Search

Two-dimensional (2D) transition metal dichalcogenides are potential candidates for ultrathin solid-state lubricants in low-dimensional systems owing to their flatness, high in-plane mechanical strength, and low shear interlayer strength. Yet, the effects of surface topography and surface chemistry on the tribological properties of 2D layers are still unclear. In this work, we performed a comparative investigation of nanoscale tribological properties of ultra-thin highly-ordered PtSe2 layers deposited on the sapphire substrates with the in-plane and out-of-plane crystallographic orientation of the PtSe2 c-axis flakes, and epitaxial PtSe2 layers. PtSe2 c-axis orientation was found to has an impact on the nanotribological, morphological and electrical properties of PtSe2, in particular the change in the alignment of the PtSe2 flakes from vertical (VA) to horizontal (HA) led to the lowering of the coefficient of friction from 0.21 to 0.16. This observation was accompanied by an increase in the root-mean-square surface roughness from 1.0 to 1.7 nm for the HA and VA films, respectively. The epitaxial films showed lower friction caused by lowering adhesion when compared to other investigated films, whereas the friction coefficient was similar to films with HA flakes. The observed trends in nanoscale friction is attributed to a different distribution of PtSe2 structure.

Some of the distinct optical, catalytical, and electronic properties of few-layer MoS2 films arise from a specific orientation of the MoS2 layers. The growth of horizontally or vertically aligned M...

The development of defect analysis for inorganic semiconductors in the past century paved the way for the success story of today’s electronics. By analogy, defect analysis plays a critical role in ...

Recently, few-layer PtSe2 films have attracted significant attention due to their properties and promising applications in high-speed electronics, spintronics and optoelectronics. Until now, the transport properties of this material have not reached the theoretically predicted values, especially with regard to carrier mobility. In addition, it is not yet known which growth parameters (if any) can experimentally affect the carrier mobility value. This work presents the fabrication of horizontally aligned PtSe2 films using one-zone selenization of pre-deposited platinum layers. We have identified the Se : Pt ratio as a parameter controlling the charge carrier mobility in the thin films. The mobility increases more than twice as the ratio changes in a narrow interval around a value of 2. A simultaneous reduction of the carrier concentration suggests that ionized impurity scattering is responsible for the observed mobility behaviour. This significant finding may help to better understand the transport properties of few-layer PtSe2 films.

The crystallographic alignment of an ultra-thin MoS2 film strongly influences its properties and is, therefore, substantial for various applications. Developing the methods for controlled growth is...

Hybrid materials of π-conjugated molecular semiconductors combined with 2D materials exhibit significant potential for novel (opto)electronics. The overall characteristics of molecular growth, such...

Small π-conjugated organic molecules have attracted substantial attention in the past decade as they are considered as candidates for future organic-based (opto-)electronic applications. The molecular arrangement in the organic layer is one of the crucial parameters that determine the efficiency of a given device. The desired orientation of the molecules is achieved by a proper choice of the underlying substrate and growth conditions. Typically, one underlying material supports only one inherent molecular orientation at its interface. Here, we report on two different orientations of diindenoperylene (DIP) molecules on the same underlayer, i.e. on a few-layer MoS2 substrate. We show that DIP molecules adopt a lying-down orientation when deposited on few-layer MoS2 with horizontally oriented layers. In contrast, for vertically aligned MoS2 layers, DIP molecules are arranged in a standing-up manner. Employing in situ and real-time grazing-incidence wide-angle X-ray scattering (GIWAXS), we monitored the stress evolution within the thin DIP layer from the early stages of the growth, revealing different substrate-induced phases for the two molecular orientations. Our study opens up new possibilities for the next-generation of flexible electronics, which might benefit from the combination of MoS2 layers with unique optical and electronic properties and an extensive reservoir of small organic molecules.

