Angle-resolved Ultraviolet Photoelectron Spectroscopy Research Articles

Structural and electronic properties of MoS2 and MoSe2 monolayers grown by chemical vapor deposition on Au(111).

The exceptional electronic and photonic properties of the monolayers of transition metal dichalcogenides including the spin-orbit splitting of the valence and conduction bands at the K points of the Brillouin zone make them promising for novel applications in electronics, photonics and optoelectronics. Scalable growth of these materials and understanding of their interaction with the substrate is crucial for these applications. Here we report the growth of MoS2 and MoSe2 monolayers on Au(111) by chemical vapor deposition at ambient pressure as well as the analysis of their structural and electronic properties down to the atomic scale. To this aim, we apply ultrahigh vacuum surface sensitive techniques including scanning tunneling microscopy and spectroscopy, low-energy electron diffraction, X-ray and angle-resolved ultraviolet photoelectron spectroscopy in combination with Raman spectroscopy at ambient conditions. We demonstrate the growth of high-quality epitaxial single crystalline MoS2 and MoSe2 monolayers on Au(111) and show the impact of annealing on the monolayer/substrate interaction. Thus, as-grown and moderately annealed (<100 °C) MoSe2 monolayers are decoupled from the substrate by excess Se atoms, whereas annealing at higher temperatures (>250 °C) results in their strong coupling with the substrate caused by desorption of the excess Se. The MoS2 monolayers are strongly coupled to the substrate and the interaction remains almost unchanged even after annealing up to 450 °C.

Investigation of the effect of different types of SiN layers and cap-GaN on the surface electronic states of AlGaN/GaN heterostructures with 2DEG using X-ray and UV photoelectron spectroscopy

Determining the effect of different crystalline and amorphous passivation layers on the electronic states and surface properties of III-nitride heterostructures is an important task from the point of view of improving the performance of modern microwave and power electronic devices. In the present work the effect of in situ deposited gallium nitride and silicon nitride passivation layers on the surface electronic states of AlGaN/GaN heterostructures with a two-dimensional electron gas is investigated using the X-ray, ultraviolet and angle-resolved ultraviolet photoelectron spectroscopy (XPS, UPS, ARUPS) techniques. A thorough analysis of the experimental UPS spectra was carried out, the primary photoelectrons were extracted from the experimental spectra and the contribution of the electron states on the surface in the energy range of the AlGaN bandgap was identified. It was found that the density of states on the surface decreases by about an order of magnitude in the AlGaN bandgap during the formation of a silicon nitride layer. For the first time, it was found that a significant decrease in the density of surface states (up to a factor of 6) is observed even at extremely low thickness of the SiN ordered layer formation with the (√3 × √3)R30° superstructure.

Ultrafast Hidden Spin Polarization Dynamics of Bright and Dark Excitons in 2H-WSe_{2}.

We performed spin-, time- and angle-resolved extreme ultraviolet photoemission spectroscopy of excitons prepared by photoexcitation of inversion-symmetric 2H-WSe_{2} with circularly polarized light. The very short probing depth of XUV photoemission permits selective measurement of photoelectrons originating from the top-most WSe_{2} layer, allowing for direct measurement of hidden spin polarization of bright and momentum-forbidden dark excitons. Our results reveal efficient chiroptical control of bright excitons' hidden spin polarization. Following optical photoexcitation, intervalley scattering between nonequivalent K-K^{'} valleys leads to a decay of bright excitons' hidden spin polarization. Conversely, the ultrafast formation of momentum-forbidden dark excitons acts as a local spin polarization reservoir, which could be used for spin injection in van der Waals heterostructures involving multilayer transition metal dichalcogenides.

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.

