arXiv subjects

Raul Arenal

Publications and source records attributed to Raul Arenal.

31 records · Page 2Linked to original sources

arXiv2022

Aberration-corrected transmission electron microscopy with Zernike phase plates

We explore the possibility of applying physical phase plates (PPs) in combination with aberration-corrected transmission electron microscopy. Phase-contrast transfer characteristics are calculated and compared for a thin-film based Zernike PP, a hole-free (HF) or Volta PP and an electrostatic Zach PP, considering their phase-shifting properties in combination with partial spatial coherence. The effect of slightly converging illumination conditions, often used in high-resolution applications, on imaging with PPs is discussed. Experiments with an unheated Zernike PP applied to various nanomaterial specimens and a qualitative analysis clearly demonstrates the general compatibility of PPs and aberration-corrected transmission electron microscopy. Calculations and experiments show the benefits of the approach, among which is a strong phase-contrast enhancement of a large range of spatial frequencies. This allows the simultaneous imaging of atomic-resolution structures and morphological features at the nanometer scale, with maximum phase contrast. The calculations can explain why the HFPP damps contrast transfer at higher spatial frequencies.

physics.ins-det↗

arXiv2022

On the Existence of Photoluminescence and Room-Temperature Spin Polarization in Ambipolar V doped MoS$_2$ Monolayers

Opto-spintronics is an emerging field where ultra-thin magnetic-semiconductors having high spin-valley coupling play an important role. Here, we demonstrate substitutional vanadium (V) doping in MoS$_2$ lattice in different extent, leading to the coexistence of photoluminescence (PL), valleypolarization (~32%), and valley splitting (~28 meV shift in PL with helicity $σ^+$ and $σ^-$ of light excitation). A large V doping causes semiconductor to metal transition in MoS$_2$ but with medium level causing the existence of photoluminescence with high spin polarization. The ambipolar nature of medium level V doped MoS$_2$ is shown here indicating its potential as an opto-electronic material. The presence of V-dopants and their different level of content are proven by both spectroscopic and microscopic methods.A detailed temperature and power dependent photoluminescence studies along with density functional theory-based calculations in support unravels the emergence of the co-existence of spin-valley coupling and photoluminescence. This study shows the potential of doping MoS$_2$ for deriving new materials for next generation room temperature opto-spintronics.

physics.app-ph↗

arXiv2022

A Detailed Investigation of the Onion Structure of Exchanged Coupled Magnetic Fe(3-delta)O4@CoFe2O4@Fe(3-delta)O4 Nanoparticles

Nanoparticles (NPs) which combine several magnetic phases offer wide perspectives for cutting edge applications because of the high modularity of their magnetic properties. Besides the addition of the magnetic characteristics intrinsic to each phase, the interface that results from core-shell and, further, from onion structures leads to synergistic properties such as magnetic exchange coupling. Such a phenomenon is of high interest to overcome the superparamagnetic limit of iron oxide NPs which hampers potential applications. In this manuscript, we report on the design of NPs with an onion-like structure which have been scarcely reported yet. These NPs consist in a Fe(3_delta)O4 core covered by a first shell of CoFe2O4 and a second shell of Fe(3-delta)O4. They were synthesized by a multi-step seed mediated growth approach. Although TEM micrographs clearly show the growth of each shell from the iron oxide core, core sizes and shell thicknesses markedly differ from what is suggested by the size increase. We investigated very precisely the structure of NPs in performing high resolution (scanning) TEM imaging and GPA. The chemical composition and spatial distribution of atoms were studied by EELS. The chemical environment and oxidation state of cations were investigated by Mössbauer spectrometry, soft XAS and XMCD. These techniques allowed us to estimate the increase of Fe2+ content in the iron oxide core of the core@shell structure and the increase of the cobalt ferrite shell thickness in the core@shell@shell one, while the iron oxide shell appears to be much thinner than expected. Thus, the modification of the chemical composition as well as the size of the Fe(3-delta)O4 core and the thickness of the cobalt ferrite shell have a high impact on the magnetic properties. Furthermore, the growth of the iron oxide shell also markedly modifies the magnetic properties of the core-shell NPs.

cond-mat.mtrl-sci↗

arXiv2022

Controlled chemical functionalization toward 3D-2D carbon nanohorn-MoS2 heterostructures with enhanced electrocatalytic activity for protons reduction

