Growth Of Multilayer Graphene Research Articles

A scalable top-gate graphene field effect transistor with a polydimethylsiloxane dielectric

The limitations of modern CMOS technology have created a call to action for novel devices with great scalability potential. Graphene has been recognized as a suitable material for an enhanced transistor channel based on its incredibly large conductivity while also being easily scaled. Previous research has noted the importance of a top gate device structure, which is difficult to accomplish for graphene transistors due to graphene's incompatibility with oxide growth processes. A novel process flow for graphene field effect transistors with scalability is presented. The emphasis is on the growth of multilayer graphene using chemical vapor deposition and the implementation of polydimethylsiloxane as a gate dielectric. Polydimethylsiloxane gate insulator thickness of 815 nm and 570 nm were successfully developed on 4in large-scale wafers. Two devices of similar channel dimensions and different dielectric were compared and mobilities of 14.57cm2V−1s−1 and 0.44 cm2V−1s−1 were measured. Gate voltage sweeps from -20 V to 20 V also demonstrated channel current modulation with a charge neutrality point between 5 V and 8 V, indicating achievement of expected device operation.

Optimization for Current-Enhanced Solid-Phase Deposition of Multi-Layer Graphene with High Crystallinity and Uniformity

Solid-phase deposition (SPD) is a preferable method to fabricate multi-layer graphene (MLG) for device applications since MLG can be deposited directly on substrates without transfer. Previously, we reported that current application during SPD enhances the MLG growth. In this study, we investigated the effects of applied current and heating rate on the crystallinity and uniformity of MLG precipitated from carbon (C) / nickel (Ni) structures. It was found that higher current and slower heating rate lead to improve the MLG crystallinity. Moreover, the uniformity was improved by the optimization of Ni and C thickness which could control the nucleation and growth of MLG during SPD. As a result, a uniform MLG with a high G/D ratio of 5.5 was obtained at a low temperature of 365 °C.

Thickness-Controlled Growth of Multilayer Graphene on Ni(111) Using an Approximate Equilibrium Segregation Method for Applications in Spintronic Devices

Thickness-controlled multilayer graphene has been attracting wide interest in electronic and spintronic devices due to its tunable electronic structure and spin transport properties. In particular, the strong spin filtering effect in a lattice-matched Ni(111)/multilayer graphene heterostructure provides an ideal platform for developing high-performance spin valves and magnetic tunnel junctions. However, the thickness-controlled synthesis of multilayer graphene on Ni(111) substrates is still a large challenge, which seriously restricts the development of graphene spintronic devices. Here, we report an approximate equilibrium segregation method for large-area multilayer graphene films on Ni(111) substrates by regulating the growth process of carbon dissolution and segregation. The uniformity and coverage of multilayer graphene films are controlled precisely in an atmospheric tube furnace system by modulating the Ni film thickness, gas atmosphere, and cooling rate. Characterization results systematically confirm the production of high-coverage, high-quality multilayer graphene on Ni(111) substrates. This work provides a reliable method for the controllable fabrication of multilayer graphene on single-crystal ferromagnetic substrates, which would contribute to the application of graphene spintronic devices.

Low-Temperature Direct Growth of Nanocrystalline Multilayer Graphene on Silver with Long-Term Surface Passivation.

A wide variety of transition metals, including copper and gold, have been successfully used as substrates for graphene growth. On the other hand, it has been challenging to grow graphene on silver, so realistic applications by combining graphene and silver for improved electrode stability and enhanced surface plasmon resonance in organic light-emitting diodes and biosensing have not been realized to date. Here, we demonstrate the surface passivation of silver through the single-step rapid growth of nanocrystalline multilayer graphene on silver via low-temperature plasma-enhanced chemical vapor deposition (PECVD). The effect of the growth time on the graphene quality and the underlying silver characteristics is investigated by Raman spectroscopy, X-ray diffraction, atomic force microscopy, X-ray photoelectron spectroscopy (XPS), and cross-sectional annular dark-field scanning transmission electron microscopy (ADF-STEM). These results reveal nanocrystalline graphene structures with turbostratic layer stacking. Based on the XPS and ADF-STEM results, a PECVD growth mechanism of graphene on silver is proposed. The multilayer graphene also provides excellent long-term protection of the underlying silver surface from oxidation after 5 months of air exposure. This development thus paves the way toward realizing technological applications based on graphene-protected silver surfaces and electrodes as well as hybrid graphene-silver plasmonics.

