Absorption Edge Measurements Research Articles

Pressure-Driven Energy Band Gap Narrowing of λ-N2

Probing the energy band gap of solid nitrogen at high pressures is of importance for understanding pressure-driven changes in electronic structures and insulator-to-metal transitions under high pressure. The λ-N2 formed by cold compression is known to be the most stable one in all solid nitrogen phases observed so far. By optimizing the optical system, we successfully measured the high-pressure absorption spectra of λ-N2 covering the polymeric-nitrogen synthetic pressures (124 GPa–165 GPa). The measured optical band gap decreases with increasing pressure, from 2.23 eV at 124 GPa to 1.55 eV at 165 GPa, with a negative pressure coefficient of −18.4 meV/GPa, which is consistent with the result from our ab initio total-energy calculations (−22.6 meV/GPa). The extrapolative metallization pressure for the λ-N2 is around 288(18) GPa, which is close to the metallization pressure (280 GPa) for the η-N2 expected by previous absorption edge and direct electrical measurements. Our results provide a direct spectroscopic evidence for the pressure-driven band gap narrowing of solid nitrogen.

Ferroelastic phase transition in Cu6PS5Br1-xClx mixed crystals

ABSTRACTCu6PS5Br1-xClx mixed crystals were grown by chemical vapour transport. Optical absorption edge and optical polarization measurements were performed in the temperature range 77–320 K. The influence of anionic Br→Cl substitution on the phase transitions in Cu6PS5Br1-xClx mixed crystals is studied. Compositional dependence of the ferroelastic phase transition temperature for Cu6PS5Br1-xClx mixed crystals is obtained. The variation of optical parameters at ferroelastic phase transition is analyzed.

Optical band gap and the Burstein–Moss effect in iodine doped PbTe using diffuse reflectance infrared Fourier transform spectroscopy

Optical absorption edge measurements are performed on I doped PbTe using diffuse reflectance infrared Fourier transform spectroscopy. The Burstein–Moss shift, an increase in the absorption edge (optical band gap) with increasing doping level, is explored. The optical gap increases on the order of 0.1 eV for doping levels ranging from 3 × 1018 to 2 × 1020 cm−3, relevant doping levels for good thermoelectric materials. Chemical potential is estimated from transport measurements—specifically, Hall effect and Seebeck coefficient—using a single band Kane model. In heavily doped semiconductors, it is well-known that the band gap shrinks with increasing doping level. This effect, known as band gap renormalization, is fit here using an n1/3 scaling law which reflects an electron–electron exchange interaction. The renormalization effect in these samples is shown to be more than 0.1 eV, on the same order of magnitude as the band gap itself. Existing models do not explain such large relative changes in band gap and are not entirely self-consistent. An improved theory for the renormalization in narrow gap semiconductors is required.

Influence of anionic substitution on phase transitions in Cu6PS5I1− x Cl x mixed crystals

Cu6PS5I1 − x Cl x mixed crystals were grown using chemical vapour transport. Isoabsorption studies of optical absorption edge and optical polarization measurements were performed in the temperature range 77–320 K. The influence of anionic I → Cl substitution on the phase transitions in Cu6PS5I1 − x Cl x mixed crystals is studied. Compositional dependence of the phase-transition temperatures is obtained and the x,T-diagram for Cu6PS5I1 − x Cl x mixed crystals is constructed.

Growth and characterization of congruent lithium isotope niobate (7LiNbO3) single crystal

For further investigation on the defect structure of lithium niobate (LiNbO3—hereafter LN) crystals using the neutron scattering technique, a congruent lithium isotope niobate (C7LN) crystal was grown by the Czochralski method from a 48.4% 7Li melt. The X-ray powder diffraction (XRPD) results show that the as-grown crystal is a single-phased (R3c) LN. We observed 4A1(TO)+7E(TO) Raman modes. The crystal composition (Li mol%) determined from UV absorption edge and Raman measurements is 47.47% and 47.34%, respectively. The specific heat (Cp), thermal diffusion coefficients (λa, λc), thermal conductivities (κa, κc) and the average thermal expansion coefficients (αa, αc) were determined. The effect of Li vacancy content is analyzed by comparing the results with those obtained for stoichiometric lithium isotope niobate (S7LN) and congruent natural lithium niobate (CLN) crystals. The values of all thermal parameters of C7LN are smaller than those of S7LN and CLN, which may be related to either the difference of vacancy content or isotope substitution.

