e - Alessandro Baraldi

Transcription

1 High energy resolution core level photoelectron spectroscopy and diffraction: powerful tools to probe physical and chemical properties of solid surfaces. Alessandro Baraldi e - hν

2 For info, do not hesitate to contact me! Prof. Alessandro Baraldi Physics Department, University of Trieste, ITALY Deputy-chair Doctorate School in Nanotechnology, University fo Trieste Head of the Surface Science Laboratory, Elettra-Sincrotrone Trieste website: alessandrobaraldi.weebly.com Phone: (off) 8331 (lab)

3 Outline Core-level spectroscopy is used to... probe the properties of first-layer surface atoms. determine energetics and atomic mechanisms in diffusion processes. identify in a direct way the adsorption site of atoms and molecules. evaluate the chemical reactivity trends on model surfaces. investigate the role of surface defects. study molecular dissociation processes in nanostructured surfaces. monitor the surface chemical composition during chemical reactions. investigate the growth mechanism of graphene probe the unsual physical and chemical properties of graphene.

4 One of the most widely employed experimental techniques Number of papers published in the last twenty years in peer-review international scientific journals on X-ray Photoelectron Spectroscopy, according to the Thomson Reuters ISI Web of KnowledgeR.

5 The main advantages Down to the 100 ms time-scale In the range mev Between 20 K and 1300 K 0.5% of ML

6 Ir(111) Details from the core level lineshape Core-level photoemission lineshape is usually described by using the Doniach-Šùnjić function J. Phys. C: Sol. State Phys. 3, 285 (1970) A. Baraldi et al., Phys. Rev. B 67, (2003).

7 Many-body effects in Doniach-Šùnjić Lorentzian distribution arising from the finite corehole lifetime. A convolution of a Doniach- Šùnjić function and a Gaussian, which account for the vibration/phonon and the contribution of the instrumental resolution. Asymmetry parameter, describing the contribution of electron-hole pairs excitation.

8 Core-level shift and atomic coordination number Surface Core Level Shift of single crystal surfaces A. Baraldi et al., J. Phys.: Condens. Matter 20, (2008).

9 Surface reconstruction and atomic diffusion Oxygen-induced reconstruction on Rh(110) (2x2)pg (1x2) c(2x6) (1x3) c(2x8) (1x4) Q=0.5 ML Q=0.66 ML Q=0.75 ML A. Baraldi et al., Phys. Rev. B 72, (2005).

10 bulk surface (1x1) Photoemission Intensity [arb. units] (1x2) (1x4)+(1x1) Atomic diffusion and surface reconstruction Rh3d5/ Binding Energy [ev]

11 Atomic diffusion and surface reconstruction

12 Normalized (1x2) Rh Population Atomic diffusion and surface reconstruction K Q t Ie t e E A / K B T ln(k) [ x10-4 ] K 451 K /T [ x 10-3 ] Time [s] 1500 E A ev

13 Mechanism of surface deconstruction HOPPING EXCHANGE TS TS = 2.56 ev Final state= 1.09 ev Fs TS= 2.03 ev Final state= 0.99 ev

14 Defect mediated diffusion mechanism Step 1.14 ev 1.16 ev

15 Molecular adsorbates on surfaces Carbon Monoxide adsorption on Rh(111) on-top bridge bridge on-top A. Baraldi et al., Surf. Sci. Rep. 49, 169 (2003).

16 Localized vibrations in adsorbed molecules Harmonic approximation C*O Probing the vibration of C*O excited molecules O Photoemission Intensity C CO

17 C1s core-level spectrum of adsorbed CO CO on Ir(111) ( 3x 3)R30 ΔE 0,1 =231.2 mev ΔE 0,2 =461.7 mev c(2 3x4)rect ΔE 0,1 =232.8 mev ΔE 0,2 =466.2 mev

18 (4x4) Q=0.06 ML Atomic and molecular adsorption. on transition metals CO su Pt(111) c(4x2) Q=0.50 ML h O1s C1s

19 Carbon monoxide adsorption on Pt(111) h Pt4f

20 Carbon monoxide adsorption on Pt(111)

21 Molecular adsorption site determination dpt 0 dq 1 dpt 1 dq 1

22 Oxygen adsorption on Rh(100) MOVIE Rh-1O Rh-2O

23 Atomic adsorption site determination drh0 4 dq four-fold hollow adsorption site A. Baraldi et al., Phys. Rev. Lett. 93, (2004).

24 SCLS (ev) NO Adsorption energy [ev] SCLS vs chemical reactivity C 3, Rh(111) 0.5 ML C 2,4 C 2, ML ML 0.2 C 1,4 C 1, ML 0.25 ML O/Rh(100) E d (ev) ML SCLS (ev) 0.8 A. Baraldi et al., Phys. Rev. Lett. 93, (2004).

