Atomically Substitutional Engineering of Transition Metal Dichalcogenide Layers for Enhancing Tailored Properties and Superior Applications

doi:10.1007/s40820-023-01315-y

Abstract

Abstract:

Transition metal dichalcogenides (TMDs) are a promising class of layered materials in the post-graphene era, with extensive research attention due to their diverse alternative elements and fascinating semiconductor behavior. Binary MX₂ layers with different metal and/or chalcogen elements have similar structural parameters but varied optoelectronic properties, providing opportunities for atomically substitutional engineering via partial alteration of metal or/and chalcogenide atoms to produce ternary or quaternary TMDs. The resulting multinary TMD layers still maintain structural integrity and homogeneity while achieving tunable (opto)electronic properties across a full range of composition with arbitrary ratios of introduced metal or chalcogen to original counterparts (0-100%). Atomic substitution in TMD layers offers new adjustable degrees of freedom for tailoring crystal phase, band alignment/structure, carrier density, and surface reactive activity, enabling novel and promising applications. This review comprehensively elaborates on atomically substitutional engineering in TMD layers, including theoretical foundations, synthetic strategies, tailored properties, and superior applications. The emerging type of ternary TMDs, Janus TMDs, is presented specifically to highlight their typical compounds, fabrication methods, and potential applications. Finally, opportunities and challenges for further development of multinary TMDs are envisioned to expedite the evolution of this pivotal field.

Key words: Transition metal dichalcogenides, Atomic substitution, Tailored structure, Tunable bandgap, Enhanced applications

Zhaosu Liu, Si Yin Tee, Guijian Guan, Ming-Yong Han. Atomically Substitutional Engineering of Transition Metal Dichalcogenide Layers for Enhancing Tailored Properties and Superior Applications[J]. Nano-Micro Letters, 2024, 16(1): 95-.

Zhaosu Liu, Si Yin Tee, Guijian Guan, Ming-Yong Han. [J]. Nano-Micro Letters, 2024, 16(1): 95-.

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Figures/Tables 21

Fig. 1 a Construction of binary TMD monolayer, b synthetic strategies for atomic substitution in TMD layer, c representative structures after partial modification of metal (M, N) or/and chalcogen (X, Y) atoms in TMDs, and d tunable/enhanced properties and improved performance provided by atomic substitution in TMD layers

Table 1 Description and comparison of functionalizations on 2D TMDs and their impacts

Functionalizations	Involved species	External ratio	Description	Enhanced properties and performances
Doping	TMDs and other elements^a	< 10%	Trace amount of atoms in lattice or surface adsorption	Changed carrier type/density; limited tuning of bandgap and band structure; high reactive ability
Hybridization	Different types of TMDs; TMDs and other nanomaterials (0D to 2D)	20-80%	Vertical/lateral heterostructure via stacking, surface adsorption or covalent linkage	New channel for separating and transferring carriers; high catalytic activity
Atomic substitution	Ternary or quaternary TMDs	0-100%	Uniform random solid solution; structural integrity and homogeneousness	Modulated crystal phase; continuously tunable bandgap; tailored band alignment/structure; controlled carrier type/density; adjusted electroconductibility; increased surface activity

Fig. 2 Principles/rules and theoretical guidance for synthesizing ternary TMDs. a Lattice constant matching for metal-metal pairs of 2H TMDs based on lattice constants. The shaded blue area in the top left corner is populated with vanadium-molybdenum and vanadium-tungsten dichalcogenides, which are metal-semiconductor alloys. The shaded pink area in the bottom left corner contains semiconductor-semiconductor alloys of molybdenum and tungsten dichalcogenides. Reproduced with permission from Ref. [65]. Copyright 2014, Royal Society of Chemistry. Calculated mixing enthalpies for b MoS₂₍₁₋_x₎Se₂_x and c MoS₂₍₁₋_x₎Te₂_x via different simulation methods (all symmetry-inequivalent alloy configurations in a 24-atom cell are used for the calculation by CE). Reproduced with permission from Ref. [66]. Copyright 2013, AIP Publishing. d DFT-calculated and fitted mixing enthalpy (ΔH_mix, solid line) of W₁₋_xNb_xS₂, along with the entropy contribution (ΔS, dashed line) to the free energy at temperatures between 0 and 600 K. e x-dependent free energy of W₁₋_xNb_xS₂ obtained by merging the enthalpy and entropy terms from d. f Horizontal lines “one-to-one” correspond with the free energies in e for revealing stability. The blue and gray diamonds correspond to boundary points between different stability regions in the equilibrium phase diagram. g Miscibility temperatures of all the 20 MNX- and 5 MXY-type TMDs in the 2H phase. The top right and bottom left triangles correspond to sulfide and selenide TMDs, respectively, while the diagonal corresponds to MXY-type TMDs. Dark blue spaces indicate miscible TMDs. Reproduced with permission from Ref. [68]. Copyright 2020, Wiley-VCH

