Solid State Commun Research Articles

Superconductivity in monolayer Janus titanium-sulfur hydride (TiSH) at ambient pressure

Janus two dimensional (2D) materials are new and novel materials. As they break out-of-plane symmetry, they possess several fascinating properties which can be applied in catalytic reactions and opto-electronics. Recent synthesis of MoSH and the prediction of phonon-mediated superconductivity have opened a new way to investigate the properties of hydrogenated Janus materials (Novoselov et al 2004 Science 306 666–9; Mehta et al 2023 Solid State Commun. 375 115347; Naik et al 2023 Comput. Theor. Chem. 1228 114278). In this work, we performed the density functional theory calculations to demonstrate that titanium sulfur hydride (TiSH) is dynamically stable and becomes phonon-mediated superconductor with the superconducting critical temperature, Tc = 9.24 K with the corresponding value of electron–phonon coupling constant, λ = 0.71, in the weak interaction limits, under ambient conditions. Eliashberg spectral function α2F(ω) was well converged for dense grid of q1 × q2 × q3 = 12 × 12 × 1 and nk1 × nk2 × nk3 = 140 × 140 × 1. The effect of smearing broadening was also considered for determining well converged value of Tc and λ. Figure (b) shows that after smearing broadening of 0.02 Ry, λ shows convergent values, and subsequent changes are as low as less that 5% of the peak value. Overall, our findings predicted a new member in the 2D Janus hydride family with possible applications in 2D nanomaterials and superconducting devices applications.

Erbium-Excess Gallium Garnets.

A series of garnets of formula Er3+xGa5-xO12 are described, for which we report the crystal structures for both polycrystalline and single-crystal samples. The x limit in the garnet phase is between 0.5 and 0.6 under our conditions, with the Er fully occupying the dodecahedral (24c) garnet site plus some of the octahedral site (16a) in place of the Ga normally present. Long-range antiferromagnetic order with spin-ice-like frustration is suggested by the transition temperature (TN ≈ 0.8 K) being lower than the Curie-Weiss theta. The magnetic ordering temperature does not depend on the Er excess, but there is increasing residual entropy as the Er excess is increased, highlighting the potential for unusual magnetic behavior in this system. The field-dependent magnetic entropy trend is consistent with the reported behavior for frustrated triangular magnetic systems: an increasing transition temperature with a broader hump as the applied field increases [Xing, J.; Phys. Rev. Mater. 2019, 3(11), 114413;Filippi, J.; Solid State Commun. 1977, 23(9), 613-616; Bloxsom, J. A. Thermal and Magnetic Studies of Spin Ice Compounds. University College London, 2016].

Monoclinic symmetry of the hcp-type ordered areas in bulk cobalt

The gradual ferromagnetic spin reorientation in hexagonal close packed cobalt (hcp-Co) phase between $230{\phantom{\rule{0.16em}{0ex}}}^{\ensuremath{\circ}}\mathrm{C}$ and $330{\phantom{\rule{0.16em}{0ex}}}^{\ensuremath{\circ}}\mathrm{C}$ reported for a Co single crystal [Bertaut et al., Solid State Commun. 1, 81 (1963)] suggests that this phase could not have a hexagonal symmetry. This hypothesis is verified positively by synchrotron radiation diffraction and neutron diffraction on polycrystalline powder cobalt. The analysis of diffraction data has been done by using a specific set of Bragg peaks, which are not sensitive to the stacking faults present in abundance in hcp-Co. The crystal structure of the hcp-type ordered areas of cobalt is described by the monoclinic symmetry with the magnetic space group $C{2}^{\ensuremath{'}}/{m}^{\ensuremath{'}}$. In this monoclinic crystal structure the former hexagonal [001] axis is no longer perpendicular to the hexagonal layers. The hexagonal [001] and [010] axes make an angle equal $\ensuremath{\alpha}\ensuremath{\approx}$ 90.10(1)${\phantom{\rule{0.16em}{0ex}}}^{\ensuremath{\circ}}$, while the angle between in-plane [100] and [010] axes equals $\ensuremath{\gamma}\ensuremath{\approx}$ 120.11(1)${\phantom{\rule{0.16em}{0ex}}}^{\ensuremath{\circ}}$. The monoclinic symmetry provides an approximate description of the crystal structure of the stacking faulted hcp-Co areas coexisting with fcc-Co areas.

