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[Display omitted] • High-quality PtSe 2 was synthesized on hBN using MBE. • PtSe 2 synthesized on hBN has higher crystallinity and less grain boundaries than that on SiO 2 /Si. • Fabricated a PtSe 2 -based photodetector on hBN shows a higher response to the illuminated light with a broadband wavelength. • The PtSe 2 -based photodetector fabricated on hBN shows higher photocurrent, faster response speed, and superior broadband detection range. Two-dimensional (2D) van der Waals materials are potential candidates for high-performance optoelectrical devices owing to the bandgap modulation, high on/off ratio, and ultra-stability. However, the challenge of synthesizing large-area, uniform, and high-quality 2D van der Waals materials should be addressed to manufacture large-scale and uniform optoelectrical devices using these materials. Although several previous studies investigated synthesis methods using chemical vapor deposition and molecular beam epitaxy (MBE), there are certain limitations such as the non-uniformity, lots of grain boundaries, and low carrier mobility. Here, by studying a method for synthesizing PtSe 2 on hBN buffer layer using MBE, we fabricated a PtSe 2 -based broadband photodetector. The high-quality PtSe 2 synthesized on hBN was evaluated using Raman spectroscopy and scanning transmission electron spectroscopy. These revealed that the synthesized PtSe 2 had a high quality and small grain boundary compared with PtSe 2 synthesized on SiO 2. Owing to the reduction in the carrier scattering and carrier recombination because of the high-quality and small grain boundaries, the photodetector using PtSe 2 synthesized on hBN displayed optoelectrical properties that were significantly better than those of PtSe 2 synthesized on SiO 2. Therefore, using hBN as a buffer layer for PtSe 2 growth, a remarkable performance of complex optoelectrical devices could be achieved. [ABSTRACT FROM AUTHOR]

[Display omitted] • Positive charge-mediated phase modulation of the crystal structure in 2D MoTe 2 synthesized by molecular beam epitaxy. • Structural phase of MoTe 2 is significantly influenced by the stoichiometric ratio of molybdenum to tellurium, and the tellurium adatoms make the 1T' phase more stable than the 2H phase. • Based on the energy difference and energy barrier between the two phases, the effects of doping on the structure during and after MoTe 2 synthesis were verified. Polymorphic phase transition between semi-conducting 2H and semi-metallic 1T′ in MoTe 2 has garnered significant interest due to its wide applicability for memory device. Recently, it has been reported that charge doping is a prospective method for facilitating practical application of phase transition. In particular, the positively doped system with lower energy barrier between two phases is more advantageous than the negatively doped system. However, although various methods for negative charge-mediated phase transition have been developed, study on positive charge-mediated phase transition is scarce due to the requirement of high charge density. Herein, we report positive charge-mediated phase modulation in MoTe 2 synthesized by molecular beam epitaxy. The structural phase of MoTe 2 is significantly influenced by the stoichiometric ratio of molybdenum to tellurium, and the tellurium adatoms make 1T' phase more stable than 2H phase. Photoelectron spectroscopy reveals that positive charge doping by electron transfer to tellurium adatoms is crucial in the determination of phase. Finally, based on the energy difference and energy barrier between two phases, the doping effects during and after synthesis are verified. This study on the structure modulation of MoTe 2 provides physical insight for phase transition as well as a basis for the development of nanoscale electronic devices. [ABSTRACT FROM AUTHOR]

A PtSe2-based edge-contact field-effect transistor (FET) incorporating PtSe2 as the channel material and PtTe2 as the contact metal is fabricated using tellurization on only a selected region. The higher on–off ratio of the PtSe2-based edge-contact device compared to other synthesized PtSe2-based FETs is successfully achieved by reducing the contact resistance. [Display omitted] One-dimensional (1D) edge contact is considered critical to reducing contact resistance between two-dimensional transition-metal dichalcogenides (TMDs) and contact metal and overcoming surface contact effects, such as surface damage and orbital hybridization. Although several studies have investigated edge contact, some problems such as inevitable etching damage and a limited area persist. Herein, a tellurization method is applied to a specific lateral region to fabricate PtTe 2 /PtSe 2 /PtTe 2 edge-contact field-effect transistors (FETs) without etching damage. High-resolution X-ray photoelectron spectroscopy and Raman spectroscopy are used to verify tellurization completion, which is evidenced by the complete transformation of PtSe 2 to PtTe 2 by the substitution of Se atoms with evaporated Te atoms. Furthermore, tellurization is applied to a specific lateral region by utilizing the hexagonal boron nitride to block the interdiffusion of evaporated Te atoms, which is confirmed by Raman spectroscopy and cross-sectional annular dark-field scanning transmission electron microscopy. Finally, the 1D edge-contact FETs fabricated by tellurization demonstrate a higher on–off ratio and carrier mobility than the surface-contact FET. Therefore, this method can be applied to various FETs based on TMDs for improving electrical performance through a reduction in contact resistance. [ABSTRACT FROM AUTHOR]

Relation between FET performance and initial growth stages of PtSe 2 film is analyzed for device application. In initial growth stages, platinum nucleus which degrade device performance co-exist in thin PtSe 2. Controlling growth kinetics, suppression of platinum nucleus and improvement of on–off ratio and mobility are achieved, which contribute to understand growth mechanism and optimize 2D TMD growth on three-dimensional substrate. [Display omitted] • PtSe 2 films with 1T structure were synthesized on SiO 2 /Si and sapphire (0 0 0 1) substrates using MBE. • Thickness- and substrate-dependent properties of synthesized PtSe 2 films using HR-XPS and STEM showed that metallic Pt co-existed in thin PtSe 2 films on SiO 2 /Si substrates. • Thickness- and substrate-dependent properties of synthesized PtSe 2 films showed that metallic Pt co-existed in thin PtSe 2 films on SiO 2 /Si substrates. • Electrical performance of thin PtSe 2 according to thicknesses and substrates was verified, which illustrating that co-existence of metallic Pt caused electrical degradation. • Suppression of metallic Pt by the change in interface energy between PtSe 2 and substrate affects the increased field effect mobility and on–off ratio of PtSe 2 -based electrical devices. Platinum diselenide (PtSe 2) is an emerging transition-metal dichalcogenide showing both strong interlayer coupling and remarkable electrical properties. Because field-effect transistors (FETs) recently fabricated using PtSe 2 films grown by various methods have not shown the high mobilities and on–off ratios compared with exfoliated PtSe 2 and predicted by theoretical calculations, the PtSe 2 growth mechanism must be better understood to prevent the degradation of electrical properties and improve the electrical performances of PtSe 2 -based devices. Therefore, molecular beam epitaxy is used in the current study to grow PtSe 2 films, and a correlation is demonstrated between the initial PtSe 2 growth kinetics and the electrical performances of PtSe 2 -based devices. X-ray photoelectron spectroscopy and annular dark field–scanning transmission electron microscopy show that metallic Platinum (Pt) is present in the thin PtSe 2 films, as explained by the density functional theory calculations comparing Pt–Pt bond with Pt–Se bond. Electrical measurements of the PtSe 2 -based FETs show that the electrical performances are closely correlated with the initial growth kinetics of the PtSe 2 films, indicating that removing Pt (which degrades device performance) is the most important aspect of growing thin PtSe 2 films to optimize device performance. [ABSTRACT FROM AUTHOR]