Source: https://pubs.rsc.org/en/content/articlehtml/2019/nh/c8nh00150b?page=search
Timestamp: 2019-04-25 12:33:13+00:00

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Two-dimensional (2D) layered metal chalcogenides (MXs) have significant potential for use in flexible transistors, optoelectronics, sensing and memory devices beyond the state-of-the-art technology. To pursue ultimate performance, precisely controlled doping engineering of 2D MXs is desired for tailoring their physical and chemical properties in functional devices. In this review, we highlight the recent progress in the doping engineering of 2D MXs, covering that enabled by substitution, exterior charge transfer, intercalation and the electrostatic doping mechanism. A variety of novel doping engineering examples leading to Janus structures, defect curing effects, zero-valent intercalation and deliberately devised floating gate modulation will be discussed together with their intriguing application prospects. The choice of doping strategies and sources for functionalizing MXs will be provided to facilitate ongoing research in this field toward multifunctional applications.
Peng Luo received his BS degree from Northeastern University (NEU), China, in 2016. He is currently a PhD candidate at the School of Materials Science and Engineering, Huazhong University of Science and Technology (HUST). His research concentrates on the synthesis of 2D transition metal chalcogenides by CVD methods and their doping engineering for high performance electronic and optoelectronic applications.
Fuwei Zhuge received his PhD degree from Shanghai Institute of Ceramics, Chinese Academic of Sciences in 2011. He then joined Osaka University and Kyushu University as a postdoctoral researcher. He is now an Associate Professor at the School of Materials Science and Engineering, Huazhong University of Science and Technology (HUST). His research interests have been focused on the controllable fabrication and application of various low-dimensional nanostructured materials in photodetection, energy storage and conversion.
Tianyou Zhai received his BS degree in Chemistry from Zhengzhou University in 2003 and then received his PhD degree in Physical Chemistry from the Institute of Chemistry, Chinese Academy of Sciences (ICCAS) under the supervision of Prof. Jiannian Yao in 2008. Afterwards, he joined in National Institute for Materials Science (NIMS) as a JSPS postdoctoral fellow of Prof. Yoshio Bando's group and then as an ICYS-MANA researcher within NIMS. Currently, he is a Chief Professor at the School of Materials Science and Engineering, Huazhong University of Science and Technology (HUST). His research interests include the controlled synthesis and exploration of the fundamental physical properties of inorganic functional nanomaterials, as well as their promising applications in energy science, electronics and optoelectronics.
The prevailing 2D MXs compounds are generally comprised of cationic elements in the transition metal group (Mo, W, Ta, Nb, etc.), group IIIA (Ga, In) and IVA (Sn, Pb), and anionic elements of chalcogenides (O, S, Se, Te), which are bonded in the chemical formula MXn with n = 2, for the most part, e.g. for MoS2, WS2, etc., and n = 1, e.g. InSe, SnS, SnSe, etc.24,26,27 Because of the rich electron configurations in the d-orbitals of both the metal elements and chalcogenides (Se, Te),28 diverse electrical properties have been found in MXs compounds, ranging from semiconductors (MoS2, WS2)29 to insulators (HfS2)30 and even metals (NbS2).31 In particular, their characteristics, including doping polarity,32,33 optical luminescence34,35 and catalytic activity,36–38 have been demonstrated to be widely tuneable by a variety of doping engineering methods. Novel phase transition phenomena in terms of both crystal structure and electrical properties were also found in MoTe2 and TaS2 under high concentration charge injection.39–41 The rich properties of 2D MXs thus offered a wealth of opportunities for both fundamental studies and technological applications of their functional devices.
In this review, we summarize the recent progress in the doping engineering of 2D MXs and highlight their essential functions in various kinds of devices. The principle of each doping strategy, based on substitutional doping, charge transfer doping, intercalation doping and electrostatic doping, will be presented, followed by an in-depth discussion of their latest progress toward functional engineering and the remaining challenges for ongoing research efforts.
Fig. 1 Doping strategies to 2D MXs and their potential function applications.
The feasibility in scaling and patterning the doping process is another important factor, especially for integrated devices that require selective area doping, like detectors, logic inverters, memories, and transistors.44 Electrostatic doping has been generally adopted in such devices, which relies on the local patterning of gate dielectrics and electrodes by using lithography processes.43 However, keeping the field effects in devices draws continuous energy consumption in operation. Recently, substantial improvements have been made by using a floating gate and ferroelectric dielectrics in devices,67 which introduces retentivity to electrostatic doping. On the other hand, surface charge transfer doping of 2D MXs could be also feasibly patterned using the existing complementary metal oxide semiconductor (CMOS) technology,51 displaying bright potential in the assembly of functional devices.
There are many versatile possibilities of the various doping strategies and their functions. In the following sections, we generally classify the doping strategies into substitutional doping, charge transfer doping, intercalation doping and electrostatic doping. In each part, an in-depth survey will be provided regarding the latest progress in the doping engineering of 2D MXs. The advances in optimizing their performances in various functional devices will be also highlighted.
In MXs, both cationic and anionic elements can be substitutionally replaced by foreign atoms with comparable radii.68,69 Depending on the number of valence electrons in the dopant atom, the MXs can be doped into n-type70 or p-type conductance,32 and even with new optical emission.71,72 In the following, the theoretical aspects of substitutional doping will be firstly introduced by taking MoS2 as the example, and then the recent advances in doping metal-chalcogenides at cation and anion sites will be discussed.
Fig. 2 Substitutional doping of MoS2: (a) the crystal structure of MoS2 with Mo sandwiched between the S outer layers, (b) the electron density mapping of MoS2 with one Mo substituted by an Nb atom, reprinted with permission from ref. 79, Copyright 2008 American Physical Society. The electronic DOS of (c) Re, Ru, Rh and (d) Nb, Zr, Y substitutionally doped MoS2, reprinted with permission from ref. 69, Copyright 2013 American Physical Society.
