Institute of Solid State Physics Adolf Winkler EMRS-08 Strasbourg 1 Thermal desorption of organic materials (Is there a wetting layer?) Adolf Winkler Graz University of Technology Austria Outline:  F undamentals of Thermal Desorption Spectroscopy (TDS)  Experimental Details  Results (Oligo-phenylenes (4P, 6P) on Au, mica, KCl)  Characterization of the wetting layer  Determination of adsorption energies for wetting- and multilayers  Decomposition of organic molecules in the wetting layer  Summary and conclusions

Institute of Solid State Physics Adolf Winkler EMRS-08 Strasbourg 2 Fundamentals of Thermal Desorption Spectroscopy (TDS) QMS sample Knudsen cell UHV pump QMS sample Knudsen cell UHV pump QMS signal T, t a)Put material on T<, gas dosing, evaporation (Knudsen cell) b)Heat sample (linearly) and measure desorbing particles (QMS, pressure gauge) c)Signal vs. time or temperature yields desorption spectrum

Institute of Solid State Physics Adolf Winkler EMRS-08 Strasbourg 3 Desorption rate (Polanyi - Wigner equation): β : heating rate N: number of adsorbed particles : pre-exponential factor x: desorption order E des : desorption energy  : coverage k: Boltzmann factor Coverage:  = N ads /N surface (for small inorganic molecules and atoms),  max = 1 monolayer (ML)  * = N ads /N ads, max (for large organic molecules),  max << 1 ML,  * max = 1 ML (  *max: physical monolayer)

Institute of Solid State Physics Adolf Winkler EMRS-08 Strasbourg 4 Desorption order: 0 th order: (for condensed phases, multilayers) 1 st order: (for monolayers, non-dissociative) 2 nd order: (for associative desorption, e.g. H 2 ) 0. order 1. order

Institute of Solid State Physics Adolf Winkler EMRS-08 Strasbourg 5 Desorption energy: 0. order: Plot of lnR des vs. 1/T yields desorption energy (heat of evaporation) If R des and N max is known quantitatively, also can be determined. 1. order: T m determines the desorption energy (Redhead formula) If  s -1 then: Redhead, Vacuum 12 (1962) 203.

Institute of Solid State Physics Adolf Winkler EMRS-08 Strasbourg 6 Pre-exponential factor: For atoms and small (inorganic) molecules:  s -1 (attempt frequency) From transition state theory (TST) follows: : Partition function of particles in gas phase : Partition function of adsorbed particles For atoms and small molecules →  For large (inorganic) molecules → → typically much larger than s -1 Attention by using Redhead formula: >> !!!

Institute of Solid State Physics Adolf Winkler EMRS-08 Strasbourg 7 Coverage dependence of desorption energy and desorption order: 0. order changes to 1. order for last layer (wetting layer). 1. order desorption energy often coverage dependent: Attractive interaction: E(  ) = E 0 +  T m shifts to higher T Repulsive interaction: E(  ) = E 0 -  T m shifts to lower T repulsive

Institute of Solid State Physics Adolf Winkler EMRS-08 Strasbourg 8 Experimental details (1): Measurements under ultra-high vacuum conditions (UHV) Desorption rate has to be proportional to detected signal In case of volatile molecules gas density (pressure) changes with R des : S: pumping speed p: pressure V: chamber volume A: sample size K: constant Only if S >> then R des  p For condensable gases: S >> In line-of-sight detection with mass spectrometer (QMS) necessary Typically multiplexing of QMS: reaction products detectable Cracking pattern of molecules Attention: in-line detection requires velocity correction!!

Institute of Solid State Physics Adolf Winkler EMRS-08 Strasbourg 9 Experimental details (2) Resistive heating of single crystal samples Evaporation of material from Knudsen cell Quantitative determination of evaporated material by quartz microbalance Surface cleaning by Ar + ion sputtering Use of additional surface analytical techniques: In-situ: - Auger electron spectroscopy (AES) - X-ray photoelectron spectroscopy (XPS) - Low energy electron diffraction (LEED) Ex-situ: - Atomic force microscopy (AFM) - Scanning tunneling microscopy (STM) - Near edge X-ray absorption fine structure spectroscopy (NEXAFS) - X-ray diffraction (XRD) - Secondary electron microscopy (SEM)

Institute of Solid State Physics Adolf Winkler EMRS-08 Strasbourg 10 Characterization of the wetting layer p-Quaterphenyl (4P) on Au(111) β 1 and β 2 : wetting layer (mean thickness: 0.3 nm) α: multilayer From LEED and NEXAFS: Müllegger et al. J. Chem. Phys. 121 (2004) 2272 Müllegger et al., ChemPhysChem 7 (2006) 2552 From XRD: 4P(211) ║ Au(111) Results:

