| 1. |
- Atwa, Mohamed M., et al.
(författare)
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Trilayer Graphene as a Candidate Material for Phase-Change Memory Applications
- 2016
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Ingår i: MRS Advances. - : Cambridge University Press. - 2059-8521. ; 1:20, s. 1487-1494
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Tidskriftsartikel (refereegranskat)abstract
- There is pressing need in computation of a universal phase change memory consolidating the speed of RAM with the permanency of hard disk storage. A potentiated scanning tunneling microscope tip traversing the soliton separating a metallic, ABA-stacked phase and a semiconducting ABC-stacked phase in trilayer graphene has been shown to permanently transform ABA-stacked regions to ABC-stacked regions. In this study, we used density functional theory (DFT) calculations to assess the energetics of this phase-change and explore the possibility of organic functionalization using s-triazine to facilitate a reverse phase-change from rhombohedral back to Bernal in graphene trilayers. A significant deviation in the energy per simulated atom arises when s-triazine is adsorbed, favoring the transformation of the ABC phase to the ABA phase once more. A phase change memory device utilizing rapid, energy-efficient, reversible, field-induced phase-change in graphene trilayers could potentially revolutionize digital memory industry.
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| 2. |
- El-Sayed, R., et al.
(författare)
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Degradation of pristine and oxidized single wall carbon nanotubes by CYP3A4
- 2019
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Ingår i: Biochemical and Biophysical Research Communications. - : Elsevier BV. - 0006-291X .- 1090-2104. ; 515:3, s. 487-492
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Tidskriftsartikel (refereegranskat)abstract
- Carbon nanotubes (CNTs) are a class of carbon based nanomaterials which have attracted substantial attention in recent years as they exhibit outstanding physical, mechanical and optical properties. In the last decade many studies have emerged of the underlying mechanisms behind CNT toxicity including malignant transformation, the formation of granulomas, inflammatory responses, oxidative stress, DNA damage and mutation. In the present investigation, we studied the biodegradation of single-walled carbon nanotubes (SWCNTs) by Cytochrome P450 enzymes (CYP3A4) through using Raman spectroscopy. CYP3A4 is known isozyme accountable for metabolizing various endogenous and exogenous xenobiotics. CYP3A4 is expressed dominantly in the liver and other organs including the lungs. Our results suggest that CYP3A4 has a higher affinity for p-SWNTs compared to c-SWNTs. HEK293 cellular viability was not compromised when incubated with SWNT. However, CYP3A4 transfected HEK293 cell line showed no digestion of c-SWNTs after incubation for 96 h. Cellular uptake of c-SWNTs was observed by electron microscopy and localization of c-SWNTs was confirmed in endosomal vesicles and in the cytoplasm. This is the first study CYP3A4 degrading both p-SWNTs and c-SWNTs in an in vitro setup. Interestingly, our results show that CYP3A4 is more proficient in degrading p-SWNTs than c-SWNTs. We also employed computational modeling and docking assessments to develop a further understanding of the molecular interaction mechanism. © 2019 Elsevier Inc.
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| 3. |
- Elgammal, Karim, 1986-, et al.
(författare)
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Adsorption of carbon dioxide and water molecules on graphene on top of silica substrates: dispersion corrected density functional calculations
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Annan publikation (övrigt vetenskapligt/konstnärligt)abstract
- We report on systematic computational studies of carbon dioxide and water molecule adsorption on graphene, with the graphene layer deposited on top of a substrate. Specifically, we address the influence of cristobalite and quartz substrates, i.e. two different types of silicon dioxide. The computations are based on density functional theory (DFT), with a nonempirical nonlocal van der Waals density functional included to account for dispersion forces.We calculate the binding energies and equilibrium positions of the molecules, as well as charge transfer and how the charge density of the graphene layer changes due to the interactions with the substrate and the molecules. The molecule-graphene bonding distances are found to be in the range 3.3-3.4 Å, and the graphene-substrate bonding distances around 3.6 Å. These values are slightly larger than what we have found previously, using an empirical expression for the van der Waals density functional. At the same time, the values for the binding energies are increased, compared to what we have obtained in a previous study. We find, in all cases, a net electron transfer from the adsorbed molecule to the graphene+substrate system. For quartz, the total charge transfer is between 0.1 and 0.2 electrons per adsorbed molecule. For cristobalite, it is only about a tenth of that. Our findings are consistent with earlier calculations as well as experimental data.