Fig. 3 Li-ion intercalation-assisted liquid exfoliation of 2H-phase MoS₂_xSe₂₍₁₋_x₎ bulk crystal to form 1T-phase MoS₂_xSe₂₍₁₋_x₎ monolayer. a Schematic illustration for preparing single-layered MoS₂_xSe₂₍₁₋_x₎ nanosheets. b-d Characterization of MoS₂_xSe₂₍₁₋_x₎ nanosheets for demonstrating the formation of 1T-phase in the exfoliated MoS₂_xSe₂₍₁₋_x₎ nanosheets: b SEM image of exfoliated high-concentration MoS₂_xSe₂₍₁₋_x₎ nanosheets (scale bar, 2 µm). c Atomic STEM image of a typical MoS₂_xSe₂₍₁₋_x₎ nanosheet with 1T-phase (scale bar, 1 nm). d High-resolution XPS Mo 3d spectrum of 2H-phase MoS₂_xSe₂₍₁₋_x₎ bulk crystal, and the exfoliated (1T) and annealed (2H) MoS₂_xSe₂₍₁₋_x₎ nanosheets. Reproduced with permission from Ref. [80]. Copyright 2016, Wiley-VCH

Fig. 4 a Schematic diagram for the synthesis of Mo₁₋_xNb_xSe₂ nanosheets through one-pot solvothermal reaction using MoCl₅, NbCl₅, (PhCH₂)₂Se₂ as precursors with different ratios and oleylamine as solvent. b HRTEM image for showing the assembly of nanosheets into flower-like spheres. c HAADF-STEM image and EDX elemental mapping of Mo (L shell), Nb (L shell), and Se (L shell) for Mo_0.5Nb_0.5Se₂. Reprinted with permission from Ref. [93]. Copyright 2021, American Chemical Society

Fig. 5 Schematic diagrams of a typical CVD process for the controllable fabrication of high-quality ATMD monolayers. a Typical tube-furnace set-up employed for the growth of ternary monolayers (left figure), the two-dimensional structures of Mo₍₁₋_x₎W_xS₂ monolayer and MoS₂₍₁₋_x₎Se₂_x monolayer (middle figure), and the corresponding SEM morphology of Mo₍₁₋_x₎W_xS₂ and MoS₂₍₁₋_x₎Se₂_x (right figure). Reprinted with permission from Ref. [95]. Copyright 2015, Royal Society of Chemistry. b Proposed process for decreasing the melting point of the precursors after the addition of salt, the growth process of the 2D atomic layer, and the optical images of 13 ATMD layers synthesized using the molten-salt-assisted CVD method. The SEM images of the Nb nucleus with and without salt are also presented. Right figures are optical images of 13 ATMD layers synthesized using the molten-salt-assisted CVD method. Reprinted with permission from Ref. [33]. Copyright 2018, Nature Publishing Group. c Solution-processed precursor deposition for the CVD growth of MoS₂/WS₂ lateral heterostructures and alloys (left), the reaction conditions for the CVD growth of each monolayer structure (middle), and the optical images of each monolayer structure (right). Reprinted with permission from Ref. [106]. Copyright 2019, American Chemical Society

Table 2 The strength and weakness of different bottom-up strategies

Parameters	Hydro/solvothermal reaction	CVD	PLD	MBE
Uniformity	Medium	High	High	High
Controllability	High	High	High	High
Scalability	Medium	High	High	High
Efficiency	Medium	High	Medium	Medium
Cost effectiveness	High	Medium	Medium	Low

