Binding Energy Scale Research Articles

3D hydrogen-like screening effect on excitons in hBN-encapsulated monolayer transition metal dichalcogenides

We observe both s-series and p-series excitons by using sum frequency generation spectroscopy on monolayer (1L-)MX2 (M = Mo, W, X = S, Se) encapsulated by hexagonal boron nitride (hBN). Moreover, we perform numerical calculations with the Rytova-Keldysh potential and obtain the relative dielectric constant of hBN among other parameters. The obtained relative dielectric constant can be approximated by the high-frequency limit of the infrared dispersion even though the exciton binding energies are almost on the phonon resonances in hBN. This suggests that the theoretically indicated modification of the exciton level structure due to the phonon resonances is negligible. The power-law scaling of exciton binding energies indicates that dielectric screening of 1L-MX2 exciton levels other than 1s can be approximated by that of a 3D hydrogen model with the dielectric constant of hBN.

Binding energy referencing in X-ray photoelectron spectroscopy: Expanded data set confirms that adventitious carbon aligns to the sample vacuum level

The correct referencing of the binding energy scale is essential for the accuracy of chemical analysis by X-ray photoelectron spectroscopy. The C 1s C–C/C–H peak from adventitious carbon (AdC), most commonly used for that purpose, was previously shown to shift by several eVs following changes in the sample work function ϕSA, thus indicating that AdC aligns to the sample vacuum level (VL). Here, results from a much larger sample set including 360 specimens comprising metals, nitrides, carbides, borides, oxides, carbonitrides, and oxynitrides are presented. Irrespective of the material system the C 1s peak of AdC is found to follow changes in ϕSA fully confirming previous results. Several observations exclude differential charging as plausible explanation for the C 1s peak shifts. All experimental evidence points instead to the VL alignment at the AdC/sample interface as the main reason. Should the C 1s peak of AdC be used for spectra referencing the measurement of sample work function is necessary, irrespective of whether samples are measured grounded or insulated from the spectrometer.

Attraction from kinetic frustration in ladder systems

We analyze the formation of multiparticle bound states in ladders with frustrated kinetic energy in two-component bosonic and two-component fermionic systems. We focus on the regime of light doping relative to insulating states at half-filling, spin polarization close to 100%, and strong repulsive interactions. A special feature of these systems is that the binding energy scales with single-particle tunneling t rather than exchange interactions, since effective attraction arises from alleviating kinetic frustration. For two-component Fermi systems on a zigzag ladder we find a bound state between a hole and a flipped spin (magnon) with a binding energy that can be as large as 0.6t. We demonstrate that magnon-hole attraction leads to formation of clusters comprising several holes and magnons, and we expound on antiferromagentic correlations for the transverse spin components inside the clusters. We identify several many-body states that result from self-organization of multiparticle bound states, including a Luttinger liquid of hole-magnon pairs and a density wave state of two-hole–three-magnon composites. We establish a symmetry between the spectra of Bose and Fermi systems and use it to establish the existence of antibound states in two-component Bose mixtures with SU(2) symmetric repulsion on a zigzag ladder. We also consider Bose and Fermi systems on a square ladder with flux and demonstrate that both systems support bound states. We discuss experimental signatures of multiparticle bound states in both equilibrium and dynamical experiments. We point out intriguing connections between these systems and the quark bag model in QCD. Published by the American Physical Society 2024

HAXPES reference spectra of bulk Mo and MoSe2 with Cr Kα excitation

Monochromatic Cr Kα radiation (5414.8 eV) was used to acquire high-energy photoelectron spectroscopy (HAXPES) data on pure Mo and bulk MoSe2 compound with special attention paid to binding energy scale correction and quantification. The reported spectra include a survey scan and high-resolution Mo 2s, Mo 2p1/2, Mo 2p3/2, Mo 3d, Mo 3p1/2, Mo 3p3/2, Mo 3s, Mo 4s, Mo4p, Se2s, Se 2p1/2, Se 2p3/2, Se 3s, Se 3p1/2, Se 3p3/2, and Se 3d core-levels. The data will be useful as reference core-level spectra for HAXPES studies on molybdenum and its compounds.

