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Local Optical Spectroscopy using Photon-Scanning Tunneling Microscopy
and Beyond
Fernando Stavale and Niklas Nilius Scanning Probe Spectroscopy Group
Department of Chemical Physics Fritz-Haber Institut
Averaging technique
Inte
nsi
ty
Energy
Inhomogeneous spectral broadening
Local technique
Inte
nsi
ty
Energy
Homogeneous spectral broadening
Photons Electrons
Size and shape distribution of particles in an ensemble leads to inhomogeneous spectral broadening
Classical Spectroscopy versus Local optical Spectroscopy
Introduction
Background
Experimental setup
Tip-sample resonator
Excitation
mechanisms
Tip-induced Plasmons
FER
Metal particles
Semi-
conductors
Molecules
Oxides
Conclusions
Haes, Haynes, McFarland, Zou, Schatz, Van Duyne, MRS Bulletin 30, 368 (2005)
No correlation between optical and structural data of nano-particles
Rayleigh scattering of differently-sized Ag particles with confocal microscopy
(130 x 130 µm2)
1 2
1 2
Electro-luminescence from p-conjugated polymers on ITO
Lupton, Pogantsch, Piok, List, Patil, Scherf, Phys. Rev. Lett. 89 (2002) 167401
No information on binding properties and conformation of molecules
Classical Spectroscopy versus Local optical Spectroscopy
Introduction
Background
Experimental setup
Tip-sample resonator
Excitation
mechanisms
Tip-induced Plasmons
FER
Metal particles
Semi-
conductors
Molecules
Oxides
Conclusions
Local optical techniques
Spectroscopy of single objects (clusters, molecules, quantum wells)
no inhomogeneous broadening due to ensemble properties
no background effects due to statistical disorder and defects
Correlation with structural information:
• geometry (size/shape of particles)
• chemistry (composition)
• environment (binding conditions, coupling to neighbors)
Spatial resolution of optical microscopy:
• restricted by Abbe’s diffraction limit:
• resultion approximately 250-500 nm
• with tricks (Confocal and Laser microscopy) 100nm
nd
2
Introduction
Background
Experimental setup
Tip-sample resonator
Excitation
mechanisms
Tip-induced Plasmons
FER
Metal particles
Semi-
conductors
Molecules
Oxides
Conclusions
Near-Field Optical Microscopy and Spectroscopy
Lukas Novotny et al Annu. Rev. Phys. Chem. 2006. 57:303–31
Introduction
Background
Experimental setup
Tip-sample resonator
Excitation
mechanisms
Tip-induced Plasmons
FER
Metal particles
Semi-
conductors
Molecules
Oxides
Conclusions
Approach
Exploration of the optical near field
Scanning near-field optical microscopy
d 50nm
Spatially resolved excitation of optical modes & far-field detection
Cathodoluminescence STM-based techniques
d 0.5 nm
Introduction
Background
Experimental setup
Tip-sample resonator
Excitation
mechanisms
Tip-induced Plasmons
FER
Metal particles
Semi-
conductors
Molecules
Oxides
Conclusions
Light Emission from Inelastic Electron Tunneling
John Lambe and S. L. McCarthy Phys. Rev. Lett. 37, 923–925 (1976)
Introduction
Background
Experimental setup
Tip-sample resonator
Excitation
mechanisms
Tip-induced Plasmons
FER
Metal particles
Semi-
conductors
Molecules
Oxides
Conclusions
Enhanced Photon Emission in Scanning Tunnelling Microscopy
J. K. Gimzewski, J-K. Sass et al, Europhys. Lett., 8 (1989) 435.
Optical spectra recorded at constant tunnel current at a series of tunnel voltages as indicated
a) and b) refer to elastic (hot electron) tunnel injection. c) and d ) refer to inelastic tunnelling processes.
