A Delicate Balance between Antiferromagnetism and Ferromagnetism: Theoretical and Experimental Studies of A2 MRu5 B2 (A=Zr, Hf; M=Fe, Mn) Metal Borides.

Metal-rich borides with the Ti3 Co5 B2 -type structure represent an ideal playground for tuning magnetic interactions through chemical substitutions. In this work, density functional theory (DFT) and experimental studies of Ru-rich quaternary borides with the general composition A2 MRu5 B2 (A=Zr, Hf, M=Fe, Mn) are presented. Total energy calculations show that the phases Zr2 FeRu5 B2 and Hf2 FeRu5 B2 prefer ground states with strong antiferromagnetic (AFM) interactions between ferromagnetic (FM) M-chains. Manganese substitution for iron lowers these antiferromagnetic interchain interactions dramatically and creates a strong competition between FM and AFM states with a slight preference for AFM in Zr2 MnRu5 B2 and for FM in Hf2 MnRu5 B2 . Magnetic property measurements show a field dependence of the AFM transition (TN ): TN is found at 0.1 T for all phases with predicted AFM states whereas for the predicted FM phase it is found at a much lower magnetic field (0.005 T). Furthermore, TN is lowest for a Hf-based phase (20 K) and highest for a Zr-based one (28 K), in accordance with DFT predictions of weaker AFM interactions in the Hf-based phases. Interestingly, the AFM transitions vanish in all compounds at higher fields (>1 T) in favor of FM transitions, indicating metamagnetic behaviors for these Ru-rich phases.

Covalent Atomic Bridges Enable Unidirectional Enhancement of Electronic Transport in Aligned Carbon Nanotubes.

Interconnecting the surfaces of nanomaterials without compromising their outstanding mechanical, thermal, and electronic properties is critical in the design of advanced bulk structures that still preserve the novel properties of their nanoscale constituents. As such, bridging the π-conjugated carbon surfaces of single-walled carbon nanotubes (SWNTs) has special implications in next-generation electronics. This study presents a rational path toward the improvement of the electrical transport in aligned semiconducting SWNT films by deposition of metal atoms. The formation of conducting Cr-mediated pathways between the parallel SWNTs increases the transverse (intertube) conductance while having a negligible effect on the parallel (intratube) transport. In contrast, doping with Li has a predominant effect on the intratube electrical transport of aligned SWNT films. Large-scale first-principles calculations of electrical transport on aligned SWNTs show good agreement with the experimental electrical measurements and provide insight into the changes that different metal atoms exert on the density of states near the Fermi level of the SWNTs and the formation of transport channels.

MoS2-Based Optoelectronic Gas Sensor with Sub-parts-per-billion Limit of NO2 Gas Detection.

Red light illumination with photon energy matching the direct band gap of chemical vapor deposition grown single-layer MoS2 with Au metal electrodes was used to induce a photocurrent which was employed instead of dark current for NO2 gas sensing. The resulting Au/MoS2/Au optoelectronic gas sensor showed a significant enhancement of the device sensitivity S toward ppb level of NO2 gas exposure reaching S = 4.9%/ppb (4900%/ppm), where S is a slope of dependence of relative change of the sensor resistance on NO2 concentration. Further optimization of the MoS2-based optoelectronic gas sensor by using graphene (Gr) with a work function lower than that of Au for the electrical contacts to the MoS2 channel allowed an increase of photocurrent. The limit of detection of NO2 gas at the level of 0.1 ppb was obtained for the MoS2 channel with graphene electrodes coated by Au. This value was calculated using experimentally obtained sensitivity and noise values and exceeds the U.S. Environment Protection Agency requirement for NO2 gas detection at ppb level.

Effect of Substitution on the Hysteretic Phase Transition in a Bistable Phenalenyl-Based Neutral Radical Molecular Conductor.

The ability to tune the physical properties of bistable organic functional materials by means of chemistry can facilitate their development for molecular electronic switching components. The butylamine-containing biphenalenyl boron neutral radical, [Bu]2 B, crystalline compound has recently attracted significant attention by displaying a hysteretic phase transition accompanied by simultaneous bistability in magnetic, electrical, and optical properties close to room temperature. In this report, substitutional doping was applied to [Bu]2 B by crystallizing solid solutions of bistable [Bu]2 B and its non-radical-containing counterpart [Bu]2 Be. With increasing doping degree, the hysteretic phase transition is gradually suppressed in terms of reducing the height, but conserves the width of the hysteresis loop as observed through magnetic susceptibility and electrical conductivity measurements. At the critical doping level of about 6 %, the abrupt transformation of the crystal structure to that of the pure [Bu]2 Be crystal packing was observed, accompanied by a complete collapse of the hysteresis loop. Further study of the structure-properties relationships of bistable neutral radical conductors based on the [Bu]2 B host can be conducted utilizing a variety of biphenalenyl-based molecular conductors.

