Fig 1

Illustration of the oxygen-plasma-enhanced exfoliation process for graphene. (a) Optical image of the SiO2/Si substrate and adhesive tape with graphite flakes. (b) Oxygen plasma cleaning of the substrate. (c) Contact between the graphite decorated tape and the substrate surface, followed by heating of the substrate (with tape) on a hot plate in air at ~100 ℃ for 2 min. (d) Removal of the substrate from the hot plate and peeling off the tape. (e) Optical image of the substrate after graphene exfoliation. (f) Optical micrograph of one large scale graphene flake on the substrate. (a-e) Reproduced with permission from Ref. 120, Copyright 2015 American Chemical Society. (f) Reproduced with permission from Ref. 122, Copyright 2018 Acta Physica Sinica."

Fig 2

(a) Schematic of the Gold-assisted mechanical exfoliation process of different monolayer materials. (b-d) Optical images of exfoliated MoS2 on SiO2/Si, sapphire, and plastic film. (e) 2-inch CVD-grown monolayer MoS2 film transferred onto a 4 inch SiO2/Si substrate. (f-g) Optical images of large exfoliated 2D crystals: BP, RuCl3, Fe3GeTe2, FeSe, CrSiTe3, PtSe2, PtTe2, and PdTe2. Those exfoliated monolayers highlighted in the red box are, so far, not accessible using other mechanical exfoliate method. (h) Optical image and Raman spectra of a MoS2/WSe2 heterostructure. (i) Raman and photoluminescence (PL) spectra of suspended monolayer WSe2. (j) Optical image of suspended WSe2 with different thicknesses (1L to 3L) and a PL intensity map of the suspended monolayer. Reproduced from Ref. 126."

Fig 3

(a) Layered crystal structure and CVD growth of Bi2O2Se with Bi2Se3 and Bi2O3 as precursors 201, 202. (b) Fast growth of millimeter-sized graphene within confined space; reproduced with permission from Ref. 204, Copyright 2016 John Wiley and Sons. (c) Experimental design of Cu foil-foam stacked structure for the growth of super-clean graphene 206. (d) Epitaxial growth of single-crystal graphene wafer with free wrinkles on as-deposited Cu (111) thin film; reproduced with permission from Ref. 209, Copyright 2017 American Chemical Society."

Fig 4

(a) Schematic illustration of the synthetic route of 2D Au nanosheets using LDH as template 225. (b) The synthesis process of ultrathin Al nanosheets 228."

Fig 5

Illustration, synthesis and applications of LDHs."

Fig 6

(a) 2D oxide sacrifice preparation of MOF-74 nanosheets; reproduced with permission from Ref. 289, Copyright 2019 John Wiley and Sons. (b) Structural units of CuBDC and CO2 along [20${\rm{\bar 1}}$] axis (left) and (20${\rm{\bar 1}}$) plane (right) 290. (c) Alkali etching exfoliation for the preparation of MOF nanosheets; reproduced with permission from Ref. 291, Copyright 2019 American Chemical Society. (d) Stepwise expansion and exfoliation of layered kgmSMe MOF; reproduced with permission from Ref. 292, Copyright 2018 American Chemical Society."

Fig 7

(a) Schematic model of one example of Xenes (borophene). (b) The synthesis process of Si nanosheets through chemical etching. Reproduced with permission from Ref. 317, Copyright 2017 John Wiley and Sons. (c) HAADF-STEM image and (d) AFM image of Bi2O2Se nanosheets. Reproduced with permission from Ref. 318, Copyright 2019 American Chemical Society. (e) TEM images of PtSe2 nanosheets 319."

Table 1

Overview of elemental 2D materials explored by experimental and theoretical methods."

Fig 8

Schematic models of examples of (a) 2D covalent organic frameworks (COFs), (b) 2D perovskite and (c) 2D Mxenes (Ti3C2Tx)."

Fig 9

(a) Schematic illustration of the intercalation of TMD crystals with alkali metal to exfoliate 2D nanosheets while induce phase transition. Reproduced with permission from Ref. 105, Copyright 2020 Springer Nature. (b) Schematic illustration of laser-induced phase transition of MoTe2 at the top layer. Reproduced with permission from Ref. 381, Copyright 2015 Association for the Advancement of Science. (c) Schematic illustration of the gas-solid reaction for the synthesis of 1T'-WS2 crystals. Reproduced with permission from Ref. 101, Copyright 2021 Springer Nature."

Fig 10

(a) Schematic illustration of ligand exchange induced phase transformation of Au square sheets from hcp to fcc. Reproduced with permission from Ref. 393, Copyright 2015 Springer Nature. (b) Schematic illustration of the general synthetic process for amorphous noble metal nanosheets. M.T.: the melting point of metal acetylacetonate 226."

Fig 11

(a) The Mn K-edge XANES and EXAFS spectra. Inset (top left) the X-ray absorption spectrum from a sample showing the XANES and EXAFS regions of the spectrum. The enlargements show the Mn K-edge XANES and the k-space EXAFS data with the corresponding Fourier transform (shown on the right) 403. (b) The FT-EXFAS plots of NV-W2N3 and W2N3. Reproduced with permission from the Ref. 405, Copyright 2019 John Wiley and Sons. (c) k2-weighted Mo K-edge EXAFS spectra of Pt-MoS2 in comparison to Mo foil, bulk MoS2 and LixMoS2 406. (d) Zr K-edge XANES spectra of ZrTe2(1-x)Se2x single crystals 407."

