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