Tip-Enhanced Raman Spectroscopy and Mapping of Graphene Sheets

Abstract: Single graphene sheets, a few graphene layers, and bulk graphite, obtained via both micromechanical cleavage of highly oriented pyrolytic graphite and carbon vapor deposition methods, were deposited on a thin glass substrate without the use of any chemical treatment. Micro-Raman spectroscopy, tip-enhanced Raman spectroscopy (TERS), and tip-enhanced Raman spectroscopy mapping (TERM) were used for characterization of the graphene layers. In particular, TERM allows for the investigation of individual graphene sheets with high Raman signal enhancement factors and allows for imaging of local defects with nanometer resolution. Enhancement up to 560% of the graphene Raman band intensity was obtained using TERS. TERM (with resolution better than 100 nm) showed an increase in the number of structural defects (D band) on the edges of both graphene and graphite regions.     

Observation of low‐wavenumber out‐of‐plane optical phonon in few‐layer graphene

Few‐layer graphene grown by chemical vapor deposition has been studied by Raman and ultrafast laser spectroscopy. A low‐wavenumber Raman peak of ~120 cm⁻¹ and a phonon‐induced oscillation in the kinetic curve of electron–phonon relaxation process have been observed, respectively. The Raman peak is assigned to the low‐wavenumber out‐of‐plane optical mode in the few‐layer graphene. The phonon band shows an asymmetric shape, a consequence of so‐called Breit‐Wigner‐Fano resonance, resulting from the coupling between the low‐wavenumber phonon and electron transitions. The obtained oscillation wavenumber from the kinetic curve is consistent with the detected low‐wavenumber phonon by Raman scattering. The origin of this oscillation is attributed to the generation of coherent phonons and their interactions with photoinduced electrons. Copyright © 2012 John Wiley & Sons, Ltd.     

Effect of Thermal Treatment on the Structure of Multi-walled Carbon Nanotubes

The effects of vacuum annealing and oxidation in air on the structure of multi-walled carbon nanotubes (MWCNTs) produced by a large-scale catalytic chemical vapor deposition (CCVD) process are studied using Raman spectroscopy and transmission electron microscopy (TEM). A detailed Raman spectroscopic study of as-produced nanotubes has also been conducted. While oxidation in air up to 400°C removes disordered carbon, defects in tube walls are produced at higher temperatures. TEM reveals that MWCNTs annealed at 1,800°C and above become more ordered than as-received tubes, while the tubes annealed at 2,000°C exhibit polygonalization, mass transfer and over growth. The change in structure is observable by the separation of the Raman G and D′ peaks, a lower R-value (I _D/I _G ratio), and an increase in the intensity of the second order peaks. Using wavelengths from the deep ultraviolet (UV) range (5.08 eV) extending into the visible near infrared (IR) (1.59 eV), the Raman spectra of MWCNTs reveal a dependence of the D-band position proportional to the excitation energy of the incident laser energies.     

Raman imaging on high‐quality graphene grown by hot‐filament chemical vapor deposition

We report the synthesis of high‐quality graphene on Cu foils using hot‐filament chemical vapor deposition technique and demonstrate that by suitably varying the CH₄ and H₂ flow rates, one can also obtain hydrogenated graphene. Micro‐Raman spectroscopy studies confirm the growth of monolayer graphene as inferred from the intensity ratio of 2D to G peak which is nearly four in unhydrogenated samples. Detailed Raman area mapping confirms the uniform coverage of monolayer graphene. The grown layer is also transferred onto a Si substrate over ~10 × 10 mm sq. area. The present results provide a leap in synthesis technology of high‐quality graphene and pave way for scaling up the process. Copyright © 2012 John Wiley & Sons, Ltd.     

