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2D hybrid organic–inorganic perovskites are valued in optoelectronic applications for their tunable bandgap and excellent moisture and irradiation stability. These properties stem from both the chemical composition and crystallinity of the layer formed. Defects in the lattice, impurities, and crystal grain boundaries generally introduce trap states and surface energy pinning, limiting the ultimate performance of the perovskite; hence, an in‐depth understanding of the crystallization process is indispensable. Here, a kinetic and thermodynamic study of 2D perovskite layer crystallization on transparent conductive substrates are provided—fluorine‐doped tin oxide and graphene. Due to markedly different surface structure and chemistry, the two substrates interact differently with the perovskite layer. A time‐resolved grazing‐incidence wide‐angle X‐ray scattering (GIWAXS) is used to monitor the crystallization on the two substrates. Molecular dynamics simulations are employed to explain the experimental data and to rationalize the perovskite layer formation. The findings assist substrate selection based on the required film morphology, revealing the structural dynamics during the crystallization process, thus helping to tackle the technological challenges of structure formation of 2D perovskites for optoelectronic devices. [ABSTRACT FROM AUTHOR]

Surface‐enhanced Raman spectroscopy is a valuable tool for inspection of trace concentrations of various molecules; hence, this method has a great potential for characterization of functionalised graphene. However, to make this method a reliable analytical tool, the influence of the metal–graphene interactions on Raman spectra of the graphene must be understood. Here, the surface‐enhanced Raman spectra of exfoliated single‐layer graphene covered with gold or silver thin layers were studied. The metal–graphene interactions resulted in the broadening of the G mode and the 2D mode of graphene. A change of the 2D mode dispersion was also observed. The effects were found to be weaker for the silver layer; however, the Raman signal enhancement of the graphene features was found to be significantly stronger in case of the silver layer. Various scenarios of the observed effects are discussed: graphene–plasmon interaction, charge transfer between the metal and graphene, and selective enhancement at the lattice and topographic defects. Copyright © 2017 John Wiley & Sons, Ltd. [ABSTRACT FROM AUTHOR]

Twisted bilayer graphene, a promising material for optoelectronic applications, offers several effective routes toward electronic structure engineering, for example via tuning the relative orientation of the two graphene layers or by varying the distance between them. The so-called collapsed wrinkles, standing, or folded, combine both of these properties. The mutual lattice alignment of the sidewalls of wrinkle leads to formation of a graphene superlattice. Using Raman spectroscopy, we show that the interaction between the sidewalls of the wrinkle and thereby the properties of the twisted multilayer wrinkle can be well tuned by temperature. [ABSTRACT FROM AUTHOR]

We have investigated the temperature evolution (100-300 K) of the 2D′ mode of isotopically labelled double layer graphene using in situ Raman spectro-microscopy. The strain values obtained from the 2D′ mode analysis were confronted with those obtained from the correlation analysis of the G and 2D modes. We find that the strain landscape derived from the 2D′ mode is comparable with the one obtained from the G and 2D modes with similar temperature evolution for both layers. However; the magnitude of the strain evaluated by the two different approaches does not fully coincide, which can be attributed to the admixture of uniaxial strain to the dominant biaxial component. In addition, we observed that the 2D′ is also sensitive to the strain inhomogeneities at nanometre scale. The 2D′ mode thus serves as a valuable complementary tool for detection of the character of strain in single and double layer graphene down to helium temperatures. [ABSTRACT FROM AUTHOR]

Mechanical properties of graphene prepared by chemical vapour deposition (CVD) are not easily comparable to the properties of nearly perfect graphene prepared by mechanical cleavage. In this work, we attempt to investigate the mechanical performance of CVD graphene (simply supported or embedded in polymer matrix), transferred by two different techniques, under uniaxial loading with simultaneous in situ monitoring by Raman microspectroscopy. The level of charge transfer doping and strain is assessed using the vector analysis modified for uniaxial strain. The strain distribution across the samples varies significantly, owing to the growth and transfer process, which induces wrinkles and faults in the CVD graphene. In simply supported specimens, the stress transfer efficiency is generally very low and the changes in Raman spectra are dominated by variations in the charge transfer originating from the realignment of the domains on the substrate upon the application of strain. In contrast, samples covered with an additional polymer layer exhibit an improved stress transfer efficiency, and the alterations of charge doping levels are negligible. In fully embedded specimens, the variations in stress transfer efficiencies are caused by the size of the effective graphene domains defined by cracks, folds and/or wrinkles. [ABSTRACT FROM AUTHOR]