Introduction
Background
Experimental setup
Tip-sample resonator
Excitation
mechanisms
Tip-induced Plasmons
FER
Metal particles
Semi-
conductors
Molecules
Oxides
Conclusions
J.K. Gimzewski et al, Z. Phys. B - Condensed Matter 72, 497 501 (1988)
Photon emission with the scanning tunneling microscope Introduction
Background
Experimental
setup
Tip-sample resonator
Excitation
mechanisms
Tip-induced Plasmons
FER
Metal particles
Semi-
conductors
Molecules
Oxides
Conclusions
Photon emission in sacnning tunneling microscopy
R. Berndt and J. K. Gimzewski, Phys. Rev. B 48 (1993)
Introduction
Background
Experimental setup
Tip-sample resonator
Excitation
mechanisms
Tip-induced Plasmons
FER
Metal particles
Semi-
conductors
Molecules
Oxides
Conclusions
S. Ushioda, Journal of Electron Spectroscopy and Related Phenomena 109 (2000) 169
STM-light emission spectroscopy of surface nanostructures Introduction
Background
Experimental
setup
Tip-sample resonator
Excitation
mechanisms
Tip-induced Plasmons
FER
Metal particles
Semi-
conductors
Molecules
Oxides
Conclusions
R. Vogelgesang and K. Kern, Rev. Sci. Instr., Vol. 81, Nov, 2010, pp. 113102.
Versatile optical access to the tunnel gap in a LT-STM Introduction
Background
Experimental
setup
Tip-sample resonator
Excitation
mechanisms
Tip-induced Plasmons
FER
Metal particles
Semi-
conductors
Molecules
Oxides
Conclusions
Photon emission spectroscopy of individual oxide-supported silver clusters in a scanning tunnelling microscope
N. Nilius, Dissertation (2001) H.-M. Benia, Dissertation (2008) Innovative Measurement Techniques in Surface Science, H.-J. Freund et al ChemPhysChem 12 79 (2010)
Introduction
Background
Experimental setup
Tip-sample resonator
Excitation
mechanisms
Tip-induced Plasmons
FER
Metal particles
Semi-
conductors
Molecules
Oxides
Conclusions
Without Tip Electromagnetic point
source of unity strength
Einc(Q,r,w)
101
01
0
)sin(2
qG
E0(r’,w)
Field enhancement: G(Q,r’,w) = Eind(r’,w) / Einc(Q,r,w)
Without tip - Fresnel formula:
Q
Theory of light emission from a STM, P. Johansson, R. Monreal, P. Apell, Phys. Rev. B 42 (1990) 9210
Sample e1
q - wave vector of electromagn. waves
z
Field Enhancement Introduction
Background
Experimental
setup
Tip-sample resonator
Excitation
mechanisms
Tip-induced Plasmons
FER
Metal particles
Semi-
conductors
Molecules
Oxides
Conclusions
With Tip
Sample
Eind(r’,w)
Total field enhancement:
Scalar el.magn. potentials:
Bispherical coordinates (b,a,d) cylinder symmetry
Find(1)
Find(0)
Find(2)
R
indzz
rG 0),',(
),',(),',(),',( 0 rGrGrG ind
Einc(Q,r,w)
e1
e2
d
z
)(cos0
))(2
1())(
2
1(
)0( 00
n
n
n
n
n
nind PeBeA
Field Enhancement Introduction
Background
Experimental
setup
Tip-sample resonator
Excitation
mechanisms
Tip-induced Plasmons
FER
Metal particles
Semi-
conductors
Molecules
Oxides
Conclusions
With Tip
Eind(r’,w)
Determination of F(0,1,2) by solving Laplace equation: Appropriate boundary conditions:
(Etan and D continuous at interface)
Find(1)
Find(0)
Find(2)
0
e1
)0(
0
)2(
02
indind
)0(
0
)1(
01
indind
E0(Q,r,w)
e2
Sample
R
z
Field Enhancement Introduction
Background
Experimental
setup
Tip-sample resonator
Excitation
mechanisms
Tip-induced Plasmons
FER
Metal particles
Semi-
conductors
Molecules
Oxides
Conclusions
e1
e2
d
200 400 600 800
0
50
100
150
200
250
Fi
eld
en
han
cem
ent
Wavelength (nm)
