Metastable Atom Electron Spectroscopy Research Articles

Tripodal Triazatruxene Derivative as a Face-On Oriented Hole-Collecting Monolayer for Efficient and Stable Inverted Perovskite Solar Cells.

Hole-collecting monolayers have drawn attention in perovskite solar cell research due to their ease of processing, high performance, and good durability. Since molecules in the hole-collecting monolayer are typically composed of functionalized π-conjugated structures, hole extraction is expected to be more efficient when the π-cores are oriented face-on with respect to the adjacent surfaces. However, strategies for reliably controlling the molecular orientation in monolayers remain elusive. In this work, multiple phosphonic acid anchoring groups were used to control the molecular orientation of a series of triazatruxene derivatives chemisorbed on a transparent conducting oxide electrode surface. Using infrared reflection absorption spectroscopy and metastable atom electron spectroscopy, we found that multipodal derivatives align face-on to the electrode surface, while the monopodal counterpart adopts a more tilted configuration. The face-on orientation was found to facilitate hole extraction, leading to inverted perovskite solar cells with enhanced stability and high-power conversion efficiencies up to 23.0%.

(Invited) Surface/Interface Structures of Perovskite Films Studied By Electron Spectroscopies

Interface of perovskite layer play a crucial role in the charge extraction and recombination process of perovskite solar cell. It is, therefore, of great importance to examine the geometrical and electronic structures of surface/interface of perovskite. Unfortunately, standard experimental techniques such as X-ray diffraction (XRD) and scanning tunneling microscopy (STM) are not suitable to examine practical perovskite films for solar cells. XRD observes the lattice periodicity and is not sensitive to the surface structure. Although STM can precisely observe the surface atoms, it cannot be applied to rough surface or thick films, e.g. solution-processed perovskite surface.We have demonstrated that a combination of electron spectroscopies is powerful and versatile tool to examine both geometrical and electronic structures of perovskite surfaces. Figure 1 shows principles of ultraviolet photoelectron spectroscopy (UPS) and metastable atom electron spectroscopy (MAES). In UPS, the valence electrons are excited by ultraviolet photons (typically with the energy of 21.22 eV) and kinetic energies of photoelectrons are analyzed to measure the valence electronic states as a difference between the photon and electron kinetic energies. The probing depth of UPS is limited by the mean free path of electrons which is usually several layers a few nanometers (several layers) in the energy range of UPS. In MAES, the excitation source is replaced with the metastable helium atom (with the energy of 19.82 eV). Although the process as electron spectroscopy is similar to that of UPS, MAES is extremely surface sensitive because the helium atom can only interact with the outermost atomic orbitals of the sample surface. By comparing the UPS and MAES spectra, we can obtain information about the surface structure as well as the electronic states. In this presentation, we will demonstrate the following two topics:1. Surface termination of CH3NH3PbI3 [1]The properties of the interface of perovskite ABX3 should be strongly affected by the surface termination, i.e. whether the surface is terminated with AX or BX layer. We examined the surface termination of a CH3NH3PbI3 thin film prepared by the standard one-step solution process. The results show that surface of the perovskite film is terminated with the layer consisting of CH3NH3 and I.2. Surface structure of quasi-2D perovskite films, PEA2m(CH3NH3)n-2mPbnI3n [2]A small amount of large organic cations incorporated in perovskite film (quasi-2D) passivates the surface and enhances the stability of the perovskite solar cell against moisture. We examined surface structure of PEA2m(CH3NH3)n-2mPbnI3n, and confirmed that the PEA segregates to the surface and that the phenyl group covers the outermost surface of the quasi-2D perovskite. From the concentration dependence, we also examined the plausible structure of the quasi-2D film.1) A. Mirzehmet, T. Ohtsuka, S. A. Abd. Rahman, T. Yuyama, P. Krüger, H. Yoshida, Adv. Mater. 33, 2004981 (2021).2) A. Mirzehmet, T. Ohtsuka, S. A. Abd. Rahman, T. Aihara, M. A. Kamarudin, S. R. Sahamir, S. Hayase, T. Yuyama, P. Krüger, H. Yoshida, Appl. Phys. Express 14, 031006 (2021).

