Crystals single

Single crystals are essential in some cases or to study anisotropic properties fields as semiconductors and optics require large-size, highly pure, defect-free. 

In the Czochralski method, the material is heated in a crucible just above its melting point. A seed crystal is immersed in the melt and it is slowly pulled out (5mm/min) from the melt vertically while being slowly twisted (2-20 rpm), resulting in continuous growth at the interface. The system is within a chamber with argon to avoid contamination. 

In the Kyropoulos method, the crystal-liquid interface moves into the melt as 

In the Bridgman-Strockbarger method, the crucible is displaced along a furnace with a temperature gradient the temperature is higher at the top when it moves downward, the melt becomes solidified at the bottom of the crucible, which is conic-shaped to decrease the number of nuclei. 

In the floating zone method, a rod-shaped starting material is oriented vertically and heated by a ring-shaped furnace moving along the rod the material melts and recrystallizes if a seed crystal is put on one end of the rod. This method can be used to purify the solid, as the solubility of the impurities is usually different in the solid or in the melted state. 

Single crystals of AI2O3 have been used as windows for heat resistance and good infrared transmission. They have also been used as high-density light-bulb enclosures, and as electronic device substrates. Another application of these have been in the form of rods and other special shapes as high-temperature refractory materials. 

Single crystals of rutile (Ti02), spinel (MgAl204), strontium titanate (SrTiOJ, and ruby (AI2O3 with some Cr203 in solid solution) have been used as synthetic jewel materials. Lithium niobate (LiNbOg) is used as a laser host and as a substrate. The use of alkali and alkaline earth halide crystals for prisms 

The dominant single crystal for solid-state lasers is YAG, which is produced using the Cz melt-growth process [29]. Transition metal elements or lanthanide rare earth elements are used as laser active ions that are doped in YAG host material. Due to its narrow spectral width and high quantum efficiency, Nd ion, a four-level laser system has been acknowledged to be the most popular active ion. 

There is a large demand nowadays for single, pure and defect-free crystals of innumerable substances, e.g. for the scientific appraisal of the chemical and physical properties of pure solids, for use in solid-state electronics, where crystals of certain substances are required for their dielectric, piezoelectric, paramagnetic and semiconductor properties, and for optical and laser materials and synthetic gem-stones (see Table 7.1). 

This subject is somewhat outside the main theme of the present book, so only a brief summary will be attempted here. However, several comprehensive accounts have been made of this specialized aspect of crystallization practice (Laudise, 1970 Faktor and Garrett, 1974 Pamplin, 1980 Hurle, 1993). 

The actual operating conditions vary according to the nature of the crystallizing substance and solvent the optimum supersaturation and solution movement 

Piezoelectric quartz, Rochelle salt, ammonium and potassium dihydrogen phosphates, ethylenediamine tartrate, triglycine phosphate 

Hydrothermal and flux growth and their industrial applications have been the subjects of several comprehensive reviews (Lobachev, 1971 Elwell and Scheel, 1975 Elwell, 1980 Wanklyn, 1983). 

Let us next be more specific and describe in some detail the fundamentals of the most common preparation methods, for the production of both single crystals and thin films, illustrated with relevant case examples discussed in the book. Key references will also be given. 

Perhaps the most simple way of obtaining small single crystals, albeit in an uncontrolled way, is the drop casting teclmique. This method consists in the deposition of a drop of a saturated solution of a soluble material on a clean substrate prior to the evaporation of the solvent in air, usually at RT. Some interesting examples are shown next. 

When the solvent evaporation is performed in a well-controlled way, sufficiently large single crystals can be obtained. This is the case for pentacene, where single crystals are grown from solution in TCB by slowly evaporating the solvent over a period of four weeks at 450 K, under a stream of ultrapure N2 gas. 

Some binary MOMs form very stable phases in solution, which translates into rapid synthetic processes. TTF-TCNQ belongs to this class of materials and can be prepared quite simply as a black powder. In fact when highly purified TTF and TCNQ are combined in CH3CN, the 1 1 complex precipitates from the solution. 