Sodiation energetics in pore size controlled hard carbons determined via entropy profiling

Cell temperature of hard carbons is varied under open circuit conditions (entropy profiling) during sodiation. Features vary with pore size, revealing the nanopore filling onset. Sodium binding energy scales inversely with pore curvature radius.

Undressing the myth of apparent constant binding energy of the C 1 s peak from adventitious carbon in x-ray photoelectron spectroscopy

As evidenced by more than 12,000 papers published annually, X-ray photoelectron spectroscopy (XPS) is an indispensable technique in modern materials science for the analysis of surface chemistry. However, a literature survey reveals that in the vast majority of cases an incorrect referencing of the binding energy scale is used, neglecting warnings that have been formulated from the early days of the technique. Consequences for the data reliability are disastrous and decades of XPS work may require revisiting.

Scaling of phonon frequencies and electron binding energies with interatomic distances in InxGa1−xN

In-K-edge x-ray absorption fine structure (EXAFS), x-ray photoelectron, and Raman spectroscopy results are combined for a comprehensive study of InxGa1−xN layers with energy gaps extending over nearly the whole visible spectrum. The In–N and In–(In,Ga) distances determined by EXAFS are used for the derivation of the In–N bond ionicity as well as for the phonon frequency dependence of the LO and B12 modes, assessed by Raman, and the N 1s, In 3d5/2, Ga 3s, and Ga 2p1/2 electron binding energies on those distances. Phonon confinement due to perturbation of the periodic potential caused by the alloying is also quantified.

The same chemical state of carbon gives rise to two peaks in X-ray photoelectron spectroscopy

Chemical state analysis in X-ray photoelectron spectroscopy (XPS) relies on assigning well-defined binding energy values to core level electrons originating from atoms in particular bonding configurations. Here, we present direct evidence for the violation of this paradigm. It is shown that the C 1s peak due to C–C/C–H bonded atoms from adventitious carbon (AdC) layers accumulating on Al and Au foils splits into two distinctly different contributions, as a result of vacuum level alignment at the AdC/foil interface. The phenomenon is observed while simultaneously recording the spectrum from two metal foils in electric contact with each other. This finding exposes fundamental problems with the reliability of reported XPS data as C 1s peak of AdC is routinely used for binding energy scale referencing. The use of adventitious carbon in XPS should thus be discontinued as it leads to nonsense results. Consequently, ISO and ASTM charge referencing guides need to be rewritten.

Unraveling spectral shapes of adventitious carbon on gold using a time-resolved high-resolution X-ray photoelectron spectroscopy and principal component analysis

To extract chemical information reliably, we propose a new data processing method based both on the creation of information vectors and on a vector base change. The originality of this method is the combination of the different core XPS peaks, Auger and/or valence bands in a single vector. We show that the measurement of a sample upon its progressive chemical change, such as accumulation of the adventitious carbon, allows us to create a new vector basis using linear algebra and Principal Component Analysis. We demonstrate the application of this method using adventitious carbon films on Au foil utilizing Au 4f, C 1s and O 1s spectral envelopes. This method expands the possibilities of XPS measured chemical environment analysis and is an operator-unbiased solution for materials for which the reference XPS spectra are not available. In this demonstration, the emphasis is placed on identifying changes in a C 1s peak assumed to be adventitious carbon and highlight uncertainties associated with calibrating the binding energy scale using a simple peak model to identify a component C 1s assumed to be saturated hydrocarbon in origin.

