Ultraviolet Photoemission Spectroscopy Measurements Research Articles

Non-covalent functionalized Schottky interface at Ti3C2Tx/c-Si van der Waals heterojunction

Synergistic interaction between 2D materials and organic molecules presents an additional dimension for tuning their intrinsic properties. Herein, we aim totune the work function of 2D Ti3C2Txby introducing ultrathin interlayers of organic dipoles (O.D.) with a defined dipole moment value. Interface engineering is achieved through the inclusion of poly(ethylene)amine (PEI 0.1 %) and third generation poly(amido-)amine (PAMAM G3), between the Ti3C2TXand c-Si. c-Si/O.D./Ti3C2Tx heterostructures were fabricated by simple drop casting of the aqueous MXene solution on O.D. coated c-Si substrates. Charge transport properties of the fabricated Schottky diodes with structure of c-Si/O.D./Ti3C2TXwere evaluated through systematic analysis of the I-V and C-V characteristics. Our investigations reveal that diodes featuring O.D. as interlayers exhibit substantially reduced reverse saturation current density (J0) and enhanced built-in potential (Vbi).Work function of the fabricated c-Si/MXene/O.D. structures were evaluated from the ultraviolet photo-emission spectroscopy (UPS) measurements. We report a significant reduction in the work function value of Ti3C2Txfrom 5.8 eV to 4.2 eV for Ti3C2Tx/PEI 0.1 % and 3.3 eV for Ti3C2Tx/PAMAM-G3 heterostructures. Our studyintroduces an innovative approach for modifying the work function of Ti3C2Tx through the incorporation of O.D., highlighting the versatility of MXene electrodes to shape the future optoelectronic devices.

Toward Low‐Power Cryogenic Metal‐Oxide Semiconductor Field‐Effect Transistors

Herein, cryogenic field‐effect transistors (FETs) are discussed. In particular, the saturation of the subthreshold swing due to band tailing is studied. It is shown with simulations and experiments that engineering of the oxide‐channel interfaces and a strong increase of the gate oxide capacitance are effective in improving the switching behavior of the device. The implication of scaling the oxide capacitance on the power consumption of cryogenic devices is investigated, too. Furthermore, an alternative for conventional doping in cryogenic transistors is discussed. Based on synchrotron X‐Ray absorption spectroscopy at total fluorescence (XAS‐TFY) and ultraviolet photoemission spectroscopy (UPS) measurements, it is shown experimentally that in true nanoscale devices, a simple coating yields a shift of the conduction band that is equivalent to a very high dopant concentration. As a result, nanoscale cryogenic steep slope FETs with strongly improved electrical characteristics become feasible.

Bandgap Tunable Oxynitride LaNb2 O7-x Nx Nanosheets.

Bandgap tunable lanthanum niobium oxynitride [LaNb2 O7-x Nx ](1+x)- nanosheet is prepared by the delamination of a Ruddlesden-Popper phase perovskite oxynitride via ion-exchange and two-step intercalation processes. The lanthanum niobium oxynitride nanosheets have a homogeneous thickness of 1.6nm and exhibit a variety of chromatic colors depending on the nitridation temperature of the parent-layered oxynitride. The bandgap energy of the nanosheets is determined by ultraviolet photoemission spectroscopy, Mott-Schottky, and photoelectrochemical measurements and is found to be tunable in the range of 2.03-2.63eV. Furthermore, the oxide/oxynitride superlattice structures are fabricated by face-to-face stacking of 2D crystals using oxynitride [LaNb2 O7-x Nx ](1+x)- and oxide [Ca2 Nb3 O10 ]- nanosheets as building blocks. Moreover, the superlattices-like restacked oxynitride/oxide nanosheets hybrid exhibits unique proton conductivity and dielectric properties strongly influenced by the oxynitride nanosheets and enhanced photocatalytic activity under visible light irradiation.

