Raman spectroscopy of InN films grown on Si

We have used Raman spectroscopy to study indium nitride thin films grown by molecular beam epitaxy on (111) silicon substrates at temperatures between 450 and 550 C. The Raman spectra show well defined peaks at 443, 475, 491, and 591 cm{-1}, which correspond to the A_1(TO), E_1(TO), E_2^{high}, and A_1(LO) phonons of the wurtzite structure, respectively. In backscattering normal to the surface the A_1(TO) and E_1(TO) peaks are very weak, indicating that the films grow along the hexagonal c axis. The dependence of the peak width on growth temperature reveals that the optimum temperature is 500 C, for which the fullwidth of the E_2^{high} peak has the minimum value of 7 cm{-1}. This small value, comparable to previous results for InN films grown on sapphire, is evidence of the good crystallinity of the films.Comment: 3 pages, 1 eps figure, RevTe

Growth of out-of-plane standing MoTe2(1-x)Se2x/MoSe2 composite flake films by sol–gel nucleation of MoOy and isothermal closed space telluro-selenization

This study describes the sol–gel processing of MoOy on Si (1 0 0) to subsequently achieve out-of-plane MoTe2/MoSe2 flake composite films by an isothermal closed space vapor transformation. The oxide precursor films have been prepared from a Mo isopropoxide solution in isopropanol and acid catalysis induced by HCl. Thermal annealing at 200, 400 and 600 °C enhanced the condensation after xerogel formation. An x-ray absorption analysis demonstrates that films condensed at 200 °C are at an intermediate chemical state between MoO3 and MoO2. To achieve MoTe2/MoSe2 composite films, the precursor oxide films were reduced in H2 and exposed to the chalcogenides by isothermal closed space vapor transport at 600 °C. The multilayered nanocomposite films grow with an out-of-plane flake-like structure and an evident integration of Se in the MoTe2 phase according to a MoTe2(1-x)Se2x alloy, with an estimation of x of 0.25. The alloy and the orientation of the flakes are consistent with the bands present in the Raman spectrum. These films are attractive for applications requiring high surface area interfaces favoring gas or ion exchange reactions with transition metal dichalcogenidesThe current research was funded by grant CTQ2017-84309-C2-2-R from Ministerio de Ciencia e Innovación (Spain). The authors acknowledge the ESRF and the Ministerio de Ciencia, Innovación y Universidades (Spain), for provision of synchrotron radiation facilities and the Consejo Superior de Investigaciones Científicas (Spain) financial support for the operation of the beamline under Grant No. PIE 2010 6 OE 01

Femtosecond laser thinning for resistivity control of tungsten ditelluride thin-films synthesized from sol-gel deposited tungsten oxide

In this work we present a route for fabricating WTe2 thin-films together with femtosecond laser post processing, enabling to finely control the conductivity. First, we produce amorphous films of WO3 on Si by spin-coating a sol-gel precursor followed by a consolidating annealing and a reduction process in partial H2 atmosphere, leading to porous metallic tungsten cluster layers. To achieve WTe2, the films were exposed to the chalcogen vapours by isothermal closed space vapor transport. The formation of a tungsten ditelluride film composed of piled crystals could be confirmed and a gradient of surface rich Te identified through hard X-ray photoelectron spectroscopy. Finally, it is demonstrated that resistivity can be changed from 0.2 mΩ.m to 1 mΩ.m, while keeping the material characteristics. An anisotropic conductivity can be induced by direct selective thinning with fs laser writing (350 fs pulse duration, 515 nm laser wavelength) of 1D stripes. The obtained results, demonstrate that laser processing is a promising thin-film post-processing technique that can be applied to 2D transition metal dichalcogenide thin filmsPID2020–112770RB-C22, S2018/NMT-4291 TEC2SPACE, CSIC13-4E-179

