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Register a new userLow-temperature synthesis and electrocatalytic application of large-area PtTe2 thin films
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The synthesis of transition metal dichalcogenides (TMDs) has been a primary focus for 2D nanomaterial research over the last 10 years, however, only a small fraction of this research has been concentrated on transition metal ditellurides. In particular, nanoscale platinum ditelluride (PtTe2) has rarely been investigated, despite its potential applications in catalysis, photonics and spintronics. Of the reports published, the majority examine mechanically-exfoliated flakes from chemical vapor transport (CVT) grown crystals. While this production method is ideal for fundamental studies, it is very resource intensive therefore rendering this process unsuitable for large scale applications. In this report, the synthesis of thin films of PtTe2 through the reaction of solid-phase precursor films is described. This offers a production method for large-area, thickness-controlled PtTe2, suitable for a range of applications. These polycrystalline PtTe2 films were grown at temperatures as low as 450 degC, significantly below the typical temperatures used in the CVT synthesis methods. To investigate their potential applicability, these films were examined as electrocatalysts for the hydrogen evolution reaction (HER) and oxygen reduction reaction (ORR). The films showed promising catalytic behavior, however, the PtTe2 was found to undergo chemical transformation to a substoichiometric chalcogenide compound under ORR conditions. This study shows while PtTe2 is stable and highly useful for HER, this property does not apply to ORR, which undergoes a fundamentally different mechanism. This study broadens our knowledge of the electrocatalysis of TMDs.
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Metallic transition metal dichalcogenides (TMDs) have exhibited various exotic physical properties and hold the promise of novel optoelectronic and topological devices applications. However, the synthesis of metallic TMDs is based on gas-phase methods and requires high temperature condition. As an alternative to the gas-phase synthetic approach, lower temperature eutectic liquid-phase synthesis presents a very promising approach with the potential for larger-scale and controllable growth of high-quality thin metallic TMDs single crystals. Herein, we report the first realization of low-temperature eutectic liquid-phase synthesis of type-II Dirac semimetal PtTe2 single crystals with thickness ranging from 2 to 200 nm. The electrical measurement of synthesized PtTe2 reveals a record-high conductivity of as high as 3.3*106 S/m at room temperature. Besides, we experimentally identify the weak antilocalization behavior in the type-II Dirac semimetal PtTe2 for the first time. Furthermore, we develop a simple and general strategy to obtain atomically-thin PtTe2 crystal by thinning as-synthesized bulk samples, which can still retain highly crystalline and exhibits excellent electric conductivity. Our results of controllable and scalable low-temperature eutectic liquid-phase synthesis and layer-by-layer thinning of high-quality thin PtTe2 single crystals offer a simple and general approach for obtaining different thickness metallic TMDs with high-melting point transition metal.
As a newly emergent type-II Dirac semimetal, Platinum Telluride (PtTe2) stands out from other 2D noble-transition-metal dichalcogenides for the unique structure and novel physical properties, such as high carrier mobility, strong electron-phonon coupling and tunable bandgap, which make the PtTe2 a good candidate for applications in optoelectronics, valleytronics and far infrared detectors. Although the transport properties of PtTe2 have been studied extensively, the dynamics of the nonequilibrium carriers remain nearly uninvestigated. Herein we employ optical pump-terahertz (THz) probe spectroscopy (OPTP) to systematically study the photocarrier dynamics of PtTe2 thin films with varying pump fluence, temperature, and film thickness. Upon photoexcitation the THz photoconductivity (PC) of 5 nm PtTe2 film shows abrupt increase initially, while the THz PC changes into negative value in a subpicosecond time scale, followed by a prolonged recovery process that lasted hundreds of picoseconds (ps). This unusual THz PC response observed in the 5 nm PtTe2 film was found to be absent in a 2 nm PtTe2 film. We assign the unexpected negative THz PC as the small polaron formation due to the strong electron-Eg-mode phonon coupling, which is further substantiated by pump fluence- and temperature-dependent measurements as well as the Raman spectroscopy. Moreover, our investigations give a subpicosecond time scale of sequential carrier cooling and polaron formation. The present study provides deep insights into the underlying dynamics evolution mechanisms of photocarrier in type-II Dirac semimetal upon photoexcitation, which is fundamental importance for designing PtTe2-based optoelectronic devices.
For generation of sustainable, clean and highly efficient energy, the electrocatalytic oxygen evolution reaction represents an attractive platform, thus inviting immense research activities in recent years. However, designing the catalyst with enhanced electrocatalytic activity remains one of the major challenges. Here, we examined the oxygen evolution reaction activities of geometrically designed (with and without step-textured morphology) thin films of an electrocatalytically active correlated metallic SrRuO3 perovskite grown on c- and r-plane sapphire substrates. On c-plane sapphire, as compared to the uniform surface, the step-textured films endowed with active Ru-sites show remarkable decrease in the overpotential (25 mV). Interestingly, the behavior is opposite for the r-plane case, highlighting the significance of the active sites, in addition with the polar surface termination of selective crystal facets. Density functional theory calculation confirms the favorable energy reaction pathway for the active site dependent enhancement in OER. Our strategy might pave the way towards designing the surfaces of various oxide thin films for high performance energy conversion based devices.
The monolayer transition metal dichalcogenides have recently attracted much attention owing to their potential in valleytronics, flexible and low-power electronics and optoelectronic devices. Recent reports have demonstrated the growth of large-size 2-dimensional MoS2 layers by the sulfurization of molybdenum oxides. However, the growth of transition metal selenide monolayer has still been a challenge. Here we report that the introduction of hydrogen in the reaction chamber helps to activate the selenization of WO3, where large-size WSe2 monolayer flakes or thin films can be successfully grown.
Great achievements have been made in alloying of two-dimensional (2D) semiconducting transition metal dichalcogenides (TMDs), which can allow tunable band gaps for practical applications in optoelectronic devices. However, telluride-based TMDs alloys were less studied due to the difficulties of sample synthesis. Here, in this work we report the large-area synthesis of 2D MoTexSe2-x alloy films with controllable Te composition by a modified alkali metal halides assisted chemical vapor deposition method. The as-prepared films have millimeter-scale transverse size. Raman spectra experiments combining calculated Raman spectra and vibrational images obtained by density functional theory (DFT) confirmed the 2H-phase of the MoTexSe2-x alloys. The A1g mode of MoSe2 shows a significant downshift accompanied by asymmetric broadening to lower wavenumber with increasing value of x, while E12g mode seems unchanged, which were well explained by a phonon confinement model. Our work provides a simple method to synthesize large-scale 2H phase Te-based 2D TMDs alloys for their further applications.
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