Generation of red light with intense photoluminescence assisted by Forster resonance energy transfer from Znq2 and DCM thin films

In this work, a novel experimental investigation of photoluminescence properties of Znq2 thin films co-doped with different concentrations of DCM were performed. The thin films were successfully deposited on glass substrates with different compositions, under high vacuum, by using the vacuum evaporation technique. For all compositions, the photoluminescence was measured at room temperature and also at low temperature in a wide range from 77 to 300 K with a step of 25 K in a high vacuum. The lifetime of the sample studied in real time was also measured using the decay time technique. The results obtained confirm that the doping influences the intensity of the DCM photoluminescence and also shows a complete energy transfer occurred from Znq2 to DCM which may have shifted the photoluminescence peak from Znq2 to the orange wavelength region which is related to DCM. The lifetime of the sample studied in real time was about 4.47 ns for Znq2 and while all the other samples showed two decay time components. As a result, the doping influences the optical properties of Znq2 and makes it a potential candidate for optoelectronic applications. Graphical abstract


Introduction
Organic materials have become among the major research topics lately (Data and Takeda 2019;; its use has given rise to promising applications at the industrial scale and opened up new perspectives for fundamental physics (engineering at the molecular scale), because of their broad potential benefits in various fields such as optoelectronics (Kukhta and Bryce 2021), environmental purification (Xie et al. 2020;Wang et al. 2019), biomaterials (Zhou et al. 2020), solar energy conversion (Lee et al. 2017), sensors (Pięk et al. 2018), and in the field of medicine (Alam et al. 2018). These materials can combine very interesting optical (absorption, emission), electronic (insulating, semiconductor, metallic, or superconducting), and mechanical properties in the form of a single molecule, self-assembling single layers, or thin crystalline or amorphous films. The manufacture of thin-film has attracted considerable attention in the world in recent years due to its uses in different applications for different fields of research (Pan et al. 2019;Saadiah et al. 2019).
Organic dyes, in particular merocyanines dyes (Schembri et al. 2021), (Kulinich et al. 2018), and the metal-organic materials in the form of thin films, have been widely used in the last decades in various applications as energy transport materials, in particular for optoelectronic devices, dye Responsible Editor: Philippe Garrigues 1 3 lasers, organic light-emitting diode (OLED), and solar cells, because of their interesting electrical and optical properties (Liess et al. 2019;Li et al. 2021;Pozin et al. 2021;Saeed et al. 2020;Gu and Zhang 2019;Ou et al. 2019).
4-(Dicyanomethylene)-2-methyl-6-(4-dimethylaminostyryl)-4H-pyran (DCM) is an organic coloring molecule that belongs to the class of merocyanines; it was first used by Eastman Kodak Company in 1997 as a doping material in the red color laser (Hong et al. 2021). This molecule contains a push-pull system which generally consists of an electron donor part and an electron acceptor part that are separated by a conjugated bridge, and this structure has attracted the attention of several research groups. It has shown great potential in its field due to its very interesting photophysical  properties like its high fluorescence quantum efficiency, its absorption, and emission spectra, and also it has shown a large Stokes shift (Laouid et al. 2022). Therefore, it received great interest, which made it a basic material for several applications such as luminescent solar concentrators, photovoltaic, inorganic light-emitting diodes, and NLO applications (Popczyk et al. 2019;Waszkowska et al. 2020). This molecule contains a push-pull system which generally consists of a donor and an acceptor part of electrons (Noirbent et al. 2021). It has shown great potential in its field because of its very interesting photophysical properties (Weishäupl et al. 2021).
Studies on metal-organic materials, particularly metal-organic materials based on 8-hydroxyquinoline, remain a subject of interest due to their interesting and unique properties known for their fluorescence strength and significant non-linear optical properties (Kutluay 2021;Chen et al. 2022;About et al. 2021). Among these hydroxyquinoline compounds, we have a green luminescent material called bis(8-hydroxyquinoline) zinc (Znq 2 ). It is a semiconductor material that has attracted great interest from several researches' teams because of its mechanical, electrical, thermal, and chemical properties (Saito et al. 2021;Shinde et al. 2018;Li et al. 2019;Lougdali et al. 2022). Znq 2 has shown in front of aluminum tris (8-hydroxyquinoline) (Alq 3 ), which is the leader in its field in terms of luminescence efficiency and high quantum efficiency at a low operating voltage (Shahedi et al. 2017). This makes it an attractive material in various applications, such as medicine, photovoltaics, photonics, optoelectronics, and especially for OLED applications (Yuan et al. 2021;Shahedi et al. 2021). On the other part, photoluminescence is a field that has been expanding and refining since the 1970s (Gilliland 1997). Photoluminescence spectroscopy is a powerful nondestructive optical technique, which allows to characterize semiconductor materials and also insulating materials (Yu and McCluskey 2021). Because of its good resolution, high precision, and high sensitivity, it has become an important technique in the study and engineering of materials. Timeresolved photoluminescence (TRPL) is a technique adapted to study the quick electronic deactivation processes that lead to the emission of photons in many types of materials such as metal-organic complexes and dyes. This fluorescence lifetime can be influenced by several parameters such as the molecular environment as well as by interactions with other molecules. This technique makes it possible to measure life in real time.
The main objective of this work is to manufacture organic thin films that are both environmentally friendly and represent a suitable candidate for optoelectronic applications; for this purpose, thin films of the different compositions of DCM and Znq 2 were developed using the vacuum evaporation technique in order to take advantage of the properties presented by these two materials. This paper is centered on the study of the impact of doping and temperature on photoluminescence and decay time, and also studied the transitions made to its various samples. The photoluminescence results obtained showed that the mixture of these organic compounds (DCM and Znq 2 ) present a very good alternative to produce optoelectronic devices and more particularly a very good one to generate red color lasers of good quality and very low cost.

