Evolution of the surface atomic structure of multielement oxide films: curse or blessing?

Atomically resolved scanning tunneling microscopy (STM) and X-ray photoelectron spectroscopy (XPS) are used to gain atomic-scale insights into the heteroepitaxy of lanthanum-strontium manganite (LSMO, La1-xSrxMnO3-δ, x ≈ 0.2) on SrTiO3(110). LSMO is a perovskite oxide characterized by several composition-dependent surface reconstructions. The flexibility of the surface allows it to incorporate nonstoichiometries during growth, which causes the structure of the surface to evolve accordingly. This happens up to a critical point, where phase separation occurs, clusters rich in the excess cations form at the surface, and films show a rough morphology. To limit the nonstoichiometry introduced by non-optimal growth conditions, it proves useful to monitor the changes in surface atomic structures as a function of the PLD parameters and tune the latter accordingly.

Variations in climate habitability parameters and their effect on Earth's biosphere during the Phanerozoic Eon.

Essential insights on the characterization and quality of a detectable biosphere are gained by analyzing the effects of its environmental parameters. We compiled environmental and biological properties of the Phanerozoic Eon from various published data sets and conducted a correlation analysis to assess variations in parameters relevant to the habitability of Earth's biosphere. We showed that environmental parameters such as oxygen, global average surface temperatures, runoff rates and carbon dioxide are interrelated and play a key role in the changes of biomass and biodiversity. We showed that there were several periods with a highly thriving biosphere, with one even surpassing present day biodiversity and biomass. Those periods were characterized by increased oxygen levels and global runoff rates, as well as moderate global average surface temperatures, as long as no large or rapid positive and/or negative temperature excursions occurred. High oxygen contents are diagnostic of biomass production by continental plant life. We find that exceptionally high oxygen levels can at least in one instance compensate for decreased relative humidities, providing an even more habitable environment compared to today. Beyond Earth, these results will help us to understand how environmental parameters affect biospheres on extrasolar planets and guide us in our search for extraterrestrial life.

Enhanced Luminescence of Yb3+ Ions Implanted to ZnO through the Selection of Optimal Implantation and Annealing Conditions.

Rare earth-doped zinc oxide (ZnO:RE) systems are attractive for future optoelectronic devices such as phosphors, displays, and LEDs with emission in the visible spectral range, working even in a radiation-intense environment. The technology of these systems is currently under development, opening up new fields of application due to the low-cost production. Ion implantation is a very promising technique to incorporate rare-earth dopants into ZnO. However, the ballistic nature of this process makes the use of annealing essential. The selection of implantation parameters, as well as post-implantation annealing, turns out to be non-trivial because they determine the luminous efficiency of the ZnO:RE system. This paper presents a comprehensive study of the optimal implantation and annealing conditions, ensuring the most efficient luminescence of RE3+ ions in the ZnO matrix. Deep and shallow implantations, implantations performed at high and room temperature with various fluencies, as well as a range of post-RT implantation annealing processes are tested: rapid thermal annealing (minute duration) under different temperatures, times, and atmospheres (O2, N2, and Ar), flash lamp annealing (millisecond duration) and pulse plasma annealing (microsecond duration). It is shown that the highest luminescence efficiency of RE3+ is obtained for the shallow implantation at RT with the optimal fluence of 1.0 × 1015 RE ions/cm2 followed by a 10 min annealing in oxygen at 800 °C, and the light emission from such a ZnO:RE system is so bright that can be observed with the naked eye.

Ion-Induced Surface Charge Dynamics in Freestanding Monolayers of Graphene and MoS_{2} Probed by the Emission of Electrons.

We compare the ion-induced electron emission from freestanding monolayers of graphene and MoS_{2} to find a sixfold higher number of emitted electrons for graphene even though both materials have similar work functions. An effective single-band Hubbard model explains this finding by a charge-up in MoS_{2} that prevents low energy electrons from escaping the surface within a period of a few femtoseconds after ion impact. We support these results by measuring the electron energy distribution for correlated pairs of electrons and transmitted ions. The majority of emitted primary electrons have an energy below 10 eV and are therefore subject to the dynamic charge-up effects at surfaces.