Fluorination modifies the electronic properties of graphene, and thus it can be used to provide material with on-demand properties. However, the thermal stability of fluorinated graphene is crucial for any application in electronic devices. Herein, X-ray photoelectron spectroscopy (XPS), temperature-programmed desorption (TPD), and Raman spectroscopy were used to address the impact of the thermal treatment on fluorinated graphene. The annealing, at up to 700 K, caused gradual loss of fluorine and carbon, as was demonstrated by XPS. This loss was associated with broad desorption of CO and HF species, as monitored by TPD. The minor single desorption peak of CF species at 670 K is suggested to rationalize defect formation in the fluorinated graphene layer during the heating. However, fluorine removal from graphene was not complete, as some fraction of strongly bonded fluorine can persist despite heating to 1000 K. The role of intercalated H2O and OH species in the defluorination process is emphasised. [ABSTRACT FROM AUTHOR]

Graphene is a material of unmatched properties and eminent potential in disciplines ranging from physics, to chemistry, to biology. Its advancement to applications with a specific function requires rational design and fine tuning of its properties, and covalent introduction of various substituents answers this requirement. We challenged the obstacle of non-trivial and harsh procedures for covalent functionalization of pristine graphene and developed a protocol for mild nucleophilic introduction of organic groups in the gas phase. The painstaking analysis problem of monolayered materials was addressed by using surface-enhanced Raman spectroscopy, which allowed us to monitor and characterize in detail the surface composition. These deliverables provide a toolbox for reactivity of fluorinated graphene under mild reaction conditions, providing structural freedom of the species to-be-grafted to the single-layer graphene. [ABSTRACT FROM AUTHOR]

Using Raman spectral-mapping, we investigated in situ temperature dependence (100-300 K) of strain and doping of isotopically labelled double-layer graphene, which allows us to simultaneously and independently follow the strain and doping in both layers of the double layer. We find that the strain in both layers of the double-layer graphene is almost the same and becomes larger when the temperature is decreased. The bottom graphene layer, in contact with the Si/SiO2 substrate has a stronger doping than the top graphene layer which is in contact with the graphene bottom layer. We also observed that the spatial distribution of the temperature-induced strain and doping is not homogeneously distributed within the substrate. [ABSTRACT FROM AUTHOR]

Fluorination of graphene opens up a bandgap, which creates opportunities for optoelectronics, and also paves the way for the creation of extremely thin insulating layers, which can be important for applications in devices. However, in spite of many interesting features offered by, for example, unequally doped layers in multilayered systems, most of the work has concerned the fluorination of graphene monolayers. Here, the fluorination process of graphene bilayers is investigated through high-resolution Raman mapping followed by analysis of more than 10 000 spectra of bilayer graphene. Isotopically labeled bilayers are used, allowing each individual layer in bilayer graphene to be addressed unambiguously. The fluorinated graphene is prepared through exposure to XeF2. Monolayer graphene is found to be significantly more sensitive to fluorination than bilayer graphene. Through comparison of the D/G area ratio and the position of the G band for turbostratic and Bernal stacked (AB) bilayers, it is found that the fluorination process is more effective for turbostratic than for AB-stacked bilayer graphene. The fluorination changes the electronic structure similarly for the top and bottom layers in turbostratic bilayers. However, the top layer is more sensitive than the bottom layer in AB-stacked bilayers. [ABSTRACT FROM AUTHOR]

Chemical vapor deposition on copper is currently one of the most prospective preparation techniques of large area graphene films. There are still many undisclosed questions regarding the growth process and its conditions. The adlayer formation belongs to those, not only because of the need of uniform monolayers, but also due to the increasing demand for high-coverage bilayers. The combination of Raman spectroscopy, isotopic labeling and functionalization using fluorine can provide the necessary insight into the CVD growth process. The disorder degree caused by the fluorination is unambiguously distinguished by Raman mapping and evaluation of more than 1000 spectra for the individual layers, suggesting the shielding of the 13C adlayer underneath the continuous 12C top layer. High resolution Raman map of the D/G peak intensity ratio of the 12C component in the 13C/12C region of the studied isotopically labeled bilayer graphene and a scheme depicting the adlayer growth at the bottom. [ABSTRACT FROM AUTHOR]

