Ultralow contact resistance between semimetal and monolayer semiconductors.

Advanced beyond-silicon electronic technology requires both channel materials and also ultralow-resistance contacts to be discovered1,2. Atomically thin two-dimensional semiconductors have great potential for realizing high-performance electronic devices1,3. However, owing to metal-induced gap states (MIGS)4-7, energy barriers at the metal-semiconductor interface-which fundamentally lead to high contact resistance and poor current-delivery capability-have constrained the improvement of two-dimensional semiconductor transistors so far2,8,9. Here we report ohmic contact between semimetallic bismuth and semiconducting monolayer transition metal dichalcogenides (TMDs) where the MIGS are sufficiently suppressed and degenerate states in the TMD are spontaneously formed in contact with bismuth. Through this approach, we achieve zero Schottky barrier height, a contact resistance of 123 ohm micrometres and an on-state current density of 1,135 microamps per micrometre on monolayer MoS2; these two values are, to the best of our knowledge, the lowest and highest yet recorded, respectively. We also demonstrate that excellent ohmic contacts can be formed on various monolayer semiconductors, including MoS2, WS2 and WSe2. Our reported contact resistances are a substantial improvement for two-dimensional semiconductors, and approach the quantum limit. This technology unveils the potential of high-performance monolayer transistors that are on par with state-of-the-art three-dimensional semiconductors, enabling further device downscaling and extending Moore's law.

Growth Optimization and Device Integration of Narrow-Bandgap Graphene Nanoribbons.

The electronic, optical, and magnetic properties of graphene nanoribbons (GNRs) can be engineered by controlling their edge structure and width with atomic precision through bottom-up fabrication based on molecular precursors. This approach offers a unique platform for all-carbon electronic devices but requires careful optimization of the growth conditions to match structural requirements for successful device integration, with GNR length being the most critical parameter. In this work, the growth, characterization, and device integration of 5-atom wide armchair GNRs (5-AGNRs) are studied, which are expected to have an optimal bandgap as active material in switching devices. 5-AGNRs are obtained via on-surface synthesis under ultrahigh vacuum conditions from Br- and I-substituted precursors. It is shown that the use of I-substituted precursors and the optimization of the initial precursor coverage quintupled the average 5-AGNR length. This significant length increase allowed the integration of 5-AGNRs into devices and the realization of the first field-effect transistor based on narrow bandgap AGNRs that shows switching behavior at room temperature. The study highlights that the optimized growth protocols can successfully bridge between the sub-nanometer scale, where atomic precision is needed to control the electronic properties, and the scale of tens of nanometers relevant for successful device integration of GNRs.

Synergetic Bottom-Up Synthesis of Graphene Nanoribbons by Matrix-Assisted Direct Transfer.

The scope of graphene nanoribbon (GNR) structures accessible through bottom-up approaches is defined by the intrinsic limitations of either all-on-surface or all-solution-based synthesis. Here, we report a hybrid bottom-up synthesis of GNRs based on a Matrix-Assisted Direct (MAD) transfer technique that successfully leverages technical advantages inherent to both solution-based and on-surface synthesis while sidestepping their drawbacks. Critical structural parameters tightly controlled in solution-based polymerization reactions can seamlessly be translated into the structure of the corresponding GNRs. The transformative potential of the synergetic bottom-up approaches facilitated by the MAD transfer techniques is highlighted by the synthesis of chevron-type GNRs (cGNRs) featuring narrow length distributions and a nitrogen core-doped armchair GNR (N4-7-ANGR) that remains inaccessible using either a solution-based or an on-surface bottom-up approach alone.

Toward Intrinsic Ferroelectric Switching in Multiferroic BiFeO_{3}.

Using pulsed ferroelectric measurements, we probe switching dynamics in multiferroic BiFeO_{3}, revealing low-ns switching times and a clear pathway to sub-ns switching. Our data is well described by a nucleation and growth model, which accounts for the various timescales in the switching process, namely (1) the ferroelectric polarization switching (bound-charge) dynamics and (2) the RC-limited movement of free charge in the circuit. Our model shows good agreement with observed data and begins to bridge the gap between experiment and theory, indicating pathways to study ferroelectric switching on intrinsic timescales.

Low-Temperature Side Contact to Carbon Nanotube Transistors: Resistance Distributions Down to 10 nm Contact Length.

