a These data represent the surface measured "as is", and likely contains carbon and oxygen from atmospheric sources. b These species were below the detection limit of XPS.Communication pubs.acs.org/cm

The properties of the high order cladding modes of standard optical fibers are measured in real-time during the deposition of gold nanoparticle layers by chemical vapor deposition (CVD). Using a tilted fiber Bragg grating (TFBG), the resonance wavelength and peak-to-peak amplitude of a radially polarized cladding mode resonance located 51 nm away from the core mode reflection resonance shift by 0.17 nm and 13.54 dB respectively during the formation of a ~200 nm thick layer. For the spectrally adjacent azimuthally polarized resonance, the corresponding shifts are 0.45 nm and 16.34 dB. In both cases, the amplitudes of the resonance go through a pronounced minimum of about 5 dB for thickness between 80 and 100 nm and at the same time the wavelengths shift discontinuously. These effects are discussed in terms of the evolving metallic boundary conditions perceived by the cladding modes as the nanoparticles grow. Scanning Electron Micrographs and observations of cladding mode light scattering by nanoparticle layers of various thicknesses reveal a strong correlation between the TFBG polarized transmission spectra, the grain size and fill factor of the nanoparticles, and the scattering efficiency. This allows the preparation of gold nanoparticle layers that strongly discriminate between radially and azimuthally polarized cladding mode evanescent fields, with important consequences in the plasmonic properties of these layers.

The geometry-and size-dependent effective medium properties of ultrathin gold films deposited on the bare cladding of single mode optical fibers by chemical vapor deposition are characterized by measuring the polarized transmission spectra of in-fiber gratings at wavelengths near 1550 nm. The real part of the complex refractive indices of films with average thicknesses ranging from 6 to 65 nm are about 10 times higher than that of bulk gold at these wavelengths, while the imaginary part values are 2 orders of magnitude lower. The films are essentially isotropic, apart from a small increasing dichroism between the in-plane and out-of-plane component of the imaginary part of the refractive index at thicknesses larger than 25 nm. Unlike gold films prepared by other means, the optical properties of the coatings do not converge rapidly toward bulk values at thicknesses larger than 10 nm but remain characteristic of gold films prepared by very slow physical deposition processes. The modified Clausius−Mossotti theory for anisotropic structures was used to confirm that the observed properties arise from a persistent granularity of the film at larger thicknesses, with metal filling fractions increasing from 30% to 68% and particle aspect ratios from 0.8 to 1.0 (spherical). These conclusions are supported by nanoparticle shape measurements obtained by atomic force microscopy and scanning electron microscope images.

Surface-supported, low-dimensional gold nanostructures are of interest for plasmonic applications. Low dimensional nanostructures are readily accessible by solution-phase growth, where shape control through the addition of growthdirecting surfactants is well established. Yet, shape control in chemical vapour deposition (CVD) has not been well explored, and metallic gold films are typically limited to nanoparticulate or thin film morphologies. This article describes the self-seeded growth of high aspect ratio gold plates and wires by CVD. A directed growth mechanism is proposed, where growth is directed by the coordinating N-heterocyclic carbene (NHC) and phosphine ligands originating from the thermal decomposition of the two gas-phase precursors, [Au(HMDS)(NHC)] (HMDS = hexamethyldisilazide, NHC = 1,3-diisopropyl-imidazolidin-2-ylidene) and [Au(HMDS)(PMe 3 )]. These ligands acted as transient surfactants for plate growth between 370 and 460°C and at high precursor flux. Energy dispersive X-ray spectroscopy (EDS) and X-ray photoelectron spectroscopy (XPS) evidence indicates that hydroxyl terminated substrate surfaces are passivated with trimethylsilyl (TMS) moieties originating from the HMDS ligand in both precursors, which promoted island type growth and directed precursor decomposition to occur on gold surfaces. Secondary nucleation is observed on all gold structures, and is a crucial component to gas-phase surfactant-mediated CVD growth. This work identifies the potential to use precursor-bound coordinative ligands or gas-phase surfactants to direct growth of metal nanocrystals by CVD.

Atomic layer deposition (ALD) of gold is being studied by multiple research groups, but to date no process using non‐energetic co‐reactants has been demonstrated. In order to access milder co‐reactants, precursors with higher thermal stability are required. We set out to uncover how structure and bonding affect the stability and volatility of a family of twelve organogold(I) compounds using a combination of techniques: X‐ray diffraction (XRD), thermogravimetric analysis (TGA), differential scanning calorimetry (DSC), and density functional theory (DFT). Small, unsubstituted phosphonium ylide ligands bind more strongly to Au(I) than their silyl‐substituted analogues, but the utility of both these ligands suffers due to their poor volatility and substantial thermal decomposition. Pentafluorophenyl (C6F5) is introduced as a new, very electronegative ligand for gold vapor deposition precursors, and it was found that the disadvantage to volatility due to π‐stacking and other intermolecular interactions in the solid state was overshadowed by dramatic improvements to kinetic and thermodynamic stability. We introduce a new figure of merit to compare and rank the suitability of these and other complexes as precursors for vapor deposition. Finally, DFT calculations on four compounds that have high figures of merit show a linear correlation between the gold‐coordinative ligand bond dissociation energies and the observed decomposition temperatures, highlighting and justifying this design strategy.