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Gong, Yu

Publications and source records attributed to Gong, Yu.

31 records · Page 2
All sourcesNASA NTRSDOE OSTI
DOE OSTI2016

Synthesis and Hydrolysis of Uranyl, Neptunyl, and Plutonyl Gas-Phase Complexes Exhibiting Discrete Actinide–Carbon Bonds

In this study, gas-phase organoactinyl complexes possessing discrete An–C bonds (An = U, Np, Pu) were synthesized in a quadrupole ion trap by endothermic decarboxylation of [AnO2(O2C–R)3] - anion complexes in which a formally AnO 2 2+ actinyl core is coordinated by three carboxylate ligands, with R = CH 3 (methyl), CH 3 CC (1-propynyl), C 6 H 5 (phenyl), C 6 F 5 (pentafluorophenyl). Decarboxylation and competing ligand loss were studied computationally by density functional theory complementing experiment. Although decarboxylation was computed to be the energetically most favorable process in all cases, reduction from An(VI) to An(V) via neutral ligand loss was often prevalent, particularly for An = Np, Pu, presumably resulting from barriers associated with decarboxylation. Comparative hydrolysis rates of the An–C bonds were experimentally determined, and the chemical properties of these bonds were analyzed by the quantum theory of atoms in molecules. The measured hydrolysis rates differed by up to 3 orders of magnitude: the fastest was for [(CH 3 CC)UO 2 (O 2 C–CCCH 3 ) 2 ] - and the slowest for [(C 6 F 5 )PuO 2 (O 2 C–C 6 F 5 ) 2 ] - . There is a general correlation between hydrolysis exothermicity and hydrolysis rate. Prototypical hydrolysis reaction pathways computed for R = CH 3 (An = U, Np) reveal a mechanism in which an outer-sphere water becomes inner-sphere concomitant with transfer of an H atom to yield an OH ligand and CH 4 , with a net energy release of 170 kJ mol –1 and a transition state barrier of 45 kJ mol –1 for An = U. Infrared multiphoton dissociation spectra of selected complexes were acquired to confirm the predicted structures by agreement between the computed and observed vibrational frequencies. Lastly, the experiment and theory results provide an evaluation of the comparative propensities for formation of the organoactinyls as a function of actinide and carboxylate and an assessment of the nature and stability toward hydrolysis of the primarily ionic An–C bonds.

36 MATERIALS SCIENCE↗
DOE OSTI2015

Electrospray production and collisional dissociation of lanthanide/methylsulfonyl anion complexes: Sulfur dioxide anion as a ligand

For this study, gas-phase lanthanide-SO 2 complexes, Ln(CH 3 SO 2 ) 3 (SO 2 ) - , were produced by collision induced dissociation (CID) of Ln(CH 3 SO 2 ) 4 - precursors prepared by electrospray ionization. For all lanthanides except Eu, CID of Ln(CH 3 SO 2 ) 4 - resulted in CH 3 loss to form Ln(CH 3 SO 2 ) 3 (SO 2 ) - , which spontaneously react with O 2 to form Ln(CH 3 SO 2 ) 3 (O 2 ) - . CID of Eu(CH 3 SO 2 ) 4 - produced only Eu(CH 3 SO 2 ) 3 - , with reduction from Eu(III) to Eu(II). For Ln = Yb and Sm, the Ln(CH 3 SO 2 ) 4 - underwent neutral ligand loss to form Ln(CH 3 SO 2 ) 3 - , which reacted with O 2 to yield Ln(CH 3 SO 2 ) 3 (O 2 ) - , recovering the Ln(III) oxidation state. The CID results show parallels to condensed-phase Ln 3+ /Ln 2+ redox chemistry. Density functional theory (DFT) calculations on Ln(CH 3 SO 2 ) 3 (SO 2 ) - for Ln = La, Yb and Lu reveal that SO 2 acts as a bidentate oxygen bound ligand for doublet ground-state La(CH 3 SO 2 ) 3 (SO 2 ) - and Lu(CH 3 SO 2 ) 3 (SO 2 ) - , while the ground state for Yb(CH 3 SO 2 ) 3 (SO 2 ) - is an open-shell singlet with a monodentate SO 2 ligand. Loss of CH 3 is computed to be much more favorable than neutral ligand loss for La(CH 3 SO 2 ) 4 - and Lu(CH 3 SO 2 ) 4 - , whereas both channels are comparable in energy for Yb(CH 3 SO 2 ) 4 - , in accord with the experiments. DFT results for fragmentation of Cu(CH 3 SO 2 ) 2 - reveal that formation of the organometallic complex, Cu(CH 3 SO 2 )(CH 3 ) - , is energetically most favorable, in agreement with contrasting fragmentation pathways of copper and lanthanide complexes.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2015

Efficient Coupling and Transport of a Surface Plasmon at 780 nm in a Gold Nanostructure

We studied plasmonic nanostructures in single-crystal gold with scanning electron and femtosecond photoemission electron microscopies. We designed an integrated laser coupling and nanowire waveguide structure by focused ion beam lithography in single-crystal gold flakes. The photoemission results show that the laser field is efficiently coupled into a propagating surface plasmon by a simple hole structure and propagates efficiently in an adjacent nano-bar waveguide. A strong local field is created by the propagating surface plasmon at the nano-bar tip. A similar structure, with a decreased waveguide width and thickness, displayed significantly more intense photoemission indicating enhanced local electric field at the sharper tip.