Tracer diffusion in proton-exchanged congruent LiNbO3 crystals as a function of hydrogen content.

The proton-exchange process is an effective method of fabricating low-loss waveguides based on LiNbO3 crystals. During proton-exchange, lithium is replaced by hydrogen and Li1-xHxNbO3 is formed. Currently, mechanisms and kinetics of the proton-exchange process are unclear, primarily due to a lack in reliable tracer diffusion data. We studied lithium and hydrogen tracer diffusion in proton-exchanged congruent LiNbO3 single crystals in the temperature range between 130-230 °C. Proton-exchange was done in benzoic acid with 0, 1, 2, or 3.6 mol% lithium benzoate added, resulting in micrometre thick surface layers where Li is substituted by H with relative fractions between x = 0.45 and 0.85 as determined by Nuclear Reaction Analysis. For the diffusion experiments, ion-beam sputtered isotope enriched 6LiNbO3 was used as a Li tracer source and deuterated benzoic acid as a H tracer source. Isotope depth profile analysis was carried out by secondary ion mass spectrometry. From the experimental results, effective diffusivities governing the lithium/hydrogen exchange as well as individual hydrogen and lithium tracer diffusivities are extracted. All three types of diffusivities can be described by the Arrhenius law with an activation enthalpy of about 1.0-1.2 eV and increase as a function of hydrogen content nearly independent of temperature. The effective diffusivities and the lithium tracer diffusivities are identical within a factor of two to five, while the hydrogen diffusivities are higher by three orders of magnitude. The results show that the diffusion of Li is the rate determining step governing the proton-exchange process. Exponential dependencies between diffusivities and hydrogen concentrations are determined. The observed increase of Li tracer diffusivities and effective diffusivities as a function of hydrogen concentration is attributed to a continuous reduction of the migration enthalpy of diffusion by a maximum factor of about 0.2 eV. Simulations based on the determined diffusivities can reproduce the step-like profile of hydrogen penetration during proton-exchange.

The Al Doping Effect on Epitaxial (In,Mn)As Dilute Magnetic Semiconductors Prepared by Ion Implantation and Pulsed Laser Melting.

One of the most attractive characteristics of diluted ferromagnetic semiconductors is the possibility to modulate their electronic and ferromagnetic properties, coupled by itinerant holes through various means. A prominent example is the modification of Curie temperature and magnetic anisotropy by ion implantation and pulsed laser melting in III-V diluted magnetic semiconductors. In this study, to the best of our knowledge, we performed, for the first time, the co-doping of (In,Mn)As diluted magnetic semiconductors by Al by co-implantation subsequently combined with a pulsed laser annealing technique. Additionally, the structural and magnetic properties were systematically investigated by gradually raising the Al implantation fluence. Unexpectedly, under a well-preserved epitaxial structure, all samples presented weaken Curie temperature, magnetization, as well as uniaxial magnetic anisotropies when more aluminum was involved. Such a phenomenon is probably due to enhanced carrier localization introduced by Al or the suppression of substitutional Mn atoms.

Habitability Models for Astrobiology.

Habitability has been generally defined as the capability of an environment to support life. Ecologists have been using Habitat Suitability Models (HSMs) for more than four decades to study the habitability of Earth from local to global scales. Astrobiologists have been proposing different habitability models for some time, with little integration and consistency among them, being different in function to those used by ecologists. Habitability models are not only used to determine whether environments are habitable, but they also are used to characterize what key factors are responsible for the gradual transition from low to high habitability states. Here we review and compare some of the different models used by ecologists and astrobiologists and suggest how they could be integrated into new habitability standards. Such standards will help improve the comparison and characterization of potentially habitable environments, prioritize target selections, and study correlations between habitability and biosignatures. Habitability models are the foundation of planetary habitability science, and the synergy between ecologists and astrobiologists is necessary to expand our understanding of the habitability of Earth, the Solar System, and extrasolar planets.

Visualization and Chemical Characterization of the Cathode Electrolyte Interphase Using He-Ion Microscopy and In Situ Time-of-Flight Secondary Ion Mass Spectrometry.

