Tuning Ag⁺ and Mn²⁺ doping in ZnS:Ag,Mn embedded polymers for flexible white light emitting films

Synthesis

ZnS (99.99%), manganese chloride (MnCl₂, 99%), and silver chloride (AgCl, 99.9%) were purchased from Sigma-Aldrich in the USA. All the chemicals were used as obtained. The ZnS:Mn,Ag, ZnS:Mn (Mn-doped ZnS), and ZnS:Ag phosphors are prepared using solid-state diffusion methods. Briefly, calculated amounts of ZnS, MnCl₂, and/or AgCl were ground together thoroughly and loaded into ceramic crucibles. The crucibles were then embedded in carbon charcoal and sintered at 800 ºC for 3 h before cooling to room temperature.

Characterization

The identity, crystalline structure, and size of the ZnS:Ag,Mn phosphors were determined by X-ray diffraction (XRD), scanning electron microscopy (SEM), and transmission electron microscopy (TEM). Energy dispersive X-ray spectroscopy (EDS) was used to carry out the chemical characterization. The X-ray powder diffraction was recorded in a Bruker D8 X-ray diffractometer with a radiation beam of λ = 1.5406 Å. The ZnS:Ag,Mn phosphors were first dispersed in pure water and then placed onto holey carbon-covered copper grids for TEM observations. The TEM and SEM images and EDS spectra of the particles were obtained with a Hitachi 9500 electron microscope with an accelerating voltage of 300 kV. The photoluminescence spectra were measured by using a Shimadzu RF-5301PC fluorescence spectrophotometer.

The decay lifetime data was acquired using a 355 nm pulsed laser source DPLS-355/30, Rapp Optoelectronic GmbH as an excitation source having a pulse duration of 1 ns FWHM. A CM112 monochromator, Spectral Products, was used to select the observation wavelength. A cooled photomultiplier detector head, PMC-100, Becker & Hickl GmbH, connected to a gated photon counter SR400, Stanford Research System, was used to record the decay curves.

Structure

ZnS forms a sphalerite (β-ZnS) or wurtzite (α-ZnS) structure, depending on the synthesis conditions such as reaction temperatures and precursor concentrations. To reveal the effect of Ag and/or Mn doping on ZnS crystal structures, the pure, as purchased, ZnS was also calcined at the same condition as other samples for comparison. Figure 1 shows the XRD patterns of ZnS, ZnS:Ag(1%), ZnS:Mn(1%), and ZnS:Ag(1%),Mn(1%) samples. Both sphalerite and wurtzite structures have been observed in all the synthesized samples. The displayed diffraction peaks fit very well with standard XRD peak positions of pure sphalerite ZnS (JCPDS, No. 05-0566) and wurtzite ZnS (JCPDS, No. 36-1450), which were indexed and shown with their lattice planes in Figure 1A. A portion of the XRD patterns of the sample is enlarged [Figure 1B] and shows that the diffraction peaks of Ag⁺ and/or Mn²⁺-doped samples have shifted to the left-hand side compared to the ZnS sample with no doping. The observation of the XRD peak shifting of doped samples is expected. Since the ion radius of Ag⁺ (1.26 Å) or Mn²⁺ (0.83 Å) is larger than Zn²⁺ (0.74 Å)^[19,20], successful Ag⁺ and/or Mn²⁺ doping can enlarge the lattice parameter of ZnS. Compared to the Ag⁺ or Mn²⁺ doping along (ZnS:Ag or ZnS:Mn), the diffraction peaks from the ZnS:Ag,Mn sample display the largest shift. Additionally, there are two more wurtzite structure peaks appearing [W(101) and W(103)]. The W(101) peak is much more intense in the ZnS:Ag,Mn sample than in the ZnS sample, indicating that the co-doping of Ag⁺ or Mn²⁺ results in more wurtzite phase. For the Ag⁺ or Mn²⁺ solo-doped ZnS:Ag and ZnS:Mn samples, there are no other differences except for the previously discussed diffraction peak shifts observed when compared to the ZnS sample.

