EEW508 II. Structure of Surfaces Reconstruction – (2x1) Reconstruction of Si(100) The (2x1) reconstruction of Si (100) crystal structure as obtained by LEED crystallography. Note that the surface relaxation extends to three atomic layer into the bulk

EEW508 II. Structure of Surfaces (7x7) Reconstruction of Si (111) LEED and STM image of (7x7) reconstructed structure of Si (111) The total number of dangling bonds is reduced from 49 to 19 through this reconstruction. DAS structure: dimer, adatom, and stacking fault

EEW508 II. Structure of Surfaces (7x7) Reconstruction of Si (111) 19 dangling bonds of (7x7) reconstructed surface (12 adatom, 6 rest atom, 1 corner hole)

EEW508 II. Structure of Surfaces Reconstruction on metallic surface – Ir(100) Bulk structure:the square lattice Surface structure: hexagonally close packed layer (5x1) reconstruction

EEW508 II. Structure of Surfaces Reconstruction on metallic surface –Ir (110) missing dimer row (2x1) reconstruction structure

EEW508 II. Structure of Surfaces Reconstruction – Ionic crystal Ionic crystal consists of charged spheres stacked in a lattice.

Surfaces with strong chemical bonds exhibits more drastic rearrangement of surface atoms Generally speaking, surfaces with weak chemical bonds (van der Waals, hydrogen, dipole-dipole and ion-dipole) exhibits less pronounced reconstructed structure -- for example, Graphite (0001) surface EEW508 II. Structure of Surfaces

EEW508 II. Structure of Surfaces Reconstruction of high-Miller-index surfaces Roughening transition: If the surface is heated near the melting temperature, the steps become curved and break up into small islands Reconstruction at Cu(410) stepped surface. Atoms in the first row at the each step become adatoms which are pointed out in the side view of the reconstructed surface.

III. Molecular and Atomic Process on Surfaces

EEW508 Structure of ordered monolayer When atoms or molecules adsorb on ordered crystal surface, they usually form ordered surface structure over a wide range of temperature and surface coverages. Two factors which decide the surface ordering of adsorbates are Adsorbate-adsorbate(AA) interaction and adsorbate-substrate(AS) interaction Chemisorption – adsorbate-substrate interaction is stronger than adsorbate- adsorbate interaction, so the adsorbate locations are determined by the optimum adsorbate-substrate bonding, while adsorbate-adsorbate interaction decides the long-range ordering of the overlayer. Physisorption or physical adsorption – AA interaction dominates the AS interaction –the surface could exhibit incommensurate structures. III. Molecular and Atomic Process on Surfaces

EEW508 Coverage of adsorbate molecules Definition of coverage: one monolayer corresponds to one adsorbate atom or molecules for each unit cell of the clean, unreconstructed substrate surface. For example, the surface coverage of atom on fcc(100) is one-half a monolayer. III. Molecular and Atomic Process on Surfaces “Introduction to Surface Chemistry and Catalysis” G. A. Somorjai and Y. Li

Atomic oxygen on Ni (100) Up to one quarter of the coverage: Ni(100)-(2x2)-O Between one quarter and one half Ni(100)-c(2x2)-O EEW508 Ordering of adsorbate molecules III. Molecular and Atomic Process on Surfaces “Introduction to Surface Chemistry and Catalysis” G. A. Somorjai and Y. Li

EEW508 Epitaxial Growth With metallic adsorbates, very close packed overlayers can form because of attractive force among adsorbed metal atoms. When the atomic sizes of the overlayer and substrate metals are nearly the same, we can observe a one-monolayer (1x1) surface. This is called epitaxial growth. III. Molecular and Atomic Process on Surfaces

EEW508 Adsorbate-induced reconstructuring III. Molecular and Atomic Process on Surfaces

EEW508 Adsorbate induced restructuring – Ni (100) – c(2x2) - C Carbon chemisorption induced restructuring of the Ni (100) surface. Four Ni atoms surrounding each carbon atom rotate to form reconstructed substrate. III. Molecular and Atomic Process on Surfaces “Introduction to Surface Chemistry and Catalysis” G. A. Somorjai and Y. Li

EEW508 Adsorbate induced restructuring – Fe (110) – (2x2)-S S-Fe (110), Sulfur-chemisorption-induced restructuring of the Fe(110) surface. III. Molecular and Atomic Process on Surfaces “Introduction to Surface Chemistry and Catalysis” G. A. Somorjai and Y. Li

EEW508 Adsorbate induced restructuring of steps to multiple-height step – terrace configuration III. Molecular and Atomic Process on Surfaces

