Surface science

Surface chemistry (English surface chemistry, surface science) is a branch of physical chemistry, in which the chemical and structural processes are examined, which are usually solid / gas, play at interfaces. This surface-sensitive analytical methods are used for the past several decades in the Nobel prizes were awarded. Since the structures under investigation are in the nanometer range, count the surface chemistry to nanoscience.

3.1 Methods for surface cleaning
3.2 techniques for the application of further layers

6.1 microscopy
6.2 spectroscopy
6.3 diffraction
6.4 Kinetic methods
6.5 Sorptive methods
6.6 combinations
6.7 The "Big Four"

Basics

As the surface ( engl.: surface ) while the area of a solid is defined in which the physical and chemical properties (eg structure, electronic properties ) from the rest yourself (English: bulk) differ, and the difference of the bulk properties i.a. exponentially with the distance from the surface subsides (proportional to ). The ideal image of a surface is analogous to the ideal solid-state a strictly periodic infinite in two directions, eg as arrangement of atoms or molecules.

Bravais lattice

A periodic arrangement of atoms or molecules on a surface can be described analog to the solid state in two dimensions with a Bravais lattice. In two dimensions, there are five Bravais lattice, the square, the rectangular, the rectangular -centered, the diamond and the hexagonal structure, the hexagonal or rectangular -centered structures can be viewed as special cases of the rhombic structure with certain angles.

Unit cell

A unit cell reflects the symmetry of the Bravais lattice, it has the same symmetry elements. Due to the periodicity of the lattice, the unit cell can be represented by a translation vector to each other. The unit cells themselves are spanned by linearly independent unit vectors and. The following applies:

To the grating in a different room with other basis vectors and transform. If you work eg with diffraction methods, we measure the unit cell in reciprocal space, even

The vectors of the unit cell in real space can be determined may by scanning tunneling microscopy. The average size of the unit cell in reciprocal space are obtained for example with the low energy electron diffraction (LEED ) at the surface.

A special type of unit cell is the Wigner -Seitz cell. Your corresponds to the Brillouin zone first order in k-space.

Points and lines in the grid

A point in the grid is described to the point of origin by a vector. A straight line is described by a vector that is parallel to the grating lines.

Lattice planes

When a single crystal stops, which happens frequently along the grating plane. This results in surfaces which differ depending on the 3-dimensional crystal structure, and in the cutting direction of its 2-dimensional surface structure. The cutting planes can be described by the intersection points of the plane with the axes of the coordinate system. However, more common is the notation specifying the Miller indices, which are the integer multiple of the reciprocal axis portions. for example, ( 111) (110) (100)

Superstructures

Via structures are additional, larger structures, which are formed by rearranging or adsorption to the surface. They can be described as vectors, and multiples of the base vectors and the Wood's nomenclature or matrix representation.

Surface preparation

Before the surface can be analyzed on a microscopic scale reproducible, they must be freed of contaminants. In order to protect it from further contamination, it is handled in an ultrahigh vacuum (UHV ) (). Thus, the surface collision rate of molecules impinging from the gas phase is reduced. This is for a gas particle of type

In a study of a Ag (111 ) adsorbed organic molecule layer is a reaction with oxygen gas by means of scanning tunneling microscopy could be visualized directly in the spatial domain.

Possible causes of surface contamination include:

Adsorption of air molecules
Dust
Migration of particles from the sample interior to the surface

Surface defects

Typical nanoscale defects on single crystal surfaces [eg As the Ag ( 111) surface ] are steps, kinks, and extracted from terraces atoms. This can be visualized at the atomic scale by scanning tunneling microscopy and are generally more reactive than atomically flat terraces.

Methods for surface cleaning

Workpieces wear after machining ( eg grinding, turning) in general, residues such as oils, dust, abrasion or abrasives. These residues have a mostly negative effect on the processing steps and therefore must be removed. Typical processes are:

Oxidation or reduction of the surface of: converting the impurities into volatile compounds. Oxidation can cause a chemical conversion of adsorbates, which are then more easily desorbed. For example, strongly bound to a surface CO are oxidized to CO2, which is bound only weakly due to its chemical structure.
Sputtering with argon ions: During sputtering, the sample is bombarded with ions which are accelerated in an electric field. However, formed on the substrate larger or smaller " craters " may be smoothed by heating the sample, for example.
Annealing (heating of the sample): When heating the sample to a certain temperature (about 1000 K) can adjust the thermodynamic equilibrium, the surface is minimized, which is equivalent to a lowering of the surface energy. This is dependent on the temperature reconstructions or structures may be formed. These may be in domains of different orientation. During tempering, it can also lead to desorption of adsorbates.

