Rb Layer Research Articles

Rubrene Thin Films with Viably Enhanced Charge Transport Fabricated by Cryo-Matrix-Assisted Laser Evaporation.

Among organic semiconductors, rubrene (RB; C42H28) is of rapidly growing interest for the development of organic and hybrid electronics due to exceptionally long spin diffusion length and carrier mobility up to 20 cm2V−1s−1 in single crystals. However, the fabrication of RB thin films resembling properties of the bulk remains challenging, mainly because of the RB molecule’s twisted conformation. This hinders the formation of orthorhombic crystals with strong π–π interactions that support the band transport. In this work, RB films with a high crystalline content were fabricated by matrix-assisted laser evaporation and the associated structure, composition, and transport properties are investigated. Enhanced charge transport is ascribed to the crystalline content of the film. Spherulitic structures are observed on top of an amorphous RB layer formed in the initial deposition stage. In spherulites, orthorhombic crystals dominate, as confirmed by X-ray diffraction and the absorption and Raman spectra. Surprisingly, nanowires several microns in length are also detected. The desorption/ionization mass and X-ray photoelectron spectra consistently show minimal material decomposition and absence of RB peroxides. The observed carrier mobility up to 0.13 cm2V−1s−1, is close to the technologically accepted level, making these rubrene films attractive for spintronic and optoelectronic applications.

Reconstructing Mousterian landscapes in the southeastern Pyrenees (Roca dels Bous site, Pre-Pyrenees ranges, Spain)

AbstractLandscape evolution studies enable us to understand site formation processes affecting past hunter-gatherer settlements. This work presents a landscape reconstruction of Roca dels Bous site (RB), which is a reference site for the Late Mousterian occupation of the incised valleys of the southeastern Pyrenees. For this purpose, we combined geomorphological studies, stratigraphic descriptions, new single-grain optically stimulated luminescence datasets, statistical methods, and geophysical surveys. RB formed by gravitational processes induced by fluvial undermining of the Segre River during changing late Pleistocene climatic conditions. Geomorphological and chronological data combined with fluvial age-incision models suggest that, during Late Mousterian occupation, RB was located very near the Segre floodplain level and closer to water and raw material natural resources than at present. The accumulation of gravitational deposits associated with the archaeological levels occurred at rates of 0.16–0.44 m ka-1, between 55 and 47 ka, coinciding with Marine Oxygen Isotope Stage 3 (MIS3). More detailed comparison with available climatic curves suggests that the dated RB layers were potentially deposited during cold phases within MIS3. This work provides new landscape-based evidence to examine the paleoenvironmental context of Neanderthal presence in the southeastern Pre-Pyrenees, an important region in the debate regarding Neanderthal demise in Western Europe around 40 ka.

Observation of two collapsed phases in CaRbFe4As4

We report the observation of the pressure-induced, fully-collapsed tetragonal phase of CaRbFe4As4 for P~ 22 GPa via high-pressure x-ray diffraction and magnetotransport measurements. The x-ray measurements, along with resistivity measurements, show that there is an initial half-collapsed tetragonal phase for 6 < P < 22 GPa, in which superconductivity is continuously suppressed from Tc= 35K at P= 3.1 GPa to Tc <2K for P >17.2 GPa, as well as signs of the fully-collapsed tetragonal phase near P=22 GPa. Density functional calculations suggest that both of these transitions are driven by increased As-As bonding, first across the Ca layer, and then at the second transition, across the Rb layer. Although electrical resistivity measurements in the fully-collapsed tetragonal phase do not show superconductivity, there is a change in the slope of both the Hall coefficient and the longitudinal resistance near 22 GPa, suggesting a strong correlation between the electronic and lattice degrees of freedom in this new iron-based superconductor.

Sodium dirubidium citrate, NaRb2C6H5O7, and sodium dirubidium citrate dihydrate, NaRb2C6H5O7(H2O)2.

The crystal structures of sodium dirubidium citrate {poly[μ-citrato-dirubi-dium(I)sodium(I)], [NaRb2(C6H5O7)] n } and sodium dirubidium citrate dihydrate {poly[di-aqua-(μ-citrato)dirubidium(I)sodium(I)], [NaRb2(C6H5O7)(H2O)2] n } have been solved and refined using laboratory X-ray powder diffraction data, and optimized using density functional techniques. Both structures contain Na chains and Rb layers, which link to form different three-dimensional frameworks. In each structure, the citrate triply chelates to the Na+ cation. Each citrate also chelates to Rb+ cations. In the dihydrate structure, the water mol-ecules are bonded to the Rb+ cations; the Na+ cation is coordinated only to citrate O atoms. Both structures contain an intra-molecular O-H⋯O hydrogen bond between the hy-droxy group and one of the terminal carboxyl-ate groups. In the structure of the dihydrate, each hydrogen atom of the water mol-ecules participates in a hydrogen bond to an ionized carboxyl-ate group.

