Liquid flow and control without solid walls.

When miniaturizing fluidic circuitry, the solid walls of the fluid channels become increasingly important1 because they limit the flow rates achievable for a given pressure drop, and they are prone to fouling2. Approaches for reducing the wall interactions include hydrophobic coatings3, liquid-infused porous surfaces4-6, nanoparticle surfactant jamming7, changes to surface electronic structure8, electrowetting9,10, surface tension pinning11,12 and use of atomically flat channels13. A better solution may be to avoid the solid walls altogether. Droplet microfluidics and sheath flow achieve this but require continuous flow of the central liquid and the surrounding liquid1,14. Here we demonstrate an approach in which aqueous liquid channels are surrounded by an immiscible magnetic liquid, both of which are stabilized by a quadrupolar magnetic field. This creates self-healing, non-clogging, anti-fouling and near-frictionless liquid-in-liquid fluidic channels. Manipulation of the field provides flow control, such as valving, splitting, merging and pumping. The latter is achieved by moving permanent magnets that have no physical contact with the liquid channel. We show that this magnetostaltic pumping method can be used to transport whole human blood with very little damage due to shear forces. Haemolysis (rupture of blood cells) is reduced by an order of magnitude compared with traditional peristaltic pumping, in which blood is mechanically squeezed through a plastic tube. Our liquid-in-liquid approach provides new ways to transport delicate liquids, particularly when scaling channels down to the micrometre scale, with no need for high pressures, and could also be used for microfluidic circuitry.

Fluid Drag Reduction by Magnetic Confinement.

The frictional forces of a viscous liquid flow are a major energy loss issue and severely limit microfluidics practical use. Reducing this drag by more than a few tens of percent remain elusive. Here, we show how cylindrical liquid-in-liquid flow leads to drag reduction of 60-99% for sub-mm and mm-sized channels, regardless of whether the viscosity of the transported liquid is larger or smaller than that of the confining one. In contrast to lubrication or sheath flow, we do not require a continuous flow of the confining lubricant, here made of a ferrofluid held in place by magnetic forces. In a laminar flow model with appropriate boundary conditions, we introduce a modified Reynolds number with a scaling that depends on geometrical factors and viscosity ratio of the two liquids. It explains our whole range of data and reveals the key design parameters for optimizing the drag reduction values. Our approach promises a new route for microfluidics designs with pressure gradient reduced by orders of magnitude.

Helium Ion Microscopy for Reduced Spin Orbit Torque Switching Currents.

Spin orbit torque driven switching is a favorable way to manipulate nanoscale magnetic objects for both memory and wireless communication devices. The critical current required to switch from one magnetic state to another depends on the geometry and the intrinsic properties of the materials used, which are difficult to control locally. Here, we demonstrate how focused helium ion beam irradiation can modulate the local magnetic anisotropy of a Co thin film at the microscopic scale. Real-time in situ characterization using the anomalous Hall effect showed up to an order of magnitude reduction of the magnetic anisotropy under irradiation, with multilevel switching demonstrated. The result is that spin-switching current densities, down to 800 kA cm-2, can be achieved on predetermined areas of the film, without the need for lithography. The ability to vary critical currents spatially has implications not only for storage elements but also neuromorphic and probabilistic computing.

Current crowding issues on nanoscale planar organic transistors for spintronic applications.

The predominance of interface resistance makes current crowding ubiquitous in short channel organic electronics devices but its impact on spin transport has never been considered. We investigate electrochemically doped nanoscale PBTTT short channel devices and observe the smallest reported values of crowding lengths, found for sub-100 nm electrodes separation. These observed values are nevertheless exceeding the spin diffusion lengths reported in the literature. We discuss here how current crowding can be taken into account in the framework of the Fert-Jaffrès model of spin current propagation in heterostructures, and predict that the anticipated resulting values of magnetoresistance can be significantly reduced. Current crowding therefore impacts spin transport applications and interpretation of the results on spin valve devices.

The magnetoelectrochemical switch.

