Tunable n‐ and p‐type doping of single‐layer graphene by engineering its interaction with the SiO2 support

We report a technique to tune the excess charge concentration in single‐layer graphene from p‐ to n‐type up to densities of |n | ～ 1.2 × 10¹³ cm^–2, corresponding to a displacement electric field of ～2.5 V/nm. The tuning is achieved by engineering the interaction between graphene and the underlying Si/SiO₂ substrate with an amino group‐terminated self‐assembled monolayer, and subsequent rinsing in aqueous solutions at controlled pH. Raman spectroscopy and electrical measurements on treated graphene devices confirm the occurrence of doping. Interestingly, we found the field‐effect mobility not to be significantly affected by the procedure. (© 2012 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Raman spectroscopic studies of pulsed laser‐induced defect evolution in graphene

Raman spectra were obtained for graphene after irradiating the samples by pulsed laser (λ = 248 nm). Changes in the spectra were observed as the pulse laser energy density (PLED) was varied from 0.1 to 0.25 J/cm². Changes in bilayer graphene were accompanied by the appearance of the D peak and the broadening of the G peak. Changes in multilayer graphene are more profound as the Raman spectra changes from a multilayer to bilayer and subsequently to monolayer graphene in response to a slow increase in the PLED. The threshold PLED was found to be dependent on the number of graphene layers. We also irradiate graphene with very high PLED (much above the threshold), and the Raman spectra were found to be significantly changed. The G‐band became broader, and red shifted, while the intensity of the 2D‐band was drastically reduced and an intense defect‐related D peak appeared at about 1350 cm⁻¹. The laser ablation of graphene, both with low‐ and high‐energy intensity, is consistent with the reported theoretical predictions. Copyright © 2013 John Wiley & Sons, Ltd.     

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.     

A high‐throughput method for controlled hot‐spot fabrication in SERS‐active gold nanoparticle dimer arrays

We present a high‐throughput method for fabricating large arrays of surface‐enhanced Raman scattering (SERS) active gold dimers. Using a large‐area/low‐cost nanopatterning method in conjunction with a meniscus force deposition technique, we were able to create large arrays of uniformly spaced nanoclusters comprising two 60‐nm gold nanospheres. Raman measurements of a thiophenol monolayer deposited on smaller scale arrays of aligned dimers yielded enhancement factors as high as 10⁹. Polarization‐controlled measurements show spectral peak heights to be 10–100 times smaller when the incident beam is polarized perpendicularly to the dimer axis, confirming that the measured enhancements arise from the ‘hot spots’ between the two nanospheres. Copyright © 2009 John Wiley & Sons, Ltd.     

Fracture‐induced nanoscrolls from CVD‐grown monolayer molybdenum disulfide

We report a simple and effective way of fabricating molybdenum disulfide (MoS₂) nanoscrolls by self‐rolling up fractured monolayer CVD‐grown MoS₂ microflakes. Morphological results reveal that MoS₂ nanoscrolls are formed only at newly formed edges, owing to an orientation‐specific fracture behavior. Using Raman spectroscopy, we show that the E¹_2g Raman peak (A_1g peak) for MoS₂ nanoscrolls significantly red‐shifts (blue‐shifts), indicating structural change. The proposed mechanism is that the newly formed edges induced by fracture behavior self‐roll up to nanoscrolls to minimize the surface free energy, meanwhile, the serious lattice contradiction of upper sulfur plane controls the rolling directions. (© 2016 WILEY‐VCH Verlag GmbH &Co. KGaA, Weinheim)     

Comparative study of single and multi domain CVD graphene using large‐area Raman mapping and electrical transport characterization

We systematically investigate the impact of granularity in CVD graphene films by performing Raman mapping and electrical characterization of single (SD) and multi domain (MD) graphene. In order to elucidate the quality of the graphene film, we study its regional variations using large‐area Raman mapping and compare the G and 2D peak positions of as‐transferred chemical vapor deposited (CVD) graphene on SiO₂ substrate. We find a similar upshift in wavenumber in both SD and MD graphene in comparison to freshly exfoliated graphene. In our case, doping could play the dominant role behind the observation of such upshifts rather than the influence due to strain. Interestingly, the impact of the polymer‐assisted wet transfer process is the same in both the CVD graphene types. The electrical characterization shows that SD graphene exhibits a substantially higher (a factor 5) field‐effect mobility when compared to MD graphene. We attribute the low sheet resistance and mobility enhancement to a decrease in charge carrier scattering thanks to a reduction of the number of grain boundaries and defects in SD graphene.     

