This photo provided by IBM shows a scanning electron micrograph of an image of the sun motif printed with approximately 20,000 gold particles, which are 60 nm in diameter and shows Robert Fludd's famous 17th-century drawing of the sun - the alchemists' symbol for gold.

IBM researchers are touting one of the tiniest pieces of art ever made - an image of the sun made from 20,000 microscopic particles of gold.

A fly coated in gold, having been prepared for viewing with a scanning electron microscope.

Ant_eye

Scanning electron micrographs of barium titanate (BaTiO3) nanocomposites with polycarbonate (left, top and bottom) and Viton (right, top and bottom) polymer matrices. The images show the dramatic improvement in film uniformity through the use of phosphonic acid coated BaTiO3 nanoparticles (bottom images) as compared to uncoated nanoparticles (top images). The higher uniformity results in greatly improved dielectric properties.

Internal SurfacesInternal Surfaces

Microporous solids are materials that are full of pores of molecular Microporous solids are materials that are full of pores of molecular dimensions or larger.dimensions or larger.

These materials have large internal surface areas.These materials have large internal surface areas.

Many clays have layer structures that can accommodate molecules Many clays have layer structures that can accommodate molecules between the layers by a process called between the layers by a process called intercalationintercalation..Eg Graphite will swell with water vapour to several times its original Eg Graphite will swell with water vapour to several times its original thickness as water molecules become incorporated between the thickness as water molecules become incorporated between the graphitic layers.graphitic layers.

ZeolitesZeolitesZeolitesZeolites (Greek, (Greek, zeinzein, "to boil"; , "to boil"; lithoslithos, "a stone") are hydrated aluminosilicate minerals and have a micro-, "a stone") are hydrated aluminosilicate minerals and have a micro-

porous structure.porous structure.

The term was originally coined in the 18th century by a Swedish mineralogist named Axel Fredrik The term was originally coined in the 18th century by a Swedish mineralogist named Axel Fredrik

Cronstedt who observed, upon rapidly heating a natural mineral, that the stones began to dance about as Cronstedt who observed, upon rapidly heating a natural mineral, that the stones began to dance about as

the water evaporated. Using the Greek words which mean "stone that boils," he called this material zeolite.the water evaporated. Using the Greek words which mean "stone that boils," he called this material zeolite.

More than 150 zeolite types have been synthesized and 48 naturally occurring zeolites are known. Zeolites More than 150 zeolite types have been synthesized and 48 naturally occurring zeolites are known. Zeolites

have an "open" structure that can accommodate a wide variety of have an "open" structure that can accommodate a wide variety of cationscations, such as Na, such as Na++, K, K++, Ca, Ca2+2+, Mg, Mg2+2+ and and

others. These positive ions are rather loosely held and can readily be exchanged for others in a contact others. These positive ions are rather loosely held and can readily be exchanged for others in a contact

solution. Some of the more common mineral zeolites are: solution. Some of the more common mineral zeolites are: analcimeanalcime, , chabazitechabazite, , heulanditeheulandite, , natrolitenatrolite, ,

phillipsitephillipsite, and , and stilbitestilbite. An example mineral formula is: Na. An example mineral formula is: Na22AlAl22SiSi33OO1010-2H-2H22O, the formula for natrolite.O, the formula for natrolite.

Natural zeolites form where volcanic rocks and ash layers react with alkaline groundwater. Zeolites also Natural zeolites form where volcanic rocks and ash layers react with alkaline groundwater. Zeolites also

crystallized in post-depositional environments over periods ranging from thousands to millions of years in crystallized in post-depositional environments over periods ranging from thousands to millions of years in

shallow marine basins. Naturally occurring zeolites are rarely pure and are contaminated to varying shallow marine basins. Naturally occurring zeolites are rarely pure and are contaminated to varying

degrees by other minerals, metals, quartz or other zeolites. For this reason, naturally occurring zeolites are degrees by other minerals, metals, quartz or other zeolites. For this reason, naturally occurring zeolites are

excluded from many important commercial applications where uniformity and purity are essential.excluded from many important commercial applications where uniformity and purity are essential.

Zeolites are the aluminosilicate members of the family of microporous solids known as "molecular sieves". Zeolites are the aluminosilicate members of the family of microporous solids known as "molecular sieves".

