Ultraviolet Photoemission Spectroscopy Research Articles

Damaging Effect of Hot Metal Atoms on Organic Semiconducting Films during Top Contact Formation

The formation of a high quality interface between metallic and organic semiconducting thin films is critically important in achieving high-performance organic electronics devices. In this regard, the importance of understanding the multifaceted issue of structure damage incurred to organic films by the evaporated metal atoms cannot be overstated. In the present study, we have investigated the change of a structurally ordered, organic semiconducting (o.s.) thin film of 5,11-bis(triethylsilylethynyl)anthradithiophene (TESADT) effected by gold atoms by means of synchrotron-based soft X-ray spectroscopies including ultraviolet photoemission spectroscopy (UPS) and X-ray photoemission spectroscopies (XPS) with imaging capability, near edge X-ray absorption fine structure (NEXAFS) spectroscopy, and atomic force microscopy (AFM). This work shows that gold atoms readily diffuse into the organic films and nucleate into nanometer-size clusters, damage chemical structure, destroy structural ordering of the organic fi...

Electronic structure and transport properties of CeNi9In2

We investigated CeNi9In2 compound, which has been considered as a mixed valence (MV) system. Electrical resistivity vs. temperature variation was analyzed in terms of the model proposed by Freimuth for systems with unstable 4f shell. At low temperature the resistivity dependence is consistent with a Fermi liquid state with a contribution characteristic of electron–phonon interaction. Ultraviolet photoemission spectroscopy (UPS) studies of the valence band did not reveal a Kondo peak down to 14K. A difference of the spectra obtained with photon energies of low and high photoionization cross sections for Ce 4f electrons indicated that 4f states are located mainly close to the Fermi energy. The peaks related to f5/21 and f7/21 final states cannot be resolved but form a plateau between −0.3eV and the Fermi energy. X-ray photoemission spectroscopy (XPS) studies were realized for the cerium 3d level. The analysis of XPS spectra within the Gunnarsson-Schönhammer theory yielded a hybridization parameter of 104meV and non-integer f level occupation, being close to 3. Calculations of partial densities of states were realized by a full potential local orbital (FPLO) method. They confirm that the valence band is dominated by Ni 3d states and are in general agreement with the experiment except for the behavior of f-electrons.

Electronic and structural properties at the interface between CuPc and graphene

The electronic and structural properties at Copper phthalocyanine (CuPc)/graphene have been studied using ultraviolet photoemission spectroscopy and first-principles density function theory calculation. The five emission features α, β, γ, δ, and ε originating from the CuPc molecules locate at 1.48, 3.66, 4.98, 6.90, and 9.04 eV, respectively. These features shift in binding energy with the increasing CuPc coverage. The feature α is mostly deriving from Cu 3d orbital with some contributions from C 2p orbital. Further theoretical calculation indicates that the adsorption of CuPc on a top site is the most favorable configuration, and the separation between the adsorbate and graphene is about 3.47 Å. According to the density of states before and after CuPc adsorption, the LUMO of CuPc is slightly occupied, while the Dirac point of graphene slightly shift towards higher energy, suggesting that a small amount of electron transfer from graphene to CuPc upon contact.

Alcohol/water-soluble porphyrins as cathode interlayers in high-performance polymer solar cells

Three alcohol/water-soluble porphyrins, Zn-TPyPMeI: zinc(II) meso -tetra( N -methyl-4-pyridyl) porphyrin tetra-iodide, Zn- TPyPAdBr: zinc(II) meso -tetra[1-(1-adamantylmethyl ketone)-4-pyridyl] porphyrin tetra-bromide and MnCl-TPyPAdBr: man- ganese(III) meso -tetra[1-(1-adamantylmethyl ketone)-4-pyridyl] porphyrin tetra-bromide were employed as cathode interlayers to fabricate polymer solar cells (PSCs). The PC71BM ([6,6]-phenyl C71 butyric acid methyl ester) and PCDTBT (poly[ N -9"- hepta-decanyl-2,7-carbazole- alt -5,5-(4,7-di-2-thienyl-2,1,3-benzothiadiazole)])-blend films were used as active layers in polymer solar cells (PSCs). The PSCs with alcohol/water-soluble porphyrins interlayer showed obviously higher power conversion efficiency (PCE) than those without interlayers. The highest PCE, 6.86%, was achieved for the device with MnCl- TPyPAdBr as an interlayer. Ultraviolet photoemission spectroscopic (UPS), carrier mobility, atomic force microscopy (AFM) and contact angle ( θ ) characterizations demonstrated that the porphyrin molecules can result in the formation of interfacial dipole layer between active layer and cathode. The interfacial dipole layer can obviously improve the open-circuit voltage ( V oc) and charge extraction, and sequentially lead to the increase of PCE.