Carbon nanotube field-effect transistors (CNFETs) promise to improve the energy efficiency, speed, and transistor density of very large scale integration circuits owing to the intrinsic thin channel body and excellent charge transport properties of carbon nanotubes. Low-temperature fabrication (e.g., <400 °C) is a key enabler for the monolithic three-dimensional (3D) integration of CNFET digital logic into a device technology platform that overcomes memory bandwidth bottlenecks for data-abundant applications such as big-data analytics and machine learning. However, high contact resistance for short CNFET contacts has been a major roadblock to establishing CNFETs as a viable technology because the contact resistance, in series with the channel resistance, reduces the on-state current of CNFETs. Additionally, the variation in contact resistance remains unstudied for short contacts and will further degrade the energy efficiency and speed of CNFET circuits. In this work, we investigate by experiments the contact resistance and statistical variation of room-temperature fabricated CNFET contacts down to 10 nm contact lengths. These CNFET contacts are ∼15 nm shorter than the state-of-the-art Si CMOS "7 nm node" contact length, allowing for multiple generations of future scaling of the transistor-contacted gate pitch. For the 10 nm contacts, we report contact resistance values down to 6.5 kΩ per source/drain contact for a single carbon nanotube (CNT) with a median contact resistance of 18.2 kΩ. The 10 nm contacts reduce the CNFET current by as little as 13% at VDS = 0.7 V compared with the best reported 200 nm contacts to date, corroborated by results in this work. Our analysis of RC from 232 single-CNT CNFETs between the long-contact (e.g., 200 nm) and short-contact (e.g., 10 nm) regimes quantifies the resistance variation and projects the impact on CNFET current variability versus the number of CNT in the transistor. The resistance distribution reveals contact-length-dependent RC variations become significant below 20 nm contact length. However, a larger source of CNFET resistance variation is apparent at all contact lengths used in this work. To further investigate the origins of this contact-length-independent resistance variation, we analyze the variation of RC in arrays of identical CNFETs along a single CNT of constant diameter and observe the random occurrence of high  RC, even on correlated CNFETs.

Influence of Nonuniform Micron-Scale Strain Distributions on the Electrical Reorientation of Magnetic Microstructures in a Composite Multiferroic Heterostructure.

Composite multiferroic systems, consisting of a piezoelectric substrate coupled with a ferromagnetic thin film, are of great interest from a technological point of view because they offer a path toward the development of ultralow power magnetoelectric devices. The key aspect of those systems is the possibility to control magnetization via an electric field, relying on the magneto-elastic coupling at the interface between the piezoelectric and the ferromagnetic components. Accordingly, a direct measurement of both the electrically induced magnetic behavior and of the piezo-strain driving such behavior is crucial for better understanding and further developing these materials systems. In this work, we measure and characterize the micron-scale strain and magnetic response, as a function of an applied electric field, in a composite multiferroic system composed of 1 and 2 µm squares of Ni fabricated on a prepoled [Pb(Mg1/3Nb2/3)O3]0.69-[PbTiO3]0.31 (PMN-PT) single crystal substrate by X-ray microdiffraction and X-ray photoemission electron microscopy, respectively. These two complementary measurements of the same area on the sample indicate the presence of a nonuniform strain which strongly influences the reorientation of the magnetic state within identical Ni microstructures along the surface of the sample. Micromagnetic simulations confirm these experimental observations. This study emphasizes the critical importance of surface and interface engineering on the micron-scale in composite multiferroic structures and introduces a robust method to characterize future devices on these length scales.

Switching of perpendicularly polarized nanomagnets with spin orbit torque without an external magnetic field by engineering a tilted anisotropy.

Spin orbit torque (SOT) provides an efficient way to significantly reduce the current required for switching nanomagnets. However, SOT generated by an in-plane current cannot deterministically switch a perpendicularly polarized magnet due to symmetry reasons. On the other hand, perpendicularly polarized magnets are preferred over in-plane magnets for high-density data storage applications due to their significantly larger thermal stability in ultrascaled dimensions. Here, we show that it is possible to switch a perpendicularly polarized magnet by SOT without needing an external magnetic field. This is accomplished by engineering an anisotropy in the magnets such that the magnetic easy axis slightly tilts away from the direction, normal to the film plane. Such a tilted anisotropy breaks the symmetry of the problem and makes it possible to switch the magnet deterministically. Using a simple Ta/CoFeB/MgO/Ta heterostructure, we demonstrate reversible switching of the magnetization by reversing the polarity of the applied current. This demonstration presents a previously unidentified approach for controlling nanomagnets with SOT.

Interface Engineering of Domain Structures in BiFeO3 Thin Films.

