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Site: http://phys.org/nanotech-news/

Observing the location and angle of the GBs and the atomic structure. Credit: IBS The Center for Integrated Nanostructure Physics (CINAP) within IBS has reported results correlating the flake merging angle with grain boundary (GBs) properties, and proven that increasing the merging angle of GBs drastically improves the flow of electrons. This correlates to an increase in the carrier mobility from less than 1 cm2V-1s-1 for small angles, to 16cm2 V-1s-1 for angles greater than 20°. The paper, entitled, 'Misorientation-angle-dependent electrical transport across molybdenum disulfide grain boundaries' is published in the journal Nature Communications. According to the paper, it is essential to understand the atomic structures of GBs in order to control and improve electrical transport properties in both bulk and low-dimensional materials. Grain boundaries are the direction that atoms are arranged in a material. For the experiments undertaken by scientists at CINAP, a monolayer molybdenum disulfide (MoS2) was grown by chemical vapour deposition (CVD) and subsequently transferred to a substrate of silicon dioxide (SiO2). The team's reasoning for using MoS is twofold: firstly, it is a 2D semiconductor that features high electrical conductance and, crucially, has a natural bandgap, which enables it to be tuned on and off and; secondly, the grain boundaries are well-defined. This is paramount for successful experiments. Previous research from Northwestern University found that the GBs of MoS provided a unique way to modulate resistance; this was achieved by using a large electric field to spatially modulate the location of the grain boundaries. The Northwestern results, published last year in Nature Nanotechnology, opened a pathway for future research, but the debate regarding the transport physics at the GB is still under dispute. This is due to a large device-to-device performance variation, poor single-domain carrier mobility, and, most importantly, a lack of correlation between transport properties and GB atomic structures in MoS research. The CINAP team, headed by the Center's director Young Hee Lee, overcame these obstacles by directly correlating four-probe transport measurements across single GBs with both high-resolution transmission electron microscopy (TEM) imaging and first-principles calculations. TEM is a microscopy technique whereby a beam of electrons is transmitted through an ultra-thin specimen, interacting with the specimen as it passes through. An exact atomic-scale image is formed from the interaction of the electrons transmitted through the specimen. GBs in the MoS2 layers were identified and regions with no sign of wrinkling or multilayers were then selected to prevent misinterpretations. Four-probe transport measurements were then performed on the substrate with surprising results; when measuring flake misorientations of 8-20o, mobility increased from much less than 1 cm2V-1s-1 up to 16cm2 V-1s-1. Above 20o field effect mobility saturates at a 16cm2 V-1s-1 intra-domain cutoff. Thus, GBs between flakes having a misorientation angle of 20-60o show better transport properties. The team has, as reported in their paper, "provided a more unified picture of the relationship between mobility, merging angle and atomistic structures of the GBs of monolayer MoS ." The results provide practical expectations regarding transport properties in large-area films, which will be restricted largely by the poor mobility across GBs. The results obtained in this work are applicable to other similar 2D systems, and contribute to the fundamental understanding of transport in semiconductors. Explore further: New study gives insight into graphene grain boundaries More information: Thuc Hue Ly et al. Misorientation-angle-dependent electrical transport across molybdenum disulfide grain boundaries, Nature Communications (2016). DOI: 10.1038/ncomms10426


