Papers by Alessandro Fantoni
This paper presents an analysis of the material quality influence for amorphous silicon waveguide... more This paper presents an analysis of the material quality influence for amorphous silicon waveguides for microphotonic applications. Material quality is taken into account by a model based on the absorption coefficient data obtained by Constant Photocurrent Measurement (CPM) in the near infrared region. The GUTL (Gauss-Urbach-Tauc-Lorentz) model has been presented as an extension of the standard Urbach-Tauc-Lorentz model and proposed as a predictor for the wavelength dependent optical constants of amorphous silicon in the near infrared spectra. Values produced for the GUTL model have been used as input for a set of FDTD simulations, taking in consideration different material qualities and waveguide dimensions directed to study the characteristics of amorphous silicon waveguides embedded in a SiO2 cladding. I.
2019 PhotonIcs & Electromagnetics Research Symposium - Spring (PIERS-Spring)
This work deals with the production of a low-cost disposable biosensor for point of care applicat... more This work deals with the production of a low-cost disposable biosensor for point of care applications. The proposed sensor is a plasmonic structure based on the Localized Surface Plasmon Resonance (LSPR) interaction of metal nanoparticles (MNPs), embedded into a matrix of reduced Graphene Oxide (rGO). After proper functionalization with selective antibodies (Ab), the efficiency of light extinction is controlled by slight changes of the refractive coefficient induced by the concentration of biomarkers trapped by the antibodies on the sensor surface. This work reports a study about the applicability of rGO as a support for gold nanoparticles (AuNPs) for preparing the functionalized LSPR sensing layer. AuNPs are prepared with an economic and eco-friendly method using phytochemicals present in tea extract at room temperature, while a modified Hummer's method is used to synthesize rGO. The resulting AuNPs-rGO composites are studied in terms of UV-VIS spectroscopy spectral light transmission and plasmonic resonance. The overall analysis is supported by simulation results, obtained by Mie analysis, about the LSPR effect in AuNPs-rGO and its dependence on the biomarker concentration.
Biosensors, 2022
Aiming to develop a nanoparticle-based optical biosensor using gold nanoparticles (AuNPs) synthes... more Aiming to develop a nanoparticle-based optical biosensor using gold nanoparticles (AuNPs) synthesized using green methods and supported by carbon-based nanomaterials, we studied the role of carbon derivatives in promoting AuNPs localized surface plasmon resonance (LSPR), as well as their morphology, dispersion, and stability. Carbon derivatives are expected to work as immobilization platforms for AuNPs, improving their analytical performance. Gold nanoparticles (AuNPs) were prepared using an eco-friendly approach in a single step by reduction of HAuCl4·3H2O using phytochemicals (from tea) which act as both reducing and capping agents. UV–Vis spectroscopy, transmission electron microscopy (TEM), zeta potential (ζ-potential), and X-ray photoelectron spectroscopy (XPS) were used to characterize the AuNPs and nanocomposites. The addition of reduced graphene oxide (rGO) resulted in greater dispersion of AuNPs on the rGO surface compared with carbon-based nanomaterials used as a support. ...
2019 IEEE 6th Portuguese Meeting on Bioengineering (ENBENG), 2019
Graphene-based materials have been extensively explored in recent years as valuable candidates as... more Graphene-based materials have been extensively explored in recent years as valuable candidates as the key material for novel structures in the field, among many other applications, of sensing devices. Reduced Graphene Oxide (rGO) is a type of chemically derived graphene, with equivalent optical properties but easier to be synthetized. This work reports a study about the applicability of rGO as a support for gold nanoparticles (AuNPs). The resulting AuNPs-rGO composites are studied in terms of spectral light transmission and plasmonic resonance as a possible sensing element for a photonic protein sensor device.
Materials Letters: X, 2020
This is a PDF file of an article that has undergone enhancements after acceptance, such as the ad... more This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Photonics, 2019
In this paper, we present a simulation study that intends to characterize the influence of defect... more In this paper, we present a simulation study that intends to characterize the influence of defects introduced by manufacturing processes on the geometry of a semiconductor structure suitable to be used as a multimode interference (MMI) 3 dB power splitter. Consequently, these defects will represent refractive index fluctuations which, on their turn, will drastically affect the propagation conditions within the structure. Our simulations were conducted on a software platform that implements the Beam Propagation numerical method. This work supports the development of a biomedical plasmonic sensor, which is based on the coupling between propagating modes in a dielectric waveguide and the surface plasmon mode that is generated on an overlaid metallic thin film, and where the output readout is achieved through an a-Si:H photodiode. By using a multimode interference 1 × 2 power splitter, this sensor device can utilize the non-sensing arm as a reference one, greatly facilitating its calibr...