To determine the influence of substitutional doping, theoretical calculations based on first principles and density functional theory (DFT) are particularly instructive as the reference. The formation energy of substitutional doping and the resulting band structure, electronic density of states (DOS), and magnetic moments can be utilized to predict the thermal stability of the doping and the final electronic68,69,73 or magnetic properties.74,75 It has been shown that in MoS2, the formation energy of the S vacancy is considerably more negative than Mo vacancy,76,77 thus contributing to the usual n-type behaviour in the as-fabricated samples.78 Notably, the presence of native Schottky defects in MXs facilitates the formation of substitutional dopants at the vacancy position. As an example, the study on metal-sulfides showed that the doping of halogens (F, Cl, Br) and group V elements (N, P, As) at anion sites is more thermodynamically favourable in the presence of sulphur vacancies.69 At the metal sites, the formation of some dopants, such as Re in MoS2, also depends highly on the concentration of the metal vacancy. Doping of these elements into MXs compounds is, therefore, more favourably achieved in experiments using in situ methods during growth or post-treatments that create vacancy defects.
In addition to the modulation of electronic properties, the 2D MXs can also be doped by using magnetic elements such as Mn, Fe, Co.74,75 The ultrathin MXs can be engineered into dilute magnetic semiconductors that could find application in spintronic devices with feasible spin injection.82–84 However, the experimental realization of high-performance magnetic doping in MXs encounters the challenge of maintaining the electronic performances,45,85 because the incorporation of most magnetic dopant atoms still relies on vacancy defects in the lattice of MXs,83 which tend to limit the electronic performance.
In the past, the use of calculation power has greatly accelerated the screening of effective doping elements and designing the functionality of MXs. It should be however noted that the realistic material performance in experimental studies could be dramatically modulated by the co-existence of unguarded defect formation, which can, unfortunately, be required for the efficient incorporation of substitutional dopants. In Table 1, we summarize the electrical performance of several substitution-doped MXs at both the cation and anion sites, using the carrier concentration and mobility as the criteria.32,33,45,80,86–91 The effects of substitutional doping were seen as consistent with theoretical predictions, whereas their performances, e.g. the carrier mobility in devices, were greatly scattered for various reasons.
Using exfoliation methods, the ultrathin MXs with dopant modulation can be obtained from the doped bulk materials. Suh et al. first synthesized Nb-doped MoS2 bulk crystals using Mo, Nb, S and the source material.32 The nature of Nb substitutional doping at Mo sites (schematically illustrated in Fig. 3a) was confirmed by the study of the chemical bond environment by extended X-ray absorption fine structure (EXAFS) analysis. When exfoliated into few layers, the 0.5% Nb-doped MoS2 displayed strong p-type electrical conductance, as had been predicted.69 When assembled with non-doped MoS2, a vertical p–n junction with well-defined rectification characteristics was formed under positive gate modulation (Fig. 3b), which tuned the non-doped MoS2 into the n-type via the electrostatic effect. Notably, such degenerate Nb doping of >0.1% may cause structural transformation of MoS2 from the usual 2H (ABABAB) to 3R mode (ABCABC) stacking (Fig. 3c),49 which is asymmetric and inherently supports the SHG as a nonlinear optic material.98 It was also found to exhibit better catalytic activity in HER.99 Besides, after degenerate Nb doping, the PL properties of single-layer MoS2 were drastically shifted within 1.3 to 1.9 eV as displayed in Fig. 3d, due to the strong d-orbital coupling between the foreign dopants and the host Mo atoms.
Fig. 3 Substitutional doping at cation sites: (a) schematic illustration of substitutional Nb doping in MoS2 in partial replacement of Mo atoms, (b) the resulting rectification behaviour in a gate modulated junction formed between Nb-doped and undoped MoS2, reprinted with permission from ref. 32, Copyright 2014 American Chemical Society. (c) High resolution TEM image of bilayer MoS2 in the 2H and 3R phases induced by Nb doping and (d) their PL characteristics; reprinted with permission from ref. 49, Copyright 2018 Nature Publishing group. (e) A typical CVD configuration for in situ doping during growth using multiple source evaporation and the TEM image of the resulting Re doped MoS2, reprinted with permission from ref. 80, Copyright 2018, Wiley-VCH. (f) Illustration of hydrothermal procedures for achieving Mn doped MoS2 in supercritical conditions, reprinted with permission from ref. 108, Copyright 2017, Wiley-VCH.
In addition to vapour phase methods, cation substitution doping can be also achieved in solution methods. However, because of the formation energy issue, the incorporation of cation substitutional dopants other than Nb is still generally facilitated by creating vacancies in the host lattice. This fabrication can be achieved by choosing metal deficient or chalcogenide rich conditions. Recently, Tan et al. successfully doped magnetic Mn into MoS2 by using the supercritical hydrothermal method (Fig. 3f).108 Under high temperature and pressure, the concentration of Mn in MoS2 reached 3% while having the Mn2+ valence for room temperature ferromagnetism.
Though cation substitutional doping enriches the functionality of MXs, a critical challenge lies in the preferential patterning of the dopant for junction type electronic devices, since all the present methods ubiquitously rely on transfer methods that are unlikely to be scalable for integrated designs in the future. Developing substitutional doping methods that are temperature benign is however hindered by the high formation energy at cation sites, which deserves further attention.
Fig. 4 Substitutional doping at anion sites: (a) the PL of composition-tuned MoS2xSe2(1−x) with uniform distribution and (b) with composition gradient in a single flake, reprinted with permission from ref. 111 and 118, Copyright 2014, 2015 American Chemical Society. (c) Laser assisted selective area doping by PH3, reprinted with permission from ref. 109, Copyright 2016 American Chemical Society. (d) Schematic of N2 plasma doping of WS2, reprinted with permission from ref. 87, Copyright 2018 American Chemical Society. (e and f) 2D Janus structure of SMoSe from gentle sulfidation and the obtained HER performance, reprinted with permission from ref. 57, Copyright 2018 American Chemical Society. (g) H2 plasma-assisted fabrication of Janus type SeMoS, (h) the HRTEM image, (i) SHG, (j) piezoelectric properties, reprinted with permission from ref. 55, Copyright 2017 Nature Publishing group.