Unstable cathode electrolyte interphase (CEI) formation increases degradation in high voltage Li-ion battery materials. Few techniques couple characterization of nano-scale CEI layers on the macroscale with in situ chemical characterization, and thus, information on how the underlying microstructure affects CEI formation is lost. Here, the process of CEI formation in a high voltage cathode material, LiCoPO4, has been investigated for the first time using helium ion microscopy (HIM) and in situ time-of-flight (ToF) secondary ion mass spectrometry (SIMS). The combination of HIM and Ne-ion ToF-SIMS has been used to correlate the cycle-dependent morphology of the CEI layer on LiCoPO4 with a local cathode microstructure, including position, thickness, and chemistry. HIM imaging identified partial dissolution of the CEI layer on discharge resulting in in-homogenous CEI coverage on larger LiCoPO4 agglomerates. Ne-ion ToF-SIMS characterization identified oxyfluorophosphates from HF attack by the electrolyte and a Li-rich surface region. Variable thickness of the CEI layer coupled with inactive Li on the surface of LiCoPO4 electrodes contributes to severe degradation over the course of 10 cycles. The HIM-SIMS technique has potential to further investigate the effect of microstructures on CEI formation in cathode materials or solid electrolyte interphase formation in anodes, thus aiding future electrode development.

In Search for a Planet Better than Earth: Top Contenders for a Superhabitable World.

The fact that Earth is teeming with life makes it appear odd to ask whether there could be other planets in our galaxy that may be even more suitable for life. Neglecting this possible class of "superhabitable" planets, however, could be considered anthropocentric and geocentric biases. Most important from the perspective of an observer searching for extrasolar life is that such a search might be executed most effectively with a focus on superhabitable planets instead of Earth-like planets. We argue that there could be regions of astrophysical parameter space of star-planet systems that could allow for planets to be even better for life than our Earth. We aim to identify those parameters and their optimal ranges, some of which are astrophysically motivated, whereas others are based on the varying habitability of the natural history of our planet. Some of these conditions are far from being observationally testable on planets outside the solar system. Still, we can distill a short list of 24 top contenders among the >4000 exoplanets known today that could be candidates for a superhabitable planet. In fact, we argue that, with regard to the search for extrasolar life, potentially superhabitable planets may deserve higher priority for follow-up observations than most Earth-like planets.

Revealing the formation dynamics of the electric double layer by means of in-situ Rutherford backscattering spectrometry.

We report on a new versatile experimental setup for in situ Rutherford backscattering spectrometry at solid-liquid interfaces which enables investigations of electric double layers directly and in a quantitative manner. A liquid cell with a three-electrode arrangement is mounted in front of the beam line, and a thin Si3N4 window (thickness down to 150 nm) separates the vacuum of the detector chamber from the electrolyte in the cell. By minimizing the contribution of the window to the measurement, a large variety of elements at the solid-liquid interface with sensitivities far below one monolayer can be monitored. The attachment of Ba onto the Si3N4 surface as a function of contact time and pH value of the electrolyte solution was chosen as an example system. From our measurement, we can not only follow the evolution of the double layer but also derive limits for the point of zero charge for the Si3N4 surface. Our findings of 5.7≤pHPZC≤6.2 are in good agreement with values found in the literature obtained by other techniques. Despite focusing on a specific system in this work, the presented setup allows for a large variety of in situ investigations at solid-liquid interfaces such as, but not limited to, tracing electrochemical reactions and monitoring segregation, adsorption, and dissolution and corrosion processes.

Charge-Exchange-Driven Low-Energy Electron Splash Induced by Heavy Ion Impact on Condensed Matter.

Low-energy electrons (LEEs) are of great relevance for ion-induced radiation damage in cells and genes. We show that charge exchange of ions leads to LEE emission upon impact on condensed matter. By using a graphene monolayer as a simple model system for condensed organic matter and utilizing slow highly charged ions (HCIs) as projectiles, we highlight the importance of charge exchange alone for LEE emission. We find a large number of ejected electrons resulting from individual ion impacts (up to 80 electrons/ion for Xe40+). More than 90% of emitted electrons have energies well below 15 eV. This "splash" of low-energy electrons is interpreted as the consequence of ion deexcitation via an interatomic Coulombic decay (ICD) process.

