of 30
1
Femtosecond time
-
resolved two
-
photon photoemission studies of
ultrafast carrier relaxation in Cu
2
O photoelectrodes
Borgwardt
et al.
2
Supplementary
Figures
Supplementary
Fig
ure
1.
Measurement and optimization of the
instrument response
function (IRF)
.
Autocorrelation (AC, left) and crosscorrelation (CC, right) traces obtained at
single crystal Cu
(111) samples
. To achieve satisfactory time resolution, the pump and probe
pulses were compressed with prism pairs. To veri
fy optimized temporal resolution of the 2PPE
set
-
up, auto
-
(AC) and cross correlation (CC) measurements were performed on Cu (111) single
crystals using the response of a two
-
photon process from the occupied surface
-
state mediated via
a virtual intermediat
e state in the sp
-
band gap
1
. Pulse
durations of 23 fs for the pump and 25 fs
for the probe pulses were found.
3
Supplementary
Fig
ure
2.
Single
-
color steady state spectra
.
(a)
Single
-
color steady state
spectra of the reconstructed sample for different probe intensities plotted on a logarithmic scale.
(b)
Two
-
color spectrum (Δt=0) (PP, yellow) and single
-
color probe spectrum (black) of the
reconstructed sample. The difference spe
ctrum (green line) depicts the signal originating from
the two photon transitions involving one pump and one probe photon. The inset shows the
measurement of the valence
-
band maximum (VBM).
a
b
4
Supplementary
Fig
ure
3.
Power dependence
. Total
electron yield (TEY)
as a function of laser
power
for
the
probe only (black, bottom scale) and
for
pump
-
probe spectra (blue, top scale). In
the latter case the TEY
was corrected by the
yield
measured when only the probe beam was used
.
μ
5
Supplementary
Fi
g
ure
4
.
Excitation scheme for two
-
photon photoemission
.
The schematic
shows the photoemission process involving multiple photons. In a single
-
color experiment
(probe only, two blue arrows, right) a two
-
photon transition lifts an electron from the VBM 0.5
e
V below the Fermi level to above the vacuum level reaching the VBM
(1)
as indicated level on
the kinetic energy scale. For the two
-
color experiment (probe and pump, blue and green arrow,
center) the VBM is projected onto a lower level indicated as VBM
(2)
du
e to the lower combined
photon energy. Both cases are mediated via virtual states and the energetic band alignment of the
occupied states (valence band) is measured.
In contrast,
two photon transitions via resonant
intermediates probe the energetic structu
re of the unoccupied states. This
process is
illustrated by
Sample
(Cu
2
O)
V
BM
C
BM
E
Fermi
E
Vacuum
VBM
(2)
VBM
(1)
CBM
Binding Energy (E
B
)
Kinetic Energy (E
Kin
)
0
0
1
2
3
4
1
2
3
4
-1
6
the two
-
photon transition (pump and probe, green and blue arrow, left) lifting first an electron
from t
he VB above the band gap and subsequently exciting the photoelectron
above the vacuum
level.
7
Supplementary
Fig
ure
5. Low
-
energy electron diffraction (LEED)
and
X
-
ray photoelectron
spectroscopy
.
(a)
S
tructure of the reconstructed Cu
2
O
(1
00) surface. LEED
data showed
that the
reconstructed surface
exhibited
a (1×1) periodicity with ½ monolayer of terminating atomic
oxygen adsorbed as a c(2×2) structure (or, in Wood notation, as √2×√2 R45°);
(b)
Pt 4f
photoemission spectra, acquired for different Pt thickness
es
on the Cu
2
O
(
100) surface (hv (Al
K
α
) = 1486.7 e
V).
Area-Normalized Intensity (arb. units)
85
80
75
70
65
Binding Energy (eV)
C. Pt 1 nm
D. Pt 0.5 nm
B. Pt 15nm
Pt 4f
A. Pt 30nm
Cu 3p
Pt 4f
5/2
7/2
Pt 4f
Pt sat.