Concepts for chemical state analysis at constant probing depth by lab‐based XPS/HAXPES combining soft and hard X‐ray sources

The greater information depth provided in hard X‐ray photoelectron spectroscopy (HAXPES) enables nondestructive analyses of the chemistry and electronic structure of buried interfaces. Moreover, for industrially relevant elements like Al, Si, and Ti, the combined access to the Al 1s, Si 1s, or Ti 1s photoelectron line and its associated Al KLL, Si KLL, or Ti KLL Auger transition, as required for local chemical state analysis on the basis of the Auger parameter, is only possible with hard X‐rays. Until now, such photoemission studies were only possible at synchrotron facilities. Recently, however, the first commercial XPS/HAXPES systems, equipped with both soft and hard X‐ray sources, have entered the market, providing unique opportunities for monitoring the local chemical state of all constituent ions in functional oxides at different probing depths, in a routine laboratory environment. Bulk‐sensitive shallow core levels can be excited using either the hard or soft X‐ray source, whereas more surface‐sensitive deep core‐level photoelectron lines and associated Auger transitions can be measured using the hard X‐ray source. As demonstrated for thin Al2O3, SiO2, and TiO2 films, the local chemical state of the constituting ions in the oxide may even be probed at near‐constant probing depth by careful selection of sets of photoelectron and Auger lines, as excited with the combined soft and hard X‐ray sources. We highlight the potential of lab‐based HAXPES for the research on functional oxides and also discuss relevant technical details regarding the calibration of the kinetic binding energy scale.

Compromising Science by Ignorant Instrument Calibration—Need to Revisit Half a Century of Published XPS Data

AbstractX‐ray photoelectron spectroscopy (XPS) is an indispensable technique in modern materials science for the determination of chemical bonding as evidenced by more than 10 000 XPS papers published annually. A literature survey reveals that in the vast majority of cases an incorrect referencing of the binding energy scale is used, neglecting warnings that have been formulated from the early days of the technique. Consequences for the data reliability are disastrous and decades of XPS work require revisiting. The purpose of this Viewpoint is to highlight the existing problems, review the criticism and suggest ways forward.

Compromising Science by Ignorant Instrument Calibration-Need to Revisit Half a Century of Published XPS Data.

X-ray photoelectron spectroscopy (XPS) is an indispensable technique in modern materials science for the determination of chemical bonding as evidenced by more than 10 000 XPS papers published annually. A literature survey reveals that in the vast majority of cases an incorrect referencing of the binding energy scale is used, neglecting warnings that have been formulated from the early days of the technique. Consequences for the data reliability are disastrous and decades of XPS work require revisiting. The purpose of this Viewpoint is to highlight the existing problems, review the criticism and suggest ways forward.

Exploration of the bright and dark exciton landscape and fine structure of MoS2 using G0W0 -BSE

Spectral ordering between dark and bright excitons in transition metal dichalcogenides is of increasing interest for optoelectronic applications. However, little is known about dark exciton energies and their binding energies. We report the exciton landscape including momentum-forbidden dark excitons of MoS$_{2}$ monolayer using single shot GW-Bethe Salpeter equation (G$_{0}$W$_{0}$-BSE) calculations. We find the lowest-energy exciton to be indirect at ($\textrm K'_{v} \rightarrow \textrm K_{c}$) in agreement with recent GdW-BSE calculations [2D Mater. 6, 035003 (2019)]. We also find that by large, dark exciton binding energies ($E_b$) scale with the quasiparticle energies ($E_g$) according to the $E_b/E_g=0.25$ rule. Differences in exciton binding energies are explained using an orbital theory.

Scaling Relations on Basal Plane Vacancies of Transition Metal Dichalcogenides for CO2 Reduction

Transition metal dichalcogenides (TMDs) have shown promising electrocatalytic performance for CO2 reduction (CO2R) recently. However, the development of efficient and selective catalysts remains a major challenge. Although recent studies have suggested the importance of activation energies as activity descriptors for CO2R beyond CO, the scaling of intermediate binding energies presents the first step in computational catalyst screening. Here, we investigate the basal vacancy on 2H and 1T/1T′ phase group V, VI, and X TMDs for CO2 reduction. We find that the change of oxophicility and carbophilicity on each group of TMDs follows different trends, which leads to different scaling relations amongst key intermediates. Our thermochemical analysis also suggests group V and VI TMDs to be either selective for hydrogen evolution reaction or prone to OH poisoning. However, the initial analysis suggests group X TMDs to be possible candidates for active and selective CO2 reduction without suffering from OH poisoning, ...