Hydrogenated amorphous silicon thin film transistors have been used as switching elements in liqu... more Hydrogenated amorphous silicon thin film transistors have been used as switching elements in liquid crystal displays and large area matrix addressed sensor arrays. Later, these devices have also been used as analogue active elements in organic light emitting diode displays. However, this technology suffers from bias induced meta-stability. This issue introduces both threshold voltage and subthreshold slope shifts over time when gate bias is applied. Such instabilities jeopardize long term performance of circuits that rely on these components. Nevertheless, hydrogenated amorphous silicon thin film transistors present an exponential transfer characteristic when operating on subthreshold region and their typical power consumption is under 1 µW. This low power characteristic makes these devices ideally suited for low power electronic design. This work demonstrates, through transient analysis of a well-established simulation model for hydrogenated amorphous silicon, the viability of thin...
Ciência & Tecnologia dos Materiais
Abstract This work reports a theoretical study aimed to identify the plasmonic resonance conditio... more Abstract This work reports a theoretical study aimed to identify the plasmonic resonance condition for a system formed by metallic nanoparticles embedded in an a-Si:H matrix. The study is based on a Tauc-Lorentz model for the electrical permittivity of a-Si:H and a Drude model for the metallic nanoparticles and the polarizability of an aluminium sphereshaped particle with radius of 10-20 nm. We also performed FDTD simulations of light propagation inside this structure reporting about the effects caused by a single nanosphere of aluminium, silver and, as a comparison, an ideally perfectly conductor. The simulation results show that it is possible to obtain a plasmonic resonance in the red part of the spectrum (600-650 nm) when 10-20 nm radius aluminium spheres are embedded into a-Si:H.
Amorphous Si/SiC stacked photodiodes working as photonic devices are reviewed. Several applicatio... more Amorphous Si/SiC stacked photodiodes working as photonic devices are reviewed. Several applications (imagers, wavelength division demultiplexing devices and optical amplifiers) are proposed. In the imagers scans speeds up to 10 K lines per second were achieved without degradation in resolution. In the Wavelength Division Demultiplexing devices transmission rates are of the order of 4 kbit/s. Optical amplification occurs under steady state irradiation where the polychromatic mixture of different colors is higher than the sum of the individual monochromatic contributions. Electrical models are present to support the sensing methodologies. Experimental and simulated results show that the tandem devices act as charge transfer systems. They filter, amplify, store and transport the photogenerated carriers, keeping its memory (colour, intensity and bit rate) without adding any optical pre-amplifier or optical filter as in the standard p-in cells.
Amorphous Si/SiC stacked photodiodes working as photonic devices are reviewed. Several applicatio... more Amorphous Si/SiC stacked photodiodes working as photonic devices are reviewed. Several applications (imagers, wavelength division demultiplexing devices and optical amplifiers) are proposed. In the imagers scans speeds up to 10 K lines per second were achieved without degradation in resolution. In the Wavelength Division Demultiplexing devices transmission rates are of the order of 4 kbit/s. Optical amplification occurs under steady state irradiation where the polychromatic mixture of different colors is higher than the sum of the individual monochromatic contributions. Electrical models are present to support the sensing methodologies. Experimental and simulated results show that the tandem devices act as charge transfer systems. They filter, amplify, store and transport the photogenerated carriers, keeping its memory (colour, intensity and bit rate) without adding any optical pre-amplifier or optical filter as in the standard p-in cells.
