36 research outputs found
Ultrafast generation and decay of a surface metal
Band bending at semiconductor surfaces induced by chemical doping or electric
fields can create metallic surfaces with properties not found in the bulk, such
as high electron mobility, magnetism or superconductivity. Optical generation
of such metallic surfaces via BB on ultrafast timescales would facilitate a
drastic manipulation of the conduction, magnetic and optical properties of
semiconductors for high-speed electronics. Here, we demonstrate the ultrafast
generation of a metal at the (10-10) surface of ZnO upon photoexcitation.
Compared to hitherto known ultrafast photoinduced semiconductor-to-metal
transitions that occur in the bulk of inorganic semiconductors, the
metallization of the ZnO surface is launched by 3-4 orders of magnitude lower
photon fluxes. Using time- and angle-resolved photoelectron spectroscopy, we
show that the phase transition is caused by photoinduced downward surface band
bending due to photodepletion of donor-type deep surface defects. At low photon
flux, surface-confined excitons are formed. Above a critical exciton density, a
Mott transition occurs, leading to a partially filled metallic band below the
equilibrium Fermi energy. This process is in analogy to chemical doping of
semiconductor surfaces. The discovered mechanism is not material-specific and
presents a general route for controlling metallicity confined to semiconductor
interfaces on ultrafast timescales
Localization-dependent charge separation efficiency at an organic/inorganic hybrid interface
By combining complementary optical techniques, photoluminescence and
time-resolved excited state absorption, we achieve a comprehensive picture of
the relaxation processes in the organic/inorganic hybrid system SP6/ZnO. We
identify two long-lived excited states of the organic molecules of which only
the lowest energy one, localized on the sexiphenyl backbone of the molecule, is
found to efficiently charge separate to the ZnO conduction band or radiatively
recombine. The other state, most likely localized on the spiro-linked biphenyl,
relaxes only by intersystem crossing to a long-lived, probably triplet state,
thus acting as a sink of the excitation and limiting the charge separation
efficiency.Comment: 6 pages, 5 figure
Revealing the Competing Contributions of Charge Carriers, Excitons, and Defects to the Non-Equilibrium Optical Properties of ZnO
Due to its wide band gap and high carrier mobility, ZnO is an attractive
material for light-harvesting and optoelectronic applications. Its functional
efficiency, however, is strongly affected by defect-related in-gap states that
open up extrinsic decay channels and modify relaxation timescales. As a
consequence, almost every ZnO sample behaves differently, leading to
irreproducible or even contradicting observations. Here, a complementary set of
time-resolved spectroscopies is applied to two ZnO samples of different defect
density to disentangle the competing contributions of charge carriers,
excitons, and defects to the non-equilibrium dynamics after photoexcitation:
Time-resolved photoluminescence, excited state transmission, and electronic sum
frequency generation. Remarkably, defects affect the transient optical
properties of ZnO across more than eight orders of magnitude in time, starting
with photodepletion of normally occupied defect states on femtosecond
timescales, followed by the competition of free exciton emission and exciton
trapping at defect sites within picoseconds, photoluminescence of defect-bound
and free excitons on nanosecond timescales, and deeply trapped holes with
microsecond lifetimes. These findings do not only provide the first
comprehensive picture of charge and exciton relaxation pathways in ZnO, but
also uncover the microscopic origin of previous conflicting observations in
this challenging material and thereby offer means of overcoming its
difficulties
Dynamic screening of quasiparticles in WS monolayers
We unravel the influence of quasiparticle screening in the non-equilibrium
exciton dynamics of monolayer WS. We report pump photon energy-dependent
exciton blue/red-shifts from time-resolved-reflectance contrast measurements.
