Solution-processable 2D semiconductors for high-performance large-area electronics

ConspectusTwo-dimensional (2D) semiconducting transition metal dichalcogenides (TMDs) have demonstrated exceptional potential as materials for future complementary metal-oxide-semiconductor (CMOS) technology. This is primarily because of their atomic thickness and excellent electrical and mechanical properties. With advancements in fabrication technology, electronic devices based on 2D TMD materials have rapidly progressed from isolated units for scientific experimentation to integrated circuits with practical applications. Among the different production methods, the solution-processing of 2D TMD nanomaterial dispersions offers the distinct advantages of low-temperature processing and cost-effective manufacturing for large-scale flexible and wearable electronics. A wide range of 2D nanoflake inks with versatile electronic properties can be assembled into atomic-thick thin films with dangling-bond-free van der Waals interfaces between adjacent nanoflakes. Furthermore, direct printing techniques can easily integrate multifunctional devices, such as n-type and p-type transistors, into CMOS devices and more complex integrated circuits. Despite these benefits and previous accomplishments, the field of solution-processed CMOS electronics using 2D TMD semiconducting materials is in its early stages of development and requires further research. One of the current challenges is the production of scalable and high-purity 2D semiconductor mono- and few layers with large lateral sizes and narrow thickness distribution. The field-effect mobility of solution-processed 2D TMD transistors remains lower than that of the transistors manufactured using mechanical exfoliation and chemical vapor deposition methods. In particular, limited research has been conducted on solution-processed p-type 2D TMD transistors. As a result, solution-processed CMOS devices using n-type and p-type 2D TMD transistors are scarce. In this Account, we provide an overview of the recent progress in the field of solution-processed CMOS electronics employing 2D TMD materials. First, we introduce the basic liquid exfoliation methods, such as sonication-assisted exfoliation and molecular intercalation methods, that are commonly utilized to prepare 2D TMD dispersions. In addition, we discuss the production of monolayer 2D materials, which serve as the building blocks for fabricating atomic-thick thin films. Subsequently, we review the typical techniques for depositing 2D inks, including spin coating, drop casting, and inkjet printing. Furthermore, we outline the thin-film patterning process for each technique, which is crucial for integrating multifunctional materials in CMOS devices. Subsequently, we focus on the recent advancements in solution-processed 2D TMD transistors. Furthermore, we explore the various factors that can improve the performance of the devices with regard to charge transport and charge traps. Afterward, we highlight notable applications of solution-processed CMOS technology, such as logic circuits and ring oscillators. Finally, we provide an overview of the challenges and opportunities in the development of solution-processed 2D materials and the integration of multifunctional devices for the advancement of CMOS electronics. This Account aims to provide a comprehensive guide for readers, offering both a broad overview and an in-depth insight into solution-processed 2D material-based electronics, covering a wide range of topics from the preparation of 2D TMD ink to device fabrication and CMOS applications. Therefore, this Account is expected to drive further progress and advancements in this field and promote the realization of practical applications.

Solution-based processing of two-dimensional (2D) materials has garnered significant interest as a facile and versatile route for the large-scalable production of 2D material films. Despite the benefits in process, these films were not considered suitable for device applications during the early stages of research because their electronic properties were far from those of 2D materials obtained through micromechanical exfoliation or chemical vapor deposition. Due to the small lateral dimensions and polydisperse thickness of constituent 2D nanosheets, the resulting film tends to be porous and exhibits numerous inter-sheet junctions, primarily contacting edge-to-edge. This nanosheet morphology leads to poor electrical conductivity of the network, and also hinders the film functioning as a semiconductor or an insulator. To produce ultrathin 2D nanosheets with narrow thickness distribution and large lateral sizes, various chemical exfoliation strategies have been explored, but these are limited by long process times, involvement of harsh chemicals, and/or undesired structural damage or phase changes. Recent breakthroughs in electrochemical exfoliation using tetraalkylammonium intercalants enabled the production of high-quality 2D nanosheets with structural characteristics favorable for producing ultrathin, conformal films of 2D materials, which allow for scalable production of high-performance electronic components that can readily be assembled into functional devices via solution-processing. In this review article, we aim to offer an extensive introduction solution-based processing techniques for acquiring 2D nanosheets, their subsequent assembly into thin films, and their diverse applications, primarily focusing on electronics and optoelectronics but also extending to other fields. Remaining challenges and potential avenues for advancement will also be discussed.