Theoretical proposal and material realization of ferromagnetic negative charge-transfer energy insulator

Here we propose another type of ferromagnetic semiconductors: ferromagnetic negative charge-transfer insulator (FNCTI). In FNCTI, the negative charge-transfer states strongly enhance the ferromagnetic (FM) exchange interactions and the orbital hybridization gap permits the magnetic molecular orbitals as the underlying magnetic units rather than local atomic orbitals. Thus the FM exchange interactions are rather strong and decay slowly due to the large spearding of magnetic molecular orbitals. This is distinct from the superexchange mechanism where FM exchange interactions are quite weak as summarized in the well-known Goodenough-Kanamori-Anderson semi-empirical rules.Through first-principle calculations with the hybrid functional, PbO-type CrAs monolayer is mapped out to be a FNCTI, which possesses a band gap $\sim$ 0.35 eV, FM nearest-/next-nearest-neighbor exchange coupling strength $\sim$ 57/40 meV, and a high $T_c$ $\sim$ 1500 K respectively. It is believed that the existence of FNCTI validates the long-pending hypothesis by D. I. Khomskii and G. A. Sawatzky in 1997 [Solid State Commun. 102, 87 (1997)].

Threading dislocation lines and their impact on bulk cubic boron nitride’s low-field electron transport response

Building upon Chilleri et al.’s [Solid State Commun. 352 (2022) 114776] relaxation-time approximation based low-field electron drift mobility formalism, we incorporate treatment of threading dislocation related scattering into this analytical framework. Then, for a reasonable range of threading dislocation line density selections, representative of boron nitride’s zinc-blende phase, the role that threading dislocation lines play in shaping the low-field electron transport response of this material is examined. The six scattering processes considered in this analysis include the ionized impurity, polar optical phonon, acoustic deformation potential, piezoelectric, inter-valley, and threading dislocation line related scattering processes. The impact of threading dislocations is assessed through evaluations of the scattering rates, the fractional scattering contributions, and the low-field electron drift mobility contributions associated with the various scattering processes. The dependence of the total low-field electron drift mobility on the threading dislocation line density is also examined. We conclude the analysis with a comparison with the results of experiment. Threading dislocation lines are found to be a significant contributor to bulk zinc-blende boron nitride’s low-field electron transport response.

Redetermination of the crystal structures of rare-earth trirhodium diboride RERh3B2 (RE = Pr, Nd and Sm) from single-crystal X-ray data.

The crystal structures of the rare-earth (RE) trirhodium diborides praseo-dymium trirhodium diboride, PrRh3B2, neodymium trirhodium diboride, NdRh3B2, and samarium trirhodium diboride, SmRh3B2, were refined on the basis of single-crystal X-ray diffraction data. The crystal chemistry of RERh3B2 (RE: Pr, Nd, and Sm) compounds has previously been analyzed mainly on the basis of powder samples [Ku et al. (1980 ▸). Solid State Commun. 35, 91-96], and no structural investigation by single-crystal X-ray diffraction has been reported so far. The crystal structures of the three hexa-gonal RERh3B2 compounds are isotypic with that of CeRh3B2; RE, Rh and B sites are situated on special positions with site symmetry 6/mmm (Wyckoff position 1a), mmm (3g) and m2 (2c), respectively. In comparison with the previous powder X-ray study of hexa-gonal RERh3B2, the present redetermination against single-crystal X-ray data has allowed for the modeling of all atoms with anisotropic displacement parameters (ADPs). The ADPs of the Rh atom in each of the structures result in an elongated displacement ellipsoid in the direction of the stacking of the Rh kagomé-type layer. The features of obtained ADPs of atoms are discussed in relation to RERh3B2-type and analogous structures.

Coupling of Vibrational Modes to the Electron-Hole Continuum in Doped Single-Wall Carbon Nanotubes

