Progress in the room-temperature optical functions of semiconductors

Optical functions of a specific semiconductor in a particular wavelength region are often needed in optics and optoelectronics research. Basic optical properties of some materials are available in handbooks, but recent advances in the experimental techniques for growth, sample preparation and determination of optical functions, as well as theoretical advances in modeling the optical functions, prompted demand for a review of the state of the art of the optical functions of some important semiconductor materials. This paper gives an overview of the experimental methods for the determination of optical functions in the 1–6 eV spectral range and the available models for calculating the optical functions. Also, the paper summarizes the progress in the study of optical functions of several important semiconductors such as GaP, InP, InAs, GaSb, InSb, AlSb, Al_xGa_1−xAs, Hg_xCd_1−xTe, Si_xGe_1−x, GaN, InN, AlN, 6H–SiC, and ZnO. Besides discussing the available data and expected positions of optical transitions, the paper focuses on the model of the dielectric function which allows reproducing accurately the experimental dielectric function values for all the materials considered here.     

Synthesis and cathode luminescence properties of ZnO multipod nanoneedles

Zinc oxide (ZnO) multipod nanoneedles over a large area have been synthesized on silicon substrate by thermally oxidizing zinc foil at 650 °C. These nanoneedles have sharp tails with diameter down to less than 100 nm, with length of 10 μm, growing from the surface of the silicon substrate and legs connected at a common base. X-ray diffraction (XRD) confirmed the sample as pure ZnO nanostructures with growth direction of [002]. The cathode luminescence behaviors at different regions of an individual nanoneedle of these multipod ZnO nanostructures were characterized. It is shown the whole nanostructures are luminescent, while the tips have relative higher visible emission than the bottom. The cathode luminescence mechanisms were also discussed.     

Optical properties and characterization of zinc nitride nanoneedles prepared from ball-milled Zn powders

Zinc nitride nanoneedles (ZNNs) with diameters at stem and tip parts as 200-300 nm and 30-70 nm respectively have been prepared by the nitridation of ball-milled zinc powders at 600 °C for 120 min under NH₃ gas environment. The structural, compositional and morphological characterizations of the product were conducted by X-ray diffraction, energy dispersive X-ray spectroscopy, scanning electron microscopy and transmission electron microscopy. From transmission spectrum data, an indirect band gap of 2.72 eV has been calculated for ZNNs whereas photoluminescence studies exhibited a strong UV excitonic mission band at 395 nm as well as two weak defect related blue emissions at 453 and 465 nm. A vapor-solid (VS) process based growth mechanism for the formation of ZNNs has also been discussed briefly.     

Microwave-assisted hydrothermal synthesis and optical property of Co3O4 nanorods

Single crystalline Co₃O₄ nanorods have been successfully synthesized through thermal decomposition of the precursor, which was obtained by the microwave-assisted hydrothermal route. The obtained sample was characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), Fourier transform infrared spectrometer (FT-IR), and X-ray photoelectron spectroscopy (XPS). The results confirm that the resulting oxide was pure single-crystalline Co₃O₄ nanorods. The optical property test indicates that the absorption peak of the nanorods shifts towards short wavelength. And the blue shift phenomenon might be ascribed to the quantum effect.     

Size dependent electron transfer from CdTe quantum dots linked to TiO2 thin films in quantum dot sensitized solar cells

In this present study, we demonstrate the size dependent charge transfer from CdTe quantum dots (QDs) into TiO₂ substrate and relate this charge transfer to the actual behavior of a CdTe sensitized solar cell. CdTe QDs was synthesized using mercaptopropionic acid as the capping agent. The conduction band offset for TiO₂ and CdTe QDs indicates thermodynamically favorable band edge positions for smaller QDs for the electron-transfer at the QD–TiO₂ interface. Time-resolved emission studies were carried out for CdTe QD on glass and CdTe QD on TiO₂ substrates. Results on the quenching of QD luminescence, which relates to the transfer kinetics of electrons from the QD to the TiO₂ film, showed that at the smaller QD sizes the transfer kinetics are much more rapid than at the larger sizes. I–V characteristics of quantum dot sensitized solar cells (QDSSC) with different sized QDs were also investigated indicating higher current densities at smaller QD sizes consistent with the charge transfer results. The maximum injection rate constant and photocurrent were obtained for 2.5 nm CdTe QDs. We have been able to construct a solar cell with reasonable characteristics (V_oc = 0.8 V, J_sc = 1 mA cm⁻², FF = 60%, η = 0.5%).     

