Double-Shelled InP/ZnMnS/ZnS Quantum Dots for Light-Emitting Devices.

Traditionally, ZnS or ZnSe is chosen as the shell material for InP quantum dots (QDs). However, for green or blue InP QDs, the ZnSe shell will form a type-II structure resulting in a redshift of the emission spectrum. Although the band gap of ZnS is wider, its lattice mismatch with InP is larger (∼7.7%), resulting in more defect states and lowered quantum yield (QY). To overcome the above problems, we introduced the intermediate ZnMnS layer in InP/ZnMnS/ZnS QDs. The wide band gap of the intermediate layer (3.7 eV) can confine the electrons and holes in the core completely, and the formation of the type-II structure is avoided. As a result, green InP-based QDs with QY up to 80% were obtained. By adjusting the halogen ratios of the ZnX2 precursor, the minimum and maximum emission peaks are 470 and 620 nm, respectively, covering the whole visible range. Finally, after optimizing the coating shell process, the maximum external quantum efficiency of QD light-emitting diodes fabricated from this InP-based green light QDs can reach 2.7%.

Introduction

Quantum dots (QDs) are nanoscale semiconducting materials. Due to the quantum confinement effect, the emission wavelength of QDs gradually blueshifts with the decrease of size, so QDs covering the whole visible range can be obtained by adjusting the size. Because QDs are inorganic compounds, they have good stability. At the same time, narrow full width at half-maximum (FWHM) can be obtained by controlling reaction conditions[1,2] which have started to be deployed in liquid crystal display (LCD) backlight to achieve the high color gamut.[3,4] Ultimately, with a similar structure to the organic light-emitting diode (OLED), the electroluminescence QD light-emitting diode (QLED) is being considered as the next-generation display technology featuring all the advantages of QD, with the similar form factor of OLED.[5−9]

After nearly 2 decades of research, the QY of cadmium-based QDs is approaching 100% with an FWHM of less than 30 nm.[10] However, cadmium is a class A heavy metal contaminant. It is harmful to human health. At the same time, the European Union announced that televisions and displays sold in Europe should be banned from using cadmium since October 2019. Therefore, the prospect of cadmium-based QLED is quite hazy. InP-based QDs are relatively environment-friendly.[11] They have a larger exciton Bohr radius and adjustable luminescence peak from blue to near-infrared. Thus, InP QDs have a brighter future for displays. But compared with the traditional cadmium-based QDs, the QY and FWHM of InP QDs are both far from satisfaction.[12]

At present, there are two hypotheses explaining the wider FWHM of InP QDs. One is that the reactivity of the phosphorus precursor is too high and the reaction only needs a few seconds at high temperature, which leads to insufficient monomer supply during the growth stage. Thus, the reaction goes into the Ostwald ripening stage prematurely, resulting in large size distribution.[13] In spite of a variety of P precursors with weak reactivity tested, the homogeneity is still not ideal.[14] Another hypothesis is that the magic size cluster (MSC) appears during the nucleation and growth of nanocrystals, which makes that the nucleation process deviates from the classical nucleation theory.[15−17] Anyway, the mechanism is not clear yet. So far, the FWHM of green-emitting InP QDs can be reduced to 36 nm by optimizing experimental conditions, and its FWHM is close to the level of Cd-based QDs.[18] Recently, Yang et al.[35] reports ∼15nm InP/ZnSe/ZnS QDs, and a record EQE of 6.6% was achieved. Peng et al.[36] introduced stoichiometric control in InP/ZnSe/ZnS QDs. QLED showed a peak EQE of 12.2% and a maximum brightness of 10,000 cd m–2. The above InP QDs all use tris(trimethylsilyl)phosphine [(TMS)3P] as the P precursor since the use of (TMS)3P can synthesize InP QDs with relatively good performance. However, the price of (TMS)3P is 80 times that of tris(dimethylamino)phosphine [(DMA)3P] and produces highly toxic phosphine (PH3, safe concentration of <0.3 ppm) gas upon contact with air (phosphine can affect the heart, respiratory system, kidney, stomach, nervous system, and liver). Compared with (TMS)3P, (DMA)3P is more stable and does not produce toxic gas when exposed to air. Recently, more and more researchers are using (DMA)3P to synthesize InP QDs.[19,20] In this work, we also synthesized InP QDs using this cheap and low-toxicity P precursor.

