Continuous Graphene Films Research Articles

Improved Conductivity of Low‐Temperature‐Synthesized Graphene/Cu for CMOS Backend‐of‐Line Interconnect Applications

AbstractThis study proposes a synthesis strategy of high‐quality graphene films on the copper foil at a temperature of 400 °C throughout the graphene growth process without employing high‐temperature annealing. Through continuous CO2 laser pretreatment of the copper foil, the surface smoothness improves, and the removal of copper particles and copper oxide results in fewer defects on the foil. Therefore, the nucleation density of graphene is reduced, leading to a more uniform and continuous graphene film and showing an outstanding quality of graphene with low defects and low resistivity compared with other groups. After laser treatment, the copper foil's resistivity decreases from 1.71 ×10−8 to 1.51 ×10−8 Ω·m. The graphene‐coated on laser‐treated foil experiences an even more substantial decrease in resistivity, from 1.34 ×10−8 to 1.18 ×10−8 Ω·m, marking a significant 11.94% reduction. Excitingly, the groundbreaking technique is taken to the next level by applying it to fine copper interconnects as narrow as 0.4 µm. The experiments confirm the successful cultivation of graphene on these miniature scales, showing the immense potential of the approach. The proposed approach aligns with the demands of contemporary CMOS backend‐of‐line processes, facilitating the seamless incorporation of graphene in advanced chip technologies.

Silicon oxide particles size evolution during CVD graphene growth on Cu substrates

Chemical vapor deposition (CVD) on metal substrates is presently the remarkably route for the synthesis of high-quality continuous graphene films at a large scale. Whereas, during the thermal processing procedure in the quartz tube, the formation of silicon oxide particles would degrade the performance of graphene. To better understand the origin of these particles , a series of CVD graphene growth experiments with were conducted on the Cu substrates. In this paper, we investigated the main component and morphology of the particles by X-Ray photoelectron spectroscopy (XPS), energy dispersive spectrometry (EDS) measurement in scanning electron microscopy (SEM) and revealed that the silicon oxide particles gradually increased in diameter across the graphene films on the Cu surface, which was found to be correlated with the working time of the quartz products. Raman spectroscopy was also applied to prove the drawback consequence of the contamination on the quality of the CVD graphene film. To mitigate this issue, we further propose a modification of the CVD system that a coaxial alumina or BN tube inside the quartz tube to screen the contaminants bringing about improved graphene growth.

Graphene films grown by chemical vapor deposition and their applications

In this article we provide the results of the synthesis of graphene films and discuss their potential applications in electronic structures. Graphene films were synthesized on copper foil using the CVD method at 1050 °C. During the initial stage of synthesis, graphene domains with hexagonal shapes and an average size of 10 μm were formed. The orientation and size of the graphene domains are based on the synthesis parameters and the copper foil. As the synthesis time increases, domain cross-linking occurs, resulting in polycrystalline continuous graphene film formation. Graphene films have areas up to 100 cm2 and thicknesses ~ 1 nm to 5 nm. To measure the Raman spectra, graphene films were transferred to SiO2/Si substrates. Graphene films exhibit an intense 2D peak that significantly exceeds the G peak of crystalline graphite. Flexible transparent conductive touch panels were created on the basis of the grown graphene films. A lamination method has been used to create graphene films that can be transferred from copper foil to polymer substrates. A laboratory touch screen with a graphene film acting as a capacitive touch sensor was constructed on the basis of the transferred film, and transparent electrodes for molybdenum disulfide-based photosensitive elements were also created. Resistive humidity sensors based on graphene films were developed and transferred to SiO2/Si and epoxy/Si substrates. The graphene humidity sensor has a low response, high temperature stability, and is highly reliable.

Direct Graphene Deposition via a Modified Laser-Assisted Method for Interdigitated Microflexible Supercapacitors.

