Band gaps of ZnS

These results suggest that the most significant relaxation of the ZnS (110) surface is a downward displacement of the surface Zn atoms by approximately 0.02 nm. The surface S atoms relax out the surface by about 0.01 nm. The band structure and partial density of state (PDOS) of relaxed ZnS (110) surface are illustrated in Fig. 9.13. The atomic and bond overlap population analysis of ZnS (110) surface is listed in Table 9.6. It shows that the band gap of ZnS (110) surface is 1.5 eV, and it is smaller than that of bulk ZnS. The reason for band gap... 

Photoluminescence of ZnS Mn occurs when the phosphor absorbs photon energy corresponding to the band gap of ZnS and relaxes to release the excess energy of the exciton (a pair of an s-p electron and a hole). Based on the selection rule of Laporte, the symmetrical field of 6-coordinated Mn(ll) does not allow the d-d transition since it is not associated with the change in the parity. The 4-coordinated Mn(lI), in contrast, allows a partial d-p hybridization, enabling the d-d transition. 

Figure 1.11 shows an experimental determination of the band alignment at the ZnO/(Zn,Mg)0 interface using optical spectroscopy of quantum well structures [100]. The data indicate that the larger band gap of (Zn,Mg)0 is... 

Interestingly, a reversed type non-monotonic variation in the band gap has been observed with Mn concentration in Mn-doped ZnS nanocrystals [14]. This is shown in Figure 11.4 the main frame shows the UV-Vis absorption spectra and the inset shows an initial increase in the band gap, also seen in the UV-Vis absorption edges, with values of x in Zni- Mn S up to x = 6% followed by a decrease on further increase in x. It is to be noted here that the band gap of ZnS, in contrast to that of CdS, is larger than that in MnS, accounting for the reversed... 

A film of zinc sulphide, ZnS, 4 nm in thickness, forms on metallic zinc. Treating the film as a quantum well, what is the wavelength of the transition Am = 2 The band-gap of ZnS is 3.54 eV, and the effective mass of electrons and holes is 0.4mi . 

Numerous ternary systems are known for II-VI structures incorporating elements from other groups of the Periodic Table. One example is the Zn-Fe-S system Zn(II) and Fe(II) may substimte each other in chalcogenide structures as both are divalent and have similar radii. The cubic polymorphs of ZnS and FeS have almost identical lattice constant a = 5.3 A) and form solid solutions in the entire range of composition. The optical band gap of these alloys varies (rather anomalously) within the limits of the ZnS (3.6 eV) and FeS (0.95 eV) values. The properties of Zn Fei-xS are well suited for thin film heterojunction-based solar cells as well as for photoluminescent and electroluminescent devices. 

Lead(II) sulfide occurs widely as the black opaque mineral galena, which is the principal ore of lead. The bulk material has a band gap of 0.41 eV, and it is used as a Pb " ion-selective sensor and IR detector. PbS may become suitable for optoelectronic applications upon tailoring its band gap by alloying with II-VI compounds like ZnS or CdS. Importantly, PbS allows strong size-quantization effects due to a high dielectric constant and small effective mass of electrons and holes. It is considered that its band gap energy should be easily modulated from the bulk value to a few electron volts, solely by changing the material s dimensionality. 

Single-phase ZnS films of a fine grain size (no XRD shown) and a band gap of 3.7 eV were electrodeposited from aqueous alkaline (pH 8-10) solutions of zinc complexed with EDTA, and thiosulfate as a sulfur source [101]. The voltammet-ric data implied that deposition occurred either by S-induced UPD of Zn or by a pathway involving both Zn " and thiosulfate concurrently. 

