Publications

Phys. Rev. Applied 7 , 14009 (2017) .

Carbon 115 , 347-362 (2017) .
[Abstract]

Abstract Composite materials based on carbon fiber (CF) are prone to failure at the fiber-matrix interface upon compression or stress transverse to the fiber axis. The direct growth of carbon nanotubes on \{CF\} constitutes a novel approach to enhance the mechanical properties of the interface. However, the challenge is that, during the growth, tensile properties of the fiber are altered due to the diffusion effect of iron nanoparticles used in the process, leading to \{CF\} surface defect formation. In this work, we deliver and discuss an analysis methodology on ptychographic X-ray computed tomography (PXCT) images in order to assess the iron nanoparticle abundance within CFs. \{PXCT\} provides 50 nm – resolved 3D electron density maps of the CFs. We evidence the protective effect of an ultrathin alumina film against iron infiltration into \{CF\} during the \{CNT\} growth. This method potentially allows to evaluate the efficiency of other diffusion-minimizing approaches. The conclusions of the \{PXCT\} examination are validated by energy-dispersive X-ray spectroscopy and scanning transmission electron microscopy carried out on thin sample slices cut with a focused ion beam. The results provide a new insight into the mechanical performance of \{CFs\} and therefore constitute valuable knowledge for the development of hierarchical composites.

physica status solidi (RRL) – Rapid Research Letters 10 (11) , 807-811 (2016) .

Chimia 70 (6) , 402-412 (2016) .
[Abstract]

This article presents design strategies to demonstrate approaches to generate functionalized surfaces which have the potential for application in molecular systems; sensing and chemical reactivity applications are exemplified. Some applications are proven, while others are still under active investigation. Adaptation and extension of our strategies will lead to interfacing of different type of surfaces, specific interactions at a molecular level, and possible exchange of signals/cargoes between them. Optimization of the present approaches from each of five research groups within the NCCR will be directed towards expanding the types of functional surfaces and the properties that they exhibit.

ACS Sensors 1 (6) , 781–788 (2016) .
[Abstract]

The detection of biomarkers at very low concentration and low cost is increasingly important for clinical diagnosis. Moreover, monitoring affinities for receptor-antagonist interactions by time-resolved measurements is crucial for drug discovery and development. Biosensors based on ion-sensitive field-effect transistors (BioFETs) are promising candidates for being integrated into CMOS structures and cost-effective production. The detection of DNA and proteins with silicon nanowires has been successfully demonstrated using high affinity systems such as the biotin–streptavidin interaction. Here, we show the time-resolved label-free detection of the interaction of the bacterial FimH lectin with an immobilized mannose ligand on gold-coated silicon nanoribbon BioFETs. By comparing our results with a commercial state of the art surface plasmon resonance system, additional surface effects become visible when using this charge based detection method. Furthermore, we demonstrate the effect of sensor area on signal-to-noise ratio and estimate the theoretical limit of detection.

Biosensors 6 (2) , 21 (2016) .
[Abstract]

Ionic gradients play a crucial role in the physiology of the human body, ranging from metabolism in cells to muscle contractions or brain activities. To monitor these ions, inexpensive, label-free chemical sensing devices are needed. Field-effect transistors (FETs) based on silicon (Si) nanowires or nanoribbons (NRs) have a great potential as future biochemical sensors as they allow for the integration in microscopic devices at low production costs. Integrating NRs in dense arrays on a single chip expands the field of applications to implantable electrodes or multifunctional chemical sensing platforms. Ideally, such a platform is capable of detecting numerous species in a complex analyte. Here, we demonstrate the basis for simultaneous sodium and fluoride ion detection with a single sensor chip consisting of arrays of gold-coated SiNR FETs. A microfluidic system with individual channels allows modifying the NR surfaces with self-assembled monolayers of two types of ion receptors sensitive to sodium and fluoride ions. The functionalization procedure results in a differential setup having active fluoride- and sodium-sensitive NRs together with bare gold control NRs on the same chip. Comparing functionalized NRs with control NRs allows the compensation of non-specific contributions from changes in the background electrolyte concentration and reveals the response to the targeted species.

Optics Letters 41(7) , 1325-1328 (2016) .
[Abstract]

We used a confocal laser microscope to investigate the one-photon photoluminescence (PL) of gold antennas.The PL spectra can precisely be fitted to a plasmon enhanced photoluminescence model. For increasing antenna length, the energy peak position decreases continuously until it reaches a value of 1.7 – 1.8 eV. For longer antennas and smaller plasmon energies, we observe an additional, persistent shoulder in the PL spectra, which we explain by a Gaussian-shaped peak at4X 1.78 – 1.79 eV. We attribute this behavior to the opening of an additional decay path for electrons at the gold interband transition edge which we observe only for long antennas.

