7 research outputs found
Partial dislocations interactions with symmetrical-tilt grain boundaries containing e-structural units: Local stress analysis with molecular dynamics
No full textThis Thesis was approved for publication on 2018-04-24 at 12:25.DSpace SAF Submission Ingestion Package generated from Vireo submission #12413 on 2018-08-31 at 17:21:10Made available in DSpace on 2018-09-04T20:36:50Z (GMT). No. of bitstreams: 2
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Previous issue date: 2018-04-24Embargo set by: Seth Robbins for item 107290
Lift date: 2020-09-04T20:37:00Z
Reason: Author requested U of Illinois access only (OA after 2yrs) in Vireo ETD systemGrain boundaries containing porous E-structural units (SUs) are known to readily emit dislocations under tension. This work establishes a correlation between the atomic structure, evolution of interfacial stresses and slip transfer mechanisms at grain boundaries containing E-structural units. Using molecular dynamics simulations, we study the interactions between {111} Shockley partial dislocations and symmetrical-tilt Ni grain boundaries containing E-SUs. We show that the incoming Shockley partials can be accommodated by porous E-SUs along the grain boundary. However, the partial-absorption process disrupts the short-range interactions of incipient dislocations along the boundary, which generates high local tensile and compressive stress regimes emanating from the impingement site. For the favored Σ9(221) grain boundary comprising only of E-SUs, Shockley partials originating from E-SUs located within the tensile stress regime are subsequently re-emitted into the neighboring grain. We demonstrate that the critical strength for re-emission of Shockley partials can be delineated into contributions from tensile stress generated by partial-absorption, intrinsic grain boundary tractions, as well as external loading. In the presence of other types of SUs, the incoming Shockley partial can also be transmitted through the boundary or be stably absorbed by the boundary with no subsequent re-emission, depending on the impingement site.Submission published under a 24 month embargo labeled 'U of I Access', the embargo will last until 2020-05-01The student, Sivasakthya Mohan, accepted the attached license on 2018-04-23 at 12:14.The student, Sivasakthya Mohan, submitted this Thesis for approval on 2018-04-23 at 12:19.Embargo set by: Seth Robbins for item 107290
Lift date: 2020-09-04T20:42:08Z
Reason: Author requested U of Illinois access only (OA after 2yrs) in Vireo ETD systemU of I Only Restriction Lifted for Item 107290 on 2020-09-05T09:15:17Z
Partial dislocations interactions with symmetrical-tilt grain boundaries containing e-structural units: Local stress analysis with molecular dynamics
No full textGrain boundaries containing porous E-structural units (SUs) are known to readily emit dislocations under tension. This work establishes a correlation between the atomic structure, evolution of interfacial stresses and slip transfer mechanisms at grain boundaries containing E-structural units. Using molecular dynamics simulations, we study the interactions between {111} Shockley partial dislocations and symmetrical-tilt Ni grain boundaries containing E-SUs. We show that the incoming Shockley partials can be accommodated by porous E-SUs along the grain boundary. However, the partial-absorption process disrupts the short-range interactions of incipient dislocations along the boundary, which generates high local tensile and compressive stress regimes emanating from the impingement site. For the favored Σ9(221) grain boundary comprising only of E-SUs, Shockley partials originating from E-SUs located within the tensile stress regime are subsequently re-emitted into the neighboring grain. We demonstrate that the critical strength for re-emission of Shockley partials can be delineated into contributions from tensile stress generated by partial-absorption, intrinsic grain boundary tractions, as well as external loading. In the presence of other types of SUs, the incoming Shockley partial can also be transmitted through the boundary or be stably absorbed by the boundary with no subsequent re-emission, depending on the impingement site.U of I OnlyAuthor requested U of Illinois access only (OA after 2yrs) in Vireo ETD syste
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Characterization of the normal and shear interactions between bilayer graphene and sapphire following a novel CVD growth process
No full textBeing the pioneer 2D material, graphene has been extensively researched owing to its excellent electronic, mechanical, thermal, and optical properties. Copper foil, which is one of the most common substrates employed in graphene CVD (chemical vapor deposition), provides a catalytic surface for the large-area growth of graphene. However, graphene cannot be directly used on copper and needs to be transferred to suitable substrates. Wet transfer methods, which use polymeric support, pose serious challenges such as long processing times, residual organic contamination, and doping from copper
etchants. Dry transfer methods that rely on mechanical delamination, on the other hand, allow for recyclability of the growth substrate, eliminate the use of etchants, and provide interfacial mechanical properties between graphene and substrates. Using dry transfer for Cu/graphene has been challenging due to the high adhesion between graphene and copper, and the lack of mechanical rigidity of the foils. To circumvent these challenges, this study
focuses on the use of sapphire for the direct, metal-free growth of graphene.
