Luminescence from wide band gap materials and their applications^*

SEM images of ZnO microcrystals and ZnO microcrystals embedded in PMMA are shown in figures 1(a) and (b), respectively. The average size of the microcrystals of ZnO is ∼6 ± 0.2 μm (figure 1(a)). The synthesis and characterization of green-luminescent ZnO has been reported in [21]. The temperature-dependent PL spectra recorded in a broad range of cryogenic temperature (83–473 K) for ZnO and PMMA-ZnO is shown in figures 1(c) and (d), respectively. A strong defect-related green and a very weak (almost negligible) UV band centered around 394 and 515 nm are observed. For ZnO, the UV emission is due to the recombination of free excitons, while the green emission is attributed to transitions at a V_O defect [21, 22]. Quenched ZnO has excess oxygen vacancies resulting in strong green emission. It is interesting to note that the intensity of both green as well as UV band varies with temperature and the dependence is different, which is one of the key issues for ratiometric temperature sensing.

Zoom In Zoom Out Reset image size

Figures 2(a) and (b) show the dependence of green to UV band intensity ratio with temperature for ZnO and PMMA-ZnO, respectively. A monotonic increase in the intensity ratio is evident as the temperature is decreased. The variation of intensity ratio with temperature suggests the possibility of ratiometric temperature sensing.

Zoom In Zoom Out Reset image size

Optical temperature measurement may be realized by taking the intensity ratio R of UV (I_UV) and the green band (I_G). The sensitivity (S) of ratiometric thermometer is defined as [6, 7]

$$\begin{eqnarray*}&&S=dR/dT.\end{eqnarray*}$$

The variation of S with temperature for ZnO and PMMA-ZnO are shown in figure 2(c). It may be noted that the sensitivity of ZnO microcrystals is 67.3 − 0.87% in the temperature range of 83–473 K, while the value ranges from 17.7 − 6.9% for PMMA-ZnO in the temperature range of 83–373 K. Because of the high stability and moderate sensitivity, the application of PMMA-ZnO in cryogenic thermometry is highly recommended.

Figure 3(a) shows SEM image of In₂O₃ octahedrons which clearly reveals average size of ∼1.5 μm. Figure 3(b) shows the typical XRD pattern of In₂O₃. All the peaks are indexed to the body centered cubic (bcc) In₂O₃ with lattice parameter 1.012 nm (JCPDS 76-0152). PL studies on In₂O₃ octahedrons are carried out using 355 nm laser as excitation source. Temperature dependent PL of In₂O₃ powder sample in 293–453 K range is shown in figure 3(c). The PL intensity decreases with increase of temperature as shown in figure 3(d). With the increase of temperature, electron–phonon interaction increases and new defect levels are created that cause the change in the peak position [23]. Furthermore, non-radiative recombination among electrons and holes increases that reduces the PL intensity with the increase of temperature [24]. It is well known that bulk In₂O₃ does not show any luminescence at room temperature. The nano/micro structures exhibit luminescence due to the presence of different defects such as oxygen and indium vacancy, oxygen and indium interstitials, etc in the materials [25]. Overall, the defect mediated emission is very much essential for different optical sensing applications.

Zoom In Zoom Out Reset image size

SEM image of Ge-GeO₂ microcrystals is shown in figure 4(a) that clearly reveals holes with an average diameter of ∼150 nm. The synthesis and characterization of porous Ge-GeO₂ has been reported in references [18, 20]. Figure 4(b) shows the PL spectra of porous Ge-GeO₂ microcrystals, O-CdTe QDs and the Ge-GeO₂ sensitized with O-CdTe QDs. It is interesting to note that the overall intensity in the visible region increases 10-fold by the addition of one layer of O-CdTe QDs on Ge-GeO₂ pellet, while the intensity of orange luminescence corresponding to O-CdTe QDs disappears. This confirms that the visible light emission intensity of Ge-GeO₂ increases when sensitized by O-CdTe QDs. Interestingly, the intensity of PL spectra increases with increasing the number of O-CdTe QDs layers. It may be noted that the intensity initially increases 40-fold and then decreases. The optical photographs of white light emission from Ge-GeO₂ without and with O-CdTe layers are shown in figures 4(c) and (d), respectively. Bright white light is clearly seen as compared to the weak bluish-white light emitted only from Ge-GeO₂. The blue-green-orange (white light) emission is attributed to the defects in oxides and interfacial defects between Ge-GeO₂ [14, 15].

Zoom In Zoom Out Reset image size

Forster resonance energy transfer (FRET) is one of the possible mechanisms for the enhancement in the PL intensity. In FRET, the donor molecules with higher band gap transfer the energy non-radiatively to the acceptor molecule of lower band gap [26]. The efficiency of energy transfer depends on the interparticle distance (typically <10 nm), and the overlap between donor emission and acceptor absorption spectra.

The absorption spectrum of Ge-GeO₂ and the emission spectrum of CdTe QDs is shown in figure 5(a). Interestingly, the absorption spectrum of Ge-GeO₂ overlaps with the emission spectra of O-CdTe QDs. As there is no signature of CdTe QDs in the PL spectra of Ge-GeO₂/CdTe QDs, it is concluded that O-CdTe acts as the donor and Ge-GeO₂ acts as the acceptor. When excited with 355 nm UV light, the Ge-GeO₂ absorbs significantly, but the luminescence is very weak. At the same wavelength, CdTe QDs also absorb significantly and transfer the energy to Ge-GeO₂. As a consequence, the luminescence of Ge-GeO₂ increases and that of CdTe decreases. Overall, the experimental results support the FRET mechanism where CdTe acts as the donor and Ge-GeO₂ as the acceptor.

Zoom In Zoom Out Reset image size

The FRET efficiency (η) can be estimated if the luminescence lifetimes of donor with and without the acceptor are known. This is given by [26]

$$\begin{eqnarray*}&&\eta =1-\frac{\tau }{{{\tau }_{0}}}\;{\rm or}\;\eta =1-\frac{{{\tau }_{{\rm ave}}}}{{{\tau }_{{\rm ave}\,{\rm 0}}}},\end{eqnarray*}$$

where τ, τ₀ are the luminescence lifetimes of donor (CdTe) with and without the acceptor (Ge-GeO₂), respectively. The average lifetime is calculated by τ_ave = (A₁τ₁ + A₂τ₂)/(A₁ + A₂), where A₁ and A₂ are the amplitude of the decay with lifetime τ₁ and τ₂, respectively. The luminescence lifetime values τ and τ₀ obtained from the time resolved PL spectra (figure 5(b)) are 0.25 and 1.57 ns, respectively. Based on the above equation, the FRET efficiency is estimated to be ∼84% for Ge-GeO₂/O-CdTe.