Microstructure and properties of Se-deficient 2D MoSe2 [1]


Figure 1. Grain boundary in 2D MoSe2

March 24, 2015 - Se-deficit in single layers of MoSe2 grown by molecular beam epitaxy gives rise to a dense network of mirror-twin-boundaries (MTBs) decorating the 2D-grains, according to a new study by a team of researchers led by the University of Ulm, Germany and Aalto University, Finland. With the use of aberration-corrected high-resolution low-voltage transmission electron microscopy the atomic scale microstructure of this two-dimensional (2D) transition metal dichalcogenide was analyzed and by density functional theory calculations it was demonstrated that MTBs are thermodynamically stable structures in Se-deficient sheets. These line defects host spatially localized states with energies close to the valence band minimum, thus giving rise to enhanced conductance along straight MTBs. However, electronic transport calculations show that the transmission of hole charge carriers across MTBs is strongly suppressed due to band bending effects.[1]

The change in the atomic composition of a material is a frequently used method to specifically modify its properties. Perhaps the most famous example is the rich phase diagram of the Fe-F3C system, where smallest changes in the carbon content have a large effect on the properties of processed iron.[2] For the new class of 2D materials this area is, however, still largely unexplored.

By observing the formation of MTBs during in situ removal of Se atoms by the electron beam in the microscope, the authors confirmed that MTBs appear due to Se-deficit, and not coalescence of individual grains during growth. At a very high local Se-deficit, the 2D sheet becomes unstable and transforms to a nanowire.

Recently TMDs, which are technologically very interesting due to their electronic properties[3] have been produced by CVD.[4-6] The goal, however, was mostly to produce high-purity crystals with very low concentrations of impurities or alloys with 3 different elements,[7, 8] where the additional element is replacing the atoms of pure crystal either in the chalcogen[8, 9] or in the metal[10] sublattice. However, defects and dopant atoms can also be used to modify the properties of these materials.[11, 12]

Thus a recent study[13] demonstrates that the mobility of the charge carriers can be increased by reducing the proportion of sulfur in 2D MoS2. This is consistent with studies of 3D MoSe2, where it was shown that charge carrier concentrations[14] and mass density[15] is influenced by small deviations from the perfect crystalline composition. However for 2D structures certain restrictions of stable compositions arise, since some structures are only to be found 3-dimensional.[16]

Lehtinen and his co-authors investigated the structures of single layer 2D MoSe2 samples, which were grown by Andras Kis and co-workers (EPFL Lausanne), in an aberration-corrected high-resolution transmission electron microscope (FEI Titan 80-300), operated at 80 kV. The layers were composed of grains stitched together by tilt-grain-boundaries consisting of rows of dislocations (Figure 1), similar to what has been observed earlier for MoS2,[6] and graphene.[17, 18] The so-called mirror twin boundaries (MTBs) had also been used to explain features in a MoSe2 thin film grown on top of a cleaved MoS2 surface[19, 20] earlier.


Figure 2. Heats of formation in defective 2D MoSe2

In the new paper, the authors determine the growth mechanism of MTBs. As they have confirmed by DFT calculations, moving to Mo-rich growth conditions significantly lowers the formation energy of MTBs, giving rise to an increased density during growth (Figure 2). Similar to what was previously observed by some authors of [1], however for the case of MoS2[21, 22], vacancy line structures are found to be energetically favorable, as compared to isolated vacancies. Also, Se has a low sticking coefficient,[23] which can result in a local Se-deficit even at excess deposition rates.

In a second step the authors analyzed the electronic structure of MTBs. Density-functional theory calculations were carried out as implemented in 1996,[24] by using the exchange-correlation functional proposed by Perdew, Burke, and Ernzerhof (PBE) in the same year.[25] The wave functions indicate that additional conduction channels might originate from the surrounding Mo atoms of a boundary. However in a more realistic picture, one has to include also backscattering of 1D conductance channels on the turns and junctions of domain boundaries. There is a chance that such scattering effects would considerably reduce the conductance due to the extra channels. Such a suppression of localized electronic states at the turns of periodic grain boundaries had already last year been observed in polycrystalline graphene by the Swiss group, who is also co-author of this publication.[26]


Figure 3. 2D to 1D transition

The new study is one of a series of experiments by the research team dedicated to the understanding of radiation damage of materials in situ under the beam. In two papers in 2012 in Physical Review Letters, the authors characterize defect production and doping in two-dimensional transition metal dichalcogenides under electron irradiation[21] and measured the electron beam induced displacement cross sections for single-layer graphene.[27] One year later the pristine atomic structure of MoS2 monolayer protected from electron radiation damage by graphene was revealed.[28] By analyzing the effect of radiation damage in the images in detail, the authors now found that due to the relatively high mass of Se, the removal of the Se atoms is not direct knock-on damage (a collision of an electron from the microscope beam with the atom), and occurs due to electron-electron interaction such as ionization/radiolysis or beam mediated chemical etching, which is in good agreement with the previous studies By measuring the rate at which vacancies are formed, and taking the electron dose rate into account, the authors determined a vacancy production cross section, which is comparable to the value obtained earlier for freestanding MoS2 at 80 keV.[29]

With further removal of Se, the 2D sheet is transformed into a nanowire, that is, a 2D to 1D transition is observed, when neighboring growing holes form constrictions in the suspended layer and eventually the material self-organizes into nanowires (see Figure 3), which has been observed also earlier in MoS2.[30]

"What we’ve shown here is how the atomic scale microstructure of the binary 2D TMDs can be manipulated by the means of varying the exact composition" said Ute Kaiser, director of the group of EMMS at Ulm University. Drawing a parallel to the long tradition of materials science of bulk solids, this would open up vast possibilities for engineering the properties of this new class of 2D systems.

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