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Photoluminescence quenching in gold - MoS2 hybrid nanoflakes
Udai Bhanu, Muhammad R. Islam, Laurene Tetard, Saiful I. Khondaker
TL;DR
The paper investigates how Au nanostructures interact electronically with few-layer MoS2 and whether this hybrid interface can tune MoS2 properties. Using controlled Au deposition, localized PL measurements, mapping, and structural characterization, it finds strong PL quenching associated with electron transfer from MoS2 to Au and p-doping. The authors propose localized Au patterning as a route to tune 2D TMD properties, while noting that geometry-dependent behavior requires further study.
Problem
The study addresses how localized Au-MoS2 interactions affect MoS2 photoluminescence and electronic structure, a question relevant to tunable hybrid 2D materials.
Method
The authors thermally deposit controlled 1.0 nm and 2.0 nm Au layers on exfoliated MoS2 and compare PL spectra and maps with pristine flakes using AFM, SEM, and XPS characterization.
Results
Au deposition causes drastic PL quenching across 2L–4L MoS2, attributed to electron transfer from MoS2 to Au and consequent p-doping.
Takeaways & Limitations
Patterned Au nanostructures may enable localized tuning of 2D TMD properties for nanoelectronics and nano-optics.
Takeaways & Limitations
PL and doping responses may depend on Au nanostructure geometry, requiring further theoretical and experimental study.
Abstract
from arXiv · showhide
Achieving tunability of two dimensional (2D) transition metal dichalcogenides (TMDs) functions calls for the introduction of hybrid 2D materials by means of localized interactions with zero dimensional (0D) materials. A metal-semiconductor interface, as in gold (Au) - molybdenum disulfide (MoS2), is of great interest from the standpoint of fundamental science as it constitutes an outstanding platform to investigate plasmonic-exciton interactions and charge transfer. The applied aspects of such systems introduce new options for electronics, photovoltaics, detectors, gas sensing, catalysis, and biosensing. Here we consider pristine MoS2 and study its interaction with Au nanoislands, resulting in local variations of photoluminescence (PL) associated with various Au-MoS2 hybrid configurations. By controllably depositing monolayers of Au on MoS2 to form Au nanostructures of given size and thickness, we investigate the electronic structure of the resulting hybrid systems. We present strong evidence of PL quenching of MoS2 as a result of charge transfer from MoS2 to Au: p-doping of MoS2. The results suggest new avenues for 2D nanoelectronics, active control of transport or catalytic properties.
Results
Au deposition produces strong, spatially extended PL quenching in MoS2, while control measurements exclude substrate-induced defects and structural changes as the cause. The authors attribute the quenching to electron transfer from MoS2 to Au, which p-dopes MoS2.
- Photoluminescence quenching: 2.0 nm Au deposition eliminates both A1 and B1 exciton peaks from 2L MoS2 PL spectra.The pristine 2L sample has an A1/B1 intensity ratio of about 2; 4L has a ratio of about 1.7 and 20% lower peak amplitude than 2L.
- Photoluminescence quenching: PL mapping shows no quenching across pristine MoS2, but no A1 or B1 peaks after Au deposition across the flake.The mapped peak ranges are 1.82–1.86 eV for A1 and 1.95–2.05 eV for B1.
- Controls: The observed quenching is not attributed to substrate defects or structural changes because pristine MoS2 on the same substrate does not quench and XPS shows no structural change.The control comparison and XPS peak-position and FWHM comparison support an electronic origin.
- Mechanism: Electron transfer from excited MoS2 states to Au leaves holes in MoS2, producing p-doping and preventing radiative return to the ground state.The proposed mechanism follows a reported 0.4 eV offset between the MoS2 and Au Fermi levels and associated band bending.
- Deposition dependence: 1.0 nm Au forms isolated triangular nanostructures 5–17 nm in size and significantly quenches A1 while only slightly reducing B1 locally.PL maps show quenching on both 2L and 3L regions, confirming p-doping from thermally deposited Au.
