Using Ultraviolet Photoemission Spectroscopy Research Articles

Spectroscopic signature of negative electronic compressibility from the Ti core-level of titanium carbonitride MXene

Two-dimensional transition metal carbides, carbonitrides, and nitrides, called MXenes, exhibit high metallic conductivity, ion intercalation capability, and reversible redox activity, prompting their applications in energy storage and conversion, electromagnetic interference (EMI) shielding, and electronics, among many other fields. It has been shown that replacement of ∼50% of carbon atoms in the most popular MXene family member, titanium carbide (Ti3C2Tx), by nitrogen atoms, forming titanium carbonitride (Ti3CNTx), leads to drastically different properties. Such properties include very high negative charge in solution and extreme EMI shielding effectiveness, exceeding all known materials, even metals at comparable thicknesses. Here, by using ultraviolet photoemission spectroscopy (UPS), the electronic structures of Ti3CNTx and Ti3C2Tx are systematically investigated and compared as a function of charge carrier density. We observe that, in contrast to Ti3C2Tx, the Ti 3p core-level of Ti3CNTx exhibits a counterintuitive shift to a lower binding energy of up to ∼250 meV upon increasing the electron density, which is a spectroscopic signature of negative electronic compressibility (NEC). These experimentally measured chemical potential shifts are well captured by the density functional theory (DFT) calculation. The DFT results also further suggest that the hybridization of titanium–nitrogen bonding in Ti3CNTx helps to promote the available states of Ti atoms for receiving more electrons above the fermi level and leads to the observed NEC. Our findings explain the differences in electronic properties between the two very important and widely studied MXenes and also suggest a new strategy to apply the NEC effect of Ti3CNTx in energy and charge storage applications.

Work function measurements of clean and modified carbon nanospikes

Carbon surfaces incorporating densely packed spikes with nanometer sharpness have shown exceptional electrocatalytic properties attributed to the enhanced electric field from the topography of the spikes. In this article, the composition of nanospike surfaces is identified by x-ray photoelectron spectroscopy (XPS) and their absolute work function determined using ultraviolet photoemission spectroscopy (UPS). Low temperature annealing of as-grown samples above 275 °C is required to produce a clean surface which has a 4.13 eV work function, a half volt lower than that of flat graphite. Treatments that contaminate the spiked surface, including exposure to air, oxidation at elevated temperature, or immersion in water or hydrocarbons, all increase the work function. Processing that blunts the spikes, including exposure to an oxygen plasma, argon sputtering, or high temperature annealing (800 °C) result in a work function close to that of flat graphite. An unusual double onset in the UPS secondary electron intensity is observed on as-grown nanospike samples and is reproduced by absorbing hydrocarbons on the surface. This double onset appears to be unique to carbon substrates and may originate in inelastic scattering of photoelectrons.

Interplay of negative electronic compressibility and capacitance enhancement in lightly-doped metal oxide Bi0.95La0.05FeO3 by quantum capacitance model

Light-sensitive capacitance variation of Bi0.95La0.05FeO3 (BLFO) ceramics has been studied under violet to UV irradiation. The reversible capacitance enhancement up to 21% under 405 nm violet laser irradiation has been observed, suggesting a possible degree of freedom to dynamically control this in high dielectric materials for light-sensitive capacitance applications. By using ultraviolet photoemission spectroscopy (UPS), we show here that exposure of BLFO surfaces to UV light induces a counterintuitive shift of the O2p valence state to lower binding energy of up to 243 meV which is a direct signature of negative electronic compressibility (NEC). A decrease of BLFO electrical resistance agrees strongly with the UPS data suggesting the creation of a thin conductive layer on its insulating bulk under light irradiation. By exploiting the quantum capacitance model, we find that the negative quantum capacitance due to this NEC effect plays an important role in this capacitance enhancement

Electronic and spin structure of O- and H-adsorbed Fe3O4 (111) surfaces

The adsorption of O and H atoms on the ${\mathrm{Fe}}_{3}{\mathrm{O}}_{4}$(111) surface was investigated using ultraviolet photoemission spectroscopy (UPS) and first-principles calculations. On the FeA1-terminated surface, H adsorption decreases the work function without changing the density of states near the Fermi level. The density of states near the Fermi level decreases by O adsorption. On the O/FeA1-terminated surface, H adsorption dramatically increases the density of states near the Fermi level, indicating that the Fe $3d$ state is modified. The first-principles calculations showed that the electronic structure of the surface FeA on the FeA1-terminated surface is half metallic with its spin up forming an isolated spin-polarized conductive layer. By O adsorption, the up-spin band at the Fermi level of the surface FeA site is removed, and the Fermi level of the surface FeA moves to the down-spin ${t}_{2g}$ band. By subsequent H adsorption, electrons are doped to the FeA1 site, and the surface FeA1 layer becomes semiconducting. These results indicate that the charge and spin structure is modulated by O and H adsorption.

