Machine Learning Determination of the Twist Angle of Bilayer Graphene by Raman Spectroscopy: Implications for van der Waals Heterostructures

With the increasing interest in twisted bilayer graphene (tBLG) of the past years, fast, reliable, and nondestructive methods to precisely determine the twist angle are required. Raman spectroscopy potentially provides such a method, given the large amount of information about the state of the graphene that is encoded in its Raman spectrum. However, changes in the Raman spectra induced by the stacking order can be very subtle, thus making the angle identification tedious. In this work, we propose the use of machine learning (ML) analysis techniques for the automated classification of the Raman spectrum of tBLG into a selected range of twist angles. The ML classification proposed here is low computationally demanding, providing fast and accurate results with a ∼99% agreement with the manual labeling of the spectra. The flexibility and noninvasive nature of the Raman measurements, paired with the predictive accuracy of the ML, is expected to facilitate the exploration of the emerging research field of twisted van der Waals heterostructures. Moreover, the present work showcases how the currently available open-source tools facilitate the study and integration of ML-based techniques

Chemistry of Water-Assisted Carbon Nanotube Growth over Fe−Mo/MgO Catalyst

Introducing a small amount of water vapor in the catalytic chemical vapor deposition (CVD) growth of single- and double-walled carbon nanotubes (SWNTs and DWNTs) extends the catalyst lifetime and increases the nanotube yield. We study the mechanism of this water-assisted nanotube growth over a Fe−Mo/MgO catalyst, based on analysis of the effluent gas, in terms of chemistry of the water-induced oxidation and its effects on catalytic activity. Water vapor was found to etch away carbon precipitate covering the metal catalyst, based primarily on the chemical reaction C + H2O → CO + H2, thus maintaining the catalytic activity. This oxidative etching was strongly dependent on the CVD temperature, and the balance between the etching and carbon precipitation was important for effective nanotube growth. With an optimized water concentration, the etching rate of the carbon precipitate was estimated to be ca. 1/1000 of the formation rate of carbon precipitate consisting mainly of SWNTs and DWNTs

Mechanical Strain of Chemically Functionalized Chemical Vapor Deposition Grown Graphene

Chemical functionalization and mechanical strain of graphene are both important for the optimization of flexible electronic devices as both can alter the electronic structure of graphene. Here, we investigate the combined effects of covalent aryl diazonium functionalization and mechanical strain on graphene by Raman spectroscopy. Raman spectroscopy provides a wealth of information regarding the electronic structure of graphene and can be easily applied to flexible device architectures. The use of chemical vapor deposition (CVD) grown polycrystalline graphene is found to exhibit increased reactivity toward diazonium functionalization. This is attributed to the increased reactivity of defects predominantly present along domain boundaries. Functionalization with nitrobenzene diazonium molecules causes p-type doping to occur in the CVD graphene. The combined effects of mechanical strain and chemical functionalization on the graphene are also investigated. The Raman peak width is affected because of phonon splitting when under strain as well as an increase in frequency because of doping. Interestingly, we also observe a decrease in the <i>I</i>_D/<i>I</i>_G ratio when strain is applied to the chemically functionalized graphene indicating a possible morphological change to the surface

Roles of Metal−Support Interaction in Growth of Single- and Double-Walled Carbon Nanotubes Studied with Diameter-Controlled Iron Particles Supported on MgO

The catalytic growth of single- and double-walled carbon nanotubes (SWNTs and DWNTs) by chemical vapor deposition (CVD) strongly depends on the support material. Not only the presence of nanopores in the support material but also the interaction with metal particles is believed to play an important role. Here, we study the roles of the metal−support interaction by employing iron particles with a mean diameter of 9.8 nm supported on crystalline magnesia (MgO) powder. The crystalline MgO powder has a flat surface with few nanopores so that the change of the metal particle size and the growth of nanotubes can be observed with a transmission electron microscope. We found that the original iron particles became smaller during CVD and that these small particles gave SWNTs and DWNTs when reacted with methane. The decrease in the iron particle size is likely due to thermal diffusion of iron atoms into the MgO matrix, representing strong interaction between the iron particles and the MgO support. Reduction of the catalyst under a hydrogen flow at high temperature was found to suppress nanotube growth due to the increase in the particle size. The present results provide valuable information on the nanotube growth mechanism and could be applied to the diameter-controlled or large-scale synthesis of nanotubes on support materials

Ultrahigh-Vacuum-Assisted Control of Metal Nanoparticles for Horizontally Aligned Single-Walled Carbon Nanotubes with Extraordinary Uniform Diameters

Uniform diameter, horizontally aligned single-walled carbon nanotubes (SWNTs) are grown on sapphire substrate through the control of Fe nanoparticle size by annealing in ultrahigh vacuum (UHV). We find that thermal annealing in UHV reduces the Fe nanoparticle size and significantly narrows their size distribution. These size-controlled Fe nanoparticles enable the catalytic growth of uniform-diameter SWNTs while maintaining their horizontal alignment. Systematic analyses of Raman radial breathing modes with three laser wavelengths indicate that ∼76% of the horizontally aligned SWNTs have diameters in the range of 1.3–1.4 nm under optimized annealing conditions. Longer UHV annealing induces the increase in the Fe nanoparticle size, giving large-diameter SWNTs. Precise correlation between the metal nanoparticle size and SWNT diameter is demonstrated. Effects of UHV annealing on the nanoparticle size are discussed in terms of evaporation, subsurface diffusion, and surface precipitation of catalyst metals