The term molecular sieve refers to a particular property of these materials, i.e. the ability to selectively sort The term molecular sieve refers to a particular property of these materials, i.e. the ability to selectively sort

molecules based primarily on a size exclusion process. This is due to a very regular pore structure of molecules based primarily on a size exclusion process. This is due to a very regular pore structure of

molecular dimensions. The maximum size of the molecular or ionic species that can enter the pores of a molecular dimensions. The maximum size of the molecular or ionic species that can enter the pores of a

zeolite is controlled by the diameters of the tunnelszeolite is controlled by the diameters of the tunnels

Molecular sieves - ZeolitesMolecular sieves - Zeolites

Have ordered cages of molecular dimensions where molecules can Have ordered cages of molecular dimensions where molecules can absorb or undergo chemical reactions.absorb or undergo chemical reactions.

The micro-porous molecular structure of a zeolite, ZSM-5

Mesoporous materialMesoporous material

A A mesoporous materialmesoporous material is a material containing pores with diameters is a material containing pores with diameters between 2 and 50 nm.between 2 and 50 nm.

Porous materials are classified into several kinds by their size. According to Porous materials are classified into several kinds by their size. According to IUPAC notation (see J. Rouquerol et al., IUPAC notation (see J. Rouquerol et al., Pure & Appl. ChemPure & Appl. Chem, , 6666 (1994) (1994) 1739-1758), microporous materials have pore diameters of less than 2 nm 1739-1758), microporous materials have pore diameters of less than 2 nm and macroporous materials have pore diameters of greater than 50 nm; the and macroporous materials have pore diameters of greater than 50 nm; the mesoporous category thus lies in the middle.mesoporous category thus lies in the middle.

Typical mesoporous materials include some kinds of silica and alumina that Typical mesoporous materials include some kinds of silica and alumina that have similarly-sized fine mesopores. Mesoporous oxides of niobium, have similarly-sized fine mesopores. Mesoporous oxides of niobium, tantalum, titanium, zirconium, cerium and tin have also been reported. tantalum, titanium, zirconium, cerium and tin have also been reported. According the IUPAC notation, a mesoporous material can be disordered or According the IUPAC notation, a mesoporous material can be disordered or ordered in a mesostructure.ordered in a mesostructure.

The first mesoporous material, with a long range order, was synthesized in The first mesoporous material, with a long range order, was synthesized in the late 80:ies/ early 90:ies, by a research group of the former Mobil Oil the late 80:ies/ early 90:ies, by a research group of the former Mobil Oil Company (see Kresge et al., Company (see Kresge et al., NatureNature 359359 (1992) 710). Since then, research (1992) 710). Since then, research in this field has steadily grown. Notable examples of prospective in this field has steadily grown. Notable examples of prospective applications are catalysis, sorption, gas sensing, optics, and applications are catalysis, sorption, gas sensing, optics, and photovoltaicsphotovoltaics..

The Structure of SurfacesThe Structure of Surfaces

On closer inspection, most On closer inspection, most surfaces are not smooth.surfaces are not smooth.

Most imperfections are due to Most imperfections are due to dislocations.dislocations.

Dislocation densities on the order of 10Dislocation densities on the order of 1066 – 10 – 1088 cm cm-2-2 are common at are common at metal or ionic surfaces.metal or ionic surfaces.

Smaller dislocation densities are common in most semiconductor Smaller dislocation densities are common in most semiconductor and insulator crystals.and insulator crystals.

cf. surface concentration of atoms of the order of 10cf. surface concentration of atoms of the order of 101515 cm cm-2-2..

STM – capable of atomic STM – capable of atomic scale resolution.scale resolution.

Can see steps and kinks – Can see steps and kinks – also known as line defects.also known as line defects.

Also get point defects.Also get point defects.

Miller Indices Miller Indices

Unit meshesUnit meshes

The unit cell fo a 2-d surface structure is called a unit mesh and the The unit cell fo a 2-d surface structure is called a unit mesh and the corresponding 2-d lattice is called a surface net.corresponding 2-d lattice is called a surface net.

Adsorption vs AbsortionAdsorption vs Absortion

AdsorptionAdsorption The process in which a molecule The process in which a molecule

forms a bond with a surfaceforms a bond with a surface

AbsorptionAbsorption The process in which molecules The process in which molecules

enter the bulk of a solidenter the bulk of a solid

Associative adsorption vs dissociative adsorption : whether the molecule fragments on the surface or not.