Confined Lithium-Sulfur Reactions in Narrow-Diameter Carbon Nanotubes Reveal Enhanced Electrochemical Reactivity.

We demonstrate an unusual electrochemical reaction of sulfur with lithium upon encapsulation in narrow-diameter (subnanometer) single-walled carbon nanotubes (SWNTs). Our study provides mechanistic insight on the synergistic effects of sulfur confinement and Li+ ion solvation properties that culminate in a new mechanism of these sub-nanoscale-enabled reactions (which cannot be solely attributed to the lithiation-delithiation of conventional sulfur). Two types of SWNTs with distinct diameters, produced by electric arc (EA-SWNTs, average diameter 1.55 nm) or high-pressure carbon monoxide (HiPco-SWNTs, average diameter 1.0 nm), are investigated with two comparable electrolyte systems based on tetraethylene glycol dimethyl ether (TEGDME) and 1,4,7,10,13-pentaoxacyclopentadecane (15-crown-5). Electrochemical analyses indicate that a conventional solution-phase Li-S reaction occurs in EA-SWNTs, which can be attributed to the smaller solvated [Li(TEGDME)]+ and [Li(15-crown-5)]+ ions within the EA-SWNT diameter. In stark contrast, the Li-S confined in narrower diameter HiPco-SWNTs exhibits unusual electrochemical behavior that can be attributed to a solid-state reaction enabled by the smaller HiPco-SWNT diameter compared to the size of solvated Li+ ions. Our results of the electrochemical analyses are corroborated and supported with various spectroscopic analyses including operando Raman, X-ray photoelectron spectroscopy, and first-principles calculations from density functional theory. Taken together, our findings demonstrate that the controlled solid-state lithiation-delithiation of sulfur and an enhanced electrochemical reactivity can be achieved by sub-nanoscale encapsulation and one-dimensional confinement in narrow-diameter SWNTs.

Phenalenyl based neutral radical as a novel electrochromic material modulating visible to short-wave infrared light.

Applications for energy saving smart windows require materials which can switch from transmissive to black states in both the visible and short-wave IR range. We introduce an electrochromic phenalenyl based neutral radical small molecule and design devices capable of modulating light in both of these ranges.

Unexpected Competition between Antiferromagnetic and Ferromagnetic States in Hf2MnRu5B2: Predicted and Realized.

Materials "design" is increasingly gaining importance in the solid-state materials community in general and in the field of magnetic materials in particular. Density functional theory (DFT) predicted the competition between ferromagnetic (FM) and antiferromagnetic (AFM) ground states in a ruthenium-rich Ti3Co5B2-type boride (Hf2MnRu5B2) for the first time. Vienna ab initio simulation package (VASP) total energy calculations indicated that the FM model was marginally more stable than one of the AFM models (AFM1), indicating very weak interactions between magnetic 1D Mn chains that can be easily perturbated by external means (magnetic field or composition). The predicted phase was then synthesized by arc-melting and characterized as Hf2Mn1-xRu5+xB2 (x = 0.27). Vibrating-scanning magnetometry shows an AFM ground state with TN ≈ 20 K under low magnetic field (0.005 T). At moderate-to-higher fields, AFM ordering vanishes while FM ordering emerges with a Curie temperature of 115 K. These experimental outcomes confirm the weak nature of the interchain interactions, as predicted by DFT calculations.

Visible-Blind UV Photodetector Based on Single-Walled Carbon Nanotube Thin Film/ZnO Vertical Heterostructures.

Ultraviolet (UV) photodetectors based on heterojunctions of conventional (Ge, Si, and GaAs) and wide bandgap semiconductors have been recently demonstrated, but achieving high UV sensitivity and visible-blind photodetection still remains a challenge. Here, we utilized a semitransparent film of p-type semiconducting single-walled carbon nanotubes (SC-SWNTs) with an energy gap of 0.68 ± 0.07 eV in combination with a molecular beam epitaxy grown n-ZnO layer to build a vertical p-SC-SWNT/n-ZnO heterojunction-based UV photodetector. The resulting device shows a current rectification ratio of 103, a current photoresponsivity up to 400 A/W in the UV spectral range from 370 to 230 nm, and a low dark current. The detector is practically visible-blind with the UV-to-visible photoresponsivity ratio of 105 due to extremely short photocarrier lifetimes in the one-dimensional SWNTs because of strong electron-phonon interactions leading to exciton formation. In this vertical configuration, UV radiation penetrates the top semitransparent SC-SWNT layer with low losses (10-20%) and excites photocarriers within the n-ZnO layer in close proximity to the p-SC-SWNT/n-ZnO interface, where electron-hole pairs are efficiently separated by a high built-in electric field associated with the heterojunction.