Deposition technique of thin films
Since the glass surface quality has an impact on the properties of the thin films, cleaning is of considerable importance before starting the deposition. The cleaning method used consists of using an acetone bath with ultrasound for 15 min, then an ethanol bath, and finally, cleaning with isopropanol using flow synthesis.
The different samples were deposited on glass substrates using the vacuum evaporation technique under high vacuum 5 × 10 −6 ; the high vacuum is created by using two-pump diffusion pump and roughing pump is using the following system System-NANO 36™ (Kurt J. Lesker Company) (Popczyk et al. 2019). Figure 1 shows the diagram of the co-deposition vacuum evaporation process. Powdered materials were purchased from Sigma-Aldrich and used without any purification. In order to obtain the optimal conditions for the deposition carried out in this experiment, the powders were placed separately in two aluminum oxide crucibles. Each crucible is placed in turn in a shielded tungsten crucible heater to control the temperature of the materials so that they have different deposition temperatures. The temperatures were controlled manually. The substrates are placed in a rotating holder. This study aims to deposit samples with different compositions and therefore the adjustment of the rates is a very important element; for this reason, the crucibles were covered until the rate control to ensure that the desired percentages were deposited, after adjusting all the parameters including density, Z-factor, and thickness of each element; checking the machine parameters, and also reaching the secondary vacuum which is another necessary parameter for the deposition as it provides a guarantee of film purity. The deposition process starts, and the powder starts to evaporate to form the thin film on the substrates. After obtaining the desired thickness (100 nm) with the help of the sensors, the deposition process stops automatically.

