Angle-resolved Ultraviolet Photoelectron Research Articles

Low temperature epitaxial graphene on 3C-SiC surface induced by carbon evaporation

With the carbon bombardment, the procedure of producing graphene on SiC surfaces is modified. The carbon evaporation influences the dynamical growth of graphene on the hexagonal SiC. The epitaxial graphene induced by carbon evaporation is obtained at lower annealing temperatures of about 950 oC in case of SiC(0001). By this method, we were able to construct a monolayer graphene which is confirmed by low energy electron diffraction (LEED) and angle resolved ultraviolet photoelectron spectroscopy (ARUPS). On the 3C-SiC surface, zero-layer graphene is obtained at annealing temperatures of about 1000 oC as this evident by X-ray photoelectron spectroscopy (XPS) and LEED. Similarly, carbon evaporation on the 3C-SiC surface can be applied in order to get graphene at reduced temperatures.

An alternative route towards the fabrication of 2D blue phosphorene

Blue phosphorene (BlueP) is a novel two-dimensional material that shares properties with black phosphorene and is potentially even more interesting for opto-electronic applications because of its layer dependent wide band gap of ≈ 2 to 3 eV and superior charge carrier mobility. It was first fabricated on Au(111), where, however, a network consisting of BlueP subunits and Au-linker atoms is formed. The physical properties of such an arrangement strongly differ from a freestanding BlueP monolayer. Here, we report on the growth of epitaxial BlueP on the Au(100) surface, which is an interesting alternative when aiming at quasi-freestanding BlueP domains. We find two different phosphorus phases by means of scanning tunneling microscopy and distortion-corrected low-energy electron diffraction. In the low coverage regime, we observe a commensurate (2 × 2) phase, whereas for higher coverage, a nearly hexagonal structure is formed. For the latter, the lattice parameters measured via atomically resolved scanning tunneling hydrogen microscopy closely resemble those of freestanding BlueP, and the typical height modulation of the phosphorus atoms is verified in our layers by means of x-ray photoelectron diffraction. We further analyze the chemical and electronic properties of these films by means of x-ray and (angle resolved) ultraviolet photoelectron spectroscopy.

The adsorption geometry and molecular self-assembly of graphene for 1,3,5-triphenylbenzene on Cu(111)

Electronic structures and adsorption geometry of 1,3,5-triphenylbenzene (TPB) on the Cu(111) surface, and followed by formation of graphene nano flakes (GNFs) upon annealing the sample were investigated by using angle resolved ultraviolet photoelectron spectroscopy (ARUPS) and density functional theory (DFT) calculations. The emission features d, e and f originated from TPB molecule at a coverage of 1.3 monolayer (ML) in UPS spectra are located at 4.65 eV, 6.76 eV and 8.57 eV below Fermi level, and the decrease of the work function upon increasing of TPB coverage on Cu(111) is attributed to the interfacial dipole formed between the adsorbate and the substrate. Formation of GNFs upon annealing of TPB overlayer on Cu(111) is approved by appearance of new features g, h, i, j and k originating from GNFs, and located at 1.09, 4.97, 6.64, 9.23 and 11.23 eV below Fermi level. The ARUPS measurements and DFT calculations indicate that both TPB molecules and GNFs planes are nearly parallel to the Cu(111) substrate in the monolayer regime. Most importantly, the results show that, with decreasing of the coverage of TPB as precursor, the formation of GNFs, or graphitization temperature can be decreased down to 350 °C, indicating the crucial influence of intermolecular interaction on the dehydrogenation process.

