Cs Intercalation Research Articles

Building High-Density Polar Hybrid Perovskites via Intercalation of Cs+ and Aromatic Diamine for Passive X-ray Detection.

2D organic-inorganic hybrid perovskites (OIHPs) have shown great promise in direct X-ray detection. The development of high-performance passive X-ray detectors in 2D OIHPs calls for an increase in material density while maintaining structural polarity, which is becoming quite challenging. Here, a high-density, polar 2D alternating-cation-intercalated (ACI) perovskite, (4-AP)Cs2Pb2I8 (B, 4-AP = 4-amidinopyridinium), capable of addressing this problem is successfully constructed by introducing heavy Cs+ into the interlayer space of an aromatic Dion-Jacobson (DJ) perovskite (4-AP)PbI4 (A). Through such a DJ-to-ACI design, the newly developed 2D OIHP B not only significantly increases its density to 4.23 g cm-3 (even higher than that of 3D MAPbI3) but also crystallizes in a polar space group (Ama2), which further leads to enhanced X-ray attenuation and an obvious polar photovoltage (1.1 V) under X-ray irradiation. As a result, X-ray detectors fabricated by high-quality single crystals of B exhibit excellent and stable detection performance under self-powered mode with a high sensitivity of 107 μC Gy-1 cm-2 and a low detection limit of 289 nGy s-1. This work provides implications for the future exploration and regulation of novel ACI OIHPs for high-performance photoelectronic devices.

Work function tuned, surface Cs intercalated BiVO4 for enhanced photoelectrochemical water splitting reactions

Monoclinic BiVO4 is a widely researched semiconductor in solar water splitting owing to its suitable characteristics. However, BiVO4 faces limitations, such as the inefficient separation and transportation of photogenerated charges in the bulk and poor catalytic water oxidation reactions at the surface that affect the water-splitting efficiency. In this work, the Cs intercalation strategy at the surface of BiVO4 is proposed for the enhanced water splitting to H2 and O2 productions via the effective separation and transportation photogenerated charges and improved surface catalytic water oxidation reactions. The Cs ions are found to intercalate at the surface of BiVO4 and regulate the oxygen vacancies to provide active O2 production sites and stability. The surface intercalation of Cs boosts the photocurrent to 1.89 mA cm−2 at 1.23 V vs. reference hydrogen electrode (RHE). A stoichiometric evolution of H2 and O2 is recorded with a faradaic efficiency of 92%. The open-circuit voltage measurements confirmed the increase in the carrier lifetime with the work function tuning upon Cs intercalation. The proposed Cs intercalation strategy suggests an effective route to suppress the charge recombination with an increase in carrier lifetime and charge separation in BiVO4 for the enhanced PEC application.

Alkali-metal induced electronic structure evolution in Sn4Sb3 studied by angle-resolved photoemission spectroscopy

We report on an angle-resolved photoemission spectroscopy study of the effect of in-situ cesium (Cs) dosing on the surface electronic structure of Sn4Sb3, a newly emerging candidate of topological semimetallic superconductor. As the chemical potential of the system increases upon Cs dosing, we observed an electronic structure evolution that features an increasing sharpness of band features as well as a momentum- and band-dependent energy shift of the bands at low binding energies. These observations go beyond a simple rigid band shift model and are manifestations of quantum confinement of electronic states near the surface due to Cs decoration. By studying the evolution of Cs 5p core level as functions of Cs dosage and photoelectron emission angle, we found Cs atoms deposit on the sample surface at low dosages and tend to intercalate underneath the surface at high dosages. This process is accompanied by a gradual suppression of a surface band splitting, an observation that can be captured by simplified first-principles calculations in both cases with the deposition/intercalation occurring on top of/underneath the first Sn–Sb bilayer. Our finding highlights an intriguing possibility of an unexpected dimensionality reduction in Sn4Sb3 caused by Cs intercalation at interstitial sites that promises its novel surface properties different from the bulk.

