Inverse Photoemission Research Articles

Powerful Organic Molecular Oxidants and Reductants Enable Ambipolar Injection in a Large-Gap Organic Homojunction Diode.

Doping has proven to be a critical tool for enhancing the performance of organic semiconductors in devices like organic light-emitting diodes. However, the challenge in working with high-ionization-energy (IE) organic semiconductors is to find p-dopants with correspondingly high electron affinity (EA) that will improve the conductivity and charge carrier transport in a film. Here, we use an oxidant that has been recently recognized to be a very strong p-type dopant, hexacyano-1,2,3-trimethylene-cyclopropane (CN6-CP). The EA of CN6-CP has been previously estimated via cyclic voltammetry to be 5.87 eV, almost 300 meV higher than other known high-EA organic molecular oxidants. We measure the frontier orbitals of CN6-CP using ultraviolet and inverse photoemission spectroscopy techniques and confirm a high EA value of 5.88 eV in the condensed phase. The introduction of CN6-CP in a film of large-band-gap, large-IE phenyldi(pyren-1-yl)phosphine oxide (POPy2) leads to a significant shift of the Fermi level toward the highest occupied molecular orbital and a 2 orders of magnitude increase in conductivity. Using CN6-CP and n-dopant (pentamethylcyclopentadienyl)(1,3,5-trimethylbenzene)ruthenium (RuCp*Mes)2, we fabricate a POPy2-based rectifying p-i-n homojunction diode with a 2.9 V built-in potential. Blue light emission is achieved under forward bias. This effect demonstrates the dopant-enabled hole injection from the CN6-CP-doped layer and electron injection from the (RuCp*Mes)2-doped layer in the diode.

On the energy resolution of a GaAs-based electron source for spin-resolved inverse photoemission

The spin resolution in inverse photoemission spectroscopy is achieved by injecting spin-polarized electrons, usually produced by GaAs-based cold cathodes that replace hot-filament electron guns of spin-integrated setups. The overall energy resolution of the system can be enhanced by adjusting either the optical bandpass of the photon detector or the energy distribution of the electron beam. Here we discuss the influence of the photocurrent and the photocathode temperature on the energy broadening of the electron beam through the inverse photoemission spectra of the spin-splitted Shockley surface state of Au(111). First, we find that cooling down the GaAs photocathode to 77 K increases the band gap and reduces the number of allowed vertical transitions, monochromatizing the electron beam with an enhancement of about 30 meV for the energy resolution. Second, we observe a correlation between the generated photocurrent at the electron source, and the space-charge effects at the sample as a reduction of lifetime and spin asymmetry of a polarized bulk state. These observations allow defining a threshold of current density for the optimum acquisition in the measurements of spin-resolved inverse photoemission in Au.

A novel energy level detector for molecular semiconductors.

The multifunction of molecule-based devices is always achieved by improving their charge transport characteristics. These characteristics depend strongly on the energy levels of molecular semiconductors, which fundamentally govern the working principle and device performance. Therefore, an accurate measurement of these energy levels is crucial for evaluating the availability of the prepared materials and thus optimizing the device performance. Here, an easy-to-operate three-terminal hot electron transistor has been developed, which comprises a molecular optoelectronic device that records the charge transport. It achieves exceptional properties including the lowest unoccupied molecular orbit level, highest occupied molecular orbit level, higher energy states, and higher electronic bandgap. When compared with existing techniques such as cyclic voltammetry, inverse photoemission spectroscopy, and ultraviolet photoemission spectroscopy, the hot electron transistor provides in-situ characterization and categorizes the measured energy information as intrinsic properties of the molecular semiconductor. Furthermore, we provide an in-depth understanding of the fundamental device-physics, which provides promising guidance for performance optimization.

