Heteroatom doping of two-dimensional materials: From graphene to chalcogenides

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Review

Heteroatom doping of two-dimensional materials: From graphene to chalcogenides Haoyue Zhu a , Xin Gan b , Amber McCreary c,1 , Ruitao Lv b , Zhong Lin c , Mauricio Terrones a,c,d,e,∗ a

Department of Chemistry, Pennsylvania State University, University Park, PA 16802, USA Key Laboratory of Advanced Materials (MOE), School of Materials Science and Engineering, Tsinghua University, Beijing 100084, China c Department of Physics, Pennsylvania State University, University Park, PA 16802, USA d Department of Materials Science and Engineering, and Center for Atomically Thin Multifunctional Coatings (ATOMIC). The Pennsylvania State University, University Park, PA 16802 USA e Research Initiative for Supra-Materials, Shinshu University, 4-17-1 Wakasato, Nagano 380-8553, Japan b

a r t i c l e

i n f o

Article history: Received 6 May 2019 Received in revised form 23 October 2019 Accepted 9 December 2019 Available online xxx Keywords: 2D materials Transition metal dichalcogenides MoS2 WS2 Graphene Sensing Sensor Electronics TMD Doping Molybdenum disulfide Tungsten disulfide

a b s t r a c t In recent years, research on two-dimensional (2D) materials including graphene and transition metal dichalcogenides (TMDCs), especially molybdenum and tungsten disulfides (MoS2 and WS2 ), has rapidly developed. In order to meet the increasing demands of using these 2D materials in fields as diverse as optoelectronics and sensing, heteroatom doping has become an effective method to tune their electronic and physico-chemical properties. This review discusses versatile doping methods applied to graphene and TMDCs, the corresponding changes to their properties, and their potential applications. Future perspectives and new emerging areas are also presented. © 2019 Elsevier Ltd. All rights reserved.

Contents Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 00 Heteroatom doping of graphene: Synthesis and characterizations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 00 Nitrogen-doped graphene . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 00 Boron-doped graphene. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .00 Silicon-doped graphene . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 00 Phosphorus-doped graphene . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 00 Sulfur-doped graphene . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 00 Heteroatom doped semiconducting TMDCs: Synthesis and characterization . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 00 W-doped MoS2 (Mo1-x Wx S2 ) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 00 Co-doped MoS2 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 00 Fe-doped MoS2 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 00

∗ Corresponding author. E-mail address: [email protected] (M. Terrones). 1 Current address: Nanoscale Device Characterization Division, Physical Measurement Laboratory, National Institute of Standards and Technology, Gaithersburg, Maryland 20899, USA https://doi.org/10.1016/j.nantod.2019.100829 1748-0132/© 2019 Elsevier Ltd. All rights reserved.

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Mn-doped MoS2 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 00 Nb-doped MoS2 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 00 Se-doped MoS2 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 00 Applications of heteroatom-doped graphene . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 00 Gas sensing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 00 Graphene Enhanced Raman Spectroscopy . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 00 Applications of heteroatom-doped TMDCs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 00 Optoelectronics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 00 Gas sensing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 00 Perspectives and future work . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 00 Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 00 References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 00

Introduction Long before the discovery of two-dimensional (2D) materials such as graphene and monolayer transition metal dichalcogenides (TMDCs), their bulk counterparts have found vast applications in industry. For example, graphite was used as anode materials in lithium-ion batteries (LIBs) because of its excellent Li+ insertion/extraction capability [1], and bulk MoS2 was used as a dry lubricant originating from the weak van der Waals forces between individual MoS2 layers [2]. After Novoselov and Geim reported the electronic properties of mechanically exfoliated monolayer graphene using the “Scotch tape” method in 2004 [3], graphene and other 2D semiconductors began to attract increasing attention worldwide. Graphene is an atomically thin sp2 -hybridized carbon material with a honeycomb lattice, where the in-plane  c-c bond contributes to its mechanical robustness such as extraordinary Young’s modulus (≈1.1 TPa) and fracture strength (125 GPa), and the out-of-plane ␲-conjugation is responsible for its remarkable electronic properties, including high carrier mobilities at room temperature (>200,000 cm2 V−1 s−1 ) and strong ambipolar electric field effect [4,5]. In addition to the abovementioned mechanical and electrical properties, graphene is also attractive in terms of its extremely high thermal conductivity (3000−5000 W m−1 K−1 ), high optical transmittance (97.7%), and high surface area (2630 m2 g−1 ) [6,7]. With the fascinating properties of graphene, scalable synthesis methods were developed in order to achieve higher yield and larger sizes, including “top-down” and “bottom-up” routes [8]. A representative “top-down” route to synthesize graphene is the Hummer’s method, consisting of oxidizing/exfoliating graphite flakes with H2 SO4 and KMnO4 , and reducing the exfoliated graphite oxide into reduced graphene oxide (rGO) sheets [9,10]. One representative “bottom-up” route for graphene synthesis is chemical vapor deposition (CVD), where hydrocarbons are passed over Cu or Ni substrates at high temperatures [11]. When compared to mechanical exfoliation, CVD synthesis of graphene is not only scalable, but also offers excellent control of its structural homogeneity, thus enabling graphene to be used as a transparent conducting electrode [12,13], as well as in the fabrication of photoelectronic devices and supercapacitors [14,15]. Although the intrinsic properties of graphene make it an appealing 2D material, the touching of the conduction and valence bands of graphene at the Dirac point makes it gapless (Fig. 1) [16], and it is not suitable to construct complex electronic devices that require both p-type and n-type semiconductors. In addition, from a chemical perspective, graphene is relatively inert, thus hindering its use in applications that require high chemical reactivity such as sensing and catalysis [17]. However, an effective method to tune both the chemical and physical properties of graphene is heteroatom doping, i.e., the covalent bonding of carbon atoms within graphene with foreign atoms. For graphene synthesized by bottom-up routes

Fig. 1. The band diagram of graphene, in which the conduction band and valence band touch at the Dirac point [16]. (Reprinted with permission from reference 16. Copyright 2019 by the American Physical Society.).

including CVD, heteroatomic doping is usually carried out in-situ. For example, using heteroatom-containing precursors during the synthesis results in dopants incorporated into the lattice when graphene is grown on the substrate. In addition, for graphene synthesized by top-down routes such as the Hummer’s method, doping is more commonly carried out post-synthesis, e.g., thermal annealing of graphene oxide (GO) in reactive gases, or by treating GO in plasmas [18,19]. For the purpose of brevity, this review mainly focuses on in-situ graphene doping via CVD. For thorough discussions on heteroatom doping related to graphene synthesized by top-down methods, see several excellent reviews [20,21]. Due to the lack of a bandgap in graphene, semiconducting TMDCs, a class of materials with similar dimensionality but larger bandgaps (≈1 eV to 2 eV), have also caught the attention of scientists [22–24]. In this review, we focus on TMDCs that follow the MX2 chemical formulas, where M represents a transition metal atom and X represents a chalcogen atom (S, Se, Te). The structure of these TMDCs are similar to graphite and consist of stacked MX2 layers by van der Waals forces. An individual MX2 layer contains one layer of metal atoms sandwiched between two layers of chalcogen atoms; the metal atoms covalently bond to the nearest chalcogen atoms, generally forming a metal-centered triangular prism or octahedral configurations. The intralayer configuration determines the interlayer spacing of TMDCs, and the triangular prism configuration corresponds to the 2H or 3R stacking, and the octahedron to the 1 T stacking (Fig. 2). Because of the non-bonding d-orbit electrons and the coordination in the X-M-X layer structure, the energy bands of d electrons are positioned between the bonding and anti-bonding bands. The triangularly prismatic and octahedral configuration correspond to two and three d energy bands, respectively, while the number of d electrons of the transition metal atoms determines the relative location of the Fermi level to the d bands. Thus, the electronic, optical and magnetic properties are mainly affected by the electron configuration of the metal and intralayer structure. In