Study on interface engineering of layer-by-layer structure for applications in organic photodetector

The interfacial electronic structure of layer-by-layer 4-(dicyanomethylene)-2-t-butyl-6-(1,1,7,7-tetramethyljulolidyl-9-enyl)-4H-pyran (DCJTB)/fullerene C60 was investigated using ultra-violet photoemission spectroscopy (UPS). Photoemission data of period of DCJTB/C60/DCJTB films suggested the formation of surface dipole and interfacial band bending across the interfaces, which greatly facilitates the charge transfer from DCJTB to C60 and from C60 to DCJTB layer as well. When applied this layer-by-layer structure to a near-infrared photodetector, a maximum of photocurrent was achieved by the device with 3 periods of DCJTB/C60 thin films. Finally, the detailed work mechanism of this detector was discussed.

Intermixing Effect on Electronic Structures of TQ1:PC71BM Bulk Heterojunction in Organic Photovoltaics

The interface energetics and intermixing effects of the donor/acceptor bulk heterojunction (BHJ) blends of poly[2,3‐bis‐(3‐octyloxyphenyl) quinoxaline‐5, 8‐dilyl‐alt‐thiophene‐2, 5‐diyl]: [6,6]‐phenyl C71butyric acid methyl ester (TQ1:PC71BM) have been investigated using ultraviolet photoemission spectroscopy (UPS) in combination with the integer charge transfer model. The TQ1:PC71BM represents the useful model system for BHJ organic photovoltaics featuring effective charge generation and transport. It finds out that the positive integer charge state of TQ1 are equal in energy to the negative integer charge state of PC71BM, leading to a negligible potential step at TQ1:PC71BM interface and thus the vacuum level alignment. It is observed that the TQ1 accumulates on the top of TQ1:PC71BM BHJ and UPS spectra as function of various blend ratios suggest that the TQ1 mixes finely with PC71BM with the little work function modification in a wide range. In addition, no significant influence of the long‐range Coulomb interactions or the intermolecular hybridization on the occupied electronic structures is present for the well‐intermixed TQ1:PC71BM BHJs. These findings provide deep insights into the properties of BHJ blends and are beneficial for the performance optimization in organic photovoltaics.

Adsorption, film growth, and electronic structures of 2,7-dioctyl[1]benzothieno-[3,2-b][1]benzothiophene (C8-BTBT) on Cu (100)

Using ultraviolet photoemission spectroscopy (UPS), X-ray photoemission spectroscopy (XPS), atomic force microscopy (AFM), and grazing X-ray diffraction measurement(GIXRD), we systematically investigate the correlations of interface energy level structure, film growth and the molecular orientation of 2, 7-dioctyl[1]benzothieno-[3, 2-b][1]benzothiophene (C8-BTBT) on Cu(100). We find that the adsorption of the first layer of C8-BTBT molecules on Cu(100) is a stable physical one, and there is no chemical shift of the S 2p peaks of XPS and the ratio of the output of C to that of S is the same as the stoichiometric value of the molecular C8-BTBT. The heights of the steps of the upper layers of C8-BTBT in the AFM images are ～ 30 , close to the length of the molecular long c-axis, indicating the standing-up configuration of the upper molecules. AFM image shows that the upper molecules tend to grow into islands while the bottom molecules tend to grow into layer, suggesting an Stranski-Krastanov growth mode of multilayer C8-BTBT on Cu(100). The GIXRD shows an out-of-plane period of 30.21 , which consistently proves the standing-up configuration of the outer molecule layer. There is an electric dipole of 0.41 eV at the very interface pointing from the substrate copper to C8-BTBT, which will reduce the barrier for electron transport and increase the barrier for hole transport from Cu to C8-BTBT. The vacuum level (Evac) starts to bend downward after 16 deposition, and with the increase of the thickness of the film, a total downward shift of 0.42 eV is observed. The downward shift is ascribed to the changing of molecular orientation from lying down before 16 to standing up after 16 , which establishes an outward-pointing layer of C-H bonds and accordingly forms a dipole layer depressing the surface barrier. The shape and leading edge of the hightest occupied molecular orbit (HOMO) also change with the increase of film thickness. These changes are due to the anisotropy of electron ionization from molecular orbit. The total downward shift of the HOMO is about 0.63 eV. The downward bending of 0.42 eV for Evac and 0.63 eV for HOMO with increasing film thickness lead to a slightly decreasing ionization potential (IP) about 0.1 eV before 32 and then an increasing IP about 0.31 eV, which finally results in a total increase of 0.21 eV for IP. The bending electronic structures facilitate electron transport from interface to surface and hole transport from surface to interface. Our Investigation provides valuable information for relevant device design.