2,7-二辛基[1]苯并噻吩并\[3,2-b\]苯并噻吩(C8-BTBT)与MoS2界面的能级匹配与薄膜生长

结合紫外光电子能谱(UPS)、X射线光电子能谱(XPS)和原子力显微镜(AFM)等实验手段系统研究了C8-BTBT沉积在层状Mo S2基底上的界面能级匹配、薄膜生长和分子取向。研究发现C8-BTBT分子竖直生长在Mo S2上,生长过程中界面的真空能级(VL)、最高占据态轨道(HOMO)和电离能(IP)都出现了非常规的弯曲现象。这种能级弯曲行为可归因于直立分子从界面相到体相的转变过程中,其分子倾斜角(θ)存在一定的渐变,这种渐变会在沿表面法线方向诱导出一系列的层间电偶极,最终导致能级的弯曲。同时θ的变化也会改变薄膜的表面极化强度,引起IP的逐渐减小。能级的弯曲在界面处形成类P-N结的效应会对C8-BTBT基电子器件的性能有很大的影响。

Evolution of the surface state in Bi2Se2Te thin films during phase transition.

Topological insulators, a new quantum state of matter, have created exciting opportunities for studies in topological quantum physics and for exploring spintronics applications due to their gapless helical metallic surface states. In this study, thin films composed of alternate layers of Bi and Se (Te) ({Bi(3 Å)Te(9 Å)}n/{Bi(3 Å)Se(9 Å)}n) were fabricated by controlling the layer thickness within the atomic scale using thermal evaporation techniques. The high-purity growth of uniform Bi2Se2Te1 thin films has not yet been achieved using a thermal evaporation method. However, as a result of a self-ordering process during annealing, an as-grown amorphous film with p-type polarity could transform into single crystalline Bi2Se2Te1 with n-type polarity. Using THz-time domain spectroscopy (THz-TDS) and ultraviolet photoemission spectroscopy (UPS), we concluded that the conductivity is dominated by the Drude contribution, suggesting the presence of a quantum well state and surface states. Moreover we demonstrated that the emission of terahertz waves from the (001) surface of the single crystalline Bi2Se2Te1 thin film would be possible under the excitation of 790 nm femtosecond optical pulses, indicating the presence of a Dirac-fermion, a photo-Dember effect at the surface state and the transient current within the surface depletion region. The results reported herein provide useful information regarding a valuable deposition method that can be useful in studies of the evolution of surface state electrons in topological insulators.

Decoupling the Effects of Self‐Assembled Monolayers on Gold, Silver, and Copper Organic Transistor Contacts

In bottom‐contact organic field‐effect transistors (OFETs), the functionalization of source/drain electrodes leads to a tailored surface chemistry for film growth and controlled interface energetics for charge injection. This report describes a comprehensive investigation into separating and correlating the energetic and morphological effects of a self‐assembled monolayers (SAMs) treatment on Au, Ag, and Cu electrodes. Fluorinated 5,11‐bis(triethylsilylethynyl) anthradithiophene (diF‐TES‐ADT) and pentafluorobenzenethiol (PFBT) are employed as a soluble small‐molecule semiconductor and a SAM material, respectively. Upon SAM modification, the Cu electrode devices benefit from a particularly dramatic performance improvement, closely approaching the performance of OFETs with PFBT‐Au and PFBT‐Ag. Ultraviolet photoemission spectroscopy, polarized optical microscopy, grazing‐incidence wide‐angle X‐ray scattering elucidate the metal work function change and templated crystal growth with high crystallinity resulting from SAMs. The transmission‐line method separates the channel and contact properties from the measured OFET current–voltage data, which conclusively describes the impact of the SAMs on charge injection and transport behavior.

