HRTEM and FE-AEM Investigation of
Non-Aggregated Nanoparticles of Noble
Metals Produced by Pulsed Laser
Deposition
C. E. Allmond, V. P. Oleshko, J. M. Howe, and J. M. Fitz-Gerald
Department of Material Science and Engineering
University of Virginia
Microscopy and Microanalysis 2004
Savannah, GA
August 3, 2004
Outline
Introduction & Motivation
Experimental pulsed laser deposition (PLD)
Set-Up and TEM and FE-AEM techniques
Conventional TEM and HRTEM of Ag and Pd
nanoparticles deposited onto amorphous carbon
films and crystalline oxide supports
EDXS nanoanalysis of individual particles
PEEL band structure fingerprints of PLD deposited
particles
Conclusions
Introduction & Motivation
Ag and Pd particles in the 1-20 nm range often exhibit significant deviations from bulk
materials which are of primary importance for heterogeneous catalysis. Metal particle
size, which is directly connected with the number of active catalytic surface sites present,
is one key factor influencing the properties and performance of supported metal
catalysts. 1
Variation in the aggregate dimension deposited onto supports induces a change in
number and coordination states and electronic configuration of surface atoms. 1
Ag ([Kr]4d105s1) binds molecular oxygen to its surface at elevated temperatures by forming
Me-O bonds on the surface, thus acting as a unique oxidation catalyst in ethylene epoxidation2.
The 5s orbital of Pd ([Kr]4d10) is empty because the 4d orbital is complete. Pd serves as a
typical [de]hydrogenation catalyst because hydrogen atoms diffuse through Pd and fill the 5s
orbital with their electrons. Pd is also active in oxidation of CO and NO.
PLD on bulk Pd and Ag targets was used to deposit nanoparticles onto various materials
WITHOUT the use of secondary media (sol-gel, colloidal suspension, etc)
1Bertani,
2
Valeria; Cavallotti, Carlo; Masi, Maurizio; Carra, Sergio. Journal of Molecular Catalysis A: Chemical (2003), 204-205 771-778.
W. Li, C. Stampfl, M. Scheffler, Phy. Rev. Letters. 90 (2003) 1.
Experimental Design
KrF excimer laser ( =248 nm, 25 ns
FWHM) operating between 5-20Hz
Base pressure of 5x10-7 Torr
Target rotating motor
and assembly
Load Lock Door
Fluences: 2.5–4 J/cm2
Target
Gate Valve
KrF Laser
248 nm
Turbomolecular
pump
Substrate holder
and heater assembly
Laser induced plasma plume
Targets: Ag (99.99% at.) and Pd (99.95%
at.) ablated in an inert backfill gas of Ar at
pressures of 3, 10, 100, and 200 mTorr
Formation of nanoparticles and molecular
clusters is largely facilitated by collisions
both inter-plume and with the inert gas on
the leading edge of the laser induced plume.
Metal nanoparticles were collected on
carbon grids and examined using brightfield (BF) and dark -field (DF)
Conventional TEM, HRTEM and AEM
(EDXS,PEELS).
CTEM, selected-area electron diffraction
(SAED) and HRTEM: a JEOL 4000 EX
TEM at 400 kV, a LaB6 filament.
CTEM/HRTEM and EDXS and PEELS: a
JEOL 2010F AEM equipped with a
Schottky field-emission (FE) gun and
operating at 200 kV.
Ag Nanoparticles Deposited onto Carbon Films
Mean dia = 31.3 nm
Range = 78.4 nm
StdDev = 13.0 nm
Sample = 711
Ag particles, 5K pulses, 200 mTorr Ar:
(a) BF-TEM and SAED pattern (b) DF-TEM displayed of 8-75 nm sized nanocrystalline particles.
Histogram displays the multimodal behavior of the particle population with maxima at 18 nm,
25 nm and 39 nm, respectively.
A 200 nm splash particle displays a 5-15 nm-thick non-uniform carbon shell.