Structural evolution in CVD graphene chemically oxidized by sulphuric acid

The role of sulphuric acid (H₂SO₄) in fabrication graphene oxide besides as intercalant has not been well addressed. In this work, Raman spectroscopy is used to monitor structural evolution in chemical vapor deposition (CVD) graphene chemically oxidized by dilute H₂SO₄. From the analysis of Raman spectra of oxidized graphene, we propose that oxidation first initiates at preexisting defects, and vacancy‐like defects are formed. Following is the radial growth of the vacancy, and oxidation pits appear in graphene. This assumption is further confirmed by atomic force microscope measurement. It is also found that with increase of amounts of defects, G peak is blue shift, and this is explained by defect and hole doping effect. Hole doping in graphene is much stronger at hexagon regions near the oxidation pits. This work helps in understanding the role of H₂SO₄ in fabrication graphene oxide as oxidizer as well as helps in obtaining structure information of graphene oxide. Copyright © 2015 John Wiley & Sons, Ltd.     

Atmospheric pressure chemical vapor deposition (APCVD) grown bi‐layer graphene transistor characteristics at high temperature

We report the characteristics of atmospheric chemical vapor deposition grown bilayer graphene transistors fabricated on ultra‐scaled (10 nm) high‐κ dielectric aluminum oxide (Al₂O₃) at elevated temperatures. We observed that the drive current increased by >400% as temperature increased from room temperature to 250 °C. Low gate leakage was maintained for prolonged exposure at 100 °C but increased significantly at temperatures >200 °C. These results provide important insights for considering chemical vapor deposition graphene on aluminum oxide for high temperature applications where low power and high frequency operation are required. (© 2014 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Nanocarbon hybrids of graphene‐based materials and ultradispersed diamond: investigating structure and hierarchical defects evolution with electron‐beam irradiation

We report the influence of electron‐beam (E‐beam) irradiation on the structural and physical properties modification of monolayer graphene (Gr), reduced graphene oxide (rGO) and graphene oxide (GO) with ultradispersed diamond (UDD) forming novel hybrid composite ensembles. The films were subjected to a constant energy of 200 keV (40 nA over 100 nm region or electron flux of 3.9 × 10¹⁹ cm⁻²s⁻¹) from a transmission electron microscope gun for 0 (pristine) to 20 min with an interval of 2.5 min continuously – such conditions resemble increased temperature and/or pressure regime, enabling a degree of structural fluidity. To assess the modifications induced by E‐beam, the films were analyzed prior to and post‐irradiation. We focus on the characterization of hierarchical defects evolution using in situ transmission electron microscopy combined with selected area electron diffraction, Raman spectroscopy (RS) and Raman mapping techniques. The experiments showed that the E‐beam irradiation generates microscopic defects (most likely, interstitials and vacancies) in a hierarchical manner much below the amorphization threshold and hybrids stabilized with UDD becomes radiation resilient, elucidated through the intensity, bandwidth, and position variation in prominent RS signatures and mapping, revealing the defects density distribution. The graphene sheet edges start bending, shrinking, and generating gaps (holes) at ~10–12.5 min owing to E‐beam surface sputtering and primary knock‐on damage mechanisms that suffer catastrophic destruction at ~20 min. The microscopic point defects are stabilized by UDD for hybrids in the order of GO > rGO ≥ Gr besides geometric influence, i.e. the int erplay of curvature‐induced (planar vs curved) energy dispersion/absorption effects. Furthermore, an attempt was made to identify the nature of defects (charged vs residual) through inter‐defect distance (i.e. L_D). The trends of L_D for graphene‐based hybrids with E‐beam irradiation implies charged defects described in terms of dangling bonds in contrast to passivated residual or neutral defects. More importantly, they provided a contrasting comparison among variants of graphene and their hybrids with UDD. Copyright © 2015 John Wiley & Sons, Ltd.     