Hole-phonon coupling effect on the band dispersion of organic molecular semiconductors

The dynamic interaction between the traveling charges and the molecular vibrations is critical for the charge transport in organic semiconductors. However, a direct evidence of the expected impact of the charge-phonon coupling on the band dispersion of organic semiconductors is yet to be provided. Here, we report on the electronic properties of rubrene single crystal as investigated by angle resolved ultraviolet photoelectron spectroscopy. A gap opening and kink-like features in the rubrene electronic band dispersion are observed. In particular, the latter results in a large enhancement of the hole effective mass (> 1.4), well above the limit of the theoretical estimations. The results are consistent with the expected modifications of the band structures in organic semiconductors as introduced by hole-phonon coupling effects and represent an important experimental step toward the understanding of the charge localization phenomena in organic materials.

Charge Reorganization Energy and Small Polaron Binding Energy of Rubrene Thin Films by Ultraviolet Photoelectron Spectroscopy

The hole–phonon coupling of a rubrene monolayer on graphite is measured by means of angle resolved ultraviolet photoelectron spectroscopy. Thus, the charge reorganization energy λ and the small polaron binding energy is determined, which allows insight into the nature of charge transport in condensed rubrene.

Strain induced modification of quasi-two-dimensional electron gas state on √3×√3-Ag structure

To establish fundamental understanding of the influence of lattice strain to a quasi-two-dimensional electron gas state (2DEG), the both effects of compressive and tensile strains induced in the √3×√3-Ag structure formed on the Ge/Si(111) and the Si/Ge(111) surfaces were investigated with scanning tunneling microscopy and angle resolved ultraviolet photoelectron spectroscopy. The effective mass of the 2DEG is decreased by the compressive strain and that is increased by the tensile strain. The results indicate the dispersion of the electronic state will be modified by controlling the lattice strain.

LOCAL ELECTRONIC STATES ON TWO-DIMENSIONAL NANOSCALE ISLAND OF Si AND Ge FABRICATED ON Si(111) 7 × 7 SUBSTRATE

Nanoscale islands on semiconductor are a strong candidate as building block in nanodevices. In the nanoisland, some local deformation is induced by the surface tension, which has a great influence on the electronic property of the nanoislands. To study the electronic structure of two-dimensional (2D) nanoislands of Si and Ge on the Si(111) 7 × 7 surface, we formed nanoislands of the same size and measured with angle resolved ultraviolet photoelectron spectroscopy (ARUPS), scanning tunneling microscopy (STM) and scanning tunneling spectroscopy (STS). We found that the nanoisland shows a characteristic fine structure due to some strain. We also calculated a relation between the strain and electronic state based on the density-functional theory. In the calculation, the dangling-bond state at the strained adatom on the nanoisland (SR state) shifts to lower energy, which has liner dependence with the height of the adatoms. The ARUPS spectrum and the STS show characteristic peaks corresponding to the SR state, whose energy depends on the deformation of the adatom. The height of the adatom on the nanoisland estimated from the energy difference is consistent with a result of the STM measurement. The strain of adatoms can be estimated from the electronic structure.

Modification of electronic states of √3×√3-Ag structure by strained Ge/Si(111) substrate

To make clear the relationship between strain and electronic stricture, the strain-induced √3×√3-Ag structure formed on the Ge/Si(111) surface was investigated by using scanning tunneling microscopy and angle resolved ultraviolet photoelectron spectroscopy. The compressive strain is induced by epitaxial growth of Ge on the Si(111) substrate. The interatomic spacing in the surface plane is altered with the coverage of the Ge, and the strain of the surface is also varied. The stress reaches to a maximum value when the Ge layer covers up the surface completely. The band dispersion of the two dimensional metallic state on the √3×√3-Ag surface becomes steeper with the compressive strain, which means a reduction in the effective mass. It is confirmed that the electronic states of the √3×√3-Ag surface could be modified by using the lattice strain.