Two-dimensional (2D) materials have attracted significant attention as resistive switching materials for two-terminal non-volatile memory devices, often referred to as memristors, due to their potential for achieving fast switching speeds and low power consumption. Their excellent gate tunability in electronic properties also enables hybrid devices combining the functionality of memory devices and transistors, with the possibility of realizing large-scale memristive crossbar arrays with high integration density. To facilitate the use of 2D materials in practical memristor applications, scalable synthesis of 2D materials with high electronic quality is critical. In addition, low-temperature integration for complementary metal oxide semiconductor (CMOS) back-end-of-line (BEOL) integration is important for embedded memory applications. Solution-based exfoliation has been actively explored as a facile, cost-effective method for the mass production and low-temperature integration of 2D materials. However, the films produced from the resulting 2D nanosheet dispersions exhibited poor electrical properties in the early stages of research, thereby hindering their use in electronic devices. Recent progress in the exfoliation process and post-processing has led to significant improvements in the electronic performance of solution-processed 2D materials, driving increased adoption of these materials in memristor research. In this review article, we provide a thorough overview of the progress and current status of memristive devices utilizing solution-processed 2D resistive switching layers. We begin by introducing the electrical characteristics and resistive switching mechanisms of memristors fabricated with conventional materials to lay the groundwork for understanding memristive behavior in 2D materials. Representative solution-based exfoliation and film formation techniques are also introduced, emphasizing the benefits of these approaches for obtaining scalable 2D material films compared to conventional methods such as mechanical exfoliation and chemical vapor deposition. Finally, we explore the electrical characteristics, resistive switching mechanisms, and applications of solution-processed 2D memristive devices, discussing their advantages and remaining challenges.