The influence of the gradual polarization of a ferroelectric polymer on isotopically labeled bi-layer graphene has been investigated by Raman spectroscopy. The Raman frequencies of the 13C graphene modes are downshifted with respect to the Raman frequencies of 12C graphene, which enabled us to study the individual layer components of bi-layer graphene. The polarization of the ferroelectric polymer has a similar influence on the electron and hole concentration at the 13C graphene and the 12C graphene layer despite the 13C graphene layer being only in direct contact with the ferroelectric polymer. In the Raman experiment the doping of graphene was confirmed by a similar frequency shift of the G modes and similar changes in the intensities of the G′ modes during the electrochemical charging of the ferroelectric polymer. [ABSTRACT FROM AUTHOR]

The presented study focuses on the appearance of defects in graphene grown on Cu(100), (110) and (111) single crystals by low pressure chemical vapour deposition (CVD) from methane with either 12C or 13C isotope. By analysing the Raman D band, we assume the defects originate mainly on boundaries between tilted graphene domains grown on Cu(100) and Cu(110), in contrast to graphene grown on Cu(111), where the D band is scarce in the Raman spectra. The other main source of defects may come from graphene edges around small adlayers randomly present in the samples. [ABSTRACT FROM AUTHOR]

A mass-related symmetry breaking in isotopically labeled bilayer graphene (2LG) was investigated during in-situ electrochemical charging of AB stacked (AB-2LG) and turbostratic (t-2LG) layers. The overlap of the two approaches, isotopic labeling and electronic doping, is powerful tool and allows to tailor, independently and distinctly, the thermal-related and transport-related phenomena in materials, since one can impose different symmetries for electrons and phonons in these systems. Variations in the system's phonon self-energy renormalizations due to the charge distribution and doping changes could be analyzed separately for each individual layer. Symmetry arguments together with first-order Raman spectra show that the single layer graphene (1LG), which is directly contacted to the electrode, has a higher concentration of charge carriers than the second graphene layer, which is not contacted by the electrode. These different charge distributions are reflected and demonstrated by different phonon self-energy renormalizations of the G modes for AB-2LG and for t-2LG. [ABSTRACT FROM AUTHOR]

The influence of differently treated graphene on human osteoblasts after 2 h of incubation with regard to the presence/absence of fetal bovine serum (FBS) was investigated. Cell adhesion plays an important role in further cell fate and it is influenced by cell surrounding. It was found that treatment of graphene (by hydrogen or oxygen) does not play role in number of cells which adhere to substrate after 2 h of incubation. However, it is important for cell size - cells are larger on the hydrogen treated graphene than on the oxygen treated graphene. The presence of FBS is crucial for a type of interaction between cells and their substrate - in the presence of FBS, interactions are mediated by specific proteins and thus formation of focal adhesions (FAs) can occur. However, in the absence of FBS, a contact is carried out by non-specific bonds without FAs formation. It was observed that cells on graphene samples without FBS have star-like shape and larger area in contrast to cells adhering with FBS which have round shape and are smaller. [ABSTRACT FROM AUTHOR]

The influence of single-layer graphene produced by chemical vapor deposition on human osteoblast cells under different conditions was studied. Measurements probed the ability of cells to adhere and proliferate on graphene compared with SiO2/Si substrates and standard tissue culture plastic when cells were incubated for the first 2 h in the presence or the absence of fetal bovine serum (FBS), thus influencing the initial, direct interaction of cells with the substrate. It was found that after 48 h of human osteoblast incubation on graphene films, there were a comparable number of cells of a similar size irrespective of the presence or the absence of serum proteins. On the other hand, a strong initial influence through the presence of FBS proteins on cell number and cell size was observed in the case of the SiO2/Si substrate and control plastic. Thus, our study showed that the initial presence/absence of FBS in the medium does not determine cell fate in the case of a graphene substrate, which is very unusual and different from the behavior of cells on other materials. © 2012 Wiley Periodicals, Inc. J Biomed Mater Res Part A: 100A:3001-3007, 2012. [ABSTRACT FROM AUTHOR]

