Electronic Decoupling Research Articles

Frequency-Chirped Magic Angle Spinning Dynamic Nuclear Polarization Combined with Electron Decoupling.

Magic angle spinning (MAS) dynamic nuclear polarization (DNP) increases the signal intensity of solid-state nuclear magnetic resonance. DNP typically uses continuous wave (CW) microwave irradiation close to the resonance frequency of unpaired electron spins. In this study, we demonstrate that frequency-chirped microwaves improve DNP performance under MAS. By modulating the gyrotron anode potential, we generate a train of microwave chirps with a maximum bandwidth of 310 MHz and a maximum incident power on the spinning sample of 18 W. We characterize the efficiency of chirped DNP using the following polarizing agents: TEMTriPol-1, AsymPolPOK, AMUPol, and Finland trityl. The effects of different chirp widths and periods are analyzed at different MAS frequencies and microwave powers. Furthermore, we show that chirped DNP can be combined with electron decoupling to improve signal intensity by 59%, compared to CW DNP without electron decoupling, using Finland trityl as a polarizing agent.

Electronic Decoupling and Hole-Doping of Graphene Nanoribbons on Metal Substrates by Chloride Intercalation.

Atomically precise graphene nanoribbons (GNRs) have a wide range of electronic properties that depend sensitively on their chemical structure. Several types of GNRs have been synthesized on metal surfaces through selective surface-catalyzed reactions. The resulting GNRs are adsorbed on the metal surface, which may lead to hybridization between the GNR orbitals and those of the substrate. This makes investigation of the intrinsic electronic properties of GNRs more difficult and also rules out capacitive gating. Here, we demonstrate the formation of a dielectric gold chloride adlayer that can intercalate underneath GNRs on the Au(111) surface. The intercalated gold chloride adlayer electronically decouples the GNRs from the metal and leads to a substantial hole-doping of the GNRs. Our results introduce an easily accessible tool in the in situ characterization of GNRs grown on Au(111) that allows for exploration of their electronic properties in a heavily hole-doped regime.

Spirobifluorene-based hole-transporting materials for RGB OLEDs with high efficiency and low efficiency roll-off.

In this work, we designed and synthesized three spirobifluorene (SBF)-based hole-transporting materials (HTMs) by incorporating the di-4-tolylamino group at different positions of the SBF skeleton. These materials demonstrate excellent thermal stability with thermal decomposition temperatures (T d) up to 506 °C and outstanding morphological stability with a glass transition temperature (T g) exceeding 145 °C. The meta-linkage mode between the conjugated skeleton and functional groups in the molecular structure results in electronic decoupling, giving these 3,6-substituted SBFs higher triplet energies (E T) compared to 2,7-substituted SBFs. This makes the 3,6-substituted SBFs suitable as universal HTMs for red, green, and blue (RGB) organic light emitting diodes (OLEDs). Among the three HTMs, 3,3',6,6'-tetra(N,N-ditolylamino)-9,9'-spirobifluorene (3,3',6,6'-TDTA-SBF) exhibits the best device performance, achieving maximum external quantum efficiencies (EQEmax) of 26.1%, 26.4%, and 25.4% for RGB phosphorescent OLEDs, with extremely low efficiency roll-off in both green and blue devices. Utilizing 3,3',6,6'-TDTA-SBF as the HTM, we have also fabricated narrowband blue OLEDs based on the widely used multiple resonance emitter BCz-BN, which exhibits a EQEmax of 29.8% and low efficiency roll-off.

Orientation, electronic decoupling and band dispersion of heptacene on modified and nanopatterned copper surfaces.

The adsorption of heptacene (7 A) on Cu(110) and Cu(110)-(2 × 1)-O was studied with scanning tunneling microscopy, photoemission orbital tomography and density functional calculations to reveal the influence of surface passivation on the molecular geometry and electronic states. We found that the charge transfer into the 7 A molecules on Cu(110) is completely suppressed for the oxygen-modified Cu surface. The molecules are aligned along the Cu-O rows and uncharged. They are tilted due to the geometry enforced by the substrate and the ability to maximize intermolecular π-π overlap, which leads to strong π-band dispersion. The HOMO-LUMO gap of these decoupled molecules is significantly larger than that reported on weakly interacting metal surfaces. Finally, the Cu-O stripe phase was used as a template for nanostructured molecular growth and to assess possible confinement effects.

