All-quantum-dot emission tuning and multicolored optical films using layer-by-layer assembly method

Chemical Engineering Journal 324 (2017) 19–25

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Chemical Engineering Journal journal homepage: www.elsevier.com/locate/cej

All-quantum-dot emission tuning and multicolored optical films using layer-by-layer assembly method Xueping Liu a, Xuejing Zhang a, Ruili Wu a, Huaibin Shen a,⇑, Changhua Zhou a, Xintong Zhang b,⇑, Li-Jun Guo c, Lin Song Li a,⇑ a

Key Laboratory for Special Functional Materials of the Ministry of Education, Henan University, Kaifeng 475004, PR China Center for Advanced Optoelectronic Functional Materials Research, and Key Laboratory of UV-Emitting Materials and Technology of Ministry of Education, Northeast Normal University, Changchun 130024, PR China c Institute of Photo-biophysics, School of Physics and Electronics, Henan University, Kaifeng 475004, PR China b

h i g h l i g h t s  A novel all-quantum-dot multilayer photoluminescent film (PLF) was fabricated.  The color and PL intensities of these hybrid films can be precisely controlled.  The prepared PLFs were uniform and smooth with high visible light transmittance.  They can be potentially used in the lighting and display fields.

a r t i c l e

i n f o

Article history: Received 5 March 2017 Received in revised form 28 April 2017 Accepted 29 April 2017 Available online 2 May 2017 Keywords: Quantum dots Layer-by-layer assembly Surface modification Multicolor Photoluminescent films

a b s t r a c t In this report, all-quantum-dot multilayer photoluminescent films (PLFs) based on two types of modified quantum dots (MA-C8-QDs and PEI-QDs) were fabricated through the layer-by-layer (LBL) self-assembly method, providing a new kind of luminescent material with emission color covering blue to red spectral region. Aqueous QDs with high stability and photoluminescence properties were obtained by an efficient phase transfer, followed by fabrication of (PEI-QDs/MA-C8-QDs)n PLFs with alternate adsorbing the layer of the MA-C8-QDs endowed with negative (ACOO ) charges and PEI-QDs endowed with positive (ANH+3) charges using electrostatic interactions between each layer. The resulting single color films preserved good color purity and strong luminescence of original QDs, PL intensities increased linearly with the number of bilayers n, which indicated that growth of the film is regular and uniform. In addition, the uniform and smooth PLFs as prepared have high visible light transmittance. Furthermore, by emission tuning, multicolor and white light-emitting PLFs with the color coordinates at (0.3292, 0.3418) have been easily obtained by assembly of two types of modified red, green, blue QDs. Therefore, these PLFs (especially white-light PLFs) show promise for the development of novel multiplexed biological sensors, fullcolor displays, intelligent response, photonic, and optoelectronic devices. Ó 2017 Elsevier B.V. All rights reserved.

1. Introduction Novel materials based on colloidal semiconductor nanocrystal quantum dots (QDs) are attracting considerable interest of applications in the optoelectronic, including light-emitting diodes (LEDs) [1–8], solar cells [9], optical modulators [10], photoconductors [11], and lasers [12]. For the inherent merits of QDs, QD films have the advantageous features in the field of luminescent materials. Much effort was undertaken to prepare well-defined ultrathin ⇑ Corresponding authors. E-mail addresses: [email protected] (H. Shen), [email protected] (X. Zhang), [email protected] (L.S. Li). http://dx.doi.org/10.1016/j.cej.2017.04.137 1385-8947/Ó 2017 Elsevier B.V. All rights reserved.

QD films that may be used to explore potential applications in solid-state lighting and displays [13–16]. In order to realize fullcolor displays and lighting applications with QDs films, a method for the deposition of homogeneous and uniform QD layers with well-defined internal structures over a large area with patterning capability should be developed. As a facile, viable, and versatile technology for the preparation of many different multi-layers with nanosized objects such as nanocrystallites, polymers, nanoparticles, nanosheets, other functional components on various substrates in desired configurations, the layer-by-layer (LBL) selfassembly method is considered to be one of the most promising and feasible techniques [17–24]. Based on specific interaction forces between each deposited layer such as electrostatic