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Fig. 2. Structures of pristine TMDC layers with different phases, where chalcogen atoms are shown in yellow and transition metal atoms are shown in blue. (a) The 1H phase, (b) the 1 T phase, (c) the distorted 1 T, or 1 T’ phase, (d) the 2H phase, (e) the 3R phase [26]. (Reprinted with permission from reference 26. Copyright 2019 by the Royal Society of Chemistry.).

addition, the atomic orbitals of chalcogen atoms also affect the band gap widths of TMDCs [25]. Among the TMDCs, group VIB metal dichalcogenides (e.g., molybdenum and tungsten disulfides (MoS2 and WS2 )) appear to be the most studied candidates for digital electronics because of their moderate band gaps of 1−2 eV, which are close to those of the conventional semiconductors such as Si and GaAs [23,24,27]. It is worth mentioning that only the monolayer thicknesses of MoS2 (WS2 ) exhibit a direct bandgap of 1.8 eV (1.97 eV), while bulk MoS2 (WS2 ) has an indirect bandgap of 1.3 eV (1.35 eV) [24,28,29]. Such 2D semiconductors with direct bandgaps can be used as channel materials in more compact, integrated circuits, as their atomically smooth surface, free from dangling bonds, can overcome the issue of surface impurities when decreasing the size of transistors [30]. Recently, semiconducting 2D transistors with a high carrier mobility of 34,000 cm2 V−1 s−1 has been fabricated by utilizing six-layered MoS2 on hexagonal boron nitride (h-BN) as the insulating layer [31]. In order for these semiconducting TMDCs to be realized in digital electronics, it is necessary to develop large-scale synthesis techniques, either by top-down or bottom-up approaches. Representative top-down routes involve exploiting the weak van der Waals bonding between stacked layers. Examples include sonicating bulk TMDCs with the help of lithium intercalation [32,33] or using proper dispersing agents such as aqueous surfactants and organic solvents to facilitate the exfoliation under sonication [34–36]. A representative bottom-up approach for synthesizing TMDCs is CVD and powder vaporization, but in this case both the metal and chalcogen sources are needed [37,38]. Similar to heteroatom doping of graphene, TMDCs can also be doped with heteroatoms, thus leading to various modifications of their properties, such as adjusted band gaps and Fermi levels, altered photoluminescence (PL), ferromagnetism and improved chemical activity [39,40]. This review focuses on doped TMDCs synthesized by bottom-up routes since the changes of electronic and physical properties with heteroatoms are easier to identify when TMDCs are reduced to monolayers. For the purpose of brevity, we focus on doped MoS2 as a representative system to illustrate the versatility of heteroatom doping in TMDCs. To date, a review featuring CVD synthesis and heteroatom doping of 2D materials are rare [20,41,42]. Hereby, we provide a summary of the recent developments in CVD and powder vaporization synthesis of graphene and MoS2 , both related to substitutional doping approaches, their characterization and applications.

Heteroatom doping of graphene: Synthesis and characterizations Early theoretical investigations on doped graphene suggest that the optimal dopants for graphene primarily include neighboring 2p and 3p elements to carbon in the periodic table, which are boron, nitrogen, silicon, phosphorus and sulfur [39,43]. Given the gapless nature of graphene, controlled heteroatom doping is intended to introduce n-type or p-type carriers, or even open its bandgap, thus making doped graphene more suitable for fabricating molecular sensors (see below). In a typical CVD synthesis, heteroatomcontaining compounds are catalytically decomposed on the surface of Cu foils, and doped graphene is then synthesized following a diffusion and nucleation process [44]. Because heteroatom doping in graphene disrupts the ␲−␲ symmetry, doped graphene is less thermodynamically stable than pristine graphene (PrG). The thermodynamic instability of doped graphene increases with the dopant size and its concentration. For example, a low synthetic temperature is observed for effectively increasing the dopant concentrations of phosphorus- and sulfur-doped graphene [45–47]. Depending on the chosen heteroatom-containing precursor, along with other synthetic parameters including but not limited to temperature and pressure of the CVD system, dopant concentration and configuration can be affected in ways that are beneficial for different applications; these will be discussed in more details later in this section. A good understanding on how heteroatom doping in graphene can be affected by synthetic parameters is critical to alter the physico-chemical properties of graphene-related materials. Herein, we briefly summarize the synthesis of doped graphene, and how different precursors lead to different dopant configurations. Nitrogen-doped graphene Nitrogen (2s2 2p3 ), an electron-rich atom, has been intensively studied as dopant in sp2 hybridized carbon systems such as carbon nanotubes [48–50]. The similar sizes of the C N (1.41 Å) and C C (1.42 Å) bonds makes it possible for N atoms to remain within the basal plane of nitrogen-doped graphene (NG) as graphitic, pyridinic or nitrilic dopants (Fig. 3a) [49]. It was suggested by Schiros et al. that the effect of N on the electronic structure of NG depends on the exact C N bonding pattern. While it is predicted that n-type doping can be brought by graphitic N dopant, pyridinic and nitrilic N dopants carry out p-type doping to graphene [51].

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Fig. 3. (a) An illustration of N atom dopants in the NG lattice [52]. (Reprinted with permission from reference 52. Copyright 2019 American Chemical Society.) (b) The Raman spectra of NG and PrG on SiO2 /Si substrate, in which NG showed visible D and D’ peaks [53]. (Reprinted with permission from reference 53. Copyright 2019, Nature Publishing Group.) (c) The transport curve (source-drain current (Ids ) with respect to the backgate voltage (Vg )) of an NG device, with the neutrality point at −10 V caused by N doping. Inset: a top-view Scanning Electron Microscopy (SEM) image of the actual device [54]. (Reprinted with permission from reference 54. Copyright 2019 American Chemical Society.) (d) Scanning tunneling microscopy (STM) image of a single-substituted N atom in NG. (Inset) A line scan across the dopant shows atomic corrugation and the height of the dopant [55]. (From reference 55. Reprinted with permission from AAAS.) (e) Simulated STM image of graphitic N dopant based on density functional theory (DFT) calculations. The graphene lattice with a single N dopant is superimposed [55]. (From reference 55. Reprinted with permission from AAAS.) (f) Ball-stick model showing the double substitution of N dopant and simulated STM image obtained using first-principles calculations. The carbon and nitrogen atoms are illustrated using gray and cyan balls, respectively [53]. (Reprinted with permission from reference 53. Copyright 2019, Nature Publishing Group.).

In terms of NG synthesis, an early work published in 2009 suggested that NG could be synthesized using NH3 and CH4 , with N concentrations of ≈8.9 at%, consisting of a combination of pyridinic, graphitic, and pyrrolic N, as revealed by X-ray photoelectron spectroscopy (XPS) [52]. The visualization of these single-substituted N dopants using scanning tunneling microscopy (STM) were also reported (Fig. 3d–e) [55]. Importantly, by tuning the ratio of NH3 and CH4 in the synthesis, the N concentration could be varied accordingly. In this context, Lv et al. later synthesized mostly monolayer NG using NH3 and CH4 (Fig. 3b) in 2012, and took one step further by showing the double substitution of N dopants that existed in the same graphene sub-lattice using STM (Fig. 3f) [53]. The existence of such double substitution of N was possibly due to the high intermolecular collisions in the atmospheric-pressure CVD (APCVD) system used in this work and the optimization of the synthetic conditions. Liquid-based NG synthesis was also demonstrated by Jin et al. using pyridine in the low-pressure CVD (LPCVD) system in 2011. The N concentration was ≈2.4 at% with primarily “pyridinic” and “graphitic” N dopants, resulting in a negative neutrality point in the transport curve of the field effect transistor (FET) fabricated from such NG (Fig. 3c) [54]. A similar work was reported by Gao et al. in 2012 using dimethylformamide (DMF), and the N concentration in NG was ≈3.4 at%, as pyridinic, graphitic, and pyrrolic types [56]. Katoh et al. showed in their most recent work on synthesizing NG using four different liquid precursors, which were namely quinoline, pyridine, pyrrole, and pyrimidine [57]. Interestingly, NG with different N configurations can be synthesized by different precursors, and while quinoline only yielded pyridinic N, pyrrole only yielded to pyrrolic N. The discovery of Katoh et al. suggested a novel possibility of preferentially determining N configurations by choosing specific liquid precursors.