Metal screening effect on energy levels at metal/organic interface: Precise determination of screening energy using photoelectron and inverse-photoelectron spectroscopies

The energy level alignment at the metal/organic interface greatly affects the charge injection/extraction efficiency of electrodes in organic semiconductor devices. The charge carrier in the vicinity of the metal surface should be stabilized by the screening effect of the metal surface, thereby resulting in the narrowing of the energy gap (highest occupied molecular orbital (HOMO)--lowest unoccupied molecular orbital (LUMO) gap) and modifying the energy level alignment at the metal/organic interface. However, this metal screening effect has not been fully clarified because there has been no precise way to quantitatively evaluate the experimental values. In this study, we employed ultraviolet photoelectron spectroscopy (UPS), metastable atom electron spectroscopy (MAES), and low-energy inverse photoelectron spectroscopy (LEIPS) to examine the quasi--layer-by-layer grown film of perylene-3,4,9,10-tetracarboxylic dianhydride (PTCDA) on the atomically flat metal surface of Ag(111) and Au(111). By applying a procedure we established recently, we precisely evaluated the energy of the metal screening effect. The results demonstrate that the screening effect of the metal surface modifies the charge injection/collection barrier at the metal/organic interface by as much as 0.25 eV. We found that the PTCDA thickness dependent screening effect is precisely reproduced by the image charge model.

Transformation of Alkatetrayne Monolayers into Nanoflatcables Studied by Ultraviolet Photoelectron Spectroscopy and Metastable Atom Electron Spectroscopy

Nanoflatcables (NFCs) are an array of all-trans π-conjugated chains periodically bridged with and maintained at regular intervals by alkyl chains. The prototype NFC containing alternately arranged ...

Modification of TiO2(1 1 0)/organic hole transport layer interface energy levels by a dipolar perylene derivative

Our photoemission study reveals that the work function of TiO2(1 1 0) decreases by up to 1.5 eV upon deposition of 9-(bis-(p-(tert-octyl)phenyl)amino)-perylene-3,4-dicarboxylic anhydride (BOPA-PDCA). This effect is attributed to a chemical reaction of TiO2(1 1 0) and the molecular anhydride group, as well as the molecular dipole. Analysis of the film thickness dependent photoemission and metastable atom electron spectroscopy data reveals that for low coverage the perylene backbone of BOPA-PDCA is almost parallel to the substrate surface and higher coverage leads to an orientational transition to essentially upright standing molecules. Comparing the energy-level alignment between TiO2(1 1 0) and the hole transport materials N,N′-bis(1-naphthyl)-N,N′-diphenyl-1,1′-biphenyl-4,4′-diamine (NPB) without and with the BOPA-PDCA interlayer, we find that the perylene derivative has a positive impact on the level alignment for dye-sensitized solar cells with high open-circuit voltages.

Spectroscopic Identification of Ag-Terminated “Multilayer Silicene” Grown on Ag(111)

The electronic structure of the outermost layer of “multilayer silicene” was investigated by metastable atom electron spectroscopy (MAES). It is usually difficult to elucidate the electronic structure of an ultrathin film grown on a solid substrate excluding the contribution from the substrate, especially such as “multilayer silicene” grown on a Ag(111) substrate. MAES used in this study thus provides a proper solution because the excitation source, He*(23S) atom, cannot penetrate through the first layer. Comparing the MAES spectra of “multilayer silicene” and of the Si(111)√3 × √3-Ag surface where the Ag atoms are arranged to form a superlattice on the (111) surface of the Si diamond crystal, we find that these spectra are essentially identical to each other. This result indicates that the so-called “multilayer silicene” is actually not multilayered, i.e., a stack of honeycomb lattice layers.

