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

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.


INTRODUCTION
Two-dimensional (2D) materials have an essential place in nanoscience and technology because they have a wide range of compositions and properties across the full range wavelength from near-infrared to deep ultraviolet. 1,2The successful development of graphene and hexagonal boron nitride (hBN) monolayers encouraged extensive research on other 2D materials, such as transition metal dichalcogenides (TMDCs), 3 and elemental 2D materials like phosphorene. 4ne reason for the early success of graphene and hBN is their excellent thermal stability in ambient conditions, 5,6 which eases the environmental conditions and enables rapid exploration of their properties and applications.While graphene (hBN) can be produced in a very large area with high quality, its semimetallic (insulating) band structure offers only a few applications in the visible wavelength range if they are not integrated with other semiconducting materials.Therefore, in the past decade, 2D materials research has continuously shifted to "beyond graphene (or hBN)" 2D materials 7 including TMDC monolayers with electronic bandgaps. 8ecently, 2D metals have attracted tremendous research enthusiasm in exploring their novel fundamental physics phenomena and fascinating physiochemical properties.Some nonmagnetic elemental metals are predicted to exhibit magnetism in their 2D forms due to decreased coordination number and energy band narrowing in the out-of-plane orbitals. 9One-atom-thick Pb and In films are experimentally verified to be superconducting. 10Furthermore, their superconductivity in monolayer films originates from electronphoton interactions provided by both interlayer metallic and, more importantly, metal-substrate bonds.−14 All indicate that 2D metals have promising potential in quantum devices and spintronics, catalysis, sensing, energy storage, etc.However, compared to graphene, many beyond-graphene 2D materials are inferior in ambient stability because their constituents can react with water or oxygen and produce oxide compounds. 15Therefore, to fully utilize their potential, it is necessary to identify stable 2D materials and their stability via theory, 16 understand the roles of defects and reactivity of 2D materials in surface oxidation, 17 and establish strategies to preserve the quality of metastable 2D materials.For example, surface passivation/ encapsulation is the most common measure against surface degradation for air-sensitive materials, as it has been demonstrated with graphene and hBN on metal surfaces. 6,18,19emoving surface defects and grain boundaries is also important since they likely will draw oxygen and carbon impurities to attach without being passivated.
Here, we will review the recent progress in making large-area air-stable atomically thin metals and TMDCs by bottom-up and top-down techniques.First, we provide a brief overview of 2D metal research and introduce confined heteroepitaxy for the intercalation of a series of 2D transition and group-III to -V metal elements at the epitaxial graphene/SiC interface that isolates the atomically thin metals from the ambient environment. 20,21Due to graphene's transparency, we can characterize the intrinsic properties of "half-van der Waals" 2D metals through the topmost encapsulating epitaxial graphene.Next, the recent development of scalable, high-quality 2D TMDC semiconductors grown by chemical vapor deposition (CVD) will be reviewed.First, we briefly review the stability of 2D TMDCs with different chalcogen elements (O, S, Se, and Te).Second, we show that, by controlling the parameters of CVD and choosing appropriate precursors and templates, grain boundary and intrinsic defects of deposited TMDC films can be suppressed and, in turn, their ability against oxidation and impurity incorporation can be improved.At the end of this review, we will discuss their future opportunities and possible applications of integrating both materials.

