Valence Band Level Research Articles

Role of ZnO nanoparticle‐layers in enhancement of the performance of organic light‐emitting diodes on plastics

AbstractWe investigate roles of ZnO nanoparticle (NP)‐layers in the enhancement of the emission from organic light‐emitting diodes (OLEDs) with amorphous ZnO‐doped In2O3 (a ‐IZO)/poly(3,4‐ethylenedioxythiophene) poly(styrenesulfonate) (PEDOT:PSS)/poly[(9,9‐di‐n ‐octylfluorenyl‐2,7‐diyl)‐alt ‐(benzo[2,1,3]thiadiazol‐4,8‐diyl)] (F8BT)/ZnO NP‐layer/LiF/Al structure on plastic substrates. Compared to a Reference OLED without any ZnO NP‐layer, the electroluminescence peak intensity of the OLED employing the thermally‐annealed ZnO NP‐layer spin‐coated one time is remarkably enhanced by ∼283% at a wavelength of ∼535 nm owing to the effective hole blocking and good electron transporting properties of the ZnO NP‐layer between the organic emissive layer (EML) and the electron injection layer (EIL). This enhancement is associated with the high junction potential barrier between the highest occupied molecular orbital level of the F8BT and the valence band level of the ZnO NP‐layer, which contributes to the effective hole blocking. Our study clearly demonstrates that the ZnO NP‐layer sandwiched between the EML and the EIL is a promising electron transport layer as well as an efficient hole blocking layer and enhances the light emission of OLEDs. (© 2014 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Hard x-ray photoelectron spectroscopy as a probe of the intrinsic electronic properties of CdO

Hard x-ray photoelectron spectroscopy (HAXPES) is used to investigate the intrinsic electronic properties of single crystal epitaxial CdO(100) thin films grown by metal organic vapor phase epitaxy (MOVPE). The reduced surface sensitivity of the HAXPES technique relaxes stringent surface preparation requirements, thereby allowing the measurement of as-grown samples with intrinsically higher carrier concentration ($n=2.4\ifmmode\times\else\texttimes\fi{}{10}^{20}\phantom{\rule{0.222222em}{0ex}}{\text{cm}}^{\ensuremath{-}3}$). High-resolution HAXPES spectra of the valence band and core levels measured at photon energy of 6054 eV are presented. The effects of conduction band filling and band gap renormalization are discussed to explain the observed binding energy shifts. The measured bandwidth of the partially occupied conduction band feature indicates that a plasmon contribution may be present at higher carrier concentrations. The Cd 3$d$${}_{5/2}$ and O 1$s$ core-level line shapes are found to exhibit an increased asymmetry with increased carrier concentration, interpreted as evidence for final state screening effects from the carriers in the conduction band. Alternatively the core-level line shape is interpreted as arising from strong conduction electron plasmon satellites. The nature of these two competing models to describe core-level line shapes in metallic oxides is explored.

Photoelectron spectroscopy study of Mn/n-Si interfacial structure

The Mn/n-Si interfacial structure is susceptible to intermixing even at room temperature. To investigate the chemistry as a result of the intermixing, valence band and core level photoelectron spectroscopy of Mn/Si has been carried out using synchrotron radiation of 134 eV energy and Al Kα X-ray (λ = 1,486.6 A) source. The fabricated structures have also been irradiated from swift heavy ions (Fe7+ of ~100 MeV) to investigate the ion beam mixing in such structures. Valence band photoelectron spectroscopy with 134 eV photons shows the evolution of Mn3d, Mn3p and Si2p levels with a shifting towards lower binding energy side compared to their elemental values of the binding energy. This binding energy shift shows the formation of chemical compound of Si and Mn. Evolution of Si2p core level prior to and after the swift heavy ion irradiation shows strong chemical reactivity of manganese thin film with silicon. Deconvolution of Mn3d valence band has shown the formation of silicide phase due to the hybridization of Mn3d and Si3sp states. Mn2p core level study shows that the oxide and silicide formation takes place during the growth and for successive etching, oxide part is decreasing whereas silicide part is increasing.

