High-resolution Soft X-ray Photoelectron Research Articles

Interface Energy Alignment between Lead Halide Perovskite Single Crystals and TIPS-Pentacene.

At present, there is a huge development in optoelectronic applications using lead halide perovskites. Considering that device performance is largely governed by the transport of charges across interfaces and, therefore, the interfacial electronic structure, fundamental investigations of perovskite interfaces are highly necessary. In this study, we use high-resolution soft X-ray photoelectron spectroscopy based on synchrotron radiation to explore the interfacial energetics for the molecular layer of TIPS-pentacene and lead halide perovskite single crystals. We perform ultrahigh vacuum studies on multiple thicknesses of an in situ formed interface of TIPS-pentacene with four different in situ cleaved perovskite single crystals (MAPbI3, MAPbBr3, FAPbBr3, and CsxFA1-xPbBryI3-y). Our findings reveal a substantial shift of the TIPS-pentacene energy levels toward higher binding energies with increasing thickness, while the perovskite energy levels remain largely unaffected regardless of their composition. These shifts can be interpreted as band bending in the TIPS-pentacene, and such effects should be considered when assessing the energy alignment at perovskite/organic transport material interfaces. Furthermore, we were able to follow a reorganization on the MAPbI3 surface with the transformation of the surface C 1s into bulk C 1s.

Band alignment of wide bandgap NiO/MoO3 and NiO/WO3p-n heterojunctions studied by high-resolution X-ray photoelectron spectroscopy

NiO is one of the few transition metal oxides exhibiting p-type conductivity and visible light transparency. On the other hand, MoO3 and WO3 are high work function, wide gap metal oxides which show predominantly n-type character and have been widely applied in carrier selective contacts, heterojunction diodes, electrochromic and sensing applications. Hence, detailed understandings of the optical properties and electronic band offsets of the NiO/MoO3 and NiO/WO3p-n heterojunctions are crucial for the design and application of transparent oxide-based optoelectronic devices utilizing these heterojunctions. In this work, we synthesized thin films of the three oxides and characterized their structural and optical properties. In particular, the band alignment of NiO/MoO3 and NiO/WO3p-n heterojunctions were studied by high-resolution soft x-ray photoelectron spectroscopy (XPS). We found that both heterojunctions have a type-II band offset with the NiO/MoO3 exhibiting a larger valence band offset of 2.02 eV compared to 1.71 eV for NiO/WO3. The band positions of the three oxides with respect to the vacuum level obtained by Ultraviolet-Ambient Photoemission spectroscopy suggest that MoO3 and WO3 have a propensity to be n-type due to the easy formation of native donor defects. In contrast, NiO has a p-type character due to the high position of its VBM which favors the formation of native acceptor defects.

High-resolution soft X-ray photoelectron spectroscopy of liquid water

High-resolution soft X-ray photoelectron spectra of liquid water (H(2)O and D(2)O) were measured using a liquid beam photoelectron spectrometer. The 1a(1) (O1s) band and the lowest valence 1b(1) band had single peaks, which is not consistent with the split 1b(1)→ 1a(1) of the X-ray emission band of liquid water if the splitting is assumed to originate from level shifts in two different hydrogen bonding structures. The second valence 3a(1) band of liquid water exhibited a flat top implying that two bands exist underneath a broad feature, which is similar to the case of the 3a(1) band of amorphous ice. The energy splitting between the two 3a(1) bands is estimated to be 1.38 eV (H(2)O) and 1.39 eV (D(2)O). Ab initio calculations suggest that the large splitting of the 3a(1) band is characteristic of water molecules that function as both proton donor and acceptor. The overall result is consistent with the conventional model of a tetrahedral hydrogen-bonding network in liquid water.

Internal Structure of InP/ZnS Nanocrystals Unraveled by High-Resolution Soft X-ray Photoelectron Spectroscopy

High-energy resolution photoelectron spectroscopy (DeltaE < 200 meV) is used to investigate the internal structure of semiconductor quantum dots containing low Z-contrast elements. In InP/ZnS core/shell nanocrystals synthesized using a single-step procedure (core and shell precursors added at the same time), a homogeneously alloyed InPZnS core structure is evidenced by quantitative analysis of their In3d(5/2) spectra recorded at variable excitation energy. When using a two-step method (core InP nanocrystal synthesis followed by subsequent ZnS shell growth), XPS analysis reveals a graded core/shell interface. We demonstrate the existence of In-S and S(x)-In-P(1-x) bonding states in both types of InP/ZnS nanocrystals, which allows a refined view on the underlying reaction mechanisms.