Fig 12

(a) Doping evolutions of Fermi surface and band structure in the surface-doped 2H-MoTe2. (ⅰ-ⅳ) Fermi surface mapping taken in the as-grown sample. (ⅴ-ⅵ) the energy-momentum cut taken along the G-K direction in the as-grown sample. (ⅱ) and (ⅵ), (ⅲ) and (ⅶ), (ⅳ) and (ⅶ) are the same as (ⅰ) and (ⅴ). Reproduced with permission from Ref. 409, Copyright 2021 American Physical Society. (b) (ⅰ) and (ⅱ) shows the photoemission intensity plots along the G-M direction in the pristine ZrSe2 and Cu intercalated ZrSe2 single crystals, respectively. (b) (ⅲ) and (ⅳ) shows the photoemission intensity plots along the A-L direction in the pristine ZrSe2 and Cu intercalated ZrSe2 single crystals, respectively 410."

Fig 13

Reflection contrast spectra of monolayer (a) and bilayer (b) BP 413. (c) 1s and 2s exciton states of E11 Exciton absorption spectra for bilayer BP 414. (d, e) Extraordinarily large magneto-optical Raman effects of A1g phonon due to spin-phonon coupling 441. Single photon emission (f) and photon correlation measurements (g) 445. Schematics of valley polarization of excitons (h) and the exciton Hall effect in monolayer MoS2 (i) 446. (j) Fluorescence blinking effect in a WS2/MoSe2 heterobilyer 447. (k) Schematics of the moiré superlattice and moiré excitons formed in a heterobilayer 448. (h, i) reproduced with permission from Ref. 446, Copyright 2018 John Wiley and Sons."

Fig 14

(a-d) Exciton-polariton condensation and lasing of monolayer WS2 480: (a) Sample structure of monolayer WS2 embedded in an optical cavity formed by two DBRs; (b) Angle-resolved reflectivity map of the polariton in a monolayer WS2; (c) Above-threshold angle-resolved PL map; (d) Real space PL spectrum. (e-h) Pulse Q-switching characteristics when the pump power is 263.1 mW 492: (e) The mode-locked spectrum of borophene; (f) Typical oscilloscope pulse trains of mode-locking; (g) Autocorrelation race fitted by a sech2 profile; (h) RF spectrum of the mode-locked pulses."

Fig 15

Schematic of (a) thermoelectric module, (b) electrical and thermal transports in a given superlattice structure; (c) advanced thermoelectric parameters are also elucidated in the bulk materials with a layered structure."

Fig 16

(a) Crystal structure of SnSe. (b) Phonon dispersions measured by inelastic neutron scattering; reproduced with permission from Ref. 583, Copyright 2015 Springer Nature. (c) Band structure of SnSe, green arrows (1, 2) indicate the conduction bands, red arrows (1, 2, 3) indicate the valence bands. (d) Temperature-dependent energy offset in the valence band and conduction band, which enable the high electrical performance through reasonable manipulating band structures. (e) Valence band structure measured by angle-resolved photoemission spectroscopy. Reproduced with permission from Ref. 586, Copyright 2016 American Physical Society."

Fig 17

(a) Schematics of the SnTe crystal structure (upper) and the SnTe film (lower). (b) The stripe domain of a1-UC SnTe film (imaging conditions: -0.2 V, 30 pA, 4.7 K). The arrows in each domain indicate the direction of lattice distortion. (Upper inset) Topography image across a domain boundary (-0.2 V, 100 pA). (Lower inset) The graphene substrate (-0.1 V, 200 pA). (c) Temperature dependence of the distortion angle for the 1- to 4-UC SnTe films. (Inset) The distortion angle near Tc = 270 K for the 1-UC film exhibiting the behavior of a second-order phase transition. (d) Schematic side view of a bilayer SnS structure. Each SnS layer is ferroelectrically polarized with a net dipole moment aligned with the x-axis. Adjacent SnS layers are anti-ferroelectrically coupled due to the inversion symmetry in the AB-stacked structure (top panel); Schematic view of the wedding-cake morphology of MBE-grown SnS crystals, which are inversion-symmetry broken with non-zero net polarizations at the terraces (bottom panel). (e) (left) STM image of one ML SnS island with the nanoripple pattern. The in-plane armchair and zigzag directions are determined from the lattice constants and marked by the black arrows. The net ferroelectric polarization of this island, determined by the LCPD measurements, is marked by the white arrow. (right) Frequency shift measured as a function of the voltage, collected above the two edges marked by red and blue triangles respectively. Measurements are performed in constant-height mode after compensating the surface tilting and thermal drifting effects. Parabolic fits and corresponding parabola peaks are indicated. (f) The corresponding P-V hysteresis curves of the memory device at different gate voltages.(a-c) Reproduced with permission from Ref. 597, Copyright 2016 American Association for the Advancement of Science. (d-f) Reproduced with permission from Ref. 599, Copyright 2019 American Chemical Society."

Fig 18

(a, b) Top-view and side-view of charge density difference between ferroelectric MoTe2 d1T and paraelectric 1T phases (green, purple, cyan, orange, and pink colors denote negative charge, positive charge, Mo atom, Te atom, and polarization, respectively); (c) PFM phase hysteretic and butterfly loops of monolayer d1T-MoTe2; (d) I-V characteristic of monolayer d1T-MoTe2 on Pt and inset shows the energy diagram of ferroelectric tunneling junction devices 616."

Fig 19

(a) The moiré patterns formed in the twisted bilayer graphene (left). The calculated band energy of the twisted bilayer graphene with twist angle of 1.05° using an ab initio tight-binding method (right) 646. (b) Schematic illustration of the "tear and stack" approach for fabricating the MATBG; (c) measured conductance of MATBG device with a twist angle of 1.08° at the temperature of 0.3 K, the lighter-shaded regions denote the locations of superlattice gaps, while the darker-shaded regions highlight the half-filling states; (d) measured Resistance as function of temperature and carrier densities of the MATBG device with twist angle of 1.08°; (e) measured Rxx as function of carrier density and perpendicular displacement field (D) (Top), inset: the OM image of as-fabricated MATBG device, line cut of Rxx as function of carrier density at D/D0 = -0.22 V?nm-1 (bottom); (f) magnetic field dependence of Rxx and Rxy with n/ns = 0.746 (three-quarter filling) and D/D0 = -0.62 V?nm-1 at 30 mK, the solid and dash line correspond to the results measured while sweeping the magnetic field up and down, respectively 647."