J. Phys. Chem. C 119 (33) , 19438-19451 (2015) .
[Abstract]

Activation energies, Ea, measured from molecular exchange experiments are combined with atomic-scale calculations to describe the migration of bare Au atoms and Au–alkanethiolate species on gold nanoparticle surfaces during ligand exchange for the creation of metal–molecule–metal junctions. It is well-known that Au atoms and alkanethiol–Au species can diffuse on gold with sub-1 eV barriers, and surface restructuring is crucial for self-assembled monolayer (SAM) formation for interlinking nanoparticles and in contacting nanoparticles to electrodes. In the present work, computer simulations reveal that naturally occurring ridges and adlayers on Au(111) are etched and resculpted by migration of alkanethiolate–Au species toward high coordination kink sites at surface step edges. The calculated energy barrier, Eb, for diffusion via step edges is 0.4–0.7 eV, close to the experimentally measured Ea of 0.5–0.7 eV. By contrast, putative migration from isolated nine-coordinated terrace sites and complete Au unbinding from the surface incur significantly larger barriers of +1 and +3 eV, respectively. Molecular van der Waals packing energies are calculated to have negligible effect on migration barriers for typically used molecules (length < 2.5 nm), indicating that migration inside SAMs does not change the size of the migration barrier. We use the computational methodology to propose a means of creating Au nanoparticle arrays via selective replacement of citrate protector molecules by thiocyanate linker molecules on surface step sites. This work also outlines the possibility of using Au/Pt alloys as possible candidates for creation of contacts that are well-formed and long-lived because of the superior stability of Pt interfaces against atomic migration.

Sensors and Actuators B 220 , 500-507 (2015) .

ACS Nano 9 (5) , 4872-4881 (2015) .

J. Phys. Chem. C 119 (11) , 6344-6355 (2015) .
[Abstract]

Functionalized nanoparticle networks offer a model system for the study of charge transport in low-dimensional systems as well as a potential platform to implement and test electronic functionalities. The electrical response of a nanoparticle network is expected to sensitively depend on the molecular inter-connects, i.e. on the linker chemistry. If these linkers have complex charge transport properties, then phenomenological models addressing the large scale properties of the network need to be complemented with microscopic calculations of the network building blocks. In this study we focus on the interplay between conformational fluctuations and electronic $\pi$-stacking in single molecule junctions and use the obtained microscopic information on their electrical transport properties to parametrize transition rates describing charge diffusion in mesoscopic nanoparticle networks. Our results point out at the strong influence of mechanical degrees of freedom on the electronic transport signatures of the studied molecules. This is then reflected in the varying charge diffusion at the network level. The modeling studies are complemented with first charge transport measurements at the single-molecule level of $\pi$-stacked molecular dimers using state of the art mechanically controllable break junction techniques in a liquid environment.

Appl. Phys. Lett. 106 , 11904 (2015) .
[Abstract]

In this work, the optical response of graphene to hydrogen plasma treatment is investigated with spectroscopic ellipsometry measurements.Although the electronic transport properties and Raman spectrum of graphene change after plasma hydrogenation, ellipsometric parameters of the Si/SiO2/graphene tri-layer system do not change.This is attributed to plasma hydrogenated graphene still being electrically conductive, since the light absorption of conducting 2D materials does not depend on the electronic band structure. A change in the light transmission can only be observed when higher energy hydrogen ions (30eV) are employed, which chemically sputter the graphene layer. An optical contrast is still apparent after sputtering due to the remaining traces of graphene and hydrocarbons on the surface. In brief, plasma treatment does not change the light transmission of graphene; and when it does, this is actually due to plasma damage rather than plasma hydrogenation.

Chem. Soc. Rev. 44 (4) , 999-1014 (2015) .
[Abstract]

Arrays of metal nanoparticles in an organic matrix have attracted a lot of interest due to their diverse electronic and optoelectronic properties. Recent work demonstrates that nanoparticle arrays can be utilized as a template structure to incorporate single molecules. In this arrangement{,} the nanoparticles act as electronic contacts to the molecules. By varying parameters such as the nanoparticle material{,} the matrix material{,} the nanoparticle size{,} and the interparticle distance{,} the electronic behavior of the nanoparticle arrays can be substantially tuned and controlled. Furthermore{,} via the excitation of surface plasmon polaritons{,} the nanoparticles can be optically excited and electronically read-out. The versatility and possible applications of well-ordered nanoparticle arrays has been demonstrated by the realization of switching devices triggered optically or chemically and by the demonstration of chemical and mechanical sensing. Interestingly{,} hexagonal nanoparticle arrays may also become a useful platform to study the physics of collective plasmon resonances that can be described as Dirac-like bosonic excitations.

