Jobin Cyriac

Low-energy ionic collisions at molecular solids.

ion mechanism, discussed in section 1.1.2. Figure 27 illustrates the reactive scattering mechanism with four representative snapshots of a Cs scattering trajectory in a classical MD simulation. The abstraction reaction is driven by the ion−dipole attraction force between the Cs ion and an adsorbate molecule. The impinging projectile first releases part of its initial energy to the surface (Figure 27b) even without direct collision with the adsorbate. Subsequently, the projectile pulls the adsorbate gently away from the surface in its outgoing trajectory (parts c and d of Figures 27 in sequence), leading to the formation of a Cs−molecule complex. The velocity of the outgoing Cs must be slow enough to accommodate the inertia of the adsorbate. As a result, adsorbates of low mass and small binding energy are efficiently abstracted. A heavier projectile like Cs transfers more energy to the target surface, and its lower velocity in the outgoing trajectory enhances the efficiency of reactive scattering events. Detailed aspects of Cs reactive scattering and its application for surface analysis have been reviewed. Table 9. Hyperthermal Energy Collisions at Condensed Molecular Solids method (projectile ion) system aim/observations refs reactive scattering and LES (Cs) H2O−D2O rate and activation energy of self-diffusion and H/D exchange of water 462, 476, 479, 496 H3O −water ice affinity of protons for the ice surface and proton transfer mechanism 478−480 H3O −H2O−D2O hydronium ion-mediated proton transfer at the ice surface 495 OH−H2O−D2O hydroxide ion-mediated proton transfer at the ice surface 497 HCl−water ice molecular and ionized states of HCl on ice 457, 477 Na−water ice hydrolysis of Na 484 H3O −NH3−water ice incomplete proton transfer from H3O to NH3 on the ice surface 454, 458 H3O −amine−water ice proton transfer efficiency on ice is reversed from the order of amine basicity 502 CO2−Na−water ice CO2 hydrolysis is not facilitated by a hydroxide ion 463 NO2−water ice NO2 hydrolysis produces nitrous acid 465 SO2−water ice SO2 hydrolysis occurs through various intermediates 511 C2H4−HCl−water ice electrophilic addition reaction mechanism at the condensed molecular surface 466 ethanol/2-methylpropan-2-ol−water ice SN1 and SN2 mechanisms at the condensed molecular surface 505 NH3−water ice and UV irradiation ammonium ion formation 608 CH3NH2−water ice and UV irradiation protonated methylamine formation 483 CH3NH2−CO2−water ice and UV irradiation glycine and carbamic acid formation 464 NaX−water ice (X = F, Cl, Br) surface/bulk segregation and transport properties of electrolyte ions 472−474 reactive scattering (Cs) CO and CO2 on Pt(111) mechanism of Cs + reactive ion scattering 89 Ar, Kr, Xe, and N2 on Pt(111) adsorbate mass effect on the reactive ion scattering cross-section 609 C2H4 on Pt(111) dehydrogenation mechanism of ethylene to ethylidyne 459, 610 C2D4 and H on Pt(111) ethylidene intermediate in H/D exchange reaction with ethylene 80, 610 reactive scattering (H) water ice and alcohol H2 + formation 469 CS (Ar) water ice−chloromethanes (CCl4, CHCl3, CH2Cl2) except CCl4, others undergo diffusive mixing 174 water ice−simple carboxylic acids structural reorganization on the ice film 175 water ice micropore collapse in the top layers of the ice film 176 water ice−butanol 494 Figure 27. Illustration of the reactive scattering mechanism of a Cs ion in four snapshots of a scattering trajectory from a Pt(111) surface: (a) initial positions before impact, (b) impact of the Cs and energy release to the surface, (c) Cs pulling the adsorbate away in its outgoing trajectory, (d) slow outgoing Cs dragging the adsorbate along and forming a Cs−molecule association product. Reprinted with permission from ref 88. Copyright 2004 John Wiley and Sons, Inc. Chemical Reviews Review dx.doi.org/10.1021/cr200384k | Chem. Rev. 2012, 112, 5356−5411 5388 Figure 28 shows an example of reactive collision mass spectra, which were obtained on a D2O ice film exposed first to 0.5 L of HCl gas and then to varying amounts of NH3 gas at 140 K. The spectra show peaks at higher masses than Cs (m/z 133), viz., CsNH3 + at m/z 150, Cs(D2O)n + (n = 1, 2) at m/z 153 and 173, and CsHCl at m/z 168, indicating the presence of the corresponding molecules on the surface. The intensities of H/D-exchanged species represent their original concentrations on the surface, because H/D isotopic scrambling does not occur during the ion/surface collision time (<1 × 10−12 s). The conversion efficiency of a neutral adsorbate (X) into a gaseous ion (CsX) ranges from ∼10−4 for chemisorbed species to ∼0.1 for physisorbed small molecules. Typical product ion signal intensities for ice film surfaces are much stronger than those for chemisorbed species. Also, it is worthwhile to point out that reactive collisions of Cs are ineffective for detecting large molecules such as polymers or long-chain SAM molecules. The mass spectra in Figure 28 also