Single-crystal Monolayer Graphene Research Articles

Liquid Cu–Zn catalyzed growth of graphene single-crystals

The controllable synthesis of millimeter-sized single-crystal monolayer graphene on a liquid Cu–Zn alloy by suppressing nucleation is reported.

Wafer-scale single-crystal monolayer graphene grown on sapphire substrate.

The growth of inch-scale high-quality graphene on insulating substrates is desirable for electronic and optoelectronic applications, but remains challenging due to the lack of metal catalysis. Here we demonstrate the wafer-scale synthesis of adlayer-free ultra-flat single-crystal monolayer graphene on sapphire substrates. We converted polycrystalline Cu foil placed on Al2O3(0001) into single-crystal Cu(111) film via annealing, and then achieved epitaxial growth of graphene at the interface between Cu(111) and Al2O3(0001) by multi-cycle plasma etching-assisted-chemical vapour deposition. Immersion in liquid nitrogen followed by rapid heating causes the Cu(111) film to bulge and peel off easily, while the graphene film remains on the sapphire substrate without degradation. Field-effect transistors fabricated on as-grown graphene exhibited good electronic transport properties with high carrier mobilities. This work breaks a bottleneck of synthesizing wafer-scale single-crystal monolayer graphene on insulating substrates and could contribute to next-generation graphene-based nanodevices.

Synthesis of Single-Crystal Graphene on Copper Foils Using a Low-Nucleation-Density Region in a Quartz Boat.

The nucleation of graphene at different locations in the quartz boat was studied, and the lowest nucleation density of graphene in the quartz boat was found. The nucleation density of graphene is the lowest at the bottom of the quartz boat near the gas inlet side. Based on the above results, a simple and reproducible way is proposed to significantly suppress the nucleation density of graphene on the copper foil during the chemical vapor deposition process. Placing the copper foil with an area of 1.3 cm × 1 cm in the middle of the bottom of the quartz boat or further back, and placing two copper pockets in front of the copper foil, an ultra-low nucleation density of ~42 nucleus/cm2 was achieved on the back of the copper foil. Single-crystal monolayer graphene with a lateral size of 800 μm can be grown on the back of copper foils after 60 min of growth. Raman spectroscopy revealed the single-crystal graphene to be in uniform monolayers with a low D-band intensity.

Single-crystal, large-area, fold-free monolayer graphene.

Chemical vapour deposition of carbon-containing precursors on metal substrates is currently the most promising route for the scalable synthesis of large-area, high-quality graphene films1. However, there are usually some imperfections present in the resulting films: grain boundaries, regions with additional layers (adlayers), and wrinkles or folds, all of which can degrade the performance of graphene in various applications2-7. There have been numerous studies on ways to eliminate grain boundaries8,9 and adlayers10-12, but graphene folds have been less investigated. Here we explore the wrinkling/folding process for graphene films grown from an ethylene precursor on single-crystal Cu-Ni(111) foils. We identify a critical growth temperature (1,030kelvin) above which folds will naturally form during the subsequent cooling process. Specifically, the compressive stress that builds up owing to thermal contraction during cooling is released by the abrupt onset of step bunching in the foil at about 1,030kelvin, triggering the formation of graphene folds perpendicular to the step edge direction. By restricting the initial growth temperature to between 1,000kelvin and 1,030kelvin, we can produce large areas of single-crystal monolayer graphene films that are high-quality and fold-free. The resulting films show highly uniform transport properties: field-effect transistors prepared from these films exhibit average room-temperature carrier mobilities of around (7.0±1.0)×103 centimetres squared per volt per second for both holes and electrons. The process is also scalable, permitting simultaneous growth of graphene of the same quality on multiple foils stacked in parallel. After electrochemical transfer of the graphene films from the foils, the foils themselves can be reused essentially indefinitely for further graphene growth.