Locating the bandgap edges of eumelanin thin films for applications in organic electronics

AbstractBACKGROUNDBio‐sourced (natural) organic materials are often chemically and structurally disordered, such that their structure‐property relationships must be explored using model systems. Eumelanin is an interesting candidate among natural organic materials.RESULTSIn this work, the locations of the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) energy levels of 5,6‐dihydroxyindole (DHI) and 5,6‐dihydroxyindole‐2‐carboxylic acid (DHICA) building blocks (monomers) of the black‐brown biopigment eumelanin, in film form, are studied. The films are fabricated by the spin‐coating technique (i.e., here indicated as DHI‐ and DHICA‐films), which is sometimes followed by ammonia‐induced solid‐state polymerization (i.e., indicated as AISSP‐DHI and AISSP‐DHICA films), as well as by thermal evaporation (i.e., evaporated DHI and DHICA films). From Ultraviolet photoemission spectroscopy (UPS), we deduced the ionization energies (EI) for all DHI‐ and DHICA‐based films to be in the range of 5.34‐5.56 eV and 5.35‐5.80 eV, respectively. The electron affinities (χE) are measurable using inverse photoemission spectroscopy (IPES) for DHI films (3.80 eV) and both evaporated DHI (4.0 eV) and DHICA (3.9 eV) films. On the other hand, the χE values of DHICA, AISSP‐DHI, and AISSP‐DHICA films are estimated with about 0.5 eV of uncertainty. UV‐visible spectroscopy reveals the preferred chromophoric bands for DHICA, AISSP‐DHICA, and DHI are in the range of 300‐330 nm, while AISSP‐DHI exhibits a broadened UV‐visible absorbance.CONCLUSIONOur study paves the way for the design of suitable metal‐eumelanin interfaces for electronic applications. © 2021 Society of Chemical Industry (SCI).

Rashba-split surface state and spin-dependent photon emission from Re(0001) at Γ¯

The unoccupied electronic structure of the Re(0001) surface is investigated by spin- and angle-resolved inverse photoemission, experimentally and theoretically. The work is focused on the states around the center of the surface Brillouin zone $\overline{\mathrm{\ensuremath{\Gamma}}}$, where a hole pocket of a surface state with Rashba-type spin splitting is detected. Furthermore, we observe spin-dependent photon emission from unpolarized surface and bulk states at $\overline{\mathrm{\ensuremath{\Gamma}}}$. The size and sign of the spin asymmetry depends on experimental parameters such as the direction of the electron spin polarization and the photon detection angle. Maximum (zero) spin asymmetry is detected if the electron spin polarization and the plane of photon emission are perpendicular (parallel). The effect is traced back to spin-orbit-induced hybridization of the involved states.

Rashba-type splitting of the Au(110) surface state: A combined inverse and direct photoemission study

The Shockley surface state located at $\overline{\mathrm{Y}}$ on the $(1\ifmmode\times\else\texttimes\fi{}2)$-reconstructed Au(110) surface is predicted to exhibit a Rashba-type spin splitting. Previous photoemission experiments searched for this splitting but it could not be resolved yet. In order to uncover a possible splitting, the unoccupied surface state on Au(110) is examined with spin- and angle-resolved inverse photoemission, whereas Na-covered Au(110) allows for investigation of the now occupied surface state by means of spin- and angle-resolved direct photoemission. Our data show clear spin splittings in the order of $100\phantom{\rule{0.16em}{0ex}}\mathrm{meV}$ with a sign reversal at $\overline{\mathrm{Y}}$ in the surface state's in-plane spin components which is characteristic for a Rashba-type behavior. Furthermore, we deduce an effective mass of ${m}^{*}=(0.27\ifmmode\pm\else\textpm\fi{}0.02){m}_{e}$ and a Rashba parameter of ${\ensuremath{\alpha}}_{R}=(0.46\ifmmode\pm\else\textpm\fi{}0.04)\phantom{\rule{0.16em}{0ex}}\mathrm{eV}\phantom{\rule{0.16em}{0ex}}\AA{}$ from direct photoemission measurements.

