Effects of particle size and plasmon excitation on the photochemistry of
(NO)2 adsorbed on alumina supported Ag nanoparticles
Daniel Mulugeta
2
Introduction
• Chemical properties
• Optical properties
• Electronic properties
Atoms /Molecules Metal nanoparticles
Bulk materials
Size dependent
Evolution of Electronic Structure
BE
BEAtom
I1φ
BEMetal
EFermi
Evacφ
Atom
Metal
1
2 ~ 105
~NA
MNP
Formation of metallic band from atomic orbital
Shift of core and valence bands
NP
e-
E
E’ field enhancement effect
Landau Damping ?
Plasmon Lifetime < ~10 fs
Can plasmonic electrons drive photochemistry?
Particle plasmon (Mie plasmon)
Collective motion of conduction electrons
5
130 nm × 130 nm
Ag NPs on thin alumina film
• Particle density ~ 4×1011 cm-2
• Standard deviation of the size distribution~ 20 to 30 %
NiAl(110)
Alumina film
Ag NPs
W. Drachsel, etal. J. ele. Spec. rela. Phenom., 122 (2002) 239
6
Plasmon on Ag NPs Photon-STM
Nilius et al., Phys. Rev. Lett. 84, 3994 (2000).
Excitation of (1,0) mode of Mie plasmon at ~ 3.6 eV
∼3.5 eV ∼2.4 eV
ps
∼3.5 eV ∼2.4 eV
ps
7
Experimental setup -1
For photon induced desorption (PID) and MS-TOF measurements
Preamplifier
Discriminator
UHV chamberSignal
Stop
Nd:YAG (2.3, 3.5, 4.7 eV)
QMS Head
LN2 cooling
MCS for PID (2 ms)
Fast MCS for MS-TOF (0.8 µs)Sample:Ag / Al2O3 / NiAl(110)40 K
Pulse doser (NO)ps
p- or s-pol.
0 200 400 600 800 10000
10
20
30
40
50
60
Coun
ts
Time-of-flight (µs)
0 50 100 150 200
0
200
400
600
800
1000
1200
Irradiance [x1015 photons/cm2]
Multichannel scaler (MCS)
85 10 15 20
0
200
400
600
800
PID
Inte
nsity
[a.u
.]
Time [s]
Data acquisition time : 2ms
fPht ⋅⋅=
1
1σ
MS TOF PID data analysis
0 500 1000 1500 20000
10
20
30
40
50
60
Coun
t [a.
u]
Time of Flight [µs]
Time resolution = 0.8 µs
l=19 cm
QuadrupoleIonizer Detector
Laser
Sample
B
transk
ET 2=
Shifted Maxwell-Boltzmann distribution
−−+
−−∝
2
2242
2
1141 expexp)( v
tlb
tav
tlb
tatf
Fitted by single exponential (until 50%)
−−⋅+=
1
00
)(exp1 txxAyy
9100 200 300 400 500
QM
S Si
gnal
[a.u
.]
Temperature [K]
NO-α
NO-β
N2O
m/e = 30
m/e = 44
(NO)2 N2O + O
(NO)2 2 NO
Thermal reactions of NO dimer
TPD from Ag NPs (10 nm) dosed with NO at 75 K
10
Size dependence
TPD: NO from Ag NPs
0.0 0.2 0.4 0.6 0.8 1.060
70
80
90
100
110
120
60 80 100 120
0
3
6
9
12
15
18 ∞
N2O
NO
(b)(a)
D (nm)
Ag(111)
10.5
9.7
7.7
4.96.1
2.8
QM
S Si
gnal
at m
/e=3
0 (a
rb. u
nits
)
Temperature (K)
N2O
NO
Ag(111)
TPD
Peak
Tem
pera
ture
(K)
1/ Radius (nm-1)
108 6 4 23
Diameter (nm)
Mulugeta et al., Phys. Rev. Lett. 101, 146103 (2008).
11
Dispersion force (van der Waals force)∝ total number of Ag atoms
Possible origins of the size dependent adsorption energy of NO
TPD: NO from Ag NPs
Surface structure of Ag NPseg. contraction of lattice parameters ∝ 1/R
Electronic structure of Ag NPseg. d-band shift,
change of electron population near EF
12
0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8
0
5
10
15
2010 8 6 4
2.3 eV p-pol. 3.5 eV p-pol. 4.7 eV p-pol.