Journal of Physics: Cond. Matt. 26 , 474202 (2014) .

Sensors and Actuators B: Chemical 204 , 568-577 (2014) .

J. Am. Chem. Soc. 136 (25) , 8867-8870 (2014) .

Nanoscale 6 , 7249-7254 (2014) .

Appl. Phys. Lett. 104 , 13102 (2014) .

Sensors and Actuators B 191 , 270-275 (2014) .

Sensors and Materials 25 (8) , 567-576 (2013) .

Nanoscale 5 , 12104-12110 (2013) .

ACS Nano 7 (7) , 5978-5983 (2013) .

Nature Nanotechnology 8 , 385-389 (2013) .

Appl. Phys. Lett. 102 , 71602 (2013) .

ACS Nano 6 (10) , 9291-9298 (2012) .

Nanotechnology 23 , 365201 (2012) .
[Abstract]

Conducting atomic force microscopy is an attractive approach enabling the correlation of mechanical and electrical properties in individual molecular junctions. Here we report on measurements of gold–gold and gold–octanedithiol–gold junctions. We introduce two-dimensional histograms in the form of scatter plots to better analyze the correlation between force and conductance. In this representation, the junction-forming octanedithiol compounds lead to a very clear step in the force–conductance data, which is not observed for control monothiol compounds. The conductance found for octanedithiols is in agreement with the idea that junction conductance is dominated by a single molecule.

Int. J. Unconv. Comp. 8 (4) , 325-332 (2012) .

Langmuir 28 (25) , 9899-9905 (2012) .
[Abstract]

Conventional gate oxide layers (e.g., SiO2, Al2O3, or HfO2) in silicon field-effect transistors (FETs) provide highly active surfaces, which can be exploited for electronic pH sensing. Recently, great progress has been achieved in pH sensing using compact integrateable nanowire FETs. However, it has turned out to be much harder to realize a true reference electrode, which – while sensing the electrostatic potential – does not respond to the proton concentration. In this work, we demonstrate a highly effective reference sensor, a so-called reference FET, whose proton sensitivity is suppressed by as much as 2 orders of magnitude. To do so, the Al2O3 surface of a nanowire FET was passivated with a self-assembled monolayer of silanes with a long alkyl chain. We have found that a full passivation can be achieved only after an extended period of self-assembling lasting several days at 80 °C. We use this slow process to measure the number of active proton binding sites as a function of time by a quantitative comparison of the measured nonlinear pH-sensitivities to a theoretical model (site-binding model). Furthermore, we have found that a partially passivated surface can sense small changes in the number of active binding sites reaching a detection limit of ?Ns ? 170 ?m–2Hz–1/2 at 10 Hz and pH 3.

ACS Nano 6 (5) , 4181-4189 (2012) .
[Abstract]

We investigate the photoconductance of gold nanoparticle arrays in the Coulomb blockade regime. Two-dimensional, hexagonal crystals of nanoparticles are produced by self-assembly. The nanoparticles are weakly coupled to their neighbors by a tunneling conductance. At low temperatures, the single electron charging energy of the nanoparticles dominates the conductance properties of the array. The Coulomb blockade of the nanoparticles can be lifted by optical excitation with a laser beam. The optical excitation leads to a localized heating of the arrays, which in turn gives rise to a local change in conductance and a redistribution of the overall electrical potential in the arrays. We introduce a dual-beam optical excitation technique to probe the distribution of the electrical potential in the nanoparticle array. A negative differential photoconductance is the direct consequence of the redistribution of the electrical potential upon lifting of the Coulomb blockade. On the basis of our model, we calculate the optically induced current from the dark current–voltage characteristics of the nanoparticle array. The calculations closely reproduce the experimental observations.

ChemPhysChem 13 (5) , 1157-1160 (2012) .
[Abstract]

The response of (miniaturized) ion-sensitive field-effect transistors (ISFETs) with oxide interfaces to changes in the electrolyte concentration is discussed. It is shown that FETs covered with a thin alumina layer are almost insensitive to changes in the ionic strength of the electrolyte, while being extremely pH-sensitive.

