Controlling composition and plasmonic response of bimetallic nanoparticles (NPs) is of great relevance to tune their catalytic activity. Herein, we demonstrate reversible composition and plasmonic response transitions from a core/shell to a bimetallic alloyed palladium/gold NP triggered by CO adsorption and sample temperature. The use of self-organized growth on alumina template film allows scrutinizing the impact of core size and shell thickness onto NP geometry and plasmonic response. Topography, molecular adsorption, and plasmonic response are addressed by scanning tunneling microscopy, vibrational sum frequency generation (SFG) spectroscopy, and surface differential reflectance spectroscopy, respectively. Modeling CO dipolar interaction and optical reflectivity corroborate the experimental findings. We demonstrate that probing CO adsorption sites by SFG is a remarkably sensitive and relevant method to investigate shell composition and follow in real-time Pd atom migration between the core and the shell. Pd-Au alloying is limited to the first two monolayers of the shell and no plasmonic response is found, while for a thicker shell, a plasmonic response is observed, concomitant with a lower Pd concentration in the shell. Above 10-4 mbar, at room temperature, CO adsorption triggers the shell restructuration, forming a Pd-Au alloy that weakens the plasmonic response via Pd migration from the core to the shell. NP annealing at 550 K, after pumping CO, leads to the desorption of remaining CO and gives enough mobility for Pd to migrate back inside the core and recover a pure gold shell with its original plasmonic response. This work demonstrates that surface stoichiometry and plasmonic response can be tuned by using CO adsorption and NP annealing.

Controlling the reactive sites of nanoparticles (NPs) is crucial to improve catalyst efficiency. In this work, sum-frequency generation is used to probe CO vibrational spectra on MgO(100) ultrathin film/Ag(100) supported Pd nanoparticles ranging from 3 to 6 nm in diameter and compared to those of coalesced Pd NPs and Pd(100) single crystals. We aim to demonstrate in situ the role played by active adsorption sites in the catalytic CO oxidation reactivity trends varying with the NP size. From ultrahigh vacuum to the mbar range and temperatures from 293 K to 340 K, our observations suggest that bridge sites are the main active sites for CO adsorption and catalytic oxidation. On Pd(100) single crystals at 293 K, CO oxidation predominates over CO poisoning at a pressure ratio of O2/CO greater than 300; on Pd NPs, both the site coordination due to NP geometry and MgO-induced Pd-Pd interatomic distance change impact the reactivity trend varying with size in different ways. Edge sites with low coordination are more reactive than facet sites, while facet sites with a smaller Pd-Pd atomic length are more reactive than that with a larger length. The interplay of both site and size effects gives rise to a non-monotonic reactivity trend of CO on the MgO(100) ultrathin film supported Pd NPs: the reactivity of Pd NPs increases for the smaller NP size side due to a higher edge/facet ratio and meanwhile increases for the larger NP size side due to the terrace facet with a smaller Pd-Pd atomic length at the NP surface and a lower diffusion barrier.

One of the main challenges when building antibacterial surfaces with antimicrobial peptides (AMPs) is to preserve their antimicrobial activity after stable immobilization of the peptides. Among all parameters, order/conformation of self-assembled monolayers, used as spacer, is one the most important. Herein we report the covalent immobilization of the nisin Z peptide on a gold surface functionalized with a self-assembled monolayer of 11-mercaptoundecanoic acid (MUA) alone or mixed with 6-mercaptohexanol, used as a spacer. The MUA acid is activated by treatment with carbodiimide/N-hydroxysuccinimidine and then reacts with nisin Z to form amide bonds via the N terminal part of the peptide. We have characterized each step of the surface modification using X-ray photoelectron spectroscopy, FTIR-ATR spectroscopy and contact angle measurements. The combined results show the success of each functionalization step. Additionally, SFG brings information on the orientation and conformational ordering of the self-assembled monolayers. Indeed, a better order of MUA25 layers compared to MUA was observed due to the spacing of carboxylic acid groups. The antibacterial activity of the immobilized AMPs against Staphylococcus aureus is evaluated using confocal microscopy and bacterial counting: it increases with a better order of the SAMs rather than a greater peptide concentration. This study provides fundamental insights on how to engineer AMPs and substrate to produce efficient biocidal surfaces.