Shift = 0.35 eV
ab
2
2
x
()
R
45°
(00)
LEED Pattern (KE = 38 eV)
Cu
O
O
sur
(10)
(00)
(01)
(10)
(01)
(1x1)
8
Supplementary Figure 6.
Core
-
level analysis of the Cu
2
O (100) surface for pristine
conditions (clean and reconstructed surface) and after the deposition of 1.0 nm of Pt.
The
analysis was performed using the Al Kα photon energy (hv = 1486.7 eV).
(a)
Cu 2p,
(b)
O 1s.
b
BE
C
=
5
3
0
.
1
8
e
V
F
W
H
M
=
0
.
6
9
eV
BE
C
=
5
3
0
.
6
2
e
V
F
W
H
M
=
0
.
7
7
eV
B
E
C
=
9
32
.
2
6
e
V
F
W
H
M
=
0
.
9
8
eV
BE
C
=
9
3
2
.
5
2
e
V
F
W
H
M
=
1
.
1
6
eV
Cu
2
p
3/
2
a
9
Supplementary Figure 7
.
Simulation of the Cu 2p3/2 and O 1s core levels within the
semiconductor.
The simulations have been performed using a full
-
width at
half
-
maximum of 1
eV for the single spectral components, described by a Voigt
line shape
with a Gaussian
-
to
-
Lorentzian ratio (G/L) equal to 0.75.
(a)
Semiconductor (SC) potential profile and potential
profiles probed using the Cu 2p
3/2
and O 1s core level
s, with a band bending (BB) potential of
0.45 V and a space charge layer of 7.5 nm. The SC potential profile has been modelled with an
exponential function, to take into account temperature effects;
(b)
simulated Cu 2p
3/2
and O 1s
core levels under BB and
flat
-
band conditions. When a downward BB is present, the core level
binding energies shift to higher values (∆BE
Cu
2p = 0.32 eV, ∆BE
O
1s = 0.28 eV);
(c), (d)
2D
plots reporting the simulated Cu 2p
3/2
and O 1s core levels, respectively, as a function of t
he
distance from the surface (i.e. as a function of the probed SC potential profile and photoelectron
-0.5
-0.4
-0.3
-0.2
-0.1
0.0
Band Bending Potential (V)
10
8
6
4
2
0
Distance from Surface (nm)
1.0
0.8
0.6
0.4
0.2
0.0
Photoelectron attenuation
SC Potential
profile
O 1s
Cu 2p
3/2
PV
O 1s
SSC
PV
Cu 2p
PE attenuation
Potential Profile
a
3.0
2.0
1.0
0.0
Distance from Surface (nm)
940
938
936
934
932
930
928
Binding Energy (eV)
Cu 2p 3/2
PE KE = 555 eV
IMFP = 1.2 nm
PV = 3.6 nm
c
5.0
4.0
3.0
2.0
1.0
0.0
Distance from Surface (nm)
532
530
528
Binding Energy (eV)
O 1s
PE KE = 955 eV
IMFP = 1.8 nm
PV = 5.4 nm
d
937
935
933
931
Binding Energy (eV)
929
533
532
531
530
529
528
527
Binding Energy (eV)
Cu 2p
3/2
O 1s
Normalized Intensity
BB potential = 0.45 V
SSC width = 7.5 nm
FB
b
BE = 0.28 eV
BE = 0.32 eV
10
exponential decay). SC: semiconductor; SSC: semiconductor space charge; FB: flat band
conditions; BB: band bending conditions; PE KE: photoelectron kineti
c energy; IMFP: inelastic
mean free path; PV: probed volume.
11
Supplementary Figure
8.
Atomic force microscopy (AFM).
AFM height (
a,c
) and phase (
b,d
)
information for the Pt covered reconstructed Cu
2
O (100) surface.
a
c
b
Pt-covered
d