Band-edge emission

Micic O I ef a/1996 Highly efficient band-edge emission from InP quantum dots Appi. Phys. Lett. 68 3150... 

After postdeposition Ar and Ar/S-annealing, the films were studied again using PL. S-anneals reduced the relative intensities of the PL1, PL4, and broad emission bands, whereas Ar-anneals increased the relative intensity of the PL1 band. This can be seen in Fig. 6.28. We can also see in this figure that S-anneals suppressed the broad near-band-edge emission from the trailing edge samples. When EDS measurements were performed on the films after S-anneals, an... 

Figure 11.2. Nanowire electronic and optical properties, (a) Schematic of an NW-FET used to characterize electrical transport properties of individual NWs. (inset) SEM image of an NW-FET two metal electrodes, which correspond to source and drain, are visible at the left and right sides of the image, (b) Current versus voltage for an n-type InP NW-FET. The numbers inside the plot indicate the corresponding gate voltages (Vg). The inset shows current versus Vg for Fsd of 0.1 V. (c) Real-color photoluminescence image of various NWs shows different color emissions, (d) Spectra of individual NW photoluminescence. All NW materials show a clean band-edge emission spectrum with narrow FWHM around 20nm. (See color insert.)...

Figure 9 (A) Typical TEM image of a CdSe nanoparticle synthesized by the direct reaction of Se and CdO (see main text). (B) Typical emission spectrum of a CdSe nanoparticle. The spectrum exhibits two main features (1) a sharp Gaussian-shaped peak due to band-edge emission and (2) a broad feature at lower energies due to defect emission. (C) The size-dependence of the peak wavelength for CdSe nanoparticles, determined using data provided in Murray et al. (1993).

The CL spectrum of the ZnO film consisted of intense, near-band-edge ultraviolet emission with a wavelength maximum at 387 nm and a full width at half maximum of 21 nm. This emission is of excitonic nature and is a result of the radiative annihilation of free and bound excitons. A broad defect-related green band with much lower intensity near 510 nm, typical for ZnO, was also observed (Fig. 1). The CL spectrum of the p-Alo. i2Gao 8sN(Mg) film consisted of a very weak near-band-edge emission with at 356 nm, and of a more intense broad band with a maximum at 410 nm. 

Figure 8 PL spectra for CdSe NQDs and (CdSe)ZnS (core)shell NQDs. Core diameters are (a) 2.3, (b) 4.2, (c) 4.8, and (d) 5.5 rnn. (Core)shell PL QYs are (a) 40, (b) 50, (c) 35, and (d) 30%. Trap-state emission is evident in the (a) core-particle PL spectrum as a broadband to the red of the band-edge emission and absent in the respective (core)shell spectrum. (Reprinted with permission from Ref 30. 1997 American Chemical Society)...

To support our theoretical prediction. Fig. 13.3 shows the PL spectra taken from bare and metal-capped ZnO films. The strong emission peak observed at 3.24 eV is identified as the band-edge emission of ZnO. Expectedly, 15- and 9-fold of integrated emission enhancement are observed from Ag and Al capped ZnO while negligible enhancement is seen from Au-capped sample [15]. These experimental results support the emission enhancements are due to SPP mediation. 

Other dipolar interactions also exist that interfere SPP mediated emission enhancement. Fig. 13.4(a) displays the plot of band-edge emission enhancement ratio of Al / AlO / ZnO as a function of AlO thickness [17], The enhancement ratio... 

Figure 133 The experimental band-edge emission spectra of Al, Ag and Au capped ZnO. The backward photoluminescence geometry is shown in flie inset. The emission of bare ZnO is also displayed as reference.

Figure 13.4 a) The dependence of integrated band-edge emission enhancement ratio of Al / AlOx / ZnO on AlO thickness (square dot). The dependence of Tp on AlO thickness (solid line), b) The dispersion relation of Al / AlOx / ZnO at different AlOx thickness, c) The plasmonic DOS of Al / AlO / ZnO at different AlOx thickness, d) The dependence of normalized quenching efficiency on Al / AlOx ZnO (solid circle). The best fit of experimental data using Fbrster energy transfer [17]. 

Figure 13.5 Temperature-dependent PL spectra of (a) bare ZnO and (b) metal-capped ZnO obtained from 10 to 300 K. (c) The variation of integrated band edge emission intensity of bare (square) and c ped (circle) ZnO as a function of temperature, (d) The dependence of emission enhancement on temperature [14].

Figure 13.8 The contour plots of a) band-edge emission enhancement ratio and b) deq>level emission suppression ratio with Ar working pressure and target-sample distance.

By using metal as the capping layer, SPPs have been demonstrated as an effective means for increasing the external quantum efficiency of ZnO. The use of thin spacer and metal alloy can eliminate the unwanted Forster energy transfer and support on-resonance SPP coupling. In addition, nanocrystalline Au can on one hand suppress the deep-level emission while on the other hand increase the band-edge emission of ZnO. Finally, the radiative SPP arising from MIM can be used to increase the forward emission of ZnO. 

The emission-lifetime measurements in ns-time region were also carried out for the two emission bands. Multi-exponential decay behavior was observed for both the emission bands. Fast decay component at >.=480 nm less than the order of ns was attributed to the recombination of electrons and holes. Slow decay component at >,=480 nm in the order of a few ns was attributed to thermal detrapping of the electron from the surface states to the conduction band since such thermal activation could enhance the lifetime at the band-edge emission. The emission lifetime at >.=480 nm increased as excess Cshallow trap sites. 

The relative solubility of inorganic salts can be used to prepare more complex structures by such methods and examples indude CdS/ZnS [24], CdSe/AgS [25] HgS/CdS [26], PbS/CdS [27, 28], CdS/HgS [29], ZnS/CdSe [30] and ZnSe/CdSe [31] particles. The main constraints on the production of such structures involve the relative solubility of the solids and lattice mismatches between the phases. The preparation of quantum dot quantum well systems such as CdS/HgS/CdS [32, 33], has also been reported, in which a HgS quantum well of 1-3 monolayers is capped by 1-5 monolayers of CdS. The synthesis grows less soluble HgS on CdS (5.2 nm) by ion-replacement. The solubility products of CdS and HgS are 5 X 10 and 1.6 x 10 respectively. The authors reported fluorescence measurements in which the band edge emission for CdS/HgS/CdS is shifted to lower energy values with increasing thickness of the HgS well [33]. 

Fig. 11.5. Fluorescence excitation and emission spectra of ZnSe nanocrystals showing the band edge emission. Adapted from [17].

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