Fig. 6 Transition of crystal phase in 2D ATMDs. a-b Structure of Re_xMo₁₋_xS₂ monolayers at different Re concentrations, as shown in the experimental STEM-ADF images (left) paired with the corresponding atom mapping images (right). To enhance clarity, S atoms are excluded from the atom mapping images. c Calculated mixing/formation energy in 2H- and 1T’-phase Re_xMo₁₋_xS₂ alloy at different Re concentrations. Reproduced with permission from Ref. [120]. Copyright 2018, Wiley-VCH. d Crystal structure of 2H WSe₂ and 1T’ WTe₂, along with the composition-dependent phases in WSe₂₍₁₋_x₎Te₂_x. e Atomic resolution STEM characterization of WSe₂₍₁₋_x₎Te₂_x (x = 0-1) monolayers with different Te concentration. Z-contrast STEM images reveal the atomic structure of pristine WSe₂ monolayer in the 2H phase, alloyed WSe_1.0Te_1.0 monolayer in the 2H phase, alloyed WSe_1.0Te_1.0 monolayer in the 1T’ phase, and monolayer WTe₂ in the 1T’ phase. Corresponding FFT patterns are shown in the inset. f Composition-dependent band gaps (x) and the photoluminescence spectra (PL, inset) of the monolayer WSe₂₍₁₋_x₎Te₂_x alloys. Reproduced with permission from Ref. [48]. Copyright 2016, Wiley-VCH

Table 3 Atomic substitution-induced phase transition in special ternary TMDs

Ternary TMDs	Phase transition as x increases	Turning point of phase transition	Refs
Mo₁₋_xRe_xSe₂	2H→1T’	~ 0.42	[47]
WSe₂₍₁₋_x₎Te₂_x	2H→1T’	~ 0.5-0.6	[48]
Mo₁₋_xV_xSe₂	2H→1T	~ 0.7	[54]
WTe₂_xS₂₍₁₋_x₎	2H→1T’	~ 0.45	[74]
Re_xMo₁₋_xS₂	2H→1T’	~ 0.5	[120]
W₁₋_xV_xSe₂	2H→1T	~ 0.7	[121]
W₁₋_xRe_xS₂	2H→1T’	~ 0.4375	[124]
Nb₁₋_xV_xSe₂	2H→1T	~ 0.3	[126]

Fig. 7 Composition-dependent band gap/edge in ATMDs. a Typical SEM morphology of the CVD-synthesized MoS₂_xSe₂₍₁₋_x₎ nanosheets. b PL spectrum of the MoS₂_xSe₂₍₁₋_x₎ nanosheets and a typical PL mapping of a single ternary nanosheet (inset) excited with a 488-nm laser. c Composition (x)-dependent bandgaps of the alloy nanosheets. Reproduced with permission from Ref. [44]. Copyright 2014 American Chemical Society. d PL spectra of Mo₁₋_xW_xS₂ monolayers with the controlled compositions. e Measured optical bandgap of Mo₁₋_xW_xS₂ monolayers (x = 0-1). Reproduced with permission from Ref. [131]. Copyright 2021, American Chemical Society. f Electronic band structures of few-layer MoS₂₍₁₋_x₎Se₂_x films, with all energetic levels relative to the reduction potentials (vs. RHE) for aqueous CO₂ reduction reaction (pH = 6.8). The valence band edges are derived by XPS, while optical band gaps are measured via ellipsometry. Reprinted with permission from Ref. [132]. Copyright 2020, American Chemical Society

Fig. 8 Composition-dependent Raman scattering in ATMD monolayers. a Raman spectra of Mo₁₋_xW_xS₂ monolayers with different W compositions. The three blue lines show frequency shift of E’ and A₁’ peaks with W composition. b Composition-dependent Raman frequencies of Mo₁₋_xW_xS₂ monolayers. Reprinted with permission from Ref. [142]. Copyright 2014, Royal Society of Chemistry. c Raman spectrum of MoS₂_xSe₂₍₁₋_x₎ nanosheets excited with a 488-nm argon ion laser. d-e Raman shift of S-Mo related modes [E_2g(S-Mo), A_1g(S-Mo)] and Se-Mo related modes [A_1g(Se-Mo), E_2g(Se-Mo)] as S mole fraction. Reproduced with permission from Ref. [44]. Copyright 2014, American Chemical Society. f Schematic illustration of displacing atoms for the Raman active E’ and A₁’ modes in Mo₁₋_xW_xS₂ monolayer and force constants used in MREI model. g Composition-dependent Raman frequencies of E’ and A₁’ (E_2g¹ and A_1g for bulk) modes in Mo₁₋_xW_xS₂ alloys. The solid and dashed lines represent the fitting results of Mo₁₋_xW_xS₂ monolayers and bulks via the MREI, respectively, while the scattered square and triangle points are the experimental data of Mo₁₋_xW_xS₂ monolayers and bulks, respectively. Reprinted with permission from Ref. [142]. Copyright 2014, Royal Society of Chemistry

