Type II Dichalcogenides

Dichalcogenides, sometimes denoted TX2, have layers made up of a sheet of metal atoms (T)

Figure 2 The staging phenomenon as exhibited by graphite (for stage 3, d = carbon interlayer distance, dic = intercalate distance and dr = repeat basal distance).
Figure 3 Schematic illustration of intercalant concentration dependent intercalation of type II layered materials.

sandwiched between two sheets of chalcogen (X) atoms. T is usually a transition metal and X may be S, Se or Te. The layers are largely neutral and separated by a van der Waals gap. As mentioned above, dichalcogenides are able to accept guest species into random interlayer sites and may ultimately form a saturated stage 1 expanded material at sufficient guest species concentrations (Figure 3). During intercalation, the guest species are inserted in the van der Waals gap and in most cases occupy interstitial sites. The intercalation is generally accompanied by charge transfer between the intercalant species and the host layers and therefore intercalation complexes are formed with electron donor species only. Such species include alkali metal atoms, transition metal atoms and organic molecules. The intercalation of metal atoms (especially alkali metal atoms) results in efficient transfer of electrons to the host compound resulting in unique electronic properties. For metal atom intercalation the increase in layer separation is not large but the weak host layer interactions are replaced by strong Coulomb (alkali metal) and covalent (transition metal) interactions yielding a quasi three-dimensional solid. The intercalation of organic molecules results in much larger layer separations. An example is the intercalation of amines in which the orientation of the amines in the van der Waals gap depends on the number of carbon atoms. Short chain amines, such as methylamine, pack parallel to the layers whereas intermediate chain amines (e.g. C4_C9) orient at an angle to the layers with the nitrogen with its lone pair of electrons adjacent to the layer. The angle of inclination generally increases with chain length and for chain lengths > C16, the amines are arranged perpendicular to the host layers and form bilayers resulting in layer separations as high as 57 A for stage 1 intercalation.

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Solar Panel Basics

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