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interactions [25], covalent bonding [26], or hydrogen bonding [27], each QDs layer attracts only the layer with opposite charges by LBL self-assembly method. It can finely control the tuneable design of different QDs with desired structures and properties [23,28,29], and maintain QDs’s excellent luminescence properties from each layers. Therefore, this method can be used to get nano-films with uniform internal structure with controllable structure and color over a large scale [30]. Some researchers have fabricated hybrid films composed of QDs and polymers recently based on LBL method [31–34]. For example, monoalkyl maleate amphiphilic surfactants were used to obtain water soluble QDs with high fluorescence intensity, good stability and surface functionalization, moreover, an inspiring luminescent planar plate was obtained by LBL assembly of multilayer photoluminescent films [35]. The optical and thermal stability of QDs are not changed during the assembly process, suggesting that this kind of hybrid quantum dot light emitting film has the properties of UV resistance and thermal resistance. However, for these hybrid films, several challenges remain to be resolved, for instance, this method usually requires a polymer layer with opposite charges, which may affect the optical properties of the entire film. And the miscibility, aggregation problems of QDs with polymers, intertwist of flexible polymer chains, can lead to difficulties in controlling homogeneous architecture and fluorescence concentration quenching of QDs in the polymer matrix. In regard with these problems, there have been some attempts to create QDs-based hybrid films combining QDs with inorganic matrices to avoid the above problems by the LBL self-assembly method [36–40]. However, if all QD multilayer films can be assembled, the above-mentioned process is complex, cumbersome, and pointless. Therefore, how to fabricate a QDsbased luminescent film with desirable compatibility, stability, and processing properties remains a challenge and it is particularly necessary to make all the quantum dots in order to improve the application potential of quantum dots in optoelectronic devices. Aiming at a simple preparation of such films, we fabricate multilayer photoluminescent films (PLFs) which just rely on modified quantum dots rather than organic polymer or inorganic layers. All QD multilayer PLFs were fabricated by LBL assembly method using electrostatic interactions between each layer through the sequential deposition of oppositely charged QDs onto the substrates. Using the monoalkyl maleate (MA-C8) to surface modify pristine QDs (capped with oleic acid), we obtain the MA-C8-QDs capped with negative (ACOO ) charges dispersing in water (pH 8). The hydrophobic layer of the QDs was exchanged with amphi-

philic hyperbranched polyethyleneimine (PEI) to get PEI-QDs endowed with positive (ANH+3) charges dispersing in water (pH 5) [41] (Fig. 1). QDs were fabricated into functional (PEI-QDs/MAC8-QDs)n PLFs by alternate adsorbing the layer of the MA-C8-QDs and PEI-QDs through electrostatic interactions between each layer. By doing so, mono-color PLFs were fabricated by assembly of same color PEI-QDs and MA-C8-QDs. Multicolour and white-light PLFs were further obtained by assembly of variously color PEI-QDs and MA-C8-QDs (red, green and blue emission). Therefore, the work presented here provides a simple and convenient approach for the design and fabrication of all QD PLFs with high performance, which can be served as promising materials for the integration of multicolor optical and display devices. 2. Experimental 2.1. Materials The raw materials used in the experiment were as follows: cadmium oxide (CdO, 99.99%), zinc oxide (ZnO, 99.9%, powder), sulfur (S, 99.98%, powder), 1-octadecene (ODE, 90%), paraffin oil (99.5%), oleic acid (OA, 90%), and selenium (Se, 99.99%, powder) were purchased from Aldrich. Maleic anhydride (MA, analytical reagent (AR)), polyethyleneimine (PEI, AR), n-octanol (AR), hexanes (AR), acetone (AR), acetic acid (AR), methanol (AR), chloroform (AR), sodium hydroxide (AR), ammonia water (28%), and concentrated sulfuric acid (98%) were purchased from Beijing Chemical Reagent Ltd., China. 2.2. Synthesis of hydrophobic QDs Red CdSe/ZnS QDs (PL kmax. = 607 nm) and green CdSe/ZnS QDs (PL kmax. = 525 nm) were prepared by reported method in the previous literature [42]. Blue Cd1-xZnxSe QDs (PL kmax. = 455 nm) were synthesized according to the previous literature [43,44]. 2.3. Surface modification of hydrophobic QDs Monooctyl maleate (MA) was used as surface modifier to make hydrophobic QDs water-soluble in a phase transfer procedure. The synthesize of MA was carried out according to the previous report [45]. Briefly, mixed maleic anhydride (49.03 g, 0.50 mol) with Noctanol (65.12 g, 0.50 mol), after the mixture was heated at 80 °C for 1 h, the heptane (120 ml) was added into the reaction system, under stirring at 80 °C for 15 min. Finally, kept the solution static for 2 h at 15 °C, the crystals obtained were recrystallized in the same way, and the high purity of MA was obtained. Using the monoalkyl maleate and a typical procedure, finally aqueous MAC8-QDs with the ability of being dispersed in water (pH 8) at different concentration (0.5, 1, 2 mg/ml) were obtained. By the similar method, using amphiphilic hyperbranched poly ethyleneimine (PEI) for phase-transfer of QDs into water (pH 5) through a simple, rapid method, we prepared aqueous PEI-QDs dispersed in water at different concentration (0.5, 1, 2 mg/ml) respectively [41]. The transfer is simple and quantitative, yielding a colorless phase and no aggregated particles, proving the success of the experiment. 2.4. LBL assembly of (PEI-QDs/MA-C8-QDs)n PLFs

