Giant Stability Enhancement of CsPbX3 Nanocrystal Films by Plasma

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Functional Nanostructured Materials (including low-D carbon)

Giant Stability Enhancement of CsPbX3 Nanocrystal Films by Plasma Induced Ligand Polymerization Li Wang, Yiyuan Zhu, Hu Liu, Jinhui Gong, Wei Wang, Siyu Guo, Yao Yu, Haiyan Peng, and Yonggui Liao ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.9b12591 • Publication Date (Web): 03 Sep 2019 Downloaded from pubs.acs.org on September 3, 2019

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ACS Applied Materials & Interfaces

Giant Stability Enhancement of CsPbX3 Nanocrystal Films by Plasma Induced Ligand Polymerization

Li Wang,*,† Yiyuan Zhu,† Hu Liu,† Jinhui Gong,† Wei Wang,† Siyu Guo,† Yao Yu,‡ Haiyan Peng,§ and Yonggui Liao*,§

†School

of Materials Science and Engineering, Nanchang University, Nanchang

330031, P. R. China ‡State

Key Lab for Materials Processing and Die & Mould Technology, Wuhan

National High Magnetic Field Center, School of Materials Science and Engineering, Huazhong University of Science and Technology, Wuhan 430074, P. R. China §Key

Laboratory of Material Chemistry for Energy Conversion and Storage, Ministry

of Education, Hubei Key Laboratory of Material Chemistry and Service Failure, School of Chemistry and Chemical Engineering, Huazhong University of Science and Technology, Wuhan 430074, P. R. China

ACS Paragon Plus Environment

ACS Applied Materials & Interfaces 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Abstract All-inorganic CsPbX3 (X = Cl, Br and I) nanocrystals (NCs) are emerging as attractive semiconductor material due to their outstanding optical properties. The low resistance of CsPbX3 NCs to light, heat, oxygen and water, has been recognized as a major obstacle to their practical applications. Here, we demonstrate that the stability of CsPbX3 NC films can be dramatically enhanced by Ar plasma treatment. It is revealed that plasma irradiation can induce ligand polymerization in the NC films if the ligands contain unsaturated carbon bonds. The ligand polymerization leads to encapsulation of the NCs in the ligand polymers. Owing to the precise localization of the in-situ ligand polymerization under plasma irradiation and the high NC content in the films without extra additives, the polymerized area can be precisely defined down to several micrometers. This enables easy fabrication of high resolution NC pixels for next generation displays.

Keywords: perovskite nanocrystal, plasma treatment, polymerization, stability, quantum dot display


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INTRODUCTION In the past few years, all-inorganic cesium lead halide nanocrystals (CsPbX3 NCs) have garnered tremendous interest owing to their attractive optical properties.1-3 Even without surface passivation shells, the colloidal CsPbX3 NCs still present high photoluminescence quantum yield (PLQY) up to 90%.1 By tuning the halide composition and/or crystal size, the emission wavelength of the CsPbX3 NCs can be adjusted from ultraviolet to near-infrared. Moreover, they can be synthesized by facile and low-cost methods, and can be easily processed to form large-area flexible thin films. These advantages make them promising materials for high-definition displays and next generation lighting technologies.2 However, it remains formidable challenges for the CsPbX3 NCs to practical applications. The biggest challenge lies in their poor stability against air, light irradiation and water.4-7 When they are prepared into solid thin films, the stability problem becomes more severe due to the absence of solvent protection. To improve the stability of the CsPbX3 NCs, a variety of approaches have been proposed. Using polyhedral oligomeric silsesquioxane,8 silica,9 alumina,10 or polymer matrices,11 as a surface coat to form NC composites have been demonstrated to be effective ways to protect the CsPbX3 NCs from erosion by moisture and oxygen. However, vast of these encapsulating materials are often necessary to completely barrier the permeation of oxygen and water, so that the efficient volume fraction of CsPbX3 NCs in the films is greatly reduced. On the other hand, the additives and encapsulating materials are usually insulating, so they are disadvantageous to device fabrication. Additive-free approaches to enhance the