The transcendence toward smarter technologies and the rapid expansion of the Internet of Things requires miniaturized energy storage systems, which may also be shape-conformable, such as microflexible supercapacitors. Their fabrication must be compatible with emerging manufacturing platforms with regard to scalability and sustainability. Here, we modify a laser-based method we recently developed for simultaneously synthesizing and transferring graphene onto a selected substrate. The modification of the method lies in the tuning of two key parameters, namely, the inclination of the laser beam and the distance between the precursor material and the acceptor substrate. A proper combination of these parameters enables the displacement of the trace of the transmitted laser beam from the deposited graphene film area. This mitigates the negative effects that arise from the laser-induced ablation of graphene on heat-sensitive substrates and significantly improves the electrical conductivity of the graphene films. The optimized graphene exhibits very high C/O (36) and sp2/sp3 (13) ratios. Post-transport irradiation was used to transform the continuous graphene films to interdigitated electrodes. The capacitance of the microflexible supercapacitor was measured to be among the highest reported ones in relation to interdigitated supercapacitors with electrodes based on laser-grown graphene. The device shows good cycling stability, retaining 91% of its capacitance after 10,000 cycles, showing no substantial degradation after applying bending conditions. This promising laser-based approach emerges as a viable alternative for the fabrication of microflexible interdigitated supercapacitors for paper electronics and smart textiles.

Super Graphene-Skinned Materials: An Innovative Strategy toward Graphene Applications.

Super graphene-skinned materials are emerging members of the graphene composite materials family, which are produced through the high-temperature chemical deposition of continuous graphene layers on traditional engineering materials. The high-performance graphene "skin" endows the traditional engineering materials with additional functionalities, and atomically thin graphene films enter the market by hitching a ride on traditional material carriers. Beyond the physical coating of graphene powders onto engineering materials, the directly grown continuous graphene skin keeps its excellent intrinsic properties to a great extent and holds promise for future applications. Super graphene-skinned material is an innovative pathway for applications of continuous graphene films, which avoids the challenging peeling-transfer process and solves the non-self-supporting issue of ultrathin graphene film. It is a big family, including graphene-skinned powders, fibers, foils, and foams. With further processing and molding, we can obtain graphene-dispersed bulk materials, especially for metal-based graphene-skinned materials, which provides a creative pathway for uniformly dispersing graphene into a metal matrix. In practical applications, graphene-skinned materials would exhibit excellent performance with perfect processing compatibility with current engineering materials and be pushed to real industrial applications relying on the broad market of engineering materials.

Influence of morphological characteristics of graphene on its field emission properties

Graphene is one of the most potential field emission cathode materials due to its excellent electrical, thermal, and mechanical properties, as well as rich edge structures. In this paper, we study the growth parameters of graphene prepared by chemical vapor deposition, and prepare three kinds of morphologies of graphene: single-layer graphene, graphene islands, and graphene with buffer layers, and then we explore the influence of the morphological characteristics of graphene on its field emission properties, and analyze the mechanism of influence of the morphological characteristics of graphene on its field emission properties through COMSOL. Comparing with single-layer graphene, the turn-on field of graphene islands and that of graphene with buffer layers decrease to 5.55 V/μm and 5.85 V/μm, respectively. The current densities also increase to 40.3 μA/cm<sup>2</sup> and 26.4 μA/cm<sup>2</sup>, respectively. On the other hand, the field emission currents of single-layer graphene and graphene with buffer layers are more stable. In a 5-hour test, the current densities only decrease by 2% and 4%, respectively. COMSOL simulation shows that the morphological characteristics of graphene have significant influences on the electric field distribution characteristics and heat dissipation capacity. Graphene islands and graphene with buffer layers have exposed edges, leading to local electric field concentration, and thus improving field emission properties. The graphene islands are distributed discretely on the substrate, forming no continuous graphene film and lacking transverse heat dissipation channels, so the accumulation of heat will cause damage to the graphene emitter, and affect the stability of its field emission current. This study will be of great benefit to the understanding of the influence of the morphological characteristics of graphene on its field emission properties, and improving the field emission properties of graphene materials.

Low T direct plasma assisted growth of graphene on sapphire and its integration in graphene/MoS2 heterostructure-based photodetectors

We report on outstanding photo-responsivity, R > 103 A/W, fast response (~0.1 s), and broadband sensitivity ranging from the UV to the NIR in two terminal graphene/MoS2 photodetectors. Our devices are based on the deterministic transfer of MoS2 on top of directly grown graphene on sapphire, and their performance outperforms previous similar photodetectors using large-scale grown graphene. Here we devise a protocol for the direct growth of transparent (transmittance, Tr > 90%), highly conductive (sheet resistance, R□ < 1 kΩ) uniform and continuous graphene films on sapphire at 700 °C by using plasma-assisted chemical vapor deposition (CVD) with C2H2/H2 gas mixtures. Our study demonstrates the successful use of plasma-assisted low-temperature CVD techniques to directly grow graphene on insulators for optoelectronic applications.