In view of the challenge in patterned doping, Kim et al. developed a laser ablation assisted doping process.109 As displayed in Fig. 4c, the laser spot was focused to heat up the lattice (MoSe2) to generate anion vacancies and in the meantime cause the dissociation of PH3, which acted as the dopant source. The substitutional doping by the supervalent P occupying anion sites compensated for the intrinsic n-type doping in MoS2 while it enhanced the p-type conductance of WSe2. It is noteworthy that the doped 2D MXs displayed excellent ambient stability for over 30 days, having potential in selective area doping and contact engineering.109 An alternative solution is based on the plasma technique, which could be operated under mild temperature conditions and is compatible with the existing CMOS techniques. Various gas molecules including N2,117 O2,123 SF6,110 could be used to generate highly energetic ions using radio frequency methods. The energetic ions bombarding the surface of the 2D material are prone to create vacancies that facilitate dopant incorporation, while in some cases, they can be directly implanted into the lattice of MXs.
Due to the broken symmetry, the Janus type MXs exhibited inherently dipolar characteristics and other distinct properties compared to the randomly formed alloy, including piezoelectric and SHG.55,56 Lu et al. fabricated the SeMoS Janus structure by first stripping off the surface S atoms from MoS2 using remote H2 plasma treatment, followed by a thermal selenization as indicated in Fig. 4g.55 Precisely controlled plasma power was used to preserve the underlying Mo–S structure. The vertical asymmetric Se–Mo–S (Fig. 4h) was demonstrated to be active for optical dipole transition, leading to the out of plane SHG. In the experiment, such out of plane SHG activity could be identified from an angular dependent measurement, as displayed in Fig. 4i. The SHG intensity was scaled with the projected vertical electric field of incident optical excitation.55 The Janus structure clearly displayed the piezoelectric response in experiments,55 which in principle could be engineered by increasing the dipole contrast between the upper and lower chemical bonds.56,124 The precisely controlled substitutional doping at anion sites may, therefore, have the potential for creating new materials with novel functional properties.
Charge transfer doping has attracted much attention in modulating the electronic behaviour of semiconductors.51 In contrast to substitutional doping with foreign dopant atoms incorporated into the lattice, the charge transfer doping takes advantage of the charge transfer interaction between the host material and any adjacent mediums, including surface adatoms,125 ions,126 molecules,127,128 particles129–131 and supporting substrates.132–135 Since the dopants lie out of the transport pathways of electrical carriers, such modulated doping avoids lattice distortion and enables high mobility transport in low dimensional materials.51 Significant research efforts have therefore been devoted to exploring ideal dopants that offer efficient charge transfer doping and superior functional performances.
Fig. 5 (a) Acceptor or donor behaviour of charge transfer doping of the semiconductor according to the Fermi level difference. (b) The summary of n-type and p-type dopants of typical MXs.
Theoretical methods based on first principles and density functional theory can instead be used to predict the surface charge transfer to semiconductors and the resulting surface electronic states. The formation energy of surface adsorbents can verify the spontaneity of the adsorption process and predict the most thermally stable surface arrangement; the resulting electronic DOS then depicts the final electrical properties. The charge transfer direction between the host and surface adsorbents is directly visualized by the charge density distribution and their difference before and after the adsorption.138 Such theoretical methods are particularly viable for complicated molecules.139 However, for semiconductor organic molecules or others, the situation is much more complicated since the work function of the adsorbent and the resulting charge transfer behaviour are also influenced by the native doping and interface conditions.
For 2D materials, the ultrathin thickness and the exposed basal plane make their electrical properties especially sensitive through such charge transfer doping.51 The representative charge transfer dopants of several typical MXs, namely MoS2,126,131,134,138,140–142 WSe2,128,142–148 MoTe2,149,150 and InSe,151,152 are summarized in Table 2. In the following, we discuss their realization and performances using the classification of dopant sources as gaseous molecules, metals and metal compounds, organic small and super-molecules.
Fig. 6 Gas molecules bound to 2D MXs: (a) the modulation of PL properties of MoS2 by exposure to various gas atmospheres; (b) the change in exciton and non-radiation recombination in p-type WSe2 and n-type MoS2, MoSe2, reprinted with permission from ref. 138, Copyright 2013 American Chemical Society. (c) Preferential NO2 binding to WSe2 at vacancy sites, forming WSe2−x−yOxNy species on the surface, and greatly reduced the contact resistance in transistors (d), reprinted with permission from ref. 155, Copyright 2014 American Chemical Society. (e) The engineering of gas molecules adsorption by electrothermal (left) and gate (right) modulation, reprinted with permission from ref. 127, Copyright 2018 Wiley-CH.
Beyond the lithography patterning, the gas phase adsorption can be controlled by using electrostatic fields from gate bias or electrothermal effects for advanced manipulation in devices. For example, Feng et al. found that under gate bias stress, O2 and H2O were attracted to the surface of InSe at Vg > 0, but were repelled when Vg < 0.151 The process was controlled not only by the polarity of the applied gate bias but was also related to the bias voltage and duration. By combining the gate effect and electrothermal effect, which desorbed surface adsorbents by Joule heating the 2D MoTe2 channel (Fig. 6e),127 Chang et al. recently demonstrated a facile construction of p/n-type doping in MoTe2, and also inverters, NOR and NAND logic devices. These results revealed the potential for adapting surface gas molecules adsorption induced doping in electronic devices.
By decorating metal144,156,157 or metal compounds, typically their ion complexes152 and oxides147,158,159 at the surface, 2D MXs can be intentionally doped. The decoration can be routinely made on 2D MXs by using evaporation and solution cast methods that are compatible with existing patterning techniques. As the essential criteria, it has been demonstrated that the work function of dopant sources (Fig. 5b) can predict well the resulting electrical doping behaviour, providing the guidelines for choosing dopants.
