Abstract
As novel classes of two-dimensional (2D) materials and heterostructures continue to emerge at an increasing pace, methods are being sought for elucidating their electronic properties rapidly, non-destructively, and sensitively. Terahertz (THz) time-domain spectroscopy is a well-established method for characterizing charge carriers in a contactless fashion, but its sensitivity is limited, making it a challenge to study atomically thin materials, which often have low conductivities. Here, we employ THz parallel-plate waveguides to study monolayer graphene with low carrier densities. We demonstrate that a carrier density of ~2 × 10(11) cm(-2), which induces less than 1% absorption in conventional THz transmission spectroscopy, exhibits ~30% absorption in our waveguide geometry. The amount of absorption exponentially increases with both the sheet conductivity and the waveguide length. Therefore, the minimum detectable conductivity of this method sensitively increases by simply increasing the length of the waveguide along which the THz wave propagates. In turn, enabling the detection of low-conductivity carriers in a straightforward, macroscopic configuration that is compatible with any standard time-domain THz spectroscopy setup. These results are promising for further studies of charge carriers in a diverse range of emerging 2D materials.
Highlights
There is much interest in atomically thin, layered materials, including graphene [1], hexagonal BN [2], transition metal dichalcogenides (e.g., MoS2, WSe2) [3, 4], III-VI layered semiconductors (e.g., InSe, GaSe) [5, 6], and black phosphorous [7]
In the terahertz (THz) frequency regime, the complex conductivity, σ, of charge carriers can be conveniently determined without any contacts by using THz time-domain spectroscopy (THz-TDS) [8]
One of the widely used methods to increase the response of 2D materials to THz radiation is by increasing the carrier density through “gating”, i.e., by applying an external electric field in a transistor configuration [9, 10]
Summary
There is much interest in atomically thin, layered materials, including graphene [1], hexagonal BN [2], transition metal dichalcogenides (e.g., MoS2, WSe2) [3, 4], III-VI layered semiconductors (e.g., InSe, GaSe) [5, 6], and black phosphorous [7] These novel materials host two-dimensional (2D) carriers with unconventional properties that are promising for a wide range of applications in electronics, photonics, and optoelectronics. One of the widely used methods to increase the response of 2D materials to THz radiation is by increasing the carrier density through “gating”, i.e., by applying an external electric field in a transistor configuration [9, 10] This is not always possible for general 2D materials and, more importantly, requires gate electrodes. The transverse magnetic (TM) mode did not show any sign of interaction with the graphene monolayer, as expected, since the electric field in this mode is perpendicular to the 2D layer
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