Solution-Processed Zinc Phosphide (α-Zn3P2) Colloidal Semiconducting Nanocrystals for Thin Film Photovoltaic Applications

Zinc phosphide (Zn3P2) is a promising earth-abundant material for thin film photovoltaic applications, due to strong optical absorption and near ideal band gap. In this work, crystalline zinc phosphide nanoparticles are synthesized using dimethylzinc and tri-n-octylphosphine as precursors. Transmission electron microscopy and X-ray diffraction data show that these nanoparticles have an average diameter of ∼8 nm and adopt the crystalline structure of tetragonal α-Zn3P2. The optical band gap is found to increase by 0.5 eV relative to bulk Zn3P2, while there is an asymmetric shift in the conduction and valence band levels. Utilizing layer-by-layer deposition of Zn3P2 nanoparticle films, heterojunction devices consisting of ITO/ZnO/Zn3P2/MoO3/Ag are fabricated and tested for photovoltaic performance. The devices are found to exhibit excellent rectification behavior (rectification ratio of 600) and strong photosensitivity (on/off ratio of ∼10(2)). X-ray photoelectron spectroscopy and ultraviolet photoemission spectroscopy analyses reveal the presence of a thin 1.5 nm phosphorus shell passivating the surface of the Zn3P2 nanoparticles. This shell is believed to form during the nanoparticle synthesis.

Resonant enhancement of third-order nonlinear optical susceptibilities of Cd-free chalcopyrite nanocrystals within quantum confinement regime

Third-order nonlinear optical susceptibilities (χ(3)) have been investigated for chalcopyrite CuInS2 and AgInS2 nanocrystals within a strong confinement regime. The imaginary part of χ(3) (Imχ(3)) of 2.0- and 4.9-nm-sized CuInS2 nanocrystals and 2.6- and 4.3-nm-sized AgInS2 nanocrystals are negative and exhibit resonant enhancement around the absorption between the highest quantized levels of valence band and the lowest conduction band due to the state-filling effect. Figure of merit of |Imχ(3)| ranges 10−20–10−19 m3/V2, which is comparable to those of CdSSe nanocrystals.

Effect of valence band energy on the photocatalytic performance of N-doped TiO2 for the production of O2 via the oxidation of water by visible light

Five types of N-doped TiO2 (TiO2−xNx, x=0, 0.005, 0.015, 0.03, and 0.06) nanoparticles were prepared and evaluated for use as a catalyst for production of O2 via the oxidation of water by irradiation with visible light. The resulting materials were characterized using ultraviolet diffuse reflectance spectroscopy (UV-DRS), electrochemical impedance spectroscopy (EIS), and photoluminescence (PL) techniques. The results indicate that a change in band gap energy (Eg) as a function of N-doping level in the materials resulted in a change in the valence band (VB) level, but not the conduction band (CB) level. The very small upward shift in VB enhanced the photocatalytic formation of OH radicals produced by irradiation with visible light. In addition, the small amount N-dopant (up to 3at%) incorporated into the samples accelerated the evolution of O2, rather than the formation of OH species. The incorporation of larger amounts of N-dopant in the TiO2−xNx reduced the rates of both O2 evolution and OH formation, due to the small energy level of VB and the short life time of the photo-generated charges. The performance of TiO1.97N0.03 nanoparticles with an Eg value of 2.8eV was outstanding for the evolution of O2 from the water under the experimental conditions used.

Differences in the adsorption of FePc on coinage metal surfaces

A study of the electronic and structural properties of iron phthalocyanine (FePc) molecules adsorbed on coinage metal surfaces Cu (100) and Cu (110) has been conducted by means of density functional theory calculations. The strength of the molecule-substrate interactions is interpreted in terms of the lateral adsorption geometry and the site specific electronic structure of the molecule. In the case of FePc on a (100)-oriented copper surface, the benzopyrrole leg is found to be oriented at an angle of 9° or 3° from the [01-1] substrate direction. Further, an upward bend in the molecular plane ranging from 7 to 10° is also observed; giving an almost buckled shape to the molecule. However, in the case of FePc on Cu (110), neither a bend nor a sizable rotation is observed. From the knowledge of the principle structural and electronic properties, it is concluded that FePc—Cu (100) interaction is relatively stronger than FePc—Cu (110) interaction, which is further evidenced by the charge transfer, work function changes, changes in the shape of the adsorbed molecular orbitals, and the orbital shifts. Furthermore, density of states analysis shows that the valence band level shift is surface- and site-dependent.

