Encapsulation of MoS₂ in graphene: Controlling radiation damage

November 13, 2013 - Radiation damage in MoS₂ under the electron beam at electron energies of 60 and 80 keV can be largely avoided by encapsulation between 2 graphene layers. As two groups of researchers at the University of Ulm (Germany) [1] and at the University of Manchester in collaboration with the SuperSTEM laboratory (both UK) [2] independently and consistently reported, the radiation damage in the beam-sensitive 2-dimensional transistion metal dichalgonide (TMD) is reduced so that the defect-free imaging at the atomic level and the spectroscopic investigation becomes possible.

Reducing radiation damage by sample preparation has been possible for classical 3-dimensional materials by deposition of thin layers of amorphous carbon. However, this technique has the disadvantage of reducing the contrast of the image of the sample. For 2D materials that are one or a few atoms thick, such an approach would be almost impossible. An optimal protective material would have excellent electrical and thermal conductivity, would be durable and highly transparent to electrons at the energy used, chemically inert and crystalline. The studies show that such a material has indeed been found and an efficient protection against radiation damage through sample preparation is now also available for 2D materials.

In conducting materials such as graphene, "knock-on" (the displacement of atoms from their original positions in the lattice) is the dominant mechanism of radiation damage during observation in the electron microscope, while ionization damage (radiolysis) predominates in semiconductors and insulators [3]. For graphene, work carried out at the University of Ulm has recently shown that the calculation of the knock-on radiation damage is in agreement with the experiments if the thermal motion of the atoms is included in the calculations [4] (see this News Article on the SALVE website). If knock-on is the dominant mechanism of damage, reducing the primary beam energy below the knock-on threshold will prevent this [5], 6].

In fact, graphene is extremely stable at electron energies of 80 keV and below. Clean layers of pure monolayer graphene can be observed for extremely long periods of time with very high electron doses and no detectable damage formation. Scientists have now shown that inclusion in graphene actually provides a very effective solution for reducing radiation damage in other 2-dimensional materials that are instable in the electron microscope if they are free-standing.

In addition, only a slight loss of contrast is noticeable. If the MoS₂ monolayer is encapsulated with graphene, as shown in Figure 1d, the contrast is reduced by about 15% [2]. The graphene lattice is not clearly visible in the images of the sandwich structure, but it can be verified by the diffraction pattern, where there are two distinct groups of diffraction points (Figure 1b). The crystallinity of graphene also allows its contribution to imaging to be removed almost completely by filtering.

The comparison of the high-resolution TEM images of free-standing MoS₂ and the sandwich configuration (G/MoS₂/G) after irradiation showed a reduction in radiation damage of up to 3 orders of magnitude. The radiation damage could be quantified by HRTEM (Ulm University) (Figure 1c, d) at the atomic level. Here, 2D materials offer significant advantages over 3D materials where the radiation damage is only measurable indirect via second order phenomena such as the bleaching of diffraction spots associated with the loss of sample crystallinity, mass loss, changes in energy loss spectra, and electron beam induced X-ray yield.

Categories of radiation damage

Radiation damage in monolayer MoS₂ has not been extensively studied, although control of defect formation is critical to its properties. Most of the theoretical studies on radiation damage are limited to knock-on damage. For MoS₂, a theoretical study leaded by Arkady Krashenninikov from Aalto University, Finlanf together with the University of Ulm showed that the energy transfer from the electrons of the electron beam to the S atoms is about 4.3 eV at 60 keV, which is below the knock-on threshold [8]. At 80 keV, the energy transferred to 32S atoms roughly corresponds to the knock-on energy of these atoms in 2D MoS₂ of about 6.5 eV [2]. Due to the higher mass of Mo over S, the direct removal of Mo is unlikely at both energies.

Due to the lower conductivity than graphene, it is to be expected that the excitation of the electronic system of the material should also contribute to the radiation damage. The calculation of this second category of processes represents a much greater challenge. It involves processes such as ionization damage, damage by heating, or the emission of secondary electrons. The latter process can lead to significant charge of the sample, which can cause a Coulomb explosion effect that destroys the materials due to internal electrostatic repulsion. If radiolysis is the main damage process, there is no sharp energy barrier below which no damage occurs.

Further radiation damage results from the interaction with free radicals, which can be caused by impurities on the material under the electron beam. An interaction of damage mechanisms is possible, for example, when the stability of the system with respect to electron excitation is different in an electronically excited or charged state. This is obviously the case, for example, when the electrical conductivity of the material changes.

