Conductivity Enhancement in Transition Metal Dichalcogenides: A Complex Water Intercalation and Desorption Mechanism.

The complexity of the water adsorption-desorption mechanism at the interface of transition metal dichalcogenides (TMDs) and its impact on their current transport are not yet fully understood. Here, our work investigates the swift intercalation of atmospheric adsorbates at the TMD and sapphire interface and between two TMD monolayers and probes its influence on their electrical properties. The adsorbates consist mainly of hydroxyl-based (OH) species in the subsurface region suggesting persistent water intercalation even under vacuum conditions, as determined by time-of-flight-secondary ion mass spectrometry (ToF-SIMS) and scanning tunneling microscopy (STM). Water intercalates there rapidly, within the order of a few minutes after being exposed to ambient atmosphere, this process tends to be partly reversible under (ultra)high vacuum, as observed by time-dependent scanning probe microscopy (SPM) based conductivity and ToF-SIMS measurements. A significant enhancement of the electronic properties is observed with the complete desorption of intercalated water clusters because of the pressure-induced melting effect under the tip of the SPM probe. Conversely, it also indicates that the characterization of TMD samples is substantially affected in air, in inert environments, and to some extent even in a vacuum if water intercalation is present. More importantly, STM analysis has uncovered a correlation between water intercalation and the presence of defects, showcasing their role in the gradual degradation of the material as it ages.

SIMS Analysis of Thin EUV Photoresist Films.

This study reports on the application of secondary ion mass spectrometry (SIMS) for examining thin (20-50 nm) chemically amplified resist films on silicon. SIMS depth profiling was carried out using a gas cluster ion beam to ensure minimal sputter-induced damage to the organic constituents of interest. Specific attention concerned the distribution of the photo acid generator (PAG) molecule within these films, along with the photo-induced fragmentation occurring on extreme ultra-violet photo exposure. Positive secondary ion spectra were collected using a traditional time of flight (ToF)-SIMS and the latest generation IONTOF Hybrid SIMS instrumentation equipped with an OrbitrapTM mass analyzer. Tandem mass spectrometry (MS/MS) capability within the OrbitrapTM secondary ion column was utilized to verify that the C19H17S+ secondary ion did indeed have the molecular structure consistent with the PAG structure. The superior mass resolving power of the OrbitrapTM mass analyzer (∼20× of the ToF mass analyzer) along with improved mass accuracy (a few ppm) proved pivotal in the mass spectral and depth profile analysis of these films. This was not the case for the ToF-SIMS experiments, as the mass spectra, as well as the associated depth profiles, exhibited severe molecular (isobaric) interferences.

Efficient long-range conduction in cable bacteria through nickel protein wires.

Filamentous cable bacteria display long-range electron transport, generating electrical currents over centimeter distances through a highly ordered network of fibers embedded in their cell envelope. The conductivity of these periplasmic wires is exceptionally high for a biological material, but their chemical structure and underlying electron transport mechanism remain unresolved. Here, we combine high-resolution microscopy, spectroscopy, and chemical imaging on individual cable bacterium filaments to demonstrate that the periplasmic wires consist of a conductive protein core surrounded by an insulating protein shell layer. The core proteins contain a sulfur-ligated nickel cofactor, and conductivity decreases when nickel is oxidized or selectively removed. The involvement of nickel as the active metal in biological conduction is remarkable, and suggests a hitherto unknown form of electron transport that enables efficient conduction in centimeter-long protein structures.

Enhanced Laterally Resolved ToF-SIMS and AFM Imaging of the Electrically Conductive Structures in Cable Bacteria.

Cable bacteria are electroactive bacteria that form a long, linear chain of ridged cylindrical cells. These filamentous bacteria conduct centimeter-scale long-range electron transport through parallel, interconnected conductive pathways of which the detailed chemical and electrical properties are still unclear. Here, we combine time-of-flight secondary-ion mass spectrometry (ToF-SIMS) and atomic force microscopy (AFM) to investigate the structure and composition of this naturally occurring electrical network. The enhanced lateral resolution achieved allows differentiation between the cell body and the cell-cell junctions that contain a conspicuous cartwheel structure. Three ToF-SIMS modes were compared in the study of so-called fiber sheaths (i.e., the cell material that remains after the removal of cytoplasm and membranes, and which embeds the electrical network). Among these, fast imaging delayed extraction (FI-DE) was found to balance lateral and mass resolution, thus yielding the following multiple benefits in the study of structure-composition relations in cable bacteria: (i) it enables the separate study of the cell body and cell-cell junctions; (ii) by combining FI-DE with in situ AFM, the depth of Ni-containing protein-key in the electrical transport-is determined with greater precision; and (iii) this combination prevents contamination, which is possible when using an ex situ AFM. Our results imply that the interconnects in extracted fiber sheaths are either damaged during extraction, or that their composition is different from fibers, or both. From a more general analytical perspective, the proposed methodology of ToF-SIMS in the FI-DE mode combined with in situ AFM holds great promise for studying the chemical structure of other biological systems.