Development of a laboratory system hard X-ray photoelectron spectroscopy and its applications

Development of a laboratory hard X-ray photoelectron spectrometer using excitations by monochromatic Cr Kα X-rays of 5.4keV and a high energy analyzer with a wide acceptance angle resolved objective lens is introduced. Wide applicability of the system as a powerful tool for the investigations of electronic and chemical states of materials are demonstrated by various examples including bulk sensitive valence band and core level spectroscopy, overlayer thickness determination by photoelectron take off angle dependence measurements, buried layer analysis, bulk sensitive photoelectron diffraction to determine surface polarity of compound single and poly crystalline films, interface state spectroscopy in MOS structures by applying voltage bias, and environmental cell for high pressure photoelectron spectroscopy. These results evidently show the laboratory HXPES system is going to be indispensable in wide varieties of targets. It also opens up opportunities of the analysis of materials which are not accessible to beamlines due to limitations by safety control regulation and avoidance of risks to the beamlines.

Band Structure Engineering of Carbon Nitride: In Search of a Polymer Photocatalyst with High Photooxidation Property

The electronic band structure of a semiconductor photocatalyst intrinsically controls its level of conduction band (CB) and valence band (VB) and, thus, influences its activity for different photocatalytic reactions. Here, we report a simple bottom-up strategy to rationally tune the band structure of graphitic carbon nitride (g-C3N4). By incorporating electron-deficient pyromellitic dianhydride (PMDA) monomer into the network of g-C3N4, the VB position can be largely decreased and, thus, gives a strong photooxidation capability. Consequently, the modified photocatalyst shows preferential activity for water oxidation over water reduction in comparison with g-C3N4. More strikingly, the active species involved in the photodegradation of methyl orange switches from photogenerated electrons to holes after band structure engineering. This work may provide guidance on designing efficient polymer photocatalysts with the desirable electronic structure for specific photoreactions.

Design of visible-light photocatalysts by coupling of narrow bandgap semiconductors and TiO2: effect of their relative energy band positions on the photocatalytic efficiency

According to relative energy band positions between TiO2 and visible-light-absorbing semiconductors, three different types of heterojunction were designed, and their visible-light photocatalytic efficiencies were analyzed. In Type-A heterojunction, the conduction band (CB) level of sensitizer is positioned at a more negative side than that of TiO2, whereas in Type-B system its valence band (VB) level is more positive than that of TiO2 and in Type-C system the sensitizer energy level is located between the CB and VB of TiO2. In evolving CO2 from gaseous 2-propanol (IP) under visible-light irradiation, the Type-B systems such as FeTiO3/TiO2, Ag3PO4/TiO2, W18O49/TiO2, and Sb-doped SnO2 (ATO)/TiO2 demonstrated noticeably higher photocatalytic efficiency than the Type-A such as CdS/TiO2 and CdSe/TiO2, while the Type-C such as NiTiO3/TiO2, CoTiO3/TiO2, and Fe2O3/TiO2 did not show any appreciable improvement. Remarkably high visible-light photocatalytic activity of Type-B heterojunction structures could be explained by inter-semiconductor hole-transfer mechanism between the VB of the sensitizer and that of TiO2. The evidence for the hole-transport between sensitizer and TiO2 was also obtained by monitoring the hole-scavenging reactions with iodide (I−) and 1,4-terephthalic acid (TA).

