Two-dimensional (2D) semiconductors, such as ultrathin layers of transition metal dichalcogenides (TMDs), offer a unique combination of electronic, optical and mechanical properties, and hold ...potential to enable a host of new device applications spanning from flexible/wearable (opto)electronics to energy-harvesting and sensing technologies. A critical requirement for developing practical and reliable electronic devices based on semiconducting TMDs consists in achieving a full control over their charge-carrier polarity and doping. Inconveniently, such a challenging task cannot be accomplished by means of well-established doping techniques (
e.g.
ion implantation and diffusion), which unavoidably damage the 2D crystals resulting in degraded device performances. Nowadays, a number of alternatives are being investigated, including various (supra)molecular chemistry approaches relying on the combination of 2D semiconductors with electroactive donor/acceptor molecules. As yet, a large variety of molecular systems have been utilized for functionalizing 2D TMDs
via
both covalent and non-covalent interactions. Such research endeavours enabled not only the tuning of the charge-carrier doping but also the engineering of the optical, electronic, magnetic, thermal and sensing properties of semiconducting TMDs for specific device applications. Here, we will review the most enlightening recent advancements in experimental (supra)molecular chemistry methods for tailoring the properties of atomically-thin TMDs - in the form of substrate-supported or solution-dispersed nanosheets - and we will discuss the opportunities and the challenges towards the realization of novel hybrid materials and devices based on 2D semiconductors and molecular systems.
A variety of molecular chemistry approaches are currently investigated for tailoring the physico-chemical properties of ultrathin transition metal dichalcogenides towards novel hybrid multifunctional materials and devices.
Owing to an ultrathin body, atomic scale smoothness, dangling bond‐free surface, and sizable bandgap, transistors based on two‐dimensional (2D) layered semiconductors show the potential of ...scalability down to the nanoscale, high‐density three‐dimensional integration, and superior performance in terms of better electrostatic control and smaller power consumption compared with conventional three‐dimensional semiconductors (Si, Ge, and III‐V compound materials). To apply 2D layered materials into complementary metal‐oxide‐semiconductor logic circuits, it is important to modulate the carrier type and density in a controllable manner, and engineer the contact (between metal electrode and 2D semiconductor) and the interface (between dielectrics and semiconducting channel) to get close to their intrinsic carrier mobility. In this review, the most widely studied 2D transition metal dichalcogenides (TMD) are focused on, and an overview of recent progress on doping, contact, and interface engineering of the TMD‐based field‐effect transistors is provided.
Field‐effect transistors based on two‐dimensional transition metal dichalcogenides (TMDs) show the potential for next‐generation electronic devices. Here, the state‐of‐the‐art methods to control the carrier type, engineer the metal/TMDs contact, and optimize the dielectric/TMDs interface are reviewed.
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BFBNIB, FZAB, GIS, IJS, KILJ, NLZOH, NUK, OILJ, SBCE, SBMB, UL, UM, UPUK
Stimuli-responsive hybrid van der Waals heterostructures (vdWHs), composed of organic molecular switches superimposed on inorganic 2D materials (2DMs), can combine the outstanding physical properties ...of the latter components with the virtually infinite variety of tunable functionality of molecules, thereby offering an efficient protocol for the development of high-performance multifunctional materials and devices. The use of light as a remote control to modulate the properties of semiconducting 2DMs when interfaced with photochromic molecules suffers from both the limitation associated with the persistent photoconductivity characterizing the 2DMs and the finite thermal stability of the photochromic molecule in its different states. Here, we have devised a universal approach toward the fabrication of optically switchable electronic devices comprising a few nanometers thick azobenzene (AZO) layer physisorbed on 2D semiconductors supported on a trap-free polymer dielectric. The joint effect of the improved 2D/dielectric interface, the molecule’s light-modulated dipolar doping, and the high thermal stability of cis-AZO offers the highest control over the reversible and efficient charge carrier tuning in 2D semiconductors with a preserved high performance in 2D field-effect transistors, as quantified in terms of carrier mobility and I on/I off ratio. The device has the potential to operate as an optical memory with four current levels and long retention time (>15 h). Furthermore, by using a CMOS-compatible micropatterning process, the photoswitchable resistor–diode transition has been achieved on hybrid lateral heterojunction devices. Our approach is of general applicability toward the generation of high-performance hybrid vdWHs for the emergence of functional and responsive devices.
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IJS, KILJ, NUK, PNG, UL, UM
Platinum disulfide (PtS2), a new member of the group‐10 transition‐metal dichalcogenides, is studied experimentally and theoretically. The indirect bandgap of PtS2 can be drastically tuned from 1.6 ...eV (monolayer) to 0.25 eV (bulk counterpart), and the interlayer mechanical coupling is almost isotropic. It can be explained by strongly interlayer interaction from the pz orbital hybridization of S atoms.
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Control of the carrier type in 2D materials is critical for realizing complementary logic computation. Carrier type control in WSe2 field‐effect transistors (FETs) is presented via thickness ...engineering and solid‐state oxide doping, which are compatible with state‐of‐the‐art integrated circuit (IC) processing. It is found that the carrier type of WSe2 FETs evolves with its thickness, namely, p‐type (<4 nm), ambipolar (≈6 nm), and n‐type (>15 nm). This layer‐dependent carrier type can be understood as a result of drastic change of the band edge of WSe2 as a function of the thickness and their band offsets to the metal contacts. The strong carrier type tuning by solid‐state oxide doping is also demonstrated, in which ambipolar characteristics of WSe2 FETs are converted into pure p‐type, and the field‐effect hole mobility is enhanced by two orders of magnitude. The studies not only provide IC‐compatible processing method to control the carrier type in 2D semiconductor, but also enable to build functional devices, such as, a tunable diode formed with an asymmetrical‐thick WSe2 flake for fast photodetectors.
