Probing inhomogeneous doping in overlapped graphene grain boundaries by Raman spectroscopy

The effect of grain boundaries and wrinkles on the electrical properties of polycrystalline graphene is pronounced. Here we investigate the stitching between grains of polycrystalline graphene, specifically, overlapping of layers at the boundaries, grown by chemical vapor deposition (CVD) and subsequently doped by the oxidized Cu substrate. We analyze overlapped regions between 60 and 220 nm wide via Raman spectroscopy, and find that some of these overlapped boundaries contain AB–stacked bilayers. The Raman spectra from the overlapped grain boundaries are distinctly different from bilayer graphene and exhibit splitting of the G band peak. The degree of splitting, peak widths, as well as peak intensities depend on the width of the overlap. We attribute these features to inhomogeneous doping by charge carriers (holes) across the overlapped regions via the oxidized Cu substrate. As a result, the Fermi level at the overlapped grain boundaries lies between 0.3 and 0.4 eV below the charge neutrality point. Our results suggest an enhancement of electrical conductivity across overlapped grain boundaries, similar to previously observed measurements (Tsen et al., 2012). The dependence of charge distribution on the width of overlapping of grain boundaries may have strong implications for the growth of large-area graphene with enhanced conductivity.     

Numerical experiments on phonon properties of isotope and vacancy-type disordered graphene

We have studied phonon properties of graphene theoretically with different concentrations of ¹³C isotope and vacancy-type defects. The forced vibrational method, which is based on the mechanical resonance to extract the pure vibrational eigenmodes by numerical simulation, has been employed to compute the phonon density of states (PDOSs) and mode pattern of isotope-disordered graphene as well as a combined isotope and vacancy-type defective graphene structure. We observe a linear reduction of the E_2g mode frequencies with an increase in ¹³C concentration due to the reduced mass variation of the isotope mixture. We find a downshift of the E_2g mode of 65 cm^− 1, which is a very good agreement with the experimental results, and the phonon frequencies described by the simple harmonic oscillator model. The vacancy-type defects break down the phonon degeneracy at the Г point of the LO and TO modes, distort and shift down the phonon density of states significantly. The PDOS peaks for the combined isotope and vacancy-type defects show the remarkable increase in the low-frequency region induced by their defect formations. Due to phonon scattering by ¹³C isotope or vacancies, some graphene phonon wave functions become localized in the real space. Our numerical experiments reveal that the lattice vibrations in the defective graphene show the remarkably different properties such as spatial localization of lattice vibrations due to their random structures from those in the perfect graphene. The calculated typical mode patterns for in-plane K point optical phonon modes indicate that the features of strongly localized state depend on the defect density, and the phonon is localized strongly within a region of several nanometers in the random percolation network structures. In particular, for in-plane K point optical phonon modes, a typical localization length is on the order of ≈ 7 nm for isotope impurities, ≈ 5 nm for vacancy-type defects and ≈ 6 nm for mixed-type defects at high defect concentrations of 30%. Our findings can be useful for the interpretation of experiments on infrared, Raman, and neutron-diffraction spectra of defective graphene, as well as in the study of a wide variety of other physical properties such as thermal conductivity, specific heat capacity, and electron–phonon interaction.     

Fano resonance in Raman scattering of graphene

Fano resonances and their strong doping dependence are observed in Raman scattering of single-layer graphene (SLG). As the Fermi level is varied by a back-gate bias, the Raman G band of SLG exhibits an asymmetric line shape near the charge neutrality point as a manifestation of a Fano resonance, whereas the line shape is symmetric when the graphene sample is electron or hole doped. However, the G band of bilayer graphene (BLG) does not exhibit any Fano resonance regardless of doping. The observed Fano resonance can be interpreted as interferences between the phonon and excitonic many-body spectra in SLG. The absence of a Fano resonance in the Raman G band of BLG can be explained in the same framework since excitonic interactions are not expected in BLG.     

