Spin-Phonon Coupling and Magnetic Transition in an Organic Molecule Intercalated Cr2Ge2Te6.

The manipulation of spin-phonon coupling in both formations and explorations of magnetism in two-dimensional van der Waals ferromagnetic semiconductors facilitates unprecedented prospects for spintronic devices. The interlayer engineering with spin-phonon coupling promises controllable magnetism via organic cation intercalation. Here, spectroscopic evidence reveals the intercalation effect on the intrinsic magnetic and electronic transitions in quasi-two-dimensional Cr2Ge2Te6 using tetrabutyl ammonium (TBA+) as the intercalant. The temperature evolution of Raman modes, Eg3 and Ag1, along with the magnetization measurements, unambiguously captures the enhancement of the ferromagnetic Curie temperature in the intercalated heterostructure. Moreover, the Eg4 mode highlights the increased effect of spin-phonon interaction in magnetic-order-induced lattice distortion. Combined with the first-principle calculations, we observed a substantial number of electrons transferred from TBA+ to Cr through the interface. The interplay between spin-phonon coupling and magnetic ordering in van der Waals magnets appeals for further understanding of the manipulation of magnetism in layered heterostructures.

Reproducible graphene synthesis by oxygen-free chemical vapour deposition.

Chemical vapour deposition (CVD) synthesis of graphene on copper has been broadly adopted since the first demonstration of this process1. However, widespread use of CVD-grown graphene for basic science and applications has been hindered by challenges with reproducibility2 and quality3. Here we identify trace oxygen as a key factor determining the growth trajectory and quality for graphene grown by low-pressure CVD. Oxygen-free chemical vapour deposition (OF-CVD) synthesis is fast and highly reproducible, with kinetics that can be described by a compact model, whereas adding trace oxygen leads to suppressed nucleation and slower/incomplete growth. Oxygen affects graphene quality as assessed by surface contamination, emergence of the Raman D peak and decrease in electrical conductivity. Epitaxial graphene grown in oxygen-free conditions is contamination-free and shows no detectable D peak. After dry transfer and boron nitride encapsulation, it shows room-temperature electrical-transport behaviour close to that of exfoliated graphene. A graphite-gated device shows well-developed integer and fractional quantum Hall effects. By highlighting the importance of eliminating trace oxygen, this work provides guidance for future CVD system design and operation. The increased reproducibility and quality afforded by OF-CVD synthesis will broadly influence basic research and applications of graphene.

Thermosensitivity through Exchange Coupling in Ferrimagnetic/Antiferromagnetic Nano-Objects for Magnetic-Based Thermometry.

Temperature is a fundamental physical quantity important to the physical and biological sciences. Measurement of temperature within an optically inaccessible three-dimensional (3D) volume at microscale resolution is currently limited. Thermal magnetic particle imaging (T-MPI), a temperature variant of magnetic particle imaging (MPI), hopes to solve this deficiency. For this thermometry technique, magnetic nano-objects (MNOs) with strong temperature-dependent magnetization (thermosensitivity) around the temperature of interest are required; here, we focus between 200 K and 310 K. We demonstrate that thermosensitivity can be amplified in MNOs consisting of ferrimagnetic (FiM) iron oxide (ferrite) and antiferromagnetic (AFM) cobalt oxide (CoO) through interface effects. The FiM/AFM MNOs are characterized by X-ray diffraction (XRD), (scanning) transmission electron microscopy (STEM/TEM), dynamic light scattering (DLS), and Raman spectroscopy. Thermosensitivity is evaluated and quantified by temperature-dependent magnetic measurements. The FiM/AFM exchange coupling is confirmed by field-cooled (FC) hysteresis loops measured at 100 K. Magnetic particle spectroscopy (MPS) measurements were performed at room temperature to evaluate the MNOs MPI response. This initial study shows that FiM/AFM interfacial magnetic coupling is a viable method to increase thermosensitivity in MNOs for T-MPI.

AtomVision: A Machine Vision Library for Atomistic Images.

