590 research outputs found
Comparing quasiparticle HO level alignment on anatase and rutile TiO
Knowledge of the molecular frontier levels' alignment in the ground state can
be used to predict the photocatalytic activity of an interface. The position of
the adsorbate's highest occupied molecular orbital (HOMO) levels relative to
the substrate's valence band maximum (VBM) in the interface describes the
favorability of photogenerated hole transfer from the VBM to the adsorbed
molecule. This is a key quantity for assessing and comparing HO
photooxidation activities on two prototypical photocatalytic TiO surfaces:
anatase (A)-TiO(101) and rutile (R)-TiO(110). Using the projected
density of states (DOS) from state-of-the-art quasiparticle (QP)
calculations, we assess the relative photocatalytic activity of intact and
dissociated HO on coordinately unsaturated (Ti) sites of
idealized stoichiometric A-TiO(101)/R-TiO(110) and bridging O vacancies
(O) of defective
A-TiO(101)/R-TiO(110) surfaces ()
for various coverages. Such a many-body treatment is necessary to correctly
describe the anisotropic screening of electron-electron interactions at a
photocatalytic interface, and hence obtain accurate interfacial level
alignments. The more favorable ground state HOMO level alignment for
A-TiO(101) may explain why the anatase polymorph shows higher
photocatalytic activities than the rutile polymorph. Our results indicate that
(1) hole trapping is more favored on A-TiO(101) than R-TiO(110) and (2)
HO@Ti is more photocatalytically active than intact
HO@Ti
Level alignment of a prototypical photocatalytic system: Methanol on TiO2(110)
Photocatalytic and photovoltaic activity depends on the optimal alignment of
electronic levels at the molecule/semiconductor interface. Establishing level
alignment experimentally is complicated by the uncertain chemical identity of
the surface species. We address the assignment of the occupied and empty
electronic levels for the prototypical photocatalytic system of methanol on a
rutile TiO2 (110) surface. Using many-body quasiparticle (QP) techniques we
show that the frontier levels measured in ultraviolet photoelectron and two
photon photoemission spectroscopy experiments can be assigned with confidence
to the molecularly chemisorbed methanol, rather than its decomposition product,
the methoxy species. We find the highest occupied molecular orbital (HOMO) of
the methoxy species is much closer to the valence band maximum, suggesting why
it is more photocatalytically active than the methanol molecule. We develop a
general semi-quantitative model for predicting many-body QP energies based on
the appropriate description of electronic screening within the bulk, molecular
or vacuum regions of the wavefunctions at molecule/semiconductor interfaces.Comment: 5 pages, 5 figure
Dynamics of Excited Electrons in Copper: Role of Auger Electrons
Within a theoretical model based on the Boltzmann equation, we analyze in
detail the structure of the unusual peak recently observed in the relaxation
time in Cu. In particular, we discuss the role of Auger electrons in the
electron dynamics and its dependence on the d-hole lifetime, the optical
transition matrix elements and the laser pulse duration. We find that the Auger
contribution to the distribution is very sensitive to both the d-hole lifetime
tau_h and the laser pulse duration tau_l and can be expressed as a monotonic
function of tau_l/tau_h. We have found that for a given tau_h, the Auger
contribution is significantly smaller for a short pulse duration than for a
longer one. We show that the relaxation time at the peak depends linearly on
the d-hole lifetime, but interestingly not on the amount of Auger electrons
generated. We provide a simple expression for the relaxation time of excited
electrons which shows that its shape can be understood by a phase space
argument and its amplitude is governed by the d-hole lifetime. We also find
that the height of the peak depends on both the ratio of the optical transition
matrix elements R=|M_{d \to sp}|^2/|M_{sp \to sp}|^2 and the laser pulse
duration. Assuming a reasonable value for the ratio, namely R = 2, and a d-hole
lifetime of tau_h=35 fs, we obtain for the calculated height of the peak Delta
tau_{th}=14 fs, in fair agreement with Delta tau_{exp} \approx 17 fs measured
for polycrystalline Cu.Comment: 6 pages, 6 figure
Quasiparticle level alignment for photocatalytic interfaces
arXiv:1404.5166v1Electronic level alignment at the interface between an adsorbed molecular layer and a semiconducting substrate determines the activity and efficiency of many photocatalytic materials. Standard density functional theory (DFT)-based methods have proven unable to provide a quantitative description of this level alignment. This requires a proper treatment of the anisotropic screening, necessitating the use of quasiparticle (QP) techniques. However, the computational complexity of QP algorithms has meant a quantitative description of interfacial levels has remained elusive. We provide a systematic study of a prototypical interface, bare and methanol-covered rutile TiO2(110) surfaces, to determine the type of many-body theory required to obtain an accurate description of the level alignment. This is accomplished via a direct comparison with metastable impact electron spectroscopy (MIES), ultraviolet photoelectron spectroscopy (UPS), and two-photon photoemission (2PP) spectroscopy. We consider GGA DFT, hybrid DFT, and G0W0, scQPGW1, scQPGW0, and scQPGW QP calculations. Our results demonstrate that G0W0, or our recently introduced scQPGW1 approach, are required to obtain the correct alignment of both the highest occupied and lowest unoccupied interfacial molecular levels (HOMO/LUMO). These calculations set a new standard in the interpretation of electronic structure probe experiments of complex organic molecule/semiconductor interfaces.We acknowledge funding from the European Projects DYNamo (No. ERC-2010-
AdG-267374), and CRONOS (No. 280879-2 CRONOS CPFP7); Spanish Grants (Nos. FIS2012-37549-C05-02, FIS2010-21282-C02-01, PIB2010US-00652, RYC-2011-09582, JAE DOC, JCI-2010-08156); Grupos Consolidados UPV/EHU del Gobierno Vasco (No. IT-319-07); NSFC (Nos. 21003113 and 21121003); MOST (No. 2011CB921404); and NSF Grant No. CHE-1213189.Peer Reviewe
Lifetime of d-holes at Cu surfaces: Theory and experiment
We have investigated the hole dynamics at copper surfaces by high-resolution
angle-resolved photoemission experiments and many-body quasiparticle GW
calculations. Large deviations from a free-electron-like picture are observed
both in the magnitude and the energy dependence of the lifetimes, with a clear
indication that holes exhibit longer lifetimes than electrons with the same
excitation energy. Our calculations show that the small overlap of d- and
sp-states below the Fermi level is responsible for the observed enhancement.