The Raman spectra of two-layered graphene on a silicon substrate were studied in the temperature range from 298 to 1073 K in an inert atmosphere. Isotopic engineering was used to fabricate two-layer graphene specimens containing 13C atoms in the top layer and 12C atoms in the bottom layer, which allowed the behavior of each particular layer to be distinguished as a function of temperature. It is demonstrated that the top layer exhibits much lower Raman temperature coefficients than the bottom one for both the G and the G′ modes. We suggest that the changes in the Raman spectra of graphene observed during thermal cycling are predominantly caused by a superposition of two effects, namely, the mechanical stress in graphene exerted by the substrate and the intrinsic changes in the graphene lattice caused by the temperature itself. The top graphene layer is proposed to be more relaxed than the bottom graphene layer and thus reflects almost exclusively the temperature variations as a freestanding graphene layer would. [ABSTRACT FROM AUTHOR]

Abstract: The growth of graphene during Cu-catalyzed chemical vapor deposition was studied using 12CH4 and 13CH4 precursor gasses. We suggest that the growth begins by the formation of a multilayer cluster. This seed increases its size but the growth speed of a particular layer depends on its proximity to the copper surface. The layer closest to the substrate grows fastest and thus further limits the growth rate of the upper layers. Nevertheless, the growth of the upper layers continues until the copper surface is completely blocked. It is shown that the upper layers can be removed by modification of the conditions of the growth by hydrogen etching. [Copyright &y& Elsevier]

A detailed understanding of graphene properties both in its neutral and doped states is an important prerequisite for applications of this new material in electronic devices. We used electrochemical doping to study the influence of charge on isotopically labeled two-layer graphene. No change of the G mode intensity was observed at electrode potentials between −1.5 and 1 V. At high positive electrode potentials (>1 V) we observed enhancement of the G mode intensity due to partial removal of interfering resonant transitions. Hence, we confirmed that in case of absence of the enhancement due to a specific orientation between the two layers the Raman spectroelectrochemistry of non-stacked two-layer graphene mimics that of one-layer graphene. [ABSTRACT FROM AUTHOR]

The control of graphene's topography at the nanoscale level opens up the possibility to greatly improve the surface functionalization, change the doping level or create nanoscale reservoirs. However, the ability to control the modification of the topography of graphene on a wafer scale is still rather challenging. Here we present an approach to create well-defined nanowrinkles on a wafer scale using nitrocellulose as the polymer to transfer chemical vapor deposition grown graphene from the copper foil to a substrate. During the transfer process, the complex tertiary nitrocellulose structure is imprinted into the graphene area layer. When the graphene layer is put onto a substrate this will result in a well-defined nanowrinkle pattern, which can be subsequently further processed. Using atomic force and Raman microscopy, we characterized the generated nanowrinkles in graphene. [ABSTRACT FROM AUTHOR]

Controlled wrinkling of single-layer graphene (1-LG) at nanometer scale was achieved by introducing monodisperse nanoparticles (NPs), with size comparable to the strain coherence length, underneath the 1-LG. Typical fingerprint of the delaminated fraction is identified as substantial contribution to the principal Raman modes of the 1-LG (G and G'). Correlation analysis of the Raman shift of the G and G' modes clearly resolved the 1-LG in contact and delaminated from the substrate, respectively. Intensity of Raman features of the delaminated 1-LG increases linearly with the amount of the wrinkles, as determined by advanced processing of atomic force microscopy data. Our study thus offers universal approach for both fine tuning and facile quantification of the graphene topography up to ~60% of wrinkling. [ABSTRACT FROM AUTHOR]

The special properties of graphene offer immense opportunitiesfor applications to many scientific fields, as well as societal needs,beyond our present imagination. One of the important features of grapheneis the relatively simple tunability of its electronic structure, anasset that extends the usability of graphene even further beyond presentexperience. A direct injection of charge carriers into the conductionor valence bands, that is, doping, represents a viable way of shiftingthe Fermi level. In particular, electrochemical doping should be themethod of choice, when higher doping levels are desired and when afirm control of experimental conditions is needed. [ABSTRACT FROM AUTHOR]