stability of CsPbX3 NC films have also been studied. Manna and co-workers have demonstrated an elegant approach to cross-link the ligands of NCs through intermolecular C=C bonds in a specific small area using a focused X-ray beam.12 But, for large area films, this approach is time-consuming due to its point-by-point manipulation. In addition, a large X-ray absorption cross section for the heavy atoms (Pb, Br and I) in the NCs also causes them easy to decompose and quench their fluorescence.12,13 In this work, we demonstrate a simple additive-free approach to enhance the stability of CsPbX3 NC films. As illustrated in Scheme 1, the polymerization of double bonds in the typical ligands of oleic acid (OA) and oleylamine (OAm) on the compact NCs surface can be induced by mild plasma irradiation. The ligand polymers can dramatically enhance the stability of CsPbBr3 NC films. Meanwhile, the NC films can be protected by the thin layer of ligand polymers without any additional encapsulating materials, so the efficient luminescent NC content is as high as possible. Here, the unsaturated carbon bond in ligands plays a key role in the plasma induced polymerization. Moreover, because the ligand polymerization is precisely localized in the area under plasma irradiation, the polymerized areas can be precisely defined down to several micrometers. As a result, patterning of the NC films with a resolution over 2000 pixels per inch is easily achieved through selected-area treatment with a common mask technique. Furthermore, due to the excellent stability of plasma treated NC films to water and other solvents, multi-color NC pixels on the same substrate can be easily fabricated by alternating spin coating and selected-area plasma treatment of


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different color NCs.

Scheme 1. Schematic of the plasma induced ligand polymerization.

RESULTS AND DISCUSSION Typically, the as-obtained CsPbBr3 NCs with OA and OAm ligands in Figure 1a are cubic and with a uniform size of about 10 nm. As shown in Figure 1b, their photoluminescence peak appears at around 517 nm with a PLQY of 87%. Two sets of CsPbBr3 NC films were prepared by spin-coating method. The film thickness is about 110 nm from cross-sectional SEM measurements. One set of the films were treated by a 100 W remote Ar plasma for 30 min in a plasma enhanced atomic layer deposition system (Figure S1). The other set was left untreated for comparison. We simultaneously immersed one plasma treated film as well as one pristine film in deionized water and put them under a 365nm UV lamp to observe the change of fluorescence over time. As shown in Figure 1c, The untreated film was completely decomposed and the fluorescence disappeared within 5 min. While the plasma treated NC film maintained strong fluorescence after 100 h. That is to say, the water



resistance of plasma treated CsPbBr3 NC film is increased by at least 1000 times. Figure 1d shows a comparison of the PLQY maintenance rate in ambient air of the plasma treated and pristine CsPbBr3 NC films. After 2 d, the untreated sample loses about 50% of its original PLQY. The plasma treated NC films shows much improved stability. After 11 d, it maintains 90% of its efficiency. Furthermore, the stability of NC films against UV light was evaluated, as displayed in Figure 1e. Both pristine and treated samples were illuminated under a UV lamp (365 nm, 6W; fastened 6 cm above the NC films). With continuous UV light illumination, the pristine NC film shows dramatic PL drop and obvious PL red shift owning to the labile surface of CsPbBr3 NCs.4,14 While 95% of initial PL intensity of the treated NC film was maintained and no obvious PL peak shift was detected after 8 h strong UV light illumination. The results manifest that the stability of NC film after plasma treatment is markedly enhanced.

Figure 1. (a) TEM image of CsPbBr3 NCs. (b) UV-vis absorption and PL spectra of the CsPbBr3 NC film. The insets show the photographs of CsPbBr3 NC solution under


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ambient light and UV lamp. (c) Stability study of NC films in water. The plasma treated NC film exhibits markedly enhanced stability. (d) Normalized PLQY of pristine and plasma treated NC films, plotted against the storage time in ambient condition. (e) PL properties evolution of pristine and treated NC films under UV light irradiation.