In the field of spintronics, the archetype solid-state two-terminal device is the spin valve, where the resistance is controlled by the magnetization configuration. We show here how this concept of spin-dependent switch can be extended to magnetic electrodes in solution, by magnetic control of their chemical environment. Appropriate nanoscale design allows a huge enhancement of the magnetic force field experienced by paramagnetic molecular species in solutions, which changes between repulsive and attractive on changing the electrodes' magnetic orientations. Specifically, the field gradient force created within a sub-100-nm-sized nanogap separating two magnetic electrodes can be reversed by changing the orientation of the electrodes' magnetization relative to the current flowing between the electrodes. This can result in a breaking or making of an electric nanocontact, with a change of resistance by a factor of up to 10(3). The results reveal how an external field can impact chemical equilibrium in the vicinity of nanoscale magnetic circuits.

Voltage-dependent magnetic phase transition in magneto-electric epitaxial Cr2O3 nanoclusters.

We observe, as a function of temperature, a second order magnetic phase transition in nanometric Cr2O3 clusters that are epitaxially embedded in an insulating MgO matrix. They are investigated through their tunnel magneto-resistance signature, the MgO layer being used as a tunnel barrier. We infer the small magnetic dipoles carried by the Cr2O3 clusters and provide evidence of a magnetic phase transition at low temperature in those clusters: they evolve from an anti ferromagnetic state, with zero net moment close to 0 K, to a weak ferromagnetic state that saturates above about 10 K. The influence of magneto-electric effects on the weak ferromagnetic phase is also striking: the second order transition temperature turns out to be linearly dependent on the applied electric field.

Nanoplatelets bridging a nanotrench: a new architecture for photodetectors with increased sensitivity.

Interparticle charge hopping severely limits the integration of colloidal nanocrystals films for optoelectronic device applications. We propose here to overcome this problem by using high aspect ratio interconnects made of wide electrodes separated by a few tens of namometers, a distance matching the size of a single nanoplatelet. The semiconducting CdSe/CdS nanoplatelet coupling with such electrodes allows an efficient electron-hole pair dissociation despite the large binding energy of the exciton, resulting in optimal photoconductance responsivity. We report the highest responsivity obtained so far for CdSe colloidal material with values reaching kA·W(-1), corresponding to eight decades of enhancement compared to usual micrometer-scaled architectures. In addition, a decrease of 1 order of magnitude of the current noise is observed, revealing the reduced influence of the surface traps on transport. The nanotrench geometry provides top access to ion gel electrolyte gating, allowing for a photoresponsive transistor with 10(4) on/off ratio. A simple analytical model reproduces the device behavior and underlines the key parameters related to its performance.

Distance dependence of the energy transfer rate from a single semiconductor nanostructure to graphene.

The near-field Coulomb interaction between a nanoemitter and a graphene monolayer results in strong Förster-type resonant energy transfer and subsequent fluorescence quenching. Here, we investigate the distance dependence of the energy transfer rate from individual, (i) zero-dimensional CdSe/CdS nanocrystals and (ii) two-dimensional CdSe/CdS/ZnS nanoplatelets to a graphene monolayer. For increasing distances d, the energy transfer rate from individual nanocrystals to graphene decays as 1/d(4). In contrast, the distance dependence of the energy transfer rate from a two-dimensional nanoplatelet to graphene deviates from a simple power law but is well described by a theoretical model, which considers a thermal distribution of free excitons in a two-dimensional quantum well. Our results show that accurate distance measurements can be performed at the single particle level using graphene-based molecular rulers and that energy transfer allows probing dimensionality effects at the nanoscale.

Self-assembly of strongly dipolar molecules on metal surfaces.

