Abstract
1. Introduction
Polymer-assisted deposition (PAD), which was first reported in 2004[
Figure 1.(Color online) Timeline showing key development by polymer-assisted deposition. Metal oxides; metal nitrides; metal carbides; glassy-graphene; MoS2; MoS2/glassy-graphene heterostructure.
Figure 2.(Color online) Application of as-grown thin films by PAD.
The discovery of single-layer graphene via mechanical exfoliation in 2004[
2. Polymer-assisted deposition
2.1. Development of polymer-assisted deposition
The past 15 years have witnessed rapid developments in the preparation of epitaxial thin films by using PAD. Thus far, PAD has been successfully used to grow metal-oxides[
Metal oxides have received considerable attention because of their potential applications in nuclear targets[
Metal nitrides are used in many fields due to their hardness, electronic properties[
Transition-metal carbides exhibit high melting point, high electrical conductivity[
Two-dimensional layered materials, such as graphene and MoS2, are another type of materials prepared by PAD. In the past few years, glassy graphene[
2.2. Principles and processing steps of PAD
In the PAD process, metal ions are coordinated to the polymer as the precursor. Covalent complexes are formed between the metal cations and the lone pair on the nitrogen atoms of the polymer. Thus, the oligomerization reaction will not occur unless certain conditions are satisfied. Hence, the solutions are stable for months. At approximately 450 to 500 °C, the polyethyleneimine (PEI) polymer undergoes thermal depolymerization back to NH2CH=CH2. The ethylenediaminetetra-acetic acid (EDTA) decomposes to acetic acid, formic acid, and ethylenediamine even in inert or H2 atmospheres.
The main processing steps involve the preparation of the metal-precursor solution, ultrafiltration, coating, and annealing. Fig. 3 illustrates the typical PAD steps for the growth of thin films. We will describe the unique chemistry and basic steps of PAD in the following part.
Figure 3.(Color online) Schematic illustration of the main processing steps used to grow thin films by PAD.
2.2.1. Preparation of metal-polymer solution
Table 1 summarizes over 45 different elements that can be coordinated with polymers to form a stable polymer precursor solution. In PAD, the polymer in the solution binds to the metal ions via electrostatic attraction, hydrogen bonding, and/or covalent bonding. The first-row transition metals, using nitrates, acetates or chlorides, bind easily to the simple PEI polymer. Other hard-to-bind metals, such as Sn2+ and Ti2+, need the PEI to be functionalized with carboxylic acids to provide a stable coordination environment. The third method for binding metals utilizes the ability of protonated PEI to coordinate anionic metal complexes. For instance, EDTA could form stable complexes with almost all metals, and then the complexes successfully bind to the PEI.
In Table 2, we summarize 40 different elements that bind well to the polymer. These metal-polymer solutions have been reported in previous works. Interestingly, one metal elemental may be bound with different polymers (PEI or PEI-EDTA).
2.2.2. Ultrafiltration
The metal-polymer solution passes through a filter or membrane to remove cations and anions that are not coordinated polymers, as shown in Fig. 3(b). In the ultrafiltration process, Amicon® ultra centrifugal filtration units and a centrifugal apparatus are used for filtration in our experiments.
2.2.3. Coating
After ultrafiltration, the polymer solution is coated onto different conformation substrates via various methods, including spin, dip, spray, and inkjet. Therefore, the substrate need not to be flat, such as AnodiscTM membranes[
2.2.4. Thermal de-polymerization and crystallization
To depolymerize the polymer and enable the crystallization of the film, the coated substrate is then treated in a controlled environment at the desired temperature. The water is driven out at moderate temperature (approximately 120 °C). Furthermore, the PAD process involves high -temperature (approximately 500 °C) exposure in a controlled environment to remove the polymer. The PEI and EDTA in the precursor film do not undergo combustion, but rather thermal depolymerization back to NH2CH = CH2, acetic acid, formic acid, and ethylenediamine. Notably, this non-combustive process can lead to reduced carbon contamination in the synthesized thin films.
The thin films may be single-crystal, polycrystalline, or amorphous, depending on the annealing temperature and substrate used. Importantly, the composition of as-grown materials is determined by the metal precursor, temperature, and atmospheric environment. For example: (1) the thermal treatment of the precursor film containing Ti ions in a reducing atmosphere, such as argon mixed with hydrogen, will result in pure Ti. (2) the precursor film will be converted to TiO2 if the thermal treatment is performed in pure oxygen[
3. Large-scale 2D materials by PAD
3.1. Transparent carbon films and glassy graphene thin films
Transparent conducting films are highly important to electronic, flexible, and transparent devices. Graphene has potential applications in solar cells, touch panels, wearable electronics, and flexible displays[
Cao et al.[
As shown in Figs. 4 and 5, glucose (C6H12O6) was utilized by Dai et al.[
Figure 4.(Color online) Evolution from glassy carbon to glassy-graphene and graphene[
Figure 5.(Color online) Preparation of glassy graphene-based circuits and the flexibility test[
The three types of carbon-based thin films are deposited by PAD under different catalysis conditions, as shown in Fig. 4. (1) The glassy carbon film, which is partially crystallized and disordered, is grown in Figs. 4(a)–4(c). (2) Glassy graphene, an intermediate state between glassy carbon and graphene, is obtained at 850 °C as shown in Fig. 4(d). TEM studies in Fig. 4(f) show twisted lattice planes. The bent and curved lattice plane is one of the distinguishing features of glassy graphene. (3) When the annealing temperature is increased to 1000 °C, graphene evolves from glassy graphene. From the HRTEM lattice image and the six reflex spots in the SAED, we could confirm the high-quality of graphene in Fig. 4(i). Based on the above analysis, the structural evolution of the three types of material is described in Fig. 4(j). In addition, glassy carbon is partially crystallized and disordered as shown in Fig. 4(c); the glassy graphene in Fig. 4(f) shows a high crystal quality but has twisted, bent lattice planes; and the graphene in Fig. 4(i) has perfect lattices.
