ABSTRACT:Boosting nonlinear frequency conversion in extremely confined volumes remains a key challenge in nano-optics, nanomedicine, photocatalysis, and background-free biosensing. To this aim, field enhancements in plasmonic nanostructures are often exploited to effectively compensate for the lack of phase-matching at the nanoscale. Second harmonic generation (SHG) is, however, strongly quenched by the high degree of symmetry in plasmonic materials at the atomic scale and in nanoantenna designs.Here, we devise a plasmonic nanoantenna lacking axial symmetry, which exhibits spatial and frequency mode overlap at both the excitation and the SHG wavelengths. The effective combination of these features in a single device allows obtaining unprecedented SHG conversion efficiency. Our results shed new light on the optimization of SHG at the nanoscale, paving the way to new classes of nanoscale coherent light sources and molecular sensing devices based on nonlinear plasmonic platforms.

We study the exciton valley relaxation dynamics in single-layer MoS 2 by a combination of two nonequilibrium optical techniques: time-resolved Faraday rotation and time-resolved circular dichroism. The depolarization dynamics, measured at 77 K, exhibits a peculiar biexponential decay, characterized by two distinct time scales of 200 fs and 5 ps. The fast relaxation of the valley polarization is in good agreement with a model including the intervalley electron-hole Coulomb exchange as the dominating mechanism. The valley relaxation dynamics is further investigated as a function of temperature and photoinduced exciton density. We measure a strong exciton density dependence of the transient Faraday rotation signal. This indicates the key role of exciton-exciton interactions in MoS 2 valley relaxation dynamics.

We demonstrate optical orientation in Ge/SiGe quantum wells and study their spin properties. The ultrafast electron transfer from the center of the Brillouin zone to its edge allows us to achieve high spin polarizations and to resolve the spin dynamics of holes and electrons. The circular polarization degree of the direct gap photoluminescence exceeds the theoretical bulk limit, yielding ∼37% and ∼85% for transitions with heavy and light holes states, respectively. The spin lifetime of holes at the top of the valence band is estimated to be ∼0.5  ps and it is governed by transitions between light and heavy hole states. Electrons at the bottom of the conduction band, on the other hand, have a spin lifetime that exceeds 5 ns below 150 K. Theoretical analysis of the spin relaxation indicates that phonon-induced intervalley scattering dictates the spin lifetime of electrons.

A major challenge in molecular electronics is to develop logic devices based on a truly intramolecular switching mechanism. Recently, a new type of molecular device has been proposed where the switching characteristic is mediated by the bistability in the position of the two hydrogen atoms which can occupy different, energetically equivalent positions (tautomerization) in the inner cavity of porphyrins and naphthalocyanines. Up to now, such a reaction has only been exploited at low temperatures and induced or detected through atomic scale manipulation. In addition, the unpredictability of the tautomer orientation currently excludes molecular interconnection to functional electronic circuits. Here, full evidence is provided that, following a newly proposed growth strategy, 2D layers of metal-free tetraphenylporphyrins (H2TPP) show frozen tautomerization even at room temperature on macroscopic domains, with the H atoms aligned along a direction settled a priori. This behavior is ascribed to the buckling of the molecule, anchored to the substrate, which removes the degeneracy between the two tautomer alignments. On this basis, a new way to exploit uniaxially oriented H2TPP tautomers in a first elementary logic device is proposed

In view of large-scale applications, electrochemical exfoliation of graphite for the production of graphene sheets must follow chemical processes that ensure high quality of the products -wide-size graphene foils, single- or few-layer thickness, and low level of defectivity -in order to guarantee high electrical transport and good mechanical properties. Understanding the exfoliation process of graphite at the atomic scale, that is, the intercalation of graphene layers in the electrolyte solution, is fundamental to really be able to control and optimize such processes. This can be obtained, for instance, by investigation of the exfoliated graphite -the surface of the original crystal left behind in the chemical solution- and by real-time monitoring of graphite surface morphological and structural modifications during the exfoliation process. Here, we monitor graphite surface changes as a function of the electrochemical potential by both electrochemical (EC) atomic force microscopy and EC scanning tunneling microscopy coupled with cyclic voltammetry. Following this strategy, we disclose the surface modifications encountered during the early stages of anion intercalation, for different electrolytes: surface faceting, step erosion, terrace damages, and nanoprotrusions, all affecting the graphite surface and therefore the exfoliation process. Our results represent a key step toward a full investigation of the intercalation process in graphite. Within the current debate on the exfoliation of layered crystals, these data potentially represent important information for investigation of the intercalation process in graphite and, on the other hand, for further optimization of the electrochemical protocol for graphene production