Effect of uniaxial stress on electroluminescence, valence band modification, optical gain, and polarization modes in tensile strained p-AlGaAs/GaAsP/n-AlGaAs laser diode structures: Numerical calculations and experimental results

The effects of uniaxial compression in [110] direction on energy-band structures, heavy and light hole mixing, optical matrix elements, and gain in laser diodes with “light hole up” configuration of valence band levels in GaAsP quantum wells with different widths and phosphorus contents are numerically calculated. The development of light and heavy hole mixing caused by symmetry lowering and converging behavior of light and heavy hole levels in such quantum wells under uniaxial compression is displayed. The light or heavy hole nature of each level is established for all considered values of uniaxial stress. The results of optical gain calculations for TM and TE polarization modes show that uniaxial compression leads to a significant increase of the TE mode and a minor decrease of the TM mode. Electroluminescence experiments were performed under uniaxial compression up to 5 kbar at 77 K on a model laser diode structure (p-AlxGa1−xAs/GaAs1−yPy/n-AlxGa1−xAs) with y = 0.16 and a quantum well width of 14 nm. They reveal a maximum blue shift of 27 meV of the electroluminescence spectra that is well described by the calculated change of the optical gap and the increase of the intensity being referred to a TE mode enhancement. Numerical calculations and electroluminescence data indicate that uniaxial compression may be used for a moderate wavelength and TM/TE intensity ratio tuning.

Optical absorption in semiconductor quantum dots coupling to dispersive phonons of infinite modes

Optical absorption spectrum of semiconductor quantum dot is investigated by means of an analytical approach based on the Green's function for different forms of coupling strength in an unified method by using the standard model with valence and conduction band levels coupled to dispersive quantum phonons of infinite modes. The analytical expression of the optical absorption coefficient in semiconductor quantum dots is obtained and by this expression the line shape and the peak position of the absorption spectrum are procured. The relation between the properties of absorption spectrum and the forms of coupling strength is clarified, which can be referenced for choosing the proper form of the coupling strength or spectral density to control the features of absorption spectrum of quantum dot. The coupling and confinement induced energy shift and intensity decrease in the absorption spectrum are determined precisely for a wide range of parameters. The results show that the activation energy of the optical absorption is reduced by the effect of exciton-phonon coupling and photons with lower frequencies could also be absorbed in absorption process. With increase of the coupling constant, the line shape of optical absorption spectrum broadens and the peak position moves to lower photon energy with a rapid decrease in intensity at the same time. Both the coupling induced red shift and the confinement induced blue shift conduce to decrease in the intensity of absorption spectrum. Furthermore, this method may have application potential to other confined quantum systems.

Tungsten and nitrogen co-doped TiO2 electrode sensitized with Fe–chlorophyllin for visible light photoelectrocatalysis

Sol-hydrothermal method was used to fabricate tungsten (W) and nitrogen (N) co-doped titanium oxide (TiO2) nanoparticles. The nanoparticles were sensitized with a natural dye, sodium–iron–chlorophyllin (Fe–chl). The W and N co-doped TiO2 nanoparticles were analyzed using X-ray diffraction (XRD), filed scanning electron microscopy (FSEM), X-ray photoelectron spectroscopy (XPS) and high resolution transmission electron microscopy (HRTEM). This catalyst was sensitized with different concentrations of Fe–chl to prepare a photoelectrochemical electrode that was characterized by electrochemistry analysis and UV–visible diffuse reflectance spectra. The photoelectrocatalytic (PEC) degradation of methyl orange (MO) under visible light illumination on the obtained electrodes was investigated. The conduction and valence bands of TiO2 were respectively modified with W and N. The 3d orbit of W lowered the conduction band level of TiO2 and the 2p orbit of N shifts the valence band level of TiO2 upward. This condition, along with the excitation of the TiO2 electrode to electron–hole that was induced by porphyrin in Fe–chl under visible illumination, resulted in the high PEC activity of the W/N–TiO2–Fe–chl electrode. The highest degradation (64.3%) and CODCr removal (48.2%) efficiencies of MO on W/N–TiO2–Fe–chl-2mM under visible light illumination for 4h were obtained by applying bias potential +1.5V.

