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Abstract
We describe, for the first time, the effects of Mn co-doping on the upconversion emission of Yb, Er - NaGdF4 nanoparticles using an ns pulsed laser with variable energy density and pulse width modulation. Unlike cw excitation that leaves unchanged the emission colour with power density, ns pulsed excitation induced a remarkable green to orange colour tuning with the increase of the energy density from 3 to 70 mJ/cm2. Pulse width modulation from 0.02 to 5 ms determines green to yellow colour tuning for 10% Mn, which is well-correlated with the built and decay stages of Er green and red emissions. Our study gives new insights into Mn role in colour tuning of Mn, Yb, Er - NaGdF4 nanoparticles and highlights the potential of these systems for anti-counterfeiting, bioimaging and lifetime multiplex applications.
© 2022 Optica Publishing Group under the terms of the Optica Open Access Publishing Agreement
1. Introduction
Colour tuning of upconversion (UPC) emission of lanthanide (Ln) based nanoparticles have attracted tremendous attention over the years [1–8]. Colour tuning is achieved by physical approaches (such as pulse modulation [1,3–5,7,8], dual-wavelength excitation, power density [9] and energy density variation), chemical approaches (by modifying the Ln concentration, co-doping with a transition metal (TM), changing the size/shape of the nanoparticles, using core-shell engineering or local structure manipulation [2,3,6,10–12]) or even a combination of the two approaches (e.g., core-shell engineering and power density variation [13]).
A frequently employed chemical approach uses a transitional metal (TM) as co-dopant in the well-known upconverting systems Yb, Er - NaREF4 [2,14–17] (where the rare-earth, RE is Y, Lu or Gd). In Ref. [2], for Yb, Er - NaYF4, increasing concentration of Mn2+ from 0 to 30%, the red to green emission ratio (R/G) increased spectacularly from 0.83 to 163.78. In further experiments, the authors have found that the single-band emission of 30% Mn2+-doped Yb, Er - NaYF4 was independent of Yb amount, excitation laser power, and environmental temperature, confirming a highly efficient Er to divalent Mn energy transfer process. In Ref. [15], the authors reported the simultaneous phase/size manipulation, upconversion luminescence colour tuning, and intensity enhancement by Mn2+ doping into Yb, Er - NaLuF4.
Recently, physical approaches have been desirable as they prove to be easier to reproduce and implement in anti-counterfeiting applications. So far, the main physical approaches for upconversion colour tunning are either by controlling laser pulse modulation (frequency and pulse width or using dual excitation wavelengths [18]) or pump power density. In Ref. [1], the emission colour of Yb, Er - NaYF4 nanoparticles was tuned from an R/G value of ∼1 to ∼3.5 by increasing the 1 µs pulse frequency from 0.1 to 10 kHz. In Ref. [4], the emission colour of NaYF4-based core-shell (with 5 Ln ions: Yb, Nd, Tm, Ho and Ce: Nd, Yb-NaYF4@Yb, Tm - NaYF4@NaYF4@Yb, Ho, Ce - NaYF4@NaYF4) was tuned from an R/G of 0.5 to 6.97 by modulating the 980 nm pulse width from 0.2 to 6 ms at 100 Hz frequency. In Ref [5]. the upconversion emission colour of core-shell Yb, Er- Yb-NaYF4 nanoparticles was tuned from an R/G of ∼1 to ∼5 by either modulating the 980 nm pulse width from 0.1 to 6 ms at 100 Hz or by increasing the frequency from 100 to 1000 Hz at 0.5 ms pulse width. Finally, in Ref. [9], the upconversion emission colour of Yb, Er- Yb-NaYF4 nanoparticles was tuned from red to green by increasing the laser power from 100 to 350 mW and to white emission at 1400 mW.
