Interfaces of room temperature ionic liquids (RTILs) are important for both applications and basic science and are therefore intensely studied. However, the evolution of their interface structure with the cation's alkyl chain length [Formula: see text] from Coulomb to van der Waals interaction domination has not yet been studied for even a single broad homologous RTIL series. We present here such a study of the liquid-air interface for [Formula: see text], using angstrom-resolution X-ray methods. For [Formula: see text], a typical "simple liquid" monotonic surface-normal electron density profile [Formula: see text] is obtained, like those of water and organic solvents. For [Formula: see text], increasingly more pronounced nanoscale self-segregation of the molecules' charged moieties and apolar chains yields surface layering with alternating regions of headgroups and chains. The layering decays into the bulk over a few, to a few tens, of nanometers. The layering periods and decay lengths, their linear [Formula: see text] dependence, and slopes are discussed within two models, one with partial-chain interdigitation and the other with liquid-like chains. No surface-parallel long-range order is found within the surface layer. For [Formula: see text], a different surface phase is observed above melting. Our results also impact general liquid-phase issues like supramolecular self-aggregation and bulk-surface structure relations.

, 63.20.kd, 41.60.Cr Symmetry breaking and the emergence of order is one of the most fascinating phenomena in condensed matter physics. It leads to a plethora of intriguing ground states found in antiferromagnets, Mott insulators, superconductors, and density-wave systems. Exploiting states of matter far from equilibrium can provide even more striking routes to symmetry-lowered, ordered states. Here, we demonstrate for the case of elemental chromium that moderate ultrafast photo-excitation can transiently enhance the charge-density-wave (CDW) amplitude by up to 30% above its equilibrium value, while strong excitations lead to an oscillating, large-amplitude CDW state that persists above the equilibrium transition temperature. Both effects result from dynamic electron-phonon interactions, providing an efficient mechanism to selectively transform a broad excitation of the electronic order into a well defined, long-lived coherent lattice vibration. This mechanism may be exploited to transiently enhance order parameters in other systems with coupled degrees of freedom.

Crystal nucleation and growth at a liquid-liquid interface is studied on the atomic scale by in situ Å-resolution X-ray scattering methods for the case of liquid Hg and an electrochemical dilute electrolyte containing Pb 2+ , F − , and Br − ions. In the regime negative of the Pb amalgamation potential Φ rp = − 0:70 V, no change is observed from the surface-layered structure of pure Hg. Upon potential-induced release of Pb 2+ from the Hg bulk at Φ > Φ rp , the formation of an intriguing interface structure is observed, comprising a well-defined 7.6-Å-thick adlayer, decorated with structurally related 3D crystallites. Both are identified by their diffraction peaks as PbFBr, preferentially aligned with theirc axis along the interface normal. X-ray reflectivity shows the adlayer to consist of a stack of five ionic layers, forming a single-unit-cellthick crystalline PbFBr precursor film, which acts as a template for the subsequent quasiepitaxial 3D crystal growth. This growth behavior is assigned to the combined action of electrostatic and shortrange chemical interactions.electrochemistry | liquid metal L iquid-liquid and liquid-gas interfaces provide exciting new possibilities for material synthesis (1, 2). Contrary to solid interfaces, which exhibit strain and stress, heterogeneities, and defects such as steps, which all strongly affect growth processes, fluid systems provide soft, defect-and stress-free interfaces. The high mobility of reagents, products, and deposited particles in liquid phases facilitates the growth process as well as the selfassembly of ordered particle arrays at the interface.A large variety of materials has been prepared via deposition at liquid-liquid interfaces, such as metals (1), oxides (3, 4), chalcogenides (5, 6), polymers (7), plasmonic materials (2), and nanoparticle catalysts of ceria (3), Pd (8, 9), and Pt (10). As demonstrated by Carim et al., deposition at liquid-liquid interfaces even allows the synthesis of group IV semiconductors such as Ge from oxide materials via a simple one-step, room-temperature electrochemical process (11). Different methods for nanoparticle manufacturing, such as deposition by reduction of metal ions (12) or electrochemical deposition (11), are available at the liquidliquid interface, allowing for particle modification and growth control via adjustment of concentration or interfacial potential.Despite the absence of long-range order, liquid interfaces provide the possibility to control the crystallinity, shape, and orientation of deposits. Examples are the growth of single-crystalline CuO and CuS films (4), the surfactant-induced oriented growth of calcite crystals (13), and the formation of pyramidal PbS crystallites with defined, high surface area facets (5). These phenomena were rationalized by energetic effects, such as the interface energies, surface charges, and specific chemical interactions, as well as by the growth kinetics. However, detailed insight into the phase formation mechanisms is generally precluded by lack of atomicscale data on the in...