Fig. 5(a) indicates that a circuit pattern is obtained after the laser writing and rinsing process. The glassy graphene circuits may be easily transferred to any substrate after annealing, as shown in Fig. 5(b), such as a flexible or rigid substrate. In Fig. 5(c), the sheet resistance is mediated with the bending radius. In addition, the vibration is anisotropic. After repeated bending or twisting, the resistance does not show any obvious changes as shown in Fig. 5(d). In this work, graphene FET is also fabricated to explore its potential application.
For the first time, an ultra-smooth glassy graphene thin film is grown by PAD at the inch scale. The thin film exhibits excellent conductivity, transparency, flexibility, and mechanical and chemical stability. Most importantly, as-deposited thin films are imprinted in flexible and transparent devices.
3.2. Highly scalable synthesis of MoS2 thin films
Two-dimensional semiconductors MoS2 are attracting a wide range of research interest due to their potential applications. MoS2 thin films are also prepared through PAD. Furthermore, as-deposited thin films are fabricated into a photodetector with a broad spectral response and excellent performance.
Zhu et al.[
Figs. 6(a)–6(d) shows that the thin film is smooth, continuous, homogeneous, and dense. The thickness and root-mean-square (RMS) surface roughness of the MoS2 thin film are approximately 90 and 10.7 nm, respectively. The HRTEM image and the SAED pattern suggest that the film has high crystallinity.
Figure 6.(Color online) Thickness-dependent bandgap tunable MoS2 thin films for optoelectronics[
In addition, the optical band gap energies for the films with different thicknesses are estimated by the UV–vis absorption spectra. The PL responses are the intensity decay and red-shifted in thicker films in Fig. 6(g). The A1g peak shows a blue-shift, which is consistent with the previous report. Notably, the thickness is mediated by the concentration of the Mo precursor.
To explore their photoresponse properties, MoS2 films are fabricated into photoconductors, and then characterized under simulated AM 1.2 illumination. Interestingly, the ratio of conductivity under illumination to dark conductivity is near 3, and the average response time is approximately 0.3 s, as shown in Table 3.
Yang et al.[
Figure 7.(Color online) Wafer-scale synthesis of MoS2 thin films via polymer-assisted deposition[
Large-scale and controllable thickness is the main characteristic of MoS2 thin films from PAD. Given that most of as-grown thin films are polycrystalline and have many defects, the photodetector fabricated by as-deposited MoS2 thin films did not exhibit excellent properties, as shown in Table 3.
Ren et al.[
Figure 8.(Color online) Formation of large-area web buckles[
3.3. MoS2/glassy-graphene heterostructures as transparent photodetectors
Based on the previous experiment results[
Figure 9.(Color online) Schematic of MGH preparation and 3D view of the transparent photodetector, photoresponsivity and time-resolved photoresponse of photodetectors under different illuminations[
In summary, the heterostructures are synthesized by a layer-by-layer transfer technique, and their application as transparent photodetectors are reported for the first time[
4. Prospective and challenges
Compared with the experimental methods, such as chemical vapor deposition (CVD) and atomic layer deposition (ALD), polymer-assisted deposition (PAD) has the advantages of low cost, large scale, easy doping and conformal coatings.
Some novel 2D semiconductors and various functional thin films have been successfully deposited by PAD, but it still remains several challenges on the synthesis of mono-layer thin films. Most of current MoS2 thin films synthesized by PAD still have few layers and are polycrystalline with many defects. The growth of large-scale mono-layer thin films may be difficult to realize by PAD, thereby affecting the transport performance of the corresponding devices. In the future, if large single crystals can be realized by PAD, their device applications will be fully extended to mass production.
The other challenge is the preparation of 2D materials with doping by PAD. Doping, which is the intentional introduction of impurities into a parent material, plays a significant role in functionalizing 2D materials. For example, the wolfram and selenium chemical doping of MoS2 is an effective way of engineering the optical bandgap[
As a characteristic of PAD, conformal coatings have been grown on non-planar surfaces such as quartz fibers[
Acknowledgments
L.Z. acknowledges support from the National Natural Science Foundation of China (Grant No.11774279), the Young Talent Support Plan of Xi’an Jiaotong University, and the Instrument Analysis Center of Xi’an Jiaotong University. K.L. acknowledges the support from National Key R&D Program of China (No. 2018YFA0208400), National Natural Science Foundation of China (Nos. 51602173 and 11774191), and Fok Ying-Tong Education Foundation (No. 161042).
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