Characterization of thin films
In the present work, the atomic force microscopy technique (AFM) was used to examine the morphological properties of DCM co-doped Znq 2 thin films deposited on glass substrates. AFM imaging was studied in tapping mode, using the fiber-lite MI-150 equipment.
The photoluminescence of the thin layers grown on the glass substrate was measured by using FluoroMax-4 spectrofluorometer using FluorEssence software, and the source of excitation was a xenon lamp (Anoua et al. 2021;Zawadzka et al. 2019). This machine allows the measurement of excitation and emission as a function of the wavelength.
In order to study carefully the influence of the low temperature on the photoluminescence intensity of the samples, the measurements were carried out in the temperature interval of 77 to 300 K with a step of 25 K. The cooling was performed using liquid nitrogen. The photoluminescence measurements were made inside a vacuum chamber under vacuum 10 −3 torr as it is presented in Fig. 2. The substrate was attached to a temperature controller to adjust the temperature at each time.
The AFM pictures demonstrate that doping influences the surface of the samples. For the images of the pure DCM sample, they showed a highly roughened (Average value ≈ 1.12 μm ) surface with microscopic grains as shown in the film profile, while the sample of the pure Znq 2 showed a sponge structure with low thickness and manometric size granules are particularly noticeable. The other samples showed homogeneous morphology with a tendency to form nano spherical grains of a few nanometers in height as shown in the film's profiles. The samples of 90%DCM-10%Znq 2 , 70%DCM-30%Znq 2 , and 50%DCM-50%Znq 2 showed a flat surface with average roughness values respectively 12.9 nm, 9 nm, and 10 nm, while the samples of 20%DCM-80%Znq 2 , 10%DCM-90%Znq 2 , and 5%DCM-95%Znq 2 showed a sponge surface structure as in the case of pure Znq 2 with average roughness values respectively 7 nm, 10 nm, and 12 nm.

Photoluminescence at room temperature
For all samples except Znq 2 , the emission spectrum measurements were studied from 500 to 850 nm, excited by 450 nm, while the excitation spectrum was measured in the range 300 to 600 nm at the maximum value of the emission intensity of about 645 nm. For thin films of Znq 2 , its emission spectrum was studied from 450 to 700 nm, excited by 395 nm, while the excitation spectrum was measured in the range 200 to 500 nm at the maximum value of the emission intensity of about 536 nm.
The fluorescence changes of Znq 2 samples co-doped with DCM are shown in real images in Fig. 11.
The photoluminescence spectra of Znq 2 -doped DCM thin films with different percentages are shown in Fig. 12.
The pure Znq 2 thin film showed strong luminescence in advance of the pure DCM thin film, while the highest luminescence intensity is that of the 5%DCM-95%Znq 2 sample. For the other samples, the intensity of photoluminescence is diminished compared to the pure Znq 2 thin film as the percentage of Znq 2 increases in the structure. For samples with a large percentage of Znq 2 (5%DCM-95%Znq 2 , 10%DCM-20%Znq 2 , 20%DCM-80%Znq 2 ), their PL spectra were shifted to orange (41 nm ± 5 nm) relative to the pure Znq 2 PL spectrum, and present three emission bands, as in the case of pure DCM. The first two bands had a low intensity at 490 nm and 504 nm respectively; however, the third most intense band is 614 nm. While in the samples with a large percentage of DCM (70%DCM-30%Znq 2 , 90%DCM-10%Znq 2 ) their spectra were shifted to red (with 116 nm), 50%DCM-50%Znq 2 sample has taken the character of pure DCM.
The samples with the different compositions of DCM and Znq 2 show peaks in the orange color emission region which is related to DCM; this result indicates that a complete energy transfer has occurred from Znq 2 to DCM due to the overlap between PL Znq 2 emission and DCM absorption Fig. 13. This is the same result found by Abedi et al. (2014) when they combined DCM with Alq 3 .
As the observed PL spectra consist of a large band of visible luminescence, each spectrum was fitted with the Gaussian function in order to carefully study the transitions that occurred in the samples. The deconvoluted spectra involve several emission peaks, and each emission peak corresponds to a specific transition present in the particular sample; furthermore, the global spectrum is the sum of the corresponding Gaussian peaks. Figure 14 shows the deconvoluted PL spectra of all samples.
Through analysis of the fitted spectra, the pure DCM sample has presented five distinct characteristic peaks at or around 503 nm, 527 nm, 608 nm, 640 nm, and 692 nm. The pure Znq 2 thin film shows three distinct characteristic peaks at or around 515 nm, 546 nm, and 579 nm. Samples with a large percentage of DCM showed three emission bands around 601 ± 4 nm, 648 nm, and 695 nm ± 5 nm, while samples with a large percentage of Znq 2 have 4 emission bands around 490 nm, 506 ± 1 nm, 600 ± 1 3 The emission bands of the pure DCM sample peaking at 503 nm, 527 nm, and 646 nm represent the characteristic bands, it can be attributed to the transition from HOMO → LUMO, HOMO-1 → LUMO, and HOMO → LUMO + 1. The other emission bands can be attributed to the excited and defect levels created in the film during deposition. The other thin films showed a dominance of red emission bands that are characteristic of pure DCM. They also showed new emission bands that depend on the percentage of Znq 2 in the structure.
The excitation spectra were also fitted by the Gaussian function, as shown in Fig. 15. The fitting of the excitations graphs allows to identify the transitions carried out inside  14 Curves fitting for emissions spectra at room temperature of a Znq 2 pure, b DCM pure, c 10%DCM90%Znq 2 , d 90%DCM10%Znq 2 , e 50%DCM50%Znq 2 the thin film; more precisely, it gives a piece of information on the energy of gape of the thin films. Table 2 below summarizes the peaks' values found and their equivalent energies for all samples.
The pure Znq 2 film showed the presence of 4 peaks, while pure DCM film showed six peaks. Pure Znq 2 film has shown the transition that indicates their gape energy 2.9 eV (Monzon et al. 2011); this result is found by several research teams. The same for the pure DCM film has presented the transition of its gape energy 2.2 eV (Fujii et al. 1997;Qin et al. 2016).
The presence of the peak at 270 nm in all samples is well noted. The samples with different compositions, they presented from their turn six peaks. These peaks are also distinguished by the presence of the characteristic peaks of pure DCM and pure Znq 2 and also distinguished by the presence of new peaks which are the characteristic peaks of these samples. These results allowed us to confirm the presence of several transitions in the thin films. The fact that DCM belongs to the push-pull system which is both electron donor and acceptor, and Znq 2 has the character of an electron donor, the deposited films likely have a donor1acceptor-donor2 chain form. And consequently, the structure has characteristic energy of pure DCM, of pure Znq 2 , and more energies that are most likely related to the transition due to the interaction between A-D1 and the interaction between A-D2.

