Electron beam engineering of 2D materials for integration into electronic and optical sensing devices

August 15, 2022 - For electronic and optical applications of two-dimensional (2D) materials and their vertical heterostructures, it is important to know the positions, densities and atomic structures of crystallographic defects. Researchers from the Universities of Ulm and Erlangen-Nuremberg in Germany and the Hebrew University of Jerusalem (Israel) have enabled TEM imaging of free-standing 2D materials, followed by experiments on the same sample placed on any substrate and integration into devices for electronic and optical sensing.

For electronic and optical applications of two-dimensional (2D) materials and their vertical heterostructures, it is important to know the positions, densities, and atomic structures of the crystallographic defects. Therefore, to understand the role of these well-defined defects on the properties of 2D heterostructures, it is desirable to combine measurements on the devices with atomic-resolution transmission electron microscopy (TEM) experiments. Here, the electron beam is used not only to image atomic defects, but also to create and manipulate them. Proof-of-principle experiments show that signatures of electron beam-induced defects can be measured in electrical tunneling measurements and photoluminescence. Our transfer method works reliably for single and multilayer transition metal dichalcogenides such as MoS2, MoSe2, WSe2, MoTe2, hBN, and graphene. It may also be suitable for building defect-based sensors and photon sources.

Two-dimensional (2D) materials exhibit unique electrical and optical properties that differ from those of their solid counterparts. (1,2) There are several classes of 2D materials ranging from insulators to semiconductors and metals that exhibit exceptional properties such as superconductivity or charge density waves. (3,4) Group-6 transition metal dichalcogenides (TMDs) such as MoS2 exhibit a transition from an indirect bandgap semiconductor to a direct bandgap semiconductor when bulk-tuned to a monolayer, enabling new optoelectronic applications. (3,5) Combined in vertically stacked heterostructures, tailored properties can be obtained, (1) promising for future technological applications in nanodevices such as tunneling transistors, (7) light-emitting diodes, or photodetectors. (8) In such 2D devices, in addition to the number of layers and doping, (9) atomic defects are a promising "control lever" for tailoring the electrical and optical properties of 2D materials. (12)

Electronic transport measurements of tunneling heterostructure devices can be very sensitive to the presence of individual defects. In these devices, tunneling is mediated by quantum dots (QDs) associated with defects in the barrier. (14,15) These QDs have atomic dimensions and are considered highly sensitive electrometers and promising platforms for quantum information storage. (17)

As for optical phenomena related to applications in optoelectronic devices, Raman and photoluminescence (PL) spectroscopy are widely used techniques to measure the effects of defects in semiconducting 2D materials. (18)

However, in order to directly relate atomic defects to specific properties, the discussed measurement techniques have the common drawback that they lack information about the positions and structures of the atomic defects that cause specific optical properties.

In atomic-scale transmission electron microscopy (TEM) of 2D materials, the electron beam can perform two essential tasks simultaneously: (1) imaging of atoms in the vicinity of the defect and (2) active introduction or manipulation of defects. (20) By using accelerating voltages below the material-specific knock-on damage threshold, unwanted electron beam-induced damage to the material is drastically reduced. The ability to set accelerating voltages within the desired voltage range (typically between 20 and 80 kV for 2D materials) allows a fine balance between imaging the original structure and dose-controlled defect processing. (21,22) The SALVE microscope with chromatic and spherical abberation correction, which achieves atomic resolution between 20 and 80 kV, is well suited for this purpose. (23)

2D transfer method

We have developed a TEM sample preparation method to transfer a 2D flake onto a holey carbon TEM grid and release it after TEM examination. So far, we have performed the experiments on single- and few-layer MoS2, MoSe2, WSe2, MoTe2, hBN, and graphene (Figure 1). In the following, we refer to the placement of the platelets on the lattice as "forward" transfer and the subsequent detachment as "backward" transfer. The forward transfer is performed before the TEM experiment. It is performed using a standard technique with spin-coated PVA on SiO2/Si24, as shown in Figure 2. In anticipation of the reverse transfer, we have modified this technique by introducing an additional sputtering step of a (oxygen sensitive) copper layer. As shown below, this copper layer will later allow the flakes to detach from the grid. In the next step, the 2D flake is imaged and manipulated in the TEM. After that, the reverse transfer process is performed. This step is the focus of this work and will be explained in detail below. It is a challenging process and, to our knowledge, is reported here for the first time. After the scale is transferred back to a substrate, it can be integrated into a van der Waals stack. Importantly, the same areas that were manipulated or imaged by TEM can now be studied by electronic transport, AFM, optical measurements, or other property determination techniques.

