I.J. Ferrer

About the band gap nature of FeS2 as determined from optical and photoelectrochemical measurements

Abstract We discuss in this paper the validity of some approaches currently used to determine the energy gap value (Eg) of semiconducting pyrite (FeS2) from optical and photoelectrochemical measurements. It is concluded that the wide dispersion of the reported experimental values of Eg might be due to the inadequacy of those approaches.

TiS3 Transistors with Tailored Morphology and Electrical Properties

Control over the morphology of TiS3 is demonstrated by synthesizing 1D nanoribbons and 2D nanosheets. The nanosheets can be exfoliated down to a single layer. Through extensive characterization of the two morphologies, differences in the electronic properties are found and attributed to a higher density of sulphur vacancies in nanosheets, which, according to density functional theory calculations, leads to an n-type doping.

Thermoelectric power of bulk black-phosphorus

The potential of bulk black-phosphorus, a layered semiconducting material with a direct band gap of ∼0.3 eV, for thermoelectric applications has been experimentally studied. The Seebeck Coefficient (S) has been measured in the temperature range from 300 K to 385 K, finding a value of S = +335 ± 10 μV/K at room temperature (indicating a naturally occurring p-type conductivity). S increases with temperature, as expected for p-type semiconductors, which can be attributed to an increase of the charge carrier density. The electrical resistance drops up to a 40% while heating in the studied temperature range. As a consequence, the power factor at 385 K is 2.7 times higher than that at room temperature. This work indicates the prospective use of black-phosphorus in thermoelectric applications such as thermal energy scavenging, which typically require devices with high performance at temperatures near room temperature.

Preparation of n-type doped FeS2 thin films

Abstract N-type pyrite thin films have been prepared by sulfuration at 350°C of flash evaporated Fe−X (X =Al, Cu, Ni) layers. Physical and structural properties have been investigated. XRD showed pyrite structure. Room temperature dark resistivity of the films varies from 0.21 to 7·10 -4 Ω·cm depending on the impurity concentration. Hall mobilities are between 1 and 10 4 cm 2 /V·s. Optical properties are also drastically affected by doping.

Temperature-Dependent Raman Spectroscopy of Titanium Trisulfide (TiS3) Nanoribbons and Nanosheets

Titanium trisulfide (TiS3) has recently attracted the interest of the 2D community because it presents a direct bandgap of ∼1.0 eV, shows remarkable photoresponse, and has a predicted carrier mobility up to 10000 cm(2) V(-1) s(-1). However, a study of the vibrational properties of TiS3, relevant to understanding the electron-phonon interaction that can be the main mechanism limiting the charge carrier mobility, is still lacking. In this work, we take the first steps to study the vibrational properties of TiS3 through temperature-dependent Raman spectroscopy measurements of TiS3 nanoribbons and nanosheets. Our investigation shows that all the Raman modes linearly soften (red shift) as the temperature increases from 88 to 570 K due to anharmonic vibrations of the lattice, which also includes contributions from the lattice thermal expansion. This softening with the temperature of the TiS3 modes is more pronounced than that observed in other 2D semiconductors, such as MoS2, MoSe2, WSe2, and black phosphorus (BP). This marked temperature dependence of the Raman spectra could be exploited to determine the temperature of TiS3 nanodevices by using Raman spectroscopy as a noninvasive and local thermal probe. Interestingly, the TiS3 nanosheets show a stronger temperature dependence of the Raman modes than the nanoribbons, which we attribute to lower interlayer coupling in the nanosheets.

