Showing papers on "Photoemission spectroscopy published in 2010"

Charge neutrality and band-gap tuning of epitaxial graphene on SiC by molecular doping

Abstract: Epitaxial graphene on SiC(0001) suffers from strong intrinsic $n$-type doping We demonstrate that the excess negative charge can be fully compensated by noncovalently functionalizing graphene with the strong electron-acceptor tetrafluorotetracyanoquinodimethane (F4-TCNQ) Charge neutrality can be reached in monolayer graphene as shown in electron-dispersion spectra from angular-resolved photoemission spectroscopy In bilayer graphene the band-gap that originates from the SiC/graphene interface dipole increases with increasing F4-TCNQ deposition and, as a consequence of the molecular doping, the Fermi level is shifted into the band-gap The reduction in the charge-carrier density upon molecular deposition is quantified using electronic Fermi surfaces and Raman spectroscopy The structural and electronic characteristics of the graphene/F4-TCNQ charge-transfer complex are investigated by x-ray photoelectron spectroscopy and ultraviolet photoelectron spectroscopy The doping effect on graphene is preserved in air and is temperature resistant up to $200\text{ }\ifmmode^\circ\else\textdegree\fi{}\text{C}$ Furthermore, graphene noncovalent functionalization with F4-TCNQ can be implemented not only via evaporation in ultrahigh vacuum but also by wet chemistry

STM imaging of electronic waves on the surface of Bi2Te3: topologically protected surface states and hexagonal warping effects.

Abstract: Scanning tunneling spectroscopy studies on high-quality Bi2Te3 crystals exhibit perfect correspondence to angle-resolved photoemission spectroscopy data, hence enabling identification of different regimes measured in the local density of states (LDOS). Oscillations of LDOS near a step are analyzed. Within the main part of the surface band oscillations are strongly damped, supporting the hypothesis of topological protection. At higher energies, as the surface band becomes concave, oscillations appear, dispersing with a wave vector that may result from a hexagonal warping term.

Creation and control of a two-dimensional electron liquid at the bare SrTiO3 surface

Abstract: Many-body interactions in transition-metal oxides give rise to a wide range of functional properties, such as high-temperature superconductivity, colossal magnetoresistance, or multiferroicity. The seminal recent discovery of a two-dimensional electron gas (2DEG) at the interface of the insulating oxides LaAlO3 and SrTiO3 represents an important milestone towards exploiting such properties in all-oxide devices. This conducting interface shows a number of appealing properties, including a high electron mobility, superconductivity, and large magnetoresistance and can be patterned on the few-nanometer length scale. However, the microscopic origin of the interface 2DEG is poorly understood. Here, we show that a similar 2DEG, with an electron density as large as 8x10^13 cm^-2, can be formed at the bare SrTiO3 surface. Furthermore, we find that the 2DEG density can be controlled through exposure of the surface to intense ultraviolet (UV) light. Subsequent angle-resolved photoemission spectroscopy (ARPES) measurements reveal an unusual coexistence of a light quasiparticle mass and signatures of strong many-body interactions.

The structure of graphite oxide: Investigation of its surface chemical groups

Abstract: The structure of graphite oxide (GO) has been systematically studied using various tools such as SEM, TEM, XRD, Fourier transform infrared spectroscopy (FT-IR), X-ray photoemission spectroscopy (XPS), 13C solid state NMR, and O K-edge X-ray absorption near edge structure (XANES). The TEM data reveal that GO consists of amorphous and crystalline phases. The XPS data show that some carbon atoms have sp3 orbitals and others have sp2 orbitals. The ratio of sp2 to sp3 bonded carbon atoms decreases as sample preparation times increase. The 13C solid-state NMR spectra of GO indicate the existence of -OH and -O- groups for which peaks appear at 60 and 70 ppm, respectively. FT-IR results corroborate these findings. The existence of ketone groups is also implied by FT-IR, which is verified by O K-edge XANES and 13C solid-state NMR. We propose a new model for GO based on the results; -O-, -OH, and -C=O groups are on the surface.

