IR reflectivity shows a Drude-like behavior implying metallic conductivity. However, as the compound fully transforms to the orthorhombic phase, the bandgap reopens and the Drude behavior in the reflectivity disappears.
    Physical Review B 06/2014; 89(24):245109. DOI:10.1103/PhysRevB.89.245109 · 3.66 Impact Factor
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: The mineralogical constitution of the Earth's mantle dictates the geophysical and geochemical properties of this region. Previous models of a perovskite-dominant lower mantle have been built on the assumption that the entire lower mantle down to the top of the D″ layer contains ferromagnesian silicate [(Mg,Fe)SiO3] with nominally 10 mole percent Fe. On the basis of experiments in laser-heated diamond anvil cells, at pressures of 95 to 101 gigapascals and temperatures of 2200 to 2400 kelvin, we found that such perovskite is unstable; it loses its Fe and disproportionates to a nearly Fe-free MgSiO3 perovskite phase and an Fe-rich phase with a hexagonal structure. This observation has implications for enigmatic seismic features beyond ~2000 kilometers depth and suggests that the lower mantle may contain previously unidentified major phases.
    Science 05/2014; 344(6186):877-82. DOI:10.1126/science.1250274 · 31.48 Impact Factor
  • [Show abstract] [Hide abstract]
    ABSTRACT: As a fundamental property of a material, density is controlled by the interatomic distances and the packing of microscopic constituents. The most prominent atomistic feature in a metallic glass (MG) that can be measured is its principal diffraction peak position (q_{1}) observable by x-ray, electron, or neutron diffraction, which is closely associated with the average interatomic distance in the first shell. Density (and volume) would naturally be expected to vary under compression in proportion to the cube of the one-dimensional interatomic distance. However, by using high pressure as a clean tuning parameter and high-resolution in situ techniques developed specifically for probing the density of amorphous materials, we surprisingly found that the density of a MG varies with the 5/2 power of q_{1}, instead of the expected cubic relationship. Further studies of MGs of different compositions repeatedly produced the same fractional power law of 5/2 in all three MGs we investigated, suggesting a universal feature in MG.
    Physical Review Letters 05/2014; 112(18):185502. DOI:10.1103/PhysRevLett.112.185502 · 7.73 Impact Factor
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: We conducted in situ angle dispersive high pressure x-ray diffraction experiments on Sr3Ir2O7 up to 23.1 GPa at 25 K with neon as the pressure transmitting medium. Pressure induces a highly anisotropic compressional behavior seen where the tetragonal plane is compressed much faster than the perpendicular direction. By analyzing different aspects of the diffraction data, a second-order structural transition is observed at approximately 14 GPa, which is accompanied by the insulating state to nearly metallic state at 13.2 GPa observed previously (Li et al 2013 Phys. Rev. B 87 235127). Our results highlight the coupling between electronic state and lattice structure in Sr3Ir2O7 under pressure.
    Journal of Physics Condensed Matter 05/2014; 26(21):215402. DOI:10.1088/0953-8984/26/21/215402 · 2.22 Impact Factor
  • Journal of Power Sources 05/2014; 253:431. DOI:10.1016/j.jpowsour.2013.12.052 · 5.21 Impact Factor
  • [Show abstract] [Hide abstract]
    ABSTRACT: We investigated the effect of pressure on the structure and bonding of [121] tetramantane up to 20 GPa via in situ angle-dispersive synchrotron powder X-ray diffraction (XRD) and Raman spectroscopy in diamond anvil cells (DACs). A phase transition from the starting monoclinic P21/n structure to a triclinic P1 structure was observed beginning at 13 GPa. Upon decompression, the transition pressure showed large hysteresis. After fully releasing pressure, [121] tetramantane was found to recover into a different polymorph from the starting phase. Continuous changes of the vibration modes associated with the CCC bending and CC stretching regions suggest that this phase transition was induced by the intramolecular level distortions and modifications in the molecular packing. These results provide guidance for understanding the inter- and intramolecular interactions in diamondoids under pressure and shed light on the possibility of using pressure as a tuning parameter for synthesizing higher diamondoids.
    The Journal of Physical Chemistry C 03/2014; 118(14):7683–7689. DOI:10.1021/jp500431k · 4.84 Impact Factor
  • [Show abstract] [Hide abstract]
    ABSTRACT: Phase transitions in indentation induced Si-III/XII phases were investigated using a diamond anvil cell and nanoindentation combined with micro-Raman spectroscopy. The in situ high pressure Raman results demonstrate that the Si-III and Si-XII phases have very similar Raman spectra, indicating their relative amount cannot be determined if they are both present in a sample. The Si-III and Si-XII phases coexist in the indentations produced by a nanoindenter on a single crystalline silicon wafer as a result of the local residual compressive stresses near 1 GPa. High power laser annealing on the indentations can initiate a rapid Si-III/XII → Si-I phase transition. The newly formed polycrystalline Si-I phase initially has very small grain size, and the grains grow when the annealing time is extended. Si-IV phase was not observed in our experiment.
    Journal of Applied Physics 02/2014; 115(10). DOI:10.1063/1.4868156 · 2.19 Impact Factor

Publication Stats

2k Citations
832.71 Total Impact Points

Institutions

  • 2008–2015
    • Stanford University
      • Department of Geological and Environmental Sciences
      Palo Alto, California, United States
  • 2010–2011
    • Zhejiang University
      • Department of Material Science and Engineering
      Hangzhou, Zhejiang Sheng, China
  • 2007
    • Los Alamos Medical Center
      Los Alamos, New Mexico, United States
    • Carnegie Institute
      Washington, Washington, D.C., United States
  • 2006
    • James Cook University
      Townsville, Queensland, Australia
    • Los Alamos National Laboratory
      • Los Alamos Neutron Science Center
      Los Alamos, CA, United States
  • 2002–2005
    • University of Chicago
      • Department of Geophysical Sciences
      Chicago, Illinois, United States
  • 2004
    • University of Illinois at Chicago
      Chicago, Illinois, United States
  • 2002–2004
    • Carnegie Institution for Science
      • Geophysical Laboratory
      Washington, West Virginia, United States