Making Air-Stable 2D Metals
Due to the nondirectional metallic bonding, monoelemental metals or alloys always prefer to form three-dimensional (3D) structures, resulting in a big challenge to synthesize their twodimensional (2D) counterparts. 22,23−60 For instance, solution-based chemical methods, as the most popular methods to synthesize 2D metals for sensing, catalysis, and other applications due to their high yield and low cost, can offer reliable approaches via complex chemical procedures to prepare metal films with mono/multiple atomic thickness. 11,61owever, the lateral size of 2D metals is always less than micrometers which limits their applications.Additionally, the surfactants or solution used in the procedure may functionalize these metal films, resulting in the degradation of their properties.Notably, all the aforementioned methods have their own merits but share the same shortcoming that 2D metals are sensitive to the ambient atmosphere and can be easily oxidized. 62Although postdeposition or coating of the protective layer can isolate metal films from air or chemical environments, the additional step will bring up more issues in terms of accessibility of 2D metals, contamination, and cost.Owing to their impermeability to liquid and gas, chemical and thermal stability, graphene and its derivatives are widely explored as corrosion protecting coatings on metals. 63oreover, graphene is well-known as an optically transparent film for optical devices. 64,65Recent works also verified that graphene could act as a transparent layer to the potential field of many substrates, which enables remote epitaxial growth of III−V semiconductors, 66,67 halide perovskites, 68 and others. 69,70−73 The buffer layer is underneath graphene and partially bonded to SiC.When epitaxial graphene (EG) on SiC is annealed in H 2 , H atoms will penetrate through defects and boundaries in EG and intercalate into the space between the buffer layer and SiC, decoupling the buffer layer from SiC and forming an additional graphene layer. 74,75In this way, the intercalants will be sealed in the interface and isolated from ambient by capping graphene.Inspired by the hydrogen intercalation at EG/SiC interface, many efforts have been made to intercalate metal atoms into the EG/SiC interface to form confined 2D metals.−117 First, metal atoms are deposited on the buffer or EG surface via thermal or E-beam evaporation, sputtering, or MBE.Then metal/EG/SiC is annealed under vacuum and metal atoms will enter EG/SiC interface.−78 Taking advantage of in situ characterization methods, such as scanning tunneling microscopy (STM), low-energy electron diffraction (LEED), and angle-resolved photoemission spectroscopy (ARPES), the structures of 2D metals and band structure evolution of EG/2D metal heterostructures have been extensively explored.For example, Rosenzweig et al. revealed that intercalated Yb at 250 °C forms a disordered phase and transitions to a 3 ) 30 × °configuration with respect to the graphene layer when intercalating at 450 °C. 76oreover, Yb intercalation can induce an extreme n-type doping level of 3.5 × 10 14 cm −2 in EG, which is high enough to reach van Hove singularity.However, 2D metals prepared by confined growth under UHV are always patches with lateral scale less than micrometers due to the limitations of defect density.
To synthesize large-scale, single-crystal, and environmentally stable atomically thin metals, a growth strategy, dubbed "Confinement Heteroepitaxy (CHet)", was developed to synthesize 2D metals. 77In CHet (shown in Figure 1b), atomic vacancies are introduced in epitaxial graphene by O 2 plasma treatment.Elemental metals are subsequently intercalated into the EG/SiC interface through these vacancies.Finally, these defects will be healed during the process, and graphene becomes a confining layer for the intercalated 2D metals.XPS and Raman spectroscopy data (Figure 1c, d) can be used to describe the process.XPS spectra indicate the formation of C�O, C−OH, and C−O−C bonds in the C 1s spectrum after O 2 plasma treatment.In addition, the D band of graphene Raman spectra arises after the plasma treatment due to defect formation.After metals are intercalated, the oxygen functional groups attached to graphene are removed and the graphene D bands are suppressed, indicating the quality of graphene is recovered by the end of the CHet process.Like H intercalation, decoupling of Si−C and C−C bonds in the C 1s spectrum also indicates the buffer layer is released by metal intercalation.After examining the XPS spectrum of a Ga intercalated EG sample placed in ambient for 8 months, 2D Ga prepared via CHet shows minimal change, which reveals the excellent air-stability of CHet-synthesized 2D metals.The summary of recent progress in fabrication of 2D metals via CHet and confined growth under UHV is shown in Figure 1a.Furthermore, the properties of these air-stable 2D metals prepared via confined growth will be discussed in the following sections.

Electronic Properties.
Compared to their 3D counterpart, the confined 2D metal layers between EG/SiC interface display intriguingly electronic properties due to their half-vdW structure, in which the bonding character transits with high internal gradient from covalent bonding between bottom metal layer and SiC, to interlayer metallic bonding and to vdW bonding between top metal layer and EG.Exemplarily, first-principles calculations have predicted that monolayer silver (Ag) and gold (Au) are semiconductors owing to hybridization with the SiC surface. 78Recently, Lee et al. experimentally measured the band gap of monolayer Ag confined between bilayer graphene and SiC after the CHet process. 79According to static-ARPES data on occupied band dispersions of monolayer Ag (Figure 2a, b), the valence band maximum (VBM) is identified at K̅ Ag with a binding energy of ∼0.45 eV below the Fermi level (E F ). Then the unoccupied band dispersions near Γ̅ revealed by time-resolved ARPES shows the conduction band minimum (CBM) is identified at ∼0.56 eV above E F .Therefore, the monolayer Ag is verified to be a semiconductor with an indirect band gap of ∼1 eV, which is significantly higher than the earlier density functional theory (DFT) prediction of 0.2 eV.Forti et al. also experimentally verified that monolayer Au confined at the EG/SiC interface is a semiconductor for which the VBM is located 50 meV below E F (Figure 2c−f). 80The monolayer Au acts as the electron donor to graphene and drives the Dirac point E 0 to 700 meV under E F .Notably, 2D semiconducting Au would become metallic when the thickness of Au increases by one atomic layer, lifting the Dirac cone of graphene 150 meV above E F .