Fig. 1. Key steps in the formation of MA-C8-QDs (upper) and PEI-QDs (bottom).

For the layer-by-layer assembly of QDs on flat surfaces of glass slides, the substrates were cleaned with H2SO4 for 30 min followed by washing thoroughly with deionic water, immediately, after treatment with concentrated NH3/30% H2O2 (7:3) for 30 min, the substrate surface was negatively charged. (PEI-QDs/MA-C8-QDs)n PLFs were fabricated according to the following cyclic procedure:

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the substrates were alternately dipped into the solution of PEI-QDs (pH 5) and MA-C8-QDs (pH 8) for 5 min, along with washing steps with deionized water after the adsorption of each QD layer. According to the desired thickness of the films, the repeat numbers of above adsorption process were determined. The resulting All-QD PLFs were dried with a nitrogen gas flow for 3 min at 25 °C. 3. Characterization The size and morphology of PL QDs were investigated using a JEM 100CX-II transmission electron microscope (TEM) at 100 kV. Regular TEM specimens were made by evaporating one drop of QDs solution on carbon-coated copper grids. UV–vis absorption and PL spectra were measured at room temperature with an Ocean Optics spectrophotometer (mode PC2000-ISA). PL spectra were taken using an excitation wavelength of 365 nm. The fluorescence quantum yield (QY) of the hydrophobic PL QDs and aqueous PL QDs were measured relative to Rhodamine 6G dye solutions with known emission efficiencies. Zeta-potential data were collected using a Zetasizer Nano-ZS (Malvern Instruments, U.K.) at 25 °C. 4. Results and discussion

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after the phase transition. A colloid that is readily precipitated from a solution and can be re-dispersed by an addition of the acetic acid to get PEI-QDs solution with pH value at 5. The transmission electron microscope (TEM) images (Fig. 2b, c) indicated that the MA-C8-QDs and the PEI-QDs are nearly spherical particles with homogeneous disperse in water. The MA-C8-QDs with an average diameters of 16.08 nm and the PEI-QDs with an average diameters of 14.58 nm, the size distribution data shows that there is no significant change of size compared to the original hydrophobic QDs after modification, this is consistent with the observation from TEM, suggesting the non-aggregated nature of the two types of modified QDs in water. For electrostatically stabilized colloids, the zeta potential of colloids with more than +30 mV or less than 30 mV was widely regarded as stable colloids in water [46]. In order to verify the stability of quantum dots obtained, we adopt the zeta potential measurement to monitor the colloidal stability. As can be seen from Fig. 3, the zeta potential of MA-C8-QDs is always negatively charged and shows a downward trend from pH 4 to pH 12. Along with the increasing of pH value, more carboxylic acid on the surface of colloidal MA-C8QDs were deprotonated, thus colloidal MA-C8-QDs carries more negative charges. On the other hand, the colloidal PEI-QDs shows positively charged characteristics in the pH ranges from 4 to 12.