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| 4. |
- Elgammal, Karim, 1986-
(författare)
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Density Functional Theory Calculations for Graphene-based Gas Sensor Technology
- 2018
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Nowadays, electronic devices span a diverse pool of applications, especially when getting smaller and smaller satisfying the more than Moore paradigm. To further develop this, studies focusing on material design toward electronic devices are crucial. Accordingly, we present a theoretical study investigating the possibility of graphene as a promising material for such electronic devices design. We focus on graphene and graphene-based sensors. Graphene is known to have outstanding electronic and mechanical properties making it a game changer in the electronic design in the so-called 'post-silicon' industry. It is stronger than steel yet the thinnest material ever known while overstepping copper regarding electronic conductivity.In this thesis, we perform first-principle ab-initio density functional theory (DFT) calculations of graphene in different sensing ambient conditions, which allows fast, accurate and efficient investigations of the electronic structure properties. Principally, we centre our attention on the arising interactions between the adsorbates on top of the graphene sheet and the underlying substrates' surface defects. The combined effect of the impurity bands arising from these defects and the adsorbates reveals a doping influence within the graphene sheet. This doping behaviour is responsible for different equilibrium distances and binding energies for different adsorbate types as well as substrates. Moreover, we briefly investigate the same effect on double layered graphene under the same ambient conditions.We extend the studies to involve various types of substrates with different surface conditions and different adhesion nature to graphene. We take into consideration the governing van der Waals interactions in describing the electronic structure properties taking place at the graphene sheet interfacing both with the substrates below and the adsorbates above. Furthermore, we investigate the possibility of passivating such action of graphene sensing towards adsorbates to inhibit the graphene's sensing action as devices passivation becomes a necessity for the ultimate purpose of achieving more than Moore applications. Which in turn result in the optimal integration of graphene-based devices with different other devices functionalities on the same resultant chip.In summary, graphene, by means of first-principle calculations verification, shows a promising behaviour in the sensor functionality enabling more than Moore applications for further advances.
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| 5. |
- Elgammal, Karim, 1986-
(författare)
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Density Functional Theory Calculations of Graphene based Humidity and Carbon Dioxide Sensors
- 2016
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Licentiatavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Graphene has many interesting physical properties which makes it useful for plenty of applications. In this work we investigate the possibility of using graphene as a carbon dioxide and humidity sensor. Carbon dioxide and water adsorbates are modeled on top of the surface of a graphene sheet, which themselves lie on one of two types of silica substrates or sapphire substrate. We evaluate the changes in the electronic and structural properties of the graphene sheet in the presence of the described adsorbates as well as the accompanying substrate. We perform the study using ab-initio calculations based on density functional theory (DFT), that allows fast, accurate and efficient investigations. In particular, we focus our attention on investigating the effects of defects in the substrate and how it influences the properties of the graphene sheet. The defects of the substrate contribute with impurity bands leading to doping effects on the graphene sheet, which in turn together with the presence of the adsorbates result in changes of the electronic charge distribution in the system. We provide charge density difference plots to visualize these changes and also determine the relaxed minimum distances of the adsorbates from the graphene sheet together with the respective minimum energy configurations. We also include the density of states, Löwdin charges and work functions for further investigations.
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| 6. |
- Elgammal, Karim, 1986-, et al.
(författare)
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Graphene adhesion on surfaces: a van der Waals density functional study
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Annan publikation (övrigt vetenskapligt/konstnärligt)abstract
- We present a van der Waals density functional (vdW-DF) calculations study of graphene adhesion to different types of substrates with different surface conditions. The study expands to both metal and semiconductor substrates with different surface endings. All substrate surfaces were the 111 surfaces where they have hexagonal lattice parameters perfectly matching with the graphene's. Adsorption geometries, energies, bader charges, dipole moments and electronic structure in terms of density of states are investigated. The results are showing a general agrement with both experimental results as well as theoritical findings done with similar setup. The results reveal that the degree of adhesive of graphene to different surfaces can affect the electronic structure of graphene ending in having different applications when designing graphene in building nano-electronic devices.
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| 7. |
- Fan, Xuge, 1985-, et al.
(författare)
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Humidity and CO2 gas sensing properties of double-layer graphene
- 2018
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Ingår i: Carbon. - Netherlands : Elsevier. - 0008-6223 .- 1873-3891. ; 127, s. 576-587
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Tidskriftsartikel (refereegranskat)abstract
- Graphene has interesting gas sensing properties with strong responses of the graphene resistance when exposed to gases. However, the resistance response of double-layer graphene when exposed to humidity and gasses has not yet been characterized and understood. In this paper we study the resistance response of double-layer graphene when exposed to humidity and CO2, respectively. The measured response and recovery times of the graphene resistance to humidity are on the order of several hundred milliseconds. For relative humidity levels of less than ~ 3% RH, the resistance of double-layer graphene is not significantly influenced by the humidity variation. We use such a low humidity atmosphere to investigate the resistance response of double-layer graphene that is exposed to pure CO2 gas, showing a consistent response and recovery behaviour. The resistance of the double-layer graphene decreases linearly with increase of the concentration of pure CO2 gas. Density functional theory simulations indicate that double-layer graphene has a weaker gas response compared to single-layer graphene, which is in agreement with our experimental data. Our investigations contribute to improved understanding of the humidity and CO2 gas sensing properties of double-layer graphene which is important for realizing viable graphene-based gas sensors in the future.
