Three-dimensional direct lithographic growth of stable perovskite nanocrystals inside glass

Source: Ke Song, Dezhi Tang, Xinyuan Fang, Xingtao Xia, Dajun Lin, Huang Song, Yonghong Lin, Zhaojun Liu, Ming Gu, Yuanzheng Yue, Jianrong Qiu. Three-dimensional direct lithography of stable perovskite nanocrystals in glass. Science 2022, 375, 307–310.

Summary: The development of the composition of materials and the manufacture of devices from perovskite nanocrystals (PNC) in solution introduce organic pollution and require several stages of synthesis, processing and stabilization. The authors report 3D direct PNC lithography with customizable composition and bandgap. Controlling the distribution of halide ions on the nanoscale using ultrafast laser-induced liquid-phase separation of nanophases. PNAs obtained by direct 3D lithography have excellent resistance to UV irradiation, organic solutions, and high temperatures (up to 250°C). Photolithographic 3D structures inside glass can be used for optical storage, miniature LEDs and holographic displays. The experimental results confirm the proposed mechanism of PNA formation and compositional tunability.

Compositional tuning of the optical properties of perovskites is often performed in solution to create materials with long-term stability for high performance devices such as mixed bromide-chloroperovskites and bromide-iodide perovskites, respectively. Spectrally stable and efficient blue and red light-emitting diodes (LED). Despite recent advances in optoelectronic performance, low structural stability has been a barrier to practical perovskite devices and many strategies have been developed such as surface passivation or device encapsulation. In these approaches, stabilization requires additional processing steps at the film or device level and is not a necessary condition for tuning the properties of nanocrystals (NCs).

The combination of freshly synthesized nanosilicon carbide and glass provides enhanced photonic functionality. However, the three-dimensional adaptation of the chemical composition and band gap of nanocarbons inside glasses, as well as the functional tuning of nanocarbon-based photonic devices, is a difficult task. In recent years, people have used ultrafast lasers to create 3D functional structures in transparent solids, but the intrinsic compositional tunability of functional structures is very limited.

The authors report a different strategy for processing the chemical properties of NCs. In particular, ultrafast laser pulses introduce energy in an ultrashort time, which leads to intense heat accumulation, which increases local pressure and temperature, which allows 3D direct lithography of composition-controlled perovskite nanocrystals (PNCs) inside glasses (Fig. 1A). The mechanism of modulation of the PN composition was elucidatedK by liquid-phase nanophase separation. In addition, the proprietary method can well protect PNA from intense ultraviolet (UV) irradiation, organic solutions, or temperatures up to 250 oC.

As a medium for direct PNA lithography, the authors used glass oxide containing cesium, lead, and halide elements. Borophosphate glass is a typical oxide glass with a molar composition 40B2O3-15P2O5-10Al2O3-10ZnO-5Na2O-5K2O-7Cs2O-3PbX2-5NaX (where X is Cl, Br or I). The high mobility of cesium, lead, and halides promotes the separation of perovskite nanophases from the glass matrix and the subsequent formation of PNA with a controlled composition. By optimizing pulse duration, repetition rate, and pulse energy, the authors have realized an ideal ultra-fast direct laser lithography process. Photoluminescence (PL) of freshly prepared PNAs was modulated in the wavelength range of 520–690 nm (Fig. 1B). The light emission at 520 and 690 nm is due to the exciton recombination of CsPbBr3 and CsPbI3 NCs, respectively, and the emission between these two wavelengths comes from the mixed halide CsPb(Br1-xI x ) 3 NC, where x is determined by Vegard's law. Transmission electron microscope (TEM) images and Raman spectra confirmed the existence of PNA, and the average size of PNA was 1-4 nm.

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The author controlled the kinetic process of liquid-phase separation of nanophases by modulating the time of ultrafast laser irradiation (ti) (Fig. 2A). The migration rate of the halide ion depends on the complex formation of Pb2+ with the halide ion, as well as on the radius and weight of the ion. Compared with I-, the complexation between Pb2+ and Br- is stronger, the ion mass is lighter, and the radius is smaller, so that Br- diffuses faster and more easily to form Br-rich liquid perovskite through nanophase separation. Continued irradiation can cause more I ions to diffuse from the liquid glass region into the liquid perovskite region, and by increasing ti, the emitted light of the final PNCs changes from green to red (Fig. 2B). ).

To test the author's method, CsPb(Cl1-xBrx)3 NCs were created in glass, and the emission light can be modulated in a wide wavelength range of 450-514 nm by adjusting the laser parameters. In addition, the authors successfully developed the composition and band gap of PNA in glasses doped with Cl-Br-I. Thus, full-color printing of PNA was achieved by PL modulation in the range of 480–700 nm (Fig. 2C), which reflects the conversion of CsPb(Cl1-xBrx)3 to CsPbI3, thus confirming the compositional processing of PNA. Whether in Cl-Br doped glass or Cl-Br-I doped glass, PNC photoluminescence is continuously modulated by changing ti, in particular as ti, the main photoluminescence peak shifts to the long wavelength region. The distribution of halides in PNA in glass cannot be controlled by conventional uniform heat treatment.