Electron-phonon coupling is at the heart of many physical phenomena like superconductivity, charge transport and the Raman effect (Ferrari2007). Previously, it has been shown that a coupling of vibrational modes to the low-energy electronic continuum absorption in graphene and carbon nanotubes results in transparency windows in the mid-infrared spectral region (Lapointe2012, Lapointe2017). The reduced absorption at the vibrational energies can be attributed to so-called Fano (anti-)resonances (Fano1961).Here, we present a systematic study of the infrared continuum absorption and such antiresonances in purely semiconducting, monochiral carbon nanotube (CNT) thin-films. We observe strong antiresonances for the Raman active D- and G-phonons, which acquire oscillator strength via coupling to the bright Drude-like intraband continuum in doped CNTs. Both the spectrum of the continuum absorption and the amplitude of the Fano resonances critically depend on the doping level. With increasing carrier concentration, we find a redshift of the Drude peak accompanied with a non-monotonic doping level dependence of the antiresonance intensity. Our observations show that carrier localization and delocalization as well as disorder induced by defects play a critical role for both charge transport and electron-phonon coupling as evidenced by the Fano resonances. We suggest that our findings also have implications for other low-dimensional systems with a low-energy electronic continuum absorption.C. Ferrari, Solid State Commun. 2007, 143, 47-57.Lapointe et al., Phys. Rev. Lett. 2012, 109, 097402.Lapointe et al., J. Phys. Chem. C 2017, 121, 9053-9062.Fano, Phys. Rev. 1961, 124, 1866-1878.

Possibility of small electron states

Some authors have claimed that there exists a minimum size (on the order of the Compton radius) for electron states composed entirely of positive-frequency solutions to the free Dirac equation. Other authors have put forward counterexamples to such claims. This article asks how the counterexamples of A. J. Bracken and G. F. Melloy [J. Phys. A. 32, 6127 (1999)] bypass two arguments against their possibility. The first is an old argument that, because of the prohibition on faster-than-light motion, the electron must be larger than a certain minimum size if it is to have the correct angular momentum and magnetic moment. This challenge can be addressed by analyzing the flow of energy and charge for the counterexample states. The second argument is an explicit proof (presented in C.-P. Chuu et al., [Solid State Commun. 150, 533 (2010)]) that there is a minimum size for purely positive-frequency electron states. This proof hinges on the assumption of a small spread in momentum space, which is violated by the counterexamples that have been put forward.

Hiroshi Kitagawa.

"The secrets of being a successful scientist are dreams, imagination, and lots of effort. The biggest challenge facing scientists is the room-temperature superconductor …" Find out more about Hiroshi Kitagawa in his Author Profile.

Rotating magnetocaloric effect in the ferromagnetic Weyl semi-metal Co3Sn2S2

The rotating magnetocaloric effect is of recent interest in the magnetic refrigeration technique, in which the cooling effect is attained by rotating the anisotropic magnetocaloric material from one orientation to the other in a fixed magnetic field. In this work, we report the anisotropic magnetocaloric properties of single crystals of the ferromagnetic Weyl semimetal Co3Sn2S2 for magnetic field H along the c-axis (H∥c) and magnetic field H along the ab-plane (H∥ab). We observed a significant (factor of 2) difference between the magnetocaloric effect measured in both orientations. The rotating magnetocaloric effect has been extracted by taking the difference of the magnetic entropy change (ΔSM) for fields applied in the two crystallographic orientations. Scaling analysis of ΔSM, the rescaled ΔSM(T,H) vs reduced temperature θ curves collapse onto a single universal curve, indicates that the transition from paramagnetic to ferromagnetic phases at 174 K is a second-order magnetic phase transition. Furthermore, using the power law dependence of ΔSM and relative cooling power RCP, the critical exponents β and γ are calculated, which are consistent with the recent critical behavior study on polycrystalline samples of this compound [Yan et al., Solid State Commun. 281, 57 (2018)].

Optimization of Cation Composition on Charge-Discharge Behavior in Rock-Salt Type Lithium Titanium Oxynitride Anode Material