EEW508 Sulfur-chemisorption-induced restructuring of the Ir (110) surface fcc(111) surface restructure more frequently upon chemisorption than do the closer- packed crystal faces. III. Molecular and Atomic Process on Surfaces “Introduction to Surface Chemistry and Catalysis” G. A. Somorjai and Y. Li

EEW508 Penetration of atoms through or below the first layer “Introduction to Surface Chemistry and Catalysis” G. A. Somorjai and Y. Li III. Molecular and Atomic Process on Surfaces

EEW508 III. Molecular and Atomic Process on Surfaces Growth modes of metal surfaces

EEW508 Growth modes of metal surfaces Auger signal of adsorbate Auger signal of substrate III. Molecular and Atomic Process on Surfaces

EEW508 Adsorption of CO on transition metal CO is found to adsorb dissociatively on the early transition metals (to the left of the periodic table) and molecularly on the late transition. III. Molecular and Atomic Process on Surfaces

EEW508 Adsorption of CO on transition metal The preferred adsorption site of CO depends on three factors: The metal, the crystallographic face, and the CO coverage Ni (111) face: CO occupies the bridge sites first Rh(111), Pt(111) the top sites are preferred at low coverages. The threefold site is occupied first on Pd(111). III. Molecular and Atomic Process on Surfaces

EEW508 Adsorption of Ethylene on metal Unsaturated hydrocarbon adsorption on clean transition metal is mainly irreversible. Once unsaturated hydrocarbon molecules are adsorbed on the surface, if the surface is heated, then the adsorbed molecules will decompose to evolve hydrogen and leave the surface covered with the partially dehydrogenated fragments or carbon. III. Molecular and Atomic Process on Surfaces

EEW508 Desorption of Ethylene on metal Thermal desorption of hydrogen from chemisorbed ethylene on Rh(111) due to thermal dehydrogenation for several coverages. To determine the structure and bonding of these various surface fragments, vibration spectroscopy or HREELS over a temperature range can be used. III. Molecular and Atomic Process on Surfaces Surface Chemistry and Catalysis, Gabor Somorjai (1994)

Adsorption of ethylene on Pt(111) EEW508 (Left) SFG (Sum frequency generation) spectroscopy revealing di-  bonded ethylene at 202 K on Pt(111), (right) ethylidyne at 300K on Pt(111). III. Molecular and Atomic Process on Surfaces Surface Chemistry and Catalysis, Gabor Somorjai (1994)

EEW508 Formation of ethylidyne (CCH 3 at high temperature (> 220K) Bonding geometry of ethylidyne on the Rh(111) and Pt(111) crystal surface. III. Molecular and Atomic Process on Surfaces

EEW508 Ethylidyne-chemisorption-induced restructuring of the Rh(111) surface Metal-metal distances expand for those Rh atoms that bind to the carbon of the ethylidyne molecule located in the three fold site. Rh atoms in the second layer moves also upwards, closer to the organic molecules. III. Molecular and Atomic Process on Surfaces

EEW508 Case study – formation of graphene on Pt(111) surface The clean surface was then exposed to ethylene at room temperature by backfilling the chamber with ethylene. Exposures were typically greater than 10 Langmuir to ensure saturation of the Pt(111) surface. After exposure, the sample was heated to about 1250 K, resulting in the decomposition of ethylene and formation of a single monolayer of graphite on the Pt(111) surface. M. Enachescu et al. Phys. Rev. B (1999). III. Molecular and Atomic Process on Surfaces

Image size is 10 nm x 10 nm. EEW508 Case study – formation of graphene on Pt(111) surface III. Molecular and Atomic Process on Surfaces

EEW508 III. Molecular and Atomic Process on Surfaces Field Ion Microscopy Field ion microscope (FIM) was invented by Erwin E. Mueller in The instrument features a specimen in the form of a sharp needle mounted on a electrically insulated stage in a ultrahigh vacuum chamber. The field ion image of the specimen is formed on a microchannel plate and phosphor screen assembly. To produce a field ion image, controlled amounts of image gas (neon, helium, hydrogen and argon) at Pa pressure are admitted into the vacuum system.

inaba.nims.go.jp/G7/ap/fim.html EEW508 III. Molecular and Atomic Process on Surfaces Principle of Field Ion Microscopy

EEW508 III. Molecular and Atomic Process on Surfaces Field Ion Microscopy Images of W Single crystal tip Field ion micrograph of a tungsten tip and the ball model of a FIM tip surface. The (110) plane of the crystal is perpendicular to the axis of the tip. Surface Science, An Introduction, J. B. Hudson (1992)

SFG (Sum-frequency generation) vibrational spectroscopy EEW508 Physics Today, Somorjai and Park, Oct (2007) III. Molecular and Atomic Process on Surfaces

Schematic of SFG (Sum-frequency generation) vibrational spectroscopy system EEW508 III. Molecular and Atomic Process on Surfaces

