Vacancy-Engineered Nickel Ferrite Forming-Free Low-Voltage Resistive Switches for Neuromorphic Circuits.

Innovations in resistive switching devices constitute a core objective for the development of ultralow-power computing devices. Forming-free resistive switching is a type of resistive switching that eliminates the need for an initial high voltage for the formation of conductive filaments and offers promising opportunities to overcome the limitations of traditional resistive switching devices. Here, we demonstrate mixed charge state oxygen vacancy-engineered electroforming-free resistive switching in NiFe2O4 (NFO) thin films, fabricated as asymmetric Ti/NFO/Pt heterostructures, for the first time. Using pulsed laser deposition in a controlled oxygen atmosphere, we tune the oxygen vacancies together with the cationic valence state in the nickel ferrite phase, with the latter directly affecting the charge state of the oxygen vacancies. The structural integrity and chemical composition of the films are confirmed by X-ray diffraction and hard X-ray photoelectron spectroscopy, respectively. Electrical transport studies reveal that resistive switching characteristics in the films can be significantly altered by tuning the amount and charge state of the oxygen vacancy concentration during the deposition of the films. The resistive switching mechanism is seen to depend upon the migration of both singly and doubly charged oxygen vacancies formed as a result of changes in the nickel valence state and the consequent formation/rupture of conducting filaments in the switching layer. This is supported by the existence of an optimum oxygen vacancy concentration for efficient low-voltage resistive switching, below or above which the switching process is inhibited. Along with the filamentary switching mechanism, the Ti top electrode also enhances the resistive switching performance due to interfacial effects. Time-resolved measurements on the devices display both long- and short-term potentiation in the optimized vacancy-engineered NFO resistive switches, ideal for solid-state synapses achieved in a single system. Our work on correlated oxide forming-free resistive switches holds significant potential for CMOS-compatible low-power, nonvolatile resistive memory and neuromorphic circuits.

Exploiting hot electrons from a plasmon nanohybrid system for the photoelectroreduction of CO2.

Plasmonic materials convert light into hot carriers and heat to mediate catalytic transformation. The participation of hot carriers (photocatalysis) remains a subject of vigorous debate, often argued on the basis that carriers have ultrashort lifetime incompatible with drive photochemical processes. This study utilises plasmon hot electrons directly in the photoelectrocatalytic reduction of CO2 to CO via a Ppasmonic nanohybrid. Through the deliberate construction of a plasmonic nanohybrid system comprising NiO/Au/ReI(phen-NH2)(CO)3Cl (phen-NH2 = 1,10-Phenanthrolin-5-amine) that is unstable above 580 K; it was possible to demonstrate hot electrons are the main culprit in CO2 reduction. The engagement of hot electrons in the catalytic process is derived from many approaches that cover the processes in real-time, from ultrafast charge generation and separation to catalysis occurring on the minute scale. Unbiased in situ FTIR spectroscopy confirmed the stepwise reduction of the catalytic system. This, coupled with the low thermal stability of the ReI(phen-NH2)(CO)3Cl complex, explicitly establishes plasmonic hot carriers as the primary contributors to the process. Therefore, mediating catalytic reactions by plasmon hot carriers is feasible and holds promise for further exploration. Plasmonic nanohybrid systems can leverage plasmon's unique photophysics and capabilities because they expedite the carrier's lifetime.

Unravelling the ultrafast charge dynamics in PbS quantum dots through resonant Auger mapping of the sulfur K-edge.

There is a great fundamental interest in charge dynamics of PbS quantum dots, as they are promising for application in photovoltaics and other optoelectronic devices. The ultrafast charge transport is intriguing, offering insight into the mechanism of electron tunneling processes within the material. In this study, we investigated the charge transfer times of PbS quantum dots of different sizes and non-quantized PbS reference materials by comparing the propensity of localized or delocalized decays of sulfur 1s core hole states excited by X-rays. We show that charge transfer times in PbS quantum dots decrease with excitation energy and are similar at high excitation energy for quantum dots and non-quantized PbS. However, at low excitation energies a distinct difference in charge transfer time is observed with the fastest charge transfer in non-quantized PbS and the slowest in the smallest quantum dots. Our observations can be explained by iodide ligands on the quantum dots creating a barrier for charge transfer, which reduces the probability of interparticle transfer at low excitation energies. The probability of intraparticle charge transfer is limited by the density of available states which we describe according to a wave function in a quantum well model. The stronger quantum confinement effect in smaller PbS quantum dots is manifested as longer charge transfer times relative to the larger quantum dots at low excitation energies.

