Reentrant features on a surface are key to render it omniphobic, i.e., the apparent contact angle, ?r > 90�, especially when the intrinsic contact angle, ?o < 90�. However, a complete theoretical understanding of the wetting behavior of surfaces with reentrant features has remained unclear. Here, we present a unified and general model that can predict apparent contact angles for surfaces with reentrant features also. Unlike previous models, this model does not include the �roughness� of surfaces, but rather the specific geometry of the surface features (protrusions, cavities, etc.) to determine the apparent contact angle in terms of the �intrinsic� or �Young� contact angle (on a smooth planar surface) and two geometrydependent parameters. In particular, we find that cavities with reentrant walls (i.e., concave interior walls) result in important new conditions. The results show that both thermodynamic equilibrium and metastable states can arise, depending on the geometry (shape) and size of the cavities. We have fabricated some surfaces with micron-scale reentrant features (some of these structures mimic the surface texture of animals, birds, and plant leaves, e.g., lotus leaf, that render them hydrophobic or superhydrophobic), and have tested them with liquids of varying surface tensions, such as water, canola oil, and ethanol. We found that depending on the geometry of the surface features, the apparent contact angles span from the intrinsic angle to values that are very different and are in excellent quantitative agreement with the predictions of our model.

Several III-V nanowire materials systems exhibit features of polytypism, which is a kind of polymorphism, where the polymorphs differ only in the layer stacking sequence. In bulk, all III-V semiconductors, except nitrides, exhibit the zinc blende structure (3C polytype). However, when these materials are grown as nanowires, they often exhibit a seemingly random crystal structure and by tuning the growth parameters, more or less pure 3C or 2H can be fabricated. Sometimes often higher order polytypes, such as 4H and 6H form. In order to use III-V nanowires in electronic and optoelectronic applications, it is of highest importance to control and possibly also take advantage of the polytypism. In our current investigations, we take a classical nucleation approach to explain the phenomenon of polytypism in metal particle-seeded III-V nanowires, including polytypes up to 6H. In order to describe the formation of higher order polytypes, interaction between the stacked layers, which goes beyond nearest neighbor interactions must be taken into account. For this purpose we use the axial next nearest neighbor Ising (ANNNI) model, which I introduce before I describe our specific approach. In the ANNNI model, the stacking sequence is treated as a sequence of generalized spins and different sequences give different total energies, depending on the interlayer interaction parameters. The total energies for several polytypes can be calculated by ab initio techniques for any given material. In addition, from the total energy expressions a phase diagram can be constructed, in which the ab initio results can be visualized. Using this approach, it has been shown that 6H is the most stable SiC polytype and it has been verified that the III-V semiconductors are very stable in 3C. Another, more kinetic approach to the ANNNI modeling of polytypism is to keep track of the incremental energy change due to the addition of single layers. This approach has been used to explain the preference of SiC to grow in the 3C polytype during CVD. In our approach to polytypism in nanowires, we use the ANNNI model to express the interface energy between the forming nucleus and the underlying layers for the 3C, 6H, 4H, and 2H polytypes. I will show how to combine this interface energy with our nucleation theoretical framework and describe how we can use this model to calculate the formation probabilities of these four polytypes as functions of supersaturation. Depending on the interaction parameters, the range of attainable polytypes as a function of supersaturation can vary, and this can be graphically represented. I will introduce such polytype attainability diagrams and discuss their experimental relevance.

The control of gold nanoparticles� surface chemistry is a fundamental prerequisite to tailor their properties for biological applications such as bioimaging and sensing. Short peptides have been specifically designed to form self-assembled monolayers on gold nanoparticles surface1,2 and used to increase their stability3 and enable functionalization with biomolecule at a stoichiometric level.2,4 Here, we propose an experimental and computational approach to characterize the molecular organization and define the secondary structure of self-assembled monolayers of peptides on gold nanoparticles surface. Experimentally, we are focusing on the benzophenone-derivative peptides, aiming to characterize the organization of mixed ligand shells on the gold nanoparticles surface. Under excitation at 350 nm, the carbonyl group of the benzophenone moiety crosslinks to an adjacent molecule. The cross-linking between the two adjacent ligands on the surface of the nanoparticles is monitored by Fourier Transform Infrared (FTIR), Ultraviolet-Visible (UV-Vis) spectroscopies and Mass Spectrometry (MS), providing insight into the molecular organization and structural response of these ligand shells upon light irradiation. We are using Molecular Dynamics simulations to investigate the secondary structure of two short peptides, i.e. CALNN and CFGAILSS, on spherical GNPs of different sizes, i.e. 5, 10, 25 nm. The model is validated on the basis of Shaw et al. work5 where they have shown, using a combination of Fourier Transform Infrared (FTIR) spectroscopy and Solid-State Nuclear Magnetic Resonance (SSNMR), that the CFGAILSS� conformation on the gold nanoparticles surface depends on the size of the particle, hence on its curvature. Both these approaches are necessary in order to have an insight at the molecular level into the organization and structure of self-assembled monolayers of peptides coating the gold nanoparticles surface. References: 1. L�vy, R. et al. Rational and combinatorial design of peptide capping ligands for gold nanoparticles. J. Am. Chem. Soc. 126, 10076�84 (2004). 2. Duchesne, L., Gentili, D., Comes-Franchini, M. & Fernig, D. G. Robust Ligand Shells for Biological Applications of Gold Nanoparticles. Langmuir 24, 13572�13580 (2008). 3. Chen, X. Y. et al. Features of Thiolated Ligands Promoting Resistance to Ligand Exchange in Self-Assembled Monolayers on Gold Nanoparticles. Aust. J. Chem. 65, 266�274 (2012). 4. Duchesne, L. et al. Transport of fibroblast growth factor 2 in the pericellular matrix is controlled by the spatial distribution of its binding sites in heparan sulfate. PLoS Biol. 10, 16 (2012). 5. Shaw, C. P., Middleton, D. a, Volk, M. & L�vy, R. Amyloid-derived peptide forms self-assembled monolayers on gold nanoparticle with a curvature-dependent ?-sheet structure. ACS Nano 6, 1416�26 (2012)