Temperature effect
The photoluminescence at low temperatures of thin films grown on a glass substrate with different compositions was also studied. Figure 16 shows the photoluminescence of all samples at low temperatures.
The 5%DCM-95%Znq 2 , 10%DCM-90%Znq 2 , and 20%DCM-80%Znq 2 samples showed a decrease in photoluminescence ranging from 77 to 175 k, then they increased sharply 810 3 CPS, 5 10 3 CPS, and 4 10 4 CPS respectively in the temperature range 200 to 275 K to reach the maximum, then it decreases while keeping the same position: 605 nm for 5%DCM-95%Znq 2 , 600 nm for 10%DCM-90%Znq 2 , and 612 nm for 20%DCM-80%Znq 2 . While for the other samples, the most intense peak is obtained with the lowest temperature (77 k) and as the temperature increases the intensity of the peaks decreases; this decrease is accompanied by a change in the position of the peak maximum (1 nm for each 25 K).
The intensity of the photoluminescence of the elaborated samples is much higher at low temperatures; this dependence on the temperature can be explained by the non-radiative 1 3

Fig. 15
Curves fitting for excitations spectra at room temperature of a Znq 2 pure, b DCM pure, c 10%DCM90%Znq 2 , d 90%DCM10%Znq 2 , e 50%DCM50%Znq 2 recombination of the photoexcited carriers which becomes more effective when we increase the temperature. Table 3 combines the data for the photoluminescence measurement of all films. The most intense photoluminescence is obtained by the pure Znq 2 sample at low temperature, while for all other samples the DCM character dominates.