The role of dipole-dipole interactions in the self-assembly of dipolar organic molecules on surfaces is investigated. As a model system, strongly dipolar model molecules, p-benzoquinonemonoimine zwitterions (ZI) of type C6H2(⋯ NHR)2(⋯ O)2 on crystalline coinage metal surfaces were investigated with scanning tunneling microscopy and first principles calculations. Depending on the substrate, the molecules assemble into small clusters, nano gratings, and stripes, as well as in two-dimensional islands. The alignment of the molecular dipoles in those assemblies only rarely assumes the lowest electrostatic energy configuration. Based on calculations of the electrostatic energy for various experimentally observed molecular arrangements and under consideration of computed dipole moments of adsorbed molecules, the electrostatic energy minimization is ruled out as the driving force in the self-assembly. The structures observed are mainly the result of a competition between chemical interactions and substrate effects. The substrate's role in the self-assembly is to (i) reduce and realign the molecular dipole through charge donation and back donation involving both the molecular HOMO and LUMO, (ii) dictate the epitaxial orientation of the adsorbates, specifically so on Cu(111), and (iii) inhibit attractive forces between neighboring chains in the system ZI/Cu(111), which results in regularly spaced molecular gratings.

Janus microspheres for visual assessment of molecular interconnects.

A rigid S-functionalized metalloligand is used to pair Janus Au-coated silica microspheres and the resulting assemblies are assessed with optical microscopy. New Pd complexes provide stable molecular interconnects, and the metal centre controls the structure of the linker and provides the desired rigidity, by virtue of its well-established coordination chemistry.

Graphene Magnetoresistance Control by Photoferroelectric Substrate.

Ultralow dimensionality of 2D layers magnifies their sensitivity to adjacent charges enabling even postprocessing electric control of multifunctional structures. However, functionalizing 2D layers remains an important challenge for on-demand device-property exploitation. Here we report that an electrical and even fully optical way to control and write modifications to the magnetoresistive response of CVD-deposited graphene is achievable through the electrostatics of the photoferroelectric substrate. For electrical control, the ferroelectric polarization switch modifies graphene magnetoresistance by 67% due to a Fermi level shift with related modification in charge mobility. A similar function is also attained entirely by bandgap light due to the substrate photovoltaic effect. Moreover, an all-optical way to imprint and recover graphene magnetoresistance by light is reported as well as magnetic control of graphene transconductance. These findings extend photoferroelectric control in 2D structures to magnetic dimensions and advance wireless operation for sensors and field-effect transistors.

Single Wavelength Operating Neuromorphic Device Based on a Graphene-Ferroelectric Transistor.

As global data generation continues to rise, there is an increasing demand for revolutionary in-memory computing methodologies and efficient machine learning solutions. Despite recent progress in electrical and electro-optical simulations of machine learning devices, the all-optical nonthermal function remains challenging, with single wavelength operation still elusive. Here we report on an optical and monochromatic way of neuromorphic signal processing for brain-inspired functions, eliminating the need for electrical pulses. Multilevel synaptic potentiation-depression cycles are successfully achieved optically by leveraging photovoltaic charge generation and polarization within the photoferroelectric substrate interfaced with the graphene sensor. Furthermore, the demonstrated low-power prototype device is able to reproduce exact signal profile of brain tissues yet with more than 2 orders of magnitude faster response. The reported properties should trigger all-optical and low power artificial neuromorphic development based on photoferroelectric structures.

Photoferroelectric All-van-der-Waals Heterostructure for Multimode Neuromorphic Ferroelectric Transistors.

Interface-driven effects in ferroelectric van der Waals (vdW) heterostructures provide fresh opportunities in the search for alternative device architectures toward overcoming the von Neumann bottleneck. However, their implementation is still in its infancy, mostly by electrical control. It is of utmost interest to develop strategies for additional optical and multistate control in the quest for novel neuromorphic architectures. Here, we demonstrate the electrical and optical control of the ferroelectric polarization states of ferroelectric field effect transistors (FeFET). The FeFETs, fully made of ReS2/hBN/CuInP2S6 vdW materials, achieve an on/off ratio exceeding 107, a hysteresis memory window up to 7 V wide, and multiple remanent states with a lifetime exceeding 103 s. Moreover, the ferroelectric polarization of the CuInP2S6 (CIPS) layer can be controlled by photoexciting the vdW heterostructure. We perform wavelength-dependent studies, which allow for identifying two mechanisms at play in the optical control of the polarization: band-to-band photocarrier generation into the 2D semiconductor ReS2 and photovoltaic voltage into the 2D ferroelectric CIPS. Finally, heterosynaptic plasticity is demonstrated by operating our FeFET in three different synaptic modes: electrically stimulated, optically stimulated, and optically assisted synapse. Key synaptic functionalities are emulated including electrical long-term plasticity, optoelectrical plasticity, optical potentiation, and spike rate-dependent plasticity. The simulated artificial neural networks demonstrate an excellent accuracy level of 91% close to ideal-model synapses. These results provide a fresh background for future research on photoferroelectric vdW systems and put ferroelectric vdW heterostructures on the roadmap for the next neuromorphic computing architectures.

