Excitonic peak

Figure 4.14 The absorption spectrum of cuprous oxide at 77 K, showing the exciton peaks n = 2, 3, 4, and 5.

When CTAC is solubilized in micellar solution with cadmium Cd2+ ions, a better resolution in the excitonic peak with increasing CTAC concentration is observed. The sharp peak is more intense for low water content and for high CTAC concentration. This clearly shows a narrow size distribution. 

Beyond the exciton peak in the far ultraviolet there is a series of other absorption features in MgO, which are more distinct in Fig. 9.5. These features, a theoretical treatment of which requires electron energy band calculations, are caused by maxima in the combined density of available states for electrons in the ground and excited states. 

An example of a spectral feature of possible interest is the exciton peak in MgO near 7.6 eV (see Figs. 9.5 and 10.1). This peak has been studied primarily by reflection from cleaved crystals of MgO. But careful inspection of Fig. 11.2 reveals that the exciton peak is also expected to appear clearly in extinction by MgO particles provided that they are sufficiently small. 

Fig.1. (a) Absorption spectra and (b) 2D-IR pump probe spectra of the C=0 mode of crystalline ACN. 2D-IR spectra record the absorption change as a function of probe frequency and the center frequency of a narrow band pump pulse. The contour intervals represent a linear scale. Response of the amide I band upon selective excitation of the self-trapped states (c) and the free exciton peak (d) for two different delay times. The arrows indicate the position of the narrow band pump pulse. 

We used short broadband pump pulses (spectral width 200 cm 1, pulse duration 130 fs FWHM) to excite impulsively the section of the NH absorption spectrum which includes the ffec-exciton peak and the first three satellite peaks [4], The transient absorbance change signal shows pronounced oscillations that persist up to about 15ps and contain two distinct frequency components whose temperature dependence and frequencies match perfectly with two phonon bands in the non-resonant electronic Raman spectrum of ACN [3] (Fig. 2a, b). Therefore the oscillations are assigned to the excitation of phonon wavepackets in the ground state. The corresponding excitation process is only possible if the phonon modes are coupled to the NH mode. Self trapping theory says that these are the phonon modes, which induce the self localization. 

In a second experiment, narrow band pump pulses (spectral width 30 cm 1, pulse duration 250 fs FWHM) were used to selectively excite individual sub-levels of the NH band (Fig. 2e, g) [4]. On the sub-picosecond time scale, the free-exciton and the lower lying self-trapped states behave distinctly differently. When exciting the free-exciton (Fig 2e), a strong bleach and stimulated emission signal is observed which recovers on a 400 fs time scale. Simultaneously, population is transferred into lower lying self-trapped states. On the other hand, when pumping one of the self-trapped states directly (Fig. 2g), population within all self-trapped states equilibrates essentially instantaneously, but the free exciton peak is not back-populated. This is the direct observation of ultrafast self-trapping Excitation of the free-exciton leads to an irreversible population of self-trapped states, but not vice versa. 

It is interesting to note that the HOMO-LUMO transition in the emission spectrum at 2.20 eV is almost dark while an important excitonic peak is evident at about 2.75 eV (see Figure 18), red-shifted with respect to the first absorption peak. 

Helical polysilanes where the side groups are partly substituted with Rhodamine B dye molecules and chiral groups (Fig. 12) have been synthesized and spread onto quartz plates by vertical dipping.77 A weak absorption peak due to the dye is observed around 2 eV in addition to the sharp exciton peak at 3.85 eV. The PL spectrum shows a new peak at 2 eV, while the original peak at 4 eV for the polysilane without the dye is greatly decreased. Strong red PL is observed. The introduction of only a few percent of dye modifies the absorption... 

Shell growth is accompanied by a small red shift (5 to 10 nm) of the excitonic peak in the UV-vis absorption spectrum and the PL wavelength. This observation is attributed to a partial leakage of the exciton into the shell material. 

NCs is indispensable. In the case of cadmium chalcogenide NCs, the concentration of a colloidal solution can be determined in good approximation by means of UV-vis absorption spectroscopy thanks to tabulated relationships between the excitonic peak, the NC size, and the molar absorption coefficient.96 An advanced approach for shell growth derived from chemical bath deposition techniques and aiming at the precise control of the shell thickness is the so-called SILAR (successive ion layer adsorption and reaction) method.97 It is based on the formation of one monolayer at a time by alternating the injections of cationic and anionic precursors and has been applied first for the synthesis of CdSe/CdS CS NCs. Monodispersity of the samples was maintained for CdS shell thicknesses of up to five monolayers on 3.5 nm core CdSe NCs, as reflected by the narrow PL linewidths obtained in the range of 23 to 26 nm FWHM. 

Fig. 7.19. Photoluminescence spectra (2K) of PLD ZnO thin films on a-plane, c-plane, and r-plane sapphire substrates [63]. All films were grown at about 650°C and at 1.6 x 10 2 mbar oxygen pressure. The FWHM of the most intense donor bound exciton peaks D°X of the ZnO films are 1.4 meV on a-plane sapphire, 1.7 meV on c-plane sapphire, and 2.6 meV on r-plane sapphire. The spectral resolution of the PL setup was 1 meV at 3.35 eV...

Figure 7.19 shows PL spectra recorded at 2K for 2.2, 0.7, and 1.5 pm thick PLD ZnO films on a-plane, c-plane, and r-plane sapphire, respectively [63], The full widths at half maximum (FWHM) of the most intense bound exciton peaks are 1.4, 1.7, and 2.6 meV for a-, c-, and r-sapphire, respectively. The film on a-plane sapphire shows the narrowest FWHM among the films under investigation and the free A-exciton (Xa) is most clearly resolved, thus indicating best structural properties of ZnO on a-plane sapphire. The ZnO films on a- and c-plane sapphire grow c-axis textured, whereas films on r-plane sapphire grow a-axis oriented with the ZnO c-axis being in-plane, as demonstrated already in Fig. 7.4. The PL spectrum of the film on r-plane sapphire shows no phonon replica, probably due to the changed ZnO orientation.

Fig. 7.26. Typical room temperature CL spectra of ZnO thin films grown at optimized O2, N2O, and N2 background gas pressure of 0.9mbar [89], The film grown in O2 shows splitting and broadening of the excitonic peak. Only the film grown in N2 shows nearly no green luminescence around 2.3 eV photon energy...

Fig. 7.30. Photoluminescence spectra (2K) of PLD MgZnO-ZnO-MgZnO quantum well heterostructures on sapphire with nominal thickness of the ZnO quantum well of 25, 12, 6, and 3nm [53]. The blueshift of the excitonic peak combined with the intensity enhancement is a clear indication of optical confinement in the ZnO layer...

W/cm2. The FWHM of the strong peak (I2) was as small as 1.6 meV. The free-exciton peaks (A, B) could also clearly be observed. In addition, emission denoted by A2 in the higher energy region could clearly be seen and it is well-resolved into three peaks. FIGURE 5 shows a reflectance spectrum for the same sample at 5 K. Two sharp dispersive structures of A, B and C excitons can be seen. Furthermore, the first excited state (n = 2) transition of A and B excitons (A2, B2) can also be clearly observed. 

As was shown for polydiacetylene [15] the presence of the exciton peak at the energy of 1.6 eV provides evidence for the high degree of ordering of carbon chains in the film. Therefore we can conclude that the sp -hybridized carbon films also consist of a very highly ordered material. 

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