Fig. 9 Composition-dependent electronic properties in ternary TMDs. a-c Electrical transport of WS₂_xSe₂₋₂_x nanosheets: a optical microscopy of a typical back-gated field-effect transistor made of a WS₂_xSe₂₋₂_x nanosheet (scale bar, 5 μm), b transfer characteristics (I_d^1/2-V_g plot) of WS₂_xSe₂₋₂_x nanosheet transistors with different S atomic ratios from nearly pure WSe₂ (brown curve) to nearly pure WS₂ (black curve), and c field-effect mobility vs S atomic ratio in WS₂_xSe₂₋₂_x alloy nanosheets, where the blue dots represent the hole mobility in WSe₂-rich alloys and red dots represent electron mobility in WS₂-rich alloys. Reproduced with permission from Ref. [46]. Copyright 2015, American Chemical Society. d Transfer curves (I_ds-V_g) of Mo_0.98Re_0.02S₂ alloy FET, which presents a “bipolar-like” conduction behavior. Reproduced with permission from Ref. [45]. Copyright 2020, Wiley-VCH. e-h Electrical characterization of WSe₂₋₂_xTe₂_x/WSe₂₋₂_yTe₂_y core/shell structure-based transistors: e optical image of the core/shell structure-based transistors. f Band alignments of core and shell transistors along the channel. The shell WSe₂₋₂_yTe₂_y (Te poor) transistor has smaller hole Schottky barrier to facilitate hole transport, while the core WSe₂₋₂_xTe₂_x (Te rich) has more balanced electron/hole Schottky barriers to favor ambipolar transport. g Effective Schottky barrier height as a function of gate voltage for shell and core transistors. h Electrical transport properties of the homogeneous transistors. The homogeneous shell transistor shows strong p-type transport, while the core transistor displays ambipolar behavior. The difference between the core and shell regions can be attributed to the bandgap difference between these two materials. Reproduced with permission from Ref. [157]. Copyright 2020, Wiley-VCH

Fig. 10 Improved device performance using ATMDs as interfaces. a-b Current enhancement and bipolar current modulation of top-gate transistors based on monolayer MoS₂ on three-layer W_xMo_1−xS₂: a schematic illustration of the top-gate transistor. The gate electrode is separated from the conducting channel by an insulating layer of Al₂O₃. The panel shows the layer structure of the channel material, which is composed of monolayer MoS₂ grown on Mo_xW₁₋_xS₂. b I_D-V_GS curves of the devices with monolayer MoS₂ on three-layer WS₂ (left) and Mo_0.3W_0.7S₂ (right), respectively. The voltage V_DS was set to 2 V for all measurements. Reprinted with permission from Ref. [158]. Copyright 2018, American Chemical Society. c-d The interfacial transition region of W_xNb₁₋_xSe₂ for lowering the potential barrier height between the WSe₂ channel and NbSe₂ electrode in the WSe₂-based bottom-gate FET with NbSe₂ electrode: c optical image and schematic drawing of WSe₂-based bottom-gate FET with NbSe₂ electrode (scale bar, 100 μm). d Transfer characteristics (I_DS-V_BG) of FET devices with different electrode-channel structures: MS (metal-semiconductor, Pd-WSe₂), vdW (NbSe₂-WSe₂), and m-vdW (mixed layer containing NbSe₂-W_xNb₁₋_xSe₂-WSe₂): 1, 3, and 5 cycle junction devices. Data were fitted by averages and standard deviations of 10 devices with each junction type. I_DS was measured at V_DS of − 5 V in devices with channel length of 50 μm and width of 100 μm. Reproduced with permission from Ref. [153]. Copyright 2016, American Chemical Society

