ARTICLE
Femtosecond time-resolved two-photon
photoemission studies of ultrafast carrier relaxation
in Cu
2
O photoelectrodes
Mario Borgwardt
1
, Stefan T. Omelchenko
2,3
, Marco Favaro
1
, Paul Plate
1
, Christian Höhn
1
, Daniel Abou-Ras
4
,
Klaus Schwarzburg
4
, Roel van de Krol
1
, Harry A. Atwater
2,3,5
, Nathan S. Lewis
3,5,6
, Rainer Eichberger
1
&
Dennis Friedrich
1
Cuprous oxide (Cu
2
O) is a promising material for solar-driven water splitting to produce
hydrogen. However, the relatively small accessible photovoltage limits the development of
ef
fi
cient Cu
2
O based photocathodes. Here, femtosecond time-resolved two-photon photo-
emission spectroscopy has been used to probe the electronic structure and dynamics of
photoexcited charge carriers at the Cu
2
O surface as well as the interface between Cu
2
O and
a platinum (Pt) adlayer. By referencing ultrafast energy-resolved surface sensitive spectro-
scopy to bulk data we identify the full bulk to surface transport dynamics for excited electrons
rapidly localized within an intrinsic deep continuous defect band ranging from the whole
crystal volume to the surface. No evidence of bulk electrons reaching the surface at the
conduction band level is found resulting into a substantial loss of their energy through
ultrafast trapping. Our results uncover main factors limiting the energy conversion processes
in Cu
2
O and provide guidance for future material development.
https://doi.org/10.1038/s41467-019-10143-x
OPEN
1
Institute for Solar Fuels, Helmholtz-Zentrum Berlin für Materialien und Energie GmbH, Hahn-Meitner-Platz 1, 14109 Berlin, Germany.
2
Division of
Engineering and Applied Sciences, California Institute of Technology, Pasadena, CA 91125, USA.
3
The Joint Center for Arti
fi
cial Photosynthesis, California
Institute of Technology, Pasadena, CA 91125, USA.
4
Department Nanoscale Structures and Microscopic Analysis, Helmholtz-Zentrum Berlin für Materialien
und Energie GmbH, Hahn-Meitner-Platz 1, 14109 Berlin, Germany.
5
Kavli Nanoscience Institute, California Institute of Technology, Pasadena, CA 91125, USA.
6
Division of Chemistry and Chemical Engineering, California Institute of Technology, Pasadena, CA 91125, USA. Correspondence and requests for mate
rials
should be addressed to R.E. (email:
eichberger@helmholtz-berlin.de
) or to D.F. (email:
friedrich@helmholtz-berlin.de
)
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1
1234567890():,;
P
hotoelectrochemical (PEC) splitting of water into hydrogen
and oxygen is a renewable method of hydrogen production
that combines solar energy collection and water electrolysis
into a single process
1
. In this process, sunlight absorbed by a
semiconductor generates photoexcited charge carriers (electrons
and holes), which drive the water-splitting (reduction or oxida-
tion) reactions on the surface of the photoelectrode
2
. Solar-to-
hydrogen ef
fi
ciencies exceeding > 20% have been realized by use
of high-quality III
–
V semiconductor photoelectrodes (e.g., GaAs,
GaInP
2
, etc.)
3
–
6
, but the lack of long-term stability in aqueous
electrolytes, as well as the materials scarcity and manufacturing
cost of these photoelectrodes remain barriers to practical imple-
mentation of such systems. Metal oxide semiconductors provide
potentially cheap and abundant candidate photoelectrodes
7
–
9
.
Speci
fi
cally, cuprous oxide (Cu
2
O), a p-type semiconductor with
a bandgap of
E
g
=
1.9
–
2.2 eV, has a maximum theoretical water-
splitting ef
fi
ciency of ~18% under air mass (AM) 1.5 illumina-
tion
10
–
13
.Cu
2
O photocathodes have shown high photocurrent
densities (~8 mA/cm
2
, which could potentially lead to ef
fi
ciencies
of ~10%) but exhibit lower-than-optimal photovoltages. Buffer
layers, such as ZnO and Ga
2
O
3
, increase the photovoltage by
tuning the junction potential and optimizing the conduction-
band alignment at the Cu
2
O/buffer layer interface
14
–
16
.To
enhance the kinetics of the desired surface electrochemical reac-
tions, such as hydrogen production or CO
2
reduction, noble
metals such as Pt are usually deposited as co-catalysts either on
the buffer layer or directly onto the surface of the
photoelectrodes
11
,
17
–
19
.
