Manganese Doping of Monolayer MoS2: The Substrate Is Critical

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Manganese Doping of Monolayer MoS2: The Substrate is Critical Kehao Zhang, Simin Feng, JUNJIE WANG, Angelica Azcatl, Ning Lu, Rafik Addou, Nan Wang, Chanjing Zhou, Jordan Oswald Lerach, Vincent Bojan, Moon J. Kim, Long-Qing Chen, Robert M. Wallace, Mauricio Terrones, Jun Zhu, and Joshua Alexander Robinson Nano Lett., Just Accepted Manuscript • DOI: 10.1021/acs.nanolett.5b02315 • Publication Date (Web): 09 Sep 2015 Downloaded from http://pubs.acs.org on September 9, 2015

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Manganese Doping of Monolayer MoS2: The Substrate is Critical Kehao Zhang1,2, Simin Feng2,3, Junjie Wang3, Angelica Azcatl4, Ning Lu4, Rafik Addou4, Nan Wang1, Chanjing Zhou1,3, Jordan Lerach5, Vincent Bojan5, Moon J. Kim4, Long-Qing Chen1, Robert M. Wallace4, Mauricio Terrones1,2,3, Jun Zhu2,3 and Joshua A. Robinson1,2 1. 2. 3. 4. 5.

Department of Materials Science and Engineering, The Pennsylvania State University, University Park, PA 16802 Center for 2-Dimensional and Layered Materials, The Pennsylvania State University, University Park, PA 16802 Department of Physics, The Pennsylvania State University, University Park, PA 16802 Department of Materials Science and Engineering, The University of Texas at Dallas, Richardson, TX, 75080, USA Materials Characterization Laboratory, The Pennsylvania State University, University Park, PA 16802

Key words: Transition metal dichalcogenide, molybdenum disulfide, manganese, two-dimensional heterostructure, in-situ doping

Abstract Substitutional doping of transition metal dichalcogenides (TMDs) may provide routes to achieving tunable p-n junctions, bandgaps, chemical sensitivity, and magnetism in these materials. In this study, we demonstrate in-situ doping of monolayer molybdenum disulfide (MoS2) with manganese (Mn) via vapor phase deposition techniques. Successful incorporation of Mn in MoS2 leads to modifications of the band structure as evidenced by photoluminescence and x-ray photoelectron spectroscopy, but this is heavily dependent on the choice of substrate. We show that inert substrates (i.e. graphene) permit the incorporation of several percent Mn in MoS2, while substrates with reactive surface terminations (i.e. SiO2 and sapphire) preclude Mn incorporation and merely lead to defective MoS2. The results presented here demonstrate that tailoring the substrate surface could be the most significant factor in substitutional doping of TMDs with non-TMD elements.

The unique properties and great potential of transition metal dichalcogenides (TMDs) in electronic and optoelectronic applications1–4 have established these materials as the next family in 2D materials research beyond graphene. Doping could further increase the functionality of 2D-TMDs by providing routes to tune their intrinsic properties. For example, selenium and other chemical doping of MoS2 is an effective way to engineer the optical band gap;5,6 and bulk Nbdoped MoS2 exfoliated to monolayers exhibit p-type transport properties.7 However, chemical doping is less stable than substitutional doping, therefore, establishing methods for robust substitutional doping is essential for next generation applications.8 To-date, alloying and substitutional doping of TMDs has been limited to mixing of elements with similar valence and atomic arrangement, such as MoS2+WS2 and MoS2+MoSe2 (like-elements).9–11 While alloying of like-elements can lead to novel physical phenomenon, substitutional doping of TMDs with elements that exhibit a competing bonding coordination (non-like elements) could further enhance the community’s ability to develop novel devices. For instance, manganese (Mn) typically bonds with sulfur (S) in the rock-salt (α-MnS), zinc-blende (β-MnS) or wurtzite (γ-MnS) structure,12–14 not a hexagonal layered structure like that of TMDs, which limits the amount of doping/alloying one can do before morphing the structure from 2D to 3D. However, if one is able to achieve 10 at% Mn in the structure, it is could lead to measureable magnetism in these