Electronic Decoupling and Single‐Molecule Charging of C60 on h‐BN/Rh(111)

AbstractA detailed understanding of the interaction between molecules and 2D materials is crucial to implement molecular films into next‐generation 2D material‐organic hybrid devices effectively. In this regard, energy level alignment and charge transfer processes are particularly relevant. This work investigates the interplay between a hexagonal boron nitride (h‐BN) monolayer on an Rh(111) single crystal and self‐assembled C60 thin films. The influence of the corrugated topography and electrostatic surface potential originating from the h‐BN/Rh(111) Moiré superstructure on the electronic level alignment and charging characteristics of C60 is being studied. A combination of scanning tunneling microscopy/spectroscopy (STM/STS) and a theoretical tight‐binding approach is used to gain insight into the C60 bandstructure formation and electronic decoupling of specific C60. This decoupling results from adsorption site‐dependent variations of the molecular energy level alignment, which controls the strength of intermolecular hybridization. The decoupling of specific C60 enables the direct observation of single‐electron charging processes via STS and Kelvin probe force microscopy. The charging of the C60 is enabled by combining two gating mechanisms: the electrostatic surface potential of the monolayer h‐BN/Rh(111) Moiré and the electric field of the STM tip.

Decoupled electron and phonon transport in thermoelectric GeTe compounded with multi-walled carbon nanotubes

With developing of thermoelectric (TE) performance, GeTe-based TE materials present great promising application at medium temperatures as a replacement for toxic PbTe. However, the further improvement of TE properties is severely constrained by the coupling of electron and phonon transport. In this study, we achieve decoupled electron and phonon transport by effectively creating a special phase boundary to scatter phonon but transmit electron by implanting multi-walled carbon nanotubes (MWCNTs) into Bi-doped GeTe. With the help of this decoupling mechanism, the power factor is effectively improved and reaches up to ∼4482 μW−1 m−1 K−2 at 673 K while the thermal conductivity shows an obvious drop to ∼1.27 W m−1 K−1 at 723 K, leading to an impressive increase in zT to a maximum value of ∼2.3 at 723 K. In particular, a competitive average zT value of ∼1.43 in the temperature range of 323–773 K is obtained, which is at a high level in lead-free GeTe-based materials. Our method could be used as a guidance for the decoupling of electron and phonon transport in other TE materials.

Effect of Residual Carbon on Spin‐Polarized Coupling at a Graphene/Ferromagnet Interface

AbstractVertical stacks of graphene and ferromagnetic layers are predicted to be efficient spin filters, while the experimentally observed figures of merit systematically remain below the theoretical predictions. According to general consensus, a vaguely defined interface contamination is found responsible for this discrepancy. Here, it is demonstrated how the spin‐polarized electronic structure of single‐layer graphene supported on a ferromagnetic cobalt substrate is affected by the presence of an interfacial carbidic buffer layer, formed by residual carbon present in the Co substrate. It is found that the Co‐C hybridized single‐spin state near the Fermi level disappears upon thermal segregation of bulk carbon at the graphene–Co interface, which determines the electronic decoupling of graphene from the ferromagnetic support and consequently, the suppression of net spin polarization. These observations are shown to be independent of the graphene azimuthal orientation with respect to the high symmetry directions of the substrate. The findings provide clear evidence that the realization of highly polarized spin currents in graphene/ferromagnet heterostacks depends on careful control of the graphene growth process in order to eliminate interfacial carbon.

Improving MgO/Fe insulator-metal interface structure through oxygen-precoating of Fe(0 0 1)

We grew atomically flat and electronically uniform MgO island terraces on a Fe(001) whisker substrate precoated by a p(1 × 1) oxygen monolayer film. This interface engineering process increased about 10 times the size of MgO islands compared to the size of MgO islands directly grown on Fe(001), showing that the interface oxygen monolayer plays an essential role in MgO growth. Importantly, each squared-shape atomically flat MgO island includes only a few atomic defects, which provides high-quality insulating layers. Further, the MgO islands have an electronic decoupling functionality from the metal substrate. Surface morphology and local electronic structures were studied using a home-built ultrahigh vacuum scanning tunneling microscopy (STM) at a temperature of 5 K.