The effect of vacuum annealing on graphene

The effect of vacuum annealing on the properties of graphene is investigated by using Raman spectroscopy and electrical measurement. Heavy hole doping on graphene with concentration as high as 1.5 × 10¹³ cm⁻² is observed after vacuum annealing and exposed to an air ambient. This doping is due to the H₂O and O₂ adsorption on graphene, and graphene is believed to be more active to molecular adsorption after annealing. Such observation calls for special attention in the process of fabricating graphene‐based electronic devices and gas sensors. On the other hand, because the quality of graphene remains high after the doping process, this would be an efficient and controllable method to introduce heavy doping in graphene, which would greatly help on its application in future electronic devices. Copyright © 2009 John Wiley & Sons, Ltd.     

Synthesis of graphene oxide and graphene quantum dots from miscanthus via ultrasound-assisted mechano-chemical cracking method

Whilst graphene materials have become increasingly popular in recent years, the followed synthesis strategies face sustainability, environmental and quality challenges. This study proposes an effective, sustainable and scalable ultrasound-assisted mechano-chemical cracking method to produce graphene oxide (GO). A typical energy crop, miscanthus, was used as a carbon precursor and pyrolysed at 1200 °C before subjecting to edge-carboxylation via ball-milling in a CO₂-induced environment. The resultant functionalised biochar was ultrasonically exfoliated in N-Methyl-2-pyrrolidone (NMP) and water to form GOs. The intermediate and end-products were characterised via X-ray diffraction (XRD), Raman, high-resolution transmission electron microscopy (HR-TEM) and atomic force microscopy (AFM) analyses. Results show that the proposed synthesis route can produce good quality and uniform GOs (8–10% monolayer), with up to 96% of GOs having three layers or lesser when NMP is used. Ultrasonication proved to be effective in propagating the self-repulsion of negatively-charged functional groups. Moreover, small amounts of graphene quantum dots were observed, illustrating the potential of producing various graphene materials via a single-step method. Whilst this study has only investigated utilising miscanthus, the current findings are promising and could expand the potential of producing good quality graphene materials from renewable sources via green synthesis routes.     

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)     

Triple‐resonant two‐phonon Raman scattering in graphene

The triple‐resonant (TR) second‐order Raman scattering mechanism in graphene is re‐examined. It is shown that the magnitude of the TR contribution to the photon‐G′ mode coupling function in graphene is one order of magnitude larger than the widely accepted two‐resonant coupling. Enhancement of the order of 100 in the Raman intensity, with respect to the usual double‐resonant model, is found for the G′ band in graphene, and is expected in the related sp²‐based carbon materials, as well. Copyright © 2011 John Wiley & Sons, Ltd.     

Measurement of the absolute Raman scattering cross section of the 1584‐cm−1 band of benzenethiol and the surface‐enhanced Raman scattering cross section enhancement factor for femtosecond laser‐nanostructured substrates

The absolute Raman scattering cross section (σ_RS) for the 1584‐cm⁻¹ band of benzenethiol at 897 nm (1.383 eV) has been measured to be 8.9 ± 1.8 × 10⁻³⁰ cm² using a 785‐nm pump laser. A temperature‐controlled, small‐cavity blackbody source was used to calibrate the signal output of the Raman spectrometer. We also measured the absolute surface‐enhanced Raman scattering cross section (σ_SERS) of benzenethiol adsorbed onto a silver‐coated, femtosecond laser‐nanostructured substrate. Using the measured values of 8.9 ± 1.8 × 10⁻³⁰ and 6.6 ± 1.3 × 10⁻²⁴ cm² for σ_RS and σ_SERS respectively, we calculate an average cross‐section enhancement factor (EF) of 0.8 ± 0.3 × 10⁶. Copyright © 2009 John Wiley & Sons, Ltd.     

Rapid synthesis and characterization of ultra‐thin shell Au@SiO2 nanorods with tunable SPR for shell‐isolated nanoparticle‐enhanced Raman spectroscopy (SHINERS)

The recently reported shell‐isolated nanoparticle‐enhanced Raman spectroscopy (SHINERS) is considered as the next generation of advanced spectroscopy for its surface and molecular generality. With the aim to utilize the virtues of shell‐isolated strategy and advance the SHINERS technique, we introduce a silane‐based rapid synthesis method of silica‐coating Au nanorods (Au@SiO₂ NRs) with manoeuvrable ultra‐thin shell and tunable SPR. The results demonstrate that the SPR of Au NRs could be optimized to obtain large Raman enhancement using either 633 nm or 785 nm laser. Differing from previously reported Au@SiO₂ NRs synthesis method, we can tune the silica shell thickness within several nanometers to maximize the Raman signal while effectively eliminating the exterior interference. And this advanced synthesis method has also significantly reduced the silica‐coating time from one day to ca. 1 h. This method as a new development of SHINERS technique has successfully got enhanced signal in solution Raman tests of malachite green, giving a great potential to be extended to in‐situ measurement for daily life detection. Copyright © 2013 John Wiley & Sons, Ltd.     

