J.R. Ares

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.

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.

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).

Titanium trisulphide (TiS3) nanoribbons for easy hydrogen photogeneration under visible light

First evidence of hydrogen evolution by using titanium trisulphide (TiS3) as a photoanode in a photoelectrochemical cell (PEC) is reported. Synthesized TiS3, composed of numerous nanoribbons, has been structurally, morphologically and photoelectrochemically characterized. Moreover, the value of its flat band potential has been estimated (Vfb = −0.68 ± 0.05 V vs. Ag/AgCl) by Electrochemical Impedance Spectroscopy (EIS) measurements. This value has been used to depict the band energy levels of the TiS3/electrolyte interface. Finally, flows of photogenerated hydrogen up to 1.80 ± 0.05 μmol H2 min−1 have been quantified by Mass Spectrometry (MS) at a (Ag/AgCl) bias potential of 0.3 V, yielding a photoconversion efficiency of about 7%.

Electronic Bandgap and Exciton Binding Energy of Layered Semiconductor TiS3

A study of the electronic and optical bandgap is presented in layered TiS3, an almost unexplored semiconductor that has attracted recent attention because of its large carrier mobility and inplane anisotropic properties, to determine its exciton binding energy. Scanning tunneling spectroscopy and photoelectrochemical measurements are combined with random phase approximation and Bethe–Salpeter equation calculations to obtain the electronic and optical bandgaps and thus the exciton binding energy. Experimental values are found for the electronic bandgap, optical bandgap, and exciton binding energy of 1.2 eV, 1.07 eV, and 130 meV, respectively, and 1.15 eV, 1.05 eV, and 100 meV for the corresponding theoretical results. The exciton binding energy is orders of magnitude larger than that of common semiconductors and comparable to bulk transition metal dichalcogenides, making TiS3 ribbons a highly interesting material for optoelectronic applications and for studying excitonic phenomena even at room temperature.

Hydrogen evolution using palladium sulfide (PdS) nanocorals as photoanodes in aqueous solution.

Palladium sulfide (PdS) nanostructures are proposed to be used as photoanodes in photoelectrochemical cells (PECs) for hydrogen evolution due to their adequate transport and optical properties shown in previous works. Here, a complete morphological and electrochemical characterization of PdS films has been performed by different techniques. PdS flatband potential (Vfb=-0.65±0.05 V vs NHE) was determined by electrochemical impedance spectroscopy measurements in aqueous Na2SO3 electrolyte, providing a description of the energy levels scheme at the electrolyte-semiconductor interface. This energy levels scheme confirms PdS as a compound able to photogenerate hydrogen in a PEC. At last, photogenerated hydrogen rates are measured continuously by mass spectrometry as a function of the external bias potential under illumination, reaching values up to 4.4 μmolH2/h at 0.3 V vs Ag/AgCl.

Growth of pyrite thin-films investigated by thermoelectric measurements

Fe films have been sulfurised to be transformed into pyrite films and their Seebeck coefficient has been measured at room temperature. Comparison of the Seebeck coefficient values with XRD patterns allows us to propose that the measurement of the Seebeck coefficient could be a good tool to investigate the kinetics and energy of the pyrite formation process.

Role of cation diffusion in the formation mechanism and properties of cobalt-doped n-type pyrite thin films

Binary and ternary chalcogenides are relevant compounds for photovoltaic and thermoelectric energy conversion processes. The prototype material in this wide family is FeS2 (pyrite). In this work, we have investigated the simultaneous formation and doping (n-type) of FeS2 thin films. Sulphuration of Fe over Co metallic bilayers at constant pressure and temperature has allowed to get the necessary experimental information. Through the analysis of the chemical composition (Rutherford backscattering/elastic recoil detection analysis, energy dispersive X-ray) and structural (XRD) experimental data we have been able to propose a detailed mechanism to explain the formation of pyrite doped with Co thin films. In this mechanism the most relevant role is played by the diffusion of cationic (Fe and Co) species. Measured transport properties of the films reflect the different steps of the formation and doping processes.