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

qq

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