Calibrations and checks of the binding-energy scales of X-ray photoelectron spectrometers

A recent paper in this journal [B. V. Crist, J. Electron Spectrosc. Relat. Phenom. (in press)] describes current and historical calibration problems with core-electron binding energies (BEs) measured by X-ray photoelectron spectrometers and how these problems affect BE data in databases. Although Crist provides useful information about the variability of reported BE values, his article is incomplete or incorrect in some historical and other details. The present article corrects and extends information in the Crist paper and provides additional information on how calibrations and checks of the BE scale should be made.

The physics of single-side fluorination of graphene: DFT and DFT + U studies

We present density functional theory (DFT) calculations of the electronic and magnetic properties of fluorine adatoms on a single side of a graphene monolayer. By extrapolating the results, the binding energy of a single fluorine adatom on graphene in the dilute limit is calculated. Our results confirm that the finite-size error in the binding energy scales inversely with the cube of the linear size of the simulation cell. We establish relationships between stability and CF bond nature, diffusion of fluorine adatoms and total magnetization in different configurations of adatoms. For single-side fluorination, sp2.33 is the maximum p-content re-hybridization found in the CF bond. We show that semilocal DFT cannot predict correctly the magnetic properties of fluorinated graphene and a higher level theory, such as DFT + U is needed. The results indicate a tendency of graphene to reduce the imbalance between adsorption on the two sublattices, and therefore total magnetization, through low-energy-barrier pathways on a time scale of ∼10 ps at room temperature. The thermodynamically favored arrangements are those with the smallest total magnetization. Indeed, the electronic structure is intimately related to the magnetic properties and changes from semi-metallic to p-type half-metallic or semiconducting features, depending on the adatoms arrangement.

KL double core hole pre-edge states of HCl.

The formation of double core hole pre-edge states of the form 1s-12p-1(1,3P)σ*,nl for HCl, located on the binding energy scale as deep as 3 keV, has been investigated by means of a high resolution single channel electron spectroscopy technique recently developed for the hard X-ray region. A detailed spectroscopic assignment is performed based on ab initio quantum chemical calculations and by using a sophisticated fit model comprising regular Rydberg series. Quantum defects for the different Rydberg series are extracted and the energies for the associated double core hole ionization continua are extrapolated. Dynamical information such as the lifetime width of these double-core-hole pre-edge states and the slope of the related dissociative potential energy curves are also obtained. In addition, 1s-12p-1V-1nlλn'l'λ' double shake-up transitions and double core hole states of the form 1s-12s-1(1,3S)σ*,4s are observed.

(Invited) Redox Chemistry of Li-Sulfur Batteries Investigated Via Electrochemistry, X-Ray Photoelectron Spectroscopy and Electron Paramagnetic Resonance

