Interfaces depth

The chemical and electronic properties of elements at the interfaces between very thin films and bulk substrates are important in several technological areas, particularly microelectronics, sensors, catalysis, metal protection, and solar cells. To study conditions at an interface, depth profiling by ion bombardment is inadvisable, because both composition and chemical state can be altered by interaction with energetic positive ions. The normal procedure is, therefore, to start with a clean or other well-characterized substrate and deposit the thin film on to it slowly at a chosen temperature while XPS is used to monitor the composition and chemical state by recording selected characteristic spectra. The procedure continues until no further spectral changes occur, as a function of film thickness, of time elapsed since deposition, or of changes in substrate temperature. 

The sputter rate is then derived for the matrix of interest by dividing the derived thickness (depth) by the time required to sputter down to the respective interface. The interface depth is usually taken as the position in which some secondary ion signal representative of layer either rises or drops by 50% in intensity. As matrix effects can modify secondary ion intensities over such regions, care must be taken in signal selection. 

An increase in this parameter causes the interface depth to... 

Fig. 10.10 Aspect ratio (costal diameter/interface depth) as a function of oxygen partial pressure for sapphire. Data are from reference [4].

Fig. 11.7 Interface depths measured in quenched crystals of 13 mm diameter with critical rotation rates noted according to Table (11-2) (the numbers along the top of the figure are for the 20-mm diameter case). (Modified from Capper et al. J. Cryst. Growth 89 (1988) 171, copyright (1988) reproduced with permission from Elsevier Science.)...

Application of ACRT opened up a large number of possible parameter combinations. Quenching studies of crystals grown under a wide variety of conditions showed interface depths of mm for x = 0.12 and 0.19 start crystals, unlike the values of 1 and 4 mm, respectively, seen in equivalent standard Bridgman crystals. 

Increasing the maximum rotation rate produced the interface depths shown in Fig. 11.7. The preferred region in which to operate is clearly 25 < < 60rpm. 

Fig. 1.13 a The two-dimensional election gas stiucture of heteiojunction interface in Ti02/ SrTiOs, b the change of carriCT concentration in two-dimensionai eiectron gas with the interface depth, reprinted with the permission from Ref. [147], copyright 2008 American Chemical Society... 

If a pressure measuring device were run inside the capillary, an oil gradient would be measured in the oil column. A pressure discontinuity would be apparent across the interface (the difference being the capillary pressure), and a water gradient would be measured below the interface. If the device also measured resistivity, a contact would be determined at this interface, and would be described as the oil-water contact (OWC). Note that if oil and water pressure measurements alone were used to construct a pressure-depth plot, and the gradient intercept technigue was used to determine an interface, it is the free water level which would be determined, not the OWC. 

Both the Monte Carlo and the molecular dynamics methods (see Section III-2B) have been used to obtain theoretical density-versus-depth profiles for a hypothetical liquid-vapor interface. Rice and co-workers (see Refs. 72 and 121) have found that density along the normal to the surface tends to be a... 

A film at low densities and pressures obeys the equations of state described in Section III-7. The available area per molecule is laige compared to the cross-sectional area. The film pressure can be described as the difference in osmotic pressure acting over a depth, r, between the interface containing the film and the pure solvent interface [188-190]. 

The behavior of insoluble monolayers at the hydrocarbon-water interface has been studied to some extent. In general, a values for straight-chain acids and alcohols are greater at a given film pressure than if spread at the water-air interface. This is perhaps to be expected since the nonpolar phase should tend to reduce the cohesion between the hydrocarbon tails. See Ref. 91 for early reviews. Takenaka [92] has reported polarized resonance Raman spectra for an azo dye monolayer at the CCl4-water interface some conclusions as to orientation were possible. A mean-held theory based on Lennard-Jones potentials has been used to model an amphiphile at an oil-water interface one conclusion was that the depth of the interfacial region can be relatively large [93]. 

Finally, it is difficult to caUbrate the depth scale in a depth profile. This situation is made more compHcated by different sputtering rates of materials. Despite these shortcomings, depth profiling by simultaneous ion sputtering/aes is commonly employed, because it is one of the few techniques that can provide information about buried interfaces, albeit in a destmctive manner. 

Optical Techniques. The most important tool in a museum laboratory is the low power stereomicroscope. This instmment, usually used at magnifications of 3—50 x, has enough depth of field to be useful for the study of surface phenomena on many types of objects without the need for removal and preparation of a sample. The information thus obtained can relate to toohnarks and manufacturing techniques, wear patterns, the stmcture of corrosion, artificial patination techniques, the stmcture of paint layers, or previous restorations. Any art object coming into a museum laboratory is examined by this microscope (see Microscopy Surface and interface analysis). 

Detention efficiency. Conversion from the ideal basin sized by detention-time procedures to an actual clarifier requires the inclusion of an efficiency factor to account for the effects of turbulence and nonuniform flow. Efficiencies vaiy greatly, being dependent not only on the relative dimensions of the clarifier and the means of feeding but also on the characteristics of the particles. The cui ve shown in Fig. 18-83 can be used to scale up laboratoiy data in sizing circular clarifiers. The static detention time determined from a test to produce a specific effluent sohds concentration is divided by the efficiency (expressed as a fraction) to determine the nominal detention time, which represents the volume of the clarifier above the settled pulp interface divided by the overflow rate. Different diameter-depth combinations are considered by using the corresponding efficiency factor. In most cases, area may be determined by factors other than the bulksettling rate, such as practical tank-depth limitations. 

---