Finally, solid precursors are also used to synthesize NG. In this context, Sun et al. synthesized NG on Cu foils by spin-coating melamine dissolved in poly(methyl methacrylate) (PMMA), followed by a thermal treatment in a CVD system [58]. The N concentrations in this work ranges from 2 at% to 3.5 at%, as shown by XPS, and only graphitic N dopants were detected. Similar works also suggested melamine itself could act as both C and N sources, and melamine could be mixed with other polymers to synthesize NG [59,60] (Table 1). Boron-doped graphene Boron (2s2 2p1 ), of similar size when compared to carbon, also remains in the graphene plane when incorporated into the carbon lattice, and the C B bond is approximately the same as the C C bond [39]. While there have been many studies carried out on NG, boron-doped graphene (BG) is much less explored, especially reports on BG grown by CVD. Early works on BG using density functional theory (DFT) calculations and exfoliated boron-doped graphite both indicated that most B dopants exist as boron clusters and boron carbides within BG, while small amounts of B dopants could substitute carbon atoms within the graphene lattice [62,63]. The boron incorporation also induces a significant D peak in the Raman spectrum of graphene due to a broken hexagonal symmetry of the lattice (Fig. 4a–b). Since boron has one less valence electron than carbon, boron doping introduces p-type carriers in BG (Fig. 4c) [62,63]. Given that CVD is the state-of-the-art synthesis route to grow large area films of graphene with the potential for industrial scale applications, researchers have synthesized BG using CVD with different boron precursors in the last few years (see Table 2). Cattelan et al. reported BG synthesis using gaseous precursors [64], includ-

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Table 1 Summary of the conditions to synthesize NG by CVD. Precursors

Synthetic parameters

Layer numbers

Dopants forms and concentrations

References

CH4 +NH3 CH4 +NH3

800◦ , APCVD 980◦ for CH4 entry, 850◦ for NH3 entry, APCVD 1000◦ , LPCVD 950◦ , LPCVD 1000◦ , APCVD 500◦ , Ultrahigh vacuum CVD

Few-layer Monolayer

Pyridinic N, pyrrolic N, graphitic N, 8.9 at% Pyridinic N, graphitic N, double substitution of N, 0.25 at% Pyridinic N, graphitic N, 2.4 at% Pyridinic N, pyrrolic N, graphitic N, 3.4 at% Graphitic N, 2 at%-3.5 at% Pyridinic N for Quinoline-based NG (0.35 at%), pyridinic and graphitic N for pyridine-based NG (2.41 at%), pyrrolic N for pyrrole-based NG (4.28 at%), pyridinic and graphitic N for pyrimidine-based NG (1.78 at%) Pyridinic N, pyrrolic N, graphitic N, 5.6 at%

[52] [53]

Pyridine Dimethylformamide PMMA + melamine Quinoline, pyridine, pyrrole, and pyrimidine

930◦ -1050◦ ,

Melamine

Monolayer Monolayer Monolayer Not provided

4 layers

[54] [56] [58] [57]

[59]

Fig. 4. (a) An illustration of B atoms in the BG lattice [68]. (Reprinted with permission from reference 68. Copyright 2019 Royal Society of Chemistry.) (b) Raman spectra of monolayer BG exfoliated from bulk graphite doped with boron [63]. (Reprinted with permission from reference 63. Copyright 2019 American Chemical Society.) (c) Source-drain current as a function of backgate voltage for intrinsic (undoped, black) and BG (red), showing the p-type behavior of BG [65]. (Reprinted with permission from reference 65. Copyright 2019 John Wiley and Sons, Inc.) (d–g) STM images of two monolayer BG grown using different growth conditions. For BG grown using diborane, single graphitic substitution is observed (d,e) while for BG grown using TEB, the dominating dopant arrangement is more complicated (f, g) [67]. (Reprinted with permission from reference 67, Copyright 2019 American Chemical Society.).

Table 2 Summary of the conditions to synthesize BG by CVD. Precursors

Synthetic parameters

Layer numbers

Dopants forms and concentrations

References

Boron powder + ethanol CH4 + B2 H6 Phenylboronic acid Hexane + triethylborane

950◦ 1000◦ , LPCVD 950◦ , LPCVD 1000◦ , APCVD

3-5 layers Monolayer Monolayer Monolayer

Graphitic B, “boron silane” (BC2 ) B, 0.5 at% Graphitic B, B4 C, B-O-C compounds, 2.5 at% Graphitic B, 1.5 at% “Croissant”-shaped, 1.75 at%

[66] [64] [65] [67]

ing methane and diborane, with B concentrations of ≈2.5 at%, and B configurations ranging from graphitic B components to boron oxides as shown by XPS and STM (Fig. 4d–e). Solid precursors such as phenylboronic acid or even boron powders dissolved in ethanol have been used for BG synthesis. BG based on phenylboronic acid exhibited a B concentration of ≈1.5 at%, and only graphitic B was detected by XPS [65,66]. Lv et al. also reported the BG synthesis using liquid precursors involving hexane and triethylborane, with a B concentration of ≈1.75 at% that primarily existed as “croissantlike” doping patterns - corresponding to three boron atoms and a di-vacancy as shown by STM images and simulations (Fig. 4f–g) [67].

Silicon-doped graphene Although Si (3s2 3p2 ) is an element in the same group as C, it is less investigated as a graphene dopant when compared to N or B. Unlike N or B, however, Si would protrude out of the plane in silicondoped graphene (SiG) due to a larger bond length of the Si-C bond (1.767 Å) when compared to the C C bond (1.41 Å; Fig. 5a,b). Such deviation from a perfectly flat sp2 -hybridized carbon layer would theoretically cause SiG to be more reactive than PrG, and possibly leads to a bandgap opening in SiG (Fig. 5c) [43]. The increased chemical reactivity of SiG owing to the structural change brought by Si dopants was studied theoretically, where ab initio calculations

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Fig. 5. (a) An optimized unit cell for 6 × 6 graphene doped with a single dopant of aluminum, silicon, phosphorus or sulfur with the bond lengths (with units in Å) shown [43]. (Reprinted from reference 43. Copyright 2019, with permission from Elsevier.) (b) Atomic configuration of SiG in which the Si concentration is 3.1% and (c) the calculated band structure of SiG in (b) [72]. (Reprinted with permission from reference 72. Copyright 2019 RSC Pub.) (d) Transmission electron microscopy (TEM) image of SiG, showing the selected-area electron diffraction (SAED) pattern of monolayer SiG (Inset), and the monolayer (e) and bi-layer (f) region of SiG [73]. (Reprinted with permission from reference 73. Copyright 2019 John Wiley and Sons, Inc.) (g) Raman spectra of SiG sheet on SiO2 /Si substrate when compared to NG and PrG [73]. (Reprinted with permission from reference 73. Copyright 2019 John Wiley and Sons, Inc.).