Asymmetric Growth of Monolayer Graphene on Cu Foil by Space‐Confined CVD

AbstractChemical vapor deposition (CVD) is the most efficient method for growing graphene while its performance heavily relies on the growth environment. In this work, we examine the growth of continuous monolayer graphene films using CVD in an asymmetric environment by placing the Cu foil substrate on a specially designed semicircular quartz boat with grooves. We find that the asymmetry plays a key role in the growth of graphene and the quality of graphene on the bottom surface of Cu foil is higher than that on the top surface. This difference is attributed to different oxygen and carbon source concentration on the two surfaces of Cu foil, which leads to different nucleation density, grain size, and growth rate. COMSOL simulation confirms this hypothesis and reveals that the lower gas flow velocity on the bottom is responsible for the higher oxygen content and less carbon source supply, promoting higher‐quality graphene. Based on this asymmetric mechanism, we achieve stable and efficient batch production of high‐quality graphene using a special substrate support. Our method has guiding significance for the stable and batch growth of high‐quality graphene or other two dimensional (2D) materials in spatial design strategies.

Increasing coverage of mono-layer graphene grown on hexagonal boron nitride

Graphene sitting on hexagonal boron nitride (h-BN) always exhibits excellent electrical properties. And the properties of graphene on h-BN are often dominated by its domain size and boundaries. Chemical vapor deposition (CVD) is a promising approach to achieve large size graphene crystal. However, the CVD growth of graphene on h-BN still faces challenges in increasing coverage of monolayer graphene because of a weak control on nucleation and vertical growth. Here, an auxiliary source strategy is adapted to increase the nucleation density of graphene on h-BN and synthesis continuous graphene films. It is found that both silicon carbide and organic polymer e.g. methyl methacrylate can assist the nucleation of graphene, and then increases the coverage of graphene on h-BN. By optimizing the growth temperature, vertical accumulation of graphitic materials can be greatly suppressed. This work provides an effective approach for preparing continuous graphene film on h-BN, and may bring a new sight for the growth of high quality graphene.

Cobalt-Activated Transfer-Free Synthesis of the Graphene on Si(100) by Anode Layer Ion Source

In this research, the graphene was grown directly on the Si(100) surface at 600 °C temperature using an anode layer ion source. The sacrificial catalytic cobalt interlayer assisted hydrocarbon ion beam synthesis was applied. Overall, two synthesis process modifications with a single-step graphene growth at elevated temperature and two-step synthesis, including graphite-like carbon growth on a catalytic Co film and subsequent annealing at elevated temperature, were applied. The growth of the graphene was confirmed by Raman scattering spectroscopy and X-ray photoelectron spectroscopy. The atomic force microscopy and scanning electron microscopy were used to study samples’ surface morphology. The temperature, hydrocarbon ion beam energy, and catalytic Co film thickness effects on the structure and thickness of the graphene were investigated. The graphene growth on Si(100) by two-step synthesis was beneficial due to the continuous and homogeneous graphene film formation. The observed results were explained by peculiarities of the thermally, ion beam, and catalytic metal activated hydrocarbon species dissociation. The changes of the cobalt grain size, Co film roughness, and dewetting were taken into account.

Molten Ga-Pd alloy catalyzed interfacial growth of graphene on dielectric substrates

Direct growth of graphene on insulating substrate is highly desired for practical applications in two-dimensional electronics and optoelectronics. However, the controllable synthesis of large scale graphene films on dielectrics is still limited in thickness and crystallinity by the absence of catalyst. Here we develop a transfer-free method to synthesize continuous graphene films on various insulating substrates by using molten Ga-Pd alloy and silicon carbide (SiC) as catalyst and carbon source. The catalytic Ga-Pd alloy induces the decomposition of SiC bonds and promotes the formation of C-C sp2 at relatively low temperature around 1000 °C. The suitable carbon solubility of Ga-Pd alloy could also promote the fabrication of few-layer graphene films on the interface between alloy and SiC substrate as well as the upper surface of alloy. Furthermore, this strategy could be extended to the interfacial growth of graphene on other dielectrics by simply placing the substrate face down to the surface of molten alloy. The progress of this work may help to understand the mechanism of molten Ga-Pd alloy catalyzed interfacial growth of transfer-free graphene and benefit the application of graphene-based electronics and photonics in the future.