Heterostructures composed of two-dimensional (2D) materials are already opening many new possibilities in such fields of technology as electronics and magnonics, but far more could be achieved if the number and diversity of 2D materials is increased. So far, only a few dozen 2D crystals have been extracted from materials that exhibit a layered phase in ambient conditions, omitting entirely the large number of layered materials that may exist in other temperatures and pressures. Here, we demonstrate how these structures can be stabilized in 2D van der Waals stacks under room temperature via growing them directly in graphene encapsulation by using graphene oxide as the template material. Specifically, we produce an ambient stable 2D structure of copper and iodine, a material that normally only occurs in layered form at elevated temperatures between 645 and 675 K. Our results establish a route to the production of more exotic phases of materials that would otherwise be difficult or impossible to stabilize for experiments in ambient., Comment: 15 pages, 6 figures, separate supplementary material included

The few-layer transition metal dichalcogenides (TMD) are an attractive class of materials due to their unique and tunable electronic, optical, and chemical properties, controlled by the layer number, crystal orientation, grain size, and morphology. One of the most commonly used methods for synthesizing the few-layer TMD materials is the chemical vapor deposition (CVD) technique. Therefore, it is crucial to develop in situ inspection techniques to observe the growth of the few-layer TMD materials directly in the CVD chamber environment. We demonstrate such an in situ observation on the growth of the vertically aligned few-layer MoS2 in a one-zone CVD chamber using a laboratory table-top grazing-incidence wide-angle X-ray scattering (GIWAXS) setup. The advantages of using a microfocus X-ray source with focusing Montel optics and a single-photon counting 2D X-ray detector are discussed. Due to the position-sensitive 2D X-ray detector, the orientation of MoS2 layers can be easily distinguished. The performance of the GIWAXS setup is further improved by suppressing the background scattering using a guarding slit, an appropriately placed beamstop, and He gas in the CVD reactor. The layer growth can be monitored by tracking the width of the MoS2 diffraction peak in real time. The temporal evolution of the crystallization kinetics can be satisfactorily described by the Avrami model, employing the normalized diffraction peak area. In this way, the activation energy of the particular chemical reaction occurring in the CVD chamber can be determined.

The last few decades faced on the fabrication of advanced engineering materials involving also different composites. Here, we report on the fabrication of few-layer molybdenum disulfide on top of thin polycrystalline diamond substrates with a high specific surface area. In the method, pre-deposited molybdenum coatings were sulfurized in a one-zone furnace at ambient pressure. As-prepared MoS2 layers were characterized by several techniques including grazing-incidence wide-angle X-ray scattering, atomic force microscopy, scanning electron microscopy, Raman spectroscopy and X-ray photoelectron spectroscopy. We found out that the initial thickness of Mo films determined the final c-axis crystallographic orientation of MoS2 layer as previously observed on other substrates. Even though it is well-known that Mo diffuses into diamond at elevated temperatures, the competing sulfurization applied effectively suppressed the diffusion and a chemical reaction between molybdenum and diamond. In particular, a Mo2C layer does not form at the interface between the Mo film and diamond substrate. The combination of diamond high specific surface area along with a controllable layer orientation might be attractive for applications, such as water splitting or water disinfection.

Recently, few-layer PtSe

Few-layer PtSe2 films are promising candidates for applications in high-speed electronics, spintronics and photodetectors. Reproducible fabrication of large-area highly crystalline films is, however, still a challenge. Here, we report the fabrication of epitaxially aligned PtSe2 films using one-zone selenization of pre-sputtered platinum layers. We have studied the influence of the growth conditions on the structural and electrical properties of the films prepared from Pt layers with different initial thickness. The best results were obtained for PtSe2 layers grown at elevated temperatures (600 {\deg}C). The films exhibit signatures for a long-range in-plane ordering resembling an epitaxial growth. Charge carrier mobility determined by Hall-effect measurements is up to 24 cm2/V.s in these films.

The principles of Raman scattering and the properties of phonons and electrons in graphene are reviewed. The focus is on Raman spectroscopy of mono- and bilayer graphene, particularly perturbations that have a significant influence on the Raman spectra. The distinct behaviour of Raman modes of different orders is discussed.