Reconfigurable Multifunctional van der Waals Ferroelectric Devices and Logic Circuits.

Emerging reconfigurable devices are fast gaining popularity in the search for next-generation computing hardware, while ferroelectric engineering of the doping state in semiconductor materials has the potential to offer alternatives to traditional von-Neumann architecture. In this work, we combine these concepts and demonstrate the suitability of reconfigurable ferroelectric field-effect transistors (Re-FeFETs) for designing nonvolatile reconfigurable logic-in-memory circuits with multifunctional capabilities. Modulation of the energy landscape within a homojunction of a 2D tungsten diselenide (WSe2) layer is achieved by independently controlling two split-gate electrodes made of a ferroelectric 2D copper indium thiophosphate (CuInP2S6) layer. Controlling the state encoded in the program gate enables switching between p, n, and ambipolar FeFET operating modes. The transistors exhibit on-off ratios exceeding 106 and hysteresis windows of up to 10 V width. The homojunction can change from Ohmic-like to diode behavior with a large rectification ratio of 104. When programmed in the diode mode, the large built-in p-n junction electric field enables efficient separation of photogenerated carriers, making the device attractive for energy-harvesting applications. The implementation of the Re-FeFET for reconfigurable logic functions shows how a circuit can be reconfigured to emulate either polymorphic ferroelectric NAND/AND logic-in-memory or electronic XNOR logic with a long retention time exceeding 104 s. We also illustrate how a circuit design made of just two Re-FeFETs exhibits high logic expressivity with reconfigurability at runtime to implement several key nonvolatile 2-input logic functions. Moreover, the Re-FeFET circuit demonstrates high compactness, with an up to 80% reduction in transistor count compared to standard CMOS design. The 2D van de Waals Re-FeFET devices therefore exhibit promising potential for both More-than-Moore and beyond-Moore future of electronics, in particular for an energy-efficient implementation of in-memory computing and machine learning hardware, due to their multifunctionality and design compactness.

Altering the static dipole on surfaces through chemistry: molecular films of zwitterionic quinonoids.

The adsorption of molecular films made of small molecules with a large intrinsic electrical dipole has been explored. The data indicate that such dipolar molecules may be used for altering the interface dipole screening at the metal electrode interface in organic electronics. More specifically, we have investigated the surface electronic spectroscopic properties of zwitterionic molecules containing 12π electrons of the p-benzoquinonemonoimine type, C(6)H(2)(···NHR)(2)(···O)(2)(R = H (1), n-C(4)H(9) (2), C(3)H(6)-S-CH(3) (3), C(3)H(6)-O-CH(3) (4), CH(2)-C(6)H(5) (5)), adsorbed on Au. These molecules are stable zwitterions by virtue of the meta positions occupied by the nitrogen and oxygen substituents on the central ring, respectively. The structures of 2-4 have been determined by single crystal X-ray diffraction and indicate that in these molecules, two chemically connected but electronically not conjugated 6π electron subunits are present, which explains their strong dipolar character. We systematically observed that homogeneous molecular films with thickness as small as 1 nm were formed on Au, which fully cover the surface, even for a variety of R substituents. Preferential adsorption toward the patterned gold areas on SiO(2) substrates was found with 4. Optimum self-assembling of 2 and 5 results in ordered close packed films, which exhibit n-type character, based on the position of the Fermi level close to the conduction band minimum, suggesting high conductivity properties. This new type of self-assembled molecular films offers interesting possibilities for engineering metal-organic interfaces, of critical importance for organic electronics.