Computer vision techniques have immense potential for materials design applications. In this work, we introduce an integrated and general-purpose AtomVision library that can be used to generate and curate microscopy image (such as scanning tunneling microscopy and scanning transmission electron microscopy) data sets and apply a variety of machine learning techniques. To demonstrate the applicability of this library, we (1) establish an atomistic image data set of about 10 000 materials with large structural and chemical diversity, (2) develop and compare convolutional and atomistic line graph neural network models to classify the Bravais lattices, (3) demonstrate the application of fully convolutional neural networks using U-Net architecture to pixelwise classify atom versus background, (4) use a generative adversarial network for super resolution, (5) curate an image data set on the basis of natural language processing using an open-access arXiv data set, and (6) integrate the computational framework with experimental microscopy images for Rh, Fe3O4, and SnS systems. The AtomVision library is available at https://github.com/usnistgov/atomvision.

Thermal and Electronic Properties of Ba2MnSe3.

The structural, thermal, and electronic properties of Ba2MnSe3 were investigated. Analysis of the low-temperature heat capacity revealed a low Debye temperature and a low average speed of sound that, together with the bonding in this material, result in a low thermal conductivity over a relatively large temperature range. Density functional theory and calculated electron localization were employed to investigate the electronic structure and bonding. Absorption and photoluminescence spectroscopy measurements corroborated our calculations and revealed a direct band gap of 1.75 eV. This study expands on our understanding of the physical properties of this material and reveals previously unascertained properties, the knowledge of which is imperative for any potential application of interest.

High-Throughput DFT-Based Discovery of Next Generation Two-Dimensional (2D) Superconductors.

High-throughput density functional theory (DFT) calculations allow for a systematic search for conventional superconductors. With the recent interest in two-dimensional (2D) superconductors, we used a high-throughput workflow to screen over 1000 2D materials in the JARVIS-DFT database and performed electron-phonon coupling calculations, using the McMillan-Allen-Dynes formula to calculate the superconducting transition temperature (Tc) for 165 of them. Of these 165 materials, we identify 34 dynamically stable structures with transition temperatures above 5 K, including materials such as W2N3, NbO2, ZrBrO, TiClO, NaSn2S4, Mg2B4C2, and the previously unreported Mg2B4N2 (Tc = 21.8 K). Finally, we performed experiments to determine the Tc of selected layered superconductors (2H-NbSe2, 2H-NbS2, ZrSiS, FeSe) and discuss the measured results within the context of our DFT results. We aim that the outcome of this workflow can guide future computational and experimental studies of new and emerging 2D superconductors by providing a roadmap of high-throughput DFT data.

Dependence of Single-Wall Carbon Nanotube Alignment on the Filter Membrane Interface in Slow Vacuum Filtration.

The recent introduction of slow vacuum filtration (SVF) technology has shown great promise for reproducibly creating high-quality, large-area aligned films of single-wall carbon nanotubes (SWCNTs) from solution-based dispersions. Despite clear advantages over other SWCNT alignment techniques, SVF remains in the developmental stages due to a lack of an agreed-upon alignment mechanism, a hurdle which hinders SVF optimization. In this work, the filter membrane surface is modified to show how the resulting SWCNT nematic order can be significantly enhanced. It is observed that directional mechanical grooving on filter membranes does not play a significant role in SWCNT alignment, despite the tendency for nanotubes to follow the groove direction. Chemical treatments to the filter membrane are shown to increase SWCNT alignment by nearly 1/3. These findings suggest that membrane surface structure acts to create a directional flow along the filter membrane surface that can produce global SWCNT alignment during SVF, rather serving as an alignment template.

Advanced characterization of magnetization dynamics in iron oxide magnetic nanoparticle tracers.