Tunable Band Gap in Hydrogenated Quasi-Free-Standing Graphene

Abstract: We show by angle-resolved photoemission spectroscopy that a tunable gap in quasi-free-standing monolayer graphene on Au can be induced by hydrogenation. The size of the gap can be controlled via hydrogen loading and reaches ∼1.0 eV for a hydrogen coverage of 8%. The local rehybridization from sp2 to sp3 in the chemical bonding is observed by X-ray photoelectron spectroscopy and X-ray absorption and allows for a determination of the amount of chemisorbed hydrogen. The hydrogen induced gap formation is completely reversible by annealing without damaging the graphene. Calculations of the hydrogen loading dependent core level binding energies and the spectral function of graphene are in excellent agreement with photoemission experiments. Hydrogenation of graphene gives access to tunable electronic and optical properties and thereby provides a model system to study hydrogen storage in carbon materials.

The structure of graphite oxide: investigation of its surface chemical groups.

Abstract: The structure of graphite oxide (GO) has been systematically studied using various tools such as SEM, TEM, XRD, Fourier transform infrared spectroscopy (FT-IR), X-ray photoemission spectroscopy (XPS), 13C solid-state NMR, and O K-edge X-ray absorption near edge structure (XANES). The TEM data reveal that GO consists of amorphous and crystalline phases. The XPS data show that some carbon atoms have sp3 orbitals and others have sp2 orbitals. The ratio of sp2 to sp3 bonded carbon atoms decreases as sample preparation times increase. The 13C solid-state NMR spectra of GO indicate the existence of —OH and —O— groups for which peaks appear at 60 and 70 ppm, respectively. FT-IR results corroborate these findings. The existence of ketone groups is also implied by FT-IR, which is verified by O K-edge XANES and 13C solid-state NMR. We propose a new model for GO based on the results; —O—, —OH, and —C═O groups are on the surface.

Charge-transfer-induced structural rearrangements at both sides of organic/metal interfaces

Abstract: Organic/metal interfaces control the performance of many optoelectronic organic devices, including organic light-emitting diodes or field-effect transistors. Using scanning tunnelling microscopy, low-energy electron diffraction, X-ray photoemission spectroscopy, near-edge X-ray absorption fine structure spectroscopy and density functional theory calculations, we show that electron transfer at the interface between a metal surface and the organic electron acceptor tetracyano-p-quinodimethane leads to substantial structural rearrangements on both the organic and metallic sides of the interface. These structural modifications mediate new intermolecular interactions through the creation of stress fields that could not have been predicted on the basis of gas-phase neutral tetracyano-p-quinodimethane conformation.

Oxygen vacancy origin of the surface band-gap state of TiO2(110).

Abstract: Scanning tunneling microscopy and photoemission spectroscopy have been used to determine the origin of the band-gap state in rutile TiO2(110). This state has long been attributed to oxygen vacancies (O{b} vac). However, recently an alternative origin has been suggested, namely, subsurface interstitial Ti species. Here, we use electron bombardment to vary the O{b} vac density while monitoring the band-gap state with photoemission spectroscopy. Our results show that O{b} vac make the dominant contribution to the photoemission peak and that its magnitude is directly proportional to the O{b} vac density.

Evidence for a Lifshitz transition in electron-doped iron arsenic superconductors at the onset of superconductivity

Abstract: A study shows that in electron-doped Ba(Fe1−xCox)2As2, the presence or absence of superconductivity and antiferromagnetism can be linked directly to changes in the Fermi surface—as measured by angle-resolved photoemission spectroscopy.