The contribution of quantum confinement to optical anisotropy of a-plane Cd0.06Zn0.94O/ZnO quantum wells

We investigated quantum size effects on polarized photoluminescence (PL) from a-plane Cd0.06Zn0.94O/ZnO quantum wells (QWs) with different well widths (LW). The degree of polarized PL at 300 K increased with a narrowing of LW, which obeyed the polarization selection rule based on a Boltzmann-like distribution. Furthermore, the narrowing of LW enhanced the anisotropic lattice distortions in well layers along the growing direction, which resulted in an increase of energy separation between the two topmost valence band levels. The effect of quantum confinement on polarized PL of QWs was accompanied by a change of structural symmetry of the well layers.

Observation of band structure on amorphous silicon surface and performance of solar cells with controlled band offsets on hetero-junctions

Control of band offsets on hetero-junctions is important to achieve a higher efficiency amorphous silicon-based solar cell. It is expected to improve cell's properties of Voc and FF by suppressing band offsets on the junction. The configuration of conduction band level (Ec), valence band level (Ev) and Fermi level (Ef) based on vacuum level governs the formation of band offset on the junction. We show that the configuration of the band structure on the surface of amorphous Si thin film is deposited by plasma-CVD equipment with a VHF power supply. Ec and Ev were obtained from ionization energy and energy band gap observed by ultraviolet photoemission spectroscopy and optical measurement, respectively. Ef was measured as work function by means of Kelvin probe method. As a result, it is found that shifts of energy levels originate from change of bonding characteristics in the amorphous Si network. This phenomenon is peculiar to amorphous Si-based materials. The results indicate that the upper shift of Ec and Ev are effective in suppressing band offsets on the hetero-junction. We show the performance of solar cells with controlled band offsets on hetero-junctions and discuss the possibility of higher Voc and FF by employing amorphous Si-based materials.

Electronic Structure of Rare-Earth Scandates from X-Ray Spectroscopy and First-Principles Calculations

Combined spectroscopic techniques were applied to the study of valence band, conduction band, and core levels (by X-ray emission, absorption, and X-ray photoelectron spectroscopy, correspondingly) of seven scandium / lanthanide (Pr to Dy) double oxides. A comparison of O K-emission and absorption spectra, brought to the same energy scale, permits to extract the band gap values by a method different from optical spectroscopy. The variation of band gap value over the row of scandates with different lanthanides is not monotonous. An inverse correlation is detected between the band gap value and the mean Sc-O bond length.

Design of energy band alignment at the Zn1−xMgxO/Cu(In,Ga)Se2 interface for Cd-free Cu(In,Ga)Se2 solar cells

The electronic band structure at the Zn(1-x)Mg(x)O/Cu(In(0.7)Ga(0.3))Se(2) interface was investigated for its potential application in Cd-free Cu(In,Ga)Se(2) thin film solar cells. Zn(1-x)Mg(x)O thin films with various Mg contents were grown by atomic layer deposition on Cu(In(0.7)Ga(0.3))Se(2) absorbers, which were deposited by the co-evaporation of Cu, In, Ga, and Se elemental sources. The electron emissions from the valence band and core levels were measured by a depth profile technique using X-ray and ultraviolet photoelectron spectroscopy. The valence band maximum positions are around 3.17 eV for both Zn(0.9)Mg(0.1)O and Zn(0.8)Mg(0.2)O films, while the valence band maximum value for CIGS is 0.48 eV. As a result, the valence band offset value between the bulk Zn(1-x)Mg(x)O (x = 0.1 and x = 0.2) region and the bulk CIGS region was 2.69 eV. The valence band offset value at the Zn(1-x)Mg(x)O/CIGS interface was found to be 2.55 eV after considering a small band bending in the interface region. The bandgap energy of Zn(1-x)Mg(x)O films increased from 3.25 to 3.76 eV as the Mg content increased from 0% to 25%. The combination of the valence band offset values and the bandgap energy of Zn(1-x)Mg(x)O films results in the flat (0 eV) and cliff (-0.23 eV) conduction band alignments at the Zn(0.8)Mg(0.2)O/Cu(In(0.7)Ga(0.3))Se(2) and Zn(0.9)Mg(0.1)O/Cu(In(0.7)Ga(0.3))Se(2) interfaces, respectively. The experimental results suggest that the bandgap energy of Zn(1-x)Mg(x)O films is the main factor that determines the conduction band offset at the Zn(1-x)Mg(x)O/Cu(In(0.7)Ga(0.3))Se(2) interface. Based on these results, we conclude that a Zn(1-x)Mg(x)O film with a relatively high bandgap energy is necessary to create a suitable conduction band offset at the Zn(1-x)Mg(x)O/CIGS interface to obtain a robust heterojunction. Also, ALD Zn(1-x)Mg(x)O films can be considered as a promising alternative buffer material to replace the toxic CdS for environmental safety.