Since potassium has a low ionization energy and work function (2.3 eV), it was previously demonstrated to be very efficient for n-type doping in carbon nanotubes, graphene and recently in 2D MXs.51 By using a Joule heated K dispenser, Fang et al. investigated K doped MoS2 and WSe2. The amount of dopant was carefully controlled by the exposure time to K vapour and electrical characteristics of the device were observed in situ.144 The resulting electron concentration in MoS2 and WSe2 reached 1 × 1013 cm−2 and 2.5 × 1012 cm−2, respectively. Combined with a top-gate underlap structure (Fig. 7a), a selective area doped n+/i/n+ structure was demonstrated with remarkably suppressed electrical contact resistance in transistors, leading to improvement of the ON current by 4–5 orders (Fig. 7b). The extracted electron mobility in degenerately doped WSe2 exceeded 100 cm2 V−1 s−1. Later, a CMOS inverter was demonstrated in a single flake of WSe2 by combined contact electrode engineering (Pt for hole transport, Au for electrons) and K vapour doping.160 Other low work function dopants, such as amorphous TiO2 (ATO),140 Cs2CO3,148,161 and 2D electrides like Ca2N,149 have also been explored for n-type charge transfer doping of 2D MXs. When the single crystalline [Ca2N]+·e− electride was brought in close contact with MoTe2, ultrahigh degenerate (1.6 × 1014 cm−2) and long-range electron doping, penetrating 100 nm thickness, were recently demonstrated, leading even to a partial lattice symmetry change from 2H to 1T′. However, in spite of the strong electron doping effect, the poor air stability of K, Ca2N and similar low work function dopants in ambient conditions remains to be solved for integrated device fabrication.
Fig. 7 Charge transfer doping of MXs by metal and metal compounds: (a) K doping of WSe2 in a top gate transistor and (b) the resulting transfer curve, reprinted with permission from ref. 144, Copyright 2013 American Chemical Society. (c) Lateral PN junction enabled by AuCl3 doping of MoS2, reprinted with permission from ref. 129, Copyright 2014 American Chemical Society. (d) Schematic of Ti4+ Lewis acid bonding to the surface of InSe, (e) the transfer curve before and after doping, and (f) the schematic of a Lewis acid bridged connection to N719 dye molecules, reprinted with permission from ref. 152, Copyright 2017 Nature Publishing group.
Due to the large positive reduction potential (1.4 V), AuCl3 is one of the most commonly used p-type dopants for 2D MXs.129–131,162 Though it was initially used to explore graphene,163 it has been widely adopted for tuning the doping polarity of n-type MXs. Liu et al. demonstrated the p-type doping of 2D MoS2 by spin-coating AuCl3 solution on it.131 The AuCl3 was later transformed into Au particles by accepting electrons from adjacent MXs. When other reductants such as NH2OH were included in the solution, the doping behaviour disappeared,156 demonstrating the critical role of surface reactions in the resulting doping behaviour. By using a partially pre-stacked h-BN as the mask (Fig. 7c), the group was able to fabricate a lateral MoS2 p–n junction with an ideal factor close to 1.129 Later, they further introduced a graphene buffer layer to the contact as the hole injection layer, then greatly lowered the contact resistance to 2.3 kΩ μm in the p-type transistor; the obtained hole mobility in MoS2 reached 72 cm2 V−1 s−1 at room temperature. From there on, a CMOS inverter was successfully fabricated based on the pristine n-type MoS2 and the degenerately p-doped MoS2.131 Recently, a vertical homogeneous MoS2 p–n junction was also reported based on diluted AuCl3 coating. The device upon illumination displayed ultrasensitive response to sub fW incident light power, reaching not only a high responsivity of 7 × 104 A W−1 and superior detectivity of 3.5 × 1014 Jones, but also the fast response time on the scale of tens of ms.130 The intriguing performance demonstrated the potential for controllable surface chemical doping in the versatile engineering of device performances.
In addition to elemental metal nanoparticles and ions, metal compounds including oxides have also been explored as dopants.140,146,147,158,159,164 Because of the large band gaps of these metal compounds, their work functions are often close to the valence band of MXs and therefore cause p-type doping behaviour (Fig. 5b). In 2013, Chen et al. observed a dramatic electron depletion in MoS2 FET after the in situ evaporation of a thin MoO3 layer (∼0.1 nm) with high work function (Φ = 6.8 eV).165 The MoO3 with high work function also changed the ambipolar WSe2 into pure p-type characteristics with the hole mobility enhanced by almost two orders of magnitude (1.38–98.66 cm2 V−1 s−1).159 The high work function of MoO3 made it ideal as a hole injection layer, achieving low contact resistance in p-type transistors. To this end, McDonnell et al. investigated the detailed interfacial band alignment between MoS2 (WSe2) and MoOx (x < 3) from electron beam evaporation, and pointed out that the MoOx purity (by carbon contaminates) and stoichiometry are essential for efficient Ohmic hole injection.158 Recently, a rapid flame synthesis of MoO3 was demonstrated to directly prepare atomically thin 2D MoO3 on WSe2, which led to a record low sheet resistance (6.5 kΩ square−1) and contact resistance (0.8 kΩ μm).147 Encouragingly, the prepared p-type doping by 2D MoO3 displayed excellent ambient stability that lasted more than 20 days, implying the bright potential for device integration.
In Fig. 5b, the work functions of typical metals and metal compounds, compared to the conduction bands and valence bands of several representative MXs, are summarized. Importantly, the facile preparation of metal nanoparticles and their compounds using evaporation or thermal oxidation methods is highly compatible with existing CMOS technology, making them particularly attractive for functionalizing 2D MXs in devices.
Fig. 8 Charge transfer doping of 2D MXs by organic small molecules: (a) SAM to modulate the interfacial Schottky barrier for electrical contact to 2D TMDs, reprinted with permission from ref. 142, Copyright 2015 Wiley-VCH. (b) Reductive n-type doping of MoS2 by BV molecules and (c) the related redox processes, reprinted with permission from ref. 126, Copyright 2014 American Chemical Society. (d) Vertical and (e) lateral PN junctions enabled by combined BV induced doping and other doping methods, reprinted with permission from ref. 162, Copyright 2015 Nature publishing group, and ref. 150 Copyright 2017 Wiley-VCH.