Insights and Implications of Intricate Surface Charge Transfer and sp3-Defects in Graphene/Metal Oxide Interfaces.

Adherence of metal oxides to graphene is of fundamental significance to graphene nanoelectronic and spintronic interfaces. Titanium oxide and aluminum oxide are two widely used tunnel barriers in such devices, which offer optimum interface resistance and distinct interface conditions that govern transport parameters and device performance. Here, we reveal a fundamental difference in how these metal oxides interface with graphene through electrical transport measurements and Raman and photoelectron spectroscopies, combined with ab initio electronic structure calculations of such interfaces. While both oxide layers cause surface charge transfer induced p-type doping in graphene, in sharp contrast to TiOx, the AlOx/graphene interface shows the presence of appreciable sp3 defects. Electronic structure calculations disclose that significant p-type doping occurs due to a combination of sp3 bonds formed between C and O atoms at the interface and possible slightly off-stoichiometric defects of the aluminum oxide layer. Furthermore, the sp3 hybridization at the AlOx/graphene interface leads to distinct magnetic moments of unsaturated bonds, which not only explicates the widely observed low spin-lifetimes in AlOx barrier graphene spintronic devices but also suggests possibilities for new hybrid resistive switching and spin valves.

The impact of chemical composition of halide surface ligands on the electronic structure and stability of lead sulfide quantum dot materials.

There is a high fundamental interest in the surface and bulk chemistry of quantum dot (QD) solids, as they have proven to be very promising materials in optoelectronic devices. The choice of surface ligands for quantum dots in solid devices determines many of the film properties, as the ligands influence for example the doping density, chemical stability and charge transport. Lead halide ligands have developed as the main ligand of choice for lead sulfide quantum dots, as they have been shown to passivate quantum dot surfaces and enhance the chemical stability. In this study, we successfully varied the ligand composition on the surface of PbS quantum dot films from pure lead iodide to pure lead bromide and investigated its influence on the chemical and electronic structure of the QD solids using hard X-ray photoelectron spectroscopy (HAXPES). Furthermore, we developed a surface treatment to prevent the surface oxidation of a bulk PbS reference sample. Through measurements of this sample and of lead halide reference samples, we were able to assign the contributions of different chemical bonding to the Pb 4f core level and of different atomic orbitals to the valence band spectral shape of the QD materials. Overall, we found that the valence band edge position was very similar for all different iodide:bromide ratios and that all investigated compositions were able to protect the quantum dot surfaces within solid films from oxidation. However, the ligand composition significantly influences the sample stability under X-rays. The iodide rich QD solids showed the highest stability with very little to no chemical changes over several hours of X-ray exposure, while the bromide rich QD solids changed already within the first hour of exposure.

Wafer-sized WS2 monolayer deposition by sputtering.

We demonstrate that tungsten disulphide (WS2) with thicknesses ranging from monolayer (ML) to several monolayers can be grown on SiO2/Si, Si, and Al2O3 by pulsed direct current-sputtering. The presence of high quality monolayer and multilayered WS2 on the substrates is confirmed by Raman spectroscopy since the peak separations between the A1g-E2g and A1g-2LA vibration modes exhibit a gradual increase depending on the number of layers. X-ray diffraction confirms a textured (001) growth of WS2 films. The surface roughness measured with atomic force microscopy is between 1.5 and 3 Å for the ML films. The chemical composition WSx (x = 2.03 ± 0.05) was determined from X-ray Photoelectron Spectroscopy. Transmission electron microscopy was performed on a multilayer film to show the 2D layered structure. A unique method for growing 2D layers directly by sputtering opens up the way for designing 2D materials and batch production of high-uniformity and high-quality (stochiometric, large grain sizes, flatness) WS2 films, which will advance their practical applications in various fields.

X-ray Induced Fragmentation of Protonated Cystine.

We demonstrate site-specific X-ray induced fragmentation across the sulfur L-edge of protonated cystine, the dimer of the amino acid cysteine. Ion yield NEXAFS were performed in the gas phase using electrospray ionization (ESI) in combination with an ion trap. The interpretation of the sulfur L-edge NEXAFS spectrum is supported by Restricted Open-Shell Configuration Interaction (ROCIS) calculations. The fragmentation pathway of triply charged cystine ions was modeled by Molecular Dynamics (MD) simulations. We have deduced a possible pathway of fragmentation upon excitation and ionization of S 2p electrons. The disulfide bridge breaks for resonant excitation at lower photon energies but remains intact upon higher energy resonant excitation and upon ionization of S 2p. The larger fragments initially formed subsequently break into smaller fragments.