Energy level evolution of air and oxygen exposed molybdenum trioxide films

Abstract: The evolution of electronic energy levels of controlled air and oxygen exposed molybdenum trioxide (MoO3) films has been investigated with ultraviolet photoemission spectroscopy, inverse photoemission spectroscopy, and x-ray photoemission spectroscopy. We found that while most of the electronic levels of as deposited MoO3 films remained largely intact, the reduction in the work function (WF) was substantial. The gradual surface WF change from 6.8 to 5.3 eV was observed for air exposed film, while oxygen exposed film the surface WF saturated at ∼5.7 eV. Two distinct stages of exposure are observed, the first dominated by oxygen adsorption for 1013 L.

The formation of an energy gap in graphene on ruthenium by controlling the interface

Abstract: In this work, we have investigated the spectral function of graphene on a monolayer of intercalated gold on Ru(0001) using angle-resolved photoemission spectroscopy (ARPES). The intercalation leads to a decoupling of the graphene film, as documented by emergence of the characteristic linear π-bands near the Fermi level. However, a band gap at the band crossing is observed. We relate this gap opening to the broken symmetry of the two carbon sublattices, induced by the special lattice mismatch of the graphene and the intercalated gold monolayer.

Effects of processing conditions on the work function and energy-level alignment of NiO thin films

Abstract: We have investigated NiO thin films prepared by in situ and ex situ ozone oxidation, as well as air-exposed and vacuum-annealed NiO films. The core-level and valence-level photoemission spectra, as well as the work function and energy-level alignment with a common hole-injection material, have been measured using X-ray photoemission spectroscopy and ultraviolet photoemission spectroscopy. We found that in situ oxidation results in the formation of a purely NiO film, while ex situ oxidation and air exposure result in a hydroxide-terminated NiO film. Work functions as high as 6.7 eV can be achieved for in situ-oxidized NiO; however, the work function decreases rapidly with time due to adsorption of residual gases in vacuum. The work functions of ex situ and air-exposed NiO were significantly lower, between 5.2 and 5.6 eV, due to hydroxylation of the oxide surface. We have examined the rate at which the work function decreases with air exposure and found there to be a very rapid initial decrease in work func...

Dimensional-Crossover-Driven Metal-Insulator Transition in SrVO3 Ultrathin Films

Abstract: We have investigated the changes occurring in the electronic structure of digitally controlled SrVO(3) ultrathin films across the metal-insulator transition (MIT) by the film thickness using in situ photoemission spectroscopy. With decreasing film thickness, a pseudogap is formed at E(F) through spectral weight transfer from the coherent part to the incoherent part. The pseudogap finally evolves into an energy gap that is indicative of the MIT in a SrVO(3) ultrathin film. The observed spectral behavior is reproduced by layer dynamical-mean-field-theory calculations, and it indicates that the observed MIT is caused by the reduction in the bandwidth due to the dimensional crossover.

Energy level evolution of molybdenum trioxide interlayer between indium tin oxide and organic semiconductor

Abstract: The thickness dependance of molybdenum trioxide (MoO3) interlayer between conducting indium tin oxide (ITO) and chloro-aluminum pthalocyanine (AlPc-Cl) has been investigated with ultraviolet photoemission spectroscopy (UPS) and inverse photoemission spectroscopy. It was found that the MoO3 interlayer substantially increased the surface workfunction (WF). The increase was observed to saturate at 20 A of MoO3 coverage. The increased WF results in hole accumulation and a band-bendinglike situation in the subsequently deposited AlPc-Cl. From these observations, a possible explanation is deduced for the observed reduction in series resistance by the insertion of the MoO3 insulating layer.

Angle-resolved photoemission spectroscopy and imaging with a submicrometre probe at the SPECTROMICROSCOPY-3.2L beamline of Elettra

Abstract: The extensive upgrade of the experimental end-station of the SPECTROMICROSCOPY-3.2L beamline at Elettra synchrotron light source is reported. After the upgrade, angle-resolved photoemission spectroscopy from a submicrometre spot and scanning microscopy images monitoring the photoelectron signal inside selected acquisition angle and energy windows can be performed. As a test case, angle-resolved photoemission spectroscopy from single flakes of highly oriented pyrolitic graphite and imaging of the flakes with image contrast owing to rotation of the band dispersion of different flakes are presented.

