Electromodulation

For characterization purposes the most useful form of external modulation is electromodulation, because it provides the sharpest structure (third derivative of R in bulk or thin films) and is sensitive to surface or interface electric fields. The most widely used contacdess mode of electromodulation is termed Photoreflectance (PR) 5.7.8... 

An example of a contact mode of electromodulation would be the semiconduc-tor-insulator-metal configuration, which consists of a semiconductor, about 200 A of an insulator like AI2O3, and a semitransparent metal (about 50 A of Ni or Au). Modulating (ac) and bias (dc) voltages are applied between the front semitransparent metal and a contact on the back of the sample. To employ this mode the sample must be conducting. 

The most complicated form of Modulation Spectroscopy is electromodulation, since in certain cases it can accelerate the electron-hole pairs created by the light. If the electric field is not too large the quantity AR/Rcan be written as ... 

For sufficiendy high built-in electric fields the electromodulation spectrum can... 

Shown in Figure 3 is the variation of the fundamental direct band gap (Fq) of Gai j j(As as a function of A1 composition (jt). These results were obtained at 300 K using electromodulation. Thus it would be possible to evaluate the A1 composition of this alloy from the position of Fq. 

Apart from the above techniques, the electromodulated reflectance spectroscopy combined with cyclic voltammetry has been utilized by Gaigalas et al. [14] in the investigations of electron transfer between the 2Fe-2S protein putidaredoxin and either bare or bekanamycin-modified Ag electrode. Of the two models considered, the free diffusion model, as compared to the adsorbed layer model, exhibited better concordance with the experimental data. After modification of the Ag electrode with bekanamycin, it exhibited only a small increase in the observed redox reaction... 

The abrupt increase in slope at 3.37 eV signals the presence of a higher-energy transition, in accord with the increased absorption found by electromodulation measurement (15). The decrease in slope at 3.5 eV corresponds to a saturation of this contribution to the total absorption and is not understood. Nevertheless, several other materials give evidence of similar behavior. 

Fig. 64. Typical electromodulated spectrum of a silicon/acetonitrile+0.1 M TBAP interface. Electromodulation potentials -0.3 V, +0.3 V vs. Ag/AgCl. Modulated charge 5xl0 Ccm". (After [180]). Reprinted by permission of Elsevier Science.

ELECTROMODULATION SPECTROSCOPY OF SEMICONDUCTOR NANOSTRUCTURES III-NITRIDES AND DILUTE NITRIDES... 

The condition to obtain electromodulation (electro- and photo-reflectance) spectra is the existence of a built-in electric field in a structure under investigations. This condition is usually fulfilled in majority structures. Typical CER method utilizes a capacitor-like system with one top semitransparent electrode and one bottom copper-block electrode. The sample is glued to the bottom electrode by using a silver pasta. The front electrode is separated from the sample surface by a spacer (e.g. 0.1 mm). Thus there is nothing in direct contact with the sample. It means that the sample does not conduct any currents and the external electric field is able to change the carriers distribution inside it. Note that the voltage drop appears mainly in the air gap between the front electrode and the sample. The limit for the applied voltage is the electric breakdown in this air gap. It means that the maximal amplitude of EM in the CER technique usually is more limited than the EM amplitude in ER or PR techniques. 

PL of CdSe/ZnS nanorods is partially polarized and the degree of polarization is nearly independent of external electric field. The P-polarized component of the PL is quenched more efficiently than the 5-component. Spectral positions of PL maxima for both components have been deviated weakly with the voltage increased. The effect of sensitivity of PL properties of CdSe/ZnS nanorods with respect to electric field may find application in luminescent electromodulators. 

Electromodulation spectroscopy of semiconductor nanostructures Ill-nitrides and dilute nitrides (invited).11... 

The fact that both the COad bands and the CO2 band appeared in the same direction on an nm-Ru/GC electrode illustrates one of the main characteristics of the AIREs. It may be worth noting that the COl band center measured in Figure 11(a) is close to that reported by Lin et al. but is about 15cm lower than the value for CO adsorbed on a massive Ru electrode in CO saturated 0.5 M HCIO4 solution studied by using electromodulated infrared spectroscopy (EMIRS) [50]. It is evident that the appearance of the COb band in the spectrum of the nm-Ru/GC electrode is due to significant enhancement of IR absorption. 

A. V. Rao, F. Ozanam, and J. N. Chazalviel, In situ fourier-transform electromodulated infrared study of porous silicon formation Evidence for solvent effects on the vibrational linewidths, J. Electrochem. Soc. 138 153, 1991. 

Dhoot AS, Hogan JA, Morteani AC, Greenham NC (2004) Electromodulation of photoinduced charge transfer in polyfluorene bilayer devices. Appl Phys Lett 85 2256... 

ELECTROMODULATION OF PHOTOLUMINESCENCE FROM CdSe NANORODS FILM... 

Deore, B., Yakabe, H., Shiigi, H., and Nagaoka, T. (2002). Enantioselective uptake of amino acids using an electromodulated column packed with carbon fibres modified with overoxidised polypyrrole, 127,935-939. 

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