Hybrid approaches that use both chemical (such as using divalent Mn as codopant in Ln based upconversion systems) and physical approaches present clear advantages for use in anti-counterfeiting applications [13,19]. It was found in Ref. [13] that the colour output of Yb, Tm, Mn- NaGdF4 can be readily tuned from whitish-blue to bright green by varying the power density of the excitation source. In Ref. [19] the Mn long lifetime was exploited for multilevel anti-counterfeiting, tuning the upconversion emission colour from blue, red and white to Mn green emission for 30 mol% Mn-NaGdF4@49 mol% Yb, 1 mol% Tm-NaGdF4 @A-NaYF4 (A = 20 mol% Eu, 5 mol% Eu/ 15 mol% Tb or 20 mol%Tb), NaGdF4@49 mol% Yb, 1 mol% Tm-NaGdF4 @NaYF4@ Yb/Er (5/0.05, 20/2 or 50/0.05 mol%) - NaYF4 and 30 mol% Mn-NaGdF4@49 mol% Yb, 1 mol% Tm-NaGdF4 @Nd/Yb/Er (1/30/0.5 or 2/10/1 mol%)-NaYF4 under 980 and 808 nm excitation. According to the literature, the role of Mn is to enhance the Er red emission via the energy transfer (ET) mechanisms, as follows: Er 2H9/2, 4S3/2 →Mn→Er 4F9/2 [2,16,17] or to change the structural phase from hexagonal to cubic [2] in case fluoride based nanoparticles. So far, most of such studies using hybrid approaches have employed cw excitation [2,16,17], with punctual use of pulsed excitation for upconversion emission decays measurements [16,17].
Here, we investigate, for the first time, the effects of Mn co-doping (10 and 20%) on the upconversion emission of Yb, Er - NaGdF4 nanoparticles excited by a ns pulsed laser with variable energy density and cw laser diode with modulated pulse width. The energy density was varied from 3 to 70 mJ/cm2, and the pulse width was modulated from 0.02 to 5 ms. The evolution of the energy density dependencies of Er upconversion emission, colour and decays with Mn concentration sustain that Mn enhances the 3-photon order contribution to Er red emission. This way, the red to green emission intensity ratio (R/G) is enhanced from a subunitary value (no Mn) up to 6–7 (10, 20%Mn) at the maximum energy density leading to green to orange colour tunability. Pulse modulation induces green to yellow colour change for 10%Mn, which effect is correlated with the built and decay stages of Er green and red emission. Our study reveals the key role of Mn in the colour variation of upconversion emission of Yb, Er - NaGdF4 nanoparticles, which is attractive for anti-counterfeiting, bioimaging and lifetime multiplex applications.
2. Materials and methods
2.1 Synthesis of xMn, 20Yb, 2Er- β -NaGdF4 (x = 0, 10 and 20) nanoparticles
Hexagonal xMn, 20Yb, 2Er - β-NaGdF4: (x = 0, 10, and 20 mol%) nanoparticles were synthesized by a hydrothermal method. The typical synthesis consists of the addition of 2.5 mL of ethanol to an aqueous solution (0.5 mL) of NaOH (0.3 g) under stirring until a homogeneous solution is formed. After that, 5 mL of oleic acid was added in order to form a sodium-oleic acid complex. The oleic acid acts as both a stabilizing and a chelating agent. To the obtained solution, 2 mL ethanol solutions containing 0.25 mmol of RE(NO3)3 (RE = Gd, Yb, and Er with designed molar ratios) were added together with a stoichiometric ratio of MnCl2 water solutions under vigorous stirring for 30 minutes. Subsequently, 2 mL aqueous solution of 1.0M NaF was added and allowed to stir for another 30 minutes. The obtained solution was transferred into a 20 mL stainless Teflon-lined autoclaves, which were kept at 180 °C for 8 h. After the reaction was complete and cooled down to room temperature, the resulting precipitates were collected with chloroform:ethanol (80:20 volume ratio), sonicated, centrifugated and washed three times with ethanol and DI water (1:1 volume ratio) to remove the oleic acid and other residues, and dried at 60 °C overnight. Additionally, cubic 20Yb, 2Er - α-NaGdF4 was synthesized following the same procedure, except for the reaction conditions, which were modified to 150 °C for 24 h.
