Midgap Absorption Band Research Articles

One-Pot Synthesis of Monodisperse Colloidal Copper-Doped CdSe Nanocrystals Mediated by Ligand–Copper Interactions

We report a one-pot synthesis of high-quality colloidal copper-doped cadmium selenide nanocrystals (Cu+:CdSe NCs) by injection of a mixture of copper iodide (CuI) and trioctylphosphine (TOP) into solutions containing preformed CdSe NCs. This method allows NC doping to be separated from nucleation and growth, thereby simultaneously achieving large size tunability, narrow size dispersion, and exclusively copper-based photoluminescence (PL). The copper doping level is affected by both the reaction time and the relative concentrations of the cadmium precursor, CuI, and TOP. A correlation is demonstrated between the copper dopant concentration and the intensities of the characteristic near-IR PL and midgap absorption bands, both associated with metal-to-ligand (conduction band) charge-transfer (MLCBCT) excitation of Cu+ dopants. Mechanistic studies reveal that Cu2–xSe NCs are easily formed as kinetic intermediates under reaction conditions involving substantial copper and that these NCs then act as a copper so...

Structure, band gap, and Mn-related mid-gap states in epitaxial single crystal (Zn1−xMgx)1−yMnyO thin films

Epitaxial (Zn1−xMgx)1−yMnyO thin films were grown on c-Al2O3 substrates by radio frequency oxygen plasma assisted molecular beam epitaxy. Single crystal structure of the (Zn1−xMgx)1−yMnyO films was revealed by reflection high energy electron diffraction and X-ray diffraction. The band gap of the films can be tuned dramatically with increasing the Mg concentration, while the onset energy of Mn-related mid-gap absorption band only shows a small blue shift. Photoconductivity measurements indicate the Mn-related mid-gap states in (Zn1−xMgx)1−yMnyO films can create free carriers and contribute to charge transfer transitions. The conduction band offset ΔEC = 0.13 eV and valence band offset ΔEV = 0.1 eV were obtained for ZnO/Zn0.8Mg0.2O heterostructures, which increase to ΔEC = 0.21 eV and ΔEV = 0.14 eV for ZnO/Zn0.7Mg0.3O heterostructures.

Decay Kinetics of Long-Lived Photogenerated Kinks in anMXChain Compound

The time evolution of the thermal bleaching of the long-lived, photogenerated midgap absorption band in a PtCl chain compound is examined in a temperature range 220--330 K. The long-lived absorption arises from kinks each confined in a segment of the PtCl chain by energy barriers of the order of 0.5 eV. Because of a random distribution of the barrier height the pair coalescence decay shows a $\mathrm{ln}t$ dependence at all temperatures in the experimental range, 0--60 min, of time $t$ after discontinuation of laser pumping.

Resonance Raman scattering for mid-gap absorption bands in halogen-doped MX chains

Resonance Raman spectra for the mid-gap absorption bands were measured in chlorine-doped quasi-one dimensional [Pt(en) 2][Pt(en) 2Cl 2](ClO 4) 4 (en = ethylenediamine) single crystals, in order to classify the origin of the Raman modes relevant to the photo-induced defect states. We found that the intensity of the Raman modes located between 270 and 285 cm −1 relative to that of the Raman modes between 285 and 295 cm −1 increases with increasing the chlorine doping time. This fact strongly suggests that the origin of the former Raman modes is attributed to the hole polarons and that of the latter ones is assigned to the electron polarons.