indicate that for Si incorporated in the graphene lattice, the adsorption energy increases for probing gas molecules such as CO, O2 and NO2 [69–71]. Experimentally, the synthesis of SiG was not reported until 2014, when Lv et al. synthesized SiG in an APCVD system using methoxytrimethylsilane (MTMS) and hexane [73]. Monolayer SiG was synthesized in this work as suggested by transmission electron microscopy (TEM) images (Fig. 5d–f), and the XPS data indicated the Si bonded to C in the form of a single Si atom or small clusters, with a Si concentration of ≈1.75 at%. Interestingly, the Si concentration can be tuned by varying the amount of MTMS in hexane during the synthesis. Besides the pioneering work of Lv et al., a few other experimental works in SiG were reported after 2014, demonstrating SiG can also be synthesized by powder vaporization using a single solid precursor triphenylsilane, or by using a mixture of gaseous precursors of SiH4 and CH4 [72,74], where the Si concentration in SiG can be tuned by changing the ratio of SiH4 and CH4 , and the highest Si concentration reported is ≈4.5 at%, while for SiG synthesized using triphenylsilane, the Si concentration is ≈2.63 at% (Table 3). Phosphorus-doped graphene Given the larger radius of phosphorus (3s2 3p3 ) when compared to that of N, P would protrude out of the plane when doping graphene, preserving its sp3 character and bonds with three neighboring C atoms in a pyramidal-like configuration (Fig. 6) [75,76]. It was also predicted that substitutional doping of P would increase the chemical reactivity of phosphorus-doped graphene (PG). For example, PG could effectively reduce NO2 into NO, and NO could chemisorb on PG with a moderate adsorption energy [75]. However, the bottom-up synthesis of PG remains challenging. It was suggested by Mastrapa et al. that PG could be synthesized using

a LPCVD system involving triphenylphosphine (TPP) as both the C and P sources [45], but XPS data suggest that the P concentration in this study was ≈0.68 at% at the highest, primarily existing as P O, P P or P H bonds. In the work of Shin et al. [77], PG was synthesized by treating PrG with TPP coupled to an induction plasma. The results from spatially resolved electron energy loss spectroscopy (EELS) suggested that P was evenly distributed in the lattice without segregation or agglomeration, but the P concentration was only ≈0.26 at% as shown by XPS (Table 4). Sulfur-doped graphene Due to the larger size of sulfur (3s2 3p4 ) when compared to carbon, it is predicted that the formation energy of incorporating S into graphene would be larger than that of sulfur-doped carbon nanotubes, but sulfur-doped graphene (SG) is very interesting because it could be a small band-gap semiconductor, and the structural changes brought by S could improve the metallic properties of graphene [79]. SG was also predicted to have significant potential for gas sensing, where it only selectively binds to NO2 with a significantly large adsorption energy (Ea = -0.83 eV). A charge transfer would occur from SG to NO2 , thus triggering a change of conductivity of the system [80]. The earliest work on SG synthesis was reported by Gao et al., in which SG was synthesized using a LPCVD system with S powder dissolved in hexane [46]. The XPS analysis, however, indicated that the S concentration was ≈0.6 at%, and most of the S formed aggregates on the surface of SG. On the contrary, SG synthesized at low temperatures exhibits higher S concentrations. In 2014, Zhang et al. synthesized SG using tetrabromothiophene, a solid precursor that could vaporize at low temperatures, using a LPCVD system (Fig. 7a) [47]. In this study, SG was formed via a free radical coupling reaction from tetrabromothiophene, favoring the formation

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Table 3 Summary of the conditions to synthesize SiG by CVD. Precursors

Synthetic parameters

Layer numbers

Dopants forms and concentrations

References

Hexane + methoxytrimethylsilane Triphenylsilane CH4 + SiH4

1000◦ , APCVD 800◦ , LPCVD 1015◦

Monolayer Monolayer Monolayer

1.75 at% ≈2.63 at% 2.7 at% to 4.5 at%

[73] [74] [72]

Fig. 6. Optimized structure of P dopant in PG by DFT calculations [75]. The distance of P dopant to the basal plane of PG and C–P bond length are 1.33 Å and 1.77 Å, respectively. (Reprinted from reference 75, Copyright 2019, with permission from Elsevier.). Table 4 Summary of the conditions to synthesize PG by CVD. Precursors

Synthetic parameters

Graphene layer numbers

Dopants forms and concentrations

References

Triphenylphosphine Triphenylphosphine

100◦ -250◦ annealing in CVD system 1000◦ , LPCVD

N.A. Monolayer

[78] [45]

Triphenylphosphine

PrG treated with triphenylphosphine and inductively coupled plasma

Monolayer

4.96 at% 0.12 at%-0.68 at% (total P%), 0.09 at%-0.14 at% (P-C bonding) 0.26 at%

[77]

Fig. 7. (a) A typical Raman spectrum of SG. (b) Transfer curve (Vgs −Ids ) of SG FETs with neutrality point around −35 V, showing an n-type behavior. (c) Output characteristics (Vds −Ids ) of SG based FETs [47]. (Reprinted with permission from reference 47. Copyright 2019 American Chemical Society.). Table 5 Summary of the conditions to synthesize SG by CVD. Precursors

Synthetic parameters

Graphene layer numbers

Dopants forms and concentrations

References

Hexane + S Tetrabromothiophene

950◦ , LPCVD 300◦ , LPCVD

Monolayer Monolayer

C–S bond, 0.6 at% C − S bond and Cu − S bond in CuS and Cu2 S, 1.54 at%

[46] [47]

and protection of weak C–S bonds at low synthetic temperatures. Here, S formed covalent bonds with neighboring C atoms in SG, and the S concentration was ≈1.54 at%. The improved S concentration in the study enabled SG to show an n-type behavior in fabricated FETs (Fig. 7b–c), (Table 5). Heteroatom doped semiconducting TMDCs: Synthesis and characterization In terms of MoS2 doping, possible dopants primarily include transition metal and chalcogen atoms. Because monolayer MoS2 is a direct bandgap semiconductor, doped MoS2 primarily leads to bandgap tuning, which can be characterized by the shifting of PL peaks [81,82]. The electronic transport of TMDCs can also be

altered by doping. For example, the electronic properties of MoS2 is changed to p-type when doped with Nb [83], while the electronic transport of WS2 is gradually tuned from n-type to ambipolar, and eventually to p-type, by increasing the concentration of carbonhydrogen (CH) units within WS2 [84]. Details of these studies will be discussed below. The CVD or powder vaporization synthesis of doped MoS2 is usually carried out on insulating substrates such as SiO2 /Si, sapphire, or h-BN, following a vapor-solid growth mechanism [85–87]. Although the free energy of formation (Gf o ) for S-doped phases discussed in this review (Nb-S, Mn-S, Co-S and Fe-S phases) are negative [88–93], and the growth of W- and Sedoped MoS2 are facilitated due to isomorphism [94–96], synthetic parameters, such as temperature, pressure, and atmosphere, are key when controlling the concentration and distribution of the

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Fig. 8. (a) STEM image of Mo0.47 W0.53 S2 monolayer in which bright atoms are W and dark atoms are Mo. (b, c) Evolution of A and B exciton peak of Mo1-x Wx S2 monolayers with different W compositions. (d) Composition-dependent bandgap (Eg ) of Mo1-x Wx S2 monolayers. The red line is the parabola fitting, giving a bowing parameter b of 0.28 ± 0.04 eV. (e) Energy shift of the HOMO and LUMO of Mo1-x Wx S2 monolayers as a function of W composition. While the energy level of the LUMO increases with a bowing behavior, the energy level of HOMO increases linearly. [81]. (Reprinted with permission from reference 81. Copyright 2019 American Chemical Society.).

Table 6 Summary of the conditions to synthesize W-doped MoS2 by CVD. Precursors

Synthetic parameters

Layer numbers

References

Mo1-x Wx film, S powder MoS2 , WO3 , S powder MoO3 , WCl6 , S powder Mox W1-x Oy , H2 S

950◦ 800◦ 700◦ Two-step process, with first sulfurization at 600◦ or 800◦ , and second sulfurization at 1000◦

2-4 layers Monolayer Monolayer and multilayered triangles Layer-number-controlled monolayer to multilayered Mox W1-x S2

[99] [100] [101] [104]

dopants within the MoS2 lattice. In this section, we summarize the synthesis of doped MoS2 , and how different precursors alter its physical and chemical properties.