A second example is the synthesis of the TTF[M(dmit)2]j compounds, where M = Ni, Pd or Pt. Metathesis reaction under inert atmosphere of CH3CN solutions of (TTF)3(BP4)2 and the appropriate metal salt TBA[M(dmit)2] immediately yields insoluble, black, shiny microcrystalline powders. Because TTF[M(dmit)2]j is virtually insoluble in common solvents, its recrystallization from solution is precluded. Therefore, when single crystals are needed some alternative synthesis routes have to be undertaken. 

For a crystalline solid, when the periodic and repeated arrangement of atoms is perfect or extends throughout the entirety of the specimen without interruption, the result is a single crystal. All unit cells interlock in the same way and have the same orientation. Single crystals exist in nature, but they can also be produced artificially. They are ordinarily difficult to grow because the environment must be carefully controlled. 

As mentioned previously, SOAz was prepared following the procedure which was recently described . When repeating such a preparation many times, we 

Chemically speaking, SOAz (I) and SOAz (II) are strictly identical and pure their actions on animal tumors are also identical. Thus, we would expect that the difference in their melting points to be due to some structural peculiarities, either with regard to their space group (if the two kinds of crystals do not contain any insertion of solvent) or possibly by some inclusion of solvent in the unit cell (clathrate structure) as in the case of MYKO 63 when crystallized from C Hg or CCI4 (see above). 

Electrostatic bonding of the crystal on top of a previously prepared gate-insulator-source-drain structure [92-94]. Sometimes the source and drain contacts have been deposited afterwards, on top of the crystal [90]. 

Direct deposition of the contacts and gate insulator on to the crystal [95, 96], In this technique the gate dielectric is the polymer parylene, which forms conformal coatings with good dielectric and mechanical properties. The polymer is deposited in a three-zone reactor, in which the deposition zone can be kept at room temperature. 

These techniques have recently been reviewed [97] and will not be discussed further here. Instead, we will focus on the electrical characteristics of single-crystal OTFTs. 

Comparing single crystal and vapor-grown devices for these two compounds is difficult, because reports on evaporated tetracene OTFTs are rather scarce [99-101], and despite several (unpublished) attempts, fabrication of an operating thin-film device from rubrene has not yet been successfully achieved. For both compounds the problem seems to arise from an improper deposition mechanism, which, in contrast with experience with pentacene and sexithiophene, does not favor two-dimensional growth. 

Prominent features of these single crystal devices are [102]  

The maximal concentrations of adsorbed sulfur on different metalsd preceding the appearance of solid sulfide were determined. It has been stated that in the case of both platinum and nickel, the saturation states are different for different crystal faces, i.e., the metal S atomic ratios are 2 on 100 and slightly lower than 1 2 on the 111 face. On the 110 face of Ni and Pt, the ratios are 1.41 and 1.23 respectively. 

These data indicate that sulfur preferentiaUy adsorbs on sites of lowest coordination. Different types of sulfur adsorption were also observed by the Berkeley group on Mo 100 single metal surfaces. deposited on 

Sulfur coverage resulted in a decrease in HDS activity of Mo and the catalyst lost its HDS activity after prolonged ( 12 hr) thiophene treatment. In the author s opinion, this is a consequence of M0S2 overlayer formation. 

This contradicts, however, the HDS activity of sulfides including M0S2, observed by a high number of authors (see e.g., Refs. 20-25) long before this study on molybdenum single crystals. The loss of activity upon the long thiophene treatment was possibly a consequence of the formation of carbonaceous deposits, the by-products of thiophene HDS. 

Summing up, deposition studies on single crystal with radiosulfur indicated that the sulfur uptake and the metahsulfur ratio is different on crystal planes with different indices, and a part of the adsorbed sulfur is easily removable, whereas another part is held more strongly by the surface metal atoms. 

When a polymer is crystaUized from the melt, imprafect polycrystalline aggregations are formed in association with substantial amorphous content This is a consequence of chain entanglement, the high viscosity of the melt combining to hindm the diffusion of chains into the ordraed arrays necessary for crystaUite formation. 