Understanding the role of nanoparticle size and shape in the binding of molecules is very relevant for heterogeneous catalysis and molecular electronics. The geometry of Pd nanoparticles (NPs) has been studied from very small clusters containing 4 atoms up to large (>500 atoms), well-faceted NPs. Their geometry was retrieved by combining scanning tunneling microscopy and vibrational sum frequency generation (SFG) spectroscopy of adsorbed CO. SFG has been revealed to be highly sensitive to the geometry of NPs smaller than 100 atoms by identifying the nature of CO adsorption sites. NP growth could be followed layer by layer in the critical size range corresponding to the transition from a nonmetallic to a metallic state and to oscillations of CO adsorption energy. NP height remained at two Pd planes up to 30 atoms, and adsorption energy minima correspond to the completion of successive layers.

Combining in a synergic way inorganic and organic matter at a nanometer level has become a key research pathway for nanoelectronics, light harvesting, energy storage, or sensing. Herein, we demonstrate the nanoscale ordering at room temperature of a two-dimensional hybrid assembly combining a long-range ordered array of Pd nanoparticles (NPs) with perylene molecules. The ordering, driven by the periodic superstructure of the Al2O3 ultrathin layer support, has been achieved for 0.9 nm diameter NPs and flat-lying molecules with a 2:1 perylene/NP relative ratio. At a larger NP size and perylene coverage, molecules tilt up on alumina and adsorb on NPs. Combined near-field microscopy and optical spectroscopies provide a detailed understanding of the structural properties as a function of NP size and molecular coverage. This hybrid assembly opens the way to study at the molecular level the optical and electronic properties resulting from the coupling of organic molecules and nanoparticles using multiscale surface sensitive techniques.

Many classical symmetrical designs have desirable characteristics, one of which is called D-optimality. The D-optimality concept can also be applied to select a design when the classical symmetrical designs cannot be used, such as when the experimental region is not regular in shape, when the number of experiments chosen by a classical design is too large or when one wants to apply models that deviate from the usual first or second order ones. The D-optimality concept is developed and it is also explained that D-optimality is only one possible criterion to choose a particular design. A few other criteria are also given that complement the information obtained by the D-criterion.

Obtaining insight into the type of surface sites involved in a reaction is essential to understand catalytic mechanisms at the atomic level and a key for understanding selectivity in surface-catalyzed reactions. Here we use ultrafast broad-band vibrational spectroscopy to follow in real-time diffusion of CO molecules over a palladium nanoparticle surface toward an active site. Site-to-site hopping is triggered by laser excitation of electrons and followed in real-time from subpicosecond changes in the vibrational spectra. CO photoexcitation occurs in 400 fs and hopping from NP facets to edges follows within ∼1 ps. Kinetic modeling allows to quantify the contribution of different facet sites to the catalytic reaction. These results provide useful insights for understanding the mechanism of chemical reactions catalyzed by metal NPs.

Application of preparation recipes of the literature failed to produce an ordered array of NPs on our particular Ni3Al sample. This has motivated a systematic survey of Pd NP nucleation as a function of experimental parameters. We have shown that the increase of oxidation temperature during the preparation of Al2O3 ultra-thin film on Ni3Al(111) leads to a transition from disordered to long-range ordered Pd nanoparticle (NP) nucleation. Alumina films were prepared at different temperatures ranging from 990 to 1140 K. Crystallinity, electronic structure of the alumina film and Pd nucleation and growth have been investigated using Low Energy Electron Diffraction and Scanning Tunnelling Microscopy. NP density and long-range order nucleation along the so-called "dot structure" of 4.2 nm periodicity, strongly increase for temperatures higher than a threshold value of 1070 +/- 20 K. This transition relies on the alumina film improvement and suggests that the modulation of Pd adsorption energy at nucleation centres which is necessary to nucleate NPs at ordered sites, requires higher preparation temperature. Long-range ordered NPs with a high density were obtained 140 K above reported recipes in the literature. This optimized temperature has been tested on a fresh sample (issued from the same supplier) for which just a few cleanings were enough to obtain long-range ordered NPs. Presumably the variability of the optimal oxidation temperature for our samples with respect to the literature is related to fluctuations of the stoichiometry from sample to sample. (C) 2018 Elsevier B.V. All rights reserved.

