A novel circuit for measuring 2D materials

Figure 1. (a) Mechanical parts which comprise the vacuum chamber. (b) The opto-coupler electronic circuit board. (c) Model of the sample holder. (d) Non-local measurement in Graphene showing the removal of artefacts (negative resistance) using the circuit in (b). Adapted with permission from Review of Scientific Instruments 89, 024705 (2018).

After more than 50 years of technological advancements in microelectronics, the measurement of the properties of electronic devices is a well-established field. However, graphene and two-dimensional materials are revolutionising this field, presenting new challenges for scientists and electronic engineers. Some peculiar phenomena observed in graphene require a careful choice of measurement apparatus in order to avoid that artefacts, i.e. spurious signals coming from the apparatus and not from the device under test, mask the true nature of the signal.

Such artefacts are more visible in devices with a high impedance (known as Hi-Z devices). In direct-current (DC), high impedance corresponds to high resistance. When  the value of such resistance is above the value of the input impedance of the measuring instrument, all the voltage drops at the input of the instrument and the measurement of the device becomes impossible since there will be no current flowing into it. A similar problem appears in alternate-current (AC) measurements. In this case, a so-called common-mode-voltage (CMV), can be present at the input of a measuring instrument working in AC mode, such as a Lock-In amplifier. The CMV presents itself as a common signal between two inputs of the amplifier and it is due to coupling between the signal and ground lines. Such coupling, in the presence of a high-Z devices, can lead to wrong measurements or even unphysical results. Graphene and other 2D materials are a class of devices with high impedances in particular contacts configurations, such as in non-local measurements. This kind of measurement is very important as it reveals peculiar properties of graphene, such as the fact that electrons “flow” in this material as they were a fluid.

In a recent work published in Review of Scientific Instruments, our group in the University of Exeter tackled such issue by designing an electronic circuit which is able to reveal the presence or artefacts and, therefore, unveil the true nature of the signals measured in graphene and other two-dimensional materials, such as transition-metal dichalcogenides (TMDs) monolayers. The core of the circuit is an optical-coupler, a device which transmits a signal between two parts of a circuit using light instead of an electrical connection. The use of two of such devices allows the complete decoupling of the two parts of the measuring apparatus, effectively removing the artefacts. The circuit, shown in Figure 1b, is coupled to a measuring chamber, shown in Figure 1a, which is designed to reduce spurious signals arising from the use of coaxial lines. The application is shown in Figure 1d where the measurement of the non-local resistance of a graphene transistor is reported. The blue trace corresponds to a traditional measurement, which displays a negative value. Although negative bending resistance is a physical phenomenon, in this case the signal is clearly an artefact as negative resistance is impossible in the geometry used in our device. The use of the optical-coupling circuit removes such artefacts and unveils the true nature of the signal, as shown in Figure 1d by the red trace.

This work represents a step forward in the understanding of the electrical properties of 2D materials and it will help many researchers and scientists in developing integrated solutions based on such materials. The work is published in and Open-Source and Open-Hardware spirit for the benefit of the whole scientific community.

You can read the full paper at the following link (Open-Access): De Sanctis, A. et al. “Novel circuit design for high-impedance and non-local electrical measurements of two-dimensional materials”, Review of Scientific Instruments 89, 024705 (2018).

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