Although substantial progress has been made in optimizing
Cu
2
O photocathodes by using a p
–
n heterojunction strategy
together with a buffer layer, bene
fi
cial advances in the funda-
mental understanding of the underlying processes would include
elucidation of the carrier dynamics, charge separation and
recombination in the bulk material, as well as across junctions,
and additional information on the mechanisms by which these
processes limit the overall energy conversion
20
,
21
. Accordingly,
we describe herein the use of time-resolved two-photon photo-
emission spectroscopy (tr-2PPE) to investigate under ultrahigh
vacuum (UHV) conditions the electron dynamics and energetics
at the surface of Cu
2
O, as well as at the interface of Cu
2
O with
thin Pt adlayers. The method allows investigation of the energetic
positions of occupied and unoccupied states, as well as the tem-
poral evolution of transiently populated states.
2PPE spectroscopy has been previously successfully applied to
study electron dynamics at surfaces and charge transfer processes
across interfaces of metals and semiconductor model systems
22
–
24
.
Only recently has its capabilities been utilized to study processes in
emerging photovoltaic material systems, such as hot electron
relaxation dynamics in hybrid metal
–
organic perovskite semi-
conductors
25
–
27
. We have now extended these efforts to the group
of metal oxide semiconductors by including Cu
2
O
—
one of the most
promising metal oxide candidates for solar water splitting
28
.
The dynamics of surface-trapped photogenerated electrons, as
well as diffusion of photogenerated electrons from the bulk
toward the surface, have been evaluated by comparing the spec-
tral and dynamic signatures of reconstructed Cu
2
O (100) single
crystals before and after deposition of ultrathin adlayers of Pt.
The results indicate that photoexcited electrons in Cu
2
O lose a
substantial fraction of their energy through ultrafast trapping in
bulk defect states before arriving at the surface. The data also
suggest that the Pt adlayer suppresses the slow accumulation of
electrons into the surface defect states and that the Pt is capable of
mediating charge transfer at the semiconductor/metal interface.
Hence, our
fi
ndings imply that the modest photovoltages that can
be obtained from these Cu
2
O samples primarily result from losses
derived from bulk recombination processes into bulk defect
states.
Results
Time-resolved two-photon photoemission spectroscopy
. Time-
resolved 2PPE is a pump
–
probe technique in which one of the
beams is guided through an optical delay stage of variable length
to generate an adjustable time delay between two femtosecond
laser pulses (Fig.
1
a). The laser pulse arriving
fi
rst at the sample is
used to photoexcite carriers to intermediate states (pump pulse),
and the second pulse (probe pulse) promotes the photoexcited
electrons to a
fi
nal state above the vacuum level. The kinetic
energy of these electrons provides direct information on the
energy of the intermediate, normally unoccupied, states at the
surface (Fig.
1
b). The temporal evolution of the photoexcited
electron distribution can moreover be evaluated by recording
spectra as a function of the pump
–
probe delay. In the experi-
mental con
fi
guration used herein, the temporal resolution was
about ~35 fs (see Supplementary Fig. 1 for details). Arti
fi
cially
synthesized Cu
2
O crystals were chosen, and an extensive surface
reconstruction procedure was employed that ensures high
reproducibility accounting for the surface sensitivity of the tr-
2PPE technique. Investigations conducted at crystals without any
surface treatment (as-received) yielded qualitatively similar
results and are not shown.
The tr-2PPE study was performed with a pump pulse (494 nm,
24 μJ/cm
2
) that induced a resonant excitation above the bandgap
of Cu
2
O. The electron population distribution among the
transient states was probed by a subsequent laser pulse with a
wavelength of 274 nm. The transient signal (TS) was derived by
subtracting the photoelectron spectra recorded at negative delay
times, representing a background spectrum, from the
pump
–
probe spectra.
