Near Room-temperature Synthesis of Transfer-free Graphene Films

Materials Science EngineeringGraphene is a single layer of only carbon atoms tightly packed into a two-dimensional (2D) honeycomb lattice and is a basic building block for graphitic materials of all other dimensionalities and is the basis for understanding of physical or chemical properties of the various carbon-based materials. In graphene lattice, the carbon bonds are sp2 hybridized, where the in-plane σ bond is one of the strongest bonds in materials and the out-of-plane π bond, which contributes to a delocalized network of electrons, is responsible for the electron conduction of graphene and provides the weak interaction between graphene and substrates. In addition, the energy dispersion at K and K’ points in the first Brillouin zone is linear, which closely resembles the Dirac spectrum for massless fermions. With these unique structural characteristics and the band structure, graphene has shown exceptional physical properties, which have attracted enormous research interest in both scientific and engineering fields. One of the most remarkable properties of graphene is that the charge carriers behave as Dirac fermions, which give rise to extraordinary effects such as mobility up to 200,000 cm2V-1s-1, ballistic transport distances of up to a micron at room temperature, half-integer quantum Hall effect. Graphene also possesses the excellent mechanical strength, such as the breaking strength of ~ 42 Nm-1 and the Young’s modulus of 1.0 TPa. Its thermal conductivity is measured with a value of ~ 5,000WmK-1. In addition, graphene is highly transparent, with absorption of ~ 2.3% towards visible light. For applying these outstanding properties of graphene to various fields, the development of various methods has stimulated a vast amount of research in recent years and thus there are four different methods; the mechanical or chemical exfoliation of graphite, sublimation of SiC, and CVD growth on metal substrates. Among these, large-area graphene films are currently best synthesized via the CVD process onto polycrystalline metal surfaces and this method is the most promising method for realization of graphene-based flexible optoelectronic display technology. However, even in the CVD process, there are several problems for direct device applications, such as additional transfer process, introduction of high process temperature and high process costs. Therefore, in this study, we describe a very low-temperature and transfer-free approach to controllably deposit graphene films onto desired substrates, which we refer to as Diffusion-Assisted Synthesis (DAS) method. Our synthesis methodology exploits the properties of a ‘diffusion couple’, wherein a Ni thin film is deposited first on the substrate, and solid carbon (graphite powders) is then deposited on top of the Ni, and allowed to diffuse along the Ni layer to create a thin graphene film at the Ni-substrate interface. First we conducted our DAS process on the hard substrate, such as SiO2 layers, at temperatures below 260 °C. In this case, the as-synthesized graphene films are wrinkle-free and smooth over large areas. Interestingly, we find that the morphologies of regions covered with mono- and bi-layer graphene resemble those of the grains, and the multi-layer graphene ridges, the grain boundaries in the Ni thin films. The electrical properties of graphene layers on SiO2/Si obtained at low-temperature (T ≤ 260 °C) have been evaluated with back-gated graphene-based field-effect transistor (FET) devices and by using transmission line model method. The estimated hole mobility is ~667cm2V-1s-1 at room temperature in ambient conditions and the sheet resistance is found to be ~1,000Ω per square, suggesting that the as-synthesized graphene films are of reasonable quality. We also find that graphene films obtained range from 25 °C to 260 °C have similar structural quality, but the surface coverage of graphene on SiO2 shows a strong dependence on the growth temperature. Furthermore, we have explored the possibility of using our approach to grow graphene in air instead of inert Ar atmospheres. Surprisingly, we find that the surface morphology, areal coverage and Raman structure of the graphene films grown in Ar as well as in air are similar. In addition, we studied the characteristics of the DAS-graphene grown on SiO2/Si substrates at high-temperature growth regime (300 °C ≤ T ≤ 600 °C). In this study, we observe the formation of nanocrystalline graphene layers by precipitation and the morphologies of graphene films are largely independent of process temperature, time and microstructure of poly-Ni films in this process regime. Also we find that the layers contain no graphene ridges at all. From above experimental results and theoretical estimation using Fisher model and DFT calculations, we propose a mechanism for the growth of graphene layers in the DAS process as follows: (1) the resulting C atoms from solid carbon source are transported across the Ni film primarily along the grain boundaries to the Ni-SiO2 interface at low-temperatures and (2) upon reaching the Ni-SiO2 interface, C atoms precipitate out as graphene at the grain boundaries and (3) excess C atoms reaching the graphene ridges, diffuse laterally along the graphene-Ni (111) interface and lead to the growth of graphene over large areas, driven by the strong affinity of C atoms to self-assemble and expand the sp2 lattice. Finally, we demonstrated the applicability of our approach to prepare large-area graphene on the soft material substrates, such as PDMS, PMMA, and glass. To this purpose, we use T ≤ 160 °C and do not anneal the Ni thin films so as to minimize thermal degradation of the substrates. In contrast to graphene on SiO2, the graphene films on plastic and glass substrates are continuous over large areas at all temperatures, possibly due to the decrease in distance between grain boundaries. The as-grown layers on the soft material substrates are nanocrystalline graphene.ope

