Miroslav Haluska

Hydrogen storage in carbon nanostructures

Abstract The paper gives a critical review of the literature on hydrogen storage in carbon nanostructures. Furthermore, the hydrogen storage of graphite, graphite nanofibers (GNFs), and single-walled carbon nanotubes (SWNTs) was measured by thermal desorption spectroscopy (TDS). The samples were ball milled under Ar or D2 atmosphere in order to modify the microstructure which was characterized by X-ray diffraction, scanning electron microscopy, and transmission electron microscopy. These investigations show a reversible hydrogen storage only for SWNTs and in addition indicate that an opening of the SWNTs is essential to reach high storage capacities.

Laser-induced disassembly of a graphene single crystal into a nanocrystalline network

We report about investigations of time-dependent structural modifications in single-crystal graphene due to laser irradiation even at moderate power levels of 1 mW in a diffraction-limited spot. The modifications have been characterized by in situ scanning confocal Raman spectroscopy, atomic force height microscopy, and transport studies. The time evolution of the Raman spectrum reveals two different effects: on a short-time scale, dopants, initially present on the flake, are removed. The longer time scale behavior points to a laser induced gradual local decomposition of single-crystal graphene into a network of interconnected nanocrystallites with a characteristic length scale of approximately 10 nm due to bond breaking. The broken bonds offer additional docking sites for adsorbates as confirmed in transport and AFM height studies. These controlled structural modifications may for instance be valuable for enhancing the local reactivity, trimming graphene based gas sensors and generating spatially varying doping patterns.

Are carbon nanostructures an efficient hydrogen storage medium

Literature data on the storage capacities of hydrogen in carbon nanostructures show a scatter over several orders of magnitude which cannot be solely explained by the limited quantity or purity of these novel nanoscale materials. With this in mind, this article revisits important experiments. Thermal desorption spectroscopy as a quantitative tool to measure the hydrogen storage capacity needs an appropriate calibration using a suitable hydride. Single-walled carbon nanotubes that have been treated by ultra-sonication show hydrogen uptake at room temperature. However, this storage can be assigned to metal particles incorporated during the sonication treatment. Reactive high-energy ball milling of graphite leads to a high hydrogen loading, however the temperatures for hydrogen release are far too high for application. In view of today’s knowledge, which is mainly based on experiments with small quantities and poorly characterised samples, carbon nanostructures at around room temperature cannot store the amount of hydrogen required for automotive applications.

Thermal desorption spectroscopy as a quantitative tool to determine the hydrogen content in solids

Thermal desorption spectroscopy (TDS) utilising a quadrupolar mass spectrometer is a highly sensitive and selective method to study the hydrogen desorption of hydrogen storage materials. For a quantitative analysis of the hydrogen storage capacity, the TDS apparatus can be consistently calibrated by using hydrogenated PdGd alloys or TiH2 as standards. Owing to its hygroscopic nature, the use of CaH2 as calibration standard leads to erroneous results. The chemical reactions during the thermal desorption of CaH2 are analysed in detail.

Observations of Chemical Reactions at the Atomic Scale: Dynamics of Metal-Mediated Fullerene Coalescence and Nanotube Rupture**

the chemicalreactivity of their interior has been considered to be very low.TEM is the only method that allows direct visualizationandstudyofthemoleculesinsidenanotubes.Wearecurrentlyexploiting the capabilities of an aberration-corrected TEM,which can record atomic resolution images in a time muchshorter than needed for the electron beam to promotestructural transformations inside SWNTs. Under our imagingconditions,thesetransformationsusuallyoccuronatimescaleof seconds, which enables us to capture atomic images of theintermediates. These images can then be combined into amovie to follow the chemical transformations as they happen.Both walls and interiors of SWNTs can be seen on TEMmicrographs, such that the positions and orientations of theencapsulated molecules can be readily determined from theimages (Figure 1c). Accelerated electrons interact with thespecimen and transfer their energy and momentum to theatoms of the specimen, causing knock-on damage, ionization,and/or heat-induced damage, the extent of which depends onthe nature of the material. With this in mind, we carried outour TEM studies with an accelerating voltage of only 80 kV,which is well below the threshold for knock-on damage incarbon nanotubes (86 kV).