Local electronic structure at organic–metal interface studied by UPS, MAES, and first-principles calculation

Understanding and controlling local electronic structures at organic–metal interfaces are crucial for fabricating novel organic-based electronics, as in the case of heterojunctions in semiconductor devices. Here, we report recent studies of valence electronic states at organic–metal interfaces (especially those near the Fermi level of a metal substrate) by the combined analysis of ultraviolet photoemission spectroscopy (UPS), metastable atom electron spectroscopy (MAES), and first-principles calculations. New electronic states in the HOMO (highest occupied molecular orbital)–LUMO (lowest unoccupied molecular orbital) gap formed at an organic–metal interface are classified as a chemisorption-induced gap state (CIGS) and a complex-based gap state (CBGS). The CIGS is further characterized by an asymptotic feature of the metal wave function in the chemisorbed species. The CIGSs in alkanethiolates on Pt(111) and C60 on Pt(111) can be regarded as damping and propagating types, respectively. The CBGSs in K-doped dibenzopentacene (DBP) are composed of DBP-derived MOs and K sp states and distributed over the complex film. No metallic structures were found in the K1DBP and K3DBP phases, suggesting that they are Mott–Hubbard insulators due to strong electron correlation. The local electronic structures of a pentacene film bridged by Au electrodes under bias voltages were examined by an FET-like specimen. The pentacene-derived bands were steeply shifted at the positively biased electrode, reflecting the p-type character of the film.

Characterization of 2,9-Dihexylpentacene Films on Graphite (0001) by Metastable Atom Electron Spectroscopy and Ultraviolet Photoelectron Spectroscopy

Abstract Using metastable atom electron spectroscopy and ultra-violet photoelectron spectroscopy, the electronic structure and the molecular orientation of 2,9-dihexylpentacene are studied for two kinds of films prepared by vapor deposition onto a graphite (0001) substrate. It is found that the molecules lie flat in a monolayer of 0.4 nm thickness but stand up in a 16-nm film with the methyl ends of the chains exposed outside.

Electron Spectroscopy of Ultrathin Cycloalkane Films on Graphite (0001): Molecular Orbitals, Conformation, and Orientation

Abstract The electronic structures, conformation, and orientation of cyclooctatetracontane were studied by ultraviolet photoelectron and metastable atom electron spectroscopy for ultrathin films prepared on cooled graphite (0001) surfaces. It was found that molecules in a long and thin rectangular form are laid flat with the long sides placed with edge-on orientation up to room temperature. The local electron distributions of molecular orbitals were probed outside the edge-on surface for the first time. The geometric structures are consistent with the STM image.

Geometric and Electronic Structures of Hydrogen-Bonded Warren Truss Networks Comprising Planar Triamide Molecules on Graphite (0001)

When a very small amount of 1,3,5-benzenetricarboxamide (BTCA) is deposited onto a graphite (0001) surface held at 120 K, an amorphous monolayer is formed owing to the low mobility of the frozen molecules. Upon raising the substrate temperature, they become laid flat and then gradually transformed into peculiar aggregates with slightly tilted orientation, which are completed after annealing at 320 K. Metastable atom electron spectroscopy sensitively probes the changes in the molecular orientation, while ultraviolet photoelectron spectroscopy detects the changes in the electronic structures. Scanning tunneling microscopy affords a clue to the molecular arrangement in the annealed monolayer, for which the first-principles calculations are carried out under periodic boundary conditions and the density of states in line with the photoelectron spectra is obtained. It is revealed that hydrogen bonds are introduced among the molecules to provide an anisotropic network of a novel Warren truss structure.

