Low Intercalation Levels Research Articles

Charge coloration dynamics of electrochromic amorphous tungsten oxide studied by simultaneous electrochemical and color impedance measurements

The coloration mechanisms in electrochromic systems can be probed by comparing the dynamics of the electrical and optical responses. In this paper, the linear frequency-dependent electrical and optical responses of an amorphous tungsten oxide thin film were measured simultaneously by a combination of two techniques—that is, electrochemical impedance spectroscopy (EIS) and the so-called color impedance spectroscopy. This was done at different bias potentials and their associated intercalation levels. Equivalent circuit fitting to the EIS spectra was used to extract the Faradaic components from the total impedance response. The latter were assigned to an intermediate adsorption step before the intercalation and to the diffusion of the electron–ion couple in the film. A quantity denoted complex optical capacitance was compared to the complex electrical capacitance—particularly, their expressions are related to the Faradaic processes. The coloration at low intercalation levels followed both the adsorption and diffusion phenomena. Conversely, the diffusion contribution was dominant at high intercalation levels and the adsorption one seemed to be negligible in this case. The complex spectra of perfectly synchronized electrical and optical responses are expected to differ only by a multiplying factor. This was the case at low intercalation levels, apart from small deviations at high frequencies. A clear departure from this behavior was observed as the intercalation level increased. A combination of frequency-dependent techniques, as presented here, can help to elucidate the dynamics of the coloration mechanisms in electrochromic materials at various conditions—for example, at different intercalation levels and optical wavelengths.

Coloration of tungsten oxide electrochromic thin films at high bias potentials and low intercalation levels

The optical absorption of amorphous tungsten oxide thin films was studied at low intercalation levels. The electronic density of states was obtained by an electrochemical method and bias potential regions were assigned to the conduction band and gap states according to the experimentally estimated conduction band edge. Differences between the coloration in the conduction band and gap states were observed when comparing the experimental results to a theoretical site-saturation model that considers electronic transitions between localized tungsten sites. The model could reproduce the optical response due to conduction band states. However, it underestimated the rate of change of the optical absorption coefficient with intercalation level for gap states. This discrepancy is discussed in the context of small-polaron optical absorption.

Potassium-intercalated bulk HfS2 and HfSe2 : Phase stability, structure, and electronic structure

We have studied potassium-intercalated bulk HfS$_2$ and HfSe$_2$ by combining transmission electron energy loss spectroscopy, angle-resolved photoemission spectroscopy and density functional theory calculations. Calculations of the formation energies and the evolution of the energies of the charge carrier plasmons as a function of the potassium content show that certain, low potassium concentrations $x$ are thermodynamically unstable. This leads to the coexistence of undoped and doped domains if the provided amount of the alkali metal is insufficient to saturate the whole crystal with the minimum thermodynamically stable potassium stoichiometry. Beyond this threshold concentration the domains disappear, while the alkali metal and charge carrier concentrations increase continuously upon further addition of potassium. At low intercalation levels, electron diffraction patterns indicate a significant degree of disorder in the crystal structure. The initial order in the out-of-plane direction is restored at high $x$ while the crystal layer thicknesses expand by 33-36%. Superstructures emerge parallel to the planes which we attribute to the distribution of the alkali metal rather than structural changes of the host materials. The in-plane lattice parameters change by not more than 1%. The introduction of potassium causes the formation of charge carrier plasmons. The observation of this semiconductor-to-metal transition is supported by calculations of the density of states (DOS) and band structures as well as angle-resolved photoemission spectroscopy. The calculated DOS hint at the presence of an almost ideal two-dimensional electron gas at the Fermi level for $x<0.6$. The plasmons exhibit quadratic momentum dispersions which is in agreement with the behavior expected for an ideal electron gas.

The photoemission study of Sr-intercalated C60

Abstract Motivated by the renaissance of fullerene superconductivity, research on fullerides has once more become a hot topic in physics and material science. Here, we investigate the electronic properties of the phases that are formed upon metal Sr intercalated C60 film in ultra-high vacuum by synchrotron-radiation photoemission spectroscopy. In order to obtain the phase pure sample, special attentions were taken to the annealing procedure during the sample preparation. We find that the sample changes from semiconductor (C60) to weak metallicity (Sr2C60) at low intercalation levels, and then to semiconductor (Sr3C60) at intermediate intercalation levels. At last, the sample turns out to be metallicity (Sr6C60) at high intercalation levels. These observations along with the previously electrical resistivity and magnetic susceptibility measurements shed light on the electronic structure of Sr fullerides.