Possible electric field induced indirect to direct band gap transition in MoSe2

Direct band-gap semiconductors play the central role in optoelectronics. In this regard, monolayer (ML) MX2 (M = Mo, W; X = S, Se) has drawn increasing attention due to its novel optoelectronic properties stemming from the direct band-gap and valley degeneracy. Unfortunately, the more practically usable bulk and multilayer MX2 have indirect-gaps. It is thus highly desired to turn bulk and multilayer MX2 into direct band-gap semiconductors by controlling external parameters. Here, we report angle-resolved photoemission spectroscopy (ARPES) results from Rb dosed MoSe2 that suggest possibility for electric field induced indirect to direct band-gap transition in bulk MoSe2. The Rb concentration dependent data show detailed evolution of the band-gap, approaching a direct band-gap state. As ionized Rb layer on the surface provides a strong electric field perpendicular to the surface within a few surface layers of MoSe2, our data suggest that direct band-gap in MoSe2 can be achieved if a strong electric field is applied, which is a step towards optoelectronic application of bulk materials.

Selective reflection from an Rb layer with a thickness below λ/12 and applications.

We have studied the peculiarities of selective reflection from an Rb vapor cell with a thickness L<70 nm, which is smaller than the length scale of evanescent fields λ/2π and more than an order of magnitude smaller than the optical wavelength. A 240 MHz redshift due to the atom-surface interaction is observed for a cell thickness of L=40 nm. In addition, complete frequency-resolved hyperfine Paschen-Back splitting of atomic transitions to four components for Rb87 and six components for Rb87 is recorded in a strong magnetic field (B>2 kG).

Direct evidence for a metallic interlayer band in Rb-intercalated bilayer graphene

We have fabricated Rb-intercalated bilayer graphene on 6H-SiC(0001) and performed low-energy electron diffraction (LEED) and high-resolution angle-resolved photoemission spectroscopy (ARPES). In LEED experiments, we observed a 2 $\ifmmode\times\else\texttimes\fi{}$ 2 pattern identical to that of the bulk graphite intercalation compound C${}_{8}$Rb. The existence of a 2 $\ifmmode\times\else\texttimes\fi{}$ 2 Rb layer is also confirmed by ARPES, where band folding due to the periodic potential of intercalated atoms was observed. All these results indicate that Rb atoms are intercalated between the two graphene monolayers in a regular manner similar to that of bulk C${}_{8}$Rb. Further, we observed a parabolic metallic band at the Brillouin-zone center, analogous to the so-called interlayer band, which is thought to play an important role for the superconductivity in graphite intercalation compounds.

Photoemission studies of Mg and Rb layers on Zn(0 0 0 1)

The electronic structure of the clean Zn(0 0 0 1) surface is studied by angle resolved photoemission. An earlier detected surface state at the surface Brillouin zone centre is confirmed and a new surface state is found at the surface Brilluoin zone boundary. The surface electronic structure of Zn is found to be similar to the that of Cd. Evaporation of thin films of Mg and Rb onto the Zn(0 0 0 1) surface quenches the emission from both surface states and reduces the intensity of the bulk related structures.

Second Stage Physintercalation of N2 Molecules into C24Rb

Using neutron diffraction, a second stage physintercalation of molecular N2 into C24Rb has been observed in which the N2 molecules reside inside every second Rb layer. The new phase, C48Rb2(N2)x, coexists below 175 K with pure C24Rb and the 1st stage C24Rb(N2)x. The tilt of the N2 molecular axes relative to the graphene planes was determined versus T using the nuclear resonance photon scattering (NRPS) technique and found to be almost parallel to the graphene planes of the C24Rb sample.

Observation of a second stage compound in the intercalation of N 2 in C 24Rb: A neutron diffraction study

Upon cooling a single-phase sample of C 24Rb under N 2 pressure, intercalation of N 2 into C 24Rb starts immediately below RT. A single phase of C 24Rb(N 2) x is formed, where N 2 resides at every Rb layer, and x increases monotonically as temperature decreases. In the temperature range 175 > T > 140 K a phase transition occurs, and for T ⩽ 140 K three phase co-exist: C 24Rb, C 24Rb(N 2) x . The third phase, has a stacking where N 2 at every second Rb layer, hence, it is second stage compound in the intercalation of N 2 in C 24Rb.

Rb/β-SiC(100) surface and catalytic oxidation of the substrate

We investigate the Rb/β-SiC(100) surface and the subsequent promoted SiC oxidation using photoemission spectroscopy with the AlKα (1486.6 eV) and ZrMζ (151.4 eV) X-ray lines at Si 2p, C 1s, O 1s and Rb 3p core levels, and work-function measurements. Clean and stoichiometric β-SiC(100) surfaces are obtained by thermal annealing procedures only. Rb deposition on the clean β-SiC(100) surface induces a large decrease of the work function reaching a minimum at Δф = − 3.3 eV. The Rb layer enhances the room temperature oxygen uptake by 4 orders of magnitude. Upon subsequent annealing, high silicon oxidation states are formed, leading to SiO2/β-SiC(100) interface formation with no apparent carbon atoms present on the surface and/or in the oxide layer. Further heating below 800°C removes the Rb atoms completely by thermal desorption.