Differences in n-type doping efficiency between Al- and Ga-ZnO films

A careful and wide comparison between Al and Ga as substitutional dopants in the ZnO wurtzite structure is presented. Both cations behave as n-type dopants and their inclusion improves the optical and electrical properties of the ZnO matrix, making it more transparent in the visible range and rising up its electrical conductivity. However, the same dopant/Zn ratio leads to a very different doping efficiency when comparing Al and Ga, being the Ga cation a more effective dopant of the ZnO film. The measured differences between Al- and Ga-doped films are explained with the hypothesis that different quantities of these dopant cations are able to enter substitutionally in the ZnO matrix. Ga cations seem to behave as perfect substitutional dopants, while Al cation might occupy either substitutional or interstitial sites. Moreover, the subsequent charge balance after doping appear to be related with the formation of different intrinsic defects that depends on the dopant cation. The knowledge of the doped-ZnO films microstructure is a crucial step to optimize the deposition of transparent conducting electrodes for solar cells, displays, and other photoelectronic devices.Ministerio de Ciencia e Innovación TEC2007-60996, MAT2008-06858-C02-02, MAT2008- 06330, TEC2010-16700FUNCOAT CSD2008-00023- CONSOLIDER INGENIOSonderforschungsbereich SFB 76

Raman scattering by coupled plasmon-LO phonons in InN nanocolumns

Raman measurements on high quality, relaxed InN nanocolumns grown on Si(001) and Si(111) substrates by plasma-assisted molecular beam epitaxy are reported. A coupled LO phonon-plasmon mode around 430 cm-1, together with the uncoupled LO phonon appears in the nanocolumnar samples. The coupled mode is attributed to spontaneous accumulation of electrons at the lateral surfaces of the nanocolumns, while the uncoupled phonon originates from their inner part. Infrared reflectance measurements confirm the presence of electrons in the nanocolumns. The electron density in the accumulation layer depends on the growth temperature and is sensitive to exposure of HCl. Our results indicate that accumulation of intrinsic electrons occurs not only at the polar surfaces of InN layers, but also on non-polar lateral surfaces of InN nanocolumns. Its origin is attributed to an In-rich surface reconstruction of the nanocolumns sidewalls

Fabrication and characterization of nanostructured porous silicon-silver composite layers by cyclic deposition: Dip-coating vs spin-coating

“This is the Accepted Manuscript version of an article accepted for publication in Nanotechnology. IOP Publishing Ltd is not responsible for any errors or omissions in this version of the manuscript or any version derived from it. The Version of Record is available online at https://doi.org/10.1088/1361-6528/ab96e5”Composites of nanostructured porous silicon and silver (nPSi-Ag) have attracted great attention due to the wide spectrum of applications in fields such as microelectronics, photonics, photocatalysis and bioengineering, Among the different methods for the fabrication of nanostructured composite materials, dip and spin-coating are simple, versatile, and cost-effective bottom-up technologies to provide functional coatings. In that sense, we aimed at fabricating nPSi-Ag composite layers. Using nPSi layers with pore diameter of 30 nm, two types of thin-film techniques were systematically compared: cyclic dip-coating (CDC) and cyclic spin-coating (CSC). CDC technique formed a mix of granular and flake-like structures of metallic Ag, and CSC method favored the synthesis of flake-like structures with Ag and Ag2O phases. Flakes obtained by CDC and CSC presented a width of 110 nm and 70 nm, respectively. Particles also showed a nanostructure surface with features around 25 nm. According to the results of EDX and RBS, integration of Ag into nPSi was better achieved using the CDC technique. SERS peaks related to chitosan adsorbed on Ag nanostructures were enhanced, especially in the nPSi-Ag composite layers fabricated by CSC compared to CDC, which was confirmed by FTDT simulations. These results show that CDC and CSC produce different nPSi-Ag composite layers for potential applications in bioengineering and photonicsThis work was financially supported by Fondo Nacional de Desarrollo Científico y Tecnológico FONDECYT–Chile (grant number 11180395), FONDEQUIP–Chile (project 160152

Phonon structure, infra-red and raman spectra of Li2MnO3 by first-principles calculations

