Open Access
Add to BibSonomy
Abstract
In this paper, a InP/ZnSe/ZnS quantum dots photoresist (QDPR) film has been successfully prepared after the as-synthesized InP/ZnSe/ZnS quantum dots (QDs) were mixed with the photoresist, where the molar ratio of P3- : In3+: Se2- was 6:1:3 and the reaction time the ZnS shell was 60 min. The influence of the thickness of the film and the mass ratio of InP/ZnS QDs to photoresist on the photoluminescence quantum yield (PLQY) was investigated. The results show that the PLQY changes from 39.9% to 52.6% and the CIE color coordinates could vary from (0.28, 0. 10) to (0.65, 0.32) with the assistance of 5.5 pairs of DBR structures when the thickness of the InP/ZnSe/ZnS QDPR films is 7.82 µm and the mass ratio (wt%) of the InP/ZnSe/ZnS QDs to the photoresist is 25%. Compared with the InP/ZnSe/ZnS QDs film, the PLQY of the patterned InP/ZnSe/ZnS QDs layer can reach to be 47.2% and the CIE color coordinates of is varied from (0.28, 0.10) to (0.35, 0.14). Furthermore, the PLQY of the patterned InP/ZnSe/ZnS QDs layer can be improved to 65.1% at 5.5 pairs of DBR structures. Based on these results, the InP/ZnSe/ZnS QDs films, especially patterned QD layer, show great potential for the fabrication of high-quality QD color filter and full-color displays.
© 2022 Optica Publishing Group under the terms of the Optica Open Access Publishing Agreement
1. Introduction
Quantum dots (QDs) are a kind of nano-sized semiconductor crystals and their physical form and particle size have a strong modulating effect on their own electronic structure. QDs have attracted great attention due to their excellent photoelectric characteristics [1–3]. In the past two decades, the synthesis and characterization of Cd-based QDs is one of the major advances because they have superior photoluminescence (PL) characteristics, including high quantum yield (QY), narrow full width at half maximum (FWHM) and easy band gap energy by controlling their sizes and shapes [4,5]. Moreover, these Cd-based QDs can play an important role in numbers of technological applications, such as the light-emitting diodes and color conversion [6,7], photodetector [8] and nanocavity [9]. However, with the practical application of QDs in commercial products, the demand has shifted the focus from the Cd-based QDs with high toxicities to non-toxic, environmentally friendly QDs [10,11]. Indium phosphide (InP) QDs is of particular interest due to its size-tunable emission over the visible and near-infrared spectral range (bulk band gap energy ∼1.35 eV) and lower intrinsic toxicity [12,13]. Compared with Cd-based QDs, pure InP QDs suffer from synthetically induced broad size distributions, low photoluminescence (PL) quantum yields (QYs) and poor environmental stability although significant attention has been given to improve these aspects of InP QDs [14,15]. Compared to the bare InP QDs, the InP/ZnS core-shell QDs have higher PLQY and outstanding stability [16]. Due to the lattice mismatch between the InP core and ZnS shell, the photoluminescence-full width at half maximum (PL-FWHM) and PLQY of InP/ZnS QDs is much lower than that of Cd-based QDs [17]. Therefore, low-toxic and inexpensive InP/ZnSe/ZnS QDs with good optical performance and stability can be synthesized after the ZnSe intermediate shell as a lattice mediator is grown between InP core and ZnS shell [18–21].
In recent years, the composite gradient ZnSe is inserted between ZnS and InP as the intermediate shell to enhance the optical performance of the quantum dot. At the same time, InP/ZnSe/ZnS is the most promising structure in the construction of lighting and display devices [22,23]. Xiao et al. synthesized high-quality InP/ZnSe/ZnS QDs with a half-peak width of 42 nm and a QY of 93% by controlling the stoichiometry [24]. Kim et al. prepared bright and uniform InP/ZnSe/ZnS QDs by optimizing the reaction parameters of InP nucleus synthesis and growing a uniform ZnSe/ZnS shell layer at 320°C [20]. In 2020, Lai et al. inserted an intermediate ZnSe shell between the InP core and the outermost ZnS shell to achieve triplet excited quantum dots [25]. Han et al. proposed a red-green-blue micro-LED array based on quantum dots full-color luminescence, using quantum dots as a color transfer layer to prepare an anti-crosstalk barrier layer, combined with a DBR structure, to enhance the luminous efficiency [26]. In 2020, Liang's research group used InP QDs as the inverted color conversion layer to successfully realize the full-colorization of micro-LED array devices [27]. Recently, Han et al. reported a uniformly mixed InP/ZnSe/ZnS QDs with improved hole transport performance, thereby improving the efficiency and lifetime of LED devices [28]. These results indicate that the InP/ZnSe/ZnS QDs can synthesize quantum dots with high performance and great stability. Compared with traditional display methods, preparing quantum dots into a color conversion film can reduce the energy loss of the device and have good photoelectric performance.
Patterned QDs films are of great significance for the manufacture of next-generation full-color displays, full-color imaging, black light units, color filters, color conversion [29–37]. At present, several methods are used to make patterned quantum dot films, such as photolithography, inkjet printing, spray coating and so on [38–40]. In 2017, Lin et al. deposited a photoresist (PR) mold between the layers of each color to reduce the optical crosstalk between them [39]. In 2020, Hu et al. can change the light conversion efficiency (LCE) from blue to green and red by changing the thickness of the QD layer [41]. Wu et al. fabricated a patterned InP/ZnS quantum dots photoresist (QDPR) film via a laser-assisted route and investigated the effects of the thickness of the film and the mass ratio of InP/ZnS QDs to photoresist on the photoluminescent quantum yield (PLQY) to confirm the potential application of InP/ZnS QDPR film in color conversion [42]. These results show the potential applications of the patterned QDs in light-emitting sources, especially in the photo-emissive color conversion field.
