Spectroscopic Techniques for Probing Solid Surfaces

Part II of Surface Primer

Characterization of a Surface

To fully characterize a surface, we need to know: What types of atoms are present at a surface and

what is their surface concentration? Where are the atoms/molecules located on a

surface? How strong is the bonding of adsorbate atoms

and how does the nature of the surface bond influence surface reactivity?

LEED (low energy electron diffraction) Incident e-’s, which are

elastically back scattered from a surface, are analyzed from 20-100eV.

The e-’s possess de Broglie wavelengths of the same order of magnitude as the interatomic spacing between atoms/molecules at surfaces. undergo diffraction if the

atoms in the surface are arranged periodically

LEED patterns

From the position of the diffracted beams, the 2D periodicity of the surface unit cell may be deduced as well as variations in the unit cell size induced by adsorption.

From the variation of spot intensities with beam energy, the complete surface geometry, including bond lengths and angles, can be obtained. Si(111) 7 X 7 LEED Pattern

Work Function

The work function (Ф) is the minimum energy required to remove an electron from the Fermi level of a solid to a sufficient distance outside the surface such that it no longer feels long-range Coulomb interactions with the positive hole left on the surface.

Fermi E of a solid is bulk property electrostatic attraction

between atomic nuclei and valence elections

Work Function Cont’d.

Atomically clean single crystal surfaces of differing geometric structure have different work functions. A surface does not present an infinite potential

energy barrier to the e-’s within a solid. The e- wavefunctions may have a non-zero

amplitude ‘just outside’ (within 10 Å) of the surface.

Dipole Layer

Electron wavefunctions are exponentially damped as they penetrate outside the surface and give rise to ‘e- overspill’. The excess (-) charge is

balanced by a corresponding excess (+) charge at the solid surface Formation of a dipole layer The greater the ‘overspill’

the larger the surface dipole Extent of ‘overspill’ is a

function of surface geometry

Adsorption

Adsorption induces changes in the work function modifications of the surface dipolar layer particularly if significant charge transfer occurs

between the adsorbate and surface measurements of ΔΦ yield critical information on

the degree of charge reorganization upon adsorption

ΔФ = Фadsorbate covered - Фclean

Electropositive Adsorbates

Because alkali metals exhibit low IE’s, they tend to transfer electron charge from their outer valence shell to the substrate. This process ceases when the

Fermi level of the substrate e-’s = highest occupied electron state in the broadened valence level of the adsorbate.

A dipole layer is formed in which net (+) charge resides on the adsorbate

The dipole layer is in the opposite direction to the dipole layer at a clean surface, so a lowering in the work function is expected.

Electronegative Adsorbates

These possess an unfilled affinity level that is situated largely or entirely below the highest occupied state of the substrate. charge transfer occurs from

substrate to adsorbate The dipole layer has an

outermost (-) charge, which is in the same direction as the dipole on a clean surface, so there is a work function increase upon adsorption of EN elements.

STM

An atomically sharp tip is brought within a few nm. of a conducting surface and a small potential difference is applied between tip and sample.

If the tip has a positive bias relative to the sample, an energetic incentive is provided for e-’s from the surface to flow to the tip.

The tip is mounted on a piezoelectic tube scanner, which expands or contracts when a voltage is applied across it.

Tunnelling

In an energy level diagram for a tip close to a conducting surface, for a finite barrier (like a real metal), the wavefunction of an e- in the Fermi level penetrates beyond the sample such that e- density drops to 0 at distances of nm. from the surface.

The e- can tunnel to the tip where it will lower its E due to the positive potential applied to the tip.

The positive potential shifts the electronic E levels on the tip to lower E, thus facilitating e- transfer into unoccupied states of lower E.

Work Function and STM

Tunnelling current (I) depends exponentially on the sample to tip gap (W) and the sample work function (Φ):

I(W) = Cexp(-WΦ^(1/2))

The tunnelling current will increase in areas where protrusions exist because of a lowering in the gap distance.

Modes of Scanning in STM

Constant height mode: the tip is scanned in the xy-plane of the surface while remaining stationary in the z-direction. Image is produced consisting of tunnelling current variations as a function of position. Good for flat surfaces

because rapid scanning is possible

Modes of Scanning in STM

Constant current mode: the value of W (tip gap) is fixed by movement of the tip in the z-direction, while scanning in the xy-plane. A plot of the z-piezo electric voltage vs. lateral position is created. Good for rough surfaces to avoid tip-surface

collisions

HOPG Surface Geometry

STM current depends on lateral variation of e- density of the sample, which is dependent on surface geometry.

In a sample of HOPG, there are atoms (B) with no neighbors directly below and atoms (A) with an atom directly below.

These atoms have different local bonding environments and therefore e- density will vary, thus creating a different tunnelling current.

Atomic Force Microscopy

Allows for imaging of non-conducting surfaces

A tip, usually silicon nitride (diameter of 1-20 nm.), is mounted on a cantilever with a force constant between 0.001 and 0.2 Nm-1

The cantilever has a natural resonant vibration frequency as far removed from those experienced in the building as possible.

Resonant frequency (ν) :ν = 1/2π√K/m

To obtain high v, cantilevers are made with very low masses (1 µg.) with low force constants.

AFM Cont’d.

Contact Mode: The tip is scanned at tip-sample separation corresponding to a chemical bonding length of the tip/sample combination. This leads to the cantilever being attracted (e- overlap) or repelled (closed electronic shells) as it scans the surface.

AFM Cont’d.

Non-contact (tapping) mode: The tip vibrates close to its resonance frequency. Variations in the sample-tip forces alter the resonance frequency, and the shift is used to measure the magnitude of the forces in action. Topographic images of surface force vs. lateral

position Good for delicate samples