Co-tunneling enhancement of the electrical response of nanoparticle networks.

A co-tunneling charge-transfer process dominates the electrical properties of a nanometer-sized "slice" in a nanoparticle network, which results in universal scaling of the conductance with temperature and bias voltage, as well as enhanced spintronics properties. By designing two large (10 µm) electrodes with short (60 nm) separation, access is obtained to transport dominated by charge transfer involving "nanoslices" made of three nanoparticles only. Magnetic iron oxide nanoparticle networks exhibit a magnetoresistance ratio that is not reachable by tunneling or hopping processes, thereby illustrating how such a size-matched planar device with dominant co-tunneling charge-transfer process is optimal for realizing multifunctional devices with enhanced change of conductance under external stimulus.

Approaching an organic semimetal: Electron pockets at the Fermi level for a p-benzoquinonemonoimine zwitterion.

There is compelling evidence of electron pockets, at the Fermi level, in the band structure for an organic zwitterion molecule of the p-benzoquinonemonoimine type. The electronic structure of the zwitterion molecular film has a definite, although small, density of states evident at the Fermi level as well as a nonzero inner potential and thus is very different from a true insulator. In spite of a small Brillouin zone, significant band width is observed in the intermolecular band dispersion. The results demonstrate that Bloch's theorem applies to the wave vector dependence of the electronic band structure formed from the molecular orbitals of adjacent molecules in a molecular thin film of a p-benzoquinonemonoimine type zwitterion.

Dynamics of Spin Crossover Molecular Complexes.

We review the current understanding of the time scale and mechanisms associated with the change in spin state in transition metal-based spin crossover (SCO) molecular complexes. Most time resolved experiments, performed by optical techniques, rely on the intrinsic light-induced switching properties of this class of materials. The optically driven spin state transition can be mediated by a rich interplay of complexities including intermediate states in the spin state transition process, as well as intermolecular interactions, temperature, and strain. We emphasize here that the size reduction down to the nanoscale is essential for designing SCO systems that switch quickly as well as possibly retaining the memory of the light-driven state. We argue that SCO nano-sized systems are the key to device applications where the "write" speed is an important criterion.

Room temperature optoelectronic devices operating with spin crossover nanoparticles.

Molecular systems can exhibit multi-stimuli switching of their properties, with spin crossover materials having unique magnetic transition triggered by temperature and light, among others. Light-induced room temperature operation is however elusive, as optical changes between metastable spin states require cryogenic temperatures. Furthermore, electrical detection is hampered by the intrinsic low conductivity properties of these materials. We show here how a graphene underlayer reveals the light-induced heating that triggers a spin transition, paving the way for using these molecules for room temperature optoelectronic applications.

The interface bonding and orientation of a quinonoid zwitterion.

We have investigated the interaction and orientation of a strongly dipolar zwitterionic p-benzoquinonemonoimine-type molecule, with a large intrinsic dipole of 10 Debye, on both conducting and on polar insulating substrates. Specifically, we deposited (6Z)-4-(butylamino)-6-(butyliminio)-3-oxocyclohexa-1,4-dien-1-olate C(6)H(2)([horiz bar, triple dot above]NHR)(2)([horiz bar, triple dot above]O)(2) where R = n-C(4)H(9), on both gold and ferroelectric lithium niobate surfaces. An influence of both transient and static electric dipoles on the zwitterionic adsorbate has been observed. For adsorption on gold, we find that the molecule bonds to the surface through the nitrogen atoms, forming films that remain fairly uniform down to thicknesses in the 1 nm range. Adsorption of this zwitterionic compound from solution on insulating, periodically poled ferroelectric lithium niobate substrates, showed preferential adsorption on one type of ferroelectric domain. For both gold and the lithium niobate substrates, the zwitterion adopts a preferential orientation with the plane of its "C(6) core" along the surface normal. This simplified geometry of strong dipole alignment provides a symmetry simplification allowing better identification of the vibrational modes responsible for Frank-Condon scattering revealed in the fine spectroscopic signature in the photoemission spectrum.