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monolayers (i.e. 2D magnets).15–17 Here, we demonstrate that Mn doping of MoS2 is possible via utilization of in-situ vapor-phase Mn precursors during MoS2 synthesis. Molecular spectroscopy, x-ray photoelectron spectroscopy (XPS), and high-resolution transmission electron microscopy (HRTEM), provide evidence that up to two atomic percent (2% at.) doping of Mn can occur within MoS2 when graphene is used as substrate; however, beyond 2 at%, maintaining a 2D morphology becomes quite challenging – a byproduct of the competing MoS2-MnS bonding coordination. Furthermore, attempts at doping MoS2 on traditional substrates (SiO2, sapphire) provides strong evidence that the substrate surface chemistry can be detrimental to the doping process, and will ultimately be the dominant factor when attempting to dope TMDs with non-like elements. Pristine and Mn-doped MoS2 monolayers were grown via elemental and oxide power vaporization (PV) and metal-organic powder vaporization. Figure 1a illustrates the furnace set up, in which 2 mg of MoO3 (99.8 % Sigma Aldrich) is vaporized at 725 °C in the center of the furnace. Simultaneously, 700 mg of S powder and 0.1 mg of dimanganese decacarbonyl Mn2(CO)10 powder (Mn/Mo precursor ratio of 1:50), placed just upstream of the furnace, is heated to 200°C and 70°C, respectively. The Mn and S vapors are introduced into the furnace using 400 sccm of argon (Ar) gas. Three substrates (sapphire, SiO2, graphene) are placed facedown just above the MoO3 powder to ensure maximum exposure of MoO3 vapor to the substrate surface. Using the typical oxide vaporization technique,18,21 the grain size of pristine MoS2 on sapphire is 10-20 µm, while it is approximately 1 µm on SiO2 (Figure S1a,b). The growth on sapphire is larger than the growth on SiO2 due to the lower nucleation density on sapphire under our given conditions.22 Raman spectroscopy (Figure S1c) and atomic force microscopy (AFM) confirm that each domain is monolayer (thickness is 7Å, and A1g/E2g separation is ~19cm-1) (Figure S2).23–25 Graphene is grown via silicon sublimation (epitaxial graphene)26 and chemical vapor deposition (CVD graphene)27. Epitaxial graphene is utilized for chemical analysis of pristine and doped MoS2, while CVD graphene is used for direct growth of MoS2 and doped MoS2 on TEM grids.28 In this study, we find no statistically significant difference in the MoS2 properties based on the choice of graphene.


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Figure 1: Synthesis of pristine and manganese-doped MoS2 is achieved using (a) traditional powder vaporization techniques. Dimanganese decacarbonyl (Mn2(CO)10) and sulfur (S) are located upstream of the hotzone, and molybdenum trioxide (MoO3) and growth substrate are located in the hot zone during synthesis. The substrates utilized for this work include (b) graphene and (c) traditional insulating substrates - sapphire and SiO2.

Graphene (epitaxial and CVD) provides an ideal substrate for the synthesis of highly crystalline MoS2 as well as the ability to directly image TMDs on the atomic scale without layer transfer processes.28–30 Therefore, we use a graphene substrate to establish the efficacy of Mn doping of MoS2. Generally, the PL of TMDs on graphene is quenched compared to traditional substrates,28,30,31 and the introduction of Mn completely quenches the PL (Figure 2a) on epitaxial graphene, suggesting enhanced coupling between the MoS2 and graphene or an increase in MoS2 defect density. This is accompanied by clear evidence of Mn-S bonding (Figure 2b,c) in XPS, and the generation of S-Mn-O bonding, which is likely the result of ex-situ oxidation of the doped MoS2.32,33 Based on the Mn 2p core level (Figure 2d), the Mn exhibits various chemical states: Mn metal, MnSx, MnOx, where the peak positions and spin-orbit splitting in Figure 2d is in agreement with reports in the literature.33,34 Based on XPS, the total incorporation of Mn into the MoS2 is approximately 2%, coincidently similar to the Mn/Mo powder precursor ratio. The formation of Mn-S alloys in bulk reactions of Mn with MoS2 has previously been attributed to the negative Gibbs free energy of the reaction of Mn with MoS2 to form MnS2, MnOx with residual metallic Mo.32,35 However, in contrast to Lince et al.,32 no metallic Mo was detected in the current work. Finally, Mn doping of MoS2 on graphene also shifts the valence band edge (Figure 2e) by 150 meV from 0.76 eV to 0.61 eV, indicating that the electronic structure of the MoS2 has been altered by the addition of Mn. It is also worth noting that no Mn signature was detected via XPS or identified in TEM in the graphene substrate itself (this is true for epitaxial graphene (EG) and CVD graphene) (Figure S7), verifying that there is no doping of the graphene during the process. As a result, the Mn signal identified in experiments with a graphene substrate is isolated in the MoS2 lattice.