Small π-conjugated organic molecules have attracted substantial attention in the past decade as they are considered as candidates for future organic-based (opto-)electronic applications. The molecular arrangement in the organic layer is one of the crucial parameters that determine the efficiency of a given device. The desired orientation of the molecules is achieved by a proper choice of the underlying substrate and growth conditions. Typically, one underlying material supports only one inherent molecular orientation at its interface. Here, we report on two different orientations of diindenoperylene (DIP) molecules on the same underlayer, i.e. on a few-layer MoS

Oligothiophenes and their functionalized derivatives have been shown to be a viable option for high-performance organic electronic devices. The functionalization of oligothiophene-based materials allows further tailoring of their properties for specific applications. We have synthesized a new thiophene-based molecule 1-[5′-(2-naphthyl)-2,2′-bithiophen-5-yl]hexan-1-one (NCOH), and we have studied the optical and structural properties of NCOH thin films. NCOH is a highly substituted member of the oligothiophene family, designed to improve its molecular stacking, where the presence of an electron-withdrawing group enhances its electron transport capabilities. Employing in situ and time-resolved grazing-incidence wide-angle X-ray scattering (GIWAXS) measurements, we determined the NCOH thin film crystallographic structure and its evolution starting from the early stages of the film growth. We observed strong optical anisotropy resulting from a highly oriented crystallographic structure. Additionally, we investigated the substrate-induced changes of the molecular orientation utilizing the few-layer MoS2 with different orientations of the atomic layers. This study, with its primary focus on the fundamentally important n-type molecular semiconductor, contributes to the field of organic-based (opto-)electronics.

The frictional properties of two-dimensional (2D) materials are strongly dependent on the crystallographic orientation and number of layers. Although friction anisotropy caused by crystallographic orientations has been reported for various 2D materials, the flexural deformations and different defects complicate the insight into the mechanism of the in-plane friction anisotropy of these materials. Here, the anisotropic friction behavior between an atomic force microscopy tip and monolayer (ML) and few-layer (FL) MoSe2 flakes grown by CVD was performed by nanofriction measurements at different crystallographic directions, applied loads (FA), and tip scanning velocities. Our results reveal that the angular dependence of the friction forces is highly anisotropic for both types of MoSe2 flakes, and the anisotropy decreases with FA. Importantly, the anisotropies demonstrate opposite angular dependence for ML and FL MoSe2 flakes. As confirmed by the friction scanning velocity dependences, this difference seems to originate from different friction mechanisms for ML and FL MoSe2 flakes. The friction of FL flakes was predominantly influenced by atomic stick–slip motion. In contrast, ML MoSe2 is characterized by a coexistence of deformation-induced and atomic stick–slip motion. The experimental results presented here extend the understanding of the tribological properties of dry lubricants operating at the nanoscale.

Few-layer films of transition metal dichalcogenides have emerged as promising candidates for applications in electronics. Within this group of 2D materials, platinum diselenide (PtSe2) was predicted to be a compound with one of the highest charge carrier mobility. Recently, the successful integration of group III–V nitride semiconductors with NbNx-based superconductors was reported with a semiconductor transistor grown directly on a crystalline superconductor. This opens up the possibility of combining the macroscopic quantum effects of superconductors with the electronic, photonic, and piezoelectric properties of the semiconducting material. Here, we report on the fabrication of a few-layer PtSe2 film on top of an NbN substrate layer by selenization of pre-deposited 3 nm thick Pt layers. We found the selenization parameters preserving the chemical and structural integrity of both the PtSe2 and NbN films. The PtSe2 film alignment can be tuned by varying the nitrogen flow rate through the reaction chamber. The superconducting critical temperature of NbN is only slightly reduced in the optimized samples compared to pristine NbN. The carrier mobility in PtSe2 layers determined from Hall measurements is below 1 cm2/V s.

Graphene oxide is a complex material whose synthesis is still incompletely understood. To study the time evolution of structural and chemical properties of oxidized graphite, samples at different temporal stages of oxidation were selected and characterized through a number of techniques: X-ray photoelectron spectroscopy for the content and bonding of oxygen, X-ray diffraction for the level of intercalation, Raman spectroscopy for the detection of structural changes, electrical resistivity measurements for probing charge localization on the macroscopic scale, and scanning transmission electron microscopy for the atomic structure of the graphene oxide flakes. We found a nonlinear behavior of oxygen uptake with time where two concentration plateaus were identified: Uptake reached 20 at % in the first 15 min, and after 1 h a second uptake started, reaching a highest oxygen concentration of >30 at % after 2 h of oxidation. At the same time, the interlayer distance expanded to more than twice the value of graph...