The adsorption of CO on Pd nanoparticles (NP) supported on MgO ultrathin films is studied by sum frequency generation (SFG) vibrational spectroscopy at room temperature as a function of CO coverage for NP lateral sizes ranging from 3 to 6 nm. While the spectroscopic signature of CO on (100) top facets dominates the spectra, other new bands are attributed to (111) facets, edges, and defects. CO remains strongly bonded for all sizes, but spectroscopic and kinetic parameters evolve as NP size decreases. It is only for the smallest NPs ∼3 nm in diameter that the deviation from Pd(100) single crystal is important. Modeling of adsorption kinetics, dipolar coupling and DFT calculations allow rationalizing the observations: the size dependent Pd–Pd bond extension by the substrate is the main parameter that controls CO frequency in the low coverage limit. SFG sensitivity is found to increase up to 10–4 ML as NP size decreases. This makes SFG particularly well suited to probe molecular adsorption in the submonolaye...

Confinement of hot electrons in metal nanoparticles (NPs) is expected to lead to increased reactivity in heterogeneous catalysis. NP size as well as support may influence molecule-NP coupling. Here, we use ultrafast nonlinear vibrational spectroscopy to follow energy transfer from hot electrons generated in Pd NP/MgO/Ag(100) to chemisorbed CO. Photoexcitation and photodesorption occur on an ultrashort time scale and are selective according to adsorption site. When the MgO layer is thick enough, it becomes NP size-dependent. Hot electron confinement within NPs is unfavorable for photodesorption, presumably because its dominant effect is to increase relaxation to phonons. An avenue of research is open where NP size and support thickness, photon energy, and molecular electronic structure will be tuned to obtain either molecular stability or reactivity in response to photon excitation.

The paper addresses the creation of circular optical properties from a femtosecond laser light beam with a linear polarization in an achiral material (glass) under an orthogonal incidence. In this situation, all aspects of the experiment are achiral and therefore should not give rise to chiral property creation. From that observation, we propose an interpretation that involves the action of a light-induced torque on the matter carrying a light-induced dielectric moment. We found that a direct current (DC) electric field could be produced in the lattice by the femtosecond laser in our conditions and that a non-collinear dielectric moment is created by a nonlinear effect between the DC electric field and the stress field due to the transformation of the material. We reveal that it is possible to break the chiral symmetry of glass using an intense, ultrashort laser light pulse.

Methane solids present more than one accessible crystalline phase at low temperature at zero pressure. We trap W(CO)6 in CH4 and CD4 matrices between 8 and 35 K to probe the interaction between an impurity and its surrounding molecular solid under various physical conditions. Linear and nonlinear vibrational spectroscopies of W(CO)6 highlight different kinds of interaction and reveal new and remarkable signatures of the phase transition of methane. The structures in the absorption band of the antisymmetric CO stretching mode exhibit a clear modification at the transition between phase II and phase I in CH4 and motional narrowing is observed upon temperature increase. The vibrational dynamics of this mode is probed in stimulated photon echo experiments performed with a femtosecond IR laser. A short component around 10 ps is detected in the population relaxation lifetime in the high temperature phase of solid CH4 (phase I) and disappears at lower temperatures (phase II) where the vibrational lifetime is in the hundreds of ps. The analysis of the nonlinear time-resolved results suggests that the short component comes from a fast energy transfer between the vibrational excitation of the guest and the lattice in specific families of sites. Such fast transfers are observed in the case of W(CO)6 trapped in CD4 because of an energy overlap of the excitation of W(CO)6 and a lattice vibron. In solid CH4, even when these V-V transfers are not efficient, pure dephasing processes due to the molecular nature of the host occur: they are temperature dependent without a clear modification at the phase transition.

