CdSe-ZnS Core-Shell Quantum Dots : Surface Plasmons Effect and Optical Spectra

Absorption and photoluminescence (PL) spectra of a colloidal CdSe-ZnS core-shell quantum dots (QDs) were measured within the strong confinement regime. The QDs were casted on half-coated quartz substrates with 50 nm of gold and prepared for studying the surface plasmons effect. The samples were optically arranged and pumped by different wavelengths, and the PL spectra were detected. Excitation with a wavelength of 400 nm reveals a structure fluorescence spectrum which consists of five distinct bands. These bands are more intense and resolved than the corresponding traditional weak absorption bands. They were detected for the first time and assigned according to the theoretical predictions of excitons in a spherical potential with Coulomb interactions and valence bands mixing. A scheme based on multiple exciton generation (MEG) for the appearance of the fluorescence bands was proposed. This scheme was confirmed by the recent theoretical prediction using the state-of-the art time domain ab initio density functional theory and the atomistic pseudopotential calculations. The detected surface plasmons effect in QDs enhances the intensity of the fluorescence bands. This surface plasmons effect facilitated the appearance of these mentioned bands in more intensive features than the corresponding traditional weak absorption bands. Furthermore, the reduction in the measured lifetime of the first excited electronic state from 20 ns for the QDs deposited directly on the quartz substrate to 7 ns for the QDs casted on the gold film, gives a further evidence of the surface plasmons effect in the (PL) of the CdSe-ZnS QDs.


Introduction
Among many of the new inorganic semiconductor crystals that reduced to a nanometer size, CdSe quantum dots represent the prototype and most interesting material.They have therefore attracted considerable attention from the viewpoints of fundamental physics and functional device applications.ZnS-capped CdSe QDs properties have generated a great deal of interest due to the scientific aspects that are involved in these studies.Examples of interesting and significant physics include: fluorescent biological imaging probes (Mukherjee & Ghost, 2012), solar cell devices (Kashyout et al., 2012), white light emitting diodes (Nizamoglu & Demir, 2009), nano lasers (Yan et al., 2012), and optical fiber sensors (Jorge et al., 2009).In all these technological applications, the unique optical properties of QDs are size dependent.In addition, quantum dots are an excellent testing ground for the applicability of various theoretical models.For improving the optical properties of QDs through increasing the PL intensity, surface plasmons effect is the optimum choice.This work focuses on the surface plasmons coupled emission of CdSe-ZnS core-shell for the purpose of increasing the emission intensity and enhancing the bands that are unobservable by the traditional detection technique.

Experimental Details
A core-shell CdSe-ZnS based QDs in a toluene solution of (50 mg/ml) were purchased from Evident technologies.The QD toluene solutions were diluted five times and drop casted on half coated substrates with 50 nm gold by thermal evaporation.After the solutions evaporated, a thick layer of the QD nanocrystals remained on the substrates.Time-resolved fluorescence decay data were collected using the picosecond time-correlated single photon counting (TCSPC) technique (instrumental resolution function IRF = 23 ps).The excitation source is a picosecond Ti: sapphire laser (MIRA) from Coherent Inc. (vertical polarization, wavelength range of 720-1000 nm, and 76 MHz repetition rate) coupled to a second harmonic generator (360-500 nm).Emission   Where m e CdSe, the medium ε r of free spa The estima is 2.30 eV following approxima the QDs l mechanism 3 along w assignmen diameter.assignmen Bawendi ( description previous 3  (1996).This th n of the excit 3S 1/2 -1S (e) tra   In order to distinguish the fluorescence bands from the excitation source bands, the excitation source emission bands are presented in Figure 4.The PL spectrum is shown in Figure 5 for two cases: QDs deposited on gold and on quartz respectively.The PL spectrum is more intense for QDs deposited on gold than for QDs deposited on quartz due to surface plasmons effect.These spectra were repeated many times and found reproducible.The first excitonic peak situated at 660 nm is the traditional peak usually observed for such QDs.The other peaks were never detected.By investigating these features, the most interesting point to be considered is that the spacing between the level S(e) and P(e) is large.The density of states feature is shown to be in coincidence with the recent time domain density functional theory of CdSe-ZnS QDs developed by Kilina, Killin, and Prezhdo (2009).According to this approach the CdSe-ZnS density of states (DOS) shows asymmetry across the band gap, with the hole states having a higher density than the electron states and a larger spacing between 1S(e) and 1P(e) levels.The distinct and well separated peaks observed for the first time in fluorescence shown in Figure 5 is in the contrary to effective mass description, which predicts that electronic energy levels are highly degenerate.Figure 6 shows the energy levels involved in the absorption and fluorescence of CdSe-ZnS core-shell quantum dots.The atomistic simulation of Kilina et al. (2009) shows that the underlying atomic structure, surface effects, core-shell interaction, as well as spin-orbit interaction, break the degeneracy and create a multilevel electronic structure.These simulation results are in a good agreement with the present fluorescence experimental data.Another interesting feature, which supports the observed structure spectrum in fluorescence, is that simulation results reported by Puzder et al. (2004).This simulation proves that the DOS for unrelaxed CdSe QDs is identical to the corresponding relaxed QDs.This theoretical prediction gives a strong evidence for the observation of the fluorescence structure as approximate mirror image to the absorption features.The discrete bands observed in fluorescence can be explained by the multiple exciton generation (MEG).Excitons in CdSe-ZnS QDs relax via multiple pathways according to the Scheme 1.

Figure 4 .
Figure 4.The spectrum of the excitation source for fluorescence measurements

Figure 5 .
Figure 5.The PL spectrum of a-CdSe-ZnS QDs on gold film b-on quartz substrate

Table 1 .
Pe reasons: (i) th ation with e-h levels populat m illustrated in with the assignm nts are in agre The fifth wea nt is based on