Characterization of the magnetization dynamics of single-domain magnetic nanoparticles (MNPs) is important for magnetic particle imaging (MPI), magnetic resonance imaging (MRI), and emerging medical diagnostic/therapeutic technologies. Depending on particle size and temperature, nanoparticle magnetization relaxation time constants span from nanoseconds to seconds. In solution, relaxation occurs via coupled Brownian and Néel relaxation mechanisms. Even though their coexistence complicates analysis, the presence of two timescales presents opportunities for more direct control of magnetization behavior if the two processes can be understood, isolated, and tuned. Using high frequency coils and sample temperature tunability, we demonstrate unambiguous determination of the specific relaxation processes for iron oxide nanoparticles using both time and frequency domain techniques. Furthermore, we study the evolution of the fast dynamics at ≈ 10 nanosecond timescales, for magnetic field amplitudes relevant to MPI.

Synthesis of Mixed-Valent Lanthanide Sulfide Nanoparticles.

In targeting reduced valent lanthanide chalcogenides, we report the first nanoparticle synthesis of the mixed-valent ferromagnets Eu3 S4 and EuSm2 S4 . Using divalent lanthanide halides with bis(trimethylsilyl)sulfide and oleylamine, we prepared nanoparticles of EuS, Eu3 S4 , EuSm2 S4 , SmS1.9 , and Sm3 S4 . All nanoparticle phases were identified using powder X-ray diffraction, transmission electron microscopy was used to confirm morphology and nanoparticle size, and magnetic susceptibility measurements for determining the ordering temperatures and valence. The UV/Vis, Raman and X-ray photoelectron spectroscopies for each phase were compared. Surprisingly, the phase is influenced by the halide and the reaction temperature, where EuCl2 formed EuS while EuI2 formed Eu3 S4 , highlighting the role of kinetics in phase stabilization. Interestingly, at lower temperatures EuI2 initially forms EuS, and converts over time to Eu3 S4 .

Global Alignment of Solution-Based Single-Wall Carbon Nanotube Films via Machine-Vision Controlled Filtration.

Over the past decade, substantial progress has been made in the chemical control (chiral enrichment, length sorting, handedness selectivity, and filling substance) of single-wall carbon nanotubes (SWCNTs). Recently, it was shown that large, horizontally aligned films can be created out of postprocessed SWCNT solutions. Here, we use machine-vision automation and parallelization to simultaneously produce globally aligned SWCNT films using pressure-driven filtration. Feedback control enables filtration to occur with a constant flow rate that not only improves the nematic ordering of the SWCNT films but also provides the ability to align a wide range of SWCNT types and on a variety of nanoporous membranes using the same filtration parameters. Using polarized optical spectroscopic techniques, we show that under standard implementation, meniscus combing produces a two-dimensional radial SWCNT alignment on one side of the film. After we flatten the meniscus through silanization, spatially resolved nematicity maps on both sides of the SWCNT film reveal global alignment across the entire structure. From experiments changing ionic strength and membrane charging, we provide evidence that the SWCNT alignment mechanism stems from an interplay of intertube interactions and ordered membrane charging. This work opens up the possibility of creating globally aligned SWCNT film structures for a new generation of nanotube electronics and optical control elements.

Electrical Detection of Singlet Fission in Single Crystal Tetracene Transistors.

We present the electrical detection of singlet fission in tetracene by using a field-effect transistor (FET). Singlet fission is a photoinduced spin-dependent process, yielding two triplet excitons from the absorption of a single photon. In this study, we engineered a more deterministic platform composed of an organic single crystal FET rather than amorphous or polycrystalline FETs to elucidate spin-dependent processes under magnetic fields. Despite the unipolar operation and relatively high mobility of single crystal tetracene FETs, we were able to manipulate spin dependent processes to detect magnetoconductance (MC) at room temperature by illuminating the FETs and tuning the bias voltage to adjust majority charge carrier density and trap occupancy. In considering the crystalline direction and magnetic field interactions in tetracene, we show the MC response observed in tetracene FETs to be the result of the singlet fission process.

Contact and Noncontact Measurement of Electronic Transport in Individual 2D SnS Colloidal Semiconductor Nanocrystals.