Based on a phenomenological model, we isolate the effective impact of excitons
and free carriers on the renormalization of the quasi-free particle band gap,
exciton binding energy and linewidth broadening. This work provides a
comprehensive picture of the competing phenomena governing the exciton dynamics
in WS upon photoexcitation
Inhibition of the photoinduced structural phase transition in the excitonic insulator TaNiSe
Femtosecond time-resolved mid-infrared reflectivity is used to investigate
the electron and phonon dynamics occurring at the direct band gap of the
excitonic insulator TaNiSe below the critical temperature of its
structural phase transition. We find that the phonon dynamics show a strong
coupling to the excitation of free carriers at the \Gamma\ point of the
Brillouin zone. The optical response saturates at a critical excitation fluence
~mJ/cm due to optical absorption saturation. This
limits the optical excitation density in TaNiSe so that the system
cannot be pumped sufficiently strongly to undergo the structural change to the
high-temperature phase. We thereby demonstrate that TaNiSe exhibits a
blocking mechanism when pumped in the near-infrared regime, preventing a
nonthermal structural phase transition
Ultrafast Electronic Band Gap Control in an Excitonic Insulator
We report on the nonequilibrium dynamics of the electronic structure of the
layered semiconductor TaNiSe investigated by time- and angle-resolved
photoelectron spectroscopy. We show that below the critical excitation density
of mJ cm, the band gap transiently, while it is
above . Hartree-Fock calculations reveal that this effect can
be explained by the presence of the low-temperature excitonic insulator phase
of TaNiSe, whose order parameter is connected to the gap size. This
work demonstrates the ability to manipulate the band gap of TaNiSe with
light on the femtosecond time scale
Instantaneous band gap collapse in photoexcited monoclinic VO due to photocarrier doping
Using femtosecond time-resolved photoelectron spectroscopy we demonstrate
that photoexcitation transforms monoclinic VO quasi-instantaneously into a
metal. Thereby, we exclude an 80 femtosecond structural bottleneck for the
photoinduced electronic phase transition of VO. First-principles many-body
perturbation theory calculations reveal a high sensitivity of the VO
bandgap to variations of the dynamically screened Coulomb interaction,
supporting a fully electronically driven isostructral insulator-to-metal
transition. We thus conclude that the ultrafast band structure renormalization
is caused by photoexcitation of carriers from localized V 3d valence states,
strongly changing the screening \emph{before} significant hot-carrier
relaxation or ionic motion has occurred
How hybrid excitons suppress charge separation: ultrafast, but delayed
Inorganic/organic hybrid systems offer great technological potential for
novel solar cell design due to the combination of high charge carrier
mobilities in the inorganic semiconductor with the chemical tuneability of
organic chromophore absorption properties. While ZnO basically exhibits all
necessary properties for a successful application in light-harvesting, it was
clearly outpaced by TiO in terms of charge separation efficiency. The
physical origin of this deficiency is still under debate. Here, we use a
combination of femtosecond time-resolved photoelectron spectroscopy with
many-body ab initio calculations to demonstrate that optical excitation of the
chromophore is followed by (1) ultrafast electron transfer into the ZnO bulk
(350 fs), (2) electron relaxation, and (3) delayed (100 ps) recapture of the
electrons at a 1 nm distance from the interface in (4) a strongly bound (0.7
eV) hybrid exciton state with a lifetime exceeding 5 s that is analysed by
taking into account pump-probe delay-dependent photostationary population
dynamics. Beyond this identification and quantification of all elementary steps
leading to the suppression of charge separation at ZnO interfaces, our key
finding is the substantially delayed hybrid exciton formation. It opens up a
sufficiently large time window for counter-measures with the potential to
finally successfully implement ZnO in light-harvesting or optoelectronic
devices without significant efficiency losses.Comment: main: 11 pages, 3 figures supporting: 6 pages, 3 figure
Inhibition of the photoinduced structural phase transition in the excitonic insulator
Femtosecond time-resolved midinfrared reflectivity is used to investigate the electron and phonon dynamics occurring at the direct band gap of the excitonic insulator Ta2NiSe5 below the critical temperature of its structural phase transition. We find that the phonon dynamics show a strong coupling to the excitation of free carriers at the Γ point of the Brillouin zone. The optical response saturates at a critical excitation fluence FC=0.30±0.08 mJ/cm2 due to optical absorption saturation. This limits the optical excitation density in Ta2NiSe5 so that the system cannot be pumped sufficiently strongly to undergo the structural change to the high-temperature phase. We thereby demonstrate that Ta2NiSe5 exhibits a blocking mechanism when pumped in the near-infrared regime, preventing a nonthermal structural phase transition