The back-gated field-effect transistor (FET) configuration has been central to the study of the electronic transport properties of organic and nanoscale materials. [1][2][3] Three-terminal transport measurements using this geometry are facilitated by using a degenerately doped silicon wafer covered with a thin silicon oxide layer as the substrate. It has been widely assumed that the substrate did not influence the measurement of the intrinsic properties of the material under study. However, Chua et al. [4] recently demonstrated that changing the nature of the dielectric led to ambipolar behavior in FETs made from organic semiconductors that were previously thought to be exclusively hole (p-type) conductors. Silanol groups present on the SiO 2 surface were singled out as the culprits for generating electron traps responsible for suppressing electron (n-type) conduction in these devices. Here we show that the substrate-induced suppression of n-type behavior is not unique to organic FETs, but influences the measurements of all devices fabricated on SiO 2 /Si substrates. By using carbon nanotubes as the testbed, we investigated the impact of the chemical nature of the substrate and of ambient adsorbates on the field-effect switching behavior of both nanoscale and thin-film FETs. Our study revealed that the reduction of n-type conduction occurs when an adsorbed water layer containing solvated oxygen is present on the SiO 2 surface. This finding demonstrates that an electrochemical charge transfer reaction between the semiconducting channel and the aqueous oxygen redox couple is the underlying phenomenon governing the suppression of electron conduction in carbon nanotube devices. This effect should be taken into account when interpreting three-terminal measurements conducted on SiO 2 /Si substrates. We anticipate that the design of electronic devices, [5] chemical sensors, [6] and biosensors [7] that are based on the FET configuration will be largely influenced by the charge transfer mechanism that has been brought to light by this study.Individual carbon nanotube field-effect transistors (CFETs) are the most extensively studied molecular-scale FETs to date.

Raman spectroscopy uses visible light to acquire vibrational fingerprints of molecules, thus making it a powerful tool for chemical analysis in a wide range of media. Its potential for optical imaging at high resolution is, however, severely limited by the fact that the Raman effect is weak. Here, we report the discovery of a giant Raman scattering effect from encapsulated and aggregated dye molecules inside single-walled carbon nanotubes (SWNTs). Measurements performed on rod-like dyes, such as α-sexithiophene and βcarotene, assembled inside SWNTs as highly polarizable J-aggregates indicate a resonant Raman cross-section (CS) of ~10-21 cm 2 /sr, which is well above the CS required for detecting individual aggregates at the highest optical resolution. Free from fluorescence background and photobleaching, this giant Raman effect allows the realization of a library of functionalized and biocompatible nanoprobe labels for Raman imaging with robust detection using multispectral analysis.

BackgroundCADM is a statistical test used to estimate the level of Congruence Among Distance Matrices. It has been shown in previous studies to have a correct rate of type I error and good power when applied to dissimilarity matrices and to ultrametric distance matrices. Contrary to most other tests of incongruence used in phylogenetic analysis, the null hypothesis of the CADM test assumes complete incongruence of the phylogenetic trees instead of congruence. In this study, we performed computer simulations to assess the type I error rate and power of the test. It was applied to additive distance matrices representing phylogenies and to genetic distance matrices obtained from nucleotide sequences of different lengths that were simulated on randomly generated trees of varying sizes, and under different evolutionary conditions.ResultsOur results showed that the test has an accurate type I error rate and good power. As expected, power increased with the number of objects (i.e., taxa), the number of partially or completely congruent matrices and the level of congruence among distance matrices.ConclusionsBased on our results, we suggest that CADM is an excellent candidate to test for congruence and, when present, to estimate its level in phylogenomic studies where numerous genes are analysed simultaneously.

Quantitative description of reaction mechanisms in aqueous phase electrochemistry requires experimental characterization of local water structure at the electrode/aqueous interface and its evolution with changing potential. Gaining such insight experimentally under electrochemical conditions is a formidable task. The potential-dependent structure of a subpopulation of interfacial water with one OH group pointing towards a gold working electrode is characterized using interface specific vibrational spectroscopy in a thin film electrochemical cell. Such free-OH groups are the molecular level observable of an extended hydrophobic interface. This free-OH interacts only weakly with the Au surface at all potentials, has an orientational distribution that narrows approaching the potential of zero charge, and disappears on oxidation of the gold electrode.