Layered two-dimensional (2D) nanomaterials interact primarily via van der Waals bonding, which has created new opportunities for nanoelectronic heterostructures that are not constrained by epitaxial growth [1]. However, it is important to acknowledge that van der Waals interactions are not limited to interplanar interactions in 2D materials. In principle, any passivated, dangling bond-free surface interacts with another via non-covalent forces. Consequently, the emerging layered 2D nanomaterials can be integrated with a diverse range of other materials [2-4], including those of different dimensionality, to form van der Waals heterostructures. This talk will explore mixed dimensional combinations of 2D + n-D (n = 0, 1 and 3) materials, thus significantly expanding the van der Waals heterostructure concept. In order to efficiently explore the vast phase space for mixed dimensional heterostructures, our laboratory employs solution-based additive assembly. In particular, constituent nanomaterials (e.g., carbon nanotubes, graphene, transition metal dichalcogenides, black phosphorus, and boron nitride) are isolated in solution [5-9], and then deposited into thin films with scalable additive manufacturing methods (e.g., inkjet [10], gravure [11], and screen printing [12]). By achieving high levels of nanomaterial monodispersity and printing fidelity, large-area device arrays can realize complex electronic signal conditioning such as frequency and phase shift keying [4]. Furthermore, by integrating multiple nanomaterial inks into heterostructures, unprecedented device function has been demonstrated including anti-ambipolar p-n heterojunctions [2-4] and gate-tunable memristors [13]. In addition to technological implications for nanoelectronics and optoelectronics, this work allows the exploration of several fundamental issues including band alignment, doping, trap states, and charge/energy transfer across previously unexplored mixed dimensional heterointerfaces. [1] D. Jariwala, V. K. Sangwan, L. J. Lauhon, T. J. Marks, and M. C. Hersam, “Emerging device applications for semiconducting two-dimensional transition metal dichalcogenides,” ACS Nano, 8, 1102 (2014). [2] D. Jariwala, S. L. Howell, K.-S. Chen, J. Kang, V. K. Sangwan, S. A. Filippone, R. Turrisi, T. J. Marks, L. J. Lauhon, and M. C. Hersam, “Hybrid, gate-tunable, van der Waals p-n heterojunctions from pentacene and MoS2,” Nano Lett., DOI: 10.1021/acs.nanolett.5b04141 (2015). [3] D. Jariwala, V. K. Sangwan, C.-C. Wu, P. L. Prabhumirashi, M. L. Geier, T. J. Marks, L. J. Lauhon, and M. C. Hersam, “Gate-tunable carbon nanotube-MoS2 heterojunction p-n diode,” Proc. Nat. Acad. Sci. USA, 110, 18076 (2013). [4] D. Jariwala, V. K. Sangwan, J.-W. T. Seo, W. Xu, J. Smith, C. H. Kim, L. J. Lauhon, T. J. Marks, and M. C. Hersam, “Large-area, low-voltage, antiambipolar heterojunctions from solution-processed semiconductors,” Nano Lett., 15, 416 (2015). [5] N. D. Mansukhani, L. M. Guiney, P. J. Kim, Y. Zhao, D. Alducin, A. Ponce, E. Larios, M. J. Yacaman, and M. C. Hersam, “High-concentration aqueous dispersions of nanoscale two-dimensional materials using nonionic, biocompatible block copolymers,” Small, DOI: 10.1002/smll.201503082 (2015). [6] J. Zhu, X. Liu, M. L. Geier, J. J. McMorrow, D. Jariwala, M. E. Beck, W. Huang, T. J. Marks, and M. C. Hersam, “Layer-by-layer assembled two-dimensional montmorillonite dielectrics for solution-processed electronics,” Adv. Mater., DOI: 10.1002/adma.201504501 (2015). [7] J. Zhu, J. Kang, J. Kang, D. Jariwala, J. D. Wood, J.-W. T. Seo, K.-S. Chen, T. J. Marks, and M. C. Hersam, “Solution-processed dielectrics based on thickness-sorted two-dimensional hexagonal boron nitride nanosheets,” Nano Lett., 15, 7029 (2015). [8] J. Kang, J. D. Wood, S. A. Wells, J.-H. Lee, X. Liu, K.-S. Chen, and M. C. Hersam, “Solvent exfoliation of electronic-grade, two-dimensional black phosphorus,” ACS Nano, 9, 3596 (2015). [9] J. Kang, J.-W. T. Seo, D. Alducin, A. Ponce, M. J. Yacaman, and M. C. Hersam, “Thickness sorting of two-dimensional transition metal dichalcogenides via copolymer-assisted density gradient ultracentrifugation,” Nature Communications, 5, 5478 (2014). [10] E. B. Secor, P. L. Prabhumirashi, K. Puntambekar, M. L. Geier, and M. C. Hersam, “Inkjet printing of high conductivity, flexible graphene patterns,” J. Phys. Chem. Lett., 4, 1347 (2013). [11] E. B. Secor, S. Lim, H. Zhang, C. D. Frisbie, L. F. Francis, and M. C. Hersam, “Gravure printing of graphene for large-area flexible electronics,” Adv. Mater., 26, 4533 (2014). [12] W. J. Hyun, E. B. Secor, M. C. Hersam, C. D. Frisbie, and L. F. Francis, “High-resolution patterning of graphene by screen printing with a silicon stencil for highly flexible printed electronics,” Adv. Mater., 27, 109 (2015). [13] V. K. Sangwan, D. Jariwala, I. S. Kim, K.-S. Chen, T. J. Marks, L. J. Lauhon, and M. C. Hersam, “Gate-tunable memristive phenomena mediated by grain boundaries in single-layer MoS2,” Nature Nanotechnology, 10, 403 (2015).

Two-dimensional (2D) materials have drawn tremendous attention in recent years. Being atomically thin, stacked with van der Waals force and free of surface chemical dangling bonds, 2D materials exhibit several distinct physical properties. To date, 2D materials include graphene, transition metal dichalcogenides (TMDS), black phosphorus, black P(1-x) Asx , boron nitride (BN) and so forth. Owing to their various bandgaps, 2D materials have been utilized for photonics and optoelectronics. Photodetectors based on 2D materials with different structures and detection mechanisms have been established and present excellent performance. In this Review, localized field enhanced 2D material photodetectors (2DPDs) are introduced with sensitivity over the spectrum from ultraviolet, visible to infrared in the sight of the influence of device structure on photodetector performance instead of directly illustrating the detection mechanisms. Six types of localized fields are summarized. They are: ferroelectric field, photogating electric field, floating gate induced electrostatic field, interlayer built-in field, localized optical field, and photo-induced temperature gradient field, respectively. These localized fields are proved to effectively promote the detection ability of 2DPDs by suppressing background noise, enhancing optical absorption, improving electron-hole separation efficiency, amplifying photoelectric gain and/or extending the detection range.