Very thin MoS2 films were prepared on hexagonal GaN/AlGaN/GaN (0001) and Si (100) substrates from a stoichiometric target by a pulsed laser deposition. Combined results from Raman and X-ray reflectivity measurements have shown that the thinnest samples are 2–2.5 nm thick. The thickness increases with the number of laser pulses applied albeit no simple direct proportion between the two quantities has been observed. Concerning the stoichiometry, the distribution of Mo and S elements within as-deposited films is rather complex. The stoichiometric MoS2 is making-up only a part of the film. In spite of this, selected area electron diffraction studies have clearly confirmed that the films deposited on Si (100) are nanocrystalline and oriented perpendicularly to the substrate surface while an epitaxial growth of MoS2 films was observed on GaN/AlGaN/GaN (0001) substrates.

As long as the carbon family materials are pristine, powerful methods for their individual characterization have been established and well understood. However, as soon as these materials contain a mixture of sp2 and sp3 hybridized carbons, the spectroscopic characterization becomes challenging. In this paper we present on some aspects of multiply spectroscopic characterizations of sp2-sp3 carbon heterosystems, which can play an important role for their correct structural assignment. We study a composite of single-wall carbon nanotubes (SWCNT) with diamond nanoparticles and a SWCNT paper coated with nanocrystalline diamond films. The SWCNT paper serves here as a reference. We evaluate the surface free energy measured by contact angle technique, morphology imaged by scanning electron microscopy and the chemical composition determined by X-ray photoelectron spectroscopy, Raman spectroscopy at different excitation wavelengths and by attenuated total reflection (ATR) Fourier Transform Infrared spectroscopy (FTIR) using different ZnSe ATR prisms. We demonstrate that, because of the resonant nature of Raman process in SWCNTs, the particular conditions at Raman measurement (laser excitation wavelength, laser intensity) strongly influence the measured spectra of the sp2-sp3 hybridized heterosystems. We also show how the dimensions of the ATR prism affect the results of the FTIR measurements of the SWCNT papers.

Few-layer MoS

Few-layer MoS2 films are promising candidates for applications in numerous areas, such as photovoltaics, photocatalysis, nanotribology, lithium batteries, hydro-desulfurization catalysis and dry lubricants, especially due to their distinctive electronic, optical, and catalytic properties. In general, two alignments of MoS2 layers are possible – the horizontal and the vertical one, having different physicochemical properties. Layers of both orientations are conventionally fabricated by a sulfurization of pre-deposited Mo films. So far, the Mo thickness was considered as a critical parameter influencing the final orientation of MoS2 layers with horizontally and vertically aligned MoS2 grown from thin (1 nm) and thick (3 nm) Mo films, respectively. Here, we present a fabrication protocol enabling the growth of horizontally or vertically aligned few-layer MoS2 films utilizing the same Mo thickness of 3 nm. We show that the sulfur vapor is another parameter influencing the growth mechanism, where a sulfurization with higher sulfur vapor pressure leads to vertical MoS2 layers and slow sulfur evaporation results in horizontally aligned layers for a thicker Mo starting layer.

Ubiquitous carbon nanomaterials have great potential for emerging technologies, but they also pose a threat to human health at the end of their lifecycle, especially when they are introduced into waste or ground waters. Graphene field-effect transistors (GFET) and real time in situ confocal Raman microscopy (CRM) were employed to detect a submonolayer of carbon quantum dots (CQDs) in water. An effective GFET channel was formed on exfoliated and chemical vapor deposited (CVD) graphene. The adsorption of CQDs was monitored by measuring conductance changes in GFETs. The graphene channel was functionalized with (3-aminopropyl) triethoxysilane (APTES), which allowed for easy observation of a shift in the charge neutrality point (CNP) when the graphene channel was exposed to CQDs. The affinity of the CQD's carboxyl terminal groups to the aminofunctionalized channel enabled a highly sensitive CQD detection based on changes in the GFET conductivity. The adsorption of the CQDs induced a positive shift of the CNP with a limit of detection at concentrations of 239 ppm and 62 ppm for the exfoliated and CVD graphene, respectively. A supporting study of graphene functionalization and CQD adsorption was performed by real time in situ CRM, ellipsometry, and ex situ X-ray photoelectron spectroscopy. © 2019 Author(s).

