PacSurf2022 Session NM-WeP: Nanomaterials Poster Session

Exploring light-weight electromagnetic interference (EMI) shielding material with high EMI shielding effectiveness (SE) is of great significance to alleviate the growing EMI pollution problem, prevent electronic instruments from the EMI, and protect human health. Hence, several methods are being explored to efficiently restrain EM pollution. Among these studies, carbon nanotubes (CNT) and graphene nanoplates are of particular favor on account of their unique structures and remarkable conductivities.

In the frame of this work, hybrid reduced graphene oxide (rGO) and multi-walled carbon nanotube (MWCNT) nanofiller were designed and covalently bonded through an amide bond. A hybrid nanofiller was prepared through the reaction of modified GO with ethylenediamine and oxidized MWCNT and reduced to hydrogen iodide. Another design is 3D-foam of modified GO and oxidized MWCNT, which is mixed, and 3D-foam is manufactured using various surfactants and used as a nanofiller of the epoxy and polydimethylsiloxane (PDMS) matrix.

The chemical and electrical properties of the hybrid and 3D-foam nanofillers are characterized to establish the correlation between the material characteristics and the EMI shielding performance of the nanocomposites. The mechanical and electrical properties and EMI shielding effectiveness of nanocomposites as functions of hybrid and 3D-foam nanofillers types and contents were investigated in detail.

The addition of rGO significantly increases the electrical conductivity, because CNTs can fill the gaps between rGOsheets [https://www.sciencedirect.com/topics/engineering/graphene-sheet], and bridge the neighbor graphene sheets to form a preferable conductive network. In a low areal density 3D-foamed MWCNT/rGO aerogel, vein-structured MWCNT expands the conductive network of mesophyll-structured rGO, which promotes reflection and absorption of electromagnetic waves inside the materials, resulting in excellent EMI shielding effectiveness of 65dB in C-band and absorption-dominate shielding mechanism.

Recently, vertically aligned carbon nanotubes (VACNT) grown vertically from substrates have been attracting attention in fields such as Through Silicon Via (TSV). Although hydrocarbons such as ethylene and acetylene have been mainly used as carbon sources for the synthesis of VACNT, there have been few reports of VACNT growth using carbon monoxide as a carbon source. This study is aimed at searching for and modeling the optimum conditions for aligned growth of CNTs from carbon monoxide. As the catalyst preparation method, we use the dip-coating method, which has been reported for the aligned growth of CNTs from carbon monoxide. The sputtering method, which has been used in many reports on the aligned growth of CNTs from hydrocarbons, was also investigated.

In the dip coating method, cobalt and molybdenum were used as catalysts. Cobalt acetate was used as the source of cobalt and molybdenum acetate as the source of molybdenum, and ethanol was used as the solvent. The optimum conditions for dip coating were investigated using the lifting speed of the dip-coating and the concentration of the solution as parameters. Experiments were conducted in the range of 0.03~0.9 cm/s as the lifting speed of the dip-coating. The concentrations of cobalt and molybdenum in the solution were 0.01~1.2 wt%. Quartz substrates were dipped in a mixed solution of cobalt and molybdenum and pulled up to coat the surface with cobalt and molybdenum. Subsequently, calcination was performed at 400°C for 5 minutes in an atmosphere. The substrate after calcination was inserted into a quartz tube reactor, and after being evacuated with a vacuum pump, a mixture of Ar and H2 flowed into the reactor for reduction treatment at 700°C for 10 minutes under atmospheric pressure. After that, the feed gas was switched to CO and synthesis was performed at 700°C for 60 minutes under atmospheric pressure. The surface and cross-section of the substrate after synthesis was observed by SEM. The crystallinity of the CNTs was evaluated using Raman spectroscopy. The results showed that CNTs grew randomly on the surface of the substrate in most conditions; the Raman spectroscopy analysis indicated that single-walled CNTs were synthesized since the strong RBM peak was observed. VACNT was observed when the lifting speed was 0.9 cm/s and the concentrations of cobalt and molybdenum were 0.2 wt%. Based on the results, the conditions necessary for aligned growth and the model were discussed. We also tried to specify the conditions for aligned growth of CNTs using the sputtering method and its optimum condition and growth model are discussed.

Abstract

Various ovonic threshold switching (OTS) materials with unique insulator-metal transition (IMT) characteristics are being actively studied for applications in phase-change memories [1].Germanium telluride is one of the most actively studied materials and some variants, including germanium-antimony-telluride, are already in commercial use [2]. To improve the switching characteristics of germanium telluride, many approaches, such as stoichiometric control, doping, and process optimization, have been proposed [3,4].

In this presentation, we propose a multi-layer GeTe₆/GeTe structure to increase the on/off current ratio of OTS devices with a single GeTe₆ layer. The GeTe₆ layers with distinctive IMT characteristics were deposited by adjusting the power ratio during co-sputtering of GeTe and Te targets, and the GeTe layers were deposited in situ by sputtering the GeTe target. For fabrication of the switching device with a metal-insulator-metal structure, tungsten was used for both top and bottom electrodes having a crossbar shape. The on/off ratio of the multi-layer structure was increased compared to that of the single-layer structure (GeTe₆). Additionally, the threshold voltage was increased, and cycle stability was improved. The detailed origins for the improved characteristics will be discussed based on the experiments with various GeTe₆/GeTe stack numbers.

References

[1]D. Kau et al., “A stackable cross point phase change memory,” IEEE International Electron Devices Meeting (2009).