Ambient Synthesis and Characterization of High-Quality CdSe Quantum Dots by an Aqueous Route

Herein, we report the ambient synthesis of CdSe nanoparticles of widely tunable particle size by a solution route. The proposed protocol uses hydrazine hydrate to form an air-stable complex of selenium. These nanoparticles are characterized by X-ray diffraction, FTIR, optical absorption, photoluminescence, and transmission electron microcopy measurements. By varying the molarities of Cd(2+) and Se(2-) ions in solution with 3-mercaptopropionic acid as the capping ligand, the method permits us to synthesize nanoparticles of size ranging from 1.58 to 3.42 nm (estimated from optical absorption edge measurements) by controlling the annealing time of the starting colloid at 100 degrees C. The extracted quantum dots are of high quality (40% photoluminescence quantum yield) and exhibit colors ranging from deep blue to red. The resulting colloids are very stable, and no precipitate is observed over a period of 6 months. Thus, the method is simple and easily scalable to synthesize fluorescent CdSe nanoparticles.

Determination of the crystal composition from the OH- vibrational spectrum in lithium niobate

A simple and accurate spectroscopic method is proposed to determine the composition of LiNbO3 crystals close to the stoichiometric composition. The method is based on the evaluation of the vibrational spectrum of hydroxyl ions that are always present in air-grown crystals. The intensity ratio R=I3480/I3465 of two band components peaking at about 3480 and 3465 cm-1 changes linearly with Li2O content in the 49.7–50.0 mol % range. Thermal effects influencing the redistribution of hydroxyl ions among the different lattice sites have also been taken into account. On comparing the results with the calibration based on ultraviolet absorption edge measurements, an absolute accuracy of at about 0.02 mol % was obtained.

Stoichiometry unbalance and photoluminescence emission in Ga 1− XAs X films prepared by flash evaporation

Both narrow and broad photoluminescence bands were observed in Ga 1− X As X films prepared by flash evaporation of polycrystalline GaAs containing native C impurities. The observed narrow crystalline-like bands are similar to band-to-band and C acceptor impurity emissions in crystalline GaAs. The narrow bands are evidence that the As excess favors the PL active GaAs crystallite formation in films deposited onto silicon (1 0 0) substrate, even when the As excess is very large ( X=0.84). This favoring is not observed in twin samples grown on silica glass substrates nor on Ga rich samples, indicating the important role of the combined effect of the As excess and Si substrate in the GaAs crystallite formation. The broad amorphous-like bands were observed in Ga rich and in moderately As rich samples. The photoluminescence emission is compared with the microstructure of the material as determined from the micro-Raman, absorption edge and reflectance measurements. The volume fraction of the crystallites formed is small and PL emission indicates that the crystallite electronic quality is much better than the ones formed heat treating films grown on silica glass substrates.

Investigations on the variable large bandgap semiconductor compound HgBrI

A new ternary compound, HgBrI, having a variable, large bandgap was investigated using different nondestructive methods. The homogeneity of the crystal composition along the growth axis was given from absorption edge measurements performed on different parts of the crystals using classical optical measurements. The atomic composition was determined by x-ray energy dispersive spectroscopy. Based on photoluminescence and reflection measurements using Confocal Scanning Laser Microscopy, the exact position and dimensions of the inhomogeneity regions were established.

Semiconductive and photoconductive properties of the single-molecule magnetsMn12-acetate andFe8Br8

Resistivity measurements are reported for single crystals of Mn$_{12}$-Acetate and Fe$_8$Br$_8$. Both materials exhibit a semiconductor-like, thermally activated behavior over the 200-300 K range. The activation energy, $E_a$, obtained for Mn$_{12}$-Acetate was 0.37 $\pm$ 0.05 eV, which is to be contrasted with the value of 0.55 eV deduced from the earlier reported absorption edge measurements and the range of 0.3-1 eV from intramolecular density of states calculations, assuming $2E_a$= $E_g$, the optical band gap. For Fe$_8$Br$_8$, $E_a$ was measured as 0.73 $\pm$ 0.1 eV, and is discussed in light of the available approximate band structure calculations. Some plausible pathways are indicated based on the crystal structures of both lattices. For Mn$_{12}$-Acetate, we also measured photoconductivity in the visible range; the conductivity increased by a factor of about eight on increasing the photon energy from 632.8 nm (red) to 488 nm (blue). X-ray irradiation increased the resistivity, but $E_a$ was insensitive to exposure.