For a polymer such as polyethylraie, the fold in the ehain is completed using only three or four monomer units with bonds in the gauehe conformation. The extended portions in between have about 40 monomer units all in the trans conformation. 

Yet another tactic involves perturbing the electrostatics at the semiconductor-electrolyte interface by adsorbing 

Modification agent(s) Semiconductor(s) Modification objective Sample R erence(s) 

B electrostatic modification C catalysis of multielectron photoprocesses (refer to text). 

Native semiconductor surfaces are fairly inactive from a catalysis perspective. Thus, noble metal or metal oxide islands have been implanted on photoelectrode surfaces as electron storage centers to drive multielectron redox processes such 

Alkanethiol-based or alkylsiloxane-based SAMs have been profitably employed in all these instances to probe the distance effect in electron transfer dynamics. The thiol-based SAMs have the virtue that the spacer length can be predictably altered simply by varying the number of methylene units in the chain. The distance dependence of ket is embodied in the parameter fi, the decay coefficient. Eor a critical discussion of the subtleties involved in the extraction and interpretation of this parameter, we refer to Ref. 262. A value of 0.49 0.07 has been reported for this parameter for n-InP-alkanethiol-ferrocyanide interfaces [266]. This value is smaller than its counterpart for corresponding films on gold surfaces, which range from 0.6 to 1.1 per methylene unit. The reason for this difference is not entirely clear, although several hypotheses were advanced by the authors [266]. 

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A Technique of Ultrasonic Testing without Dead Zone for Coarse-Grained TC4 Extrusion Pipe. - The Development of Single Crystal Creeping Wave Prohe. 

KEYWORDS ultrasonic testing coares grain pipe single crystal creeping wave probe... 

The properties of a single crystal creeping wave probe The a single crystal creeping wave probe is suitable for testing various artificial defects such as surface cracks, FBH, columned hole and SDH etc. and its distance amplitude cruve is shown in Fig.6... 

Rehbinder and co-workers were pioneers in the study of environmental effects on the strength of solids [144], As discussed by Frumkin and others [143-145], the measured hardness of a metal immersed in an electrolyte solution varies with applied potential in the manner of an electrocapillary curve (see Section V-7). A dramatic demonstration of this so-called Rehbinder effect is the easy deformation of single crystals of tin and of zinc if the surface is coated with an oleic acid monolayer [144]. 

In principle, then, small crystals should show a higher solubility in a given solvent than should large ones. A corollary is that a mass of small crystals should eventually recrystallize to a single crystal (see Ostwald ripening. Section IX-4). 

The contact angle for water on single-crystal naphthalene is 87.7° at 35°C, and ddjdT is -0.13 deg/K. Using data from Table III-l as necessary, calculate the heat of immersion of naphthalene in water in cal/g if a sample of powdered naphthalene of 10 m /g is used for the immersion study. (Note Ref. 135.)... 

As on previous occasions, the reader is reminded that no very extensive coverage of the literature is possible in a textbook such as this one and that the emphasis is primarily on principles and their illustration. Several monographs are available for more detailed information (see General References). Useful reviews are on future directions and anunonia synthesis [2], surface analysis [3], surface mechanisms [4], dynamics of surface reactions [5], single-crystal versus actual catalysts [6], oscillatory kinetics [7], fractals [8], surface electrochemistry [9], particle size effects [10], and supported metals [11, 12]. 

Mention was made in Section XVIII-2E of programmed desorption this technique gives specific information about both the adsorption and the desorption of specific molecular states, at least when applied to single-crystal surfaces. The kinetic theory involved is essentially that used in Section XVI-3A. It will be recalled that the adsorption rate was there taken to be simply the rate at which molecules from the gas phase would strike a site area times the fraction of unoccupied sites. If the adsorption is activated, the fraction of molecules hitting and sticking that can proceed to a chemisorbed state is given by exp(-E /RT). The adsorption rate constant of Eq. XVII-13 becomes... 

Fig. XVIII-27. Specific rates of CO oxidation on single crystal and supported catalysts as a function of temperature. (From Ref 308. Reprinted with permission from American Chemical Society, copyright 1988.)...