The evolution of Ga and As core levels in the formation of Fe∕GaAs (001): A high resolution soft x-ray photoelectron spectroscopic study

A high resolution soft x-ray photoelectron spectroscopic study of Ga and As 3d core levels has been conducted for Fe∕GaAs (001) as a function of Fe thickness. This work has provided unambiguous evidence of substrate disrupting chemical reactions induced by the Fe overlayer—a quantitative analysis of the acquired spectra indicates significantly differing behavior of Ga and As during Fe growth, and our observations have been compared with existing theoretical models. Our results demonstrate that the outdiffusing Ga and As remain largely confined to the interface region, forming a thin intermixed layer. Whereas at low coverages Fe has little influence on the underlying GaAs substrate, the onset of substrate disruption when the Fe thickness reaches 3.5Å results in major changes in the energy distribution curves (EDCs) of both As and Ga 3d cores. Our quantitative analysis suggests the presence of two additional As environments of metallic character: one bound to the interfacial region and another which, as confirmed by in situ oxidation experiments, surface segregates and persists over a wide range of overlayer thickness. Analysis of the corresponding Ga 3d EDCs found not two, but three additional environments—also metallic in nature. Two of the three are interface resident whereas the third undergoes outdiffusion at low Fe coverages. Based on the variations of the integrated intensities of each component, we present a schematic of the proposed chemical makeup of the Fe∕GaAs (001) system.

Reduction of Kondo lattice effects in Yb1−xLuxAl3 observed by soft x-ray photoelectron spectroscopy

We have carried out the bulk-sensitive and high-resolution soft x-ray photoelectron spectroscopy on Lu substituted intermediate-valence compound Yb1−xLuxAl3 (x = 0.4) at temperatures from 200 to 20 K. The temperature dependences of the bulk Yb 4f photoelectron spectra revealed in our preceding works on high purity YbAl3 have not been observed in this Lu substituted system. The temperature dependences of the bulk Yb 4f peak positions and the Yb valence in this system can be well reproduced by the single impurity Anderson model (SIAM), whereas the spectral behaviors in YbAl3 were not at all reproduced by the SIAM. These results confirm the importance of the Kondo lattice effects for YbAl3, for which the coherent lattice periodicity plays essential roles.

Nanoscopic nickel aluminate spinel (NiAl 2O 4) formation during NiAl(1 1 1) oxidation

Oxidation of a NiAl(1 1 1) single crystal surface was investigated using high resolution soft X-ray photoelectron spectroscopy (HRSXPS), high resolution scanning electron microscopy (HRSEM), energy dispersive X-ray spectroscopy (EDS), X-ray mapping, and atomic force microscopy (AFM). After repeated oxygen exposure, annealing, and cleaning cycles under ultrahigh vacuum conditions, a new oxide phase in the form of tiny 3-dimensional surface structures was detected. These features are several micrometers long and ∼300 nm high and oriented along low index 〈 1 ¯ 0 1 〉 directions in the plane of the substrate; they have nickel aluminate spinel (NiAl 2O 4) stoichiometry. We propose that repeated cycles of oxygen dosing and annealing of the NiAl(1 1 1) surface leads to oxygen diffusion into the bulk and nucleation of spinel below the surface.

High-Resolution Soft X-ray Photoelectron Spectroscopic Studies and Scanning Auger Microscopy Studies of the Air Oxidation of Alkylated Silicon(111) Surfaces

High-resolution soft X-ray photoelectron spectroscopy was used to investigate the oxidation of alkylated silicon(111) surfaces under ambient conditions. Silicon(111) surfaces were functionalized through a two-step route involving radical chlorination of the H-terminated surface followed by alkylation with alkylmagnesium halide reagents. After 24 h in air, surface species representing Si(+), Si(2+), Si(3+), and Si(4+) were detected on the Cl-terminated surface, with the highest oxidation state (Si(4+)) oxide signal appearing at +3.79 eV higher in energy than the bulk Si 2p(3/2) peak. The growth of silicon oxide was accompanied by a reduction in the surface-bound Cl signal. After 48 h of exposure to air, the Cl-terminated Si(111) surface exhibited 3.63 equivalent monoleyers (ML) of silicon oxides. In contrast, after exposure to air for 48 h, CH(3)-, C(2)H(5)-, or C(6)H(5)CH(2)-terminated Si surfaces displayed <0.4 ML of surface oxide, and in most cases only displayed approximately 0.20 ML of oxide. This oxide was principally composed of Si(+) and Si(3+) species with peaks centered at +0.8 and +3.2 eV above the bulk Si 2p(3/2) peak, respectively. The silicon 2p SXPS peaks that have previously been assigned to surface Si-C bonds did not change significantly, either in binding energy or in relative intensity, during such air exposure. Use of a high miscut-angle surface (7 degrees vs < or =0.5 degrees off of the (111) surface orientation) yielded no increase in the rate of oxidation nor change in binding energy of the resultant oxide that formed on the alkylated Si surfaces. Scanning Auger microscopy indicated that the alkylated surfaces formed oxide in isolated, inhomogeneous patches on the surface.