In this work, we have synthesized the InP/ZnSe/ZnS QDs by optimizing the reaction parameters for the InP core synthesis and the growth of uniform ZnSe and ZnS shells. The injection temperature of the P source was set to 180°C to control the reaction rate and form uniform InP cores. To prevent the anisotropic shell growth, the ZnSe and ZnS shell was performed at high temperature of 300°C, which is beneficial to form highly crystalline and well passivating layer. A InP/ZnSe/ZnS QDs photoresist (QDPR) film was prepared via a spin-coating method after the as-synthesized InP/ZnSe/ZnS QDs solution was mixed with the photoresist. And then, the InP/ZnSe/ZnS QDs QDPR solution was inkjet into the matrix metal bank by using an inkjet printing method and form the patterned InP/ZnSe/ZnS QDPR layer. The effects of the QDPR film thickness and the mass ratio of InP/ZnSe/ZnS QDs solution to the photoresist on the PLQY were measured, where the InP/ZnSe/ZnS QDPR film was used as the color conversion layer and blue LEDs was used as the excitation light source. The PLQY improvement was also investigated with the assistance of 5.5 pairs of DBR structures to confirm the potential application for the fabrication of high-quality QD color filter and full-color displays.
2. Experimental method
2.1 As-synthesized InP/ZnSe/ZnS QDs
Figure 1 depicts the schematic illustration of nontoxic InP/ZnSe/ZnS QDs by a thermal injection method. The experimental process mainly includes three steps: InP QDs colloidal solution, and InP/ZnSe QDs colloidal solution and InP/ZnSe/ZnS QDs solution. First, InCl3 and ZnCl2 are added to a three-necked flask and completely mixed with oleylamine (OLA) by stirring at 140 °C for 0.5 h under nitrogen flow. When the temperature of the mixture solution was reached to 180 °C, the mixture with Tris (dimethylamino) phosphine (P[N(CH3)2]3) and OLA was rapidly injected the three-necked flask. After reacting for 0.5 h at 180°C, the three-necked flask was immediately cooled to room temperature and the InP QDs colloidal solution was obtained. Second, the mixture with Zn(ST)2 and 1-Octadecene (ODE) was injected into the three-necked flask after Se precursor solution mixed with Se and Tri-N-Octylphosphine (TOP) was injected into the three-necked flask. After reacting for 0.5 h at 300°C, the three-necked flask was immediately cooled down to room temperature and the InP/ZnSe QDs colloidal solution was prepared. Thirdly, the mixture with Zn(OAc)2•2H2O, OLA and 1-Dodecanethiol (DDT) was slowly injected into the three-necked flask. After reacting for 1 h at 300°C, the three-necked flask was immediately cooled to room temperature and the InP/ZnSe/ZnS colloid solution was produced. Finally, InP/ZnSe/ZnS QDs was acquired by the centrifugation after adding toluene and ethanol into the colloid solution.
2.2 Preparation of InP/ZnSe/ZnS QDPR film
InP/ZnSe/ZnS QDPR film were prepared by using a spin-coating method. Firstly, the as-synthesized InP/ZnSe/ZnS QDs was put into the centrifuge tube and the ethanol was added to the QDs solution, where the volume ratio of the QDs solution to ethanol was 1:3. After standing and layering, the InP/ZnSe/ZnS QDs were obtained by centrifuging at a high speed of 12,000 rpm for 8 min. Secondly, the InP/ZnSe/ZnS QDs were completely mixed and dissolved with toluene and were obtained InP/ZnSe/ZnS QDs toluene solutions after sonicating for 10 min. The InP/ZnSe/ZnS QDs toluene solutions were injected into the photoresist by a syringe and InP/ZnSe/ZnS QDPR solutions were obtained after stirring for 30 min by the agitator. Finally, the InP/ZnSe/ZnS QDPR film was successfully prepared via a spin-coating method at the room temperature after dropping the InP/ZnSe/ZnS QDPR solution on a clean glass substrate.
2.3. Structural and optical characterization
The PL spectra and UV-vis absorption spectra of the QDs samples were investigated from a diluted aqueous solution in a quartz cuvette at a room temperature via a fluorescence spectrophotometer (Shimadzu, F-4600) and a spectrophotometer (Hitachi, UV-3600), respectively. High resolution transmission electron microscopy (HRTEM) was carried out a Tecnai G2 F20 S-Twin TEM (American, FEI) with an operating voltage of 200 kV. X-ray difraction (XRD) spectrum of QDs sample were taken on a Bruker AXS D8-advance X-ray difractometer with Cu Kα radiation (λ =1.54 Å). The chemical bonds of the QDs sample were measured using a Fourier transform infrared spectrometer (FT-IR, Nicolet is50). The elemental characterization of QDs sample was analyzed via the energy dispersive X-ray spectrometer (EDS) (ESCALAB 250, American, FEI). X-ray photoelectron spectroscopy (XPS) was obtained by using a Kratos Axis Ultra X-ray photoelectron spectrometer with Al Kα (1486.6 eV) with the X-ray excitation source. The microscopic morphology of the patterned QDPR sample was measured by optical microscopy (Olympus, OLS-3000/4000). The color coordinate of the QDs sample was characterized using a spectrometer (Everfine, SRC-200). The PLQY of QDPR sample excited by blue LEDs was measured using a fluorescence spectrometer with an integrating sphere (FLS980, Edinburgh) at room temperature, as illustrated for the proposed report [16].
3. Results and discussion
Figure 2(a) depicts the molar ratios of P3-/In3+ versus the PL spectra of as-synthesized InP/ZnSe/ZnS QDs and the inset presents the corresponding fluorescence images under UV excitation with 365 nm wavelength, where the molar ratio of Se2-/In3+ is 3:1 and the reaction time of ZnS shell is 60 min. With the increase of P3- source as a precursor solution, the excessive P(N(CH3)2)3 can effectively improve the formation of InP core and the PL intensities gradually increase. UV-vis absorption spectra in Fig. 2(b) presents a clear red-shift, which mainly attributes to the size effect of QDs because the size of InP/ZnSe/ZnS QDs gradually increases and the bandwidth become smaller. As the molar ration of P3-/In3+=6:1, the PL intensity of the emission peak located at 633 nm become strongest and the PL-FWHM is 39.5 nm. When the molar ration of P3-/In3+ is higher than 6:1, the InP core continues to grow, resulting that the PL-FWHM becomes wider and the PL intensity gradually decreases due to the core-shell mismatch.