In glass-forming systems, phase separation occurs in the presence of a chemical potential gradient. Based on the experimental results of the authors, this paper uses Br-I doped glass as an example to propose a mechanism for PNA formation by nanophase separation (Fig. 2A). First, the formation of immiscible phases leads to the separation of liquid phases at the nanoscale. Thus, separation of the high-bromine halide phase from the glass matrix phase occurs at temperatures above the liquidus temperature of the glass constituents. Second, continuous ultrafast laser irradiation not only increases the size of the liquid perovskite domains, but also causes an exchange of I for Br positions due to the chemical potential gradient (Fig. 2A). As laser irradiation progresses, I- ions gradually diffuse from the surrounding liquid into relatively ordered liquid perovskite domains and finally form liquid perovskite nanodomains containing I. Nanoscale phase separation lowers the energy barrier to form ordered domains with a perovskite-like structure. Third, during cooling, the liquid-phase ordered perovskite domains become more ordered and form crystallization nuclei that grow into PNA under conditions of limited diffusion and reaction.

During the ultra-fast laserIn direct lithography, the temperature of the laser impact region rises rapidly with increasing number of pulses and remains stable at its maximum value after tens of pulses (usually less than 100 pulses, which corresponds to 100°C). i ultrafast laser kHz is 1 ms). After turning off the ultrafast laser radiation, the hardening process took place. The ultrafast laser-induced temperature (> 1000 oC) in the modified region is higher than the liquidus of the glass composition. Thus, the dependence of the PNA radiation wavelength on ti confirms the presence of liquid-phase nanoseparation. With an increase in ti from 350 ms to 1200 ms, the average size of CsPb(Cl1-xBrx)3 NCs increases from 1.9 nm to 3.6 nm, which has obvious characteristics of a continuous partial liquid phase. nanoseparation.

When adjusting the parameters of the ultrafast laser, the apparent evolution of the PNC (Fig. 2B) can be attributed to differences in temperature, pressure, and illumination of the ultrafast laser. For example, phase diagrams are pressure dependent, and increasing the pressure to billions of Pascals can help separate the liquid phase. These features partly explain why ultrafast laser heating can lead to liquid-phase separation of nanophases, while conventional heat treatment cannot.

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PNA instability can occur due to chemically and thermally induced degradation and light-induced phase segregation. The authors studied the resistance of PNA to UV irradiation, heat treatment, or solvent (ethanol). All PNAs emitting green, yellow, orange, and red PL were stable; the PL intensity did not change after 12 h of UV irradiation (Fig. 3A). In addition, the PL peak does not shift when CsPb(Cl1-xBrx)3 NCs are irradiated with UV light with a power density (IUV) of 2 W/cm2 or even 32 W/cm2. (Fig. 3B), which means no phase segregation. For reference: UV light with I 0.1 W/cm2 UV radiation can cause significant phase separation in mixed halide perovskites.

The PNA dispersion in ethanol remained stable, and the PL quantum yield did not change after 6 months (Fig. 3C). The PL intensity and PNA location did not change after heat treatment at 85°C for 960 h and even after heat treatment at 250°C for 2 h under atmospheric conditions (Fig. 3d). The high stability of the PNC is due to the effective protection of the glass matrix, which prevents the recorded PNC from being exposed to environmental molecules at various temperatures. In addition, factors such as nanometer confinement, laser-induced ultra-rapid set, high surface-to-volume ratio, high cohesive energy, and limited carrier diffusion length in smaller PNAs can lead to ion diffusion and UV-induced radiation. The phase separation is strongly suppressed.

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Writing synthetic custom PNCs allows for multidimensional information coding and forgery protection (Figure 1A). For example, the green, yellow, and red logos of Zhejiang University are written directly on the glass (Figures 4A-4C). On fig. 4D and 4E are respectively CsPb(Br1-xIx)3 NA and CsPb(Cl1-xBrx > em>)3 NCs created colored patterns on the respective glasses. The authors also demonstrated full-color PNC printing in Cl-Br-I doped glass (Fig. 4F) and 3D microspiral patterning of PNC (Fig. 4G).

Micro-sized LEDs (μ-LEDs) for high-resolution displays were fabricated using wet chemical NCs. For standard NC-based devices, dots with different wavelengths of radiation are printed or transferred onto a substrate; the preparation of NCs and the manufacture of the device become more complicated. Therefore, the manufacturing cost of the device is high, the stability of CNC processing is low, and CNC forming is difficult. These shortcomings severely limit the widespread use of CNC-based devices. In addition, despite significant efforts to develop glass as light-emitting materials and devices, it has not yet been possible to endow a single glass chip with wide and continuously adjustable colors or to print CNC-based devices such as mu-LEDs and solid-state LEDs. status displays.

PNC glasses can be used as key components of photonic devices, not just transparent protective layers or substrates. The authors demonstrate one-step 3D printing of color PNC patterns with a dot size of <10 µm, which can be used to create micrometer-sized devices. Combined with commercially available UV or blue LED arrays, this method can be used to make mu LEDs, high resolution displays, and even white LEDs. In addition, PNC arrays are used as holographic display devices. By switching holograms, a dynamic holographic display based on PNC devices was realized (Fig. 43). In addition, three holographic images (letter "Z", "J", and "U") were simultaneously reconstructed in multiple planes along the light propagation direction, indicating that the excitation of specific PNC patterns can realize three-dimensional holographic display.

Because liquid nanophase separation occurs only locally within the glass, direct 3D lithography is a completely dry fabrication technology that improves the fabrication efficiency of structures and devices with high throughput and high scalability. Contamination of organic components (reagents and solvents) during the synthesis of materials and processing of devices is not taken into account. In addition, the high temperature stability of the PNC-based glass device suggests that the device canbut use for a long time. This study demonstrates that the composition and band gap of PNC can be tuned over the entire tunable range on a single solid crystal.

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