2.2.2.Superconductivity.While pristine EG on SiC and bulk metals exhibit poor superconductivity, some 2D metals confined in EG/SiC exhibit enhanced superconductivity.Ca intercalated EG is one of these systems that has stimulated considerable research interest.−85 Wang et al. revealed that the emerging superconductivity in C 6 CaC 6 is mainly due to the free-electron-like interlayer band merging into π* bands around E F . 85Notably, Figure 2g shows that the critical temperature at 8.83 K is obtained via intercalating Ca into the top two graphene layers in a trilayer EG system and can be attributed to the suppression of charge-density wave by screening C 6 CaC 6 film from the buffer layer with a graphene layer.Briggs et al. developed CHet to successfully obtain 2−3 layers of Ga, which is covalently bonded to the bottom SiC and has vdW mediated bonds with the graphene overlayer. 77The novel "half-vdW" Ga is verified to have a 4 times higher critical temperature close to 4 K than its bulk counterpart, as shown in Figure 2h.

Optical Properties.
Raman spectroscopy is one of the most important characterization techniques, which can supply abundant information on 2D materials including layer numbers, interlayer coupling and layer stacking configuration. 86Wetherington et al. 87 explored the unique lowfrequency (LF, < 100 cm −1 ) Raman features of 2D polar metals (Ag, Cu, Pb, Bi, Ga, In) and alloys (In x Ga 1−x ) synthesized via CHet, displayed in Figure 2i. 123It is revealed that these LF Raman features are related to metal atoms stabilized in the interface and also the phase of 2D metals.Taking 2D Ga as an example and combining with DFT studies, the origination of these peaks contributes to the surface resonance mode coupled to SiC phonons in EG/2D metal/SiC system. 88,89The covalent bond between the bottom 2D metal layer and SiC also plays a significant role in the resonant Raman enhancement of the shear modes.−92 Steves et al. reported that 2D Ga and In are two of the most efficient second harmonic generation systems, with room-temperature near-infrared χ (2) values approaching 10 nm/V. 90The surprisingly large χ (2) value, which is over 100× larger than 3D gold nanorods, graphene-coated Au films, and LiNbO 3 , is due to symmetry breaking in atomically thin 2D metal films as a result of the bonding environment transition from the covalent bonds at the SiC/2D metal interface to the vdW ones at the 2D metal/graphene interface (Figure 2j).

Improving Air-Stability of 2D Semiconductors
2.3.1.Previous Investigations.Two-dimensional (2D) semiconductors are promising candidates for next-generation transistors and diodes because they can overcome the fundamental material and device challenges that Si is facing when the channel body needs to be thinner than 5 nm. 93To adopt them in practical optoelectronic applications in the foreseeable future, 2D semiconductors that can be synthesized and stable enough in ambient conditions should be first to be considered.There is a large variety of 2D semiconductors with bandgaps ranging from the infrared to the ultraviolet, including TMDCs, 3 elemental monolayers (phosphorene and silicene), 4 and group III−VI metal chalcogenides (GaSe and InSe). 94omputational study is helpful for understanding the ambient stability of some of 2D semiconductors. 16,95Rasmussen et al. 16 tested 2H-and 1T-phase TMDCs with a variety of combinations of transition metal and chalcogen atoms using density functional theory calculations (Figure 3a).Taking Hfand Zr-based TMDCs as an example, their formation energies indicate that their oxide form is the most stable.Compared to other transition metals, their formation energy with oxide is more negative than that of other transition metals, such as W and Mo.In 2016, Chael et al. already observed oxidation on HfS 2 crystals under ambient conditions within a few hours and found that oxidation starts from edge part of HfS 2 in TEM experiments (Figure 3b). 96The work by Mleczko et al. 97 found that exfoliated ZrSe 2 and HfSe 2 crystals oxidize rapidly even in a diluted O 2 environment to form ZrO x /ZrSe 2 and HfO x / HfSe 2 .The rapid oxidation of ZrS x Se 2-x alloys was investigated with real-time spectroscopic ellipsometry measurements (Figure 3c) and molecular dynamic simulations by Jo et   They also found that oxidation of ZrS x Se 2-x exacerbates with increasing Se fraction.Taking MoTe 2 in H-and T-phase as another example in Figure 3a, they are less stable compared to their MoO 2 counterparts and can be oxidized faster than MoS 2 and MoSe 2 in ambient conditions.Pace et al. 99 synthesized 1T-MoTe 2 on SiO 2 /Si and used optical microscopy (OM) image contrast to observe real-time oxidation of MoTe 2 flakes in air (Figure 3d).They saw noticeable optical contrast change and increased roughness in air within 10 min and severe material degradation in 3 h.The above examples show that layered materials may be driven to oxidation by their thermodynamic instability even when they are made with the highest quality. 