Fig 20

Main topics related to the chiral 2D nanomaterials.ORD: optical rotatory dispersion; ECD: electronic circular dichroism; CPL: circularly polarized luminescence; VCD: vibration circular dichroism; ROA: Raman optical activity; SHG: Second-harmonic generation."

Fig 21

Physical properties of chiral perovskites.Top left panel reproduced from Ref. 697. Top right panel reproduced with permission from Ref. 698, Copyright 2021 American Association for the Advancement of Science. Bottom left panel reproduced with permission from Ref. 699, Copyright 2019 John Wiley and Sons. Bottom right penal reproduced with permission from Ref. 700, Copyright 2020 American Chemical Society."

Fig 22

(a) Schematic of a 2D FET with a monolayer semiconductor (MoS2) channel and semimetal (Bi) contacts. The degenerate part of Bi-contacted MoS2 due to gap-state saturation (GSS) is marked in orange colour. (b) The DOS of semimetal and semiconductor contact. Because the Fermi level of the semimetal aligns with the conduction band of the semiconductor, and the DOS at the Fermi level of the semimetal is near-zero, conduction-band contributed metal-induced gap states (MIGS) are suppressed and the branching point is elevated into the conduction band. The MIGS, now mostly contributed by the valence band, are saturated, leading to gap-state saturation. (c) Arrhenius plots of the Ohmic Bi-MoS2 (blue) and Schottky Ni-MoS2 (red) FETs at a carrier density (n2D) of 1.5 × 1012 cm-2 and VDS of 1 V. A 30-meV and negligible barrier for Ni-MoS2 and Bi-MoS2 FETs, respectively, are extracted. (d) Contact resistance (Rc) extraction using the transfer-length method (TLM) for Bi-MoS2 FETs on 100-nm-thick SiNx dielectrics. Blue squares and black circles are total resistance vs. channel length at carrier densities of 5.6 × 1012 and 1.5 × 1013 cm-2, respectively. Inset, False colour SEM image of the TLM structure. Scale bar, 1 μm. (e) IDS-VDS curves of a 35 nm LCH Bi-MoS2 FET. VGS changes from -10 V to 30 V with steps of 10 V. Inset, SEM image of the 35 nm LCH device. (f) State-of-the-art contact technology for MoS2 transistors plotted as a function of carrier density, showing the respective Rc of various semiconductor technologies (Si, III-Vs, and MoS2). The black line represents the quantum limit of Rc 736."

Fig 23

(a) Electron transport mechanism in MoS2 channel devices, including electron-phonon scattering, remote phonon scattering, charge impurity scattering, defection scattering and electron trap. Reproduced with permission from Ref. 758, Copyright 2017 Acta Physica Sinica. (b) Field-effect mobility as a function of temperature for two monolayer MoS2 FETs on HfO2 substrate with thiol treatment under n = 10.5 × 1012 cm-2 (symbols), together with the best theoretical ?ttings (solid lines), the calculated CI-limited mobility (dashed lines), and the calculated phonon-limited mobility (black dotted line). Reproduced with permission from Ref. 757, Copyright 2018 John Wiley and Sons. (c) Mobility vs. semiconductor channel thickness, covering silicon on insulator, some promising 2D materials 5."

Fig 24

(a) Schematic layer structure, AFM topography image and the TEM cross-sectional image of the Al2O3 nucleation layer grown on WS2 layers with 20 ALD cycles at 50 ℃ with a short N2 purge time of 1 s. Reproduced with permission from Ref. 763, Copyright 2020 American Chemical Society. (b) Schematic of using evaporated PTCDA molecular layer as the seeding layer for growing ultrathin HfO2 ALD oxides as shown on the cross-sectional TEM images. Reproduced with permission from Ref. 766, Copyright 2019 Springer Nature."

Fig 25

Logic circuits based on 2D materials. (a) The schematic of a typical n-type FET. (b) A 1-bit microprocessor based on 115 CVD-grown MoS2 transistors 773. (c) High-efficiency logic gate circuit based on 2D materials 787. (d) Reconfigurable logic circuit based on 2D ambipolar transistors 783."

Fig 26

Neuromorphic computing based on 2D materials. (a) The schematic of a typical floating gate transistor 791. (b) Robust memristor based on graphene/MoS2-xOx/graphene vdWH 795. (c) Atomristor based on monolayer MoS2; reproduced with permission from Ref. 799, Copyright 2018 American Chemical Society."

Fig 27

(a) Schematic image of PC detector. (b) The energy band structure of PC devices with and (c) without illumination. (c) Typical output characteristic (I-V) curves of PC device in dark and light states and photogenerated current Ip as a function of bias voltage. (d) Typical transfer characteristic (I-Vg) curves of PC devices in dark and light states. (e) Two key parameters (Responsivity and Detectivity) of a detector as a function of wavelength. (f) A key parameter (Response speed) of a detector with two typical presentations."

Fig 28

(a, e, f) Schematics of band structure of semiconductor and semimetal. Calculated band structures of GeS2 (b), monolayer GaPS4 (c), Te (d); (b) reproduced with permission from Ref. 822, Copyright 2019 John Wiley and Sons; (c) reproduced with permission from Ref. 823, Copyright 2021 John Wiley and Sons. (d) Schematic band structure of trigonal tellurium 842."

