0000000000004240
AUTHOR
Gerard Tobias
showing 12 related works from this author
Synthesis and crystal structure of novel Ruddlesden–Popper strontium niobium oxynitrides
2002
Abstract The family (SrO)(SrNbO 2-x N) n is the first example of Ruddlesden-Popper strontium niobium oxynitrides and provides layered structures to obtain mixed valence niobium compounds with a variety of oxidation states and transport properties. We have prepared members with n =1 (Sr 2 NbO 3 N) and n =2 (Sr 3 Nb 2 O 5 N 2 ) by solid state reaction of Nb 2 O 5 and SrCO 3 at 900–1050 °C under ammonia flow. These compounds crystallize in the I4/mmm space group and show a range of anionic stoichiometry that together with the N/O ratio determines the formal oxidation state for niobium and therefore the resultant physical properties. Samples of the n =1 member show mixed valence Nb 5+ /Nb 4+ an…
Nanotexturing To Enhance Photoluminescent Response of Atomically Thin Indium Selenide with Highly Tunable Band Gap.
2016
Brotons-Gisbert, Mauro et al.
Crystal symmetry and pressure effects on the valence band structure ofγ-InSe andε-GaSe: Transport measurements and electronic structure calculations
2005
This paper reports on Hall effect and resistivity measurements under high pressure up to 3--4 GPa in $p$-type $\ensuremath{\gamma}$-indium selenide (InSe) (doped with As, Cd, or Zn) and $\ensuremath{\epsilon}$-gallium selenide (GaSe) (doped with N or Sn). The pressure behavior of the hole concentration and mobility exhibits dramatic differences between the two layered compounds. While the hole concentration and mobility increase moderately and monotonously in $\ensuremath{\epsilon}$-GaSe, a large increase of the hole concentration near 0.8 GPa and a large continuous increase of the hole mobility, which doubled its ambient pressure value by 3.2 GPa, is observed in $\ensuremath{\gamma}$-InSe.…
Experimental and theoretical study of band structure of InSe andIn1−xGaxSe(x<0.2)under high pressure: Direct to indirect crossovers
2001
This paper reports on the pressure dependence of the absorption edge of indium selenide and ${\mathrm{In}}_{1\ensuremath{-}x}{\mathrm{Ga}}_{x}\mathrm{Se}$ alloys $(xl0.2)$ up to the pressure at which precursor effects of the phase transition prevent further transmission measurements. The absorption edge could be divided into three components exhibiting different pressure coefficients: one corresponding to a direct transition that could be analyzed through the Elliot-Toyozawa theory, and two supplementary edges with quadratic dependence on the photon energy. The first component is attributed to the direct transition at the Z point of the rhombohedral Brillouin zone. One of the quadratic abso…
New Family of Ruddlesden−Popper Strontium Niobium Oxynitrides: (SrO)(SrNbO2-xN)n (n = 1, 2)
2002
The new family (SrO)(SrNbO2-xN)n (n = 1, n = 2) constitutes the first example of strontium niobium Ruddlesden−Popper oxynitrides and provides layered structures to obtain mixed valence niobium comp...