137asmussen et al. 16 also reported that the formation energies of group-VI TMDC including MoS 2 , MoSe 2 , WS 2 , and WSe 2 are not largely different from their oxide form, indicating their oxidation may not occur as rapidly as Zr-and Hf-based TMDC.Their excellent stability was experimentally confirmed by their device performance in ambient conditions in numerous works. 100Jo et al. demonstrated the evidence of the air stability via spectral ellipsometry, no spontaneous native oxidized layer was observed over 6 days, and they conducted XPS measurement on MoS 2 cleaved for over a year and found that no elevated oxygen levels were observed. 98However, a surface science study performed on group-VI TMDC indicates that they are not always free from oxidation.Petőet al. 101 studied surface oxidation of MoS 2 and MoSe 2 in a year with scanning tunneling microscopy found that oxygen gradually reacts with MoS 2 surface to form volatile gaseous SO 2 and heavily oxidize MoS 2 into MoO x over time (Figure 3e) while MoSe 2 remains mostly intact due to a high energy barrier for SeO 2 formation.In addition, oxidation and impurity substitution can occur at chalcogen vacancies and grain boundaries where chemical reactivity is high. 102Petőet al. 101 also demonstrated that the surface oxidation and point defects of 2D MoS 2 can be healed by conducting thermal annealing with H 2 S at 200 °C (Figure 3f), which can be integrated into the thin film processes of these 2D semiconductors.Despite the DFT calculation guideline revealing that the stability of sulfur-based TMDC is better than selenium-based TMDC, the formation energy of S−O and Se−O bonds also needs to be carefully considered when exposed in an ambient condition.

Bottom-Up Approaches to Improve the Air Stability of 2D
Semiconductors.Among all 2D semiconductors, W-and Mo-based 2D TMDCs have been at the center of the research and development for the implementation of practical 2D semiconductors because they have intriguing properties including high ON/OFF ratios, excellent flexibility, and direct bandgaps and can grow on various substrates.Although there are other stable TMDC semiconductors like PdSe 2 103 and PtSe 2 , 104 large-area controllable growth and characterization of group-VI TMDC films are so far the most advanced and mature.−107 Growth substrate plays an important role in making good and air-stable films.C-plane sapphire is an ideal template for MoS 2 and WSe 2 growth because it can help TMDC grow into a film with long-range order and minimize grain boundaries through vdW epitaxy. 107Precursor consideration is not trivial.For example, Eichfeld et al. 106 reported a MOCVD process for WSe 2 crystals, in which W(CO) 6 and (CH 3 ) 2 Se were chosen as the precursors to provide W and Se.However, the purity of the precursors and carbon from C 2 H 6 Se were found to impact the film quality drastically by promoting defect densities, grain boundaries, and impurities that will eventually oxidize.Therefore, for the next generation of MOCVD for WSe 2 , the Se supply was switched from (CH 3 ) 2 Se to H 2 Se gas that was proved to be cleaner and release less carbon during the process. 107ne reason that the electronic industry may prefer MOCVD over powder-based CVD is its scalability.For small-scale labbased research, powder-based CVD (PCVD) is still useful because it can provide prototypical experiments in a short time and is budget-friendly.The common precursors for PCVD of group-VI TMDC include MoO 3 , WO 3 , S, Se, and Te powders.However, TMDC flakes grown with oxide precursors typically have abundant defects and oxidation and ultimately exhibit insufficient performance for advanced devices.A recent result shows that using H 2 O as the transport agent during synthesis of WS 2 can provide positive outcomes.Wan et al. 108 developed hydroxide vapor phase deposition (OHVPD) that includes H 2 O in the growth of 2D WS 2 on sapphire (Figure 4a).The authors found that W−OH bond in the hydroxide intermediates produced from the reaction of H 2 O and a high-purity W foil makes WS 2 formation more energetically favorable than W−O bond from the use of WO 3 .Unlike the direct reaction of WO 3 and S vapors, the OHVPD relies on H 2 O vapors to slowly oxidize W foil and carry high-purity W clusters to reduce excessive oxygen and other impurities in the deposited WS 2 crystals.To compare the quality between traditional CVD-and OHVPD-grown WS 2 , STM and low temperature (4 K) photoluminescence (PL) experiments were performed on both samples (Figure 4b).The defect count carried out in their STM images (inset, Figure 4b) shows the total atomic defect number was reduced from near 21 000 in CVD-grown WS 2 to 8000 in the OHVPD-grown one.Furthermore, low-temperature PL spectra corroborate with the STM data, showing that the emission of defect-bound exciton (X D ) of OHVPD-grown WS 2 at 2 eV is suppressed by at least 4 folds and less emission of trions, compared to CVDgrown WS 2 .