Vapor-Phase Intercalation of Cesium into Black Phosphorous

Cesium vapors were charged into black phosphorous (BP) flakes at varied times and at a temperature gradient of 150 °C. The X-ray diffraction (XRD) measurements of these samples suggest a reduction in the strength of van der Waal interactions between BP layers leading to the loss of coherence of out-of-plane peaks. At the same time, the three main Raman modes of BP (Ag1, B2g, and Ag2) steadily redshifted as exposure times were increased, with modes B2g and Ag2 shifting faster than Ag1. After initial rapid downshifts of active BP phonon modes, this intercalation strategy showed its limits following prolonged exposure times. Saturation of BP flakes by Cs vapors ensued and the kinetics was fitted with an exponential decay function. Furthermore, the thermoelectric power (TEP) of cesiated BP exhibited an inversion in sign from positive to negative around 400 K, lending credence to the transformation of as-prepared BP which is a p-type semiconductor to an n-type equivalent due to Cs atom intercalation driven shifting of the Fermi level toward the conduction band of BP and the donation of electrons from Cs. Density functional theory (DFT) calculations were used to delve deeper into understanding Cs intercalation on the structural evolution of BP.

Alkali Metal Intercalation and Reduction of Layered WO2Cl2

Li, Na, K, Rb, and Cs have been intercalated into the two-dimensional (2D) van der Waals material WO2Cl2, with K, Rb, and Cs intercalation being reported for the first time. The topochemical reacti...

Investigation on Strontium Adsorption Selectivity of Hydrothermally Synthesized Layered Sodium Titanates

Two layered sodium titanate phases, sodium nonatitanate (Na4Ti9O20) and sodium trititanate (Na2Ti3O7), have been hydrothermally synthesized and their Sr2+ adsorption selectivity was investigated in the coexistence of Cs+ with ionic equivalent concentration. Although both phases exhibit Sr2+ selective adsorption, Na4Ti9O20 adsorbed both Sr2+ and Cs+, while the adsorption of Cs+ was not detected on Na2Ti3O7, despite its higher adsorption capacity. To investigate the causes for the high Sr2+ selectivity of Na2Ti3O7, additional adsorption tests were carried out in different pH, which can be interpreted as the Sr2+ –H+ binary system, and in single and ternary systems of Al3+, Sr2+ and K+ with ionic equivalent concentrations. When changing the pH, the adsorption amount of Sr2+ showed a high and nearly constant value at pH above 4 and drastically decreased at pH below 3, reaching nearly zero at pH 2. In the Al3+–Sr2+ –K+ ternary system, the adsorption amount decreased in the order of Sr2+, Al3+ and K+. The adsorption amount of K+ was low compared to that of Sr2+ and Al3+ in both the single and ternary systems. Meanwhile, the adsorption amount of Sr2+ significantly decreased compared to that in the single system, unlike in the Sr2+–Cs+ binary system where the adsorption of Sr2+ was almost the same. From these results, the high Sr2+ selectivity of Na2Ti3O7 in the Sr2+–Cs+ binary system was anticipated to be due to the size effect. The smaller interlayer spacing of Na2Ti3O7 compared to that of Na4Ti9O20 appears to inhibit the intercalation of Cs+ due to its large ionic radius.

Modifying the geometric and electronic structure of hexagonal boron nitride on Ir(111) by Cs adsorption and intercalation

Epitaxial hexagonal boron nitride on Ir(111) is significantly modified by adsorption and intercalation of alkali-metal atoms. Regarding geometry, intercalation lifts the two-dimensional layer from its substrate and reduces the characteristic corrugation imprinted by direct contact with the metal substrate. Moreover, the presence of charged species in close proximity to the hexagonal boron nitride (hBN) layer strongly shifts the electronic structure (valence bands and core levels). We used scanning tunneling microscopy, low-energy electron diffraction, x-ray photoelectron spectroscopy (XPS), and the x-ray standing wave technique to study changes in the atomic structure induced by Cs adsorption and intercalation. Depending on the preparation, the alkali-metal atoms can be found on top and underneath the hexagonal boron nitride in ordered and disordered arrangements. Adsorbed Cs does not change the morphology of hBN/Ir(111) significantly, whereas an intercalated layer of Cs decouples the two-dimensional sheet and irons out its corrugation. XPS and angle-resolved photoelectron spectroscopy reveal a shift of the electronic states to higher binding energies, which increases with increasing density of the adsorbed and intercalated Cs. In the densest phase, Cs both intercalates and adsorbs on hBN and shifts the electronic states of hexagonal boron nitride by 3.56 eV. As this shift is not sufficient to move the conduction band below the Fermi energy, the electronic band gap must be larger than 5.85 eV.

