Workfunction

Field emission applicable to flat-panel display is one of the most advanced and energetically studied applications of CNTs. The apparatus is illustrated in Fig. 12 along with pictures of closed-tips of MWCNTs. In spite of their high workfunction (4..3 eV for MWCNTs [39]), CNTs emit electrons from their tips when high voltages (100-1000 V) are applied between the metal grid of the accelerator and the CNT film which are separated by 20 im [38]. 

For a junction of a conjugated polymer, which has an energy gap of around 3 eV (around the value of the polyphenylenes) and a free carrier concentration n> 1017 cm-3, with a low workfunction metal (e.g. when [Pg.155]

Figure 9-22. Energy diagram ol a metal/ scmiconductor/meta Schottky barrier (0... workfunction, x,. electron affinity, /,... ionization potential, . ..bandgap, W... depletion width).

The metallic electrode materials are characterized by their Fermi levels. The position of the Fermi level relative to the eneigetic levels of the organic layer determines the potential barrier for charge carrier injection. The workfunction of most metal electrodes relative to vacuum are tabulated [103]. However, this nominal value will usually strongly differ from the effective workfunction in the device due to interactions of the metallic- with the organic material, which can be of physical or chemical nature [104-106]. Therefore, to calculate the potential barrier height at the interface, the effective work function of the metal and the effective ionization potential and electron affinity of the organic material at the interface have to be measured [55, 107],... 

A semiconductor can be described as a material with a Fermi energy, which typically is located within the energy gap region at any temperature. If a semiconductor is brought into electrical contact with a metal, either an ohmic or a rectifying Schouky contact is formed at the interface. The nature of the contact is determined by the workfunction, (the energetic difference between the Fermi level and the vacuum level), of the semiconductor relative to the mclal (if interface effects are neglected - see below) 47J. 

In a first approximation, conjugated polymers with a high charge earner concentration can be described as p-lype semiconductors whereby holes seem to be the majority charge earners. Depending on the workfunction of the polymer rela-... 

Consider the solid electrolyte cell shown in Figure 5.20. For simplicity we consider only a working (W) and reference (R) electrode deposited on a solid electrolyte, such as YSZ or p"-Al203. The two electrodes are made of the same metal or of two different metals, M and M. The partial pressures of 02 on the two sides of the cell are p02 and po2 Oxygen may chemisorb on the metal surfaces so that the workfunctions w and R(p 02). 

Figure 5.46 shows clearly how the application of potential changes the brightness and thus the workfunction O, of the grounded Pt catalyst-electrode (windows 2 and 3) and of the YSZ surface, (window 1), in accordance to the above discussed alignment (pinning) of the two Fermi levels. 

It is clearly the workfunction O. By just charging a metal in vacuum (which affects J(= Ep)) one cannot induce NEMCA, This has been shown by charging one electrode of Pt/YSZ at potentials up to 104 V relative to earth (thus decreasing/increasing jl by 104 eV) without observing any change in kinetics. 

The Vacuum Reference The first reference in the double-reference method enables the surface potential of the metal slab to be related to the vacuum scale. This relationship is determined by calculating the workfunction of the model metal/water/adsorbate interface, including a few layers of water molecules. The workfunction, — < ermi. is then used to calibrate the system Fermi level to an electrochemical reference electrode. It is convenient to choose the normal hydrogen electrode (NHE), as it has been experimentally and theoretically determined that the NHE potential is —4.8 V with respect to the free electron in a vacuum [Wagner, 1993]. We therefore apply the relationship... 

Higher water coverages and the presence of solution both act to lower the barriers to activate water. The intermolecular interactions that result from hydrogen bonding with other water molecules stabilize the activated HO—H complex over the entire dissociation reaction coordinate. For metals with high workfunctions, the aqueous phase can enable heterolytic water activation... 

At first sight it would look as if the definition of surface potential (x) described in Section 6.4.8 would overlap with the definition of the workfunction. Does this mean that both quantities are the same but with opposite signs To answer this question, let us look closer to the trajectory of the electron as defined in the work function (Fig. 6.45). The electron starts in a point deep inside the metal, where all different types of chemical bondings and interactions exist. After breaking all these forces, the electron moves itself free from inside the metal to a point close to the surface. Then, from here it has to cross the barrier of dipoles (see Section 6.3.8) to reach a point just outside the metal. 

The workfunction w is a spectrometer constant and represents mainly the work necessary to excite the electron from the Fermi-level to the free electron level. Bearing in mind the experimental set-up, where EK = EK> is the constant analyzer energy, the complete equation reads... 

The workfunction y introduced in Eq. 5 actually contains three contributions ... 

In the chapter on device motivation to the study of polymer surfaces and interfaces, a diagram similar to Fig. 8.3 appears. At first glance, there is an apparent discrepancy between the UPS model of Fig. 8.2 (bottom), and LED mode 2>26, in Fig. 8.3. If a second metal, with workfunction less than that of the metal shown in Fig. 8.2, is used as the counter electrode on the polymer film in the UPS model, assuming the direct alignment of the various energy band edges, the level must move, such that Ajt = 0 throughout the sandwich, upon... 

The most prominent and most frequently studied electropositive additives are alkali metals. Several comprehensive reviews have been published on the subject which provide more detailed information [260— 262]. Characteristically, adsorption of alkali leads to dramatic changes of the work function of the system [260-262], An example, K on Pt(l 11), is shown in Fig. 33 [263]. In general, small alkali coverages already lower the workfunction considerably before monolayer coverage is reached (in the present case more than... 

Table 12.1 Solution Cyclic Voltammetric Half-Wave Potentials 1/2 (V versus SCE), Gas-Phase Ionization Potentials/D(eV), and Electron Affinities AA(eV) for Selected Donors D and Acceptors A, along with Metal Workfunctions [4]...

The photoelectron signal as a function of the laser intensity was measured at two temperatures for the LC film and is shown in Fig. 9. At room temperature the photoemission signal depends linearly on the laser intensity, indicating a single photon photoemission process. From the high energy cut-off in Fig. 9a we deduce that the workfunction is about 3.9 eV, much lower than the workfunction of the bare... 

Park C, Cho BJ, Tang LJ, Kwong DL. Substituted aluminum metal gate on high-k dielectric for low workfunction and fermi-level pinning free. lEDM tech digest 2004. p 299-302. 

Fig. 18. (a) Negative ion fraction of scattered O versus exit angle for 1 keV oxygen atoms incident on Mg, Al, and Ag. (b) Ion fractions for different metals as a function of the metal s workfunction (from Ref. 78). 

The calculated energy of the highest occupied orbitals (HOMO), Ehomo, of the M015O56H22 cluster amounts to 6.1 eV. Based on general DFT theory [35] the quantity -Ehomo represents the first cluster ionization potential which converges with increasing cluster size towards the workfunction of the Mo03(010) surface. The theoretical value of -Ehomo is reasonable in view of typical work-... 

---