Fig. 2. Optical properties of as-synthesized InP/ZnSe/ZnS QDs: (a) PL spectra at various P3-/In3 + molar ratios and the inset presents the corresponding fluorescent image; (b) UV-vis absorption spectra versus P3-/In3 + molar ratios; (c) PL spectra at different Se2-/In3 + molar ratios and the inset depicts the corresponding fluorescent image; (d) UV-vis absorption spectra versus Se2-/In3 + molar ratios; (d) PL spectra at different reaction times of ZnS and the inset shows the corresponding fluorescent image and (f) UV-vis absorption spectra versus ZnS reaction time.
The PL spectra and UV-vis absorption spectra of as-synthesized InP/ZnSe/ZnS QDs at different Se2-/In3+ molar ratios are shown in Fig. 2(c) and Fig. 2(d), respectively. The inset in Fig. 2(c) depicts the corresponding fluorescence image under UV excitation. As the Se2-/In3+ molar ratios increase, it is beneficial to improve the growth of the ZnSe shell. With the assistance of 1-octadecene (ODE), the ZnSe shell is induced to grow on the surface of the InP core, which can reduce the defects of InP QDs and improve its optical performance. When Se2-/In3+ molar ratio is 3:1, the as-synthesized InP/ZnSe/ZnS QDs have strong PL intensity and small PL-FDHM. With the increase of Se2-/In3+ molar ratios, the continual growth of ZnSe shell may cause the core-shell mismatch and result in redundant defects. Moreover, the Se on the surface of QDs is easily oxidized and many defects of non-radioactive binding is generated due to the increase of Se2- content [43]. So, the PL intensity of InP/ZnSe/ZnS QDs decreases and the PL emission peak shows a slight red-shift, as shown in Fig. 2(d).
Figure 2(e) presents the effect of the reaction time on the PL spectrum of as-synthesized InP/ZnSe/ZnS QDs and the inset is the corresponding emission images under UV excitation, where the reaction time was varied from 30 min to 120 min and the molar ration of P3- : Se2+: In3+=6:3:1. The PL peak intensity is the strongest and the half width of the emission peak is the smallest at the reaction time of 60 min due to the growth of ZnS shell. When the growth time of ZnS shell is less than 60 min, the surface passivation of the InP/ZnSe/ZnS QDs may be incomplete and the PL-FWHM becomes large due to the uneven particle size. With the growth time of ZnS shell, the ZnS shell becomes thicker and the bonding of the core-shell interface is mismatched. Moreover, the size of InP/ZnSe/ZnS QDs becomes large because the monomers in the solution are completely consumed and the small size QDs are dissolved, which results that PL peaks occur a red-shift. Figure 2(f) shows the corresponding UV-Vis absorption spectra of the as-synthesized InP/ZnSe/ZnS QDs under various reaction times. The absorption peak shows a slight blue-shift in emission due to the growth of the ZnS shells as the reaction time is higher than 60 min. The phenomenon may attribute to the energy-band shift of the ZnS shell because of the diffusion of S source into Se-rich region.
HRTEM and selected area electron diffraction (SAED) were performed to characterize the morphology and crystal phase of the as-synthesized InP/ZnSe/ZnS QDs samples, where the molar ratio of P3- : In3+: Se2- is 6:1:3 and the reaction time of ZnS shell was 60 min. The HRTEM image in Fig. 3(a) shows that the InP/ZnSe/ZnS QDs possess a nearly spherical shape and QDs samples have good dispersion and uniformity. The particle size distribution histogram in Fig. 3(b) further confirms that InP/ZnSe/ZnS QDs have sizes ranging between 11 nm and 12 nm. The inset.1 presents a high magnification image of boxed area in Fig. 3(a), which indicates that InP/ZnSe/ZnS QDs with an interplanar distance of d = 0.318 nm has great crystallinities and basically assign to the literature value for the (111) plane. These results confirm that the as-synthesized InP/ZnSe/ZnS QDs belong to the zinc-blende phase and possess a crystalline nature. The corresponding SAED pattern (Inset. 2) in Fig. 3(a) depicts three concentric rings corresponding to the (111), (220) and (311) plane.
Fig. 3. (a) HRTEM image of InP/ZnSe/ZnS QDs. Inset 1 presents a high magnification image of boxed area and Inset 2 shows its corresponding SAED image; (b) Distribution histogram of the particle size; and (c) XRD pattern of InP/ZnSe/ZnS QDs.
Figure 3(c) presents the XRD pattern (black solid line) of the as-synthesized InP/ZnSe/ZnS QDs, where red, green and blue solid line present three diffraction peaks of standard cubic zinc blende structure, corresponding to ZnSe (JCPDS 80-2220), InP (JCPDS 32-0452) and ZnS (JCPDS 05-0566), respectively. Three characteristic peaks are located at 27.5°, 45.9° and 54.5° and corresponded to (1 1 1), (2 2 0) and (3 1 1) plane, respectively. Compared with the XRD pattern (black) of InP/ZnSe QDs, the diffraction peaks of InP/ZnSe/ZnS QDs transfer to higher angle and become stronger. In addition, no extra diffraction peaks indicate that the ZnS shell is successfully grown on InP/ZnSe QDs.
