Charging of highly resistive granular metal films

We have used the Scanning Kelvin probe microscopy technique to monitor the charging process of highly resistive granular thin films. The sample is connected to two leads and is separated by an insulator layer from a gate electrode. When a gate voltage is applied, charges enter from the leads and rearrange across the sample. We find very slow processes with characteristic charging times exponentially distributed over a wide range of values, resulting in a logarithmic relaxation to equilibrium. After the gate voltage has been switched off, the system again relaxes logarithmically slowly to the new equilibrium. The results cannot be explained with diffusion models, but most of them can be understood with a hopping percolation model, in which the localization length is shorter than the typical site separation. The technique is very promising for the study of slow phenomena in highly resistive systems and will be able to estimate the conductance of these systems when direct macroscopic measurement techniques are not sensitive enough.Comment: 8 pages, 7 figure

Growth and formation of inverse GaP and InP opals

Opals consist of an ordered array of SiO2 spheres. This leads to a modulation of the refractive index and hence photonic stop bands behaviour over the visible/IR range of the electro-magnetic spectrum. The exact position of the stop bands depends on the size of the silica spheres. However, the refractive index contrast between the SiO2 spheres and air spaces is not great enough to open up a full photonic band gap (PBG), only the pseudogap. To increase the contrast the air spaces are filled with a material of high refractive index such as InP or GaP. To further increase the contrast the SiO2 is removed leaving a III-V framework as the inverse opal structure. By use of MOCVD we have been able to infill opals with InP and GaP to such a level that has supported the inversion of the composite forming a structure of air holes within a III-V lattice. XRD and Raman confirmed the quality of the III-V infill, while the extent of the infill was studied by SEM and reflectance measurements

Nanoscale Charge Density and Dynamics in Graphene Oxide

Graphene oxide (GO) is widely used as a component in thin film optoelectronic device structures for practical reasons because its electronic and optical properties can be controlled. Progress critically depends on elucidating the nanoscale electronic structure of GO. However, direct experimental access is challenging because of its disordered and nonconductive character. Here, we quantitatively mapped the nanoscopic charge distribution and charge dynamics of an individual GO sheet by using Kelvin probe force microscopy (KPFM). Charge domains are identified, presenting important charge interactions below distances of 20 nm. Charge dynamics with very long relaxation times of at least several hours and a logarithmic decay of the time correlation function are in excellent agreement with Monte Carlo simulations, revealing an universal hopping transport mechanism best described by Efros-Shklovskii''s law. © 2021 The Authors. Published by American Chemical Society

Graphene oxide: key to efficient charge extraction and suppression of polaronic transport in hybrids with poly (3-hexylthiophene) nanoparticles

Nanoparticles (NPs) of conjugated polymers in intimate contact with sheets of graphene oxide (GO) constitute a promising class of water-dispersible nanohybrid materials of increased interest for the design of sustainable and improved optoelectronic thin-film devices, revealing properties exclusively pre-established upon their liquid-phase synthesis. In this context, we report for the first time the preparation of a P3HTNPs–GO nanohybrid employing a miniemulsion synthesis approach, whereby GO sheets dispersed in the aqueous phase serve as a surfactant. We show that this process uniquely favors a quinoid-like conformation of the P3HT chains of the resulting NPs well located onto individual GO sheets. The accompanied change in the electronic behavior of these P3HTNPs, consistently confirmed by the photoluminescence and Raman response of the hybrid in the liquid and solid states, respectively, as well as by the properties of the surface potential of isolated individual P3HTNPs–GO nano-objects, facilitates unprecedented charge transfer interactions between the two constituents. While the electrochemical performance of nanohybrid films is featured by fast charge transfer processes, compared to those taking place in pure P3HTNPs films, the loss of electrochromic effects in P3HTNPs–GO films additionally indicates the unusual suppression of polaronic charge transport processes typically encountered in P3HT. Thus, the established interface interactions in the P3HTNPs–GO hybrid enable a direct and highly efficient charge extraction channel via GO sheets. These findings are of relevance for the sustainable design of novel high-performance optoelectronic device structures based on water-dispersible conjugated polymer nanoparticles

Morphology Analysis of Si Island Arrays on Si(001)

The formation of nanometer-scale islands is an important issue for bottom-up-based schemes in novel electronic, optoelectronic and magnetoelectronic devices technology. In this work, we present a detailed atomic force microscopy analysis of Si island arrays grown by molecular beam epitaxy. Recent reports have shown that self-assembled distributions of fourfold pyramid-like islands develop in 5-nm thick Si layers grown at substrate temperatures of 650 and 750°C on HF-prepared Si(001) substrates. Looking for wielding control and understanding the phenomena involved in this surface nanostructuring, we develop and apply a formalism that allows for processing large area AFM topographic images in a shot, obtaining surface orientation maps with specific information on facets population. The procedure reveals some noticeable features of these Si island arrays, e.g. a clear anisotropy of the in-plane local slope distributions. Total island volume analysis also indicates mass transport from the substrate surface to the 3D islands, a process presumably related to the presence of trenches around some of the pyramids. Results are discussed within the framework of similar island arrays in homoepitaxial and heteroepitaxial semiconductor systems