+-+-+
-+ +- - +++++
- - ---
Development of strong electromagnetic field in tip-sample cavity
induced by collective electronic excitations in tip and sample
Resonance conditions determined by dielectric tip-sample properties Cut-off frequency: plasmon in Ag sphere
Optical mode
Acoustic mode R = 100Å
d = 5Å
Ag
Ag-sample
Tip-induced plasmons (TIP)
2real(lcut-off = 350nm)
Field Enhancement Introduction
Background
Experimental
setup
Tip-sample resonator
Excitation
mechanisms
Tip-induced Plasmons
FER
Metal particles
Semi-
conductors
Molecules
Oxides
Conclusions
Plasmons and Surface Plasmons
Dispersion Relation
Plasmons are associated with the collective oscillation of conduction electrons in the simple Drude-type model (in the tip-sample junction along the tip-sample axis, where the maxima in the emission spectra corresponding to the resonance modes)
Surface plasmons are confined electromagnetic waves that propagate along the metal-dielectric interface
Introduction
Background
Experimental setup
Tip-sample resonator
Excitation
mechanisms
Tip-induced Plasmons
FER
Metal particles
Semi-
conductors
Molecules
Oxides
Conclusions
Wavelength (nm)
Fiel
d e
nh
ance
men
t Distance Dependence
200 300 400 500 600 700 800 9000,1
1
10
100
10005 Å 9 Å
13 Å 100 Å
Field enhancement increases with decreasing tip-sample distance
Enhanced electromagnetic coupling
Ag-Ag
Fiel
d e
nh
ance
men
t
Wavelength (nm) 200 300 400 500 600 700 800 900
0,1
1
10
100
1000
Radius Dependence
300 Å 200 Å 100 Å 40 Å
Field enhancement increases with tip radius
Enhanced polarizability of tip-sample contact
Ag-Ag
Field Enhancement Introduction
Background
Experimental
setup
Tip-sample resonator
Excitation
mechanisms
Tip-induced Plasmons
FER
Metal particles
Semi-
conductors
Molecules
Oxides
Conclusions
Wavelength (nm)
Fiel
d e
nh
ance
men
t
Material Dependence
200 300 400 500 600 700 800 900 10000,01
0,1
1
Frequency course of TIP’s depends on dielectric tip-sample properties
Narrow and intense modes only for small imaginary parts of dielectric functions
W-tip / Pt-sample
W-tip NiAl-sample
Ag-tip / Ag-sample
x 250
Field Enhancement Introduction
Background
Experimental
setup
Tip-sample resonator
Excitation
mechanisms
Tip-induced Plasmons
FER
Metal particles
Semi-
conductors
Molecules
Oxides
Conclusions
L. Limot, T. Maroutian, P. Johansson, R. Berndt, PhysRevLett. 91 196801 (2003) S. Cramp, PhysRevLett. 95 046801 (2005)
dI/dV spectrum taken on Ag(111) (T 4:6 K).
Stark effect—the shift in energy due to the electric field—has been identified in scanning tunneling spectroscopy (STS) of surface-state electrons at a metal surface
Surface-State Stark Shift in a Scanning Tunneling Microscope Lifetimes of Stark-Shifted Image States
Tip-sample resonator Introduction
Background
Experimental
setup
Tip-sample resonator
Excitation
mechanisms
Tip-induced Plasmons
FER
Metal particles
Semi-
conductors
Molecules
Oxides
Conclusions
Roger M. Macfarlane , Journal of Luminescence 125 (2007) 156
Optical Stark spectroscopy of solids
Screening by metal surfaces can reduce the oscillator frequency at short distance, a red
Shift in the meV range. Also the stark effect play a role, as a shift and or splitting
The Stark effect measures the electric dipole moment of a particular quantum state (analogous to the Zeeman effect) The optical Stark effect measures the change in frequency of an optical transition, with respect an external electric field
