Mariam Barawi

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.

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.

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.

Dual Band Electrochromic Devices Based on Nb-Doped TiO2 Nanocrystalline Electrodes

The reliable exploitation of localized surface plasmon resonance in transparent conductive oxides is being pursued to push the developement of an emerging class of advanced dynamic windows, which offer the opportunity to selectively and dynamically control the intensity of the incoming thermal radiation without affecting visible transparency. In this view, Nb-doped TiO2 colloidal nanocrystals are particularly promising, as they have a wide band gap and their plasmonic features can be finely tailored across the near-infrared region by varying the concentration of dopants. Four batches of Nb-doped TiO2 nanocrystals with different doping levels (from 0% to 15% of niobium content) have been used here to prepare highly transparent mesoporous electrodes for near-infrared selective electrochromic devices, capable of dynamically modulating the intensity of the transmitted radiation upon the application of a relatively small bias voltage. An engineered dual band electrochromic device (made of 10%-Nb-doped TiO2 nanocrystals) has been eventually fabricated. It was shown to provide two complementary spectroelectrochemical responses, which can be independently controlled through the intensity of the applied potential: a large variation of the optical transmittance in the near-infrared region (by the intensification of the localized surface plasmon scattering) was achievable in the 0-3 V voltage window, reaching values greater than 64% in the spectral range from 800 to 2000 nm, whereas the visible absorption could also be intensively varied at higher potentials (from 3 to 4 V), driven by Li intercalation into the TiO2 anatase lattice.

Colloidal Synthesis of Bipolar Off-Stoichiometric Gallium Iron Oxide Spinel-Type Nanocrystals with Near-IR Plasmon Resonance

We report the colloidal synthesis of ∼5.5 nm inverse spinel-type oxide Ga2FeO4 (GFO) nanocrystals (NCs) with control over the gallium and iron content. As recently theoretically predicted, some classes of spinel-type oxide materials can be intrinsically doped by means of structural disorder and/or change in stoichiometry. Here we show that, indeed, while stoichiometric Ga2FeO4 NCs are intrinsic small bandgap semiconductors, off-stoichiometric GFO NCs, produced under either Fe-rich or Ga-rich conditions, behave as degenerately doped semiconductors. As a consequence of the generation of free carriers, both Fe-rich and Ga-rich GFO NCs exhibit a localized surface plasmon resonance in the near-infrared at ∼1000 nm, as confirmed by our pump–probe absorption measurements. Noteworthy, the photoelectrochemical characterization of our GFO NCs reveal that the majority carriers are holes in Fe-rich samples, and electrons in Ga-rich ones, highlighting the bipolar nature of this material. The behavior of such off-stoichiometric NCs was explained by our density functional theory calculations as follows: the substitution of Ga3+ by Fe2+ ions, occurring in Fe-rich conditions, can generate free holes (p-type doping), while the replacement of Fe2+ by Ga3+ cations, taking place in Ga-rich samples, produces free electrons (n-type doping). These findings underscore the potential relevance of spinel-type oxides as p-type transparent conductive oxides and as plasmonic semiconductors.

A dual band electrochromic device switchable across four distinct optical modes

The paper reveals a dual-band electrochromic device capable of selectively controlling the transmitted sunlight in the visible and near-infrared regions. It exploits the peculiar spectro-electrochemical features of vanadium-modified titanium oxide colloidal nanocrystals, which exhibit a distinctive electrochromic response at visible wavelengths upon the application of a small cathodic potential. They have been used as an active electrode in a sandwich cell in combination with a nanocrystalline tungsten oxide layer, which indeed allows selective control over the incoming near-infrared radiation at low applied potentials, until turning into a deep blue colored coating at higher potentials. These two coatings have been conveniently combined to realize an electrochromic device capable of separately operating over four different optical regimes, namely, fully transparent, VIS blocking, NIR blocking, and VIS + NIR blocking. Great benefits are anticipated in the field of glazing elements for buildings and transportation, where the solution proposed here may pave the way for the implementation of a novel class of “zero-energy” smart windows that self-adapt to both external climatic conditions and interior visual and thermal comfort requirements.