Graphene oxide and poly(3-hexylthiophene) nanoscale interface interactions probed by KPFM

Graphene oxide (GO) is a chemically modified form of graphene containing many different types of oxygen functional groups on its basal plane and edges. Their presence imparts hydrophilicity and allows for ease of processing from water dispersions. We recently have shown that sheets of graphene oxide critically affect the aggregate structure of water dispersible nanoparticles P3HTNPs and thus the performance of optoelectronic thin film devices. In this work we elucidate the photogenerated charge tranfer dynamics of (P3HTNPs-GO) ensembles with defined aggregate structure across their interface by Kelvin Probe Force microscopy (KPFM).EU Horizon 2020 research and innovation programme Marie Sklodowska-Curie grant agreement No 642742. Project ENE2016-79282-C5-1-R. Grupo Reconocido DGA T03_17R.Peer reviewe

Photobleaching of MEH-PPV thin films: Correlation between optical properties and the nanoscale surface photovoltage

The electro-optical properties of Poly[2-methoxy-5-(2-ethylhexyloxy)-1,4- phenylenevinylene] (MEH-PPV) thin films have been studied while the sample is illuminated with blue light. By combining Kelvin probe microscopy with classical optical techniques it has been possible to monitor the evolution of the nanoscale contact potential, the surface photovoltage and the optical properties of the material establishing a correlation between them. Contrary to what is usually accepted, it has been found that the nanoscale surface photovoltage is larger when the polymer is already photobleached. In addition, it has been possible to distinguish between the photophysical processes that govern the working principle of a polymer-based device and the photochemical reactions that degrade the material. Although complex charges trapping phenomena take place during the irradiation, the surface photovoltage effect depends only on the number of photons that have reached the polymer. © 2013 Elsevier B.V.This work was supported by the MICINN thought the projects Force for Future CSD2009-Force For Future-00024, FIS2009-07657, MAT2010-21267-C02-01, the Comunidad Autónoma de la Región de Murcia through the project “Células solares orgánicas: de la estructura molecular y nanométrica a dispositivos operativos macroscópicos“and the Fundacion Seneca 15324/PI/10. EE thanks the MICINN for the FPI program and EPL thanks the Ramon y Cajal program for financial support. This work has also been co-funded by the European Union (Fondos FEDER).Peer Reviewe

Photonic band gap properties of GaP opals with a new topology

In this paper, we propose that the “anomalous” optical response exhibited by GaP and InP infiltrated opals is due to the peculiar morphology shown by these materials when grown within the pores. In order to account for their optical response, we propose a new structural model consisting of a network of high dielectric spheres located in the pores of the bare opal, interconnected by cylinders of the same material. A fair agreement between the theoretical predictions using this model and the experimental measurements has been found. We also show that the inverse structure presents very interesting optical properties

Optical and morphological study of disorder in opals

7 pages, 8 figures, 2 tables.-- PACS: 42.70.Qs; 78.66.Qn; 68.55.Jk; 68.55.Ln; 68.35.Bs; 61.72.Ji; 61.72.Ss; 61.72.Hh; 68.37.Hk; 78.20.CiAn optical and morphological study has been carried out to understand the role of intrinsic defects in the optical properties of opal-based photonic crystals. By doping poly(methylmethacrylate) (PMMA) thin-film opals with larger polystyrene (PS) spheres, structural disorder has being generated perturbing the PMMA matrix periodicity. It is shown that this disorder dramatically affects the optical response of the system worsening its photonic properties. It has been found that the effect of doping is highly dependent not only on the concentration but also on the relative size of the dopant with reference to the matrix. Through a detailed scanning electron microscopy inspection, the sort of structural defects involved, derived from the different particle size used, has been characterized. A direct relationship between the observed optical response with the different perturbations generated in the lattice has been found. In addition, from this study it can be concluded that it is possible to grow high quality alloyed photonic crystals, exhibiting intermediate photonic properties between pure PMMA and pure PS opals by simple sphere size matching and variation of the relative concentration of both components.This work is partially financed by the Comunidad Autónoma de Madrid through a PhD grant (E. P.-L.) and 07T/ 0048/2003 project and the Spanish CICyT through MAT2003-01237 project.Peer reviewe

Optical study of the pseudogap in thickness and orientation controlled artificial opals

Artificial opals consisting of polystyrene spheres ordered into a fcc lattice are grown with control over orientation and thickness. A thorough study of the formation of the pseudogap in these structures is carried out by means of reflectivity and transmission spectra. Special attention is paid to discriminate those features inherent to the photonic crystal, and those to the substrate on which the crystal is grown. Further, a characterization of the photonic band structure in the hexagonal facet of the Brillouin zone is made using angle resolved reflectivity measurements.Peer reviewe