Figure 1. XRD patterns of the ZnS, ZnS:Ag, ZnS:Mn, and ZnS:Ag,Mn samples. Both sphalerite and wurtzite structures are observed (A). The plot (B) is an enlarged portion of the XRD patterns displaying the peak shifting due to Ag⁺ and/or Mn²⁺ doping. XRD: X-ray diffraction.

Morphology

Figure 2A displays a typical SEM image of ZnS:Ag,Mn phosphors. The average diameter of the ZnS:Ag,Mn particles was estimated to be 4~5 µm. Interestingly, in the high-magnification SEM image [Figure 2B], smaller particles are observed on these large ZnS:Ag,Mn particle surfaces. High resolution electron transmission microscopy (HRTEM) was further conducted and focused on the smaller particles. The HRTEM image [Figure 2C] clearly shows the existence of small spherical particles that are about 0.5 µm. The EDS measurement on these small particles confirms that they are composed of Zn and S [Figure 2D]. Low levels of Cu, C, and O can be seen due to the copper grid sample holder composition. These elements are common impurities in sample preparation. Due to their low doping concentrations, neither Ag nor Mn was observed. A co-existence of large (4~5 µm) and small (~0.5 µm) ZnS:Ag,Mn particles may have been established in this material. From the SEM and TEM observation, it is probable that the formation of larger ZnS:Ag,Mn particles results from the aggregation of smaller particles.

Figure 2. SEM images (A) and (B) and TEM image (C) display the co-existence of large (4~5 µm) and small (~0.5 µm) particle sizes of ZnS:Ag,Mn phosphors. Image (D) is an EDS spectrum measured on a small particle shown in the image (C). EDS: Energy dispersive X-ray spectroscopy; SEM: scanning electron microscopy; TEM: transmission electron microscopy.

Photoluminescence

The emission (PL) and excitation (PLE) spectra of synthesized ZnS:Ag phosphors are shown in Figure 3. Blue photoluminescence from ZnS:Ag phosphors is attributed to electron transition from sulfur vacancies to Ag luminescence centers^[21-23]. At multiple excitation wavelengths (330, 340, and 350 nm), ZnS:Ag phosphors display the same blue emission peaked at 450 nm but at different intensities [Figure 3A], which is consistent with previous reports^[24-28]. The excitation spectrum is narrow, peaked at about 340 nm (3.8 eV), and suggests that the band edge absorption of ZnS plays a significant role in the emission intensities observed from the excitation. Figure 3B shows the ZnS:Ag PL and PLE spectra using different Ag doping concentrations (λ_ex = 340 nm). It can be seen that the emission peak remains at 450 nm, but its absorption and emission intensities vary in a way that is not expected. The inset in Figure 3B reveals a relationship between the intensity of this blue emission and Ag⁺ doping concentration. Compared to other doping concentrations, the 2% Ag⁺ doping results in the highest absorption and fluorescence intensity.

Figure 3. (A) The emission and excitation spectra of ZnS:Ag(2%) phosphors. The emission spectra are monitored by different excitation wavelengths; (B) The emission and excitation spectra of ZnS:Ag phosphors at different Ag⁺ doping concentrations (λ_ex = 340 nm, λ_em = 450 nm). The inset is a plot of the ZnS:Ag emission intensities as a function of Ag⁺ doping concentrations.

ZnS:Mn phosphors were prepared using the same experimental conditions as the ZnS:Ag. Figure 4 shows similar optical property data for the prepared ZnS:Mn phosphors. The red emission centered at 586 nm is from the ⁴T₁-⁶A₁ transition of Mn²⁺ ions^[29-31], while a shoulder located at about 640 nm on the emission spectrum is due to interactions of Mn²⁺ ions [Figure 4A]. The ZnS:Mn excitation spectrum also peaked at approximately 340 nm and is attributed to the band-edge absorption of ZnS. We also prepared ZnS:Mn samples by varying Mn doping concentrations to determine an optimized Mn²⁺ concentration, which may lead to the most intense red emission in solo-doped ZnS:Mn phosphors. Figure 4B shows that the ZnS:Mn phosphor emits the highest red luminescence when Mn²⁺ is doped at 4%. A weak blue band (when Mn = 1%) is observed and assigned to defect luminescence centers^[32]. However, its contribution would be insignificant since it is very weak and disappears quickly as the Mn²⁺ doping concentration increases.