Structure transformation of LiAl 0.15Mn 1.85O 4 cathode material during charging and discharging in aqueous solution

The electrochemical lithium-ion extraction/insertion properties of a spinel LiAl 0.15Mn 1.85O 4 material were studied. Cyclic voltammetry shows two-step reactions for lithium-ion extraction/insertion at its 4 V plateau. Synchrotron in situ X-ray diffraction studies show that lithium ions are extracted and inserted from the LiAl 0.15Mn 1.85O 4 material, respectively, from two to three different environments. The different environments may be due to different Li–Li interactions and/or different intensities of interactions with the nearest neighbors in the spinel LiAl 0.15Mn 1.85O 4 material frame work. From the Mn absorption edge measurements as a function of potential, during lithium extraction, it can be suggested that the charge transfer rate is faster than that of structural transformations. However, during lithium insertion, the rate of both charge transfer and structural transformations are more or less similar.

Electron–hole reduced effective mass in monoatomic …–O–Ti–O–Ti–O–… quantum wires embedded in the siliceous crystalline matrix of ETS-10

Optical absorption edge measurements on a new microporous crystalline material, Engelhard titanosilicate (ETS-10), containing monoatomic …–O–Ti–O–Ti–O–… wires embedded in an insulating SiO 2 guest allowed us to determine the bandgap blue shift due to the charge carrier confinement. This has enabled the determination of the effective reduced mass μ of electrons and holes in a one-dimensional quantum sized …–O–Ti–O–Ti–O–… wire having unprecedented geometrical definition (1.66 m e< μ<1.97 m e). Besides this relevant finding, related to the peculiar structure of ETS-10, the present work underlines the future possible role that micro- and mesoporous materials could play in the field of optoelectronic and non-linear optics, dominated up to now by quantum confinement in III–V and II–VI semiconductors.

Miscibility gap in ZnxSr1 − xS, ZnxCa1 − xS and ZnxSr1 − xSe thin films

The miscibility gap in ZnxSr1 − xS, ZnxCa1 − xS and ZnxSr1 − xSe thin film mixed systems has been studied from the results of film composition, lattice constant and absorption edge measurements. The phase transition in the immiscible range is discussed in terms of the fraction of ionic character based on the Phillips dielectric theory for binary compounds.

Temperature dependence of the optical absorption edge of pyrite FeS2 thin films

Optical absorption edge measurements of pyrite thin films prepared by flash evaporation have been made at different temperatures in the range 10-300 K. Absorption edge values vary from 1.05 to 0.99 eV in that temperature interval. Experimental data have been fitted to the Varshni formula and to an expression proportional to the statistical Bose-Einstein factor used previously by other workers. The main phonon energy calculated from the fitting of the experimental points to the Bose-Einstein expression is in good agreement with the average energy of the active optical phonons as determined by Raman spectroscopy carried out on the same pyrite thin film.

Gaussian interpretation of the optical absorption edge tail in amorphous hydrogenated carbon a-C:H

Amorphous hydrogenated carbon films of thickness from 260 to 320 nm were obtained by glow discharge decomposition of benzene vapours under pressure 0.05, 0.2 and 0.5 Torr. Changes introduced by annealing to temperatures 200, 300, 350, 400, 450, 500, 550°C were studied by optical absorption edge measurements. A Gaussian plot (insteadof Urbach law) was found to be useful for interpretation of the lower part of the edge (for α < 10 4 cm −1). Such a character of the optical absorption edge tail was theoretically derived from normal distribution of sp 2-cluster sizes. The Gaussian plot parameters E π , ϵ and α give information about the sizes, variability and number of sp 2 clusters in a-C:H.