Fast Electrochromic Device Based on Single-Walled Carbon Nanotube Thin Films.

Optical properties of electrochromic materials can be controlled by the application of an electric field allowing recent development of new applications such as smart windows technology for indoor climate control and energy conservation. We report the fabrication of a single-walled nanotube (SWNT) thin film based electro-optical modulator controlled by ionic liquid polarization in which the active electrochromic layer is made of a film of semiconducting (SC-) SWNTs and the counter-electrode is composed of a film of metallic (MT-) SWNTs. Optimization of this electro-optical cell allows the operations with an optical modulation depth of 3.7 dB and a response time in the millisecond range, which is thousands of times faster than typical electrolyte-controlled devices. In addition, a dual electro-optical device was built utilizing electro-optically active SC-SWNT films for each electrode that allowed increasing modulation depth of 6.7 dB while preserving the speed of the response.

Giant Raman Response to the Encapsulation of Sulfur in Narrow Diameter Single-Walled Carbon Nanotubes.

Encapsulation of sulfur in HiPCO-SWNTs leads to large changes in the Raman spectra with the appearance of new peaks at 319, 395, and 715 cm(-1) which originate from the sulfur species within the SWNTs, while the high frequency SWNT bands (ν > 1200 cm(-1)) are decreased in intensity. The encapsulated species also shifts the near-IR interband electronic transitions to lower energy by more than 10%. These effects seem to originate with the van der Waals interaction of the confined sulfur species with the walls of the SWNTs which are not expected to be significant in the case of the previously studied large diameter SWNTs. We suggest that sulfur in the small diameter SWNTs exists as a helical polymeric sulfur chain that enters the SWNT interior in the form of S2 ((3)Σ(g)(-)) molecules which undergo polymerization to linear diradicals.

Application of Hybrid Fillers for Improving the Through-Plane Heat Transport in Graphite Nanoplatelet-Based Thermal Interface Layers.

The in-plane alignment of graphite nanoplatelets (GNPs) in thin thermal interface material (TIM) layers suppresses the though-plane heat transport thus limiting the performance of GNPs in the geometry normally required for thermal management applications. Here we report a disruption of the GNP in-plane alignment by addition of spherical microparticles. The degree of GNP alignment was monitored by measurement of the anisotropy of electrical conductivity which is extremely sensitive to the orientation of high aspect ratio filler particles. Scanning Electron Microscopy images of TIM layer cross-sections confirmed the suppression of the in-plane alignment. The hybrid filler formulations reported herein resulted in a synergistic enhancement of the through-plane thermal conductivity of GNP/Al2O3 and GNP/Al filled TIM layers confirming that the control of GNP alignment is an important parameter in the development of highly efficient GNP and graphene-based TIMs.

Networks of semiconducting SWNTs: contribution of midgap electronic states to the electrical transport.

Single-walled carbon nanotube (SWNT) thin films provide a unique platform for the development of electronic and photonic devices because they combine the advantages of the outstanding physical properties of individual SWNTs with the capabilities of large area thin film manufacturing and patterning technologies. Flexible SWNT thin film based field-effect transistors, sensors, detectors, photovoltaic cells, and light emitting diodes have been already demonstrated, and SWNT thin film transparent, conductive coatings for large area displays and smart windows are under development. While chirally pure SWNTs are not yet commercially available, the marketing of semiconducting (SC) and metallic (MT) SWNTs has facilitated progress toward applications by making available materials of consistent electronic structure. Nevertheless the electrical transport properties of networks of separated SWNTs are inferior to those of individual SWNTs. In particular, for semiconducting SWNTs, which are the subject of this Account, the electrical transport drastically differs from the behavior of traditional semiconductors: for example, the bandgap of germanium (E = 0.66 eV) roughly matches that of individual SC-SWNTs of diameter 1.5 nm, but in the range 300-100 K, the intrinsic carrier concentration in Ge decreases by more than 10 orders of magnitude while the conductivity of a typical SC-SWNT network decreases by less than a factor of 4. Clearly this weak modulation of the conductivity hinders the application of SC-SWNT films as field effect transistors and photodetectors, and it is the purpose of this Account to analyze the mechanism of the electrical transport leading to the unusually weak temperature dependence of the electrical conductivity of such networks. Extrinsic factors such as the contribution of residual amounts of MT-SWNTs arising from incomplete separation and doping of SWNTs are evaluated. However, the observed temperature dependence of the conductivity indicates the presence of midgap electronic states in the semiconducting SWNTs, which provide a source of low-energy excitations, which can contribute to hopping conductance along the nanotubes following fluctuation induced tunneling across the internanotube junctions, which together dominate the low temperature transport and limit the resistivity of the films. At high temperatures, the intrinsic carriers thermally activated across the bandgap as in a traditional semiconductor became available for band transport. The midgap states pin the Fermi level to the middle of the bandgap, and their origin is ascribed to defects in the SWNT walls. The presence of such midgap states has been reported in connection with scanning tunneling spectroscopy experiments, Coulomb blockade observations in low temperature electrical measurements, selective electrochemical deposition imaging, tip-enhanced Raman spectroscopy, high resolution photocurrent spectroscopy, and the modeling of the electronic density of states associated with various defects. Midgap states are present in conventional semiconductors, but what is unusual in the present context is the extent of their contribution to the electrical transport in networks of semiconducting SWNTs. In this Account, we sharpen the focus on the midgap states in SC-SWNTs, their effect on the electronic properties of SC-SWNT networks, and the importance of these effects on efforts to develop electronic and photonic applications of SC-SWNTs.