Institute of Solid State Physics Adolf Winkler EMRS-08 Strasbourg 11 p-Hexaphenyl (6P) on Au(111) 1 st wetting layer: β 1 + β 2 ( nm) 2 nd wetting layer: β 3 ( nm) Multilayer: α Müllegger, Winkler, Surf. Sci. 600 (2006) 1290 Again flat lying and side tilted molecules in wetting layer. From XRD: 6P(21-3) ║ Au(111)

Institute of Solid State Physics Adolf Winkler EMRS-08 Strasbourg 12 6P on mica(0001) β: wetting layer ( nm) α: multilayer 1 nm thick film Frank et al., Surf. Sci. 601 (2007) 2152 From XRD: 6P(11-1) ║mica(0001)

Institute of Solid State Physics Adolf Winkler EMRS-08 Strasbourg 13 6P on KCl(001) Only a single desorption peak No wetting 400 K first strong de-wetting already at low coverage, <0.5 nm Needles (lying m.) + terraces (standing m.) 100 K 400 K Frank et al. Thin Solid Films 516 (2008) 2939 hνhν e-e- hv e-e- 1.1 nm thick From XRD: 6P(20-3) ║ KCl(001)

Institute of Solid State Physics Adolf Winkler EMRS-08 Strasbourg 14 Influence of surface impurities (carbon) on wetting layer 4P-Au(111) 6P-Au(111) 6P-mica(0001)

Institute of Solid State Physics Adolf Winkler EMRS-08 Strasbourg 15 Influence of wetting layer on structure and morphology on 4P thin film on Au(111) TDS LEED XRD AFM clean 0.15 ML C 0.5 ML C 4P(211) 4P(201) 4P(001) Mülleger, Winkler, Surf. Sci. 574(2005)322, Resel et al. J. Cryst Growth 283 (2005) nm 30 nm

Institute of Solid State Physics Adolf Winkler EMRS-08 Strasbourg 16 1 nm thick Substrate impurities and substrate structure influence on the wetting layer (6P on mica(0001)) 1 nm thick Frank et al. Surf. Sci. 601 (2007) nm thick C-contamination sputtering

Institute of Solid State Physics Adolf Winkler EMRS-08 Strasbourg 17 Determination of desorption energy Multilayer (p-Quaterphenyl): From slope: → E = 1.5 eV From Y-axis intercept: → = 1.6x10 21 s -1 Monolayer: E mono ( β 1 )  103·T 1 = 2.6 eV E mono (β 2 )  103·T 2 = 2.1 eV Repulsive interaction energy  = eV/monolayer 4P-Au(111):

Institute of Solid State Physics Adolf Winkler EMRS-08 Strasbourg 18 Compilation of desorption energies and pre-exponential factors System (multi) [s-1] E des (multi) [eV] E des (mono)-β 2 [eV] E des (mono)-β 1 [eV] p-4P/Au(poly) 5x p-4P/Au(111) 1.6x p-6P/Au(111) 5.6x p-6P/Mica(0001) 3.7x p-6P/KCl(001) 3x Pre-exponential factor increases with increasing molecule size Has also been shown theoretically for n-alkanes by: Fichthorn, Miron, PRL 89 (2002) and experimentally by: Tait et al. J. Chem. Phys. 122 (2005)

Institute of Solid State Physics Adolf Winkler EMRS-08 Strasbourg 19 Decomposition of organic molecules in the wetting layer In addition to 4P (6P) desorption also H 2 desorption at high T. → Stems from partial dehydrogenation High temperature peak (B) indicates intermediate of polycyclic aromatic hydrocarbon (PAH) Decomposition increases with nP and surface roughness Müllegger, Winkler Surf. Sci. 600(2006)3982

Institute of Solid State Physics Adolf Winkler EMRS-08 Strasbourg 20 Summary and conclusions TDS can be applied successfully to determine wetting layers Oligo-phenylenes form wetting layers of lying molecules on gold and mica, but not on KCl The wetting layer acts as a template for multilayer formation (Influences structure and morphology) Carbon on the surface strongly influences the wetting layer (decrease of adsorption energy, total removal of wetting layer) TDS allows determination of desorption energies (be aware of a large pre-exponential factors) With TDS decomposition of organic molecules in wetting layer can be detected

Institute of Solid State Physics Adolf Winkler EMRS-08 Strasbourg 21 Acknowledgements Stefan Müllegger Paul Frank Johanna Stettner Ondrey Stranik Roland Resel Thomas Haber Martin Oehzelt Ingo Salzmann Ondrej Lengyel Egbert Zojer Christian Teichert Gregor Hlawacek Christof Wöll Katrin Hänel Thomas Strunskus Peter Pölt Stefan Mitsche Helmut Sitter Gerardo Hernandes-Soza Financial support by the Austrian Science Fund Uni Leoben Electron microscopy Center Graz Uni Bochum Uni Linz