Nano Today
ReviewTwo-dimensional nanomaterials with engineered bandgap: Synthesis, properties, applications
Graphical Abstract
Introduction
Two-dimensional (2D) nanomaterials, a new type of nanomaterial that demonstrates nanosheet structures with significantly large lateral size of up to a few centimeters but with only a single or few atomic layers of thickness, revealed excellent opportunities to venture into largely unexplored fields of materials. In 2004, the emergence of a single-layer graphene, which was exfoliated from graphite using a Scotch tape, reawakened the enthusiasm for research on ultrathin 2D nanomaterials, despite the fact that scientific research on layered graphite had been conducted for over one hundred years [1], [2], [3]. The unique 2D planar structure provides graphene with several excellent properties, such as half-integer quantum Hall effect, ultrahigh carrier mobility, which was obtained from theoretical calculations and electronic devices, high thermal conductivity, high specific surface area, and the highest mechanical strength [1], [4], [5], [6], [7], [8], [9], [10], [11], [12], [13]. Consequently, graphene quickly became popular in materials science and it has initiated a significant increase in research on 2D nanomaterials over the last decade and more. Similar to graphite, long and productive research was conducted on layered transition metal dichalcogenides (TMDs). As early as in 1923, the structure of TMDs was first identified by Linus Pauling and coworkers [14]. By the end of the 1960 s, approximately 60 TMDs had been discovered, of which at least 40 exhibited layered structures [15]. Subsequently, the discovery of ultrathin and monolayer MoS2, the most representative TMD, was reported [16], [17]. In the past few years, there have been considerable developments in 2D TMDs, which were spurred by the demonstration of various applications ranging from nanoelectronic and nanooptoelectronic devices to nanosensors and nanoactuators [18]. With the development of graphene and TMDs, various graphene-like 2D nanomaterials were prepared from their layered bulk materials, such as phosphorene, arsenene, hexagonal boron nitride (h-BN), g-C3N4 [19], [20], [21], [22], [23], [24]. In 2011, Naguib et al. [25] reported the selective etching of aluminum from nonlayered precursor Ti3AlC2 using aqueous HF at room temperature, leading to a new class of 2D nanomaterial (Ti3C2), named MXenes. Besides, organic and organic-inorganic layered materials, such as 2D covalent organic frameworks (COFs), metal-organic frameworks (MOFs) and organic-inorganic hybrid perovskites, can be prepared by molecular design [26], [27], [28]. These new 2D structures further enriched the varieties and quantities of 2D nanomaterials. The discovery of new 2D nanomaterials is inseparable from the innovation of preparation methods; moreover, the well-established synthesis methods of 2D nanomaterials can be primarily classified into two categories: top-down and bottom-up methods. The former method prepares 2D nanomaterials from bulk materials using different approaches, including micromechanical cleavage (Scotch tape, ball milling), liquid exfoliation (sonication), and chemical exfoliation (oxidation, ion-intercalation, ion exchange, selective etching), while the bottom-up methods start with atomic ingredients and assemble them together, such as chemical vapor deposition (CVD), atomic layer deposition (ALD), molecular beam epitaxy (MBE), wet-chemical syntheses (hydrothermal synthesis, colloidal synthesis, etc.) [29], [30], [31], [32]. The development of various synthesis methods to prepare 2D nanomaterials and construct diverse nanostructures has provided a significant opportunity to understand property modulations.