The role of etching on growth of adlayer graphene by chemical vapor deposition

Understanding the growth mechanism of adlayer graphene is key to preparing a large-area uniform and continuous bilayer or multilayer graphene film, which is essential for electronic and photonic device applications. We synthesized high-quality single-layer graphene (SLG) film with numerous adlayer grains on liquid copper by controllable chemical vapor deposition (CVD). By adjusting parameters related to hydrogen and methane flow, we confirmed a competitive relationship between the etching and the growth of the adlayer, and they can reach dynamic equilibrium during the CVD process. Through the reasonable design of growth time, more details about adlayer growth were revealed. The etched defect is used as the center of carbon injection to grow adlayer under the first layer. For the first time, this research proposes and proves the growth mechanism of etching controlled adlayer growth, which opens a new door for the controllable synthesis of bilayer or multilayer graphene.

Batch production of uniform graphene films via controlling gas-phase dynamics in confined space

Batch production of continuous and uniform graphene films is critical for the application of graphene. Chemical vapor deposition (CVD) has shown great promise for mass producing high-quality graphene films. However, the critical factors affected the uniformity of graphene films during the batch production need to be further studied. Herein, we propose a method for batch production of uniform graphene films by controlling the gaseous carbon source to be uniformly distributed near the substrate surface. By designing the growth space of graphene into a rectangular channel structure, we adjusted the velocity of feedstock gas flow to be uniformly distributed in the channel, which is critical for uniform graphene growth. The monolayer graphene film grown inside the rectangular channel structure shows high uniformity with average sheet resistance of 345 Ω sq−1 without doping. The experimental and simulation results show that the placement of the substrates during batch growth of graphene films will greatly affect the distribution of gas-phase dynamics near the substrate surface and the growth process of graphene. Uniform graphene films with large-scale can be prepared in batches by adjusting the distribution of gas-phase dynamics.

Scrolled Production of Large-Scale Continuous Graphene on Copper Foils*Supported by the Beijing Natural Science Foundation (Grant No. JQ19004), the Key R&D Program of Guangdong Province (Grant Nos. 2019B010931001, 2020B010189001, 2018B010109009 and 2018B030327001), Bureau of Industry and Information Technology of Shenzhen (Graphene platform 201901161512), the National Natural Science Foundation of China (Grant Nos. 51991340, 51991342 and 51522201), the National Key R&D

We report an efficient and economical way for mass production of large-scale graphene films with high quality and uniformity. By using the designed scrolled copper-graphite structure, a continuous graphene film with typical area of 200 × 39 cm2 could be obtained in 15 min, and the production rate of the graphene film and space utilization rate of the CVD reactor can reach 520 cm2⋅min−1 and 0.38 cm−1⋅min−1, respectively. Our method provides a guidance for the industrial production of graphene films, and may also accelerate its large-scale applications.

Fast Growth of Continuous Single-Crystal Graphene Film on Copper by Low-Pressure Chemical Vapor Deposition

In this study, continuous and single-crystal graphene films on copper (Cu) are synthesized by low-pressure chemical vapor deposition in a growth time of 40 min. Meticulous modulation of the oxygen and carbon supply in multistage synthesis processing and dependence of graphene properties on the oxygen and carbon supply has been investigated. It is found that double-oxygen passivation together with the optimal methane (CH4):hydrogen (H2) ratio leads to a balance between nucleation and graphene growth rate to accomplish continuous single-crystal bilayer graphene films in a short time. Moreover, the graphene-based field effect transistor reveals the superior hole mobility of 5565 cm2 V−1 s−1. Our study provides a simple chemical vapor deposition procedure to achieve high quality graphene film on Cu in a short time.

Fluid-Assisted Sorted Assembly of Graphene on Polymer.