4.1. Synthesis of MA-C8-QDs and PEI-QDs Fig. 1 (upper) shows the scheme of experimental processes to prepare aqueous MA-C8-QDs. In this process, the monoalkyl maleate (MA-C8) was used as a surface modifier, which contains both hydrophobic chains (AOCnH2n+1) and free carboxylic acid groups. In which AOCnH2n+1 was connected to QDs while ACOOH was bare in solution. Owing to the van der Waals interactions, hydrophobic chains (AOCnH2n+1) and hydrophobic coatings already present on the QDs were connected together. As synthesized MA-C8-QDs capped with free carboxylic acid groups were available for dissolving in water [34,43]. This kind of structure is beneficial to the phase transition, leading to colloidal nanocrystals with negative charges, and therefore ensuring QDs were dispersed in the aqueous solution and not aggregated. Fig. 1 (bottom) depicts the proposed mechanism of using PEI to accomplish phase-transfer. As PEI is soluble in chloroform and insoluble in cyclohexane, while the hydrophobic quantum dots are soluble in both chloroform and cyclohexane, so we use the method of Fig. 1 (bottom) to prepare water-soluble PEI-QDs. In chloroform, PEI, used as a replacement, displaced the original surface ligand of the QDs during ligand-exchange. Thus, a stable chloroform colloid system was built with PEI-QDs. Subsequent adding of cyclohexane, a precipitation of the PEI-QDs was received. Because there are a large number of water-soluble amino groups at the end of PEI, PEI-QDs can be dissolved in water, and it becomes a water-soluble QD with positive charges on the surface

Fig. 3. pH-dependent zeta potential of red MA-C8-QDs and red PEI-QDs. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

Fig. 2. TEM images of (a) hydrophobic QDs (red color with emission peak at 607 nm) in chloroform, (b) MA-C8-QDs, and (c) PEI-QDs in water. Inset: size distribution data. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

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And this could be interpreted that as the increase of the pH value of solution, more protonated amino on the surface of colloidal PEIQDs lead to more positive charges. As can be captured from Fig. 3, the MA-C8-QDs were stable colloids in pH > 8 aqueous solutions with zeta potentials of < 65.5 and the PEI-QDs were stable colloids in pH < 8 aqueous solutions with zeta potentials of >38.8. This is of great significance for the electrostatic LBL assembly. 4.2. Assembly of red, green, and blue (PEI-QDs/MA-C8-QDs)n PLFs The LBL assembly method, considered as one of the most promising and practicable technologies, provides uniform films with regular and consistent internal structures. All-QD multilayer PLFs were prepared by LBL assembly of oppositely charged QDs

onto the substrates. MA-C8-QDs and PEI-QDs dispersed in water (pH 8 or pH 5) were endowed with negative (ACOO ) charges or positive (ANH+3) charges, respectively, on the surfaces of the QDs. QDs were fabricated into functional (PEI-QDs/MA-C8-QDs)n PLFs by alternate adsorbing the layer of the MA-C8-QDs and PEI-QDs via electrostatic interactions between each layer. Additionally, increasing the concentration of the PEI-QDs or MA-C8-QDs solution can make more adsorption quantity of each layer. Scheme 1 shows the detailed assembly process of All QD PLFs. In the following section, we illustrate the stepwise LBL assembly of red, green, and blue emitting QDs for fabricating uniform mono-color, multicolor as well as white color PLFs respectively. Using red PLFs as an example to illustrate, red (PEI-QDs/MA-C8QDs)n PLFs were fabricated by alternate absorbing the layer of the red MA-C8-QDs and red PEI-QDs. The red PLFs are highly transparent in normal daylight, as shown in photographs (Fig. 4a, upper). The red PLFs deposited on quartz substrates were monitored by fluorescence spectrometer. Moreover, fluorescence spectra of the red PLFs showed that the PL intensities of the fluorescence bands at about 607 nm increase gradually with the number of bilayers n (Fig. 4d), which indicates the successful deposition of each layer. The linear relationship between averaged PL intensity measured from 5 parts of the same film and number of bilayers of red (PEIQDs/MA-C8-QDs)n PLFs also demonstrates that growth of the film is regular and uniform (Fig. 4g). Additionally, it is noteworthy that no significant shift or broadening of the emission band for different values of n illustrates that no obvious changes in intermolecular interactions or for the nature of QDs in the whole assembly process. The explanation above is also applicable to the green and blue PLFs. 4.3. Morphological and optical characterization of red PLFs

Scheme 1. Schematic representation for the LBL fabrication of multilayer PLFs based MA-C8-QDs and PEI-QDs.

