Compared with bulk materials, 2D materials exhibit extraordinary and novel properties, such as dangling-bond free surface and atomically-thin thickness, making them potential materials in electronics, optoelectronics, and energy applications. 2D semiconductors are viewed as potential “next-generation” materials. Currently, most 2D semiconducting materials studied now are small band-gap (<2 eV) materials, such as transition metal dichalcogenides (TMDCs) and black phosphorous (BP), which constricts their applications in the high-power electronics, low energy consumption devices, ultraviolet optoelectronics, among many others. Exploring air-stable wide band-gap (>3 eV) 2D semiconductors and figuring out their fundamental properties are potential and significant research directions.
Bulk β-ZrNCl is an air-stable and wide band-gap semiconductor, but few studies have been conducted on its properties when it is scaled down to atomically thin thickness. Recently, Associate Professor Bilu LIU’s research group, from Tsinghua Shenzhen International Graduate School (Tsinghua SIGS), has successfully isolated monolayer and few-layer β-ZrNCl and studied its layer-dependent properties for the first time. It was found that 2D ZrNCl exhibits good air stability and its Raman scattering exhibits an obvious layer dependent behavior. In addition, the back-gate transistors based on 2D ZrNCl shows a high on/off ratio of 108.
In this research, β-ZrNCl crystals were synthesized by a modified chemical vapor transport method, and then were isolated to monolayer and few layers by scotch tape exfoliation. Raman spectroscopy was conducted to further study the properties of 2D ZrNCl. The researchers observed a blue shift of ~3 cm-1 for the out-of-plane A1g peak (189 cm-1) of ZrNCl with the increasing thickness. Importantly, this A1g peak is absent in monolayer ZrNCl, which is a fingerprint for its thickness identification. Thus, Raman spectroscopy is a fast and non-destructive method to identify their layer numbers.
Meanwhile, few-layer ZrNCl is found to be stable in ambient condition and shows good electrical properties. The researchers found that the flakes before and after air exposure for 1 month and 4 months show similar surface roughness and clear atom arrangement, as revealed by atomic-resolved atomic force microscopy images. The position of the characteristic Raman peak does not show a noticeable difference after months. To further explore the electronic application of 2D β-ZrNCl as a channel material, the researchers fabricated back-gate FETs using electron-beam lithography to study its electrical properties. The ZrNCl FETs show high on/off ratios of 108, which is much higher than previously reported back-gate FET devices based on wide band-gap 2D materials. In the present research, the mobility is only 3.15 cm2 V-1 s-1, which is much lower than the theoretical value. Improving the contact between electrode and channel material as well as optimizing the interface of the devices may further improve the electronic performance of 2D β-ZrNCl.
Figure 1. Structure, exfoliation, and characterization of 2D β-ZrNCl. In this structure, Zr and N atoms form an AB type six-member-ring structure.
Figure 2. Layer-dependent Raman spectra of 2D β-ZrNCl
Figure 3.Environmental stability of 2D β-ZrNCl
Figure 4. Electrical transport measurements of few-layer β-ZrNCl back-gate FETs
The research article “Layer-dependent Raman Spectroscopy and Electronic Applications of Wide Band-Gap 2D Semiconductor β-ZrNCl” was published in Small recently. The corresponding author of this paper is Associate Prof. Bilu LIU, and the first author is Huiyu NONG, a doctoral student at Tsinghua SIGS. The authors of the paper also include Dr. Qinke WU, Junyang TAN, Yujie SUN, Rongxu ZHENG, Rongjie ZHANG, and Dr. Shilong ZHAO. This research has been supported by the National Science Fund for Distinguished Young Scholars, the Major Research Plan of the National Natural Science Foundation of China, the Innovation and Entrepreneurship Team Project of Guangdong Province, the Science, Technology, and Innovation Commission of Shenzhen Municipality, and the Development and Reform Commission of Shenzhen Municipality.
Link to full article: https://onlinelibrary.wiley.com/doi/10.1002/smll.202107490
Written by Huiyu Nong & Liqiong He
Edited by Alena Shish & Yuan Yang