Owing to their atomic thickness, 2D nanomaterials show many unprecedented mechanical, electronic, chemical, and optical properties that are unattainable in their bulk materials. In traditional electronic devices, the size of silicon-based transistors exhibits a limitation, i.e., the minimum channel length can only reach 5 nm; otherwise, the devices cannot be switched off owing to short channel effect. The advent of 2D nanomaterials has presented a method to overcome this limitation as the confinement of electrons in 2D planes effectively suppresses short channel effect, leading to smaller channel length of electronic devices and higher integration of integrated circuits [33]. Xie et al. [34] reported that MoS2 field-effect transistors (FETs) with channel lengths scaling down to approximately 4 nm could be achieved reliably. Moreover, many 2D nanomaterial devices demonstrated significantly high carrier mobility, such as graphene (≥15,000 cm2/(V·s)) and phosphorene FETs (≥ 1000 cm2/(V·s)), leading to promising materials for the next generation of electronic devices [1], [35]. Another striking feature in 2D nanomaterials is its ultrahigh specific surface area, which can provide more active sites and contacts, leading to excellent performance in several applications, such as batteries, supercapacitors, catalysis, and molecular sieving [36]. For example, a theoretical specific surface area as high as 2630 m2/g was demonstrated in graphene [37]. Thus, Cui and coworkers [38] fabricated a hybrid electrode created from graphene and few layers of phosphorene nanosheets for application in a sodium-ion battery, which demonstrated a specific capacity of 2440 mA·h/g at a current density of 0.05 A/g. Reddy et al. [39] reported an ultrahigh photocatalytic hydrogen production rate of 183.24 mmol/(h·g) in a system with few layers of black phosphorene, MoS2, and CdS nanohybrid. In addition, 2D nanomaterials demonstrate maximum mechanical strength and flexibility, high optical transparency resulting from strong chemical bonds in the plane, and atomic thickness. For instance, graphene is reported to be the stiffest material with a Young’s moduli of 1 TPa and its optical transparency can reach 97.7% [10], [12]. The measured Young’s modulus of 270 GPa in monolayer MoS2 was stronger than that of its bulk MoS2 (∼240 GPa) or steel (∼205 GPa) [40]. Therefore, 2D nanomaterials are advantageous in the construction of wearable, flexible, and transparent electronic and optoelectronic devices [41], [42]. Last but not the least, 2D nanomaterials can serve as ideal platforms for improving their properties and designing their functionalities at the atomic level through surface chemical modification or functionalization, element doping or molecular absorption and so on, which is difficult to achieve in their bulk materials [43], [44]. Thus, owing to their excellent properties, 2D nanomaterials are widely studied in many applications, such as solar cells, FETs, light-emitting devices, photodetectors, displays, sensors, lasers, and photocatalysis (Fig. 1) [45], [46], [47], [48], [49], [50], [51], [52], [53], [54], [55].
Bandgap, which represents the width of the forbidden band between the top of the valence band (the highest occupied molecular orbital) and the bottom of the conduction band (the lowest unoccupied molecular orbital), is arguably the most critical parameter in many applications of 2D nanomaterials. For example, on/off ratio of FETs is proportional to the logarithm of bandgap in an ideal case [64]. An effective photocatalyst must have a suitable bandgap to absorb photon energy and a suitable band structure to match target substances. Solar cells can only absorb photons, whose energy is close to or beyond the bandgap, to convert them into electricity, as higher-energy photons lose their redundant energy through thermalization and phonon generation while lower-energy photons cannot be absorbed and are simply transmitted through the solar cell or reflected back. Direct red-green-blue or multicolor emission in solid-state lighting, illumination, and displays requires semiconductors with a wide bandgap ranging from 1.77 eV to 3.1 eV. To further expand the potential and applications, bandgap engineering in 2D nanomaterials was performed by various methods, such as chemical modification, doping, hybridizing domains structures, phase transition, controlling thickness, controlling width and edge structure of nanoribbons and twisting vertical heterostructure. Thickness-dependence of bandgap is a prominent feature in 2D nanomaterials, such as phosphorene, which shows a varying bandgap of 0.3–1.7 eV from the bulk material to a single layer [65]. Bandgap opening of graphene by chemical modification can achieve a gradual transition from metal to insulator, leading to significantly increasing on/off ratio of up to 105, while the zero bandgap of graphene results in on/off current ratios in FETs of approximately 100, which is significantly lower than the 103 to 106 range required for mainstream applications [66], [67]. Meanwhile, hybridizing h-BN domains into graphene can not only tune bandgap in a wide range, leading to the combination of a relatively high on/off ratio and mobility, but also obtain a suitable band structure that can satisfy the requirements of photocatalysis [68], [69], [70]. Phase transition between metallic and semiconducting structures is common in TMDs. Semiconducting (2H) phase MoS2 with a moderate bandgap of 1.82 eV is considered as an alternative channel material to graphene in FETs; however, 2H-phase MoS2 FETs demonstrated low carrier mobility [71]. By converting the 2H- to 1T-phase (metallic) TMDs, contact resistances in FETs can be significantly reduced, thereby increasing the mobility by several times [72], [73]. The 1T-phase TMDs can also be used as a cocatalyst, thereby increasing the photocatalytic efficiency by dozens of times, when compared to that of 2H-phase TMDs [74], [75]. Alloying, where the similar structures are doped with each other, can tune the bandgap continuously and provide a favorable structure, leading to less vacancies and enhanced application performance [76]. In addition, controlling width and edge structure of nanoribbons and twisting vertical heterostructure can also realize the effective regulation of bandgap and the optimization of application performance.