The effects of interlayer mismatch on electronic properties of bilayer armchair graphene nanoribbons

We investigate the impact of interlayer mismatch on the electronic properties of bilayer graphene nanoribbons (BGNRs) with armchair-edges in terms of the total energy and electronic structures by first principle calculations. Simulation results show that in-plane misalignments require little energy and a large variation in the energy bandgap (E_G) can be observed. Based on the resulting atomic configurations due to the misalignments, the details of the observed relationship between bandgap and the lattice mismatch are investigated. It is observed that in general, misalignment in the transverse direction results in a decrease in the interaction between the two layers, giving rise to a larger E_G. On the other hand, misalignment in the longitudinal direction, i.e. along the edges, leads to an oscillation in E_G due to the periodic change of the GNR stacking order. A combination of these movements results in a complex variation of E_G, which introduces great uncertainty in electronic devices. However, such a phenomenon could also be used in various kinds of nanoelectromechanical systems as it provides a large change in electronic properties with a small movement.     

Identification of nano-sized holes by TEM in the graphene layer of graphite and the high rate discharge capability of Li-ion battery anodes

SEM images of round-shaped natural graphite, currently widely used as the anode active material of Li-ion batteries, show that the surface mainly consists of the basal plane, which suggests that the Li insertion/extraction reaction rate is quite limited. In contrast to this suggestion, however, the anode of commercial Li-ion batteries is capable of high rate charging/discharging. In order to explain this inconsistency, we propose that there are nano-holes in the graphene layers of the graphite allowing Li to be very easily inserted and extracted via the holes.Prior to the measurements a quantum chemical investigation was performed on the energy required for Li to pass through the hole in a graphene layer (E_act). The results showed that the E_act value is too high when the size is smaller than pyrene, but is fairly low for holes of the size of coronene, implying that Li can pass through the basal plane layer if there is a hole larger than coronene.Characterization of the rounded graphite sample and flaky natural graphite was conducted by constant-current charge/discharge cycle tests, X-ray diffraction (XRD), Raman spectroscopy, scanning electron microscopy (SEM), and high-resolution transmission electron microscopy (HRTEM). XRD revealed no appreciable difference between the rounded graphite and flaky natural graphite, in agreement with Raman data.A detailed analysis of the HRTEM results revealed the presence of a number of variously sized circular images. We believe that these are holes in the graphene layer through which Li can pass. The mechanism of formation of the holes is discussed.     

Temperature-dependent Raman scattering in round pit of 4H-SiC

The Raman spectra of N-doped 4H-SiC single crystal films is investigated between 100 and 600 K. The temperature dependence of the three optical modes is obtained. These measurements reveal that all Raman peaks shift to lower frequencies with increasing temperature, except A₁(LO). The temperature dependence of A₁(LO) phonon modes in the round pit also manifests different features with temperature increasing, but the demarcation temperature point of the blueshift and the redshift in the round pit is higher than that in the outer area. At high temperature, all active phonon modes clearly become broader, but the linewidth of the E₁(TO) phonon mode from round pit increases with temperature more rapidly than that from the outer area, this indicates that the lifetime of the E₁(TO) phonon in round pit is more sensitive than that in the outer area.     

Surface-enhanced Raman scattering of suspended monolayer graphene

Abstract

The interactions between phonons and electrons induced by the dopants or the substrate of graphene in spectroscopic investigation reveal a rich source of interesting physics. Raman spectra and surface-enhanced Raman spectra of supported and suspended monolayer graphenes were measured and analyzed systemically with different approaches. The weak Raman signals are greatly enhanced by the ability of surface-enhanced Raman spectroscopy which has attracted considerable interests. The technique is regarded as wonderful and useful tool, but the dopants that are produced by depositing metallic nanoparticles may affect the electron scattering processes of graphene. Therefore, the doping and substrate influences on graphene are also important issues to be investigated. In this work, the peak positions of G peak and 2D peak, the I_2D/I_G ratios, and enhancements of G and 2D bands with suspended and supported graphene flakes were measured and analyzed. The peak shifts of G and 2D bands between the Raman and SERS signals demonstrate the doping effect induced by silver nanoparticles by n-doping. The I_2D/I_G ratio can provide a more sensitive method to carry out the doping effect on the graphene surface than the peak shifts of G and 2D bands. The enhancements of 2D band of suspended and supported graphenes reached 138, and those of G band reached at least 169. Their good enhancements are helpful to measure the optical properties of graphene. The different substrates that covered the graphene surface with doping effect are more sensitive to the enhancements of G band with respect to 2D band. It provides us a new method to distinguish the substrate and doping effect on graphene.