Tip-sample resonator Introduction
Background
Experimental
setup
Tip-sample resonator
Excitation
mechanisms
Tip-induced Plasmons
FER
Metal particles
Semi-
conductors
Molecules
Oxides
Conclusions
jelast
jinelast
Tip Sample
TIP
E -eVF
EF
Ene
rgy
Tip-induced plasmons
TIP modes excited by inelasti-cally tunneling electrons
Energy loss occurs in gap (effect of tip and sample material)
Light emission following the radiative decay of TIP modes
jelast
Tip Sample
E -eVF
EF
Electro-luminescence
Injection of hot electrons (holes) into sample surface
Optically active modes localized exclusively in sample
Emission properties dominated by sample material
Excitation Mechanisms Introduction
Background
Experimental
setup
Tip-sample resonator
Excitation
mechanisms
Tip-induced Plasmons
FER
Metal particles
Semi-
conductors
Molecules
Oxides
Conclusions
Spontaneous Emission Probabilities at Radio Frequencies
E. M. Purcell, Harvard University
The observation that atomic decay rates are dependent on the local environment
where P and P0 are the power dipole radiations in the presence of the optical antenna and in free space
Excitation Mechanisms in the Tip-sample resonator Introduction
Background
Experimental
setup
Tip-sample resonator
Excitation
mechanisms
Tip-induced Plasmons
FER
Metal particles
Semi-
conductors
Molecules
Oxides
Conclusions
200 400 600
Wavelength (nm)
Inte
nsi
ty
Spectroscopy mode
Photon mapping
Topography Optical signal
Ag particles on alumina/NiAl(110)
Experiment Introduction
Background
Experimental
setup
Tip-sample resonator
Excitation
mechanisms
Tip-induced Plasmons
FER
Metal particles
Semi-
conductors
Molecules
Oxides
Conclusions
Photo- multiplier
Polarization Prism
Spectrograph & CCD
Primary mirror and microscope head
Secondary mirror
Excitation bias: 3.0-20 V
Electron current: 1-10 nA
Wavelength range: 200-1200 nm (1-6 eV)
Spectral acquisition time: 1-25 min
Experiment Introduction
Background
Experimental
setup
Tip-sample resonator
Excitation
mechanisms
Tip-induced Plasmons
FER
Metal particles
Semi-
conductors
Molecules
Oxides
Conclusions
7V
5V
4V 3V 2V
1.5V
Tip-induced Plasmons
Inte
nsi
ty
Inte
nsi
ty
9V
7V
5V
4V
3V
2V
W-tip / NiAl(110)-sample PtIr-tip / NiAl(110)-sample
Exp
erim
ent
Th
eory
Calculated emission cross section: 10-7 photons per electron
4V
6V
8V
2V
Wavelength (nm)
4V
6V
8V
2V
Wavelength (nm)
Inte
nsi
ty
Inte
nsi
ty
200 400 600 800 1000 200 400 600 800 1000
11nm
Introduction
Background
Experimental setup
Tip-sample resonator
Excitation
mechanisms
Tip-induced Plasmons
FER
Metal particles
Semi-
conductors
Molecules
Oxides
Conclusions
Inte
nsi
ty
4V
6V
8V
2V
4V
6V
8V
2V
Dielectric properties W, PtIr, NiAl
TIP spectrum determined by NiAl & PtIr dielectric properties
W: not actively participating in emission process
e1 = -2 resonance condition for metal sphere TIP-active
regions
PtIr
NiAl W
Wavelenght (nm)
Tip-induced Plasmons Introduction
Background
Experimental
setup
Tip-sample resonator
Excitation
mechanisms
Tip-induced Plasmons
FER
Metal particles
Semi-
conductors
Molecules
Oxides
Conclusions
MgO thin films on Mo(100) / Au-tip
Intense light emission from highest MgO islands
MgO insulator: no contribution to plasmonic excitations ??