Table 7 Summary of the conditions to synthesize Co-doped MoS2 by CVD. Precursors

Synthetic parameters

Layer numbers

References

MoO3 , Co3 O4, S powder

680◦ and 750◦

2 layers

[109]

W-doped MoS2 (Mo1-x Wx S2 ) The growth of Mo1-x Wx S2 is facilitated by the isomorphism between WS2 and MoS2 , thus enabling the doping to be achieved without phase separation [94]. Chen et al. reported an early study of Mo1-x Wx S2 exfoliated from its bulk crystals synthesized by chemical vapor transport (CVT) [81]. The W atoms were homogeneously introduced into the MoS2 lattice as shown by the scanning transmission electron microscopy (STEM; Fig. 8a). Two prominent features were observed in the PL spectrum of Mo1-x Wx S2 monolayers: the A exciton (lower energy) and the B exciton (higher energy one) emissions (Fig. 8b,c).The exfoliated Mo1-x Wx S2 monolayers revealed a bowing effect in A exciton, i.e., the peak emission first red-shifted, then blue-shifted, and overall varied from 1.82 eV (x = 0) to 1.99 eV (x = 1) (Fig. 8b). Such a bowing effect is related to the different contribution of the d orbitals of Mo and W to the highest occupied molecular orbital (HOMO), and the lowest unoccupied molecular orbital (LUMO) of Mo1-x Wx S2 , as shown by DFT calculations (Fig. 8d–e) [81,97]. Similar analysis of exfoliated Mo1-x Wx S2 from their corresponding bulk crystals further suggested that the A–B energy separation increases significantly with increasing W concentration, thus reflecting the stronger spin-orbit interaction of W when compared to Mo [98]. Interestingly, monolayer Mo1-x Wx S2 can also be directly synthesized by powder vaporization, where the synthetic parameters have led to interesting variations on its properties [99–103] (Table 6). For example, Lin et al. synthesized Mo1-x Wx S2 by sulfurizing MoS2 and WO3 powders observing a gradual increase of W concentration from the edge to the center of the Mo1-x Wx S2 triangle, as evidenced in the evolution of the PL spectra (Fig. 9a–c) [100]. Wang et al. synthesized Mo1-x Wx S2 using WCl6 , S and MoO3 , demonstrating a semiconducting 1H phase and a metallic 1 T phase by high-angle annular dark-field STEM (HAADF-STEM, Fig. 9d) [101]. Song et al. were able to control the layer number and

the concentration of Mo and W in Mo1-x Wx S2 by first synthesizing Mo1-x Wx O2 using super-cycle atomic layer deposition (ALD), and then sulfurize the Mo1-x Wx O2 using H2 S [104]. Besides the variations of phases and W distribution mentioned above, further research suggested that Mo1-x Wx S2 showed an n-type semiconducting behavior with a field effect mobility of ≈30 cm2 ·V−1 ·s−1 at 300 K [102]. However, subsequent works are needed to elucidate the growth mechanism of Mo1-x Wx S2 with different synthetic parameters, such that an optimized synthesis would lead to effective phase engineering and hybrid structure control of Mo1-x Wx S2 , as described in the works of Lin et al. and Wang et al. Co-doped MoS2 Co, along with Fe and Mn, have been intensively studied theoretically as dopants to transform MoS2 into a dilute magnetic semiconductor [105,106]. From a thermodynamic perspective, early literature suggested that various Co-S phases exist, such as CoS2 , Co3 S4 and Co9 S8 , and their Gf o at room temperature are all greater than −50 kJ/(mol of atoms), which is much less negative than that for MoS2 [88,89]. Industrially, Co-doped MoS2 powder has been utilized as an effective catalyst for hydrodesulfurization process in the oil industry, and STM illustrated that Co is preferentially introduced along the edges of MoS2 , thus changing MoS2 from triangles to hexagons [107,108]. In terms of the CVD synthesis of Co-doped MoS2 , Li et al. synthesized bi-layer Co0.16 Mo0.84 S2 by sulfurizing MoO3 and Co3 O4 powders [109] (Table 7). The A1g and E2g Raman-active peaks corresponding to MoS2 and CoS2 were observed across the entire sample (Fig. 10a). The E2g peak related to CoS2 , on the other hand, was strong at the edge and weak at the center of the sample (Fig. 10b,c), in agreement with earlier studies indicating that Co atoms mainly

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Fig. 9. (a) PL spectra acquired from the center, edge and a position between the center and the edge of one Mox W1-x S2 triangular monolayer. As a reference, PL spectra of single phase MoS2 and WS2 monolayers are also plotted with dashed lines. PL intensity mapping of one Mox W1-x S2 triangular monolayer at (b) 1.81 eV, same as that for single phase monolayer MoS2 , and (c) 1.97 eV, same as that for single phase monolayer WS2 [100]. (Reprinted from reference 100, with the permission of AIP Publishing, licensed under a Creative Commons Attribution license.) (d) Phase boundary (blue dashed line) for 1 T and 1H phases of Mox W1-x S2 sample, with the intensity profile along the orange dashed line overlaid on the image [101]. (Reprinted with permission from reference 101. Copyright 2019, Nature Publishing Group.).

Fig. 10. (a) Raman spectrum of Co0.16 Mo0.84 S2 sample, where the Eg mode at ∼290 cm−1 , Tg (1) mode at ∼311 cm−1 , and Ag mode ∼395 cm−1 belong to CoS2 , and A1g mode at ∼405 cm-1 , E2g modes at ∼379 cm-1 and 374 cm-1 belong to Cox Mo1-x S2 . Raman peak intensity mappings at (b) 374 cm-1 , associated with CoS2 , and (c) 379 cm-1 , associated with MoS2 [109]. (Reprinted with permission from reference 109. Copyright 2019 American Chemical Society.).

grow at edge of Co-doped MoS2 [107,108]. The amount of S used and the temperature both affect the morphology and composition of the Co-doped MoS2 . For example, by increasing the amount of S at low temperature, the Cox Mo1-x S2 layers changed from David’s star shapes into hexagons, and by increasing the temperature Cox Mo1-x S2 transformed into separate layers of MoS2 and CoS2 . A FET fabricated by Co-doped MoS2 revealed an n-type behavior, and a mobility of ≈0.52 cm2 V−1 s−1 .

in 2H phase as confirmed by X-ray Diffraction (XRD). In terms of transport properties, Fe-doped MoS2 exhibits an n-type behavior, and its carrier mobilities were lower than those of pure MoS2 , originating from the lattice distortions brought by Fe dopants. It is clear that alternative routes to these magnetic systems are needed as well as a systematic study on their layer dependent properties. A possible approach would be the use of suitable atomic surfactants during the growth of these doped monolayers.

Fe-doped MoS2

Mn-doped MoS2

Similar to Co, Fe is also predicted to change the magnetic properties of MoS2 , but DFT calculations suggested the effect of doping was layer-dependent [110]. While Fe doping in monolayer MoS2 leads to ferromagnetic coupling, Fe doping in bi-layer or multilayered MoS2 leads to antiferromagnetic coupling. Though the Gf o for FeS2 and other Fe-S phases are all negative, the experimental synthesis of Fe-doped MoS2 with high Fe content (e.g. > 10 %at.) remains a challenge [90,91]. In this context, Wang et al. reported the synthesis of multilayered Fe-doped MoS2 single crystal using CVT with Mo, S and Fe powders [111]. The Energy Dispersive X-ray Spectroscopy (EDS) measurements suggested the Fe concentration was ≈0.53 at% in the sample, and the obtained Fe-doped MoS2 was

As the third possible magnetic dopant to MoS2 after Fe and Co, Mn has also been intensively investigated using first-principles calculations, DFT and Monte Carlo simulations [112,113]. Mn doping is interesting as there exists a long-range ferromagnetism of Mn spins mediated by an antiferromagnetic exchange between the localized d-states of Mn and the delocalized p-states of the chalcogen atoms, while the coupling of Fe and Co with the chalcogen atoms are generally weaker [114]. It is also suggested that the magnetic moment induced by Mn could be significantly enhanced by applying a tensile strain [115]. Although the Gf o for MnS and MnS2 are both negative, the factors related to the synthesis of Mn-doped MoS2 are not only limited

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Fig. 11. (a) The PL spectrum of monolayer pristine and Mn-doped MoS2 grown on epitaxial grapheme. The addition of Mn leads to enhanced nonradiative recombination in Mn-doped MoS2 , causing the absence of A- and B- exciton peaks. (b) The STEM image of Mn-doped MoS2 grown on epitaxial graphene, showing the substitution of Mn atoms in the lattice as illustrated by the red circles and blue rectangles. (c) Room-temperature current − voltage curve of MoS2 samples, where the incorporation of Mn leads to a lower saturation conductance, possibly due to an increase in the density of states in the bandgap of the MoS2 [116]. (Reprinted with permission from reference 116. Copyright 2019 American Chemical Society.).