PACS

78.67.Wj (optical properties of graphene); 74.25.nd (Raman and optical spectroscopy); 63.22.Rc (phonons in graphene)     

Evolution of Raman spectra in nitrogen doped graphene

We present systematical Raman studies of nitrogen doped graphene (NG). Defective graphene by Ar⁺ ion bombardment was also studied for comparison. It was found that the defects/nitrogen dopants in NG are not homogenous. Our results also suggest that the G peak position and I_2D/I_G ratio cannot be simply used as fingerprint of doping concentration in NG. Both doping and compressive strain (as verified by transmission electron microscope) contribute to the shift of Raman peaks, while both doping and lattice defects contribute to the attenuation of 2D peak. Finally, the nature of defects in NG was probed and found that they are boundary defects. The detail analysis of the evolution of Raman spectra in NG would greatly help on the characterization and future application of this novel material.     

Structural and optical characterization of Zn0.95−xMg0.05AlxO nanoparticles

The structural and optical properties of ZnO nanoparticles doped simultaneously with Mg and Al were investigated. XRD results revealed the hexagonal wurtzite crystalline structure of ZnO. The FE-SEM study confirmed the formation of nano-sized homogeneous grains whose sizes decreased monotonously with increasing doping concentrations of Mg and Al. The absorption spectra showed that band gap increased from 3.20 to 3.31 eV with Mg doping. As the Al concentration changed from x=0.01 to x=0.06 mol% at constant Mg concentration the band gap observed to be decreased. Particle sizes estimated from effective mass approximation using absorption data and these values are in good agreement with the crystallite sizes calculated from XRD data. Raman spectra of ZnO showed a characteristic peak at 436 cm⁻¹ correspond to a non-polar optical phonon E₂ (high). With increase of the Al doping concentrations, E₂ (high) phonon frequency shifted to 439 cm⁻¹ from to 436 cm⁻¹. The origin of E₂ (high) peak shift in ZnO nanoparticles is attributed to optical phonon confinement effects or the presence of intrinsic defects on the nanoparticles. PL spectra indicated that with increase of Al co-doping along with Mg into ZnO, intensity of the peak positioned at 395 nm was initially increased at x=0 and then decreased with increase of the Al concentrations from x=0.01 to x=0.06 mol%.     

Effect of yttria on thermal transport and vibrational modes in yttria-stabilized hafnia

Low thermal conductivity plays an essential role in application relevant to thermal energy conversion and management. In this paper, we utilize molecular dynamics to investigate the thermal transport and lattice variation modes in yttria-stabilized hafnia, which only contains binary oxides of Y₂O₃ and HfO₂. It is found that the thermal conductivity κ of yttria-stabilized hafnia decreases significantly with the increase of doping ratio of Y₂O₃, and then reaches a limiting value (～2.1 W m^?1K^?1), because of the strong phonon scattering of oxygen vacancies. Importantly, a glass-like thermal conductivity κ is achieved in yttria-stabilized hafnia samples when the content of Y₂O₃ exceeds 15 mol%. By decomposing the phonon vibrational modes, we find that most of the heat is transported by diffusive modes. As a result, the κ exhibits a glass-like feature in yttria-stabilized hafnia samples with high content of Y₂O₃. Notably, the κ of yttria-stabilized hafnia is much lower than those of classical functional ceramics materials. The insight into the κ in yttria-stabilized hafnia system is beneficial for understanding and reducing the κ of materials through defect engineering. Despite its simple composition, yttria-stabilized hafnia with different doping ratios demonstrates unexpected high scattering rate of phonon vibration density states, which is confirmed by the diffused wavevector-frequency dispersion. Eigenvector periodicity and phonon participation ratio of phonon have been visualized to capture the distribution of phonon modes in yttria-stabilized hafnia with various dopant. This work investigates into the details of phonon vibrational modes in yttria-stabilized hafnia, which would be valuable for conducting experiments to acquire low thermal conductivity materials in laboratory.     

Raman spectroscopy at the edges of multilayer graphene

Individual graphene layers in a multilayer graphene sample contribute their own edges. The edge of a graphene layer laid on an n layer graphene (nLG) is a building block for the edges of multilayer graphenes. We found that the D band observed from the edge of the top graphene layer laid on the nLG exhibits an identical line shape to that of disordered (n + 1)LG. Based on the spectral features of the D and 2D bands, we identified two types of alignment configurations at the edges of bilayer and trilayer graphenes, whose edges are well-aligned from their optical images.     

