Chemical vapor deposition of 3D transition metal oxide and chalcogenide nanocrystals

This thesis aims to prepare 3d transition metal oxide and chalcogenide nanocrystals by the chemical vapor deposition (CVD) method and study their magnetic properties at a single nanocrystal level. Typically, the nanomaterials prepared in this thesis are two-dimensional (2D) structures, with the thickness down to several nanometers and the lateral size as large as tens of micrometers. The prepared materials include layered FeTe1-xSex, and nonlayered Fe3O4, MnS and MnSe. For the layered FeTe1-xSex, its 2D nanocrystals can be directly synthesized on the SiO2/Si substrate. A mixed precursor strategy was developed and enabled the tuning of composition and control of thickness. The size of the prepared nanosheets can be as large as 40 µm, and the thickness can be as low as 3.8 nm. The prepared FeTe1-xSex nanocrystals exhibited good quality with a superconducting transition temperature (Tc) close to the bulk counterparts. The study of using different precursors showed the relation between the defects and the quality of this unconventional superconductor. Besides, a stacking method utilizing the “stand-on” FeTe1-xSex samples can be easily used to prepare FeTe1-xSex-based van der Waals structures, giving hope to make use of these FeTe1-xSex nanosheets for fabricating complicated devices for topological quantum computing. For the nonlayered Fe3O4, MnS, and MnSe, the usage of mica, a van der Waals material, as the growth substrate was critical to growing the 2D crystals. The free of dangling bonding of the van der Waals substrate enables the easy migration of the absorbed species, thus facilitating the lateral growth and resulting in the formation of 2D crystals. Besides, in preparing 2D Fe3O4 nanocrystals, a water-assisted CVD method using CaSO4∙2H2O as the water precursor was developed, and it is capable of preparing ultrathin Fe3O4 nanocrystals. The prepared Fe3O4 nanocrystals possess very few antiphase boundaries (APBs), as confirmed by transmission electron microscope (TEM) results. This feature enables the study of the magnetic property of the single APB. In the preparation of the 2D MnS and MnSe nanocrystals, the mica also plays a significant role. The synthesis parameters were studied detailly, and their magnetic properties were also characterized. In addition, the morphology of the prepared MnSe nanocrystals can also change from triangular to dendritic shape by tuning the hydrogen concentration, which can also give new magnetic properties. The CVD synthesis strategies mentioned above are not limited to growing the materials mentioned above. They can also be used to synthesize other large-sized 3d transition metal oxide and chalcogenide nanocrystals with controllable thickness and defects, which can serve as a platform for further study the relation between their structures and properties on a single nanocrystal level.Doctor of Philosoph

Recent developments in chemical vapor deposition of 2D magnetic transition metal chalcogenides

In recent years, two-dimensional (2D) magnetic transition metal chalcogenides (TMCs) have attracted tremendous research interests thanks to their intriguing properties that are essential in developing future electronic and spintronic devices in this modernizing era. This review aims to introduce recent developments in the preparation of 2D magnetic TMCs, especially chromium and iron-based chalcogenides, their structures, as well as the related intriguing magnetic phenomena. First, the common crystal structures of magnetic TMCs including both layered and nonlayered structures are introduced. Various chemical vapor deposition strategies for synthesizing 2D magnetic TMCs are then introduced with emphasis on the key synthesis parameters. Moreover, the intriguing physical properties associated with 2D TMCs such as magnetic anisotropy, thickness, and phase-dependent magnetic response as well as stability are summarized. Last but not least, challenges and future research directions are briefly discussed in light of recent advances in the field.Ministry of Education (MOE)National Research Foundation (NRF)Submitted/Accepted versionZ.L. acknowledges support from National Research Foundation Singapore Programme Grants NRF-CRP22-2019-0007, NRF-CRP21-2018-0007, and NRF-CRP22-2019-0004. This research is also supported by the Ministry of Education, Singapore, under its AcRF Tier 3 Programme “Geometrical Quantum Materials” (Grant MOE2018-T3-1-002), and AcRF Tier 1 Grant RG161/19

Two-dimensional cobalt ferrite through direct chemical vapor deposition for efficient oxygen evolution reaction

