Local Optical Spectroscopy using Photon-Scanning Tunneling Microscopy and Beyond

Transcription

1 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

2 Intensity Intensity Introduction Classical Spectroscopy versus Local optical Spectroscopy Averaging technique Local technique Photons Electrons Energy Energy Inhomogeneous spectral broadening Homogeneous spectral broadening Size and shape distribution of in an ensemble leads to inhomogeneous spectral broadening

3 1 Classical Spectroscopy versus Local optical Spectroscopy (130 x 130 µm 2 ) Rayleigh scattering of differently-sized Ag with confocal microscopy Haes, Haynes, McFarland, Zou, Schatz, Van Duyne, MRS Bulletin 30, 368 (2005) No correlation between optical and structural data of nano- Electro-luminescence from p- conjugated polymers on ITO Lupton, Pogantsch, Piok, List, Patil, Scherf, Phys. Rev. Lett. 89 (2002) No information on binding properties and conformation of molecules

4 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 ) chemistry (composition) environment (binding conditions, coupling to neighbors) Spatial resolution of optical microscopy: restricted by Abbe s diffraction limit: resultion approximately nm 2n with tricks (Confocal and Laser microscopy) 100nm d

5 Near-Field Optical Microscopy and Spectroscopy Lukas Novotny et al Annu. Rev. Phys. Chem :303 31

6 Exploration of the optical near field Scanning near-field optical microscopy d 50nm Approach Spatially resolved excitation of optical modes & far-field detection Cathodoluminescence STM-based techniques d 0.5 nm

7 Light Emission from Inelastic Electron Tunneling John Lambe and S. L. McCarthy Phys. Rev. Lett. 37, (1976)

8 Enhanced Photon Emission in Scanning Tunnelling Microscopy a) and b) refer to elastic (hot electron) tunnel injection. c) and d ) refer to inelastic tunnelling processes. Optical spectra recorded at constant tunnel current at a series of tunnel voltages as indicated J. K. Gimzewski, J-K. Sass et al, Europhys. Lett., 8 (1989) 435.

9 Photon emission with the scanning tunneling microscope J.K. Gimzewski et al, Z. Phys. B - Condensed Matter 72, (1988)

10 Photon emission in sacnning tunneling microscopy R. Berndt and J. K. Gimzewski, Phys. Rev. B 48 (1993)

11 STM-light emission spectroscopy of surface nanostructures S. Ushioda, Journal of Electron Spectroscopy and Related Phenomena 109 (2000) 169

12 Versatile optical access to the tunnel gap in a LT-STM R. Vogelgesang and K. Kern, Rev. Sci. Instr., Vol. 81, Nov, 2010, pp

13

14 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 (2010)

15 Field Enhancement z Without Tip Q Electromagnetic point source of unity strength E inc (Q,r,w) E 0 (r,w) Sample e 1 Field enhancement: G(Q,r,w) = E ind (r,w) / E inc (Q,r,w) Without tip - Fresnel formula: G q 1 0 q 0 sin( Theory of light emission from a STM, P. Johansson, R. Monreal, P. Apell, Phys. Rev. B 42 (1990) 9210 q 1 ) q - wave vector of electromagn. waves

16 Field Enhancement z With Tip e 2 E inc (Q,r,w) F (2) ind F (0) ind F (1) ind d R Sample E ind (r,w) e 1 Total field enhancement: G(, r', ) G0(, r', ) G (, r', ind ) Scalar el.magn. potentials: Bispherical coordinates (b,a,d) cylinder symmetry G(, r', ) 0 z z ind 1 1 ( n )( ) ( )( ) ( 0) 0 n ind Ane Bne n 0 P n (cos )

17 Field Enhancement z With Tip e 2 E 0 (Q,r,w) F ind (2) R F (0) ind F (1) ind Sample E ind (r,w) e 1 Determination of F (0,1,2) by solving Laplace equation: 0 Appropriate boundary conditions: 2 0 (2) ind 0 (0) ind (E tan and D continuous at interface) 1 0 (1) ind 0 (0) ind

18 Field enhancement Introduction Field Enhancement Ag d Ag-sample e 2 R = 100Å d = 5Å e Optical mode Acoustic mode Wavelength (nm) Development of strong electromagnetic field in tip-sample cavity induced by collective electronic excitations in tip and sample plasmons (TIP) Resonance conditions determined by dielectric tip-sample properties Cut-off frequency: plasmon in Ag sphere real 2 (l cut-off = 350nm)