Band Structure Engineering by Substitutional Doping in Solid-State Solutions of [5-Me-PLY(O,O)]2B(1-x)Be(x) Radical Crystals.

We report the substitutional doping of solid-state spiro-bis(5-methyl-1,9-oxido-phenalenyl)boron radical ([2]2B) by co-crystallization of this radical with the corresponding spiro-bis(5-methyl-1,9-oxido-phenalenyl)beryllium compound ([2]2Be). The pure compounds crystallize in different space groups ([2]2B, P1̅, Z = 2; [2]2Be, P21/c, Z = 4) with distinct packing arrangements, yet we are able to isolate crystals of composition [2]2B(1-x)Be(x), where x = 0-0.59. The phase transition from the P1̅ to the P21/c space group occurs at x = 0.1, but the conductivities of the solid solutions are enhanced and the activation energies reduced for values of x = 0-0.25. The molecular packing is driven by the relative concentration of the spin-bearing ([2]2B) and spin-free ([2]2Be) molecules in the crystals, and the extended Hückel theory band structures show that the progressive incorporation of spin-free [2]2Be in the lattice of the [2]2B radical (overall bandwidth, W = 1.4 eV, in the pure compound) leads to very strong narrowing of the bandwidth, which reaches a minimum at [2]2Be (W = 0.3 eV). The results provide a graphic picture of the structural transformations undergone by the lattice, and at certain compositions we are able to identify distinct structures for the [2]2B and [2]2Be molecules in a single crystalline phase.

Ionic Liquid Gating of Suspended MoS2 Field Effect Transistor Devices.

We demonstrate ionic liquid (IL) gating of suspended few-layer MoS2 transistors, where ions can accumulate on both exposed surfaces. Upon application of IL, all free-standing samples consistently display more significant improvement in conductance than substrate-supported devices. The measured IL gate coupling efficiency is up to 4.6 × 10(13) cm(-2) V(-1). Electrical transport data reveal contact-dominated electrical transport properties and the Schottky emission as the underlying mechanism. By modulating IL gate voltage, the suspended MoS2 devices display metal-insulator transition. Our results demonstrate that more efficient charge induction can be achieved in suspended two-dimensional (2D) materials, which with further optimization, may enable extremely high charge density and novel phase transition.

Effect of Lanthanide Metal Complexation on the Properties and Electronic Structure of Single-Walled Carbon Nanotube Films.

We spectroscopically analyze the effect of e-beam deposition of lanthanide metals on the electronic structure and conductivities of films of semiconducting (SC) single-walled carbon nanotubes (SWNTs) in high vacuum. We employ near-infrared and Raman spectroscopy to interpret the changes in the electronic structure of SWNTs on exposure to small amounts of the lanthanides (Ln = Sm, Eu, Gd, Dy, Ho, Yb), based on the behavior of the reference metals (M = Li, Cr) which are taken to exemplify ionic and covalent bonding, respectively. The analysis shows that while the lanthanides are more electropositive than the transition metals, in most cases they exhibit similar conductivity behavior which we interpret in terms of the formation of covalent bis-hexahapto bonds [(η(6)-SWNT)M(η(6)-SWNT), where M = La, Nd, Gd, Dy, Ho]. However, only M = Eu, Sm, Yb show the continually increasing conductivity characteristic of Li, and this supports our contention that these metals provide the first examples of mixed covalent-ionic bis-hexahapto bonds [(η(6)-SWNT)M(η(6)-SWNT), where M = Sm, Eu, Yb].

