Z3 Exciton Research Articles

Preparation of transparent CuI–CuBr alloy thin films by solution processing

Transparent p-type CuBr1−xIx semiconducting thin films with a variable band gap were prepared using spin coating, which is a non-vacuum process. The dependence of their structural and optical properties on the CuBr1−xIx composition was investigated. In X-ray diffraction patterns, the dominant CuBr1−xIx peak was found to shift to higher angles with decreasing mixing ratio x. The Z1,2 and Z3 exciton absorption bands were observed from 300 to 400 nm, and the absorption wavelength varied with x. The ionization potential was determined using photo yield spectroscopy, and was found to increase with increasing x. These results suggest that the valence band maximum for CuBr1−xIx depends on the composition. The results obtained in the present study allowed the energy band structure for this material to be proposed.

Characteristics of cavity polaritons in a CuBr microcavity

We report the characteristics of cavity polaritons in a CuBr microcavity consisting of a λ/2-thick CuBr active layer and HfO2/SiO2 distributed Bragg reflectors: λ corresponds to an effective resonant wavelength of the lowest-lying exciton. The excitonic system of a CuBr crystal has three kinds of excitons labeled Zf, Z1,2, and Z3 in which the Zf exciton originates from a triplet state. We have investigated the dispersion relations of the cavity polaritons in the CuBr microcavity with the use of angle-resolved reflectance spectroscopy. The experimental results demonstrate the formation of four cavity-polariton branches due to the strong coupling between the Zf, Z1,2, and Z3 excitons and cavity photon. The cavity-polariton dispersions were well analyzed with a phenomenological Hamiltonian for the strong coupling. The evaluated Rabi-splitting energies are 28, 95, and 74 meV for the Zf, Z1,2, and Z3 excitons, respectively. These Rabi-splitting energies reflect the magnitudes of the oscillator strengths of the relevant excitons. Furthermore, it was confirmed that the cavity polaritons are fully stable at room temperature. We discuss the temperature dependence of the cavity-polariton energies and detuning, comparing with that of the bare exciton.

Control of Rabi Splitting Energies in CuCl Microcavities with HfO2/SiO2 Distributed Bragg Reflectors

Abstract The exciton-photon interaction in CuCl microcavities with HfO2/SiO2 distributed Bragg reflectors has been investigated from the viewpoint of the active-layer-thickness dependence of the interaction energy, the so-called vacuum Rabi splitting energy. The active layer thickness was changed from A/32 to A/5, where A corresponds to an effective resonant wavelength of the lowest-lying Z3 exciton. We performed angle-resolved reflectance measurements, and clearly detected three cavity-polariton modes originating from the lower, middle, and upper polariton branches in a strong coupling regime of the Z3 and Z1,2 excitons, and cavity photon. The incidence-angle dependence of the cavity-polariton energies was analyzed with a phenomenological Hamiltonian for the strong coupling. The vacuum Rabi splitting energies are systematically controlled from 22 (37) to 68 (122) meV for the Z3 (Z1,2) exciton with an increase in the active layer thickness. The active-layer-thickness dependence of the Rabi splitting energy is quantitatively explained using a simple model for quantum-well microcavities.

Piezo-Optical Effect of Excitons in CuCl

ABSTRACT We calculated the stress-induced variations in real part and imaginary part of the dielectric functions in copper chloride (CuCl) by Kramers-Kronig analysis from the piezoreflectance measurements of the 1s Z3 and 1s Z12 excitons in two configurations of the applied stress X // [001] and X // [110]. From the data of the deduced stress-induced variations in real part and imaginary part of the dielectric functions, the three independent complex components W 11 , W 12 , and W 44 of the fourth-rank linear piezo-optical tensor were determined.

Atomic layer deposition of CuCl nanoparticles

We report the growth of copper (I) chloride by atomic layer deposition. CuCl was deposited as nanoparticle arrays whose size and density were controlled by the process conditions. The nanoparticles were deposited using the self-limiting reaction of [bis(trimethylsilyl)acetylene]-(hexafluoroacetylacetonato)-copper(I) and hydrogen chloride. UV absorption measurements showed the characteristic Z1,2 and Z3 exciton absorption bands of CuCl. A strong UV emission was observed at 5 K from the free exciton Z3 and bound exciton I1 at 386.7 and 390.6 nm, respectively. A previously unreported visible emission band at 408 nm was also observed and attributed to the acceptor level of Cu vacancies.

