The electron dynamics in heterostructures formed by multilayer graphite and monolayer or bulk MoS 2 were studied by femtosecond transient absorption measurements. Samples of monolayer MoS 2 -multilayer graphite and bulk MoS 2 -multilayer graphite were fabricated by exfoliation and dry transfer techniques. Ultrafast laser pulses were used to inject electron–hole pairs into monolayer or bulk MoS 2 . The transfer of these photocarriers to the adjacent multilayer graphite was time resolved by measuring the differential reflection of a probe pulse. We found that photocarriers injected into monolayer MoS 2 transfer to graphite on an ultrafast time scale shorter than 400 fs. Such an efficient charge transfer is key to the development of high performance optoelectronic devices with MoS 2 as the light absorbing layer and graphite as electrodes. The absorption coefficient of monolayer MoS 2 can be controlled by the carriers in graphite. This process can be used for interlayer coupling and control. In a bulk MoS 2 -graphite heterostructure, the photocarrier transfer time is about 220 ps, due to the inefficient interlayer charge transport in bulk MoS 2 . These results provide useful information for developing optoelectronic devices based on MoS 2 -graphite heterostructures.
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Effects of rhenium dopants on photocarrier dynamics and optical properties of monolayer, few-layer, and bulk MoS 2
We report a comprehensive study on the effects of rhenium doping on optical properties and photocarrier dynamics of MoS 2 monolayer, few-layer, and bulk samples. Monolayer and few-layer samples of Re-doped (0.6%) and undoped MoS 2 were fabricated by mechanical exfoliation, and were studied by Raman spectroscopy, optical absorption, photoluminescence, and time-resolved differential reflection measurements. Similar Raman, absorption, and photoluminescence spectra were obtained from doped and undoped samples, indicating that the Re doping at this level does not significantly alter the lattice and electronic structures. Red-shift and broadening of the two phonon Raman modes were observed, showing the lattice strain and carrier doping induced by Re. The photoluminescence yield of the doped monolayer is about 15 times lower than that of the undoped sample, while the photocarrier lifetime is about 20 times shorter in the doped monolayer. Both observations can be attributed to diffusion-limited Auger nonradiative recombination of photocarriers at Re dopants. These results provide useful information for developing a doping strategy of MoS 2 for optoelectronic applications.
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- Award ID(s):
- 1505852
- PAR ID:
- 10058205
- Date Published:
- Journal Name:
- Nanoscale
- Volume:
- 9
- Issue:
- 48
- ISSN:
- 2040-3364
- Page Range / eLocation ID:
- 19360 to 19366
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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Abstract Doping is a fundamental requirement for tuning and improving the properties of conventional semiconductors. Recent doping studies including niobium (Nb) doping of molybdenum disulfide (MoS2) and tungsten (W) doping of molybdenum diselenide (MoSe2) have suggested that substitutional doping may provide an efficient route to tune the doping type and suppress deep trap levels of 2D materials. To date, the impact of the doping on the structural, electronic, and photonic properties of in situ‐doped monolayers remains unanswered due to challenges including strong film substrate charge transfer, and difficulty achieving doping concentrations greater than 0.3 at%. Here, in situ rhenium (Re) doping of synthetic monolayer MoS2with ≈1 at% Re is demonstrated. To limit substrate film charge transfer,
r ‐plane sapphire is used. Electronic measurements demonstrate that 1 at% Re doping achieves nearly degenerate n‐type doping, which agrees with density functional theory calculations. Moreover, low‐temperature photoluminescence indicates a significant quench of the defect‐bound emission when Re is introduced, which is attributed to the MoO bond and sulfur vacancies passivation and reduction in gap states due to the presence of Re. The work presented here demonstrates that Re doping of MoS2is a promising route toward electronic and photonic engineering of 2D materials. -