Gong, Yu↗
DOE OSTI2015

Ultrafast Imaging of Surface Plasmons Propagating on a Gold Surface

We record time-resolved nonlinear photoemission electron microscopy (tr-PEEM) images of propagating surface plasmons (PSPs) launched from a lithographically patterned rectangular trench on a flat gold surface. Our tr-PEEM scheme involves a pair of identical, spatially separated, and interferometrically-locked femtosecond laser pulses. Power dependent PEEM images provide experimental evidence for a sequential coherent nonlinear photoemission process, in which one laser source creates a PSP polarization state through a linear interaction, and the second subsequently probes the prepared state via two photon photoemission. The recorded time-resolved movies of a PSP allow us to directly measure various properties of the surface-bound wave packet, including its carrier wavelength (785 nm) and group velocity (0.95c). In addition, tr-PEEM in concert with finite-difference time domain simulations together allow us to set a lower limit of 75 μm for the decay length of the PSP on a 100 nm thick gold film.

Environmental Molecular Sciences Laboratory↗
DOE OSTI2015

Gas Phase Uranyl Activation: Formation of a Uranium Nitrosyl Complex from Uranyl Azide

Activation of the oxo bond of uranyl, UO22+, was achieved by collision induced dissociation (CID) of UO2(N3)Cl2– in a quadrupole ion trap mass spectrometer. The gas phase complex UO2(N3)Cl2– was produced by electrospray ionization of solutions of UO2Cl2 and NaN3. CID of UO2(N3)Cl2– resulted in the loss of N2 to form UO(NO)Cl2–, in which the “inert” uranyl oxo bond has been activated. Formation of UO2Cl2– via N3 loss was also observed. Density functional theory computations predict that the UO(NO)Cl2– complex has nonplanar Cs symmetry and a singlet ground state. Analysis of the bonding of the UO(NO)Cl2– complex shows that the side-on bonded NO moiety can be considered as NO3–, suggesting a formal oxidation state of U(VI). Activation of the uranyl oxo bond in UO2(N3)Cl2– to form UO(NO)Cl2– and N2 was computed to be endothermic by 169 kJ/mol, which is energetically more favorable than formation of NUOCl2– and UO2Cl2–. The observation of UO2Cl2– during CID is most likely due to the absence of an energy barrier for neutral ligand loss.

Environmental Molecular Sciences Laboratory↗
DOE OSTI2015

Efficient forward second-harmonic generation from planar archimedean nanospirals

Here, the enhanced electric field at plasmonic resonances in nanoscale antennas can lead to efficient harmonic generation, especially when the plasmonic geometry is asymmetric on either inter-particle or intra-particle levels. The planar Archimedean nanospiral offers a unique geometrical asymmetry for second-harmonic generation (SHG) because the SHG results neither from arranging centrosymmetric nanoparticles in asymmetric groupings, nor from non-centrosymmetric nanoparticles that retain a local axis of symmetry. Here, we report forward SHG from planar arrays of Archimedean nanospirals using 15 fs pulses from a Ti:sapphire oscillator tuned to 800 nm wavelength.

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗
DOE OSTI2015

Enhanced Raman scattering from aromatic dithiols electrosprayed into plasmonic nanojunctions