Surface roughness is an important index to quantitatively evaluate the characteristics of surface micro topography. The atomic

Fig. 4. Photographs of red PLFs, green PLFs, blue PLFs (a, b, c) on glass slides. Fluorescence spectra of red PLFs, green PLFs, blue PLFs (d, e, f, g, h, i). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

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Fig. 5. Tapping-mode AFM images of 12, 20 bilayers red PLFs. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

Fig. 6. Optical characteristics of films measured by UV–vis spectrophotometry.

force microscope (AFM) images of the red PLFs were taken to provide detailed information about the surface morphology and the homogeneity of the deposited films. The AFM topographical images (1 lm  1 lm) of the red PLFs are illustrated in Fig. 5 with low value of root-mean-square (RMS) roughness, indicating that

the obtained multilayer red PLFs have a flat, compact, and smooth surface morphology. There is neither obvious bulge caused by the large aggregation of quantum dots, nor large voids are found, indicating the homogeneous distribution of QDs in each layer. However, from the images the small vacancies between adjacent QDs and the aggregation of the QDs still can be seen. We attribute this phenomenon to the spherical geometry of the QDs and imperfection in the QD layer caused those minor defects to appear. With the increase of the bilayer number, the RMS increases slowly without great change, this also suggests that growth of the films is regular and uniform throughout the fabrication process. Another important indicator of photoluminescence is the transparency of the luminescent materials, optical characteristics of films were investigated by UV–vis spectrophotometry. Fig. 6 displays that transmission of these blue PLFs decreases slowly with the bilayer number varies from 4 to 20. When the bilayer number n is 4, the transparency is close to 100%. And that all the transmittance above 80% indicates the blue PLFs are transparent, such a high value is beneficial to the absorption of light.

4.4. Assembly and characterization of multi-color and white-color PLFs Multicolor and white PLFs were fabricated on the glass slides by patterning and placing red, green, blue color QDs. Red/blue PLFs were prepared through LBL assembly of red PEI-QDs and blue

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Fig. 7. Fluorescence spectra of red/blue PLFs, red/green PLFs, green/blue PLFs (a, b, c), the inset shows the photographs of these PLFs under UV irradiation (365 nm). The color coordinates of red/blue PLFs, red/green PLFs, green/blue PLFs (d). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

(0.3144, 0.2401), n = 20), red/green PLFs (CIE 1931: (0.3672, 0.3741), n = 20), green/blue PLFs (CIE 1931: (0.2384, 0.3281), n = 20). Besides, a white light emitting PLF was also fabricated based on LBL assembly of (blue PEI-QDs/green MA-C8-QDs)20, (red PEI-QDs/ red MA-C8-QDs)4 and (red PEI-QDs/green MA-C8-QDs)8 with the color coordinates at (0.3292, 0.3418) (Fig. 8), which is rather close to the standard coordinates of white light (0.333, 0.333). After a year of storage under room temperature, the emission peak of these films remains unchanged and the decrease of photoluminescence intensity is less than 5%, providing the good photostability of the PLFs. It is expected that these hybrid films can be potentially used as a color conversion layer for illumination and display. 5. Conclusions

Fig. 8. Fluorescence spectra of white-color PLFs, the inset shows the photographs of the white-color PLFs under UV irradiation and the color coordinates of white-color PLFs.

MA-C8-QDs. Red/green PLFs were achieved by assembly of red PEIQDs and green MA-C8-QDs. Green/blue PLFs were obtained by assembly of blue PEI-QDs and green MA-C8-QDs respectively. Compared with the fluorescence spectra of mono-color film, the fluorescence peak width and peak position of corresponding color did not change obviously in the fluorescence spectra of these multi-color PLFs (Fig. 7a, b and c). To take the red/blue PLFs as an example, Fig. 7a illustrates the bright emission of the film. The peak emission at 455 nm attributed to the MA-C8-QDs layer, and the peak wavelength at 610 nm attributed to the PEI-QDs layer. Similarly, the red/green and green/blue PLFs were obtained. Fig. 7d displays the color coordinates of red/blue PLFs (CIE 1931:

In this work, the MA-C8-QDs capped with negative (ACOO ) charges and PEI-QDs endowed with positive (ANH+3) charges were obtained. All QD PLFs can be successfully fabricated on the glass slides by layer-by-layer assembly of above oppositely charged QDs using electrostatic interactions between each layer. This LBL self-assembly method offer a facile and cost-effective strategy for accurately fabricating multicolor and white-light PLFs, which make a significant impact on the development of new materials and devices. Furthermore, it has been shown that highly transparent multilayers. Using the same color MA-C8-QDs and PEI-QDs to get a single color (red, green, blue) QDs/QDs film, and in particular, by accurately controlling the ratio between three monochromatic thin films, we got white light emitting PLFs with the color coordinates at (0.3292, 0.3418). The result indicated that growth of the film is regular and uniform and the prepared PLFs were uniform and smooth with a high visible light transmittance. These highly ordered all-QD PLFs would have potential applications in biological sensors, multicolor photoemission devices, and optoelectronic devices.

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