Electron dynamics and electronic structure
. The color maps of
Figs
2
a, c show the two-dimensional dependence of the TS on the
electron kinetic energy and on the pump
–
probe delay, for the
reconstructed and Pt-covered Cu
2
O samples, respectively.
Representative spectra at selected pump
–
probe delays up to 1 ns
are shown in Figs
2
b, d, in which the bottom panels show the data
after background subtraction. The steady-state energy structure
(i.e., the occupied states) of the clean and Pt-deposited Cu
2
O
samples was inferred from spectra recorded using only the probe
Pump
Continuum
CB
VB
VB
Delay
Energy
CB
Cu
2
O
Pt
e
–
e
–
e
–
e
–
Delay
TOF
UV
probe
ab
Fig. 1
tr-2PPE working principle.
a
Schematic illustration of time-resolved
two-photon photoemission spectroscopy (tr-2PPE) applied to Cu
2
O
utilizing a time-of-
fl
ight (TOF) spectrometer.
b
The transient PES study was
performed with a pump laser intensity of 24
μ
J/cm
2
at 494 nm wavelength
(~2.5 eV energy), giving rise to resonant excitation above the bandgap. The
electron population distribution among the transient states was probed by a
274 nm laser pulse (~4.5 eV energy) and was recorded as a function of
pump
–
probe delay
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beam (see Supplementary Fig. 2). We point out that the recorded
spectra consist of a combination from two different ionization
processes involving different photoionization energies and origi-
nate from initially occupied and unoccupied states. For example,
the stationary background (Fig.
2
, black line) arises from a
transition comprising two probe photons, whereas the actual
2PPE signal is composed of a mixed transition involving one
pump and one probe photon (see Supplementary Fig. 3). Hence,
the de
fi
nition of a single binding energy scale representing all
signals is not feasible and the directly measured electron kinetic
energy scale was chosen. However, each spectral component can
be referenced to the system
’
s internal Fermi energy level with
E
F
=
0 and the calibration of the binding energy scale was performed
by measuring the Fermi edge of a Cu reference sample (see
Supplementary Fig. 4 and Supplementary Note 1). The valence-
band maximum (VBM) was found to be 0.5 eV below the Fermi
energy of the reconstructed Cu
2
O surface. The Pt-covered Cu
2
O
sample exhibited a similar steady-state energy structure to that of
the reconstructed Cu
2
O surface, except that the Pt adlayer pro-
vided an extension of the signal up to the Fermi level, providing a
clear signature of the metallic character of the adlayer. XPS, as
well as SEM measurements were indicative of a non-conformal
structure of the Pt adlayer (see Supplementary Methods and
Supplementary Figs. 5
–
10 for details). The island-like growth of
the Pt adlayer is consistent with a non-uniform distribution of
pinholes at the reconstructed surface.
For low photoelectron kinetic energies, the inelastic mean-free
paths are relatively large, exceeding tens of nanometers
29
. However,
because elastic electron-acoustic phonon scattering overcomes the
energy loss scattering events, the photoelectron escape depth is
substantially reduced for such low kinetic energies
30
,
31
. Hence, the
thickness region sampled by 2PPE is typically on the order of a few
nanometers
25
,
32
,
33
. In addition to the above-mentioned surface
sensitivity of 2PPE, it is important to note that due to the non-
conformal coverage of the Pt adlayer and the surface area accessible
by the probe pulse with a diameter of ~50 μm, both Pt-covered and
uncovered Cu
2
O areas were simultaneously probed. We therefore
expect that the Cu
2
O-derived features on the Pt-covered sample
primarily originate from a combination of directly emitted electrons
from the Cu
2
O substrate in conjunction with electrons probed
through the Pt top layer.