Phase Transition and Generation Efficiency of Mismatch of Valence-Alternation in the Neutral MX Chain System, [PtX2(en)][PtX4(en)] (X = Cl, Br, I)

Abstract We have investigated mid-gap absorption bands, their photo-induced absorption effect at various temperatures between liquid helium temperature and room temperature, and crystal structures between 150 K and 300 K in a neutral MX chain system [PtX2(en)][PtX4(en)] (X=Cl, Br, I), which have one-dimensional structure, ···PrII ···X–PrIV–X···. Temperature dependence of the intensity of the mid-gap absorption bands and the growth of the photo-induced absorption in [PtCl2(en)][PtCl4(en)] show that the generation efficiency of the mismatch of valence-alternation is changed at 200–220 K. Temperature dependence of lattice parameters of [PtCl2(en)][PtCl4(en)] shows that the structural phase transition occurs at 220 K from the orthorhombic system to the monoclinic system by cooling. Comparison of the crystal structure between 195 K and 296 K shows that the basic structure does not change at the phase transition. The conformation of the chelate ring of the ethylenediamine is changed from the disordered state to the ordered state at the transition point by cooling. This result suggests that the order-disorder transition of the conformation of the ethylenediamine would be responsible for the generation efficiency of the mismatch of the valence alternation.

Optical Studies of MX Chain Complex [Pt(en)2] [PtBr2(en)2] (SO4)2-6H2O Under High Pressure

Abstract We have measured the absorption and resonance Raman spectra in single crystals of the MX chain complex [Pt(en)2][PtBr2(en)2](SO4)2.6H2O in order to study the mixed-valence state, the midgap state and the phase transition under various hydrostatic pressures up to 3.7 GPa at room temperature. Red shifts of both the intervalence charge-transfer absorption edge and the Pt-Br stretching mode Raman peak with increasing pressure show that the electronic state of PtII and PtIV in the mixed-valence state approaches that of PtIII. An absorption band appearing above 2.5 GPa and a Raman peak appearing above 1.6 GPa suggest that a new phase coexists at pressures above 1.6 or 2.5 GPa. On the other hand, no enhancement of the optical absorption in the region of the midgap absorption bands (A-, B- and C-bands) is observed. This result shows that the midgap state is not generated by applying pressure at room temperature.

Spectroscopic Study of Fluctuations of Kinks Photoinduced in an MX Chain Complex

Abstract We report the experimental results on the temperaturte dependence of the photoinduced midgap absorption bands in [Pt(en)2][Pt(en)2Cl2](CIO4)4. The behaviors of the spectral position and line shape of the A band are found to be explained well in terms of the quantum fluctuation and translational motion of solitons.

特集 半導体 擬一次元白金錯体の電子構造と高圧効果

The relationship is outlined between the mixed-valence state and the electronic structure in the halogen-bridged platinum compounds. Pressure dependences of the optical absorption edge and the Raman frequency of halide stretching mode are interpreted in terms of the electron-lattice coupling characteristic of the PtX chains. Presented are the results of the recent optical experiments on the pressure-induced phase transition in PtCl and PtBr, demonstrating the role of the long range order among the chains. Also discussed is the behavior of the midgap absorption band, which is attributable to solitons, in PtCl under high pressures.

Controlling the mixed-valence state of halogen-bridged one-dimensional platinum complexes by the substitution of counter ions

The mixed-valence states of halogen-bridged one dimensional platinum complexes have been investigated by the substitution of counter ions. The crystal structures of [Pt(en) 2][PtCl 2(en) 2](PF 6) 4 (ClPF 6), [Pt(en) 2][PtCl 2(en) 2](HSO 4) 4 (ClHSO 4) and [Pt(en) 2][PtBr 2(en) 2](H 2PO 4) 4 · 6H 2O (BrH 2PO 4) (en=ethylenediamine) have been determined by single-crystal X-ray diffraction, and their absorption edges have been measured at room temperature. The crystal data are: for ClPF 6, monoclinic, P2/m, a=9.204(3), b=5.506(2), c=8.185(2) A ̊ , β=107.29(2)° and Z= 1 2 ; for ClHSO 4, orthorhombic, Ibam, a=9.261(1), b=14.422(2), c=10.929(1) A ̊ , and Z=2; for BrH 2PO 4, monoclinic, P2/a, a=19.463(2), b=5.722(1), c=8.325(1) A ̊ , ß=101.59(1)° and Z=1. The absorption edegs of the intervalence charge-transfer bands are c.a. 17,500cm −1 for ClPF 6, c.a. 16,500cm −1 for ClHSO 4 and c.a. 14,800cm −1 for BrH 2PO 4. These results show that the counter ion size and the hydrogen atom of the counter ion influence the mixed-valence state. A mid-gap absorption band below the absorption edge is found at c.a. 14,700cm −1 in ClHSO 4.