Due to the large lattice mismatch between InP and ZnS (7.7%), for InP/ZnS QDs, there will be a large number of defects trapping excitons at the core/shell interface, resulting in only 60% QY of InP/ZnS QDs.[21−23] While the lattice mismatch between InP and ZnSe is small (3.3%), the ZnSe intermediate layer is usually introduced between the InP core and ZnS shell to reduce the defects caused by core/shell lattice mismatch. However, for green- or blue-emitting InP/ZnSe QDs, due to the lowest conduction band energy level of the shell, ZnSe is lower than that of the core InP, the electrons in the core can easily transition to the shell, and the peak position of InP/ZnSe QDs will redshift relative to the InP/ZnS QDs.[24] Since the band gap of MnS is 3.7 eV,[41] which is wider than the green- or blue-emitting InP QDs, this shell can effectively limit electrons and holes in the core, type-I structure QDs are formed, and there is no redshift of the PL peak compared to InP/ZnSe/ZnS QDs with the same core size. Meanwhile, because the lattice mismatch between MnS and InP is small (mismatch is about 4.3%), we choose ZnMnS alloy as the intermediate layer can reduce the lattice mismatch between the core InP and shell ZnS. For InP/ZnMnS/ZnS QDs, QY can reach up to 80%. By adjusting the halogen ratios of the ZnX2 precursor, the minimum and maximum emission peaks are 470 and 620 nm, respectively, covering the whole visible range. By energy-dispersive spectrometry (EDX) element mapping, high-resolution transmission electron microscopy (HRTEM), and X-ray diffraction (XRD) characterization, the results show that Mn2+ has incorporated into QDs and the doping content has been determined by inductively coupled plasma mass spectrometry (ICP-MS) quantitatively. Finally, using these InP/ZnMnS/ZnS QDs, we fabricated QLED with 2.7% external quantum efficiency (EQE), which consists of ITO/PEDOT:PSS/PVK/QDs/ZnMg1–ONPs/Al.

Results and Discussion

Due to the promulgation of the cadmium restriction order, researchers pay more and more attention on InP QDs. At present, the improvement of QY of InP QDs is mainly through increasing the thickness of the shell and reducing lattice mismatch between the core and shell. By increasing the thickness of QDs, the energy transfer between QDs can be effectively reduced, thus increasing QY. At the same time, by using the intermediate layers such as ZnSe or GaP to reduce the lattice mismatch between the core InP and shell ZnS, finally, defects can be reduced. In this work, we have developed a new intermediate layer material ZnMnS. Previous studies on Mn-doped QDs mainly focused on their effects on the stability of materials. Due to the pinning effect of doped Mn2+, the stability of QDs can be improved significantly.[31] At the same time, Mn2+ emits 600 nm red light and has a wide FWHM (>60 nm), which can be used in the field of lighting. In this paper, we used Mn as an alloying element in the ZnMnS intermediate layer. The ZnMnS intermediate layer has the same effect as ZnSe in reducing lattice mismatch; at the same time, it has a wide band gap, which can limit electrons and holes in the core completely and overcome the shortcomings of ZnSe and ZnS shells. The general experimental steps are as follows: the InP core was first obtained by reacting (DMA)3P (2.4 mmol) with InX3 (0.34 mmol) at 200 °C about 20 min in 5 mL of oleylamine solution. MnCl2 was added and reacted for another 5 min at 250 °C. Then, the mixture was heated to 300 °C, and 6.6 mmol of DDT and 6 mL of zinc precursor were injected immediately into the above mixture for shell coating. After 45 min of reaction, we obtained InP/ZnMnS core/shell QDs. After purification of the QDs, the ZnS shell was continued to cover by injecting Zn(OA)2 and S-TOP. It can be seen from TEM that the size of InP/ZnMnS QDs is 5.3 nm; after coating the ZnS shell, the size increases to 7.2 nm (see Figure ). By coating the ZnS shell, the energy transfer between QDs is reduced and the stability of QDs can be improved at the same time.[34,35] Finally, we obtained InP/ZnMnS/ZnS core/shell/shell QDs. Using these InP/ZnMnS/ZnS QDs, we fabricated QLED, which consists of indium tin oxide (ITO)/PEDOT:PSS (30 nm)/PVK (30 nm)/QDs (40 nm)/ZnMg1–ONPs (40 nm)/Al (100 nm).

