FUNCTIONAL NANOSTRUCTURES FROM CLUSTERS
A. PEREZ, P. MELINON, V. DUPUIS, B. MASENELLI, L. BARDOTTI,
B. PREVEL, J. TUAILLON-COMBES, E. BERNSTEIN, F. TOURNUS,
I. WANG, A. TAMION, D. NICOLAS, C. RAUFAST, D. TAINOFF, N. BLANC
Laboratory of Condensed Matter Physics and Nanostructures
University Claude Bernard-Lyon 1 and CNRS
Lyon - France
Particles 2007 – Toronto-Canada – 19-21 july 2007
¬
GENERAL CONTEXT
Two main approaches to nanostructure preparation
" Top Down "
" Bottom Up "
Elementary Bricks :
- atoms, molecules
Functionalized film
Substrate
¬ Nano-Engraving Technique :
- Nano-lithography,
- Nano-imprint,
- FIB…
¬ Deposition :
- Nucleation
- Growth
Clusters
preformed :
- i.e. in the
Gas-phase
¬ Deposition :
- Nucleation
- Growth
Substrate
Couche Fonctionnalisée
Substrate
Substrat
Substrate
OUTLINE
¾ Free cluster production and deposition on substrates using the :
Low Energy Cluster Beam Deposition technique (LECBD)
characteristic examples of functionalized cluster-assembled
¾ Some
nanostructures :
- Magnetic from TM (i.e. Co, Fe,…) and mixed TM-X clusters (i.e. Co-Sm,
Co-Pt)
- Optical from photoluminescent sesquioxide clusters (i.e. Gd2O3:Eu3+)
¾ Preparation of 2D–organized arrays of cluster-assembled dots on
FIB-functionalized substrates
- Application to very high integration-density devices (~ Gbits/in2 -Tbits/in2)
¾ Conclusion and prospects
NANOSTRUCTURE PREPARATION FROM CLUSTERS
Pulsed valve
(He : 4 – 6 bars)
Laser-vaporization cluster
Source
He
¬ Typical cluster sizes : a
few tens to a few thousands
of atoms (~ 1 to 4 nm)
YAG Laser N° 1
¬Very high cooling rate :
~ 108 - 1010 K/s
YAG Laser N° 2
Target rod
Supersonic
expansion
TOF- mass spectrometer
Mass
spectrum
UHV-Deposition
chamber
Ã
à Neutrals
Ions
Free cluster studies
RHEED
Diffractometer
¬ LECBD regime :
clusters are not
fragmented upon
impact on the substrate
Evaporation
cell
XPS-ISS
Analyzer
Substrate
STM-AFM
Microscope
Eximer Laser
OUTLINE
¾ Free cluster production and deposition on substrates using the :
Low Energy Cluster Beam Deposition technique (LECBD)
characteristic examples of functionalized cluster-assembled
¾ Some
nanostructures :
- Magnetic from TM (i.e. Co, Fe…) and mixed TM-X clusters (i.e. Co-Sm,
Co-Pt)
- Optical from photoluminescent sesquioxide clusters (i.e. Gd2O3:Eu3+)
¾ Preparation of 2D–organized arrays of cluster-assembled dots on
FIB-functionalized substrates
- Application to very high integration-density devices (~ Tbits/in2)
¾ Conclusion and prospects
FUNCTIONAL MAGNETIC NANOSTRUCTURES (*)
To overcome the superparamagnetic limit
¬ High magnetic anisotropy nanoclusters
, High magnetic-blocking Temperature ( ≥ 300 K)
, Application to high density data-storage systems (~ Tbits/in2)
1,5
Normalized Magnetization
M/Ms
Cobalt-Samarium System
300 K
20 K
1,0
SmCo5-clusters
0,5
20 K
0,0
300 K
-0,5
-1,0
2 nm
2 nm
-1,5
-3
Cobalt-Platinum System
CoPt _ L10-Phase
CoPt _ A1-Phase (fcc)
[
-2
-1
0
1
2
3
Magnetic Field (kOe)
CoPt3 _ fcc-Phase
]
2 nm
2 nm
(*) see : "Functionalized cluster-assembled magnetic nanostructures for applications to high
integration-density devices", A. Perez et al., Adv. Engineer. Mat., 7(6), 475 (2005).