Synthesis of Multilayer Graphene with Controlled C Supply

Because of the very low C solubility, Cu substrates possess a great advantage in growing the graphene film of monolayer or few layers with well‐developed uniformity. The self‐limited growth manner ensures the growth of nearly pure monolayer graphene, but it prevents the supply of C for the growth of adlayers after the complete growth of the top layer. Herein, a method to continuously supply C from one side of Cu substrate while maintaining sustained growth of multilayer graphene on the other side by separately controlling the atmosphere on two sides is studied. The growth rate and thickness of graphene can be controlled by tuning the oxygen flow and postgrowth time. Multilayer graphene with thickness (t) up to 100 nm is achieved. It demonstrates a potential application in terahertz shielding with a high shielding effectiveness (SE) of 25 dB and accordingly a high SE/t of 2.5 × 105 dB/mm. The study contributes to the controllable growth of graphene films in terms of layer numbers and can be scaled up to large‐area growth through the appropriate design of equipment.

Fast spatially resolved optical characterization of multi-layer CVD graphene

A new quantitative and spatially resolved optical method was developed to quickly analyze the spatial distribution of multilayer graphene grown on large flat substrates, i.e., on one or more 4″ Si wafers. An image processing analysis shows that the background-corrected reflection image of graphene on the flat substrate can be normalized so that the highest image intensity is related to the thinnest or absent layer of graphene. Which of these two most typical cases is in effect for a given sample can be easily determined by a calibration wafer using laser Raman spectroscopy 2D/G peak intensity ratio. We show that the spatial distribution of multilayer graphene can be determined by relating reflected light intensity from an intensity-calibrated reflection image to the layer numbers of a local graphene film. By dividing the greyscale reflection intensity into bins, the spatial multilayer graphene distribution can be visualized with a false-colored contour plot where each graphene layer quantity is represented by a unique color for easy visualization of their spatial distribution regions. The spatial distribution over extended spatial regions of multilayer graphene grown on a flat substrate, for example, a Ni or Cu films on a 4ʺ Si or Ge wafer, can quickly be analyzed and visualized through this non-destructive characterization method by locally sampling and analyzing multiple regions of interest across a full spatially extended large substrate. Selected applications of this method are, for example, process uniformity and quality optimization for one or more large Si wafers grown in a single batch or different batches, as well as for fast and non-destructive automated multilayer graphene growth quality control methods.

Multilayer graphene sunk growth on Cu(111) surface

The controllable growth of multilayer graphene is a challenging research topic. Prior results show graphene adlayers can grow beneath pre-existing graphene layers on a Cu(111). The conventional inverted-wedding-cake (IWC) model used to describe this incurs an energy disadvantage due to deformation in the overlying graphene. We propose an alternative theoretical model, the sunk growth mode, for understanding multilayer graphene growth on Cu substrates. Extensive density functional theory (DFT) calculations show that multilayer graphene grown via this sunk mode is energetically favourable compared to the on-terrace growth mode for Cu(111). These results reveal that graphene underlayers tend to grow in a sunk growth mode, minimizing deformation in the overlayers, reducing deformation energy. Further density functional tight binding-molecular dynamic (DFTB-MD) simulations on Cu(111) substrates yield sunken structures consistent with our sunk growth mode. Moreover, AFM investigations of experimentally grown multilayer graphene on polycrystaline Cu show that while friction data indicates multiple graphene edges in the sample, the topological height measurement indicates flat graphitic sheets, further confirming our sunk growth mode. This discovery provides a novel and more reasonable model for the “underlayer” growth of multilayer graphene and can be extended to a general theory for the multilayer graphene growth on various substrates.

Unconventional Reaction Phase Diagram for the Penetration Etching/Growth of Graphene Adlayers

Cu has shown an advantage in growing monolayer graphene due to the very low C solubility and surface-mediated self-limiting growth, which hinders the growth of multilayer graphene. This work reports an unconventional penetration etching/growth of graphene adlayers tuned by oxygen beyond the self-limiting growth, supported by the C isotope labeling results. The effect of oxygen is nonmonotonic, i.e., with the increase of oxygen, graphene adlayers are etched without damaging the top layer, then shift to growth, and finally, all layers are etched. In addition, the reaction did not seem to reach equilibrium in the time range of the experiment but continued as if oxygen was increasing with respect to time. An oxygen-assisted exchange penetration model is proposed to interpret the growth mechanism. Oxygen etches the top shield layer, which is simultaneously healed by consuming the C species around adlayers and results in adlayer decomposition due to the break of equilibrium. Additionally, oxygen assists penetration of C from the gaseous agents into the shield layer for adlayer growth. The domination of one over the other depends on the concentration of oxygen, resulting in overall etching or growth. Finally, the synthesis of large-area monolayer and bilayer graphene films with good uniformity is demonstrated.

