Core Level Binding Energy Shifts Research Articles

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.

Chemical Doping by Fluorination and Its Impact on All Energy Levels of π-Conjugated Systems.

Halogenation of organic molecules causes chemical shifts of C1s core-level binding energies that are commonly used as fingerprints to identify chemical species. Here, we use synchrotron-based X-ray photoelectron spectroscopy and density functional theory calculations to unravel such chemical shifts by examining different partially fluorinated pentacene derivatives. Core-level shifts occur even for carbon atoms distant from the fluorination positions, yielding a continuous shift of about 1.8 eV with increasing degree of fluorination for pentacenes. Since also their LUMO energies shift markedly with the degree of fluorination of the acenes, core-level shifts result in a nearly constant excitation energy of the leading π* resonance as obtained in complementary recorded K-edge X-ray absorption spectra, hence demonstrating that local fluorination affects the entire π-system, including valence and core levels. Our results thus challenge the common picture of characteristic chemical core-level energies as fingerprint signatures of fluorinated π-conjugated molecules.

In English

The hexagon-shape graphene nanoflakes (GNFs) limited by zigzag edges only (with doubly and triply coordinated atoms) have unique increased reactivity. Despite the high systems symmetry (D6h) the Carbon atoms in GNFs occupy non-equivalent positions. Can such physical and chemical characteristics of GNFs, which depend of the atom position in the cluster, definition? This characteristic together with the simplicity of its calculation makes it possible to predict the properties of nanoflakes obtained from GNFs by introducing single and multiatomic vacancies into them or by replacing Carbon atoms with electron withdrawing and electron donating atoms. This characteristic includes the C1s core-level binding energy shifts, the maxima of which characterize the C atoms of a certain type. The proposed work is devoted to quantum chemical calculations of the electronic density of states (DOS) of pristine hexagon-shape GNF C96 (multiplicity, M=5), their saturated counterpart –polycyclic aromatic hydrocarbon(PAH) C96H24 (M=1) and their derivatives with one and two single vacancies in the ground electronic state (GES). All calculations were performed using the density functional theory (DFT) method with the involvement of the valence-split basis set 6-31G (d,p). Systems with open shells were considered using the UB3LYP exchange-correlation functional. The obtained spectra were fitted using Gaussian curve fitting program to determine the binding energy for each peak. The Gaussian function distribution of the theoretically calculated C1s core-level binding energy shifts of GNFs testified the presence of six peaks, each of which refers to a certain type of Carbon atoms. The C1s peak with the highest binding energy (-285.57 eV) is caused by contributions from the doubly coordinated edge cyclic chain (ECC) Carbon atoms. The C1s orbitals of the central hexagon (CHex) atoms and the first cyclic chain (FCC) atoms form delocalized molecular orbitals (MOs) in different parts of the cluster. The analogous spectrum of PAH C96H24 is slightly shifted to the region of lower binding energies and contains only two well-defined peaks. The peak with a higher binding energy (-284.36 eV) is generated by the 1s states of the CHex atoms and the atoms of the FCC, which are bounded to the CHex atoms. The electronic DOS difference in C1s core-level spectra of GNF C96 (M=5) and their saturated counterpart PAH C96H24 is established due to the presence of two weakly bounded π-systems in GNF and common conjugated system in PAH. The electronic DOS of defect-containing cluster C96-1(1) (M=3) (one CHex atom has been removed from the C96nanoflake) is generated by the C1s core-level atoms of the second cyclic chain (SCC), which are located at the different distances from the center of the nanoflake. The peak of the lowest intensity (-284.63 eV) appears in the spectrum as a reflection of the appearance of doubly coordinated Carbon atoms surrounding the single vacancy in the C96-1(1) nanoflake. The analysis of the electronic DOS of the C1s core-level spectrum of the C96-2(1) nanoflakeis shown, that doubly coordinated Carbon atoms, concentrated around two single vacancies, are essentially non-equivalent. If the MO with the lowest binding energy is localized on two of them – the MO with the highest binding energy is localized on the third atoms (one around each single vacancy). The electronic C1s core-level DOS spectrum of defect-containing molecular systems with one C96-1(1)H24 and two C96‑2(1)H24 single vacancies are similar to the analogous spectrum of PAH C96H24. In the first of them – one additional maximum appears due to C1s atoms surrounding the single vacancy. In the second – there are two additional maxima, each of which is generated by C1s core-level atoms adjacent to individual vacancies.