Two-dimensional (2D) materials such as graphene, transition metal dichalcogenides (TMDs), and hexagonal boron nitride (hBN) have attracted a large amount of interests due to their extraordinary electrical, optical, and mechanical properties comparing with their bulk counterparts. Recently, black phosphorus (BP) has emerged as a new 2D material with demonstrated high hole mobility, showing great potential in electronic applications such as field-effect transistors (FETs). In this talk we will first review our recent works on the electric, photo-electrical, mechanical, and thermal behaviors of few-layer black phosphorus [1-4]. We will then discuss the photonics applications of black phosphorus. 2D black phosphorus is an excellent candidate for use in photodetection devices due to its direct and thickness-dependent bandgap [1]. However, light absorptions in these 2D materials are often very low due to its ultra- thin nature. For example, the visible light absorption in single layer graphene is only 2.3%. Making plasmonic structures, such as nano disks and rods, on top of the 2D material can be a possible way to enhance the absorption. In our work, a new bowtie-like plasmonic structure was proposed, and numerical simulations were used to design and optimize the plasmonic structures that are used to enhance both absorption and polarization selectivity in black phosphorus photodetection devices. The optimized structure devices were then fabricated on black phosphorus on transparent substrate. Photocurrent measurements showed strong light polarization dependence/selectivity in the fabricated device as well as much stronger photo responsivity compared with devices without plasmonic enhancement. Our study demonstrated the potentials of black phosphorus for photodetection and other light harvesting applications. [1] Liu, H., Neal, A., Zhu, Z., Luo, Z., Xu, X., Tomanek, D., Ye, P.D., 2014, “Phosphorene: An Unexplored 2D Semiconductor with a High Hole Mobility,” ACS Nano, 8(4), pp. 4033–4041. [2] Deng, Y., Luo, Z., Conrad, N. J., Liu, H., Gon,g Y., Najmaei, S., Ajayan, P. M., Lou, J., Xu, X., and Ye, P. D., 2014, “Black Phosphorus-Monolayer MoS2 van der Waals Heterojunction P-N Diode.,” ACS Nano, 8(8), pp. 8292–8299. [3] Luo, Z., Maassen, J., Deng, Y., Du, Y., Garrelts, R. P., Lundstrom, M. S., Ye, P. D., and Xu, X., 2015, Anisotropic in-plane thermal conductivity observed in few-layer black phosphorus, Nat. Commun. 6:8572, pp. 9572-1-8. [4] Du, Y., Maassen, J., Wu, W., Luo, Z., Xu, X., and Ye, P., 2016, Auxetic Black Phosphorus: A 2D Material with Negative Poisson’s Ratio, Nano Lett., DOI: 10.1021/acs.nanolett.6b03607.