Surface induced smectic order was found for the ionic liquid 1-methyl-3-docosylimidazolium bis(trifluoromethlysulfonyl)imide by X-ray reflectivity and grazing incidence scattering experiments. Near the free liquid surface, an ordered structure of alternating layers composed of polar and non-polar moieties is observed. This leads to an oscillatory interfacial profile perpendicular to the liquid surface with a periodicity of 3.7 nm. Small angle X-ray scattering and polarized light microscopy measurements suggest that the observed surface structure is related to fluctuations into a metastable liquid crystalline SmA phase that was found by supercooling the bulk liquid. The observed surface ordering persists up to 157 °C, i.e. more than 88 K above the bulk melting temperature of 68.1 °C. Close to the bulk melting point, we find a thickness of the ordered layer of L = 30 nm. The dependency of L(τ) = Λ ln(τ/τ) vs. reduced temperature τ follows a logarithmic growth law. In agreement with theory, the pre-factor Λ is governed by the correlation length of the isotropic bulk phase.

Molecular self-assembly is a key to wide-ranging nano-and micro-scale applications in numerous fields. Understanding its underlying molecular level science is therefore of prime importance. This study resolves theÅ-scale structure of the earliest and simplest self-assembled monolayer (SAM): octadecanol on amorphous-SiO 2 -terminated Si (001) substrate, and determines the structure's temperature evolution. At low temperatures lateral hexagonal order exists, with close-packed, surface-normal molecules. ∼ 12 • C above the alkanol's bulk melting a fully-reversible disordering transition occurs to a novel "stretched liquid" phase, laterally disordered, but only ∼ 15% thinner SAM than the crystalline phase. The SAM persists to ≥ 100 • C. A thermodynamic model yields here a headgroup-substrate bond energy ∼ 40% lower than on crystalline sapphire, highlighting the importance of the substrate's order, and near-epitaxy, for the SAM's ordering and stability.

The atomic-scale structure of the mercury-electrolyte (0.01 M NaF) interface was studied as a function of temperature and potential by x-ray reflectivity and x-ray diffuse scattering measurements. The capillary wave contribution is determined and removed from the data, giving access to the intrinsic surface-normal electron density profile at the interface, especially to the surface layering in the Hg phase. A temperature dependent roughness anomaly known from the Hg-air interface is found to persist also at the Hg-electrolyte interface. Additionally, a temperature dependence of the layering period was discovered. The increase in the layer spacing with increasing temperature is approximately four times lager than the increase expected from thermal expansion. Finally, the interface is found to broaden towards the electrolyte side as the potential becomes more negative, in agreement with the Schmickler-Henderson theory. Our results favor a model for the interface structure, which is different to the model formerly used in comparable studies.

Correction for 'Surface induced smectic order in ionic liquids - an X-ray reflectivity study of [CCim][NTf]' by Julian Mars et al., Phys. Chem. Chem. Phys., 2017, 19, 26651-26661.

Room temperature ionic liquids (RTILs), a novel class of liquid salts, are intensively studied for their basic science and numerous emerging applications. When undercooled, RTILs comprising long alkyl chains often exhibit liquid crystal (LC) bulk phases. However, only one molecular-resolution experimental structure study was published for their LC surface phases. We measured the temperature evolution of another LC surface phase, using surface specificÅ-resolution x-ray methods. This phase's existence range, 90 • C, much exceeds the corresponding bulk phase's 3 • C. Its thickness, L, confirms the theory-predicted logarithmic temperature dependence, with an amplitude equalling the bulk correlation length. Surprisingly, at L's divergence temperature, a ∼ 20Å-thick, hexagonally-packed, crystalline monolayer forms at, and fully covers, the sample's surface. It is identified as a surface-frozen Langmuir-Gibbs film, and fundamentally differs from the only reported RTIL surface crystal, a Coulomb-dominated, four-layer, island phase, covering only 5%-15% of the surface.