The presence of a direct band gap and in an ultrathin form factor has caused a considerable interest in two-dimensional semiconductors from the transition metal dichalcogenides (TMD) family with molybdenum disulphide (MoS2) being the most studied representative of this family of materials. While diverse electronic elements1, integrated circuits2 and optoelectronic devices3 have been demonstrated using ultrathin MoS2 and related materials, very little is known about their performance at high frequencies where commercial devices are expected to function. We fabricated top-gated MoS2 transistors operating in the gigahertz range of frequencies. The presence of a band gap also gives rise to current saturation,4 allowing voltage gain higher than 1. The MoS2 FETs are fabricated from exfoliated MoS25. We have fabricated RF transistors based on MoS2 layers with different thickness. Electrical contacts were patterned using electron-beam lithography and by depositing Au electrodes. Atomic layer deposition (ALD) was used to deposit HfO2 as a gate dielectric. All our devices presented transconductance typical of n-type materials with on-state current reaching ~300 �A/�m for Vds = 2 V and gate voltage Vtg = 10 V in the case of monolayer MoS2. The current gain of the MoS2 FETs decreases with increasing frequency and shows the typical 1/f dependence for different thicknesses of 2D MoS2 crystals. We realized MoS2 FETs showing current saturation. We found the behavior of the cut-off frequency as a function of the number of layers of MoS2 FETs. The cut-off frequency rises with increasing number of layers in the ambient atmosphere. In conclusion, we studied top-gated MoS2 transistors with a 240 nm gate length. Our MoS2 RF-FETs show an intrinsic transconductance higher than 50 �S/�m and a drain-source current saturation with a voltage gain higher than 1. All these features allow the operation of MoS2 transistors in the GHz range of frequencies. Our devices show cut-off frequencies in the GHz range and are able not only to amplify current in this frequency range but also power and voltage, with the maximum operating frequency fmax = 8.2 GHz. 1 Bertolazzi, S., Krasnozhon, D. & Kis, A. ACS Nano 7, 3246-3252, (2013). 2 Radisavljevic, B., Whitwick, M. B. & Kis, A. ACS Nano 5, 9934-9938, (2011). 3 Lopez-Sanchez, O., Lembke, D., Kayci, M., Radenovic, A. & Kis, A. Nat Nano 8, 497-501, (2013). 4 Lembke, D. & Kis, A. ACS Nano 6, 10070-10075, (2012). 5 Benameur, M. M. et al. Nanotechnology 22, 125706, (2011).

Nanomaterials such as nanowires and nanopillars have been shown to interact strongly with biological cells. Such 1D nanomaterials can enable the penetration of cell membrane by electroporation which is useful in the area of molecular biology to deliver polar substances into cells. Here we introduce a new application, water disinfection, enabled by 1D nanomaterial-assisted electroporation. The high electric field induced by the sharp 1D nanomaterials can be used to damage the membrane of the microorganisms and inactivate the microorganisms by disrupting the inner cellular environment. The 1D nanowires can enhance the local electric field 2-3 orders of magnitudes higher than that of planer structure. Using flow devices with filter electrodes made from silver nanowires or copper oxide nanowires, we demonstrate a high efficiency inactivation of both model bacteria and virus, of > 6log (>99.9999%) removal, with a fast treatment speed of 3000-15000 L/h-m2, which is equivalent to only 1s of contact time of microorganism with filter electrodes. During operation, the flow device can be powered by a small voltage < 20V and the energy consumption was very low of only