The transient features (unoccupied states) can be inferred from
the color maps, which showed several distinct features for both
samples. In the vicinity of zero-time delay (± 100 fs), the TS
exhibited a strong signal at kinetic energies between 0.5 and 2.5
eV. This feature showed a sharp trailing edge at the end of the
pump pulse (~100 fs) and exhibited an increase in asymmetry, as
the kinetic energies decreased and as the time delay became
increasingly negative. The symmetric part of this feature can be
consistently attributed to the cross-correlation (CC) signal
originating from the sample, and is limited by the instrument
response. The asymmetric part of the signal at negative time
2.5
ac
bd
Reconstructed
Pt-covered
Pt-covered
CB
DB
CB
DB
2.0
Electron kinetic energy (eV)
Electron kinetic energy (eV)
1.5
1.0
0.5
100
Reconstructed
1.62 eV
1.62 eV
Background
CC (t0)
Delay 180 fs
Delay 100 ps
Background
CC (t0)
Delay 180 fs
Delay 8 ps
Delay 1 ns
CB
DB
CB
DB
Signal (arb. u.)
Signal (normalized)
10
1
0.1
0.01
1.0
0.8
0.6
0.4
0.2
0.0
100
Signal (arb. u.)
Signal (normalized)
10
1
0.1
0.01
1.0
0.8
0.6
0.4
0.2
0.0
0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5
Electron kinetic energy (eV)
0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5
2.5
1.0
0.75
0.5
0.25
0.0
2.0
Electron kinetic energy (eV)
1.5
1.0
0.5
0
500
Delay (fs)
1000
0
500
Delay (fs)
1000
Fig. 2
tr-2PPE spectra. Color maps of the transient photoemission signal as a function of the electron kinetic energy and the pump
–
probe time delay for the
reconstructed Cu
2
O (100) surface (
a
), and for Pt-covered Cu
2
O(
c
). In
b
and
d
, selected spectra are shown at the speci
fi
ed pump
–
probe delays before
(top) and after (bottom) background subtraction. In
a
–
d
the positions of the conduction band and defect band are indicated (CC: cross-correlation, DB:
defect band, CB: conduction band). Source data are provided as a Source Data
fi
le
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3
delays can be consistently assigned to thermal cooling of an
initially hot electron distribution that relaxed in ~50 fs toward the
conduction-band minimum (further details in Supplementary
Fig. 11).
For the Pt-covered sample, a resonant feature centered at
~1.62 eV kinetic energy corresponded to a binding energy of
−
1.65 eV, and disappeared within 1 ps. The same feature was also
present in the signal of the reconstructed Cu
2
O sample, but in
that case, the signal was dif
fi
cult to resolve unambiguously, due to
an adjacent intense signal at lower kinetic energies that partially
overlapped with the resonance of interest. In both cases, the
energy difference between the resonance of interest and the
position of the VBM was 2.15 eV, in excellent accordance with
the expected electronic bandgap energy of Cu
2
O. This resonance
can thus consistently be assigned to the conduction-band (CB)
level. The appearance of this characteristic feature on both
samples (reconstructed and Pt covered) strongly indicates that
electrons originating from Cu
2
O can be probed on both samples,
and provides a clear basis for understanding the semiconductor
band alignment.
In the reconstructed sample, an intense, long-lived signal at
energies of ~1 eV was observed ~200 fs after the initial pump
pulse (see Fig.
2
a), with the signal shifting to lower kinetic
energies at time delays larger than 1 ps. This spectral region
encompasses states located within the bandgap, and these signals
are therefore likely to originate from defects.
Origin of defect levels by bulk-sensitive photoluminescence
.In
order to gain additional information about the origin of the
fi
lling
of these defect levels located within the bandgap, photo-
luminescence (PL) measurements were conducted exhibiting pri-
marily bulk sensitivity and, therefore, represent a well-suited
complementary method to the surface-sensitive 2PPE technique.
The experiments were conducted at the same stages of sample
preparation (reconstructed, Pt deposited) without breaking UHV
conditions and applying identical pump laser pulse conditions as
utilized in the 2PPE measurements (Supplementary Fig. 12 and
Supplementary Methods). Both stationary and time-resolved pho-
toluminescence measurements (tr-PL) strongly suggest that Cu
vacancies act as the dominant defect type in the investigated
material, and the photoemission energy is in close accordance with
the energetic position of the defect state in the bandgap. The dif-
ferent surface treatments did not alter the PL signals, suggesting a
relatively high concentration of Cu vacancies in the bulk, and that
the surface contribution, as well as in
fl
uence to the charge recom-
bination is negligible. Therefore, additional surface-sensitive
experiments (LEED, XPS) were performed to obtain information
about the stoichiometry, structure, and energetics (band bending,
band alignment) of the material (see Supplementary Figs. 5
–
7and
Supplementary Discussion and the
“
Methods
”
section).