Pyrite thin films: Improvements in their optical and electrical properties by annealing at different temperatures in a sulfur atmosphere

Iron pyrite thin films prepared by flash evaporation of pyrite powder have been annealed at different temperatures in a sulfur atmosphere. We present some results on the influence of the annealing temperature (from 250 to 450 °C) on the optical and electrical properties of three groups of samples with different thicknesses (≂0.3, 0.6, and 1 μm, respectively). Sulfuration temperature has a clear influence on the optical absorption and electrical resistivity of the films, with some differences in their behavior depending on the film thickness. In light of the available present knowledge of pyrite thin films, interpretation of the obtained results is difficult, it suggests that the shape of the optical absorption curves (and their absorption edge) at low photon energies is determined by the density of point defects, which decreases on increasing the annealing temperature. On the other hand, the electrical resistivity seems to be influenced by both the film grain size and point defect density.

Titanium trisulfide (TiS3) : A 2D semiconductor with quasi-1D optical and electronic properties

We present characterizations of few-layer titanium trisulfide (TiS3) flakes which, due to their reduced in-plane structural symmetry, display strong anisotropy in their electrical and optical properties. Exfoliated few-layer flakes show marked anisotropy of their in-plane mobilities reaching ratios as high as 7.6 at low temperatures. Based on the preferential growth axis of TiS3 nanoribbons, we develop a simple method to identify the in-plane crystalline axes of exfoliated few-layer flakes through angle resolved polarization Raman spectroscopy. Optical transmission measurements show that TiS3 flakes display strong linear dichroism with a magnitude (transmission ratios up to 30) much greater than that observed for other anisotropic two-dimensional (2D) materials. Finally, we calculate the absorption and transmittance spectra of TiS3 in the random-phase-approximation (RPA) and find that the calculations are in qualitative agreement with the observed experimental optical transmittance.

A note on the Hall mobility and carrier concentration in pyrite thin films

Abstract This note presents results on Hall effect characteristics (mobility and density of majority charge carriers) of undoped p-type and n-doped pyrite thin films. Carrier mobilities between 200 and 0.07 cm 2 / V s have been measured in undoped p-type films. Doped n-type films present values which vary from 200 to 0.1 cm 2 / V s . The corresponding carrier densities change from 2×10 18 to 10 22 cm −3 in p-type films and from 6×10 17 to 10 21 cm −3 in n-type films. These results reinforce the physical basis and conclusions of the modelling of pyrite solar cells accomplished by Altermatt et al. (Sol. Energy Mater. Sol. Cells 71 (2002) 181).

Electronics and optoelectronics of quasi-1D layered transition metal trichalcogenides

This work was supported by the Netherlands Organization for Scientific Research (NWO/FOM). AJM-M acknowledges the financial support of MINISTERIO DE CIENCIA E INNOVACION (MICINN) (Spain) through the scholarship BES2012–057346. R.D’A and RB acknowledge financial support by the DYN-XC-TRANS (Grant No. FIS2013-43130-P), NanoTHERM (Grant No. CSD2010- 00044), and SElecT-DFT (FIS2016-79464-P) of the Ministerio de Economia y Competitividad (MINECO), and Grupo Consolidado UPV/EHU del Gobierno Basco (Grant No. IT578-13). RB acknowledges the financial support of the Ministerio de Educacion, Cultura y Deporte (Grant No. FPU12/01576). AC-G acknowledges financial support from the European Commission under the Graphene Flagship, contract CNECTICT-604391, from the MINECO (Ramon y Cajal 2014 program, RYC-2014-01406) and from the MICINN (MAT2014-58399-JIN). MIRE Group thanks MINECO (MAT2015-65203R) for financial support. E Flores also acknowledges the Mexican National Council for Science and Technology (CONACyT).

Temperature dependence of the optical absorption edge of pyrite FeS2 thin films

Optical absorption edge measurements of pyrite thin films prepared by flash evaporation have been made at different temperatures in the range 10-300 K. Absorption edge values vary from 1.05 to 0.99 eV in that temperature interval. Experimental data have been fitted to the Varshni formula and to an expression proportional to the statistical Bose-Einstein factor used previously by other workers. The main phonon energy calculated from the fitting of the experimental points to the Bose-Einstein expression is in good agreement with the average energy of the active optical phonons as determined by Raman spectroscopy carried out on the same pyrite thin film.