CVD graphene-based flexible and transparent SERS substrate towards L-tyrosine detection

The simple, rapid, reproducible, and sensitive detection of trace amounts of molecules is of great interest in terms of early diagnosis of diseases and food safety in daily-life applications. In this study, low-pressure chemical vapor deposited (LP-CVD) graphene(G)-based flexible and transparent AgNPs/CVDG@AryLite™ nanostructured surface enhanced Raman scattering (SERS) substrate has been demonstrated towards the detection of L-Tyrosine (L-Tyr) amino acid molecules. The synthesize of high-quality, single-crystal monolayer graphene films was achieved with considerably low (62.81 Ω sq.−1) sheet resistance and 94% transmittance at 550 nm wavelength. AgNPs are uniformly formed with a size distribution of about 60 nm on the CVDG surface. The fabricated substrates have been investigated by Rhodamine 6G (R6G) as probe molecule and L-Tyr amino acid molecules with different concentrations to understand the SERS properties. Significant enhancement of R6G and L-Tyr on the fabricated AgNPs/CVDG@AryLite™ SERS substrates obtained even if the 10−8 M R6G and L-Tyr solutions are used. The enhancement factors (EFs) are calculated as 109 and 7.5 × 109 for R6G and L-Tyr, respectively. The results in this research show that the fabricated nanostructured SERS substrates might open up a new path to design flexible and transparent optoelectronic devices towards detection of trace amount of molecules particularly portable spectroscopic instruments.

Adlayer-free large-area single-crystal CVD graphene growth on copper

Chemical vapor deposition (CVD) graphene growth on copper foil has the potential to produce high-quality graphene at the industrial scale to realize its applications in future electronics and optoelectronic devices. During the graphene growth process, various parameters such as catalytic substrate, temperature, flow rate of precursor gases (methane and hydrogen) and presence of oxygen on the Cu substrate, directly affect graphene grain size, thickness and crystallinity. Researchers have been able to achieve large-area single-crystal monolayer graphene on Cu substrate. However, small fractions of bilayer or multilayer (adlayer) regions also appear along with single-crystal monolayer graphene. In recent years, researchers have been studying the effect of various growth parameters on adlayer formation. In this review, we have discussed the role and optimization of these growth parameters to achieve adlayer-free large-area single-crystal graphene on copper foil.

Scalable and ultrafast epitaxial growth of single-crystal graphene wafers for electrically tunable liquid-crystal microlens arrays

The scalable growth of wafer-sized single-crystal graphene in an energy-efficient manner and compatible with wafer process is critical for the killer applications of graphene in high-performance electronics and optoelectronics. Here, ultrafast epitaxial growth of single-crystal graphene wafers is realized on single-crystal Cu90Ni10(1 1 1) thin films fabricated by a tailored two-step magnetron sputtering and recrystallization process. The minor nickel (Ni) content greatly enhances the catalytic activity of Cu, rendering the growth of a 4 in. single-crystal monolayer graphene wafer in 10 min on Cu90Ni10(1 1 1), 50 folds faster than graphene growth on Cu(1 1 1). Through the carbon isotope labeling experiments, graphene growth on Cu90Ni10(1 1 1) is proved to be exclusively surface-reaction dominated, which is ascribed to the Cu surface enrichment in the CuNi alloy, as indicated by element in-depth profile. One of the best benefits of our protocol is the compatibility with wafer process and excellent scalability. A pilot-scale chemical vapor deposition (CVD) system is designed and built for the mass production of single-crystal graphene wafers, with productivity of 25 pieces in one process cycle. Furthermore, we demonstrate the application of single-crystal graphene in electrically controlled liquid-crystal microlens arrays (LCMLA), which exhibit highly tunable focal lengths near 2 mm under small driving voltages. By integration of the graphene based LCMLA and a CMOS sensor, a prototype camera is proposed that is available for simultaneous light-field and light intensity imaging. The single-crystal graphene wafers could hold great promising for high-performance electronics and optoelectronics that are compatible with wafer process.