Fig 29

Polarized-sensitive photodetectors with broad-spectrum. (a) Obvious anisotropic photocurrent of photodetector based on (BA)2(GA)Pb2I7, inset are the crystal structure of material and the image of device; reproduced with permission from Ref. 876, Copyright 2021 John Wiley and Sons. (b) 2D absorption spectrum of PdSe2 flake from 300 to 1100 nm, and the performance of polarization-sensitive photodetector; reproduced with permission from Ref. 877, Copyright 2021 John Wiley and Sons. (c) Wavelength-dependent responsivity, specific detectivity and the polarimetric imaging of the Gr/PdSe2/Ge heterojunction device, inset is schematic diagram of the device; reproduced with permission from Ref. 878, Copyright 2019 American Chemical Society. (d) Schematic layout of the image scanning system and the NIR polarimetric imaging of GeSe-based image sensor; reproduced with permission from Ref. 879, Copyright 2021 Science China Press."

Fig 30

Near-/In-sensor computing based on 2D materials. (a) Optic-neural synaptic device based on h-BN/WSe2 heterostructure 883. (b) 1 Kb vision processor based on MoS2 photosensitive FET crossbar array; reproduced with permission from ref. 886, Copyright 2020 John Wiley and Sons. (c) Ultrafast machine vision sensor based on split-gate WSe2 transistors. (d) Reconfigurable vision sensor based on vdWHs 885. (e) A prototype neuromorphic vision system based on the retinomorphic sensor and the memristive crossbar array 891."

Fig 31

(a) Schematic illustration for the preparation and (b) the rotating ring-disk electrode polarization curves in 0.1 mol?L-1 KOH of Fe SAs/GO; reproduced with permission from Ref. 909, Copyright 2019 John Wiley and Sons. (c) Schematic illustration of 2D coplanar Pt-C nanomeshes. (d) H2-O2 fuel cell i-V polarization and power density plots recorded under different O2 pressure with the cathode Pt loading of 0.10 mg?cm-2 for coplanar Pt/C NMs; reproduced with permission from Ref. 910, Copyright 2020 John Wiley and Sons. (e) Schematic structure of PcCu-O8-M, (f) ORR polarization curves of PcCu-O8-Co/CNT in 0.1 mol?L-1 KOH; reproduced with permission from Ref. 913, Copyright 2019 John Wiley and Sons. (g) Schematic illustration and (h) LSV plots at the scan rate of 5 mV?s-1 in O2-saturated in 0.1 mol?L-1 KOH electrolyte for Fe-N-C/MXene superlattice-like heterostructure; reproduced with permission from Ref. 915, Copyright 2020 American Chemical Society."

Fig 32

(a) Proposed mechanism for CO2RR to CO on Tri-Ag-NPs. (b) Faradaic efficiencies of CO at various applied potentials (the inset shows the CO, CH4 and H2 overall FE for Tri-Ag-NPs); reproduced with permission from Ref. 922, Copyright 2017 American Chemical Society. (c) Schematic illustration of holey NF-C layers. (d) CO FEs over N-C and NF-C; reproduced with permission from Ref. 925, Copyright 2019 American Chemical Society. (e) Illustration of prepared Cu2O@CuHHTP. (f) Faradaic efficiencies of different products for Cu2O@CuHHTP. (g) Comparison of CH4 faradaic efficiencies between Cu2O@CuHHTP, Cu2O@CCB, and commercial Cu2O; reproduced with permission from Ref. 926, Copyright 2020 John Wiley and Sons. (h) Calculated free energy diagrams for CO2 electroreduction to CO on Ag(111), Ag55 nanoplates, MoS2, WS2, MoSe2, and WSe2 nanoflakes at 0 V vs. RHE; reproduced with permission from Ref. 931, Copyright 2016 Association for the Advancement of Science."

Fig 33

(a) An illustration of SA-Mo/NPC and its corresponding atomic structure model. (b) A TEM image of SA-Mo/NPC. (c) Mo EDS mapping reveals the homogeneous distribution of Mo on the carbon support; the figure inset is the corresponding HAADF-STEM image. (d) Atomic-resolution HAADF-STEM image. (e) EELS spectra from areas A and B in the atomic-resolution HAADF-STEM image of the inset; area A does contain single Mo atoms whereas area B does not. The two orange arrows point to the signals of the Mo M4, 5 and M2, 3 edges, respectively. Reproduced with permission from Ref. 945, Copyright 2019 John Wiley and Sons."

Fig 34

Fabrication and structure characteristics of SA Ru-Mo2CTX. (a) Schematic illustration of the fabrication mechanism. (b) HAADF-STEM image of SA Ru-Mo2CTX, insert is the corresponding SAED pattern. (c) The magnified HAADF STEM image of SA Ru-Mo2CTX. (d) The STEM-EDS elemental mapping of SA Ru-Mo2CTX. (e) XPS spectra of SA Ru-Mo2CTX and commercial Ru/C in Ru 3p regions. (f) The normalized XANES spectra at the Ru K-edge of RuO2, RuCl3, Ru foil and SA Ru-Mo2CTX. (g) The FT-EXAFS spectra derived from EXAFS of Ru K-edge of RuO2, RuCl3, Ru foil and SA Ru-Mo2CTX. Scale bars: (b) 5 nm, (c) 1 nm, (d) 50 nm. Reproduced with permission from Ref. 954, Copyright 2020 John Wiley and Sons."

Fig 35

(a) Schematic illustration of the preparation of PdCu NPs/rGO composites; reproduced with permission from Ref. 941, Copyright 2018 John Wiley and Sons. (b) Optimized structure of FeN3-graphene, (c) moieties of the Fe-N=NH2 intermediate; reproduced with permission from Ref. 978, Copyright 2016 American Chemical Society. (d) Schematic of the atomic orbital of BC3 for binding N2 and LUMO and HOMO of undoped graphene and B-doped graphene, (e) the NH3 production rates and FE of B-doped graphene, the error bars represent the average of three measurements 983."