Inverse photoemission of the light actinide metals and dioxides

Inverse photoemission spectroscopy (IPES) operating in the isochomat mode (≈10 eV) has been used to probe the unoccupied states in the actinide dioxides of thorium, uranium and plutonium, and the actinide metals of thorium, uranium, α-plutonium and a plutonium-gallium alloy, δ-Pu(Ga). The IPE spectra of the dioxides show a signal predominantly occurring from the 5f unoccupied states and where possible have been curve fit to obtain the 5f5/2 – 5f7/2 spin orbit coupling and intensity values. Combined with the HeI Ultraviolet Photoelectron Spectroscopy of the actinide dioxides the band gap values have been determined. The IPE spectra of the metals show a weak signal on a large background contribution with little difference between the α- and δ-Pu(Ga) phases. The IPE spectra of the metals have been used to aid interpretation of the possible screening mechanisms in the 4f core level photoelectron spectra. Photoelectric work functions for these materials are also reported. UK Ministry of Defence © Crown Owned Copyright 2021/AWE

Renormalization of the valence and conduction bands of (C6H5C2H4NH3)2PbI4 hybrid perovskite

Quasi two-dimensional hybrid organic-inorganic perovskites (HOIPs) have been rediscovered recently for photovoltaics due to a higher stability than other HOIPs. We focus here on the electronic structure of the 2D perovskite (C6H5C2H4NH3)2PbI4. We perform an experimental k-resolved determination of the valence and conduction bands by angle-resolved photoemission spectroscopy (ARPES) and inverse photoemission spectroscopy (IPES). The experimental and theoretical dispersions are compared. The valence band width is in agreement with that of renormalized theoretical bands, while no significative renormalization is observed for the conduction band. The effect of the spin–orbit coupling in the conduction band is also experimentally observed.

Tunable crystal structure of Cu2SnS3 deposited by spray pyrolysis and its impact on the chemistry and electronic structure

Cu2SnS3 (CTS) thin films were grown by spray pyrolysis using two synthetic routes: stoichiometric and substoichiometric in an aprotic media. The use of dimethyl sulfoxide (DMSO) as a solvent improved the solubility of metal chlorides at room temperature, which was crucial to avoid metal-hydroxylation. In this sense, a mechanism for the solvation of metal chloride precursors in an aprotic media is proposed. Different sulfurization temperatures allowed the presence of diverse crystalline structures of Cu2SnS3, corroborated by Raman Spectroscopy and X-ray diffraction. For the substoichiometric composition Cu/Sn = 1.76 and a temperature annealing interval of 350–600 °C, the films showed the monoclinic-Cu2SnS3 phase. While for the stoichiometric composition Cu/Sn = 2, the tetragonal-Cu2SnS3 phase was present between 300 and 400 °C. At 500 °C, the Cu content increase allows the orthorhombic-Cu3SnS4 phase formation. The presence of Sn4+ and Cu+ species was confirmed by X-ray photoelectron spectroscopy (XPS) measurements. Simultaneously, electronic structures of the surface of the film were obtained by XPS combined with inverse photoemission spectroscopy (IPES). Bandgap values were 1.33 eV, 1.25 eV, and 1.15 eV for those films with tetragonal, orthorhombic, and monoclinic phases. The proximity of the valence band maximum (VBM) to the Fermi level causes that these compounds have a p-type nature, producing a small cliff in the conduction band alignment at the CdS/CTS interface. Photoresponse measurements revealed that all the samples are photoconductive to be considered as an option for solar applications.