Instead of being drafted on the surface, the SAMs can be firstly prepared on the substrate, and then 2D MXs can be transferred on top of them.132,134 This not only brings the bottom layer of 2D MXs in contact with the functional groups of SAMs but also screens the substrate-induced charge transfer doping. Using patterned SAMs on substrates, the selective modification of MoS2 in terms of the electrical and PL performances has been demonstrated.132 Among the various functional groups in SAMs, –NH2, –SH are prone to donate electrons to 2D MXs, while –CF3 are likely to induce hole-doping.134,142,170 To name a few examples, 3-(trimethoxysilyl)-1-propanamine (APTMS) terminated with –NH2 showed electron-donating characteristics, while trichloro-(1H,1H,2H,2H-perfluorooctyl)silane (FOTS) with –CF3 displayed a large p-type doping effect to MoS2 due to the large electronegativity of the F element.134 A similar effect was also observed for thiol bonded SAMs.62 In the case of the abovementioned interfacial SAM layer between the contact electrode and TMDs (Fig. 8a), the chosen SAMs, octadecyltrichlorosilane (OTS) terminated with –CH3 and 3-aminopropyltriethoxysilane (APTES) terminated with –NH2, displayed the opposite non-degenerate doping of MoS2 and WSe2. The selective engineering of electron and hole transport across the interface enabled remarkably enhanced mobilities of 142 and 168.9 cm2 V−1 s−1 for MoS2 and WSe2, respectively,142 because of the lowering of the Schottky barrier height (SBH) at the prepared Pt–WSe2 and Ti–MoS2 contact.
Though SAMs exhibit chemical stability in ambient environments, they only cause slight non-degenerate doping of the adjacent MXs in experiments. Reductive and oxidative organic molecules can be used as degenerate charge transfer dopants,35,126,150,166,167,175–177 taking advantage of their electrochemical redox potential in solution as the criteria. Starting with the highly reductive benzyl viologen (BV), illustrated in Fig. 8b, for the replacement of hazardous hydrazine,175 changing the majority carriers in MXs and building p–n junctions by selective modification were previously demonstrated in MoS2126 and MoTe2150 with air stability. According to the electrochemical potential (Fig. 8c), each BV molecule may donate 2 electrons to MoS2via the successive BV0/BV+ and BV+/BV2+ transition.126 The degenerate doping (ne > 1013 cm−2) with BV molecule treatment near the contact area allows the significant reduction of contact resistance to 1.1 kΩ μm in the top gated MoS2 transistor. Within this regime, Tarasov et al. were able to realize large-scale work function change (±1 eV) in MoS2 using benzimidazoline radicals as the n-type dopants, and “magic blue” as the p-type dopant.178 The doping by BV molecules was also combined with other doping methods to fabricate both vertical162 and lateral homojunctions150 as indicated in Fig. 8d and e.
The rich choice of organic molecules is undoubtedly advantageous for the controllable doping engineering of 2D MXs from non-degenerate to degenerate levels. However, at the present stage, a critical issue is that n-type dopants are generally non-stable in ambient conditions due to the negative redox potential versus the air exposure. Nevertheless, one should be optimistic, given the versatile potential by incorporating different functional groups into organic molecules. In addition to electronic modification, the accompanied tailoring of the optical, HER, and magnetic properties remain to be explored for enriching the function of 2D MXs.
Due to the small radii of H and alkali metal ions, they were most easily intercalated in layer structured materials.196 Their reductive nature allows them to donate electrons to the lattice of 2D MXs, thereby causing lattice distortion and electronic structure change. Taking advantage of the intercalation-induced interlayer expansion (Fig. 9a),193 the ion intercalation has been widely used for the liquid phase exfoliation of 2D materials;19 using electrochemical cells and similar setups, ion intercalation and de-intercalation in 2D MXs can be easily modulated by an external electric field.198 This renders the reversible engineering of material properties and revolutionizes its prospects in functional devices.
Fig. 9 Intercalation modulation of MXs: (a) intercalation of MoS2 by alkali ions (Li+, Na+, K+) with solvent molecules, and by TEA+, reprinted with permission from ref. 193, Copyright 2015 Nature Publishing group. (b) Sonication assisted exfoliation of MoS2 in the 1T phase and IR irradiation assisted restoration; (c) the resulting reflectance spectral change, reprinted with permission from ref. 200, Copyright 2015 American Chemical Society. (d) The electrochemical setup and (e) the illustration of Li intercalation in TaS2; (f) patterned control of Li+ intercalation into devices and the resulting gate modulation characteristics, reprinted with permission from ref. 40, Copyright 2015 Nature Publishing group.
To control the extent of intercalation, Zhang et al. applied an electrochemical setup with high-yield exfoliation efficiency for transition metal dichalcogenides (MoS2, WS2, TiS2, TaS2 and ZrS2).191 The method has been also applied to NbSe2.203 Using Li metal foil as the ion source and layered MXs as the cathode, the extent of intercalation was monitored in the electrochemical cell and well-controlled during the discharge process. The electrochemical cell setup for ion intercalation was later reassembled on microscale devices using side gate structures, with ion containing electrolytes as the intercalation source, as illustrated in Fig. 9d.40 The electrical performances of host 2D materials were monitored in situ and controlled during the gate bias driven intercalation.40,195,203 In Li intercalated 1T-TaS2 (Fig. 9e), Yu et al. observed the alteration of charge-ordered states and 5 orders of magnitude change in resistance with multiple phase transitions from Mott-insulator to metal, and further to superconductor at low temperature.40 By a patterned PMMA blocking layer, the selective area intercalation and modulation to material performances was demonstrated, as indicated in Fig. 9f, thereby opening up possibilities for gate-controlled intercalation in devices.
