AC-Modulated XPS Enables to Externally Control the Electrical Field Distributions on Metal Electrode/Ionic Liquid Devices.

X-Ray Photoelectron Spectroscopy (XPS) has been utilized to extract local electrical potential profiles by recording core-level binding energy shifts upon application of the AC [square-wave (SQW)] bias with different frequencies. An electrochemical system consisting of a coplanar capacitor with a polyethylene membrane (PEM) coated with the Ionic Liquid (IL) N,N-diethyl-N-methyl-N-(2-methoxyethyl) ammonium bis(trifluoromethanesulfonyl)imide (DEME-TFSI) as the electrolyte is investigated. Analyses are carried out in operando, such that XPS measurements are recorded simultaneously with current measurements. ILs have complex charging/discharging processes, in addition to the formation of Electrical Double Layers (EDL) at the interfaces, and certain properties of these processes can be captured using AC modulation within appropriate time windows of observation. Herein, we select two frequencies, namely, 10 kHz and 0.1 Hz, to separate effects of the fast polarization and slow migratory motions, respectively. Moreover, the local potential developments after adding two equivalent series resistors at three different physical positions of the device have been carefully evaluated from the binding energy shifts in the F 1s peak representing the anion of the IL. This circuit modification allows us to quantify the AC currents passing through the device, as well as the system's impedance, in addition to revealing the potential variations due the IR drops. The complex AC-modulated local XPS data recorded can also be faithfully reproduced using the unmodulated F 1s spectrum and by convoluting it with electrical circuit output provided by the LT-Spice software. The outcome of these efforts is a more realistic equivalent circuit model, which can be related to chemical/physical makeup of the electrochemical system. An important finding of this methodology emerges as the possibility to induce additional local electrical field developments within the device, the directions of which can be reversed controllably.

Assessing Local Electrical Properties of Ionic Liquid/Metal Interfaces with Operando-XPS and by Incorporating Additional Circuit Elements.

X-ray photoelectron spectroscopy (XPS) has been utilized to record binding energy changes upon applying direct current (DC) and/or alternating current (AC) (square-wave) bias with different frequencies on a coplanar capacitor, having an ionic liquid (IL) film as the electrolyte. Electrical potential developments in numerous locations on the device are extracted from the variations in binding energy positions of the atomic core levels, which together with electrochemical measurements are used to extract local information before and after insertion of additional resistors in series. The presence of the IL introduces complex charging/discharging processes with a direct influence on the electrical double layer (EDL) formation, some of which can be untangled from each other via AC modulation by choosing appropriate time windows of observation. Accordingly, under 10 kHz modulation, fast processes are sampled, which are associated with electronic currents, and effects of slow migratory currents can be measured using 0.1 Hz. The addition of serial resistors allows us to quantify AC currents passing through, which reveals the magnitude of the system's impedance under different conditions. This process surprisingly reverses differences(s) in the voltage developments between the low and high frequencies over the electrified electrodes compared to those over the porous membrane in between. Our approach turns XPS into a powerful electrical and surface-sensitive tool for extracting localized electrochemical properties in a noninvasive and direct way. We expect that a wider utilization of the technique will lead to better identification of the obstacles for developing the next-generation sensing, energy harvesting, and storage systems as well as devices for iontronic/neuromorphic applications.

Localized X-ray photoelectron impedance spectroscopy (LoXPIS) for capturing charge dynamics of an ionic liquid electrolyte within an energy storage device.