Structural and Electronic Properties of Epitaxial Graphene on SiC (0001)

The structural and electronic properties of epitaxial graphene on SiC(0001) are investigated by low energy electron diffraction (LEED) and angle resolved ultraviolet photoelectron spectroscopy (ARUPS). Fingerprints in the spot intensity spectra in LEED allow for the exact determination of the number of layers for the first three graphene layers after being correlated with the electronic bandstructure obtained from ARUPS using He II excitation. Our analysis includes the consideration of samples with different doping levels. A possible influence of the polytype 4H- or 6H-SiC is discussed. LEED by itself turns out to be an easy and practical method for the thickness analysis of epitaxial graphene on SiC(0001).

Light Effective Mass in the Widely-Dispersed Valence Band of Single Crystalline Rubrene Observed by High-Resolution Angle-Resolved Ultraviolet Photoelectron Spectroscopy

AbstractHigh-resolution angle resolved ultraviolet photoelectron spectroscopy measurements were conducted on rubrene single crystals successfully through relief of the sample charging assisted by a laser illumination. Significant dispersion of the valence band was clearly resolved. The band width W and the hole effective mass mh* were estimated to be 0.4 eV and 0.7m0, respectively, along the most conductive direction. The present results strongly suggest that the transport nature in rubrene single crystals should be described in the band transport framework of a delocalized charge carrier.

Precise in situ thickness analysis of epitaxial graphene layers on SiC(0001) using low-energy electron diffraction and angle resolved ultraviolet photoelectron spectroscopy

We demonstrate an easy and practical method for the thickness analysis of epitaxial graphene on SiC(0001) that can be applied continuously during the preparation procedure. Fingerprints in the spot intensity spectra in low energy electron diffraction (LEED) allow for the exact determination of the number of layers for the first three graphene layers. The LEED data have been correlated with the electronic bandstructure around the K¯-point of the graphene Brillouin zone as investigated by laboratory based angle resolved ultraviolet photoelectron spectroscopy using He II excitation. The morphology and homogeneity of the graphene layers can be analyzed by low energy electron microscopy.

シリコンおよびゲルマニュームナノアイランドの微細構造と局所電子状態

To study the relation between the strain and the electronic structure in two-dimensional (2D) nanoislands of Si and Ge on the Si(111)-7×7 surface, we made the nanoislands the same size and measured with the angle resolved ultraviolet photoelectron spectroscopy (ARUPS), the scanning tunneling microscopy (STM) and scanning tunneling spectroscopy (STS). We also calculated the relation based on the density-functional theory. In the calculation, the dangling-bond state at the strained adatom on the nanoisland (SR state) shifts to lower energy, which has a liner dependence with the height of the adatoms. The ARUPS spectrum and the STS show characteristic peaks corresponding to the SR state, whose energy depends on the deformation of the adatom. The height of the adatom on the nanoisland estimated from the energy difference is consistent with a result of the STM measurement. The strain of adatoms can be estimated from the electronic structure.

Lithium-induced dimer reconstructions on Si(001) studied by photoelectron spectroscopy and band-structure calculations

The electronic and atomic structure of Si(001) with 0.5 and 1 ML of lithium have been studied experimentally using angle resolved ultraviolet photoelectron spectroscopy, Si $2p$ core level spectroscopy, and low energy electron diffraction. Experimental surface state dispersions are compared with recent theoretical results in the literature and with results from additional density functional theory calculations. Four adsorption configurations for the 0.5 ML $2\ifmmode\times\else\texttimes\fi{}2$ surface and three configurations for the 1 ML $2\ifmmode\times\else\texttimes\fi{}1$ surface are compared. Fittings of Si $2p$ core level data support the alternation of strongly and weakly buckled Si dimers of the $2\ifmmode\times\else\texttimes\fi{}2$ models and symmetric Si dimers of the $2\ifmmode\times\else\texttimes\fi{}1$ models based on the relative intensities of the surface components. As a tool to differentiate between the different $2\ifmmode\times\else\texttimes\fi{}2$ and $2\ifmmode\times\else\texttimes\fi{}1$ models surface state dispersions are used since they are sensitive to the positions of the Li adatoms.