Understanding and Engineering Phonon-Mediated Tunneling into Graphene on Metal Surfaces

Metal-intercalated graphene on Ir(111) exhibits phonon signatures in inelastic electron tunneling spectroscopy with strengths that depend on the intercalant. Extraordinarily strong graphene phonon signals are observed for Cs intercalation. Li intercalation likewise induces clearly discriminable phonon signatures, albeit less pronounced than observed for Cs. The signal can be finely tuned by the alkali metal coverage and gradually disappears upon increasing the junction conductance from tunneling to contact ranges. In contrast to Cs and Li, for Ni-intercalated graphene the phonon signals stay below the detection limit in all transport ranges. Going beyond the conventional two-terminal approach, transport calculations provide a comprehensive understanding of the subtle interplay between the graphene-electrode coupling and the observation of graphene phonon spectroscopic signatures.

Application of Cesium on the Restriction of Precursor Crystallization for Highly Reproducible Perovskite Solar Cells Exceeding 20% Efficiency

In this study, we systematically explored the mixed-cation perovskite Cs x(MA0.4FA0.6)1- xPbI3 fabricated via sequential introduction of cations. The details of the effects of Cs+ on the fabrication and performance of inorganic-organic mixed-cation perovskite solar cells examined in detail in this study are beyond the normal understanding of the adjusting band gap. It is found that a combined intercalation of Cs+ and dimethyl sulfoxide (DMSO) in PbI2-DMSO precursor film formed a strong and steady coordinated intermediate phase to retard PbI2 crystallization, suppress yellow nonperovskite δ-phase, and obtain a highly reproducible perovskite film with less defects and larger grains. The Cs-contained triple-cation-mixed perovskite Cs0.1(MA0.4FA0.6)0.9PbI3 devices yield over 20% reproducible efficiencies, superior stabilities, and fill factors of around 0.8 with a very narrow distribution.

Bentonite/chitosan nanocomposite: Preparation, characterization and kinetic study of its thermal degradation

The nanocomposite 5%Bentonite/Chitosan (5%Bt/CS) has been prepared and characterized by the means of Fourier transform infrared (FTIR) spectroscopy, X-ray diffraction (XRD), transmission electron microscopy (TEM) and Brunauer, Emmett and Teller method (BET). These techniques confirmed the formation of the 5%Bt/CS, via the intercalation of CS in the interlayer space of Bt leading to the exfoliation of its structure. The non-isothermal degradation process of 5%Bt/CS under air atmosphere was investigated by thermogravimetric analysis (TGA/DTG) and differential scanning calorimetry (DSC), at various heating rates. The appearance of multi-peaks in the DTG curves of 5%Bt/CS, revealed that the nanocomposite underwent a complex degradation mechanism with at least two steps, and greater stability compared to those of neat CS. The thermal degradation steps kinetic parameters were calculated using Kissinger (KIS) and isoconversional Flynn–Wall–Ozawa (FWO), Kissinger-Akahira-Sunose (KAS), Tang and Friedman (FR) methods It was found that the average values of activation energies were Ea = 210.53 kJ/mol for the first step and Ea = 345.28 kJ/mol for the second step. These values were used to match the experimental and simulated y(α) and z(α) master plots. It was found that the modified Sestak-Berggren f(α) = c(1 − α)nαmmodel allowed reproducing qualitatively the Y(α) curves, giving the same maximums, αm, for each value of β, which are consistent with the thermal degradation mechanisms of 5%Bt/CS.

Vacancy clusters as entry ports for cesium intercalation in graphite

Graphite has been subjected to surface damage by Ar + ion bombardment. It has been found by scanning tunneling microscopy (STM) that deposited Cs atoms preferentially cluster at the artificially-produced vacancy clusters in the basal plane at 300 K. Density functional theory calculations show that Cs is more strongly bound at these defect sites than at basal plane step edges, providing a plausible explanation for the experimental observation. STM reveals that Cs intercalation into graphite occurs by diffusion through these vacancy clusters, acting as entry ports to the interior, at 700 K. DFT calculations show that these defects must consist of more than four contiguous atoms in order for Cs intercalation to be energetically easy. Experimentally, Cs atom nucleation at defect sites seems to culminate at Cs cluster heights near 1 nm and cluster diameters near 5 nm. Intercalated Cs produces coherent single Cs islands in the galleries as well as layered structures caused by superposition of overlapping individual islands. Oxygen exposure to graphite containing Ar +-produced defects does not influence Cs clustering at the defects or Cs entry into galleries beneath the defect. However, large oxygen exposures these clusters results in diminution of Cs intercalation, probably due to surface oxidation of Cs clusters.