FUNCTIONAL MAGNETIC NANOSTRUCTURES
¬
Size control
Without mass-selection :
, size dispersion ~ 40 %
Mass-selected cluster deposition using
an electrostatic mass-analyzing system :
, size dispersion ~ 5 %
As deposited
CoPt-clusters
on a-C substrates
at 300 K.
CoPt _ A1-Phase
(fcc)
Fit
Log-normal
Diameter (nm)
Size Histogram
Number of particles
Number of particles
Size Histogram
Ø = 2 nm
Diameter (nm)
OUTLINE
¾ Free cluster production and deposition on substrates using the :
Low Energy Cluster Beam Deposition technique (LECBD)
characteristic examples of functionalized cluster-assembled
¾ Some
nanostructures :
- Magnetic from TM (i.e. Co, Fe…) and mixed TM-X clusters (i.e. Co-Sm,
Co-Pt)
- Optical from photoluminescent sesquioxide clusters (i.e. Gd2O3:Eu3+)
¾ Preparation of 2D–organized arrays of cluster-assembled dots on
FIB-functionalized substrates
- Application to very high integration-density devices (~ Tbits/in2)
¾ Conclusion and prospects
FUNCTIONAL OPTICAL NANOSTRUCTURES (*)
¬ Preparation and characterization of novel photoluminescent nanostructures exhibiting :
- A high-emission efficiency
- A good stability under high-power excitation
- Emission wavelength adjustable in a wide range of visible
- Potential applications to nano-optics devices
- Alternative to photoluminescent semiconducting-nanostructures
¬ Rare-earth doped sesquioxyde nanoparticles, i.e. :
- Gd2O3:Eu3+, Y2O3:Eu3+ (red emission)
- Gd2O3:Tb3+ (green emission)
- GdBO3:Pr3+ (blue emission)
Luminescent materials commonly
used for TV-screen coatings
¬ In such materials, the doping element is responsible of the light emission and not the
sesquioxide matrix
¬ Fundamental aspects :
- Confinement effects in strongly ionic nanocrystals ? ? ?
- Comparison to the well known effects in semiconducting nanoparticles
(*) See : "Quantum confinement effect on Gd2O3 clusters"
B. Mercier et al., J. of Chem. Phys., 126, 044507 (2007).
PHOTOLUMINESCENTE NANOSTRUCTURES FROM Gd2O3:Eu3+ - CLUSTERS
HRTEM-image of a Gd2O3:Eu3+ cluster
deposited on an a-C coated grid at 300 K
d22-2
d400 = 2,67 Ǻ
30
Number of particles
d222 = 3,08 Ǻ
Size distribution
<d> = 3,2 nm
25
20
2.5 nm ≤ 80% ≤ 3.6 nm
15
10
5
0
0
2
4
6
8
Diameter (nm)
2,7 nm
Cubic structure with : a = 10.7 Å
Bulk phase (bixbyite, bcc, Ia3) : a = 10.8 Å
O2Gd3+
Composition
¾Target-rod mounted in the cluster source :
Gd2O3 doped Eu3+ (10 %)
¾ Average composition deduced from
XPS-measurements on a thick clusterassembled film : Eu3+ ≈ 13 %
Rhombic dodecahedron
10
PHOTOLUMINESCENCE PROPERTIES OF Gd2O3:Eu3+ - NANOCLUSTERS
CB
Red emission : transition between
the 4f-levels of Eu3+-impurities
Laser excitation : λexc
Variation of the gap of the Gd2O3-matrix
as a function of the size (*)
ΔEg
ΔEg (eV)
(eV.)