2.2 Structural characterization
The crystalline structure was studied by X-Ray Diffraction (XRD) using a Shimadzu XRD-7000 diffractometer (Shimadzu Corp., Kyoto, Japan) (Cu Kα radiation-40Kv, 40mA, Bragg-Bretano geometry, at a scanning speed of 0.1°/min, in the 2θ range 10-90 with a step size of 0.02° and a step time of 2 seconds). Particle size was determined using Scherrer equation [20](1) $\textrm{L} = \frac{{\mathrm{K\lambda }}}{{{\mathrm{\beta} \mathrm{cos\theta} }}}$ where λ is the wavelength in nanometer, β is the peak width of the diffraction peak profile at half maximum height resulting from small crystallite size in radians, K is the shape factor which was considered 0.9 in our study, and θ is the Bragg angle. Diffuse reflectance optical (DR-UV-Vis) spectra were recorded at room temperature on an Analytik Jena Specord 250 spectrophotometer with an integrating sphere for reflectance measurements and MgO as the reflectance standard. DR-UV-Vis spectra of the materials were recorded in reflectance units and were transformed in Kubelka–Munk remission function F(R). For Transmission Electron Microscopy (TEM) analysis, the samples were prepared by powder immersion in ethanol, and the obtained suspension was dropped on a microscopy grid deposited initially on a carbon membrane. The TEM results were obtained using the analytical transmission electron microscopy JEM ARM 200F, at an acceleration voltage of 200 kV.
2.3 Luminescence measurements
Time-resolved emission spectra were recorded on an intensified CCD (iCCD) camera (Andor Technology, iStar iCCD DH720) coupled to a spectrograph (Shamrock 303i, Andor) upon excitation using wavelength-tunable NT340 Series EKSPLA OPO (Optical Parametric Oscillator) operated at 10 Hz as an ns pulsed excitation light source (with a pulse duration of 4 ns). The energy of the laser pulse was modified using a linear neutral density filter and measured with a Coherent Energy Max Laser Energy Sensor (J-10MB-HE Energy Max Sensor). And the cw laser diode power density was measured using Coherent PowerMax USB (PM USB PM10). A PMT module (PMA-C 192-N-M, PicoQuant GmbH) as a detector coupled to a spectrograph (Shamrock 500, Andor) monitored the visible emission decays. All average decay times were estimated by integrating the area of the emission decays normalized at maximum intensity. In cw excitation, a fibre-coupled 973 nm laser diode (RLTMFC-980-4W-5, ROITHNER LASERTECHNIK GmbH) was used. A signal generator (TGP 3121, Thurlby Thandar Instruments Ltd.) modulated the laser pulses using the TTL signal on the fibre optic laser diode (RLTMFC-980-4W-5, ROITHNER LASERTECHNIK GmbH) for pulse modulation experiments.
3. Results and discussion
3.1 Overview of the structural and cw excited upconversion emission properties: comparison with literature
Figure 1 gathers the X-ray diffraction (XRD) patterns, diffuse reflectance spectra, and selected transmission electron microscopy (TEM) images of Mn, Yb, Er- NaGdF4 nanoparticles. XRD patterns (Fig. 1(a)) show that Mn addition induces incomplete phase transition from the hexagonal (C3h) [21] to cubic phase (Oh) [21], which is in agreement with previous reports [2,19]. In addition, a small content of unreacted precursor NaF was also detected (peak at 38.8 °). The estimation of the crystallite size (beta phase) using the Scherrer equation gives values of ca. 20 nm (0Mn) that remains relatively invariant to Mn addition as previously observed [2,17]. The diffuse reflectance spectra (Fig. 1(b)) show a broad absorption peak around 350 nm and tails that extend up to 1100 nm, which apparently enhances with Mn addition. Superimposed on this broad absorption are some weak Er f-f absorptions together with a more significant absorption around 973 nm corresponding to Yb 2F7/2-2F5/2 absorption transition. TEM images in Fig. 1(c) show a relatively broad distribution of the nanoparticle size ranging from 5 to 40 nm, and with only few particles larger than 40 nm.