Rabi splitting in CuCl microcavity with DBR consisting of PbCl2/NaF multilayers

AbstractWe have investigated exciton‐photon coupling phenomena in bulk CuCl microcavities with distributed Bragg reflectors consisting of PbCl2 and NaF multilayers. The CuCl microcavities, which have the cavity‐length of half‐integer multiples of Z3 exciton wavelength, were fabricated by the vacuum deposition in the consistent process. From angle‐resolved reflectance spectra, the cavity polaritons originating from Z3, Z1,2 excitons and cavity photon were clearly obtained in each sample. We assumed the three polariton modes derived from 3×3 phenomenological Hamiltonian with two vacuum Rabi splitting energies. From the analysis, the obtained Rabi splitting energies for λ‐cavity are 110 and 195 meV for Z3 and Z1,2 excitons, respectively. On the other hand, we have determined the complex refractive indices of PbCl2/NaF layers by spectral ellipsometry. Using these optical constants, the reflectance spectrum of the sample was calculated by nonlocal theory with spatial structures of wavefunction of excitons. The spectrum and Rabi splitting energies estimated by the calculations agree well with the experimental results (© 2011 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

ChemInform Abstract: Preparation and Nonlinear Optical Properties of Quantum-Sized CuCl- Doped Silica Glass by the Sol-Gel Process.

The sol–gel process has been applied successfully to the preparation of small-sized CuCl-doped silica glass with a significant quantum-sized effect. Gel synthesized by the hydrolysis of a complex solution of Si(OC2H5)4 and CuCl with an HCl catalyst is heated to 900°C to form fine, cubic CuCl crystals. Above 700°C, absorption peaks observed at ∼370 and ∼380 nm are attributed to the excitation of confined Z1,2 and Z3 excitons, respectively, in the CuCl microcrystals. As the size of the CuCl crystals decreases, exciton energy shifts to the higher-energy side. The resonant third-order nonlinear susceptibility at 77 K is 1.1 × 10−8 esu, which originates from the enhancement by the quantum-sized effect of the exciton.

Effects of Distributed Bragg Reflectors on Temporal Stability of CuCl Microcavities

We have investigated the characteristics of exciton polaritons in a CuCl microcavity with distributed Bragg reflectors (DBRs). Two sets of multilayers, PbBr2/PbF2 and HfO2/SiO2, were adopted as the DBRs in order to study the temporal stability of the CuCl microcavity. The thickness of the CuCl active layer was fixed to an effective 3λ/2 length. Angle-resolved reflectance spectra clearly demonstrate the formation of the cavity polaritons. From the phenomenological analysis with a 3×3 Hamiltonian for the cavity-polariton modes originating from the Z3 exciton, Z1,2 exciton, and cavity photon, the Rabi splitting energies are evaluated to be 97 and 162 meV for the Z3 and Z1,2 excitons, respectively, in the fresh CuCl microcavity with the PbBr2/PbF2 DBR. However, the Rabi splitting energies remarkably decrease within 6 days from the sample preparation, which is due to the degradation of the DBR resulting from alloying of PbBr2 and PbF2. On the other hand, in the CuCl microcavity with the HfO2/SiO2 DBR, the Rabi splitting energies of 105 and 168 meV for the Z3 and Z1,2 excitons, respectively, hardly change during 360 days from the sample preparation. This indicates that the stability of the oxide materials of HfO2 and SiO2 prevents the degradation of the DBR and CuCl active layer. Thus, a stable CuCl microcavity can be prepared by adopting the multilayer of HfO2/SiO2 as the DBR, which is a merit in applications.

Exciton polaritons in bulk CuCl microcavities grown by vacuum deposition

AbstractWe demonstrate the realization of cavity polariton states in bulk CuCl microcavities. The CuCl microcavities made from a bulk CuCl layer and distributed Bragg reflectors consisting of PbF2 and PbBr2 layers prepared by vacuum deposition. Angle‐resolved reflectance spectra show the strong coupling characteristics of the excitons and photon mode; namely, the energies of reflectance structures exhibit incident‐angle dependence accompanying with an anticrossing behaviour peculiar to the cavity polaritons. From the phenomenological analysis with a 3×3 Hamiltonian of the cavity‐polariton modes originating from the Z3 exciton, Z1,2 exciton, and cavity photon, it has been revealed that the vacuum Rabi splitting energies in the λ‐thick CuCl active layer are 97 meV and 162 meV for the Z3 and Z1,2 excitons, respectively. We also indicate the potential to control the Rabi‐splitting energy by changing the active layer thickness. Furthermore, the spectral profile of reflectance is reasonably explained by calculations based on a theory of non‐local linear optical response. (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Stress-Induced Changes in the Optical Parameters in Copper Chloride

ABSTRACT We have calculated the stress-induced variations in refractive indices (of the index of refraction Δn and of the extinction coefficient Δk) of CuCl by the means of the Kramers–Kronig relationship from the piezoreflectance spectra of the 1s Z3 and 1s Z12 excitons. In the two studied configurations corresponding with the applied stress p //[001] and p //[111] the spectrum of the differential parameters Δk consist of two principals absorption lines in the region of the Z3 band. The corresponding energetic separation is comparable with the transverse-longitudinal splitting. The longitudinal Z3 exciton Γ5L is mixed with the transverse exciten Γ5T by virtue of the stress-induced k-linear term and became optically active.