Separation of surface oxide from bulk Ni by selective Ni 3p photoelectron spectroscopy for chemical analysis in coincidence with Ni M-edge Auger electrons.

The chemical shift of core level binding energies makes electron spectroscopy for chemical analysis (ESCA) a workhorse analytical tool for science and industry. For some elements, close lying and overlapping spectral features within the natural life time broadening restrict applications. We establish how the core level binding energy chemical shift can be picked up experimentally by the additional selectivity through Auger electron photoelectron coincidence spectroscopy (APECS). Coincident measurement of Ni 3p photoemission with different MVV Auger regions from specific decay channels, narrows the 3p core-levels to a width of 1.2 eV, resolves the spin-orbit splitting of 1.6 eV and determines the chemical shift of Ni 3p levels of a Ni(111) single crystal and its oxidized surface layer to 0.6 eV.

Hard x-ray photoelectron spectroscopy: a snapshot of the state-of-the-art in 2020.

Hard x-ray photoelectron spectroscopy (HAXPES) is establishing itself as an essential technique for the characterisation of materials. The number of specialised photoelectron spectroscopy techniques making use of hard x-rays is steadily increasing and ever more complex experimental designs enable truly transformative insights into the chemical, electronic, magnetic, and structural nature of materials. This paper begins with a short historic perspective of HAXPES and spans from developments in the early days of photoelectron spectroscopy to provide an understanding of the origin and initial development of the technique to state-of-the-art instrumentation and experimental capabilities. The main motivation for and focus of this paper is to provide a picture of the technique in 2020, including a detailed overview of available experimental systems worldwide and insights into a range of specific measurement modi and approaches. We also aim to provide a glimpse into the future of the technique including possible developments and opportunities.

Tailoring ultra-fast charge transfer in MoS2.

Charge transfer dynamics are of importance in functional materials used in devices ranging from transistors to photovoltaics. The understanding of charge transfer in particular of how fast electrons tunnel away from an excited state and where they end up, is necessary to tailor materials used in devices. We have investigated charge transfer dynamics in different forms of the layered two-dimensional material molybdenum disulphide (MoS2, in single crystal, nanocrystalline particles and crystallites in a reduced graphene oxide network) using core-hole clock spectroscopy. By recording the electrons in the sulphur KLL Auger electron kinetic energy range we have measured the prevalence of localised and delocalised decays from a state created by core excitation using X-rays. We show that breaking the crystal symmetry of the single crystal into either particles or sheets causes the charge transfer from the excited state to occur faster, even more so when incorporating it in a graphene oxide network. The interface between the MoS2 and the reduced graphene oxide forms a Schottky barrier which changes the ratio between local and delocalised decays creating two distinct regions in the charge transfer dependent on the energy of the excited electron. Thereby we show that ultra-fast charge transfer in MoS2 can be tailored, a result which can be used in the design of emergent devices.

A method for studying pico to microsecond time-resolved core-level spectroscopy used to investigate electron dynamics in quantum dots.

Time-resolved photoelectron spectroscopy can give insights into carrier dynamics and offers the possibility of element and site-specific information through the measurements of core levels. In this paper, we demonstrate that this method can access electrons dynamics in PbS quantum dots over a wide time window spanning from pico- to microseconds in a single experiment carried out at the synchrotron facility BESSY II. The method is sensitive to small changes in core level positions. Fast measurements at low pump fluences are enabled by the use of a pump laser at a lower repetition frequency than the repetition frequency of the X-ray pulses used to probe the core level electrons: Through the use of a time-resolved spectrometer, time-dependent analysis of data from all synchrotron pulses is possible. Furthermore, by picosecond control of the pump laser arrival at the sample relative to the X-ray pulses, a time-resolution limited only by the length of the X-ray pulses is achieved. Using this method, we studied the charge dynamics in thin film samples of PbS quantum dots on n-type MgZnO substrates through time-resolved measurements of the Pb 5d core level. We found a time-resolved core level shift, which we could assign to electron injection and charge accumulation at the MgZnO/PbS quantum dots interface. This assignment was confirmed through the measurement of PbS films with different thicknesses. Our results therefore give insight into the magnitude of the photovoltage generated specifically at the MgZnO/PbS interface and into the timescale of charge transport and electron injection, as well as into the timescale of charge recombination at this interface. It is a unique feature of our method that the timescale of both these processes can be accessed in a single experiment and investigated for a specific interface.

In Situ Formation of Ge Nanoparticles by Annealing of Al-Ge-N Thin Films Followed by HAXPES and XRD.