Due to weak light-matter interaction, standard chemical vapor deposition (CVD)/exfoliated single-layer graphene-based photodetectors show low photoresponsivity (on the order of mA/W). However, epitaxial graphene (EG) offers a more viable approach for obtaining devices with good photoresponsivity. EG on 4H–SiC also hosts an interfacial buffer layer (IBL), which is the source of electron carriers applicable to quantum optoelectronic devices. We utilize these properties to demonstrate a gate-free, planar EG/4H–SiC-based device that enables us to observe the positive photoresponse for (405–532) nm and negative photoresponse for (632–980) nm laser excitation. The broadband binary photoresponse mainly originates from the energy band alignment of the IBL/EG interface and the highly sensitive work function of the EG. We find that the photoresponsivity of the device is > 10 A/W under 405 nm of power density 7.96 mW/cm2 at 1 V applied bias, which is three orders of magnitude greater than the obtained values of CVD/exfoliated graphene and higher than the required value for practical applications. These results path the way for selective light-triggered logic devices based on EG and can open a new window for broadband photodetection. [Display omitted] [ABSTRACT FROM AUTHOR]

Wrinkles are one possible rugae phase that can be observed in two-dimensional crystals. Those topographic features, with dimensions ranging from nanometers to micrometers, can be identified in single-layer graphene transferred onto various substrates. When the temperature of the sample is varied, the different thermal expansion coefficients of graphene and its supporting substrate lead to reconstruction of the graphene's topography, which results in spatially inhomogeneous strain and doping. Using Raman spectral mapping, we investigated in situ the temperature dependence (50–300 K) of topographic features in a graphene monolayer grown by chemical vapor deposition (CVD) and transferred onto a Si/SiO 2 substrate. We find that the temperature-induced strain follows the temperature variation of the cubic lattice parameter of the Si substrate. Furthermore, the temperature-induced strain has an unambiguous relation to the topographic reconstruction of the graphene monolayer. We focused on the behavior of two types of rugae : 1) large wrinkles with dimensions that are larger than the laser spot size and can be traced directly and 2) nanoscale wrinkles and ripples, which can be identified by changes in the parameters of the principal Raman active bands of the graphene monolayer. In addition, we observed that temperature-induced reconstructions are accompanied by local variation of doping. [ABSTRACT FROM AUTHOR]

Correct defect quantification in graphene samples is crucial both for fundamental and applied research. Raman spectroscopy represents the most widely used tool to identify defects in graphene. However, despite its extreme importance the relation between the Raman features and the amount of defects in multilayered graphene samples has not been experimentally verified. In this study we intentionally created defects in single layer graphene, turbostratic bilayer graphene and Bernal stacked bilayer graphene by oxygen plasma. By employing isotopic labelling, our study reveals substantial differences of the effects of plasma treatment on individual layers in bilayer graphene with different stacking orders. In addition Raman spectroscopy evidences scattering of phonons in the bottom layer by defects in the top layer for Bernal-stacked samples, which can in general lead to overestimation of the number of defects by as much as a factor of two. [ABSTRACT FROM AUTHOR]

The interaction between graphene and metals represents an important issue for the large-area preparation of graphene, graphene transfer and the contact quality in graphene devices. We demonstrate a simple method for estimating and manipulating the level of interaction between graphene and copper single crystals through heat treatment, at temperatures from 298 K to 1073 K. We performed in-situ Raman spectroscopy showing Cu face-specific behavior of the overlying graphene during the heat treatment. On Cu(1 1 1) the interaction is consistent with theoretical predictions and remains stable, whereas on Cu(1 0 0) and Cu(1 1 0), the initially very weak interaction and charge transfer can be tuned by heating. Our results also suggest that graphene grown on Cu(1 0 0) and Cu(1 1 0) is detached from the copper substrate, thereby possibly enabling an easier graphene transfer process as compared to Cu(1 1 1). [ABSTRACT FROM AUTHOR]