Figure 4 indicates a high-resolution energy dispersive X-ray (EDX) chemical map of the InP/ZnSe/ZnS QDs. The map reveals in great details the location of the core and shell elements of the particle and clearly reveals that the InP core tends to be centered in a symmetrically grown ZnSe and ZnS shells. EDX spectra in Fig. 4(a) presents that the relative atomic percentages of In, P, Zn, Se and S are approximately 3.78%, 33.51%, 49.75%, 10.07% and 2.89%. Figure 4(b) presents the high-angle annular dark-field (HAADF) image of InP/ZnSe/ZnS QD and the corresponding scanning transmission electron microscopy (STEM) EDX maps are shown in Fig. 4(c)-(g), whereas Zn, Se and S elements are homogeneously covered on the surface of InP core
Fig. 4. Elemental characterization of InP/ZnSe/ZnS QDs: (a) EDX spectra of the InP/ZnSe/ZnS QDs and the relative atomic percentages: In: 3.78%, P: 33.51%, Zn: 49.75%, Se: 10.07%, S: 2.89%; (b) HAADF image of InP/ZnSe/ZnS QD and STEM maps of (c) Phosphorus, (d) Indium, (e) Sulfur, (f) Selenium and (g) Zinc.
The InP/ZnSe/ZnS QDs were further demonstrated by XPS spectrum, as shown in Fig. 5(a) and the high-resolution XPS spectra of In-3d, P-2p, Zn-2p, Se-3d and S-2p are present in Fig. 5(b)-(f). The deconvoluted core-level spectrum of In-3d is shown in Fig. 5(b). The bivalent In3+ state is confirmed by peaks at 451.38 eV (3d3/2) and 443.90 eV (2p5/2) with its characteristic peak separation of 7.48 eV, corresponding to that of In3+ [44]. The P core-shell spectrum in Fig. 5(c) has been split into P-2p1/2 and P-2p3/2 located at 128.71 eV and 127.80 eV, respectively and their spin-orbit splitting energy is approximately 0.9 eV, which indicate that the chemical state of P element is −3 [45]. The Zn core-level spectrum in Fig. 5(d) has been split into Zn-2p1/2 (1044.91 eV) and Zn-2p3/2 (1021.86 eV) with a spin-orbit splitting energy of 23.05 eV, implying that the chemical state of Zn is +2 [46]. The Se-3d core-level spectrum in Fig. 5(e) is deconvoluted into two Gaussian components located at 54.73 eV and 53.85 eV, which is corresponding to the characteristics peak of 3d3/2 and 3d5/2, respectively. Their energy splitting is approximately 0.88 eV, which is corresponds to that of Se2- [47]. The deconvoluted core-level spectrum of S-2p in Fig. 5(f) presents a spin-orbit doublet composed of S-2p1/2 and S-2p3/2. The bivalent S2- state is confirmed by peaks at 161.69 eV (2p1/2) and 160.23 eV (2p3/2) and their energy splitting of 1.46 eV is mainly attributed to the Zn-S bond. In addition, S-2p spectrum has been split into S-2p1/2 (166.69 eV) and S-2p3/2 (165.73 eV) with a spin-orbit splitting energy of 0.96 eV, implying that the chemical state of S is oxidized [48].
Fig. 5. (a) XPS spectrum of InP/ZnSe/ZnS QDs; (b) XPS spectrum of In-3d; (c) XPS spectrum of P-2p; (d) XPS spectrum of Zn-2p; (e) XPS spectrum of Se-3d and (f) XPS spectrum of S-2p.
To confirm the potential application of InP/ZnSe/ZnS QDPRs film in color conversion, a blue LEDs is used to excite the InP/ZnSe/ZnS QDPRs film with different thicknesses and mass ratios (wt%) of InP/ZnSe/ZnS QDs to the photoresist. The photoluminescence quantum yield (PLQY) is a key parameter to estimate the conversion efficiency of QDs film and can be calculated from the PL spectra by using the following equation:
Figure 6(a) shows the PL spectra of the InP/ZnSe/ZnS QDPR films, where the thickness of the InP/ZnSe/ZnS QDPR films changes from 2.37 µm to 7.82 µm and the mass ratio (wt%) of the InP/ZnSe/ZnS QDs to the photoresist is approximately 25%. As shown in Fig. 6(a), the PL intensities of the red-band emission gradually improve, and the PL intensities of the blue-band emission gradually decrease with the increase of the QDPR film thicknesses. Calculated by Eq. (1), the PLQY increases as the thickness of the InP/ZnSe/ZnS QDPR films varies from 2.37 µm to 7.82 µm, as shown in the inset in Fig. 6(a). In addition to thickness, the mass ratio also has a significant effect on color conversion. Figure 6(b) shows the PL spectra of the InP/ZnSe/ZnS QDPR films, where the mass ratio (wt%) changes from 10% to 25% and the thickness of the InP/ZnSe/ZnS QDPR films is approximately 7.82 µm. The PL intensities of the blue-band emission gradually decrease, and the PL intensities of the red-band emission and PLQY gradually improve with the increase of the mass ratio, whose variation trend is the same as Fig. 6(a).The PLQY can be enhanced by up to 39.9% and the Commission Internationale de l’Eclairage (CIE) color coordinates transferred to (0.28, 0.10) when the thickness of the QDPR film and the mass ratio (wt%) is approximately 7.82 µm and 25%, respectively. However, these results indicate that the blue lights from LEDs cannot be completely absorbed by the color conversion layer and pass through the InP/ZnSe/ZnS QDPR films even though the mass ratio (wt%) is approximately 25% and the thickness of the QDPR film is 7.82 µm. Therefore, some blue light is strongly leaked and the outgoing light is not exactly red light. Seen from the calculation formula of PLQY, the absorption of blue light should be reduced and the intensity of converted light should be increased in order to further improve the PLQY. According to the previous reports, increasing the thickness or the mass ratio of QDPR can improve the PLQY to some extent, but if the mass ratio is too high or the thickness is too thick, then the PLQY will fall, which is due to the self-absorption by the QDPR film [42,49]. Therefore, we choose to use the distributed Bragg reflector (DBR) structures to further improve the PLQY, where the thickness of the InP/ZnSe/ZnS QDPR film is fixed to 7.82 µm in our experiment.