Fig. 1. XRD patterns (a), diffuse reflectance spectra (b) and selected TEM images (c) of xMn, Yb,Er - NaGdF4 (x = 0, 10 and 20).
Figure S1 (a, b) presents the evolution of the Er upconversion emission spectra with power density upon excitation into Yb 2F7/2 - 2F5/2 absorption at 973 nm together with the power density dependencies of (0/20)Mn, Yb, Er-NaGdF4 nanoparticles. The emission spectra show the green and red emissions corresponding to Er 2H11/2, 4S3/2 - 4I15/2, and 4F9/2 - 4I15/2 transitions. Power density dependencies show that irrespective of Mn concentration, the Er green and red emissions present a slope of ∼2, which is in good agreement with the literature [2,16,17]. A laser heating effect is observed from the 2H11/2, 4S3/2 - 4I15/2 emission shape, which is likely to distort the R/G value at higher power densities (>5 W/cm2). The red to green emission ratio (R/G, calculated as the ratio of the integrated emissions of the above transitions) increases slightly from 1.6 to 1.84 for 0Mn from 1 to 33 W/cm2, Fig. S1c. In the presence of 20Mn, R/G is increased to 5 at 1 W/cm2 in agreement with previous reports [2,15,22]. Increasing the power density at 33 W/cm2 reduces the R/G value to 3.5.
3.2 Tuning the upconversion emission colour by energy density variation
Figure 2(a-c) presents the evolution of the Er upconversion emission spectra with the energy density upon ns pulse excitation at 973 nm together with the energy density dependencies of xMn, Yb, Er-NaGdF4 (x = 0, 10 and 20% at.) nanoparticles. The energy density varied from 3 to 70 mJ/cm2 for xMn, Yb, Er-NaGdF4. Without Mn, R/G remains below unitary, increasing from 0.15 to 0.61. In contrast, the addition of 10 and 20Mn significantly enhances R/G from 1.4/1.3 to 6/6.5 in the same energy density range. The relative increase of the red emission intensity is correlated with the contribution increase of the Er 2H9/2 - 4I13/2 emission transition at ∼560 nm (green highlighted in Fig. 2(a-c)). Such behaviour correlates with the observed energy density dependencies of Er green and red emission energy dependencies, also included in Fig. 2(a-c). The Er green emission is a 2-photon order process up to 10–15 mJ/cm2 with a slight slope variation from 2.08 (0Mn), 2 (10 Mn sample) to 2.08 (20Mn). In contrast, the Er red emission evolves from a mixture of 2- and 3-photon order processes (2.35 for 0Mn) to a predominant 3-photon order process (2.65 for 10 Mn and 2.77 for 20Mn) in the same energy density range. Therefore, the evolution of Er red emission can be assigned to the increased relative contribution of the 3-photon order process via 2H9/2 - 4F9/2 → 2F7/2 - 2F5/2 [23–25]. Above 10–15 mJ/cm2, the slope values diminish due to the saturation of the intermediary levels (Yb 2F5/2 and Er 4I11/2) [26], Fig. 2(a-c).
Fig. 2. Evolution of Er upconversion emission spectra with energy density together with energy density dependencies of xMn, Yb, Er-NaGdF4 (x= 0, 10 and 20Mn), (a-c). The emission spectra were normalized at the maximum green emission (∼540 nm). (d) Dependence of R/G on the energy density. Excitation was performed at 975 nm, and the energy density varied from 3 to 70 mJ/cm2.
Unlike cw excitation, which leaves unchanged the upconversion emission shape (i.e., R/G) with power density (Fig. S1), the upconversion emission shape is dramatically affected by the energy density of the ns pulsed laser (Fig. 2(d)). As observed from Fig. 2(d), Mn significantly enhances R/G across the energy density range (3 - 70 mJ/cm2) from green (for 10 and 20Mn samples, R/G= 1.3 and 1.4) to orange (for 10 and 20Mn samples, R/G= 6 and 6.5).