Evaluation of oxidation suppression of multilayer graphene synthesized using fluorene as a solid source

Multilayer graphene (MLG) on a Cu substrate was successfully fabricated via a solid-source chemical vapor deposition process using fluorene as the solid source. The surface coverage of graphene was greater than 98% even at 6 min, and the MLG ratio was ∼97%, indicating that our growth method selectively promoted MLG growth from the beginning of graphene growth. The oxidation suppression characteristics of MLG were confirmed using electron back-scattered diffraction (EBSD) analysis. Cu crystal orientations were identified only under areas covered by graphene, indicating that the oxidation of Cu did not proceed at the interface between Cu and graphene. We confirmed using EBSD techniques that MLG suppresses the surface oxidation of a Cu substrate. Furthermore, we showed that EBSD was an effective tool for evaluating surface oxidation as well as surface crystal orientations.

Bifunctional catalytic effect of Mo2C/oxide interface on multi-layer graphene growth

The role of the Mo2C/oxide interface on multi-layer graphene (MLG) nucleation during a chemical vapor deposition (CVD) process is investigated. During the CVD process, MLG growth is only observed in the presence of a Mo2C/SiO2 interface, indicating that the chemical reactions occurring at this interface trigger the nucleation of MLG. The chemical reaction pathway is explained in four steps as (1) creation of H radicals, (2) reduction of the oxide surface, (3) formation of C–C bonds at O–H sites, and (4) expansion of graphitic domains on the Mo2C catalyst. Different Mo2C/oxide interfaces are investigated, with varying affinity for reduction in a hydrogen environment. The results demonstrate a catalyst/oxide bifunctionality on MLG nucleation, comprising of CH4 dehydrogenation by Mo2C and initial C–C bond formation at the oxide interface.

Growth of monolayer and multilayer graphene on glassy carbon electrode for simultaneous determination of guanine, adenine, thymine, and cytosine

• Graphene (Gr) was grown vertically on a glassy carbon (GC) electrode with Au catalyst. • Monolayer and multilayer Gr films were deposited on GC to form a novel carbon electrode. • The carbon electrode responds directly to all four DNA bases and various other biomarkers. • The carbon electrode has excellent stability and reliability in DNA base analysis. A novel carbon film composed of glassy carbon (GC), monolayer graphene (Gr), and multilayer Gr was prepared by continuously growing Gr sheets on a GC substrate, using Au nanoparticles as catalysts. The formation of the monolayer Gr sheets and the vertical growth mechanism of the multilayer Gr sheets were investigated by analyzing the morphology, microstructure, and composition variations. The carbon multilayer film can recognize biomolecules and inorganic ions, and can respond to up to 10 species. This function is based on the inherent electrocatalytic activity of the carbon film without modification using additional sensitive materials. The use of the multilayer carbon film as an electrochemical biosensor was investigated with the four DNA bases: guanine (G), adenine (A), thymine (T), and cytosine (C). The carbon film clearly identified G, A, T, and C in a neutral electrolyte environment, with detection limits of 0.3, 0.4, 6.5, and 4.5 μM, respectively; the peak potential differences for A–G, T–A, and C–T were 164, 216, and 184 mV, respectively. This carbon film can replace commercial GC, Au, and carbon paste electrodes and function as a basic electrode to explore new electrochemical sensors with high functionality and sensitivity.

The utilization of waste cooking palm oil as a green carbon source for the growth of multilayer graphene

Waste cooking palm oil (WCPO) has been utilized as a green carbon source for the synthesization of graphene by double thermal chemical vapor deposition. The WCPO was placed in the first furnace (precursor furnace) whereas nickel was placed in the second furnace (deposition furnace). The deposition temperatures were varied between 850 and 1100 °C. Raman results reveal the highest 2-D peak for the sample synthesized at 1000 °C, which indicates the high-quality formation of graphene. Besides, the sample also shows good crystallinity with a sharp peak at 26.8° which represents the hexagonal graphite structure and the introduction of graphene sheet formation. On the other hand, the FESEM image displays hexagonal structures since the graphene layers were formed after the precipitation of the carbon. Meanwhile, the UV-Vis result shows the highest reflectance in the visible light region which indicates the presence of the graphene layer on Ni.

