Two-dimensional (2D) molybdenum ditelluride (MoTe 2 ) is an interesting material for fundamental study and applications, due to its ability to exist in different polymorphs of 2H, 1T, and 1T′, their phase change behavior, and unique electronic properties. Although much progress has been made in the growth of high-quality flakes and films of 2H and 1T′-MoTe 2 phases, phase-selective growth of all three phases remains a huge challenge, due to the lack of enough information on their growth mechanism. Herein, we present a novel approach to growing films and geometrical-shaped few-layer flakes of 2D 2H-, 1T-, and 1T′-MoTe 2 by atmospheric-pressure chemical vapor deposition (APCVD) and present a thorough understanding of the phase-selective growth mechanism by employing the concept of thermodynamics and chemical kinetics involved in the growth processes. Our approach involves optimization of growth parameters and understanding using thermodynamical software, HSC Chemistry. A lattice strain-mediated mechanism has been proposed to explain the phase selective growth of 2D MoTe 2 , and different chemical kinetics-guided strategies have been developed to grow MoTe 2 flakes and films.
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Dynamic adhesion of 2D materials to mixed-phase BiFeO 3 structural phase transitions
Two-dimensional materials, such as transition metal dichalcogenides, have generated much interest due to their strain-sensitive electronic, optical, magnetic, superconducting, or topological properties. Harnessing control over their strain state may enable new technologies that operate by controlling these materials’ properties in devices such as straintronic transistors. Piezoelectric oxides have been proposed as one method to control such strain states on the device scale. However, there are few studies of how conformal 2D materials remain on oxide materials with respect to dynamic applications of the strain. Non-conformality may lead to non-optimal strain transfer. In this work, we explore this aspect of oxide-2D adhesion in the nanoscale switching of the substrate structural phase in thin 1T′-MoTe 2 attached to a mixed-phase thin-film BiFeO 3 (BFO), a multiferroic oxide with an electric-field induced structural phase transition that can generate mechanical strains of up to 2%. We observe that flake thickness impacts the conformality of 1T′-MoTe 2 to structural changes in BFO, but below four layers, 1T′-MoTe 2 fully conforms to the nanoscale BFO structural changes. The conformality of few-layer 1T′-MoTe 2 suggests that BFO is an excellent candidate for deterministic, nanoscale strain control for 2D materials.
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- NSF-PAR ID:
- 10352583
- Date Published:
- Journal Name:
- Journal of Applied Physics
- Volume:
- 132
- Issue:
- 4
- ISSN:
- 0021-8979
- Page Range / eLocation ID:
- 045301
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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Abstract The layer stacking order in 2D materials strongly affects functional properties and holds promise for next-generation electronic devices. In bulk, octahedral MoTe2possesses two stacking arrangements, the ferroelectric Weyl semimetal Tdphase and the higher-order topological insulator 1T′ phase. However, in thin flakes of MoTe2, it is unclear if the layer stacking follows the Td, 1T′, or an alternative stacking sequence. Here, we use atomic-resolution scanning transmission electron microscopy to directly visualize the MoTe2layer stacking. In thin flakes, we observe highly disordered stacking, with nanoscale 1T′ and Tddomains, as well as alternative stacking arrangements not found in the bulk. We attribute these findings to intrinsic confinement effects on the MoTe2stacking-dependent free energy. Our results are important for the understanding of exotic physics displayed in MoTe2flakes. More broadly, this work suggests
c -axis confinement as a method to influence layer stacking in other 2D materials. -
Abstract Monolayer molybdenum disulfide (MoS 2 ) is one of the most studied two-dimensional (2D) transition metal dichalcogenides that is being investigated for various optoelectronic properties, such as catalysis, sensors, photovoltaics, and batteries. One such property that makes this material attractive is the ease in which 2D MoS 2 can be converted between the semiconducting (2H) and metallic/semi-metallic (1T/1T′) phases or heavily n-type doped 2H phase with ion intercalation, strain, or excess negative charge. Using n -butyl lithium (BuLi) immersion treatments, we achieve 2H MoS 2 monolayers that are heavily n-type doped with shorter immersion times (10–120 mins) or conversion to the 1T/1T′ phase with longer immersion times (6–24 h); however, these doped/converted monolayers are not stable and promptly revert back to the initial 2H phase upon exposure to air. To overcome this issue and maintain the modification of the monolayer MoS 2 upon air exposure, we use BuLi treatments plus surface functionalization p-(CH 3 CH 2 ) 2 NPh-MoS 2 (Et 2 N-MoS 2 )—to maintain heavily n-type doped 2H phase or the 1T/1T′ phase, which is preserved for over two weeks when on indium tin oxide or sapphire substrates. We also determine that the low sheet resistance and metallic-like properties correlate with the BuLi immersion times. These modified MoS 2 materials are characterized with confocal Raman/photoluminescence, absorption, x-ray photoelectron spectroscopy as well as scanning Kelvin probe microscopy, scanning electrochemical microscopy, and four-point probe sheet resistance measurements to quantify the differences in the monolayer optoelectronic properties. We will demonstrate chemical methodologies to control the modified monolayer MoS 2 that likely extend to other 2D transition metal dichalcogenides, which will greatly expand the uses for these nanomaterials.more » « less
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Among group VI transition metal dichalcogenides, MoTe 2 is predicted to have the smallest energy offset between semiconducting 2H and semimetallic 1T′ states. This makes it an attractive phase change material for both electronic and optoelectronic applications. Here, we report fast, nondestructive, and full phase change in Al 2 O 3 -encapsulated 2H-MoTe 2 thin films to 1T′-MoTe 2 using rapid thermal annealing at 900 °C. Phase change was confirmed using Raman spectroscopy after a short annealing duration of 10 s in both vacuum and nitrogen ambient. No thickness dependence of the transition temperatures was observed for flake thickness ranging from 1.5 to 8 nm. These results represent a major step forward in understanding the structural phase transition properties of MoTe 2 thin films using external heating and underline the importance of surface encapsulation for avoiding thin film degradation.more » « less
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Abstract The phase transitions of two-dimensional (2D) materials are key to the operation of many devices with applications including energy storage and low power electronics. Nanoscale confinement in the form of reduced thickness can modulate the phase transitions of 2D materials both in their thermodynamics and kinetics. Here, using in situ Raman spectroscopy we demonstrate that reducing the thickness of MoS 2 below five layers slows the kinetics of the phase transition from 2H- to 1T′-MoS 2 induced by the electrochemical intercalation of lithium. We observe that the growth rate of 1T′ domains is suppressed in thin MoS 2 supported by SiO 2 , and attribute this growth suppression to increased interfacial effects as the thickness is reduced below 5 nm. The suppressed kinetics can be reversed by placing MoS 2 on a 2D hexagonal boron nitride ( h BN) support, which readily facilitates the release of strain induced by the phase transition. Additionally, we show that the irreversible conversion of intercalated 1T′-MoS 2 into Li 2 S and Mo is also thickness-dependent and the stability of 1T′-MoS 2 is significantly increased below five layers, requiring a much higher applied electrochemical potential to break down 1T′-MoS 2 into Li 2 S and Mo nanoclusters.more » « less
- Free Publicly Accessible Full Text
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Accepted Manuscript1.0
- Journal Article:
- https://doi.org/10.1063/5.0096686