Figure 4. (A) The emission and excitation spectra of ZnS:Mn(4%) phosphors. The emission spectra are obtained by different excitation wavelengths; (B) The emission and excitation spectra of ZnS:Mn phosphors at different Mn²⁺ doping concentrations (λ_ex = 340 nm, λ_em = 586 nm). The inset is a plot of the ZnS:Mn emission intensities as a function of Mn²⁺ doping concentrations. The intensity is reduced at a factor of 1/3 in measurement.

It was seen that both ZnS:Ag and ZnS:Mn are excited by ZnS band-edge absorption and emit intense fluorescence. Moreover, as the Ag⁺ blue emission and Mn²⁺ red emission cover from violet to green (400~560 nm) and green to red (540 to 680 nm) in luminescence spectra, it presents the opportunity to investigate Ag and Mn co-doped ZnS (ZnS:Ag,Mn) phosphors for white light production. Considerations of possible quenching effect at high Ag and/or Mn concentrations were addressed by preparing ZnS:Ag,Mn samples at a relatively low Ag concentration of 0.5% and varied co-doped Mn concentration from 0.1% to 1%. The photoluminescence spectra of the samples prepared are revealed in Figure 5. Under different excitation wavelengths from 320 to 350 nm, all the samples show both blue and red emission peaks at the same fluorescence wavelengths as displayed in solo-doped ZnS:Ag and ZnS:Mn samples. Therefore, transitions can be assigned to the same luminescence mechanisms previously discussed, i.e., two-electron transition pathways emitting blue and red light. The blue emission was assigned to be from defects to Ag⁺ luminescence centers, the other from the Mn²⁺ d-d transition (⁴T₁-⁶A₁). With a fixed Ag⁺ doping concentration of 0.5%, Figure 5A-D displays the emission and excitation spectra of ZnS:Ag,Mn samples when Mn²⁺ doping were 0.1%, 0.2%, 0.5%, and 1%, respectively. It was observed that the blue emission was significantly quenched when Mn doping was relatively high (0.5% Mn or 1% Mn). Particularly, the blue emission due to Ag luminescence centers almost disappeared at 1% Mn²⁺ co-doping, while the red emission due to Mn²⁺ doping showed similar intensity in comparison to the solo-doped ZnS:Mn(1%). This evidence revealed that the Mn²⁺ d-d transition must take precedence in the two-electron transition pathways and can suppress the transition from defects to Ag luminescence centers. In consequence, the Mn concentration may need to be kept at low levels to have an intensive Ag⁺ blue emission in co-doped ZnS:Ag,Mn phosphors. Here, the blue emission was more intense for the samples with lower Mn concentrations of both 0.1% and 0.2%. However, the red emissions in the Mn(0.1%) sample were much weaker than the Mn(0.2%) sample under different excitation wavelengths. Considerations from the above characteristics led to the ZnS:Ag(0.5%),Mn(0.2%) phosphors being selected as a white light emitting candidate for further investigation.

Figure 5. The emission and excitation spectra of ZnS:Ag(0.5%),Mn monitored by different wavelengths. The doping concentration of Mn²⁺ is (A) 0.1%, (B) 0.2%, (C) 0.5%, and (D) 1.0%, respectively.

