Dark current/response

A spectrum in a specified ranalogue signals from eadi photodiode are digitised and transferred to a computer, where they e corrected for dark current response and transformed to absorbance. A number of digital techniques are available to increase sensitivity and to extend the use of rapid-scanning detectors to multicomponent analysis, reaction kinetics, tablet dissolution tests, process control, and detection in HPLC (A. F. Fell et al, Chrom-atographia, 1982, 16, 69-78). 

Figure 12. Dark-current response to step field excitation in 44 wt % TPD-PC film with Au (ohmic) contacts. T = 333 K, and L = 26 xm. A 500-Vfield was applied. The cusp occurs at approximately 0.8 Ur, as determined independently with the TOF technique. The current equilibrium achieved in several transit times indicates the absence of deep trapping. (Reproduced with permission from reference 43. Copyright 1986 Taylor and Francis.)...

Many detectors exhibit a small constant response K known as a dark current, even when no radiation strikes their surfaces. Instruments with detectors that have a significant dark-current response are ordinarily capable of compensation so that the dark current is automatically subtracted. Thus, under ordinary circumstances, we can simplify Equation 25-3 to... 

It is possible to extract a maximum theoretical jgc (and consequently STH) from three-electrode measurements, but only if at least TWO measurements are made for both the cathode and anode. Figure 6.8c-h depicts several typical scenarios. In Fig. 6.8c, the photoresponse of a photoanode is overlayed with the dark current response of a cathode catalyst (note that the sign of the current response for... 

Figure 2. The photocurrent and dark current response of compound 6 as a function of the external electric field.

Record the dark current response of the detector over the region of interest. 

Figure 15-12. Spectrally resolved pliotocurrent of a ITO/PPV/Mg photodiode at dilTcrcnt bias after correction for dark current, light source, and monochromator response, and normalization to the same peak value. The broken line is the normalized absorption spectrum of PPV (reproduced by permission of the Institute of Physics from Ref. 143)).

FIG. 14 On-off photocurrent responses (a) associated with the reaction in Eq. (41) at Ao0 = —0.225 V. In this figure, positive currents correspond to the transfer of a negative charge from water to DCE. The potential dependence of the photocurrent (b) was obtained under chopped illumination and lock-in detection. The maximum in the photocurrent-potential curve contrasts with the small changes in the dark current shown in (c). These responses are developed within the polarizable window described in (d). (From Ref. 49. Reproduced by permission of The Royal Society of Chemistry.)... 

In the manufacturing process inhomogeneities arise in the silicon substrate and all the pixels in the array are not exactly identical. They may show variations in their response to the same photon flux. Even in the absence of a light signal, differences occur in the dark current in a group of pixels. However, these differ-... 

The spectral response of photoemissive tubes depends upon the composition of the cathode and the use of various mixtures of elements permits the production of a wide range of tubes of varying responsiveness (Figure 2.27). Photomultiplier tubes can be used to detect low intensity radiation and even in the absence of any light will still generate a small current due to various emissions from the material of the tube, etc. This dark current has to be compensated for in any measurements that are made. 

In one of the most common types of photodiodes used for time-resolved work, the p-i-n photodiode (see Figure 12.24), the depletion layer thickness (i for intrinsic) is fabricated to obtain this optimum performance. Manufacturers usually give full specification sheets detailing, active area, time/frequency response, responsivity amps/watt (AAV) at a given wavelength, dark current, depletion layer capacitance, and bias volts such that with minimal external electronics devices can be made operative. 

The slow-scan CCD, also called the scientific CCD, or in the spectroscopy literature simply CCD, is the detector of choice for most applications of Raman spectroscopy. A well-designed CCD has essentially zero dark current, very low readout noise, and high quantum efficiency (peak 45—70% near 700 nm) in the visible region of the spectrum. However, the response drops quickly above 800 nm and there is no photon response above 1.05 J m. For routine spectroscopy or process control, thermoelectrically cooled (to about —40° C) CCDs are adequate. Although these detectors are somewhat noisier than detectors operated at —100° C or lower, the former do not require liquid nitrogen cooling. The general properties and spectroscopic applications of the CCD have been reviewed (22). 

A shutter and pure solvent in the sample compartment of this pseudo-double beam spectrometer permits acquisition of both a dark current (0% T) and an emmisivity/responsivity (100% T) data array. These consist of digitized responses from each of the pixels in the array under conditions where the integration time and the speed of read-out is identical to that planned for the measurement of the spectrum of a sample. The dark-current data will provide information as to the shot-noise and other inherent... 

Two hypotheses have been forwarded to account for this blocking of the Na + channels ([1] for a recent review, see [2]). The first is that Ca2+ ions, which are released from the inside of the discs into the cytoplasmic space upon illumination, are responsible for the shut off of the dark current. The second hypothesis casts cyclic GMP into the role of the transmitter upon absorption of light by a visual pigment, activation of a phosphodiesterase occurs which reduces the levels of cyclic GMP (5 X 104 molecules of cyclic GMP disappear upon bleaching of one molecule of pigment). This in turn results in a reduced activity of a protein kinase which is... 

The IR, UV-VIS, and XPS, dark current and photocurrent have been compared for a number of the polymorphs of CuPc (Knudsen 1966 Enokida and Hirohashi 1991). The most important characteristic of a pigment is its colour the variation in spectral response five CuPc polymorphs, presented in Fig. 6.9, shows considerable... 

The partially oxidized sihcon species are responsible for the anodic current transient measured at the end of etching of an anodic oxide film-covered n-Si electrode in the dark as shown in Fig. 3.25. For a clean n-Si surface, the anodic current is very small. This dark current during the etch-back experiment, whose peak position depends on the thickness, occurs on anodic oxide as well as on thermal oxide. The data shown in Fig. 3.25 indicate that the anodic reaction proceeds by injection of electrons from the partially oxidized sihcon species at the sihcon/oxide interface. The amount of charge associated with the current transient, which is similar for anodically and thermally oxidized surface, is about 4.4 x 10 C/cm corresponding to two monolayers of on a (100) surface. The partially oxidized species may extend to a number of atomic layers, fewer for thermal oxide than anodic oxide as shown in Fig. 3.25. 

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