Ultraviolet work function measurement

Fig. 3.14 Work function measure of polycrystalline Ag using ultraviolet photoelectron spectroscopy...

Ultraviolet photoelectron spectroscopy is capable of providing chemical state and electronic structure information from materials. However, due to the complex nature of the density-of-states (DOS) in the valence band, it is more difficult to extract this information, as compared to XPS, usually requiring band-structure calculations and other spectroscopies. By observation of the onset of photoelectron emission, work function measurements may be made using UPS. Like XPS, UPS is non-destructive. However, UPS cannot typically provide quantitative information. 

The ultraviolet and x-ray photoelectron spectroscopy (UPS and XPS) measurements are used to calculate /P of PFO at — 5.6 +0.05 eV, and the band gap at 3.1 +0.1 eV, which is also much closer to the optical band gap than to the value deduced from the electrochemistry in films [254]. Thus, the HOMO LUMO levels of PF can be reasonably well-matched by work functions of ITO/PEDOT (—5.1eV) and Ca electrode (ca. — 2.9 eV), respectively. However,... 

In the photoelectric method, the measured average work function is always less than the true since patches of high work function tend to be excluded from the emission process. Thus, the nonuniform distribution of adsorbate on a patch surface may cause a slight discrepancy in the evaluation of A. Experimentally, the photoelectric method has various limitations. Photocurrents of the order of 10 A. must be measured accurately in the region of vo, and for films of work function greater than 5 v., the threshold frequency lies in the far ultraviolet—a practical disadvantage. Furthermore, the method is inapplicable at pressures in excess of 10 mm. Hg because of ionization of the gas by collision. 

In this context it is instructive to mention the work function 4>. Here, eFermi level of a solid into vacuum far away from the surface [107], The work function can be measured for example by ultraviolet photon spectroscopy (for a discussion see Ref. [108]). 

There are two practical difficulties. One is the measurement of the very small photocurrent, which may be as low as 10" amp, requiring the use of a vibrating reed electrometer or similar instrument. The second is that, for work functions above 5 eV, Vq lies in the far ultraviolet. This makes the study of some adsorptions very difficult and 6 eV is about the practical limit of such measurements. A suitable light source is the quartz mercury arc. The energy of the incident beam can be measured with a calibrated photocell, a vacuum thermopile or a radiometer. A suitable cell for adsorption studies is shown in Fig. 11. The sample being studied forms the cathode B. It can be a metal foil or a film formed by evaporation from the filament E. A wire C is fused through the glass to make contact with the... 

When ultraviolet light of wavelength of 131 nm strikes a polished nickel surface, the maximum kinetic energy of ejected electrons is measured to be 7.04 X 10 J. Calculate the work function of nickel. 

For photoelectron spectroscopy organic layers (neat films and mixtures) with a nominal thickness of 25 nm were deposited on 100 nm thick gold films which were thermally evaporated onto oxidised Si wafers. The electronic properties of the films were characterised using X-ray and ultraviolet photoelectron spectroscopy (XPS, UPS) by employing monochromated A1 Ka radiation (hv = 1486.7 eV) for measurement of the core levels as well as ultraviolet radiation [He I (hv = 21.2 eV) and He II (hv = 40.8 eV)] for an analysis of the occupied states near the Fermi level. For a measurement of the secondary electron cut-off to determine the sample work function exactly, an additional bias (-2 V and -5 V) was applied to the sample. 

OSITs based on pentacene thin films have been fabricated on TTO formed on glass substrafes. It is well known that the work function of TTO is con-frolled by the method used to clean its surface. OSITs were fabricated based on pentacene thin films wifh a high-work function ITO of 5.3 eV and a low-work function of 4.2 eV. The effect of the work function of ITO on the static characteristics of fhe OSITs was investigated using I-V measurements and ultraviolet photoemission spectroscopy (UPS) [34]. These results provided an important clue, in that the characteristics of the OSITs were strongly associated with the work function of ITO used as a source electrode. In general, the hole injection barrier at the organic semiconductor/metal interface is influenced by the work function of fhe metal. 

The minimum energy required to eject a photoelectron is the work function of the metal and the ejected photoelectrons come from the top of the Fermi surface. By increasing the photon energy, electrons that lie deeper in the Fermi well can be ejected and the numbers and energies of the photoelectrons coming off can be measured. This technique is called photoelectron spectroscopy. If an ultraviolet lamp is used as the photon source, the technique is called ultraviolet photoelectron spectroscopy or UPS. Similarly, if x-rays are used as the photons, the technique is called x-ray photoelectron spectroscopy or XPS. 

The delta-function deconvolution method (FFT) was used to improve the spectral resolution and to remove the plural scattering effect at the core-loss edge in electron energy-loss spectroscopy (EELS). The zero-loss peak (used as an instrumental resolution function) works as a nonattenuation high-pass filter in this technique [17]. Reflectance spectra in the vacuum ultraviolet of microcrystalline 3-BN, prepared by plasma CVD (chemical vapor deposition), and of sintered (3-BN measured with synchrotron radiation in the energy range from 5 to 25 eV, show reflectance peaks near 11.4 and 14 eV and a broad peak near 18 eV. The peaks at 11.4 and 14.0 eV are assigned to the E and E2 peaks of the sphalerite-type semiconductor [18]. 

The characterization of additive diffusivity in polymers is an important area of study in order to understand the performance of a material. There has been much work in diffusivity measurement by mapping concentration profiles through various techniques, such as FTIR [2, 3], fluorescence recovery [4, 5], ultraviolet microscopy [6, 7], and refractive index-related optical spectroscopy [8]. The process usually involves preparation of standard samples, and correlation of concentration profiles with a mathematical model that is a function of distance and time. 

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