Electronic structure evolution in doping of fullerene (C60) by ultra-thin layer molybdenum trioxide

Ultra-thin layer molybdenum oxide doping of fullerene has been investigated using ultraviolet photoemission spectroscopy (UPS) and X-ray photoemission spectroscopy (XPS). The highest occupied molecular orbital (HOMO) can be observed directly with UPS. It is observed that the Fermi level position in fullerene is modified by ultra-thin-layer molybdenum oxide doping, and the HOMO onset is shifted to less than 1.3 eV below the Fermi level. The XPS results indicate that charge transfer was observed from the C60 to MoOx and Mo6+ oxides is the basis as hole dopants.

Electronic properties and adsorption structures of tetracene on the Ag(110) surface

The adsorption of tetracene on Ag(110) surface has been studied using ultraviolet photoemission spectroscopy (UPS) and first-principles density function theory (DFT) calculation. The five emission features originating from the organic molecules locate at 3.4, 5.7, 6.3, 7.2, and 8.8eV below the Fermi level, respectively. There is no apparent binding energy shift of these features with the increasing coverage of tetracene, indicating a relatively weak interaction between tetracene and the Ag(110) substrate. Moreover, our DFT calculation shows that tetracene molecules prefer to adsorb between two Ag atomic rows of the first layer with its molecular axis along the [001] direction of the substrate, and the center of its second benzene ring locates at top site of Ag atoms of the second layer. With this optimized adsorption configuration, the π–d interactions between tetracene and Ag(110) are maximized. Comparing with the densities of states (DOS) before and after tetracene adsorption, no apparent intensity change can be observed for the DOS from p-orbital of tetracene, suggesting that the coupling between the substrate d-band and the filled p-orbitals of tetracene are relatively weak.

Surface analytical investigation on organometal triiodide perovskite

In a little over a year, there has been an unexpected breakthrough and rapid evolution of highly efficient solid-state hybrid solar cells based on organometal trihalide perovskite materials. This technology has the potential to produce solar cells with the very highest efficiencies while retaining the very lowest cost. The authors have measured the electronic density of states of CH3NH3PbI3 using ultraviolet photoemission spectroscopy (UPS), inverse photoemission spectroscopy (IPES), and x-ray photoemission spectroscopy (XPS). The valence band maximum and conduction band minimum positions are obtained from the UPS and IPES spectra, respectively, by linear extrapolation of the leading edges. The authors investigate the Au/perovskite and C60/perovskite interfaces by UPS and XPS. An interface dipole of 0.1 eV is observed at Au/perovskite interface. The energy levels of perovskite shift upward by ca.0.4 eV with Au coverage of 64 Å upon it, resulting in band bending, hence a built-in field in perovskite that encourages hole transport to the interface. The XPS results show a strong initial shift of core levels to lower binding energy in the perovskite, which indicates that electrons transfer from the perovskite film to fullerene molecules. Further deposition of fullerene forms C60 solid, accompanied by the reduction of the electron transfer. The strongest electron transfer happened at 1/4 monolayer of fullerene.

Electronic structure evolution of fullerene on CH3NH3PbI3

The thickness dependence of fullerene on CH3NH3PbI3 perovskite film surface has been investigated by using ultraviolet photoemission spectroscopy (UPS), X-ray photoemission spectroscopy (XPS), and inverse photoemission spectroscopy (IPES). The lowest unoccupied molecular orbital and highest occupied molecular orbital (HOMO) can be observed directly with IPES and UPS. It is observed that the HOMO level in fullerene shifts to lower binding energy. The XPS results show a strong initial shift of core levels to lower binding energy in the perovskite, which indicates that electrons transfer from the perovskite film to fullerene molecules. Further deposition of fullerene forms C60 solid, accompanied by the reduction of the electron transfer. The strongest electron transfer happened at 1/4 monolayer of fullerene.