Pt deposition reduces the 2PPE signal from defect states
.
Surface analysis of the reconstructed and Pt-covered Cu
2
O
sample by means of atomic-force and scanning electron micro-
scopy (AFM, SEM; see Supplementary Figs. 8
–
10 and Supple-
mentary Methods) revealed a non-uniform defect distribution;
hence, the long-lived signal can consistently be ascribed to sites
associated with a high dislocation density. When the defect band
intensity was normalized to the amplitude of the conduction-
band signal (bottom panel of Fig.
2
b, d), the 2PPE signal from the
defect states in the reconstructed Cu
2
O was much more intense
than the analogous signal exhibited by the Pt-covered Cu
2
O
sample. This behavior suggests that either the defect states dis-
appeared after deposition of Pt, or that the Pt provided an
alternative pathway for the photoexcited electrons that prevented
accumulation of charge carriers in the defect states.
Temporal evolution of electron density at the interfaces
. Two
main spectral regions were integrated to facilitate comparison of
the temporal dependence of the electron concentration at both
interfaces. The low-energy region (0.5
–
1.3 eV) corresponds to the
defect states, whereas the high-energy region (1.7
–
2.5 eV) corre-
sponds to the CB (Fig.
2
c). Figure
3
shows the data normalized to
the CC peak at zero-time delay. The most substantial difference
between both samples was observed in the low-energy region,
which corresponds to the defect states (orange curves). The low-
energy electron yield of the reconstructed surface exhibited a
pronounced increase vs. time, whereas the electron signal only
decayed on Pt-covered Cu
2
O surfaces. At ~200 fs after the pump
pulse excitation, the initial occupation level for the reconstructed
surface increased by more than one order of magnitude and
extended into the nanosecond domain (not shown here, for
extended transients, see Supplementary Fig. 13). This substantial
increase in electron density is consistent with a diffusion process
of photoexcited electrons from the bulk to the surface. Similar
fi
ndings have been reported for tr-2PPE studies on GaAs (100)
surfaces, where a rising electron population on a ps timescale has
been assigned to scattering of electrons into low-energy states in
the band-bending region
34
. The considerable temporal delay after
the excitation pulse supports this assignment, in that the majority
of electrons detected at the defect level were not initially created
at the surface. The electron yield rise was
fi
tted with a biexpo-
nential model, yielding a rapid time constant on the order of 1 ps
and a second, slower component in the range of 80 ps. Intensity-
dependent measurements (Supplementary Fig. 13) revealed that
the time constant of the fast-rise component of the signal
decreased with increasing pump pulse intensity, whereas the
second component of the signal was independent of the pump/
pulse intensity. The Pt-covered sample exhibited no change in
2PPE transients over the same range of pump powers.
The integrated signal from the conduction-band electrons
(1.7
–
2.5 eV, blue curves in Fig.
3
) exhibited identical behavior for
both the Pt-coated and reconstructed Cu
2
O samples. The initial
CC peak was followed by a single exponential decay, with a decay
constant of 110 ± 10 fs in both cases. Thus, surface electrons
initially generated in the CBM relax on this ultrafast timescale
into lower-lying defect states. No evidence was obtained for a
subsequent diffusion process of charge carriers that were excited
in the bulk reaching the surface at the CB level.
Discussion
Time-resolved 2PPE spectroscopy provides detailed insight into the
ultrafast charge-carrier dynamics and relaxation processes at the
surface of reconstructed Cu
2
O (100) electrodes, as well as at Cu
2
O
(100)/Pt interfaces. The energetic data indicate that downward band
bending
35
(Fig.
4
a) in ultrahigh vacuum provides a driving force for
electrons to move to the Cu
2
O surface, where they contribute to the
2PPE signals (compare Fig.