Study on the optical absorption of oxygen-doped polysilicon thin films

Optical absorption measurements were used to study the optical properties of semi-insulating polycrystalline silicon (SIPOS) with different levels of oxygen dopings and a variety of annealing conditions. The optical properties of SIPOS were found to be closely related to the SIPOS microstructure. The absorption coefficient in SIPOS films with a high oxygen content displays a blue shift. From the absorption edge measurements, it has been observed that more band-tail states were introduced forthe higher oxygen contentsamples. During the heat-annealing processes, changes occur in the absorption spectra of SIPOS: the absorption coefficient of SIPOS displays a blue shift, whereas after high-temperature (<i>T</i> > 900°C) treatments, the absorption coefficient of SIPOS shows a red shift. These features of the optical properties of SIPOS are interpreted as the contribution of the distribution of Si-O bonds in the films and the recrystallization of SIPOS during low-temperature and high-temperature annealing processes, respectively.

The effect of hydrogenation on the optical bandgaps of a-SiO x:H y thin films

We have carried out optical absorption edge and IR measurements on four series of room-temperature r.f.-sputtered a-SiO x:H y thin films corresponding to values of x equal to 0, 0.7, 1.1 and 1.5. The Tauc gaps exhibit a widening with hydrogenation similar to that found for a-Si:H, but this becomes less pronounced as x increasses. Integrated IR absorption peak areas indicate that the hydrogen content of the films reduces rapidly with x above 0.7. The systematic widening of the optical gaps is interpreted in terms of the theoretical electronic densities-of-states of the films.

Structure of Ag, Fe and Ge microclusters

The structures of Ag, Fe and Ge microclusters were determined using EXAFS. The measurements were performed over a wide range of clusters sizes. The clusters were prepared using the gas aggregation technique and isolated in solid argon at 4.2 K. The measurements were performed at the National Synchrotron Light Source (NSLS) at beam line X-18B. A strong contraction of the interatomic distances was observed for Ag dimers and multimers. Silver clusters larger than 12 A mean diameter show a small contraction of thenn distance and a structure consistent with an fcc lattice. By contrast clusters smaller than 12 A show the presence of a small expansion and a strong reduction or absence ofnnn in the EXAFS signal. This points towards a different crystallographic structure for Ag microclusters with diameter less than 12 A. In iron clusters we observe a gradual reduction of thenn distance as the cluster size decreases. The interatomic distance for iron dimers was determined to be 1.94 A, in good agreement with earlier measurements. The iron microclusters show a bcc structure down to a mean diameter of 9 A. Iron clusters with 9 A mean diameter show a structure inconsistent with a bcc lattice. The new structure is consistent with an fcc or hcp lattice. The measurements on Ge clusters show the presence of only nearest neighbors. There was clear evidence of temporal annealing as determined by variations in the near edge structure of the K-absorption edge. Absorption edge measurements were also performed for free Ge clusters travelling perpendicular to the direction of the synchrotron radiation beam. The measurements performed on the free clusters were consistent with those obtained for matrix isolated clusters.

Time-Resolved Luminescence of Oxygen Related Defects in Aluminum Nitride

AbtractA charged defect model for the formation of band-tail states, first proposed by Redfield et al. to understand the optical properties of Si doped GaAs [1], is applied to oxygen doped aluminum nitride (AIN) ceramics. In this case, oxygen substituting on a nitrogen site and (subsequently formed) aluminum vacancies comprise the charged centers. The results of optical absorption edge measurements as well as steady state and time-resolved luminescence between 10 and 300K are used to support this model. Finally, it is noted that the mixed covalent and ionic nature of bonding in AIN make it a model system for this charge defect picture, and thus explains the extremely large extent of the band-tail states in this material.

Magnetism in an Yb intermetallic compound: YbPt

The authors report structural, magnetic and valence properties of the intermetallic compound YbPt, which belongs to the orthorhombic FeB structure, with only one site for Yb ion. Susceptibility measurements in the range (5–300K) show Curie-Weiss behavior with a negative intercept down to 40K, then some tendency to increased ordering at lower temperature. L iii absorption edge measurements on YbPt are consistent with Yb ion in fully trivalent state. Low energy neutron inelastic scattering of the central quasielastic component reveals the onset of magnetic ordering with the appearance of a magnon peak below 6K. These results are of particular interest since relatively few Yb-based materials become magnetically ordered.