Many electrochemical devices are based on the fundamental process of ion migration and accumulation on surfaces. Complex interplay of molecular properties of ions and device dimensions control the entire process and define the overall dynamics of the system. Particularly, for ionic liquid-based electrolytes it is often not clear which property, and to what extent, contributes to the overall performance of the device. Herein we use X-ray photoelectron spectroscopy (XPS) while the device is under electrical bias. Such a procedure reveals localized electrical potential developments, through binding energy shifts of the atomic core levels, in a chemically specific fashion. Combining it with square-wave AC modulation, the information can also be extended to time domain, and we investigate devices configured as a coplanar capacitor, with an ionic liquid as the electrolyte, in macro-dimensions. Our analysis reveals that a nonlinear voltage profile across the device emerges from spatially non-uniform electrical double layer formation on electrode surfaces. Interestingly the coplanar capacitor has an extremely slow time response which is particularly controlled by IL film thickness. XPS measurements can capture the ion dynamics in the tens of seconds to microseconds range, and reveal that ionic motion is all over the device, including on metallic electrode regions. This behavior can only be attributed to motion in more than one dimension. The ion dynamics can also be faithfully simulated by using a modified PNP equation, taking into account steric effects, and device dimensions. XPS measurements on two devices with different dimensions corroborated and validated the simulation results. The present results propose a new experimental approach and provide new insights into the dynamics of ions across electrochemical devices.

Comparative Operando XPS and SEM Spatiotemporal Potential Mapping of Ionic Liquid Polarization in a Coplanar Electrochemical Device.

The polarization response of a coplanar electrochemical capacitor covered with an ionic liquid as the electrolyte has been examined using a combination of two powerful analytic techniques, X-ray photoelectron spectroscopy (XPS) and scanning electron microcopy (SEM). Spatiotemporal distribution of the ionic liquid surface potential, upon DC or AC (square wave) biasing, has been monitored via chemical element binding energy shifts using XPS and secondary electron intensity variations using SEM. SEM's high spatial resolution and speedy imaging together with application of a data mining algorithm made mapping of the surface potential distribution across the capacitor possible. Interestingly, despite the differences in the detection principles, both techniques yield similar polarization relaxation time constants. The results demonstrate the power of a synergistic combination of the two techniques with complementary capabilities and pave the way to a deeper understanding of liquid/solid interfaces and for performance evaluation and diagnostics of electrochemical devices.

Surface Propensity of Anions in a Binary Ionic-Liquid Mixture Assessed by Full-Range Angle-Resolved X-ray Photoelectron Spectroscopy and Surface-Tension Measurements.

Angle-resolved X-ray photoelectron spectroscopy and contact-angle measurements guided by a signal attenuation model are utilized to extract molar composition and anion enrichment in the vacuum interface of a binary ionic liquid mixture, having a common quaternary ammonium cation and two different anions. By using the intensity ratio of the F1s peaks belonging to the two different anions recorded at the full electron take-off angle range, from 0° to 80°, we have determined that only a fractionally covered and anion enriched surface layer can predict the AR-XPS data, which is also consistent with surface tension measurements. Moreover, the more bulky and non-spherical anion enrichment is evident even at the conventional and the so assumed bulk sensitive take-off angle of 0°. This methodology provides a surface enrichment factor of the molecular ions and clearly serves as an experimental evidence for recently debated surface layering and/or island structure in ionic liquid systems.

X-ray Photoelectron Spectroscopy with Electrical Modulation Can Be Used to Probe Electrical Properties of Liquids and Their Interfaces at Different Stages.

Operando X-ray photoelectron spectroscopy (o-XPS) has been used to record the binding energy shifts in the C 1s peak of a pristine poly(ethylene glycol) (PEG) liquid drop in an electrowetting on dielectric (EWOD) geometry and after exposing it to several high-voltage breakdown processes. This was achieved by recording XPS data while the samples were subjected to 10 V dc and ac (square-wave modulation) actuations to extract electrical information related to the liquid and its interface with the dielectric. Through analysis of the XPS data under ac actuation, a critical frequency of 170 Hz is extracted for the pristine PEG, which is translated to a resistance value of 14 MΩ for the liquid and a capacitance value of 60 pF for the dielectric, by the help of simulations using an equivalent circuit model and also by XPS analyses of a mimicking device under similar conditions. The same measurements yield an increased value of 23 MΩ for the resistance of the liquid after the breakdown by assuming that the capacitance of the dielectric stays constant. In addition, an asymmetry in polarity dependence is observed with respect to both the onset of the breakdown voltage and also the leakage behavior of the deteriorated (PEG + dielectric) system such that deviations are more pronounced at positive voltages. Both dc and ac behaviors of the postbreakdown system can also be simulated, but only by introducing an additional element, a diode or a polarity- and magnitude-dependent voltage source (VCVS), which might be attributed to negative charge accumulation at the interface. Measurements for a liquid mixture of PEG with 8% ionic liquid yields an almost 2 orders of magnitude smaller resistance for the drop as a result of the enhanced conductivity by the ions. Coupled with modeling, XPS measurements under dc and ac modulations enable probing unique electrochemical properties of liquid/solid interfaces.