Electronic structure evolution in doping of fullerene (C60) by molybdenum trioxide

Molybdenum oxide doping of fullerene has been investigated using ultraviolet photoemission spectroscopy (UPS), X-ray photoemission spectroscopy (XPS), and inverse photoemission spectroscopy (IPES). The lowest unoccupied molecular orbital and the highest occupied molecular orbital (HOMO) can be observed directly with IPES and UPS. It is observed that the Fermi level position in fullerene is modified by molybdenum oxide doping, and the HOMO onset is shifted to less than 0.3 eV below the Fermi level. The energy level shift is found to saturate at doping ratio of 18%. Till this stage, the shift depends on the doping concentration in a semi-logarithmic scale, with a slope substantially higher than that of the traditional semiconductor theory. The XPS results indicate that charge transfer continues beyond the energy level shift saturation till the doping ratio reaches 66% as evidenced by the Mo5+ component. At higher doping concentration, there is more Mo6+ component, which indicates the saturation of the charge transfer between MoOx and C60.

Electronic structure and carrier transport at laminated polymer homojunctions

Soft contact lamination, whereby films prepared separately from solution are brought into contact to form a single device, was used here to form homojunctions comprising two identical layers of poly(3-hexylthiophene) (P3HT) or two layers of poly[(9,9-dioctylfluorenyl-2,7-diyl)-co-(1,4-benzo-{2,1′-3}-thiadiazole)] (F8BT). Using ultraviolet photoemission spectroscopy (UPS), Kelvin Probe Force Microscopy (KPFM), and current–voltage (I–V) measurements, the electronic structure of, and carrier transport across, these homojunctions were investigated. UPS and KPFM show that lamination does not introduce any significant offset in the molecular levels across the interface. The I–V characteristics confirm this result by showing that transport across the film is largely unaffected by the presence of the laminated interface. This important result means that lamination could become a versatile tool for constructing multi-layer polymer devices.

Photoemission study of electronic structure evolution across the metal–insulator transition of heavily B-doped diamond

We studied the electronic structure evolution of heavily B-doped diamond films across the metal–insulator transition (MIT) using ultraviolet photoemission spectroscopy (UPS). From high-temperature UPS, through which electronic states near the Fermi level (EF) up to ∼5kBT can be observed (kB is the Boltzmann constant and T the temperature), we observed the carrier concentration dependence of spectral shapes near EF. Using another carrier concentration dependent UPS, we found that the change in energy position of sp-band of the diamond valence band, which corresponds to the shift of EF, can be explained by the degenerate semiconductor model, indicating that the diamond valence band is responsible for the metallic states for samples with concentrations above MIT. We discuss a possible electronic structure evolution across MIT.

In-gap state and effect of light illumination inCuIr2S4probed by photoemission spectroscopy

We have studied disorder-induced in-gap states and effect of light illumination in the insulating phase of spinel-type ${\text{CuIr}}_{2}{\text{S}}_{4}$ using ultraviolet photoemission spectroscopy (UPS). The ${\text{Ir}}^{3+}/{\text{Ir}}^{4+}$ charge-ordered gap appears below the metal-insulator transition temperature. However, in the insulating phase, in-gap spectral features with softgap are observed in UPS just below the Fermi level $({E}_{F})$, corresponding to the variable range hopping transport observed in resistivity. The spectral weight at ${E}_{F}$ is not increased by light illumination, indicating that the ${\text{Ir}}^{4+}{\text{-Ir}}^{4+}$ dimer is very robust, although the long-range octamer order would be destructed by the photoexcitation. Present results suggest that the ${\text{Ir}}^{4+}{\text{-Ir}}^{4+}$ bipolaronic hopping and disorder effects are responsible for the conductivity of ${\text{CuIr}}_{2}{\text{S}}_{4}$.