Structure of copper halide nanocrystals in photochromic glasses

The crystal structure of CuCl nanocrystals with effective radii ranging from 2.4 to 18.5 nm in photochromic glasses heat treated under different conditions is investigated using exciton spectroscopy, x-ray powder diffraction, and small-angle x-ray scattering. It is revealed that the size dependence of the energy of the Z3 exciton exhibits an anomalous behavior. The results obtained suggest that small-sized CuCl nanocrystals can crystallize in a modification different from the stable cubic modification.

AgI nanostructure development in sputter-disordered and Al-doped Ag films probed by XRD, SEM, optical absorption and photoluminescence

By use of thermal evaporation and rf magnetron sputtering 300 and 500 A thick Ag and Ag (Al) films were prepared. γ-AgInanoparticles were formed during (a) short time (2–5 min) iodization of undoped thermally evaporated Ag films, (b) longer time (12 h) iodization of undoped rf sputtered Ag films and (c) short time (2–15 min) iodization of thermally evaporated Ag0.95Al0.05 and Ag0.90Al0.10 films of 500 and 300 A thickness respectively. Both rf sputtered and Al doped Ag films yielded ∼20 to ∼60 nm sized γ-AgI particles upon iodization. Optical absorption spectra reveal Z1,2 and Z3 exciton transitions with increased broadening of γ-AgI nanoparticles, suggesting the effect of disorder produced during film formation. Blue shift observed with increasing film thickness could be the effect of decreasing particle size, thereby increasing the quantum confinement effects. Photoluminescence studies show that the donor-acceptor recombination rate, enhanced by 25% for Ag0.95Al0.05I film relative to that of undoped AgI, is due to the tight binding of Al to surface defect sites.

Iodization of rf sputter induced disordered Ag thin films reveals volume plasmon-exciton “transition”

Quasiamorphous Ag films of thicknesses ranging from 5to30nm were prepared using rf magnetron sputtering technique and their controlled iodization was carried out for selected durations in the range of 15min–60h at room temperature. As deposited Ag and iodized films were characterized using x-ray diffraction (XRD), atomic force microscope (AFM), and optical absorption techniques. From XRD, γ and β+γ (mixed) phases of AgI nanoparticles have been observed for 5–10 and 20–30nm thick films, respectively. Lattice parameters (a and c) and average strain (ε) were calculated versus iodization time for γ and β-AgI nanoparticles. Uniform and nonuniform spherically shaped AgI nanoparticles (∼20–130nm) are realized through AFM for 5–10 and 20–30nm thick films. Optical absorption shows volume plasmons (classified as PR1) for short duration iodization, which “decay” upon further iodization to convert to Z1,2 and Z3 excitons at 420 and 330nm, respectively, in the manner of a metal-semiconductor/dielectric phase transition. Ag “colloidal” particles (classified as PR2) are formed for 5–10nm thick films and thereby control the γ phase—a significant and applicable effect attributed to critical film thickness. With increasing thickness, a surface strain field lifting the degeneracy of the valence band results in Z1,2 and Z3 exciton formation at room temperature. Blueshift in the exciton absorption with decreasing film thickness implies the progressive quantum confinement due to decrease in the particle size. A thickness induced phase transition from γ-AgI to β-AgI is discussed by means of x-ray diffraction and optical absorption studies.

Piezomodulation of excitonic reflectance spectra in CuCl

We have investigated the piezoreflectance spectra of the 1s Z3 and Z12 excitons in single crystals of copper chloride CuCl at 95 K with linearly polarized light. The spectra were studied for the applied low pressure p (p lower than 1 MPa) directed along the [001], [111] and [110] axis with the wave vector k of the incident light parallel to the [110] direction. A strong piezo-optical response is observed for the Z3 exciton to regard to the one observed for Z12. From the stress-induced shifts and splitting of the excitonic levels, we have been able to deduce the shear excitonic deformation potentials: b = (−0.18 ± 0.02) eV and d = (0.30 ± 0.03) eV.