25 Surface defects vs chemical reactivity Role of steps in N 2 activation on Ru(0001) S. Dhal et al., Phys. Rev. Lett. 83, 1814 (1999) Controlling the catalytic bondbreaking selectivity on Ni surfaces by step blocking R.T. Vang et al., Nature 4, 260 (2005) Catalytic activity of gold nanoparticles, B. Hvolbæk et al., Nanotoday 2, 14 (2007)

26 The role of adatoms on solid surfaces 2D adatom lattice-gas Pd, Cu and Ag adatoms densities are 1-6 % on W at 700 K. Acetylene Cyclotrimerization

27 Adatom and addimer-induced Rh3d5/2 SCLS A. Baraldi et al., New J. Phys. 9, 143 (2007).

28 Adatom and addimer local geometrical structure d=2.50 Å -6.4 % Rh(100) 2.67 Å d 12 =1.82 Å -3.7 % d 1 d 1 =2.59 Å -3.0 % d 2 =2.51 Å -6.2 % d 3 =2.55 Å -4.8 % d 3 d Å d A =1.60 Å d D =1.63 Å d D =1.60 Å d 12 =1.84 Å -2.7 % d 12 =1.87 Å 2.59 Å -3.0 %

29 Adatom and addimer-induced Rh3d 5/2 SCLS bulk II-layer Rh(111) II-layer Rh(100)

30 Undercoordinated atoms and surface chemical reactivity A careful analysis of CLSs provides a spectroscopic measure of chemical reactivity changes at the atomic level. A. Baraldi et al., J. Phys. Chem. C 115, 3378 (2011).

31 Nanostructured surfaces and chemical reactivity Tuning the Rh(110) surface morphology [110] [001]

32 Carbon Monoxide adsorption on Rh surfaces CO C+O flat step kink (111) (211) step E diss E ads CN Rh(111) Rh-step Rh-kink

33 K K <1-10> (% BZ) The rhomboidal nanopyramids Energy 700 ev 400 ev 250 ev a) 700 ev b) 400 ev c) 220 ev K <00 1> (% BZ) Terrace width G=6.6 Å Facet slope τ=11±2 Spatial Periodicity L=14.7 nm Thermal stability up to ~500 K

34 Thermal dissociation of Carbon Monoxide 9.4±0.5 % F. Buatier de Mongeot et al., Phys. Rev. Lett. 97, (2006). 22±3 % 80±14 %

35 Coverage [ML] Surface Concentration [%] Surface segregation during chemical reactions Adsorbates-induced surface segegation. H 2 +O 2 su Pt 50 Rh 50 (100) Q O Q H H 2 O T=330 K T=400 K T=520 K Pt surf Rh surf A. Baraldi et al., J. Am. Chem. Soc. 127, (2005). Time [s]

36 A new dimension for carbon Two dimensional allotrope of carbon Basic-building block of graphite, carbon nanotubes and large fullerenes New electronic structural properties Single electron transistors Hydrogen storage devices Chemical sensors Ultracapacitors

37 The methods for graphene production Mechanical exfoliation Liquid suspension graphene oxide followed by chemical reduction Epitaxial growth by thermal desorption of Si atoms from the SiC surface Unzipping carbon nanotubes Epitaxial growth by chemical vapor deposition on transition metals High quality carbon layers Tunable properteis Ethylene adsorption on Transition Metals

38 Graphene-induced Surface Core Level Shift C 1s Ir 4f 7/2 SCLS= -545 mev EXP -550 mev THEO HCP TOP SCLS= -535 mev EXP -549 mev THEO -551 mev THEO P. Lacovig et al., Phys. Rev. Lett. 103, (2009).

39 Kinetics of C 2 H 4 adsorption Time-lapsed C 1s spectra at 1273 K 400 ms/spectrum Time-lapsed C 1s spectra at 823 K

40 Temperature evolution of the adsorbate layer C 1s components C A = ev C B = ev ev C C = ev

41 Morphology of carbon nano-islands from DFT 1.62 Å 2.53 Å Θ=0 Θ=11

42 Dome-shaped carbon nano-islands formation 2.63 Å 3.13 Å Θ=16 Θ=21

43 Nano-islands induced Ir4f 7/2 core-level shifts -225/-390 mev Bulk Surface = -550 mev Ir 1 = +498 mev Ir 2 = -325 mev Ir 3 = -551 mev Ir 4 = -132 mev Ir 5 = -270 mev

44 Nano-islands induced C 1s core-level shifts C 4 C 1 = +268 mev C 2 = -348 mev C 3 = -439 mev

45 The energetics of nano-islands Clusters formed with different number P of Honeycomb Rings (HRs) P=19, n=54 P=7, n=24 P=3, n=13 graphene P=1, n=6

46 Thermal expansion of graphene K.V. Zakharchenko et al. Phys. Rev. Lett. 102, (2009) C1s temperature dependence Graphene is strongly anharmonic due to soft bending modes. Up to 900 K, graphene is anomalous since its lattice parameter decreases going over to normal behavior at higher temperatures. Phonon-induced broadening Core level binding energy shift M. Pozzo et al., Phys. Rev. Lett. 106, (2010).