Intrinsic Organic Semiconductors as Hole Transport Layers in p–i–n Perovskite Solar Cells

Thin polymeric and small‐molecular‐weight organic semiconductors are widely employed as hole transport layers (HTLs) in perovskite solar cells. To ensure ohmic contact with the electrodes, the use of doping or additional high work function (WF) interlayer is common. In some cases, however, intrinsic organic semiconductors can be used without any additive or buffer layers, although their thickness must be tuned to ensure selective and ohmic hole transport. Herein, the characteristics of thin HTLs in vacuum‐deposited perovskite solar cells are studied, and it is found that only very thin (<5 nm) HTLs readily result in high‐performing devices, as the HTL acts as a WF enhancer while still ensuring selective hole transfer, as suggested by ultraviolet photoemission spectroscopy and Kelvin probe measurements. For thicker films (≥5 nm), a dynamic behavior for consecutive electrical measurements is observed, a phenomenon which is also common to other widely used HTLs. Finally, it is found that despite their glass transition temperature, small‐molecule HTLs lead to thermally unstable solar cells, as opposed to polymeric materials. The origin of the degradation is still not clear, but might be related to chemical reactions/diffusion at the HTL/perovskite interface, in detriment of the device stability.

Light-Induced Charge Accumulation in PTCDI/Pentacene/Ag(111) Heterojunctions

The incorporation of singlet fission (SF) chromophores in solar cells is expected to bring significant increases in the power conversion efficiency thanks to multiexciton generation. However, efficient charge generation in the device is determined by the energy level alignment (ELA) between the active materials, which should favor exciton transport and separation under illumination. By combining ultraviolet photoemission spectroscopy and optical differential reflectance measurements, we determine the ELA in a prototypical SF heterojunction between pentacene (Pc) and perylene-tetracarboxylic-diimide (PTCDI) grown on Ag(111). Time-resolved X-ray photoelectron spectroscopy on such a system reveals light-induced modifications of the ELA; by measuring the transient shift of the core level photoemission lines we observe an accumulation of long-lived holes in the PTCDI within the first hundred picoseconds after the optical pump.

Surfactant-Encapsulated Polyoxometalate Complex as a Cathode Interlayer for Nonfullerene Polymer Solar Cells

Surfactant-Encapsulated Polyoxometalate Complex as a Cathode Interlayer for Nonfullerene Polymer Solar Cells

Gap States in Methylammonium Lead Halides: The Link to Dimethylsulfoxide?

Understanding the origin and distribution of electronic gap states in metal halide perovskite (MHP) thin films is crucial to the further improvement of the efficiency and long-term stability of MHP-based optoelectronic devices. In this work, the impact of Lewis-basic additives introduced in the precursor solution on the density of states in the perovskite bandgap is investigated. Ultraviolet photoemission spectroscopy and contact potential difference measurements are conducted on MHP thin films processed from dimethylformamide (DMF)-based solutions to which either no additive, dimethylsulfoxide (DMSO), or N-methylpyrrolidine-2-thione (NMPT) isadded. The results show the presence of a density of states in the gap of methylammonium lead halide films processed from DMSO-containing solution. The density of gap states is either suppressed when the methylammonium concentration in mixed cation films is reduced or when NMPT is used as an additive, and eliminated when methylammonium (MA) is replaced with cesium or formamidinium (FA). These results are consistent with the notion that reaction products that result from DMSO reacting with MA+ in the precursor solution are responsible for the formation of gap states.

Ultra-thin films of barium fluoride with low work function for thermionic-thermophotovoltaic applications

The deposition of barium fluoride thin and ultra-thin films on gallium arsenide substrates was performed by electron beam evaporation for analyzing the influence of film thickness and chemical composition on the work function of the resulting heterostructure. X-ray photoemission spectroscopy combined with ultraviolet photoemission spectroscopy measurements reveals that films of 2 nm nominal thickness and Ba/F = 1.0 stoichiometry ratio induce the achievement of a significantly low work function of 2.1 eV to the BaFx/GaAs heterostructure. The significant reduction of the work function at least down to 3.0 eV is confirmed by a test thermionic converter operating at a cathode temperature of 1385 °C, where the heterostructure was applied as anode. The low work function, together with a negligible optical absorption, makes feasible the practical application of barium fluoride coatings on GaAs within hybrid thermionic-thermophotovoltaic devices.