In 2012, Koski et al. successfully realized the high concentration zerovalent intercalation of Cu into the layered Bi2Se3, a well-known topological insulator with large interlayer space (9.53 Å), using a disproportionate redox reaction of monovalent copper (2Cu+(aq) ↔ Cu2+(aq) + Cu0) that leaves zerovalent Cu intercalated in the interlayer space, as indicated in Fig. 10a.206 The intercalated concentration of Cu in Bi2Se3 nanoribbons reached 60% of the host lattice atoms, leading to the stoichiometry of Cu7.5Bi2Se3, which is considerably higher than the usual cases of ion intercalation by electrochemical methods. It was later demonstrated that the method could be extended to other host materials210 including Bi2Te3, In2Se3, GaSe, Sb2Te3, MoO3, and using other guest atoms like Ag, Au, Co, Fe, In, Ni, Pd and Sn.188 The features of zerovalent intercalation and superstoichiometric intercalation are clearly evidenced from the composition analysis displayed in Fig. 10c.188 In particular, substantial electron doping from the superstoichiometric intercalation resulted in a peculiar enhancement of the optical transmission of the host lattice by causing the huge Burstein–Moss shift in the bandgap, at a much larger scale than degenerate doping.211 Recently, a vapour phase intercalation of Cu in Bi2Se3 and Bi2Te3 was demonstrated using Cu nanocrystals as the source, where high concentration Cu (>35%) incorporation was found to lead to the cation exchange of Cu2−xSe.212 This is likely a result of the corrupted thermal stability of the MXs lattice at a higher temperature (250–450 °C).
Fig. 10 Zerovalent intercalation of MXs: (a) Cu intercalation in Bi2Se3, (b) 2D TMDs available for zerovalent intercalation, reprinted with permission from ref. 188 and 210 Copyright 2012, 2014 American Chemical Society. (c) Bi:Se composition under superstoichiometric intercalation of In, Sn, Co, Fe, Ag, Cu, Ni elements, reprinted with permission from ref. 188 Copyright 2012 American Chemical Society.
To control the intercalation concentration of atomic species, Zhang et al. further devised an electrochemical setup using Bi2Se3 as the working electrode (WE) and Cu foil as the counter electrodes (CE).198 The copper intercalation was found to occur at a well-defined potential plateau of ∼17 mV over CE, thereby enabling the accurate control of Cu incorporation of up to 57% into the host lattice through a coulometric strategy. Another meaningful advance to the zerovalent intercalation was made through the stepwise combination of a disproportionate redox reaction, hydrazine reduction or carbonyl decomposition, by which Koski et al. were able to introduce multiple zerovalent species (Co/Ni, Cu/Ni, Fe/Ni, etc.) into the Bi2Se3 nanoribbons.207 The prepared materials exhibited a variety of superlattice diffraction patterns from the ordered structures of intercalated species and charge density wave characteristics. The versatile atom species and their combination in such multiple intercalations will undoubtedly create enormous opportunities for the creation of novel materials with intriguing optical and electrical properties.
Fig. 11 Zerovalent interaction of MXs: (a) the zerovalent intercalation of SnS2 by Cu and Co, enabling facile p, n, and metallic modulation. (b) The illustration of mask protected intercalation for fabricating lateral junctions, reprinted with permission from ref. 187, Copyright 2018 Nature Publishing group. (c) The lithium intercalation assisted zerovalent intercalation of noble metals into MoS2, reprinted with permission from ref. 36, Copyright 2017 Nature Publishing group.
In addition to the above-mentioned ions and elemental atoms with small sizes, larger molecules could also be used as the intercalants. Though they were initially explored for liquid phase exfoliation purposes, there has been increasing attention to the modulation of application performances of MXs.214,215 For example, Wan et al. prepared a multiple molecule intercalated TiS2/[(hexylammonium)x(H2O)y(DMSO)z] through successive electrochemical intercalation and ion exchange processes,216 reaching a promising thermoelectric figure of merit, ZT, of 0.28 at 373 K. With the enormous choices of organic molecules and their functional groups, the organic molecules as intercalants will for sure promote the discovery of new intercalation systems and engineering of their functions.
According to the driving gate biases, the floating gate configurations are further classified into three-terminal16 and two-terminal227,228 structures with or without a separate control gate. Fig. 12a illustrates a typical three-terminal structure where the floating gate is inserted in a MoSe2 field-effect transistor.238 To fabricate the floating gate device, graphene, h-BN and MoSe2 were successively transferred above the SiO2/Si substrate, in which h-BN was taken as the tunnelling barrier layer while graphene acted as the floating gate. The charge trapping and modulation of channel conductance could be identified from the measured transfer curves, as indicated in Fig. 12b. The tunnelling of free charges into the floating graphene layer caused dramatic modulation of the threshold voltage of the MoSe2 layer, which resulted in the appearance of two electrical conductance states, namely the ON and OFF states. Due to the persistent characteristics of the trapped charges, the ON and OFF states were non-volatile and they can be used in memory devices.224,227,238 Under opposite gate biases, the electrical conductance in the semiconductor channel can be written (programmed) or erased.
Fig. 12 Floating gate modulation of MXs: (a) the typical floating gate structure is integrated into a transistor; (b) the resulting hysteresis under control gate modulation due to (c) charge trapping of the floating gate under control gate bias, reprinted with permission from ref. 238, Copyright 2017 American Physical Society. (d) Semi-floating gate configuration and (e) the resulting rectification characteristics in the WSe2 transistor, reprinted with permission from ref. 243, Copyright 2017 Nature Publishing group. (f) A semi-floating gate integrated in parallel with a vertical PN junction, reprinted with permission from ref. 226, Copyright 2018 Nature Publishing group. (g)–(i) The typical two terminal floating gate configuration and its current–voltage and memory characteristics, reprinted with permission from ref. 228, Copyright 2016 Nature Publishing group.