Electrochemical doping of single-walled carbon nanotubes in double layer capacitors studied by in situ Raman spectroscopy

The electrochemical doping of single-walled carbon nanotubes (SWCNTs) in 1 M Et₄NBF₄ in acetonitrile was investigated by in situ Raman spectroscopy. The capacitance was determined to be 82 F/g for the positive and 71 F/g for the negative SWCNT electrode, respectively, which approaches the typical values for microporous activated carbons used in supercapacitors. The changes in the Raman intensities and shifts of the D and G⁺ bands as well as of the radial breathing modes (RBMs) during electron and hole injection were studied as a function of the electrode potential. For the D and G⁺ bands, hole doping leads to strong upshifts which can be attributed to a stiffening of C-C bonds and the corresponding phonon modes. Electron doping results in much less pronounced changes in the band positions. The intensity attenuation of the RBM bands was found to be markedly different for semi-conducting and metallic SWCNTs, whereby sufficiently high doping leads to a loss of Raman intensity due to bleaching of electronic transitions. The main RBM bands upshift upon both electron and hole doping, which is attributed to changes in the chemical environment of individual SWCNTs upon charging and discharging of the electrochemical double layer within SWCNT bundles.     

Probing substrate influence on graphene by analyzing Raman lineshapes

We provide a new approach to identify the substrate influence on graphene surface. Distinguishing the substrate influences or the doping effects of charged impurities on graphene can be realized by optically probing the graphene surfaces, included the suspended and supported graphene. In this work, the line scan of Raman spectroscopy was performed across the graphene surface on the ordered square hole. Then, the bandwidths of G-band and 2D-band were fitted into the Voigt profile, a convolution of Gaussian and Lorentzian profiles. The bandwidths of Lorentzian parts were kept as constant whether it is the suspended and supported graphene. For the Gaussian part, the suspended graphene exhibits much greater Gaussian bandwidths than those of the supported graphene. It reveals that the doping effect on supported graphene is stronger than that of suspended graphene. Compared with the previous studies, we also used the peak positions of G bands, and I_2D/I_G ratios to confirm that our method really works. For the suspended graphene, the peak positions of G band are downshifted with respect to supported graphene, and the I_2D/I_G ratios of suspended graphene are larger than those of supported graphene. With data fitting into Voigt profile, one can find out the information behind the lineshapes.     

Monolayer graphene/hexagonal boron nitride heterostructure

A buried metal-gate field-effect transistor (FET) using a stacked hexagonal boron nitride (h-BN) and chemically vapor deposited (CVD) graphene heterostructure is demonstrated. A thin h-BN multilayer serves as both gate dielectric and supporting layer for the monolayer graphene channel. It is observed that electrical stressing could significantly improve graphene conduction, similar to the effect reported in the graphene/SiO₂ system. In the graphene/h-BN/TiN FET structure, p-type doping behavior in graphene is observed, possibly attributed to spontaneous doping due to the work function difference between the graphene channel and the metal gate electrode. At a high-level of stress, graphene exhibits n-type doping behavior due to charge transfer across the thin h-BN multilayer. The dielectric strength and tunneling behavior of h-BN are investigated, showing the robust nature of the layer-structured insulator.     

A preparation approach of exploring cluster ion implantation: from ultra-thin carbon film to graphene

Based on the extensive application of 2 × 1.7MV Tandetron accelerator, a low-energy cluster chamber has been built to explore for synthesizing graphene. Raman spectrum and atomic force microscopy (AFM) show that an amorphous carbon film in nanometer was deposited on the silicon by C₄ cluster implantation. And we replaced the substrate with Ni/SiO₂/Si and measured the thickness of Ni film by Rutherford backscattering spectrometry (RBS). Combined with suitable anneal conditions, these samples implanted by various small carbon clusters were made to grow graphene. Results from Raman spectrum reveal that few-layer graphene were obtained and discuss whether I_G/I_2D can contribute to explain the relationship between the number of graphene layers and cluster implantation dosage.     