Layer‐dependent resonant Raman scattering of a few layer MoS2

We report resonant Raman scattering of MoS₂ layers comprising of single, bi, four and seven layers, showing a strong dependence on the layer thickness. Indirect band gap MoS₂ in bulk becomes a direct band gap semiconductor in the monolayer form. New Raman modes are seen in the spectra of single‐ and few‐layer MoS₂ samples which are absent in the bulk. The Raman mode at ~230 cm⁻¹ appears for two, four and seven layers. This mode has been attributed to the longitudinal acoustic phonon branch at the M point (LA(M)) of the Brillouin zone. The mode at ~179 cm⁻¹ shows asymmetric character for a few‐layer sample. The asymmetry is explained by the dispersion of the LA(M) branch along the Γ‐M direction. The most intense spectral region near 455 cm⁻¹ shows a layer‐dependent variation of peak positions and relative intensities. The high energy region between 510 and 645 cm⁻¹ is marked by the appearance of prominent new Raman bands, varying in intensity with layer numbers. Resonant Raman spectroscopy thus serves as a promising non invasive technique to accurately estimate the thickness of MoS₂ layers down to a few atoms thick. Copyright © 2012 John Wiley & Sons, Ltd.     

High‐Responsivity Photodetectors Based on Formamidinium Lead Halide Perovskite Quantum Dot–Graphene Hybrid

Highly performance photodetector requires a wide range of responses of the incident photons and converts them to electrical signals efficiently. Here, a photodetector based on formamidinium lead halide perovskite quantum dots (e.g., FAPbBr₃ QDs)–graphene hybrid, aiming to take the both advantages of the two constituents. The FAPbBr₃ QD–graphene layer not only benefits from the high mobility and wide spectral absorption of the graphene material but also from the long charge carrier lifetime and low dark carrier concentration of the FAPbBr₃ QDs. The photodetector based on FAPbBr₃ QD–graphene hybrid exhibits a broad spectral photoresponse ranging from 405 to 980 nm. A photoresponsivity of 1.15 × 10⁵AW⁻¹ and an external quantum efficiency as high as 3.42 × 10⁷% are obtained under an illumination power of 3 µW at 520 nm wavelength. In detail, a high responsivity is achieved in 405–538 nm, while a relatively low but fast response is observed in 538–980 nm. The photoelectric conversion mechanism of this hybrid photodetector is investigated in the view of built‐in electric field from the QD–graphene contact which improves the photoconductive gain.     

Strained graphene as a local probe for plasmon‐enhanced Raman scattering by gold nanostructures

We investigate with Raman spectroscopy how gold nanostructures of different shape, size and geometry locally modify a graphene cover layer through strain. The resulting phonon softening translates into frequency downshifts of up to 85 cm^–1 for the 2D‐mode of graphene. With spatially resolved and excitation dependent Raman measurements we demonstrate that the downshifted Raman peaks exclusively arise from strained graphene subject to plasmonic enhancement by the nanostructures. The signals arise from an area well below the size of the laser spot. They serve as a local probe for the interaction between graphene and intense light fields. (© 2013 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

The interaction of iron and metal‐free meso‐tetra (4‐sulfonatophenyl) porphines with CVD graphene: a comparative Raman spectroscopy study

In this work, a non‐covalent interaction of iron and metal‐free meso‐tetra (4‐sulfonatophenyl) porphines (FeTPPS and TPPS, respectively) with high‐quality single‐layer graphene is studied by Raman spectroscopy. Such a kind of graphene functionalization is promising for a development of novel optoelectronic devices and sensors. Our results show that the central metal atom of porphyrin macrocycle, iron particularly, plays an important role in the integrity of FeTPPS on graphene surface; however, the predicted Raman enhancement is not significant. The interaction of metal‐free TPPS with graphene leads to the deprotonation of TPPS molecules and higher Raman enhancement values. Moreover, initially deprotonated TPPS solutions after the adsorption onto the graphene surface demonstrate the appearance of new Raman bands and significantly enhanced Raman signals. We propose that a strong interaction between deprotonated TPPS and graphene is realized through pyrrole and desulfonated phenyl rings of closely located planar TPPS molecules on the graphene surface. The results show that both the protonation of porphyrin macrocycle and the existence of central metal atom are crucial for a formation of nanocomposites with defined electronic properties. Copyright © 2014 John Wiley & Sons, Ltd.     

