arXiv subjects

Marta De Luca

Publications and source records attributed to Marta De Luca.

8 recordsLinked to original sources

arXiv2024

Single photon emitters in thin GaAsN nanowire tubes grown on Si

III-V nanowire heterostructures can act as sources of single and entangled photons and are enabling technologies for on-chip applications in future quantum photonic devices. The unique geometry of nanowires allows to integrate lattice-mismatched components beyond the limits of planar epilayers and to create radially and axially confined quantum structures. Here, we report the plasma-assisted molecular beam epitaxy growth of thin GaAs/GaAsN/GaAs core-multishell nanowires monolithically integrated on Si (111) substrates, overcoming the challenges caused by the low solubility of N and a high lattice mismatch. The nanowires have a GaAsN shell of 10 nm containing 2.7% N, which reduces the GaAs bandgap drastically by 400 meV. They have a symmetric core-shell structure with sharp boundaries and a defect-free zincblende phase. The high structural quality reflects in their excellent opto-electroinic properties, including remarkable single photon emission from quantum confined states in the thin GaAsN shell with a second-order autocorrelation function at zero time delay as low as 0.056.

physics.optics↗

arXiv2021

Low charge-noise nitrogen-vacancy centers in diamond created using laser writing with a solid-immersion lens

We report on pulsed-laser induced generation of nitrogen-vacancy (NV) centers in diamond facilitated by a solid-immersion lens (SIL). The SIL enables laser writing at energies as low as 5.8 nJ per pulse and allows vacancies to be formed close to a diamond surface without inducing surface graphitization. We operate in the previously unexplored regime where lattice vacancies are created following tunneling breakdown rather than multiphoton ionization. We present three samples in which NV-center arrays were laser-written at distances between ~1 $μ$m and 40 $μ$m from a diamond surface, all presenting narrow distributions of optical linewidths with means between 62.1 MHz and 74.5 MHz. The linewidths include the effect of long-term spectral diffusion induced by a 532 nm repump laser for charge-state stabilization, thereby emphasizing the particularly low charge-noise environment of the created color centers. Such high-quality NV centers are excellent candidates for practical applications employing two-photon quantum interference with separate NV centers. Finally, we propose a model for disentangling power broadening from inhomogeneous broadening in the NV center optical linewidth.

physics.optics↗

arXiv2021

Probing Lattice Dynamics and Electronic Resonances in Hexagonal Ge and SixGe1-x Alloys in Nanowires by Raman Spectroscopy

Recent advances in nanowire synthesis have enabled the realization of crystal phases that in bulk are attainable only under extreme conditions, i.e. high temperature and/or high pressure. For group IV semiconductors this means access to hexagonal-phase SixGe1-x nanostructures (with a 2H type of symmetry), which are predicted to have a direct band gap for x up to 0.5 - 0.6 and would allow the realization of easily processable optoelectronic devices. Exploiting the quasi-perfect lattice matching between GaAs and Ge, we synthesized hexagonal phase GaAs-Ge and GaAs-SixGe1-x core-shell nanowires with x up to 0.59. By combining position-, polarization- and excitation wavelength-dependent u-Raman spectroscopy studies with first-principles calculations, we explore the full lattice dynamics of these materials. In particular, by obtaining frequency-composition calibration curves for the phonon modes, investigating the dependence of the phononic modes on the position along the nanowire, and exploiting resonant Raman conditions to unveil the coupling between lattice vibrations and electronic transitions, we lay the grounds for a deep understanding of the phononic properties of 2H-SixGe1-x nanostructured alloys and of their relationship with crystal quality, chemical composition, and electronic band structure.

cond-mat.mtrl-sci↗

arXiv2020

Experimental demonstration of the suppression of optical phonon splitting in 2D materials by Raman spectroscopy

Raman spectroscopy is one of the most extended experimental techniques to investigate thin-layered 2D materials. For a complete understanding and modeling of the Raman spectrum of a novel 2D material, it is often necessary to combine the experimental investigation to density-functional-theory calculations. We provide the experimental proof of the fundamentally different behavior of polar 2D vs 3D systems regarding the effect of the dipole-dipole interactions, which in 2D systems ultimately lead to the absence of optical phonons splitting, otherwise present in 3D materials. We demonstrate that non-analytical corrections (NACs) should not be applied to properly model the Raman spectra of few-layered 2D materials, such as WSe$_{2}$ and h-BN, corroborating recent theoretical predictions [Nano Lett. 2017, 17 (6), 3758-3763]. Our findings are supported by measurements performed on tilted samples that allow increasing the component of photon momenta in the plane of the flake, thus unambiguously setting the direction of an eventual NAC. We also investigate the influence of the parity of the number of layers and of the type of layer-by-layer stacking on the effect of NACs on the Raman spectra.