20nm 20nm
Usample=5V, I=1 nA, 100x100 nm2
Topography Photon map
Introduction
Background
Experimental setup
Tip-sample resonator
Excitation
mechanisms
Tip-induced Plasmons
FER
Metal particles
Semi-
conductors
Molecules
Oxides
Conclusions
Field-Emission Resonances
4.6 V 4.9 V 5.2 V
6.2 V 6.6 V5.8 V5.4 V
4.6 V
MgO on Mo(100) / Au-tip - Bias Dependence
Emission yield depends on applied bias and MgO island height
High islands emit at lower bias voltage
MgO
Mo(100)
Topo
Field-Emission Resonances Introduction
Background
Experimental
setup
Tip-sample resonator
Excitation
mechanisms
Tip-induced Plasmons
FER
Metal particles
Semi-
conductors
Molecules
Oxides
Conclusions
F large F small
Tip Sample
E -eVF
Ener
gy
n=1
n=2
n=3n=4
EF
Tip
Sam
ple
Tip Sample
E -eVF
Ener
gy
n=1
n=2
n=3
EF
Tip
Sam
ple
Thin MgO Thick MgO Thin MgO Thick MgO
F large F small
Drop of work-function with MgO thickness
• compression of surface dipole layer
• reduced image potential interaction due to dielectric layer
Field-Emission Resonances Introduction
Background
Experimental
setup
Tip-sample resonator
Excitation
mechanisms
Tip-induced Plasmons
FER
Metal particles
Semi-
conductors
Molecules
Oxides
Conclusions
Photon emission spectroscopy of thin MgO films with the STM
H-M Benia, P Myrach and N Nilius New Journal of Physics 10 (2008) 013010
Field-Emission Resonances Introduction
Background
Experimental
setup
Tip-sample resonator
Excitation
mechanisms
Tip-induced Plasmons
FER
Metal particles
Semi-
conductors
Molecules
Oxides
Conclusions
Metal particles
Wavelenght (nm)
Lig
ht
inte
nsi
ty 1
23
4
56
Light emission only for electron injection into metal particle
Spatial resolution of the method better than 1nm
Spectroscopy of single metal particles
Emission originates from radiative decays of Mie-plasmons
1 2 3 4 5 6
9nm
Au particles on TiO2(110)
Usample= 15 V, I = 2nA
Introduction
Background
Experimental setup
Tip-sample resonator
Excitation
mechanisms
Tip-induced Plasmons
FER
Metal particles
Semi-
conductors
Molecules
Oxides
Conclusions
collective oscillations of the particle’s free-electron gas excited by electrons or photons
determine absorption & emission properties
• Particle size and shape • Chemical composition (dielectric properties) • Particle environment
+ -
e- ħ
Tip
Sample
Particle
Mie-Plasmons
Energy depends on
Metal particles Introduction
Background
Experimental
setup
Tip-sample resonator
Excitation
mechanisms
Tip-induced Plasmons
FER
Metal particles
Semi-
conductors
Molecules
Oxides
Conclusions
1.0 2.0 3.0 4.0 5.0 6.0 7.0 Photon Energy (eV)
Inte
nsi
ty (
arb
. Un
its)
U= -10 V, I = 5 nA, 30nm x 30nm
Mie plasmon energy decreases with increasing particle diameter
Emission yield proportional to number of electron involved in plasmon excitations
Ag particles on Al2O3 / NiAl(110)
Metal particles Introduction
Background
Experimental
setup
Tip-sample resonator
Excitation
mechanisms
Tip-induced Plasmons
FER
Metal particles
Semi-
conductors
Molecules
Oxides
Conclusions
Ag-Au alloy particles on Al2O3 / NiAl(110)
200 400 600 800 Wavelength in nm
200 400 600 800 Wavelength in nm
200 400 600 800 Wavelength in nm
200 400 600 800 Wavelength in nm
100% Ag 50% Ag 25% Ag 10% Ag
(75x75nm)
Continuous red shift of plasmon with increasing Au content in particles
Metal particles Introduction
Background
Experimental
setup
Tip-sample resonator
Excitation
mechanisms
Tip-induced Plasmons
FER
Metal particles
Semi-
conductors
Molecules
Oxides
Conclusions
