Hyeonsik M. Cheong

Electrochromic mechanism in a-WO3−y thin films

The electrochromic mechanism in amorphous tungsten oxide films is studied using Raman scattering measurements. The Raman spectra of as-deposited films show two strong peaks at 770 and 950 cm−1 due to vibrations of the W6+–O and W6+=O bonds, respectively, and a weaker peak at 220 cm−1 that we attribute to the W4+–O bonds. When lithium or hydrogen ions and electrons are inserted, extra Raman peaks due to W5+–O and W5+=O bonds appear at 330 and 450 cm−1, respectively. Comparison of the Raman spectra of sputtered isotopic a-W16O3−y and a-W18O3−y films confirms these assignments. We conclude that the as-deposited films contain mainly the W4+ and W6+ states, and the W5+ states are generated as a result of reduction of the W6+ states when lithium or hydrogen ions and electrons are inserted. We propose that the optical absorption in the colored films is caused by transitions between the W6+ and W5+, and W5+ and W4+ states.

Electrochromic coloration efficiency of a-WO3−y thin films as a function of oxygen deficiency

We report on how electrochromic coloration is affected by oxygen deficient stoichiometries in sputtered amorphous tungsten oxide (a-WO3−y) films. The electrochromic coloration efficiency increases with increasing oxygen deficiency in (a-WO3−y) films. No coloration is observed in nearly stoichiometric WO3 films. Raman spectroscopic studies reveal that the number of W5+ states generated with lithium insertion increases with the oxygen deficiency. Furthermore, there are no Raman peaks resulting from W5+ states in lithiated a-WO3−y films with near perfect stoichiometry, which is consistent with the absence of electrochromic coloration in those films. We conclude that the coloration efficiency of a-WO3−y films depends on the number of the W5+ states generated by lithium insertion and that the oxygen deficiency plays an important role in generating the W5+ states with lithium insertion.

Raman spectroscopic studies of electrochromic a-WO3

We report on the eAect of electrochromic coloration on the Raman spectra of evaporated and sputtered a-WO3ˇy films. The Raman spectra of as-deposited films show two strong peaks at 770 and 950 cm ˇ1 due to vibrations of the W 6+ ‐O and W 6+ .O bonds, respectively, and a weaker peak at 220 cm ˇ1 that we attribute to the vibrations of the W 4+ ‐O bonds. When lithium or hydrogen ions are inserted, extra Raman peaks due to W 5+ ‐O and W 5+ .O bonds appear at 330 and 450 cm ˇ1 , respectively. Comparison of the Raman spectra of sputtered a-W 16 O3ˇy and aW 18 O3ˇy isotopic films confirms these assignments. We conclude that the as-deposited films contain mainly the W 4+ and W 6+ states, and the W 5+ states are generated as a result of reduction of the W 6+ states when ions are inserted. We propose that the electrochromism in a-WO3ˇy films originates from reduction of the W 6+ states to W 5+ due to the double insertion/extraction of ions and electrons and the optical absorption in the colored films is caused by transitions between the W 6+ and W 5+ , and W 5+ and W 4+ states. # 1999 Elsevier Science Ltd. All rights reserved.

Alternating current impedance and Raman spectroscopic study on electrochromic a-WO3 films

The chemical diffusion of lithium ions in a-LixWO3 films is investigated using alternating current impedance spectroscopy and Raman scattering measurements. The diffusion coefficients increase with increasing x in a-LixWO3 up to x=0.072 and then decrease. Raman measurements show that the W6+=O/O–W6+–O ratio also increases at the early stage of lithium insertion and then decreases with further lithium insertion. We conclude that the diffusion kinetics of lithium ions in a-LixWO3 films is very closely related to the W6+=O/O–W6+–O ratio.

Size-dependent Raman study of InP quantum dots

Raman spectrum of a quantum dot (QD) is characterized by transverse (TO) and longitudinal (LO) optical modes as well as surface optical modes, occurring between the TO and LO modes. We have studied in detail the size-dependence of the Raman spectrum of InP QD of diameter larger than 35 A. The LO phonon frequency decreases while the TO phonon frequency increases with decreasing QD size. The linewidth of the LO phonon broadens and the broadening becomes increasingly asymmetrical towards the low frequency side as the QD size decreases. By analyzing the Raman intensity ratio of the LO phonon to its overtone, we find that the electron-phonon coupling decreases with decreasing QD size.