The realization of novel heterostructures arising from the combination of nanomaterials is an effective way to modify their physicochemical and electrocatalytic properties, giving them enhanced characteristics stemming from their individual constituents. Interfacing carbon nanohorns (CNHs) possessing high porosity, large specific surface area and good electrical conductivity, with MoS2 owning multiple electrocatalytic active sites but lacking significant conductivity, robust interactions and effective structure, can be a strategy to boost the electrocatalytic reduction of protons to molecular hydrogen. Herein, we covalently introduce, in a stepwise approach, complementary functional groups at the conical tips and sidewalls of CNHs, along with the basal plane of MoS2, en route the construction of 3D-2D CNH-MoS2 heterostructures. The increased MoS2 loading onto CNHs, improving and facilitating charge delocalization and transfer in neighboring CNHs, along with the plethora of active sites, results in excellent electrocatalytic activity for protons reduction same to that of commercial Pt/C. We have registered minute overpotential, low Tafel slope and small charge-transfer resistance for electrocatalyzing the evolution of hydrogen from the newly prepared heterostructure of 0.029 V, 71 mV/dec and 34.5 Ω, respectively. Furthermore, the stability of the 3D-2D CNH-MoS2 heterostructure was validated after performing 10,000 ongoing electrocatalytic cycles.

cond-mat.mtrl-sci↗

arXiv2021

Shelling with MoS$_2$ Functional CuS@MoS2 Hybrids as Electrocatalysts for the Oxygen Reduction and Hydrogen Evolution Reactions

The development of noble-metal free electrocatalysts is of high importance for clean energy conversion applications. MoS$_2$ has been considered as a promising low cost catalyst for the hydrogen evolution reaction (HER), however its activity is limited by poor conductivity and low abundance of active sites. Moreover, its suitability as an effective catalyst for other reactions, in particluar the oxygen reduction reaction (ORR), was hardly explored to date. Herein, we show hybrid nanostructures of shelled CuS particles with MoS$_2$ layers, which produces several outcomes: The MoS$_2$ shell is strained and defective, and charge transfer from the core to MoS$_2$ occurs, enabling activation of the basal plane of MoS$_2$. Changing the feed ratio of the precursors led to control over morphology, such that the wrapping of the cores with the shell was continuously varied and characterized. We found an optimal hybrid structure, which provided high electrochemical active surface area and fast charge transfer kinetics, leading to improved activity not only towards HER (overpotential of 225 mV at 10 mA cm$^{-1}$), but also for the sluggish ORR (onset potential 0.87 V vs RHE).

physics.app-ph↗

arXiv2021

Comparative image simulations for phase-plate transmission electron microscopy

Numerous physical phase plates (PP) for phase-contrast enhancement in transmission electron microscopy (TEM) have been proposed and studied with the hole-free or Volta PP having a high impact and interest in recent years. This study is concerned with comparative TEM image simulations considering realistic descriptions of various PP approaches and samples from three different fields of application covering a large range of object sizes. The simulated images provide an illustrative characterization of the typical image appearance and common artifacts of the dfferent PPs and the influence of simulation parameters especially important for PP simulations. A quantitative contrast analysis shows the superior phase-shifting properties of the hole-free phase plate for biological applications and the benefits of adjustable phase plates. The application of PPs in high-resolution TEM imaging, especially of weak-phase objects such as (atomically thin) 2D materials, is shown to increase image interpretability. The software with graphical user interface written and used for the presented simulations is available for free usage.

physics.ins-det↗

arXiv2021

In-situ reduction by Joule heating and measurement of electrical conductivity of graphene oxide in a transmission electron microscope

Graphene oxide (GO) is reduced by Joule heating using in-situ transmission electron microscopy (TEM). The approach allows the simultaneous study of GO conductivity by electrical measurements and of its composition and structural properties throughout the reduction process by TEM, electron diffraction and electron energy-loss spectroscopy. The small changes of GO properties observed at low applied electric currents are attributed to the promotion of diffusion processes. The actual reduction process starts from an applied power density of about 2 1014 Wm-3 and occurs in a highly uniform and localized manner. The conductivity increases more than 4 orders of magnitude reaching a value of 3 103 Sm-1 with a final O content of less than 1%. We discuss differences between the reduction by thermal annealing and Joule heating.

cond-mat.mes-hall↗

arXiv2019

Coaxial nanowires as plasmon-mediated remote nanosensors

This study reports on the plasmon-mediated remote Raman sensing promoted by specially designed coaxial nanowires. This unusual geometry for Raman study is based on the separation, by several micrometres, of the excitation laser spot, on one tip of the nanowire, and the Raman detection at the other tip. The very weak efficiency of Raman emission makes it challenging in a remote configuration. For the proof-of-concept, we designed coaxial nanowires consisting in a gold core to propagate the surface plasmon polaritons and a Raman-emitting shell of poly(3,4-ethylene-dioxythiophene). The success of the fabrication was demonstrated by correlating, for the same single nanowire, a morphological analysis by electron microscopy and Raman spectroscopy analysis. Importantly for probing remote-Raman effect, the original hard template-based process allows to control the location of the polymer shell all along the nanowire, or only close to one or the two nanowire tips. Such all-in-one single nanowires could have applications in the remote detection of photo-degradable substances and for exploring 1D nanosources for integrated photonic and plasmonic systems.