Oxyhydride Nature of Rare-Earth-Based Photochromic Thin Films.

Thin films of rare-earth (RE)-oxygen-hydrogen compounds prepared by reactive magnetron sputtering show a unique color-neutral photochromic effect at ambient conditions. While their optical properties have been studied extensively, the understanding of the relationship between photochromism, chemical composition, and structure is limited. Here we establish a ternary RE-O-H composition-phase diagram based on chemical composition analysis by a combination of Rutherford backscattering and elastic recoil detection. The photochromic films are identified as oxyhydrides with a wide composition range described by the formula REO xH3-2 x where 0.5 ≤ x ≤ 1.5. We propose an anion-disordered structure model based on the face-centered cubic unit cell where the O2- and H- anions occupy tetrahedral and octahedral interstices. The optical band gap varies continuously with the anion ratio, demonstrating the potential of band gap tuning for reversible optical switching applications.

A versatile ion beam spectrometer for studies of ion interaction with 2D materials.

We present an ultrahigh vacuum setup for ion spectroscopy of freestanding two-dimensional solid targets. An ion beam of different ion species (e.g., Xe with charge states from 1 to 44 and Ar with charge states from 1 to 18) and kinetic energies ranging from a few 10 eV to 400 keV is produced in an electron beam ion source. Ions are detected after their transmission through the 2D target with a position sensitive microchannel plate detector allowing the determination of the ion's exit charge state as well as the scattering angle with a resolution of approximately 0.04°. Furthermore, the spectrometer is mounted on a swiveling frame covering a scattering angle of ±8° with respect to the incoming beam direction. By utilizing a beam chopper, we measure the time-of-flight of the projectiles and determine the energy loss when passing a 2D target with an energy uncertainty of about 2%. Additional detectors are mounted close to the target to observe emitted secondary particles and are read-out in coincidence with the position and time information of the ion detector. A signal in these detectors can also be used as a start trigger for time-of-flight measurements, which then yield an energy resolution of 1% and an approximately 1000-fold larger duty cycle. First results on the interaction of slow Xe30+ ions with a freestanding single layer of graphene obtained with the new setup are compared to recently published data where charge exchange and energy were measured by means of an electrostatic analyzer.

Cluster Tool for In Situ Processing and Comprehensive Characterization of Thin Films at High Temperatures.

A new cluster tool for in situ real-time processing and depth-resolved compositional, structural and optical characterization of thin films at temperatures from -100 to 800 °C is described. The implemented techniques comprise magnetron sputtering, ion irradiation, Rutherford backscattering spectrometry, Raman spectroscopy, and spectroscopic ellipsometry. The capability of the cluster tool is demonstrated for a layer stack MgO/amorphous Si (∼60 nm)/Ag (∼30 nm), deposited at room temperature and crystallized with partial layer exchange by heating up to 650 °C. Its initial and final composition, stacking order, and structure were monitored in situ in real time and a reaction progress was defined as a function of time and temperature.

On the insulator-to-metal transition in titanium-implanted silicon.

Hyperdoped silicon with deep level impurities has attracted much research interest due to its promising optical and electrical properties. In this work, single crystalline silicon supersaturated with titanium is fabricated by ion implantation followed by both pulsed laser melting and flash lamp annealing. The decrease of sheet resistance with increasing Ti concentration is attributed to a surface morphology effect due to the formation of cellular breakdown at the surface and the percolation conduction at high Ti concentration is responsible for the metallic-like conductivity. The insulator-to-metal transition does not happen. However, the doping effect of Ti incorporation at low concentration is not excluded, which might be responsible for the sub-bandgap optical absorption reported in literature.

Tuning the Fabrication of Nanostructures by Low-Energy Highly Charged Ions.

