Chapter 21 Surface Characterization by Spectroscopy and Microscopy

Introduction n The most inclusive way to define a surface is to state that a surface of interface exists in any case where there is an abrupt change in the system properties with distance, with many degrees of abruptness.

A crystalline solid in contact with its own vapor at low temperature, effectively has an interface that is one atomic distance in width. A more diffuse interface is present in the extreme case, where we may consider a system near its critical point, such as a liquid in contact and hence at equilibrium with its own vapor at high temperature and pressure. Typical properties exhibiting abrupt change at an interface are density, crystal structure, crystal orientation, chemical composition and ferromagnetic or paramagnetic ordering.

Surface of a Solid The surface of a solid in contact with a liquid or gaseous phase usually has very different chemical composition and physical properties from the interior of the solid n Characterization of these surface properties is often important in many fields, including heterogeneous catalysis, semiconductor thin-film technology, corrosion and adhesion mechanisms, activity of metal surfaces, embrittlement properties, and studies of the n behavior and functions of biological membranes

Surface Measurements n Classical methods n useful information about the physical nature of surfaces but less about their chemical nature n They involve obtaining optical and electron microscopic images, as well as measurements of adsorption isotherms, surface areas, surface roughness, pore sizes and reflectivity

Measurements (cont. ) n Spectroscopic n provided methods information about the chemical nature of surfaces, as well as determine their concentration n began in the 1950 s

Measurements (cont. ) n Microscopic n imaging methods surfaces and determining their morphology, or physical features

The figure below illustrates the general principle by which a spectroscopic examination of surface is performed.

Three types of sampling methods are used regardless of the spectroscopic surface method. Primary beam focused on a single small area of the sample and observing the secondary beam. n The surface can be mapped, by moving the primary beam across the surface in a raster patter of measured increments and observing changes in the secondary beam. n A beam of ions from an ion gun is used to etch a hole in the surface by sputtering. During this process a finer primary beam is used to produce a secondary beam from the center of the hole. This provides analytical data on the surface composition as a function of depth. This method is known as depth profiling. n

Spectroscopic Surface Methods n The chemical composition of a surface of a solid is often different from the interior of the solid n One should not focus solely on this interior bulk composition because the chemical composition of the surface layer of a solid is sometimes much more important

Electron spectroscopy n The first three methods listed in the above table are based upon the analysis of emitted electrons produced by various incident beams. In electron spectroscopy, the spectroscopic measurement consists of the determination of the power of the electron beam as a function of the energy (or frequency hv) of the electrons.

n The most common type is based upon the irradiation of the sample surface with monochromatic X-radiation. This is called X-ray photoelectron spectroscopy (XPS). This method is also known as Electron spectroscopy for chemical analysis (ESCA).

n The second type of electron spectroscopy is called Auger electron spectroscopy (AES). Auger spectra are most commonly excited by a beam of electrons, although X-rays are also used.

n The third type of electron spectroscopy is ultraviolet photoelectron spectroscopy (UPS). In this method, a monochromatic beam of ultraviolet radiation causes the ejection of electrons form the analyte. This method is not as common as the first two methods.

n Electron spectroscopy can be used for the identification of all of the elements in the periodic table except for helium and hydrogen. The method also permits the determination of the oxidation state of an element and the type of species to which it is bonded. This technique also provides useful information about the electronic structure of molecules.

Understanding how the SEM works

SEM Setup

n X-Ray Photoelectron Spectroscopy (XPS), not only provided information about the atomic composition of the a sample, but also information about the structure and oxidation state of the compounds being examined

n The kinetic energy of the emitted electron Ek is measured in an electron spectrometer. The binding energy of the electron Eb can be calculated Eb = hv – Ek - w where, w is the work function of the spectrometer, a factor that corrects for the electrostatic environment in which the electron is formed and measure.

n Secondary-ion mass spectrometry (SIMS) is the most highly developed of the mass spectrometric surface methods, with several manufacturers offering instruments for this technique. SIMS is useful from determining both atomic and the molecular composition of solid surfaces.

n In secondary-ion mass analyzers that serve for general surface analysis and for depth profiling, the primary ion beam diameter from 0. 3 to 0. 5 mm. Doublefocusing, single-focusing, time-of-flight and quadrapole spectrometers are used for mass determination.

n Ion microprobe analyzers are more sophisticated (and thus more expensive) instruments that are based upon a focused beam of primary ions that has a diameter of 1 to 2 m. This beam can be moved across a surface for about 300 m in both x and y directions.

Scanning Electron Microscopy In many fields of chemistry, material science, geology and biology, detailed knowledge of the physical nature of the surface of solids is of great importance. The classical method of obtaining this information was optical microscopy. the resolution of optical microscopy is limited by diffraction effects to about the wavelength of light.

Current surface information at considerably higher resolution is obtained by three techniques: Scanning electron microscopy (SEM) · Scanning tunneling microscopy (STM) · Atomic force microscopy (AFM) ·

A raster is a scanning pattern similar to that used in a cathode-ray tube or in a television set in which the electron beam is: n swept across a surface in a straight line (x direction) n returned to its starting position n shifted downward (y direction) by a standard increment This process is repeated until a desired area for the sample has been scanned

Below is a schematic of a combined instrument that is both a scanning electron microscope and a scanning electron microprobe.

Scanning Probe Microscopes n Scanning probe microscopes (SPMs) are capable of resolving details or surfaces down to the atomic level. Unlike optical and electron microscopes, scanning probe microscopes reveal details not only on the lateral x and y axis of a sample but also the z axis.

The figure below is a contour map

The figure below shows the most common method of detecting the deflection of the cantilever holding the tip.

The diagram below shows a common design for an AFM.

Below is a micrograph of an Si. O 2 cantilever

n One disadvantage of the contact mode scanning is that the downward force of the tip may not be low enough to avoid damage to the sample surface, causing image distortion. This effect can be overcome by using a tapping mode of operation in which the cantilever is oscillated at a frequency of a few hundred kilohertz.

CFM studies provide specific qualitative analytical information as well as information on the spatial arrangement of analytes on the surface.

References. . . n n n http: //www. mcs. com/~wbstine/spm. html http: //www. chem. wisc. edu/~hamers/gallery/in dex. html http: //www. molec. com/gallery/ http: //www. csc. fi/lul/chem/graphics. html http: //www. kri. physik. unimuenchen. de/crystal/stm/ http: //www. imb-jena. de/IMAGE. html

References (cont. ) n n n http: //www. people. vcu. edu/~srutan/chem 4 09/pp 222_231/sld 002. htm http: //www. ipfdd. de/research/res 15. html http: //www. uksaf. org/tech/sem. html http: //www. mse. iastate. edu/microscopy/ho me. html http: //www. llnl. gov/str/Scan. html http: //stm 2. nrl. navy. mil/how-afm/howafm. html