Abstract
1. Introduction
Inorganic oxide thin films have attracted extensive attention thanks to their rich electrical, optical, thermal, mechanical, magnetic and other properties[
Benefiting from the rapid development of materials, mechanics and manufacturing science, flexible oxide thin films have been enabled to achieve various deformations, such as bending[
Figure 1.(Color online) Advanced strategies for high quality inorganic oxide thin-film flexibility. In particular, the physical flexible strategies include LLO and van der Waal epitaxy, while the chemical flexible strategies include transferring films from silicon-based substrates and traditional single crystal oxide substrates to flexible substrates after etching the sacrificial layer.
2. Physical strategies
Physical strategies refer to thin films that can be mechanically stripped to achieve macro-scale flexibility. LLO technique is a selective technology to remove one material from another[
2.1. Mechanical peeling through LLO
In the LLO technique, a transparent substrate is required to allow the laser beam to scan the entire interfacial film. Because the laser beam energy is lower than the band gap of substrate material and higher than that of upper film material, the laser beam energy can be absorbed only by the upper film, which results in damage-free separation of the upper film from the substrate[
Figure 2.(Color online) Mechanical peeling through LLO. (a–d) Schematic illustrations of the LLO process in fabricating flexible memory, skin-like transistor, thermoelectric generator, NG[
Lee’s group has fabricated flexible nanogenerator (NG) by growing the PbZr0.52Ti0.48O3 (PZT) film by sol-gel method with a subsequent crystallization step at 650 °C on sapphire substrate[
The LLO technique can realize the large area transfer of thin films with low damage. In addition, the substrate (such as sapphire used in LLO technique) can be reused[
2.2. Mechanical peeling by van der Waal epitaxy on mica
Micas are well-known phyllosilicates belonging to monoclinic structure. Muscovite (M-Mica) and fluorphlogopite (F-Mica) are common substrates in the thin film fabrication process. The M-Mica can be perfectly split along (001) plane by mechanical exfoliation, resulting in atomically smooth surface. M-Mica also possesses rich properties of chemical inertness, high transparency, and mechanical flexibility. The F-Mica has the similar properties to M-Mica, but it has lower flexibility than M-Mica, while its thermal stability is better than that of M-mica[
Figure 3.(Color online) Mechanical peeling by van der Waal epitaxy on mica. (a) Schematic of deposition process for flexible CFO film monitored by RHEED in real time (left), and the magnetic hysteresis loops (right) show that the flexible CFO/mica exhibits solid magnetic properties regardless of bending[
In the process of promoting device flexibility, the coupling between material properties and mechanical deformation should be of concern, including magnetostrictive properties, piezoelectric effects, and so on. Magnetostriction represents the relationship between the strain state and the magnetic state of a ferromagnet, which can be quantified by magnetostrictive coefficient (λ). To achieve higher response to magnetic field, Chu’s group has grown van der Waals epitaxial CoFe2O4 (CFO) thin films on M-Mica by the pulsed laser deposition (PLD) method. The reflective high energy electron diffraction (RHEED) is adopted to in situ monitor the deposition[
The clamping effect caused by rigid substrate has always been an important factor to weaken the magnetoelectric coupling. To overcome this problem, Chu’s group has prepared van der Waals epitaxy self-assembled BiFeO3 (BFO)-CFO bulk heterojunctions on M-Mica[
A flexible memory unit is an important part of flexible display screens and smart wearable devices. Although organic memory is more flexible than inorganic memory, it generally has poor high temperature stability and is easy to oxidize under illumination. Thus, it is important to develop inorganic memory with mechanical flexibility and high temperature stability. Chu’s group has grown van der Waals epitaxial PbZr0.2Ti0.8O3 thin films on M-Mica[
