Sefik Suzer

There has been a strong recent interest in the large spin–orbit coupling (SOC), a relativistic effect, in Pb moieties adsorbed on graphene and related 2D materials. Since the SOC mainly originates from the strong nuclear potential of heavy atoms, a conceptually simple approach is adopted to relate the underlying physical/chemical origin of the large spin–orbit splitting to the lead atom’s outermost valence 6p atomic level. To examine this possible origin at the most fundamental level, we investigated a minimal heavy-adatom−π model system consisting of a single Pb atom positioned at various distances above the center of a benzene (C6H6) molecule. In order to analyze the evolution of the electronic structure of this model system, fully relativistic four-component Dirac–Hartree–Fock (4c-DHF) calculations, which took account of the angular momentum coupling of the open shell, Pb 6p, electrons, as well as quasi-relativistic approaches are used. Our results show that the ligand field effects exerted by benzene on the excited states of Pb are comparable in magnitude to the intrinsic 6p SOC of the Pb atom when it is at short distances above the C6H6. Indeed, in this regime, the SOC and ligand-field interactions cannot be cleanly separated. In addition, single-determinant and quasi-relativistic SOC treatments are shown to overestimate the Pb–benzene interaction and the associated SOC transfer to the π system, whereas the inclusion of multiconfigurational effects significantly reduces SOC within the open-shell space. A key result of our findings is that the SOC in heavy-adatom−π systems is not a simple projection of atomic SOC, but rather a consequence of hybridization, which depends critically on the correlated description of the heavy-atom valence shell.

X-ray Photoelectron Spectroscopy under bias tracks surface population and electrical potentials on graphene electrodes with an ionic liquid solution, composed of one anion (TFSI^-) and two cations (DEME⁺ and Rb⁺). As bias increases, ions are enriched at the analyzed vacuum/graphene interface due to electrosorption, and the measured binding energies shift due to both bias and ion screening. The relative population of Rb⁺ increases, but that of DEME⁺ decreases to maintain electroneutrality. After prolonged biasing, induced currents increase by more than 1 order of magnitude, attributed to a significant contribution from electrosorbed, smaller, and mobile Rb⁺ cations, causing a sizable asymmetry with respect to the polarity of the bias. The evolving relative concentrations of Rb⁺ and DEME⁺ allow us to elaborate on the dynamics of the ionic structure inside the electrolyte. These findings bring a new understanding of the surface composition of electrified interfaces in terms of anion and cation balance.

Use of FTIR spectroscopy together with isotopic exchange offers a powerful approach towards understanding cation dependence of water uptake in alkali perchlorate salts at room temperature.

Isotope effects are widely used in infrared analyses of a range of chemical systems. Water molecules with three active modes are one of the most studied for testing the applicability of theoretical models. Whereas the IR spectrum of free gaseous water is very complex due to the accompanying rotational spectral features, the ATR-FTIR spectrum of a semidry solid LiCO4 sample reveals well-resolved 3 vibrational features of seemingly isolated molecules trapped within the solid matrix. Exposure of that sample to additional H2O vapors causes an increase in the intensity of these features, while further exposure causes them to broaden, red-shift, and intensify due to formation of H-bonding-networked water aggregates. Exposure of a different sample to D2O vapors enables the recording of new and well-resolved peaks assignable to the normal vibrational modes of HDO and D2O molecules. The position and relative intensity of these peaks can be used for testing theoretical predictions, based on coupled-oscillators model, together with the approach of force constants and geometrical factors of a bent triatomic molecule. By using the ratio of the experimentally determined positions of the antisymmetric and symmetric stretching peaks of H2O, and computing the ratio of geometric factors, one can estimate ratio of the force constants within the solid and extend it to estimate the same for D2O molecule to compare with experimental data. Extracting force constant variations is a very crucial step for tracing the chemical nature of intermolecular interactions within various systems and materials. Through repeating these simple measurements, one would not only gain a deeper understanding of several advanced concepts in IR spectroscopy and isotope effects but also learn how to simplify difficult mathematical procedures. Simple measurements outlined herein would also enable visualization of gas phase H/D exchange in real-time.

Time-dependent X-ray photoelectron spectroscopy (XPS) data are recorded for capturing dynamics of the electrical double layer (EDL) formation on two multilayered graphene electrodes, configured as a coplanar-capacitor, with an ionic liquid (IL) as the electrolyte. The device is subjected to 2 V biasing cycles, reversing its polarity every hour, while iteratively recording the O 1s peak representing the IL with 0.5 s steps, which turns XPS into a recording chemical voltmeter, since variations in binding energy position of the O 1s peak on electrodes report directly the local electrical potential of the IL medium. If the device is physically shorted during measurements, the potential suddenly jumps to the opposite polarization since this process nulls the electronic component of the potential, retaining the ionic one. Complementing action is captured on the drain electrode, which exhibits voltage decay(s) toward the system's ground. On the other hand, data recorded when the circuit is temporarily opened reveal only decays without polarity reversal. Generally, these potentials capture decay with relatively long and multiple time constants, associated with ionic motion. The polarity reversal is explained by a simple model, considering the huge time constant difference between electronic and ionic motions.

The ability to control heat transport with electrical signals has been an outstanding challenge due to the lack of efficient electrothermal materials. Previous attempts have mainly concentrated on low–thermal conductivity materials and encountered various problems such as narrow dynamic range and modest on/off ratios. Here, using high–thermal conductivity graphite films, we demonstrate an electrothermal switch enabling electrically tunable heat flow at the device level. The device uses reversible electro-intercalation of ions to modulate the in-plane thermal conductivity of graphite film by more than 13-fold via tunable phonon scattering, enabling observable modulation of the thermal conductivity at the device level. We anticipate that our results could provide a realistic pathway for adaptive thermal transport, enabling electrically driven thermal devices that would find a broad spectrum of applications in aerospace and microelectronics.

Because of the importance of surfaces and interfaces in many scientific and technological areas, the use of x-ray photoelectron spectroscopy (XPS) has been growing exponentially. Although XPS is being used to obtain useful information about the surface composition of samples, much more information about materials and their properties can be extracted from XPS data than commonly obtained. This paper describes some of the areas where alternative analysis methods or experimental design can obtain information about the near-surface region of a sample, often information not available in other ways. Experienced XPS analysts are familiar with many of these methods, but they may not be known to new or casual XPS users, and sometimes, they have not been used because of an inappropriately assumed complexity. The information available includes optical, electronic, and electrical properties; nanostructure; expanded chemical information; and enhanced analysis of biological materials and solid/liquid interfaces. Many of these analyses can be conducted on standard laboratory XPS systems, with either no or relatively minor system alterations. Topics discussed include (1) considerations beyond the “traditional” uniform surface layer composition calculation, (2) using the Auger parameter to determine a sample property, (3) use of the D parameter to identify sp2 and sp3 carbon information, (4) information from the XPS valence band, (5) using cryocooling to expand range of samples that can be analyzed and minimize damage, and (6) using electrical potential effects on XPS signals to extract chemically resolved electrical measurements including band alignment and electrical property information.

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.