Core-level binding energy shifts between interior, terrace and edge atoms in MnO(001) thin films

Understanding the physical origin of core-level photoemission line shapes can offer valuable information about the chemical and physical properties of surfaces. For instance, in a large number of transition metals oxides, changes in the line shape allow the accurate determination of their oxidation states and cation site symmetry. Yet, despite this importance, experimental investigations on core-level shifts have been much neglected in recent years. In order to provide further evidence of the physical relevance, we have, in this contribution, introduced a new aspect of interior-, terrace- and edge- atom core-level binding energy shifts to describe monolayers of MnO(001) films grown on an Au(111) substrate. By this means we were able to distinguish the line shape contributions related to different types of atomic sites. We show that their relative intensities and energy shifts are able to provide information about the relative amount of under coordinated atoms on the surface and, thus give insights into their catalytic properties. Our findings reveal the importance of a detailed surface science characterization to provide the correct interpretation of distinct photoemission line shapes when considering thin film samples as well as nanostructures in general.

Hydrogen absorption and diffusion behaviors in cube-shaped palladium nanoparticles revealed by ambient-pressure X-ray photoelectron spectroscopy

The hydrogen absorption and diffusion processes in cube-shaped palladium (Pd) nanoparticles (NPs) were studied by the combination of ambient-pressure X-ray photoelectron spectroscopy (AP-XPS) and density functional theory (DFT) calculations. A size dependence of the subsurface hydrogen absorption was observed. More hydrogen atoms were absorbed in the subsurface of the smaller-sized Pd NPs owing to the enhancement of the diffusivity of H atoms from the surface into the subsurface rather than the adsorption or absorption rates at the initial stages. This results from the weakened Pd-H bond caused by surface disordering of Pd NPs with the reduction of the size of the particles. Furthermore, we discuss the H absorption sites in the bulk by comparing the relative Pd 3d core-level binding energy shifts of the Pd atoms obtained from the AP-XPS results and the theoretical calculations. The octahedral (O) sites are shown to be more favorable than the tetrahedral (T) sites for hydrogen occupation by comparing the experimental results and theoretical calculations. Finally, we proposed an interaction model between hydrogen and the Pd NPs during H2 absorption and diffusion to provide new insights into the hydrogen absorption process.

X-ray photoemission studies of BiInO3: Surface termination and effective Debye temperature

BiInO3 is a potentially polar oxide with distinct optical properties whose origin could result from a surface that has not been well characterized. The surface properties of BiInO3 thin films have been characterized here by temperature dependent angle-resolved x-ray photoemission spectroscopy. A large surface to bulk core-level binding energy shift for the In 3d5/2 core-level is identified, indicating a surface very different from the bulk. BiInO3 terminates in indium oxide and loses bismuth from the surface of the film at T = 573 K. The Debye–Waller plots suggest effective Debye temperatures of 263 ± 10 and 556 ± 27 K for the surface and bulk components of In 3d core-level, respectively.

The Role of Native Oxide in Silicon Anode Lithiation Investigated By in-Situ X-Ray Photoelectron Spectroscopy

Understanding fundamental aspects of silicon anode lithiation is important for designing improved battery chemistries with more stable interfaces. The present study focuses on the role that native SiO2 plays in early-stage lithiation and its conversion into relevant inorganic components of the solid-electrolyte interphase (SEI) on a silicon anode. The study uses the virtual-electrode (VE) approach to deliver a square-wave pulsed flux of Li+ ions to a native-SiO2/Si(001) wafer anode surface while monitoring compositional and chemical-state changes via in situ X-ray photoelectron spectroscopy (XPS). Lithiation proceeds initially by conversion of the native SiO2 layer into two distinct LiySiOx phases. Consistent with the Li-Si-O equilibrium phase diagram, the onset of LixSi formation is observed only after all SiO2 has been converted to lithium silicate phases. As lithiation proceeds and an increasing fraction of the Si0 observable within the XPS information depth (~10nm) is converted to LixSi, evidence for a third LiySiOx phase was observed. As these phase transformations occurred, the pulsed VE-XPS approach revealed chemically resolved changes in overpotential via transient Li+ion-induced shifts of core-level binding-energies (BEs) associated with the various phases. Through this analysis, we show how nanoscale native SiO2 significantly contributes to net overpotential during the initial stages of lithiation, and how overpotential dramatically decreases as SiO2 converts to LiySiOx.