Models of chemisorbed ethene and ethyne on Cu(111)Models of chemisorbed ethene and ethyne on Cu(111)

Studies of ethylene (ethene) and acetylene (ethyne) chemisorbed on Cu(111), Studies of ethylene (ethene) and acetylene (ethyne) chemisorbed on Cu(111), a surface with which they interact relatively weakly, have led to the following a surface with which they interact relatively weakly, have led to the following proposed structures for the molecular adsorption complexes.proposed structures for the molecular adsorption complexes.

A. Plan view of the Cu(111) surface A. Plan view of the Cu(111) surface showing the adsorption geometry of showing the adsorption geometry of athylene and acetylene molecules at low athylene and acetylene molecules at low temperatures.temperatures.

B. Perspective view of the Cu(111) surface B. Perspective view of the Cu(111) surface with an adsorbed acetylene molecule, with an adsorbed acetylene molecule, illustrating the re-hybridisation of the carbon illustrating the re-hybridisation of the carbon centres to a geometry more closely centres to a geometry more closely associated with a C=C double-bond associated with a C=C double-bond configuration. configuration.

                                                                   

                                                           

The structure of clean surfaceThe structure of clean surface

Two important structural changes that occur at solid surfaces are Two important structural changes that occur at solid surfaces are unique and are associated with the two dimensional and anisotropic unique and are associated with the two dimensional and anisotropic environment to which the surface atoms must adjust. These environment to which the surface atoms must adjust. These structural changes are :structural changes are :

Surface relaxationSurface relaxation Surface reconstructionSurface reconstruction

The phenomena of relaxation and reconstruction involve The phenomena of relaxation and reconstruction involve rearrangements of surface ( and near surface ) atoms, this process rearrangements of surface ( and near surface ) atoms, this process being driven by the energetics of the system i.e. the desire to being driven by the energetics of the system i.e. the desire to reduce the surface free energy. As with all processes, there may be reduce the surface free energy. As with all processes, there may be kinetic limitations which prevent or hinder these rearrangements at kinetic limitations which prevent or hinder these rearrangements at low temperatures. low temperatures.

Both processes may occur with clean surfaces in ultrahigh vacuum, Both processes may occur with clean surfaces in ultrahigh vacuum, but it must be remembered that adsorption of species onto the but it must be remembered that adsorption of species onto the surface may enhance, alter or even reverse the process ! surface may enhance, alter or even reverse the process !

RelaxationRelaxation

                          

                                                                

 

Unrelaxed Surface

                          

                                                                

 

Relaxed Surface (d1-2 < dbulk )

Relaxation is a small and subtle rearrangement of the surface layers which may nevertheless be significant energetically, and seems to be commonplace for metal surfaces. It involves adjustments in the layer spacings perpendicular to the surface , there is no change either in the periodicity parallel to the surface or to the symmetry of the surface

The lower picture shows the relaxed surface : the first layer of atoms is typically drawn in slightly towards the second layer (i.e. d1-2 < dbulk )

Relaxation – possible driving forceRelaxation – possible driving force

If we use a localised model for the bonding in the solid then it is clear that an atom in the bulk is acted upon by a balanced, symmetrical set of forces.

On the other hand, an atom at the unrelaxed surface suffers from an imbalance of forces and the surface layer of atoms may therefore be pulled in towards the second layer.

(Whether this is a reasonable model for bonding in a metal is open to question !)

The magnitude of the contraction in the first layer spacing is generally small ( < 10 % )- compensating adjustments to other layer spacings may extend several layers into the solid.

ReconstructionReconstructionThe reconstruction of surfaces is a much more readily observable effect, involving larger (yet still atomic scale) displacements of the surface atoms. It occurs with many of the less stable metal surfaces (e.g. it is frequently observed on fcc(110) surfaces), but is much more prevalent on semiconductor surfaces.

Unlike relaxation, the phenomenon of reconstruction involves a change in the periodicity of the surface structure - the diagram below shows a surface, viewed from the side, which corresponds to an unreconstructed termination of the bulk structure.

This may be contrasted with the following picture which shows a schematic of a reconstructed surface - this particular example is similar to the "missing row model" proposed for the structure of a number of reconstructed (110) fcc metal surfaces.

Since reconstruction involves a change in the periodicity of the surface and in some cases also a change Since reconstruction involves a change in the periodicity of the surface and in some cases also a change in surface symmetry, it is readily detected using surface diffraction techniques (e.g. LEED & RHEED ). in surface symmetry, it is readily detected using surface diffraction techniques (e.g. LEED & RHEED ).