Most fiindamental surface science investigations employ single-crystal samples cut along a low-index plane. The single-crystal surface is prepared to be nearly atomically flat. The surface may also be modified in vacuum. For example, it may be exposed to a gas that adsorbs (sticks) to the surface, or a film can be grown onto a sample by evaporation of material. In addition to single-crystal surfaces, many researchers have investigated vicinal, i.e. stepped, surfaces as well as the surfaces of polycrystalline and disordered materials. 

To first approximation, a single-crystal surface is atomically flat and unifonu, and is composed of a regular... 

For many studies of single-crystal surfaces, it is sufficient to consider the surface as consisting of a single domain of a unifonn, well ordered atomic structure based on a particular low-Miller-mdex orientation. However, real materials are not so flawless. It is therefore usefril to consider how real surfaces differ from the ideal case, so that the behaviour that is intrinsic to a single domain of the well ordered orientation can be distinguished from tliat caused by defects. 

When atoms, molecules, or molecular fragments adsorb onto a single-crystal surface, they often arrange themselves into an ordered pattern. Generally, the size of the adsorbate-induced two-dimensional surface unit cell is larger than that of the clean surface. The same nomenclature is used to describe the surface unit cell of an adsorbate system as is used to describe a reconstructed surface, i.e. the synmietry is given with respect to the bulk tenninated (unreconstructed) two-dimensional surface unit cell. 

Figure Al.7.10. STM image (1000 A x 1000 A) of the (111) surface of a tungsten single crystal, after it had been coated with a very thin film of palladium and heated to about 800 K (courtesy of Ted Madey).

Electrons interact with solid surfaces by elastic and inelastic scattering, and these interactions are employed in electron spectroscopy. For example, electrons that elastically scatter will diffract from a single-crystal lattice. The diffraction pattern can be used as a means of stnictural detenuination, as in FEED. Electrons scatter inelastically by inducing electronic and vibrational excitations in the surface region. These losses fonu the basis of electron energy loss spectroscopy (EELS). An incident electron can also knock out an iimer-shell, or core, electron from an atom in the solid that will, in turn, initiate an Auger process. Electrons can also be used to induce stimulated desorption, as described in section Al.7.5.6. 

One of the main uses of these wet cells is to investigate surface electrochemistry [94, 95]. In these experiments, a single-crystal surface is prepared by UFIV teclmiqiies and then transferred into an electrochemical cell. An electrochemical reaction is then run and characterized using cyclic voltaimnetry, with the sample itself being one of the electrodes. In order to be sure that the electrochemical measurements all involved the same crystal face, for some experiments a single-crystal cube was actually oriented and polished on all six sides Following surface modification by electrochemistry, the sample is returned to UFIV for... 

Soriaga M P 1992 Ultra-high vacuum techniques in the study of single-crystal electrode surfaces Prog. Surf. Sc/. 39 325... 

Easier V, Schweiss P, Meingast C, Obst B, Wuhl H, Rykov A I and Tajima S 1998 3D-XY critical fluctuations of the thermal expansivity in detwinned YBa2Cu30y g single crystals near optimal doping Phys. Rev. Lett. 81 1094-7... 

How are fiindamental aspects of surface reactions studied The surface science approach uses a simplified system to model the more complicated real-world systems. At the heart of this simplified system is the use of well defined surfaces, typically in the fonn of oriented single crystals. A thorough description of these surfaces should include composition, electronic structure and geometric structure measurements, as well as an evaluation of reactivity towards different adsorbates. Furthemiore, the system should be constructed such that it can be made increasingly more complex to more closely mimic macroscopic systems. However, relating surface science results to the corresponding real-world problems often proves to be a stumbling block because of the sheer complexity of these real-world systems. 

Figure A3.10.15 NFl synthesis activity of different Fe single-crystal orientations [32]. Reaction conditions were 20 atm and 600-700 K.

Figure A3.10.18 Surface concentration of nitrogen on different Fe single crystals following N2 exposure at elevated temperatures in UHV [48],...

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