Fig 36

Morphology and structure characterization of PtPb hexagonal nanoplates. Representative (a) HAADF-STEM image, (b) TEM image, (c) TEM-EDX, and (d) PXRD pattern of PtPb hexagonal nanoplates. (e) SAED and (f) HRTEM of one single hexagonal nanoplate; insets in (f) are the FFT patterns from the white squares at the edge of and inside the nanoplate, respectively. (g) STEM-EELS elemental mapping of PtPb hexagonal nanoplates: HAADF-STEM image, Pt mapping in green, Pb mapping in red, and integrated mapping of Pt and Pb are shown. The compositional ratio between Pt/Pb is 55.9/44.1, as revealed by ICP-AES. Reproduced with permission from Ref. 989, Copyright 2015 Association for the Advancement of Science."

Fig 37

(a) CVs of PdIr bimetallene/C, PdIr NP/C and Pd/C for FAOR in 0.1 mol?L-1 HClO4 containing 0.5 mol?L-1 HCOOH at the scan rate of 50 mV?s-1; (b) mass activities of different catalysts at the potential of 0.5 V vs. RHE; (c) chronoamperometric curves of the PdIr bimetallene/C, PdIr NP/C and Pd/C catalysts at 0.5 V vs. RHE; (d) CO stripping tests of PdIr bimetallene/C, PdIr NP/C and Pd/C catalysts conducted in 0.1 mol?L-1 HClO4 at the scan rate of 50 mV?s-1; (e) free energy pathway (ΔG) for FAOR under acidic conditions; (f) chemisorption of H and CO2 on the surface (111) 1000."

Fig 38

(a) Diagram of CoNi alloy encapsulated by graphene with three-layer. (b) Plots of △G(H*) (red line) and electronic potential (blue line) vs. the graphene layers. (c) Electron density redistribution of CoNi covered by one to three layers of graphene. Reproduced with permission from Ref. 1039, Copyright 2015 John Wiley and Sons."

Fig 39

Illustration of electronic coupling between Co and Ni 1059."

Fig 40

Schematic illustration of the basic principles of photocatalysis (Eg: bandgap, CB: conduction band, VB: valence band)."

Fig 41

(a) The constructed X-doped Bi3O4Cl crystal structure and corresponding electrostatic potential diagrams of X-doped Bi3O4Cl (X = P, S, N, B, F, Br, C); reproduced with permission from Ref. 1089, Copyright 2016 John Wiley and Sons. (b) Side-view 3D AFM images and (c) corresponding height profiles along the lines in (b); (d, e) the average measured thicknesses and theoretical thicknesses of mono-layered Bi12O17Cl2 and MoS2 in 2D Janus; (f) the corresponding EELS elemental maps of 2D Janus (Cl2)-(Bi12O17)-(MoS2) bilayer junctions; scale bar is 10 nm 1090."

Fig 42

(a) Illustration of the different CoFe-x catalysts formed by hydrogen reduction of a CoFeAl-LDH nanosheet precursor at different temperatures and the CO2 hydrogenation selectivity of each CoFe-x catalyst; reproduced with permission from Ref. 1097, Copyright 2017 John Wiley and Sons. (b) Illustration of the variation tendencies of different polymeric C3N4/CdSe QDs; reproduced with permission from Ref. 1121, Copyright 2019 John Wiley and Sons. (c) Schematic illustration of TiO2 and MoS2 heterojunction: charge transfer and separation under UV-visible light irradiation for CO2 reduction. Reproduced with permission from Ref. 1132, Copyright 2018 John Wiley and Sons. (d) Mechanism diagram of the photocatalytic CO2 reduction reaction over a Pt/BP-Bi2WO6 heterojunction under solar light irradiation. Reproduced with permission from Ref. 1126, Copyright 2021 American Chemical Society."

Fig 43

(a) Side and top view of (001) surface of BiOBr with a Vo. (b) The adsorption geometry of N2 on the Vo of BiOBr (001) surface and corresponding charge density distribution; the yellow and blue contour represent charge accumulation and depletion in the space respectively 1149. (c) Schematic polyhedral representation of the ultrathin LDH structure with defective MO6 octahedra at the nanosheet edge or surface and the distortion structure induced by Vo (Vo marked by the yellow dot); reproduced with permission from Ref. 1153, Copyright 2017 John Wiley and Sons. (d) Schematic illustration and HRTEM images of the supported ultrafine Cu2O prepared through in situ reduction of LDH using different concentrations of ascorbic acid; reproduced with permission from Ref. 1170, Copyright 2020 John Wiley and Sons."

Fig 44

High-reactivity oxygen species generated in photocatalysis. Reproduced with permission from Ref. 1178, Copyright 2017 American Chemical Society."

Fig 45

(a) The consumption of dissolved O2 under irradiation using various photocatalysts determined by in situ MS. (b) Post-mortem TPD spectra revealing the desorption of Hads from the g-C3N4 and A-g-C3N4 surfaces. (c) Scheme of the suggested reaction pathway; the reduction reaction will be hindered if Hads binds strongly to the catalyst. (d) Reaction path for the photoconversion of nitroaromatic compounds to azoxy-, azo-aromatic compounds, and amines. NBS = Nitrosobenzene, NPH = N-phenylhydroxylamine 1190. (e) AFM and (f) TEM image of the few-layer C3N4; reproduced with permission from Ref. 1191, Copyright 2019 American Chemical Society. (g) Working principle of the Bi24O31Br10(OH)δ 1193."

Fig 46

Classification of 2D materials and modification strategies in application for rechargeable batteries."

Fig 47

Supercapacitors based on 2D material electrodes: (a) electrochemical double layer mechanism and (b) pseudocapacitance. (c) Various strategies to enhance the electrochemical properties of 2D materials. (d) Unique mechanical properties of 2D materials compared to bulk electrode materials."

