Deposition on Nonreconstructing Metallic Surfaces

1
Deposition on Non-reconstructing Metallic
Surfaces

• Ag/Cu on Ru(0001) substrates
• Stevens and Hwang PRL 74, 2078 (1995)
• Ag, Cu, and Ru are immiscible in bulk.
• Lattices aCu(111) lt aRu(0001) lt aAg(111) .
• Large misfit strain on Ru(0001) surface.

2
Ru(0001) Metallic Surfaces
Ru(0001) substrate
Ru(0001)

• Reconstruction happens if there are a means that
  the system can lower its energy via additional
  surface bonding (e.g, by Peierls distortion).
• With Ru, what d-states are involved? bonding,
  anti-bonding, non-bonding
• You expect NO reconstruction. Why? (Consider
  bonds and surface.)
• Hence it is a good growth substrate.

• If no reconstructive bonding, then what affects
  arrangements of an overlayer?
• Bonding effects can be overwhelmed by
• Electrostatic Effects can be large (Coulomb,
  ionic surfaces, )
• Strain Effects can be large (mismatch, misfit
  dislocations, )

3
Relaxation of Metallic Surfaces warning on
potentials
1st surface layer, e.g., Cu(001)
metal/Cu

• From a theoretical perspective Simple pairwise
  potentials cannot generally describe proper
  relaxation of metallic surfaces.
• Expect metal surfaces generally to contract
  bonds to increase electronic density around
  surface atoms, which mimics better the bulk
  density and makes the surfaces atoms happier.
• Pairwise potentials have 2nd n.n. atoms relax
  outward, no matter what.
• For (001) 2nd n.n. controlling surface
  relaxations.
• not physically correct relaxations.
• need multibody interactions
• Pairwise interactions too simple.

4
Deposition on Ru(0001) Metallic Surfaces Ordering
can effectively remove strain!
Ag
Ag-Cu alloy

• Ag/Cu on Ru(0001) substrates
• Ag, Cu, and Ru are immiscible in bulk.
• What states are involved for Ag/Cu?
• aCu(111) lt aRu(0001) lt aAg(111)
• Large misfit strain on Ru(0001) surface.

• e.g., bonding overwhelmed by Strain Effects
  (mismatch, misfit dislocations, )
• Consider alloying at surface with aCu(111) lt
  aRu(0001) lt aAg(111).
• Formation Energy dictates alloy stability
  relative to phase segregation.
• Emix Ealloy(aalloy) cAg EAg (aAg )
  (1cAg) ECu (aCu )
• Emix gt 0 phase segregation Emix lt 0 ordering

5
Ag/Cu on Ru(0001) substrates Ordering can remove
large misfit strain!

• Consider alloying at surface with aCu(111) lt
  aRu(0001) lt aAg(111).
• What if, roughly, (aCu(111) aAg(111))/2
  aRu(0001) at the surface?
• Formation Energy
• Constraint at surface can induce ordering where
  none was possible in bulk.
• Emix Ealloy(aalloy) cAg EAg (aalloy )
  (1cAg) ECu (aalloy)
• cAg(EAg (aalloy ) EAg (aAg ))
  (1cAg) (ECu(aalloy) ECu (aCu ))
• Top RHS chemical mixing energy at alloy
  lattice constant.
• Possibly Emix(aalloy) gt 0 (segregation)
  or Emix (aalloy) lt 0 (ordering)
• Bottom RHS relaxation energy at alloy lattice
  constant. ALWAYS positive!
• This contribution always promotes segregation.
  
• Surface constrains the Ag/Cu alloy bonds so
  that strain almost vanish!
• That is, at surface aCu aAg aalloy
  aRu and bottom (underlined) RHS terms is 0!

6
Ag/Co on Ru(0001) substrates Ordering can remove
large misfit strain! Potential to control size of
domains via surface alloying and misfit
dislocations.

• Consider alloying at surface with aCo(111) lt
  aRu(0001) lt aAg(111) Co is magnetic.
• Without alloying, Co forms hexagonal domains.
• e.g, M.C. Barltelt et al., PRL 81, 1901
  (1998)
• Without Surface constraint magnetic domains
• form with misfit dislocation as an organizing
  agent.
• Co on reconstructed Au(111).
• - e.g. self-organized Co pillars on Au(111).
• B. Voigtländer et al., Phys. Rev. B 44, 10354
  (1991)
• Short-range order of Pd-Ag/Ru(0001)
• due to elimination of strain via surface
  alloying.
• Pd-Ag in bulk phase segregate.
• e.g., Sadigh et al., PRL 83, 1379 (1999)

7
How do Molecules Bond to Surfaces? There are
two principal modes of adsorption of molecules on
surfaces Physical Adsorption ( Physisorption
) Chemical Adsorption ( Chemisorption )
Buffer-layer assisted growth of Ag-Xe-Si(111) Lin
Huang, Jay Chen, J.H. Weaver PRL 80, 4095
(1998) Evaporating Xe leads to soft landing of
Ag clusters on Si(111). of Ag in cluster
related to Xe ML.
O
O
C
C
Ni (001)
The basis of distinction is the nature of the
bonding between the molecule and the surface.
Physical Adsorption the only bonding is by
weak Van der Waals-type forces. There is no
significant redistribution of electron density in
either the molecule or at the substrate surface.
Chemisorption a chemical bond is formed. There
is substantial rearrangement of electron density
involved between the adsorbate and substrate. The
nature of this bond may lie anywhere between the
extremes of virtually complete ionic or complete
covalent character.
8
Characteristics of Adsorption Processes
The most definitive method for establishing the
formation of a chemical bond between the
adsorbing molecule and the substrate ( i.e.
chemisorption ) is to use an appropriate
spectroscopic technique, for example IR - to
observe the vibrational frequency of the
substrate/adsorbate bond UPS - to monitor
intensity energy shifts in the valence orbitals
of the adsorbate and substrate
9
Potential Energy Energetics of Adsorption
It should be remembered that this is a very
simplistic model that neglects many other
parameters that influence the energy of the
system (a single molecule approaching a clean
surface), including for example the angular
orientation of the molecule changes in the
internal bond angles and bond lengths of the
molecule the position of the molecule parallel
to the surface plane The interaction of a
molecule with a given surface will also clearly
be dependent upon the presence of any existing
adsorbed species, whether these be surface
impurities or simply pre-adsorbed molecules of
the same type (in the latter case we are starting
to consider the effect of surface coverage on the
adsorption characteristics). Nevertheless it is
useful to first consider the interaction of an
isolated molecule with a clean surface using the
simple 1D model. Thus, we will also not be overly
concerned whether the "energy" being referred to
should strictly be the internal energy, the
enthalpy or free energy of the system.
10
Collective Motion and Structural Order in
Adsorbate Vibrational Dynamics
e.g, Pykhtin et al., PRL 81, 5940 (1998) Lewis
and Rappe, J. Chem Phys 108, 1157 (1998) ibid
110 4619 (1999)
Low-frequency adsorbate (CO) vibrations couple
via metal substrate phonon (acoustic) modes. CO
on Cu(001) surface vs coverage.(in-plane
frustrated translations) Ordered 1/2 monolayer
2.3 ps Proportional to coverage. Disordered 3
coverage 8 ps Ordered 3 layer 35 ps Not
same as isolated CO on Cu. Bulk excitation
provide a decay channel for the vibrational modes
in CO, e.g. Propogating elastic waves in Cu.
Cu
In-phase or out-of-phase modes that couple
to elastic waves in Cu
Cu (001)
Cu (001)