Figure 6 reveals the intensities of ZnS:Ag(0.5%),Mn(0.2%) blue and red emissions using different excitation wavelengths. The intensity ratio of the average blue emission (450 nm) to red emission (586 nm) showed to be about 0.8 and suggested that the ZnS:Ag(0.5%), Mn(0.2%) overall blue emission was less intensive than its red emission when a practical excitation source was used. Hence, it would be necessary to increase the blue emission intensity in the ZnS:Ag(0.5%),Mn(0.2%) phosphor for white light applications. For further investigation, ZnS:Ag(0.2%),Mn and ZnS:Ag(1%),Mn phosphors were prepared with varying Mn concentrations (0.1%, 0.2%, 0.5%, and 1%). Similarly, at 0.1% Mn²⁺ doping, the red emission was much weaker than the blue emission so that it was not proper for white light applications. As for the 0.5% or 1% Mn²⁺ doping, the red emission was much more intensive than the blue emission, as was seen for the ZnS:Ag(0.5%),Mn(0.5%) and ZnS:Ag(0.5%),Mn(1%) samples discussed in Figure 5. Figure 6 displays the emission and excitation spectra of ZnS:Ag(0.2%),Mn(0.2%), ZnS:Ag(0.5%),Mn(0.2%), and ZnS:Ag(1%),Mn(0.2%) phosphors. Compared to the ZnS:Ag(0.5%),Mn(0.2%) sample, the blue and red emissions from the ZnS:Ag(0.2%),Mn(0.2%) sample are getting stronger and weaker, respectively [Figure 6A], while the emissions from ZnS:Ag(1%),Mn(0.2%) seem to remain at similar intensities [Figure 6B]. In order to figure out the best candidate for white light applications, the intensities of blue (450 nm) and red (586 nm) emissions from ZnS:Ag,Mn phosphors under different excitation wavelengths are plotted together with varying Ag⁺ doping concentrations (0.2%, 0.5%, and 1%), as shown in Figure 6C and D. The Mn²⁺ concentration is fixed at 0.2% for all the samples. Unlike the solo-doped ZnS:Ag phosphors, the most intensive blue emission is found when Ag is at 0.2%, indicating that Ag⁺ concentration quenching has occurred at a significantly lower concentration level in the presence of Mn²⁺ ions. On the other hand, it is seen that the red emissions from ZnS:Ag(0.2%),Mn(0.2%) are weaker than from the ZnS:Ag(0.5%),Mn(0.2%) and ZnS:Ag(1%),Mn(0.2%) samples. This is reasonable due to its high blue emission. The ratio of average blue/red emission intensities of the ZnS:Ag(0.2%),Mn(0.2%) is found to be 1.29 with different excitation wavelengths. In comparison to the ZnS:Ag(0.5%),Mn(0.2%) phosphor, the increased Ag concentration in ZnS:Ag(1%),Mn(0.2%) has almost no change on the blue emission intensities but has reduced the red emission intensities. Calculated in the same method, the ratio of average blue/red emission intensities of the ZnS:Ag(1%),Mn(0.2%) is 0.92. Thus, the blue and red emissions from the ZnS:Ag(1%),Mn(0.2%) phosphor have closer intensities, and this sample is more appropriate for white light generation in our study.

Figure 6. (A) and (B) The intensities of ZnS:Ag(0.5%), (0.2%) blue (450 nm) and red (586 nm) emissions under different excitation wavelengths; (C) The emission spectra of ZnS:Ag(0.2%),Mn(0.2%), ZnS:Ag(0.5%),Mn(0.2%) and ZnS:Ag(1%),Mn(0.2%) monitored by different wavelengths; (D) The intensities of ZnS:Ag,Mn blue (450 nm) and red (586 nm) emissions under different excitation wavelengths when Ag⁺ doping concentration is 0.2%, 0.5%, and 1%, respectively. The Mn²⁺ concentration is 0.2% for all the samples.