Fig. 11 Enhanced photoelectronic performance of ternary TMDs over binary TMDs, making MoS₂₍₁₋_x₎Se₂_x a promising candidate for use in industrial applications in nanophotonic devices. a Photograph of a large-scale, continuous multilayer MoS_1.15Se_0.85 film on a 4-inch quartz wafer. b Schematic illustration of MoS_1.15Se_0.85-based visible light photodetectors, including 20 devices with identical geometry. c Suggested band diagrams with localized defect states for MoS₂, MoS_1.15Se_0.85, and MoSe₂. d Plots of photocurrent of MoS₂, MoSe₂, and MoS_1.15Se_0.85 as a function of bias voltage. e Device-to-device variations in photocurrent extracted from 20 devices for MoS₂-, MoSe₂-, and MoS_1.15Se_0.85-based photodetectors. Reprinted with permission from Ref. [163]. Copyright 2019, Wiley-VCH

Fig. 12 Anisotropic and broadened photoresponse. a-d Anisotropic photoresponse provided by ReS₂_xSe₂₍₁₋_x₎ alloys: a typical optical microscopy of monolayer ReS₂_xSe₂₍₁₋_x₎ alloys on SiO₂/Si substrate and AFM image of as-grown ReS₂_xSe₂₍₁₋_x₎ alloys on mica substrate (inset). b ADF-STEM image of monolayer ReS_0.98Se_1.02 alloy. c Photocurrent response of ReS_1.06Se_0.94 device under light on and off irradiation, and the light with different polarization direction. The direction of b-axis is determined via ARPRS (angle-resolved polarized Raman spectra). d Polar plots for the photocurrent with respect to the polarization angle of the incident light. Reproduced with permission from Ref. [130]. Copyright 2017, Wiley-VCH. e-h Broadened photoresponse provided by Mo_xRe₁₋_xS₂ alloys: e schematic diagram for the photoelectric measurement of the Mo_xRe₁₋_xS₂ alloy device. f Atomic structure of 2H and 1T’ Mo_xRe₁₋_xS₂ alloys. g Plots of bandgap as a function of the Mo composition x. h Plots of bandgap as a function of the Mo composition x. i Photocurrent as a function of time for ReS₂ (the first row), MoS₂ (the second row), and Mo_xRe₁₋_xS₂ alloy (the third row) devices under light illumination with different wavelength: 532, 633, 980, and 1550 nm. The photoresponse measurements of these devices were taken at V_ds = 2 V, V_g = 0 V, the laser power density is ~ 5 mW mm⁻². Reproduced with permission from Ref. [45]. Copyright 2020, Wiley-VCH

Fig. 13 Enhanced HER performance provided by Li intercalation-exfoliated 1T-phase MoSSe nanodots. a Atomic resolution L2D-WF-filtered HAADF-STEM of MoSSe nanodots. Inset in a: photograph of aqueous solution of MoSSe nanodots (scale bar, 5 nm). b Enlarged L2D-WF-filtered image of MoSSe nanodots shown in the dotted square in a (scale bar, 1 nm). c Brightness profiles along the dotted lines in b. V_Se in the brightness profiles represents the Se vacancy. d Atomic models for hydrogen atoms adsorbing at the active sites of basal planes of 1 T-phase MoS₂, 1T-phase MoSSe, 1T-phase MoS₂ with S vacancy, and 1T-phase MoSSe with Se vacancy. e Calculated free energy versus the reaction coordinates of HER for the basal planes of various catalysts. f iR-corrected polarization curves of Li intercalation-exfoliated 1T-phase MoSSe, MoS₂, Mo_0.5W_0.5S₂, WS₂, and MoSe₂ nanodots, and the commercial Pt/C catalysts. g The corresponding Tafel slopes of these catalysts derived from f. Reproduced with permission from Ref. [53]. Copyright 2018, Wiley-VCH

Fig. 14 Enhanced HER performance provided by vacancy-rich Mo₁₋_xV_xSe₂ nanosheets. a Atomically resolved HAADF-STEM images of vacancy-rich Mo_0.6V_0.4Se₂ (with intensity profile along the white line). The darker regions contain the V vacancies (marked by magenta circles). The V atom sites are labeled by yellow circles, and the Se vacancies (1Se: monovacancy, 2Se: divacancy) are marked by cyan circles and diamonds, respectively. In the line profiles, V_M and V_1Se represent the metal vacancy and Se monovacancy sites, respectively. b Gibbs free energies (ΔG_H*) for x = 0, 0.5, and 1 with different vacancy structures. c LSV curves with scan rate of 2 mV s⁻¹ for Mo₁₋_xV_xSe₂ and Pt/C catalysts in 0.5 M H₂SO₄. d Tafel plots derived from the LSV data measured at low potential region. Reproduced with permission from Ref. [54]. Copyright 2021, American Chemical Society