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Figure 2. Synthesis of MoS2 and doped MoS2 on epitaxial graphene indicates that the incorporation of Mn can significantly affect (a) the photoluminescence of monolayer MoS2, indicating that the addition of Mn leads to enhanced non-radiative recombination in Mn-doped MoS2. High resolution x-ray photoelectron spectroscopy (XPS) (b-d) of the pristine and Mn-doped MoS2 confirms the formation of Mn-S bonding. Finally, (e) XPS also confirms that the valence band maximum (VBM) shifts from 0.76eV for pristine MoS2 to 0.61eV for Mn-doped MoS2. This VBM shift is in good agreement with theoretical work.46

To further identify the location of Mn within the MoS2, doped layers were directly grown on CVD graphene on a Au quantifoil TEM grid, and subsequently characterized by HRTEM and scanning TEM (STEM). Mn atoms can readily be imaged (Figure 3b & c) and detected using energy dispersive spectroscopy (EDS) (Figure 3d) for doped MoS2 directly grown on a graphene TEM grid. Using high angle annular dark field-scanning transmission electron microscopy (HAADF-STEM) the S (Z=14), Mn (Z=25), and Mo (Z=42) atoms were clearly distinguished, as shown in Figure 3b. Compared to Mo, the Mn atoms are expected to have ~50% intensity in HAADF, where the positions of doped Mn atoms are indicated by red circles in Figure 3b and 3c. Figure 3c is the contrast-corrected STEM image, optimized to maximize the contrast between Mo and Mn (the image before contrast correction is in the supplemental information, Figure S8e) Additionally, the cross-sectional intensity of the atom contrast (Figure 3d) verifies the intensity ratio of ~1.5:1 for Mo:Mn. Importantly, HRTEM provides evidence that Mn is not merely segregated to domain boundaries, but also exist in the MoS2 lattice. Based on image analysis, the ratio of Mn:Mo is approximately 1.2:50, which is verified by energy dispersive x-ray spectroscopy (EDS) (Figure 3e), and similar to that discussed earlier for doped MoS2 on epitaxial graphene. We note that attempts to increase the doping concentration beyond 2 at% Mn (via additional Mn precursor) resulted significant degradation of domain morphology, and an inability to achieve 2D structures (see supplemental information, Figure S3a) due to the competing MoS2-MnS bonding coordination.


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Figure 3. Direct growth of MoS2 on suspended graphene TEM grids (a) provides a facile route to investigating the atomic-scale incorporation of Mn in the MoS2 domains. The average MoS2 domain size is ~200nm in this sample, providing an ideal platform for imaging the Mn atoms in the MoS2 monolayer lattice. The Mn is shown to be incorporated at (b) the MoS2 domain boundary as well as substitutionallly (c) within the MoS2 matrix. Each Mn atom is identified from the (d) intensity spectra of the selected area, where the Mn exhibits approximately 50% lower intensity. Finally, (e) energy dispersive x-ray spectroscopy (EDS) of the Mn region exhibits a weak Mn signal, which is expected due to the detection limit of EDS (≤ 5%). The existence of Co and Cu comes from the grid background.