Tribological properties of a surface are controlled by its topography and chemical structure as suggested by theoretical studies. However, the effect of crystallographic orientation on the tribological properties is still controversial. By tuning the heating rate during sulfurization of CVD Mo films, we prepared the few-layer MoS2 films with similar surface topography but different crystallographic c-axis orientation. This resulted in distinctly different tribological behavior both on the nanoscale and macroscale that can be attributed to different surface chemistry and surface electronic and phonon structures. In particular, horizontally aligned MoS2 sheets with c-axis oriented along the surface normal favor the weak van der Waals forces which resulted in a twice lower coefficient of friction comparing to the vertically aligned MoS2 sheets. These results demonstrate the possibility to optimize tribological properties by crystallographic orientation as required for the thin solid-state lubricating layers for extreme conditions.

In recent years, molybdenum disulfide (MoS2) has been investigated due to its unique electronic, optical, and mechanical properties with a variety of applications. Sulfurization of pre-deposited MoO3 layers is one of the methods of the preparation of large-area MoS2 thin films. The MoO3 layers have been grown on c-sapphire substrates, using two different techniques (rf sputtering, pulsed laser deposition). The films were subsequently annealed in vapors of sulfur at high temperatures what converted them to MoS2 films. The quality of MoS2 is strongly influenced by the properties of the precursor MoO3 layers. The pre-deposited MoO3, as well as the sulfurized MoS2, have been characterized by several techniques including Raman, Rutherford backscattering spectroscopy, atomic force microscopy, scanning electron microscopy, and X-ray diffraction. Here we compare two types of MoS2 films prepared from different MoO3 layers to determine the most suitable MoO3 layer properties providing good quality MoS2 films for future applications.

Correction for ‘Tuning the charge carrier mobility in few-layer PtSe2 films by Se : Pt ratio’ by Jana Hrdá et al., RSC Adv., 2021, 11, 27292–27297, DOI: 10.1039/D1RA04507E.

The preparation of continous three-dimensional carbon foam composed of a few-layer graphene film decorated with carbon nanowalls standing perpendicular to its surface is presented. The synthesis is done by the direct-current PECVD method using nickel foam as a catalyst substrate. Variable curvature of the surface of the Ni foam causes a variation in electric field during the dcPECVD synthesis, which results in forming the foam of hybrid carbon structures. The material was characterized by scanning electron microscope images and by Raman spectra. The effect of plasma penetration depth on growth of carbon nanowalls is also discussed. Macropores and nanopores coexisting in the structure might offer new functionalities particularly for charge storage applications.

Carbon nanowalls (CNWs) were grown by atmospheric dc plasma enhanced chemical vapour deposition method. Maintaining plasma at atmospheric pressure give us possibility to synthetize CNWs at high rate. By using two different liquid carbon sources we can control CNWs density. Growth of sparsely distributed CNWs can be achieved by using ethanol, while hexane gives densely packed CNWs films.

Thermal expansion exhibits considerable challenges developing residual stresses at the interfaces of different materials treated at high temperature. Electrical devices containing materials with different thermal expansion behaviour very often suffer this problem. Thermal expansion co-efficient (TEC) of different metals can be tuned by using carbon nanotube (CNT). Metal matrix composites (MMCs) using CNT are fabricated by hot-press sintering method and TEC of the composites are investigated throughout a wide range of temperature (-155 to 275 °C). Reduction of TEC of the composite materials was observed up to 20% compared to that of pure metals. The effect of CNTs in the matrix materials and the mechanism behind the improvement are explained from the microscopic investigation of the composites.

Molecular nanostructures are excellent systems to study surface enhanced Raman scattering (SERS), since monolayers or sub-monolayers are expected to provide maximum scattering enhancement. The SERS enhancement is due to a plasma induced field enhancement by metallic nanoparticles. By this effect the Raman response of molecules in the immediate neighborhood of the particles can be dramatically increased. We investigated the SERS effect of Ag colloids on glass substrates and on lithographically produced nanostructures of gold and aluminum on silicon and ITO-glass. For testing extremely thin layers (monolayers and submonolayers) of 4-octyl-4'-cyanobiphenyl (liquid crystal 8CB) were prepared. The observed enhancement for 8CB on Ag colloids was about 103. Enhancement factors for the lithographic structures were initially below 100. To increase the enhancement the An nanostructures were coated with a 20 nm thick Ag-layer. Optimisation of the Ag nanoparticle geometry was investigated as an alternative SERS improvement routine. The viability of iR the chip structures for multiple usage was tested. The experimental analysis was supported by calculations of the) SERS cross-section for different structure geometries.