Figure 1

TEM images of (A) InP/ZnMnS QDs and (B) InP/ZnMnS/ZnS QDs. Size distribution histograms of (C) InP/ZnMnS QDs and (D) InP/ZnMnS/ZnS QDs.

For the control of the QD size, most research studies adjust the size of QDs by controlling the reaction temperature, reaction time, or ratio of precursors. In this work, we adjust the size of QDs by controlling different halogens of the precursor. Due to the halogen ions have strong coordination ability with indium ions, the InP monomer can combine with a halogen to form an intermediate during the reaction, and then the intermediate can form the InP core by dehalogenation. Since different halogens have different coordination abilities with the intermediate, therefore, different halogen ions can be used to adjust the InP monomer formation rate.[11] Finally, the InP core with different particle sizes can be obtained by changing different halogen ratios.[25] By changing the ratio of I, Br, and Cl, the wavelength of InP QDs can be adjusted from 470 to 620 nm (see Figure ). The FWHM of blue, green, and red QDs are 47, 62, and 81 nm, respectively, and QY can reach 38, 80, and 41%, respectively. It is more effective and simpler than traditional methods to obtain QDs with different sizes by controlling reaction time and temperature, so this method has the potential to realize the mass production of high-quality InP QDs.

Figure 2

(A) PL spectra of InP/ZnS QDs with different ratios of I, Br, and Cl. (B) PL spectra of InP/ZnMnS QDs with varying Mn2+ contents.

After synthesizing the InP core, the temperature was raised to 250 °C and MnCl2 was added for reaction for 5 min. During this process, MnCl2 reacted with OLA to form the Mn precursor. Then, the temperature was raised to 300 °C, and the Zn precursor (zinc stearate mixed with ODE) and S precursor (DDT) were added to coat the shell. In this process, the S precursor reacted with Zn and Mn precursors to form the alloyed ZnMnS shell, and the content of Mn2+ is measured by ICP-MS. It can be seen from ICP-MS that the amount of added Mn2+ is different from that of measured, which is mainly due to the different reactivity of each precursor, and the final product proportion is far from that of actual addition. With the continuous addition of MnCl2, the amount of Mn2+ in the shell gradually increases, when the added MnCl2 exceeds 1.1 mmol, the amount of Mn2+ in the shell will exceed 14% (see Table ), and the distance between the Mn2+ is decreasing gradually. When the distance between Mn2+ is less than the critical distance, energy transfer will occur between them, that is, from one center to the next, until they enter a quenching center, resulting in the quenching of luminescence.[26,27] For InP/ZnMnS QDs (see Figure ), the ion diffusion will occur at high temperature. Because the valence of Mn2+ is different from that of In3+ and the radius of Mn2+ is 0.66 Å, it is easier to replace Zn2+ (0.74 Å) than In3+ (0.8 Å), and it is difficult for Mn2+ to diffuse to the core InP. The maximum doping amount of Mn2+ in the InP core is only 0.07% (24 h reaction at 260 °C) according to previous reports.[28] For this experimental condition (45 min reaction at 250 °C), it is difficult for Mn2+ to diffuse to the core.[27] The luminescence intensity of Mn2+ is gradually increase with the increase of shell Mn2+ content (see Figure ). When the amount of Mn2+ added exceeds 3.3 mmol, the emission peak intensity of Mn2+ remains unchanged. This is due to two reasons: one is due to the saturation of Mn2+ at the surface of the InP core, and the other is due to Mn2+ reaching the maximum doping level in the shell (see Table ).

Table 1

Ratio of Zn to Mn in InP/ZnMnS QDs Measured by ICP-MS

QDs	Zn/Mn feed ratio	Zn/Mn ratio measured in QDs	percentage of Mn in shell	τavg (ns)
InP/ZnS			0%	46.6
InP/Zn0.69Mn0.31S	0.5	2.23	30.9%	35.7
InP/Zn0.48Mn0.52S	1	0.92	52.1%	16.0
InP/Zn0.44Mn0.56S	1.5	0.79	55.8%	10.3
InP/Zn0.43Mn0.57S	2	0.75	57.1%	10.2

