Time-Resolved X-ray Emission Spectroscopy and Resonant Inelastic X-ray Scattering Spectroscopy of Laser Irradiated Carbon.

The existence of liquid carbon as an intermediate phase preceding the formation of novel carbon materials has been a point of contention for several decades. Experimental observation of such a liquid state requires nonthermal melting of solid carbon materials at various laser fluences and pulse properties. Reflectivity experiments performed in the mid-1980s reached opposing conclusions regarding the metallic or insulating properties of the purported liquid state. Time-resolved X-ray absorption studies showed shortening of C-C bonds and increasing diffraction densities, thought to evidence a liquid or glassy carbon state, respectively. Nevertheless, none of these experiments provided information on the electronic structure of the proposed liquid state. Herein, we report the results of time-resolved resonant inelastic X-ray scattering (RIXS) and time-resolved X-ray emission spectroscopy (XES) studies on amorphous carbon (a-C) and ultrananocrystalline diamond (UNCD) as a function of delay time between the irradiating pulse and X-ray probe. For both a-C and UNCD, we attribute decreases in RIXS or XES signals to transition blocking, relaxation, and finally, ablation. Increased signal at 20 ps following the irradiation of the UNCD is attributed to the probable formation of nanoscale structures in the ablation plume. Differences in the amount of signal observed between a-C and UNCD are explained by the difference in sample thickness and, specifically, incomplete melting of the UNCD film. Comparisons to spectral simulations based on MD trajectories at extreme conditions indicate that the carbon state in our experiments is crystalline. Normal mode analysis confirmed that symmetrical bending or stretching of the C-C bonds in the diamond lattice results in XES spectra with small intensity differences. Overall, we observed no evidence of melting to a liquid state, as determined by the lack of changes in the spectral properties for up to 100 ps delays following the melting pulses.

Probing lithium mobility at a solid electrolyte surface.

Solid-state electrolytes overcome many challenges of present-day lithium ion batteries, such as safety hazards and dendrite formation1,2. However, detailed understanding of the involved lithium dynamics is missing due to a lack of in operando measurements with chemical and interfacial specificity. Here we investigate a prototypical solid-state electrolyte using linear and nonlinear extreme-ultraviolet spectroscopies. Leveraging the surface sensitivity of extreme-ultraviolet-second-harmonic-generation spectroscopy, we obtained a direct spectral signature of surface lithium ions, showing a distinct blueshift relative to bulk absorption spectra. First-principles simulations attributed the shift to transitions from the lithium 1 s state to hybridized Li-s/Ti-d orbitals at the surface. Our calculations further suggest a reduction in lithium interfacial mobility due to suppressed low-frequency rattling modes, which is the fundamental origin of the large interfacial resistance in this material. Our findings pave the way for new optimization strategies to develop these electrochemical devices via interfacial engineering of lithium ions.

Saturable Absorption of Free-Electron Laser Radiation by Graphite near the Carbon K-Edge.

The interaction of intense light with matter gives rise to competing nonlinear responses that can dynamically change material properties. Prominent examples are saturable absorption (SA) and two-photon absorption (TPA), which dynamically increase and decrease the transmission of a sample depending on pulse intensity, respectively. The availability of intense soft X-ray pulses from free-electron lasers (FELs) has led to observations of SA and TPA in separate experiments, leaving open questions about the possible interplay between and relative strength of the two phenomena. Here, we systematically study both phenomena in one experiment by exposing graphite films to soft X-ray FEL pulses of varying intensity. By applying real-time electronic structure calculations, we find that for lower intensities the nonlinear contribution to the absorption is dominated by SA attributed to ground-state depletion; our model suggests that TPA becomes more dominant for larger intensities (>1014 W/cm2). Our results demonstrate an approach of general utility for interpreting FEL spectroscopies.

Pressure-induced metallization and 3d-like behavior in TcS2.

TcS2 undergoes a charge transfer insulator to metal transition above 28 GPa. Laser annealing reveals a kinetically hindered high pressure arsenopyrite phase that is recoverable to ambient. The new phase is similar to the Mn-dichalcogenides rather than the expected Re-dichalcogenides and involves the formation of S-S and Tc-Tc bonds.