AC Electrowetting Modulation of Low-Volatile Liquids Probed by XPS: Dipolar vs Ionic Screening.

X-ray photoelectron spectroscopic (XPS) data have been recorded for a low-molecular-weight poly(ethylene glycol) microliter-sized sessile liquid drops sitting on a dielectric covered planar electrode while imposing a ±6 V square-wave actuation with varying frequencies between 10-1 and 105 Hz to tap into the information derivable from (AC) electrowetting. We show that this time-varying XPS spectra reveal two distinct behaviors of the device under investigation, below and above a critical frequency, measured as ∼70 Hz for the liquid poly(ethylene glycol) with a 600 Da molecular weight. Below the critical frequency, the liquid complies faithfully to the applied bias, as determined by the constant shift in the binding energy position of the XPS peaks representative of the liquid throughout its entire surface. The liquid completely screens the applied electrical field and the entire potential drop takes place at the liquid/dielectric interface. However, for frequencies above the critical value, the resistive component of the system dominates, resulting in the formation of equipotential surface contours, which are derived from the differences in the positions of the twinned O 1s peaks under AC application. This critical frequency is independent of the size of the liquid drop, and the amplitude of the excitation, but increases when ionic moieties are introduced. The XP spectra under AC actuation is also faithfully simulated using an equivalent circuit model consisting of only resistors and capacitors and using an electrical circuit simulation software. Moreover, a mimicking device is fabricated and its XP spectra are recorded using the Sn 3d peaks of the solder joints at different points on the circuit to confirm the reliability of the measured and simulated AC behaviors of the liquid. These new findings indicate that in contrast to direct current case, XPS measurements under variable frequency AC actuation reveal (through differences in the frequency response) information related to the chemical makeup of the liquid(s) and brings the laboratory-based XPS as a powerful complimentary arsenal to electrochemical analyses of liquids and their interfaces.

Chemically addressed switching measurements in graphene electrode memristive devices using in situ XPS.

The memristor has been the topic of heated research activity since the Pt/TiO2/TiOx/Pt structure was reported by Hewlett-Packard Labs. The characteristics of memristors such as the pinched hysteresis loops and time and input signal-dependent memristance are due to the drift of positively charged oxygen vacancies in the TiOx layer. While different modes of switching behaviour have been characterized, observing the switching as it happens with in situ measurements using X-ray photoelectron spectroscopy (XPS) can allow a better understanding of the device operation. The setup used in this work enables the application of voltage signals of different frequencies and amplitudes and observing the hysteresis seen in the I-V plane through chemical addressing. The measurements were conducted on Pt/TiO2/TiOx/graphene structures. The single layer graphene, utilized as a top electrode, effectively acts as a transparent electrode in that the layer beneath it can be observed to a depth of ∼10 nm in XPS. This allows for the observation of the changes in the binding energies of C 1s, Ti 2p and O 1s. By comparing the variation in the binding energy of Ti 2p to that of C 1s, and observing how the variation changes for different excitation signals (at different frequency and amplitudes), it is possible to inspect the effect of the oxygen vacancy drift. We employed a variety of input signals with varying frequency and amplitudes in order to test the memristive devices thoroughly: sine wave, triangular wave and DC bias. Graphene has been attracting attention due to its intriguing optoelectronic properties. This study utilizes graphene as a transparent top electrode for in situ measurements in XPS to observe chemically-addressed memristive hysteresis while an excitation signal is being applied to the device.