1
CdS
- Krishna et. al
- Ledoux et. al
- Viswanatha et. al
- Nanda et. al
CdS - Our results
Si
0,1
0,01
α=
ZnO
ΔE g =
α
Gd2O3
dγ
CuBr
Gd2O3
with γ = 1.39 (**)
2
5
10
α=
20
1,0
0,8
- Bulk Gd2O3:Eu3+ (5 %)
- Cluster film (<Ф> = 3.2 nm)
- Cluster film (<Ф> = 2.8 nm)
0,6
0,4
8.7
8
0,2
3 .7
3
α=
1
α = .67
1 .4
α=
7
0.5
6
nanocrystals diameter (nm)
Intensity (a.u)
VB
0,0
580 600 620 640 660 680 700 720
Wavelength (nm)
50
Nanocrystal diameter : d (nm)
(*) Deduced from excitation measurements in VUV
on cluster films (Gd2O3:Eu3+ (10 %), at 10 K)
using the synchrotron radiation at DESY
(**) See : C. Delerue et.al., Phys. Rev. B 48, 11024 (1993).
OUTLINE
¾ Free cluster production and deposition on substrates using the :
Low Energy Cluster Beam Deposition technique (LECBD)
characteristic examples of functionalized cluster-assembled
¾ Some
nanostructures :
- Magnetic from TM (i.e. Co, Fe…) and mixed TM-X clusters (i.e. Co-Sm,
Co-Pt)
- Optical from photoluminescent sesquioxide clusters (i.e. Gd2O3:Eu3+)
¾ Preparation of 2D–organized arrays of cluster-assembled dots on
FIB-functionalized substrates
- Application to very high integration-density devices (~ Tbits/in2)
¾ Conclusion and prospects
DIFFUSION OF DEPOSITED CLUSTERS ON THE SUBSTRATE
EXPERIMENTAL EVIDENCE (*) :
⇓⇓⇓⇓⇓⇓
Two extreme cases depending on the cluster-surface interaction
Easy diffusion of clusters :
No cluster diffusion :
i.e. Gold clusters (Au750) on HOPG at 300K
i.e. Gold clusters (A750) on Au(111) at 300K
t=0.01 nm
t=0.03 nm
t=0.08 nm
t=3.4 nm
100 nm
1μm
150 nm
(*) See : L. Bardotti et al., Phys. Rev. B, 62, 2835 (2000).
150 nm
ISLAND DENSITY
NUCLEATION AND GROWTH PROCESS OF LECBD FILMS : SUMMARY
Nucleation
Growth
Coalescence
COVERAGE
RATE
PREPARATION OF 2D-ORGANIZED ARRAYS OF MAGNETIC CLUSTER-DOTS(*)
¬ Application to high integration-density devices (~ 100 Gbits/in2 - 1 Tbits/in2)
for data storage systems and spintronics
TMAFM images
Nano-craters
(2,5 μm x 2,5 μm)
Nano-hillocks
Functionalized
HOPG-substrates
using the
FIB-nanoengraving
technique
- Ga+-ions 30 keV
- Periodicity 300 nm
50.103 ions/point
10.103 ions/point
5.103 ions/point
2D-arrays of magnetic
CoPt-cluster dots on
FIB-HOPG substrates
nm
300
- Periodicity : 300 nm
, ~ 10 Gbits/in2
3,5 μm
2 μm
0,5 μm
(*) See : "2D arrays of CoPt nanocluster assemblies" A. Hannour et al., Surf. Sci., 594, 1-11 (2005).
CONCLUSION
¾ The preparation of original / functional nanostructures from clusters preformed in the
gas phase using the LECBD technique seems promising :
¬ Model nano-systems well suited for fundamental studies, as well as
functionalized ones well suited for applications are easily synthesized.
¾ The control of the nucleation and growth process of cluster- assembled nanostructures
on functionalized substrates is used to prepare 2D-organized arrays of cluster-dots
¬ Applications to high integration-density devices (, Tbits/in2).