We further investigated how Mn addition affects the Er upconversion emission dynamics associated with green and red-emitting levels upon excitation at 975 nm. As shown in Fig. 3(a), the green emission is accelerated with Mn addition with an average decay time decreasing from 0.119 ms (0Mn) to 0.078 ms (10Mn) to 0.041 ms (20Mn). By contrast, the average decay time of red emission is only modified from 0.332 ms (0Mn) to 0.392 ms (10Mn) and 0.321 ms (20Mn), Fig. 3(b) (see also Table 1). The acceleration of Er green emission decay with Mn addition may sustain an energy transfer from Er donor to Mn acceptor (from Er(2H11/2, 4S3/2) → Mn(4T1)), Fig. 4. To further validate this, we investigated the evolution of Er green and red emission dynamics with Mn addition using direct excitation into Er 4I15/2 - 4F5/2 absorption at 455 nm. As shown in Fig. 3(c), the Er green emission accelerates significantly, with the average decay decreasing by a factor of 27 from 0.055ms (0Mn) to 0.002 ms (20Mn) (Table 1). Unlike Er green emission, the red emission is moderately accelerated, with the average decay time reduced from 0.037 (0Mn) to 0.016 ms (10Mn) to 0.014 ms (20Mn) (Table 1).
Fig. 3. Evolution of Er green and red emission decays with Mn addition upon upconversion (975 nm) (a,b) and direct excitation (455 nm) (c,d)
Fig. 4. Schematic representation of the relevant energy levels and upconversion mechanisms occurring in Mn, Yb, Er-NaGdF4. The blue arrows denoted as 1, 2 and 3 represent the Yb to Er energy transfer upconversion (ETU) processes. The grey arrows denoted as 4, 5 and 6 refer to the energy transfer (ET) between Er and Mn.
Table 1. Average decay times of Er green and red emission upon upconversion and direct excitation.
3.3 Role of Mn in upconversion emission colour tunability
It is well-established that upon excitation at ∼975 nm into Yb absorption (2F7/2-2F5/2 transition), the Er green and red upconversion emission is induced by sequential energy transfer (labelled with ETU steps 1 - 3 in Fig. 4) by 2 and mixed 3 and 2-photon order processes, respectively. The mixed 2 and 3-photon order processes that populate the 4F9/2 red-emitting level are well supported by the photon order of 2.35 (compared to 2.08 for the green emission) determined from the energy density dependencies in Fig. 2(a-c).
The addition of Mn significantly stimulates the contribution of the 3-photon order process to red emission, as illustrated by the R/G evolutions with the energy density in Fig. 2(d). The R/G is considerably enhanced, starting already from 1.4 / 1.3 at the minimum energy density reaching 6 / 6.5 at the end of the energy density interval for 10 / 20Mn. In contrast, without Mn, R/G remains subunitary irrespective of the energy density. The fact that R/G is enhanced with Mn addition suggests that Mn plays an intermediary donor level for 2H9/2, known to feed the 4F9/2 via a 3-photon order process [27,28].
Considering the acceleration of the Er green emission with Mn addition measured under both upconversion and direct excitation (Fig. 3), we may conclude that Mn establishes an Er-Mn-Er energy transfer pathway in which the Er 2H9/2 and 4S3/2 levels serve first as a donor for the Mn 4T1 level followed by a second energy transfer step in which Mn plays the role of donor for Er 4F9/2 level. Such a pathway was previously suggested in the literature [2,16,17], but the role of the Er 2H9/2 level could not be properly identified using cw experiments.
We should note that, although the photon density in pulsed excitation is higher than in cw excitation (at 10 mJ/cm2, the equivalent power density during the excitation pulse or the peak power density is ∼106 W/cm2 magnitude), the short pulse duration (<5 ns) allows a higher saturation level of the intermediary levels. This further determines an increased population of Er 2H9/2 when compared with the population of 2H11/2, 4S3/2 level (Fig. 4), which further enhances the relative contribution of the 3-photon order process to red emission [27,28].