The operating principle of the floating gate device is explained in Fig. 12c from the band diagrams.238 Under the positive control gate bias, the majority of electron carriers in MoSe2 are forced to tunnel into the intermediate graphene layer through Fowler–Nordheim (FN) tunnelling, in which the tunnelling barrier is lowered by the strong external electric field.227 When electrons are trapped in the floating gate, the semiconductor channel is p-doped with the same amount of holes due to the charge compensation rule. Conversely, the trapped charges can be repelled into the semiconductor channel under negative gate biases. Thus, the doping concentration and polarity in semiconductor MXs can be feasibly tuned by choosing the magnitude and duration of the gate bias.16 Because the insulator barrier surrounds the floating gate layer, the trapped charges can persist for non-volatile doping for years, while the proper bandgap (1–2 eV) of semiconductor MXs (MoS2, MoSe2, WSe2, etc.) enables large on–off ratios (103–105) in memory devices.16 For a higher doping level or memory ratio, the thickness of the charge trapping layer or tunnelling barriers could be reduced to allow more charge trapping,16 while a too thin dielectric layer would cause leaking current and poor retention characteristics.239 In addition to the conventional high-k dielectrics and 2D h-BN, polymer and SAM insulators such as PVP,240 pV3D3229 and alkyl chain SAMs241 have also been explored as the tunnelling insulators in floating devices. This could potentially relieve the surface stability issue of MXs during the high-temperature processing of high-k oxides and the scalable issue when using the mechanically exfoliated and transferred h-BN.
Notably, the floating gate induced electrical doping and its persistent characteristics have been applied to other functional devices beyond memories. By applying the floating gate structure to phototransistors, Hu et al. were able to remarkably suppress the dark current in MoS2 and WS2 channels, thereby enhancing the photodetectivity to 3.5 × 1011 Jones while maintaining a high responsivity over 103 A W−1.242 However, the persistent charge trapping characteristics in floating gate configuration generally resulted in the persistent photoconductance (PPC) effect in photodetection that was usually refreshed using reversed gate pulses.
Similar to other doping methods, the floating gate structure could also be used for selective region doping, e.g. by using a partially stacked or underlapped floating gate. A typical structure is illustrated in Fig. 12d, by which Zhang et al. realized a semi-floating gate-controlled transistor with rectification characteristics.243 Due to the charge trapping in the underlap floating graphene layer, the doping concentration and polarity in the WSe2 channel were partially modulated beneath the drain electrode. With the inherent p-type conduction in WSe2, a PN junction was realized by applying a negative gate pulse, which induced positive charge trapping into the floating gate and n-type conductance in the above WSe2 channel. The resulting homojunction displayed apparent rectification (Fig. 12e), photovoltaic conversion and long retention characteristics (>103 s). Similarly, non-volatile PNP junctions were also realized with the middle part of the semiconductor channel coupled with the floating gate.244 Further, an ingenious parallel integration of the semi-floating gate (MoS2/h-BN/HfS2) with a vertical WSe2/MoS2 pn junction (Fig. 12f), which exhibited fast charge injection yet low reverse current leakage, was recently reported to enable both tuneable charge persistency and ultrafast operation speed on a nanosecond scale.226 It is apparent that the feasible but versatile integration configurations of floating gates in devices have greatly promoted the brilliant development of multifunctional devices based on MXs.
Recently, two-terminal floating gate devices were also invented to simplify the charge trapping operation through the control gate and the reliance on gate blocking insulators. As shown in Fig. 12g, the devices exhibited only source and drain terminals with a floating gate beneath them.228 By reducing the thickness of the tunnelling insulator (<10 nm), the applied source-drain potential bias was sufficient to induce charge injection to the floating layer. The charge trapping caused a large hysteresis and well-defined ON/OFF states in a simple cycle of current–voltage sweep (Fig. 12h). This characteristic enabled a two-terminal operation of memory devices, in which the write, read, erase, read were respectively achieved by separate source–drain biases denoted as (i), (ii), (iii), (iv) in Fig. 12h. Such two terminal devices not only exhibited stable operation voltages but could also maintain high on/off ratios (>104) (Fig. 12i), long retentivity (>104 s), stable endurance (>105 cycles) and multilevel capacity. The absence of a rigid dielectric blocking layer made it stretchable up to a strain of 19% without deterioration of electrical performances. When combined with a large area MoS2 film and ALD grown dielectrics, a flexible memory array and 6-levels of discriminate operation were later achieved,227 implying the great potential of such two-terminal floating gate devices in wearable electronics and neuromorphic memories.
Lead zirconium titanate (Pb(Zr,Ti)O3, PZT) has been the most studied ferroelectric material with high remnant polarization (20–80 μC cm−2).248 The value is orders of magnitude larger than the charge density induced by capacitance coupling in field-effect transistors, which is typically 0.345 μC cm−2 for 100 nm SiO2 under a gate bias of 10 V. The large remnant polarization enabled the dramatic modulation of the doping concentration and polarity of 2D materials by simple ferroelectric polarization.222,231,232,246,250–252 Due to the limitation in preparing high quality PZT films, i.e. high temperature, the integration of 2D MXs was generally realized by transferring and placing them on top of PZT substrates.222,231,248,251 A typical structure is illustrated in Fig. 13a, in which the semiconductor channel worked together with the back gate as the two poling electrodes.248 The polarization loop measured at varied poling voltages indicated tailorable remnant polarization in the ferroelectric layer under appreciably low operation voltages below 4 V (Fig. 13b). By using such bottom gated ferroelectric coupling, Lipatov et al. successfully reversed the doping polarity of MoS2 to p-type.222 Their device structure and ferroelectric polarization effects are indicated in Fig. 13c and d. The persistent ferroelectric modulation of MoS2 was implied by the electrical conductance change of MoS2 5 min after resetting the gate bias (Vg) to 0 V (Fig. 13d). It was found that the ferroelectric polarization could be depolarized by shedding light on the MoS2 area, thus allowing feasible writing and erasing operations in memories using the combined optical and electrical methods indicated in Fig. 13e. It should be noted that the retention characteristics were found to be related to the surface roughness of the ferroelectrics,248 and also the ambient environments,231 where both interfacial contaminants and humidity were prone to screen out the polarization.
Fig. 13 Ferroelectric doping of MXs with PZT at the bottom: (a) the typical configuration of ferroelectric transistor with 2D MoSe2 placed on top of ferroelectrics, and (b) the typical polarization characteristic of PZT ferroelectrics, reprinted with permission from ref. 248, Copyright 2017 American Physical Society. (c) MoS2 ferroelectric transistor and (d) the polarization induced conductance change in MoS2, (e) the potential write and erase procedures that combines optical and electrical operation, reprinted with permission from ref. 222, Copyright 2015 American Chemical Society.