Ge nanoparticles embedded in thin films have attracted a lot of attention due to their promising optical and electronic properties that can be tuned by varying the particle size and choice of matrix material. In this study, Ge nanoparticle formation was investigated for Al-Ge-N based thin films by simultaneous measurements of HAXPES and grazing incidence XRD during in situ annealing in vacuum conditions. As-deposited Al-Ge-N thin films, synthesized by reactive dc magnetron sputtering, consisted of a nanocrystalline (Al1-xGex)Ny solid solution and an amorphous tissue phase of Ge3Ny. Upon annealing to 750 °C, elemental Ge was formed shown by both HAXPES and XRD measurements, and N2 gas was released as measured by a mass spectrometer. Postannealed ex situ analysis by SEM and TEM showed that the elemental Ge phase formed spherical nanoparticles on the surface of the film, with an average size of 210 nm. As the annealing temperature increased further to 850 °C, the Ge particles on the film surface evaporated, while the phase segregation of Ge still could be observed within the film. Thus, these results show the possibility for a controlled synthesis of Ge nanoparticles through annealing of Al-Ge-N thin films to produce materials suitable for use in electronic or optoelectronic devices.

Energy-Dependent Relative Cross Sections in Carbon 1s Photoionization: Separation of Direct Shake and Inelastic Scattering Effects in Single Molecules.

We demonstrate that the possibility of monitoring relative photoionization cross sections over a large photon energy range allows us to study and disentangle shake processes and intramolecular inelastic scattering effects. In this gas-phase study, relative intensities of the carbon 1s photoelectron lines from chemically inequivalent carbon atoms in the same molecule have been measured as a function of the incident photon energy in the range of 300-6000 eV. We present relative cross sections for the chemically shifted carbon 1s lines in the photoelectron spectra of ethyl trifluoroacetate (the "ESCA" molecule). The results are compared with those of methyl trifluoroacetate and S-ethyl trifluorothioacetate as well as a series of chloro-substituted ethanes and 2-butyne. In the soft X-ray energy range, the cross sections show an extended X-ray absorption fine structure type of wiggles, as was previously observed for a series of chloroethanes. The oscillations are damped in the hard X-ray energy range, but deviations of cross-section ratios from stoichiometry persist, even at high energies. The current findings are supported by theoretical calculations based on a multiple scattering model. The use of soft and tender X-rays provides a more complete picture of the dominant processes accompanying photoionization. Such processes reduce the main photoelectron line intensities by 20-60%. Using both energy ranges enabled us to discern the process of intramolecular inelastic scattering of the outgoing electron, whose significance is otherwise difficult to assess for isolated molecules. This effect relates to the notion of the inelastic mean free path commonly used in photoemission studies of clusters and condensed matter.

Investigation of the surface species during temperature dependent dehydrogenation of naphthalene on Ni(111).

The temperature dependent dehydrogenation of naphthalene on Ni(111) has been investigated using vibrational sum-frequency generation spectroscopy, X-ray photoelectron spectroscopy, scanning tunneling microscopy, and density functional theory with the aim of discerning the reaction mechanism and the intermediates on the surface. At 110 K, multiple layers of naphthalene adsorb on Ni(111); the first layer is a flat lying chemisorbed monolayer, whereas the next layer(s) consist of physisorbed naphthalene. The aromaticity of the carbon rings in the first layer is reduced due to bonding to the surface Ni-atoms. Heating at 200 K causes desorption of the multilayers. At 360 K, the chemisorbed naphthalene monolayer starts dehydrogenating and the geometry of the molecules changes as the dehydrogenated carbon atoms coordinate to the nickel surface; thus, the molecule tilts with respect to the surface, recovering some of its original aromaticity. This effect peaks at 400 K and coincides with hydrogen desorption. Increasing the temperature leads to further dehydrogenation and production of H2 gas, as well as the formation of carbidic and graphitic surface carbon.

Electronic Structure Characterization of Cross-Linked Sulfur Polymers.

Cross-linked polymers of elemental sulfur are of potential interest for electronic applications as they enable facile thin-film processing of an abundant and inexpensive starting material. Here, we characterize the electronic structure of a cross-linked sulfur/diisopropenyl benzene (DIB) polymer by a combination of soft and hard X-ray photoelectron spectroscopy (SOXPES and HAXPES). Two different approaches for enhancing the conductivity of the polymer are compared: the addition of selenium in the polymer synthesis and the addition of lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) during film preparation. For the former, we observe the incorporation of Se into the polymer structure resulting in a changed valence-band structure. For the latter, a Fermi level shift in agreement with p-type doping of the polymer is observed and also the formation of a surface layer consisting mostly of TFSI anions.

