Electric Field Mapping in 2D Heterostructures

The K3® IS camera from Gatan provides synchronous low-dose imaging via real-time electron counting, rapid continuous data capture and a large field of view.

In a 4D STEM experiment, the speed of the scanning probe was precisely synchronized to the camera frame rate using the STEMx® system to facilitate high-speed data acquisition and eradicate the potential for data loss.

Context

Two-dimensional (2D) material heterostructures have given the next generation of optoelectronic architectures and quantum information science an extensive range of features. These heterostructures are made up of various 2D materials, including graphene, boron nitride (h-BN) and transition metal dichalcogenides.

Their performance and functionalities are typically a result of the charge transport dynamics present at numerous 2D or contact interfaces. Besides information about a structure, short- and long-range electrostatic fields and charge distributions can be captured using 4D STEM by measuring the intensity fluctuations of the CBED patterns.

The momentum transfer from the electron beam interactions with the sample’s electrostatic fields is represented by the consequent differential phase contrast images by measuring the deflection of an electron beam due to the field at each probe point.

With a high-speed camera, these methods can also be applied in a dynamic framework where the sample is exposed to external biases to measure the evolution of the electrostatic profile.

a) Schematic of MoS2/hBN sample analyzed. Inset shows an optical image of the final architecture with MoS2 (green) on top of hBN (light purple). b) Scanning area – full dataset was 680 x 60 x 1048 x 1048 pixels, took 136 s to acquire. c) CoMx and electrostatic field maps of the sample without external biasing. d) CoMx and electrostatic field maps of the sample under external biasing of 5V. Color wheel scale bars show the normalized magnitude and direction of the E-field extracted from CoM.

Figure 1. a) Schematic of MoS2/hBN sample analyzed. Inset shows an optical image of the final architecture with MoS2 (green) on top of hBN (light purple). b) Scanning area – full dataset was 680 x 60 x 1048 x 1048 pixels, took 136 s to acquire. c) CoMx and electrostatic field maps of the sample without external biasing. d) CoMx and electrostatic field maps of the sample under external biasing of 5 V. Color wheel scale bars show the normalized magnitude and direction of the E-field extracted from CoM. Image Credit: Gatan Inc.

Materials and Methods

The K3 IS camera and a STEMx system were used to obtain 4D STEM diffraction datasets at around 300 frames per second in electron counting mode on a JEOL JEM-ARM 300F (S)TEM running at 300 kV and with a probe convergence angle of 30 mrad.

Utilizing gold interdigitated electrodes, the probing of a sample made up of a monolayer of MoS2 on hBN substrate was conducted (Fig 1a). Charge carriers have the capacity to quantum mechanically tunnel through the insulating hBN layer and are introduced into the semiconducting MoS2 layer.

CoM along the scanning direction (CoMx) at 0 and 5 V and their respective electrostatic field maps were produced employing the Gatan Microscopy Suite® plug-in. A distinct difference in the E-field distribution in the semiconductor region of the tunnel junction is seen when there is an application of the external bias (Figures 1c and 1d).

Summary

The K3 IS with STEMx facilitates hardware synchronized 4D STEM diffraction experiments at rapid speeds (>3500 fps at 256 x 256-pixel resolution) with the greatest possible signal-to-noise ratio (using electron counting).

This allows such multi-dimensional experiments, as demonstrated here: using the combination of electrical biasing and 4D STEM DPC to establish how the electrostatic profile in a heterostructure of MoS2/hBN evolves when subjected to an external electric field can be explored.

This information has been sourced, reviewed and adapted from materials provided by Gatan, Inc.

For more information on this source, please visit Gatan, Inc.

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