Quasi van der Waals epitaxy of copper thin film on single-crystal graphene monolayer buffer

Quasi van der Waals epitaxial growth of face-centered cubic Cu (~100 nm) thin films on single-crystal monolayer graphene is demonstrated using thermal evaporation at an elevated substrate temperature of 250 °C. The single-crystal graphene was transferred to amorphous (glass) and crystalline (quartz) SiO2 substrates for epitaxy study. Raman analysis showed that the thermal evaporation method had minimal damage to the graphene lattice during the Cu deposition. X-ray diffraction and electron backscatter diffraction analyses revealed that both Cu films are single-crystal with (1 1 1) out-of-plane orientation and in-plane Σ3 twin domains of 60° rotation. The crystallinity of the SiO2 substrates has a negligible effect on the Cu crystal orientation during the epitaxial growth, implying the strong screening effect of graphene. We also demonstrate the epitaxial growth of polycrystalline Cu on a commercial polycrystalline monolayer graphene consisting of two orientation domains offset 30° to each other. It confirms that the crystal orientation of the epitaxial Cu film follows that of graphene, i.e. the Cu film consists of two orientation domains offset 30° to each other when deposited on polycrystalline graphene. Finally, on the contrary to the report in the literature, we show that the direct current and radio frequency flip sputtering method causes significant damage to the graphene lattice during the Cu deposition process, and therefore neither is a suitable method for Cu epitaxial growth on graphene.

Chemical Stability of Graphene Coated Silver Substrates for Surface-Enhanced Raman Scattering

Surface enhanced Raman spectroscopy (SERS) is a novel method to sense molecular and lattice vibrations at a high sensitivity. Although nanostructured silver surface provides intense SERS signals, the silver surface is unstable under acidic environment and heated environment. Graphene, a single atomic carbon layer, has a prominent stability for chemical agents, and its honeycomb lattice completely prevents the penetration of small molecules. Here, we fabricated a SERS substrate by combining nanostructured silver surface and single-crystal monolayer graphene (G-SERS), and focused on its chemical stability. The G-SERS substrate showed SERS even in concentrated hydrochloric acid (35–37%) and heated air up to 400 °C, which is hardly obtainable by normal silver SERS substrates. The chemically stable G-SERS substrate posesses a practical and feasible application, and its high chemical stability provides a new type of SERS technique such as molecular detections at high temperatures or in extreme acidic conditions.

Interedge backscattering in buried split-gate-defined graphene quantum point contacts

Quantum Hall effects offer a formidable playground for the investigation of quantum transport phenomena. Edge modes can be detected, branched, and mixed by designing a suitable potential landscape in a two-dimensional conducting system subject to a strong magnetic field. In the present work, we demonstrate a buried split-gate architecture and use it to control electron conduction in large-scale single-crystal monolayer graphene grown by chemical vapor deposition. The control of the edge trajectories is demonstrated by the observation of various fractional quantum resistances, as a result of a controllable inter-edge scattering. Experimental data are successfully modeled both numerically and within the Landauer-Buettiker formalism. Our architecture is particularly promising and unique in view of the investigation of quantum transport via scanning probe microscopy, since graphene constitutes the topmost layer of the device. For this reason, it can be approached and perturbed by a scanning probe down to the limit of mechanical contact.