The Commission Internationale de l’Elcairage (CIE) chromaticity coordinates of the ZnS:Ag(1%),Mn(0.2%) phosphor by varying excitation wavelengths are displayed in Figure 7. We can find that the test chromaticity coordinates corresponding with excitation wavelengths 320 and 330 nm are located in the white light region. With the increase of excitation wavelength from 320 to 340 nm, the emission color shifts from bluish white light to reddish white light. Although the 350 nm excitation generates chromaticity coordinates in a more reddish region, its emission intensity is much lower than that using excitation wavelengths from 320 to 340 nm due to the sharp ZnS:Ag,Mn excitation bands. The inset in Figure 7 displays a photo image of ZnS:Ag(1%),Mn(0.2%) under a UV lamp with an emission peak at 365 nm, which shows the overall white light output from the ZnS:Ag(1%),Mn(0.2%) and the images of flexible ZnS:Ag(1%),Mn(0.1%) infused PDMS. The right chromaticity plot shows the color of the samples listed in Table 1.

Figure 7. (A) The CIE chromaticity coordinates (displayed as dark dots from left to right hand side) of ZnS:Ag(1%),Mn(0.2%) under different excitation wavelengths at 320, 330, 340, and 350 nm, respectively. The left inset is a photo image of ZnS:Ag(1%),Mn(0.2%) under a UV lamp (peaked at 365 nm); (B) CIE coordinates of samples in Table 1. Far right: Images of flexible ZnS:Ag(1%),Mn(0.1%) infused PDMS. CIE: Commission Internationale de l’Elcairage; PDMS: polydimethylsiloxane.

Quantum yield

External photoluminescent quantum yield (EQY) measurements were performed using a Thorlabs integrating sphere to observe the changes in emission yield as different dopant concentrations of Ag⁺ and Mn²⁺ were varied in the ZnS. The chosen samples were mixed in PDMS polymers and placed onto a quartz slide of 1 cm² area, each containing 15 mg of sample mass. Samples were then cured at 125 °C for 30 min until solid. A 365 nm UV-LED pen was used as an excitation source to measure the EQY. EQY was calculated as follows:

Where A₁ is the sum of the photon counts from 400-700 nm (emission), and A₀ is the sum of the photon counts from 350-400 nm (incident). The EQY of ZnS:Mn, ZnS:Ag, and ZnS:Ag,Mn Phosphors are shown in Table 2.

Photoluminescence decay lifetime

The Photoluminescence Decay (PLD) measurements were conducted at room temperature on four samples of ZnS:2%Ag with different Mn doping levels: 0.1%, 0.2%, 0.5%, and 1%, respectively [Figure 8A]. As can be seen from Figure 8B and C, the PLD curves that were recorded at 450 and 590 nm occur on very different time scales. The average lifetime for the 450 nm emission was measured at around 1 μs, while for the 590 nm region, the average lifetime was approximated at more than three orders of magnitude longer (several ms). This means that 10 μs after the end of the excitation pulse, all the light emitted by the samples was in the region centered on 590 nm, in agreement with the data about similar samples^[24-28]. Most researchers usually associate the prompt luminescence (450 nm region) in ZnS:Ag powdered scintillators with excitonic recombination. The initial part of the decay can be fitted with two exponentials, and that suggests that two types of recombination centers with different oscillator strengths would be involved in the process^[20,21]. The decay constants of 26 ns and 130 ns were previously reported^[20,21]. In our experiments [Figure 8B], the first decay constant was outside of our experimentally accessible time scale. For the second time constant, we measured a value of 122 ns, which is close to the reported values^[20,21]. The fast initial decay in the 450 nm region was followed by an afterglow governed by different electron and hole traps present in the material. This part of the decay can be approximated with a hyperbolic function or, in simple cases, by an exponential function^[20]. We chose to fit the afterglow to a single exponential function, which was appropriate due to the short time scale of our experiment. The afterglow decay constant value of 1μs that we obtained [Figure 8B] was in good agreement with the data published^[20].

