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.

SHINERS Study of Chloride Order-Disorder Phase Transition and Solvation of Cu(100).

Shell-isolated nanoparticle enhanced Raman spectroscopy (SHINERS) and density functional theory (DFT) are used to probe Cl- adsorption and the order-disorder phase transition associated with the c(2 × 2) Cl- adlayer on Cu(100) in acid media. A two-component ν(Cu-Cl) vibrational band centered near 260 ± 1 cm-1 is used to track the potential dependence of Cl- adsorption. The potential dependence of the dominant 260 cm-1 component tracks the coverage of the fluctional c(2 × 2) Cl- phase on terraces in good agreement with the normalized intensity of the c(2 × 2) superstructure rods in prior surface X-ray diffraction (SXRD) studies. As the c(2 × 2) Cl- coverage approaches saturation, a second ν(Cu-Cl) component mode emerges between 290 and 300 cm-1 that coincides with the onset and stiffening of step faceting where Cl- occupies the threefold hollow sites to stabilize the metal kink saturated Cu <100> step edge. The formation of the c(2 × 2) Cl- adlayer is accompanied by the strengthening of ν(O-H) stretching modes in the adjacent non-hydrogen-bonded water at 3600 cm-1 and an increase in hydronium concentration evident in the flanking H2O modes at 3100 cm-1. The polarization of the water molecules and enrichment of hydronium arise from the combination of Cl- anionic character and lateral templating provided by the c(2 × 2) adlayer, consistent with SXRD studies. At negative potentials, Cl- desorption occurs followed by development of a sulfate νs(S═O) band. Below -1.1 V vs Hg/HgSO4, a new 200 cm-1 mode emerges congruent with hydride formation and surface reconstruction reported in electrochemical scanning tunneling microscopy studies.

Bridging Structure, Magnetism, and Disorder in Iron-Intercalated Niobium Diselenide, FexNbSe2, below x = 0.25.

Transition-metal dichalcogenides (TMDs) intercalated with magnetic ions serve as a promising materials platform for developing next-generation, spin-based electronic technologies. In these materials, one can access a rich magnetic phase space depending on the choice of intercalant, host lattice, and relative stoichiometry. The distribution of these intercalant ions across given crystals, however, is less well defined-particularly away from ideal packing stoichiometries-and a convenient probe to assess potential longer-range ordering of intercalants is lacking. Here, we demonstrate that confocal Raman spectroscopy is a powerful tool for mapping the onset of intercalant superlattice formation in Fe-intercalated NbSe2 (FexNbSe2) for 0.14 ≤ x < 0.25. We use single-crystal X-ray diffraction to confirm the presence of longer-range intercalant superstructure and employ polarization-, temperature-, and magnetic field-dependent Raman measurements to examine both the symmetry of emergent phonon modes in the intercalated material and potential magnetoelastic coupling. Magnetometry measurements further indicate a correlation between the onset of magnetic ordering and the relative degree of intercalant superlattice formation. These results show Raman spectroscopy to be an expedient, local probe for mapping intercalant ordering in this class of magnetic materials.

Noncryogenic Air Separation Using Aluminum Formate Al(HCOO)3 (ALF).

Separating oxygen from air to create oxygen-enriched gas streams is a process that is significant in both industrial and medical fields. However, the prominent technologies for creating oxygen-enriched gas streams are both energy and infrastructure intensive as they use cryogenic temperatures or materials that adsorb N2 from air. The latter method is less efficient than the methods that adsorb O2 directly. Herein, we show, via a combination of gas adsorption isotherms, gas breakthrough experiments, neutron and synchrotron X-ray powder diffraction, Raman spectroscopy, and computational studies, that the metal-organic framework, Al(HCOO)3 (ALF), which is easily prepared at low cost from commodity chemicals, exhibits substantial O2 adsorption and excellent time-dependent O2/N2 selectivity in a range of 50-125 near dry ice/solvent (≈190 K) temperatures. The effective O2 adsorption with ALF at ≈190 K and ≈0.21 bar (the partial pressure of O2 in air) is ≈1.7 mmol/g, and at ice/salt temperatures (≈250 K), it is ≈0.3 mmol/g. Though the kinetics for full adsorption of O2 near 190 K are slower than at temperatures nearer 250 K, the kinetics for initial O2 adsorption are fast, suggesting that O2 separation using ALF with rapid temperature swings at ambient pressures is a potentially viable choice for low-cost air separation applications. We also present synthetic strategies for improving the kinetics of this family of compounds, namely, via Al/Fe solid solutions. To the best of our knowledge, ALF has the highest O2/N2 sorption selectivity among MOF adsorbents without open metal sites as verified by co-adsorption experiments..

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.

DNA-guided lattice remodeling of carbon nanotubes.