Techniques for applying further layers

On a surface of other layers of atoms or molecules can be applied to modify the properties of the interface. This allows, for example, semiconductor elements in three-dimensional form in an integrated circuit (IC ) to accommodate because they are separated by the layers. An important in basic research tool is the chemisorption of probe molecules, give their vibrational properties, for example information about the surface. The layers happens i a with one of the following methods of thin film technology:

Chemical vapor deposition (CVD)
Plasma enhanced chemical vapor deposition ( PECVD)
Physical vapor deposition (PVD)
Molecular beam epitaxy (MBE)
Electroplating deposition
Oxidation of the surface with oxygen
Sol -gel process

Examples of questions

Examples of problems in surface chemistry are: the elemental compositions of surfaces, concentration of elements in the surface region, the distribution of elements in the depth profile of the surface and the chemical bonding of adsorbates. The study of the kinetics of adsorption, the adsorption and the desorption, and the ( e ) structure at the interface, and the vibration characteristics are functions of the surface chemistry. Furthermore, the surface chemistry employed with reaction mechanisms of heterogeneously catalyzed reactions, models created for catalytic reactions for the development of the industrial catalysts and studied the diffusion of adsorbates on surfaces ( surface dynamics) as well as the oxidation state of the surface atoms.

Surface coordination chemistry

The coordination chemistry of metal oxide surfaces has many parallels to the complex chemistry in solutions. Here are oxide ions and hydroxide groups in particular that form on the metal oxide surface by dissociative adsorption of water molecules as ligands for metal ions or metal ion complexes from an adjacent phase. These metal complexes may be bound by weak interactions ( outer-sphere complexes) or the binding is via exchange reactions of ligands ( inner-sphere complexes). Example of an inner-sphere complex formation:

The production of surface complexes is for heterogeneous catalysts of great importance.

On the surface, particularly expire and acid -base reactions. The hydroxide can react either as a Bronsted acid or Bronsted base. Depending on the metal is present here a different acidity of the Bronsted acid. Such surfaces play an important role as a catalyst for acid-catalyzed reactions in non-aqueous solvents and in the gas phase. Also playing in catalysis centers on metal oxide surfaces play a role that can react as Lewis acids. This increases the number of metal cations, and thus the Lewis acidity, especially at higher temperatures.

Surface-sensitive methods

Surface analytical methods are used in industry and in basic research.

Heterogeneous catalysis (for example, the Haber- Bosch process for the production of ammonia)
Semiconductor technology
Fuel cell research
Processes at electrodes in electrochemical reactions
Nanoelectronics, ie production of electronic components on a nanometer scale
Adhesives
Resistant surface coatings ( eg corrosion protection)
Medical Applications
Materials research, such as atomic composition of surface alloys
Certain biological questions

In order to investigate the interfacial processes, methods must be used that can "see " only processes in the region of a sample, which differs in its properties from the remaining solids. To this end, the interactions of the following waves / particles are used with matter:

The mean free path of the charged particles due to Coulomb interactions i.a. is much smaller than that of neutral. Another strong impact, the kinetic energy of the particles; in certain energy ranges processes can be initiated, which reduces the mean free path. It is crucial for the surface sensitivity of a method that either the interacting with the sample or the detected particle or wave has a small mean free path in matter. Therefore, an ultra-high vacuum is required for many methods. The method chosen depends on the question. The following overview is intended to give only an overview. For several methods, various spatially resolving techniques exist. For more description, see their articles. Each method has advantages and disadvantages that need to be Also taken into account in the experiment.

Microscopy

Spectroscopy

In spectroscopy is generally a method in which a spectrum is produced, i.e., an intensity is applied against one of the energy equivalent size, such as the frequency. In the electron spectroscopy is the energy of electrons, that size is applied to the intensity. There are the following methods:

Diffraction

Kinetic methods

Sorptive methods

Combinations

Certain types of radiation can stimulate several processes that can bring for each method pros and cons. For example, with ionization by X-rays at the same time Auger electrons and photoelectrons resulting from potentially overlap in the spectrum and thus complicate the analysis. On the other hand be obtained in an apparatus in the TEM by additional emission of Auger - and photo- electrons backscattered electrons emitted particles and EELS additional information about the sample.

The "Big Four"

When the "Big Four " (Eng. "the big four " ), the measurement methods XPS, AES, SIMS and ISS respectively.