Films size quantization

Nafion films Size-quantized CdS in Nafion films... 

Ethylene-15% methacrylic acid copolymer Polyvinylcarbazole CdS and PbS incorporated into polymer films Size-quantized CdS particles incorporated into polymers... 

Electrodeposition of Nanostructures Size-Quantized Films on Metal Substrates... 

Torimoto T, Nagakubo S, Nishizawa M, Yoneyama H (1998) Photoelectrochemical properties of size-quantized CdS thin films prepared by an electrochemical method. Langmuir 14 7077-7081... 

Torimoto T, Obayashi A, Kuwabata S, Yasuda H, Mori H, Yoneyama H (2000) Preparation of size-quantized ZnS thin films using electrochemical atomic layer epitaxy and their photoelectrochemical properties. Langmuir 16 5820-5824... 

Mastai Y, Hodes G (1997) Size quantization in electrodeposited CdTe nanocrystalline films. 

Finally, the tendency for CD films to be nanocrystalline and often to exhibit quantum-size effects is treated in the final chapter, Chapter 10, Nanocrystallinity and Size Quantization in CD Semiconductor Films. 

The layout of this book means that there will be some overlap between sections. However, this system should allow those readers interested in one or more specific sections to skip the others, thereby making the book more efficient for the individual reader. An example of this is the use of quantum-size effects to elucidate CD mechanisms. This is treated, with different emphasis, both in Chapter 3 (Mechanisms of Chemical Deposition) and in Chapter 10 (Nanocrystallinity and Size Quantization in CD Semiconductor Films). 

Nanocrystallinity and Size Quantization in Chemical Deposited Semiconductor Films... 

Optical absorption spectroscopy is often carried out on CD films to verify that the films have a bandgap expected from the deposited semiconductor. Additionally, since CD films are often nanocrystaUine and the most apparent effect of very small crystal size is the increasing bandgap due to size quantization (the effect is visible to the eye if the bandgap is in the visible region of the spectrum), absorption (or transmission) optical spectroscopy is clearly a fast and simple pointer to crystal size, since bandgap-size correlations have been made for a number of semiconductor colloids and films. 

Another cause of interest in this technique is due to the fact that the crystals in most as-deposited CD fdms are very small. Considering the current interest in nanoparticles, CD is an excellent technique to deposit nanocrystalline fdms. More specifically, if the nanocrystals are small enough, they exhibit size quantization, the most obvious manifestation of which is an increase in the optical bandgap with decrease in crystal size, as was shown for CD CdSe [17] and later for CD PbSe [18,19]. In fact, the changes in optical spectra that occurred in these films as a function of nanocrystal size were exploited to provide information on the different mechanisms of the deposition process [20]. 

OPTICAL PROPERTIES. CD CdSc films often exhibit size quantization, with a blue shift in their absorption spectrum of as much as 0.6 eV. 

There are many studies on photoconductivity in these films, many of them early ones and focused on annealed films (since air annealing is necessary for optimal photoconductivity). The use of a chemical oxidant (which never seems to be specified) gives much higher photosensitivity for as-deposited films than for films deposited without oxidant, although even here annealing is used to obtain maximum performance. Some studies on photoconductivity in as-deposited PbSe films have shown shifts in photoconductivity spectral response, with onsets shifted to 2.2 (xm instead of the ca. 4.5 xm more typical of annealed films. As with optical absorption studies, these shifts can be attributed to size quantization. 

The size of the crystals formed in CD films is often small enough that quantum-size effects become apparent. The terms quantum size effect and size quantization are normally used to describe a material whose energy structure differs from that of the bulk material. As crystal (or, more generally, particle) size decreases, charges (electrons and holes) in the particles are constrained in an increasingly small volume. When the particle size becomes smaller than the Bohr diameter of the charges in the bulk material (between 2 and 20 nm for many ma-... 

While shifts due to size quantization have most commonly been seen in absorption spectroscopy, other spectroscopies, such as photoelectrochemical photocurrent, photovoltage (using a vibrating Kelvin probe), photoluminescence, and photoconductivity spectroscopies have all shown quantum shifts in various CD films. 

Another possibility that could explain the effect of illumination is a change in the electric double layer surrounding the CdSe particles, either adsorbed on the substrate or in the solution, which could lower a potential barrier to adsorption and coalescence, as suggested previously for film formation from Se colloids under illumination [93]. Partial coalescence would reduce the blue spectral shift due to size quantization. However, the spectral shape is not expected to undergo a fundamental change in this case. The photoelectrochemical explanation therefore appears more reasonable. 

Optical transmission of the films was a spectral average of ca. 85% beyond the bandgap. The bandgap measured from the spectrum was 3.76 eV (literature for zincblende ZnS = 3.6 eV), suggesting size quantization, although the measured crystal (ca. 10 nm) is at least twice the size required for such an effect. 

The wide range of very small crystal sizes in these films gives rise to strong blue shifts in their optical spectra due to size quantization [65], This aspect of these films is dealt with in detail in Chapter 10. 

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