Covalent modification of carbon nanotubes is a promising strategy for engineering their electronic structures. However, keeping modification sites in registration with a nanotube lattice is challenging. We report a solution using DNA-directed, guanine (G)-specific cross-linking chemistry. Through DNA screening we identify a sequence, C3GC7GC3, whose reaction with an (8,3) enantiomer yields minimum disorder-induced Raman mode intensities and photoluminescence Stokes shift, suggesting ordered defect array formation. Single-particle cryo-electron microscopy shows that the C3GC7GC3 functionalized (8,3) has an ordered helical structure with a 6.5 angstroms periodicity. Reaction mechanism analysis suggests that the helical periodicity arises from an array of G-modified carbon-carbon bonds separated by a fixed distance along an armchair helical line. Our findings may be used to remodel nanotube lattices for novel electronic properties.

Computational Methods for Charge Density Waves in 2D Materials.

Two-dimensional (2D) materials that exhibit charge density waves (CDWs)-spontaneous reorganization of their electrons into a periodic modulation-have generated many research endeavors in the hopes of employing their exotic properties for various quantum-based technologies. Early investigations surrounding CDWs were mostly focused on bulk materials. However, applications for quantum devices require few-layer materials to fully utilize the emergent phenomena. The CDW field has greatly expanded over the decades, warranting a focus on the computational efforts surrounding them specifically in 2D materials. In this review, we cover ground in the following relevant theory-driven subtopics for TaS2 and TaSe2: summary of general computational techniques and methods, resulting atomic structures, the effect of electron-phonon interaction of the Raman scattering modes, the effects of confinement and dimensionality on the CDW, and we end with a future outlook. Through understanding how the computational methods have enabled incredible advancements in quantum materials, one may anticipate the ever-expanding directions available for continued pursuit as the field brings us through the 21st century.

Surface Hydride Formation on Cu(111) and Its Decomposition to Form H2 in Acid Electrolytes.

Mass spectrometry and Raman vibrational spectroscopy were used to follow competitive dynamics between adsorption and desorption of H and anions during potential cycling of three low-index Cu surfaces in acid electrolytes. Unique to Cu(111) is a redox wave for surface hydride formation coincident with anion desorption, while the reverse reaction of hydride decomposition with anion adsorption yields H2 by recombination rather than oxidation to H3O+. Charge imbalance between the reactions accounts for the asymmetric voltammetry in SO42-, ClO4-, PO43-, and Cl- electrolytes with pH 0.68-4.5. Two-dimensional hydride formation is evidenced by the reduction wave prior to H2 evolution and vibrational bands between 995 and 1130 cm-1. In contrast to Cu(111), no distinct voltammetric signature of surface hydride formation is observed on Cu(110) and Cu(100). The Cu(111) hydride surface phase may serve to catalyze hydrofunctionalization reactions such as CO2 reduction to CH4 and should be broadly useful in electro-organic synthesis.

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 .

Comparing polarized Raman spectroscopy and birefringence as probes of molecular scale alignment in 3D printed thermoplastics.

Polymer chain orientation is crucial to understanding the polymer dynamics at interfaces formed during thermoplastic material extrusion additive manufacturing. The flow field and rapid cooling produced during material extrusion can result in chains which are oriented and stretched, which has implications for interdiffusion and crystallization. Polarized Raman spectroscopy offers a non-destructive and surface sensitive method to quantify chain orientation. To study orientation and alignment of chains in 3D printed polycarbonate filaments, we used a combination of polarized Raman spectroscopy and birefringence (Δn) measurements. By changing the orientation of the sample with respect to polarization of incident radiation, we probe changes in the ratio between orientation-dependent vibration modes and orientation-independent modes. We used principal component analysis (PCA) and partial least squares (PLS) regression to develop correlations for birefringence and Raman measurements in samples that were pulled at different draw ratios (DRs). PCA was used to differentiate between orientation-dependent and orientation-independent modes, while PLS regression was used to calculate birefringence from Raman measurements of 3D printed samples. Birefringence measurements were compared to the polycarbonate intrinsic birefringence of 0.2, to estimate the degree of orientation. We find that measured values of birefringence underestimate orientation compared to Raman measurements.

Comparable Enhancement of TERS Signals from WSe2 on Chromium and Gold.

Plasmonic tip-sample junctions, at which the incident and scattered optical fields are localized and optimally enhanced, are often exploited to achieve ultrasensitive and highly spatially localized tip-enhanced Raman scattering (TERS). Recent work has demonstrated that the sensitivity and spatial resolution that are required to probe single molecules are attainable in such platforms. In this work, we observe and rationalize comparable TERS from few-layer WSe2 single crystals exfoliated onto Au- and Cr-coated Si substrates, using a plasmonic TERS probe excited with a 638 nm laser. Our experimental observations are supported by finite-difference time-domain simulations that illustrate that the attainable field enhancement factors at the Au-Au and Au-Cr tip-sample junctions are comparable in magnitude. Through a combined experimental and theoretical analysis, we propose that besides Au/Ag, several metallic substrates may be used to record bright TERS spectral images.