The PL peak of InP QDs is gradually redshifted with the increase of Mn2+ content. This is due to Mn2+ absorbs the light emitted by the InP core, leading to fluorescence resonance energy transfer (FRET) between the InP core and ZnMnS shell, and hence in the redshift of the core InP emission peak.[29−31] The fluorescence lifetime of InP QDs decreases gradually with the increase of Mn2+ (see Table ), the fluorescence lifetime is 46.6 ns for InP/ZnS QDs, with increasing Mn2+ in the shell, and the lifetime of fluorescence decreases gradually. When the content of Mn2+ reaches 57.1% of the maximum doping content, the fluorescence lifetime decreases to 10.2 ns, and this change is consistent with the peak redshifted of InP QDs (see Figure ). It is because the distance between the core InP and the shell Mn2+ is less than 10 nm, and the green light emitted by InP can be absorbed by the shell Mn2+, resulting in energy transfer from the InP core to the shell.[34] We estimate the efficiency for InP to Mn2+ (InP/Zn0.48Mn0.52S, the case in Figure B) to be E = 1 – τdonor – acceptor/τdonor = 1 – 15.96/46.65 = 65.79%. Because the luminescence of Mn2+ in QDs belongs to the discrete center luminescence,[32] it is generated by the electron transition in the 3d5 orbital (4T1-6A1),[32] so the emission of Mn2+ has no significant correlation with its location (see Figure ).The emission peak remains unchanged with the increase of the Mn2+ doping amount.

Figure 3

Time-resolved photoluminescence lifetimes of InP/ZnMnS QDs with different shell structures.

Figure 4

Luminescence process of Mn2+ dopants.

It can be seen from film XRD patterns that for InP/ZnS QDs, the three strong peaks are 28.56°, 47.52°, and 56.29 °, which correspond to the X-ray diffraction peaks of ZnS. With the increase of Mn2+, the peak position of ZnS is shifted slightly (see Figure ). This is because Mn2+ enters the lattice of ZnS, resulting in a shift in the peak position.[32,33] From the element mapping results of InP/ZnMnS QDs (see Figure ), it can be seen that Mn2+ enter QDs, which are consistent with the characterization of film XRD patterns.

Figure 5

Film XRD patterns of the sample with different Mn2+ contents.

Figure 6

Element mapping results of InP/ZnMnS QDs.

It can also be seen by lattice parameters of QDs that Mn2+ was incorporated into QDs (see Figure ). The lattice parameter of InP/ZnS is 3.12 Å, which is matched with the lattice parameter of ZnS (3.123 Å). When Mn2+ was added, the lattice parameter of InP/ZnMnS QDs is 3.20 Å, which is between MnS (3.24 Å) and ZnS (3.123 Å), indicating that Mn2+ was doped into QDs.

Figure 7

HRTEM images of (A) InP/ZnS QDs and corresponding lattice fringes and (B) InP/ZnMnS QDs and corresponding lattice fringes.

Due to the narrow band gap of red InP QDs, the ZnSe, ZnS, or MnS shell can confine electrons and holes in the core completely, and the peak positions of InP QDs are all 610 nm for these three kinds of shell structures. But for green and blue InP QDs, only ZnS or MnS shell with the wide band gap can restrict electrons and holes in the core completely. However, for the ZnSe shell, its lowest conduction band energy level is lower than that of InP, which results in the electrons in the core transition to the shell layer and leads to the redshift of the PL spectrum. The green InP QDs are redshifted from 507 to 558 nm, and the blue InP QDs are redshifted from 470 to 525 nm (see Figure ). As an intermediate layer, ZnMnS can reduce the lattice mismatch between the core InP and shell ZnS, thus reducing the defects caused by lattice mismatch and improving the luminescence efficiency of QDs. QY can increase from 60% of InP/ZnS to 80% of InP/ZnMnS/ZnS QDs.[21]

Figure 8

Band gap and lattice mismatch of InP, ZnSe, ZnS, and MnS material. (A) Red InP QDs, (B) green InP QDs, and (C) blue InP QDs. (D–F) The photoluminescence spectra of InP QDs with different shells.