High-resolution soft x-ray photoelectron study of density of states and thermoelectric properties of the Heusler-type alloys(Fe2∕3V1∕3)100−yAly

The high-resolution soft x-ray photoelectron study of HeuslerL21-type Fe2/3V1/3100�yAly y = 23.8� 25.8 alloys directly confirms the correlation between their thermoelectric properties and the electronic structures near the Fermi level. It shows the rigid-band-like shift of the main 3d bands and the increase of the photoelectron intensity at the Fermi edge with the small deviation of the Al content from stoichiometry, i.e., y = 25.0. This implies the existence of the pseudogap across the Fermi level in Fe2VAl. The Seebeck coefficients evaluated from the present spectroscopic data agree well with the experimental ones as well as the values estimated from a theoretical density of states in a rigid-band model, while the shift is much smaller than what is expected in the rigid-band model. The small modification of the electronic structure in the offstoichiometric Fe2/3V1/3100�yAly alloys may be attributed to the lower contribution of Al to the electronic states near EF.

High resolution photoemission study of growth, alloying, and intermixing of ultrathin ruthenium films on W(1 1 1) and W(2 1 1)

The alloying, intermixing, growth and faceting of ruthenium overlayers on tungsten single crystals have been studied with high resolution soft X-ray photoelectron spectroscopy, SXPS, (using synchrotron radiation) and low energy electron diffraction (LEED). W4f core-level photoemission spectra and W valence band spectra have been measured at various photon energies, and at normal and grazing emission angles, for W(1 1 1) and W(2 1 1). These two surfaces were chosen for this study because it has been shown previously that W(1 1 1) covered with monolayer films of several different 4d and 5d transition metals develop nanoscale pyramidal facets with (2 1 1) faces, upon annealing. The present work extends the measurement to another catalytically active overlayer metal, Ru. The growth and evolution for dosing and annealing on both W surfaces are investigated for coverages from 0 to greater than 3 monolayers of Ru (1 ML=1.7 × 10 15 atoms/cm 2). Incremental dosing of Ru causes intermixing of the Ru and W atoms at the interface, even at fractional monolayer coverages. Annealing of surfaces with Ru coverages > 1 ML produces a complex set of SXPS results that indicate the formation of surface Ru/W alloys. Faceting of Ru/W(1 1 1) is observed by LEED for Ru coverages > 1 ML, after annealing at temperatures between 700 and 1000 K. Upon annealing to temperatures higher than 1000 K, the SXPS data indicate that clustering of the Ru–W surface alloys may occur.

Chemistry of sulfur-containing molecules on Au( [formula omitted]): thiophene, sulfur dioxide, and methanethiol adsorption

The interactions of three sulfur-containing molecules (C 4H 4S, SO 2, CH 3SH) with a clean Au(1 1 1) surface have been studied with a combination of thermal desorption spectroscopy (TDS) and synchrotron-based high-resolution soft X-ray photoelectron spectroscopy. The adsorption and reactivity of the three molecules on Au(1 1 1) are very different. Thiophene adsorbs molecularly on Au(1 1 1) at 100 K and desorbs completely below 330 K without further decomposition. In the submonolayer range, three different adsorption states for chemisorbed thiophene are identified in TDS. It is suggested that thiophene preferably adsorbs on the defect sites at the lowest exposure. After the defect sites are saturated, the change from a flat-lying geometry to a tilted adsorption configuration follows as the exposure increases. Sulfur dioxide also does not decompose on Au(1 1 1). For SO 2 adsorption at 100 K, in addition to the multilayer desorption feature (∼130 K), only one distinct monolayer peak with a tail extending to higher temperature appears in TDS. The desorption temperature difference between the SO 2 monolayer and multilayer is only 15 K, indicating a weak binding between SO 2 and Au. For methanethiol adsorption on Au(1 1 1) at 100 K, three desorption states appear in the submonolayer range for the parent thiol. All of them appear below 300 K. The only desorption products at higher temperature are methane or methyl radicals (∼540 K), and dimethyl disulfide (∼470 K). Apart from the intact methyl thiol molecule, which exists at low temperatures (⩽150 K), two inequivalent intermediate thiolates, are seen to coexist on Au(1 1 1) in the 150–400 K temperature range, with one of them existing as low as 100 K. Atomic sulfur is present on the surface from 200 to 950 K.