Electrical measurements of Mn-doped MoS2 on graphene is not possible due to shorting of devices through the graphene substrate, and delamination techniques can lead to extensive damage to the MoS2 domains. Therefore, it was necessary to extend the doping of monolayer MoS2 to traditional insulating substrates such as SiO2 and sapphire. Interestingly, the incorporation of Mn into MoS2 grown on traditional (SiO2 and sapphire) substrates is drastically different than that on graphene. This is evident simply by viewing the MoS2 domain morphology (Figure 4). Here, the introduction of Mn leads to a domain size increase of 3-10× (Figure 4a-d), accompanied by the edge termination being transposed from Mo to S (based on domain shape analysis).36 The increase of the domain size is likely due to the reduction of the nucleation density and a seeding effect.37 The partially-decomposed Mn2(CO)10 may potentially be acting as a pre-seed, thereby reducing the nucleation density and increasing domain size.18,38 The doped MoS2 exhibits a dendritic-like morphology (Figure 4b,d), with the edge of the domains being concave. In addition to switching of the edge termination,36 phase field modelling39 (Figure 4e-h) indicates that morphology evolution is due to modifications of atomic diffusion rates along the domain edge. For 2D hexagonal materials, two primary edges occur (A and B, Figure 4e), with specific growth velocities (νa and νb, Figure 4e). While the island growth is fed by a diffusion flux from the substrate normal to the island edge, the edge diffusion flux (DE) is a critical mechanism that dictates the growth rate of individual domain edges and determines the edge shape.39 In the case of MoS2, the growth rate is captured using diffusive time scales since the


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commonly observed dendritic island shape in CVD growth indicates diffusion limited growth.40 There are a variety of scenarios in the growth of the domains: 1) νa = νb (Figure 4f), 2) νa > νb with high DE (Figure 4g), and 3) νa > νb with low DE (Figure 4h). Evaluation of domain shape indicates that the domain morphology of doped MoS2 is due to νa > νb, with a reduction in the DE along the B edge when compared to the pristine case. Interestingly, the synthesis of pristine and Mn-doped MoS2 on graphene does not result in a morphological change from the typical triangular shape, indicating that DE is not effected when using an inert substrate. As a result, it is hypothesized that the substrate surface termination could be the dominating factor in controlling the DE, and thus the morphology, of 2D domains.

Figure 4. The morphology of MoS2 is heavily impacted by the incorporation of Mn during synthesis. Scanning electron microscopy (a-d) of pristine (a,c) and Mn-doped (b-d) MoS2 on SiO2 and sapphire clearly demonstrates a domain size increase of 3~10X, with domain termination switching from Mo to S. The growth can be explained via phase field modelling (e-h), where the edge growth rates (va, vb) and diffusion rate along the edge (DE) determine the morphology. In the case where (f) the edge velocity is equivalent, the domain grows as a hexagon; while if (g) va>vb with high DE, the shape is triangular; but if (g) va>vb with low DE, then the growth is dendritic-like. This indicates that the addition of Mn slows diffusion along the MoS2 domain edge, impeding growth of the domain edge in some directions.

Beyond the morphology, the introduction of Mn into the vapor phase alters the optical and electronic properties of monolayer MoS2 on traditional substrates (sapphire and SiO2). The PL of MoS2 on sapphire exhibits a 90meV red shift and >10× intensity quench, while there is a 50 meV red shift with >3× intensity quench on SiO2 (Figure 5a, Figure S4a,b). Additionally, the shift and quench for doped MoS2 is quite uniform across the triangular monolayer domain (Figure S4c, d) regardless of substrate. This PL signal shift and intensity quenching may be the result of: 1) Mn incorporation within the MoS2 lattice;41 2) the formation of lattice defects caused by Mn that generate bound excitons;42 3) the introduction of local strain due to modifications of MoS2/substrate interactions;43,44 or 4) introduction of local electronic states that affect the electronic localized band formation due to Mn doping.45–48 The PL comparison before and after transfer suggests the shift may not be related to the tensile strain from the substrate (Figure S5).43 Room temperature current-voltage measurements suggest that the incorporation of Mn in the vapor phase leads to a modification of electronic properties. The gate-dependent conductance G(Vbg) of a pristine device is similar to that reported in the literature, where G (Vbg) initially rises rapidly with increasing back gate voltage (Vbg), followed by a gradual slope change as the Fermi level approaches the bottom of the conduction band.3 In contrast, G(Vbg) of the device with


