The growth direction of the GR (rose arrow marked in (c)) was strictly along the graphene zigzag direction, as shown in the SAED pattern. The composition of the particle (as determined from EDS elemental analysis) also shows no change.
Background
- Introduction of carbon materials
- Dissolution of diamond in metal films
- Synthesis of graphene ribbon
- Vapor-liquid-solid (VLS) growth of ribbons
- Kinetics study
We have separately studied the particle-mediated growth rates of graphene ribbons on copper foil and described this in detail in Chapter 4. We studied the growth of graphene ribbons mediated by silica particles, VLS: growth rates at different temperatures and thus obtained the corresponding values for the activation enthalpy of longitudinal growth and the activation enthalpy for lateral growth of such graphene ribbons.
Experimental techniques
Characterization techniques
- Optical microscopy (OM)
- Scanning electron microscopy (SEM)
- Transmission electron microscopy (TEM)
- Atomic force microscopy (AFM)
- Raman spectroscopy
- Time-of-flight secondary ion mass spectrometry (ToF-SIMS)
- Residual gas analyzer (RGA)
- X-ray diffraction (XRD) and in situ XRD
We used AFM (Bruker Dimension Icon) to study the morphology of graphene ribbons on Cu foils. We used Powder XRD (Rigaku SmartLab) to analyze the crystalline structure of the Ni and Co films on the single crystal diamond samples and of the Cu foil used in our graphene ribbon studies.
Experiment and sample preparation
- Temperature calibration in quartz tube furnace and cold wall system
- Diamond dissolution
- Graphene ribbon growth
In more detail, a 2 inch diameter quartz tube CVD furnace was used, equipped with a water bubbler placed upstream and a gas switch used to control the water vapor supply (Figure 2.7a). A rapid Joule heating system with an 8 cm diameter quartz window on top of a stainless steel water jacket was used for rapid heating and cooling experiments to allow visual observation and video recording of the sample during heating and cooling (Figure 2.7). b); The system was also equipped with a water bubbler that was in a constant temperature water bath.
Introduction
Results and discussion
- Quartz tube furnace experiments
- Cold wall system (RSR-M) experiments
- XRD characterization
- Theoretical modeling of reaction pathways and potential energy barriers
The configuration of the Ni diamond sample and the direction of C diffusion are shown schematically in Figure 3.2b. The concentration gradient of C in the Ni or Co film established by the continuous removal of C at the surface drives the continuous dissolution of the diamond at the M/D(100) interface as shown schematically in Figure 3.2e . The process of dissolution of diamond in Ni or Co films in the presence of water vapor involves 3 primary steps: (i) C-C bond breaking, followed by the diffusion of C atoms in Ni or Co films to the metal /diamond interface, (ii) ) C diffusion through the 500-nm thick metal film, and (iii) the reaction of C atoms with a thin layer of metal oxide on the metal surface to form CO(g), as shown schematically in Figure 3.4f shown.
These images show that on the surface of the diamond, a small amount of carbon was dissolved in the not fully oxidized Ni film at the beginning of the exposure to water vapor. In short, too high a concentration of water vapor quickly turns the Ni film into an oxidized one, which completely inhibits the dissolution of diamond. The potential energy barriers of (i) the oxidation reaction at exposed metal surfaces and (ii) carbon "dissolving" at the metal-diamond interfaces were calculated at ~1273 K and are shown in Fig. 3.23 (see details in the Experimental section).
The potential energy curves describing the formation of the –Ni-C-O configuration on the Ni(100) surface and the decomposition to release CO into the gas phase are given in Figure 3.25. For the M-D(110) interface, in the diamond dissolution process, the permeability of C through the metal film starts with the dissociation of the C-C bond at the metal-diamond interface.
Conclusion
On the other hand, under ideal conditions, based on the highest dissolution rates of Ni and separate Co, a C atom will typically jump from one octahedral site to another, to allow the C atom that came from the D/M- interface, to cross through the film and thus be eliminated as CO and/or CO2 at the top of the film, are 1.5E-6 s and 4.9E-7 s, respectively, for Ni, Co films (we only treated the unoccupied octahedral site case). Without water vapor, thin graphite films form on the metal surface and also at the metal/diamond interface during the heat treatment at temperatures around 1000°C. In regime I (lower partial pressure of H2O(g)), the rate-limiting step was the removal of C, which diffused through the metal film to the free surface, and in regime II (higher partial pressure of H2O(g)), the rate-limiting step was dissolution of the diamond by the diamond/metal interface.
A concentration gradient of C in the metal films was found by ToF-SIMS depth profiling with a high concentration at the metal-diamond interface and a low concentration at the free surface. The experimental and theoretical (DFT calculated) values for the activation enthalpies of the relevant reactions at the metal surface and at the metal–diamond interface and for the diffusion of C through each metal are in relatively close agreement. Knowing the kinetics and thus also being able to model the dissolution rates can open up new opportunities to "preshape" single crystal diamond in ways that can replace or augment other methods such as RIE, casting, and laser patterning to achieve low-cost, high-efficiency microfabrication and in a very controllable way, including for new 3D structures such as quantum devices,38 MEMS,39 and current devices40 and without "plasma damage" that occurs with RIE methods.41 This study also shows the possibility of tuning the resolution of diamond via rational control of the thermochemical parameters at the free surface and the metal/diamond interface.