(Invited) Graphene Synthesis and Processing on Ge Substrates

Graphene is considered as a material which can enable new functionalities and performance improvements in a large variety of applications, among them in microelectronics [1, 2]. In microelectronics, techniques required for commercial large scale fabrication of graphene devices are not yet in place and further progress towards wafer-scale processing is required. Development of a wafer-scale Si technology-compatible graphene synthesis method and a toolbox of processes dedicated to handling, cleaning, patterning as well as integration of graphene with semiconductors, insulators, and metals is viewed as a prerequisite to practical applications of this material in electronic and photonic devices [3]. Semiconducting Ge surfaces appeared recently as attractive substrates for chemical vapor deposition (CVD) of graphene. There are several advantages of using them instead of metals. Firstly, the risk of metallic contaminations [4] is eliminated, ensuring the front-end-of-line compatibility in the device manufacturing process. Secondly, the thermal expansion coefficient mismatch between the Ge substrate and graphene is significantly lower than in the Cu-graphene system. This provides an opportunity to reduce the density of wrinkles which originate from thermal stress release and cause scattering of charge carriers in graphene. Furthermore, as a result of very low carbon solubility in Ge, the graphene growth on Ge can proceed, similarly to Cu substrates, in a “self-limiting” manner enabling more straightforward process control than for example on Ni. Finally, synthesis on patterned semiconducting Ge may enable direct use of graphene in some device concepts without the need of transfer which is inevitable in case of metallic substrates. In this talk, we will review the recent progress in graphene synthesis on Ge achieved by IHP and our collaborators starting from proof-of-concept molecular beam growth experiments [5], through low- [6] and high-pressure [7] CVD on Ge single crystals and Ge epi-layers on Si, to 200 mm wafer-scale CVD synthesis on Ge(100)/Si(100) virtual substrates. As an optimization approach towards improved rotational alignment of graphene on Ge [8,9] we will present results from the preparation of Ge(110)/Si(110) wafers and graphene growth on them. Beyond growth we will look at selected aspects of subsequent processing in a 200 mm Si wafer pilot line including patterning and interaction with photoresists as well as the deposition of other materials on graphene. Among the challenges ahead, re-usability of the Ge virtual substrates and wafer-scale graphene transfer procedures eliminating disadvantages of currently existing noble metal-assisted techniques [8] will be addressed. [1] A.C. Ferrari et al., “Science and technology roadmap for graphene, related two-dimensional crystals, and hybrid systems”, Nanoscale, 2015, 7, 4598-4810 [2] G. Fiori et al., “Electronics based on two-dimensional materials”, Nature Nanotechnology 2014, 9, 768–779 [3] L. Colombo et al., “Graphene growth and device integration”, IEEE Proceedings, 2013, 101, 1536-1556 [4] G. Lupina et al., “Residual metallic contamination of transferred chemical vapor deposited graphene”, ACS Nano, 2015, 9, 4776-4785 [5] G. Lippert et al., “Graphene grown on Ge(001) from atomic source”, Carbon, 2014, 75, 104-112 [6] J. Dabrowski et al., “Direct growth of low-doped graphene on Ge/Si(100) surfaces”, 2016, arXiv:1604.02315 [7] I. Pasternak et al., “Graphene growth on Ge(100)/Si(100) substrates by CVD method”, Scientific Reports, 2016, 6, 21773 [8] J.-H. Lee et al., “Wafer scale growth of single-crystal monolayer graphene on reusable hydrogen-terminated germanium”, Science, 2014, 344, 286-289 [9] B. Kiraly et al., “Electronic and mechanical properties of graphene-germanium interfaces grown by chemical vapor deposition”, Nano Lett., 2015, 15, 7414-7420

Low-temperature quantum transport in CVD-grown single crystal graphene

Chemical vapor deposition (CVD) is typically used for large-scale graphene synthesis for practical applications. However, the inferior electronic properties of CVD graphene are one of the key problems to be solved. Therefore, we present a detailed study on the electronic properties of high-quality single-crystal monolayer graphene. The graphene is grown via CVD on copper, by using a cold-wall reactor, and then transferred to Si/SiO2. Our low-temperature magneto-transport data demonstrate that the characteristics of the single-crystal CVD graphene samples are superior to those of polycrystalline graphene and have a quality which is comparable to that of exfoliated graphene on Si/SiO2. The Dirac point in our best samples occurs at back-gate voltages lower than 10 V, and a maximum mobility of 11,000 cm2/(V·s) is attained. More than 12 flat and discernible half-integer quantum Hall plateaus occur under a high magnetic field on both the electron and hole sides of the Dirac point. At a low magnetic field, the magnetoresistance exhibits a weak localization peak. Using the theory of McCann et al., we obtain inelastic scattering lengths of >1 µm, even at the charge neutrality point of the samples.

Controllable Growth of the Graphene from Millimeter-Sized Monolayer to Multilayer on Cu by Chemical Vapor Deposition.