Hot electron spectroscopy: A novel method to study molecular semiconductor

Molecular semiconductors, as the core component, determine the performance of different devices. It is essential to gain more knowledge of such materials. Hot electron spectroscopy, by applying a ballistic electron emission microscope and hot-electron transistor, has been demonstrated to be the most powerful method. This review concentrates on the achievements in several emerging fields, such as the interfacial energy level alignment and the charge transport characteristics in molecular film. As a versatile technique, hot electron spectroscopy has been found to be helpful in exploring the metal/molecule interface energy, the molecular levels, the electronic transport gap, and even the group structures. In comparison to the traditional techniques, such as ultraviolet photoemission spectroscopy, inverse photoemission spectroscopy, and scanning tunneling spectroscopy, this method offers in-situ characterizations, which makes the detection much more accurate and convenient. This review contributes to a deep comprehension of the intrinsic characteristics of molecular semiconductors and molecule-based devices. • Hot electron spectroscopy has recently been employed to determine the intrinsic lowest unoccupied molecular orbital. • Hot electron spectroscopy has been developed to determine the highest occupied molecular orbital. • Hot electron spectroscopy has applied to acquire more informations about molecular levels, such as the LUMO+1 level. • Hot electron spectroscopy provides a powerful approach to achieve a high-efficient spin injection.

Unoccupied electronic states of 2D Si on Ag--Si(111)

Optimizing substrate characterization to grow 2D Si layers on surfaces is a major issue toward the development of synthesis techniques of the promising silicene. We have used inverse photoemission spectroscopy (IPES) to study the electronic band structure of an ordered 2D Si layer on the -Ag/Si(111) surface (-Ag). Exploiting the large upwards band bending of the -Ag substrate, we could investigate the evolution of the unoccupied surface and interface states in most of the Si band gap. In particular, the k ∥-dispersion of the -Ag free-electron-like S 1 surface state measured by IPES, is reported for the first time. Upon deposition of ∼1 ML Si on -Ag maintained at ∼200 °C, the interface undergoes a metal-insulator transition with the complete disappearance of the S 1 state. The latter is replaced by a higher-lying state U 0 with a minimum at 1.0 eV above E F. The origin of this new state is discussed in terms of various Si 2D structures including silicene.

Mapping the evolution of Bi/Ge(111) empty states: From the wetting layer to pseudo-cubic islands

Semiconductors interfaced with heavy elements possessing a strong atomic spin–orbit coupling are important building blocks for the development of new spintronic devices. Here, we present a microscopic and spin-resolved spectroscopic investigation of ultrathin Bi films grown onto a Ge(111) substrate. At monolayer coverage, a Bi wetting layer is formed, characterized by a semiconducting behavior and a (3×3)R30° superstructure. The wetting layer supports the subsequent growth of Bi islands with a pseudo-cubic structure similar to that of Bi(110), showing a well-defined orientation with respect to the substrate high-symmetry directions. We performed photoemission and spin-resolved inverse photoemission experiments at off-normal electron emission and incidence, respectively, along the substrate Γ¯K¯ direction. Inverse photoemission, in particular, highlights the presence of a spin-polarized empty Bi state, not reported so far, due to the strong spin–orbit effects characteristic of the Bi surface and thin layers. Finally, scanning tunneling spectroscopy is employed to link the observed spectroscopic features to either the wetting layer or the Bi islands.

Voltage controlled N\xe9el vector rotation in zero magnetic field

Multi-functional thin films of boron (B) doped Cr2O3 exhibit voltage-controlled and nonvolatile Néel vector reorientation in the absence of an applied magnetic field, H. Toggling of antiferromagnetic states is demonstrated in prototype device structures at CMOS compatible temperatures between 300 and 400 K. The boundary magnetization associated with the Néel vector orientation serves as state variable which is read via magnetoresistive detection in a Pt Hall bar adjacent to the B:Cr2O3 film. Switching of the Hall voltage between zero and non-zero values implies Néel vector rotation by 90 degrees. Combined magnetometry, spin resolved inverse photoemission, electric transport and scanning probe microscopy measurements reveal B-dependent TN and resistivity enhancement, spin-canting, anisotropy reduction, dynamic polarization hysteresis and gate voltage dependent orientation of boundary magnetization. The combined effect enables H = 0, voltage controlled, nonvolatile Néel vector rotation at high-temperature. Theoretical modeling estimates switching speeds of about 100 ps making B:Cr2O3 a promising multifunctional single-phase material for energy efficient nonvolatile CMOS compatible memory applications.