Polarization-Resolved Extreme-Ultraviolet Second-Harmonic Generation from LiNbO_{3}.

Second harmonic generation (SHG) spectroscopy ubiquitously enables the investigation of surface chemistry, interfacial chemistry, as well as symmetry properties in solids. Polarization-resolved SHG spectroscopy in the visible to infrared regime is regularly used to investigate electronic and magnetic order through their angular anisotropies within the crystal structure. However, the increasing complexity of novel materials and emerging phenomena hampers the interpretation of experiments solely based on the investigation of hybridized valence states. Here, polarization-resolved SHG in the extreme ultraviolet (XUV-SHG) is demonstrated for the first time, enabling element-resolved angular anisotropy investigations. In noncentrosymmetric LiNbO_{3}, elemental contributions by lithium and niobium are clearly distinguished by energy dependent XUV-SHG measurements. This element-resolved and symmetry-sensitive experiment suggests that the displacement of Li ions in LiNbO_{3}, which is known to lead to ferroelectricity, is accompanied by distortions to the Nb ion environment that breaks the inversion symmetry of the NbO_{6} octahedron as well. Our simulations show that the measured second harmonic spectrum is consistent with Li ion displacements from the centrosymmetric position while the Nb─O bonds are elongated and contracted by displacements of the O atoms. In addition, the polarization-resolved measurement of XUV-SHG shows excellent agreement with numerical predictions based on dipole-induced SHG commonly used in the optical wavelengths. Our result constitutes the first verification of the dipole-based SHG model in the XUV regime. The findings of this work pave the way for future angle and time-resolved XUV-SHG studies with elemental specificity in condensed matter systems.

Angstrom-Resolved Interfacial Structure in Buried Organic-Inorganic Junctions.

Charge transport processes at interfaces play a crucial role in many processes. Here, the first soft x-ray second harmonic generation (SXR SHG) interfacial spectrum of a buried interface (boron-Parylene N) is reported. SXR SHG shows distinct spectral features that are not observed in x-ray absorption spectra, demonstrating its extraordinary interfacial sensitivity. Comparison to electronic structure calculations indicates a boron-organic separation distance of 1.9 Å, with changes of less than 1 Å resulting in easily detectable SXR SHG spectral shifts (ca. hundreds of milli-electron volts).

Extreme Ultraviolet Second Harmonic Generation Spectroscopy in a Polar Metal.

The coexistence of ferroelectricity and metallicity seems paradoxical, since the itinerant electrons in metals should screen the long-range dipole interactions necessary for dipole ordering. The recent discovery of the polar metal LiOsO3 was therefore surprising [as discussed earlier in Y. Shi et al., Nat. Mater. 2013, 12, 1024]. It is thought that the coordination preferences of the Li play a key role in stabilizing the LiOsO3 polar metal phase, but an investigation from the combined viewpoints of core-state specificity and symmetry has yet to be done. Here, we apply the novel technique of extreme ultraviolet second harmonic generation (XUV-SHG) and find a sensitivity to the broken inversion symmetry in the polar metal phase of LiOsO3 with an enhanced feature above the Li K-edge that reflects the degree of Li atom displacement as corroborated by density functional theory calculations. These results pave the way for time-resolved probing of symmetry-breaking structural phase transitions on femtosecond time scales with element specificity.

The purported square ice in bilayer graphene is a nanoscale, monolayer object.

The phase diagram of water is complex, and interfacial effects can stabilize unusual structures at the nanoscale. Here, we employ bond order accelerated molecular dynamics simulations to show that upon encapsulation within bilayer graphene, water can spontaneously adopt a two-dimensional (monomolecular) layer of "square ice" at ambient conditions, instead of an encapsulated water droplet. Free energy calculations show that this motif is thermodynamically stable up to diameters of approximately 15 nm due to enhanced hydrogen bonding and favorable binding to the graphene sheets. Entropic losses due to solidification and reduced graphene-graphene binding enthalpy are opposing thermodynamic forces that conspire to limit the maximum size, but modification of any of these thermodynamic factors should change the range of stability. Simulated core-level spectroscopy reveals unambiguous orientation dependent signatures of square ice that should be discernable in experiments.