In the thickness‐dependent transport behavior of WSe2 transistors, it is revealed that all p‐type, ambipolar, and n‐type characteristics are obtained by merely varying the thickness of WSe2 flakes. The layer‐dependent band structure is a determining factor in achieving this strong thickness‐dependent transport behavior in WSe2 field‐effect transistors. The unique band structure of WSe2 also enables efficient p‐type doping in WSe2 by solid‐state oxide.
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The electrical and optical measurements, in combination with density functional theory calculations, show distinct layer‐dependent semiconductor‐to‐semimetal evolution of 2D layered PtSe2. The high ...room‐temperature electron mobility and near‐infrared photoresponse, together with much better air‐stability, make PtSe2 a versatile electronic 2D layered material.
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BFBNIB, FZAB, GIS, IJS, KILJ, NLZOH, NUK, OILJ, SBCE, SBMB, UL, UM, UPUK
Metallic nanostructures are widely used for surface-enhanced Raman spectroscopy (SERS). Nanoscale surface corrugation significantly affects the localized plasmon response and the subsequent Raman ...intensity of the molecules in close proximity to the nanostructures. Experimentally, the surface roughness of metal films can be controlled by adjusting the deposition conditions, and the resulting localized near-field properties can be probed by measuring the Raman spectrum of the conformally coated monolayer graphene. The well-known Raman characteristics of graphene and its atomic-level 2D nature make it an ideal test-bed for SERS measurements on corrugated metal films. In this work, we experimentally and theoretically study the effects of surface roughness of Ag thin films on the SERS of graphene. We find that the nonlocality effect of the metal dielectric response has to be taken into account for more accurate prediction of the SERS enhancement at large surface roughness. Our results also reveal that the effect of physisorption strain should be included to understand the Raman peak shift and spectral broadening. These observations are fundamentally important for understanding the SERS from metallic nanostructures with sub-nanoscale corrugation.
The electronic properties of two-dimensional semiconductors can be strongly modulated by interfacing them with atomically precise self-assembled molecular lattices, yielding hybrid van der Waals ...heterostructures (vdWHs). While proof-of-concepts exploited molecular assemblies held together by lateral unspecific van der Waals interactions, the use of 2D supramolecular networks relying on specific non-covalent forces is still unexplored. Herein, prototypical hydrogen-bonded 2D networks of cyanuric acid (CA) and melamine (M) are self-assembled onto MoS
and WSe
forming hybrid organic/inorganic vdWHs. The charge carrier density of monolayer MoS
exhibits an exponential increase with the decreasing area occupied by the CA·M unit cell, in a cooperatively amplified process, reaching 2.7 × 10
cm
and thereby demonstrating strong n-doping. When the 2D CA·M network is used as buffer layer, a stark enhancement in the catalytic activity of monolayer MoS
for hydrogen evolution reactions is observed, outperforming the platinum (Pt) catalyst via gate modulation.
The controlled functionalization of semiconducting 2D materials (2DMs) with photoresponsive molecules enables the generation of novel hybrid structures as active components for the fabrication of ...high‐performance multifunctional field‐effect transistors (FETs) and memories. This study reports the realization of optically switchable FETs by decorating the surface of the semiconducting 2DMs such as WSe2 and black phosphorus with suitably designed diarylethene (DAE) molecules to modulate their electron and hole transport, respectively, without sacrificing their pristine electrical performance. The efficient and reversible photochemical isomerization of the DAEs between the open and the closed isomer, featuring different energy levels, makes it possible to generate photoswitchable charge trapping levels, resulting in the tuning of charge transport through the 2DMs by alternating illumination with UV and visible light. The device reveals excellent data‐retention capacity combined with multiple and well‐distinguished accessible current levels, paving the way for its use as an active element in multilevel memories.
Optically switchable field‐effect transistors (FETs) are realized by decorating the surface of 2D WSe2 and black phosphorene with photochromic diarylethenes. Both electron and hole transport can be efficiently and reversibly modulated as a result of light irradiation at different wavelengths. These FETs also show five distinguishable output current levels with high‐accuracy readout, demonstrating their huge potential for multilevel memories.
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BFBNIB, FZAB, GIS, IJS, KILJ, NLZOH, NUK, OILJ, SBCE, SBMB, UL, UM, UPUK
Control of carrier type and carrier density provides a way to tune the physical properties of two-dimensional (2D) semiconductors. Modulation doping of heterostructures can effectively inject ...carriers into or extract carriers from the 2D semiconductors, and eliminate the adverse effect from the ionized dopants. Here we first investigate the layer-dependent negative trion PL of 2D MoS 2 , and further construct heterostructures with transition metal dichalcogenides (TMDs) and transition metal oxides (TMOs). By choosing the oxide with different charge neutrality levels (CNLs), we demonstrate effective electron injection into MoS 2 by TiO 2 doping, and electron extraction from MoS 2 by MoO 3 doping. Photoluminescence (PL) spectra and electrical characterization show that thicker MoS 2 flakes are more easily n-doped by TiO 2 , while thinner MoS 2 flakes are more easily p-doped by MoO 3 . Our experimental results are in good agreement with theoretical calculations. The modulation doping with TMO is compatible with conventional Si processing and highly air-stable. This method can also be extended for the controllable doping of other 2D materials.
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