A wealth of fascinating phenomena have been discovered at the BiFeO3 domain walls, examples such as domain wall conductivity, photovoltaic effects, and magnetoelectric coupling. Thus, the ability to precisely control the domain structures and accurately study their switching behaviors is critical to realize the next generation of novel devices based on domain wall functionalities. In this work, the introduction of a dielectric layer leads to the tunability of the depolarization field both in the multilayers and superlattices, which provides a novel approach to control the domain patterns of BiFeO3 films. Moreover, we are able to study the switching behavior of the first time obtained periodic 109° stripe domains with a thick bottom electrode. Besides, the precise controlling of pure 71° and 109° periodic stripe domain walls enable us to make a clear demonstration that the exchange bias in the ferromagnet/BiFeO3 system originates from 109° domain walls. Our findings provide future directions to study the room temperature electric field control of exchange bias and open a new pathway to explore the room temperature multiferroic vortices in the BiFeO3 system.

Imaging of voltage-controlled switching of magnetization in highly magnetostrictive epitaxial Fe-Ga microstructures.

The magnetoelectric behavior of epitaxial Fe-Ga microstructures on top of a (001)-oriented PMN-PT piezoelectric substrate is imaged with magnetic X-ray microscopy. Additionally, the micron-scale strain distribution in PMN-PT is characterized by X-ray microdiffraction and examined with respect to the results of the Fe-Ga magnetoelectric switching. The magnetic reorientation of Fe-Ga is found to be strongly correlated with size, shape, and crystallographic orientation of the microstructures. In the case of square-shaped structures, size dictates the influence of the strain distribution on both the initialization of the ground state and on the magnetic reorientation during application of voltage. On the other hand, elliptical microstructures demonstrate completely different magnetic responses depending on the relative orientation of their long axis with respect to the crystallographic directions of the PMN-PT. This study demonstrates that engineering the behavior of highly magnetostrictive epitaxial microdevices is possible. It further elucidates that voltage-induced actuation can be largely tuned to achieve the desired type of magnetic switching ranging from vortex circulation reversal, domain wall motion, to a large rotation of magnetization. Because of the outstanding properties of the investigated material system, the reported findings are expected to be of great interest for the realization of next-generation energy-efficient magnetic memory and logic devices.

Plasmonic near-field probes: a comparison of the campanile geometry with other sharp tips.

Efficient conversion of photonic to plasmonic energy is important for nano-optical applications, particularly imaging and spectroscopy. Recently a new generation of photonic/plasmonic transducers, the 'campanile' probes, has been developed that overcomes many shortcomings of previous near-field probes by efficiently merging broadband field enhancement with bidirectional coupling of far- to near-field electromagnetic modes. In this work we compare the properties of the campanile structure with those of current NSOM tips using finite element simulations. Field confinement, enhancement, and polarization near the apex of the probe are evaluated relative to local fields created by conical tapered tips in vacuum and in tip-substrate gap mode. We show that the campanile design has similar field enhancement and bandwidth capabilities as those of ultra-sharp metallized tips, but without the substrate and sample restrictions inherent in the tip-surface gap mode operation often required by those tips. In addition, we show for the first time that this campanile probe structure also significantly enhances the radiative rate of any dipole emitter located near the probe apex, quantifying the enhanced decay rate and demonstrating that over 90% of the light radiated by the emitter is "captured" by this probe. This is equivalent to collecting the light from a solid angle of ~3.6 pi. These advantages are crucial for performing techniques such as Raman and IR spectroscopy, white-light nano-ellipsometry and ultrafast pump-probe studies at the nanoscale.

Picosecond spin-orbit torque-induced coherent magnetization switching in a ferromagnet.

Electrically controllable nonvolatile magnetic memories show great potential for the replacement of conventional semiconductor-based memory technologies. Here, we experimentally demonstrate ultrafast spin-orbit torque (SOT)-induced coherent magnetization switching dynamics in a ferromagnet. We use an ultrafast photoconducting switch and a coplanar strip line to generate and guide a ~9-picosecond electrical pulse into a heavy metal/ferromagnet multilayer to induce ultrafast SOT. We then use magneto-optical probing to investigate the magnetization dynamics with sub-picosecond resolution. Ultrafast heating by the approximately 9 picosecond current pulse induces a thermal anisotropy torque which, in combination with the damping-like torque, coherently rotates the magnetization to obtain zero-crossing of magnetization in ~70 picoseconds. A macro-magnetic simulation coupled with an ultrafast heating model agrees well with the experiment and suggests coherent magnetization switching without any incubation delay on an unprecedented time scale. Our work proposes a unique magnetization switching mechanism toward markedly increasing the writing speed of SOT magnetic random-access memory devices.

Radiation engineering of optical antennas for maximum field enhancement.