Alternating current driven organic light emitting diodes using lithium fluoride insulating layers.

We demonstrate an alternating current (AC)-driven organic light emitting diodes (OLED) with lithium fluoride (LiF) insulating layers fabricated using simple thermal evaporation. Thermal evaporated LiF provides high stability and excellent capacitance for insulating layers in AC devices. The device requires a relatively low turn-on voltage of 7.1 V with maximum luminance of 87 cd/m2 obtained at 10 kHz and 15 Vrms. Ultraviolet photoemission spectroscopy and inverse photoemission spectroscopy are employed simultaneously to examine the electronic band structure of the materials in AC-driven OLED and to elucidate the operating mechanism, optical properties and electrical characteristics. The time-resolved luminance is also used to verify the device performance when driven by AC voltage.

Quasiparticle interfacial level alignment of highly hybridized frontier levels: H2O on TiO2(110).

Knowledge of the frontier levels' alignment prior to photoirradiation is necessary to achieve a complete quantitative description of H2O photocatalysis on TiO2(110). Although H2O on rutile TiO2(110) has been thoroughly studied both experimentally and theoretically, a quantitative value for the energy of the highest H2O occupied levels is still lacking. For experiment, this is due to the H2O levels being obscured by hybridization with TiO2(110) levels in the difference spectra obtained via ultraviolet photoemission spectroscopy (UPS). For theory, this is due to inherent difficulties in properly describing many-body effects at the H2O-TiO2(110) interface. Using the projected density of states (DOS) from state-of-the-art quasiparticle (QP) G0W0, we disentangle the adsorbate and surface contributions to the complex UPS spectra of H2O on TiO2(110). We perform this separation as a function of H2O coverage and dissociation on stoichiometric and reduced surfaces. Due to hybridization with the TiO2(110) surface, the H2O 3a1 and 1b1 levels are broadened into several peaks between 5 and 1 eV below the TiO2(110) valence band maximum (VBM). These peaks have both intermolecular and interfacial bonding and antibonding character. We find the highest occupied levels of H2O adsorbed intact and dissociated on stoichiometric TiO2(110) are 1.1 and 0.9 eV below the VBM. We also find a similar energy of 1.1 eV for the highest occupied levels of H2O when adsorbed dissociatively on a bridging O vacancy of the reduced surface. In both cases, these energies are significantly higher (by 0.6 to 2.6 eV) than those estimated from UPS difference spectra, which are inconclusive in this energy region. Finally, we apply self-consistent QPGW (scQPGW1) to obtain the ionization potential of the H2O-TiO2(110) interface.

Interface properties of FeMgOCu-phthalocyanine metal-insulator-organic semiconductor structures

Hybrid interface structures consisting of organic copper-phthalocyanine (CuPc) and ferromagnetic metal Fe(001) with and without a MgO(001) cover were investigated by using surface sensitive techniques of X-ray photoemission spectroscopy and ultraviolet photoemission spectroscopy. A systematic study of the energy level alignment at the interfaces was carried out. For the hybrid interfaces considered here, our results indicate that the insertion of an artificially-grown ultra-thin oxide layer MgO(001) can prevent Femi level pinning and induce a rather large interface dipole, thereby resulting in remarkable CuPc Fermi level shifts when the thickness of the CuPc film is less than 3 nm. This study provides a better understanding of spin filtering in MgO-based organic spin devices and a new way to alter the interface electronic structure of metalorganic semiconductor hybrid systems.