Preparation and electronic structure of the WSe2/graphene/NiSex/Ni(111) heterostructure

In this work, a stacked heterostructure made up of single-layer WSe2 and graphene was created through a scalable and efficient way. Graphene was grown on a Ni (111) single crystal, producing an ordered and well-defined carbon overlayer that is strongly hybridized with the support, disrupting its peculiar conductive properties. A monolayer WSe2 was deposited on top of graphene by the simultaneous evaporation of metal W and elemental Se. We demonstrate here that the chalcogen can efficiently intercalate between graphene and the Ni surface, decoupling the two materials and forming a buffering NiSex layer. The concurrent selenization of both W and Ni effectively eliminates the need for an additional decoupling step in the synthesis of a free-standing graphene/Ni heterostructure. The formation process of the complex WSe2/Graphene/NiSex/Ni(111) heterostructure was studied by means of low-energy electron diffraction, x-ray photoelectron spectroscopy, angle-resolved ultraviolet photoelectron spectroscopy, and ex situ Raman spectroscopy. These analyses confirm the presence of single-layer WSe2 on top of a free-standing graphene.

Experimental evidence for a metastable state in FeTe1−xSex following coherent-phonon excitation

Changes of the electronic structure of FeTe1−xSex (x = 0, 0.3) upon ultrafast excitation with near-infrared laser pulses are investigated using time- and angle-resolved extreme ultraviolet photoemission spectroscopy. At different temperatures and for different doping levels we observe a global oscillation of energy bands near the Fermi energy driven by the coherent excitation of the A1g(Te) phonon mode in FeTe1−xSex. Prominently, photo-induced band shifts and a spectral weight reduction persist for at least 300 ps, long after the decay of the photo-excited carriers and the damping of the coherent phonons. We argue that the system escapes from the original equilibrium state and is transiently trapped in a possibly metastable state envisioning a purely optical means of manipulating the electronic structure in an iron-based superconductor.

Demonstrating the Impact of the Adsorbate Orientation on the Charge Transfer at Organic-Metal Interfaces.

Charge-transfer processes at molecule–metal interfaces play a key role in tuning the charge injection properties in organic-based devices and thus, ultimately, the device performance. Here, the metal’s work function and the adsorbate’s electron affinity are the key factors that govern the electron transfer at the organic/metal interface. In our combined experimental and theoretical work, we demonstrate that the adsorbate’s orientation may also be decisive for the charge transfer. By thermal cycloreversion of diheptacene isomers, we manage to produce highly oriented monolayers of the rodlike, electron-acceptor molecule heptacene on a Cu(110) surface with molecules oriented either along or perpendicular to the close-packed metal rows. This is confirmed by scanning tunneling microscopy (STM) images as well as by angle-resolved ultraviolet photoemission spectroscopy (ARUPS). By utilizing photoemission tomography momentum maps, we show that the lowest unoccupied molecular orbital (LUMO) is fully occupied and also, the LUMO + 1 gets significantly filled when heptacene is oriented along the Cu rows. Conversely, for perpendicularly aligned heptacene, the molecular energy levels are shifted significantly toward the Fermi energy, preventing charge transfer to the LUMO + 1. These findings are fully confirmed by our density functional calculations and demonstrate the possibility to tune the charge transfer and level alignment at organic–metal interfaces through the adjustable molecular alignment.

Demonstration of the key substrate-dependent charge transfer mechanisms between monolayer MoS2 and molecular dopants

Tuning the Fermi level (EF) in two-dimensional transition metal dichalcogenide (TMDC) semiconductors is crucial for optimizing their application in (opto-)electronic devices. Doping by molecular electron acceptors and donors has been suggested as a promising method to achieve EF-adjustment. Here, we demonstrate that the charge transfer (CT) mechanism between TMDC and molecular dopant depends critically on the electrical nature of the substrate as well as its electronic coupling with the TMDC. Using angle-resolved ultraviolet and X-ray photoelectron spectroscopy, we reveal three fundamentally different, substrate-dependent CT mechanisms between the molecular electron acceptor 1,3,4,5,7,8-hexafluoro-tetracyano-naphthoquinodimethane (F6TCNNQ) and a MoS2 monolayer. Our results demonstrate that any substrate that acts as charge reservoir for dopant molecules can prohibit factual doping of a TMDC monolayer. On the other hand, the three different CT mechanisms can be exploited for the design of advanced heterostructures, exhibiting tailored electronic properties in (opto-)electronic devices based on two-dimensional semiconductors.

