EXAMPLE 1. Core level spectroscopy (XPS): And how much? What do we have on the surface?

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1 1... Examples We will discuss a few examples from current research with emphasis on in-situ and real-time characterisation of growth phenomena using spectroscopy. For details, we refer to dedicated lecture courses on methods and the references given. θ n+1 θ n θ n-1 F D 1 EXAMPLE 1 >> 1eV inner shells (core levels) XPS, etc Core level spectroscopy (XPS): What do we have on the surface? And how much? XPS: Spectra are established and are used to characterise an unknown sample.

2 Photo-Effect: History 1. H. Hertz, Ann. Physik 31, 983 (1887).. A. Einstein, Ann. Physik 17, 13 (1905) Albert Einstein explains photoelectric effect 1, 191 Nobel prize 3 Photo-Effect: History Mid1960s Kai Siegbahn and his group develop photoelectron spectroscopy 1 1. K. Siegbahn, Et. Al., Nova Acta Regiae Soc.Sci., Ser. IV, Vol. 0 (1967) Nobel prize 4

3 XPS: Element-specific information of coverage Determination of the stoichiometry PEN : PFP mixtures XPS intensity proportional to coverage Ch. Frank, Diploma thesis, Tübingen XPS: Core level chemical shifts Information about chemical environment of the atoms Core level chemical shifts caused by overall charge on atom moderated by environment (initial state effect) Charge withdrawal increases BE by reducing nuclear shielding 6

4 Inelastic mean free pathλ e "universal curve" of IMFP versus E kin XPS: Sampling depth Probability of escape without loss determines Sampling depth z of XPS Maximum surface sensitivity at ev (Λ e ~ 5 Å) 95 % of electrons from 3Λ e depth 7 EXAMPLE Optical spectroscopy during growth in situ and in real time Optical spectra: Assume that structure is established and optical spectroscopy is used to explore optical properties and their changes during growth. 8

5 Intensity In-Situ Growth Studies a b c d e f g h q z (Å -1 ) out-of-plane structure in-plane structure 9 Real-time growth studies: Optics Proehl et al., PRB 71 (005) Hosokai et al., APL 97 (010) Heinemeyer et al., PRL 104 (010)

6 Real-time growth studies: Optics Differential reflectance spectroscopy (DRS) (a very simple and efficient technique) R R R s = I r s I 0 R Rs = R s R = I r f +s I 0 light source CCD detector optical fiber g lens UV-vis window b sample UHV chamber Measure reflected Normal incidence (sensitive to in-plane component only) In-situ Proehl et al., PRB 71 (005) Hosokai et al., APL 97 (010) Heinemeyer et al., PRL 104 (010) Real-time growth studies: Optics Differential reflectance spectroscopy (DRS) (a very simple and efficient technique) 3-phase system Fresnel coefficient (1) ambient ε a () film ε f (3) substrate ε s Simplifications (very thin films): E + r = + r r iβ = r 0 13 = iβ E 1+ r1 r3e R = (r ) 1 Thin film limit d «λ (expand to first order in β) transparent substrate normal incidence (AOI = 0 ) r e πn f d cos( AOI) β = λ R R 8πd = λ ε 1 n substrate Proehl et al., PRB 71 (005) Hosokai et al., APL 97 (010) Heinemeyer et al., PRL 104 (010)

7 Real-time growth studies: Optics follow in real time during growth (DIP, PEN, PFP) spectra coupled to structure (Franck-Condon etc.) coupling to vibrations coupling to neighbours LUMO+1 LUMO monomer spectrum HOMO HOMO-1 Zhang et al., PRL 104 (010) Hosokai et al., APL 97 (010) Heinemeyer et al., PRL 104 (010) Optical Properties: Theory Electron-phonon coupling (Huang-Rhys-parameter S) energy levels resulting spectrum Mω S = Q h S 1,n S 0, 0 S e I( 0 n ) = n! n S If structure changes (DQ) (e.g. during growth) then S will change (and spectrum) 14

8 Optical Properties During Growth using differential reflectance spectroscopy (DRS) and spectroscopic ellipsometry with CCD detection θ n+1 θ n θ n-1 F D Zhang et al., PRL 104 (010) Hosokai et al., APL 97 (010) Heinemeyer et al., PRL 104 (010) Optical Properties During Growth Forth mode I 4 due to intermolecular coupling growing with coverage (up to ~ 15 nm) Shift ~ 50 mev(similar to solvent shift ) Interaction between molecules within one ML. Interaction between molecules in different ML s 3. Interaction between molecules and substrate monomer 0.7 nm 14.6 nm I 4 I 4 Zhang et al., PRL 104 (010) Hosokai et al., APL 97 (010) Heinemeyer et al., PRL 104 (010)

9 Optical Properties During Growth Forth mode I 4 due to intermolecular coupling growing with coverage (up to ~ 15 nm) Shift ~ 50 mev(similar to solvent shift ) Oil WaxPolym ers Amph Langm uirfilms iphilicsyst ems Self-A Mon sembling olayers(sa Ms) Micel les Biolog icalmemb ranes LiquidCr Molecula ystals rcrystals Interaction between molecules within one ML. Interaction between molecules in different ML s 3. Interaction between molecules and substrate In analogy to, e.g., magnetism expect surface contribution to enter in first approximation as Energy density = (E bulk ) + (E surface / d) Zhang et al., PRL 104 (010) Hosokai et al., APL 97 (010) Heinemeyer et al., PRL 104 (010) Conclusions There are things happening during growth, structurally (out of plane and also in plane) and optically 14.6 nm I 4 18