Tuning electronic properties of carbon nanotubes by nitrogen grafting: Chemistry and chemical stability

Plasma-based methods were used to graft nitrogen atoms to the hexagonal lattice of vertically aligned carbon nanotubes (v-CNTs). The nitrogen grafting (as pyridinic, pyrrolic and graphitic) was mediated by the creation of defects induced by energetic species present in the nitrogen plasma. We investigated the effect of adding nitrogen atoms via plasma treatment on the electronic properties of both v-CNT tips and sidewalls using ultraviolet and X-ray photoemission spectroscopy and spectromicroscopy. Site selective nitrogen grafting near the tips, up to a depth of 4μm, was evaluated, beyond which the properties of the v-CNTs remain unperturbed. The N 1s XPS spectra recorded on the v-CNT tips showed three components related to nitrogen grafted as pyridinic, pyrrolic or graphitic. During thermal heating, we observed variations in the intensity ratio of these components due to the different thermal stability of the nitrogen grafting configurations; the most stable were the sp2 pyridinic and graphitic nitrogen. The area ratio variation of these components was accompanied by a change in the density of states at the Fermi energy level, thus suggesting that the nitrogen functionalization strategy employed can be used to activate the v-CNT tips allowing the tuning of electronic properties by controlling the grafting of different nitrogen species.

Study of the oxygen vacancies changes in SnO2 polycrystalline thick films using impedance and photoemission spectroscopies

Changes in the concentration of oxygen vacancies within the grains of polycrystalline SnO2, due to different atmosphere exposures, were detected using impedance and photoemission spectroscopies. From measured capacitance values, variations of the potential barrier widths could be determined. It is shown that under the presence of an oxygen rich atmosphere, at relatively low temperature, the width of intergranular potential barriers increase to the point that grains become completely depleted of carriers. With subsequent exposure to vacuum, capacitance adopts a higher value, indicative of intergranular barriers and quasi-neutral regions at the center of the grains. X-ray and ultraviolet photoemission spectroscopy measurements showed that SnO2 samples treated in oxidizing or reducing environments have similar barrier heights and different work functions. Results are especially relevant in the study of mechanisms responsible for metal oxide gas sensing.

Energy barrier at the N719-dye/CsSnI₃ interface for photogenerated holes in dye-sensitized solar cells.

This report is to address the question if black γ-polymorph of cesium tin tri-iodide (B-γ-CsSnI3) can be used as a solid-state hole-transport material in the conventional DSSCs with the N719 dye to replace the liquid electrolyte as reported by I. Chung et al. on Nature 485, 486, (2012). Here we demonstrate rigorously that B-γ-CsSnI3 is not energetically possible to collect photogenerated holes because of the large energy barrier at the interface of N719/B-γ-CsSnI3. Therefore, it cannot serve as a hole-transporter for the conventional DSSCs although it is a good hole-conducting material. A solution-based method was employed to synthesize the B-γ-CsSnI3 polycrystalline thin-films used for this work. These thin-films were then characterized by X-ray diffraction, Hall measurements, optical reflection, and photoluminescence (PL). Particularly, spatially resolved PL intensity images were taken after B-γ-CsSnI3 was incorporated in the DSSC structure to insure the material integrity. The means of ultraviolet photoemission spectroscopy (UPS) was used to reveal why B-γ-CsSnI3 could not act as the substitute of liquid electrolyte in the conventional DSSCs. For the completeness, other two related compounds, one is the yellow polymorph of CsSnI3 and other is Cs2SnI6 with tetravalent tin instead of double-valent tin in CsSnI3 were also investigated by UPS.

Protection of MoO3 high work function by organic thin film

The effects of air exposure are investigated for molybdenum trioxide (MoO3) covered with organic thin films using ultraviolet photoemission spectroscopy. It is found that the severe drop of the work function of MoO3 by air exposure is substantially reduced by the organic thin films. Both CuPc and C60 are used for the investigations. The results indicate that the MoO3 surface can be passivated by approximately two monolayers of organic thin films against exposure to air.