Doping and alloying TMDC could help to reduce the chalcogen point defects of TMDC.Li et al. 109 conducted powder-based CVD using mixture of WO 3 and MoO 3 with predetermined ratios to synthesize W 1−x Mo x Se 2 and found that adding W into MoSe 2 can suppress the original Se vacancies by nearly half.STEM analysis performed on 5000 Se sites in the STEM images of MoSe 2 and W 0.18 Mo 0.82 Se 2 shows that the percentage of Se vacancies was reduced from (4 ± 0.06)% to (2 ± 0.08)%. 109This phenomenon was attributed to a stronger bonding strength and higher formation energy of Se vacancy in the presence of W in MoSe 2 . 110However, it was not rigorously verified.Low-temperature PL measurements were carried out on MoSe 2 and W 0.18 Mo 0.82 Se 2 to understand the impact of the W dopant on the optical properties.While doping W does not change the emission of free excitons and trions (band 1,2 at 1.6 and 1.58 eV, respectively), it reduces the emission of X D at around 1.5 eV by 4-fold. 109A probable mechanism for defect reduction in TMDCs enabled by dopant incorporation was recently examined in another work using Re-doped MoS 2 (Re-MoS 2 ).Torsi et al. 110 synthesized Re-MoS 2 via MOCVD using Re 2 (CO) 10 and Mo(CO) 6 on sapphire (Figure 4c).They produced Re substitutional doping ranging from 0.1 to 5 atomic percent (atom %) and found that the sulfur vacancy (S v ) can be reduced when Re is present during growth.The statistical study shows that the density can be reduced from 3 × 10 13 cm −2 in MoS 2 to 5 × 10 12 cm −2 in Re-MoS 2 .The TEM experiments found that the sample had a high density of singleand double-S vacancies in undoped MoS 2 (Figure 4e), while there's only few single sulfur vacancies were found when Re was introduced into the sample (Figure 4f).To explain the defect reduction that occurs during Re-MoS 2 growth, energetic models to study the influence of present Re in MoS 2 lattice to sulfur vacancy formation were constructed by DFT calculations (Figure 4g).The theory shows that the formation energy for single-sulfur vacancy is the largest when the Re is present at the nearest cation site and can apply to the actual growth in which Re attached to the growing edges of MoS 2 domains can make sulfur vacancy harder to incorporate into the edges.and a p-type material 112 and can form when oxygen fills into sulfur vacancies of MoS 2 . 113Figure 5a shows transfer characteristics of 3 types of MoS 2 FET that depend on backgate voltage (V BG ): undoped, annealed with H 2 S, and 0.1 atom % Re-doped MoS 2 under a drain voltage (V d ) of 1 V.The undoped MoS 2 FET has a smaller on-current density because its defect density is the highest without intentional defect suppression.H 2 S-annealed MoS 2 FET was made of undoped MoS 2 that was postgrowth annealed at 500 °C with H 2 S to heal sulfur vacancy with excessive sulfur.Compared to undoped MoS 2 FET, it has a slightly better on-current density and more negative threshold voltage (V th ) (Figure 5b); both should be considered as evidence for sulfur vacancy removal. 110,1130.1 atom % Re-MoS 2 FET also exhibits a V th distribution similar to the H 2 S-annealed FET and the highest on-current density, indicating that Re not only helps suppress sulfur vacancy (similar to the effect achieved by H 2 S postgrowth annealing) but also improves the contact resistance by electrical doping.Encapsulation is useful to isolate 2D materials from the ambient and can be conveniently integrated into device fabrication processes.To prevent the direct contact of metastable 2D materials such as ZrSe 2 , HfSe 2 , MoTe 2 and black phosphorus (BP) with the ambient, surface encapsulation with hBN or oxides can be deposited on the surfaces by atomic layer deposition (ALD) at low temperature (≤200 °C).An experiment by Kim et al. 114 observed the change in the sheet resistance (R SH ) of two BP flakes with and without Al 2 O 3 encapsulation in air over 1 week.Their result indicates the R SH of unencapsulated BP increases as a result