Colloidal-based solution syntheses offer a scalable and cost-efficient means of producing 2D nanomaterials in high yield. While much progress has been made toward the controlled and tailorable synthesis of semiconductor nanocrystals in solution, it remains a substantial challenge to fully characterize the products' inherent electronic transport properties. This is often due to their irregular morphology or small dimensions, which demand the formation of colloidal assemblies or films as a prerequisite to performing electrical measurements. Here, we report the synthesis of nearly monodisperse 2D colloidal nanocrystals of semiconductor SnS and a thorough investigation of the intrinsic electronic transport properties of single crystals. We utilize a combination of multipoint contact probe measurements and ultrafast terahertz spectroscopy to determine the carrier concentration, carrier mobility, conductivity/resistivity, and majority carrier type of individual colloidal semiconductor nanocrystals. Employing this metrological approach, we compare the electronic properties extracted for distinct morphologies of 2D SnS and relate them to literature values. Our results indicate that the electronic transport of colloidal semiconductors may be tuned through prudent selection of the synthetic conditions. We find that these properties compare favorably to SnS grown using vapor deposition techniques, illustrating that colloidal solution synthesis is a promising route to scalable production of nanoscale 2D materials.

High-throughput assessment of vacancy formation and surface energies of materials using classical force-fields.

In this work, we present an open access database for surface and vacancy-formation energies using classical force-fields (FFs). These quantities are essential in understanding diffusion behavior, nanoparticle formation and catalytic activities. FFs are often designed for a specific application, hence, this database allows the user to understand whether a FF is suitable for investigating particular defect and surface-related material properties. The FF results are compared to density functional theory and experimental data whenever applicable for validation. At present, we have 17 506 surface energies and 1000 vacancy formation energies calculation in our database and the database is still growing. All the data generated, and the computational tools used, are shared publicly at the following websites: www.ctcms.nist.gov/~knc6/periodic.html, https://jarvis.nist.gov and https://github.com/usnistgov/jarvis. Approximations used during the high-throughput calculations are clearly mentioned. Using some of the example cases, we show how our data can be used to directly compare different FFs for a material and to interpret experimental findings such as using Wulff construction for predicting equilibrium shape of nanoparticles. Similarly, the vacancy formation energies data can be useful in understanding diffusion related properties.

Contacts to solution-synthesized SnS nanoribbons: dependence of barrier height on metal work function.

Tin(ii) monosulfide (SnS) is a layered, anisotropic material that is of interest as a two-dimensional semiconductor for opto-electronic, thermoelectric, and piezoelectric applications. In this study, the effect of work function on contact behavior was investigated. Ni/Au, Pd/Au, Cr/Au, and Ti/Au contacts were fabricated onto individual, solution-synthesized, p-type SnS nanoribbons. The lower work function metals (Cr and Ti) formed Schottky contacts, whereas the higher work function metals (Ni and Pd) formed ohmic or semi-ohmic contacts. Of the ohmic contacts, Ni was found to have a lower contact resistance (∼10-4 Ω cm2 or lower) than Pd (∼10-3 Ω cm2 or lower). Both the calculated Schottky barriers (0.39 and 0.50 eV) for Cr and Ti, respectively, and the ohmic behavior for Ni and Pd agree with behavior predicted by Schottky-Mott theory. The results indicate that high work function metals should be considered to form low resistance contacts to SnS multilayers.

Charge Carrier Dynamics and Mobility Determined by Time-Resolved Terahertz Spectroscopy on Films of Nano-to-Micrometer-Sized Colloidal Tin(II) Monosulfide.