The layer-structured monoclinic Li2MnO3 is a key material, mainly due to its role in Li-ion batteries and as a precursor for adsorbent used in lithium recovery from aqueous solutions. In the present work, we used first-principles calculations based on density functional theory (DFT) to study the crystal structure, optical phonon frequencies, infra-red (IR), and Raman active modes and compared the results with experimental data. First, Li2MnO3 powder was synthesized by the hydrothermal method and successively characterized by XRD, TEM, FTIR, and Raman spectroscopy. Secondly, by using Local Density Approximation (LDA), we carried out a DFT study of the crystal structure and electronic properties of Li2MnO3. Finally, we calculated the vibrational properties using Density Functional Perturbation Theory (DFPT). Our results show that simulated IR and Raman spectra agree well with the observed phonon structure. Additionally, the IR and Raman theoretical spectra show similar features compared to the experimental ones. This research is useful in investigations involving the physicochemical characterization of Li2MnO3 materia

Enhancement of Rabi Splitting in a Microcavity with an Embedded Superlattice

We have observed a large coupling between the excitonic and photonic modes of an AlAs/AlGaAs microcavity filled with an 84-({\rm {\AA}})/20({\rm {\AA}}) GaAs/AlGaAs superlattice. Reflectivity measurements on the coupled cavity-superlattice system in the presence of a moderate electric field yielded a Rabi splitting of 9.5 meV at T = 238 K. This splitting is almost 50% larger than that found in comparable microcavities with quantum wells placed at the antinodes only. We explain the enhancement by the larger density of optical absorbers in the superlattice, combined with the quasi-two-dimensional binding energy of field-localized excitons.Comment: 5 pages, 4 figures, submitted to PR

Antibacterial films of silver nanoparticles embedded into carboxymethylcellulose/chitosan multilayers on nanoporous silicon: A layer-by-layer assembly approach comparing dip and spin coating

The design and engineering of antibacterial materials are key for preventing bacterial adherence and proliferation in biomedical and household instruments. Silver nanoparticles (AgNPs) and chitosan (CHI) are broad-spectrum antibacterial materials with different properties whose combined application is currently under optimization. This study proposes the formation of antibacterial films with AgNPs embedded in carboxymethylcellulose/chitosan multilayers by the layer-by-layer (LbL) method. The films were deposited onto nanoporous silicon (nPSi), an ideal platform for bioengineering applications due to its biocompatibility, biodegradability, and bioresorbability. We focused on two alternative multilayer deposition processes: cyclic dip coating (CDC) and cyclic spin coating (CSC). The physicochemical properties of the films were the subject of microscopic, microstructural, and surface–interface analyses. The antibacterial activity of each film was investigated against Escherichia coli (Gram-negative) and Staphylococcus aureus (Gram-positive) bacteria strains as model microorganisms. According to the findings, the CDC technique produced multilayer films with higher antibacterial activity for both bacteria compared to the CSC method. Bacteria adhesion inhibition was observed from only three cycles. The developed AgNPs–multilayer composite film offers advantageous antibacterial properties for biomedical applicationsPID2020-112770RB-C2

Raman scattering by longitudinal optical phonons in InN nanocolumns grown on Si(111) and Si(001) substrates

Raman measurements in high-quality InN nanocolumns and thin films grown on both Si(1 1 1) and Si(1 0 0) substrates display a low-energy coupled LO phonon–plasmon mode together with uncoupled longitudinal optical (LO) phonons. The coupled mode is attributed to the spontaneous accumulation of electrons on the lateral surfaces of the nanocolumns, while the uncoupled ones originates from the inner part of the nanocolumns. The LO mode in the columnar samples appears close to the E1(LO) frequency. This indicates that most of the incident light is entering through the lateral surfaces of the nanocolumns, resulting in pure longitudinal–optical mode with quasi-E1 symmetry. For increasing growth temperature, the electron density decreases as the growth rate increases. The present results indicate that electron accumulation layers do not only form on polar surfaces of InN, but also occur on non-polar ones. According to recent calculations, we attribute the electron surface accumulation to the temperature dependent In-rich surface reconstruction on the nanocolumns sidewalls