Optical investigations on sputtered CuCl thin films

ABSTRACTCopper (I) chloride (CuCl) is a potential candidate for ultra violet optoelectronics due to the fact that it is closely lattice matched with silicon, which makes it readily integrable with silicon device technology. The structural and optoelectronic properties of CuCl thin films deposited by RF magnetron sputtering are investigated. The crystallinity is studied using X-ray diffraction which confirms the growth of CuCl thin films with cubic zinc blende structure predominantly orientated in <111> direction. Excitonic transitions in the thin films were thoroughly investigated using optical absorbance and luminescence spectroscopies. Room temperature absorption spectroscopic analysis confirms the existence of two exciton peaks namely Z12 and Z3 at 372 and 380 nm respectively. A strong UV emission is observed at room temperature in cathodoluminescence and photoluminescence spectra due to the recombination of Z3 exciton at approximately 384 nm. In the low temperature photoluminescence spectrum, a free exciton (Z3) and a bound exciton (I1) are observed. A variation of 1.3 nm to 10 nm was observed in the Z3 exciton line width from 10 K to 300 K.

Sol–gel synthesis of cuprous halide nanoparticles in a glassy matrix and their characterization

A sol–gel route to synthesize semiconductor cuprous halide CuX (X = Cl, Br) nanoparticles in a glassy matrix has been developed. Cu2O and HCl or HBr were used as the precursors for CuX and the matrix was prepared from 3-glycidoxypropyl trimethoxysilane (GPTS)/tetraethoxysilane (TEOS). Samples were dip-coated and cured either by ultraviolet (UV) irradiation at a temperature of 150 °C or in an oven at temperatures of 150–250 °C. Thin films (thicknesses 0.7–1.0 µm) containing cubic CuCl and CuBr nanoparticles with diameters ranging from 4–16 nm and 2–28 nm, respectively, were obtained. The ultraviolet-visible (UV-Vis) absorption spectra recorded at room temperature exhibited peaks at 371 and 379 nm for CuCl and at 410 and 391 nm for CuBr, corresponding to the Z1,2 and Z3 excitons respectively. X-Ray diffraction patterns showed the characteristic (111), (220) and (311) peaks of cuprous halides. High resolution transmission electron microscopy (HR-TEM) characterization proved that the lattice plane spacings correspond to copper halide nanocrystals.

On the cathodoluminescence of a CuCl powder

The electron-beam-excited luminescence spectra of a CuCl powder were studied in a temperature interval from 80 to 400 K. New emission peaks were observed at 393.5, 700, 780, and 830 nm. The emission intensity distribution in the spectrum significantly varies with the temperature. The Z3 exciton emission band exhibits an anomalous temperature-induced displacement.

Energy fluctuations in optical transitions in CuHal crystals

This paper reports on a comprehensive study of fundamental-absorption spectra of CuCl and CuBr nanocrystals measured in the temperature range extending from 80 K up to the melting point. The temperature dependences of the energy E, oscillator strength f, and temperature-broadening coefficient σ of optical-transition spectral to excitonic states were determined. A new approach to the calculation of the shape and temperature-broadening of absorption spectra, based on quantum statistics, was employed. The crystal is treated as a quantum statistical ensemble. The set of atoms involved in the electronic transition initiated by a photon absorption is taken as an element of the ensemble. Energy fluctuations of electronic states occurring in the elements of the ensemble give rise to temperature broadening of the optical-transition spectra. The temperature dependence of the χ coefficient of thermodynamic fluctuations of the Z1, 2 and Z3 exciton states was found. A theoretical substantiation of the Urbach rule was obtained in terms of the model proposed.

Cathodoluminescence of CuCl Powder

The cathodoluminescence behavior of CuCl powder was studied in the temperature range 80–400 K. The cathodoluminescence spectrum of CuCl was found to contain bands peaked at 393.5, 700, 780, and 830 nm and to vary significantly with temperature. An anomalous temperature-induced shift of the Z3 exciton emission was revealed.

Observation of Excited State Excitons in CuCl Quantum Cubes

Persistent spectral hole burning (PSHB) spectroscopy was utilized to investigate excited state excitons confined in CuCl quantum dots embedded in NaCl crystals. In the PSHB spectra of CuCl quantum dots, a resonantly burned hole and lower energy satellite holes were observed. These satellite holes are thought to originate from hole burning of the ground state excitons which results from site-selective excitation of excited state excitons confined in CuCl quantum dots. Relations between energy positions of the resonantly burned hole and that of each satellite hole are explained well in terms of the simple concept of a particle in a quantum cube with infinitely high potential barriers. Spectral profiles of the ground state excitons and the first excited state excitons confined in CuCl quantum dots were obtained. The Z3 exciton luminescence spectrum showed that the excited state excitons are in a majority in the higher energy region of the Z3 exciton absorption band.