With specific energies of 400–500 Wh kg-1 Li-Sulphur (Li-S) batteries are promising candidates for energy storage applications. Challenges that still impede their commercialization are the expansion of the cathode during discharge, low conductivity, operational instability due to side reactions and slow Li+ transport [1]. In the presence of Li+, sulphur undergoes a multistep reduction, yielding Li2S as an end product. Practically, however, the highly reactive and soluble sulphur intermediates (polysulfides) inevitably form and start their activity as a redox shuttle or/and participate in side reactions. These processes follow a complicated mechanism, which clarification requires a comprehensive analysis [1-3]. This paper will discuss recent results from the authors' labs on the multi-electron transfer of sulfur as a function of the electrolyte composition. Additionally, we have investigated the influence of cathode binder type (e.g. Polyvinylidene difluoride-PVDF, Carboxymethylcellulose-CMC, Guar gum - GG) on the cycling performance of the cathodes. Our methodology involves a combination of electrochemical (galvanostatic cycling, cyclic voltammetry, impedance spectroscopy- EIS) and spectroscopic methods (X-ray Photoelectron Spectroscopy, XPS; Electron Paramagnetic Resonance, EPR), useful for analysis of electrolyte and interface stability. We will show that the increase of lithium bis(trifluoromethylsulfonyl)imide (Li[Tf2N]) concentration displays a valuable impact on the cycling stability(Fig1A,B), suppressing the shuttle effect and improving capacity retention of the sulfur-lithium cells. Moreover, the polysaccharide type binders present better cycling performance in terms of capacity retention and current efficiency (Fig1A,B). To understand the origin of this improvement we characterized the chemical composition of the cycled electrodes by means of XPS. Additionally, the role of small amount 1-butyl-1-methyl-pyrrolidinium bis(trifluoromethylsulfonyl)imide ([BMP][Tf2N]) was also studied. Fig.1C,D compares the Li1s and S2p experimental core level peaks from three different anode samples. Sulphur displays multicomponent spectra, in which Li2S (S2p3/2~160eV), disulfides (S2p3/2~162eV), sulfites (S2p3/2~167eV) and sulfates (S2p3/2~169eV) can be distinguished [4]. Their comparison reveals difference in the composition on the surface and in the bulk of the passivating layer, when lower concentration of Li[Tf2N] was used, independent on the presence of [BMP][Tf2N] (Spectra 1 and 3). The later however, facilitate the overall decrease of sulphur quantity in the bulk (Spectra 3). Contrary, higher Li[Tf2N] concentration shows nearly identical spectral features before and after sputtering (Spectra 2), suggesting homogeneous in-depth elemental distribution in the interfacial layer. We anticipate that the Li1s core level peaks originate from at least three Li-containing species (Fig.1C), which close proximity in the binding energy scale leads to spectral broadening (FWHM ~3eV, Fig1, Li1s-Panel). The influence of the electrolyte composition on the electrical properties of Li-electrolyte interface was studied by impedance spectroscopy using a symmetric cell [5]. This method allows an individual assessment of the Li|electrolyte interface without interferences from the reference electrode position and the presence of cathode. The properties of the anode interface are correlated with their surface chemical composition, analyzed by XPS. By using EPR we studied the paramagnetic species formed at the early lithiation stage, where typically just long chain polysulfide anions are present. Our studies demonstrate generation of at least one sulphur-centered radical, which suggests that disproportionation accompany the sulfur lithiation reaction [6]. We will discuss the quantity, the life-time and the coordination geometry of the detected radicals.

Room temperature observation of biexcitons in exfoliated WS2 monolayers

Single layers of WS2 are direct gap semiconductors with high photoluminescence (PL) yield holding great promise for emerging applications in optoelectronics. The spatial confinement in a two-dimensional monolayer together with the weak dielectric screening leads to huge binding energies for the neutral excitons as well as other excitonic complexes, such as trions and biexcitons whose binding energies scale accordingly. Here, we report on the existence of biexcitons in mechanically exfoliated WS2 flakes from 78 K up to room temperature. Performing temperature and power dependent PL measurements, we identify the biexciton emission channel through the superlinear behavior of the integrated PL intensity as a function of the excitation power density. On the contrary, neutral and charged excitons show a linear to sublinear dependence in the whole temperature range. From the energy difference between the emission channels of the biexciton and neutral exciton, a biexciton binding energy of 65–70 meV is determined.

Excitonic absorption spectra in graphene nanoflakes: Tuning of exciton binding energy by dielectric environments.

By solving the Bethe-Salpeter equation within the Hartree-Fock formalism, we study the excitonic absorption spectra of graphene nanoflakes embedded in various dielectric environments. With the excitonic effects fully taken into account, the exciton binding energy as a function of the dielectric constant is found to be well described by a single scaling rule in which the scaling factor is found to vary slowly with the size of the nanoflakes. Furthermore, it is revealed that the exciton binding energy scales almost linearly with the on-site interaction energy and exhibits more sensitive dependence in smaller nanoflakes. Our results are found to agree well with the recent experiment.