The overall driving force for reconstruction is once again the minimization of the surface free energy - at The overall driving force for reconstruction is once again the minimization of the surface free energy - at the atomic level, however, it is not always clear why the reconstruction should reduce the surface free the atomic level, however, it is not always clear why the reconstruction should reduce the surface free energy. For some metallic surfaces, it may be that the change in periodicity of the surface induces a energy. For some metallic surfaces, it may be that the change in periodicity of the surface induces a splitting in surface-localized bands of energy levels and that this can lead to a lowering of the total splitting in surface-localized bands of energy levels and that this can lead to a lowering of the total electronic energy when the band is initially only partly full. electronic energy when the band is initially only partly full.

In the case of many semiconductors, the simple reconstructions can often be explained in terms of a In the case of many semiconductors, the simple reconstructions can often be explained in terms of a "surface healing" process in which the co-ordinative unsaturation of the surface atoms is reduced by bond "surface healing" process in which the co-ordinative unsaturation of the surface atoms is reduced by bond formation between adjacent atoms. For example, the formation of a Si(100) surface requires that the formation between adjacent atoms. For example, the formation of a Si(100) surface requires that the bonds between the Si atoms that form the new surface layer and those that were in the layer immediately bonds between the Si atoms that form the new surface layer and those that were in the layer immediately above in the solid are broken - this leaves two "dangling bonds" per surface Si atom. above in the solid are broken - this leaves two "dangling bonds" per surface Si atom.

A relatively small co-ordinated movement of the atoms in the topmost layer can reduce this unsatisfied co-A relatively small co-ordinated movement of the atoms in the topmost layer can reduce this unsatisfied co-ordination - pairs of Si atoms come together to form surface "Si dimers", leaving only one dangling bond ordination - pairs of Si atoms come together to form surface "Si dimers", leaving only one dangling bond per Si atom. This process leads to a change in the surface periodicity : the period of the surface structure per Si atom. This process leads to a change in the surface periodicity : the period of the surface structure is doubled in one direction giving rise to the so-called (2x1) reconstruction observed on all clean Si(100) is doubled in one direction giving rise to the so-called (2x1) reconstruction observed on all clean Si(100) surfaces [ Si(100)-(2x1) ]. surfaces [ Si(100)-(2x1) ].

Reconstruction – cond.Reconstruction – cond.

SummarySummary

The minimisation of surface energy means that even single crystal surfaces will The minimisation of surface energy means that even single crystal surfaces will not exhibit the ideal geometry of atoms to be expected by truncating the bulk not exhibit the ideal geometry of atoms to be expected by truncating the bulk structure of the solid parallel to a particular plane. The differences between the structure of the solid parallel to a particular plane. The differences between the real structure of the clean surface and the ideal structure may be imperceptibly real structure of the clean surface and the ideal structure may be imperceptibly small (e.g. a very slight small (e.g. a very slight surface relaxation surface relaxation ) or much more marked and involving ) or much more marked and involving a change in the surface periodicity in one or more of the main symmetry a change in the surface periodicity in one or more of the main symmetry directions ( directions ( surface reconstruction surface reconstruction ). ).

Describing crystallite sitesDescribing crystallite sites

While in many instances surfaces are associated with ‘pure’ materials, there are also While in many instances surfaces are associated with ‘pure’ materials, there are also a range of circumstances when the surface relates to a small particle.a range of circumstances when the surface relates to a small particle.

Particle size distribution is also important.Particle size distribution is also important.

XRD does NOT give this information!!XRD does NOT give this information!!Use SEM, TEM and count particles.Use SEM, TEM and count particles.Is this representative?Is this representative?

Why is UHV required for surface studies ?Why is UHV required for surface studies ?Ultra high vacuum is required for most surface science experiments Ultra high vacuum is required for most surface science experiments for two principal reasons : for two principal reasons :

To enable atomically clean surfaces to be prepared for study, and such To enable atomically clean surfaces to be prepared for study, and such surfaces to be maintained in a contamination-free state for the duration of surfaces to be maintained in a contamination-free state for the duration of the experiment. the experiment.

To permit the use of low energy electron and ion-based experimental To permit the use of low energy electron and ion-based experimental techniques without undue interference from gas phase scattering.techniques without undue interference from gas phase scattering.

