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Transition metal dichalcogenide growth via close proximity precursor supply.

O'Brien M, McEvoy N, Hallam T, Kim HY, Berner NC, Hanlon D, Lee K, Coleman JN, Duesberg GS - Sci Rep (2014)

Bottom Line: TMD monolayers were realized using a close proximity precursor supply in a CVD microreactor setup.A model describing the growth mechanism, which is capable of producing TMD monolayers on arbitrary substrates, is presented.Furthermore, through patterning of the precursor supply, we achieve patterned growth of monolayer TMDs in defined locations, which could be adapted for the facile production of electronic device components.

View Article: PubMed Central - PubMed

Affiliation: 1] School of Chemistry, Trinity College Dublin, Dublin 2, Ireland [2] Centre for Research on Adaptive Nanostructures and Nanodevices (CRANN) and Advanced Materials and BioEngineering Research (AMBER) Centre, Trinity College Dublin, Dublin 2, Ireland.

ABSTRACT
Reliable chemical vapour deposition (CVD) of transition metal dichalcogenides (TMDs) is currently a highly pressing research field, as numerous potential applications rely on the production of high quality films on a macroscopic scale. Here, we show the use of liquid phase exfoliated nanosheets and patterned sputter deposited layers as solid precursors for chemical vapour deposition. TMD monolayers were realized using a close proximity precursor supply in a CVD microreactor setup. A model describing the growth mechanism, which is capable of producing TMD monolayers on arbitrary substrates, is presented. Raman spectroscopy, photoluminescence, X-ray photoelectron spectroscopy, atomic force microscopy, transmission electron microscopy, scanning electron microscopy and electrical transport measurements reveal the high quality of the TMD samples produced. Furthermore, through patterning of the precursor supply, we achieve patterned growth of monolayer TMDs in defined locations, which could be adapted for the facile production of electronic device components.

No MeSH data available.


(a) Optical image of an MoS2 film. Scale bar is 6 µm (b) Map of PL maximum intensity of the same area as in (a). The dark areas show a decrease in PL intensity in the vicinity of grain boundaries (c) Average PL spectrum over the scanned area of 14,400 individual point spectra. (d) Map of E' Raman peak maximum intensity (e) Map of A'1 Raman peak maximum intensity (f) Average Raman spectra over grain boundary and non-grain boundary regions. Additional maps of this region and masks used to extract average spectra over grain boundary and non-grain boundary regions can be found in Section S5 of the Supporting Information.
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f2: (a) Optical image of an MoS2 film. Scale bar is 6 µm (b) Map of PL maximum intensity of the same area as in (a). The dark areas show a decrease in PL intensity in the vicinity of grain boundaries (c) Average PL spectrum over the scanned area of 14,400 individual point spectra. (d) Map of E' Raman peak maximum intensity (e) Map of A'1 Raman peak maximum intensity (f) Average Raman spectra over grain boundary and non-grain boundary regions. Additional maps of this region and masks used to extract average spectra over grain boundary and non-grain boundary regions can be found in Section S5 of the Supporting Information.

Mentions: It has been reported that the intensity of photoluminescence (PL) in MoS2 increases dramatically with decreasing layer number and that luminescence from a monolayer is the most prominent, while being absent in bulk material4. This is because the absence of interlayer coupling of electronic states at the Γ point of single layer MoS2's Brillouin zone4 results in enhanced absorption and PL bands at a direct bandgap of ~1.8 eV, as opposed to an indirect bandgap of ~1.3 eV in the bulk34. This results in two resonances that have been well established to be direct excitonic transitions, known as the A1 and B1 excitons4, at ~1.85 and 1.98 eV, respectively. Fig. 2(a) shows a magnified optical image of an area of MoS2 monolayer over which PL and Raman maps were obtained. The average PL spectrum is shown in Fig. 2(c) with the relevant photoluminescence intensity map shown in Fig. 2(b). This map shows the emergence of PL peaks expected for monolayer MoS23, verifying the high crystallinity and monolayer nature of the material. Raman spectroscopy was also used to evaluate the quality of the as-grown material. The MoS2 Raman spectrum displays two characteristic Raman active modes, which in the case of a monolayer crystal are E' at ~385 cm−1, and A'1 at ~403 cm−142. These arise from in-plane vibrations of Mo and S atoms and out-of-plane vibrations of S atoms in different directions only, respectively42. These peaks have been shown to shift in position with layer number43, allowing monolayer MoS2 to be identified easily and quickly. The maps of E' in Fig. 2(d) and A'1 in Fig. 2(e) show little variation in intensity over the entire area, apart from in the presence of grain boundaries. The average Raman spectrum for the scanned area in Fig. 2(f) shows peak positions of ~385 and 403 cm−1 for E' and A'1, respectively for non-grain boundary areas4243, giving a separation of 18 cm−1 as expected for monolayer MoS24243. In the vicinity of the grain boundaries, both the A'1 mode intensity and the peak separation of E' and A'1 decreases. This can be explained by correlating the Raman peak change to the PL map. The decrease in intensity at grain boundaries of the PL map suggests they are molybdenum rich, and therefore n-doped36, quenching the photoluminescence. Previous reports show that n-doping results in softening of the A'1 phonon, meaning a decrease of relative intensity and peak frequency difference between E' and A'1 Raman modes4445. This implies that the grain boundaries shown here are n-doped, due to the decrease in A'1 peak intensity, the red shift in A'1 peak position, and the localized decrease in PL intensity. Additional Raman and PL maps from this region can be found in Section S5 of the Supporting Information, along with the masks used to extract average spectra from grain boundary and non-grain boundary regions.


Transition metal dichalcogenide growth via close proximity precursor supply.