Although there is qualitative good agreement of our theoretical predictions and
the measured lifetimes, there still exist some discrepancies pointing to the
need of a better description of the actual band structure of the solid.Comment: 15 pages, 7 figures, 1 table, to appear in Phys. Rev.
Leaf physiological and morphological constraints of water-use efficiency in C plants
The increasing evaporative demand due to climate change will significantly affect the balance of carbon assimilation and water losses of plants worldwide. The development of crop varieties with improved water-use efficiency (WUE) will be critical for adapting agricultural strategies under predicted future climates. This review aims to summarize the most important leaf morpho-physiological constraints of WUE in C3 plants and identify gaps in knowledge. From the carbon gain side of the WUE, the discussed parameters are mesophyll conductance, carboxylation efficiency and respiratory losses. The traits and parameters affecting the waterside of WUE balance discussed in this review are stomatal size and density, stomatal control and residual water losses (cuticular and bark conductance), nocturnal conductance and leaf hydraulic conductance. In addition, we discussed the impact of leaf anatomy and crown architecture on both the carbon gain and water loss components of WUE. There are multiple possible targets for future development in understanding sources of WUE variability in plants. We identified residual water losses and respiratory carbon losses as the greatest knowledge gaps of whole-plant WUE assessments. Moreover, the impact of trichomes, leaf hydraulic conductance and canopy structure on plants’ WUE is still not well understood. The development of a multi-trait approach is urgently needed for a better understanding of WUE dynamics and optimization
Dynamics of Excited Electrons in Copper and Ferromagnetic Transition Metals: Theory and Experiment
Both theoretical and experimental results for the dynamics of photoexcited
electrons at surfaces of Cu and the ferromagnetic transition metals Fe, Co, and
Ni are presented. A model for the dynamics of excited electrons is developed,
which is based on the Boltzmann equation and includes effects of
photoexcitation, electron-electron scattering, secondary electrons (cascade and
Auger electrons), and transport of excited carriers out of the detection
region. From this we determine the time-resolved two-photon photoemission
(TR-2PPE). Thus a direct comparison of calculated relaxation times with
experimental results by means of TR-2PPE becomes possible. The comparison
indicates that the magnitudes of the spin-averaged relaxation time \tau and of
the ratio \tau_\uparrow/\tau_\downarrow of majority and minority relaxation
times for the different ferromagnetic transition metals result not only from
density-of-states effects, but also from different Coulomb matrix elements M.
Taking M_Fe > M_Cu > M_Ni = M_Co we get reasonable agreement with experiments.Comment: 23 pages, 11 figures, added a figure and an appendix, updated
reference
Hole dynamics in noble metals
We present a detailed analysis of hole dynamics in noble metals (Cu and Au),
by means of first-principles many-body calculations. While holes in a
free-electron gas are known to live shorter than electrons with the same
excitation energy, our results indicate that d-holes in noble metals exhibit
longer inelastic lifetimes than excited sp-electrons, in agreement with
experiment. The density of states available for d-hole decay is larger than
that for the decay of excited electrons; however, the small overlap between d-
and sp-states below the Fermi level increases the d-hole lifetime. The impact
of d-hole dynamics on electron-hole correlation effects, which are of relevance
in the analysis of time-resolved two-photon photoemission experiments, is also
addressed.Comment: 4 pages, 2 figures, to appear in Phys. Rev. Let
Ultrafast Spin Dynamics in Nickel
The spin dynamics in Ni is studied by an exact diagonalization method on the
ultrafast time scale. It is shown that the femtosecond relaxation of the
magneto-optical response results from exchange interaction and spin-orbit
coupling. Each of the two mechanisms affects the relaxation process
differently. We find that the intrinsic spin dynamics occurs during about 10 fs
while extrinsic effects such as laser-pulse duration and spectral width can
slow down the observed dynamics considerably. Thus, our theory indicates that
there is still room to accelerate the spin dynamics in experiments.Comment: 4 pages, Latex, 4 postscript figure
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