Two-dimensional (2D) transition metal oxides (TMOs) are promising electrocatalysts for the new energy industry, owing to their earth-abundancy, excellent performance, and unique physicochemical properties. However, microscopic electrochemical study for 2D TMOs is still lacking to provide detailed electrocatalytic mechanisms due to the challenges in synthesizing 2D TMOs with high quality and controlled thickness, which is indispensable for the microscopic studies. In this study, we report the direct synthesis of 2D cobalt ferrite (CoFeO) using a chemical vapor deposition (CVD) method. The as-synthesized 2D CoFeO possesses a well-crystallized spinel structure with an ultrathin thickness of 6.8 nm. Its oxygen evolution reaction (OER) properties under alkaline conditions were accurately assessed using an ultra-microelectrode testing platform. The (111) facet of the 2D CoFeO exhibits a low overpotential of 330 mV at a current density of 10 mA cm–2 and a high current density of ~142 mA cm–2 at an overpotential of 570 mV. The OER mechanism of the 2D CoFeO was analyzed using density functional theory (DFT) calculations, which reveal the bimetallic sites on the surface reduce the energy barrier and facilitate the reaction. Moreover, we demonstrate the reduced thickness of 2D CoFeO improves the OER activity by lowering the bulk resistance and improving the utilization of active sites, which was confirmed by the thickness-activity dependency (6.8 to 35 nm) tests using the ultra-microelectrode platform. Furthermore, the practical values of the as-prepared 2D CoFeO was demonstrated by synthesizing a large-area continuous film and collecting high OER activity and superb durability from macro-electrochemical experiments. Our study provides new solutions for the controlled synthesis of 2D TMOs electrocatalysts and uncovers the electrocatalytic mechanisms with the ultra-microelectrode platform, which provides new insights for exploring the inherent properties and applications of 2D materials in electrocatalysis.Ministry of Education (MOE)This work was supported by Singapore Ministry of Education AcRF Tier 2 (MOE2019-T2-2-105), AcRF Tier 1 RG4/17 and RG161/19

Controlled growth of ultrathin ferromagnetic β‐MnSe semiconductor

Two-dimensional (2D) magnetic crystals with intrinsic ferromagnetism are highly desirable for novel spin-electronic devices. However, the controllable synthesis of 2D magnets, especially the direct growth of 2D magnets on substrate surfaces, is still a challenge. Here, we demonstrate the synthesis of ultrathin zinc-blende phase manganese selenide (β-MnSe) nanosheets using the chemical vapor deposition (CVD) technique. The 2D β-MnSe crystals exhibit distinct ferromagnetic properties with a Curie temperature of 42.3 K. Density functional theory (DFT) calculations suggest that the ferromagnetic order in β-MnSe originates from the exchange coupling between the unsaturated Se and Mn atoms. This study presents significant progress in the CVD growth of ultrathin 2D magnetic materials by thinning bulk magnets, and it will pave the way for the building of energy-efficient spintronic devices in the future.Ministry of Education (MOE)National Research Foundation (NRF)Published versionThis study was supported by the National Research Foundation–Competitive Research Program of Singapore (Nos. NRF-CRP21-2018-0007, CRP22-2019-0060, and NRF2017-ANR002 2DPS); MOE Tier 2 (No. MOE2017-T2-2-136) and Tier 3 (No. MOE2018-T3-1-002); National Natural Science Foundation of China (Nos. 11828401, 11964024, and 21971113); Startup Project of Inner Mongolia University (No. 21200-5175101); Fund of University of Macau (Nos. MYRG2018-00079-IAPME and MYRG2019-00115-IAPME); Science and Technology Development Fund of Macau SAR (Nos. FDCT0059/2018/A2 and FDCT009/2017/AMJ); Fund of Shenzhen Science and Technology Innovation Committee (No. SGDX20201103093600003); Shanghai Pujiang Program (No. 20PJ1411500)

In‐Plane Anisotropic Properties of 1T′‐MoS 2

Crystal phases play a key role in determining the physicochemical properties of a material. To date, many phases of transition metal dichalcogenides have been discovered, such as octahedral (1T), distorted octahedral (1T′), and trigonal prismatic (2H) phases. Among these, the 1T′ phase offers unique properties and advantages in various applications. Moreover, the 1T′ phase consists of unique zigzag chains of the transition metals, giving rise to interesting in-plane anisotropic properties. Herein, the in-plane optical and electrical anisotropies of metastable 1T′-MoS2 layers are investigated by the angle-resolved Raman spectroscopy and electrical measurements, respectively. The deconvolution of J1 and J2 peaks in the angle-resolved Raman spectra is a key characteristic of high-quality 1T′-MoS2 crystal. Moreover, it is found that its electrocatalytic performance may be affected by the crystal orientation of anisotropic material due to the anisotropic charge transport.Ministry of Education (MOE)Nanyang Technological UniversityG.-H.N. and Q.H. contributed equally to this work. This work was supported by MOE under AcRF Tier 2 (MOE2015-T2-2-057, MOE2016-T2-2-103, MOE2017-T2-1-162) and AcRF Tier 1 (2017-T1-001-150, 2017-T1-002-119), and NTU under Start-Up Grant (M4081296.070.500000) in Singapore. The authors acknowledge the Facility for Analysis, Characterization, Testing and Simulation, Nanyang Technological University, Singapore, for use of their electron microscopy (and/or X-ray) facilities. H.Z. thanks the support from ITC via Hong Kong Branch of National Precious Metals Material Engineering Research Center, and the Start-Up Grant from City University of Hong Kong