Relating X-ray photoelectron spectroscopy data to chemical bonding in MXenes.

The relationship between core level binding energy shifts (ΔCLBEs), that can be experimentally determined by X-ray photoelectron spectroscopy, and chemical bonding is analyzed for a series of MXenes, a new family of two-dimensional materials with a broad number of applications in nanotechnology. Based on first-principles calculations, the atomic and electronic structure of bare and O-terminated carbide MXene with M2C and M2CO2 (M = Ti, Zr, Hf, V, Nb, Ta, Cr, Mo, and W) stoichiometries are investigated with a focus on trends in the C(1s) and O(1s) ΔCLBEs, including initial and final state effects, along with the series. A rather good linear correlation between the available experimental and calculated C(1s) and O(1s) ΔCLBEs exists, with quantitative agreement when final state effects are included, that validates the conclusions from the present computational approach. The present study shows that ΔCLBEs of bare MXenes are governed by the initial state effects and directly correlate with the net charge on the C atoms. However, for the case of O-terminated MXenes, C(1s) and O(1s) ΔCLBEs exhibit a much less significant correlation with the net charge of either C or O atoms which is attributed to the structural changes induced on the M2C moiety by the presence of the O layers and the different stacking sequence observed depending on the MXene composition. The present study shows how and when XPS can be used to extract information regarding the nature of the chemical bond in bare or functionalized MXenes.

Potential Screening at Electrode/Ionic Liquid Interfaces from In Situ X-ray Photoelectron Spectroscopy.

A new approach to investigate potential screening at the interface of ionic liquids (ILs) and charged electrodes in a two‐electrode electrochemical cell by in situ X‐ray photoelectron spectroscopy has been introduced. Using identical electrodes, we deduce the potential screening at the working and the counter electrodes as a function of applied voltage from the potential change of the bulk IL, as derived from corresponding core level binding energy shifts for different IL/electrode combinations. For imidazolium‐based ILs and Pt electrodes, we find a significantly larger potential screening at the anode than at the cathode, which we attribute to strong attractive interactions between the imidazolium cation and Pt. In the absence of specific ion/electrode interactions, asymmetric potential screening only occurs for ILs with different cation and anion sizes as demonstrated for an imidazolium chloride IL and Au electrodes, which we assign to the different thicknesses of the electrical double layers. Our results imply that potential screening in ILs is mainly established by a single layer of counterions at the electrode.

Revisiting surface core-level shifts for ionic compounds

The shifts of core-level binding energies can provide powerful information about the electronic structure of a material. Understanding the physical origin of these shifts for catalytically relevant oxides may provide important insight into their properties. This requires reliable theoretical methods which are able to relate the binding energy shifts to the electronic structure. In order to establish such a methodology, the CaO(100) surface to bulk core-level binding energy shifts have been studied with Hartree-Fock and density-functional theory methods using both cluster and periodic slab models. The shifts obtained from the different theoretical methods are compared with each other and with data from synchrotron-based x-ray photoelectron spectroscopy (XPS) measurements. With a common approximation for the slab model treatment of XPS, the predicted binding energy shifts are seriously in error. The origin of the error is identified as arising from a flawed treatment of the surface atom binding energies, and a method for correcting the failure is presented.

First-principles calculation of X-ray photoelectron spectroscopy binding energy shift for nitrogen and phosphorus defects in 3C-silicon carbide

We systematically investigated the formation energies and the core-level X-ray photoelectron spectroscopy binding energy (XPSBE) shifts of nitrogen (N) 1s and phosphorus (P) 2p for defects including N and P in 3C-SiC by a first-principles calculation using the generalized gradient approximation, whose reliability for n-type defects was confirmed by some tests using the HSE06 hybrid functional. XPSBEs were separated into the local potential average around the impurity and the relaxation energy of the wave function to analyze the relationship between the XPSBE shift and the defect structures. It is difficult to understand the relaxation energy intuitively. The electrons localized around the impurity atom, which have energy levels in energy gaps, make a large contribution to the relaxation energies. Considering the formation energies, we predicted some XPS peaks expected to be found.

Effect of electron correlation in the decomposition of core level binding energy shifts into initial and final state contributions.

