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Visualizing nanoscale excitonic relaxation properties of disordered edges and grain boundaries in monolayer molybdenum disulfide.

Bao W, Borys NJ, Ko C, Suh J, Fan W, Thron A, Zhang Y, Buyanin A, Zhang J, Cabrini S, Ashby PD, Weber-Bargioni A, Tongay S, Aloni S, Ogletree DF, Wu J, Salmeron MB, Schuck PJ - Nat Commun (2015)

Bottom Line: Although challenging for two-dimensional systems, sub-diffraction optical microscopy provides a nanoscale material understanding that is vital for optimizing their optoelectronic properties.Synthetic monolayer MoS2 is found to be composed of two distinct optoelectronic regions: an interior, locally ordered but mesoscopically heterogeneous two-dimensional quantum well and an unexpected ∼300-nm wide, energetically disordered edge region.The nanoscale structure-property relationships established here are critical for the interpretation of edge- and boundary-related phenomena and the development of next-generation two-dimensional optoelectronic devices.

View Article: PubMed Central - PubMed

Affiliation: 1] Molecular Foundry, Lawrence Berkeley National Laboratory, 1 Cyclotron Road, Berkeley, California 94720, USA [2] Materials Sciences Division, Lawrence Berkeley National Laboratory, 1 Cyclotron Road, Berkeley, California 94720, USA [3] Department of Materials Science and Engineering, University of California Berkeley, 210 Hearst Mining Building, Berkeley, California 94720, USA.

ABSTRACT
Two-dimensional monolayer transition metal dichalcogenide semiconductors are ideal building blocks for atomically thin, flexible optoelectronic and catalytic devices. Although challenging for two-dimensional systems, sub-diffraction optical microscopy provides a nanoscale material understanding that is vital for optimizing their optoelectronic properties. Here we use the 'Campanile' nano-optical probe to spectroscopically image exciton recombination within monolayer MoS2 with sub-wavelength resolution (60 nm), at the length scale relevant to many critical optoelectronic processes. Synthetic monolayer MoS2 is found to be composed of two distinct optoelectronic regions: an interior, locally ordered but mesoscopically heterogeneous two-dimensional quantum well and an unexpected ∼300-nm wide, energetically disordered edge region. Further, grain boundaries are imaged with sufficient resolution to quantify local exciton-quenching phenomena, and complimentary nano-Auger microscopy reveals that the optically defective grain boundary and edge regions are sulfur deficient. The nanoscale structure-property relationships established here are critical for the interpretation of edge- and boundary-related phenomena and the development of next-generation two-dimensional optoelectronic devices.

No MeSH data available.


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Optoelectronic discrimination between edge and interior regions of ML-MoS2.(a,b) Nano-PL images of emission intensity and SM (defined in Fig. 1d), respectively, of a single flake of ML-MoS2. The dotted line marks the boundary between the interior of the flake and a ∼300-nm wide periphery edge. Scale bars, 1 μm. (c) Emission intensity of each pixel plotted against its spectral median value for the interior (orange data points) and the edge regions (blue data points). Each data point in c corresponds to an emission spectrum recorded at a different position. The emission spectra from the interior and edges can be further grouped by their total emission intensity into one of five ranges (I, II, III, IV and V). (d) Averaged emission spectra of the interior data points for the intensity ranges II, III, IV and V (range I does not contain any interior data points) plotted on a normalized, semi-log scale for comparison. (e) Likewise, average emission spectra for the data points from the edge region for the intensity ranges I, II and III (ranges IV and V do not contain any data points from the edge region).
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f2: Optoelectronic discrimination between edge and interior regions of ML-MoS2.(a,b) Nano-PL images of emission intensity and SM (defined in Fig. 1d), respectively, of a single flake of ML-MoS2. The dotted line marks the boundary between the interior of the flake and a ∼300-nm wide periphery edge. Scale bars, 1 μm. (c) Emission intensity of each pixel plotted against its spectral median value for the interior (orange data points) and the edge regions (blue data points). Each data point in c corresponds to an emission spectrum recorded at a different position. The emission spectra from the interior and edges can be further grouped by their total emission intensity into one of five ranges (I, II, III, IV and V). (d) Averaged emission spectra of the interior data points for the intensity ranges II, III, IV and V (range I does not contain any interior data points) plotted on a normalized, semi-log scale for comparison. (e) Likewise, average emission spectra for the data points from the edge region for the intensity ranges I, II and III (ranges IV and V do not contain any data points from the edge region).