Effects of Charge Impurities and Laser Energy on Raman Spectra of Graphene

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.

Advances in NO2 sensing with individual single-walled carbon nanotube transistors

Summary The charge carrier transport in carbon nanotubes is highly sensitive to certain molecules attached to their surface. This property has generated interest for their application in sensing gases, chemicals and biomolecules. With over a decade of research, a clearer picture of the interactions between the carbon nanotube and its surroundings has been achieved. In this review, we intend to summarize the current knowledge on this topic, focusing not only on the effect of adsorbates but also the effect of dielectric charge traps on the electrical transport in single-walled carbon nanotube transistors that are to be used in sensing applications. Recently, contact-passivated, open-channel individual single-walled carbon nanotube field-effect transistors have been shown to be operational at room temperature with ultra-low power consumption. Sensor recovery within minutes through UV illumination or self-heating has been shown. Improvements in fabrication processes aimed at reducing the impact of charge traps have reduced the hysteresis, drift and low-frequency noise in carbon nanotube transistors. While open challenges such as large-scale fabrication, selectivity tuning and noise reduction still remain, these results demonstrate considerable progress in transforming the promise of carbon nanotube properties into functional ultra-low power, highly sensitive gas sensors.

Hydrogen storage in mechanically treated single wall carbon nanotrubes

The hydrogen storage capacity in mechanically treated single wall carbon nanotubes (SWNTs) is investigated in this paper. In order to open the nanotubes three mechanical methods were applied: ball milling, ultrasonication and rubbing. Changes induced by the treatment were analyzed by Raman spectroscopy. The amount of hydrogen stored by the SWNTs reaches fractions of weight percents only. In the case of sonicated specimens, the hydrogen storage can be ascribed to titanium particles incorporated during the treatment.

Atomic-resolution three-dimensional force and damping maps of carbon nanotube peapods

Atomic force microscopy (AFM) has become a versatile and powerful method for imaging both insulating and conducting objects down to the atomic scale. By extending the high spatial resolution and sensitivity of AFM to the force spectroscopy dimension, oscillations of individual molecules can be studied with atomic resolution. Using three-dimensional mapping of the force and damping fields we address individual Dy@C(82) metallofullerene molecules confined inside single-walled carbon nanotubes (so-called metallofullerene peapods) and reveal their oscillatory behaviour via attractive interactions with the AFM probe tip. The damping energy DeltaE signals, generated in very close proximity of the tip and nanotube peapod, show a close relationship with hysteresis in the short-range forces, thereby indicating that a soft vibrational (phonon) mode is site-specifically (i.e., atom-by-atom) induced by the AFM tip.

Electrical contacts to individual SWCNTs: A review.

Summary Owing to their superior electrical characteristics, nanometer dimensions and definable lengths, single-walled carbon nanotubes (SWCNTs) are considered as one of the most promising materials for various types of nanodevices. Additionally, they can be used as either passive or active elements. To be integrated into circuitry or devices, they are typically connected with metal leads to provide electrical contacts. The properties and quality of these electrical contacts are important for the function and performance of SWCNT-based devices. Since carbon nanotubes are quasi-one-dimensional structures, contacts to them are different from those for bulk semiconductors. Additionally, some techniques used in Si-based technology are not compatible with SWCNT-based device fabrication, such as the contact area cleaning technique. In this review, an overview of the investigations of metal–SWCNT contacts is presented, including the principle of charge carrier injection through the metal–SWCNT contacts and experimental achievements. The methods for characterizing the electrical contacts are discussed as well. The parameters which influence the contact properties are summarized, mainly focusing on the contact geometry, metal type and the cleanliness of the SWCNT surface affected by the fabrication processes. Moreover, the challenges for widespread application of CNFETs are additionally discussed.