Area-Selective Atomic Layer Deposition of TiN Using Trimethoxy(octadecyl)silane as a Passivation Layer.

Area-selective deposition (ASD) offers tremendous advantages when compared with conventional patterning processes, such as the possibility of achieving three-dimensional features in a bottom-up additive fashion. Recently, ASD is gaining more and more attention from IC manufacturers and equipment and material suppliers. Through combination of self-assembled monolayer (SAM) surface passivation of the nongrowth substrate area and atomic layer deposition (ALD) on the growth area, ASD selective to the growth area can be achieved. With the purpose of screening SAM precursors to provide optimal passivation performance on SiO2, various siloxane precursors with different terminal groups and alkyl chains were investigated. Additionally, the surface dependence and growth inhibition of TiN ALD on -NH2, -CF3, and -CH3 terminations is investigated. We demonstrated the methyl termination of the SAM precursor combined with a C18 alkyl chain plays an important role in broadening the ALD selectivity window by suppressing precursor adsorption. Owing to the high surface coverage, excellent thermal stability and longer carbon chain length, an optimized trimethoxy(octadecyl)silane (TMODS) film deposited from liquid phase was able to provide a selectivity higher than 0.99 up to 20 nm ALD film deposited on hydroxyl-terminated Si oxide. The approach followed in this work can allow extending the ASD process window, and it is relevant for a wide variety of applications.

A Correlative ToF-SIMS/SPM Methodology for Probing 3D Devices.

With the continuous miniaturization and increasing complexity of the devices used in nanotechnology, there is a pressing need for characterization techniques with nm-scale 3D-spatial resolution. Unfortunately, techniques like Secondary Ion Mass Spectrometry (SIMS) fail to reach the required lateral resolution. For this reason, new concepts and approaches, including the combination of different complementary techniques, have been developed in over the past years to try to overcome some of the challenges. Beyond the problem of spatial resolution in a 3D SIMS experiment, one is also faced with the impact of changes in topography during the analysis. These are quite difficult to identify because they originate from the different sputter rates of the various materials and or phases in a heterogeneous system and are notorious at the interfaces between organic and inorganic layers. As each of these materials will erode at a different velocity, accurate 3D-analysis will require means to establish a spatially resolved relation between ion bombardment time and depth. Inevitably such a nonhomogeneous erosion will lead to the development of surface topography. The impact of these effects can be overcome provided one can capture the time and spatially dependent surface erosion (velocity) with high spatial resolution during the course of a profiling experiment. Incorporating a Scanning Probe Microscope (SPM) unit which provides topography measurements with high spatial resolution, into a SIMS tool (e.g., Time of Flight (ToF) SIMS) with means to alternate between SPM and SIMS measurements, is one approach to meet that demand for complementary topographical information allowing accurate 3D chemical imaging. In this paper, the result of integrating a SPM module into a ToF-SIMS system is presented illustrating the improvements in 3D data accuracy which can be obtained when analyzing complex 3D-systems.

Hybrid Perovskites Depth Profiling with Variable-Size Argon Clusters and Monatomic Ions Beams.

