Band Gap Of Transition Metal Dichalcogenides Research Articles

Advances in Two-Dimensional Magnetic Semiconductors via Substitutional Doping of Transition Metal Dichalcogenides.

Transition metal dichalcogenides (TMDs) are two-dimensional (2D) materials with remarkable electrical, optical, and chemical properties. One promising strategy to tailor the properties of TMDs is to create alloys through a dopant-induced modification. Dopants can introduce additional states within the bandgap of TMDs, leading to changes in their optical, electronic, and magnetic properties. This paper overviews chemical vapor deposition (CVD) methods to introduce dopants into TMD monolayers, and discusses the advantages, limitations, and their impacts on the structural, electrical, optical, and magnetic properties of substitutionally doped TMDs. The dopants in TMDs modify the density and type of carriers in the material, thereby influencing the optical properties of the materials. The magnetic moment and circular dichroism in magnetic TMDs are also strongly affected by doping, which enhances the magnetic signal in the material. Finally, we highlight the different doping-induced magnetic properties of TMDs, including superexchange-induced ferromagnetism and valley Zeeman shift. Overall, this review paper provides a comprehensive summary of magnetic TMDs synthesized via CVD, which can guide future research on doped TMDs for various applications, such as spintronics, optoelectronics, and magnetic memory devices.

Scalable fabrication of long-wave infrared PtSe2-G heterostructure array photodetectors

Two-dimensional (2D) materials with excellent optoelectronic properties have attracted tremendous research interest in recent years. The promising performances of photodetectors based on 2D materials, such as ultrafast photoresponse and ultrahigh photoresponsivity, have been demonstrated in the visible to short-wavelength infrared spectrum range (0.8–2 μm). However, high performance, room temperature operation long-wavelength infrared (LWIR) photodetection is challenging. The detectors based on graphene usually exhibit low photoresponsivity due to the low optical absorption and short carrier lifetime. In addition, the relatively large bandgap of transition metal dichalcogenides limited the photoresponse bandwidth. Here, we report a way to fabricate a scalable device array of room-temperature operation LWIR PtSe2-G heterostructure detectors. The photoresponsivity at 10.6 μm up to ∼300 mA/W is obtained. The long-wave infrared light in the pico-watt range could be detected at room temperature by the PtSe2-G heterostructure detector. This result indicates that the PtSe2-G heterostructure device could be a highly competitive candidate for an uncooled LWIR detector. It also opens a way for a scalable array infrared focus plane device for the LWIR image.

Bandgap Tunability of Transition Metal Dichalcogenide Atomic Layers.

The temperature-dependent bandgap of transition metal dichalcogenides (TMDCs, MX2; M = Mo or W; X = S, Se, or Te) is analyzed using the O'Donnell and Chen relation with parameters including the average acoustic phonon energy (〈ħω〉) and the electron-phonon coupling strength (s). Wider (narrower) tunability of the bandgap results from the larger (smaller) electron-phonon coupling strength for a constant acoustic phonon energy. A 1.5 eV bandgap change was observed for weak electron-phonon coupling (s = 2) as well as with the strong electron-phonon coupling (s = 30). However, the weak electron-phonon coupling leads to a linear decrease in the bandgap energy as a function of temperature above ~85 K while the strong coupling exhibits similar behavior after ~60 K. Narrower (wider) tunability of the bandgap results from the larger (smaller) acoustic phonon energy for a constant electron-phonon coupling strength. The slope of negative entropy of exciton formation is large (small) at lower (higher) temperature. The management of the electron-phonon interaction as well as the average acoustic phonon energy indicates the ability to control the bandgap.

Universal Mechanism of Band-Gap Engineering in Transition-Metal Dichalcogenides.

van der Waals two-dimensional (2D) semiconductors have emerged as a class of materials with promising device characteristics owing to the intrinsic band gap. For realistic applications, the ideal is to modify the band gap in a controlled manner by a mechanism that can be generally applied to this class of materials. Here, we report the observation of a universally tunable band gap in the family of bulk 2H transition metal dichalcogenides (TMDs) by in situ surface doping of Rb atoms. A series of angle-resolved photoemission spectra unexceptionally shows that the band gap of TMDs at the zone corners is modulated in the range of 0.8-2.0 eV, which covers a wide spectral range from visible to near-infrared, with a tendency from indirect to direct band gap. A key clue to understanding the mechanism of this band-gap engineering is provided by the spectroscopic signature of symmetry breaking and resultant spin-splitting, which can be explained by the formation of 2D electric dipole layers within the surface bilayer of TMDs. Our results establish the surface Stark effect as a universal mechanism of band-gap engineering on the basis of the strong 2D nature of van der Waals semiconductors.