Fig. 6. (a) PL spectra of InP/ZnSe/ZnS QDPR film with different thicknesses at the mass ratio (wt%) of 25%. The inset presents the film thickness versus PLQY based on color conversion; (b) PL spectra of InP/ZnSe/ZnS QDPR film with different mass ratios at the film thickness of 7.82 µm. The inset depicts the mass ratio (wt%) versus PLQY based the color conversion; (c) Total transmittance versus the number of DBR pairs; (d) Color conversion based on InP/ZnSe/ZnS QDPR film with 0 and 5.5 pairs of DBR structures; and (e) CIE Color coordinate of the InP/ZnSe/ZnS QDPR film with 0 and 5.5 pairs of DBR structures.
As known that the DBR structures are usually employed to control the management of the incident and outgoing lights. These DBR structures possess high reflectance in the exciting-band wavelength and the high transmittance in the emission-band wavelength [50,51]. In general, the DBR structures consist of two dielectric layers with high and low refractive indices, which is the same as photonic crystals with strong interference effects [52]. In our experiments, the stacked DBR structures are composed of alternate TiO2 and Al2O3 layer with high and low refractive indices, and the thicknesses of the stacked TiO2 and Al2O3 layers are 45 nm and 67 nm, respectively. Figure 6(c) indicates the effect of the number of DBR layers on the total transmission by using TFCalc software. The DBR structures with 3, 5, 7, 9, 11, and 13 layers are used as 1.5, 2.5, 3.5, 4.5, 5.5, and 6.5 pairs of DBRs. As shown in Fig. 6(c), the average transmittance is approximately 0.65% at 450 nm and the total reflectance in the range of 420-480 nm can reach 99.13% for 6.5 pairs of DBRs, while the average transmittance is close to 99.5% at ∼640 nm. These results indicate that 6.5 pairs of DBRs can effectively reflect the blue light of ∼450 nm and improve the transmittance in the wavelength of ∼640 nm.
Figure 6(d) shows the color conversion based on the InP/ZnSe/ZnS QDPR film excited by blue LEDs, where the thickness of QDPR film is 7.82 µm and the mass ratio (wt%) of the InP/ZnSe/ZnS QDs to the photoresist is 25%. As show in Fig. 6(d), the PLQY can be enhanced by up to 52.6% with the assistance of 5.5 pairs of DBR structures, which is much higher than that InP/ZnSe/ZnS QDPR film without the DBR structures. Because the DBR structures can obviously integrate the red lights from the color converter layer and strongly reduce the transmittance of the blue lights. Figure 6(e) presents CIE color coordinates of the InP/ZnSe/ZnS QDPR films with 0 DBRs and 5.5 pairs of DBRs. The CIE color coordinates changes from (0.28, 0. 10) and (0.65, 0.32) with the assistance of DBR structures, indicating that the fluorescent images can be transferred to a much deeper emission basically corresponded to the PL peak wavelength.
To improve the PLQY of the InP/ZnSe/ZnS QDPR film and its potential application in the full-color displays, a patterned InP/ZnSe/ZnS QDPR layer was successfully prepared by an inkjet printing method with the assistance of a matrix metal bank and its color conversion characteristics were also investigated.
The optical image of the matrix metal bank is shown in Fig. 7(a). The light-transmitting area of the matrix metal bank is 40 µm×40 µm (blue matrix) and the length and width of the adjacent matrix metal bank is 50 µm (red line), respectively. The InP/ZnSe/ZnS QDPR is deposited into the matrix metal bank by using the inkjet printing. The inset in Fig. 7(b) depicts the corresponding fluorescent images excited by UV lamp with the wavelength of 365 nm. Figure 7(b) shows the reflectance versus the metal film and the transmittance versus the matrix metal bank. In the visible light band, the reflectance of the metal film is close to 95% and the transmittance of the matrix metal bank is higher than 80%. These results indicate that the matrix metal bank (red matrix) has excellent shielding-light properties and prevents light-crosstalk between adjacent pixels form the patterned InP/ZnSe/ZnS QDPR layer. At the same time, the matrix metal bank can reflect the incident light and realize the reuse of blue light.
Fig. 7. (a) Optical image of the matrix metal bank; (b) Reflectance versus the metal film and the transmittance versus the matrix metal bank. The inset depicts the fluorescent images of the InP/ZnSe/ZnS QDPR layer excited by UV lamp with the wavelength of 365 nm; (c) Color conversion characteristics of patterned InP/ZnSe/ZnS QDPR layer; (d) CIE Color coordinate of the InP/ZnSe/ZnS QDPR layer.
Figure 7(c) depicts the color conversion characteristics of patterned InP/ZnSe/ZnS QDPR layer. With the assistance of the patterned matrix metal bank, the light intensity of the InP/ZnSe/ZnS QDPR film is significantly enhanced and the overall luminous color of the sample gradually shifts to long wavelengths. This phenomenon is mainly attributed to the fact that the metal matrix can reduce a reabsorption between QDs samples and improve the reuse of blue LEDs, which can effectively prevent the reduction of the light radiation power of QDs samples [53]. With the assistance of the matrix metal bank, the PLQY of the InP/ZnSe/ZnS QDPR film can change from 39.9% to 47.2% and the color conversion efficiency can be improved by up to 18.3%. Figure 6(d) presents the CIE color coordinates of the patterned InP/ZnSe/ZnS QDPR layer can be transferred to (0.35, 0.14). Of course, which can be improved with the assistance of 5.5 pairs DBRs. The PLQY of the patterned InP/ZnSe/ZnS QDPR layer can reach to be 65.1% and the CIE color coordinates can be changed from (0.35, 0.14) to (0.65, 0.32). These results show that the patterned QDs layer with the matrix metal bank can reduce the reabsorption and improve the brightness and efficiency of the color conversion layer. And then, it is beneficial to realize a controllable single pixel and reduce crosstalk between adjacent pixels, thus showing great potential for the fabrication of high-quality full-color displays.