Optical antennas have generated much interest in recent years due to their ability to focus optical energy beyond the diffraction limit, benefiting a broad range of applications such as sensitive photodetection, magnetic storage, and surface-enhanced Raman spectroscopy. To achieve the maximum field enhancement for an optical antenna, parameters such as the antenna dimensions, loading conditions, and coupling efficiency have been previously studied. Here, we present a framework, based on coupled-mode theory, to achieve maximum field enhancement in optical antennas through optimization of optical antennas' radiation characteristics. We demonstrate that the optimum condition is achieved when the radiation quality factor (Q(rad)) of optical antennas is matched to their absorption quality factor (Q(abs)). We achieve this condition experimentally by fabricating the optical antennas on a dielectric (SiO(2)) coated ground plane (metal substrate) and controlling the antenna radiation through optimizing the dielectric thickness. The dielectric thickness at which the matching condition occurs is approximately half of the quarter-wavelength thickness, typically used to achieve constructive interference, and leads to ∼20% higher field enhancement relative to a quarter-wavelength thick dielectric layer.

Hyperspectral nanoscale imaging on dielectric substrates with coaxial optical antenna scan probes.

We have demonstrated hyperspectral tip-enhanced Raman imaging on dielectric substrates using linearly polarized light and nanofabricated coaxial antenna tips. A full Raman spectrum was acquired at each pixel of a 256 by 256 pixel contact-mode atomic force microscope image of carbon nanotubes grown on a fused silica microscope coverslip, allowing D and G mode intensity and D-mode peak shifts to be measured with ∼20 nm spatial resolution. Tip enhancement was sufficient to acquire useful Raman spectra in 50-100 ms. Coaxial scan probes combine the efficiency and enhanced, ultralocalized optical fields of plasmonically coupled antennae with the superior topographical imaging properties of sharp metal tips. The yield of the coaxial tip fabrication process is close to 100%, and the tips are sufficiently durable to support hours of contact-mode force microscope imaging. Our coaxial probes avoid the limitations associated with the "gap-mode" imaging geometry used in most tip-enhanced Raman studies to date, where a sharp metal tip is held ∼1 nm above a metallic substrate with the sample located in the gap.

Accelerated Ultrafast Magnetization Dynamics at Graphene/CoGd Interfaces.

Atomically thin graphene layers can act as a spin-sink material when adjacent to a nanoscale magnetic surface. The enhancement in the extrinsic spin-orbit coupling (SOC) strength of graphene plays an important role in absorbing the spin angular momentum injected from the magnetic surface after perturbation with an external stimulus. As a result, the dynamics of the excited spin system is modified within the magnetic layer. In this paper, we demonstrate the modulation of ultrafast magnetization dynamics at graphene/ferrimagnet interfaces using the time-resolved magneto-optical Kerr effect (TRMOKE) technique. Magnetically modified interfaces with a systematic increase in the number of graphene layers coupled with the 10 nm-thick Co74Gd26 layer are studied. We find that the variation in the dynamical parameters, i.e., ultrafast demagnetization time, remagnetization times, decay time, effective damping, precessional frequency, etc., observed at different time scales is interconnected. The demagnetization time and decay time for the ferrimagnet become approximately two times faster than the corresponding intrinsic values. We found a possible correlation between the demagnetization time and damping. The effect is more pronounced for the interfaces with monolayer graphene and graphite. The spin-mixing conductance is found to be approximately 0.8 × 1015 cm-2. The effect of SOC, pure spin current, the appearance of structural defects, and thermal properties at the graphene/ferrimagnet interface are responsible for the modifications of several dynamical parameters. This work demonstrates some important properties of the graphene/ferrimagnet interface which may unravel the possibilities of designing spintronic devices with elevated performance in the future.

Exploring the thermodynamic limits of computation in integrated systems: magnetic memory, nanomagnetic logic, and the Landauer limit.

Nanomagnetic memory and logic circuits are attractive integrated platforms for studying the fundamental thermodynamic limits of computation. Using the stochastic Landau-Lifshitz-Gilbert equation, we show by direct calculation that the amount of energy dissipated during nanomagnet erasure approaches Landauer's thermodynamic limit of kTln(2) with high precision when the external magnetic fields are applied slowly. In addition, we find that nanomagnet systems behave according to generalized formulations of Landauer's principle that hold for small systems and generic logic operations. In all cases, the results are independent of the anisotropy energy of the nanomagnet. Lastly, we apply our computational approach to a nanomagnet majority logic gate, where we find that dissipationless, reversible computation can be achieved when the magnetic fields are applied in the appropriate order.