In Situ Spectroscopic Identification of the Electron-Transfer Intermediates of Photoelectrochemical Proton-Coupled Electron Transfer of Water Oxidation on Au

Experimental elucidation of the decoupling of electron and proton transfer at a molecular level is essential for thoroughly understanding the kinetics of heterogeneous (photo)electrochemical proton-coupled electron transfer water oxidation. Here we illustrate the electron-transfer intermediates of positively charged surface oxygenated species on Au (Au-OH+) and their correlations with the rate of water oxidation by in situ microphotoelectrochemical surface-enhanced Raman spectroscopy (SERS) and ambient-pressure X-ray photoelectron spectroscopy. At the intermediate stage of water oxidation, a characteristic blue shift of the vibration of Au-OH species in laser-power-density-dependent measurements was assigned to the light-induced production of Au-OH+ in water oxidation. The photothermal effect was excluded according to the vibrational frequencies of Au-OH species as the temperature was increased in a variable-temperature SERS measurement. Density functional theory calculations evidenced that the frequency blue shift is from the positively charged Au-OH species. The photocurrent-dependent frequency blue shift indicated that Au-OH+ is the key electron-transfer intermediate in water oxidation by decoupled electron and proton transfer.

Mechanochemical Synthesis of Sustainable Ternary and Quaternary Nanostructured Cu2SnS3, Cu2ZnSnS4, and Cu2ZnSnSe4 Chalcogenides for Thermoelectric Applications

Copper-based chalcogenides have emerged as promising thermoelectric materials due to their high thermoelectric performance, tunable transport properties, earth abundance and low toxicity. We have presented an overview of experimental results and first-principal calculations investigating the thermoelectric properties of various polymorphs of Cu2SnS3 (CTS), Cu2ZnSnS4 (CZTS), and Cu2ZnSnSe4 (CZTSe) synthesized by high-energy reactive mechanical alloying (ball milling). Of particular interest are the disordered polymorphs of these materials, which exhibit phonon-glass-electron-crystal behavior-a decoupling of electron and phonon transport properties. The interplay of cationic disorder and nanostructuring leads to ultra-low thermal conductivities while enhancing electronic transport. These beneficial transport properties are the consequence of a plethora of features, including trap states, anharmonicity, rattling, and conductive surface states, both topologically trivial and non-trivial. Based on experimental results and computational methods, this report aims to elucidate the details of the electronic and lattice transport properties, thereby confirming that the higher thermoelectric (TE) performance of disordered polymorphs is essentially due to their complex crystallographic structures. In addition, we have presented synchrotron X-ray diffraction (SR-XRD) measurements and ab initio molecular dynamics (AIMD) simulations of the root-mean-square displacement (RMSD) in these materials, confirming anharmonicity and bond inhomogeneity for disordered polymorphs.

Electron-decoupled MAS DNP with N@C60.

Frequency-chirped microwaves decouple electron- and 13C-spins in magic-angle spinning N@C60:C60 powder, improving DNP-enhanced 13C NMR signal intensity by 12% for 7 s polarization, and 5% for 30 s polarization. This electron decoupling demonstration is a step toward utilizing N@C60 as a controllable electron-spin source for magic-angle spinning magnetic resonance experiments.

A versatile platform for graphene nanoribbon synthesis, electronic decoupling, and spin polarized measurements.

The on-surface synthesis of nano-graphenes has led the charge in prototyping structures with perspectives beyond silicon-based technology. Following reports of open-shell systems in graphene-nanoribbons (GNRs), a flurry of research activity was directed at investigating their magnetic properties with a keen eye for spintronic applications. Although the synthesis of nano-graphenes is usually carried out on Au(111), the substrate is difficult to use for electronic decoupling and spin-polarized measurements. Using a binary alloy Cu3Au(111), we show possibilities for gold-like on-surface synthesis compatible with spin polarization and electronic decoupling known from copper. We prepare copper oxide layers, demonstrate the synthesis of GNRs, and grow thermally stable magnetic Co islands. We functionalize the tip of a scanning tunneling microscope with carbon-monoxide, nickelocene, or attach Co clusters for high-resolution imaging, magnetic sensing, or spin-polarized measurements. This versatile platform will be a valuable tool in the advanced study of magnetic nano-graphenes.

Observing Nearby Nuclei on Paramagnetic Trityls and MOFs via DNP and Electron Decoupling.