Effects of Water Vapor on Diameter Distribution of SWNTs Grown over Fe/MgO-Based Catalysts

With an interest in the H2O-assisted chemical vapor deposition (CVD) process, roles of H2O in tuning the diameter and chirality distributions of single-walled carbon nanotubes (SWNTs) were studied using Fe/MgO-based catalysts. A controlled amount of H2O was found to narrow the diameter distribution of SWNTs by reducing the populations of small- and large-diameter nanotubes. The addition timing of H2O vapor was investigated to understand the interaction between H2O and growing nanotubes or catalyst nanoparticles. Also, the addition of a small amount of Mo in the Fe/MgO catalyst was observed to reduce the diameter distribution of SWNTs, and H2O further promoted the formation of smaller-diameter SWNTs over the Fe−Mo/MgO catalyst. These results elucidate that the surface chemistry of catalyst nanoparticles exerted significant effects on the behaviors of H2O in the CVD process. Consequently, it is proposed that H2O first reacts with the surface of catalyst nanoparticle, and successively plays two aspects of roles, inhibiting the agglomeration of nanoparticles and preferential etching to small-diameter nanotubes, which determine the net diameter distribution of as-grown SWNTs

Spatially Controlled Nucleation of Single-Crystal Graphene on Cu Assisted by Stacked Ni

In spite of recent progress of graphene growth using chemical vapor deposition, it is still a challenge to precisely control the nucleation site of graphene for the development of wafer-scale single-crystal graphene. In addition, the postgrowth patterning used for device fabrication deteriorates the quality of graphene. Herein we demonstrate the site-selective nucleation of single-crystal graphene on Cu foil based on spatial control of the local CH₄ concentration by a perforated Ni foil. The catalytically active Ni foil acts as a CH₄ modulator, resulting in millimeter-scale single-crystal grains at desired positions. The perforated Ni foil also allows to synthesize patterned graphene without any postgrowth processing. Furthermore, the uniformity of monolayer graphene is significantly improved when a plain Ni foil is placed below the Cu. Our findings offer a facile and effective way to control the nucleation of high-quality graphene, meeting the requirements of industrial processing

Effect of Domain Boundaries on the Raman Spectra of Mechanically Strained Graphene

We investigate the effect of mechanical strain on graphene synthesized by chemical vapor deposition (CVD) transferred onto flexible polymer substrates by observing the change in the Raman spectrum and then compare this to the behavior of exfoliated graphene. Previous studies into the effect of strain on graphene have focused on mechanically exfoliated graphene, which consists of large single domains. However, for wide scale applications CVD produced films are more applicable, and these differ in morphology, instead consisting of a patchwork of smaller domains separated by domain boundaries. We find that under strain the Raman spectra of CVD graphene transferred onto a silicone elastomer exhibits unusual behavior, with the G and 2D band frequencies decreasing and increasing respectively with applied strain. This unusual Raman behavior is attributed to the presence of domain boundaries in polycrystalline graphene causing unexpected shifts in the electronic structure. This was confirmed by the lack of such behavior in mechanically exfoliated large domain graphene and also in large single-crystal graphene domains grown by CVD. Theoretical calculation of G band for a given large shear strain may explain the unexpected shifts while the shift of the Dirac points from the K point explain the conventional behavior of a 2D band under the strain

Formation of Oriented Graphene Nanoribbons over Heteroepitaxial Cu Surfaces by Chemical Vapor Deposition

We demonstrate a new bottom-up approach to synthesize graphene nanoribbons (GNRs) on a Cu(100) film by chemical vapor deposition (CVD) without the use of any lithography and etching processes. Ambient pressure CVD with a low concentration CH₄ feedstock produced a number of GNRs with widths of 40–50 nm on a heteroepitaxial Cu(100)/MgO(100) substrate. These nanoribbons are confined inside the nanoscale trenches formed on the Cu surface, and their orientations are highly controlled by the crystallographic orientation of the Cu(100) lattice. Raman spectra taken after the transfer indicated the growth of high-quality, single-layer GNRs. Moreover, low-energy electron microscopy revealed that all these aligned GNRs have the hexagonal orientations whose edges are terminated with zigzag edges. The GNR growth was not observed on Cu foil, and we discuss the growth mechanism of the oriented GNRs over epitaxial Cu(100) film. Our bottom-up approach offers a new method to grow single-layer GNRs which are oriented in a specific directions for future carbon-based nanoelectronics and spintronics applications

Hole Doping to Aligned Single-Walled Carbon Nanotubes from Sapphire Substrate Induced by Heat Treatment

We studied the effects of heat treatment on single-walled carbon nanotubes (SWNTs) aligned on a sapphire (α-Al2O3) substrate to understand the interaction between the SWNTs and sapphire. The Raman measurements showed a clear upshift of the G-band after heat treatment at 1000 °C in a high vacuum. Furthermore, Auger spectroscopy showed an increase of the [Al]/[O] atomic ratio of the sapphire surface upon heat treatment, indicating the removal of oxygen atoms from the sapphire surface. The observed upshift of the G-band is accounted for by the hole doping to the aligned SWNTs from the oxygen-deficient sapphire substrate. This annealing-induced carrier doping from the underlying substrate would offer a new and unique approach to modify the electronic structure of SWNTs