Table 4 Comparison of HER performance of diverse TMDs with different morphologies and crystalline phases with their corresponding end-materials

2D (A)TMDs	Synthetic methods	Morphology	Phase	Tafel slope	Overpotential	References
MoS_0.98Se_1.02	CVD	Monolayer films	2H	119	273	[134]
MoS₂			2H	134	335
MoSe₂			2H	134	303
MoSSe	Li-ion intercalation	Nanodots	1T	40	140	[53]
MoS₂			1T	53	173
MoSe₂			1T	54	209
ReSSe	Li-ion intercalation	Nanodots	1T’	50.1	84	[169]
ReS₂			1T’	106.9	320
ReSe₂			1T’	50.8	123
WS_1.14Se_0.86	CVD	Triangular monolayer nanosheets	2H	85	80	[59]
WS₂			2H	95	100
WSe₂			2H	100	150
rGO/W_0.4Mo_0.6S₂	Wet chemical method	Heterojunction thin films	2H	38.7	96	[170]
rGO/WS₂			2H	68.4	150
rGO/MoS₂			2H	82.0	197
Mo_0.37W_0.63S₂/C	CVD	Membranes	2H	53	137	[143]
MoS₂/C			2H	68	178
WS₂/C			2H	60	166
Re_0.55Mo_0.45S₂	CVD	Monolayer flakes	1T’	56	147	[120]
ReS₂			1T’	200	438
MoS₂			2H	134	475
MoS₂			1T	77	216
Re_0.9Mo_0.1Se₂	Hydrothermal reaction	Nanosheets	1T’	42	77	[55]
ReSe₂			1T’	61	107
MoSe₂			2H	77	188
Mo_0.5Nb_0.5Se₂	Solvothermal reaction	Nanosheets	2H	46	140	[93]
MoSe₂			2H	56	207
NbSe₂			2H	85	292
Mo_0.7V_0.3Se₂	Colloidal reaction	Nanosheets	2H	43	114	[54]
MoSe₂			2H	54	195
VSe₂			1T	76	330
Nb_0.7V_0.3Se₂	Colloidal reaction	Nanosheets	2H/1T	72	236	[126]
NbSe₂			2H	86	295
VSe₂			1T	107	386
W_0.9V_0.1Se₂	Colloidal reaction	Nanosheets	2H	80	128	[121]
WSe₂			2H	83	168
VSe₂			1T	108	387

Fig. 15 a-c Enhanced gas-sensing performance provided by WS₂_xSe₂₋₂_x layers: a schematic for the fabrication process of the WS₂_xSe₂₋₂_x alloy-based gas sensors. b HAADF TEM image of WS_0.96Se_1.04 alloy and EDS elemental maps of W, S, and Se in HAADF images. c Response of WSe₂, WS_0.66Se_1.34 alloy, WS_0.96Se_1.04 alloy, and WS₂ gas sensor for NO₂ exposure as a function of gas concentration. Reproduced with permission from Ref. [127]. Copyright 2018, American Chemical Society. d-e Enhanced photoelectrocatalytic performance provided by Sn_xMo₁₋_xS₂/MoS₂ heterostructure as a photoanode: d schematic illustration of the photoelectrocatalysis measurements. The Sn_xMo₁₋_xS₂/MoS₂-on-quartz sample was connected in the external circuit with Ti electrodes. All the measurements were taken in 0.5 M Na₂SO₄ solutions. The light illumination was produced using a 300-W Xe lamp unless otherwise specified. Inset in d: atomic structure diagram of the lateral Sn_xMo₁₋_xS₂/MoS₂ heterostructure. e Atomic resolution STEM image taken from the epitaxial metal-semiconductor heterostructure at the interface. The yellow dotted line indicates the atomic interface. Inset is a typical SEM image of Sn_xMo₁₋_xS₂/MoS₂ heterostructure. f Linear-sweep voltammogram curves of different anodes. Solid line: light irradiation. Dashed line: dark. Reprinted under the terms of the Creative Commons Attribution License from Ref. [172]. Copyright 2020, The Authors, published by Wiley-VCH