Direct comparative study on the energy level alignments in unoccupied/occupied states of organic semiconductor/electrode interface by constructing in-situ photoemission spectroscopy and Ar gas cluster ion beam sputtering integrated analysis system

Through the installation of electron gun and photon detector, an in-situ photoemission and damage-free sputtering integrated analysis system is completely constructed. Therefore, this system enables to accurately characterize the energy level alignments including unoccupied/occupied molecular orbital (LUMO/HOMO) levels at interface region of organic semiconductor/electrode according to depth position. Based on Ultraviolet Photoemission Spectroscopy (UPS), Inverse Photoemission Spectroscopy (IPES), and reflective electron energy loss spectroscopy, the occupied/unoccupied state of in-situ deposited Tris[4-(carbazol-9-yl)phenyl]amine (TCTA) organic semiconductors on Au (ELUMO: 2.51 eV and EHOMO: 1.35 eV) and Ti (ELUMO: 2.19 eV and EHOMO: 1.69 eV) electrodes are investigated, and the variation of energy level alignments according to work function of electrode (Au: 4.81 eV and Ti: 4.19 eV) is clearly verified. Subsequently, under the same analysis condition, the unoccupied/occupied states at bulk region of TCTA/Au structures are characterized using different Ar gas cluster ion beam (Ar GCIB) and Ar ion sputtering processes, respectively. While the Ar ion sputtering process critically distorts both occupied and unoccupied states in UPS/IPES spectra, the Ar GCIB sputtering process does not give rise to damage on them. Therefore, we clearly confirm that the in-situ photoemission spectroscopy in combination with Ar GCIB sputtering allows of investigating accurate energy level alignments at bulk/interface region as well as surface region of organic semiconductor/electrode structure.

An XPS and UPS Study on the Electronic Structure of ThOx (x ≤ 2) Thin Films

Model systems are needed for surface corrosion studies of spent nuclear oxide fuels. For this purpose ThO2 films have been prepared in situ by adsorption of molecular and atomic oxygen on Th metal films and by sputter deposition of Th metal in an Ar/O2 gas mixture. Surface compositions and electronic structure were compared to the bulk oxide and oxygen substoichiometry effects investigated. X-ray and ultraviolet photoemission spectroscopy (XPS and UPS, respectively) were used to measure to Th-4f, O-1s core levels and the valence band region. The Th-4f line was analyzed in terms of the final-state screening model. The evolution of the binding energies with oxygen concentration has been studied. On Th metal, adsorption of molecular oxygen ceased after the formation of a ThO2 surface layer. In the presence of atomic oxygen, the oxidation proceeded into the underlying bulk. The formation of oxygen interstitials was shown by the broadening of the O-2p and O-1s lines and by the increase of the O-1s/Th-4f ratio. Once ThO2 is formed, all photoemission peaks from Th and O undergo a rigid shift to low binding energy.

Electronic Structure of the SrTiO3(001) Surfaces: Effects of the Oxygen Vacancy and Hydrogen Adsorption

The influence of electron irradiation and hydrogen adsorption on the electronic structure of the SrTiO3 (001) surface was investigated by ultraviolet photoemission spectroscopy (UPS). Upon electron irradiation of the surface, UPS revealed an electronic state within the band gap (in-gap state: IGS) with the surface kept at 1×1. This is considered to originate from oxygen vacancies at the topmost surface formed by electron-stimulated desorption of oxygen. Electron irradiation also caused a downward shift of the valence band maximum indicating downward band-bending and formation of a conductive layer on the surface. With oxygen dosage on the electron-irradiated surface, on the other hand, the IGS intensity was decreased along with upward band-bending, which points to disappearance of the conductive layer. The results indicate that electron irradiation and oxygen dosage allow us to control the surface electronic structure between semiconducting (nearly-vacancy free: NVF) and metallic (oxygen de cient: OD) regimes by changing the density of the oxygen vacancy. When the NVF surface was exposed to atomic hydrogen, in-gap states were induced along with downward band bending. The hydrogen saturation coverage was evaluated to be 3.1±0.8×10 14 cm ?2 with nuclear reaction analysis. From the IGS intensity and H coverage, we argue that H is positively charged as H ~0:3+ on the NVF surface. On the OD surface, on the other hand, the IGS intensity due to oxygen vacancies was found to decrease to half the initial value with molecular hydrogen dosage. H is expected to be negatively charged as H? on the OD surface by occupying the oxygen vacancy site.