Ultraviolet Photoelectron Spectroscopy and Metastable Atom Electron Spectroscopy of a Sashlike Polydiacetylene (Atomic Sash) Monolayer: Observation of the π Electronic Structures Peculiar to Two Major Conformers

Atomic sash (AS) is a single sheet of a sashlike polydiacetylene (PD) made from conjugated alkadiyne molecules laid flat on a graphite (0001) surface. When they are irradiated with UV light, one of the major AS conformers, AS-I, with the PD chain held higher than the alkyl chains in contact with the substrate is formed first but transformed to another conformer AS-II with all of the carbon atoms placed in a common plane. Changes in the π electronic structures upon polymerization and structural transformation are investigated by ultraviolet photoelectron spectroscopy (UPS) and metastable atom electron spectroscopy (MAES), and the spectra are compared with the results of our preceding calculations. The π electronic structures peculiar to each conformer are successfully discriminated: UPS detects differences in the density of states, whereas MAES probes a drastic decrease in the local electron distribution of the π wave functions at the monolayer surface due to the foundering of the PD chain in the lying alkyl chains. The threshold ionization potential 5.0 eV obtained for the PD of the AS-I exposed outside the surface corresponds to the highest HOMO energy in an ideal isolated PD chain.

Chemisorption-induced gap states at organic–metal interfaces: benzenethiol and benzeneselenol on metal surfaces

Valence electronic states of benzenethiol (C(6)H(5)SH) and benzeneselenol (C(6)H(5)SeH) in the gas, condensed, and chemisorbed phases were examined by ultraviolet photoemission spectroscopy, metastable atom electron spectroscopy, and first-principles calculations using density functional theory. C(6)H(5)SH is chemisorbed on Pt(111) and Au(111) substrates to form a thiolate (C(6)H(5)S), and C(6)H(5)SeH is bound on Pt(111) substrate to form a selenolate (C(6)H(5)Se). In all cases, chemisorption-induced gap states (CIGSs) appear just below the Fermi level (E(F)) of the substrate, yielding a metallic character around the anchor S and Se atoms. However, the local density at E(F) decreases considerably from the anchor atom to the benzene ring, because strong coupling between benzene π(1e(1g)) and S 3p(or Se 4p) in free molecules is apparently lifted upon chemisorption. In other words, thiolates and selenolates (especially C(6)H(5)S on Au(111)) act as poor mediators of the metal wave functions at E(F), which is closely related to electric conductance in the relevant metal-organic-metal junctions at zero bias.

Local electronic properties at organic–metal interfaces: thiophene derivatives on Pt(111)

The valence electronic states of thiophene (TP), 2-thiophenethiol (TT), 2,2'-bithiophene (BTP), and 2,2'-bithiophene-5-thiol (BTT) on Pt(111) were measured by ultraviolet photoemission spectroscopy (UPS) and metastable atom electron spectroscopy (MAES) to elucidate how the local electronic properties at the organic-metal interface are altered by the extent of π-conjugation and substituent effects. First-principles calculations using density functional theory (DFT) were used to assign the observed spectra. TP and BTP chemisorb weakly on Pt(111), whereas TT and BTT are strongly bound to Pt(111) through the S atom with the cleavage of the S-H bond, forming a thiolate. In the MAES spectra, weak emission just below the Fermi level (E(F)) was attributed to a chemisorption-induced gap state (CIGS) produced by orbital mixing between the organic species and Pt(111). The formation of CIGS is responsible for a metallic structure at the organic-metal interface. The relative intensities of CIGSs at E(F) were in the order of TP (flat-lying configuration) > TT > TP (inclined configuration), indicating that the spatial distribution of CIGSs is drastically altered by the strength of the organic-metal bond and the adsorption geometry. In other words, TP (flat-lying geometry) and TT serve as good mediators of the extension of the metal wave function at E(F), which would be closely related to charge transport at organic-metal interfaces.