2
). Two distinct photoemission regions
were observed for both samples, ascribable to conduction-band
states at higher kinetic energy and mid-gap states at lower kinetic
energy, respectively. Due to the high pump photon energy com-
pared with the optical bandgap, we expect to generate initially free
electrons in the CB followed by subsequent formation of excitons
being well known to be present in Cu
2
O and exhibiting large
exciton-binding energies of about 150 meV
13
. The relaxation of free
electrons into excitons would lead to a spectral feature slightly
below the initial level, although we point out that both contributions
would be dif
fi
cult to distinguish. However, at CB levels ascribed to
either species, the signal was relatively weak, and disappeared within
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the
fi
rst ps after excitation (Fig.
3
), consistent with ultrafast capture
of conduction-band electrons or excitons into the broad mid-gap
band presumably originating from Cu vacancies
36
–
38
(dotted green
arrows in Fig.
4
).
The reconstructed sample exhibited a much higher photo-
electron yield at the mid-gap levels, consistent with carriers from
these defects populating a large density of long-lived surface
acceptor states. The long rise time of the integrated 2PPE signal,
which extended into the ns time domain, is consistent with these
states being
fi
lled with bulk electrons that drift or diffuse toward
the surface with the electrons then primarily accumulated at sites
with a high dislocation density. The width of the space-charge
layer is only ~8 nm (Supplementary Discussion, Surface band
bending), and most of the electrons are slow to arrive at the
surface, as evidenced by the PL results, consistent with most CB
electrons being trapped in the putative Cu vacancy states well
before the electrons reach the surface. This
fi
ts well to the low
electron mobility estimated by Paracchino et al.
39
between 2.7
and 6.3 cm
2
V
−
1
s
−
1
in electrodeposited Cu
2
O using time-
resolved terahertz spectroscopy. Considering the position of the
V
Cu
defect band located deep below the conduction band mini-
mum, bulk electron transport to the surface would consistently
occur via thermally assisted hopping within the defect band, in
accordance with the moderate values for minority-carrier mobi-
lity and diffusion length from other reports for Cu
2
O
11
,
40
–
42
.
Assuming that surface accumulation of the electrons occurs at
levels isoenergetic with the bulk trap band implies the presence of
a continuous defect band up to the crystal surface.
The mid-gap level for the Pt-covered sample (Figs
2
,
4
b and
bottom of Fig.
3
) exhibited weak photoemission signals ascribable
to defect states directly populated by the optical pump pulse.
Deposition of the noble metal adlayer moreover suppressed the
delayed electron signal rise. The surface treatment did not how-
ever substantially affect the measured band bending (Supple-
mentary Discussion, Surface band bending). Accordingly, the
transport of bulk electrons toward the surface is not changed due
to Pt deposition. These data are consistent with the electrons
being rapidly transferred into the continuum of Pt states upon
arrival at the Cu
2
O/Pt interface. The carriers would then
promptly relax toward the system Fermi level, where they would
not be detected experimentally due to the energy cutoff of the
2PPE setup for low kinetic energies.
Noble metal co-catalysts are often deposited onto the surface of
photoabsorbers to enhance water splitting or CO
2
reduction
17
,
43
.
Pt is a material of choice for such processes and the Pt can be
deposited either directly onto the semiconductor surface or onto
an intermediate buffer layer
11
,
18
,
43
,
44
. Chatchai et al.
45
deposited
Pt on Cu
2
O for hydrogen evolution and observed higher photo-
catalytic activity for decorated vs. bare Cu
2
O surfaces. To the
extent that the surfaces in UHV provide useful comparisons to
those in contact with an electrolyte, the observations herein
suggest that the Pt may not primarily act as a passivation layer
that inhibits recombination, but instead Pt primarily can mediate
ultrafast carrier transfer across the internal interface by direct
coupling to the Cu
2
O defect band.