Since the average decay time of red emission presents a non-monotonous evolution with Mn concentration upon upconversion excitation (Fig. 3(b, d) and Table 1), the Er-Mn interaction may be not the only variable that determines such trend. The heterovalent doping with low solubility metals such as Mn2+ can induce Ln multisite distribution and dopant or phase segregation, impacting the Ln luminescence intensity, colour, and dynamics. Few reports of quantitative determination of Mn concentration indicate a difference of two orders of magnitude between the designed and final Mn concentration [19]. To check the possible heterogeneity effects, we measured the upconversion emission spectra for 10 and 20 Mn samples using site-selective excitation spanning with 2–5 nm step the Yb absorption between 910 and 1050 nm. We found that the upconversion emission shape remained unchanged with the excitation wavelength, which means that Er is homogenously distributed on Gd sites in fluoride lattice. Furthermore, to exclude the interference from cubic NaGdF4 (revealed by the XRD patterns in Fig. 1(a)), we have synthesized cubic (Mn free) NaGdF4 co-doped with Yb and Er (see the Experimental Section) and compared the upconversion emission shapes of the two Mn free systems. Compared to the hexagonal Yb, Er - NaGdF4, the cubic counterpart present an overall broader emission with a slightly altered shape of red emission (Fig. S2). Such shape could not be revealed in the Mn, Yb, Er - NaGdF4. We conclude that the overall luminescence behaviour of xMn, Yb, Er- NaGdF4. is exclusively related to the hexagonal phase with no contribution from the cubic phase.
3.4 Tuning the upconversion emission colour by pulse width modulation
Due to differences in the dynamics of Er green and red emissions, it is expected that a variable excitation pulse width induces a significant colour tuning which is an attractive property for bioimaging and anti-counterfeiting applications [1,4,29]. A short excitation (tens of µs) pulse favours the faster decay green-emitting level, while long excitation pulse width (few ms) favours the relative slow red decaying level leading thus to green-red colour tuning.
Here we investigate, for the first time, the evolution of the upconversion emission colour and dynamics with Mn concentration using pulse modulation. To this aim, we modulated the diode laser pulse at 973 nm from 0.02 ms to 5 ms at 100 Hz frequency. The pulse range was selected upon considering the longest timescale of the upconversion emission (5 ms for the red emission at 650 nm) and the minimum signal to noise ratio at 0.02 ms. A frequency of 100 Hz allows a 10 ms window to measure the longest red emission decay upon the longest excitation pulse (5 ms).
Figure 5 illustrate the evolution of Er upconversion emission spectra and dynamics with pulse modulation for 0, 10 and 20Mn. With increasing the 973 nm pulse width from 0.02 to 5 ms, the upconversion emission shape changes with an increased relative contribution of the red emission. The most significant change is observed for 20Mn, Yb, Er- NaGdF4, as it will be further analyzed in Fig. 6. When monitoring the green and red emission decays, it can be observed that with increasing the excitation pulse, the green emission decays is lengthened while the red emission decays remain almost constant.
Fig. 5. Evolution of the upconversion emission spectra and emission decays of xMn, Yb, Er - NaGdF4 (x = 0, 10 and 20) upon excitation at 973 nm. All upconversion emission spectra were normalized at the maximum intensity of green emission (540 nm). The pulse width of the cw laser diode was varied from 0.05 to 5 ms at 100 Hz frequency. The emission decays were monitored at 540 nm (2H11/2, 4S3/2 - 4I15/2) and 650 nm (4F9/2 - 4I15/2).
Fig. 6. Evolution of R/G (a) and average decay time (b) of Er green and red emission of xMn, Yb, Er - NaGdF4 (x = 0, 10 and 20) with diode pulse width. The pulse width varied from 0.02 to 5 ms at 100 Hz frequency. The emission was monitored at 540 nm (2H11/2, 4S3/2 - 4I15/2) and 650 nm (4F9/2 - 4I15/2) upon excitation at 973 nm.
For 0 and 10Mn, the average decay time of green emission increases from 0.124 to 0.184 ms and from 0.09 to 0.176 ms, respectively. For 20Mn, the average decay time of green emission increases from 0.043 to 0.207 ms. Pulse modulation only slightly affects the red emission decay (the average decay time remains within the 0.3–0.4 ms range). For 20Mn, the Er green emission dynamics present a distinct behaviour with a clearly slower component in the built time (observable after 0.5 ms pulse width) that translates to longer emission decay times when increasing the pulse width (Fig. S3) [30].