The reverse transfer process is based on the use of polystyrene as the transfer medium. A step-by-step sketch of the process is shown in figure 2a: First, a Si/SiO2 wafer is coated with polystyrene using a spin coater. Polystyrene is used as a substrate because it strongly adheres to the grating and flakes. The TEM grid with the electron beam exposed flake is placed on the substrate so that the flake faces the polystyrene layer (step 1). Isopropyl alcohol (IPA) is dropped onto the TEM grid (step 2). Evaporation of the IPA firmly bonds the grid to the substrate. At this point, the exfoliated platelet is in intimate contact with both the sputtered copper layer on its top surface and the polystyrene on its bottom surface (step 3). The next steps aim at detaching the platelet from the grid. For this purpose, a drop of ammonium peroxodisulfate (APS) solution (0.3 mol/l) is applied to the top of the grid at the position of the platelet to etch the copper layer. At this point, the progress of the process can be followed under the optical microscope, where the APS can be seen flowing under the grid and the process of detachment of the carbon support film from the substrate and the flake can be followed. Finally, the grid can be removed with tweezers (step 4). The remaining APS is rinsed with ultrapure water, and the entire sample is air dried (step 5). Optical micrographs taken during this process are shown in Figure 2b,c, and a backward transferred scale on polystyrene is shown in Figure 2d. Our transfer method should be applicable to all 2D materials that are insensitive to acids and water.

The reverse transfer process is based on the use of polystyrene as the transfer medium. A step-by-step sketch of the process is shown in Figure 3a: First, a Si/SiO2 wafer is coated with polystyrene using a spin coater. Polystyrene is used as a substrate because it strongly adheres to the grating and flakes. The TEM grating with the electron beam exposed flake is placed on the substrate with the flake facing the polystyrene layer (step 1). Isopropyl alcohol (IPA) is dropped onto the TEM grid (step 2). Evaporation of the IPA firmly bonds the grid to the substrate. At this point, the exfoliated platelet is in intimate contact with both the sputtered copper layer on its top surface and the polystyrene on its bottom surface (step 3). The next steps aim at detaching the platelet from the grid. For this purpose, a drop of ammonium peroxodisulfate (APS) solution (0.3 mol/l) is applied to the top of the grid at the position of the platelet to etch the copper layer. At this point, the progress of the process can be followed under the optical microscope, where the APS can be seen flowing under the grid and the process of detachment of the carbon support film from the substrate and the flake can be followed. Finally, the grid can be removed with tweezers (step 4). The remaining APS is rinsed with ultrapure water, and the entire sample is air dried (step 5). Optical micrographs taken during this process are shown in Figure 3b,c, and a backward transferred scale on polystyrene is shown in Figure 3d. Our transfer method should be applicable to all 2D materials that are insensitive to acids and water. The surface tension of the materials involved and the aqueous APS solution plays an important role in the detachment of the carrier film from the platelet. A hydrophobic substrate is required for successful transfer of the flakes from the grid to the substrate. We used polystyrene as the substrate because it is the most hydrophobic compared to common substrate materials such as poly(methyl methacrylate) or SiO2.25 When APS drips onto the copper-polystyrene interface, we assume that two processes occur. First, the copper is etched, and second, capillary force pulls the aqueous APS solution between the carrier film and the polystyrene substrate. The stiffness of the gold mesh of the TEM grid, combined with this capillary force, results in a mechanical stress that pulls the carrier film off the substrate. The reason for using a hydrophobic substrate is to reduce capillary forces and prevent the carrier film from being torn off along with the platelet before the copper etching process can separate them.