Organo-lead halide perovskite solar cells have gained enormous significance and approached the power conversion efficiencies of commercialized c-Si solar cells and some other thin film photovoltaic solar cells recently. However, one major concern comes from the potential toxicology of the soluble lead. Efficiency boosts for the lead-free perovskite solar cells are thus highly desirable in this regard. Here we show that the crystallization of the methylammonium tin triiodide (CH3NH3SnI3) perovskite can be controlled through the choice of solvents used in the spin coating process, thus enabling a faster crystallization rate in comparison with the CH3NH3PbI3 analog. Careful evaporation of the solvents and the convective self-assembly process during spinning effectively assist the formation of well-crystallized perovskite films due to the favorable interactions between the solvent molecules and the solvated CH3NH3+ and [SnI3]-. The crystallization processes in the selected polar solvents (N,N-dimethylmethanamide (DMF), ?-butyrolactone(GBL) and dimethylsulfoxide (DMSO)) have been investigated and compared. Intermmediate compounds of the perovskite withcrystallized solvent molecules were recognized to be important for the resultant film morphology. Efficient lead-free pervoskite solar cells have been realized with a conductive poly(triarylamine) hole-transporting material. Our results provide important progress towards the understanding of the role of crystallization engineering in the realization of low-cost and highly efficient lead-free perovskite solar cells.

Building practical devices from 2D semiconducting crystals and their heterostructures is an intensively pursued research area. Currently, chemical vapor deposition (CVD) of monolayer semiconductors is the only practical way to synthesis these materials at industrial scale. However, elastic properties of CVD-grown 2D semiconductors and their heterostructures have not been measured, although their less-defective, exfoliated counterparts have. In this work we experimentally and theoretically characterized the elastic modulus of CVD-grown MoS2, WS2 as well as their heterostructures with each other and with graphene. The 2D moduli of heterostructures are slightly lower than the sum of 2D modulus of each layer, but comparable to the corresponding bilayer homo structures, implying similar interactions between hetero monolayers compared to between homo monolayers. The interlayer coupling of different bilayer homo or hetero structures is also qualitatively compared by introducing a sliding coefficient. Our results provide calibrated values of elastic modulus of these structures for various applications, especially in flexible electronic and mechanical devices.

The past five years have witnessed a tremendous surge in the popularity of hybrid organic-inorganic halide perovskites among several research groups around the globe. The impulse for this trend can be largely attributed to CH3NH3PbI3, a compound semiconductor adopting a distorted perovskite crystal structure that was originally discovered in the late 70�s. CH3NH3PbI3 has been successfully used for the fabrication of inexpensive, high-efficiency solar cells when used as a light absorber and it has been shown to operate under various device architectures. CH3NH3PbI3 is a member of a wider class of halide perovskites, AMX3, where A is a univalent cation able to stabilize the perovskite structure, M is typically a group 14 bivalent metal ion and X is a halide anion. Other than CH3NH3PbI3, various line compounds and solid solutions within the AMX3 system have been demonstrated as efficient photosensitizers, thus highlighting the universally good semiconducting properties for this class of compounds. Despite the fact that the perovskite compounds are efficient photosensitizers and promise further improvements in the photovoltaic efficiency in the near future, the fundamental optical and electronic properties of the compounds themselves are not yet properly understood. In particular, we observe a significant variation in the photoluminescence (PL) properties of the compounds depending on whether the bulk material was isolated by means of a solution- or solid-state-based process. In the present work we study the properties of selected compounds from the APbI3 and ASnI3 systems as a function of the preparation method and we evaluate the resulting materials in terms of vacancy formation by employing a combination of single-crystal X-ray diffraction and theoretical DFT calculations. We further attempt to rationalize on the PL properties (PL emission vs. PL quenching) and electrical properties (resistivity, Hall effect) of these materials and correlate the estimated carrier density with the observed effects.

Many active biomolecules are available to perturb cell function by direct addition to cell culture media, including receptor ligands and antibodies. Drug delivery mechanisms interact with the cell membrane to deliver molecules directly into cells and generate exponentially more possibilities for cell perturbation, especially the option for effecting permanent change in cells by altering their genome, and the ability to directly perturb the central dogma by altering mRNA transcription and translation into protein. The majority of these active biomolecules have similar molecular structure, which led to the development of delivery vehicles specific for delivering DNA, peptides, and other classes of molecules. Recently, synthetic biomolecules have been developed which have hybrid characteristics, often due to the addition of functionality such as fluorescence or tailored binding sites. These synthetic functionalities could represent another exponential leap forward for studying cells in theory, but these molecules often lack an option for intracellular delivery due to their hybrid structure, limiting their practical use. Here we show the use of a microfabricated supported nanotube structure, called nanostraws, to deliver synthetic biomolecules into cells. The nanostraws are a nanobio interface, which create fluidic conduits into the cell for molecular transport. We focus on two types of molecules: synthetic carbohydrates with click chemistry moieties that, when delivered into cells, can be used to track metabolic pathways, and modified peptides with active sites that can be used to track enzyme activity. The delivery of these disparate molecules using the same technique demonstrates the utility of nanostraws as a non-specific delivery mechanism, capable of delivering synthetic molecules of diverse structures.