Dipolar hole-blocking layers for inverted perovskite solar cells: effects of aggregation and electron transport levels

Herein, we report on the synthesis and investigation of two triazino-isoquinoline tetrafluoroborate electrolytes as hole-blocking layers in methylammonium triiodide perovskite photovoltaic devices with fullerene electron extraction layer. We find that increasing the thickness of the dipolar hole-blocking layer results in a gradual increase in the open-circuit voltage suggesting that aggregation of the molecules can enhance the dipole induced by the layer. This finding is confirmed by theoretical calculations demonstrating that while both molecules exhibit a similar dipole moment in their isolated state, this dipole is significantly enhanced when they aggregate. Ultra-violet photoemission spectroscopy measurements show that both derivatives exhibit a high ionization potential of 7 eV, in agreement with their effective hole-blocking nature demonstrated by the devices. However, each of the molecules shows a different electron affinity due to the increased conjugation of one of the derivatives. While the change in electron transport level between the two derivatives is as high as 0.3 eV, the difference in the open-circuit voltage of both types of devices is negligible, suggesting that the electron transport level plays only a minor role in determining the open-circuit voltage of the device. Numerical device simulations confirm that the increase in built-in potential, arising from the high dipole of the electrolyte layer, compensates for the non-ideal energetic alignment of the charge transport levels, resulting in high open-circuit voltages for a range of electron transport levels. Our study demonstrates that the application of small molecule electrolytes as hole-blocking layer in inverted architecture perovskite solar cells is a powerful tool to enhance the open-circuit voltage and provides useful guidelines for designing future generations of such compounds.

Titanium-carbide MXenes for work function and interface engineering in perovskite solar cells.

To improve the efficiency of perovskite solar cells, careful device design and tailored interface engineering are needed to enhance optoelectronic properties and the charge extraction process at the selective electrodes. Here, we use two-dimensional transition metal carbides (MXene Ti3C2Tx) with various termination groups (Tx) to tune the work function (WF) of the perovskite absorber and the TiO2 electron transport layer (ETL), and to engineer the perovskite/ETL interface. Ultraviolet photoemission spectroscopy measurements and density functional theory calculations show that the addition of Ti3C2Tx to halide perovskite and TiO2 layers permits the tuning of the materials' WFs without affecting other electronic properties. Moreover, the dipole induced by the Ti3C2Tx at the perovskite/ETL interface can be used to change the band alignment between these layers. The combined action of WF tuning and interface engineering can lead to substantial performance improvements in MXene-modified perovskite solar cells, as shown by the 26% increase of power conversion efficiency and hysteresis reduction with respect to reference cells without MXene.

PbI2-MoS2 Heterojunction: van der Waals Epitaxial Growth and Energy Band Alignment.

van der Waals (vdW) epitaxy offers a promising strategy without lattice and processing constraints to prepare atomically clean and electronically sharp interfaces for fundamental studies and electronic device demonstrations. Herein, PbI2 was thermally deposited at high-vacuum conditions onto CVD-grown monolayer MoS2 flakes in a vdW epitaxial manner to form 3D-2D heterojunctions, which are promising for vdW epitaxial growth of perovskite films. X-ray diffraction, X-ray photoemission spectroscopy, Raman, and atomic force microscopy measurements reveal the structural properties of the high-quality heterojunctions. Photoluminescence (PL) measurements reveal that the PL emissions from the bottom MoS2 flakes are greatly quenched compared to their as-grown counterparts, which can be ascribed to the band alignment-induced distinct interfacial charge-transfer behaviors. Strong interlayer excitons can be detected at the PbI2/MoS2 interface, indicating an effective type II band alignment, which can be further confirmed by ultraviolet photoemission spectroscopy measurements. The results provide a new material platform for the application of the vdW heterojunctions in electronic and optoelectronic devices.

Upshift of the d Band Center toward the Fermi Level for Promoting Silver Ion Release, Bacteria Inactivation, and Wound Healing of Alloy Silver Nanoparticles.

Silver (Ag)-based nanoparticles (NPs) with a high potential of Ag+ release have been known to be capable of promoting bacteria inactivation and the wound healing process; however, keeping a steady flux of high levels of Ag+ in Ag-based NPs is still challenging. Herein, a novel strategy in terms of altering the intrinsic electronic structure of Ag NPs was attempted to facilitate Ag oxidation and boost the Ag+ flux, as results of improved antibacterial and wound healing performance of Ag NPs. Gold (Au), platinum (Pt), and palladium (Pd) were doped into Ag NPs to tune their d band centers to upshift toward the Fermi level, and the formed Pd-Ag alloy NPs showed the largest shift, followed by Pt-Ag and Au-Ag NPs, as determined by density function theory calculation and ultraviolet photoemission spectroscopy measurement. Further X-ray photoelectron spectroscopy analysis indicates that a larger upshift could induce less electron filling in the antibonding orbital and a higher Ag oxidation level, leading to the more remarkable Ag+ release as determined by inductively coupled plasma optical emission spectrometry. All these alloy Ag NPs could more efficiently inhibit bacterial growth and accelerate the wound healing process than pure Ag NPs, and their antibacterial activity and wound healing performance were progressively proportional to the upshift values of the d band center. Taken together, tuning the d band center to upshift toward the Fermi level becomes a feasible strategy for designing therapeutic Ag-based NPs with a promising antibacterial and wound healing performance.