Fig 48

Factors of 2D materials that are suitable micro-supercapacitors. High conductivity and fast ionic diffusion are essential for high power performance. The inherently large surface area and engineerable pores increase the possibility to pack high energy density. The mechanical strength and out-of-plane flexibility are attractive for all-weather, wearable MSCs for integrated electronics. Doping, composites, and or heterostructures are general and effective strategies to render high-performance and multifunctional MSC electrodes."

Fig 49

Device structure and work mechanism of solar cells, and the application of 2D materials in solar cells."

Fig 50

Biomedical applications of 2D biomaterials. (a) Multifunctional GO-based stimuli-responsive nanotheranostics. (a1) The scheme of double redox strategy to fabricate Fe3O4/MnOx-GO composite nanosystems. (a2) The scheme of MnOx decomposition and Mn2+ release under either mild acidic or redox environment. (b) Schematic representation of the synthesis regarding BP-BG scaffold and the therapeutic countermeasure for osteosarcoma based on osteogenesis by BP-BG. (a) Reproduced with permission from Ref. 1366, Copyright 2014 John Wiley and Sons. (b) Reproduced with permission from Ref. 1367, Copyright 2018 John Wiley and Sons."

Fig 51

Scheme of the MoS2-based fluorescence sensing platform for detection and imaging of miRNA-21 in cells. Reproduced with permission from Ref. 1387, Copyright 2019 American Chemical Society."

Fig 52

Fabrication of antimonene-based SPR sensor for miRNA detection 1390."

Fig 53

Schematic illustration of SERS sensing platform for organic pollutants detection based on the MXene/AuNR composite. Reproduced with permission from Ref. 1400, Copyright 2019 American Chemical Society."

Fig 54

Schematic of graphene-based FET sensor for COVID-19 detection 1406."

Fig 55

Construction of electrochemical sensing platform for the detection of miRNA-21 based on MoS2-based nanoprobes 1427."

Fig 56

Construction of colorimetric sensor for CEA detection based on Au/Bi2Se3 nanocomposites. Reproduced with permission from Ref. 1437, Copyright 2017 American Chemical Society."

Fig 57

(a) Scheme of the concept of graphene-metal bottom contacts to achieve ultralow contact resistance; reproduced with permission from Ref. 1453, Copyright 2019 American Chemistry Society. (b) Scheme of all-carbon stretchable transistor, with graphene/graphene scrolls as source and drain electrodes and single-walled carbon nanotubes (SWNTs) as channel, (c) transfer curves, on and off current, on/off ratio, and mobility of such all-carbon transistor at different strains 1454. (d) Left: scheme of fabrication of non-disposable graphene-based skin-electrodes by semi-embedding graphitized electrospun fiber/monolayer graphene into SEBS elastomer; right: such electrode as a robust and non-disposable skin-electrode for sEMG, ECG, and EEG detection; reproduced with permission from Ref. 1455, Copyright 2020 American Chemistry Society. (e) Optical images of the on-skin bioelectronic sensing systems, including electrophysiological sensors, hydration sensors, and temperature sensors; reproduced with permission from Ref. 1462, Copyright 2018 John Wiley and Sons. (f) Scheme of the MoS2 /Au junction measured using Kelvin probe force microscope (KPFM); reproduced with permission from Ref. 1473, Copyright 2020 John Wiley and Sons. (g) Left: a schematic layout of the ultrasound detector combined with the flexible MoS2 FET and piezoelectric device based on P(VDF-TrFE); right: the pulsed-gate switching responses of the ultrasound detector under the applied frequencies of 10 kHz; reproduced with permission from Ref. 1474, Copyright 2021 John Wiley and Sons."

Fig 58

(a) Schematic illustration of adsorption mechanisms by GO; reproduced with permission from Ref. 1511, Copyright 2020 American Chemical Society. (b) Schematic showing the formation of GA-500, inset: snapshots of stepwise fabrication process; reproduced with permission from Ref. 1513, Copyright 2018 John Wiley and Sons."

Fig 59

(a) Schematic illustration of solar-thermal steam generator based on GO film; reproduced with permission from Ref. 1499, Copyright 2016 National Academy of Sciences. (b) The temperature raise for various solar-thermal steam generation materials under solar irradiation of 0.1 W?cm-2; reproduced with permission from Ref. 1522, Copyright 2021 John Wiley and Sons. (c) Schematic illustrations of the separation mechanism of 2D material membranes; reproduced with permission from Ref. 1524, Copyright 2020 John Wiley and Sons. (d) The size-dependent molecular sieve based on GO lamellar; reproduced with permission from Ref. 1497, Copyright 2014 Association for the Advancement of Science. (e) Comparison of the long-term operation times for state-of-the-art two-dimensional material membranes applied in water separation; reproduced with permission from Ref. 1529, Copyright 2020 John Wiley and Sons. (f) CNT-assisted plasma perforation on CVD-growth graphene can provide a desalination membrane with high salt rejection and fast water permeation; reproduced with permission from Ref. 1532, Copyright 2019 Association for the Advancement of Science."

Fig 60

(a) External force driven assembly approach for fabricating 2D channels, (b) H2 /CO2 separation performance of EFDA-GO membranes compared with state-of-the-art gas separation membranes; reproduced with permission from Ref. 1538, Copyright 2016 American Chemical Society. (c) Influence of temperature on GO-piperazine (GOP) hollow fiber membrane performance for mixed gas (CO2 : N2 =15% : 85%, volume fraction) separation under the wet condition 1539. (d) H2 /CO2 separation performance of the MXene membrane compared with state-of-the-art gas separation membranes; the black line indicates the Robeson 2008 upper bound of polymeric membranes for H2/CO2 separation, and the orange dashed line represents the 2017 upper bound of the best current membranes for H2 /CO2 separation 1540. (e) Schematic of the organic/water (butanol/water) separation process using the synergistic effect of a hydrophilic polymer and GO laminates, (f) comparison of GO membranes and the CS@GO membrane with state-of-the-art membranes for water/butanol dehydration; reproduced with permission from Ref. 1547, Copyright 2015 John Wiley and Sons."