To study what is the cause of the giant stability enhancement of the plasma treated CsPbBr3 NC film, we observed the surface morphologies of the treated and the pristine films by scanning electron microscope (SEM). Figure 2a gives the SEM image of a NC film without plasma treatment. It shows a mudcrack-like surface, which should be a result of the film contraction during solvent evaporation. These mudcracks disappeared in the plasma treated NC films, as shown in Figure 2b. In order to check whether the enhancement of water resistance of the plasma treated film comes from the modification of surface hydrophobicity, we examined the hydrophilicity of the films by contact angle measurements. As shown in Figure 2c and 2d, the contact angle of water drop on film surface slightly decreased after plasma treatment, indicating a slight enhancement of the hydrophilicity. Modification of the surface hydrophilicity can be commonly observed in plasma processes, due to adsorption or removal of hydrophilic/ hydrophobic groups on the surface. 15-17 XPS measurements showed that hydrophilic groups on our NC films were increased after plasma treatment (Figure S2). These results show that the improvement of water resistance is not an effect from the modification of the surface hydrophilicity, but



from the bulk of the films. We have tried to exfoliate the NC films from the glass substrates to gain more information about the bulk of the films. Under optical microscope observation, free-standing NC film pieces can be seen peeled off the substrate (Figure 2e). Figure 2f shows the NC film pieces scraped from the glass substrate and suspended in cyclohexane. They still can emit bright green light under UV lamp irradiation. These results clearly show that a polymer-like thin film has formed after the plasma treatment.

Figure 2. Top-view SEM images of (a) pristine NC film and (b) treated NC film. Water contact angles of (c) pristine NC film and (d) treated NC film. (e) Photograph of the treated NC film on glass substrate after stripped. (f) Photograph of NC film pieces scraped from glass substrate and suspended in cyclohexane under UV lamp. The treated NC film shows a feature of polymer, which stems from ligand polymerization.


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In order to study whether a ligand polymerization reaction has taken place in the plasma treated NC films, we performed nuclear magnetic resonance (NMR) measurements. Figure 3a shows the

13C

spectra of pure OA, pure OAm, untreated

and treated NCs. Spectra of pure OA, pure OAm and untreated CsPbBr3 NCs are measured using liquid samples. As for the plasma treated NCs, we measured the powders formed by scraped films from the glass substrates using a cross-polarization magic angle spinning nuclear magnetic resonance spectrometer

(CPMAS-NMR).

The resonance peak appears at ~ 130 ppm in the spectra of pure OA, pure OAm and CsPbBr3 NCs is assigned to typical C=C double bonds.18 Notably, around 130 ppm no peak is observed for the plasma treated NC films. This results show that the C=C bonds in the ligands disappeared after plasma treatment, proving that the ligand polymerization had taken place in the plasma treated NC films. We further studied the change of the ligands using Fourier transform infrared spectrometer (FTIR). Figure 3b shows the FTIR spectra of a pristine CsPbBr3 NC film as well as a plasma treated film. After plasma treatment, the most prominent change of FTIR spectrum is the disappearance of the peak at 3006 cm-1. This peak has been assigned to the C-H stretching mode in C=C-H by previous studies.19-21 The result confirms that the C=C bonds disappeared, in accordance with the NMR results. Further proof of ligand polymerization comes from the X-ray induced auger electron spectroscopy (XAES) analysis.12 As shown in Figure 3c, the D parameter (see characterization section) decreased from 13.6 eV for the pristine sample to 12.8 eV for the plasma treated film,



implying a reduction of sp2/sp3 ratio. In other words, the amount of C=C bonds reduced after plasma treatment.22,23 This result firmly confirms the occurrence of ligand polymerization again.

Figure 3. (a)

13C

NMR spectrum of pure OA (oleic acid), pure OAm (oleylamine),

pristine CsPbBr3 NCs and plasma treated NCs. The signal at 130 ppm from C=C is indicated. (b) FTIR spectra of pristine and treated NC films. (c) First derivative of XAES C KLL spectra.