The position and width of the Raman G-line was analyzed for unintentionally doped single-layered graphene samples. Results indicate a significant heating of the monolayer by the laser beam. Moreover, a weak additional component was resolved in the G-band. The position of the line is independent of the level of doping of the sample. We conclude that this new component is due to the phonons coupled to the intraband electronic transitions.

Hexa-perihexabenzocarbonate (HBC) are small cluster of a graphene plane with various side groups for stabilization. Due to theire ate geometry they are ideal candidates to study surface enhanced Raman spectroscopy (SERS). Very thin films HBC were vaopur deposited to different SERS-active Ag-surfaces. The enhancement of the Raman signal was studied with the goal to develop an Ag-surface of optimum enhancement, for multiple use and very simple sample preparation. For the current surface the observed enhancement is up to 10 6 . For SWNT filled with Ag-particles the SERS effect could also be shown - but enhancement was only a factor 10.

In the present work the way to grow SWCNTs, large enough to accommodate C 82 fullerenes, using non-magnetic metal catalysts is demonstrated. The growth was carried out in a water-cooled dc arc-discharge Kratschmer generator with horizontally aligned electrodes. Different methods such as Raman and optical spectroscopy, TGA and TEM were used for characterization of the materials produced under various growth conditions. The concentration of sulphur promoter strongly influences the yield of SWNTs and affects their diameter distribution. The highest relative concentration of SWCNTs with diameters larger than 1.5 nm was found for an addition of 0.9-1.3 at% of sulphur to PtRh catalysts.

Single wall carbon nanotubes with diameter 0.4 nm grown in the channels of AlPO4-5 crystals were studied by Raman spectroscopy. Up to 21 different laser lines were used to characterize vibrational properties. Spectra depend strongly on the energy of the laser line used for excitation. It was found that only two types of nanotubes with different chiralities, (5,0) and (4,2), are responsible for the spectra observed. The frequencies of the radial breathing modes were reliable assigned. A strong response was also observed for frequencies ∼1250 cm−1. The positions of two such peaks assigned to the (5,0) do not depend on the laser energy whereas only one such a peak was observed for the (4,2) nanotube. Its frequency shifts with the laser energy like the D line of large diameter nanotubes, but the rate of the shift is only a half of the value known for the latter. We did not observe a radial breathing mode of the (3,3) tube even though a laser line close to the predicted resonance energy was used. On the other hand, we detected a possible presence of the (5,1) tube in the spectrum excited with the UV laser line.

Single wall carbon nanotubes with diameter 0.4 nm grown in the channels of AlPO4-5 crystals were studied by Raman spectroscopy and ab initio density functional calculations. In the experiment up to 19 different laser lines were used to characterize vibrational properties. Spectra depend strongly on the energy of the laser line used for excitation. Even though the observed Raman spectra were very rich on lines only two types of nanotubes with different chiralities, (5,0) and (4,2), were found to be responsible for the observed response. The frequencies of the radial breathing modes were reliably assigned. Even though the (5,0) is metallic, the A1g mode does not couple to the electronic continuum and the Peierls-type mechanism does not shift the mode toward lower frequencies. A strong response was also observed for frequencies around 1250 cm−1. The positions of two peaks assigned to the (5,0) do not depend on the laser energy whereas only one peak was observed for the (4,2) nanotube. Its frequency shifts wi...

Metal-induced crystallisation with layer exchange is applied on Ni/C bilayer stacks deposited by three PVD techniques on SiO2. The layer stacks were deposited at room temperature by either ion beam sputtering, direct current magnetron sputtering or high-power impulse magnetron sputtering. The influence of the Ni morphology on the layer exchange degree αLE and the resulting graphitic ordering is studied by atomic force microscopy, Rutherford backscattering spectrometry and Raman spectroscopy. The initial RMS roughness of the Ni top layer varied with the deposition technique by a factor of 20, from 0.3 to 6.1 nm. After annealing in UHV at up to 700 °C, layer exchange was observed for all samples. Still, the layer exchange degree was affected by the roughness of the initial bilayer stack. The highest value of 96% was achieved for magnetron-sputtered samples, what is by ∼35% higher than for the initially roughest Ni surfaces. Raman spectroscopy showed the formation of graphitic carbon, characterised by a strong 2D line, for all three bilayer stacks. The degree of graphitic ordering increased with decreasing Ni surface roughness.

