Ultrafast Dynamics of Nonequilibrium Electrons in Metal/Adsorbate Nanosystems
Christophe Bauer, Jean-Pierre Abid and Hubert GiraultLaboratoire d’Electrochimie Physique et Analytique,
ISIC, SB, EPFL, CH-1015 Lausanne, Switzerland
Summary
Metal/molecule interfaces play a crucial role in topics such asmolecular electronics, surface femtochemistry, organic light emitting diodes,catalysis and solar cells. Here, we use adsorbates-covered metalnanoparticles to investigate by femtosecond transient absorptionspectroscopy the dynamical behaviour of nonequilibrum electrons (NEs) atmetal/molecule interfaces. This approach allows: the separation of internalfrom external thermalization, the investigation of the size behaviour ofinternal thermalization and hot electron cooling dynamics, the study ofmolecular vibrations effect on electron transfer process, the identification ofthe mechanism of internal thermalization retardation, the observation ofcomplex nonlinear dynamics with feedback loops.
Ultrafast Electron Dynamics in Metals
Dynamical events: The picture
Spectral Map:Nonthermal regime and hot electron gas
A: Energy redistribution processes in a metal. B: Metal/molecule/metal interfaces: Heart ofmolecular electronics
Femtosecond Pump-Probe Spectroscopy
Metal moleculeMetal molecule Metal
e-
ph
e-
MV
Light
Metal moleculeMetal molecule Metal
e-
ph
e-
MV
Light
e-
ph
e-
MV
LightP
I
Nascent nonthermal electrons (NNEs)
Hot electron gas
Hot lattice
Surrounding medium
electron-electron scattering
electron-Molecular Vibration scattering
PII
PIII
electron-phonon interaction
phonon-phonon interaction
PI
Nascent nonthermal electrons (NNEs)
Hot electron gas
Hot lattice
Surrounding medium
electron-electron scattering
electron-Molecular Vibration scattering
PII
PIII
electron-phonon interaction
phonon-phonon interaction
500 550 600 650
-1.0
-0.5
0.0
0.5
Absorb
ance C
hange (
a.
u.)
Wavelength (nm)
R e g i o n I R e g i o n I I R e g i o n I I I
200 fs
4 ps
500 5 5 0 6 0 0 6 5 0
-1.0
- 0 . 5
0 . 0
0 . 5
Absorb
ance C
hange (
a.
u.)
Wavelength (nm)
Region I Region II Region III
200 fs
4 ps
Gold NP Sulfate
Excitation
Feedback Energy dissipation
toward surrounding medium
Gold NP Sulfate
Excitation
Feedback Energy dissipation
toward surrounding medium
0 2 4 6 8 10 12
1.0
1.5
2.0Lifetim
e (
ps)
Nanoparticles Diameter (nm)
0 2 4 6 8 10 12
1.0
1.5
2.0
L i f e t i m e ( p s )
Nanoparticles Diameter (nm)
E = Ei- Ef
LUMO
h
EF
Adsorbates
ini
fin
Density of
states
Nonthermal
electrons
LUMO
Metal
h
EF
ini
fin
Density of
states
Nonthermal
electrons
E = Ei- Ef
LUMO
h
EF
Adsorbates
ini
fin
Density of
states
Nonthermal
electrons
LUMO
Metal
h
EF
ini
fin
Density of
states
Nonthermal
electrons
E = Ei- Ef
LUMO
h
EF
Adsorbates
ini
fin
Density of
states
Nonthermal
electrons
LUMO
Metal
h
EF
ini
fin
Density of
states
Nonthermal
electrons
-1 0 1 2 3 4 5 6 7
-1.0
-0.8
-0.6
-0.4
-0.2
0.0
0.2
Ab
so
rba
nc
e C
ha
ng
e (
a.
u.)
Delay Time (ps)
50 100 150 200 250 3001.5
2.0
2.5
3.0
3.5
4.0
Life
tim
e (
ps)
Pump Fluence (nJ/pulse)
Inelastic electron tunneling at metal/molecule interface
Build-up of a population of hot adsorbates (highly vibrationnaly excited adsorbates)
Size effect on hot electron cooling
LW
k k
d-band
Conduction
band
Ener
gy
EF
Emax
=
Emax=
LW
k k
d-band
Conduction
band
Ener
gy
EF
Emax
=
Emax=Emax=
NNEs (Metal) Adsorbates Hot adsorbates
Electron tunneling e-MV interaction
Negative feedback
NNEs (Metal) Adsorbates Hot adsorbates
Electron tunneling e-MV interaction
Hot electron emission
Negative feedback
Retardation
Internal
thermalization
External
thermalizationUCIS
e-e scatteringAdsorbates
e-phinteraction
e-e scatteringAdsorbates
e-ph
interaction
Retardation
Build-up hot
adsorbates
Retardation
Internal
thermalization
External
thermalizationUCIS
e-e scatteringAdsorbates
e-phinteraction
e-e scatteringAdsorbates
e-ph
interaction
Retardation
Build-up hot
adsorbates
BA
Gold band structure around L pointof Brillouin zone
Nonthermal
Thermal
Nonthermal regime controls external thermalization
530 nm
Probing the d-band to Fermi surface transition:Accordance with Fermi liquid theory
Non-adiabatic surface reaction:Break-down of Born-Oppenheimer
approximation
Low-perturbation regime
Nonlinear dynamics
Interconnection between the dynamical processes
625 nm
615 nm
605 nm
590 nm
0 2 4 6 8 10 12
-0.04
-0.02
0.00
0.02
0.04
0.06
Ab
so
rba
nce
Ch
an
ge
(a
. u
.)
Delay Time (ps)
6 2 5 n m
6 1 5 n m
6 0 5 n m
5 9 0 n m
0 2 4 6 8 10 12
-0.04
-0.02
0.00
0.02
0.04
0.06
Ab
so
rba
nce
Ch
an
ge
(a
. u
.)
Delay Time (ps)
0 2-2
0
1
-1
Electron energy
f
Thermal
Nonthermal
0 2-2
0
1
-1
Electron energy
f
Thermal
Nonthermal
Possible when the system exhibits chemical interface damping:Electrons tunnel back and forth between metal and adsorbates Additional damping channel for surface plasmon
References
Transient absorption data for 2.5 nm
Pump fluence dependence for 4.2 nm
J. Chem. Phys. 120, 2004, 9302Chem. Phys. 319, 2005, 409