Electronic structures and optical properties of transition metals (Fe, Co, Ni, Zn) doped rutile TiO2

The geometric structures of transition metals (Fe, Co, Ni and Zn) doped rutile TiO2 are studied using the first-principles method based on the density functional theory. The lattice parameters, the electronic energy band structure, and the optical properties are calculated and discussed. The results show that the errors between calculated and experimental values of lattice parameters are less than 3.6%. Appropriate dopants of transition metal ions not only influence the band structure of rutile TiO2 system and broaden the scope of light absorption, but also play an important role in trapping electrons, improving the effective separation of electronic-hole pair and enhancing light absorption ability. The optimum Fe, Co, Ni, Zn doped rutile TiO2 systems in theory are Ti0.75Fe0.25O2, Ti0.75Co0.25O2, Ti0.75Ni0.25O2, Ti0.17Zn0.17O2, respectively. The 3d orbits of Fe, Co, Ni split into two groups of energy bands, t2g and eg states contribute to the higher level of valence band and the lower level of conduction band, respectively, which is conducive to the generation of electronic-hole pair and the enhancement of photocatalytic performance of rutile TiO2. Zn 3d orbit is completely filled with electrons, and the electrons are hardly excited, so the photocatalytic activity of rutile TiO2 is not obviously improved.

Photoemission and LEED study of the Sn/Rh(111) surface—early oxidation steps and thermal stability

We have deposited two monolayers of Sn onto Rh(111) single crystal. After the deposition, no ordered structure was revealed by low energy electron diffraction (LEED). We oxidized the obtained system in a low-pressure oxygen atmosphere at 420 K. The oxidized sample was then gradually heated to study the thermal stability of the oxide layer. We characterized the system by synchrotron radiation stimulated photoelectron spectroscopy and LEED. Valence band and core level photoelectron spectra of rhodium, tin and oxygen were used to study the oxidation of the Sn–Rh(111) surface and its behaviour upon annealing. A low stoichiometric oxide of Sn was created on the surface. The oxidation process did not continue towards creation of SnO2 with higher oxygen dose. The annealing at 970 K caused decomposition of the surface oxide of Sn and creation of an ordered (√3 × √3)R30° Sn–Rh(111) surface alloy.

Synthesis and characterization of tin disulfide hexagonal nanoflakes via solvothermal decomposition

Tin disulfide (SnS 2) hexagonal flakes with diameters in the range of 50−150 nm are synthesized by using SnCl 2.2H 2O and sodium diethyldithiocabamate as source materials via a solvothermal decomposition route. As-prepared SnS 2 hexagonal nanoflakes are characterized by X-ray diffraction (XRD), transmission electron microscopy (TEM), field emission scanning electron microscopy (FESEM), energy dispersive spectroscopy (EDS) and ultraviolet–visible (UV–vis) spectroscopy. The band gap energy of the SnS 2 nanoflakes is measured to be 2.17 eV, and the conduction band (CB) and valence band (VB) levels of the SnS 2 nanoflakes are calculated to be − 4.34 eV and − 6.55 eV respectively, showing them to be suitable for optional and electronic applications.