show LES signals corresponding to pre-existing ions on the surface. The hydronium ions seen are produced by the spontaneous ionization of HCl on the ice surface, and they undergo proton transfer reactions with NH3 to generate ammonium ions. The spectra show characteristic H/D isotopomers of each species produced by H/D exchange reactions with D2O molecules. The LES signals due to preformed hydronium and ammonium ions exhibited sputtering thresholds at Cs impact energies of 17 and 19 eV, respectively. On the other hand, on pure H2O and NH3 surfaces, these ions were emitted only above ∼60 eV due to their formation during secondary ion emission. It was also found that ultra-low-energy (a few electronvolts) collision of H with the ice surface can produce H2 +. The reaction proceeds more efficiently on amorphous solid water than crystalline water, reflecting differences in the surface concentration of dangling O−H bonds. Simple alkanols also behave in the same manner. The combined occurrence of reactive scattering and LES provides a powerful means to probe both neutral molecules and ions on surfaces and, therefore, to follow reactions on ice surfaces such as the ionization of electrolytes and acid−base reactions, which are described below. 7.2. Surface Composition and Structure Impurities in ice become concentrated in the quasi-liquid layers in the surface and at grain boundary regions due to the “freeze concentration effect”, and this has important consequences for atmospheric reactions on ice surfaces. However, there appear to be numerous exceptions to this general trend, where the surface segregation behavior of the dissolving species and their bulk solubility are determined by thermodynamic factors specific to individual chemical species. A good example is the formation of stable bulk phases of clathrate hydrates. Chemical specificity in the segregation phenomena can be studied by monitoring the surface populations of the dissolving species during the slow annealing of ice samples. Kang and coworkers examined these propensities in Na and halide ions at the surface and in the interior of ice films. They ionized NaF, NaCl, and NaBr molecules on ice films by the vapor deposition of the salts, and the variation in the surface population of the ions was monitored as a function of the ice temperature for 100−140 K by using LES. As shown in Figure 29, the LES intensities of Na and F− ions decrease with an increase in temperature above ∼120 K, whereas the Cl− and Br− intensities remain unchanged. The results indicate that Na and F− ions migrate from the ice surface to the interior at the elevated temperatures. The migration process is driven Figure 28. Cs reactive scattering and LES spectra monitoring the H3O −NH3 reaction on ice. The D2O film [3−4 bilayers (BLs), 1 BL = 1.1 × 10 water molecules cm−2] was exposed first to 0.5 L of HCl to generate hydronium ions and then to NH3 at varying exposures: (a) 0.02 L, (b) 0.3 L, (c) 0.7 L. The sample temperature was 100 K. The Cs collision energy was 30 eV. Reprinted with permission from ref 454. Copyright 2001 John Wiley and Sons, Inc. Figure 29. Surface populations of Na (□), F− (▲), Cl− (◇), and Br− (●) ions as a function of the ice film temperature measured from LES intensities of the ions. NaF, NaCl, and NaBr were deposited for a coverage of 0.8 ML for each salt on a D2O ice film grown at 130 K. The LES signals were measured at the indicated temperatures of salt adsorption. The LES intensities are shown on the normalized scale with the intensity at 100−105 K as a reference. The Cs beam energy was 35 eV. The figure is drawn on the basis of the data in refs 473 and 474. Chemical Reviews Review dx.doi.org/10.1021/cr200384k | Chem. Rev. 2012, 112, 5356−5411 5389 by the ion solvation energy, and it requires that surface water molecules have enough mobility to facilitate ion passage at temperatures above 120 K. It is worth noting that such a segregation behavior for ice agrees with the negative adsorption energy of these ions at water surfaces predicted by the Gibbs surface tension equation and MD simulations. An interesting property of hydronium ions observed in recent studies is that they preferentially reside at the surface of ice rather than in its interior. Evidence of this property has come from a variety of experimental observations over the past decade. The adsorption and ionization of HCl on an ice film promotes H/D exchange on the surface. However, vertical proton transfer to the film interior is inefficient. Continuous exposure of HCl gas on the ice film led to saturation in the hydronium ion population at the surface, and the amount of HCl uptake required for this saturation was independent of the thickness of the ice film. These observations suggest that protons stay at the ice surface and hardly migrate to the interior. This behavior can be attributed either to the active trapping of protons at the surface or to the lack of proton mobility to the ice interior. The observation of asymmetric