Raman scattering studies on manganese ion-implanted GaN

This paper reports that the Raman spectra have been recorded on the metal-organic chemical vapour deposition epitaxially grown GaN before and after the Mn ions implanted. Several Raman defect modes have emerged from the implanted samples. The structures around 182 cm^-1 modes are attributed to the disorder-activated Raman scattering, whereas the 361 cm^-1 and 660 cm^-1 peaks are assigned to nitrogen vacancy-related defect scattering. One additional peak at 280 cm^-1 is attributed to the vibrational mode of gallium vacancy-related defects and/or to disorder activated Raman scattering. A Raman-scattering study of lattice recovery is also presented by rapid thermal annealing at different temperatures between 700 °C and 1050 °C on Mn implanted GaN epilayers. The behaviour of peak-shape change and full width at half maximum (FWHM) of the A₁(LO) (733 cm^-1) and E^H₂ (566 cm^-1) Raman modes are explained on the basis of implantation-induced lattice damage in GaN epilayers.     

Diamond film coated on WC/Co tools by double bias-assisted hot filament CVD

WC–6%Co cutting tool inserts were coated with diamond films using a double bias-assisted hot-filament chemical vapor deposition method. Coating of the cutting tools with chemical vapor deposition diamond is taken as a three-step process in which the growth of diamond follows the pretreatment and nucleation of the substrate. The presented operating parameters allow to substantially suppress the presence of amorphous carbon and/or graphite phases in the diamond films deposited on WC/Co tools. The substrate temperature of ∼700 °C, and a low methane concentration result in a sharp diamond Raman peak centered at 1333–34 cm⁻¹ with FWHM of 6–7 cm⁻¹ as detected by micro-Raman spectroscopy. The diamond morphology is characterized by scanning electron microscopy, optical microscopy, and micro-Raman spectroscopy.     

In‐situ Raman spectroscopy of current‐carrying graphene microbridge

In‐situ Raman spectroscopy was performed on chemical vapor deposited graphene microbridge (3 μm × 80 μm) under electrical current density up to 2.58 × 10⁸ A/cm² in ambient conditions. We found that both the G and the G′ peak of the Raman spectra do not restore back to the initial values at zero current, but to slightly higher values after switching off the current through the microbridge. The up‐shift of the G peak and the G′ peak, after switching off the electrical current, is believed to be due to p‐doping by oxygen adsorption, which is confirmed by scanning photoemission microscopy. Both C–O and C=O bond components in the C1s spectra from the microbridge were found to be significantly increased after high electrical current density was flown. The C=O bond is likely the main source of the p‐doping according to our density functional theory calculation of the electronic structure. Copyright © 2014 John Wiley & Sons, Ltd.     

Transport properties of graphene in the region of its interface with water surface

The graphene growth by thermal decomposition of silicon carbide at the temperature of ~1400°C in a high vacuum of ~10^–6 Torr has been optimized. By Raman spectroscopy, the mean thickness of obtained graphene (2–4 single layers) has been estimated and the presence of high-quality graphene areas in the samples has been demonstrated. It has been found out that the four-point resistance of graphene increases in the region of its interface with water approximately by 25%. For the graphene–water interface in the transistor geometry, with variation in the gate-to-source voltage, the field effect corresponding to the hole type of charge carries in graphene has been revealed.     

The influence of chemical solvents on the properties of CVD graphene

The ultra‐clean and defect‐free transfer of chemical vapor deposition (CVD) graphene is essential for its application in electronic devices. Here, we study the influence of commonly used etching solvents during the transfer process, i.e. ammonium persulfate, ferric chloride, and ferric nitrate, on the properties of CVD graphene by Raman spectroscopy. Obvious blue shift and broadening of Raman G and 2D peaks were observed for graphene transferred by ferric chloride and ferric nitrate, as compared to that transferred by ammonium persulfate. These changes are attributed to p‐doping as well as reduction of phonon lifetime of graphene in the presence of residue iron compounds during the transfer process. The latter would also introduce a great reduction of thermal conductivity of graphene, e.g. with 76% reduction for graphene transferred by ferric nitrate as compared to that transferred by ammonium persulfate. This work would provide valuable information for the transfer of high‐quality CVD graphene. Copyright © 2014 John Wiley & Sons, Ltd.     

