Understanding the plasmonics of nanostructured atomic force microscopy tips

Structured metallic tips are increasingly important for optical spectroscopies such as tip-enhanced Raman spectroscopy, with plasmonic resonances frequently cited as a mechanism for electric field enhancement. We probe the local optical response of sharp and spherical-tipped atomic force microscopy (AFM) tips using a scanning hyperspectral imaging technique to identify the plasmonic behaviour. Localised surface plasmon resonances which radiatively couple with far-field light are found only for spherical AFM tips, with little response for sharp AFM tips, in agreement with numerical simulations of the near-field response. The precise tip geometry is thus crucial for plasmon-enhanced spectroscopies, and the typical sharp cones are not preferred.The authors thank EPSRC Grant Nos. EP/G060649/1, EP/K028510/1, and EP/L027151/1, and ERC Grant No. LINASS 320503 for funding and NanoTools for their services providing Au-coated spherical AFM tips. R.W.B. thanks Queens' College and the Royal Commission for the Exhibition of 1851 for financial support

Plasmonic tunnel junctions for single-molecule redox chemistry

Nanoparticles attached just above a flat metallic surface can trap optical fields in the nanoscale gap. This enables local spectroscopy of a few molecules within each coupled plasmonic hotspot, with near thousand-fold enhancement of the incident fields. As a result of non-radiative relaxation pathways, the plasmons in such sub-nanometre cavities generate hot charge carriers, which can catalyse chemical reactions or induce redox processes in molecules located within the plasmonic hotspots. Here, surface-enhanced Raman spectroscopy allows us to track these hot-electron-induced chemical reduction processes in a series of different aromatic molecules. We demonstrate that by increasing the tunnelling barrier height and the dephasing strength, a transition from coherent to hopping electron transport occurs, enabling observation of redox processes in real time at the single-molecule level.We acknowledge financial support from EPSRC grants EP/G060649/1, EP/I012060/1, EP/L027151/1, ERC grant LINASS 320503. F.B. acknowledges support from the Winton Programme for the Physics of Sustainability. S.J.B. thanks the European Commission for a Marie Curie Fellowship (NANOSPHERE, 658360). M.K. thanks the European Commission for a Marie Curie Fellowship (SPARCLEs, 7020005). P.N. acknowledges support from the Harvard University Center for the Environment (HUCE). R.C. acknowledges support from the Dr Manmohan Singh scholarship from St John’s College. C.C. acknowledges support from the UK National Physical Laboratories. R.S. acknowledges computational resources provided by the Center for Computational Innovations (CCI) at Rensselaer Polytechnic Institute

Childbearing postponement and child well-being: a complex and varied relationship?

Over the past several decades, U.S. fertility has followed a trend toward the postponement of motherhood. The socioeconomic causes and consequences of this trend have been the focus of attention in the demographic literature. Given the socioeconomic advantages of those who postpone having children, some authors have argued that the disadvantage experienced by certain groups would be reduced if they postponed their births. The weathering hypothesis literature, by integrating a biosocial perspective, complicates this argument and posits that the costs and benefits of postponement may vary systematically across population subgroups. In particular, the literature on the weathering hypothesis argues that as a consequence of their unique experiences of racism and disadvantage, African American women may experience a more rapid deterioration of their health, which could offset or eventually reverse any socioeconomic benefit of postponement. But because very few African American women postpone motherhood, efforts to find compelling evidence to support the arguments of this perspective rely on a strategy of comparison that is problematic because a potentially selected group of older black mothers are used to represent the costs of postponement. This might explain why the weathering hypothesis has played a rather limited role in the way demographers conceptualize postponement and its consequences for well-being. In order to explore the potential utility of this perspective, we turn our attention to the UK context. Because first-birth fertility schedules are similar for black and white women, we can observe (rather than assume) whether the meaning and consequences of postponement vary across these population subgroups. The results, obtained using linked UK census and birth record data, reveal evidence consistent with the weathering hypothesis in the United Kingdom and lend support to the arguments that the demographic literature would benefit from integrating insights from this biosocial perspective

Understanding the plasmonics of nanostructured atomic force microscopy tips

Structured metallic tips are increasingly important for optical spectroscopies such as tip-enhanced Raman spectroscopy, with plasmonic resonances frequently cited as a mechanism for electric field enhancement. We probe the local optical response of sharp and spherical-tipped atomic force microscopy (AFM) tips using a scanning hyperspectral imaging technique to identify the plasmonic behaviour. Localised surface plasmon resonances which radiatively couple with far-field light are found only for spherical AFM tips, with little response for sharp AFM tips, in agreement with numerical simulations of the near-field response. The precise tip geometry is thus crucial for plasmon-enhanced spectroscopies, and the typical sharp cones are not preferred

Unfolding the contents of sub-nm plasmonic gaps using normalising plasmon resonance spectroscopy

Plasmonic coupling of gold nanoparticles to a gold surface creates intense plasmonic hot spots with large electromagnetic field-enhancements within the cavity formed by the two metallic surfaces. The localised field in such structures is extremely sensitive to morphological fluctuations and subtle changes in the dielectric properties of the cavity contents. Here, we present an optical method that pins down the properties of the gap contents with high sensitivity, termed normalising plasmon resonance (NPR) spectroscopy. We use this on a variety of ultrathin molecular spacers such as filled and empty cucurbiturils, and graphene. Clear differences in the spectral positions and intensities of plasmonic modes observed in the scattering spectrum resolve thickness differences of 0.1 nm, and refractive index changes from molecular filling

Research data supporting "Plasmonic Tunnel Junctions for Single-Molecule Redox Chemistry"

Research data supporting the publication "Plasmonic Tunnel Junctions for Single-Molecule Redox Chemistry", including FDTD, and DFT simulation results and experimental SERS data in text form. Description of how data were collected is available in the associated publication

Controlling Sub-nm Gaps in Plasmonic Dimers using Graphene

Graphene is used as the thinnest possible spacer between gold nanoparticles and a gold substrate. This creates a robust, repeatable, and stable sub-nanometre gap for massive plasmonic field enhancements. White light spectroscopy of single 80 nm gold nanoparticles reveals plasmonic coupling between the particle and its image within the gold substrate. While for a single graphene layer, spectral doublets from coupled dimer modes are observed shifted into the near infra-red, these disappear for increasing numbers of layers. These doublets arise from plasmonic charge transfer, allowing the direct optical measurement of out-of-plane conductivity in such layered systems. Gating the graphene can thus directly produce plasmon tuning