Transparent conducting oxides (TCO) have been widely used in solar cells, light emitting diodes, photodiodes, thin film transistors, photocatalysis, flat panel displays, gas sensors and energy-saving windows, and have become the basic components of advanced technology and equipment. With the development of portable and flexible electronic devices, it is particularly important to find a transparent and mechanically flexible base for TCO. As mentioned earlier, the commonly used transparent and flexible substrates are problematic; for example, ultra-thin glass is fragile and expensive, while polymer substrates such as PET and PI are not thermally stable, which impedes the growth of high-quality films. Meanwhile, micas are the ideal substrate for TCO preparation because of their high transparency, smooth atomic surface, thermal stability, chemical stability, flexibility, and mechanical durability. Chu’s group has grown van der Waals epitaxy TCO including ITO and Al-doped ZnO (AZO) films on M-Mica. A yttria-stabilized zirconia (YSZ) buffer layer was deposited before the ITO deposition, which is confirmed later that YSZ/mica heterostructure has good ionic conductivity and a high transmittance of more than 90% in the visible (380–800 nm) range[
In summary, micas can be widely used in various flexible devices. Generally, M-Mica has higher transparency, slightly better electrical properties and better flexibility than F-Mica, while F-Mica has better thermal stability than M-Mica, thus F-Mica is more suitable for the films which need higher annealing temperature for better crystallinity, and M-Mica is more suitable to increase the flexibility of films or devices. However, even though the advantages of micas enable the epitaxial layer to grow with its bulk lattice, it can hardly control the orientation of epitaxy layer, thus a thin buffer layer on the surface of mica or surface treatment for mica is usually needed[
3. Chemical strategies
For chemical strategies to make flexible thin film devices, the basic idea is transferring films from rigid substrates to flexible substrates after etching the sacrificial layer. In 2004, Rogers’s group first proposed microstructured silicon (μs-Si) technology, which involves photolithography, dry etching, wet etching and transfer printing process[
Figure 4.(Color online) Transfer printing technology. (a) Schematic illustration of the generic process flow for transfer printing[
Rogers’s group and Huang’s group have analyzed the dynamic control of the adhesive force of elastic stamps[
3.1. Transferring of oxide thin films grown on silicon-based substrates
Based on μs-SC technology, Lee’s group has deposited BTO films on prepared Pt/Ti/SiO2/Si substrate by radio frequency magnetron sputtering, following by annealing process above 600 °C[
Figure 5.(Color online) Transferring of oxide thin films grown on silicon-based substrates. (a–c) A cross sectional image (a), optical image (b) and dielectric properties (c) of the bendable PEO/Au/BTO/Pt/TiO2/SiO2/Si structure[
Metal–insulator transition (MIT) refers to the phenomenon of transforming or reversing from metal to insulator under certain external conditions, which has attracted considerable attention. Vanadium dioxide (VO2), as a well-known MIT material, will change from insulating monoclinic phase at low temperature to metal tetragonal phase at high temperature, and its optical and electrical properties will change dramatically at the same time. Its phase transition temperature (Tc) is close to room temperature and the resistance difference before and after phase transformation can reach 105. Moreover, VO2 has a high temperature coefficient of resistance (TCR) near room temperature, so it is very suitable for sensitive materials in temperature sensors. Lin’s group has prepared VO2 thin films on SiO2/Si substrates by the polymer-assisted deposition (PAD) method[
Figure 6.(Color online) Flexible sensors based on VO2/PDMS structure. (a) The resistance versus temperature curves for the VO2 film in different states[
Benefiting from the high TCR of VO2 near room temperature, Lin’s group has designed and fabricated a flexible breathing sensor with ultra-fast response based on PI/VO2/PDMS structure[
The transfer based on silicon-based substrates is compatible with the modern electronic industry. However, it is also obvious that epitaxial growth of most of the oxide thin films on silicon-based substrates is difficult because of the amorphous nature of the SiO2 layer. Therefore, it is very important to extend the transfer-printing technology used in silicon-based substrates to other single crystalline oxide substrates.