News Article
Site: http://phys.org/nanotech-news/

Comparison between the synapse and the two-terminal tunnelling random access memory (TRAM). In the junctions (synapses) between neurons, signals are transmitted from one neuron to the next. TRAM is made by a stack of different layers: A semiconductor molybdenum disulfide (MoS2) layer with two electrodes (drain and source), an insulating hexagonal boron nitride (h-BN) layer and graphene layer. This two-terminal architecture simulates the two neurons that made up to the synaptic structure. When the difference in the voltage of the drain and the source is sufficiently high, electrons from the drain electrode tunnel through the insulating h-BN and reach the graphene layer. Memory is written when electrons are stored in the graphene layer, and it is erased by the introduction of positive charges in the graphene layer. Credit: Institute for Basic Science Last March, the artificial intelligence (AI) program AlphaGo beat Korean Go champion LEE Se-Dol at the Asian board game. "The game was quite tight, but AlphaGo used 1200 CPUs and 56,000 watts per hour, while Lee used only 20 watts. If a hardware that mimics the human brain structure is developed, we can operate artificial intelligence with less power," points out Professor YU Woo Jong. In collaboration with Sungkyunkwan University, researchers from the Center for Integrated Nanostructure Physics within the Institute for Basic Science (IBS), have devised a new memory device inspired by the neuron connections of the human brain. The research, published in Nature Communications, highlights the devise's highly reliable performance, long retention time and endurance. Moreover, its stretchability and flexibility makes it a promising tool for the next-generation soft electronics attached to clothes or body. The brain is able to learn and memorize thanks to a huge number of connections between neurons. The information you memorize is transmitted through synapses from one neuron to the next as an electro-chemical signal. Inspired by these connections, IBS scientists constructed a memory called two-terminal tunnelling random access memory (TRAM), where two electrodes, referred to as drain and source, resemble the two communicating neurons of the synapse. While mainstream mobile electronics, like digital cameras and mobile phones use the so-called three-terminal flash memory, the advantage of two-terminal memories like TRAM is that two-terminal memories do not need a thick and rigid oxide layer. "Flash memory is still more reliable and has better performance, but TRAM is more flexible and can be scalable," explains Professor Yu. TRAM is made up of a stack of one-atom-thick or a few atom-thick 2D crystal layers: One layer of the semiconductor molybdenum disulfide (MoS2) with two electrodes (drain and source), an insulating layer of hexagonal boron nitride (h-BN) and a graphene layer. In simple terms, memory is created (logical-0), read and erased (logical-1) by the flowing of charges through these layers. TRAM stores data by keeping electrons on its graphene layer. By applying different voltages between the electrodes, electrons flow from the drain to the graphene layer tunnelling through the insulating h-BN layer. The graphene layer becomes negatively charged and memory is written and stored and vice versa, when positive charges are introduced in the graphene layer, memory is erased. IBS scientists carefully selected the thickness of the insulating h-BN layer as they found that a thickness of 7.5 nanometers allows the electrons to tunnel from the drain electrode to the graphene layer without leakages and without losing flexibility. Flexibility and stretchability are indeed two key features of TRAM. When TRAM was fabricated on flexible plastic (PET) and stretachable silicone materials (PDMS), it could be strained up to 0.5% and 20%, respectively. In the future, TRAM can be useful to save data from flexible or wearable smartphones, eye cameras, smart surgical gloves, and body-attachable biomedical devices. Last but not least, TRAM has better performance than other types of two-terminal memories known as phase-change random-access memory (PRAM) and resistive random-access memory (RRAM).


Home > Press > Memory for future wearable electronics: Stretchable, flexible, reliable memory device inspired by the brain Abstract: Last March, the artificial intelligence (AI) program AlphaGo beat Korean Go champion LEE Se-Dol at the Asian board game. "The game was quite tight, but AlphaGo used 1200 CPUs and 56,000 watts per hour, while Lee used only 20 watts. If a hardware that mimics the human brain structure is developed, we can operate artificial intelligence with less power," points out Professor YU Woo Jong. In collaboration with Sungkyunkwan University, researchers from the Center for Integrated Nanostructure Physics within the Institute for Basic Science (IBS), have devised a new memory device inspired by the neuron connections of the human brain. The research, published in Nature Communications, highlights the devise's highly reliable performance, long retention time and endurance. Moreover, its stretchability and flexibility makes it a promising tool for the next-generation soft electronics attached to clothes or body. The brain is able to learn and memorize thanks to a huge number of connections between neurons. The information you memorize is transmitted through synapses from one neuron to the next as an electro-chemical signal. Inspired by these connections, IBS scientists constructed a memory called two-terminal tunnelling random access memory (TRAM), where two electrodes, referred to as drain and source, resemble the two communicating neurons of the synapse. While mainstream mobile electronics, like digital cameras and mobile phones use the so-called three-terminal flash memory, the advantage of two-terminal memories like TRAM is that two-terminal memories do not need a thick and rigid oxide layer. "Flash memory is still more reliable and has better performance, but TRAM is more flexible and can be scalable," explains Professor Yu. TRAM is made up of a stack of one-atom-thick or a few atom-thick 2D crystal layers: One layer of the semiconductor molybdenum disulfide (MoS2) with two electrodes (drain and source), an insulating layer of hexagonal boron nitride (h-BN) and a graphene layer. In simple terms, memory is created (logical-0), read and erased (logical-1) by the flowing of charges through these layers. TRAM stores data by keeping electrons on its graphene layer. By applying different voltages between the electrodes, electrons flow from the drain to the graphene layer tunnelling through the insulating h-BN layer. The graphene layer becomes negatively charged and memory is written and stored and vice versa, when positive charges are introduced in the graphene layer, memory is erased. IBS scientists carefully selected the thickness of the insulating h-BN layer as they found that a thickness of 7.5 nanometers allows the electrons to tunnel from the drain electrode to the graphene layer without leakages and without losing flexibility. Flexibility and stretchability are indeed two key features of TRAM. When TRAM was fabricated on flexible plastic (PET) and stretachable silicone materials (PDMS), it could be strained up to 0.5% and 20%, respectively. In the future, TRAM can be useful to save data from flexible or wearable smartphones, eye cameras, smart surgical gloves, and body-attachable biomedical devices. Last but not least, TRAM has better performance than other types of two-terminal memories known as phase-change random-access memory (PRAM) and resistive random-access memory (RRAM). For more information, please click If you have a comment, please us. Issuers of news releases, not 7th Wave, Inc. or Nanotechnology Now, are solely responsible for the accuracy of the content.