Fig 61

(a) Effect of pH on the separation for NDNG-1 membrane, (b) permeation flux of REEs as a function of the NDNG-1 membrane thickness 1555. (c) Schematic diagram Zn-BTC MOF/nanoporous graphene in the separation of REEs, (d) adsorption of ZnO/NG, MOF, and MOF/NG to RE3+ (Ce) at the same time (160 min) and pH (2.03); reproduced with permission from Ref. 1556, Copyright 2021 American Chemical Society. (e) Effect of concentration on the separation for GO nanoparticles (GONPs), (f) effect of solution chemistry on the separation for GO nanoparticles (GONPs); reproduced with permission from Ref. 1559, Copyright 2014 American Chemical Society."

Fig 62

(a) Schematic of proton transport mechanism. Bottom right: proton conductivity through varies 2D materials with distinct electron density in their lattice 1565. Typical applications of proton transport: (b) proton exchange membrane fuel cells made by few-layer h-BN, reproduced with permission from Ref. 1568, Copyright 2018 American Chemical Society; (c) comparison of 2D material-based H/D separation membrane performance with other techniques for heavy water separation 1569; (d) isolating hydrogen gas using h-BN 1570."

Fig 63

(a, b) Schematic illustration of the method for planar stacking composites and the transverse shear method for scrolled nanocomposite fiber; reproduced with permission from Ref. 1598, Copyright 2016 Association for the Advancement of Science. (c, d) The TEM image of B4C-NWs@graphene, (e) flexural stress-strain curves of epoxy and B4C-NWs@graphene (0.1%, 0.2%, and 0.3%, volume percentage) reinforced composites 1600."

Fig 64

(a) The oxygen transmission rate (OTR) plot against coating gap, (b) the water vapor transmission rate (WVTR) plot against PET, 10% (w) PVA-C, and 10%-60%LDH-C 1606."

Fig 65

The epitaxial growth of thin films vs. 2D materials. (a) Schematic of the chemical binding between a thin film and a substrate. (b) Illustration of a polycrystalline thin film growth on a polycrystalline substrate. (c) Schematic of the weak vdW interaction between a 2D material and a substrate. (d) Illustration of the growth of a single crystalline 2D material on a polycrystalline substrate via nucleation control. (d) Reproduced with permission from Ref. 1633. Copyright 2012 American Chemical Society."

Fig 66

The interaction between a 2D material and a substrate. (a) Model of a graphene zigzag edge on a Cu(111) surface and the binding energy of the edge on the substrate as a function of the alignment angle, θ. (b) Charge density differences with isovalue of 0.01 Bohr-3 of graphene on the Cu(111) surface with misorientation angles of 0° and 15.3°, where electron accumulation and depletion are denoted by Blue and red colors 1640."

Fig 67

Alignment of 2D islands with various symmetries on the three low-index FCC crystal surfaces and the hBN (0001) surface. The high-symmetry crystallographic orientations of substrates are denoted by green lines. The symmetry groups of the 2D materials, substrates, and the systems of 2D islands on substrates are provided. Reproduced with permission from Ref. 1625.Copyright 2021 American Chemical Society."

Fig 68

Schematic of parallelly aligned 2D materials with different symmetries grown on a high-index substrate with steps 1640."

Fig 69

Alignment of graphene on high-index substrates. (a) Formation energies of various graphene edges attaching to a Cu < 110> step edge as a function of the graphene edge's tilt angle, θ, where θ = 0 and 30 degrees represent zigzag and armchair edges, respectively. (b) On a vicinal Cu(111) surface, all the nucleated graphene islands, either nucleated on the Cu(111) terrace or nearby the possible step edges, could be parallelly aligned. Reproduced with permission from Ref. 1660. Copyright 2014 American Chemical Society."

Fig 70

Alignment of h-BN on high-index metal substrates. (a) Schematic of hBN on various Cu surfaces and the configurations of various vicinal FCC(100) surfaces. (b-c) Schematic and experimental observation of triangular h-BN clusters on different terraces of the Ni(111) substrate. (d-e) Schematic and experimental observation of triangular h-BN clusters on different terraces of the Ru(0001) substrate. (f-g) Model of a h-BN zigzag edge attaching to the Cu < 211> step of a vicinal Cu(110) surface and the experimental observation of h-BN epitaxial growth on vicinal Cu (110) surface with < 211> step edges. (h-i) Model of h-BN clusters attaching to Cu < 110> steps of a vicinal Cu(111) surface and the experimental observation of h-BN epitaxial growth. (j) Schematic of a tilted hBN edge attaching to a tilted step edge, where the kink heights of the h-BN edge and the step edge are shown. (k-m) Illustration of experimentally observed h-BN orientations on various high-index Cu surfaces, which agree well with theoretical predictions. (n) Schematic of h-BN alignment along a tilted step edge. (o-p) The alignment of h-BN islands on various high-index Cu surface under the same surface roughness. (a, j-m, o-p) Reproduced with permission from Ref. 1655, Copyright 2021 John Wiley and Sons. (b-e) Reproduced from Ref. 1665. (f-g) Reproduced from Ref. 214. (h-i) Reproduced from Ref. 99. (n) Reproduced with permission from Ref. 1667, Copyright 2019 American Chemical Society."

Fig 71

Growth mechanism of TMD on various substrates. (a) First principal molecular dynamic (MD) simulations showing that the nucleation of T-phase MoS2 on the Au(111) surface is preferred and the transformation from T-phase to H-phase will occur during the further growth process. (b) Schematic of the sulfidation of a MoO3 surface to synthesize a MoS2 film. (c) Close-up of Mo-S termination and Mo-S-Mo bridge structures created during reactive MD simulation. (a) Reproduced with permission from Ref. 1672, Copyright 2021 American Chemical Society. (b, c) Reproduced with permission from Ref. 1677, Copyright 2017 American Chemical Society."