At defects or at the edge of the 2D material, the atoms are less coordinated in the lattice, which facilitates the ionization damage, but also reduces the knock-on threshold.

Quantifying knock-on damage experimentally

The findings published in the journals ACS Nano and Applied Physics Letters, now experimentally answer the question, what role knock-on damage plays in the 2D material at the different voltages. While the studies from UK were conducted at an energy of 60 keV, the scientists from Germany used an energy of 80 keV.

The graphene layer serves as a source of electrons, which can compensate the charging effect in the specimen, serve as a thermal conductor, and protect the sample from chemical etching [9]. When the exit surface of the sample is coated (MoS₂/G configuration), knock-on damage of the material can be inhibited in addition. To quantify the level of this type of damage, the scientists hypothesized that the difference between the G/MoS₂ and MoS₂/G configuration is precisely the knock-on damage.

Significant differences in the radiation damage between the G/MoS₂ and the MoS₂/G configuration at 80 keV give a knock-on scattering cross-section of 1.2 b corresponding to 24% of the total radiation damage for free-standing MoS₂ [1] (Fig. 2). This result is in good agreement with the previous ab initio calculations [8]. For the sulfur atoms of the bottom layer, a value of 0.8 b was calculated here.

At 60 keV inside the material, no significant difference was found between the radiation damage in the G/MoS₂ and the MoS₂/G configuration [2]. This result is also in agreement with the results of the simulation, which had found no appreciable knock-on damage for this voltage [8].

"The study demonstrates the need for voltages less than 80 kV when it comes to analyzing many 2D materials," said SALVE project director Ute Kaiser, who lead the study at the University of Ulm. "Further the two studies show that it is beneficial to investigate the specimen at different excitation energies due to the electron beam in order to understand radiation damage."

Study co-authors of the study based from the UK include Konstantin Novoselov who supervised the study at the University of Manchester and Ursel Bangert from the SuperSTEM lab.

Quantifying chemical etching, and electronic excitation damage

The comparison of the free-standing MoS₂ samples with the G/MoS₂ configuration allows the effects of other damage mechanisms to be quantified, although the mechanisms cannot be isolated as clearly as in the case of knock-on damage. Assuming that even the coverage of the upper side of the MoS₂ layer with graphene is sufficient to completely prevent radiation damage by electronic excitation and charging, only chemical etching on a surface is active in the MoS₂/G configuration. This results in a contribution of 2.4 b from electronic excitation and charging to total radiation damage, which corresponds to 55% of the total radiation damage in free-standing MoS₂ [1].

If both sides are protected with graphene, chemical etching is also greatly reduced, which should lead to a dramatic reduction of radiation damage. And indeed, both studies consistently report a strong reduction in radiation damage in this case. The authors also noted that the close proximity between the graphene and MoS₂ layers can lead to the formation of interlayer bonds (and thus a modification of the electronic structure of the encapsulated material). Such bond formation would favor electron transport between the graphene and MoS₂ layers, thereby contributing to the attenuation of ionization. [2]

Edge effects of radiation damage

The lower binding energy of the peripheral atoms should facilitate both ionization damage and knock-on damage. In fact, the faster growth of already existing holes was observed in comparison to the generation of new radiation damage inside the material. Moreover, due to Mo's higher knock-on threshold compared to S and the propensity of Mo atoms to form metallic clusters, Mo atoms at the edges of the damaged region appear to aggregate, while S atoms are simply sputtered away (Fig. 3) [8]. These results were also confirmed spectroscopically. In EELS spectra, there was an increased concentration of Mo in the peripheral areas than in the interior of the material, which has already been observed in previous studies [10].

The entire vacancy-forming cross-section was reduced by 600 times compared to a free-standing MoS₂ layer to the sandwich configuration [1]. Thus, also the spectroscopic analysis, which requires much longer irradiation times, is possible (Fig. 4) [2].

Heterostructures of various materials would be useful to treat the effect more reliably. A possible follow-up experiment would be to replace the graphene layer on one or both surfaces with a hexagonal boron nitride layer which is chemically inert, has comparable mechanical properties to graphene, but is an insulating material. Such additional studies would allow further separation of the various damage mechanisms.

Other types of radiation have not been explicitly explored here, but the results suggest that graphene can also be used more generally to protect surfaces from radiation damage.

"The reduction of radiation damage by encapsulation not least extends the applicability of AC-HRTEM considerably, as radiation damage poses a serious obstacle to enabling high resolution imaging in the aberration-corrected electron microscope." Kaiser said. "The technique also has great potential for imaging molecules and nano- or bioparticles."