Graphene-Based Adaptive Thermal Camouflage.

In nature, adaptive coloration has been effectively utilized for concealment and signaling. Various biological mechanisms have evolved to tune the reflectivity for visible and ultraviolet light. These examples inspire many artificial systems for mimicking adaptive coloration to match the visual appearance to their surroundings. Thermal camouflage, however, has been an outstanding challenge which requires an ability to control the emitted thermal radiation from the surface. Here we report a new class of active thermal surfaces capable of efficient real-time electrical-control of thermal emission over the full infrared (IR) spectrum without changing the temperature of the surface. Our approach relies on electro-modulation of IR absorptivity and emissivity of multilayer graphene via reversible intercalation of nonvolatile ionic liquids. The demonstrated devices are light (30 g/m2), thin (<50 µm), and ultraflexible, which can conformably coat their environment. In addition, by combining active thermal surfaces with a feedback mechanism, we demonstrate realization of an adaptive thermal camouflage system which can reconfigure its thermal appearance and blend itself with the varying thermal background in a few seconds. Furthermore, we show that these devices can disguise hot objects as cold and cold ones as hot in a thermal imaging system. We anticipate that, the electrical control of thermal radiation would impact on a variety of new technologies ranging from adaptive IR optics to heat management for outer space applications.

DC Electrowetting of Nonaqueous Liquid Revisited by XPS.

Liquid poly(ethylene glycol) (molecular weight, ∼600 Da) with a low vapor pressure is used as droplets in an ultrahigh-vacuum X-ray photoelectron spectrometer (XPS) chamber with traditional electrowetting on dielectric (EWOD) device geometry. We demonstrate that, using XPS data, independent of the sign of the applied voltage, the droplet expands on the substrate with the application of a nonzero voltage and contracts back when the voltage is brought back to zero. However, the main focus of the present investigation is about tracing the electrical potential developments on and around the droplet, using the shifts in the binding energy positions of the core levels representative of the liquid and/or the substrate in an noninvasive and chemically specific fashion, under imposed electrical fields, with an aim of shedding light on numerous models employed for simulating EWOD phenomenon, as well as on certain properties of liquid/solid interfaces. While the lateral resolution of XPS does not permit to interrogate the interface directly, we explicitly show that critical information can be extracted by probing both sides of the interface simultaneously under external bias in the form of potential steps or direct current. We find that, even though no potential drop is observed at the metal-wire electrode/liquid interface, the entire potential drop develops across the liquid/solid-substrate interface, which is faster than our probe time window (∼100 ms) and is promptly complying with the applied bias until breakdown. No indication of band bending nor additional broadening can be observed in the C 1s peak of the liquid, even under electrical field strengths exceeding 107 V/m. Moreover and surprisingly, the liquid recovers within seconds after each catastrophic breakdown. All of these findings are new and expected to contribute significantly to a better understanding of certain physicochemical properties of liquid/solid interfaces.

In-Situ XPS Monitoring and Characterization of Electrochemically Prepared Au Nanoparticles in an Ionic Liquid.

Gold nanoparticles (Au NPs) have been electrochemically prepared in situ and in vacuo using two different electrochemical device configurations, containing an ionic liquid (IL), N-N-diethyl-N-methyl-N-(2-methoxyethyl)ammonium bis(trifluoromethanesulfonyl)imide, that serves both as reaction and as stabilizing media for the NPs. It was observed in both devices that Au NPs were created using an anodically triggered route. The created Au NPs are relatively small (3-7 nm) and reside within the IL medium. X-ray photoelectron spectroscopy is utilized to follow not only the formation of the NPs but also their charging/discharging properties, by monitoring the charging shifts of the Au4f peak representing the electrodes and also the Au NPs as well as the F1s peak of the IL after polarizing one of the electrodes. Accordingly, DC polarization across the electrodes leads to a uniform binding energy shift of F1s of the IL along with that of Au4f of the NPs within. Moreover, this shift corresponds to only half of the applied potential. AC polarization brings out another dimension for demonstrating further the harmony between the charging/discharging property of the IL medium and the Au NPs in temporally and laterally resolved fashions. Polarization of the electrodes result in perfect spectral separation of the Au4f peaks of the NPs from those of the metal in both static (DC) and in time- and position-dependent (AC) modes.