Dynamic nuclear polarization (DNP) is an NMR sensitivity enhancement technique that mediates polarization transfer from unpaired electrons to NMR-active nuclei. Despite its success in elucidating important structural information on biological and inorganic materials, the detailed polarization-transfer pathway from the electrons to the nearby and then the bulk solvent nuclei, and finally to the molecules of interest-remains unclear. In particular, the nuclei in the paramagnetic polarizing agent play significant roles in relaying the enhanced NMR polarizations to more remote nuclei. Despite their importance, the direct NMR observation of these nuclei is challenging because of poor sensitivity. Here, we show that a combined DNP and electron decoupling approach can facilitate direct NMR detection of these nuclei. We achieved an ∼80 % improvement in NMR intensity via electron decoupling at 0.35 T and 80 K on trityl radicals. Moreover, we recorded a DNP enhancement factor of ∼90 and ∼11 % higher NMR intensity using electron decoupling on paramagnetic metal-organic framework, magnesium hexaoxytriphenylene (MgHOTP MOF).

Efficient Short-Wave Infrared Light-Emitting Diodes Based on Heavy-Metal-Free Quantum Dots.

Short-wave infrared (SWIR) light emission is important for a diverse range of modern applications, such as eye-safe depth sensing, light detection and ranging (LiDAR), facial recognition, eye tracking, optical communication, and health-monitoring technologies. However, there are a very limited number of known semiconductors that can emit efficiently in the SWIR spectral range. Presently, SWIR light-emitting diodes (LEDs) based on colloidal quantum dots (CQD) are dominated by lead chalcogenide systems, despite the presence of heavy metal and modest efficiencies. Here, a highly efficient SWIR LED based on heavy-metal-free indium arsenide (InAs) core-shell CQDs is presented. In the LED design, the implementation of an otherwise hole-transporting poly(vinylcarbazole) (PVK) layer on the electron-injecting side of the device stack leads to a surprising enhancement in device performance, giving remarkably high external quantum efficiencies (EQEs) of 13.3% at 1006 nm. Single-carrier device and optical investigations reveal the origins of enhancement to be the electronic decoupling of the CQD layer with the electron-injecting zinc oxide (ZnO) layer, which mitigates luminescence quenching and improves charge balance. This work marks one of the highest efficiencies reported for heavy-metal-free solution-processed LEDs in the SWIR spectral region, and can find significant applications in emerging consumer electronic technologies.

A single solvating benzene molecule decouples the mixed-valence complex through intermolecular orbital interactions

SummaryCharacterization of covalency of intermolecular interactions in the van der Waals distance limit remains challenging because the interactions between molecules are weak, dynamic, and not measurable. Herein, we approach this issue in a series of supramolecular mixed-valence (MV) donor(D)-bridge(B)-acceptor(A) systems consisting of two bridged Mo2 units with a C6H6 molecule encapsulated, as characterized by the X-ray crystal structures. Comparative analysis of the intervalence charge transfer spectra in benzene and dichloromethane substantiates the strong electronic decoupling effect of the solvating C6H6 molecule that breaks down the dielectric solvation theory. Ab initio and DFT calculations unravel that the intermolecular orbital overlaps between the complex bridge and the C6H6 molecule alter the electronic states of the D-B-A molecule through intermolecular nuclear dynamics. This work exemplifies that site-specific intermolecular interaction can be exploited to control the chemical property of supramolecular systems and to elucidate the functionalities of side-chains in biological systems.

Super-structured defects modulation for synergistically optimizing thermoelectric property in SnTe-based materials

Among the intriguing p-type thermoelectric IV-VI compounds, SnTe is an important alternative to both PbTe and GeTe for free of toxicity of Pb and scarcity of Ge. However, pristine SnTe has poor thermoelectric performance because of its intrinsic high hole concentration, big energy difference between L and Σ bands and high lattice thermal conductivity. In this work, In and Cd co-doping SnTe material, which combines the effect of resonant level and band convergence together, was obtained by self-propagating high-temperature synthesis (SHS) and plasma activated sintering (PAS). The generated Te vacancies are regularly arranged to form super-structured defects in the subsequent constrained hot compressing process, which include regular array of atomic defects and nanoscale precipitates. As a result, the hole concentration of the compressed SnTe-based material is reduced due to the donor doping effect of Te vacancies, and the hole mobility is remarkably improved to 372.94 cm2V−1s−1 and 66.19 cm2V−1s−1 at room temperature for the SnTe and In0.01Cd0.02Sn0.97Te material with compressibility of 35%, respectively. These atomic and nanoscale super-structured defects not only ensure superior electrical transport properties, but also bring much stronger scattering effect on phonons, thus leading to extremely low lattice thermal conductivity of 1.2 Wm−1K−1 at 888 K for the 35% compressed In0.01Cd0.02Sn0.97Te material. With synergistically optimized carrier and phonon transport behavior through resonant level engineering, band convergence engineering and super-structured defects engineering, a peak ZT value of 0.8 is achieved in In0.01Cd0.02Sn0.97Te material with compressibility of 35%. This work provides an effective strategy to realize the decoupling of electron and phonon scattering through Te vacancy-based super-structured defects modulation, and makes up an important part of multiscale microstructure tailoring for thermoelectric materials.

