Quantum well electrode

Charge Transfer Processes at Quantum Well Electrodes (MQW, SQW)... 

Photo-induced electron transfer reactions from quantum well electrodes into a redox system in solution represent an intriguing research area of photoelectrochemistry. Several aspects of quantized semiconductor electrodes are of interest, including the question of hot carrier transfer from quantum well electrodes into solution. The most interesting question here is whether an electron transfer from higher quantized levels to the oxidized species of the redox system can occur, as illustrated in Fig. 9.31. In order to accomplish such a hot electron transfer, the rate of electron transfer must be competitive with the rate of electron relaxation. It has been shown that quantization can slow down the carrier cooling dynamics and make hot carrier transfer competitive with carrier cooling. 

Short wavelength photons (ofenergymuch greater than Eg) create hot carriers. If, somehow, thermalization of these carriers can be avoided, photoelectrochemical reactions that would otherwise be impossible with the cooled counterparts, that is, at very negative potentials for n-type semiconductors, would be an intriguing possibility. The key issue here is whether the rate of electron transfer across the interface can exceed the rate of hot electron cooling. The observation of hot carrier effects at semiconductor-electrolyte interfaces is a controversial matter [3,7,11,171] and practical difficulties include problems with band edge movement at the interface and the like [4]. Under certain circumstances (e.g. quantum-well electrodes, oxide film-covered metallic electrodes), it has been claimed that hot carrier transfer can indeed be sustained across the semiconductor-electrolyte interface [7,172,173]. 

In MBE, ultra-high vacuum chambers are outfitted with a number of evaporation cells. Each cell has a shutter to control the molecular flux. The molecular beam can be pulsed within 0.1 sec and the growth rate can be controlled within a few A sec f During the growth, the substrate temperature is maintained at ca. 500-700 °C. Atomic species most commonly used include Al, In, Sb, Be, Ge, Se, Te, Cd, Hg, Zn, Mn, Pb, and Si. The most common quantum well electrodes produced by MBE are the III-V semiconductors binary and ternary compounds. Some II-VI quantum well structures have also been prepared. 

Nanocrystalline particulate films, which exhibit pronounced quantum size effects in three dimensions, are of great interest due to applications in solar cell (108-112) and sensor (57, 113-115) applications. They exhibit novel properties due to not only the SQE manifested by individual nanoparticles but also the total surface area. Unlike MBE and MOCVD methods used to prepare quantum well electrodes, these electrodes can be prepared by conventional chemical routes described in Section 9.5.2.2. For example, II-VI semiconductor particulate films were prepared by using low concentrations of precursors and by controlling the temperature of the deposition bath. Nodes demonstrated the SQE for CdSe thin films deposited by an electroless method (98). The blue shift in the spectra of CdSe films has been demonstrated to be a function of bath temperature. As described in Section 9.5.2.1, electrodeposition of semiconductors in non-aqueous solvents leads to the formation of size-quantized semiconductor particles. On a single-crystal substrate, electrodeposition methods result in epitaxial growth (116, 117), and danonstrate quantum well properties. 

Photo-induced electron transfer reactions from quantum well electrodes into a redox system in solution represent an intriguing research area of photoelectrochemistry. Several aspects of quantized semiconductor electrodes are of interest, including the question of hot carrier transfer from quantum well electrodes into... 

Experts in the field of biology, chemistry, and physics know very well that boundaries must be crossed, which can only be done effectively with the present and emerging concepts of dynamical systems, quantum theory, electrodics, and the solid state, among others. Submolecular biology is becoming more accessible by means of sophisticated research methods based on microtechniques, microcircuitry, and computers. The steps from in vitro to in vivo experimentation, although still tentative, have already taken place and will accelerate rapidly. 

Fig. 9.31 Kinetic model for electron transfer from a quantum well (QW) electrode into a redttx electrolyte. = standard potential of the redox couple A the reorganization energy. The acceptor states correspond to the oxidized species of the redox system. (After ref. [2J)...

Time-resolved measurements of electron transfer times for quantum well photoelectrodes which can be compared with hot electron relaxation times, have not yet been reported. Only some excitation spectra, i.e. photocurrent vs. photon energy for MQWs and single quantum wells (SQWs), have been published so far [2]. In both cases, the photocurrent spectra show distinct structures corresponding to transitions between the hole and electron wells as shown for SQW electrodes in Fig. 9.32. The... 

Fig. 9. 32 Photocurrent action spectrum for GaAs single quantum well (SOW) electrodes at room temperature as a function of inner barrier thickness, Lf. a) L, = 170 A b) = 1.5 pm c) L, = 2.5 pm. For all three samples, the outer barrier thickness is 270 A and the nominal well width is 130 A. For (a) and (b), the peak at about 1.43 eV is due to the GaAs buffer layer. The zero baseline for curve (a) is offset for clarity. (After ref. [79])...

Fig. 9.33 a) Single quantum well (SOW) electrode structure (not to scale) b) energy level diagram for SQW under reverse bias. 

The effect of size quantization on the electronic properties of semiconductors, discussed in Section 9.2.2, demonstrates that semiconductor electrodes made of nanostructured particles are of great practical interest. Based on size quantization, these films can be categorized into (a) thin semiconductor films deposited or epitaxial growth on a substrate where the SQE is due to the space confinement in two dimensions (i.e., a quantum well) and (b) particulate films of size-quantized nanoparticles that may be several micrometers thick their properties are due to the combined effect of film and isolated size-quantized particles. Both the situations are illustrated in Figure 9.41. 

Nozik and Memming have worked extensively on the electrochemistry and photoelectrochemistry of quantum well semiconductor electrodes (14). As discussed in Section 9.5.1, the quantum wells are produced by either MBE or MOCVD method. Both of these techniques are capable of creating epitaxial layers exhibiting quantum size effects. Their properties can be varied by film thickness, interfacial abruptness, and crystalline perfection. 

In electroluminescence devices (LEDs) ionized traps form space charges, which govern the charge carrier injection from metal electrodes into the active material [21]. The same states that trap charge carriers may also act as a recombination center for the non-radiative decay of excitons. Therefore, the luminescence efficiency as well as charge earner transport in LEDs are influenced by traps. Both factors determine the quantum efficiency of LEDs. 

According to Vitanov et a/.,61,151 C,- varies in the order Ag(100) < Ag(lll), i.e., in the reverse order with respect to that of Valette and Hamelin.24 63 67 150 383-390 The order of electrolytically grown planes clashes with the results of quantum-chemical calculations,436 439 as well as with the results of the jellium/hard sphere model for the metal/electro-lyte interface.428 429 435 A comparison of C, values for quasi-perfect Ag planes with the data of real Ag planes shows that for quasi-perfect Ag planes, the values of Cf 0 are remarkably higher than those for real Ag planes. A definite difference between real and quasi-perfect Ag electrodes may be the higher number of defects expected for a real Ag crystal. 15 32 i25 401407 10-416-422 since the defects seem to be the sites of stronger adsorption, one would expect that quasi-perfect surfaces would have a smaller surface activity toward H20 molecules and so lower Cf"0 values. The influence of the surface defects on H20 adsorption at Ag from a gas phase has been demonstrated by Klaua and Madey.445... 

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