“…Since thiomolybdates are strong nucleophiles, filling the surface levels acting as electron traps can be suggested as the mechanism of emission enhancement. 35,36 The red shift of the excitonic emission, 15 nm, is comparable to that of the absorption spectrum, which is 7 nm (Figure 1). The discrepancy between these two shifts is quite substantial and cannot be dismissed as an experimental error.…”
Section: Resultsmentioning
confidence: 51%
“…The elevated quantum yield of the excitonic emission indicates partial elimination of intra-band-gap electronic levels stimulating radiationless recombination. Since thiomolybdates are strong nucleophiles, filling the surface levels acting as electron traps can be suggested as the mechanism of emission enhancement. , 3 Luminescence spectra of MoS 4 2- (1) and ripened CdS nanoparticles modified with MoS 4 2- in a molecular ratio of 0:1 (2), 10:1 (3), 50:1 (4), and 100:1 (5) MoS 4 2- units per nanoparticle; λ ex = 340 nm.…”
Modification with thiomolybdate results in the attachment of spatially separated MoS 4 2groups to the surface of CdS. This produces the red shift of excitonic absorption and emission bands and splitting of the trapped emission band. Because of the spatial gap between the thiomolybdate molecules of the modifying layer, the interaction of individual groups of the modifier with the CdS core must be considered. This was accomplished by using the density functional theory (DFT) description of a MoS 4 2--CdS model cluster. Geometry, dipole moment, and localization of calculated molecular orbitals allow for the consistent description of optical effects observed for thiomolybdate-modified CdS nanoparticles, such as the red shift of adsorption and emission bands, enhancement of excitonic emission, and splitting of the trapped emission band. A good agreement between DFT calculated and experimental FTIR spectra has been obtained.
“…Since thiomolybdates are strong nucleophiles, filling the surface levels acting as electron traps can be suggested as the mechanism of emission enhancement. 35,36 The red shift of the excitonic emission, 15 nm, is comparable to that of the absorption spectrum, which is 7 nm (Figure 1). The discrepancy between these two shifts is quite substantial and cannot be dismissed as an experimental error.…”
Section: Resultsmentioning
confidence: 51%
“…The elevated quantum yield of the excitonic emission indicates partial elimination of intra-band-gap electronic levels stimulating radiationless recombination. Since thiomolybdates are strong nucleophiles, filling the surface levels acting as electron traps can be suggested as the mechanism of emission enhancement. , 3 Luminescence spectra of MoS 4 2- (1) and ripened CdS nanoparticles modified with MoS 4 2- in a molecular ratio of 0:1 (2), 10:1 (3), 50:1 (4), and 100:1 (5) MoS 4 2- units per nanoparticle; λ ex = 340 nm.…”
Modification with thiomolybdate results in the attachment of spatially separated MoS 4 2groups to the surface of CdS. This produces the red shift of excitonic absorption and emission bands and splitting of the trapped emission band. Because of the spatial gap between the thiomolybdate molecules of the modifying layer, the interaction of individual groups of the modifier with the CdS core must be considered. This was accomplished by using the density functional theory (DFT) description of a MoS 4 2--CdS model cluster. Geometry, dipole moment, and localization of calculated molecular orbitals allow for the consistent description of optical effects observed for thiomolybdate-modified CdS nanoparticles, such as the red shift of adsorption and emission bands, enhancement of excitonic emission, and splitting of the trapped emission band. A good agreement between DFT calculated and experimental FTIR spectra has been obtained.
“…Interactions of semiconductor nanoparticles with metal ions can also be used to probe the dynamics of photoinduced interfacial electron transfer (i.e., trivalent lanthanide ions 16 ). The effect of metal ions adsorbed onto the surface of semiconductor nanoparticles may be different from that expected in bulk semiconductors …”
Nonstoichiometric cadmium sulfide nanoparticles ([Cd 2+ ]/[S 2-] ) 3) in 2-propanol were surface-modified with Cu 2+ ions. Addition of copper(II) perchlorate to CdS nanoparticles leads to binding of copper ions onto the surface of the semiconductor, accompanied by rapid reduction of Cu 2+ to Cu + , as confirmed by EPR and absorption spectra. Copper(II) perchlorate also quenches the recombination luminescence of CdS nanoparticles effectively. The quenching data obey a static interaction model, which confirms the binding of copper ions onto CdS. The latter was confirmed also by ultrafiltration and ICP spectroscopy. Copper ions bound onto the surface of CdS lead to formation of a new, red-shifted, luminescence band. The maximum of the new band is at 14 700 cm -1 compared to that of the original band at 17 900 cm -1 . It is suggested that, at low copper ion concentrations, copper ions bound onto the surface of CdS nanoparticles exist as isolated Cu + ions. They create a new energy level in the bandgap at about 1.2 eV below the conduction band, which is responsible for the new emission band (14 700 cm -1 ). Higher copper(II) perchlorate concentrations give rise to the formation of ultrasmall particles of CuxS (x ) 1-2) on the surface of CdS, which eventually lead to precipitation. Both isolated Cu + ions and ultrasmall particles of CuxS quench the original recombination luminescence of CdS nanoparticles by facilitating e -/h + nonradiative annihilation. The presence of copper ions bound onto the surface of CdS nanoparticles retard both the photocorrosion of the latter and the photodecomposition of copper(II) tetraphenylporphyrin induced by CdS nanoparticles. Appropriate mechanisms are presented.
“…We and others have shown that surface-adsorbate adduct formation can be monitored by changes in the steady-state photoluminescence (PL) intensity emitted from n-VI semiconductors like CdS and CdSe [CdS(e)]. [1][2][3] In cases where adsorption and desorption of molecular species are sufficiently rapid, the reversibility of the PL changes may permit use of this effect for on-line chemical sensing. 4 For single-crystal substrates, we have modeled these reversible interactions as being characteristic of a weak charge-transfer complex for which the adduct-induced PL changes reflect the nature of the adsorbing molecule's interaction with the surface: Adsorbing Lewis acids, which draw electrons from the semiconductor bulk to surface states, quench PL intensity, and adsorbing Lewis bases, which push electrons from the surface states back into the bulk semiconductor, are observed to enhance the PL intensity.1 These effects on PL intensity can be modeled as resulting from a change in the depletion width of the semiconductor caused by the shifts in electron distribution that occur during adduct formation: Photogenerated electron-hole pairs are separated by the electric field characterizing the depletion width, resulting in a nonemissive zone or "dead-layer" whose thickness is roughly on the order of the depletion width.5 The concentration dependence of PL changes has been used to estimate adduct binding constants using the Langmuir adsorption isotherm model.…”
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