Well-Ordered Monolayer Growth of Crown-Ether Ring Molecules on Cu(111) in Ultra-High Vacuum: An STM, UPS, and DFT Study

Crown-ether (CR) ring molecules are known as host molecules for capturing guest species inside the ring. So far CR molecular films have only been grown by drop-casting a CR solution on an inert substrate in air which offers little control over the molecular structure. Here we report the successful growth of a well-ordered CR molecular array on an atomically flat and clean noble metal Cu(111) substrate at 300 K in ultra-high vacuum (UHV), using 4,4′,5,5′-tetrabromodibenzo[18] crown-6 ether (Br-CR). The adsorption, self-assembly, and electronic structures of Br-CR were studied by means of UHV low-temperature scanning tunneling microscopy and spectroscopy, low electron energy diffraction, and angle-resolved ultraviolet photoemission spectroscopy. We found that (1) the Br-CR ring, which is bent both in the crystal and gas phase, flattens upon adsorption on Cu(111). Density functional theory reveals that the two benzene groups of the molecule lie flat on the surface such as to maximize the substrate-molecule interaction. (2) The moderate molecule–substrate interaction allows thermal diffusion of the Br-CR molecules, resulting in the formation of self-assembled monolayer islands with 7 × 4 superstructure. (3) While the deposition of 0.05 ML Br-CR forms multidomain islands with disordered defects, a drastic improvement occurred at 0.25 ML, where only atomically flat single-domain islands were grown. This Br-CR flat ring cavity array could become a template for designing novel two-dimensional arrays of desired guest atoms, ions, or functionalized molecules.

Widely Dispersed Intermolecular Valence Bands of Epitaxially Grown Perfluoropentacene on Pentacene Single Crystals.

Strong intermolecular electronic coupling and well-ordered molecular arrangements enable efficient transport of both charge carriers and excitons in semiconducting π-conjugated molecular solids. Thus, molecular heteroepitaxy to form crystallized donor-acceptor molecular interfaces potentially leads to a novel strategy for creating efficient organic optoelectronic devices via the concomitance of these two requirements. In the present study, the crystallographic and electronic structures of a heteroepitaxial molecular interface, perfluoropentacene (PFP, C22F14) grown on pentacene single crystals (Pn-SCs, C22H14), were determined by means of grazing-incidence X-ray diffraction (GIXD) and angle-resolved ultraviolet photoelectron spectroscopy (ARUPS), respectively. GIXD revealed that PFP uniquely aligned its primary axis along the [11̅0] axis of crystalline pentacene to form well-crystallized overlayers. Valence band dispersion (at least 0.49 eV wide) was successfully resolved by ARUPS. This indicated a significant transfer integral between the frontier molecular orbitals of the nearest-neighbor PFP molecules.

Band Dispersion and Hole Effective Mass of Methylammonium Lead Iodide Perovskite (Solar RRL 10∕2018)

In article no. 1800132, Jin-Peng Yang, Nobuo Ueno, Satoshi Kera, and co-workers perform observations of the top valence band structure of CH3NH3PbI3 single crystals using angle-resolved ultraviolet photoelectron spectroscopy of the cleaved single-crystal surfaces. The combination of freshly cleaved crystal surfaces and determination of the exact orientation of the crystal axes using diffraction techniques successfully measures well-determined crystal directions.

Anisotropic valence band dispersion of single crystal pentacene as measured by angle-resolved ultraviolet photoelectron spectroscopy

Abstract

π-Like band structure observed for coronene monolayers deposited on Ag(111)

Angle-resolved ultraviolet photoelectron spectroscopy (ARUPS) was performed for coronene monolayers deposited on Ag(111) showing that the polarization dependence of the silver sp-bands is changed by the molecular adlayer. The change was observed at back-folded replicas of the substrate bands and is reminiscent of the π-bands of epitaxial graphene. A transfer of the molecule's π-character to the silver sp-bands is concluded to cause the graphene-like properties observed.

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.