Ion beam depth profiling is increasingly used to investigate layers and interfaces in complex multilayered devices, including solar cells. This approach is particularly challenging on hybrid perovskite layers and perovskite solar cells because of the presence of organic/inorganic interfaces requiring the fine optimization of the sputtering beam conditions. The ion beam sputtering must ensure a viable sputtering rate on hard inorganic materials while limiting the chemical (fragmentation), compositional (preferential sputtering) or topographical (roughening and intermixing) modifications on soft organic layers. In this work, model (Csx(MA0.17FA0.83)100-xPb(I0.83Br0.17)3/cTiO2/Glass) samples and full mesoscopic perovskite solar cells are profiled using low-energy (500 and 1000 eV) monatomic beams (Ar⁺ and Cs⁺) and variable-size argon clusters (Arn⁺, 75 < n < 4000) with energy up to 20 keV. The ion beam conditions are optimized by systematically comparing the sputtering rates and the surface modifications associated with each sputtering beam. X-ray photoelectron spectroscopy, time-of-flight secondary ion mass spectrometry, and in-situ scanning probe microscopy are combined to characterize the interfaces and evidence sputtering-related artifacts. Within monatomic beams, 500 eV Cs⁺ results in the most intense and stable ToF-SIMS molecular profiles, almost material-independent sputtering rates and sharp interfaces. Large argon clusters (n > 500) with insufficient energy (E < 10 keV) result in the preferential sputtering of organic molecules and are highly ineffective to sputter small metal clusters (Pb and Au), which tend to artificially accumulate during the depth profile. This is not the case for the optimized cluster ions having a few hundred argon atoms (300 < n < 500) and an energy-per-atom value of at least 20 eV. In these conditions, we obtain (i) the low fragmentation of organic molecules, (ii) convenient erosion rates on soft and hard layers (but still different), and (iii) constant molecular profiles in the perovskite layer, i.e., no accumulation of damages.

Nanoscale electrochemical response of lithium-ion cathodes: a combined study using C-AFM and SIMS.

The continuous demand for improved performance in energy storage is driving the evolution of Li-ion battery technology toward emerging battery architectures such as 3D all-solid-state microbatteries (ASB). Being based on solid-state ionic processes in thin films, these new energy storage devices require adequate materials analysis techniques to study ionic and electronic phenomena. This is key to facilitate their commercial introduction. For example, in the case of cathode materials, structural, electrical and chemical information must be probed at the nanoscale and in the same area, to identify the ionic processes occurring inside each individual layer and understand the impact on the entire battery cell. In this work, we pursue this objective by using two well established nanoscale analysis techniques namely conductive atomic force microscopy (C-AFM) and secondary ion mass spectrometry (SIMS). We present a platform to study Li-ion composites with nanometer resolution that allows one to sense a multitude of key characteristics including structural, electrical and chemical information. First, we demonstrate the capability of a biased AFM tip to perform field-induced ionic migration in thin (cathode) films and its diagnosis through the observation of the local resistance change. The latter is ascribed to the internal rearrangement of Li-ions under the effect of a strong and localized electric field. Second, the combination of C-AFM and SIMS is used to correlate electrical conductivity and local chemistry in different cathodes for application in ASB. Finally, a promising starting point towards quantitative electrochemical information starting from C-AFM is indicated.

Influence of different cleaning processes on the surface chemistry of gold nanoparticles.

In this paper, the authors have investigated the effects of different cleaning methods (centrifugation and dialysis) on the surface chemistry and composition of 15 nm sodium citrate stabilized gold nanoparticles. The nuclear magnetic resonance (NMR) results indicate that three centrifugation cycles are sufficient to remove most of the citrate molecules, while centrifuged liquid sedimentation and dynamic light scattering data reveal some degree of nanoparticle aggregation when three centrifugation cycles are exceeded. Regarding the dialysis procedure, NMR analysis demonstrated that after nine cleaning cycles, the citrate concentration is comparable to that measured after the first centrifugation (about 6 × 10-4 M) but with an increase in the dispersion polydispersivity index as determined by dynamic light scattering. X-ray photoelectron spectroscopy results support the NMR findings and revealed a major hydrocarbon contamination after the nanoparticles cleaning process. The impact of cleaning on surface functionalization was tested using 1H,1H,2H,2H-perfluorodecanethiol hydrophobic thiols (PFT) to test thiol-citrate substitution. After 24 h exposure, the PFT coverage was less than 0.6 monolayer (ML) for both pristine nanoparticles and particles after three dialysis cycles, but about 0.8 ML after two centrifugation washes.

Characterisation of nanomaterial hydrophobicity using engineered surfaces.

Characterisation of engineered nanomaterials (NMs) is of outmost importance for the assessment of the potential risks arising from their extensive use. NMs display indeed a large variety of physico-chemical properties that drastically affect their interaction with biological systems. Among them, hydrophobicity is an important property that is nevertheless only slightly covered by the current physico-chemical characterisation techniques. In this work, we developed a method for the direct characterisation of NM hydrophobicity. The determination of the nanomaterial hydrophobic character is carried out by the direct measurement of the affinity of the NMs for different collectors. Each collector is an engineered surface designed in order to present specific surface charge and hydrophobicity degrees. Being thus characterised by a combination of surface energy components, the collectors enable the NM immobilisation with surface coverage in relation to their hydrophobicity. The experimental results are explained by using the extended DLVO theory, which takes into account the hydrophobic forces acting between NMs and collectors. Graphical abstractDetermination of hydrophobicity character of nanomaterials by measuring their affinity to engineered surfaces.