Location and Visualization of Working p-n and/or n-p Junctions by XPS.

X-ray photoelectron spectroscopy (XPS) is used to follow some of the electrical properties of a segmented silicon photodetector, fabricated in a p-n-p configuration, during operation under various biasing configurations. Mapping of the binding energy position of Si2p reveals the shift in the position of the junctions with respect to the polarity of the DC bias applied. Use of squared and triangular shaped wave excitations, while recording XPS data, allows tapping different electrical properties of the device under normal operational conditions, as well as after exposing parts of it to harsh physical and chemical treatments. Unique and chemically specific electrical information can be gained with this noninvasive approach which can be useful especially for localized device characterization and failure analyses.

Chemical Visualization of a GaN p-n junction by XPS.

We report on an operando XPS investigation of a GaN diode, by recording the Ga2p3/2 peak position under both forward and reverse bias. Areal maps of the peak positions under reverse bias are completely decoupled with respect to doped regions and allow a novel chemical visualization of the p-n junction in a 2-D fashion. Other electrical properties of the device, such as leakage current, resistivity of the domains are also tapped via recording line-scan spectra. Application of a triangular voltage excitation enables probing photoresponse of the device.

Gate-tunable photoemission from graphene transistors.

In this Letter, we report gate-tunable X-ray photoelectron emission from back-gated graphene transistors. The back-gated transistor geometry allows us to study photoemission from graphene layer and the dielectric substrate at various gate voltages. Application of gate voltage electrostatically dopes graphene and shifts the binding energy of photoelectrons in various ways depending on the origin and the generation mechanism(s) of the emitted electrons. The gate-induced shift of the Fermi energy of graphene alters the binding energy of the C 1s electrons, whereas the electric field of the gate electrodes shift the binding energy of core electrons emitted from the gate dielectric underneath the graphene layer. The gradual change of the local potential through depths of the gate dielectric provides quantitative electrical information about buried interfaces. Our results suggest that gate-tunable photoemission spectra with chemically specific information linked with local electrical properties opens new routes to elucidating operation of devices based especially on layered materials.

Band-bending at buried SiO2/Si interface as probed by XPS.

X-ray photoelectron spectroscopy is used to probe the photoinduced shifts in the binding energies of Si2p, O1s, and C1s of the SiO2/Si interfaces of a number of samples having oxide and/or thin organic layers on top of p- and n-Si wafers. Whereas the photoinduced shifts, in each and every peak related, vary from 0.2 to 0.5 eV for the p-type samples, the corresponding shifts are substantially smaller (<0.1 eV) for the n-type, regardless of (i) oxidation route (thermal or anodic), (ii) thickness of oxide layer, (iii) nature of organic layer, or (iv) color of three illuminating sources we have used. This leads us to conclude that these particular photoshifts reflect the charge state of the SiO2/Si interface, even in the case of a 20 nm thick oxide, where the interface is buried and cannot be probed directly by XPS.

Probing voltage drop variations in graphene with photoelectron spectroscopy.

We use X-ray photoelectron spectroscopy (XPS) for characterization of voltage drop variations of large area single-layer graphene on quartz substrates, by application of a voltage bias across two gold electrodes deposited on top. By monitoring the spatial variation of the kinetic energies of emitted C1s photoelectrons, we extract voltage variations in the graphene layer in a chemically specific format. The potential drop is uniform across the entire layer in the pristine sample but is not uniform in the oxidized one, due to cracks and/or morphological defects created during the oxidation process. This new way of data gathering reintroduces XPS as a major analytical tool for extracting electrical as well as chemical information about surface and/or nanostructured materials.