To further investigate the role of C=C in the plasma induced polymerization, we tested a variety of ligands. As shown in Figure S3, all CsPbBr3 NCs with ligands containing a C=C bond, like 3-(N,N-dimethyloleylammonio) propanesulfonate and lecithin showed a great enhancement of water resistance after plasma treatment. But no obvious change of the NCs without C=C bond in their ligands, including octanoic acid and octylamine,24 3-(N,N-dimethyloctadecylammonio) propanesulfonate,25 was observed after plasma treatment. The effects of plasma treatment on the water


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resistance of the NCs with different ligands are summarized in Table 1. These results proved that the presence of double bonds in ligands plays a key role in the plasma induced ligand polymerization. Table 1. Effects of plasma treatment on the water resistance of various ligands capped CsPbBr3 NCs.

NC ligands

ligand structure

C=C

Stability enhancement

double

against water after

bond

plasma treatment

oleic acid contain oleylamine octanoic acid none octylamine 3-(N,N-dimethyloctadecylammonio) propanesulfonate 3-(N,N-dimethyloleylammonio) propanesulfonate

lecithin

O

O O−

S

none

N+

contain

O

O O−

N+

S

contain

The above results have showed that the mechanism of ligand polymerization in this work is distinctly different from that of the NC films irradiated by X-ray.12, 26 In the X-ray irradiated NC films, the ligands are cross-linked by formation of C=C double bonds.27 The polymerization mechanism in this work involves the breaking of C=C bonds, which can be considered as a traditional polymerization. It is known that the plasma is a mixture of photons, electrons, ions and neutral atoms in ground and



excited states. To get more insight into the initiation mechanism of the plasma induced polymerization, we have tried to separate the factors of plasma. We used a quartz glass to cover the NC film during plasma treatment, so all particles except photons are blocked by the quartz glass. The film showed no stability enhancement after treatment, indicating that photon is not the initiator of the ligand polymerization. We used the electron beam of a SEM to irradiate the NC film and no water resistance enhancement was observed either, ruling out the role of electrons in the ligand polymerization (see Figure S4). In order to further distinguish whether the Ar+ ions or the excited Ar atoms acts as the initiator for polymerization, we performed a plasma confinement treatment for the NC films. We put the NC film inside an open cylindrical quartz tube and cover the opening using a steel shield plate containing multiple holes. The diameter of the holes was 0.3 mm and the plate was connected to the ground (Figure S4). Because the radius of the holes is smaller than the thickness of plasma sheath,28-30 the plate can block charged particles and allow only neutral atoms, including excited atoms, to pass through. The NC films treated using the confined plasma still showed an obvious enhancement of the water resistance comparing to the pristine films (Figure S5). But the enhancement effect is much weaker than that of the NC films treated without plasma confinement. These results indicate that the polymerization of ligands was mainly initiated by Ar+ ions, but the excited neutral atoms also played a role. Because the NC films subjected to plasma treatment are just spin-coated and vacuum-dried, they are actually in the gel state. The initiators, i.e. Ar+ ions and


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excited Ar atoms, can not directly enter the inner part of the films but interact only with the film surface. This means that the carbon chain growth process in the NC films should be very different from that of gas or liquid monomer polymerization. In the NC films, the ligands are bound to the NC surfaces and the thermal motion of the NCs should be very slow and localized in a small region. Thus the polymerization can be regarded as in-situ polymerization. Therefore, the carbon chain growth should be slow and cannot reach a long distance. As a result, the degree of ligand polymerization depends strongly on the supply of initiators. We have tested the water resistance of the NC films as a function of the plasma treatment time. The water resistance of the treated NC films did show a gradually increase when the treatment time increased from 5 min to 30 min and saturated after then. Owing to its in-situ nature, the polymerization reaction of the ligands is highly localized. This feature makes it easy to enhance stability of the NC films in small selected areas. Moreover, the content of luminescent NCs is high due to the absence of extra additives in the films. Therefore, the NC films are explored to the application of high definition display using a mask during plasma treatment (Figure 4a). The holes in the mask act as the windows where the plasma can directly contact the NC films. Thus, only the ligands in the window areas can be polymerized while other areas are left unchanged during treatment. When the selected-area polymerized NC films are subjected to water soaking, the un-polymerized area will dissolve and the polymerized areas are left. After plasma treated for 10 min and soaked in water, well-defined NC-dot arrays can be obtained on the substrates. Because the plasma