Understanding and Control of Bipolar Self-Doping in Copper Nitride.

Semiconductor materials that can be doped both n-type and p-type are desirable for diode-based applications and transistor technology. Copper nitride (Cu3N) is a metastable semiconductor with a solar-relevant bandgap that has been reported to exhibit bipolar doping behavior. However, deeper understanding and better control of the mechanism behind this behavior in Cu3N is currently lacking in the literature. In this work, we use combinatorial growth with a temperature gradient to demonstrate both conduction types of phase-pure, sputter-deposited Cu3N thin films. Room temperature Hall effect and Seebeck effect measurements show n-type Cu3N with an electron density of 1017 cm-3 for low growth temperature (≈ 35 °C) and p-type with a hole density between 1015 cm-3 and 1016 cm-3 for elevated growth temperatures (50 °C to 120 °C). Mobility for both types of Cu3N was ≈ 0.1 cm2/Vs to 1 cm2/V. Additionally, temperature-dependent Hall effect measurements indicate that ionized defects are an important scattering mechanism in p-type films. By combining X-ray absorption spectroscopy and first-principles defect theory, we determined that VCu defects form preferentially in p-type Cu3N while Cui defects form preferentially in n-type Cu3N; suggesting that Cu3N is a compensated semiconductor with conductivity type resulting from a balance between donor and acceptor defects. Based on these theoretical and experimental results, we propose a kinetic defect formation mechanism for bipolar doping in Cu3N, that is also supported by positron annihilation experiments. Overall, the results of this work highlight the importance of kinetic processes in the defect physics of metastable materials, and provide a framework that can be applied when considering the properties of such materials in general.

Synthesis of a mixed-valent tin nitride and considerations of its possible crystal structures.

Recent advances in theoretical structure prediction methods and high-throughput computational techniques are revolutionizing experimental discovery of the thermodynamically stable inorganic materials. Metastable materials represent a new frontier for these studies, since even simple binary non-ground state compounds of common elements may be awaiting discovery. However, there are significant research challenges related to non-equilibrium thin film synthesis and crystal structure predictions, such as small strained crystals in the experimental samples and energy minimization based theoretical algorithms. Here, we report on experimental synthesis and characterization, as well as theoretical first-principles calculations of a previously unreported mixed-valent binary tin nitride. Thin film experiments indicate that this novel material is N-deficient SnN with tin in the mixed ii/iv valence state and a small low-symmetry unit cell. Theoretical calculations suggest that the most likely crystal structure has the space group 2 (SG2) related to the distorted delafossite (SG166), which is nearly 0.1 eV/atom above the ground state SnN polymorph. This observation is rationalized by the structural similarity of the SnN distorted delafossite to the chemically related Sn3N4 spinel compound, which provides a fresh scientific insight into the reasons for growth of polymorphs of metastable materials. In addition to reporting on the discovery of the simple binary SnN compound, this paper illustrates a possible way of combining a wide range of advanced characterization techniques with the first-principle property calculation methods, to elucidate the most likely crystal structure of the previously unreported metastable materials.

Communication: On the difficulty of reproducibly measuring PbCl2 X-ray absorption spectra.

Previous measurements of the X-ray absorption spectra of PbCl2 at the chlorine K-edge have shown significant variation between different studies. Herein, using first principles simulations of X-ray absorption spectroscopy, we show that the observed spectral variations are due to the generation of Cl2 gas and depletion of chlorine from PbCl2, consistent with what is observed during ultraviolet absorption for the same compound. We note that Cl2 gas generation can also be initiated using higher resonant X-ray energies, including Pb X-ray absorption edges. While this casts doubt on previous interpretations of certain measurements, it does indicate a means of generating chlorine gas during in situ experiments by passing high energy x-rays through a hard x-ray transparent medium and onto PbCl2.

Electronic structure of aqueous borohydride: a potential hydrogen storage medium.