Chemical vapor deposition of superconducting FeTe1-xSex nanosheets

FeTe1-xSe x, a promising layered material used to realize Majorana zero modes, has attracted enormous attention in recent years. Pulsed laser deposition (PLD) and molecular-beam epitaxy (MBE) are the routine growth methods used to prepare FeTe1-xSexthin films. However, both methods require high-vacuum conditions and polished crystalline substrates, which hinder the exploration of the topological superconductivity and related nanodevices of this material. Here we demonstrate the growth of the ultrathin FeTe1-xSex superconductor by a facile, atmospheric pressure chemical vapor deposition (CVD) method. The composition and thickness of the two-dimensional (2D) FeTe1-xSex nanosheets are well controlled by tuning the experimental conditions. The as-prepared FeTe0.8Se0.2 nanosheet exhibits an onset superconducting transition temperature of 12.4 K, proving its high quality. Our work offers an effective strategy for preparing the ultrathin FeTe1-xSex superconductor, which could become a promising platform for further study of the unconventional superconductivity in the FeTe1-xSex system.Accepted versio

Chemical vapor deposition of phase-pure 2D 1T-CrS₂

2D transition-metal dichalcogenides of chromium are technologically important due to their diverse electrical and magnetic properties. Herein this study, 1T-CrS2 nanosheets as thin as 2 nm are successfully synthesized by an atmospheric pressure space-confined chemical vapor deposition method. The prepared CrS2 nanosheets are stable at room temperature. Electrical measurement shows a quasi n-type semiconductor behavior. In addition, terahertz characterizations on 1T-CrS2 reveal its potential to integrate with the ultrathin infrared sensor platforms.Ministry of Education (MOE)This work was sup-ported by the Ministry of Education, Singapore, under its AcRF Tier 3 Programme ‘Geometrical Quantum Materials’ (MOE2018-T3-1-002), AcRF Tier 2 (MOE2019-T2-105) and AcRF Tier 1 RG161/19

Metastable 1T′-phase group VIB transition metal dichalcogenide crystals

Metastable 1T′-phase transition metal dichalcogenides (1T′-TMDs) with semi-metallic natures have attracted increasing interest owing to their uniquely distorted structures and fascinating phase-dependent physicochemical properties. However, the synthesis of high-quality metastable 1T′-TMD crystals, especially for the group VIB TMDs, remains a challenge. Here, we report a general synthetic method for the large-scale preparation of metastable 1T′-phase group VIB TMDs, including WS2, WSe2, MoS2, MoSe2, WS2xSe2(1−x) and MoS2xSe2(1−x). We solve the crystal structures of 1T′-WS2, -WSe2, -MoS2 and -MoSe2 with single-crystal X-ray diffraction. The as-prepared 1T′-WS2 exhibits thickness-dependent intrinsic superconductivity, showing critical transition temperatures of 8.6 K for the thickness of 90.1 nm and 5.7 K for the single layer, which we attribute to the high intrinsic carrier concentration and the semi-metallic nature of 1T′-WS2. This synthesis method will allow a more systematic investigation of the intrinsic properties of metastable TMDs.Agency for Science, Technology and Research (A*STAR)Ministry of Education (MOE)National Research Foundation (NRF)Submitted/Accepted versionH.Z. acknowledges support from ITC via the Hong Kong Branch of National Precious Metals Material Engineering Research Center (NPMM), the Start-Up Grant (project no. 9380100) and grants (project nos. 9610478 and 1886921) from the City University of Hong Kong and the Science Technology and Innovation Committee of Shenzhen Municipality (grant no. JCYJ20200109143412311). Q.H. acknowledges the funding support from the Start-Up Grant (project no. 9610482) from the City University of Hong Kong. Y.S. and Y.M. acknowledge the funding support from the National Natural Science Foundation of China (under grant no. 11534003) and the Program for JLU Science and Technology Innovative Research Team and Science Challenge Project (no. TZ2016001). K.H. and D.V.M.R. acknowledge funding from the Accelerated Materials Development for Manufacturing Program at A*STAR via the AME Programmatic Fund by the Agency for Science, Technology and Research under grant no. A1898b0043. R.V.R. and V.S. acknowledge support by grants from the National Research Foundation, Prime Minister’s Office, Singapore, under its Campus for Research Excellence and Technological Enterprise (CREATE) programme. X.R.W. acknowledges supports from Academic Research Fund Tier 2 (grant no. MOE-T2EP50120-006) from Singapore Ministry of Education

Phase-controllable growth of ultrathin 2D magnetic FeTe crystals

10.1038/s41467-020-17253-xNature Communication