O'Brien M, McEvoy N, Hallam T, Kim HY, Berner NC, Hanlon D, Lee K, Coleman JN, Duesberg GS - Sci Rep (2014)

(a) Optical image of an MoS2 film. Scale bar is 6 µm (b) Map of PL maximum intensity of the same area as in (a). The dark areas show a decrease in PL intensity in the vicinity of grain boundaries (c) Average PL spectrum over the scanned area of 14,400 individual point spectra. (d) Map of E' Raman peak maximum intensity (e) Map of A'1 Raman peak maximum intensity (f) Average Raman spectra over grain boundary and non-grain boundary regions. Additional maps of this region and masks used to extract average spectra over grain boundary and non-grain boundary regions can be found in Section S5 of the Supporting Information.
© Copyright Policy - open-access
Related In: Results  -  Collection

License
Show All Figures
getmorefigures.php?uid=PMC4260218&req=5

f2: (a) Optical image of an MoS2 film. Scale bar is 6 µm (b) Map of PL maximum intensity of the same area as in (a). The dark areas show a decrease in PL intensity in the vicinity of grain boundaries (c) Average PL spectrum over the scanned area of 14,400 individual point spectra. (d) Map of E' Raman peak maximum intensity (e) Map of A'1 Raman peak maximum intensity (f) Average Raman spectra over grain boundary and non-grain boundary regions. Additional maps of this region and masks used to extract average spectra over grain boundary and non-grain boundary regions can be found in Section S5 of the Supporting Information.
Mentions: It has been reported that the intensity of photoluminescence (PL) in MoS2 increases dramatically with decreasing layer number and that luminescence from a monolayer is the most prominent, while being absent in bulk material4. This is because the absence of interlayer coupling of electronic states at the Γ point of single layer MoS2's Brillouin zone4 results in enhanced absorption and PL bands at a direct bandgap of ~1.8 eV, as opposed to an indirect bandgap of ~1.3 eV in the bulk34. This results in two resonances that have been well established to be direct excitonic transitions, known as the A1 and B1 excitons4, at ~1.85 and 1.98 eV, respectively. Fig. 2(a) shows a magnified optical image of an area of MoS2 monolayer over which PL and Raman maps were obtained. The average PL spectrum is shown in Fig. 2(c) with the relevant photoluminescence intensity map shown in Fig. 2(b). This map shows the emergence of PL peaks expected for monolayer MoS23, verifying the high crystallinity and monolayer nature of the material. Raman spectroscopy was also used to evaluate the quality of the as-grown material. The MoS2 Raman spectrum displays two characteristic Raman active modes, which in the case of a monolayer crystal are E' at ~385 cm−1, and A'1 at ~403 cm−142. These arise from in-plane vibrations of Mo and S atoms and out-of-plane vibrations of S atoms in different directions only, respectively42. These peaks have been shown to shift in position with layer number43, allowing monolayer MoS2 to be identified easily and quickly. The maps of E' in Fig. 2(d) and A'1 in Fig. 2(e) show little variation in intensity over the entire area, apart from in the presence of grain boundaries. The average Raman spectrum for the scanned area in Fig. 2(f) shows peak positions of ~385 and 403 cm−1 for E' and A'1, respectively for non-grain boundary areas4243, giving a separation of 18 cm−1 as expected for monolayer MoS24243. In the vicinity of the grain boundaries, both the A'1 mode intensity and the peak separation of E' and A'1 decreases. This can be explained by correlating the Raman peak change to the PL map. The decrease in intensity at grain boundaries of the PL map suggests they are molybdenum rich, and therefore n-doped36, quenching the photoluminescence. Previous reports show that n-doping results in softening of the A'1 phonon, meaning a decrease of relative intensity and peak frequency difference between E' and A'1 Raman modes4445. This implies that the grain boundaries shown here are n-doped, due to the decrease in A'1 peak intensity, the red shift in A'1 peak position, and the localized decrease in PL intensity. Additional Raman and PL maps from this region can be found in Section S5 of the Supporting Information, along with the masks used to extract average spectra from grain boundary and non-grain boundary regions.

Bottom Line: TMD monolayers were realized using a close proximity precursor supply in a CVD microreactor setup.A model describing the growth mechanism, which is capable of producing TMD monolayers on arbitrary substrates, is presented.Furthermore, through patterning of the precursor supply, we achieve patterned growth of monolayer TMDs in defined locations, which could be adapted for the facile production of electronic device components.

View Article: PubMed Central - PubMed

Affiliation: 1] School of Chemistry, Trinity College Dublin, Dublin 2, Ireland [2] Centre for Research on Adaptive Nanostructures and Nanodevices (CRANN) and Advanced Materials and BioEngineering Research (AMBER) Centre, Trinity College Dublin, Dublin 2, Ireland.

ABSTRACT
Reliable chemical vapour deposition (CVD) of transition metal dichalcogenides (TMDs) is currently a highly pressing research field, as numerous potential applications rely on the production of high quality films on a macroscopic scale. Here, we show the use of liquid phase exfoliated nanosheets and patterned sputter deposited layers as solid precursors for chemical vapour deposition. TMD monolayers were realized using a close proximity precursor supply in a CVD microreactor setup. A model describing the growth mechanism, which is capable of producing TMD monolayers on arbitrary substrates, is presented. Raman spectroscopy, photoluminescence, X-ray photoelectron spectroscopy, atomic force microscopy, transmission electron microscopy, scanning electron microscopy and electrical transport measurements reveal the high quality of the TMD samples produced. Furthermore, through patterning of the precursor supply, we achieve patterned growth of monolayer TMDs in defined locations, which could be adapted for the facile production of electronic device components.

No MeSH data available.