treatment is conducted under room temperature, it has no extreme restriction on substrate choosing for fabricating the NC pixels. As shown in Figure 4b, we have successfully obtained the NC pixels on a flexible polyethylene terephthalate film. To further explore the ability of the selective area ligand polymerization in high resolution pixel defining, we used a 2000-mesh copper grid as the mask to treat the NC films. Figure 4c shows that the well-defined NC-dot arrays with 2000 pixel per inch (ppi) resolution have been achieved. These NC pixels can endure long time water soaking, facilitating the subsequent process in device fabrication (Figure S6). By repeating the spin-coating, mask plasma treating and water-soaking processes using CsPbCl1.5Br1.5, CsPbBr3 and CsPbBr1I2 NCs in sequence, we can fabricate blue, green and red pixels on the same substrate. This opens the possibility for application in future full-color high-definition quantum dot displays. It is worth noting that, plasma sources are widely used in industries and source with area up to 1 m2 has already been available. This endorses a great potential of the plasma induced in-situ ligand polymerization in application for high definition quantum dot displays.

Figure 4. (a) Schematic of the NC film patterning via selected-area plasma induced ligand polymerization. (b) Photograph of patterned NC film under UV lamp on a


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flexible polyethylene terephthalate substrate. (c) Photograph of the green NC-dot arrays with scale bar of 25 μm. A 2000-mesh copper grid has been used as the mask. (d) Photograph of the blue, green and red NC-dot arrays on a 5 × 5 cm2 glass substrate. The diameter of NC-dot in (b) and (d) is 1 mm.

CONCLUSIONS In summary, we demonstrated a facile approach to dramatically improve the stability of the lead halide perovskite NC films against air, UV exposure, and water. By using Ar plasma to treat the NC films, the ligands containing a C=C bond can be polymerized in situ. As a result, the NCs are encapsulated and isolated from the ambient condition by the ligand polymers. After treated for 30 min, the NC films can endure water soaking for over 100 h. Due to the precise localization of the in-situ polymerization and high content of luminescent NCs without extra additives, the polymerized area can be precisely defined down to micrometer-scale, which enables high-resolution patterning of the NC films. These advantages, combining with the low process temperature and the availability of large area plasma sources, offer a powerful technology for NC films processing in mass production scale. Experimental section Chemicals and Materials: Lead (Ⅱ) chloride (PbCl2, 99.999%, Macklin), lead (Ⅱ) bromide (PbBr2, 99.999%, Yingkou Youxuan Trade Co. Ltd.), lead (Ⅱ) iodide (PbI2, 99.9985%, Energy Chemical), cesium carbonate (Cs2CO3, 99.999%, Macklin), cesium acetate (CsAc, 99.999%, Macklin), 1-octadecene (ODE, 90%, Macklin), lead



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(Ⅱ) acetate trihydrate (99.99%, Macklin), lecithin (98%, Macklin), oleic acid (OA, 90%, Shanghai D&B Biological Science and Technology Co. Ltd.), oleylamine (OAm, 90%,

Shanghai

D&B

Biological

Science

and

Technology

Co.