Graphene growth with no intended carbon precursor feeding into the LPCVD process: causes, solutions, and effects

In this research, we have investigated the unintended graphene nucleation problem and its damaging effects on monolayer graphene synthesis in low-pressure chemical vapor deposition (LPCVD) process. This problem is the growth of graphene on the copper surface with no carbon feedstock. A new source of undesired carbon species was identified which has not been addressed so far. The hydrogen-rich heating stage was diagnosed as the onset of the unintended nucleation for the first time owing to the determinant catalytic role of hydrogen in this stage. It was found out that this problem leads to uncontrollable growth of multilayer graphene, growth of defective graphene film and also inhibition of the reliable synthesis of monolayer graphene. We managed to grow enhanced-quality monolayer graphene by developing some innovative solutions to the problem containing a general solution based on the hydrogen effects in the heating stage. The results reveal a significant decrease in the unintended nucleation density from ∼2000 to almost zero domains per 100 × 100 μm2 copper area. Furthermore, Raman, HRTEM and SAED analysis confirm the defect-free growth of monolayer graphene after employing the solutions. These findings could pave the way for the reliable synthesis of high-quality monolayer graphene as well as large-sized graphene domains.

Cu‐Phosphorus Eutectic Solid Solution for Growth of Multilayer Graphene with Widely Tunable Doping

AbstractA one‐step chemical vapor deposition (CVD) is proposed to grow multilayer graphene (MLG) with tunable doping types using a copper–phosphorus eutectic system as a catalyst. At the growth temperature, the phosphorus‐dissolved copper forms a liquid phase, which promotes the formation of phosphorus‐doped MLG. With this method, the thickness and doping level of graphene are simultaneously controlled at the synthesis stage. Moreover, the proposed CVD method enables patterned growth of MLG at the microscale. The resultant phosphorus‐doped graphene demonstrates a tunable doping state from large n‐type doping to p‐type doping because of the high affinity of phosphorus to water molecules. Finally, stable n‐type doping of MLG by passivating it with a parylene thin film is demonstrated.

Twistronics in Graphene, from Transfer Assembly to Epitaxy

The twistronics, which is arising from the moiré superlattice of the small angle between twisted bilayers of 2D materials like graphene, has attracted much attention in the field of 2D materials and condensed matter physics. The novel physical properties in such systems, like unconventional superconductivity, come from the dispersionless flat band that appears when the twist reaches some magic angles. By tuning the filling of the fourfold degeneracy flat bands, the desired effects are induced due to the strong correlation of the degenerated Bloch electrons. In this article, we review the twistronics in twisted bi- and multi-layer graphene (TBG and TMG), which is formed both by transfer assembly of exfoliated monolayer graphene and epitaxial growth of multilayer graphene on SiC substrates. Starting from a brief history, we then introduce the theory of flat band in TBG. In the following, we focus on the major achievements in this field: (a) van Hove singularities and charge order; (b) superconductivity and Mott insulator in TBG and (c) transport properties in TBG. In the end, we give the perspective of the rising materials system of twistronics, epitaxial multilayer graphene on the SiC.

Growth of Multilayer Graphene with a Built-in Vertical Electric Field

Multilayer graphene is considered a promising material for various optoelectronic devices. To exploit its intriguing electronic properties, an electric field must be achieved inside this material. ...

Growth of Single-Layer and Multilayer Graphene on Cu/Ni Alloy Substrates

ConspectusGraphene, a one-atom-thick layer of carbon with a honeycomb lattice, has drawn great attention due to its outstanding properties and its various applications in electronic and photonic devices. Mechanical exfoliation has been used for preparing graphene flakes (from monolayer to multilayer with thick pieces also typically present), but with sizes limited typically to less than millimeters, its usefulness is limited. Chemical vapor deposition (CVD) has been shown to be the most effective technique for the scalable preparation of graphene films with high quality and uniformity. To date, CVD growth of graphene on the most commonly used substrates (Cu and Ni foils) has been demonstrated and intensively studied. However, a survey of the existing literature and earlier work using Cu or Ni substrates for CVD growth indicates that the bilayer and multilayer graphene over a large area, particularly single crystals, have not been obtained.In this Account, we review current progress and development in the CVD growth of graphene and highlight the important challenges that need to be addressed, for example, how to achieve large single crystal graphene films with a controlled number of layers. A single-layer graphene film grown on polycrystalline Cu foil was first reported by our group, and since then various techniques have been devoted to achieving the fast growth of large-area graphene films with high quality. Commercially available Cu/Ni foils, sputtered Cu/Ni thin films, and polycrystalline Cu/Ni foils have been used for the CVD synthesis of bilayer, trilayer, and multilayer graphene. Cu/Ni alloy substrates are particularly interesting due to their greater carbon solubility than pure Cu substrates and this solubility can be finely controlled by changing the alloy composition. These substrates with controlled compositions have shown the potential for the growth of layer-tunable graphene films in addition to providing a much higher growth rate due to their stronger catalytic activity. However, the well-controlled preparation of single crystal graphene with a defined number of layers on Cu/Ni substrates is still challenging.Due to its small lattice mismatch with graphene, a single crystal Cu(111) foil has been shown to be an ideal substrate for the epitaxial growth of graphene. Our group has reported the synthesis of large-size single crystal Cu(111) foils by the contact-free annealing of commercial Cu foils, and single crystal Cu/Ni(111) alloy foils have also been obtained after the heat-treatment of Ni-coated Cu(111) foils. The use of these single crystal foils (especially the Cu/Ni alloy foils) as growth substrates has enabled the fast growth of single crystal single-layer graphene films. By increase of the Ni content, single crystal bilayer, trilayer, and even multilayer graphene films have been synthesized. In addition, we also discuss the wafer-scale growth of single-layer graphene on the single crystalline Cu/Ni(111) thin films.Recent research results on the large-scale preparation of single crystal graphene films with different numbers of layers on various types of Cu/Ni alloy substrates with different compositions are reviewed and discussed in detail. Despite the remarkable progress in this field, further challenges, such as the wafer-scale synthesis of single crystal graphene with a controlled number of layers and a deeper understanding of the growth mechanism of bilayer and multilayer graphene growth on Cu/Ni substrates, still need to be addressed.