Home > Press > IBS report electric transport across molybdenum disulfide grain boundaries: Scientific team from CINAP/IBS identifies previously undiscovered differences in grain boundaries Abstract: The Center for Integrated Nanostructure Physics (CINAP) within IBS has reported results correlating the flake merging angle with grain boundary (GBs) properties, and proven that increasing the merging angle of GBs drastically improves the flow of electrons. This correlates to an increase in the carrier mobility from less than 1 cm2V-1s-1 for small angles, to 16cm2 V-1s-1 for angles greater than 20°. The paper, entitled, 'Misorientation-angle-dependent electrical transport across molybdenum disulfide grain boundaries' is published in the journal Nature Communications. According to the paper, it is essential to understand the atomic structures of GBs in order to control and improve electrical transport properties in both bulk and low-dimensional materials. Grain boundaries are the direction that atoms are arranged in a material. For the experiments undertaken by scientists at CINAP, a monolayer molybdenum disulfide (MoS2) was grown by chemical vapour deposition (CVD) and subsequently transferred to a substrate of silicon dioxide (SiO2). The team's reasoning for using MoS2 is twofold: firstly, it is a 2D semiconductor that features high electrical conductance and, crucially, has a natural bandgap, which enables it to be tuned on and off and; secondly, the grain boundaries are well-defined. This is paramount for successful experiments. Previous research from Northwestern University found that the GBs of MoS2 provided a unique way to modulate resistance; this was achieved by using a large electric field to spatially modulate the location of the grain boundaries. The Northwestern results, published last year in Nature Nanotechnology, opened a pathway for future research, but the debate regarding the transport physics at the GB is still under dispute. This is due to a large device-to-device performance variation, poor single-domain carrier mobility, and, most importantly, a lack of correlation between transport properties and GB atomic structures in MoS2 research. The CINAP team, headed by the Center's director Young Hee Lee, overcame these obstacles by directly correlating four-probe transport measurements across single GBs with both high-resolution transmission electron microscopy (TEM) imaging and first-principles calculations. TEM is a microscopy technique whereby a beam of electrons is transmitted through an ultra-thin specimen, interacting with the specimen as it passes through. An exact atomic-scale image is formed from the interaction of the electrons transmitted through the specimen. Identifying Grain Boundaries GBs in the MoS2 layers were identified and regions with no sign of wrinkling or multilayers were then selected to prevent misinterpretations. Four-probe transport measurements were then performed on the substrate with surprising results; when measuring flake misorientations of 8-20o, mobility increased from much less than 1 cm2V-1s-1 up to 16cm2 V-1s-1. Above 20o field effect mobility saturates at a 16cm2 V-1s-1 intra-domain cutoff. Thus, GBs between flakes having a misorientation angle of 20-60o show better transport properties. The team has, as reported in their paper, "provided a more unified picture of the relationship between mobility, merging angle and atomistic structures of the GBs of monolayer MoS2." The results provide practical expectations regarding transport properties in large-area films, which will be restricted largely by the poor mobility across GBs. The results obtained in this work are applicable to other similar 2D systems, and contribute to the fundamental understanding of transport in semiconductors. For more information, please click If you have a comment, please us. Issuers of news releases, not 7th Wave, Inc. or Nanotechnology Now, are solely responsible for the accuracy of the content.


Hong S.,SKKU Advanced Institute of Nanotechnology SAINT | Hong S.,Samsung | Kim W.,Samsung | Kim W.,Center for Integrated Nanostructure Physics | And 10 more authors.
Journal of Physical Chemistry C | Year: 2013

Thermoelectric power waves, where multiwalled carbon nanotubes coated with cyclotrimethylene trinitramine (MWCNT/TNA) directly convert chemical energy to electricity, have received considerable attention recently. However, the low Seebeck coefficient of carbon nanotubes has been regarded as a hurdle to increasing the electrical potential. Here, we present Sb2Te 3-coated MWCNT arrays prepared by a sputtering method. An analytical model predicts an increase in Seebeck coefficient of the annular multishell structure by ∼75%. The heterostructure coupled with exothermic chemical reaction of TNA demonstrates an increase in peak electrical potential of 175% (∼198 mV), compared with typical outputs of bare MWCNT/TNA (∼72 mV). A serial connection of two repeating units increased the peak potential by up to 406 mV. © 2013 American Chemical Society. Source

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