Ltd.),

3-(N,N-dimethyloctadecylammonio) propanesulfonate (97%, TCI), formaldehyde (37% in water, TCI), trioctylphosphine (TOP, 90%, Energy Chemical), 1-propanol (99.5%, Energy Chemical), octanoic acid (OC, 98%, Energy Chemical), octylamine (OCm, 98%, Energy Chemical), hexane (97%, Energy Chemical), ethyl acetate (99.8%, Energy Chemical), cyclohexane (99.5%, Energy Chemical), formic acid (99%, Energy Chemical), 1,3-propanesultone (98%, Energy Chemical), acetonitrile (99.9%, Energy Chemical). All chemicals were used as accepted without further purification. Preparation of Cs-oleate: Cs2CO3 (0.324 g), OA (1 mL), and ODE (16 mL) were loaded into a 50 mL two-neck flask and degassed for 20 min at 120 °C, and then heated to 160 °C under N2 until all Cs2CO3 reacted with OA. The solution was kept at 160 °C to avoid solidification before injection. Synthesis of OA/OAm capped CsPbX3 NCs: The method was followed a previous report developed by Kovalenko and co-workers with slight modification.1 Typically, ODE (15 mL), OA (1.5 mL), OAm (1.5 mL) and PbBr2 (0.207 g) were loaded into a 50 mL three-neck flask, degassed for 20 min at 120 °C, mixed for 30 min at 120 °C, and heated to 170 °C under N2. Cs-oleate solution (1.2 mL of stock solution prepared as described above) was swiftly injected. After 5 s, the reaction mixture was cooled by the ice-water bath. The obtained NCs were precipitated by adding equal volume of acetone and separated via centrifugation. The separated NCs were redispersed in 6


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mL of cyclohexane and centrifugated again to remove large NCs. The concentration of CsPbBr3 NCs in cyclohexane was about 8 mg/mL. It should be noted that multiple centrifugations should be avoided due to the facile loss of ligands. For chloride containing NCs, TOP was used to solubilize PbCl2. For purification of CsPbCl1.5Br1.5 and CsPbBr1I2 NCs, no external solvent was added. Synthesis of OC/OCm capped CsPbBr3 NCs: The OC/OCm CsPbBr3 NCs were synthesized according to the synthetic approach reported by Konstantatos et al.24 Briefly, 6 mL of hexane, 3 mL of 1-propanol and 32 mg of CsAc were mixed in a vial with constant stirring and 0.9 mL of PbBr2 solution (dissolved in 1:1:1 OcAc, OcAm and 1-propanol in volume with a concentration of 185 mg/mL) were injected. The NCs were centrifuged and redispersed in cyclohexane. Synthesis

of

3-(N,N-dimethyloctadecylammonio)

propanesulfonate,

3-(N,N-dimethyloleylammonio) propanesulfonate or lecithin capped CsPbBr3 NCs: The method was followed a previous reportedm method developed by Kovalenko with some modifications.25 Cs2CO3 (0.25 g), lead (Ⅱ) acetate trihydrate (0.095 g), ODE (5 mL), and OC (0.3 mL) were loaded into a 50 mL three-neck flask, degassed for 30 min at 120 °C. Then 0.1 mmol of 3-(N,N-dimethyloctadecylammonio) propanesulfonate, 3-(N,N-dimethyloleylammonio) propanesulfonate or lecithin was added and the mixture was heated to 160 °C under N2. 1 mL of TOP-Br2 stock solution (prepared by dissolving TOP-Br2 adduct in toluene to create a 0.25 M solution) was swiftly injected. After injection, the solution was immediately cooled by ice-water bath. The crude solution was centrifugated to remove large NCs and washed



with ethyl acetate. The final NCs were dispersed in cyclohexane to obtaind a clear solution. Film Preparation: CsPbX3 NCs were spin-coated on different substrates such as glass and silicon wafer at a speed rate of 500 rpm for 6s and 2000 rpm for 40s subsequently. 130 μL of NC solution was dropped on the substrate for spin-coating. Then the substrate was placed in a vacuum chamber for 30 min to obtain a dried film. The thickness of obtained NC film is about 110 nm from cross-sectional SEM analysis. Characterization: The transmission electron microscopy (TEM) images were taken on a JEM-2100 TEM instrument operated at 200 kV. The sample in the cyclohexane solution was dispersed onto a copper grid. Absorption spectra were collected using a Lambda 750S UV-VIS-NIR spectrometer. Scanning electron microscopy (SEM) images were obtained on a JSM 6701F system. The PLQY data of NC films were collected using an Edinburgh instruments FS5 spectrofluorometer equipped with an integrating sphere at the excitation wavelength of 400 nm. The contact angles of water were measured using a JC2000C1 contact angle meter. Fourier transform infrared spectroscopy (FTIR) was recorded using a Nicolet iS50 FT-IR spectrometer. Samples were prepared on the silicon wafers. The background was collected from the bare silicon. X-ray induced Auger electron spectroscopy (XAES) characterization was performed by an ESCALAB 250Xi XPS with a monochromatic Al Kα source (hν = 1486.6 eV). The take-off angle was 0° (angle between the electron optical axis of the spectrometer and sample surface normal). The spectra were obtained at the pass