Borohydride salts have been considered as good prospects for transportable hydrogen storage materials, with molecular hydrogen released via hydrolysis. We examine details of the hydration of sodium borohydride by the combination of X-ray absorption spectroscopy and first principles' theory. Compared to solid sodium borohydride, the aqueous sample exhibits an uncharacteristically narrow absorption feature that is shifted to lower energy, and ascribed to the formation of dihydrogen bonds between borohydride and water that weaken the boron-hydrogen covalent bonds. Water also acts to localize the highly excited molecular orbitals of borohydride, causing transitions to excited states with p character to become more intense and a sharp feature, uncharacteristic of tetrahedral molecules, to emerge. The simulations indicate that water preferentially associates with borohydride on the tetrahedral corners and edges.

pH-dependent x-ray absorption spectra of aqueous boron oxides.

Near edge x-ray absorption fine structure (NEXAFS) spectra at the boron K-edge were measured for aqueous boric acid, borate, and polyborate ions, using liquid microjet technology, and compared with simulated spectra calculated from first principles density functional theory in the excited electron and core hole (XCH) approximation. Thermal motion in both hydrated and isolated molecules was incorporated into the calculations by sampling trajectories from quantum mechanics∕molecular mechanics simulations at the experimental temperature. The boron oxide molecules exhibit little spectral change upon hydration, relative to mineral samples. Simulations reveal that water is arranged nearly isotropically around boric acid and sodium borate, but the calculations also indicate that the boron K-edge NEXAFS spectra are insensitive to hydrogen bonding, molecular environment, or salt interactions.

Communication: Near edge x-ray absorption fine structure spectroscopy of aqueous adenosine triphosphate at the carbon and nitrogen K-edges.

Near edge x-ray absorption fine structure (NEXAFS) spectroscopy at the nitrogen and carbon K-edges was used to study the hydration of adenosine triphosphate in liquid microjets. The total electron yield spectra were recorded as a function of concentration, pH, and the presence of sodium, magnesium, and copper ions (Na(+)/Mg(2+)/Cu(2+)). Significant spectral changes were observed upon protonation of the adenine ring, but not under conditions that promote π-stacking, such as high concentration or presence of Mg(2+), indicating that NEXAFS is insensitive to the phenomenon. Intramolecular inner-sphere association of Cu(2+) did create observable broadening of the nitrogen spectrum, whereas outer-sphere association with Mg(2+) did not.

An analysis of the NEXAFS spectra of a molecular crystal: alpha-glycine.

The nitrogen K-edge near edge x-ray absorption fine structure spectrum of alpha-crystalline glycine has been calculated for temperatures ranging from 0 to 450 K. Significant temperature dependent spectral changes are predicted. The calculated room temperature spectrum is in good agreement with the experiment. At high temperatures, molecular motions strongly influence the spectrum, as any unique spectrum from an individual instantaneous configuration does not resemble the experimental result or the average calculated spectrum; complex coupled motions in this prototypical molecular crystal underlie the observed spectral changes.

Investigation of protein conformation and interactions with salts via X-ray absorption spectroscopy.

Nitrogen K-edge spectra of aqueous triglycine were measured using liquid microjets, and the effects of Hofmeister-active salts on the spectra were observed. Spectra simulated using density functional theory, sampled from room temperature classical molecular dynamics trajectories, capture all major features in the measured spectra. The spectrum of triglycine in water is quite similar to that in the presence of chaotropic sodium bromide (and other halides), which raises the solubility of proteins. However, a new feature is found when kosmotropic Na(2)SO(3), which lowers solubility, is present; this feature results from excitations of the nitrogen atom in the terminal amino group of triglycine. Both direct interactions between this salt and the protonated amino terminus, as well as corresponding changes in the conformational dynamics of the system, contribute to this new feature. These molecular measurements support a different mechanism for the Hofmeister effect than has previously been suggested based on thermodynamic measurements. It is also shown that near edge X-ray absorption fine structure (NEXAFS) is sensitive to strong direct interaction between certain salts and charged peptides. However, by investigating the sensitivity of NEXAFS to the extreme structural differences between model beta-sheets and alpha-helices, we conclude that this technique is relatively insensitive to secondary structure of peptides and proteins.