Tin(II) monosulfide (SnS) is a semiconductor material with an intermediate band gap, high absorption coefficient in the visible range, and earth abundant, non-toxic constituent elements. For these reasons, SnS has generated much interest for incorporation into optoelectronic devices, but little is known concerning the charge carrier dynamics, especially as measured by optical techniques. Here, as opposed to prior studies of vapor deposited films, phase-pure colloidal SnS was synthesized by solution chemistry in three size regimes, ranging from nanometer- to micron-scale (SnS small nanoparticles, SnS medium 2D nanosheets, and SnS large 2D µm-sheets), and evaluated by time-resolved terahertz spectroscopy (TRTS); an optical, non-contact probe of the photoconductivity. Dropcast films of the SnS colloids were studied by TRTS and compared to both thermally annealed films and dispersed suspensions of the same colloids. TRTS results revealed that the micron-scale SnS crystals and all of the annealed films undergo decay mechanisms during the first 200 ps following photoexcitation at 800 nm assigned to hot carrier cooling and carrier trapping. The charge carrier mobility of both the dropcast and annealed samples depends strongly on the size of the constituent colloids. The mobility of the SnS colloidal films, following the completion of the initial decays, ranged from 0.14 cm2/V·s for the smallest SnS crystals to 20.3 cm2/V·s for the largest. Annealing the colloidal films resulted in a ~ 20 % improvement in mobility for the large SnS 2D µm-sheets and a ~ 5-fold increase for the small nanoparticles and medium nanosheets.

Ligand-induced fate of embryonic species in the shape-controlled synthesis of rhodium nanoparticles.

The shapes of noble metal nanoparticles directly impact their properties and applications, including in catalysis and plasmonics, and it is therefore important to understand how multiple distinct morphologies can be controllably synthesized. Solution routes offer powerful capabilities for shape-controlled nanoparticle synthesis, but the earliest stages of the reaction are difficult to interrogate experimentally and much remains unknown about how metal nanoparticle morphologies emerge and evolve. Here, we use a well-established polyol process to synthesize uniform rhodium nanoparticle cubes, icosahedra, and triangular plates using bromide, trifluoroacetate, and chloride ligands, respectively. In all of these systems, we identified rhodium clusters with diameters of 1-2 nm that form early in the reactions. The colloidally stable metal cluster intermediates served as a stock solution of embryonic species that could be transformed predictably into each type of nanoparticle morphology. The anionic ligands that were added to the embryonic species determined their eventual fate, e.g., the morphologies into which they would ultimately evolve. Extensive high-resolution transmission electron microscopy experiments revealed that the growth pathway-monomer addition, coalescence, or a combination of the two-was different for each of the morphologies, and was likely controlled by the interactions of each specific anionic adsorbate with the embryonic species. Similar phenomena were observed for related palladium and platinum nanoparticle systems. These studies provide important insights into how noble metal nanoparticles nucleate, the pathways by which they grow into several distinct morphologies, and the imperative role of the anonic ligand in controlling which route predominates in a particular system.

Electrocatalytic and photocatalytic hydrogen production from acidic and neutral-pH aqueous solutions using iron phosphide nanoparticles.

Nanostructured transition-metal phosphides have recently emerged as Earth-abundant alternatives to platinum for catalyzing the hydrogen-evolution reaction (HER), which is central to several clean energy technologies because it produces molecular hydrogen through the electrochemical reduction of water. Iron-based catalysts are very attractive targets because iron is the most abundant and least expensive transition metal. We report herein that iron phosphide (FeP), synthesized as nanoparticles having a uniform, hollow morphology, exhibits among the highest HER activities reported to date in both acidic and neutral-pH aqueous solutions. As an electrocatalyst operating at a current density of -10 mA cm(-2), FeP nanoparticles deposited at a mass loading of ∼1 mg cm(-2) on Ti substrates exhibited overpotentials of -50 mV in 0.50 M H2SO4 and -102 mV in 1.0 M phosphate buffered saline. The FeP nanoparticles supported sustained hydrogen production with essentially quantitative faradaic yields for extended time periods under galvanostatic control. Under UV illumination in both acidic and neutral-pH solutions, FeP nanoparticles deposited on TiO2 produced H2 at rates and amounts that begin to approach those of Pt/TiO2. FeP therefore is a highly Earth-abundant material for efficiently facilitating the HER both electrocatalytically and photocatalytically.

Ternary hybrid nanoparticle isomers: directing the nucleation of Ag on Pt-Fe(3)O(4) using a solid-state protecting group.