Small Change—Great Effect: Steep Increase of Li Ion Dynamics in Li4Ti5O12 at the Early Stages of Chemical Li Insertion

Lithium titanate (LTO) is one of the most promising anode materials for large-scale stationary electrochemical storage of energy produced from renewable sources. Besides many other aspects, such as negligible formation of passivation layers and no volume expansion during lithiation, the success of LTO is mainly based on its ability to easily accommodate and release Li ions in a fully reversible way. This feature is tightly connected with Li self-diffusion. As yet, little information is available about microscopic Li diffusion properties and elementary steps of Li hopping at low intercalation levels, i.e., at values of x being significantly smaller than 1. Here, we used 7Li spin-locking NMR relaxometry to probe absolute hopping rates of LTO (homogeneous) solid solutions in quasi-thermodynamic equilibrium. As a result, the largest increase of Li diffusivity is observed when small amounts of Li are inserted. Strong Coulomb repulsions caused by the simultaneous occupation of neighboring 8a and 16c sites serve as an explanation for the enhanced Li diffusivity found. At even larger values of x, Li mobility slows down but is still much faster than in the host material with x = 0. Our results experimentally corroborate the outcome of recently published calculations on the DFT level focusing on both dynamic and structural aspects. The findings favor the formation of LTO solid solutions upon chemical lithiation; the steep increase in Li diffusivity found might also help with understanding the flat insertion potential observed.

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.

Electronic structure of Eu–C70 fullerides

The electronic structure of Eu-intercalatedC70 has been studied by a synchrotron radiation photoemission spectroscopytechnique. At low intercalation levels (below the stoichiometry ofEu3C70), the photoemission data clearly exhibit charge transfer from Eu 6sstates to the lowest-unoccupied-molecular-orbital (LUMO) and theLUMO + 1 ofC70. The amount of charge transfer reaches its maximum far before intercalationsaturation. Detailed analysis reveals that most of the 5d6s electrons ofEu occupy the so-called interstitial states in the saturation phase (Eu9C70). The interstitial states are induced by a Eu sub-lattice formedat heavy intercalation levels, and comprise substantial 6s-π hybridizedstates. The π states participating in the hybridization are mainly theHOMO − n (n = 6–10) orbitals. The PES data also reveal the semiconducting property for bothEu3C70 andEu9C70. The6s–(HOMO − n) hybridization and the semiconducting property should playimportant roles in understanding the ferromagnetic mechanism forEu9C70.

Hybrid inorganic/organic composites of Mg–Al layered double hydroxides intercalated with citrate, malate, and tartrate prepared by co-precipitation

Inorganic/organic composite materials composed of Mg–Al layered double hydroxides (Mg–Al LDHs) intercalated with citrate (C 6H 5O 7 3−), malate (C 4H 4O 5 2−), or tartrate (C 4H 4O 6 2−) anions were prepared by a co-precipitation technique and characterized by X-ray diffraction and compositional analyses. The molecular orientation of the organic component in the interlayer space was determined. At low intercalation levels, citrate ion was inclined at an angle of 26° relative to the brucite-like layers of Mg–Al LDH. At higher solution concentrations, both the Mg(C 6H 5O 7) − complex and free C 6H 5O 7 3− were isotropically oriented in the interlayer space. For both malate·Mg–Al LDH and tartrate·Mg–Al LDH, the organic anions were inclined at 26° relative to the Mg–Al LDH layers regardless of anion concentration.

Preparation and characterization of Mg–Al layered double hydroxides intercalated with benzenesulfonate and benzenedisulfonate

Mg–Al layered double hydroxides (Mg–Al LDHs) intercalated with benzenesulfonate (BS –) and benzenedisulfonate (BDS 2–) ions were prepared by coprecipitation and characterized by X-ray diffraction, FT-IR spectroscopy, and chemical analyses. The intercalated BS – and BDS 2– maintained their intrinsic molecular structures within the Mg–Al LDH interlayers. At low intercalation levels, the benzene ring of BS – in BS · Mg–Al LDH was inclined at 30° relative to the plane of the brucite-like layers of Mg–Al LDH. With increasing BS – content, the benzene ring adopted an additional configuration perpendicular to the Mg–Al LDH layers. In BDS-intercalated Mg–Al LDH, the benzene ring of BDS 2– was tilted at 26° relative to the plane of the Mg–Al LDH layers. Intercalation levels of BDS 2– were smaller than those of BS – despite the greater charge density of BDS 2–, which was likely attributable to a greater degree of electrostatic repulsion between intercalated anions.