The lithium-ion batteries have been widely used in portable electronic devices such as laptop computers, mobile phones and so on. Despite lithium batteries possess obvious advantages over nickel–cadmium and lead–acid cells with regards to high energy density, there are still some safety concerns with the carbon anode and a safer alternative which can offer the same performance would be preferable. Bonding in metal oxynitrides show lower ionicity and higher covalency than that in the metal oxides, since nitrogen possesses lower electronegativity than oxygen and the difference in electronegativity between metal and anions (oxygen and nitrogen) should be smaller in oxynitrides. Covalent bond can be featured by a directed bond as a results of forming an electron pair between the bond whereas ionic bond can be regarded as a non-directional bond due to electrostatic attraction between metal cations and anions. When temperature rises, durability of bond against heat would be stronger in the more covalent oxynitrides than in the more ionic oxides. In this study, we focused on rock-salt type oxynitride Li0.30Ti0.60O0.50N0.50. Although there is a synthetic report on this material[1], there is no evaluation report as an electrode material. Li rich oxynitrides such as Li7.9MnN3.2O1.6 have already been reported as superior anode materials[2]. We referred to this idea and tried to obtain a better electrochemical performance in Li0.30Ti0.60O0.50N0.50 as an anode material by enriching Li against Ti. We investigated the effects of different Ti/Li ratio in LTON on the electrochemical performance.As the result, we successfully synthesized the rock-salt type LTON (lithium titanium oxynitride, with Ti/Li=1.25, 1.5, 1.75, 2.0) without impurity phase by solid state reaction technique as shown in Fig.1. XPS spectra revealed that the valence state of Ti increased as Ti/Li ratio decreased. Electrochemical charge–discharge cycle tests were performed at various current density of 20–200 mAg−1 with the cutoff voltage of 0.01–3.2 V. At relatively low current density (20 mA g-1), the corresponding initial discharge capacity of LTON with Ti/Li=1.25 reached about 140 mAh g-1, which was the highest capacity of all the LTON electrodes, and smaller charge-discharge hysteresis was observed. Moreover, the LTON with Ti/Li=1.25 showed excellent performance at all of current density compared with other LTON electrodes tested, as shown in Fig.2. For further understanding of the improved rate capability by changing the Ti/Li ratio, EIS measurements for LTON electrodes (Ti/Li=1.25, 2.0) were carried out, and it was found that the lithium ion diffusion coefficient increased as Ti/Li ratio decreased. In addition, In-situ XRD result suggested that the discharge and charge reactions should be reversible. XPS analysis for the pristine and discharged LTONs demonstrated that the charge compensation due to Li insertion would be performed by a reduction of Ti. The decrease of Ti/Li ratio in the oxynitrides would affect on the existence of the more number of high valent Ti such as Ti4+, which would possess a larger allowance to compensate the reduction during the discharging to show the better electrochemical performance.[1] T. Katsumata et al., Solid State Commun. 132, 583-587 (2004).[2] J. Cabana et al., Electrochem. Commun. 12, 315-318 (2010). Figure 1

Stripe instabilities in menisci of free-standing smectic films with a direct transition from smectic C to an isotropic or nematic phase

ABSTRACTRegular periodic stripe patterns that are found in the menisci of smectic C and C freely suspended films were described first by Meyer and Pershan (Solid State Commun. 13 (1973)) more than 40 years ago, but their physical origin is still under debate. Loudet et al. (Phys. Rev. Lett. 106 117, 802 (2011)) proposed a layer dilatation model to explain their formation. Our observation of such patterns in films with a direct transition from smectic C to a nematic or isotropic phase is in sharp conflict with that model. Moreover, we demonstrate the disappearance of the patterns at the transition from synclinic to anticlinic smectic C phases. An essentially different explanation of the physical mechanism of the stripe formation must be sought. We propose potential sources of the instability.

Delta-doping-controllable magnetoresistance device in a magnetically modulated semiconductor nanostructure

A magnetoresistance (MR) device was proposed by depositing two parallel ferromagnetic stripes on top and bottom of a semiconductor heterostructure [Solid State Commun. 141 (2007) 248]. In order to manipulate its performance, we dope a tunable δ-potential into the device by atomic layer doping technique. Transmission, conductance and MR ratio are calculated for the δ-doped MR device. It is confirmed that an obvious MR effect still exists in the device even though a δ-doping is comprised. Results show that the MR ratio varies intensely with the weight and/or the position of the δ-doping. Therefore, one can manipulate structurally the MR device by altering the δ-doping, and a tunable MR device can be obtained for magnetic information storage.