Nano Lett. 11 (9) , 3597-3600 (2011) .
[Abstract]

We observe very small gate-voltage shifts in the transfer characteristic of as-prepared graphene field-effect transistors (GFETs) when the pH of the buffer is changed. This observation is in strong contrast to Si-based ion-sensitive FETs. The low gate-shift of a GFET can be further reduced if the graphene surface is covered with a hydrophobic fluorobenzene layer. If a thin Al-oxide layer is applied instead, the opposite happens. This suggests that clean graphene does not sense the chemical potential of protons. A GFET can therefore be used as a reference electrode in an aqueous electrolyte. Our finding sheds light on the large variety of pH-induced gate shifts that have been published for GFETs in the recent literature.

J. Am. Chem. Soc. 133 (31) , 12185-12191 (2011) .

Appl. Phys. Lett. 98 , 12114 (2011) .
[Abstract]

Recent studies on nanoscale field-effect sensors reveal the crucial importance of the low-frequency noise for determining the ultimate detection limit. In this letter, the 1/f-type noise of Si nanoribbon field-effect sensors is investigated. We demonstrate that the signal-to-noise ratio can be increased by almost two orders of magnitude if the nanoribbon is operated in an optimal gate voltage range. In this case, the additional noise contribution from the contact regions is minimized, and an accuracy of 0.5‰ of a pH shift in 1 Hz bandwidth can be reached.

Analytical Chemistry 82 (14) , 6299-6302 (2010) .

CHIMIA Int. J. Chem 64 (6) , 391-397 (2010) .

Nano Letters 10 , 2268-2274 (2010) .

CHIMIA Int. J. Chem 64 , 140-144 (2010) .

Nano Letters 10 (3) , 759-764 (2010) .

Eur. J. Org. Chem. 5 , 833-845 (2010) .

Procedia Chemistry 1 (1) , 678-681 (2009) .

Appl. Phys. Lett. 94 , 161104 (2009) .
[Abstract]

We report on a photoconductive gain effect in two-dimensional arrays of gold nanoparticles (NPs) in which alkane molecules are inserted. The NP arrays are formed by a self-assembly process from alkanethiol-coated gold NPs, and subsequently they are patterned on a Si/SiO2 chip by a microcontact printing technique. We find that the photoconductance of the arrays is strongly enhanced at the frequency of the surface plasmon of the NPs. We interpret the observation as a bolometric enhancement in the conductance of the NP arrays upon excitation of the surface plasmon resonance

Swiss Physical Society Communications 26 , 10 (2009) .
[Abstract]

If individual molecules are to be used as building blocks for electronic devices, it will be essential to understand charge transport at the level of single molecules. Most existing experiments rely on the synthesis of functional rod-like molecules with chemical linker groups at both ends to provide strong, covalent anchoring to the source and drain contacts. This approach has proved very successful, providing quantitative measures of single-molecule conductance, and demonstrating rectification and switching at the single-molecule level. However, the influence of intermolecular interactions on the formation and operation of molecular junctions has been overlooked. Here we report the use of oligophenylene ethynylene molecules as a model system, and establish that molecular junctions can still form when one of the chemical linker groups is displaced or even fully removed. Our results demonstrate that aromatic pi-coupling between adjacent molecules is efficient enough to allow for the controlled formation of molecular bridges between nearby electrodes.

Nano Letters 9 , 76-80 (2009) .

Phys. Rev. B 78 , 235421 (2008) .

Nature Nanotechnology 3 (9) , 569-574 (2008) .

New J. Phys. 10 , 65018 (2008) .

New J. Phys. 10 , 65019 (2008) .

J. Phys. Chem. B 112 , 7140 (2008) .

J. Am. Chem. Soc. 130 (3) , 1080-1084 (2008) .

J. Phys. Chem. C 111 (50) , 18445-18450 (2007) .

Chem. Comm. 46 , 4854-4856 (2007) .

J. Org. Chem. 72 (22) , 8337-8344 (2007) .

App. Phys. Lett. 91 (5) , 53118 (2007) .

Nanotech. 18 (23) , 235202 (2007) .

Nano Lett. 6(10) , 2238-2242 (2006) .

Adv. Mat. 18 (18) , 2444-2447 (2006) .

UniNova, Wissenschaftsmagazin der Universität Basel 103 , 22-24 (2006) .

Imaging & Microscopy 8 , 36 (2006) .

NanoNews, Newsletter of the NCCR Nanoscale Science 01 , 26-27 (2006) .

SMALL 1 (11) , 1067-1070 (2005) .

Nanotech. 16 , 2143 (2005) .

Single Mol. 3 (4) , 189-193 (2002) .

Phys. Rev. Lett. 86 (16) , 3630-3633 (2001) .

Nature Biotech. 19 (4) , 365-370 (2001) .

Physica B 284 , 891-892 (2000) .

NEC Technical Report 99 , 138 (1999) .

Nature Communications 10 (1) , 262 (2019) .