3.2. Transferring oxide thin films grown on single crystalline oxide substrates
With the development of strain engineering, researchers have done a lot of work on introducing large biaxial strain through heteroepitaxy. As shown in Fig. 7(a), following the energy preference of the substrate material or the lower layer epitaxial material, when the film material is deposited on the substrate with a larger lattice constant, the deposited atoms can cause the film to begin biaxial stretching. In contrast, when the film material is deposited on the substrate with a smaller lattice constant, the deposited atoms can cause the film to start biaxial compression[
Figure 7.(Color online) Transferring oxide thin films grown on traditional oxide substrates. (a) Crystal structure of biaxial compression and stretching process during depositing films on substrate with a smaller or larger lattice[
In recent years, many researchers have tried to transfer inorganic oxide films grown on single crystalline oxide substrates to other substrates using the approach of growing and etching a sacrificial layer. The sacrificial layers on single crystal oxide substrates correspond to the different etching solutions, which are often buffer solutions of different acids and salts[
After successfully transferring inorganic oxide films grown on single crystalline oxide substrates, researchers have found that the properties of inorganic oxide films can also be tailored with the stress regulation of flexibility, tension and torsion of inorganic oxide films transferred onto flexible substrates. Qi’s group has prepared PZT nanoribbons on MgO substrates[
Unlike etching with acid or acid salt solutions, Sr3Al2O6 (SAO) as sacrificial layer can be etched by water. Nie’s group has prepared free-standing BFO film with atomic layer thickness by etching the water-soluble SAO buffer layer, which opened the door of two-dimensional quantum phenomena with strong correlation[
Transferring the films grown on single crystalline oxide substrates broadens the application field of the transfer-printing technology, and develops strain engineering and the applications of oxide thin films at the same time. It is worth noting that the selection of sacrificial layers and the corresponding etching solutions play crucial roles in achieving flexible inorganic oxide thin films from the epitaxial layers on single crystalline oxide substrates.
4. Conclusion
Inorganic oxides have already occupied an important position in various applications because of their rich physical properties. By achieving the flexibility of inorganic oxides films, they also show great potential in wearable devices. At present, the main approaches to the flexibility of high quality inorganic oxide film can be divided into the physical approach of mechanical peeling films from rigid substrates, and the chemical approach of transferring films from rigid substrates to flexible substrates after etching the sacrificial layer. However, there are still many problems in the preparation of stable and high-performance inorganic oxide films. In particular, typical physical approaches may rely on LLO and van der Waal epitaxy on mica. In the LLO technique, the consumption of epitaxial layer and uneven heating or cooling of the upper layer during laser irradiation may affect the performance. Van der Waal epitaxy on mica is difficult to achieve films with good stretchability. For the chemical approach, after extending the flexible technologies used in silicon-based substrates to other single crystalline oxide substrates, the selections of sacrificial layers and corresponding etching solutions should also be broadened.
Flexible inorganic oxide films and devices have exhibited great performance, and they will play key roles in the next generation of flexible wearable devices. To meet the market’s requirements, the universality and practicability of methods for achieving flexible high quality inorganic oxide films should be further improved. Meanwhile, various flexible high quality inorganic oxide films should be integrated for practical applications. With the rapid development of materials, mechanics, manufacturing science and the rise of flexible devices, people are increasingly looking forward to the application of inorganic oxide functional films in flexible devices. This can not only greatly reduce the size of traditional devices, which will hopefully break Moore's law and greatly improve device performance, but can also expand the application field of flexible devices. At the same time, the development of inorganic oxides in rigid devices has matured and it is bound to accelerate with the development of flexible devices. Because the core of realizing inorganic oxide thin film flexible devices is to realize flexible inorganic oxide thin film, getting free-standing or self-supporting inorganic oxide thin film and realizing the integration of inorganic oxide thin film on flexible substrates have become the research focus. It is believed that flexible, high quality, inorganic oxide films and devices will promote the development of flexible wearable devices, and make our life more efficient and convenient in the electronic information era.
Acknowledgements
We acknowledge the financial support from the National Basic Research Program of China (973 Program) under Grant No. 2015CB351905, the Technology Innovative Research Team of Sichuan Province of China (No. 2015TD0005), "111" project (No. B13042), China National Funds for Distinguished Young Scientists (No. 61825102).
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