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| 8. |
- Quellmalz, Arne, et al.
(författare)
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Influence of Humidity on Contact Resistance in Graphene Devices
- 2018
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Ingår i: ACS Applied Materials and Interfaces. - : American Chemical Society (ACS). - 1944-8244 .- 1944-8252. ; 10:48, s. 41738-41746
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Tidskriftsartikel (refereegranskat)abstract
- The electrical contact resistance at metal–graphene interfaces can significantly degrade the properties of graphene devices and is currently hindering the full exploitation of graphene’s potential. Therefore, the influence of environmental factors, such as humidity, on the metal–graphene contact resistance is of interest for all graphene devices that operate without hermetic packaging. We experimentally studied the influence of humidity on bottom-contacted chemical-vapor-deposited (CVD) graphene–gold contacts, by extracting the contact resistance from transmission line model (TLM) test structures. Our results indicate that the contact resistance is not significantly affected by changes in relative humidity (RH). This behavior is in contrast to the measured humidity sensitivity of graphene’s sheet resistance. In addition, we employ density functional theory (DFT) simulations to support our experimental observations. Our DFT simulation results demonstrate that the electronic structure of the graphene sheet on top of silica is much more sensitive to adsorbed water molecules than the charge density at the interface between gold and graphene. Thus, we predict no degradation of device performance by alterations in contact resistance when such contacts are exposed to humidity. This knowledge underlines that bottom-contacting of graphene is a viable approach for a variety of graphene devices and the back end of the line integration on top of conventional integrated circuits.
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| 9. |
- Smith, Anderson D., et al.
(författare)
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Graphene-based CO2 sensing and its cross-sensitivity with humidity
- 2017
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Ingår i: RSC Advances. - : Royal Society of Chemistry. - 2046-2069. ; 7:36, s. 22329-22339
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Tidskriftsartikel (refereegranskat)abstract
- We present graphene-based CO2 sensing and analyze its cross-sensitivity with humidity. In order to assess the selectivity of graphene-based gas sensing to various gases, measurements are performed in argon (Ar), nitrogen (N2), oxygen (O2), carbon dioxide (CO2), and air by selectively venting the desired gas from compressed gas bottles into an evacuated vacuum chamber. The sensors provide a direct electrical readout in response to changes in high concentrations, from these bottles, of CO2, O2, nitrogen and argon, as well as changes in humidity from venting atmospheric air. From the signal response to each gas species, the relative graphene sensitivity to each gas is extracted as a relationship between the percentage-change in graphene's resistance response to changes in vacuum chamber pressure. Although there is virtually no response from O2, N2 and Ar, there is a sizeable cross-sensitivity between CO2 and humidity occurring at high CO2 concentrations. However, under atmospheric concentrations of CO2, this cross-sensitivity effect is negligible – allowing for the use of graphene-based humidity sensing in atmospheric environments. Finally, charge density difference calculations, computed using density functional theory (DFT) are presented in order to illustrate the bonding of CO2 and water molecules on graphene and the alterations of the graphene electronic structure due to the interactions with the substrate and the molecules.
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| 10. |
- Smith, Anderson D., et al.
(författare)
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Resistive graphene humidity sensors with rapid and direct electrical readout
- 2015
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Ingår i: Nanoscale. - : Royal Society of Chemistry (RSC). - 2040-3364 .- 2040-3372. ; 7:45, s. 19099-19109
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Tidskriftsartikel (refereegranskat)abstract
- We demonstrate humidity sensing using a change of the electrical resistance of single-layer chemical vapor deposited (CVD) graphene that is placed on top of a SiO2 layer on a Si wafer. To investigate the selectivity of the sensor towards the most common constituents in air, its signal response was characterized individually for water vapor (H2O), nitrogen (N-2), oxygen (O-2), and argon (Ar). In order to assess the humidity sensing effect for a range from 1% relative humidity (RH) to 96% RH, the devices were characterized both in a vacuum chamber and in a humidity chamber at atmospheric pressure. The measured response and recovery times of the graphene humidity sensors are on the order of several hundred milliseconds. Density functional theory simulations are employed to further investigate the sensitivity of the graphene devices towards water vapor. The interaction between the electrostatic dipole moment of the water and the impurity bands in the SiO(2)d substrate leads to electrostatic doping of the graphene layer. The proposed graphene sensor provides rapid response direct electrical readout and is compatible with back end of the line (BEOL) integration on top of CMOS-based integrated circuits.
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