PROSPECTS
¾ Control/modification/combination of core/surface/interface effects to realize functional
nanostructures with unique properties :
¬ mixed clusters : alloying effects, surface effects, segregation effects…
¾ 2D-organized arrays of functionalized nanoclusters on functionalized substrates :
¬ Study of the organization-properties relationship
¬ very high integration densities ( ≥ Tbits/in2)
RESEARCH GROUP ON CLUSTERS AND NANOSTRUCTURES
AT LPMCN – Univ. Lyon 1
O. BOISRON
Engineer
V. DUPUIS
P. MELINON
B. MASENELLI
G. GUIRAUD
Engineer
L. FAVRE
PhD
L. BARDOTTI
A. PEREZ
J. TUAILLON
B. PREVEL
F. TOURNUS
E. BERNSTEIN
2D-PERCOLATION THRESHOLD OF CLUSTER - ASSEMBLED
FILMS PREPARED BY LECBD
V
Cr - electrode
A
Corning-glass
substrate
Electrical-conductivity
measurements in situ
during cluster deposition
1.5 mm
2 mm
-6
10
current intensity (A)
, Coverage rate ≈ 50 %
current intensity (A)
Equivalent deposited
thickness at the
percolation threshold :
t ≈ 2 nm
0.001
-4
10
1.8 nm
-8
10
-10
10
Co-clusters
T = 80 K
-12
10
-14
10
0
2
4
6
8
10
Deposited thickness (nm)
-5
10
2.5 nm
-7
10
-9
10
Ni-clusters
T = 300 K
-11
10
-13
10
0
2
4
6
8
10
12
Deposited thickness (nm)
14
NUCLEATION AND GROWTH MECHANISM
CHARACTERISTIC OF LECBD(*)
incident cluster, size Ni ≈ 102 to 103 atoms
(a)
(c)
(e)
(b)
(d)
(a) Deposition
(b) Diffusion
(c) Nucleation
(d) Coalescence
(e) Growth
Substrate
Clusters are not fragmented
upon impact on the substrate
in the LECBD - regime
Coalescence is limited
⇒ 2D-Growth
(*) Review article : P. JENSEN, Rev. Mod. Phys., 71, 1695 (1999).
NANOSTRUCTURED MORPHOLOGY OF A THICK CLUSTER FILM
Sb-cluster
Film
Si-substrate
TEM cross section view of a thick (~80 nm)
antimony-cluster film deposited on a silicon substrate
at room temperature :
¬
density ≈ 50 to 60 % of the bulk phase
MD-SIMULATIONS OF THE CLUSTER DEPOSITION
C28-Fullerenes on a
semiconducting substrate
Mo1043-Clusters on Mo(001) surface
LECBD
regime
A. Canning et al,
Phys. Rev. Lett, 78, 4442 (1997).
H. Haberland et al.,
Phys. Rev. B, 51, 11061 (1995).
MD-SIMULATIONS OF THE CLUSTER DIFFUSION
ON A CRYSTALLINE SURFACE(*)
Top view
Cross section
view
D=f(Misfit)
D=f(Size N)
, D~Nα
-0.66 < α < -1.4
D=f(Temperature)
(*) P. Deltour et al., Phys. Rev. Lett., 78, 4597 (1997)
EXPERIMENT-SIMULATION OF THE CLUSTER-ASSEMBLED
NANOSTRUCTURE MORPHOLOGIES
¬ DDA - model (Deposition - Diffusion - Aggregation)(*)
¬ Kinetic Monte Carlo (KMC) model
Experiment :
Sb2300-clusters on HOPG at 400 K
DDA - Simulation :
h Incident clusters can diffuse on the
HOPG-surface.
h Dcluster ≈ 10-8 cm2/s
χ
χ
Nislands ~ (F/D) = (F/D0) exp(χ Ea /kT)
h
with χ = 1/3, Ea ≈ 0.7 eV and D0 ≈ 104 cm2/s
Cluster fluence (F) and Temperature (T) allow to control Nislands
(*) See : P. JENSEN, Rev. Mod. Phys., 71, 1695 (1999).