Ag Nanoparticles Deposited onto Carbon Films
HRTEM of 2-20 nm-sized Ag particles, 5k pulses, 200 mTorr. Enlarged inserts show multiple
twinned crystalline nanoparticles
Pd Nanoparticles Deposited onto Carbon Grids
Mean dia. = 2.081 nm
StdDev = 0.563 nm
Sample = 936
5 nm
Deposited Pd particles, 250 pulses, 3 mTorr Ar.
The SAED displays the diffuse ring corresponding to most probable reflecting spacing of
0.23 nm
The Pd nanoparticles exhibit preferentially monomodal size distribution with a mean diameter of 2 nm.
Pd Nanoparticles Deposited onto Carbon Grids
PLD deposited 2-15 nm sized Pd
particles:dmean = 4.1 nm, SD = 1.4 nm, 5k
pulses, 100 mTorr.
Spherical, elongated and irregular
shapes influenced by melting, fracturing,
fragmentation and complex multiple
twinning.
Lattice fringes (HRTEM) and partially
discrete Debye - Scherrer rings (SAED)
assigned to the fcc structure.
Pd Deposited onto TiO2 Support
Mean dia. = 2.26 nm
Range = 1.71 nm
StdDev = 0.23 nm
Sample = 943
10 nm
2-5 nm-sized Pd deposited onto TiO2, 250 pulses, 3 mTorr Ar.
The Pd particles have a mean diameter of 2 nm both with or without carrier and
display a preferentially monomodal distribution with a long asymmetric right-side tale.
The SAED pattern displays point Bragg reflections and discrete rings corresponding to
anatase phase of TiO2.
Pd Deposited onto Al2O3 Support
Mean = 1.999 nm
Range = 2.843 nm
StdDev = 0.4161 nm
Sample = 112
10nm
Pd was deposited onto polycrystalline Al2O3: 250 pulses 3mTorr Ar
The Pd particles have a mean diameter of 2 nm with or without carrier and display a
preferentially monomodal distribution with maximum at 2 nm.
The SAED pattern displays the discrete rings corresponding to the nanocrystallineAl2O3.
EDXS Nanoprobe Analyses of Individual
Nanoparticles
5 nm Ag particle, 10k pulse
x1M, 100s
5 nm Pd particle, 10k pulse
x1M, 100s
EDX spectra of a Ag nanoparticle correspond to 1.3x10-20 g or 1 atomic
column for a 0.5 nm probe and of 10-19 g or 37 atomic columns for a 2.4
nm probe, respectively.
EDX spectra of a Pd nanoparticle correspond to 2.0x10-20 g or 1 atomic
column for a 0.5 nm probe and 1.6x10-19 g or 17 atomic columns for a 1.6
nm probe, respectively. Au and Ag represent 500 ppm impurities in the
Pd target.
Band Structure Fingerprints
valence to conduction-band transitions inner-shell excitations
Ag
surface + volume plasmon
Pd
+ interband edge
volume plasmon
free resonance
interband transitions
Interband
transition
interband
transition
volume plasmon
interband
transition
Pd, Ag 4p3/2,1/2
Pd, Ag, 5s, p
transition
surface + volume plasmons
PEEL spectra demonstrate distinct differences between Pd and Ag particles below 10 eV: a sharp
intensity onset at 3.7 eV (Ag) and slowly increasing intensity reaching a maximum at 7 eV
because of a strongly dumped resonance.
For energies above 10 eV, the spectra reveal some similarities because of similar electron
configuration of Ag and Pd with maxima between 17-44 eV corresponding to bulk plasmons at
26.1-26.2 eV and interband transitions from low-lying d-band to conduction band.