Impact of structural imperfections on the energy-level alignment in organic films

This paper reports that structural imperfection in an organic thin film modulates the electronic structure to result in a serious band bending and change in the energy-level alignment (ELA) at the organic-conductor interface. Ultraviolet photoelectron spectroscopy (UPS) and metastable atom electron spectroscopy (MAES) were adopted to investigate thickness dependences of the electronic structure of polar phthalocyanine (chlorogallium phthalocyanine) thin films grown on graphite with respect to the film structure. We observed a large band-bending--like shift of occupied molecular-orbital bands toward the Fermi level and a continuous increase in the vacuum level for the as-grown film, whereas these phenomena were considerably suppressed by annealing the film. Both the as-grown and annealed films were characterized as essentially the same stacked bilayer film structure; however, high-resolution UPS and MAES measurements evidenced that there are structural defects in the as-grown film but not clearly in the annealed film, indicating that the defects are the origin of the modulation of the ELA and the band bending. Controlling the structural imperfections is a key issue for the desired ELA in organic devices.

Observation and Analysis of Small Inclination of Thymine Molecules on Graphite

The orientation of vacuum-sublimated thymine molecules on graphite was studied by ultraviolet photoelectron spectroscopy (UPS) and metastable atom electron spectroscopy (MAES). MAES gave pronounced band intensity enhancement for π molecular orbitals, which confirms the nearly lying-down orientation of the molecules proposed by Uchihashi et al. (Appl. Surf. Sci. 2000, 157, 244). In UPS, a lowering of the vacuum level was observed upon monolayer formation and attributed to inclination of the molecular dipoles from the surface. The tilt angle of the molecular plane was estimated to be 2.1−4.4° by applying the Helmholtz equation to the vacuum level shift.

Potassium-benzene interactions on Pt(111) studied by metastable atom electron spectroscopy

Electron emission spectra obtained by thermal collisions of He(∗)(2(3)S) metastable atoms with C(6)H(6)/Pt(111), C(6)H(6)/K/Pt(111), and K/C(6)H(6)/Pt(111) were measured in the temperature range of 50-200 K to elucidate the adsorption/aggregation states, thermal stabilities of pure and binary films, and local electronic properties at the organic-metal interface. For C(6)H(6)/Pt(111), the He(∗)(2(3)S) atoms de-excite on the chemisorbed overlayer predominantly via resonance ionization followed by Auger neutralization and partly via Penning ionization (PI) yielding weak emission just below the Fermi level (E(F)). We assigned this emission to the C(6)H(6) π-derived states delocalized over the Pt 5d bands on the basis of recent density functional calculations. During the layer-by-layer growth, the C(6)H(6)-derived bands via PI reveal a characteristic shift caused by the final-state effect (hole response at the topmost layer). C(6)H(6) molecules chemisorb weakly on the bimetallic Pt(111) (θ(K)=0.1) and physisorb on the K multilayer. In both cases, the sum rule was found to be valid between the K 4s and C(6)H(6)-derived bands. The band intensity versus exposure plot indicates that the C(6)H(6) film grows on the K multilayer by the Volmer-Weber mechanism (island growth), reflecting the weak K-C(6)H(6) interactions. In case of K/C(6)H(6)/Pt(111), the K atoms are trapped on the topmost C(6)H(6) layer at 65 K, forming particlelike clusters. The surface plasmon satellite was identified for the first time and the loss energy increases with increasing cluster size. The K clusters are unstable above ∼100 K due to thermal migration into the C(6)H(6) film. When the cluster coverage is low, the K 4s band extends below and above E(F) of the Pt substrate and the anomaly is discussed in terms of vacuum level bending around the cluster.

Thickness-dependent electronic properties and molecular orientation of diradical metal complex thin films grown on SiO 2

We studied the thickness dependence of electronic structure and molecular orientation of neutral, square planar, diamagnetic diradical metal complex, bis( o-diiminobenzosemiquinonate) nickel(II) complexes (Ni(DIBSQ) 2), thin films grown on a SiO 2 substrate by using ultraviolet photoelectron spectroscopy in combination with metastable atom electron spectroscopy. We observed that the small hole-injection barrier of 0.46 eV for the monolayer film, and the barrier increases continuously with the film thickness and becomes larger than the electron-injection barrier. We further found that molecules are oriented with their long-axes nearly perpendicular to the surface both in the monolayer and multilayer films independent of the film thickness.