19 and Surface Surface plasmons are confined electromagnetic waves that propagate along the metal-dielectric interface Dispersion Relation 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)

20 Field enhancement Field enhancement Introduction Field Enhancement Distance Dependence Ag-Ag 0, Wavelength (nm) 5 Å 9 Å 13 Å 100 Å Field enhancement increases with decreasing tip-sample distance Enhanced electromagnetic coupling Radius Dependence Ag-Ag 300 Å 200 Å 100 Å 40 Å 0, Wavelength (nm) Field enhancement increases with tip radius Enhanced polarizability of tipsample contact

21 Field enhancement Introduction Field Enhancement Material Dependence 1 Ag-tip / Ag-sample x 250 W-tip / Pt-sample 0,1 W-tip NiAl-sample 0, Wavelength (nm) Frequency course of TIP s depends on dielectric tip-sample properties Narrow and intense modes only for small imaginary parts of dielectric functions

22 Surface-State Stark Shift in a Scanning Tunneling Microscope Lifetimes of Stark-Shifted Image States 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 di/dv spectrum taken on Ag(111) (T 4:6 K). L. Limot, T. Maroutian, P. Johansson, R. Berndt, PhysRevLett (2003) S. Cramp, PhysRevLett (2005)

23 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 Roger M. Macfarlane, Journal of Luminescence 125 (2007) 156

24 Energy TIP Introduction Mechanisms plasmons Electro-luminescence E -ev F j elast j inelast E F E -ev F j elast E F Tip Sample 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 Tip Sample Injection of hot electrons (holes) into sample surface Optically active modes localized exclusively in sample Emission properties dominated by sample material

25 Mechanisms in the Spontaneous Emission Probabilities at Radio Frequencies The observation that atomic decay rates are dependent on the local environment where P and P 0 are the power dipole radiations in the presence of the optical antenna and in free space E. M. Purcell, Harvard University

26 Intensity Introduction Experiment Spectroscopy mode Photon mapping Topography Optical signal Wavelength (nm) Ag on alumina/nial(110)

27 Experiment Photomultiplier Spectrograph & CCD Polarization Prism Primary mirror and microscope head bias: V Electron current: 1-10 na Wavelength range: nm (1-6 ev) Spectral acquisition time: 1-25 min Secondary mirror

28 Theory Intensity Intensity Experiment Intensity Intensity Introduction W-tip / NiAl(110)-sample PtIr-tip / NiAl(110)-sample 7V 5V 4V 3V 2V 1.5V 9V 7V 5V 4V 3V 2V 11nm 4V 6V 8V 2V 4V 6V 8V 2V Wavelength (nm) Wavelength (nm) Calculated emission cross section: 10-7 photons per electron

29 Intensity Introduction Dielectric properties W, PtIr, NiAl TIP-active regions PtIr 4V 6V 8V 2V NiAl Wavelenght (nm) W 4V 6V 8V 2V e 1 = -2 resonance condition for metal sphere TIP spectrum determined by NiAl & PtIr dielectric properties W: not actively participating in emission process

30 Field-Emission Resonances MgO thin films on Mo(100) / Au-tip 20nm 20nm Topography Photon map U sample =5V, I=1 na, 100x100 nm 2 Intense light emission from highest MgO islands MgO insulator: no contribution to plasmonic excitations??

31 Field-Emission Resonances Topo MgO on Mo(100) / Au-tip - Bias Dependence 4.6 V 4.6 V 4.9 V 5.2 V 5.4 V 5.8 V Emission yield depends on applied bias and MgO island height High islands emit at lower bias voltage 6.2 V 6.6 V MgO Mo(100)

32 Energy Sample Energy Sample Tip Tip Introduction Field-Emission Resonances E -ev F F large n=3 n=2 n=1 E -ev F F small n=4 n=3 n=2 n=1 E F E F Tip Sample Tip Sample 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

33 Field-Emission Resonances Photon emission spectroscopy of thin MgO films with the STM H-M Benia, P Myrach and N Nilius New Journal of Physics 10 (2008)

34 Light intensity Introduction Wavelenght (nm) Au on TiO 2 (110) 9nm Light emission only for electron injection into metal particle Spatial resolution of the method better than 1nm Spectroscopy of single metal U sample = 15 V, I = 2nA Emission originates from radiative decays of Mie-plasmons