As is well established, mastery to precise control of the layer number, stacking order of graphene, and the size of single-crystal monolayer graphene is very important for both fundamental interest and practical applications. In this report, millimeter-sized single-crystal monolayer graphene has been synthesized to multilayer graphene on Cu by chemical vapor deposition. The relationship of the growth process between monolayer graphene and multilayer graphene is investigated carefully. Besides the general multilayer graphene with Bernal stacking order, parts of multilayer graphene with non-Bernal stacking order were modulated under optimized growth conditions. The oxide nanoparticle on the Cu surface derived from annealing has been found to play the key role in nucleation. In addition, the hydrogen concentration impacts significantly on the layer number and shape of the graphene. Moreover, a possible mechanism was proposed to understand the growth process discussed above, which may provide an instruction to graphene growth on Cu by chemical vapor deposition.Electronic supplementary materialThe online version of this article (doi:10.1186/s11671-015-1164-0) contains supplementary material, which is available to authorized users.

The Role of Pre Hydrogen Flow in Nucleation of Graphene on Silicon Nitride

Graphene has been considered as one of most attractive materials in nano-scaled electronic devices. Graphene film has been mainly grown on many metal (eg., Ni , Cu, Fe) by chemical vapor deposition (CVD) process and transferred on dielectric substrate. However, the complicated transfer techniques for metal-catalyzed graphene inevitably have resulted in wrinkles, holes, and metal etching residues. Thus, there is a substantial need to develop a scalable method for reliable production of large-area graphene directly on insulating substrates. We synthesized the nucleation of graphene on silicon nitride (Si3N4) substrate and report the influence of pre hydrogen flow during CVD process on silicon nitride which is the stable dielectric and suitable for carbon diffusion barrier. Test wafer was cleaned in 10:1 HF solution and nucleation of graphene was synthesized by CVD process using CH4, Ar, H2gas at 1000°C. We evaluated the graphene nucleation with and without pre hydrogen flow on silicon nitride film as shown in Fig.1. Although amorphous carbon cluster was formed on silicon nitride without pre hydrogen flow as shown in Fig. 2(a), Raman shift peak related graphene was obtained with pre hydrogen flow condition as shown in Fig. 2(b). It is assumed that hydrogen ions are bonded with silicon atom on silicon nitride surface during pre hydrogen process and then chemisorptions of carbon precursors on the H-terminated surface are easily made. Fig.3 shows the role of hydrogen [1]. To synthesize the graphene film on silicon nitride, it is essential to be terminated with hydrogen at silicon nitride surface. Reference [1] Jae-Hyun Lee et al, “Wafer-Scale Growth of Single-Crystal Monolayer Graphene on Reusable Hydrogen-Terminated Germanium”, Science, Vol. 344, 2014 Figure 1

Scalable Growth of Single-Crystal Graphene on Hydrogen-Terminated Germanium

The uniform growth of single-crystal graphene over wafer-scale area remains a challenge in the commercial-level manufacturability of various electronic, photonic, mechanical, and other devices based on graphene. Here, I present scalable growth of wrinkle-free single-crystal monolayer graphene on a hydrogen-terminated germanium substrate. The anisotropic twofold symmetry of the Ge(110) surface allowed unidirectional alignment of multiple seeds, which were merged to uniform single-crystal graphene with pre-defined orientation. Furthermore, the weak interaction between graphene and underlying H-terminated Ge surface enabled the facile etch-free dry transfer of graphene and the recycling of the Ge substrate for continual graphene growth.

ChemInform Abstract: Chemical Vapor Deposition of Graphene Single Crystals

AbstractReview: 46 refs.

Wafer-Scale Growth of Single-Crystal Monolayer Graphene on Reusable Hydrogen-Terminated Germanium

The uniform growth of single-crystal graphene over wafer-scale areas remains a challenge in the commercial-level manufacturability of various electronic, photonic, mechanical, and other devices based on graphene. Here, we describe wafer-scale growth of wrinkle-free single-crystal monolayer graphene on silicon wafer using a hydrogen-terminated germanium buffer layer. The anisotropic twofold symmetry of the germanium (110) surface allowed unidirectional alignment of multiple seeds, which were merged to uniform single-crystal graphene with predefined orientation. Furthermore, the weak interaction between graphene and underlying hydrogen-terminated germanium surface enabled the facile etch-free dry transfer of graphene and the recycling of the germanium substrate for continual graphene growth.