Artificially controlled synthesis of graphene intramolecular heterojunctions for phonon engineering

Because phonons are the main carriers for graphene heat transfer, modifying the dynamic properties of the crystal lattice by isotopes modulates the phonon behavior and alters the thermal properties. Here we demonstrate an artificially controlled texture synthesis of ¹²C‐graphene/¹³C‐graphene heterostructures via chemical vapor deposition and an O₂ plasma etching. The electrical and thermal properties of the graphene across the heterojunction show that ¹²C‐graphene and ¹³C‐ graphene are electronically connected as resistors in series, while the thermal conductivity across the junction is dramatically reduced due to the suppressed phonon propagation, which causes the conductivity across the junction to be lower than that of graphene sheets with randomly mixed isotopes. These findings should help realize novel two‐dimensional graphene thermoelectric devices where phonon modulation controls the electrons and heat transport independently. (© 2014 WILEY‐VCH Verlag GmbH &Co. KGaA, Weinheim)     

Stress relaxation of GaN microstructures on a graphene‐buffered Al2O3 substrate

GaN microstructures were grown on c‐Al₂O₃ with a multi‐stacked graphene buffered layer using metal metal‐organic chemical‐vapor deposition. Under the same growth conditions, the nucleation of GaN was suppressed by the low surface energy of graphene, resulting in a much lower density of microstructures relative to those grown on c‐Al₂O₃. Residual stress in the GaN microstructures was estimated from the peak shift of the E₂ phonon using micro‐Raman spectroscopy. The results showed that the compressive stress of approximately 0.36 GPa in GaN on c‐Al₂O₃ caused by lattice mismatch and the difference in the thermal expansion coefficient was relaxed by introducing the graphene layer. (© 2014 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Influence of topological defects on the structure of G and D spectral bands of a single-layer carbon nanotube

A topological defect in a carbon nanotube grown by chemical vapor deposition from methane onto a silicon substrate with thermal oxide has been investigated and visualized (with a resolution of about 1.5 μm) by confocal Raman spectroscopy. Vibrational Raman spectra of molecular fragments of a single-wall carbon nanotube (SWCNT) without a defect and with Stone–Wales defects (two pentagonal and two heptagonal cells) are calculated. The influence of defects on the shape of G-band components (G⁺ and G^–), which makes it possible to determine the nanotube conductivity type, is considered.     

Metallic/semiconducting ratio of carbon nanotubes in a bundle prepared using CVD technique

We present an investigation of the nature of single-walled carbon nanotubes (SWCNTs) in a bundle by resonant Raman spectroscopy. The calculation has been done for the three peak positions in radial breathing mode (RBM) spectra obtained by using a laser excitation wavelength (γ) of 633 nm from He-Ne laser on SWNT bundle sample prepared by chemical vapor deposition (CVD) technique using iron catalyst at 800°C. The detailed analysis in the present study is focused on peak positions 162 cm⁻¹, 186 cm⁻¹, and 216 cm⁻¹. The firs step of the analysis is to construct a list of possible (n, m) pairs from the diameters calculated from the RBM peak positions. The parameters of SWNTs studied gives in-depth understanding of many symmetry, resonance and characteristic properties of SWNT bundles. Our results indicate that the contribution of metallic SWNTs in the bundle is small at RBM peak positions 162 cm⁻¹, 186 cm⁻¹ and in agreement with pervious results at peak position 216 cm⁻¹.     

Electrochemical characterization of ionic liquid based gel polymer electrolyte for lithium battery application