Amorphous Si/SiC stacked photodiodes working as photonic devices are reviewed. Several applicatio... more Amorphous Si/SiC stacked photodiodes working as photonic devices are reviewed. Several applications (imagers, wavelength division demultiplexing devices and optical amplifiers) are proposed. In the imagers scans speeds up to 10 K lines per second were achieved without degradation in resolution. In the Wavelength Division Demultiplexing devices transmission rates are of the order of 4 kbit/s. Optical amplification occurs under steady state irradiation where the polychromatic mixture of different colors is higher than the sum of the individual monochromatic contributions. Electrical models are present to support the sensing methodologies. Experimental and simulated results show that the tandem devices act as charge transfer systems. They filter, amplify, store and transport the photogenerated carriers, keeping its memory (colour, intensity and bit rate) without adding any optical pre-amplifier or optical filter as in the standard p-in cells.
Solar Energy Materials and Solar Cells, Jun 1, 2002
In this paper a set of one-dimensional simulations of a-Si:H pin junctions under different illu... more In this paper a set of one-dimensional simulations of a-Si:H pin junctions under different illumination conditions and with different intrinsic layer are presented. The simulation program ASCA permits the analysis of the internal electrical behaviour of the cell allowing a comparison among ...
The bias voltage dependent spectral response (with and without steady state bias light) and the c... more The bias voltage dependent spectral response (with and without steady state bias light) and the current voltage dependence has been simulated and compared to experimentally obtained values. Results show that in the heterostructures the bias voltage influences differently the field and the diffusion part of the photocurrent. The interchange between primary and secondary photocurrent (i. e. between generator and load device operation) is explained by the interaction of the field and the diffusion components of the photocurrent. A field reversal that depends on the light bias conditions (wavelength and intensity) explains the photocurrent reversal. The field reversal leads to the collapse of the diode regime (primary photocurrent) launches surface recombination at the p-i and i-n interfaces which is responsible for a double-injection regime (secondary photocurrent). Considerations about conduction band offsets, electrical field profiles and inversion layers will be taken into account to explain the optical and voltage bias dependence of the spectral response.
Philosophical Magazine B
ABSTRACT
Light-activated multiplexer/demultiplexer silicon-carbon devices are analyzed. An electrical mode... more Light-activated multiplexer/demultiplexer silicon-carbon devices are analyzed. An electrical model for the device operation is presented and used to compare output signals with experimental data. An algorithm that takes into accounts the voltage and the optical bias controlled sensitivities was developed. The device is a double pi'n/pin a-SiC:H heterostructure with two optical gate connections for light triggering in different spectral regions. Multiple monochromatic pulsed communication channels were transmitted together, each one with a specific bit sequence. The combined optical signal was analyzed by reading out, under different applied voltages and optical bias, the generated photocurrent across the device. Experimental and simulated results show that the output multiplexed signal has a strong nonlinear dependence on the light absorption profile, i.e., on the incident light wavelength, bit rate and intensity under unbalanced light generation of carriers. By switching betwee...
Thin Solid Films
We present measurements and numerical simulation of a-Si:H p–i–n detectors with a wide range of i... more We present measurements and numerical simulation of a-Si:H p–i–n detectors with a wide range of intrinsic layer thickness between 2 and 10 µm. Such a large active layer thickness is required in applications like elementary particle detectors or X-ray detectors. For large thickness and depending on the applied bias, we observe a sharp peak in the spectral response in the red region near 700 nm. Simulation results obtained with the program ASCA are in agreement with the measurement and permit the explanation of the experimental data. In thick samples holes recombine or are trapped before reaching the contacts, and the conduction mechanism is fully electron dominated. As a consequence, the peak position in the spectral response is located near the optical band gap of the a-Si:H i-layer.
MRS Proceedings, 1998
ABSTRACTThe photocurrent delivered by microcrystalline hydrogenated silicon p-i-n devices and the... more ABSTRACTThe photocurrent delivered by microcrystalline hydrogenated silicon p-i-n devices and the spectral response are analysed under different applied bias voltage. The spectral response is extended far beyond 900 nm. Under reverse bias the spectral response is high and essentially unchanged while under increasing forward bias it decreases continuously. For forward bias higher than the open circuit voltage, the usual reversal of the spectral response is not observed and the photocurrent remains negative and almost independent of the bias voltage. A heterojunction model based on the presence of additional local electric fields near the grain boundaries is presented to explain this behaviour. Those fields lead to a strong increase of the recombination at the grain boundaries decreasing the contribution from the photogenerated carriers for the secondary photocurrent. Reverse bias restores the electric field at the interfaces minimizing the influence of the local barriers.
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Papers by Alessandro Fantoni