To put these points in context we shall now look at the variation of To put these points in context we shall now look at the variation of various parameters with pressure various parameters with pressure

1. Gas Density1. Gas Density

2. Mean Free Path of Particles in the Gas Phase2. Mean Free Path of Particles in the Gas Phase

3. Incident Molecular Flux on Surfaces3. Incident Molecular Flux on Surfaces

4. Gas Exposure - the "Langmuir" 4. Gas Exposure - the "Langmuir"

5. Sticking Coefficient & Surface Coverage5. Sticking Coefficient & Surface Coverage

How long will it take for a clean surface to become covered with a How long will it take for a clean surface to become covered with a complete monolayer of adsorbate ? complete monolayer of adsorbate ?

       Time / ML ~ ( 1019 / F )        [ s ]

This is dependent upon the flux of gas phase molecules incident upon the surface, This is dependent upon the flux of gas phase molecules incident upon the surface, the actual coverage corresponding to the monolayer and the coverage-dependent the actual coverage corresponding to the monolayer and the coverage-dependent sticking probability….sticking probability….

however , it is possible to get a minimum estimate of the time required by however , it is possible to get a minimum estimate of the time required by assuming a unit sticking probability (i.e. assuming a unit sticking probability (i.e. SS = 1) and noting that monolayer = 1) and noting that monolayer coverages are generally of the order of 10coverages are generally of the order of 101515 per cm per cm22 or 10 or 101919 per m per m22 . Then . Then

Summary - Variation of Parameters with PressureSummary - Variation of Parameters with Pressure

For most surface science experiments there are a number of factors For most surface science experiments there are a number of factors necessitating a high vacuum environment : necessitating a high vacuum environment :

For surface spectroscopy, the mean free path of probe and detected For surface spectroscopy, the mean free path of probe and detected particles (ions, atoms, electrons) in the vacuum environment must particles (ions, atoms, electrons) in the vacuum environment must be significantly greater than the dimensions of the apparatus in be significantly greater than the dimensions of the apparatus in order that these particles may travel to the surface and from the order that these particles may travel to the surface and from the surface to detector without undergoing any interaction with residual surface to detector without undergoing any interaction with residual gas phase molecules. This requires pressures better than 10-4 Torr. gas phase molecules. This requires pressures better than 10-4 Torr. There are, however, some techniques, such as IR spectroscopy, There are, however, some techniques, such as IR spectroscopy, which are "photon-in/photon-out" techniques and do not suffer from which are "photon-in/photon-out" techniques and do not suffer from this requirement.this requirement.(On a practical level, it is also the case that the lifetime of (On a practical level, it is also the case that the lifetime of channeltron and multiplier detectors used to detect charged channeltron and multiplier detectors used to detect charged particles is substantially reduced by operation at pressures above particles is substantially reduced by operation at pressures above 10-6 Torr). 10-6 Torr).

Most spectroscopic techniques are also capable of detecting Most spectroscopic techniques are also capable of detecting molecules in the gas phase; in these cases it is preferable that the molecules in the gas phase; in these cases it is preferable that the number of species present on the surface substantially exceeds number of species present on the surface substantially exceeds those present in the gas phase immediately above the surface - to those present in the gas phase immediately above the surface - to achieve a surface/gas phase discrimination of better than 10:1 when achieve a surface/gas phase discrimination of better than 10:1 when analysing ca. 1% of a monolayer on a flat surface this requires that analysing ca. 1% of a monolayer on a flat surface this requires that the gas phase concentration is less than ca. 1012 molecules cm-3 the gas phase concentration is less than ca. 1012 molecules cm-3 ( = 1018 molecules m-3), i.e. that the (partial) pressure is of the ( = 1018 molecules m-3), i.e. that the (partial) pressure is of the order of 10-4 Torr or lower.order of 10-4 Torr or lower.

In order to begin experiments with a reproducibly clean surface, and In order to begin experiments with a reproducibly clean surface, and to ensure that significant contamination by background gases does to ensure that significant contamination by background gases does not occur during an experiment, the background pressure must be not occur during an experiment, the background pressure must be such that the time required for contaminant build-up is substantially such that the time required for contaminant build-up is substantially greater than that required to conduct the experiment i.e. of the order greater than that required to conduct the experiment i.e. of the order of hours. The implication with regard to the required pressure of hours. The implication with regard to the required pressure depends upon the nature of the surface, but for the more reactive depends upon the nature of the surface, but for the more reactive surfaces this necessitates the use of UHV (i.e. < 1 x 10-9 Torr). surfaces this necessitates the use of UHV (i.e. < 1 x 10-9 Torr).