The current energy needs have put the focus on highly efficient energy storage systems such as supercapacitors. At present, much attention focuses on graphene-like materials as promising supercapacitor electrodes. Here we show that reduced graphite oxide offers a very interesting potential. Materials obtained by oxidation of natural graphite and subsequent sonication and reduction by hydrazine achieve specific capacitances as high as 170 F/g in H2SO4 and 84 F/g in (C2H5)4NBF4/acetonitrile. Although the particle size of the raw graphite has no significant effect on the physico-chemical characteristics of the reduced materials, that exfoliated from smaller particles (, Financial support from EU 7FP [Project Electrograph-266391], the project VEGA 1254/12 (M.H.) and MICINN [MAT 2011-25198] (T.A.C.) is gratefully acknowledged. The authors are grateful to Prof. Viera Skákalová for the useful discussions and critical remarks.

We report on the evaluation of the distribution of diameters for nanotube samples with a wide variation of mean diameters. Such results were obtained from a detailed analysis of the radial breathing mode Raman response and compared to results obtained from an evaluation of optical spectra and X-ray diffraction pattern. The evaluation of the Raman data needs a well refined analysis as the experimental analysis exhibits a rather complicated and oscillating relation between response and exciting laser. Both, an exact calculation where the density of states was considered explicitly and an approximate calculation were applied. Both models used for the analysis are able to explain several unexpected results from the experiment such as the oscillating behavior of the spectral moments, unusual discontinuities in the first moments of the Raman response for excitation in the IR, a fine structure for the response in optics and Raman, and an up shift of the RBM frequency as compared to qualified ab initio calculations. In detail the first moment and the variance of the spectra were used for the evaluation of the diameter distribution. To obtain good results between experimental and theoretical oscillation pattern the transition energy between the first two van Hove singularities had to be scaled up which is considered as a result from coulomb interaction of the electrons in the tubular material. On the other hand the analysis does not only allow to determine the mean value and the width of the diameter distribution but yields also a value for the average bundle diameters or, alternatively, the strength of the tube-tube interaction. The model used for the analysis of the Raman data is also appropriate to analyze the optical response, at least for the spectral range from 0.5 eV to 3.5 eV. The fine structure in the response for the transitions between the three lowest van Hove singularities is well reproduced and the mean tube diameters and their distribution is obtained in very good agreement with the results from the Raman analysis. From the X-ray analysis the same mean values and comparable distributions for the tube diameters were received whereas the bundle diameters could not be retained with high precision in this case.

Raman scattering and infrared absorption is reported for various empty and filled fullerene isomers in the low-energy range below 600 cm−1 to clarify the influence of different carbon cage structures on the bonding strength between encapsulated metal ion and fullerene cage and to check the potential of vibrational spectroscopy for isomer identification. The spectra of three isomers of Tm@C82 and three isomers of Sc2@C84 were measured at room temperature. The results are compared to the response of the most abundant isomers of empty C84 as well as to data for C82 and C60. The vibrational structure of the higher empty fullerene cages C82 and C84 resembles a downshifted and split C60 spectrum. Moreover the spectra of the two C84 isomers exhibited only small differences due to the very similar molecular structure, i.e. identical hexagon indices and direct neighbourhood in the Stone–Wales conversion map. Larger differences of the low-energy cage modes were found for the Tm@C82 isomers and in particular for the Sc2@C84 isomers. This goes along with an increasing difference in hexagon indices and a larger distance on the Stone–Wales conversion map. Due to the charge transfer from the endohedral metal to the fullerene the low-energy cage modes are shifted in the same direction as the modes of C60 during the exohedral doping process with alkali metals. New lines induced by the endohedral scandium and thulium ions with almost complementary Raman and infrared intensities were found for Sc2@C84 below 200, around 250 and 260 cm−1 and for Tm@C82 at 116 to 118 cm−1 and at 42 cm−1. These vibrations were further identified as M–C2n stretching and M–C2n deformation modes. Only a minor influence of the cage isomerism on these modes was observed. This is consistent with a simple ionic picture for the interaction between carbon cage and encapsulated metal ion. It is the amount of metal to fullerene charge transfer and the distance of the oppositely charge centres which determine the carbon cage–metal ion bond strength.