The influence of electron correlation into the decomposition of core level binding energy shifts, measured by X-ray photoelectron spectroscopy (XPS), into initial and final effects is analysed for a series of molecules where these effects are noticeable. Moreover, the series of molecules is chosen in such a way that electron delocalization and increasing number of electrons may provide a large screening of the core hole. A detailed analysis shows that the Hartree-Fock decomposition is biased whereas a physically meaningful decomposition is obtained when electron correlation effects are taken into account. The results show that in this case, trends in core level binding energy shifts are driven by initial state effects thus providing further support to the use of these observable quantities to interpret changes in the chemical bond in the neutral molecule rather than on the core ionized cation. Consequences for the theoretical interpretation of XPS data in materials and surface science are discussed.

Correlation between Ethylene Adsorption Energies and Core-Level Shifts for Pd Nanoclusters

Density functional theory calculations have been used to investigate the adsorption of ethylene on Pd nanoclusters together with shifts in core-level binding energies of Pd atoms bonded to the adsorbate. The adsorption energy is found to correlate with the core-level shifts (CLS), which is consistent with the notion that the core-level binding energy is a measure of differences in cohesion. The correlation between adsorption energies and core-level shifts is found to be stronger than the correlation between adsorption energies and generalized coordination numbers, indicating that descriptors preferably should account for electronic effects explicitly. The advantages of CLS as a descriptor for the screening of adsorption properties is discussed.

Interface properties and built-in potential profile of a LaCrO3/SrTiO3 superlattice determined by standing-wave excited photoemission spectroscopy

LaCrO$_3$ (LCO) / SrTiO$_3$ (STO) heterojunctions are intriguing due to a polar discontinuity along (001), two distinct and controllable interface structures [(LaO)$^+$/(TiO$_2$)$^0$ and (SrO)$^0$/(CrO$_2$)$^-$], and interface-induced polarization. In this study, we have used soft- and hard x-ray standing-wave excited photoemission spectroscopy (SW-XPS) to generate a quantitative determination of the elemental depth profiles and interface properties, band alignments, and the depth distribution of the interface-induced built-in potentials in the two constituent oxides. We observe an alternating charged interface configuration: a positively charged (LaO)$^+$/(TiO$_2$)$^0$ intermediate layer at the LCO$_\textbf{top}$/STO$_\textbf{bottom}$ interface and a negatively charged (SrO)$^0$/(CrO$_2$)$^-$ intermediate layer at the STO$_\textbf{top}$/LCO$_\textbf{bottom}$ interface. Using core-level SW data, we have determined the depth distribution of species, including through the interfaces, and these results are in excellent agreement with scanning transmission electron microscopy and electron energy loss spectroscopy (STEM-EELS) mapping of local structure and composition. SW-XPS also enabled deconvolution of the LCO-contributed and STO- contributed matrix-element-weighted density of states (MEWDOSs) from the valence band (VB) spectra for the LCO/STO superlattice (SL). Monitoring the VB edges of the deconvoluted MEWDOS shifts with a change in probing profile, the alternating charge- induced built-in potentials are observed in both constituent oxides. Finally, using a two-step simulation approach involving first core-level binding energy shifts and then valence-band modeling, the built-in potential gradients across the SL are resolved in detail and represented by the depth distribution of VB edges.

Time-resolved electron spectroscopy for chemical analysis of photodissociation: Photoelectron spectra of Fe(CO)5, Fe(CO)4, and Fe(CO)3.

The prototypical photoinduced dissociation of Fe(CO)5 in the gas phase is used to test time-resolved x-ray photoelectron spectroscopy for studying photochemical reactions. Upon one-photon excitation at 266 nm, Fe(CO)5 successively dissociates to Fe(CO)4 and Fe(CO)3 along a pathway where both fragments retain the singlet multiplicity of Fe(CO)5. The x-ray free-electron laser FLASH is used to probe the reaction intermediates Fe(CO)4 and Fe(CO)3 with time-resolved valence and core-level photoelectron spectroscopy, and experimental results are interpreted with ab initio quantum chemical calculations. Changes in the valence photoelectron spectra are shown to reflect changes in the valence-orbital interactions upon Fe-CO dissociation, thereby validating fundamental theoretical concepts in Fe-CO bonding. Chemical shifts of CO 3σ inner-valence and Fe 3p core-level binding energies are shown to correlate with changes in the coordination number of the Fe center. We interpret this with coordination-dependent charge localization and core-hole screening based on calculated changes in electron densities upon core-hole creation in the final ionic states. This extends the established capabilities of steady-state electron spectroscopy for chemical analysis to time-resolved investigations. It could also serve as a benchmark for how charge and spin density changes in molecular dissociation and excited-state dynamics are expressed in valence and core-level photoelectron spectroscopy.