Characterization of electronic structure of Cu2ZnSn(SxSe1 − x)4 absorber layer and CdS/Cu2ZnSn(SxSe1 − x)4 interfaces by in-situ photoemission and inverse photoemission spectroscopies

The dependences of electronic structure of CZTSxSe1−x (CZTSSe) layers synthesized by sulfurization and/or selenization of the vacuum-deposited metal precursors on the anion mixing ratio x=S/(S+Se) have been studied by in-situ ultraviolet and X-ray photoemission spectroscopies (UPS, XPS) and inverse photoemission spectroscopy (IPES). The band alignment at interfaces between the CdS buffer by the sequential evaporation and the CZTSSe (x=0.28 and 1.0) has also been investigated by the in-situ measurements of these spectroscopies. The UPS/IPES results of the CZTSSe surfaces have revealed linear expansion of band gap energy Eg with an increase of x: Eg(CZTSe; x=0)=0.9–1.0eV and Eg(CZTS; x=1)=1.5–1.6eV. This expansion mainly originates in the rise of conduction band minimum CBM: CBM(CZTSe; x=0)=0.45–0.50eV and CBM(CZTS; x=1)=0.95–1.05eV. The in-situ measurements of the interface electronic structure have revealed that the CdS/CZTSSe (x=0.28) interface has a so-called “type I” band alignment with a conduction band offset CBO about +0.2eV which is favorable to high cell performance. A negative CBO was distinguished for the CdS/CZTS (x=1.0) interface, and the observed change in the band alignment with the anion mixing ratio was consistent with that of the variation in cell-performances.

Bridging donor–acceptor energy offset using organic dopants as energy ladders to improve open-circuit voltages in bulk-heterojunction solar cells

A systematic way to improve the open circuit voltage of bulk-heterojunction (BHJ) organic photovoltaics is demonstrated by incorporating energy ladder material to form a ternary blend with cascade energy structures. We propose that doping organic molecules with appropriate energetic alignment to bridge the donor–acceptor energy offset can facilitate exciton dissociation and hence improve open-circuit voltage (Voc) and power conversion efficiency. The cascade energy alignment structure is measured via ultraviolet photoemission spectroscopy. The Voc and power conversion efficiency of devices using the proposed schemes can be enhanced by 10% and 20%, respectively. The results provide a new path toward higher power conversion efficiency in BHJ systems by engineering the energy structures of the materials in the active layers.

Induced work function changes at Mg-doped MgO/Ag(001) interfaces: Combined Auger electron diffraction and density functional study

The properties of MgO/Ag(001) ultrathin films with substitutional Mg atoms in the interface metal layer have been investigated by means of Auger electron diffraction experiments, ultraviolet photoemission spectroscopy, and density functional theory (DFT) calculations. Exploiting the layer-by-layer resolution of the Mg KL_23 L_23 Auger spectra and using multiple scattering calculations, we first determine the interlayer distances as well as the morphological parameters of the MgO/Ag(001) system with and without Mg atoms incorporated at the interface. We find that the Mg atoms incorporation drives a strong distortion of the interface layers and that its impact on the metal/oxide electronic structure is an important reduction of the work function (0.5 eV) related to band-offset variations at the interface. These experimental observations are in very good agreement with our DFT calculations which reproduce the induced lattice distortion and which reveal (through a Bader analysis) that the increase of the interface Mg concentration results in an electron transfer from Mg to Ag atoms of the metallic interface layer. Although the local lattice distortion appears as a consequence of the attractive (repulsive) Coulomb interaction between O2- ions of the MgO interface layer and the nearest positively (negatively) charged Mg (Ag) neighbors of the metallic interface layer, its effect on the work function reduction is only limited. Finally, an analysis of the induced work function changes in terms of charge transfer, rumpling, and electrostatic compression contributions is attempted and reveals that the metal/oxide work function changes induced by interface Mg atoms incorporation are essentially driven by the increase of the electrostatic compression effect.