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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: 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.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.


Excited-state quenching of GBs and elemental mapping of ML-MoS2.Far-field confocal micro-PL (a) and nano-PL (b) images of an aggregate of three flakes (labelled 1, 2 and 3) forming three interflake GBs. In the interior of flake 1, radial intraflake GBs are observed extending from the centre towards the apexes of the triangular flake. The interflake GB quenches the PL intensity by 50–80%, whereas the intraflake GB quenches the PL intensity by ∼20%. Scale bars, 1 μm. (c) Map of the emission energy (that is, the spectral median value defined in Fig. 1d). Scale bar, 1 μm. (d) Histograms of the half width at half max sizes of the interflake and intraflake GBs, which are measured from the spatial extent of the PL reduction and sampled semi-equidistantly along the respective features (see Supplementary Fig. 9 for more details). (e) Nano-Auger elemental mapping of S and Mo on a similar multiflake aggregate from the same growth run. Both the edge region and GBs are S-deficient, while the Mo composition is uniform over the flakes. Scale bar, 2 μm.
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f4: Excited-state quenching of GBs and elemental mapping of ML-MoS2.Far-field confocal micro-PL (a) and nano-PL (b) images of an aggregate of three flakes (labelled 1, 2 and 3) forming three interflake GBs. In the interior of flake 1, radial intraflake GBs are observed extending from the centre towards the apexes of the triangular flake. The interflake GB quenches the PL intensity by 50–80%, whereas the intraflake GB quenches the PL intensity by ∼20%. Scale bars, 1 μm. (c) Map of the emission energy (that is, the spectral median value defined in Fig. 1d). Scale bar, 1 μm. (d) Histograms of the half width at half max sizes of the interflake and intraflake GBs, which are measured from the spatial extent of the PL reduction and sampled semi-equidistantly along the respective features (see Supplementary Fig. 9 for more details). (e) Nano-Auger elemental mapping of S and Mo on a similar multiflake aggregate from the same growth run. Both the edge region and GBs are S-deficient, while the Mo composition is uniform over the flakes. Scale bar, 2 μm.

Mentions: Whereas single-crystalline flakes of ML-MoS2 can be anatomized into an interior and edge, disruptions to the crystalline structure can also occur during the CVD growth process. Isolated polycrystalline MoS2 flakes can form intricate star-like structures while aggregates of flakes that merge during growth form complex polycrystalline patchworks1923. The resulting GBs can significantly alter the local optoelectronic properties of the flake interior1932. For example, some types of GBs are known to quench excitons, locally reducing the PL quantum yield19, but the limited resolution of conventional optical microscopy fails to precisely resolve the quenching phenomena. In Fig. 4, confocal micro-PL (Fig. 4a) and Campanile nano-PL (Fig. 4b) maps of three MoS2 flakes (labelled 1, 2 and 3) that merged during growth are compared. Although the confocal optical microscopy image (Fig. 4a) exhibits a mostly uniform PL intensity distribution within each flake, indicating a ‘high-quality' sample, substantial nanoscale fluctuations are observed in the nano-PL image. Furthermore, a reduction in the PL intensity that corresponds to an exciton-quenching region marks the boundaries between flakes 1 and 2 as well as flakes 2 and 3. This effect is better resolved by the Campanile probe, more precisely quantifying the reduction in PL intensity. In addition to these interflake GBs, three narrow regions where the PL is quenched by ∼20% extending radially from the centre of flake 1 (also in flake 3 and others shown in the Supplementary Fig. 5) are also better resolved in the nano-PL map. Interestingly, these GBs do not seem to alter the energetics of the PL, as can be observed in the nano-PL map of the emission energy in Fig. 4c, which is mostly devoid of systematic variations in the vicinity of the intra- and interflake GBs.


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)

Excited-state quenching of GBs and elemental mapping of ML-MoS2.Far-field confocal micro-PL (a) and nano-PL (b) images of an aggregate of three flakes (labelled 1, 2 and 3) forming three interflake GBs. In the interior of flake 1, radial intraflake GBs are observed extending from the centre towards the apexes of the triangular flake. The interflake GB quenches the PL intensity by 50–80%, whereas the intraflake GB quenches the PL intensity by ∼20%. Scale bars, 1 μm. (c) Map of the emission energy (that is, the spectral median value defined in Fig. 1d). Scale bar, 1 μm. (d) Histograms of the half width at half max sizes of the interflake and intraflake GBs, which are measured from the spatial extent of the PL reduction and sampled semi-equidistantly along the respective features (see Supplementary Fig. 9 for more details). (e) Nano-Auger elemental mapping of S and Mo on a similar multiflake aggregate from the same growth run. Both the edge region and GBs are S-deficient, while the Mo composition is uniform over the flakes. Scale bar, 2 μm.
© Copyright Policy - open-access
Related In: Results  -  Collection

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

f4: Excited-state quenching of GBs and elemental mapping of ML-MoS2.Far-field confocal micro-PL (a) and nano-PL (b) images of an aggregate of three flakes (labelled 1, 2 and 3) forming three interflake GBs. In the interior of flake 1, radial intraflake GBs are observed extending from the centre towards the apexes of the triangular flake. The interflake GB quenches the PL intensity by 50–80%, whereas the intraflake GB quenches the PL intensity by ∼20%. Scale bars, 1 μm. (c) Map of the emission energy (that is, the spectral median value defined in Fig. 1d). Scale bar, 1 μm. (d) Histograms of the half width at half max sizes of the interflake and intraflake GBs, which are measured from the spatial extent of the PL reduction and sampled semi-equidistantly along the respective features (see Supplementary Fig. 9 for more details). (e) Nano-Auger elemental mapping of S and Mo on a similar multiflake aggregate from the same growth run. Both the edge region and GBs are S-deficient, while the Mo composition is uniform over the flakes. Scale bar, 2 μm.
Mentions: Whereas single-crystalline flakes of ML-MoS2 can be anatomized into an interior and edge, disruptions to the crystalline structure can also occur during the CVD growth process. Isolated polycrystalline MoS2 flakes can form intricate star-like structures while aggregates of flakes that merge during growth form complex polycrystalline patchworks1923. The resulting GBs can significantly alter the local optoelectronic properties of the flake interior1932. For example, some types of GBs are known to quench excitons, locally reducing the PL quantum yield19, but the limited resolution of conventional optical microscopy fails to precisely resolve the quenching phenomena. In Fig. 4, confocal micro-PL (Fig. 4a) and Campanile nano-PL (Fig. 4b) maps of three MoS2 flakes (labelled 1, 2 and 3) that merged during growth are compared. Although the confocal optical microscopy image (Fig. 4a) exhibits a mostly uniform PL intensity distribution within each flake, indicating a ‘high-quality' sample, substantial nanoscale fluctuations are observed in the nano-PL image. Furthermore, a reduction in the PL intensity that corresponds to an exciton-quenching region marks the boundaries between flakes 1 and 2 as well as flakes 2 and 3. This effect is better resolved by the Campanile probe, more precisely quantifying the reduction in PL intensity. In addition to these interflake GBs, three narrow regions where the PL is quenched by ∼20% extending radially from the centre of flake 1 (also in flake 3 and others shown in the Supplementary Fig. 5) are also better resolved in the nano-PL map. Interestingly, these GBs do not seem to alter the energetics of the PL, as can be observed in the nano-PL map of the emission energy in Fig. 4c, which is mostly devoid of systematic variations in the vicinity of the intra- and interflake GBs.

Bottom Line: 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.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.