Highly Efficient and Stable Perovskite Solar Cells via Modification of Energy Levels at the Perovskite/Carbon Electrode Interface.

Perovskite solar cells (PSCs) have attracted great attention in the past few years due to their rapid increase in efficiency and low-cost fabrication. However, instability against thermal stress and humidity is a big issue hindering their commercialization and practical applications. Here, by combining thermally stable formamidinium-cesium-based perovskite and a moisture-resistant carbon electrode, successful fabrication of stable PSCs is reported, which maintain on average 77% of the initial value after being aged for 192 h under conditions of 85 °C and 85% relative humidity (the "double 85" aging condition) without encapsulation. However, the mismatch of energy levels at the interface between the perovskite and the carbon electrode limits charge collection and leads to poor device performance. To address this issue, a thin-layer of poly(ethylene oxide) (PEO) is introduced to achieve improved interfacial energy level alignment, which is verified by ultraviolet photoemission spectroscopy measurements. Indeed as a result, power conversion efficiency increases from 12.2% to 14.9% after suitable energy level modification by intentionally introducing a thin layer of PEO at the perovskite/carbon interface.

Energy disorder and energy level alignment between host and dopant in organic semiconductors

Energy level alignment between host and dopant molecules plays a critical role in exciton formation and harvesting in light emission zone of organic light-emitting diodes. Understanding the mechanism for predicting energy level alignment is thus important in materials selection for fabricating high-performance organic light-emitting devices. Here we show that host-dopant energy level alignment strongly depends on film thickness and substrate work function by using X-ray and ultraviolet photoemission spectroscopy. Invariant Gaussian density of states fails to explain the experimental data. We speculate that energy disorder in molecules next to the surface dictates the alignment. Ultraviolet photoemission spectroscopy measurements of several archetypical organic semiconductors confirm our speculation. An empirical interface disorder function is derived and used to construct a functional Gaussian density of states to compute host energy levels. Host-dopant energy level alignment is then computed by applying the universal energy alignment rule and is found in excellent agreement with the experimental data.

Catalytic Intermediates of CO2 Hydrogenation on Cu(111) Probed by In Operando Near-Ambient Pressure Technique.

The in operando monitoring of catalytic intermediates is crucial for understanding the reaction mechanism and for optimizing the reaction conditions to improve the efficiency of the catalytic protocol; however, until now, this has remained a daunting challenge. Herein, we investigated the interaction of CO2 and H2 with the Cu(111) surface in a CO2 hydrogenation model system by using the in operando technique of near-ambient pressure X-ray photoelectron spectroscopy, which is further assisted by ultraviolet photoemission spectroscopy and low-energy electron diffraction (LEED) measurements. These techniques allowed the direct observation of CO2 dissociation into CO+O on the Cu(111) surface and the adsorption of O on the surface at room temperature. The intermediate HCOO- was unambiguously detected in the CO2 +H2 environment, which corroborated the formate pathway for methanol formation on the Cu(111) surface. We further found that O coverage can prevent the build up of graphitic carbon on the Cu surface. By taking advantage of the competitive interplay between Cu-O and graphitic carbon, we have proposed a feasible strategy for inhibition of the formation of graphitic carbon by tuning the CO2 and H2 partial pressures, which may contribute to sustaining the active Cu catalyst under the reaction conditions.