Observation of filled states at the Fermi-level in alkali-metal intercalated organic films: dependence on substrate work function

We present ultraviolet photoelectron spectroscopy studies on alkali metal-intercalated thin films of electroluminescent p-sexiphenyl (6P) and the organic hole transporting material N, N′-bis-(1-naphthyl)- N, N′-diphenyl1-1,1-biphenyl1-4,4′-diamine (α-NPD). The formation of new occupied electronic states in the otherwise empty energy gap of 6P and α-NPD is observed, accompanied by a significant shift of the Fermi-level. Depending on the work function ϕ of the substrate used for the organic thin films, we observe two different scenarios at low intercalation levels: the new electronic states are observed at (and even above) E F for high ϕ substrates, whereas they are observed below E F for low ϕ substrates. At high intercalation levels, all the states are observed below E F. The observation of states at and above E F is attributed to a lack of thermodynamic equilibrium across the organic thin film.

Optical absorption of Li-intercalated polycrystalline tungsten oxide films: comparison to large polaron theory

Thin films of polycrystalline tungsten trioxide were manufactured using DC magnetron sputtering. Films of different thickness were deposited onto glass substrates coated with indium tin oxide (ITO). The crystallinity was confirmed by X-ray diffraction, and the grain size was found to be 30 nm. Li ions and electrons were intercalated into the sample using a three-electrode setup. The samples were submitted to optical characterization by spectrophotometry, in the visible and infrared ranges. The optical spectra were recorded at different intercalation states, and the absorption of the films was obtained. At low intercalation levels, a pronounced absorption peak was observed to be centered at a wavelength of 1.8 μm. Upon intercalation, the inserted electrons enter the conduction band, but due to a strong electron–phonon interaction, they are believed to form localized polarons. Calculations of optical absorption by large polaron theory were carried out and the position of the observed peak was in good agreement with the theory. A crossover from dielectric (low reflectance and clear phonon absorption bands) to metallic (high infrared reflectance) occurred at a Li intercalation level of around 0.05–0.15 Li/W. This may be due to overlap of the polaron states.

CHARGE TRANSFER IN DOPED SINGLE WALL CARBON NANOTUBES

We report on the charge transfer in intercalated SWCNT and C 60 peapods. Differences between n-doping (K-intercalation) and p-doping (FeCl 3 -intercalation) are discussed. For the peapods a competition between a charge transfer to the SWCNT pods and one to the C 60 peas is observed. At low intercalation levels the charge transfer is predominantly to the SWCNTs. In addition at high levels of potassium intercalation the C 60 pods are charged up to C 60 6- and form a metallic one dimensional polymer.

Quasicontinuous electron and hole doping ofC60peapods

We report on the doping dependence of so-called peapods where ${\mathrm{C}}_{60}$ is encapsulated inside single-wall carbon nanotubes (SWCNT's) using Raman spectroscopy as probe. The charge transfer in intercalated ${\mathrm{C}}_{60}$ peapods is analyzed as a function of doping with ${\mathrm{FeCl}}_{3}$ $(p$ type) and potassium $(n$ type). A competition between a charge transfer to the SWCNT pods and to the ${\mathrm{C}}_{60}$ peas is observed. At low intercalation levels, the charge transfer is predominantly to the SWCNT pods. The Fermi level shifts beyond the second van Hove singularity of the semiconducting tubes and below the first singularity of the metallic tubes, respectively. The doping of the valence band for ${\mathrm{FeCl}}_{3}$ intercalation $(p$ doping) and of the conduction band for K intercalation $(n$ type) is symmetric. At high levels of potassium intercalation, the Fermi level shifts beyond the third singularity of the semiconducting tubes and the ${\mathrm{C}}_{60}$ molecules are charged as well finally yielding a ${\mathrm{C}}_{60}^{6\ensuremath{-}}$ state that forms a one-dimensional metallic single-bonded ${\mathrm{C}}_{60}$ polymer. For intermediate charging levels of the ${\mathrm{C}}_{60}$ molecules, no line phases are observed in contrast to the fullerides. Rather, a quasicontinuous charge transfer is observed. This provides a unique procedure to establish a continuous charge transfer to ${\mathrm{C}}_{60}$ by intercalation.