35 Particle Tip e - ħ + - Sample Mie- Energy depends on collective oscillations of the particle s freeelectron gas excited by electrons or photons determine absorption & emission properties Particle size and shape Chemical composition (dielectric properties) Particle environment

36 Intensity (arb. Units) Introduction Ag on Al 2 O 3 / NiAl(110) Photon Energy (ev) 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

37 Ag-Au alloy on Al 2 O 3 / NiAl(110) (75x75nm) 100% Ag 50% Ag 25% Ag 10% Ag Wavelength in nm Wavelength in nm Wavelength in nm Wavelength in nm Continuous red shift of plasmon with increasing Au content in

38 Photon mapping of individual Ag on MgO/Mo(001) PRB 83, (2011) P. Myrach, N. Nilius, H-J. Freund

39 Energy 1.5 ev 1.6 ev 1.9 ev Introduction Structure InP Quantum dots GaInP InP GaAs GaAs InP GaInP Potential diagram GaAs InP GaInP 1.57eV Energy (ev) 1.94eV Discrimination between quantum dot and capping material via local luminescence measurements (different band gap energies) Håkanson, Johansson, Holm, Pryor, Samuelson, Seifert, Pistol; Appl. Phys.Lett. 81 (2002) 4443

40 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

41 Zn-Etioporphyrin on Al 2 O 3 / NiAl(110) Alumina NiAl 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

42 Intensity Mechanism Introduction Zn-Etioporphyrin on Al 2 O 3 / NiAl(110) Tip Mole cule Oxide NiAl Emission fine structure due to coupling of electronic transitions and vibrational progression of the molecule

43 Plasmon enhanced luminescence from fullerene molecules excited by local electron tunneling Wolf-Dieter Schneider et al. Surface Science Reports 65 (2010) 129

44 Cathodoluminescence of near-surface centres in Cr-doped MgO(001) annealing at 1000K in UHV 0.05% Cr 0.5% Cr 1% Cr Morphology: atom-sized dark features increase Mg vacancy F. Stavale, N. Nilius, H-J. Freund, New J. Physics (2012)

45 Optical Properties E xc. = 200 V I = 5 na t= 300 s

46 Energy Introduction Bulk Cr-doped MgO Mg +2 Cr O -2 Octahedral field +2 Mg +2 O -2 Cr +3 Crystal Field Theory Free ion Linear Tetrahedral Octahedral charge compensation mechanism xy z 2 xz yz x 2 -y 2 xy xz yz z 2 x 2 -y 2 Δ z 2 x 2 -y 2 xy xz yz Δ z 2 x 2 -y 2 z 2 x 2 -y 2 Cr-vacancy tetragonal Cr-vacancy-Cr tetragonal Cr-vacancy rhombic xy xz yz xy xz yz G. F. Imbusch and co-workers, Luminescence of Inorganic Solids

47 Cathodoluminescence results: diffusion towards the surface Zero-phonon line Phononic sidebands

48 Cathodoluminescence results: excitation mechanism F. Stavale, N. Nilius, H-J. Freund, New J. Physics (2012)

49 The Influence of an Electric Field on the Auger Recombination in 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 10 3 V/cm or 10 5 V/cm enhance the recombination probability remarkably U. Gebranzig, A. Haug, W. Rosenthal, phys. stat. sol. (b) 68, 749 (1975)

50 Energy Introduction Europium-doped Oxide: background...4f 7 6s 2 RE Free ion hu 5 D 0 Octahedral field A 1 O 7 F 2 E T f 7 4f 6 5d 7 F 1 7 F 0 T 1 A f 7 4f 7

51 Eu-doped MgO (001): morphology and optical properties bare MgO (001) mixed-system 1100 K 40x40nm ad-system 100x100nm As evaporated 800 K 80x80nm 800 K 1100 K 40x40nm 40x40nm 40x40nm F. Stavale, L. Pascua, N. Nilius, H.-J. Freund, Phys. Rev. B (2012 )

52 Eu +3 site symmetry: resolved spectra

53 Eu 2 O 3 clusters on MgO: local luminescence spectroscopy 40x40nm F. Stavale, N. Nilius, H.-J. Freund Appl. Phys. Lett (2012 )

54 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-, 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