Abstract: In recent years graphene has become a very popular material in biomedicine studies. Its potential use varies from implant coating to biosensor fabrication. For application of graphene in this field an understanding of the interactions between the substrate and various types of cells is necessary. This work is focused on early behavior of a human osteoblastic cell line on graphene sheets with different surface treatments (hydrogen or oxygen). The cell adhesion was investigated at an early time point of incubation (2h) and a late one (48h). Studies were performed in the presence and the absence of fetal bovine serum (FBS), as the proteins contained in the FBS play a crucial role in the cell adhesion. Despite the fact that the graphene substrates used had a comparable surface topography, their different wettability caused an unexpected inverse effect on cell adhesion and growth. The single-layer graphene treated with hydrogen (1-LG, hydrophobic surface) enhanced cell proliferation, whereas the single-layer graphene treated with oxygen (1-LG-O, hydrophilic surface) caused cell response as good as tissue-culture plastic specially treated for cell cultivation. [Copyright &y& Elsevier]

Abstract: We present a comprehensive study of graphene grown by chemical vapor deposition on copper single crystals with exposed (100), (110) and (111) faces. Direct examination of the as-grown graphene by Raman spectroscopy using a range of visible excitation energies and microRaman mapping shows distinct strain and doping levels for individual Cu surfaces. Comparison of results from Raman mapping with X-ray diffraction techniques and atomic force microscopy shows it is neither the crystal quality nor the surface topography responsible for the specific strain and doping values, but it is the Cu lattice orientation itself. We also report an exceptionally narrow Raman 2D band width caused by the interaction between graphene and metallic substrate. The appearance of this extremely narrow 2D band with full-width-at-half maximum (FWHM) as low as 16cm−1 is correlated with flat and undoped regions on the Cu(100) and (110) surfaces. The generally compressed (∼0.3% of strain) and n-doped (Fermi level shift of ∼250meV) graphene on Cu(111) shows the 2D band FWHM minimum of ∼20cm−1. In contrast, graphene grown on Cu foil under the same conditions reflects the heterogeneity of the polycrystalline surface and its 2D band is accordingly broader with FWHM >24cm−1. [Copyright &y& Elsevier]

The growth of large-area bilayergraphene has been of technologicalimportance for graphene electronics. The successful application ofgraphene bilayers critically relies on the precise control of thestacking orientation, which determines both electronic and vibrationalproperties of the bilayer system. Toward this goal, an effective characterizationmethod is critically needed to allow researchers to easily distinguishthe bilayer stacking orientation (i.e., AB stacked or turbostratic).In this work, we developed such a method to provide facile identificationof the stacking orientation by isotope labeling. Raman spectroscopyof these isotopically labeled bilayer samples shows a clear signatureassociated with AB stacking between layers, enabling rapid differentiationbetween turbostratic and AB-stacked bilayer regions. Using this method,we were able to characterize the stacking orientation in bilayer graphenegrown through Low Pressure Chemical Vapor Deposition (LPCVD) withenclosed Cu foils, achieving almost 70% AB-stacked bilayer graphene.Furthermore, by combining surface sensitive fluorination with suchhybrid 12C/13C bilayer samples, we are ableto identify that the second layer grows underneath the first-grownlayer, which is similar to a recently reported observation. [ABSTRACT FROM AUTHOR]

Abstract: In this paper we present the results of morphological, mechanical and electrical investigation of the properties of prepared graphene flakes and graphene-based quantum Hall devices. AFM imaging allowed us to identify the local imperfections and unintentional modifications of the graphene sheets which had caused severe deterioration of the device electrical performance. Utilizing the NanoSwing imaging method, based on the time-resolved tapping mode, we could observe non-homogeneities of the structural and mechanical properties. We also diagnosed the device under working conditions by Kelvin probe microscopy and detected its local electric field distribution. [Copyright &y& Elsevier]

Abstract: The biocompatibility of large single layer graphene produced by chemical vapour deposition was investigated using human osteoblasts and mesenchymal stromal cells. The study was focused on cellular adhesion, morphology and the ability to proliferate on graphene substrates. It was found that both of the cell types which were tested adhered and proliferated better when cultured on graphene films than on a SiO2 substrate. [ABSTRACT FROM AUTHOR]

Twisted bilayer graphene (tBLG) represents a family of unique materials with optoelectronic properties tuned by the rotation angle between the two layers. The presented work shows an additional way of tweaking the electronic structure of tBLG by modifying the interlayer distance, for example by a small uniaxial out-of-plane compression. We have focused on the optical transition energy, which shows a clear dependence on the interlayer distance, both experimentally and theoretically. [ABSTRACT FROM AUTHOR]