Electronic properties of Cs-intercalated single-walled carbon nanotubes derived from nuclear magnetic resonance

We report on the electronic properties of Cs-intercalated single-walled carbon nanotubes (SWNTs). A detailed analysis of the 13C and 133Cs nuclear magnetic resonance (NMR) spectra reveals an increased metallization of the pristine SWNTs under Cs intercalation. The ‘metallization’ of CsxC materials where x=0–0.144 is evidenced from the increased local electronic density of states (DOS) n(EF) at the Fermi level of the SWNTs as determined from spin–lattice relaxation measurements. In particular, there are two distinct electronic phases called α and β and the transition between these occurs around x=0.05. The electronic DOS at the Fermi level increases monotonically at low intercalation levels x<0.05 (α-phase), whereas it reaches a plateau in the range 0.05⩽x⩽0.143 at high intercalation levels (β-phase). The new β-phase is accompanied by a hybridization of Cs(6s) orbitals with C(sp2) orbitals of the SWNTs. In both phases, two types of metallic nanotubes are found with a low and a high local n(EF), corresponding to different local electronic band structures of the SWNTs.

Pressure-induced deformation of theC60fullerene inRb6C60andCs6C60

We report a detailed experimental and theoretical study of the ${\mathrm{Rb}}_{6}{\mathrm{C}}_{60}$ and ${\mathrm{Cs}}_{6}{\mathrm{C}}_{60}$ systems under pressure. X-ray diffraction and x-ray absorption experiments have been coupled with ab initio calculations in order to understand the mechanisms taking place during the compression of these intercalated systems. The whole data set is consistent with a pressure-induced modification of the shape of the ${\mathrm{C}}_{60}$ molecule. This deformation constitutes an enhancement of the calculated distortion of the molecule in these intercalated compounds at ambient conditions, being equivalent to pulling the molecule through the three orthogonal axes pointing toward the bcc faces containing the alkali metals. Both experiments and calculations show that the pressure-induced deformation of the fullerene molecule is more important for Rb than it is for Cs intercalation.

A theoretical study of Cs adsorption at tips of single-wall carbon nanotubes: field emission properties

Cs intercalation has demonstrated experimentally a significant reduction of the work function of carbon nanotubes, thus improving field emission properties. In this paper, we report a density functional theory (DFT) study within the generalized gradient approximation (GGA), regarding the effects of Cs on field emission of single-wall carbon nanotubes (SWCNTs). Specifically, a comprehensive examination was carried out to investigate the effects of Cs adsorbed on capped and open-ended C(5,5) armchair SWCNT tips. Our calculations showed a reduction in the ionization potential (IP) upon Cs physisorption, thus improving the emission properties of carbon nanotubes, as reported experimentally. The structure of the adsorbed Cs-cluster, the corresponding adsorption energies, and the IPs, were altered upon the inclusion of a local electric field in the calculations to mimic the emission environment.

Extremely small diffusion constant of Cs in multiwalled carbon nanotubes

The Cs intercalation process in multiwalled carbon nanotubes (MWNTs) was studied by cross-sectional scanning photoemission microscopy. Cs atoms initially deposited on the tips of aligned nanotubes diffused toward their roots. The Cs diffusion constant for the MWNTs at room temperature was evaluated from the Cs distribution measured along the axes of the tubes. The value of 2×10−12 cm2/s obtained is seven orders of magnitude smaller than that in graphite, although the local atomic structure of an intercalated MWNT is very similar to that of intercalated graphite.

Characterization of CsC 24 prepared from carbon materials with different graphitization degree

Samples of CsC 24 were prepared from carbon materials (derived from petroleum cokes) with different graphitization degree and were characterized by X-ray diffraction (XRD), electron spin resonance (ESR) and Raman spectroscopy. The XRD measurement showed that all the host carbon samples heat treated from 1300 to 2800°C being contacted with CsC 8 planar specimen allowed the intercalation of Cs. Correspondingly, the resulting CsC 24 samples were able to sorb hydrogen molecules, which is a clear evidence of the formation of the nanospace. The broad ESR signal due to conduction electron observed for carbon samples (heat-treatment temperature, HTT above 2200°C) disappeared after intercalation of Cs because of the spin–orbit interaction caused by the intercalated Cs. The Raman G-band of the CsC 24 samples (HTT above 1750°C) shifted from 1584 cm −1 of the host carbon to higher wave number (∼1602 cm −1) in agreement with the reported data for CsC 24. In addition, it was confirmed that the D-band signal disappeared for the CsC 24 samples (HTT above 2000°C).