Table 8 Summary of the conditions to synthesize Mn-doped MoS2 by CVD. Precursors MoO3 , Mn2 (CO)10 , S powder

Synthetic parameters

Layer numbers

725◦

Monolayer

Table 9 Summary of the conditions to synthesize Nb-doped MoS2 by CVD. References

Precursors

Synthetic parameters

Layer numbers

References

Mo, Nb, S

900◦

N.A.

[83]

[116]

to thermodynamics [92]. Experimentally, Zhang et al. synthesized Mn-doped MoS2 by treating MoO3 with S vapor and vaporized Mn2 (CO)10 powder, suggesting the growth is highly dependent on the substrate [116] (Table 8). When the synthesis was carried out on substrates covered by graphene, the PL of the obtained Mndoped MoS2 was completely quenched, indicating the existence of Mn dopants, which were also directly observed by high-resolution TEM (HRTEM) and STEM (Fig. 11a–b). However, when the same synthesis was performed on insulating substrates such as SiO2 and sapphire, the Mn-S bonding was not found via XPS or HAADF-STEM, regardless of increases in the Mn/Mo ratio of the precursors, indicating that the Mn doping was below the detection limits of these techniques. Despite the low Mn concentration of Mn-doped MoS2 grown on insulating substrates, the gate-dependent conductance curve of Mn-doped MoS2 showed a much lower saturated conductance, thus indicating the slow movement of the Fermi level inside the band gap, possibly due to the presence of a higher density of localized states originating from Mn doping (Fig. 11c). Further experiments along these lines should be carried out in the future and higher concentrations of Mn dopants need to be embedded within the TMD monolayers. It is possible that other routes besides CVD can be used in these attempts.

While there are studies of Nb-doped MoS2 flakes produced by mechanical exfoliation of bulk crystals, reports demonstrating the growth of Nb-doped MoS2 by CVD are scarce [120,121] (Table 9). In this context, Zhu et al. synthesized Nb-doped WS2 nanotubes by heating Nb2 O5 -coated W18 O49 nanorods in H2 S. The Nb dopants inhibited WS2 nanotube formation, invariably closeed the tube tips, and created structural defects within the layers of WS2 nanotube [122]. More recently, Laskar et al. were able to use CVD to induce p-type doping in few-layer MoS2 films by evaporating a Mo/Nb/Mo stack onto sapphire substrates and then sulfurizing these stacks at 900◦ [83]. The Nb acted as an efficient acceptor in which a Hall mobility of 8.5 cm2 V−1 s−1 was demonstrated for a hole density of 3.1 × 1020 cm-3 . In addition, the films displayed a low contact resistance of 0.6  mm using Ni/Au/Ni contacts. Through aberration-corrected STEM, the highly crystalline nature of the MoS2 was shown to be preserved after Nb doping (Fig. 12b). Furthermore, in terms of the Raman spectra, the ratio of the A1g to E1 2g was observed to be dependent on the Nb doping concentration (Fig. 12a). To the best of our knowledge, the CVD growth of monolayer Nb-doped MoS2 has not yet been reported and remains a challenge in the field. Novel alternative routes to Nb-doped MoS2 should be developed in the future as doping clearly alters the electronic transport of TMDCs.

Nb-doped MoS2

Se-doped MoS2

Although hole conduction in MoS2 has been demonstrated using ionic liquid gating and high work function MoOx contacts [117,118], an alternative method is the substitutional doping of MoS2 with Nb, which contains one electron less than Mo. Using DFT, Dolui et al. demonstrated that doping MoS2 with Nb serves as an attractive way to induce p-type conduction as there is minimal changes in the bond lengths and the density of states, yet the Fermi energy is shifted to below the valence band maximum [119]. With Morich conditions during growth, the authors estimated the formation energy for Nb doping to be -0.19 eV, with an even more negative formation energy in the S-rich limit. Similar results of Shatynski et al. suggest that NbS2 has a Gf o of -149.30 kJ/(mol of atoms) between 1123 K–1373 K [93].

As a chalcogen atom in the same group with S, Se could be doped into MoS2 without causing phase separation, and DFT studies suggested the bandgap of the material could be continuously tuned by Se dopants [95,96]. Experimentally, Su et al. reported a facile synthesis of MoS2x Se2(1−x) by selenizing as-grown MoS2 monolayers using Se vapors [123]. The Se substitution started to occur at 600◦ , and by 900◦ the S were completely substituted by Se. Although Se doping did not result in obvious changes to the morphology of the flakes, characteristic Raman signals of MoSe2 began to appear when selenization was carried out above 700◦ (Fig. 13a). The selenization of MoS2 was also demonstrated by Ma, et al., using diselenodiphenyl (DS), an organic Se source [124]. By cycling between mildly sputtering MoS2 and exposing it to DS vapor

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Fig. 12. (a) Raman spectra of MoS2 with different concentration of Nb dopants. The relative intensities of E1 2g and A1g vibrational modes observed at 381 cm−1 and 407 cm−1 changed with varying Nb concentration, respectively. (b) A STEM image of Nb-doped MoS2 film on Al2 O3 substrate showing the stacking of MoS2 layers in the [0001] direction [83]. (Reprinted from reference 83, with the permission of AIP Publishing.).

Fig. 13. (a) Raman spectra for the MoS2 flakes before and after selenization at different temperatures. When selenization is carried out at above 700◦ , new peaks at 287.1 cm−1 and 239 cm−1 are observed and are assigned to E1 2g and A1g modes of MoSe2 respectively [123]. (Reprinted with permission from reference 123. Copyright 2019 John Wiley and Sons, Inc.) (b) Normalized room-temperature PL spectra of a monolayer MoS2 film after sputtering and DS insertion cycles. The PL peak red shifts and broadens until reaching a saturated Se concentration [124]. (Reprinted with permission from reference 124. Copyright 2019 American Chemical Society.) (c) Room-temperature PL intensities and peak positions of a monolayer MoS2 film after cycles of sputtering and DS exposure. Sputtering reduces the PL intensity and blue-shifts the PL peak (blue curve), while DS exposure almost recovers the PL intensity to the initial level after 5 cycles, and red-shifts the PL peak (red curve) [124]. (Reprinted with permission from reference 124. Copyright 2019 American Chemical Society.).

followed by a low temperature annealing, a cycle of decrease and recovery of PL intensity, and a gradual red-shift of the PL emission were observed (Fig. 13b,c). Along with XPS data showing the existence of Se dopants, the recovery and red-shift of the PL signal indicates the bandgap tuning of Se [124]. In terms of the CVD synthesis of Se-doped MoS2 , Feng et al. demonstrated the growth of MoS2(1-x) Se2x with vaporized MoS2 and MoSe2 powders [82], where the Se and S atoms were randomly distributed within the lattice as indicated by HAADF-STEM (Fig. 14). Similarly, Se-doped MoS2 can be synthesized by treating MoO3 powders with solid chalcogen sources such as S and Se powders, or liquid chalcogen sources such as diphenyl-diselenide and thiophenol dissolved in tetrahydrofuran [125,126]. In both cases, the synthesized MoS2(1-x) Se2x showed tunable bandgaps with varying Se concentrations. It is also possible to co-introduce Se, S and Te

in these TMDCs, where further theoretical and experimental work along this direction is needed (Table 10).