Micro-Raman and micro-transmission imaging of epitaxial graphene grown on the Si and C faces of 6H-SiC

Micro-Raman and micro-transmission imaging experiments have been done on epitaxial graphene grown on the C- and Si-faces of on-axis 6H-SiC substrates. On the C-face it is shown that the SiC sublimation process results in the growth of long and isolated graphene ribbons (up to 600 μm) that are strain-relaxed and lightly p-type doped. In this case, combining the results of micro-Raman spectroscopy with micro-transmission measurements, we were able to ascertain that uniform monolayer ribbons were grown and found also Bernal stacked and misoriented bilayer ribbons. On the Si-face, the situation is completely different. A full graphene coverage of the SiC surface is achieved but anisotropic growth still occurs, because of the step-bunched SiC surface reconstruction. While in the middle of reconstructed terraces thin graphene stacks (up to 5 layers) are grown, thicker graphene stripes appear at step edges. In both the cases, the strong interaction between the graphene layers and the underlying SiC substrate induces a high compressive thermal strain and n-type doping.     

Formation of high-quality quasi-free-standing bilayer graphene on SiC(0 0 0 1) by oxygen intercalation upon annealing in air

We report on the conversion of epitaxial monolayer graphene on SiC(0 0 0 1) into decoupled bilayer graphene by performing an annealing step in air. We prove by Raman scattering and photoemission experiments that it has structural and electronic properties that characterize its quasi-free-standing nature. The (6√3 × 6√3)R30° buffer layer underneath the monolayer graphene loses its covalent bonding to the substrate and is converted into a graphene layer due to the oxidation of the SiC surface. The oxygen reacts with the SiC surface without inducing defects in the topmost carbon layers. The high-quality bilayer graphene obtained after air annealing is p-doped and homogeneous over a large area.     

Graphene as electronic structure modifier of nanostructured Pt film for enhanced methanol oxidation reaction electrocatalysis

We report a novel, nanostructured Pt-graphene (Pt-G) hybrid, which is composed of Pt nanowires (NWs) grown directly on intrinsic graphene by using a mesoporous silica thin film as a nanotemplate. The direct junction between the Pt NWs and graphene, as well as the defect-free nature of the graphene grown via chemical vapor deposition, enables the charge transfer from graphene to Pt in Pt-G. The Raman, ultraviolet photoelectron spectroscopy (UPS) and X-ray photoelectron spectroscopy (XPS) data on Pt-G show clear evidence for the charge transfer from graphene to Pt. Through the interaction, the Fermi level of Pt is raised by 0.3 eV. With the electronic structure altered, Pt-G shows an increased tolerance to CO-poisoning and, hence, enhanced methanol oxidation reaction (MOR) performance with increased I_f/I_b ratio and cycle stability. The present data demonstrate a novel way to exploit the unusual characteristics of graphene for useful applications.     

Quasi-free-standing monolayer and bilayer graphene growth on homoepitaxial on-axis 4H-SiC(0 0 0 1) layers

Quasi-free-standing monolayer and bilayer graphene is grown on homoepitaxial layers of 4H-SiC. The SiC epilayers themselves are grown on the Si-face of nominally on-axis semi-insulating substrates using a conventional SiC hot-wall chemical vapor deposition reactor. The epilayers were confirmed to consist entirely of the 4H polytype by low temperature photoluminescence. The doping of the SiC epilayers may be modified allowing for graphene to be grown on a conducing substrate. Graphene growth was performed via thermal decomposition of the surface of the SiC epilayers under Si background pressure in order to achieve control on thickness uniformity over large area. Monolayer and bilayer samples were prepared through the conversion of a carbon buffer layer and monolayer graphene respectively using hydrogen intercalation process. Micro-Raman and reflectance mappings confirmed predominantly quasi-free-standing monolayer and bilayer graphene on samples grown under optimized growth conditions. Measurements of the Hall properties of Van der Pauw structures fabricated on these layers show high charge carrier mobility (>2000 cm²/Vs) and low carrier density (<0.9 × 10¹³ cm⁻²) in quasi-free-standing bilayer samples relative to monolayer samples. Also, bilayers on homoepitaxial layers are found to be superior in quality compared to bilayers grown directly on SI substrates.     

Brillouin scattering study of low-frequency bulk acoustic phonons in multilayer graphene

The low-frequency acoustic dynamics of multilayer graphene flakes, on SiO₂/Si substrate, have been studied by polarized Brillouin light scattering. Interference enhancement affords the first observation of both the in-plane longitudinal and transverse acoustic bulk modes near the Γ-point of the Brillouin zone. The experimental data yielded information on the acoustic and elastic properties of multilayer graphene. Additionally, the measured phonon dispersion is found to be in good accordance with that evaluated for graphene, based on first-principles calculations within the generalized gradient approximation. Our findings strongly suggest that the interaction between layers in multilayer graphene is weak.