Polaronic defect states in quasi one-dimensional solid with medium electron-lattice coupling system [Pt(en) 2][Pt(en) 2Br 2](ClO 4) 4

Photoinduced defect states have been studied for [Pt(en) 2][Pt(en) 2Br 2](ClO 4) 4 (en=ethylene-diamine. hereafter abbreviated as PtBr)) and chlorine doped PtBr single crystals synthesized by the improved method. The PtBr which belongs to the medium electron-lattice coupling system has absorption edge of the CT band at 1.45eV at 2K. Two midgap absorption bands located at 1.10eV and 1.23eV due to polaron and small hump (1.36eV) are produced by the photoexcitation. Polarons in the PtBr are trapped near halogen impurities and/or lattice defects with a trapping energy of about 11meV, showing that the polarons are mobile at the temperature as low as 20K. The polarons are delocalized over a significantly large number of PtBr units in the chain, which may cause a broad linewidth in ESR spectrum and also motional narrowing.

Gap states of iodine-doped β-carotene

The absorption spectra of both neutral and iodine-doped β-carotene in p-xylene solution are reported for the spectral range 0.5 to 4 eV. The neutral β-carotene spectrum exhibits a π–π ∗ absorption band with a clear vibronic structure. The 0–0 transition appears at 2.53 eV and the vibronic separation is 0.17 eV. The iodine-doped absorption spectrum shows a reduced π–π ∗ absorption band and, in addition, a mid-gap absorption band centered at 1.3 eV. No ESR signal is observed for either the neutral or doped β-carotene in p-xylene solution. We discuss doping models of the β-carotene molecule to account for the observed data.

Experimental evidence of polaronic states in halogen bridged mixed valence platinum complexes

Two midgap absorption bands are observed in the quasi one-dimensional semiconductor (Pt(en) 2) (Pt(en) 2X 2) (C10 4) 4 by the photo-excitation of the light of charge transfer absorption band or higher at nitrogen temperature. The photo-induced absorption bands are active only for the light polarized parallel to the chain axis, showing that the photo-induced defect states are not associated with interchain defects. The defect states have spin and charge. These suggest that the photo-induced defect states are polaronic ones.

Soliton-to-band optical absorption in a quasi-one-dimensional PtII-PtIV mixed-valence complex under hydrostatic pressure.

A midgap absorption band is observed in the quasi one-dimensional semiconductor [Pt${(\mathrm{en})}_{2}$]-[Pt${(\mathrm{en})}_{2}$${\mathrm{Cl}}_{2}$]${(\mathrm{C}\mathrm{l}{\mathrm{O}}_{4})}_{4}$ under hydrostatic pressures at room temperature, where en is ethylenediamine. The transition is allowed only for the polarization parallel to the -Cl-${\mathrm{Pt}}^{\mathrm{II}}$-Cl-${\mathrm{Pt}}^{\mathrm{IV}}$- chain. The peak position remains near the middle of the Peierls gap at any pressure up to 2.2 GPa. The intensity increases exponentially with the peak shift. The gap states responsible for this band are attributed to soliton excitations corresponding to kinks of the charge-density wave.

Photoinduced absorption study of recombination in amorphous Si: H doped with P or B

The decay of the photoinduced midgap absorption band was studied in a-Si: H doped with P and B and compared with the decay in undoped a-Si: H and in a-H with a high concentration of dangling bonds produced by electron irradiation. At high temperatures, recombination controlled by multiple trapping transport is observed in most samples. At low temperatures an additional recombination process is observed and attributed to tunneling which increases with doping. The temperature dependence of the decays correlates with dangling bond density produced by doping or irradiation.