Fig 72

Growth of graphene polycrystals on liquid Cu surface. (a) Formation energy of graphene grain boundary as a function of the misalignment angle of the two graphene grains. (b) Schematic showing two routes of graphene coalescence on liquid Cu surface, a single crystal via seamless stitching of two aligned graphene islands or a graphene polycrystal with a 30-degree GB. (c) The prediction and corresponding experimental observation of graphene polycrystals via coalescence of multi graphene islands on liquid Cu surfaces. Reproduced with permission from Ref. 1679, Copyright 2019 John Wiley and Sons."

Fig 73

The mechanism of graphene growth on insulating substrates. (a) Summary of the experimental data (growth rates and sizes of grown graphene islands) of graphene growth on the insulating substrates and metal substrates. (b) Schematic of vapor-solid growth of graphene on insulating substrates. (c) Energy profile and reaction geometries of the threshold step for a graphene zigzag and armchair edge. (d) Morphology of a growing graphene island on the insulating substrates obtained by the kinetic Wulff plots in theory. (e) Comparison of the graphene growth rate between experimental results and calculated results based on the experimental conditions and Eq. (3). Reproduced with permission from Ref. 1686, Copyright 2021 American Chemical Society."

Fig 74

(a) Light induced oxidation: the light-induced ambient degradation process of BP and the electronic structures of BP with the redox potential of O2/O2-; reproduced with permission from Ref. 1700, Copyright 2016 John Wiley and Sons. (b) Water catalyzed oxidation: the polarization effect of H2O on O2 illustrated by theoretical calculations; reproduced with permission from Ref. 1702, Copyright 2017 John Wiley and Sons. (c) Defect induced oxidation: schematic of the degradation mechanism for InSe; reproduced with permission from Ref. 1703, Copyright 2017 American Chemical Society. (d) Several proposed protection strategies by theoretical calculations and simulations; reproduced with permission from Ref. 1700, 1704-1706, Copyright 2016 John Wiley and Sons, Copyright 2016 American Chemical Society, Copyright 2017 Royal Society of Chemistry."

Fig 75

(a) Band structure and partial density of states for single-layer MoS2 with an SV. The localized states are highlighted by red lines; (b) schematics of electron transport mechanism in perfect and defective MoS2 750. (c) Two competing Reaction mechanisms between a thiol molecule and SV-MoS2, energy units are in kJ?mol-1 and the distances are in ?; reproduced with permission from Ref. 1722, Copyright 2017 John Wiley and Sons."

Fig 76

(a) Calculated free energy change for each reaction of OER on different SACs@β12-BM as a function of the d band center; reproduced with permission from Ref. 1730, Copyright 2017 American Chemical Society. (b) Calculated ΔGO-ΔGOH on Ti3C2O2-based SACs catalysts as a function of the corresponding DF (defined as $D_{\mathrm{F}}=\frac{\int_{-\infty}^{+\infty} E D(E) W(E) \mathrm{d} E}{\int_{-\infty}^{+\infty} D(E) W(E) \mathrm{d} E}$, where D(E) and E are the DOS and energy of d states of transition metal atoms, and W(E) is the weight factor 1728. (c) Simplified schematic of N2 bonding to transition metals and B atom with sp3 hybridization; reproduced with permission from Ref. 1733, Copyright 2018 American Chemical Society."

Fig 77

Energy difference between AFM and FM spin configurations of bilayer CrI3 as a function of (a) on-site U, (b) interlayer stacking orders and (c) vdW functional corrections. (a) Reproduced with permission from Ref. 1745, Copyright 2019 American Physical Society. (b) Reproduced with permission from Ref. 1748, Copyright 2018 American Chemical Society. (c) Reproduced with permission from Ref. 1750, Copyright 2019 American Physical Society."

Fig 78

(a) Bethe-Slater-curve-like behavior in bilayer CrSe2; reproduced with permission from Ref. 1759, Copyright 2020 American Physical Society. (b) Skyrmions in the Moire? of vdW 2D Magnets composed by a FM monolayer on an AFM substrate; reproduced with permission from Ref. 1761, Copyright 2018 American Chemical Society. (c) 2D multiferroicity in vdW CrGeTe3/In2Se3 heterostructure 572. (d) Evolution of total valley splitting (ΔK'K, upper panels) and valance and conduction band valley splitting (ΔVB and ΔCB, bottom panels) for different displacements along lattice a of CrI3/WSe2 heterostructure; reproduced with permission from Ref. 1763, Copyright 2019 American Physical Society."

Fig 79

(a) Tensile strain switched ferromagnetism in layered NbS2 and NbSe2; reproduced with permission from Ref. 1769, Copyright 2012 American Chemical Society. (b) Bilayer CrI3 based spin field-effect transistor with generation of spin-polarized current controlled by an electric field; reproduced with permission from Ref. 1771, Copyright 2020 American Chemical Society. (c) Band structures of monolayer CrI3 with magnetic moment along the out-of-plane and in-plane, respectively; reproduced with permission from Ref. 1777, Copyright 2018 American Chemical Society. (d) Schematic plot of optically modulated magnetism in RuCl3 monolayer; reproduced with permission from Ref. 565, Copyright 2019 American Chemical Society."

Fig 80

(a) Schematic diagram of the search procedure for 2D FM materials; reproduced with permission from Ref. 1783, Copyright 2018 American Physical Society. (b) Quantum anomalous Hall state in FM monolayer RuCl3, including atomic structure (upper left) and band structure with (blue line) and without spin orbital coupling (gray line) (upper right) of monolayer RuCl3, Berry curvature in first Brillouin zone (left lower), and band structure of RuCl3 nanoribbon with a width of 386 ? (low right); reproduced with permission from Ref. 1784, Copyright 2018 American Chemical Society. (c) Magnetic topological materials 1786."