Colloidal hybrid nanoparticles are an important class of materials that incorporate multiple nanoparticles into a single system through solid-state interfaces, which can result in multifunctionality and the emergence of synergistic properties not found in the individual components. These hybrid structures are typically produced using seeded-growth methods, where preformed nanoparticles serve as seeds onto which additional domains are added through subsequent reactions. For hybrid nanoparticles that contain more than two domains, multiple configurations with distinct connectivities and functionalities are possible, and these can be considered as nanoparticle analogues of molecular isomers. However, accessing one isomer relative to others in the same hybrid nanoparticle system is challenging, particularly when the formation of a target isomer is disfavored relative to more stable or synthetically accessible configurations. Here, we show that an iron oxide shell installed onto the Pt domain of Pt-Fe3O4 hybrid nanoparticles serves as a solid-state protecting group that can direct the nucleation of a third domain to an otherwise disfavored site. Under traditional conditions, Ag nucleates exclusively onto the Pt domain of Pt-Fe3O4 heterodimers, resulting in the formation of the Ag-Pt-Fe3O4 heterotrimer isomer. When the Pt surface is covered with an iron oxide protecting group, the nucleation of Ag is redirected onto the Fe3O4 domain, producing the distinct and otherwise inaccessible Pt-Fe3O4-Ag isomer. Similar results are obtained for the Au-Pt-Fe3O4 system, where formation of the favored Au-Pt-Fe3O4 configuration is blocked by the iron oxide protecting group. The thickness of the iron oxide shell that protects the Pt domain can be systematically tuned by adjusting the ratio of oleic acid to iron pentacarbonyl during the synthesis of the Pt-Fe3O4 heterodimers, and this insight is important for controllably implementing the protecting group chemistry.

Synthesis and crystallographic analysis of shape-controlled SnS nanocrystal photocatalysts: evidence for a pseudotetragonal structural modification.

Tin sulfide, SnS, is a narrow band gap semiconductor comprised of inexpensive, earth abundant, and environmentally benign elements that is emerging as an important material for a diverse range of applications in solar energy conversion, energy storage, and electronics. Relative to many comparable systems, much less is known about the factors that influence the synthesis or morphology-dependent properties of SnS nanostructures. Here, we report the synthesis of colloidal SnS cubes, spherical polyhedra, and sheets and demonstrate their activity for the photocatalytic degradation of methylene blue. We also study their morphology-dependent polymorphism using an in-depth crystallographic analysis that correlates high-resolution TEM data of individual nanocrystals with ensemble-based electron diffraction and powder XRD data. These studies reveal that the crystal structure adopted by the SnS cubes and spherical polyhedra is expanded along the a and b axes and contracted along c, converging on a pseudotetragonal cell that is distinct from that of orthorhombic α-SnS, the most stable polymorph. All of the peaks observed in powder XRD patterns that are often interpreted as originating from a mixture of metastable zincblende-type SnS and α-SnS can instead be accounted for by this single-phase pseudotetragonal modification, and this helps to rationalize discrepancies that exist between theoretical predictions of SnS polymorph stability and interpretations of experimental diffraction data. This same crystallographic analysis also indicates the morphologies of the nanocrystals and the facets by which they are bound, and it reveals that the SnS cubes form through selective overgrowth of spherical polyhedral seeds.

Nanostructured nickel phosphide as an electrocatalyst for the hydrogen evolution reaction.

Nanoparticles of nickel phosphide (Ni2P) have been investigated for electrocatalytic activity and stability for the hydrogen evolution reaction (HER) in acidic solutions, under which proton exchange membrane-based electrolysis is operational. The catalytically active Ni2P nanoparticles were hollow and faceted to expose a high density of the Ni2P(001) surface, which has previously been predicted based on theory to be an active HER catalyst. The Ni2P nanoparticles had among the highest HER activity of any non-noble metal electrocatalyst reported to date, producing H2(g) with nearly quantitative faradaic yield, while also affording stability in aqueous acidic media.