In situ SIMS analysis and reactions of surfaces prepared by soft landing of mass-selected cations and anions using an ion trap mass spectrometer

Mass-selected polyatomic cations and anions, produced by electrosonic spray ionization (ESSI), were deposited onto polycrystalline Au or fluorinated self-assembled monolayer (FSAM) surfaces by soft landing (SL), using a rectilinear ion trap (RIT) mass spectrometer. Protonated and deprotonated molecules, as well as intact cations and anions generated from such molecules as peptides, inorganic catalysts, and fluorescent dyes, were soft-landed onto the surfaces. Analysis of the modified surfaces was performed in situ by Cs+ secondary ion mass spectrometry (SIMS) using the same RIT mass analyzer to characterize the sputtered ions as that used to mass select the primary ions for SL. Soft-landing times as short as 30 s provided surfaces that yielded good quality SIMS spectra. Chemical reactions of the surfaces modified by SL were generated in an attached reaction chamber into which the surface was transferred under vacuum. For example, a surface on which protonated triethanolamine had been soft landed was silylated using vapor-phase chlorotrimethylsilane before being returned still under vacuum to the preparation chamber where SIMS analysis revealed the silyloxy functionalization. SL and vapor-phase reactions are complementary methods of surface modification and in situ surface analysis by SIMS is a simple way to characterize the products produced by either technique.

Vibrational spectroscopy and mass spectrometry for characterization of soft landed polyatomic molecules.

We report implementation of two powerful characterization tools, in situ secondary ion mass spectrometry (SIMS) and ex situ surface enhanced Raman spectroscopy (SERS), in analyzing surfaces modified by ion soft landing (SL). Cations derived from Rhodamine 6G are soft landed onto Raman-active silver colloidal substrates and detected using SERS. Alternatively and more conveniently, high-quality SERS data are obtained by spin coating a silver colloidal solution over the modified surface once SL is complete. Well-defined SERS features are observed for Rhodamine 6G in as little as 15 min of ion deposition. Deposition of ~3 pmo1 gave high-quality SERS spectra with characteristic spectroscopic responses being derived from just ~0.5 fmol of material. Confocal SERS imaging allowed the enhancement to be followed in different parts of deposited dried droplets on surfaces. Characteristic changes in Raman spectral features occur when Rhodamine 6G is deposited under conditions that favor gas-phase ion fragmentation. Simultaneous deposition of both the intact dye and its fragment ion occurs and is confirmed by SIMS analysis. The study was extended to other Raman active surfaces, including Au nanostar and Au coated Ni nanocarpet surfaces and to SL of other molecules including fluorescein and methyl red. Overall, the results suggest that combination of SERS and SIMS measurements are effective in the characterization of surfaces produced by ion SL with significantly enhanced molecular specificity.