Solvothermal synthesis and photoluminescence properties of ZnO nanorods and nanorod assemblies from ZnO2 nanoparticles

Without the use of any extra surfactant or template, hexagonal phase ZnO crystallites consisting of individual nanorods or nanorod assemblies were synthesized simply by solvothermal treatment of several nanometer ZnO₂ nanoparticles in three different solvents (including ethanol, 80 wt.% hydrazine hydrate aqueous solution and ethylenediamine) at 150 °C for 24 h. The structures and optical properties of the resultant products were characterized by means of X-ray powder diffraction (XRD), scanning electron microscope (SEM), and room temperature photoluminescence (RTPL) spectra. The RTPL spectra of the resultant products all showed a much stronger ultraviolet bandgap emission peaking at around 387 nm and a weaker emission associated with the defect level. The as-synthesized ZnO crystallites are promising materials for the optoelectronic devices due to their excellent UV emission properties.     

Fabrication and characterization of ordered macroporous semiconductors CdS by colloidal crystal template

The synthesis and characterization of a highly ordered macroporous CdS with regular arrays of spherical pores comparable to optical wavelengths are discussed. The sample has been successfully fabricated using colloidal crystal of poly (styrene-acrylic) (PSA) spheres as templates. The pore size is tunable in the range of 100-400 nm based on the size of PSA spheres. Transmission electron microscope (TEM) and scanning electron microscope (SEM) show that the exactly three-dimensional structure of the template has been imprinted in the final materials. XRD pattern indicates that the walls of the macroporous material are composed of 4 nm CdS nanoparticles making the absorption spectrum shift blue. The sample was also characterized by Raman spectroscopy and photo luminescent spectra (PL).     

Correlation between structural and opto-electronic properties of a-Si1 − xCx:H films deposited by plasma enhanced chemical vapour deposition

Hydrogenated amorphous silicon carbon alloy films of different carbon content were prepared by Plasma Enhanced Chemical Vapor Deposition using silane and methane with helium dilution and were characterized to study their opto-electronic, structural and defective properties. A linear correlation between micro structural disorder and overall disorder has been demonstrated. Further, it has been shown that the increase in the intrinsic disorder leads to an increase in the defect density while the increase in voids results in the decrease in the mass density for the studied films.     

Characterization and optical properties of ZnO nanoparticles obtained by oxidation of Zn nanoparticles

Strong UV emission at about 399 nm (3.1 eV) and multiple blue emissions at 440-490 nm (2.70-2.53 eV) by ZnO nanoparticles have been observed. The characterizations of ZnO nanoparticles, obtained by oxidizing Zn nanoparticles prepared by arc-discharging, were investigated. The multiple blue PL becomes stronger as the oxidation temperature and time increase, which is attributed to the existence of various defects, in particular to interstitial zinc at the surface of the ZnO nanoparticles.     

Energy transfer between lead sulfide quantum dots in the liquid phase

Förster resonant energy transfer (FRET) from smaller sized PbS Quantum Dots (QDs) to larger sized ones occurs in the liquid phase when adjacent QDs are brought in close proximity by the bridging action of the two SH groups in aromatic or aliphatic dithiols. Signatures of FRET were observed by the lowering of the fluorescence peak corresponding to the smaller particle group and intensification of the peak corresponding to the larger particle group in a mixture of the two in the liquid phase. The suspensions of the two QDs size distributions were mixed in a ratio such that total surface area of the smaller QDs was equal to that of the larger QDs. A 10–13% size deviation in each PbS QDs group also allows this phenomenon to be observed in these samples and is manifested by red-shift and broadening of the fluorescence peak. The ratio of the absorption peak intensity corresponding to the two groups of PbS QDs in the mixture remains the same.     

