Q-particles

Nanoclusters/Polymer Composites. The principle for developing a new class of photoconductive materials, consisting of charge-transporting polymers such as PVK doped with semiconductor nanoclusters, sometimes called nanoparticles, Q-particles, or quantum dots, has been demonstrated (26,27). 

The properties of a tungsten oxide monolayer on 0.18 pm silica particles were studied by Leland and Bard The material was prepared by a controlled WCI hydrolysis technique. These particles differ from the colloidal Q-particles that they are small in only one dimension thus these layers are related to the semiconductor... 

Fig. 11 Effect of particle size of phenacetin on dissolution of drug from granules containing starch and gelatin. Q, particle size 0.11-0.15mm A, particle size 0.15-0.21 mm , particle size 0.21-0.30mm , particle size 0.30-0.50mm , particle size 0.50-0.71 mm. (From Ref. 17.).

Colloidal CdS particles 2-7 nm in diameter exhibit a blue shift in their absorption and luminescence characteristics due to quantum confinement effects [45,46]. It is known that particle size has a pronounced effect on semiconductor spectral properties when their size becomes comparable with that of an exciton. This so called quantum size effect occurs when R < as (R = particle radius, ub = Bohr radius see Chapter 4, coinciding with a gradual change in the energy bands of a semiconductor into a set of discrete electronic levels. The observation of a discrete excitonic transition in the absorption and luminescence spectra of such particles, so called Q-particles, requires samples of very narrow size distribution and well-defined crystal structure [47,48]. Semiconductor nanocrystals, or... 

Here again an equation is established (2) to describe the trajectory of a particle under the combined effect of the liquid transport velocity acting in the. v-direction and the centrifugal settling velocity in they-direction. Equation 13 determines the minimum particle size which originates from a position on the outer radius, r2, and the midpoint of the space,, between two adjacent disks, and just reaches the upper disk at the inner radius, q. Particles of this size initially located above the midpoint of space a are all collected on the underside of the upper disk those particles initially located below the midpoint escape capture. This condition defines the throughput, for which a 50% recovery of the entering particles is achieved. That is,... 

More detailed investigations [38,39] have shown the kinetics of low-temperature electron transfer reaction (1) in bacteria to have a biphase character, i.e. to consist of two sections, one with a faster and the other with a slower decay of the Pf centres. Also, the type of kinetics of reaction (1) in bacteria at low temperatures has been found to depend on the conditions of sample preparation. The region of fast (t 50 ms) charge recombination at T < 230 K was observed only for the samples frozen in the dark. The extent of P decay was observed to decrease upon freezing the samples in the light. These results were explained by the presence of two channels for the decay of P+ centres by reactions with particles A and Q". The faster decay of P+ was assumed to be due to its reaction with A and the slower decay of P4 to its reaction with Q . The relative amounts of A and Q particles (i.e. the extent of electron transfer from the reduced form of the primary acceptor A to the secondary acceptor Q) was assumed to depend on temperature. This assumption explains why the character of P decay depends on whether P+ species are formed after or in the process of freezing the sample. 

At T = 77 K in MTHF, the kinetics of fluorescence decay of P-L-Q with a bridge containing one bicyclo[2.2.2]octyl is of a non-exponential character. This effect can be explained by the coexistence in the frozen solution of several rotational conformations of the P-L-Q molecule (rotation of the porphyrin fragment around the a bond in its meso position is meant here). The characteristic time of the fluorescence decay for the predominant portion of the P-L-Q particles at 77 K, r 1.1 x 10 1°s, virtually coincides with the value of r = l/k(e1 at 298 K, i.e. the rate of tunneling from P to Q is independent of temperature. The exponential character of the fluorescence decay curve at 298 K indicates that, at this temperature, the rate of rotation exceeds k(e1. ... 

Figure 8.1 Cyclic voltammetric response in the absence and presence of thioglycol-capped CdS Q-particles (1 mg/mL of fraction IV) at a Pt electrode. 50 mM tetrahexylammonium perchlorate in DMF, sweep rate = 50 mV s 1. Cl, C2, and C3 refer to cathodic peaks 1, 2, and 3, respectively. Al, A2, and A3 refer to anodic peaks 1, 2, and 3 respectively.8 (Reprinted with permission from S. K. Haram et al.,...

Colloidal solutions of semiconductor particles are of great interest mainly as photocatalysts of various processes. Of special interest are the so-called Q-particles of semiconductors (for CdS with the size 2R < 50 A), some of their properties being considerably different from... 

The equation gives the chamber diameter required to separate the so-called df,Q particle diameter, as a function of the slurry flow rate and the liquid and solid physical properties. The particle diameter is the diameter of the particle, 50 per cent of which will appear in the overflow, and 50 per cent in the underflow. The separating efficiency for other particles is related to the dso diameter by Figure 10.22, which is based on a formula by Bennett (1936). 

This new definition of normal ordering changes our analysis of the Wick s theorem contractions only slightly. Whereas before, the only nonzero pairwise contraction required the annihilation operator to be to the left of the creation operator (cf. Eq. [84]), now the only nonzero contractions place the q -particle operator to the left of the -particle creation operator. There are only two ways this can occur, namely. 

Q Particles at the surface are drawn toward the interior of a liquid until attractive and repulsive forces are balanced, o How does the spider s structure help it stay afloat on water ... 

These drawings show three of the ways particles are arranged in crystal lattices. Each sphere represents a particle. Q Particles are arranged only at the corners of the cube. o There Is a particle In the center of the cube. 

Holmes JD, Bhargava PA, Korgel BA, Johnston KP. Synthesis of cadmium sulfide Q-particles in water-in-C02 microemulsions. Langmuir 1999 15 6613-6615. 

The reactants, 4Be and H, have a combined nuclear charge of 5 and mass number of 10. In addition to the Q-particle, a product will be formed of charge 5-2 = 3, and mass number 10-4 = 6. This is fLi, since lithium is the element of atomic number 3. 

During the past decade, a new focus has developed. It was found that semiconductor particles can be made so small, typically into the nanometer size regime, that a quantum confinement effect occurs [6-15]. Particles of this size are often referred to as nanoclusters, nanoparticles, quantum dots, or Q-particles. The structures of these nanometer-sized semiconductor clusters are usually similar to those of the bulk crystals, yet their properties are remarkably different. With the proper surface-capping agents, clusters of varying sizes can be isolated as powders and redissolved into various organic solvents just like molecules. The availability of this new class of materials allows us to study the transition of a material from molecule to bulk solid, as well as its various properties and applications. 

Electrodes can be prepared with films of semiconductor particles. A straightforward approach involves suitable chemical treatment of a substrate metal, e.g., chemical or anodic oxidation of Ti to form a film of Ti02, which is made up of many small crystals, called grains. An alternative is to spread a film of semiconductor particles on an electrode surface. Films of nm-size semiconductor particles nanocrystalline films) have been of special interest. Such small particles (variously called quantum particles, Q-particles, or quantum dots), have properties that differ from those of larger (/xm) dimension (86, 87). 

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