Mentions: A nano-PL map of another ML-MoS2 flake is shown in Fig. 2. Spatial variations in the intensity (Fig. 2a) and emission energy (Fig. 2b) are observed across the flake in addition to a decrease in PL intensity near the edges. These trends are observed in all of the ML-MoS2 flakes that were investigated in this study (Supplementary Fig. 5) and do not change over an order of magnitude of excitation power (Supplementary Fig. 6). In ML-MoS2, increased relative emission of the trion is correlated with a reduction in the overall PL intensity15. To explore this type of behaviour in our spatially resolved data, we plot the integrated intensity of the PL acquired at each spatial position versus its emission energy (that is, its spectral median) in Fig. 2c. Two distinct clusters are observed. By systematically segregating the data points into either an internal interior region or a peripheral edge region (see Supplementary Fig. 7 for details), we find that the brighter cluster of correlated data points (orange data points) belongs to the interior of the ML-MoS2, while the significantly more scattered, dimmer cluster of points (blue data points) belongs to an ∼300-nm peripheral edge region. In the interior, the intensity and energy of the PL are correlated, with the higher-energy emission tending to be brightest. Such behaviour is consistent with increased emission intensity from regions with reduced trion populations. In contrast, the emission energy of the PL from the edge region is more disordered, spanning nearly the entire range of emission energies at the lower emission intensities.


Visualizing nanoscale excitonic relaxation properties of disordered edges and grain boundaries in monolayer molybdenum disulfide.

Bao W, Borys NJ, Ko C, Suh J, Fan W, Thron A, Zhang Y, Buyanin A, Zhang J, Cabrini S, Ashby PD, Weber-Bargioni A, Tongay S, Aloni S, Ogletree DF, Wu J, Salmeron MB, Schuck PJ - Nat Commun (2015)

Optoelectronic discrimination between edge and interior regions of ML-MoS2.(a,b) Nano-PL images of emission intensity and SM (defined in Fig. 1d), respectively, of a single flake of ML-MoS2. The dotted line marks the boundary between the interior of the flake and a ∼300-nm wide periphery edge. Scale bars, 1 μm. (c) Emission intensity of each pixel plotted against its spectral median value for the interior (orange data points) and the edge regions (blue data points). Each data point in c corresponds to an emission spectrum recorded at a different position. The emission spectra from the interior and edges can be further grouped by their total emission intensity into one of five ranges (I, II, III, IV and V). (d) Averaged emission spectra of the interior data points for the intensity ranges II, III, IV and V (range I does not contain any interior data points) plotted on a normalized, semi-log scale for comparison. (e) Likewise, average emission spectra for the data points from the edge region for the intensity ranges I, II and III (ranges IV and V do not contain any data points from the edge region).
© Copyright Policy - open-access
Related In: Results  -  Collection

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Show All Figures
getmorefigures.php?uid=PMC4557266&req=5