Electronic properties of carbon nanotubes as detected by photoemission and inverse photoemission

The relation between morphology and energy level alignment in carbon nanotubes (CNT) is a crucial information for the optimization of applications in nanoelectronics, optics, mechanics and (bio)chemistry. Here we present a study of the relation between the electronic properties and the morphology of single wall CNT (SWCNT), aligned multi wall CNT (MWCNT) and unaligned MWCNT. The CNT were synthesized via catalytic chemical vapor deposition in ultra-high vacuum conditions. Combined ultraviolet photoemission and inverse photoemission (IPES) spectra reveal a high sensitivity to the nanotube morphology. In the case of unaligned SWCNT the distinctive unoccupied Van Hove singularities (vHs) features are observed in the high resolution IPES spectra. Those features are assigned to semiconducting and metallic SWCNT states, according to calculated vHs DOS. The two MWCNT samples are similar in the diameter of the tube (about 15 nm) and present similar filled and empty electronic states, although the measured features in the aligned MWCNT are more defined. Noteworthy, interlayer states are also revealed. Their intensities are directly related to the MWCNT alignment. Focussing and geometrical effects associated to the MWCNT alignment are discussed to account the spectral differences.

Mapping the Density of States Distribution of Organic Semiconductors by Employing Energy Resolved–Electrochemical Impedance Spectroscopy

AbstractAlthough the density of states (DOS) distribution of charge transporting states in an organic semiconductor is vital for device operation, its experimental assessment is not at all straightforward. In this work, the technique of energy resolved–electrochemical impedance spectroscopy (ER‐EIS) is employed to determine the DOS distributions of valence (highest occupied molecular orbital (HOMO)) as well as electron (lowest unoccupied molecular orbital (LUMO)) states in several organic semiconductors in the form of neat and blended films. In all cases, the core of the inferred DOS distributions are Gaussians that sometimes carry low energy tails. A comparison of the HOMO and LUMO DOS of P3HT inferred from ER‐EIS and photoemission (PE) or inverse PE (IPE) spectroscopy indicates that the PE/IPE spectra are by a factor of 2–3 broader than the ER‐EIS spectra, implying that they overestimate the width of the distributions. A comparison of neat films of MeLPPP and SF‐PDI2 or PC(61)BM with corresponding blends reveals an increased width of the DOS in the blends. The results demonstrate that this technique does not only allow mapping the DOS distributions over five orders of magnitude and over a wide energy window of 7 eV, but can also delineate changes that occur upon blending.

Empty electron states in cobalt-intercalated graphene.

The dispersion of the electronic states of epitaxial graphene (Gr) depends significantly on the strength of the bonding with the underlying substrate. We report on empty electron states in cobalt-intercalated Gr grown on Ir(111), studied by angle-resolved inverse photoemission spectroscopy and x-ray absorption spectroscopy, complemented with density functional theory calculations. The weakly bonded Gr on Ir preserves the peculiar spectroscopic features of the Gr band structure, and the empty spectral densities are almost unperturbed. Upon intercalation of a Co layer, the electronic response of the interface changes, with an intermixing of the Gr π* bands and Co d states, which breaks the symmetry of π/σ states, and a downshift of the upper part of the Gr Dirac cone. Similarly, the image potential of Ir(111) is unaltered by the Gr layer, while a downward shift is induced upon Co intercalation, as unveiled by the image state energy dispersion mapped in a large region of the surface Brillouin zone.

Inverse photoemission measurements of plutonium metal and oxides

The unoccupied density of states of α-plutonium metal, plutonium sesquioxide (α-Pu2O3) and plutonium dioxide (PuO2) have been measured using inverse photoemission spectroscopy (IPS) in the isochromat mode (9.7 eV). Ultraviolet Photoelectron spectroscopy (UPS) at four different photon energies between 11 and 41 eV are also reported to support the interpretation of the IPS data. The band gap energies of the oxides have been determined from the combination of UPS and IPS spectra; the photoelectric work function was established from the UPS data for the metal and oxides and information from the UPS and IPS spectra have been used to aid interpretation of the core level Pu 4f XPS data. Crown copyright © 2020 Published by Elsevier B.V. All rights reserved.

