Date Published: December 20, 2018
Publisher: John Wiley and Sons Inc.
Author(s): Yusi Yang, Xia Wang, Shun‐Chang Liu, Zongbao Li, Zhaoyang Sun, Chunguang Hu, Ding‐Jiang Xue, Gengmin Zhang, Jin‐Song Hu.
Germanium diselenide (GeSe2) has recently emerged as a new member of in‐plane anisotropic 2D materials, notable for its wide bandgap of 2.74 eV, excellent air stability, and high performance in polarization‐sensitive photodetection. However, the interlayer interaction in GeSe2 has never been reported, which usually plays an important role in layer‐number‐dependent physical properties. Here, the interlayer coupling in GeSe2 is systematically investigated from theory to experiment. Unexpectedly, all of density functional theory (DFT) calculations about layer‐dependent band structures, cleavage energy, binding energy, translation energy, and interlayer differential charge density demonstrate the much weaker interlayer interaction in GeSe2 when compared with black phosphorus (BP). Furthermore, both thickness‐dependent and temperature‐dependent Raman spectra of GeSe2 flakes, which exhibit no detectable changes of Raman peaks with the increase in thickness and a small first‐order temperature coefficient of −0.0095 cm−1 K−1, respectively, experimentally confirm the weakly coupled layers in GeSe2. The results establish GeSe2 as an unusual member of in‐plane anisotropic 2D materials with weak interlayer interaction.
The interlayer interaction in 2D‐layered materials attracts increasing interest as an efficient tool for controlling the optical, electronic, thermal, vibrational, and mechanical properties of 2D materials through precise modulation of the number of layers.1, 2, 3 For example, MoS2 undergoes a crossover from indirect bandgap in the bulk or multilayers to direct bandgap in monolayer limit, enabling MoS2 to absorb and emit light efficiently.3, 4, 5 In particular, the recently isolated black phosphorus (BP) with a unique in‐plane anisotropy demonstrates an exceptional tunability of its bandgap ranging from 0.3 eV for bulk to 2.0 eV for monolayer due to its strong interlayer interaction explained by quantum confinement in out‐of‐plane direction,6, 7, 8, 9, 10 making it especially suitable for optoelectronic applications such as photodetection ranging from far‐infrared to visible regime.9, 11
Before studying the interlayer interaction in GeSe2, we first investigated the in‐plane anisotropy of GeSe2 by ADRDM, which was a nondestructive, surface‐sensitive, rapid and directly visualizing detection technique to characterize the optical anisotropy of the sample. The detection principle of ADRDM was to directly measure the normalized difference in reflectance (ΔR) between two arbitrary orthogonal directions in the surface plane (a and b) when the sample is irradiated by incident polarized light, which can be defined as18(1)ΔRR=2Ra−RbRa+Rb=2Nwhere Ra and Rb are the intensities of reflectance along a and b directions. The dimensionless value N(θ) alters as incident direction of linearly polarized light changes, which can be described as18, 19, 20(2)Nθ=Rx−RyRx+Rycos2θ−θ0where θ represents the angle between incident polarized light and x axis of GeSe2. Therefore, from the above equation, we could see that the value of N(θ) can be varied periodically with the changing azimuth angle of incident polarized light toward materials with in‐plane anisotropy. In particular, ADRDM can collect N(θ) at all pixels and thus be used as an imaging tool, offering a direct visualization of the anisotropic contrast.
In summary, we have systematically studied the interlayer interaction in GeSe2 from theory to experiment. ADRDM measurement clearly indicated the in‐plane optical anisotropy of GeSe2. DFT calculations about layer‐dependent band structures, cleavage energy, binding energy, translation energy, and interlayer differential charge density demonstrated the weak interlayer interaction in GeSe2. Room temperature thickness‐dependent and temperature‐dependent Raman spectra of GeSe2 flakes, which exhibited no detectable changes of Raman peaks with the increase in thickness and a small first‐order temperature coefficient of −0.0095 cm−1 K−1, respectively, further verified the weak interlayer coupling in GeSe2 from experiment, highly consistent with our theoretical predications. Our combined theoretical and experimental results introduce a new member of in‐plane anisotropic 2D materials with weak interlayer interaction.
The authors declare no conflict of interest.