Photoelectron spectroscopy studies of growth, alloying, and segregation for transition-metal films on tungsten (211)

High resolution soft x-ray photoelectron spectroscopy using synchrotron radiation and Auger-electron spectroscopy are used to study late-transition-metal films (Pt, Pd, Ir, Rh, Au,) on W(211). It is found that the films grow in a layer mode at 300 K. As a function of the film thickness, different ${4f}_{7/2}$ photoemission peaks are observed, corresponding to a single monolayer, to an interface layer, to bulk atoms and to surface atoms. Single physical monolayers of these late-transition metals on tungsten are stable against thermal rearrangement. In contrast, when multilayer films of Pd, Pt, Ir, Rh are annealed above 700--1000 K, tungsten atoms diffuse into the overlayer to form an alloy film. Gold constitutes a different case; upon annealing it does not alloy with tungsten but the metal in excess of one monolayer forms clusters. The evolution of the bimetallic systems as a function of coverage and annealing temperature is interpreted by analysis of intensities and shapes of the ${4f}_{7/2}$ features. Born-Haber cycles and the equivalent core approximation are used to extract thermochemical data concerning energetics of adhesion, segregation, and alloying in these early-late transition-metal systems.

Band offsets for ultrathin SiO2 and Si3N4 films on Si(111) and Si(100) from photoemission spectroscopy

High resolution soft x-ray photoelectron spectroscopy with synchrotron radiation is used to study the interfaces of SiO2/Si(111), SiO2/Si(100), Si(111)/Si3N4, and SiO2/Si3N4 for device-quality ultrathin gate oxides and nitrides. The thin oxides and nitrides were grown by remote plasma deposition at a temperature of 300 °C. Aftergrowth samples were further processed by rapid thermal annealing for 30 s at various temperatures from 700 to 950 °C. The Si(111)/Si3N4 samples were air exposed and formed a thin ∼6 Å SiO2 layer with a Si(2p) core-level shift of 3.9 eV, thus allowing us to study both the Si(111)/Si3N4 and SiO2/Si3N4 interfaces with a single type of sample. We obtain band offsets of 4.54±0.06 eV for SiO2/Si(111) and 4.35±0.06 eV for SiO2/Si(100) with film thicknesses in the range 8–12 Å. The Si(111)/Si3N4 nitrides show 1.78±0.09 eV valence-band offset for 15–21 Å films. This value agrees using the additivity relationship with our independent photoemission measurements of the nitride–oxide valence-band offset of 2.66±0.14 eV. However, we measure a substantially larger SiO2/Si3N4 ΔEV value of 3.05 eV for thicker (∼60 Å) films, and this indicates substantial differences in core-hole screening for films of different thickness due to additional silicon substrate screening in the thinner (15–21 Å) films.

Substrate disorder induced by a surface chemical reaction: the fluorine-silicon interaction

Si(111) surfaces etched by XeF 2 are studied with high-resolution soft X-ray photoelectron spectroscopy. After a sufficient exposure to XeF 2, a thick fluorosilyl reaction layer forms on the surface. An inelastic loss feature in the Si2p core-level spectra collected from these surfaces indicates the presence of Si defects that have been created by the reaction. These defects are located both at the reaction layer/substrate interface and in the near-surface region of the substrate. As suggested by earlier theoretical studies, it is proposed that the defects in the substrate are created by the excess heat liberated from the formation of exothermic Si-F bonds. Reaction-induced defects are an integral and essential part of the etching process, as they enable F to bond with subsurface Si and form the characteristic fluorosilyl reaction layer.

The structure of sodium adsorption phases on Al(111)

The structures formed by the adsorption of Na and Al(111) have been studied by normal incidence standing X-ray wavefield absorption using both the (111) and (1̄11) Bragg reflections in order to obtain not only the Na-Al layer spacings, but also the adsorption sites. In the case of the ordered (√3 × √3)R30°-Na phase, at a nominal coverage of 0.33 ML, and at lower coverages down to approximately 0.12 ML, Na is found to adsorb in a coverage independent site which involves a substitution of some of the top layer Al atoms; if the substrate layer spacings remain unchanged by this reconstruction, the effective radius of the adsorbed Na species is found to be 1.67 Å. The higher coverage (2 × 2)-Na phase is found to involve (at least) two distinct Na adsorption sites, but each of these has a very similar layer spacing relative to the extended (111) substrate scatterer lattice planes. A specific model of this phase, involving two reconstructed layers each of stoichiometry NaAl 2 is proposed, which accounts in a quantitative way for the present X-ray standing wave data and previous X-ray absorption data, and in a qualitative way for published high resolution soft X-ray photoelectron spectroscopy results. As Al and Na are essentially immiscible in the bulk, this two-layer structure appears to be an entirely new kind of surface phase.