Si(O,N)C Ceramics of Controlled Porosity for Storage of Lithium Ions

Porous carbons have been investigated as high capacity electrodes for lithium storage for more than 10 years [ 1 ]. Realization of active materials with high surface to volume ratio leads to intensification of the presence of active sites for lithium storage. Although high active electrode leads to significant losses during SEI formation, it is recompensed by a considerable increase of the specific capacity. Moreover, the high surface area implies a high contact area with the electrolyte, hence leading to high lithium-ion flux across the electrode/electrolyte interface. Moreover, controlled porosity leads to an improved lithium diffusion due to the reduction of its path length, hence batteries with enhanced power capability and higher electron transfer rates[ 2 ]. Within this work we present a novel approach to prepare porous, polymer-derived Si(O,N)C ceramic for lithium storage. Through controlled crosslinking/pyrolysis process, materials with significantly higher surface area compared to standard polymer derived ceramic (PDC) route are synthesised. Free carbon phase formed after pyrolysis of the polymeric samples is characterized by means of Raman spectroscopy. The characteristic carbon vibrations appear in all spectra [ 3 ]. Unusually high I(D)/I(G) ratio ranging from 1.8 to 2 is calculated, whereas fitting[ 4 ] reveals the presence of strong D3 band at 1540 cm-1, originating from the presence of amorphous carbon. In parallel, the intensity of G band attributed to an ideal graphitic lattice vibration mode with E2g symmetry remains low. In general, the increase of pyrolysis temperature leads to increase of ordering in free carbon phase. Elemental analysis reveals similar composition with average free carbon content of ~45 wt.%. Galvanostatic cycling with potential limitation show a significant improvement in capacity and rate capability (Figure1) for porous samples compared to dense sample. Namely discharge capacity increases from about 160 mAh·g-1 (Poly1_1100) to 320 mAh·g-1 for samples pyrolysed at 1100 °C (Poly2_1100). The sample exposed to 900 °C (Poly2_900) reveals very high capacities around 520 mAh·g-1 after 100 cycles (Figure1, insert). Furthermore, the porous materials demonstrate high and stable capacities when subjected to higher currents, namely 180 mAh·g-1 is recovered with the rate of 5C (1860 mA·g-1). In order to understand the differences in the ion storage mechanism in dense and porous materials solid state NMR study is in progress. Acknowledgments: We gratefully acknowledge the financial support of the German Research Foundation (DFG) SPP1473/JP8. Authors thank to Christina Schitco for fruitful discussions. Literature: 1. Zhou, H.; Zhu, S.; Hibino, M.; Honma, I.; Ichihara, M. Advanced Materials 2003, 15, 2107–2111. 2. Roy, P.; Srivastava, S. K. J. Mater. Chem. A 2015, 3, 2454–2484. 3. Ferrari, A. C. Solid State Commun. 2007, 143, 47–57. 4. Sadezky, A.; Muckenhuber, H.; Grothe, H.; Niessner, R.; Poschl, U. Carbon 2005, 43, 1731–1742. Figure 1. Recovered capacity of Poly2_1100 at different C-rates ranging from C/2 up to 5C. Insert: Extended cycling curves for Poly1_1100, Poly2_1100, Poly2_900 at current of 72 mA·g-1. Figure 1

Magnetization plateaus in the antiferromagnetic Ising chain with single-ion anisotropy and quenched disorder

We have studied the presence of plateaus on the low-temperature magnetization of an antiferromagnetic spin-1 chain, as an external uniform magnetic field is varied. A crystal-field interaction is present in the model and the exchange constants follow a random quenched (Bernoulli or Gaussian) distribution. Using a transfer-matrix technique we calculate the largest Lyapunov exponent and, from it, the magnetization at low temperatures as a function of the magnetic field, for different values of the crystal field and the width of the distributions. For the Bernoulli distribution, the number of plateaus increases, with respect to the uniform case [Litaiff etal., Solid State Commun. 147, 494 (2008)] and their presence can be linked to different ground states, when the magnetic field is varied. For the Gaussian distributions, the uniform scenario is maintained, for small widths, but the plateaus structure disappears as the width increases.

Study on multicaloric effect of CuO induced multiferroic

One of the induced multiferroic materials, CuO has the magnetic as well as ferroelectric phase transition at same temperature (TN1 ∼ 213 K, TC and TN2 ∼ 230 K). These type of materials can show two types of entropy; magnetic field induced entropy (Magnetocaloric Effect) as well as electeric field induced entropy (Electrocaloric Effect). The presence of both type of entropy may be called “Multicaloric Entropy” (M. Vopson, Solid State Commun. 152, 2067 (2012) and Meng et al., Phys. Lett. A 377, 567 (2013)). We observed that “Multicaloric Entropy” in the induced multiferroic materials also depends on the magnetoelectric interaction (γ). Therofore, this numerical attempt to calculate the entropy of CuO may be useful for the future “electro-magnetic” based refrigerator technology.