Fig. 14 Ferroelectric doping of MXs using the top gate configuration: (a) the typical configuration of the MoSe2 based ferroelectric transistor with Al as the top gate; (b) the measured hysteresis and (c) retention characteristics for memory, reprinted with permission from ref. 252, Copyright 2017 IOP Publishing group. (d) Ferroelectric phototransistor and (e) its photodetection performance under different polarization states, reprinted with permission from ref. 223, Copyright 2015 Wiley-VCH. (f) Schematic of the AFM probe setup for the selective area polarization of ferroelectric polymer on MoS2 channel, reprinted with permission from ref. 233, Copyright 2017 American Physical Society.
The ferroelectric polymer coupling to 2D MXs has also been utilized to boost the photodetection performances in detectors. By using a patterned Al gate electrode on top of the p(VDF-TrFE) layer displayed in Fig. 14d,223 Hu et al. successfully achieved the dramatic suppression of the photodetection dark current by almost 3 orders in a MoS2 detector polarized to the Pup state, as indicated in Fig. 14e. Compared to the fresh device, the photocurrent also showed improvement after the ferroelectric polarization. The hybrid structure thus rendered an overall lift of the photodetection performance compared to bare MoS2, yielding high responsivity (>2500 A W−1), fast response speed (<2 ms), as well excellent detectivity up to 2.2 × 1012 Jones. In particular, the strong polarization field imposed on MoS2 even led to the modulation of its electronic band structure, enabling near-infrared photodetection at the considerably longer wavelengths than normally allowed.223 The structure was recently successfully extended to MoTe2,255 WSe2,234 In2Se3,249 making possible the optimization of photodetection via facile ferroelectric polarization.
Through a careful analysis of the carrier mobility in p(VDF-TrFE) coupled MoS2 by correcting the free carrier density, Xiao et al. found that the integration of the ferroelectric polymer interface does not impose additional scattering or degrade the inherent carrier mobility of 2D MoS2.233 On the other hand, the ferroelectric polarization-induced doping modulation of MXs could be utilized to optimize the carrier mobility in 2D MXs. For example, Wang et al. found that after coating p(VDF-TrFE-CFE), the hole mobility in WSe2 was improved from 27 to 170 cm2 V−1 s−1 at room temperature,234 which could be attributed to both the improved dielectric and free carrier screening of Coulomb scattering.233 In MoS2, a superior effective field-effect electron mobility of 625 cm2 V−1 s−1 was even achieved when MoS2 was placed on top of the 150 nm thick p(VDF-TrFE). The high carrier mobility in 2D MXs helped to decrease the sub-threshold swing in transistors, which is desired for low power devices.234 To this end, the negative capacitance effect in ferroelectric materials has also been explored. An example was recently reported by Wang et al., in which an ultralow SS of 24 mV dec−1 was demonstrated in p(VDF-TrFE) coupled MoS2.232 The crystallinity of the ferroelectric polymer was found to be critical to optimize the SS below 60 mV dec−1 since it determines the polarization intensity and switching characteristics. This, however, poses a challenge to the integration of ultrathin ferroelectric polymers.
In addition to the top gate or bottom gate polarization, the ferroelectric materials can also be polarized by using scanning probes. As indicated in Fig. 14f, an AFM probe with an applied voltage was used to polarize the ferroelectric polymer on top of MoS2.233 The selective area scanning allowed patterned modulation of the electrical conductance in MoS2, and further, the facile preparation of Schottky junctions across the channel. Though time-consuming, the method is still a good choice for understanding the effect of the ferroelectric modulation of 2D MXs, especially in terms of local effects, e.g. during the construction of planar or vertical p–n junctions.
In summary, we have given in the above an inclusive review of the strategies and recent advances towards the doping engineering and functionalization of 2D MXs. Compared to the mainstream Si, III–V semiconductors, the tailorable doping engineering remains to be developed and maturated for the solid foundation of practical applications in transistors, memories, detectors and even electrochemical devices. At present, by using substitutional doping, surface charge transfer doping, intercalation and electrostatic doping, one could achieve both non-degenerate and degenerate modulation of 2D MXs. This not only enabled the realization of widely tuned semiconductor properties but also allowed the appearance of metallic behaviours and phase transition under the intensive charge injection. However, there are still challenges mainly arising from the demand of addressing the precisely controlled dopant incorporation, activation and long-term stability issues, and also the resulted function performances. More attention should be therefore paid on clarifying the dopant interaction with various 2D MXs in substitutional doping, charge transfer doping and intercalation doping, thus to maintain the crystalline integrity of host materials.
In Table 3, we finally summarize the main strategies and the related doping sources available for functionalization of 2D MXs. Though each strategy may have its own limitations, enormous new properties and functions have been enabled in the past by such doping engineering, which greatly promoted the wealthy evolving of various kinds of functional devices. Moving forward, optimizing existing or finding new dopant sources is still important to improve the performance and enrich their application functions. As a non-destructive modulation method, the electrostatic doping has emerged as a convenient method to tune the electrical performances of 2D MXs. The facile construction of patterned floating gate or ferroelectric coupling is going to allow more and more sophisticated doping control of 2D MXs with synergistic functions integrated. One can therefore clearly foresee the wealthy potential of 2D MXs based on the increasingly developed doping processes with convenient, stable, flexible, and precise characteristics, by which intriguing multifunctional devices that integrate two or more functions can thrive.
This work was supported by National Natural Science Foundation of China (Grant No. 91622117, 51472097, and 51727809), National Key Research and Development Program of “Strategic Advanced Electronic Materials” (Grant No. 2016YFB0401100), and the Fundamental Research Funds for the Central University (Grant No. 2017KFKJXX007 and 2015ZDTD038). The authors are indebted for the kind permission from the corresponding publishers/authors to reproduce their materials, especially figures, used in this article.
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