Formation of bandgap and subbands in graphene nanomeshes with sub-10 nm ribbon width fabricated via nanoimprint lithography.

We fabricated hexagonal graphene nanomeshes (GNMs) with sub-10 nm ribbon width. The fabrication combines nanoimprint lithography, block-copolymer self-assembly for high-resolution nanoimprint template patterning, and electrostatic printing of graphene. Graphene field-effect transistors (GFETs) made from GNMs exhibit very different electronic characteristics in comparison with unpatterned GFETs even at room temperature. We observed multiplateaus in the drain current-gate voltage dependence as well as an enhancement of ON/OFF current ratio with reduction of the average ribbon width of GNMs. These effects are attributed to the formation of electronic subbands and a bandgap in GNMs. Such mesoscopic graphene structures and the nanofabrication methods could be employed to construct future electronic devices based on graphene superlattices.

Direct chemical vapor deposition of graphene on dielectric surfaces.

Direct deposition of graphene on various dielectric substrates is demonstrated using a single-step chemical vapor deposition process. Single-layer graphene is formed through surface catalytic decomposition of hydrocarbon precursors on thin copper films predeposited on dielectric substrates. The copper films dewet and evaporate during or immediately after graphene growth, resulting in graphene deposition directly on the bare dielectric substrates. Scanning Raman mapping and spectroscopy, scanning electron microscopy, and atomic force microscopy confirm the presence of continuous graphene layers on tens of micrometer square metal-free areas. The revealed growth mechanism opens new opportunities for deposition of higher quality graphene films on dielectric materials.

Transfer-Free Synthesis of Atomically Precise Graphene Nanoribbons on Insulating Substrates.

The rational bottom-up synthesis of graphene nanoribbons (GNRs) provides atomically precise control of widths and edges that give rise to a wide range of electronic properties promising for electronic devices such as field-effect transistors (FETs). Since the bottom-up synthesis commonly takes place on catalytic metallic surfaces, the integration of GNRs into such devices requires their transfer onto insulating substrates, which remains one of the bottlenecks in the development of GNR-based electronics. Herein, we report on a method for the transfer-free placement of GNRs on insulators. This involves growing GNRs on a gold film deposited onto an insulating layer followed by gentle wet etching of the gold, which leaves the nanoribbons to settle in place on the underlying insulating substrate. Scanning tunneling microscopy and Raman spectroscopy confirm that atomically precise GNRs of high density uniformly grow on the gold films deposited onto SiO2/Si substrates and remain structurally intact after the etching process. We have also demonstrated transfer-free fabrication of ultrashort channel GNR FETs using this process. A very important aspect of the present work is that the method can scale up well to 12 in. wafers, which is extremely difficult for previous techniques. Our work here thus represents an important step toward large-scale integration of GNRs into electronic devices.

Single-Domain Multiferroic Array-Addressable Terfenol-D (SMArT) Micromagnets for Programmable Single-Cell Capture and Release.

Programming magnetic fields with microscale control can enable automation at the scale of single cells ≈10 µm. Most magnetic materials provide a consistent magnetic field over time but the direction or field strength at the microscale is not easily modulated. However, magnetostrictive materials, when coupled with ferroelectric material (i.e., strain-mediated multiferroics), can undergo magnetization reorientation due to voltage-induced strain, promising refined control of magnetization at the micrometer-scale. This work demonstrates the largest single-domain microstructures (20 µm) of Terfenol-D (Tb0.3 Dy0.7 Fe1.92 ), a material that has the highest magnetostrictive strain of any known soft magnetoelastic material. These Terfenol-D microstructures enable controlled localization of magnetic beads with sub-micrometer precision. Magnetically labeled cells are captured by the field gradients generated from the single-domain microstructures without an external magnetic field. The magnetic state on these microstructures is switched through voltage-induced strain, as a result of the strain-mediated converse magnetoelectric effect, to release individual cells using a multiferroic approach. These electronically addressable micromagnets pave the way for parallelized multiferroics-based single-cell sorting under digital control for biotechnology applications.

Gold nanoparticle self-similar chain structure organized by DNA origami.

Here we demonstrate Au nanoparticle self-similar chain structure organized by triangle DNA origami with well-controlled orientation and <10 nm spacing. We show for the first time that a large DNA complex (origami) and multiple AuNP conjugates can be well-assembled and purified with reliable yields. The assembled structure could be used to generate high local-field enhancement. The same method can be used to precisely localize multiple components on a DNA template for potential applications in nanophotonic, nanomagnetic, and nanoelectronic devices.