Na and Cs intercalation of 2H-TaSe2 studied by photoemission

The electronic structure of the layered compound 2H-TaSe2 has been studied using angle-resolved photoemission before and after in situ intercalation with Na and Cs. Core level spectra verified that Na and Cs both intercalate easily at room temperature, with only small amounts remaining on the surface. Valence band spectra revealed changes in the electronic band structure which were much more extensive than predicted by the rigid band model, but which were in reasonable agreement with theoretical bands calculated by the LAPW method. Some discrepancies between the experimental and calculated results are probably due to intercalation induced changes in the stacking of host layers. A general similarity with results from transition metal dichalcogenides with 1T structure indicates that the intercalation properties are not critically dependent on the internal structure of the host layers.

Calculation and XPS measurements of the Ta4f CDW splitting in Cu, Cs and Li intercalation phases of 1T-TaX 2 (X=S, Se)

Metallic quasi-two-dimensional transition metal dichalcogenides (TMDCs) 1T-TaS 2 and 1T-TaSe 2 show charge density wave (CDW) at room temperature. These show up as a superstructure in low energy electron diffraction (LEED) and as a distinct splitting of the Ta4f core level and a part of the Ta d z 2 derived valence band. The changes in this Ta4f CDW splitting are investigated for in situ Cu, Li and Cs intercalated crystals by photoelectron spectroscopy (PES, XPS, SXPS) and LEED. The intercalation effects a certain number of CDW phase transitions, where each CDW phase is related to a characteristic CDW related splitting of the Ta4f core level. We extend the simple electrostatic model of Hughes and Pollak [ 1] to calculate the form of the splitting for different CDW phases and find a good correlation to the XPS measurements. This agreement confirms that the Ta4f CDW splitting is brought about by an electrostatic interaction between the Ta atoms and the (free) metallic charge density. The comparison of the phase transition sequence for three different intercalants indicates that a three-dimensional rather than a two-dimensional Fermi surface nesting mechanism is responsible for the phase transitions, as originally proposed by Wooley and Wexler [ 2].

Photoemission spectroscopy of single-walled carbon nanotube bundles

Abstract The electronic structures and the work functions of pristine and Cs-intercalated single-walled carbon nanotube bundles were investigated by C 1 s and valence band photoemission spectroscopy. The C 1 s spectrum of the pristine material showed a Doniach–Sunjic type asymmetry, indicating the existence of metallic tubes. The work function of the pristine bundles was found to be 4.8 eV, which is about 0.2 eV larger than that of graphite. A drastic decrease of the work function to about 2.0 eV was observed in the Cs-intercalated sample. The Cs intercalation also caused a nearly two-order increase in the spectral intensity at the Fermi level.

Synthesis and Structure of BxC1−x Intercalation Compounds with Heavy Alkali Metals (K, Rb, and Cs)

BxC1−x (x = 0.1 and 0.25) oriented platelets were intercalated with alkali metal vapor (M = K, Rb, Cs), giving first-stage M(B0.1C0.9)8 and M(B0.25C0.75)10. The presence of M(BxC1−x)5 dense domains interstratified in the first-stage structure were brought out from the 00.ℓ simulations. The presence of these domains is attributed to the acceptor electron effect of boron, which slightly enhances the intercalation rate as compared to pure carbon. Intercalation of Cs in liquid ammonia is improved using 1600 °C heat-treated B0.25C0.75 as a host material, and the composition Cs(B0.25C0.75)12 is reached after intercalation. In intercalation compounds of Cs in liquid ammonia obtained from heat-treated B0.25C0.75, as the heat-treatment temperature (HTT) was increased from 1600 to 2000 °C, the segregation of first stage was observed in two structures Cs(BxC1−x)8 and Cs(BxC1−x)10 with the respective 2 × 2 0° and 2.23 × 2.23 two-dimensional lattices of the cesium atoms. The presence of these two structures is assigned to the heterogeneity of the host material induced by the formation of B4C boron carbide domains and the consecutive boron elimination of the BxC1−x lamellar phase with increasing HTT.