Up to now, various growth approaches have been used for the synthesis of Alq3 nano- or microstructures [23–31]. Partially, different variants of vacuum evaporation of Alq3 have resulted in the creation of fine nanowires and microcrystals with a clear hexagonal morphology [23, 26, 29]. The problem is that Alq3 molecule has two different geometric isomers: meridional (mer-) and facial (fac-) . Five crystalline phases of Alq3, α-, β-, γ-, δ- and ε-, have been observed . α- and β- phases have two mer-Alq3 molecules in a unit cell, while δ-phase has four fac-Alq3 per unit cell . Also, the morphology of vacuum deposited organic films depends strongly on the substrate . Zinc oxide nanostructure was employed as catalyst in organic syntheses and transformations . ZnO microneedles can be expected to act as catalysts and nucleation centres for the growth of specific Alq3 structures.
X-ray diffraction pattern of the initial Alq3 powder is shown in Fig. 1. The X-ray diffraction analysis of OLEDs materials, including Alq3, is often rather difficult, because the crystals of OLEDs materials are sometimes disordered and contaminated by other polymorphs. We compared this pattern with the XRD patterns of α-, β-, γ-, δ- and ε-Alq3 [27, 28, 34–37]. The powder exhibits quite similar XRD pattern to that of α-phase Alq3. This is important to know since photoluminescence properties for different phases of Alq3 may vary.
The morphology of the investigated ZnO microneedles and ZnO-AlQ3 composite is presented in Fig. 2. ZnO microneedles with an average diameter of about 0.5–6 µm and height of ~ 4 µm were grown on the (100) silicon substrate. Figure 2, a confirm the hexagonal nature of grown ZnO structures. The morphology of Alq3 film deposited on ZnO microneedles is presented in Fig. 2, b. These structures with the thickness of 50 nm seem to cover outside surfaces of ZnO microneedles.
The room-temperature PL spectrum of the ZnO microneedles (Fig. 3) consists of the two weak bands in the ultraviolet (UV) and visible regions. The UV band at 390 nm is typical for ZnO and arises due to recombination of the free excitons, bound excitons and transitions in the donor-acceptor pairs . The wide green band in the range from approximately 450 nm to 650 nm is caused by defects, first of all, by uncontrolled impurities and stoichiometry defects .
The room-temperature PL emission spectrum of pure Alq3 film with the same thickness as in composite film (they were obtained at the same deposition process) exhibits a characteristic green emission at around 525 nm [28, 39] when excited at 266 nm is shown in Fig. 3. Alq3 is characterized by crystallization in polymorphism both under vapor deposition and solvent evaporation .
As you can see in Fig. 3, the intensity of the UV PL band of the composite is more than 10 times higher than that of ZnO microneedles, and the intensity of the band with a maximum at 525 nm is approximately twice as high than that of this band in the PL spectrum of the Alq3 thin film. It can be seen that both bands are not relatively narrow and shoulders of both bands do not increase the intensity of each other. Our results correlate satisfactorily with the data reported in Refs. [10–12, 14]. According to that data, compared with PL of pure Alq3 and ZnO, PL of the composite sample based on ZnO and Alq3 has higher intensities due to the processes of energy transfer between inorganic and organic materials. According to , when composite material based on ZnO and Alq3 is excited with 266 nm wavelength, both ZnO and Alq3 molecules are excited simultaneously. The excited state energy of Alq3 molecule can be absorbed by the luminescent quencher and then the absorbed energy may eventually be non-radiatively transferred to ZnO, giving rise to an increase in UV emission (band edge emission) of ZnO in the composite.
However, in the case of the energy transfer the luminescence intensity of component from which energy transfer occurs, has to be decreased. But, in our case, the luminescence enhancement of ZnO and Alq3 is observed in both bands. To address the question about energy transfer between inorganic and organic counterparts in our system, excitation functions of Alq3 and the composite have been measured (see Fig. 4). They characterize the efficiency of energy transformation in the considered system, taking into account the efficiency of absorption, luminescence quantum yield, and luminescence spectrum shift. In the case of the absence of changes in quantum yield and spectral shifts, excitation functions correspond to the absorption spectra. The shape of the obtained function for pure Alq3 is similar to the reported literature . In the excitation function of the composite both components can be excited simultaneously (Fig. 4). Probability of excitations is changed over the spectrum, as a result the ratio of the enhanced emission will depend on the excitation wavelength. Since no emission of Alq3 in the region of 390 nm is observed, curve 3 in Fig. 4 belongs only to radiation of ZnO microneedles. The energy in the region of 390 nm ZnO band can be transferred to Alq3. However, comparing energy diagrams of ZnO (4.2 and 7.6 eV ) and Alq3 (3.2 and 5.7 eV ), it can be seen that conditions for energy transfer from Alq3 to ZnO UV band and even from this ZnO band to Alq3 are not appropriate. Nevertheless, the radiative energy transfer can be observed, when Alq3 absorbs UV luminescence of ZnO. But, luminescence lifetime of ZnO UV band (1.56 ns)  is an order of magnitude less than that of Alq3 (12 ns) . As a result, this process can’t be efficient. The shape of excitation function registered at 520 nm is slightly differed for composite as compared to pure Alq3 film. It means that some energy transfer from ZnO green band to Alq3 occurs. It can be noted that some difference in PL intensity of Alq3 band may also be due to the difference in Alq3 morphology (pure Alq3 film is amorphous and Alq3 in composite may form specific structures with higher quantum yield). No evidence of energy transfer from AlQ3 to ZnO is observed.
On the other side, the observed enhancement of luminescence intensity of both components under Alq3 deposition can be caused by mutual influence of each other. It has been found that passivation of ZnO films by coatings with metal oxides  and doping with hydrogen  result in the strong UV ZnO luminescence enhancement. Alq3 contains oxygen atoms and a lot of hydrogen atoms to promote this process. Besides, interaction of Zn ions with Alq3 was found to enhance luminescence of Alq3 . The luminescence intensity of green band is also enhanced that can be noticed from the shape changes of Alq3 band.