Enhanced electrical conductivity in a substitutionally doped spiro-bis(phenalenyl)boron radical molecular solid.

We report the crystallization of a subsitutionally doped organic conductor based on a host lattice composed of spiro-bis(phenalenyl)boron radicals. Co-crystallization of solutions of spiro-bis(9-oxidophenalenone)boron radical [PLY(O,O)]2B mixed with selected amounts of spiro-bis(9-oxidophenalenone)beryllium [PLY(O,O)]2Be leads to the formation of a series of solid-state solutions of composition [PLY(O,O)]2B(1-x)Be(x). The dopant molecules [PLY(O,O)]2Be serve to introduce holes into the lattice of spins provided by the [PLY(O,O)]2B radicals and lead to a systematic increase in the conductivity while decreasing the activation energy of the conduction process and leaving the solid-state structure relatively unperturbed. While the energies of the hole sites are expected to be high, the results are consistent with the interpretation of the electronic structure of [PLY(O,O)]2B in terms of the resonating valence bond model.

Effect of atomic interconnects on percolation in single-walled carbon nanotube thin film networks.

The formation of covalent bonds to single-walled carbon nanotube (SWNT) or graphene surfaces usually leads to a decrease in the electrical conductivity and mobility as a result of the structural rehybridization of the functionalized carbon atoms from sp(2) to sp(3). In the present study, we explore the effect of metal deposition on semiconducting (SC-) and metallic (MT-) SWNT thin films in the vicinity of the percolation threshold and we are able to clearly delineate the effects of weak physisorption, ionic chemisorption with charge transfer, and covalent hexahapto (η(6)) chemisorption on these percolating networks. The results support the idea that for those metals capable of forming bis-hexahapto-bonds, the generation of covalent (η(6)-SWNT)M(η(6)-SWNT) interconnects provides a conducting pathway in the SWNT films and establishes the transition metal bis-hexahapto organometallic bond as an electronically conjugating linkage between graphene surfaces.

Hexahapto-lanthanide interconnects between the conjugated surfaces of single-walled carbon nanotubes.

We report the response of the electrical conductivity of semiconducting single-walled carbon nanotube (SWNT) thin films on exposure to metal vapors of the early lanthanides under high vacuum conditions. We attribute the strongly enhanced conductivities observed on deposition of samarium and europium to charge transfer from the metals to the SWNT backbone, thereby leading to the first examples of mixed covalent-ionic bis-hexahapto bonds [(η(6)-SWNT)M(η(6)-SWNT), where M = Sm, Eu].

Synthesis of tetrachalcogenide-substituted phenalenyl derivatives: preparation and solid-state characterization of bis(3,4,6,7-tetrathioalkyl-phenalenyl)boron radicals.

We report the synthesis and properties of a series of spiro-bis(3,4,6,7-tetrachalcogenide-substituted-phenalenyl)boron salts and two of the corresponding tetrathioalkyl-substituted spiro-bis(phenalenyl)boron radicals [tetrathiomethyl (10) and tetrathioethyl (11)] in which all of the active positions of the phenalenyl (PLY) nucleus are functionalized. In the solid state, radicals 10 and 11 exist as a weak π-dimers due to the steric congestion of the thioalkyl groups in the superimposed PLY units. As a result, the spins are localized in the isolated (nonsuperimposed) PLY rings, and the structure, magnetic susceptibility measurements, and band structure calculations confirm that these PLY units are unable to undergo strong intermolecular interaction as a result of the orientation of the thioalkyl groups.

Functionalized single-walled carbon nanotube-based fuel cell benchmarked against US DOE 2017 technical targets.

Chemically modified single-walled carbon nanotubes (SWNTs) with varying degrees of functionalization were utilized for the fabrication of SWNT thin film catalyst support layers (CSLs) in polymer electrolyte membrane fuel cells (PEMFCs), which were suitable for benchmarking against the US DOE 2017 targets. Use of the optimum level of SWNT -COOH functionality allowed the construction of a prototype SWNT-based PEMFC with total Pt loading of 0.06 mg(Pt)/cm²--well below the value of 0.125 mg(Pt)/cm² set as the US DOE 2017 technical target for total Pt group metals (PGM) loading. This prototype PEMFC also approaches the technical target for the total Pt content per kW of power (<0.125 g(PGM)/kW) at cell potential 0.65 V: a value of 0.15 g(Pt)/kW was achieved at 80°C/22 psig testing conditions, which was further reduced to 0.12 g(Pt)/kW at 35 psig back pressure.