CARACTERISTIC EXAMPLES OF FUNCTIONALIZED
CLUSTER-ASSEMBLED NANOSTRUCTURES
1- Preparation of original semiconducting nanostructures
¬ from
silicon and mixed silicon-carbon cage like clusters :
(*)
C59Si
Si-Fullerenes
Si stuffedFullerènes
C58Si2
Si-C
Heterofullerenes
(C60)13 - Si2
Icosahedral
edifice
Ä Original electronic structures, différent from the bulk-Si one, mainly
due to the presence of large numbers of pentagonal rings
Ä Large quasi direct gap (~ 1.6 to 2 eV) , photoluminescence
Ä
Applications to nano electronics / opto-electronics
(*) See : P. Mélinon et al., in "Clusters as precursors of nano objects",
Eds C. Brechignac et al., Comptes Rendus de Physique, 3 (2002) pp. 273-288.
CLUSTER DIFFUSION AND TRAPPING AT DEFECTS
¬ Application to the preparation of 2D-organized arrays
of cluster-assembled dots
Au750-clusters
on HOPG
at 300K
Trapping at
step edges
1μm
1μm
Au750-clusters on
ion-irradiated
HOPG at 300 K
* Ar+-1,5 keV
25 nm
1μm
¬
KINETIC MONTE CARLO (KMC) SIMULATIONS
Ballistic model DDA (Deposition – Diffusion – Aggregation)(*)
Experiment :
¬ 10-2 ML of Au750-clusters deposited
on FIB-functionalized HOPG at 373 K
Distance between defects : 300 nm
KMC-Simulation considering :
¬ Diffusion of incident clusters and
compact islands with sizes up to 20
clusters.
¬ No evaporation
¬ Introduction of specific trapping
sites (¦) with irreversible sticking
(ideal traps)
(*) See : P. Jensen, Rev. Mod. Phys., 71, 1695 (1999).
SUMMARY OF KMC-SIMULATIONS
Nislands / functionalized substrate (Nisl/fs)
Ndefects / functionalized substrate (Ndef/fs)
100
1
Existence of
islands between
defects
All islands are
Created on defects
N isl / fs / N def / fs
No creation of
Cluster - islands
between defects
10
100
80
60
40
1
20
0.1
0.01
0.1
1
N isl / non-fs / N def / fs
10
0
Mean size of of islands
(number of clusters)
All defects must
be occupied
COMPETITION BETWEEN 2 KINETIC PROCESSES
¬ capture at defects and nucleation outside of defects
Lisl / non-fs
h 2 characteristic lengths
: Mean distance between islands
on virgin substrates (non-functionalized)
, Cluster-cluster aggregation kinetic ~ (F/D)Х
with Х = - 1/6
Ldef / fs
: Mean distance between defects
on functionalized substrates
, define the mean time for a diffusing cluster
to be captured at a defect ~ (Ldef / fs)2 / D
h "Growth on defects" regime (Ldef / fs << Lisl / non-fs)
, A diffusing clusters is captured at a defect before meeting another
cluster to form an island
, filling of defects by a Poisson law , Lowest island-size = 5 clusters
h Other regimes (Ldef / fs >> Lisl / non-fs)
, Nucleation outside of defects is preponderant
ADJUSTMENTS OF THE PARAMETERS OF THE 2D - ARRAYS
¬ Lattice parameter : Ldef / fs
h Adjustment of
Ldef / fs , Adjustment of Ndef / fs
h Adjust. of F (cluster fluence)