Using these InP/ZnMnS/ZnS QDs, we fabricated QLED, which consists of indium tin oxide (ITO)/PEDOT:PSS (30 nm)/PVK (30 nm)/QDs (40 nm)/ZnMg1–ONPs (40 nm)/Al (100 nm). The aluminum cathode layer is deposited by vacuum thermal evaporation, and other layers are obtained by spin coating. PEDOT:PSS was spin-coated onto ITO glass and heat-treated at 130 °C for 15 min and then PVK, QDs were spin-coated layer by layer, the PVK layer was heat-treated at 120 °C for 15 min, and QDs layer were heat-treated at 100 °C for 5 min. The ZnMgO nanoparticle was spin-coated and then heat-treated at 80 °C for 20 min; after that, the device was transferred into a vacuum chamber, and Al (100 nm) electrodes were deposited on the device.

Figure A show the energy level diagram for the QLED we tested using these InP QDs. The Tauc plot of InP/ZnMnS/ZnS was measured by ultraviolet–visible absorption spectra. The band gap of InP/ZnMnS/ZnS (2.3 eV) QDs can be calculated by the intercept between the tangent and abscissa of the absorption spectrum. The CBM and VBM values of InP/ZnMnS/ZnS QDs were determined to be −5.1 and −7.4 eV by an ultraviolet photoelectron spectrometer (UPS), respectively (see Figure S2, Supporting Information). Figure B shows the PL and EL spectrum of the InP/ZnMnS/ZnS QDs. The peak of InP/ZnMnS/ZnS QLED is at 517 nm. There is a redshift of QLED compared with the PL peak at 513 nm, which is due to the Förster energy transfer in the InP/ZnMnS/ZnS QLEDs films. Figure C shows that when the voltage is 6 V, the maximum brightness can reach 420 cd/m2, and when the voltage is 8 V, the maximum current density can reach 105 mA/cm2. The maximum EQE can up to 2.7% (see Figure D).

Figure 9

(A) Energy level diagram for InP/ZnMnS/ZnS QLED. (B) PL and EL spectrum of QDs. (C) Current density and luminance versus voltage. (D) CE and EQE versus current density.

Conclusions

In summary, covering the whole visible range, InP/ZnMnS/ZnS QDs were synthesized by controlling the ratio of I to Cl. The wavelength of InP QDs can be adjusted from 470 to 620 nm. Because the valence of Mn2+ is different from that of In3+ and the radius of Mn2+ (0.66 Å) is different from that of In3+ (0.8 Å), it is difficult for Mn2+ to diffuse to the core InP. Due to the luminescence of Mn2+ in QDs belongs to the discrete center luminescence, it is generated by the electron transition in the 3d5 orbital (4T1-6A1), so the emission of Mn2+ has no significant correlation with its location. At the same time, XRD and mapping characterization were used to determine whether Mn2+ was doped into the shell. For the ZnSe shell, the lowest conduction band level is lower than that of green and blue InP QDs, which results in the transition of the core InP electrons to the shell and the redshift of the emission spectrum. For the ZnMnS shell, the lowest conduction band level of ZnMnS is higher than that of the core InP, and the electrons can be confined in the core and no redshift of the PL spectrum. As an intermediate layer, ZnMnS can reduce the lattice mismatch between the core InP and shell ZnS, thus reducing the defects caused by lattice mismatch and improving the luminescence efficiency of QDs. QY can increase from 60% of InP/ZnS to 80% of InP/ZnMnS/ZnS QDs. The maximum EQE of this QLED fabricated from this Cd-free green light QDs can reach 2.7%.

Experimental Section

Chemicals

InBr3 (99%), InCl3 (99.9%), tris(dimethylamino)phosphine ((DMA)3P, 97%), and ZnBr2 (99.9%) were purchased from Aldrich. Zinc stearate (Zn 10–12%), 1-octadecene (ODE, 90%), 1-dodecanethiol (DDT, 98%), oleylamine (OLA, 80–90%), MnCl2 (99.99%), ZnCl2 (99.9%), oleic acid (OA 90%), zinc oxide (99.999%), and trioctylphosphine (97%) were purchased from Aladdin.

Synthesis of Zn and S Precursors

In a typical reaction, Zn(OA)2 (0.2 M) was synthesized by mixed ZnO, OA (1:4 molar ratio), and ODE. The mixture was degassed for 60 min at 150 °C and then heated at 310 °C until the solution turned clear.

S-TOP (0.2 M) was made by adding pure S powder in trioctylphosphine (TOP) and heated at 100 °C until the solution turned clear.