Decay time
Time-resolved photoluminescence (TRPL) is a technique adapted to study the quick electronic deactivation processes that lead to the emission of photons in many types of materials such as metal-organic complexes and dyes. This fluorescence lifetime can be influenced by several parameters such as the molecular environment as well as by interactions with other molecules. This technique makes it possible to measure life in real time.
The lifetime decay of the samples was measured at room temperature (300 K) using the FluoroMax-4 spectrofluorometer, which is connected to the FluoroHub single-photon counting controller using a pulsed diode as the excitation source (Zawadzka et al. 2014). The choice of the diode is related to the excitation wavelength. Figures 17 and 18 show the decay time curves of all studied samples at different compositions. These curves show a transition between the ground state and the excited state and therefore are the results of the interaction between the emitting residues and their excited state of the films.
The spectra of all the films studied showed a decay with a nature of two exponentials. To properly analyze the lifetime decay of the samples, all the curves obtained are fitted with the bi-exponential function described by the following equation: where A 1 and A 2 are constants, t is pulse time, and 1 and 2 are decay values.
All samples showed two decay time components one short and one long. The Znq 2 film showed a longer exciton lifetime than the other samples 1Znq2 = 3.47 ns and 2Znq2 = 10.96 ns, while the samples with different compositions showed a dominance of the pure DCM film character even in the samples with a high percentage of Znq 2 as is presented in Fig. 18. The obtained values of the slowest decay components for the other samples are 1 = 0.09 ns, while the values obtained of the highest decay component are 2 = 2.27 ns. The average sample lifetime was also calculated av = 1.18 ns; consequently, the emission is fluorescence.
In polar environments, the DCM undergoes an intramolecular charge transfer (Kumar Kanaparthi et al. 2021 . 16 Photoluminescence emission spectra at different temperatures for a Znq 2 pure, b DCM pure, c 90%DCM10%Znq 2 , d 10%DCM90%Znq 2 , e 50%DCM50%Znq 2 et al. 2018) but in the following study we work on thin films, and due to the rigid environment hindering the movement of the molecules it is impossible that this phenomenon will occur, and therefore it is very likely that there is a Forster resonance energy transfer phenomena which results in nonradiative energy transfer from the donor fluorophore in the excited state to an acceptor molecule via a Coulomb interaction (Sasaki et al. 2016); this hypothesis can be reinforced by the spectral overlap of the emission spectra of the donor and the absorption spectra of the acceptor. The presence of the two decay times in pure DCM can be explained by two different physical processes illustrated in the transitions that occur from the ground state to the excited state of the donor and the transition from the ground state to the excited state of the acceptor, while for pure Znq 2 samples, a study suggests that the presence of the two decomposition rates is made up of two distinct physical processes that take place during the transfer of energy from the ground state to a higher state or a central metal atom to a quinoline ligand (Painuly et al. 2020).

Conclusion
In summary, DCM and Znq 2 , which are red and green luminescent materials, respectively, were deposited under a high vacuum as thin films on glass substrates by the vacuum evaporation method. The emission spectra data shows that a small percentage of DCM inside Znq 2 can increase the photoluminescence intensity of the film; it also showed that a shift in the peak positions of the samples was also carried out which confirm that a complete energy transfer occurred from Znq 2 to DCM due to the overlap between the PL emission of Znq 2 and the absorption of DCM. Excitation spectra data showed in its turns the presence of more than one transition in the different samples. Low-temperature photoluminescence spectra showed that the photoluminescence intensity of the processed samples is much higher at low temperature; this temperature dependence can be explained by the non-radiative recombination of the photoexcited carriers which becomes more efficient when the temperature is increased. The time-resolved photoluminescence (TRPL) technique showed the presence of a single decay time in pure Znq 2 and two decay times for the other samples. The results obtained show that the combinations of these two materials are a good choice for optoelectronic applications.