of oxidation that occurs from the edge of the flake and propagates into the center of the flake.On the other hand, the BP whose edges and surface are encapsulated shows that its R SH remains constant over a week.In a recent study by Zhao et al., 115 a MoS 2 FET capped with 24 nm Al 2 O 3 prepared by ALD exhibits unchanged transport characteristics over 6 months (Figure 5c).This encapsulation can prevent 2D material surface oxidation and block water and oxygen intercalating into the device interfaces of 2D materials/dielectric 115 and/2D materials/substrate.There is a valid concern about using water in ALD of oxides (Al 2 O 3 and HfO 2 ) for encapsulation since there is a chance that water may react with surface defects of 2D materials even at room temperature.The alternative to oxide is hBN that can be prepared on 2D surfaces by transfer.Pace et al. 99 transferred CVD-grown hBN onto 1T-MoTe 2 devices to demonstrate the benefit of encapsulation (Figure 5d).The current monitored by two-terminal electrical measurement dropped 1000 times within 150 min without hBN encapsulation, while the current with encapsulation remained constant.BN can also be put down by pulsed lased positioning or CVD so that any contact with solvent can be avoided.

SUMMARY AND FUTURE WORK
In this Review, we discuss the instability of large-area 2D metals and 2D semiconductors in air and review the various approaches for improving their stability in air.The first part is focusing on 2D metals.The confined growth of 2D metals at graphene/SiC interface paves a way to synthesize air-stable 2D metals with unique optical and electronic properties.Due to the antioxidation of 2D metals under graphene, more ex situ characterization methods can be applied to explore more exciting properties.CHet process enables synthesis of millimeter-scale 2D metals, 77,79,117 including Ga, In, Ag, and Pb, which is vital to further device fabrication.Furthermore, the capping graphene layer is well-known as a good platform to integrate other materials to form novel heterostructures. 118,119ntegrating these air-stable 2D metals/EG systems with other materials may also lead to unexpected phenomena and performance.There are already some pioneering works in this field. 116,120For example, Li et al. 116 grew (Bi,Sb) 2 Te 3 on top of graphene/Ga by molecular beam epitaxy to create an epitaxial (Bi,Sb) 2 Te 3 /graphene/Ga heterostructure (Figure 5e) and verified the atomically sharp interfaces between each material with TEM (Figure 5f).Due to the protection of graphene, 2D Ga was not degraded after the growth of (Bi,Sb) 2 Te 3 .The proximity-induced superconductivity of (Bi,Sb) 2 Te 3 /graphene/Ga heterostructure was investigated with graphite and hBN-integrated tunnel junction devices and transport tunneling spectroscopy measurements. 116They found a robust proximity-induced superconducting gap in the Dirac surface states of topological insulating (Bi,Sb) 2 Te 3 films and the presence of Abrikosov vortices in tunneling conductance down to a single vortex in the heterostructure. 116dditionally, there are still a lot of 2D metals, which have been predicted to have intriguing properties, not experimentally synthesized.For example, DFT results show that some nonmagnetic 3D elemental metals (such as Ru, Pd and Ti) will become magnetic in their 2D counterparts due to coordination number decreases and energy band narrowing of the out-of-plane orbitals. 121These confined 2D monoelemental metals can also be a platform to prepare new confined 2D alloys, oxide and nitride.For instance, Rajabpour et al. 122 synthesized 2D In x Ga 1−x (0 < x < 1) alloys with controllable composition via changing the elemental concentration in the precursor during CHet.The 2D In x Ga 1−x alloys are verified to be continuous across microsize terraces of SiC and uniformly distributed in the interface without segregation.Notably, the dielectric function and superconductivity of 2D In x Ga 1−x alloys can be tuned by alloy compositions.