Low-temperature deposition of multilayer graphene with continuous morphology and few defects

Multilayer graphene has increasingly attracted significant research interest owing to its high conductivity with an available band gap. Chemical vapor deposition (CVD) has shown a great promise for large-scale multilayer graphene growth with a high temperature (typically 1000 °C). Here, we adopt camphor as the hydrocarbon feedstock and nickel as the catalyst to grow multilayer graphene at a low temperature down to 550 °C. The transferred multilayer graphene shows a uniform and continuous surface morphology with a low surface roughness down to 0.6 nm and few defects. In addition, the low temperature CVD technique can be applied in the three-dimensional architecture of multilayer graphene. Our work may pave a way to an undemanding route for economical and convenient multilayer graphene growth and its applications in microelectronic devices.

Barrier-assisted ion beam synthesis of transfer-free graphene on an arbitrary substrate

In distinction to the generally utilized chemical vapor deposition (CVD) synthesis that leads to multilayer graphene growth by carbon (C) synthesis from nickel (Ni), we proposed a controllable strategy to synthesize graphene on an arbitrary substrate through ion implantation technology, where the layer number of the obtained graphene film is accurately controlled by the corresponding dose of implanted C ions. To be specific, an oxide layer (NiO) was introduced as the barrier to prevent implanted C atom precipitation at the surface but at the interface. Various unusual substrates (such as sapphire, glass, SiO2, and Si), in terms of traditional CVD, have been utilized for growing high-quality graphene. Employing the as-grown graphene/Si, Schottky junction-based photodetectors with high responsivity (63 mA W−1) and high detectivity (∼1.4 × 1010 cm Hz1/2 W−1) at 1550 nm are demonstrated without requiring any post-transfer process, thus avoiding additional contaminations, complexities, and costs during device fabrications. Our works afford a versatile technique for growing graphene on arbitrary substrates, with controllable layer numbers and transfer-free optoelectronic device fabrications, thus accelerating their further practical applications in electro-optical devices.

Large-scale chemical vapor deposition of graphene on polycrystalline nickel films: Effect of annealing conditions

In the present study, 8-in. silicon substrates, covered with thin (200 nm) polycrystalline nickel films have been employed for the growth of graphene by chemical vapor deposition. In order to control the uniformity and coverage of the graphene, thin nickel layers were used due to their less deep grain boundary grooves and ability to store less carbon in comparison with thick nickel films (>500 nm). The preferential sites for the growth of multilayer graphene were influenced by the surface pretreatment of the polycrystalline nickel films at 1025 °C under different ambient conditions (hydrogen and vacuum). Significant differences in the surface morphologies were observed for the annealed nickel films. The growth of larger grains up to ~6 μm for the films annealed in hydrogen could be attributed to hydrogen interstitials. On the other hand, grains up to ~3 μm were extracted for the films annealed in vacuum. Graphene was grown after exposing the annealed Ni films to ethylene at 925 °C. The lower range (42–106 cm−1) of full width at half maxima of the 2D band as determined by Raman spectroscopy was obtained for the films annealed in hydrogen as compared to the ones annealed in vacuum (51–128 cm−1), indicating that the thickness uniformity of graphene was strongly influenced by the surface modifications of nickel films.