Tobias Blom

Conductivity engineering of graphene by defect formation

Transport measurements have revealed several exotic electronic properties of graphene. The possibility to influence the electronic structure and hence control the conductivity by adsorption or doping with adatoms is crucial in view of electronics applications. Here, we show that in contrast to expectation, the conductivity of graphene increases with increasing concentration of vacancy defects, by more than one order of magnitude. We obtain a pronounced enhancement of the conductivity after insertion of defects by both quantum mechanical transport calculations as well as experimental studies of carbon nano-sheets. Our finding is attributed to the defect induced mid-gap states, which create a region exhibiting metallic behaviour around the vacancy defects. The modification of the conductivity of graphene by the implementation of stable defects is crucial for the creation of electronic junctions in graphene-based electronics devices.

Improved gas sensing activity in structurally defected bilayer graphene

Graphene is a two-dimensional material with a capability of gas sensing, which is here shown to be drastically improved by inducing gentle disorder in the lattice. We report that by using a focused ion beam technique, controlled disorder can be introduced into the graphene structure through Ga(+) ion irradiation. This disorder leads to an increase in the electrical response of graphene to NO(2) gas molecules by a factor of three in an ambient environment (air). Ab initio density functional calculations indicate that NO(2) molecules bind strongly to Stone-Wales defects, where they modify electronic states close to the Fermi level, which in turn influence the transport properties. The demonstrated gas sensor, utilizing structurally defected graphene, shows faster response, higher conductivity changes and thus higher sensitivity to NO(2) as compared to pristine graphene.

Fabrication and characterization of highly reproducible, high resistance nanogaps made by focused ion beam milling

Nanoelectrodes were fabricated combining photolithography, electron beam lithography and focused ion beam milling allowing for large scale integration and nanoengineering of the electrode propertie ...

Biomimetic calcium phosphate coatings on recombinant spider silk fibres

Calcium phosphate ceramic coatings, applied on surfaces of metallic and polymeric biomaterials, can improve their performance in bone repair and regeneration. Spider silk is biocompatible, strong and elastic, and hence an attractive biomaterial for applications in connective tissue repair. Recently, artificial spider silk, with mechanical and structural characteristics similar to those of native spider silk, has been produced from recombinant minispidroins. In the present study, supersaturated simulated body fluid was used to deposit calcium phosphate coatings on recombinant spider silk fibres. The mineralization process was followed in time using scanning electron microscopy equipped with an energy dispersive x-ray (EDX) detector and Raman spectroscope. Focused ion beam technology was used to produce a cross section of a coated fibre, which was further analysed by EDX. Preliminary in vitro experiments using a culture of bone marrow-derived human mesenchymal stem cells (hMSCs) on coated fibres were also performed. This study showed that recombinant spider silk fibres were successfully coated with a homogeneous and thick crystalline calcium phosphate layer. In the course of the mineralization process from modified simulated body fluid, sodium chloride crystals were first deposited on the silk surface, followed by the deposition of a calcium phosphate layer. The coated silk fibres supported the attachment and growth of hMSCs.

Formation and NMR Spectroscopy of ω-Thiol Protected α,ω-Alkanedithiol-Coated Gold Nanoparticles and Their Usage in Molecular Charge Transport Junctions

Gold nanoparticles (AuNPs) coated with stabilizing molecular monolayers are utilized in areas ranging from life sciences to nanoelectronics. Here we present a novel and facile one-pot single phase procedure for the preparation of stable AuNPs with good dispersity, which are coated with α,ω-alkanedithiols whose outer ω-thiol is protected by a triphenylmethyl group. Using dielectrophoresis we were able to trap these AuNPs, coated with ω-thiol protecting groups, in a 20 nm gold electrode nanogap. The ω-thiol group was then deprotected under acidic conditions in situ once the AuNPs were correctly positioned in the device. The subsequent deprotection resulted in an increase of conductance by up to 3 orders of magnitude, indicating that the isolated dithiol-coated AuNPs were fused into a covalently bonded network with AuNP-molecule-AuNP as well as electrode-molecule-AuNP linkages. Furthermore, complete characterization of the AuNP surface-bonded alkanedithiols was achieved using a series of one- and two-dimensional NMR spectroscopy techniques. Our spectra of the molecule-coated AuNPs show well-resolved signals, only slightly broader than for free molecules in solution, which is in contrast to many earlier reported NMR spectral data of molecules attached to AuNPs. Complementary diffusion NMR spectroscopic experiments were performed to prove that the observed alkanedithiols are definitely surface-bonded species and do not exist in free and unattached form.