Versailles Project on Advanced Materials and Standards Interlaboratory Study on Measuring the Thickness and Chemistry of Nanoparticle Coatings Using XPS and LEIS.

We report the results of a VAMAS (Versailles Project on Advanced Materials and Standards) inter-laboratory study on the measurement of the shell thickness and chemistry of nanoparticle coatings. Peptide-coated gold particles were supplied to laboratories in two forms: a colloidal suspension in pure water and; particles dried onto a silicon wafer. Participants prepared and analyzed these samples using either X-ray photoelectron spectroscopy (XPS) or low energy ion scattering (LEIS). Careful data analysis revealed some significant sources of discrepancy, particularly for XPS. Degradation during transportation, storage or sample preparation resulted in a variability in thickness of 53 %. The calculation method chosen by XPS participants contributed a variability of 67 %. However, variability of 12 % was achieved for the samples deposited using a single method and by choosing photoelectron peaks that were not adversely affected by instrumental transmission effects. The study identified a need for more consistency in instrumental transmission functions and relative sensitivity factors, since this contributed a variability of 33 %. The results from the LEIS participants were more consistent, with variability of less than 10 % in thickness and this is mostly due to a common method of data analysis. The calculation was performed using a model developed for uniform, flat films and some participants employed a correction factor to account for the sample geometry, which appears warranted based upon a simulation of LEIS data from one of the participants and comparison to the XPS results.

Surface Analysis of Gold Nanoparticles Functionalized with Thiol-Modified Glucose SAMs for Biosensor Applications.

In this work, Time of Flight Secondary Ion Mass Spectrometry (ToF-SIMS), Principal Component Analysis (PCA) and X-ray Photoelectron Spectroscopy (XPS) have been used to characterize the surface chemistry of gold substrates before and after functionalization with thiol-modified glucose self-assembled monolayers and subsequent biochemical specific recognition of maltose binding protein (MBP). The results indicate that the surface functionalization is achieved both on flat and nanoparticles gold substrates thus showing the potential of the developed system as biodetection platform. Moreover, the method presented here has been found to be a sound and valid approach to characterize the surface chemistry of nanoparticles functionalized with large molecules. Both techniques were proved to be very useful tools for monitoring all the functionalization steps, including the investigation of the biological behavior of the glucose-modified particles in the presence of the maltose binding protein.

Multi-Functionalized Carbon Nano-onions as Imaging Probes for Cancer Cells.

Carbon-based nanomaterials have attracted much interest during the last decade for biomedical applications. Multimodal imaging probes based on carbon nano-onions (CNOs) have emerged as a platform for bioimaging because of their cell-penetration properties and minimal systemic toxicity. Here, we describe the covalent functionalization of CNOs with fluorescein and folic acid moieties for both imaging and targeting cancer cells. The modified CNOs display high brightness and photostability in aqueous solutions and their selective and rapid uptake in two different cancer cell lines without significant cytotoxicity was demonstrated. The localization of the functionalized CNOs in late-endosomes cell compartments was revealed by a correlative approach with confocal and transmission electron microscopy. Understanding the biological response of functionalized CNOs with the capability to target cancer cells and localize the nanoparticles in the cellular environment, will pave the way for the development of a new generation of imaging probes for future biomedical studies.

Measuring Compositions in Organic Depth Profiling: Results from a VAMAS Interlaboratory Study.

We report the results of a VAMAS (Versailles Project on Advanced Materials and Standards) interlaboratory study on the measurement of composition in organic depth profiling. Layered samples with known binary compositions of Irganox 1010 and either Irganox 1098 or Fmoc-pentafluoro-l-phenylalanine in each layer were manufactured in a single batch and distributed to more than 20 participating laboratories. The samples were analyzed using argon cluster ion sputtering and either X-ray photoelectron spectroscopy (XPS) or time-of-flight secondary ion mass spectrometry (ToF-SIMS) to generate depth profiles. Participants were asked to estimate the volume fractions in two of the layers and were provided with the compositions of all other layers. Participants using XPS provided volume fractions within 0.03 of the nominal values. Participants using ToF-SIMS either made no attempt, or used various methods that gave results ranging in error from 0.02 to over 0.10 in volume fraction, the latter representing a 50% relative error for a nominal volume fraction of 0.2. Error was predominantly caused by inadequacy in the ability to compensate for primary ion intensity variations and the matrix effect in SIMS. Matrix effects in these materials appear to be more pronounced as the number of atoms in both the primary analytical ion and the secondary ion increase. Using the participants' data we show that organic SIMS matrix effects can be measured and are remarkably consistent between instruments. We provide recommendations for identifying and compensating for matrix effects. Finally, we demonstrate, using a simple normalization method, that virtually all ToF-SIMS participants could have obtained estimates of volume fraction that were at least as accurate and consistent as XPS.