In monolayer (1L) transition metal dichalcogenides (TMDs) the valence and conduction bands are spin-split because of the strong spin−orbit interaction. In tungsten-based TMDs the spin-ordering of the conduction band is such that the so-called dark excitons, consisting of electrons and holes with opposite spin orientation, have lower energy than A excitons. The transition from bright to dark excitons involves the scattering of electrons from the upper to the lower conduction band at the K point of the Brillouin zone, with detrimental effects for the optoelectronic response of 1L-TMDs, since this reduces their light emission efficiency. Here, we exploit the valley selective optical selection rules and use two-color helicity-resolved pump−probe spectroscopy to directly measure the intravalley spin−flip relaxation dynamics in 1L-WS 2 . This occurs on a sub-ps time scale, and it is significantly dependent on temperature, indicative of phonon-assisted relaxation. Time-dependent ab initio calculations show that intravalley spin−flip scattering occurs on significantly longer time scales only at the K point, while the occupation of states away from the minimum of the conduction band significantly reduces the scattering time. Our results shed light on the scattering processes determining the light emission efficiency in optoelectronic and photonic devices based on 1L-TMDs.

We report on a combined X-ray and UV photoemission spectroscopy study (XPS and UPS) of organicinorganic perovskites prepared from a solution of lead chloride (PbCl 2 ) and methylammonium iodide (CH 3 NH 3 I). The XPS intensities are consistent with a pure iodide perovskite (CH 3 NH 3 PbI 3 ), with no detectable chloride left. However, we found that the elimination of chloride results in residual methylamine molecules (CH 3 NH 2 ) trapped within the perovskite crystal lattice. Furthermore, we show that vacuum annealing or sputtering induce the formation of a thin PbI 2 layer at the crystal surface which acts as a surface barrier blocking electron transfer from the underlying perovskite film.

We have studied the magnetoresistance (TMR) of tunnel junctions with electrodes of La 2/3 Sr 1/3 MnO3 and we show how the variation of the conductance and TMR with the bias voltage can be exploited to obtain a precise information on the spin and energy dependence of the density of states. Our analysis leads to a quantitative description of the band structure of La 2/3 Sr 1/3 MnO3 and allows the determination of the gap δ between the Fermi level and the bottom of the t2g minority spin band, in good agreement with data from spin-polarized inverse photoemission experiments. This shows the potential of magnetic tunnel junctions with half-metallic electrodes for spin-resolved spectroscopic studies.PACS numbers: 75.47. Lx, 79.60.Jv A magnetic tunnel junction (MTJ) is composed of two conducting ferromagnetic electrodes separated by a thin insulating barrier. Its resistance depends on the relative orientation of the magnetizations of the electrodes, a property which is called TMR (Tunneling Magnetoresistance). The TMR ratio is defined aswhere R P and R AP are the junction resistances in the parallel (P) and antiparallel (AP) configuration respectively. The MTJs are extensively investigated for the interest of the TMR in spintronic devices such as MRAM (Magnetic Random Access Memory) or magnetic sensors [1], but they also raise interesting fundamental problems. In this Letter, we present an example of exploitation of the TMR to obtain a precise information on the spin and energy dependence of the density of states (DOS) of a ferromagnetic conductor. This shows the potential of MTJs for spin-resolved spectroscopic studies. Electron tunneling at different bias voltages (V DC ) probes different energy ranges of the DOS and this was first used to extract information on the electronic structure of a superconducting electrode by Giaever in 1960 [2] : the presence of a quasi-particle gap in the DOS of the superconductor is reflected in the voltage dependence of the current tunneling into the superconductor thus allowing a quantitative determination of this gap. More recently, Xiang et al [3] have also performed a numerical analysis of TMR vs V DC curves in transition metal based MTJs to estimate the spin-dependent DOS of a Co collecting electrode. However, the relatively narrow energy range (E F ±0.4 eV) probed by TMR is not well suited to investigate the DOS of a wide band transition metal. We will see that the technique is more appropriate for narrow band metallic oxides.Also, conceptually, to extract information on the DOS above the Fermi level of a ferromagnetic collecting electrode from the bias dependence of the TMR, it is highly desirable to use a fully spin-polarized emitting electrode, i.e. a half-metal [4]. Supposing, for example, a halfmetal (HM) electrode emitting only electrons of its majority spin direction, the tunneling will probe separately the majority-spin DOS of the collecting electrode in the P configuration (mainly at an energy eV DC above E F ) and the minority one in the AP configuration.The MTJs of...