Applications of heteroatom-doped graphene Gas sensing One important application of doped graphene is gas sensing. Gas sensors have been widely used, ranging from detecting toxic or combustible gases in industries, to work as smoke detectors or monitoring common air pollutants at home. The key component of a gas sensor is an active sensing layer, where the conductivity of the material should be highly sensitive to the different gaseous species. As a novel carbon material, graphene is one of the most

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Fig. 14. (a) HAADF-STEM image of MoS2(1-x) Se2x in false color, showing the random distribution of Se dopants in the lattice. (b) PL spectra of monolayer MoS2(1−x) Se2x , where a red-shift is observed with the increasing x value [82]. (Reprinted with permission from reference 82. Copyright 2019 John Wiley and Sons, Inc.).

Fig. 15. (a) Ball-stick model of NO2 molecule interacting with BG. Yellow, red, grey, and blue atoms represent C, O, B, and N atoms, respectively. The B atom protrudes out of the plane when it binds to one O atom of the NO2 molecule. (b) Isosurface plot of the electron charge density difference for NO2 on BG. The charge density piles between the O atom of NO2 and the B atom of the BG, indicating a strong bonding due to a hybridization of the orbital between NO2 and BG [130]. (Reprinted from reference 130, with the permission of AIP Publishing.).

Table 10 Summary of the conditions to synthesize Se-doped MoS2 by CVD. Precursors

Synthetic parameters

Layer numbers

References

MoS2 , MoSe2 MoO3 , S, Se MoO3 , diphenyl-diselenide, thiophenol

600◦ -700◦ 800◦ 650◦ -700◦

Monolayer Monolayer Monolayer

[82] [125] [126]

ideal candidates for gas sensing applications due to its high surface area and electrical conductivity at room temperature [4–7]. It was suggested that graphene could detect the adsorption and desorption of individual gas molecules, based on the change of the local carrier concentration during such events [127]. However, it was later discovered that the sensitive response of graphene to gas molecules may come from the contamination layer, originating from electron-beam lithography [128]. Such a contamination layer is responsible for concentrating gaseous analytes at the graphene surface, thus enhancing the response, while the intrinsic response of PrG to vapors is surprisingly small [128].

Alternatively, doped graphene was shown to be more competitive in gas sensing when compared to its pristine counterpart. For example, first-principle studies suggested that common gases, such as CO, NO, NH3 and NO2 , show higher adsorption energies when interacting with BG and NG than that with PrG due to the stronger interaction between the gases and the dopant atoms (Fig. 15) [129]. Larger heteroatoms such as S and Si were also predicted to increase adsorption when embedded in graphene, for gases such as CO, O2 , NO2 and H2 O [69,130]. Experimentally, BG was shown to be more effective in gas sensing than PrG. It was demonstrated by Lv et al. that BG synthesized

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Fig. 16. The comparison of the response of sensors, measured by the changes of drain-to-source current (Ids ) with respect to the time, between PrG (a & b) and BG (c & d) to NO2 and NH3 . For both NH3 and NO2 detection, BG displays more clearly resolved signals with bigger s/n ratios [67]. (Reprinted with permission from reference 67).

using triethylborane and hexane could detect NO2 at concentrations as low as 1 ppb with a signal-to-noise (s/n) ratio of 31.5, while PrG synthesized by hexane was only able to detect NO2 at 8 ppb, with a s/n ratio of 9.4. BG could also sense NH3 at concentrations of 1 ppm with a s/n ratio of 50.1, while PrG showed a response when NH3 concentration reached 20 ppm with a s/n ratio of 9.5 (Fig. 16) [67]. The improved gas sensing performance of BG in this work is related to strong molecule and graphene interaction, originating from the B3 -dopants (three boron atoms and a vacancy) that have a large affinity to both donor and acceptor molecules, such as NH3 and NO2 . Graphene Enhanced Raman Spectroscopy Fluorescent organic molecules are known to be difficult to analyze using Raman spectroscopy because their Raman signals are invisible in the high fluorescence background. It was shown previously that graphene effectively quenches the fluorescence of Rhodamine 6 G (R6 G) and Protopphyrin IX (PPP) after they were adsorbed on graphene [131]. Considering graphene possesses an atomically smooth surface, high transparency to visible light, and surface plasmons in the terahertz range, the fluorescence quenching of graphene is mostly based on a chemical mechanism, where the electron transfers between the molecules and graphene, quenching the fluorescence cross section [6,132–134] (Fig. 17). Two factors are important to facilitate graphene enhanced Raman spectroscopy (GERS): First is the orientation of the molecule, where GERS is only significant when the molecule stays parallel and close to graphene as the electron transfer is a short-range process [135]. The second factor is the relative position of the Fermi level of graphene with respect to the HOMO or LUMO. When the energy gap between the Fermi level of graphene and the molecular orbital of the molecule is close to the energy of the incident photon, the Raman intensity of the molecule is enhanced, originating from sufficient electron transfer between graphene and the molecule [136–138]. In addition to varying the Fermi level of graphene by gating, heteroatom doping also becomes a powerful method to optimize

the Raman enhancement of graphene. In this context, Lv. et al. reported NG synthesized by CVD in the presence of NH3 and CH4 demonstrates a significantly enhanced Raman scattering effect on Rhodamine B (RhB) than PrG [53]. The intensities of all RhB peaks increased and were more clearly resolved when RhB was adsorbed on NG, and new fingerprints of RhB at 1282 cm−1 , 1531 cm−1 and 1567 cm−1 were also observed on NG (Fig. 18b). The signal enhancement of RhB on NG is due to the tuning of the Fermi level caused by N dopants. For example, Feng et al. showed that the detection limit of RhB using NG synthesized by NH3 and CH4 can be as low as 5 × 10-11 M [140]. Through ab initio calculations, Feng et al. suggested that N doping shifts the Fermi level of NG into higher energy and shifts the EF of NG closer to the LUMO of RhB, thus enhancing the RhB signals on NG (Fig. 18c,d). Besides N doping, heteroatom doping of graphene with larger atoms can lead to a modified surface structure, thus improving the GERS performance. Lv et al. reported an enhancement in GERS performance of SiG synthesized by MTMS and hexane [73]. The signals of RhB on SiG were even higher compared to those of NG. Besides the shifting of the Fermi level of SiG and the LUMO of RhB caused by Si-O bond and Si substitution, Si doping induces a convex curvature around the dopant, and such curvature might be beneficial for the interaction of RhB with SiG as it allows a “wrapping” of RhB around the dopant. Applications of heteroatom-doped TMDCs Optoelectronics Zhang et al. investigated CVD monolayer MoS2 as phototransistors in 2013, revealing n-type behavior, a high photoresponsivity of 2200 AW−1 and a high photogain of 5000 in vacuum [86]. Adsorbates from air results in p-type doping to the MoS2 -based phototransistor, thus increasing the possibility of photoexcited carriers’ recombination and resulting in a decrease of the photoresponsivity and photogain of MoS2 . At the same time, ambient p-type doping assists photocurrent relaxation and decreases the photocurrent decay time. Zhang et al. also indicated the existence of trap

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Fig. 17. (a) Schematic illustration of graphene quenching the fluorescence of R6 G molecules [139]. (Reprinted with permission from reference 139. Copyright 2019 John Wiley and Sons, Inc.) (b) Comparison of Raman spectra of R6 G in water (10 ␮M) and on monolayer graphene, in which “∗” marks the Raman signals of the SiO2 /Si substrate. (c) & (d) Estimated fluorescence (FL) and resonance Raman spectroscopy (RRS) cross-sections of R6 G in solution and on graphene, respectively. The FL cross-section was decreased significantly on graphene, allowing the Raman signals of R6 G to be present [131]. (Reprinted with permission from reference 131. Copyright 2019 American Chemical Society.).