(Invited) Platinum Group Metal-Free Oxygen Reduction Electrocatalysts: Structure-to-Property Relationships and Design Directions

Platinum group metal-free (PGM-free) electrocatalysts draw much attention these days as low-temperature (polymer electrolyte) fuel cell technology is approaching its broad market introduction. Among several competing chemistries, catalysts based on metal-nitrogen-carbon (M-N-C) materials system are by far the most popular choice. UNM have developed and commercialized catalysts from this class obtained by pyrolysis of N-containing organic precursor (N and C source) and transition metal salt (Fe, Co, Mn or combination of more than one metal). The pyrolysis is carried after integration of the precursors with a sacrificial support (usually mono-dispersed silica powder) through ball-milling. After the pyrolysis, the silica support is removed by leaching with HF. The catalysts are usually re-pyrolyzed and a second acid wash is carried out to remove remaining of metallic, oxide or carbide particles from the nano-composite. The resulting catalysts show high graphitic content (exceeding 80%) and atomically dispersed transition metal (usually less than 1%). Both nitrogen and transition metal moieties are integrated into the carbonaceous structure as defects in graphene. Design for performance of this type of catalysts depends on an understanding the nature and of these transition metal and nitrogen sites and the ways they participate in the oxygen reduction reaction (ORR). The structure-to-property relationships in these materials will be discussed here through correlating of theirs observed electrocatalytic activity in Rotating Ring-Disk Electrode (RRDE) and performance in single Membrane Electrode Assembly (MEA) polymer electrolyte fuel cells with results of X-ray photoelectron (XPS), X-ray absorption (XAS) and Mössbauer spectroscopy charaterization. We have introduced Density Functional Theory (DFT) calculations of core level binding energy shifts as a powerful tool for interpretation of XPS spectra of such catalysts.1-3 A combination of XPS and first principles calculations of nitrogen-containing model electrocatalysts4-5 was further used to elucidate the chemical nature of the nitrogen defects. We will discuss here how hydrogenation and protonation of different nitrogen-containing defects play a major role in ORR activity of the PGM-free catalysts across different pH ranges. This paper will place a particular emphasis on elucidating the role of nitrogen- and Fe-N-containing in-plane and edge defects on the acid-base steps in ORR catalysis and thus regulation PGM-free catalysts reactivity and selectivity. The nature of Fe-N-containing in-plane and edge defects pre-determines their association with the micro-pores (edge defects) or with the extended graphene surface (in-plane defects). This sets the stage for a “first order” dependence of chemistry of the active site and the morphology of the carbonaceous material, which is the catalysts’ backbone (see the Figure for conceptual illustration). We have shown that these correlations are more pronounced when MEA performance data are being used. In this case, we observe a strong anti-correlation of the overall performance with the “stacking number” of the graphitic domains in these catalysts, which lead us to conclude that in-plane defects are the predominant contributors to catalytic activity in fuel cells.6 Further discussion will bring the importance of these two classes of active sites in different modes of MEA performance loss and will serve as a guide for both active and durable PGM-free Polymer Electrolyte Fuel Cell (PEMFC) cathode catalysts and catalyst layer design. K. Artyushkova, et al., Chem. Commun., 49 (2013) 2539-2541S. Kabir, et al., Phys. Chem. Chem. Phys., 17 (2015) 17785-17789S. Kabir, et al., Surf. Interface Anal., 48 (2016) 293-300I. Matanovic, et al., J. Phys. Chem. C, 120 (2016) 29225-29232K. Artyushkova, et al., J. Phys. Chem. C, 121 (2017) 2836–2843M. Workman, et al., ACS Energy Letters, 2 (2017) 1489-1493 Figure 1

Core electron binding energies of adsorbates on Cu(111) from first-principles calculations.