An atmospheric pressure CVD process was optimized for obtaining uniform bilayer graphene on sapphire. It was found that thermally annealing the sapphire prior to growth is a key factor in obtaining a suitable surface reconstruction required for growing high quality graphene at conventional metal CVD temperatures. The growth and annealing parameters were optimized, and the graphene was characterized using Raman spectroscopy, scanning electron microscopy (SEM), and atomic force microscopy (AFM).
Additionally, the number of layers of synthesized graphene was determined using Raman mapping and high-resolution transmission electron microscopy. Following growth, dry transfer was carried out on laminated beam specimens using a dual actuator loading device. The transfer was carried out under both nominally mode I and mixed mode loading conditions, and the quality and yield of transfer were determined using Raman, SEM and AFM. Using a beam-on-elastic foundation analysis, the adhesion energy and strength of the graphene-sapphire interface was determined experimentally, and it was found that the overall toughness decreased in going from Mode I to Mode II loading. This is the first-time dry transfer of graphene has been carried out under such a range of mixed-mode conditions, and while the current studies have been carried out on strips, these results have significant implications in employing other loading configurations for the transfer which may be easier to employ at wafer-scales. To explore other loading configurations, normal and shear
interactions were first experimentally determined at the transferred graphene-silicon interface. A cohesive surface based finite element model was developed on Abaqus to obtain a Mode I transfer map that showed that with competing interfaces, the crack grew along the interface with lower strength rather than lower fracture energy. Finally, using the mixed-mode results of graphene-sapphire and graphene-silicon interfaces, a three-point bending model was proposed on Abaqus to explore the feasibility of a direct, polymer-free, one-step dry transfer of graphene from sapphire to silicon.Materials Science and Engineerin
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Giant memory window performance and low power consumption of hexagonal boron nitride monolayer atomristor
No full textTwo-dimensional (2D) monolayers have gained significant attention as ultrathin active layers for fabricating atomic-scale memristor (atomristor) structures due to their crystalline structures and clean surfaces. This study reports on the giant memory window performance and low power consumption of the atomristor structures using a hexagonal boron nitride (h-BN) monolayer and symmetric silver (Ag) metal electrodes through a polypropylene carbonate (PPC) assisted transfer method. The h-BN atomristor exhibits the highest memory window (~4 × 109), the lowest leakage current (~0.24 pA), and the lowest power consumption (~3 × 10−14 W) compared to the other 2D atomristors. Furthermore, the h-BN atomristor achieves significant endurances and yields of up to 10,000 switching cycles and 77%, respectively, due to the superior thermomechanical properties of the PPC support layer for transferring ultrathin and large-area h-BN monolayers. These results represent a significant step toward the realization of high-performance and energy-efficient neuromorphic computing circuits based on 2D monolayers.Center for Dynamics and Control of Material
Direct Metal-Free Growth and Dry Separation of Bilayer Graphene on Sapphire: Implications for Electronic Applications
No full textThe rate at which graphene is used in different fields
of science
and engineering has only increased over the past decade and shows
no indication of saturating. At the same time, the most common source
of high-quality graphene is through chemical vapor deposition (CVD)
growth on copper foils with subsequent wet transfer steps that bring
environmental problems and technical challenges due to the compliance
of copper foils. To overcome these issues, thin copper films deposited
on silicon wafers have been used, but the high temperatures required
for graphene growth can cause dewetting of the copper film and consequent
challenges in obtaining uniform growth. In this work, we explore sapphire
as a substrate for the direct growth of graphene without any metal
catalyst at conventional metal CVD temperatures. First, we found that
annealing the substrate prior to growth was a crucial step to improve
the quality of graphene that can be grown directly on such substrates.