Slow highly charged ions have been utilized recently for the creation of monotype surface nanostructures (craters, calderas, or hillocks) in different materials. In the present study, we report on the ability of slow highly charged xenon ions (^{129}Xe^{Q+}) to form three different types of nanostructures on the LiF(100) surface. By increasing the charge state from Q=15 to Q=36, the shape of the impact induced nanostructures changes from craters to hillocks crossing an intermediate stage of caldera structures. A dimensional analysis of the nanostructures reveals an increase of the height up to 1.5 nm as a function of the potential energy of the incident ions. Based on the evolution of both the geometry and size of the created nanostructures, defect-mediated desorption and the development of a thermal spike are utilized as creation mechanisms of the nanostructures at low and high charge states, respectively.

The Search for Extraterrestrial Intelligence in Earth's Solar Transit Zone.

Over the past few years, astronomers have detected thousands of planets and candidate planets by observing their periodic transits in front of their host stars. A related method, called transit spectroscopy, might soon allow studies of the chemical imprints of life in extrasolar planetary atmospheres. Here, we address the reciprocal question, namely, from where is Earth detectable by extrasolar observers using similar methods. We explore Earth's transit zone (ETZ), the projection of a band around Earth's ecliptic onto the celestial plane, where observers can detect Earth transits across the Sun. ETZ is between 0.520° and 0.537° wide due to the noncircular Earth orbit. The restricted Earth transit zone (rETZ), where Earth transits the Sun less than 0.5 solar radii from its center, is about 0.262° wide. We first compile a target list of 45 K and 37 G dwarf stars inside the rETZ and within 1 kpc (about 3260 light-years) using the Hipparcos catalogue. We then greatly enlarge the number of potential targets by constructing an analytic galactic disk model and find that about 10(5) K and G dwarf stars should reside within the rETZ. The ongoing Gaia space mission can potentially discover all G dwarfs among them (several 10(4)) within the next 5 years. Many more potentially habitable planets orbit dim, unknown M stars in ETZ and other stars that traversed ETZ thousands of years ago. If any of these planets host intelligent observers, they could have identified Earth as a habitable, or even as a living, world long ago, and we could be receiving their broadcasts today. The K2 mission, the Allen Telescope Array, the upcoming Square Kilometer Array, or the Green Bank Telescope might detect such deliberate extraterrestrial messages. Ultimately, ETZ would be an ideal region to be monitored by the Breakthrough Listen Initiatives, an upcoming survey that will constitute the most comprehensive search for extraterrestrial intelligence so far.

Formation, habitability, and detection of extrasolar moons.

The diversity and quantity of moons in the Solar System suggest a manifold population of natural satellites exist around extrasolar planets. Of peculiar interest from an astrobiological perspective, the number of sizable moons in the stellar habitable zones may outnumber planets in these circumstellar regions. With technological and theoretical methods now allowing for the detection of sub-Earth-sized extrasolar planets, the first detection of an extrasolar moon appears feasible. In this review, we summarize formation channels of massive exomoons that are potentially detectable with current or near-future instruments. We discuss the orbital effects that govern exomoon evolution, we present a framework to characterize an exomoon's stellar plus planetary illumination as well as its tidal heating, and we address the techniques that have been proposed to search for exomoons. Most notably, we show that natural satellites in the range of 0.1-0.5 Earth mass (i) are potentially habitable, (ii) can form within the circumplanetary debris and gas disk or via capture from a binary, and (iii) are detectable with current technology.

Charge exchange and energy loss of slow highly charged ions in 1 nm thick carbon nanomembranes.

Experimental charge exchange and energy loss data for the transmission of slow highly charged Xe ions through ultrathin polymeric carbon membranes are presented. Surprisingly, two distinct exit charge state distributions accompanied by charge exchange dependent energy losses are observed. The energy loss for ions exhibiting large charge loss shows a quadratic dependency on the incident charge state indicating that equilibrium stopping force values do not apply in this case. Additional angle resolved transmission measurements point on a significant contribution of elastic energy loss. The observations show that regimes of different impact parameters can be separated and thus a particle's energy deposition in an ultrathin solid target may not be described in terms of an averaged energy loss per unit length.