A thermoluminescence study on the state of Cl in ZnS:Ag by electron beam

The degradation behavior of ZnS:Ag, Cl as a phosphor for CL by EB irradiation at 7 kV was examined by TL measurement. After EB irradiation, TL intensity decreased and the TL peak shifted to the lower temperature side. By comparing TL thermograms of mechanically damaged ZnS:Ag, Cl, ZnS:Cl with varying Cl concentration as well as ZnS:Ag, Al after EB irradiation, we conclude that the decrease in the effective concentration of Cl⁻, serving as active luminescence center, is responsible for the CL degradation of ZnS:Ag, Cl by EB irradiation.     

Low temperature synthesis and optical properties of CaTiO3 nanoparticles from Ca(NO3)2·4H2O and TiO2 nanocrystals

Choosing low-melting-point Ca(NO₃)₂·4H₂O and high-reactive-activity TiO₂ nanocrystals as the raw materials, a simple and cost-effective route was developed for the synthesis of CaTiO₃ nanoparticles at 600 °C, which is much lower than that (about 1350 °C) used in the conventional solid state reaction methods. X-ray diffraction, energy dispersive X-ray spectroscopy and field emission scanning electron microscopy revealed the formation of orthorhombic phase CaTiO₃ nanoparticles with oxygen-deficiency at the surface. UV-vis absorption spectrum of the as-obtained CaTiO₃ nanoparticles displayed an absorption peak centered at around 325 nm (3.8 eV), together with a tail at lower energy side. Room temperature photoluminescence spectrum of the as-obtained CaTiO₃ nanoparticles upon laser excitation at 325 nm demonstrated a strong and broad visible light emission ranging from about 527 to 568 nm, which may be originated from the surface states and defect levels.     

Synthesis of AgBr/ZnO nanocomposite with visible light-driven photocatalytic activity

AgBr/ZnO nanocomposite was synthesized via chemical precipitation from pure ZnO nanowires, AgNO₃, and NaBr. Inductively coupled plasma optical emission spectroscopy, X-ray diffraction, and high resolution transmission electron microscopy results confirmed the forming of AgBr/ZnO nanocomposite. High resolution transmission electron microscopy results of the as-synthesized AgBr/ZnO nanocomposite revealed that AgBr nanoparticles were attached to the surface of ZnO nanowires. UV-vis diffuse reflectance spectra of both pure ZnO and AgBr/ZnO nanocomposite displayed a band gap edge at about 350-380 nm. However, compared with pure ZnO, an additional broad tail from approximately 400 nm to 700 nm appeared in the UV-vis diffuse reflectance spectrum of AgBr/ZnO nanocomposite. The photocatalytic studies indicated that the as-synthesized AgBr/ZnO nanocomposite was a kind of promising photocatalyst in remediation of water polluted by some chemically stable azo dyes under visible light.     

Solvothermal synthesis of Cr-doped ZnO nanowires with visible light-driven photocatalytic activity

Cr-doped ZnO nanowires were fabricated by a solvothermal route from Zn(NO₃)₂6H₂O, Cr(NO₃)₃9H₂O and NaOH. Inductively coupled plasma optical emission spectroscopy, X-ray diffraction, transmission electron microscopy, and high resolution transmission electron microscopy results confirmed the doping of Cr into ZnO lattices. UV-vis absorption spectra of both pure ZnO and Cr-doped ZnO displayed a band gap absorption peak at about 365 nm. However, compared with pure ZnO, an additional broad tail from approximately 400 nm to 750 nm appeared in the UV-vis absorption spectrum of Cr-doped ZnO. The photocatalytic studies indicated that the as-synthesized Cr-doped ZnO nanowires were a kind of promising photocatalyst in remediation of water polluted by some chemically stable azo dyes under visible light.     

Hydrothermal synthesis and characterization of Bi2O3 nanowires

An alternative two-step method has been proposed for the synthesis of Bi₂O₃ nanowires with a diameter of about 40 nm from common and cost-effective Bi(NO₃)₃·5H₂O, Na₂SO₄, and NaOH. That is, first, Bi₂O(OH)SO₄ nanowires were prepared through the precipitation reaction of Bi(NO₃)₃·5H₂O and Na₂SO₄ in distilled water under the ambient condition and second, monoclinic phase Bi₂O₃ nanowires were prepared via the hydrothermal reaction of Bi₂O(OH)SO₄ and NaOH at 120 °C for 12 h. The resultant products were characterized by X-ray diffraction, field emission scanning electron microscope, and high resolution transmission electron microscopy. In addition, the photocatalytic studies indicated that the as-synthesized Bi₂O₃ nanowires were a kind of promising photocatalyst in remediation of water polluted by some chemically stable azo dyes.     