f2: Optoelectronic discrimination between edge and interior regions of ML-MoS2.(a,b) Nano-PL images of emission intensity and SM (defined in Fig. 1d), respectively, of a single flake of ML-MoS2. The dotted line marks the boundary between the interior of the flake and a ∼300-nm wide periphery edge. Scale bars, 1 μm. (c) Emission intensity of each pixel plotted against its spectral median value for the interior (orange data points) and the edge regions (blue data points). Each data point in c corresponds to an emission spectrum recorded at a different position. The emission spectra from the interior and edges can be further grouped by their total emission intensity into one of five ranges (I, II, III, IV and V). (d) Averaged emission spectra of the interior data points for the intensity ranges II, III, IV and V (range I does not contain any interior data points) plotted on a normalized, semi-log scale for comparison. (e) Likewise, average emission spectra for the data points from the edge region for the intensity ranges I, II and III (ranges IV and V do not contain any data points from the edge region).
Mentions: A nano-PL map of another ML-MoS2 flake is shown in Fig. 2. Spatial variations in the intensity (Fig. 2a) and emission energy (Fig. 2b) are observed across the flake in addition to a decrease in PL intensity near the edges. These trends are observed in all of the ML-MoS2 flakes that were investigated in this study (Supplementary Fig. 5) and do not change over an order of magnitude of excitation power (Supplementary Fig. 6). In ML-MoS2, increased relative emission of the trion is correlated with a reduction in the overall PL intensity15. To explore this type of behaviour in our spatially resolved data, we plot the integrated intensity of the PL acquired at each spatial position versus its emission energy (that is, its spectral median) in Fig. 2c. Two distinct clusters are observed. By systematically segregating the data points into either an internal interior region or a peripheral edge region (see Supplementary Fig. 7 for details), we find that the brighter cluster of correlated data points (orange data points) belongs to the interior of the ML-MoS2, while the significantly more scattered, dimmer cluster of points (blue data points) belongs to an ∼300-nm peripheral edge region. In the interior, the intensity and energy of the PL are correlated, with the higher-energy emission tending to be brightest. Such behaviour is consistent with increased emission intensity from regions with reduced trion populations. In contrast, the emission energy of the PL from the edge region is more disordered, spanning nearly the entire range of emission energies at the lower emission intensities.

Bottom Line: Although challenging for two-dimensional systems, sub-diffraction optical microscopy provides a nanoscale material understanding that is vital for optimizing their optoelectronic properties.Synthetic monolayer MoS2 is found to be composed of two distinct optoelectronic regions: an interior, locally ordered but mesoscopically heterogeneous two-dimensional quantum well and an unexpected ∼300-nm wide, energetically disordered edge region.The nanoscale structure-property relationships established here are critical for the interpretation of edge- and boundary-related phenomena and the development of next-generation two-dimensional optoelectronic devices.

View Article: PubMed Central - PubMed

Affiliation: 1] Molecular Foundry, Lawrence Berkeley National Laboratory, 1 Cyclotron Road, Berkeley, California 94720, USA [2] Materials Sciences Division, Lawrence Berkeley National Laboratory, 1 Cyclotron Road, Berkeley, California 94720, USA [3] Department of Materials Science and Engineering, University of California Berkeley, 210 Hearst Mining Building, Berkeley, California 94720, USA.

ABSTRACT
Two-dimensional monolayer transition metal dichalcogenide semiconductors are ideal building blocks for atomically thin, flexible optoelectronic and catalytic devices. Although challenging for two-dimensional systems, sub-diffraction optical microscopy provides a nanoscale material understanding that is vital for optimizing their optoelectronic properties. Here we use the 'Campanile' nano-optical probe to spectroscopically image exciton recombination within monolayer MoS2 with sub-wavelength resolution (60 nm), at the length scale relevant to many critical optoelectronic processes. Synthetic monolayer MoS2 is found to be composed of two distinct optoelectronic regions: an interior, locally ordered but mesoscopically heterogeneous two-dimensional quantum well and an unexpected ∼300-nm wide, energetically disordered edge region. Further, grain boundaries are imaged with sufficient resolution to quantify local exciton-quenching phenomena, and complimentary nano-Auger microscopy reveals that the optically defective grain boundary and edge regions are sulfur deficient. The nanoscale structure-property relationships established here are critical for the interpretation of edge- and boundary-related phenomena and the development of next-generation two-dimensional optoelectronic devices.

No MeSH data available.


Related in: MedlinePlus