h Adjust. of D (diffusion coeff.,
T-dependent)
Adjust. of
Nisl / non-fs ~ (F / D)
Nisl / non-fs
Ndef / fs
χ
(χ= 1/3)
<< 1
¬ Mean Island size
h Defects = exclusive nucleation centres
h Above tc, islands grow by capture of clusters
t = tc
t > tc
¬ Nanoparticle morphology
h Compact rather than ramified
Annealings :
activation of the
ramified / compact
transition
i.e. Au-clusters – 150 °C
1 hour in situ in the TEM
¬ Nature of the clusters
h General behaviour of LECBD films whatever is the nature of the clusters
Au-clusters / HOPG
Sb-clusters / HOPG
Co-clusters / HOPG
MAGNETIC NANOSTRUCTURES FROM CLUSTERS
1- Pure cobalt clusters(*)
X-rays diffraction at grazing incidence on a
80 nm thick Co-cluster film deposited on a
Si-substrate at 300K
* Incident free clusters : Co300 ⇒ Φ ≈ 2 nm
TEM-image
fcc-Co
fcc-Co
1 nm
Fourier transform of the X-rays absorption
spectrum (EXAFS)
Nearest
neighbours
fcc-cobalt cluster
* Truncated octahedron
* Diameter ≈ 3 nm
⇒ 1388 atoms
1 → Co-Co core
(d ≈ 0.25 nm)
[001]
2 → Co-Co surface
(d ≈ 0.26 nm)
3 → Co-0 surface
(d ≈ 0.22 nm)
[100]
[010]
(*) See i.e. J. TUAILLON et al., Phil. Mag., 76, 493 (1997).
MAGNETIC PROPERTIES OF ONE INDIVIDUAL Co-CLUSTER (*)
Using the microSQUID technique developed at LLN - Grenoble
μ0 Η y
Highest sensitivity obtained with the
Co-cluster embedded in the Nb-film at
a micro-bridge :
⇒ ≈ 10-17 emu
⇒ ≈ 103 Bohr-magneton
⇒ ≈ One Co-cluster with Φ ≈ 3 nm
Hz (T)
Hx (T)
Hy (T)
1 μm
1 μm
μ0 Η x
1 μm
micro-bridge
junctions
cluster
Cluster
20 nm
200 nm
Niobium
loop
9
3D-switching field distribution
measured at 35 mK for a Co-cluster :
Φ ≈ 3 nm ⇒ ≈ 1000 Co-atoms
⇒ 2 anisotropy axes :
- Hard // Hy
- Easy // Hz
Hz (T)
Simulation using the StonerWohlfarth uniform rotation model :
E(m)/V = - K1mZ2 + K2mY2
⇒ K1 = 2.1 105 J/m3
⇒ K2 = 0.5 105 J/m3
Hx (T)
Hy (T)
(*) See : M. JAMET et al., Phys. Rev. Lett., 86, 4676 (2001).
EVOLUTION WITH TEMPERATURE
2D-switching field distributions in the yz-plane
measured at different temperatures
0.3
0.04 K
T B - 14 K
1K
0.2
2K
0.1
8K
12 K
μ0
μ0Hz (Tesla)
4K
0
-0.1
-0.2
-0.3
-0.3
-0.2
-0.1
0
0.1
0.2
0.3
μ0Hy (Tesla)
⇓⇓⇓⇓⇓
Blocking temperature TB ≈ 14 K for a Co-cluster (Φ ≈ 3 nm)
MIXED COBALT – SAMARIUM CLUSTERS(*)
h High magnetic anisotropy Ä High blocking temperature (~ 400 K)
h Applications to high density memory devices and spin electronics
Deposited on a-C at 300K
Annealed in UHV at 570 °C
1.27 Å
2.09 Å
2.92 Å
1.07 Å
1.82 Å
2 nm
15 nm
100
Φ ≈ 3.5 nm
80
60
40
20
0
0
1
2
3
4
5
Diamètre (nm)
6
Diameter (nm)
7
8
Nombre de particules
120
Number of particles
of particles
Number
Nombre de particules
10 nm
80
70
Φ ≈ 6 nm
60
50
40
30
20
10
0
0
2
4
6
8
10
12
14
Diamètre (nm)
Diameter (nm)
(*) See M. NEGRIER et al., Europ. Phys. J. D, 9, 475 (2000).