The graphene grown on annealed sapphire was uniformly bilayer and
had some of the lowest Raman D/G ratios found in the literature.
In addition, dry transfer experiments have been performed that have
provided a direct measure of the adhesion energy, strength, and range
of interactions at the sapphire/graphene interface. The adhesion
energy of graphene to sapphire is lower than that of graphene grown
on copper, but the strength of the graphene–sapphire interaction
is higher. The quality of the several centimeter scale transfer was
evaluated using Raman, SEM, and AFM as well as fracture mechanics
concepts. Based on the evaluation of the electrical characteristics
of the graphene synthesized in this work, this work has implications
for several potential electronic applications
Thin Films for Memristor Device Applications
No full text2D materials have been of considerable interest as new materials for device applications. Non-volatile resistive switching applications of MoS2 and WS2 have been previously demonstrated; however, these applications are dramatically limited by high temperatures and extended times needed for the large-area synthesis of 2D materials on crystalline substrates. The experimental results demonstrate a one-step sulfurization method to synthesize MoS2 and WS2 at 550 °C in 15 min on sapphire wafers. Furthermore, a large area transfer of the synthesized thin films to SiO2/Si substrates is achieved. Following this, MoS2 and WS2 memristors are fabricated that exhibit stable non-volatile switching and a satisfactory large on/off current ratio (103–105) with good uniformity. Tuning the sulfurization parameters (temperature and metal precursor thickness) is found to be a straightforward and effective strategy to improve the performance of the memristors. The demonstration of large-scale MoS2 and WS2 memristors with a one-step low-temperature sulfurization method with simple strategy to tuning can lead to potential applications such as flexible memory and neuromorphic computing.This research was
primarily supported by the National Science Foundation through
the Center for Dynamics and Control of Materials: an NSF MRSEC
under Cooperative Agreement No. DMR-1720595. The work was partly
done at the Texas Nanofabrication Facility supported by NSF grant
NNCI-2025227. This work was performed in part at the Center for
Integrated Nanotechnologies, an Office of Science User Facility operated
for the U.S. Department of Energy (DOE) Office of Science. Los Alamos
National Laboratory, an affirmative action equal opportunity employer,
is managed by Triad National Security, LLC for the U.S. Department
of Energy’s NNSA, under contract 89233218CNA000001.Center for Dynamics and Control of Material
Skin Controlled Electronic and Neuromorphic Tattoos
No full textWearable human activity sensors developed in the past decade show a distinct trend of becoming thinner and more imperceptible while retaining their electrical qualities, with graphene e-tattoos, as the ultimate example. A persistent challenge in modern wearables, however, is signal degradation due to the distance between the sensor\u27s recording site and the signal transmission medium. To address this, we propose here to directly utilize human skin as a signal transmission medium as well as using low-cost gel electrodes for rapid probing of 2D transistor-based wearables. We demonstrate that the hypodermis layer of the skin can effectively serve as an electrolyte, enabling electrical potential application to semiconducting films made from graphene and other 2D materials placed on top of the skin. Graphene transistor tattoos, when biased through the body, exhibit high charge carrier mobility (up to 6500 2V-1s-1), with MoS2 and PtSe2 transistors showing mobilities up to 30 cm2V-1s-1 and 1 cm2V-1s-1, respectively. Finally, by introducing a layer of Nafion to the device structure, we observed neuromorphic functionality, transforming these e-tattoos into neuromorphic bioelectronic devices controlled through the skin itself. The neuromorphic bioelectronic tattoos have the potential for developing self-aware and stand-alone smart wearables, crucial for understanding and improving overall human performance