Low-temperature formation of epitaxial graphene on 6H-SiC induced by continuous electron beam irradiation

It is observed that epitaxial graphene forms on the surface of a 6H-SiC substrate by irradiating electron beam directly on the sample surface in high vacuum at relatively low temperature (similar to 670 degrees C). The symmetric shape and full width at half maximum of 2D peak in the Raman spectra indicate that the formed epitaxial graphene is turbostratic. The gradual change of the Raman spectra with electron beam irradiation time increasing suggests that randomly distributed small grains of epitaxial graphene form first and grow laterally to cover the entire irradiated area. The sheet resistance of epitaxial graphene film is measured to be similar to 6.7 k Omega/sq.open4

Electrically Robust Single-Crystalline WTe2 Nanobelts for Nanoscale Electrical Interconnects

As the elements of integrated circuits are downsized to the nanoscale, the current Cu-based interconnects are facing limitations due to increased resistivity and decreased current-carrying capacity because of scaling. Here, the bottom-up synthesis of single-crystalline WTe2 nanobelts and low- and high-field electrical characterization of nanoscale interconnect test structures in various ambient conditions are reported. Unlike exfoliated flakes obtained by the top-down approach, the bottom-up growth mode of WTe2 nanobelts allows systemic characterization of the electrical properties of WTe2 single crystals as a function of channel dimensions. Using a 1D heat transport model and a power law, it is determined that the breakdown of WTe2 devices under vacuum and with AlOx capping layer follows an ideal pattern for Joule heating, far from edge scattering. High-field electrical measurements and self-heating modeling demonstrate that the WTe2 nanobelts have a breakdown current density approaching approximate to 100 MA cm(-2), remarkably higher than those of conventional metals and other transition-metal chalcogenides, and sustain the highest electrical power per channel length (approximate to 16.4 W cm(-1)) among the interconnect candidates. The results suggest superior robustness of WTe2 against high-bias sweep and its possible applicability in future nanoelectronics

Enhancement in the Seismic Performance of a Nuclear Piping System using Multiple Tuned Mass Dampers

In a nuclear power plant, it is essential to improve the seismic safety of the piping system for the coolant transfer to cool the high temperature caused by the nuclear reaction. Under this background, this study makes two major contributions. The first is that though tuned mass dampers (TMDs) were originally used only to reduce the vibration of piping itself, through this research, it was first proved that it had a positive effect on the improvement of the seismic performance of nuclear piping systems. Additionally, this study proposed a design approach that effectively obtains the optimal design values of TMDs associated with seismic performance. In order to effectively derive the TMD optimum design values, we not only utilized the existing TMD optimum design formula, but also additionally proposed a frequency response analysis-based TMD optimal design method. As a result, it was seen that primary responses of system were significantly reduced under the input seismic load due to the use of TMDs for the piping system. It was also confirmed that the use of the existing TMD formula brought about a similar degree of response reduction effect, while it was possible to get the improved effect when using the proposed method