Fig. 18. (a) A schematic of the effect of the energy level alignment between the adsorbed molecule and graphene Fermi level on the charge transfer resonance [136]. (Reprinted with permission from reference 136. Copyright 2019 American Chemical Society. (b) The GERS spectra of RhB on PrG, NG and SiG. Significantly stronger RhB signals were detected on SiG when compared to those on NG and PrG. Peaks labeled with “” are from RhB [73]. (Reprinted with permission from reference 73. Copyright 2019 John Wiley and Sons, Inc.) Density of states (DOS) of the clusters representing the adsorbed RhB on (c) PrG and (d) NG. The filled areas are the DOS projected (PDOS) on the dyes. Vertical dashed lines indicate the position of the system’s Fermi level (EF ), horizontal pink arrows indicate the HOMO-LUMO gaps of the molecule (in eV), and horizontal black arrows indicate the gap of HOMO-EF (in eV) [141]. (From reference 141. Reprinted with permission from AAAS.).

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Fig. 19. PL spectra of monolayer (a) MoSe2 and (b) MoS2 , respectively, at room temperature (RT) and 12 K. While only free exciton peaks are observed in MoSe2 and MoS2 at RT, a broad defect-bound exciton peak between 1.4 and 1.9 eV is observed in MoS2 at 12 K. The large width of the bound exciton peak in MoS2 indicates the presence of different kinds of defects [142]. (Reprinted with permission from reference 142. Copyright 2019 American Chemical Society.) (c) Time-dependent response of MoS2 , MoS1.2 Se0.8 , and MoSe2 devices. The fast photocurrent decrease for a MoSe2 device is highlighted in the inset. Increasing the Se concentration effectively accelerates the photoresponse of the material [143]. (Reprinted with permission from reference 143. Copyright 2019 American Chemical Society.).

Fig. 20. (a) Change in resistance versus time for monolayer MoS2 upon NH3 exposures for 15 s at concentrations ranging from 2 ppm to 30 ppm. (b) Plot of resistance change (black solid circles) and signal-to-noise ratios (gray open boxes) vs. NH3 concentration. A linear relationship is observed in the dashed fitted line applied [144]. (Reprinted with permission from reference 144. Copyright 2019 John Wiley and Sons, Inc.) Conductance change of MoS2 transistors in the atmosphere of (c) 400 ppb of NO2 and (d) 500 ppm of NH3 with respect to the drain-to-source voltage (VDS ) [145]. (Reprinted with permission from reference 145. Copyright 2019 American Chemical Society.).

centers such as defects and charged impurities affects the performance of MoS2 phototransistors by persistent photoconductance (PPC), and a change in the concentration of free carriers, which persists after the excitation has been removed, further elongates the photocurrent decay time. Deeper understanding of PPC suggests the trap centers could be the long-range Coulomb potentials at the SiO2 /MoS2 interface [86]. Compared to MoS2 -based phototransistors, an interesting work by Chang et al., demonstrated CVD grown MoSe2 revealed a much faster photo response and decay [142]. Although MoSe2 had a smaller photogain (≈5 × 10−4 ) when compared to that of MoS2 (≈0.2), a higher electron Schottky barrier of MoSe2 produced a much faster photo response and decay (≈25 ms) when compared to that of MoS2 (30 s), and the underlying reason was illustrated in their PL spectra. While free exciton peaks, representing direct band-to-band A excitonic transitions, were observed for both MoS2 and MoSe2 at room temperatures, an additional sub-bandgap emission between 1.4 eV and 1.9 eV originating from defects or charged

impurities was observed for MoS2 at low temperatures (Fig. 19a,b). The absence of this sub-bandgap emission peak in MoSe2 suggests a better degree of crystallinity and fewer defects or charge impurity states. For example, Klee et al. further indicated the photoresponse was significantly faster with increased Se concentration in MoS2(1−x) Se2x samples (Fig. 19c) [143]. Gas sensing The possibility of using CVD monolayer MoS2 for gas sensing was first demonstrated by Lee et al. [144]. Despite possessing a Schottky barrier between the metal contacts and MoS2 , the fabricated MoS2 sensor showed sensitive and linear responses for NH3 for concentrations ranging from 2 to 30 ppm (Figs. 20a,b). The Fermi level of MoS2 was shifted to its conduction band upon exposure to NH3 and the conductivity increased, confirming n-type behavior of the MoS2 gas sensor. Significant response to NH3 was observed even when at sub-ppm levels of gas exposure, and the sen-

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sor response remains linearly proportional to the concentration of NH3 . Liu et al. further extended the target analytes of MoS2 -based gas sensors to NH3 and NO2 , two toxic gases with different electron affinities [145]. While both NH3 and NO2 triggered significant response of the MoS2 gas sensor, NH3 increased the conductance of the channel, whereas NO2 decreased the conductance (Fig. 20c,d). The conductance change of the channel was caused by both the charge transfer and Schottky barrier modulations, in which NO2 moves the Fermi level of MoS2 closer to its valence band, making the Schottky barrier width higher; NH3 results in an opposite change. NO2 also withdraws electrons from MoS2 and forms negatively charged species, increasing the Schottky barrier height. As a novel semiconductor analogue with graphene, the effectiveness of MoS2 as gas sensors were demonstrated in the works mentioned above. Although there is no report of doped-MoS2 for gas sensing, to the best of our knowledge, given the bandgap tuning ability of metal and chalcogen dopants in MoS2 as mentioned in the previous sections, it is expected that doped MoS2 will bring exciting variations in gas sensing applications and further work should be targeted along this direction.

Perspectives and future work In this review, we summarized the progress of doped graphene with elements from Group 13–16, and doped MoS2 with transition metal and chalcogen atoms. For graphene, doping mainly affects the Fermi level, bringing changes to its electronic and physical properties that leads to novel applications in gas sensing and GERS. For MoS2 , the metal dopants span from Group 5–9, while the chalcogen dopant is mainly Se thus far, but Te should be considered as a possibility. Similar to doped graphene, heteroatom doping of MoS2 tunes its bandgap and brings changes on its magnetic and physical properties. Despite the fast research progress in graphene and MoS2 , there is still much work to do regarding doping and co-doping of these and other 2D systems. Up until now, monolayer graphene is the most investigated 2D material, however sublattice control doping, resulting in the opening of a bandgap, remains a challenge. In this context, large area monolayer crystals need to be synthesized with dopants occupying one of the triangular graphene sublattices. In addition, doping few-layer graphene such as bi-layer graphene with controlled stacking has been rarely explored, and further work is warranted. Theoretical studies also suggested the possibility of bandgap opening in monolayer graphene by heavy doping, yet this has not been achieved experimentally. For doped MoS2 and other TMDCs, although varying the amount of dopant precursors enables tuning of the properties, the positions of the dopants within the doped material are still difficult to control; controlling ordered doping configurations and dopant locations is a new direction to follow. For example, a heterostructure consisting of one layer of Se atoms, one layer of Mo atoms, and one layer of S atoms, would be interesting to investigate, and the recent research on “Janus 2D materials” have revealed novel properties and great potentials of these heterostructures as sensors, actuators, and other electromechanical devices [146–148]. Currently, the dopants successfully incorporated into MoS2 by CVD or powder vaporization primarily cover a limited region of transitional metals. Lastly, with the development of CVD technologies and spectroscopic characterizations of TMDCs, it would be intriguing to expand the doping of semiconducting MoS2 into superconducting MoS2 , MoS, and MoS3 . Multi-atom doping (co-doping) constitutes another avenue to be explored and more theoretical and experimental work should be performed. Therefore, there are still many opportunities regarding the doping of 2D materials, where we foresee new methods and emergent phenomena appearing in the near future.

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