Experimental section
- Materials and chemicals
- Detailed methods
This work is a comprehensive study of the water vapor-induced dissolution kinetics of single 100 and 110 diamond in nickel and cobalt films. Five 500 nm thick nickel-coated diamond plates were placed inside the 1/2-inch quartz tube upstream of the hot zone, which was heated to 1050 ℃ (furnace temperature) for 1 hour and held in this temperature during the experiment. To obtain the background signal, we tested CVD-RGA experiments without diamond samples in the hot zone of the inner 1/2 inch tube.
Spectropyrometer measurement in the cold wall system: To determine the emissivity of the nickel-coated diamond sample, we performed spectropyrometer measurements. In Table 3.11, the supercell size, surface Miller indices, plate thickness, interlayer spacing, and K-point sampling of the simulated structures are given. 22] Beck, P.; Pommerol, A.; Schmitt, B.; Brissaud, O., Kinetics of water sorption on minerals and respiration of Martian regolith.
Vapor-liquid-solid (VLS) growth of graphene ribbons on Cu foil and its kinetics
Introduction
If the catalyst particle is not molten at the growth temperature, the growth is commonly referred to as vapor-solid-solid (VSS) growth. In VLS growth, selecting an appropriate catalytic particle and controlling the growth conditions can in some cases allow varying the morphology of the nanoobjects, including their size and shape.13. We grew GRs from silica particles dispersed on a Cu foil which formed a relatively stable Cu-Si-O alloy under the growth conditions (Si nanoparticles, as well as Ni, NiO and Cu5Si nanoparticles, were also tried, but any type was "consumed" by the Cu substrate and did not grow catalytic GRs).
The crystalline quality of the ribbons was comparable to that of the graphene islands nucleating and growing on the same single-crystal Cu surface (without the participation of silica particles) under the same conditions. Changing the growth temperature changed the ribbon morphology from a tapered shape (grown at >900 ℃) to rectangular (grown at <900). Finally, we measured the electrical properties of the mature GRs by fabricating GR back-gated field-effect transistors (“GR-FETs”).
Results and discussion
- Morphology and characterization of graphene ribbons
- Growth behavior of graphene ribbons
- Growth of graphene ribbon and its relationship with the Cu foil substrate
- Kinetics analyses of graphene ribbon growth
Si, O, Cu, and C were detected in the particle, indicating that the chemical composition of the particles changed significantly during GR growth. At the end of the GR we observed a higher silicon signal, as shown in Figure 4.11d. We observed a higher silicon signal in the depth profile at the tip of the GR, which suggested that the particles diffused into the Cu foil, which (ultimately) stopped further growth of the GR (Figure 4.11).
One possible reason for this is that the particle was moving nearby. We conclude that GR nucleated at the initial position of the particle, and then the longitudinal growth was governed by the movement of the particle on the substrate. Two parallel processes (growth of VLS at the tip and growth of VS at the sides) therefore determine the morphology (tapering angle) of the strip.
We therefore determined the growth rates of graphene islands and the average growth rates at the different temperatures were the same as for the VS growth part of the ribbons. This suggests that the carbon that builds the GR through VLS diffuses at the surface of the particle.
Conclusion
Two growth models have been used to explain particle-mediated growth with VLS.34 In one, the particle is first saturated with carbon (or another element) in its bulk (ie, the entire interior), which then precipitates. In the second growth mode, the carbon precursor diffuses only on the surface of the particle, which then acts as a carbon source for particle-mediated growth. Note that the growth rate is either particle size dependent (total diffusion) or particle size independent (surface diffusion). ) is proportional to the volume (r3) of the particle (2r is the effective diameter of the particle).
When carbon diffuses only or mainly at the surface of the particle, the diffusion rate (Rd) is proportional to the effective area (r2) of particle, and the deposition rate (Rp) of carbon is also proportional to the effective area of particle (r2) and consequently the particle-mediated growth rate is independent of the particle size. The growth rate was found to be independent of the particle size, showing that a surface diffusion particle-mediated growth model and not bulk diffusion is correct. By controlling the growth temperature, we were able to change the morphology of GRs from needle-shaped to rectangular - and to achieve fine control of the "taper" angle.
Experimental section
- Materials and chemicals
- Detailed methods
We conclude that our method provides the possibility to grow GR (sub-10 nm) on Cu(111) foil, which, if achieved, can open the band gap due to the quantum confinement effect.1,38 As the VLS (VSS) growth mechanism enables the growth of CNTs -jev39 or other nanoribbons14,15,40 on dielectric substrates, it is possible that our method will enable growth on dielectric substrates; further study of this is indicated. Then, overall mean values were obtained using the “mean ± SD” function in Microsoft Excel and plotting the figure using Origin software. On the other hand, the micrometer graphene ribbon was synthesized by a vapor-liquid-solid growth mechanism, and the taper angle could be mainly controlled by the growth temperature.
But since the width of the graphene band is larger than 200 nm, we cannot open the band gap on it. Moreover, since the growth of graphene ribbons was catalyzed by the silica particle, we can potentially grow graphene ribbons on dielectric substrate with silica particle assistance. The future work we plan to do is to grow narrower graphene ribbons to open the band gap or to grow graphene ribbons on dielectric substrate.
