【科学综述】Local electrical characterization of two-dimensional......
Local electrical characterization of two-dimensional materials with functional atomic force microscopy
Sabir Hussain1,3, Kunqi Xu1, Shili Ye1,2,3, Le Lei2, Xinmeng Liu2, Rui Xu1,2,†, Liming Xie1, Zhihai Cheng2
Abstract:
Research about two-dimensional(2D) materials is growing exponentially across various scientific and engineering disciplines due to the wealth of unusual physical phenomena that occur when charge transport is confined to a plane. The applications of 2D materials are highly affected by the electrical properties of these materials, including current distribution, surface potential, dielectric response, conductivity, permittivity, and piezoelectric response. Hence, it is very crucial to characterize these properties at the nanoscale. The Atomic Force Microscopy(AFM)-based techniques are powerful tools that can simultaneously characterize morphology and electrical properties of 2D materials with high spatial resolution, thus being more and more extensively used in this research field. Here, the principles of these AFM techniques are reviewed in detail. After that, their representative applications are further demonstrated in the local characterization of various 2D materials’ electrical properties.
1 Introduction
The atomically thin two-dimensional(2D) materials are one of the most extensively studied materials, driven by continues discoveries of new physics in them[1,2]. The single-layer graphene, known asthe first stable 2D material[3,4], exhibit unique and fascinating physical properties, such as zero-band gap[5–7], high thermal conductivity[8,9], high carrier mobility[10,11] and quantum hall effect[12,13], which are highly different from its bulk. Because of these unusual properties, graphene shows promising potentials in a wide range, including nanoelectronics[14,15], optoelectronics devices[16,17], chemical sensor[18,19], transparent electrodes for displays[20,21] and solar cells[22,23]. But its deficiency of an electronic band gap has enthused the search for others 2D materials with semiconducting characteristics. So far, a variety of 2D materials have been studied and new materials are still being discovered around the world[5,24].
2D materials beyond graphene[25,26] span the entire range of electronic structures, e.g., insulator h-BN[27,28], semiconductor MoS2, metal NbSe2, and display interesting properties. h-BN is acknowledged as one of the ideal inert substrates to investigate intrinsic properties of atomic layers[29,30]. MoS2 monolayers[31,32] can be used to prepare field effect transistor[33] and they have strong photo-luminescence[34]. NbSe2 layers exhibit unique properties such as superconductivity, quantum metallic state, and strong enhancement of the charge density wave(CDW)[36–38]. The great diversity of structure and remarkable properties of 2D atomic crystals make them suitable for applications in next generation of ultra-thin flexible electronics[39,40], catalyst[41,42], chemical sensor[43,44], lithium-ion batteries[45,46], photo sensor device[47,48] and field effect transistor[49,50].
Atomic Force Microscopy (AFM) has provided a vast and valuable contribution to the understanding of the fundamentalel ectrical properties of graphene and other 2D materials. It was invented by Nobel Prized Binnig along with his colleagues in 1986[51]. Initially, AFM was developed to investigate the surfaces of insulators at nanoscale[52]. It obtains sample surface information by detecting the interaction forces between probe and sample other than depending on electrons or photons to detect asample, so AFM can be applied to a wide range of systems and has predominant applications in many fields including physics, chemistry, material science and engineering.
In early period of AFM, the tip was always in mechanical contact mode with the sample surface. For instance, the first AFM image was taken by contact mode and showed nanoscale spatial resolution[52]. After a few years, Martin, Perez, and many other researchers applied dynamic operation in AFM[53–55]. The cantilever is deliberately excited at a single frequency while scanning on the sample surface[56,57]. And a feedback loop holds one parameter of the oscillation, either the frequency or the amplitude, at a fixed value. Therefore, in the field of dynamic AFM, there are two basic methods. One is frequency modulation AFM (FM-AFM) that uses frequency as a feedback parameter and is usually operated in vacuum[58]. The other mode is amplitude modulation AFM(AMAFM) that uses amplitude as feedback parameter and is usually operated in the atmosphere. During the last two decades, AFM and its applications have obtained broad attention from the topographic imaging to electrical mapping[59–63].
Several AFM reviews have been published to introduce the various properties of semiconductors[64,65], material science[66], food technology[67] and biomaterials[68–70]. Considering the importance of 2D materials electrical properties, this review highlights the principles of AFM based electrical characterization methods and their further applications in 2D materials. Firstly, the principles of conductive atomic force microscopy(CAFM), scanning Kelvin probe microscopy (SKPM), conventional electrostatics force microscopy (EFM), multiharmonic electrostatic force microscopy(MH-EFM), scanning microwave impedance microscopy (sMIM), agilent technology-scanning microwave microscopy (AT-SMM) and piezo force microscopy (PFM) are introduced in detail. Then, the applications of these techniques in characterization of the electrical properties of 2D materials are demonstrated. Finally, we summarize this review and present the prospects for new functional AFM technologies.
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