[2] D. Ielminia and A. L. Lacaita, Mater. Today 14, 600 (2011).

[3] K. Singh et al., Appl. Nanosci. (2021).https://doi.org/10.1007/s13204-021-01911-7

[4] G. Navarro et al., “Electrical performances of tellurium-rich Ge_xTe_1-x phase change memories,” 3rd IEEE International Memory Workshop (2011).

A highly crystalline two-dimensional (2D) ferroelectric material, α-In₂Se₃ has been extensively studied for neuromorphic, ferroelectric tunnel junction devices, and phototransistors [1-3] because of its unique material characteristic, a ferroelectric semiconductor. Here, we demonstrate, for the first time, a ferroelectric α-In₂Se₃/GaN HEMT van der waals heterostructure, in which the switchable ferroelectric polarization of α-In₂Se₃ can induce steep subthreshold switching (SS) and large memory window. In-plane polarization within the α-In₂Se₃ layer was successfully suppressed via self-aligned-gate etching process as analyzed by micro-Raman spectroscopy. On the other hand, out-of-plane polarization is strongly coupled to two-dimensional electron gas. Therefore, a record low SS of ~12 mV/dec with high on/off ratio of ~ 10¹⁰ was obtained. The transfer curve exhibits a counter-clockwise hysteretic behavior with a memory window of ~ 0.9 V, induced by the ferroelectric switching above the coercive field of α-In₂Se₃.The results show that ferroelectric polarization and semiconductor characteristic of α-In₂Se₃ is a promising for ferroelectric/III-V heterostructures, enabling emerging III-V based reconfigurable and neuromorphic applications.

This work was supported by the national R&D Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Science and ICT (2020M3F3A2A01082593, 2021R1A4A1033155)

[1] S. Wang, L. Liu, L. Gan, H. Chen, X. Hou, Y. Ding, S. Ma, D. W. Zhang, and P. Zhou, Nat. Commun., 12 (2021) 1. [2] K. Jeong, H. Lee, C. Lee, L. H. Wook, H. Kim, E. Lee, and M. H. Cho, Appl. Mater. Today, 24 (2021) 1. [3] J. Wu, H. Y. Chen, N. Yang, J. Cao, X. Yan, F. Liu, Q. Sun, X. Ling, J. Guo, and H. Wang, Nat. Electron., 3 (2020) 1.

Line pattern replication process through nanoimprint lithography (NIL) method has been used in numerous of research fields. NIL technology is not yet utilized for displays industry, and we propose an alignment layer of the sol-gel process using NIL. One-dimensionally nanopatterned by polydimethylsiloxane sheets cause surface changes in hybrid SnGaO thin films mixed in a 3:7 ratio, which aligns the liquid crystals (LCs) uniformly in the line pattern direction. These surface changes are confirmed through atomic force microscopy data analysis, and changes in surface shapes for different the curing temperatures in the furnace are analyzed. X-ray photoelectron spectroscopy (XPS) shows that the chemical composition of the thin films changes according to curing temperatures, and the intensities of SnO and GaO increase exponentially at 200°C compared to those at 50 °C. Through this, the van der Waals force increases between surface molecules, in the anisotropic direction to help align the LCs. Furthermore, we performed polarized optical microscopy and pre-tilt angle analysis confirm that the LCs are energized uniformly. Finally, the performance of an actual display device transmittance and electro-optical properties; the transmittance of SnGaO is 4.51p% higher than that of the currently commercialized PI-rubbing thin films, and the voltage-transmittance curve is a perfect graph. Thus, nanopatterned SnGaO thin films using NIL are expected to become the basis for next-generation displays.

Keywords: Nanoimprint lithography, Sol-gel method, Tin-gallium oxide, Atomic force microscopy, X-ray photoelectron spectroscopy

References

[1]O. V. Yaroshchuk, R. M. Kravchuk, S. S. Pogulay, V. V. Tsiolko, H. S. Kwok, Appl. Surf. Sci.2011, 257, 2443.

[2] G. M. Wu, C. Y. Liu, A. K. Sahoo, Appl. Surf. Sci.2015, 354, 48.

Recently, nanobots are receiving great attention due to their high potential for use in various fields such as intelligent robots, biomedical devices, and drug delivery. Nanobots are realized through the use of nanomaterials and assembly of nanomaterials, and research on driving nanobots using electromagnetic fields, electrochemistry, and specificity of enzymes is in progress. Among them, studies on actuators of carbon nanotubes and graphene based structures driven by electrochemical power have been reported. Electrochemical-based actuators have the advantage of being operated with a low voltage and can be used in places made of electrolyte, such as the human body. However, a useful electrochemical power-based nanoactuator is driven by ion entry and exit, so it is difficult to drive it with a single nanomaterial rather than structures. The development of electrochemical-powered single nanomaterial-based nanoactuators remains a challenge. In this work, we implemented an electrochemical-powered actuator by using graphene as a carbon nanoscoll (CNS) structure with a diameter of 50 nm. Unlike carbon nanotubes and graphene, CNS has a scroll structure and is a material that can be driven independently based on electrochemical power through the input and output of ions between layers. We measured the actuation performance of CNS. When the voltage was applied to CNS on electrolytes, CNS charged and it let the apparent diameter of the scroll increase significantly. Changes of the CNS diameter were measured in real-time using atomic force microscope equipment. It was confirmed that the type of electrolyte and the magnitude of the applied voltage play an important role in controlling the performance of the CNS. Additionally, we have structured the CNS and confirmed that it operates electrochemically. It is expected to see the possibility of using it as a nanobot.