J. Daniels et al. in: Springer Tracts in Modern Phys., v. 54 (Springer, Berlin, 1970), p.77.
Band Structure Fingerprints of PLD Deposited Particles
valence to conduction-band transitions inner-shell excitations
interband
transition
volume
plasmon
free
resonance
s + v.p.+ inter
band edge
interband
transitions
volume
plasmon
interband
transition
Pd,Ag 4p3/2→Pd 5s,Ag5s,5p
interband
transition
surface +
volume plasmons
PEEL spectra demonstrate distinct differences between Pd and Ag particles below 10 eV: a sharp
intensity onset at 3.7 eV (Ag) and slowly increasing intensity reaching a maximum at 7.8 eV due
to a strongly dumped resonance.
For energies above 10 eV the spectra reveal some similarities because of similar electron
configuration of Ag and Pd with maxima between 17-44 eV corresponding to bulk plasmon at
26.1-26.2 eV and interband transitions from low-lying d-band to the conduction band.
J. Daniels et al., in: Springer Tracts in Modern Phys., v.54 (Springer, Berlin, 1970), p.77
Conclusions
PLD enables altering of deposition densities, particle sizes and
morphologies of Ag and Pd with size ranging from 1-20 nm with little
evidence of particle aggregation onto amorphous films as well as onto
crystalline TiO2 and Al2O3 supports.
Particles of Ag and Pd were deposited with a mean diameter of 2 nm at 3
mTorr of Ar: larger particles with some coalescence were observed with
increased pressures.
Pd nanoparticles deposited onto the oxide carriers exhibited similar size
distributions as particles deposited onto amorphous carbon films under
the same ablation conditions.
Low-loss PEEL spectra of the PLD-deposited particles show distinct
differences between Pd and Ag due to differences in band structures below
10 eV and some similarities governed by interband transitions and manyelectron effects above 10 eV. The elemental compositions of the Ag and Pd
nanoparticles (the latter with 500 ppm admixtures of Au and Ag) were
confirmed by EDXS.
Processes in PLD
Laser Pulse
e-
ee-
ee- eee-
lattice
e-
ee-
Processes in PLD
ee- eeee- ee- eeeeee-
Electronic excitation
Processes in PLD
ee- ee- eee-e-e-elattice
eee- e-
Energy relaxation to lattice (~1 ps)
Processes in PLD
lattice
Heat diffusion (over ps - ns)
Processes in PLD
lattice
Melting (tens of ns), Evaporation, Plasma Formation
(ns), Re-solidification
Processes in PLD
lattice
If laser pulse is long (ns) or repetition rate
is high, laser may continue interactions
Processes in Pulsed Laser Deposition
1. Absorption of laser pulse in material
(Beers-Lambert Law)
Qab=(1-R)Ioe-aL
(metals, absorption depths ~ 100 nm, depends on L)
2. Relaxation of energy (~ 1 ps)
3. Heat transfer, Melting and Evaporation
when electrons and lattice at thermal equilibrium (long pulses)
use heat conduction equation:
(or heat diffusion model)
∂T
= ∇ ⋅ ( K∇T ) + Qab
ρC p
∂t
Processes in Pulsed Laser Deposition
4. Plasma creation
threshold intensity:
4 x 10 4Ws1 / 2cm −2
I threshold =
t pulse
governed by Saha equation:
Ψ
ne ni QeQi me mi
exp − ion
=
nn
Qn me + mi
kT
5. Absorption of light by plasma, ionization
(inverse Bremstrahlung)
6. Interaction of target and ablated species with plasma
7. Cooling between pulses
(Re-solidification between pulses)
PLD Background
Deposition of inorganic thin films
Laser entry
Laser target
Superconductivity in ceramic
materials and widespread availability
of UV excimer lasers
Deposition of complex, multicomponent materials in thin film form
(ceramics, metals, etc..)
Substrate
Utilizes the focused output of UV laser
(193 and 248 nm) rapidly heats, vaporizes
and creates nearly atomic (partially
ionized) vapor with preserved
stoichiometry.
The Laboratory PLD Set-Up
Lambda Physik
Compex 208
Excimer Laser
= 248 nm
Optics Table
Vacuum Chamber