The structure and photoelectrochemical properties of TiO 2 films deposited onto SnO 2 conducting glass from the ambient hydrolysis of TiCl 4 and annealed at temperatures ranging from 100 to 500 °C were studied by Raman spectroscopy, X-ray diffraction (XRD), transmission electron microscopy (TEM), intensity-modulated photovoltage spectroscopy (IMVS), and intensity-modulated photocurrent spectroscopy (IMPS) measurements. Analysis of the XRD and Raman spectra shows that TiCl 4 -produced TiO 2 films have the rutile structure, regardless of annealing temperature. The TEM reveals that the rutile TiO 2 films consist of rod-shaped particles that grow with increasing annealing temperature. The AM-1.5 short-circuit photocurrent J sc and open-circuit photovoltage V oc of Ru[LL′(NCS) 2 ]-sensitized (L ) 2,2′-bypyridyl-4,4′-dicarboxylic acid, L′ ) 2,2′-bipyridyl-4,4-ditetrabutylammoniumcarboxylate) 4.5 µm thick rutile films increase significantly with annealing temperature, from 1.1 mA/cm 2 and 602 mV at 100 °C to 8.7 mA/cm 2 and 670 mV at 500 °C. Studies of the incident photon-to-current conversion efficiency (IPCE), the photocurrent-voltage characteristics, the optical appearance, the water content, and the particle size of the films indicate that the increase of both J sc and V oc with annealing temperature is due, in part, to increased dye adsorption resulting from the evaporation of surface water and the improved light-scattering properties of the film associated with the growth of rutile particles. IMVS and IMPS measurements indicate that variations of the charge-collection efficiency of the cell, which increases from 86% for the 300 °C annealed samples to above 99% for the 500 °C annealed samples, have only a minor effect on J sc . Analysis of the time constants at open circuit and short circuit for a given electron injection current suggests that the ratio of free-to-trapped electrons at short circuit decreases and the diffusion coefficient of free electrons increases with annealing temperature. Raman and XRD measurements and other observations indicate that treating transparent nanocrystalline anatase TiO 2 electrodes with TiCl 4 produces a translucent overlayer of rutile TiO 2 . The increased film thickness and light-scattering characteristics of the rutile overlayer may explain, in part, the improved IPCE observed for dye-sensitized TiCl 4 -treated nanocrystalline anatase TiO 2 electrodes.

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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.

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.

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Magnetic ordering in the two-dimensional limit has been one of the most important issues in condensed matter physics for the past several decades. The recent discovery of new magnetic van der Waals materials heralds a much-needed easy route for the studies of two-dimensional magnetism: the thickness dependence of the magnetic ordering has been examined by using Isingand XXZ-type magnetic van der Waals materials. Here, we investigated the magnetic ordering of MnPS3, a two-dimensional antiferromagnetic material of Heisenberg-type, by Raman spectroscopy from bulk all the way down to bilayer. The phonon modes that involve the vibrations of Mn ions exhibit characteristic changes as temperature gets lowered through the Néel temperature. In bulk MnPS3, the Raman peak at ~155 cm -1 becomes considerably broadened near the Néel temperature and upon further cooling is subsequently red-shifted. The measured peak positions and polarization dependences of the Raman spectra are in excellent agreement with our first-principles calculations. In few-layer MnPS3, the peak at ~155 cm -1 exhibits the characteristic red-shift at low temperatures down to the bilayer, indicating that the magnetic ordering is surprisingly stable at such a thin limit. Our work sheds light on the hitherto unexplored magnetic ordering in the Heisenberg-type antiferromagnetic systems in the atomic-layer limit. ∑ ∑where XY J and I J are spin-exchange energies on the basal plane and along the c-axis, respectively; j S α is the α (α = x, y, or z) component of total spin; and j and δ run through all lattice sites and all nearest-neighbors, respectively. All three fundamental models can be realized with the generic Hamiltonian: 0 XY J = for the Ising model, 0 I J = for the XY model, and XY I J J = for the Heisenberg model. According to the Mermin-Wagner theorem [4], no magnetic ordering is possible at any nonzero temperature in one-or two-dimensional isotropic Heisenberg models. On the other hand, 2D Ising systems can have magnetic ordering at finite temperatures according to Onsager [5].Transition metal phosphorus trisulfides (TMPS3) belong to a class of 2D van der Waals magnetic materials that can be exfoliated to atomically thin layers [6,7]. For transition metal elements like Fe, Ni, and Mn, the materials share the same crystal structures but the magnetic phase at low temperatures vary depending on the magnetic elements: Ising (Fe), XXZ (Ni), and

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 Å. 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.

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