Surface analysis of zinc-porphyrin functionalized carbon nano-onions.

Multishell fullerenes, known as carbon nano-onions (CNOs), are an interesting class of carbon-based nanomaterials. They display several unique properties, such as a large surface area to volume ratio, a low density, and a graphitic multilayer morphology, which have made them appealing for several applications in many fields, including biology. Chemical functionalization of CNOs dramatically enhances their solubility and attenuates their inflammatory properties, thereby increasing their applicability especially in the fields of biology and medicine. CNOs functionalized with fluorescent probes can be used for cellular imaging. In this article, detailed surface characterization of CNOs functionalized with a zinc porphyrin (ZnTPP) as the fluorescent probe is presented. In particular, time-of-flight secondary ion mass spectrometry and x-ray photoelectron spectroscopy provide a detailed surface characterization of the organic functionalities introduced via "click chemistry" and clearly demonstrate the success of the CNOs functionalization process. XPS data reveal the presence of Zn and N, whilst ToF-SIMS is able to identify specific fragments related to the presence of the ZnTPP, such as the quasimolecular mass peak [C32N4H20Zn](+) and the molecular mass peak [C44N4H28Zn](+). Moreover, ions fragments deriving from the triazole ring formed by the click chemistry reaction have been identified by ToF-SIMS analysis, proving unambiguously the covalent binding of the fluorescent molecules to the CNOs surfaces.

Controlled density patterning of tolylterpyridine-tagged oligonucleotides.

An efficient surface anchoring strategy of tolylterpyridine-tagged DNA single strands (ssDNA-ttpy) synthesized on gold electrodes is reported. The method is based on exchange reactions between Fe(II)bis-terpyridine complexed SAMs and ssDNA-ttpy, and allows efficient hybrydization of the cDNA strands. Moreover, by using low-current focused ion beam lithography, micropatterned arrays are obtained, homogeneously covered with ssDNA-ttpy. The surface adsorption kinetics of ssDNA-ttpy, as well as its hybridization efficiency, was monitored by in situ quartz crystal microbalance with dissipation monitoring (QCM-D) technique. The effective confinement of the ssDNA-ttpy at the micrometer level has been monitored by time of flight secondary ion mass spectrometry (ToF-SIMS) and ellipsometric surface imaging experiments, providing laterally resolved chemical and topographic mapping.

Functionalization of oxide surfaces by terpyridine phosphonate ligands: surface reactions and anchoring geometry.

A strategy for creating a general-purposes surface functionalization platform is reported, based on direct attachment of phosphate groups onto hydroxylated surfaces and subsequent formation of a terpyridine-based monolayer. Such a platform is suitable for the construction, onto technologically relevant oxide surfaces, of single- and multilayer structures of interest in technological applications. In particular, the paper describes the successful attachment of 4-(2,2':6',2''-terpyridine-4-yl)benzenephosphonic acid (1, PPTP) onto a SiO(2) surface previously functionalized by means of Zr-phosphate groups. Two alternative anchoring strategies of the PPTP were explored: (i) a direct one-step way, implying no protection of terpyridinic functionality, and (ii) a three-step way, implying protection and successive deprotection of this group. It was found that, in the first case, the PPTP ligand anchoring to the Zr-containing phosphate layer takes place by means of terpyridinic group. At variance of this, in the second case, due to the protection of the terpyridinic functionality, the anchoring process takes place through the phosphonic group, making the terpyridinic moiety available for further reactions, i.e., multilayer constructs. X-ray photoelectron spectroscopy (XPS) and time-of-flight secondary ion mass spectrometry (ToF-SIMS) were used to study the functionalized surfaces, providing information on coverage, chemical structure, and stoichiometry of the various functionalized layers and, among the others, clear evidence of the PPTP linkage and orientation.