Cros
s se
ctio
n, σ
X10-1
7 [cm
2 ]
1/R [nm-1]
Photon energy
Mean diameter [nm]
35
Ag(111)
X 10
Photodesorption cross sectionSize and photon energy dependence
@ 2.3 eV and 4.7 eV
Linear increase of σ with 1/R
13
Photodesorption cross section
Rate of collision with the surface∝ S/V = 1 / R
σ ∝ S/V = 1/R
Surface to volume ratio
Confinement effect
e-
e-
Enhanced cross-section (σ)
Particle size ≤ mean free path of e-
14
0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8
0
5
10
15
2010 8 6 4
2.3 eV p-pol. 3.5 eV p-pol. 4.7 eV p-pol.
Cro
ss s
ectio
n, σ
X10
-17 [c
m2 ]
1/R [nm-1]
Photon energy
Mean diameter [nm]
Ag(111)
35
X 10
Photodesorption cross sectionSize and photon energy dependence
Excitation of (1,0) mode Mie plasmon
@ 3.5 eV in p-pol
∼3.5 eV ∼2.4 eV
ps
∼3.5 eV ∼2.4 eV
ps
Mulugeta et al., Phys. Rev. Lett. 101, 146103 (2008).
15
Plasmon oscillator strength
∝ number of conduction electrons
RadiationDamping
Landau Damping+
Radiation Damping
1Particle Diameter [nm]
4 20
LandauDamping
e-h pair Photon
Photochemistry
Effects of plasmon
Osc
illat
or s
treng
th
Land
au d
ampi
ng
Plasmon decay channels
16
Effects of plasmon
Osc
illat
or s
treng
th
Land
au d
ampi
ng0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8
0
5
10
15
2010 8 6 4
2.3 eV p-pol. 3.5 eV p-pol. 4.7 eV p-pol.
Cro
ss s
ectio
n, σ
X10
-17 [c
m2 ]
1/R [nm-1]
Photon energy
Mean diameter [nm]
Ag(111)
35
170.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8
600
700
800
900
1000
1100
1200
130010 8 6 4
2.3 eV p-pol. 3.5 eV p-pol. 4.7 eV p-pol.
<Et>
/2k B [
K]
1/R [nm-1]
Photon energy35
Mean Diameter (nm)
Ag(111)
Translational temperature
• Tt is constant at ~700 K
@ 2.3 and 3.5 eV
Size and photon energy dependence
• No plasmon effect on Tt
∼3.5 eV ∼2.4 eV
ps
∼3.5 eV ∼2.4 eV
ps
180.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8
600
700
800
900
1000
1100
1200
130010 8 6 4
2.3 eV p-pol. 3.5 eV p-pol. 4.7 eV p-pol.
<Et>
/2k B [
K]
1/R [nm-1]
Photon energy35
Mean Diameter (nm)
Ag(111)
Translational temperatureSize and photon energy dependence
• Tt is constant at ~700 K
• No plasmon effect on Tt
@ 4.7 eV
• Tt increases atsmall D
@ 2.3 and 3.5 eV
∼3.5 eV ∼2.4 eV
ps
∼3.5 eV ∼2.4 eV
ps
19
0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8
600
700
800
900
1000
1100
1200
130010 8 6 4
2.3 eV p-pol. 3.5 eV p-pol. 4.7 eV p-pol.
<Et>
/2k B [
K]
1/R [nm-1]
Photon energy
35Mean Diameter (nm)
Ag(111)
2710 K 747 K 72 K
0 500 1000
747 K 74 K
Time of Flight (μs)
Translational temperatureSize and photon energy dependence
Mulugeta et al., Phys. Rev. Lett. 101, 146103 (2008).
20
21225.3 225.6 225.9 226.2 226.5 226.8 227.1
0
1
2
3
X2 Π3/2A2Σ3/2 NO
+ Sign
al (a.
u.)
λ (nm)
A2Σ1/2 X2 Π1/2
Experimental setup - 2
State resolved detection of NO via (1+1) REMPI
NO+
Π2X
v
vΣ2A
To MCP
Probe laser
Pump laser
REMPI -TOF
REMPI spectrum
Ion detection
(1+1) REMPI principle
v = 0, J” = 3.5, Ω = 1/2
0 50 100 150 200 250 300
NO+
signa
l [a.u.