Role of final states in photoemission from Al(1 1 1)

Enhancement of surface state peaks in angle resolved ultraviolet photoelectron spectra (ARUPS) from the Al(1 1 1) surface is studied experimentally and theoretically within the one-step model of photoemission. The resonant enhancement of the surface state emission is explained by the crucial role of elastic scattering of the outgoing electron. Dipole transitions to evanescent states in the final bands of the crystal are shown to determine photoemission at the resonant photon energy. The band structure based explanation is confirmed by the measurements of electron reflectivity and of the fine structure of valence band spectra. The surface sensitivity of ARUPS is shown to depend strongly on the complex band structure of the crystal and to be finely tunable by the choice of photoemitted electron energy.

Transition between tetramer and monomer phases driven by vacancy configuration entropy onBi∕Ag(001)

A phase transition between low-temperature $c(4\ifmmode\times\else\texttimes\fi{}4)$ and high-temperature $c(2\ifmmode\times\else\texttimes\fi{}2)$ phases on $\mathrm{Bi}∕\mathrm{Ag}(001)$ has been investigated using low energy electron diffraction, scanning tunneling microscopy (STM), and angle resolved ultraviolet photoelectron spectroscopy (ARUPS). STM experiments show that the $c(4\ifmmode\times\else\texttimes\fi{}4)$ surface is composed of Bi tetramers arrayed on a square lattice, while the tetramers are decomposed to monomers in the $c(2\ifmmode\times\else\texttimes\fi{}2)$ phase. ARUPS shows an upward shift of a Bi-induced surface resonance band around $\overline{\ensuremath{\Gamma}}$ upon the transition from $c(4\ifmmode\times\else\texttimes\fi{}4)$ to $c(2\ifmmode\times\else\texttimes\fi{}2)$, which suggests that the Bi tetramer is stabilized by local, covalent bonding between Bi atoms constituting a tetramer. ARUPS also indicates large Rashba spin-orbit splitting of the Bi-induced surface resonance band. The phase transition is associated with an intermediate temperature region in which the two phases coexist on the surface with their area changing gradually with changing temperature. The transition temperature is lowered for surface with higher vacancy density. A model is presented to show that vacancy configuration entropy drives the phase transition. Larger vacancy configuration entropy in the $c(2\ifmmode\times\else\texttimes\fi{}2)$ phase, due to the decomposition of tetramers, accounts for the behavior of the phase transition via the two-phase coexisting region.

Scanning tunneling and photoemission spectroscopies at the PTCDA/Au(111) interface

Abstract We have studied the electronic properties at the interface of a PTCDA molecular film on Au(1 1 1) by means of scanning tunneling microscopy (STM), scanning tunneling spectroscopy (STS) and angle resolved ultraviolet photoelectron spectroscopy (ARUPS). Measurements were performed for the clean Au(1 1 1) surface as well as for 1 ML and multilayer coverage of PTCDA. STS curves recorded for 1 ML PTCDA film show a feature at the density of states inside the PTCDA gap and a metallic behavior, in contrast to higher coverage, where a semiconductor behavior is observed. By relating this information to the band dispersion of the electronic states close to the Fermi edge, we can assign this electronic feature to the Au(1 1 1) Shockley surface state. By comparison of intramolecularly resolved STM images with first-principles calculations of the free molecule we show that the electronic molecular states at the interface are not altered by the gold surface. All these facts indicate a weak interaction of the PTCDA molecules with the Au surface and we conclude that the observed metallicity of the interface is provided by the gold substrate rather than by the organic layer.

