Defects inducing anomalous exciton kinetics in monolayer WS2

Radiative and Non-Radiative Exciton Recombination Processes in a Chemical Vapor Deposition-Grown MoSe2 Film

“…The left parts of Figure a–c are optical micrographs of three continuous film samples a , b , and c of MoSe 2 . It is noted that three kinds of different regions, i.e., monolayer (1L), bilayer (2L), and multiplayer (>2L) regions, can be observed on these figures: (a) CVD as-grown sample a , where a region around point A represents the 2L region; (b) the film transferred to the SiO 2 /Si substrate by PDMS (sample b ), where a region around point B represents the 1L region (which also contains many bilayer regions); (c) another CVD as-grown sample c with a distribution of a large-area 1L region, such as a region around point C. Figure d shows the AFM image of the transferred film sample b , where the red dotted line describes the result of a height scanning, which indicates that there is a height dip of 1.5 nm, giving evidence for the existence of a 2L region. ,, To study the exciton luminescence of the films with different thicknesses, the room temperature PL intensity mapping micrographs are shown in the right parts of Figure a–c. The ellipses as marked by dashed lines in both the left and right parts of these figures clearly show the regions, which are carefully studied and identified.…”

“…The PL and TRPL spectra were performed using excitation of a 640 nm pulsed semiconductor laser (with a pulse length of 1d shows the AFM image of the transferred film sample b, where the red dotted line describes the result of a height scanning, which indicates that there is a height dip of 1.5 nm, giving evidence for the existence of a 2L region. 19,25,40 To study the exciton luminescence of the films with different thicknesses, the room temperature PL intensity mapping micrographs are shown in the right parts of Figure 1a−c. The ellipses as marked by dashed lines in both the left and right parts of these figures clearly show the regions, which are carefully studied and identified.…”

“…This may be one critical reason for the low quantum yield in most TMDs. 24,25,30,33 Compared with the transferred sample, the as-grown MoSe 2 film has tensile stress that may inhibit the tunneling process of 2e, we make an analysis of the carrier's tunneling rate from the band tail state into the deep defect levels under the influence of the stress. Here, the parabolic curve represents the energy relaxation process of the tunneling carriers; E 1 and E 2 represent the localized levels of the band tail for unstressed and stressed film samples.…”

“…Atomically thin-layered MoSe 2 , a member of the transition metal dichalcogenides (TMDs), is a quasi-two-dimensional semiconductor material with strong Coulomb interaction and whose monolayer transforms into a direct bandgap structure . These unique physical properties make them shine in the field of optoelectronics such as light emitters and photodetectors, − which have attracted widespread attention. ,,− Compared with the widely studied MoS 2 , its direct bandgap is closer to the optimal bandgap value of single-junction photovoltaic devices, and the more easily tunable bandgap gives it a greater potential for applications in photocatalysis and optoelectronics. ,, At present, chemical vapor deposition (CVD) is a mature method to grow continuous TMD thin films over a large area compared with micromechanical exfoliation; , however, more complex defects will inevitably be introduced due to the local variation of thermal strain and precursor concentration in the growth process. − These defects can trap excitons/carriers to greatly affect the kinetic process of exciton radiative recombination or carrier transport in TMDs, ,,− e.g., non-radiative recombination of defects can greatly reduce the quantum yield of photoluminescence (PL) below 0.6% . However, defects are not always detrimental and can also lead to exciting functions such as increased electrical conductivity and extended radiative luminescence lifetime of excitons. , Therefore, an in-depth study of defect engineering to fully understand and characterize the effect of various defects on the carrier dynamics process, so as to use defects to tune the optical properties of materials, is of great significance for the realization of higher-efficiency optoelectronic devices.…”

Hydrothermal synthesis and controlled growth of group-VIB W metal compound nanostructures from tungsten oxide to tungsten disulphide

“…1(a)), [21][22][23] WS 2 is fabricated by sulfiding tungsten oxide (in the lower left corner of Fig. 1(a)), [24][25][26][27][28] and the WS x O y hybrid (such as S-doped WO 3 , O-doped WS 2 and WO 3 /WS 2 heterojunction) is present in the process. [29][30][31] Fig.…”