The Intrinsic Thermodynamic Difficulty and a Step‐Guided Mechanism for the Epitaxial Growth of Uniform Multilayer MoS₂ with Controllable Thickness

Abstract

consequently low carrier density limit its performance in electronic devices. [10] The carrier mobility of monolayer MoS 2 is relatively low and can even be seriously vulnerable against the optical phonons, defect and impurities. [11][12][13] Multilayer MoS 2 exposes much higher current density (400-1500 µA µm −1 ) and mobility (200-500 cm 2 V −1 s −1 ) than monolayer MoS 2 (current density: 50-700 µA µm −1 ; mobility: 70-100 cm 2 V −1 s −1 ), [14][15][16][17] making them more suitable for electronic application such as field effect transistors (FETs) and 2D device integrated circuits. Quantum transport simulations suggested that the bilayer and trilayer MoS 2 -based FETs could show better performance than the monolayer MoS 2 . [18][19][20] Excitingly, the bilayer MoS 2 based devices have been successfully demonstrated with very promising performance in experimental works. [21,22] Liu et al. produced bilayer MoS 2 films by the Scotch-tape technique and fabricated FETs devices with high on/ off ratio of 1 × 10 8 and field effect mobility of 517 cm 2 V −1 s −1 . [23] In addition, it was shown that the layerdependent electronic structures of multilayer MoS 2 can give rise to the unique optoelectronic response in devices. [24] However, the growth of large-area uniform multilayer MoS 2 , as the prerequisite of industrial application, is still very challenging. [25] Actually, the grand breakthrough was not made until very recently in the wafer-scale growth of monolayer MoS 2 single crystal films, in comparison with which the growth of large-area uniform multilayer MoS 2 is severely lagged. [26] Presently, the growth of multilayer MoS 2 films has commonly been achieved through the layer-by-layer growth mode via chemical vapor deposition (CVD) methods. [16,27,28] However, the multilayer MoS 2 films prepared by this route show poor uniformity on the thickness and domain size. This is attributed to that the nucleation of top-layer has to be occurred on the surface of previously grown bottom MoS 2 films, making it difficult to control the thickness and domain size of grown MoS 2 domain. Besides, the layer-by-layer growth mode would suffer from the synthesis time penalty and insufficient growth time will lead to a much smaller area of the upper MoS 2 layer than that of the lower layer and consequently a much smaller usable area of the grown film. [13] Hong et al. claimed that the growth of wafer-scale MoS 2 polycrystalline films within five layers could Multilayer MoS 2 shows superior performance over the monolayer MoS 2 for electronic devices while the growth of multilayer MoS 2 with controllable and uniform thickness is still very challenging. It is revealed by calculations that monolayer MoS 2 domains are thermodynamically much more favorable than multilayer ones on epitaxial substrates due to the competition between surface interactions and edge formation, leading accordingly to a layer-bylayer growth pattern and non-continuously distributed multilayer domains with uncontrollable thickness uniformity. The thermodynamics ...

Strain Engineering of Two-Dimensional Piezophotocatalytic Materials for Improved Hydrogen Evolution Reaction

“…34−36 Nevertheless, strain engineering involves complex synthesis techniques, like the growing of thin films on top of substrates and/or mechanical nanoactuators methods that may hamper the development of novel photocatalytic applications based on TMDCs. 37 A possibility to take full advantage of strain engineering techniques for TMDC-driven hydrogen sunlight production, by getting rid of such intricate synthesis/mechanical actuation methods, may consist in exploiting external electric fields. 40−42 Most TMDC single layers are piezoelectric materials (i.e., their crystal structures do not fulfill inversion crystal symmetry�are noncentrosymmetric) and consequently react to external electric field bias by elongating/contracting their lattice parameters.…”

“…Fortunately, TMDCs are well known for their great tunabilities, and various techniques have been adopted to improve their optoelectronic and catalytic properties, − among which strain engineering has been demonstrated to be one of the most effective strategies both experimentally and theoretically. − Nevertheless, strain engineering involves complex synthesis techniques, like the growing of thin films on top of substrates and/or mechanical nanoactuators methods that may hamper the development of novel photocatalytic applications based on TMDCs. − …”

Two-Dimensional Wedge-Shaped Magnetic EuS: Insight into the Substrate Step-Guided Epitaxial Synthesis on Sapphire

“…31 Centimeter-scale bilayer MoS 2 films were obtained on c-plane sapphire with an engineered terrace height, 32 and the mechanism for the growth of multilayer MoS 2 induced by steps of sapphire was theoretically proposed. 33 However, the step-guided gradient growth during the CVD process is rarely reported. It is worth noting that the unique wedge-shaped structure can open up new possibilities for investigating novel physical phenomena or thicknessdependent properties.…”

“…The steps on the sapphire’s surface have a critical influence on the growth of materials. , For example, the modified sapphire with steps along the ⟨101̅0⟩ axis has been designed to synthesize wafer-scale MoS 2 because the step edges break the degeneracy of nucleation energy, which is essential for the unidirectional alignment . Centimeter-scale bilayer MoS 2 films were obtained on c -plane sapphire with an engineered terrace height, and the mechanism for the growth of multilayer MoS 2 induced by steps of sapphire was theoretically proposed . However, the step-guided gradient growth during the CVD process is rarely reported.…”

Nucleation and growth studies of large-area deposited WS2 on flexible substrates

“…During the WS 2 film growth the substrate has a significant impact concerning the film quality and topography. Most of the studies have been performed on amorphous SiO 2 [21,22], sapphire model substrates [23][24][25][26][27] and graphene/HOPG [28,29]. Recently, on W-Au alloy substrate Wang et al demonstrated a strategy for preferentially forming antisite defects during WS 2 growth [30].…”