- Plasmonic response: Au nanostructures show plasmonic activity but do not enhance MoS2 PL, and the authors suggest that PL and doping depend on nanostructure geometry.The paper contrasts its result with PL enhancement reported for Au-coated silica nanospheres of approximately 180 nm.
Conclusion
Thermal Au deposition on 2L–4L MoS2 causes drastic PL quenching, which the authors associate with electron transfer from MoS2 to Au and resulting p-doping. The effect may enable localized tuning where Au patches are patterned.
- Conclusion: Au nanostructure size is controlled by deposited thickness, and drastic PL quenching occurs irrespective of MoS2 thickness from 2L to 4L.The conclusion attributes the effect to electron transfer from MoS2 to Au because MoS2 has the lower work function.
- Conclusion: Electron transfer from excited MoS2 states to Au leaves holes behind and causes p-doping in MoS2.The conclusion presents this as the proposed electronic-structure change underlying quenching.
- Conclusion: Patterned Au nanostructure patches could provide localized tunability of 2D TMD properties.The authors identify possible applications in nanoelectronics and nano-optics.
Methods
The study mechanically exfoliates few-layer MoS2 onto Si/SiO2, characterizes it with optical, AFM, SEM, Raman, PL, and XPS methods, and deposits Au by thermal evaporation. PL and structural measurements compare pristine and Au-decorated flakes.
- Sample preparation: Few-layer MoS2 is mechanically exfoliated from crystalline bulk material onto highly doped Si with a 250 nm thermally grown oxide layer.Optical microscopy with a 100X objective identifies the exfoliated flakes.
- Spectroscopy: Raman and PL spectra are recorded using a confocal Raman system with 532.0 nm illumination in ambient air at room temperature.The laser power is set to 0.6 mW and integration time to 1 s for data acquisition.
- Au deposition: Au is thermally evaporated onto MoS2 at 1.5X10^-6 mBar and 0.02 Å per second to form nanostructures from 2.0 nm and 1.0 nm depositions.AFM and SEM estimate the resulting nanostructure height and size.
- Characterization: AFM, SEM, and XPS characterize the Au-MoS2 hybrid morphology and chemical structure.SEM is performed at approximately 5 kV, while XPS uses a monochromatized Al Kα source.
Author contribution statement
The supplied contribution statement assigns sample preparation, imaging, spectroscopy, experiment direction, manuscript preparation, and review across the listed authors. The accompanying figure materials document the study’s experimental results.
- Author contributions: U.B. and M.R.I. prepared samples and performed XPS, optical imaging, and SEM; U.B. also performed AFM imaging.
- Author contributions: U.B. and L.T. acquired PL and Raman data, while L.T. and S.K. directed the experiment.
- Author contributions: U.B., L.T., and S.K. contributed to manuscript preparation, and all authors reviewed the manuscript.
- Disclosure: The paper declares no competing financial interests.
Supplementary information
The supplementary information documents layer identification, Au nanostructure heights, and local PL variation in pristine and Au-deposited MoS2 flakes.
- Layer identification: Raman peak separations of 21.5 and 24.3 cm-1 identify the 2L and 4L regions of pristine MoS2, respectively.The separations are measured between the E2g and A1g peaks.
- Layer identification: Raman peak separations of 21.1 and 23.4 cm-1 agree with the 2L and 3L regions identified in the AFM image.The spectra were acquired at locations labeled 2L and 3L.
- Au nanostructures: Average Au nanostructure heights are 1.6 nm after 2.0 nm deposition and 0.6 nm after 1.0 nm deposition.The values were obtained from height profiles used for RMS analysis.
- Photoluminescence: PL intensity varies locally across pristine MoS2, with spectra distinguishing 2L regions near the flake edge, near the 3L interface, and representative 3L regions.Five points were measured along a line shown in the A1 peak intensity map.