Strain and electronic structure of Ge nanoislands on Si(111)-7×7surface

To study the relation between the strain and the electronic structure in two-dimensional (2D) Ge nanoislands on the Si(111)-$7\ifmmode\times\else\texttimes\fi{}7$ surface, we calculated the relation based on the density functional theory and measured with the angle resolved ultraviolet photoelectron spectroscopy (ARUPS) and the scanning tunneling microscopy (STM). In the calculation, the dangling-bond state at the strained adatom on the 2D Ge island (${S}_{Ge}^{R}$ state) shifts to lower energy at the rate of $\ensuremath{-}8.2\phantom{\rule{0.3em}{0ex}}\mathrm{eV}∕\mathrm{nm}$ with the height of the adatoms. The ARUPS spectrum shows a characteristic peak corresponding to the ${S}_{Ge}^{R}$ state, whose energy depends on the size of the Ge nanoisland. The state appears at $\ensuremath{-}0.7\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$ below the Fermi level for the small Ge islands (3.8--5 nm) and at $\ensuremath{-}0.6\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$ for the large Ge islands. The height of the Ge islands estimated from the energy difference is consistent with a result of the STM measurement. The strain of Ge islands can be estimated from the electronic structure.

Sexithiophene films on clean and oxidized Si(111) surfaces: Growth and electronic structure

The growth and the electronic properties of sexithiophene thin films on silicon surfaces have been studied by angle resolved ultraviolet photoelectron spectroscopy while morphology and crystalline order have been assessed by atomic force microscopy and x-ray diffraction. The influence of the surface modification of the substrate as well as of the growth temperature and the growth rate were investigated in ultrahigh vacuum through controlled multistep growth sequences ranging from (sub)monolayer coverage up to thick films. Depending on the preparation, two electronically distinct sexithiophene (6T) films could be produced, as revealed by their ionization potentials which differed by ∼0.7eV. The evolution of the electronic energy levels with film thickness is considered in terms of the concepts of interface dipole and band bending, with the latter being argued to be inappropriate. Irrespective of substrate or growth conditions the thick 6T films were found to all have the same work function of ∼0.4eV, which is related to near vertically oriented molecules on the films surface. Against general expectations 6T grown at a low rate on the reactive Si(111) substrate and those grown at high rates on the oxide surface produces films that are more uniform and crystalline than those grown at low rates on the passive oxidized Si surfaces. The latter promotes the formation of small three-dimensional crystallites and a much poorer morphology for planar devices.

ARUPS study of graphitic boron terminated WB 2(0001) surface

The electronic structure of the WB 2(0001) surface that is terminated with graphitic boron layer has been investigated by angle resolved ultraviolet photoelectron spectroscopy and the results were compared with predictions of the first-principles cluster and band calculations to determine its validity. The occurrence of the charge transfer from W to B, leading to large work function (6.1 eV) of the WB 2(0001) surface, is confirmed from Mulliken population analysis. It is concluded that the graphitic boron layer at the WB 2(0001) surface forms π bond which is largely modified due to hybridization with the W 5d orbitals. The photoelectron spectra also show that the two types of boron layers exist in the third layer of the WB 2

Angle resolved ultraviolet photoelectron spectroscopy of oriented sexiphenyl layers

Ultra-thin films of oriented p-sexiphenyl (6P) on a GaAs(001) wafer were studied by angle resolved ultraviolet photoelectron spectroscopy (ARUPS) using UV photons in the energy range between 20 and 60 eV. The samples were prepared under ultra-high vacuum by controlled evaporation onto cleaned GaAs substrates, which were thermostated at enhanced temperature during the deposition process. The thickness of the deposited films played a very important role for the orientation of 6P molecules. For layers having a thickness of about 35 Å, the molecules were oriented perpendicular to the substrate surface and for films of about 300-Å thickness the molecules lost their upright orientation. The orientation of 6P molecules was monitored both by the anisotropy as visible in the measured ARUPS spectra and independently by atomic force microscopy (AFM) investigations carried out after UPS measurements. From the experimental photoelectron spectra, the full one-dimensional valence band structure of oriented p-sexiphenyl layers has been determined. While valence states between 2 and 5 eV show only weak dispersion, the bands between 5 and 11 eV show a clear momentum dispersion, which indicates the high degree of order in the investigated 6P layers.