Trigonal (1T) and hexagonal (2H) mixed phases MoS2 thin films

Molybdenum di-sulfide thin films of thickness <1500 Å, were prepared by the dip-coating technique at different baking temperatures within the range 400 °C–450 °C using methanolic solution of ammonium molybdate and ammonium thiocyanate and hence studied their different properties using X-ray diffraction (XRD), UV-vis spectroscopy, Photoluminescence (PL), X-ray photoelectron spectroscopy (XPS), ultraviolet photoemission spectroscopy (UPS) and Current-Voltage (I-V) relationship measurements. XRD result shows that the MoS2 films are mixed metallic octahedral prismatic 1T- and trigonal prismatic bulk crystal 2H-phases. The I-V relationships show higher electrical conductivity for the films prepared at 400 °C and 450 °C than the films prepared at 425 °C. XPS measurement also shows the formation of mixed phases of MoS2 thin films as revealed by XRD. We have estimated the valence band maximum (VBM) and work functions from higher and lower energy range of UPS (He-I) spectra of MoS2 thin films prepared at 425 °C and 450 °C. The VBM are ∼4.4 eV & ∼4.2 eV and their work functions are ∼3.5 eV & ∼3.3 eV respectively that are correlated with conductivity and formation of mixed phases of MoS2 thin films.

Anomalous Ambipolar Transport of Organic Semiconducting Crystals via Control of Molecular Packing Structures.

Organic crystals deposited on 2-dimensional (2D) van der Waals substrates have been widely investigated due to their unprecedented crystal structures and electrical properties. van der Waals interaction between organic molecules and the substrate induces epitaxial growth of high quality organic crystals and their anomalous crystal morphologies. Here, we report on unique ambipolar charge transport of a "lying-down" pentacene crystal grown on a 2D hexagonal boron nitride van der Waals substrate. From in-depth analysis on crystal growth behavior and ultraviolet photoemission spectroscopy measurement, it is revealed that the pentacene crystal at the initial growth stage have a lattice-strained packing structure and unique energy band structure with a deep highest occupied molecular orbital level compared to conventional "standing-up" crystals. The lattice-strained pentacene few layers enable ambipolar charge transport in field-effect transistors with balanced hole and electron field-effect mobilities. Complementary logic circuits composed of the two identical transistors show clear inverting functionality with a high gain up to 15. The interesting crystal morphology of organic crystals on van der Waals substrates is expected to attract broad attentions on organic/2D interfaces for their electronic applications.

The thickness of the two-dimensional charge transfer state at the TTF-TCNQ interface

The properties of the interface between two materials can be significantly different to those of the constituents. One example is the tetrathiafulvalene (TTF) – 7,7,8,8-tetracyanoquinodimethane (TCNQ) interface, in which the interface can be metallic even though both TTF and TCNQ are wide bandgap semiconductors. Despite of its potential applications, the mechanism of the interfacial conduction remains unclear. In this work, we use ultraviolet photoemission spectroscopy and electrical measurement to determine the physical nature of the interfacial charges and the spatial extent of this interfacial state. The charge transfer state that is responsible for the interfacial conductivity is found to be quite localized. It only extends ≈1–2 nm away from the interface. Moreover, the electron-hole pairs at the interface are bound by Coulomb interaction to form excitons. Ohmic behavior and orders of magnitude decrease in the sheet resistance are observed by merely depositing a 2.4 nm TCNQ layer on a bulk TTF single crystal. The ultrathin nature of this highly conductive state suggests that it can be incorporated readily into ultrathin or field-effect organic devices.

Understanding the effects of electronic polarization and delocalization on charge-transport levels in oligoacene systems.

Electronic polarization and charge delocalization are important aspects that affect the charge-transport levels in organic materials. Here, using a quantum mechanical/embedded-charge (QM/EC) approach based on a combination of the long-range corrected ωB97X-D exchange-correlation functional (QM) and charge model 5 (CM5) point-charge model (EC), we evaluate the vertical detachment energies and polarization energies of various sizes of crystalline and amorphous anionic oligoacene clusters. Our results indicate that QM/EC calculations yield vertical detachment energies and polarization energies that compare well with the experimental values obtained from ultraviolet photoemission spectroscopy measurements. In order to understand the effect of charge delocalization on the transport levels, we considered crystalline naphthalene systems with QM regions including one or five-molecules. The results for these systems show that the delocalization and polarization effects are additive; therefore, allowing for electron delocalization by increasing the size of the QM region leads to the additional stabilization of the transport levels.

Electronic field effect tuning of the electronic properties of fluorinated epitaxial graphene

Ultraviolet photoemission spectroscopy measurements reveal that there is notable variation of the electron density of states in the valence band of multiple layers of fluorinated epitaxial graphene with applied electronic bias. The authors show that the tailoring of electronic band structure correlates with the decoupling of the graphene interlayers. The work function of the fluorinated graphene is tunable with the applied bias and is attributable to compensation of the fluorine-carbon dipole.