Quantitative analysis of the electrostatic and electronic polarization energies in molecularly mixed films of organic semiconductors

The energy levels of organic semiconductors are primarily determined by the molecular orbital energies of constituent molecules. Recent studies have, however, shown that the energy levels can be changed by the mixing ratio of two molecules which have different permanent quadrupole moments. From the good correlation between the magnitude of the mixed film's energy shift and the constituent molecules' permanent quadrupole moment, it was noted that the molecular quadrupole plays an important role in the energy shift. In this study, ultraviolet photoelectron spectroscopy (UPS) and low-energy inverse photoemission spectroscopy (LEIPS) are applied to the mixed films of zinc phthalocyanine (ZnPc) and perfluorinated ZnPc (${\mathrm{F}}_{16}\mathrm{ZnPc}$), which have permanent quadrupole moments with opposite directions. From the precisely determined ionization energies and electron affinities, we directly determine the electronic polarization energy $D$ and electrostatic energy $S$ as a function of mixing ratio. Furthermore, we examined the molecular orientation dependence of $S$ and $D$ values. $D$ is almost independent of the mixing ratio (the difference is less than 0.2 eV over the range of mixing ratio) whereas $S$ differs by as much as 1.6 eV. The result clearly shows that the energy levels' continuous shift by the mixing ratio originates in the electrostatic interaction, whose leading term is the charge-permanent quadrupole interaction.

Synthesis of graphitic carbon nitride under low ammonia partial pressure

Graphitic carbon nitride (g-C3N4) is a very attractive potential solar-light-activated photocatalyst for hydrogen evolution. Substantial effort has been devoted to improving the photoactivity of g-C3N4, but to date most g-C3N4 prepared by the thermal polymerization of melamine are the linear polymer carbon nitride “melon”. We thus explored a new way of synthesizing g-C3N4 under low ammonia partial pressure using an approach similar to train sublimation. The obtained product (p-CN) is reddish-orange in color and has a different chemical composition and different optical properties from those of melon and melon-based materials. These differences were characterized by ultraviolet-visible spectroscopy, Fourier transform infrared spectroscopy, X-ray diffraction measurements and X-ray photoemission spectroscopy. Our findings show that p-CN is nano-scale g-C3N4. In addition, theoretical calculations using density functional theory, as well as direct observation of the characteristic electronic structure around the energy gap of p-CN using ultraviolet photoemission spectroscopy combined with inverse photoemission spectroscopy, provide evidence that p-CN has the g-C3N4 structure. These findings shed light on the synthesis of g-C3N4 composed of stacked two-dimensional layers of heptazine networks. Furthermore, the fundamental insights into the electronic structures of melem, melon and p-CN are particularly significant for research in the field of visible light-responsive photocatalysts.

The band shifts in MoS2(0001) and WSe2(0001) induced by palladium adsorption

The band structures of the transition metal dichalcogenides (TMD's) 2H-MoS2(0001) and 2H-WSe2(0001), before and after palladium adsorption, were investigated through angle-resolved photoemission. Palladium adsorption on 2H-MoS2(0001) is seen to result in very different band shifts than seen for palladium on 2H-WSe2(0001). The angle resolved photoemission results of palladium adsorbed on WSe2(0001) indicate that palladium accepts electron density from substrate. The resulting band shift will lead to a decrease in the barriers to the hole injection. The opposite band shifts occur upon palladium adsorption between 2H-MoS2(0001). The overall trend is consistent with the deposition of other metals deposited on TMD's, except that for palladium adsorption on MoS2(0001), there is an increase in the MoS2(0001) substrate band gap with palladium adsorption, as is evident from the combination of photoemission and inverse photoemission.