The increased adhesion due to the interaction of the electron beam with hydrocarbon impurities originates from the air or solvents used during sample preparation. These impurities are located between the platelet and the carrier film and are modified by the electron beam.28 The impurities were observed mainly at exposed sites, supporting our explanation for the adhesion induced by the electron beam. The drastically increased adhesion makes the described reverse transfer technique particularly challenging. In general, electron-beam exposed flakes require the presented procedure, in which the flakes are detached by etching a sacrificial copper layer. However, unexposed flakes can also be transferred from the TEM grid to a substrate using various simpler approaches without a sacrificial copper layer.

In Figure 4, we show that microscopic defects in the 2D flake can survive our retransfer procedure by tracking a marker hole in a MoS2 flake before (a) and after (c) the retransfer preparation steps. This marker hole, created by a converged 80 kV electron beam, is about 50 nm in size and has a distinct shape. We then follow this hole further in the next preparation steps. We then transfer the same MoS2 flake again from the substrate to a new TEM grid and image it again in the TEM (Figure 4c). As can be seen, the overall structure of the hole is preserved after the transfer steps.

Experiments to prove the principle

We performed two proof-of-principle experiments. In the first experiment, we investigated the signature of defects created by electron beams in a subsequent electronic transport experiment. A platelet on a TEM grid was exposed to different electron beam doses in the TEM to investigate how the electron beam induced changes in the 2D material are reflected in the electronic signature. The platelet on the TEM grating is shown in Figure 5a. After TEM exposure, the platelet was transferred to a polystyrene substrate using the procedure described above. Then, the exposed areas on the platelet were precisely positioned on a pre-patterned gold electrode, using polystyrene as the transfer medium. Finally, gold contacts were patterned on the top of the platelet using standard electron beam lithography to contact the exposed areas. The electrodes shown in Figure 5b include both exposed and unexposed areas, with the latter used for control measurements.

The devices were measured using a voltage bias in a two-terminal geometry at T = 4 K (Figure 5c). The presence of defects in such barriers would appear as discrete jumps in the DC current associated with resonant tunneling through the discrete QD energy levels.14 The transport data shown in the figure have two main features. First, it is clear that the exposed regions are indeed characterized by the telltale discrete current jumps typical of defect-assisted transport. Second, we find that the unexposed regions exhibit a much larger tunneling conductance. While the first result is consistent with our expectation that TEM exposure would produce deep defects, the second result suggests that beam exposure also alters band alignment, possibly by doping the TMD flake and minimizing the height of the tunnel barrier formed by the thin TMD flake.

In the second fundamental experiment, we investigate electron beam induced defects with PL. Similar to the previous experiment, a MoS2 flake is exfoliated, transferred to a TEM grid, and exposed to an 80 kV electron beam at different doses (Figure 6a). The MoS2 flake is then transferred to polystyrene (Figure 6b), and the PL of the exposed monolayer regions is measured. Two peaks can be identified from the results in Figure 6c. The A0 peak (?1.85 eV) is from a recombination of free excitons at the direct band gap, while the AB peak (?1.7 eV) is from a radiative recombination of bound excitons. These excitons are bound to defects and therefore have lower energy than the free excitons. Moreover, the B exciton is observed with a weak intensity. With increasing electron irradiation and thus increasing defect density, the intensity of A0 decreases, while the intensity of AB, which is bound to defects, increases. A similar behavior is described in the literature.12

In summary, we have demonstrated a preparation technique that can reliably transfer electron-irradiated 2D material flakes from a TEM grid to any substrate. Using this method, we are now able to combine TEM with complementary characterization techniques. Here we demonstrate its application to electronic and optical measurements. Thus, we built devices from the 2D material platelets with the defect structure generated in TEM and performed optical and transport measurements whose signatures we related to the atomic defect structure. Linking the atomic defect structure determined with the electron probe in the TEM to properties determined with completely different probes opens new possibilities for understanding and improving the properties of nanomaterials at the atomic level.

Resource: Quincke, M., Lehnert, T., Keren, I., Moses Badlyan, N., Port, F., Goncalves, M., Mohn, M. J., Maultzsch, J., Steinberg, H., Kaiser, U. (2022). Transmission-electron-microscopy-generated atomic defects in two-dimensional nanosheets and their integration in devices for electronic and optical sensing. ACS Appl. Nano Mater., 5(8), 11429-11436.