(Invited) Ge1-xSnx Optical Devices: Growth and Applications

Ge has been considered as a promising candidate for the active region in optical interconnect applications such as lasers and photodetectors, since its band gap corresponds to a wavelength with less loss in SiO2 waveguides. However, to increase the efficiency of these applications, a direct band gap material is required.Incorporation of Sn into Ge has been considered to obtain a direct band gap material [1,2]. According to theoretical calculations [3], the critical Sn content to achieve the direct transition depends on the strain. When the GeSn layer is fully relaxed, around 11% of Sn is required. On the other hand, when the layer experiences compressive strain, the critical Sn content increases. One option to obtain highly strain-relaxed GeSn with high Sn content is to increase the thickness of the GeSn layer. In addition, the active region of the optical devices has to be thick to obtain a high efficiency. However, it is well known that Sn segregation to the surface easily occurs [4], which results in Sn precipitation/agglomeration on the surface when a thick layer is grown.The Sn agglomeration is the result of the collision among Sn atoms on the surface. Therefore, we focused on the freezing of Sn atoms during GeSn growth. GeSn layers were grown with Ge2H6 and SnCl4 precursors on Ge(001) substrates at 320°C by atmospheric CVD in an Epsilon like equipment from ASM. The target thickness of the GeSn layers is 200 nm. N2 and H2 with different flows ranging from 10 to 40 slm were used as carrier gases to study the impact of the partial pressure of the precursors on Sn agglomeration. It is expected that using low carrier gas flows enhances the Sn freezing due to the higher Ge2H6 and SnCl4 partial pressures. In addition, it is expected that switching the carrier gas from N2 to H2suppresses the Sn migration on the surface due to the resulting surface H-termination.We found opposite tendencies for the surface Sn agglomeration as a function of the H2 and N2 carrier gas flow. A lower N2 suppresses the surface Sn agglomeration. As we expected, a high partial pressure of the precursors enhances the freezing of Sn atoms into the subsurface. On the other hand, when H2 is used as a carrier gas, a higher H2 flow suppresses the surface Sn agglomeration. The growth temperature (320°C) is close to the temperature at which adsorbed H starts to desorb thermally from the Ge surface (340°C) [5, 6]. However, a high H2 partial pressure may suppress the desorption of surface H which is provided by the deposition by Ge2H6. The Sn segregation should be suppressed by the surfactant effect of surface H [6]. The different behavior of these two carrier gases will be discussed in detail.[1] D. W. Jenkins and J. D. Dow, Phys. Rev. B, 36, 7994 (1987).[2] M. R. Bauer, et al., Solid State Commun. 127, 355 (2003).[3] Y. Shimura, et al., J. Appl. Phys. submitted.[4] E. Kasper, et al., Thin Solid Films, 520, 3195 (2012).[5] L. Surnev and M. Tikhov, Surf. Sci. 138, 40 (1984).[6] A. Sakai and T. Tatsumi, Appl. Phys. Lett. 64, 52 (1994).

Hydrogen Evolution in a Photoelectrochemical Cell with Ti-Doped Pyrite As Photoanode