Effect of annealing on the electronic structure of poly(3-hexylthiophene) thin film.

Abstract: The electronic structure and film structure of poly(3-hexylthiophene) (P3HT) have been studied by X-ray diffraction (XRD) measurements, ultraviolet-visible (UV-vis) absorption spectroscopy, near-edge X-ray absorption fine structure (NEXAFS) measurements, ultraviolet photoemission spectroscopy (UPS) and inverse photoemission spectroscopy (IPES). As reported in previous works, XRD results show that the crystallinity of the film with regioregular P3HT is significantly improved by annealing at 170 °C. The effects of annealing on the electronic structure strongly depend on the substrate and the degree of regioregularity of the P3HT polymer backbone. Even in the case of the regiorandom sample, annealing considerably changes the vacuum level energy, which is the result of changes in the conformation of the hexyl groups at the free surface of the film. The π- and π*-band onsets uniformly shift downward by the annealing resulting in an increased hole-injection barrier at the electrode interface. The effects of annealing on the electronic structure of regioregular samples are more complex and depend on multiple factors. It is necessary to take into account variations in the π- and π*-band widths and the polarization energy to determine the effects of annealing. The former is associated with the conformation of the backbones of the polymer chains, and the latter is associated with the packing density of the conjugated polymer planes. The combination of these variations determines the effects of annealing on the electronic structure of the regioregular film. This is a possible reason for the strong dependence of the effects of annealing on the surface roughness of the substrate, since substrate roughness has a considerable effect on the morphology and crystallinity of regioregular P3HT films.

Electronic structure of In 2 O 3 and Sn-doped In 2 O 3 by hard x-ray photoemission spectroscopy

Abstract: The valence and core levels of In(2)O(3) and Sn-doped In(2)O(3) have been studied by hard x-ray photoemission spectroscopy (hv = 6000 eV) and by conventional Al K alpha (hv = 1486.6 eV) x-ray photoemission spectroscopy. The experimental spectra are compared with density-functional theory calculations. It is shown that structure deriving from electronic levels with significant In or Sn 5s character is selectively enhanced under 6000 eV excitation. This allows us to infer that conduction band states in Sn-doped samples and states at the bottom of the valence band both contain a pronounced In 5s contribution. The In 3d core line measured at hv = 1486.6 eV for both undoped and Sn-doped In(2)O(3) display an asymmetric lineshape, and may be fitted with two components associated with screened and unscreened final states. The In 3d core line spectra excited at hv = 6000 eV for the Sn-doped samples display pronounced shoulders and demand a fit with two components. The In 3d core line spectrum for the undoped sample can also be fitted with two components, although the relative intensity of the component associated with the screened final state is low, compared to excitation at 1486.6 eV. These results are consistent with a high concentration of carriers confined close to the surface of nominally undoped In(2)O(3). This conclusion is in accord with the fact that a conduction band feature observed for undoped In(2)O(3) in Al K alpha x-ray photoemission is much weaker than expected in hard x-ray photoemission.

Complete band offset characterization of the HfO2/SiO2/Si stack using charge corrected x-ray photoelectron spectroscopy