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MoS2 grown in the presence of Mn saturates more abruptly at smaller Vbg and the highest conductance reached over the same Vbg range is much smaller (GDoped ~ 5.5x10-7 S versus Gpristine ~ 4.7x10-5 S) (Figure 5b and Figure S6). The near saturation of G(Vbg) in Mn-doped devices over such a large Vbg range suggests the slow movement of the Fermi level inside the band gap, likely due to the presence of a high density of localized states.49

Figure 5: The PL spectra (a) of pristine and doped MoS2 on sapphire and SiO2 indicates that the addition of Mn during synthesis leads to a ~10X quench and ~90meV red shift on sapphire, and 3X quench and 50meV red shift on SiO2. This is accompanied by two-terminal conductance versus back gate voltage measurements (b) indicating that Mn-doping leads to an increase in the density of states in the bandgap of the MoS2 and thus lower saturation conductance. However, elemental analysis of the doped MoS2 (c,d) provides evidence that the concentration of Mn (yellow pixels) on the substrate is equivalent (or higher) to that found in the areas with MoS2, suggesting that Mn may be bound to the substrate surface rather than being incorporated into the MoS2 lattice.

Interestingly, the expected Mn-S bonding from XPS (Figure S7) and Z-contrast variations in HAADF-STEM (Figure S8) was not found in any doped MoS2 sample on SiO2 or sapphire substrates, indicating that if the Mn has been incorporated, it is below the detection limits of these techniques. This is evident in Figure S7a and Figure S7b, where the sulfur (S) 2p peak only indicates Mo-S bonding, and the Mn 2p peak is below the detection limit. Therefore, the incorporation of Mn into the MoS2 lattice is considered less probable under the current conditions. Even utilizing Mn/Mo ratios as high as 2:5 in the powder precursors, which is much higher in the vapor phase, we could not detect Mn signals via XPS (Figure S7). To further identify if any Mn was incorporated, we carried out time-of-flight secondary ion mass spectroscopy (TOF-SIMS), which is highly surface sensitive with an elemental sensitivity of 109 atoms/cm2 for Mn on Si surfaces.50 TOF-SIMS analysis of the Mn-doped MoS2 on SiO2 provides clear evidence that Mn is present in regions where MoS2 exists (identified by the Mo signal, Figure 5c). However, Mn is also present in regions where there is only SiO2 (Figure 5d, yellow pixels). While precise quantitative analysis of the Mn concentration is not possible due to the lack of standards, the relative similarity of Mn in the MoS2 versus non-MoS2 regions indicates that Mn may actually be bonding to the substrate surface and is not being incorporated into the MoS2. As the SiO2 surface is heated to growth temperature, a fraction of the hydroxyl groups are dissociated leaving behind positively charged silicon sites,51 which could lead to Mn-Si bonding that is more stable than Mn incorporation into MoS2. This ultimately suggests that the optical and


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electronic modifications in “doped” MoS2 on SiO2 and sapphire in this study is likely the result of an increased defect density in the MoS2 rather than direct incorporation of Mn into the lattice. In conclusion, doping 2D semiconductors is advantageous for achieving new functionalities in these novel materials, and by introducing magnetic elements within semiconducting TMDs, there is the potential to enable novel applications such as 2D magnonic devices.52 To date, theory has focused on doping of freestanding 2D layers,5,7,48 however, we show that the doping efficiency is highly dependent on the environment in which the doping occurs. We have demonstrated that Mn atoms can indeed be incorporated into a monolayer MoS2 on graphene via in-situ vapor phase deposition, but doping above 2 at% Mn can lead to loss of the 2D-nature of MoS2. Furthermore, doping of MoS2 on traditional (reactive) substrates is highly inefficient due to reactivity of these substrate surfaces. The contrast between graphene and SiO2 (sapphire) illustrated in this work clearly provides evidence that substrate surface chemistry, and the reaction of the substrate surface with the dopant, is perhaps the most critical experimental challenge for doping TMDs with non-TMD elements such as Mn. Finally, while the level of doping of Mn is presumably enough to alter the electronic structure of MoS2, significant work remains before magnetic measurements are achievable because techniques such as superconducting quantum interference device (SQUID) require doping levels on scales higher than that currently achievable.