Most of the current electronic devices compose of millions of transistors that are packed into an area smaller than a fraction of a fingernail and the continuing improvement of consumer devices are driven by the miniaturization of these devices for the past four decades. By shrinking down the sizes of these electronic components, device performances are improved such as reducing power consumption and increasing device speed, as well as other enhancements such as complex functionalities, portability and cost reduction. This leads to the wide-spread usage of electronic devices in every aspect of society, from increasing productivity in automations in manufacturing industries to vital signs monitoring in biomedical sector. As the channel length of silicon transistors, the work horse of modern electronic devices, shrank from few hundred micrometres to tens of nanometres, new challenges had surfaced which necessitated the development of novel device designs and fabrication techniques, such as ultrathin body (UTB) technology, high-k gate dielectrics, strained channel material and tri-gate design in FinFET. While these advances in electronics have been instrumental in sustaining the Moore’s law for the past decade, further improvement in the coming years would require adoption of novel materials and transistor geometries. Since the successful demonstration of thermodynamically stable two dimensional (2D) graphene by Novoselov, et al. and Berger, et al. in 2004, and the subsequent studies on the properties of graphene and other layered 2D materials, the idea of using these atomically thin layers as the channel material in field-effect devices has been investigated extensively. A wide range of 2D materials have been proposed and studied, including graphene, transition metal dichalcogenides (TMDs), hexagonal boron nitride (h-BN) and black phosphorus (BP), as well as 2D version of traditional material such as silicene and germanene. Experimentally, most of these materials applied into FET type devices haven been demonstrated and theoretically, it has also been intensively studied their FET performance down to sub 10nm. However, the devices simulated thus far had been restricted to double gated structures, which have not been demonstrated experimentally. Furthermore, due to the weak van der Waal (vdW) forces between the 2D material layers, the electrostatic environment of single top gate device diverges from the usual ultra-thin materials. Therefore, in this work, using BP as the example, we firstly investigate the device performance of few-layer 2D layered material FETs with a single top gate device structure MOSFET and Schottky barrier FET. The current characteristics of the devices is obtained via a non-equilibrium Green’s function (NEGF) quantum simulator using Wannier function Hamiltonians based on first principle calculations, which enables the investigation of the individual BP layers potential profile at different gate biases. Therefore, the effect of van der Waals interlayer interaction on device performance and carrier transport properties in multilayer BP FETs can be fully and properly studied. Finally, we will discuss the ultimate voltage scalability of a double-gate ultra-thin body (DG-UTB) FETs employing materials from group IV, III-V, and 2-dimensional (2D) materials, including BP, MoS2, etc., based on International Technology Roadmap for Semiconductors (ITRS) projected specifications for high performance (HP) and low power (LP) technologies. The ballistic performance of FETs designed based on the ITRS specifications for 2018 and beyond was evaluated via the semiclassical ballistic transport model. The device performance of 2D layer materials in FET applications will be compared to devices based the conversional semiconductor materials, such as Si, Ge, InGaAs and GaSb, based on ITRS roadmap requirement and their potential applications fitted into the future needed will be also addressed to provide the guidelines for future industrial development.

Emerging terahertz photodetectors based on two-dimensional materials

The isolation of graphene has sparked an unprecedented world wide research activity on the topic of two-dimensional (2D) materials. However, even though graphene has a high potential for various applications, its metallic character is a significant limitation for a large variety of electron devices, starting with the conventional field effect transistor. This limitation could be overcome in the near future by the use of layered van der Waals crystals such as black phosphorus (i.e., phosphorene, which is one to few layers of black phosphorus) and also by other semiconducting 2D materials such as transition metal dichalcogenides (TMDCs) of general formula MX2, where M is the metal and X the chalcogen (S, Se and Te). For instance, phosphorene exhibits a direct band gap varying between ~ 0.3 eV for bulk crystals to ~ 2 eV for a monolayer. In layered TMDC structures, each layer typically comprises 3 atomic planes (corresponding to a thickness below 8 Å), and consists of an hexagonally-organized atomic layer of metal atoms sandwiched between two planes of chalcogen atoms, also in hexagonal configurations. M–X bonds inside each layer are covalent; however, the sandwiching chalcogen layers are linked by weak van der Waals bonds, so that a bulk crystal is easy to cleave along the chalcogen planes. As with phosphorene, most semiconducting TMDC materials also see their band gap increasing as thickness decreases and they switch from indirect to direct band gap for a monolayer. To date, the most studied semiconductor TMDCs are MoS2 and WS2. Because they are atomically thin, 2D materials are particularly well suited for flexible electronics applications. After a brief description of the major features of phosphorene and TMDCs (crystal structures, relevant electronic properties as a function of thickness …), the talk will highlight recent results concerning field effect transistor characteristics. In particular, carrier mobility values as high as 5000 cm2/Vs have been measured in phosphorene films. Since local substitutional-type doping cannot be used in 2D materials, there is a general contacting problem (Schottky barrier formation), which is of paramount importance for device applications. In particular, two types of contacts can be formed: the top contact and the edge contact, which behave differently. Generally speaking, the semiconductor thickness is much smaller than the depletion length at the contact, which is an intriguing and novel situation. In the second part of the talk, various synthesis methods will be presented and discussed, particularly in view of their possible use in the field of large area electronics. For most devices fabricated so far, the 2D layers were obtained by exfoliation of bulk crystals. However, recent progress in synthesis include molecular beam epitaxy, controlled chemical vapour deposition and vapour phase transport. Those various approaches will be discussed, as well as the newly-introduced atomic layer deposition (ALD) process. As most 2D materials are grown on a separate substrate, the transfer problem will be highlighted, particularly concerning phosphorene which is highly hygroscopic, and sensitive to the presence of light. The presentation will be concluded by some perspectives concerning the industrialization of 2D materials.