Abstract: The HfO2–Si valence and conduction band offsets (VBO and CBO, respectively) of technologically relevant HfO2/SiO2/Si film stacks have been measured by several methods, with several groups reporting values within a range of ∼1 eV for both quantities. In this study we have used a combination of x-ray photoemission spectroscopy (XPS) and spectroscopic ellipsometry to measure the HfO2–Si VBO and CBO of both as-deposited and annealed stacks. Unlike previous XPS based measurements of the HfO2–Si VBO, we have corrected for the effect of charging in the XPS measurement. We find that after correction for charging, the HfO2–Si VBOs are decreased from their typical XPS-measured values, and agree better with values measured by UV photoemission spectroscopy and internal photoemission. We also report values for the rarely reported HfO2–SiO2 and SiO2–Si VBOs and CBOs in HfO2/SiO2/Si stacks. In addition to the band offsets, XPS was used to measure the band bending in the Si substrate of HfO2/SiO2/Si film stacks. Unannealed HfO2 stacks showed downward Si band bending of 0.4–0.5 eV, while annealed HfO2 stacks showed negligible band bending. Finally, we investigated the composition of the SiO2 layer in SiO2/Si and HfO2/SiO2/Si. By decomposing the Si 2p spectra into the spin orbit partner lines of its five oxidation states we observed that the growth of the HfO2 films resulted in the growth of the SiO2 underlayer and an increase by a factor of ∼2.3 in the density of suboxide species of SiO2. Based on the relatively high binding energy of the Si 2p4+ level with respect to the Si 2p0 level and a survey of results from literature, we conclude that the SiO2 layer in the HfO2/SiO2/Si samples we measured does not undergo significant intermixing with HfO2.

Quasifreestanding single-layer hexagonal boron nitride as a substrate for graphene synthesis

Abstract: We demonstrate that freeing a single-atom thick layer of hexagonal boron nitride (h-BN) from tight chemical bonding to a Ni(111) thin film grown on a W(110) substrate can be achieved by intercalation of Au atoms into the interface. This process has been systematically investigated using angle-resolved photoemission spectroscopy, x-ray photoemission, and absorption techniques. It has been demonstrated that the transition of the h-BN layer from the "rigid" into the "quasifreestanding" state is accompanied by a change in its lattice constant. Using chemical vapor deposition, graphene has been successfully synthesized on the insulating, quasifreestanding h-BN monolayer. We anticipate that the in situ synthesized weakly interacting graphene/h-BN double layered system could be further developed for technological applications and may provide perspectives for further inquiry into the unusual electronic properties of graphene. (Less)

Bulk transport measurements in ZnO: the effect of surface electron layers

Abstract: Magnetotransport measurements and x-ray photoemission spectroscopy were used to investigate the surface conductivity of ZnO Near-surface downward band bending, consistent with electron accumulation, was found on the polar and nonpolar faces of bulk ZnO single crystals A significant polarity effect was observed in that the downward band bending was consistently stronger on the Zn-polar face and weaker on the O-polar face The surface electron accumulation layer was found to significantly influence the electrical properties of high resistivity, hydrothermally grown bulk ZnO crystals at temperatures below 200 K, and is largely responsible for the anomalously low electron mobility reported for this material

Improved photoinduced charge carriers separation in organic-inorganic hybrid photovoltaic devices

Abstract: We demonstrate enhanced performance of a hybrid photovoltaic device, where poly[3-hexylthiophene] (P3HT) is used as active material and a solution-processed thin flat film of ZnO modified by a self-assembled monolayer (SAM) of phenyl-C61-butyric acid (PCBA) is used as electron extracting electrode. Ultraviolet photoemission spectroscopy measurements reveal an increase in the substrate work function from 3.6 to 4.1 eV upon PCBA SAM deposition due to an interfacial dipole pointing away from the ZnO. External quantum efficiency (EQE) of the SAM modified devices reached 9%, greatly improved over the 3% EQE of the unmodified devices. This corresponds to full charge separation of all photoexcitations generated in the P3HT within an exciton diffusion range from the interface.

Relating energy level alignment and amine-linked single molecule junction conductance.