Assessment of a nanoparticle bridge platform for molecular electronics measurements

A combination of electron beam lithography, photolithography and focused ion beam milling was used to create a nanogap platform, which was bridged by gold nanoparticles in order to make electrical measurements and assess the platform under ambient conditions. Non-functionalized electrodes were tested to determine the intrinsic response of the platform and it was found that creating devices in ambient conditions requires careful cleaning and awareness of the contributions contaminants may make to measurements. The platform was then used to make measurements on octanethiol (OT) and biphenyldithiol (BPDT) molecules by functionalizing the nanoelectrodes with the molecules prior to bridging the nanogap with nanoparticles. Measurements on OT show that it is possible to make measurements on relatively small numbers of molecules, but that a large variation in response can be expected when one of the metal-molecule junctions is physisorbed, which was partially explained by attachment of OT molecules to different sites on the surface of the Au electrode using a density functional theory calculation. On the other hand, when dealing with BPDT, high yields for device creation are very difficult to achieve under ambient conditions. Significant hysteresis in the I-V curves of BPDT was also observed, which was attributed primarily to voltage induced changes at the interface between the molecule and the metal.

Nano-fabrication of molecular electronic junctions by targeted modification of metal-molecule bonds

Reproducibility, stability and the coupling between electrical and molecular properties are central challenges in the field of molecular electronics. The field not only needs devices that fulfill these criteria but they also need to be up-scalable to application size. In this work, few-molecule based electronics devices with reproducible electrical characteristics are demonstrated. Our previously reported 5 nm gold nanoparticles (AuNP) coated with ω-triphenylmethyl (trityl) protected 1,8-octanedithiol molecules are trapped in between sub-20 nm gap spacing gold nanoelectrodes forming AuNP-molecule network. When the trityl groups are removed, reproducible devices and stable Au-thiol junctions are established on both ends of the alkane segment. The resistance of more than 50 devices is reduced by orders of magnitude as well as a reduction of the spread in the resistance histogram is observed. By density functional theory calculations the orders of magnitude decrease in resistance can be explained and supported by TEM observations thus indicating that the resistance changes and strongly improved resistance spread are related to the establishment of reproducible and stable metal-molecule bonds. The same experimental sequence is carried out using 1,6-hexanedithiol functionalized AuNPs. The average resistances as a function of molecular length, demonstrated herein, are comparable to the one found in single molecule devices.

Identification of vibrational signatures from short chains of interlinked molecule–nanoparticle junctions obtained by inelastic electron tunnelling spectroscopy

Short chains containing a series of metal-molecule-nanoparticle nanojunctions are a nano-material system with the potential to give electrical signatures close to those from single molecule experiments while enabling us to build portable devices on a chip. Inelastic electron tunnelling spectroscopy (IETS) measurements provide one of the most characteristic electrical signals of single and few molecules. In interlinked molecule-nanoparticle (NP) chains containing typically 5-7 molecules in a chain, the spectrum is expected to be a superposition of the vibrational signatures of individual molecules. We have established a stable and reproducible molecule-AuNP multi-junction by placing a few 1,8-octanedithiol (ODT) molecules onto a versatile and portable nanoparticle-nanoelectrode platform and measured for the first time vibrational molecular signatures at complex and coupled few-molecule-NP junctions. From quantum transport calculations, we model the IETS spectra and identify vibrational modes as well as the number of molecules contributing to the electron transport in the measured spectra.

Enabling measurements of low-conductance single molecules using gold nanoelectrodes

A high resistance nanogap platform was used to trap and electrically characterize 30 nm thiolated double-stranded DNA molecules. High resolution scanning electron microscopy was also used to image the trapped DNA strands. It was found that the surface state of the electrodes and underlying substrate could influence the measurements of trapped molecules when the measured resistances were on the order of TΩ or greater. Hydrophilic surfaces gave rise to larger leakage currents that could potentially mask the underlying signals from molecules positioned in the nanogap. Finally, the careful handling of the samples and control of the environment is essential to avoid surface charging of the oxide substrate layer as these parasitic charges affect electrical measurements of the nanogap. The presented results thus outline some important considerations when making low-conductance measurements on molecules and should prove useful for the characterization of molecules in molecular electronics or sensors employing nanogap platforms.

Nanoparticle Bridges for Studying Electrical Properties of Organic Molecules

The use of single molecules as building blocks for practical electronic devices and sensors has high potential for novel applications due to the versatility of electronic properties of the molecules. Nano-sized molecules offer great potential for further miniaturization of electronic devices. We describe a method where such molecules are used to bridge a nanoparticles-nanoelectrode interface and thus determine the electrical properties of such a junction. We describe in detail the fabrication of the platform, its functionalization with molecules, and the basics of the electrical measurements. This platform has been shown to guide electrical current through a few molecules. The versatility of such nanoparticle-molecule-nanoelectrode heterojunctions makes this platform suitable for both basic molecular electronics measurements and also for molecular sensing devices in biological and medical applications.