We describe surface enhanced Raman spectroscopy (SERS) experiments in which molecular coverage is systematically varied from 3.8 x 105 to 3.8 x 102 to 0.38 molecules/μm2 using electrospray deposition of ethanolic 4,4’-dimercaptostilbene (DMS) solutions. The plasmonic SERS substrate used herein consists of a well-characterized 2-dimensional (2D) array of silver nanospheres [see El-Khoury et al., J. Chem. Phys., 2014, 141, 214308], previously shown to feature uniform topography and plasmonic response, as well as intense SERS activity. When compared to their ensemble averaged analogues, the spatially and temporally averaged spectra of a single molecule exhibit several unique features including: (i) distinct relative intensities of the observable Raman-active vibrational states, (ii) more pronounced SERS backgrounds, and (iii) broader Raman lines indicative of faster vibrational dephasing. The first observation may be understood on the basis of an intuitive physical picture in which removal of averaging over multiple molecules exposes the tensorial nature of Raman scattering. When an oriented single molecule gives rise to the recorded SERS spectra, the relative orientation of the molecule with respect to vector components of the local electric field determines the relative intensities of the observable vibrational states. Using a single molecule SERS framework described herein, we derive a unique molecular orientation in which a single DMS molecule is isolated at a nanojunction formed between two silver nanospheres in the 2D array. The DMS molecule is found lying nearly flat with respect to the metal surface. The derived orientation of a single molecule at a plasmonic nanojunction is consistent with observations (ii) and (iii). In particular, a careful inspection of the temporal spectral variations along the recorded single molecule SERS time sequences reveals that the time-averaged SERS backgrounds arise from individual molecular events, marked by broadened SERS signatures. We assign the broadened spectra along the SERS time sequence – which sum up to a SERS background in the averaged spectra – to instances in which the π-framework of the DMS molecule is parallel to the metal surface. This also accounts for Raman line broadening as a result of fast vibrational dephasing and driven by molecular reorientation at a plasmonic nanojunction. Furthermore, we report on the orientation dependence of single molecule SERS enhancement factors. We find that in the case of a single DMS molecule isolated at a plasmonic nanojunction, molecular orientation may affect the derived single molecule SERS enhancement factor by up to 5 orders of magnitude. Taking both chemical effects as well as molecular orientation into account, we were able to estimate a single molecule enhancement factor of ~1010 in our measurements.

Environmental Molecular Sciences Laboratory↗
DOE OSTI2014

Interferometric Plasmonic Lensing with Nanohole Arrays

Nonlinear photoemission electron microscopy (PEEM) of nanohole arrays in gold films maps propagating surface plasmons (PSPs) launched from lithographically patterned structures. Strong near field photoemission patterns are observed in the PEEM images, recorded following low angle of incidence irradiation of nanohole arrays with sub-15 fs laser pulses centered at 780 nm. The recorded photoemission patterns are attributed to constructive and destructive interferences between PSPs launched from the individual nanoholes which comprise the array. By exploiting the wave nature of PSPs, we demonstrate how varying the array geometry (hole diameter, pitch, and number of rows/columns) ultimately yields intense localized photoemission. Through a combination of PEEM and finite-difference time-domain simulations, we identify the optimal array geometry for efficient light coupling and interferometric plasmonic lensing. We show a preliminary application of inteferometric plasmonic lensing by enhancing the photoemission from the vertex of a gold triangle using nanohole array.

Gong, Yu↗
DOE OSTI2014

Electric field enhancement in a self-assembled 2D array of silver nanospheres

We investigate the plasmonic properties of a self-assembled 2D array of Ag nanospheres (average particle diameter/inter-particle separation distance of 9/3.7 nm). The structures of the individual particles and their assemblies are characterized using high-resolution transmission electron microscopy (HR-TEM). The plasmonic response of the nanoparticle network is probed using two-photon photoemission electron microscopy (TP-PEEM). HR-TEM and TP-PEEM statistics reveal the structure and plasmonic response of the network to be homogeneous on average. This translates into a relatively uniform surface-enhanced Raman scattering (SERS) response from biphenyl,4-4{sup ′}-dithiol (BPDT) molecules adsorbed onto different sites of the network. Reproducible, bright, and low-background SERS spectra are recorded and assigned on the basis of density functional theory calculations in which BPDT is chemisorbed onto the vertex of a finite tetrahedral Ag cluster consisting of 20 Ag atoms. A notable agreement between experiment and theory allows us to rigorously account for the observable vibrational states of BPDT in the ∼200–2200 cm{sup −1} region of the spectrum. Finite difference time domain simulations further reveal that physical enhancement factors on the order of 10{sup 6} are attainable at the nanogaps formed between the silver nanospheres in the 2D array. Combined with modest chemical enhancement factors, this study paves the way for reproducible single molecule signals from an easily self-assembled SERS substrate.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2014

Nonlinear Photoemission Electron Micrographs of Plasmonic Nanoholes in Gold Thin Films

Nonlinear photoemission electron microscopy of isolated nanoholes in gold thin films map propagating surface plasmon polaritons (SPPs) launched from the lithographically patterned plasmonic structures. A damped sinusoidal elongated ring-like photoemission beat pattern is observed from the nanoholes, following low angle of incidence irradiation of these structures with sub-15 fs 780 nm laser pulses. A notable agreement between finite difference time domain simulations and experiment corroborates our assignment of the observed photoemission patterns to SPPs launched from isolated nanoholes and probed through nonlinear photoemission. We also demonstrate how the efficiency of coupling light waves into isolated plasmonic holes can be tuned by varying hole diameter. In this regard, a simple intuitive geometrical model, which accounts for the observed and simulated diameter dependent plasmonic response, is proposed. Overall, this study paves the way for designing nanohole assemblies where optical coupling and subsequent plasmon propagation can be rationally controlled through 2D SPP interferometry

Environmental Molecular Sciences Laboratory↗