Electronic structure of frontier states in an evaporated thin film of a highly amphoteric and polar molecule

{4-[4,5-Bis(methylsulfanyl)-1,3-dithiol-2-ylidene]cyclohexa-2,5-dien-1-ylidene}malononitrile (BMDCM) is designed toward a highly amphoteric and polar molecule (HAPM). Its electron donating and accepting abilities in the solid state were examined from the measurements of its electronic structure using ultraviolet photoemission spectroscopy (UPS) and inverse photoemission spectroscopy (IPES): The threshold ionization energy I s th and electron affinity A s th in the solid state were determined to be 5.1 ± 0.1 5 and 3.5 ± 0.4 eV, respectively. These values turn out to be comparable to the corresponding values of typical donor or acceptor compounds, indicating high abilities in both electron donating and accepting nature of BMDCM. The origin of such a nature of BMDCM is discussed in terms of aromatic stabilization in its ionic states.

Water-Induced Negative Electron Affinity on Diamond (100)

The interaction of water vapor on clean diamond (100) has been studied using ultraviolet photoemission spectroscopy (UPS), X-ray photoemission spectroscopy (XPS), X-ray-absorption near-edge structure (XANES) spectroscopy, and high-resolution electron energy loss spectroscopy (HREELS). It is shown that water dissociates at room temperature on clean diamond forming C−H and C−OH bonds, resulting in a surface dipole layer which produces the condition of negative electron affinity (NEA). The strong polarization dependence of the O K XANES could be associated with the out-of-plane orientation of the OH bonds. Density functional theory (DFT) calculations confirm the existence of NEA on this surface with a mixture of hydrogen and hydroxyl (OH) terminations.

Study on the interaction between tetracene and Cu(110) surface

The electronic structure of tetracene on Cu (110) surface has been studied by using ultraviolet photoemission spectroscopy (UPS). The emission features from the organic molecule are located from 1 to 10 eV below the Fermi level, and they shift in binding energy with increasing the coverage of the organic material. For the surface with multilayer of tetracene, six well-resolved features were found at 1.90, 3.40, 4.70, 5.95, 6.95, and 9.15 eV below the Fermi level, respectively. On the surface with a lower coverage of tetracene, angle-resolved UPS measurements suggest that the molecular plane is parallel to the substrate. Density functional theory calculation confirms the flat-lying adsorption mode and shows that the tetracene molecule prefers to be adsorbed on the long bridge site with its long axis in the [110] azimuth.

Electronic Structure of Methoxy-, Bromo-, and Nitrobenzene Grafted onto Si(111)

The properties of Si(111) surfaces grafted with benzene derivatives were investigated using ultraviolet photoemission spectroscopy (UPS) and X-ray photoelectron spectroscopy (XPS). The investigated materials were nitro-, bromo-, and methoxybenzene layers (-C(6)H(4)-X, with X = NO(2), Br, O-CH(3)) deposited from diazonium salt solutions in a potentiostatic electrochemical process. The UPS spectra of the valence band region are governed by the molecular orbital density of states of the adsorbates, which is modified from the isolated state in the gas phase due to molecule-molecule and molecule-substrate interaction. Depending on the adsorbate, clearly different emission features are observed. The analysis of XPS intensities clearly proves multilayer formation for bromo- and nitrobenzene in agreement with the amount of charge transferred during the grafting process. Methoxybenzene forms only a sub-monolayer coverage. The detailed analysis of binding energy shifts of the XPS emissions for determining the band bending and the secondary electron onset in UPS spectra for determining the work function allow one to discriminate between surface dipole layers--changing the electron affinity--and band bending, affecting only the work function. Thus, complete energy band diagrams of the grafted Si(111) surfaces can be constructed. It was found that silicon surface engineering can be accomplished by the electrochemical grafting process using nitrobenzene and bromobenzene: silicon-derived interface gap states are chemically passivated, and the adsorbate-related surface dipole effects an increase of the electron affinity.

Influence of molecular conformation on organic/metal interface energetics

Using ultraviolet photoemission spectroscopy (UPS), we show that the conformation of a conjugated organic molecule can be changed significantly upon adsorption on a metal surface, even in the case of weak interactions with the metal. This is accomplished by comparing the thickness-dependent spectral evolution for a non-rigid non-planar molecule ( p-sexiphenyl) deposited onto Ag(1 1 1), with those of a coplanar molecule (α-sexithienyl) and a rigid non-planar one (1,3,4,6,7,9,10,12,13,15,16,18-dodecamethyl-hexa- peri-hexabenzocoronene). Molecular conformation changes can influence hole injection barriers by at least 0.1 eV. These results have a direct impact on the detailed understanding of organic/metal interfaces.