Sum frequency generation (SFG) and low-energy electron diffraction (LEED) have been used to revisit CO adsorption on Pd(100) from very low coverages up to saturation at 300 K Below 0.5 ML, variations of SFG frequency and intensity with coverage are consistent with IRAS results from the literature. Novel observations are done above 0.5 ML, where the CO adlayer compression takes place. The existing compression model postulates the coexistence of compressed and uncompressed CO. We observe two bands in the spectral region of bridge sites and assign them to compressed and uncompressed CO. Both types of CO behave very differently: the molecular hyperpolarizability at compressed sites is smaller by a factor of 2 than at uncompressed sites. The frequency of uncompressed CO red-shifts during compression as the partial coverage decreases, while that of compressed CO continues to blue-shift as coverage increases. In the time domain, the coexistence of compressed and uncompressed sites results in oscillations in the decay of SFG intensity. A strong decrease from 690 to 222 Is of the phase relaxation time of uncompressed CO is observed during compression, indicating a stronger coupling to the substrate. These results are complemented by calculations of dipole-dipole interactions and DFT VASP calculations. While continuing blue-shift of compressed sites reflects a combination of increasing dipolar coupling and chemisorption change with coverage like below 0.5 ML, the very large red-shift amplitude of uncompressed CO indicates a large chemical contribution opposite to compressed CO. DFT VASP calculations allow us to follow the surface structure evolution from 0.5 to 0.67 ML and CO frequency changes with coverage. Pd atoms below compressed CO rows are pushed up, and compressed CO is tilted by 8-90 with respect to the surface normal. A frequency split between compressed and uncompressed CO is found in agreement with experimental data. These results suggest that while compressed CO is less strongly bonded as compression proceeds the remaining uncompressed CO is more strongly bonded.

Quantitative analysis of BroadBand Sum Frequency Generation (BBSFG) vibrational spectra by deconvolution into Lorentzian components has been shown recently to suffer from complications that depend on the spectro-temporal properties and delay of the visible pulse which has to be shaped in order to achieve spectral resolution and possibly temporal discrimination. We present a comprehensive spectro-temporal analysis of BBSFG in order to evaluate quantitatively the consequences of delay dependent spectral changes on the accuracy of BBSFG spectra. We compare purely spectral deconvolution of single spectra and spectro-temporal analysis of multiple spectra obtained with a picosecond visible pulse produced by either a Fabry–Perot (FP) etalon or a 4f pulse shaper (PS). The case of OctaDecaneThiol Self Assembled Monolayer is used for comparison of theory and experiment. Accurate relative intensities can only be obtained from the analysis of multiple spectra generated by the FP-produced visible pulse. Although the spectra obtained with the PS are not delay dependent contrarily to those of the FP, the purely spectral deconvolution does not provide accurate values of the relative intensities. The need for a spectro-temporal analysis is explained by the fact that the first-order IR polarization is distorted both in the time domain by the ps visible pulse shape and in the frequency domain by interferences between the bands that are complicated by a frequency and delay dependent phase shift.

Strong birefringence photo-induced by the femtosecond laser irradiation and a sensitivity to the orientation of the laser movement versus the laboratory reference (i.e., difference between rightward and leftward or between upward and downward) have been demonstrated in 32.5Li2O–27.5Nb2O5–40SiO2 glass. An asymmetry with the orientation of the laser scanning was firstly observed by one of the authors in 2003 in pure silica (Poumellec et al. in Opt Express 11:1070, 2003). Here, we show in addition that this asymmetric orientational sensitivity is dependent on the direction of laser polarization and the laser scanning (i.e., when the direction is horizontal or vertical). In particular, no asymmetric writing is detected when both laser scanning and polarization directions are vertical. These observations are not explained by the available model.

Vibrational dynamics of the T1u CO stretching mode of tungsten hexacarbonyl is explored when the molecule is embedded in a nitrogen matrix at low temperature. Experiments combined infrared (IR) absorption spectroscopy and IR stimulated photon echoes at the femtosecond time scale. W(CO)6 is found to be trapped in two main families of sites differing by their symmetry (called hereafter Oh and D2h sites). In Oh sites, the vibrational coherence is strongly temperature dependent, exhibiting a coupling with librational phonons of the nitrogen lattice. Perturbation in D2h sites results in the splitting of the T1u band in three components. Each component is inhomogeneously broadened, with dephasing times in the tens of picoseconds, and is weakly coupled to the lattice phonons. Experiments in solid krypton are performed to compare the effect of atomic and diatomic host lattices. Dephasing time in Kr does not depend on temperature and remains in the hundreds of picoseconds, highlighting the molecular origin of the dephasing process in N2. Additionally, nonlinear signals show oscillations due to quantum beats and polarization interferences between different frequency components of the induced third order polarization, giving information, in particular, on the overtone vibrational transition.