Wurtzite materials surfaces

Most surfaces of compound semiconductors are polar, that is, the number of anions and cations per surface unit cell is not balanced. While for the zinc blende materials there is only one nonpolar exception, the (110) face, for the wurtzite structures, there are two nonpolar surfaces, the m-plane (1100) and a-plane (1120) [98]. In wurtzite materials, a (110) surface does not exist because of the different crystal structure. 

Wurtzite GaN is a polar material. Therefore, along the c-axis, there are N-face (N-polar) or Ga-face (Ga-polar) orientations on the GaN surface. 

This topic will be discussed in two steps, firstly the crystals with 3-dimensional structures (metal nanoparticles and 4-coordinate semiconductors) and then those with a layered stmcture (Bi, Se, Te, etc). A decrease in the crystal sizes can result in a change of the stmcture type if the surface energy gain exceeds the enthalpy of the corresponding phase transition. Thus, Co has the structure of the hep type in the bulk, the/cc type in 10-20 nm particles and the bcc type in 2-5 nm particles [33], Particles of In with the diameter of <5 nm have the/cc structure, and those from 5 nm upward to the bulk have the fecf-lattice [34]. Agl adopts the cubic stmcture in the particles larger than 50 nm and the hexagonal one in smaller crystals [35]. In As has the wurtzite (w) stmcture up to 40 nm and the sphalerite (zb) structure in grains of >80 nm [36]. Nano-CdS has the the zb stmcture for D = 4 nm, while the w-phase is stable for the bulk material [37, 38]. On the contrary, MnSe was obtained in the w-form in nanoparticles, whereas the zb phase is stable for bulk crystals [37, 39]. 

Oxides commonly studied as catalytic materials belong to the structural classes of corundum, rocksalt, wurtzite, spinel, perovskite, rutile, and layer structure. These structures are commonly reported for oxides prepared by normal methods under mild conditions [1,5]. Many transition metal ions possess multiple stable oxidation states. The easy oxidation and reduction (redox property), and the existence of cations of different oxidation states in the intermediate oxides have been thought to be important factors for these oxides to possess desirable properties in selective oxidation and related reactions. In general terms, metal oxides are made up of metallic cations and oxygen anions. The ionicity of the lattice, which is often less than that predicted by formal oxidation states, results in the presence of charged adsorbate species and the common heterolytic dissociative adsorption of molecules (i.e., a molecule AB is adsorbed as A+ and B ). Surface exposed cations and anions form acidic and basic sites as well as acid-base pair sites [1]. The fact that the cations often have a number of commonly obtainable oxidation states has resulted in the ability of the oxides to undergo oxidation and reduction, and the possibility of the presence of rather high densities of cationic and anionic vacancies. Some of these aspects are discussed in this chapter. In particular, the participation of redox sites in oxidation and ammoxidation reactions and the role of redox sites in various oxides that are currently pursued in the literature are presented with relevant references. 

In the wurtzite structure, the polar faces are the (0001) and (OOOT) faces. In ZnO, they are terminated by a zinc and an oxygen plane, respectively. It seems that their surface energies are lower than in the rocksalt crystals. The experiments performed on these materials do not mention impurity or faceting problems, and the Zn-terminated surface was found to be unrelaxed (Sambi et al., 1994). 

The micro- and nanoindentation methods have been widely used to determine the hardness of ZnO over a wide range of size scales and temperatures. Hardness measurements are usually carried out on the (0001) surface of the crystal using the conventional pyramidal or spherical diamond tip, or alternatively, with a sharp triangular indentei The depth-sensing indentation measurements provide the complete information on the hardness and pressure-induced phase transformation of semiconductor materials. Table 1.6 shows the measured and calculated mechanical parameters for ZnO crystallized in the form of wurtzite, zinc blende, and rocksalt phases. 

The wurtzite crystal structure is prominent in the class of II-VI compounds (ZnO, CdS, etc.) and, concerning the III-V materials, the III nitrides, tliat is, GaN, AlN, InN, and their multinary compounds. In the following, we will discuss surface structures of wurtzite crystals, using III nitrides as examples. The fundamental rules of surface formation are the same as for the zinc blende structures, treated in the last chapters 12.3.2 in detail. 

For noncubic compounds, the bulk crystal may be optically anisotropic. This is the case not only for wurtzite or chalcopyrite crystal structures but also for ternary and quaternary III-V and II-VI compounds showing ordering effects on the sublattices. In aU these cases, the compound semiconductor is an optically anisotropic material. The RAS signal is then a superposition of two contributions, one originating from the bulk and the other from the surface. In order to analyze the surface, the two contributions have to be separated, a task for optical modeling. 

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