Interest in hydrogen solar production from water splitting is motivated by the need to find a green, renewable and environmentally safe fuel. Hydrogen is considered as a viable option sustainable development beyond fossil fuels, especially when it is produced from water by using only renewable energy sources. The identification of low cost materials for the photogeneration of hydrogen is recently a growing field of research. In the last years, pyrite (FeS2) has received increasing attention as a material for photoelectrochemical applications1,2 and hydrogen photogeneration. Due to its low cost and not toxicity, high optical absorption coefficient, and an adequate electronic properties (Egap=1.1±0.1eV)3, FeS2 shows some advantages compared with other semiconductor materials to be used as a photoelectrode in a PEC. Previous works3,4 about the Ti doping of pyrite thin films have been reported and their properties investigated. In this work the photogeneration of hydrogen in a photoelectrochemical cell with n-type Ti doped FeS2 thin film as photoanode will be demonstrated.Ti-FeS2 thin films have been grown by sulphuration of Fe layers (100 nm) thermally evaporated on discs in a vacuum sealed ampoule at 300 ºC. Film morphology and crystallinity were investigated by Scanning electron microscopy (SEM) and X-ray diffraction (XRD), respectively. Cubic FeS2 was identified as the unique phase at 300ºC (Fig1).Electro- and photoelectrochemical measurements have been carried out using a conventional three-electrode glass cell with a quartz window (PEC) to illuminate the photoanode, i.e. the Ti-FeS2 samples used as working electrode. As counter electrode a platinum sheet was used and the reference one was a Ag/AgCl reference electrode. 0.5M Na2SO3 aqueous solution was used as electrolyte, which serves both, to increase the conductivity and as sacrificial agent. An halogen lamp (200 W) was utilized as white light source. Photocurrents show positive values, as expected from the doping with titanium, because as far as we know, pure pyrite thin films always present p-type conductivity3, but Ti doped pyrite show n-type behavior4,5.A mass spectrometer connected to the PEC is used to measure the evolved H2 from the photoelectrochemical water splitting. Results show that Ti-FeS2 exhibit a high flow of photogenerated hydrogen from bias potential such low as 0.5V (vs. Ag/AgCl). In fact, strong bubbling hydrogen was experimentally observed in our photoelectrochemical cell and quantified with the mass spectrometer (Fig. 2).In conclusion, the hydrogen photogeneration by using Ti-FeS2in a PEC under bias potential has been demonstrated. Quantitative results will be exposed in the oral communication. Acknowledgements Financial support from MINECO (MAT2011-22780) is acknowledged. Authors thank to F. Moreno for his technical assistance. References 1 J. Jiao, L. Chen, D. Kuang, W. Gao, H. Feng, J. Xia, RSC Advances 1, 255-261 (2011). 2 H. Huang, T. Ling, S.Z. Qiao, X.W. Du, J. Mat. Chem. A 1, 11828 (2013). 3 I. J. Ferrer, D. M. Nevskaia, C. de las Heras and C. Sánchez. Solid State Commun. 74, (9) 913-916 (1990).4 I. J. Ferrer, J. R. Ares, C. Sánchez, Solid State Phenom.80-81, 281-286 (2001). 5A. Pascual, P. Díaz-Chao, I. J. Ferrer, C. Sánchez, J. R. Ares. Solar Energy Materials & Solar Cells 87 575-582 (2005).

A detailed characterization of the transient electron transport within zinc oxide, gallium nitride, and gallium arsenide

A three-valley Monte Carlo simulation approach is used in order to probe the transient electron transport that occurs within bulk wurtzite zinc oxide, bulk wurtzite gallium nitride, and bulk zinc-blende gallium arsenide. For the purposes of this analysis, we follow the approach of O'Leary et al. [O'Leary et al., Solid State Commun. 150, 2182 (2010)], and study how electrons, initially in thermal equilibrium, respond to the sudden application of a constant applied electric field. Through a determination of the dependence of the transient electron drift velocity on both the time elapsed since the onset of the applied electric field and the applied electric field strength, a complete characterization of the transient electron transport response of these materials is obtained. We then apply these results in order to estimate how the optimal cut-off frequency and the corresponding operating device voltage vary with the device length. We find that while the cut-off frequency found for the case of zinc-blende gallium arsenide, 637 GHz for a device length of 100 nm, is marginally less than that found for the cases of wurtzite zinc oxide and wurtzite gallium nitride, 1.05 and 1.32 THz, respectively, the corresponding operating voltage found for the case of zinc-blende GaAs, 0.08 V, precludes the use of this material for the operation of devices in the terahertz frequency range if higher powers are required; the corresponding operating voltages for the cases of wurtzite ZnO and wurtzite GaN are found to be 8 and 4 V, respectively. These results clearly demonstrate the compelling advantage offered by wurtzite zinc oxide and wurtzite gallium nitride, as opposed to zinc-blende gallium arsenide, for electron devices operating in the terahertz frequency range if higher powers are required.

A transient electron transport analysis of bulk wurtzite zinc oxide

A three-valley Monte Carlo simulation approach is used in order to probe the transient electron transport that occurs within bulk wurtzite zinc oxide. For the purposes of this analysis, we follow O’Leary et al. [Solid State Commun. 150, 2182 (2010)], and study how electrons, initially in thermal equilibrium, respond to the sudden application of a constant applied electric field. We find that for applied electric field strength selections in excess of 300 kV/cm that an overshoot in the electron drift velocity is observed. An undershoot in the electron drift velocity is also observed for applied electric field strength selections in excess of 700 kV/cm, this velocity undershoot not being observed for other compound semiconductors, such as gallium arsenide and gallium nitride. We employ a means of rendering transparent the electron drift velocity enhancement offered by the transient electron transport, and then use the calculated dependence of the peak transient electron drift velocity on the applied electric field for the design optimization of short-channel high-frequency electron devices.