Reactions of Organic Ions at Ambient Surfaces in a Solvent-Free Environment

Solvent-free ion/surface chemistry is studied at atmospheric pressure, specifically pyrylium cations, are reacted at ambient surfaces with organic amines to generate pyridinium ions. The dry reagent ions were generated by electrospraying a solution of the organic salt and passing the resulting electrosprayed droplets pneumatically through a heated metal drying tube. The dry ions were then passed through an electric field in air to separate the cations from anions and direct the cations onto a gold substrate coated with an amine. This nontraditional way of manipulating polyatomic ions has provided new chemical insights, for example, the surface reaction involving dry isolated 2,4,6-triphenylpyrylium cations and condensed solid-phase ethanolamine was found to produce the expected N-substituted pyridinium product ion via a pseudobase intermediate in a regiospecific fashion. In solution however, ethanolamine was observed to react through its N-centered and O-centered nucleophilic groups to generate two isomeric products via 2H-pyran intermediates. The O-centered nucleophile reacted less rapidly to give the minor product. The surface reaction product was characterized in situ by surface enhanced Raman spectroscopy, and ex situ using mass spectrometry and H/D exchange, and found to be chemically the same as the major pyridinium solution-phase reaction product.

In situ Raman spectroscopy of surfaces modified by ion soft landing.

The design and characterization of a system for in situ Raman analysis of surfaces prepared by ion soft landing (SL) is described. The performance of the new high vacuum compatible, low cost, surface analysis capability is demonstrated with surface enhanced Raman spectroscopy (SERS) of surfaces prepared by soft landing of ions of crystal violet, Rhodamine 6G, methyl orange and copper phthalocyanine. Complementary in situ mass spectrometric information is recorded for the same surfaces using a previously implemented secondary ion mass spectrometer (SIMS). Imaging of the modified surfaces is achieved using 2D Raman imaging as demonstrated for the case of Rhodamine 6G soft landing. The combination of the powerful molecular characterization tools of SERS and SIMS in a single instrument fitted with in-vacuum sample transport capabilities, facilitates in situ analysis of surfaces prepared by ion SL. In particular, information is provided on the charge state of the soft landed species. In the case of crystal violet the SERS data suggest that the positively charged ions being landed retain their charge state on the surface under vacuum. By contrast, in the case of methyl orange which is landed as an anion, the SERS spectra suggest that the SL species has been neutralized.

An ascorbic acid sensor based on cadmium sulphide quantum dots

AbstractWe present a Förster resonance energy transfer (FRET)-based fluorescence detection of vitamin C [ascorbic acid (AA)] using cadmium sulphide quantum dots (CdS QDs) and diphenylcarbazide (DPC). Initially, DPC was converted to diphenylcarbadiazone (DPCD) in the presence of CdS QDs to form QD–DPCD. This enabled excited-state energy transfer from the QDs to DPCD, which led to the fluorescence quenching of QDs. The QD–DPCD solution was used as the sensor solution. In the presence of AA, DPCD was converted back to DPC, resulting in the fluorescence recovery of CdS QDs. This fluorescence recovery can be used to detect and quantify AA. Dynamic range and detection limit of this sensing system were found to be 60–300 nM and 2 nM, respectively. We also performed fluorescence lifetime analyses to confirm existence of FRET. Finally, the sensor responded with equal accuracy to actual samples such as orange juice and vitamin C tablets. Graphical abstractSchematic showing the FRET based fluorescence detection of ascorbic acid