Advances in Two‐Dimensional Layered Materials

Solution-processed amorphous oxide semiconductors have attracted considerable interest in large-area transparent electronics. However, due to its relative low carrier mobility (∼10 cm(2) V(-1) s(-1)), the demonstrated circuit performance has been limited to 800 kHz or less. Herein, we report solution-processed high-speed thin-film transistors (TFTs) and integrated circuits with an operation frequency beyond the megahertz region on 4 inch glass. The TFTs can be fabricated from an amorphous indium gallium zinc oxide/single-walled carbon nanotube (a-IGZO/SWNT) composite thin film with high yield and high carrier mobility of >70 cm(2) V(-1) s(-1). On-chip microwave measurements demonstrate that these TFTs can deliver an unprecedented operation frequency in solution-processed semiconductors, including an extrinsic cut-off frequency (f(T) = 102 MHz) and a maximum oscillation frequency (f(max) = 122 MHz). Ring oscillators further demonstrated an oscillation frequency of 4.13 MHz, for the first time, realizing megahertz circuit operation from solution-processed semiconductors. Our studies represent an important step toward high-speed solution-processed thin film electronics.

2D semiconductor crystals offer the opportunity to further extend Moore's law to the atomic scale. For practical and low-cost electronic applications, directly printing devices on substrates is advantageous compared to conventional microfabrication techniques that utilize expensive photolithography, etching, and vacuum-metallization processes. However, the currently printed 2D transistors are plagued by unsatisfactory electrical performance, thick semiconductor layers, and low device density. Herein, a facile and scalable 2D semiconductor printing strategy is demonstrated utilizing the interface capture effect and hyperdispersed 2D nanosheet ink to fabricate high-quality and atomic-thick semiconductor thin-film arrays without additional surfactants. Printed robust thin-film transistors using 2D semiconductors (e.g., MoS2 ) and 2D conductive electrodes (e.g., graphene) exhibit high electrical performance, including a carrier mobility of up to 6.7 cm2 V-1 s-1 and an on/off ratio of 2×106 at 25°C. As a proof of concept, 2D transistors are printed with a density of ≈47000 devices per square centimeter. In addition, this method can be applied to many other 2D materials, such as NbSe2 , Bi2 Se3 , and black phosphorus, for printing diverse high-quality thin films. Thus, the strategy of printable 2D thin-film transistors provides a scalable pathway for the facile manufacturing of high-performance electronics at an affordable cost.

