6 research outputs found
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 (∼10<sup>13</sup> cm<sup>–2</sup>) 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 sp<sup>2</sup>-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 Ag<sub>2</sub>S 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 Ag<sub>2</sub>S 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/Al<sub>2</sub>O<sub>3</sub> 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<sup>+</sup>-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 MoS<sub>2</sub> Transistors by Nanoscale Tailoring of Schottky Barrier Using Oxygen Plasma Functionalization
One
of the main challenges to exploit molybdenum disulfide (MoS<sub>2</sub>) 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 MoS<sub>2</sub> 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 MoS<sub>2</sub> surface can be tailored at nanoscale using
soft O<sub>2</sub> plasma treatments. The morphological, chemical,
and electrical modifications of MoS<sub>2</sub> 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 MoS<sub>2</sub> to a broader distribution (from 0.2 to 0.8
eV) after 600 s O<sub>2</sub> 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 MoS<sub>2</sub> multilayer surface, as shown
by STEM/EELS analyses and confirmed by ab initio density functional
theory (DFT) calculations. Back-gated multilayer MoS<sub>2</sub> FETs,
fabricated by self-aligned deposition of source/drain contacts in
the O<sub>2</sub> plasma functionalized areas, exhibit ambipolar current
transport with on/off current ratio <i>I</i><sub>on</sub>/<i>I</i><sub>off</sub> ≈ 10<sup>3</sup> and field-effect
mobilities of 11.5 and 7.2 cm<sup>2</sup> V<sup>–1</sup> s<sup>–1</sup> 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 O<sub>2</sub> plasma
functionalized MoS<sub>2</sub> surface