Understanding the Photoluminescence Mechanism of Nitrogen‐Doped Carbon Dots by Selective Interaction with Copper Ions

Doped fluorescent carbon dots (CDs) have drawn widespread attention because of their diverse applications and attractive properties. The present report focusses on the origin of photoluminescence in nitrogen-doped CDs (NCDs), which is unraveled by the interaction with Cu(2+) ions. Detailed spectroscopic and microscopic studies reveal that the broad steady-state photoluminescence emission of the NCDs originates from the direct recombination of excitons (high energy) and the involvement of defect states (low energy). In addition, highly selective detection of Cu(2+) is achieved, with a detection limit of 10 μm and a dynamic range of 10 μm-0.4 mm. The feasibility of the present sensor for the detection of Cu(2+) in real water samples is also presented.

Detection of PETN and RDX using a FRET-based fluorescence sensor system

Most of the fluorescence based detection of explosives involves the detection of nitro-aromatic compounds, such as trinitrotoluene (TNT) and dinitrotoluene (DNT). Here, we report a Forster resonance energy transfer (FRET)-based nanosensor system for the highly selective detection of powerful explosives, such as PETN (pentaerythritol tetranitrate) and RDX (cyclotrimethylenetrinitramine). The nanosensor system is composed of cadmium sulfide quantum dots (CdS QDs) and diphenylamine (DPA). Initially, the inherent fluorescence of DPA was quenched by resonance energy transfer to the CdS QDs. During detection, due to the strong interaction of DPA with PETN/RDX, the FRET was turned-off and was accompanied by the recovery of the donors (DPA) fluorescence. This provides an opportunity to follow the detection in a two-way manner, either the decrease in the FRET intensity at ∼585 nm or the evolution of fluorescence at ∼355 nm. The detection limits for PETN and RDX were found to be 10 nM and 20 nM, respectively. The fluorescence lifetime measurements confirmed that the energy transfer process is effective in the CdS QD–DPA sensor system. The details of the molecular interactions, between QD–DPA and DPA–analytes, were established using infrared spectroscopy. The easy one pot synthesis method of CdS QDs, excellent selectivity and very good sensitivity make the present sensor system attractive.

Nanoparticles-chemistry, new synthetic approaches, gas phase clustering and novel applications

In this paper, an overview of the synthesis, chemistry and applications of nanosystems carried out in our laboratory is presented. The discussion is divided into four sections, namely (a) chemistry of nanoparticles, (b) development of new synthetic approaches, (c) gas phase clusters and (d) device structures and applications. In ‘chemistry of nanoparticles’ we describe a novel reaction between nanoparticles of Ag and Au with halocarbons. The reactions lead to the formation of various carbonaceous materials and metal halides. In ‘development of new synthetic approaches’ our one-pot methodologies for the synthesis of core-shell nanosystems of Au, Ag and Cu protected with TiO2 and ZrO2 as well as various polymers are discussed. Some results on the interaction of nanoparticles with biomolecules are also detailed in this section. The third section covers the formation of gas phase aggregates/clusters of thiol-protected sub-nanoparticles. Laser desorption of H2MoO4, H2WO4, MoS2, and WS2 giving novel clusters is discussed. The fourth section deals with the development of simple devices and technologies using nanomaterials described above.

Direct synthesis of highly stable nitrogen rich carbon dots toward white light emission

Here we report a single step, rapid synthetic strategy for white light emitting nitrogen rich carbon dots (NCDs) under a range of excitation wavelengths by carbonizing ethylenediamine using P2O5 and water. The NCDs show unprecedented stability towards ultraviolet (UV) irradiation, extreme pH, and oxidative conditions, which is highly desired in light emitting diode (LED) applications.