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energy of 30 eV and step of 0.1 eV. The D parameter, i.e. the energy difference between the most positive and the most negative excursions in the first derivatives of the C KLL peaks, is utilized as the indicator of sp2/sp3 ratio. According to previous studies, the D parameter is ~13 eV for diamond (pure sp3) and ~22 eV for graphite (pure sp2), respectively.22 The cross-polarization magic angle spinning nuclear magnetic resonance (CPMAS-NMR) experiments were performed on a Bruker Avance III spectrometer, which is operating on 11.5 T with 1H resonance at 500 MHz. The samples were loaded into a 2.5-mm zirconia rotor and sealed with polytetrafluoroethylene (PTFE) cap. The spin rate was set as 20 kHz. The

13C

CPMAS-NMR spectra were accumulated by 2048 scans with recycle delay time of 2.5 s. Tetramethyl silane (TMS) was used as reference for

13C

spectra. To obtain

sufficient sample for CPMAS-NMR analysis, 70 pieces of glass (8 × 8 cm2) were used for plasma treatment. The NC films on glass substrate were treated for 30 min and scraped for NMR measurement. Remote Argon Plasma Treatment: The remote Ar plasma treatment was conducted in a plasma enhanced atomic layer deposition system (PE-ALD) from Jiaxing Ke-micro Technology. At first, the prepared CsPbX3 NC film was transferred into the chamber at room temperature. Before treatment, the chamber was evacuated to a pressure below 0.001 Torr, and then filled with Ar (99.999%) to an operating pressure of 0.135 Torr. The Ar flow rate is 65 sccm (standard cubic centimeter per min) from above and a total flow rate of 20 sccm on both sides. A radio frequency power source was used to generate the Ar plasma with the plasma power of 100 W. It should be



noted that high plasma power and long-time plasma treatment are not suggested due to the etching effect. The Ar plasma was generated inside the quartz tube and reached the surface of CsPbX3 NC film to induce ligand polymerization. The scheme of remote Ar plasma treatment and the experimental images of PE-ALD are shown in Figure S1. Film Patterning: The NC film patterning, involving two steps, can be readily accomplished. i) selected-area ligand polymerization, ii) NC film water-soaking. The film patterning is achieved via selected-area remote Ar plasma treatment with a ceramic mask covering the top. The designed mask is used to define the region for remote Ar plasma treatment in the PE-ALD and realize the selected-area ligand polymerization. And the typical remote Ar plasma treatment time is 10 min. Then the treated NC film is immersed in water. The CsPbBr3 NCs in the untreated region lose their PL, while the treated ones with Ar plasma remain luminescent due to the durability against water. As a result, the NC film finally displays a certain pattern. A more detailed explanation about the process are showed in Figure S6.

ASSOCIATED CONTENT Supporting information Additional experimental section and figures.

AUTHOR INFORMATION Corresponding Authors


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*[email protected] *[email protected] ORCID Li Wang: 0000-0002-5957-7512 Yonggui Liao: 0000-0003-2943-1501 Notes The authors declare no competing financial interests. ACKNOWLEDGEMENTS This work was financially supported by National Natural Science Foundation of China (61964011), National Key Research and Development Program of China (2018YFB0406704)

and

Natural

Science

Foundation

of

Jiangxi

Province

(20165BCB18004, 20171BCB23005).

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82x43mm (300 x 300 DPI)


Giant Stability Enhancement of CsPbX3 Nanocrystal Films by Plasma

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