4. Conclusion
In summary, InP/ZnSe/ZnS QDs have been successfully synthesized by using 1-DDT as S source and Zn acetate dihydrate as Zn source at the molar ratio of P3- : In3+: Se2- = 6:1:3 and ZnS reaction time of 60 min. The emission peak of InP/ZnSe/ZnS QDs is 633 nm and the PL-FWHMs are 39.6 nm, respectively. The as-synthesized InP/ZnSe/ZnS QDs belong to the zinc-blende phase and the particle sizes of QDs sample is 11∼12 nm. The PLQY of InP/ZnSe/ZnS QDPR films by the spin-coating route can reach to be 39.9% and the CIE color coordinates are located at (0.28, 0.10). However, the PLQY of the patterned InP/ZnSe/ZnS QDs layer by the inkjet printing method reach to be 47.2% and the CIE color coordinates is transferred to (0.35, 0.14). Moreover, the CIE color coordinates is transferred to (0.65, 0.32) at 5.5 pairs of DBR structures, and the PLQY of the InP/ZnSe/ZnS QDs film can be enhanced to 52.6% and the PLQY of the patterned InP/ZnSe/ZnS QDs layer can be improved to 65.1%. These results demonstrate that the InP/ZnSe/ZnS QDs show great potential for the high-quality color filter and next generation full-color displays.
Funding
National Natural Science Foundation of China (61775038, 61904031); Fujian Science & Technology Innovation Laboratory for Optoelectronic Information of China (2020ZZ111, 2020ZZ113); National Natural Science Foundation of Fujian Province, China (No. 2019J01221).
Acknowledgements
This work was financially supported by the National Natural Science Foundation of China (61775038 and 61904031), Fujian Science & Technology Innovation Laboratory for Optoelectronic Information of China (2020ZZ111, 2020ZZ113), and the National Natural Science Foundation of Fujian Province, China (No. 2019J01221).
Disclosures
The author declares that there are no conflicts to interest to this article.
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.
References
1. A. M. Nightingale and J. C. DeMello, “Improving the ensemble optical properties of InP quantum dots by indium precursor modification,” J. Mater. Chem. C 4(36), 8454–8458 (2016). [CrossRef]
2. K. R. Reid, J. R. McBride, N. J. Freymeyer, and L. B. Thal, “Chemical structure, ensemble and single-particle spectroscopy of thick-shell InP-ZnSe quantum dots,” Nano Lett. 18(2), 709–716 (2018). [CrossRef]
3. Y. Cao, Y. Gong, W. Z. Liao, and Y. F. Luo, “A review of cardiovascular toxicity of TiO2, ZnO and Ag nanoparticles (NPs),” Biometals 31(4), 457–476 (2018). [CrossRef]
4. X. P. Zhou, J. J. Ren, X. Dong, and X. C. Wang, “Controlling nucleation process of InP/ZnS quantum dots using zeolite as a nucleation site,” CrystEngComm 22(20), 3474–3481 (2020). [CrossRef]
5. V. Brunetti, H. Chibli, R. Fiammengo, A. Galeone, M. A. Malvindi, G. Vecchio, R. Cingolani, J. L. Nadeau, and P. P. Pompa, “InP/ZnS as a safer alternative to CdSe/ZnS core/shell quantum dots: in vitro and in vivo toxicity assessment,” Nanoscale 5(1), 307–317 (2013). [CrossRef]
6. J. K. Bai, C. Chang, J. H. Wei, C. T. Dong, H. S. Peng, X. Jin, Q. Zhang, F. Li, and Q. H. Li, “High efficient light-emitting diodes with improved the balance of electron and hole transfer via optimizing quantum dot structure,” Opt. Mater. Express 9(7), 3089–3097 (2019). [CrossRef]
7. K. Han, J. Heo, and W. J. Chung, “The effect of rare earth on color conversion properties of Cd-S-Se quantum dot embedded silicate glasses for white LED,” Opt. Mater. 111, 110545 (2021). [CrossRef]
8. X. G. Yang, Y. Liu, H. X. Lei, and B. J. Li, “An organic-inorganic broadband photodetector based on a single polyaniline nanowire doped with quantum dots,” Nanoscale 8(34), 15529–15537 (2016). [CrossRef]
9. X. G. Yang, Y. C. Li, and B. J. Li, “Silicon particle as a nanocavity for stable emission of quantum dots,” ACS Photonics 4(11), 2669–2675 (2017). [CrossRef]
10. W. Shen, H. Y. Tang, and Z. T. Deng, “Progress on synthesis of indium phosphide quantum dots,” Materials China 36(2), 95–102 (2017). [CrossRef]
11. C. Shen, Y. Q. Zhu, Z. X. Li, J. L. Li, H. Tao, J. H. Zou, X. Q. Xu, and G. Xua, “Highly luminescent InP-In(Zn)P/ZnSe/ZnS core/shell/shell colloidal quantum dots with tunable emission synthesized based on growth-doping,” J. Mater. Chem. C 9(30), 9599–9609 (2021). [CrossRef]
12. F. Cao, S. Wang, F. J. Wang, Q. Q. Wu, D. W. Zhao, and X. Y. Yang, “A layer-by-layer growth strategy for large-size InP/ZnSe/ZnS core–shell quantum dots enabling high-efficiency light-emitting diodes,” Chem. Mater. 30(21), 8002–8007 (2018). [CrossRef]
13. W. Sukkabot, “Atomistic effect of internal and external shell on physical behaviours of InP/GaP/ZnS core/shell/shell nanocrystals: Empirical tight-binding theory,” Comput. Mater. Sci. 161, 46–52 (2019). [CrossRef]