Growth of Millimeter-Size Single Crystal Graphene on Cu Foils by Circumfluence Chemical Vapor Deposition

A simply and reproducible way is proposed to significantly suppress the nucleation density of graphene on the copper foil during the chemical vapor deposition process. By inserting a copper foil into a tube with one close end, the nucleation density on the copper foils can be reduced by more than five orders of magnitude and an ultra-low nucleation density of ~10 nucleus/cm2 has been achieved. The structural analyses demonstrate that single crystal monolayer graphene with a lateral size of 1.9 mm can be grown on the copper foils under the optimized growth condition. The electrical transport studies show that the mobility of such single crystal graphene is around 2400 cm2/Vs.

Chemical Vapor Deposition of Graphene Single Crystals

As a two-dimensional (2D) sp(2)-bonded carbon allotrope, graphene has attracted enormous interest over the past decade due to its unique properties, such as ultrahigh electron mobility, uniform broadband optical absorption and high tensile strength. In the initial research, graphene was isolated from natural graphite, and limited to small sizes and low yields. Recently developed chemical vapor deposition (CVD) techniques have emerged as an important method for the scalable production of large-size and high-quality graphene for various applications. However, CVD-derived graphene is polycrystalline and demonstrates degraded properties induced by grain boundaries. Thus, the next critical step of graphene growth relies on the synthesis of large graphene single crystals. In this Account, we first discuss graphene grain boundaries and their influence on graphene's properties. Mechanical and electrical behaviors of CVD-derived polycrystalline graphene are greatly reduced when compared to that of exfoliated graphene. We then review four representative pathways of pretreating Cu substrates to make millimeter-sized monolayer graphene grains: electrochemical polishing and high-pressure annealing of Cu substrate, adding of additional Cu enclosures, melting and resolidfying Cu substrates, and oxygen-rich Cu substrates. Due to these pretreatments, the nucleation site density on Cu substrates is greatly reduced, resulting in hexagonal-shaped graphene grains that show increased grain domain size and comparable electrical properties as to exfoliated graphene. Also, the properties of graphene can be engineered by its shape, thickness and spatial structure. Thus, we further discuss recently developed methods of making graphene grains with special spatial structures, including snowflakes, six-lobed flowers, pyramids and hexagonal graphene onion rings. The fundamental growth mechanism and practical applications of these well-shaped graphene structures should be interesting topics and deserves more attention in the near future. Following that, recent efforts in fabricating large single-crystal monolayer graphene on other metal substrates, including Ni, Pt, and Ru, are also described. The differences in growth conditions reveal different growth mechanisms on these metals. Another key challenge for graphene growth is to make graphene single crystals on insulating substrates, such as h-BN, SiO2, and ceramic. The recently developed plasma-enhanced CVD method can be used to directly synthesize graphene single crystals on h-BN substrates and is described in this Account as well. To summarize, recent research in synthesizing millimeter-sized monolayer graphene grains with different pretreatments, graphene grain shapes, metal catalysts, and substrates is reviewed. Although great advancements have been achieved in CVD synthesis of graphene single crystals, potential challenges still exist, such as the growth of wafer-sized graphene single crystals to further facilitate the fabrication of graphene-based devices, as well as a deeper understanding of graphene growth mechanisms and growth dynamics in order to make graphene grains with precisely controlled thicknesses and spatial structures.

Hexagonal Graphene Onion Rings

Precise spatial control of materials is the key capability of engineering their optical, electronic, and mechanical properties. However, growth of graphene on Cu was revealed to be seed-induced two-dimensional (2D) growth, limiting the synthesis of complex graphene spatial structures. In this research, we report the growth of onion ring like three-dimensional (3D) graphene structures, which are comprised of concentric one-dimensional hexagonal graphene ribbon rings grown under 2D single-crystal monolayer graphene domains. The ring formation arises from the hydrogenation-induced edge nucleation and 3D growth of a new graphene layer on the edge and under the previous one, as supported by first principles calculations. This work reveals a new graphene-nucleation mechanism and could also offer impetus for the design of new 3D spatial structures of graphene or other 2D layered materials. Additionally, in this research, two special features of this new 3D graphene structure were demonstrated, including nanoribbon fabrication and potential use in lithium storage upon scaling.