physics.app-ph↗

arXiv2018

Weakly Trapped, Charged, and Free Excitons in Single-Layer MoS2 in the Presence of Defects, Strain, and Charged Impurities

Few- and single-layer MoS2 host substantial densities of defects. They are thought to influence the doping level, the crystal structure, and the binding of electron-hole pairs. We disentangle the concomitant spectroscopic expression of all three effects and identify to what extent they are intrinsic to the material or extrinsic to it, i.e., related to its local environment. We do so by using different sources of MoS2 -- a natural one and one prepared at high pressure and high temperature -- and different substrates bringing varying amounts of charged impurities and by separating the contributions of internal strain and doping in Raman spectra. Photoluminescence unveils various optically active excitonic complexes. We discover a defect-bound state having a low binding energy of 20 meV that does not appear sensitive to strain and doping, unlike charged excitons. Conversely, the defect does not significantly dope or strain MoS2. Scanning tunneling microscopy and density functional theory simulations point to substitutional atoms, presumably individual nitrogen atoms at the sulfur site. Our work shows the way to a systematic understanding of the effect of external and internal fields on the optical properties of two-dimensional materials.

cond-mat.mtrl-sci↗

arXiv2016

Oxidation-assisted graphene heteroepitaxy on copper foil

We propose an innovative, easy-to-implement approach to synthesize large-area singlecrystalline graphene sheets by chemical vapor deposition on copper foil. This method doubly takes advantage of residual oxygen present in the gas phase. First, by slightly oxidizing the copper surface, we induce grain boundary pinning in copper and, in consequence, the freezing of the thermal recrystallization process. Subsequent reduction of copper under hydrogen suddenly unlocks the delayed reconstruction, favoring the growth of centimeter-sized copper (111) grains through the mechanism of abnormal grain growth. Second, the oxidation of the copper surface also drastically reduces the nucleation density of graphene. This oxidation/reduction sequence leads to the synthesis of aligned millimeter-sized monolayer graphene domains in epitaxial registry with copper (111). The as-grown graphene flakes are demonstrated to be both single-crystalline and of high quality.

cond-mat.mtrl-sci↗

arXiv2014

From hollow gold nanoparticles to gold nanorings: Morphological tunability of the plasmonic response

The optical and morphological properties of hollow gold nanoparticles (HGNPs) can be finely modified by employing Poly-L-Lysine hydrobromide (PLL), an homo poly aminoacid of the L-lysine, used as reducer and stabilizer. We investigate locally the plasmonic response of these nanostructures by electron energy loss spectroscopy (EELS). The plasmon excitations are interpreted by discrete dipolar approximation (DDA) simulations. We demonstrate that this controlled top-down morphological modification a fine tuning of the optical response is possible. Unlike the traditional lithographic techniques, this has been achieved in a controlled manner using wet chemistry, enabling the potential use of these nanostructures for a broad range of plasmonic applications, including biomedicine, catalysis and quantum communications.

cond-mat.mtrl-sci↗

arXiv2014

Atomic Configuration of Nitrogen Doped Single-Walled Carbon Nanotubes

Having access to the chemical environment at the atomic level of a dopant in a nanostructure is crucial for the understanding of its properties. We have performed atomically-resolved electron energy-loss spectroscopy to detect individual nitrogen dopants in single-walled carbon nanotubes and compared with first principles calculations. We demonstrate that nitrogen doping occurs as single atoms in different bonding configurations: graphitic-like and pyrrolic-like substitutional nitrogen neighbouring local lattice distortion such as Stone-Thrower-Wales defects. The stability under the electron beam of these nanotubes has been studied in two extreme cases of nitrogen incorporation content and configuration. These findings provide key information for the applications of these nanostructures.

cond-mat.mtrl-sci↗

arXiv2011

Coupled study by TEM/EELS and STM/STS of electronic properties of C- and CNx-nanotubes

Carbon nanotubes are the focus of considerable research efforts due to their fascinating physical properties. They provide an excellent model system for the study of one dimensional materials and molecular electronics. The chirality of nanotubes can lead to very different electronic behaviours, either metallic or semiconducting. Their electronic spectrum consists of a series of Van Hove singularities defining a bandgap for semiconducting tubes and molecular orbitals at the corresponding energies. A promising way to tune the nanotubes electronic properties for future applications is to use doping by heteroatoms. Here we report on experimental investigation of the role of many-body interactions in nanotube bandgaps, the visualization in direct space of the molecular orbitals of nanotubes and the properties of nitrogen doped nanotubes using scanning tunneling microscopy and transmission electron microscopy as well as electron energy loss spectroscopy.

cond-mat.mtrl-sci↗