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A multivalued optical memory composed of 2D materials

Posted By Graphene Council, Friday, September 11, 2020
The National Institute for Materials Science (NIMS) has developed a memory device capable of storing multiple values using both optical and voltage input values. This device composed of layered two-dimensional materials is able to optically control the amount of charge stored in these layers. This technology may be used to significantly increase the capacity of memory devices and applied to the development of various optoelectronic devices. The research was published in Advanced Functional Materials ("Laser-assisted multilevel non-volatile memory device based on 2D van-der-Waals fewlayer-ReS2/h-BN/Graphene Heterostructures").

Memory devices used to store information (e.g., flash memory) play an indispensable role in today’s information society. The recording density of these devices has substantially increased in the past 20 years. In anticipation of widespread adoption of IoT technologies in the near future, it is desirable to accelerate the development of higher speed, larger capacity memory devices.

However, the current approach to increasing memory capacity and energy efficiency through silicon microfabrication is about to reach its limits. Development of memory devices with different working principles therefore has been awaited.

To meet expected technology needs, this research group has developed a transistor memory device composed of layered two-dimensional materials, including rhenium disulfide (ReS2) – a semiconductor – serving as a channel transistor, hexagonal boron nitride (h-BN) used as an insulating tunnel layer and graphene functioning as a floating gate.

This device records data by storing charge carriers in the floating gate in a manner similar to conventional flash memory. Hole-electron pairs in the ReS2 layer are prone to excitation when irradiated with light. The number of these pairs can be regulated by changing the intensity of the light.

The group succeeded in creating a mechanism that allows the amount of charge in the graphene layer to gradually decrease as the exited electrons once again couple with the holes in this layer. This success enabled the device to operate as a multivalued memory capable of efficiently controlling the amount of stored charge in stages through the combined use of light and voltage.

Moreover, this device can operate energy efficiently by minimizing electric current leakage—an achievement made possible by layering two-dimensional materials, thereby smoothening the interfaces between them at an atomic level.

This technology may be used to significantly increase the capacity and energy efficiency of memory devices. It also may be applied to the development of various optoelectronic devices, including optical logic circuits and highly sensitive photosensors capable of controlling the amount of charge stored in them through combined use of light and voltage.

This project was carried out by a research group consisting of Yutaka Wakayama (Leader of the Quantum Device Engineering Group (QDEG), International Center for Materials Nanoarchitectonics (MANA), NIMS), Bablu Mukherjee (Postdoctoral Researcher, QDEG, MANA, NIMS) and Shu Nakaharai (Principal Researcher, QDEG, MANA, NIMS).

This study was conducted in conjunction with another project entitled “Development of a ultra-sensitive photosensor using two-dimensional atomic film layers” funded by the Grant-in-Aid for JSPS Fellows.

Tags:  2D materials  Bablu Mukherjee  Graphene  International Center for Materials Nanoarchitecton  optoelectronics  Semiconductor  Sensors  Shu Nakaharai  The National Institute for Materials Science  Yutaka Wakayama 

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WPI-MANA Demonstrates First Fabrication of fBBLG/hBN Superlattices

Posted By Graphene Council, Wednesday, August 26, 2020
A team at the International Center for Materials Nanoarchitectonics (WPI-MANA) has demonstrated for the first time the fabrication of folded bilayer-bilayer graphene (fBBLG)/hexagonal boron nitride (hBN) superlattices. This achievement could pave the way for expanded applications of superlattices, such as in a variety of quantum devices.

Graphene superlattices represent a novel class of quantum metamaterials that have promising prospects. They have been generating a lot of attention recently, ever since the discovery of superconductivity in twisted bilayer graphene (BLG). This was followed by studies related to twisted bilayer-bilayer graphene. Bernal-stacked BLG has a parabolic energy dispersion with a four-fold spin and valley degeneracy.

A superlattice is a periodic structure of layers of two or more materials. Typically, the width of layers is orders of magnitude larger than the lattice constant, and is limited by the growth of the structure. The WPI-MANA team's superlattices are made up of vertically stacked ultrathin/atomic-layer quasi 2D materials.

The WPI-MANA team's results point to the emergence of a unique electronic band structure in the fBBLG, which could provide a way for investigating correlated electron phenomena by performing energy-band engineering with superlattice structures.

The results of this study indicate the emergence of a unique electronic band structure in fBBLG, which could be modified by the moire superlattice potential. Although a systematic way to fold graphene is still lacking, it should be a fruitful topic of future research, leading to 2D paper-folding engineering like "origami." The team's results suggest a possible way to engineer 2D electronic systems by mechanical folding, similar to "tear and stack" for twisted heterostructures.

This work could lead the way to expanded applications of superlattices, including quantum devices such as Bloch oscillators, quantum cascade lasers and terahertz source generators.

Tags:  2D materials  Graphene  hexagonal boron nitride  International Center for Materials Nanoarchitecton 

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