47 Lattice constant versus interatomic distance Ab initio molecular dynamics calculations

48 Thermal expansion of graphene Gas-phase data CV G 1 CV G

49 Thermal properties of graphene SHIFTS EXP =+70 mev THEO =+20 mev Karl-Franzens Universität, GRAZ, April 2010

50 C1s core level shifts of epitaxial graphene C 1s A correlation between graphene-substrate bond strength and graphene corrugation weak interaction 0.2 Å Strong chemical bond 1.5 Å Why just two components?

51 Graphene growth on Re(0001) Formation of a high quality single-layer of graphene is strongly opposed by two competing processes, namely surface carbide formation and carbon bulk dissolution. Time-lapsed spectral sequence of C1s spectra taken during ethylene exposure and surface annealing to high temperature. E. Miniussi et al., submitted

52 Graphene corrugation on Re(0001) 2 μm moirè pattern strong interaction weak interaction E. Miniussi et al., Phys. Rev. Lett. 106, (2011).

53 C-C bond stretching on Re(0001) C-C bond length, obtained as the average distance from the three nearest neighbouring atoms for each C atom valleys humps Å Å (0.5 %)

54 Graphene on Ru(0001): C1s spectrum W S S W D. Alfè et al., Sci. Rep. 3, 2430 (2013)

55 Graphene growth mechanism Fast-data acquisition allows to monitor the C1s spectral evolution while dosing ethylene at high temperature. LEEM (10 μm f.o.v.)

56 The carbon lattice-gas: precursor to graphene formation E b (ev) C 1s(eV) E b (ev) C 1s(eV) M H2 M F2 M H4 M F3 M H5 M F4 M F M H D A1 D A2 D T1 D T2 D B1 D B Three-fold hcp site on the terraces (MH1) and the C monomer at the steps (MF2) have very similar adsorption energies. Monomers form a 2D lattice gas which supplies C atoms for GR formation. D. Alfè et al., Sci. Rep. 3, 2430 (2013)

57 Fine-tuning of graphene-metal adhesion Bimetallic surface alloying provides a viable route for governing the interaction between graphene and metal through the selective choice of the elemental composition of the surface alloy. The formation of PtRu surface alloys by deposition of sub-monolayer Pt films on Ru(0001) and subsequent annealing to HT C1s H. E. Hoster et al., Phys. Chem. Chem. Phys. 10, 3812 (2008).

58 Fine-tuning of graphene-metal adhesion C-substrate distance Ru(0001) 0.1 ML 0.2 ML 0.5 ML Charge density difference Simulated C1s spectra

59 For those who are interested in graphene P. Lacovig, et al., Growth of dome-shaped carbon nanoislands on Ir(111): the intermediate between carbidic clusters and quasi free-standing graphene, Phys. Rev. Lett. 103, (2009). S. Lizzit, et al., Band dispersion in the deep 1s core levels of graphene, Nature Physics 6, 345 (2010). S. Lizzit, et al., High resolution fast x-ray photoelectron spectroscopy study of ethylene interaction with Ir(111): from chemisorption to dissociation and graphene formation, Catal. Today 154, 68 (2010). M. Pozzo, et al., Thermal expansion of supported and free-standing graphene: lattice constant versus interatomic distance, Phys. Rev. Lett. 106, (2011). E. Miniussi, et al., Thermal stability of corrugated epitaxial graphene grown on Re(0001), Phys. Rev. Lett. 106, (2011) R. Larciprete, et al., Dual Path Mechanism in the thermal reduction of graphene oxide. J. Am. Chem. Soc. 133, (2011). A. Cavallin, et al., Local Electronic Structure and Density of Edge and Facet Atoms at Rh Nanoclusters Self-Assembled on a Graphene Template, ACSNano 6, 3034 (2012). S. Lizzit, et al., Transfer-free electrical insulation of epitaxial graphene from its metal substrate, Nano Letters 12, 4503 (2012). S. Ulstrup et al., High-temperature behavior of supported graphene: Electron-phonon coupling and substrate-induced doping, Phys. Rev. B 86, R (2012). R. Larciprete, et al., Oxygen switching of the epitaxial graphene metal interaction, ACS Nano 6, 9551 (2012) D. Alfè, et al., Fine-tuning of graphene-metal adhesion by surface alloy, Sci. Rep. 3, 2430 (2013)