Origins of Improved Hole‐Injection Efficiency by the Deposition of MoO3 on the Polymeric Semiconductor Poly(dioctylfluorene‐alt‐benzothiadiazole)

AbstractThe electronic structure of the interfaces formed after deposition of MoO3 hole‐injection layers on top of a polymer light‐emitting material, poly(dioctylfluorene‐alt‐benzothiadiazole) (F8BT), is studied by ultraviolet photoelectron spectroscopy (UPS), X‐ray photoelectron spectroscopy and metastable atom electron spectroscopy. Significant band bending is induced in the F8BT film by MoO3 “acceptors” that spontaneously diffuse into the F8BT “host” probably driven by kinetic energy of the deposited hot MoO3. Further deposition leads to the saturation of the band bending accompanied by the formation of MoO3 overlayers. Simultaneously, a new electronic state in the vicinity of the Fermi level appears on the UPS spectra. Since this peak does not appear in the bulk MoO3 film, it can be assigned as an interface state between the MoO3 overlayer and underlying F8BT film. Both band bending and the interface state should result from charge transfer from F8BT to MoO3, and they appear to be the origin of the hole‐injection enhancement by the insertion of MoO3 layers between the F8BT light‐emitting diodes and top anodes.

Surface Structural Study on Ionic Liquids Using Metastable Atom Electron Spectroscopy

The surface structures of ionic liquids 1-CnH2n+1-3-methylimidazolium tetrafluoroborate (BF4−), trifluoromethanesulfonate (OTf) and bis(trifluoromethylsulfonyl)amide (TFSA) with n = 4, 8, and 10 were investigated by ultraviolet photoemission spectroscopy (UPS) and metastable atom electron spectroscopy (MAES). The MAES spectra reveal that the anions and cations share the surface of the room-temperature ionic liquids (RTILs) with a shorter alkyl chain at n = 4 although the alkyl chain layer tends to cover the outermost surface in the case of RTILs with longer alkyl chains of n = 8 and 10. Comparison of UPS and MAES spectra demonstrates that the nonpolar groups such as the alkyl chain and the CF3 group point toward the vacuum side, while the polar groups such as the SO3 and SO2 groups face toward the bulk side. A double-layered structure at the surface, which consists of an alkyl chain layer and a polar layer containing anions and imidazolium rings, is proposed from these observations.

Vacuum sublimed α,ω-dihexylsexithiophene thin films: Correlating electronic structure and molecular orientation

In order to correlate the molecular orientation of organic thin films with charge injection barriers at organic/metal interfaces, the electronic structure and molecular orientation of vacuum sublimed thin films of α,ω-dihexylsexithiophene (DH6T) on the substrates Ag(111), highly oriented pyrolytic graphite (HOPG), and tetratetracontane (TTC) precovered Ag(111) were investigated. Results from metastable atom electron spectroscopy, ultraviolet photoelectron spectroscopy, and x-ray diffraction were used to derive growth models (including molecular orientation and conformation) of DH6T on the different substrates. On Ag(111), DH6T exhibits a transition from lying molecules in the monolayer/bilayer range to almost standing upright molecules in multilayers. This is accompanied by a shift of the molecular energy levels to a lower binding energy by 0.65 eV with respect to the vacuum level. The unit cell of standing DH6T on lying DH6T on Ag(111) is estimated to be similar to the DH6T bulk phase. On HOPG, DH6T grows in the bulk phase with lying orientation, starting already from the monolayer coverage. DH6T on TTC precovered Ag(111) grows in an almost lying orientation and a conformation that allows a strong overlap of the hexyl chains of DH6T with the alkyl chains of TTC. In all cases, the electronic structure and, particulary, the ionization energy of DH6T is dependent on the orientation of DH6T, i.e., lying DH6T has higher ionization energy than standing DH6T.