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Acknowledgements Work at PSU was supported by Penn State and the Penn State NSF-MRSEC Center for Nanoscale Science (NSF Grant DMR-08-20404a). The authors also acknowledge use of facilities at the PSU site of NSF NNIN. Work at UTD was supported by the Center for Low Energy Systems Technology (LEAST), one of six centers supported by the STARnet phase of the Focus Center Research Program (FCRP), a Semiconductor Research Corporation (SRC) program sponsored by MARCO and DARPA. The content of the information does not necessarily reflect the position or the policy of the Government, and no official endorsement should be inferred. Supporting Information The supporting Information is available free of charge on the ACS Publication Website. Morphology of MoS2, Trial to achieve high concentration doping, Phase field simulations, optical/electrical characterization, XPS characterization, HR(S)TEM characterization, TOF-SIMS characterization. References (1)

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Figures

Figure 1: Synthesis of pristine and manganese-doped MoS2 is achieved using (a) traditional powder vaporization techniques. Dimanganese decacarbonyl (Mn2(CO)10) and sulfur (S) are located upstream of the hotzone, and molybdenum trioxide (MoO3) and growth substrate are located in the hot zone during synthesis. The substrates utilized for this work include (b) graphene and (c) traditional insulating substrates - sapphire and SiO2.


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Figure 2. Synthesis of MoS2 and doped MoS2 on epitaxial graphene indicates that the incorporation of Mn can significantly affect (a) the photoluminescence of monolayer MoS2, indicating that the addition of Mn leads to enhanced non-radiative recombination in Mn-doped MoS2. High resolution x-ray photoelectron spectroscopy (XPS) (b-d) of the pristine and Mn-doped MoS2 confirms the formation of Mn-S bonding. Finally, (e) XPS also confirms that the valence band maximum (VBM) shifts from 0.76eV for pristine MoS2 to 0.61eV for Mn-doped MoS2. This VBM shift is in good agreement with theoretical work.46


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b

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C

O Mn Co Cu

0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0 Energy (keV)

Figure 3. Direct growth of MoS2 on suspended graphene TEM grids (a) provides a facile route to investigating the atomic-scale incorporation of Mn in the MoS2 domains. The average MoS2 domain size is ~200nm in this sample, providing an ideal platform for imaging the Mn atoms in the MoS2 monolayer lattice. The Mn is shown to be incorporated at (b) the MoS2 domain boundary as well as substitutionallly (c) within the MoS2 matrix. Each Mn atom is identified from the (d) intensity spectra of the selected area, where the Mn exhibits approximately 50% lower intensity. Finally, (e) energy dispersive x-ray spectroscopy (EDS) of the Mn region exhibits a weak Mn signal, which is expected due to the detection limit of EDS (≤ 5%). The existence of Co and Cu comes from the grid background.


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Figure 4. The morphology of MoS2 is heavily impacted by the incorporation of Mn during synthesis. Scanning electron microscopy (a-d) of pristine (a,c) and Mn-doped (b-d) MoS2 on SiO2 and sapphire clearly demonstrates a domain size increase of 3~10X, with domain termination switching from Mo to S. The growth can be explained via phase field modelling (e-h), where the edge growth rates (va, vb) and diffusion rate along the edge (DE) determine the morphology. In the case where (f) the edge velocity is equivalent, the domain grows as a hexagon; while if (g) va>vb with high DE, the shape is triangular; but if (g) va>vb with low DE, then the growth is dendritic-like. This indicates that the addition of Mn slows diffusion along the MoS2 domain edge, impeding growth of the domain edge in some directions.


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Figure 5: The PL spectra (a) of pristine and doped MoS2 on sapphire and SiO2 indicates that the addition of Mn during synthesis leads to a ~10X quench and ~90meV red shift on sapphire, and 3X quench and 50meV red shift on SiO2. This is accompanied by two-terminal conductance versus back gate voltage measurements (b) indicating that Mn-doping leads to an increase in the density of states in the bandgap of the MoS2 and thus lower saturation conductance. However, elemental analysis of the doped MoS2 (c,d) provides evidence that the concentration of Mn (yellow pixels) on the substrate is equivalent (or higher) to that found in the areas with MoS2, suggesting that Mn may be bound to the substrate surface rather than being incorporated into the MoS2 lattice.


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Manganese Doping of Monolayer MoS2: The Substrate Is Critical

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