cond-mat.mtrl-sci↗

arXiv2019

A Universal Length-Dependent Vibrational Mode in Graphene Nanoribbons

Graphene nanoribbons (GNRs) have attracted considerable interest as their atomically tunable structure makes them promising candidates for future electronic devices. However, obtaining detailed information about the length of GNRs has been challenging and typically relies on low-temperature scanning tunneling microscopy. Such methods are ill-suited for practical device application and characterization. In contrast, Raman spectroscopy is a sensitive method for the characterization of GNRs, in particular for investigating their width and structure. Here, we report on a length-dependent, Raman active low-energy vibrational mode that is present in atomically precise, bottom-up synthesized armchair graphene nanoribbons (AGNRs). Our Raman study demonstrates that this mode is present in all families of AGNRs and provides information on their length. Our spectroscopic findings are corroborated by scanning tunneling microscopy images and supported by first-principles calculations that allow us to attribute this mode to a longitudinal acoustic phonon. Finally, we show that this mode is a sensitive probe for the overall structural integrity of the ribbons and their interaction with technologically relevant substrates.

cond-mat.mes-hall↗

arXiv2019

New insights in the lattice dynamics of monolayers, bilayers, and trilayers of WSe2 and unambiguous determination of few-layer-flakes' thickness

Among the most common few-layers transition metal dichalcogenides (TMDs), WSe2 is the most challenging material from the lattice dynamics point of view. Indeed, for a long time the main two phonon modes (A1g and E12g) have been wrongly assigned. In the last few years, these two modes have been properly interpreted, and their quasi-degeneracy in the monolayer has been used for its identification. In this work, we show that this approach has a limited validity and we propose an alternative, more general approach, based on multi-phonon bands. Moreover, we show and interpret all the peaks (about 40) appearing in the Raman spectra of monolayers, bilayers, and trilayers of WSe2 by combining experimental wavelength- and polarization-dependent Raman studies with density-functional theory calculations providing the phonon dispersions, the polarization-resolved first-order Raman spectra, and the one- and two-phonon density of states. This complete study not only offers a method to distinguish between monolayers, bilayers, and trilayers with no need of optical images and atomic force microscopy, but it also sheds light on the interpretation of single and multi-phonon bands appearing in the inelastic light scattering experiments of layered WSe2; some of these bands were never observed before, and some were observed and uncertainly assigned. We promote the full understanding of the lattice dynamics of this material that is crucial for the realization of optoelectronics devices and of novel phononic metamaterials, such as TMDs superlattices.

cond-mat.mtrl-sci↗

arXiv2019

Common nonlinear features and spin-orbit coupling effects in the Zeeman splitting of novel wurtzite materials

The response of semiconductor materials to external magnetic fields is a reliable approach to probe intrinsic electronic and spin-dependent properties. In this study, we investigate the common Zeeman splitting features of novel wurtzite materials, namely InP, InAs, and GaAs. We present values for the effective g-factors of different energy bands and show that spin-orbit coupling effects, responsible for the spin splittings, also have noticeable contributions to the g-factors. Within the Landau level picture, we show that the nonlinear Zeeman splitting recently explained in magneto photoluminescence experiments for InP nanowires by Tedeschi et al. [Phys. Rev. B 99, 161204 (2019)] are also present in InAs, GaAs and even in the conventional GaN. Such nonlinear features stem from the peculiar coupling of the A and B valence bands, as a consequence of the interplay between the wurtzite crystal symmetry and the breaking of time-reversal symmetry by the external magnetic field. Moreover, we develop an analytical model to describe the experimental nonlinear Zeeman splitting and apply it to InP and GaAs data. Extrapolating our fitted results, we found that the Zeeman splitting of InP reaches a maximum value, which is a prediction that could be probed at higher magnetic fields.

cond-mat.mes-hall↗

arXiv2017

Effects of Dielectric Stoichiometry on the Photoluminescence Properties of Encapsulated WSe2 Monolayers

Two-dimensional transition-metal-dichalcogenide semiconductors have emerged as promising candidates for optoelectronic devices with unprecedented properties and ultra-compact performances. However atomically thin materials are highly sensitive to surrounding dielectric media, which imposes severe limitations to their practical applicability. Hence for their suitable integration into devices, the development of reliable encapsulation procedures that preserve their physical properties are required. Here, the excitonic photoluminescence of WSe2 monolayer flakes is assessed, at room temperature and 10 K, on mechanically exfoliated flakes encapsulated with SiOx and AlxOy layers employing chemical and physical deposition techniques. Conformal flakes coating on untreated - non-functionalized - flakes is successfully demonstrated by all the techniques except for atomic layer deposition, where a cluster-like oxide coating is observed. No significant compositional or strain state changes in the flakes are detected upon encapsulation by any of the techniques. Remarkably, our results evidence that the flakes' optical emission is strongly influenced by the quality of the encapsulating oxide - stoichiometry -. When the encapsulation is carried out with slightly sub-stoichiometric oxides two remarkable phenomena are observed. First, there is a clear electrical doping of the monolayers that is revealed through a dominant trion - charged exciton - room-temperature photoluminescence. Second, a strong decrease of the monolayers optical emission is measured attributed to non-radiative recombination processes and/or carriers transfer from the flake to the oxide. Power- and temperature-dependent photoluminescence measurements further confirm that stoichiometric oxides obtained by physical deposition lead to a successful encapsulation.

cond-mat.mes-hall↗