]
Pump-probe delay [µs]
v = 0,
Repeller
sample27 mm
22
0 300 600 900 1200 1500-8
-6
-4
-2 Ω = 1/2 Ω = 3/2
ln (I
(J")/
(2J"
+1)S
j) [a
.u.]
Erot + EΩ [cm-1]
Trot = 465 + 50 K
v = 0, Ω = 1/2 and 3/2,
REMPI - Spectrum
Pump-probe delay = 29 µs
Boltzmann Plot
v = 0
225.0 225.5 226.0 226.5 227.0
NO+
Sign
al In
tens
ity [a
.u.]
λ [nm]
R21Q21+R11
P21+Q11
R22
Q22+R12
P22+Q12
Rotational temperature8-nm Ag NPs /Al2O3/NiAl(110) @ 3.5 eV in p-pol.
23
Rotational temperatureSize and photon energy dependence
v = 1500 m/sv = 950 m/s
4 nm Ag NPs
8 nm Ag NPs
0 25 50 75 100
NO
+ Sig
nal i
nten
sity
[a.u
.]
Pump-probe delay [µs]
v = 0, J" = 25.5, Ω = 1/2
24
Vibrational temperatureSize and photon energy dependence
v = 1500 m/sv = 950 m/s
4 nm Ag NPs
8 nm Ag NPs
0 25 50 75 100
NO
+ Sig
nal i
nten
sity
[a.u
.]
Pump-probe delay [µs]
v = 0, J" = 25.5, Ω = 1/2
25
0 100 200 3000
1
2
3
4
5
NO si
gnal
inten
sity [
a.u.]
Pump-probe delay [µs]
Translation – Rotation correlation
J” = 16.5
J” = 28.5
J” = 3.5
State resolved TOF spectraNO (v = 0, Ω = ½), hv = 2.3 eV
Positive T - R correlation
0 200 400 600 800 1000 1200 1400 1600200
400
600
800
1000
1200
1400
1600
<Et>
/ 2K
B (K
)
Erot (cm-1)
4 nm8 nm11 nmAg(111)
Ag NPs diameter
+
26
Photodesorption Dynamics
Ener
gy
Distance from Surface
Ek’Ek
D0
EK’M+A- or M+A+
M+A
P. R. Antoniewicz, Phys. Rev. B 21, 3811 (1980).
Antoniewicz Model
Desorption via transient ion state
27
Photon energy dependence
Photoexcitation mechanisms
HOMO
EF
Evac
LUMO
d-band
Substrate Molecule
E
e-
e-
e-
sp-band
e-Desorption via
transient negative ion (TNI) state
Desorption via TNI or transient positive ion (TPI) state
@ 2.3 and 3.5 eV
@ 4.7 eV
28
TNI TPI
Photodesorption Dynamics
TNI versus TPI
PP
29
Ground PES (neutral state)
Rotational excitation(Impulsive model)
z
α
l
N
O
Common excitation mechanismfor T and R
L = P x l sinα
P
P
Photodesorption Dynamics
Positive T – R correlation
30
Photodesorption Dynamics
Vibrational excitation Excited PES (Transient ion state)
N
O (NO)2-
(NO)2+
31
Summary
Confinement of electrons and plasmon excitation do not alter theindividual desorption dynamics of NO, at least for hv < 3.5 eV.
At sufficiently high hv (4.7 eV) to form hot holes in the adsorbate a new desorption mechanism is found, which results in more energeticdesorbates. It is probably proceeds via TPI (Transient positive ions).
Confinement of electrons can result in an increase of the desorptioncross section (σ) with the surface to volume ratio (1/R) of the NPs.
Plasmon excitation can result in additional enhancement of σwhich has maximum at an intermediate size.
32
Metal Substrate
Adsorbatehv
Hot electrons
Plasmon field enhancement
Dielectric Substrate
Metal Substrate
MNP
Hot electrons
hv
Confinement & plasmon excitation
Confinement : Electron dynamics Transport property
Optical property : Mie plasmon → field enhancementMNPs