The Basic Significance of Oxygen Chemisorption on the Photoelectronic Properties of CdS and CdSe

Abstract

The long-known phenomenon of oxygen chemisorption on CdS and CdSe, more intensively studied recently, is shown to have a basic significance for the photoelectronic properties of thin crystals or layers. It is proposed that every "pure" crystal of CdS or CdSe is of relatively high conductivity [--~10 -6 (ohm cm) -1] and high photosensitivity (electron lifetime of 10 -3 sec) as prepared by standard techniques. If the crystal or layer is thin enough, oxygen adsorption obscures these properties and gives the appearance o5 a highly insulating insensitive material. Smaller effects are found in crystals containing deliberately incorporated impurities as long as the Fermi level at room temperature lies above the level associated with the surface state for chemisorbed oxygen. Because of the universal presence of oxygen chemisorption effects, every experiment in which the effect of heat, vacuum, or photoexcitation are investigated toward understanding the nature of photochemical. changes, must be interpreted in the light of the significant effects associated with the simple desorption of oxygen.The effect of oxygen on the photoelectronic properties of CdS and CdSe has been known from the early days of research in this area. A common prescription for improving the light-to-dark resistance ratio of photoconducting cells was to bake the cells in air to increase the dark resistance (1). The importance of adsorbed oxygen in determining measured values of dark resistance and photosensitivity in CdS (2, 3) and CdSe (4) was investigated a number of years ago, but it is only in recent years that the general importance of these effects has been realized. The effect of adsorbed oxygen on the photovoltaic effect of CdS has been studied by Williams (5) and by Haas, Fox, and Katz (6), on the lifetime and response time ~[or strongly absorbed light in CdS by Mark (7-9), on the conductivity of CdSe layers by Somorjai (10), on the photoelectronic properties of sintered layers of CdS-CdSe and CdSe by Shear, Hilton, and Bube (11), and on the photo-Hall effect of sintered layers by Robinson and Bube (12). The importance of oxygen chemisorption phenomena in simulating other types of photochemical changes has also been discussed for CdS-CdSe crystals (13), for which the electron lifetime at room temperature could be varied by a factor of 20 by varying the amount of adsorbed ox.-ygen.It now appears that what was originally believed to be an appreciable effect of importance for only a few specially prepared crystals or for particular excitation conditions is in fact a quite general effect that can have a profound influence on the photoelectronic properties of CdS and CdSe. The effects would be expected to be the most pronounced for thin crystals or layers of these materials for which surface chemisorption of oxygen could have electrical effects extending throughout the volume of the material.Mark has given a thorough report on the phenomenon of oxygen chemisorption on CdS crystals, both as to its kinetics and its energetics. He has worked with str...

Solution-processed CdS transistors with high electron mobility

“…The off-current is reduced dramatically aer air exposure; however, the overall current through the device also drops by several orders of magnitude and signicant hysteresis is apparent. This increase in resistivity is consistent with effects of oxygen adsorption on thin CdS lms previously observed by Bube, et al 31 and the observed depletion of electron density in CdS nanobelt devices upon exposure to oxygen. 32 These undesirable effects can be resolved by one nal annealing step at 300 C in a nitrogen atmosphere, leading to devices with reduced leakage currents and generally good performance.…”

Oxygen-Induced Blinking of Single CdSe Nanocrystals

“…In the experiments described here the minimum time bin was set to 100 ms, thereby limiting the accessible on'±off' kinetics to processes that occur on a longer time scale. [18] It is assumed that the adsorbed oxygen traps photogenerated electrons, which decreases the free-electron lifetime by a few orders of magnitude. Although this procedure is strictly valid only for exponential distributions, it still allows us to study the influence of oxygen on the blinking dynamics.…”

“…Ground-state triplet oxygen is known to be an efficient fluorescence quencher via different mechanisms. [18] Obviously, the creation of additional traps by oxygen influences only the charging and not the neutralization and therefore leaves the off'-time distribution unaffected. [16] In another set of experiments it was suggested that the on'-times of flickering single terrylene-diimide (TDI) molecules were shortened by formation of a non-fluorescing TDI-oxygen complex.…”

<title>Probing the interactions of single CdSe quantum dots with their local environment</title>

“…Obviously the presence of oxygen at the surface of the particles changes the electronic properties of the nanocrystals, which in turn changes their fluorescence behavior. In the past it has been shown, that adsorption of oxygen also changes the photoconductivity of thin CdS and CdSe films 29 . In this work the authors proposed that adsorbed oxygen can act as a scavenger for the charge carriers and therefore reduces the conductivity by more than one order of magnitude 30 .…”