Adsorption Characteristics of Dimethylated Arsenicals on Iron Oxide–Modified Rice Husk Biochar

In this study, the adsorption characteristics of dimethylated arsenicals to rice husk biochar (BC) and Fe/biochar composite (FeBC) were assessed through isothermal adsorption experiments and X-ray absorption spectroscopy analysis. The maximal adsorption capacities (qm) of inorganic arsenate, calculated using the Langmuir isotherm equation, were 1.28 and 6.32 mg/g for BC and FeBC, respectively. Moreover, dimethylated arsenicals did not adsorb to BC at all, and in the case of FeBC, qm values of dimethylarsinic acid (DMA(V)), dimethylmonothioarsinic acid (DMMTA(V)), and dimethyldithioarsinic acid (DMDTA(V)) were calculated to be 7.08, 0.43, and 0.28 mg/g, respectively. This was due to the formation of iron oxide (i.e., two-line ferrihydrite) on the surface of BC. Linear combination fitting using As K-edge X-ray absorption near edge structure spectra confirmed that all chemical forms of dimethylated arsenicals adsorbed on the two-line ferrihydrite were DMA(V). Thus, FeBC could retain highly mobile and toxic arsenicals such as DMMTA(V) and DMDTA(V)) in the environment, and transform them into DMA(V) with relatively low toxicity

A Resource-Efficient Keyword Spotting System Based on a One-Dimensional Binary Convolutional Neural Network

This paper proposes a resource-efficient keyword spotting (KWS) system based on a convolutional neural network (CNN). The end-to-end KWS process is performed based solely on 1D-CNN inference, where features are first extracted from a few convolutional blocks, and then the keywords are classified using a few fully connected blocks. The 1D-CNN model is binarized to reduce resource usage, and its inference is executed by employing a dedicated engine. This engine is designed to skip redundant operations, enabling high inference speed despite its low complexity. The proposed system is implemented using 6895 ALUTs in an Intel Cyclone V FPGA by integrating the essential components for performing the KWS process. In the system, the latency required to process a frame is 22 ms, and the spotting accuracy is 91.80% in an environment where the signal-to-noise ratio is 10 dB for Google speech commands dataset version 2

Preparation of Giant Quantum Dot-Liposome Complexes by the Asolectin Lipid and Theoretical Model for Stabilization of Nanoparticle Inside the Liposome

We prepare giant Quantum dot-Liposome Complexes (QLCs). Quantum dots (QDs) incorporated inside liposome above 10 mu m. QLCs is made by using the electro-swelling method combined with spin coating techniques. Three types of PC lipids and asolectin lipid are used for QLCs with HDA (hexadecylamine) coated QDs, which ranged from blue-(diameter-2.1 nm) to red-emission (diam-eter-5.0 nm). As expected, (blue-or) green-emission QDs (smaller than) comparable to the thick-ness of PC lipid bilayer (-4 nm) are successfully formed QLCs, but QDs bigger than that fail to reproduce. This observation is well-consistent with those reported by Gopakumar et al. Surprisingly, all QDs irrespective of their size are, contrary to PC lipids, successfully loaded into asolectin lipid IP: 203.8.109.20 On: Thu, 11 Aug 2022 08:59:06 bilayer. In order to understand what makes different behaviors between PC and asolectin lipids on Copyright: American Scientific Publishers QLC formation, we suggest a theoretical model based on a geometrical assumptions for deformed Delivered by Ingenta lipid bilayer surrounding QD. The main advantage of this model is that the critical size Rcr of QD radius can be decided without calculating elastic free energy. As a result, it predicts that only QDs below the critical size (diameter-3.0 nm) can be loaded in a typical PC-lipid, but all size of QDs can be incorporated into asolectin bilayer under the assumption of two different curvatures on deformed monolayer