14. R. P. Brown, M. J. Gallagher, D. H. Fairbrother, and Z. Rosenzweig, “Synthesis and degradation of cadmium-free InP and InPZn/ZnS quantum dots in solution,” Langmuir 34(46), 13924–13934 (2018). [CrossRef]
15. R. L. Wells, S. R. Aubuchon, S. S. Kher, M. S. Lube, and P. S. White, “Synthesis of nanocrystalline indium arsenide and indium phosphide from indium(III)halides and tris(trimethylsilyl)pnicogens,” Chem. Mater. 7(4), 793–800 (1995). [CrossRef]
16. T. R. Kuo, S. T. Hung, Y. T. Lin, T. L. Chou, M. C. Kuo, Y. P. Kuo, and C. C. Chen, “Green synthesis of InP/ZnS core/shell quantum dots for application in heavy-metal-free light-emitting diodes,” Nanoscale Res. Lett. 12(1), 537 (2017). [CrossRef]
17. R. G. Xie, D. Battaglia, and X. G. Peng, “Colloidal InP nanocrystals as efficient emitters covering blue to near-infrared,” J. Am. Chem. Soc. 129(50), 15432–15433 (2007). [CrossRef]
18. J. Zhang, J. Wang, T. Yan, Y. A. Peng, D. J. Xu, and D. W. Deng, “InP/ZnSe/ZnS quantum dots with strong dual emissions: visible excitonic emission and near-infrared surface defect emission and their application in in vitro and in vivo bioimaging,” J. Mater. Chem. B 5(41), 8152–8160 (2017). [CrossRef]
19. T. Greco, C. Ippen, and A. Wedel, “InP/ZnSe/ZnS core-multishell quantum dots for improved luminescence efficiency,” Proc. SPIE 8424, 842439 (2012). [CrossRef]
20. Y. Kim, S. J. Ham, H. Jang, J. H. Min, H. Chung, J. Lee, D. Kim, and E. Jang, “Bright and Uniform Green Light Emitting InP/ZnSe/ZnS Quantum Dots for Wide Color Gamut Displays,” ACS Appl. Nano Mater. 2(3), 1496–1504 (2019). [CrossRef]
21. T. Kim, Y. H. Won, E. Jang, and D. Kim, “Negative Trion Auger Recombination in Highly Luminescent InP/ZnSe/ZnS Quantum Dots,” Nano Lett. 21(5), 2111–2116 (2021). [CrossRef]
22. W. K. Bae and J. Lim, “Nanostructured colloidal quantum dots for efficient electroluminescence devices,” Korean J. Chem. Eng. 36(2), 173–185 (2019). [CrossRef]
23. E. Cho, T. Kim, S. Choi, H. Jang, K. Min, and E. Jang, “Optical characteristics of the surface defects in InP colloidal quantum dots for highly efficient light-emitting applications,” ACS Appl. Nano Mater. 1(12), 7106–7114 (2018). [CrossRef]
24. Y. Li, X. Q. Hou, X. L. Dai, Z. L. Yao, L. L. Lyu, Y. Z. Jin, and X. G. Peng, “Stoichiometry-controlled InP-based quantum dots: synthesis, photoluminescence, and electroluminescence,” J. Am. Chem. Soc. 141(16), 6448–6452 (2019). [CrossRef]
25. R. C. Lai, Y. B. Sang, Y. Zhao, and K. F. Wu, “Triplet sensitization and photon upconversion using InP-Based quantum dots,” J. Am. Chem. Soc. 142(47), 19825–19829 (2020). [CrossRef]
26. H. V. Han, H. Y. Lin, C. C. Lin, W. C. Chong, J. R. Li, K. J. Chen, P. C. Yu, T. M. Chen, H. M. Chen, K. M. Lau, and H. C. Kuo, “Resonant-enhanced full-color emission of quantum-dot-based micro LED display technology,” Opt. Express 23(25), 32504 (2015). [CrossRef]
27. J. X. Wang, J. Tao, J. G. Lyu, Y. Li, and J. Q. Liang, “Design and fabrication of color conversion layer based on InP/ZnS core-shell quantum dots,” Chin. J. Lumin. 41(5), 592–602 (2020). [CrossRef]
28. M. G. Han, Y. Lee, H. Kwon, H. Lee, T. Kim, Y. H. Won, and E. Jang, “InP-based quantum dot light-emitting diode with a blended emissive layer,” ACS Energy Lett. 6(4), 1577–1585 (2021). [CrossRef]
29. W. Y. Tian, L. Dou, Z. Jin, and J. Q. Xiao, “Full-color micro-LED displays with cadmium-free quantum dots patterned by photolithography technology,” Appl. Opt. 59(35), 11112–11122 (2020). [CrossRef]
30. X. G. Yang, L. Wen, J. H. Yan, Y. J. Bao, Q. Chen, A. Camposeo, D. Pisignano, and B. J. Li, “Energy dissipation and asymmetric excitation in hybrid waveguides for routing and coloring,” J. Phys. Chem. Lett. 12(29), 7034–7040 (2021). [CrossRef]
31. Q. Chen, X. H. Nan, M. J. Chen, D. H. Pan, X. G. Yang, and L. Wen, “Nanophotonic color routing,” Adv. Mater. 33(49), 2103815 (2021). [CrossRef]
32. M. J. Chen, L. Wen, D. H. Pan, D. R. S. Cumming, X. G. Yang, and Q. Chen, “Full-color nanorouter for high-resolution imaging,” Nanoscale 13(30), 13024–13029 (2021). [CrossRef]
33. D. W. Jeong, J. Y. Kim, H. W. Seo, K. M. Lim, M. J. Ko, T. Y. Seong, and B. S. Kim, “Characteristics of gradient-interface-structured ZnS/CdSe quantum dots with modified interface and its application to quantum-dot-sensitized solar cells,” Appl. Surf. Sci. 429, 16–22 (2018). [CrossRef]
34. G. Y. Fan, C. Y. Wang, and J. Y. Fang, “Solution-based synthesis of III-V quantum dots and their applications in gas sensing and bio-imaging,” Nano Today 9(1), 69–84 (2014). [CrossRef]