MAGNETIC PROPERTIES OF MIXED Co-Sm CLUSTERS
Magnetization measurements
As deposited at 300 K
Annealed at 770 K
1.5
300 K
20 K
1
0.5
20 K
0
300 K
-0.5
-1
-1.5
-3
-2
-1
0
1
2
3
Magnetic field (kOe)
Normalized
magnetization : M/Ms
Normalized
magnetization : M/Ms
1.5
300 K
20 K
1
0.5
20 K
0
300 K
-0.5
-1
-1.5
-3
-2
-1
0
1
2
3
Magnetic field (kOe)
Annealed at 840 K
Normalized
magnetization : M/Ms
1,5
Nanosize clusters (Φ ≈ 5 nm)
magnetically blocked at T > 300 K.
300 K
20 K
1,0
0,5
20 K
0,0
*Problem of the segregation of
300 K
-0,5
-1,0
-1,5
-3
-2
-1
0
1
Magnetic field (kOe)
2
3
samarium at the cluster surface.
*Recrystallisation after annealings
(Sm-Nb non-miscible).
SURFACE CONTAMINATION
XPS-measurements (O1s-level)
on Gd2O3:Eu3+-cluster films
Intensity (a.u.)
5
Presence of hydroxide pollution
(c) - Bulk hydroxide sample : Gd2(OH)3
4
(b) - Cluster film transferred in air
and subsequently annealed
for ½ hour at 300 °C
3
2
(a) - As deposited cluster-film
transferred in air
1
0
526
528
530
532
534
Binding energy (eV)
536
STRUCTURE
¬ Phase transition in the low cluster-size range
as observed from cathodoluminescence spectra of Eu3+-impurities
Cathodoluminescence measurements (using e- 4 keV) in situ in UHV
on 10 nm-thick Gd2O3:Eu3+-cluster films deposited on Si-passivated substrates
Mean cluster-size 3.2 nm
Mean cluster-size 2.5 nm
0,8
0,8
0,6
0,6
Cluster film
(Ф ≈ 2.5 nm)
u.a.
1,0
u.a.
1,0
Cluster film (Ф ≈ 3.2 nm)
0,4
0,4
Bulk cubic-phase
0,2
0,2
0,0
0,0
Bulk monoclinic-phase
600
610
620
630
640
600
610
w avelength (nm )
620
630
640
w avelength (nm )
Cubic
monoclinic
Transition pressure at room temperature
for the bulk phase ~ 2 GPa
¬ Possibility of a phase transition at low cluster-size (~2 nm)
SIZE EFFECT ON THE PHOTOLUMINESCENCE PROPERTIES
Particular Case of Y2O3:Ce3+
Nanocrystals
Bulk materials
¬ e--delocalization
¬ Confinement effects F Widening of the gap
No luminescence
¬ Luminescence of Ce3+ ?
CB
Ce 5d
CB
Ce 5d
X
Ce 4f
?
Ce 4f
VB
M.Raukas et. al. Appl. Phys. Lett. 69, 3300 (1997).
D.Jia et. al. Phys. Rev. B 69, 235113 (2004).
VB
PHOTOLUMINESCENCE MEASUREMENTS
¬ VUV-Synchrotron radiation at DESY
¬ Sample : Y2O3:Ce3+ (1 %)
¬T = 10 K
Appearance of an emission band
probably due to Ce3+-impurities
Nanocrystals
Ce 5d
Emission spectra
Normalized intensity (a.u.)
CB
?
Ce 4f
VB
Excitation spectra
Intensity (a.u.)
1,0
<d> = 3 nm
0,8
<d> = 30 nm
0,6
Bulk
0,4
0,2
0,0
<d> = 9 nm
0,8
1,0
250
<d> = 30 nm
0,4
0,2
0,0
180
Bulk
190
200
210
220
Wavelength (nm)
230
350
400
450
500
550
600
650
Wavelength (nm)
<d> = 15 nm
0,6
300
240
Excitation
at 200 nm
Type :
Self-Trapped
Exciton emission