Zinc sulfide, with its wide band gap of 3.66 eV, has been considered as an excellent electroluminescent (EL) material. The electroluminescence of ZnS has been used as a probe for unraveling the energetics at the ZnS/electrolyte interface and for possible application to display devices. Fan and Bard [127] examined the effect of temperature on EL of Al-doped self-activated ZnS single crystals in a persulfate-butyronitrile solution, as well as the time-resolved photoluminescence (PL) of the compound. Further [128], they investigated the PL and EL from single-crystal Mn-doped ZnS (ZnS Mn) centered at 580 nm. The PL was quenched by surface modification with U-treated poly(vinylferrocene). The effect of pH and temperature on the EL of ZnS Mn in aqueous and butyronitrile solutions upon reduction of per-oxydisulfate ion was also studied. EL of polycrystalline chemical vapor deposited (CVD) ZnS doped with Al, Cu-Al, and Mn was also observed with peaks at 430, 475, and 565 nm, respectively. High EL efficiency, comparable to that of singlecrystal ZnS, was found for the doped CVD polycrystalline ZnS. In all cases, the EL efficiency was about 0.2-0.3%. 

Ultrasonic-assisted rinsing has been tested for the growth of ZnS thin films. The growth rate clearly increased, and the morphology of the films became smoother, whereas the grains became smaller. The band gap of the ZnS films was 3.67 eV.11... 

Zinc chalcogenide thin hlms have been grown by ECALE using zinc sulphate as metal source and sodium sulphide and sodium selenite as chalcogenide precursors.145-148 The formation of the hrst layers of ZnS on (lll)Au has been analyzed by STM and XPS.145 HRSEM images showed that the him surface was very hat, even at an atomic level. On the other hand, thicker ZnS hlms were formed of well-separated crystal nuclei. The stoichiometry of a thicker ZnS him showed a slight excess of sulphur, with a Zn S ratio of 1 1.2. The band gap of a thicker him (deposition time 12 h) was 3.60eV.147... 

Similar PDOS distribution can be seen on the ZnS surface doped with Fe ions. The dominant state in valence band is Fe (3d) orbital, and the conduction band is composed of S (3p) and Zn (4p) orbital. This result indicates that doping Cu or Fe ions on the ZnS surface reduces the band gap of the ZnS. This kind of reduction will produce lot of surface state in bulk ZnS forbidden band. 

Some semiconducting compounds can be of the II-VI type, which also has an average valence of four, but these have much more ionic character than Ill-V compounds. Their band gaps are thus larger, and in some cases they may even be viewed as insulators. For example, ZnS, with a band-gap energy of 3.6 eV, is an insulator, whereas ZnSe has an band gap of 2.8 eV, which is closer to a semiconductor. A wide variety of... 

The two distinct types of interlayer reaction observed in the MPS3 componnds (electron transfer to the layers and ion transfer from the layers into the reactant solution) are partially reversible and occur with simultaneous intercalation of catioiuc species into the interlayer spaces. The balance between the two different reaction pathways appears to depend on the band gap of the particular solid. Ion transfer reactions are easier in the larger band gap compounds (for example, with M = Zn, Cd, and Mn) where the layers appear to behave as cations weakly coupled to P2S6 units. The MPS3 lattices behave very differently from the dichalcogenides where the strong covalent interactions between formally M + cations and anions preclude any ion transfer chemistry involving the layers. 

Fig. n.4. UV-Vis absorption spectra of Zni- Mn S nanocrystals with varying x. Notice the change in the band edge with change in the amount of Mn. The inset shows the shift in the band gap with reference to the band gap of undoped ZnS nanocrystals. Adapted from [14, 20]. 

Fig. 11.7. Fluorescence emission spectra of 18A Mn-doped ZnS nanocrystals. Notice the ratio of orange blue emission with change in Mn concentration. The excitation energy 4.27 eV (290 nm) corresponds to the band gap of ISA ZnS nanocrystals. Adapted from [20].

Zinc oxide may crystallize in either the wurtzite or, more rarely, the zinc blende structure, the former being more stable at lower temperatures. In both structures the zinc and oxygen ions are tetrahedrally co-ordinated to each other. The bonding is intermediate between the completely ionic and the completely covalent, both ions being more polarizable than in a perfect ionic crystal and carrying an effective charge of only 0.5e. As is well known, non-stoicheio-metric ZnO, with excess of Zn, is an n-type semiconductor with a band gap of 3.2 eV. 

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