Fig. 16 Strategic preparation of Janus TMDs. a Thermodynamic stability of the H-phase and T-phase for different Janus XMY monolayers. The colors denote the energy above the convex hull in eV/atom. Reprinted with permission from Ref. [178]. Copyright 2019, American Chemical Society. b Synthesis of the Janus SMoSe monolayer. A MoS₂ monolayer grown by CVD is exposed to H₂ plasma to strip the top-layer S. The plasma is then switched off, and a quartz boat loaded with Se powder is moved next to the SMoH sample without breaking the vacuum. Se powders are then thermally vaporized to achieve selenization and complete the synthesis of Janus SMoSe monolayers. Reprinted with permission from Ref. [50]. Copyright 2017, Nature Publishing Group. c-e PLD-based low-energy implantation into 2D TMDs to form Janus structures: c experimental setup for Se plasma generation and impingement on CVD-grown WS₂ monolayer within a vacuum chamber equipped with an ICCD camera and a translatable probe for ion-flux measurement. d R-t plots of the leading edge of the plasma (from ion probe currents, see * in inset) track the propagation and deceleration in different background Ar pressures. e Summary diagram of KE regimes for selenization of WS₂ monolayer by using Se PLD. Selenization of only the top S layer of WS₂ monolayer suitable for Janus SWSe formation occurs between 20 and 40 mTorr for Se plume KEs between 3 and 4.5 eV atom⁻¹. At low pressures (≤ 20 mTorr) and plume KEs above 5.4 eV atom⁻¹, selenization of the bottom S layer by larger Se clusters increases and completes rapidly once pressures decrease toward vacuum. Reproduced with permission from Ref. [182]. Copyright 2020, American Chemical Society. f-h Room-temperature synthesis of 2D Janus TMDs. f Schematic demonstration of the selective epitaxy atomic replacement (SEAR) process through inductively coupled plasma for the synthesis of 2D Janus TMDs. g Working scheme of room-temperature SEAR process. h The atomic representation and optical image of Janus SMoSe/SWSe lateral heterostructures, and Raman mapping at 290 and 284 cm⁻¹ for characteristic Janus SMoSe and SWSe peaks. Reproduced with permission from Ref. [101]. Copyright 2020, Wiley-VCH

Fig. 17 Characterization of built-in dipole moment in JTMD layers and ultrasensitive detection of organic molecules/biomolecules. a Angle-dependent SHG intensity ratio between p and s polarization in the Janus SMoSe and randomized alloy samples. The I_p/I_s ratio (blue circles) increases symmetrically with more tilted incidence and is fitted well by an angle-dependent SHG model (blue curve). The I_p/I_s ratio (red circles) undergoes almost no change as the incident angle varies, and the flat fitting (red curve) suggests a negligible out-of-plane dipole. Reprinted with permission from Ref. [50]. Copyright 2017, Nature Publishing Group. b Schematics of the crystalline structure (side view) of MS₂ and Janus SMSe (M = Mo, W) and the corresponding wave functions of electron (f_e) and hole (f_h) in excitons in each material. Reprinted with permission from Ref. [174]. Copyright 2021, American Chemical Society. c Calculated electron density difference (EDD) plots of acetone adsorption on S layer of Janus SMoSe and SMoTe monolayers. Green and red represent the regions of electron depletion and accumulation, respectively. Reprinted with permission under a Creative Commons Non-Commercial No Derivative Works (CC-BY-NC-ND) Attribution License from Ref. [184]. Copyright 2020, American Chemical Society. d-g SERS-based ultrasensitive detection of glucose by Janus SMoSe: d optical image of Janus SMoSe and schematic of laser irradiation of glucose on Janus SMoSe for SERS characterization. The scale bar is 20 μm. e-f DFT calculations for the glucose molecule on the Janus substrate. e Charge distribution in glucose. The light blue and the purple regions show the electron cloud distribution in the isolated single glucose molecule and in anchored glucose on Janus SMoSe, respectively. Their distinguishing shapes indicate the drastic charge redistribution after glucose is adsorbed on monolayer Janus SMoSe. f Calculated Raman peaks for isolated glucose and anchored glucose on Janus SMoSe. g Concentration-dependent Raman spectra of glucose on Janus SMoSe. The integrated peak intensity increased linearly with elevated glucose concentration in solution. Reprinted with permission from Ref. [185]. Copyright 2020, Royal Society of Chemistry

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