Thin-film transistors (TFTs) using oxide semiconductors have attracted much attention for switching and driving devices in large-screen, high-resolution liquid crystal display (LCD) and organic light-emitting diode (OLED) displays because of their high mobility1-3. As an advanced oxide TFT fabrication method, the solution-process is widely researched due to advantages such as its low process cost, large-scale fabrication capacity, and the simplicity of the process. However, several drawbacks of the solution-process must be addressed for display applications. One of the key points is the trade-off between the processing temperature and the TFT characteristics. From previous research on solution-processed oxide TFTs, it has been reported that low-temperature-processed TFTs are generally inactive below 400 °C due to a lack of metal-oxide-metal bond formation and decomposition of the residual species4,5. However, a low-temperature process is strongly required for application to plastic substrates for flexible displays and flexible electronics devices.In addition, a low-temperature process is important to achieve low fabrication costs. Here, we demonstrate improvement of the TFT characteristics by application of a hydrogen injection and oxidation (HIO) process for low-temperature solution-processed oxide TFTs. From a technical standpoint, we consider that the important factor is how to efficiently decompose the residual species. The solution-processed oxide TFTs exhibit inferior device performance when a few residual species remain. Therefore, a reduction reaction is initiated by a hydrogen injection process to decompose the residual spices. The characteristics of a TFT based on indium gallium zinc oxide (IGZO) were evaluated, such as the field effect mobility and the threshold voltage (Vth) shift under positive and negative bias stress. The maximum temperature for the entire fabrication process was as low as 300 °C. Solution-processed IGZO thin films were fabricated by a spin-coating method onto 200 nm thick thermally oxidized SiO2/n+-Si substrates and annealing at 300 °C for 1 h. The IGZO precursor was prepared from 0.3 M solutions of indium nitrate hydrate (In(NO3)3·xH2O), gallium nitrate hydrate (Ga(NO3)3·xH2O), and zinc nitrate hydrate (Zn(NO3)2·xH2O) in pure water. The composition ratio for In:Ga:Zn was set to 4:1:1. Hydrogen plasma treatment was adopted for the hydrogen injection process with subsequent annealing at 300 °C for 1 h. After fabrication of the IGZO film, the active area of the IGZO film was defined using a conventional photolithography method. Patterned source/drain (S/D) electrodes (Mo) were formed on the IGZO thin films using a shadow mask. Figure 1 shows the transfer characteristics of the IGZO TFT with and without the HIO process. The transfer characteristics of these IGZO TFTs for gate voltages (Vg) ranging from -30 to 30 V were measured at a fixed drain voltage (Vd) of 30 V. After the HIO process for the IGZO TFTs, the field effect mobility was significantly improved from 2.1 cm2/Vs to 4.8 cm2/Vs. Figure 2 shows the results for the threshold voltage shift (ΔVth) under a positive gate bias stress (PBS) of 20 V and a negative gate bias stress (NBS) of -20 V, with and without the HIO process. The stress time was set to 1 h at the maximum. Improvement of the reliability was confirmed: the Vth shift was less than 0.25 V for PBS and -2 V for NBS. The trap sites of any residual species or defect states in active channels were thus efficiently suppressed by the hydrogen injection process. In addition, the oxidation process may enhance the metal-oxide-metal bond formation. In conclusion, the HIO process as a combination of hydrogen reduction reaction and oxidation was demonstrated as effective for the improvement of solution-processed oxide TFTs. Improvement of the TFT characteristics was confirmed in terms of the field effect mobility and the Vthshift under PBS and NBS. Therefore, we consider that this new approach will lead to the development of high-performance solution-processed oxide TFTs in future. 1K. Nomura, H. Ohta, A. Takagi, T. Kamiya, M. Hirano, and H. Hosono, Nature 432, 488 (2004). 2K. Nomura, H. Ohta, K. Ueda, T. Kamiya, M. Hirano, and H. Hosono, Science 300, 1269 (2003). 3L. Wang, M. Yoon, G. Lu, Y. Yang, A. Facchetti, and T.J. Marks, Nature materials 5, 893 (2006). 4 M. G. Kim, M. G. Kanatzidis, A. Facchetti, and T. J. Marks, Nature materials 5, 382 (2011) 5 S. Jeong, Y. G. Ha, J. Moon, A. Faccchetti, T. J. Marks, Advanced materials 22, 1346 (2010) Figure 1

Two-dimensional (2D) materials are a broad family of layered crystals characterized by strong intra-layer bonds, but with weak inter-layer coupling dominated by van der Waals forces. These characteristics allow 2D materials to be either exfoliated or grown with atom-scale thickness. A wide range of 2D materials exist [1], including graphene, transition metal dichalcogenides (TMDs), black phosphorus (BP) and many others. While these materials have generated a great deal of excitement in the scientific community, many of the applications where these materials can truly provide a benefit compared to state-of-the-art solutions remain unclear. Here, I will describe our work on 2D materials, and will specifically describe how we have attempted to identify applications for which these materials are best suited.

Since graphene was first reported as a saturable absorber to achieve ultrafast pulses in fiber lasers, many other two-dimensional (2D) materials, such as topological insulators, transition metal dichalcogenides, black phosphorus, and MXenes, have been widely investigated in fiber lasers due to their broadband operation, ultrafast recovery time, and controllable modulation depth. Recently, solution-processing methods for the fabrication of 2D materials have attracted considerable interest due to their advantages of low cost, easy fabrication, and scalability. Here, we review the various solution-processed methods for the preparation of different 2D materials. Then, the applications and performance of solution-processing-based 2D materials in fiber lasers are discussed. Finally, a perspective of the solution-processed methods and 2D material-based saturable absorbers are presented.