At voltages much closer to the intrinsic bandgap energy of the
material, only electrons at higher energetic levels presumably
contribute to the photocurrent
46
–
48
. However, the intrinsic
semiconductor defect band produces substantial open-circuit
voltage losses in Cu
2
O. Hence, in addition to known issues in
Cu
2
O, such as a mismatch between the electronic band alignment
and defect states at heterojunction interfaces, bulk defect states in
the Cu
2
O may substantially limit the obtainable voltage in Cu
2
O
devices. These
fi
ndings provide insight into materials with high
trap densities that produce high photocurrents, but that have
photovoltage losses that are more dif
fi
cult to identify. Different
preparation methods reported for Cu
2
O result either in photo-
cathodes that exhibit high photovoltage
49
or high photocurrent
densities, but only recently have high-quality, thermally oxidized
Cu
2
O layers exhibited simultaneous improvement in both
quantities
40
,
50
. To evaluate the potential of a material for pho-
tovoltaic applications or subsequent chemical reactions, a surface-
sensitive technique with high time resolution is thus bene
fi
cial to
facilitate determination of the energetic distribution of charge
carriers in the light absorber. Our results suggest that 2PPE
complemented by PL spectroscopy is a useful method to
–3
CB
a
b
ab
O
sur
V
cu
V
cu
CB
Diffusion/
Transfer
Trapping
Recombination/
Relaxation
Charge accumulation
at surface states
Ultrafast relaxation
into Pt continuum
Pt
VB
Cu
2
O
VB
Cu
2
O
e
–
e
–
e
–
e
–
e
–
e
–
e
–
e
–
e
–
e
–
e
–
e
–
–
E
vac
(eV)
–4
–5
–6
–7
–8
Fig. 4
Energy-band diagram. Band bending and carrier dynamics at the
surface of the reconstructed (
a
) and Pt-deposited Cu
2
O (100) single
crystals (
b
). The energy-band positions and band bending were measured
by steady-state 2PPE and XPS measurements referenced to the vacuum
level. In
a
, charge transfer and accumulation occur at defect states (V
Cu
)
located within the bandgap. In contrast, in
b
, deposition of a Pt adlayer
leads to ultrafast charge extraction and relaxation into the Pt continuum
1.0
Reconstructed
Pt-covered
Delay (fs)
Integrated yield
Integrated yield
0.8
0.6
0.4
0.2
0.0
1.0
Energy range
[0.5;1.3]
[1.7;2.5]
0.8
0.6
0.4
0.2
0.0
0
200
400
600
800
1000
Fig. 3
Integrated electron yield of the reconstructed (top) and Pt-deposited
(bottom) samples for two different spectral regions. Fits (black, dashed)
were obtained by using exponential decay or biexponential rise models
convoluted with a symmetric Gaussian shape to account for the instrument
response. The asymmetry at negative time delays resulting from the
temporal evolution of an initially hot electron distribution relaxing toward
the conduction band minimum was added to the
fi
t. Source data are
provided as a Source Data
fi
le
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5
characterize various materials and material synthesis methods, for
photovoltaic and photoelectrochemical applications.
In conclusion, before photoexcited electrons arrive at the sur-
face, in Cu
2
O, these electrons lose a substantial fraction of their
energy through ultrafast trapping into bulk defect states. No
evidence was found for bulk electrons reaching the surface at CB
energy levels. The spectroscopy implies that the modest photo-
voltages that can be obtained from these Cu
2
O samples are not
primarily due to recombination induced by surface states, but
result from losses derived from bulk recombination processes into
bulk defect states. For reconstructed Cu
2
O (100) surfaces, the
electrons accumulated in long-lived (sub)surface states upon
arrival at the surface. In contrast, a Pt adlayer mediated ultrafast
extraction of electrons and made them available for subsequent
photochemical conversion steps. This process increased the
photocurrent, but the resultant energy loss upon transfer of the
electron to the Pt further reduced the accessible
photovoltage. The observations indicate that tr-2PPE can provide
a powerful experimental method to simultaneously unravel the
energetics and dynamics of photoexcited electrons as they arrive
at surfaces or internal interfaces of semiconductors. Tr-2PPE thus
provides explicit insight into the origin and mechanism of pho-
tovoltage losses in photovoltaic or photocatalytic materials and
can offer crucial insights at an early stage of materials
development.
Methods
Time-resolved two-photon photoemission spectroscopy
. Laser pulses were
generated with a pulsed high-repetition-rate (150 kHz) Ti:sapphire laser system
with two low-power non-colinear optical parametric ampli
fi
ers (NOPAs), that
provided light in the visible spectrum. Subsequent second-harmonic generation of
the output of one NOPA provided UV photons for the probe pulse. The time delay
between both pulse trains was produced using an electronically controlled delay
stage to vary the optical path of one of the beams. The probe beam photon energy
was maintained at
h
ν
probe
=
4.5 eV, and the pump photon energy was set at
h
ν
pump
=
2.5 eV. Satisfactory time resolution in the 2PPE experiments was obtained
by compressing the pulses with prism pairs, which resulted in a cross-/auto cor-
relation of <50 fs. The kinetic energy of the photoemitted electrons was measured
with a homemade time-of-
fl
ight spectrometer.