Molecular structure,spectroscopic (FTIR,FTIR gas phase,FT‐Raman) first‐order hyperpolarizability and HOMO–LUMO analysis of 4‐methoxy‐2‐methyl benzoic acid

Vibrational spectral analysis was carried out for 4‐methoxy‐2‐methyl benzoic acid (4M2MBA) by using Fourier transform infrared (FT‐IR) (solid, gas phase) and FT‐Raman spectroscopy in the range of 400–4000 and 10–3500 cm⁻¹ respectively. The effects of molecular association through O H···O hydrogen bonding have been described by the single dimer structure. The theoretical computational density functional theory (DFT) and Hatree‐Fock (HF) method were performed at 6–311++G(d,p) levels to derive the equilibrium geometry, vibrational wavenumbers, infrared intensities and Raman scattering activities. The scaled theoretical wavenumbers were also shown to be in good agreement with experimental data. The first‐order hyperpolarizability (β₀) of this novel molecular system and related properties (β, α₀ and Δα) of 4M2MBA are calculated using the B3LYP/cc‐pvdz basis set, based on the finite‐field approach. A detailed interpretation of the infrared and Raman spectra of 4M2MBA is reported. The theoretical spectrograms for FT‐IR and FT‐Raman spectra of the title molecule were also constructed and compared with the experimental one. Copyright © 2010 John Wiley & Sons, Ltd.     

”Eau de graphene” from a KC8 graphite intercalation compound prepared by a simple mixing of graphite and molten potassium

We synthesized KC₈ by simply mixing molten potassium and graphite at 180 °C under inert atmosphere. The KC₈ shows typical shiny bronze color, Raman characteristics and XRD pattern of an efficiently intercalated stage 1 GIC, and is of sufficient quality to produce fully exfoliated graphenide solutions in tetrahydrofuran (THF) and subsequently single layer graphene in water as ”eau de graphene” (EdG). The evolution of absorption and Raman spectroscopic signatures of the EdG as a function of processing conditions give key indications on the number of layers of the graphene flakes dispersed in EdG. (© 2016 WILEY‐VCH Verlag GmbH &Co. KGaA, Weinheim)     

Substrate‐Induced Synthesis of Nitrogen‐Doped Holey Graphene Nanocapsules for Advanced Metal‐Free Bifunctional Electrocatalysts

The development of efficient metal‐free electrocatalysts for oxygen electrocatalysis is of great significance for various energy conversion devices. Herein, novel nitrogen‐doped holey graphene nanocapsules (NHGNs) are reported prepared by self‐assembly of graphene oxide nanosheets on the surface of amino‐functionalized silica template and NH₃ activation with simultaneously enhanced nitrogen doping and etching of nanopores in graphene, followed by template etching. The silica template is demonstrated to show a substrate‐enhanced effect on nitrogen doping and etching of nanopores in graphene based on density functional theory calculations. Benefiting from the large surface area, unique pore distribution, and high surface functionality of nitrogen doping, the resulting NHGNs exhibit superior bifunctional electrocatalytic activity and durability for both oxygen reduction reaction and oxygen evolution reaction, which is similar to that of the commercial Pt/C and RuO₂ electrocatalysts, respectively. This work presents an advance in developing new nitrogen‐doped graphene species for highly efficient metal‐free electrocatalysis.     

Layer‐by‐layer AuCl3 doping of stacked graphene films

We investigated the effect of layer‐by‐layer AuCl₃ doping on the electrical and optical properties of stacked graphene films. Graphene grown by the chemical‐vapor deposition method on a Cu‐foil was chemically doped by AuCl₃ solution with a concentration of 20 mM. Eight different configurations were prepared and analyzed by using four‐point probe measurements, optical transmittance measurements, scanning electron microscopy, and micro‐Raman spectroscopy to compare the optical and electrical characteristics of the different graphene samples. In our study, the top‐layer doping method was very effective because better performances considering both sheet resistance and optical transmittance were observed from the configurations with the top‐layer doped. (© 2014 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)