Ordered Growth and Electronic Properties of 1,2:8,9-Dibenzopentacene (trans-DBPen) on Ag(111)

Herein, we have studied highly ordered two-dimensional layers of 1,2:8,9-dibenzopentacene (trans-DBPen) adsorbed on a single-crystalline silver surface. While its parent molecule pentacene is known for a relatively high hole mobility, reports on trans-DBPen are rather scarce. Furthermore, it belongs to polycyclic aromatic hydrocarbons for which superconducting properties have been observed upon intercalation with potassium. Our scanning tunneling microscopy and low-energy electron diffraction results reveal a highly ordered monolayer structure of trans-DBPen on Ag(111). We further used angle-resolved ultraviolet photoelectron spectroscopy to measure photoelectron momentum maps (PMMs). The experimental PMMs agree very nicely with simulations based on our structural data while assuming free molecules for the density functional theory calculations. A comparison with pentacene yields some insights into the properties of the two related molecules. We conclude that the degree of hybridization between the molecu...

Adsorption of 1,3,5-Triphenylbenzene Molecules and Growth of Graphene Nanoflakes on Cu(100) Surface**Supported by the National Basic Research Program of China under Grant No 2011CB921903, and the Scientific Research Fund of Zhejiang Provincial Education Department under Grant Nos Y201121234 and LQ12F04001.

Adsorption of 1,3,5-triphenylbenzene (TPB) molecules on Cu(100) surface is studied using ultraviolet photoelectron spectroscopy (UPS) and density functional theory (DFT) calculations. Researches on the bottom-up fabrication of graphene nanoflakes (GNFs) with TPB as a precursor on the Cu(100) surface are carried out based on UPS and DFT calculations. Three emission features d, e and f originating from the TPB molecules are located at 3.095, 7.326 and 9.349 eV below the Fermi level, respectively. With the increase of TPB coverage on the Cu(100) substrate, the work function decreases due to the formation of interfacial dipoles and charge (electron) rearrangement at the TPB/Cu(100) interface. Upon the formation of GNFs, five emission characteristic peaks of g, h, i, j and k originating from the GNFs are located at 1.100, 3.529, 6.984, 8.465 and 9.606 eV below the Fermi level, respectively. Angle resolved ultraviolet photoelectron spectroscopy (ARUPS) and DFT calculations indicate that TPB molecules adopt a lying-down configuration with their molecular plane nearly parallel to the Cu(100) substrate at the monolayer stage. At the same time, the lying-down configuration for the GNFs on the Cu(100) surface is also unveiled by ARUPS and DFT calculations.

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.

Influence of Film and Substrate Structure on Photoelectron Momentum Maps of Coronene Thin Films on Ag(111).

Angle-resolved ultraviolet photoelectron spectroscopy (ARUPS) was measured for one-monolayer coronene films deposited on Ag(111). The (kx,ky)-dependent photoelectron momentum maps (PMMs), which were extracted from the ARUPS data by cuts at fixed binding energies, show finely structured patterns for the highest and the second-highest occupied molecular orbitals. While the substructure of the PMM main features is related to the 4 × 4 commensurate film structure, various features with three-fold symmetry imply an additional influence of the substrate. PMM simulations on the basis of both free-standing coronene assemblies and coronene monolayers on the Ag(111) substrate confirm a sizable molecule–molecule interaction because no substructure was observed for PMM simulations using free coronene molecules.

Recovery of the bulk-like electronic structure of manganese phthalocyanine beyond the first monolayer on Bi2Te3

The evolution of electronic structure of manganese phthalocyanine on Bi2Te3 shows a transition to a bulk-like aspect abruptly after completion of the first layer. This allows the inference that, in the first layer, there is charge transfer and electronic hybridization involving the occupied Mn-derived d orbitals of the molecule into the conduction band of the substrate. The charge transfer coupling is seen using angle-resolved ultraviolet photoelectron spectroscopy by monitoring the evolution of work function and band structure with increasing molecular film thickness. The electronic structure in the second layer is more bulk-like as indicated by the reappearance of well-known low energy d orbitals that were depopulated in the first layer. Scanning tunneling microscopy shows that the transition to bulk like behavior is also reflected in film structure as a transition from a unique disordered monolayer to a locally ordered and dense second layer. These observations are relevant to ongoing efforts to control topological insulator interfaces especially for spintronics applications.