High-pressure, high-temperature phase diagram of InSe: A comprehensive study of the electronic and structural properties of the monoclinic phase of I…
2006
We report on an investigation of the high-pressure, high-temperature phase diagram of InSe. We optically observed the phase transition from the rhombohedral polytype (InSe-I) to the monoclinic phase (InSe-II) and determined the phase boundary up to $10\phantom{\rule{0.3em}{0ex}}\mathrm{GPa}$. High-pressure resistivity measurements were performed to complement the optical measurements. Monoclinic and cubic InSe (InSe-III) were observed to be metastable around $14.5\phantom{\rule{0.3em}{0ex}}\mathrm{GPa}$ and $420\phantom{\rule{0.3em}{0ex}}\mathrm{K}$, and evidence suggesting the existence of an as yet unidentified new high-pressure and high-temperature phase was found. By means of optical ab…
Role ofp-dands-dinteractions in the electronic structure and band gap of Zn1−xMxO (M=Cr, Mn, Fe, Co, Ni, and Cu): Photoelectron and optical spectrosc…
2012
We report an investigation on the effect of $p$-$d$ and $s$-$d$ interactions in the electronic structure, and especially in the band-gap value, of wurtzite wide-gap diluted magnetic semiconductors Zn${}_{1\ensuremath{-}x}$${M}_{x}$O ($M=\mathrm{Cr}$, Mn, Fe, Co, Ni, Cu). Thin films prepared by pulsed laser deposition are investigated by means of optical absorption at low-temperature and photoelectron spectroscopy. Pure wurzite phase is shown to be maintained for Co and Mn concentrations up to 25$%$ and for Cr up to 10$%$, while in the case of Fe, Ni, and Cu, other phases are present for concentrations higher than 5, 2, and 1$%$, respectively. The band gap of the Zn${}_{1\ensuremath{-}x}$${M…
Ultrafast Interface Charge Separation in Carbon Nanodot-Nanotube Hybrids
2021
Carbon dots are an emerging family of zero-dimensional nanocarbons behaving as tunable light harvesters and photoactivated charge donors. Coupling them to carbon nanotubes, which are well-known electron acceptors with excellent charge transport capabilities, is very promising for several applications. Here, we first devised a route to achieve the stable electrostatic binding of carbon dots to multi- or single-walled carbon nanotubes, as confirmed by several experimental observations. The photoluminescence of carbon dots is strongly quenched when they contact either semiconductive or conductive nanotubes, indicating a strong electronic coupling to both. Theoretical simulations predict a favo…
Specific features of the electronic structure of III–VI layered semiconductors: recent results on structural and optical measurements under pressure …
2003
In this paper we review some recent results on the electronic structure of III-VI layered semiconductors and its dependence under pressure, stressing the specific features that differentiate their behaviour from that of tetrahedrally coordinated semiconductors. We will focus on several unexpected results that have led to changes in the image that was currently accepted a few years ago. Intralayer bond angles change under pressure and the layer thickness remains virtually constant or increases. As a consequence, models based in intra- and inter-layer deformation potentials fail in explaining the low pressure nonlinearity of the band gap. Numerical-atomic-orbital/density-functional-theory ele…
Band structure of indium selenide investigated by intrinsic photoluminescence under high pressure
2004
This paper reports on photoluminescence experiments in $n$-type indium selenide $(T=300\phantom{\rule{0.3em}{0ex}}\mathrm{K})$ under hydrostatic pressure up to 7 GPa at low and high excitation densities. Photoluminescence measurements at low excitation density exhibit a broad band around the energy of the direct band gap of $\mathrm{InSe}$ and with the same pressure dependence. The reversible increase of its linewidth above $1\phantom{\rule{0.3em}{0ex}}\mathrm{GPa}$ is associated to a direct-to-indirect band-gap crossover between valence band maxima. The reversible decrease of its intensity above $4\phantom{\rule{0.3em}{0ex}}\mathrm{GPa}$ is interpreted as evidence of a direct-to-indirect b…
Anion ordering and defect structure in Ruddlesden-Popper strontium niobium oxynitrides
2004
The crystal structure of the n = 1 member of the Ruddlesden-Popper family (SrO)(SrNbO(2)N)(n) was refined by the Rietveld method using neutron powder diffraction data. This complex crystallizes in the I4/mmm space group with cell parameters a = 4.0506(2) and c = 12.5936(9) angstroms. The refined composition was Sr(2)NbO(3.28)N(0.72), which corresponds to a formal oxidation state for Nb of +4.72, meaning 72% Nb(V) and 28% Nb(IV). The nitrogen atoms order in the equatorial sites of the niobium octahedra according to Pauling's second crystal rule as the more charged anion occupies the site showing the larger bond strength sums. Pauling's second crystal rule is shown to be able to predict the d…
Transport measurements under pressure in III–IV layered semiconductors
2007
PACS 61.50.Ks, 62.50.+p, 72.15.Jf, 72.80.Jc This paper reports on Hall effect, resistivity and thermopower effect measurements under high pressure up to 12 GPa in p-type γ-indium selenide (InSe) and e-gallium selenide (GaSe). The paper focuses on two applications of transport measurements under pressure: electronic structure and phase transition studies. As concerns the electronic structure, we investigate the origin of the striking differences between the pressure behaviour of transport parameters in both layered compounds. While the hole concentration and mobility increase moderately and monotonously in e-GaSe up to 10 GPa, a large increase of the hole concentration at near 0.8 GPa and a …