And for 2D semiconductors, group-VI 2D TMDCs have emerged as relatively air-stable 2D semiconductors and have been extensively studied.The presence of grain boundaries and point defects makes these materials sensitive to air, leading to rapid oxidation and thus decreasing stability.To address these issues, several strategies have been proposed.−127 Utilizing postgrowth annealing under a chalcogen-rich environment, or substitutional doping in the lattice drastically reduced chalcogen vacancies. 107Although these approaches are wellstudied, the quality of CVD-TMDCs at the wafer scale still falls short of exfoliated materials.Hence, it is essential to study the impact of different CVD precursor species, particularly the MOCVD precursors which potentially can provide a highquality thin film at larger scales.Meanwhile, several air-stable candidates such as PtSe 2 128,129 and PdSe 2 130,131 have been demonstrated; however, the development of large-scale growth methods and quality improvement are still in early phases.Conceptionally, adding a capping layer to encapsulate the airsensitive materials is another avenue to improve their air stability, which may enable further applications on those highperformance but air-sensitive materials.Hence, to realize the electronic-grade, large-area, and air-stable semiconductors, it is crucial to not only develop growth methods for the materials, bu also t further investigate the sequential growth of scaled-up capping layers.

Figure 1 .
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. 77(b−g) Adapted with permission from ref 77.Copyright 2020 Springer Nature.
al. and was attributed to favorable O 2 adsorption on ZrS x Se 2−x and Zr−O bond switching that reduce the structural stability.98

Figure 3 .
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. 16Adapted with permission from ref 16.Copyright 2015 American Chemical Society.(b) TEM and SAED images of an oxidized few-layer HfS 2 .The oxidized HfS 2 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 x Se 2−x with x from 0 to 0.3, 0.6, 1.14, 1.51, 2, and MoS 2 crystals were studied with spectroscopic ellipsometry measurements.Oxide thickness versus exposure time plots for ZrS x Se 2−x and MoS 2 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-MoTe 2 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. 99Adapted with permission from ref 99.Copyright 2021 American Chemical Society.(e) Atomic-resolution STM images (5 mV, 2 nA) of an exfoliated MoS 2 monolayer on Au after 1 month (left) and 1 year (right) of ambient exposure, revealing a progressive defect formation. 101(f) Reduction of 2D oxidized MoS 2 to pristine MoS 2 : Representative atomic-resolution STM images (5 mV, 2 nA) of 2D MoS 2−x O x before (left) and after (right) 30 min annealing with H 2 S at 200 °C, showing the reduction of the oxy-sulfide solid solution to the pristine MoS 2 through resubstitution of O by S. 101 (e, f) Adapted with permission from ref 101.Copyright 2018 Springer Nature.

Figure 4 .
Figure 4. (a) Schematic of hydroxide vapor phase deposition (OHVPD) growth of WS 2 monolayers. 108(b) Low-temperature PL spectra and STM topography (inset) of CVD-(top) OHVPD-(bottom) WS 2 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 (X 0 ), trion (X T ), and defect-bound exciton (X D ). 108 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-MoS 2 using metalorganic precursors.Inset: a camera image of a Re-MoS 2 film on sapphire.(d) Statistic for sulfur vacancy sites in MoS 2 and Re-MoS 2 based on TEM study.(e, f) Point defect density.(e) MoS 2 , and (f) 5 atom % Re-MoS 2 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 MoS 2 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 .
Figure 5. (a) Transfer characteristics for undoped, H 2 S annealed, and 0.1 atom % Re-MoS 2 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 MoS 2 FET using 24 nm Al 2 O 3 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 MoTe 2 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) 2 Te 3 /graphene/gallium (BST/Gr/Ga) thin films.Cross-sectional TEM shows the clean interface in the heterostructures. 116(e, f) Adapted with permission from ref 116.Copyright 2023 Springer Nature.

2 . 3 . 3 .
Defect Healing and Passivation for Air Stability Improvement.Defects exacerbate oxidation of 2D materials in air and ultimately impact the electrical properties and lifetime of the devices including field-effect transistor (FET).Molybdenum oxides (MoO x ) are p-type dopants to MoS 2 111