Raman spectroscopic studies of Ni-W oxide thin films

We report on a Raman spectroscopic study of sputtered nickel–tungsten oxide films. Their crystallographic and chemical identification with electrochemical lithium insertion and extraction are obtained by Raman spectroscopy. The Raman spectra of as-deposited Ni–W oxide films show a strong peak at 525 cm−1 due to vibrations of the NiO bonds and two weaker peaks at 875 and 950 cm−1, which we attribute to the vibrations of the WO bonds. The Raman spectrum of the as-deposited Ni–W oxide film shows a blueshift of the NiO stretching mode compared to that of a NiOx film. When lithium ions and electrons are inserted, the overall Raman intensity increases due to electrochromic bleaching and the relative intensity of the peak at 875 cm−1 also increases. By comparing these results with results from the same measurements using sodium ions, we conclude that this reversible peak at 875 cm−1 is not directly related to the Li or Na ions.

Raman spectroscopic studies of gasochromic a-WO3 thin films

The gasochromic mechanism in amorphous tungsten oxide films has been investigated using Raman scattering measurements and optical cycling tests. The Raman spectra of as-deposited films show two strong peaks at 770 and 950 cm 1 due to vibrations of the OW 6 O and W 6 O bonds, respectively. When the Pd:a-WO3 films are gasochromically colored by exposure to diluted hydrogen gas, extra Raman peaks due to OW 5 Oa nd W 5 O bonds appear at 330 and 450 cm 1 , respectively. Optical cycling tests confirm that the Pd:a-WO3 films can not be bleached without oxygen. The Raman spectra of a-WO3 films bleached with isotopic heavy oxygen ( 18 O) show that the gasochromic coloration in a-WO3 films is directly related to the double injection of hydrogen ions and electrons.

Gasochromic mechanism in a-WO3 thin films based on Raman spectroscopic studies

The gasochromic mechanism in amorphous tungsten oxide films has been investigated using Raman scattering measurements. The Raman spectra of as-deposited films show two strong peaks at 770 and 950 cm−1 due to vibrations of the O–W6+–O and W6+=O bonds, respectively. When the Pd/a-WO3 films are gasochromically colored by exposure to diluted hydrogen gas, extra Raman peaks due to O–W5+–O and W5+=O bonds appear at 330 and 450 cm−1, respectively. The Raman spectra of a-WO3 films bleached with isotopic heavy oxygen (18O) show that the gasochromic coloration in a-WO3 films is directly related to the double injection of hydrogen ions and electrons.

Effect of crystallinity on electrochromic mechanism of LixWO3 thin films

Abstract Intercalation of lithium ions in amorphous tungsten oxide films and in monoclinic polycrystalline tungsten oxide films has been studied using electrochemical analyses and Raman spectroscopy. We present direct evidence for W 5+ 5d-electron localization in amorphous tungsten oxide films upon lithium ion and electron intercalation by comparing the Raman spectra of amorphous and crystalline tungsten oxide films. We observe that when lithium ions and electrons are inserted into the crystalline tungsten oxide films, the monoclinic structure of the material proceeds to progressively increased symmetry. On the other hand, when lithium ions and electrons are inserted into amorphous tungsten oxide films, the inserted electrons are localized in W 5+ sites and polarize their surrounding lattice to form small polarons. The extra Raman peaks due to the W 5+ O single bonds and W 5+ O double bonds appear at 330 and 450 cm −1 , respectively.

Microstructure study of amorphous vanadium oxide thin films using raman spectroscopy

Microstructural changes of amorphous V2O5 films with lithium intercalation are studied using Raman-scattering measurements. The Raman spectra of as-deposited films show two broad peaks around at 520 and 650 cm−1, due to the stretching modes of the V3–O and V2–O bonds, respectively, and a relatively sharp peak at 1027 cm−1 due to the V5+=O stretching mode of terminal oxygen atoms. In addition, there is a peak at 932 cm−1 that we attribute to the V4+=O bonds. Comparison of the Raman spectra of V2O5 films with different oxygen deficiencies confirms this assignment. This Raman peak due to the stretching mode of the V4+=O bonds develops and shifts toward lower frequencies with increasing lithium concentration. Comparison to results from gasochromic hydrogen insertion indicates that the 932 cm−1 Raman peak is not a result of vibrations which involve Li or H atoms. We propose that the V4+=O bonds are created by two different mechanisms: a direct conversion from V5+=O bonds and the breaking of the single oxygen b...