Super high-performance 7-atomic-layer thermoelectric material ZrGe2N4.

7-Atomic-layer materials have attracted much attention recently because of their rich structures and more-abundant properties than 3- and 5-atomic-layer materials. However, the thermoelectric properties of the new monolayer materials have not been explored yet. Here, we investigate the thermoelectric conversion efficiency of a 7-atomic-layer structure ZrGe2N4, which is selected from a series of 7-atomic-layer structures according to their stabilities and thermoelectric properties. The results indicate that this material is an excellent candidate for high-performance thermoelectric materials. Its figure of merit ZT value is close to 4.0 at high temperature. The high efficiency originates from two factors: one is the lower thermal conductivity of ZrGe2N4 and the other is the decoupling of electron and phonon transport in the 7-atomic-layer structures.

Decoupling of CVD-grown epitaxial graphene using NaCl intercalation.

The structural and electronic properties of graphene grown on catalytic metal surfaces are significantly modified via graphene-substrate interaction. To minimize the influence of the metal substrate, a dielectric buffer layer can be introduced between the graphene and metal substrate. However, the catalytic synthesis of graphene limits the potential alternatives for buffer layers. The intercalation of atoms below the graphene layer is a promising method that does not require the chemical treatment of graphene or the substrate. In this study, the electronic and structural properties of single-layer graphene (SLG) on the Cu(111) substrate intercalated with ultrathin NaCl thin films were investigated using scanning tunnelling microscopy. The intercalation of the NaCl monolayer decoupled SLG from the metal substrate, thereby producing quasi-freestanding graphene.

Investigation of the homogeneity of energy conversion processes at dipolarization fronts from MMS measurements

We report on six dipolarization fronts (DFs) embedded in fast earthward flows detected by the Magnetospheric Multiscale mission during a substorm event on 23 July 2017. We analyzed Ohm's law for each event and found that ions are mostly decoupled from the magnetic field by Hall fields. However, the electron pressure gradient term is also contributing to the ion decoupling and likely responsible for an electron decoupling at DF. We also analyzed the energy conversion process and found that the energy in the spacecraft frame is transferred from the electromagnetic field to the plasma (J·E>0) ahead or at the DF, whereas it is the opposite (J·E<0) behind the front. This reversal is mainly due to a local reversal of the cross-tail current indicating a substructure of the DF. In the fluid frame, we found that the energy is mostly transferred from the plasma to the electromagnetic field (J·E′<0) and should contribute to the deceleration of the fast flow. However, we show that the energy conversion process is not homogeneous at the electron scales due to electric field fluctuations likely related to lower-hybrid drift waves. Our results suggest that the role of DF in the global energy cycle of the magnetosphere still deserves more investigation. In particular, statistical studies on DF are required to be carried out with caution due to these electron scale substructures.

Localized Orbital Excitation Drives Bond Formation in Plasmonic Catalysis.

Localized surface plasmons generated on metallic nanostructures can be used to accelerate molecular transformations; however, the efficiency is limited by the challenge to control the energy/charge transfer at the interfaces. Here, we combine density functional theory (DFT) calculations and experiments to reveal the mechanism of nitrophenol reduction on Au nanoparticles under visible-light irradiation and propose a strategy to further enhance the reaction rates. DFT calculations show a reduced activation barrier under electronic excitation on Au(111), thus explaining the measured higher rates under visible-light irradiation. Furthermore, we propose a heterostructure with Au nanoparticles covered by a thin film of hexagonal boron nitride; the latter is used to decouple the molecular orbitals from the metal to enable charge localization in the molecule. DFT calculations show that by this electronic decoupling, the activation barrier can be lowered by a factor of five. This work thus provides a valuable strategy for optimizing catalytic efficiency in plasmonic photocatalysis.