High molecular weight polymer poly(vinylidenefluoride-co-hexafluoropropylene) (PVdF-HFP), ionic liquid 1-ethyl-3-methylimidazolium bis(fluorosulfonyl)imide (EMIMFSI), and salt lithium bis(trifluoromethanesulfonyl)imide (LiTFSI)-based free-standing and conducting ionic liquid-based gel polymer electrolytes (ILGPE) have been prepared by solution cast method. Thermal, electrical, and electrochemical properties of 80 wt% IL containing gel polymer electrolyte (GPE) are investigated by thermogravimetric (TGA), impedance spectroscopy, linear sweep voltammetry (LSV), and cyclic voltammetry (CV). The 80 wt% IL containing GPE shows good thermal stability (~?200 °C), ionic conductivity (6.42?×?10^?4 S cm^?1), lithium ion conductivity (1.40?×?10^?4 S cm^?1 at 30 °C), and wide electrochemical stability window (~?4.10 V versus Li/Li⁺ at 30 °C). Furthermore, the surface of LiFePO₄ cathode material was modified by graphene oxide, with smooth and uniform coating layer, as confirmed by scanning electron microscopy (SEM), and with element content, as confirmed by energy dispersive X-ray (EDX) spectrum. The graphene oxide-coated LiFePO₄ cathode shows improved electrochemical performance with a good charge-discharge capacity and cyclic stability up to 50 cycles at 1C rate, as compared with the without coated LiFePO₄. At 30 °C, the discharge capacity reaches a maximum value of 104.50 and 95.0 mAh g^?1 for graphene oxide-coated LiFePO₄ and without coated LiFePO₄ at 1C rate respectively. These results indicated improved electrochemical performance of pristine LiFePO₄ cathode after coating with graphene oxide.     

Theory of the evolution of 2D band in the Raman spectra of monolayer and bilayer graphene with laser excitation energy

A double‐resonance process gives rise to the 2D band in the Raman spectra of monolayer and bilayer graphene. Based on the electronic and vibrational dispersion energies of graphene, the wavenumbers of the 2D band were calculated under different laser excitation energies (from 1.0 to 4.4 eV). Calculated results are in good agreement with experimental data and reproduce the experimental dispersion slope of the 2D band very well. The calculated wavenumbers of the 2D band do not show a linear dependence on the laser excitation energies. Moreover, it is explained that the lowest wavenumber peak of the 2D band of the bilayer graphene, which is composed of four components, has the largest slope with laser excitation energy. Copyright © 2009 John Wiley & Sons, Ltd.     

Excellent organic/inorganic transparent thin film moisture barrier entirely made by hot wire CVD at 100 °C

We present a silicon nitride/polymer hybrid multilayer moisture barrier for flexible electronics made entirely by hot wire chemical vapor deposition (HWCVD) at substrate temperatures below 100 °C. Using the initiated CVD (iCVD) variant of HWCVD for the polymer layers, these can be extremely thin, while efficiently decoupling the defects in consecutive inorganic layers. Although a single layer of low temperature SiN_x is more prone to have pinholes than its state‐of‐the‐art high temperature equivalent, we have achieved a simple three‐layer structure consisting of two low‐temperature SiN_x layers with a polymer layer in between, which is pinhole free and shows a water vapor transmission rate (WVTR) as low as 5 × 10^–6 g/m²/day at a temperature of 60 °C and a relative humidity of 90%. This WVTR is low enough for organic devices. (© 2012 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Origin of the Raman mode at 379 cm−1 observed in ZnO thin films grown on sapphire

In this work, we present a detailed Raman scattering study to clarify the origin of the mode at 379 cm⁻¹ which is observed in Raman spectra of the ZnO films grown on c‐sapphire substrates and generally attributed to the A₁‐transverse optical (A₁‐TO) mode of ZnO. The studied ZnO films were deposited by metal‐organic chemical vapor deposition on c‐sapphire and (0001) ZnO substrates. In the z(−,−)z̄ backscattering configuration, the A₁‐TO mode is forbidden, while the 379 cm⁻¹ peak is still observed in the as‐deposited film grown on sapphire substrate. However, this mode is not observed in Raman spectra of the as deposited film grown on ZnO substrate. We suggest that the peak at 379 cm⁻¹ is the E_1g mode of the sapphire substrate which is allowed in z(−,−)z̄ backscattering configuration. The effects of annealing, the substrate and the collection cross‐section on Raman active modes were analyzed. Copyright © 2011 John Wiley & Sons, Ltd.