Effect of the anisotropy of the electron g-factor in spin polarization

Spin polarization in the presence of an external magnetic field and electric bias in quantum confined semiconductor structures has been studied by time- and polarization-resolved spectrometry. From measurements with angular variations of the magnetic field from the Voigt configuration (VC) it was found that both the frequency (Ω) and decay rate (β) of the oscillatory component of the polarization increase with variation of the angle from the VC. Their dependences are discussed based on the electron spin dephasing related to the spread of the electron g-factor (g_e) (i.e. unequal values of the longitudinal (g_e||) and transverse (g_e⊥) components of g_e) and the exchange interaction between the electron and hole spins. It is demonstrated that the increase in Ω upon deviation of the magnetic field from the VC relates to the anisotropy of g_e (g_e|| and g_e⊥) resulting from the quantum confinement effect. However, the angular dependence on β is related to the residual exchange interaction between the electron spin and rapidly relaxing hole spin.     

Iron pyrite films prepared by sulfur vapor transport

Iron films deposited via thermal evaporation, with a thickness between 100 and 250 nm, were converted into FeS₂ by open sulfur transport using nitrogen as a gas vector. The films thus obtained constituted a single pyrite phase and were optically highly absorbing. The sulfurization process was optimized. As a result, sample temperature and conversion time were found to be the major determining parameters. The films were characterized using several methods. The crystallinity and phase identification were determined by X-ray diffractometry. Scanning electron microscopy showed a homogeneous surface of both iron and pyrite layers. Optical transmission measurements confirmed the highly absorbing character of FeS₂ and allowed the determination of direct (1.35 eV) and indirect (0.82 eV) transitions.     

Synthesis of wurtzite ZnS nanorods by microwave assisted chemical route

ZnS nanorods were synthesized by microwave assisted chemical method. Polyvinylpyrrolidone (PVP) was used as a capping agent to stabilize the nanostructure. Synthesized ZnS nanorods were characterized by X-ray diffraction (XRD), Energy dispersive X-ray analysis (EDAX), Transmission Electron Microscopy (TEM), UV-visible spectrophotometry (UV), photoluminescence spectroscopy (PL) and Fourier transform Infra-red spectroscopy (FTIR). The results showed that the nanorods have wurtzite phase crystal structure and exhibits near band edge luminescence in the ultra violet region. The diameter of the synthesized PVP capped ZnS nanorods is about 600 nm. The possible growth mechanism of the ZnS nanorods could be attributed to the oriented attachment effect.     

Synthesis of organic ligand passivated zinc selenide nanorods via wet chemical route

Zinc selenide (ZnSe) nanorods have been synthesized by simple and inexpensive wet chemical method using N-Methylaniline as a capping agent. The morphologies and structure of ZnSe nanorods have been characterized by scanning electron microscopy (SEM), transmission electron microscopy (TEM) and X-ray diffraction (XRD). The SEM and TEM reveal the formation of nanorods. XRD shows the cubic structure with the lattice constant of 5.633 Å. Strong “blue shift” absorption is observed from UV-visible spectrophotometry. The enhanced luminescence property is measured from photoluminescence spectrophotometry. The presence of N-Methylaniline in the ZnSe nanorods is confirmed by the Fourier transformed infrared spectrum.     

Solvo-solid preparation of Zn3N2 hollow structures; their PL yellow emission and hydrogen absorption characteristics

Zinc nitride (Zn₃N₂) hollow structures with 10-25 μm size have been prepared by solvo-solid approach using aqueous ammonia treated Zn precursor at reaction temperature of 600 °C for reaction time of 240 min under ammonia gas flow. The structural, compositional and morphological characterizations of the as-obtained product were performed by XRD, EDS and SEM. Room temperature photoluminescence (PL) spectrum of zinc nitride hollow structures (ZNHSs) exhibited UV emission band at 384 nm and a defect related yellow emission band at 605 nm. The first ever studies on hydrogen absorption characteristics of ZNHSs performed at 373 K showed an absorption capacity of 1.29 wt.%. Growth mechanism proposed for the formation of ZNHSs is also discussed briefly.