Distinct magneto-Raman signatures of spin-flip phase transitions in CrI3.

The discovery of 2-dimensional (2D) materials, such as CrI3, that retain magnetic ordering at monolayer thickness has resulted in a surge of both pure and applied research in 2D magnetism. Here, we report a magneto-Raman spectroscopy study on multilayered CrI3, focusing on two additional features in the spectra that appear below the magnetic ordering temperature and were previously assigned to high frequency magnons. Instead, we conclude these modes are actually zone-folded phonons. We observe a striking evolution of the Raman spectra with increasing magnetic field applied perpendicular to the atomic layers in which clear, sudden changes in intensities of the modes are attributed to the interlayer ordering changing from antiferromagnetic to ferromagnetic at a critical magnetic field. Our work highlights the sensitivity of the Raman modes to weak interlayer spin ordering in CrI3.

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.

Gateless and reversible carrier density tunability in epitaxial graphene devices functionalized with chromium tricarbonyl.

Monolayer epitaxial graphene (EG) has been shown to have clearly superior properties for the development of quantized Hall resistance (QHR) standards. One major difficulty with QHR devices based on EG is that their electrical properties drift slowly over time if the device is stored in air due to adsorption of atmospheric molecular dopants. The crucial parameter for device stability is the charge carrier density, which helps determine the magnetic flux density required for precise QHR measurements. This work presents one solution to this problem of instability in air by functionalizing the surface of EG devices with chromium tricarbonyl -Cr(CO)3. Observations of carrier density stability in air over the course of one year are reported, as well as the ability to tune the carrier density by annealing the devices. For low temperature annealing, the presence of Cr(CO)3 stabilizes the electrical properties and allows for the reversible tuning of the carrier density in millimeter-scale graphene devices close to the Dirac point. Precision measurements in the quantum Hall regime show no detrimental effect on the carrier mobility.

Dielectric Properties of NbxW1-xSe2 Alloys.

The growth of transition metal dichalcogenide (TMDC) alloys provides an opportunity to experimentally access information elucidating how optical properties change with gradual substitutions in the lattice compared with their pure compositions. In this work, we performed growths of alloyed crystals with stoichiometric compositions between pure forms of NbSe2 and WSe2, followed by an optical analysis of those alloys by utilizing Raman spectroscopy and spectroscopic ellipsometry.

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.

Quantum Hall device data monitoring following encapsulating polymer deposition.

The information provided in this data article will cover the growth parameters for monolayer, epitaxial graphene, as well as how to verify the layer homogeneity by confocal laser scanning and optical microscopy. The characterization of the subsequently fabricated quantum Hall device is shown for example cases during a series of environmental exposures. Quantum Hall data acquired from a CYTOP encapsulation is also provided. Data from Raman spectroscopy, atomic force microscopy, and other electrical property trends are shown. Lastly, quantum Hall effect data are presented from devices with deposited Parylene C films measuring 10.7 µm and 720 nm. All data are relevant for Rigosi et al. [1].

Controllable, Wide-Ranging n-Doping and p-Doping of Monolayer Group 6 Transition-Metal Disulfides and Diselenides.

Developing processes to controllably dope transition-metal dichalcogenides (TMDs) is critical for optical and electrical applications. Here, molecular reductants and oxidants are introduced onto monolayer TMDs, specifically MoS2 , WS2 , MoSe2 , and WSe2 . Doping is achieved by exposing the TMD surface to solutions of pentamethylrhodocene dimer as the reductant (n-dopant) and "Magic Blue," [N(C6 H4 -p-Br)3 ]SbCl6 , as the oxidant (p-dopant). Current-voltage characteristics of field-effect transistors show that, regardless of their initial transport behavior, all four TMDs can be used in either p- or n-channel devices when appropriately doped. The extent of doping can be controlled by varying the concentration of dopant solutions and treatment time, and, in some cases, both nondegenerate and degenerate regimes are accessible. For all four TMD materials, the photoluminescence intensity; for all four materials the PL intensity is enhanced with p-doping but reduced with n-doping. Raman and X-ray photoelectron spectroscopy (XPS) also provide insight into the underlying physical mechanism by which the molecular dopants react with the monolayer. Estimates of changes of carrier density from electrical, PL, and XPS results are compared. Overall a simple and effective route to tailor the electrical and optical properties of TMDs is demonstrated.