Abstract: Using photoemission spectroscopy, we determine the relationship between electronic energy level alignment at a metal−molecule interface and single-molecule junction transport data. We measure the position of the highest occupied molecular orbital (HOMO) relative to the Au metal Fermi level for three 1,4-benzenediamine derivatives on Au(111) and Au(110) with ultraviolet and resonant X-ray photoemission spectroscopy. We compare these results to scanning tunnelling microscope-based break-junction measurements of single molecule conductance and to first-principles calculations. We find that the energy difference between the HOMO and Fermi level for the three molecules adsorbed on Au(111) correlate well with changes in conductance and agree well with quasiparticle energies computed from first-principles calculations incorporating self-energy corrections. On the Au(110) that presents Au atoms with lower-coordination, critical in break-junction conductance measurements, we see that the HOMO level shifts further ...

Electronic structure of Fe1.04Te0.66Se0.34

Abstract: We report the electronic structure of the iron-chalcogenide superconductor, Fe-1.04(Te0.66Se0.34), obtained with high-resolution angle-resolved photoemission spectroscopy and density-functional calculations. In photoemission measurements, various photon e

Water Adsorption on ZnO(0001): Transition from Triangular Surface Structures to a Disordered Hydroxyl Terminated phase

Abstract: We present room temperature scanning tunneling microscopy and photoemission spectroscopy studies of water adsorption on the Zn-terminated ZnO(0001) surface. Data indicates that the initial adsorpti ...

Modulating the electronic structures of graphene by controllable hydrogenation

Abstract: The evolution of electronic structures of hydrogenated graphene with different amount of hydrogen (H) coverage was investigated by ultraviolet photoemission spectroscopy and optical absorption spectroscopy. Raman spectroscopy and x-ray photoelectron spectroscopy were used to monitor and evaluate the H coverage. At low H coverage, the sp3 CH bonds embedded within a sp2 C matrix behave as defects in graphene and depress the delocalized π electron system. At high H coverage, two localized π electron states originating from the sp2 C clusters encircled by the sp3 CH matrix appear in the electronic band structures, and an opening of a band gap has been observed.

Raman spectroscopy and field emission characterization of delafossite CuFeO2

Abstract: Delafossite p-type CuFeO2 (CFO) semiconductors were synthesized by a modified solid state reaction technique and investigated by x-ray diffraction, x-ray photoemission spectroscopy (XPS), energy dispersive x-ray spectroscopy, and scanning electron microscopy, revealing the single-phase nature of CFO with 1:1 Cu/Fe atomic ratio. The valance states of CFO were examined by XPS and suggest Cu and Fe ions are in +1 and +3 valance states with high spin S=5/2. The “turn-on field” which is the macroscopic field needed to get an emission current of 9 nA, was calculated as 5.72 V/μm. Room temperature Raman spectra of CFO displayed two main Raman active modes at Eg∼351 cm−1 and Ag∼692 cm−1 in accord with other delafossite structures. Temperature dependent Raman spectra showed that both the modes shifted to lower frequency with significant decrease in intensity with increase in temperature. Frequency shift and linewidth of both phonon lines matched well with the theoretical damped harmonic oscillator model based on t...

Photoemission Spectroscopy of Tethered CdSe Nanocrystals: Shifts in Ionization Potential and Local Vacuum Level As a Function of Nanocrystal Capping Ligand

Abstract: We report the characterization of the frontier orbital energies and interface dipole effects for bare and ligand-capped 3.6 and 6.0 nm diameter CdSe nanocrystals (NC) tethered to smooth gold substrates, using He(I) and He(II) UV photoemission spectroscopy. Changes in the ionization potential (IP) of the NCs and local effective work function of the films were explored as a function of the dipolar nature of the NC capping ligands. The addition of thiol-capping ligands 1-hexanethiol, 1-benzenethiol, and 4-fluorothiophenol to both sizes of NCs produces negligible shifts in energy offset between the high kinetic energy edge of the CdSe NCs and the gold substrate Fermi energy. However, the local vacuum level and IP of the nanocrystal layer are altered by as much as 0.3 eV. We demonstrate the importance of determining both the local vacuum level and the high kinetic energy edge of a tethered NC sample. These studies demonstrate a method that can be used in the future to characterize the frontier orbital energy o...