Figure 8. (A) The emission spectra of ZnS:2%Ag samples containing 0.1%, 0.2%, 0.5%, and 1.0% Mn excited by 355 nm laser pulses with duration of 1 ns and 10 kHz repetition rate; (B) Emission decay curve for ZnS:2%Ag:1%Mn recorded with 450 nm observation wavelength at a 20 Hz repletion rate; (C) Emission decay curve for ZnS:2%Ag:1%Mn recorded with an observation wavelength of 590 nm and a 10 Hz repetition rate; (D) Dependence of the emission decay at 590 nm on the level of Mn doping. Measurements are done at a 10 Hz repetition rate.

The photoluminescence in the 590 nm region has been associated with the Mn²⁺ ions. A characteristic feature of the Mn²⁺ emission is the long decay lifetime (up to several milliseconds) due to the spin-forbidden ⁴T₁-⁶A₁ d-d transitions^[24-26], whereas the presence of defects could shorten the decay lifetime^[24].

In practice, it is hard to disperse the Mn homogeneously in the material and to keep the coordination of the Mn ions the same (always substitutional doping). For that reason, in most systems, the decay kinetics is complicated due to the presence of magnetic coupling between the Mn²⁺ ions in the material and/or due to Mn²⁺ ions residing on the surface of the powder particle, which can be easily observed at high doping concentrations and/or hosts containing particles with small size. Thus, it is difficult to assign decay channels to different light-emitting Mn²⁺ species because Mn²⁺ ions with different degrees of coupling may generate numerous relaxation pathways. In most cases, the decay can be fitted reasonably well to the sum of three exponentials, and we chose that approach for our experimental data [Figure 8C and Table 3].

For the reasons mentioned above, the decay lifetime is strongly dependent on the average distance between the Mn²⁺ ions and decreasing that distance, i.e., by increasing the Mn concentration, leads to shortening of the observed decay lifetimes. As seen in Figure 8D and Table 3, the observed decay constants are inversely proportional to Mn dopant concentrations, which is in good agreement with previously published research^[33,34].

Actually, not only the decay lifetimes but also the luminescence efficiency or intensity are highly dependent on the doping concentrations as can be seen in Figure 8A. Further, the ZnS:Ag with 2% Mn has the strongest luminescence in intensity. Usually, at low concentrations, the luminescence intensity is increased with the increasing concentrations; when it reaches the peak, the intensity is decreased with further increasing the concentration as a result of concentration quenching, as reported in other publications^[10,35]. However, the critical concentration for this kind of quenching is higher, which makes it possible to reach higher efficiency in doped nanophosphors^[10,36].

The research on ZnS:Mn,Ag nanoparticles for foldable LEDs represents a novel and groundbreaking contribution to the field of nanotechnology and optoelectronics. While extensive work has been conducted on ZnS-based nanoparticles, the introduction of manganese and silver dopants into the ZnS matrix is a unique and innovative approach that has not been extensively explored. This novel combination of materials opens up new possibilities for enhancing the performance and functionality of LEDs.

Moreover, the incorporation of ZnS:Mn,Ag nanoparticles, specifically for foldable LEDs, is a pioneering concept that, to the best of our knowledge, has not been previously reported in the literature. The development of foldable LEDs is of significant interest due to the increasing demand for flexible and portable display technologies. This research not only offers a new avenue for achieving foldable LEDs but also addresses the challenges and opportunities associated with this emerging technology.

The study of ZnS:Mn,Ag nanoparticles for foldable LEDs represents a unique and unexplored dimension in the field of nanomaterials and optoelectronics. By investigating this uncharted territory, this research promises to advance our understanding of materials science and potentially lead to the development of innovative, flexible, and high-performance lighting devices with a wide range of applications in the display industry.

Here, we present the successful synthesis of luminescent nanoparticles and their subsequent integration into flexible films, showcasing their potential applications in the development of foldable electronic devices. However, it is imperative to acknowledge that temperature and heating pose formidable challenges in this context, as elevated temperatures have the potential to significantly diminish the emission efficiency of these nanoparticles^[33,34]. Addressing this critical concern is a primary focus of our research, and we are dedicated to exploring innovative solutions to mitigate the adverse effects of heating on the luminescent properties of these materials.