Nuclear quantum effects in the structure and lineshapes of the N2 near-edge x-ray absorption fine structure spectrum.

We study the relative ability of several models of x-ray absorption spectra to capture the Franck-Condon structure apparent from an experiment on gaseous nitrogen. In doing so, we adopt the Born-Oppenheimer approximation and a constrained density functional theory method for computing the energies of the x-ray-excited molecule. Starting from an otherwise classical model for the spectrum, we systematically introduce more realistic physics, first by substituting the quantum mechanical nuclear radial density in the bond separation R for the classical radial density, then by adding the effect of zero-point energy and other level shifts, and finally by including explicit rovibrational quantization of both the ground and excited states. The quantization is determined exactly, using a discrete variable representation (DVR). We show that the near-edge x-ray absorption fine structure (NEXAFS) spectrum can be predicted semiquantitatively within this framework. We also address the possibility of non-trivial temperature dependence in the spectrum. By using constrained density functional theory in combination with more accurate potentials, we demonstrate that it is possible to improve the predicted spectrum. Ultimately, we establish the predictive limits of our method with respect to vibrational fine structure in NEXAFS spectra.

Monopeptide versus monopeptoid: insights on structure and hydration of aqueous alanine and sarcosine via X-ray absorption spectroscopy.

Despite the obvious significance, the aqueous interactions of peptides remain incompletely understood. Their synthetic analogues called peptoids (poly-N-substituted glycines) have recently emerged as a promising biomimetic material, particularly due to their robust secondary structure and resistance to denaturation. We describe comparative near-edge X-ray absorption fine structure spectroscopy studies of aqueous sarcosine, the simplest peptoid, and alanine, its peptide isomer, interpreted by density functional theory calculations. The sarcosine nitrogen K-edge spectrum is blue shifted with respect to that of alanine, in agreement with our calculations; we conclude that this shift results primarily from the methyl group substitution on the nitrogen of sarcosine. Our calculations indicate that the nitrogen K-edge spectrum of alanine differs significantly between dehydrated and hydrated scenarios, while that of the sarcosine zwitterion is less affected by hydration. In contrast, the computed sarcosine spectrum is greatly impacted by conformational variations, while the alanine spectrum is not. This relates to a predicted solvent dependence for alanine, as compared to sarcosine. Additionally, we show the theoretical nitrogen K-edge spectra to be sensitive to the degree of hydration, indicating that experimental X-ray spectroscopy may be able to distinguish between bulk and partial hydration, such as found in confined environments near proteins and in reverse micelles.

Importance of electronic relaxation for inter-coulombic decay in aqueous systems.

Inspired by recent photoelectron spectroscopy experiments on hydroxide solutions, we have examined the conditions necessary for enhanced (and, in the case of solutions, detectable) inter-Coulombic decay (ICD)--Auger emission from an atomic site other than that originally excited. We present general guidelines, based on energetic and spatial overlap of molecular orbitals, for this enhancement of inter-Coulombic decay-based energy transfer in solutions. These guidelines indicate that this decay process should be exhibited by broad classes of biomolecules and suggest a design criterion for targeted radiooncology protocols. Our findings show that photoelectron spectroscopy cannot resolve the current hydroxide coordination controversy.

Auto-oligomerization and hydration of pyrrole revealed by x-ray absorption spectroscopy.

Near edge x-ray absorption fine structure spectra have been measured at the carbon and nitrogen K-edges of the prototypical aromatic molecule, pyrrole, both in the gas phase and when solvated in water, and compared with spectra simulated using a combination of classical molecular dynamics and first principles density functional theory in the excited state core hole approximation. The excellent agreement enabled detailed assignments. Pyrrole is highly reactive, particularly in water, and reaction products formed by the auto-oligomerization of pyrrole are identified. The solvated spectra have been measured at two different temperatures, indicating that the final states remain largely unaffected by both hydration and temperature. This is somewhat unexpected, since the nitrogen in pyrrole can donate a hydrogen bond to water.