Photon mapping of individual Ag particles on MgO/Mo(001)
PRB 83, 035416 (2011) P. Myrach, N. Nilius, H-J. Freund
Metal particles Introduction
Background
Experimental
setup
Tip-sample resonator
Excitation
mechanisms
Tip-induced Plasmons
FER
Metal particles
Semi-
conductors
Molecules
Oxides
Conclusions
InP – Quantum dots
Håkanson, Johansson, Holm, Pryor, Samuelson, Seifert, Pistol; Appl. Phys.Lett. 81 (2002) 4443
Structure
Potential diagram
1.5
eV
1.6
eV
1.9
eV
Ene
rgy
Discrimination between quantum dot and capping material via local luminescence measurements (different band gap energies)
InP GaInP
GaAs
GaAs GaInP InP
GaAs InP GaInP
1.57eV
1.94eV
Energy (eV) 1.4 1.6 1.8 2.0
Semiconductors Introduction
Background
Experimental
setup
Tip-sample resonator
Excitation
mechanisms
Tip-induced Plasmons
FER
Metal particles
Semi-
conductors
Molecules
Oxides
Conclusions
InP – Quantum dots
CB VB
Emission fine-structure due to quanitization of InP quantum well states
Spectra reproduced by semi-empirical calculations considering only splitting of conduction band
Semiconductors Introduction
Background
Experimental
setup
Tip-sample resonator
Excitation
mechanisms
Tip-induced Plasmons
FER
Metal particles
Semi-
conductors
Molecules
Oxides
Conclusions
Molecules
Zn-Etioporphyrin on Al2O3 / NiAl(110)
Alumina
NiAl(110)
Insulating spacer layer
Ultra-fast quenching of excited molecular states on metal surfaces
Decoupling of molecular electronic system from support essential to observe light emission
Vibrationally Resolved Fluorescence with STM, X.H. Qiu, G.V. Nazin, W. Ho, Science 299 (2003) 542
NiAl
Alumina
Introduction
Background
Experimental setup
Tip-sample resonator
Excitation
mechanisms
Tip-induced Plasmons
FER
Metal particles
Semi-
conductors
Molecules
Oxides
Conclusions
Tip Oxide NiAlMolecule
Inte
nsi
ty
Me
chan
ism
Zn-Etioporphyrin on Al2O3 / NiAl(110)
Emission fine structure due to coupling of electronic transitions and vibrational progression of the molecule
Molecules Introduction
Background
Experimental
setup
Tip-sample resonator
Excitation
mechanisms
Tip-induced Plasmons
FER
Metal particles
Semi-
conductors
Molecules
Oxides
Conclusions
Wolf-Dieter Schneider et al. Surface Science Reports 65 (2010) 129
Plasmon enhanced luminescence from fullerene molecules excited by local electron tunneling
Molecules Introduction
Background
Experimental
setup
Tip-sample resonator
Excitation
mechanisms
Tip-induced Plasmons
FER
Metal particles
Semi-
conductors
Molecules
Oxides
Conclusions
Morphology: atom-sized dark features increase
0.05% Cr 0.5% Cr 1% Cr
annealing at 1000K in UHV
Mgvacancy
F. Stavale, N. Nilius, H-J. Freund, New J. Physics 14 033006 (2012)
Cathodoluminescence of near-surface centres in Cr-doped MgO(001)
Oxides Introduction
Background
Experimental
setup
Tip-sample resonator
Excitation
mechanisms
Tip-induced Plasmons
FER
Metal particles
Semi-
conductors
Molecules
Oxides
Conclusions
Optical Properties
Exc. = 200 V I = 5 nA t= 300 s
Oxides Introduction
Background
Experimental
setup
Tip-sample resonator
Excitation
mechanisms
Tip-induced Plasmons
FER
Metal particles
Semi-
conductors
Molecules
Oxides
Conclusions
G. F. Imbusch and co-workers, “Luminescence of Inorganic Solids”
Bulk Cr-doped MgO
Octahedral field
+2
x2-y2 z2
xy
xz
yz
+3
x2-y2 z2
xy
xz
yz
Cr-vacancytetragonal
charge compensation mechanism
Cr-vacancy-Crtetragonal Cr-vacancyrhombic
Mg+2 O-2
Cr
Mg+2
Cr
O-2
Free ion Linear Tetrahedral Octahedral
Ene
rgy
xy
z2
xz
yz