Figure 6 quantifies the evolutions in Fig. 5 as R/G (Fig. 6(a)) and average decay time, (Fig. 6(b)). At 0Mn, the R/G increases from 0.18 to 0.96 (increase by a factor of 5.3) for 0Mn. Mn addition enhances the R/G value from 0.72 to 3.6 (10Mn) and from 0.46 to 0.99 (20Mn) (see also Fig. 6(a)).
Due to the increased contribution of the slower build time, the R/G evolution drops below the unitary value approaching the R/G evolution for 0Mn, Fig. 6(a).
The 10Mn sample shows the highest R/G value starting from 0.72 (for 0.02 ms pulse width) and reaching 3.6 (5 ms pulse width). On one hand, the green emission dynamics (built and decay time, Fig. S3) is comparable with that of 0Mn, while on the other hand, the red emission dynamics (built and decay time) are lengthened by ∼25% across the complete range of pulse width, Fig. 6. Therefore, when investigating the colour tunability (R/G) by pulse modulation, it is essential to monitor the emission decays within and after the laser pulse to assess both the population and depopulation processes involved in green and red emissions.
3.5 Tuning the upconversion emission colour: pulse width modulation versus energy density variation
We summarise in Fig. 7 the two strategies of colour tuning described above, i.e., pulse width modulation versus energy density variation, using the CIE (Commission internationale de l’éclairage) coordinates of the upconversion emission spectra.
Fig. 7. CIE coordinates evolution with excitation pulse modulation and energy density for xMn, Yb, Er - NaGdF4 (x = 0, 10 and 20).
Without Mn, using either pulse width or energy density modulation, the Er upconversion colour remains essentially green, with R/G value in the sub unitary range (0.18 - 0.96). For 10Mn, the pulse width modulation varies the emission colour from green (R/G of 0.7) to yellow (R/G of 3.6), while energy density modulation shifts the colour from green (R/G of 1.3) to orange (R/G of 6). For 20Mn, the pulse modulation varies the emission colour from green to yellowish-green, while the energy density modulation varies the emission colour from green to orange, similar to 10Mn. The combination of pulse modulation and energy density variation in Mn, Yb, Er- NaGdF4 can be thus of interest for double-layer security code bars for anti-counterfeiting applications.
4. Conclusions
The effects of Mn co-doping on the upconversion emission of Yb, Er - NaGdF4 nanoparticles were investigated for the first-time using energy density variation and pulse width modulation. The energy density varied between 3 and 70 mJ/cm2 using a tunable ns laser excitation while the pulse width of a cw laser diode was modulated from 0.02 to 5 ms. The evolution of the Er upconversion emission shape (R/G) with Mn concentration and energy density indicates that Mn enhances the contribution of 3-photon order process to Er red emission. This way, R/G is enhanced from a subunitary value (no Mn) to 6–7 (10, 20%Mn) at the maximum energy density determining a green to orange colour tunability. Pulse width modulation is most effective for 10Mn, where green to yellow colour tuning is observed. Our findings suggest that Mn, Yb, Er - NaGdF4 nanoparticles represent exciting systems for anti-counterfeiting, bioimaging, and lifetime multiplex applications.
Funding
Unitatea Executiva pentru Finantarea Invatamantului Superior, a Cercetarii, Dezvoltarii si Inovarii (PN-III-P4-ID-PCE-2020-1553).
Acknowledgements
This work was supported by a grant from the Romanian Ministry of Education and Research, CNCS - UEFISCDI, project number PN-III-P4-ID-PCE-2020-1553, within PNCDI III. The authors thank Cosmin Istrate for TEM measurements.
Disclosures
The authors declare no conflicts of interest.
Data availability
Data underlying the results presented in this paper are not publicly available at this time but may be obtained from the authors upon reasonable request.
Supplemental document
See Supplement 1 for supporting content.
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