35. P. Mushonga, M. O. Onani, A. Madiehe, and M. Meyer, “Indium phosphide-based semiconductor nanocrystals and their applications,” J. Nanomater. 2012, 1–11 (2012). [CrossRef]
36. B. W. Zhang, X. W. Wang, D. K. Wang, J. L. Tang, X. Fang, D. Fang, X. H. Wang, R. Chen, T. C. He, and Z. P. Wei, “Ultrafast charge carrier dynamics and nonlinear optical absorption of InP/ZnS core-shell colloidal quantum dots,” J. Phys. Chem. 123(44), 9431–9449 (2019). [CrossRef]
37. L. Li and P. Reiss, “One-pot synthesis of highly luminescent InP/ZnS nanocrystals without precursor injection,” J. Am. Chem. Soc. 130(35), 11588–11589 (2008). [CrossRef]
38. S. Lee and C. Lee, “High-density quantum dots composites and its photolithographic patterning applications,” Polym. Adv. Technol. 30(3), 749–754 (2019). [CrossRef]
39. Y. Liu, F. Han, F. Li, Y. Zhao, M. S. Chen, Z. W. Xu, X. Zheng, H. L. Hu, J. M. Yao, T. L. Guo, W. Z. Lin, Y. H. Zheng, B. H. You, P. Liu, Y. Li, and L. Qian, “Inkjet-printed unclonable quantum dot fluorescent anti-counterfeiting labels with artificial intelligence authentication,” Nat. Commun. 10(1), 1–9 (2019). [CrossRef]
40. T. T. Xuan, S. C. Shi, L. Wang, H. C. Kuo, and R. J Xie, “Inkjet-printed quantum dot color conversion films for high-resolution and full-color micro light-emitting diode displays,” J. Phys. Chem. Lett. 11(13), 5184–5191 (2020). [CrossRef]
41. H. Y. Lin, C. W. Sher, D. H. Hsieh, X. Y. Chen, H. M. P. Chen, T. M. Chen, K. M. Lau, C. H. Chen, C. C. Lin, and H. C. Kuo, “Optical cross-talk reduction in a quantum-dot-based full-color micro-light-emitting-diode display by a lithographic-fabricated photoresist mold,” Photonics Res. 5(5), 411–416 (2017). [CrossRef]
42. Y. L. Weng, S. Y. Chen, Y. A. Zhang, L. Sun, Y. Wu, Q. Yan, T. L. Guo, X. T. Zhou, and C. X. Wu, “Fabrication and color conversion of patterned InP/ZnS quantum dots photoresist film via a laser-assisted route,” Opt. Laser Technol. 140, 107026 (2021). [CrossRef]
43. C. J. Wei, J. Y. Li, F. Gao, S. X. Guo, Y. C. Zhou, and D. Zhao, “One-step synthesis of high-quality water-soluble CdSe quantum dots capped by N-acetyl-L-cysteine via hydrothermal method, and their characterization,” J. Spectrosc. 2015, 1–7 (2015). [CrossRef]
44. X. Liu, X. H. Wang, D. K. Wang, and J. L. Tang, “Determination of band offset in MgO/InP heterostructure by X-ray photoelectron spectroscopy,” Vacuum 134, 136–140 (2016). [CrossRef]
45. H. Virieux, M. L. Troedec, A. Cros-Gagneux, W. S. Ojo, F. Delpech, C. Nayral, H. Martinez, and B. Chaudret, “InP/ZnS nanocrystals: coupling NMR and XPS for fine surface and interface description,” J. Am. Chem. Soc. 134(48), 19701–19708 (2012). [CrossRef]
46. H. Moon, C. Lee, W. Lee, J. Kim, and H. Chae, “Stability of quantum dots, quantum dot films, and quantum dot light-emitting diodes for display applications,” Adv. Mater. 31(34), 1804294 (2019). [CrossRef]
47. J. Selvaraj, A. Mahesh, V. Asokan, V. Baskaralingam, A. Dhayalan, and T. Paramasivam, “Phosphine-free, highly emissive, water-soluble Mn: ZnSe/ZnS core–shell nanorods: synthesis, characterization, and in vitro bioimaging of HEK293 and HeLa cells,” ACS Appl. Nano Mater. 1(1), 371–383 (2018). [CrossRef]
48. H. Wang, S. M. Wen, G. Han, and Q. C. Feng, “Effect of copper ions on surface properties of ZnSO4-depressed sphalerite and its response to flotation,” Sep. Purif. Technol. 228, 115756 (2019). [CrossRef]
49. Y. L. Weng, G. X. Chen, X. T. Zhou, Q. Yan, T. L. Guo, and Y. A. Zhang, “Design and fabrication of bi-functional TiO2/Al2O3 nanolaminates with selected light extraction and reliable moisture vapor barrier performance,” Nanotechnology 30(8), 085702 (2019). [CrossRef]
50. Y. H. Kim, S. Koh, H. Lee, S. M. Kang, D. C. Lee, and B. S. Bae, “Photo-patternable quantum dots/siloxane composite with long-term stability for quantum dot color filters,” ACS Appl. Mater. Interfaces 12(3), 3961–3968 (2020). [CrossRef]
51. K. J. Chen, H. C. Chen, K. A. Tsai, C. C. Lin, H. H. Tsai, S. H. Chien, B. S. Cheng, Y. J. Hsu, M. H. Shih, C. H. Tsai, H. H. Shih, and H. C. Kuo, “Resonant-enhanced full-color emission of quantum-dot based display technology using a pulsed spray method,” Adv Funct Mater. 22(24), 5138–5143 (2012). [CrossRef]
52. M. J. Bruchez, M. Moronne, P. Gin, S. Weiss, and A. P. Alivisatos, “Semiconductor nanocrystals as fluorescent biological labels,” Science 281(5385), 2013–2016 (1998). [CrossRef]
53. J. S. Li, Y. Tang, Z. T. Li, X. R. Ding, S. D. Yu, and B. H. Yu, “Improvement in color-conversion efficiency and stability for quantum-dot-based light-emitting diodes using a blue anti-transmission film,” Nanomaterials 8(7), 508 (2018). [CrossRef]