X-ray study of the liquid and solid phases of the alkali metals in KC24- and RbC24-intercalated graphite single crystals.

We report a detailed x-ray scattering study of the structure of stage-2 K- and Rb-intercalated graphite single crystals, as a function of temperature. The graphite-host samples were characterized by a low level of stage impurities but the degree of stacking disorder was high. The diffuse scattering from the room-temperature two-dimensional (2D) liquidlike structure of the alkali metals has been measured. It reveals subtle but meaningful differences between K and Rb. The in-plane liquid-solid transitions and the associated growth of interlayer correlations (2D-3D crossover) have been studied through scans of the main and satellite reflections. The stacking faults of the graphite host appear to be transferred to the intercalate. In the case of ${\mathrm{KC}}_{24}$, a model calculation based on an average hexagonal K layer, with a subsequent partial relaxation of the K atoms in the graphite sites, provides a good fit to the observed reflection intensities. At lower temperatures, the K layers are confirmed to undergo a hexagonal-oblique symmetry-breaking transition at ${\mathit{T}}_{\mathit{L}}$\ensuremath{\sim}95 K, while the Rb layers stay hexagonal down to 10 K. Moreover, complex stacking sequences of the intercalate develop as the temperature is reduced, and in ${\mathrm{RbC}}_{24}$ one observes a clear loss of the stacking correlations. This is due presumably to a frustration of the K sublattice coming from interlayer and intralayer interactions.

Dynamics of two-dimensional melting on a periodic substrate.

Quasielastic and inelastic neutron scattering has been used to study the diffusion and the layer shear elastic constant across the melting transition of incommensurate K and Rb layers intercalated in graphite. The melting of the quasi two-dimensional layers on a periodic graphite substrate is characterized by a smooth transition from a solidlike to a liquidlike dynamical behavior via unpinning of discommensuration domain walls.

Dynamical properties of alkali intercalates in graphite

We report on neutron quasi-elastic measurements of in-plane Rb dynamics in a C 24Rb compound. The spectra show diffusion broadened linewidths, yielding a diffusion constant of 2.6 × 10 −5 cm 2/s at 343 K which increases by a factor of 3 at 618 K. From the Q dependence of the linewidth the difference in the dynamics between a usual three-dimensional liquid and a single Rb layer is elucidated.

X-ray, e.p.r. and n.m.r. studies and structure of some ternary compounds of graphite

The potassium hydrogenographitides, KH 2/3C 8 and K 3H 4/3C 16, and potassium and rubidium mercurographitides have been extensively studied. In the hydrogenographitide series 1H n.m.r. is not observed, but e.p.r. shows metallic character in stage I and II compounds. 13C n.m.r. suggests charge transfer, in agreement with a negative hydrogenated ion in the lattice. A structure is proposed using the stability of the H 2 − ion when the bond is sufficiently lengthened. With mercurographitides, e.p.r. cannot be seen because of the Hg spin-orbit coupling. 13C n.m.r. agrees with a model having negatively charged Hg atoms between two K or Rb layers. The possible structure for this layer is compared with the structure of alloys of the same stoichiometry.

Phase transitions in the graphite intercalation compound C 24Rb

The phase transitions in C 24Rb have been studied by means of resistivity, a.c. calorimetry and neutron scattering measurements. Four anomalies in resistivity are observed at temperatures 60, 110, 165 and ca. 210 K, while corresponding anomalies in specific heat are observed. The neutron scattering study reveals two structural changes: (1) Above 165 K the Rb atoms have neither long-range intra- or interlayer correlation, while below 165 K they form a non-registered triangular lattice which is modulated by the graphite potential and has a preferred orientation relative to the graphite lattice. The modulation is interpreted in terms of the static distortion wave. The Rb layers have a stacking order with a period of three CRbC sandwiches. (2) Below about 110 K the stacking of Rb layers changes to that of a much longer period than the phase above 110 K.

Neutron Scattering Studies of the Ordered Structure of C24Rb

A neutron elastic scattering measurement has been performed on C 24 Rb at temperatures between 106 K and 165 K. The observed extinction rule of the diffraction can be explained on the basis of a simple model: Rb layers form a triangular lattice with fundamental wave vector | G R 0 |=1.20 A -1 , which is modulated by the graphite layers. The modulation is described by a static distortion wave, which manifests itself in the appearance of a satellite peak at | G R 0 - G 0 |=1.78 A -1 , for example, where | G 0 |=2.95 A -1 is the fundamental in-plane wave vector of graphite. This model leads to a prediction that the Rb layer is rotated relative to the graphite layer by ∼9°.