The significant size distribution of as-synthesized nanomaterials presents a challenge for reproducable and reliable applications. In this paper, we report a fluidic-assisted sorted assembly method in which nanomaterial sorting and enhanced assembly can be achieved simultaneously. As a proof of concept, a two-dimensional (2D) graphene flake, with a large size variation, was chosen as the target nanomaterial system. This study synergizes a novel fluidic assembly design, suspending a rotating disk over a polydimethylsiloxane (PDMS) substrate, and a computational fluid dynamics (CFD) model using Ansys CFX to disclose the mechanism of sorted assembly. By controlling the rotating speed and the gap between the disk and the substrate, the flow field is altered. In contrast to centrifugal sorting, where larger particles move outward, in this study, the size of assembled graphene flake (average lateral size, Xc) reduces significantly from the center (Xc = 3 μm) to the edge of the disk (Xc = 2 μm). The particle sorting process is dictated by the fluid shear-stress, with higher shear-stress leading to smaller particles, while the assembly process is mainly dominated by the pressure field with higher pressure magnitude leading to better assembly. Near the edge of the disk, enhanced particle sorting is coupled with an enhanced assembly where a continuous graphene film with smaller Xc can be formed. To prove the potential application of this method, an ultrasensitive strain sensor with one of the lowest detection limits, 0.02%, is demonstrated. This research presents a novel route toward large-scale and cost-effective manufacturing of nanomaterial-based flexible electronics.

Etching-controlled preparation of large-area fractal graphene by low-pressure CVD on polycrystalline Cu substrate

Fractal graphene can provide more active sites for electrocatalytic reactions due to its unique morphology. The preparation of large-area fractal graphene and the understanding of its morphology evolution are crucial to the improvement of catalytic performance. Chemical vapour deposition (CVD) technology is a unique method to obtain high quality fractal graphene. In this study, low-pressure CVD method was used to prepare fractal graphene on Cu substrate. Through the etching effect of hydrogen in the cooling process, the evolution process of graphene from compact to dentritic was realized and the fractal dimensions of graphene with different morphologies were calculated. It was found that the hydrogen etching reaction of graphene begins at the edges and moves towards the nucleation point gradually until the etching progress is completed. And continuous large-area fractal graphene films were obtained for the first time, which would find potential application in electrocatalysis and other fields.

Controlled doping of graphene by impurity charge compensation via a polarized ferroelectric polymer

A simple technique of doping graphene by manipulating adsorbed impurity charges is presented. Using a field effect transistor configuration, controlled polarization of a ferroelectric polymer gate is used to compensate and neutralize charges of one type. The uncompensated charges of the opposite type then dope graphene. Both n- and p-type doping are possible by this method, which is non-destructive and reversible. We observe a change in n-type dopant concentration of 8 × 1012 cm−2 and a change in electron mobility of 650%. The electron and hole mobilities are inversely proportional to the impurity concentration, as predicted by theory. Selective doping of graphene can be achieved using this method by patterning gate electrodes at strategic locations and programming them independently. Such charge control without introducing hard junctions, therefore, permits seamless integration of multiple devices on a continuous graphene film.

Characterization of graphene grown by direct-liquid-injection chemical vapor deposition with cyclohexane precursor in N2 ambient

We synthesize graphene films by direct-liquid-injection chemical vapor deposition (DLI-CVD) method with cyclohexane precursor (C6H12) in N2 ambient. This process offers a safer, and more economical route for large-scale graphene production in which hydrogen gas is not required. Graphene films are grown on Cu foil substrates at 890 to 980 °C for 10 min in the total pressure of 2 mbar. The flow rate of cyclohexane is varied between 0.2 and 0.5 g/min. The Raman results shows continuous monolayer graphene films at growth temperature of 950 °C and a flow rate of 0.5 g/min. Hall and field-effect measurements show mobilities in the range of 450–800 cm2/V·s. The relatively low D peak intensity suggests that carrier mobility is likely limited by impurities introduced to the devices during transfer process and device fabrication.

High-quality nitrogen-doped graphene films synthesized from pyridine via two-step chemical vapor deposition

Modulation of the electrical properties of graphene is of significant importance in advancing graphene electronics: it can be achieved by a Fermi level shift induced by electron acceptor/donor doping. Suitable doping methods involving low-temperature processes and offering long-term stability are imperative to practical applications for such materials. Here, we demonstrate a two-step chemical vapor deposition (CVD) technique for direct synthesis of N-doped graphene film from a pyridine feed-stock at 300 °C under ambient pressure. We extended the synthesis—classified into nucleation and lateral growth steps—by controlling the carbon partial pressure in the processing gases. This led to large-area, continuous N-doped graphene films of excellent quality with full surface coverage: for example, a film size of 2 in2, optical transmittance of 97.6%, and electron mobility of 1400 cm2 V−1 s−1. Our modified CVD method is expected to facilitate the direct synthesis of N-doped graphene in device manufacturing processes toward practical applications while keeping the underlying devices intact.