Partially Reversible Photoinduced Chemical Changes in a Mixed-Ion Perovskite Material for Solar Cells.

Metal halide perovskites have emerged as materials of high interest for solar energy-to-electricity conversion, and in particular, the use of mixed-ion structures has led to high power conversion efficiencies and improved stability. For this reason, it is important to develop means to obtain atomic level understanding of the photoinduced behavior of these materials including processes such as photoinduced phase separation and ion migration. In this paper, we implement a new methodology combining visible laser illumination of a mixed-ion perovskite ((FAPbI3)0.85(MAPbBr3)0.15) with the element specificity and chemical sensitivity of core-level photoelectron spectroscopy. By carrying out measurements at a synchrotron beamline optimized for low X-ray fluxes, we are able to avoid sample changes due to X-ray illumination and are therefore able to monitor what sample changes are induced by visible illumination only. We find that laser illumination causes partially reversible chemistry in the surface region, including enrichment of bromide at the surface, which could be related to a phase separation into bromide- and iodide-rich phases. We also observe a partially reversible formation of metallic lead in the perovskite structure. These processes occur on the time scale of minutes during illumination. The presented methodology has a large potential for understanding light-induced chemistry in photoactive materials and could specifically be extended to systematically study the impact of morphology and composition on the photostability of metal halide perovskites.

Site-selective local fluorination of graphene induced by focused ion beam irradiation.

The functionalization of graphene remains an important challenge for numerous applications expected by this fascinating material. To keep advantageous properties of graphene after modification or functionalization of its structure, local approaches are a promising road. A novel technique is reported here that allows precise site-selective fluorination of graphene. The basic idea of this approach consists in the local radicalization of graphene by focused ion beam (FIB) irradiation and simultaneous introduction of XeF2 gas. A systematic series of experiments were carried out to outline the relation between inserted defect creation and the fluorination process. Based on a subsequent X-ray photoelectron spectroscopy (XPS) analysis, a 6-fold increase of the fluorine concentration on graphene under simultaneous irradiation was observed when compared to fluorination under normal conditions. The fluorine atoms are predominately localized at the defects as indicated from scanning tunneling microscopy (STM). The experimental findings are confirmed by density functional theory which predicts a strong increase of the binding energy of fluorine atoms when bound to the defect sites. The developed technique allows for local fluorination of graphene without using resists and has potential to be a general enabler of site-selective functionalization of graphene using a wide range of gases.

A multi purpose source chamber at the PLEIADES beamline at SOLEIL for spectroscopic studies of isolated species: cold molecules, clusters, and nanoparticles.

This paper describes the philosophy and design goals regarding the construction of a versatile sample environment: a source capable of producing beams of atoms, molecules, clusters, and nanoparticles in view of studying their interaction with short wavelength (vacuum ultraviolet and x-ray) synchrotron radiation. In the design, specific care has been taken of (a) the use standard components, (b) ensuring modularity, i.e., that swiftly switching between different experimental configurations was possible. To demonstrate the efficiency of the design, proof-of-principle experiments have been conducted by recording x-ray absorption and photoelectron spectra from isolated nanoparticles (SiO2) and free mixed clusters (Ar/Xe). The results from those experiments are showcased and briefly discussed.

Circularly polarized x rays: another probe of ultrafast molecular decay dynamics.

Dissociative nuclear motion in core-excited molecular states leads to a splitting of the fragment Auger lines: the Auger-Doppler effect. We present here for the first time experimental evidence for an Auger-Doppler effect following F1s → a(1g)* inner-shell excitation by circularly polarized x rays in SF(6). In spite of a uniform distribution of the dissociating S-F bonds near the polarization plane of the light, the intersection between the subpopulation of molecules selected by the core excitation with the cone of dissociation induces a strong anisotropy in the distribution of the S-F bonds that contributes to the scattering profile measured in the polarization plane.

Auger electron spectroscopy as a probe of the solution of aqueous ions.

Aqueous potassium chloride has been studied by synchrotron-radiation excited core-level photoelectron and Auger electron spectroscopy. In the Auger spectrum of the potassium ion, the main feature comprises the final states where two outer valence holes are localized on potassium. This spectrum exhibits also another feature at a higher kinetic energy which is related to final states where outer valence holes reside on different subunits. Through ab initio calculations for microsolvated clusters, these subunits have been assigned as potassium ions and the surrounding water molecules. The situation is more complicated in the Auger spectrum of the chloride anion. One-center and multicenter final states are present here as well but overlap energetically.