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Lab Instruments

 Low Energy Electron Diffraction(LEED)

Figure : Si (111) 7X7 at 31eV

Principle	Elastic monoenergetic electros are diffracted from an ordered surface. The observed diffraction pattern can be used to determine the periodicity of the two dimensional overlayer lattice.
Applications	Determination of surface-crystallographic structures and two dimensional structures of adsorbed overlayer.

Auger electron spectroscopy(AES)

Figure : Typical Auger electron spectra obtained during metal(Pt)-on-metal(Cu) epitaxy and

         variation of Auger peak-to-peak intensities showing layer-by-layer growth mode.

Principle	The Auger process occurs after an atomic level has been ionized by incident photons or electrons. The hole in the inner shell is filled by one electron from a higher level and a secondary electron (called Auger electron) escapes into the vacuum with the remaining kinetic energy. The kinetic energy of the secondary electron is measured to give information about surface composition.
Applications	The qaulitative and quantitative analysis of surface composition, Depth profiling to know elemental concentration in a direction normal to the surface.

X-ray photoelectron spectroscopy

(XPS or ESCA)

Figure  : The C 1s XP spectra from stepwise desorption of

       1,3-disilabutane adsorbed on Si(111) surface

 

Principle	If monochromatic X-ray photons impinge on the sample surface, an electron which was bound to the solid with  energy Eb is ejected into the vacuum with kinetic energy Ek. By conservation of energy, Ek = hν- Eb - Φ Eb ; the binding energy can be deduced. Atomic and molecular species may be identified by comparision of Eb with standard values.
Applications	Determination of chemical state of elements on surface, Quantitative analysis of surface species,

Ultraviolet photoelectron spectroscopy (UPS)

Figure : UP valence band spectra from the stepwise desorption

of 5.0L CO adsorbed W(110) surface at 200K

 

Principle	If an low energy radiation source such as Helium discharge lamp (21.2 eV, 40.8 eV) is used instead of using X-rays for photoexcitation, the binding energy of an electron in valence band is measured to yield information about valence structures of surface.
Applications	Determination of valence band structures of surface, Structural information of adsorbed species, Determination of work function changes on surface.

Cs⁺-ion scattering

Figure : The thermal decomposition reaction of DSB on a Si(111) surface

with Cs⁺-ion reactive scattering in the low temperature

 

Principle	Cs+(g) + X-surface → Cs+(g) + X(g) + surface  (reaction 1)         Cs+(g) + X(g) → CsX+(g)                      (reaction 2) Collision of a low-energy Cs+ ion with a surface causes desorption of the adsorbate X from the surface (reaction 1). The desorbed X combines with the scattered Cs+ ion via electrostatic attraction forces in the gas phase, and forms a CsX+ ion complex (reaction 2).
Applications	The CsX+ ion product is detected by mass spectrometer. Note that this methodology probes neutral species (X) desorbed from a surface, which is an important advantage over secondary ion mass spectrometry

Thermal desorption spectroscopy(TDS) and Temperatue programmed desorption spectroscopy(TPD)

Figure : The thermal desorption spectra of 3.0 L CO adsorbed at 950K

with exposures of O₂ preadsorbed on W(110) at R.T

 

Principle	An adsorbed layer on a sample surface is programmed upwards in temperature, and various desorption species are simultaneously monitored in multiplex mode.
Applications	Direct information on the adsorption and desorption energy, The kinetics of adsorption and desorption,Identification of desorption products.