Short, Long and Spatial Dynamics of Informal Employment

<p>Di Caro P. and Nicotra G. Short, long and spatial dynamics of informal employment, <i>Regional Studies</i>. This paper analyses regional shadow labour markets. Cross-regional migration flows are introduced in a stochastic two-sector model used to study the effects of regional interactions on informality. Empirical results show that informal activities across Italian regions are driven by the inefficient provision of public goods and high taxes. Regional connections are found to be significant. Place-specific reactions of informal employment to national shocks in the official economy are investigated, finding that the informal sector can act as a complement to or a substitute for formal activities. The summary of the results and policy conclusions are then discussed.</p

Delaminated Graphene at Silicon Carbide Facets: Atomic Scale Imaging and Spectroscopy

Atomic-resolution structural and spectroscopic characterization techniques (scanning transmission electron microscopy and electron energy loss spectroscopy) are combined with nanoscale electrical measurements (conductive atomic force microscopy) to study at the atomic scale the properties of graphene grown epitaxially through the controlled graphitization of a hexagonal SiC(0001) substrate by high temperature annealing. This growth technique is known to result in a pronounced electron-doping (∼10¹³ cm^–2) of graphene, which is thought to originate from an interface carbon buffer layer strongly bound to the substrate. The scanning transmission electron microscopy analysis, carried out at an energy below the knock-on threshold for carbon to ensure no damage is imparted to the film by the electron beam, demonstrates that the buffer layer present on the planar SiC(0001) face delaminates from it on the (112̅<i>n</i>) facets of SiC surface steps. In addition, electron energy loss spectroscopy reveals that the delaminated layer has a similar electronic configuration to purely sp²-hybridized graphene. These observations are used to explain the local increase of the graphene sheet resistance measured around the surface steps by conductive atomic force microscopy, which we suggest is due to significantly lower substrate-induced doping and a resonant scattering mechanism at the step regions. A first-principles-calibrated theoretical model is proposed to explain the structural instability of the buffer layer on the SiC facets and the resulting delamination

Programmable Extreme Chirality in the Visible by Helix-Shaped Metamaterial Platform

The capability to fully control the chiro-optical properties of metamaterials in the visible range enables a number of applications from integrated photonics to life science. To achieve this goal, a simultaneous control over complex spatial and localized structuring as well as material composition at the nanoscale is required. Here, we demonstrate how circular dichroic bands and optical rotation can be effectively and independently tailored throughout the visible regime as a function of the fundamental meta-atoms properties and of their three dimensional architecture in a the helix-shaped metamaterials. The record chiro-optical effects obtained in the visible range are accompanied by an additional control over optical efficiency, even in the plasmonic context. These achievements pave the way toward fully integrated chiral photonic devices

Nanoscale Study of the Tarnishing Process in Electron Beam Lithography-Fabricated Silver Nanoparticles for Plasmonic Applications

Silver is the ideal material for plasmonics because of its low loss at optical frequencies, though it is often replaced by a lossier metal, gold. This is because of silver’s tendency to tarnish, an effect which is enhanced at the nanoscale due to the large surface-to-volume ratio. Despite chemical tarnishing of Ag nanoparticles (NPs) has been extensively studied for decades, it has not been well understood whether resulted by sulfidation or oxidation processes. This intriguing quest is herein rationalized by studying the atmospheric corrosion of electron beam lithography-fabricated Ag NPs, through nanoscale investigation performed by high-resolution transmission electron microscopy (HRTEM) combined with electron energy loss (EEL) and energy dispersive X-ray (EDX) spectroscopies. We demonstrate that tarnishing of Ag NPs upon exposure to indoor air of an environment located inside a rural site, not particularly influenced by naturally and human-made sulfur sources, is caused by chemisorbed sulfur-based contaminants rather than via an oxidation process. Furthermore, we show that the sulfidation occurs through the formation of crystalline Ag₂S bumps onto Ag surface in place of a homogeneous growth of a silver sulfide film. From a single 2D Z-contrast scanning transmission electron microscopy image, a method for 3D reconstruction of silver nanoparticles with extremely high spatial resolution has been derived thus establishing the preferential nucleation of Ag₂S bumps in proximity of lattice defects located on the NP surface. Finally, we also provide a straightforward and low-cost solution to achieve stable Ag NPs by passivating them with a self-assembled monolayer of hexanethiols. The sulfidation mechanism inhibition allows to prevent the increased material damping and scattering losses

Tailoring Electromagnetic Hot Spots toward Visible Frequencies in Ultra-Narrow Gap Al/Al₂O₃ Bowtie Nanoantennas

Plasmonic bowtie nanoantennas are intriguing nanostructures, capable to achieve very high local electromagnetic (EM) field confinement and enhancement in the hot spots. This effect is strongly dependent on the gap size, which in turn is related to technological limitations. Ultranarrow gap bowtie nanoantennas, operating at visible frequencies, can be of great impact in biosensing applications and in the study of strong light–matter interactions with organic molecules. Here, we present a comprehensive study on the structural and optical properties of aluminum bowties, realized with ultranarrow gap by He⁺-ion milling lithography, and operating from the near-infrared to the red part of the visible range. Most importantly, this analysis demonstrates that large EM near-field enhancement and different hot spot spatial positions, as a function of nanometer-sized gaps, are constrained by the native aluminum oxide, thus, working as hot spot ruler

Ambipolar MoS₂ Transistors by Nanoscale Tailoring of Schottky Barrier Using Oxygen Plasma Functionalization

One of the main challenges to exploit molybdenum disulfide (MoS₂) potentialities for the next-generation complementary metal oxide semiconductor (CMOS) technology is the realization of p-type or ambipolar field-effect transistors (FETs). Hole transport in MoS₂ FETs is typically hampered by the high Schottky barrier height (SBH) for holes at source/drain contacts, due to the Fermi level pinning close to the conduction band. In this work, we show that the SBH of multilayer MoS₂ surface can be tailored at nanoscale using soft O₂ plasma treatments. The morphological, chemical, and electrical modifications of MoS₂ surface under different plasma conditions were investigated by several microscopic and spectroscopic characterization techniques, including X-ray photoelectron spectroscopy (XPS), atomic force microscopy (AFM), conductive AFM (CAFM), aberration-corrected scanning transmission electron microscopy (STEM), and electron energy loss spectroscopy (EELS). Nanoscale current–voltage mapping by CAFM showed that the SBH maps can be conveniently tuned starting from a narrow SBH distribution (from 0.2 to 0.3 eV) in the case of pristine MoS₂ to a broader distribution (from 0.2 to 0.8 eV) after 600 s O₂ plasma treatment, which allows both electron and hole injection. This lateral inhomogeneity in the electrical properties was associated with variations of the incorporated oxygen concentration in the MoS₂ multilayer surface, as shown by STEM/EELS analyses and confirmed by ab initio density functional theory (DFT) calculations. Back-gated multilayer MoS₂ FETs, fabricated by self-aligned deposition of source/drain contacts in the O₂ plasma functionalized areas, exhibit ambipolar current transport with on/off current ratio <i>I</i>_on/<i>I</i>_off ≈ 10³ and field-effect mobilities of 11.5 and 7.2 cm² V^–1 s^–1 for electrons and holes, respectively. The electrical behavior of these novel ambipolar devices is discussed in terms of the peculiar current injection mechanisms in the O₂ plasma functionalized MoS₂ surface