Abstract Monolayer molybdenum disulfide (MoS 2 ) is one of the most studied two-dimensional (2D) transition metal dichalcogenides that is being investigated for various optoelectronic properties, such as catalysis, sensors, photovoltaics, and batteries. One such property that makes this material attractive is the ease in which 2D MoS 2 can be converted between the semiconducting (2H) and metallic/semi-metallic (1T/1T′) phases or heavily n-type doped 2H phase with ion intercalation, strain, or excess negative charge. Using n -butyl lithium (BuLi) immersion treatments, we achieve 2H MoS 2 monolayers that are heavily n-type doped with shorter immersion times (10–120 mins) or conversion to the 1T/1T′ phase with longer immersion times (6–24 h); however, these doped/converted monolayers are not stable and promptly revert back to the initial 2H phase upon exposure to air. To overcome this issue and maintain the modification of the monolayer MoS 2 upon air exposure, we use BuLi treatments plus surface functionalization p-(CH 3 CH 2 ) 2 NPh-MoS 2 (Et 2 N-MoS 2 )—to maintain heavily n-type doped 2H phase or the 1T/1T′ phase, which is preserved for over two weeks when on indium tin oxide or sapphire substrates. We also determine that the low sheet resistance and metallic-like properties correlate with the BuLi immersion times. These modified MoS 2 materials are characterized with confocal Raman/photoluminescence, absorption, x-ray photoelectron spectroscopy as well as scanning Kelvin probe microscopy, scanning electrochemical microscopy, and four-point probe sheet resistance measurements to quantify the differences in the monolayer optoelectronic properties. We will demonstrate chemical methodologies to control the modified monolayer MoS 2 that likely extend to other 2D transition metal dichalcogenides, which will greatly expand the uses for these nanomaterials.more » « less
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Abstract A comprehensive experimental study on optical properties and photocarrier dynamics in Bi2O2Se monolayers and nanoplates is presented. Large and uniform Bi2O2Se nanoplates with various thicknesses down to the monolayer limit are fabricated. In nanoplates, a direct optical transition near 720 nm is identified by optical transmission, photoluminescence, and transient absorption spectroscopic measurements and is attributed to the transition between the valence and conduction bands in the Γ valley. Time‐resolved differential reflection measurements reveal ultrafast carrier thermalization and energy relaxation processes and a photocarrier recombination lifetime of about 200 ps in nanoplates. Furthermore, by spatially resolving the differential reflection signal, a photocarrier diffusion coefficient of about 4.8 cm2s−1is obtained, corresponding to a mobility of about 180 cm2V−1s−1. A similar direct transition is also observed in monolayer Bi2O2Se, suggesting that the states in the Γ valley do not change significantly with the thickness. The temporal dynamics of the excitons in the monolayer is quite different from the nanoplates, with a strong saturation effect and fast exciton–exciton annihilation at high densities. Spatially and temporally resolved measurements yield an exciton diffusion coefficient of about 20 cm2s−1.
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Measuring the behavior of redox-active molecules in space and time is crucial for understanding chemical and biological systems and for developing new technologies. Optical schemes are noninvasive and scalable, but usually have a slow response compared to electrical detection methods. Furthermore, many fluorescent molecules for redox detection degrade in brightness over long exposure times. Here, we show that the photoluminescence of “pixel” arrays of monolayer MoS 2 can image spatial and temporal changes in redox molecule concentration. Because of the strong dependence of MoS 2 photoluminescence on doping, changes in the local chemical potential substantially modulate the photoluminescence of MoS 2 , with a sensitivity of 0.9 mV / Hz on a 5 μm × 5 μm pixel, corresponding to better than parts-per-hundred changes in redox molecule concentration down to nanomolar concentrations at 100-ms frame rates. This provides a new strategy for visualizing chemical reactions and biomolecules with a two-dimensional material screen.more » « less
- Free Publicly Accessible Full Text
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Accepted Manuscript1.0
- Journal Article:
- https://doi.org/10.1039/C7NR07227A