Core-level X-ray Photoelectron Spectroscopy (XPS) is often used to study the surfaces of heterogeneous copper-based catalysts, but the interpretation of measured spectra, in particular the assignment of peaks to adsorbed species, can be extremely challenging. In this study we present a computational scheme which combines the use of slab models of the surface for geometry optimization with cluster models for core electron binding energy calculation. We demonstrate that by following this modelling strategy first principles calculations can be used to guide the analysis of experimental core level spectra of complex surfaces relevant to heterogeneous catalysis. The all-electron ΔSCF method is used for the binding energy calculations. Specifically, we calculate core-level binding energy shifts for a series of adsorbates on Cu(111) and show that the resulting C1s and O1s binding energy shifts for adsorbed CO, CO2, C2H4, HCOO, CH3O, H2O, OH, and a surface oxide on Cu(111) are in good overall agreement with the experimental literature.

Monitoring the chemical and electronic properties of electrolyte-electrode interfaces in all-solid-state batteries using operando X-ray photoelectron spectroscopy.

Understanding the degradation of the solid electrolyte-electrodes interface during cycling is currently one of the most challenging obstacles in the development of all-solid-state batteries. Here, we introduce operando X-ray photoelectron spectroscopy (XPS) as a combined approach for real-time monitoring of the (i) (electro-) chemical interfacial reactions between different components of the composites electrode and (ii) surface electronic properties. The dedicated electrochemical cell, capable of maintaining high mechanical pressure, offers reliable electrochemistry and versatility in terms of materials application. We propose a fundamental physical model to explain the effect of applied cell voltage on core level binding energy shifts, which allows direct contactless measurement of the surface potentials. Such a fundamental understanding is also an essential prerequisite for (i) accurate interpretation of the XPS core level peaks, (ii) the assignment of solid electrolyte decomposition byproducts and (iii) an improved description of the interfacial reaction mechanisms in all-solid-state batteries. A mixture of LiCoO2 (LCO) and (Li2S)3-P2S5 (LPS) cycled vs. InLix is investigated revealing that the LPS oxidation begins above 2.1 V vs. InLix and the byproducts passivate the composite surface against further oxidation. Moreover, we confirm that LCO particles are electronically conductive even in the absence of conductive additives by measuring directly its surface potential during cycling.

Fluorinated Phosphorene: Electrochemical Synthesis, Atomistic Fluorination, and Enhanced Stability.

Phosphorene has attracted great interest due to its unique electronic and optoelectronic properties owing to its tunable direct and moderate band-gap in association with high carrier mobility. However, its intrinsic instability in air seriously hinders its practical applications, and problems of technical complexity and in-process degradation exist in currently proposed stabilization strategies. A facile pathway in obtaining and stabilizing phosphorene through a one-step, ionic liquid-assisted electrochemical exfoliation and synchronous fluorination process is reported in this study. This strategy enables fluorinated phosphorene (FP) to be discovered and large-scale, highly selective few-layer FP (3-6 atomic layers) to be obtained. The synthesized FP is found to exhibit unique morphological and optical characteristics. Possible atomistic fluorination configurations of FP are revealed by core-level binding energy shift calculations in combination with spectroscopic measurements, and the results indicate that electrolyte concentration significantly modulates the fluorination configurations. Furthermore, FP is found to exhibit enhanced air stability thanks to the antioxidation and antihydration effects of the introduced fluorine adatoms, and demonstrate excellent photothermal stability during a week of air exposure. These findings pave the way toward real applications of phosphorene-based nanophotonics.

Effects of rhenium on graphene grown on SiC(0001)

We study the effects of Rhenium (Re) deposited on epitaxial monolayer graphene grown on SiC(0001) and after subsequent annealing at different temperatures, by performing high resolution photoelectron spectroscopy (PES) and angle resolved photoelectron spectroscopy (ARPES). The graphene-Re system is found to be thermally stable. While no intercalation or chemical reaction of the Re is detected after deposition and subsequent annealing up to 1200°C, a gradual decrease in the binding energy of the Re 4f doublet is observed. We propose that a larger mobility of the Re atoms with increasing annealing temperature and hopping of Re atoms between different defective sites on the graphene sample could induce this decrease of Re 4f binding energy. This is corroborated by first principles density functional theory (DFT) calculations of the Re core-level binding energy shift. No change in the doping or splitting of the initial monolayer graphene electronic band structure is observed after Re deposition and annealing up to 1200°C, only a broadening of the bands.