Detection of reaction intermediates on Pt(111) with SFG EEW508 SFG spectrum of the Pt(111) surface during ethylene hydrogenation The spectrum was measured with 100 Torr of H 2, 35 Torr of C 2 H 4, and 615 Torr of He at 295 K III. Molecular and Atomic Process on Surfaces

(100 x 100) Å 2 STM images of the Pt(111) surface under different pressures: (a) 20 mtorr H 2, (b) 20 mtorr H 2 and 20 mtorr ethylene, and (c) 20 mtorr H 2, 20 mtorr ethylene, and 2.5 mtorr CO. The presence of CO induced the formation of a (  19 x  19) R23.4  structure on the surface. (d) ( ) Å 2 STM image showing two rotational domains of (  19 x  19)R23.4 . EEW508 High pressure STM and surface mobility – ethylene on Pt(111) III. Molecular and Atomic Process on Surfaces

EEW508 High pressure STM and surface mobility – ethylene on Rh(111) (100 x 100) Å 2 STM images of the Rh(111) surface under pressures of (a)20 mtorr H 2 and (b)20 mtorr H 2 and 20 mtorr ethylene. (c) (50 50) Å2 STM image of c(4 x 2)-CO + C 2 H 3 structure formed at 20 mtorr H 2, 20 mtorr ethylene, and 5.6 mtorr CO, and (d) A schematic showing the proposed structure III. Molecular and Atomic Process on Surfaces

EEW508 Reactivity of ethylene hydrogenation – with and without CO Turnover rate during ethylene hydrogenation on Pt(111) III. Molecular and Atomic Process on Surfaces

Detection of reaction intermediates on Pt nanoparticles with SFG EEW508 In situ monitoring of nanoparticles by high-pressure SFG spectroscopy. NP: nanoparticle III. Molecular and Atomic Process on Surfaces

Aliaga et al. J. Phys. Chem. C, 2009, 113 (15), TEM images of a Langmuir-Blodgett film of 10 nm platinum cubes (a) before and (b) after 2 h of UV-ozone treatment UV/Ozone cleaning removes the organic capping layers of nanoparticles EEW508 III. Molecular and Atomic Process on Surfaces

SFGVS spectra of a Langmuir-Blodgett film of 10 nm TTAB-capped platinum cubes. UV/Ozone cleaning removes the organic capping layers of nanoparticles EEW508 III. Molecular and Atomic Process on Surfaces Aliaga et al. J. Phys. Chem. C, 2009, 113 (15),

SFG spectra of a drop-cast film of a 10 nm TTAB-capped platinum cube under ethylene hydrogenation conditions. The spectrum shows contributions from ethylidine and di-σ- bonded ethylene adsorbates. A very small contribution from the intermediate π-bonded species is also visible at 760 Torr and 298 K. Detection of reaction intermediates on Pt NP with SFG EEW508 Aliaga et al. J. Phys. Chem. C, 2009, 113 (15), III. Molecular and Atomic Process on Surfaces

EEW508 III. Molecular and Atomic Process on Surfaces Low energy electron diffraction (LEED)

EEW508 III. Molecular and Atomic Process on Surfaces Concept of penetration depth

EEW508 III. Molecular and Atomic Process on Surfaces Inelastic mean free path of electrons as a function of energy Seah and Dench Surface Interface and Analysis 1,2 (1979)

EEW508 III. Molecular and Atomic Process on Surfaces LEED instrumentation An Introduction Surface Science Hudson (1992)

EEW508 Surface structure of alloy, AlCu Cu 84 Al 16 alloy (111) structure exhibiting  3 x  3 R30 o The surface composition is 50% “Introduction to Surface Chemistry and Catalysis” G. A. Somorjai and Y. Li III. Molecular and Atomic Process on Surfaces

EEW508 III. Molecular and Atomic Process on Surfaces Low Energy Electron Diffraction Principle of Low energy electron diffraction (LEED) The single crystal surfaces are used in LEED studies. After chemical or ion-bombardment cleaning in UHV, the crystal is heated to permit the ordering of surface atoms by diffusion to their equilibrium positions. The electron beam (in the range of eV) is backscattered. The elastic electrons that retain their incident kinetic energy are separated from the inelastically scattered electron by applying the reverse potential to the retarding grids. These elastic electrons are accelerated to strike a fluorescent screen and LEED pattern can be obtained.

LEED pattern of a Si(100) reconstructed surface. The underlying lattice is a square lattice while the surface reconstruction has a 2x1 periodicity. The diffraction spots are generated by acceleration of elastically scattered electrons onto a hemispherical fluorescent screen. Also seen is the electron gun which generates the primary electron beam. It covers up parts of the screen. EEW508 II. Structure of Surfaces