Cu
2
O sample preparation
. Cuprous oxide single crystals were grown by the fol-
lowing method: feed and seed rods were grown by the thermal oxidation of high-
purity Cu rods (Alfa Aesar, 99.999%) in a vertical tube furnace (Crystal Systems
Inc.) in air for 100 h at 1050 °C. The rods were then cooled in N
2
at 120 °C/h. Prior
to growth, the rods were cleaned in acetone and etched using dilute nitric acid (0.1
M) for 60 s. The rods were suspended by either Cu or Pt wire. Single crystals were
grown in an optical
fl
oating zone furnace (CSI FZ-T-4000-H-VII-VPO-PC).
Crystals were grown in air with the seed and feed rods counter-rotating at 7 rpm.
Single crystallinity was con
fi
rmed using X-ray diffractometry and pole
fi
gure
analysis. The resulting single-crystalline boules were cut to the (100) plane and
polished (SurfaceNet GmbH). Surface reconstruction of the Cu
2
O (100) single
crystal was achieved by 3
–
4 cycles of Ar
+
ion bombardement (800 V, 1 μA, 50 min)
and annealing at 550 °C for 30 min in UHV.
Photoelectron spectroscopy
. X-ray photoelectron spectroscopic (XPS) measure-
ments were performed under ultrahigh vacuum using an Al K
α
X-ray source
(1486.74 eV) equipped with a SPEC FOCUS 500 monochromator. A hemispherical
XPS analyzer supplied by SPECS (Phoibos 100) was used with a source-to-analyzer
angle of 54°. Pass energies of 30 and 10 eV and scan steps of 0.5 and 0.05 eV were
used to obtain survey and
fi
ne spectra, respectively.
Ultraviolet photoelectron spectroscopic (UPS) data were obtained using a
helium-discharge lamp emitting He I
α
(21.22 eV) radiation. The ground of the
sample holder was used to bias the sample. The analyzer equipment was the same
as that used for XPS measurements, and UPS data were collected using the same
parameters as those for XPS
fi
ne spectra.
All data were obtained at room temperature in ultrahigh vacuum (UHV)
conditions (base pressure below 1 × 10
–
10
torr).
Data availability
The source data underlying Figs.
2
a
–
d and
3
are available in Zenodo,
https://doi.org/
10.5281/zenodo.2628238
. The source data for the Supplementary Figures are available
from the corresponding author upon reasonable request.
Received: 9 November 2018 Accepted: 23 April 2019
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Acknowledgements
M.B., R.E. and D.F. thank P. Sippel for discussions. M.B. acknowledges funding from the
Helmholtz Association through the Excellence network UniSysCat (ExNet-0024-1). This
work was supported in part (S.T.O., H.A.A. and N.S.L.) through the Of
fi
ce of Science of
the U.S. Department of Energy (DOE) under award no. DE-SC0004993 to the Joint
Center for Arti
fi
cial Photosynthesis, a DOE Energy Innovation Hub. D.F. acknowledges
support by the German Research Foundation (DFG), project numbers PAK 981/1 and
FR 4025/2-1.
Author contributions
M.B., D.F. and R.E. designed the experiments on samples provided by S.T.O, H.A.A., and
N.S.L. M.B. and D.F. carried out the laser experiments and analyzed the data with the
help of R. vdK and R.E. M.B., M.F., P.P., C.H. and D.F. performed the LEED and XPS
measurements and analyzed the data. D.A.-R. performed the SEM/EDX experiments. K.
S. performed the AFM measurements. M.B., R.E. and D.F. prepared the paper. All
authors discussed the results and commented on the paper.
Additional information
Supplementary Information
accompanies this paper at
https://doi.org/10.1038/s41467-
019-10143-x
.
Competing interests:
The authors declare no competing interests.
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