xy
xz
yz
x2-y2
Δ
Δ
Crystal Field Theory
z2
x2-y2
x2-y2 z2
xy
xz
yz
Introduction
Background
Experimental setup
Tip-sample resonator
Excitation
mechanisms
Tip-induced Plasmons
FER
Metal particles
Semi-
conductors
Molecules
Oxides
Conclusions
Cathodoluminescence results: diffusion towards the surface
Zero-phonon line Phononic sidebands
Introduction
Background
Experimental setup
Tip-sample resonator
Excitation
mechanisms
Tip-induced Plasmons
FER
Metal particles
Semi-
conductors
Molecules
Oxides
Conclusions
F. Stavale, N. Nilius, H-J. Freund, New J. Physics 14 033006 (2012)
Cathodoluminescence results: excitation mechanism
Oxides Introduction
Background
Experimental
setup
Tip-sample resonator
Excitation
mechanisms
Tip-induced Plasmons
FER
Metal particles
Semi-
conductors
Molecules
Oxides
Conclusions
U. Gebranzig , A. Haug, W. Rosenthal, phys. stat. sol. (b) 68, 749 (1975)
“ Auger recombination in semiconductors is sometimes affected by an electric field. Proceeding from Bloch electrons in such a field the transition probability in this case is calculated. The result shows that electric fields of the order of 103 V/cm or 105 V/cm enhance the recombination probability remarkably “
The Influence of an Electric Field on the Auger Recombination in Semiconductors
Oxides Introduction
Background
Experimental
setup
Tip-sample resonator
Excitation
mechanisms
Tip-induced Plasmons
FER
Metal particles
Semi-
conductors
Molecules
Oxides
Conclusions
RE
O
...4f7 6s2
+2
+3 4f7 → 4f7
4f7 → 4f65d
Free ion
Ener
gy
hu
Octahedral field
7F0
7F1
7F2
5D0
A1
T1
T2
A1
E
Europium-doped Oxide: background Oxides
Introduction
Background
Experimental setup
Tip-sample resonator
Excitation
mechanisms
Tip-induced Plasmons
FER
Metal particles
Semi-
conductors
Molecules
Oxides
Conclusions
Eu-doped MgO (001): morphology and optical properties
mixed-system
ad-system
bare MgO (001)
100x100nm 80x80nm
40x40nm
40x40nm 40x40nm 40x40nm
As evaporated 800 K
800 K
1100 K
1100 K
F. Stavale, L. Pascua, N. Nilius, H.-J. Freund, Phys. Rev. B 86 0854481 (2012 )
Oxides Introduction
Background
Experimental
setup
Tip-sample resonator
Excitation
mechanisms
Tip-induced Plasmons
FER
Metal particles
Semi-
conductors
Molecules
Oxides
Conclusions
Eu+3 site symmetry: resolved spectra
Oxides Introduction
Background
Experimental
setup
Tip-sample resonator
Excitation
mechanisms
Tip-induced Plasmons
FER
Metal particles
Semi-
conductors
Molecules
Oxides
Conclusions
Eu2O3 clusters on MgO: local luminescence spectroscopy
40x40nm
F. Stavale, N. Nilius, H.-J. Freund Appl. Phys. Lett. 101 0131091 (2012 )
Oxides Introduction
Background
Experimental
setup
Tip-sample resonator
Excitation
mechanisms
Tip-induced Plasmons
FER
Metal particles
Semi-
conductors
Molecules
Oxides
Conclusions
Conclusions
Light emission spectroscopy with the STM:
Technique for optical characterization of samples with nm spatial resolution
Photon response amplified by field enhancement in tip-sample cavity
Spectroscopy and photon mapping mode
Applicable for single nano-particles, semiconductor quantum wells and molecules on various supports Also reverse approach: →Coupling laser light into STM junction
• Raman spectroscopy with an STM • Local photo-conductivity measurements • Time and spatially resolved spectroscopy using fs-Laser & STM
Introduction
Background
Experimental setup
Tip-sample resonator
Excitation
mechanisms
Tip-induced Plasmons
FER
Metal particles
Semi-
conductors
Molecules
Oxides
Conclusions