Synthesis of InP Core with Different Diameters

InBr3 (0.34 mmol, 120 mg), x mmol of ZnBr2, 2.2 – x mmol of ZnCl2, and 5 mL of OLA were added into a 50 mL three-necked bottle. The mixture was degassed for 60 min at 140 °C to remove water and oxygen. Then, the reaction was flooded with Ar and further heated to 200 °C, and 0.45 mL (2.4 mmol) of (DMA)3P mixed with 1 mL of OLA was injected into the reaction solution quickly and reacted for 20 min. Different ratios of ZnBr2:ZnCl2 got QDs with different emission wavelengths. Specific results are presented in the Results and Discussion section.

Synthesis of InP/ZnMnS QDs (515 nm)

InBr3 (0.34 mmol, 120 mg), 2.2 mmol (495 mg) of ZnBr2, and 5mL of OLA were added into a 50 mL three-neck flask. The mixture was degassed for 60 min at 140 °C to remove water and oxygen. Then, the reaction was flooded with Ar and further heated to 200 °C, 0.45 mL (2.4 mmol) of (DMA)3P mixed with 1 mL of OLA was injected into the reaction solution quickly and reacted for 20 min, and 2.2 mmol (275 mg) of MnCl2 was added and reacted for another 5 min at 250 °C. Then, the solution was heated to 300 °C and injected with 6.6 mmol (1.5 mL) of DDT and 6 mL of zinc precursor (1.5 g of zinc stearate mixed with 6 mL of ODE) immediately to coat the shell; after reacting for 45 min, the three-necked bottle was cooled down to room temperature. The obtained InP/ZnMnS QD solution was mixed with 10 mL of hexane and centrifuged at 10,000 rpm for 3 min to remove the impurities and unreactants, the obtained supernatant was mixed with ethanol 1:1 and centrifuged at 10,000 rpm for 3 min, the obtained precipitate was dissolved in hexane, and the purification process was repeated above three times. We could get InP/ZnMnS QDs.

Synthesis of InP/ZnMnS/ZnS QDs (515 nm)

The purified InP/ZnMnS QDs and 5 mL of OLA were added into a 50 mL three-neck flask, and the mixture was degassed for 60 min at 140 °C to remove water, oxygen, and hexane. Then, the reaction was flooded with Ar and further heated to 250 °C; when the temperature reached 250 °C, it was injected with 2.2 mmol of S-TOP and 2.2 mL of zinc precursor immediately to coat the shell for 60 min and then the reaction was stopped. The purification process is the same as the InP/ZnMnS QDs.

Device Fabrication

Using these InP/ZnMnS/ZnS QDs, we fabricated QLED, which consists of indium tin oxide (ITO)/PEDOT:PSS (30 nm)/PVK (30 nm)/QDs (40 nm)/ZnMg1–ONPs (40 nm)/Al (100 nm). The aluminum cathode layer is deposited by vacuum thermal evaporation, and other layers are obtained by spin coating. ITO glass was cleaned with toluene, acetone, deionized water, and isopropanol under ultrasonication successively for 20 min and then ultraviolet (UV)-light treated for 20 min before usage. PEDOT:PSS (Baytron PVP Al 4083, filtered through a 0.45 mm filter) was spin-coated onto ITO glass at 4000 rpm for 45 s and heat-treated at 130 °C for 15 min and then PVK (dissolve in chlorobenzene, 8 mg/mL), QDs (dissolve in octane, 10 mg/mL) were spin-coated layer by layer at 4000 rpm for 45 s, the PVK layer was heat-treated at 120 °C for 15 min, and QDs layer were heat-treated at 100 °C for 5 min. The ZnMgO nanoparticle (dissolve in ethanol, 20 mg/mL) was spin-coated at 3000 rmp for 45 s and then heat-treated at 80 °C for 20 min; after that, the device was transferred into a vacuum chamber, and Al (100 nm) electrodes were deposited on the device under a vacuum level of 5×10–4 Torr with a speed of 0.5–1 nm/s. The active area of the device was approximately 0.04 cm2.

Characterization of Materials and Devices

The TEM, XRD, and PL spectrum characterization can refer to our previous work.[40] The QY were taken using a Hamamatsu Quantaurus-QY, model no. C11347. PL lifetime measurements were carried on a Fluo Time 300 “Easy Tau” fluorescence lifetime spectrometer. Mapping measurements were carried on an FEI Talos F200S. The characterization of devices can refer to our previous work.[37−39]