Air-Stable, Large-Area 2D Metals and Semiconductors

Abstract

Two-dimensional (2D) materials are popular for fundamental physics study and technological applications in next-generation electronics, spintronics, and optoelectronic devices due to a wide range of intriguing physical and chemical properties. Recently, the family of 2D metals and 2D semiconductors has been expanding rapidly because they offer properties once unknown to us. One of the challenges to fully access their properties is poor stability in ambient conditions. In the first half of this Review, we briefly summarize common methods of preparing 2D metals and highlight some recent approaches for making air-stable 2D metals. Additionally, we introduce the physicochemical properties of some air-stable 2D metals recently explored. The second half discusses the air stability and oxidation mechanisms of 2D transition metal dichalcogenides and some elemental 2D semiconductors. Their air stability can be enhanced by optimizing growth temperature, substrates, and precursors during 2D material growth to improve material quality, which will be discussed. Other methods, including doping, postgrowth annealing, and encapsulation of insulators that can suppress defects and isolate the encapsulated samples from the ambient environment, will be reviewed.

Figure 1

(a) Summary of 2D metals prepared via confined growth between SiC and graphene. (b) Schematics of CHet process. (c, d) XPS and Raman evolution during CHet process, respectively. The defects generated from plasma etching are healed after metal intercalation in the interface, which will prevent oxidation of metals. (e, f) XPS spectra of Ga intercalated EG postsynthesis (within several days) and >8 months later, indicating the excellent air-stability of 2D Ga synthesized via CHet. (b–g) Adapted with permission from ref (77). Copyright 2020 Springer Nature.

Figure 2

(a ,b) Second derivative images of occupied and unoccupied electronic structures of monolayer Ag, respectively. Adapted with permission from ref (79). Copyright 2022 American Chemical Society. (c, d) Graphene and Au band dispersion after monolayer Au intercalated in EG/SiC, respectively. (e,f) Graphene and Au band dispersion after bilayer Au intercalated in EG/SiC, respectively. (c–f) Adapted with permission under a Creative Commons CC-BY License from ref (80). Copyright 2020 Springer Nature. (g, h) Superconducting properties of 2D Ca and Ga, respectively.^, Adapted with permission from refs (77) and (85). Copyright 2020 Springer Nature and 2022 American Chemical Society, respectively. (i) Ultralow frequency fingerprint of 2D metals and Ga/Ag with different phases. Adapted with permission from ref (87). Copyright 2021 IOP publishing. (j) Nonlinear spectra and of χ⁽²⁾ values for 2D metals and 3D gold nanoparticles. Adapted with permission from ref (90). Copyright 2020 American Chemical Society.

Figure 3

(a) Calculated heat of formation for monolayers in the 2H and 1T phases with selected transition metals. In all chalcogen elements, oxide provides the highest stability. Adapted with permission from ref (16). Copyright 2015 American Chemical Society. (b) TEM and SAED images of an oxidized few-layer HfS₂. The oxidized HfS₂ appears to be polycrystalline. Adapted with permission from ref (96). Copyright 2016 American Chemical Society. (c) Kinetics of native oxide formation on freshly cleaved ZrS_xSe_2–x with x from 0 to 0.3, 0.6, 1.14, 1.51, 2, and MoS₂ crystals were studied with spectroscopic ellipsometry measurements. Oxide thickness versus exposure time plots for ZrS_xSe_2–x and MoS₂ crystals show that Zr–S and Zr–Se switch to Zr–O bonds rapidly under ambient conditions. Adapted with permission from ref (98). Copyright 2020 American Chemical Society. (d) Optical images of 1T-MoTe₂ exposed in air after its growth and after 10 min, 1 h, and 3 h. A clear dimming of the contrast inside the dashed red box is visible and corresponds to about 92% contrast intensity loss in 3 h. Adapted with permission from ref (99). Copyright 2021 American Chemical Society. (e) Atomic-resolution STM images (5 mV, 2 nA) of an exfoliated MoS₂ monolayer on Au after 1 month (left) and 1 year (right) of ambient exposure, revealing a progressive defect formation. (f) Reduction of 2D oxidized MoS₂ to pristine MoS₂: Representative atomic-resolution STM images (5 mV, 2 nA) of 2D MoS_2–xO_x before (left) and after (right) 30 min annealing with H₂S at 200 °C, showing the reduction of the oxy-sulfide solid solution to the pristine MoS₂ through resubstitution of O by S. (e, f) Adapted with permission from ref (101). Copyright 2018 Springer Nature.

Figure 4

(a) Schematic of hydroxide vapor phase deposition (OHVPD) growth of WS₂ monolayers. (b) Low-temperature PL spectra and STM topography (inset) of CVD- (top) OHVPD- (bottom) WS₂ monolayers provide the comparison of their surface defect density and defect-bound exciton. The solid lines and dashed ones are the experimental and fitted peaks, respectively. The fitted peaks can be assigned to neutral exciton (X₀), trion (X_T), and defect-bound exciton (X_D). Adapted with permission under a Creative Commons CC-BY License from ref (108). Copyright 2022 Springer Nature. (c) Illustration of the growth of ML Re-MoS₂ using metalorganic precursors. Inset: a camera image of a Re-MoS₂ film on sapphire. (d) Statistic for sulfur vacancy sites in MoS₂ and Re-MoS₂ based on TEM study. (e, f) Point defect density. (e) MoS₂, and (f) 5 atom % Re-MoS₂ indicates their S-site defect (marked with red circles) densities using Z-contrast STEM image. In (e), there are 34 single-sulfur vacancies and 11 double-sulfur vacancies while in (f), there are 3 single-sulfur vacancies. The brighter atoms in (f) are Re due to its larger Z number. (g) DFT model of sulfur vacancy formation energy as a function of the distance between the sulfur vacancy at the edge of MoS₂ model and the Re position moving away from the edge. The corresponding energy values as a function of the Re position are listed. (c–g) Adapted with permission from ref (110). Copyright 2023 American Chemical Society.

Figure 5

(a) Transfer characteristics for undoped, H₂S annealed, and 0.1 atom % Re-MoS₂ FETs measured in ambient conditions and their (b) statistical analysis of V_TH at V_DS = 1 V The red horizontal line in each graph marks I_DS at 10^–5 A/mm. (a, b) Adapted and modified with permission from ref (110). Copyright 2023 American Chemical Society. (c) Time-dependent study (pristine, 2 weeks, and 6 months) of transfer characteristics of a top-gated MoS₂ FET using 24 nm Al₂O₃ as the encapsulation and dielectric. Inset shows the cross-sectional view of the device. Adapted with permission from ref (115). Copyright 2022 Elsevier. (d) Variation of the current flowing over 150 min through a single MoTe₂ crystal exposed in air (red dots) and encapsulated with hBN (black dots) at V_DS = 0.1 V. Adapted and modified with permission from ref (99). Copyright 2021 American Chemical Society. (e, f) Structure of (Bi,Sb)₂Te₃/graphene/gallium (BST/Gr/Ga) thin films. Cross-sectional TEM shows the clean interface in the heterostructures. (e, f) Adapted with permission from ref (116). Copyright 2023 Springer Nature.