Although there were many reviews regarding 2D nanomaterials and their synthesis, properties, and applications, limited reviews focused on the bandgap engineering of 2D nanomaterials. Herein, we report a systematic review of bandgap engineering in various 2D nanomaterials. We start with a brief introduction on the physical significance of bandgap in 2D nanomaterials. Then, we introduce several methods to control and regulate bandgap, primarily including chemical modification, doping, hybridizing domains structures, phase transition, controlling thickness, controlling width and edge structure of nanoribbons and twisting vertical heterostructure. Subsequently, we elaborate on bandgap engineering for different 2D nanomaterials using various methods. Further, the optimization of application performance through bandgap engineering is also emphasized. Finally, based on the current developments, we conclude this review and present the challenges and outlooks in 2D nanomaterials.
Section snippets
Bandgap in 2D nanomaterials
In general, the bandgap of 2D nanomaterials is larger than that of their bulk materials owing to quantum confinement. The bandgap of 2D nanomaterials is related to their thickness, and when further limiting one of the dimensions to nanoscale (i.e., nanoribbon), bandgap mainly varies inversely with the nanoribbon width [77], [78]. In theory, if a free electron can only move in one dimension (x), Schrodinger equation and its energy solution are described as follows: [79].
Mono-element 2D nanomaterials with engineered bandgap
Mono-element 2D nanomaterials are a kind of 2D nanomaterial with single- or few-atomic-layer hexagonal honeycomb lattice composed of only a single element containing group-ⅣA 2D nanomaterials (graphene, silicene, germanene), group-ⅤA 2D nanomaterials (phosphorene, arsenene, antimonene, and bismuthine), etc. [156] Mono-element 2D nanomaterials mainly demonstrate three kinds of configurations, that is, perfect plane structure like graphene, buckled hexagonal (chair-like) structure like germanene
Summary and outlook
For over a decade, 2D nanomaterials have generated considerable research enthusiasm in science and technology owing to their excellent properties. In many application fields of 2D nanomaterials, such as FETs, photocatalysis, photodetection, light-emitting devices, solar cells., the bandgap is an important parameter and is significantly critical to their application performance. In this review, we provided detailed information on bandgap engineering in various 2D nanomaterials using several
CRediT authorship contribution statement
Under the supervision of Prof. Wei Feng, Yu Wang investigated and summarized a large amount of literatures, then Yu Wang organized, wrote and revised this review. Ling Wang adjusted the framework and structure of this review. Xin Zhang and Xuejing Liang helped investigate the literatures and checked the spelling and formatting errors of this review, and Yiyu Feng provided some advice and help in writing this review. All authors read and contributed to the manuscript.
Declaration of competing interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgments
This work was financially supported by National Key R&D Program of China (No. 2016YFA0202302), the State Key Program of National Natural Science Foundation of China (No. 51633007), National Natural Science Funds for Distinguished Young Scholars (No. 51425306), and National Natural Science Foundation of China (No. 51573125 and 51773147).
Yu Wang received his BS degree in the School of Materials Science and Engineering at Tianjin University in 2016. He is now pursuing PhD under the tutelage of Prof. Wei Feng at the School of Materials Science and Engineering, Tianjin University. Currently, his research is focused on two-dimensional materials.
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Cited by (0)
Yu Wang received his BS degree in the School of Materials Science and Engineering at Tianjin University in 2016. He is now pursuing PhD under the tutelage of Prof. Wei Feng at the School of Materials Science and Engineering, Tianjin University. Currently, his research is focused on two-dimensional materials.
Ling Wang is a full professor at Tianjin University, Tianjin, China. He received his PhD degree in Materials Science from the University of Science and Technology Beijing (China). From October 2013 to September 2018, he worked as a postdoctoral research fellow at the Advanced Materials and Liquid Crystal Institute of Kent State University (USA) and the Artie McFerrin Department of Chemical Engineering at Texas A&M University (USA), respectively. His research interests include design, synthesis and properties of active soft materials, bioinspired materials and functional nanomaterials, as well as their emerging applications in diverse fields ranging from soft robotics, adaptive camouflage, smart windows to energy and safety issues.
Yiyu Feng is a professor in the School of Materials Science and Engineering at Tianjin University. He obtained his PhD from Tianjin University in 2009 and held an academic position at Tianjin University. He has authored and co-authored over 100 academic articles and reviews. Currently, his research is focused on carbon-based materials or composites for solar-thermal fuels, interfacial heat dissipation and structural self-healing.
Wei Feng is a professor at the School of Materials Science and Engineering in Tianjin University. He obtained his PhD degree from the Xi’an Jiaotong University (China) in 2000. Then, he worked at Osaka University and Tsinghua University as a JSPS fellow and postdoctoral researcher, respectively. In 2004, he became a full professor at Tianjin University. He has obtained the support of the National Science Fund for Distinguished Young Scholars in China. His research interests include photo-responsive organic molecules and their derivatives, thermal-conductive and high-strength carbon-based composites, two-dimensional materials, fluorinated carbon materials and polymers.